Steel material

Abstract

To provide a steel material that is capable of yielding a welded structure with superior HAZ toughness, even in cases involving large heat input welding, a slow post-weld cooling speed, and the possibility that a multiphase microstructure can be formed as the HAZ structure.SOLUTION: A steel material has a chemical composition that satisfies specified ranges for respective elements, with the balance being Fe and inevitable impurities, where the a value as represented by formula (a) is 1.60 or less and the b value as represented by formula (b) is 1.28 or less.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present disclosure relates to a steel material, and more particularly to a steel material capable of producing a welded structure having excellent toughness of a HAZ (heat affected zone) (hereinafter referred to as "HAZ toughness"). [Background technology]

[0002] In recent years, as welded structures such as bridges, high-rise buildings, and ships become larger, the plate thickness of the steel material applied to the welded structures has increased, and large heat input welding has been performed in order to improve the efficiency of welding work. In addition, from the viewpoint of ensuring the safety of the welded structures, it is required to exhibit excellent HAZ toughness even after large heat input welding. In Patent Document 1, in consideration of the problem that when large heat input welding is performed, the HAZ is heated to a high-temperature austenite region and then slowly cooled, so that the structure of the HAZ (especially in the vicinity of the bond part of the HAZ) becomes coarse and the toughness of that part is easily deteriorated, a high-strength thick steel plate with excellent HAZ toughness is proposed even when large heat input welding with a heat input of 30 to 100 kJ / mm is performed.

[0003] In detail, Patent Document 1 shows that by satisfying the relationship of the predetermined formula (1), while keeping the chemical composition of the steel plate within a predetermined range, and further controlling the dispersion state (number / density) of fine Ti-containing nitrides, it is possible to disperse fine Ti-containing nitrides in the steel that do not dissolve in the steel material during high heat input welding, thereby realizing a thick steel plate with improved HAZ toughness. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] JP 2009-167447 A Summary of the Invention [Problem to be solved by the invention]

[0005] As a weldment constituting a welded structure typified by architecture such as a high-rise building (in this specification, the weldment is collectively referred to as a "welded structure"), a box column (also referred to as a "welded assembly box section column") can be mentioned. As a box column, for example, a welded structure manufactured by joining a skin plate welded to a square steel pipe shape and a plurality of diaphragms dividing the internal space, for example, by electroslag welding can be mentioned. In conventional welding methods, the cooling rate of the HAZ after welding is fast, and the structure of the HAZ (hereinafter sometimes referred to as the "HAZ structure") is a structure mainly composed of bainite. However, in recent years, various demands have been increasing for the enlargement of welded structures, the complexity of the shape, the securing of a large space, and the like. Accordingly, in the case of a box column with an internal diaphragm, for example, depending on the combination of the plate thickness of the steel plate used for the internal diaphragm and the plate thickness of the steel plate used for the skin plate, the cooling rate of the HAZ after welding may be slow, and the HAZ structure may become a multi-phase structure of bainite and a structure other than bainite, unlike the conventional structure. However, the properties of the HAZ when it is a multi-phase structure have not been investigated.

[0006] The present disclosure has been made in consideration of the above circumstances, and one of its objectives is to provide a steel material that can obtain a welded structure with excellent HAZ toughness, even when large heat input welding is performed and the cooling rate after welding is slow, so that a multi-phase structure may be formed as the HAZ structure. [Means for solving the problem]

[0007] Aspect 1 of the present invention is The chemical composition is C: 0.02~0.06% by mass, Si:0.01~0.20% by mass, Mn: 1.00~1.60% by mass, P: 0.010% by mass or less (including 0% by mass), S: 0.005% by mass or less (including 0% by mass), Cu: 0.80~1.00% by mass, Al: 0.02~0.05% by mass, N:0.0030~0.0080% by mass, Ni: 0.90~1.10% by mass, Cr:0.25~0.70% by mass, Ti:0.005~0.020% by mass, B: 0.0005 to 0.0025 mass%, and Ca: 0.0005 to 0.0030 mass%, with the balance being Fe and unavoidable impurities; The steel material has a value a represented by the following formula (a) of 1.60 or less, and a value b represented by the following formula (b) of 1.28 or less.

