The challenge to produce consistent mechanical properties in Nb-HSLA strip steels

High strength steels have provided a unique opportunity for the automotive industry to achieve weight reduction via reduced material thickness whilst maintaining performance requirements such as crash worthiness. However, the initial success of high strength low alloy (HSLA) steels was constrained by inconsistencies in the final mechanical properties. Although significant improvements in both mechanical and dimensional properties have been made, increasingly stringent customer specifications for high strength steels mean that the challenge to reduce variability in automotive HSLA steel products and enure their continued competitiveness has never been greater. This paper addresses the issues concerning the controlled hot rolling of Nb-HSLA strip steels through a series of laboratory and commercial trials. Both sets of data established that: (i) the strength was decreased by reducing the soak temperature from 1250 to 1150 °C; (ii) the presence of furnace skid chills during soaking locally reduced the strength; (iii) lowering of the finishing temperature towards the Ar3 reduced the overall strength, but improved the coil width property consistency; and (iv) the existence of an optimum coiling temperature developed higher strengths and improved property consistency. The programme of work conducted also indicates that, by finishing near the Ar3 temperature, very fine ferrite subsurface grains are created which limit the fall in strength. Additionally, the commercial trials highlight the need to compensate for the exposure-related cooling effects of the inner and outermost laps of the hot rolled coil, thereby reducing any potential for developing further inconsistencies.