Steel sheet and method for producing same

Abstract

Provided is a steel sheet having high strength, and having good extremely-low-temperature toughness at -253°C or colder at not only a base material but also a portion affected by welding heat (CGHAZ, FGHAZ).　This steel sheet has a component composition of, by mass%, 0.01 to 0.15% C, 0.01 to 0.50% Si, 0.05 to 1.50% Mn, 10.0 to 15.5% Ni, 0.03% or less P, 0.005% or less S, 0.0010 to 0.0080% N, and 0.008 to 0.10% Al, and Fe and inevitable impurities as the balance. The structure of the steel sheet is primarily tempered martensite or tempered martensite and bainite, and the average equivalent circle diameter of the Ni-concentrated region is 1.0 μm or smaller.

Description

Steel plate and its manufacturing method

[0001] The present invention relates to a steel plate and a manufacturing method thereof, and more particularly to a steel plate having high strength and excellent cryogenic toughness and a manufacturing method thereof.The steel plate of the present invention is suitable for use at cryogenic temperatures, and can be suitably used for structures to be used in cryogenic environments, such as liquefied gas storage tanks for ships and on land.

[0002] When hot-rolled steel plates are used in structures such as liquefied gas storage tanks, the environment in which they are used is extremely low, and so they are required to have not only excellent strength but also excellent toughness at extremely low temperatures (cryogenic toughness). For example, when hot-rolled steel plates are used in liquefied hydrogen storage tanks, they must have excellent toughness at extremely low temperatures of −253°C or lower, which is the boiling point of liquefied hydrogen. If the cryogenic toughness of the steel plate is poor, there is a risk that the safety of the cryogenic storage structure cannot be maintained, so there is a strong demand for improved cryogenic toughness of the steel plates used. To meet this demand, γ (austenitic) stainless steel plates have traditionally been used.

[0003] However, γ-based materials have low strength, and in recent years, there has been a demand for higher strength materials in order to increase the size of tanks.

[0004] An example of a high-strength material that can be used at low temperatures is 9% Ni steel, which is used in liquefied natural gas tanks (see, for example, Patent Document 1).

[0005] International Publication No. 2020 / 184162

[0006] However, even with 9% Ni steel, good toughness of the base metal and weld heat affected zone at extremely low temperatures of -253°C or less, such as in liquefied hydrogen, is not guaranteed, and it has not yet been put to practical use.

[0007] The present invention has been made in view of the above circumstances, and has an object to provide a steel plate which has high strength and excellent cryogenic toughness at −253° C. or less not only in the base material but also in the weld heat affected zone (CGHAZ, FGHAZ).

[0008] The present inventors have conducted extensive research on Ni-containing steels. As a result, when the cryogenic temperature toughness of 9% Ni steel was evaluated by a Charpy test at −269°C, it was found that the toughness of the base material was good, but the toughness of the weld heat-affected zone (HAZ), particularly the CGHAZ, was poor. Here, the CGHAZ refers to a coarse-grained heat-affected zone formed in a steel plate near the fusion line during welding. The inventors have also found that the cryogenic temperature toughness of this CGHAZ can be improved by increasing the Ni content to 10.0% or more.

[0009] Furthermore, it was found that increasing the Ni content makes it easier for Ni-enriched regions to form, leading to the formation of unstable γ in the base metal and a tendency for cryogenic toughness to deteriorate, particularly a tendency for the Charpy absorbed energy to decrease. It was also found that by setting the Ni content to 15.5% or less, setting the finishing temperature in hot rolling to 850°C or higher, setting the heating rate in the two-phase region during heat treatment for quenching to 0.2°C / s or higher, and setting the tempering temperature to 650°C or lower, the size of the Ni-enriched regions can be optimized, and excellent cryogenic toughness can be obtained even in the base metal. In addition, it was found that optimizing the size of the Ni-enriched regions also improves the cryogenic toughness of the FGHAZ. Here, FGHAZ refers to a fine-grained heat-affected zone (FGHAZ) formed near the base metal during welding.

[0010] The present invention was completed based on the above findings and through further investigation, and its gist is as follows: [1] A steel sheet having a chemical composition containing, by mass%, C: 0.01 to 0.15%, Si: 0.01 to 0.50%, Mn: 0.05 to 1.50%, Ni: 10.0 to 15.5%, P: 0.03% or less, S: 0.005% or less, N: 0.0010 to 0.0080%, and Al: 0.008 to 0.10%, with the balance being Fe and unavoidable impurities, wherein the structure is mainly tempered martensite or tempered martensite and bainite, and the average circle equivalent diameter of the Ni-enriched region is 1.0 μm or less. [2] The steel sheet according to [1], wherein the chemical composition further contains, by mass%, one or more groups selected from the following group a and group b: Group a: one or more selected from Cr: 1.00% or less, Mo: 0.50% or less, Cu: 0.40% or less, Nb: 0.05% or less, V: 0.05% or less, Ti: 0.03% or less, and B: 0.0030% or less Group b: one or more selected from Ca: 0.007% or less, REM: 0.010% or less, Mg: 0.0070% or less, and Zr: 0.0050% or less [3] A steel material having the component composition described in [1] or [2] above is heated to 900 ° C. or more and 1300 ° C. or less, and then hot-rolled at a surface temperature of 850 ° C. or more to obtain a steel plate, and then The steel plate is quenched by heating the steel plate to a temperature at a position 1 / 4 of the plate thickness in a temperature range of from the Ac1 point to the Ac3 point inclusive at an average heating rate of 0.2°C / s or more, to a heating temperature of from (Ac3 point + 50°C) to 1000°C inclusive, followed by cooling at an average cooling rate of 3°C / s or more in a temperature range of from 600°C to 300°C inclusive, to a cooling stop temperature of 200°C or less, and then tempered to a temperature range of from 400°C to 650°C inclusive.

