Thick steel plate and method for manufacturing the same

Abstract

To provide a thick steel plate which is excellent in low-temperature toughness, is suppressed in stress corrosion cracking caused by liquefied ammonia, and has 490 MPa or more of tensile strength, and a method for manufacturing the thick steel plate.SOLUTION: A thick steel plate is provided, which has a chemical composition of a specific range. In the thick steel plate, the area fraction of a bainite structure from 1 / 4 thickness to 3 / 4 thickness in the sheet thickness direction from the surface is 90% or more, Vickers hardness at the 1 mm position in the sheet thickness direction from the surface is 210 or less, the maximum value of the maximum hardness test measured on the basis of JIS Z 3101:1990 is 210 or less on Vickers hardness, the tensile strength is 490 MPa or more, and absorption energy in Charpy impact test is 47 J or more at -40°C.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a high-strength thick steel plate used in welded structures that require weldability and stress corrosion cracking resistance, such as tanks and pressure vessels for storing ammonia or hydrogen sulfide, and a method for manufacturing the same. [Background technology]

[0002] In the future, the use of ammonia fuel will expand in response to carbon neutrality, and this expansion will require the installation of large tanks. High-strength thick steel plates and welded joints are required to manufacture large tanks, and the use of 490 MPa-class (SLA365, etc.), 570 MPa-class (SM570, etc.), and 610 MPa-class (SPV490, etc.) thick steel plates is being considered. However, it is known that stress corrosion cracking occurs when low-alloy steel is used in tanks that store liquefied ammonia, which is an obstacle to its expansion.

[0003] Therefore, studies are being conducted on steel plates that are resistant to stress corrosion cracking caused by liquefied ammonia. For example, Patent Document 1 discloses a method for producing steel plates that have a strength of 570 MPa or more, low-temperature toughness at -40°C, and excellent toughness in the weld heat-affected zone by reducing the amount of C in the steel and optimally controlling the amounts of Ti and N. In Patent Document 2, the amount of C in the steel is reduced to achieve a low yield ratio while maintaining a strength of 400N / mm 2 A method for producing steel plates having tensile strengths of at least grade is disclosed. In addition, Patent Document 3 discloses that the amount of C in the steel is reduced and accelerated cooling is applied to reduce the increase in hardness of the steel sheet surface while achieving a hardness of 530 to 610 N / mm 2 A method for producing a steel plate having a tensile strength of In addition, in Patent Document 4, a decarburized layer is formed on the surface of the steel sheet, and a pressure of 60 kgf / mm 2 This publication discloses a method for manufacturing a steel sheet that has high tensile strength and excellent stress corrosion cracking resistance. Furthermore, Patent Document 5 discloses a steel sheet that has excellent stress corrosion cracking resistance achieved by softening the surface of the steel sheet to a surface hardness of 190 or less in Vickers hardness. Patent Documents 6 and 7 disclose methods for manufacturing steel sheets that ensure stress corrosion cracking resistance by optimally controlling the ferrite structure fraction and crystal grain size of the steel sheets and controlling the yield strength to 440 MPa or less. Patent Document 8 discloses a method for manufacturing a steel plate that ensures stress corrosion cracking resistance by optimizing the ferrite structure fraction in the center of the steel plate thickness while suppressing the hardness of the surface layer of the steel plate to 210 or less. Patent Document 9 discloses a method for manufacturing a steel plate that ensures stress corrosion cracking resistance by optimizing the morphology of the surface layer of the steel plate and optimizing the ferrite structure fraction in the center of the steel plate thickness. Patent Document 10 discloses a method for manufacturing a steel plate suitable for a tank for transporting liquefied ammonia, in which the yield ratio of the steel plate is reduced by optimizing the structure of ferrite and the like at the quarter thickness position of the steel plate. Furthermore, Patent Document 11 discloses a method for producing a steel plate that ensures excellent stress corrosion cracking resistance by performing two-stage water cooling during quenching or direct quenching of the steel plate to generate a soft ferrite structure. Furthermore, Patent Document 12 discloses a method for manufacturing a clad steel plate that ensures excellent stress corrosion cracking resistance by joining a clad material having a low carbon and manganese content to the surface of the steel plate by cast cladding or overlay welding. In Patent Document 13, the yield strength of the weld heat affected zone is 45 kgf / mm 2 A method for manufacturing a clad steel plate that ensures excellent stress corrosion cracking resistance by joining the following clad materials is disclosed. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2001-115233 [Patent Document 2] Japanese Patent Application Publication No. 11-131178 [Patent Document 3] Japanese Patent Application Publication No. 10-195533 [Patent Document 4] Japanese Patent Publication No. 61-279631 [Patent Document 5] Japanese Patent Application Publication No. 50-085516 [Patent Document 6] Patent No. 7323090 [Patent Document 7] Patent No. 7323091 [Patent Document 8] Patent No. 7323088 [Patent Document 9] International Publication No. 2021 / 106368 [Patent Document 10] Patent Publication No. 2021-88753 [Patent Document 11] Japanese Patent Publication No. 156228 / 1983 [Patent Document 12] Japanese Patent Publication No. 57-149425 [Patent Document 13] Japanese Patent Application Publication No. 50-085546 Summary of the Invention [Problem to be solved by the invention]

[0005] However, the methods described in Patent Documents 1 to 4 are intended for steel plates having a strength of up to 590 MPa, and although they can ensure excellent stress corrosion cracking resistance in thick steel plates, they do not disclose a method for ensuring stress corrosion cracking resistance in the weld heat affected zone when a welded joint is prepared in the actual manufacture of a tank. Furthermore, Patent Document 5 does not take into consideration the low temperature toughness of the base metal or the weld metal at all. The methods described in Patent Documents 6 to 10 are based on the premise that the steel sheet contains a soft ferrite structure, and may not be able to produce the 490 MPa-class high-strength steel sheet that is the subject of the present invention. Furthermore, they do not disclose a method for ensuring the stress corrosion cracking resistance of the weld heat-affected zone. Patent Document 11 applies two-stage cooling, but requires special water cooling equipment, and does not disclose a method for ensuring the stress corrosion cracking resistance of the weld heat affected zone. Patent Document 12 requires cast cladding or build-up welding, which requires special equipment and has productivity problems. Furthermore, it does not disclose a method for ensuring stress corrosion cracking resistance of the weld heat-affected zone. In Patent Document 13, stress corrosion cracking in the heat-affected zone can be suppressed, but when rolling and heat treatment conditions are selected to increase the strength of the clad base material in order to manufacture a large tank, the hardness of the clad material before welding increases, and stress corrosion cracking performance cannot be suppressed outside the heat-affected zone.

[0006] Furthermore, the stress corrosion cracking resistance of these steels in a liquefied ammonia environment was evaluated using simple tests that reproduced the environment at the laboratory level, but it is known that the presence of carbon dioxide plays an important role in stress corrosion cracking in the environment of an actual liquefied ammonia storage tank. However, it was not possible to reproduce stress corrosion cracking in these steels at the laboratory level under conditions that fully reproduce the environment in which actual liquefied ammonia contains carbon dioxide, and it was unclear whether they could ensure performance in such an environment.

[0007] Furthermore, as a countermeasure against stress corrosion cracking, stainless steel plates or clad steel plates made of stainless steel plates can be used, but this has the problem of being significantly more expensive than using low-alloy steel plates.

[0008] As described above, the methods proposed so far have not disclosed a technology for ensuring the stress corrosion cracking resistance of the base material and weld heat-affected zone of a welded joint with a strength class of 490 MPa or higher (a technology for suppressing stress corrosion cracking caused by liquefied ammonia) without necessarily requiring special equipment.

[0009] An object of the present invention is to provide a thick steel plate having excellent low-temperature toughness, suppressing stress corrosion cracking due to liquefied ammonia, and having a tensile strength of 490 MPa or more, and a method for manufacturing the same.

[0010] Here, excellent low-temperature toughness means that the absorbed energy in a Charpy impact test of a thick steel plate is 47 J or more at -40°C in accordance with JIS Z 2242:2018. In addition, suppressing stress corrosion cracking caused by liquefied ammonia means that stress corrosion cracking does not occur in (1) a liquefied ammonia stress corrosion cracking test on thick steel plates and (2) a liquefied ammonia stress corrosion cracking test on weld heat-affected zones. (1) indicates that a 5mm thick test piece taken from the surface of a thick steel plate up to 5mm in the thickness direction is placed in an immersion cell in a state where a four-point bending test is carried out at 90% of the yield stress of the base material determined in a tensile test, and the piece is immersed in a saturated solution of liquefied NH3 + 5vol% NH4CO2NH2 at 20°C for a test period of one month, and no cracks are observed visually. (2) indicates that in the heat-affected zone obtained by welding thick steel plates together, from which excess reinforcement has been removed, a 5mm thick four-point bending test piece is taken up to 5mm in the plate thickness direction, and the test piece is placed in a state where 90% of the yield stress of the base material obtained in the tensile test has been subjected to a four-point bending test. The test piece is then immersed in a saturated solution of liquefied NH3 + 5vol% NH4CO2NH2 at 20°C, and the test period is one month. No cracks are observed in each case, and the test piece has excellent stress corrosion cracking resistance. Furthermore, having a tensile strength of 490 MPa or more means that the tensile strength of the steel plate is 490 MPa or more in accordance with JIS Z 2241:2022. [Means for solving the problem]

[0011] In order to solve the above-mentioned problems, the present inventors conducted four-point bending stress corrosion cracking tests under conditions in which liquefied ammonia was saturated with carbon dioxide on various thick steel plates and steel materials that had been subjected to heat treatment simulating their weld heat-affected zones, and as a result, they obtained the following findings. First, the conditions for suppressing the initiation and subsequent progression of liquefied ammonia stress corrosion cracking in low-alloy steel were examined, and it was found that the Vickers hardness of the steel plate on the surface that comes into contact with liquefied ammonia must be kept below 210. Furthermore, it was found that the Vickers hardness of the weld heat-affected zone after welding must also be kept below 210. It has been said that it is appropriate to carry out the maximum hardness test specified in JIS Z 3101:1990 in order to evaluate the maximum hardness of the weld heat affected zone of a steel plate. On the other hand, keeping the yield strength of steel below 440 MPa is also considered effective in preventing liquefied ammonia stress corrosion cracking, but it has been found that cracks will occur if the Vickers hardness of the steel plate on the surface that comes into contact with liquefied ammonia exceeds 210.

