steel

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] This disclosure relates to steel materials, and more particularly to steel materials that can be used to obtain welded structures with excellent heat-affected zone (HAZ) toughness (hereinafter referred to as "HAZ toughness"). [Background technology]

[0002] In recent years, with the increasing size of welded structures such as bridges, high-rise buildings, and ships, for example, the plate thickness of the steel materials used in these welded structures has increased, and high-heat input welding is being performed to improve welding efficiency. Furthermore, from the perspective of ensuring the safety of welded structures, it is required that the heat-induced azure zone (HAZ) exhibits excellent toughness even after high-heat input welding. Patent Document 1 addresses the problem that when high-heat input welding is performed, the HAZ is heated to a high-temperature austenite region and then slowly cooled, causing the structure of the HAZ (especially near the bond area of ​​the HAZ) to become coarse and the toughness of that area to deteriorate. It proposes a high-strength, thick steel plate that exhibits excellent HAZ toughness even when high-heat input welding of 30 to 100 kJ / mm is performed.

[0003] In detail, Patent Document 1 shows that by satisfying the predetermined relationship of equation (1), keeping the chemical composition of the steel sheet 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 that do not disappear in solid solution during high-heat input welding into the steel, thereby realizing a thick steel sheet with improved HAZ toughness. [Prior art documents] [Patent Documents]

[0004] [Patent Document 1] Japanese Patent Publication No. 2009-167447 [Overview of the project] [Problems that the invention aims to solve]

[0005] Box columns (also called "welded assembled box-section columns") are examples of welded structures that constitute welded structures such as high-rise buildings (in this specification, these welded structures are collectively referred to as "welded structures"). Examples of box columns include welded structures manufactured by joining, for example, a skin plate welded to a square steel pipe shape and multiple diaphragms that partition the internal space, using electroslag welding. In conventional welding methods, the cooling rate of the heat-affected zone (HAZ) after welding is fast, and the structure of the HAZ (hereinafter sometimes referred to as "HAZ structure") is mainly composed of bainite. However, in recent years, there has been an increasing demand for larger welded structures, more complex shapes, and the securing of large spaces. Accordingly, for example, in the case of a box column with an internal diaphragm, 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 differ from conventional structures, becoming a multi-phase structure of bainite and other structures. However, the characteristics of HAZ when the HAZ structure is multiphase have not been studied to date.

[0006] This disclosure has been made in view of the above circumstances, and one of its purposes is to provide a steel material that can produce a welded structure with excellent HAZ toughness even when high heat input welding is performed and the cooling rate after welding is slow, which can lead to the formation of a multi-phase structure as the HAZ structure. [Means for solving the problem]

[0007] One aspect 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: 0.0005~0.0025 mass%, and Ca: satisfies 0.0005~0.0030 mass%, with the remainder consisting of Fe and unavoidable impurities. The steel material satisfies the following conditions: the value of a, expressed by formula (a), is 1.60 or less, and the value of b, expressed by formula (b), is 1.28 or less.

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[0008] Aspect 2 of the present invention is the steel material according to Aspect 1, having the chemical composition and containing more than 0 mass% and 0.1 mass% or less of V in place of a part of Fe.

Advantages of the Invention

[0009] According to the present disclosure, it is possible to provide a steel material capable of obtaining a welded structure excellent in HAZ toughness even when large heat input welding is performed and the cooling rate after welding is slow, and a duplex structure can be formed as the HAZ structure.

Brief Description of the Drawings

[0010] [Figure 1] It is a graph showing the relationship between the a value and the ferrite fraction in the examples. [Figure 2] It is a graph showing the relationship between the b value and the MA fraction in the examples.

Embodiments for Carrying Out the Invention

[0011] When the cooling rate after large heat input welding is slow, a duplex structure of bainite, ferrite, and island martensite (MA) can be formed as the HAZ structure. So far, when the HAZ structure is a duplex structure, the influence of the duplex structure on the HAZ toughness has not been studied, and it has been difficult to reliably ensure excellent HAZ toughness when large heat input welding is performed and the cooling rate after welding is slow.

