JP2002053928A - High tensile strength steel for superlarge heat input welding - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a high tensile strength steel excellent in the toughness of a part affected by a superlarge heat input weld heat in which heat input is >=200 kJ/c (e.g. even about 1,500 kJ/c). SOLUTION: This steel contains 1.0×105 to 1.0×107 pieces per mm2 of copper sulfide grains with a grain size of 0.005 to 0.5 μm, and has a composition containing, by weight, 0.04 to 0.25% C, 0.02 to 0.5% Si, 0.02 to 2.0% Mn, <=0.02% P, 0.002 to 0.02% S, 0.03 to 2.0% Cu, 0.015 to 0.5% Al and 0.001 to 0.005% Mg, and the balance Fe with inevitable impurities. Further, as selective elements, one or more kinds among Ni, Cr, Mo, Nb, V, Ti and B are suitably contained. By this pinning action of the fine copper sulfide grains, the growth of γ grains in the superlarge heat input weld HAZ is suppressed to improve the HAZ toughness.

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 ultra-high heat input welding having excellent toughness, particularly having excellent HAZ toughness even when the heat input is 200 kJ / cm or more, for example, about 750 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 technology disclosed in "Iron and Steel", No. 11 of the 61st year (1975) is based on TiN.
In order to suppress the growth of austenite grains by utilizing nitrides, the effect is exhibited in large heat input welding, but 1350 ° C. in super large heat input welding targeted by the present invention.
Due to the extremely long residence time, most TiN
Dissolves and loses the effect of suppressing grain growth. 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.

[0013] The technique disclosed in Japanese Patent Application Laid-Open No. 60-184663 is to prevent coarsening of the HAZ during large heat input welding by using sulfides and oxides of REM. Sulfurization
Oxides have higher stability at high temperatures of 1350 ° C. or higher than nitrides, so that the effect of suppressing grain growth is maintained. However, it is difficult to finely disperse sulfur oxides. Due to the low number density of sulfur and oxides, even though the pinning effect of individual particles is maintained, high heat input welding HA
There is a limit to reducing the austenite grain size of Z, and this alone cannot improve toughness.

The technique described in Japanese Patent Application Laid-Open No. 60-245768 discloses a HAZ structure in which particles of either Ti oxide or a composite of Ti oxide and Ti nitride act as ferrite transformation nuclei. HA
It improves Z 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 is based on the present inventors, and is disclosed in Japanese Patent Application Laid-Open (JP-A) No. 9-157787.
Ultra-high heat input welding HAZ toughness can be improved by suppressing austenite grain growth by a 0.20 μm fine Mg-containing oxide and promoting intragranular ferrite transformation by a composite of 0.20-5.0 μm Ti-containing oxide and MnS. . However, in order to form a Ti-containing oxide, the amount of Al is set to 0.005.
%, Which impairs the advantages of conventional Al-added steel. That is, when the conventional Al amount is 0.010-0.
In the case of Al deoxidized steel of about 5%, the temperature of molten steel can be easily controlled by utilizing the heat generated by oxidation due to Al in the steel, thereby enabling mass production of inexpensive and stable steel. A
If the amount of l 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. Al in molten steel also has a role of preventing molten steel contamination by oxygen in the atmosphere.
It is also widely known that the formation of nitrides is effective in securing the material, and reduction of the Al content to 0.005% or less impairs the advantages of the addition of Al.

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 ultra-high heat input welding excellent in HAZ toughness in ultra-high heat input welding of not less than Al / cm 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, on the premise of Al-added steel, it has been newly demonstrated that fine copper sulfide particles are extremely stable at a high temperature of 1350 ° C. or higher and can be finely dispersed in steel containing a specific amount of Mg or more. I learned. The present invention has been made based on the finding that the austenite grain growth of the HAZ can be remarkably suppressed by this new finding, 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) By weight%, 0.04 ≦ C ≦ 0.25, 0.02 ≦ Si ≦ 0.5, 0.02 ≦ Mn ≦ 2.0, P ≦ 0.02, 0.002 ≦ S ≦ 0 .02, 0.03 ≦ Cu ≦ 2.0, 0.015 ≦ Al ≦ 0.5, 0.001 ≦ Mg ≦ 0.005, and having a particle diameter of 0.005 to 0.5 μm The particle size is 1.0 × 10 ^{5 to} 1.0 × 10 ⁷ per square mm.
A high-strength steel for ultra-high heat input welding, characterized in that the steel consists of Fe and the balance of Fe and unavoidable impurities. 4.0, 0.02 ≦ Cr ≦ 1.0, 0.02 ≦ Mo ≦ 1.0, 0.005 ≦ Nb ≦ 0.05, 0.005 ≦ V ≦ 0.1, 0.005 ≦ Ti ≦ The high tensile strength steel for ultra-high heat input welding according to (1), characterized by containing one or more of 0.025 and 0.0004 ≦ B ≦ 0.004.