[0012] The present invention was derived by further studying the above findings, namely, [1] In mass%, C: 0.010% or more and less than 0.050% Si: 0.01 to 0.55%, Mn: 1.00-1.95% P:0.030% or less, S: 0.010% or less, Al: 0.001 to 0.060%, Nb: 0.006 to 0.090%, N: 0.001 to 0.006% and has a chemical composition in which PcmN represented by the following formula (1) satisfies 0.11 to 0.17, with the remainder consisting of Fe and unavoidable impurities, The area fraction of bainite structure from the surface to 1 / 4 to 3 / 4 of the thickness in the plate thickness direction is 90% or more, The Vickers hardness at a position 1 mm from the surface in the thickness direction is 210 or less, The maximum value of the maximum hardness test measured based on JIS Z 3101:1990 is 210 or less in Vickers hardness, The tensile strength is 490 MPa or more, A thick steel plate with an absorbed energy of 47J or more at -40°C in a Charpy impact test. PcmN=C+Si / 30+Mn / 20+Cu / 20+Ni / 60+Cr / 20+Mo / 15+Nb / 2+V / 3+23×B...Formula (1) In formula (1), the element symbols indicate the content (mass%) of each element contained in the steel plate, and elements that are not contained are represented as 0. [2] The chemical composition further comprises, in mass %, Cu: 0.50% or less, Ni: 0.30% or less, Cr: 0.50% or less, Mo: 0.50% or less V: 0.08% or less, Ti: 0.05% or less, B: 0.0030% or less, Ca: 0.006% or less, Mg: 0.006% or less The steel plate according to [1] above, containing one or more selected from the following: [3] When hot rolling a slab having the chemical composition described in [1] or [2], the slab is heated to 1000 to 1250 ° C., After rolling at a cumulative reduction rate of 50% or more at 930°C or less, Hot rolling is completed at a temperature of Ar3 point + 10°C or higher. At an accelerated cooling start temperature of Ar3 point or higher, the average cooling rate from 680°C to 600°C on the steel plate surface is 10 to 80°C / s, A method for manufacturing a thick steel plate, in which the thick steel plate is obtained by performing accelerated cooling at an accelerated cooling stop temperature of 100 to 600°C. [4] The method for producing a steel plate according to [3], wherein the steel plate is cooled to room temperature immediately after the accelerated cooling is stopped or after the accelerated cooling is stopped, and then tempered at a temperature of 500 to 680°C. [Effects of the Invention]

[0013] According to the present invention, there are provided high-strength thick steel plates which are excellent in low-temperature toughness, suppress stress corrosion cracking due to liquefied ammonia, and have a tensile strength of 490 MPa or more, and methods for producing the same. DETAILED DESCRIPTION OF THE INVENTION

[0014] The steel plate of the present invention has a chemical composition containing, by mass%, C: 0.010% or more but less than 0.050%, Si: 0.01 to 0.55%, Mn: 1.00 to 1.95%, P: 0.030% or less, S: 0.010% or less, Al: 0.001 to 0.060%, Nb: 0.006 to 0.090%, and N: 0.001 to 0.006%, and wherein PcmN represented by the following formula (1) satisfies 0.11 to 0.17, with the balance being Fe and unavoidable impurities: The area fraction of bainite structure from the surface to 1 / 4 to 3 / 4 of the thickness in the plate thickness direction is 90% or more, the Vickers hardness at a position 1 mm from the surface in the plate thickness direction is 210 or less, the maximum value of the Vickers hardness in the maximum hardness test measured based on JIS Z 3101:1990 is 210 or less, the tensile strength is 490 MPa or more, and the absorbed energy in a Charpy impact test at -40°C is 47 J or more. PcmN=C+Si / 30+Mn / 20+Cu / 20+Ni / 60+Cr / 20+Mo / 15+Nb / 2+V / 3+23×B...Formula (1) In formula (1), the element symbols indicate the content (mass%) of each element contained in the steel plate, and elements that are not contained are represented as 0. The thickness of the steel plate of the present invention is not particularly limited, but is preferably 6 mm or more, more preferably 8 mm or more, from the viewpoint of use as a liquefied ammonia tank, and is preferably 60 mm or less, more preferably 40 mm or less.

[0015] (1)Chemical composition First, the reasons for limiting the chemical composition of the steel plate will be explained. Note that "%" for the content of each element means "mass %."

[0016] C: 0.010% or more and less than 0.050% C is an element effective in increasing strength, and for this purpose, the C content must be 0.010% or more. On the other hand, in terms of stress corrosion cracking resistance and weld cold cracking resistance, the upper limit of the C content is less than 0.050%. The preferred range of the C content is 0.02% or more and less than 0.050%.

[0017] Si: 0.01 to 0.55% Since Si is an effective element for deoxidation, it is added to improve the yield of Al, which is also a powerful deoxidizer. Si is also effective in increasing strength. To obtain these effects, the Si content must be 0.01% or more. The Si content is preferably 0.03% or more. However, if the Si content exceeds 0.55%, the toughness of the weld heat-affected zone decreases and cold cracking during welding is promoted, so the upper limit of the Si content is set to 0.55%. The upper limit of the Si content is preferably 0.40%.

[0018] Mn: 1.00 to 1.95% Mn is an element effective in increasing strength, and to obtain this effect, the Mn content must be 1.00% or more. However, if the Mn content exceeds 1.95%, toughness and weldability deteriorate, so the upper limit of the Mn content is set to 1.95%. To ensure higher performance in all of strength, toughness, and weldability, the upper limit of the Mn content is preferably set to 1.80%, and more preferably 1.60%.

[0019] P:0.030% or less P is an impurity element that reduces the toughness of the base material and the weld heat-affected zone. If the P content exceeds 0.030%, the desired toughness cannot be obtained, so the upper limit of the P content is set to 0.030%. The upper limit of the P content is preferably 0.025%. Although there is no particular lower limit, excessive reduction of P significantly increases the cost of refining, so the preferred lower limit of the P content is set to 0.001%.

