JP3390606B2 - Steel continuous casting method - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for preventing slab internal cracks in continuous casting. 2. Description of the Related Art As shown in FIG. 1, continuous casting of steel is performed by melting molten steel 2 in a ladle 1 from a long nozzle 3 to a tundish 4.
And then continuously inject the molten steel 2 from the immersion nozzle 5 into the mold 6. And mold 6
The molten steel 2 injected into the inside is quenched here, so that a solidified shell 7 is formed on the inner wall of the mold 6, and the solidified shell 7 is supported by the guide roll 8 while being supplied by the secondary cooling water supplied here. It is cooled and completely solidified, and is continuously drawn out as a slab 9 by a pinch roll 10. Reference numeral 11 denotes a mold powder. [0003] In recent years, for the purpose of improving productivity,
In order to achieve the production of high-temperature slabs and the stabilization of the direct-feed rolling process, continuous casting has been accelerated. In establishing this high-speed casting technique, one of quality problems is cracking inside the slab (hereinafter abbreviated as "internal cracking"). The occurrence of this internal crack is caused by an increase in strain at the solidification interface. The distortion acting on the solidification interface includes bulging distortion, bending distortion, correction distortion, roll misalignment distortion, distortion due to thermal stress, and the like. Among them, bulging distortion is considered to be the main. For this reason, especially in order to reduce bulging distortion,
Secondary cooling has been enhanced. In addition, with the recent high-speed casting, internal cracks tend to occur frequently near the short sides,
It has come to be seen as a problem. It is known that internal cracks near the short side are caused by stress due to uneven slab temperature and thermal stress at the time of recuperation, and are caused by tensile stress in the slab thickness direction. Conceivable. Several studies have been made to suppress this internal cracking. For example, Japanese Patent Application Laid-Open No. 63-154249 discloses that appropriate secondary cooling, particularly cooling of the short side of a slab, has been proposed. It is disclosed to be effective. [0005] However, in the above-mentioned prior art in which the secondary cooling on the short side is performed, if the cooling is strengthened, the temperature of the corner of the slab is lowered. In the case of a steel type with a high crack sensitivity of 1 wt% or more, corner surface cracking (key cracking;
Abbreviated. ) Is more likely to occur. As described above, none of the conventional techniques can suppress both internal cracks and corner cracks. Accordingly, an object of the present invention is to provide a continuous casting method for producing a sound slab which does not involve any of internal cracks, particularly internal cracks near the short side, and corner cracks. Another object of the present invention is to provide a steel having a high corner cracking susceptibility and a C content of 0.1 wt% or more,
It is an object of the present invention to provide a continuous casting method for producing a sound slab without any of the internal cracks and corner cracks even in the case of high-speed casting of 2.0 m / min or more. According to the present invention, in the secondary cooling of the slab, the short side of the slab is completely covered only at a position corresponding to １ ／ to ／ thickness of the short side. It was developed as a basic concept of forced cooling until solidification. That is, according to the present invention, in the secondary cooling of the slab, the surface temperature at the center of the thickness of the short side of the slab becomes 1000 ° C. or less after the bending position of the continuous casting machine, and the surface temperature of the corner portion becomes 75 at the bending and straightening positions of the continuous caster
A continuous casting method for steel, characterized by cooling to 0 ° C. or higher. As described above, the occurrence of internal cracks in continuous casting can be prevented by strengthening the secondary cooling, but on the other hand, the occurrence of corner cracks is caused. The inventors have repeated research on a secondary cooling method in order to cope with such conflicting crack behavior. As a result, in order to solve the above problems, in the secondary cooling under the mold,
As shown schematically in FIG. 2, it is extremely effective to cool the short side of the slab only at a position corresponding to 位置 to ／ thickness of the short side until it is completely solidified. I found something. By applying such a secondary cooling method, 1) the surface temperature at the center of the short side thickness can be maintained in a temperature range of 1000 ° C. or less after the bending position of the continuous casting machine, and a sufficient solidified shell can be obtained. Therefore, it is possible to prevent the occurrence of internal cracks near the short side. 2) The cooling of the short side is performed only at a position corresponding to 1/4 to 3/4 thickness of the short side, and the corner is not forcibly cooled. Can be maintained in a temperature range of 750 ° C. or higher, preferably 800 ° C. or higher, which avoids the surface temperature of the corner portion at the brittle temperature, specifically, 700 to 745 ° C., and prevents corner cracking. It becomes possible. In addition, it is necessary to perform the above-mentioned forced cooling up to complete solidification, since the occurrence of internal cracks becomes more remarkable at the end of solidification when the embrittlement zone width becomes thicker, and is necessary from the viewpoint of preventing the occurrence of thermal distortion due to reheating. Because. In addition to the above-mentioned secondary cooling of the short side of the slab,