[0012] Therefore, the inventors first investigated the influence of the ferrite and MA fractions, other than bainite, on HAZ toughness in the HAZ structure. As a result, they first clarified that in order to achieve excellent HAZ toughness in the multiphase structure, it is effective to suppress crack initiation by reducing HAZ hardness through a reduction in 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, with the aim of obtaining a welded structure that exhibits excellent HAZ toughness in high-heat-input welding, even when the above multiphase structure is formed as the HAZ structure, by controlling the chemical composition of the steel material, they diligently researched the relationship between the chemical composition of the steel material, the HAZ structure, and HAZ toughness.

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

[0014] 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 ferrite nucleation sites are effective. The inventors investigated the chemical composition from the viewpoint of suppressing hardenability and increasing ferrite nucleation sites. As a result, they found that the ferrite fraction in the HAZ structure can be organized by the composition of the steel material using the following formula (a), which is based on the generally known hardenability index (DI), includes Nb, an element that is not included in DI but affects hardenability, and further considers 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 represent the content (% by mass) of each element in the steel material, and for elements not contained, they are set to zero. When Mn is less than 1.20% by mass, the term (5.1(Mn - 1.2)+5) is replaced by (3.33Mn + 1). The steel material of the present disclosure does not contain Mo and Nb, and for these elements, the a value is calculated with zero. Also, D and E are each represented by the following formulas.

[0017]

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[0018] In the formula representing the above D, MIN represents a function that adopts the minimum value among the two arguments within the parentheses, and Ti and N represent the content (% by mass) of each element in the steel material. Also, N A represents Avogadro's constant (6.02×10 23 mol -1 ).

[0019]

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[0020] In the formula representing the above E, MIN represents a function that adopts the minimum value among the two arguments within the parentheses, and B, Ti, and N represent the content (% by mass) of each element in the steel material. Also, N A represents Avogadro's constant (6.02×10 23 mol -1 ).

[0021] Furthermore, as shown in the examples described later, the inventors have found that in order to achieve excellent HAZ toughness, such as a Charpy impact absorption energy value of 70 J or more for HAZ obtained by high heat input welding (or heating with a thermal cycle equivalent to high heat input welding) under conditions of slow cooling rate after welding, the ferrite fraction in the HAZ structure must be 15 area % or more. They have also clarified that in order to achieve a ferrite fraction of 15 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, a smaller a value is preferable, but considering the range of each component in the steel material of this disclosure, the lower limit of the a value can be around 0.80.

[0022] Next, we will explain the MA fraction. MA is a structure that is formed starting from stabilized untransformed austenite, which is concentrated with carbon during the cooling process after welding. Therefore, in order to reduce the MA fraction, it is effective to suppress the amount of carbon and promote the γ→α+θ transformation to suppress the stabilized untransformed austenite. From these perspectives, the inventors have devised equation (b) using the elements Si, Cr, and Mn, which affect the ease of the γ→α+θ transformation for a given amount of carbon. We found that the MA fraction in the HAZ structure can be organized by the composition of the steel material using the following equation (b).

[0023]

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

[0025] Furthermore, as shown in the examples described later, the inventors have found that in order to achieve excellent HAZ toughness, such as a Charpy impact absorption energy value of 70 J or more for HAZ obtained by high heat input welding (or heating with a thermal cycle equivalent to high heat input welding) under conditions of slow cooling rate after welding, the MA fraction in the HAZ structure must be 2.0 area% or less. They have also 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, a smaller b value is preferable, but considering the range of each component in the steel material of this disclosure, the lower limit of the b value can be around 1.00.

[0026] Furthermore, the inventors have found that, as a result of their investigations, if the value of a represented by formula (a) and the value of b represented by formula (b) are within a predetermined range, and the ferrite fraction and MA fraction in the HAZ structure are within the above-mentioned range, then 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 mentioned above, for example, in steel materials for construction such as box columns, in order to improve crack resistance during welding and achieve sufficient high strength, the strength characteristics of the base material (e.g., tensile strength of 60 kg class) and the toughness of the base material are improved, and each component is set within the following ranges.

[0028] The following describes each component that makes up the chemical composition.

[0029] C:0.02~0.06% by mass Carbon (C) is an essential element for ensuring strength, and if the C content is less than 0.02 mass%, the required 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 hardening of the heat-affected zone (HAZ) becomes significant, and a large amount of mineral oxide (MA) is generated in the HAZ, leading to a deterioration of the HAZ toughness. Therefore, the C content needs to be kept below 0.06 mass%, preferably below 0.05 mass%.

