STEEL SHEET

Description

FIELD

The present disclosure relates to steel sheet.

BACKGROUND

In the automobile industry, reduction of the weight of car bodies has been sought from the viewpoint of improvement of the fuel efficiency. To achieve both lighter weight of car bodies and collision safety, raising the strength of the steel sheet used is one effective method. Due to such a background, development efforts of high strength steel sheet have been proceeding.

In relation to this, PTL 1 describes high strength hot dip galvanized steel sheet comprising a base sheet of a steel sheet having a hot dip galvanized layer on its surface, in which hot dip galvanized steel sheet, the base sheet containing, by mass %, C: 0.02 to 0.20%, Si: 0.7% or less, Mn: 1.5 to 3.5%, P: 0.10% or less, S: 0.01% or less, Al: 0.1 to 1.0%, N: 0.010% or less, and Cr: 0.03 to 0.5%, an annealing surface oxide index A, defined by the formula A=400Al/(4Cr+3Si+6Mn) where the contents of Al, Cr, Si, and Mn are entered as terms, being 2.3 or more, and a balance of Fe and unavoidable impurities, having a microstructure of the base sheet comprised of ferrite and a second phase, the second phase being mainly martensite.

Further, PTL 1 describes that the high strength hot dip galvanized steel sheet has an excellent surface quality and a 590 MPa or more tensile strength suitable for applications of mainly frame members, rockers, and other structural parts of automobiles.

CITATIONS LIST

Patent Literature

• [PTL 1] Japanese Unexamined Patent Publication No. 2005-220430
• [PTL 2] WO2022/181761
• [PTL 3] WO2020/145256

SUMMARY

Technical Problem

In recent years, in response to the demands for further improvement of fuel efficiency, the need for reduction of weight has been rising—not only for the frame members and other structural parts described in PTL 1, but also for roofs, hoods, fenders, doors, and other external panel parts. The external panel parts, unlike such structural parts, are visible to the eye, therefore not only the strength and other properties, but also the design quality and surface quality are important. Therefore, excellent appearance after shaping is sought. On the other hand, along with such reduction of weight, further higher strength and lower thickness are being sought even in the steel sheet used for these external panel parts. In addition, along with the increased complexity of shape of these external panel parts, asperities tend to easily form at the steel sheet surface after shaping. If such asperities form, there is the problem that the appearance is degraded.

More specifically, for example, in the case of DP steel (dual phase steel) comprised of soft ferrite and mainly martensite hard second phases such as described in PTL 1, at the time of press-forming and other working, the soft phases comprised of ferrite and their surroundings deform with precedence resulting easily in uneven deformation. For that reason, if utilizing such a composite structure steel made of soft phases and hard phases, fine asperities form at the steel sheet surface after shaping and thereby defects in appearance called “ghost lines” sometimes occur.

As means for keeping down such asperities at the steel sheet surface, for example, PTL 3 discloses steel sheet having a specific chemical composition and metallographic structure and having a value X1 of a standard deviation in the sheet thickness direction of the average Mn concentration at the rolling direction at the ¼ position of the sheet thickness direction divided by the average Mn concentration of 0.025 or less. Further, PTL 4 discloses steel sheet having a specific chemical composition and including an aggregate structure having a metallographic structure at surface layer regions comprised of ferrite and 0.01 to 5.0% of second phases by volume fraction, having a metallographic structure at an inside region comprised of ferrite and 2.0 to 10.0% of second phases by volume fraction, having a volume fraction of second phases of the surface layer regions smaller than a volume fraction of second phases of the inside region, having an average grain size of the second phases at the surface layer regions of 0.01 to 4.0 μm, and having a strength ratio X_{ODF{001}/{111}} of a {001} direction and {111} direction of ferrite of 0.60 or more and less than 2.00.

The present disclosure has as its object to provide high strength steel sheet having improved appearance after shaping by a novel constitution.

Solution to Problem

The disclosers engaged in detailed studies focusing on the form of the hard phases in a metallographic structure for achieving the above object. As a result, the disclosers discovered that by keeping down the formation of hard phases connected in band shapes (banded hard phases) and making the hard phases disperse more evenly in the metallographic structure, the high strength due to such hard phases is maintained while defects in appearance after shaping are decreased. Specifically, the disclosers discovered that by reducing center segregation of Mn at the time of solidification, which is a factor behind formation of banded hard phases, and reducing the hard phase fraction and by making the variation smaller, it is possible to sufficiently maintain high strength while remarkably decreasing the defects in appearance after shaping.

This disclosure was completed based on these discoveries and include the following aspects.

(Aspect 1)

Steel sheet having a chemical composition comprising, by mass %,

• • C: 0.030 to 0.100%,
  • Mn: 1.00 to 2.50%,
  • Si: 0.005 to 1.500%,
  • P: 0.100% or less,
  • S: 0.0200% or less,
  • Al: 0.005 to 0.700%,
  • N: 0.0150% or less,
  • O: 0.0100% or less,
  • Cr: 0 to 0.80%,
  • Mo: 0 to 0.50%,
  • B: 0 to 0.0100%,
  • Ti: 0 to 0.100%,
  • Nb: 0 to 0.100%,
  • V: 0 to 0.50%,
  • Ni: 0 to 1.00%,
  • Cu: 0 to 1.00%,
  • W: 0 to 1.00%,
  • Sn: 0 to 1.00%,
  • Sb: 0 to 0.200%,
  • Ca: 0 to 0.0100%,
  • Mg: 0 to 0.0100%,
  • Zr: 0 to 0.0100%,
  • REM: 0 to 0.0100%, and
  • bal.: Fe and impurities, wherein
  • an index A expressed by the following formula (1) is 0.45% or more,
  • the metallographic structure comprises, by area %, ferrite: 75 to 97% and hard phase: 3 to 25%, and
  • a standard deviation of the hard phase fraction in the direction perpendicular to rolling is 0.75% or less:

A = [ Si ] + 10 [ P ] + 0.6 [ Al ] + 8 [ Ti ] + 9 [ Nb ] ( 1 )

• • where, [Si], [P], [Al], [Ti], and [Nb] are the contents of the elements in mass % units and 0% is entered if the elements are not included.

(Aspect 2)

Steel sheet of the aspect 1, wherein the chemical composition includes, by mass %, one or more elements selected from the group comprising:

• • Cr: 0.01 to 0.80%,
  • Mo: 0.01 to 0.50%,
  • B: 0.0001 to 0.0100%,
  • Ti: 0.001 to 0.100%,
  • Nb: 0.001 to 0.100%,
  • V: 0.01 to 0.50%,
  • Ni: 0.01 to 1.00%,
  • Cu: 0.01 to 1.00%,
  • W: 0.01 to 1.00%,
  • Sn: 0.01 to 1.00%,
  • Sb: 0.001 to 0.200%,
  • Ca: 0.0001 to 0.0100%,
  • Mg: 0.0001 to 0.0100%,
  • Zr: 0.0001 to 0.0100%, and
  • REM: 0.0001 to 0.0100%.

(Aspect 3)

Steel sheet of the above aspect 1 or 2, wherein the steel sheet satisfies the following formula (2):

( T ⁢ S - 180 , 000 / T ⁢ S ) / Vm ≥ 35 ( 2 )

• • where, TS is the tensile strength in MPa units and Vm is the hard phase fraction in area % units.

(Aspect 4)

Steel sheet of any one of the above aspects 1 to 3, wherein an average crystal grain size of the ferrite is 5.0 to 30.0 μm and an average crystal grain size of the hard phase is 1.0 to 5.0 μm.

(Aspect 5)

Steel sheet of any one of the above aspects 1 to 4, wherein the hard phase is comprised of at least one of martensite, bainite, tempered martensite, and pearlite.

Advantageous Effects of Invention

According to the present disclosure, it is possible to provide high strength steel sheet having an improved appearance after shaping.

DESCRIPTION OF EMBODIMENTS

Below, preferred embodiments of the steel sheet of the present disclosure will be explained in detail. Note that, in this Description, the various numerical ranges, unless otherwise indicated, mean ranges including the upper and lower limit values.

<Steel Sheet>

The steel sheet according to one embodiment of the present disclosure is characterized by having a chemical composition comprising, by mass %,

• • C: 0.030 to 0.100%,
  • Mn: 1.00 to 2.50%,
  • Si: 0.005 to 1.500%,
  • P: 0.100% or less,
  • S: 0.0200% or less,
  • Al: 0.005 to 0.700%,
  • N: 0.0150% or less,
  • O: 0.0100% or less,
  • Cr: 0 to 0.80%,
  • Mo: 0 to 0.50%,
  • B: 0 to 0.0100%,
  • Ti: 0 to 0.100%,
  • Nb: 0 to 0.100%,
  • V: 0 to 0.50%,
  • Ni: 0 to 1.00%,
  • Cu: 0 to 1.00%,
  • W: 0 to 1.00%,
  • Sn: 0 to 1.00%,
  • Sb: 0 to 0.200%,
  • Ca: 0 to 0.0100%,
  • Mg: 0 to 0.0100%,
  • Zr: 0 to 0.0100%,
  • REM: 0 to 0.0100%, and
  • bal.: Fe and impurities, wherein
  • an index A expressed by the following formula (1) is 0.45% or more,
  • the metallographic structure comprises, by area %, ferrite: 75 to 97% and hard phases: 3 to 25%, and
  • a standard deviation of the hard phase fraction in the direction perpendicular to rolling is 0.75% or less:

A = [ Si ] + 10 [ P ] + 0.6 [ Al ] + 8 [ Ti ] + 9 [ Nb ] ( 1 )

• • where, [Si], [P], [Al], [Ti], and [Nb] are the contents of the elements in mass % units and 0% is entered if the elements are not included.

In roofs, doors, and other external panel parts, from the viewpoint of avoiding surface defects called “surface distortions” formed at the time of press-forming etc., DP steel with a relatively low yield strength is often used. However, as explained above, in the case of DP steel comprised of soft phases made of ferrite and hard phases mainly made of martensite etc., at the time of press-forming or other working, uneven deformation where the soft phases and their surroundings deform with precedence easily occurs. Due to such uneven deformation, fine asperities form at the steel sheet surface after shaping whereby sometimes defects in appearance called “ghost lines” are formed. Explaining the formation of the ghost lines in more detail, first, the time of press-forming or other working, the soft phases comprised of ferrite deform and become recessed while the hard phases mainly comprised of martensite etc. deform but do not become recessed or rather be built up and project out. Due to this, fine asperities are formed at the steel sheet surface after shaping. The fine asperities are formed as projecting parts extending generally along the rolling direction and recessed parts extending generally along the rolling direction aligned in the width direction perpendicular to the rolling direction. Then, when polishing the steel sheet surface after shaping, the projecting parts of the fine asperities of the steel sheet surface are shaved away whereby stripe-shaped pattern ghost lines extending in the rolling direction of the steel sheet appear. Note that the rolling direction is easily identified based on the stretching direction of grains of the steel sheet. Further, the “direction perpendicular to rolling” is a direction vertical to the rolling direction and thickness direction.

Therefore, the disclosers engaged in detailed studies focusing on the form of the hard phases in the metallographic structure so as to decrease defects in appearance after shaping. As a result, the disclosers pinpointed the fact that, first, in steel sheet such as DP steel where there is a mix of soft layers and hard phases, the presence of hard phases connected in band shapes in the metallographic structure causes ghost lines appear more remarkably. Further, the disclosers discovered that by keeping down the formation of such banded hard phases and making the hard phases disperse more uniformly in the metallographic structure, it is possible to sufficiently maintain a high strength while keeping down the formation of fine asperities at the steel sheet surface after shaping and as a result keep down the formation of ghost lines.

More specifically, the disclosers discovered that to keep down the formation of the banded structures relating to the hard phases, it is effective to reduce the Mn segregation at the time of solidification in the slab casting step when solidifying the molten steel to cast a slab. In relation to this, the disclosers engaged in detailed studies on a technique for reducing Mn segregation from the two viewpoints of center segregation and microsegregation.

First, the disclosers thought that to reduce the center segregation of Mn, it would be effective to keep down the fluid movement of molten steel at the time of slab casting and engaged in various studies on the same. Explained more specifically, molten steel solidifies from the surface at the time of solidification. Finally, the center part solidifies. At that time, at the molten steel, solid phases are formed from the liquid phase, therefore at that stage, the Mn in the liquid phase becomes concentrated. If the molten steel fluidly moves at the time of solidification, such concentrated parts of Mn easily finally collect together at the center part in the process of solidification. As a result, the center segregation becomes remarkable. Therefore, the disclosers discovered that, when producing steel sheet, by suitably controlling the conditions at the time of solidification and keeping down such fluid motion of the molten steel, it is possible to remarkably suppress center segregation of Mn.

On the other hand, the disclosers considered it would be effective to reduce the microsegregation of Mn by promoting the diffusion of Mn at the time of solidification and engaged in various studies on the same. To promote the diffusion of Mn, it is effective to create a microstructure in which Mn easily diffuses. Therefore, the disclosers focused on the & phase with the faster speed of diffusion of Mn and investigated by experimentation the degree of impact of different elements in the steel on the microsegregation of Mn so as to make the mode of solidification 8 solidification. As a result, the disclosers discovered that if the contents of C and Mn becomes higher. 8 solidification does not result at the time of solidification, the speed of diffusion of Mn falls, and microsegregation increases, but if the contents of Si, Al. Cr. and Mo become higher, the diffusion of Mn at the time of solidification is promoted and microsegregation can be decreased.

Ghost lines can be decreased by the above such technique of reduction of Mn segregation, but to sufficiently obtain the effect of improvement at the entire length and entire width of a coil, the disclosers studied techniques for further decrease of ghost lines in addition to the above such technique of reduction of Mn segregation. As a result, the disclosers first obtained the discovery that by reduction of the hard phase fraction of the steel sheet, even if center segregation of Mn remains to a certain extent, ghost lines can be decreased. Furthermore, the disclosers obtained the discovery that the formation of ghost lines is greatly affected by the solidified structure. Even if the center segregation of Mn is small, if coarse equiaxed crystals are formed in the solidified structure, negative segregation of Mn is caused, the variation in the hard phase fraction in the direction perpendicular to rolling becomes greater, and the defects in appearance after shaping become worse. For this reason, unlike conventional center segregation countermeasures (note: the fact that for decrease of center segregation, an increase of the equiaxed crystal fraction is necessary is common sense among persons skilled in the art. For example, Takaho Kawawa: “Tetsu-to-Hagane”, vol. 60 (1974), no. 5, p. 486 to p. 500. Kou Kumai et al.: “Tetsu-to-Hagane”, vol. 60 (1974), no. 7, p. 894 to p. 914), by reducing the equiaxed crystal fraction and controlling the solidified structure to a columnar crystal structure, negative segregation of Mn is suppressed and ghost lines can be decreased. Based on these discoveries, the disclosers discovered that by controlling the solidified structure at the time of casting to a columnar crystal structure and reducing center segregation of Mn at the time of solidification, the cause of formation of banded hard phases, and reducing the hard phase fraction and making variation in the same smaller, it is possible to sufficiently maintain a high strength while remarkably decreasing defects in appearance after shaping.

In one embodiment of the present disclosure, the steel sheet, as explained above, has a specific chemical composition and has a unique metallographic structure comprised of a lower hard phase fraction than conventional DP steel and having smaller variation of the hard phase fraction in the direction perpendicular to rolling. This metallographic structure, as explained later, can be obtained by employing a specific chemical composition and casting conditions so that the solidified structure at the time of casting becomes columnar crystals.

Since the steel sheet of the present embodiment has such a unique metallographic structure, i.e., a metallographic structure with a small center segregation of Mn at the time of solidification and further with a small hard phase fraction and variation of the same, it can sufficiently maintain high strength while keeping down the formation of fine asperities at the steel sheet surface after shaping. Due to this, the steel sheet of the present embodiment can sufficiently maintain high strength while remarkably keeping down the formation of ghost lines and other defects in appearance after shaping. That is, according to the present embodiment, it is possible to provide high strength steel sheet having improved appearance after shaping.

Below, the steel sheet of the present embodiment will be explained in further detail. Note that, in the following explanation, the “%” of the contents of the elements and units of the index A, unless otherwise indicated, mean “mass %”.

