Steel sheet and method for manufacturing the same

Abstract

To provide a steel sheet and method for manufacturing the sheet.SOLUTION: A steel sheet has a chemical composition containing 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 5.0%, P:0.03% or less, S:0.0050% or less and N:0.0080% or less in mass%, and the balance Fe with inevitable impurities, a microstructure at 1 / 4 depth position of a sheet thickness is ferrite by 5% or more and 90% or less, island martensite by 1% or more and less than 5% in area ratio, and the balance with tempered martensite and / or bainite, the area ratio of island martensite is the area ratio of carbide or less, and when regions surrounded by large-angle grain boundaries with the crystal orientation difference of 15° or more are defined as 1 crystal grain, the average thickness of the crystal grain is 30 μm or less.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a steel plate that can stably ensure excellent strength, toughness, and a low yield ratio and is suitable for use at low temperatures, and a method for producing the same. [Background technology]

[0002] For shipboard liquefied gas storage tanks with independent Type-C design temperatures below -10°C, post-weld stress relief is required when using carbon steel or carbon-manganese steel. This stress relief is typically performed using post-weld heat treatment (PWHT). However, PWHT becomes more difficult as tank sizes increase. On the other hand, mechanical stress relief is possible when the steel has a yield ratio of 0.8 or less, or when the steel has a yield ratio slightly above 0.8 but is highly ductile. For this reason, it is desirable to use low-yield-ratio and high-ductility materials that allow mechanical stress relief. For example, when carbon steel is used for large-scale liquefied CO2 storage tanks, it is desirable to use high-strength steel with a tensile strength (TS) of 770 MPa or more while maintaining excellent toughness at temperatures as low as -50°C. That is, there is a demand for steel materials that have a low yield ratio, high ductility, high strength, and excellent toughness at low temperatures.

[0003] As an example of a steel plate having a low yield ratio and excellent toughness at low temperatures, Patent Document 1 discloses a steel plate having an impact toughness of 150 J or more at −75° C., a yield ratio of 0.8 or less, and a tensile strength of 530 MPa or more. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Special Publication No. 2016-507649 Summary of the Invention [Problem to be solved by the invention]

[0005] However, the steel sheet described in Patent Document 1 has a maximum tensile strength (TS) of 620 MPa, and no steel sheet with a TS of 690 MPa or more has been provided. Thus, no carbon steel has been provided to date that has a low yield ratio, excellent toughness at low temperatures, a yield stress (hereinafter also referred to as "YS") of 580 MPa or more, and high strength with a TS of 690 MPa or more. This necessitates the use of expensive nickel steel, such as 9% Ni steel, which results in a problem of high material costs.

[0006] The present invention has been made in view of the above circumstances, and aims to provide a steel plate that is a high-strength steel having YS≧580 MPa and TS≧690 MPa and that achieves excellent toughness at low temperatures and a low yield ratio, and a method for manufacturing the same. [Means for solving the problem]

[0007] In order to solve the above-mentioned problems, the present inventors have conducted extensive research into the chemical composition and microstructure of high-strength steel that achieves excellent toughness (hereinafter also simply referred to as "toughness") and a low yield ratio at low temperatures, and have made the following findings, particularly regarding the microstructure.

[0008] Specifically, it has been newly discovered that the microstructure at a depth of 1 / 4 of the plate thickness from the surface of the steel plate in the plate thickness direction has an area ratio of 5% to 90% ferrite, 1% to less than 5% island martensite, and the remainder being tempered martensite and / or bainite, and that the island martensite area ratio is equal to or less than the cementite area ratio, which is effective for improving the desired properties.

[0009] As will be explained in detail later, the above-mentioned island martensite refers to an island structure with an equivalent circle diameter of 0.5 μm or more, and the above-mentioned cementite (hereinafter also referred to as "carbide") refers to an island structure with an equivalent circle diameter of less than 0.5 μm. Furthermore, the above-mentioned ferrite refers to a BCC structure that remains without reverse transformation even when martensite or bainite is heat-treated at a temperature above the Ac1 point, and inherits the original lath structure.

[0010] The inventors have discovered that by properly performing two-phase region quenching, the area fraction of this relatively soft ferrite phase can be set to 5 to 90%, and that by properly controlling the tempering conditions, the area fraction of island martensite can be set to 1% or more and less than 5%, and the island martensite area fraction can be set to be less than or equal to the carbide area fraction.

[0011] In addition, by appropriately controlling the cooling rate and cooling end temperature of the two-phase intercritical quenching, the average thickness of the crystal grains is set to 30 μm or less, when the region surrounded by high-angle grain boundaries with a crystal orientation misorientation of 15° or more obtained by EBSD measurement described in the examples below is considered as one crystal grain. As a result, the steel has YS≧580 MPa, TS≧690 MPa, a yield ratio (hereinafter also referred to as "YR") of YR≦0.85, and low-temperature toughness of vE -50 It was found that it was possible to satisfy the condition of ≧100J.