[0030] Si:0.01~0.20% by mass Si is a useful element for ensuring strength through solid solution strengthening. To ensure the required strength of the steel sheet, it should contain 0.01% by mass or more of Si. The Si content may be 0.04% by mass or more. However, if Si is present in excess, MA formation in the HAZ will degrade the HAZ toughness. From this viewpoint, the Si content should be 0.20% by mass or less. Preferably, the Si content is 0.15% by mass or less. Furthermore, from the viewpoint of ensuring HAZ toughness, the Si content may be 0% by mass.

[0031] Mn:1.00~1.60% by mass Mn is a useful element for improving the hardenability of steel materials (e.g., steel plates) and ensuring strength. To effectively achieve these effects, it is necessary to include 1.00 mass% or more of Mn. Preferably, the Mn content is 1.05 mass% or more. However, excessive Mn content leads to the formation of MA and an increase in HAZ hardness, resulting in a deterioration of HAZ toughness. Therefore, the Mn content should be 1.60 mass% or less. Preferably, the Mn content is 1.55 mass% or less.

[0032] P: 0.010% by mass or less (including 0% by mass) P is an unavoidable impurity and an element that adversely affects the toughness of the base material and HAZ. Therefore, the amount of P must be kept below 0.010 mass%. Preferably, the amount of P is 0.008 mass% or less. The lower the amount of P, the better, but from the viewpoint of suppressing the increase in costs associated with reducing the amount of P, the lower limit of the amount of P can be around 0.001 mass%.

[0033] S: 0.005% by mass or less (including 0% by mass) S is an element that forms MnS, which degrades the toughness of the matrix and HAZ, as well as the elongation of the matrix. Therefore, a low amount of S is preferable, and should be 0.005 mass% or less. Preferably, the amount of S is 0.003 mass% or less. The lower the amount of S, the better, but from the viewpoint of suppressing the increase in costs associated with reducing the amount of S, the lower limit of the amount of S can be around 0.001 mass%.

[0034] Cu: 0.80~1.00 mass%, Ni: 0.90~1.10 mass% Both Cu and Ni are effective in 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 include 0.80% by mass or more of Cu and 0.90% by mass or more of Ni. More preferably, the Cu content is 0.85% by mass or more and the Ni content is 0.95% by mass or more. However, if the content of these elements is excessive, HAZ hardening becomes significant and the HAZ toughness deteriorates. For this reason, the Cu content should be kept to 1.00% by mass or less and the Ni content to 1.10% by mass or less. Preferably, the Cu content is 0.95% by mass or less and the Ni content is 1.05% by 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 include at least 0.02% by mass of Al. Preferably, the Al content is 0.03% by mass or more. However, if the Al content is excessive, many coarse Al-based inclusions will form in the HAZ, degrading the HAZ toughness, so it is necessary to keep it below 0.05% by mass.

[0036] N:0.0030~0.0080% by mass N is a useful element for finely dispersing B and / or Ti nitrides and ensuring a certain level of ferrite in the HAZ. To achieve these effects, the N content needs to be 0.0030 mass% or more. Preferably, the N content is 0.0040 mass% or more. However, if the N content is excessive, the amount of solid-solution B decreases, reducing hardenability and making it difficult to ensure strength. Therefore, the N content needs to be kept below 0.0080 mass%, preferably below 0.0070 mass%.

[0037] Cr:0.25~0.70% by mass Cr is a useful element for increasing hardenability and ensuring strength. To achieve this effect, it is necessary to include 0.25% by mass or more of Cr. Preferably, the Cr content is 0.30% by mass or more. However, if the Cr content is excessive, it leads to an increase in HAZ hardness and deterioration of HAZ toughness. Therefore, the Cr content should be kept below 0.70% by mass. The Cr content can also be 0.65% by mass or less.

[0038] Ti:0.005~0.020% by mass Ti reacts with N to form fine Ti-containing nitrides (e.g., TiN), which suppress the coarsening of austenite grains (γ grains) in the HAZ and act as ferrite nucleation sites within prior γ grains, making it a useful element for improving HAZ toughness. To effectively exert these effects, it is necessary to include 0.005 mass% or more of Ti. Preferably, the Ti content is 0.010 mass% or more. However, if the Ti content is excessive, the Ti-containing nitrides become coarser and their number decreases, increasing the variability in HAZ toughness. For this reason, the Ti content should be kept below 0.020 mass%. Preferably, the Ti content is 0.018 mass% or less.

