HOT ROLLED STEEL SHEET

Description

FIELD

The present invention relates to a hot rolled steel sheet.

BACKGROUND

In recent years, in the automotive industry, lighter weight of car bodies has been sought from the viewpoint of improvement of fuel efficiency. To achieve both lighter weight of car bodies and collision safety, increasing the strength of the steel sheet used is one effective method. A high strength steel sheet is being developed from this background. On the other hand, along with higher strength, the workability of steel sheet generally falls. For this reason, in development of a high strength steel sheet, it is important to secure a certain level or more of workability while raising the strength.

In relation to this, for example, PTL 1 describes a high strength hot rolled steel sheet having a predetermined chemical composition, having a Tief expressed by [Ti]−48/14×[N]−48/32×[S] of 0.01 to 0.30%, containing crystal grains with an orientation difference of grain boundaries of adjoining crystal grains of 150 or more, which crystal grains have an average of orientation differences in the crystal grains of 0 to 0.5°, in an area ratio of 50% or more, further having a total of martensite, tempered martensite, and retained austenite of an area percentage of 2% or more and 10% or less, 40% or more mass % of Ti of the Tief being present as Ti carbides, the mass of the Ti carbides with a circle equivalent diameter of 7 nm or more and 20 nm or less being 50% or more of the mass of all Ti carbides. Further, PTL 1 teaches that crystal grains with an average of orientation differences in the crystal grains of 0 to 0.5° are high in ductility and further that Ti carbides cause precipitation strengthening, and therefore by securing such crystal grains in an area ratio of 50% or more, it is possible to maintain the tensile strength (TS) at 540 MPa or more while improving the ductility.

PTL 2 describes a high strength hot rolled steel sheet having a predetermined composition of constituents and having a microstructure having a volume ratio of a total of the ferrite phase and bainite phase in the structure as a whole of 95% or more, having a volume ratio of the ferrite phase in the structure as a whole of 50 to 90%, having precipitates of a size of less than 20 nm including 650 to 1100 ppm of Ti form in the ferrite phase, and a ΔHv of the bainite phase (difference of maximum value and minimum value of the Vickers hardness of the bainite phase at 30 locations measured at a position of a sheet thickness ¼ of the sheet thickness cross-section along the rolling direction) of 150 or less. Further, PTL 2 teaches that if making the microstructure one comprised mainly of a ferrite phase and bainite phase, having precipitates of a size of less than 20 nm including 650 to 1100 ppm of Ti form in the ferrite phase, and having an ΔHv of the bainite phase of 150 or less, it is possible to secure TS of 780 MPa or more and achieve both excellent stretch flangeability (hole expandability) and impact resistance.

CITATIONS LIST

Patent Literature

• • [PTL 1] Japanese Unexamined Patent Publication No. 2016-204690
  • [PTL 2] Japanese Unexamined Patent Publication No. 2011-068945

SUMMARY

Technical Problem

In regard to the hole expandability and impact resistance described in PTL 2, for example, if the hole expandability falls, in the suspension parts of an automobile, etc., sometimes it is not possible to form the steel sheet into the desired shapes of the parts. Further, in parts where impact resistance is sought, if receiving an impact over the yield strength, plastic deformation occurs, and therefore from the viewpoint of securing collision safety of automobiles, improvement of not only the tensile strength, but also the yield strength is being sought. For this reason, raising the yield ratio of the ratio of the yield strength and tensile strength is being sought.

Therefore, the present invention has as its object to provide a hot rolled steel sheet which is high in strength and has a high hole expandability and yield ratio.

Solution to Problem

The inventors engaged in studies to achieve the above object while focusing in particular on the microstructure of a hot rolled steel sheet. As a result, the inventors discovered that by making the microstructure of a hot rolled steel sheet having a predetermined chemical composition include mainly ferrite, bainite, and martensite and controlling the bainite percentage to a relatively high range and utilizing dislocation strengthening, it is possible to remarkably raise the strength of a hot rolled steel sheet and that, further, by establishing the presence in the ferrite of TiC precipitates having a suitable diameter in a predetermined number density, it is possible to strengthen the ferrite by precipitates to make the hot rolled steel sheet further higher in strength and to reduce the hardness difference of ferrite and bainite to raise the hole expandability and yield ratio, and thereby completed the present invention.

The present invention able to achieve this object is as follows:

• • (1) A hot rolled steel sheet having a chemical composition comprising, by mass %,
    • C: 0.03 to 0.10%,
    • Si: 0.010 to 0.100%,
    • Mn: 0.50 to 3.00%,
    • Ti: 0.05 to 0.20%,
    • Al: 0.20 to 0.40%,
    • P: 0.100% or less,
    • S: 0.0100% or less,
    • N: 0.010% or less,
    • O: 0.010% or less,
    • Nb: 0 to 0.050%,
    • V: 0 to 1.000%,
    • Cr: 0 to 2.00%,
    • Ni: 0 to 2.00%,
    • Cu: 0 to 2.00%,
    • Mo: 0 to 1.000%,
    • B: 0 to 0.0100%,
    • Sn: 0 to 1.000%,
    • Sb: 0 to 1.000%,
    • Ca: 0 to 0.0100%,
    • Mg: 0 to 0.0100%,
    • Hf: 0 to 0.0100%,
    • Bi 0 to 0.010%,
    • REM: 0 to 0.0100%,
    • As: 0 to 0.010%,
    • Zr: 0 to 0.010%,
    • Co: 0 to 2.000%,
    • Zn: 0 to 0.010%,
    • W: 0 to 1.000%, and
    • balance: Fe and impurities, and
    • a microstructure comprising, by area %,
    • ferrite: 30 to 60%,
    • bainite: 30 to 60%, and
    • martensite: 5 to 20%, wherein
    • TiC precipitates having a diameter of 2.0 to 8.0 nm are present in the ferrite in a number density of 1.0×10¹⁶/cm³ or more, and
    • an average aspect ratio of prior austenite grains in a region including the bainite and martensite is 3.0 or more.
  • (2) The hot rolled steel sheet according to the above (1), wherein the chemical composition includes, by mass %, at least one of
    • Nb: 0.001 to 0.050%,
    • V: 0.001 to 1.000%,
    • Cr: 0.001 to 2.00%,
    • Ni: 0.001 to 2.00%,
    • Cu: 0.001 to 2.00%,
    • Mo: 0.001 to 1.000%,
    • B: 0.0001 to 0.0100%,
    • Sn: 0.001 to 1.000%,
    • Sb: 0.001 to 1.000%,
    • Ca: 0.0001 to 0.0100%,
    • Mg: 0.0001 to 0.0100%,
    • Hf: 0.0001 to 0.0100%,
    • Bi: 0.001 to 0.010%,
    • REM: 0.0001 to 0.0100%,
    • As: 0.001 to 0.010%,
    • Zr: 0.001 to 0.010%,
    • Co: 0.001 to 2.000%,
    • Zn: 0.001 to 0.010%, and
    • W: 0.001 to 1.000%.
  • (3) The hot rolled steel sheet according to the above (1) or (2), wherein a ratio of an average nanohardness of ferrite with respect to an average nanohardness of bainite is 0.75 to 1.20.

Advantageous Effects of Invention

According to the present invention, it is possible to provide a hot rolled steel sheet which is high in strength and has a high hole expandability and yield ratio.

DESCRIPTION OF EMBODIMENTS

<Hot Rolled Steel Sheet>

The hot rolled steel sheet according to an embodiment of the present invention has a chemical composition comprising, by mass %,

• • C: 0.03 to 0.10%,
  • Si: 0.010 to 0.100%,
  • Mn: 0.50 to 3.00%,
  • Ti: 0.05 to 0.20%,
  • Al: 0.20 to 0.40%,
  • P: 0.100% or less,
  • S: 0.0100% or less,
  • N: 0.010% or less,
  • O: 0.010% or less,
  • Nb: 0 to 0.050%,
  • V: 0 to 1.000%,
  • Cr: 0 to 2.00%,
  • Ni: 0 to 2.00%,
  • Cu: 0 to 2.00%,
  • Mo: 0 to 1.000%,
  • B: 0 to 0.0100%,
  • Sn: 0 to 1.000%,
  • Sb: 0 to 1.000%,
  • Ca: 0 to 0.0100%,
  • Mg: 0 to 0.0100%,
  • Hf: 0 to 0.0100%,
  • Bi: 0 to 0.010%,
  • REM: 0 to 0.0100%,
  • As: 0 to 0.010%,
  • Zr: 0 to 0.010%,
  • Co: 0 to 2.000%,
  • Zn: 0 to 0.010%,
  • W: 0 to 1.000%, and
  • balance: Fe and impurities, and
  • a microstructure comprising, by area %,
  • ferrite: 30 to 60%,
  • bainite: 30 to 60%, and
  • martensite: 5 to 20%, wherein
  • TiC precipitates having a diameter of 2.0 to 8.0 nm are present in the ferrite in a number density of 1.0×10¹⁶/cm³ or more, and
  • an average aspect ratio of prior austenite grains in a region including the bainite and martensite is 3.0 or more.

Along with a higher strength of steel sheet, hole expandability and other workability generally fall. Therefore, sufficiently securing strength of steel sheet while improving the hole expandability and further achieving a high yield ratio from the viewpoint of collision safety, etc., of automobiles is generally extremely difficult. Therefore, the inventors engaged in studies for focusing in particular on the microstructure of a hot rolled steel sheet in addition to making the chemical composition of a hot rolled steel sheet more suitable. Explaining this more specifically, first, the inventors discovered that by making the microstructure of a hot rolled steel sheet having a predetermined chemical composition include mainly ferrite, bainite, and martensite, it is possible to maintain the strength of a hot rolled steel sheet at a certain high level while raising the hole expandability and yield ratio. Next, the inventors discovered that by controlling the percentage of the hard phase bainite in the microstructure to a relatively high range and utilizing dislocation strengthening to make the strength further higher, it is possible to remarkably raise the strength of a hot rolled steel sheet. More specifically, the inventors discovered that by controlling the area ratio of bainite in the microstructure within a range of 30 to 60% and additionally, as explained later in detail in relation to the method of production of the hot rolled steel sheet, introducing dislocations resulting in an average aspect ratio of 3.0 or more of the prior austenite grains in the region including bainite and martensite into the steel sheet in the hot rolling, it is possible to remarkably raise the strength of a hot rolled steel sheet.

