JPH075989B2 - 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 a cold rolled steel sheet having extremely excellent deep drawability.

[Prior Art] A method for producing a steel plate having good workability by using a steel having a C content of 0.005% or less and by appropriately setting hot rolling conditions is already known.

For example, Japanese Patent Application Laid-Open No. 61-110722 discloses extremely low C-low Mn-low N.
It is a method for producing a hot-rolled steel sheet having good workability by making the hot-rolled sheet microstructure fine by controlling the hot finish rolling temperature of steel, cooling conditions and winding conditions. However, this method relates to hot-rolled steel sheets, not to deep-drawability of cold-rolled steel sheets, and as described in the publication, the finish rolling temperature is, for example, Ceq = 0.03%.
The ultra-low C-low Mn-low N steel of (1) requires a high degree of temperature control. In this invention, after hot rolling (Ar3 + 1
(0 ° C.) to 30 ° C./s or more, and the rolled material is cooled at a cooling rate of 10 to 30 ° C./s as shown in FIG. 2 of the publication. It is based on the fact that the cooling rate is remarkably high and is saturated in a high cooling rate range, and therefore, 30 ° C./s or more means, for example, 45 ° C./s in the example.

Further, for example, JP-A-61-276930 discloses that components, hot rolling conditions,
By combining cooling conditions, winding conditions, cold rolling conditions, and annealing conditions,
This is a method for producing a cold-rolled sheet having good elongation and deep drawability.
However, this method is to produce a cold-rolled steel sheet excellent in deep drawability by starting cooling immediately after hot rolling, suppressing the growth of γ grains, and performing α transformation to make the structure of the hot-rolled sheet fine. However, the α grain size of the obtained hot-rolled sheet has a lower limit, a very fine hot-rolled structure cannot be obtained, and a cold-rolled steel sheet having a very high deep drawability cannot be manufactured.

Although this method cools the hot-rolled material at an average cooling rate of 10 ° C / s or more, there is no special description on the limitation of the cooling rate, and therefore 10
C./s or higher refers to, for example, 30.degree. C./s in the examples. Therefore, in order to manufacture a cold-rolled steel sheet having further excellent deep drawability, a method of reducing C, which is known to be effective for improving deep drawability, can be considered. However, if the C in the steel is lowered, the ductility of the steel is improved, but if the C is extremely lowered, coarse columnar grains extending in the plate thickness direction are generated in the hot-rolled sheet after hot rolling, and these coarse columnar grains are generated. Would rather reduce the deep drawability of the cold-rolled steel sheet, and the effect of improving the deep drawability of conventional ultra-low carbon could not be effectively used.

The generation of these columnar grains becomes more remarkable as the C content decreases. That is, although the ductility is desirable for the ultra-low carbonized steel having a C content of 0.0015% or less, this steel produces columnar grains more remarkably than the known steel having a C content of 0.003 to 0.005%. Has not been used as a cold-rolled steel sheet for deep drawing. Iron and Steel 70th Year (1984)
No. 15, pp. 332-334 describes that C: 0.003% steel was cooled at a cooling rate of 60 ° C / s. This report is to rapidly cool from 920 ℃ to room temperature, and if the rolled material is cooled to room temperature in the manufacturing process of the hot rolled coil, winding and adjustment of the material after winding cannot be performed. Although it is not a cooling method, its behavior is significantly different from that of ultra-low carbon steel with 0.0015% or less, and this technology is different from such ultra-low carbon steel.

[Problems to be Solved by the Invention] The present invention relates to a method for producing a cold-rolled steel sheet having extremely excellent deep drawability by producing a hot-rolled sheet having no columnar grains with a steel having an extremely low level of C of 0.0015% or less. .

[Means and Actions for Solving Problems] The present invention, in% by weight, is C ≦ 0.0015, Si ≦ 0.05, Mn ≦ 0.08, P
≦ 0.005, S ≦ 0.01, SolAl ≦ 0.10, N ≦ 0.003, and Ti: 0.0
04-0.06, Nb: 0.004 ~ 0.05 containing at least one or more, the balance Fe and unavoidable impurities steel, during hot rolling, finish rolling at a temperature of Ar3 or more, then Ar3 11 to the temperature range above the point and below (Ar3-30 ℃)
Cooling at a cooling rate of 0 to 400 ° C / s, winding at a temperature of 650 to 750 ° C, and then cold rolling and annealing according to a conventional method. It is a manufacturing method.

