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Cast Steel-making Processes

1. Field of the Invention

This invention relates to a method for producing steel castings by continuous casting process.

2. Description of the Prior Art

In continuous steel casting, there arise problems of defects as detected by ultrasonic test, e.g., inclusions occurring in a sub-surface or internal portion of a continuously cast strand (hereinafter referred to as "c.c. strand" for brevity) in its solidifying stage or shrinkage cavities produced in axial center portions of the c.c. strand. In addition, strong segregation occurs in c.c. strands cast at high temperature in continuous casting operations, impairing cold forgeability due to lowered reduction ratio.

Various attempts have thus far been made to eliminate the internal defects of c.c. strands, including the center segregations and shrinkage cavities, through single electromagnetic stirring either within a mold or in a secondary cooling zone, severing tip ends of growing crystals with fluidic movements of molten steel to produce a large quantity of equiaxed crystal nuclei, Steel casting process thereby expanding the equiaxed crystal zone in the center portion of c.c. strands. However, none of them has succeeded in sufficiently reducing the rate of center segregation and irregularities of center segregation in the axial direction of c.c. strands, failing to produce steel castings of satisfactory quality.

It is a primary object of the present invention to provide a method which overcomes the above-mentioned problems and which is capable of producing steel castings of satisfactory quality with less center segregations in continuous steel casting processes.

In order to attain this object, the method of the present invention includes, in its preferred form, the step of electromagnetically stirring molten metal in at least two of three locations, viz., a casting mold and intermediate and final solidifying zones of a continuously cast strand, by application of:

for electromagnetic stirring in the casting mold, a magnetic field induced by alternate current of a frequency f=1.5.about.10 Hz and having G (Gauss) in the range of 195¡Áe -0 .18f .about.1790¡Áe -0 .2f at the inner surface of the casting mold;

for Cast steel electromagnetic stirring in the intermediate solidifying zone, a magnetic field induced by alternate current of a frequency f=1.5.about.10 Hz and having a magnetic flux density G in the range of 195¡Áe -0 .18f .about.1790¡Áe 0 .2f at the surface of the strand or a magnetic field induced by alternate current of a frequency f=50.about.60 Hz and having a magnetic flux density G in the range of 0.6¡Á10 6 /(D-107) 2 .about.1.8¡Á10 6 /(D-100) 2 (in which D= the thickness of a solidified shell layer of the strand (mm)) at the surface of the strand;

In the production of a low carbon steel by the continuous casting process, there arises a problem of shrinkage cavities occurring in the center portions of c.c. strands, which is a problem inherent in low carbon steels, in addition to the above-mentioned problem of inclusions. This problem can be eliminated by an electromagnetic stirring treatment in a final solidifying zone of the c.c. strand further to the stirring treatment in the mold and/or in the intermediate solidifying zone.