Thick steel plate

Abstract

To provide a thick steel plate with improved low-temperature toughness after a post-welding heat treatment, while suppressing a crack at the post-welding heat treatment.SOLUTION: A thick steel plate includes, by mass%, 0.080-0.170% C, 0.10-0.50% Si, 1.10-1.50% Mn, 0.020% or less P, 0.0050% or less S, 0.61-0.95% Ni, 0.10-0.90% Cr, 0.30-1.00% Mo, 0.005-0.050% V, 0.010-0.080% Al, 0.0010-0.0080% N, 0.0050% or less O, and a balance Fe with impurities. The thick steel plate has a chemical composition satisfying an expression 1 (in which each [symbol of element] denotes a content of each element.) and an average of the largest crystal grains up to 10th is 80 μm or less with regard to a circle equivalent diameter at a 1 / 4 position of a plate thickness from a surface thereof: 0.60≤VX≤1.90. Expression 1, VX=3.69-0.75×(2.7×[C]+0.4×[Si]+[Mn]+0.45×[Ni]+0.8×[Cr]+[Mo]).SELECTED DRAWING: None

Description

Technical Field

[0001] This disclosure relates to thick steel plates.

Background Art

[0002] Structures such as bridges, buildings, shipbuilding, and pressure vessels are welded. For these structures, post-weld heat treatment (hereinafter sometimes referred to as "PWHT") may be performed to remove or relieve the residual stress generated by welding. In recent years, in consideration of the enlargement of these structures and harsh usage environments, in addition to increasing the thickness of the steel plate, it has also been required to improve the strength and low-temperature toughness of the steel plate after PWHT.

[0003] In the prior art, various thick steel plates for use in these structures and manufacturing methods thereof have been proposed (see Patent Documents 1 to 7, etc.). Also, cracks may occur from the heat-affected zone of the base metal during post-weld heat treatment, and preventing this is also important. For example, Patent Document 8 describes a test method in which a heat treatment of holding a welded joint at 600°C for 2 hours is performed, and the presence or absence of cracks is observed at the cross-section on the test bead side.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Patent Document 2

Patent Document 3

Patent Document 4

Patent Document 5

Patent Document 6

Patent Document 7

[0005] When PWHT is applied to a structure, the steel sheet tends to experience a decrease in low-temperature toughness due to the concentration of impurity elements and the coarsening of alloy carbides. Therefore, improving low-temperature toughness after PWHT is generally difficult. To improve low-temperature toughness after PWHT, increasing the content of alloys that enhance hardenability is effective. However, it has been found that increasing the hardenability of the steel sheet makes it difficult to prevent cracking in the heat-affected zone that occurs during PWHT.

[0006] This disclosure has been made in view of the above circumstances, and its purpose is to provide a thick steel plate in which low-temperature toughness after PWHT is improved and cracking during PWHT can be suppressed (excellent crack resistance during post-weld heat treatment). [Means for solving the problem]

[0007] To achieve the above objective, the inventors of this disclosure investigated the chemical composition and manufacturing conditions of thick steel plates. As a result, the inventors of this disclosure found that by controlling the chemical composition of the thick steel plate within a predetermined range and appropriately controlling the hot rolling and post-hot rolling heat treatment, it is possible to refine the crystal grains in the thick steel plate, improve low-temperature toughness after PWHT, and prevent cracking during PWHT by reducing the hardness in the heat-affected zone of the weld, thus completing the invention of this disclosure.

[0008] The following are examples of thick steel plates that have achieved the above objectives. <1> In mass%, C: 0.080~0.170%, Si: 0.10~0.50%, Mn: 1.10~1.50%, P: 0.020% or less, S: 0.0050% or less, Ni: 0.61~0.95%, Cr: 0.10~0.90%, Mo: 0.30~1.00%, V: 0.005~0.050%, Al: 0.010~0.080%, N: 0.0010~0.0080%, O: 0.0050% or less, Cu: 0~0.60%, Nb: 0~0.050%, Ti: 0~0.050%, W: 0~0.50%, Ca: 0~0.0050%, Mg: 0~0.0050%, REM: 0~0.0050%, The remainder consists of Fe and impurities, and has a chemical composition that satisfies the following formula 1. A thick steel sheet in which the average of the top 10 crystal grains with the largest equivalent circular diameters, enclosed by a boundary with an orientation difference of 15° or more within a range of 1 mm in the rolling direction and 0.4 mm in the thickness direction, in a cross-section in the rolling direction at a position 1 / 4 of the sheet thickness from the surface, is 80 μm or less. 0.60≦VX≦1.90 … Formula 1 VX=3.69-0.75×(2.7×[C]+0.4×[Si]+[Mn]+0.45×[Ni]+0.8×[Cr]+[Mo]) In the formula, [C], [Si], [Mn], [Ni], [Cr], and [Mo] represent the mass %) content of C, Si, Mn, Ni, Cr, and Mo, respectively. <2> When heated to 1400°C and held for 5 seconds, then cooled from 1400°C to 800°C at 32°C / s, and further cooled from 800°C to 200°C at 8°C / s, the Vickers hardness HV10 at a position 1 / 4 of the plate thickness from the surface satisfies the following equation 2. When heat treatment is performed at a heating temperature of 650°C for a holding time of 15 hours, the tensile strength at a position 1 / 4 of the plate thickness from the surface is 600 MPa or more, and the Charpy impact absorption energy at -50°C is 70 J or more. <1> Thick steel plate as described above. 634 × √[C] + 140 - HV10 > 0 … Equation 2 <3> The thick steel plate according to <1> or <2>, wherein the chemical composition satisfies at least one of Ni: 0.61 to 0.85% and Cr: 0.10 to 0.75%.

Advantages of the Invention

[0009] According to the present disclosure, there is provided a thick steel plate in which the low-temperature toughness after PWHT is improved, and cracking during PWHT of the weld heat-affected zone can be suppressed (excellent crack resistance during post-weld heat treatment).

Brief Description of the Drawings

[0010] [Figure 1] It is a schematic diagram for explaining the stress corrosion cracking (SCC) test.

Modes for Carrying Out the Invention

[0011] <Thick Steel Plate> The thick steel plate according to the present disclosure is preferably a high-strength thick steel plate, more specifically, a thick steel plate having a tensile strength of 600 MPa or more. In particular, it is a thick steel plate having a tensile strength of 600 MPa or more when subjected to a heat treatment (hereinafter, also referred to as "650°C × 15-hour heat treatment") corresponding to post-weld heat treatment (PWHT), with a heating temperature of 650°C and a holding time of 15 hours, in which the crystal grains are refined to improve the low-temperature toughness after PWHT. Further, the thick steel plate according to the present disclosure preferably controls the chemical composition so that the hardness becomes below a predetermined value when subjected to a heat treatment corresponding to the heat cycle of the weld heat-affected zone, that is, a heat treatment in which it is heated to 1400°C and held for 5 seconds, then cooled from 1400°C to 800°C at 32°C / s, and cooled from 800°C to 200°C at 8°C / s, thereby preventing cracking during PWHT. Cracking during PWHT of the weld heat-affected zone is caused by the 650°C × 15-hour heat treatment, and the presence or absence of cracking is confirmed after cooling. Furthermore, the crack resistance of thick steel plates during PWHT (Polymer Heat-Affected Welding) can be evaluated by applying heat treatment equivalent to the thermal cycle of the heat-affected zone during welding, then increasing the temperature while applying a constant tensile load, and observing whether or not fracture occurs.

