JP2014031544A - Steel material for large heat input welding - Google Patents

Abstract

The present invention provides a steel material having a welding heat input of more than 300 kJ / cm and a yield stress of 460 MPa or more having excellent weld toughness and joint strength.
SOLUTION: In mass%, C: 0.03 to 0.08%, Si: 0.01 to 0.15%, Mn: 1.80 to 2.40%, P: 0.015% or less, S : 0.0005 to 0.0040%, Al: 0.005 to 0.100%, Nb: 0.003 to 0.030%, Ti: 0.005 to 0.050%, N: 0.0050 to 0 0.160%, B: 0.0003 to 0.0025%, optionally containing one or more of V, Cu, Ni, Cr, Mo, Ca, Mg, Zr, REM, Ti / N ratio: 2 0.0 or more and less than 4.0, A value is 10 or more and 25 or less, remaining Fe and inevitable impurities, yield stress is 460 MPa or more, welding heat input exceeds 300 kJ / cm, hardness of heat-affected zone is 160 at HV10 Steel material that is above. A = 2256 × Ti-7716 × N + 10000 × B, where each element symbol is content (% by mass).
[Selection figure] None

Description

  The present invention relates to a steel material having a yield stress of 460 MPa or more, particularly a steel material suitable for high heat input welding with a heat input of welding exceeding 300 kJ / cm, used for various steel structures in the fields of ships, buildings, civil engineering, and the like. Specifically, the present invention relates to a steel sheet having excellent weld toughness and joint strength even when the high heat input welding is performed.

  Generally, steel structures used in the fields of ships, offshore structures, architecture, steel pipes, etc. are finished into structures of a desired shape by welding. Therefore, from the viewpoint of ensuring safety, these structures are required to have excellent toughness of welds in addition to securing the base material characteristics of the steel materials used, that is, strength and toughness. .

  Furthermore, in recent years, the ships and steel structures have become increasingly larger, and the steel materials used have been actively promoted to increase strength and thickness. Along with that, high efficiency and high heat input welding methods such as submerged arc welding, electrogas welding, and electroslag welding have come to be applied. However, a steel material having excellent toughness of the welded part is required.

  Here, the structure of the high heat input weld will be described. FIG. 1 is a macro-structure photograph of a cross section of a high heat input weld zone. In the center of the weld zone, both the molten base material and the weld metal produced from the weld material are mixed almost uniformly in a molten state and solidified. There are weld metal parts, and on both sides there are heat affected zones (sometimes referred to as Heat Affected Zone; HAZ) that are affected by the heat input during welding and whose structure and properties of the base material have changed. It shows a state in which a base material exists on both sides. A portion (broken line portion in the figure) that contacts the weld metal in the heat affected zone (HAZ) is generally referred to as a “bond portion”. The HAZ in the vicinity of the bond part is heated to a high temperature in the vicinity of the melting point in the heat-affected zone, so that the crystal grains become coarse and the toughness is remarkably lowered. On the other hand, at a distance from the bond portion, it becomes a fine-grained region and softens, which is a main cause of reduced joint strength.

  Many countermeasures have been studied for the reduction of HAZ toughness associated with high heat input welding. For example, a technique for finely dispersing TiN in steel to suppress coarsening of austenite grains or to use it as a ferrite transformation nucleus has already been put into practical use. In addition, a technique aiming at the same effect as described above has been developed by dispersing an oxide of Ti.

  In the above-described technology utilizing TiN, since the heat affected zone is heated to the melting temperature range of TiN when subjected to large heat input welding, the dispersion effect disappears due to the decomposition of TiN or the decomposition of TiN. Due to the solid solution Ti and the solid solution N generated by the above, the ground structure of the steel becomes brittle and the toughness is remarkably lowered.

  Moreover, the technique using Ti oxide has a problem that it is difficult to disperse the oxide uniformly and finely. As a technique for such a problem, for example, Patent Document 1 is added for the purpose of controlling the form of sulfides in order to improve the toughness of the weld heat-affected zone subjected to high heat input welding exceeding 300 kJ / cm. A technique for optimizing the amount of Ca to crystallize CaS and effectively utilizing it as a ferrite transformation nucleus is disclosed.

