Steel plate and method of manufacturing the same

Abstract

The present invention provides a steel plate having excellent resistance to ammonia stress corrosion cracking, excellent low-temperature toughness, minimal change in mechanical properties due to PWHT treatment, and high strength, and a method for manufacturing the same. The steel has a specific mass% content of C, Si, Mn, P, S, Al, Ti, Mo, Ca, N, and O, satisfying 0.05 ≤ 2 × Cr + Mo + V + W ≤ 0.60, and the remainder being Fe and unavoidable impurities, and has a steel structure consisting of ferrite and hard structures other than ferrite at a position 1 / 4 of the plate thickness from the surface of the steel plate, the volume fraction of ferrite is 60% or more and 90% or less, the average crystal grain size of ferrite is 3 μm or more and 15 μm or less, the average hardness of the hard structure is 250 HV0.01 or more and 350 HV0.01 or less, and in terms of volume fraction, 60% or more of the hard structure is adjacent to ferrite rather than adjacent to the hard structure, and the yield strength is 325 MPa or more and 440 MPa or less.

Description

Technology Field

[0001] The present invention relates to a steel plate having excellent low-temperature toughness and resistance to stress corrosion cracking caused by ammonia, which is used in tanks for loading liquefied petroleum gas (hereinafter referred to as LPG) or liquefied ammonia, and a method for manufacturing the same. Background Technology

[0002] In order to realize a low-carbon society, ammonia, which does not emit carbon dioxide during combustion, is attracting attention as a new energy source. In addition, since hydrogen also does not emit carbon dioxide during combustion, it is considered a candidate for clean energy, and ammonia is also seen as a promising form for stably transporting such hydrogen.

[0003] Here, it is assumed that the international transport of liquefied ammonia will primarily be carried out by ships. Furthermore, in the medium term, given the demand for fossil fuels as energy sources, it is anticipated that the loading of liquefied ammonia will be diverted or transported in combination with other fuels such as LPG, liquefied butane, dimethyl ether, and butene.

[0004] Since LPG and ammonia are loaded into tanks as liquefied gases at low temperatures, excellent low-temperature toughness is required for steel. Furthermore, it is known that liquefied ammonia causes stress corrosion cracking in steel materials.

[0005] Therefore, from the perspective of ensuring the safety of the tank, it is necessary to take measures to suppress stress corrosion cracking caused by liquefied ammonia in the tank. To suppress this stress corrosion cracking caused by liquefied ammonia, the International Code for the Construction and Equipment of Ships Carrying Liquefied Gases in Bulk (IGC Code) stipulates that the yield point (hereinafter referred to as YS) of manganese carbon steel for tanks must be 440 MPa or less. In addition, tanks that pressurize and liquefy ammonia are required to perform post-weld heat treatment (hereinafter referred to as PWHT) during the manufacturing process.

[0006] Meanwhile, to improve transport efficiency, tanks are also being enlarged, and the steel used is required to have high tensile strength (hereinafter referred to as TS).

[0007] Specifically, for steel sheets for LPG or liquefied ammonia tanks, mechanical properties are required such that the ductile-brittle fracture surface transition temperature vTrs is -60°C or lower, and YS is 325 to 440 MPa and TS is 440 to 560 MPa. Furthermore, there is also a demand for steel sheets with equivalent toughness, YS is 355 to 440 MPa and TS is 490 to 610 MPa.

[0008] As described above, technology for having low-temperature toughness required for liquefied gas tanks and satisfying the requirements for YS constraints and high TS is described in Patent Documents 1 to 3.

[0009] The technology described in Patent Document 1 aims to suppress the yield ratio of a steel sheet by controlling the shape of the ferrite through hot rolling and accelerated cooling.

[0010] In addition, the technology described in Patent Document 2 achieves a low yield ratio of steel sheets by controlling the crystal grain size distribution of ferrite through hot rolling and multiple accelerated coolings.

[0011] In addition, Patent Document 3 describes a technique for lowering the yield ratio by distributing a hard structure through hot rolling and accelerated cooling, and creating a difference in hardness with ferrite. Prior art literature

[0012] Japanese Published Patent Application No. 2006-89830, Japanese Published Patent Application No. 2019-214752, Japanese Published Patent Application No. 2010-90406 The problem to be solved

[0013] As mentioned above, PWHT is sometimes performed during the drying of liquefied ammonia tanks. PWHT is a method that removes residual stress imparted by welding by heating the steel plate to a temperature below its transformation point and maintaining it for a certain period of time. Generally, it is known that the mechanical properties of steel plates change compared to the time of manufacture when heat-treated. Specifically, YS and TS may decrease, or vTrs may increase, leading to a deterioration in toughness. In particular, steel plates subjected to accelerated cooling show significant changes in mechanical properties. If the steel plate fails to satisfy the required mechanical properties due to PWHT, it becomes difficult to meet the specified strength for the tank structure, and in some cases, the tank may become unusable.

[0014] The present invention aims to solve the above problems and provide a steel plate with excellent resistance to ammonia stress corrosion cracking by ensuring YS specifications, excellent low-temperature toughness, minimal change in mechanical properties after PWHT treatment, and high strength, as well as a method for manufacturing the same.

[0015] Here, excellent resistance to ammonia stress corrosion cracking means that the yield strength YS is 325 MPa or more and 440 MPa or less according to the measurement method based on JIS Z2241 (2022).

[0016] In addition, high strength refers to a tensile strength TS of 440 MPa or more and 610 MPa or less, measured by the method in accordance with JIS Z2241 (2022).

[0017] In addition, excellent low-temperature toughness indicates that the brittle-ductile fracture surface transition temperature vTrs is -60°C or lower in the Charpy impact test in accordance with JIS Z2242 (2023).

[0018] In addition, a small change in mechanical properties after PWHT treatment indicates that the difference in average Vickers hardness of hard structures other than ferrite at a position 1 / 4 of the plate thickness from the surface of the steel plate before and after heating at 630°C for 10 hours is 20 HV 0.01 or less. means of solving the problem

[0019] To achieve the above objective, the inventors investigated a method to improve TS while maintaining the low-temperature toughness of the steel sheet, within a range where YS does not exceed 440 MPa, and also to suppress changes in mechanical properties due to heat treatment. As a result, it was found that it is effective to contain elements such as C, Si, Mn, Ti, Cr, Mo, V, and W within a predetermined range, to set the volume fraction and average crystal grain size of ferrite at a position 1 / 4 of the thickness from the surface of the steel sheet within a predetermined range, and to control the average hardness of the hard structure and the structure adjacent to the hard structure within a predetermined range.

[0020] The present invention has been completed by further examining the findings based on these findings.

[0021] In other words, the gist of the present invention is as follows.

