Research On High Speed Tensile Properties Of Medium-High Carbon Nanstructured TRIP Steels

The aim is to study the effect of different heat treatment processes on high speed tensile properties of medium-high carbon nanstructured TRIP steels in this paper.Firstly,We investigated the microstructures and mechanical properties of the experimental steel with the multi-step low temperature bainitic transformation and the quenching and partitioning(Q&P)process.Secondly,the microstructures,phase fraction were analyzed by OM(optical microscopy),SEM(scanning electron microscopy),TEM(transmission electron microscopy)and XRD(X-ray diffraction).The traditional tensile test was carried out and the micro-hardness was measured.Finally,the high speed tensile properties of three-step low temperature bainitic transformation and quenching and partitioning(Q&P)process were studied under different strain rates.The main results are as follows:1.Compared with the traditional one-step bainitic transformation,multi-step low temperature bainitic transformation effectively reduced the average thickness of bainite plates as well as the size and volume fraction of retained austenite,so that the mechanical properties can be greatly improved.The multi-step low temperature bainitic transformation can get a higher product of strength and ductility,which is close to(or higher than)that of quenching and partitioning process in traditional tensile test.2.The microstuctures of multi-step low temperature bainitic transformation and quenching and partitioning process is similar,including nanostrcutured bainite-ferrite(or martensite)plates and retained austenite.Both of them have a higher volume fraction of retained austenite and micro-hardness.3.The fracture mode of multi-step low temperature bainitic transformation and quenching and partitioning process is ductile fracture.Many dimples with different sizes in the fracture surfaces were observed by SEM microscope.4.In the high speed tensile tests,the three-step low temperature bainitictransformation of TRIP steels showed a good strain rate sensitivity,so as quenching and partitioning process of TRIP steels.Under the same strain rate,the high speed tensile properties of three-step low temperature bainitic transformation is much better than that of quenching and partitioning process.