Control of Carbide Precipitation During Electroslag Remelting-Continuous Rapid Solidification of GCr15 Steel
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DUCTION
HIGH-CARBON-CHROMIUM GCr15 bearing steels are widely used as raw materials to produce rail transportation and in the automobile industry due to their uniform hardness, good contact fatigue strength, and impact toughness. There are three main problems during the solidification of bearing steel, including inclusions,[1] alloying element segregation, and cracks.[2] The formation of these defects is closely related to smelting, pouring, and solidification. With the increasing demand of bearing steels with high grade, tight control of carbide segregation becomes important during the solidification process. Carbon and chromium are the main alloying elements in bearing steels and play an important role in strengthening the steel performance. The segregation of carbon and chromium has a strong relationship with the flow of interdendritic solute-enriched liquid in the final stage of GANG DU, JING LI, and ZHONG-BING WANG are with the State Key Laboratory of Advanced Metallurgy, University of Science and Technology Beijing (USTB), Beijing 100083, P.R. China. Contact e-mail: [email protected] Manuscript submitted March 28, 2017.
METALLURGICAL AND MATERIALS TRANSACTIONS B
solidification.[3,4] Wang et al. investigated the segregation and precipitation behavior of electroslag remelting (ESR) ingot in industrial scale.[5] They found that Cr is a positive segregation element and enriched in the interdendritic area, and the nominal segregation index of Cr is 0.91 in the hot-top region, which suffered the severest segregation. The microstructure, especially the element segregation and carbides of the GCr15 steel in various conditions, has drawn much attention and been extensively studied. Considering the high content of carbon and chromium in GCr15 bearing steels, the alloying element segregation is expected to be more serious. Such segregation cannot be eliminated by the subsequent hot working process, which is troublesome in influencing the mechanical properties of GCr15 bearing steel. Hence, it is of vital importance to investigate the segregation behavior during and after solidification of this steel. Sun and Zhang[6] investigated the macrosegregation in the bloom continuous casting (CC) processes of high-carbon GCr15-bearing steel and found that the segregation degree of solute element C at the strand center is increased from 1.0 to 1.2 as the solidification proceeds. Chang et al.[7] reported that applying relative motion between the mold and the consumable electrodes in ESR results in a more uniform chemical element distribution leading to less segregation in the GCr15 ESR ingots.
Apart from the element segregation, the primary carbide is also important for the mechanical properties of high-carbon-chromium hypereutectoid steels. Increasing carbon and chromium content can cause the segregation of carbon and chromium, leading to the generation of a higher level of larger carbides. Large primary carbides would form during the solidification process of GCr15 bearing steel due to its high content of alloy, which are difficult to remo
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