JPH0681032A - Heat treated ht590 steel excellent in uniform elongation and its production - Google Patents

Abstract

PURPOSE:To produce a heat treated HT590 steel excellent in uniform elongation by casting a slab with specific chemical composition, subjecting the resulting cast slab to thick plate rolling, and further specifying tempering temp. at the time of thermal refining treatment of hardening and tempering. CONSTITUTION:A slab having a composition consisting of, by weight, 0.03-0.20% C, 0.05-0.60% Si, 0.80-2.00% Mn, <=0.025% P, <=0.015% S, 0.10-1.0% Cu, 0.10-1.5% Ni, 0.05-0.30% Mo, 0.005-0.03% Nb, 0.01-0.09% V, 0.005-0.03% Ti, 0.010-0.10% Sol.Al, and the balance iron with inevitable impurities is cast, and the resulting cast slab is subjected, without delay or after reheating up to >=Ac3 point, to thick plate rolling. At the time of subsequent thermal refining treatment of hardening and tempering, tempering temp. is regulated to 670-730 deg.C to provide >=15% uniform elongation. By this method, the heat treated HT590 steel having carbides of a size of >=1mum in the lath interface of bainite and excellent in uniform elongation can be obtained.

Description

Detailed Description of the Invention

[0001]

FIELD OF THE INVENTION The present invention relates to HT590 steel, which requires excellent uniform elongation characteristics for safety design, as a strength member such as welded structural steel for buildings, bridges and offshore structures, and steel for pressure vessels, and its production. It is about the method. The steel material is JIS G3106 SM570, G3115.
SPV50, HBS G3104 / G3106 HT5
70, ASTM A572 / A537 and other steel materials, and steel materials such as HT590 for building structures that will be standardized in the future are typical examples.

[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 even uniform elongation, but SM490 has been developed by the recent development of TMCP steel or temper heat treatment technology. SM5
Only 70-class low YR steel has finally been put to practical use. For example, in the Ministry of Construction Comprehensive Technology Development Project,
SM490 in high-performance steel WG together with steel manufacturers
Studies are underway for steel for general use in the construction field. Therefore, with respect to uniform elongation, the actual situation is that it cannot meet strict steel material specifications, relying on design considerations. On the other hand, it was a general common sense that the uniform elongation of steel is inversely proportional to the strength, but CAMP-ISIJ, Vo
l. 6, P823 (1993), Sumitomo Metals Vol. Four
3, No. 7, P13 (1991) and Nippon Steel Pipe Technique N
o. 122, P5 (1988), the so-called quenching heat treatment from the two-phase region is introduced into the heat treatment of the above standard.
As a result of improving the low YR characteristics by the three-step heat treatment, a new manufacturing technique for HT590 steel in which the uniform elongation is slightly improved has been newly reported.

[0003]

In the above-mentioned prior art, the low YR of the steel material was achieved by the development of TMCP steel or the heat treatment technology for tempering, but it was not possible to sufficiently improve the uniform elongation of the steel material. However, due to design considerations, there was a great deal of construction loss. CAMP-ISIJ, V
ol. 6, P823 (1993), Sumitomo Metals Vol. Four
3, No. 7, P13 (1991) and Nippon Steel Pipe Technique N
o. 122, P5 (1988), the so-called three-step heat treatment in which the quenching heat treatment from the two-phase region is introduced into the heat treatment for heat treatment described above causes a great economic loss. Further, the level of uniform elongation due to the three-step heat treatment is less than 20% in the thick material, and as a result, the degree of freedom is restricted because design considerations have to be relied upon.
The object of the present invention is to improve the plastic deformability of a large structure to achieve both safety design and economic design, and to enhance the competitiveness in terms of construction period and construction. Tempered HT with excellent uniform elongation
590 steel and its rational manufacturing method.

