JPH0551694A - Low yield ratio high strength steel and method of manufacturing the same - Google Patents

Abstract

(57) [Abstract] [PROBLEMS] To provide a high-strength steel having a high tensile strength and a low yield ratio, which is preferable in use, and a method for producing the same. [Constitution] C, Si, Mn, P, S, Cr, Mo, V and B are contained in a specific range, or one or more of Cu, Ni, Ti and Nb are contained in a specific range. And has a structure in which the ferrite + bainite fraction is 50% or more, the tensile strength is 780 MPa or more, and the yield ratio is 85% or less.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a low-yield ratio high-strength steel having a tensile strength of 780 MPa or more and a yield ratio of 85% or less, which is used in the field of construction or civil engineering, and a method for producing the same.

[0002]

2. Description of the Related Art Conventionally, generally 780 MPa (80 Kgf / m
m ² : 1MPa = 0.098Kgf / mm ² ) Grade steel is often manufactured by quenching and tempering, and the yield ratio is 90-
It was about 95%, but recently, it has been disclosed in JP-A-64-5202.
3, JP-A-2-213411, JP-A-2-93020,
Japanese Unexamined Patent Publication (Kokai) No. 3-79716 proposes a method for manufacturing various high-strength steel sheets in which the yield ratio and the like are controlled.

That is, the manufacturing method disclosed in Japanese Patent Laid-Open No. 64-52023 is based on controlled rolling- (α + γ) two-phase region quenching-tempering. It strictly controls the end temperature and so on.

In Japanese Patent Laid-Open No. 2-213411, (α
+ Γ) After heating in the two-phase region, a process of cooling to a temperature 20 to 200 ° C. lower than that temperature, holding for 5 to 60 minutes, followed by quenching and tempering is adopted, but this method involves complicated heat treatment of the steel sheet. Yes, reduce production efficiency.

The yield ratio is set to 8 by the method disclosed in Japanese Patent Laid-Open No. 2-93020.
It is said that high yield strength steel with a low yield ratio of less than 5% is produced, but the heating temperature is T = 6770 / (2.26-log
[Nb%] [C%])-Limited to 273 ° C or higher, (α +
γ) Rolling from the two-phase region before direct quenching with Ac ₃ +10
Manufacturing regulations such as finishing at a high temperature of 0 ° C. or higher are strictly limited.

Japanese Patent Laid-Open No. 3-79716 discloses (α +
γ) Quenching and tempering from the two-phase region is performed, and Cu is added at about 1% in order to obtain high strength, which is not preferable from the economical point of view.

Incidentally, in Japanese Patent Application Laid-Open No. 1-176029,
A method for manufacturing a steel sheet having a high strength of 780 MPa (80 Kgf / mm ² ) or more and a yield ratio of about 80% is disclosed.

[0008]

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention
Steel sheet tensile strength prepared by those published except 76029 is a about ^{580MPa (60Kgf / mm 2),} 780MPa (80Kgf / mm 2) or more steel sheet not obtained almost, moreover, as described above There are various manufacturing difficulties.

In Japanese Patent Laid-Open No. 1-176029, a high strength of 780 MPa (80 Kgf / mm ² ) or more and a yield ratio of 80
Even if a steel sheet can be obtained with about 10%, it is necessary to strictly control the rolling conditions such as reducing the rolling ratio in the unrecrystallized austenite region to 30% or more, which is disadvantageous for industrial mass production. is doing.

[0010]

Means for Solving the Problems As a result of repeated studies on solving the problems in the conventional ones as described above, the present invention shows that tensile alloys can be formed without adding a large amount of expensive alloying elements such as Cu. Strength is 780MPa or more and yield ratio is 8
It has succeeded in providing a high-strength steel having a low yield ratio of 5% or less, and performing appropriate production thereof without strict regulation of complicated heat treatment, heating temperature, and rolling finish temperature. , As follows.

