JP3257009B2 - Manufacturing method of high workability high strength composite structure steel sheet - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a high-workability, high-strength composite structure steel sheet, and an object of the present invention is to provide a method for producing a composite structure steel sheet suitable for use in forming members such as automobiles. .

[0002]

2. Description of the Related Art In recent years, from the viewpoint of global environmental protection, it has been desired to improve the fuel efficiency of automobiles. For this reason, there is an active movement to reduce the weight of the vehicle itself by making the body material thinner by increasing the strength. . However, increasing the strength of such a thin steel sheet for automobiles leads to a decrease in ductility, that is, a reduction in formability, and therefore, development of a material having both high strength and high workability has been desired.

In response to such a demand, Japanese Patent Application Laid-Open No. 55-145
121, JP-A-62-182224, JP-A-63-24
1120, JP-A-64-79321 and the like have been proposed many steel sheets having a composite structure utilizing the work-induced transformation of retained austenite. These steels are characterized in that the retained austenite is hardened by transformation induced martensitic transformation, does not cause local strain concentration, and has high uniform deformability. Excellent. To obtain such properties, a proper combination of chemical composition and manufacturing conditions is essential. In cold-rolled steel sheets subjected to continuous annealing, the heating temperature and holding time in the ferrite-austenite two-phase region, and the subsequent intermediate holding The importance of controlling the temperature and time, as well as the cooling rate between these temperatures, is disclosed.

[0004]

However, there is a problem that high workability cannot always be stably obtained by only the control as in the above-mentioned prior art. That is, when it is assumed that the structure is complexed by heating in the two-phase region, the structure is not completely transformed into austenite, so that the structure before heating in the two-phase region in continuous annealing greatly affects the final structure and properties. Will be.

[0005]

The present invention has been studied to solve the problems in the prior art as described above, and as a result, it has been found that an appropriate solution can be attained by improving the manufacturing conditions of such a composite structure steel sheet. I confirmed that I got it.

That is, in the present invention, the ferrite-austenite is made into a two-phase state by continuous annealing heating, and is transformed into bainite and / or martensite while a part of austenite basically remains by subsequent cooling and intermediate holding. Let it. In this case, it is well known that the control of the fraction of these structures, particularly ferrite and retained austenite, is important from the viewpoint of ensuring high strength and high workability. It is being prospered. However, there is a problem that mere control of the tissue fraction alone lacks material stability, and control of the form and size of the tissue is also important for material stabilization. For the latter, not only the heating conditions during continuous annealing but also the microstructural changes during the heating process are important factors. To control this, it is necessary to optimize not only the process after the two-phase region heating but also the heating rate. As a result, it has been newly found that the retained austenite can be uniformly and finely dispersed to stabilize the material and further improve the characteristics. Japanese Patent Application Laid-Open Nos. 61-217529 and 60-17013 disclose the importance of the heating rate. The former is for heating in the austenite single phase region, and the combination with the two-phase region heating is disclosed. Is essentially different from the gist of the present invention which requires the following. In the latter, the heating rate is 1000 ° C./min (= 16.7 ° C./sec) or more, and the concept of the two-step heating rate control of the present invention and the speed range of the second step are greatly different. The present invention is as follows.

(1) C: 0.07 to 0.3% by weight,
Si: 0.7 to 2.5%, Mn: 0.7 to 2.5%, sol. Al: 0.00
Hot rolling a steel slab containing 5 to 0.10%, N: 0.0080% or less, P: 0.02% or less, S: 0.01% or less, with the balance being Fe and unavoidable impurities; After winding at 700 ° C. or less, the scale is removed, cold-rolled, and subjected to continuous annealing in a two-phase region heating, and the temperature range from 500 ° C. to the heating rate inflection point: Tt represented by the following equation: 20 ° C / sec or more, the heating rate inflection point: Tt to Ac ₁ transformation point + 25 ° C or more
A method for producing a high-workability, high-strength composite steel sheet, comprising heating to a holding temperature lower than the Ac ₃ transformation point at a rate of 1 to 5 ° C / sec. However, 675-66 [% C]-
14 [% Mn] +22 [% Si] ≦ Tt (° C.) ≦ 725-66
[% C] -14 [% Mn] +22 [% Si]

