JPS6028579B2 - Slab straightening method in continuous casting - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for straightening a slab that prevents the occurrence of roughness on the surface of the slab when the slab cast by curved continuous casting equipment is straightened horizontally.

Among the surface defects of slabs cast by curved continuous casting equipment, the most problematic are surface irregularities consisting of transverse cracks that occur when the slab is straightened horizontally. This surface roughness occurs more frequently as the target material becomes a higher grade product, and has become a key factor in the continuous casting of high grade products. When observing the microstructure of the horizontal cracking habit,
It can be seen that the texture is along the austenite grain boundaries, and pro-eutectoid ferrite precipitates from the austenite grain boundaries. In addition, the austenite grains in the cracking area are characterized by coarsening. On the other hand, in curved continuous casting equipment, when straightening a slab horizontally, a straightening stress equivalent to 1% of strain acts in the thickness direction of the slab. That is, the cause of the occurrence of transverse cracking during straightening is considered to be that straightening stress acts on the high-temperature brittle zone in which primary ferrite is precipitated at the coarsened austenite grain boundaries and is in a two-phase state.

This invention aims to control the slab surface temperature at the straightening point in order to prevent the tendency of cracking, taking into account the above-mentioned cause of the occurrence of cracks.

That is, the present invention calculates the surface temperature of a slab cast by curved continuous casting equipment just before reaching the straightening point.
From the precipitation temperature of first prayer ferrite in the continuous cooling transformation curve (hereinafter referred to as CCT curve) created by heating to the austenite region for each component material, 10
It is proposed that the slab be horizontally straightened during traversing by maintaining the temperature within a temperature range of 0.000C or less.

The inventors of the present invention have conducted dozens of tests to prevent slab surface defects on mirror pieces using CCT curves, and have found that the CCT curve for continuous casting is based on the CCT curve for heat treatment from the austenite range temperature. It was found from the test results that it was optimal.

Therefore, the precipitation temperature of the pro-eutectoid ferrite can be determined from the CCT curve determined for each component material. As mentioned above, it is necessary to control the slab surface temperature at the straightening point to be higher than the precipitation temperature of the first folded ferrite, but if the temperature is too high, the austenite will become coarser and the frequency of horizontal cracking will increase.

Therefore, the inventor conducted various tests regarding the appropriate slab surface temperature. As an example, the test results for API standard X-69/P material are shown in FIG. The figure shows the relationship between the surface temperature of the slab straightening point and the frequency of occurrence of transverse cracking, which is based on the initial ferrite precipitation temperature. It can be seen that when the ferrite precipitation temperature exceeds 10,000, the occurrence of horizontal cracks rapidly increases. Therefore, it is desirable to control the slab surface temperature at the slab straightening point to a temperature range exceeding the pro-eutectoid ferrite precipitation temperature and within 100 qo. Next, embodiments of the invention will be described.

C.O. 09%, Sio. 32%, Mnl. 39%, P.O.
．． 019%, SO. 02%, Cuo. 16%, Cro.
15%, Moo. 01%, VO. 02%, Nbo. 04
CCT of steel (API standard X-69/P material) consisting of 6%, SoA 0.047%, residual iron and unavoidable impurities
The curve obtained is as shown in Figure 2. When making a slab of dimensions 1800 x 190 from this steel using curved continuous casting equipment, temperature control at the time of straightening the slab horizontally (position with P/R pinch rolls) was performed using the conventional method 1. (b) A method of straightening at a temperature lower than the temperature (730°C), (b) By carrying out this invention, the slab surface temperature can be reduced to 78°C.
The correction was carried out by controlling the temperature within a temperature range of 000°C.

In the conventional method No. 1, as can be seen from the CCT curve in Figure 2, the correction is performed in a two-phase region where pro-eutectoid ferrite precipitates at the austenite grain boundaries, and as seen in the micrograph in Figure 3, the correction is performed at the austenite grain boundaries. Horizontal cracks had occurred along the ridge, and these cracks had reached the depth of the maximum IQ rib.

On the other hand, in the method of the present invention (b), the correction was made in the austenite one-phase region, and as shown in the micrograph of FIG. 4, no cracks were generated and no slab surface defects were observed.

This invention can be applied to all continuous casting methods, regardless of whether they are curved or vertical.

[Brief explanation of the drawing]

Figure 1 is a chart showing the relationship between the slab straightening point surface temperature and the frequency of occurrence of horizontal cracking in continuous casting of API standard X-69/P material, and Figure 2 is the CCT of the target steel in one embodiment of the present invention. 3 is a microscopic photograph of the surface of a slab that has been straightened by the conventional method, and FIG. 4 is a microscopic photograph of the surface of a slab that has been straightened by the method of the present invention. Figure 1 Figure 2 Figure 4 Figure 3

Claims (1)

[Claims] 1 Continuous cooling transformation curve (CC
The temperature range is controlled within 100℃ from the precipitation temperature of pro-eutectoid ferrite in the T curve), and the slab being poured is straightened within the above temperature range to prevent the occurrence of surface defects on the slab. A method for straightening slabs in continuous casting, characterized in that: