JP2002003986A - High tensile steel for large heat input welding - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide high tensile steel for large heat input welding, having excellent toughness in a heat affected zone when welding at a heat input of >=200 kj/cm (e.g. as high as about 1,500 kj/cm). SOLUTION: The steel has a composition consisting of, by weight, 0.04-0.25% C, 0.02-0.5% Si, 0.2-2.0% Mn, <=0.02% pP, 0.002-0.02% S, 0.015-0.5% Al, 0.0005-0.005% Mg, proper amounts of one or more elements among Cu, Ni, Cr, Mo, Nb, V, Ti and B as selective elements, and the balance Fe with inevitable impurities and also has a structure containing (Mn, Mg)S of 0.005-0.5 μm particle size in proportions of 1.0×105 to 1.0×107 pieces per square mm. The growth of γ-grains in HAZ in very large heat input welding can be suppressed by the pinning action of the fine (Mn, Mg)S particles, and toughness in HAZ can be improved.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a heat-affected zone in ultra-high heat input welding such as electroslag welding applied in assembling box columns of high-rise buildings or electrogas welding applied in ships and bridges. The present invention relates to a high-strength steel for welding having excellent toughness. Particularly, when the heat input is 200 kJ / cm or more, for example, 75 kJ / cm.
It has excellent HAZ toughness even at about 0 to 1500 kJ / cm.

[0002]

2. Description of the Related Art With the recent increase in the height of building structures, steel columns have become larger and the thickness of steel materials used for the columns has also increased. When assembling such large steel columns by welding,
It is necessary to perform welding with high efficiency, and electroslag welding, which can weld extremely thick steel plates in one pass, has been widely applied. Also, in the field of shipbuilding and bridges, electrogas welding in which a steel plate having a thickness of about 25 mm or more is welded in one pass has been widely applied. A typical heat input range is 200 to 1500 kJ / cm, and such a super large heat input welding differs from a large heat input welding such as a submerged arc welding (heat input is less than 200 kJ / cm) and a welding fusion wire (the heat input is less than 200 kJ / cm). In the thermal history near FL or HAZ, the high-temperature residence time of 1350 ° C. or more becomes extremely long (remains several to several tens of times longer in large heat input welding than in large heat input welding), and austenite grains become coarse. It was extremely remarkable, and it was difficult to secure the toughness of HAZ.
It is an urgent task to secure the reliability of building structures in the wake of the recent large earthquake, and it is extremely important to achieve such improvement in the toughness of the HAZ portion having a very large heat input weld.

[0003] Conventionally, there are many knowledge and techniques for improving the toughness of a high heat input welding HAZ as shown below.
As described above, since the heat history applied to the HAZ between the very large heat input welding and the large heat input welding, especially the residence time at 1350 ° C. or more, is greatly different, the technique for improving the large heat input welding HAZ toughness is simply applied to the present invention. It cannot be applied to the field.

[0004] The conventional high heat input welding HAZ toughness improvement is largely based on two basic technologies. One is a technique for preventing austenite grain coarsening using the pinning effect of particles in steel, and the other is an effective grain refinement technique using ferrite transformation in austenite grains.

"Iron and Steel", 61th year (1975), eleventh
No. 3 examines the effect of suppressing austenite grain growth on various types of nitrides and carbides in steel.
There is disclosed a technique in which fine particles of iN are generated in steel to effectively suppress austenite grain growth in a large heat input welding HAZ.

[0006] Japanese Patent Application Laid-Open No. 60-184663 discloses A
1 to 0.04 to 0.10%, Ti to 0.002 to 0.0
2%, rare earth element (REM) 0.003 to
Heat input of 150 kJ / c in steel containing 0.05%
A technique for improving the high heat input welding HAZ toughness of m is disclosed. This is because the REM has an action of forming sulfur / oxide and preventing the HAZ portion from coarsening during large heat input welding.

Japanese Patent Application Laid-Open No. 60-245768 discloses that the particle size is 0.1 to 3.0 μm and the number of particles is 5 × 10 ³ to 1 ×.
In steels containing either 10 ⁷ / mm ³ Ti oxide or a composite of Ti oxide and Ti nitride, these particles become ferrite in a large heat input welding HAZ with a heat input of 100 kJ / cm. HAZ by acting as a transformation nucleus
A technique capable of improving the HAZ toughness by refining the structure is disclosed.