The "high-strength steel for welding" referred to in the present invention includes, for example, JIS G3106 "rolled steel material for welded structure", JIS G3136 "rolled steel material for building structure", and JIS G3115 "steel plate for pressure vessel". ”, JIS
G3118 “Carbon steel plate for medium / normal temperature pressure vessel”, JI
SG3124 "High-strength steel plate for medium / normal temperature pressure vessel", J
It corresponds to IS G3126 “Carbon steel plate for low temperature pressure vessel” and JIS G3128 “High yield point steel plate for welded structure”.

[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 comparative studies on various particles based on the assumption that Al deoxidized steel is used. As a result, in steel containing Mg, copper sulfide particles are stable at high temperatures and suitable for fine dispersion. Was newly found. Particles that exhibit an effect of suppressing austenite grain growth of HAZ are mainly 0.1 μm.
m or less, but by controlling the amounts of Cu, S, Mg, and Al added, fine copper sulfide particles can be finely dispersed in a large amount in steel.

Conventionally, 0.03 to 2 in Al deoxidized steel
% Of Cu and 0.002 to 0.02% of S are added in many cases, but no suppression of austenite grain growth of HAZ was observed. S in these steels
Is widely known to form MnS rather than copper sulfide. This MnS was unstable at a high temperature and was dissolved, and thus could not become austenite fine particles. However, in steel containing a specific amount of Mg or more, sulfide formation behavior is completely different from conventional steel, and the formation of copper sulfide is remarkably promoted.As a result, S forms copper sulfide rather than MnS, Further, it was found that the copper sulfide particles are stable at a high temperature and are finely dispersed, so that they have a remarkable effect of suppressing austenite grain growth of HAZ. The reason why the formation of copper sulfide particles is remarkably promoted in steel containing Mg is unknown at present, but it is considered that Mg dissolved in steel has an effect of remarkably reducing the solubility product of copper sulfide.

As described above, the formation of copper sulfide particles is remarkably promoted by Mg in the steel, but copper sulfide particles that are stable at high temperatures are not often formed only by adding Mg to the steel. As a result of a detailed study of the reason, it was found that Mg is an oxide because it is a strongly deoxidized element. Mg 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
-0.005% of a small amount of Mg is prevented from being consumed as an oxide, and a solid solution M effective for promoting the formation of copper sulfide.
It is extremely important that the compound be present as g. FIG. 1 shows C
In steels in which the addition amounts of u, Mg, and S are within the range of the present invention, the contents of 0.1, 0.2, and 0.1 are preferable.
The effect of the amount of Al added on the number of copper sulfide particles having a size of 005 to 0.5 μm is shown. Al content is 0.015%
If it is less than 1, the number of copper sulfide particles is small. At this time, Mg is present as an oxide mainly as MgAl ₂ O ₄ or MgO, so that the amount of dissolved Mg is small. On the other hand, when the amount of Al added is 0.015% or more, the number of copper sulfide particles increases remarkably. Since the oxide is mainly composed of Al ₂ O ₃ , most of Mg exists as solid solution Mg. That is, 0.015%
Many fine copper sulfide particles can be generated by the addition of Al.

In the present invention, the particle size of the copper sulfide particles is set to 0.0
It was limited to 05 to 0.5 μm. If the thickness is less than 0.005 μm, the effect of suppressing austenite grain growth becomes small. Also,
If it exceeds 0.5 μm, the probability that these particles or the interface between the particles and the ground iron will be the starting point of fracture increases, and the toughness decreases. 0.
Austenite grain growth suppressing effect becomes remarkable when the number of copper sulfides particle size 005~0.5μm of 1.0 × 10 ⁵ or more per square mm, 1.0 × 10 ⁷ cells more than the steel To reduce ductility, the number of copper sulfide particles was limited to 1.0 × 10 ^{5 to} 1.0 × 10 ⁷ per square mm.