[0022] [1] Mass %,

[0023] C: 0.03% or more and 0.14% or less,

[0024] Si: 0.10% or more and 0.50% or less,

[0025] Mn : 0.70% or more, 1.70% or less,

[0026] P : 0.030 % or less,

[0027] S : 0.0030 % or less,

[0028] Al : 0.010 % or more and 0.100 % or less,

[0029] Ti : 0.010 % or more and 0.030 % or less,

[0030] Mo: 0.02% or more, 0.10% or less,

[0031] Ca : 0.0005% or more and 0.0030% or less,

[0032] N : 0.0010 % or more, 0.0070 % or less,

[0033] O : Includes 0.0040% or less,

[0034] Satisfying the following (1) equation,

[0035] It has a composition consisting of residual Fe and unavoidable impurities,

[0036] At a position 1 / 4 of the plate thickness from the surface of the steel plate,

[0037] It has a steel structure consisting of ferrite and hard structures other than the ferrite, and

[0038] The volume fraction of the above ferrite is 60% or more and 90% or less, and

[0039] The average crystal grain size of the ferrite is 3 μm or more and 15 μm or less, and

[0040] The average hardness of the above hard tissue is 250 HV0.01 or higher and 350 HV0.01 or lower, and

[0041] In terms of volume fraction, 60% or more of the hard tissue is adjacent to ferrite and not adjacent to the hard tissue, and

[0042] Steel plate having a yield strength of 325 MPa or more and 440 MPa or less.

[0043] 0.05 ≤ 2 × Cr + Mo + V + W ≤ 0.60 ···Equation (1)

[0044] The element symbols in Equation (1) represent the content (mass%) of each component, and if not contained, they are set to 0.

[0045] [2] The above composition of components is in mass %,

[0046] Cr : 0.30% or less,

[0047] V : 0.10 % or less,

[0048] W : 0.10 % or less,

[0049] Cu: 1.00% or less,

[0050] Ni: 1.00% or less,

[0051] Co : 1.00 % or less,

[0052] Nb: 0.05% or less

[0053] A steel plate described in [1] above, containing one or more types selected from among.

[0054] [3] A steel plate described in [1] or [2], wherein the difference in average Vickers hardness of the hard structure at a position 1 / 4 of the plate thickness from the surface of the steel plate before and after heating at 630 ℃ for 10 hours is 20 HV 0.01 or less.

[0055] [4] Mass %,

[0056] C: 0.03% or more and 0.14% or less,

[0057] Si: 0.10% or more and 0.50% or less,

[0058] Mn : 0.70% or more, 1.70% or less,

[0059] P : 0.030 % or less,

[0060] S : 0.0030 % or less,

[0061] Al : 0.010 % or more and 0.100 % or less,

[0062] Ti : 0.010 % or more and 0.030 % or less,

[0063] Mo: 0.02% or more, 0.10% or less,

[0064] Ca : 0.0005% or more and 0.0030% or less,

[0065] N : 0.0010 % or more, 0.0070 % or less,

[0066] O : Includes 0.0040% or less,

[0067] Satisfying the following (1) equation,

[0068] A steel material having a composition consisting of remainder Fe and unavoidable impurities,

[0069] Heating to a temperature of 1000 ℃ or higher and 1250 ℃ or lower, and

[0070] The cumulative reduction rate in the unrecrystallized temperature range is set to 30% or more and 70% or less, and

[0071] Hot rolling is performed such that the hot rolling end temperature at a position 1 / 4 of the plate thickness from the surface of the steel plate is 750 ℃ ​​or higher, and

[0072] Cooling is initiated at a cooling start temperature of 680°C or higher and 900°C or lower at a position 1 / 4 of the plate thickness from the surface of the steel plate, and

[0073] Define temperature T in the range of 600 ℃ to 750 ℃, and

[0074] At a position 1 / 4 of the plate thickness from the surface of the steel plate,

[0075] The average cooling rate from the above cooling start temperature to the above temperature T is set to be 2.0 ℃ / s or more and 10.0 ℃ / s or less, and

[0076] A method for manufacturing a steel plate, wherein the average cooling rate from the above temperature T to the cooling stop temperature: 300 ℃ or higher and 550 ℃ or lower is 20 ℃ / s or higher and 100 ℃ / s or lower.

[0077] 0.05 ≤ 2 × Cr + Mo + V + W ≤ 0.60 ···Equation (1)

[0078] The element symbols in Equation (1) represent the content (mass%) of each component, and if not contained, they are set to 0.

[0079] [5] The above composition of components is in mass %,

[0080] Cr : 0.30% or less,

[0081] V : 0.10 % or less,

[0082] W : 0.10 % or less,

[0083] Cu: 1.00% or less,

[0084] Ni: 1.00% or less,

[0085] Co : 1.00 % or less,

[0086] Nb: 0.05% or less

[0087] A method for manufacturing a steel plate as described in [4] above, containing one or more types selected from among. Effects of the invention

[0088] According to the present invention, a high-strength steel plate is obtained that has a small change in mechanical properties after PWHT treatment and excellent low-temperature toughness and ammonia stress corrosion crack resistance.

[0089] The steel plate of the present invention is preferably used in tanks provided in environments with low temperatures and corrosive atmospheres caused by liquefied ammonia. Specific details for implementing the invention

[0090] Next, the steel plate of the present invention will be described in detail.

[0091] The steel sheet of the present invention has a compositional composition comprising, in mass%, C: 0.03% or more and 0.14% or less, Si: 0.10% or more and 0.50% or less, Mn: 0.70% or more and 1.70% or less, P: 0.030% or less, S: 0.0030% or less, Al: 0.010% or more and 0.100% or less, Ti: 0.010% or more and 0.030% or less, Mo: 0.02% or more and 0.10% or less, Ca: 0.0005% or more and 0.0030% or less, N: 0.0010% or more and 0.0070% or less, and O: 0.0040% or less, satisfying the following equation (1), and the remainder being Fe and unavoidable impurities, and from the surface of the steel sheet At the position of 1 / 4 of the plate thickness, the steel structure is composed of ferrite and a hard structure other than the ferrite, the volume fraction of ferrite is 60% or more and 90% or less, the average crystal grain size of ferrite is 3 μm or more and 15 μm or less, the average hardness of the hard structure is 250 HV0.01 or more and 350 HV0.01 or less, and in terms of volume fraction, 60% or more of the hard structure is adjacent to ferrite rather than adjacent to the hard structure, and the yield strength is 325 MPa or more and 440 MPa or less.

[0092] 0.05 ≤ 2 × Cr + Mo + V + W ≤ 0.60 ···Equation (1)

[0093] The element symbols in Equation (1) represent the content (mass%) of each element, and if not contained, they are set to 0.

[0094] In the present invention, it is important that the steel plate and the steel material provided for manufacturing the same have the above-mentioned composition. Therefore, the reason for limiting the composition of the steel plate in the present invention as above is explained first. In addition, "%" regarding the composition of the composition shall mean "mass%" unless otherwise specifically stated.

[0095] [Ingredient Composition]

[0096] C: 0.03% or more, 0.14% or less

[0097] C is an element that increases the quenchability of steel and is one of the important elements that must be included to achieve high tensile strength (TS). To obtain the above effect, the C content is set to 0.03% or more. In addition, from the perspective of manufacturing at a lower cost by reducing the content of other alloying elements, it is desirable to set the C content to 0.05% or more.

[0098] On the other hand, if the carbon content exceeds 0.14%, toughness and weldability decrease. Therefore, the carbon content is kept at 0.14% or less. In addition, from the perspective of suppressing the decrease in toughness and weldability, it is desirable to keep the carbon content at 0.10% or less.