(2) C: 0.07 to 0.3% by weight,
Si: 0.7 to 2.5%, Mn: 0.7 to 2.5%, sol. Al: 0.00
5 to 0.10%, N: 0.0080% or less, P: 0.02% or less, S: 0.01% or less, and B: 0.0005 to
0.003%, Ti: 0.005 to 0.08%, Nb: 0.005 to 0.08
%, V: 0.005 to 0.08%, Zr: 0.005 to 0.08
%, Cr: 0.2-1.0%, Mo: 0.2-1.0%, Ni: 0.2-
Hot rolled steel slab containing at least one or more selected from 1.0% and Cu: 0.2 to 1.0%, the balance being Fe and unavoidable impurities, 700 ° C or less And then subjected to scale removal, cold rolling, and continuous annealing in a two-phase region heating. sec or more, the heating rate inflection point: Tt to Ac ₁ transformation point + 25 ° C or more
High workability high strength method for producing a composite steel sheet, which comprises heating Ac ₃ a to the holding temperature of less than transformation point by heating rate of 1 to 5 ° C. / sec. However, 675-66 [% C]-
14 [% Mn] +22 [% Si] -8 [% Cr] -8 [% Mo]
≤ Tt (° C) ≤ 725-66 [% C] -14 [% Mn] + 2
2 [% Si] -8 [% Cr] -8 [% Mo]

(3) The temperature is held for 15 to 240 seconds in a holding temperature range of not less than Ac ₁ transformation point + 25 ° C. and less than Ac ₃ transformation point, 5 to 30 ° C./sec from the temperature to 600 to 750 ° C. 350-500 ° C at cooling rate of ~ 300 ° C / sec
The method for producing a high-workability, high-strength composite structure steel sheet according to the above (1) or (2), wherein the steel sheet is cooled to room temperature, kept at this temperature range for 20 to 600 seconds, and then cooled to room temperature.

[0010]

In the present invention as described above, the reasons for limiting the range of components of the steel to be treated are as follows. C: 0.07 to 0.3%. C is an austenite stabilizing element as well as a strengthening element, and is concentrated in austenite during two-phase region heating and intermediate heating holding, leaving austenite to room temperature. This retained austenite is a very important element because it undergoes plasticity-induced martensitic transformation during processing and improves formability. If it is less than 0.07%, retained austenite, which only improves workability, cannot be secured. On the other hand, when the content exceeds 0.3%, the weldability deteriorates and the weldability joint strength also decreases. Therefore, the appropriate addition amount of C is 0.07 to 0.0.
3%.

Si: 0.7-2.5%. Si is a ferrite stabilizing element, and is an important element that increases the amount of retained austenite in order to increase strength, suppress carbide formation, and promote C enrichment in austenite. To obtain this effect, the addition of 0.7% or more is necessary. However, if it exceeds 2.5%, the steel is hardened and the productivity of the steel sheet itself is greatly reduced. 0.7-2.5%.

Mn: 0.7-2.5%. Mn is an important element that enhances hardenability, suppresses the formation of pearlite, and stabilizes austenite, and must be added in an amount of 0.7% or more. However, an excessive addition of more than 2.5% forms a band-like structure and deteriorates ductility. Therefore, the range of addition
It is necessary to set it to 0.7 to 2.5%.

Sol.Al: 0.005 to 0.10%. Although sol. Al needs to be added in an amount of 0.005% or more for deoxidation, an excessive addition exceeding 0.10% degrades ductility due to an increase in nitrides. % Is the appropriate addition amount.

N: 0.0080% or less. N is an austenite-stabilized interstitial solid-solution element similar to C, but if it exceeds 0.0080%, ductility is reduced. Therefore, the amount of N must be 0.0080% or less.

P: 0.02% or less. P is a ferrite stabilizing element and has the same effect as Si, but causes embrittlement due to segregation. Therefore, 0.0
Limited to 2% or less.

S: 0.01% or less. S is desirably as small as possible to reduce ductility, and the upper limit should be 0.01%.

In the present invention, the above composition is used as a basic component, but one or more of the following elements may be added as necessary. That is, B is an element that improves the hardenability in a trace amount and has an effect of suppressing the grain growth of ferrite during cooling. However, if the amount is less than 0.0005%, the effect cannot be obtained. On the contrary, if the amount exceeds 0.003%, coarse boride is formed and the hardenability is rather lowered, so that the addition amount is 0.0005 to 0.005%. 003%.

Ti, Nb, V, and Zr form fine carbonitrides, refine the structure, and uniformly distribute the retained austenite, whereby the work-induced transformation can be effectively generated. If it is less than 0.005%, the effect cannot be obtained, and if it exceeds 0.08%, the carbonitride becomes coarse,
On the contrary, since the ductility is reduced, the amount of addition is 0.005 to 0.0.
8%.