[0008] JP-A-2-254118 discloses that Ti
In a steel containing a proper amount of S and S, intragranular ferrite is formed in the large heat input welding HAZ structure by using a composite precipitate of TiN and MnS as a nucleus, and the HAZ structure is refined.
A technique capable of improving toughness is disclosed.

Japanese Patent Application Laid-Open No. 61-253344 discloses A
1 is 0.005 to 0.08%, and B is 0.0003 to 0.
Steel containing at least 0050% and at least 0.03% of at least one of Ti, Ca and REM is subjected to a cooling process starting from unmelted REM / Ca oxide / sulfide or TiN in the large heat input welding HAZ. A technique is disclosed in which a high heat input HAZ toughness is improved by forming BN and forming ferrite from the BN.

Japanese Patent Application Laid-Open No. 9-157787 discloses that Mg
The content of oxides is 40,000 to 100,
Steel containing 20 to 400 per square mm of a composite comprising Ti-containing oxide and MnS having a particle diameter of 0.20 to 5.0 μm and suppressing austenite grain growth and promoting intragranular ferrite transformation is included. High heat input welding HA
A technique capable of improving the Z toughness is disclosed.

JP-A-11-286743 discloses that MgO, MgS, Mg having a particle size of 0.005 to 0.5 μm is used.
In steels containing two or more types of (O, S), ultra-high heat input welding H
A technique capable of improving AZ toughness is disclosed.

[0012]

The technique disclosed in "Iron and Steel", No. 11, 1986 (1975) aims at suppressing austenite grain growth by using nitrides such as TiN. Yes, the effect is exhibited in large heat input welding, but in the ultra-high heat input welding targeted by the present invention, since the residence time at 1350 ° C. or more is extremely long, most of TiN forms a solid solution, and the effect of suppressing grain growth is exhibited. Lose. Therefore, this technique cannot be applied to the toughness of the ultra-high heat input welding HAZ which is the object of the present invention.

The technique disclosed in Japanese Patent Application Laid-Open No. 60-184663 is to prevent the HAZ from coarsening during large heat input welding by using REM sulfide / oxide. Sulfurized oxide has a higher stability at a high temperature of 1350 ° C. or higher than nitride, so that the effect of suppressing grain growth is maintained. However, it is difficult to finely disperse sulfur oxides. Ultra-high heat input welding HAZ even though pinning effect of individual particles is maintained due to low number density of sulfur and oxide
There is a limit to reducing the austenite grain size of the steel, and it is not possible to improve the toughness by itself.

The technique described in Japanese Patent Application Laid-Open No. 60-245768 discloses that a HAZ structure is formed by particles of either Ti oxide or a composite of Ti oxide and Ti nitride acting as ferrite transformation nuclei. HAZ
It improves toughness, and its effect is maintained even in ultra-high heat input welding in consideration of the high temperature stability of Ti oxide. However, the crystal orientation of ferrite generated from the intragranular transformation nucleus is not completely random, and is affected by the crystal orientation of the parent phase austenite. Therefore, when austenite grains are coarsened by ultra-high heat input welding, there is a limit to reducing the HAZ structure only by intragranular transformation.

The technique disclosed in Japanese Patent Application Laid-Open No. 2-254118 is for transforming ferrite from a TiN—MnS composite precipitate, and is similar to large heat input welding at 1350 ° C.
The effect is exhibited when the above residence time is relatively short. However, in ultra-high heat input welding such as electroslag or electrogas welding, the residence time of 1350 ° C. or more is long, and during this time, a large amount of TiN forms a solid solution. As a result, the ferrite transformation nuclei disappear and the effect cannot be sufficiently exhibited.

The technique disclosed in Japanese Patent Application Laid-Open No. 61-253344 discloses an oxidation / sulfide of REM / Ca or Ti
BN is formed on N and ferrite is formed from the BN to refine the HAZ structure. Similar effects can be expected in ultra-high heat input welding. However, it is difficult to increase the number of oxides and sulfides of REM / Ca, and TiN forms a solid solution, and there is a limit in improving the toughness of ultra-high heat input welding HAZ only by ferrite transformation.