[0004]

The present invention provides a low C-high C content by limiting the carbides precipitated at the lath interface of bainite to 1 μm or more, or by specifying the tempering temperature.
It is an HT590 steel excellent in uniform elongation due to the formation of soft bainite by the formation of u and the precipitation of soft ε-Cu, and a manufacturing method thereof. That is, the gist of the present invention is as follows. (1) C: 0.03 to 0.20% by weight%, Si: 0.
03-0.60%, Mn: 0.80-2.0%, P:
0.025% or less, S: 0.015% or less, Cu: 0.
03-1.0%, Ni: 0.03-1.5%, Mo:
0.03-0.30%, Nb: 0.005-0.03
%, V: 0.005-0.10%, Ti: 0.005-
0.03%, Sol. Al: Heat-treated HT590 having a uniform elongation of 15% or more, characterized by containing 0.010 to 0.10%, balance iron and unavoidable impurities, and having carbides of 1 μm or more at the lath interface of bainite. steel. (2) C: 0.02 to 0.08% by weight, Si: 0.
10 to 0.60%, Mn: 0.80 to 2.0%, P:
0.015% or less, S: 0.010% or less, Cu: 0.
60-2.5%, Ni: 0.30-2.5%, Mo:
0.03-0.50%, Nb: 0.005-0.03
%, V: 0.005-0.10%, Ti: 0.005-
0.03%, Sol. Al: Tempered HT590 having a uniform elongation of 20% or more, characterized by containing 0.010 to 0.10%, balance iron and unavoidable impurities, and having carbides of 1 μm or more at the lath interface of bainite. steel. (3) Cr: 0.03 to 1.0% by weight%, B: 0.0
003 to 0.0020%, Ca: 0.0015 to 0.0
The tempered HT590 steel excellent in uniform elongation according to the above (1) or (2), characterized in that one or more of 080% is contained in the slab. (4) 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 being reheated to Ac ₃ points or more, followed by thick plate rolling, followed by quenching and tempering heat treatment. At this time, the tempering temperature is set to 670 ° C. or higher and 730 ° C. or lower, and a method for producing a tempered HT590 steel having a uniform elongation of 15% or more. (5) Cr: 0.03 to 0.50% by weight, B: 0.
0003-0.0015%, Ca: 0.0015-0.
0080%, REM: 0.001 to 0.0050% of one or two or more kinds are contained in the slab, and the method for producing a tempered HT590 steel excellent in uniform elongation as described in (4) above. . (6) C: 0.02 to 0.08% and Si: 0.
10 to 0.60%, Mn: 0.80 to 2.0%, P:
0.015% or less, S: 0.010% or less, Cu: 0.
60-2.5%, Ni: 0.30-2.5%, Mo:
0.03-0.50%, Nb: 0.005-0.03
%, V: 0.005-0.10%, Ti: 0.005-
0.03%, Sol. Al: A slab containing 0.010 to 0.10% and the balance iron and unavoidable impurities is cast immediately after casting or after reheating to Ac ₃ points or more, followed by thick plate rolling, followed by quenching and tempering heat treatment. A method for producing a tempered HT590 steel having a uniform elongation of 20% or more, characterized in that the tempering temperature is 670 ° C. or higher and 730 ° C. or lower when performing. (7) Cr: 0.03 to 0.50% by weight%, B: 0.
0003-0.0020%, Ca: 0.0015-0.
0080%, REM: 0.001 to 0.005% of one or two or more of them are contained in the slab, and the method for producing a tempered HT590 steel excellent in uniform elongation as described in (6) above. . (8) C: 0.02 to 0.08% by weight%, Si: 0.
10 to 0.60%, Mn: 0.80 to 2.0%, P:
0.015% or less, S: 0.010% or less, Cu: 0.
60-2.5%, Ni: 0.30-2.5%, Mo:
0.03-0.50%, Nb: 0.005-0.03
%, V: 0.005-0.10%, Ti: 0.005-
0.03%, Sol. Al: A slab containing 0.010 to 0.10% and the balance iron and unavoidable impurities is cast immediately after casting or after reheating to Ac ₃ points or more, followed by thick plate rolling, followed by quenching and tempering heat treatment. A method for producing a tempered HT590 steel having a uniform elongation of 20% or more, characterized in that the tempering temperature is 670 ° C. or higher and 730 ° C. or lower when performing. (9) C: 0.02 to 0.08% by weight, Si: 0.
10 to 0.60%, Mn: 0.80 to 2.0%, P:
0.015% or less, S: 0.010% or less, Cu: 0.
60-2.5%, Ni: 0.30-2.5%, Mo:
0.03-0.50%, Nb: 0.005-0.03
%, V: 0.005-0.10%, Ti: 0.005-
0.03%, Sol. Al: A slab containing 0.010 to 0.10% and the balance of iron and inevitable impurities is cast immediately after casting or after being reheated to Ac ₃ points or more and then directly quenched and tempered from a two-phase region following thick plate rolling. Tempering heat treatment, or direct quenching following thick plate rolling, quenching from the two-phase region, tempering heat treatment for tempering, or quenching after finishing thick plate rolling, quenching from the two-phase region, tempering heat treatment for tempering On the occasion,
Tempered HT59 having uniform elongation of 25% or more, characterized by tempering temperature of 670 ° C or more and 730 ° C or less
0 Steel manufacturing method. (10) Cr: 0.03 to 1.0% by weight, B: 0.
0003-0.0020%, Ca: 0.0015-0.
HT590 excellent in uniform elongation according to the above (8) or (9), characterized in that one or more of REM: 0.001 to 0.005% is contained in the slab.
Steel manufacturing method.