[0099] Si: 0.10% or more, 0.50% or less

[0100] Si is an element that acts as a deoxidizer. If the Si content is less than 0.10%, a large amount of oxides are formed in the steel, and toughness decreases. Therefore, the Si content is set to 0.10% or more.

[0101] On the other hand, since Si is an element that causes a decrease in toughness or weldability, the Si content should be 0.50% or less. In particular, if the Si content exceeds 0.30%, there is a risk of deteriorating toughness by forming fiber martensite in the HAZ formed when welding with high heat input is performed, so it is desirable to keep the Si content 0.30% or less.

[0102] Mn: 0.70% or more, 1.70% or less

[0103] Mn is an element that increases the quenchability of steel and is one of the important elements that must be included to achieve high tensile strength (TS). To obtain the above effect, the Mn content is set to 0.70% or more. In addition, from the perspective of manufacturing steel sheets at a lower cost by reducing the content of other alloying elements, it is desirable to set the Mn content to 0.90% or more, and more desirable to set it to 1.20% or more.

[0104] On the other hand, if the Mn content exceeds 1.70%, in addition to a decrease in toughness and weldability, the alloy cost becomes excessively high. Therefore, the Mn content is set to 1.70% or less. Furthermore, from the perspective of suppressing the decrease in toughness and weldability, it is desirable to set the Mn content to 1.60% or less, and more desirable to set it to 1.50% or less.

[0105] P: 0.030% or less

[0106] P is an element contained as an unavoidable impurity and has adverse effects, such as reducing toughness or weldability by segregating at grain boundaries. Therefore, it is desirable to keep the P content as low as possible, but it is acceptable if it is 0.030% or less.

[0107] In addition, the lower limit of the P content is not specifically limited and may be 0%, but since P is an element that is inevitably contained in steel as an impurity, it may exceed 0% for industrial purposes. Since excessive reduction of P leads to a rise in refining costs, it is desirable to have a P content of 0.0005% or more.

[0108] S: 0.0030% or less

[0109] S is an element contained as an unavoidable impurity, and exists in steel as sulfide inclusions such as MnS, acting as a starting point for brittle cracking and reducing toughness. Therefore, it is desirable to keep the S content as low as possible, and to keep it 0.0030% or less.

[0110] In addition, the lower limit of the S content is not specifically limited and may be 0%. Typically, since S is an element inevitably contained in steel as an impurity, it may exceed 0% for industrial purposes. However, since excessive reduction of S leads to higher refining costs, it is desirable to keep the S content at 0.0005% or higher from a cost perspective.

[0111] Al: 0.010% or more, 0.100% or less

[0112] Al is an element that acts as a deoxidizer, refines grain size, and improves toughness. To obtain these effects, the Al content is 0.010% or more. To obtain the above effects more significantly, it is desirable to have an Al content of 0.020% or more.

[0113] Meanwhile, if the Al content exceeds 0.100%, oxide inclusions increase, which lowers the cleanliness and toughness of the steel sheet. The decrease in cleanliness leads to deterioration of surface properties due to an increase in surface scratches and a decrease in bendability. Therefore, the Al content is kept at 0.100% or less. In addition, it is preferable to keep the Al content at 0.060% or less.

[0114] Ti: 0.010% or more, 0.030% or less

[0115] Ti is an element that contributes to the improvement of the YS of steel sheets by having a strong tendency to form nitrides and precipitating as TiN upon solidification, thereby suppressing the coarsening of austenite during the heating process of steel materials or acting as a ferrite transformation nucleus. To obtain this effect, the Ti content is set to 0.010% or more. In addition, if the Ti content is 0.012% or more, a large amount of TiN precipitates, and the effect of suppressing the coarsening of austenite is significantly exhibited; therefore, it is desirable to set the Ti content to 0.012% or more.

[0116] Meanwhile, if the Ti content exceeds 0.030%, the TiN particles coarsen and become brittle fracture initiation points, which lowers toughness; therefore, the Ti content is kept at 0.030% or less. In addition, it is preferable to keep the Ti content at 0.020% or less.

[0117] Mo: 0.02% or more, 0.10% or less

[0118] Mo is an element that is dissolved in ferrite in a structure composed of ferrite and hard structure, and is an element that improves the YS and TS of steel sheets. In addition, it has a strong tendency to form fine carbides during PWHT heating, and the presence of fine carbides in the ferrite suppresses the decrease in ferrite hardness. This suppresses the decrease in YS and TS after PWHT. To obtain this effect, the Mo content is set to 0.02% or more and 0.10% or less.

[0119] When the Mo content is less than 0.02%, the hardness of the ferrite is reduced by PWHT, and YS and TS after PWHT are reduced. The Mo content is preferably 0.04% or more.

[0120] Meanwhile, if the Mo content exceeds 0.10%, the YS becomes excessively high. Therefore, the Mo content is kept below 0.10%. The Mo content is preferably below 0.07%.

[0121] Ca: 0.0005% or more, 0.0030% or less

[0122] Ca is an element that combines with S and has the effect of suppressing the formation of MnS, etc., which are elongated in the rolling direction. Therefore, by containing a predetermined amount of Ca, the shape of sulfide inclusions can be controlled so that they exhibit a spherical shape, thereby improving the toughness of the steel sheet.

[0123] If the Ca content is less than 0.0005%, the toughness of the steel sheet is inferior. Therefore, the Ca content is set to 0.0005% or more. In addition, it is desirable to set the Ca content to 0.0015% or more.

[0124] Meanwhile, if the Ca content exceeds 0.0030%, the cleanliness of the steel decreases. This decrease in cleanliness leads to deterioration of surface properties due to an increase in surface scratches and a decrease in bendability. Therefore, the Ca content is kept at 0.0030% or less. The Ca content is preferably 0.0025% or less.

[0125] N: 0.0010% or more, 0.0070% or less

[0126] N combines with Ti to form TiN, which precipitates and contributes to the refinement of the microstructure, thereby improving YS and toughness. To obtain these effects, the N content is 0.0010% or more. The N content is preferably 0.0030% or more, and more preferably 0.0050% or more.

[0127] On the other hand, if the N content exceeds 0.0070%, the amount of dissolved N increases, leading to a decrease in toughness. Also, if the N content exceeds 0.0070%, weldability decreases. Therefore, from the perspective of suppressing the decrease in toughness or weldability, the N content should be 0.0070% or less. In addition, it is desirable to keep the N content 0.0060% or less.

[0128] O : 0.0040% or less

[0129] Since O is an element contained as an unavoidable impurity that forms oxides and acts as a starting point for brittle cracking, thereby reducing toughness, the O content is limited to 0.0040% or less. It is preferable to limit the O content to 0.0030% or less.

[0130] Meanwhile, the lower limit of the O content is not specifically limited and may be 0%, but since O is an element that is inevitably contained in steel as an impurity, it may exceed 0% industrially. That is, since excessive reduction leads to higher refining costs, it is desirable to have an O content of 0.0020% or more from a cost perspective.

[0131] 0.05 ≤ 2 × Cr + Mo + V + W ≤ 0.60 ···Equation (1)

[0132] The element symbols in Equation (1) represent the content (mass%) of each element, and if not contained, they are set to 0.

