JP2007327100A - Thick steel plate having excellent toughness in high-heat input weld heat affected zone - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a thick steel plate having a thickness of 15 to 100 mm and a yield strength in a class of 325 to 500 MPa, and in which the average value of Charpy impact absorbed energy at -20 to 0°C is ≥150 J in a HAZ (Heat Affected Zone) welded by a welding heat input value of 20 to 150 kJ/mm. <P>SOLUTION: The thick steel plate having excellent toughness in the high heat input weld heat affected zone welded by a welding heat input value of 20 to 150 kJ/mm has a composition comprising, by mass, 0.03 to 0.2% C, ≤0.5% Si, 0.5 to 2.0% Mn, ≤0.02% P, 0.001 to 0.005% S, 0.001 to 0.1% Al, 0.01 to 0.1% V, 0.0001 to <0.0003% B, 0.001 to 0.006% N and ≤0.004% O, and the balance iron with inevitable impurities. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a thick steel plate excellent in the toughness of a heat affected zone (HAZ) in a high heat input welded joint. The present invention is mainly applied to thick steel plates manufactured in the steel industry. The present invention can also be applied to steel products such as H-shaped steel and steel pipe other than thick steel plates. The steel plate to which the present invention is applied is used for welding structures such as shipbuilding, architecture, bridges, tanks, offshore structures, line pipes, etc., and subjected to high heat input welding with high welding work efficiency, and welded parts It is suitable when the required level of toughness is high.

In recent years, major requirements for welded structures such as shipbuilding and construction are to increase the size of structures, increase the efficiency of construction, and improve safety against destruction. In response to such trends, thicker steel sheets for welded structures are required to have higher HAZ toughness when high-efficiency large heat input welding is applied. At the same time, as a basic characteristic of thick steel plates, there is still a strong demand for small variations in the strength of the base metal when mass-producing the same steel type and good weldability when performing small heat input welding. .
The conventional technology for increasing the high heat input welding HAZ toughness of thick steel plates is generally aimed at refining the structure of the HAZ near the fusion line. There are roughly two methods for refining the HAZ structure. The first method is to suppress the growth of austenite (γ) grains by the pinning effect, maintain fine grains γ, increase the area of γ grain boundaries as ferrite (α) transformation nuclei as much as possible, and reduce the HAZ structure. This is a method for miniaturization. The second method is a method for refining the HAZ structure by utilizing precipitates present in the grain boundaries and grains of γ as α transformation nuclei. The key point of this technique is to disperse as many precipitates as possible of the α transformation with high nucleation ability.

In the second HAZ structure refinement method, as disclosed in Patent Document 1, the effectiveness of VN is known as a precipitate having a high nucleation ability of α transformation. Further, BN is known in Non-Patent Document 1, and Fe ₂₃ (C, B) ₆ and Fe ₃ (C, B) are known in Non-Patent Document 2. It is known that these precipitates precipitate in the γ grain boundaries and γ grains during the cooling process of HAZ, and these α transformation nuclei are generated to form a large number of α transformation nuclei, making the structure finer and improving the HAZ toughness. Yes.
The problem of the prior art using VN as an α transformation nucleus in high heat input welding HAZ is that the amount of N is intentionally increased in order to deposit a sufficient amount of VN in the γ region of the cooling process of HAZ. It is that surface cracks of the continuous cast slab are likely to occur.

  For example, the amount of N in the example of Patent Document 1 has a very high characteristic of 0.0081 to 0.0156% N from Table 1. When such high N steel is continuously cast, when an external force is applied to the surface of the slab at the bending or bending back part of a continuous casting machine, a large amount of nitrides precipitated at the γ grain boundary promotes embrittlement. Γ grain boundary occurs, hindering the productivity of the casting process, reducing the yield of the slab, increasing cracks on the surface of the steel plate after rolling and increasing the maintenance load. There is a problem to do. Accordingly, it is required to draw out the VN effect (α transformation nucleus effect) on the premise of a low N amount that can avoid surface cracking of the continuous cast slab.

