JPH0551696A - High tensile strength steel excellent in weldability and sr cracking resistance - Google Patents

Abstract

PURPOSE:To obtain a high tensile strength steel having superior welding performance characteristic and free from SR cracking in a weld zone in the course of stress relief annealing treatment (SR Treatment) by reducing S content and controlling the contents of the other components to the values in proper ranges, respectively. CONSTITUTION:This steel has a composition consisting of, by weight, 0.03-0.20% C, <=1.0% Si, 0.3-2.0% Mn, <=0.02% P, <=0.001% S, 0.3-3.0% Cr, 0.2-2.0% Mo, <=0.5% V, 0.003-0.10% Al, 0.0003-0.006% B, and the balance Fe. Besides these components, <=2.0% Cu and/or <=5.0% Ni can be further incorporated. This steel is a high tensile strength steel having >=70kg/mm<2> tensile strength, excellent in weldability and SR cracking resistance, and applicable to structures, such as pressure vessel and hydraulic steel pipe. By reducing S content to the above value, intergranular strength can be relatively improved and SR cracking can be prevented, and further, weldability can be improved while maintaining high strength by controlling the contents of the other components to the values in the above-mentioned ranges, respectively.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a large-scale welded structure, in particular, stress-relief non-annealing treatment after welding assembly (hereinafter referred to as "SR treatment")
It is a high-strength steel used for pressure vessels, penstocks, bifurcations of penstocks, and offshore structures, etc., which require excellent weldability and cracks in the weld during stress relief annealing ( Good S-resistance that does not cause (SR cracking)
It relates to a high strength steel having R cracking characteristics.

[0002]

2. Description of the Related Art In recent years, welded structures such as pressure vessels, penstocks, and their branching parts have tended to become larger and larger, and accordingly high-strength steel has come to be used for these structures. It was However, from the viewpoint of safety, these welded structures are often subjected to SR treatment for the purpose of removing residual stress in the welded portion and improving the metal structure after welding and assembly, and in some cases, they may be required. However, high-strength steel generally causes SR cracks in the weld during SR treatment. Especially, high tensile steel with a tensile strength of 70 kgf / mm ² or more,
Since Cr-Mo steel has a high SR cracking susceptibility, the SR treatment may rather hinder the safety of the structure.

[0003] Therefore, one of the inventors of the present application, in order to cope with such a situation, first suppresses the content of an element that precipitates in the SR treatment process and has an intragranular strengthening effect, and further reduces the content of Cr / Mo.
Invented a high-strength steel with a tensile strength of 70 kgf / mm ² or more, which was excellent in SR cracking resistance and whose ratio was adjusted to a specific range.
57-55783). Compared with the conventional high-strength steel, the steel of this prior application does not cause SR cracks in the welded portion during the SR treatment process, so it is a material that is suitable for the above-mentioned structures that require large-scale SR treatment. Since structures such as pressure vessels are assembled by welding, it is desirable that the steel of the material has excellent weldability, but the steel of the prior invention has a higher carbon content than conventional steel of the same strength class. Since the equivalent weight (Ceq) is high, there is a drawback that the weldability is poor. For these reasons, there is a demand for the development of a high-strength steel having excellent SR cracking resistance and having weldability equal to or higher than that of conventional steel.

[0004]

SUMMARY OF THE INVENTION An object of the present invention is to meet the above-mentioned demands and has good weldability.
Moreover, it is to provide a high-strength steel having a tensile strength of 70 kgf / mm ² or more, which is excellent in SR crack resistance and does not cause SR cracks in a welded portion in the SR treatment process.

[0005]

SR cracks are intergranular fractures that occur due to the concentration of strain near the old γ grain boundaries in the weld heat affected zone during the SR treatment process. Therefore, in order to prevent SR cracking, it is effective to mitigate strain concentration near the grain boundaries or to relatively improve the grain boundary strength.

Further, in order to improve the weldability, it is effective to use steel having a low carbon equivalent chemical composition.

The inventors of the present invention have investigated the chemical composition of steel in order to develop a high-strength steel having excellent SR cracking resistance and good weldability. As a result, the S content is 0.0
When it is reduced to a level of 01% or less, the grain boundary strength is relatively improved, SR cracking is suppressed, and when the content of other components is adjusted to an appropriate range, high strength is maintained. It was found that the weldability was improved.

The present invention based on the above findings is described in the following (I) to
The high-strength steel of (IV) is the main point.

(I)% by weight, C: 0.03 to 0.20%, Si:
1.0% or less, Mn: 0.3 to 2.0%, P: 0.02% or less, S:
0.001% or less, Cr: 0.3-3.0%, Mo: 0.2-2.0%,
V: 0.5% or less, Al: 0.003 to 0.10%, B: 0.0003 to 0.
A high-strength steel containing 006%, the balance consisting of Fe and unavoidable impurities, and having a tensile strength of 70 kgf / mm ² or more with excellent weldability and SR crack resistance.

