JPS5926364B2 - Cold rolling method of thin steel strip for processing - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a tandem mill cold rolling method for producing a thin steel strip with good workability.

Conventionally, the cold rolling method using tandem mills for thin steel sheets has no special restrictions, and the total rolling reduction rate is determined mainly from the product thickness, and the total rolling reduction rate is determined at each stand based on the capacity of the tandem cold rolling mill and the stability of operation. The distribution and rolling schedule (rolling reduction rate, rolling speed, tension) is determined.

For example, in the example of rolling reduction distribution in a tandem mill on page 916 of the Steel Handbook (4th edition), when the total rolling reduction is 66%, the first stand is 27-30%, the second stand is 30-33%, and the third stand is 27-30%.
25 to 27% in the stand and 10 to 12% in the fourth stand, and regardless of the total reduction rate, the reduction rate in the first stand is small, the reduction rate in the second and subsequent stands is large, and the reduction rate in the final stand is small. This method is preferred from the viewpoint of operational stability and is being implemented.

In terms of material, if deep drawability is required for a thin steel plate, cold rolling with a total reduction of 60 to 70% will give the highest Lankford value (value 7) (and good deep drawability). R.L., Whiteley et al. have reported that a thin steel plate with a high quality can be obtained.

The present inventors have conducted extensive research on the influence of the rolling schedule (reduction ratio, rolling speed, tension distribution in each stand) in a cold rolling method, that is, a tandem mill, on the material quality of a thin steel plate.

As a result, we have invented a rolling method that dramatically improves the workability of thin steel sheets when the same material and total rolling reduction are used.

The present invention will be explained in detail below.

The present invention complies with JIS G3141. G3302 and G3314
A cold rolling method for providing good workability when manufacturing a thin steel plate defined by the following by rolling the plate thickness reduction rate before and after cold rolling, that is, the total reduction rate of 60 to 70%, using a tandem cold rolling mill. Specifically, it is characterized by rolling at 33% or more on the first stand of a tandem mill.

In order to improve the workability of thin steel sheets, especially their ductility (elongation) and yield strength, it is desirable to increase the grain size after recrystallization annealing, and to achieve this, the total rolling reduction during cold rolling should be as low as possible. However, if it is too low, the recrystallization temperature will become high, so the lower limit of the total pressure reduction is set at 60%.

On the other hand, the r value, which indicates deep drawability, improves as the total reduction rate increases up to 70%, but as the grain size becomes finer, the yield strength increases (and ductility deteriorates, so the upper limit of the total reduction rate 70
%.

Figure 1 shows normal i-killed steel hot-rolled in the normal process, then the total reduction in cold rolling kept constant at 68%, the rolling reduction in the first stand changed, and the results after annealing. This is the result of investigating mechanical properties.

There is a good correlation between the rolling reduction of the first stand and workability, and when the rolling reduction becomes 30% or more, the yield strength rapidly decreases and the elongation increases.

Based on this experimental result, the lower limit of the rolling reduction ratio of the first stand is set at 33%, which is recognized as a significant difference in the improvement in workability.

On the other hand, the higher the rolling reduction ratio of the first stand, the better the workability, so there is no need to limit the upper limit.

Rather, the upper limit is determined from the limit of the biting of the plate into the rolls, and varies depending on the specifications of the cold rolling mill.

In addition, if the rolling reduction ratio distribution & ζ total rolling reduction from the second stand to the final stand is in the range of 60 to 70%, if the rolling reduction is 33% or more in the first stand, the shape and roll of the steel plate in the final stand will change. In order to improve the transfer of surface roughness, the rolling reduction ratio of the final stand needs to be around 10 to 20%, so the rolling reduction ratio of the intermediate stand is inevitably greatly restricted, so There is little room to do so.

From the above, it is clear that FIG. 1 includes cases where the rolling reduction per stand after the second stand has changed.

This cold rolling method can be applied to thin steel sheets specified by JIS G3141, G3302 and G3314 without being restricted by the history up to the cold rolling process or the method and conditions of annealing after cold rolling. .

In addition, other conditions of cold rolling, such as rolling speed, rolling temperature, >K
The effect of the method of the present invention is not affected by roll diameter, rolling tension, lubrication, etc.

Next, embodiments of the invention will be described.

Ladle components C0,028%, Si0.02%, Mn0.2
0%, PO, o17%, 80.013%, AAO, 03
Molten steel containing 5% NO, 0,032% was made into a slab by continuous casting, and a hot rolled coil with a plate thickness of 2.3u was manufactured by hot rolling.

In this case, the heating temperature of the slab is 1250°C, the hot rolling finishing temperature is 860 to 890°C, and the winding temperature is 610 to 640°C.
Met.

After pickling, this hot-rolled coil was cold-rolled in a tandem mill under two conditions to a total reduction of 65% and a thickness of 0.8 cage.

Table 1 shows the distribution of reduction ratios in cold rolling.

A is the conventional method, and B is the method of the present invention.

Box annealing and continuous annealing of cold rolled coil to 1.0%
It was temper rolled at a reduction ratio of .

The mechanical properties of the obtained product are shown in Table 2.

The results in Table 2 show that the thin steel sheets manufactured by the cold rolling method of the present invention are soft regardless of the annealing method, have good ductility, and have good deep drawability, although the ductility is small, and has good overall press hull formability. is clearly superior.

It is not yet clear why thin steel sheets with good workability can be obtained by rolling at a high reduction rate in the No. 1 cold rolling stand as described above, but it is important to note that rolling at a high reduction rate/pass, that is, at a high strain rate, results in a thin steel sheet with good workability. It is thought that the form of dislocations introduced by rolling (compressive deformation) changes, and the form of crystal grains and the state of remaining dislocations change after recrystallization.

[Brief explanation of the drawing]

Figure 1 shows capped steel manufactured in the usual way up to hot rolling, cold rolled in a tandem mill, and then subjected to normal box annealing to produce a product with various reduction ratios in the No. 1 stand during cold rolling. FIG.

Claims (1)

[Claims] 1. When cold rolling a steel strip in a tandem mill in a range where the total reduction rate, which is the plate thickness reduction rate before and after cold rolling, is in the range of 60% or more and less than 70%, the number 1 in the reduction rate distribution for each stand. A method for cold rolling a thin steel strip for processing, characterized by rolling a stand at a rate of 33% or more.