[0020] S: 0.010% or less S is an impurity element that combines with Mn to form MnS, which reduces the toughness and through-thickness tensile performance of the base material and weld heat-affected zone. If the S content exceeds 0.010%, the desired toughness and through-thickness tensile performance cannot be obtained, so the upper limit of the S content is set to 0.010%. The upper limit of the S content is preferably 0.006%, and more preferably 0.003%. Although there is no particular lower limit, excessive reduction of S significantly increases the cost of refining, so the preferred lower limit of the S content is 0.001%.

[0021] Al: 0.001 to 0.060% Since Al has a strong deoxidizing effect and is also effective in refining crystal grains, the Al content must be 0.001% or more. The Al content is preferably 0.005% or more. However, since an excessive amount of Al reduces toughness, etc., the Al content is set to 0.060% or less. The Al content is preferably 0.040% or less.

[0022] Nb: 0.006 to 0.090% In its solid solution state, Nb improves the hardenability of steel and increases its strength. It also preserves and accumulates dislocations introduced by rolling, and these accumulated dislocations significantly refine the lath length, which is the grain size unit of bainite. Therefore, Nb is an essential element, and is also essential for ensuring the strength of welded joints. For this reason, the Nb content is set to 0.006% or more, preferably 0.010% or more. However, if the Nb content exceeds 0.090%, not only does it reduce the toughness of the weld heat-affected zone, but it may also cause cracks on the surface of the slab during continuous casting. Therefore, the upper limit of the Nb content is set to 0.090%. The preferred upper limit of the Nb content is 0.060%.

[0023] N: 0.001 to 0.006% Since the presence of N reduces the toughness of the base metal and the weld heat-affected zone, a low content is preferable. Since a N content exceeding 0.006% significantly reduces toughness, the N content is set to 0.006% or less. On the other hand, reducing the N content to less than 0.001% requires significantly high refining costs, so the lower limit of the N content is set to 0.001%. The N content is preferably 0.001 to 0.005%.

[0024] PcmN: 0.11 to 0.17 PcmN=C+Si / 30+Mn / 20+Cu / 20+Ni / 60+Cr / 20+Mo / 15+Nb / 2+V / 3+23×B...Formula (1) In formula (1), the element symbols indicate the content (mass%) of each element contained in the steel plate, and elements that are not contained are represented as 0. Even if the chemical composition is as described above, if the parameter PcmN value expressed by formula (1) exceeds 0.17, the hardness of the weld heat-affected zone increases and stress corrosion cracking resistance deteriorates, so the PcmN value is set to 0.17 or less. The PcmN value is preferably 0.16 or less. On the other hand, if the PcmN value is less than 0.11, the strength is insufficient, so the PcmN value is set to 0.11 or more. The PcmN value is preferably 0.12 or more, and more preferably 0.13 or more.

[0025] The steel plate of the present invention may further contain one or more of the following chemical compositions. Cu: 0.50% or less, Ni: 0.30% or less, Cr: 0.50% or less, Mo: 0.50% or less, V: 0.08% or less, Ti: 0.05% or less, B: 0.0030% or less, Ca: 0.006% or less, Mg: 0.006% or less

[0026] Cu: 0.50% or less Cu is effective in increasing strength, so it may be added when particularly high strength is desired. From the viewpoint of increasing strength, when Cu is contained, the lower limit of the Cu content is preferably 0.05%. The lower limit of the Cu content is more preferably 0.10%. However, if the Cu content exceeds 0.50%, the toughness may decrease and scratches may occur on the surface of the steel plate, so when Cu is contained, the Cu content is set to 0.50% or less. The Cu content is preferably 0.30% or less.

[0027] Ni: 0.30% or less Ni is effective in improving toughness and preventing cracking in steel materials containing Cu, so it is preferable to add it, especially when Cu is included. When Ni is included, the lower limit of the Ni content is preferably 0.05%. On the other hand, if Ni is included in an amount exceeding 0.30%, the stress corrosion cracking resistance of the thick steel plate (base metal) is deteriorated, and when a welded joint is obtained using the thick steel plate, Ni dissolves in the weld metal due to dilution, thereby reducing the stress corrosion cracking resistance of the weld metal. Therefore, when Ni is included, the upper limit of the Ni content is set to 0.30%. The upper limit of the Ni content is preferably 0.15%.

[0028] Cr:0.50% or less Cr improves hardenability and is effective in increasing strength, so it is advisable to add it when the plate thickness is large or when higher strength is required. When Cr is contained, the lower limit of the Cr content is preferably 0.05%, more preferably 0.10%. On the other hand, if the Cr content exceeds 0.50%, when a welded joint is obtained using the thick steel plate, the toughness of the weld heat affected zone is deteriorated. Therefore, when Cr is contained, the upper limit of the Cr content must be 0.50%. The upper limit of the Cr content is preferably 0.30%.

[0029] Mo: 0.50% or less Mo improves hardenability and is effective in increasing strength, so it is advisable to add it when the plate thickness is large or when higher strength is required. When Mo is contained, the lower limit of the Mo content is preferably 0.05%, more preferably 0.10%. On the other hand, if the Mo content exceeds 0.50%, when a welded joint is obtained using the thick steel plate, the toughness of the weld heat affected zone is deteriorated, so the upper limit of the Mo content must be 0.50%. The upper limit of the Mo content is preferably 0.30%.

[0030] V:0.08% or less V improves the strength of the base material through precipitation strengthening. From the viewpoint of base material strength, when V is contained, the lower limit of the V content is preferably set to 0.01%. On the other hand, if the V content exceeds 0.08%, when a welded joint is obtained using thick steel plates, the toughness of the base material and the weld heat affected zone deteriorates. Therefore, when V is contained, the upper limit of the V content is set to 0.08%. The upper limit of the V content is preferably 0.05%.

