JPH04272129A - Production of high tension steel having low yield ratio - Google Patents

Abstract

PURPOSE:To produce high tension steel having a low yield ratio by hot-rolling and hardening a steel slab having a specified compsn. and further carrying out hardening and tempering under specified conditions. CONSTITUTION:A steel slab having a compsn. contg., by weight, 0.06-0.20% C, 0.4-1.5% Si, 0.7-2.0% Mn, <0.020% P and <0.020% S or further contg. one or more among 0.1-1.2% Cu, 0.1-2.0% Ni, 0.1-2.0% Cr, 0.05-1.0% Mo, 0.005-0.040% Nb, 0.01-0.10% V, 0.005-0.10% Ti and 0.0005-0.0020% B is hot-rolled or hot-rolled and hardened, hardened by heating to [(1,555+33XSi%)/2]+ or -40 deg.C and tempered at <=700 deg.C. High tension steel having <=75% yield ratio and >=60kgf/mm<2> tensile strength is obtd.

Description

[Detailed description of the invention]

0001

[Industrial Application Field] The present invention is used for steel structures such as architecture and civil engineering, and has a yield ratio of 75% or less and a tensile strength of 60%.
The present invention relates to a method for producing high tensile strength steel with a low yield ratio of Kgf/mm2 or more.

0002

[Prior Art] In recent years, in steel structures such as high-rise buildings, the use of low yield ratio high tensile strength steel with excellent plastic deformability has become established in order to improve the safety of the structure against earthquakes, etc. . Regarding the manufacturing method of class 2 high tensile strength steel with a low yield ratio of 60 kgf/mm in order to simultaneously satisfy high strength and low yield ratio, please refer to JP-A-2-93020 and JP-A-2-21341.
There are reports of No. 1 etc. These conventional techniques employ a process in which hot-rolled steel is heated or maintained in the (α+γ) two-phase region and subjected to quenching treatment or direct quenching treatment.

0003

[Problems to be Solved by the Invention] It is known that such heat treatment from the two-phase region is effective in lowering the yield ratio, but as stated in JP-A-2-93020. However, the use of such a process has the problem that strength tends to decrease. Regarding the above problem, JP-A-2-9
No. 3020 deals with this by utilizing the addition of Nb, but the yield ratio is about 75 to 85% at most. Furthermore, in Japanese Patent Application Laid-Open No. 2-213411, the reduction in yield ratio is limited to about 80% due to concerns about a decrease in strength, and the manufacturing process thereof is extremely complicated from an industrial perspective.

[0004] As described above, these conventional techniques have problems in that there is a limit to the low yield ratio due to concerns about strength deterioration, and that alloying elements such as Nb are essential for low yield ratio. Some kind of countermeasure is also needed to deal with the fact that extremely complicated manufacturing processes have to be applied.

The present invention was made to solve these problems of the prior art, and does not require the addition of special alloying elements, simplifies the manufacturing process, and has a yield ratio of 75 under such conditions. % or less, tensile strength is 60Kgf/mm
It is an object of the present invention to provide a manufacturing method that can stably obtain a high tensile strength steel with a low yield ratio of 2 or more.

[0006]

[Means for Solving the Problems] In order to solve the above-mentioned problems of the prior art, the present inventors have made extensive studies, and have developed a method for producing high-strength steel with a low yield ratio of 60 Kgf/mm2 or more. It has been found that it is effective to add a large amount of Si, which exceeds the conventional wisdom.

Addition of a large amount of Si has the effect of promoting the formation of ferrite, which is effective for lowering the yield ratio, in heat treatment, and also has the effect of suppressing the formation of carbides in tempering treatment. Suppression of carbide formation suppresses a decrease in tensile strength, and high strength can be achieved. Moreover, since ferrite formation is stably obtained, the yield strength can be lowered, and as a result, the present inventors have confirmed that it is possible to achieve a stable low yield ratio.

[0008] Furthermore, it was found that the above effects could not be obtained unless the Si amount was increased and an appropriate heat treatment temperature was selected in the heat treatment.As a result of detailed study of the heat treatment conditions, it was found that the amount of Si increases. A causal relationship is recognized between the temperature and the heat treatment temperature, and the optimum heat treatment temperature is determined by the amount of Si as a factor. Formula: {(1555+33×Si%)/2
}±40°C was determined.

The contents of the present invention described above are specifically as shown below.

