Mechanism Of Deformation Induced Plasticity Enhancement On Advanced High Strength Steel

TRIP (transformation induced plasticity) steel and TWIP (twining induced plasticity) steel are the high strength steels used in automobile industry because of their excellent ductility and high strength. The principle of TRIP steel is that metastable austenite transforms to martensite in the process of deformation, which results in increase of strength as well as plasticity. TWIP steels are mainly high manganese steels with Mn mass contents of15～30%, which exhibit ultra-high plasticity by deformation twinning during plastic deformation. TRIP steel and TWIP steel meet the requirements for weight saving, fuel efficiency as well as the safety for its high energy absorption capacity, which is continuing to be a major concern on automobile manufacturing.Studies on low carbon high manganese TRIP/TWIP steels have been mainly focused on static deformation behavior. During the dynamic loading and static loading, the deformation behaviors are quite difference, and a local temperature rising and a strong effects of shock wave takes place in the dynamic loading. The development of the processing technologies on metal materials and anti-collision performance is slow, due to very few scientific researches on the dynamic deformation behavior. In addition, many researchers obtained the stress-strain curves through unidirectional continuous tensile test when they studied on TRIP steel and TWIP steel. The stress-strain curves of TRIP steel and TWIP steel can be found that there is a flat area inside the maximum load area, which covers a wide range of strain. So it is necessary to study the constant stress deformation of these steels.Two kinds of tensile tests which are high-speed tensile test and step-wise tensile test were carried out to research materials including high alumina TRIP experimental steel and high manganese TRIP/TWIP experimental steels. The influence of strain induced austenite transformation and deformation twinning on the mechanical properties of the studied steels has been investigated by using OM, XRD, and TEM techniques. The main work involved as follows:(1) The microstructure of high alumina TRIP experimental steel is mainly composed of ferrite, bainite and retained austenite after heat treatment. There is a lot of intricate dislocation inside ferrite matrix. Martensite has been generated after tensile deformation, indicating that part of the retained austenite in TRIP experimental steel transformed to the martensitie. The microstructure of high manganese TRIP/TWIP experimental steel is mainly ferrite, austenite after heat treatment. A small amount of dislocations is in the ferrite and there is a certain amount of annealing twins in the austenite matrix. A large number of deformation twins were found and the austenite transformed to martensite after tensile deformation, which indicates that TRIP effect and TWIP effect both existed.(2) Among the high alumina TRIP experimental steel in high-speed tensile deformation, with the Al content increasing the elongation of the experimental steel increased and the strength decreased, but the final mechanical property increased. The mechanical properties of1.3A1experiment steel was the best, and the highest product of tensile strength and elongation were2348.5MPa%at10-s-1and26944.5MPa%at102s-1respectively. Among the dynamic tensile strain rate range (101s-1～102s-1), with the strain rate increasing, both tensile strength and elongation of the experimental steel increased.(3) Among the high manganese TRIP/TWIP experimental steel in high-speed tensile deformation, with Mn content increasing austenitic stability has been intensified, and the ability of stress induced martensite transformation decreased, but the concentration of stress make formation ability of deformation twin strengthened. In the same breath, the tensile strength of experimental steel decreased, and the elongation increased.21Mn steel’s mechanical property was the best and the highest product of tensile strength and elongation was59270.4MPa%. Under different heat treatment processes, with the solution temperature and holding time changes, the mechanical properties of high Mn TRIP/TWIP experimental steel is greatly differentThe optimum heat treatment processes are1000℃-60min for18Mn steel and900℃-60min for21Mn steel.(4) During the constant stress deformation, the single stage load and multi-stage loads showed the same law. For high alumina experimental steel and high manganese experimental steel, the strain changed slightly with time going at a lower stress level. When the stress increased to the stage of yield stress, force holding time had a strong influence on the strain. So phase transition occurs mainly in the yield stage.