JP2002224835A - Method of welding high toughness high tension steel having excellent weld heat influence zone toughness - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method of welding for high toughness high tension steel having excellent weld heat influence zone toughness. SOLUTION: The high tension steel which contains, by mass %, 0.01 to 0.15% C, 0.005 to 1% Si, 0.1 to 3% Mn, <=0.02% P, <=0.01% S, 0.001 to 0.1% Al, 1 to 5% Ni, 0.0002 to 0.005% B, 0.001 to 0.01% N further contains one or more kinds of 0.003 to 0.05% Ti, 0.005 to 0.5% Nb, 0.02 to 1% Ta, 0.1 to 2% Mo, 0.5 to 4% W, 0.05 to 1.5% Cu, 0.05 to 2% Cr and 0.01 to 0.5% V and consists of the balance Fe and inevitable impurities, is welded by TIG welding of <=3 kJ/mm heating and <=20 g/min in deposition rate, by which a structural material having the excellent weld heat influence zone toughness is obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention
It is related to a method of welding structural steel provided to all structures requiring excellent low-temperature toughness of not less than -40 ° C or less in a toughness assurance temperature of not less than -40 ° C.
It can be applied to welded structures such as low-temperature storage tanks, low-temperature pressure vessels, marine structures, ships, bridges, line pipes, and the like. Although the form of the steel is not particularly limited, the steel is used as a structural member and is useful for a steel sheet, particularly a thick plate or a steel pipe material, which requires low-temperature toughness.

[0002]

2. Description of the Related Art Conventionally, high-strength steels having a tensile strength of 780 MPa or more have been mainly produced by reheating quenching and tempering. Reheating and quenching
In order to ensure the toughness of the tempered material, it is necessary to control the quenched structure and to heat austenite (γ).
It is necessary to reduce the particle size, and for that purpose, it is necessary to limit the heating temperature (quenching temperature or austenitizing temperature) of the reheating quenching. However, lowering the quenching temperature makes it impossible to expect a sufficient solid solution of the element effective for strengthening, which may lead to a decrease in strength and a deterioration in toughness due to undissolved carbides, thereby increasing both strength and toughness. It is not easy.

In the case of reheating quenching and tempering,
Particularly, in the case of a thick material, the quenched structure is greatly different between the surface layer portion and the inside due to a difference in cooling rate between the surface layer and the inside during quenching. As a result, there is also a problem that the material is greatly different between the surface layer portion and the inside. In other words, when the structure of the surface layer is a chemical composition that makes the lower bainite (BL) or a mixed structure of lower bainite and martensite (M) optimal from the viewpoint of strength and toughness, the central part of the sheet thickness where the cooling rate is small The upper bainite (B
U), and it becomes difficult to secure the strength and toughness of the central portion of the sheet thickness. On the other hand, if an alloy element is added to improve the structure of the central portion of the sheet thickness, the hardenability of the surface layer having a large cooling rate becomes excessive,
Since it has a martensite single phase structure, the toughness of the surface layer is insufficiently improved, and it is difficult to stably improve the material from the surface layer to the center of the sheet thickness.

[0004] As one of the measures to overcome the problems in the reheating quenching / tempering treatment, a technique using a working heat treatment is disclosed. For example, Japanese Patent Publication No. 63-5890
No. 6, in the production of a steel sheet by thermomechanical treatment (direct quenching and tempering) under conditions of chemical components that make the surface layer harden excessively, the surface layer is made into elongated austenite grains by controlled rolling. Thus, the toughness of the surface layer portion having a martensite single phase structure is improved. Furthermore, there is a possibility that the toughness can be further improved by lowering the controlled rolling temperature and directly quenching.