On the other hand, controlling the percentage of bainite to a relatively high range in a triplex structure comprised mainly of ferrite, bainite, and martensite as explained above is extremely effective from the viewpoint of raising the strength, but the hardness difference becomes relatively large in particular between the hard phase bainite and the soft phase ferrite. If the hardness difference of the phases in the microstructure becomes greater, there is a possibility that the hole expandability and yield ratio will fall due to this. For this reason, the inventors studied improvement of the hole expandability and realization of a higher yield ratio from the viewpoint of reducing the hardness difference of the phases in a microstructure containing such a triplex structure. As a result, the inventors discovered that by strengthening the softest ferrite by precipitates in the triplex structure, more specifically by establishing the presence in the ferrite of TiC precipitates having a diameter of 2.0 to 8.0 nm in a number density of 1.0×10¹⁶/cm³ or more to strengthen the ferrite by precipitates, it is possible to contribute to improvement of the strength of the hot rolled steel sheet as a whole only naturally while sufficiently reducing the hardness difference between the hard phase bainite present relatively greatly in the microstructure and the softest ferrite in the triplex structure. In the hot rolled steel sheet according to an embodiment of the present invention, in order to establish the presence in the ferrite of TiC precipitates having a diameter of 2.0 to 8.0 nm by a number density of 1.0×10¹⁶/cm³ or more, in addition to the method of production explained in detail later, it is necessary to make the chemical composition of the hot rolled steel sheet more suitable. For example, the Si and Al contained in the steel have the actions of suppressing precipitation of cementite. Therefore, by including these elements in the steel in predetermined amounts or more, more specifically by including Si and Al in respectively 0.010 mass % or more and 0.20 mass % or more, it is possible to keep the C in the steel from being consumed for formation of cementite and thereby becomes possible to promote the formation of TiC precipitates at the time of cooling after hot rolling. As a result, the inventors discovered that despite forming the microstructure by a triplex structure relatively susceptible to increase of the hardness difference by the actions of raising the bainite percentage and utilizing dislocation strengthening so as to achieve higher strength, it is possible to obtain a hot rolled steel sheet which is high in strength and has high hole expandability and yield ratio by raising the hardness of ferrite by precipitation strengthening utilizing TiC precipitates of a specific diameter and number density. Therefore, the hot rolled steel sheet according to an embodiment of the present invention can be effectively used even in members where achievement of both of the contradictory properties of high strength and excellent workability is sought and further impact resistance is sought, and therefore is particularly useful for use in the automotive field.

Below, the hot rolled steel sheet according to an embodiment of the present invention will be explained in more detail. In the following explanation, the “%” of the units of contents of the elements, unless otherwise indicated, means “mass %”. Further, in this Description, the “to” showing a numerical range, unless otherwise indicated, is used in the sense of the numerical values described before and after the same being included as the lower limit value and the upper limit value.

[C: 0.03 to 0.10%]

C is an element effective for raising the strength of steel sheet. Further, C forms carbides and/or carbonitrides with Ti and Nb in the steel and also contributes to the precipitation strengthening based on the precipitates formed and the refinement of the structure by the pinning effect of the precipitates. To sufficiently obtain these effects, the C content is 0.03% or more. The C content may also be 0.04% or more, 0.05% or more, or 0.06% or more. On the other hand, if excessively containing C, sometimes the hole expandability and yield ratio fall due to the formation of cementite. Therefore, the C content is 0.10% or less. The C content may also be 0.09% or less, 0.08% or less, or 0.07% or less.

[Si: 0.010 to 0.100%]

Si is an element effective for raising strength as a solution strengthening element. Further, Si has the action of suppressing precipitation of cementite. For this reason, by inclusion of Si, the C in the steel can be kept from being consumed for formation of cementite. Due to this, it becomes possible to promote the formation of the TiC precipitates at the time of cooling after hot rolling. To sufficiently obtain such an effect, the Si content is 0.010% or more. The Si content may also be 0.020% or more, 0.030% or more, or 0.040% or more. On the other hand, if excessively containing Si, sometimes defects in surface quality called “Si scale” are caused. Therefore, the Si content is 0.100% or less. The Si content may also be 0.090% or less, 0.080% or less, 0.070% or less, 0.060% or less, or 0.050% or less.

[Mn: 0.50 to 3.00%]

Mn is an element effective for hardenability and for raising strength as a solution strengthening element. To sufficiently obtain these effects, the Mn content is 0.50% or more. The Mn content may also be 0.70% or more, 1.00% or more, 1.20% or more, or 1.50% or more. On the other hand, if excessively containing Mn, sometimes MnS is greatly formed and the toughness is lowered. Therefore, the Mn content is 3.00% or less. The Mn content may also be 2.80% or less, 2.50% or less, 2.20% or less, or 2.00% or less.

[Ti: 0.05 to 0.20%]

Ti is an element finely precipitating in steel as a carbide (TiC) and improving the strength of the steel by precipitation strengthening and raising the hardness of ferrite. Further, Ti is an element forming carbides to fix C and suppress the formation of the cementite harmful to the hole expandability. To sufficiently obtain these effects, the Ti content is 0.05% or more. The Ti content may also be 0.08% or more, 0.10% or more, 0.12% or more, or 0.14% or more. On the other hand, if excessively containing Ti, the carbides become coarser and sometimes it is not possible to obtain the desired precipitation strengthening in ferrite. In addition, along with the coarsening of the TiC precipitates, the number density of the TiC precipitates will also fall, and therefore, in this case, it is no longer possible to sufficiently raise the hardness of the ferrite by precipitation strengthening. Therefore, the Ti content is 0.20% or less. The Ti content may also be 0.18% or less, 0.17% or less, 0.16% or less, or 0.15% or less.

[Al: 0.20 to 0.40%]

Al is an element acting as a deoxidizer of molten steel. Further, Al has the action of suppressing precipitation of cementite. For this reason, inclusion of Al enables the C in the steel to be kept from being consumed for formation of cementite and thereby enables promotion of formation of the TiC precipitates at the time of cooling after hot rolling. To sufficiently obtain these effects, the Al content is 0.20% or more. The Al content may also be 0.22% or more, 0.25% or more, or 0.28% or more. On the other hand, if excessively containing Al, coarse oxides are formed and sometimes the toughness and ductility fall. Therefore, Al content is 0.40% or less. The Al content may also be 0.38% or less, 0.35% or less, or 0.32% or less.

[P: 0.100% or Less]

P, if excessively contained, sometimes affects the weldability detrimentally. Therefore, the P content is 0.100% or less. The P content may also be 0.080% or less, 0.050% or less, 0.030% or less, or 0.020% or less. The lower limit of the P content is not particularly prescribed and may also be 0%, but excessive reduction invites a rise in costs. Therefore, the P content may also be 0.00010% or more, 0.0010% or more, or 0.0050% or more.

[S: 0.0100% or Less]

S, if excessively contained, forms a large amount of MnS and sometimes causes a drop in toughness. Therefore, the S content is 0.0100% or less. The S content may also be 0.0050% or less, 0.0030% or less, or 0.0020% or less. The lower limit of the S content is not particularly prescribed and may also be 0%, but excessive reduction invites a rise in costs. Therefore, the S content may also be 0.0001% or more, 0.0005% or more, or 0.0010% or more.

[N: 0.010% or Less]

N, if excessively contained, forms coarse nitrides and sometimes causes the toughness falls. Therefore, the N content is 0.010% or less. The N content may also be 0.008% or less, 0.005% or less, or 0.003% or less. The lower limit of the N content is not particularly prescribed and may also be 0%, but excessive reduction invites a rise in costs. Therefore, the N content may also be 0.00010% or more, 0.00050% or more, or 0.0010% or more.

[O: 0.010% or Less]

O is an element entering in the production process. If excessively containing 0, coarse inclusions are formed and sometimes the toughness of the steel sheet falls. Therefore, the O content is 0.010% or less. The O content may also be 0.008% or less, 0.006% or less, or 0.004% or less. The lower limit of the O content is not particularly prescribed and may also be 0%, but for reduction to less than 0.0001%, time is required for refining and a drop in productivity is invited. Therefore, the O content may also be 0.0001% or more or 0.0005% or more.

The basic chemical composition of the hot rolled steel sheet according to an embodiment of the present invention is as explained above. Further, the hot rolled steel sheet may, in accordance with need, contain at least one of the following optional elements in place of part of the balance of Fe.

[Nb: 0 to 0.050%]

Nb is an element forming carbides, nitrides, and/or carbonitrides in steel to contribute to refinement of the structure due to the pinning effect and in turn higher strength of the steel sheet. The Nb content may also be 0%, but to obtain these effects, the Nb content is preferably 0.001% or more. The Nb content may also be 0.005% or more, 0.010% or more, 0.012% or more, 0.015% or more, or 0.020% or more. On the other hand, if excessively containing Nb, coarse carbides, etc., are formed in the steel and sometimes the ductility of the steel sheet falls. Therefore, Nb content is preferably 0.050% or less. The Nb content may also be 0.040% or less, 0.030% or less, or 0.025% or less.

[V: 0 to 1.000%]

V is an element contributing to improvement of strength due to precipitation strengthening, etc. The V content may also be 0%, but to obtain such an effect, the V content is preferably 0.001% or more. The V content may also be 0.010% or more, 0.030% or more, or 0.050% or more. On the other hand, even if excessively including V, the effect becomes saturated and a rise in the production costs is liable to be invited. Therefore, the V content is preferably 1.000% or less. The V content may also be 0.500% or less, 0.200% or less, 0.100% or less, or 0.080% or less.

[Cr: 0 to 2.00%]

Cr is an element raising the hardenability of steel and contributing to improvement of strength. The Cr content may also be 0%, but to obtain such an effect, the Cr content is preferably 0.001% or more. The Cr content may also be 0.01% or more, 0.03% or more, or 0.05% or more. On the other hand, even if excessively including Cr, the effect becomes saturated and a rise in the production costs is liable to be invited. Therefore, the Cr content is preferably 2.00% or less. The Cr content may also be 1.50% or less, 1.00% or less, 0.50% or less, 0.30% or less, 0.15% or less, or 0.10% or less.