(II) In addition to the components described in (I) above, it further contains at least one of 2.0% by weight or less of Cu and 5.0% by weight or less of Ni, and has a tensile strength of 70 kgf / mm ^2. The high-strength steel excellent in weldability and SR cracking resistance as described above.

(III) In addition to the components described in (I) or (II) above, 0.01% by weight or less of Ca is further contained and the tensile strength is 70 kgf / mm ² or more. High strength steel with excellent properties.

(IV) In addition to the components described in (I), (II) or (III) above, it further contains 0.05% by weight or less of Nb,
A high-strength steel with a tensile strength of 70 kgf / mm ² or more and excellent in weldability and SR crack resistance.

[0013]

The reason for limiting the chemical composition of the high-strength steel in the present invention as described above will be explained below. In addition, "%" of a component content means "weight%."

C: C acts to improve the strength of steel. In order to secure the desired high strength, 0.03% or more is necessary, but if it exceeds 0.20%, the weldability is markedly deteriorated, so the content was made 0.03 to 0.20%.

Si: Si has a deoxidizing effect and an effect of improving the strength of steel. However, if it exceeds 1.0%, the weldability is significantly deteriorated, so the content is made 1.0% or less.

Mn: Mn has the effect of improving strength and toughness, but if it is less than 0.3%, the desired effect cannot be obtained.
If it exceeds 2.0%, the weldability will be impaired, so the content was made 0.3 to 2.0%.

P: P promotes SR cracking and SR embrittlement, so its content is limited to 0.02% or less.

S: It is an important point of the present invention to keep the S content as low as possible. The present inventors set the S content
It has been found that when the content is controlled to 0.001% or less, the high temperature strength of the weld heat affected zone in the SR treatment temperature range is improved and the SR crack resistance is significantly improved.

FIG. 1 shows the S content and the tensile strength of the weld heat affected zone.
It is a graph which investigated the relationship with (tensile strength at 600 ° C). The high-temperature tensile strength of the heat-affected zone of welding is produced by manufacturing the butt-welded test piece 1 shown in FIG. 2 (a), and the round bar having the shape shown in FIG. Tensile test piece 4
Was cut out, heated to 600 ° C., and subjected to a tensile test at this temperature.

From FIG. 1, it can be seen that when the S content is 0.001% or less, the high temperature strength of the weld heat affected zone is rapidly improved. From the result of FIG. 1, the S content was set to 0.001% or less. It should be noted that, when S is reduced to 0.001% or less, the reason why the high temperature strength of the weld heat affected zone in the SR treatment temperature range is improved and the SR crack resistance is improved has not been sufficiently clarified, but
It is considered that the reduction of S suppresses the interfacial delamination between the matrix and the relatively coarse grain boundary precipitate which is the origin of the grain boundary fracture.

Cr: Cr is for ensuring desired strength
0.3% or more is necessary, but if it exceeds 3.0%, not only the weldability is impaired, but also temper embrittlement easily occurs, so the content was made 0.3 to 3.0%.

Mo: Mo ensures the desired strength.
0.2% or more is necessary, but if it exceeds 2.0%, the weldability is significantly impaired, so the content was made 0.2 to 2.0%.

V: V is also necessary to secure the desired strength, but if it exceeds 0.5%, the toughness and weldability are impaired, so its content was made 0.5% or less.

Al: Al is effective for deoxidation and grain refinement of the structure. In order to obtain the desired effect, 0.003% or more is necessary, but if it exceeds 0.10%, the toughness and weldability deteriorate, so its content was made 0.003 to 0.10%.

B: B has the effect of enhancing the hardenability and improving the strength, but if it is less than 0.0003% the desired effect cannot be obtained, and if it exceeds 0.006%, the toughness of the base steel and welds Is significantly inhibited, its content is 0.0003-0.
It was 006%.

Cu and Ni: Both Cu and Ni have the function of improving the strength of steel. In particular, Ni has the effect of improving toughness as well as strength without impairing weldability. Therefore, one or more of these components may be added, if necessary, when higher strength or higher strength and toughness are required. However, in the case of Cu, if the content exceeds 2.0%, the toughness and weldability are impaired.
The content should be 2.0% or less. On the other hand, since Ni is an expensive component, it is necessary to add 5 even if it is added from the viewpoint of production cost, except for steel intended for use at low temperature.
The content is preferably less than or equal to%.

Ca: Ca has the effect of preventing the S-based inclusions from elongating by rolling, improving the toughness of the base steel, particularly the toughness in the plate thickness direction, and preventing weld cracks. When the weldability of is required, it may be added if necessary. However, if the content exceeds 0.01%, the ductility of the steel is significantly impaired. Therefore, when adding, the content should be 0.01% or less.

