JP2000212680A - Non-heat treated high tensile strength steel plate small in reduction of yield stress by bauschinger effect and its production - Google Patents

Abstract

PROBLEM TO BE SOLVED: To produce a non-heat treated high tensile strength steel plate small in the reduction of Bauschinger effect without requiring the addition of a large amt. of Cr, and to provide a method for producing the same. SOLUTION: A steel slab having a compsn. contg., by weight, 0.03 to 0.12% C, 0.05 to 0.30% Si, 0.30 to 2.00% Mn, 0.005 to 0.06% Nb, 0.01 to 0.10% Al and <=0.007% N, moreover contg. one or >= two kinds selected from 0.05 to 1.30% Cu, 0.10 to 10.0% Ni, 0.05 to 1.50% Cr, 0.03 to 0.50% Mo, 0.01 to 0.15% V, <=.070% Ti, 0.0005 to 0.0040% Ca and 0.001 to 0.020% rare earth metals, and the balance Fe with inevitable impurities is subjected to hot rolling so as to control the rolling finishing temp. to >=(Ar3-50 deg.C), and, after the completion of the rolling, it is subjected to forced cooling to the temp. range of 400 to 700 deg.C.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a non-refined high-strength steel sheet having a tensile strength of 490 MPa or higher used for line pipes, pressure vessels, construction machinery, shipbuilding, bridges, tanks, and the like, and a method for producing the same. .

[0002]

2. Description of the Related Art In general, when a steel sheet as rolled is subjected to a cold tensile process and further subjected to a compression process, the load-elongation diagram in the tensile test of the steel changes, and the yield stress (load-elongation) is changed. When the yield point does not appear clearly in the diagram, the yield strength is used instead.In the present invention, the value of 0.5% proof stress is used, the same applies hereinafter), a phenomenon called the "Bausinger effect", which is lower than that of the as-rolled state. Can be seen. Therefore, in the production of a steel sheet for line pipes, the production conditions are designed in consideration of the reduction in yield stress caused in the process of pipe making and forming. Increasing the strength of the as-rolled steel sheet in this way is effective in ensuring the strength after pipe making and forming, but on the other hand, in order to achieve higher strength, adding alloy elements or Since severe control rolling is required, the weldability and the economic efficiency are reduced. In addition, when the strength of the steel sheet is high, a greater force is required at the time of processing, which also causes a reduction in processing efficiency and processing accuracy.

As a method of reducing the amount of reduction in yield stress due to the Bauschinger effect, it is effective to pay attention to the load-elongation diagram of a steel sheet as rolled and to make this a round curve without a yield point. It is said that there is. Japanese Patent Publication No. 53-25801 proposes a means for suppressing the Bauschinger effect by employing a special component system of low C and high Cr.

[0004]

However, in order to obtain a load-elongation diagram of such a shape, it is necessary to prepare a metal structure mainly composed of bainite and martensite by accelerating cooling and preparing chemical components. This may cause an increase in surface hardness and a decrease in weldability. Further, a large amount of Cr is not a method that can be put to practical use because it has disadvantages such as deterioration of weldability and the like. Therefore, the present invention has the conventional technology,
It is an object of the present invention to improve the above-mentioned problems and to propose a non-heat treated high-strength steel sheet which does not require a large amount of Cr addition, has little yield stress reduction due to the Bauschinger effect, and a method for producing the same.

[0005]

The gist of the present invention is as follows. (1) C: 0.03 to 0.12 wt%, Si: 0.05 to 0.30 wt%, Mn:
0.30 to 2.00 wt%, Nb: 0.005 to 0.06 wt%, Al: 0.01 to
0.10wt%, N: 0.007wt% or less, and Cu: 0.05
-1.30wt%, Ni: 0.1-100.0wt%, Cr: 0.05-1.50wt
%, Mo: 0.03 ~ 0.50wt%, V: 0.01 ~ 0.15wt%, T
i: 0.070 wt% or less, Ca: 0.0005 to 0.0040 wt%, REM:
Any one or two selected from 0.001 to 0.020 wt%
Containing at least one species, the balance being composed of Fe and unavoidable impurities, characterized by a yield ratio of at least 90% and a yield elongation of at least 2.7%. Quality high tensile steel plate.

