High resistance boron treated steels for railway applications

Abstract

The present study analyses different boron contents (between 10 and 160 ppm) on the structure of a 0·2C–2Mn–1Si (wt-%) steel deformed at a starting temperature of 1050°C in a T. J. Pigott laboratory rolling mill. The steel was made in a laboratory open induction furnace using high purity raw materials and cast into metallic moulds. This experimental steel has proved to have tribological properties, under dry rolling/sliding contact, as good as those for the 0·8% pearlitic steels used in railway applications. Before thermomechanical processing, the steel ingots (70 × 70 × 70 mm) were homogenised at 1100°C for 1·5 h. The thermomechanical treatment was carried out by a reversed multipass process to reach a level of deformation of 60%. Plastic deformation was finished at ∼920°C for all the rolled steels and the plates (70 × 150 × 20 mm) were then water quenched and/or air cooled to room temperature. Results show more bainitic structures as boron content increases in the air cooled steel after hot rolling. For the quenched steels, the structure becomes more martensitic as boron content increases. The best combination of mechanical properties was obtained for the air cooled 76 ppm boron containing steel, which had a lower bainitic structure. This steel had the yield strength of 750 MPa, 15% elongation and the hardness of 40 HRC. Materials characterisation was carried out by optical and transmission electron microscopy (TEM). Results are discussed in terms of the boron segregation towards grain boundaries, the effect of boron on the steel hardenability, as well as on the boron carbonitrides (CNB) precipitation.