[0012] The present invention has been completed based on the above findings, and the gist of the present invention is as follows. [1] 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 5.0% P: 0.03% or less, S: 0.0050% or less, and N: 0.0080% or less and the balance being Fe and unavoidable impurities, The microstructure at a depth of 1 / 4 of the plate thickness from the surface of the steel plate in the plate thickness direction is as follows: In terms of area ratio, ferrite is 5% or more and 90% or less, island martensite is 1% or more and less than 5%, and the remainder is tempered martensite and / or bainite, When island structures having an equivalent circle diameter of less than 0.5 μm are considered as carbides, the area ratio of the island martensite is equal to or less than the area ratio of the carbides, A steel sheet in which the average thickness of crystal grains is 30 μm or less, when a region surrounded by high-angle grain boundaries with a crystal orientation misorientation of 15° or more is considered to be one crystal grain. [2] The component composition further includes, in mass%, Al: 0.100% or less, Nb: 0.1% or less, Cr: 2.00% or less, Mo: 1.0% or less Cu: 2.00% or less, V: 0.1% or less, Ti: 0.03% or less, B: 0.0050% or less, Ca: 0.0070% or less, REM: 0.010% or less, Mg: 0.0070% or less, and Zr: 0.0050% or less The steel sheet according to [1], containing one or more selected from the following: [3] A method for producing a steel sheet according to [1] or [2], hot-rolling a steel material having the above-described composition to form a hot-rolled steel sheet; The hot-rolled steel sheet is heated and held at a temperature in a two-phase temperature range of not less than Ac1 point and not more than Ac3 point at the center of the sheet thickness, and then subjected to a two-phase region quenching process under conditions such that the average cooling rate in the temperature range of not more than 700°C and not less than 500°C at a temperature at a depth position of 1 / 4 of the sheet thickness is 3°C / s or more, and the cooling end temperature is less than 300°C at a temperature at a depth position of 1 / 4 of the sheet thickness; Next, a tempering step is carried out under conditions in which the tempering temperature is 300°C or higher but lower than 500°C at the center of the plate thickness, and the tempering parameter TP expressed by formula (1) is 15.50 or lower. TP = T × (log(t) + 20) / 1000 … (1) Here, in the formula (1), T is the tempering temperature (K) and t is the soaking time (hours). [4] Before the two-phase region quenching process, The method for manufacturing a steel plate according to [3], wherein the hot-rolled steel plate is heated and held at a heating temperature of not less than the Ac3 point and not more than 1000°C at the center of the plate thickness, and then subjected to a γ-region quenching process under conditions such that the average cooling rate in the temperature range of not more than 700°C and not less than 500°C at a temperature at a depth of 1 / 4 of the plate thickness is 3°C / s or more, and the cooling end temperature is not more than 300°C at a temperature at a depth of 1 / 4 of the plate thickness. [Effects of the Invention]

[0013] According to the present invention, it is possible to achieve excellent toughness at low temperatures and a low yield ratio in a high-strength steel having a YS≧580 MPa and a TS≧690 MPa. Therefore, the steel plate of the present invention, although being a carbon steel or a carbon-manganese steel, can be used as a steel material for steel structures used in low-temperature environments, such as large cryogenic storage tanks for ships, e.g., liquefied CO2 tanks and LPG tanks. Furthermore, the steel plate of the present invention can reduce the construction costs of large cryogenic storage tanks for ships compared to nickel steel. Therefore, the present invention brings about significant industrial benefits. DETAILED DESCRIPTION OF THE INVENTION

[0014] Hereinafter, embodiments of the present invention will be specifically described. Note that the following description shows preferred embodiments of the present invention, and the present invention is not limited thereto.

[0015] First, the steel sheet of the present invention will be described.

[0016] [Component composition] The steel sheet of the present invention has the following predetermined chemical composition. Furthermore, the steel material used to manufacture the steel sheet of the present invention, which will be described later, also has the predetermined chemical composition. Hereinafter, each element contained in this chemical composition will be explained. Unless otherwise specified, "%" used in this specification as a unit of content of each element means "% by mass."

[0017] C: 0.02% or more and 0.15% or less C is an element that has the effect of improving the strength of steel sheet. To obtain this effect, the C content is set to 0.02% or more. The C content is preferably set to 0.03% or more. On the other hand, if the C content exceeds 0.15%, island martensite becomes excessive and the toughness of the base material decreases. Therefore, the C content is set to 0.15% or less. The C content is preferably set to 0.12% or less.

[0018] Si: 0.01% or more and 0.50% or less Si is an element that acts as a deoxidizer. To achieve this effect, the Si content is set to 0.01% or more. The Si content is preferably 0.03% or more, and more preferably 0.10% or more. On the other hand, if the Si content is excessively high, the area fraction of island martensite increases, resulting in a decrease in the toughness of the base material. Therefore, the Si content is set to 0.50% or less. The Si content is preferably 0.30% or less, and more preferably 0.25% or less.

[0019] Mn: 0.05% or more and 2.50% or less Mn is an element that improves the hardenability of steel and is effective in increasing the strength of steel sheets. To achieve this effect, the Mn content is set to 0.05% or more. The Mn content is preferably 0.10% or more, and more preferably 0.30% or more. On the other hand, if the Mn content exceeds 2.50%, temper embrittlement tends to occur, resulting in a deterioration in the toughness of the base material, so the Mn content is set to 2.50% or less. The Mn content is preferably 2.00% or less.

[0020] Ni: 0.50% or more and less than 5.0% Ni is an element effective in improving the toughness of the base material. To achieve this effect, the Ni content is set to 0.50% or more. However, because Ni is an expensive element, the higher the Ni content, the higher the steel sheet cost. Therefore, in the present invention, the Ni content is set to less than 5.0%. The Ni content is preferably 0.80% or more. The Ni content is preferably 3.5% or less, and more preferably 2.50% or less.

[0021] P:0.03% or less P is an unavoidable impurity and a harmful element that adversely affects the toughness of steel plates. For example, in order to obtain a sound base material and welded joint when steel plates are welded to form a welded structure, it is desirable to reduce the P content as much as possible. Therefore, in the present invention, the P content is limited to 0.03% or less. Furthermore, from the viewpoint of toughness (note that this toughness also includes base material toughness), the lower the P content, the better. Therefore, the lower limit of the P content is not particularly limited and may be 0%, but its inclusion as an unavoidable impurity is permitted. On the other hand, excessive reduction of P causes an increase in costs, so from the viewpoint of cost, it is preferable that the lower limit of the P content be 0.001% or more.

