JPH0681045A - Production of cold rolled steel sheet excellent in workability and baking hardenability - Google Patents

Abstract

PURPOSE:To produce a cold rolled steel sheet excellent in workability and baking hardenability by subjecting a slab of a dead-soft steel containing specific small amounts of Ti, Nb, etc., and further one or >=2 elements among Mo, W, and Cr to hot rolling at specific temp., to cold rolling, and to continuous annealing at specific temp. CONSTITUTION:A slab of a steel, prepared by adding and incorporating, by weight, 0.01-0.10%, in total, of either or both of Ti for improving workability and Nb for improving baking hardenability into a dead-soft steel containing <0.007% C, <0.8% Si, <1% Mn, <0.15% P, <0.020% S, 0.01-0.1% Al, and <0.01% N and also incorporating, as baking hardenability improving elements, 0.001-3.0% of one or >=2 elements among Mo, W, and Cr, is hot-rolled at a temp. in the range between 700 deg.C and <A3 transformation point. The resulting hot rolled plate is coiled at >=500 deg.C, cold-rolled at 50-90% draft, and continuously annealed at a temp. in the range between the recrystallization temp. and the Ar3 transformation point.

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 cold rolled steel sheet having excellent workability and bake hardenability.

[0002]

As a method for improving the workability and bake hardenability of a cold rolled steel sheet, for example, Japanese Patent Publication No. 55-1415.
No. 26, Japanese Patent Publication No. 55-141555, N
In the b-added steel, Nb is added according to the C, N, and Al contents in the steel, and at. Disclosed is a method of adjusting the solid solution C and the solid solution N in the steel sheet by limiting Nb / (solid solution C + solid solution N) in a certain range in%, and further controlling the cooling rate after annealing. ing. Further, Japanese Patent Publication No. 61-45689 discloses that a steel sheet having excellent bake hardenability is obtained by adding Ti and Nb in combination. However, even with such a method, it is still difficult to obtain a satisfactory result on an industrial scale, and the bake hardening amount is small.

Further, as a method for producing a steel plate for ultra-deep drawing having no bake hardenability, there are Ti-killed steel plate (Japanese Patent Publication No. 61-45689) and Nb-killed steel plate (Japanese Patent Publication No. 54-1245). Two strains are disclosed. However, these steel sheets have excellent workability because C and N in the steel sheet are completely fixed as precipitates such as Ti or Nb, but strain aging phenomenon occurs during coating baking after pressing. Therefore, it has no bake hardenability. Thus, there is a strong demand for improvement in both the workability and the bake hardenability of the cold rolled steel sheet.

[0004]

SUMMARY OF THE INVENTION The present invention provides a method for manufacturing a cold rolled steel sheet which is excellent in workability and bake hardenability so as to sufficiently satisfy such requirements.

[0005]

The feature of the present invention resides in that when a cold rolled steel sheet is manufactured, C:
0.007% or less, Si: 0.8% or less, Mn: 1% or less, P: 0.15% or less, S: 0.020% or less, A
L: 0.01 to 0.1%, N: 0.01% or less, and steel consisting of inevitable impurities, added with 0.01 to 0.10% in total of one or two of Ti and Nb. In addition, steel with a total amount of 0.001 to 3.0% of one or more of Mo, W and Cr added at 700 ° C. or higher, A
After hot rolling at less than ₃ transformation points, coiling at a temperature of 500 ° C. or more, then cold rolling at 50 to 90%, and continuous annealing at a temperature of not less than the recrystallization temperature and not more than A ₃ points and workability. This is a method for producing a cold rolled steel sheet having excellent bake hardenability.

The cold-rolled steel sheet to which the present invention is applied is a so-called cold-rolled steel sheet that is not plated, a plated steel sheet that is plated with zinc or the like, and any of the methods for producing steel may be a converter, an electric furnace, an open hearth furnace, or the like. The method may be any method, such as a slab that is lumped after casting with a mold, or a slab that is cast by continuous casting.

The inventors of the present invention have conducted various studies to improve the workability and bake hardenability of cold rolled steel sheets, and as a result, in% by weight, C: 0.007% or less, Si: 0. 8
% Or less, Mn: 1% or less, P: 0.15% or less, S:
0.020% or less, Al: 0.01 to 0.1%, N:
Steel containing less than 0.01% and unavoidable impurities, T
The total amount of one or two of i and Nb is 0.01 to 0.
Steel containing 10% added and further 0.001 to 3.0% of the total amount of one or more of Mo, W, and Cr added was hot rolled at 700 ° C or higher and below the A ₃ transformation point. After 50
By winding at a temperature of 0 ° C. or higher, then cold rolling at 50 to 90%, and then performing continuous annealing at a temperature of recrystallization temperature or higher and A ₃ point or lower, workability and aging are excellent, and further, It was found that the bake hardenability was significantly improved. In the conventional cold-rolled steel sheet described above, the workability is excellent but it does not have bake hardenability, or even if it has bake hardenability, its amount is small, and it also impairs aging. They were incompatible, unstable, and poor in workability.

The reasons for limiting the components of the steel of the present invention are as follows. First, C: 0.007% or less means that C is a strengthening element of steel when it exceeds that, strength is increased, workability is impaired, and one or two elements of Ti and Nb are used. The reason for this is that the amount of addition of a large amount increases the strength due to precipitates, which leads to poor workability and economical disadvantage. Further, if C is high, even if the cold rolling rate is low, a steel sheet with good workability is obtained, and low temperature finishing in hot rolling for leaving a lot of working strain is not necessary, and the effect of lowering C and improving workability is obtained. Because it is not.

Si: 0.8% or less is because Si is a strengthening element of steel and increases in strength and impairs workability if it exceeds it. This is because it is difficult to adhere and the adhesion is impaired. Mn is set to 1% or less because Mn is a strengthening element of the steel and the strength is increased and the workability is impaired if Mn is exceeded.

P is set to 0.15% or less because it is a strengthening element of steel even with a small amount of P, and the strength is increased and the workability is impaired. Moreover, P is concentrated in the grain boundary. , Is an element that easily causes grain boundary embrittlement, and if added in excess of this, workability is impaired.

S: 0.020% or less is essentially an element that is meaningless to be present in steel, and if it is added in excess of that, hot rolling will occur if there are few sulfide-forming elements such as Mn. This is because it sometimes causes red heat embrittlement and cracks at the surface, so-called hot embrittlement.

Al: 0.01 to 0.1% and the lower limit thereof is set to 0.01%, because if it is less than that, N is precipitated as AlN and it becomes difficult to improve the aging property by N. Is. Further, the upper limit is set to 0.1%, the improvement of aging is saturated even if it is added beyond that,
Moreover, the strength is increased and the workability is impaired.

[0013] N: 0.01% or less, because if added over that amount, the aging cannot be secured unless the amount of Al added is increased, and moreover, the strength becomes high and the workability is impaired. Is.

The total amount of one or two kinds of Ti and Nb is 0.01 to 0.10%, and the one or two kinds of Ti and Nb are added only Ti to improve workability. , And Nb are added, the bake hardenability is good, and the addition of two kinds is effective because both defects are compensated. The lower limit of 0.01% makes it possible to fix solid solution elements such as C and N and secure aging when the lower limit is set to 0.01%. Further, the upper limit is set to 0.10% because even if it is added more than that, the aging becomes saturated, and moreover, the strength is increased by the precipitate and the workability is deteriorated.

The total amount of Mo, W and Cr is 0.001-
The lower limit is set to 3.0%, and the lower limit is set to 0.001% because there is no effect of increasing the bake hardenability at less than 0.001%. The upper limit of 3.0% is M if the upper limit is exceeded.
This is because o, W, and Cr are reinforcing elements of steel, because the strength becomes too high and the workability is impaired, and the bake hardenability is saturated, so that it becomes expensive and economical. . Mo is known to be an element that suppresses the nucleation of Fe ₃ C and suppresses the pearlite transformation, but the reason for increasing the bake hardenability is not clear, but it was added because of the low carbon steel. Since Mo is solid-dissolved to form many strain fields, even if the temperature is about 170 ° C. at the time of baking the coating at a portion with less processing strain, it is easy to react with the remaining solid-solution carbon and solid-solution nitrogen. It is considered that precipitates are formed or clusters are formed, and mobile dislocations are fixed and hardened. On the contrary, rather than forming precipitates and clusters of Mo and C, a conglomeration of C gathered around Mo is formed, and the migration of C is suppressed at room temperature, and the baking temperature of the paint is 170 ° C to 20 ° C.
It is also considered that Mo-C separates at 0 ° C, and C fixes dislocations and hardens. In any case, it is considered that the effect of adding Mo appears as an improvement in bake hardenability. Also,
It is considered that W and Cr, like Mo, also appear as an improvement in bake hardenability due to the action of C.

