Effect of microstructural parameters, microtexture and matrix strain on the Charpy impact properties of low carbon HSLA steel containing MnS inclusions

Low-carbon micro-alloyed steel containing coarse MnS inclusions was subjected to different thermo-mechanical processing routes to evaluate the effect of microstructure on its Charpy impact properties over a range of temperatures. MnS inclusions were found to deteriorate the upper shelf energy (USE) of the steel but its effect on the impact transition temperature was not as detrimental as that due to the presence of TiN particles. MnS inclusions were responsible for the initiation of micro-voids; however, the propagation of the cracks from these depends on the effective grain size and the strength of the matrix. An increase in the density of dislocations and low-angle boundaries enhances the strength and the strain-hardening ability of the ferrite matrix. This helps in retarding the growth of micro-voids, thereby reducing USE and promoting cleavage crack propagation resulting in an increase in its impact transition temperature. Refinement of effective grain size, on the other hand, increases the crack propagation resistance and therefore, improves the low-temperature toughness of the steel. Finish rolling of the steel just above the Ar3 temperature (austenite to ferrite transformation start temperature) or a simple normalizing treatment of the as-rolled plates at a low austenitization temperature is recommended from the point of view of higher impact toughness and lower impact transition temperature.