JPH0625737A - Method for producing tempered HT590 steel with excellent uniform elongation - Google Patents

Abstract

(57) [Summary] [Purpose] To prevent the collapse of large structures, it is required to improve the plastic deformability of steel. Conventionally, low YR steel has been developed, but the present invention is a tempered HT excellent in uniform elongation.
It is possible to provide 590 steel. [Constitution] C: 0.03 to 0.20%, Si: 0.05
.About.0.60%, Mn: 0.80 to 2.00%, P: 0.
025% or less, S: 0.015% or less, Cu: 0.10.
~ 1.0%, Ni: 0.10 to 1.5%, Mo: 0.0
5 to 0.30%, Sol. Al: 0.010 to 0.10.
% Of the basic component of the tempered HT590 steel with excellent uniform elongation, characterized by setting the quenching stop temperature to 450 ° C. or higher when performing the tempering heat treatment of quenching and tempering after the plate rolling. Production method.

Description

Detailed Description of the Invention

[0001]

[Field of Industrial Application] The present invention is a tempered HT59 that requires excellent uniform elongation characteristics as a strength member for welded structural steel for buildings, bridges and offshore structures, steel for pressure vessels, etc. for safety design.
The present invention relates to a method for manufacturing 0 steel.

[0002]

2. Description of the Related Art With the recent increase in the size of structures, it has been an important issue for steel materials conventionally used for the above-mentioned applications to prevent the structures from collapsing due to an earthquake, a typhoon, or the like. In terms of safety design, the steel material used must have plastic deformability capable of absorbing huge natural energy.

Therefore, there is a movement to tighten the required values such as yield ratio (low YR), hardening gradient (N value), and uniform elongation, but in recent years, TMCP steel or temper heat treatment technology has been developed. Therefore, only SM490 to 590 class low YR steel has finally been put to practical use.

For example, in the Ministry of Construction Comprehensive Technology Development Project, SM4 in the high-performance steel WG together with steel manufacturers
90 steel is being studied for general use in the construction field. Therefore, in the case of uniform elongation, it is the actual situation that strict steel material specifications cannot be met due to design considerations.

On the other hand, a tempered HT which is the object of the present invention
Usually, various heat treatments are applied to 590 steel, and a set of quenching (Q, DQ) and tempering (T) is applied to the heat treatment subsequent to the rolling or heating / rolling (including controlled rolling) after cast casting. Matching is common. Further, the extremely thick HT590 steel used for large-scale structures may be combined with slow cooling or heat retention (T) for the purpose of dehydrogenation before heating or before and after heat treatment.

[0006]

DISCLOSURE OF THE INVENTION In the prior art,
We have achieved low YR of steel products by developing TMCP steel or heat treatment technology, but we do not improve uniform elongation of steel products and rely on design considerations. there were.

[0007] On the other hand, it was generally accepted that the uniform elongation of steel is inversely proportional to the strength, but iron and steel CAMP-
ISIJ, Vol. 4 (1991) P774, a technique for improving uniform elongation by so-called three-stage heat treatment in which a quenching heat treatment from a two-phase region is introduced into the above heat treatment has been newly reported. The economic loss caused by the change was not negligible.

The present inventor has made a detailed investigation on the uniform elongation of the tempered HT590 steel. As a result, the work hardening in the uniform plastic strain region in the tensile test, that is, the existence state of cementite in ferrite and pearlite (bainite depending on the cold speed) It has been found that there is a close relationship between and the uniform elongation. As a result, when quenching the tempered steel, tempering may be performed after the quenching stop temperature is set to 450 ° C. or higher.
We have newly found that it is extremely effective in improving uniform elongation.

From the viewpoint of dehydrogenation, in order to cool ordinary tempering heat treatment and three-step heat treatment to room temperature during quenching, a heat retention (T) for dehydrogenation is set before and after heat treatment (quenching and tempering). It was necessary to perform slab gradual cooling for a long time at the sacrifice of dehydrogenation efficiency before the heat-rolling.

On the other hand, in the method for manufacturing a high-strength steel sheet with a low yield ratio in JP-A-4-52225, in the case of direct quenching of 80 kg HT, quenching is stopped at 350 to 500 ° C., and air cooling or slow cooling to 200 ° C. is carried out for martensite. A technique is disclosed in which the amount is controlled to 95% or less and a hydrogenous UST defect is solved by cooling.

The present invention aims to further improve the uniform elongation by further increasing the quenching stop temperature, and dehydrogenates more rationally in cooling the quenching, which is a secondary problem to be solved by the present invention. Is.

[0012]

The present invention provides a method for producing a tempered HT590 steel having an excellent uniform elongation by specifying a quenching stop temperature, thereby improving the plastic deformability of a large structure and ensuring safety. Both the design and the economic design are made compatible, and the competitiveness in terms of construction period and construction is strengthened. Further, the dehydrogenation efficiency is also improved by increasing the quenching stop temperature at the time of quenching subsequent to rolling.