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, it is measured how many copper sulfide particles are present among the particles whose number has been measured. When the number of particles is at least 100 or more, and when the number is large, it is 10,000 or more. Identifying one by one is a daunting task. For this reason, at least 50 particles or more of the copper sulfide particles are identified under the following conditions and the abundance ratio is determined,
The number of copper sulfide particles is obtained by multiplying the number of particles obtained above by the existing ratio of copper sulfide particles. For example, when the existing ratio of copper sulfide particles is 90% with respect to the number of particles described above and 2 × 10 ⁵ per square mm, the number of copper sulfide particles is 1.8 × 10 ⁵ per square mm. It is assumed that there are

Next, a method for identifying copper sulfide particles will be described. The copper sulfide particles referred to in the present invention are particles in which the peaks of Cu and S are clearly detected by qualitative analysis of EDX, and are mainly composed of Cu ₂ S and Cu _2-X S (0 <x <0). .
2), Cu _1.8 S, CuS and the like. Even if Mn, Mg, Ca, etc., as elements other than Cu, for example, are detected from the particles in trace amounts of 1% or less, the sulfides mainly composed of Cu can clearly show the austenite grain refinement effect of the present invention. It is considered to be effective. Also, a small amount of O may be detected from the particles, but the ratio of S and O is 9% by weight.
If 5% ≦ S and the amount of O contained is as small as less than 5%, it is regarded as copper sulfide according to the present invention. In addition, even if the ratio of S and O is 95% ≦ S in weight% and the contained O is less than 5%, the particles are clearly a complex of copper sulfide and an oxide (for example, MgO or the like). If it can be identified as, it is not regarded as copper sulfide in the present invention.

The electron beam diameter used during the EDX qualitative and quantitative analysis is 0.001 to 0.02 μm, the TEM observation magnification is 50,000 to 1,000,000, and an arbitrary position in the fine copper sulfide particles is quantified. When preparing a sample of the extraction replica, Cu
When a mesh holder containing no Cu is used, or when a mesh holder containing Cu is used, particles at positions sufficiently separated from the Cu mesh are quantified using the Cu L line.

When an extraction replica is created from a steel sheet,
When a large number of precipitates other than copper sulfide 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 copper sulfide particles, 140
It is preferred to hold the solution at 0 ° C. for about 60 seconds to dissolve particles other than copper sulfide, and then rapidly cool to prepare a sample containing less cementite and alloy carbonitride, and to prepare an extraction replica from this.

In order to disperse particles having the above-mentioned size and number in steel, it is desirable to limit the contents of Cu, Mg, S and Al as follows. Cu is an essential element in the present invention because it is an element constituting copper sulfide. By adding 0.03% or more of Cu, a large amount of fine copper sulfide particles can be dispersed. Therefore, the lower limit is set to 0.03%. If Cu exceeds 2.0%, the copper sulfide particles tend to be coarsened and the effect of improving the HAZ toughness is reduced.
% As the upper limit.

S is an essential element for producing copper sulfide. If it is less than 0.002%, the number of copper sulfide particles becomes insufficient, so the lower limit was made 0.002%. In order to generate a larger amount of fine copper sulfide particles, addition of 0.003% or more is more preferable. If the content exceeds 0.02%, coarse copper sulfide particles are generated and the effect of refining the γ grains of the super-large heat input welding HAZ cannot be obtained, so the upper limit value is set to 0.02%.