In addition, although B is not limited to be added as an element to be contained, about 0.0010% is added for improving hardenability and strength and for improving secondary workability. You may.

Next, under hot rolling conditions, the finish rolling finish temperature is 700 ° C. or higher and lower than the A ₃ transformation point, and the lower limit is 700 ° C.
The reason is that if it is less than that, the rolling structure remains, and the amount of reduction becomes large during cold rolling reduction, which is disadvantageous.
Further, in order to improve the workability, it is said that the grain size after hot rolling is preferably as random, so that the presence of the rolling structure adversely affects the workability in terms of the texture of the crystal. Is. Further, the upper limit is set to be less than the A ₃ transformation point because the texture for improving the workability becomes high when the amount of C becomes low, and it cannot be achieved unless the cold rolling rate exceeds 90%. The reason is that strain is less likely to be stored because the amount of C is low, so strain is stored in the α grains to try to replace the lack of strain in cold rolling. That is, an appropriate strain is left in the hot-rolled steel sheet to obtain a cold-rolled texture having an optimum strain after cold-rolling.

The coiling temperature is set to 500 ° C. or higher because it is necessary to make the crystal grains large in order to improve the workability of the steel sheet, and the crystal grains are expected to grow during the cooling process from a high temperature. Is.

The cold rolling rate is 50 to 90%, and the lower limit is 50%, provided that continuous annealing is performed at a temperature not lower than the recrystallization temperature and not higher than the A ₃ point, and the cold rolling rate is the workability. There is an optimum point to improve, even if the strain is accumulated before cold rolling, if it is less than that, adjust the cold rolling texture,
This is because the annealing texture cannot be optimized to improve the drawability. The upper limit of the cold rolling rate was set to 90% because if strain is applied beyond that, the cold rolling texture (10
This is because the number of 0) planes increases, and the texture after annealing has poor (100) planes that are inferior in drawability and impairs drawability. When combined with the strain before cold rolling, strain in cold rolling Is because it can be small. In other words, the cold rolling rate has an optimum point not only to match the final thickness of the cold rolled steel sheet but also to improve the texture, and generally, the cold rolling rate is increased as the carbon content decreases. It is necessary to increase the size of the equipment. However, it is possible to reduce the cold rolling rate by allowing the strain to remain before the cold rolling, which is significant industrially.

Further, the temperature condition of the continuous annealing is set to the recrystallization temperature or higher and the A ₃ point or lower and the lower limit is set to the recrystallization temperature. Below that, the strain generated by cold rolling is not removed, and the recrystallization is not performed. This is because the crystals do not have excellent workability and the workability is poor. Also, the upper limit is A ₃
The reason for this is that if annealing is carried out beyond that, the recrystallized grains become coarse and the surface of the steel sheet after processing becomes rough, causing problems in appearance. It is said that the higher the heating rate during annealing is, the more the development of the (111) plane that improves the workability is promoted, but it is not particularly specified. The range is 5 to 5000 ° C / sec in annealing called continuous annealing.
However, the heating method is not specified. Although the annealing time is not particularly specified, it may be set to a recrystallization completion time that is shorter when the temperature is higher and longer when the temperature is lower in relation to the temperature. The cooling rate after annealing is not specified, but rapid cooling is desirable from the viewpoint of enhancing the bake hardenability.

Subsequent temper rolling may be carried out to adjust the shape, but it is also possible to produce the product as it is without temper rolling. The reason why the workability is good without yield elongation without temper rolling is because it is a method that can be manufactured by leaving a large number of mobile dislocations as described above. It is also advantageous in that it becomes easy.

The characteristics of the cold-rolled steel sheet thus manufactured, the hot-rolling finishing temperature, the cold-rolling rate, and the components Mo, W and C contained therein.
The results of examining the relationship with r are shown in FIGS. 1, 2 and 3. The steel composition used in the experiment is C: 0.0010%, S
i: 0.01%, Mn: 0.17%, P: 0.008
%, S: 0.003%, Al: 0.032%, Ti:
0.012%, Nb: 0.010%, total amount of Mo, W and Cr: 0.15%, balance consisting essentially of Fe, heat rolled at a finishing temperature of 800 ° C. and then wound at 600 ° C. The rolled sheet is cold rolled at a rolling reduction of 80%, and the cold rolled steel sheet is rolled at 800 ° C for 40%.
After annealing for 2 seconds, it was cooled at 100 ° C./second, and then temper rolling was carried out at a reduction rate of 0.4%.