(2) C: 0.03-0.12 wt%, Si: 0.05-0.3
0wt%, Mn: 0.30 ~ 2.00wt%, Nb: 0.005 ~ 0.06wt
%, Al: 0.01 to 0.10 wt%, N: 0.007 wt% or less, and Cu: 0.05 to 1.30 wt%, Ni: 0.1 to 10.0 wt%,
Cr: 0.05-1.50 wt%, Mo: 0.03-0.50 wt%, V: 0.01
~ 0.15wt%, Ti: 0.070wt% or less, Ca: 0.0005 ~ 0.00
A steel slab containing at least one selected from 40 wt% and REM: 0.001 to 0.020 wt%, the balance being Fe and unavoidable impurities, and a rolling end temperature of (Ar ₃ -50 ° C.) Production of non-heat treated high-strength steel sheets with low yield stress reduction due to the Bauschinger effect, characterized by hot rolling as described above and forcibly cooling the temperature range from 400 to 700 ° C after rolling is completed. Method.

[0007]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The reasons for limiting the composition of the components and the production conditions in the present invention to the above ranges will be described below. C: 0.03 to 0.12 wt% C is an element necessary for securing hardenability and strength. To obtain the target strength, it is necessary to add 0.03 wt% or more. On the other hand, if the addition amount exceeds 0.12 wt%, the base metal toughness and the heat affected zone (hereinafter referred to as “HA”)
Z "), the toughness deteriorates, so the upper limit is 0.12 wt%. When the amount of C increases, the ratio of the ferrite structure decreases and the yield elongation described below decreases, and the tensile strength of the second phase structure increases and the yield ratio decreases. These characteristics lead to an increase in the yield stress reduction due to the Bauschinger effect. Therefore, it is desirable to reduce the C content as much as possible within the above range.

Si: 0.05 to 0.30 wt% Si is an element effective in promoting deoxidation and increasing strength. To achieve these effects, 0.05wt%
It is necessary to add the above, but if added excessively, it deteriorates the base material toughness and the HAZ toughness.
Add within the range. Note that increasing the Si content increases the tensile strength of the second phase structure, lowers the yield ratio, and leads to an increase in the Bauschinger effect. Therefore, it is desirable to reduce the Si content within the above range as much as possible.

Mn: 0.30 to 2.00 wt% Mn is a useful element that increases strength without impairing toughness. To achieve this effect, at least 0.30wt
% Or more is required. However, if added excessively, the workability deteriorates, so the upper limit of the amount added is 2.00 wt%. When the amount of Mn added increases, the strength of the second phase structure increases and the yield ratio decreases, leading to an increase in the Bauschinger effect. Therefore, it is desirable to reduce the Mn within the above range as much as possible.

Nb: 0.005 to 0.06 wt% Nb is an element that prevents the austenite grains from becoming coarse during slab heating, and is effective in reducing the grain size and strengthening during rolling.
In order to exert these effects, it is necessary to add 0.005 wt% or more. However, if the Nb content exceeds 0.06% by weight, the HAZ toughness deteriorates. Therefore, the Nb content is added in the range of 0.005 to 0.06% by weight.

Al: 0.01-0.10 wt% Al is at least 0.01% for deoxidation of steel and refining of the structure.
It is necessary to add wt%, but if it is added excessively, a large amount of oxide-based inclusions will be generated in the steel and the toughness will be significantly deteriorated. Therefore, the upper limit is set to 0.10 wt%.

N: 0.007 wt% or less N is an element that combines with Al to form AlN and contributes to preventing the crystal grains from becoming coarse when the slab is heated. However, when N is contained in a large amount, the HAZ toughness is deteriorated.
It shall be 7wt% or less.

In addition to the basic components described above, at least one of the following elements is added for improving the strength and / or toughness. Cu: 0.05-1.30 wt% Cu is an element effective for solid solution strengthening and precipitation strengthening. In order to obtain these effects, 0.05 wt% or more (especially,
In order to utilize precipitation strengthening, it is necessary to add 0.5 wt% or more). However, even if added in excess of 1.30 wt%, no further effect can be obtained, so it is added in the range of 0.05 to 1.30 wt%. However, when the added amount of Cu increases, the strength of the second phase structure increases and the yield ratio decreases, leading to an increase in the Bauschinger effect. Therefore, it is desirable to reduce the Cu amount as much as possible within the above range.