[0022] S: 0.0050% or less Since S forms MnS in steel and significantly deteriorates toughness, the upper limit of S is set to 0.0050%, and it is desirable to reduce the S content as much as possible. Therefore, the S content is set to 0.0050% or less. The S content is preferably set to 0.0040% or less. On the other hand, the lower the S content, the better, so the lower limit of the S content is not particularly limited and may be 0%, although its inclusion as an unavoidable impurity is permitted. From the viewpoint of production load, the S content is preferably set to 0.0005% or more.

[0023] N: 0.0080% or less N forms precipitates in steel. If the N content exceeds 0.0080%, it will cause a decrease in the toughness of the base metal. Therefore, the N content is set to 0.0080% or less. The N content is preferably 0.0060% or less, and more preferably 0.0050% or less. From the viewpoint of the manufacturing load, the N content is preferably 0.0010% or more.

[0024] The basic chemical composition of the steel sheet of the present invention is such that, in addition to the predetermined amounts of the elements described above, the balance is Fe and unavoidable impurities.

[0025] With this basic composition, the steel sheet of the present invention can obtain the desired properties.

[0026] In addition to this basic component composition, the present invention may optionally contain one or more elements selected from Al, Nb, Cr, Mo, Cu, V, Ti, and B in order to further improve the properties.

[0027] Al: 0.100% or less Al is an element that acts as a deoxidizer and is most commonly used in the molten steel deoxidation process for high-tensile steel. When Al is contained to obtain this effect, the Al content is preferably 0.001% or more. The Al content is more preferably 0.010% or more. On the other hand, if the Al content exceeds 0.100%, the toughness of the base material may decrease. Therefore, when Al is contained, the Al content is preferably 0.100% or less. The Al content is more preferably 0.07% or less.

[0028] Nb: 0.1% or less Nb is an element that contributes to improving the strength of steel and can be added at any amount depending on the desired strength. However, if the Nb content exceeds 0.1%, the toughness of the base material may deteriorate. Therefore, when Nb is added, the Nb content is preferably 0.1% or less. From the viewpoint of obtaining the strength-improving effect of Nb, the Nb content is preferably 0.005% or more.

[0029] Cr:2.00% or less Cr is an element that can improve the strength of steel sheet without significantly impairing toughness. When Cr is contained to obtain the above effect, the Cr content is preferably 0.01% or more. The Cr content is more preferably 0.30% or more. However, if the Cr content exceeds 2.00%, the toughness of the base material may decrease. Therefore, when Cr is contained, the Cr content is preferably 2.00% or less. The Cr content is more preferably 1.50% or less.

[0030] Mo: 1.0% or less Mo is an element that contributes to improving the strength of steel and can be added at any amount depending on the desired strength. However, if the Mo content exceeds 1.0%, the toughness of the base material may deteriorate, so if Mo is added, the Mo content is preferably 1.0% or less. The Mo content is more preferably 0.60% or less. From the perspective of obtaining the strength-improving effect of Mo, the Mo content is preferably 0.01% or more.

[0031] Cu:2.00% or less Cu is an element that can increase strength while maintaining high toughness, and can be added at any amount depending on the desired strength. However, if the Cu content exceeds 2.00%, hot embrittlement may occur, which may deteriorate the surface quality of the steel sheet. Therefore, when Cu is added, the Cu content is preferably 2.00% or less. The Cu content is more preferably 1.00% or less. In order to achieve the above effects, when Cu is added, the Cu content is preferably 0.01% or more. The Cu content is more preferably 0.10% or more, and even more preferably 0.20% or more.

[0032] V: 0.1% or less V is an effective element that can increase the strength of steel sheet through precipitation strengthening. However, if the V content is excessively high, the toughness of the base material may decrease. Therefore, when V is contained, the V content is preferably 0.1% or less. The V content is more preferably 0.04% or less. There is no particular lower limit for the V content, but when V is contained to obtain the above effect, the V content is preferably 0.010% or more.

[0033] Ti: 0.03% or less Ti is an element that has the effect of increasing the toughness of the weld without degrading the mechanical properties of the base material when steel plates are welded to form a welded structure. When Ti is contained to obtain this effect, it is preferable that the Ti content be 0.003% or more. On the other hand, if the Ti content exceeds 0.03%, there is a risk that the toughness of the base material will be reduced. Therefore, when Ti is contained, it is preferable that the Ti content be 0.03% or less.

[0034] B: 0.0050% or less B is an element that improves hardenability when added in small amounts. When B is contained to effectively exert this effect, it is preferable that the B content be 0.0003% or more. On the other hand, if the B content exceeds 0.0050%, the toughness of the base material may deteriorate. Therefore, when B is contained, it is preferable that the B content be 0.0050% or less. The B content is more preferably 0.0030% or less.

[0035] Furthermore, in the present invention, in addition to the above-mentioned component composition, one or more elements selected from Ca, REM, Mg, and Zr may be optionally contained.

[0036] Ca:0.0070% or less Ca is an element that has the effect of improving the toughness of steel plate by controlling the morphology of inclusions in steel. However, an excessively high Ca content may impair the cleanliness of the steel and reduce the Charpy absorbed energy (hereinafter also referred to as "Charpy toughness") at low temperatures. Therefore, when Ca is contained, the Ca content is preferably 0.0070% or less. The Ca content is more preferably 0.0040% or less. There is no particular lower limit for the Ca content, but when Ca is contained to obtain the above effect, the Ca content is preferably 0.0010% or more.

[0037] REM: 0.010% or less Like Ca, REM (rare earth metals) are elements that have the effect of improving the toughness of steel sheets by controlling the morphology of inclusions in steel. However, excessively high REM content may impair the cleanliness of the steel and reduce Charpy toughness. Therefore, when REM is contained, the REM content is preferably 0.010% or less. The REM content is more preferably 0.008% or less. There is no particular lower limit for the REM content, but when REM is contained to obtain the above effect, the REM content is preferably 0.001% or more.