As described above, the base metal properties can be adjusted to a certain degree in strength and toughness by devising a method of hot rolling or heat treatment. Separate measures are needed to secure the same level as the base metal. The properties of the welded joint are governed by the properties of the heat affected zone (HAZ) where the steel material has been affected by the heat of welding. However, the steel material is extremely near the heat affected zone (fusion line: FL) adjacent to the weld metal. Because it is exposed to high temperature,
The properties created in the steel manufacturing process are almost completely eliminated. Therefore, for example, JP-A-63-79921
In Japanese Patent Application Publication, a method is disclosed in which the upper bainite in the HAZ structure is suppressed by adjusting the components of the steel material and the parameters indicating the hardenability are adjusted, thereby improving the toughness of the multi-pass weld HAZ. However, in this method, H
The steel component optimal for AZ toughness is not always optimal for base metal properties, and the reduction of alloying elements expected when steel production by working heat treatment is further strengthened is also suitable for ensuring HAZ toughness. Rather, in that case, it becomes difficult to achieve both base metal toughness and HAZ toughness. Therefore, there is a need for a welding method capable of obtaining HAZ characteristics equivalent to the base material even with the base material optimum component system.

[0006]

SUMMARY OF THE INVENTION The present invention has been made in view of the above points, and has a tensile strength of at least 780 MPa class.
The base material has excellent low-temperature toughness with a toughness guarantee temperature of -40 ° C or less.
An object of the present invention is to provide a method for welding steel materials that can be achieved with both HAZ.

[0007]

Means for Solving the Problems The present inventors have studied various means for solving the above-mentioned problems experimentally, and as a result, have found that a chemical composition capable of obtaining a steel material which satisfies the above-mentioned base material characteristics by thermomechanical treatment. Through detailed examination, it was found that, by optimizing the welding method, the welding heat input, and the welding amount in the range, HAZ characteristics equivalent to those of the base material manufactured by thermomechanical treatment can be obtained.

That is, in the HAZ of a high-strength steel having a tensile strength of 780 MPa class or higher, the toughness of a fusion line heated to just below the melting point is reduced most, but this is caused by the fact that the austenite which has been heated to a high temperature and is coarsened is cooled. This is because coarse bainite or martensite is formed by the transformation, and the coarse bainite or martensite forms a coarse brittle fracture fracture surface unit at the time of fracture.

Therefore, in the case of high-strength steel, a large heat input
Although multi-pass welding is used instead of pass welding, how the toughness of this fusion line is affected by multi-layer thermal cycling is the key to HAZ toughness. The present inventors have assumed various heat input levels and have studied in detail the toughness of the fusion line under multiple and varied thermal cycles. As a result, if the melting point of the steel to be set to T _L, T _L ~
The area heated by the thermal influence of the welding to the range of T _L -200 ° C. is at least Ac ₃ + 200 ° C. in the subsequent multiple heating.
It has been found that unless subjected to the above reheating, an initial coarse structure remains and toughness does not recover. This is the same as
Since it can occur even when the steel material component indicated as the HAZ toughness improvement method in 3-79921 and the like is adjusted, not only the steel material component but also the fundamental improvement of the HAZ toughness is required.
The inventors have found that the optimization of the welding method is indispensable, and have reached the present invention.

That is, the gist of the present invention is as follows. (1) In mass%, C: 0.01 to 0.15%, Si:
0.005 to 1%, Mn: 0.1 to 3%, P: 0.02
%, S: 0.01% or less, Al: 0.001-0.
1%, Ni: 1 to 5%, B: 0.0002 to 0.005
%, N: 0.001 to 0.01%.
i: 0.003 to 0.05%, Nb: 0.005 to 0.
5%, Ta: 0.02 to 1%, Mo: 0.1 to 2%,
W: A high-strength steel containing 0.5% to 4% of one or more kinds, the balance being Fe and unavoidable impurities, was heated to a heat input of 3 kJ / m.
A method for welding high-strength and high-strength steel having excellent toughness in the heat affected zone, characterized by welding by TIG welding with a welding amount of 20 g / min or less. (2) In mass%, C: 0.01 to 0.15%, Si:
0.005 to 1%, Mn: 0.1 to 3%, P: 0.02
%, S: 0.01% or less, Al: 0.001-0.
1%, Ni: 1 to 5%, B: 0.0002 to 0.005
%, N: 0.001 to 0.01%.
u: 0.05 to 1.5%, Cr: 0.05 to 2%, V:
A high-tensile steel containing 0.01 to 0.5% of one or more kinds, and the balance consisting of Fe and unavoidable impurities, is heated to a heat input of 3 kJ.
A method for welding high-strength and high-strength steel having excellent heat-affected zone toughness, characterized in that welding is performed by TIG welding with a welding amount of 20 g / min or less. (3) In mass%, C: 0.01 to 0.15%, Si:
0.005 to 1%, Mn: 0.1 to 3%, P: 0.02
%, S: 0.01% or less, Al: 0.001-0.
1%, Ni: 1 to 5%, B: 0.0002 to 0.005
%, N: 0.001 to 0.01%.
i: 0.003 to 0.05%, Nb: 0.005 to 0.
5%, Ta: 0.02 to 1%, Mo: 0.1 to 2%,
W: one or more of 0.5 to 4%, and C
u: 0.05 to 1.5%, Cr: 0.05 to 2%, V:
A high-tensile steel containing 0.01 to 0.5% of one or more kinds, and the balance consisting of Fe and unavoidable impurities, is heated to a heat input of 3 kJ.
A method for welding high-strength and high-strength steel having excellent heat-affected zone toughness, characterized in that welding is performed by TIG welding with a welding amount of 20 g / min or less.