Ni: 0.10 to 10.0 wt% Ni has an effect of greatly improving toughness, and is an element that is desirably added for low-temperature applications. Such an effect can be obtained by adding 0.10 wt% or more. However, if the content exceeds 10.0 wt%, further effects cannot be obtained.
The amount is in the range of 0.1 to 10.0 wt%. When the amount of Ni added increases, the strength of the second phase structure increases and the yield ratio decreases, leading to an increase in the Bauschinger effect. Therefore, it is desirable to reduce the Ni content as much as possible within the above range.

Cr: 0.05-1.50 wt% Cr is an element effective for securing the strength of steel, and its effect can be obtained by adding 0.05 wt% or more. However, excessive addition of Cr deteriorates the weldability, so the Cr content is 1.50wt%
Is the upper limit.

Mo: 0.03 to 0.50 wt% Mo is a useful element that improves the strength and toughness of steel even in a small amount. These effects are exhibited when added in an amount of 0.03 wt% or more, but if added excessively, the weldability is deteriorated. Therefore, the amount of Mo is set to 0.03 to 0.50 wt%. When the amount of Mo added increases, the strength of the second phase structure increases and the yield ratio decreases, leading to an increase in the Bauschinger effect. Therefore, it is desirable to reduce the amount of Mo as much as possible within the above range.

V: 0.01 to 0.15 wt% V is an element effective for increasing the strength by precipitation strengthening. In order to exert this effect, it is necessary to add 0.01 wt% or more. However, if it is added in excess of 0.15 wt%, the weldability and the HAZ toughness deteriorate, so it is added within the above range.

Ti: 0.070 wt% or less Ti is an element that contributes to preventing coarsening of the welded portion and improving the strength by precipitation strengthening. But the amount is 0.07
If it exceeds 0 wt%, the toughness will deteriorate.

Ca: 0.0005 to 0.0040 wt% Ca is an element that improves the toughness by making MnS spherical. This effect is exerted when 0.0005 wt% or more is added, but when it exceeds 0.0040 wt%, oxide-based inclusions increase and toughness is deteriorated. Therefore, the upper limit is 0.0040 wt%. And

REM: 0.001 to 0.020 wt% REM (rare earth) improves toughness by the same mechanism as Ca. To achieve this effect, at least 0.00
Addition of 1 wt% is necessary, but if it exceeds 0.020 wt%, oxide inclusions increase and the toughness deteriorates.
It is added in the range of 1 to 0.020 wt%.

Next, the manufacturing conditions in the present invention will be described. Generally, to ensure the strength and toughness of non-heat treated steel,
It is effective to perform hot rolling by controlled rolling. In the present invention, the rolling end temperature of the controlled rolling is set to (Ar ₃
(Point -50 ° C) or higher. Because
When rolling is completed at a temperature less than (Ar ₃ points-50 ° C),
This is because the dislocation density of the ferrite structure increases, the deformation thereof decreases, and the yield elongation value decreases. This is extremely disadvantageous in keeping the Bauschinger effect low. Therefore, the lower limit of the rolling end temperature is (Ar ₃ points−50 ° C.).

Further, after the above-mentioned controlled rolling, 400 to 700
Cool down to ℃. The method of forced cooling may be not only water cooling but also gas accelerated cooling. By performing such cooling, the strength can be secured without increasing the dislocation density of the ferrite structure.
If the cooling control range exceeds 700 ° C., the effect is not sufficient, and if it is less than 400 ° C., the surface hardness increases and the shape becomes poor after cooling.
After rolling, the temperature was raised to 400 to 700 ° C.

With the above-described steel composition and manufacturing conditions, it is possible to manufacture a non-heat treated high-strength steel sheet with a small decrease in yield stress (0.5% proof stress when no yield point occurs) due to the Bauschinger effect. As means for suppressing the Bauschinger effect to a low level, it is important to set the yield ratio to 90% or more and the yield elongation to 2.7% or more. Hereinafter, this point will be described. FIG. 1 schematically shows a load-elongation diagram measured for a comparative steel plate manufactured under conditions not following the method of the present invention and FIG. 2 shows a steel plate manufactured under conditions meeting the method of the present invention. 1 and 2, (a) is a load-elongation diagram of the as-rolled state, and (b) is a load-elongation diagram after bending and unbending. 1 and 2, the difference in yield stress due to the Bauschinger effect (the difference Δ between the as-rolled yield strength and the yield strength after bending and unbending) is shown.
When the invention example having a small amount of decrease represented by YS) is compared with the comparative example in which the Bauschinger effect is as large as the conventional example, there is a difference between the two in the as-rolled state in the following points. a) The elongation of the yield shelf from the yield point until the load starts to increase, that is, the "yield elongation" is larger in the invention examples. b) The rate of increase in load from the end point of the yield shelf to the maximum load is smaller in the invention example, and therefore, the yield ratio is large.