  Since CaS crystallizes at a lower temperature than oxides, it can be finely dispersed in the steel. Further, during cooling, this is used as a nucleus, and ferrite transformation formation nuclei such as MnS, TiN, and BN are fine. Therefore, the structure of the weld heat-affected zone can be made into a fine ferrite pearlite structure, and high toughness can be achieved. The technique of Patent Document 1 has made it possible to suppress toughness to some extent due to high heat input welding.

  However, as a result of subsequent research, when the yield stress was increased to 460 MPa or more and the steel with a relatively large amount of C and alloy elements added, welding heat input exceeding 300 kJ / cm was applied. Furthermore, it has been found that a hard embrittlement structure called island martensite (MA) is formed in HAZ near the bond part by several volume%, which prevents further improvement of the toughness of the weld.

  Therefore, in order to improve the HAZ toughness in the vicinity of the bond portion of such a high heat input weld of high strength steel, in addition to suppressing austenite grain coarsening, fine dispersion of ferrite transformation nuclei, and reduction of solute N, island-like It is necessary to suppress the formation of martensite.

  Regarding the technique for reducing island martensite, for example, Patent Document 2 discloses that in addition to reducing the C and Si contents, it is effective to reduce the P content. Furthermore, in Patent Document 3, by adding Mn actively and reducing P as much as possible, it is possible to reduce island martensite in the vicinity of the bond part HAZ, and to obtain a steel material having a yield stress of 460 MPa with excellent toughness. Yes.

  On the other hand, the technology for suppressing the HAZ softening associated with high heat input welding is not disclosed as much as the HAZ toughness countermeasure. In the above Patent Documents 1, 2, and 3, there is no description regarding the HAZ softening. This is probably because the design of high heat input welding steel is based on the premise that the joint strength can be secured. Several techniques have been disclosed for inhibiting HAZ softening.

  These techniques include a technique using precipitation strengthening elements such as Nb and V, and a technique using the hardenability of B. In Patent Document 4, it is said that HAZ softening is reduced by increasing the amount of C, reducing Si and Mn, and containing Nb and V.

  Moreover, in order to improve the hardenability by B, in Patent Document 5, by defining the component formula so as to contain a large amount of Ti, B, Nb with respect to the N amount, By prescribing, HAZ softening suppression is achieved.

Japanese Patent No. 3546308 JP 2008-163446 A JP 2011-6772 A JP 60-67622 A JP 2007-177327 A Japanese Patent No. 4233033

  The technique described in Patent Document 1 is a technique for improving the toughness of the bond part when large heat input welding is performed, particularly on a steel material with a yield stress of 390 MPa grade, but the yield strength is higher than that. It cannot sufficiently cope with high heat input HAZ toughness and HAZ softening of a steel material with a stress of 460 MPa.

  The technique described in Patent Document 2 targets a steel material with a yield stress of 460 MPa grade, reduces the content of C, Si, and P to reduce HAZ island martensite in the vicinity of the bond portion, and Ca The ferrite transformation nuclei are finely dispersed by adding a proper amount to ensure the HAZ toughness, but there is no description for the softening of the HAZ, and there is a problem that the alloy cost is high because the addition of Ni is essential. There is.

  The technology described in Patent Document 3 is also intended for steel materials with a yield stress of 460 MPa grade, and is said to reduce island-like martensite by actively using Mn, and the required steel materials can be obtained at low cost. There is no description about softening.

  The technique described in Patent Document 4 has a high C content and takes sufficient measures against HAZ softening by using precipitation strengthening elements such as Nb and V, but a large amount of HAZ in the vicinity of the bond portion during high heat input welding. There is a concern that island-shaped martensite is formed and the HAZ toughness in the vicinity of the bond portion is significantly reduced.

  The techniques described in Patent Documents 5 and 6 are techniques for suppressing HAZ softening by using the hardenability of B. However, Patent Document 5 is premised on the addition of a large amount of Ti, B, and N, and there is a problem in manufacturability. In addition, there is a concern that toughness due to solute N may be lowered in a region where TiN is melted in the vicinity of the bond portion.

  Patent Document 6 is premised on Nb-free, and when a steel material with a yield stress of 460 MPa grade is targeted, it is difficult to ensure joint strength.

  Accordingly, an object of the present invention is to provide a yield stress that is excellent in softening resistance that does not decrease the hardness of the weld heat affected zone even when a high heat input welding in which the heat input of welding exceeds 300 kJ / cm, and HAZ toughness near the bond portion is 460 MPa. The object is to provide the above high heat input welding steel materials at low cost.