[0038] Here, REM is a collective term for 17 elements, including 15 lanthanoid elements plus Y and Sc, and these elements can be contained alone or in combination. Therefore, the REM content means the total content of these elements.

[0039] Mg: 0.0070% or less Like Ca and REM, Mg is an element that has the effect of improving the toughness of steel plate by controlling the morphology of inclusions in steel. However, an excessively high Mg content may impair the cleanliness of the steel and reduce Charpy toughness. Therefore, when Mg is contained, the Mg content is preferably 0.0070% or less. The Mg content is more preferably 0.0040% or less. There is no particular lower limit for the Mg content, but when Mg is contained to obtain the above effect, the Mg content is preferably 0.0010% or more.

[0040] Zr: 0.0050% or less Like Ca and Mg, Zr is an element effective in controlling the morphology of inclusions such as MnS, and can be contained as needed. By controlling the morphology of these inclusions, toughness and resistance to sulfide stress corrosion cracking can be improved. When Zr is contained to achieve this effect, the Zr content is preferably 0.0005% or more. The Zr content is more preferably 0.0010% or more. On the other hand, a high Zr content increases the amount of nonmetallic inclusions, which may actually result in a decrease in through-thickness tensile properties at the center of the sheet thickness. Therefore, when Zr is contained, the Zr content is set to 0.0050% or less. The Zr content is more preferably 0.0040% or less.

[0041] The above-mentioned components Al, Nb, Cr, Mo, Cu, V, Ti, B, Ca, REM, Mg, and Zr can be contained as needed, and therefore the content of these components may be 0%.

[0042] [Microstructure] In the steel sheet of the present invention, the microstructure at a depth of 1 / 4 of the sheet thickness from the surface of the steel sheet in the sheet thickness direction is, in area ratio, 5% to 90% ferrite, 1% to less than 5% island martensite, and the remainder being tempered martensite and / or bainite, the area ratio of the island martensite is equal to or less than the area ratio of the carbides, and the average thickness of the crystal grains is 30 μm or less, when a region surrounded by high-angle grain boundaries with a crystal orientation misorientation of 15° or more is defined as one crystal grain.

[0043] Here, each structure in the present invention will be described. "Ferrite" refers to the BCC phase that is formed by the martensite or bainite produced during hot rolling or γ-region quenching and then thoroughly tempered through the two-phase region quenching and tempering processes. "Martensite" refers to martensite formed during the two-phase region quenching process, in which there are no island martensites between the laths and no carbide precipitation can be confirmed by structural observation. "Martensite islands" refers to martensite that is formed between laths of bainite during the two-phase intercritical quenching process and has a high carbon concentration, with an equivalent circle diameter of 0.5 μm or more. Those with an equivalent circle diameter of less than 0.5 μm are carbides. "Tempered martensite" refers to martensite that has been produced in the two-phase region quenching process and has been tempered, and in which carbide precipitation can be confirmed by structural observation. "Bainite" refers to bainite that is formed during the two-phase region quenching process, in which island martensite is confirmed between the laths and carbides are confirmed within the structure.

[0044] Ferrite: area ratio: 5% to 90% If the ferrite fraction in the microstructure is less than 5%, the yield ratio will not satisfy the requirement of 0.85 or less. This is for the following reason: if the ferrite fraction is less than 5%, the amount of the softest ferrite phase will be insufficient, and the YS will not be sufficiently reduced, resulting in a yield ratio exceeding 0.85. Therefore, the ferrite fraction is set to 5% or more. The ferrite fraction is preferably 10% or more, and more preferably 15% or more.

[0045] On the other hand, if the ferrite fraction exceeds 90%, the fraction of hard phases such as island martensite will be low, and the yield ratio will not satisfy the requirement of 0.85 or less. Therefore, the ferrite fraction is set to 90% or less. The ferrite fraction is preferably 85% or less, and more preferably 80% or less.

[0046] Island martensite: area ratio of 1% to less than 5% To achieve a YR of 0.85 or less, the hard island martensite content must be 1% or more, but because an increase in the island martensite content can impair toughness, the hard island martensite content must be less than 5%.

[0047] Remaining structure: One or two selected from tempered martensite and bainite The remainder other than ferrite and island martensite is one or two selected from tempered martensite and bainite. By making the remainder tempered martensite and / or bainite, the desired strength can be obtained, and the higher the area ratio, the higher the strength that can be obtained.

[0048] The fraction of the remaining structure, in terms of the total area ratio of each structure, is preferably 5 to 94%, and more preferably 10% or more and 85% or less.

[0049] The fractions of the above-mentioned ferrite, island martensite, tempered martensite, bainite, and martensite structures can be measured by the method described in the examples below.

[0050] Area ratio of island martensite ≦ Area ratio of carbide The desired toughness can be ensured by increasing the area ratio (unit: %) of carbides relative to island martensite, i.e., by decomposing island martensite during tempering. Therefore, the area ratio of island martensite is set equal to or less than the area ratio of carbides.

[0051] Average grain thickness: 30 μm or less The microstructure at a depth of 1 / 4 of the plate thickness from the surface of the steel plate in the plate thickness direction is set to have an average grain thickness of 30 μm or less. By controlling this average grain thickness, a fine-grained structure can be achieved, ensuring the desired strength and toughness. Here, the above-mentioned "grains" refers to the region surrounded by high-angle grain boundaries with a grain boundary misorientation of 15 degrees or more, and the above-mentioned "average thickness of grains" refers to the average length of the grains in the plate thickness direction when the above-mentioned region is considered to be one grain.

