Microstructure of Hot Rolled 1.0C-1.5Cr Bearing Steel and Subsequent Spheroidization Annealing
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AS a hypereutectoid steel, 1.0C-1.5Cr bearing steel is widely used in the bearing industry. It is usually supplied in a hot-rolled condition with a microstructure consisting of lamellar pearlite and a small amount of grain boundary cementite (i.e., proeutectoid carbide). However, the lamellar pearlite is not appropriate for machining or cold forming. In order to improve the cold workability, it is usual for the pearlite steel to be subjected to spheroidization annealing producing a mixed microstructure, in which globular cementite particles are uniformly distributed throughout the ferrite matrix.[1] Due to the high carbon content, proeutectoid carbide distributed along grain boundaries is often observed in the hypereutectoid steels. If the proeutectoid carbide impinges with each other, the carbide network will be formed.[2] The presence of carbide network will significantly decrease the ductility and fatigue properties of hypereutectoid steels.[1,3,4] The ZHEN-XING LI, JIAN ZHANG, and BIN-ZHOU LI, Ph.D. Students, and CHANG-SHENG LI, Professor, are with the State Key Laboratory of Rolling and Automation, Northeastern University, Shenyang 110819, P.R. China. Contact e-mail: [email protected] XUE-DONG PANG, Engineer, is with Fushun Special Steel Co., Ltd, Fushun 113001, P.R. China. Manuscript submitted November 15, 2015. Article published online April 20, 2016 METALLURGICAL AND MATERIALS TRANSACTIONS A
work by Luzginova et al.[5] showed that the carbide network is difficult to be removed in the subsequent spheroidization annealing. In order to decrease the degree of carbide network, sometimes hypereutectoid steels need to be subjected to normalizing treatment. However, Sun and Wu[6] reported if the bearing steel bar was water-cooled at a rate of 200 K/s to 963 K (690 C) from final rolling temperature 1233 K to 1273 K (960 C to 1000 C), the precipitation of proeutectoid carbide can be suppressed effectively on the surface of the steel bar. It should be noted that it is difficult for the steel to be cooled at such a fast cooling rate in the industry production. Therefore, apart from increasing the cooling rate, it is also necessary to control the rolling parameters, especially the final rolling temperature. The decrease of final rolling temperature is beneficial to refining the prior austenite grain size.[7] As a result, the grain boundary area per unit volume will increase with the decreasing of final rolling temperature, which could reduce the thickness of grain boundary cementite and suppress the formation of continuous network of carbide. In this work, in order to further suppress the formation of carbide network, the final rolling temperature and subsequent cooling process were both controlled. Apart from the carbide network, other microstructure parameters including pearlite interlamellar spacing, pearlite colony size and prior austenite grain size are VOLUME 47A, JULY 2016—3607
also important for the mechanical properties of hot rolled hypereutectoid steels. Daeubler et al.[8] reported that finer pearlite interlamellar
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