  The inventors of the present invention have the HAZ toughness near the bond part and the hardness of the HAZ softest part when high heat input welding with a heat input exceeding 300 kJ / cm is applied to a high strength steel material having a yield stress of 460 MPa or more. The effects of structural factors and alloying elements on the properties were investigated.

  Regarding the HAZ toughness in the vicinity of the bond part, it was found that a small amount of island martensite adversely affects the toughness. Therefore, the inventors have further studied the relationship between the alloy elements and the hardness of the HAZ island martensite near the bond portion and the hardness of the HAZ most softened portion.

  As a result, while keeping the amount of C low, and actively containing Mn to compensate for the decrease in the strength of the base metal, which is a concern due to the reduction in the amount of C, the island-like martensite that adversely affects the HAZ toughness in the vicinity of the bond portion is minimized It has been found that the strength of the base material can be effectively increased without generation.

  The HAZ softening region that has a dominant influence on the joint strength was studied as follows. As described above, the HAZ softened region, which is a main cause of joint strength reduction, is a region slightly separated from the bond portion.

  Although this region undergoes welding heat input, the steel structure is transformed into austenite, but the austenite produced is finer because it is lower in temperature than the region closer to the bond portion. For this reason, hardenability falls compared with the case of austenite having a larger particle size, transformation strengthening structures such as bainite and martensite are difficult to obtain, and ferrite is easily generated, so that softening occurs.

  Therefore, in order to increase the hardness of the HAZ softened region, it is effective to improve the hardenability of steel, and the use of B, which is known as one of the means for improving the hardenability of steel, was examined. 2. Description of the Related Art Conventionally, there has been known a technique for securing solute B by suppressing generation of BN by fixing N with Ti to generate TiN.

  However, in the conventional technique, the value of the solid solution B required to ensure hardenability is a value calculated stoichiometrically. This is based on the premise that the reaction such as TiN formation proceeds in an equilibrium manner, and the required value of solute B is the theoretical maximum value which is an ideal state. It is unlikely to be appropriate for steel that undergoes the thermal history of time.

  The present inventors have optimized the ratio of the content of Ti and N to fix N in steel with Ti, and the contents of Ti, B, and N in the steel are limited by these elements. By making the value of the formula within a certain range, the HAZ having a hardness of 160 or higher at HV10 in the HAZ softened region of the high heat input welded part is reduced without reducing the HAZ toughness in the vicinity of the bond part. It has been found that the softest part can be secured stably. If the hardness of the HAZ softest part is HV10 of 160 or more, sufficient joint strength can be given to a base material of YP460 grade (yield stress of 460 MPa or more).

The present invention has been completed by further investigation based on the obtained knowledge, that is, the present invention,
1. In mass%, C: 0.03 to 0.08%, Si: 0.01 to 0.15%, Mn: 1.80 to 2.40%, P: 0.015% or less, S: 0.0005 -0.0040%, Al: 0.005-0.100%, Nb: 0.003-0.030%, Ti: 0.010-0.050%, N: 0.0050-0.0160%, B: 0.0003 to 0.0025% is contained, Ti / N ratio (ratio of mass%) is 2.0 or more and less than 4.0, and A value defined by the following formula (1) is 10 or more and 25 or less. The hardness of the softest part of the heat-affected zone has a chemical composition of the remaining Fe and unavoidable impurities, a yield stress of 460 MPa or more, and a high heat input welding with a heat input of welding exceeding 300 kJ / cm. A steel material for high heat input welding characterized by being HV10 of 160 or more.
A = 2256 × Ti-7716 × N + 10000 × B (1)
However, each element symbol indicates the content (% by mass) of each element.
2. 2. The steel material for high heat input welding according to 1, wherein the chemical component further contains, by mass%, V: 0.20% or less.
3. The chemical component further includes at least one selected from Cu: 0.50% or less, Ni: 0.20% or less, Cr: 0.40% or less, and Mo: 0.40% or less in terms of mass%. The steel material for high heat input welding according to 1 or 2, which is contained.
4). In addition to chemical components, Ca: 0.0005 to 0.0050%, Mg: 0.0005 to 0.0050%, Zr: 0.0010 to 0.0200%, REM: 0.0010 to 0.0200% The steel material for high heat input welding according to any one of 1 to 3, wherein the steel material contains at least one selected from among them.