On the other hand, the problem of the prior art in which B precipitates are used as α transformation nuclei in high heat input welding HAZ is that the following material variations occur when mass steel plates of the same steel type are mass-produced.
(1) The strength and toughness of the base material vary when manufacturing thick steel plates.
(2) HAZ toughness varies during high heat input welding.
(3) The welding low temperature crack sensitivity and toughness of HAZ vary during small heat input welding.

These material variations occur because the hardenability of the γ grain boundaries varies greatly depending on the amount of dissolved B in γ during the cooling process. It is known that the solid solution B segregates at the γ grain boundary to reduce the energy of the γ grain boundary and suppress the α transformation from the γ grain boundary. FIG. As shown in FIG. 7, in a slight range where the amount of solute B in γ is 0 to 5 ppm (0.0005%), the hardenability is remarkably increased as the amount of solute B is increased. 0.0005%) hardens hardenability. That is, when mass-producing thick steel plates of the same steel type with B added, if the amount of solute B in γ in the base metal or HAZ varies in the range of 0 to 5 ppm (0.0005%), the hardenability varies greatly. Therefore, the control of the α transformation structure becomes unstable, and the strength and toughness vary greatly. Therefore, it is required to produce a B precipitate effect (α transformation nucleus effect) on the premise of a low amount of solute B in γ that can avoid such material variations.
In order to avoid the high hardenability (curability) caused by the solid solution B in γ, a technique for controlling B as an impurity element in a very small amount has been invented. For example, it is necessary to control the B amount to 0.0002% or less in Patent Document 2 and to less than 0.0003% in Patent Document 3. In Table 1 of the example of Patent Document 2, a very small amount of B is controlled in the range of 0.00004% (0.4 ppm) to 0.00036 (3.6 ppm).
JP 05-186848 A JP 2001-081529 A JP-A-10-008193 Funakoshi, et al., “Improvement of microstructure and toughness of high heat input weld bonds of high strength steel by adding rare earth elements and B”, Iron and Steel, 63rd (1977), p. 303-312 Ono, Y. et al., “Characteristics of HAZ microstructure of Ti-B high heat input welding steel”, Iron and Steel, 73rd (1987), p. 1010-1017 Masakatsu Ueno et al., “Optimum amount of boron that contributes to improving hardenability of steel”, Iron and Steel, 74th (1988), p. 910-917

An object of the present invention is to absorb Charpy impact at −20 ° C. to 0 ° C. in HAZ having a thickness of 15 to 100 mm and a yield strength of 325 to 500 MPa class and welded at a welding heat input of 20 to 150 kJ / mm. It is to provide a thick steel plate having an energy average value of 150 J or more.
The steel plate to which the present invention is applied can be used for shipbuilding, construction and other large welded structures, and can ensure good HAZ toughness even when subjected to high heat input welding with high welding construction efficiency. aim.

“1” The present invention is mass%, C: 0.03 to 0.2%, Si: 0.5% or less, Mn: 0.5 to 2.0%, P: 0.02% or less, S : 0.001-0.005%, Al: 0.001-0.1%, V: 0.01-0.1%, B: 0.0001% or more and less than 0.0003%, N: 0.001 ~ 0.006%, O: 0.004% or less, consisting of remaining iron and inevitable impurities, toughness of heat input heat affected zone with high heat input of 20 to 150 kJ / mm It is an excellent thick steel plate.
“2” The present invention is further characterized by containing, in mass%, Ti: 0.003 to 0.017%, and satisfying the following formula (1) calculated using mass% of N and Ti: It is a thick steel plate excellent in toughness of the high heat input welding heat-affected zone described in “1”.
N-Ti / 3.4 ≧ 0.001% (1)

“3” The present invention further includes one or two of mass%, Ca: 0.0003 to 0.003%, Mg: 0.0003 to 0.003%, La + Ce: 0.001 to 0.02%. It is a thick steel plate excellent in the toughness of the high heat input welding heat-affected zone as described in “1” or “2”, characterized by containing the above.
"4" The present invention further includes, in mass%, Cu: 0.05 to 1%, Ni: 0.05 to 3%,
One or two or more of Cr: 0.05 to 1%, Mo: 0.05 to 1%, Nb: 0.003 to 0.03%, “1” to “3” It is a thick steel plate excellent in the toughness of the high heat input welding heat affected zone of any one of these.