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[0008] Aspect 2 of the present invention is A steel material according to aspect 1, further comprising, in the chemical composition, more than 0 mass % and 0.1 mass % or less of V instead of a portion of Fe. Effect of the Invention

[0009] According to the present disclosure, it is possible to provide a steel material that can produce a welded structure with excellent HAZ toughness, even when large heat input welding is performed, the cooling rate after welding is slow, and a multi-phase structure may be formed as the HAZ structure. [Brief description of the drawings]

[0010] [Figure 1] 4 is a graph showing the relationship between the a value and the ferrite fraction in the examples. [Diagram 2] 1 is a graph showing the relationship between the b value and the MA fraction in an example. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0011] When the cooling rate after high heat input welding is slow, a multi-phase structure of bainite, ferrite, and island martensite (MA) may form as the HAZ structure. Until now, when the HAZ structure is a multi-phase structure, the effect of the multi-phase structure on the HAZ toughness has not been studied, and it has been difficult to reliably ensure excellent HAZ toughness when high heat input welding is performed and the cooling rate after welding is slow.

[0012] Therefore, the present inventor first studied the effect of each fraction of ferrite and MA other than bainite in the HAZ structure on the HAZ toughness. As a result, it was first clarified that in order to realize excellent HAZ toughness in the multi-phase structure, it is effective to suppress crack initiation through reduction of HAZ hardness by reducing the MA fraction and increasing the ferrite fraction in the HAZ structure, and to suppress crack propagation by refining the block size of bainite in the HAZ structure. Then, in order to obtain a welded structure that shows excellent HAZ toughness in large heat input welding even when the above-mentioned multi-phase structure is formed as the HAZ structure by controlling the chemical composition of the steel, the inventor conducted extensive research on the relationship between the chemical composition of the steel and the HAZ structure and HAZ toughness.

[0013] First, the relationship between the ferrite fraction in the HAZ structure and the chemical composition of the steel will be described.

[0014] In order to improve the ferrite fraction in the HAZ structure, it is necessary to promote the transformation from austenite to ferrite, and suppressing hardenability and increasing the number of ferrite nucleation sites are effective. The inventors have studied chemical compositions from the viewpoint of suppressing hardenability and increasing the number of ferrite nucleation sites. As a result, they have found that the ferrite fraction in the HAZ structure can be organized by the components of the steel material using the following formula (a), which is based on the commonly known hardenability index (DI), contains Nb, which is an element that affects hardenability but is not included in DI, and further takes into account the number (density) of TiN and BN, which act as ferrite nucleation sites.

[0015]

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[0016] In formula (a), C, Si, Mn, Cu, Ni, Cr, Mo, V, B, and Nb indicate the content (mass%) of each element in the steel material, and elements that are not included are set to zero. When Mn is less than 1.20 mass%, the term (5.1(Mn-1.2)+5) is replaced with (3.33Mn+1). The steel material of the present disclosure does not contain Mo and Nb, and these elements are set to zero to calculate the a value. In addition, D and E are respectively expressed by the following formulas.

[0017]

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[0018] In the formula expressing D, MIN indicates a function that adopts the minimum value of the two arguments in parentheses, and Ti and N indicate the contents (mass%) of each element in the steel material. A is Avogadro's constant (6.02×10 23 Mol -1 )

[0019]

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[0020] In the formula expressing E, MIN indicates a function that adopts the minimum value of the two arguments in parentheses, and B, Ti, and N indicate the contents (mass%) of each element in the steel material. A is Avogadro's constant (6.02×10 23 Mol -1 )

[0021] Furthermore, as shown in the examples below, the present inventors have found that in order to achieve excellent HAZ toughness with a Charpy impact absorption energy value of 70 J or more in the HAZ obtained by performing high heat input welding (or heating with a thermal cycle equivalent to the high heat input welding) under conditions of a slow cooling rate after welding, the ferrite fraction in the HAZ structure must be 15% by area or more. And, it has been clarified that in order to achieve a ferrite fraction of 15% by area or more, the a value represented by the above formula (a) must be 1.60 or less. The a value is preferably 1.58 or less, more preferably 1.54 or less, and even more preferably 1.50 or less. From the viewpoint of increasing the ferrite fraction, the smaller the a value, the more preferable it is, but considering the range of each component in the steel material of the present disclosure, the lower limit of the a value can be about 0.80.