[0039] B:0.0005~0.0025% by mass B is an element that contributes to improved hardenability. Furthermore, B reacts with N to produce BN, which acts as a ferrite nucleation site at prior γ grain boundaries, making it a useful element for improving HAZ toughness. To effectively exert these effects, it is necessary to include 0.0005% by mass or more of B. The B content is preferably 0.0010% by mass or more. However, if the B content is excessive, HAZ hardening becomes significant and HAZ toughness deteriorates, so the B content must be 0.0025% by mass or less. The B content is preferably 0.0023% by 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 crystallizing in combination with oxide inclusions), and is an element that contributes to improving the variability of HAZ toughness. To effectively exert this effect, the Ca content should be 0.0005% by mass or more. However, if the Ca content is excessive, the inclusions will become coarser and the HAZ toughness will deteriorate, so it is necessary to keep it below 0.0030% by mass. The Ca content is preferably 0.0025% by mass or less.

[0041] The remainder is Fe and unavoidable impurities. In one preferred embodiment, the remainder consists of Fe and unavoidable impurities. As unavoidable impurities, the inclusion of trace elements (e.g., As, Sb, Sn, etc.) introduced depending on the conditions of the raw materials, materials, manufacturing equipment, etc., is acceptable. Note that there are elements, such as P and S, which are generally preferable in smaller amounts and therefore unavoidable impurities, but whose composition range is separately defined as described above. For this reason, in this specification, when we refer to "unavoidable impurities" that constitute the remainder, we mean the concept excluding elements whose composition range is separately defined.

[0042] The steel materials of this disclosure, and the steel billets used in the manufacture of said steel materials, only need to satisfy the chemical composition of each of the above elements, and the a and b values ​​within predetermined ranges. The element V described below does not need to be included. By including V along with the above elements as needed, the following effects can be obtained.

[0043] V: More than 0% by mass, 0.1% by mass or less V, when added in small amounts exceeding 0% by mass, has the effect of increasing hardenability and tempering softening resistance. However, since the HAZ toughness decreases if the content exceeds 0.1% by mass, it is preferable to keep it at 0.1% by mass or less. The V content is more preferably 0.06% by mass or less, and even more preferably 0.04% by mass or less.

[0044] The properties of the steel material disclosed herein are described in detail below.

[0045] (1) Strength characteristics of the base material (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 later is 440 MPa or higher. The yield strength or 0.2% proof stress is preferably 460 MPa or higher. When the steel material of this disclosure is used, for example, as steel material for construction, the yield strength or 0.2% proof stress may be 540 MPa or lower. Also, the tensile strength determined by the tensile test described in the examples later is 590 MPa or higher. The tensile strength is preferably 610 MPa or higher. When the steel material of this disclosure is used, for example, as steel material for construction, the tensile strength may be 740 MPa or lower. Furthermore, the yield ratio determined from [the above (yield strength or 0.2% proof stress) / the above tensile strength] × 100 (%) is 80% or lower. The yield ratio is preferably 78% or lower.

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

[0047] (3) HAZ densitivity As described in the examples below, • The cooling rate after welding tends to be slower than conventional methods, and the HAZ obtained by welding under conditions where a multi-phase structure can be formed as the HAZ structure, or • Test specimens that have undergone a thermal cycling history corresponding to the HAZ of the above welding, The Charpy impact absorption energy value (average value) obtained in the impact test is 70 J or higher. Preferably, the Charpy impact absorption energy value is 80 J or higher, and more preferably 100 J or higher.

[0048] The shape of the steel material in this disclosure is not limited and includes, for example, steel plates (e.g., thick steel plates with a thickness of 19 mm or more), steel pipes, H-beams, etc.

[0049] The applications of the steel materials disclosed herein are not particularly limited. For example, they can be used as building materials, and for example, they can be used in the diaphragms and skin plates that constitute the box columns described above. Examples of diaphragms include outer diaphragms and inner diaphragms. The steel materials disclosed herein can be applied to inner diaphragms, which are smaller in size and have a smaller heat capacity than outer diaphragms, and as a result, heat does not escape easily, and the cooling rate after welding tends to be slow. Even in cases where the thickness of the inner diaphragm in a box column (e.g., 60-70 mm) is thicker than the thickness of the skin plate (e.g., 40-50 mm), and the cooling rate after welding tends to be slow, the box column, which is a welded structure, can exhibit excellent HAZ toughness.