[0011]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, the present invention will be described in detail. First, the reasons for limiting the chemical composition in the present invention will be described. C is contained as an effective component for improving the strength of the steel. If it is less than 0.01%, it is difficult to secure the strength required for structural steel, but if it exceeds 0.15%, the excessive content exceeds 0.15%. Since the toughness and weld cracking resistance of the material and the welded portion are reduced, the content is set to 0.01 to 0.15%.

Next, Si is an element effective as a deoxidizing element and for securing the strength of the base material. However, if the content is less than 0.005%, the deoxidation becomes insufficient and disadvantageous for securing the strength. It is. Conversely, an excessive content exceeding 1% forms a coarse oxide and causes deterioration of ductility and toughness. Therefore, the range of Si is set to 0.005 to 1%.

Mn is an element necessary for securing the strength and toughness of the base material, and must be contained at least 0.1% or more. However, if it is contained excessively, hard phases are formed and grain boundary embrittlement occurs. For example, the base material toughness, the toughness of the welded portion, the weld cracking property, and the like are deteriorated due to the above factors.

P and S are impurity elements which deteriorate ductility and toughness, and are preferably reduced as much as possible.
The upper limit of P is limited to 0.02%, and the upper limit of S is limited to 0.01%, as the amount that the material deterioration does not significantly increase and is allowable.

Al is an element effective for deoxidation, reduction of austenite grain size, and the like. In order to exhibit the effect, it is necessary to contain 0.001% or more. On the other hand, 0.1%
If it is contained in excess of more than 0.1%, a coarse oxide is formed and the ductility is extremely deteriorated, so it is necessary to limit the content to the range of 0.001 to 0.1%.

Ni is the most effective element for securing toughness. In high-strength steels having a tensile strength of 780 MPa or more, at least 1% or more including a case where the guaranteed temperature of toughness is -40 ° C. or lower. Must be contained. On the other hand, Ni is an expensive alloy element, and if the content is further increased, the hardenability in the case of producing a steel material by thermomechanical treatment becomes excessive, so the upper limit is made 5%.

B is an element capable of increasing the hardenability in a very small amount by segregating in the austenite grain boundary in a solid solution state. However, when segregated in the grain boundary, B is also effective in suppressing austenite recrystallization. It is. In order to exhibit the effect of hardenability and recrystallization suppression, it is necessary to add 0.0002% or more. On the other hand, in an excessive addition exceeding 0.005%,
Since coarse precipitates such as BN and Fe ₂₃ (C, B) ₆ are generated to deteriorate toughness, the content is limited to 0.0002 to 0.005%.

N is effective in refining austenite grains in combination with Al and Ti, so that a small amount of N contributes to improvement of mechanical properties. Further, it is impossible to industrially completely remove N in steel, and it is not preferable to reduce N more than necessary because an excessive load is applied to a manufacturing process. Therefore, the lower limit is set to 0.001% as a range in which industrial control is possible and load on the manufacturing process can be tolerated. If contained excessively, solute N increases, which may have an adverse effect on ductility and toughness.
1%.

Furthermore, in order to satisfy the structural requirements of the present invention, Ti, Nb, Ta, Mo, W effective for suppressing austenite recrystallization during thermomechanical treatment, and Cu, Cr, V effective for controlling hardenability are required. It is necessary to further contain one or more of these. The range of addition of each element is limited as follows.

