JPS5824488B2 - Method for manufacturing soft cold-rolled steel sheet with excellent ductility - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for manufacturing a low carbon cold rolled steel sheet by continuous annealing, and is intended to manufacture a continuously annealed cold rolled steel sheet with excellent ductility and a low yield point.

Cold-rolled steel sheets for press forming of automobile bodies and the like are required to have excellent deep drawability, stretchability, and aging resistance.

Conventionally, such cold-rolled steel sheets are made by forming aluminum killed steel into a slab by normal ingot formation or continuous casting, hot-rolling at a low temperature that prevents the precipitation of AIN, and after cold rolling, cylindrical annealing and recrystallization. It was manufactured by matching the precipitation of AIN well.

However, since such box-type annealing involves slow heating and slow cooling annealing, it takes a very long time of 4 to 5 days from the start to the completion of annealing, which has been a major problem in improving productivity.

For this reason, in recent years, technology for manufacturing cold-rolled steel sheets for press forming by continuous annealing (for example, Japanese Patent Publication No. 42-11911, Japanese Patent Publication No. 11911, No. 48)
-32248, etc.) have been developed and are being produced on actual lines.

According to this continuous annealing method, the process from the start of annealing to skin pass rolling can be completed in just less than 10 minutes, and compared to the box-type annealing described above, the productivity of steel plate manufacturing is dramatically improved. be.

By the way, when manufacturing a soft cold-rolled steel sheet by such a continuous annealing method, it is generally considered to be an essential condition that hot rolling and high-temperature coiling be performed.

Firstly, because continuous annealing involves heating and soaking for a short time, the growth of ferrite grains during annealing is poorer than in box-type annealing, making it difficult to produce soft cold-rolled steel sheets that can be used in presses. This is because it is necessary to perform high-temperature coiling to increase the size of ferrite grains in the hot rolled steel sheet.Secondly, in the case of aluminum killed steel, as in the case of box annealing, AIN precipitation occurs during annealing. It is difficult to match the timing of recrystallization with the timing of recrystallization, and when hot rolling is performed at low temperature as in box annealing, precipitation of AIN occurs earlier than recrystallization.
This finely precipitated AIN significantly hinders the growth of ferrite after recrystallization, resulting in a problem that the cold-rolled steel sheet produced is extremely hard.For this reason, aluminum-killed steel requires hot rolling and high-temperature coiling. This is because it is necessary to completely precipitate and grow AIN.

As described above, the higher the coiling temperature in hot rolling, the larger the ferrite grain size, and therefore the softer the produced cold rolled steel sheet. On the other hand, if the coiling temperature is too high, Abnormally coarse grains are sometimes produced in the surface layer of a hot rolled steel sheet, which not only reduces the workability of the hot rolled steel sheet, but also significantly reduces the press formability (especially ductility) of the cold rolled product.

In the case of aluminum-killed steel, AIN precipitated during high-temperature coiling has the effect of suppressing the abnormal grain growth of the ferrite, and this effect is even greater than that of other types of steel, such as semi-killed steel and rimmed steel (capped steel). Although it is strong, on the other hand, it has the disadvantage that such a suppressing effect also inhibits the normal grain growth of ferrite during hot rolling or annealing.

In this way, in the past, it was not possible to satisfy both of the two conditions of the softness of the steel sheet and the suppression of the generation of abnormally coarse grains of ferrite. Alternatively, by increasing the addition rate of N, the material quality of the steel sheet is sacrificed to some extent, and the generation of abnormally coarse grains is suppressed.

The present invention was devised in view of the current situation, and it is possible to stably produce a soft cold-rolled steel sheet for drawing to deep drawing without generating abnormally coarse grains at all. The purpose is to provide a method for manufacturing cold-rolled steel sheets.

Therefore, in the present invention, C: 0.02 to 0.06%, Mn
≦0.30%, SOI, Al: 0.02-0.1%
, Si≦0.05%, N amount 0.005%, Cr: 0.0
The aluminum killed steel containing 2 to 0.20% is finished rolled to a temperature of 630 to 750 by performing finish rolling at the A3 transformation point or higher in the hot rolling stage.
Its basic characteristics are that it is coiled at a temperature of 0.degree.

