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

High strength, high plasticity and low cost steel is thetarget which material personnel pursuit. At present, grain refinement,TRIP effect and TWIP effect are the effective ways to overcome theproblem that the plastic of steel decreases with the strength of steelincreasing. This paper studied the heat treatment process of hot rolledultrafine grained metastable steel, and finally succeeded in gaining thehot rolled ultrafine grained metastable steel, which the product ofstrength and ductility is more than30GPa·%, based on reviewing theAHSS in home&foreign, and analysising the heat treatment process.This paper mainly studies the effect of intercritical annealingprocess on the microstructure and properties of hot rolled ultrafinegrained metastable steel, mirostructure evolution laws in the annealingprocess, and the strength-plasticity mechanism of ultrafine grainedmetastable steel, through the mechanical properties experiment,microstructure observtion and phase determination of the tested steelbefore and after heat treatment. The main conclusions are as follows:The microstructure of hot rolled steel contains ultrafine lathmartensite, deformation ferrite and a little of retained austenite. Thetensile strength of hot rolled steel is1550MPa, the elongation of hotrolled steel is9.5%, and the product of strength and ductility is only14.73GPa·%. After intercritical annealing, the comprehensivemechanical properties of hot rolled steel improve. After annealing at630℃with different time from5min to360min, the microstructure oftested steel contains more than50%retained austenite and some lathferrite. The size of grain is mainly concentrated in between100nm and700nm. The tensile strength is between1108MPa and1154MPa. Theelongation is between23.5%and33.5%. And the product of strengthand ductility reaches30GPa·%. In the thermal360min, thecomprehensive mechanical properties are the best. The tensile strengthis1142MPa. The elongation is33.5%. And the product of strength andductility reaches38.5GPa·%.During the intercritical annealing, the equiaxed ferrite is due to ferrite recrystallization starts before austenite formation, because of thesufficient hot rolled stored energy and the surrounding containing poorMn. The lath ferrite is formed in Mn-rich regions. Austenitic formationprecedes ferrite recrystailization. The dislocation density and the storedenergy will decrease due to the phase transformation. And the drivingforce for recrystallization decreases. In such a situation, the ferrite onlyhappens recovery. Lath austenite nucleates in between the lathmartensite. Polygonal austenite nucleates in the large angle grainboundaries. A large number of dislocations and defects in hot rolledhigh reduction and the alloy elements add austenite nucleation rates andaustenite phase transformation, thin the original structure. While the Mnelement segregation on grain boundary can inhibite grain coarseningduring the annealing process.The combination of TRIP effects by the large fraction of metastableaustenite and the ultrafine grain size results in the tested steel havinghigh strength and high plasticity.