(Chemical Composition)

The steel sheet of the present embodiment, as explained above, has a unique chemical composition comprising, by mass %,

• • C: 0.030 to 0.100%,
  • Mn: 1.00 to 2.50%,
  • Si: 0.005 to 1.500%,
  • P: 0.100% or less,
  • S: 0.0200% or less,
  • Al: 0.005 to 0.700%,
  • N: 0.0150% or less,
  • O: 0.0100% or less,
  • Cr: 0 to 0.80%,
  • Mo: 0 to 0.50%,
  • B: 0 to 0.0100%,
  • Ti: 0 to 0.100%,
  • Nb: 0 to 0.100%,
  • V: 0 to 0.50%,
  • Ni: 0 to 1.00%,
  • Cu: 0 to 1.00%,
  • W: 0 to 1.00%,
  • Sn: 0 to 1.00%,
  • Sb: 0 to 0.200%,
  • Ca: 0 to 0.0100%,
  • Mg: 0 to 0.0100%,
  • Zr: 0 to 0.0100%,
  • REM: 0 to 0.0100%, and
  • bal.: Fe and impurities, wherein
  • an index A expressed by the following formula (1) is 0.45% or more:

A = [ Si ] + 10 [ P ] + 0.6 [ Al ] + 8 [ Ti ] + 9 [ Nb ] ( 1 )

• • where, [Si], [P], [Al], [Ti], and [Nb] are the contents of the elements in mass % units and 0% is entered if the elements are not included.

Below, the elements in this chemical composition will be explained in detail.

[C: 0.030 to 0.100%]

C is an element raising the strength of the steel sheet. To sufficiently obtain such an effect, the C content is 0.030% or more. The C content may also be 0.035% or more, 0.040% or more, or 0.050% or more. On the other hand, if excessively containing C, diffusion of Mn at the time of solidification is obstructed and sometimes microsegregation of Mn cannot be sufficiently suppressed. Therefore, the C content is 0.100% or less. The C content may also be 0.095% or less, 0.090% or less, or 0.080% or less.

[Mn: 1.00 to 2.50%]

Mn is an element raising the hardenability of steel and contributing to improvement of the strength. To sufficiently obtain such an effect, the Mn content is 1.00% or more. The Mn content may also be 1.20% or more, 1.30% or more, or 1.40% or more. On the other hand, if excessively containing Mn, the diffusion of Mn at the time of solidification is obstructed and sometimes microsegregation of Mn cannot be sufficiently suppressed. Therefore, the Mn content is 2.50% or less. The Mn content may also be 2.25% or less, 2.00% or less, or 1.85% or less.

[Si: 0.005 to 1.500%]

Si is a deoxidizing element of steel and a solution strengthening element effective for enhancing the strength without damaging the ductility of steel sheet. Further, Si is an element promoting the diffusion of Mn at the time of solidification so as to reduce the microsegregation of Mn. To sufficiently obtain these effects, the Si content is 0.005% or more. The Si content may also be 0.010% or more, 0.050% or more, or 0.100% or more. On the other hand, if excessively containing Si, the peelability of scale falls and sometimes surface defects are formed. Therefore, the Si content is 1.500% or less. The Si content may also be 1.000% or less, 0.500% or less, or 0.300% or less.

[P: 0.100% or Less]

P is an element entering in the production process. Further, P is a solution strengthening element. The P content may also be 0%. However, to reduce the P content to less than 0.0001%, time is required for refining and a drop in productivity is invited. Therefore, the P content may also be 0.0001% or more, 0.0005% or more, or 0.001% or more. On the other hand, if excessively containing P, sometimes the toughness of the steel sheet falls. Therefore, the P content is 0.100% or less. The P content may also be 0.060% or less, 0.040% or less, or 0.020% or less.

[S: 0.0200% or Less]

S is an element entering in the production process. The S content may also be 0%. However, to reduce the S content to less than 0.0001%, time is required for refining and a drop in productivity is invited. Therefore, the S content may also be 0.0001% or more, 0.0005% or more, or 0.0010% or more. On the other hand, if excessively containing S, Mn sulfides are formed and sometimes the ductility, hole expandability, stretch flangeability, and/or bendability and other shapeability of the steel sheet are lowered. Therefore, the S content is 0.0200% or less. The S content may also be 0.0100% or less, 0.0060% or less, or 0.0040% or less.

[Al: 0.005 to 0.700%]

Al is an element functioning as a deoxidizer and a solution strengthening element effective for enhancing the strength of the steel. Further, Al is an element effective for promoting the diffusion of Mn at the time of solidification so as to reduce the microsegregation of Mn. To sufficiently obtain these effects, the Al content is 0.005% or more. The Al content may also be 0.010% or more, 0.020% or more, or 0.025% or more. On the other hand, if excessively containing Al, the castability deteriorates and sometimes the productivity falls. Therefore, the Al content is 0.700% or less. The Al content may also be 0.600% or less, 0.400% or less, 0.300% or less, 0.200% or less, or 0.100% or less.

[N: 0.0150% or Less]

N is an element entering in the production process. The N content may be 0%. However, to reduce the N content to less than 0.0001%, time is required for refining and a drop in productivity is invited. Therefore, the N content may also be 0.0001% or more, 0.0005% or more, or 0.0010% or more. On the other hand, if excessively containing N, nitrides are formed and sometimes the ductility, hole expandability, stretch flangeability, and/or bendability and other shapeability of the steel sheet are lowered. Therefore, the N content is 0.0150% or less. The N content may also be 0.0100% or less, 0.0080% or less, or 0.0050% or less.

[O: 0.0100% or Less]

O is an element entering in the production process. The O content may also be 0%. However, to reduce the O content 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, 0.0005% or more, or 0.0010% or more. On the other hand, if excessively containing O, coarse oxides are formed and sometimes the ductility, hole expandability, stretch flangeability, and/or bendability and other shapeability of the steel sheet are lowered. Therefore, the O content is 0.0100% or less. The O content may also be 0.0070% or less, 0.0040% or less, or 0.0020% or less.

The basic chemical composition of the steel sheet of the present embodiment is as explained above. Furthermore, in the present embodiment, the steel sheet may contain one or more of the following optional elements in place of part of the Fe of the balance in accordance with need. Below, these optional elements will be explained in detail.

[Cr: 0 to 0.80%]

Cr is an element raising the hardenability of steel and contributing to improvement of the strength of the steel sheet. Further, Cr is an element effective for promoting the diffusion of Mn at the time of solidification and reducing the microsegregation of Mn. The Cr content may also be 0%, but to obtain these effects, the Cr content is preferably 0.001% or more, more preferably 0.01% or more. The Cr content may also be 0.10% or more, 0.20% or more, or 0.30% or more. On the other hand, if excessively containing Cr, sometimes coarse Cr carbides acting as starting points of fracture are formed. Therefore, the Cr content is preferably 0.80% or less. The Cr content may also be 0.70% or less, 0.60% or less, or 0.50% or less.

[Mo: 0 to 0.50%]

Mo is an element keeping down phase transformation at a high temperature and contributing to improvement of the strength of the steel sheet. Further, Mo is an element effective for promoting the diffusion of Mn at the time of solidification and reducing the microsegregation of Mn. The Mo content may also be 0%, but to obtain these effects, the Mo content is preferably 0.001% or more, more preferably 0.01% or more. The Mo content may also be 0.05% or more or 0.07% or more. On the other hand, if excessively containing Mo, sometimes the hot workability falls and the productivity falls. Therefore, the Mo content is preferably 0.50% or less. The Mo content may also be 0.40% or less, 0.30% or less, or 0.20% or less.

[B: 0 to 0.0100%]

B is an element keeping down phase transformation at a high temperature and contributing to improvement of the strength of the steel sheet. 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 0.0005% or more, 0.0010% or more, or 0.0015% or more. On the other hand, if excessively containing B, B precipitates are formed and sometimes the strength of the steel sheet falls. Therefore, the B content is preferably 0.0100% or less. The B content may also be 0.0080% or less, 0.0060% or less, or 0.0030% or less.