[Ni: 0 to 2.00%]

[Cu: 0 to 2.00%]

Ni and Cu are elements contributing to improvement of strength by precipitation strengthening and solution strengthening. The Ni and Cu contents may also be 0%, but to obtain such effects, the contents of these elements are preferably respectively 0.001% or more and may also be 0.01% or more, 0.03% or more, or 0.05% or more. On the other hand, even if excessively including these elements, the effect becomes saturated and a rise in the production costs is liable to be invited. Therefore, the Ni and Cu content are preferably respectively 2.00% or less and may also be 1.50% or less, 1.00% or less, 0.50% or less, 0.30% or less, 0.15% or less, or 0.10% or less.

[Mo: 0 to 1.000%]

Mo is an element raising the hardenability of steel and contributing to improvement of the strength. The Mo content may also be 0%, but to obtain such an effect, the Mo content is preferably 0.001% or more. The Mo content may also be 0.010% or more, 0.020% or more, or 0.050% or more. On the other hand, if excessively containing Mo, the deformation resistance at the time of hot working increases and sometimes the load on the facilities becomes greater. Therefore, the Mo content is preferably 1.000% or less. The Mo content may also be 0.800% or less, 0.500% or less, 0.200% or less, 0.100% or less, or 0.080% or less.

[B: 0 to 0.0100%]

B segregates at the grain boundaries to raise the intergranular strength and thereby improve the low temperature toughness. The B content may also be 0%, but to obtain such an effect, the B content is preferably 0.0001% or more. The B content may also be 0.0002% or more, 0.0003% or more, or 0.0005% or more. On the other hand, even if excessively containing B, the effect becomes saturated and a rise in production cost is liable to be invited. Therefore, the B content is preferably 0.0100% or less. The B content may also be 0.0050% or less, 0.0030% or less, 0.0015% or less, or 0.0010% or less.

[Sn: 0 to 1.000%]

[Sb: 0 to 1.000%]

Sn and Sb are elements effective for improvement of the corrosion resistance. The Sn and Sb contents may be 0%, but to obtain such an effect, the contents of these elements are respectively preferably 0.001% or more and may also be 0.010% or more, 0.020% or more, or 0.050% or more. On the other hand, if excessively containing these elements, sometimes a drop in toughness is invited. Therefore, the Sn and Sb contents are preferably 1.000% or less and may also be 0.800% or less, 0.500% or less, 0.300% or less, 0.100% or less, or 0.080% or less.

[Ca: 0 to 0.0100%]

[Mg: 0 to 0.0100%]

[Hf: 0 to 0.0100%]

Ca, Mg, and Hf are elements enabling control of the form of the nonmetallic inclusions. The Ca, Mg, and Hf contents may also be 0%, but to obtain such an effect, the contents of these elements are preferably respectively 0.0001% or more and may also be 0.0005% or more or 0.0010% or more. On the other hand, even if excessively containing these elements, the effect becomes saturated. Inclusion in the steel sheet more than necessary invites a rise in production costs. Therefore, the Ca, Mg, and Hf contents are preferably respectively 0.0100% or less and may also be 0.0050% or less, 0.0030% or less, or 0.0020% or less.

[Bi: 0 to 0.010%]

Bi is an element effective for improvement of the corrosion resistance. The Bi content may be 0%, but to obtain such an effect, the Bi content is preferably 0.001% or more. The Bi content may also be 0.001% or more or 0.002% or more. On the other hand, even if excessively including Bi, the effect becomes saturated. Inclusion in the steel sheet more than necessary invites a rise in production costs. Therefore, the Bi content is preferably respectively 0.010% or less. The Bi content may also be 0.005% or less, or 0.003% or less.

[REM: 0 to 0.0100%]

An REM is an element enabling control of the form of nonmetallic inclusion. The REM content may be 0%, but to obtain such an effect, the REM content is preferably 0.0001% or more. The REM content may also be 0.0005% or more or 0.0010% or more. On the other hand, even if excessively containing REM, the effect becomes saturated. Inclusion in the steel sheet more than necessary invites a rise in production costs. Therefore, the REM content is preferably 0.0100% or less. The REM content may also be 0.0050% or less, 0.0030% or less, or 0.0020% or less. The “REM” in this Description is the general name of the 17 elements of atomic number 21 scandium (Sc), atomic number 39 yttrium (Y), and the lanthanoid atomic number 57 lanthanum (La) to atomic number 71 lutetium (Lu). The REM content is the total content of these elements.

[As: 0 to 0.010%]

As is an element effective for improving the corrosion resistance. The As content may be 0%, but to obtain such an effect, the As content is preferably 0.001% or more. The As content may also be 0.002% or more or 0.003% or more. On the other hand, even if excessively including As, the effect becomes saturated. Inclusion in the steel sheet more than necessary invites a rise in production costs. Therefore, the As content is preferably respectively 0.010% or less. The As content may also be 0.008% or less or 0.005% or less.

[Zr: 0 to 0.010%]

Zr is an element enabling control of the form of nonmetallic inclusions. The Zr content may also be 0%, but to obtain such an effect, the Zr content is preferably 0.001% or more. The Zr content may also be 0.002% or more or 0.003% or more. On the other hand, even if excessively containing Zr, the effect becomes saturated. Inclusion in the steel sheet more than necessary invites a rise in production costs. Therefore, the Zr content is preferably 0.010% or less. The Zr content may also be 0.008% or less or 0.005% or less.

[Co: 0 to 2.000%]

Co is an element contributing to improvement of the hardenability and/or heat resistance. The Co content may also be 0%, but to obtain these effects, the Co content is preferably 0.001% or more. The Co content may also be 0.010% or more, 0.050% or more, or 0.100% or more. On the other hand, if excessively containing Co, the hot workability sometimes falls. This also leads to an increase in material costs. Therefore, the Co content is preferably 2.000% or less. The Co content may also be 1.000% or less, 0.500% or less, 0.300% or less, or 0.200% or less.

[Zn: 0 to 0.010%]

Zn is an element able to be included in steel sheet when using scrap, etc., as a steel raw material. Therefore, the Zn content is preferably 0.010% or less and may also be 0.008% or less or 0.005% or less. The Zn content may also be 0%, but for reduction to less than 0.001%, time is required for refining and the productivity falls. Therefore, Zn content may also be 0.001% or more, 0.002% or more, or 0.003% or more.

[W: 0 to 1.000%]

W is an element raising the hardenability of steel and contributes to improvement of strength. The W content may also be 0%, but to obtain such an effect, the W content is preferably 0.001% or more. The W content may also be 0.010% or more, 0.050% or more, or 0.100% or more. On the other hand, if excessively containing W, sometimes the weldability falls. Therefore, the W content is preferably 1.000% or less. The W content may also be 0.800% or less, 0.500% or less, 0.300% or less, or 0.200% or less.

In the hot rolled steel sheet according to an embodiment of the present invention, the balance aside from the above elements is comprised of Fe and impurities. The “impurities” are constituents entering the slab due to the ore, scrap, or other raw materials and other various factors in the production process when industrially producing the slab.

The chemical composition of the hot rolled steel sheet according to an embodiment of the present invention may be measured by a general analysis method. For example, the chemical composition of the hot rolled steel sheet may be measured using inductively coupled plasma-atomic emission spectrometry (ICP-AES). C and S can be measured using the combustion-infrared absorption method, N using the inert gas melting-thermal conductivity method, and O using the inert gas melting-nondispersive type infrared absorption method.

[Microstructure]

The microstructure of the hot rolled steel sheet according to an embodiment of the present invention includes, by area %, ferrite: 30 to 60%, bainite: 30 to 60%, and martensite: 5 to 20%. By the microstructure of the hot rolled steel sheet mainly containing these three structures and further containing bainite in a relatively large amount, it becomes possible to make the hot rolled steel sheet higher in strength while raising the hole expandability and yield ratio. By including these three structures in the above such specific area ratios and additionally utilizing the dislocation strengthening explained in detail later, it becomes possible to further raise the strength of the hot rolled steel sheet. Further, by including these three structures in the above such specific area ratios, it becomes possible to suitably reduce the hardness difference of the microstructure by precipitation hardening of ferrite explained in detail later. For example, if the area ratio of ferrite is small, the ratios of the hard phases of bainite and martensite, in particular the ratio of bainite, become higher and sometimes it is not possible to suitable reduce the hardness difference of the microstructure, more specifically the hardness difference of the ferrite and bainite, even by precipitation strengthening of ferrite. In such a case, it becomes no longer possible to achieve the desired hole expandability and/or yield ratio. Therefore, the area ratio of ferrite has to be 30% or more and, for example, may also be 35% or more, 40% or more, or 45% or more. On the other hand, if the area ratio of ferrite becomes higher, the ratios of the hard phases bainite and martensite become lower and, as a result, sometimes it is not possible to achieve the desired strength, for example, a 780 MPa or more tensile strength. Therefore, the area ratio of ferrite is 60% or less and for example may also be less than 60%, 59% or less, 58% or less, 55% or less, 52% or less, or 50% or less.

From the viewpoint of improving the tensile strength, the area ratios of the hard phases bainite and martensite are preferably higher. From such a viewpoint, for example, the area ratio of bainite may also be more than 30%, 31% or more, 32% or more, 35% or more, 40% or more, or 45% or more. Similarly, the area ratio of martensite may also be 8% or more, 10% or more, or 12% or more. On the other hand, from the viewpoint of reducing the hardness difference in the microstructure and further improving the hole expandability and yield ratio, the area ratios of bainite and martensite are preferably lower. From such a viewpoint, for example, the area ratio of bainite may also be 58% or less, 55% or less, 52% or less, or 50% or less. Similarly, the area ratio of martensite may also be 18% or less, 16% or less, or 14% or less.

[Balance Structures]

The microstructure of the hot rolled steel sheet according to an embodiment of the present invention, as explained above, includes ferrite, bainite, and martensite and may also contain other balance structures besides these, but the area ratio of the balance structures is preferably small and may also be 0%. The area ratio of the balance structures is not particularly limited, but, for example, may be 0 to 5%, 0 to 4%, or 0 to 3%. In other words, the total area ratio of ferrite, bainite, and martensite may also be, for example, 95 to 100%, 96 to 100%, or 97 to 100%. The lower of the balance structures may be 1% or 2%. If there are balance structures present, the balance structures include at least one of pearlite and retained austenite or can be at least one of these.