Nb: Nb has the effect of improving the toughness of the steel through grain refinement of the structure, so it may be contained in a range of 0.05% or less in order to further enhance the toughness of the steel. But 0.
If the content exceeds 05%, the weldability and the toughness of the welded part deteriorate.

The steel having the above composition is processed by hot rolling,
Quench and temper to obtain a tensile strength of 70 kgf / mm ² or more. Quenching may be performed by a so-called direct quenching method in which quenching is performed immediately after hot rolling, or after hot rolling, the material may be once cooled to room temperature and then reheated.

[0030]

EXAMPLE Steels having the chemical compositions shown in Table 1 were melted, cast, and then heated to 1150 ° C. to start hot rolling, and finished at a temperature of 900 ° C. or higher to a thickness of 50 mm. Next, the invention steels C to E and the comparative steel 4 were once water-quenched by cooling them to room temperature and then reheating to 900 ° C., and the other invention steels and the comparative steels were water-quenched immediately after rolling. It was After quenching, everything from 620
Tempering was performed at a temperature of 650 ° C.

Specimens were cut out from the steels of the present invention and comparative steels thus obtained, and subjected to a tensile test, a Y-groove restraint cracking test, a temperature rising rupture test and an SR cracking test.

In the Y-groove restraint cracking test, the preheating temperature for preventing low temperature cracking was determined and the weldability was evaluated. In the temperature rising rupture test, a butt-welding test piece 1 shown in FIG. 2 (a) was produced, and a round bar tensile test piece 4 having a dimension and shape shown in FIG. It was cut out and heated at 600 ° C. to determine the breaking strength. In the SR cracking test, a welded joint 5 having the dimensions and shape shown in FIG. 3 was prepared, and SR treatment was performed at a temperature of 600 ° C. for 2 hours, and the crack was observed on the 5 cross section of the test bead 6 side to check for the presence of cracks. .. These results are summarized in Table 2
Shown in.

[0033]

[Table 1]

[0034]

[Table 2]

From Table 2, all of the steels of the present invention have high high-temperature strength in the SR temperature range in the temperature rising rupture test and no cracks in the SR cracking test, whereas the comparative steels in the SR temperature range. Has relatively low high-temperature strength, and SR cracking has occurred (incidence rate 100%).

The preheating temperatures for preventing cold cracking are almost the same as those of the steels of the present invention and comparative steels, and the preheating temperatures of these are almost the same as those of ordinary steels. Therefore, the steels of the present invention have excellent SR resistance.
It can be said that it has good weldability as well as cracking characteristics.

[0037]

As described above, the high-strength steel of the present invention has both excellent SR crack resistance and good weldability, and is applicable to the construction of large pressure vessels and hydraulic iron pipe branches. Therefore, it is not necessary to pay special attention to welding work and SR treatment can be performed, which contributes to the improvement of safety of these structures.

[Brief description of drawings]

FIG. 1 is a graph showing the relationship between the S content and the high temperature tensile strength of the heat affected zone (tensile strength at 600 ° C.).

FIG. 2 (a) is a side view of a butt-welding test piece before cutting out a round bar tensile test piece used in a temperature rising rupture test, and FIG. 2 (b) is a side view of the round bar tensile test piece. Is.

FIG. 3 is a perspective view of a welded joint used in an SR crack test.

[Explanation of symbols]

1 is a butt welding test piece, 2 is a welding heat affected zone, 3 is a circumferential notch, 4 is a round bar tensile test piece, 5 is a welded joint, 6 is a test bead, and 7 is a restraining bead.

Claims (4)

[Claims] 1. By weight%, C: 0.03 to 0.20%, Si: 1.0%
Below, Mn: 0.3-2.0%, P: 0.02% or less, S: 0.001
% Or less, Cr: 0.3 to 3.0%, Mo: 0.2 to 2.0%, V:
0.5% or less, Al: 0.003 to 0.10%, B: 0.0003 to 0.006
%, The balance is Fe and unavoidable impurities, and the tensile strength is 70 kgf / mm ² or more. High tensile strength steel with excellent weldability and SR crack resistance. 2. In addition to the components of claim 1, 2.0
A high-strength steel containing at least one of Cu in an amount of not more than 10% by weight and Ni in an amount of not more than 5.0% by weight, having a tensile strength of not less than 70 kgf / mm ² and having excellent weldability and SR crack resistance. 3. The composition according to claim 1 or 2, further containing 0.01% by weight or less of Ca, and having a tensile strength of 70.
High-strength steel with excellent weldability and SR cracking resistance of at least kgf / mm ² . 4. The weldability and resistance which, in addition to the components of claim 1, claim 2 or claim 3, further contain 0.05% by weight or less of Nb and have a tensile strength of 70 kgf / mm ² or more. High strength steel with excellent SR cracking properties.