According to the present invention, the average Charpy impact absorption energy at −20 ° C. to 0 ° C. in HAZ having a thickness of 15 to 100 mm and a yield strength of 325 to 500 MPa class and welded at a welding heat input of 20 to 150 kJ / mm. It is possible to provide a steel plate having a value of 100 J or more at a low cost.
The steel plate to which the present invention is applied is used in shipbuilding, construction, and other welded structures, and can achieve both high welding construction efficiency in construction of the structure and high safety and reliability of the structure.

  The present invention is a technique intended to utilize the α transformation nucleus effect by VN and B precipitates in high heat input welding HAZ. One of the premise is to suppress the N content to be as low as 0.006% or less from the viewpoint of avoiding surface cracking of the continuous cast slab. Another premise is to suppress the amount of B added to a very small amount of 0.0003% or less in order to reduce the amount of solute B in γ that causes material variations as much as possible. Under these two preconditions, the idea of the present invention is to achieve good high heat input welding HAZ toughness by utilizing the α-nuclear transformation effect of VN and B precipitates in combination.

When the amount of N is reduced to 0.006% or less in the V-added steel, the size of VN precipitated in γ of HAZ is reduced, and the ability as an α transformation nucleus is reduced. However, when a very small amount of B of 0.0001% or more and less than 0.0003% is added to this low N-based V-added steel, the average cooling rate from 800 ° C. to 500 ° C. becomes 1.5 ° C./s or less. In this case, the present inventors have found a phenomenon in which the α transformation nucleus effect in the γ grain boundary and in the γ grain is remarkably accelerated.
As a result of detailed analysis of the α transformation nucleus at this time by the present inventors, the γ grain boundary and the composite grain of BN and VN or V (C, N) precipitated in the γ grain and the γ grain boundary and It was found that the α transformation nucleus effect within the γ grain was remarkably promoted. When the amount of C is relatively high, Fe ₂₃ (C, B) ₆ and Fe ₃ (C, B) may be further complex precipitated.
In other words, the present inventor has found that α-transformed nuclei are obtained by combining BN, Fe ₂₃ (C, B) ₆ , and Fe ₃ (C, B) with VN or V (C, N) in a slow cooling HAZ with a low N content. As a result, it was found that the ability as

The B precipitates and V precipitates described above are precipitated in the γ region of 800 ° C. to 700 ° C. during the cooling process, and α transformation occurs in the subsequent cooling process of 700 ° C. to 500 ° C. using these composite particles as nuclei. A cooling rate of 800 ° C. to 500 ° C. is metallurgically important. When the average cooling rate of 800 ° C. to 500 ° C. is higher than 1.5 ° C./s, B precipitates and V precipitates are not sufficiently precipitated in the γ region, so that the size is insufficient and the α transformation nuclei are reduced. On the other hand, when the average cooling rate is less than 0.3 ° C./s, the growth of α transformed from the composite particles of B precipitate and V precipitate is promoted, and the α structure becomes coarse.
Therefore, it is important to apply this knowledge based on welding conditions in which the average cooling rate of 800 to 500 ° C. in HAZ is 1.5 to 0.3 ° C./s. Industrially, there is a convention to express the magnitude of the cooling rate during welding using the magnitude of the welding heat input. Thus, when the average cooling rate of 1.5 to 0.3 ° C./s is made to correspond to the heat input of welding, it can be regarded as approximately 20 to 150 kJ / mm.