[0031] Ti: 0.05% or less Ti not only increases the strength of steel through solid solution strengthening and precipitation strengthening, but also inhibits the growth of austenite grains during slab heating. This effectively reduces the lath length of bainite grains in thick steel plates, which are difficult to refine by rolling alone, thereby improving toughness. Furthermore, when thick steel plates are used to form welded joints, Ti prevents grain coarsening in the heat-affected zone (HAZ), preventing a decrease in hardness. This effectively improves the toughness and prevents cracking in the HAZ. Furthermore, in Nb-containing steels, trace amounts of Ti are effective in suppressing cracking on the surface of continuously cast slabs, which is promoted by Nb. From this perspective, when Ti is contained, the Ti content is preferably 0.01% or more. However, if the Ti content exceeds 0.05%, toughness deteriorates due to precipitation embrittlement. Therefore, when Ti is contained, the upper limit of the Ti content is set to 0.05%. The upper limit of the Ti content is preferably 0.03%.

[0032] B: 0.0030% or less Since B is effective in improving hardenability and the resulting increase in strength, it is added when an increase in plate thickness or strength is required. When B is contained, the lower limit of the B content is preferably set to 0.0006%. However, if the B content exceeds 0.0030%, the weldability and toughness deteriorate, so when B is contained, the upper limit of the B content must be set to 0.0030%. The upper limit of the B content is preferably 0.0020%.

[0033] Ca: 0.006% or less Ca spheroidizes MnS, thereby improving the toughness of the base material, the toughness of the weld heat-affected zone, and the tensile performance in the thickness direction when a welded joint is obtained from thick steel plates. When Ca is contained, the lower limit of the Ca content is preferably 0.001%. On the other hand, if the Ca content exceeds 0.006%, the Ca clusters, which not only deteriorates the toughness but also the internal quality. Therefore, when Ca is contained, the upper limit of the Ca content is set to 0.006%. The upper limit of the Ca content is preferably 0.004%.

[0034] Mg: 0.006% or less Mg spheroidizes MnS, and when a welded joint is obtained from a thick steel plate, it improves the toughness of the base material, the toughness of the weld heat-affected zone, and the tensile performance in the thickness direction. When Mg is contained, the lower limit of the Mg content is preferably 0.001%. On the other hand, if the Mg content exceeds 0.006%, the Mg clusters, which not only deteriorates the toughness but also the internal quality. Therefore, when Mg is contained, the upper limit of the Mg content is set to 0.006%. The upper limit of the Mg content is preferably 0.004%.

[0035] In the steel plate, the balance other than the above-mentioned chemical composition consists of Fe and unavoidable impurities.

[0036] (2) Tissue morphology, mechanical properties Area fraction of bainite structure from the surface of the steel plate to 1 / 4 to 3 / 4 of the thickness in the plate thickness direction: 90% or more The present invention aims to produce a welded joint having a tensile strength of 490 MPa or more. To achieve this, the steel plate must contain 90% or more of a bainite structure from the surface to 1 / 4 to 3 / 4 of the thickness in the plate thickness direction, in order to obtain the desired strength. Therefore, the area fraction of the bainite structure is set to 90% or more. The area fraction of the bainite structure is preferably 95% or more. The area fraction of the bainite structure may be 100%. It is preferable that the microstructure fraction be the same in regions other than the 1 / 4 to 3 / 4 thickness from the surface of the steel plate in the thickness direction. However, in this region, structures other than bainite may form due to surface decarburization or air cooling between hot rolling and the start of accelerated cooling. Since this has little effect on strength, it is acceptable for the microstructure fraction to be non-equivalent. Note that bainite also includes cementite and island martensite present between the bainite laths, but does not include cementite, ferrite, or martensite outside the bainite laths. These can be distinguished by taking a sample from the base material, mirror-polishing it, and etching it with 4 to 5 vol% nital, and then observing it with an optical microscope at a magnification of approximately 200 to 500 times.

[0037] Vickers hardness at a position 1 mm from the surface of the thick steel plate in the thickness direction: 210 or less The present invention aims to produce a thick steel plate with excellent resistance to stress corrosion cracking caused by liquefied ammonia, and further to produce a welded joint obtained by welding this thick steel plate as a base material. High surface hardness of the base material makes stress corrosion cracking more likely to occur, and if the Vickers hardness exceeds 210, stress corrosion cracking caused by liquefied ammonia cannot be suppressed, which may lead to failure during tank operation. Therefore, the upper limit of the Vickers hardness at a position 1 mm from the surface in the plate thickness direction is set to 210. The upper limit of the Vickers hardness is preferably 200. While the position closer to the surface is preferred for hardness evaluation, the position 1 mm from the surface in the plate thickness direction is selected as a location where the Vickers test can be performed stably and accurate measurement values ​​can be evaluated. The Vickers test is performed in accordance with JIS Z 2244:2020.

[0038] Tensile strength: 490 MPa or more The present invention aims to manufacture welded joints suitable for large ammonia tanks of around 30,000 tons. To achieve this, a base metal strength of 490 MPa or more is required, so the lower limit of the tensile strength of the steel plate is set to 490 MPa. The tensile strength of the steel plate is preferably 570 MPa or more, and more preferably 610 MPa or more. The tensile strength is obtained by a tensile test carried out in accordance with JIS Z 2241:2022.

