JPS5852441A - Production of high strength cold rolled steel plate having good press formability - Google Patents

Abstract

PURPOSE:To obtain a cold rolled steel plate provided with good press formability and high strength at a low cost by subjecting a slab which is cast continuously under limited relations between the Ti equiv. and C equiv. in low C low N steel to cold and hot rolling and continuous annealing under limited conditions. CONSTITUTION:The steel contg. 0.001-0.008% C, 0.01-0.80% Mn, <=0.06% Sol.Al, and 0.0005-0.0030% N, contg. >=1 kind among <=0.08% Ti, <=0.01% Nb, <=0.10% Zr, having the relations between the Ti equiv. of the equationIand the C equiv. of the equation II satisfying the equation III and contg. specific contents of V, P, Si according to need is cast continuously. Such slab is heated to 1,000-1,250 deg.C and is hot-rolled at 500-780 deg.C finishing temp. and <=480 deg.C coiling temp. In succession, the hot-rolled steel plate is cold-rolled at >=50% draft and is then annealed continuously at >=660 deg.C. According to this method, the high strength cold-rolled steel plate of good press formability suited for automotive bodies, etc. is obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for manufacturing cold-rolled steel sheets with good press formability and high strength at low cost.

In recent years, various high-tensile strength cold-rolled steel sheets have been developed and put into practical use for the purpose of reducing the weight of passenger cars. but,
A steel plate that can be used in the body of a passenger car and exhibits the same performance as conventional mild steel plates while reducing vehicle weight has the following characteristics: (1) The production cost is as low as that of mild steel plates, and (2) It is press workable. It is necessary to have three characteristics: (3) the strength of the steel sheet as a panel is high, and (3) the strength of the steel sheet as a panel is high. None of the steel sheets currently in use satisfy these characteristics. It's not something to prepare for. That is, for example, as a steel sheet with good press workability, so-called IF steel (Interst.
itiatFree 5teel) has been developed.

In this IF' steel, for example, when Ti is used as a carbonitride-forming element, T1 (%) ≧ 4 (C (%)
+ -HN (%)). Therefore, the lower the C1 and the lower the N, the smaller the amount of Tl and the lower the manufacturing cost. However, in this IF steel, the Ar3 transformation temperature increases when the C becomes low, so in order to ensure good press workability as a cold-rolled steel sheet, it is necessary to increase the finishing temperature during hot rolling. In addition to conflicting with recent demands for energy conservation, the low C content reduces strength and makes the steel sheet soft, which conflicts with recent demands for high-strength steel sheets. On the other hand, to address the latter problem of strength reduction, it is possible to increase the strength of the steel plate to some extent by including solid solution strengthening elements such as P and Si, but on the other hand, it causes deterioration of weldability and surface quality. was unavoidable.

Therefore, from the above-mentioned viewpoint, the present inventors conducted research to obtain a steel plate with high strength and good press workability at a low cost.
Fine grains are produced by containing one or more of i, Zr, and Nb, precipitating these as small amounts of fine carbonitrides, and lowering the finishing temperature and coiling temperature during hot rolling. They found that if the rolling reduction ratio during cold rolling is sufficiently increased and continuous annealing is performed after cold rolling, a steel plate with high strength and r value can be obtained. .

This invention was made based on the above knowledge, and the steel is made of C: 0.001 to 0.008% and Mn: 0.
．． 01-0.80%, soL, M: 0.06%
Hereinafter, N: contains 0.0005 to O, OO30, and T1: 0.08% or less, Nb: 0.10% or less,
and zr; 0.10% or less, and T1 equivalent=Ti(@+-Nb(%)+H2r(%)-=
(1) 3 C equivalent = c (%) + 100 N (labor...
2) (C equivalent) -' (Ti equivalent) ≦0.0015 (@・
(3) The relationship between the Ti-M amount calculated by the above formula (1) and the C equivalent calculated by the above formula (2) satisfies the above formula (3), and if necessary, V : 0.01~0.
10%, P: 0.03-0.10%, and Si:
Containing one or more of 0.10 to 1.0%, and having a composition (weight% above) with the remainder consisting of Fe and unavoidable impurities, this steel is continuously cast. After forming a slab, this slab is heated to a temperature of 1000 to 1250°C, then hot rolled at a finishing temperature of 500 to 780°C and a winding temperature of 480°C or less, followed by a rolling reduction of 50 inches or more. This method is characterized by a method for manufacturing a high-strength cold-rolled steel sheet with good press formability by performing cold rolling at a high temperature and continuous annealing at a temperature of 660° C. or higher.

Next, in the method of the present invention, the reason why the component composition and manufacturing conditions are numerically limited to the above-mentioned standard will be explained.

A. Composition (a) C The C component has the effect of forming fine carbonitrides and improving the strength of the steel sheet, but if its content is less than 0.001%, the desired effect will not be achieved. On the other hand, if the content exceeds o, oos%, it will not be necessary to use many carbonitride-forming elements, but it will cause carbonitride formation. If the amount of precipitation is too large, the press workability will deteriorate, so the content was determined to be 0.001 to 0,000%.

(b) Mn The Mn component has the effect of improving the toughness of steel sheets, but if its content is less than 0.0:1%, hot embrittlement will occur, while if it exceeds O, SO%. The content was determined to be 0.01 to 0.80 because its content would make melting difficult9 and also cause an increase in cost.

(c) soL AA soLNl is contained to sufficiently deoxidize and improve the yield of carbonitride-forming elements, but in sol-M 0.
Even if the content exceeds 0.06%, no further deoxidizing effect can be obtained and the cost will increase, so the upper limit value should be set at 0.0%.
It was set at 6%.

(d) N N is preferable because the smaller the content of carbonitride-forming elements, the smaller the content of carbonitride-forming elements.
Setting the OO content to less than 5% is difficult in terms of melting, but on the other hand, if the content exceeds 0.0030%, a decrease in the r value is unavoidable even if carbonitride-forming elements are included.
Its content is 0.0005-0. OO was set at 30%.

(e) Ti, Nb, and Zr As mentioned above, these components have a uniform effect of forming fine carbonitrides and improving press formability and strength, but each of them has the following effect: 0.08%.

Even if the content exceeds 0.10% of Nb and 10% of Zr2O, no further improvement effect is observed due to the above action,
Since it also increases the cost, the upper limit of each is set to Ti.
:0.08%, Nb:0110%, and Zr:0.1
It was set as 0%. In addition, the above formulas (1) to (3) are expressed as a solid solution (
C+N) is 0.0015 (less than or equal to 0.0015), and the remaining C1N
This figure shows the relational expression for precipitating carbonitrides as carbonitrides. In addition, the reason why the upper limit value of (C equivalent, 1--(T1 equivalent)) was set as o, oox5 (@) is that when this upper limit value is exceeded,
This is because the solid solution (C+N) increases and the press formability of the steel sheet deteriorates. Furthermore, these components need to be uniformly distributed, which can be achieved by applying continuous casting with less segregation.

(f) V, P, and Sl These components have a uniform effect of further improving the strength of the steel plate, so they are included as necessary when even higher strength is required. V: 0.01
less than %, p:o, less than 03%, and 3i:O,10
%, the desired strength improvement effect cannot be obtained; on the other hand, if the content is less than v:0. lochi, P:O, 10%, and S
If the content exceeds 1.0%, the weldability and surface quality of the steel plate will deteriorate. 03～
0.10%, and f31: 0.10 to 1.0ch.

B. Manufacturing conditions (1) Slab heating temperature If the heating temperature is less than 1000°C, carbonitrides cannot be sufficiently solid-dissolved, and as a result, the steel plate cannot be strengthened. There is a need to. However, heating above 1250°C makes it difficult to refine the grains, making it difficult to improve properties and increasing costs, so heating above 1250°C is not recommended.

(2) Finishing temperature The finishing temperature in conventional hot rolling is usually 850°C or higher, but at such a high finishing temperature, the r value of the cold rolled steel sheet becomes high in the case of the steel according to the present invention. , the strength is insufficient, and it is also undesirable from the point of view of energy saving. Therefore, in the present invention, the finishing temperature during hot rolling is set at 500 to 780° C., which is lower than that of the conventional method, to impart high strength and good press formability to the steel sheet. In this case, if the finishing temperature exceeds 780°C, the ferrite phase and austenite phase will coexist, resulting in variations in properties in the longitudinal direction of the steel sheet, which is undesirable. On the other hand, if the finishing temperature is less than 500°C, the This is not preferable because it is necessary to apply a large rolling load during inter-rolling and the flatness of the steel sheet decreases.

(3) Coiling temperature When the coiling temperature during hot rolling exceeds 480°C, the precipitated carbonitrides aggregate and become coarse, making it impossible to obtain a fine grain structure. The winding temperature should not exceed 480°C, as this will reduce the strength of the material.

(4) Cold rolling reduction ratio A reduction ratio of less than 60 inches does not necessarily mean that a steel plate with a high r value cannot be obtained, but the crystal grains after continuous annealing become large and soft, and the desired high Since strength could not be obtained, the reduction ratio of cold rolling was set at 50% or more. Note that the rolling reduction ratio is desirably 73% or more.

(5) Temperature of continuous annealing For annealing, it is preferable to adopt continuous annealing in which rapid heating, short-time soaking, and rapid cooling are performed, but if the annealing temperature in this case is less than 660°C, the temperature of the cold-rolled steel sheet The continuous annealing temperature was set at 660° C. or higher because the r value was lowered and good press formability was obtained.

Next, the method of the present invention will be explained by comparing examples and comparative examples.

Example 1 C: 0.0030%, 3i: 0.010%,
Mn: 0.12%.

P: 0.0126Ib, S: 0.004%,
sot, 4: 0.001%.

Steel having a composition consisting of N: 0°0015%, Ti: 0.014%, and Fe: the remainder was continuously cast into a slab, which was then heated to 1200°C, and then finished at a finishing temperature and a coiling temperature. Hot rolling was carried out under various combinations as shown in FIG. 1 to obtain hot rolled sheets having a thickness of 4.0 mm. Note that the T1 equivalent of the steel is 0.014%.

The C equivalent is 0.0043%, which satisfies the above formula (3). Subsequently, the hot-rolled plate was pickled, cold-rolled to a thickness of 0.8 ml at a rolling reduction of 80%, and then continuously annealed at a temperature of 830°C for 1 minute. was applied. Next, the various cold-rolled steel sheets obtained as a result were subjected to a tensile test using a JI No. 85 tensile test piece to measure the tensile strength and determine the r value. These measurement results are shown in FIG. 1 in terms of the relationship between hot rolling finishing temperature and coiling temperature. In Fig. 1, ○ mark, C mark, and * mark indicate tensile strength, and ○ mark is 32 kg・f/ma.
The following items are marked C: those with a tensile strength of 32 to 34 kg・fArlt; ・marks are those with a tensile strength of 34 kg・f/++J or more; Indicates r value, 0 mark indicates 1.3 or less, 1 mark indicates 1
．． Items with an r value of 3 to 1.5, ■ mark is an r of 1.5 or more
Each item has a value. From the results shown in FIG. 1, if one or both of the finishing temperature and coiling temperature in hot rolling is out of the range of the present invention, the strength and/or r value will change. Whereas only low cold-rolled steel sheets can be obtained, if the finishing temperature and coiling temperature are within the range of this invention, high tensile strength of 34 kg・f/+j or higher and 1.5 or higher tensile strength can be obtained. It is clear that a cold-rolled steel sheet with a high r value can be obtained.

Example 2 C was varied within the range of 0.002 to o, o s%, and Si: 0.01%, Mn: 0.01%. I
B%, P:0.0:10%.

S: 0.006%, sot, M: 0.02
1%, N: 0.0020%.

Various steels containing Ti: 0.022% and the balance consisting of Fe were cast into slabs by continuous casting, and the slab was heated at a slab heating temperature of 1200°C and a finishing temperature of near 40°C.
1 coiling temperature: 4.0 by hot rolling at 400℃
A hot-rolled sheet of long thickness was prepared, and then this hot-rolled sheet was pickled, and then cold-rolled at a rolling reduction ratio of 8o% to obtain O', 8M.
Cold-rolled steel sheets with various C contents were produced by forming thick cold-rolled steel sheets and then continuously annealing them at a temperature of 800° C. for 90 seconds. The r value of this cold-rolled steel sheet is determined, and this result is used as the amount of solid solute C in the cold-rolled steel sheet, that is, the C equivalent expressed as the above equation (3). -(T1 equivalent) is shown in FIG. As shown in FIG. 2, it is clear that a high r value is exhibited when the value of equation (3) is 0.0015% or less.

Example 3 Steels having the compositions shown in Table 1 were melted and made into slabs by continuous casting. Both slabs were heated to a temperature of 1200°C, and then the steels were made into slabs having the compositions shown in Table 1. After hot rolling at finishing temperature and coiling temperature to obtain a hot-rolled sheet with a thickness of on*, it was pickled, followed by cold rolling at a rolling reduction of 280% and at a temperature of 800°C. By performing continuous annealing under the condition of holding for 90 seconds, both
．． Cold rolled steel sheets 1 to 10 of the present invention and comparative cold rolled steel sheets 1 to 6 each having a thickness of 8 mm were manufactured. In addition, comparative cold rolled steel sheets 1 to 6
All of these have either component composition or hot rolling conditions that are outside the scope of the present invention (in Table 1, applicable items are marked with *).