As explained above, with the low N content of 0.006% or less, the intention is to increase the α transformation nucleus effect of VN in the high heat input welding HAZ, but not less than 0.0001% and less than 0.0003%. Until now, no technology has been found to add a small amount of B that is strictly controlled within the above range, and to effectively use the α transformation nucleus effect of the combined precipitates of V and B.
Conventionally, in V-added steel used for high heat input welding, a trace amount B of less than 0.0003% is regarded as an inevitably mixed impurity, and the B content is less than 0.0001% or more than 0.0001%. It was never a problem. That is, there has been no technique that utilizes the metallurgical effect of a trace amount B as in the present invention in the high heat input welding HAZ of V-added steel.

In order to strictly control the B amount within a minute range as in the present invention, a method for analyzing the B amount is important. JIS G 1227 Annex 2 (normative) In methyl borate distillation separation curcumin spectrophotometry (1), B amount of 0.0001% or more (1 ppm) can be analyzed.
Further, JIS G 1227 Annex 3 (normative) methyl borate distillation separation curcumin spectrophotometry (2) can analyze B amount of 0.00005% (0.5 ppm) or more. If these analytical methods are followed, the presence of 0.0001 to 0.0003% B content defined by the present invention can be confirmed reliably and accurately. In other words, a B amount of less than 0.0001%, which is outside the scope of the present invention, can be confirmed.

The reason for limiting the chemical components in the thick steel plate according to the present invention will be described in detail below.
C is required to be 0.03% or more in order to ensure the strength of the base material, and this is the lower limit. C is also used for promoting precipitation of V (C, N), Fe ₂₃ (C, B) ₆ and Fe ₃ (C, B) in HAZ. However, if C exceeds 0.2%, the toughness of the base material and HAZ is impaired, so this is the upper limit.
Si is effective as a deoxidizing element and strengthening element, but if it exceeds 0.5%, the HAZ toughness is impaired, so this is the upper limit. Since Si tends to promote MA generation and deteriorate high heat input welding HAZ toughness, it is preferably as small as possible in the present invention.
Mn is required to be 0.5% or more in order to ensure the strength and toughness of the base material economically and to stably generate MnS that becomes a preferential precipitation site for V precipitates and B precipitates in HAZ. However, if Mn is added in excess of 2%, the hazard of central segregation becomes remarkable and the toughness of the base material and HAZ is impaired, so this is the upper limit.
P is an impurity element and needs to be reduced to 0.02% or less in order to stably secure the HAZ toughness.
S is required to be 0.001% or more in order to stably generate MnS that is a preferential precipitation site for V precipitates and B precipitates. However, if S exceeds 0.005%, coarse sulfides are generated and the toughness of the base metal and HAZ is impaired, so this is the upper limit.

Al is necessary for carrying out deoxidation and reducing O which is an impurity element to 0.004% or less. In addition to Al, Si also contributes to deoxidation, but even when Si is added, it is difficult to stably suppress O to 0.004% or less without 0.001% or more of Al. However, when Al exceeds 0.1%, an alumina-based coarse oxide or a cluster thereof is generated, and the toughness of the base material and the HAZ is impaired, so this is the upper limit.
V and B are the most important elements in the present invention. When the average cooling rate from 800 ° C. to 500 ° C. in the cooling process of high heat input welding is 1.5 to 0.3 ° C./s (corresponding generally to welding heat input 20 to 150 kJ / mm), γ grain boundaries and γ VN, V (C, N), BN, Fe ₂₃ (C, B) ₆ , and Fe ₃ (C, B) are precipitated in the grains, thereby exhibiting a high α transformation nuclear ability. Therefore, the lower limit of V required for this is 0.01%, and the lower limit of B is 0.0001%. If V exceeds 0.1%, precipitation hardening due to V carbides becomes prominent in the HAZ after α transformation and the HAZ toughness deteriorates, so this is the upper limit. When B is 0.0003% or more, the variation in the amount of solute B increases in the base material and HAZ when mass-produced thick steel plates of the same steel type are increased, and the material variation becomes large, so this is the upper limit.