[0005]

The present inventor has made a detailed investigation on the uniform elongation of the HT590 steel. As a result, the work hardening in the uniform plastic strain region in the tensile test, that is, the close relation between the existence state of carbides in bainite and the uniform elongation. Found that there is.
As a result, the carbide at the lath interface of bainite was
Precipitation is extremely effective in improving uniform elongation, and tempering temperature of tempered high-strength steel is generally 650 ° C or lower, and tempering temperature of TMCP (DQ) steel is generally 600 ° C or lower, Higher temperature tempering: 670 ° C-7
It has been found that tempering at 30 ° C. is necessary to obtain the above precipitate.

Further, it was found that the strengthening by precipitation of ε-Cu in the low carbon content compensates for the strength reduction due to high temperature tempering and has a dramatic combined effect in improving the uniform elongation. That is, even if the precipitation of carbides of 1 μm or more and the formation of soft bainite at the lath interface of bainite due to the low carbon content and high temperature tempering and the formation of soft ε-Cu are utilized, the uniform elongation is achieved in the adversity of strengthening. The present invention has led to the invention of a technique for more effectively improving

The present invention will be described in detail below. The heat-treated HT590 steel targeted by the present invention is usually subjected to various heat treatments, and the heat treatment subsequent to the rolling or heating / rolling (including controlled rolling) after the cast slab is quenched (Q,
A combination of DQ) and tempering (heat treatment T) is common. Further, when low YR characteristics are required like the steel of the present invention, quenching (L, DL) from the two-phase region is introduced into the heat treatment. Further, the extremely thick HT590 steel for large-scale structures may be combined with slow cooling or heat retention (dehydrogenation T) for the purpose of dehydrogenation before heating or before and after heat treatment.

If C exceeds 0.20%, the low temperature toughness and weldability are impaired, and if it is less than 0.03%, the required strength cannot be easily ensured, so the content is limited to 0.03 to 0.20%.
In addition, in a high Cu material of Cu ≧ 0.60%, which requires a higher uniform elongation, if it exceeds 0.08%, the low temperature toughness and weldability are impaired, and if it is less than 0.02%, the required strength cannot be secured. It is restricted to 0.02-0.08%.

From the standpoint of deoxidation and strength, Si is required to be 0.03% or more, preferably 0.05% or more, and addition of more than 0.60% significantly impairs low temperature toughness and weldability, so 0.0
It is limited to 3 to 0.60%, preferably 0.05 to 0.
Limited to 60%. Further, in order to improve the elongation, a high Cu material with a low C of C ≦ 0.08% (Cu ≧ 0.6
0%) requires 0.10% or more in terms of strength,
It is limited to 0.10 to 0.60%.

Mn must be 0.80% or more in terms of strength, and 2.
Since the addition of more than 0% deteriorates both the low temperature toughness and the weldability, it is limited to 0.80 to 2.0%. 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, and is preferably 0.015% or less. Be restricted. Although S is limited to 0.015% or less due to low temperature toughness,
A lower value is preferably restricted to 0.010%. Furthermore,
When lamellar tear resistance is required due to the shape of the structure, S is controlled to 0.003% or less, and morphology control of MnS by Ca or REM is required.