Ti is an element that contributes to the improvement of the base metal strength by precipitation strengthening and is also effective in reducing the austenite grain size by forming stable TiN even at a high temperature. It is an essential element. In order to exert the effect, the content of 0.003% or more is necessary. On the other hand, if it exceeds 0.05%, coarse precipitates and inclusions are formed to deteriorate toughness and ductility, so the upper limit is made 0.05%.

Nb is effective for suppressing the recrystallization of austenite in the form of solid solution and precipitation in the austenite phase, and improving the strength by forming Nb (C, N) during transformation or tempering. Although it is an element, if it is contained excessively, the toughness deteriorates due to precipitation embrittlement. Therefore, the effective range without deteriorating the toughness is 0.005.
Limited to the range of 0.5%.

Ta is also an element effective for suppressing and strengthening austenite recrystallization by the same mechanism as Nb. The effect is slightly weaker than that of Nb in terms of mass%, and it is necessary to contain 0.02% or more to exhibit the effect. On the other hand, if it exceeds 1%, precipitation embrittlement and toughness deterioration due to coarse precipitates / inclusions occur, so the upper limit is made 1%.

Mo is an element effective for improving hardenability, strength, tempering embrittlement resistance and SR embrittlement resistance, but is an element similar to Nb and effective for suppressing recrystallization of austenite. In order to exhibit the effect, it is necessary to add 0.1% or more. On the other hand, if it exceeds 2%, toughness and weldability are deteriorated.

W is also an element having the same effect as Mo. The range in which the effect can be exhibited and the quality does not deteriorate is limited to the range of 0.5 to 4%.

Cu has the effect of improving the hardenability, strengthening the solid solution, and strengthening the precipitation, but if it exceeds 1.5%, there is a problem in the hot workability. Therefore, in the present invention, the range in which the effect is exhibited and the problem such as hot workability does not occur is limited to the range of 0.05 to 1.5%.

Cr is an element effective for improving the strength of the base material by improving hardenability and precipitation strengthening. However, 0.05% or more is necessary to produce a clear effect. On the other hand, if added in excess of 2%, the toughness and weldability tend to deteriorate, so the content is made 0.05 to 2%.

V is an element effective for improving the strength by forming VN together with the hardenability. However, when V is contained excessively, the toughness is deteriorated due to precipitation embrittlement. Therefore, the range in which the effect can be exerted without causing significant deterioration in toughness is set to 0.01 to 0.2.
Limit to 5% range.

By manufacturing a steel material by working heat treatment under such a limitation of components, it is possible to manufacture a steel material excellent in strength and toughness balance as indicated. Typical flow of thermomechanical processing, Ac ₃ transformation point to 120 the slab
Heat to 0 ° C, start temperature is below 900 ° C, end temperature is 6
The hot rolling is performed at a temperature of 50 ° C. or higher and the cumulative draft is 30 to 95%, and then the cooling is performed at a cooling rate of 1 to 100 ° C./s, starting at 600 ° C. or higher and ending at 500 ° C. or lower. After heating, before rolling in the non-recrystallized region, thickness, rolling for adjusting the grain size of recrystallized austenite may be added,
Further, if necessary, after accelerated cooling, 400 ° C. or more, Ac
Tempering can also be performed at a temperature below _one transformation point.

Next, in order to achieve the same HAZ toughness as the base material manufactured by adjusting the components in this way, the reasons for limiting the welding method, welding heat input, and welding amount, which are the basic requirements of the present invention, will be described. . As already described above, in the HAZ of a high-strength steel having a tensile strength of 780 MPa or higher, the toughness of the fusion line heated to just below the melting point is reduced most, but this is caused by the fact that the HAZ is cooled from the austenite that has been heated to a high temperature and coarsened. This is because coarse bainite or martensite is formed by transformation, and these coarse bainite or martensite directly form a coarse brittle fracture fracture surface unit at the time of fracture.