Mg is an element essential for promoting the formation of copper sulfide particles. If it is less than 0.001%, the effect of promoting the formation of copper sulfide particles is small. In order to generate a large amount of fine copper sulfide particles, addition of 0.002% or more is more preferable. 0.
Addition of more than 005% saturates the effect of accelerating the formation of copper sulfide particles because Mg forms an oxide, saturates the effect of improving the HAZ toughness, and impairs economic efficiency. Therefore, the upper limit is 0.005%.
And

Al is an essential element in the present invention to suppress the formation of oxides by Mg and to secure the amount of solid solution Mg necessary to promote the formation of copper sulfide particles.
It is necessary to add 0.015% or more. In order to generate a larger amount of fine copper sulfide, 0.02% or more of Al
Addition is more preferred. If the content exceeds 0.5%, HAZ embrittlement due to solid solution Al occurs, so that even if the austenitic grains of the HAZ are refined by the copper sulfide particles, a large toughness improving effect cannot be obtained. Therefore, the upper limit is set to 0.5%.

The HAZ toughness varies greatly depending on the alloying element as well as the refinement of austenite grains and the grain structure. 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 the amount of cementite or island martensite which is a starting point of brittle fracture, even if the austenitic grains of HAZ are refined by copper sulfide particles, a large effect of improving toughness 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 the ferrite ground is hardened, even if the austenite grains of the HAZ are refined by the copper sulfide particles, a large effect of improving toughness cannot be obtained. Therefore, the upper limit is set to 0.5%.

P is an element that causes grain boundary embrittlement and is harmful to toughness. If the content exceeds 0.02%, even if the austenitic grains of the HAZ are refined by the copper sulfide particles, the reduction in toughness becomes remarkable, so the upper limit is made 0.02%.

Further, the limited range of the selected element effective in increasing the base metal strength was determined for the following reasons. Ni has the effect of increasing the base metal strength 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, and if the content exceeds 4.0%, economical efficiency is impaired, so the upper limit was set to 4.0%.

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 austenitic grains of the HAZ are refined by the copper sulfide particles, 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 austenitic grains of the HAZ are refined by the copper sulfide particles, 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 the copper sulfide particles, 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 carbonitride in the HAZ becomes remarkable, and even if the austenitic grains of the HAZ are refined by the copper sulfide particles, a large HAZ toughness improving effect 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 fracture. Therefore, even if the austenite grains of HAZ are refined by copper sulfide particles, a large HAZ toughness improving effect cannot be obtained. Therefore, the upper limit is set to 0.025%.

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 carbohydrates precipitate and serve as starting points of destruction. Therefore, even when the austenite grains of HAZ are refined by copper sulfide particles, a large HAZ toughness improving effect is obtained. Can not be obtained. Therefore, the upper limit is set to 0.004%.

In the present invention, it is necessary to generate fine copper sulfide particles. For this purpose, it is desirable to reduce the sulfide-forming elements other than Cu as much as possible. A typical element is Ca
And REM, which are desirably 0.0005% or less.

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 unavoidably contained in steel are acceptable. Ni, Cr, Mo,
Even if Nb, V, B, N, Ti and the like are mixed as impurities, the properties of the present invention are not impaired. For example, Ni is 0.0
Less than 5%, Cr and Mo less than 0.02%, Nb,
Even if V and Ti are contained as impurities by less than 0.005% and B is contained by less than 0.0004%, there is no particular adverse effect. For example, in the method of smelting steel, appropriate amounts of Cu, Mg, and Al are added in a state in which the temperature of molten steel is 1650 ° C. or less, the concentration of molten steel O is 0.01% or less, and the concentration of molten steel S is 0.02% or less. As a result, fine copper sulfide particles can be generated in the molten steel. By casting this molten steel by continuous casting, fine particles of copper sulfide can be contained in the steel. Since the steel production method only needs to have a predetermined amount of copper sulfide particles, 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, C
Steels in which only the amounts of u, 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 of the measurement of the number of copper sulfide particles having a particle diameter of 5 to 0.5 μm according to the method described above.

