Microstructure and mechanical properties of thermomechanically processed C-Si-Mn steels

Two TRIP steels having high and low silicon contents have been subjected to simulated thermomechanical processing (rolling and coiling) in a quench dilatometer or a laboratory rolling mill. The dilatometer specimens showed a higher level of retained austenite than the rolled specimens, which had higher contents of martensite, and also of pearlite, reflecting the less rapid cooling in the larger rolling specimens. The best combination of strength and ductility was achieved after simulated coiling at 400-450 degreesC, when the final microstructure of the high silicon steel consisted of similar to 55-60% polygonal ferrite with small amounts of martensite, retained austenite (similar to 4-5%) and carbide free bainitic phases. Martensite and retained austenite were not observed in the low silicon steel. Results from tensile and shear punch tests were found be in good agreement, confirming the viability of the micropunch method where insufficient material is available for full tensile tests (e.g. with dilatometer specimens). The scaling constant relating the tensile and punch test results is dependent on specimen composition and microstructure. (C) 2001 loM Communications Ltd.