Improvement in Properties of 301LN Austenitic Stainless Steel for Metro Coach Manufacture

Abstract

Metastable austenitic stainless steel type 301LN is widely used for fabrication of structural components of Metro Coaches. The steel exhibits both high strength and enhanced plasticity due to strain hardening as well as formation of strain-induced martensite (α) during cold deformation (TRIP effect). The current market requirement, as projected by many of the ongoing Metro Rail Projects in India, calls for manufacture of this steel with ultimate tensile strength (UTS) in excess of 1000 MPa and yield (YS/UTS) ratio of less than 0.8, as this would facilitate substantial reductions in tare weight and crash-worthiness of metro coaches. The typical property requirements in high temper (HT) as per one typical Metro Coach specification are: Yield Strength (YS) ~ 751-921 MPa, Ultimate Tensile Strength (UTS) ~ 1001-1151 MPa, Elongation ~ 22% min, Hardness ~ 36 HRC max and YS/UTS ratio <0.8. previous="" plant="" experience="" has="" shown="" that="" the="" maximum="" attainable="" uts="" in="" this="" grade="" through="" cold="" rolling="" is="" only="" to="" tune="" of="" 970="" mpa="" and="" any="" excess="" deformation="" severely="" impairs="" both="" ductility="" 22="" hardness=""> 36 HRC) beyond acceptable limits. In the present work, an innovative thermomechanical processing (TMP) methodology has been evolved for the attainment of this seemingly unlikely combination of properties through experimental cold rolling and short annealing simulations in Gleeble 3500 C thermomechanical simulator. The novel process methodology entails imparting heavy cold reductions (CR) of 45-50% in Sendzimir Mill followed by brief/ short reversion annealing treatments (80-160 s) by means of single furnace operation at 750 °C at standard line operating speeds for 300 series in Annealing Pickling Line-1 (AP Line-1) of Salem Steel Plant (SSP) in India. The improvement in properties (strength-ductility combination) has been attributed to grain refinement through formation of submicron grained austenitic (γ) microstructure by controlled reversion of strain-induced martensite (α) during the short annealing treatment. The process is distinct from conventional long annealing treatments (300-360 s), which are employed to soften the steel after cold rolling by means of recovery and recrystallization processes.