[Ti: 0 to 0.100%]

Ti is an element having the effect of reducing the amounts of S, N, and O causing formation of coarse inclusions acting as starting points of fracture. Further, Ti is a precipitation strengthening element with the effect of refining the structure and enhancing the balance of the strength of the steel sheet and shapeability. The Ti content may also be 0%, but to obtain these effects, the Ti content is preferably 0.001% or more. The Ti content may also be 0.005% or more, 0.007% or more, or 0.010% or more. On the other hand, if excessively containing Ti, coarse Ti sulfides, Ti nitrides, and/or Ti oxides are formed and sometimes the shapeability of the steel sheet falls. Therefore, the Ti content is preferably 0.100% or less. The Ti content may also be 0.080% or less, 0.060% or less, or 0.030% or less.

[Nb: 0 to 0.100%]

Nb is a precipitation strengthening element contributing to improvement of the strength of the steel sheet due to strengthening by precipitates, fine grain strengthening due to suppression of growth of ferrite crystal grains, and/or dislocation strengthening by suppression of recrystallization. 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, 0.007% or more, or 0.010% or more. On the other hand, if excessively containing Nb, sometimes the non-recrystallized ferrite increases and the shapeability of the steel sheet falls. Therefore, the Nb content is preferably 0.100% or less. The Nb content may also be 0.060% or less, 0.040% or less, or 0.030% or less.

[V: 0 to 0.50%]

V is an element contributing to improvement of the strength of the steel sheet due to strengthening by precipitates, fine grain strengthening due to suppression of growth of ferrite crystal grains, and/or dislocation strengthening by suppression of recrystallization. The V content may also be 0%, but to obtain these effects, the V content is preferably 0.001% or more, more preferably 0.005% or more, still more preferably 0.01% or more. The V content may also be 0.02% or more. On the other hand, if excessively containing V, a large amount of carbonitrides precipitates and sometimes the shapeability of the steel sheet falls. Therefore, the V content is preferably 0.50% or less. The V content may also be 0.40% or less, 0.20% or less, or 0.10% or less.

[Ni: 0 to 1.00%]

Ni is an element keeping down phase transformation at a high temperature and contributing to improvement of the strength of the steel sheet. The Ni content may also be 0%, but to obtain these effects, the Ni content is preferably 0.001% or more, more preferably 0.01% or more. The Ni content may also be 0.03% or more or 0.05% or more. On the other hand, if excessively containing Ni, sometimes the weldability of the steel sheet falls. Therefore, the Ni content is preferably 1.00% or less. Ni content may also be 0.60% or less, 0.40% or less, or 0.20% or less.

[Cu: 0 to 1.00%]

Cu is an element present in steel in the form of fine particles and contributing to improvement of the strength of the steel sheet. The Cu content may also be 0%, but to obtain such an effect, the Cu content is preferably 0.001% or more, more preferably 0.01% or more. The Cu content may also be 0.03% or more or 0.05% or more. On the other hand, if excessively containing Cu, sometimes the weldability of the steel sheet falls. Therefore, the Cu content is preferably 1.00% or less. The Cu content may also be 0.60% or less, 0.40% or less, or 0.20% or less.

[W: 0 to 1.00%]

W is an element keeping down phase transformation at a high temperature and contributing to improvement of the strength of the steel sheet. The W content may also be 0%, but to obtain such an effect, the W content is preferably 0.001% or more, more preferably 0.01% or more. The W content may also be 0.02% or more or 0.10% or more. On the other hand, if excessively containing W, sometimes the hot workability falls and the productivity falls. Therefore, the W content is preferably 1.00% or less. The W content may also be 0.80% or less, 0.50% or less, or 0.20% or less.

[Sn: 0 to 1.00%]

Sn is an element keeping down coarsening of the crystal grains and contributing to improvement of the strength of the steel sheet. The Sn content may also be 0%, but to obtain such an effect, the Sn content is preferably 0.001% or more, more preferably 0.01% or more. The Sn content may also be 0.05% or more or 0.08% or more. On the other hand, if excessively containing Sn, sometimes embrittlement of the steel sheet is triggered. Therefore, the Sn content is preferably 1.00% or less. The Sn content may also be 0.80% or less, 0.50% or less, or 0.20% or less.

[Sb: 0 to 0.200%]

Sb is an element keeping down coarsening of the crystal grains and contributing to improvement of the strength of the steel sheet. The Sb content may also be 0%, but to obtain such an effect, the Sb content is preferably 0.001% or more. The Sb content may also be 0.01% or more, 0.05% or more, or 0.08% or more. On the other hand, if excessively containing Sn, sometimes embrittlement of the steel sheet is triggered. Therefore, the Sb content is preferably 1.00% or less, more preferably 0.20% or less. The Sb content may also be 0.10% or less, 0.05% or less, or 0.01% or less.

[Ca: 0 to 0.0100%], [Mg: 0 to 0.0100%], [Zr: 0 to 0.0100%], and [REM: 0 to 0.0100%]

Ca, Mg. Zr, and REM are elements contributing to improvement of the shapeability of the steel sheet. The Ca, Mg, Zr, and REM contents may also be 0%, but to obtain such an effect, the Ca, Mg, Zr, and REM contents are respectively preferably 0.0001% or more. The Ca, Mg, Zr, and REM contents may also be respectively 0.0005% or more, 0.0010% or more, or 0.0015% or more. On the other hand, if excessively containing these elements, sometimes the ductility of the steel sheet falls. Therefore, the Ca, Mg, Zr, and REM contents are respectively preferably 0.0100% or less. The Ca, Mg. Zr, and REM contents may also be respectively 0.0080% or less, 0.0060% or less, or 0.0030% or less.

Note that, in this Description, “REM” 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.

Regarding the above optional elements, in the present embodiment, the chemical composition of the steel sheet preferably contains one or more elements selected from the group comprising:

• • Cr: 0.01 to 0.80%,
  • Mo: 0.01 to 0.50%,
  • B: 0.0001 to 0.0100%,
  • Ti: 0.001 to 0.100%,
  • Nb: 0.001 to 0.100%,
  • V: 0.01 to 0.50%,
  • Ni: 0.01 to 1.00%,
  • Cu: 0.01 to 1.00%,
  • W: 0.01 to 1.00%,
  • Sn: 0.01 to 1.00%,
  • Sb: 0.001 to 0.200%,
  • Ca: 0.0001 to 0.0100%,
  • Mg: 0.0001 to 0.0100%,
  • Zr: 0.0001 to 0.0100%, and
  • REM: 0.0001 to 0.0100%. If the steel sheet contains an optional element in this way, it is possible to more reliably maintain high strength while remarkably keeping down the formation of ghost lines and other defects in appearance after shaping.

In the steel sheet of the present embodiment, the balance besides the above elements is comprised of Fe and impurities. Here, the “impurities” are constituents entering due to various factors in the production process such as, first and foremost, ore, scrap, etc. when industrially producing steel sheet. As impurities, for example, H, Na, Cl, Co, Zn, Ga, Ge, As, Se, Y, Tc, Ru, Rh, Pd, Ag, Cd, In, Te, Cs, Ta, Re, Os, Ir, Pt, Au, Pb, Bi, and Po may be mentioned. The impurities may be included in 0.100% or less in total.

[Index A: 0.45% or More]

In the present embodiment, the chemical composition of the steel sheet has an index A expressed by the following formula (1) of 0.45% or more:

A = [ Si ] + 10 [ P ] + 0.6 [ Al ] + 8 [ Ti ] + 9 [ Nb ] ( 1 )

• • where, [Si], [P], [Al], [Ti], and [Nb] are the contents of the elements in mass % units and 0% is entered if the elements are not included.

The index A is an index determined by the contents of the solution strengthening elements of Si, P, and Al and the precipitation strengthening elements of Ti and Nb. The larger the value, the less the hard phase fraction for obtaining high strength. By making the index A 0.45% or more, it is possible to control the hard phase fraction of the steel sheet to a certain level or less while obtaining a high strength.

Note that, the index A may be 0.48% or more, 0.50% or more, or 0.52% or more. Note that, the upper limit of the index A is not particularly prescribed, but for example, the index A may also be 1.50% or less, 1.20% or less, or 1.00% or less.