As a result of various experiments regarding the softness of the steel sheet and the occurrence of abnormally coarse grains, the present inventor found that by adding a small amount of Cr to aluminum killed steel, the limit for the occurrence of abnormally coarse grains during winding was shifted to the high temperature side. However, it has been found that a steel sheet that is softer than a Cr-free steel sheet can be obtained.

Therefore, the present invention is based on the addition of a small amount of Cr.

The present invention will be described below. In the present invention, aluminum killed steel basically includes the following component ranges.

C: 0.02-0.06% Mn: 0.30% or less 5OIAI: 0.02-0.10% si: Q, Q 5% or less N: 0.005% or less Cr: 0.02-0.20 % The reason why the range of components is limited as described above in the present invention is as follows.

If C: 0.06% or more, it becomes hard, and if it is 0.02% or less, there are few places where solid solute C precipitates, so even if over-aging treatment is performed by continuous annealing, supersaturated solid solute C will not precipitate sufficiently. Therefore, the aging resistance of the cold-rolled product deteriorates and the yield stress also increases, which is not preferable.

Si: Keep it to the minimum necessary level, but if it exceeds 0.05%, it becomes hard.

Mn: At 0.3% or more, solid solution hardening occurs and becomes hard, and deep drawability is reduced.

Also, in relation to S, if it is too low, there is a risk of hot embrittlement, but as long as this hot embrittlement does not occur, a lower value is preferable.

N: Combines with AI to form AIN during hot rolling and winding, but at 50ppII1 or more, the amount of AIN precipitation increases,
Although it is advantageous in preventing abnormally coarse grains, fine AIN also hinders normal grain growth.
The ferrite grain size of the cold-rolled product becomes smaller, it becomes harder, and its deep drawability also decreases.

For this reason, it is preferable that the amount of N be small.

Cr: This is a feature of the present invention, and it is already known that addition of a small amount causes solid solution softening, and generally a solid solution softening phenomenon is observed when Cr≦1%.

On the other hand, as the amount of Cr added increases, the carbide size tends to decrease, and this tendency becomes more pronounced as the winding temperature increases.

In normal continuous annealing, the deep drawability of the continuously annealed material is improved by using a material with coarser carbide, and the refinement of the carbide inevitably reduces the deep drawability of the continuously annealed material. invite

Figure 1 shows the relationship between the amount of Cr added and the carbide size of a hot-rolled sheet of aluminum killed steel rolled at a high temperature of 700°C, and Figure 2 shows the relationship between the amount of Cr added and the carbide size of a hot-rolled sheet of aluminum-killed steel rolled at a high temperature of 700°C. It also shows the relationship between the amount of Cr added and the 〒 value, which is a measure of yield stress, total elongation, and deep drawability, when annealed at 700°C and 850°C.

As is clear from these figures, when the amount of Cr added increases, the carbide size of the hot-rolled sheet becomes smaller, which causes the lower value after cold rolling annealing to decrease, and also the yield stress to decrease.

On the other hand, the total elongation is improved by reducing the carbide size.

However, from the viewpoint of deep drawing or manufacturing cold rolled steel sheets for deep drawing, a significant decrease in deep drawability is not desirable, and if the amount of Cr added is 0.2% or more, it has the advantage of softening and improving ductility. The negative side of the decrease in the Cr value becomes more significant, and therefore the upper limit of the amount of Cr added should be suppressed to 0.2%.

Furthermore, if the amount of Cr added is less than 0.02%, mechanical properties such as total elongation will be significantly reduced as shown in Figure 2. Therefore, the lower limit of the amount of Cr added is limited to 0.02%. Should.

On the other hand, as is clear from Fig. 3, the relationship between the amount of Cr added, the winding temperature, and the abnormal coarse particle generation rate is that Cr≦0.01
In the case of normal steel with or without addition of 0.05% Cr, abnormally coarse grains occur in the coils of about 20% at CT-700℃ and about 50% at CT=720℃, whereas in the case of steel with 0.05% Cr addition. In this case, abnormally coarse grains do not occur at all at CT=700°C, and the occurrence rate is only 10% or less even at CT=720°C.

Furthermore, in the case of 0.10% Cr added steel, CT=720°C
However, in the case of 0 and 20% Cr added steel, CT-750
Abnormally coarse grains do not occur at all even at ℃.

From the above, in order to obtain a cold-rolled steel sheet with excellent softness and ductility, without generating abnormally coarse grains and without impairing deep drawability even when hot-rolled and coiled at high temperatures, it is necessary to add Cr. 0.0
It is necessary to add 2-0.2%.

