Investigations On The Evolution Of Microstructures And Properties In High Manganese TRIP Steels And The Strip Forming Techniques

In recent years, as the automotive industry faces an increasingly competitive market, it has become an inevitable development trend for the manufacturers to improve vehicle safety indicators while reducing the weight of vehicle. High manganese TRIP steel microstructure mainly composes of metastable austenite transformed to martensite in the process of deformation, which results in increase of strength as well as plasticity, obtaining optimal strength and toughness combination that other steels do not have. With great potential and promising prospect, high manganese TRIP steel is widely used in automotive structure. However, the research of high manganese TRIP steel has just started, and there are many technical problems to be solved. In the present paper, high manganese TRIP steel hot deformation behaviors and microstructure evolution were studied, and then misconstrue and mechanical properties for high manganese TRIP steel during hot rolling solution treatment process under different holding time were deeply investigated. In addition, the deformation mechanism and influence factors for cold rolled high manganese TRIP steel during tensile deformation. Moreover, twin roll casting high manganese TRIP steel strip process and microstructure, performance, and deformation mechanism were thoroughly investigated. The main original works of this paper are present as follows:(1) Two typical high manganese TRIP steels flow stress curve were obtained by high temperature uniaxial compression tests. The high manganese TRIP steel deformation behavior and microstructure evolution during high temperature compression process were studied with a constitutive equation for high manganese TRIP steel being established. The impact factors of high temperature deformation resistance for high manganese TRIP steel were analyzed, and a deformation resistance model was established. The high manganese TRIP steel deformation behavior during heat processing in the800-1050℃temperature and the0.01-10.0s-1strain rate ranges was predicted by dynamic material model and microstructure analysis with hot working equations for high manganese TRIP steels being obtained. The results show that unsafe hot working regime exists in hot deformation processing at low temperatures and high strain rates. Therefore, the deformation in the rolling process should be carried out as much as possible at high temperature and low strain rate to produce a favorable deformation recrystallization microstructure to avoid thermal cracking.(2) Different heat treatments were carried out for three typical high-manganese TRIP steels, and the uniaxial tensile tests were performed. The result indicates that increase of holding time at temperature can increase elongation and strength. Through the analysis of stress-strain curves and microstructure observation, the relationship between mechanical properties and deformation mechanism was established. The transformation routes of γâ†'εâ†'α’ and γâ†'α’ were observed by TEM. Microstructural characterization of ε-martensite and a’-martensite indicated that α’-martensite held the K-S orientation relationship with austenite, and the S-N (Shoji-Nishiyama) orientation relationship obeyed between austenite and ε-martensite, whereas s-martensite and α’-martensite held the Burgers orientation relationship. Deformation behavior of the experimental steels has been clarified through the relationship between work hardening rate and true strain based on the true stress-true stain curves. It has been found that continuous transformation from austenite or s-martensite to α’-martensite can take place with increasing strain during deformation, which increases the work-hardening rate to improve the TRIP effect.(3) The behavior and related mechanisms of work hardening for high manganese TRIP steel during uniaxial tensile deformation was studied. The nucleation site of α’-martensite was determined through HRTEM. In combination with cold-rolled high manganese TRIP steel strain hardening behavior and microstructure characteristics, tensile deformation process was divided into three stages. In deformation stage â… , transformation induced s-martensite nucleated directly by fault. In deformation stage â…¡, because of local stress concentration resulting from mutual tangling of dislocation or ε-martensite growth being prohibited, another directional s-martensite grew. α’-martensite nucleated and grew at interpenetrating of the two directional ε-martensite. Transformation induced martensite promoted work hardening rate increased with increasing of true strain in this stage. In deformation stage â…¢, work hardening rate rapidly decreased with increasing of true strain, leading to the decrease of work hardening rate. (4) The mechanical properties of high manganese TRIP steels and the microstructure evolution with temperature were investigated through tensile testing at25,100,200,400℃. The characteristic of transformation induced martensite and deformation twin were investigated by TEM. It was found that with increasing of deformation temperature, high manganese TRIP steel deformation mechanism transferred from deformation induced martensitic transformation mechanism to competition mechanism between deformation twins and the γâ†'ε phase transformation, and then to twinning and slip mechanisms. The SFE of high manganese TRIP steels, Γ, at different temperatures was estimated by thermodynamic equations. The dependence of SFE on deformation mechanism was analyzed. The effect of SFE on martensitic phase transformation was investigated. It was pointed out that when Γ <10.0mJ/m2, deformation mechanism was the deformation induced martensitic transformation; when17.7mJ/m2<Γ<36.4mJ/m2, the competition mechanism between deformation twins and the γâ†'ε phase transformation was main deformation mechanism; when17.7mJ/m2<T<36.4mJ/m2, the governing deformation mode was crystallographic slip and deformation twin; when41.9mJ/m2<Γ<78.3mJ/m2, the slipping was a predominant deformation mode.(5) By using strip casting process, excellent surface quality was obtained without cracking in hot rolling processing. The microstructures of cold rolled twin-roll strip casted high manganese TRIP steel were investigated. A good combination of strength and ductility was obtained, with the tensile strength and total elongation being approximate to the conventional processed counterpart, while the yield strength being lower appreciably. Therefore, it was feasible and advantageous to produce high manganese TRIP steel by using twin-roll strip casting process. There was also three-stage deformation behavior during tensile testing. In deformation stage â… , work hardening rate decreased with increasing of true strain. In deformation â…¡, work hardening rate increased with increasing of true strain. In deformation â…¢, work hardening rate decreased with increasing true strain, indicating that the transformation rate of martensite induced by deformation decreased.