The technique disclosed in Japanese Patent Application Laid-Open No. 9-157787 has been developed by the present inventors, and is disclosed in Japanese Patent Application Laid-Open No. H09-157787.
The super large heat input welding HAZ toughness can be improved by suppressing the austenite grain growth by the 20 μm fine Mg-containing oxide and promoting the intragranular ferrite transformation by the composite of 0.20-5.0 μm Ti-containing oxide and MnS. However, in order to generate a Ti-containing oxide, it is necessary to suppress the Al content to 0.005% or less, which impairs the advantages of the conventional Al-added steel. That is, the conventional Al content is 0.010 to 0.5%
In a degree of Al deoxidized steel, the temperature of molten steel can be easily controlled by utilizing the heat generated by oxidation of Al in the steel, and low-cost and stable mass production of steel has been enabled. If the amount of Al added is limited to about 0.005% or less, a means for controlling the temperature of molten steel by heat generated by oxidation of Al, such as heating by a molten steel heating device, is required. It is widely known that Al in molten steel also has a role of preventing molten steel from being contaminated by oxygen in the atmosphere, and that Al is effective in securing the material by forming nitrides. The problem remains that the reduction to less than% impairs the advantages of these Al additions.

The technique disclosed in Japanese Patent Application Laid-Open No. H11-286743 is also based on the present inventors, and is disclosed in US Pat.
In steel containing two or more of 0.5 μm of MgO, MgS, and Mg (O, S), the super large heat input welding HAZ toughness can be improved by suppressing austenite grain growth by these fine particles. However, in order to generate fine MgO, it is necessary to suppress the amount of Al to 0.01% or less, and the problem remains that the advantage of the above-described addition of Al is lost.

The present invention has a heat input of 200 kJ such as electroslag welding applied in assembling box columns of a high-rise building and electrogas welding applied in shipbuilding and bridges.
Another object of the present invention is to provide a high-strength steel for welding having excellent HAZ toughness in ultra-high heat input welding of at least / cm or more on the premise of Al-added steel.

[0020]

Means for Solving the Problems The inventors of the present invention have found that miniaturization of the HAZ structure is indispensable for improving the toughness of the ultra-high heat input welding HAZ, and this can be achieved by remarkably suppressing the austenite grain growth of the HAZ. In addition, assuming that Al-added steel, fine (Mn, Mg) S particles are 1350
It has been newly discovered that the composition is extremely stable at a high temperature of not less than ° C and that fine dispersion is possible. With this new finding, HA
The present invention has been made based on the finding that the austenite grain growth of Z can be remarkably suppressed, and as a result, the ultra-high heat input HAZ toughness can be greatly improved.

The gist of the present invention is as follows. (1) 0.04 ≦ C ≦ 0.25, 0.02 ≦
Si ≦ 0.5, 0.2 ≦ Mn ≦ 2.0, P ≦ 0.02,
0.002 ≦ S ≦ 0.02, 0.015 <Al ≦ 0.
5, (Mn, Mg) S containing 0.0005 ≦ Mg ≦ 0.005 and having a particle diameter of 0.005 to 0.5 μm per square mm of 1.0 × 10 ⁵ to 1.0 × 10 ⁵ Including ^seven ,
A high-strength steel for ultra-high heat input welding, characterized in that the steel is composed of a balance of Fe and unavoidable impurities. (2) Further, the group of elements for increasing the base metal strength is 0.05% by weight.
≦ Cu ≦ 1.5, 0.05 ≦ Ni ≦ 2.0, 0.02 ≦
Cr ≦ 1.0, 0.02 ≦ Mo ≦ 1.0, 0.005 ≦
Nb ≦ 0.05, 0.005 ≦ V ≦ 0.1, 0.005
≦ Ti ≦ 0.025, 0.0004 ≦ B ≦ 0.004,
(1) The high tensile strength steel for ultra-high heat input welding according to (1), characterized in that it contains one or more of the following.

The "high-strength steel for welding" referred to in the present invention includes, for example, JIS G3106 "Rolled steel material for welded structures", JIS G3115 "Steel for pressure vessels",
JIS G3118 “Carbon steel sheet for medium / normal temperature pressure vessel”, JIS G3124 “High strength steel sheet for medium / normal temperature pressure vessel”, JIS G3126 “Carbon steel sheet for low pressure pressure vessel”, and JIS G3128 “Height for welded structure” Yield point steel plate ".