[0050] The manufacturing method of the steel material described herein is not limited, but the following methods are examples of methods for obtaining a steel material that has the base material strength characteristics required for a welded structure.

[0051] A steel billet satisfying the aforementioned chemical composition is hot-rolled and then accelerated-cooled. The hot-rolling conditions include a cumulative reduction ratio of 25% or more in the temperature range of 1050°C to 900°C, and a rolling completion 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 steel may be left as is after accelerated-cooling, or, if necessary, after accelerated-cooling, it may be quenched in the temperature range of Ac1 to Ac3, and then tempered at Ac1 or below to adjust the strength. [Examples]

[0052] The steel materials of this disclosure will be described in more detail below with reference to examples. This disclosure is not limited by the following examples, and can be implemented with appropriate modifications to the extent that it is consistent with the spirit described above and below, and all such modifications are included within the technical scope of this disclosure.

[0053] 1. Preparation of steel plate (sample) Steel billets were prepared by converter-continuous casting or vacuum melting, satisfying the chemical composition shown in Table 1. These steel billets were heated in a range of 1000°C to 1200°C, then hot-rolled until the plate thickness was 19 mm to 100 mm (hot-rolling conditions: cumulative reduction ratio of 25% or more in the temperature range of 1050°C to 900°C, rolling completion temperature: 850°C to 950°C), and then cooled to below the Bf point (500°C) by accelerated cooling (accelerated cooling was performed at an average cooling rate of 1.0°C / s or more from 800°C to 500°C). Next, the steel billets were heated in a two-phase temperature range of Ac1 point (700°C) to Ac3 point (860°C), then water-quenched, and then heated to a temperature below the Ac1 point, followed by air cooling to obtain steel plates.

[0054] [Table 1]

[0055] 2. Evaluation of characteristics 2-1. Evaluation of base material strength properties (tensile test) A test specimen (JIS Z2241 No. 4 test specimen) was taken perpendicular to the rolling direction from the t / 4 position (or near the t / 4 position if it was not possible to take a specimen from the t / 4 position) of the obtained steel plate. Tensile tests were performed according to the method in accordance with JIS Z2241, and the yield strength or 0.2% proof stress and tensile strength were read. A specimen was evaluated as having excellent base material strength characteristics if 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 (ratio of yield strength or 0.2% proof stress to tensile strength) was 80% or less.

[0056] 2-2. Evaluation of base material toughness (impact test) A test specimen (V-notch Charpy standard test specimen in accordance with JIS Z2242) was taken parallel to the rolling direction from the t / 4 position of the obtained steel plate (or near the t / 4 position if it was not possible to take a specimen from the t / 4 position). Three such test specimens were prepared. The impact test was carried out in accordance with JIS Z2242, and the Charpy impact absorption energy values ​​at 0°C for the three test specimens were determined. The base material was evaluated as having excellent toughness if the average value of the Charpy impact absorption energy values ​​at 0°C for the three test specimens was 70 J or higher.

[0057] 2-3. Evaluation of HAZ toughness (impact test) In evaluating HAZ toughness, impact tests were performed using actual joints or specimens with a reproduced thermal history of the HAZ of actual joints, i.e., specimens heated under thermal cycle conditions where the cooling rate after welding was slower than conventional methods, making it easier for the HAZ structure to become multiphase.

[0058] As the actual joint, a combination of a 45 mm thick skin plate and a 65 mm thick diaphragm was used, joined by electroslag welding under conditions of a welding heat input of 840 kJ / cm. Furthermore, the post-thermal cycle test specimens were obtained as follows: From the t / 4 or 3t / 4 position of the prepared steel plate, test specimens for imparting thermal history were obtained by cutting out pieces with dimensions of 12 mm in the 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 specimen for imparting thermal history was heated from room temperature to 1420°C at 50°C / sec by high-frequency heating and held for 35 seconds. Then, it was 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. This process reproduced the thermal history of the HAZ of the welded joint in a single pass weld with a heat input of approximately 840 kJ / cm, and a specimen after thermal cycling was obtained. Three actual joints and three specimens after thermal cycling were prepared.

