STEEL PLATE AND METHOD OF PRODUCING SAME

Description

TECHNICAL FIELD

The present disclosure relates to a steel plate and a method of producing the same. The present disclosure particularly relates to a steel plate having high strength, low yield ratio, and excellent low-temperature toughness, and a method of producing the same. The steel plate according to the present disclosure can be suitably used for steel structures that are used in low-temperature environments, such as liquefied gas storage tanks for ships. For liquefied gas storage tanks for ships, for example, stress relief after welding can be performed mechanically instead of by post weld heat treatment.

BACKGROUND

When liquefied gas storage tanks for ships are independent Type-C tanks and have a design temperature below −10° C., the steel material needs to be subjected to stress relief after welding. Stress relief is usually performed by post weld heat treatment (PWHT), but mechanical stress relief is also possible in the case where the yield ratio (hereafter also referred to as YR) of the steel material is 0.8 or less. For large tanks, PWHT is difficult. It is therefore desirable to use a low yield ratio steel material that can be subjected to mechanical stress relief for large tanks. For example, for large storage tanks for liquefied CO2, the use of a low yield ratio steel material that has excellent toughness in cryogenic environments of −50° C. to −70° C. and a tensile strength (hereafter also referred to as TS) of 690 MPa or more is desired.

For example, JP 2016-507649 A (PTL 1) discloses “A high strength steel plate comprising, in wt %, carbon (C): 0.02% to 0.12%, manganese (Mn): 0.5% to 2.0%, silicon (Si): 0.05% to 0.5%, nickel (Ni): 0.05% to 1.0%, titanium (Ti): 0.005% to 0.1%, aluminum (Al): 0.005% to 0.5%, phosphorus (P): 0.015% or less, and sulfur(S): 0.015% or less with a balance consisting of Fe and other inevitable impurities, wherein a microstructure contains 70% to 90% of ultrafine ferrite and 10% to 30% of MA (martensite/austenite) microstructure in area fraction and has a yield ratio (YS/TS) of 0.8 or less.” JP 2013-57105 A (PTL 2) discloses “A steel plate for low-yield-ratio thick circular steel pipes or tubes having a tensile strength of 780 MPa or more, containing C: 0.02% to 0.15% (denoting mass %, the same applies below for chemical composition), Si: 0.10% to 0.40%, Mn: 1.5% to 2.5%, P: 0.012% or less (excluding 0%), S: 0.005% or less (excluding 0%), Ti: 0.005% to 0.02%, N: 0.002% to 0.006%, and Al: 0.02% to 0.08%, and one or more selected from the group consisting of Ni: 2.5% or less (excluding 0%), Cr: 2.0% or less (excluding 0%), and Mo: 0.5% or less (excluding 0%) with a balance consisting of iron and inevitable impurities, wherein a hardenability index DI defined by the following formula (1) is 8 inches or more:

DI ⁡ ( inches ) = { 1.16 × ( [ C ] / 10 ) ⁢ 1 / 2 } × ( 0.7 × [ Si ] + 1 ) × { 5.1 × ( [ Mn ] - 1.2 ) + 5 } × ( 0.35 × [ Cu ] + 1 ) × ( 0.36 × [ Ni ] + 1 ) × ( 2.16 × [ Cr ] + 1 ) × ( 3 × [ Mo ] + 1 ) × ( 1.75 × [ V ] + 1 ) × ( 200 × [ B ] + 1 ) ( 1 )

• • where [C], [Si], [Mn], [Cu], [Ni], [Cr], [Mo], [V], and [B] respectively denote contents (mass %) of C, Si, Mn, Cu, Ni, Cr, Mo, V, and B, and the following requirements (A), (B), and (C) are satisfied:
  • (A) in a microstructure at a site of ¼ of a plate thickness, bainite is 90 area % or more,
  • (B) in the microstructure at the site of ¼ of the plate thickness, an average equivalent circular diameter d of regions surrounded by large-angle grain boundaries with an orientation difference of 15 degrees or more is 4 μm or less,
  • (C) the microstructure at the site of ¼ of the plate thickness contains 3 area % to 10 area % of martensite austenite constituent with an average equivalent circular diameter of 0.5 μm to 3 μm and a Vickers hardness Hv of 700 or more.”

JP 2019-119934 A (PTL 3) discloses “An ultra-low yield ratio and high tensile strength steel plate comprising: a chemical composition containing, in mass %, C: 0.03% to 0.20%, Si: 0.01% to 0.50%, Mn: 0.5% to 3.0%, P: 0.015% or less, S: 0.0050% or less, Al: 0.005% to 0.1%, and N: 0.0015% to 0.0065% with a balance consisting of Fe and inevitable impurities; and a microstructure that contains bainite including martensite austenite constituent, martensite, and cementite and in which cementite is contained in either or both of bainite and martensite microstructures, a total area fraction of bainite and martensite is 50.0% or more and less than 95.0%, an area fraction of martensite austenite constituent is 5% to 20%, an average equivalent circular diameter of martensite austenite constituent is less than 5.0 μm, an area fraction of cementite is more than 0% and 5% or less, and an average equivalent circular diameter of cementite is less than 0.5 μm.”

CITATION LIST

Patent Literature

• • PTL 1: JP 2016-507649 A
  • PTL 2: JP 2013-57105 A
  • PTL 3: JP 2019-119934 A

SUMMARY

Technical Problem

However, the TS of the steel plate described in PTL 1 is 620 MPa at a maximum. Thus, PTL 1 fails to provide a steel plate with TS of 690 MPa or more. For the steel plates described in PTL 2 and PTL 3, toughness in cryogenic environments of −50° C. to −70° C. (hereafter also simply referred to as low-temperature toughness) is not taken into consideration. Moreover, although nickel steel, such as 9% Ni steel, may be able to achieve the desired properties mentioned above, high material costs are required.

There is thus a need to develop a steel plate having high strength, low yield ratio, and excellent low-temperature toughness, especially a steel plate having TS of 690 MPa or more, YR of 0.8 or less, and Charpy absorbed energy at −70° C. (hereafter also referred to as vE_{−70° C.}) of 100 J or more, which can replace expensive nickel steel.

It could therefore be helpful to provide a steel plate having high strength, low yield ratio, and excellent low-temperature toughness, and a method of producing the same.

Solution to Problem

Upon careful examination, we discovered the following.

An effective way of achieving desired property improvement is to control the microstructure at a depth position of ¼ of the plate thickness from the surface of the steel plate in the plate thickness direction (hereafter also referred to as a depth position of ¼ of the plate thickness of the steel plate) as follows:

• • (1) The ferrite fraction is 5% to 95% and the martensite austenite constituent fraction is 1% to 30% with the residual microstructure consisting of one or both of bainite and tempered martensite.
  • (2) Among crystal grains defined by large-angle grain boundaries with an orientation difference of 15 degrees or more, the number density of crystal grains having an equivalent circular diameter of more than 30 μm is 200/mm² or less.

Herein, ferrite can be regarded as BCC phase that inherits the lath-like microstructure of bainite and martensite which remain without reverse transformation during heat-treatment to a temperature of Ac₁ point or more. By setting this relatively soft ferrite phase to 5% to 95% and finely distributing martensite austenite constituent, low yield ratio can be achieved.

Toughness in cryogenic environments of −50° C. to −70° C. is greatly influenced by the number density of coarse crystal grains, especially crystal grains having an equivalent circular diameter of more than 30 μm among crystal grains defined by large-angle grain boundaries with an orientation difference of 15 degrees or more. In view of this, the desired low-temperature toughness can be achieved by consisting residual microstructure, other than ferrite and martensite austenite constituent, of one or both of bainite and tempered martensite as in (1) and also controlling the microstructure at a depth position of ¼ of the plate thickness of the steel plate as in (2).

In order to obtain the foregoing microstructure, it is important to appropriately prepare the chemical composition and appropriately control the production conditions. In particular, in order to control the microstructure as in (2), it is important to simultaneously control the following within appropriate ranges:

• • the in-furnace time in the heat treatment furnace in first heating,
  • the rolling finish temperature in hot rolling,
  • the average cooling rate in a certain temperature range and the cooling end temperature in quenching, and
  • the average cooling rate in a certain temperature range and the cooling end temperature in cooling.

The present disclosure is based on these discoveries and further studies.

We thus provide the following.

• • 1. A steel plate comprising a chemical composition containing (consisting of), in mass %, C: 0.02% or more and 0.15% or less, Si: 0.01% or more and 0.50% or less, Mn: 0.05% or more and 2.50% or less, Ni: 0.50% or more and less than 5.00%, P: 0.03% or less, S: 0.0050% or less, and N: 0.0010% or more and 0.0080% or less, with a balance consisting of Fe and inevitable impurities, wherein in a microstructure at a depth position of ¼ of a plate thickness of the steel plate, a ferrite fraction is 5% to 95% and a martensite austenite constituent fraction is 1% to 30% with a residual microstructure consisting of one or both of bainite and tempered martensite, and a number density of crystal grains having an equivalent circular diameter of more than 30 μm among crystal grains defined by large-angle grain boundaries with an orientation difference of 15 degrees or more is 200/mm² or less.
  • 2. The steel plate according to 1., wherein the chemical composition further contains, in mass %, one or more selected from Cr: 2.00% or less, Mo: 1.0% or less, Al: 0.100% or less, Cu: 2.0% or less, Nb: 0.1% or less, V: 0.05% or less, Ti: 0.03% or less, and B: 0.0030% or less.
  • 3. The steel plate according to 1, or 2., wherein the chemical composition further contains, in mass %, one or more selected from Ca: 0.007% or less, REM: 0.010% or less, and Mg: 0.007% or less.
  • 4. The steel plate according to 1., wherein the chemical composition further contains, in mass %, one or more selected from Cr: 2.00% or less, Mo: 1.0% or less, Al: 0.100% or less, Cu: 2.0% or less, Nb: 0.1% or less, V: 0.05% or less, B: 0.0030% or less, Ca: 0.007% or less, REM: 0.010% or less, and Mg: 0.007% or less.
  • 5. A method of producing a steel plate, the method comprising: performing first heating that heats a steel material having the chemical composition according to 1., 2., 3., or 4. in a heat treatment furnace; thereafter hot rolling the steel material to obtain a hot-rolled steel plate; thereafter quenching the hot-rolled steel plate; thereafter performing second heating that heats the hot-rolled steel plate; and thereafter cooling the hot-rolled steel plate, wherein in the first heating, a soaking temperature in the heat treatment furnace is 900° C. or more and 1250° C. or less, and an in-furnace time in the heat treatment furnace is 600 minutes or less, in the hot rolling, a finish temperature is 1000° C. or less and 700° C. or more at a surface of the hot-rolled steel plate, in the quenching, an average cooling rate in a temperature range from 600° C. to 300° C. is 3° C./s or more and a cooling end temperature is 300° C. or less, at a depth position of ¼ of a plate thickness of the hot-rolled steel plate, in the second heating, a heating temperature is Ac₁ point or more and less than Ac₃ point, at the depth position of ¼ of the plate thickness of the hot-rolled steel plate, and in the cooling, an average cooling rate in a temperature range from 700° C. to 500° C. is 3° C./s or more and a cooling end temperature is 500° C. or less and 200° C. or more, at the depth position of ¼ of the plate thickness of the hot-rolled steel plate.

Advantageous Effect

It is thus possible to obtain a steel plate having high strength, low yield ratio, and excellent low-temperature toughness, specifically, a steel plate having TS of 690 MPa or more, YR of 0.8 or less, and vE_{−70° C.} of 100 J or more, which can replace expensive nickel steel. The steel plate can be used for steel structures that are used in low-temperature environments, for example, large liquefied gas storage tanks such as liquefied CO2 tanks and LPG tanks for ships, and contributes to significantly lower production costs than when nickel steel is used. This yields industrially great advantageous effects.

DETAILED DESCRIPTION

(1) Steel Plate

A steel plate according to an embodiment of the present disclosure will be described in detail below. The following description shows a preferred embodiment of the present disclosure, and the present disclosure is not limited to such.

[Chemical Composition]

The chemical composition of the steel plate according to an embodiment of the present disclosure will be described. Preferably, the steel material used in the method of producing the steel plate according to an embodiment of the present disclosure also has the below-described chemical composition. In this specification, “%” as a unit of content of each element denotes “mass %” unless otherwise specified.

C: 0.02% or More and 0.15% or Less

Cis an element that has the effect of improving the strength of the steel plate. In order to achieve this effect, the C content is 0.02% or more. The C content is preferably 0.03% or more. If the C content is more than 0.15%, the amount of martensite austenite constituent in the steel plate is excessive and low-temperature toughness decreases. The C content is therefore 0.15% or less. The C content is preferably 0.12% or less.

Si: 0.01% or More and 0.50% or Less

Si is an element that acts as a deoxidizer. In order to achieve this effect, the Si content is 0.01% or more. The Si content is preferably 0.03% or more. If the Si content is excessively high, toughness decreases. The Si content is therefore 0.50% or less. The Si content is preferably 0.30% or less.

Mn: 0.05% or More and 2.50% or Less

Mn is an element that enhances the hardenability of the steel and is effective in increasing the strength of the steel plate. In order to achieve this effect, the Mn content is 0.05% or more. The Mn content is preferably 0.10% or more. If the Mn content is more than 2.50%, toughness degrades. The Mn content is therefore 2.50% or less. The Mn content is preferably 2.00% or less.

Ni: 0.50% or More and Less than 5.00%

Ni is an element effective in improving the low-temperature toughness of the steel plate. In order to achieve this effect, the Ni content is 0.50% or more. Since Ni is an expensive element, the steel plate costs increase as the Ni content increases. The Ni content is therefore less than 5.00%. The Ni content is preferably 0.80% or more. The Ni content is preferably 3.50% or less.

P: 0.03% or Less

P is an inevitable impurity, and is a harmful element that adversely affects the low-temperature toughness of the steel plate. For example, in order to obtain a sound base metal and weld joint when welding the steel plate, it is preferable to reduce P as much as possible. The P content is therefore 0.03% or less. Since lower P content is better from the viewpoint of improving low-temperature toughness, the lower limit of the P content is not set and may be 0%. P is, however, allowed to be contained as an inevitable impurity. Moreover, excessively reducing P causes an increase in cost. Accordingly, the P content is preferably 0.001% or more from the viewpoint of cost.

S: 0.0050% or Less

S forms MnS in the steel and greatly degrades low-temperature toughness. Hence, it is desirable to reduce S as much as possible with the upper limit of the S content being 0.0050%. The S content is preferably 0.0020% or less. Since lower S content is better, the lower limit of the S content is not set and may be 0%. S is, however, allowed to be contained as an inevitable impurity. Moreover, excessively reducing S causes an increase in cost. Accordingly, the S content is preferably 0.0001% or more from the viewpoint of cost.

N: 0.0010% or More and 0.0080% or Less

N forms precipitates in the steel. In particular, if the N content is more than 0.0080%, the toughness of the base metal decreases. N is also an element that forms AlN and thus contributes to grain refinement of the base metal. This effect is achieved when the N content is 0.0010% or more. The N content is therefore 0.0010% or more and 0.0080% or less. The N content is preferably 0.0020% or more. The N content is preferably 0.0060% or less.

The chemical composition of the steel plate according to one embodiment of the present disclosure may contain the certain amounts of the elements described above with the balance consisting of Fe and inevitable impurities.

In another embodiment of the present disclosure, the chemical composition may optionally further contain one or both of: one or more selected from Cr, Mo, Al, Cu, Nb, V, Ti, and B; and one or more selected from Ca, REM, and Mg, preferably in the following amounts.

In yet another embodiment of the present disclosure, the chemical composition may optionally further contain one or more selected from Cr, Mo, Al, Cu, Nb, V, B, Ca, REM, and Mg, preferably in the following amounts.

Cr: 2.00% or Less

Cr is an element that improves the strength of the steel plate without significantly impairing low-temperature toughness. In order to achieve this effect, the Cr content is preferably 0.01% or more. The Cr content is more preferably 0.30% or more. If the Cr content is more than 2.00%, the low-temperature toughness of the steel plate may decrease. Accordingly, in the case where Cr is added, the Cr content is preferably 2.00% or less. The Cr content is more preferably 0.80% or less.

Mo: 1.0% or Less

Mo is an element that contributes to improved strength of the steel, and may be optionally added depending on the desired strength. If the Mo content is more than 1.0%, toughness may degrade. Accordingly, in the case where Mo is added, the Mo content is preferably 1.0% or less. From the viewpoint of achieving the strength improving effect by Mo, the Mo content is preferably 0.01% or more.

Al: 0.100% or Less

Al is an element that acts as a deoxidizer, and is widely used in the molten steel deoxidation process for high tensile strength steel. In order to achieve this effect, the Al content is preferably 0.001% or more. The Al content is more preferably 0.010% or more. If the Al content is more than 0.100%, the toughness of the base metal may decrease. Accordingly, in the case where Al is added, the Al content is preferably 0.100% or less. The Al content is more preferably 0.070% or less.

Cu: 2.0% or Less

Cu is an element that can increase strength while maintaining high toughness, and may be optionally added depending on the desired strength. If the Cu content is more than 2.0%, hot brittleness may occur and degrade the surface characteristics of the steel plate. Accordingly, in the case where Cu is added, the Cu content is preferably 2.0% or less. The Cu content is more preferably 1.0% or less. In order to achieve the foregoing effect, the Cu content is preferably 0.01% or more. The Cu content is more preferably 0.10% or more, and further preferably 0.20% or more.

Nb: 0.1% or Less

Nb is an element that contributes to improved strength of the steel, and may be optionally added depending on the desired strength. If the Nb content is more than 0.1%, the toughness of the base metal may degrade. Accordingly, in the case where Nb is added, the Nb content is preferably 0.1% or less. From the viewpoint of achieving the strength improving effect by Nb, the Nb content is preferably 0.005% or more.

V: 0.05% or Less

V is an element effective in enhancing the strength of the steel plate through strengthening by precipitation. If the V content is excessively high, the low-temperature toughness of the steel plate may decrease. Accordingly, in the case where V is added, the V content is preferably 0.05% or less. The V content is more preferably 0.04% or less. Although no lower limit is placed on the V content, the V content is preferably 0.010% or more in order to achieve the foregoing effect.

Ti: 0.03% or Less

Ti is an element that has the effect of enhancing the toughness of the weld without degrading the mechanical properties of the base metal when welding the steel plate. In order to achieve this effect, the Ti content is preferably 0.003% or more. A Ti content of more than 0.03%, however, may cause a decrease in toughness. Accordingly, in the case where Ti is added, the Ti content is preferably 0.03% or less.

B: 0.0030% or Less

B is an element that enhances hardenability when added in a small amount. In order to sufficiently achieve this effect, the B content is preferably 0.0003% or more. If the B content is more than 0.0030%, toughness may degrade. Accordingly, in the case where B is added, the B content is preferably 0.0030% or less. The B content is more preferably less than 0.0025%.

Ca: 0.007% or Less

Ca is an element that has the effect of improving the low-temperature toughness of the steel plate by controlling the form of inclusions in the steel. If the Ca content is excessively high, the cleanliness of the steel may be impaired and low-temperature toughness, in particular Charpy absorbed energy at low temperatures, may decrease. Accordingly, in the case where Ca is added, the Ca content is preferably 0.007% or less. The Ca content is more preferably 0.004% or less. Although no lower limit is placed on the Ca content, the Ca content is preferably 0.001% or more in order to achieve the foregoing effect.

REM: 0.010% or Less

REM (rare earth metal) is an element that has the effect of improving the low-temperature toughness of the steel plate by controlling the form of inclusions in the steel, as with Ca. If the REM content is excessively high, the cleanliness of the steel may be impaired and low-temperature toughness, in particular Charpy absorbed energy at low temperatures, may decrease. Accordingly, in the case where REM is added, the REM content is preferably 0.010% or less. The REM content is more preferably 0.008% or less. Although no lower limit is placed on the REM content, the REM content is preferably 0.001% or more in order to achieve the foregoing effect.