  According to the present invention, a good joint strength and weld heat-affected zone toughness can be obtained even when a high heat input welding exceeding 300 kJ / cm is applied to a high strength steel having a yield stress of 460 MPa or more. It greatly contributes to improving the quality of ships and large structures constructed by high heat input welding such as arc welding and electroslag welding. In particular, it is effective to apply to a steel sheet having a thickness of more than 50 mm because it exhibits a more significant advantage over steel according to the prior art.

The figure explaining the structure | tissue of a welded joint part cross section.

  The form for implementing this invention is demonstrated below. The steel material which is the object of the present invention refers to a steel material manufactured by hot rolling.

In the present invention, the minimum hardness (also referred to as the hardness of the HAZ most softened portion) is defined among the component composition, strength, and softened region of the heat affected zone formed by high heat input welding exceeding 300 kJ / cm. First, the hardness of the softest part of the heat affected zone, which is a feature of the steel material of the present invention, will be described.
The joint having welded a steel material having a hardness of HV10 of 160 or more and a yield stress of 460 MPa or more requires a tensile strength equivalent to that of the base material, that is, a tensile strength of 570 MPa or more. Factors affecting the tensile strength of the joint mainly include weld metal strength, sheet thickness, hardness in the softest zone of the HAZ, and the influence of the hardness of the softened portion in the heat affected zone is particularly large. In a steel material having a yield stress of 460 MPa or more, if the minimum value of the hardness in the softened region, that is, the hardness of the softest part is less than 160 at HV10, it is difficult to obtain a required joint strength. Therefore, it is set to 160 or more at HV10. HV10 is a hardness symbol HV10 defined by JIS Z 2244 (1998), and indicates Vickers hardness measured with a test force of 98.07 N (corresponding to 10 kgf). Here, in the present invention, the softened region of the heat-affected zone refers to a heat-affected zone that becomes an austenite fine grain region separated by about 10 mm from the bond as shown in FIG. The hardness of the softest part, which is the lowest hardness of the softened region, is the minimum hardness among the hardnesses obtained by measuring the softened region at intervals of 0.5 mm.

  Next, the component composition that the steel of the present invention should have in order to control the hardness of the softest part of the HAZ within the above range and also achieve high strength will be described. In the present invention, “%” regarding chemical components means “% by mass”.

C: 0.030 to 0.080%
C is an element that increases the strength of the steel material, and in order to ensure the strength necessary for structural steel, it is necessary to contain 0.030% or more. On the other hand, if C exceeds 0.080%, island-like martensite is easily generated in the HAZ near the bond portion, so the upper limit is made 0.080%.

Si: 0.01 to 0.15%,
Si is an element added as a deoxidizer when melting steel, and it is necessary to add 0.01% or more. However, if it exceeds 0.15%, the toughness of the base material is lowered, and island martensite is generated in the vicinity of the bond portion HAZ subjected to high heat input welding, which tends to cause a reduction in toughness. Therefore, Si is taken as 0.01 to 0.15% of range.

Mn: 1.80 to 2.40%
Mn is an element that increases the strength like C, and is less expensive than alloy elements such as Mo and V, and is actively added because it does not promote the formation of MA (also called island martensite) in the HAZ near the bond. To do. In order to secure the required strength and obtain the above effect, the addition of 1.80% or more is necessary, the addition of 1.90% or more is more preferable, and the addition of 2.00% or more is more preferable. However, since an excessive content impairs the toughness of the welded portion, it is necessary to be 2.40% or less, more preferably 2.20% or less, and even more preferably 2.10% or less.

P: 0.015% or less P is made 0.015% or less in order to promote MA formation in HAZ in the vicinity of the bond portion and greatly reduce its toughness. Preferably, it is 0.010% or less.

S: 0.0005 to 0.0040%,
S is an element necessary for forming MnS or CaS that acts as a nucleation site for ferrite. For this reason, 0.0005% or more is added. However, excessive addition causes a decrease in the base material toughness, so the upper limit is made 0.0040%.

