The effect of tempering temperature on microstructure, mechanical properties and bendability of direct-quenched low-alloy strip steel

The tempering of re-austenized, quenched and tempered (RAQT) martensitic steels is an extensively studied and well understood field of metallurgy. However, a similar understanding of the effect of tempering on direct-quenched (DQ) high-strength steels has been lacking. Now, for the first time, the effect of tempering in the range of 250-650 degrees C on the strength, toughness, bendability, microstructure, crystallography and dislocation density of a DQ steel is reported. In the case of tempering at 570 degrees C, the effects of having a RAQ or DQ starting condition are compared. For the composition and thermal cycles studied, it was found that a peak tempering temperature in the range of 570-600 degrees C resulted in a DQT steel with an optimal balance of strength, bendability and toughness, i.e. a yield strength greater than 960 MPa, a minimum usable bending radius of 2 times the sheet thickness and T28J of - 50 to - 75 degrees C depending on the test direction. Crystallographic texture, dislocation density and the distribution of carbides are important factors affecting the bendability of DQT strip. Tempering had no effect on texture, but strongly influenced the size and distribution of carbides thereby resulting in differences in bendability and impact toughness transition temperature.