(1) Mass%, C: 0.05 to 0.20%,
Si: 0.05 to 1.50%, Mn: 0.50 to 2.00%, P: 0.015
% Or less, S: 0.015% or less, Cr: 0.05 to 2.00
%, Mo: 0.05 to 1.00%, V: 0.01 to 0.10%, B
: 0.0003 to 0.0020%, with the balance being Fe and inevitable impurities, and having a structure of ferrite + bainite fraction of 50% or more,
A high-strength steel with a low yield ratio, which has a tensile strength of 780 MPa or more and a yield ratio of 85% or less.

(2) Mass%, C: 0.05 to 0.20%,
Si: 0.05 to 1.50%, Mn: 0.50 to 2.00%, P: 0.015
% Or less, S: 0.015% or less, Cr: 0.05 to 2.00
%, Mo: 0.05 to 1.00%, V: 0.01 to 0.10%, B
: 0.0003 to 0.0020%, Cu: 0.10 to 0.70%, Ni: 0.10 to 2.00%, and balance of Fe and unavoidable impurities and ferrite + bainite fraction of 50 %, High tensile strength steel having a tensile strength of 780 MPa or more and a yield ratio of 85% or less.

(3) Mass%, C: 0.05 to 0.20%,
Si: 0.05 to 1.50%, Mn: 0.50 to 2.00%, P: 0.015
% Or less, S: 0.015% or less, Cr: 0.05 to 2.00
%, Mo: 0.05 to 1.00%, V: 0.01 to 0.10%, B
: 0.0003 to 0.0020%, Ti: 0.003 to 0.10%, Nb: 0.005 to 0.10%, 1 or 2 types, the balance Fe and inevitable impurities, and ferrite + bainite fraction of 50 %, High tensile strength steel having a tensile strength of 780 MPa or more and a yield ratio of 85% or less.

(4) Mass%, C: 0.05 to 0.20%,
Si: 0.05 to 1.50%, Mn: 0.50 to 2.00%, P: 0.015
% Or less, S: 0.015% or less, Cr: 0.05 to 2.00
%, Mo: 0.05 to 1.00%, V: 0.01 to 0.10%, B
: 0.0003 to 0.0020%, Cu: 0.10 to 0.70%, Ni: 0.1
Contains 0 to 2.00%, Ti: 0.003 to 0.10%, Nb: 0.005 to 0.10%, one or two, the balance Fe and inevitable impurities, and ferrite + bainite fraction of 50%. A high-strength steel having a low yield ratio and a tensile strength of 780 MPa or more and a yield ratio of 85% or less.

(5) Mass%, C: 0.05 to 0.20%,
Si: 0.05 to 1.50%, Mn: 0.50 to 2.00%, P: 0.015
% Or less, S: 0.015% or less, Cr: 0.05 to 2.00
%, Mo: 0.05 to 1.00%, V: 0.01 to 0.10%, B
: 0.0003 to 0.0020%, Ti: 0.003 to 0.10%, Nb: 0.
005 to 0.10%, Cu: 0.10 to 0.70%, Ni: 0.10 to 2.00%, 1 or 2 kinds, the balance Fe and unavoidable impurities, ferrite + bainite fraction 50% A high-strength steel having a low yield ratio and a tensile strength of 780 MPa or more and a yield ratio of 85% or less.

(6) After rolling the steel having the component composition as described in any one of the above items 1 to 5 into a structure having a ferrite + bainite fraction of 50% or more, a temperature range T:
(Ac ₁ + Ac ₃ ) / 2-20 ° C to (Ac ₁ + Ac ₃ ) / 2 + 50 ° C
A method for producing a high-strength steel having a low yield ratio, which comprises heating the steel during heating, quenching, and then tempering at a temperature of Ac ₁ or lower.

[0017]

The technical relationship of the present invention as described above will be described in detail. First, the chemical composition of the steel sheet according to the present invention will be described in terms of mass% (hereinafter simply referred to as "%").