[0052] The average thickness of the crystal grains can be controlled to fall within the above numerical range by controlling the cooling rate in the γ region quenching and two-phase region quenching steps to 3° C. / s or more.

[0053] The steel sheet of the present invention has a microstructure having the above-mentioned constitution, thereby obtaining the mechanical properties described below.

[0054] Plate Thickness The thickness of the steel plate of the present invention is not particularly limited. As described above, from the viewpoint of applying the steel plate of the present invention to steel materials for steel structures used in low-temperature environments such as large cryogenic storage tanks for ships, e.g., liquefied CO2 tanks and LPG tanks, the thickness is preferably 6 mm or more and 50 mm or less.

[0055] [Mechanical properties] The steel sheet of the present invention having the above-described component composition and microstructure has the following mechanical properties.

[0056] (yield stress) The lower limit of the yield stress (YS) of the steel plate is set to 580 MPa or more. This is because the plate thickness can be reduced when the steel plate is applied to a tank. The yield stress is more preferably set to 600 MPa or more. There is no particular upper limit to the yield stress, but it is preferably set to 800 MPa or less. The yield stress can be measured by the method described in the examples below.

[0057] (tensile strength) The lower limit of the tensile strength (TS) of the steel plate is preferably 690 MPa or more. This is because the plate thickness can be reduced when the steel plate is applied to a tank. The tensile strength is more preferably 720 MPa or more. There is no particular upper limit to the tensile strength, but it is preferably 940 MPa or less. Here, "high strength" in the present invention refers to a steel plate having a TS of 690 MPa or more and a YS of 580 MPa or more. The above tensile strength can be measured by the method described in the examples below.

[0058] (yield ratio) The upper limit of the yield ratio (YR) of the steel sheet is preferably 0.85 or less from the viewpoint of ensuring the ductility of the steel sheet. The lower limit of the yield ratio of the steel sheet is not particularly limited, but is preferably 0.60 or more. The yield ratio can be determined by the method described in the examples below.

[0059] (toughness) The toughness value of the steel plate is the Charpy absorbed energy (vE -50℃ ) is preferably 100J or more in a full-size Charpy impact test. -50℃ More preferably, the Charpy absorbed energy (vE -50℃ ) is 100 J or more. For half-size, it is 50 J or more. The Charpy absorbed energy can be measured by the method described in the examples below.

[0060] Next, an embodiment of the method for producing a steel sheet according to the present invention will be described.

[0061] In the following description of the manufacturing method, unless otherwise specified, the temperature refers to the temperature at a depth position of 1 / 4 of the plate thickness in the steel material and steel plate (i.e., the position of (1 / 4)t when the plate thickness is t (unit: mm)) or at the center of the plate thickness. The temperatures at a depth position of 1 / 4 of the plate thickness and the center of the plate thickness can be obtained by heat transfer calculation from the surface temperature of the steel plate measured with a radiation thermometer, for example.

[0062] The steel sheet of the present invention can be produced by successively carrying out a hot rolling step, a cooling step, a two-phase region quenching step, and a tempering step under the specific conditions described below. In addition, a γ region quenching step may be provided between the hot rolling step and the two-phase region quenching step.

[0063] [Hot rolling process] First, in the hot rolling process, a steel material is heated and hot rolled to form a hot-rolled steel sheet.

[0064] The method for producing the steel material is not particularly limited. For example, a steel material can be produced by melting molten steel having the above-mentioned composition and then casting it. Examples of the melting method include a converter, an electric furnace, and an induction furnace. Examples of the casting method include a continuous casting method and an ingot making-blooming rolling method, and from the viewpoint of productivity, the continuous casting method is preferred. For example, a steel slab can be used as the steel material.

[0065] The steel material may be heated after the steel material obtained by the above casting has been cooled, or the steel material may be heated directly without being cooled.

[0066] The heating temperature of the steel material can be set as appropriate. For example, when using a steel material having the above-mentioned composition, the steel material is preferably heated to a heating temperature of 900°C or higher and 1250°C or lower.

[0067] If the heating temperature of the steel material is less than 900°C, the high deformation resistance of the steel material increases the load on the rolling mill in the subsequent hot rolling, which may make it difficult to perform the hot rolling. Therefore, the heating temperature of the steel material is preferably 900°C or higher, and more preferably 950°C or higher. On the other hand, if the heating temperature of the steel material is higher than 1250°C, oxidation of the steel becomes significant, and steel loss increases due to the removal of the oxide film caused by oxidation. As a result, there is a risk of a decrease in yield. Therefore, the heating temperature of the steel material is preferably 1250°C or lower, and more preferably 1200°C or lower.

[0068] The heated steel material is then hot-rolled to produce a hot-rolled steel sheet. The final thickness of the hot-rolled steel sheet is not particularly limited, but as described above, the thickness is preferably 6 mm or more and 50 mm or less.

[0069] [Cooling process] After the hot rolling step, a cooling step is carried out. In the cooling step, the hot-rolled steel sheet is cooled after the hot rolling step and before the two-phase region quenching step described below, or after the hot rolling step and before the γ region quenching step described below. The cooling method is not particularly limited, and examples thereof include air cooling and water cooling.

[0070] [γ-region hardening process] (optional process) In the present invention, a γ-region quenching step can be performed as necessary after the completion of the cooling step and before the two-phase intercritical quenching step described below. In this step, the cooled hot-rolled steel sheet is reheated to a temperature in the γ-region, held in the γ-region for a desired period of time, and then cooled and quenched. Performing this γ-region quenching step before the two-phase intercritical quenching step is even more effective in refining the grain structure.