Although Cr and Mo are ferrite stabilizing elements, they have the effect of improving hardenability and leaving austenite. The effect can be obtained at 0.2% or more. However, when the content exceeds 1.0%, the generation of stable carbides causes the reduction of retained austenite. Therefore, the added amount is 0.2
11.0%.

Cu and Ni are austenite stabilizing elements, and are effective in retaining austenite and increasing strength. This effect cannot be obtained at less than 0.2%,
%, The strength is too high, and the moldability is rather reduced. Therefore, the amount of addition is set to 0.2 to 1.0%.

The production conditions of the present invention for steel having the above composition are described below. A slab melt-cast by a conventional method is hot-rolled and wound at 700 ° C. or less. At winding temperatures above 700 ° C,
The carbide is agglomerated and coarsened, and austenite grains generated using the nucleus as a core become large and high ductility cannot be obtained. Therefore, the winding temperature is set to 700 ° C. or lower. It should be noted that there is no particular problem whether the hot rolling is direct rolling or reheating rolling.

The hot-rolled coil thus obtained is scale-removed by pickling or the like, and is cold-rolled to a predetermined thickness. Although the cold rolling reduction at this time is not particularly specified,
In order to obtain a fine recrystallized / transformed structure, it is desirable to secure a rolling rate of 50% or more. Then, continuous annealing is performed.

The two-step heating rate control in the continuous annealing is as follows.
As can be seen from the influence of the first and second heating rates on the overhang height (LDHo) in the plane strain state, which is the main deformation mode of the press forming of the thin steel sheet in FIG. What to do. The first stage is faster, and the second stage is slower, whereby the moldability is improved. That is, the heating rate from room temperature to 500 ° C. is arbitrary, but the heating rate inflection point from 500 ° C .:
Up to Tt, it is a recovery / recrystallization area, where the heating rate is 2
If the temperature is lower than 0 ° C./sec, since slow recrystallization of ferrite proceeds, the ferrite grains are not fine, and the second phase is easily aggregated and coarsened. It becomes impossible to obtain a uniform and finely dispersed state which is desirable for causing continuous plasticity-induced transformation as the process proceeds. Also, from Tt to Ac ₁ transformation point + 25 ° C
Up to the holding temperature lower than the Ac ₃ transformation point, it is necessary to control the heating rate to 5 ° C./sec or less, to progress the transformation slowly in order to secure the retained austenite, and to concentrate C in the austenite. . Such a concentration of C may occur during the heating and holding, but in the latter case, a remarkable structural change such as grain growth is accompanied, so that the former control is important. However, if the heating rate is less than 1 ° C./sec, it takes a long time to reach the holding temperature, so that an increase in the line length and an increase in the size of the equipment are indispensable, which is a cause of impairing economic efficiency. Therefore, the heating rate is from 500 ° C to Tt
Temperature range up to 20 ° C / sec, Tt to Ac ₁ transformation point +
1 to 5 ° C. until the holding temperature below 25 ° C. or higher Ac ₃ transformation point /
Seconds.

The manufacturing conditions corresponding to the data in FIG. 1 are within the scope of the present invention, and are specified in the examples. Here, the heating rate inflection point: Tt is basically a function of the Ac ₁ transformation point and a component that governs the decomposition rate of carbide, and is approximately represented by the following equation within the component range of the present invention. 675-66 [% C] -14 [% Mn] +22 [% Si]-
8 [% Cr] -8 [% Mo] ≦ Tt (° C) ≦ 725-66%
[C] -14 [% Mn] +22 [% Si] -8 [% Cr] -8
[% Mo]

If the heating inflection point is lower or higher than the temperature represented by the above-mentioned components as shown in FIG. 2, the moldability is reduced. If the temperature is low, the concentration of C does not sufficiently occur. Conversely, if the temperature is high, the structure becomes coarse and the formability is lowered. Therefore, the temperature must be within the above range.
The vertical axis in the figure is the median Tt mean (° C.) of the heating rate inflection point expressed by the above equation = 700−66 [% C] −14.
Plotting the ratio of the temperature difference from Tt mean to LDHo when the heating temperature inflection point is changed, where LDHo in [% Mn] +22 [% Si] -8 [% Cr] -8 [% Mo] is 1 It was done. Except for the heating temperature inflection point, both the composition and the production conditions are within the scope of the present invention.