The gist of the present invention is as follows. (1) C: 0.03 to 0.20% by weight%, Si: 0.
05-0.60%, Mn: 0.80-2.00%, P:
0.025% or less, S: 0.015% or less, Cu: 0.
10-1.0%, Ni: 0.10-1.5%, Mo:
0.05-0.30%, Nb: 0.005-0.03
%, V: 0.01 to 0.09%, Ti: 0.005 to
0.03%, Sol. Al: A slab containing 0.010 to 0.10% of balance iron and unavoidable impurities is immediately after casting or after reheating to Ac ₃ points or more, followed by thick plate rolling, followed by quenching and tempering heat treatment. At this time, the quenching stop temperature is set to 450 ° C. or higher, and a method for producing a tempered HT590 steel excellent in uniform elongation.

(2) Cr in weight%: 0.50% or less,
B: 0.0015% or less, Ca: 0.0080% or less,
REM: Tempered HT excellent in uniform elongation characterized by containing one or two 0.0050% or less in a slab.
590 steel manufacturing method.

[0015]

The present invention will be described in detail below. C is 0.20
%, The low temperature toughness and weldability are impaired, and 0.03%
If less than 0.03, the required strength cannot be ensured, so 0.03 to 0.
Limited to 20%.

From the viewpoint of deoxidation and strength, Si is required to be 0.05% or more, and the addition of more than 0.60% significantly impairs the low temperature toughness and weldability, so it was limited to 0.05 to 0.60%.

Mn is required to be 0.80% or more in terms of strength, and 2.
Since the addition of more than 00% deteriorates both the low temperature toughness and the weldability, it was restricted to 0.80 to 2.00%.

Although P is limited to 0.025% or less from the viewpoint of weldability and low temperature toughness, it is preferably as low as possible from the viewpoint of preventing welding defects during large heat input welding of large structures and high-rise buildings.

From the low temperature toughness, S is limited to 0.015% or less, but it is preferable that S is lower, and it is more advantageous to add Ca and REM for controlling the morphology of MnS, if necessary. When lamellar tear resistance is required due to the shape of the structure, S is limited to 0.003% or less, and Ca and R
Morphology control of MnS by EM is required.

The contents of C, Si and Mn are component-designed in consideration of the plate thickness of the tempered HT590 steel from the required properties (strength and low temperature toughness) in a predetermined heat treatment. In the case of manufacturing extremely thick tempered HT590 steel for large-scale structures and high-rise buildings, the necessary amounts of Cu, Ni, Mo, Nb, V, and Ti in addition to the above five elements are appropriately determined to design the components. The reasons for the limitation will be described below.

Cu is added by substituting C, Si and Mn for the purpose of reducing Ceq in order to improve low temperature toughness, and is added to 0.10% or more in order to secure strength. If it exceeds 1.0%, hot brittleness is promoted. And the weldability deteriorates.
It was limited to 1.0%.

Ni is added by substituting C, Si and Mn for the purpose of reducing Ceq in order to improve low temperature toughness, and is added to 0.10% or more in order to secure the strength. If it exceeds 1.5%, the effect is saturated. Therefore, it was limited to 0.10 to 1.5%.

Mo is added in an amount of 0.05% or more to secure the strength by improving the hardenability, and if it exceeds 0.30%, the YR is low.
0.05 to 0.3 because it is difficult to secure the
Limited to 0%.

Nb is added in an amount of 0.005% or more for the purpose of improving strength and controlling crystal grains, but addition of more than 0.03% results in deterioration of weldability, low temperature toughness and low YR property, and therefore 0.0%.
It was limited to 05 to 0.03%.

V is added in an amount of 0.01% or more for improving the strength, and if it exceeds 0.09%, the weldability and the low temperature toughness are deteriorated, so V is limited to 0.01 to 0.09%.

Ti is added in an amount of 0.005% or more in order to prevent cracking during cast slab casting and to improve joint toughness during high heat input welding. Addition of more than 0.03% results in low temperature toughness, low weldability and low weldability. Since it is difficult to secure the YR property, it is limited to 0.005 to 0.03%. Most preferably, Ti is added so that the number of atoms is equal to that of N (N × 3.4).

Sol. Al is 0.0 for deoxidation and particle size adjustment.
It is necessary to be 10% or more and 0.10% or less from the viewpoint of weldability.
When B, Ca and REM are added, Sol. A
It is preferable to add 1 of 0.030% or more.

Other elements (Cr, B,
The effect of the present invention is not impaired to some extent even if Ca, REM) is added in combination of one or two in order to improve the required properties (strength, low temperature toughness, lamellar tear resistance).

Cr is added to secure the strength by improving the hardenability, and if it exceeds 0.50%, it becomes difficult to secure low YR property and uniform elongation, so it is limited to 0.50% or less.

