Research On The Effect Of Strain Modes And Strain Rates On Microstructures And Mechanical Properties Of TRIP780 Steel Sheet

The tendency of global warming and air pollution are getting worse recently, many countries have increased demands on energy saving and emission reduction. Thus the automotive lightweight progress achieved a high-speed development. Although their lower density can meet the requirement of automotive lightweight, aluminum and magnesium alloys can not ensure the safety performance of the vehicles. So the high strength steels have naturally become the best choice to realize automotive lightweight because of their great combination of ultrahigh strength and high ductility. Despite of high strength, DP steel and HSLA steel are easy to cause crack, springback, mold wear and other defects in the stamping process because their ductility and Rmx A are not good enough. Then the new generation of automotive steel represented by TWIP steel and Q&P steel were developed. Though having great strength, ductility and Rmx A, TWIP steel is rarely used to produce automotive parts, due to its complex production progress, superior difficulty in metallurgy progress and high cost. Q&P steel whose Rmx A is close to TRIP steel has not gain public recognition in automotive industry, and it is still in the improvement and trial stage. TRIP steel is now applied more widely in the automotive industry, due to its excellent strength, ductility, integrated mechanical properties and low cost. Microstructures of TRIP steel consist mostly of ferrite, bainite and retained austenite. The excellent combination of high strength and ductility of TRIP steel was obtained through transformation induced plasticity effect(TRIP effect), i.e. retained austenite transforms to martensite under external stress. TRIP effect and Rmx A separately determine TRIP steel’s mechanical properties and energy absorption ability. Therefore TRIP effect and Rmx A was took as the main mechanical properties evaluation criteria of TRIP steel. Strain Modes and strain rates of automotive parts vary widely in the forming and working period, so it is very necessary to study the effect of Strain Modes and strain rates on TRIP steel’s mechanical properties, which can provide appropriate forming processes and working conditions of TRIP steel.In this paper, mechanical properties of TRIP780 steel sheet were researched through uniaxial tension test, high speed tension test and bulging test. The effect of strain modes and rates on rupture elongation, uniform elongation, tensile strength, Rmx A and bulging height were studied. In addition, microstructures and the change of retained austenite content were studied with the help of optical microscope, scanning electron microscope and X-ray diffraction, mechanism of the effect of strain modes and rates on mechanical properties was revealed. Main conclusions are as follows:TRIP780 steel sheet has good mechanical properties, work hardening ability and energy absorption ability, its Rmx A can reach30 GPa·%. strain modes have a bigger effect on mechanical properties of TRIP steel than strain rates, The results of TRIP effect decreased with bulging test, uniaxial tension test, high speed tension test in regular order. This result shows that TRIP780 steel sheet has better integrated mechanical properties under multiaxial stress and low strain rates. The integrated mechanical properties including rupture elongation, uniform elongation and Rmx A under uniaxial tension test are better than those under high speed tension test. Because the lower strain rates are, the more dislocation slips, the better the extension rate is. Moreover, lower strain rates are conducive to the rise of phase change driving force, which determines TRIP effect, so the integrated mechanical properties of TRIP780 steel are better. The higher strain rates are, the more serious of non-uniform deformation and internal temperature rise is, which lead to shortage of phase change driving force and rise of retained austenite stability, therefore mechanical properties were weakened with strain rates increasing. But high speed tension test results are different, the rupture elongation, tensile strength and energy absorption capacity increase due to deformation resistance getting stronger when strain rates go up, thus the mechanical properties of TRIP 780 steel were enhanced. The higher high speed tension rates are, the less cross slips in the necking zone are, the more retained austenites are, and matrix soften seriously because of temperature increment effect. All of these factors lead to rupture’s delay finally.Based on the above conclusions, multi-directional deformation, lower strain rates in the forming progress and work conditions can fully exert the mechanical properties of TRIP780 steel, Crash energy absorption of TRIP780 steel sheet is excellent which can effectively improve the safety performance of vehicles.