Quantitative Relationship between Degree of Center Segregation and Large Carbide Size in Continuously Cast Bloom of High Carbon Chromium Bearing Steel

摘要

One of the disadvantages of the continuous casting process, compared to ingot casting, is the center segregation, which causes the formation of large carbides in blooms of high carbon chromium bearing steel. Many activities have been performed to minimize the center segregation by steel manufacturers, and until now, the soft reduction is chosen as the best way to control it. Large carbides formed during casting, detrimental to the rolling contact fatigue life of bearing components, can just be eliminated by holding blooms at high temperatures for a long time before hot rolling, which is called the soaking process. Therefore it is necessary to examine the relationship between the degree of center segregation and large carbide size in continuously cast blooms for a more efficient soaking process. The aim of this research is to describe the relationship quantitatively. Continuously cast blooms of high carbon chromium bearing steel, AISI 52100, were investigated, and the degree of center segregation was not defined as the ratio of carbon concentration in the segregated region to that of nominal composition (C/Co), as it is defined conventionally, but evaluated with a discrete index by comparing the macrograph of a bloom with the standard one settled arbitrarily for the study. The higher was the degree of center segregation, the bigger was the large carbide, and the quantitative relationship between the degree of center segregation and the maximum size of the large carbide was well fitted linearly with a reliability of 95.9 %. In order to apply soft reduction adequately during casting for the study, an in situ equipment to measure the real thickness of a solidifying bloom was installed in front of the soft reduction zone, and both the degree of center segregation and the large carbide size were improved.