| As automobile industries transform to lighter weight and securing, the development of auto steel is focused on the combination of attractive strength and ductility. The advanced high strength steels (AHSS) for auto manufacturing have developed to the3rd generation. Based the advanced high strength steels (AHSS), B. C. DeCooman et al of PoHang Steelers proposed the Mn-Al medium manganese steel as the3rd generation automobile steels. Mn-Al medium manganese Steel can greatly reduce the weight of auto body quality and meet the requirement of safety for its attractive strength and ductility. The application prospect of Mn-Al medium manganese Steel is bright, while the related study on Mn-Al medium manganese steel have not gone far enough. General research means for Mn-Al medium manganese steel involve experiment simulation and heat treatment process of hot rolled and cold rolled. The Mn-Al medium manganese steel with8ferrite was used in this study. Its rapid heating process was simulated with Full Automatic Transformation Measuring Apparatus. The partition of C、Mn elements on the phase interface was calculated with Thermo-Calc and Dictra. The experiment of hot rolled and cold rolled was simulated with the thermal simulation testing machine (MMS300). Heat treatment process and tensile experiment was analyzed.Following conclusions can be achieved.(1) The calculation of Thermo-Calc showed as the isothermal temperature raised, C concentration in austenite first raised and than dropped, the concentration reached most at700℃while the change of Mn concentration was lasting falling. The calculation of Dictra showed that partitioning equilibrium time of C on the phase interface between austenite and martensite was around1s, while the time of Mn was more than1000s, the phase interface of martensite moved inversely.(2) The result of rapid heating process showed that the microstructure of600℃texted steel consisted of8ferrite, martensite and retained austenite, when the temperature raised, the content of martensite dropped and the content of austenite raised, bainitic ferrite appeared; the austenite content at700～800℃reached most. The martensite content went up and the austenite content dropped at900℃. The main microstructure in30s～60s consisted of retained austenite and δ ferrite, when the austenite content reached most. The martensite content raised between60s～10min. The content of austenite was more than martensite with the cooling rate of5～50℃/s, martensite content raised between80～100℃/s.(3) The study on the heat treatment of hot rolled by the thermal simulation testing machine (MMS300) showed that the microstructure of600℃texted steel consisted of martensite and δ ferrite, the retained austenite content raised, while martensite content dropped with temperature went up and C、Mn partitioning. The retained austenite content reached most between700～800℃,while martensite content raised between800～900℃. The result of quenching and tempering process showed that the microstructure of texted steel consisted of tempering martensite,δ ferrite and less of retained austenite.(4) The study on the heat treatment of cold rolled by the thermal simulation testing machine (MMS300) showed that the microstructure of650℃texted steel consisted of martensite and δ ferrite, austenite content increased with temperature raising and reached most at750℃, the content dropped down to15%between750～850℃. The result of quenching and tempering process showed that the microstructure of texted steel consisted of tempering martensite,δ ferrite and less of retained austenite, and retained austenite content was around14%.(5) The result of heat treatment process of hot rolled showed that the retained austenite content first raised and than dropped with isothermal temperature went up. With the austenite reversed transformation advancing, the austenite content between600～700℃waved between10～15%., the content reached on35%at750℃, while dropped down to15%at900℃.(6) The result of tensile test of hot rolled showed that the elongation was increased by the augment of austenite content and its TRIP effect. The tensile strength increased from650MPa to1000MPa between600～900℃, while the change of elongation was the same as austenite, first raised and than dropped, the elongation reached on40%at750℃.(7) The result of heat treatment process of hot rolled showed that retained austenite content first raised and than dropped with isothermal temperature and time went by. The austenite content between600～900℃waved between10～25%, the content reached on25%at 750℃. The austenite content between1～10min at750℃waved between12～25%, the content reached25%at2min. Retained austenite content was around15%in the quenching and tempering process at750℃.(8) The result of tensile test of cold rolled showed that the tensile strength increased with isothermal temperature and time went by. The elongation increased with the increasing retained austenite content and TRIP effect. The tensile strength was around750MPa and the elongation was35%about at750℃in2min, which showed great mechanical properties. At750℃, tensile of quenching and tempering process was1000MPa about, but the elongation was20%below. |
PDF Full Text Request