Herein, REM is a generic term for 17 elements including 15 lanthanoid elements and Y and Sc. These 17 elements may be contained singly or in combination. The REM content is the total content of these 17 elements.

Mg: 0.007% or Less

Mg is an element that has the effect of improving the low-temperature toughness of the steel plate by controlling the form of inclusions in the steel, as with Ca and REM. If the Mg content is excessively high, the cleanliness of the steel may be impaired and low-temperature toughness, in particular Charpy absorbed energy at low temperatures, may decrease. Accordingly, in the case where Mg is added, the Mg content is preferably 0.007% or less. The Mg content is more preferably 0.004% or less. Although no lower limit is placed on the Mg content, the Mg content is preferably 0.001% or more in order to achieve the foregoing effect.

[Microstructure]

Next, the microstructure of the steel plate according to an embodiment of the present disclosure will be described. The microstructure is measured at a depth position of ¼ of the plate thickness of the steel plate as described later.

Ferrite Fraction: 5% to 95%

The steel plate according to an embodiment of the present disclosure has a phase constitution in which the ferrite fraction is 5% to 95% and the martensite austenite constituent fraction is 1% to 30% with the residual microstructure consisting of one or both of bainite and tempered martensite. If the ferrite fraction is less than 5%, the desired yield ratio cannot be achieved. If the ferrite fraction is more than 95%, the fraction of hard phase such as martensite austenite constituent is low and the desired yield ratio cannot be achieved. The ferrite fraction is therefore 5% to 95%. The ferrite fraction is preferably 10% or more, and more preferably 15% or more. The ferrite fraction is preferably 90% or less, and more preferably 85% or less.

Martensite Austenite Constituent Fraction: 1% to 30%

If the martensite austenite constituent fraction is less than 1%, the desired yield ratio cannot be achieved. If the martensite austenite constituent fraction is more than 30%, low-temperature toughness decreases. The martensite austenite constituent fraction is therefore 1% to 30%. The martensite austenite constituent fraction is preferably 2% or more, and more preferably 3% or more. The martensite austenite constituent fraction is preferably 28% or less, and more preferably 26% or less.

Residual Microstructure: One or Both of Bainite and Tempered Martensite

If the residual microstructure other than ferrite and martensite austenite constituent is not one or both of bainite and tempered martensite, for example, if the residual microstructure is as-quenched martensite (hereafter also simply referred to as martensite), the desired low-temperature toughness cannot be achieved. The residual microstructure other than ferrite and martensite austenite constituent is therefore one or both of bainite and tempered martensite.

The fraction of each phase can be measured in the manner described in the EXAMPLES section below. The fraction of each phase herein is the area proportion (area ratio) of the phase to the entire microstructure.

Number density of crystal grains having equivalent circular diameter of more than 30 μm: 200/mm² or less

Toughness in cryogenic environments of −50° C. to −70° C. is greatly influenced by the number of coarse crystal grains, especially crystal grains having an equivalent circular diameter of more than 30 μm (hereafter also referred to as coarse crystal grains), as mentioned above. In particular, low-temperature toughness is significantly improved by reducing the number of coarse crystal grains. The number density of coarse crystal grains is therefore 200/mm² or less. This can achieve the desired low-temperature toughness. The number density of coarse crystal grains is preferably 150/mm² or less. The lower limit of the number density of coarse crystal grains is not set and may be 0/mm². From the viewpoint of industrial implementation, the number density of coarse crystal grains is preferably 10/mm² or more. The term “crystal grains” herein denotes crystal grains defined by large-angle grain boundaries with an orientation difference of 15 degrees or more (i.e. each region surrounded by large-angle grain boundaries with an orientation difference of 15 degrees or more is a crystal grain). The number density of coarse crystal grains can be measured in the manner described in the EXAMPLES section below.

The number density of coarse crystal grains does not necessarily correlate with the average grain size of crystal grains. In detail, coarse crystal grains are generated as a result of crystal grains in the steel plate being locally coarsened during heat treatment and these coarsened crystal grains remaining in the microstructure of the steel plate in the finished product, rather than as a result of crystal grains in the steel plate being uniformly coarsened. Hence, even when the average grain size of crystal grains is 5 μm or less, coarse crystal grains may exist locally and their number density may be more than 200/mm². In this case, excellent toughness in cryogenic environments, for example, at −70° C., cannot be achieved. In order to reduce the number of coarse crystal grains, it is very important to appropriately prepare the chemical composition as described above and appropriately control the production conditions, in particular, appropriately control the following simultaneously:

• • the in-furnace time in the heat treatment furnace in first heating,
  • the rolling finish temperature in hot rolling,
  • the average cooling rate in a certain temperature range and the cooling end temperature in quenching, and
  • the average cooling rate in a certain temperature range and the cooling end temperature in cooling.

The plate thickness of the steel plate according to an embodiment of the present disclosure is not limited, but is preferably 6 mm or more and 50 mm or less, for example.

[Mechanical Properties]

(Tensile Strength)

The tensile strength of the steel plate is preferably 690 MPa or more, for example. With a tensile strength of 690 MPa or more, the plate thickness can be reduced when the steel plate is used for tanks. The tensile strength of the steel plate is more preferably 720 MPa or more. Although no upper limit is placed on the tensile strength of the steel plate, the tensile strength is preferably 1000 MPa or less, for example.

The tensile strength can be measured by the method described in the EXAMPLES section below.

(Yield Ratio)

The yield ratio of the steel plate is preferably 0.80 or less, for example. With a yield ratio of 0.80 or less, mechanical stress relief can be performed instead of post weld heat treatment.

The yield ratio can be measured by the method described in the EXAMPLES section below.

(Low-Temperature Toughness)

For the low-temperature toughness of the steel plate, vE_{−70° C.} is preferably 100 J or more. vE_{−70° C.} is more preferably 150 J or more.

vE_{−70° C.} can be measured by a full-size Charpy impact test, for example, the method described in the EXAMPLES section below.

(2) Method of Producing Steel Plate

Next, the method of producing the steel plate according to an embodiment of the present disclosure will be described in detail. In the following description, the term “temperature” refers to the temperature at the center of the plate thickness of the steel plate or steel material unless otherwise specified. The temperature at each of the center of the plate thickness and a depth position of ¼ of the plate thickness can be obtained, for example, by heat transfer calculation from the surface temperature of the steel plate measured with a radiation thermometer.

The steel plate according to the present disclosure can be suitably produced by sequentially performing the following steps:

• • (1) first heating
  • (2) hot rolling
  • (3) quenching (accelerated cooling)
  • (4) second heating
  • (5) cooling.

