JPH0830210B2 - Manufacturing method of cold rolled steel sheet with excellent deep drawability - Google Patents

Description

TECHNICAL FIELD The present invention relates to a method for producing an Al-killed cold-rolled steel sheet, which comprises subjecting a continuous casting slab to a specific temperature history and then directly rolling at a specific temperature. It is intended to provide a method for improving the deep drawability of a cold rolled steel sheet by starting the method.

(Prior art) Conventionally, when producing an Al-killed steel sheet for deep drawing by continuous annealing, it is usually produced by heating a cold piece slab to 1050-1300 ° C, and then hot rolling and cold rolling. However, much energy is required to heat the cold piece slab to a high temperature.

On the other hand, from the viewpoint of energy saving, a direct hot rolling process is being developed in which the continuous casting slab is hot rolled as it is without forming cold pieces.

As a prior art for improving the workability of a cold rolled steel sheet manufactured by the direct rolling process, there are technologies disclosed in JP-A-60-43432 and JP-A-60-228617.

However, the former relates to a method for producing a material for box annealing, and is not a process for producing a cold rolled sheet by continuous annealing.

The latter relates to a method for producing a cold-rolled sheet by continuous annealing, but it is aimed at improving the aging characteristics and the deep drawability is not sufficient. A deep drawing steel sheet is required to have a large r value, a large ductility and a low yield strength. However, the cold-rolled sheet produced by the method disclosed in JP-A-60-228617 has finer precipitates than those produced by reheating a cold piece slab, and these are less likely to occur during low temperature annealing. Since it is inferior in properties, the high temperature annealing is essential.
However, when high temperature annealing is carried out, build-up of hearth rolls or defective shapes of cold-rolled sheets are likely to occur at the time of passing, so that there is a problem that not only poor stripability but also surface defects of the product and cost increase. .

Currently, to avoid this problem, deep drawing steel sheets are made of ultra-low carbon steel with Ti or Nb added, and low-temperature annealing is also used.However, low-carbon Al-killed steel is used. Compared with, the material cost will be significantly increased.

As described above, the direct rolling process, which has a great advantage in terms of energy saving, is not fully utilized due to the material cost or high temperature annealing.

(Problems to be solved by the invention) Therefore, an object of the present invention is to produce a deep-drawable cold-rolled steel sheet by using a low-carbon Al-killed steel by a direct rolling process and low-temperature annealing.

(Means for Solving the Problems) The present inventors cooled the slab immediately after continuous casting at a cooling rate of 50 ° C./min or less until the slab surface temperature became 1100 to 1200 ° C., and then the slab surface temperature was 1000. It was found that the above object can be achieved by cooling to ~ 1100 ° C and starting hot rolling at a slab surface temperature of 1000 ~ 1100 ° C (hereinafter, all those with no particular description of temperature indicate "surface temperature"). ).

 The present invention will be described in detail below.

C: 0.010-0.040% and Mn: 0.05-0.25% by weight. S:
When continuously casting a steel containing 0.004 to 0.020%, Al: 0.010 to 0.080%, N: 0.0030% or less, the balance being Fe and unavoidable impurities, the slab immediately after continuous casting is 50 ° C.
Cool at a cooling rate of / min or less until the slab surface temperature reaches 1100 to 1200 ° C. After that, retention within this temperature range,
Alternatively, either cooling to the hot rolling start temperature without holding may be performed, or the slab surface temperature
It may be heated to a temperature not exceeding 11200 ° C. Then, after cooling until the surface temperature of the slab reaches 1000 to 1100 ° C., hot rolling is started from this temperature range to manufacture a hot rolled steel sheet.

The hot-rolled steel sheet after this step is wound at a temperature of 650 ° C. or higher, followed by cold rolling and continuous annealing to obtain a steel sheet having excellent deep drawability.

First, the reasons for limiting the chemical composition of steel to which the method of the present invention is applied will be described.

When C is less than 0.010%, aging deterioration after continuous annealing is large, which is not desirable. If it exceeds 0.040%, the deep drawability of the product deteriorates. Therefore, the amount of C is limited to 0.010 to 0.040%.

Mn is a component necessary to prevent hot brittleness, but 0.
If it is less than 05%, FeS is generated and its effect is not obtained. Also, 0.25
If it exceeds%, the deep drawability deteriorates. Therefore, set the amount of Mn to 0.
Limited to 05-0.25%.