[0012] Herein, in this disclosure, a crystal grain refers to a region enclosed by a boundary where the orientation difference between adjacent grains is 15° or more, as measured by electron backscatter diffraction (EBSD). Furthermore, as will be explained in detail later, in this disclosure, coarse grain size and average grain size refer to grain sizes calculated based on the equivalent circular diameter of each crystal grain measured by the above EBSD.

[0013] The thick steel plates relating to this disclosure can be realized by the specific embodiments shown below. The following will describe in more detail specific embodiments for realizing the thick steel plates relating to this disclosure. However, these descriptions are intended merely as examples of preferred embodiments of this disclosure and are not intended to limit this disclosure to such specific embodiments.

[0014] The thick steel plate relating to this disclosure is, by mass %, C: 0.080~0.170%, Si: 0.10~0.50%, Mn: 1.10~1.50%, P: 0.020% or less, S: 0.0050% or less, Ni: 0.61~0.95%, Cr: 0.10~0.90%, Mo: 0.30~1.00%, V: 0.005~0.050%, Al: 0.010~0.080%, N: 0.0010~0.0080%, O: 0.0050% or less, Cu: 0~0.60%, Nb: 0~0.050%, Ti: 0~0.050%, W: 0~0.50%, Ca: 0~0.0050%, Mg: 0~0.0050%, REM: 0~0.0050%, The remainder consists of Fe and impurities, and has a chemical composition that satisfies the following formula 1. The average of the top 10 crystal grains with the largest equivalent circular diameters (sometimes referred to as "coarse grain size" in this specification) enclosed by a boundary with an orientation difference of 15° or more within a range of 1 mm in the rolling direction and 0.4 mm in the thickness direction, in a cross-section in the rolling direction at a position 1 / 4 of the thickness from the surface, is 80 μm or less. 0.60≦VX≦1.90 … Formula 1 VX=3.69-0.75×(2.7×[C]+0.4×[Si]+[Mn]+0.45×[Ni]+0.8×[Cr]+[Mo]) Here, [C], [Si], [Mn], [Ni], [Cr], and [Mo] represent the mass %) content of C, Si, Mn, Ni, Cr, and Mo, respectively. Furthermore, according to the thick steel plate of this disclosure, when the plate is held at 1400°C for 5 seconds, then cooled from 1400°C to 800°C at 32°C / s, and further cooled from 800°C to 200°C at 8°C / s, the Vickers hardness HV10 at a position 1 / 4 of the plate thickness from the surface satisfies the following formula 2, and when heat treatment is performed at 650°C for 15 hours, it is possible to achieve a tensile strength of 600 MPa or more at a position 1 / 4 of the plate thickness from the surface and a Charpy impact absorption energy of 70 J or more at -50°C. 634×√[C]+140-HV10>0 … Formula 2

[0015] As mentioned earlier, post-weld heat treatment (PWHT) tends to increase the concentration of impurity elements and coarseen alloy carbides, reducing the low-temperature toughness of thick steel plates. Therefore, improving low-temperature toughness after PWHT is generally difficult. In this regard, the inventors of this disclosure investigated a method to suppress the formation of a coarse microstructure. As a result, they found that finer grain formation can be achieved by controlling the chemical composition so that VX = 3.69 - 0.75 × (2.7 × [C] + 0.4 × [Si] + [Mn] + 0.45 × [Ni] + 0.8 × [Cr] + [Mo]) is 1.90 or less.

[0016] However, if the VX value is made too low, cracks occur in the heat-affected zone during post-weld heat treatment, making it unsuitable for use as a structural member. Therefore, the inventors of this disclosure investigated the relationship between crack occurrence and the hardness of the heat-affected zone. As a result, they found that the relationship between the hardness of the heat-affected zone and the carbon content affects crack occurrence during post-weld heat heating (PWHT). Further investigation revealed that crack occurrence during PWHT can be evaluated by the relationship between the Vickers hardness HV10 and carbon content of a thick steel plate that has undergone a heat treatment equivalent to the thermal cycle of the heat-affected zone, namely, heating to 1400°C and holding for 5 seconds, then cooling from 1400°C to 800°C at 32°C / s, and then cooling from 800°C to 200°C at 8°C / s. Specifically, it was found that if the Vickers hardness HV10 at a position 1 / 4 of the plate thickness from the surface satisfies 634 × √[C] + 140 - HV10 > 0, crack occurrence during PWHT can be prevented. Furthermore, we found that in order to achieve this condition, the VX value needs to be 0.60 or higher.

[0017] The thick steel plates related to this disclosure will be described in detail below. In this disclosure, the unit "%" for the content of each element means "mass%" unless otherwise specified. When the lower limit for the content of each element in the chemical composition is stated as "0," it means that the element is an optional component and does not need to be included. In numerical ranges represented using "~", if the numbers before and after "~" are not preceded by "greater than" or "less than", it means that the range includes those numbers as the lower and upper limits, respectively. In the numerical ranges described in stages, the upper limit of one stage may be replaced with the upper limit of another stage, or with the value shown in the example. Similarly, the lower limit of one stage may be replaced with the lower limit of another stage, or with the value shown in the example. Furthermore, if the lower and upper limits for the content of each element are listed separately, a numerical range obtained by arbitrarily combining the lower and upper limits may be used as the content of that element.

[0018] The chemical composition of the thick steel plate relating to this disclosure is, in mass%, C: 0.080~0.170%, Si: 0.10~0.50%, Mn: 1.10~1.50%, P: 0.020% or less, S: 0.0050% or less, Ni: 0.61~0.95%, Cr: 0.10~0.90%, Mo: 0.30~1.00%, V: 0.005~0.050%, Al: 0.010~0.080%, N: 0.0010~0.0080%, O: 0.0050% or less, The remainder may consist of Fe and impurities, and may have a chemical composition that satisfies formula 1. Furthermore, the chemical composition may include one or more elements selected from the groups A and B described below. Group A mainly consists of elements that improve mechanical properties such as strength and toughness. Group B mainly consists of elements that control the form of oxides and sulfides. In the thick steel sheet according to this disclosure, boron (B) is an impurity, and a content of 0.0003% or more is undesirable. [Group A] One or more elements selected from the group consisting of Cu: 0.60% or less, Nb: 0.050% or less, Ti: 0.050% or less, and W: 0.50% or less. [Group B] One or more selected from the group consisting of Ca: 0.0050% or less, Mg: 0.0050% or less, and REM: 0.0050% or less.