C is an element effective for increasing the strength, and
If it is less than 05%, it is difficult to obtain the target strength in the present invention, and if it exceeds 0.20%, the carbon equivalent is increased and the weldability is impaired. Therefore, 0.05 to 0.20
%.

Si is effective in deoxidizing effect and increasing strength by solid solution strengthening, but if less than 0.05%, its effect is poor, while excessive addition exceeding 1.50% deteriorates weldability. Based on these relationships, the addition amount was set to 0.05 to 1.50%.

Mn is an element effective for increasing strength, but
If it is less than 50%, the effect is insufficient. In contrast,
If it exceeds 2.00%, the weldability will decrease, so 0.50-
2.00%

P and S are both impurity elements and elements that cause important characteristic deterioration in use such as ductility, workability and weldability, so it is desirable to reduce them as much as possible. However, since the significant reduction of these causes an increase in cost, the upper limit of 0.015% is set as the upper limit which does not cause remarkable deterioration of the material.

Cr is an element effective in increasing strength, and is 0.0
The effect is recognized at 5% or more. But this Cr is 2.00
%, The weldability will be reduced, so this should be the upper limit,
The amount added is set to 0.05 to 2.00%.

Mo enhances hardenability and strength, but if it is less than 0.05%, its effect is insufficient, while 1.
Even if it exceeds 00%, the effect is saturated and it is economically disadvantageous, so it is necessary to set it to 0.05 to 1.00%.

V is effective for increasing the strength and has an effect of increasing the temper softening resistance, but if it is less than 0.01%, such an effect is not exhibited, so the content is made 0.01% or more.
Even if it exceeds 0.10%, the effect will be saturated, so 0.01-0.10
%.

B is a very important element for improving the strength by improving the hardenability by adding a trace amount. However, if it is less than 0.0003%, there is almost no effect. On the other hand, if it exceeds 0.002%, the effect is saturated, and on the contrary, the strength is lowered. Therefore, the addition amount is 0.0003 to 0.002%. It is necessary to.

Cu is an element effective in increasing the strength and also improving the toughness in such a case, but if it is less than 0.10%, these effects are hardly recognized, and 0 Excessive addition exceeding 0.70% is not preferable from the viewpoint of both hot workability and economy. Therefore, the addition amount is set to 0.10 to 0.70%.

Ni enhances hardenability and strength,
The toughness is also improved, but if it is less than 0.10%, its effect is small and insufficient. Further, if it exceeds 2.00%, it is economically unfavorable, so it is necessary to make the addition amount 0.10 to 2.00%.

Nb increases the strength by precipitation strengthening, but if it is less than 0.005%, its effect can hardly be expected. Moreover, since the addition of more than 0.10% deteriorates the weldability and the toughness of the weld zone, the range is 0.005 to 0.10%.

Ti strengthens similarly to Nb by precipitation strengthening, but if its amount is less than 0.03%, its effect is hardly obtained. Further, if it exceeds 0.10%, the weldability is deteriorated, so the addition amount should be 0.003 to 0.10%.

As described above, the present invention requires a ferrite + bainite fraction of 50% or more. Even if a method for obtaining such a ferrite + bainite fraction can be considered as such, the present invention is applicable. The ferrite + bainite fraction of 50% or more is based on the result obtained by measuring the tissue fraction by the linear analysis method using a 100 × photograph observed with an optical microscope. The requirement of the present invention is satisfied when the ferrite + bainite fraction at that time is 50% or more.

When the tensile strength is 780 MPa or more, the yield ratio is 85%.
By the following, a steel material having a high strength and a low yield ratio can be provided without adding a special alloying element, and a steel material product that can be easily machined and has excellent strength can be obtained.

