Hot rolled HSLA strip steels for automotive and construction applications

High strength low alloy (HSLA) steels offer the advantage of weight and cost savings compared to mild steel. Therefore they are widely used in the automotive and the construction industry. The hot strip mill typically hot rolls gauges ranging from about 2 to 10mm. Processing on a hot strip mill is especially favourable to maximise the strength during thermomechanical rolling. The retardation of recrystallisation, responsible for grain refinement, occurs naturally in niobium microalloyed steel at the typical rolling temperatures found in the finishing train. The high deformation speed and short interpass time allows for about 50% of the total niobium content to remain in solid solution, thus adding to retardation of transformation during accelerating cooling, which also supports grain refinement, and permits additional precipitation hardening in the coil. As the final product often undergoes a cold forming operation, the required ductility is obtained by producing low carbon (<0.08%) and low sulphur (<0.005%) steel, including sulphide shape control. With the additions of microalloys and other alloying elements, HSLA strip steels up to a minimum yield strength of 690MPa (Grade 100) is widely applied, even substituting traditional quenched and tempered steels. All these modern steel grades are based on niobium as a first choice microalloy. The (semi)-continuous rolling on a hot strip mill asks for specific efforts to guarantee a high uniformity of properties over the width and length of the coiled hot band. Thus the processing parameters and their influence on the final properties are analysed in detail. (AU)