Abstract: In this paper, low- and high-strength lath martensite (350 and 640 HV) was fabricated in an IF steel via high pressure martensitic transformation. The microstructure and the softening during their tempering from 200 °C to 800 °C for 1 h were systematically investigated. A carbon-irrelevant tempering process was proposed, exhibiting a three-stage structural evolution pattern depending upon the tempering degree (1-(HV-HV_FP)/(HV_NP-HV_FP), where the HV is the instant hardness, HV_NP is the non-tempered hardness and HV_FP is the fully tempered hardness): (1) low tempered (<10%), removing the loose dislocations and dislocation boundaries within martensitic variants; (2) medium tempered (10%-50%), eliminating the martensitic variant laths via the migration of their terminal tips; (3) highly tempered (>50%), clearing up the remained variant laths via the migration of the triple junctions. Martensite-type microstructure is tailored by low-index lamellar variant boundaries and is thus intrinsically thermally stable, whereas the mobile terminal tips decrease the tempering resistance. The underlying mechanism for such carbon-irrelevant process was discussed and the potential effect on the tempering behavior of carbon-contained martensite was highlighted.

Key words: Tempering mechanism, Lath martensite, IF steel, Terminal tip migration, High pressure