B is added as needed to improve the strength and low temperature toughness. If it exceeds 0.0015%, the low temperature toughness is impaired, so B is limited to 0.0015% or less.

Ca is added from the viewpoint of low temperature toughness and lamellar tear resistance, but if it is added in excess of 0.0080%, inclusions increase, so it is limited to 0.0080% or less.

REM is added in place of Ca as required in order to improve low temperature toughness, lamellar tear resistance, and joint toughness during high heat input welding, and inclusion of more than 0.0050% increases inclusions. It was limited to 0050% or less.

Next, the reason for limiting the quenching stop temperature, which is the most important technical idea of the present invention, will be described. The quenching stop temperature is determined so as to optimize the strength-toughness balance. However, the solid solution carbon supersaturated in the steel when quenched (DQ, Q) to room temperature is quenched at a high temperature in the present invention. 15% after being discharged from the matrix during cooling after the suspension and growing and coagulating into stable carbide (cementite)
In order to stably obtain the above uniform elongation, 450 ° C or higher is required.

When the quenching stop temperature is 500 ° C. or higher, the uniform elongation is further improved, but when it becomes difficult to secure the strength, 650 ° C. or lower is preferable. Therefore, there is no reason to restrict the temperature to 650 ° C. or lower if strength can be secured, from the viewpoint of improving uniform elongation. Needless to say, the strength reduction due to the increase in the quenching stop temperature is compensated by the component design.

[0035]

EXAMPLES Examples of the present invention are shown in Tables 1 and 2 together with comparative examples. Table 1 shows the chemical composition of the examples of the present invention (Steels A, B, D) and the comparative example (Steel C). Steels A and D are added with Ca or REM for improving the lamellar tear resistance. Steel C
Cr and Mo exceed the upper limit of the component range of the present invention.

[0036]

[Table 1]

[0037]

[Table 2]

Both the tensile test and the impact test are performed according to JIS4.
Table 2 shows the manufacturing results of the present invention example and the comparative example performed on the No. test piece. In the examples of the present invention (steels A, B, D), the uniform elongation improves as the quenching stop temperature increases. Further, at the quenching stop temperature of 500 ° C. or higher, further improvement in uniform elongation is recognized.
It is noted that the strength tends to decrease sharply when the quenching stop temperature exceeds 650 ° C.

Therefore, from the viewpoint of ensuring uniform elongation of 15% or more, the quenching stop temperature is limited to 450 ° C. or more.
From the viewpoint of ensuring strength, caution is required when the quenching stop temperature is 650 ° C or higher.

On the other hand, in Comparative Example (Steel C), the properties of YR and uniform elongation are inferior to the Examples of the present invention, although the quenching stop temperature is within the range of the Examples of the present invention.

The yield ratio (YR), which is another important evaluation index for plastic deformability, is 80% or less, which is a recently severely required value, and all of the examples of the present invention are satisfied. At a temperature of 590 ° C or higher, the YR improving effect is remarkable.

[0042]

EFFECTS OF THE INVENTION The steel of the present invention can improve the uniform elongation of the tempered HT590 steel by specifying the quenching stopping temperature at the time of quenching to be a high temperature of 450 ° C. or higher rather than the usual ordinary temperature. Further, if the quenching stop temperature is controlled within a narrow range, the present invention also provides a method for producing a tempered HT590 steel excellent in uniform elongation as well as low YR which is the two major factors of plastic deformability.

As a result, not only is the safety design of large structures possible not only from the construction side but also from the steel material side, a tempered 590 steel having a high uniform elongation that replaces the three-step heat treatment and the conventional tempered heat treatment. Dehydrogenation is performed more rationally in the production of. Therefore, according to the present invention, not only the safety design and the economic design that can be enjoyed by the industrial world are compatible, but also the construction period,
The economic benefits are considered to be enormous.

Claims (2)

[Claims] Baked 1. By weight%, C: 0.03 to 0.20%, Si: 0.05 to 0.60%, Mn: 0.80 to 2.00%, P: 0.025% or less, S : 0.015% or less, Cu: 0.10 to 1.0%, Ni: 0.10 to 1.5%, Mo: 0.05 to 0.30%, Nb: 0.005 to 0.03% , V: 0.01 to 0.09%, Ti: 0.005 to 0.03%, Sol. Al: 0.010 to 0.10%, when performing a tempering heat treatment of quenching and tempering immediately after casting of a slab composed of balance iron and unavoidable impurities or after reheating to Ac ₃ points or more and subsequent to plate rolling. A method for producing a tempered HT590 steel excellent in uniform elongation, characterized in that the quenching stop temperature is 450 ° C. or higher. 2. One or two kinds of Cr: 0.50% or less, B: 0.0015% or less, Ca: 0.0080% or less, REM: 0.0050% or less are contained in a slab by weight. The method for producing a tempered HT590 steel having excellent uniform elongation according to claim 1.