[0023]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Such a high-tensile steel for ultra-high heat input welding is manufactured on the premise of Al deoxidation, which is an excellent mass production process with a large production track record. The present inventors have conducted detailed investigations and studies on the relationship between the structure and toughness of the ultra-high heat input welding HAZ. Heat input welding H
AZ toughness was limited, and it was concluded that austenitic grains of HAZ had to be remarkably refined to improve toughness.

First, it is effective to use the pinning effect of the particles in the steel to refine the austenite grains, but TiN is considered to be the most thermally stable among nitrides.
However, if it is heated to 1350 ° C. or more for a long time, most of it melts and loses the pinning effect, so that there is a limit to its application to ultra-high heat input welding. Therefore, the use of particles that are stable at high temperatures is essential. However, with conventional REM or Ca oxides (including oxides and sulfides), it is extremely difficult to finely disperse these oxides in steel to an extent sufficient to suppress austenite grain coarsening in ultra-high heat input welding HAZs. It is.

The inventors of the present invention have already found that fine Mg-containing oxides are effective as a result of comparative studies on various particles. However, in order to generate a large amount of these fine oxides in steel, A
It is necessary to suppress the l content to, for example, about 0.005% or less, which impairs the advantage of Al addition as described above.

The present inventors have conducted a comparative study on various particles based on the assumption that Al deoxidized steel is used. As a result, it has been newly found that (Mn, Mg) S particles are stable at high temperatures and suitable for fine dispersion. I learned. Particles that exhibit an effect of suppressing austenite grain growth of HAZ are mainly 0.1 μm or less, but fine (Mn, Mg) S can be produced by controlling the amounts of Mn, Mg, S, and Al added. It can be finely dispersed in a large amount in steel.

Conventionally, Mn of about 0.2 to 2% and S of about 0.002 to 0.02% have been added to Al deoxidized steel, and it is widely known that MnS is formed.
Since this MnS is unstable at high temperature and is dissolved,
Austenitic grains could not be refined. However, (Mn, Mg) S, which is considered to have some of Mn in MnS replaced by Mg, is completely different from MnS in its properties, is extremely stable at high temperatures, and can be easily finely dispersed. The reason why (Mn, Mg) S is stable at high temperature and easily dispersed finely is unknown at present.

Simply adding Mg to steel (M
n, Mg) S is hardly formed. The reason is that Mg is a strong deoxidizing element and becomes an oxide. M
g is an element that has a high vapor pressure and is difficult to yield in molten steel even when added in a large amount. Therefore, 0.0005 to 0.005
It is extremely important to prevent a trace amount of Mg of about% from being consumed as an oxide and to generate (Mn, Mg) S. FIG. 1 shows (M) having a size of 0.005 to 0.5 μm in a steel in which the added amounts of Mn, Mg and S are within the range of the present invention.
The effect of the amount of Al added on the number of (n, Mg) S particles is shown. When the amount of Al added is less than 0.015%, (Mn, Mg)
The number of S particles is small. Mg at this time is mainly MgAl ₂
It exists as an oxide as O ₄ or MgO. On the other hand, when the amount of Al added is 0.015% or more, (Mn, M
g) The number of S particles is significantly increased, and the oxide is Al ₂ O ₃
Most of Mg mainly exists as (Mn, Mg) S.
That is, a large number of fine (Mn, Mg) S particles can be generated by adding 0.015% or more of Al.

In the present invention, the particle diameter of (Mn, Mg) S is limited to 0.005 to 0.5 μm. If the thickness is less than 0.005 μm, the effect of suppressing austenite grain growth becomes small. On the other hand, if it exceeds 0.5 μm, the probability that these particles or the interface between the particles and the base iron become fracture starting points increases, and the toughness decreases.
When the number of (Mn, Mg) S particles having a size of 0.005 to 0.5 μm is 1.0 × 10 ⁵ or more per 1 mm 2, the effect of suppressing austenite grain growth becomes remarkable.
If it exceeds 0 × 10 ^7, the ductility of the steel will be reduced.
The number of (Mn, Mg) S particles is set to 1.0 per square mm.
× 10 ^{5 to} 1.0 × 10 ⁷ .

