The effect of finish rolling temperature and tempering on the microstructure, mechanical properties and dislocation density of direct-quenched steel

A unique batch tempering treatment for industrial scale direct-quenched steel coils has been studied using laboratory simulations. The tempering treatment was non-isothermal with slow heating to 570 degrees C and slow cooling to simulate the tempering of large steel coils. The paper presents the effect of finishing rolling temperature (FRT) relative to the non-recrystallization temperature (T-NR) and the effect of long time tempering on the micro-structure, dislocation density and mechanical properties of direct-quenched coiled strips. Conditioning austenite below the recrystallization stop temperature resulted in a finer effective grain size distribution, which correlated strongly with the impact toughness of the final product. Furthermore low finish rolling temperature resulted in partially ferritic microstructures while higher finishing rolling temperatures led to mixtures of bainite and martensite. Dislocation densities determined with TEM and XRD showed somewhat different trends regarding the effect of tempering: infra-lath dislocation density, as measured with TEM, showed a statistically significant drop in only one case, while XRD analysis indicated a drop in all cases. Furthermore, no significant correlation between finishing rolling temperature and dislocation density existed in XRD studies. The XRD results indicate that the decrease in dislocation density corresponds to about 100 MPa lower dislocation strengthening. However, precipitation hardening and potential internal micro stress relief compensates this as yield strength remains unchanged or even increases during tempering.