Here, the chemical composition of the steel sheet may be measured by a general analysis method. For example, the chemical composition of steel sheet may be measured using inductively coupled plasma-atomic emission spectrometry (ICP-AES). C and S may be measured using the combustion-infrared absorption method, N can be measured using the inert gas fusion-thermal conductivity method, and O may be measured using the inert gas fusion-nondispersive infrared absorption method.

(Metallographic Structure)

[Ferrite: 75 to 97% and Hard Phases: 3 to 25%]

In the present embodiment, the metallographic structure of the steel sheet is comprised of, by area %, 75 to 97% of ferrite and hard phases in 3 to 25%. By making the metallographic structure of the steel sheet such a composite structure, it is possible to maintain the strength of the steel sheet within a suitable range, more specifically reach a 500 MPa or more tensile strength, while keeping down defects in appearance after shaping. From the viewpoint of further enhancing the strength of the steel sheet, the area fraction of the hard phases may be 7% or more, 10% or more, 12% or more, or 15% or more. Similarly, the area fraction of ferrite may be 93% or less, 90% or less, 88% or less, or 85% or less. On the other hand, from the viewpoint of further enhancing the appearance after shaping, the area fraction of the hard phases may be 24% or less, 22% or less, or 20% or less. Similarly, the area fraction of ferrite may be 76% or more, 78% or more, or 80% or more.

In the steel sheet of the present embodiment, “hard phases” mean structures harder than ferrite and, for example, are comprised of at least one type of structure of martensite, bainite, tempered martensite, and pearlite. From the viewpoint of enhancement of the strength of the steel sheet, the hard phases are preferably comprised of at least one of martensite, bainite, and tempered martensite, more preferably are comprised of martensite. In the present embodiment, the metallographic structure of the steel sheet preferably has little retained austenite. Specifically, the retained austenite is preferably, by area %, less than 1% or less than 0.5%, more preferably 0%.

(Identification of Metallographic Structure and Calculation of Area Fraction)

The metallographic structure is identified and the area fraction is calculated in the following way. First, a sample for examination of the metallographic structure (microstructure) (size of, for example, 20 mm in rolling direction×20 mm in width direction×thickness of steel sheet) is taken from a W/4 position or 3W/4 position of the sheet width W of the obtained steel sheet (i.e., position of W/4 in width direction from any width direction end part of the steel sheet). Next, a scan electron microscope (SEM) is used to examine the metallographic structure (microstructure). At this time, the examined fields include, at positions in the sheet thickness direction of the steel sheet, (i) one field having the position of 125 μm from the steel sheet surface (if there is plating, the surface minus the plating layer) (below, called the “sheet thickness 125 μm position”) as the center in the sheet thickness direction, (ii) one field having the position of the sheet thickness ½ thickness from the steel sheet surface (below, called the “sheet thickness ½ thickness position”) as the center in the sheet thickness direction, and (iii) eight fields between the above (i) and the above (ii) dividing the distance from the sheet thickness 125 μm position to the sheet thickness ½ thickness position into nine equal parts, for a total of 10 fields. As preparation for the sample, the sheet thickness cross-section in the direction perpendicular to rolling is polished as the examined surface and etched by Nital corrosion. Next, the “microstructure” is classified from a power 500 or 1000× SEM photograph. The ferrite and hard phases can be discriminated by the difference in brightness.

The steel sheet etched by Nital corrosion is examined at the above 10 locations of examined fields by a 500× or 1000× power. “Photoshop™ CS5” image analysis software made by Adobe is used for image analysis to find the area fraction of the hard phases. The ferrite and hard phases are binarized by the differences in brightness and the area fraction of the hard phases is calculated. The total 10 locations of examined fields are analyzed by image analysis in the same way as above to measure the area fractions of the hard phases. These area fractions are averaged to calculate the average value. This average value is made the area fraction of the hard phases. The remainder is made the area fraction of the ferrite. Note that the examined areas of the fields are respectively 150 μm in the sheet thickness direction and 250 μm in the rolling direction (the examined areas in this case being 150×250−37500 μm²).

Further, if measurement of the area fraction of the retained austenite is necessary. X-ray diffraction of the above examined surface can be used to measure the area fraction of the retained austenite. Specifically, Co-Kα rays are used to find the integrated intensities of the total six peaks of α (110), α (200), α (211), γ (111), γ (200), γ (220) at the sheet thickness direction ¼ position. The intensity averaging method is used to calculate the volume fraction of retained austenite. The obtained volume fraction of the obtained retained austenite is made the area fraction of the retained austenite.

[Standard Deviation of Hard Phase Fraction in Direction Perpendicular to Rolling: 0.75% or Less]

In the present embodiment, the metallographic structure of the steel sheet has a standard deviation of the hard phase fraction in the direction perpendicular to rolling of 0.75% or less. Note that the “standard deviation of the hard phase fraction” means a standard deviation of the area fraction itself of the hard phases. As explained above, ghost lines and other defects in appearance after shaping relate the result of not only Mn segregation, but also the effects of the solidified structure in large part. For example, even if the center segregation of Mn is small, if coarse equiaxed crystals are formed in the solidified structure, negative segregation of Mn occurs, the variation in hard phase fraction in the direction perpendicular to rolling becomes greater, and sometimes defects in appearance after shaping become worse. However, in the steel sheet of the present embodiment, the standard deviation of the hard phase fraction in the direction perpendicular to rolling is 0.75% or less, i.e., the variation in the hard phase fraction in the direction perpendicular to rolling is a fixed level or less, therefore it is possible to remarkably suppress defects in appearance after shaping. Note that, in the hard phase fraction, the ratio of the average value of the area ratios of hard phases and the standard deviation of the same preferably becomes 0.10 or less (i.e., standard deviation of hard phase fraction/average value of area ratio of hard phases≤0.10). The ratio is preferably 0.09 or less. 0.08 or less, or 0.07 or less. The lower limit of the ratio is 0, but the lower limit may also be 0.01 in accordance with need.

The standard deviation of the hard phase fraction in the direction perpendicular to rolling of the metallographic structure can be found in the following way. First, a range between a position of 50 μm from one surface of the steel sheet at a cross-section parallel to the direction perpendicular to rolling of the steel sheet and vertical to the steel sheet surface and a position of 50 μm from the other surface of the steel sheet is examined by a scan electron microscope (SEM) by a 500× or 1000× power to obtain an SEM photograph image. This SEM photograph image is analyzed by image analysis using image analysis software in the same way as the area fraction of the hard phases explained above. The area fraction of the hard phases at every 100 μm in the range of 8 mm in the direction perpendicular to rolling of the steel sheet is measured and the standard deviation is calculated. Note that, the examined range in the direction perpendicular to rolling may be less than 8 mm and may be more than 8 mm as well. However, the lower limit of the examined range of the standard deviation of the hard phase fraction in the direction perpendicular to rolling is 4 mm, while the upper limit is 12 mm.

From the viewpoint of further improving the appearance after shaping, the standard deviation of the hard phase fraction in the direction perpendicular to rolling may be 0.65% or less, 0.55% or less, or 0.45% or less. Note that, the lower limit of the standard deviation is not particularly prescribed, but, for example, the standard deviation of the hard phase fraction may be 0.01% or more, 0.05% or more, 0.10% or more, 0.15% or more, or 0.20% or more.

[Relationship of Tensile Strength of Steel Sheet and Hard Phase Fraction: (TS-180,000/TS)/Vm≥35]

In the present embodiment, the steel sheet preferably satisfies the following formula (2).

( T ⁢ S - 180 , 000 / T ⁢ S ) / Vm ≥ 35 ( 2 )

• • where, TS is the tensile strength (MPa) and Vm is the hard phase fraction (area %).

If the steel sheet satisfies such a formula (2), even in a case where center segregation of Mn remains to a certain extent, it is easy to suppress variation in the hard phase fraction in the direction perpendicular to rolling, therefore as a result, defects in appearance of the steel sheet after shaping can be made harder to arise.

The tensile strength (TS) of the steel sheet can be measured by taking from the steel sheet a No. 5 test piece of JIS Z 2241:2011 having a direction perpendicular to the rolling direction as a longitudinal direction and conducting a tensile test compliant with JIS Z 2241:2011.

