Investigation On The Microstructure And Mechanical Properties Of Selected Area In Microalloy High-strength Steel

The microstructures evolution and matrix hardness of a low carbon Mn-Mo-Nb and Mn-Vsteel during continuously transformation was investigated by using light microscopy, SEM andEBSD and nano indentation method.(1)The microstructures and morphology of microalloy high-strength steel in differentaustenitizing temperature was obtained by water quenching and air cooling, it shows that themajor microstructures of D40in water quenching were martensites, and a few acicular ferrite(AF) was discovered. But in X80steel AF was discovered in air cooling.(2)Evaluation of microstructures formed during continuously cooling under different coolingrates was made. It shows that As the cooling speed increased the amount of acicular ferrite is onthe rise, and deformation temperature has little impact on microstructure in different coolingspeed. With the deformation temperature reducing, D40steel can obtain AF in a wide coolingrate range.(3)Use nano indentation apparatus and vickers hardness meter research nanohardness andvickers hardness of different phase structure, It is found that martensites of two kinds of steelhave the same hardness at complete austenitizing temperature, and the proeutectoid ferrite ofD40has minimum nanohardness. Evaluation nanohardness-displacement curves of the typicalphases, it is found that nanohardness of AF was6.28GPa, which was between nanohardness ofmartensites and nanohardness of other ferrites. It shows that the formation of acicular ferrite byshear transformation mechanism point of view is consistent with the experiment.(4) EBSD analysis showed that when deformation temperature increased from800℃to900℃in the high cooling speed, the date of Band Slope has a decreasing trend. Found that withrising deformation temperature the non-equilibrium phase of AF was produced easily in thehigh-strength steel. There was a biger influence on X80steel than D40steel when thedeformation temperature was improved in high cooling speed. It shows that AF was producedeasily in ultimate rolling temperature of900℃, as a result improving composite properties ofmicroalloy high-strength steel.