[0039] Absorbed energy in Charpy impact test of thick steel plate: 47J or more at -40℃ Ammonia liquefies at -35°C under normal pressure. Therefore, the low-temperature toughness of steel must be lower than -35°C, so the Charpy impact test is conducted at -40°C. The test temperature is preferably -50°C, more preferably -60°C. The absorbed energy is 47J or more, as specified in JIS G 3115:2016, the main standard used for high-strength tanks, and is the average value of three test pieces. Preferably, the average absorbed energy is 47J or more, with each test piece being 27J or more. While the Charpy impact test method is not specifically specified, the same method as specified in JIS G 3115:2016, the main standard used for high-strength tanks, is usually used, with samples taken at 1 / 4 the thickness of the steel plate, perpendicular to the rolling direction. The absorbed energy in the Charpy impact test is measured in accordance with JIS Z 2242:2018.

[0040] The maximum value of the highest hardness test measured based on JIS Z 3101:1990 is Vickers hardness: 210 or less When welded joints are obtained using thick steel plates, investigations into the liquefied ammonia stress corrosion cracking in the weld heat affected zone revealed that the cracking can be prevented by limiting the maximum hardness of the weld heat affected zone to 210 or less in Vickers hardness, and therefore the upper limit of the maximum hardness of the weld heat affected zone is set to 210 in Vickers hardness. The maximum hardness of this weld heat affected zone is preferably 200 or less. Furthermore, when the maximum hardness of the weld heat affected zone generated by various types of welding applied to actual welded joints was compared with the maximum hardness evaluated by JIS Z 3101:1990, it was found that JIS Z 3101:1990 is stricter, so the maximum hardness of the weld heat affected zone of clad materials will be evaluated by JIS Z 3101:1990. Note that, because the maximum hardness evaluated by JIS Z 3101:1990 is affected by the chemical composition of the steel material, it may be evaluated using another steel plate as long as the molten steel is the same (for example, if the product thickness is 9 mm and testing is not possible, the test may be performed on a thick steel plate of 20 mm or more produced from the same molten steel).

[0041] (3) Manufacturing method of thick steel plate Next, the method for manufacturing a steel plate according to the present invention will be described. In the method for producing a steel plate of the present invention, when hot rolling a slab having the above-mentioned chemical composition, the slab is heated to 1000 to 1250°C, and is rolled at a cumulative reduction rate of 50% or more at 930°C or less, and then the hot rolling is completed at a temperature of Ar3 point + 10°C or more, The accelerated cooling is started at an Ar3 point or higher, and the average cooling rate on the steel plate surface from 680°C to 600°C is 10 to 80°C / s. Accelerated cooling is then performed at an accelerated cooling stop temperature of 100 to 600°C to produce a thick steel plate. Unless otherwise specified, the temperatures described below refer to the surface temperatures of the slab or steel plate.

[0042] Heating temperature: 1000~1250℃ If the heating temperature is less than 1000°C, the crystal grains before rolling become too fine and Nb does not dissolve, making it impossible for the base material to obtain the desired strength, so the lower limit of the heating temperature is set to 1000°C. If the heating temperature exceeds 1250°C, the crystal grains before rolling become too coarse, reducing the toughness of the base material, so the upper limit of the heating temperature is set to 1250°C. The heating temperature is preferably 1050 to 1200°C.

[0043] Cumulative reduction rate at 930°C or less in hot rolling: 50% or more The presence of high-density dislocations is desirable to nucleate ferrite from within austenite grains and suppress the growth of bainite laths. To achieve this, rolling is performed with a cumulative reduction of 50% or more in the partial recrystallization and non-recrystallization temperature ranges of austenite. When the Nb-added steel of the present invention is produced at the above heating temperature, the upper limit of the partial recrystallization range is 930°C, so the upper limit is set to 930°C. The larger the reduction, the better, but a cumulative reduction of 50% or more is necessary to obtain the desired strength and toughness, so the lower limit of the cumulative reduction is set to 50%. The lower limit of the cumulative reduction is preferably 55%, more preferably 60%.

[0044] Rolling end temperature: Ar3 point + 10℃ or more The steel plate manufactured in accordance with the present invention has various base material composition restrictions in order to ensure a strength of 490 MPa class while reducing the hardness of the weld heat-affected zone when a welded joint is obtained. Therefore, it is necessary to select hot rolling conditions that facilitate ensuring strength. Since the desired strength cannot be obtained if the rolling end temperature is below the Ar3 point + 10°C, the lower limit of the rolling end temperature is set to the Ar3 point + 10°C. While there is no particular upper limit, the rolling end temperature is preferably 930°C or lower, and more preferably 910°C or lower. The Ar3 point (°C) is calculated using the formula based on the chemical composition of the steel: Ar3 = 910-310C-80Mn-20Cu-55Ni-15Cr-80Mo (°C) (wherein the formula, the element symbols indicate the content (mass%) of each element in the steel, and elements that are not contained are set to 0). As the hot rolling conditions of the present invention, the cumulative reduction rate at 930°C or less and the rolling end temperature are specified as described above, but other conditions such as the reduction rate and temperature are not particularly limited and may be set appropriately based on publicly known information.

[0045] Accelerated cooling start temperature: Ar3 point or higher The steel plate manufactured in accordance with the present invention has various base material components restricted to reduce the hardness of the weld heat-affected zone while maintaining a tensile strength of 490 MPa. Therefore, it is necessary to select accelerated cooling conditions that facilitate maintaining strength. If the accelerated cooling start temperature is below the Ar3 point, a large amount of ferrite forms, preventing the desired strength from being achieved. Therefore, the lower limit of the accelerated cooling start temperature is set to the Ar3 point. While there is no particular upper limit, the accelerated cooling start temperature is preferably 900°C or lower, and more preferably 850°C or lower.