As a result of detailed studies by the present inventors, it has been found that this embrittlement structure is restored in toughness by being subjected to reheating of at least Ac ₃ + 200 ° C. or more by a subsequent multiple heating cycle during multi-layer welding. Was done. Conventionally, in multi-pass welding of high-strength steel, a subsequent pass temper,
It was found that the toughness of the preceding pass was restored by reheating at 00 ° C. to Ac ₁ , but this was due to the fact that the component system having high hardenability
That is, it is effective in the case where the steel material is manufactured by the reheating quenching / tempering treatment, and does not apply to the steel manufactured by the thermomechanical heat treatment targeted by the present invention.

That is, the feature of the production of high-strength steel by thermomechanical treatment is that even if the hardenability is lower than that of the conventional high-strength steel, both high strength and high toughness can be achieved. There is an advantage that steps such as re-quenching can be omitted, but on the other hand, the welding HAZ structure is a structure mainly composed of bainite rather than a completely quenched structure. This is because the effect is low. Therefore, as the HAZ toughness recovery means in this case, it is essential to control the reheat temperature by the subsequent welding pass different from the conventional knowledge.

Based on such knowledge, TIG welding, MIG welding, submerge (SAW) welding,
Each welding method of covered arc welding and CO ₂ (MAG) welding was examined under various welding conditions. As a result, TIG
T _L ~T _L -200 ℃ range of fusion line vicinity to be heated to the tissue of the welding susceptible to multiple thermal cycles in comparison with other welding methods, in particular, the heat input 3 kJ / mm
Hereinafter, the T _{L is set} by setting the welding amount to 20 g / min or less.
~ T _L- The tissue near the fusion line heated to the range of -200 ° C is almost entirely Ac ₃ by the subsequent pass.
It was found that it was reheated to + 200 ° C. or higher.

When the heat input is further increased, T _L
When the region heated to the range of _{TL to} -200 ° C is expanded, the γ-grain growth in this region becomes remarkable, and the reheating effect by the subsequent pass becomes insufficient. it was also found that the proportion of area to be heated in the range of T _L ~T _L -200 ℃ is reheated to Ac ₃ + 200 ° C. or higher by subsequent passes is considerably reduced.

From the above, in order to obtain the same HAZ toughness as the base metal when welding the indicated steel, the heat input should be 3 kJ / mm or less.
It was concluded that it was extremely important to perform welding by TIG welding with a welding amount of 20 g / min or less. The other is a relatively deep shape penetration in the welding method, if heat input is low rate to be reheated is remarkably reduced to Ac ₃ + 200 ° C. or higher by a subsequent pass, when high Ac subsequent pass ₃ +
Since reheated fraction is also occur area to be heated again to T _L range through T _L -200 ° C. simultaneously but increased to 200 ° C. or higher, after all, the area to decrease the toughness remains, the base material equivalent HAZ toughness It does not reach recovery.

[0035]

The above has been a description of the requirements of the present invention. The effects of the present invention will be further shown based on examples. Using four test steels having the chemical compositions shown in Table 1, welded joints were prepared according to the welding methods and welding conditions shown in Table 2, and the HAZ toughness of the fusion line was evaluated. Steels A, B and C are 780 MPa steels within the range of the present invention, steel D is 950 MPa steel, and steels E and F are 780 MPa steels outside the range of the present invention. All the test steels were 240mm thick slabs at 1100 ℃
And rough-rolled to a thickness of 50 mm, finish-rolled to a thickness of 25 mm at 750 to 800 ° C., and then water-cooled. It was previously confirmed that all the base materials satisfied a predetermined strength and also achieved a predetermined level of toughness.

A bevel (angle 45 °) groove was machined on this steel sheet, and welding was performed by multi-layer welding with a root gap of 3 mm. The welding method used was TIG welding, MIG welding, or SAW welding. As the welding material and the flux in SAW welding, a commercially available welding material for 780 MPa was used. 1.6mm for TIG welding, 2.4 for MIG welding
mm, and 3.2 mm for SAW welding. The shielding gas is pure Ar for TIG welding and Ar + 2% O for MIG welding.
₂ was used. The toughness was evaluated by a Charpy test on the vertical fusion line of the wrench-welded portion, and the ductility-brittle transition temperature (vTrs) was determined from the relationship between the test temperature and the brittle fracture rate.