[0019] [C:0.080~0.170%] Carbon (C) is an essential element for ensuring the strength of the base material (thick steel plate). To obtain this effect sufficiently, the C content should be 0.080% or higher. The C content may be 0.085% or higher, 0.090% or higher, or 0.095% or higher. On the other hand, excessive C content can lead to significant deterioration of toughness and a tendency towards excessive strength. Therefore, the C content should be 0.170% or lower. The C content may be 0.165% or lower, 0.160% or lower, or 0.155% or lower.

[0020] [Si: 0.10~0.50%] Silicon (Si) is a deoxidizing element and also contributes to improving strength. To fully obtain these effects, the Si content should be 0.10% or more. The Si content may be 0.12% or more, 0.14% or more, or 0.16% or more. On the other hand, if the Si content is excessive, island-like martensite may form, which may reduce toughness. Therefore, the Si content should be 0.50% or less. The Si content may be 0.45% or less, 0.40% or less, or 0.35% or less.

[0021] [Mn: 1.10~1.50%] Manganese (Mn) is a deoxidizing element and also an element that improves hardenability. To ensure sufficient strength of the base material, the Mn content should be 1.10% or more. The Mn content may be 1.12% or more, 1.14% or more, or 1.16% or more. On the other hand, if the Mn content is excessive, the hardenability will be excessive, leading to an excessive increase in strength and a decrease in toughness. Therefore, the Mn content should be 1.50% or less. The Mn content may be 1.48% or less, 1.46% or less, or 1.44% or less.

[0022] [P:0.020% or less] Phosphorus (P) is an impurity that segregates at grain boundaries and reduces toughness. For this reason, the P content should be 0.020% or less. The P content may also be 0.015% or less, 0.010% or less, or 0.008% or less. A lower P content is preferable, so no lower limit is specifically defined. However, from the viewpoint of manufacturing costs, the P content may be 0.0001% or more, 0.0005% or more, or 0.0010% or more.

[0023] [S:0.0050% or less] Sulfur (S) is an impurity that promotes central segregation and can cause the formation of stretched MnS, which can be the initiation point for brittle fracture. For this reason, the S content should be 0.0050% or less. The S content may also be 0.0045% or less, 0.0040% or less, or 0.0035% or less. A lower S content is preferable, so no lower limit is specifically specified. However, from the viewpoint of manufacturing costs, the S content may be 0.0001% or more, 0.0005% or more, or 0.0010% or more.

[0024] [Ni: 0.61~0.95%] Nickel (Ni) is an effective element for ensuring toughness. To achieve this effect, the Ni content should be 0.61% or higher. The Ni content may be 0.62% or higher, 0.63% or higher, or 0.64% or higher. On the other hand, excessive Ni content can increase manufacturing costs and lead to excessive hardenability, which can actually decrease the toughness of the base material. For this reason, the Ni content should be 0.95% or lower. The Ni content may be 0.90% or lower, 0.85% or lower, 0.80% or lower, or 0.75% or lower.

[0025] [Cr: 0.10~0.90%] Chromium (Cr) is an element that contributes to improved resistance to carbon dioxide corrosion and hardenability, and affects strength. To obtain these effects, the Cr content should be 0.10% or more. The Cr content may be 0.15% or more, 0.20% or more, or 0.25% or more. On the other hand, excessive Cr content may reduce the toughness of the base material. For this reason, the Cr content should be 0.90% or less. The Cr content may be 0.85% or less, 0.80% or less, 0.75% or less, 0.70% or less, or 0.65% or less.

[0026] [Mo: 0.30~1.00%] Molybdenum (Mo) is an element that improves the strength of the base material. Furthermore, Mo suppresses the coarsening of carbides, contributing to the prevention of stress corrosion cracking (SCC) through selective corrosion at the interface between coarse carbides and the matrix phase. To fully obtain these effects, the Mo content should be 0.30% or higher. The Mo content may also be 0.35% or higher, 0.40% or higher, or 0.45% or higher. On the other hand, excessive Mo content can lead to an excessive increase in the strength of the base material, impairing its toughness. Therefore, the Mo content should be 1.00% or lower. The Mo content may also be 0.95% or lower, 0.90% or lower, or 0.85% or lower.

[0027] [V:0.005 ~0.050%] Vanadium (V) is an element that contributes to improving the strength of the base material by precipitating carbonitrides during the tempering process and post-weld heat treatment. To obtain these effects sufficiently, the V content should be 0.005% or more. The V content may also be 0.007% or more, 0.009% or more, or 0.011% or more. On the other hand, excessive V content can lead to saturation of the effect and deterioration of toughness. For this reason, the V content should be 0.050% or less. The V content may also be 0.048% or less, 0.046% or less, or 0.044% or less.

[0028] [Al:0.010~0.080%] Aluminum (Al) is a deoxidizing element and also an element that suppresses cementite formation. Furthermore, Al contributes to finer particle formation as pinning particles AlN. To obtain at least one of these effects, the Al content should be 0.010% or more. The Al content may also be 0.015% or more, 0.020% or more, or 0.025% or more. On the other hand, excessive Al content can lead to an increase in inclusions and a decrease in toughness. For this reason, the Al content should be 0.080% or less. The Al content may also be 0.070% or less, 0.060% or less, or 0.050% or less.

[0029] [N: 0.0010~0.0080%] Nitrogen (N) is an essential element for refining crystal grains and improving toughness by precipitating as AlN. To obtain this effect sufficiently, the N content should be 0.0010% or more. The N content may also be 0.0015% or more, 0.0020% or more, or 0.0030% or more. On the other hand, if the N content is excessive, AlN may precipitate excessively, which can reduce toughness by acting as a fracture initiation point. Therefore, the N content should be 0.0080% or less. The N content may also be 0.0070% or less, 0.0060% or less, or 0.0050% or less.

[0030] [O:0.0050% or less] Oxygen (O) is an impurity, and the O content should be 0.0050% or less. The O content may also be 0.0045% or less, 0.0040% or less, or 0.0030% or less. While it is preferable to reduce the O content, from the viewpoint of deoxidation costs, the O content may be 0.0001% or more, 0.0002% or more, or 0.0003% or more.

[0031] The basic chemical composition of the thick steel sheet relating to this disclosure is as described above. Furthermore, the thick steel sheet relating to this disclosure may contain one or more of the following optional elements as needed. [Group A] One or more elements selected from the group consisting of Cu: 0.60% or less, Nb: 0.050% or less, Ti: 0.050% or less, and W: 0.50% or less. [Group B] One or more selected from the group consisting of Ca: 0.0050% or less, Mg: 0.0050% or less, and REM: 0.0050% or less. The following provides a detailed explanation of these optional elements.