Al: 0.005 to 0.100%
Al is an element added for deoxidation of steel, and it is necessary to contain 0.005% or more. However, if the content exceeds 0.100%, not only the toughness of the base metal but also the toughness of the weld metal is lowered. Therefore, Al is taken as 0.005 to 0.100% of range. Preferably it is 0.010 to 0.100% of range.

Nb: 0.003% to 0.030%
Nb is an element effective for ensuring the strength of the base material and the hardness of the HAZ softened portion, and consequently the weld joint strength. However, when the content is less than 0.003%, the above effect is small. On the other hand, when the content exceeds 0.030%, island martensite is generated in the HAZ in the vicinity of the bond portion and the toughness is lowered. Therefore, Nb is set to a range of 0.003 to 0.030%.

Ti: 0.010 to 0.050%,
Ti precipitates as TiN during solidification, suppresses the coarsening of austenite grains in the vicinity of the bond HAZ, and becomes a transformation nucleus of ferrite, contributing to its high toughness and simultaneously bonding with B By reducing N and securing the solid solution B, it effectively works in securing the hardness of the HAZ softened part and consequently the weld joint strength. In order to acquire such an effect, 0.010% or more of addition is required, and it is preferable to add 0.015% or more. On the other hand, if the content exceeds 0.050%, the precipitated TiN becomes coarse and the above effect cannot be obtained. Therefore, Ti is set to a range of 0.010 to 0.050%.

N: 0.0050 to 0.0160%,
N forms TiN during solidification and contributes to the suppression of coarsening of HAZ austenite grains in the vicinity of the bond portion. At the same time, N forms BN, and the BN acts as a ferrite transformation nucleus, thereby forming the HAZ structure in the vicinity of the bond portion. Contributes to miniaturization and high toughness. In order to secure the necessary amount of TiN, it is necessary to contain 0.0050% or more of N, and preferably 0.0070% or more. However, if excessively contained, the effect of improving the hardenability by B is impaired, the strength of the HAZ softened portion is greatly impaired, and the HAZ toughness is deteriorated, so it is necessary to be 0.0160% or less, and 0.0120% or less. It is preferable to do.

B: 0.0003 to 0.0025%
B is an element that improves the hardenability of steel and promotes the formation of a hard structure such as bainite and martensite by lowering the transformation temperature of austenite and contributes to increasing the strength of the base steel sheet. Similarly, in the HAZ softened portion, the formation of ferrite which is a soft phase is suppressed and the strength of the HAZ softened portion is improved. In order to acquire such an effect, it is necessary to contain 0.0003% or more. However, when it contains exceeding 0.0025%, hardenability will increase excessively and the toughness of a base material steel plate and HAZ will be reduced. For this reason, B is taken as 0.0003 to 0.0025% of range.

Ti / N ratio (ratio by mass%): 2.0 or more and less than 4.0 The Ti / N ratio is an important requirement in the present invention, together with the definition of the A value described later. Since the Ti / N ratio greatly affects the fine dispersion state of TiN and the toughness deterioration due to solute N in the HAZ bond portion, it is necessary to appropriately control the Ti / N ratio. When Ti / N is 4.0 or more, BN does not precipitate, and Ti borate and the like are precipitated, resulting in a significant reduction in HAZ toughness. On the other hand, if it is less than 2.0, the HAZ toughness is lowered by the solute N, and the hardenability of the B cannot be ensured due to the BN precipitation in the HAZ part, making it difficult to secure the required hardness of the HAZ softened part. Therefore, it is set to 2.0 or more and less than 4.0. Preferably, it exists in the range of 2.5 or more and 3.5 or less.

A value: 10 or more and 25 or less The A value defined by the following formula (1) is an important requirement in the present invention. When the steel material receives a thermal history corresponding to the heat-affected zone of high heat input welding, even when the formation reaction of TiN, BN, etc. does not proceed in equilibrium, other invention specifics relating to Ti, N, and B If the A value is 10 or more after satisfying the requirements, the effect of improving the hardenability by the solid solution B is sufficiently exerted, so the A value needs to be 10 or more. If the A value exceeds 25, the hardenability of the steel material becomes excessive and the HAZ toughness in the vicinity of the bond portion decreases, so the A value is 25 or less.

