JPH0692619B2 - Manufacturing method of cold rolled steel sheet with excellent workability - Google Patents

Description

The present invention relates to a method for producing a cold rolled steel sheet having excellent workability.

[Conventional technology]

Cold-rolled steel sheets with excellent workability are steels with limited steel components, which are conventionally limited to continuous cast slabs or agglomerate slabs with a thickness of 200 to 250 mm, which are rolled to a thickness of 2 to 6 mm, pickled and cold rolled.
It is manufactured through an annealing process. In this case, hot rolling
It needs a large rolling mill, and it consumes a lot of energy, because it rolls from 200 to 250 mm thick to 2 to 6 mm thick. Recently, as a method for overcoming the above-mentioned drawbacks, attempts are being made to make the thickness of the cast piece as close as possible to the thickness of the product. As a specific method, there is a method of directly manufacturing a cold-rolled material by continuous casting without performing hot rolling, and a method of manufacturing a slab of an intermediate thickness in the hot rolling step to simplify hot rolling. It is considered.

The cold-rolled steel sheet produced by the former method is hard and inferior in workability. In addition, roughening occurs during processing and it cannot be used for processing. On the other hand, when manufactured by the latter method,
Although it does not have rough skin during processing, it is still hard and inferior in workability. In addition, in the case where the solidified slab is directly hot-rolled, the steel sheet is cracked during rolling.

[Problems to be solved by the invention]

The present invention has been made for the purpose of providing a method for manufacturing a cold-rolled steel sheet having excellent workability, which solves the above-mentioned problems in the prior art.

[Means for solving problems]

The inventors of the present invention have variously investigated the cast slab thickness and the heat history after solidification, limiting the steel components, and by controlling the heat history after solidification, it is possible to omit the rough rolling step in hot rolling and at the time of hot rolling. The present invention has been completed by finding that a cold rolled steel sheet having no cracks and excellent workability can be produced.

The gist of the present invention is that C: 0.070% or less by weight%, Mn: 0.07 to 0.50%, S: 0.015% or less, Al: 0.005 to 0.10.
Molten steel containing 0% and, if necessary, B in the range of 0.3 ≦ B / N ≦ 1.5, with the balance being iron and inevitable impurities, 20 to 80
When making a slab having a thickness of mm, and hot rolling the slab immediately after solidification, the slab is allowed to stay in a temperature range of 1100 to 950 ° C. for 3 to 60 minutes and then hot rolled, followed by cold rolling, It is a method for producing a cold rolled steel sheet having excellent workability, which is characterized by annealing.

First, the component limitation of the cold-rolled steel sheet for processing of the present invention will be described.

It is well known that C is an element which becomes hard when the added amount is large and deteriorates workability.
%, The workability deteriorates even with the method of the present invention, so the upper limit was made 0.070%. The preferable range is 0.05% or less. The lower the C content, the better the workability, so the lower limit of the C content was made 0.0010% which is industrially practicable.

Mn is required to be 0.07% or more in the method of the present invention in order to prevent hot brittleness induced by S during hot working. Mn amount is 0.
If it exceeds 50%, the workability is deteriorated and the characteristics of the present invention are impaired. Therefore, the Mn content was limited to 0.07 to 0.50%. A preferred range is 0.10 to 0.30%.

Since S is an element that induces cracking during hot working, its lower content is preferable. In the present invention, S is a component that greatly affects the workability. If the S content exceeds 0.015%, the steel becomes hard and the workability deteriorates. The preferred range is 0.010% or less. The lower limit of the S content is preferably lower from the viewpoint of workability, but it is set to 0.0005% which is industrially achievable.

Since Al is a killed steel, at least 0.005% is required. On the other hand, when the Al content exceeds 0.100%, the steel sheet becomes hard and at the same time the surface flaws of the steel sheet increase, which further increases the cost. Therefore, the Al content is limited to 0.005 to 0.100%.
A preferred range is 0.010 to 0.080%.

The reasons for limiting the basic components of the present invention have been described above. The feature of the present invention is further exerted by adding B, which is a nitride forming element, to this basic component. When B is added, the range is 0.3 ≦ B / N ≦ 1.5. If the B / N is less than 0.3, there is no workability improving effect due to the addition of B, and if the B / N exceeds 1.5, the workability is deteriorated.