[0032] [Cu: 0~0.60%] Copper (Cu) is an element that contributes to increased strength. While the Cu content may be 0%, it is preferable that the Cu content be 0.05% or more to obtain this effect. The Cu content may also be 0.15% or more, 0.20% or more, or 0.25% or more. On the other hand, excessive Cu content may reduce the toughness of the base material. For this reason, the Cu content should be 0.60% or less. The Cu content may also be 0.55% or less, 0.50% or less, or 0.45% or less.

[0033] [Nb:0~0.050%] Niobium (Nb) is an element that refines the microstructure and improves low-temperature toughness through its pinning effect in the NbCN morphology. While the Nb content may be 0%, it is preferable that the Nb content be 0.003% or higher to obtain this effect. The Nb content may also be 0.006% or higher, 0.010% or higher, or 0.015% or higher. On the other hand, excessive Nb content can lead to saturation of the pinning effect and deterioration of toughness due to the precipitation of coarse carbides and nitrides. For this reason, the Nb content should be 0.050% or lower. The Nb content may also be 0.045% or lower, 0.040% or lower, or 0.035% or lower.

[0034] [Ti: 0~0.050%] When titanium (Ti) is used in deoxidation, it forms an oxide phase consisting of Al, Ti, and Mn, which refines the structure and affects strength. The Ti content may be 0%, but to obtain the above effect, a Ti content of 0.003% or more is preferable. The Ti content may be 0.006% or more, 0.010% or more, or 0.015% or more. On the other hand, if the Ti content is excessive, Ti oxides or Ti carbonitrides may be formed, leading to a deterioration of toughness. For this reason, the Ti content should be 0.050% or less. The Ti content may be 0.040% or less, 0.035% or less, or 0.030% or less.

[0035] [W: 0~0.50%] Tungsten (W) is an element that contributes to improved corrosion resistance and affects strength. While the W content may be 0%, it is preferable that the W content be 0.05% or more to obtain these effects. The W content may also be 0.10% or more, 0.15% or more, or 0.20% or more. On the other hand, excessive W content may reduce the toughness of the HAZ (heat-affected zone). For this reason, the W content should be 0.50% or less. The W content may also be 0.45% or less, 0.40% or less, or 0.35% or less.

[0036] [Ca: 0~0.0050%] Calcium (Ca) is an element that controls the form of oxides and sulfides. The Ca content may be 0%, but it is preferable to have 0.0001% or more to obtain such effects. The Ca content may be 0.0005% or more, 0.0010% or more, or 0.0015% or more. If the Ca content is excessive, the above effects will saturate, and toughness may be impaired due to the formation of inclusions. For this reason, the Ca content should be 0.0050% or less. The Ca content may be 0.0045% or less, 0.0040% or less, or 0.0035% or less.

[0037] [Mg: 0~0.0050%] Magnesium (Mg) is an element that controls the form of oxides and sulfides. The Mg content may be 0%, but it is preferable to have 0.0001% or more to obtain such effects. The Mg content may be 0.0005% or more, 0.0010% or more, or 0.0015% or more. If the Mg content is excessive, the above effects will saturate, and toughness may be impaired due to the formation of inclusions. For this reason, the Mg content should be 0.0050% or less. The Mg content may be 0.0045% or less, 0.0040% or less, or 0.0035% or less.

[0038] [REM:0~0.0050%] Rare earth metals (REMs) refer to a total of 17 elements: the two elements Sc and Y, and the 15 lanthanide elements such as La, Ce, and Nd. REM content refers to the total content of the aforementioned 17 elements. REMs are elements that control the form of oxides and sulfides. The REM content may be 0%, but to obtain such effects, it is preferable to have 0.0001% or more. The REM content may be 0.0005% or more, 0.0010% or more, or 0.0015% or more. If the REM content is excessive, the above effects will saturate, and toughness may be impaired due to the formation of inclusions. For this reason, the REM content should be 0.0050% or less. The REM content may be 0.0045% or less, 0.0040% or less, or 0.0035% or less.

[0039] In the thick steel sheet relating to this disclosure, the remainder of the elements other than those mentioned above consists of Fe and impurities. Impurities include components that are mixed in during the industrial production of thick steel sheets due to various factors in the manufacturing process, such as raw materials like ore and scrap, and are not components that were intentionally added to the thick steel sheet relating to this disclosure.

[0040] The following describes the chemical composition formula (VX), average grain size, microstructure, thickness, and mechanical properties of the thick steel sheet related to this disclosure.

[0041] [0.60≦VX≦1.90] VX is an index of hardenability. VX is an index of the critical cooling rate of martensitic transformation, and the lower VX, the lower the transformation temperature due to quenching, which allows for a finer microstructure. The VX of thick steel plates shall be 1.90 or less. The VX of thick steel plates may be 1.80 or less, 1.70 or less, or 1.60 or less. On the other hand, the lower VX, the higher the hardness of the thick steel plate tends to be, making it more prone to cracking from the heat-affected zone during PWHT. The VX of thick steel plates shall be 0.60 or more. The VX of thick steel plates may be 0.70 or more, 0.80 or more, or 0.90 or more. VX in this disclosure can be calculated by the following formula. VX=3.69-0.75×(2.7×[C]+0.4×[Si]+[Mn]+0.45×[Ni]+0.8×[Cr]+[Mo]) In the formula, [C], [Si], [Mn], [Ni], [Cr], and [Mo] represent the mass %) of each element, and 0 is used if the element is not present.

[0042] In this disclosure, it is important to control the chemical composition within the aforementioned range in order to achieve an appropriate average grain size (coarse grain size) and to prevent the occurrence of cracks from the weld-affected zone during post-weld heat treatment.

[0043] [Coarse grain size: 80 μm or less] Coarse structures in thick steel plates serve as initiation points for brittle fracture, thereby reducing the toughness of the thick steel plate, particularly its low-temperature toughness. According to this disclosure, in order to achieve significantly improved low-temperature toughness in a thick steel plate, 10 crystal grains with large equivalent circle diameters are selected from among the crystal grains surrounded by boundaries with an orientation difference of 15° or more, and the average value of their equivalent circle diameters (hereinafter sometimes referred to as "coarse grain size") is controlled to 80 μm or less. The coarse grain size is preferably 70 μm or less, more preferably 65 μm or less, and most preferably 60 μm or less. Since a smaller coarse grain size is preferable, no lower limit is particularly specified. The coarse grain size may be 5 μm or more or 8 μm or more.

[0044] In this disclosure, the coarse grain size of the crystal grains is determined as follows. First, an L-shaped cross section (a cross section parallel to the rolling direction and thickness direction of the thick steel plate) at a position 1 / 4 of the plate thickness from the surface of the thick steel plate in the thickness direction (1 / 4 plate thickness position) is mirror polished. Then, the crystal orientation of an arbitrary region of 1.0 mm in the rolling direction × 0.4 mm in the thickness direction is measured by electron beam backscatter diffraction (EBSD). A region where the orientation difference between adjacent grains is 15° or more is defined as one crystal grain, and the grain size of each crystal grain is calculated as the equivalent diameter of a circle. The crystal orientation measurement can be performed at any one location. Ten of these crystal grains with large equivalent diameters are selected, and the average value of their equivalent diameters is determined as the "coarse grain size of the crystal grains".

