A study on phase transformation and deformation behaviors considering transformation plasticity in steels

Abstract

The recent trends in the automotive industry have mainly focused on increasing the crashworthiness properties of automobiles and at the same time decreasing the fuel consumption and gas emissions. In this point of view, Advanced High Strength Steel (AHSS) offers an opportunity for the development of cost effective and light weight parts with improved safety and optimized environmental performance for automotive applications. Starter of AHSS, for example dual phase (DP) and transformation induced plasticity (TRIP) steels, has a higher strength than previous conventional steel. Next stage, Ultra - Advanced High Strength Steel (U-AHSS) is developed. The representative of the U-AHSS is the twinning induced plasticity (TWIP) steels, which has a much strength and elongation. However this steel are yet to be commercialized in use, mainly because of the high level of Mn, which leads to associated processing problems. Thus, X-AHSS steels, with properties of both AHSS and U-AHSS, are now consider as a new option for commercialized steels. Phase transformation behaviors play an important role to decide material properties of the AHSS. Therefore it is necessary to predict accurate phase transformation behaviors using appropriate model. In this paper, there are various attempts in order to predict phase transformation behaviors of AHSS which included complex transformation behaviors. Firstly, method of dilatometric analysis to analyze the phase transformation behaviors of AHHS with retained austenite was developed. Developed dilatometric analysis method is based on lattice parameter and atomic volume of each phase and additionally consider transformation plasticity and enrich of alloying element. Using the thermodynamic data and equations embedded in the method, it can distinguish the type of BCC structure. Analysis of two specimens which has different Mn, the strong austenite stabilizer, we can verify the accuracy of developed method and find various parameters concerning transformation plasticity and fraction of retained austenite. This method is preferred to obtain the fraction of final fraction of retained austenite very simply and phase transformation kinetics of retained austenite steel. Secondly, the dissolution and precipitation of Nb, which has been known as strong carbide-forming element, play a key role in controlling phase transformation kinetics of microalloyed steels. In this study, we analyzed both numerically and experimentally the precipitation behavior of Nb-microalloyed steel and its effect on the austenite decomposition during cooling. Nb precipitation in austenite matrix could be predicted by the thermo-kinetic software MatCalc, in which interfacial energy between precipitate and matrix is calculated. The simulated precipitation kinetics were fairly well agreement with the experimental observations by TEM. Austenite decomposition, which is strongly affected by Nb precipitaion during cooling, was measured by dilatometry and was modeled on the basis of a Johnson–Mehl–Avrami–Kolmorgorov (JMAK) equation. It was confirmed that the dissolved Nb delays the austenite decomposition, whereas, the precipitated Nb performs as nucleation sites of phase transformation during the austenite decomposition.. Lastly, a finite element model was developed to predict the deformation, temperature history, carbon diffusion, phase fraction, and hardness during the carburizing heat treatment of automotive annulus gear ring, initially made of a medium carbon steel. Carburizing gas with a constant carbon potential for entire surfaces of the gear was assumed. The temperature and pressure driven carbon diffusion was solved by the finite element simulation based on Ficks law. Both the diffusional and displacive phase transformations during the heat treatment were modeled incorporating the carbon concentration inside the gear. The constitutive equation of the transformation plasticity was incorporated into the finite element model. Strains due to the phase transformation, transformation plasticity, and thermal expansion/contraction were calculated by the finite element model. The prediction accuracy for the phase evolution, hardness distribution, and dimensional change of the gear ring was verified with the measurement data. From this study, phase transformation model in X-AHSS, which has not been clear up to now, is described well. The developed model and suggested analysis method lead to a clearer understanding about phase transformation behaviors and related phenomenon of ferrous alloys. Furthermore, using developed model coupled with finite element simulation or material property prediction model, we can predict the changes of microstructural characteristics and thermal-mechanical behaviors during complex phase transformation.