Atsushi Ito

Ultrafine ferrite + austenite steels with the chemical composition of 0.1%C-2%Si-5wt%Mn show excellent strength (TS=1200 MPa) and high ductility (TEl=25%) balance, compared to conventional TRIP steels. This steel is expected as the third generation advanced high-tensile strength steels (AHSS). This steel can be produced by a simple intercritical annealing, however, longer annealing time is necessary to obtain appropriate ferrite + austenite structure. It is difficult to produce this steel by continuous annealing process. If the annealing time can be drastically reduced, this new TRIP steels can be commercialized. We focused on the effect of the prior microstructures before annealing on the formation of ferrite + austenite structure. The effect of the prior structure is not clear. Therefore, in this study, two kind of prior structures, ultrafine grained ferrite + cementite and martensite were used in 0.1%C-2%Si-5wt%Mn steels. It was found that the prior structure of ferrite + cementite can form large amount (20%) of austenite in a very short time (600 s). This is because cementite finely dispersed in the structure effectively acts as a preferential nucleation site of reverse transformed austenite and C and Mn are concentrated in cementite to enable a short time formation of austenite. Excellent strength-ductility balance (32000 MPa%) which is superior to conventional TRIP steels is also obtained.

Austempered Ductile Cast Iron (ADI) has outstanding strength and elongation balance because the retained austenite enhances ductility by strain induced transformation. The effects of fine pearlite as the prior structure for heat treatment on the formation of fine ferrite, bainitic ferrite, and retained austenite and its mechanical properties were investigated in our previous research. However, the effects of chemical composition on strain induced transformation have yet to be sufficiently studied. Thus in this study, fine ferrite, bainitic ferrite and retained austenite structures were formed by combining different types and addition amounts of austenite stabilizing elements. The influence of strain induced transformation of retained austenite in these fine structures for strength and ductility was also investigated. The dynamic change of retained austenite volume fraction during tensile deformation was measured with synchrotron radiation in SPring-8. The behaviors of strain induced transformation after initiation of plastic deformation were varied from the chemical composition. High strength and low ductility was obtained when the strain induced transformation occurred rapidly after the initiation of plastic deformation, and low strength and high ductility were obtained when the strain induced transformation hardly occurred. On the other hand, high strength and high ductility were obtained when the strain induced transformation gradually occurred. The stability of retained austenite, which affects strain induced transformation behavior in the specimen with various chemical compositions, was considered in terms of Md₃₀. Md₃₀ indicates austenite stability during deformation in metastable austenite. The strain induced transformation behaviors of the specimens were in good agreement with the tendency of Md₃₀. Since the effect of the solute carbon content is the largest in the equation of Md₃₀, the concentration of solute carbon into austenite with proceeding bainite transformation was considered. Mn is also thought to have influenced bainite transformation.