The number of particles is measured by preparing an extraction replica from a steel plate and measuring the number of particles having a size of 0.005 to 0.5 μm with a transmission electron microscope (TEM) equipped with a characteristic X-ray detector (EDX). Is measured for an area of at least 1000 μm ² and converted to the number per unit area. For example, one field of view is 100 mm x 80 mm at a magnification of 20,000 times.
Observed area per visual field is 20μ
Observe at least 50 fields of view because of m ² .
If the number of particles of 0.005 to 0.5 μm at this time is 200 in 50 visual fields (1000 μm ² ), the number of particles can be converted to 2 × 10 ⁵ per square mm.

Next, among the particles whose number was measured, (M
Measure how much n, Mg) S particles were present,
The number of particles should be at least 100 or more,
Since the number of particles is 00 or more, identifying all the particles one by one is a serious task. For this reason, (Mn, Mg) S was identified for at least 50 or more particles under the following conditions, and its abundance was determined.
The number of (Mn, Mg) S is obtained by multiplying the existence ratio of S. For example, for the number of particles described above and 2 × 10 ⁵ per square mm, the existing ratio of (Mn, Mg) S is 9
If it is 0%, the number of (Mn, Mg) S is assumed to be 1.8 × 10 ⁵ per square mm.

Next, a method for identifying (Mn, Mg) S will be described. In the present invention, the ratio of Mn and Mg in (Mn, Mg) S is 60% ≦ Mn ≦ 95% and 5% ≦ Mg ≦ 4 by weight%.
Limited to 0%. Even if an element other than Mn and Mg is detected, it is considered that a sulfide mainly composed of Mn and Mg exerts the austenite grain refinement effect of the present invention. In addition, a small amount of O may be detected in the particles. However, if the ratio of S and O is 95% ≦ S in weight% and the contained O is less than 5% (Mn , Mg) S
Is assumed to be. The ratio of S and O is 95% by weight.
If ≦ S and the particles are clearly identified as a complex of MnS and MgO, even if the contained O is less than 5%, they are not regarded as (Mn, Mg) S. The ratios of Mn and Mg and the ratios of S and O are determined quantitatively by EDX. The electron beam diameter used for this determination is 0.00
1 to 0.02 μm, the TEM observation magnification is 50,000 to 1,000,000 times, and an arbitrary position in fine (Mn, Mg) S particles is quantified.

When an extraction replica is made from a steel sheet,
When a large number of precipitates other than (Mn, Mg) S having a size of 0.005 to 0.5 μm, such as cementite and alloy carbonitride, are formed, and it is difficult to measure the number of (Mn, Mg) S particles, , Held at 1400 ° C for about 60 seconds (M
n, Mg) Solution of particles other than S, and then quenching to make a sample with less cementite and alloy carbonitride,
It is good to make an extraction replica from now on.

In order to disperse particles having the above-mentioned size and number in steel, Mn, Mg, S, and Al
Is desirably limited as follows. Mn
Is an element constituting (Mn, Mg) S and is an essential element in the present invention. When Mn is added in an amount of 0.2% or more, a large amount of fine (Mn, Mg) S can be dispersed, so the lower limit is made 0.2%. When Mn exceeds 2.0% (M
Since (n, Mg) S tends to become coarse and the effect of improving the HAZ toughness is reduced, the upper limit is set to 2.0%.

Mg is an element essential for producing (Mn, Mg) S. If it is less than 0.0005%, the required number of (M
n, Mg) S particles cannot be obtained. In order to generate a larger amount of fine (Mn, Mg) S particles, 0.0
Addition of 015% or more is more preferable. If the addition exceeds 0.005%, Mg forms an oxide, so that the amount of (Mn, Mg) S is saturated, the effect of improving the HAZ toughness is saturated, and the economy is impaired. Therefore, the upper limit is made 0.005%. .

S is an essential element for producing (Mn, Mg) S. If less than 0.002%, (Mn, M
g) Since the amount of S is insufficient, the lower limit was made 0.002%. In order to generate a larger amount of fine (Mn, Mg) S particles, 0.003% or more is more preferable.
If the content exceeds 0.02%, coarse (Mn, Mg) S is generated and the effect of refining the γ grains of the ultra-high heat input welding HAZ cannot be obtained, so the upper limit is set to 0.02%.