[0011] According to the present invention, it is possible to provide a steel plate that has high strength and excellent cryogenic toughness at -253°C or lower not only in the base material but also in the weld heat affected zone (CGHAZ, FGHAZ). By using the steel plate of the present invention in steel structures used in low-temperature and cryogenic environments, such as liquefied gas storage tanks, for example, LNG tanks and liquefied hydrogen tanks, the safety of the steel structures can be improved, bringing about significant industrial benefits.

[0012] Hereinafter, embodiments of the present invention will be specifically described. Note that the following description shows preferred embodiments of the present invention, and the present invention is not limited thereto.

[0013] [Composition] The steel sheet of the present invention has a predetermined composition. Furthermore, it is preferable that the steel material used to manufacture the steel sheet of the present invention also has the above-mentioned predetermined composition. Hereinafter, each element contained in this composition will be explained. In addition, unless otherwise specified, in this specification, "%" as a unit of the content of each element means "mass%".

[0014] C: 0.01 to 0.15% C is an element that has the effect of improving the strength of steel plate. To achieve this effect, the C content is set to 0.01% or more. The C content is preferably 0.03% or more. On the other hand, if the C content exceeds 0.15%, the toughness of the steel plate (base metal and weld heat affected zone (CGHAZ, FGHAZ)) at cryogenic temperatures of -253°C or less decreases. Therefore, the C content is set to 0.15% or less. The C content is preferably 0.12% or less. Hereinafter, toughness at cryogenic temperatures of -253°C or less will be simply referred to as cryogenic temperature toughness.

[0015] Si: 0.01 to 0.50% Si is also an element that acts as a deoxidizer. To achieve this effect, the Si content is set to 0.01% or more. The Si content is preferably 0.03% or more. On the other hand, if the Si content exceeds 0.50%, the cryogenic toughness of the base metal and the weld heat affected zone (CGHAZ, FGHAZ) decreases. Therefore, the Si content is set to 0.50% or less. The Si content is preferably 0.30% or less.

[0016] Mn: 0.05 to 1.50% Mn is an element that improves the hardenability of steel and is effective in increasing the strength of steel plate. To achieve this effect, the Mn content is set to 0.05% or more. The Mn content is preferably 0.10% or more. On the other hand, if the Mn content exceeds 1.50%, unstable γ (austenite) is likely to be formed, which may reduce the cryogenic toughness of the base material and weld heat-affected zone (CGHAZ, FGHAZ), so the Mn content is limited to 1.50% or less. The Mn content is preferably 1.0% or less.

[0017] Ni: 10.0 to 15.5% Ni is an element that is extremely effective in improving the cryogenic temperature toughness of steel plate. To obtain the desired cryogenic temperature toughness, the Ni content is set to 10.0% or more. The Ni content is preferably 11.0% or more. On the other hand, a Ni content exceeding 15.5% leads to the formation of unstable γ, which actually reduces the cryogenic temperature toughness. Therefore, the Ni content is set to 15.5% or less.

[0018] P: 0.03% or less P is an inevitable impurity and a harmful element that adversely affects the cryogenic toughness of steel sheets. For example, in order to obtain sound cryogenic toughness of the base material and weld heat-affected zones (CGHAZ, FGHAZ) when steel sheets are welded to form a welded structure, it is preferable to reduce the P content as much as possible. Therefore, the P content is set to 0.03% or less. Furthermore, from the viewpoint of cryogenic toughness, the lower the P content, the better, so the lower limit is not particularly limited and may be 0%. However, even in this case, P is allowed to be contained as an inevitable impurity. On the other hand, excessive reduction of P causes an increase in costs, so from the viewpoint of cost, the lower limit of the P content is preferably set to 0.001%.

[0019] S: 0.005% or less S forms MnS in the steel and significantly deteriorates the cryogenic toughness of the base material and the weld heat-affected zone (CGHAZ, FGHAZ). Therefore, the upper limit is set to 0.005%, and it is desirable to reduce the content as much as possible. The S content is preferably set to 0.002% or less. On the other hand, the lower the S content, the better, so the lower limit is not particularly limited and may be 0%. However, even in this case, it is allowed to be contained as an unavoidable impurity. On the other hand, excessive reduction of S causes an increase in costs, so from a cost perspective, the lower limit of the S content is preferably set to 0.0005%.