Secondary cooling is also performed on the long side of the slab. The condition of the secondary cooling on the long side of the slab is not particularly defined because the effect of the present invention can be obtained if it is performed according to a conventional method, but for example, in order to prevent a decrease in corner temperature, 100 to 150m
In the range of m, it is preferable to cut the cooling water. The specific water amount for cooling was 0.0% on the short side of the slab.
4 to 0.08 l / kg, 1.8 to 1.8 mm on long side of slab
It is preferable to carry out in the range of 2.0 liter / kg. As other casting conditions, the casting speed is 1.0 to 2.
It is good to be 0 m / min. EXAMPLES Using a vertical bending-type continuous casting machine having a radius of curvature of 10 m, aluminum-killed steel having the composition shown in Table 1 was produced at various casting speeds into cast slabs having a width of 1250 mm and a thickness of 220 mm. At this time, the cooling condition on the long side was made constant at a specific water volume of 1.8 liter / kg, and the cooling condition on the short side was as shown in Table 1 (however, the specific water volume during cooling was 0.05 liter / kg). The experiment was performed by changing Here, the cooling on the short side of the invention example was performed only at a position corresponding to 1/4 to 3/4 thickness of the short side until a position 10 m from the meniscus, that is, until the solidification was completed. For comparison, in accordance with the conventional process, cooling on the short side was performed from the meniscus to the position of the support roll 2.5 m (Comparative Examples A to C), and over the entire thickness of the short side, 10 m from the meniscus. (Comparative Example D) was also carried out. [Table 1] The slab continuously cast by the above method is
In addition to measuring the surface temperature at the center and corners in the thickness direction of the short side of the slab, internal cracking was investigated by sulfur printing of a sample perpendicular to the casting direction in the casting direction, and corner cracking was investigated by color checking the slab corner surface. . Table 1 also shows the results. As shown in Table 1, in the invention example, the surface temperatures of the central portion and the corner portion were within a predetermined temperature range, and there were no internal cracks or corner cracks. However, in the comparative example, one of the above-mentioned temperatures was out of the predetermined temperature range, and either internal cracks or corner cracks occurred, and a sound slab could not be obtained. FIGS. 3 and 4 show the results of measuring the surface temperatures of the central portion and the corner portion in the thickness direction of the short side of the slab in the above experiment. As can be seen from FIG. 3, the surface temperature at the center of the short side increases with an increase in the casting speed. Has become. On the other hand, the cooling of the short side is performed by the
m, the casting speed is 2.0 m
Despite the high speed, the surface temperature at the center of the short side is 1000 ° C. or less after the bending position. Such a difference in the surface temperature at the center of the short side corresponds well to the behavior of the internal crack of the slab, and the occurrence of the internal crack depends on the surface temperature of 1000 ° C. Further, as can be seen from FIG. 4, in Comparative Example D in which cooling was performed over the entire thickness of the short side, the surface temperature of the corner portion at the bending position and the correction position entered the brittle temperature range of 700 to 745 ° C. Is consistent with what occurred. On the other hand, in the invention example in which only the position corresponding to １ ／ to ／ thickness of the short side is cooled, the surface temperature of the corner becomes relatively high, and the embrittlement occurs at any of the bending position and the correction position. The temperature range could be avoided on the high temperature side, which is in good agreement with the fact that corner cracking did not occur. According to the present invention, it is possible to obtain a sound continuous cast slab which does not generate any of internal cracks, especially internal cracks near the short sides and corner cracks. Further, according to the present invention, it is possible to obtain a continuously cast slab which does not generate both internal cracks and corner cracks, even if the C content is 0.1 wt% or more. Further according to the invention, 2.0
Since the above-mentioned effects can be obtained even in high-speed casting at m / min or more, the productivity of continuous casting is dramatically improved.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic view showing a continuous casting process. FIG. 2 is a schematic diagram showing a cooling state on a short side of a slab according to the present invention. FIG. 3 is a diagram showing a transition of a slab surface temperature in a central portion in a thickness direction of a short side. FIG. 4 is a diagram showing a transition of a slab surface temperature at a corner portion. [Description of Signs] 1 Ladle 2 Molten steel 3 Long nozzle 4 Tundish 5 Immersion nozzle 6 Mold 7 Solidified shell 8 Guide roll 9 Slab 10 Pinch roll 11 Mold powder 12 Ca

Continuation of the front page (72) Inventor Kenichi Sorimachi 1-chome, Mizushima-Kawasaki-dori, Kurashiki-shi, Okayama Pref. Kawasaki Steel Corporation Mizushima Works (56) References JP-A-60-40658 (JP, A) (58) ) Surveyed field (Int.Cl. ⁷ , DB name) B22D 11/124 B22D 11/22

Claims (1)

(57) [Claims 1] In the secondary cooling of the slab, the surface temperature at the center of the thickness of the short side of the slab becomes 1 after the bending position of the continuous casting machine.
A continuous casting method for steel, characterized in that the steel is cooled so as to have a temperature of 000 ° C or less and a surface temperature of a corner portion of 750 ° C or more at a bending position and a straightening position of the continuous casting machine.