Influence of boron addition on the hot ductility of medium carbon spring steel

Abstract

In this article, the influence of boron addition (0.0005%, 0.0020% and 0.0047%) on the hot ductility of the medium carbon spring steel 60Si2Mn is studied. Hot tensile experiments in the temperature range of 873–1573 K (600-1300℃) are conducted using Gleeble 1500D. Stress-elongation curves are measured to describe the dynamic recrystallization (DRX) behavior. Reduction of area (R.A.) until fracture is taken to characterize the steel hot ductility. Sample microstructures and fracture morphologies are analyzed using optical microscopy (OM), scanning electron microscopy (SEM) and electron probe microanalyser (EPMA). Besides, the evolution of the secondary phase precipitations are studied by CALPHAD modelling. Results show that the influence of boron addition on steel hot ductility varies with test temperatures: i) in the temperature range of 1323–1573 K (1050-1300℃), boron segregation at austenite grain boundaries leads to BN precipitation which would pin the grain boundaries and deteriorate hot ductility, ii) in the temperature range of 1073–1273 K (800-1000℃), increase of boron addition lowers the DRX onset temperature thus refining grain size and improving the steel hot ductility, iii) in the temperature range of 873–1023 K (600-750℃), the precipitation and distribution of BN along austenite grain boundaries inhibits the formation of grain boundary ferrite, hence the toughness of the austenite grain boundaries and the steel hot ductility are improved.