[Average Crystal Grain Size of Ferrite: 5.0 to 30.0 μm]

In the steel sheet of the present embodiment, the average crystal grain size of ferrite in the metallographic structure is preferably 5.0 to 30.0 μm. By controlling the average crystal grain size of ferrite to such a fine range, the appearance after shaping of the steel sheet can be further improved. The average crystal grain size of ferrite may also be 7.0 μm or more, 8.0 μm or more, 9.0 μm or more, or 10.0 μm or more. Similarly, the average crystal grain size of ferrite may be 27.0 μm or less, 25.0 μm or less, 20.0 μm or less, or 16.0 μm or less.

The average crystal grain size of ferrite in the steel sheet is determined in the following way. First, in the region of the steel sheet etched by a Nital reagent from the surface down to the sheet thickness ½ position in the sheet thickness direction. 10 fields in the sheet thickness direction the same as with measurement of the area fractions of ferrite and hard phases are examined by a power of 500×. “Photoshop™ CS5” image analysis software made by Adobe is used for image analysis and the area fraction of ferrite and the number of particles of ferrite are calculated at the different fields. Next, the area fractions of ferrite and the numbers of particles of ferrite at the 10 fields are respectively totaled and the total area fraction of the ferrite is divided by the total number of particles of the ferrite to calculate the average area fraction per ferrite particle. The circle equivalent diameter is calculated from this average area fraction and number of particles. The obtained circle equivalent diameter is determined as the average crystal grain size of the ferrite. Note that the examined areas of the fields are respectively 150 μm in the sheet thickness direction and 250 μm in the rolling direction (the examined areas in this case being 150×250−37500 μm²).

[Average Crystal Grain Size of Hard Phases: 1.0 to 5.0 μm]

In the steel sheet of the present embodiment, the average crystal grain size of the hard phases in the metallographic structure is preferably 1.0 to 5.0 μm. If controlling the average crystal grain size of the hard phases to within such a fine range, it is possible to further improve the appearance after shaping of the steel sheet. The average crystal grain size of the hard phases may be 1.2 μm or more, 1.5 μm or more, 1.7 μm or more, or 2.0 μm or more. Similarly, the average crystal grain size of the hard phases may be 4.7 μm or less, 4.5 μm or less, 4.2 μm or less, or 4.0 μm or less.

The average crystal grain size of the hard phases is determined in the following way. First, in the region of the steel sheet etched by a Nital reagent from the surface down to the sheet thickness ½ position in the sheet thickness direction. 10 fields in the sheet thickness direction the same as with measurement of the area fractions of ferrite and hard phases are examined by a power of 500×. “Photoshop™ CS5” image analysis software made by Adobe is used for image analysis and the area fraction of the hard phases and the number of particles of the hard phases are calculated at the different fields. Next, the area fractions of the hard phases and numbers of particles of the hard phases in the 10 fields are respectively totaled. The total area fraction of the hard phases is divided by the total number of particles of the hard phases to calculate the average area fraction per hard phase particle. The circle equivalent diameter is calculated from the average area fraction and number of particles. The obtained circle equivalent diameter is determined as the average crystal grain size of the hard phases. Note that the examined areas of the fields were respectively 150 μm in the sheet thickness direction and 250 μm in the rolling direction (the examined areas in this case being 150×250−37500 μm²).

(Sheet Thickness)

In the present embodiment, the sheet thickness of the steel sheet is not particularly limited, but, for example, the steel sheet may also have a 0.1 to 2.0 mm sheet thickness. Steel sheet having such a sheet thickness is suitable in the case of use as a material of a door, hood, or other cover member. Note that, the sheet thickness of the steel sheet may also be 0.2 mm or more, 0.3 mm or more, or 0.4 mm or more. Similarly, the sheet thickness of the steel sheet may be 1.8 mm or less, 1.5 mm or less, 1.2 mm or less, or 1.0 mm or less. For example, by making the sheet thickness of the steel sheet 0.2 mm or more, the additional effect can be obtained that the shaped article can be maintained flat in shape and the dimensional precision and shape precision are improved. On the other hand, by making the sheet thickness 1.0 mm or less, the effect of reducing the weight of a member becomes remarkable. The sheet thickness of the steel sheet is measured by a micrometer.

(Plating)

In the steel sheet of the present embodiment, for the purpose of improving the corrosion resistance etc., the surface may further have a plating layer. The plating layer may be either a hot dip coated layer or electroplated layer. As the hot dip coating layer, for example, a hot dip galvanized layer (GI), hot dip galvannealed layer (GA), hot dip aluminum coated layer, hot dip Zn—Al alloy coated layer, hot dip Zn—Al—Mg alloy coated layer, hot dip Zn—Al—Mg—Si alloy coated layer, etc. may be mentioned. As an electroplated layer, for example, an electrogalvanized layer (EG), electro Zn—Ni alloy plated layer, etc. may be mentioned. Among these, the plated layer is preferably a hot dip galvanized layer, hot dip galvannealed layer, or electrogalvanized layer. The amount of deposition of the plated layer is not particularly limited and may be a general amount of deposition.

(Mechanical Properties)

According to the steel sheet of the present embodiment having the above specific chemical composition and metallographic structure, it is possible to achieve a high tensile strength, specifically a 500 MPa or more tensile strength. The tensile strength of the steel sheet is preferably 540 MPa or more, more preferably 600 MPa or more. The upper limit of the tensile strength is not particularly prescribed, but, for example, the tensile strength may also be 980 MPa or less or 850 MPa or less. By making the tensile strength 850 MPa or less, there is the advantage that it is easy to secure shapeability when press-forming steel sheet.

The steel sheet of the present embodiment, in spite of having a high strength, specifically a 500 MPa or more tensile strength, can maintain an excellent appearance even after press-forming or other shaping. For this reason, the steel sheet of the present embodiment is, for example, extremely useful in automobiles for usage as roofs, hoods, fenders, doors, and other external panel parts where high design quality is sought

<Method of Production of Steel Sheet>

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

The method of production of steel sheet of the present embodiment includes a casting step of casting a slab having the above specific chemical composition. This casting step includes small reduction rolling using a continuous caster provided with a plurality of reduction rolls adjoining each other in the direction of conveyance of the slab. The roll pitch of adjoining reduction rolls is 290 mm or less. Note that, in this Description, “small reduction rolling” indicates rolling reduction having a rolling reduction gradient of 0.6 mm or more per meter in the direction of casting progression.

(Casting Step)

The steel sheet of the present embodiment, as explained above, must have a unique metallographic structure comprised of a hard phase fraction less than conventional DP steel and having a smaller variation of the hard phase fraction in the direction perpendicular to rolling. To obtain such a metallographic structure, it is important to control the solidified structure at the time of casting to columnar crystals. Specifically, in the casting step, by making a superheat ΔT (difference of molten steel temperature and solidification temperature of the molten steel) of the molten steel having the above specific chemical composition 25° C. or more and, furthermore, making a segment pressing force 450) tons or more, it is possible to even suppress center segregation while using a method different from the conventional center segregation countermeasure of controlling the solidified structure to a columnar crystal structure with an equiaxed crystal rate of 15% or less. The superheat ΔT is more preferably 30° C. or more. Further, the superheat ΔT is preferably 40° C. or less. Note that, the molten steel temperature is the molten steel temperature inside a tundish and can be found by actual measurement. The solidification temperature can be found from the chemical composition of the molten steel by utilizing a known solidification temperature estimation formula.

Note that, the conventional countermeasures for improvement of center segregation greatly reduces the superheat ΔT (at least to less than 25° C.) and causes the equiaxed crystal rate to increase (at least make it increase to more than 15%), but with such a conventional countermeasure, a sufficient effect of improvement is not obtained. In the present embodiment, by employing casting conditions completely different from the conventional countermeasure, i.e., unique casting conditions making the superheat ΔT 25° C. or more and making the segment pressing force 450 tons or more and controlling the solidified structure to a columnar crystal structure, negative segregation of Mn is suppressed. As a result, microsegregation of Mn is mitigated and ghost lines can be sufficiently decreased.