[0022] Next, the MA fraction will be explained. MA is a structure formed from untransformed austenite stabilized by C concentration during the cooling process after welding. Therefore, in order to reduce the MA fraction, it is effective to suppress the stabilized untransformed austenite by suppressing the C content and promoting the γ→α+θ transformation. From these points of view, the inventor devised formula (b) using the elements Si, Cr, and Mn that affect the ease of the γ→α+θ transformation for a certain C content. It was found that the MA fraction in the HAZ structure can be organized by the components of the steel material using the following formula (b).

[0023]

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[0024] In formula (b), Si, Cr, and Mn represent the content (mass%) of each element in the steel material.

[0025] Furthermore, as shown in the examples below, the inventors have found that in order to achieve excellent HAZ toughness with a Charpy impact absorption energy value of 70 J or more in the HAZ obtained by performing high heat input welding (or heating with a thermal cycle equivalent to the high heat input welding) under conditions of a slow cooling rate after welding, the MA fraction in the HAZ structure must be 2.0 area % or less. And, it has been clarified that in order to achieve an MA fraction of 2.0 area % or less, the b value represented by the above formula (b) must be 1.28 or less. The b value is preferably 1.25 or less, more preferably 1.22 or less, and even more preferably 1.20 or less. From the viewpoint of reducing the MA fraction, the smaller the b value, the more preferable it is, but considering the range of each component in the steel material of the present disclosure, the lower limit of the b value can be about 1.00.

[0026] Furthermore, as a result of investigations, the present inventors have clarified that, when the value a represented by the above formula (a) and the value b represented by the above formula (b) are within predetermined ranges, and the ferrite fraction and the MA fraction in the HAZ structure are within the above-mentioned ranges, excellent HAZ toughness can be stably ensured regardless of the block size of bainite in the HAZ structure.

[0027] In addition to the HAZ toughness described above, in order to achieve resistance to cracking during welding and sufficient strength for architectural steel materials such as box columns, each component is set within the following ranges to improve the strength characteristics of the base material (e.g., tensile strength of 60 kg class) and toughness of the base material.

[0028] Each component constituting the chemical composition will be described below.

[0029] C:0.02~0.06% by mass C is an essential element for ensuring strength, and if the C content is less than 0.02 mass%, the specified strength of the steel material (e.g., steel plate) cannot be ensured. The C content is preferably 0.03 mass% or more. However, if the C content is excessive, the HAZ becomes significantly hardened, and a large amount of MA is generated in the HAZ, which leads to deterioration of the HAZ toughness. Therefore, the C content needs to be suppressed to 0.06 mass% or less, preferably 0.05 mass% or less.

[0030] Si:0.01~0.20% by mass Silicon is a useful element for ensuring strength by solid solution strengthening. In order to ensure a predetermined strength of the steel plate, the silicon content is set to 0.01 mass% or more. The silicon content may be 0.04 mass% or more. However, if silicon is contained in excess, the HAZ toughness deteriorates due to the generation of MA in the HAZ. From this viewpoint, the silicon content must be 0.20 mass% or less. The silicon content is preferably 0.15 mass% or less. From the viewpoint of ensuring the HAZ toughness, the silicon content may be 0 mass%.

[0031] Mn:1.00~1.60% by mass Mn is a useful element for improving the hardenability of steel materials (e.g., steel plate) and ensuring strength. To effectively exert this effect, it is necessary to contain 1.00 mass% or more of Mn. The Mn content is preferably 1.05 mass% or more. However, excessive Mn content leads to the formation of MA and an increase in HAZ hardness, which leads to deterioration of HAZ toughness. Therefore, the Mn content is set to 1.60 mass% or less. The Mn content is preferably 1.55 mass% or less.

[0032] P: 0.010% by mass or less (including 0% by mass) P is an inevitable impurity and an element that adversely affects the toughness of the base material and HAZ. Therefore, the P content must be suppressed to 0.010 mass% or less. The P content is preferably 0.008 mass% or less. The lower the P content, the better, but from the viewpoint of suppressing the increase in costs associated with reducing the P content, the lower limit of the P content may be about 0.001 mass%.

[0033] S: 0.005% by mass or less (including 0% by mass) S is an element that forms MnS and deteriorates the toughness of the base material and HAZ, and the elongation of the base material. Therefore, the S content is preferably small, and is set to 0.005 mass% or less. The S content is preferably 0.003 mass% or less. The smaller the S content, the better, but from the viewpoint of suppressing the increase in costs associated with reducing the S content, the lower limit of the S content may be about 0.001 mass%.