As can be seen from Table 2, excellent HAZ toughness was obtained with the combinations of the steel materials of the present invention # 1 to # 4 and the welding conditions, whereas the steel had low welding heat input and welding amount within the range of the present invention. # 5 to # 10 outside the range of the present invention have low toughness. Steel components # 11 to # 14 outside the present invention do not fall under the present invention even if the welding heat input and wire welding amount are within the present invention, and the toughness is further deteriorated when the welding heat input and the welding amount are outside the range of the present invention. descend. Than this,
The effect of the present invention is evident in welding steel sheets having excellent strength and toughness produced by thermomechanical treatment.

[0038]

[Table 1]

[0039]

[Table 2]

[0040]

According to the present invention, it is possible to produce a steel having an excellent low-temperature toughness having a tensile strength of 780 MPa or higher and a toughness assurance temperature of the base material and HAZ of -40 ° C. or less. Furthermore, the present invention can be applied to steel having a toughness assurance temperature of -100 ° C. or lower for a base material and a welded portion, and steel requiring a brittle crack propagation stopping property of the base material. As a result, it is possible to provide extremely safe structural materials for low-temperature storage tanks, low-temperature pressure vessels, marine structures, ships, bridges, line pipes, etc.
The industrial effect is extremely large.

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Toshinaga Hasegawa 1 Oshinozu, Oita-shi, Oita Pref. Inside Nippon Steel Corporation Oita Works (72) Inventor Manabu Hoshino 5-3 Tokai-cho, Tokai-shi, Aichi Prefecture New Japan Nagoya Steel Works, Ltd. (72) Naoki Saito, Inventor 5-3 Tokai-cho, Tokai City, Aichi Prefecture Nippon Steel Corporation, Nagoya Works, (72) Yoichi Tanaka 5, Tokai-cho, Tokai City, Aichi Prefecture -3 F-term in Nippon Steel Corporation Nagoya Works (reference) 4E001 AA03 BB07 CA02 DD02 EA02

Claims (3)

[Claims] 1. Mass%: C: 0.01 to 0.15%, Si: 0.005 to 1%, Mn: 0.1 to 3%, P: 0.02% or less, S: 0. 01% or less, Al: 0.001 to 0.1%, Ni: 1 to 5%, B: 0.0002 to 0.005%, N: 0.001 to 0.01%, and further Ti : 0.003 to 0.05%, Nb: 0.005 to 0.5%, Ta: 0.02 to 1%, Mo: 0.1 to 2%, W: 0.5 to 4% Or high-strength steel containing two or more types and the balance of Fe and unavoidable impurities
A method for welding a high toughness and high tensile strength steel having excellent toughness in a heat affected zone, characterized by welding by TIG welding at a welding rate of 20 g / min or less. 2. In mass%, C: 0.01 to 0.15%, Si: 0.005 to 1%, Mn: 0.1 to 3%, P: 0.02% or less, S: 0. 01% or less, Al: 0.001 to 0.1%, Ni: 1 to 5%, B: 0.0002 to 0.005%, N: 0.001 to 0.01%, and further Cu : High-strength steel containing one or more of 0.05 to 1.5%, Cr: 0.05 to 2%, V: 0.01 to 0.5%, the balance being Fe and unavoidable impurities Welding of high toughness and high strength steel excellent in the heat affected zone toughness, characterized in that TIG welding is performed at a heat input of 3 kJ / mm or less and a welding amount of 20 g / min or less. 3. In mass%, C: 0.01 to 0.15%, Si: 0.005 to 1%, Mn: 0.1 to 3%, P: 0.02% or less, S: 0. 01% or less, Al: 0.001 to 0.1%, Ni: 1 to 5%, B: 0.0002 to 0.005%, N: 0.001 to 0.01%, and further Ti : 0.003 to 0.05%, Nb: 0.005 to 0.5%, Ta: 0.02 to 1%, Mo: 0.1 to 2%, W: 0.5 to 4% Or one or more of Cu: 0.05 to 1.5%, Cr: 0.05 to 2%, V: 0.01 to 0.5%, and the balance Fe High toughness, high toughness with excellent toughness in the heat affected zone, characterized by welding TIG welding of high tensile steel consisting of unavoidable impurities and heat input of 3 kJ / mm or less and welding amount of 20 g / min or less. Welding method for power steel.