Low carbon niobium alloyed high strength steel for automotive hot strip

Hot rolled strip from microalloyed high strength steel typically has a carbon content up to 0.12%. A carbon content close to the peritectic range enhances the susceptibility to surface defects such as edge and transverse cracks, which have to be removed by, for example, scarfing and grinding before hot rolling. These defects can be avoided when a low carbon content is applied. It has been demonstrated that by reducing the carbon content in linepipe steel to similar to 0.03%, further improvements in toughness, ductility and weldability are achieved. Furthermore, a low carbon content promotes niobium carbide solution during reheating of the slab before hot rolling. This permits the application of higher niobium contents than are usually applied. Niobium contents up to 0.10% have recently been adopted in low carbon pipe steel. Besides refining the grain size during austenite processing, niobium in solid solution leads to an additional strength increase by delaying the gamma ->alpha transformation, thus promoting a higher volume fraction of bainite. In addition, NbC precipitation in ferrite gives a further strength increase. This concept is especially effective in producing high strength when accelerated cooling is applied after hot rolling. Based on this experience, a new hot strip steel concept has been developed. Besides its good surface properties, this steel shows high strength as well as excellent toughness and ductility. The properties achieved are homogeneous over the length and width of the hot strip. Changes in processing conditions have only a minor influence on the mechanical properties. In addition, this steel shows exceptional processing properties, for example in cold forming, welding and fine cutting as applied in the automotive industry.