[0034] Cu: 0.80~1.00 mass%, Ni: 0.90~1.10 mass% Both Cu and Ni have the effect of improving the strength and toughness of the base material, and are also effective elements in improving HAZ toughness. To achieve these effects, it is necessary to contain 0.80 mass% or more of Cu and 0.90 mass% or more of Ni. More preferably, the Cu content is 0.85 mass% or more, and the Ni content is 0.95 mass% or more. However, if the contents of these elements are excessive, the HAZ hardening becomes significant and the HAZ toughness deteriorates. For this reason, it is necessary to suppress the Cu content to 1.00 mass% or less, and the Ni content to 1.10 mass% or less. Preferably, the Cu content is 0.95 mass% or less, and the Ni content is 1.05 mass% or less.

[0035] Al: 0.02~0.05% by mass Al is useful as a deoxidizing element. To achieve this effect, it is necessary to contain 0.02 mass% or more of Al. The Al content is preferably 0.03 mass% or more. However, if the Al content is excessive, many coarse Al-based inclusions are generated in the HAZ, deteriorating the HAZ toughness, so it is necessary to suppress the Al content to 0.05 mass% or less.

[0036] N:0.0030~0.0080% by mass N is a useful element for finely dispersing nitrides of B and / or Ti and ensuring a certain amount of ferrite in the HAZ. To achieve this effect, the N content must be 0.0030% by mass or more. The N content is preferably 0.0040% by mass or more. However, if the N content is excessive, the amount of dissolved B is reduced, which reduces the hardenability and makes it difficult to ensure strength. Therefore, the N content must be kept to 0.0080% by mass or less, and is preferably 0.0070% by mass or less.

[0037] Cr:0.25~0.70% by mass Cr is a useful element for improving hardenability and ensuring strength. To achieve this effect, Cr must be contained in an amount of 0.25 mass% or more. The Cr content is preferably 0.30 mass% or more. However, excessive Cr content increases the HAZ hardness and deteriorates the HAZ toughness. Therefore, the Cr content is limited to 0.70 mass% or less. The Cr content can also be 0.65 mass% or less.

[0038] Ti:0.005~0.020% by mass Ti reacts with N to form fine Ti-containing nitrides (e.g., TiN), suppresses the coarsening of austenite grains (γ grains) in the HAZ, and acts as a ferrite nucleation site in the prior γ grains, making it a useful element for improving HAZ toughness. To effectively exert such effects, it is necessary to contain Ti at 0.005 mass% or more. The Ti content is preferably 0.010 mass% or more. However, if the Ti content is excessive, the Ti-containing nitrides become coarse and their number decreases, increasing the variation in HAZ toughness. For this reason, the Ti content is limited to 0.020 mass% or less. The Ti content is preferably 0.018 mass% or less.

[0039] B:0.0005~0.0025% by mass B is an element that contributes to improving hardenability. In addition, B reacts with N to generate BN, which acts as a ferrite nucleation site at the prior γ grain boundary, and is also a useful element for improving HAZ toughness. To effectively exert such effects, B must be contained in an amount of 0.0005 mass% or more. The B content is preferably 0.0010 mass% or more. However, if the B content is excessive, the HAZ hardening becomes significant and the HAZ toughness deteriorates, so the B content must be 0.0025 mass% or less. The B content is preferably 0.0023 mass% or less.

[0040] Ca:0.0005~0.0030% by mass Ca has the effect of reducing coarse Ti-containing nitrides (reducing the amount of coarse nitrides crystallized in combination with oxide-based inclusions), and is an element that contributes to improving the variation in HAZ toughness. In order to effectively exert this effect, Ca is contained at 0.0005 mass% or more. However, if the Ca content is excessive, the inclusions become coarse and the HAZ toughness deteriorates, so it is necessary to suppress it to 0.0030 mass% or less. The Ca content is preferably 0.0025 mass% or less.

[0041] The balance is Fe and unavoidable impurities. In a preferred embodiment, the balance is Fe and inevitable impurities. As inevitable impurities, the inclusion of trace elements (e.g., As, Sb, Sn, etc.) brought in due to the conditions of raw materials, materials, manufacturing facilities, etc. is permitted. Note that, for example, there are elements such as P and S, which are usually preferable to have a smaller content and are therefore inevitable impurities, but whose composition ranges are separately specified as described above. For this reason, in this specification, when referring to the "unavoidable impurities" constituting the balance, it is a concept excluding elements whose composition ranges are separately specified.