[0133] Cr is an element dissolved in the hard structure in a structure composed of ferrite and hard structure, and it suppresses the decrease in hardness of the hard structure during PWHT heating. In addition, V and W are elements dissolved in ferrite in the same way as Mo, and they suppress the hardness of the ferrite during PWHT heating. The combination of Mo and the above elements suppresses the decrease in YS and TS after PWHT. To satisfy the desired mechanical properties after PWHT, it is necessary to ensure that 0.05 ≤ 2 × Cr + Mo + V + W ≤ 0.60. If 2 × Cr + Mo + V + W is less than 0.05, the hardness of the ferrite and hard structure decreases due to PWHT, and YS and TS after PWHT decrease significantly. Therefore, 2 × Cr + Mo + V + W is set to 0.05 or higher. 2 × Cr + Mo + V + W is preferably 0.10 or higher, and more preferably 0.20 or higher.

[0134] Meanwhile, if 2 × Cr + Mo + V + W exceeds 0.60, the ferrite and hard structures become excessively hardened, the TS becomes excessively high, and the toughness deteriorates. Therefore, 2 × Cr + Mo + V + W is set to 0.60 or less. 2 × Cr + Mo + V + W is preferably 0.50 or less, and more preferably 0.30 or less.

[0135] The composition of the above components, with the remainder being Fe and unavoidable impurities, is the basic composition of the present invention. This composition may additionally contain one or more selected from the group consisting of Cr: 0.30% or less, V: 0.10% or less, W: 0.10% or less, Cu: 1.00% or less, Ni: 1.00% or less, Co: 1.00% or less, and Nb: 0.05% or less, for the purpose of further improving strength characteristics or toughness.

[0136] Cr: 0.30% or less

[0137] Cr is an element that dissolves into the hard structure in a structure composed of ferrite and hard structure, and suppresses the decrease in hardness of the hard structure during heating of PWHT. It is preferable that the Cr content be 0.05% or more.

[0138] Meanwhile, if the Cr content exceeds 0.30%, it may cause a deterioration in toughness. Therefore, when Cr is included, the Cr content is kept at 0.30% or less. More preferably, the Cr content is 0.20% or less.

[0139] V: 0.10% or less

[0140] V is an element that is dissolved in ferrite in a structure composed of ferrite and hard structure, and is an element that improves the YS and TS of the steel sheet. In addition, it has a strong tendency to form carbides during the heating of PWHT and suppresses the reduction of ferrite hardness by existing in the ferrite as fine carbides. It is preferable that the V content be 0.03% or more.

[0141] Meanwhile, if the V content exceeds 0.10%, the YS may become excessively high. Therefore, when V is included, the V content is kept at 0.10% or less. More preferably, the V content is 0.05% or less.

[0142] W: 0.10% or less

[0143] W is an element that is dissolved in ferrite in a structure composed of ferrite and hard structure, and is an element that improves the YS and TS of the steel sheet. In addition, it has a strong tendency to form carbides during the heating of PWHT and suppresses the reduction of ferrite hardness by existing in the ferrite as fine carbides. It is preferable that the W content be 0.02% or more.

[0144] Meanwhile, if the W content exceeds 0.10%, a large amount of high-hardness carbides is produced, and TS may become excessive. Therefore, when W is included, the W content is kept at 0.10% or less. More preferably, the W content is 0.04% or less.

[0145] Cu: 1.00% or less

[0146] Cu is an element that improves the strength of the steel sheet by increasing the quenchability of the steel, and may be optionally included. It is preferable that the Cu content be 0.15% or more.

[0147] On the other hand, if the Cu content exceeds 1.00%, it may lead to a deterioration in toughness or an increase in alloy costs. Therefore, when Cu is included, the Cu content is kept at 1.00% or less. More preferably, the Cu content is 0.50% or less.

[0148] Ni: 1.00% or less

[0149] Ni is an element that has the same effect as Cu in improving the strength of the steel sheet and can be optionally included. It is preferable that the Ni content be 0.15% or more.

[0150] On the other hand, if the Ni content exceeds 1.00%, it may lead to a deterioration in weldability or an increase in alloy costs. Therefore, when Ni is included, the Ni content is kept at 1.00% or less. More preferably, the Ni content is 0.50% or less.

[0151] Co: 1.00% or less

[0152] Co is an element that has the same effect as Cu in improving the strength of the steel sheet and may be optionally included. It is preferable that the Co content be 0.20% or more.

[0153] On the other hand, if the Co content exceeds 1.00%, it may lead to a deterioration in weldability or an increase in alloy costs. Therefore, when Co is included, the Co content is kept at 1.00% or less. More preferably, the Co content is 0.60% or less.

[0154] Nb: 0.05% or less

[0155] Nb is an element that reduces the size of the old austenite grains by precipitating as carbonitrides, contributes to microstructure refinement by hot rolling, and improves YS and TS. It is desirable to have an Nb content of 0.020% or more.

[0156] Meanwhile, if the Nb content exceeds 0.05%, the YS exceeds 440 MPa due to excessive micronization. Therefore, the Nb content is kept below 0.05%. More preferably, the Nb content is below 0.030%.

[0157] In addition to having the above-mentioned composition, the steel sheet of the present invention has a microstructure in which, at a position 1 / 4 of the sheet thickness from the surface of the steel sheet, the volume fraction of ferrite is 60% or more and 90% or less, the average crystal grain size of the ferrite is 3 μm or more and 15 μm or less, the average hardness of the hard structure other than ferrite is 250 HV0.01 or more and 350 HV0.01 or less, and 60% or more of the hard structure is adjacent to ferrite rather than adjacent to the hard structure. The reason for limiting the microstructure as above is explained below.

[0158] [Microtissue]

[0159] The microstructure of the steel plate of the present invention is described.

[0160] [At a position 1 / 4 of the plate thickness from the surface of the steel plate, the volume fraction of ferrite is 60% or more and 90% or less]

[0161] In the steel sheet of the present invention, the volume fraction of ferrite at a position 1 / 4 of the sheet thickness from the surface of the steel sheet is 60% or more and 90% or less. If the volume fraction of ferrite exceeds 90%, the hard structure is insufficient and the desired TS and YS are not obtained.

[0162] On the other hand, if the volume fraction of ferrite is less than 60%, hard structures such as bainite and martensite are abundant in the remainder of the structure, causing TS and YS to exceed the specified range and also reducing toughness. The volume fraction of ferrite is preferably 60% or more and 70% or less.

[0163] As for the remainder structure, as a hard structure, structures such as bainite, pearlite, austenite, and martensite may be mixed. Although the volume fraction of each structure in the remainder structure does not need to be specifically limited, from the perspective of toughness, it is desirable that the volume fraction of pearlite or bainite in the remainder structure be high following that of ferrite. In addition, the volume fraction of various microstructures can be measured by the method described in the examples to be described later.

[0164] [Average crystal grain size of ferrite is 3 µm or more and 15 µm or less]

[0165] In the steel sheet of the present invention, the average crystal grain size of ferrite at a position 1 / 4 of the sheet thickness from the surface of the steel sheet is 3 μm or more and 15 μm or less. If the average crystal grain size of ferrite exceeds 15 μm, YS does not reach a predetermined range, and toughness is reduced. Therefore, the average crystal grain size of ferrite is 15 μm or less, preferably 12 μm or less, and more preferably 9 μm or less.

