The Control Of Microstructures And Mechanical Properties Of Hot-rolled Ultra-low Carbon Titanium Low Alloyed Automobile Strip

The development of modern steel industry not only emphasized the growth in quantity, but also focused on innovation on the technical level. The low-carbon low alloyed steel with high strength and excellent ductility combination is always the research focus during developing the novel steels. Cr, Ni, Cu and other expensive alloying elements are added widely, which is lead to the cost of steels increased. In this paper, we followed the principle of low-cost high-performance, and on the basic of C-Mn steel, low alloyed element Ti was appropriately added. Moreover, the content of Mn(steel A (1.05%Mn), steel B(1.5%Mn)) was adjusted, and element B(steel B(1.5%Mn-B)) or element Mo(steel D(1.5%Mn-Mo)) was added. The simple and reasonable thermo mechanical control process (TMCP) was taken, and good microstructural morphologies and nano-scale precipitates were obtained. The steel strips would acquire the high strength, excellent impact property and reaming property, which met the requirements of automobile strips. The main work and conclusions of this paper were as follows.(1) By using the thermal simulation experiment technology, the study of austenitic high temperature deformation behavior of tested steel was conducted by single pass compression experiments. The main conclusions of these experiments were as follows:(a) With the increase of deformation temperature, deformation resistance gradually decreased under the same strain;(b) When the temperature was constant, with strain rate increasing, deformation resistance increased;(c) As the deformation increasing, the deformation resistance increased;(d) Recrystallization activation energy of tested steel B was329.8kJ/mol. At the same time, the dynamic recrystallization mathematical model and deformation resistance model were determined, and the mathematical models had good precision.(2) By using the thermal simulation experiment technology, we studied the austenite continuous cooling phase transition behaviors of tested steel A, B, C, and drew static and dynamic CCT curves. We can acquire results from the curves:with cooling rate increasing, the ferrite transformation start temperature decreased, and bainite phase transformation start temperature emerged phenomenon of increased and then decreased due to the effect of ferrite phase transformation. Deformation promoted the ferrite phase transformation, so that the ferrite region in dynamic CCT curves was larger than that in static CCT curves. At the same time, we used OM, SEM, and macro hardness to analyze the effect of alloying elements, cooling rate and deformation on austenitic continuous cooling transformation behaviors.(3) Rolling experiments was carried out the Φ450hot rolling mill, microstructures of rolled tested steel A, B, C and D were detected and analyzed by OM, SEM, TEM and other equipments. The results were as follow:(a) The room temperature microstructures of tested steel A, B and D were ferritic organization, their average sizes were6.4μm,5.2γm and1.9μm respectively. Tested steel C was bainite.(b) Nanoparticles in ferrite of tested steel A, B, D were shown two types of nano size, while nanoparticles in tested steel C were uniform. Larger size precipitates nucleared in the process of controlled rolling and the rolling interval stage, while smaller precipitates formed in the slow cooling process after the rapid cooling.(4) Tensile test, impact test and hole expanding test of hot-rolled steels were studied, tested steel A had the lowest yield strength(492.4MPa) and the highest elongation(23.6%), while tested steel C had the highest yield strength(653.4MPa) and a lower elongation(19.0%). Impact energy of four tested steels were51.64J,50.17J,53.24J and43.10J respectively at-40℃, and low-temperature impact performance were good. Their hole expanding rates were around40%, so they can meet the performance requirements of commercial vehicles rims steel. At the same time, fracture surface morphology and fracture crack propagation of tensile test, impact test and reaming test specimen were analyzed to get the failure factors which affected the mechanical properties of tested steels by OM, SEM and electron probe.