[0045] [Aspect ratio of old austenite grains: 1.0~1.8] In the thick steel sheet according to this disclosure, it is sufficient to define the coarse grain size of crystal grains surrounded by a boundary with an orientation difference of 15° or more at the 1 / 4 thickness position, and the morphology of such crystal grains and other crystal grains such as prior austenite grains, such as aspect ratio, is not particularly limited. However, for example, the aspect ratio of prior austenite grains at a position 2 mm from the surface in the thickness direction may be 1.0 to 1.8. When hot rolling and quenching processes are performed, as in the manufacturing method described later, the aspect ratio of prior austenite grains at a depth of 2 mm from the surface tends to be close to 1.0. If quenching is performed immediately after hot rolling, the prior austenite may stretch in the rolling direction, and the aspect ratio may exceed 1.8. The aspect ratio of the prior austenite grain is determined as follows: First, an L-shaped section (a section parallel to the rolling direction and thickness direction of the steel plate) located 2 mm from the surface of the thick steel plate in the thickness direction is mirror-polished. Then, the crystal orientation of an arbitrary region of 1.0 mm in the rolling direction and 0.4 mm in the thickness direction is measured by electron beam backscatter diffraction (EBSD). A region enclosed by a boundary where the orientation difference is between 20° and 45° is defined as one prior austenite grain. The definition of a prior austenite grain by measuring the crystal orientation only needs to be done at any one location. Next, the length in the rolling direction and the length in the thickness direction of each prior austenite grain are measured, and the aspect ratio of each prior austenite grain is calculated. The arithmetic mean of all the calculated aspect ratios of the prior austenite grains is determined as the "aspect ratio of the prior austenite grain".

[0046] [Vickers hardness HV10 after heat treatment corresponding to the thermal cycle of the heat-affected zone during welding: 634 × √[C] + 140 - HV10 > 0] By setting the Vickers hardness HV10 after heat treatment corresponding to the thermal cycle of the heat-affected zone of the weld to 634 × √[C] + 140 - HV10 > 0, the difference in hardness between the base material and the heat-affected zone of the welded joint of the structure can be reduced, thereby preventing cracking during PWHT. In this disclosure, the Vickers hardness HV10 after heat treatment corresponding to the thermal cycle of the heat-affected zone of the weld of a thick steel plate is determined as follows. First, a test piece (for example, 12 mm × 12 mm × 70 mm) of an arbitrary size is taken from the surface of the thick steel plate at a position 1 / 4 of the plate thickness in the plate thickness direction (1 / 4 plate thickness position). Specifically, if the thickness of the original thick steel plate is t, a 12 mm × 12 mm × 70 mm test piece is taken such that the center of the 12 mm thick test piece is at the 1 / 4 t position of the original thick steel plate, and the length of the test piece 70 mm is in the rolling direction of the original thick steel plate. The test specimen is subjected to a heat treatment in which it is heated to 1400°C and held for 5 seconds, then cooled from 1400°C to 800°C at 32°C / s, and then cooled from 800°C to 200°C at 8°C / s. This heat treatment corresponds to the thermal history of the heat-affected zone of a submerged arc welded joint. Subsequently, the Vickers hardness is measured at three points within the 12 mm square contact section of the test specimen at a position 1 / 4 tangential to the original thick steel plate, under a load of 10 kgf, and the arithmetic mean is determined as the "Vickers hardness HV10 after heat treatment corresponding to the thermal cycle of the heat-affected zone of the weld". The Vickers hardness is measured according to JIS Z 2244:2009, and no heat treatment at 650°C for 15 hours is performed prior to measurement. Here, the heat treatment temperature corresponding to the thermal cycle of the heat-affected zone is the surface temperature of the thick steel plate, and is measured by a thermocouple attached to the surface of the test piece.

[0047] [plate thickness] The thick steel plates described herein are manufactured by hot rolling and heat treatment, and do not include thin steel plates or hot-rolled steel strips manufactured by winding them into coils after hot rolling. The thickness of the thick steel plates described herein is not particularly limited, but is, for example, 30 mm or more, preferably 40 mm or more, and preferably 70 mm or more. By controlling the chemical composition and microstructure of the thick steel plates within the aforementioned range, it is possible to achieve excellent low-temperature toughness, particularly excellent low-temperature toughness after PWHT, while maintaining high strength, even in such thick steel plates. In this disclosure, the thickness of the thick steel plates is not particularly limited, but may be 75 mm or more, 80 mm or more, or 90 mm or more. There is no particular upper limit, but the thickness of the thick steel plates may be 150 mm or less.

[0048] [Mechanical properties] The thick steel plate according to this disclosure can achieve excellent mechanical properties, such as high strength, more specifically a tensile strength (TS) of 600 MPa or higher. The tensile strength is preferably 610 MPa or higher, and more preferably 620 MPa or higher. The tensile strength may be 800 MPa or lower, or 780 MPa or lower. Furthermore, the thick steel plate according to this disclosure can maintain high strength even after PWHT, and can achieve a tensile strength (TS) of 600 MPa or more even after heat treatment at 650°C for 15 hours (equivalent to PWHT). The tensile strength after heat treatment at 650°C for 15 hours is preferably 600 MPa or more, more preferably 620 MPa or more, and may be 800 MPa or less or 780 MPa or less. Here, the heating temperature (650°C) for the 650°C x 15-hour heat treatment, which corresponds to PWHT, is the surface temperature of the thick steel plate from which the test specimen is taken, and is measured using a thermocouple.

[0049] Furthermore, the thick steel sheet according to this disclosure can similarly achieve excellent yield strength (YS) with or without PWHT. More specifically, the thick steel sheet according to this disclosure can achieve a yield strength of 450 MPa or more, preferably 470 MPa or more, and more preferably 480 MPa or more, both when subjected to heat treatment at 650°C for 15 hours (equivalent to PWHT) and when such heat treatment is not performed.

[0050] In addition, the thick steel plate according to this disclosure can similarly achieve excellent low-temperature toughness with or without PWHT. More specifically, the thick steel plate according to this disclosure can achieve low-temperature toughness of 70 J or more, preferably 100 J or more, and more preferably 120 J or more, in both cases: when heat-treated at 650°C for 15 hours (equivalent to PWHT) and when such heat treatment is not performed.

[0051] As described above, the thick steel plates according to this disclosure exhibit excellent strength and low-temperature toughness not only before PWHT but also after PWHT, making them very suitable for use in structures such as bridges, buildings, shipbuilding, and pressure vessels. In particular, the thick steel plates according to this disclosure are extremely useful as thick steel plates for pressure vessels, such as in applications where various gases are reacted in low-temperature regions below -10°C.

