Effect Of Cooling Path To Microstructure And Mechanical Properties Of Hot Rolled Multiphase Steel

As the development of society the demand of high quality to structural steels is higher and higher, the question of how to increase plasticity while the strength is kept is always an important subject that concerned by many materials researchers. Among many high strength steels the TRIP (Transformation Induced Plasticity) steel is known as the most prospecting multiphase steel. The essence of TRIP steel is that by inducing martensite transformation of retained austenite in steel results in the increase of strength and plasticity during plastic deformation. There is an important actual significance to research and develop hot-rolled multiphase steel for the worldwide aims of resources saving, reduction of energy consumption and the sustainable development of society.The rules of effect of cooling path to microstructure and mechanical property for three different chemical composition low carbon multiphase steels with Si and Mn content after hot rolling were investigated. Through the thermal simulation the transformation rules in continous cooling and high temperature deformation behavior were researched and based these results the hot rolling experiments in laboratory were carried out. After rolling the three sections cooling and two sections cooling pattern were applied and the effect of different final cooling temperature to steels were studied. The microstructure was observed by scan electron microscope (SEM) and transmission electron microscope (TEM). The X-ray diffraction was used to meter the mount of retained austenite. The effect of different parameters of hot rolling to properties of multiphase steels was analysed, and the main results of the paper shown as follow:(1) The continuous cooling transformation (CCT) diagrams of the tested steels were acquired in the undeformed and deformed conditions by thermal-mechanical simulator of MMS-300with the method of thermal expansion and observe of micrographs. The effect of some alloys and different deformation parameters to phase transformation and the microstructure were investigated. These research given an important basis to the hot rolling and controlled cooling of the tested multiphase steels.(2) The experiment of high temperature austenite deformation behavior was carried out through the single pass deformation on thermal-mechanical simulator. Through the test the effect of different deformation temperatures, deformation rates, and the deformation mount to strain-stress curves were analysed, and that the model of deformation stress in high temperature was obtained.(3) With the results of thermal simulation the hot rolling experiments were carried out in laboratory. After the hot rolling the three sections cooling pattern (Laminar cooling+Air cooling+Ultra-fast cooling) and the two sections cooling pattern (Air cooling+Ultra-fast cooling) were applicated and the effect of different final cooling temperatures to microstructure and mechanical properties was investigated. The results of the experiments showed that through the three sections cooling and the two sections cooling after hot rolling can obtain multiphase steels which content ferrite, bainite and retained austenite or some martensite.(4)The best properties of two sections cooling pattern can reach a high level such as the tension strength is820MPa, the elongation is35%, the volume of retained austenite is13.74%and the tension strength multiply elongation is26240MPa%while those properties in three sections cooling pattern were lower such as the tension strength is810MPa, the elongation is31%, the volume of retained austenite is9.19%and the tension strength multiply elongation is25110MPa%. So the mechanical properties under two sections cooling is better than three sections cooling pattern after hot rolling and those results given an important basis to industrial production of multiphase steel.