That is, in the method of the present invention, C: 0.06% by weight
~0.20%, Si: 0.4-1.5%, Mn: 0.7
~2.0%, P: 0.020% or less, S: 0.020%
Steel containing the following, with the remainder consisting of Fe and unavoidable impurities, is heated as hot rolled or after hot rolling and quenching treatment at {(1555+33×Si%)/2}±40°C.
The basic feature is that the product is heated to a temperature of 700°C, quenched, and then tempered at a temperature of 700°C or less, and with this configuration, it has a tensile strength of 60 kgf/mm2 or more and a low yield ratio of 75% or less. It becomes possible to stably obtain specific high tensile strength steel.

Further, in the second invention, in addition to the above-mentioned component composition, Cu, Ni, Cr, Mo, Nb, V, Ti, B
The composition includes the selective addition of components such as
As for its specific composition, in weight%, C: 0.06 to 0
．． 20%, Si: 0.4-1.5%, Mn: 0.7-2
．． 0%, P: 0.020% or less, S: 0.020% or less, Cu: 0.1 to 1.2%, Ni: 0.1
~2.0%, Cr:0.1~2.0%, Mo:0.05
~1.0%, Nb: 0.005~0.040%, V: 0
．． 01-0.10%, Ti: 0.005-0.10%,
B: Steel containing one or more of 0.0005 to 0.0020%, with the remainder consisting of Fe and unavoidable impurities, as hot-rolled, or after hot-rolling and quenching. , {(1555+33×Si%)/2}±40℃
The material is heated to a temperature of 700° C., quenched, and then tempered at a temperature of 700° C. or less.

[0012] Below, the reasons for these limitations will be explained by dividing them into additive elements and manufacturing processes.

[Additional element] C: 0.06% to 0.20%
C is an effective element for stably ensuring the room-temperature strength of steel; if it is less than 0.06%, it is difficult to obtain the desired sufficient strength, and if it is added more than 0.20%, it is Since weldability deteriorates, the amount of C was set to 0.06% to 0.20%.

Si: 0.4% to 1.5%Si is the most important additive element in the present invention, suppressing the increase in YS,
It has the effect of increasing TS and is an essential element for stably lowering the yield ratio as a result. In order to exhibit such an effect of Si, it is necessary to add 0.4% or more.Addition of more than 1.5% deteriorates weldability, so the addition range is 0.4% to 1.5%. It was set at 5%.

Mn: 0.7% to 2.0% Mn is an effective element for ensuring strength, and must be added in an amount of 0.7% or more. Moreover, since weldability deteriorates if the addition exceeds 2.0%, the content is set in the range of 0.7% to 2.0%.

[0016] P: 0.020% or less, S: 0.020%
Hereinafter, P and S are impurity elements that cause problems such as a decrease in ductility, workability, and weldability, and are preferably reduced as much as possible. However, since a significant reduction would lead to an increase in cost, the upper limit of the amount that would not cause significant material deterioration was set at 0.020% or less.

[0017] Cu: 0.1% to 1.2% Cu is an effective element for increasing strength and also has the effect of improving corrosion resistance. Moreover, addition of more than 1.2% deteriorates hot workability and increases costs, so the addition range was set to 0.1% to 1.2%.

Ni: 0.1% to 2.0% Ni is an effective element for increasing strength and also has the effect of improving low-temperature toughness. Since adding more than 2.0% increases the cost, the addition range was set to 0.1% to 2.0%.

Cr: 0.1% to 2.0% Cr is an effective element for increasing strength and also has the effect of improving corrosion resistance. Moreover, addition of more than 2.0% deteriorates weldability and increases costs.
The addition range was 0.1% to 2.0%.

Mo: 0.05% to 1.0% Mo is an effective element for increasing strength, and in order to exhibit its effect, it must be
It is necessary to add 0.05% or more, and addition of more than 1.0% deteriorates weldability and increases costs, so the addition range was set from 0.05% to 1.0%. .

[0021] Nb: 0.005% to 0.040% Nb is an element effective in increasing strength through precipitation strengthening, and in order to exhibit its effect, it is necessary to add 0.005% or more. , addition of more than 0.040% deteriorates weldability and weld toughness, so the addition range is 0.005%.
% to 0.040%.

V: 0.01% to 0.10% V is an element effective in increasing strength through precipitation strengthening, and in order to exhibit its effect, it is necessary to add 0.01% or more. , since addition of more than 0.10% deteriorates weldability and weld toughness, the addition range is 0.01% to 0.10%.
And so.