The contents of C, Si, Mn, P, and S depend on the necessary characteristics (strength and low temperature toughness) in a predetermined heat treatment, and the temper HT.
The composition is designed in consideration of the plate thickness of 590 steel. For the production of extra-thick tempered HT590 steel for large structures and high-rise buildings,
In addition to the above five elements, the necessary amounts of Cu, Ni, Mo, Nb, V, and Ti 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 the low temperature toughness, and is added in an amount of 0.03% or more in order to secure the strength, and preferably 0.00.
10% or more is added, but if it exceeds 1.0%, hot brittleness is promoted and weldability deteriorates, so 0.03 to 1.
It is limited to 0% and is preferably restricted to 0.10 to 1.0%. Further, when C ≦ 0.08%, it is necessary to add 0.60% or more in order to improve uniform elongation due to precipitation of soft ε-Cu, but if it exceeds 2.5%, hot brittleness may occur. The amount of Ni is limited to 0.60 to 2.5% because it promotes the addition of an equal amount of Ni and deteriorates the weldability.

Ni is added to 0.03% or more by substituting C, Si and Mn for the purpose of reducing Ceq in order to improve low temperature toughness, and 0.10% or more is preferable for ensuring strength.
If it is 1.5% or more, the effect is saturated, so 0.03 ~
It is limited to 1.5% and is preferably restricted to 0.10 to 1.5%. Furthermore, when C ≦ 0.08%, soft ε-Cu
When Cu is added in an amount of 0.60% or more for the purpose of improving the uniform elongation due to the precipitation of Cu, it is necessary to add at least 1/2 or more of the Cu addition amount in order to prevent hot embrittlement due to Cu. 0.30% or more was added, and the effect was saturated at more than 2.5%, so the content was limited to 0.30 to 2.5%.

Mo is added in an amount of 0.03% or more, preferably 0.05% or more in order to secure the strength by improving the hardenability. Addition of more than 0.50% makes it difficult to secure low YR property and uniform elongation. In the case of C ≧ 0.08%, the addition of more than 0.30% is further disadvantageous, so the content is limited to 0.03 to 0.30%, and in the case of C ≧ 0.08%, 0.03 to 0.30%. 0.3
It is restricted to 0%, preferably 0.05 to 0.30%.

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% deteriorates weldability, low temperature toughness, low YR property and uniform elongation. It was limited to 0.005-0.03%. V is added in an amount of 0.005% or more, preferably 0.01% or more for improving the strength, and if it is added over 0.10%, preferably over 0.09%, weldability and low temperature toughness are deteriorated. 0.005-
It is limited to 0.10%, preferably 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, weldability, and low weldability. Since it becomes difficult to secure YR property and uniform elongation, 0.005
It was limited 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 needs to be 0.010% or more for deoxidation and particle size adjustment, and needs to be 0.10% or less from the viewpoint of weldability, and is limited to 0.010 to 0.10%. In addition,
When B, Ca and REM are added, Sol. It is preferable to add 0.030% or more of Al.

Other elements (Cr, B,
(Ca, REM) may be added singly or in combination of two or more in order to improve properties such as strength, low temperature toughness, lamellar tear resistance, etc., but the effect of the present invention is not impaired to some extent. Cr is added in an amount of 0.03% or more to secure the strength by improving the hardenability, and if it exceeds 1.0%, it becomes difficult to secure low YR property and uniform elongation, so 0.03 to 1.0%. Limited to C
When ≧ 0.08%, addition of more than 0.50% further lowers Y
0.03 to 0.50 because R property and uniform elongation are disadvantageous
A% constraint is preferred.

If necessary, B is added in an amount of 0.0003% or more to improve strength and low temperature toughness, and if added in excess of 0.0020%, preferably in excess of 0.0015%, low YR property is impaired, so that B. 0003 to 0.0020%, preferably 0.0003 to 0.0015% is preferable. Ca is 0.0015 when low temperature toughness and lamellar tear resistance are required.
%, But inclusions exceeding 0.008% increase the inclusions, so it is desirable to limit the content to 0.0015 to 0.0080%.

REM is added in an amount of 0.001% or more in place of Ca as needed in order to improve low temperature toughness, lamellar tear resistance, and joint toughness during high heat input welding, and inclusions exceeding 0.0050% increase inclusions. Therefore, the content is limited to 0.0050% or less, and the constraint of 0.001 to 0.005% is preferable.

Next, the reason for limiting the size of the carbide at the lath interface of bainite, which is the most important technical idea in the present invention, will be described. If the size of the carbides precipitated at the lath interface of bainite is less than 1 μ, HT590 steel having a uniform elongation of 25% or more cannot be obtained. From the viewpoint of uniform elongation, it is not necessary to limit the upper limit of the size of the carbide, but from the viewpoint of low temperature toughness, it is preferably 20 μm or less.

