STEEL GRIPS online - Studies on solidification of boron containing alloy steels via thermodynamic modelling

STEEL GRIPS online
ISSN 1866-8453

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May 10, 2012

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Akula Durgavara Prasad and Sankara Raman Sankaranarayanan:
Studies on solidification of boron containing alloy steels via thermodynamic modelling

Boron steel, solidification, retrograde melting, thermodynamic modelling, Thermo-Calc, AHSS

Current steel market for the automobile industry is moving towards increased use of advanced high-strength steels (AHSS), including boron containing steels. It is well known that the addition of boron to steel enhances its strength. However, difficulties are encountered during continuous casting of boron steels, i.e., surface cracks, internal half-way cracks, and breakouts. In the present work, pseudo-binary phase diagrams are generated using Thermo-Calc to study the effect of boron on the Fe-B alloys at various carbon levels. It was found that, in the absence of carbon, a metatectic reaction occurred at 1360 °C when the B mass content is between 0.022 and 0.045 %. The addition results in the formation of a knee, below which retrograde melting occurs at around 1350 °C. And, the final solidification occurred at around 1150 °C. Because of this retrograde melting behaviour, cracks tend to develop in the interdendritic regions. To prevent this situation, the boron content should be relatively low; and it is found that the boron content should not exceed 90 ppm for carbon mass contents of up to 0.4 %. However, in case of sulfur additions (mass contents of 0.05 %), this problem may become much more severe, as the final solidification temperature is depressed to 1070 °C and the maximum boron content is restricted to 50 ppm only.