Accordingly, in the present invention, aluminum killed steel containing the above-mentioned components is finish rolled at a temperature of A3 transformation point or higher in the hot rolling stage, and then coiled at 630°C to 750°C.

As is clear from Figure 3, when the amount of addition is 0.02%≦C≦0.2%, no abnormally coarse particles are observed at all until the winding temperature is 700°C or lower; Above that, the occurrence of abnormally coarse grains is observed in increasing order of the amount of Cr added.

Therefore, when the winding temperature is set to 700 t' or more, it is preferable to set the temperature according to the amount of Cr added.

Further, in the present invention, after this winding, cold rolling is performed at a rolling reduction of 30% or more, and recrystallization annealing and overaging treatment are performed by a continuous annealing method after this cold rolling.

Next, the present invention will be explained in detail.

Tables 1 and 2 shown below show examples and comparative examples according to the present invention, in which aluminum killed steel having the components shown in Table 1 is melted and made into a slab by continuous casting. After soaking this slab at 1250°C, it was finished at a high temperature of 860'C in the hot rolling stage, and the plate thickness was 2.8 mm.
A hot-rolled sheet was obtained, and this was wound up at 700°C.

Further, this was pickled, cold-rolled to a thickness of 0.8 mm, and then continuously annealed.

This continuous annealing is performed at heating and soaking temperatures of 700℃ and 850℃.
Performed at C2 level, after soaking, forced air cooling to 560°C, then cooling from this temperature at a cooling rate of 1000°C/se, and then overaging treatment for 1 to 2 minutes at a temperature of 300 to 400°C. I did it.

Table 2 shows the occurrence of abnormally coarse grains in hot rolled steel sheets in the above Examples and Comparative Examples, and 0.7 to 1.0% after continuous annealing.
This shows the mechanical test values (yield point, total elongation rank-hold value) of the pressure-adjusted strip.

In the above clothing, (2) to 5) are examples according to the present invention, and (1) and (6) to ←υ are comparative examples with respect to the present invention, but in the examples according to the present invention, there were no abnormally coarse particles. It does not occur, and the mechanical test values show good results that are equal to or better than ordinary box-type annealed materials (
The continuous annealing material according to the present invention has a 2 to 3% better total elongation.

). On the other hand, in Comparative Example (1) without the addition of Cr, abnormally coarse grains were observed during hot rolling and winding, and this effect caused roughness in the cold rolled product after cold rolling annealing.

In addition, Comparative Example (6) of Cr-added steel or Cr in αυ
In Comparative Examples (6) and (7) in which Cr was added in an amount exceeding the component range of the present invention, the rank-hord value (T) decreased, and in comparisons in which components other than Cr were outside the component range of the present invention, In Example (8) or αυ, the yield point increases,
The overall elongation and rank hoard value G) have decreased.

As described above, according to the method for producing a soft cold-rolled steel sheet according to the present invention, it is possible to produce a soft cold-rolled steel sheet for drawing to deep drawing with excellent ductility without generating abnormally coarse grains during hot rolling. It has the excellent effect of being able to be manufactured stably.

[Brief explanation of drawings]

FIG. 1 shows the relationship between the amount of Cr added and the carbide size of a hot-rolled sheet of aluminum killed steel rolled at a high temperature of 700°C. Figure 2 shows the amount of Cr added, yield stress, total elongation, and Lankford value when a hot-rolled sheet with the amount of Cr added in the range shown in Figure 1 is annealed at 700°C and 850°C using a continuous annealing method after cold rolling. It shows the relationship between FIG. 3 shows the relationship between hot rolling coiling temperature and abnormal coarse grain generation rate.

Claims (1)

[Claims] IC; 0.02-0.06%, Seed≦0.30%, S
O1, Al: 0.02-0.10%, Si≦0.0
5%, N≦O, OO5%, Cr: 0.02-0.2
At the hot rolling stage, aluminum killed steel containing 0% aluminum is subjected to finish rolling at a temperature above the transformation point, then coiled at 630-750°C, then cold rolled at a rolling reduction of 30% or above, and then further annealed by continuous annealing. A method for producing a soft cold-rolled steel sheet with excellent ductility, characterized by performing recrystallization annealing and over-aging treatment.