[0042] The steel material of the present disclosure and the steel slabs used in the manufacture of said steel material only need to have a chemical composition that satisfies the above-mentioned ranges of each element and the predetermined ranges of a and b values. V, which will be described below, does not have to be included. By including V as necessary together with the above elements, the following effects can be obtained.

[0043] V: More than 0% by mass, 0.1% by mass or less Addition of a small amount of V, more than 0 mass%, has the effect of improving hardenability and temper softening resistance. However, if the content exceeds 0.1 mass%, the HAZ toughness decreases, so it is preferable to keep the V content at 0.1 mass% or less. The V content is more preferably 0.06 mass% or less, and even more preferably 0.04 mass% or less.

[0044] The properties of the steel material of the present disclosure are described in detail below.

[0045] (1) Base material strength characteristics (yield strength or 0.2% proof stress, tensile strength, yield ratio) The yield strength or 0.2% proof stress determined by the tensile test described in the Examples below is 440 MPa or more. The yield strength or 0.2% proof stress is preferably 460 MPa or more. When the steel material of the present disclosure is used, for example, as a steel material for construction, the yield strength or 0.2% proof stress can be 540 MPa or less. Furthermore, the tensile strength determined by the tensile test described in the Examples below is 590 MPa or more. The tensile strength is preferably 610 MPa or more. When the steel material of the present disclosure is used, for example, as a steel material for construction, the tensile strength can be 740 MPa or less. Furthermore, the yield ratio determined from [the (yield strength or 0.2% proof stress) / the tensile strength]×100(%) is 80% or less. The yield ratio is preferably 78% or less.

[0046] (2) Base material toughness The Charpy impact absorption energy value obtained in the impact test described in the Examples below is at least 70 J. The Charpy impact absorption energy value is preferably at least 80 J, and more preferably at least 100 J.

[0047] (3) HAZ toughness As described in the following examples, - HAZ obtained by welding under conditions where the cooling rate after welding is slower than usual and a multi-phase structure can form as the HAZ structure, or -The test piece was subjected to a thermal cycle history equivalent to the HAZ of the above weld. The Charpy impact absorption energy value (average value) obtained in the impact test is at least 70 J. The Charpy impact absorption energy value is preferably at least 80 J, and more preferably at least 100 J.

[0048] The shape of the steel material of the present disclosure is not limited, and examples include steel plates (for example, thick steel plates having a plate thickness of 19 mm or more), steel pipes, H-shaped steel, and the like.

[0049] The use of the steel material of the present disclosure is not particularly limited. For example, it can be used as a steel material for construction, for example, it can be used for the diaphragm and skin plate constituting the box column described above. Examples of the diaphragm include an outer diaphragm and an inner diaphragm. The steel material of the present disclosure can be applied to the inner diaphragm, which is smaller in size and has a smaller heat capacity than the outer diaphragm, and as a result, the heat is less likely to escape and the cooling rate after welding tends to be slow. Even in the case of a combination of an inner diaphragm and a skin plate in which the cooling rate after welding tends to be slow, such as when the plate thickness of the inner diaphragm in the box column (for example, plate thickness 60 to 70 mm) is thicker than the plate thickness of the skin plate (for example, plate thickness 40 to 50 mm), the box column, which is a welded structure, can exhibit excellent HAZ toughness.

[0050] The method for producing the steel material of the present disclosure is not limited, but examples of methods for obtaining a steel material having the base metal strength characteristics required for a welded structure include the following methods.

[0051] The steel slab satisfying the above chemical composition is hot-rolled and then accelerated cooled. The hot-rolling conditions include a cumulative reduction rate of 25% or more in the temperature range of 1050°C to 900°C, and a rolling end temperature of 850°C to 950°C. The accelerated cooling conditions include an average cooling rate of 1.0°C / s or more from 800°C to 500°C. The accelerated cooling may be performed as is, or, if necessary, the accelerated cooling may be followed by quenching in a temperature range of Ac1 point to Ac3 point, and then tempering at Ac1 point or less to adjust the strength. EXAMPLES

[0052] The steel material of the present disclosure will be described in more detail below with reference to examples. The present disclosure is not limited to the following examples, and may be modified as appropriate within the scope of the above and below-described aims, and all such modifications are within the technical scope of the present disclosure.