Al suppresses the formation of oxides by Mg, and Mg is an essential element for forming (Mn, Mg) S, and it is necessary to add 0.015% or more. In order to generate a larger amount of fine (Mn, Mg) S particles, Al addition of 0.02% or more is more preferable. 0.5
%, HAZ embrittlement due to solid-solution Al occurs, so that even if the austenitic grains of HAZ are refined by (Mn, Mg) S, a large effect of improving toughness cannot be obtained.
Therefore, the upper limit is set to 0.5%.

The HAZ toughness varies not only with the refinement of austenite grains and with the refinement of the intragranular structure, but also with the alloy elements. Further, since an appropriate alloy element may be contained in order to secure the strength of the base material, the amount of the alloy element added is limited for the following reasons.

C is an element capable of increasing the strength of the base material.
If it is less than 0.04%, the base material strength cannot be secured, so 0.04% was made the lower limit. Conversely, if a large amount of C is contained,
In order to increase cementite and island-like martensite which are the starting points of brittle fracture, HAZ is formed by (Mn, Mg) S.
However, even if the austenite grains are refined, a large toughness improving effect cannot be obtained. If it exceeds 0.25%, the toughness is significantly reduced.

Si is an element effective for increasing the strength of the base material.
If less than 0.02%, this effect cannot be obtained, so the lower limit is set to 0.02%. Conversely, if the content exceeds 0.5%, HA
Since a large amount of island martensite is generated in the Z structure and hardens the ferrite ground, (Mn, Mg) S
Therefore, even if the austenite grains of the HAZ are made fine, a large toughness improving effect cannot be obtained. Therefore, the upper limit is 0.5%
And

P is an element that causes grain boundary embrittlement and is harmful to toughness. If the content exceeds 0.02%, even if the austenite grains of the HAZ are refined by (Mn, Mg) S, the reduction in toughness becomes remarkable, so the upper limit is made 0.02%.

Further, the limited range of the selected element effective for increasing the base material strength was determined for the following reason. Cu is an element effective in increasing the strength of the base material. In particular, a remarkable increase in strength can be obtained by precipitating a fine Cu phase by aging heat treatment. If it is less than 0.05%, the strength cannot be increased,
0.05% was made the lower limit. Conversely, if the content exceeds 1.5%, the embrittlement of the base material and HAZ becomes remarkable, so the upper limit value is set to 1.5.
%.

Ni has the effect of increasing the strength of the base metal by increasing the hardenability, and further improves the toughness.
If the content is less than 0.05%, these effects cannot be obtained, so the lower limit is set to 0.05%. Ni is an expensive element;
If the content exceeds 0%, the economical efficiency is impaired, so the upper limit is 2.0%.
And

Cr is effective in increasing the strength of the base material. 0.
If it is less than 02%, this effect cannot be obtained, so the lower limit is set to 0.
02%. Conversely, if the content exceeds 1.0%, a hardened structure is formed in the HAZ, and even if the austenite grains of the HAZ are refined by (Mn, Mg) S, a large HAZ toughness improving effect cannot be obtained. Therefore, the upper limit was set to 1.0%.

Mo is effective in increasing the strength of the base material. 0.
If it is less than 02%, this effect cannot be obtained, so the lower limit is set to 0.
02%. Conversely, if the content exceeds 1.0%, a hardened structure is formed in the HAZ, and even if the austenite grains of the HAZ are refined by (Mn, Mg) S, a large HAZ toughness improving effect cannot be obtained. Therefore, the upper limit was set to 1.0%.

Nb is an element effective for increasing the strength and refining the base material. If the content is less than 0.005%, these effects cannot be obtained, so the lower limit is set to 0.005%. Conversely, 0.
If the content exceeds 0.05%, precipitation of Nb carbonitride in the HAZ becomes remarkable, and even if the austenite grains of the HAZ are refined by (Mn, Mg) S, a large effect of improving the HAZ toughness cannot be obtained. Therefore, the upper limit is set to 0.05%.

V is an element effective for increasing the strength and refining the base material. If the content is less than 0.005%, these effects cannot be obtained, so the lower limit is set to 0.005%. Conversely, 0.1
%, The precipitation of carbonitrides in the HAZ becomes remarkable, and even if the austenite grains of the HAZ are refined by (Mn, Mg) S, a large effect of improving the HAZ toughness cannot be obtained. Therefore, the upper limit is set to 0.1%.