[0071] Specifically, in the γ-region quenching process, it is preferable to quench the hot-rolled steel sheet by heating and holding the sheet at a heating temperature of not less than the Ac3 point and not more than 1000°C at the temperature at the center of the sheet thickness, and then performing accelerated cooling under conditions such that the average cooling rate in the temperature range of not more than 700°C and not less than 500°C at a temperature at a depth of ¼ of the sheet thickness from the surface of the steel sheet in the sheet thickness direction is 3°C / s or more, and the cooling end temperature at the temperature at a depth of ¼ of the sheet thickness is less than 300°C.

[0072] Heating temperature: Ac3 point or higher and less than 1000°C The heating temperature in the γ-region quenching step is preferably equal to or higher than the Ac3 point and lower than 1000°C. In order to prevent the formation of an excessively soft ferrite phase and suppress a lack of strength, the heating temperature (°C) is preferably equal to or higher than the Ac3 point. The heating temperature is more preferably equal to or higher than (Ac3 point + 30)°C. Furthermore, in order to prevent excessive grain coarsening that would prevent the desired toughness from being obtained, the heating temperature is preferably lower than 1000°C. The heating temperature is more preferably equal to or lower than 950°C.

[0073] After the temperature at the center of the thickness of the hot-rolled steel sheet reaches the heating temperature, it is held for a predetermined time. The holding time is not particularly specified, but from the viewpoint of material uniformity (i.e., stabilization of toughness), the holding time is preferably set to 10 to 60 minutes. By setting the holding time to 10 minutes or more, material uniformity can be achieved. By setting the holding time to 60 minutes or less, an effect of not significantly impairing production efficiency can be obtained.

[0074] Average cooling rate in the temperature range of 500°C or above and below 700°C: 3°C / s or above The average cooling rate in the above temperature range is preferably 3°C / s or more. If the average cooling rate in the above temperature range during γ-phase quenching is less than 3°C / s, a transformed structure with the desired average grain thickness cannot be obtained, and it may be difficult to obtain sufficient strength and toughness. The average cooling rate is more preferably 5°C / s or more.

[0075] The upper limit of the average cooling rate in the above temperature range is not particularly limited. If the average cooling rate is higher than 200°C / s, it becomes difficult to control the temperature at each position in the steel sheet, and the material properties tend to vary in the sheet width direction and rolling direction. As a result, the material properties such as tensile properties and toughness tend to vary. For these reasons, the average cooling rate is preferably 200°C / s or less, and more preferably 100°C / s or less.

[0076] Cooling end temperature: less than 300°C The cooling end temperature in the γ-region quenching process is preferably less than 300°C. If the cooling end temperature at a depth of 1 / 4 of the plate thickness (hereinafter sometimes referred to as "(1 / 4)t") in γ-region quenching is 300°C or higher, there is a risk that the desired average thickness of the crystal grains will not be obtained. Therefore, the cooling end temperature at (1 / 4)t is preferably less than 300°C. It is more preferable that the cooling end temperature be 250°C or lower.

[0077] The cooling treatment in quenching is not particularly limited and can be carried out by any method. For example, air cooling and / or water cooling can be used. For water cooling, any cooling method using water can be used, such as spray cooling, mist cooling, and laminar cooling.

[0078] In the γ-region quenching step, the hot-rolled steel sheet can be well quenched by performing accelerated cooling under the conditions described above.

[0079] [Two-phase hardening process] After the cooling step, a two-phase region quenching step is performed. If the above-mentioned γ region quenching step is performed after the cooling step, the two-phase region quenching step is performed after the γ region quenching step. In this two-phase region quenching step, the cooled hot-rolled steel sheet or the γ region quenched hot-rolled steel sheet is heated to a two-phase region temperature, held at the two-phase region temperature for a desired period of time, and then cooled and quenched.

[0080] Specifically, in the two-phase region quenching process, the hot-rolled steel sheet is heated and held at a two-phase region temperature range of not less than Ac1 and not more than Ac3 at the center of the sheet thickness, and then cooled under conditions such that the average cooling rate in the temperature range of not more than 700°C and not less than 500°C at a temperature at a depth of 1 / 4 of the sheet thickness is 3°C / s or more, and the cooling end temperature at a temperature at a depth of 1 / 4 of the sheet thickness is less than 300°C, thereby quenching the hot-rolled steel sheet.

[0081] Heating temperature: Ac1 point or higher and less than Ac3 point The heating temperature (°C) in the two-phase region quenching step is set to be equal to or higher than the Ac1 point and lower than the Ac3 point. If the heating temperature is lower than the Ac1 point, reverse transformation does not occur sufficiently, making it impossible to obtain island martensite, and as a result, it is not possible to achieve a low yield ratio. The heating temperature is preferably set to be equal to or higher than the Ac1 point + 30°C.

[0082] On the other hand, if the heating temperature is higher than the Ac3 point, the entire structure will undergo reverse transformation, resulting in a ferrite fraction of less than 5%, making it impossible to achieve a low yield ratio.The heating temperature is preferably set to (Ac3 point - 10)°C or lower.

[0083] Any heating method can be used in the two-phase region quenching step as long as it can control the heating temperature as described above. One example of the heating method is furnace heating. This furnace heating is not particularly limited, and a general heat treatment furnace can be used.

[0084] After the heating temperature in the two-phase temperature range is reached, the heating temperature (hereinafter also referred to as the "tempering temperature") may be held for any time, followed by cooling. The holding time is not particularly limited, but from the viewpoint of material uniformity, the holding time at the tempering temperature is preferably 5 minutes or more. From the viewpoint of operational load, the holding time is preferably 120 minutes or less.

[0085] Here, the Ac1 point (also referred to as the Ac1 transformation point) can be determined by the following formula (2). Ac1 point (℃) = 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 … (2) The Ac3 point (also referred to as the Ac3 transformation point) can be calculated by the following formula (3). Ac3(℃)=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 …(3) In the formulas (2) and (3), the symbols of the elements represent the content (mass%) of the element in the steel, and the content of an element that is not contained in the steel is set to 0.