[Identification of Microstructure and Calculation of Area Ratios]

The structures are observed by a scan type electron microscope. Before observation, the sample for observation of the structures is wet polished by emery paper and polished by a diamond abrasive having an average grain size of 1 μm so as to finish the observed surface to a mirror finish, then the structures are etched by a 3% nitric acid alcohol solution. The power of the observation is made 2000×. Ten 30 μm×40 μm fields at a ¼ position of sheet thickness from the surface are randomly photographed. The ratios of the structures are found by the point count method. Over the obtained structural image, a total of 225 lattice points are arranged at intervals of vertically 3 μm and horizontally 4 μm, the structures present under the lattice points are judged, and the ratios of structures contained in the steel material are found from the average values of the 10 photos. Ferrite is a clumpy crystal grain not including inside it long axis 100 nm or more iron-based carbides. Bainite is a collection of lath shaped crystal grains not including inside it long axis 20 nm or more iron-based carbides or containing inside it long axis 20 nm or more iron-based carbides but the carbides belonging to a single variant, i.e., a group of iron-based carbides stretching in the same direction. Here, “group of iron-based carbides stretching in the same direction” means a difference in stretching direction of the group of iron-based carbides of within 5°. In bainite, bainite surrounded by grain boundaries with an orientation difference of 150 or more is counted as a single bainite grain. Further, martensite, which contains a large amount of dissolved carbon, is higher in brightness compared with other structures and appears white, and therefore can be differentiated from other structures. If there are structures other than ferrite, bainite, and martensite, the area ratio of balance structures is determined by subtracting from 100% the total area ratio of ferrite, bainite, and martensite. There is no need to specifically identify the balance structures, but if the balance structures include pearlite and retained austenite, etc., since pearlite has a unique structure of cementite precipitated in a lamellar manner, it can be discriminated by a scan type electron microscope. Further, for retained austenite, the volume ratio can be calculated by measurement by X-ray diffraction. The volume ratio of retained austenite is equal to the area ratio, and therefore this can be made the area ratio of the retained austenite.

[Number Density of TiC Precipitates Having Diameter of 2.0 to 8.0 nm in Ferrite: 1.0×10¹⁶/cm³ or More]

In the hot rolled steel sheet according to an embodiment of the present invention, TiC precipitates having a diameter of 2.0 to 8.0 nm are present in the ferrite in a number density of 1.0×10¹⁶/cm³ or more. Here, the “TiC precipitates” include not only TiC, but also encompass complex carbides including Ti and elements besides Ti, for example, V and Nb. By establishing the presence in the ferrite of TiC precipitates having a diameter of 2.0 to 8.0 nm in such a number density, it is possible to raise the hardness of the ferrite by precipitation strengthening. More specifically, by raising the hardness of ferrite to reduce the hardness difference with the hard phase bainite present in a relatively large amount in the microstructure, it is possible to reduce the hardness difference in the microstructure mainly comprised of ferrite, bainite, and martensite. As a result, it becomes possible to remarkably raise the hole expandability and yield ratio of the hot rolled steel sheet, for example, becomes possible to achieve a 60.0% or more hole expansion rate (λ) and 0.70 or more yield ratio (YR). Only naturally, precipitation strengthening by TiC precipitates also contributes to improvement of the hot rolled steel sheet as a whole. If the diameter of the TiC precipitates is smaller than 2.0 nm, the TiC precipitates cannot sufficiently act as obstacles to dislocation motion and therefore the effect of improvement of hardness of the ferrite by precipitation strengthening cannot be sufficiently obtained. In addition, sometimes the effect of improvement of strength of the hot rolled steel sheet also cannot be sufficiently obtained. On the other hand, even if the TiC precipitates become too large in diameter, sometimes it is not possible to obtain the desired precipitation strengthening in the ferrite.

While not intending to be bound by any specific theory, it is believed that this is because by the TiC precipitates becoming coarser, the strengthening mechanism changes in relation to dislocation motion and that, for example, the dislocation line does not pass cutting across the TiC precipitates but passes leaving behind a loop of dislocation line around the coarse TiC precipitates and therefore the amount of precipitation strengthening becomes small. In addition, along with coarsening of the TiC precipitates, the number density of the TiC precipitates also greatly falls, and therefore it becomes no longer possible to sufficiently raise the hardness of the ferrite by precipitation strengthening. Therefore, to effectively raise the hardness of ferrite by precipitation strengthening, it is effective to control the diameter of TiC precipitates to a range of 2.0 to 8.0 nm. To increase the hardness of ferrite by precipitation strengthening to the desired level, it becomes important to control the number density of TiC precipitates having such a diameter to within a predetermined range. From such a viewpoint, it is necessary to establish the presence in the ferrite of TiC precipitates having a diameter of 2.0 to 8.0 nm, as explained above, in a number density of 1.0×10¹⁶/cm³ or more. To further enhance the effect or improvement of hardness of the ferrite, the higher the number density, the more preferable. For example, it may be 1.2×10¹⁶/cm³ or more, 1.5×10¹⁶/cm³ or more, 2.0×10¹⁶/cm³ or more, 5.0×10¹⁶/cm³ or more, or 10.0×10¹⁶/cm³ or more. On the other hand, there is a limit to the contents of C and Ti of the sources of supply of the TiC precipitates, and therefore if the number density becomes too high, sometimes controlling the diameter of the TiC precipitates to within a predetermined range becomes difficult. Therefore, the number density is not particularly limited so long as the diameter of 2.0 to 8.0 nm is satisfied, but for example may be 75.0×10¹⁶/cm³ or less, 50.0×10¹⁶/cm³ or less, 30.0×10¹⁶/cm³ or less, or 20.0×10¹⁶/cm³ or less. In the hot rolled steel sheet according to an embodiment of the present invention, if performing measurement by the 3D atom probe measurement method explained in detail later, it is sufficient that TiC precipitates having a diameter of 2.0 to 8.0 nm be present in the ferrite in a number density of 1.0×10¹⁶/cm³ or more. Therefore, so long as satisfying the above diameter and number density requirements, for example, coarse TiC precipitates may also be present in the ferrite.

[Calculation of Diameter and Number Density of TiC Precipitates]

The diameter and number density of TiC precipitates are calculated by the 3D atom probe measurement method as follows: First, from a sample for measurement, a needle-shaped sample is prepared by cutting and electrolytic polishing making use of, if necessary, the focused ion beam processing method along with electrolytic polishing. In the 3D atom probe measurement method, it is possible to reconstruct cumulative data to find an image of distribution of atoms in real space. In the case of fine TiC precipitates of Na—Cl structures, the unit lattice is 4.33 Å, and therefore the distance between atoms of Ti and Ti is deemed to be 4.33×√2=6.1 Å. Therefore, if there are a plurality of Ti atoms at substantially the same coordinate position (7 Å or less), it is judged that these Ti atoms are in the same precipitates and the number of Ti atoms judged to be in the same precipitates is counted. If the number is 50 or more, these precipitates are defined as “fine TiC precipitates”. The diameter of the fine TiC precipitates is made the circle equivalent diameter calculated from the number of atoms of Ti forming the observed fine Ti precipitates and the lattice constant of the fine Ti precipitates assuming the fine Ti precipitates to be spherical. The method of using the number of Ti atoms of the fine TiC precipitates obtained by the 3D atom probe measurement method so as to find the diameter (circle equivalent diameter) R of the fine TiC precipitates is shown below: The number N of all of the atoms of the sample covered is measured by the 3D atom probe measurement method, but in actuality, it is not possible to detect the number N of all of the atoms of the sample covered by the 3D atom probe measurement method. There is a detection rate a (=number of atoms detected/total number of atoms) of atoms inherent to each device, and therefore the number N of atoms probably present is calculated from the actual measurement value “n”. That is, the total number N of atoms=n/a. Next, for the total number N of atoms, Na—Cl structure TiC precipitates are assumed to have eight Ti atoms present in a unit lattice. Further, the lattice constant “a” of the Na—Cl structure is deemed 4.33 Å and the following formula is used to calculate the diameter R (circle equivalent diameter) of the TiC precipitates.

Diameter ⁢ R ⁢ of ⁢ TiC ⁢ precipitates = { ( 6 / 8 ) · ( 1 / π ) · N · a 3 } ( 1 / 3 )

Finally, the number density of the TiC precipitates is calculated using the measurement field as the denominator and the number of fine TiC precipitates as the numerator.

[Ratio of Average Nanohardness of Bainite/Average Nanohardness of Ferrite: 0.75 to 1.20]

In a preferred embodiment of the present invention, the ratio of an average nanohardness of bainite/average nanohardness of ferrite, i.e., the (average nanohardness of ferrite)/(average nanohardness of bainite), is controlled to within a range of 0.75 to 1.20. By controlling the ratio of an average nanohardness of bainite/average nanohardness of ferrite to within such a range, it is possible to further reduce the hardness difference of ferrite and bainite in the microstructure. As a result, it becomes possible to further raise the hole expandability and yield ratio of the hot rolled steel sheet, for example, it becomes possible to achieve a 65.0% or more hole expansion rate (λ) and 0.75 or more yield ratio (YR). From the viewpoint of further raising the hole expansion rate and yield ratio, the ratio of an average nanohardness of bainite/average nanohardness of ferrite may also be 0.80 or more, 0.85 or more, or 0.90 or more and similarly may also be 1.15 or less, 1.10 or less, 1.05 or less, or 1.00 or less. The average nanohardness of bainite is not particularly limited, but, for example, may also be 3.0 to 5.0 GPa, 3.2 to 4.8 GPa, or 3.5 to 4.5 GPa. Similarly, the average nanohardness of ferrite is not particularly limited, but, for example, may also be 2.5 to 5.0 GPa, 2.8 to 4.8 GPa, or 3.0 to 4.5 GPa.