Next, regarding the heating temperature, the Ac ₁ transformation point +
When the temperature is lower than 25 ° C., the amount of austenite in the two-phase region is insufficient, so that the amount of austenite finally remaining does not reach an amount sufficient to exert the effect of the work-induced transformation. Conversely, when the temperature is equal to or higher than the Ac ₃ transformation point, the austenite single phase region is formed, so that austenite does not remain without concentration of C occurring. Therefore, the heating temperature is set to be at least the Ac ₁ transformation point + 25 ° C and less than the Ac ₃ transformation point.

If the holding time in this temperature range is less than 15 seconds, the concentration of C and Mn in austenite is not sufficiently performed, so that even if a low heating rate is assumed, the final residual austenite is not removed. It becomes impossible to secure. Meanwhile, 24
Holding for more than 0 seconds must be avoided because it leads to coarsening of crystal grains and ultimately lowers ductility. Therefore,
The holding time is 15 to 240 seconds.

In the primary cooling from this temperature to 600 to 750 ° C., ferrite transformation proceeds, and C is further enriched in austenite. If the cooling rate is lower than 5 ° C./sec, austenite is decomposed into pearlite, so that retained austenite cannot be secured. Conversely, if the rate is higher than 30 ° C./sec, the sufficient austenite C concentration does not occur, so that the retained austenite decreases. If the end temperature of the primary cooling is higher than 750 ° C., the secondary rapid cooling starts without sufficiently enriching C in austenite, and if it is lower than 600 ° C., the austenite is decomposed into pearlite. , High ductility cannot be obtained. Thus, Ac ₁ + 50 ℃ above Ac ₃
Primary cooling from a holding temperature of less than 600 to 750 ° C at a rate of 5 to 30 ° C / sec.

Next, as a secondary cooling, cooling is performed from 600 to 750 ° C. to 350 to 500 ° C. at a rate of 30 to 300 ° C./sec. This is to transform austenite sufficiently enriched in C into bainite without decomposing into pearlite. If the cooling rate is lower than 30 ° C./sec, pearlite decomposition occurs, and if it exceeds 300 ° C./sec, the ferrite becomes acicular and the formability decreases. If the cooling is stopped at a temperature higher than 500 ° C., carbides are generated, and austenite does not remain at room temperature. Conversely, if the temperature is lower than 350 ° C., austenite is transformed into martensite, so that a work-induced transformation effect cannot be obtained.

After this secondary cooling, in this temperature range, 20 to 60
Hold for 0 seconds. More C during bainite transformation here
Such an austenite stabilizing element is distributed, so that bainite containing almost no C and residual austenite in which the austenite stabilizing element is concentrated until the Ms point becomes lower than room temperature. If the holding time is shorter than 20 seconds, sufficient transformation of bainite does not proceed, so that austenite undergoes martensite transformation during cooling to room temperature. on the other hand,
If the time is longer than 600 seconds, the C-enriched austenite is decomposed into bainite and carbide, and as a result, the work-induced transformation effect cannot be obtained.

The heating rate, the holding temperature and the cooling rate do not need to be constant within the range of the steps described above, and the effects of the present invention are not impaired even if they are varied within the range. .

[0032]

EXAMPLES Specific examples of the present invention will now be described. Some examples of steels according to the present invention and comparative steels prepared specifically by the present inventors are shown in Table 1 below. In Table 2, the transformation point and the plate thickness are also shown.

[0033]

[Table 1]

The hot rolling for each steel in Table 1 was 9
Finished at 0.05 to 920 ° C and reduced to 3.2 mm. After the scale was removed, the sheet thickness was as shown in Table 1 by cold rolling, and continuous annealing was performed. The thermal cycle of the continuous annealing is schematically shown in FIG. 3, and the specific production process and the characteristic results after the temper rolling of 0.8% are shown in Tables 2 to 5 (Tables 2 and 3 below). Inventive steels, Tables 4 and 5 show comparative steels)
As characteristics, strength was evaluated by a JIS No. 5 tensile test piece, and formability was evaluated by a plane strain forming height: LDHo (test conditions are shown in FIG. 1).

[0035]

[Table 2]

[0036]

[Table 3]

[0037]

[Table 4]

[0038]

[Table 5]

That is, according to the results of Tables 2 to 5, the steels of the present invention are more excellent in formability than comparative steels of the same strength class. In addition, 44 of the comparative steels,
45, 47 to 52, 55, 56, 60, 63, 68, 7
0, 73, and 74 are heating rates outside the range of the present invention.
3, 57, 58, 64-69, 71, 72 have heating rate inflection points outside the range of the present invention, and 54, 62 have first stage heating temperatures.
Ac ₃ or more and out of the range, 59 has a winding temperature out of the range, and 61 has a low heating temperature and is out of the range, resulting in poor moldability. FIG.
The data in Table 5 are arranged as a balance between the tensile strength and the formed height of plane strain, and it can be confirmed that the steel of the present invention is clearly superior to the comparative steel in any case.