(1) First Heating

First, a steel material having the above-described chemical composition is prepared. The method of preparing the steel material is not limited. For example, molten steel having the above-described chemical composition may be prepared by steelmaking using a conventional method and cast to produce the steel material. The steelmaking may be performed using any method such as a converter, an electric furnace, or an induction furnace. The casting is preferably performed by continuous casting from the viewpoint of productivity, but may be performed by ingot casting and blooming. An example of the steel material is a steel slab.

The prepared steel material is then heated in a heat treatment furnace under the following conditions.

[Soaking Temperature in Heating Furnace: 900° C. or More and 1250° C. or Less]

The soaking temperature in the heating furnace (hereafter also referred to as soaking temperature) is 900° C. or more and 1250° C. or less. If the soaking temperature is less than 900° C., due to high deformation resistance of the steel material, the load on the mill in the subsequent hot rolling may increase and hinder the hot rolling. The soaking temperature is therefore 900° C. or more.

The soaking temperature is preferably 950° C. or more. If the soaking temperature is more than 1250° C., due to noticeable oxidation of the steel, the loss incurred by the removal of the oxide film caused by the oxidation may increase, resulting in a decrease in yield rate. The soaking temperature is therefore 1250° C. or less. The soaking temperature is preferably 1200° C. or less.

The soaking temperature herein is the (target) temperature reached by the entire steel material in the heating furnace, and can also be called the set temperature of the heating furnace.

[In-Furnace Time in Heating Furnace: 600 Minutes or Less]

The in-furnace time in the heating furnace (hereafter also referred to as in-furnace time) is 600 minutes or less. If the in-furnace time is more than 600 minutes, coarse crystal grains form locally and eventually remain in the finished product. This makes it impossible to achieve the desired number density of coarse crystal grains. The in-furnace time is therefore 600 minutes or less. The in-furnace time is preferably 580 minutes or less, and more preferably 560 minutes or less. Although no lower limit is placed on the in-furnace time, for example, the in-furnace time is preferably 60 minutes or more from the viewpoint of the operating load of the heating furnace.

The in-furnace time herein is the time from when the steel material is conveyed to the heat treatment furnace for performing heating (i.e. heating before hot rolling) for hot rolling to when the steel material is discharged. A typical heat treatment furnace can be used as the heat treatment furnace.

The soaking time is not limited. For example, from the viewpoint of reducing the deformation resistance of the steel material and enhancing rollability, the soaking time is preferably 10 minutes to 200 minutes. The soaking time herein is the time during which the temperature of the entire steel material is maintained within the range of the soaking temperature ±30° C.

Here, the steel material obtained as a result of casting and the like may be heated after cooling, or directly heated without cooling.

(2) Hot Rolling

The steel material is then hot rolled under the following conditions to obtain a hot-rolled steel plate.

[Finish Temperature (Rolling Finish Temperature): 1000° C. or Less and 700° C. or More]

If the finish temperature in the hot rolling is less than 700° C., due to high deformation resistance of the steel material, the load on the mill increases and hinders the hot rolling. If the finish temperature is more than 1000° C., fine microstructure cannot be obtained and coarse crystal grains remain, causing a decrease in low-temperature toughness. The finish temperature is therefore 1000° C. or less and 700° C. or more. The finish temperature is preferably 980° C. or less, and more preferably 960° C. or less. The finish temperature herein is the temperature at the surface of the hot-rolled steel plate.

The final plate thickness of the hot-rolled steel plate is not limited. For example, the final plate thickness of the hot-rolled steel plate is preferably 6 mm or more and 50 mm or less as mentioned above.

(3) Quenching (Accelerated Cooling)

The hot-rolled steel plate is then quenched (accelerated cooling). Here, it is important to set the average cooling rate in the temperature range from 600° C. to 300° C. (hereafter also referred to as quenching rate) to 3° C./s or more and the cooling end temperature to 300° C. or less.

[Quenching Rate: 3° C./s or More]

If the quenching rate is less than 3° C./s, it is difficult to obtain the desired transformed microstructure, so that sufficient strength cannot be achieved. The quenching rate is therefore 3° C./s or more. The quenching rate is preferably 4° C./s or more, and more preferably 5° C./s or more. Although no upper limit is placed on the quenching rate, if the quenching rate is more than 200° C./s, it is difficult to control the temperature at each position in the steel plate, so that the material quality tends to vary in the plate transverse direction and the rolling direction of the steel plate. This is likely to cause variation in material properties such as tensile property and toughness. The quenching rate is therefore preferably 200° C./s or less. The temperature and the quenching rate herein are respectively the temperature at a depth position of ¼ of the plate thickness of the hot-rolled steel plate and the rate calculated from the temperature changes at the position.

[Cooling End Temperature: 300° C. or Less]

If the cooling stop temperature (i.e. cooling end temperature) in the quenching is more than 300° C., the desired transformed microstructure cannot be obtained. The cooling stop temperature is therefore 300° C. or less. By accelerated cooling under such conditions, the hot-rolled steel plate is quenched well. Although no lower limit is placed on the cooling stop temperature, the cooling stop temperature is preferably 0° C. or more, for example. The temperature herein is the temperature at a depth position of ¼ of the plate thickness of the hot-rolled steel plate.

The cooling treatment in the quenching may be performed by any method without limitation. For example, one or both of air cooling and water cooling may be used. For water cooling, any cooling method (for example, spray cooling, mist cooling, laminar cooling, etc.) using water may be used.

(4) Second Heating

The hot-rolled steel plate is then heated under the following conditions.

[Heating Temperature: Ac₁ Point or More and Less than Ac₃ Point]

The heating temperature in the second heating is Ac₁ point or more and less than Ac₃ point. In other words, heating to a dual-phase temperature range is performed in the second heating. If the heating temperature is less than Ac₁ point, a sufficient amount of martensite austenite constituent cannot be obtained and the desired low yield ratio cannot be achieved. If the heating temperature is Ac₃ point or more, a ferrite fraction of less than 5% and a tempered martensite fraction of more than 90% will result, and the desired low yield ratio cannot be achieved. The temperature herein is the temperature at a depth position of ¼ of the plate thickness of the hot-rolled steel plate.

For heating in the second heating, any heating method may be used as long as the heating temperature can be controlled in the foregoing range. An example of the heating method is heating using a heat treatment furnace (hereafter also referred to as furnace heating). The heat treatment furnace used for furnace heating is not limited, and a typical heat treatment furnace may be used.

After the heating temperature is reached, the hot-rolled steel plate may be held in the dual-phase temperature range of Ac₁ point or more and less than Ac₃ point for any period of time, for example, 10 minutes to 120 minutes, before cooling.