The equiaxed crystal rate (%) can be calculated by taking an etch print of the sheet thickness cross-section in the width direction of the slab, determining the boundary between the columnar crystal structure and equiaxed crystal structure by visual examination, measuring the thickness (mm) of the equiaxed crystal structure at the sheet thickness center of the slab and the thickness (mm) of the slab, dividing the thickness of the equiaxed crystal structure by the thickness of the slab, and multiplying the result by 100.

Further, in the casting step, small reduction rolling is performed using a continuous caster with a roll pitch of each adjoining two of the plurality of reduction rolls of 290 mm or less so as be able to suppress the fluid motion of the molten steel at the time of solidification and reduce the concentration of Mn at the center part. Due to this, it is possible to suppress center segregation of Mn. Note that, the roll pitch of each adjoining two of the plurality of reduction rolls is more preferably 280 mm or less.

The method of production of the steel sheet of the present embodiment may further include, in addition to the above-mentioned casting step, a hot rolling step, cold rolling step, annealing step, cooling step, and other steps. Furthermore, the present method of production may optionally include a plating step. These steps are not particularly limited. They may be performed by suitably selecting any suitable conditions so that a metallographic structure including the above-mentioned ferrite and hard phases in predetermined area fractions in the steel sheet is obtained. Below, the preferred conditions of these steps will be simply explained.

(Hot Rolling Step)

Before the hot rolling, the slab is preferably heated to 1100° C. or more. By making the heating temperature 1100° C. or more, the rolling reaction force does not become excessively large in the hot rolling and the target product thickness can be made easier to obtain. The upper limit of the heating temperature is not particularly limited, but from the economic viewpoint, the heating temperature is preferably less than 1300° C. In the hot rolling step, the heated slab is rough rolled and finish rolled. The thus obtained hot rolled steel sheet is coiled at, for example, a 450 to 650° C. coiling temperature.

Note that, the finish rolling end temperature is preferably 950° C. or less. By making the finish rolling end temperature 950° C. or less, it is possible to make the average crystal grain size of the hot rolled steel sheet and average crystal grain size of the final product smaller and secure a sufficient yield strength and high surface quality after shaping. Further, by making the coiling temperature 450 to 650° C., it is possible to make the average crystal grain size smaller and suppress growth of scale.

(Cold Rolling Step)

The hot rolled steel sheet obtained by the hot rolling step is suitably pickled for removing the scale, then is sent on to the cold rolling step. In the cold rolling step, for example, the hot rolled steel sheet is preferably cold rolled so that the cumulative rolling reduction becomes 50 to 90%. By controlling the cumulative rolling reduction to such a range, it is possible to secure the desired sheet thickness and, furthermore, sufficiently secure uniformity of quality in the sheet width direction while preventing the rolling load from becoming excessive and rolling from becoming difficult.

(Annealing Step)

In the annealing step, the cold rolled steel sheet is preferably annealed by heating it to and holding it at the 750 to 900° C. soaking temperature. By making the soaking temperature 750° C. or more, it is possible to make the recrystallization of ferrite and reverse transformation from ferrite to austenite sufficiently proceed and obtain the desired metallographic structure at the final product. On the other hand, by making the soaking temperature 900° C. or less, it is possible to pack the crystal grains closer to obtain sufficient strength.

(Cooling Step)

In the cooling step, the cold rolled steel sheet after the annealing step is cooled. In the cooling step, the cold rolled steel sheet is preferably cooled so that the average cooling speed from the soaking temperature becomes 5 to 50° C./s. By making the average cooling speed 5° C./s or more to suppress excessive transformation to ferrite, it is possible to increase the amount of formation of martensite and other hard phases to obtain the desired strength. Further, by making the average cooling speed 50° C./s or less, it is possible to more uniformly cool the steel sheet in the width direction.

(Plating Step)

For the purpose of improving the corrosion resistance etc., the surface of the obtained cold rolled steel sheet may also be plated. As the plating treatment, for example, hot dip coating, alloyed hot dip coating, electroplating, and other treatment may be mentioned. For example, as plating treatment, the steel sheet surface may be hot dip galvanized. After the hot dip galvanization, alloying treatment may be performed. The specific conditions of the plating treatment and the alloying treatment are not particularly limited. Any suitable conditions known to persons skilled in the art can be employed. For example, the alloying temperature may be 450 to 600° C.

Note that, the present disclosure is not limited to the above-mentioned embodiments or the following examples etc. Suitable combinations or substitutions, changes, etc. are possible within a scope not departing from the object and gist of the present disclosure.

Below, examples will be illustrated to explain the present disclosure more concretely, but the present disclosure is not limited to only these examples.

EXAMPLES

In the following examples, steel sheets according to one embodiment of the present disclosure were produced under various conditions and the obtained steel sheets were investigated for tensile strength, properties of the appearance after shaping, etc.

First, the continuous casting method of using a continuous caster provided with a plurality of rolling reduction rolls arranged at a 290 mm or less roll pitch for small reduction rolling having a rolling reduction gradient of 0.6 mm or more per meter in the direction of casting progression was used so as to cast a slab having each of the chemical compositions shown in Table 1 and having a 200 to 300 mm thickness. The balance other than the constituents shown in Table 1 was Fe and impurities. In each example, the casting condition (I) was the condition of the superheat ΔT≥25° C. or more while the casting condition (II) was the condition of the segment pressing force≥450 tons. In each example, cases where these conditions are satisfied (labeled as “OK”) and cases where they are not satisfied (labeled as “NG”) are respectively shown in Table 2.

Next, each obtained slab was subjected to a hot rolling step (heating temperature 1200° C., finish rolling end temperature 900° C., and coiling temperature 550° C.), cold rolling step (cumulative rolling reduction 80%), and annealing step (soaking temperature 800° C.) and cooling step (average cooling speed 10° C./s) to produce sheet thickness 0.4 mm cold rolled steel sheet. The surface of each obtained cold rolled steel sheet was suitably plated to form a hot dip galvanized layer (GI), hot dip galvannealed layer (GA), or electrogalvanized layer (EG). Further, a sample obtained from the produced cold rolled steel sheet was analyzed for chemical composition, whereupon there was no change from the chemical composition of the slab shown in Table 1.

	TABLE 1
	Chemical composition (mass %), bal.: Fe and impurities

Index

Steel

C

Mn

Si

P

S

Al

N

O

Cr

Mo

B

Ti

Nb

Others

A

Remarks

A	0.046	1.62	0.158	0.026	0.0018	0.031	0.0024	0.0009	0.41	0.06		0.012			0.53	Inv. ex.
B	0.052	1.15	0.352	0.021	0.0018	0.031	0.0035	0.0009	0.29	0.27		0.019			0.73	Inv. ex.
C	0.061	1.75	0.612	0.008	0.0026	0.026	0.0045	0.0010		0.07	0.0020				0.71	Inv. ex.
D	0.069	1.86	0.013	0.058	0.0011	0.302	0.0035	0.0019	0.36	0.11			0.011	Sb: 0.005	0.87	Inv. ex.
E	0.044	1.86	0.233	0.042	0.0008	0.657	0.0019	0.0015			0.0018	0.021			1.22	Inv. ex.
F	0.035	1.09	0.292	0.042	0.0020	0.034	0.0031	0.0012	0.08	0.28				V: 0.12	0.73	Inv. ex.
G	0.060	1.62	0.154	0.029	0.0021	0.035	0.0044	0.0009	0.58		0.0019	0.011	0.006	W: 0.02, Cu: 0.05	0.61	Inv. ex.
H	0.047	2.03	0.086	0.067	0.0013	0.105	0.0052	0.0019							0.82	Inv. ex.
I	0.051	1.21	0.184	0.019	0.0017	0.254	0.0035	0.0015	0.42	0.07			0.015	Ni: 0.05, Sn:	0.66	Inv. ex.
														0.008
J	0.060	1.72	0.151	0.020	0.0028	0.054	0.0042	0.0015		0.07		0.035		Zr: 0.0015, REM:	0.66	Inv. ex.
														0.0020
K	0.060	1.70	0.285	0.026	0.0021	0.051	0.0038	0.0015	0.53	0.05		0.010		Mg: 0.0034	0.66	Inv. ex.
L	0.062	1.73	0.153	0.024	0.0024	0.057	0.0035	0.0015	0.48	0.07	0.0013	0.023		Ca: 0.0018	0.61	Inv. ex.
M	0.082	1.25	0.112	0.020	0.0054	0.243	0.0022	0.0012	0.25	0.34		0.014			0.57	Inv. ex.
N	0.048	1.31	0.260	0.031	0.0015	0.026	0.0041	0.0024			0.0026	0.015		Cu: 0.21, W: 0.12	0.71	Inv. ex.
O	0.055	1.26	0.153	0.019	0.0032	0.154	0.0061	0.0008		0.21		0.062			0.93	Inv. ex.
P	0.049	1.34	0.078	0.041	0.0015	0.054	0.0049	0.0015	0.18	0.12			0.051		0.98	Inv. ex.
Q	0.057	1.05	0.294	0.035	0.0041	0.078	0.0035	0.0011	0.61			0.012	0.008	Cu: 0.39, Ni: 0.22	0.86	Inv. ex.
R	0.051	1.68	0.254	0.013	0.0020	0.054	0.0036	0.0013	0.18	0.09					0.42	Comp. ex.
S	0.082	1.71	0.085	0.012	0.0033	0.030	0.0033	0.0015					0.020		0.40	Comp. ex.
T	0.110	1.31	0.291	0.025	0.0028	0.034	0.0035	0.0015	0.40	0.10					0.56	Comp. ex.
U	0.084	2.61	0.125	0.038	0.0026	0.033	0.0030	0.0014		0.07		0.013			0.63	Comp. ex.
V	0.052	1.35	1.560	0.008	0.0013	0.291	0.0041	0.0011	0.25		0.0013	0.012			1.91	Comp. ex.
W	0.026	1.81	0.351	0.019	0.0029	0.030	0.0033	0.0015		0.12			0.007		0.62	Comp. ex.
Underlines indicate outside scope of present invention.