[0166] On the other hand, if the average crystal grain size of the ferrite is less than 3 μm, YS exceeds a predetermined range, and thus the occurrence of stress corrosion cracking caused by ammonia cannot be prevented. Therefore, the average crystal grain size of the ferrite is 3 μm or more, preferably 5 μm or more, and more preferably 7 μm or more.

[0167] The crystal grain size of ferrite refers to the circle equivalent diameter, which is the diameter of a circle with an area equal to that of a single ferrite crystal grain projected onto a two-dimensional plane. The circle equivalent diameter is calculated by performing image analysis processing on an image of a microstructure.

[0168] [Average hardness of hard tissue is 250 HV 0.01 or higher and 350 HV 0.01 or lower]

[0169] In the steel sheet of the present invention, the average hardness of the hard structure other than ferrite at a position 1 / 4 of the sheet thickness from the surface of the steel sheet is 250 HV0.01 or higher and 350 HV0.01 or lower. If the average hardness of the hard structure exceeds 350 HV0.01, the TS exceeds a predetermined range and the toughness decreases. Therefore, the average hardness of the hard structure is 350 HV0.01 or lower, preferably 330 HV0.01 or lower, and more preferably 320 HV0.01 or lower.

[0170] On the other hand, if the average hardness of the hard tissue is less than 250 HV0.01, the TS does not reach the predetermined range. Therefore, the average hardness of the hard tissue is 250 HV0.01 or higher, preferably 270 HV0.01 or higher, and more preferably 290 HV0.01 or higher.

[0171] In addition, the average hardness of the hard tissue can be measured by the method described in the examples below.

[0172] [More than 60% of the volume fraction of hard tissue is adjacent to ferrite rather than to hard tissue]

[0173] Carbides are generated within the hard structure by PWHT. When hard structures are adjacent, C diffuses between them, forming coarser carbides. This lowers the hardness of the hard structure and reduces the TS of the steel sheet. In addition, toughness is also reduced.

[0174] When more than 60% of the volume fraction of hard structure is adjacent to ferrite rather than to hard structure, the coarsening of carbides is suppressed, so the decrease in YS and TS before and after PWHT is small and the required mechanical properties are satisfied.

[0175] On the other hand, if the volume fraction of the hard structure adjacent to ferrite, rather than adjacent to the hard structure, is less than 60%, the decrease in YS and TS due to PWHT is significant, and the desired mechanical properties are not satisfied. Therefore, it is necessary for at least 60% of the hard structure to be adjacent to ferrite rather than adjacent to the hard structure. Accordingly, the volume fraction of the hard structure adjacent to ferrite rather than adjacent to the hard structure is 60% or more, preferably 70% or more, and more preferably 80% or more.

[0176] The upper limit of the volume fraction of the hard structure adjacent to ferrite and not adjacent to the hard structure is not specifically limited and may be 100%, and the volume fraction of the hard structure adjacent to ferrite and not adjacent to the hard structure may be 90% or less.

[0177] In addition, the volume fraction of the hard tissue can be measured by the method described in the examples below.

[0178] [Difference in average Vickers hardness of hard structures other than ferrite at a position 1 / 4 of the plate thickness from the surface of the steel plate before and after heating at 630 ℃ for 10 hours: 20 HV 0.01 or less]

[0179] Carbides are formed within the hard structure other than ferrite by PWHT. When C dissolved in the hard structure becomes a carbide, the deformation formed within the hard structure due to elemental dissolution is lost, so the hardness of the hard structure decreases and the TS of the steel sheet decreases.

[0180] If the difference in Vickers hardness obtained by subtracting "hardness before heating" from "hardness after heating" is 20 HV 0.01 or less before and after heating at 630 ℃ for 10 hours, the decrease in TS before and after PWHT is small, satisfying the required mechanical properties.

[0181] On the other hand, if the decrease in Vickers hardness of the hard structure after PWHT exceeds 20 HV 0.01, the decrease in TS due to PWHT is large, and even if the mechanical properties required before PWHT are satisfied, the TS after PWHT is insufficient. Alternatively, if the TS is satisfied after PWHT, the TS before PWHT is excessively high. Therefore, it is necessary that the difference in Vickers hardness of the hard structure before and after PWHT, that is, the difference in average Vickers hardness of the hard structure other than ferrite at a position 1 / 4 of the plate thickness from the surface of the steel sheet before and after heating at 630°C for 10 hours, be 20 HV 0.01 or less. Accordingly, before and after heating at 630°C for 10 hours, the difference in average Vickers hardness of the hard structure other than ferrite at a position 1 / 4 of the plate thickness from the surface of the steel plate is preferably 20 HV0.01 or less, 15 HV0.01 or less, and more preferably 10 HV0.01 or less.

[0182] The lower limit is not specifically limited, and the difference in average Vickers hardness of hard structures other than ferrite at a position 1 / 4 of the plate thickness from the surface of the steel plate may be 0 HV 0.01 or 5 HV 0.01 or more.

[0183] In addition, the average hardness of the hard tissue can be measured by the method described in the examples below. Also, the difference in the average Vickers hardness can be defined as “(the average Vickers hardness before heating at 630°C for 10 hours) - (the average Vickers hardness after heating at 630°C for 10 hours)”.

[0184] In addition, even if the PWHT conditions are different, an equivalent effect can be expected if the hardness difference requirement is satisfied under the aforementioned heating conditions.

[0185] Next, the method for manufacturing the steel plate of the present invention will be described.

[0186] A steel material having the above-mentioned composition is heated and hot-rolled to form a hot-rolled steel sheet, and then cooled under the conditions described below to form a steel sheet.

[0187] The manufacturing conditions are explained in detail below.

[0188] Although the method of manufacturing the steel material does not need to be specifically limited, it is preferable to melt the molten steel having the above-mentioned composition using a known melting method, such as a converter, and to produce a steel material, such as a slab of a predetermined size, using a known casting method, such as a continuous casting method. Furthermore, there is absolutely no problem with producing a steel material, such as a slab of a predetermined size, by means of the ingot-deconstruction rolling method.

[0189] The steel material obtained in this way is either hot-rolled directly without cooling, or heated after cooling and then provided for hot-rolling.

[0190] This hot rolling is performed by heating to a temperature of 1000°C or higher and 1250°C or lower, setting the cumulative reduction rate in the unrecrystallized temperature range of austenite to 30% or higher and 70% or lower, and terminating the hot rolling at 750°C or higher at a position 1 / 4 of the plate thickness from the surface of the steel plate. Next, cooling is initiated at a cooling start temperature of 680°C or higher and 900°C or lower at a position 1 / 4 of the plate thickness from the surface of the steel plate, and for a temperature T defined in the range of 600°C or higher and 750°C or lower, the average cooling rate from the cooling start temperature to T at a position 1 / 4 of the plate thickness from the surface of the steel plate is set to 2.0°C / s or higher and 10.0°C / s or lower, and the average cooling rate from temperature T to the cooling stop temperature of 300°C or higher and 550°C or lower is set to 20°C / s or higher and 100°C / s or lower.

