Investigation of hot ductility in Al-killed boron steels

The influence of boron to nitrogen ratio, strain rate and cooling rate on hot ductility of aluminium-killed, low carbon, boron microalloyed steel was investigated. Hot tensile testing was performed on steel samples reheated in argon to 1300 degrees C, cooled at rates of 0.3, 1.2 and 3.0 degrees Cs-1 to temperatures in the range 750-1050 degrees C, and then Strained to failure at initial strain rates of 1 x 10(-4) or 1 x 10(-3)s(-1). It was found that the steel with a B:N ratio of 0.19 showed deep hot ductility troughs for all tested conditions; the steel with a B:N ratio of 0.47 showed a deep ductility trough for a high cooling rate of 3.0 degrees Cs-1 and the steel with a near-stoichiometric B:N ratio of 0.75 showed no ductility troughs for the tested conditions. The ductility troughs extended from similar to 900 degrees C (near the Ae(3) temperature) to similar to 1000 or 1050 degrees C in the singlephase austenite region. The proposed mechanism of hot ductility improvement with increase in B:N ratio in these steels is that the B removes N from solution, thus reducing the strain-induced precipitation of AlN. Additionally, BN co-precipitates with sulphides, preventing precipitation of fine MnS, CuS and FeS, and forming large, complex precipitates that have no effect on hot ductility. (C) 2008 Elsevier B.V. All rights reserved.