Research On Microstructure Refinement For Hot Forming Process Of Low Carbon Structural Steel

With the development of modern industry and scientific technology, steel structural parts have been evolved towards the trend of giant and high performance. Grain refining strengthening has been proved to be the optimum way that can improve both the strength and the toughness. Therefore, the study of ultra-fine steel is becoming one of the most heated topics in the field of structural materials. The works of this dissertation are carried out integrating with the National Natural Science Foundation of China-Microstructure refinement mechanism for hot forming process of structural steel. The austenite recrystallizaiton behavior and strain induced ferrite transformation (SIFT) during hot forming for low carbon structural steel SPHC are investigated. The effects of hot working processing parameters on microstructure refinement are investigated also, and the integrated mathematical models for prediction of microstructure refinement are developed. The research results are summarized as the following:The dynamic recrystallization (DRX) behaviors of SPHC steel during hot deformation are investigated using single pass compression test. The metallographic analysis shows that grain size of DRX decreases with increasing strain rate or decreasing deformation temperature. The critical condition models for the initiation of DRX are built by analyzing stress-strain curves of DRX, and fittingθ-σcurves and ln (θ)-σcurves with cubic polynomial. The calculated activation energy of DRX is 299.4 kJ/mol, and the dynamic recrystallization kinetics and grain size models are established.The static recrystallization (SRX) and metadynamic recrystallizaiton (MDRX) behaviors of SPHC steel are investigated using double pass compression test. The results show that the volume fractions of SRX rapidly increase with the increase of the deformation temperature, strain, and strain rate. Also, the volume fractions increase with the decrease of initial austenite grain sizes. The grain size after SRX is affected mainly by strain and initial austenite grain size. The grain size of SRX decreases with increasing strain or decreasing initial austenite grain size. The metadynamic recrystallizaiton is affected mainly by strain rate and deformation temperature. With increasing strain rate, the MDRX progress accelerates and the grain size of MDRX decrease. As the deformation temperature decreases, the MDRX progress becomes difficult, but the grain size of MDRX decreases. The calculated activation energies of SRX and MDRX are 284.1kJ/mol and 195.6 kJ/mol respectively. Their kinetics and grain size models are built, respectively.The effects of hot working processing parameters on strain induced ferrite (SIF) transformation behavior are investigated on Gleeble 1500 hot simulator. The results show that SIF amount increases and SIF grain size decreases with increasing strain or decreasing deformation temperature. SIF amount and SIF grain size decreases with increasing strain rate. SIF amount increases with decreasing the initial austenite grain size. The mechanism of SIFT is mainly a diffusional transformation.A method for calculating deformation stored energy of austenite is proposed. On the thermodynamic basis of superelement mode and modified KRC mode, the thermodynamics parameters of multicomponent steels for SIFT are calculated, and the effects of deformation stored energy on the thermodynamics parameters are analyzed. The results of the calculations show that the value of deformation stored energy increases with lowering the deformation temperature or increasing strain rate. Deformation stored energy enhance the driving force of SIFT, which is relevant to deformation temperature, and the nearer the equilibrium temperature of austenite to ferrite transformation Ae3, the more obvious impact effect. The equilibrium temperature of austenite to ferrite transformation Ae3 rises with increasing deformation stored energy.The critical condition models for the initiation of SIFT are established by analyzing stress-strain curves of SIFT and fittingθ-σcurves and ln(θ)-σcurves with cubic polynomial.By the experimental analysis, nucleation is consider to be a dominant course during SIFT. Therefore, the model of SIFT is suggested on the view that SIFT is a single course of nucleation and growth. A method for calculating SIFT time is proposed, and SIFT kinetics model is suggested based on Cahn's isothermal transformation kinetics theory of grain boundary nucleation and growth mechanism. The simulations of some experiments prove that calculated results agree with the measured results in the effect of hot working processing parameters on transformed fraction and ferrite grain size. So the SIFT model of present paper is valuable for analyzing the process of SIFT.