[0046] Accelerated cooling rate: Average cooling rate on the surface of the steel plate from 680°C to 600°C: 10 to 80°C / s The cooling rate in accelerated cooling affects the surface hardness and strength. After examining the definition of a cooling rate highly correlated with surface hardness, it was found that the average cooling rate defined from 680°C to 600°C on the surface of the steel plate is highly correlated with surface hardness. As mentioned above, in the present invention, it is important that the hardness of the surface layer of the steel plate is not too high in order to prevent the occurrence of stress corrosion cracking. Here, in the present invention, the steel plate surface and its vicinity are assumed to be bainite. After extensive investigation into factors determining the hardness of this bainite surface and its vicinity, it was found that there is a high correlation with the cooling rate when passing through the bainite transformation point during accelerated cooling. The bainite transformation point of the steel plate of the present invention is in the range of 600°C to 680°C. Therefore, the inventors have come to specify the average cooling rate in this temperature range, i.e., from 680°C to 600°C. Since the desired strength cannot be obtained if the average cooling rate is less than 10°C / s, the lower limit of the cooling rate is set to 10°C / s. The lower limit of the average cooling rate is preferably 20°C / s. On the other hand, if the average cooling rate exceeds 80°C / s, the desired surface hardness cannot be obtained, so the upper limit of the average cooling rate is set to 80°C / s. The upper limit of the cooling rate is preferably 70°C / s, and more preferably 60°C / s. The average cooling rate is derived by heat conduction calculation from the relationship between the cooling start temperature, the cooling stop temperature, and the time required between them. Specifically, the average cooling rate is obtained by dividing the difference (°C) between the cooling start temperature and the cooling stop temperature by the cooling time (s). In the present invention, the cooling rate in a temperature range other than 680 to 600° C. is not particularly limited, and the cooling conditions may be set appropriately based on publicly known information.

[0047] Accelerated cooling stop temperature: 100~600℃ The accelerated cooling stop temperature affects the strength of the base material. If the accelerated cooling stop temperature exceeds 600°C, the desired strength cannot be obtained, so the accelerated cooling stop temperature is set to 600°C or less. If the accelerated cooling stop temperature is below 100°C, strain due to cooling increases and cracks occur after shear cutting due to hydrogen that has condensed during rolling, so the lower limit of the accelerated cooling stop temperature is set to 100°C. The accelerated cooling stop temperature is preferably 100 to 550°C. The accelerated cooling stop temperature is set to the temperature at which the temperature distribution in the plate thickness direction becomes approximately uniform due to reheating after accelerated cooling.

[0048] Tempering temperature: 500~680℃ The steel plate produced in the present invention can be tempered after the accelerated cooling is stopped. Tempering can be performed by reheating the plate by a method such as induction heating immediately after the accelerated cooling is stopped, or by cooling (air-cooling) the plate to room temperature (-5 to 50°C) and then reheating it in a heating furnace or the like. Specifically, in the present invention, the steel plate may be tempered to a temperature of 500 to 680°C immediately after the accelerated cooling is stopped, or after the accelerated cooling is stopped and the steel plate is cooled to room temperature. Here, there is no particular limitation on the term "immediately reheating", but it is preferable to reheat within 1000 seconds after the accelerated cooling is stopped. If the tempering temperature is less than 500°C, the toughness of the base material will actually deteriorate, so the lower limit of the tempering temperature is set to 500°C. The lower limit of the tempering temperature is preferably 520°C. Furthermore, if the tempering temperature exceeds 680°C, the desired strength will not be obtained, so the upper limit of the tempering temperature is set to 680°C. The upper limit of the tempering temperature is preferably 650°C. The cooling after tempering is not particularly limited, and for example, the material may be left to cool naturally without active water cooling or air cooling, that is, the material may be allowed to cool naturally until the temperature drops. [Example]

[0049] Next, the present invention will be described in more detail based on examples. The following examples are intended to illustrate preferred examples of the present invention, and the present invention is not limited to these examples in any way. First, slabs having the chemical compositions shown in Table 1 were produced by continuous casting. Next, the obtained slabs were reheated under the conditions shown in Table 2, hot-rolled, and immediately subjected to accelerated cooling. Some of the steel sheets after accelerated cooling (water cooling) were tempered.

[0050] [Table 1]

[0051] [Table 2]

[0052] Material test pieces were taken from the manufactured steel plates and tested, and the results are summarized in Table 3.

[0053] [Table 3]

[0054] [Vickers hardness] A sample was taken from the obtained thick steel plate at a cross section perpendicular to the rolling direction so that the test surface was at a depth of 1 mm from the surface, and after the surface of the test piece was mirror-polished, the Vickers hardness was measured in accordance with JIS Z 2244: 2020. A load of 10 kgf was used for the measurement, and the test was carried out at five points, with the maximum value being taken as the Vickers hardness.

[0055] [Tensile strength] Test specimens were taken from the obtained steel plates and processed into No. 5 tensile test specimens according to JIS Z 2241:2022. The test specimens were taken in a direction perpendicular to the rolling direction, with the longitudinal direction of the specimen perpendicular to the rolling direction. A tensile test was conducted on the test specimens to measure their tensile strength.

[0056] [Absorbed energy in Charpy impact test] Test specimens were taken from the resulting steel plates and processed into 2 mm V-notch Charpy impact test specimens according to JIS Z 2242:2018. The specimens were taken perpendicular to the rolling direction and from a position at or close to 1 / 4 of the thickness in the plate thickness direction. For steel plates with a thickness of 12 mm or more, 10 mm x 10 mm specimens were used, and for steel plates with a thickness of less than 12 mm, 10 mm x 7.5 mm specimens were used. The test was carried out at -40°C, and the absorbed energy was measured.