[0020] N: 0.0010 to 0.0080% N forms precipitates in steel, and if the N content exceeds 0.0080%, it will cause a decrease in the cryogenic toughness of the base material. Therefore, the N content is set to 0.0080% or less. The N content is preferably 0.0060% or less. On the other hand, excessive denitrification leads to an increase in costs, so the N content is set to 0.0010% or more. The N content is preferably 0.0020% or more.

[0021] Al: 0.008 to 0.10% Al is an element that acts as a deoxidizer. If the Al content is less than 0.008%, the effect as a deoxidizer is poor. Therefore, the Al content is set to 0.008% or more. The Al content is preferably set to 0.02% or more. On the other hand, if the Al content exceeds 0.10%, the cleanliness of the steel is impaired, and the toughness, particularly the cryogenic toughness, of the base metal and weld heat affected zone (CGHAZ, FGHAZ) decreases. Therefore, the Al content is set to 0.10% or less. The Al content is preferably set to 0.05% or less.

[0022] In one embodiment of the present invention, the composition of the alloy may be such that the balance is composed of Fe and inevitable impurities in addition to the predetermined amounts of the elements described above. The inevitable impurities are impurities that are inevitably mixed in from raw materials, the manufacturing process, manufacturing equipment, etc., and are allowed to be present to the extent that they do not impair the objectives of the present invention. Examples of raw materials include iron ore, reduced iron, and scrap.

[0023] In another embodiment of the present invention, the above-mentioned chemical composition may optionally contain one or more elements selected from the following group a and group b: Group a: one or more elements selected from Cr: 1.00% or less, Mo: 0.50% or less, Cu: 0.40% or less, Nb: 0.05% or less, V: 0.05% or less, Ti: 0.03% or less, and B: 0.0030% or less; Group b: one or more elements selected from Ca: 0.007% or less, REM: 0.010% or less, Mg: 0.0070% or less, and Zr: 0.0050% or less.

[0024] Cr: 1.00% or less Cr is an element that can improve the strength of a steel plate without significantly impairing the cryogenic toughness. However, if the Cr content exceeds 1.00%, the cryogenic toughness of the steel plate decreases. Therefore, when Cr is contained, the Cr content is set to 1.00% or less. The Cr content is preferably 0.80% or less. On the other hand, although there is no particular limitation on the lower limit of the Cr content, when Cr is contained, in order to obtain the above-mentioned effect, the Cr content is preferably 0.01% or more, and more preferably 0.30% or more.

[0025] Mo: 0.50% or less Like Cr, Mo is an element that can improve the strength of steel plate without significantly impairing cryogenic toughness. However, if the Mo content exceeds 0.50%, the cryogenic toughness actually decreases. Therefore, when Mo is contained, the Mo content is set to 0.50% or less. The Mo content is preferably 0.30% or less, and more preferably 0.25% or less. On the other hand, although there is no particular lower limit for the Mo content, when Mo is contained, in order to obtain the above-mentioned effect, the Mo content is preferably 0.01% or more, and more preferably more than 0.10%.

[0026] Cu: 0.40% or less Cu is an element that has the effect of increasing the strength of steel sheet by improving hardenability. However, if the Cu content exceeds 0.40%, the cryogenic toughness of the steel sheet decreases, and the surface properties of the steel (slab) after casting deteriorate. Therefore, when Cu is contained, the Cu content is set to 0.40% or less. The Cu content is preferably set to 0.30% or less. On the other hand, although there is no particular limitation on the lower limit of the Cu content, when Cu is contained, it is preferable that the Cu content be 0.10% or more to obtain the above-mentioned effect.

[0027] Nb: 0.05% or less Nb is an effective element for increasing the strength of steel sheet through precipitation strengthening. However, if the Nb content is excessively high, the cryogenic toughness of the steel sheet decreases. Therefore, when Nb is contained, the Nb content is set to 0.05% or less. The Nb content is preferably set to 0.03% or less. On the other hand, although there is no particular lower limit for the Nb content, when Nb is contained, the Nb content is preferably set to 0.01% or more in order to obtain the above-mentioned effect.

[0028] V: 0.05% or less Like Nb, V is an effective element for increasing the strength of steel sheet by precipitation strengthening. However, if the V content exceeds 0.05%, the cryogenic toughness of the steel sheet decreases. Therefore, when V is contained, the V content is set to 0.05% or less. The V content is preferably set to 0.04% or less. On the other hand, although there is no particular lower limit for the V content, when V is contained, the V content is preferably set to 0.01% or more in order to obtain the above-mentioned effect.