N is an important element in the present invention. In order to stably avoid the surface cracking of the continuous cast slab, it is necessary to suppress it to 0.006% or less, which is the upper limit. Such a low N content also has an effect of reducing solid solution N embrittlement of HAZ. Further, in order to precipitate BN, VN, and V (C, N) in HAZ, 0.001% or more is necessary, which is the lower limit. If N is less than 0.001%, it is difficult to form BN, so the amount of solute B in γ of the base material and HAZ increases, and there is a risk of inducing material variations. In addition, when Ti is added, Ti is combined with N to form TiN in preference to B and V in the cooling process, and consumes N. Therefore, BN, VN and V (C, N It is necessary to secure an extra N for forming (). For that purpose, it is necessary to secure N defined by the following formula (1). If the following formula (1) is not satisfied, precipitation of BN, VN, and V (C, N) in HAZ is insufficient, and the α transformation nucleus effect is lowered. Furthermore, there is a risk that the amount of solid solution B in γ of the base material or HAZ increases and induces material variations.
N-Ti / 3.4 ≧ 0.001% (1)

O constitutes an oxide, and there is concern about the harmful effect that some coarse oxides act as a starting point for brittle fracture. Therefore, it is necessary to suppress O to 0.004% or less.
Ti is added to increase the toughness of the base material and the HAZ. TiN is formed to suppress the growth of γ grains during slab heating or HAZ (the region where the maximum heating temperature ≦ 1350 ° C.), and has the effect of increasing the toughness by refining the α structure after transformation. The lower limit for exerting this effect is 0.003%. However, if Ti is added in excess of 0.017%, in addition to the HAZ toughness being cured and embrittled, the N amount satisfying the above formula (1) exceeds 0.006%. This is the upper limit.
Equation (1) expresses that BN, VN, and V (C, N) are not stably precipitated in γ in the subsequent cooling process unless the solute N during welding heating is 0.001% or more. ing. When N is less than this, N for constituting BN, VN, and V (C, N) becomes quantitatively insufficient. In the case of Ti addition, TiN is generated at a high temperature in preference to BN, VN and V (C, N), so it is necessary to secure 0.001% or more of the remaining N after subtracting N fixed to Ti as TiN. There is. Here, the amount of N as TiN is calculated as Ti / 3.4 using mass% from the ratio of atomic weight 48 of Ti and atomic weight 14 of N. (1) indicates that the remaining N amount obtained by subtracting the N amount (Ti / 3.4) as TiN from the total N amount needs to be 0.001% or more.

Ca, Mg, La + Ce improves the quality of the base material and HAZ by increasing the cleanliness of the steel by deoxidation or desulfurization, or by forming fine oxides and sulfides to contribute to the refinement of the structure. There is. The lower limit for exerting the effect is 0.0003% for Ca and Mg, and 0.001% for La + Ce. However, even if these elements are added in a large amount, the effect is saturated, so from the economical viewpoint, the upper limit is 0.003% for Ca and Mg and 0.02% for La + Ce.
Cu, Ni, Cr, and Mo are added to ensure the required strength of the base material. Each element has the effect of increasing the hardenability during the γ → α transformation in the cooling process after thick plate rolling and increasing the strength of the base material. The lower limit for the effect of these elements is 0.05% for Cu, Ni, Cr, and Mo. However, if there are too many of these elements, the HAZ toughness and weldability deteriorate, so an upper limit must be provided. The upper limit of Cu, Cr, and Mo is 1%, and the upper limit of Ni is 3%.
Nb is added in order to ensure both the strength and toughness of the base material. Nb refines the rolled γ structure, increases hardenability during the γ → α transformation, and precipitates after the α transformation, thereby contributing to the toughening of the base material. The lower limit for exerting this effect is 0.003%. However, if Nb is added over 0.03%, the HAZ hardens and becomes brittle, so this is the upper limit.