If S exceeds 0.020%, it causes hot embrittlement, so it must be less than this. In order to use MnS as a precipitation nucleus of cementite during overaging of continuous annealing,
It is necessary to contain 0.004% or more.

Al requires at least 0.010% in order to precipitate N as AlN after deoxidation and winding. However, if it exceeds 0.080%, the workability is deteriorated.

Further, AlN is also preferable because it deteriorates the workability, and the N content is set to 0.0030% or less.

The inventors of the present invention melted steel in the above-mentioned composition range, casted it, cooled it in various temperature ranges, and then started hot rolling at various temperatures. Further, cold rolling and continuous annealing with a recrystallization temperature of 800 ° C. were performed to investigate the material. Typical results are shown in FIGS. 1 and 2. Fig. 1 shows a continuous cast slab containing 0.015% S by weight immediately after solidification, cooled at 20 ℃ / min and 50 ℃ / min until the slab surface temperature reaches 1200 ℃. 3 shows the effect on the value of the hot rolling start temperature when hot rolling was started at various slab surface temperatures, followed by cold rolling and continuous annealing. Fig. 2 shows that the slab surface temperature is 1100 ℃ at various cooling rates immediately after continuous casting of the above steels.
Fig. 3 shows the effect on the value of the cooling rate when hot rolling is started from the slab surface temperature of 1050 ° C after cooling to 0 ° C.

From these results, in order to obtain a steel sheet with excellent deep drawability, the slab after continuous casting is cooled at a cooling rate of 50 ° C / min or less, and hot rolling is started when the slab surface temperature is 1000 to 1100 ° C. I knew I had to do it.

When the precipitates of the cold-rolled sheet were examined by a transmission electron microscope, the number of fine MnS according to the conditions of the present invention was small, but the number of fine MnS deviated from this range was large.
Therefore, the deep drawability is considered to be greatly affected by these fine MnS.

Furthermore, the slab used in the manufacturing method of the present invention is made of Si, P
And other elements can be included.

The coiling temperature is 650 ° C for sufficient precipitation of AlN.
Although the above is required, the range of 680 to 780 ° C is preferable in order to coarsen AlN and agglomerate cementite. The cold rolling reduction may be the same as usual, but (11
1) To develop a texture and improve deep drawability 7
A cold rolling rate under high pressure of 0% or more is preferable.

Next, the continuous annealing conditions will be described. The heating temperature needs to be higher than the recrystallization temperature. As long as hot rolling, cold rolling and the like are performed according to the present invention, sufficient deep drawability can be provided even by low temperature annealing. Therefore, at present, the temperature is mainly heated to 700 to 830 ° C., but annealing at a temperature higher than this temperature does not impair the deep drawability. In order to improve the aging characteristics, the primary cooling rate should be 50 ° C / sec or more, then 25
It is better to perform overaging treatment in the temperature range of 0 to 350 ° C. This is a distribution state in which MnS in steel acts as a precipitation nucleus of cementite during overaging due to the slab temperature history condition, and the action is particularly effectively exhibited by this overaging condition. Because.

The cold-rolled sheet produced as described above has excellent deep drawability as compared with the conventional material that is directly hot-rolled.

(Example) Example 1 C: 0.020%, Si: 0.010%, Mn: 0.15%, P: 0.007%, S:
After continuously casting a steel with a composition of 0.011%, sol.Al:0.050%, N: 0.0015%, Fe: balance, 1200 ° C at 30 ° C / min
Then, hot rolling was performed. Due to the solidification of samples A, B and C, the slab surface temperature between 1100 ° C and 1200 ° C is within the range of the present invention, and the slab surface temperature at the start of hot rolling is within the temperature range of the present invention. is there. Samples D, E and F have either the slab cooling rate or the hot rolling start temperature outside the temperature range according to the invention. The above samples have a finishing temperature of 900 ° C and a final hot-rolled sheet thickness of 4.0 mm. The winding temperature was 720 ° C. After completion of winding at this temperature, N in the steel was precipitated as AlN and coarsened in all the samples, so that the solid solution N and AlN did not adversely affect the workability of the product sheet. Cementite is also sufficiently agglomerated, and the difference in the materials is due only to the MnS distribution state.