	TABLE 2
	Metallographic structure

Hard

	Slab				Ferrite	phase
	structure				average	average	Standard

Equiaxed

Hard

crystal

crystal

deviation

Properties

Steel

Casting

crystal

Ferrite

phase

grain

grain

of hard

(TS-

Tensile

Appearance

sheet

conditions

rate

Plating

fraction

fraction

size

size

phase

180000/

strength

after

no.

Steel

(I)

(II)

(%)

type

(%)

(%)

(μm)

(μm)

fraction

TS)/Vm

(MPa)

shaping

Remarks

1	A	OK	OK	0	GA	93	7	12.4	3.3	0.34	42	596	1	Inv. ex.
2	B	OK	OK	5	GA	94	6	10.4	2.6	0.41	46	585	2	Inv. ex.
3	B	NG	OK	21	GA	94	6	9.2	2.5	0.76	45	581	4	Comp. ex.
4	B	NG	NG	29	GA	94	6	9.6	2.4	0.88	45	582	5	Comp. ex.
5	C	OK	OK	6	GA	91	9	19.5	4.3	0.56	46	677	3	Inv. ex.
6	D	OK	OK	0	GI	86	14	8.9	2.9	0.61	40	788	3	Inv. ex.
7	E	OK	OK	10	GA	92	8	9.0	2.6	0.54	48	655	3	Inv. ex.
8	F	OK	OK	4	None	96	4	14.2	3.7	0.38	60	560	1	Inv. ex.
9	G	OK	OK	0	EG	92	8	7.5	1.9	0.32	40	612	2	Inv. ex.
10	H	OK	OK	6	GA	92	8	20.2	4.6	0.44	39	609	2	Inv. ex.
11	I	OK	OK	7	GA	91	7	8.9	2.9	0.45	39	581	2	Inv. ex.
12	I	OK	NG	18	GA	92	7	8.4	2.5	0.82	39	582	4	Comp. ex.
13	J	OK	OK	2	GA	92	8	10.0	3.1	0.43	42	624	3	Inv. ex.
14	K	OK	OK	0	GA	92	8	11.5	3.8	0.40	40	616	2	Inv. ex.
15	L	OK	OK	4	GI	92	8	8.3	2.5	0.55	40	613	3	Inv. ex.
16	L	NG	OK	17	GA	91	9	9.1	2.9	0.81	37	621	4	Comp. ex.
17	L	NG	NG	25	GA	92	8	8.6	2.6	0.89	39	606	5	Comp. ex.
18	M	OK	OK	0	GA	82	18	8.4	2.4	0.62	35	845	3	Inv. ex.
19	N	OK	OK	9	GA	91	9	13.5	3.6	0.55	40	643	3	Inv. ex.
20	O	OK	OK	5	GA	93	7	7.3	1.8	0.41	41	591	3	Inv. ex.
21	P	OK	OK	0	GA	93	7	6.9	2.0	0.33	43	600	2	Inv. ex.
22	Q	OK	OK	8	GA	91	9	13.4	3.9	0.45	36	618	3	Inv. ex.
23	R	OK	OK	2	GI	87	13	16.5	4.0	0.78	27	634	5	Comp. ex.
24	S	OK	OK	0	GA	84	16	10.4	3.0	0.82	23	652	5	Comp. ex.
25	T	OK	OK	4	GA	89	11	10.1	3.1	0.92	37	675	5	Comp. ex.
26	U	OK	OK	5	GA	74	26	8.9	2.4	0.91	29	945	5	Comp. ex.
27	V	OK	OK	0	None	89	9	15.6	3.3	0.54	42	654	5	Comp. ex.
28	W	OK	OK	3	GA	98	2	18.5	3.4	0.43	53	480	2	Comp. ex.
Underlines indicate outside scope of present invention or not preferable properties.

The properties of each obtained steel sheet were measured and evaluated by the following methods:

(Tensile Strength)

The tensile strength of the steel sheet was measured by taking from the steel sheet a No. 5 test piece of JIS Z 2241:2011 having a direction perpendicular to the rolling direction as a longitudinal direction and conducting a tensile test compliant with JIS Z 2241:2011.

Furthermore, the tensile strength of the steel sheet measured was used to calculate the value of the relationship (TS-180,000/TS)/Vm of the tensile strength (TS) of the steel sheet and the hard phase fraction (Vm).

(Appearance after Shaping)

The appearance after shaping of the steel sheet was evaluated by the extent of ghost lines formed at the surface of an outer door member after shaping. The surface after the press-forming was ground by a grindstone. Stripe patterns formed at the surface at an order of several mm distance were judged as ghost lines. These were scored as 1 to 5 points based on the following criteria corresponding to the extent of formation of the stripe patterns. Any 100 mm×100 mm region was visually checked. Cases where no stripe patterns at all were confirmed were evaluated as “1”, cases where the maximum length of the stripe patterns was 20 mm or less were evaluated as “2”, cases where the maximum length of the stripe patterns was more than 20 mm and 50 nm or less were evaluated as “3”, cases where the maximum length of the stripe patterns was more than 50 mm and 70 mm or less were evaluated as “4”, and cases where the maximum length of the stripe patterns was more than 70 mm were evaluated as “5”. Cases evaluated as “3” or less were judged as passing as being excellent in appearance after shaping. On the other hand, cases evaluated as “4” or more were judged as failing as being inferior in appearance after shaping.

Cases where the tensile strength was 500 MPa or more and the appearance after shaping was evaluated as “3” or less were evaluated as high strength steel sheets having improved appearance after shaping. The results are shown in Table 2. I

Referring to Table 2, in the comparative examples of Steel Sheet Nos. 3, 4, 12, 16, 17, and 23 to 27 with the chemical composition or standard deviation of the hard phase fraction of the metallographic structure outside the scope of the present invention, in each case, the appearance after shaping deteriorated. Further, in the comparative example of Steel Sheet No. 28 with the chemical composition and ferrite fraction and hard phase fraction of the metallographic structure outside the scope of the present invention, the appearance after shaping was excellent, but the C content was low, therefore the hard phase fraction became low and sufficient strength could not be obtained.

On the other hand, in the invention examples of Steel Sheet Nos. 1, 2, 5 to 11, 13 to 15, and 18 to 22, even in cases where a tensile strength 500 MPa or more high strength was maintained while strain was imparted by press-forming, it was possible to suppress the formation of fine asperities at the steel sheet surface and remarkably suppress the formation of ghost lines.