Next, an example of the manufacturing method of the thick steel plate to which this invention is applied is demonstrated.
In the steelmaking process in the steel industry, the impurity elements of molten steel are reduced and the necessary alloying elements are added appropriately, and slabs are made by continuous casting. In adjusting the composition of molten steel, it is important to control the amount of B in a very small range, so that B mixed from steelmaking raw materials and refractories is suppressed as much as possible, and a high-grade B alloy with a stable B content is prepared. Weigh with high accuracy and add to molten steel.
In order to specify the B content, an analysis method such as methyl borate distillation separation curcumin absorption spectrophotometry (1) according to JIS G 1227 described above or methyl borate distillation separation curcumin absorption spectrophotometry (2) according to JIS G 1227 is appropriately used. Select and use. The method for specifying the B content is not limited to these methods, and it is needless to say that other analysis methods that can be used for specifying the content at the level defined in the present invention may be used.
The steel slab is reheated during or after cooling during casting, a steel plate having a thickness of 15 to 100 mm is produced by thick plate rolling, and air-cooled or water-cooled after rolling. In the middle of water cooling, water cooling may be stopped and air cooling may be performed. The strength and toughness of the base material may be adjusted by performing various heat treatments as necessary.

The steel plate manufactured as described above has an α transformation nucleus effect obtained by adding a specified amount of V and N together with avoiding surface cracks with an N amount as low as 0.006% or less as an additive element, When the average cooling rate during welding is low (0.3 to 1.5 ° C./sec) due to the strict regulation of the B content, in other words, in the range where the welding heat input is approximately 20 to 150 kJ / mm, γ grain boundaries and composite particles of BN and VN or V (C, N) precipitated in the γ grains significantly promote the α transformation nucleus effect in the γ grain boundaries and γ grains, and provide good toughness. High heat input welding with no variation in base material strength and base material toughness when mass-produced thick steel plates with an average value of Charpy impact absorption energy at −20 ° C. to 0 ° C. of 150 J or more. HAZ with small heat input without variation in HAZ toughness It can be obtained in the absence of variations in weld cold cracking susceptibility and toughness.
In addition, when the amount of C is relatively high, in addition to these, Fe ₂₃ (C, B) ₆ and Fe ₃ (C, B) are further precipitated together to promote the α transformation nucleus effect.
That is, if it is the above-mentioned thick steel plate, it is used for shipbuilding, construction, and other welded structures, and it is possible to achieve both high welding construction efficiency in construction of the structure and high safety and reliability of the structure.

Steel slabs were produced by continuous casting while controlling the chemical components of the molten steel in the steel making process, and this was reheated to form steel plates having a thickness of 15 to 100 mm by thick plate rolling, and air cooling or water cooling was performed after rolling. Heat treatment was performed as necessary to produce a thick steel plate having a yield strength of 325 to 500 MPa class.
Table 1 shows the chemical composition of the steel, and Table 2 shows the mechanical properties of the thick steel plate and the HAZ toughness of the one-pass welded joint. A butt joint as shown in FIG. 1 is produced by electrogas welding (EGW) and a T-joint as shown in FIG. 2 is produced by electroslag welding (ESW). One-pass large heat input welding of ˜150 kJ / mm was applied.
FIG. 1 shows a butt weld, which includes base materials 1 and 2 of thick steel plates and a weld metal portion 3 obtained by butt welding them, and a weld line (FL) from the weld metal portion 3 along the plate thickness center line S. The test piece 5 was sampled from a portion passing through the weld heat affected zone 4 beyond the range.
FIG. 2 shows a T-jointed welded portion, and a back surface in which a diaphragm 8 made of a thick steel plate is arranged in a T-shape with a gap formed in a skin plate 7 made of a thick steel plate, and is arranged along the gap. Enclose the molten slag and molten metal so that they do not flow out of the welded portion by the metal 9, 10, supply the welding wire into the molten slag bath, and melt the welding wire mainly by the resistance heat of the molten slag, This is a welded portion formed by forming the weld metal portion 12. In this welded portion, along the plate thickness center line of the diaphragm 8, the portion that passes from the weld metal portion 12 to the inner side of the skin plate 7 through the weld heat affected zone 13 on the skin plate side, passing through the fusion line (FL). The test piece 15 was extract | collected from.