[0029] Ti: 0.03% or less Ti is an element that has the effect of increasing the toughness of the weld without degrading the mechanical properties of the base material when steel plates are welded to form a welded structure. However, if the Ti content exceeds 0.03%, the toughness is actually reduced. Therefore, when Ti is contained, the Ti content is set to 0.03% or less. On the other hand, there is no particular lower limit for the Ti content, but when Ti is contained, it is preferable that the Ti content be 0.003% or more to obtain the above effect.

[0030] B: 0.0030% or less B is an element that improves hardenability when added in small amounts. However, if the B content exceeds 0.0030%, toughness deteriorates. Therefore, when B is contained, the B content is set to 0.0030% or less. On the other hand, there is no particular lower limit for the B content, but when B is contained, it is preferable that the B content be 0.0003% or more in order to effectively exert the above-mentioned effects.

[0031] Ca: 0.007% or less Ca is an element that has the effect of improving the cryogenic temperature toughness of steel plate by controlling the morphology of inclusions in steel. However, if the Ca content exceeds 0.007%, the cleanliness of the steel is impaired, reducing the cryogenic temperature toughness. Therefore, when Ca is contained, the Ca content is set to 0.007% or less. The Ca content is preferably set to 0.004% or less. On the other hand, although there is no particular lower limit for the Ca content, when Ca is contained, the Ca content is preferably set to 0.001% or more to obtain the above effect.

[0032] REM: 0.010% or less Like Ca, REM (rare earth metals) are elements that improve the cryogenic toughness of steel sheets by controlling the morphology of inclusions in steel. However, if the REM content exceeds 0.010%, the cleanliness of the steel is impaired and the cryogenic toughness decreases. Therefore, when REM is contained, the REM content is set to 0.010% or less. The REM content is preferably set to 0.008% or less. On the other hand, there is no particular lower limit for the REM content, but when REM is contained, to achieve the above-mentioned effect, the REM content is preferably set to 0.001% or more. Here, REM is a collective term for 17 elements, including 15 lanthanoid elements plus Y and Sc, and these elements can be contained alone or in combination. Note that the REM content refers to the total content of these elements.

[0033] Mg: 0.0070% or less Like Ca and REM, Mg is an element that has the effect of improving the cryogenic temperature toughness of steel plate by controlling the morphology of inclusions in steel. However, if the Mg content exceeds 0.0070%, the cleanliness of the steel is impaired and the cryogenic temperature toughness decreases. Therefore, when Mg is contained, the Mg content is set to 0.0070% or less. The Mg content is preferably set to 0.004% or less. On the other hand, although there is no particular lower limit for the Mg content, when Mg is contained, the Mg content is preferably set to 0.001% or more to obtain the above-mentioned effect.

[0034] Zr: 0.0050% or less Like Ca and Mg, Zr is an element that has the effect of improving the cryogenic temperature toughness of steel sheets by controlling the morphology of inclusions in steel. However, if the Zr content exceeds 0.0050%, the cleanliness of the steel is impaired and the cryogenic temperature toughness decreases. Therefore, when Zr is contained, the Zr content is set to 0.0050% or less. The Zr content is preferably set to 0.004% or less. On the other hand, although there is no particular limitation on the lower limit of the Zr content, when Zr is contained, the Zr content is preferably set to 0.001% or more to obtain the above-mentioned effect.

[0035] [Microstructure] The steel plate of the present invention has a structure mainly composed of tempered martensite or mainly composed of tempered martensite and bainite at a depth position of ¼ of the plate thickness t (hereinafter also referred to as ¼t) from the surface of the steel plate in the plate thickness direction. The steel plate also has a structure in which the amount of austenite is 20% or less by volume. If the austenite volume fraction exceeds 20%, the cryogenic toughness decreases. The lower limit of the austenite volume fraction is not particularly limited, and the austenite volume fraction may be 0% or more than 0%.

[0036] Here, "mainly composed" means that the main phase occupies 80% or more of the steel sheet structure. The area ratio of the main phase is preferably 90% or more, more preferably 98% or more. The upper limit of the area ratio of the main phase is not particularly limited, and the area ratio of the main phase may be 100% or less. The main phase may be a phase consisting of tempered martensite, or a phase consisting of tempered martensite and bainite. By using a structure consisting mainly of tempered martensite or tempered martensite and bainite, it is possible to obtain sufficient strength while ensuring excellent cryogenic toughness. Note that when the main phase is a phase consisting of tempered martensite and bainite, the area ratio of tempered martensite to bainite may be any.

[0037] The steel sheet of the present invention is characterized in that the average equivalent circle diameter of the Ni-enriched region is 1.0 μm or less. If the average equivalent circle diameter of the Ni-enriched region exceeds 1.0 μm, unstable γ forms in the base metal or FGHAZ, resulting in a decrease in cryogenic toughness. The average equivalent circle diameter of the Ni-enriched region is determined by the method described in the Examples.