Ti is an element effective for increasing the strength and reducing the grain size of the base material. If the content is less than 0.005%, these effects cannot be obtained, so the lower limit is set to 0.005%. Conversely, 0.
If the content exceeds 025%, coarse TiN is generated and this becomes a starting point of the destruction, so that HA is formed by (Mn, Mg) S.
Even if the austenite grains of Z are refined, a large HAZ toughness improving effect cannot be obtained. Therefore, the upper limit is set to 0.025%
And

B exerts a particularly remarkable increase in strength when subjected to controlled cooling and quenching heat treatment. 0.000
If the content is less than 4%, the effect of increasing the strength cannot be obtained, so the lower limit was made 0.0004%. Conversely, if the content exceeds 0.004%, coarse B nitrides and carbides are precipitated and serve as starting points of destruction.
Even if the austenite grains of Z are refined, a large HAZ toughness improving effect cannot be obtained. Therefore, the upper limit is 0.004%
And

In the present invention, it is necessary to generate fine (Mn, Mg) S. For this reason, it is desirable to reduce sulfide-forming elements other than Mn and Mg as much as possible. Representative elements are Ca and REM, which are 0.0005
% Is desirable.

In the present invention, the oxygen content in steel is not particularly limited. In the case of the Al-added steel of 0.015 to 0.5%, the oxygen content in the steel is about 0.0003 to 0.0040%. However, if the oxygen content is within this range, the grain refining effect of the present invention is impaired. There is no. In the present invention, the amount of nitrogen in steel is not particularly limited. If the amount of nitrogen is about 0.0010 to 0.010% of the normal amount, the effect of the present invention on grain refinement is not impaired. The effect of improving the HAZ toughness according to the present invention is not only super large heat input welding but also large heat input welding (for example, 100 to less than 200 kJ /
cm) is also effective. In the present invention, impurity elements usually inevitably contained in steel are acceptable. C
Even if u, Ni, Cr, Mo, Nb, V, B, N, Ti and the like are mixed as impurities, the properties of the present invention are not impaired. For example, Cu and Ni are less than 0.05%, Cr and Mo are less than 0.02%, Nb, V, and Ti are 0.005%.
%, And B does not adversely affect even if it is contained as an impurity up to less than 0.0004%.

The method of smelting steel includes, for example, setting the temperature of molten steel to 165.
0 ° C or less, molten steel O concentration 0.01% or less, molten steel S
With the concentration set to 0.02% or less, appropriate amounts of Mn and M
By adding g and Al, fine (Mn, Mg) S can be generated in the molten steel. By casting the molten steel by continuous casting, fine particles of (Mn, Mg) S can be contained in the steel. The steel production method is (M
Since it is sufficient that a predetermined amount of (n, Mg) S is present, heating, rolling and heat treatment conditions after casting may be appropriately selected according to the mechanical properties of the base steel material.

[0053]

Examples of the present invention will be described below. Steel was melted by a converter, and a slab having a thickness of 240 to 400 mm was manufactured by continuous casting. Table 1 shows the chemical components of the steel materials. HA
Since the Z toughness greatly depends on the carbon equivalent of steel, in order to confirm the effect of the present invention, M
Steels in which only n, Mg, S, and Al were changed were melted and compared.

Table 2 shows the steel sheet manufacturing method, the sheet thickness, and the mechanical properties of the base material. As shown in the table, steel sheets were produced by a controlled rolling / controlled cooling method, a quenching / tempering method, a direct quenching / tempering method, and a direct quenching / two-phase region heat treatment / tempering method. The plate thickness was 40 to 100 mm. Weld specimens were prepared by electrogas welding shown in FIG. 2 and electroslag welding shown in FIG. The plate thickness was adjusted to 35 mm, and electrogas welding with a heat input of 310 kJ / cm was performed. Here, the welding current was 610 A and the voltage was 35.
V, the speed was 4.1 cm / min. As shown in the figure,
3mm from welding fusion line (FL) and welding fusion line (HA
A Charpy impact test piece was collected so that the position of Z3) coincided with the notch position. The current of the electroslag welding was 380 A, the voltage was 46 V, and the speed was 1.14 cm /.
Minutes. The heat input is 920 kJ / cm. A Charpy impact test specimen was collected so as to be at the same notch position as in electroslag welding. The impact test was performed at −5 ° C., and the toughness was evaluated by the average value of three repetitions. Table 3 shows the results.
In addition, the microstructure of the HAZ near the electroslag weld FL was observed to measure the γ particle size, and further, 0.00
Table 3 also shows the results obtained by measuring the number of (Mn, Mg) S particles having a particle diameter of 5 to 0.5 μm according to the above method.

