Formation of Ultrafine Single-Variant Martensite from Prior Ferrite

In the present investigation, changes in the single-variant martensite structure of 0.1%C-2%Si-5%Mn steel with a decrease in the prior austenite grain size have been investigated. While prior austenite grain size in the range of 150 to 24 mu m led to the formation of martensite with a multi-packet and multi-block structure, a single packet and multi-block martensite structure formed from the prior austenite grain size of about 7 mu m. Furthermore, the prior austenite grain size of about 2 mu m led to the formation of a single-variant (single-block) martensite structure. This microstructural analysis indicates that the critical prior austenite grain size to form martensite with a single-variant structure is about 2 mu m. The tensile strength (TS, 1,490 MPa), uniform elongation (UEL, 7.1 %), and total elongation (TEL, 14.2 %) of the single-variant martensite was found to be higher than that of the multi-packet multi-block martensite (TS = 1,270 MPa, UEL = 6.2 %, and TEL =13.6 %). The improvement in the mechanical properties of the single-variant martensite was attributed to enhancement in its strain-hardening rate as a result of change in the structure from the multivariant to the single variant.