  As shown in Fig. 1 and Fig. 2, a notch is made in HAZ or FL 1mm away from the fusion line (FL), EGW joint is -20 ℃, ESW joint is Charpy impact test at 0 ℃. went. Three tests were conducted for one notch position and one test temperature, and the average absorbed energy was adopted. The Charpy impact test was based on JIS Z 2242, and a V-notch test piece of JIS Z 2202 was used.

  Steels 1 to 18 shown in Tables 1 and 2 are steels of the present invention, and the chemical composition of the steel is steel in which the V and B contents are appropriately adjusted under a low N content of 0.006% or less. The surface crack of the continuous cast slab did not occur, and good HAZ toughness exceeding 150 J at 0 ° C. or −20 ° C. was achieved. On the other hand, Steels 19 to 26 are comparative steels, and the HAZ toughness was insufficient because the chemical components of the steel were not appropriate. Since the steel 19 has a small amount of S, the number of MnS in the HAZ γ grains is insufficient, the precipitation of V precipitates and B precipitates is suppressed, the α transformation nucleus effect cannot be sufficiently obtained, and the HAZ structure is refined. Was insufficient, and good HAZ toughness was not obtained. Steel 20 has a small amount of Al, so deoxidation is insufficient, so O cannot be reduced to an appropriate level, and coarse oxides acting as starting points for brittle fracture increase, resulting in poor HAZ toughness. It was. Since Steel 21 has a small amount of V, Steel 23 has a small amount of B, and Steel 23 and Steel 25 have a small amount of N, VN, V (C, N) and BN are sufficiently precipitated in the γ region of HAZ. In addition, since the α transformation nucleus effect cannot be sufficiently extracted, the HAZ structure is not sufficiently refined and good HAZ toughness cannot be obtained. Since the steel 22 had a large amount of V, precipitation hardening occurred and good HAZ toughness could not be obtained. Since Steel 24 has a large amount of B, the solid solution B of HAZ increased, and the HAZ after transformation became hard, and good HAZ toughness was not obtained. The comparative steel 4 'and the comparative steel 11' in Table 2 are samples using the same base material as the steels 4 and 11 of the present invention, but the HAZ toughness was lowered because the welding heat input was too little or too much. .

It is a figure which shows the extraction | collection point of the Charpy test piece in an electrogas welding butt joint. It is a figure which shows the extraction | collection point of the Charpy test piece in an electroslag welding T-shaped joint.

Explanation of symbols

3 weld metal parts,
4 Welding heat affected zone (HAZ),
5 specimens,
12 Weld metal part,
13 Weld heat affected zone (HAZ),
15 specimens,

Claims (4)

% By mass
C: 0.03-0.2%,
Si: 0.5% or less,
Mn: 0.5 to 2.0%
P: 0.02% or less,
S: 0.001 to 0.005%,
Al: 0.001 to 0.1%,
V: 0.01-0.1%
B: 0.0001% or more and less than 0.0003%,
N: 0.001 to 0.006%,
A thick steel plate excellent in toughness of a high heat input welding heat-affected zone having a welding heat input of 20 to 150 kJ / mm, characterized by containing O 4: 0.004% or less and remaining iron and inevitable impurities. Furthermore, in mass%,
Ti: 0.003 to 0.017%
The thick steel plate excellent in toughness of the high heat input welding heat-affected zone according to claim 1, wherein the following formula (1) calculated using mass% of N and Ti is satisfied.
N-Ti / 3.4 ≧ 0.001% (1) Furthermore, in mass%,
Ca: 0.0003 to 0.003%,
Mg: 0.0003 to 0.003%,
La + Ce: 0.001 to 0.02%
The thick steel plate excellent in toughness of the high heat input welding heat-affected zone according to claim 1 or 2, characterized by containing one or more of the following. Furthermore, in mass%,
Cu: 0.05 to 1%,
Ni: 0.05-3%,
Cr: 0.05 to 1%,
Mo: 0.05 to 1%
Nb: 0.003 to 0.03%
The thick steel plate excellent in toughness of the high heat input welding heat-affected zone according to any one of claims 1 to 3, characterized by containing one or more of the following.

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