Hot ductility of Nb and Al microalloyed steels follovving high-temperature solution treatment

Abstract

C–Mn–Al, C–Mn–Nb, and C–Mn–Nb–Al steels are known to exhibit troughs in their hot ductility behaviour. This paper attempts to explain the high-temperature brittleness of these steels by examining their hot deformation behaviour. Slow strain-rate, hot tensile testing of laboratory-melted and commercial C–Mn steels containing niobium and/or soluble aluminium following solution treatment at 1300°C for 1·5 h has been performed. The occurrence (or otherwise) of dynamic recrystallization, hot strength, and elongation to fracture were noted as a function of deformation temperature, and the austenite-ferrite equilibrium transformation temperature determined by slow-heating dilatometry. The addition of niobium and/or soluble aluminium to C–Mn–N steel produced a hot ductility trough, failure being intergranular when ductility was poor and by dimpled rupture and tensile necking when ductility was good. Poor ductility was associated with a fully austenitic microstructure. Small amounts of ferrite did not appear to affect ductility although larger amounts were beneficial. In aluminium-bearing steels, poor ductility is thought to be the result of embrittlement before dynamic recrystallization could occur. In the C–Mn–Nb steel ductility seemed to be controlled entirely by precipitation and dynamic recrystallization did not appear to playa part. No ductility trough was observed in the C–Mn steel. In this steel, the austenite continued to recrystallize during deformation below the Ae 3 temperature, so that it remained sofi and fine grained.