Ac₁ point can be calculated according to the following formula (1):

Ac 1 ⁢ point ⁢ ( °C . ) = 750.8 - 26.6 × C + 17.6 × Si - 11.6 × Mn - 22.9 × Cu - 23 × Ni + 24.1 × Cr + 22.5 × Mo - 39.7 × V - 5.7 × Ti + 232.4 × Nb - 169.4 × Al . ( 1 )

Ac₃ point can be calculated according to the following formula (2):

Ac 3 ⁢ point ⁢ ( °C . ) = 937.2 - 436.5 × C + 56 × Si - 19.7 × Mn - 16.3 × Cu - 26.6 × Ni - 4.9 × Cr + 38.1 × Mo + 124.8 × V + 136.3 × Ti - 19.1 × Nb + 198.4 × Al + 3315 × B . ( 2 )

The element symbols in the formulas (1) and (2) each represent the content (mass %) of the corresponding element in the chemical composition of the steel plate. If the element is not contained, the content of the element is set to “0”.

(5) Cooling

The hot-rolled steel plate is then cooled under the following conditions.

[Average Cooling Rate in Temperature Range from 700° C. to 500° C.: 3° C./s or More]

If the average cooling rate in the temperature range from 700° C. to 500° C. (hereafter also simply referred to as average cooling rate) is less than 3° C./s, there is a possibility that the desired transformed microstructure cannot be obtained and strength and low-temperature toughness decrease. The average cooling rate is therefore 3° C./s or more. The average cooling rate is preferably 4° C./s or more, and more preferably 5° C./s or more. Although no upper limit is placed on the average cooling rate, the average cooling rate is preferably 200° C./s or less, for example. The temperature and the average cooling rate herein are respectively the temperature at a depth position of ¼ of the plate thickness of the hot-rolled steel plate and the rate calculated from the temperature changes at the position.

[Cooling End Temperature: 500° C. or Less and 200° C. or More]

If the cooling end temperature is more than 500° C., martensite austenite constituent decomposes during cooling to room temperature after the end of the cooling, for example, during cooling by air cooling (hereafter also simply referred to as air cooling), and the desired low yield ratio cannot be achieved. If the cooling end temperature is less than 200° C., the desired tempering effect cannot be achieved during air cooling and toughness degrades. The cooling end temperature is therefore 500° C. or less and 200° C. or more. The temperature herein is the temperature at a depth position of ¼ of the plate thickness of the hot-rolled steel plate.

(6) Cooling to Room Temperature (Self-Tempering)

After the cooling, the hot-rolled steel plate is cooled to room temperature. The cooling method is not limited, and may be air cooling, for example. This causes self-tempering, which further improves toughness. The cooling rate in the air cooling is typically 1° C./s or less in the case where the plate thickness of the hot-rolled steel plate is about 6 mm to 50 mm. The cooling rate herein is the rate calculated from the temperature changes at a depth position of ¼ of the plate thickness of the hot-rolled steel plate.

Conditions other than those described above are not limited and may be in accordance with conventional methods.

EXAMPLES

Steel plates were each produced according to the following procedure, and their properties were evaluated.

First, molten steel having the chemical composition shown in Table 1 (with the balance consisting of Fe and inevitable impurities) was prepared by steelmaking using a converter, and subjected to continuous casting to produce a steel slab (thickness: 200 mm) as a steel material. Ac₁ point (C) calculated according to the foregoing formula (1) and Ac₃ point (° C.) calculated according to the foregoing formula (2) are shown in Table 1.

Steel

sample

Chemical composition (mass %)

ID	C	Si	Mn	Ni	P	S	N	Cr	Mo	Al
A	0.08	0.22	1.21	1.45	0.01	0.0010	0.0025
B	0.02	0.30	1.95	2.21	0.01	0.0005	0.0028
C	0.03	0.15	2.45	1.85	0.01	0.0006	0.0022
D	0.14	0.15	0.81	1.65	0.01	0.0008	0.0027
E	0.07	0.01	1.35	1.75	0.01	0.0010	0.0031
F	0.07	0.45	0.89	1.85	0.01	0.0010	0.0032
G	0.12	0.35	0.06	3.50	0.01	0.0009	0.0025
H	0.08	0.02	1.21	2.11	0.03	0.0010	0.0025
I	0.11	0.12	1.10	1.84	0.01	0.0050	0.0021
J	0.09	0.05	1.85	0.50	0.01	0.0010	0.0025
K	0.06	0.05	1.20	4.50	0.01	0.0010	0.0019
L	0.06	0.15	1.21	1.25	0.01	0.0010	0.0025	0.55	0.45	0.008
M	0.08	0.21	0.94	2.05	0.01	0.0010	0.0022	0.35	0.35
N	0.18	0.31	0.89	1.15	0.01	0.0009	0.0026
O	0.01	0.30	1.95	2.21	0.01	0.0005	0.0028
P	0.07	0.62	0.89	1.85	0.01	0.0010	0.0032
Q	0.14	0.32	2.25	0.35	0.01	0.0010	0.0024
R	0.11	0.21	0.02	1.25	0.01	0.0010	0.0024
S	0.09	0.26	2.80	1.11	0.01	0.0010	0.0028
T	0.11	0.05	1.35	0.85	0.04	0.0005	0.0019
U	0.11	0.05	1.35	1.31	0.04	0.0060	0.0025
V	0.09	0.15	0.87	1.85	0.01	0.0010	0.0105
W	0.09	0.15	0.87	1.85	0.01	0.0010	0.0005			0.008
X	0.09	0.25	1.55	2.00	0.01	0.0008	0.0032	0.45
Y	0.07	0.15	1.85	2.25	0.01	0.0010	0.0026		0.40
Z	0.06	0.25	1.55	2.55	0.01	0.0009	0.0039			0.030
AA	0.08	0.25	1.35	1.95	0.01	0.0010	0.0025
BB	0.09	0.15	1.45	2.15	0.01	0.0010	0.0030
CC	0.07	0.15	1.75	1.95	0.01	0.0010	0.0029
DD	0.08	0.20	1.75	1.45	0.01	0.0009	0.0027
EE	0.06	0.15	1.85	2.35	0.01	0.0008	0.0026
FF	0.09	0.20	1.70	2.15	0.01	0.0009	0.0028	0.45	0.35
Steel									Ac1

sample

Chemical composition (mass %)

point

Ac3 point

ID	Cu	Nb	V	Ti	B	Ca	REM	Mg	(° C.)	(° C.)
A									705	852
B									682	848
C									682	835
D									702	825
E									693	834
F									704	865
G									673	810
H									686	823
I									695	825
J									716	851
K									633	770
L			0.03	0.01	0.0010	0.0015			730	883
M	0.5	0.01	0.05				0.001	0.0021	700	851
N									715	828
O									682	852
P									707	875
Q									719	840
R									723	867
S									695	828
T									714	843
U									703	831
V									698	840
W									697	842
X									700	826
Y									687	834
Z									672	833
AA	0.35								685	832
BB		0.01							687	820
CC			0.05						684	835
DD				0.01					698	842
EE					0.0009				676	823
FF						0.0015			701	830
Underlines indicate outside the scope of the present disclosure.