  By controlling the A value to 10 or more and 25 or less, the hardness of the HAZ softest part is stably secured to a hardness of 160 or more at HV10, which is a hardness necessary for securing the joint strength in the base material of YP460 grade. be able to. From the viewpoint of achieving both HAZ toughness and strength in the vicinity of the bond part, the preferable range of the A value is 12 or more and 18 or less.

A = 2256 × Ti-7716 × N + 10000 × B (1)
However, each element symbol indicates the content (% by mass) of each element.

  The above is the basic component composition of the present invention, and the balance is Fe and inevitable impurities. As an unavoidable impurity, O: 0.0050% or less is acceptable. Further, the steel material of the present invention can contain V as a selective element in the following range in addition to the above essential components.

V: 0.20% or less V precipitates as VN, contributes to the improvement of the strength and toughness of the base material, and also acts as a ferrite-forming nucleus, and can be contained as necessary. In order to exhibit this effect, 0.005% or more of addition is preferable. However, excessive addition causes a decrease in toughness, so the upper limit is preferably 0.20%.

  In addition to the above components, the steel material of the present invention can further contain one or more selected from Cu, Ni, Cr and Mo as selective elements in the following range for the purpose of improving the strength.

Cu: 0.50% or less, Ni: 0.20% or less, Cr: 0.40% or less and Mo: 0.40% or less Cu, Ni, Cr and Mo are effective elements for increasing the strength of the base material. In order to obtain the effect, it is preferable to add 0.05% or more of Cu and Ni and 0.02% or more of Cr and Mo. However, if any of these elements is added in a large amount, the toughness is adversely affected. Also, Ni leads to an increase in alloy cost. Therefore, when it is contained, Cu is 0.50% or less, and Ni is 0.20. % Or less, and Cr and Mo are preferably 0.40% or less.

  In addition to the above components, the steel material of the present invention can further contain one or more selected from Ca, Mg, Zr and REM as selective elements in the following range.

Ca: 0.0005 to 0.0050%
Ca can be contained in order to obtain an effect of improving toughness by fixing S and dispersing oxides and sulfides. In order to acquire the said effect, it is preferable to contain at least 0.0005%. However, even if added over 0.0050%, the above effect is only saturated. Therefore, when it contains Ca, it is preferable to set it as 0.0005 to 0.0050% of range.

Mg: 0.0005-0.0050%, Zr: 0.0010-0.0200%, REM: 0.0010-0.0200%
Mg, Zr, and REM are all elements having an effect of improving toughness due to oxide dispersion. In order to exhibit such an effect, it is preferable to contain 0.0005% or more of Mg and 0.0010% or more of Zr and REM. On the other hand, even if Mg exceeds 0.0050% and Zr and REM exceed 0.0200%, the effect is only saturated. Therefore, when it contains these elements, it is preferable to set it as the said range.

  If the steel material which concerns on this invention is a manufacturing method which makes a yield stress 460 Mpa or more, it can manufacture with a conventionally well-known method, and there is no restriction | limiting in particular in manufacturing conditions. For example, after the hot metal is made into molten steel with a converter or the like, the steel components are adjusted to the appropriate range by RH degassing or the like, and then the steel piece is made through a continuous casting or ingot-bundling process. Next, the steel slab is reheated and hot rolled to obtain a steel material having a desired size, and then allowed to cool, or after the hot rolling, accelerated cooling, direct quenching-tempering, reheating quenching-tempering, re-heating. It can be manufactured through a process such as heat normalization-tempering.

  As described above, the yield stress is 460 MPa or more, and the hardness of the softest part of the heat affected zone when the large heat input welding with a welding heat input exceeding 300 kJ / cm is performed is 160 or more in HV10, A steel material for high heat input welding can be obtained.

  No. having the component composition shown in Table 1 using a 150 kg high-frequency melting furnace. Steels 1 to 27 were melted and cast to form a steel ingot, and then hot rolled to form a steel piece having a thickness of 120 mm. The obtained steel slab was heated at 1150 ° C. for 2 hours, and then hot-rolled in a state where the plate thickness center temperature was 850 ° C. to 900 ° C. to obtain a thick steel plate having a plate thickness of 60 mm. The plate was accelerated and cooled until the plate thickness center temperature became 350 ° C. so that the cooling rate at the position was 8 ° C./sec, and then allowed to cool.