[0053] 1. Preparation of steel plate (sample) A billet produced by converter-continuous casting or a billet produced by vacuum smelting, which satisfies the chemical composition shown in Table 1, was prepared. The billet was heated in the range of 1000°C to 1200°C, and then hot-rolled until the plate thickness was 19mm to 100mm (hot-rolling conditions: cumulative reduction in the temperature range of 1050°C to 900°C: 25% or more, rolling end temperature: 850°C to 950°C), and then cooled to the Bf point (500°C) or less by accelerated cooling (accelerated cooling is performed at an average cooling rate of 1.0°C / s or more from 800°C to 500°C). Next, the billet was heated in the range of Ac1 point (700°C) to Ac3 point (860°C), which is a two-phase region temperature, and then water quenched, and then heated at a temperature of Ac1 point or less, and air-cooled to obtain a steel plate as a steel material.

[0054] [Table 1]

[0055] 2. Characterization 2-1. Evaluation of base material strength characteristics (tensile test) A test piece (JIS Z2241 No. 4 test piece) was taken perpendicular to the rolling direction from the t (t is plate thickness, the same below) / 4 position (when it was not possible to take from the t / 4 position, it was taken near the t / 4 position). A tensile test was carried out according to a method conforming to JIS Z2241, and the yield strength or 0.2% proof stress and tensile strength were read. Then, a steel sheet was evaluated as having excellent base material strength characteristics when the yield strength or 0.2% proof stress was 440 MPa or more and 540 MPa or less, the tensile strength was 590 MPa or more and 740 MPa or less, and the yield ratio (the ratio of the yield strength or 0.2% proof stress to the tensile strength) was 80% or less.

[0056] 2-2. Evaluation of base material toughness (impact test) A test piece (a V-notch Charpy standard test piece according to JIS Z2242) was taken parallel to the rolling direction from the t / 4 position of the obtained steel plate (when it was not possible to take from the t / 4 position, it was taken from the vicinity of the t / 4 position). Three such test pieces were prepared. An impact test was carried out according to a method conforming to JIS Z2242, and the Charpy impact absorption energy values ​​at 0°C of the three test pieces were obtained. Then, when the average value of the Charpy impact absorption energy values ​​at 0°C of the three test pieces was 70J or more, it was evaluated that the base material toughness was excellent.

[0057] 2-3. Evaluation of HAZ toughness (impact test) To evaluate the HAZ toughness, impact tests were performed using actual joints or test pieces that had been given a simulated thermal history of the HAZ of an actual joint, i.e., test pieces that had been subjected to a thermal cycle in which the cooling rate after welding was slower than conventional conditions and heating was performed under conditions that made the HAZ structure more likely to become multiphase.

[0058] The actual joint used was a combination of a 45 mm thick skin plate and a 65 mm thick diaphragm, joined by electroslag welding under conditions of a welding heat input of 840 kJ / cm. The test pieces after the thermal cycle were obtained as follows. Test pieces for imparting thermal history were obtained by cutting out the steel plate from the t / 4 or 3t / 4 position with a size of 12 mm in the plate thickness direction, 33 mm or 55 mm in the rolling direction, and 55 mm or 33 mm in the direction perpendicular to the rolling direction. Next, the test piece for imparting thermal history was heated from room temperature to 1420°C at 50°C / sec by high-frequency heating, held at that temperature for 35 seconds, and then cooled from 1420°C to 1000°C in 64 seconds, from 1000°C to 800°C in 220 seconds, from 800°C to 600°C in 700 seconds, and from 600°C to 500°C in 700 seconds, thereby giving a simulated thermal history to the HAZ of a welded joint in one pass welding with a heat input of about 840kJ / cm, and obtaining a test piece after thermal cycle. Three actual joints and three test pieces after thermal cycle were prepared.

[0059] Using three actual joints and three test pieces after the thermal cycle, V-notch Charpy standard test pieces were prepared in accordance with JIS Z2242, and impact tests were conducted to measure the Charpy impact absorbed energy value at 0℃. For the actual joints, the V-notch was introduced in the fusion line (the boundary between the weld metal and the base metal). If the average Charpy impact absorbed energy value at 0℃ of the three V-notch Charpy standard test pieces was 70J or more, it was evaluated as having excellent HAZ toughness.

