Effect of boron on microstructure and mechanical properties of Al-killed high-strength low-alloy steel

Through the application of experimental analysis and mechanical characterisation, the influence of boron on the microstructure, strength and impact toughness of Al-killed high-strength low-alloy steel was examined. The results showed that with increasing boron content, the area fraction and average size of martensite–austenite constituents increased by 11.8% and 2.04 μm, respectively. Owing to the strengthening and combining effects of boron, the proportion of low angle grain boundaries in the boron-containing steel increased from 67.94% to 79.41% in comparison to the boron-free steel. After adding 0.0014% boron, the tensile and yield strength increased by 14.65% and 3.66%, respectively, due to the increasing content of martensite–austenite constituents. The long strip and large-sized martensite–austenite constituents of the steel matrix with 0.003% boron easily became the source of cracks and facilitated crack propagation, resulting in a decrease in strength. The toughness tested at −20 °C of the steel was severely worsened after adding boron, and the impact absorbed energy decreased from 105 to 7.4 J. This was because boron increased the martensite–austenite constituents, low angle grain boundaries and random boundaries, which favoured crack generation and propagation during the impact process, resulting in a sharp deterioration of toughness.