[0038] [Mechanical Properties] (Tensile Strength) The tensile strength of the steel sheet of the present invention is preferably 690 MPa or more, more preferably 720 MPa or more. There is no particular upper limit to the tensile strength of the steel sheet of the present invention, but the tensile strength of the steel sheet of the present invention is preferably 930 MPa or less, more preferably 900 MPa or less. The tensile strength can be measured by the method described in the Examples below.

[0039] (Cryogenic Toughness) The steel plate of the present invention has good cryogenic toughness in the base material and the weld heat affected zone (CGHAZ, FGHAZ). The cryogenic toughness value of the steel plate (base material) of the present invention is Charpy absorbed energy (vE －２６９℃ ) is preferably 100 J or more in a full-size Charpy impact test. －２６９℃ In addition, the cryogenic toughness value of the weld heat affected zone (CGHAZ, FGHAZ) of the steel plate of the present invention is preferably vE －２６９℃is preferably 27J or more in a full-size Charpy impact test. －２６９℃ However, it is preferable that the cryogenic toughness value in a half-size Charpy impact test is 14 J or more. The cryogenic toughness values ​​of the base material and the weld heat affected zone of the steel plate can be measured by the method described in the examples below.

[0040] The thickness of the steel plate of the present invention is not particularly limited and can be any thickness. The thickness of the steel plate of the present invention is, for example, preferably 6 mm or more, more preferably 10 mm or more. Furthermore, the thickness of the steel plate of the present invention is, for example, preferably 50 mm or less, more preferably 40 mm or less.

[0041] [Manufacturing Method] Next, an embodiment of a method for manufacturing a steel plate according to the present invention will be described. The temperature at the 1 / 4 plate thickness position described below can be determined by heat transfer calculation from the surface temperature of the steel plate measured with a radiation thermometer, for example.

[0042] The steel sheet of the present invention can be suitably produced by sequentially carrying out the following steps (1) to (3): (1) hot rolling, (2) quenching (heat treatment), and (3) tempering.

[0043] (1) Hot Rolling First, a steel material having the above-described composition is heated to a surface temperature of 900°C or more and 1300°C or less. There are no particular limitations on the method for producing the steel material, but, for example, the steel material can be produced by melting molten steel having the above-described composition by a conventional method and casting it. Melting can be carried out by any method, such as a converter, an electric furnace, or an induction furnace. Furthermore, from the viewpoint of productivity, casting is preferably carried out by a continuous casting method, but it can also be carried out by an ingot making-blooming rolling method. As the steel material, for example, a steel slab can be used.

[0044] Here, the heating of the steel material may be carried out after the steel material obtained by a method such as casting has been cooled, or the obtained steel material may be directly heated without being cooled. If the heating temperature of the steel material is less than 900°C, the deformation resistance of the steel material is high, which increases the load on the rolling mill in the subsequent hot rolling, making it difficult to perform the hot rolling. Therefore, the heating temperature of the steel material is set to 900°C or higher. On the other hand, if the heating temperature of the steel material is higher than 1300°C, oxidation of the steel becomes significant, and loss due to removal of the oxide film caused by oxidation increases, resulting in a decrease in yield. Therefore, the heating temperature of the steel material is set to 1300°C or lower.

[0045] Next, the steel material heated as described above is subjected to hot rolling. At this time, the finishing temperature of the hot rolling (the temperature of the steel sheet immediately after final rolling) is 850°C or higher in terms of the surface temperature of the steel sheet. If the finishing temperature is lower than 850°C, the formation of Ni-enriched regions progresses, making it impossible to obtain the desired average circle equivalent diameter of the Ni-enriched regions, and the cryogenic toughness of the base material and the FGHAZ deteriorates. The finishing temperature is preferably 900°C or higher. Furthermore, the upper limit of the finishing temperature is not particularly limited, but as an example, the finishing temperature can be 1000°C or lower. Conditions other than those described above are not particularly limited, as long as the desired sheet thickness is achieved. Furthermore, cooling after hot rolling may be performed arbitrarily, for example, by air cooling or water cooling.

[0046] (2) Quenching (heat treatment) The hot-rolled steel sheet is subjected to quenching (heat treatment). This quenching is performed by heating the steel sheet at a temperature at 1 / 4 of the sheet thickness (a depth position at 1 / 4 of the sheet thickness t from the surface of the steel sheet) in the temperature range of Ac1 point to Ac3 point inclusive at an average heating rate of 0.2°C / s or more, to a heating temperature of (Ac3 point + 50°C) to 1000°C or less, and then cooling the steel sheet at an average cooling rate of 3°C / s or more in the temperature range of 600°C to 300°C or more, to a cooling stop temperature of 200°C or less.

[0047] During quenching, if the average heating rate in the temperature range from the Ac1 point to the Ac3 point is less than 0.2°C / s, the formation of Ni-enriched regions will progress, making it impossible to obtain the desired average equivalent circle diameter of the Ni-enriched regions, and will also lead to the formation of unstable γ, which will result in a decrease in the cryogenic toughness of the base material and the FGHAZ. Therefore, the average heating rate in this temperature range is set to 0.2°C / s or more. Furthermore, there is no particular upper limit to the average heating rate in this temperature range, but as an example, the average heating rate in this temperature range can be set to 1.0°C / s or less.