Other impurity elements such as P, Si and O are not particularly limited, but it is preferable that the impurity elements are as small as possible from the viewpoint of improving the workability of the steel sheet.

Next, a method for manufacturing a cold-rolled steel sheet having the above-described limited components and excellent workability will be described.

In the laboratory: C: 0.03%, Mn: 0.20%, S: 0.006%, Al: 0.040
%, N: 0.0021%, P: 0.008%, Si: 0.010% molten steel 40 mm
A thick slab was used, the cold speed and cooling pattern after solidification of the slab were changed variously, and then hot rolled steel plate of 3.7 mm was obtained by hot rolling. After descaling, this was cold-rolled to 0.80 mm, annealed at 725 ° C x 1 min + 350 ° C x 3 min, and tempered at 1.0%, and the material properties were investigated. The degree of edge cracking of the hot rolled sheet was also investigated before cold rolling.

Fig. 1 shows the thermal history of a slab until the start of rolling, at a temperature range of 50 ° C with a staying time of 5 minutes or more in that temperature range (including all temperature zones having a staying time of 5 minutes or more) and cold. The relationship of elongation of the rolled steel sheet is shown. As you can see from Figure 1, 11
It can be seen that the material hot-rolled after being left in the temperature range of 50 to 950 ° C for 5 minutes or more has excellent elongation. Those with a thermal history having a staying period of 5 minutes or more in a temperature range of higher than 1150 ° C or lower than 950 ° C have large variations in elongation. There 11
The thermal hysteresis material that stays in the temperature range of 50 to 950 ° C at intervals of 50 ° C for 5 minutes or more is plotted in this temperature range, and the results excluding the other temperature ranges are shown by the dotted line in the figure. This figure shows 1150-95
It can be seen that the residence time in the temperature range of 0 ° C is important.
Therefore, the relationship between the staying time from the same experiment until the start of hot rolling in the temperature range of 1150 to 950 ° C and the elongation and value of the cold rolled steel sheet is shown in Fig. 2. Similarly, FIG. 3 shows the relationship between the aging index and the residence time in the temperature range of 1150 to 950 ° C. It can be seen from FIG. 2 that the properties that correlate with the value and the workability of elongation are good at a residence time of 3 minutes or more in the temperature range of 1150 to 950 ° C. On the other hand, it can be seen that the aging index increases when the residence time in the temperature range of 1150 to 950 ° C is 60 minutes or more, and the aging property of the steel sheet deteriorates.

Fig. 4 shows the relationship between the degree of edge cracking on a hot-rolled sheet and the residence time at 1100 to 950 ° C. As for the score of cracking, 1 was no cracking, 2 was some cracking, and 3 was 3 levels of cracking. It can be seen from FIG. 4 that cracks occurred within a residence time of 3 minutes within the temperature range of 1100 to 950 ° C., and there was almost no crack in the rolled material after 3 minutes or more.

Based on the above experimental facts, when hot-rolling a solidified slab, the condition of hot-rolling after allowing the slab to stay in the temperature range of 1150 to 950 ° C for 3 to 60 minutes is excellent in workability and aging. It was defined as having excellent properties and not causing cracks in the hot rolled sheet. By setting the residence time in this temperature range to 5 minutes or more, the object of the present invention can be stably achieved.

Fig. 5 shows the relationship between the thickness of the cast piece and the value of the cold rolled steel sheet. Conditions other than the thickness of the slab are as follows.

C: 0.025%, Mn: 0.20%, Al: 0.036%, S: 0.005%, N: 0.0
A slab is manufactured from 020% molten steel, and the heat history of the slab is 15 minutes for the stay time at 1100 to 950 ° C, hot rolled to 3.7 mm thickness, and after descaling, cold rolled to 0.80 mm , 750 ℃ x 1
Min + 350 ° C. × 3 min annealing, 1.5% skin pass rolling. As can be seen from Fig. 5, the value is low when the cast piece thickness is less than 20 mm. Based on this fact, the lower limit thickness of the slab was limited. The preferred range is 30 mm or more. The upper limit of the thickness of the slab is up to 80 mm from the viewpoints of the object of the present invention, that is, the simplicity of hot rolling equipment and the reduction of hot rolling energy consumption.