[0052] Tensile strength (TS) and yield strength (YS) are measured by tensile testing in accordance with JIS Z2241:2011. For tensile testing, a JIS 14A test specimen is used, taken from the 1 / 4 thickness position, with the longitudinal direction parallel to the width direction of the thick steel plate (C direction). The average Charpy impact energy is measured by a Charpy impact test at -50°C using an impact blade with a radius of 2 mm, in accordance with the provisions of JIS Z2242:2018. The Charpy impact energy is measured using three test specimens and calculated by averaging them. For the Charpy impact test, a V-notch test specimen is used, taken from the 1 / 4 thickness position of the thick steel plate, with the longitudinal direction parallel to the width direction of the thick steel plate (C direction). No heat treatment equivalent to a thermal cycle in the heat-affected zone of the weld is performed prior to tensile and Charpy impact tests.

[0053] [Method for manufacturing thick steel plates related to this disclosure] Next, a method for manufacturing thick steel plates related to this disclosure will be described. The term "process" includes not only independent processes, but also any process that cannot be clearly distinguished from other processes, as long as its intended purpose is achieved. The following description is intended to illustrate characteristic methods for manufacturing the thick steel plates relating to this disclosure, and is not intended to limit the thick steel plates relating to this disclosure to those manufactured by the manufacturing methods described below.

[0054] The method for manufacturing thick steel plates according to this disclosure includes a hot rolling step, a quenching step, and a tempering step, but does not include a winding step in which the hot-rolled steel is wound into a coil. Each step will be described in more detail below. The steel billet used in this manufacturing method is not particularly limited as long as it is within the range of chemical composition of this disclosure, and any steel billet manufactured under any suitable casting conditions known to those skilled in the art can be used. For example, the steel billet may be a block-to-block slab or a continuously cast slab. From the viewpoint of manufacturing efficiency, yield, and energy saving, it is preferable to use a continuously cast slab as the steel billet.

[0055] [Hot rolling process] First, a steel slab having a chemical composition defined in the present disclosure is reheated in a hot rolling process and then hot rolled with a reduction ratio of 40% or more. The reheating temperature is preferably 1000°C or higher from the viewpoint of reducing the load on the rolling rolls, and preferably 1250°C or lower from the viewpoint of suppressing the coarsening of the structure. The finishing temperature of the hot rolling is preferably at or above the temperature at which the transformation from austenite to ferrite starts during cooling, for example, 800°C or higher. The finishing temperature of the hot rolling may be 1000°C or lower.

[0056] [Quenching process] After the hot rolling process, the steel plate is once cooled to 150°C or lower, then reheated to a temperature (quenching temperature) of 800°C or higher, and then cooled to 200°C or lower at an average cooling rate of 1.0°C / s or more. Here, by controlling the reheating temperature T (°C) so as to satisfy the following formulas 3 and 4, the coarsening of the metal structure can be suppressed, that is, the average circle equivalent diameter (coarse grain size) of crystal grains surrounded by boundaries with an orientation difference of 15° or more at the 1 / 4 plate thickness position of the thick steel plate can be surely made 80 μm or less. When the parameter R is less than 1.0, low-temperature toughness can be ensured, and when the parameter R exceeds 0, tensile strength can be ensured. 0 < R < 1.0 ··· Formula 3 R = (T - AC) / 130 ··· Formula 4 Here, AC is a coefficient related to the phase transformation determined from the alloy elements in the steel plate and is obtained by the following formula 5. AC = 937 - 476×[C] + 56×[Si] - 20×[Mn] - 16×[Cu] - 27×[Ni] - 5×[Cr] + 38×[Mo] + 125×[V] + 136×[Ti] - 19×[Nb] ··· Formula 5 Here, [C], [Si], [Mn], [Cu], [Ni], [Cr], [Mo], [V], [Ti] and [Nb] are the contents (mass%) of the respective elements, and are 0 when the corresponding element is not contained. If the parameter R is too large, the crystal grains may coarseen, and sufficient low-temperature toughness may not be achieved. On the other hand, if the parameter R is too small, austenitization during reheating may be insufficient, and sufficient tensile strength of the steel sheet may not be achieved. The cooling rate during the quenching process should be 1.0°C / s or higher to ensure tensile strength. The cooling rate during the quenching process may be 20.0°C / s or lower, or 10.0°C / s or lower.

[0057] [Tempering process] Finally, the steel sheet is tempered in a tempering process, specifically by heating it at a tempering temperature of 550-700°C for 30 minutes to 2 hours. This tempering process allows for adjusting the strength to an appropriate range and improving toughness. The cooling rate after tempering is not particularly limited; for example, it can be cooled by air cooling. [Examples]

[0058] The thick steel plates relating to this disclosure will be described in more detail below with reference to examples, but the thick steel plates relating to this disclosure are not limited to these examples in any way. In the following embodiments, a thick steel plate according to the present disclosure was manufactured, and the mechanical properties of the obtained thick steel plate were investigated. [Examples]

[0059] [Manufacturing of thick steel plates] First, slabs with the chemical composition shown in Table 1 were cast using a continuous casting method. The remainder, other than the components shown in Table 1, consists of Fe and impurities. Blank spaces indicate that certain components were not intentionally added.

[0060] [Table 1]

[0061] Next, thick steel plates with a thickness of 70 mm or more were manufactured from these slabs according to the manufacturing conditions shown in Table 2. Hot rolling was carried out at the reduction ratio shown in Table 2. After the hot rolling process, the steel plates were cooled to below 150°C, then reheated to the reheating temperature T (°C) shown in Table 2, followed by quenching and tempering. The tempering time was 1 hour.

[0062] [Table 2]

[0063] [Measurement and Evaluation] The average grain size and aspect ratio of the crystal grains, mechanical properties, and crack resistance after post-weld heat treatment of the obtained thick steel plates were determined by the following method.

[0064] [Coarse grain size of crystal grains] The coarse grain size of the crystal grains was determined as follows. First, the L-shaped section at the 1 / 4 thickness position of the thick steel plate (a section parallel to the rolling direction and thickness direction of the thick steel plate) was mirror-polished. Then, the crystal orientation of an arbitrary region of 1.0 mm in the rolling direction × 0.4 mm in the thickness direction (the center of the 0.4 mm thickness direction is at the 1 / 4 thickness position of the original thick steel plate) was measured by electron beam backscatter diffraction (EBSD). A region where the orientation difference between adjacent grains is 15° or more was defined as one crystal grain, and the grain size of each crystal grain was calculated as the equivalent diameter of a circle. Next, 10 crystal grains with the largest equivalent diameters were selected from these crystal grains, and the average value of their equivalent diameters was determined as the "coarse grain size of the crystal grains".