[0191] (a) Heating temperature of steel material: 1000 ℃ or higher and 1250 ℃ or lower

[0192] When the heating temperature of the steel material is below 1000°C, the excessively low heating temperature results in high deformation resistance of the steel material and an increased load on the hot rolling mill, which may make hot rolling difficult. On the other hand, if the heating temperature of the steel material exceeds 1250°C, oxidation of the material surface becomes significant, and there is a risk that the yield will decrease due to increased oxidation loss of the material. For these reasons, the heating temperature is set to be between 1000°C and 1250°C. The heating temperature is preferably 1050°C or higher. In addition, the heating temperature is preferably 1150°C or lower.

[0193] (b) Cumulative reduction rate in the non-recrystallization temperature range: 30% or more and 70% or less

[0194] In the unrecrystallized temperature range (in the present invention, this refers to a temperature range of less than the Ar3 transformation point + 150°C for the steel material), if the cumulative reduction rate is less than 30%, the austenite is not sufficiently processed during hot rolling. If the austenite is not sufficiently processed, the number of ferrite nucleation sites decreases during the cooling process described later, the ferrite becomes coarsened, and the average grain size exceeds 15 μm. As a result, YS does not reach a predetermined range, and toughness is reduced.

[0195] On the other hand, if the cumulative reduction rate in the unrecrystallized temperature range exceeds 70%, the nucleation sites of ferrite during the cooling process become excessive, the average crystal grain size of the ferrite becomes less than 3 μm, and YS exceeds the predetermined range. Therefore, the cumulative reduction rate in the unrecrystallized temperature range is specified to be 30% or more and 70% or less. It is preferable that the cumulative reduction rate in the unrecrystallized temperature range be 40% or more, and more preferable that it be 50% or more. Furthermore, it is preferable that the cumulative reduction rate in the unrecrystallized temperature range be 67% or less, and more preferable that it be 60% or less.

[0196] Here, the Ar3 transformation point (°C) can be calculated, for example, using the following formula.

[0197] Ar3 transformation point (°C) = 910 - 273 × C - 74 × Mn - 57 × Ni - 16 × Cr - 9 × Mo - 5 × Cu

[0198] However, each element symbol indicates the content (mass%) of the corresponding element in the steel.

[0199] (c) Hot rolling finishing temperature: 750°C or higher at a position 1 / 4 of the plate thickness from the surface of the steel plate

[0200] Hot rolling is terminated at a temperature of 750°C or higher. If the hot rolling termination temperature is lower than 750°C, the ferrite generated during hot rolling is hardened by the processing of rolling, causing YS to exceed a predetermined range and toughness to decrease. Furthermore, the load on the hot rolling mill increases. Therefore, the hot rolling termination temperature is set to 750°C or higher. The hot rolling termination temperature is preferably 780°C or higher.

[0201] Although the upper limit of the hot rolling end temperature is not specifically limited, it is desirable to keep it below 950°C because if it exceeds 950°C, the ferrite may coarsen and the toughness may deteriorate.

[0202] (d) Cooling onset temperature: 680°C or higher and 900°C or lower at a position 1 / 4 of the plate thickness from the surface of the steel plate

[0203] If the cooling start temperature of the steel sheet is less than 680°C, a large amount of ferrite is generated before cooling begins, and the volume fraction of ferrite after cooling is completed exceeds 90%. If the volume fraction of ferrite exceeds 90%, the hard structure is insufficient, and the required YS and TS are not obtained. Therefore, the cooling start temperature is set to 680°C or higher. The cooling start temperature is preferably 750°C or higher.

[0204] On the other hand, if the cooling start temperature of the steel sheet exceeds 900°C, the volume fraction of ferrite becomes less than 60%, TS and YS exceed the specified range, and toughness is reduced. Therefore, the cooling start temperature is set to 900°C or lower. The cooling start temperature is preferably 830°C or lower.

[0205] (e) Average cooling rate from the surface of the steel plate to a temperature T defined in the range of 600 ℃ to 750 ℃ ​​from the cooling start temperature (first average cooling rate) at a position 1 / 4 of the plate thickness: 2.0 ℃ / s to 10.0 ℃ / s

[0206] The average cooling rate (first average cooling rate) in the temperature range from the cooling start temperature at a position 1 / 4 of the plate thickness from the surface of the steel plate to a temperature T defined in the range of 600 ℃ to 750 ℃ ​​is 2.0 ℃ / s to 10.0 ℃ / s. Before the cooling start or during the cooling process, ferrite is generated from the grain boundaries of austenite, and the distribution state of the hard structure and ferrite is determined by cooling to temperature T. If the temperature T exceeds 750 ℃, or if the first average cooling rate at a position 1 / 4 of the plate thickness from the surface of the steel plate exceeds 10.0 ℃ / s, the generation of ferrite from the austenite grain boundaries is insufficient, and a structure in which hard structures are adjacent to each other is formed. Specifically, the volume fraction of the hard structure surrounded by ferrite becomes less than 60% of the total hard structure, and the decrease in hardness after PWHT is significant.

[0207] Meanwhile, if the temperature T is less than 600 ℃, or if the first average cooling rate is less than 2.0 ℃ / s, the generation of ferrite in this process becomes excessive, the volume fraction of ferrite exceeds 90%, and YS and TS decrease. The first average cooling rate of the above temperature range is preferably 4 ℃ / s or higher. Also, the first average cooling rate of the above temperature range is preferably 8 ℃ / s or lower.

[0208] Here, the first average cooling rate (°C / s) is obtained by (cooling start temperature - temperature T) (°C) / (cooling time from cooling start temperature to temperature T (s)).

[0209] (f) Average cooling rate from temperature T to cooling stop temperature at a position 1 / 4 of the plate thickness from the surface of the steel plate (second average cooling rate): 20 ℃ / s or more and 100 ℃ / s or less

[0210] The average cooling rate (second average cooling rate) in the temperature range from temperature T to the cooling stop temperature is 20 ℃ / s or more and 100 ℃ / s or less. During this process, ferrite grains grow, and the volume fraction of the final microstructure is determined. If the second average cooling rate in the above temperature range exceeds 100 ℃ / s, the volume fraction of ferrite becomes less than 60%, and YS and TS exceed the predetermined range. In addition, as a large amount of filamentous martensite is generated, the hardness of the hard microstructure increases, and the toughness decreases.

[0211] Meanwhile, if the second average cooling rate is less than 20 ℃ / s, ferrite growth proceeds, and the volume fraction of ferrite exceeds 90%, causing YS and TS to decrease. The second average cooling rate of the above temperature range is preferably 30 ℃ / s or higher. Also, the second average cooling rate of the above temperature range is preferably 70 ℃ / s or lower.

[0212] Here, the second average cooling rate (°C / s) is obtained by (temperature T - cooling stop temperature) (°C) / (cooling time from temperature T to cooling stop temperature (s)).

[0213] (g) At a position 1 / 4 of the plate thickness from the surface of the steel plate, cooling stop temperature: 300 ℃ or higher and 550 ℃ or lower

[0214] The cooling stop temperature is set to be between 300°C and 550°C. If the cooling stop temperature exceeds 550°C, the hardness of the hard structure becomes less than 250 HV0.01, and the TS decreases.