The hot rolling finishing temperature is preferably set to ₃ or more Ar points, as in the usual process. After hot rolling, control-cooled as usual, 600-780 ℃ for continuous annealed material, 500 for box annealed material.
It is preferable to wind at a temperature of ˜710 ° C.

The hot-rolled sheet is cold-rolled at 60 to 90% after descaling. The annealing exhibits the features of the present invention in both continuous annealing and box annealing. The annealed plate is temper-rolled as necessary and then used as a finished product.

Even if the steel sheet manufactured by the method of the present invention is used as the surface-treated steel sheet, the characteristics of the present invention are not impaired.

〔Example〕

A cold rolled steel sheet is manufactured under the components and manufacturing conditions shown in Table 1,
Evaluate the material characteristics and the degree of edge cracking on the hot rolled sheet,
It is shown in the table. The evaluation of the edge cracking of the hot-rolled sheet was made into three grades: (1) No cracking, (2) Some cracking, and (3) Large cracking.

From the results in Table 1, it can be seen that those manufactured by the method of the present invention exhibit higher ductility, higher value, and superior aging than those manufactured by the methods other than the present invention. In addition, the ear cracks on the hot rolled sheet did not occur.

Coil Nos. 10, 11, 12, and 13 are compositionally outside the scope of the present invention, but all have low elongation and values that correlate with workability.
Coil No. 14 has a slab thickness of 15 mm, which is outside the scope of the present invention.
No. 15 was produced when the residence time in the temperature range of 1150 ° C. to 950 ° C. before the start of hot rolling was out of the range of the present invention, and all of them had poor workability. Coil No. 16 has an excellent value when the stay time at 1150 to 950 ° C. is too long, and has an excellent aging index and aging.

[Effects of the Invention] According to the present invention, as is clear from the above examples, steel with limited components is continuously cast into thin slabs, and by controlling the cooling process of the slabs, it is possible to obtain strong strength as in the conventional process. Even if hot rolling under a cumulative large pressure is not performed by a series of hot rolling mills,
It becomes possible to manufacture a cold-rolled steel sheet which has excellent workability and also has a good aging property. Thus, the present invention is an industrially remarkably useful invention because it is possible to save energy and to significantly reduce costs with labor saving in the process.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a diagram showing the relationship between the elongation of a cold-rolled steel sheet and a temperature range having a stay time of 5 minutes or more at 50 ° C. intervals in the heat history until the start of rolling of a slab, Fig. 2 is a diagram showing the relationship between El and the residence time at 1150 to 950 ° C until the hot rolling of the slab is started, and Fig. 3 is the temperature at 1150 to 950 ° C until the hot rolling of the slab is started. Fig. 4 is a diagram showing the relationship between the residence time and the aging index, Fig. 4 is a diagram showing the relation between the residence time at 1150 to 950 ° C until the start of hot rolling of the slab and the edge cracking of the hot rolled sheet, and Fig. 5 is the cast It is a figure which shows the relationship between a piece thickness and the value of a cold rolled steel plate.

Claims (2)

[Claims] 1. C: 0.070% or less by weight%, Mn: 0.07 to 0.50
%, S: 0.015% or less, Al: 0.005 to 0.100%, molten steel consisting of iron and inevitable impurities in the balance is made into a slab with a thickness of 20 to 80 mm, and when the slab immediately after solidification is hot rolled, 1150 to
A method for producing a cold-rolled steel sheet having excellent workability, which comprises allowing the material to stay in a temperature range of 950 ° C for 3 to 60 minutes, followed by hot rolling, followed by cold rolling and annealing. 2. C: 0.070% or less by weight%, Mn: 0.07 to 0.50
%, S: 0.015% or less, Al: 0.005 to 0.100%, and B
It is contained in the range of 0.3 ≦ B / N ≦ 1.5, and the balance is molten steel consisting of iron and unavoidable impurities as a cast piece with a thickness of 20 to 80 mm, and when hot rolling the cast piece immediately after solidification, 1150 to 950 ° C. The method for producing a cold-rolled steel sheet having excellent workability, which comprises allowing the material to stay in the temperature range of 3 to 60 minutes, hot rolling, followed by cold rolling and annealing.