[Method of Determination of Ratio of Average Nanohardness of Ferrite to Average Nanohardness of Bainite]

The ratio of the average nanohardness of ferrite to the average nanohardness of bainite is determined in the following way. First, a sample is cut out from the hot rolled steel sheet so that a sheet thickness cross-section vertical to the surface can be observed. The cross-section of the sample is wet polished by emery paper and polished by a diamond abrasive having an average grain size of 1 μm to finish it to a mirror surface. Using a nanohardness tester, the mirror finished cross-section is dented by a test load of 3 gf at a ¼ depth position of sheet thickness from the surface to measure the nanohardness and obtain measurement values of a total of 100 points or more. Next, a scan type electron microscope is used to measure the same sample. Referring to the obtained results of structural analysis, only the measurement points with dents at the inside of the ferrite crystal grains and inside of the bainite are extracted. Finally, the arithmetic average of the nanohardnesses of 10 or more ferrite crystal grains extracted is determined as the average nanohardness of the ferrite, the arithmetic average of the nanohardnesses of 10 or more bainite similarly extracted is determined as the average nanohardness of the bainite, and the ratio (average nanohardness of ferrite)/(average nanohardness of bainite) of these is determined as a ratio of the average nanohardness of ferrite with respect to the average nanohardness of bainite.

[Average Aspect Ratio of Prior Austenite Grains of Region Including Bainite and Martensite: 3.0 or More]

In the hot rolled steel sheet according to an embodiment of the present invention, the average aspect ratio of prior austenite grains of a region including bainite and martensite has to be 3.0 or more. As explained above, by configuring the microstructure to include hard phase bainite in a relatively large amount of 30 to 60 area %, it is possible to raise the strength of the hot rolled steel sheet. However, if just raising the area ratio of bainite, sometimes it is not possible to reliably achieve the desired high strength. Therefore, in the hot rolled steel sheet according to an embodiment of the present invention, in addition to including bainite in a relatively large amount, dislocation strengthening is utilized to enable a further higher strength. Explained more specifically, by performing suitable rolling reduction at the time of hot rolling, it is possible to suppress recrystallization of the microstructure while introducing dislocations into the steel sheet. A microstructure in which dislocations are suitably introduced in this way is suppressed in recrystallization, and therefore becomes a structure with a relatively large aspect ratio. In other words, by suitably controlling the average aspect ratio of prior austenite grains of a region including bainite and martensite, it becomes possible to further enhance the effect of improvement of strength obtained by inclusion of a relatively large amount of bainite due to the dislocation strengthening. In the hot rolled steel sheet according to an embodiment of the present invention, by making the microstructure contain 30 to 60 area % of bainite and controlling the average aspect ratio of prior austenite grains of a region including bainite and martensite to 3.0 or more, it becomes possible to remarkably raise the strength of the hot rolled steel sheet due to the combination of the effect of improvement of strength by bainite and dislocation strengthening. From the viewpoint of further raising the strength of the hot rolled steel sheet, the larger the average aspect ratio, the more preferable. For example, it may also be 3.2 or more, 3.5 or more, 3.8 or more, or 4.0 or more. The upper limit of the average aspect ratio is not particularly prescribed, but, for example, the average aspect ratio may also be 6.0 or less, 5.5 or less, or 5.0 or less.

[Measurement of Average Aspect Ratio of Prior Austenite Grains of Region Including Bainite and Martensite]

The average aspect ratio of prior austenite grains of a region including bainite and martensite is measured is by a scan type electron microscope. Before measurement, the sample for observation of the structures is wet polished by emery paper and polished by a diamond abrasive having an average grain size of 1 m. The sample is finished to a mirror surface using the L direction cross-section (cross-section parallel to rolling direction and sheet thickness direction) as the observed surface, then the structure is etched by a picric acid solution. The power of the observation is made 1000×. Ten 60 μm×80 μm fields at a ¼ position of sheet thickness from the surface are randomly photographed. Covering the crystal grain boundaries appearing due to corrosion by picric acid, the slice method was used to find the crystal grain sizes in the sheet thickness direction and the rolling direction. In the slice method, five lines are drawn at equal intervals in the sheet thickness direction and rolling direction of the photographed image and the points intersecting the crystal grain boundaries are counted. The value of the total length of the five lines divided by the number of intersecting points is made the crystal grain size. The value of the crystal grain size in the rolling direction divided by the crystal grain size in the sheet thickness direction is determined as the average aspect ratio of the prior austenite grains of a region including bainite and martensite.

[Sheet Thickness]

The hot rolled steel sheet according to an embodiment of the present invention is not particularly limited, but in general has a 1.0 to 6.0 mm sheet thickness. For example, the sheet thickness may also be 1.2 mm or more, 1.6 mm or more, or 2.0 mm or more and/or may also be 5.0 mm or less or 4.0 mm or less.

[Mechanical Properties]

[Tensile Strength: TS]

According to the hot rolled steel sheet having the above chemical composition and microstructure, it is possible to achieve a high tensile strength, specifically a 780 MPa or more tensile strength. The tensile strength is preferably 800 MPa or more, 820 MPa or more, or 840 MPa or more. According to the hot rolled steel sheet according to an embodiment of the present invention, despite having such an extremely high tensile strength, by the specific combination of the chemical composition and microstructure explained above, it is possible to achieve both improvement of the hole expandability and realization of a high yield ratio. The upper limit of the tensile strength is not particularly prescribed, but for example the tensile strength of the hot rolled steel sheet may be 1180 MPa or less, 980 MPa or less, 940 MPa or less, 900 MPa or less, or 860 MPa or less. The tensile strength is measured by taking a JIS No. 5 test piece from an orientation (C direction) where the longitudinal direction of the test piece becomes parallel to the rolling perpendicular direction of the hot rolled steel sheet and conducting a tensile test based on JIS Z 2241: 2011.

[Yield Ratio: YR]

According to the hot rolled steel sheet having the above chemical composition and microstructure, in addition to a high tensile strength, it is possible to raise the yield ratio, more specifically to achieve a 0.70 or more yield ratio. The yield ratio is preferably 0.75 or more, more preferably 0.80 or more. The upper limit is not particularly prescribed, but for example the yield ratio may also be 0.90 or less or 0.85 or less. The yield ratio is determined by the following formula based on the tensile strength and 0.2% yield strength measured by taking a JIS No. 5 test piece from an orientation (C direction) where the longitudinal direction of the test piece becomes parallel to the rolling perpendicular direction of the hot rolled steel sheet and conducting a tensile test based on JIS Z 2241: 2011.

Yield ⁢ ratio ⁢ YR = 0.2 % ⁢ yield ⁢ strength / tensile ⁢ strength ⁢ TS

[Hole Expansion Rate: λ]

According to the hot rolled steel sheet having the above chemical composition and microstructure, it is possible to achieve a high hole expandability, specifically a 60.0% or more hole expansion rate. The hole expansion rate is preferably 65.0% or more, more preferably 70.0% or more or 80% or more. The upper limit of the hole expansion rate is not particularly prescribed, but, for example, the hole expansion rate may also be 120% or less, 110% or less, or 100% or less. The hole expansion rate is determined in the following way. First a width 100 mm×length 100 mm test piece is taken from the hot rolled steel sheet. A punch diameter: 10 mm and die diameter: 10.25 to 11.5 mm (clearance 12.5%) punching tool is used to prepare a punched hole (initial hole: hole diameter d0=10 mm). Next, with the burr becoming the die side, the initial hole is pushed open by a conical punch of a vertex angle of 60° until a crack is formed passing through the sheet thickness. The hole size d1 mm at the time of cracking is measured to find the hole expansion rate λ (%) of each test piece by the following formula. This hole expansion test is conducted three times and the average value of these is determined as the hole expansion rate λ.

λ = 100 × { ( d ⁢ 1 - d ⁢ 0 ) / d ⁢ 0 }

<Method of Production of Hot Rolled Steel Sheet>

Next, a preferable method of production of the hot rolled steel sheet according to an embodiment of the present invention will be explained. The following explanation is intended to illustrate the characteristic method for production of the hot rolled steel sheet according to an embodiment of the present invention and is not intended to limit the hot rolled steel sheet to one produced by the method of production explained below.

The method of production of the hot rolled steel sheet according to an embodiment of the present invention is characterized by comprising

• • hot rolling including heating a slab having the chemical composition explained above in relation to the hot rolled steel sheet, then finish rolling it, wherein the conditions of the following (a) to (e) are satisfied:
  • (a) a heating temperature of the slab is 1200 to 1300° C.,
  • (b) a holding time at a temperature region of 1200 to 1300° C. is 1000 to 4000 seconds,
  • (c) the finish rolling is performed using a tandem rolling machine comprised of five or more rolling stands and a total rolling reduction at prior stages of rolling passes besides final three stages is 60 to 90%,
  • (d) rolling reductions in the final three stages of individual rolling passes is 10% or more and a total rolling reduction in the final three stages of rolling passes is 30 to 50%, and
  • (e) an end temperature of the finish rolling is 900 to 1000° C.,
  • primary cooling the finish rolled steel sheet by an average cooling speed of 50 to 200° C./s down to an intermediate air-cooling temperature of 670 to 750° C., then intermediate cooling it over 3 to 10 seconds, and
  • secondary cooling the intermediate air-cooled steel sheet by an average cooling speed of 50 to 200° C./s, then coiling it at a coiling temperature of 20 to 200° C. Below, the steps will be explained in detail.

[Hot Rolling Step]

[(a) Heating Temperature of Slab: 1200 to 1300° C.]

[(b) Holding Time at Temperature Region of 1200 to 1300° C.: 1000 to 4000 Seconds]

First, a slab having the chemical composition explained above in relation to the hot rolled steel sheet is heated. The slab used is preferably cast by a continuous casting method from the viewpoint of productivity, but may also be produced by an ingot making method or a thin slab casting method. The slab used contains relatively large amounts of alloy elements for obtaining high strength steel sheet. For this reason, before being supplied for hot rolling, the slab has to be heated to make the alloy elements dissolve in the slab. If the heating temperature is low, sometimes the alloy elements will not sufficiently dissolve in the slab but coarse alloy carbides will remain and brittle fracture will occur during the hot rolling. For this reason, the heating temperature is preferably 1200° C. or more. The upper limit of the heating temperature is not particularly prescribed, but 1300° C. or less is preferable from the viewpoint of the capacity of the heating facilities and productivity. Further, by making the holding time in the temperature region of 1200 to 1300° C. 1000 seconds or more, it is possible to reliably make the alloy elements dissolve in the slab. The upper limit of the holding time is not particularly prescribed, but is preferably 4000 seconds or less from the viewpoint of the productivity, etc. If performing rough rolling, it is also possible to hold the steel sheet at the temperature region of 1200 to 1300° C. after the rough rolling.

