Study of factors contributing to decrease of carbide heterogeneity in bearing grades of steel
At present to meet requirement to the quality of bearing steels it is necessary of accomplish estimation of carbide heterogeneity by SEP 1520 and provide the level of carbide network, carbide liquation and streaking, not exceeding adjusted limits, required by customer. To estimate possibility to decrease carbide heterogeneity (segregation), factors influencing its value in the rolling production considered. Description of the process of long products of bearing steels production at the rolling mill 370/150 of the OJSC “BMZ - managing company of the holding “BMK” presented. Possibilities of the existing mill equipment considered for 804 accomplishing of technological operations aimed at decrease of carbide heterogeneity in the finished product. The factors studied, contributing to decrease of carbide heterogeneity under conditions of mill 370/150 operation. To determine the temperature modes effect on the level of carbide heterogeneity in bearing steels, in the process of three campaigns phased decrease of the temperature of end of rolling was accomplished. Results of data analysis obtained at the production of long products profiles of 34-50 diameter of bearing steel grades presented. Conclusion on rolled stock production of bearing steel grades with high requirements to carbide heterogeneity value was formed. It was shown that to obtain high-quality long products it is appropriate to accomplish the heating in the furnace before the rolling no less than 600 min at the temperature in the soaking zone 1150-1220°С followed by application of technology of normalizing rolling and control of temperature of the end of rolling 750°С. For profiles with drawing less 25% it is recommended to accomplish additional heat treatment of the finished profile - normalization. Long time heating of workpieces before the rolling at the temperature 1150-1220°С enables to decrease the carbide heterogeneity down to acceptable level (carbide liquation no more than 3 points and carbide streaking no more than 4 points). The value of carbide network is decreasing at decreasing of temperature of the end of the rolling and increasing deformation value in the last stands. A stable satisfactory result (CN no more than 5.4) was reached at the temperature of the end of rolling 750°С and drawing more than 25%. It was established that the higher the total drawing, the lower the point of carbide network and liquation in the finished profile, which is caused by crushing of nondissolved carbides to separate fragments.
Improvement of the technological process of manufacturing bearing grades of steel at the 370/150 mill.
