The Influence of Peritectic Reaction/Transformation on Crack Susceptibility in the Continuous Casting of Steels
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THE surface cracking of continuously cast steel has drawn the attention and focus of many researchers, since it affects the efficiency of the casting process. Continuous research has helped to understand the phenomenon of crack formation at the early stages of continuous casting (CC) and has improved the process quality considerably.[1–3] In earlier work,[1,4] Brimacombe et al. investigated the crack formation in different steel grades during continuous casting and explained the theory behind it. They related crack formation with the surface depressions, while the effect of melt stream impingement was found to cause the so-called white bands and structure changes. Harada et al.[2] investigated the role of the surface segregation in the formation and propagation of the cracks during CC and proposed a mechanism based upon these observations. Takeuchi et al.[3] observed transverse cracks under the oscillation mark (OM) depressions and along the austenite grain boundaries. Maehara et al.[5] considered the effect of local delay of solidification due to the uneven surface cooling on low-alloyed peritectic steels during continuous casting and suggested that the crack susceptibility will be largely accelerated by this mechanism. Steels that undergo peritectic reaction and transformation complicate the CC process. Hypo-peritectic steels (C < 0.16 wt pct) are particularly susceptible to SAUD SALEEM, MICHAEL VYNNYCKY, and HASSE FREDRIKSSON are with the KTH Royal Institute of Technology, Brinellvgen 23, SE-10044 Stockholm, Sweden. Contact e-mail: [email protected] Manuscript submitted March 4, 2016. METALLURGICAL AND MATERIALS TRANSACTIONS B
crack formation.[4] The crack susceptibility of the initially formed shell increases due to the additional stresses and strains resulting from the fact that the primary and secondary solid phases have different densities.[6–10] Peritectic transformation that follows upon completion of the peritectic reaction generates high tensile strain and a contraction which can lead to crack formation in the initially solidified brittle material.[1,11–13] Fredriksson[14] utilized the method of unidirectional solidification to investigate the peritectic transformation in high-alloyed steels containing ferrite stabilizers. Based on his experimental results, he concluded that the peritectic temperature and extent of the peritectic transformation increase with increasing cooling rate. Moreover, at high cooling rates, the transformation occurred very rapidly without any diffusion of the alloying elements into the interior of the ferrite. Nassar and Fredriksson[15] performed many DTA experiments on low-alloy steels and observed a massive transformation of d to c, which they attributed to stress relief relaxation, since such a transformation rate cannot be explained by a diffusion-driven transformation. The surface oxidation of the strand outside the mold is an inevitable phenomenon. An oxide skin forms on the surface of the strand due to the prolonged exposure of the surface to atmosphere at higher temperatures. Oxida
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