[0086] Average cooling rate in the temperature range of 500°C or above and below 700°C: 3°C / s or above The average cooling rate in the temperature range of 700°C or less and 500°C or more at a depth of 1 / 4 of the plate thickness is 3°C / s or more. If the average cooling rate is less than 3°C / s, a transformed structure with the desired average grain thickness cannot be obtained, which may result in a decrease in strength and toughness. The average cooling rate is preferably 5°C / s or more.

[0087] The upper limit of the average cooling rate in the above temperature range is not particularly limited. From the viewpoint of uniformity of the material, the average cooling rate is preferably 200°C / s or less, and more preferably 100°C / s or less.

[0088] Cooling end temperature: less than 300°C The cooling end temperature in the two-phase region quenching process is set to less than 300°C at a depth of 1 / 4 of the plate thickness. If the cooling end temperature is 300°C or higher, it may not be possible to obtain the desired average crystal grain thickness. The cooling end temperature is preferably set to 250°C or lower.

[0089] The lower limit of the cooling end temperature in the two-phase region quenching step is not particularly limited. The cooling treatment in quenching is the same as that described above, and therefore a description thereof will be omitted.

[0090] [Tempering process] After the two-phase region quenching step is completed, a tempering step is carried out.

[0091] Specifically, the hot-rolled steel sheet that has been quenched in the two-phase region is tempered at a temperature of 300°C or higher but lower than 500°C at the center of the sheet thickness (hereinafter referred to as the "tempering temperature") under conditions where the tempering parameter TP expressed by equation (1) is 15.50 or lower. TP = T × (log(t) + 20) / 1000 … (1) Here, in the formula (1), T is the tempering temperature (K) and t is the soaking time (hours).

[0092] Temperature rise rate: 0.2°C / s or more and 2.0°C / s or less (preferred embodiment) In the present invention, it is preferable to control the rate of temperature rise up to the tempering temperature. If the rate of temperature rise is less than 0.2°C / s, the area fraction of island martensite will be less than 1%, and the desired yield ratio may not be obtained. Therefore, the rate of temperature rise is preferably 0.2°C / s or more, and more preferably 0.3°C / s or more. On the other hand, if the rate of temperature rise exceeds 2.0°C / s, the area fraction of island martensite will be greater than the area fraction of carbide, and the desired toughness may not be obtained. Therefore, the rate of temperature rise is preferably 2.0°C / s or less, and more preferably 1.8°C / s or less. The temperature rise rate refers to the average temperature rise rate in the temperature range from the start of the tempering temperature rise until the tempering temperature is reached.

[0093] Tempering temperature: 300℃ or higher and less than 500℃ If the tempering temperature is less than 300°C, the island martensite will be 5% or more, or the island martensite area ratio will be greater than the carbide area ratio, and the desired yield stress will not be obtained. The tempering temperature is preferably 350°C or higher. On the other hand, if the tempering temperature is 500°C or higher, the island martensite will be less than 1%, and the desired low yield ratio will not be obtained. The tempering temperature is preferably 450°C or lower.

[0094] The holding time at the tempering temperature is not particularly specified. It may be 0 minutes at the above tempering temperature. From the viewpoint of operational load, the holding time is preferably 60 minutes or less. The holding time at this tempering temperature is t (soaking holding time) in the above formula (1).

[0095] Tempering parameters: TP≦15.50 The tempering parameter (TP) shown in the above formula (1) is an index of tempering temperature and hardness. If TP exceeds 15.50, the island martensite content will be less than 1%, and as a result, the desired yield ratio will not be obtained. TP is preferably 15.00 or less. There is no particular lower limit for TP. From the viewpoint of material uniformity, TP is preferably 10.00 or more, and more preferably 10.30 or more.

[0096] As explained above, when the steel plate of the present invention having the above-mentioned microstructure and properties is used for manufacturing, for example, a tank for storing liquefied gas on a ship, mechanical stress removal can be performed instead of post-weld heat treatment as a method for stress relief after welding of the steel plate. Therefore, the steel plate of the present invention can be suitably used as a steel material for a large-scale tank for storing low-temperature liquefied gas. [Example]

[0097] The present invention will be specifically described below based on examples. Note that the present invention is not limited to these embodiments. In these examples, steel sheets were manufactured according to the following procedures, and the properties of the obtained steel sheets were evaluated.

[0098] First, molten steel having the chemical composition shown in Table 1 was melted in a converter, and a steel slab (thickness: 200 mm) was produced as the steel material by continuous casting. Note that blank spaces in Table 1 indicate that no element was intentionally added, and include not only cases where no element was contained (i.e., a content of 0%), but also cases where an element was unavoidably contained. Table 1 also lists the Ac1 point (°C) and Ac3 point (°C) calculated using the above-mentioned formulas (2) and (3).

[0099] [Table 1]

[0100] Next, the obtained steel slabs were heated and hot-rolled according to the manufacturing conditions shown in Table 2 to obtain hot-rolled steel sheets having the thicknesses (final thicknesses) shown in Table 2. Thereafter, the hot-rolled steel sheets were cooled and subjected to a quenching process and a tempering process as heat treatments to produce the steel sheets.

[0101] [Table 2]

[0102] The steel sheets thus obtained were evaluated for (1) microstructure, (2) tensile strength and yield stress, (3) yield ratio, and (4) toughness according to the following methods. The evaluation results are shown in Table 3.

[0103] (1) Microstructure A test piece for microstructure observation was taken from each steel plate so that the observation position was a depth position of 1 / 4 of the steel plate thickness (i.e., the (1 / 4)t position). This test piece was embedded in resin so that the cross section perpendicular to the rolling direction was the observation surface, and was mirror-polished. Next, after performing nital etching, the specimen was observed with a scanning electron microscope at a magnification of 5000 times and images of the structure were taken. The obtained images were analyzed to identify the microstructure fraction. Note that a description of each structure has been omitted here, as it has been described above.