[Rough Rolling]

In the present method, for example, the heated slab may be rough rolled before the finish rolling so as to adjust the sheet thickness, etc. The rough rolling need only secure the desired sheet bar dimensions. The conditions are not particularly limited.

[(c) Total Rolling Reduction in Rolling Passes of Prior Stages of Finish Rolling: 60 to 90%]

The heated slab or the slab additionally rough rolled according to need is next finish rolled. In the present method of production, the finish rolling is performed by a tandem rolling machine comprised of five or more rolling stands, more specifically five to eight rolling stands. In the present method of production, in the finish rolling performed on the heated slab, the total rolling reduction at the prior stages of rolling passes besides the final three stages (last three stages) is controlled to 60 to 90%. In the prior stages of individual rolling passes, it is possible to perform rolling by such a high rolling reduction so as to promote recrystallization and refinement of the microstructure. However, the recrystallization rates in the prior stages of individual rolling passes do not have to be 100%. Refinement of the microstructure due to such recrystallization in extremely advantageous for forming the desired microstructure and improving the hole expandability and other properties. If the total rolling reduction of the prior stages of rolling passes is less than 60%, the desired microstructure including ferrite, bainite, and martensite in specific ratios cannot be obtained and sometimes the hole expandability and other properties fall. Therefore, the total rolling reduction of the prior stages of rolling passes is 60% or more, preferably 70% or more. On the other hand, if the total rolling reduction of the prior stages of rolling passes is too high, the rolling load becomes excessive and the load of the rolling machine becomes high. For this reason, the total rolling reduction of the prior stages of rolling passes is 90% or less.

[(d) Rolling Reductions in Final Three Stages of Individual Rolling Passes: 10% or More and Total Rolling Reduction of Final Three Stages of Rolling Passes of Finish Rolling: 30 to 50%]

In the finish rolling of the present method of production, the individual rolling reductions at the final three stages of rolling passes are controlled to 10% or more and the total rolling reduction of the final three stages of rolling passes is controlled within a range of 30 to 50%. At the later stages of rolling passes of finish rolling, unlike the case of the prior stages of rolling passes, it is necessary to suppress recrystallization. By controlling the rolling reductions in the final stages of individual rolling passes to 10% or more while controlling the total rolling reduction in the final three stages of rolling passes within a range of 30 to 50%, the rolling reductions in the individual rolling passes are limited within a range of 10 to 30%. By performing the final three stages of rolling passes under such relatively light rolling reduction, it is possible to suppress recrystallization and reliably introduce dislocations at the individual rolling passes and possible to control the average aspect ratio of the prior austenite grains in a region including bainite and martensite in the microstructure of the finally obtained hot rolled steel sheet to 3.0 or more.

If the rolling reductions in the individual final three stages of rolling passes are less than 10%, recrystallization is suppressed in the individual rolling passes, but in the rolling passes, it is not possible to sufficiently introduce dislocations and it becomes no longer possible to achieve the desired aspect ratio in the finally obtained microstructure. Similarly, even if the total rolling reduction in the final three stages of rolling passes is less than 30%, it is not possible to sufficiently introduce dislocations in at least one rolling pass of the final three stages and it becomes no longer possible to achieve the average aspect ratio of the prior austenite grains in a region including bainite and martensite of 3.0 or more in the finally obtained microstructure. On the other hand, if the rolling reductions in the final three stages of individual rolling passes are more than 30% or the total rolling reduction in the final three stages of rolling passes is more than 50%, recrystallization is promoted and dislocations cannot be sufficiently introduced and similarly it becomes no longer possible to achieve the average aspect ratio of the prior austenite grains in the region including bainite and martensite of 3.0 or more in the finally obtained microstructure. The total rolling reduction at the final three stages of rolling passes of the finish rolling is preferably controlled within a range of 35 to 50%.

[(e) End Temperature of Finish Rolling: 900 to 1000° C.]

In the present method of production, in addition to control of the rolling reductions at prior stages and later stages after the finish rolling, the end temperature of the finish rolling is also important in controlling the microstructure of steel sheet. If the end temperature of the finish rolling is low, sometimes the microstructure becomes uneven and the strength and/or hole expandability, etc., fall. For this reason, the end temperature of the finish rolling is 900° C. or more. On the other hand, if the end temperature of the finish rolling is high, in the final three stages of individual rolling passes of the finish rolling, recrystallization is promoted and it becomes no longer possible to sufficiently introduce dislocations. As a result, in the finally obtained microstructure, it becomes no longer possible to achieve the average aspect ratio of the prior austenite grains in the region including bainite and martensite of 3.0 or more. Therefore, the end temperature of the finish rolling is 1000° C. or less.

[Intermediate Air-Cooling Step]

In the next intermediate air-cooling step, the finish rolled steel sheet is primary cooled on a runout table (ROT) at an average cooling speed of 50 to 200° C./s down to an intermediate air-cooling temperature of 670 to 750° C., then intermediate air-cooled over 3 to 10 seconds. By primary cooling by an average cooling speed of 50 to 200° C./s down to an intermediate air-cooling temperature of 670 to 750° C., it is possible to keep ferrite from being excessively formed while promoting the formation of TiC precipitates by the following intermediate air-cooling under a high temperature. As a result, ferrite is sufficiently strengthened by precipitates and thereby it is possible to reduce the hardness difference between the ferrite and bainite and raise the hole expandability and yield ratio. Explained in more detail, to promote the formation and grain growth of TiC precipitates at the time of intermediate air-cooling and sufficiently strength the ferrite by precipitates, the intermediate air-cooling temperature has to be set to a relatively high temperature region, i.e., a temperature region of 670 to 750° C. However, in this case, ferrite is excessively produced and in the final microstructure, the area ratio of ferrite exceeds 60% and the desired properties can no longer be obtained. Therefore, in the present method of production, by making the average cooling speed at the primary cooling from after the finish rolling down to the intermediate air-cooling temperature 50° C./s or more, excessive formation of ferrite is suppressed and TiC precipitates are sufficiently formed by the next high temperature intermediate air-cooling. On the other hand, if the average cooling speed of the primary cooling is more than 200° C., formation of ferrite is excessively suppressed, the area ratio of ferrite at the final microstructure becomes less than 30%, and the hole expandability and other properties fall. Therefore, the average cooling speed of the primary cooling is 200° C./s or less, preferably is 160° C./s or less.

If the intermediate air-cooling temperature is more than 750° C. or the duration of the intermediate air-cooling is more than 10 seconds, ferrite is excessively formed or the TiC precipitates coarsen. If ferrite is excessively formed, in the finally obtained hot rolled steel sheet, it becomes no longer possible to form the desired microstructure including ferrite, bainite, and martensite in specific ratios. Further, if the TiC precipitates coarsen, the number density of the TiC precipitates also greatly falls, and therefore the effect of improvement of hardness of the ferrite by precipitation strengthening can no longer be sufficiently obtained. On the other hand, if the intermediate air-cooling temperature is less than 670° C. or the duration of the intermediate air-cooling is less than 3 seconds, the formation and grain growth of the TiC precipitates are suppressed and the desired diameter and/or number density cannot be obtained. In this case as well, in the same way, the effect of improvement of hardness of the ferrite by precipitation strengthening cannot be sufficiently obtained. In addition, formation of ferrite is also excessively suppressed. In such a case, in the finally obtained hot rolled steel sheet, it becomes no longer possible to form the desired microstructure including ferrite, bainite, and martensite in specific ratios.

As opposed to this, by primary cooling in the intermediate air-cooling step by an average cooling speed of 50 to 200° C./s, preferably 50 to 160° C./s, down to an intermediate air-cooling temperature of 670 to 750° C., preferably 690 to 750° C., then intermediate air-cooling over 3 to 10 seconds, preferably 4 to 9 seconds, it becomes possible to make ferrite precipitate in the desired ratio and to form TiC precipitates in the ferrite and suitably grow the grains to finally establish the presence of TiC precipitates having a diameter of 2.0 to 8.0 nm in a number density of 1.0×10¹⁶/cm³ or more. As a result, it becomes possible to sufficiently obtain the effect of improvement of hardness of the ferrite by precipitation strengthening and thereby reduce the hardness difference of ferrite and bainite in the microstructure and remarkably raise the hole expandability and yield ratio.

[Cooling Step]

At the next cooling step, the intermediate air-cooled steel sheet is secondary cooled by an average cooling speed of 50 to 200° C./s, then is coiled at a coiling temperature of 20 to 200° C. By secondary cooling the intermediate air-cooled steel sheet by such a relatively fast average cooling speed, it is possible to suitably make the bainite and martensite precipitate, and therefore in the finally obtained hot rolled steel sheet, it becomes possible to form a microstructure containing ferrite, bainite, and martensite in specific ratios. As opposed to this, if the average cooling speed of the secondary cooling is less than 50° C./s, it is not possible to make the bainite and/or martensite suitably precipitate. Therefore, in the finally obtained hot rolled steel sheet, it is no longer possible to obtain the desired microstructure. In such a case, it is no longer possible to achieve a 780 MPa or more tensile strength. On the other hand, if the average cooling speed of the secondary cooling is more than 200° C., bainite is not sufficiently formed and/or martensite is excessively formed and similarly the desired microstructure can no longer be obtained at the finally obtained hot rolled steel sheet. Therefore, the average cooling speed of the secondary cooling is 200° C./s or less, preferably is 180° C./s or less.

On the other hand, if the coiling temperature is more than 200° C., sometimes cementite precipitates in a relatively large amount. In such a case, the C in the steel is consumed for formation of cementite. As a result, formation of the TiC precipitates is suppressed and the effect of improvement of hardness of the ferrite by precipitation strengthening utilizing the TiC precipitates can no longer be sufficiently obtained. In addition, sometimes the effect of improvement of strength of the hot rolled steel sheet also cannot be sufficiently exhibited. On the other hand, if the coiling temperature is too low, excessive water-cooling, etc., becomes necessary and the productivity falls. Further, embrittlement of the hot rolled steel sheet is also sometimes triggered. Therefore, the coiling temperature is 20° C. or more.

