Influence of different heat treatment parameters on microstructure and mechanical properties of C–Mn strapping quality steels

Strap steel produced at Bokaro Steel Plant is a plain C–Mn (nominal chemistry C: 0.35%, Mn: 1.35% and Si: 0.25%) cold rolled steel that is austenitised at 930 °C and austempered in a lead bath at 450 °C. The microstructure consists of ferrite, pearlite and bainite also to achieve ultimate tensile strength (UTS) 900 MPa min and elongation 8% min on 150 mm gauge length. In order to achieve such a microstructure consistently, influence of chemistry and different heat treatment parameters on strapping quality steels have been studied. It has been found in the laboratory that increase in Mn (from 1.35 to 1.55%) and Si (from 0.25 to 0.45%) in a 0.32% C strap steel results in 100% bainite in the microstructure after austenitisation and subsequent air cooling. The austempering of the modified strap steel (0.4% C, 1.65% Mn and 0.5% Si) at Bokaro Steel Plant results in excessively high UTS (1040–1100 MPa) without affecting the elongation value (8–10%, 150 mm GL). The microstructure of the austempered straps of modified chemistry consists of cent percent bainite. The austenitisation and subsequent air cooling without austempering of the modified strap steel result in a microstructure consisting of ferrite, pearlite and bainite. The UTS (867–875 MPa) achieved marginally falls short of the specified values (900 MPa min) while % elongation values (8–10%, 150 mm GL) conform to the IRS: P-41 specification.