Next, in the present invention, the temperature range T: (Ac ₁ + Ac ₃ ) / 2-20 ° C. to (Ac ₁ + Ac ₃ ) / 2 + 50 ° C. is used as the manufacturing method.
The reason for this limitation is described below. By allowing ferrite and bainite to appear in the pre-structure in the as-rolled state or by various heat treatments, two-phase separation of ferrite and austenite is promoted when heated to the temperature range T. This is because bainite is composed of carbide and ferrite which are austenite nucleation sites, and quenching from the temperature range T,
The final structure after tempering becomes martensite or bainite transformed from ferrite and austenite, and high strength and low yield ratio are achieved. However, if the structure before quenching does not satisfy the ferrite + bainite fraction of 50% or more, sufficient transformation does not occur when heated to the temperature range T, and the final structure has the above-mentioned high strength and low yield ratio. It does not become an effective organization. Therefore, as the structure before quenching from the temperature range T, the ferrite + bainite fraction must be 50% or more.

Further, the heating temperature range of quenching is T: (Ac ₁ +
Ac ₃ ) / 2-20 ° C. to (Ac ₁ + Ac ₃ ) / 2-50 ° C. is because it is possible to obtain a low yield ratio by quenching from a temperature lower than this temperature range. Can not achieve high strength. It is possible to achieve high strength by quenching from a temperature higher than this temperature range, but the yield ratio is 85%.
As described above, the low yield ratio targeted by the present invention cannot be obtained. Therefore, the heating temperature range before quenching is T: (Ac
_{1 + Ac 3) / 2-20 ℃} ~ (Ac 1 + Ac 3) / 2 + 50 ℃ and it is necessary to.

[0034]

EXAMPLE A specific example of the method for producing a high-strength steel having a low yield ratio according to the present invention will be described. Table 1 below shows the chemical composition of the sample steel adopted by the inventors. Sample steels A to O are steels having the compositional range of the present invention, while P, Q and R are comparative steels, each having a plate thickness of 25 mm.
Was constant.

[0035]

[Table 1]

For each of the test steels shown in Table 1, the pretreatment conditions for quenching, the ferrite + bainite fraction and the mechanical performance are summarized as shown in Table 2 below. A1, C1, J1 and O1 are steels having the same composition as A, C, J and O in Table 1 above.

[0037]

[Table 2]

As is clear from Table 2, the steels A to O produced by the method of the present invention have a tensile strength of 780 MPa or more,
The yield ratio is 82.4% or less, and high strength and low yield ratio are achieved. On the other hand, although the comparative steels A1 and J1 satisfy the specified range of the present invention as their components, the quenching temperature is out of the specified temperature range T of the present invention, and the comparative steels C1 and O1 are quenched from the temperature range T. The previous structure does not have a predetermined structure fraction, and the comparative steels P, Q and R1,
R2 is out of the range of the component of the present invention, and the structure before quenching from the temperature range T does not have a predetermined microstructure fraction for R2, and all of these have strength and yield as targeted in the present invention. Not satisfied with the ratio.

Especially, the ferrite + bainite fraction is 60%.
By setting it to be about the above, TS is 820 MPa or more,
Moreover, the yield ratio of 82.5% or less was sufficiently achieved, which was confirmed to be industrially significant.

[0040]

According to the present invention as described above, a low yield ratio high strength steel sheet having a tensile strength of 780 MPa or more and a yield ratio of 85% or less is obtained, and a steel material that is easy to work and has high strength is provided. In addition, such steel materials can be stably manufactured without complicated heat treatment, or by strictly defining the heating temperature and rolling finish temperature, or by adding a large amount of expensive alloying elements such as Cu. Since it can be widely provided in the fields of construction and civil engineering, it is an invention that has a great industrial effect.