[0215] Meanwhile, when the cooling stop temperature is below 300 ℃, the hardness of the hard structure becomes excessive, and in addition to the increase in TS, the toughness decreases. The cooling stop temperature is 350 ℃ or higher. Also, the cooling stop temperature is preferably 500 ℃ or lower.

[0216] By manufacturing a steel material having the above-described composition according to the above-described manufacturing conditions, a steel plate having the above-described structure (steel plate according to the present invention) can be obtained. The steel plate according to the present invention obtained in this way is equipped with excellent strength characteristics and low-temperature toughness. Here, excellent strength characteristics are defined as YS (upper yield point when there is a yield point, 0.2% yield strength when there is no yield point) being 325 MPa or more and 440 MPa or less, and TS being 440 MPa or more and 610 MPa or less.

[0217] In addition, the IMO Gas Code and classification rules by the International Maritime Organization specify that the yield point of steel plates must be 440 MPa or less to minimize the risk of ammonia stress corrosion cracking. Therefore, if the YS is 440 MPa or less, it can be said that it has excellent ammonia stress corrosion cracking resistance.

[0218] Basically, a higher TS of a steel plate is preferable, but steel plates with a TS exceeding 610 MPa are more likely to cause cracking during processing or welding. Alternatively, a large amount of alloy may be added, which is likely to increase costs. Furthermore, since it is difficult to achieve both the goal of controlling YS to 440 MPa or less to ensure resistance to ammonia stress corrosion cracking and the improvement of TS, it is desirable to keep the TS of the steel plate 610 MPa or less. Additionally, the TS of the steel plate obtained in the present invention is substantially 610 MPa or less.

[0219] Although not specifically limited, the thickness of the steel plate of the present invention is preferably 12 mm or more, and more preferably 20 mm or more. In addition, the thickness of the steel plate of the present invention is preferably 50 mm or less, and more preferably 40 mm or less.

[0220] In the steel plate and the method for manufacturing the same according to the present invention, for items not described in this specification, all known regulations and conventional methods related to steel plates may be used.

[0221] Examples

[0222] Molten steel with the composition shown in Table 1 was melted to form steel materials (slabs). These steel materials (slabs) were subjected to hot rolling and cooling under the conditions shown in Table 2 to obtain steel plates. Elements in blank spaces in the table indicate that they were intentionally not added.

[0223] A heat treatment simulating PWHT was performed on the obtained steel plate. Using a heating furnace, the steel plate was heated to a temperature of 630°C at a position 1 / 4 of the plate thickness from the surface, and the temperature was maintained for 10 hours. In addition, the influence of the heat treatment temperature and the holding time of the heat treatment temperature on the mechanical properties of the steel material is summarized by the following tempering parameter P.

[0224] P = T{log (t) + 20} × 10 -3

[0225] Here, T is the heat treatment temperature (K), and t is the holding time at the heat treatment temperature (hour).

[0226] The tempering parameter P in the heat treatment simulating the above PWHT is 18.963. Since the effect on mechanical properties is smaller as P decreases, the inventive example in the present embodiment can be expected to exhibit an equivalent or greater effect under PWHT conditions where P ≤ 18.963.

[0227] For the obtained steel plates, the volume fraction of the microstructure at a position 1 / 4 of the plate thickness from the surface, the ferrite grain size distribution, the hardness of the hard structure, and the tensile properties and toughness were evaluated. In addition, for the steel plates after heat treatment simulating PWHT, the hardness at a position 1 / 4 of the plate thickness from the surface was investigated. Each test method is as follows.

[0228] [Measurement of volume fraction of microstructure and ferrite grain size distribution]

[0229] Each observation sample was taken with the center positioned at 1 / 4 of the plate thickness from the surface of the obtained steel plate. The surface of the sample was mirror-polished and etched with Nital, and then photographed using a scanning electron microscope (SEM) (magnification: 1000x). The imaging range was set to 4 mm × 3 mm. The area fraction of the microstructure was obtained by analyzing the captured image using an image analysis device. Since the steel plate of the present invention has low anisotropy of the microstructure and the two-dimensional information obtained by analysis has universality, the area fraction of the microstructure was equated to the volume fraction, and the area fraction was defined as the volume fraction.

[0230] When determining the volume fraction of microtissues, the identification of each tissue was performed as follows.

[0231] Ferrite is a structure surrounded by white lines without containing isotropically grown carbides, and pearlite is massive or flat in shape, with dark ferrite and white carbides appearing in a striped pattern.

[0232] Bainite was defined as a structure having a slender, elongated lath-like ferrite structure containing carbides with an equivalent diameter of 0.05 μm or more, and martensite was defined as a structure having the same slender, elongated lath-like ferrite structure as bainite but not containing carbides with an equivalent diameter of 0.05 μm or more. Additionally, the carbides were defined as appearing as white dots.

[0233] In addition, austenite was defined as a non-carbide structure with an equivalent diameter of 0.50 μm or more, existing between the Rath-phase ferrite structures of bainite or martensite.

[0234] The circular equivalent diameter of the ferrite was calculated by counting the pixels within the region enclosed by grain boundaries as a single crystal in an image captured by a scanning electron microscope (SEM) to determine the area, and then converting the number of pixels into an actual length to calculate the circular equivalent diameter.

[0235] There are cases where noise caused by insufficient resolution of the image to be analyzed or lens focus misalignment during shooting is misidentified as a single crystal grain, leading to an incorrect calculation that a vast amount of minute ferrite exists. Therefore, the grain size distribution was obtained by randomly selecting more than 4,000 ferrites with dimensions that can be identified as crystals by the naked eye, that is, a circular equivalent diameter of 2 μm or more.

[0236] The average crystal grain size of ferrite was obtained by dividing the sum of these grain sizes (circle equivalent diameters) by the number.

[0237] The volume fraction of the hard structure adjacent only to ferrite was determined by calculating the ratio of the area of ​​the microstructure adjacent to the hard structure, which was determined to be only ferrite, to the total area of ​​the hard structure identified in the scanning electron microscope (SEM) image. Since the two-dimensional information obtained through analysis has universality, the area fraction of the microstructure was considered to correspond to the volume fraction, and the obtained area fraction was defined as the volume fraction.

[0238] [Hardness of hard tissue (average hardness)]

[0239] The hardness of the hard structure (average hardness) was measured by hardness measurement using a micro-indenter. A sample was taken from a position 1 / 4 of the plate thickness from the surface of the steel plate, the surface was polished and then etched with Nital solution, observed using an optical microscope to determine the hard structure targeted for hardness measurement, a square pyramidal indenter was pressed with a load of 10 gf, and Vickers hardness was measured from the indentation dimensions. The hardness of 20 hard structures was measured, and the average value was taken as the average hardness of the hard structure. The hardness measurement method is the same for the obtained steel plate and the steel plate after heat treatment simulating PWHT.

[0240] In addition, in Table 2, the change in hardness (difference in Vickers hardness) before and after PWHT represents “(average Vickers hardness before heating at 630°C for 10 hours) - (average Vickers hardness after heating at 630°C for 10 hours)”.