[0048] Furthermore, if the heating temperature is less than (Ac3 point + 50°C), unstable γ is likely to remain after quenching, and the specified average equivalent circle diameter of the Ni-enriched region cannot be obtained, resulting in a decrease in the cryogenic toughness of the base material and FGHAZ. Therefore, the heating temperature is set to (Ac3 point + 50°C) or higher. The heating temperature is preferably set to (Ac3 point + 100°C) or higher. On the other hand, if the heating temperature exceeds 1000°C, the operating load increases, so the heating temperature is set to 1000°C or lower. The heating temperature is preferably 900°C or lower.

[0049] During cooling after heating, if the average cooling rate in the temperature range from 600°C to 300°C is less than 3°C / s, it is difficult to obtain the desired transformed structure. Furthermore, the specified average equivalent circle diameter of the Ni-enriched region cannot be obtained, making it difficult to obtain sufficient strength and cryogenic toughness of the base material and FGHAZ. Therefore, the average cooling rate in this temperature range is set to 3°C / s or more. The average cooling rate in this temperature range is preferably 5°C / s or more, more preferably 7°C / s or more, and even more preferably 10°C / s or more. On the other hand, while there is no particular upper limit for the average cooling rate in this temperature range, if the average cooling rate is higher than 200°C / s, it becomes difficult to control the temperature at each position within the steel sheet, which tends to result in variations in the material properties in the sheet width direction and rolling direction. As a result, variations in material properties such as tensile properties and toughness tend to occur. Therefore, it is preferable that the average cooling rate in this temperature range be 200°C / s or less. The average cooling rate in the above temperature range is more preferably 100° C. / s or less, and even more preferably 50° C. / s or less.

[0050] Furthermore, if the cooling stop temperature during quenching is higher than 200°C, unstable residual γ is likely to be generated, and the specified average equivalent circle diameter of the Ni-enriched region cannot be obtained, resulting in a decrease in the cryogenic toughness of the base material and the FGHAZ. Therefore, the cooling stop temperature is set to 200°C or lower. By performing cooling (accelerated cooling) under these conditions, the hot-rolled steel sheet is well quenched. The cooling stop temperature is preferably 150°C or lower. Furthermore, the lower limit of the cooling stop temperature is not particularly limited, but as an example, the cooling stop temperature can be set to 50°C or higher.

[0051] The cooling process in quenching is not particularly limited and can be performed by any method. For example, air cooling and / or water cooling can be used. As water cooling, any cooling method using water (e.g., spray cooling, mist cooling, laminar cooling, etc.) can be used.

[0052] (3) Tempering Next, the quenched steel plate is tempered. The tempering temperature is 400°C or higher and 650°C or lower. If the tempering temperature is lower than 400°C, the tempering is insufficient and the cryogenic temperature toughness of the base material is reduced. If the tempering temperature exceeds 650°C, coarse Ni-enriched regions are formed, which leads to the generation of unstable γ, and the cryogenic temperature toughness of the base material and FGHAZ is reduced. The tempering temperature is preferably 450°C or higher. Furthermore, the tempering temperature is preferably 630°C or lower.

[0053] In the present invention, the Ac1 point (Ac1 transformation point) and the Ac3 point (Ac3 transformation point) can be calculated by the following formulas (1) and (2), respectively. Ac1 point (°C) = 750.8-26.6×C+17.6×Si-11.6×Mn-22.9×Cu-23×Ni+24.1×Cr+22.5×Mo-39.7×V-5.7×Ti+232.4×Nb-169.4×Al...(1) Ac3 point (°C) = 937.2-436.5×C+56×Si-19.7×Mn-16.3×Cu-26.6×Ni-4.9×Cr+38.1×Mo+124.8×V+136.3×Ti-19.1×Nb+198.4×Al+3315×B...(2) However, the element symbols in the above formulas (1) and (2) represent the content (mass %) of each element, and are set to 0 when the element is not contained.

[0054] Any heating method can be used in the tempering process as long as it can control the heating temperature as described above. One example of the heating method is furnace heating. The furnace heating is not particularly limited, and a general heat treatment furnace can be used.

[0055] After the tempering temperature is reached, the material may be held at the tempering temperature for any desired time, after which any desired cooling may be initiated. When the material is held at the tempering temperature, the holding time is not particularly limited, but is preferably 5 minutes or more.

[0056] Steel plates were produced according to the procedure described below, and their properties were evaluated. First, molten steel having the chemical composition shown in Table 1 was melted in a converter, and steel slabs (thickness: 200 mm) were produced as steel materials by continuous casting. The Ac1 point (°C) and Ac3 point (°C) calculated using the above-mentioned formulas (1) and (2), respectively, are also shown in Table 1. Blank cells in Table 1 indicate that the element was not intentionally added, and include not only cases where the element was not contained (0 mass%), but also cases where the element was unavoidably contained.

