Microstructure Characteristic and Strengthening Mechanism of Ti-Microalloyed Automobile Beam Steel Produced by Compact Strip Production

Abstract

The microstructure characteristics, precipitation behavior and misorientation of automobile beam steels produced by Compact Strip Production (CSP) were investigated by use of scanning electron microscopy (SEM), transmission electron microscopy (TEM) and electron back scattered diffraction (EBSD), etc. It is shown that the final microstructure is mainly of polygonal ferrite and some pearlite, the average ferrite grain size is about 4-5μm. It is also noticed that some fraction of granular bainite appears in the microstructure. The precipitation behavior of Ti(C,N) precipitates was studied by using TEM and electrolytic dissolution technique analysis. The experimental results show that large amount of fine Ti(C,N) precipitates with about 10-30nm in size exist in Ti-microalloyed steel. It is proposed that besides the grain refinement, the nano-precipitates play an important role in precipitation strengthening. The EBSD results show that grain boundaries in ferrite are basically high-angle grain boundaries (HAGBs). So the hot rolled austenite is deduced to be comprised mostly recrystallized austenite. Refinement strengthening is the predominant strengthening mode, solid solution strengthening, precipitation strengthening and dislocation strengthening are second to it.