Research On The Heat Treatment Process And The Strength-plasticity Mechanism Of Cold Rolled Ultrafine Grained Metastable Steel

Cold rolled ultrafine grained metastable steel is one kind of advanced high strength steels developed on the base of ultrafine grain and TRIP effect. It has excellent comprehensive mechanical properties. This paper studied the heat treatment process of cold rolled ultrafine grained metastable steel, and gained the cold rolled ultrafine grained metastable steel, which the product of strength and ductility is more than 30GPa·%.This paper mainly studied the effect of intercritical annealing process on the microstructure and properties of cold rolled ultrafine grained metastable steel, strength-plasticity mechanism of ultrafine grained metastable steel, diffusion and distribution of Mn, stability of retained austenite through the mechanical properties experiment, microstructure observtion and phase determination of the tested steel before and after heat treatment. The main conclusions are as follows:The microstructure of cold rolled steel is ultrafine lath martensite. The tensile strength of cold rolled steel is 1608.43 MPa, the elongation of cold rolled steel is 5.25%, and the product of strength and ductility is low. After intercritical annealing, the comprehensive mechanical properties improve. After annealing time at 20 min with different temperature from 550℃ to 780℃, the tensile strength is between 1012 MPa and 1494 MPa, and the elongation is between 11% and 33%, and the product of strength and ductility is between 12% and 33.4%. At 650℃, the tensile strength is 1012 MPa. The elongation reaches 33%. And the product of strength and ductility reaches a maximum value of 33.4%. The comprehensive mechanical properties are the best. After annealing, the microstructure of tested steel contains lath martensite, blocky ferrite and equiaxed austenite. The size of grain is mainly concentrated in between 100 nm and 500 nm. At the initial stage of annealing, the amount of retained austenite increases rapidly. With the heating temperature increasing, the amount of retained austenite first increases and then decreases. At 700℃, the amount of residual austenite is up to 22.66%The nucleation mechanism of lath retained austenite and block retained austenite are analysed. It can be seen clearly that most of the lath austenite develops in between the martensitic laths whereas the block austenite grains forms in martensitic packet boundary or the original austenite grain boundary. After fusing, the equiaxed austenite formead. In the annealing process, because the solubility in γ-Fe is greater than the solubility of α-Fe, Mn enriched and improves the hardenability and the stability of austenite.The grain size of microstructure, the amount of metastable austenite, organization structure and the existence of the state are the key to obtain high strength and ductility. High strength and ductility are mainly realized by ultrafine grains, solid solution strengthening and TRIP effects.