Based on these phenomena, the relationship between the yield stress reduction ΔYS due to the Bauschinger effect and the yield elongation or yield ratio was investigated. The results are shown in FIGS. 3 to 5 show that the yield ratio is 90% or more and the yield elongation is 2.7%.
%, The Bauschinger effect can be suppressed to a low level. From the above, according to the present invention, it is possible to achieve a yield ratio of 90% or more and a yield elongation of 2.7% or more by appropriately controlling the chemical components, the rolling end temperature, and the cooling after rolling. It can be said that through this characteristic, it is possible to manufacture a non-heat treated high-strength steel sheet with a small decrease in yield stress due to the Bauschinger effect. Further, according to the production method of the present invention, it is not necessary to use a metal structure mainly composed of bainite or martensite. Is possible.

[0025]

EXAMPLE Steel having the composition shown in Table 1 was subjected to controlled rolling and forced cooling by water cooling according to the production conditions shown in Table 2. A rectangular specimen was sampled from the direction perpendicular to the rolling direction of the steel sheet thus obtained and subjected to a tensile test. A 30-inch (762 mm) φ pipe was formed using this steel sheet, and a similar test piece was collected from the pipe and subjected to a test.
The results are also shown in Table 2. From Table 2, it can be seen that the invention example has a smaller decrease in YS as compared with the comparative example having the same level of strength. The inventors conducted further experiments,
It has been confirmed that such effects can be widely applied to 490 to 700 MPa grade steel.

[0026]

[Table 1]

[0027]

[Table 2]

[0028]

As described above, according to the present invention,
By optimizing both the chemical composition, the controlled rolling end temperature, and the cooling method, the non-heat treated high-strength steel with reduced Bauschinger effect is provided with a yield ratio of 90% or more and a yield elongation of 2.7% or more. can do.

[Brief description of the drawings]

FIG. 1 is a load-elongation diagram of a comparative steel as-rolled and after a bending-bending process.

FIG. 2 is a load-elongation diagram of the invention steel as-rolled and after bending-unbending.

FIG. 3 is a diagram showing the relationship between yield elongation, yield ratio, and ΔYS.

FIG. 4 is a diagram showing a relationship between a yield ratio and ΔYS.

FIG. 5 is a diagram showing a relationship between yield elongation and ΔYS.

Claims (2)

[Claims] 1. C: 0.03 to 0.12 wt%, Si: 0.05 to 0.30 wt%, Mn: 0.30 to 2.00 wt%, Nb: 0.005 to 0.06 wt%, Al: 0.01 to 0.10 wt%, N: 0.007 wt% Including the following, Cu: 0.05 to 1.30 wt%, Ni: 0.1 to 10.0 wt%, Cr: 0.05 to 1.50 wt%, Mo: 0.03 to 0.50 wt%, V: 0.01 to 0.15 wt%, Ti: 0.070 wt% Hereinafter, one or more selected from Ca: 0.0005 to 0.0040 wt% and REM: 0.001 to 0.020 wt% are contained, and the balance is composed of Fe and unavoidable impurities.
A non-heat treated, high-strength steel sheet having a yield stress of at least 90% and a yield elongation of at least 2.7% with little decrease in yield stress due to the Bauschinger effect. 2. C: 0.03 to 0.12 wt%, Si: 0.05 to 0.30 wt%, Mn: 0.30 to 2.00 wt%, Nb: 0.005 to 0.06 wt%, Al: 0.01 to 0.10 wt%, N: 0.007 wt% Including: Cu: 0.05-1.30 wt%, Ni: 0.110-0.0 wt%, Cr: 0.05-1.50 wt%, Mo: 0.03-0.50 wt%, V: 0.01-0.15 wt%, Ti: 0.070 wt% A steel slab containing one or more selected from the group consisting of Ca: 0.0005 to 0.0040 wt% and REM: 0.001 to 0.020 wt%, with the balance being Fe and unavoidable impurities, is subjected to a rolling end temperature. Is hot rolled so as to be equal to or more than (Ar ₃ -50 ° C.), and after the rolling is completed, the temperature range from 400 to 700 ° C. is forcibly cooled. Manufacturing method of tempered high strength steel sheet.