Subsequently, (1) first heating, (2) hot rolling, (3) quenching (accelerated cooling), (4) second heating, and (5) cooling were performed under the conditions shown in Table 2 to obtain each steel plate (hot-rolled steel plate) having the corresponding plate thickness (final plate thickness). After (5) cooling, each hot-rolled steel plate was cooled to room temperature by air cooling. Conditions not specified were in accordance with the above detailed description and conventional methods. In No. 10 in Table 2, after (2) hot rolling, the hot-rolled steel plate was allowed to naturally cool to room temperature.

TABLE 2
		Production conditions

(4)

(1) First

(3) Quenching

Second

(5) Cooling

heating

(2) Hot rolling

Cooling

heating

Cooling

		Soaking	In-		Finish		end	Heating	Aver-	end
	Steel	tem-	fur-	Plate	tem-	Quench-	tem-	tem-	age	tem-
	sam-	per-	nace	thick-	per-	ing	per-	per-	cooling	per-
	ple	ature	time	ness	ature	rate	ature	ature	rate	ature
No.	ID	(° C.)	(min)	(mm)	(° C.)	(° C./s)	(° C.)	(° C.)	(° C./s)	(° C.)	Remarks

1	A	1150	250	40	850	15	200	800	15	350	Example
2	A	1250	500	25	950	30	200	790	15	350	Example
3	A	950	160	25	750	30	200	810	15	350	Example
4	A	1120	450	30	900	25	150	730	25	450	Example
5	A	1050	240	30	850	25	150	840	25	300	Example
6	A	1200	550	40	1050	15	200	790	15	400	Comparative Example
7	A	1100	550	40	800	15	450	810	15	450	Comparative Example
8	A	1050	450	40	900	15	250	690	15	350	Comparative Example
9	A	1100	400	40	870	15	150	870	15	300	Comparative Example
10	A	1100	550	40	950	0.5	—	790	15	300	Comparative Example
11	A	1150	550	40	750	15	150	820	0.5	300	Comparative Example
12	A	1090	300	40	800	15	200	810	15	100	Comparative Example
13	A	1120	550	40	820	15	200	830	15	550	Comparative Example
14	B	1120	250	12	800	5	200	800	50	450	Example
15	C	1100	200	40	820	15	150	780	15	400	Example
16	D	1070	170	40	810	15	200	770	15	350	Example
17	E	1120	500	30	900	25	200	790	25	400	Example
18	F	1170	450	40	870	15	200	800	15	360	Example
19	G	1100	400	20	800	35	150	790	35	400	Example
20	H	1200	400	40	850	15	200	780	15	320	Example
21	I	1100	350	40	820	15	100	800	15	350	Example
22	J	1150	400	40	870	15	200	830	15	350	Example
23	K	1150	250	40	800	15	200	720	15	230	Example
24	L	1100	250	40	800	15	200	830	15	350	Example
25	M	1080	250	40	820	15	200	810	15	300	Example
26	N	1100	200	40	800	15	200	790	15	300	Comparative Example
27	O	1080	200	40	750	15	250	820	15	350	Comparative Example
28	P	1120	300	40	850	15	200	830	15	400	Comparative Example
29	Q	1070	250	40	850	15	200	800	15	400	Comparative Example
30	R	1150	380	40	880	15	200	830	15	350	Comparative Example
31	S	1100	250	40	820	15	200	790	15	320	Comparative Example
32	T	1160	300	40	860	15	200	820	15	400	Comparative Example
33	U	1200	200	40	800	15	200	810	15	300	Comparative Example
34	V	1150	300	40	850	15	200	820	15	300	Comparative Example
35	W	1150	550	40	950	15	150	830	15	350	Comparative Example
36	A	1250	700	40	900	15	150	800	15	300	Comparative Example
37	X	1150	300	40	900	15	200	770	15	350	Example
38	Y	1150	300	40	850	15	200	800	15	300	Example
39	Z	1100	450	40	900	15	150	790	15	300	Example
40	AA	1150	300	40	800	15	150	780	15	350	Example
41	BB	1100	500	40	950	15	200	770	15	400	Example
42	CC	1150	350	40	900	15	150	810	15	300	Example
43	DD	1150	300	40	900	15	150	800	15	350	Example
44	EE	1100	300	40	850	15	200	780	15	400	Example
45	FF	1150	500	40	900	15	150	800	15	350	Example
Underlines indicate outside the scope of the present disclosure.

For each of the obtained steel plates, the microstructure, tensile strength (TS), yield ratio (YR), and Charpy absorbed energy at −70° C. (vE. 70° C.) were measured in the following manner. The measurement results are shown in Table 3.

[Microstructure]

A test piece for microstructure observation was collected from the steel plate so that a depth position of ¼ of the plate thickness of the steel plate would be the observation position. The test piece was embedded in resin so that a cross section perpendicular to the rolling direction would be the observation plane. The observation plane of the test piece was then mirror-polished and thereafter etched with nital. After this, the observation plane of the test piece was observed using a scanning electron microscope with 5000 magnification, and an image of microstructure was taken. The obtained image was analyzed to calculate the fraction of each phase. The phases were identified as follows:

• • tempered martensite: matrix phase containing cementite
  • martensite austenite constituent: hard phase not containing cementite and having an equivalent circular diameter of 1 μm or less
  • as-quenched martensite: hard phase not containing cementite and having an equivalent circular diameter of more than 1 μm
  • bainite: matrix phase in which martensite austenite constituent is formed in microstructure
  • ferrite: matrix phase other than the above.

Martensite austenite constituent may contain retained austenite. Each of the foregoing phases may contain precipitates. The fraction of each phase is calculated including these.

Further, microstructure analysis was conducted by electron backscatter diffraction measurement (hereafter also referred to as EBSD measurement) using the foregoing test piece. In the EBSD measurement, the step size was 0.1 μm, and the measurement region was 1 mm×1 mm in total. From the obtained crystal orientation data, each crystal grain was defined with large-angle grain boundaries with an orientation difference of 15 degrees or more being set as crystal grain boundaries. The equivalent circular diameter of each crystal grain was then calculated from the area of the crystal grain. After this, the number of crystal grains having an equivalent circular diameter of more than 30 μm was counted, and the number was divided by the total area of the measurement region to determine the number density of coarse crystal grains.

[Tensile Strength] [Yield Ratio]

A JIS No. 4 tensile test piece perpendicular to the rolling direction was collected from a depth position of ¼ of the plate thickness of the steel plate. Using the tensile test piece, a tensile test was conducted in accordance with JIS Z 2241 to measure the tensile strength (TS) and yield stress (YS) of the steel plate. In addition, the yield ratio (YR) was calculated according to the following formula. The measurement results are shown in Table 3.