According to the hot rolled steel sheet produced by the above-mentioned method of production, by the area ratio of the hard phase bainite in the microstructure being controlled to within the 30 to 60% relatively high range and additionally the average aspect ratio of the prior austenite grains in a region including bainite and martensite becoming 3.0 or more, it is possible to utilize the dislocation strengthening relating to this and as a result possible to remarkably raise the strength of the hot rolled steel sheet. Furthermore, since TiC precipitates having a diameter of 2.0 to 8.0 nm are present in the ferrite in a number density of 1.0×10¹⁶/cm³ or more, while improvement of the strength of the hot rolled steel sheet as a whole is naturally contributed to due to precipitation strengthening, it is possible to sufficiently reduce the hardness difference of the hard phase bainite present in a relatively large amount in the microstructure and the softest phase ferrite in the triplex structure and, as a result, it becomes possible to remarkably raise the hole expandability and yield ratio of the hot rolled steel sheet. Therefore, the hot rolled steel sheet produced by the above method of production can be effectively used even in members where achievement of both the contradictory properties of high strength and excellent workability is sought and, further, impact resistance is sought, and therefore is particularly useful for the automobile industry.

Below, examples will be used to explain the present invention in more detail, but the present invention is not limited to these examples in any way.

EXAMPLES

In the following examples, hot rolled steel sheets according to an embodiment of the present invention were produced under various conditions. The obtained hot rolled steel sheets were investigated for tensile strength (TS), hole expansion rate (λ), and yield ratio (YR).

First, slabs having various chemical compositions shown in Table 1 were produced in a vacuum melting furnace. Next, these were reheated to the heating temperatures shown in Table 2, then were rough rolled to produce 30 mm thick coarse bars. The coarse bars were held at the temperature region of 1200 to 1300° C. for 3600 seconds, then were finish rolled using a rolling machine comprised of a plurality of rolling stands by two or more prior stages of rolling passes and three later stages of rolling passes under the conditions shown in Table 2. The end temperatures of the finish rolling were as shown in Table 2. Next, the finish rolled steel sheets were primary cooled under the conditions shown in Table 2 down to the intermediate air-cooling temperature, then were intermediate air-cooled. Finally, the intermediate cooled steel sheets were secondary cooled under the conditions shown in Table 2 down to the coiling temperature, then were cooled at the coiling temperatures to obtain hot rolled steel sheets having 2.5 mm sheet thicknesses.

TABLE 1
Steel	Chemical composition (mass %), balance: Fe and impurities

no.	C	Si	Mn	Ti	Al	P	S	N	O	Nb	V	Cr	Ni	Cu
A	0.04	0.050	1.49	0.14	0.35	0.020	0.0010	0.003	0.001	0.013
B	0.07	0.090	2.50	0.18	0.39	0.020	0.0010	0.003	0.001	0.020
C	0.04	0.020	0.70	0.15	0.25	0.020	0.0010	0.004	0.001	0.016	0.220	0.10
D	0.03	0.060	0.50	0.09	0.36	0.020	0.0010	0.003	0.001	0.012			0.21	0.20
E	0.05	0.040	1.35	0.16	0.37	0.010	0.0020	0.003	0.001	0.047
F	0.07	0.070	1.72	0.19	0.31	0.010	0.0020	0.004	0.001	0.014
G	0.10	0.050	1.93	0.20	0.29	0.010	0.0010	0.004	0.001	0.015
H	0.10	0.100	3.00	0.20	0.30	0.020	0.0020	0.003	0.001
I	0.20	0.070	1.49	0.14	0.35	0.010	0.0010	0.003	0.001	0.011
J	0.10	0.090	2.50	0.02	0.39	0.010	0.0010	0.003	0.001	0.014
K	0.04	0.005	2.10	0.15	0.28	0.010	0.0010	0.003	0.001	0.013
L	0.05	0.050	2.10	0.14	0.10	0.010	0.0010	0.003	0.001	0.013

Steel

Chemical composition (mass %), balance: Fe and impurities

no.

Mo

B

Sn

Sb

Ca

Mg

Hf

Bi

REM

As

Zr

Co

Zn

W

A

B

C

0.001

D

0.100

0.0010

0.001

E

0.0020

0.100

0.100

F

0.002

0.0010

0.002

G

0.0010

0.003

0.0010

0.001

H

I

J

K

L

Underlines indicate outside scope of present invention.

	TABLE 2
	Hot rolling step

(c)

(d)

Intermediate air-cooling step

Cooling step

			(b)	Total	Minimum	Total	(e)	Primary	Inter-	Inter-	Secondary
			1200 to	rolling	rolling	rolling	Finish	cooling	mediate	mediate	cooling
		(a)	1300° C.	reduction	reduction	reduction	rolling	average	air-	air-	average
		Heating	holding	of prior	of later	of later	end	cooling	cooling	cooling	cooling	Coiling
Prod.	Steel	temp.	time	stages	three stages	three stages	temp.	speed	temp.	duration	speed	temp.
no.	no.	° C.	s	%	%	%	° C.	° C./s	° C.	s	° C./s	° C.	Remarks
1	A	1250	3600	84	10	46	950	90	720	9	100	190	Inv. ex.
2	A	1250	3600	82	10	45	940	85	700	7	90	200	Inv. ex.
3	A	1200	3600	79	12	40	950	90	690	6	55	150	Inv. ex.
4	A	1240	3600	70	11	38	930	120	710	6	100	100	Inv. ex.
5	A	1220	3600	80	12	50	960	100	690	7	120	50	Inv. ex.
6	A	1230	3600	85	10	42	950	150	740	4	180	30	Inv. ex.
7	B	1250	3600	84	10	45	1000	65	730	8	100	200	Inv. ex.
8	B	1200	3600	77	10	48	970	55	750	9	115	150	Inv. ex.
9	B	1200	3600	82	10	40	950	100	690	4	90	100	Inv. ex.
10	B	1200	3600	79	10	35	920	160	690	5	105	50	Inv. ex.
11	B	1200	3600	70	11	46	900	60	700	6	95	30	Inv. ex.
12	C	1250	3600	80	10	42	960	100	690	6	100	200	Inv. ex.
13	D	1250	3600	85	11	45	980	110	700	5	120	180	Inv. ex.
14	E	1250	3600	84	12	48	1000	105	710	4	180	190	Inv. ex.
15	F	1250	3600	77	10	40	925	90	730	8	150	200	Inv. ex.
16	G	1250	3600	88	10	38	950	95	750	9	115	180	Inv. ex.
17	H	1250	3600	90	12	50	900	180	730	9	180	90	Inv. ex.
18	A	1250	3600	80	10	46	950	250	700	5	100	190	Comp. ex.
19	A	1250	3600	40	10	45	940	95	690	6	120	150	Comp. ex.
20	A	1250	3600	78	18	70	960	105	700	5	100	160	Comp. ex.
21	B	1250	3600	79	10	33	930	100	710	7	100	400	Comp. ex.
22	B	1250	3600	82	10	40	1000	85	800	5	150	150	Comp. ex.
23	C	1270	3600	80	13	45	1100	84	680	6	120	150	Comp. ex.
24	D	1250	3600	82	10	39	980	40	690	3	80	140	Comp. ex.
25	E	1250	3600	84	10	48	960	72	685	4	38	130	Comp. ex.
26	F	1250	3600	77	11	40	1000	100	400	5	60	150	Comp. ex.
27	G	1250	3600	88	10	38	920	130	730	16	60	140	Comp. ex.
28	I	1250	3600	80	10	42	960	120	700	6	100	130	Comp. ex.
29	J	1250	3600	85	12	45	925	65	730	8	200	150	Comp. ex.
30	K	1250	3600	84	10	48	930	70	670	7	80	140	Comp. ex.
31	L	1250	3600	77	11	40	930	70	670	7	80	140	Comp. ex.

The properties of each obtained hot rolled steel sheet were measured and evaluated by the following methods.

[Calculation of Diameter and Number Density of TiC Precipitates]

The diameter and number density of TiC precipitates were calculated by the 3D atom probe measurement method explained in detail in this Description while deeming the detection rate a of atoms unique to the apparatus as 0.35.

[Tensile Strength (TS) and Yield Ratio (YR)]

The tensile strength (TS) was measured by taking a length 200 mm and thickness 2.5 mm JIS No. 5 test piece from an orientation (C direction) of the long direction of the test piece becoming parallel to the rolling perpendicular direction of the hot rolled steel sheet and conducting a tensile test based on JIS Z 2241: 2011. More specifically, a test was performed at room temperature in a range of 10 to 35° C. The test piece was given a tensile test force and given strain until break. Further, the yield ratio (YR) was determined by the following formula based on the tensile strength (TS) and 0.2% yield strength measured by similarly performing a tensile test using a JIS No. 5 test piece based on JIS Z 2241: 2011:

Yield ⁢ ratio ⁢ YR = 0.2 % ⁢ yield ⁢ strength / tensile ⁢ strength ⁢ TS

[Hole Expansion Rate: λ]

The hole expansion rate was determined in the following way. First a thickness 2.5 mm×width 100 mm×length 100 mm test piece was taken from the hot rolled steel sheet. A punch diameter: 10 mm and die diameter: 10.25 to 11.5 mm (clearance 12.5%) punching tool was used to prepare a punched hole (initial hole: hole diameter d0=10 mm). Next, with the rising edge (burr) becoming the die side, the initial hole was pushed open by a conical punch of a vertex angle of 60° until a crack was formed passing through the sheet thickness. The hole size d1 mm at the time of cracking was measured to find the hole expansion rate λ (%) of each test piece by the following formula. This hole expansion test was conducted three times and the average value of these was determined as the hole expansion rate λ.

