Influence of soft reduction on internal qualityof GCr15 bearing steel bloom castings
QI Meng1, LI Liang1, TIE Zhan-peng1,2, WANG Pu1, MIAO Hong-sheng2, ZHANG Jia-quan1
1. School of Metallurgical and Ecological Engineering, University of Science and Technology Beijing, Beijing 100083, China; 2. Xining Special Steel Co. , Ltd. , Xining 810000, Qinghai, China
Abstract:Central soundness and carbide defects of bearing steel bars are closely related to the internal quality control of bloom castings. Taking GCr15 bearing steel as the research object, a two-dimensional longitudinal solidification and heat transfer model was established for the casting process of the bloom semi-products. The accuracy of solidification model is verified by field temperature measurement. Based on the theory of compensating the local solidification shrinkage and controlling the central shrinkage cavity under the soft reduction at the end of solidification, the reasonable soft reduction interval in the pasty region can be revealed through the accurate prediction of the solidification process. Among them, under the conditions of casting tests, the effective soft reduction interval corresponding to the solid phase ratio of the center of the casting is from 0.30 to 0.75, which refers to 16.4-22.5 m from the meniscus. The test results showed that the solidification structure and morphology of the bloom castings are little affected by soft reduction. However, after soft reduction, the central porosity index drops from greater than or equal to 1.5 to 0.5-1.5, which can meet the rolling requirements, and the central shrinkage cavity is basically disappeared, indicating that reasonable soft reduction position and moderate soft reduction amount can obviously improve the degree of central porosity and central shrinkage cavity of the bearing steel bloom castings, so as to improve the qualified rate of the following rolled material. However, the unreasonable or unstable distribution of the position and amount of press-down may lead to the occurrence of mid-way cracks, thus reducing the stability and consistency of the rolled material quality. For the present production, it is recommended to keep the constant casting speed and press down at No.3 to No.6 pressure roller to assure the internal quality of the steel.
Tamura A, Suzuki M, Kimura K, et al. Application of a soft reduction process with the bloom continuous caster in Kokura steel works[J]. Sumitomo Metals, 1993, 45: 103.
SUN Hai-bo, LI Lie-jun, YE De-xin, et al. On the alternate stirring mode of F-EMS for bloom continuous castings[J]. Metallurgical and Materials Transactions: B, 2018,49(4):1909.
[7]
JIANG D, ZHU M. Solidification structure and macrosegregation of billet continuous casting process with dual electromagnetic stirrings in mold and final stage of solidification: A numerical study[J]. Metallurgical and Materials Transactions:B, 2016, 47(6):3446.
[8]
Louhenkilpi S, Laitinen E, Nieminen R. Real-time simulation of heat transfer in continuous casting[J]. Metallurgical Transactions:B, 1993, 24(4):685.
Lally B, Biegler L, Henein H. Finite difference heat-transfer modeling for continuous casting[J]. Metallurgical Transactions:B, 1990,21(4):761.
[12]
WANG H, LI G, LEI Y, et al. Mathematical heat transfer model research for the improvement of continuous casting slab temperature[J]. Transactions of the Iron and Steel Institute of Japan, 2006,45(9):1291.
[13]
陶文铨. 数值传热学[M]. 西安:西安交通大学出版社,2001.
[14]
LIU H, YANG C, ZHANG H, et al. Numerical simulation of fluid flow and thermal characteristics of thin slab in the funnel-type molds of two casters[J]. ISIJ International, 2011, 51(3): 392.
WANG Pu, ZHANG Zhuang, TIE Zhan-peng, et al. Initial transfer behavior and solidification structure evolution in a large continuously cast bloom with a combination of nozzle injection mode and M-EMS[J]. Metals,2019, 9(10):1083.