Abstract: The microstructure and the characteristics of the inclusions embedded in ferrite matrix in simulated coarse-grain heat affected zone (CGHAZ) of a Ti-Zr-treated high strength low alloy (HSLA) steel have been investigated. The microstructure of the simulated CGHAZ dominantly Consisted of intragranular acicular ferrite (IAF) combining with a small amount of polygonal ferrite (PF), widmanstiitten ferrite (WF), bainite ferrite (BF), pearlite and martensite-austenite (M-A) islands. The PF, WF and BF were generally observed at the prior austenite grain boundaries and the interlocking acicular ferrite was usually found intragranularly. It was found that the inclusions were composed of Ti2O3 ZrO2, Al2O3 locating at the center of the particles and MnS lying on the surface layer of the inclusions. The intragranular complex inclusions prorooted the acicular ferrite formation and the refinement of microstructure whilst those at prior austenite grain boundaries caused PF formation on the inclusions. The simulated CGHAZ consisting of such complicated microstructure exhibited desired mechanical properties.

Key words: microalloyed HSLA steel, heat affected zone, microstructure