JPH0514010B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for manufacturing cold rolled steel sheets using thin steel strips directly cast from molten steel.

Various proposals have been made in the past regarding methods of manufacturing cold-rolled steel sheets using thin steel strips directly cast from molten steel. However, there are currently no reports that steel plates with excellent formability can be obtained by these methods.

As a result of studying the problems with the material quality of thin sheets cast by the above method, we found that the r-value of cold-rolled sheets is lower than that of sheets produced through normal blooming or continuous casting-hot rolling processes. It became clear that it was an eye.

Therefore, research was conducted to overcome this drawback, but it was found that the above-mentioned values of the cold rolled sheet could not be improved no matter how the cold rolling conditions, annealing conditions, etc. were changed. Therefore, we have come to the realization that the previously proposed direct casting method itself, that is, the method in which only cooling is performed after casting until winding, is flawed.

An object of the present invention is to provide a method for manufacturing a cold-rolled steel sheet that can improve the properties of the cold-rolled sheet by applying an appropriate thermal cycle to the steel strip after casting.

[Summary of the invention]

The feature of the method for producing cold rolled steel sheets of this invention is that the C content of molten steel is adjusted to 0.015% or more, a thin steel strip for cold rolling is directly cast from this molten steel, and the temperature of the steel strip after solidification is at least 800°C. After cooling to a temperature below 900°C, the steel strip is reheated to 900°C or higher, cooled again to 800°C or lower, and wound into a steel strip, followed by pickling, cold rolling, and annealing.

FIG. 1 shows the thermal cycle of a steel strip directly cast from molten steel according to the present invention. The effects of cooling temperature T ₁ , reheating temperature T ₂ , and winding temperature T ₃ shown in the figure were investigated. As a result of our research, we succeeded in manufacturing a cold-rolled sheet with excellent formability using an appropriate combination of these.

The specific conditions will be described below.

(1) Cooling temperature (T ₁ ) The value is improved by setting it to 800°C or less, preferably 750°C or less. At temperatures above this temperature, reheating becomes ineffective and the value decreases.

(1) Reheating temperature (T ₂ ): 900℃ or higher and 1200℃ or lower. In a temperature range lower than this, reheating has no effect. The value also decreases at higher temperatures. This decrease in value may be due to the fact that the reheating temperature is too high and the cooling effect before reheating is lost.

If the reheating temperature is too low, the effect of cooling before reheating cannot be achieved, and if it is too high, the effect will be negated, so cooling and reheating are inseparable from each other. It is in.

The reheating holding time may be at least 1 second, preferably at least 2 seconds. This holding time may be maintained at a constant temperature, but may also be a time during which the temperature remains within the reheating temperature range of the present application, and in reality, the apparatus may be designed accordingly.

The cooling rate in recooling may be either water cooling or air cooling, but the effect of reheating is lost in furnace cooling. In the case of furnace cooling, this has a negative effect because the thermal cycle is similar to that of increasing the winding temperature.

The metallurgical reason why a steel plate with excellent formability can be obtained by using the steel strip manufactured by the casting method of the present application is unknown, but as described later, the effect of the present application is lost when the C content is low. Therefore, it is inferred that this is a phenomenon in which C was involved. In addition, if the reheating temperature is too high, the value decreases, but this seems to be because in this temperature range (1200°C or higher) the diffusion of alloying elements cannot be ignored, which has a negative effect on the value.

(3) Winding temperature (T ₃ ) 800°C or less, preferably 750°C or less. In a temperature range higher than this, the cooling and reheating effects described above are lost and the value decreases. Therefore, it is necessary to re-cool it below the above temperature.

Note that at temperatures below 600°C, the value decreases slightly in the case of rapid heating annealing, but this is due to the same effect as the winding temperature of hot rolled sheets in continuous annealing, and the winding conditions are normally called low temperature winding. Conceivable. When casting Al-killed steel for box annealing, it is sufficient to wind it at 600°C or lower, similar to winding in normal hot rolling.

