Steel sheet and method for producing same

Abstract

A steel plate comprises a certain chemical composition, wherein in a microstructure at a depth position of 1 / 4 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.

Description

TECHNICAL FIELD

[0001] 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

[0002] 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 CO 2 , 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.

[0003] 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."

[0004] 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: 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 1 / 4 of a plate thickness, bainite is 90 area% or more, (B) in the microstructure at the site of 1 / 4 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 1 / 4 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."

[0005] 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 LISTPatent Literature

[0006] PTL 1: JP 2016-507649 A PTL 2: JP 2013-57105 A PTL 3: JP 2019-119934 A SUMMARY(Technical Problem)

[0007] 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.

[0008] 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.

[0009] 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)

[0010] Upon careful examination, we discovered the following.

[0011] An effective way of achieving desired property improvement is to control the microstructure at a depth position of 1 / 4 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 1 / 4 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 / mm 2< or less.

[0012] 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 Ac 1 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.

[0013] 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 1 / 4 of the plate thickness of the steel plate as in (2).

[0014] 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.

[0015] The present disclosure is based on these discoveries and further studies.

[0016] 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 1 / 4 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 / mm 2< 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 1 / 4 of a plate thickness of the hot-rolled steel plate, in the second heating, a heating temperature is Ac 1 point or more and less than Ac 3 point, at the depth position of 1 / 4 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 1 / 4 of the plate thickness of the hot-rolled steel plate. (Advantageous Effect)

[0017] 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 CO 2 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

[0018] 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]

[0019] 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

[0020] C is 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

[0021] 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

[0022] 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 %

[0023] 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

[0024] 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

[0025] 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

[0026] 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.

[0027] 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.

[0028] 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.

[0029] 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

[0030] 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

[0031] 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

[0032] 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

[0033] 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

[0034] 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

[0035] 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

[0036] 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

[0037] 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

[0038] 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

[0039] 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.

[0040] 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

[0041] 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]

[0042] 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 1 / 4 of the plate thickness of the steel plate as described later.Ferrite fraction: 5 % to 95 %

[0043] 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 %

[0044] 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

[0045] 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.

[0046] 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.

[0047] Number density of crystal grains having equivalent circular diameter of more than 30 µm: 200 / mm 2< or less

[0048] 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 / mm 2< or less. This can achieve the desired low-temperature toughness. The number density of coarse crystal grains is preferably 150 / mm 2< or less. The lower limit of the number density of coarse crystal grains is not set and may be 0 / mm 2< . From the viewpoint of industrial implementation, the number density of coarse crystal grains is preferably 10 / mm 2< 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.

[0049] 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 / mm 2< . 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.

[0050] 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)

[0051] 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.

[0052] The tensile strength can be measured by the method described in the EXAMPLES section below.(Yield ratio)

[0053] 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.

[0054] The yield ratio can be measured by the method described in the EXAMPLES section below.(Low-temperature toughness)

[0055] 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.

[0056] 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

[0057] 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 1 / 4 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.

[0058] 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

[0059] 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.

[0060] 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]

[0061] 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.

[0062] 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]

[0063] 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.

[0064] 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.

[0065] 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.

[0066] 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

[0067] 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]

[0068] 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.

[0069] 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)

[0070] 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]

[0071] 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 1 / 4 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]

[0072] 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 1 / 4 of the plate thickness of the hot-rolled steel plate.

[0073] 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

[0074] The hot-rolled steel plate is then heated under the following conditions.[Heating temperature: A C1 point or more and less than Ac 3 point]

[0075] The heating temperature in the second heating is Ac 1 point or more and less than Ac 3 point. In other words, heating to a dual-phase temperature range is performed in the second heating. If the heating temperature is less than Ac 1 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 Ac 3 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 1 / 4 of the plate thickness of the hot-rolled steel plate.

[0076] 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.

[0077] After the heating temperature is reached, the hot-rolled steel plate may be held in the dual-phase temperature range of Ac 1 point or more and less than Ac 3 point for any period of time, for example, 10 minutes to 120 minutes, before cooling.

[0078] Ac 1 point can be calculated according to the following formula (1):

[0079] Ac 3 point can be calculated according to the following formula (2):

[0080] 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

[0081] 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]

[0082] 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 1 / 4 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]

[0083] 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 1 / 4 of the plate thickness of the hot-rolled steel plate.(6) Cooling to room temperature (self-tempering)

[0084] 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 1 / 4 of the plate thickness of the hot-rolled steel plate.

[0085] Conditions other than those described above are not limited and may be in accordance with conventional methods.EXAMPLES

[0086] Steel plates were each produced according to the following procedure, and their properties were evaluated.

[0087] 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. Ac 1 point (°C) calculated according to the foregoing formula (1) and Ac 3 point (°C) calculated according to the foregoing formula (2) are shown in Table 1.

[0088] 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]

