Cold-rolled multiphase boron steels: microstructure and mechanical properties

The influence of the boron concentration on phase transformation characteristics, microstructure and mechanical properties of multiphase steels was investigated using computational thermodynamics (Thermo-Calc®), dilatometry, quantitative metallography and tensile tests. Pilot scale 50 kg steel ingots were prepared in an induction furnace operating under an argon gas atmosphere with boron contents between 0 and 47 ppm. The ingots were cut into 35 mm thick blocks, which were reheated to 1250 °C for 1 h and hot rolled for seven passes to attain a thickness of 7.0 mm. The hot-rolled sheets were machined and then cold rolled to a final thickness of 1.2 mm. Continuous annealing cycles were performed in a Bähr dilatomer and in a Gleeble machine. Continuous annealing laboratory simulations showed that boron did not significantly influence the amount of austenite formed during heating and soaking steps. However, boron influenced austenite transformation during the cooling step, which reduced the amount of ferrite and increased the amount of bainite. Regarding the mechanical properties, adding boron increased strength and decreased ductility of the product. The steels with boron concentrations up to 27 ppm exhibited the greatest effect. The amount of austenite, which was calculated using Thermo-Calc®, was slightly overestimated compared with that obtained by dilatometry and metallography, particularly for soaking temperatures lower than 800 °C.