Next, the reasons for limiting the heat treatment for improving the uniform elongation as well as the YR reduction in the present invention will be described. Quenching (Q, DQ) may be general quenching that is performed to secure the strength and toughness of HT590 steel, and the quenching temperature is Ac ₃ or more in the case of Q for quenching from the complete austenite region, In case of DQ, Ar ₃ or more, 950
Above ₃ ° C, the ductility and low temperature toughness deteriorate due to the coarsening of the austenite structure, so Ac _{3 to} 950 ° C (Ar _{3 to} 95 ° C).
0 ° C).

Quenching (L, DL) from the two-phase region which improves the ferrite fraction or promotes the soft bainite to improve the uniform elongation may be the two-phase region quenching which is common in the production of low YR steel. In order to completely set the quenching temperature to the (γ + α) two-phase region, in the case of L, (Ac ₁ + 40 ° C.) to (Ac ₃
-30 ° C.), and in the case of DL, the temperature at 1/4 t of the plate thickness is limited to (Ar ₁ + 70 ° C.) to (Ar ₃ −60 ° C.).

The tempering temperature is generally determined in order to optimize the strength-toughness balance, but in the present invention, in the quenched (Q, DQ, L, DL) state, the bainite lath interface is finely divided. In order to coarsen the carbides precipitated in 1 μm or more to 670 ° C. or more, and above 730 ° C., the uniform elongation is saturated and the effect is not improved, but the yield point and low temperature toughness decrease. It was limited to 670 ° C to 730 ° C.

The reason why a stable uniform elongation can be obtained when carbides of 1 μm or more are precipitated (aggregated or coarsened) at the lath interface of bainite is that carbides finely precipitated at the lath interface of bainite which hinder the movement of dislocations. It is considered that the plastic deformability is improved as a result of the decrease or disappearance of the solid solution carbon existing in the matrix in a supersaturated state and the decrease of the dislocation density of the lath. Needless to say, it is necessary to compensate the decrease in strength due to the increase in tempering temperature in the component design.

The uniform elongation is defined as the elongation when the load is reduced by 5% over the maximum load in a JIS No. 4 tensile test piece (at 95% load). The elongation at the highest load may be defined as uniform elongation, but this value is about 5% lower than the uniform elongation of the present invention.

[0025]

【Example】

[Example 1] Examples of the present invention in a heat treatment for tempering after quenching (Q, DQ) and comparative examples, together with Tables 1 and 2
Shown in. 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.

[0026]

[Table 1]

[0027]

[Table 2]

Production results (t = 40) of the example of the present invention and the comparative example in which both a tensile test and an impact test were performed on JIS No. 4 test pieces.
˜80 mm) is shown in Table 2. Examples of the present invention (Steels A, B, D)
The uniform elongation improves with increasing tempering temperature.
Saturation occurs at 730 ° C or higher, and no further improvement is observed. Further, the low temperature toughness rapidly deteriorates when the tempering temperature exceeds 730 ° C.

Therefore, the tempering temperature is limited to 670 ° C. or more from the viewpoint of ensuring uniform elongation of 15% or more, and the tempering temperature is limited to 730 ° C. or less from the viewpoint of ensuring low temperature toughness.

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

The carbides precipitated on the lath interface of bainite increased with the increase of the tempering temperature and increased to 1 μ or more at 670 ° C or higher, and the uniform elongation was also improved. It dissolves in the matrix and becomes 1 μm or less, but the area ratio of ferrite increases and the deterioration of uniform elongation is apparently small. Incidentally, the yield ratio (YR), which is another important evaluation index for plastic deformability, is 80% or less, which is a recently severely required value. The YR improvement effect by tempering is remarkable.

Example 2 Quenching with a high Cu material (Q,
Table 3 and Table 4 show the production results (t = 80 mm) of the example of the present invention (steel E) in the heat treatment of tempering after DQ) together with the steel B of example 1 for comparison. Compared with Steel B, Example E in which the present invention was applied to a high Cu material satisfied all the properties of HT590 steel with a uniform elongation of 20% or more, and the size of the carbide precipitated at the lath interface of bainite was 1 μm or more. Has become.