[0104] The area fraction of the island martensite mentioned above was determined by calculating the area fraction of the observed island structures with a size of 0.5 μm or more. The area fraction of the carbides mentioned above was determined by calculating the area fraction of the observed island structures with a size of less than 0.5 μm. In the column "Island martensite area fraction ≦ Carbide area fraction" in Table 3, the symbol "◯" is entered when this relationship is satisfied, and the symbol "×" is entered when this relationship is not satisfied.

[0105] The average thickness of the crystal grains was determined as follows: Structural analysis was performed using EBSD, and the region surrounded by high-angle grain boundaries with a misorientation of 15 degrees or more in a total area of ​​1 mm x 1 mm was defined as a crystal grain. Using the intercepting method, the thickness of each crystal grain (i.e., the length of each crystal grain in the thickness direction) was determined at 50 μm intervals, and the average value of these was taken as the average thickness of the crystal grains. If the average thickness of these crystal grains was 30 μm or less, it was determined that no coarse structure had formed.

[0106] (2) Tensile strength and yield stress A JIS No. 4 tensile test specimen was taken from the (1 / 4)t position of the steel plate so that the longitudinal direction of the test specimen coincided with the plate width direction. Using this tensile test specimen, a tensile test was conducted in accordance with the provisions of JIS Z2241 (2022) to evaluate the tensile strength (TS) and yield stress (YS) of the steel plate. Here, a tensile strength of 690 MPa or more and a yield stress of 580 MPa or more were evaluated as high strength and "passed."

[0107] (3) Yield ratio Using the tensile test results obtained in (2) above, the yield ratio (YR) expressed as YS / TS was evaluated. Here, a yield ratio of 0.85 or less was considered "pass."

[0108] (4) Toughness The Charpy absorbed energy can be regarded as an index of the toughness of a steel plate. Therefore, in the present invention, the Charpy absorbed energy (vE -50℃ ) was used.

[0109] V-notch test pieces were taken from the (1 / 4)t position of the steel plate parallel to the rolling direction in accordance with the provisions of JIS Z2202 (2022). Using these V-notch test pieces, Charpy impact tests were carried out in accordance with the provisions of JIS Z2242 (2023), and the Charpy absorbed energy (vE -50℃ ) was sought.

[0110] The Charpy impact test was carried out by taking three test pieces from each steel plate. The absorbed energy of each test piece was measured, and the average of the three measurements was recorded as "vE -50℃ In this full-size Charpy impact test, the vE of the three specimens -50℃ If the average value of 100 J or more was 100 J or more, the test piece was evaluated as having excellent Charpy toughness (that is, excellent toughness at low temperatures), and was rated as "passed." In this example, No. 34 in Table 3 is the Charpy absorbed energy (vE -50℃ ) and as mentioned above, vE -50℃ If it was 50J or more, it was considered a "pass."

[0111] Table 3

Claims

1. 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 5.0% P: 0.03% or less, S: 0.0050% or less, and N: 0.0080% or less and the balance being Fe and unavoidable impurities, The microstructure at a depth of 1 / 4 of the plate thickness from the surface of the steel plate in the plate thickness direction is In terms of area ratio, ferrite is 5% or more and 90% or less, island martensite is 1% or more and less than 5%, and the remainder is tempered martensite and / or bainite, When island structures having an equivalent circle diameter of less than 0.5 μm are considered as carbides, the area ratio of the island martensite is equal to or less than the area ratio of the carbides, A steel sheet having an average thickness of crystal grains of 30 μm or less, when a region surrounded by high-angle grain boundaries with a crystal orientation misorientation of 15° or more is defined as one crystal grain.

2. The component composition further includes, in mass %, Al: 0.100% or less, Nb: 0.1% or less, Cr: 2.00% or less, Mo: 1.0% or less, Cu: 2.00% or less, V: 0.1% or less, Ti: 0.03% or less, B: 0.0050% or less, Ca: 0.0070% or less, REM: 0.010% or less, Mg: 0.0070% or less, and Zr: 0.0050% or less The steel sheet according to claim 1, comprising one or more selected from the following:

3. The method for producing a steel sheet according to claim 1 or 2, hot-rolling a steel material having the above-described composition to form a hot-rolled steel sheet; The hot-rolled steel sheet is heated to a temperature of Ac １ Point or more Ac ３ The material is heated and held in a two-phase temperature range below the temperature point, and then the material is subjected to a two-phase region quenching process under the conditions that the average cooling rate in the temperature range of 700°C to 500°C at a temperature of 1 / 4 of the plate thickness is 3°C / s or more, and the cooling end temperature is less than 300°C at a temperature of 1 / 4 of the plate thickness. Next, a tempering step is performed under conditions in which the tempering temperature is 300°C or higher but lower than 500°C at the center of the plate thickness, and the tempering parameter T.P. expressed by formula (1) is 15.50 or lower. T. P. =T×(log(t)+20) / 1000…(1) Here, in the formula (1), T is the tempering temperature (K), and t is the soaking time (hours).

4. Before the two-phase region quenching step, The hot-rolled steel sheet is heated to a temperature of Ac ３ 4. The method for producing a steel plate according to claim 3, wherein the steel plate is heated and held at a heating temperature of not less than 700°C and not more than 1000°C, and then a γ-region quenching step is performed under conditions such that the average cooling rate in a temperature range of not more than 700°C and not less than 500°C at a temperature at a depth of ¼ of the plate thickness is 3°C / s or more, and the cooling end temperature is less than 300°C at a temperature at a depth of ¼ of the plate thickness.

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