As is clear from Table 3, the steel of the present invention has a large number of copper sulfide particles, and the γ of electroslag welding HAZ is large.
Small particle size. As a result, the toughness of the ultra-high heat input welding HAZ is high. Similarly, the HA of the steel of the present invention is also used in electrogas welding.
The improvement in Z toughness is apparent. In contrast, comparative steel 9,
In 10, 18, 20, 22, and 24, the addition amounts of Cu, S, and Al are appropriate, but the addition amount of Mg is lower than the range of the present invention, so that the number of copper sulfide particles is small, the effect of suppressing the growth of γ grains is small, and the effect of improving the HAZ toughness is reduced. Is small. In comparative steel 5, Cu, M
Although the addition amounts of g and Al are appropriate, the addition amount of S is lower than the range of the present invention, so that the number of copper sulfide particles is small, the effect of suppressing the growth of γ grains and the effect of improving the HAZ toughness are small. In Comparative Steel 6, since the amount of S added is higher than the range of the present invention, the number of fine copper sulfide particles is small, the effect of suppressing the growth of γ grains is small, and the effect of improving the HAZ toughness is small. In comparative steels 7, 8, 26 and 28, Cu,
Although the addition amounts of Mg and S are appropriate, the addition amount of Al is lower than the range of the present invention, so that the number of copper sulfide particles is small, the effect of suppressing the growth of γ grains is small, and the effect of improving the HAZ toughness is small.

[0056]

[Table 1]

[0057]

[Table 2]

[0058]

[Table 3]

[0059]

As described above, in the steel of the present invention, Al
In the case of deoxidized steel, the heat input is 200 kJ / cm or more by dispersing copper sulfide particles finely in the steel.
The effective crystal grains of HAZ are refined by the action of L and HAZ to suppress the growth of γ grains, and the HAZ toughness can be remarkably improved. By applying the present invention to a structure to which ultra-high heat input welding is applied, it is possible to manufacture a highly reliable welded structure. Therefore, the present invention is extremely effective industrially.

[Brief description of the drawings]

FIG. 1 is a graph showing the effect of the amount of Al added on the number of copper sulfide particles having a size of 0.005 to 0.5 μm.

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

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[Procedure amendment]

[Submission date] August 3, 2000 (2008.3.3)

[Procedure amendment 1]

[Document name to be amended] Statement

[Correction target item name] 0052

[Correction method] Change

[Correction contents]

In the present invention, impurity elements usually unavoidably contained in steel are acceptable. Ni, Cr, Mo,
Even if Nb, V, B, N, Ti and the like are mixed as impurities, the properties of the present invention are not impaired. For example, Ni is 0.0
Less than 5%, Cr and Mo less than 0.02%, Nb,
Even if V and Ti are contained as impurities by less than 0.005% and B is contained by less than 0.0004%, there is no particular adverse effect. In the method of smelting steel, appropriate amounts of Cu, Mg and Al are added, for example, with the molten steel temperature set to 1650 ° C. or less, the molten steel O concentration set to 0.01% or less, and the molten steel S concentration set to 0.02% or less . By casting steel by continuous casting, fine particles of copper sulfide can be contained in the steel. Since the steel production method only needs to have a predetermined amount of copper sulfide particles, heating, rolling, and heat treatment conditions after casting may be appropriately selected according to the mechanical properties of the base steel material.

 ──────────────────────────────────────────────────続 き Continued on the front page (72) Inventor Ryuji Uemori 20-1 Shintomi, Futtsu City, Chiba Prefecture Nippon Steel Corporation Technology Development Division (72) Inventor Toshimichi Nagao 5-3 Tokaicho, Tokai City, Aichi Prefecture Nippon Steel Corporation Nagoya Works

Claims (2)

[Claims] 1. In% by weight, 0.04 ≦ C ≦ 0.25, 0.02 ≦ Si ≦ 0.5, 0.02 ≦ Mn ≦ 2.0, P ≦ 0.02, 0.002 ≦ S ≦ 0.02, 0.03 ≦ Cu ≦ 2.0, 0.015 ≦ Al ≦ 0.5, 0.001 ≦ Mg ≦ 0.005, having a particle diameter of 0.005 to 0.5 μm. 1.0 × 10 ^{5 to} 1.0 × 10 ⁷ copper sulfide particles per square mm
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 element group for increasing the strength of the base material, in terms of% by weight, 0.05 ≦ Ni ≦ 4.0, 0.02 ≦ Cr ≦ 1.0, 0.02 ≦ Mo ≦ 1.0. 005 ≦ Nb ≦ 0.05, 0.005 ≦ V ≦ 0.1, 0.005 ≦ Ti ≦ 0.025, 0.0004 ≦ B ≦ 0.004, or one or more of the following: The high-strength steel for ultra-high heat input welding according to claim 1, characterized in that: