Sensitivity of novel 460 MPa proof strength titanium-vanadium microalloyed high strength low alloy steels to annealing linespeed and soak temperature

摘要

High strength low alloy steels are characterised by predominantly ferritic microstructures, strengthened by grain boundary and precipitation strengthening. Both of these strengthening mechanisms traditionally arise from the niobium addition. Increasing the niobium addition would theoretically increase strength. However, increasing niobium content above ～0.04 wt.% is not recommended in industrial practice due to narrowing of the annealing process window. Two novel grades exhibiting different additions of titanium and vanadium in place of the traditional niobium addition were investigated. Sensitivity to annealing linespeed and soak temperature was investigated to conclude whether a practically achievable process window exists and moreover, to conclude whether proof strength in excess of 420 MPa could be achieved while satisfying the maximum ultimate tensile strength and minimum total elongation specifications of CEN Grade HC420LA under European Standard EN 10268:2006. One of the two novel grades, exhibiting higher manganese and vanadium contents, met the minimum proof strength target, while almost satisfying the maximum ultimate tensile strength and minimum total elongation specifications. However, the annealed microstructure was found to be partially recrystallised, which is not recommended in industrial practice. Moreover, sensitivity to annealing linespeed and soak temperature was considered too great to obtain a practically achievable process window.