JPS6199630A - Manufacture of cold-rolled steel sheet - Google Patents

Abstract

PURPOSE:To obtain a steel sheet having superior formability, by heat-treating a cast thin steel plate contg. a specific percentage of C at a prescribed temp. CONSTITUTION:The steel contg. >=0.015% C is refined. This molten steel is directly cast into a thin steel strip for cold rolling. The solidified strip is cooled to <=800 deg.C, reheated to >=900 deg.C, and cooled to <=800 deg.C to be wound up. After that, pickling, cold-rolling, and annealing are carried out. In this way, the steel sheet having superior formability can be obtained.

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 examining 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 process. It became clear that.

Therefore, research was conducted to overcome this drawback, but it was found that the value 7 of the cold-rolled sheet described above could not be improved no matter how the cold rolling conditions, annealing conditions, etc. were changed. Therefore, it has been recognized that the direct casting method itself proposed in the past, that is, the method in which only cooling is performed after casting until winding, is prone to defects.

An object of the present invention is to provide a method for producing a cold-rolled steel sheet with K value such that the 7 value of the cold-rolled sheet can be improved by applying an appropriate heat cycle to the steel strip after casting.

[Summary of the invention]

The feature of the method for manufacturing cold rolled steel sheets of this invention is that the C content of molten steel is reduced to 0.
．． 015- or higher, and from this molten steel, thin steel for cold rolling is produced.
Directly cast the steel strip after solidification to at least 8 degrees
After cooling to below 00°C, reheating to above 900°C, cooling again to below 800°C and winding the steel strip, then pickling,
Cold rolling and annealing are performed.

FIG. 1 shows the thermal cycle of a steel strip directly cast from molten steel according to the present invention. The effects of the cooling temperature Tl, reheating temperature T8, and winding temperature T3 shown in the figure were investigated. Research results: Using an appropriate combination of these, we succeeded in producing a cold-rolled sheet with excellent formability.

The specific conditions will be described below.

(1) Cooling temperature (T1) 800°C or less, preferably 750°C or less, improves the r value. If the temperature is higher than this, the reheating effect will be lost and the value will decrease.

(1) Reheating temperature (1) Set at 900° C. or higher and 1200° C. or lower. In a lower temperature range, reheating has no effect. In addition, even in a higher temperature range, the r value decreases.

This decrease in r value is thought to 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. Nichiru.

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.

A steel strip manufactured by the casting method of the present application is used.

The metallurgical reason why a steel plate with particularly excellent formability is obtained is unknown, but as described later, the effect of the present application is lost when the amount of C is low, so it is a phenomenon that C is involved. It is inferred that. Also, if the reheating temperature is too high, the 7 value decreases, but this is because diffusion of alloying elements cannot be ignored in this temperature range (1200°C or higher), and this may have a negative effect on the r value. Seem.

(3) Winding temperature (TI) The temperature is 800°C or lower, preferably 750°C or lower.

In a temperature range higher than this, the cooling and reheating effects described above are lost and the r value decreases. Therefore, it is necessary to re-cool it to below the above temperature.

Note that at temperatures below 600°C, the r value decreases slightly in the case of rapid heating annealing, but this is the same effect as the winding temperature of hot rolled sheets in continuous annealing, and the winding conditions are normally called low-temperature winding. it is conceivable that. When casting kt killed steel for box annealing, 600 mm is the same as for winding in normal hot rolling.
It is sufficient to wind it at a temperature below ℃.

(4) Cooling rate and processing time The cooling rate after solidification is not particularly affected by 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 invention are instantaneously met, so it may be set for 3 hours. 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) Influence of Ci Cfk shall be 0.0151 or more. Below this, a decrease in the r value is unavoidable even with the method of the present application, ie, an appropriate combination of cooling, reheating, and recooling.

Note that the r 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 The amounts of components other than the amount of C are similar to those in a cold rolled sheet using a normal hot rolled sheet. For example, the F)r value can be improved by reducing the amount of Mn.

[Embodiments of the invention] Example 1 Material CSt Mn P SO, 022tr O, 130, 0170, 0133,
Solidify to a thickness of 2m, cool by water cooling to 00-950℃, reheat with an open flame burner and hold at 950℃ for 5 seconds,
It was water-cooled to 700'C and wound up. After pickling the cast steel strip,
After cold rolling to a thickness of 0.8 m, continuous annealing was performed at 700° C. for 30 seconds, and the pressure was adjusted to an elongation rate of 1. JIS
A No. S tensile test piece was prepared and the r value was measured. The results are shown in FIG.

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

Example 2 The cooling temperature was kept constant at 750°C, and the reheating temperature was set at 750-1
The r value was determined under the same conditions as in Example 1 except that the temperature was changed to 300°C. The results are shown in Figure 3.

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

Example 3 Sample 7 was measured under the same conditions as 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.

7 value increases when the winding temperature is 800°C or less.

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

When the reheating time is 0.5 seconds, the r value is low, but when the reheating time is 1 second or more, a high r(]!E is obtained.

Example 5 Material CSt Mn P 8 A cold-rolled sheet was produced under the same conditions as Example 1, except that the cooling temperature of the above material was 750° C., and the r value was obtained.

C Effects of the invention] The method for manufacturing a cold rolled steel sheet of this invention is as described above,
When producing a cold rolled steel sheet using a steel strip directly cast from molten steel, the r value of the cold rolled sheet can be improved by heat treating the cast steel strip.

[Brief explanation of drawings]

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 r value,
Figure 3 is an explanatory diagram showing the relationship between reheating temperature and r value, Figure 4 is an explanatory diagram showing the relationship between winding temperature and r value, and Figure 5 is an explanatory diagram showing the relationship between reheating holding time and r value. Figure 6 shows the amount of C and r
It is an explanatory diagram showing the relationship of values.

Claims (1)

[Claims] The C content of the 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 cooled to at least 800°C or lower, and then raised to 900°C or higher. A method for producing a cold-rolled steel sheet, which comprises reheating the steel strip, cooling it again to 800° C. or lower, winding the steel strip, and then performing pickling, cold rolling, and annealing.