That is, according to the present invention, C: 0.0015% by weight or less of ultra-low carbonized steel is added with one or more kinds of Ti and Nb as carbonitride forming elements, and most of C and N harmful to deep drawability are precipitated and fixed. And reduce the amount to obtain high deep drawability,
The cause of reducing the deep drawability of these steels was found to be the coarse columnar structure of the hot-rolled sheet, and after this hot rolling was completed, the temperature range from Ar3 point or higher to (Ar3-30 ° C) or lower was 110
We have found a new finding that a hot-rolled sheet consisting of fine equiaxed grains can be obtained by ultra-rapid cooling of ℃ / sec or more, and when such hot-rolled sheet is cold-rolled and annealed, it has excellent deep drawability It was confirmed that it was possible to manufacture cold-rolled steel sheets and the invention was completed.

The present invention will be specifically described below.

It is desirable that the C content be low in order to deteriorate the deep drawability and ductility, and the lower the C content, the smaller the amount of Ti and Nb added to fix it and the amount of carbides formed. Is obtained. Note that 0.0015% by weight or less is a content that can be achieved by recent refining technology.

Since Si hardens the steel and reduces the deep drawability,
The lower the better, the more preferable is 0.05%.

Mn also hardens the steel and lowers the deep drawability, so it is desirable that it be low, and the upper limit is 0.08%.

Since P deteriorates the ductility, a lower value is preferable and 0.005
% Or less.

S forms Mn and MnS and precipitates. If it is present in a large amount, it hardens the steel and deteriorates the press formability, which is not preferable. 0.
01% or less.

The lower N increases the ductility of steel. Further, when N is low, the amount of added Ti or Nb and the amount of nitride produced are small, so that excellent deep drawability can be obtained. Therefore, in the present invention, the upper limit of N is 0.003%.

SolAl is added to deoxidize molten steel and improve the yield of Ti and Nb. However, if added excessively, the press formability of the steel sheet is impaired, so 0.10% is made the upper limit.

In the present invention, Ti and Nb are added in order to precipitate and fix C and N in steel and to obtain good press formability.

When Ti is 0.004% or less, C and N are not sufficiently precipitated and fixed. Further, this effect of Ti is sufficient if its content is 0.06%.

Nb is also contained in an amount of 0.004 to 0.05% by weight for the same reason as Ti.

The finish rolling temperature of the hot rolling of the present invention is Ar3 point or higher. If the number of Ar points is 3 or less, coarse grains will be generated in the hot rolled sheet and the work structure will remain, impairing the deep drawability of the steel sheet after cold rolling and annealing.

Next, the cooling rate of the present invention will be described. In the present invention, the temperature range from Ar3 point to (Ar3-30 ° C) is cooled at a cooling rate of 110 ° C / s to 400 ° C / s. The cooling start temperature is above the Ar3 point and the end of the cooling is below (Ar3-30 ° C), and it is sufficient to cool the temperature range from the Ar3 point to (Ar3-30 ° C) as described above. This cooling method has not been known so far,
It has a remarkable effect of preventing the generation of coarse columnar grains on the hot-rolled sheet to form a fine equiaxed grain structure. In particular, the effect of making the crystal grains fine without generating columnar grains becomes remarkable at a very high cooling rate.

Also, start cooling from Ar3 point or below, (Ar3-30 ℃)
When cooling is completed as described above, columnar grains are generated, and the deep drawability after cold rolling and annealing deteriorates.

The reason for this is that if the rolled material is strongly cooled in the temperature range of Ar3 to (Ar3-30 ° C), it will be in a supercooled state, the transformation point will drop, and the generation of α-grain nuclei will increase. It is considered that a fine equiaxed structure can be obtained. Therefore, this effect cannot be achieved at the conventional cooling rates of 30 ° C./s and 45 ° C./s, and becomes more remarkable at 110 ° C. as the critical cooling rate.

In the present invention, the upper limit of the cooling rate is 400 ° C / s. The cooling rate may be higher, but this range is easy to achieve.