(4) Cooling rate and processing time There is no particular effect on the cooling rate after solidification, whether air cooling or water cooling.

The holding time after cooling has no essential effect in this technique, and in the extreme, the cooling temperature conditions of the present application may be instantaneously satisfied. Extending this holding time has no effect on the cooling effect, but it can actually be determined based on the arrangement of the apparatus.

(5) Effect of C content The C content shall be 0.015% or more. Below this, even if the method of the present application is used, that is, an appropriate combination of cooling, reheating, and recooling, a decrease in the value is unavoidable.

Note that the value decreases slightly on the high C side, but this is the same tendency as that of continuous annealing materials manufactured from ordinary hot-rolled sheets.

(6) Other component system Regarding the amount of components other than the amount of C, the effect is similar to that in a cold rolled sheet using a normal hot rolled sheet. for example
The value can be improved by reducing the amount of Mn.

[Embodiments of the invention]

Example 1 Material C Si Mn P S 0.022 tr 0.13 0.017 0.013 Solidified to a thickness of 3.3 mm, cooled to 700 to 950°C by water cooling, reheated with a direct burner, held at 950°C for 5 seconds, and water-cooled to 700°C. I rolled it up. After pickling the cast steel strip, it was cold rolled to a thickness of 0.8 mm, and then continuously annealed at 700°C for 30 seconds to adjust the pressure to an elongation rate of 1%. Figure 2 shows the results of making JIS No. 5 tensile test pieces and measuring the values.

The value increases as the cooling temperature decreases, and high values are obtained at 800°C or lower, preferably 750°C or lower.

Example 2 Cooling temperature is constant at 750℃, reheating temperature is 750~
Values were determined under the same conditions as in Example 1 except that the temperature was changed to 1300°C. The results are shown in Figure 3.

The value increases in the reheating temperature range of 900℃ or higher and 1200℃ or lower.

Example 3 Values were measured under the same conditions as in Example 1, except that the cooling temperature was 750°C and the winding temperature was 550 to 950°C. The results are shown in Figure 4.

The value increases when the winding temperature is below 800℃.

Example 4 Values were measured under the same conditions as in Example 1, except that the cooling temperature was 750°C and the reheating time was 0.5 to 100 seconds. The results are shown in Figure 5.

A low value is obtained when the reheating time is 0.5 seconds, but a high value is obtained when the reheating time is 1 second or more.

Example 5 Material C Si Mn P S 0.007 tr 0.12 0.011 0.009 ~ ~ ~ ~ 0.067 0.24 0.023 0.018 Except that the cooling temperature of the above material was 750°C.
A cold-rolled plate was manufactured under the same conditions as in Example 1, and the values were measured.

The results are shown in Figure 6. A high value can be obtained at 0.015% C.

〔Effect of the invention〕

The method for manufacturing a cold rolled steel sheet of the present invention is as described above, and when manufacturing a cold rolled steel sheet using a steel sheet directly cast from molten steel, the value of the cold rolled sheet is improved by heat treating the cast steel strip. can be done.

[Brief explanation of the drawing]

FIG. 1 is an explanatory diagram of the heat treatment cycle of cast steel strip in the present invention, FIG. 2 is an explanatory diagram showing the relationship between cooling temperature and value, FIG. 3 is an explanatory diagram showing the relationship between reheating temperature and value, and FIG. FIG. 5 is an explanatory diagram showing the relationship between winding temperature and value, FIG. 5 is an explanatory diagram showing the relationship between reheating holding time and value, and FIG. 6 is an explanatory diagram showing the relationship between C amount and value.

Claims (1)

[Claims] 1 Adjust the C content of molten steel to 0.015% or more, directly cast a thin steel strip for cold rolling from this molten steel, cool the steel strip temperature after solidification to at least 800 ° C.,
A method for producing a cold-rolled steel sheet, which comprises reheating the steel strip to 900°C or higher, cooling it again to 800°C or lower, winding the steel strip, and then performing pickling, cold rolling, and annealing.