REGULATION OF MULTI-PHASE MICROSTRUCTURE AND MECHANICAL PROPERTIES IN A 700 MPa GRADE LOW CARBON LOW ALLOY STEEL WITH GOOD DUCTILITY

Abstract

Low carbon and low alloy steels require good combination of strength and ductility to ensure safety and stability of structures. Heat treatment in intercritical area can not only produce multi-phase microstructure,but also lead to the redistribution of alloying elements in different phases. Multi-step intercritical heat treatment is favorable to obtain retained austenite that is stabilized by repeated enrichment of alloying elements in reversed austenite and nanometer-sized precipitate that are primarily formed during tempering. Excellent mechanical properties are contributed by transformation-induced-plasticity effect of retained austenite and precipitation hardening effect of nanometer-size precipitates. In this work, the microstructural evolution and relative mechanical properties were investigated in a low carbon low alloy steel processed by a three-step heat treatment, namely, intercritical annealing, intercritical tempering and tempering. The microstructure was a typical dual- phase microstructure consisting of intercritical ferrite and bainite/martensite after intercritical annealing, and primarily comprised of intercritical ferrite, tempered bainite/martensite and retained austenite after intercritical tempering. Retained austenite with volume fraction of 29% distributed at the ferrite/bainite(martensite) boundaries and betweent bainitic/martensitic laths. Retained austenite was stabilized by enrichment of C, Mn, Ni and Cu in reversed austenite during the reversion transformation process. Nb C precipitates with average size of 10 nm was formed in ferrite matrix and bainite/martensite, while Cu- containing particles in size range of 10~30 nm precipitated in ferrite and retained austenite during intercritical tempering and tempering process. The morphology of Nb C precipitates was spherical, elliptical and irregular, and copper precipitates were spherical. With the combination of transformation- induced- plasticity(TRIP) effect of retained austenite and precipitation hardening, the steel possessed outstanding mechanical properties: yield strength 700 MPa, tensile strength 900 MPa, uniform elongation 20%, and total elongation 30%.