[0089] Table 2No.Steel sample IDProduction conditionsRemarks(1) First heating(2) Hot rolling(3) Quenching(4) Second heating(5) CoolingSoaking temperature (°C)In-furnace time (min)Plate thickness (mm)Finish temperature (°C)Quenching rate (°C / s)Cooling end temperature (°C)Heating temperature (°C)Average cooling rate (°C / s)Cooling end temperature (°C)1A1150250408501520080015350Example2A1250500259503020079015350Example3A950160257503020081015350Example4A1120450309002515073025450Example5A1050240308502515084025300Example6A12005504010501520079015400Comparative Example7A1100550408001545081015450Comparative Example8A1050450409001525069015350Comparative Example9A1100400408701515087015300Comparative Example10A1100550409500.5-79015300Comparative Example11A115055040750151508200.5300Comparative Example12A1090300408001520081015100Comparative Example13A1120550408201520083015550Comparative Example14B112025012800520080050450Example15C1100200408201515078015400Example16D1070170408101520077015350Example17E1120500309002520079025400Example18F1170450408701520080015360Example19G1100400208003515079035400Example20H1200400408501520078015320Example21I1100350408201510080015350Example22J1150400408701520083015350Example23K1150250408001520072015230Example24L1100250408001520083015350Example25M1080250408201520081015300Example26N1100200408001520079015300Comparative Example27O1080200407501525082015350Comparative Example28P1120300408501520083015400Comparative Example29Q1070250408501520080015400Comparative Example30R1150380408801520083015350Comparative Example31S1100250408201520079015320Comparative Example32T1160300408601520082015400Comparative Example33U1200200408001520081015300Comparative Example34V1150300408501520082015300Comparative Example35W1150550409501515083015350Comparative Example36A1250700409001515080015300Comparative Example37X1150300409001520077015350Example38Y1150300408501520080015300Example39Z1100450409001515079015300Example40AA1150300408001515078015350Example41BB1100500409501520077015400Example42CC1150350409001515081015300Example43DD1150300409001515080015350Example44EE1100300408501520078015400Example45FF1150500409001515080015350ExampleUnderlines indicate outside the scope of the present disclosure.

[0090] 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]

[0091] A test piece for microstructure observation was collected from the steel plate so that a depth position of 1 / 4 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.

[0092] Martensite austenite constituent may contain retained austenite. Each of the foregoing phases may contain precipitates. The fraction of each phase is calculated including these.

[0093] 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]

[0094] A JIS No. 4 tensile test piece perpendicular to the rolling direction was collected from a depth position of 1 / 4 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 .

[0095] 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]

[0096] V-notched test pieces parallel to the rolling direction were collected from a depth position of 1 / 4 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]

[0097] Table 3No.Steel sample IDMicrostructureMechanical propertiesRemarksFerrite fraction (%)Martensite austenite constituert fraction (%)Residual microstructure*Number density of coarse crystal grains ( / mm 2< )TS (MPa)YR (%)vE -70°C (J) Individual value (first)vE -70°C (J) Individual value (second)vE -70°C (J) Individual value (third)vE -70°C (J) Average value1A4015Bainite / tempered martensite507500.71255245250250Example2A4520Bainite / tempered martensite1607000.71190180200190Example3A3515Bainite / tempered martensite30780074202220230217Example4A8010Bainite / tempered martensite1107100.72180170190180Example5A510Bainite / tempered martensite507800.78210220230220Example6A4520Bainite / tempered martensite2107000.719016070107Comparative Example7A3515Bainite / tempered martensite2107200.7180180150137Comparative Example8A1000-1107200.95180170190180Comparative Example9A05Bainite / tempered martensite907400.85210220220217Comparative Example10A4510Bainite / tempered martensite2056500.72110160120130Comparative Example11A4015Bainite / tempered martensite210640074120160110130Comparative Example12A3515Bainite / martensite457700.6880210120137Comparative Example13A255Bainite / tempered martensite2107100.758021070120Comparative Example14B4010Bainite / tempered martensite508200.78200210220210Example15C408Bainite / tempered martensite457300.73220230240230Example16D5525Bainite / tempered martensite357400.71220210240223Example17E4015Bainite / tempered martensite1207700.73180170190180Example18F3510Bainite / tempered martensite807500.72210230220220Example19G2010Bainite / tempered martensite607200.75220240240233Example20H4015Bainite / tempered martensite607300.72160170160163Example21I2510Bainite / tempered martensite457400.73160170150160Example22J3010Bainite / tempered martensite807500.74210200200203Example23K3515Bainite / tempered martensite407700.75255260256257Example24L4520Bainite / tempered martensite509000.71245250260252Example25M3515Bainite / tempered martensite55920069255260245253Example26N3535Bainite / tempered martensite507800.6880708077Comparative Example27Q2510Bainite / tempered martensite456500.78240235240238Comparative Example28P3015Bainite / tempered martensite607300.7390706073Comparative Example29Q3510Bainite / tempered martensite507200.7290909090Comparative Example30R3010Bainite / tempered martensite906400.71180160170170Comparative Example31S4015Bainite / tempered martensite458200.7370606565Comparative Example32T1010Bainite / tempered martensite707500.7260505053Comparative Example33U2010Bainite / tempered martensite407600.7160506057Comparative Example34V1010Bainite / tempered martensite607400.7190607073Comparative Example35W1010Bainite / tempered martensite2057300.7080150150127Comparative Example36A3020Bainite / tempered martensite2207600.7290160150133Comparative Example37X3015Bainite / tempered martensite90760074220230250233Example38Y2010Bainite / tempered martensite808000.71215255220230Example39Z3010Bainite / tempered martensite907500.71220240230230Example40AA3515Bainite / tempered martensite907600.72240210200217Example41BB3015Bainite / tempered martensite100780074220210210213Example42CC1010Bainite / tempered martensite807600.76230220220223Example43DD2015Bainite / tempered martensite707500.71250240200230Example44EE2520Bainite / tempered martensite508000.77240210230227Example45FF2015Bainite / tempered martensite1508300.75230210210217Example* 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

[0098] 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.

[0099] In each Comparative Example, at least one of TS, YR, and vE -70°C was poor.

Claims

1. A steel plate comprising a chemical composition containing, 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 1 / 4 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 claim 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 claim 1, 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 claim 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.

5. The steel plate according to claim 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.

6. A method of producing a steel plate, the method comprising: performing first heating that heats a steel material having the chemical composition according to claim 1, 2, 3, 4, or 5 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 1 / 4 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 1 / 4 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 1 / 4 of the plate thickness of the hot-rolled steel plate.

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