Study of Ferrite During Refinement of Prior Austenite Grains in Microalloyed Steel Continuous Casting
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THE quality of microalloyed steel slab in continuous casting has always been plagued by surface transverse cracks.[1–5] With respect to the formation mechanism of surface transverse cracks, a large number of studies have shown that the slab’s microstructure has an intimate relationship with the formation of transverse cracks. Coarse austenite grains, the carbonitride of microalloy elements and the film-like ferrite clustered in large quantities along the austenite grain boundaries, are the main sources that induce transverse cracks. Dippenaar et al.[6] found that the coarse austenite grains were the key factor, affecting the occurrence of transverse cracks; therefore, refining the austenite grains could reduce the occurrence of transverse cracks. In view of this, Lee et al.[7] reported that the prior austenite grains of hypoeutectoid steels could be refined through double phase-transformation method controlled by a thermal
JIANG LIU, GUANGHUA WEN, and PING TANG are with the The State Key Laboratory of Mechanical Transmission, College of Materials Science and Engineering, Chongqing University, Chongqing 40004 P.R.China. Contact e-mail: [email protected] Manuscript submitted May 13, 2016.
METALLURGICAL AND MATERIALS TRANSACTIONS B
cycle (i.e., phase change from austenite to ferrite by rapid cooling was regarded as the first phase transformation; and the phase change from ferrite to austenite by reheating was regarded as the secondary phase transformation). During the double phase-transformation technology, the phase-transformation process of cfiafic is the key factor that determines the degree of grain refinement. Therefore, providing better conditions for refining the grains by controlling the occurrence of the two phase transformations is of great significance. A considerable amount of research has been conducted on the phase-transformation processes of cfia and afic. Kato et al.[8] found that during cfia phase-transformation process, the ferrite formed under different cooling rates showed different morphologies. Especially, the film-like proeutectoid ferrite at the austenite grain boundaries developed very well under low cooling rates. Ray et al.[9] investigated the cfia texture transformation process and discovered some orientation relationship between austenite (parent phase) and ferrite (generated phase). For carbon steels, the orientation relationship obeys or approaches the Kurdjumov–Sachs (K–S) relationship.[10,11] Lischewski et al.[12] studied the afic phase-change process and found that the nucleation sites of austenitic grains mainly located at the triple junctions of ferrite grains; moreover, the smaller the orientation difference between
austenite grain,[7] the premise of this experiment is to ensure that the distributions of microalloying elements in TH0 and TH1 are the same. According to the thermodynamic calculation, the second-phase precipitate containing Ti or Nb can precipitate nearly completely at >1073 K (800 C).[16,17] Thus, the distribution of the second-phase precipitates has the same effect on both th
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