[0033]

[Table 3]

[0034]

[Table 4]

[Example 3] Tables 5 and 6 show examples of the present invention in a tempering heat treatment in which quenching is introduced from a two-phase region, together with comparative examples. Table 5 shows examples of the present invention (steels F, G, I)
Further, as a chemical composition of Comparative Example (Steel H), Ca or REM is added to Steels G, H, and I to improve lamellar tear resistance. Steel H has Nb and Ti exceeding the upper limit of the composition range of the present invention.

Steels obtained by rolling the steels shown in Table 5 to t = 45 to 80 mm were subjected to the heat treatment for tempering of the present invention, and the mechanical test results (tensile test and impact test were carried out according to JIS
Table 6 shows the results of Examples of the present invention and Comparative Examples for No. 4 test piece). Except for Steel I-2, all of the steels have sufficient strength characteristics and low temperature toughness as general HT590 steel.

[0037]

[Table 5]

[0038]

[Table 6]

In the examples of the present invention (steels F, G, I), the carbides precipitated at the lath interface of bainite increased and the uniform elongation improved, and when the size of the carbides was 1 μ or more, it was stable at 20% or more. A uniform elongation of is obtained. Therefore, 20
%, The size of carbides precipitated at the lath interface of bainite is limited to 1 μm or more.

On the other hand, in Comparative Example (Steel H), although the carbide of 1 μm or more is deposited, Steel H is mainly composed of upper bainite because Nb and Ti are added in excess of the composition range of the present invention. Has a low plastic deformability and low temperature toughness, and YR is as high as 80% or more, and uniform elongation of 20% or more is not obtained.

The carbides deposited on the lath interface of bainite increased with the increase of the tempering temperature, and carbides of 1 μ or more were obtained at temperatures above 670 ° C., but above 730 ° C., the tempering temperature became coarse in the two-phase region. The carbide is solutionized into austenite and is quenched, so that the size of the carbide becomes again less than 1 μm. Therefore, in order to secure the carbide of 1 μm or more according to the present invention, the tempering temperature is 670 ° C. or more and 7
It is limited to 30 ° C or lower.

The uniform elongation improves as the tempering temperature rises, but the uniform elongation decreases again above 730 ° C., which is the two-phase heat treatment. On the other hand, if the tempering temperature exceeds 730 ° C., a two-phase region heat treatment is performed and the low temperature toughness of the steel, such as I-2, deteriorates, and the YR becomes too low, resulting in an insufficient yield point of the HT590 steel. Therefore, the tempering temperature is limited to 670 ° C. or more and 730 ° C. or less from the viewpoint of ensuring uniform elongation of 20% or more.

The yield ratio (YR), which is another important evaluation index for plastic deformability, is 80% or less, which is a recently severely required value. In the comparative examples except 2, YR is not less than 80%. That is, the conventional low YR technology does not necessarily lead to a sufficient improvement of uniform elongation even if the low YR is achieved, but the uniform elongation improvement technology of the present invention improves the uniform elongation to low YR. Is also easily achieved.

Example 4 An example of the present invention in a heat treatment for heat treatment by introducing quenching from a two-phase region using a high Cu material is shown in Tables 7 and 8 together with the steel G of Example 3 for comparison. Table 7 shows the chemical composition of the invention examples (Steels J, K, L) and Steel G. Steels G, L are C for improving the lamellar tear resistance.
a or REM is added.

[0046]

[Table 7]

[0047]

[Table 8]

Steels obtained by rolling the steels shown in Table 7 to t = 45 to 80 mm were subjected to the heat treatment for tempering according to the present invention, and the mechanical test results (the tensile test and the impact test were carried out according to JIS
Table 8 also shows the actual results (t = 80 mm) of Steel G for the No. 4 test piece. Except for steel L-2, all of the steels have sufficient strength characteristics and low temperature toughness as general HT590 steel.

In the examples of the present invention (steels J, K, L), the carbide precipitated at the lath interface of bainite increased and the uniform elongation improved, and when the size of the carbide was 1 μ or more, it was stable at 25% or more. A uniform elongation of is obtained. Therefore, 25
%, The size of carbides precipitated at the lath interface of bainite is limited to 1 μm or more. FIG. 1 shows transmission electron micrographs of carbides precipitated at the lath interface of bainite of steel J-1 and steel K-1. On the other hand, in Steel G, despite the precipitation of carbides of 1 μm or more, formation of soft bainite and soft ε-Cu due to low carbon content
Precipitation is insufficient and uniform elongation of 25% or more is not obtained.