[0241] [Strength Characteristics]

[0242] For each steel plate, a tensile test specimen was taken in accordance with JIS Z2241 (2022) such that the direction perpendicular to the rolling direction, that is, the plate width direction and the length direction of the tensile test specimen coincided, and a tensile test was performed in accordance with the guidelines of JIS Z2241 (2022) to measure YS and TS. Yield strength YS is closely related to resistance to ammonia stress corrosion cracking, and since it is necessary to minimize the risk of ammonia stress corrosion cracking as a structural member of a liquefied gas bulk carrier, the IMO Gas Code and classification society rules specify that the yield point of the steel plate must be 440 MPa or less. In this regard, in this embodiment, a steel plate with a YS of 325 MPa or more and 440 MPa or less was evaluated as a steel plate with excellent resistance to ammonia stress corrosion cracking. In addition, steel plates with a TS of 440 MPa or more and 610 MPa or less were evaluated as steel plates with excellent tensile strength.

[0243] [tenacity]

[0244] A test specimen in accordance with JIS Z2242 (2023) was taken from a section 0.5 mm removed from the surface side of each steel plate, such that the direction perpendicular to the rolling direction, i.e., the plate width direction, coincides with the length direction of the tensile test specimen. Then, a Charpy impact test was performed in accordance with JIS Z2242 (2023) to measure the brittle-ductile fracture surface transition temperature vTrs. Then, steel plates with a vTrs of -60°C or lower were evaluated as having excellent low-temperature toughness.

[0245] The evaluation results obtained in this way are listed in Table 2.

[0246]

[0247]

[0248] As can be seen from Tables 1 and 2, all of the inventive examples have a yield strength YS of 325 MPa or more and 440 MPa or less and a tensile strength TS of 440 MPa or more and 610 MPa or less, a brittle-ductile fracture surface transition temperature vTrs of -60 ℃ or less, excellent toughness at low temperatures and ammonia stress corrosion cracking resistance, and high-strength steel sheets with small changes in mechanical properties before and after PWHT treatment are obtained.

[0249] Meanwhile, steel plates No. 4, 5, 7, 8, 9, 11, 13, and 14 corresponding to comparative examples have a ferrite volume fraction or average ferrite grain size outside the range of the present invention, and at least one of yield strength YS, tensile strength TS, and toughness is inferior compared to the inventive example.

[0250] In addition, steel plate No. 6, which corresponds to the comparative example, has a rolling end temperature outside the range of the present invention, and its yield strength YS and toughness are lower compared to the inventive example.

[0251] Steel plates No. 10 and 12 have a volume fraction of hard structure adjacent only to ferrite that is outside the scope of the present invention, and the change in hardness before and after PWHT is large.

[0252] In addition, steel plates No. 15 and 16 have an average hardness of the hard structure outside the range of the present invention, and the TS is lower compared to the invention example, and No. 15 also has lower toughness.

[0253] In addition, steel plates No. 22 to 41 have different content of various elements compared to the invention example, and at least one of the yield strength YS, tensile strength TS, toughness, and change in mechanical properties before and after PWHT treatment is lower compared to the invention example.

Claims

Claim 1 In mass%, it has a compositional composition consisting of C: 0.03% or more and 0.14% or less, Si: 0.10% or more and 0.50% or less, Mn: 0.70% or more and 1.70% or less, P: 0.030% or less, S: 0.0030% or less, Al: 0.010% or more and 0.100% or less, Ti: 0.010% or more and 0.030% or less, Mo: 0.02% or more and 0.10% or less, Ca: 0.0005% or more and 0.0030% or less, N: 0.0010% or more and 0.0070% or less, and O: 0.0040% or less, satisfying the following equation (1), and the remainder being Fe and unavoidable impurities, and the thickness from the surface of the steel plate A steel plate having a steel structure composed of ferrite and a hard structure other than the ferrite at the 1 / 4 position, wherein the volume fraction of the ferrite is 60% or more and 90% or less, the average crystal grain size of the ferrite is 3 μm or more and 15 μm or less, the average hardness of the hard structure is 250 HV 0.01 or more and 350 HV 0.01 or less, and in terms of volume fraction, 60% or more of the hard structure is adjacent to the ferrite rather than adjacent to the hard structure, and the yield strength is 325 MPa or more and 440 MPa or less. 0.05 ≤ 2 × Cr + Mo + V + W ≤ 0.60 ···Equation (1) The element symbols in Equation (1) represent the content (mass%) of each element, and are set to 0 if not contained. Claim 2 A steel plate according to claim 1, wherein the composition of the above components contains, in mass%, one or more selected from Cr: 0.30% or less, V: 0.10% or less, W: 0.10% or less, Cu: 1.00% or less, Ni: 1.00% or less, Co: 1.00% or less, and Nb: 0.05% or less. Claim 3 A steel plate according to claim 1 or 2, wherein, before and after heating at 630°C for 10 hours, the difference in average Vickers hardness of the hard structure at a position 1 / 4 of the plate thickness from the surface of the steel plate is 20 HV 0.01 or less. Claim 4 A steel material having a compositional composition consisting of, in mass%, C: 0.03% or more and 0.14% or less, Si: 0.10% or more and 0.50% or less, Mn: 0.70% or more and 1.70% or less, P: 0.030% or less, S: 0.0030% or less, Al: 0.010% or more and 0.100% or less, Ti: 0.010% or more and 0.030% or less, Mo: 0.02% or more and 0.10% or less, Ca: 0.0005% or more and 0.0030% or less, N: 0.0010% or more and 0.0070% or less, and O: 0.0040% or less, satisfying the following equation (1), and the remainder being Fe and unavoidable impurities, is subjected to a temperature of 1000°C or higher. Manufacturing of a steel plate, wherein the steel plate is heated to a temperature of 1250 ℃ or lower, the cumulative reduction rate in the unrecrystallized temperature region is 30% or more and 70% or less, and hot rolling is performed such that the hot rolling end temperature at a position 1 / 4 of the plate thickness from the surface of the steel plate is 750 ℃ ​​or higher, cooling is initiated at a cooling start temperature of 680 ℃ or more and 900 ℃ or less at a position 1 / 4 of the plate thickness from the surface of the steel plate, a temperature T is defined in the range of 600 ℃ or more and 750 ℃ ​​or less, the average cooling rate from the cooling start temperature to the temperature T at a position 1 / 4 of the plate thickness from the surface of the steel plate is 2.0 ℃ / s or more and 10.0 ℃ / s or less, and subsequently, the average cooling rate from the temperature T to the cooling stop temperature: 300 ℃ or more and 550 ℃ or less is 20 ℃ / s or more and 100 ℃ / s or less. Method. 0.05 ≤ 2 × Cr + Mo + V + W ≤ 0.60 ···Equation (1) The element symbols in Equation (1) represent the content (mass%) of each element, and if not contained, they are set to 0. Claim 5 A method for manufacturing a steel plate according to claim 4, wherein the above component composition contains, in mass%, one or more selected from Cr: 0.30% or less, V: 0.10% or less, W: 0.10% or less, Cu: 1.00% or less, Ni: 1.00% or less, Co: 1.00% or less, and Nb: 0.05% or less.

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