Next, the tensile properties, r value, and BH value were measured for the cold rolled steel sheets 1 to 10 of the present invention and comparative cold rolled steel sheets 1 to 6 obtained as a result, and the measurement results are shown in Table 1. Ta. The BH value is determined by applying a 2% pre-strain to the steel plate, then heat-treating it at a temperature of 170°C for 20 minutes, performing a tensile test in this state, and calculating the increase in yield stress caused by the heat treatment. The larger the value, the more it can contribute to weight reduction when used in automobiles and the like.

As shown in Table 1, the cold rolled steel sheets 1 to 10 of the present invention are:
All of them have high strength and high r value, that is, good press formability, whereas comparative cold rolled steel sheet 1 has a low C content, comparative cold rolled steel sheet 2 has a low N content, and comparative cold rolled steel sheet 3 has a low C content. Since the equivalent weight -7 (Ti equivalent weight) is higher than the scope of this invention, the elongation and r value are low. Comparative cold-rolled steel sheet 4 does not contain any carbonitride-forming elements, so its strength and r value are low, and comparative cold-rolled steel sheet 5 has a finishing temperature higher than the range of the present invention, so its characteristic values are low. The variation is large, and the r value is lower than that of the cold rolled steel sheets 9 and 10 of the present invention, which have the same composition. Also,
Comparative cold-rolled steel sheet 6 has a coiling temperature that is similarly high and outside the range of the present invention, and therefore has significantly low strength.

As described above, according to the method of the present invention, cold-rolled steel sheets with good press formability and high strength can be produced at low cost while saving energy, and moreover, this cold-rolled steel sheet can be used in automobiles. When used in car bodies such as cars, it can bring about industrially useful effects such as reducing the weight of the car bodies.

[Brief explanation of drawings]

Figure 1 is a graph showing the influence of finishing temperature and coiling temperature on tensile strength and r value during hot rolling, and Figure 2 is the relationship between C equivalent -4 (T 1 equivalent) and r value of steel plate. FIG. Applicant Sumitomo Metal Industries Co., Ltd. Agent Tomi 1) Kazuo

Claims (2)

[Claims] (1) C:O, OO1-0.008%, Mn-0
,01~0.80%, sol, AQ: 0.06%
The following contains N: 0.0005 to 0.0030%, and one or two of Ti: 0.08% or less, Nb: 0.10% or less, and Zr: 0.10% or less.
If more than one species is contained, T1 equivalent = 'c'i (@+
−Nb(99+ ■zr(%) −−−−−−(1) 3 2 C equivalent=C←) 10−fτN(chi) ・・・・・・
(2) (C equivalent) --! L (Ti equivalent)≦0.0015
(H)・・・・・・(3) T calculated by the above formula (1)
1 equivalent and the C equivalent calculated by the above formula (2) satisfies the above formula (3), and the steel has a composition (more than 1% by weight) consisting of Fe and unavoidable impurities, After continuous casting to form a slab, this slab is heated to a temperature of 1000 to 1250°C, then hot rolled at a finishing temperature of 500 to 780°C, 9 coiling temperature of 480°C or less, and then heated to a temperature of 50°C or less. A method for producing a high-strength cold-rolled steel sheet with good press formability, characterized by cold rolling at a rolling reduction of 660° C. or higher and continuous annealing at a temperature of 660° C. or higher. (2) C: 0.001-0.008%, Mn: 0.
01 to 0.80%, sot, AA: 0.06% or less, N: 0.0005 to 0.0030%, and T170.08% or less, Nb: 0.10% or less: and Zr: 0 Contains one or more of .10% or less, and 8 Ti-equivalent = T1 (%) + Nb (@+ 91
Zr (%) =...(1)3 C equivalent=C←)+πN)......(2
) (C equivalent) -4 (T1 equivalent)≦o, o 015
(%)...(3) T calculated using formula (1) above
1 equivalent and the C equivalent calculated by the above formula (2) satisfies the above formula (3), and further V: 0.01 to O,
, 10%, p: 0.03-0.10%. and one or two of Si:O, lO~1.0%
After continuous casting of steel containing Fe and unavoidable impurities, the slab is cast at 100%
Heating to a temperature of 0 to 1250℃, then finishing temperature: 50℃
Hot rolling at 0~'/80°C, coiling temperature: 48°C or less, followed by cold rolling at a rolling reduction of 50 inches or more,
A method for producing a high-strength cold-rolled steel sheet with good press formability, characterized by carrying out continuous annealing at a temperature of 660° C. or higher.