[0057]

[0058] Next, according to the conditions shown in Table 2, the obtained steel slabs were heated, hot rolled, quenched (heat treated), and tempered to obtain hot-rolled steel sheets having various thicknesses (final thicknesses).

[0059] Next, for each of the obtained steel sheets, the microstructure, the amount of residual γ, the average circle equivalent diameter of the Ni-enriched region, the tensile strength (TS), and the Charpy absorbed energy at −269°C (vE －２６９℃ The evaluation results are shown in Table 2.

[0060] [Microstructure] Test specimens for microstructure observation were taken from each steel sheet so that the observation position was the 1 / 4t position. These test specimens were embedded in resin so that the cross section perpendicular to the rolling direction was the observation surface, and mirror-polished. Next, after performing nital etching, the specimens were observed using a scanning electron microscope at magnifications of 2000x and 10000x, and images of the structure were taken. The obtained images were analyzed to identify the microstructure. In the images, austenite is a precipitated phase that appears as fine islands, and the rest is tempered martensite or bainite.

[0061] Average Equivalent Circular Diameter of Ni-Enriched Regions Test specimens for measuring the average equivalent circular diameter of Ni-enriched regions were taken from each steel sheet so that the measurement position was the 1 / 4t position (a cross section perpendicular to the steel sheet surface and parallel to the rolling direction). Ni concentration distribution was measured using area analysis of EDX (energy dispersive X-ray analysis) with a STEM (scanning transmission electron microscope) device, and a Ni concentration map was created. In this case, for two regions with different Ni concentrations in the Ni concentration map, the region with the lower Ni concentration was designated as a Ni-dilute region, and the region with the higher Ni concentration was designated as a Ni-enriched region. The Ni-enriched region is a region that exists locally in the structure of the steel sheet of the present invention. Two fields of view of 10 μm × 10 μm were observed, and the Ni-enriched regions were identified by image analysis. The equivalent circular diameters of the identified Ni-enriched regions were determined, and the average value was designated as the average equivalent circular diameter of the Ni-enriched regions. The Ni-enriched region is a region where the Ni concentration is 1.1 times or more higher than that of the Ni-depleted region. The Ni concentrations of the Ni-enriched region and the Ni-depleted region were determined by point analysis using an EPMA (field emission electron probe microanalyzer) and were averaged values ​​measured at 10 arbitrary points in each region. In the structure of the steel sheet of the present invention, the Ni-depleted region is a region corresponding to tempered martensite or bainite (BCC phase), and the Ni-enriched region is a region corresponding to austenite (FCC phase).

[0062] Five test pieces for X-ray diffraction were taken from each steel plate at the 1 / 4t position parallel to the steel plate surface, and after immersion in liquid nitrogen for 30 minutes, the test pieces were ground and chemically polished so that the 1 / 4t position became the measurement surface, and then subjected to X-ray diffraction. The diffraction intensities of the (200) and (211) planes of α-Fe and the (200), (220), and (311) planes of γ-Fe, which appear in the symmetric reflection X-ray diffraction pattern, were determined, and the volume fraction of γ-Fe was calculated. The average value of the volume fractions of γ-Fe for the five test pieces was calculated and used as the amount of retained γ (volume fraction).

[0063] (Tensile strength) A JIS No. 4 tensile test piece was taken from the 1 / 2t position of the steel plate. A tensile test was carried out on the tensile test piece in accordance with the provisions of JIS Z 2241 (2022) to evaluate the tensile strength (TS) of the steel plate. A tensile strength of 690 MPa or more was deemed to be high strength and passed.

[0064] (Cryogenic Toughness) <Cryogenic Toughness of Base Material> A V-notch test piece was taken from the 1 / 2t position of the steel plate in accordance with the provisions of JIS Z 2242 (2023). A Charpy impact test was carried out on the V-notch test piece in accordance with the provisions of JIS Z 2242 (2023), and the Charpy absorbed energy (vE －２６９℃ ) was determined. The Charpy absorbed energy can be regarded as an index of the cryogenic temperature toughness of the steel plate (base metal). In the Charpy impact test, three test specimens were taken from each steel plate at different positions along the rolling direction, and measurements were performed once for each test specimen, a total of three times, and the average value was determined as the cryogenic temperature toughness of the base metal. Note that for No. 9, No. 37, and No. 38, which had thin plate thicknesses (plate thickness of 10 mm or less), half-size Charpy impact tests were performed using half-size test specimens (sub-size test specimens), and for the other examples, full-size Charpy impact tests were performed using full-size test specimens. In the full-size Charpy impact test, vE －２６９℃ A specimen with a vE of 100J or more was evaluated as having excellent Charpy toughness and good cryogenic toughness of the base material, and was therefore deemed to have passed. －２６９℃ A value of 50J or higher was considered a pass.