YR=YS/TS.

If TS was 690 MPa or more, the steel plate was rated as “pass”. If YR was 0.80 or less, the steel plate was rated as “pass”

[Low-Temperature Toughness]

V-notched test pieces parallel to the rolling direction were collected from a depth position of ¼ of the plate thickness of the steel plate in accordance with JIS Z 2202. Using the V-notched test pieces, a Charpy impact test was conducted in accordance with JIS Z 2242 to determine the Charpy absorbed energy at −70° C. (vE_{−70° C.}). The Charpy absorbed energy serves as an index of the low-temperature toughness of the steel plate. In the Charpy impact test, three test pieces were collected per steel plate and measured. The individual measured values and the average value are shown in Table 3. In the full-size Charpy impact test, if vE_{−70° C.} of all test pieces was 100 J or more, the steel plate was evaluated as having excellent low-temperature toughness and rated as “pass”.

TABLE 3
		Microstructure

Mar-

Number

Mechanical properties

tensite

density

vE-70° C.

vE-70° C.

vE-70° C.

austenite

of

(J)

(J)

(J)

vE-70° C.

Steel

Ferrite

con-

coarse

Indi-

Indi-

Indi-

(J)

sam-

frac-

stituent

crystal

vidual

vidual

vidual

Aver-

ple

tion

fraction

Residual

grains

TS

YR

value

value

value

age

No.

ID

(%)

(%)

microstructure*

(/mm2)

(MPa)

(%)

(first)

(second)

(third)

value

Remarks

1	A	40	15	Bainite/tempered martensite	50	750	0.71	255	245	250	250	Example
2	A	45	20	Bainite/tempered martensite	160	700	0.71	190	180	200	190	Example
3	A	35	15	Bainite/tempered martensite	30	780	0.74	202	220	230	217	Example
4	A	80	10	Bainite/tempered martensite	110	710	0.72	180	170	190	180	Example
5	A	5	10	Bainite/tempered martensite	50	780	0.78	210	220	230	220	Example
6	A	45	20	Bainite/tempered martensite	210	700	0.71	90	160	70	107	Comparative Example
7	A	35	15	Bainite/tempered martensite	210	720	0.71	80	180	150	137	Comparative Example
8	A	100	0	-	110	720	0.95	180	170	190	180	Comparative Example
9	A	0	5	Bainite/tempered martensite	90	740	0.85	210	220	220	217	Comparative Example
10	A	45	10	Bainite/tempered martensite	205	650	0.72	110	160	120	130	Comparative Example
11	A	40	15	Bainite/tempered martensite	210	640	0.74	120	160	110	130	Comparative Example
12	A	35	15	Bainite/martensite	45	770	0.68	80	210	120	137	Comparative Example
13	A	25	5	Bainite/tempered martensite	210	710	0.75	80	210	70	120	Comparative Example
14	B	40	10	Bainite/tempered martensite	50	820	0.78	200	210	220	210	Example
15	C	40	8	Bainite/tempered martensite	45	730	0.73	220	230	240	230	Example
16	D	55	25	Bainite/tempered martensite	35	740	0.71	220	210	240	223	Example
17	F	40	15	Bainite/tempered martensite	120	770	0.73	180	170	190	180	Example
18	F	35	10	Bainite/tempered martensite	80	750	0.72	210	230	220	220	Example
19	G	20	10	Bainite/tempered martensite	60	720	0.75	220	240	240	233	Example
20	H	40	15	Bainite/tempered martensite	60	730	0.72	160	170	160	163	Example
21	T	25	10	Bainite/tempered martensite	45	740	0.73	160	170	150	160	Example
22	J	30	10	Bainite/tempered martensite	80	750	0.74	210	200	200	203	Example
23	K	35	15	Bainite/tempered martensite	40	770	0.75	255	260	256	257	Example
24	L	45	20	Bainite/tempered martensite	50	900	0.71	245	250	260	252	Example
25	M	35	15	Bainite/tempered martensite	55	920	0.69	255	260	245	253	Example
26	N	35	35	Bainite/tempered martensite	50	780	0.68	80	70	80	77	Comparative Example
27	O	25	10	Bainite/tempered martensite	45	650	0.78	240	235	240	238	Comparative Example
28	P	30	15	Bainite/tempered martensite	60	730	0.73	90	70	60	73	Comparative Example
29	Q	35	10	Bainite/tempered martensite	50	720	0.72	90	90	90	90	Comparative Example
30	R	30	10	Bainite/tempered martensite	90	640	0.71	180	160	170	170	Comparative Example
31	S	40	15	Bainite/tempered martensite	45	820	0.73	70	60	65	65	Comparative Example
32	T	10	10	Bainite/tempered martensite	70	750	0.72	60	50	50	53	Comparative Example
33	U	20	10	Bainite/tempered martensite	40	760	0.71	60	50	60	57	Comparative Example
34	V	10	10	Bainite/tempered martensite	60	740	0.71	90	60	70	73	Comparative Example
35	W	10	10	Bainite/tempered martensite	205	730	0.70	80	150	150	127	Comparative Example
36	A	30	20	Bainite/tempered martensite	220	760	0.72	90	160	150	133	Comparative Example
37	X	30	15	Bainite/tempered martensite	90	760	0.74	220	230	250	233	Example
38	Y	20	10	Bainite/tempered martensite	80	800	0.71	215	255	220	230	Example
39	Z	30	10	Bainite/tempered martensite	90	750	0.71	220	240	230	230	Example
40	AA	35	15	Bainite/tempered martensite	90	760	0.72	240	210	200	217	Example
41	BB	30	15	Bainite/tempered martensite	100	780	0.74	220	210	210	213	Example
42	CC	10	10	Bainite/tempered martensite	80	760	0.76	230	220	220	223	Example
43	DD	20	15	Bainite/tempered martensite	70	750	0.71	250	240	200	230	Example
44	EE	25	20	Bainite/tempered martensite	50	800	0.77	240	210	230	227	Example
45	FF	20	15	Bainite/tempered martensite	150	830	0.75	230	210	210	217	Example
*Tempered martensite: matrix phase containing cementite (state in which carbides are precipitated in microstructure)
Martensite: hard phase not containing cementite and having equivalent circular diameter of more than 1 μm (state in which carbides are not precipitated in microstructure)
Bainite: microstructure in which martensite austenite constituent is formed

As shown in Table 3, in each Example, a steel plate having high strength, low yield ratio, and excellent low-temperature toughness, specifically, a steel plate having TS of 690 MPa or more, YR of 0.80 or less, and vE_{−70° C.} of 100 J or more, was obtained.

In each Comparative Example, at least one of TS, YR, and vE_{−70° C.} was poor.