Ti: 0.005% to 0.10% Ti is an element effective in increasing strength through precipitation strengthening, and
Although it is effective in forming iN and refining crystal grains, it is necessary to add 0.005% or more to achieve this effect, and addition of more than 0.10% deteriorates weldability and Since it deteriorates the toughness of the weld, its addition range is 0.0
05% to 0.10%.

B: 0.0005% to 0.0020% B is an element effective in increasing strength by improving hardenability, and in order to exhibit its effect, addition of 0.0005% or more is required. Moreover, addition of more than 0.0020% deteriorates weldability and weld zone toughness, so the addition range was set to 0.0005% to 0.0020%.

<<Manufacturing process>> ・Heating to {(1555+33×Si%)/2}±40°C and quenching. In order to stably achieve a low yield ratio, it is necessary to apply high Si steel and As mentioned above, it is necessary to select an appropriate heat treatment temperature. {(1555+33×Si%)/2}-4
If the temperature is below 0°C, the strength will decrease and high strength will not be obtained.
If it exceeds 1555+33×Si%)/2}+40°C, a low yield ratio cannot be stably achieved, so the heat treatment temperature should be {(1555+33×Si%)/2}±40°C.
The range of

- Tempering treatment 700 at a temperature of 700°C or less
Tempering at a temperature exceeding 70°C may cause partial austenite transformation, making it impossible to obtain the desired properties and significantly reducing toughness.
The temperature was below 0°C.

[0027]

[Examples] Specific examples of the method of the present invention will be described below.

Table 1 below shows the chemical composition of the test steel. Among these, the test steels with codes A to H are steels manufactured by the method of the present invention, and the test steels with codes I and J have a Si content lower than that specified in the present invention and are outside the specified range. ing.

[0029]

[Table 1]

Table 2 below also shows the conditions under which steel plates of various thicknesses were manufactured and their tensile properties. Of these, code A-
The test steels labeled as 1, A-2, F-1, and F-2 are shown in Table 1.
This indicates that the steels were manufactured using test steels A and F, respectively.

[0031]

[Table 2]

A-1, B, produced by the method of the present invention
The test steels C, D, E, F-1, G, and H all have a tensile strength of 60 Kgf/mm2 or more and a yield ratio of 75%.
The following results satisfy the purpose of the present invention. Further, test steel A-2 is test steel A- manufactured by the method of the present invention.
This is a comparative example of No. 1, and the temperature range of quenching (1) of the manufacturing conditions is outside the specified range of the present invention. Therefore, the tensile strength is low, less than 60 Kgf/mm2, and the yield ratio is over 75%. Furthermore, test steel F-2 is also a comparative example of F-1, and the temperature conditions for quenching (■) are outside the specified range of the present invention.
The yield ratio is significantly higher.

On the other hand, test steels I and J exhibit high yield ratios even though their manufacturing conditions satisfy the provisions of the present invention because their chemical compositions are originally outside the specified range of the present invention. In addition, test steel I is test steel A-1 manufactured by the method of the present invention,
This is a comparative example of B and C, and sample steel J is a comparative example of sample steel F-1.

[0034]

[Effects of the Invention] By implementing the method of the present invention, a high tensile strength steel having a high strength of 60 Kgf/mm2 class or higher can be stably achieved on an industrial scale, which has been extremely difficult in the past. By applying this steel to steel structures, it is expected that the safety of the structures will be further improved.

Claims (2)

[Claims] [Claim 1] C: 0.06-0.20% in weight%
, Si: 0.4 to 1.5%, Mn: 0.7 to 2.0%,
Steel containing P: 0.020% or less, S: 0.020% or less, and the remainder consisting of Fe and unavoidable impurities, is hot-rolled as it is, or after hot-rolling and quenching. (
1555+33×Si%)/2}±40°C, quenching, and tempering at a temperature of 700°C or less. [Claim 2] C: 0.06-0.20% in weight%
, Si: 0.4 to 1.5%, Mn: 0.7 to 2.0%,
Contains P: 0.020% or less, S: 0.020% or less, Cu: 0.1 to 1.2%, Ni: 0.1 to 2.0
%, Cr: 0.1-2.0%, Mo: 0.05-1.0
%, Nb: 0.005-0.040%, V: 0.01-
0.10%, Ti: 0.005-0.10%, B: 0.
Steel containing one or more of 0005 to 0.0020% and the remainder consisting of Fe and unavoidable impurities is hot-rolled as it is, or after hot-rolling and quenching,
A method for producing a low yield ratio high tensile strength steel, which comprises heating to {(1555+33×Si%)/2}±40°C, quenching, and tempering at a temperature of 700°C or lower.