The Effect of Titanium on the Thermomechanical Processing of a Dual Phase Steel

The present work studies the microalloying effect of titanium on the distribution and features of martensite of a dual-phase steel after a thermomechanical processing, and an intercritical heat treatment. The steel composition is 0.05C-1.2Mn-0.5Si-0.2Cr and titanium contents up to 0.06%. The steel was made in a laboratory and cast into small ingots, then heat treated at 1200C to dissolve any carbides formed during solidification. After solubilisation, the steel was cooled down to 1050C and hot rolled in a multi-pass reversible mill to get a reduction of 80%. Just at the end of hot rolling, the measured temperature was about 950C for all the steel plates. Intercritical heat treatments were undertaken at 710, 715 y 720C for 30 min and then water quenched. A grain refinement was observed due to the effect of titanium during the thermomechancial processing; grain size went from 25 microns for the steel without titanium to 12 microns for the 0.06%Ti steel. In addition, a diminution in the pearlite fraction from 0.12 to 0.06 was observed when titanium content increased. On the other hand, after the intercritical heat treatment the martensite fraction decreases as titanium increases for a constant temperature. By correlating the microstructure and the mechanical properties, it is assumed that titanium reduces the grain size during the thermomechanical processing, but also reduces the martensite fraction for a constant temperature during the intercritical heat treatment. The grain refinement and the presence of martensitecontribute to a good combination of strength and ductility in this kind of steels, 680 MPa and 35%.