Formation Mechanism of Slab Surface Transverse Cracking in Continuously Cast Ni Bearing Steel

KATO Toru, KASAI Norifumi, KAWAMOTO Masayuki, YAMANAKA Akihiro, WATANABE Tadao

doi:10.2355/tetsutohagane1955.84.12_856

The hot ductility behavior of low-alloy steels with Ni of 0-4.9mass% was investigated in the temperature range from 873K to 1273K. The mechanism of forming slab surface transverse cracking in continuously cast Ni bearing steel was discussed in terms of the behavior and heat transfer properties previously reported.<BR>For the hot ductility, significant influence of Ni content was observed below 1050K, while slight influence was observed over 1100K. The width of the ductility trough spread toward the lower temperature as the Ni content increase. The precipitates consist of Ni did not form even in Ni bearing steel. According to thermodynamic calculation, precipitation behavior such as carbide or nitride did not change depending on Ni content, therefore, embrittlement in Ni bearing steel was not caused by precipitates. The spread of ductility depending on Ni content could be attributed to the A₃ transformation temperature change, and brought about the slab surface transverse cracking. In the temperature range over 1100K, corresponding to the low temperature austenite region, ductility loss was induced by precipitates, e.g. NbC and/or AIN. Then hot ductility was independent of Ni content in that temperature range.<BR>On the other hand, Ni content had influence not only on hot ductility but also on cooling property as previously reported. The breakdown temperature of film boiling became higher with Ni content because of adherent scale layer, thereby transition boiling begins at higher temperature. After all, uniformity of cooling intensity was impeded, and susceptibility to cracking was increased because of thermal stress and falling into ductility trough.

Formation Mechanism of Slab Surface Transverse Cracking in Continuously Cast Ni Bearing Steel

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