[0060] 3. Observation of HAZ structure The ferrite fraction and MA fraction of the test piece after the thermal cycle were easily distinguished by the contrast of the area except for the end 3 mm of the test piece after the thermal cycle to which the simulated thermal history was given, which was wet polished and then subjected to LePeller corrosion, and observed with an optical microscope at a magnification of ×400, and the area fractions of ferrite and MA were obtained from the contrast. The ferrite fraction and MA fraction of the actual joint were obtained by wet polishing and LePeller corrosion in the same manner as above, and then observed with an optical microscope at a magnification of ×400, and the area fractions of ferrite and MA were obtained from the contrast.

[0061] The results are shown in Table 2.

[0062] [Table 2]

[0063] The results in Tables 1 and 2 reveal the following. The steel plates No. 1 to 5, which satisfy the chemical composition of the present disclosure including the a and b values, have HAZ structures that satisfy the specified ferrite and MA fractions, and have Charpy impact absorption energy values ​​of 70 J or more, showing excellent HAZ toughness. On the other hand, the steel plates No. 6 to 12, which do not satisfy the chemical composition of the present disclosure, have poor HAZ toughness. In detail, the a values ​​of No. 6 to 11 are particularly large beyond the specified range, and as a result, a HAZ structure with a certain or higher ferrite fraction cannot be obtained, resulting in poor HAZ toughness. Furthermore, the b value of No. 12 is particularly large beyond the specified range, and as a result, a HAZ structure with a high MA fraction is obtained, resulting in poor HAZ toughness.

[0064] FIG. 1 shows a graph illustrating the relationship between the a-value and the ferrite fraction using the data of this embodiment. From FIG. 1, it can be seen that the fraction of ferrite in the HAZ structure can be sufficiently increased to 15 area% or more by setting the a-value to 1.60 or less. In FIG. 1, the comparative example in which the a-value is 1.60 or less is No. 12, in which the b-value is larger than the specified range. Also, FIG. 2 shows a graph illustrating the relationship between the b-value and the MA fraction using the data of this embodiment. From FIG. 2, it can be seen that the fraction of MA in the HAZ structure can be sufficiently suppressed to 2.0 area% or less by setting the b-value to 1.28 or less. In FIG. 2, the comparative examples in which the b-value is 1.28 or less are No. 6 to 11, in which the a-value is larger than the specified range. By setting the ferrite fraction to 15 area% or more and the MA fraction to 2.0 area% or less in the HAZ structure as described above, it is possible to reliably ensure excellent HAZ toughness even when large heat input welding is performed, the cooling rate after welding is slow, and a multi-phase structure can be formed as the HAZ structure.

Claims

1. The chemical composition is C: 0.02 to 0.06% by mass, Si: 0.01 to 0.20% by mass, Mn: 1.00 to 1.60% by mass, P: 0.010% by mass or less (including 0% by mass), S: 0.005% by mass or less (including 0% by mass), Cu: 0.80 to 1.00% by mass, Al: 0.02 to 0.05% by mass, N: 0.0030 to 0.0080% by mass, Ni: 0.90 to 1.10% by mass, Cr: 0.25 to 0.70% by mass, Ti: 0.005 to 0.020% by mass, B: 0.0005 to 0.0025% by mass, and Ca: 0.0005 to 0.0030 mass%, with the balance being Fe and unavoidable impurities; A steel material having an a-value represented by the following formula (a) of 1.60 or less and a b-value represented by the following formula (b) of 1.28 or less. [0010] In formula (a), C, Si, Mn, Cu, Ni, Cr, Mo, V, B, and Nb indicate the content (mass%) of each element in the steel material, and elements that are not included are set to zero. When Mn is less than 1.20 mass%, the term (5.1(Mn-1.2)+5) is replaced with (3.33Mn+1). In addition, D and E are respectively expressed by the following formulas. [0025] In the above formula expressing D, MIN indicates a function that adopts the minimum value of the two arguments in parentheses, and Ti and N indicate the contents (mass%) of each element in the steel material. Ａ is the Avogadro constant (6.02 x 10 ２３ Mol －１ ) [0030] In the formula expressing E, MIN indicates a function that adopts the minimum value of the two arguments in parentheses, and B, Ti, and N indicate the contents (mass%) of each element in the steel material. Ａ is the Avogadro constant (6.02 x 10 ２３ Mol －１ ) [0045] In formula (b), Si, Cr, and Mn represent the content (mass%) of each element in the steel material.

2. The steel material according to claim 1, further comprising, in the chemical composition, V: more than 0 mass % and 0.1 mass % or less, instead of a portion of Fe.

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