[0065] <Cryogenic Toughness of Weld Heat Affected Zone (CGHAZ)> The cryogenic toughness of the weld heat affected zone (CGHAZ) was evaluated by reproducing the thermal cycle of the CGHAZ formed by the final welding pass in the weld. Specifically, a thermal cycle reproducing the CGHAZ (holding at 1350°C for 1 second, cooling at a rate of 20°C / s between 800 and 500°C) was applied to the steel plate, and then test specimens were taken from the steel plate in the same manner as above, and the Charpy absorbed energy (vE) at -269°C was measured in the same manner as above. －２６９℃ ) was determined. The Charpy absorbed energy can be regarded as an index of the cryogenic temperature toughness of the weld heat affected zone (CGHAZ) of the steel plate. In the Charpy impact test, three test specimens were taken from each steel plate, as described above, and the average value of the measurement results of the three test specimens was determined as the cryogenic temperature toughness of the weld heat affected zone (reproduced CGHAZ). As described above, for No. 9, No. 37, and No. 38, which have thin plate thicknesses, half-size Charpy impact tests were conducted using half-size test specimens (sub-size test specimens), and full-size Charpy impact tests were conducted using full-size test specimens in the other examples. In the full-size Charpy impact test, vE －２６９℃ A specimen with a vE of 27J or more was evaluated as having excellent Charpy toughness and good cryogenic toughness of the weld heat affected zone (CGHAZ), and was therefore deemed to have passed the test. －２６９℃ A score of 14J or above was considered a pass.

[0066] <Cryogenic Toughness of Weld Heat Affected Zone (FGHAZ)> The cryogenic toughness of the weld heat affected zone (FGHAZ) was evaluated by reproducing the thermal cycle of the FGHAZ formed by the final welding pass in the weld. Specifically, a thermal cycle reproducing the FGHAZ (holding at 900°C for 1 second, cooling at a rate of 20°C / s between 800 and 500°C) was applied to the steel plate, and then test pieces were taken from the steel plate in the same manner as above, and the Charpy absorbed energy (vE) at -269°C was measured in the same manner as above. －２６９℃) was determined. The Charpy absorbed energy can be regarded as an index of the cryogenic temperature toughness of the weld heat affected zone (FGHAZ) of the steel plate. In the Charpy impact test, three test specimens were taken from each steel plate, as described above, and the average value of the measurement results of the three test specimens was determined as the cryogenic temperature toughness of the weld heat affected zone (reproduced FGHAZ). Note that, as described above, for No. 9, No. 37, and No. 38, which have thin plate thicknesses, half-size Charpy impact tests were conducted using half-size test specimens (sub-size test specimens), and full-size Charpy impact tests were conducted using full-size test specimens in the other examples. As described above, in the full-size Charpy impact test, vE －２６９℃ A specimen with a vE of 27J or more was evaluated as having excellent Charpy toughness and good cryogenic toughness of the weld heat affected zone (FGHAZ), and was therefore deemed to have passed the test. －２６９℃ A score of 14J or above was considered a pass.

[0067] As shown in Table 2, the examples of the present invention have high strength and vE －２６９℃ As described above, the steel plate according to the present invention has high strength and excellent cryogenic toughness at -253°C or less not only in the base material but also in the weld heat affected zone.

[0068]

Claims

1. A steel plate having a composition containing, by mass%, the following: C: 0.01-0.15%, Si: 0.01-0.50%, Mn: 0.05-1.50%, Ni: 10.0-15.5%, P: 0.03% or less, S: 0.005% or less, N: 0.0010-0.0080%, and Al: 0.008-0.10%, with the balance being Fe and unavoidable impurities; its structure is mainly composed of tempered martensite or tempered martensite and bainite, and the average equivalent circle diameter of the Ni-enriched region is 1.0 μm or less.

2. The steel sheet according to claim 1, wherein the chemical composition further contains, in mass%, one or more elements selected from the following group a and group b: Group a: one or more elements selected from Cr: 1.00% or less, Mo: 0.50% or less, Cu: 0.40% or less, Nb: 0.05% or less, V: 0.05% or less, Ti: 0.03% or less, and B: 0.0030% or less Group b: one or more elements selected from Ca: 0.007% or less, REM: 0.010% or less, Mg: 0.0070% or less, and Zr: 0.0050% or less 3. A method for manufacturing a steel plate, comprising heating a steel material having the composition as set forth in claim 1 or 2 to 900°C or higher and 1300°C or lower, hot rolling the steel plate at a surface finishing temperature of 850°C or higher, and then quenching the steel plate, at a temperature at a position 1 / 4 of the plate thickness of the steel plate, with an average heating rate of 0.2°C / s or higher in the temperature range from Ac1 point to Ac3 point to a heating temperature of (Ac3 point + 50°C) to 1000°C, followed by cooling at an average cooling rate of 3°C / s or higher in the temperature range from 600°C to 300°C and higher, with a cooling stop temperature of 200°C or lower.

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