[0040]

According to the present invention as described above, it is possible to accurately manufacture and provide a composite structure steel sheet having high workability and high strength. The present invention is industrially effective because it provides stable and advantageous use.

[Brief description of the drawings]

FIG. 1 is a table summarizing the effects of the first and second heating rates on the forming height (LDHo) of plane distortion, which is the main deformation mode of press forming of a steel sheet.

FIG. 2 is a chart showing a distribution range of a heating rate inflection point.

FIG. 3 is an explanatory view of a continuous annealing heat cycle according to the present invention.

FIG. 4 is a table showing the relationship between tensile strength and plane strain forming height for the steel according to the present invention and comparative steel.

──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Yuki Osaki 1-1-2 Marunouchi, Chiyoda-ku, Tokyo Nippon Kokan Co., Ltd. (72) Inventor Yasuyuki Takada 1-1-2 Marunouchi, Chiyoda-ku, Tokyo Japan (56) References JP-A-2-217425 (JP, A) JP-A-1-184226 (JP, A) JP-A-63-195221 (JP, A) (58) Fields investigated (Int .Cl. ⁷ , DB name) C21D 9/46-9/48 C21D 8/00-8/04

Claims (3)

(57) [Claims] C .: 0.07 to 0.3% by weight, Si: 0.3% by weight.
7 to 2.5%, Mn: 0.7 to 2.5%, sol.Al: 0.005 to 0.10%, N:
0.0080% or less, P: 0.02% or less, S: 0.01% or less, with the balance being
Hot rolling steel slab consisting of Fe and unavoidable impurities,
After winding at 700 ° C. or less, the scale is removed, cold-rolled, and subjected to continuous annealing in a two-phase region heating, and the temperature range from 500 ° C. to the heating rate inflection point: Tt represented by the following equation: 20 ° C./sec or more, the heating rate inflection point: from Tt to the holding temperature lower than the Ac ₁ transformation point + 25 ° C. and less than the Ac ₃ transformation point is 1 to 5.
A method for producing a high-workability, high-strength composite structure steel sheet, wherein the steel sheet is heated at a heating rate of ° C / sec. However, 675-
66 [% C] -14 [% Mn] +22 [% Si] ≦ Tt (℃)
≤725-66 [% C] -14 [% Mn] +22 [% Si] 2. C: 0.07 to 0.3% by weight, Si: 0.3% by weight.
7 to 2.5%, Mn: 0.7 to 2.5%, sol.Al: 0.005 to 0.10%, N:
0.0080% or less, P: 0.02% or less, S: 0.01% or less, B: 0.0005 to 0.003%, Ti: 0.005 to 0.08%, Nb: 0.00
5 to 0.08%, V: 0.005 to 0.08%, Zr: 0.005 to 0.08%, C
r: 0.2 to 1.0%, Mo: 0.2 to 1.0%, Ni: 0.2 to 1.0%, Cu: 0.2 to 1.0%
%, At least one selected from the group consisting of Fe and inevitable impurities is hot-rolled, rolled up at 700 ° C. or less, scale removed, cold-rolled, In performing the continuous annealing in the two-phase zone heating, the temperature range from 500 ° C. to the heating rate inflection point: Tt represented by the following formula is 20 ° C./sec or more, and the heating rate inflection point:
High workability high strength method for producing a composite steel sheet, which comprises heating a heating rate of 1 to 5 ° C. / sec from Tt to the holding temperature of Ac less than ₁ transformation point + 25 ° C. or higher Ac ₃ transformation point.
However, 675-66 [% C] -14 [% Mn] +22 [%
Si] -8 [% Cr] -8 [% Mo] ≦ Tt (° C.) ≦ 725-6
6 [% C] -14 [% Mn] +22 [% Si] -8 [% Cr]
-8 [% Mo] 3. The temperature is kept for 15 to 240 seconds in a holding temperature range of not less than Ac ₁ transformation point + 25 ° C. and less than Ac ₃ transformation point.
5 to 30 ° C / sec from 00 to 750 ° C, 3
3. The method according to claim 1, wherein the cooling is performed at a cooling rate of 0 to 300 [deg.] C./sec to 350 to 500 [deg.] C., the temperature is maintained in the temperature range for 20 to 600 seconds, and then cooled to room temperature. A method for producing a high workability, high strength composite structure steel sheet