FIG. 1 shows the relationship with the r value when the hot rolling finishing temperature is changed in the above treatment. As shown in the figure, when the hot rolling finishing temperature is 700 ° C or higher and less than the A ₃ transformation point, r
Cold rolled steel sheet with high value and good workability. Also,
FIG. 2 shows the relationship with the r value when the cold rolling rate is changed. The cold rolling rate is 50 to 90%, the r value is high, and a cold rolled steel sheet with good workability is obtained. There is. Furthermore, FIG.
Shows the relationship of the bake hardenability when the addition amount of Mo, W, Cr is changed, but the addition amount of Mo, W, Cr is 0.0
In the range of 01 to 3.0%, the bake-hardening amount is high, showing excellent bake-hardening property. In addition, it is possible to obtain a cold-rolled steel sheet having no yield elongation and good workability and excellent bake hardenability.

Thus, the composition of the steel is adjusted, the hot rolling conditions,
By adjusting the cold rolling conditions and the annealing conditions, a cold rolled steel sheet having excellent workability and bake hardenability can be obtained. Then, as a method for producing a cold rolled steel sheet having excellent bake hardenability as described above, continuous casting is performed, and C: 0.00
7% or less, Si: 0.8% or less, Mn: 1% or less, P:
0.15% or less, S: 0.020% or less, Al: 0.0
1 to 0.1%, N: 0.01% or less and 0.01% to 0.10% of the total amount of one or two of Ti and Nb are added to steel consisting of inevitable impurities. Mo,
The total amount of one or more of W and Cr is 0.001
~ 3.0% of steel is hot-rolled at 700 ° C or higher and lower than the A ₃ transformation point, then wound at a temperature of 500 ° C or higher, and then cold-rolled at 50-90%, and then recrystallization temperature. Above A ₃
By carrying out continuous annealing at a temperature below the point, workability is excellent, stretcher strain-free aging is excellent, and bake hardenability can be greatly improved.

The steel sheet thus manufactured is, for example, Z
n can be electroplated to form a rust-preventing steel plate, which can be a bake-hardening rust-preventing steel plate. Further, after cold rolling of such a steel plate, the recrystallization temperature or higher and A ₃ point or lower. Immediately after performing continuous annealing at the temperature of, the hot-dip galvanized immediately, in addition, the alloying treatment, to make an alloyed hot-dip galvanized steel sheet, and to make a steel sheet that can contribute to high strength, high rust prevention Is possible.

[0026]

EXAMPLES Examples of the present invention are shown in Table 1 together with comparative examples.
And shown in Table 2. Table 1 shows the components of the steel, and Table 2 shows the production conditions and the characteristic values of the steel sheet. As is clear from Table 2, it can be seen that the examples of the present invention all have excellent characteristics.

[0027]

[Table 1]

[0028]

[Table 2]

[0029]

As described above, the bake hardenability of the cold rolled steel sheet is improved, and for example, it is used for the outer panel of automobiles, has excellent dent resistance, is soft during processing, and is harder during use. It is possible to obtain excellent effects such as being able to.

[Brief description of drawings]

FIG. 1 is a diagram showing a relationship between a hot rolling finish temperature of a cold rolled steel sheet and an r (average) value.

FIG. 2 is a diagram showing a relationship between a cold rolling rate of a cold rolled steel sheet and an r (average) value.

FIG. 3 is a diagram showing the relationship between the amounts of Mo, W and Cr added to a cold rolled steel sheet and bake hardenability.

Claims (1)

[Claims] 1. When manufacturing a cold-rolled steel sheet, in weight%, C: 0.007% or less, Si: 0.8% or less, Mn: 1% or less, P: 0.15% or less, S: 0.020% or less, Al: 0.01 to 0.1%, N: 0.01% or less, and steel consisting of unavoidable impurities, the total amount of one or two of Ti and Nb being 0.01 to.
0.10% is added, and the total amount of one or more of Mo, W and Cr is 0.1.
Steel to which 001 to 3.0% is added is hot-rolled at 700 ° C. or higher and lower than the A ₃ transformation point, then wound at a temperature of 500 ° C. or higher, and then cold-rolled at 50 to 90% and then recrystallized. A method for producing a cold rolled steel sheet having excellent workability and bake hardenability, which comprises performing continuous annealing at a temperature of not less than A _{3 and not} more than A ₃ .