Next, in the sample G, both the slab cooling rate and the hot rolling start temperature are within the range according to the present invention, but the winding temperature is out of the range.

These hot-rolled sheets were pickled, cold-rolled to a thickness of 0.8 mm at a rolling reduction of 80%, and further continuously annealed at a temperature of 800 ° C. for 1 minute.

The cold-rolled steel sheet obtained as a result was subjected to a tensile test using a JIS No. 5 tensile test piece to measure r value, yield strength and elongation. The results of these measurements are shown in Table 1.

From the results shown in Table 1, when the slab surface temperature at the start of hot rolling is out of the range of the present invention, any or all of the r value, the yield strength and the elongation can provide a material sufficient for deep drawing. On the other hand, when it is within the range of the present invention, it is clear that the r value, the yield strength and the elongation are all materials sufficient for deep drawing.

Investigation of the MnS distribution in the cold rolled steel sheet revealed the following.

MnS when the cooling rate of the slab and the surface temperature of the slab at the start of hot rolling are outside the scope of the present invention (Samples D, E and F)
When the distribution is within the range of the present invention (Samples A, B, and C), the fine particles with a size of less than 0.05 μm occupy the majority, but with MnS of 0.05 to 0.2 μm as the main component, The number of fine MnS is greatly reduced, and the total number of MnS is also significantly reduced. Sample G has the same MnS distribution as in the range of the present invention, but since the winding temperature is lower than the range of the present invention, AlN in the hot-rolled sheet was wound at 720 ° C. In comparison, finer precipitation was found, and it was found that the degree of aggregation of cementite was insufficient.

That is, the cooling rate of the slab and the slab surface temperature at the start of hot rolling are set within the range of the present invention, whereby Mn in the steel is
By controlling the size and distribution of S, it is possible to manufacture cold-rolled steel sheets with excellent deep drawability.

Example 2 20 ° C. after continuous casting of steel having the composition shown in Table 2
After cooling the slab surface temperature to 1200 ℃ / min,
Hot rolling was started when the slab surface temperature was 1100 ° C. sample
The compositions of H, I and J are within the scope of the present invention, except for sample K,
L, M, N and O are all outside the scope of the invention for the underlined components. The above samples have a finishing temperature of 90.
At 0 ° C, the final thickness of the hot rolled sheet is 4.0 mm. The winding temperature is
It was set to 720 ° C. These hot-rolled sheets were pickled, cold-rolled to a thickness of 0.8 mm with a reduction rate of 80%, and the temperature of the cold-rolled sheets at 800 ° C.
Was annealed continuously for 1 minute.

The cold-rolled steel sheet obtained as a result was subjected to a tensile test using a JIS No. 5 tensile test piece to measure r value, yield strength and elongation. The results of these measurements are shown in Table 3.

From the results shown in Table 3, it can be seen that when the composition is out of the range of the present invention, any or all of the r value, the yield strength and the elongation cannot obtain a sufficient material for deep drawing.

(Effects of the Invention) As is clear from the above description, according to the method of the present invention, a cold rolled steel sheet having excellent deep drawability can be manufactured. Also,
In this method, since the direct hot rolling process and the continuous annealing process are applied, the productivity is high, and the energy saving and the labor saving are possible.

[Brief description of drawings]

FIG. 1 shows the hot rolling start temperature range in which a good deep drawability can be obtained by hatching. FIG. 2 shows the cooling rate range of the slab in which a good deep drawability can be obtained by hatching.

Claims (1)

[Claims] 1. A weight ratio of C: 0.010 to 0.040%, Mn: 0.05 to
0.25%, S: 0.004-0.020%, Al: 0.010-0.080%, N: 0.0
Steel containing 030% or less with the balance Fe and unavoidable impurities is continuously cast, and the slab immediately after that is 50
Slab surface temperature is 1100 ~ 1200 ℃ at cooling rate of ℃ / min or less
After cooling until it becomes, the slab surface temperature is 1000 ~ 1100 ℃
The deep drawability is characterized in that hot rolling is started when the temperature is within the temperature range, the surface temperature of the obtained hot rolled steel sheet is taken up in the temperature range of 650 ° C or higher, and then cold rolling and continuous annealing are performed. Of excellent cold rolled steel sheet.