[0065] [Aspect ratio of old austenite grains] The aspect ratio of the prior austenite grains was determined as follows: An L-shaped section (a section parallel to the rolling direction and thickness direction of the steel plate) located 2 mm from the surface of the steel plate in the thickness direction was mirror-polished. Then, the crystal orientation of an arbitrary region of 1.0 mm in the rolling direction × 0.4 mm in the thickness direction was measured by electron beam backscatter diffraction (EBSD). A region enclosed by a boundary where the orientation difference is between 20° and 45° was defined as one prior austenite grain. The rolling direction length and thickness direction length of each prior austenite grain were measured, and the aspect ratio of each prior austenite grain was calculated. The arithmetic mean of all the calculated aspect ratios of the prior austenite grains was determined as the "aspect ratio of the prior austenite grain".

[0066] [Mechanical properties] To evaluate the mechanical properties of thick steel plates after heat treatment (PWHT), the tensile strength (TS), yield strength (YS), and average Charpy impact absorption energy (KV2) at -50°C were measured after heat treatment equivalent to PWHT (650°C for 15 hours) was performed on the obtained thick steel plates. The specimens used for the tensile and Charpy impact tests were taken from the 1 / 4 thickness position of the thick steel plate after heat treatment at 650°C for 15 hours. TS and YS were measured by performing a tensile test in accordance with JIS Z2241:2011 based on a JIS 14A test specimen in which the longitudinal direction was parallel to the width direction (C direction) of the heat-treated thick steel plate. The average Charpy impact absorption energy at -50°C was calculated by measuring the Charpy impact absorption energy at -50°C using a 2 mm radius impact blade, in accordance with the provisions of JIS Z2242:2018, based on a V-notch test piece of thick steel plate that had undergone the same heat treatment as described above, with the C direction as the longitudinal direction.

[0067] In the following, a high-strength thick steel plate with improved low-temperature toughness after PWHT was evaluated as having a TS of 600 MPa or higher and an average Charpy impact absorption energy (KV2) of 70 J or higher at -50°C.

[0068] [Vickers hardness HV10 after heat treatment equivalent to the thermal cycle of the heat-affected zone during welding] The test specimens used to measure the Vickers hardness after heat treatment corresponding to the thermal cycle of the heat-affected zone during welding are taken from a 1 / 4 thickness position of the thick steel plate. The test specimen (12 mm × 12 mm × 70 mm) is subjected to a heat treatment in which it is heated to 1400°C and held for 5 seconds, then cooled from 1400°C to 800°C at 32°C / s, and then cooled from 800°C to 200°C at 8°C / s. Subsequently, the Vickers hardness was measured at three points with a load of 10 kgf in accordance with JIS 2244:2009, and the average was calculated (HV10). In addition, HY, which is obtained by the following formula, was calculated. HY = 634 × √[C] + 140 - HV10

[0069] [Crack resistance during post-weld heat treatment] For evaluating crack resistance during post-weld heat treatment, a round bar specimen (φ10mm, φ6mm in the center) taken from the 1 / 4 thickness position of the thick steel plate is used. The round bar specimen is subjected to a heat treatment in which it is heated to 1400°C and held for 5 seconds, then cooled from 1400°C to 800°C at 32°C / s, and then cooled from 800°C to 200°C at 8°C / s. After that, the round bar specimen is heated at 200°C / hr while a constant tensile load of 200 MPa is applied, and the presence or absence of fracture is evaluated by 650°C. If no fracture occurred, it was evaluated as a high-strength thick steel plate that can prevent cracking during post-weld heat treatment. If no fracture occurred, it was classified as G, and if fracture occurred, it was classified as B.

[0070] The results are shown in Table 3. The Vickers hardness HV10 and HY after heat treatment corresponding to the thermal cycle of the weld heat-affected zone are shown in the "Properties corresponding to the weld heat-affected zone" column of Table 3. Although not shown in Table 3, the aspect ratio of the prior austenite grains was 1.8 or less for steel plates 1 to 54 in Table 3. The aspect ratio of the prior austenite grains for steel plate 55 was 2.1 because it was manufactured by direct quenching.

[0071] [Table 3]

[0072] Referring to Tables 1-3, in Comparative Examples 28-30 and 32-34, sufficient TS could not be obtained due to low C, Si, Mn, Cr, Mo, or V content. In Comparative Example 31, sufficient low-temperature toughness could not be obtained due to the low Ni content. In comparative examples 35 and 36, the low content of Al or N resulted in insufficient pinning effect by AlN, leading to grain coarsening and reduced low-temperature toughness. In comparative examples 37, 39, and 42-45, the high content of C, Mn, Ni, Cr, Mo, or V resulted in excessive strength and reduced low-temperature toughness. In Comparative Example 38, island-like martensite was formed due to the high Si content, resulting in a decrease in low-temperature toughness.

[0073] In comparative examples 40 and 41, the low-temperature toughness decreased due to the high content of P or S. In Comparative Example 46, the high Al content resulted in the precipitation of coarse precipitates and an increase in the amount of inclusions, leading to a decrease in low-temperature toughness. In comparative examples 47 and 48, the high content of N or O resulted in the formation of many inclusions and other impurities, leading to a decrease in low-temperature toughness. In Comparative Example 49, the addition of B resulted in increased Vickers hardness after heat treatment corresponding to the thermal cycle in the heat-affected zone during welding, leading to decreased crack resistance. In Comparative Example 50, the low VX value resulted in a high Vickers hardness after heat treatment corresponding to the thermal cycle in the heat-affected zone of the weld, leading to a decrease in crack resistance. In Comparative Example 51, the high VX value resulted in grain coarsening and a decrease in low-temperature toughness. In Comparative Example 52, the parameter R in the quenching process was 0.0 or less, resulting in insufficient austenitization during reheating, and consequently, insufficient yield strength, tensile strength, and low-temperature toughness. In Comparative Example 53, the parameter R during the quenching process exceeded 1.0, resulting in grain coarsening and a decrease in low-temperature toughness. In Comparative Example 54, sufficient strength could not be obtained because the cooling rate during the quenching process was too slow. In Comparative Example 55, direct quenching was performed after hot rolling, resulting in grain coarsening and a decrease in low-temperature toughness.