Front page continuation (72) Inventor Hiroshi Ishikawa 1-2 1-2 Marunouchi, Chiyoda-ku, Tokyo Nihon Kokan Co., Ltd. (72) Inventor Kotaro Hatakeyama 1-2 1-2 Marunouchi, Chiyoda-ku, Tokyo In the company

Claims (6)

[Claims] 1. In mass%, C: 0.05 to 0.20%, Si: 0.05 to 1.50%, Mn: 0.
50 to 2.00%, P: 0.015% or less, S: 0.015% or less, Cr: 0.05 to 2.00%, Mo: 0.05 to 1.00%, V
: 0.01 to 0.10%, B: 0.0003 to 0.0020%, the balance consisting of Fe and unavoidable impurities, and a structure of ferrite + bainite fraction of 50% or more,
A high tensile steel with a low yield ratio, which has a tensile strength of 780 MPa or more and a yield ratio of 85% or less. 2. Mass%, C: 0.05 to 0.20%, Si: 0.05 to 1.50%, Mn: 0.
50 to 2.00%, P: 0.015% or less, S: 0.015% or less, Cr: 0.05 to 2.00%, Mo: 0.05 to 1.00%, V
: 0.01 to 0.10%, B: 0.0003 to 0.0020%, Cu: 0.10 to 0.70%, Ni: 0.10 to 2.00%, one or two, and the balance Fe and unavoidable impurities. A low yield ratio high tensile steel having a structure of ferrite + bainite fraction of 50% or more, a tensile strength of 780 MPa or more and a yield ratio of 85% or less. 3. Mass%, C: 0.05 to 0.20%, Si: 0.05 to 1.50%, Mn: 0.
50 to 2.00%, P: 0.015% or less, S: 0.015% or less, Cr: 0.05 to 2.00%, Mo: 0.05 to 1.00%, V
: 0.01 to 0.10%, B: 0.0003 to 0.0020%, Ti: 0.003 to 0.10%, Nb: 0.005 to 0.10%, one or two, and the balance Fe and inevitable impurities. A low yield ratio high tensile steel having a structure of ferrite + bainite fraction of 50% or more, a tensile strength of 780 MPa or more and a yield ratio of 85% or less. 4. Mass%, C: 0.05 to 0.20%, Si: 0.05 to 1.50%, Mn: 0.
50 to 2.00%, P: 0.015% or less, S: 0.015% or less, Cr: 0.05 to 2.00%, Mo: 0.05 to 1.00%, V
: 0.01 to 0.10%, B: 0.0003 to 0.0020%, Cu: 0.1
In addition to containing 0 to 0.70%, Ni: 0.10 to 2.00%, Ti: 0.003 to 0.10%, Nb: 0.005 to 0.10%, one or two kinds, and the balance Fe and unavoidable impurities. + Low yield ratio high tensile steel having a structure with a bainite fraction of 50% or more, a tensile strength of 780 MPa or more and a yield ratio of 85% or less. 5. Mass%, C: 0.05 to 0.20%, Si: 0.05 to 1.50%, Mn: 0.
50 to 2.00%, P: 0.015% or less, S: 0.015% or less, Cr: 0.05 to 2.00%, Mo: 0.05 to 1.00%, V
: 0.01 to 0.10%, B: 0.0003 to 0.0020%, Ti: 0.
In addition to containing 003 to 0.10%, Nb: 0.005 to 0.10%, Cu: 0.10 to 0.70%, Ni: 0.10 to 2.00%, one or two, with the balance being Fe and inevitable impurities, and ferrite. + Low yield ratio high tensile steel having a structure with a bainite fraction of 50% or more, a tensile strength of 780 MPa or more and a yield ratio of 85% or less. 6. A steel having the composition of any one of claims 1 to 5 is rolled into a structure having a ferrite + bainite fraction of 50% or more, and then a temperature range T: (Ac ₁ +
Ac ₃ ) / 2-20 ° C. to (Ac ₁ + Ac ₃ ) / 2 + 50 ° C., quenching is performed, and then tempering is performed at a temperature of Ac ₁ or lower. Manufacturing method.