  Next, a round bar tensile test piece having a parallel portion of 14 mmΦ × 85 mm and a distance between gauge points of 70 mm is taken so that the longitudinal direction of the test piece coincides with the plate width direction from the ¼ thickness position of the 60 mm thick steel plate. Then, a tensile test was performed, and the base material strength (yield stress YS and tensile strength TS) was measured.

  The hardness of the heat-affected zone softened part greatly affects the welded joint strength, and the higher the softened part hardness is, the higher the welded joint strength is. In order to evaluate the hardness of the HAZ softest part, a small test piece of 3 mmΦ × 10 mm was taken from the thick steel plate, heated to 900 ° C. corresponding to the austenite fine grain region immediately above the transformation point, and then 800 to 500 ° C. in 390 sec. A cooling heat treatment was performed. The Vickers hardness HV10 (JIS Z 2244 (1998)) of the small test pieces subjected to these treatments was measured, and the lowest hardness was taken as the softest part hardness.

  Moreover, about the said small test piece corresponding to a HAZ softening part, the structure was revealed by etching the cross section of the test piece with nital. Using SEM, three-view tissue photographs were taken at a magnification of 1000, and the images were analyzed to determine the average area fraction of martensite, which was used as the martensite volume fraction of the HAZ softest part.

  In order to evaluate the toughness in the vicinity of the bond part, a test piece having a width of 80 mm, a length of 80 mm and a thickness of 15 mm was taken from the thick steel plate, heated to 1450 ° C., and cooled between 800 to 500 ° C. in 390 sec. From these test pieces, 2 mmV notch Charpy test pieces were collected.

  The Charpy test piece obtained was appropriately subjected to a Charpy impact test in the range of −100 to 40 ° C. to determine the fracture surface transition temperature vTrs at which the ductile fracture surface ratio was 50%, and the toughness in the vicinity of the bond part was evaluated. The heat treatment conditions correspond to large heat input welding by electrogas welding with a heat input of 500 kJ / cm.

  Table 2 shows the measurement results of tensile properties (YS, TS) of the base material (thick steel plate) evaluated by the above procedure, hardness and martensite volume fraction of the HAZ softened part, and HAZ toughness in the vicinity of the bond part. It was. From Table 2, No. of invention example. The thick steel plates 1 to 12 have a yield stress YS of 460 MPa or more, a tensile strength TS of 570 MPa or more, and a softest part hardness of HV10, which is as high as 160 or more. It can be seen that excellent toughness is obtained. In these steels, the fraction of martensite in the HAZ softened part was 5 to 15% by volume.

  In contrast, no. Nos. 13 to 27 have any component, or the Ti / N ratio or the A value is outside the range defined by the present invention, and any of YS, HAZ softened part hardness or bond vicinity HAZ toughness vTrs is low. Yes.

Claims (4)

In mass%, C: 0.03 to 0.08%, Si: 0.01 to 0.15%, Mn: 1.80 to 2.40%, P: 0.015% or less, S: 0.0005 -0.0040%, Al: 0.005-0.100%, Nb: 0.003-0.030%, Ti: 0.010-0.050%, N: 0.0050-0.0160%, B: 0.0003 to 0.0025% is contained, Ti / N ratio (ratio of mass%) is 2.0 or more and less than 4.0, and A value defined by the following formula (1) is 10 or more and 25 or less. The hardness of the softest part of the heat affected zone is HV10 when it has a high heat input welding having a residual Fe and inevitable impurity chemical components, a yield stress of 460 MPa or more, and a welding heat input exceeding 300 kJ / cm. A steel material for high heat input welding characterized by being 160 or more.
A = 2256 × Ti-7716 × N + 10000 × B (1)
However, each element symbol indicates the content (% by mass) of each element.   The steel material for high heat input welding according to claim 1, wherein the chemical component further contains, in mass%, V: 0.20% or less.   The chemical component further includes at least one selected from Cu: 0.50% or less, Ni: 0.20% or less, Cr: 0.40% or less, and Mo: 0.40% or less in terms of mass%. The steel material for high heat input welding according to claim 1 or 2, wherein the steel material is contained.   In addition to chemical components, Ca: 0.0005 to 0.0050%, Mg: 0.0005 to 0.0050%, Zr: 0.0010 to 0.0200%, REM: 0.0010 to 0.0200% The steel material for high heat input welding according to any one of claims 1 to 3, comprising at least one selected from among them.