The carbides precipitated at the lath interface of bainite increased with the increase of the tempering temperature, and carbides of 1 μ or more were obtained at temperatures above 670 ° C., but above 730 ° C., the tempering temperature became a two-phase region and became coarse. The carbide is solutionized into austenite and is quenched, so that the size of the carbide becomes again less than 1 μm. Therefore, in order to secure the carbide of 1 μm or more according to the present invention, the tempering temperature is 670 ° C. or more and 7
It is limited to 30 ° C or lower.

The uniform elongation improves as the tempering temperature rises, but the uniform elongation decreases again above 730 ° C., which is the two-phase heat treatment. On the other hand, if the tempering temperature exceeds 730 ° C., a two-phase region heat treatment is performed and the low temperature toughness deteriorates like steel L-2, resulting in too low YR, resulting in an insufficient yield point of HT590 steel. Therefore, the tempering temperature is limited to 670 ° C. or more and 730 ° C. or less from the viewpoint of ensuring uniform elongation of 25% or more.

The yield ratio (YR), which is another important evaluation index for plastic deformability, is 80% or less, which is a recently severely required value. Even if YR is 80% or less, uniform elongation is 25%
Not over. That is, the conventional low YR technology does not necessarily lead to a sufficient improvement of uniform elongation even if the low YR is achieved, but the uniform elongation improvement technology of the present invention improves the uniform elongation to low YR. Is also easily achieved.

[0053]

INDUSTRIAL APPLICABILITY The steel of the present invention limits the carbides precipitated at the lath interface of bainite to 1 μm or more, or specifies the tempering temperature in the heat treatment for refining to a high temperature higher than the conventional temperature, and has a low C- Due to the formation of soft bainite and the precipitation of soft ε-Cu by increasing the Cu content, temper HT5
It was possible to further improve the uniform elongation of 90 steel. Further, the present invention also provides a tempered HT590 steel having a high uniform elongation and a method for producing the same as well as a reduction in YR which is the two major factors of plastic deformability. As a result, not only is the safety design of large-scale structures possible not only from the construction side but also from the steel side, a tempered HT590 with a high uniform elongation that replaces the three-step heat treatment.
It also enables the rational production of steel. Therefore,
According to the present invention, it is considered that not only the safety design and the economic design that can be enjoyed by the industrial world are compatible, but also the construction period and the economic profit are great.

[Brief description of drawings]

FIG. 1 (a) is a micrograph of the present invention steel (steel J-1), and (b) is a micrograph of the present invention steel (steel K-1).

Front page continuation (72) Inventor Yoshiki Kawashima 1 Nishinosu, Oita-shi, Oita-shi Nippon Steel Corporation Oita Works

Claims (10)