λ = 100 × { ( d ⁢ 1 - d ⁢ 0 ) / d ⁢ 0 }

A case with a tensile strength (TS) of the hot rolled steel sheet of 780 MPa or more, a hole expansion rate (λ) of 60.0% or more, and a yield ratio (YR) of 0.70 or more was evaluated as a hot rolled steel sheet which is high in strength and has high hole expandability and yield ratio. The results are shown in Table 3.

	TABLE 3
	Composition

TiC

Prior γ

Mechanical properties

precipitates

A

B

grain

Hole

Pro-					Balance		Number	Ferrite	Bainite		average	Tensile	expand-	Yield
duc-				Mar-	struc-	Diam-	density	nanohard-	nanohard-		aspect	strength	ability	ratio
tion	Steel	Ferrite	Bainite	tensite	tures	eter	×1016/	ness	ness	A/B	ratio	TS	λ	YR
no.	no.	Area %	Area %	Area %	Area %	nm	cm3	GPa	GPa	—	—	MPa	%	—	Remarks
1	A	42	50	7	1	7.4	1.0	2.9	4.0	0.73	3.9	795	60.0	0.71	Inv. ex.
2	A	40	52	7	1	5.5	2.5	3.6	3.7	0.97	4.0	800	77.8	0.80	Inv. ex.
3	A	45	48	5	2	5.3	2.8	3.5	3.8	0.92	3.8	795	73.7	0.78	Inv. ex.
4	A	33	48	18	1	5.7	2.2	3.4	3.8	0.89	3.5	820	71.6	0.79	Inv. ex.
5	A	35	55	8	2	5.4	2.6	3.7	3.9	0.95	4.2	812	75.9	0.78	Inv. ex.
6	A	31	59	7	3	7.3	1.1	3.2	3.8	0.84	3.9	825	67.4	0.75	Inv. ex.
7	B	51	42	6	1	7.0	1.5	3.5	4.2	0.83	4.0	789	66.7	0.76	Inv. ex.
8	B	53	37	8	2	7.5	1.2	3.3	4.3	0.77	4.2	782	65.0	0.75	Inv. ex.
9	B	54	40	5	1	3.2	16.0	4.5	4.0	1.13	3.9	830	90.0	0.83	Inv. ex.
10	B	32	55	12	1	4.5	5.8	4.4	4.0	1.10	3.2	840	88.0	0.85	Inv. ex.
11	B	51	42	5	2	5.6	3.0	4.0	4.1	0.98	4.5	810	78.0	0.80	Inv. ex.
12	C	43	50	6	1	5.0	3.3	4.2	3.8	1.10	3.9	815	88.0	0.83	Inv. ex.
13	D	46	42	10	2	3.4	6.7	4.3	3.7	1.16	3.6	808	93.0	0.88	Inv. ex.
14	E	43	40	17	0	7.0	1.3	3.5	3.7	0.95	3.4	830	75.7	0.78	Inv. ex.
15	F	52	40	6	2	7.5	1.3	3.3	3.5	0.94	3.9	795	75.4	0.78	Inv. ex.
16	G	50	42	7	1	8.0	1.2	3.1	3.8	0.82	4.3	790	65.3	0.75	Inv. ex.
17	H	58	36	5	1	2.3	50.0	5.5	4.6	1.20	5.8	880	86.3	0.85	Inv. ex.
18	A	25	60	14	1	4.4	0.9	2.6	3.8	0.68	3.4	820	54.7	0.70	Comp. ex.
19	A	24	66	8	2	7.3	1.1	3.0	3.5	0.86	3.5	810	52.0	0.77	Comp. ex.
20	A	22	70	5	3	6.2	1.7	3.3	3.9	0.85	1.5	775	67.7	0.76	Comp. ex.
21	B	60	30	9	1	8.5	0.8	2.2	4.0	0.55	4.6	740	44.0	0.60	Comp. ex.
22	B	80	15	4	1	9.0	0.7	2.2	4.0	0.55	4.0	730	45.0	0.65	Comp. ex.
23	C	42	43	14	1	4.5	4.9	3.6	3.9	0.92	1.7	770	73.8	0.77	Comp. ex.
24	D	70	20	9	1	7.0	1.7	3.4	4.0	0.85	4.2	745	68.0	0.78	Comp. ex.
25	E	43	54	1	2	3.7	9.3	3.2	3.7	0.86	4.1	776	69.2	0.76	Comp. ex.
26	F	17	75	6	2	1.9	65.0	2.7	4.7	0.57	4.5	825	46.0	0.59	Comp. ex.
27	G	72	20	7	1	9.5	0.6	2.6	4.2	0.62	4.6	741	49.5	0.61	Comp. ex.
28	I	40	52	7	1	5.0	3.0	3.2	5.0	0.64	4.0	805	51.2	0.62	Comp. ex.
29	J	47	43	8	2	3.4	0.2	2.5	4.7	0.53	3.7	765	42.6	0.55	Comp. ex.
30	K	27	66	6	1	7.0	0.8	2.8	3.8	0.74	3.5	780	61.0	0.67	Comp. ex.
31	L	50	41	7	2	7.5	0.9	2.9	4.0	0.73	3.8	777	60.0	0.68	Comp. ex.

Referring to Tables 1 to 3, in Comparative Example 18, the average cooling speed down to the intermediate air-cooling temperature at the primary cooling was fast, and therefore formation of ferrite was excessively suppressed and at the final microstructure, the area ratio of ferrite became less than 30%. As a result, the λ fell. In Comparative Example 19, the total rolling reduction of the prior stages of rolling passes of the finish rolling was low, and therefore it is believed that recrystallization was suppressed at the prior stages of rolling passes and the microstructure could not be refined. As a result, the desired microstructure could not be obtained and the λ fell. In Comparative Example 20, the total rolling reduction at the final three stages of rolling passes of the finish rolling was high, and therefore it is believed that at the later stages of rolling passes, recrystallization was promoted and dislocations could not be sufficiently introduced. As a result, the average aspect ratio of the prior austenite grains in a region including bainite and martensite became less than 3.0, and the TS fell. In Comparative Example 21, the coiling temperature was high, and therefore it is believed cementite precipitated relatively largely and the C in the steel was consumed for formation of cementite. As a result, formation of the TiC precipitates was suppressed and the number density of the TiC precipitates became less than 1.0×10¹⁶/cm³, the effect of improvement of hardness of the ferrite by precipitation strengthening and further the effect of improvement of strength of the hot rolled steel sheet could not be sufficiently obtained, and the TS, λ, and YR fell. In Comparative Example 22, the intermediate air-cooling temperature was high, and therefore ferrite was excessively formed, further the TiC precipitates coarsened, and in relation to this, the number density of the TiC precipitates also fell. As a result, the TS, λ, and YR fell. In Comparative Example 23, the end temperature of the finish rolling was high, and therefore it is believed that in the final three stages of rolling passes of the finish rolling, recrystallization was promoted and dislocations could not be sufficiently introduced. As a result, the average aspect ratio of the prior austenite grains in a region including bainite and martensite became less than 3.0 and the TS fell. In Comparative Example 24, the average cooling speed at the primary cooling down to the intermediate air-cooling temperature was slow, and therefore ferrite was excessively formed and the area ratio of ferrite at the final microstructure became more than 60%. As a result, the TS fell.

In Comparative Example 25, the average cooling speed in the secondary cooling after the intermediate cooling was slow, and therefore the area ratio of martensite became less than 5% and the TS fell. In Comparative Example 26, the intermediate air-cooling temperature was low, and therefore formation and grain growth of the TiC precipitates were suppressed and the desired diameter of the TiC precipitates could not be obtained. As a result, the effect of improvement of hardness of the ferrite by precipitation strengthening could not be sufficiently obtained and the 2 and YR fell. In Comparative Example 27, the duration of the intermediate air-cooling was long, and therefore ferrite was excessively formed, the TiC precipitates coarsened, and, relating to this, the number density of the TiC precipitates also fell. As a result, the TS, λ, and YR fell. In Comparative Example 28, the C content was high, and therefore the λ and YR fell due to the formation of cementite. In Comparative Example 29, the Ti content was low, and therefore the TiC precipitates could not be made to precipitate at a sufficient number density. As a result, the TS, λ, and YR fell. In Comparative Example 30, the Si content was low, and therefore it is believed precipitation of cementite could not be sufficiently suppressed and the C in the steel was consumed for formation of cementite. As a result, formation of the TiC precipitates was suppressed, the number density of the TiC precipitates became less than 1.0×10¹⁶/cm³, the effect of improvement of hardness of the ferrite by precipitation strengthening could not be sufficiently obtained, and the YR fell. In Comparative Example 31, the Al content was low, and therefore similarly it is believed the formation of cementite could not be sufficiently suppressed and the C in the steel was consumed for formation of cementite. As a result, the formation of the TiC precipitates was suppressed, the number density of the TiC precipitates became less than 1.0×10¹⁶/cm³, the effect of improvement of hardness of the ferrite by precipitation strengthening and further the effect of improvement of strength of the hot rolled steel sheet could not be sufficiently obtained, and the TS and YR fell.

In contrast to this, in each of the hot rolled steel sheets of all of the invention examples, by having a predetermined chemical composition and further suitably controlling the conditions at the method of production, it was possible to obtain hot rolled steel sheets having a microstructure containing, by area ratio, ferrite: 30 to 60%, bainite: 30 to 60%, and martensite: 5 to 20%, wherein TiC precipitates having a diameter of 2.0 to 8.0 nm are present in the ferrite in a number density of 1.0×10¹⁶/cm³ or more, and an average aspect ratio of the prior austenite grains in a region including the bainite and martensite is 3.0 or more. Further, as a result, due to the relatively high bainite percentage and the dislocation strengthening and the precipitation strengthening by TiC precipitates, it was possible to achieve a high strength of the tensile strength of 780 MPa or more while reducing the hardness difference of ferrite and bainite to raise the hole expandability and yield ratio. Further, in the hot rolled steel sheets of Invention Examples 2 to 17 with ratios of average nanohardness of bainite/average nanohardness of ferrite controlled to within a range of 0.75 to 1.20, it was possible to achieve a 65.0% or more hole expansion rate and 0.75 or more yield ratio and therefore possible to further remarkably raise the hole expandability and yield ratio of the hot rolled steel sheet. Further, if there were balance structures present in the invention examples, the balance structures were at least one of pearlite and retained austenite.