In the present invention, the cooling can be started at an appropriate temperature of Ar3 or higher. That is, the cooling start timing does not have to be immediately after the finish rolling, and the cooling can be started at an appropriate position on the runout table. Therefore, in the present invention, even with a normal rolling mill in which a strip thickness gauge and a thermometer are arranged after the finishing rolling mill, the strip thickness and temperature of the rolled material can be measured without being affected by water vapor due to cooling, and therefore hot rolling can be performed. It is also easy to control.

Further, it is desirable that the cooling device is arranged as close as possible to the finishing mill within a range that does not hinder the operation of a thermometer and a plate thickness gauge that are usually arranged after the finishing mill. this is
This is because cooling is started from Ar 3 points or higher. Even when finishing rolling is finished near the Ar3 point, cooling can be performed from the Ar3 point or higher. Next, in the present invention, the winding temperature is 650 to 750 ° C.

If the temperature is 650 ° C or lower, the winding shape is easily damaged, and if it is 750 ° C or higher, the pickling property is deteriorated.

The hot rolling slab heating temperature is not particularly limited, but 10
Good materials can be obtained at temperatures above 00 ° C and below 1300 ° C.
It is preferable that the temperature is 1000 ° C. or higher and 1100 ° C. or lower because a better material can be obtained. In addition, a good material can be obtained even in the case of direct rolling after continuous casting. The hot rolled steel sheet produced by this method is cold rolled or annealed at room temperature. The conditions of cold rolling and annealing are not particularly limited, but the cold rolling rate is 40 to 95%,
Desirably, if it is 70 to 90%, a cold rolled steel sheet having very high deep drawability can be obtained. Also annealing is not preferable too high annealing temperature or too low temperature below the recrystallization temperature, continuous annealing, any method of box-type annealing,
The cold-rolled steel sheet having excellent deep drawability can be obtained by each ordinary annealing condition.

The cooling after the finish rolling may be any method such as water cooling or gas cooling.

Various platings such as zinc plating, tin plating, and chrome plating may be performed according to the application during continuous annealing or in the process thereafter.

Further, temper rolling, anticorrosion treatment, application of lubricant, etc. may be carried out as necessary.

[Example] A 245 mm slab was produced by continuous casting of molten steel according to a normal process. Table 1 shows the chemical composition of steel.

Then, after soaking at 1150 ° C. for 1.5 hours, rough rolling and finish rolling were obtained and wound at a predetermined temperature to form a hot coil. After that, pickling, cold rolling to 80%,
Continuous annealing was performed for 0 seconds, and temper rolling was performed at 0.6% to manufacture a cold rolled steel sheet.

Table 2 shows the r value of the cold rolled steel sheet, the crystal grain size of the hot coil, that is, the hot rolled sheet, and the presence or absence of columnar grains.

As shown in Table 2, of the chemical components within the scope of the present invention. To produce a cold-rolled steel sheet having extremely excellent deep drawability by using steel and cooling at a rolling end temperature and a cooling start temperature, a cooling end temperature, and a cooling rate in hot rolling within the scope of the present invention. You can see that

The Rankford value (r value) was used as a reference for deep drawability. r value is the rolling direction, the direction tilted ± 45 ° from the rolling direction,
It is the average of the values in the direction perpendicular to the rolling.

[Effects of the Invention] By doing so, it is possible to manufacture a cold-rolled steel sheet having extremely excellent deep drawability.

Claims (1)

[Claims] 1. At% by weight, C ≤ 0.0015 Si ≤ 0.05 Mn ≤ 0.08 P ≤ 0.005 S ≤ 0.01 Sol Al ≤ 0.10 N ≤ 0.003 and at least one of Ti: 0.004 to 0.06, Nb: 0.004 to 0.05 Containing the balance, the balance Fe and inevitable impurities, during hot rolling, finish rolling at a temperature of Ar3 points or more, then Ar3 points or more ~ (Ar3-30 ℃) below the temperature range of 110 Cooling at a cooling rate of ~ 400 ° C / s, 650 ~ 750
A method for producing a cold-rolled steel sheet having extremely excellent deep drawability, which comprises winding at a temperature of ℃, followed by cold rolling and annealing according to a conventional method.