[0059] Using three actual joints and post-thermal-cycle test specimens, V-notch Charpy standard test specimens were prepared in accordance with JIS Z2242, and impact tests were conducted to measure the Charpy impact absorption energy value at 0°C. For the actual joints, the V-notch was introduced in the Fusion Line section (the boundary between the weld metal and the base metal). A value of 70J or higher as the average of the Charpy impact absorption energy values ​​at 0°C for the three V-notch Charpy standard test specimens was evaluated as having excellent HAZ toughness.

[0060] 3. Observation of HAZ tissue The ferrite and MA fractions of the thermally cycled specimens were easily distinguished by contrast after wet polishing and repeller etching were performed on the area excluding the 3 mm end of the thermally cycled specimens to which the reproduced thermal history had been applied, and then observed with an optical microscope at ×400x magnification. From this contrast, the area fractions of ferrite and MA were determined, respectively. Similarly, the ferrite and MA fractions of the actual joints were determined by wet polishing and repeller etching on the Fusion Line portion in the same manner as above, and then observed with an optical microscope at ×400x magnification, and the area fractions of ferrite and MA were determined, respectively, from the contrast.

[0061] The results are shown in Table 2.

[0062] [Table 2]

[0063] The results in Tables 1 and 2 show the following: Steel sheets No. 1 to 5 that satisfy the chemical composition of this disclosure, including the a and b values, have HAZ structures that satisfy the predetermined ferrite and MA fractions, and exhibit excellent HAZ toughness with a Charpy impact absorption energy value of 70 J or more. On the other hand, steel sheets No. 6 to 12 that do not satisfy the chemical composition of this disclosure showed inferior HAZ toughness. In detail, Nos. 6 to 11 had a particularly large a value that exceeded the specified range, resulting in an HAZ structure with a ferrite fraction above a certain level and thus inferior HAZ toughness. Also, No. 12 had a particularly large b value that exceeded the specified range, resulting in a HAZ structure with a high MA fraction and thus inferior HAZ toughness.

[0064] Using data from this embodiment, Figure 1 shows a graph illustrating the relationship between the a value and the ferrite fraction. From Figure 1, it can be seen that by setting the a value to 1.60 or less, the ferrite fraction in the HAZ structure can be sufficiently increased to 15 area % or more. In Figure 1, the comparative example with an a value of 1.60 or less is No. 12, where the b value is larger than the specified range. Furthermore, using data from this embodiment, Figure 2 shows a graph illustrating the relationship between the b value and the MA fraction. From Figure 2, it can be seen that by setting the b value to 1.28 or less, the MA fraction in the HAZ structure can be sufficiently suppressed to 2.0 area % or less. In Figure 2, the comparative examples with a b value of 1.28 or less are Nos. 6 to 11, where the a value is larger than the specified range. In the HAZ structure, by setting the ferrite fraction to 15 area % or more and the MA fraction to 2.0 area % or less as described above, excellent HAZ toughness can be reliably ensured even when high heat input welding is performed and the cooling rate after welding is slow, potentially leading to the formation of a multi-phase structure in 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 mass%, and Ca: satisfies 0.0005 to 0.0030 mass%, with the remainder being Fe and unavoidable impurities. A steel material that satisfies the following conditions: the value of a, expressed by formula (a), is 1.60 or less, and the value of b, expressed by formula (b), is 1.28 or less. [Math 1] In equation (a), C, Si, Mn, Cu, Ni, Cr, Mo, V, B, and Nb represent the mass %) content of each element in the steel, and elements that are not present are represented as zero. If Mn is less than 1.20 mass%, the term (5.1(Mn-1.2)+5) is replaced with (3.33Mn+1). Also, D and E are expressed by the following formulas, respectively. [Math 2] In the above formula representing D, MIN indicates a function that takes the minimum value of the two arguments in parentheses, and Ti and N indicate the content (mass %) of each element in the steel. Ａ Avogadro's number (6.02 × 10⁻¹⁰) is Avogadro' ２３ mol －１ ) represents. [Math 3] In the above formula representing E, MIN indicates a function that takes the minimum value of the two arguments in parentheses, and B, Ti, and N indicate the content (mass %) of each element in the steel. Ａ Avogadro's number (6.02 × 10⁻¹⁰) is Avogadro' ２３ mol －１ ) represents. [Math 4] In formula (b), Si, Cr, and Mn represent the mass percentage content of each element in the steel.

2. The steel material according to claim 1, wherein, in the above chemical composition, V: more than 0% by mass and 0.1% by mass or less is included in place of a portion of Fe.

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