[Claims] 1. C: 0.03 to 0.20% by weight%, Si: 0.03 to 0.60%, Mn: 0.80 to 2.0%, P: 0.025% or less, S : 0.015% or less, Cu: 0.03 to 1.0%, Ni: 0.03 to 1.5%, Mo: 0.03 to 0.30%, Nb: 0.005 to 0.03% , V: 0.005-0.10%, Ti: 0.005-0.03%, Sol. Al: 0.010 to 0.10% A balance consisting of balance iron and unavoidable impurities, having a carbide of 1 μ or more at the lath interface of bainite, and having a uniform elongation of 15% or more. Excellent tempering HT5
90 steel. 2. C: 0.02 to 0.08%, Si: 0.10 to 0.60%, Mn: 0.80 to 2.0%, P: 0.015% or less, S by weight% : 0.010% or less, Cu: 0.60 to 2.5%, Ni: 0.30 to 2.5%, Mo: 0.03 to 0.50%, Nb: 0.005 to 0.03% , V: 0.005 to 0.10%, Ti: 0.005 to 0.03%, Sol. Al: 0.010 to 0.10% A balance consisting of balance iron and unavoidable impurities, having a carbide of 1 μ or more at the lath interface of bainite, and having a uniform elongation of 20% or more. Excellent tempering HT5
90 steel. 3. By weight%, Cr: 0.03-1.0%, B: 0.0003-0.0020%, Ca: 0.0015-0.0080%, REM: 0.001-0.005. %, One or two or more of them are contained in the slab, and the tempered HT5 excellent in uniform elongation according to claim 1 or 2, characterized in that
90 steel. 4. C: 0.03 to 0.20% by weight%, 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% Immediately after casting of a slab composed of balance iron and unavoidable impurities, or after reheating to a point of Ac ₃ or more and subsequent to thick plate rolling, when performing heat treatment for quenching and tempering, A method for producing a tempered HT590 steel, characterized in that the tempering temperature is 670 ° C. or higher and 730 ° C. or lower, and uniform elongation of 15% or more is given. 5. By weight%, Cr: 0.03 to 0.50%, B: 0.003 to 0.0015%, Ca: 0.0015 to 0.0080%, REM: 0.001 to 0.0050. 5. The method for producing a tempered HT590 steel according to claim 4, wherein the slab contains 1% or 2% or more of H 2 O 3 and gives a uniform elongation of 15% or more. 6. C: 0.03 to 0.20% by weight%, Si: 0.03 to 0.60%, Mn: 0.80 to 2.0%, P: 0.025% or less, S : 0.015% or less, Cu: 0.03 to 1.0%, Ni: 0.03 to 1.5%, Mo: 0.03 to 0.30%, Nb: 0.005 to 0.03% , V: 0.005 to 0.10%, Ti: 0.005 to 0.03%, Sol. Al: 0.010 to 0.10% Immediately after casting of a slab consisting of balance iron and inevitable impurities or after reheating to Ac ₃ points or more, direct quenching from a two-phase region following thick plate rolling, tempering When performing direct heat treatment after heat treatment or plate rolling, quenching heat treatment from two-phase region, temper heat treatment for tempering, or quenching after completion of plate rolling, quenching from two-phase region, temper heat treatment for tempering, tempering A method for producing a tempered HT590 steel, characterized in that the temperature is 670 ° C. or higher and 730 ° C. or lower and a uniform elongation of 20% or more is given. 7. By weight%, Cr: 0.03 to 0.50%, B: 0.0003 to 0.0020%, Ca: 0.0015 to 0.0080%, REM: 0.001 to 0.005. 7. The method for producing a heat-treated HT590 steel according to claim 6, wherein one or two or more types are contained in the slab to give a uniform elongation of 20% or more. 8. C: 0.02 to 0.08%, Si: 0.10 to 0.60%, Mn: 0.80 to 2.0%, P: 0.015% or less, S by weight% : 0.010% or less, Cu: 0.60 to 2.5%, Ni: 0.30 to 2.5%, Mo: 0.03 to 0.50%, Nb: 0.005 to 0.03% , V: 0.005 to 0.10%, Ti: 0.005 to 0.03%, Sol. Al: 0.010 to 0.10% Immediately after casting of a slab composed of balance iron and unavoidable impurities, or after reheating to a point of Ac ₃ or more and subsequent to thick plate rolling, when performing heat treatment for quenching and tempering, A tempering HT590 steel manufacturing method, characterized in that a tempering temperature is set to 670 ° C. or higher and 730 ° C. or lower and a uniform elongation of 20% or more is given. 9. C: 0.02 to 0.08%, Si: 0.10 to 0.60%, Mn: 0.80 to 2.0%, P: 0.015% or less, S by weight% : 0.010% or less, Cu: 0.60 to 2.5%, Ni: 0.30 to 2.5%, Mo: 0.03 to 0.50%, Nb: 0.005 to 0.03% , V: 0.005 to 0.10%, Ti: 0.005 to 0.03%, Sol. Al: 0.010 to 0.10% Immediately after casting of a slab consisting of balance iron and inevitable impurities or after reheating to Ac ₃ points or more, direct quenching from a two-phase region following thick plate rolling, tempering When performing direct heat treatment after heat treatment or plate rolling, quenching heat treatment from two-phase region, temper heat treatment for tempering, or quenching after completion of plate rolling, quenching from two-phase region, temper heat treatment for tempering, tempering A method for producing a tempered HT590 steel, characterized in that the temperature is 670 ° C. or higher and 730 ° C. or lower and a uniform elongation of 25% or more is given. 10. By weight%, Cr: 0.03 to 1.0%, B: 0.0003 to 0.0020%, Ca: 0.0015 to 0.0080%, REM: 0.001 to 0.005. %, One or two or more of them are contained in the cast slab, and the temper HT590 according to claim 8 or 9, characterized in that
Steel manufacturing method.