[0057] [Microstructure] A sample was taken from the resulting steel plate so that the cross section parallel to the rolling direction served as the observation surface. The observation surface of the sample was mirror-polished and then etched with 5 vol% nital. A 1000 μm × 1000 μm region of the etched surface was photographed using an optical microscope at 200x magnification, and the area fraction of the bainite structure was calculated from the surface to the 1 / 4 to 3 / 4 thickness in the plate thickness direction by image processing. Specifically, the area fraction of the bainite structure was calculated at three locations in the plate thickness direction (1 / 4, 1 / 2, and 3 / 4 thickness) by image processing, and the average value was calculated and listed in Table 3. Note that bainite includes cementite and island martensite present between the bainite laths, but does not include cementite, ferrite, or martensite outside the bainite laths. Image processing was performed by tracing the areas of the microstructure other than the bainite structure in black on the photographed image and calculating the ratio of the area of ​​the black portion to the area of ​​the entire image.

[0058] [Maximum hardness] The maximum hardness test, measured based on JIS Z 3101:1990, was conducted on the produced steel plate itself or scrap material taken from another steel plate produced from the same molten steel. The welding material was LB26 manufactured by Kobe Steel, Ltd., and the test was conducted at a welding current of 170±10A and a welding speed of 150±10mm / s.

[0059] [Liquid ammonia stress corrosion cracking test for thick steel plate] Stress corrosion tests using liquefied ammonia were performed using a four-point bending test with a constant load applied to the solution. 5-mm-thick specimens were taken from thick steel plates, positioned 5 mm from the surface of the plate in the thickness direction. Next, the specimens were subjected to four-point bending to impart 90% of the base metal's yield stress determined in the tensile test. The specimens were then placed in an immersion cell and immersed in a saturated solution of liquefied NH3 + 5 vol% NH4CO2NH2 at 20°C for one month. The specimens were then visually inspected. Those with cracks were deemed "fail" (in Table 3, marked "crack"), and those without cracks were deemed "pass" (in Table 3, marked "no crack").

[0060] [Liquid ammonia stress corrosion cracking test of weld heat affected zone] The stress corrosion test using liquefied ammonia was carried out using a four-point bending test with a constant load applied to the solution. Test specimens were prepared by applying shielded metal arc welding to the surface of the thick steel plate, removing the excess weld, and taking a 5mm thick test specimen from a position 5mm from the surface of the thick steel plate in the thickness direction. The specimens were then processed under conditions where the center of the back side of the four-point bending test was the weld heat-affected zone. The welding was performed using LB26 welding material manufactured by Kobe Steel, Ltd., with a welding current of 170±10 A and a welding speed of 150±10 mm / s. Next, the test was performed by applying 90% of the base metal's yield stress determined in the tensile test to the test specimen by four-point bending. The test specimen was then placed in an immersion cell and immersed in a saturated solution of liquefied NH3 + 5 vol% NH4CO2NH2 at 20°C for one month. After visual inspection, the test specimens were evaluated as failing if cracks were observed in the weld heat-affected zone (marked "cracks" in Table 3), and passing if no cracks were observed in the weld heat-affected zone (marked "no cracks" in Table 3).

Claims

1. In mass%, C: 0.010% or more and less than 0.050%; Si: 0.01 to 0.55%, Mn: 1.00-1.95%, P: 0.030% or less, S: 0.010% or less, Al: 0.001-0.060%, Nb: 0.006-0.090%, N: 0.001-0.006% and has a chemical composition in which PcmN represented by the following formula (1) satisfies 0.11 to 0.17, with the remainder consisting of Fe and unavoidable impurities, The area fraction of bainite structure from the surface to 1 / 4 thickness to 3 / 4 thickness in the plate thickness direction is 90% or more, The Vickers hardness at a position 1 mm from the surface in the plate thickness direction is 210 or less, The maximum value of the maximum hardness test measured based on JIS Z 3101:1990 is 210 or less in Vickers hardness, The tensile strength is 490 MPa or more, A thick steel plate having an absorbed energy of 47 J or more in a Charpy impact test at -40°C. PcmN=C+Si / 30+Mn / 20+Cu / 20+Ni / 60+Cr / 20+Mo / 15+Nb / 2+V / 3+23×B...Formula (1) In formula (1), the element symbols indicate the content (mass%) of each element contained in the steel plate, and elements that are not contained are represented as 0.

2. The chemical composition further comprises, in mass %, Cu: 0.50% or less, Ni: 0.30% or less, Cr: 0.50% or less, Mo: 0.50% or less, V: 0.08% or less, Ti: 0.05% or less, B: 0.0030% or less, Ca: 0.006% or less, Mg: 0.006% or less The steel plate according to claim 1, comprising one or more selected from the group consisting of:

3. When hot rolling a slab having the chemical composition according to claim 1 or 2, the slab is heated to 1000 to 1250°C, After rolling at a temperature of 930°C or less with a cumulative reduction rate of 50% or more, The hot rolling is completed at a temperature of Ar3 point + 10°C or higher. At an accelerated cooling start temperature of Ar3 point or higher, the average cooling rate from 680 ° C. to 600 ° C. on the steel plate surface is 10 to 80 ° C. / s, A method for manufacturing a thick steel plate, in which the thick steel plate is obtained by performing accelerated cooling at an accelerated cooling stop temperature of 100 to 600°C.

4. The method for producing a thick steel plate according to claim 3, wherein the steel plate is tempered to a temperature of 500 to 680°C immediately after the accelerated cooling is stopped or after the accelerated cooling is stopped and the steel plate is cooled to room temperature.

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