[0074] In contrast, in all of the examples of the invention (the examples disclosed herein), by appropriately controlling the chemical composition and grain size of the thick steel plate, even when heat treatment equivalent to PWHT was performed at 650°C for 15 hours, cracking from the heat-affected zone of the weld was suppressed, and a thick steel plate with high strength (TS) of 600 MPa or more and excellent low-temperature toughness (Charpy impact absorption energy at -50°C of 70 J or more) was obtained. Furthermore, although not shown in Table 3, we also measured the mechanical properties before PWHT, and found that all of the thick steel plates before PWHT related to the invention examples achieved a TS of 600 MPa or higher and an average Charpy impact absorption energy of 70 J or higher at -50°C. [Examples]

[0075] [SCC resistance] Furthermore, submerged arc welded joints were fabricated from steel plates having the chemical composition shown in Table 1 and obtained by the manufacturing process shown in Table 2, specifically steel plate number 1 (this disclosure example) and steel plate number 33 (comparative example). The coarse grain size of the crystal grains in the heat-affected zone of the weld was measured in the same manner as in Example 1 at the L-section at a 1 / 4 plate thickness position within 1 to 3 mm from the fusion line. After PWHT was applied to the submerged arc welded joints, their SCC resistance was evaluated. For the SCC resistance evaluation test, strip-shaped test specimens (3 mm thick, 10 mm wide, 115 mm long) taken from near the surface of a submerged arc welded joint were used. Figure 1 schematically shows the SCC resistance evaluation test that was performed. In Figure 1, 1 is an amine aqueous solution, 2 is CO2 bubbles, 3 is a strip-shaped test specimen, 4 is a four-point bending jig, 5 is a reference electrode (SSE), 6 is a counter electrode (Pt), 7 is an oil bath, and 8 is a heater. The strip-shaped test specimen, with the weld located in the center of its longitudinal direction, was held at a constant displacement by a four-point bending jig as shown in Figure 1, and the tensile stress on the surface of the test specimen between the two central points was set to 300 MPa. Specifically, strain gauges were attached to the surface of the test specimen, and displacement was applied from the opposite side with screws to adjust the strain to correspond to 300 MPa in the stress-strain curve (tensile test) of the steel plate. The test specimen was immersed in a corrosive environment together with the jig and given a potential to accelerate SCC (Figure 1). After a 720-hour test period, the specimens were removed from the corrosive environment and detached from the jig. After cleaning, multiple cross-sections were cut between two central points. Each cross-section was polished and etched, and the toxicity was determined by observation with an optical microscope, based on whether the maximum depth of the SCC exceeded the measured coarse grain size in each weld heat-affected zone.

[0076] The SCC test was conducted under the following conditions to simulate an acidic gas absorption vessel using an amine. Corrosive environment: Aluminium aqueous solution saturated with CO2 gas Test solution (amine aqueous solution): An aqueous solution containing 20% ​​by mass of monoethanolamine. Test solution temperature: 70℃ Applied voltage: -0.5V (SSE) Exam period: 720 hours

[0077] In steel plate number 33 (comparative example), SCCs with depths exceeding the coarse grain size occurred in the heat-affected zone of the weld. Such deep SCCs are undesirable for maintaining the integrity of acidic gas absorption containers using amines because their growth is accelerated due to stress concentration at the crack tip. On the other hand, in steel plate number 1 (present disclosure example), no SCCs exceeding the coarse grain size occurred, and good SCC resistance was confirmed. [Explanation of Symbols]

[0078] 1. Amine aqueous solution 2 CO2 bubbles 3. Strip-shaped test specimens 4-point bending jig 5 Reference electrode (SSE) 6. Opposite pole (Pt) 7. Oil Bath 8 Heater

Claims

1. In mass percent, C: 0.080-0.170%, Si: 0.10 to 0.50%, Mn: 1.10 to 1.50%, P: 0.020% or less, S: 0.0050% or less, Ni: 0.61-0.95%, Cr: 0.10-0.90%, Mo: 0.30-1.00%, V: 0.005-0.050%, Al: 0.010-0.080%, N: 0.0010-0.0080%, O: 0.0050% or less, The remainder consists of Fe and impurities, and has a chemical composition that satisfies the following formula 1. In a cross-section in the rolling direction at a position 1 / 4 of the thickness from the surface, within a range of 1 mm in the rolling direction and 0.4 mm in the thickness direction, the average of the top 10 crystal grains with the largest equivalent circle diameters enclosed by boundaries with an orientation difference of 15° or more is 80 μm or less, and when heated to 1400°C and held for 5 seconds, then cooled from 1400°C to 800°C at 32°C / s, and further cooled from 800°C to 200°C at 8°C / s, the Vickers hardness HV10 at a position 1 / 4 of the thickness from the surface satisfies the following equation 2. A thick steel plate in which, after heat treatment performed at a heating temperature of 650°C for a holding time of 15 hours, the tensile strength at a position 1 / 4 of the plate thickness from the surface is 600 MPa or more, and the Charpy impact absorption energy at -50°C is 70 J or more. 0.60≦VX≦1.90 … Formula 1 VX=3.69-0.75×(2.7×[C]+0.4×[Si]+[Mn]+0.45×[Ni]+0.8×[Cr]+[Mo]) In the formula, [C], [Si], [Mn], [Ni], [Cr], and [Mo] represent the mass %) content of C, Si, Mn, Ni, Cr, and Mo, respectively. 634×√[C]+140−HV10>0 … Formula 2

2. In mass percent, C: 0.080-0.170%, Si: 0.10 to 0.50%, Mn: 1.10 to 1.50%, P: 0.020% or less, S: 0.0050% or less, Ni: 0.61-0.95%, Cr: 0.10-0.90%, Mo: 0.30-1.00%, V: 0.005-0.050%, Al: 0.010-0.080%, N: 0.0010-0.0080%, O: Contains 0.0050% or less, and further contains one or two types selected from the groups A and B described below. [Group A] One or more selected from the group consisting of Cu: 0.60% or less, Nb: 0.050% or less, Ti: 0.050% or less, and W: 0.50% or less. [Group B] One or more selected from the group consisting of Ca: 0.0050% or less, Mg: 0.0050% or less, and REM: 0.0050% or less. The remainder consists of Fe and impurities, and has a chemical composition that satisfies the following formula 1. In a cross-section in the rolling direction at a position 1 / 4 of the thickness from the surface, within a range of 1 mm in the rolling direction and 0.4 mm in the thickness direction, the average of the top 10 crystal grains with the largest equivalent circle diameters enclosed by boundaries with an orientation difference of 15° or more is 80 μm or less, and when heated to 1400°C and held for 5 seconds, then cooled from 1400°C to 800°C at 32°C / s, and further cooled from 800°C to 200°C at 8°C / s, the Vickers hardness HV10 at a position 1 / 4 of the thickness from the surface satisfies the following equation 2. A thick steel plate in which, after heat treatment performed at a heating temperature of 650°C for a holding time of 15 hours, the tensile strength at a position 1 / 4 of the plate thickness from the surface is 600 MPa or more, and the Charpy impact absorption energy at -50°C is 70 J or more. 0.60≦VX≦1.90 … Formula 1 VX=3.69-0.75×(2.7×[C]+0.4×[Si]+[Mn]+0.45×[Ni]+0.8×[Cr]+[Mo]) In the formula, [C], [Si], [Mn], [Ni], [Cr], and [Mo] represent the mass %) content of C, Si, Mn, Ni, Cr, and Mo, respectively. 634×√[C]+140−HV10>0 … Formula 2

3. The thick steel plate according to claim 2, wherein the chemical composition includes the group A.

4. The thick steel plate according to claim 2, wherein the chemical composition includes the group B.

5. The thick steel plate according to any one of claims 1 to 4, wherein the chemical composition satisfies at least one of Ni: 0.61 to 0.85% and Cr: 0.10 to 0.75%.

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