As is evident from Table 3, the steel of the present invention (M
The number of particles of n, Mg) S is large, and the γ particle size of the electroslag welding HAZ is small. As a result, very large heat input welding HA
Z has high toughness. Similarly, the improvement of the HAZ toughness of the steel of the present invention is evident also by electrogas welding. On the contrary,
For comparative steels 9, 10, 18, 20, 24, 26, 28, M
Although the addition amounts of n, S, and Al are appropriate, the addition amount of Mg is lower than the range of the present invention, so that the number of (Mn, Mg) S particles is small, the effect of suppressing γ grain growth is small, and the effect of improving HAZ toughness is small. In Comparative Steels 5, 16, and 22, the addition amounts of Mn, Mg, and Al are appropriate, but the addition amount of S is lower than the range of the present invention. The improvement effect is small. In Comparative Steel 6, since the amount of S added is higher than the range of the present invention, fine (Mn, M
g) The number of S particles is small and the effect of suppressing the
The effect of improving AZ toughness is small. In comparative steels 7 and 8, Mn, M
Although the amounts of g and S are appropriate, the number of (Mn, Mg) S particles is small because the amount of Al added is lower than the range of the present invention.
The effect of suppressing grain growth is small and the effect of improving HAZ toughness is small.
In Comparative Steel 15, although the added amounts of Mg, S, and Al were appropriate, the added amount of Mn was lower than the range of the present invention, so that (Mn, Mg)
The number of S particles is small and the effect of suppressing the growth of γ grains is small, and HAZ
The effect of improving toughness is small.

[0056]

[Table 1]

[0057]

[Table 2]

[0058]

[Table 3]

[0059]

As described above, in the steel of the present invention, Al
Fine particles of (Mn, Mg) S are dispersed in steel in deoxidized steel so that the heat input is 200 kJ / cm or more.
Effective crystal grains of AZ are refined, HAZ toughness can be remarkably improved, and by applying the present invention to a structure to which ultra-high heat input welding is applied, a highly reliable welded structure is manufactured. It is possible to Therefore, the present invention is extremely effective industrially.

[Brief description of the drawings]

FIG. 1: (Mn, M) with a size of 0.005 to 0.5 μm
g) is a diagram showing the effect of the amount of Al added on the number of S particles.

FIG. 2 is a diagram showing conditions of electrogas welding.

FIG. 3 is a diagram showing conditions of electroslag welding.

[Explanation of symbols]

 1 Charpy test piece 2 Notch position of Charpy test piece: FL 3 Notch position of Charpy test piece: HAZ 3 mm

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Ryuji Uemori 20-1 Shintomi, Futtsu-shi, Chiba Nippon Steel Corporation Technology Development Division (72) Inventor Tomoo Nagao 5-3 Tokai-cho, Tokai-shi, Aichi Nippon Steel Corporation Nagoya Works

Claims (2)

[Claims] (1) 0.04 ≦ C ≦ 0.25 by weight,
0.02 ≦ Si ≦ 0.5, 0.2 ≦ Mn ≦ 2.0, P ≦
0.02, 0.002 ≦ S ≦ 0.02, 0.015 <A
(Mn, Mg) containing l ≦ 0.5, 0.0005 ≦ Mg ≦ 0.005, and having a particle size of 0.005 to 0.5 μm.
S is set to 1.0 × 10 ^{5 to} 1.0 × 10 ⁷ per 1 mm 2.
A high-strength steel for ultra-high heat input welding, characterized in that it is a steel containing, in balance, Fe and unavoidable impurities. 2. The method of claim 1, further comprising:
0.05 ≦ Cu ≦ 1.5, 0.05 ≦ Ni ≦ 2.0,
0.02 ≦ Cr ≦ 1.0, 0.02 ≦ Mo ≦ 1.0,
0.005 ≦ Nb ≦ 0.05, 0.005 ≦ V ≦ 0.
1, 0.005 ≦ Ti ≦ 0.025, 0.0004 ≦ B
The high-strength steel for ultra-high heat input welding according to claim 1, comprising one or more of ≦ 0.004.