Influence of Composition and Hot Rolling on the Subsurface Microstructure and Bendability of Ultrahigh-Strength Strip

The effect of subsurface microstructure on the bendability of three 8-mm-thick low-alloyed hot-rolled and direct-quenched ultrahigh-strength strip steels with yield strengths in the range 800 to 1100 MPa has been investigated. Rolling to lower finish rolling temperatures increased austenite pancaking, leading to the formation of ferritic/granular bainitic subsurface microstructures that are softer than the upper bainitic microstructures found with higher finish rolling temperature. In addition, increased austenite pancaking was found to increase the intensities of ~{112}〈111〉 α and ~{110}〈112〉 α to {110}〈111〉 α texture components in the surface layers, especially in upper bainitic microstructures. It is shown that the bendability of ultrahigh-strength steels is governed by subsurface hardness and crystallographic texture. Bendability was found to be related to mean microhardness 0.1 to 0.4 mm below the surface, such that excellent bendability was achieved with a relatively soft subsurface layer down to a depth of 0.4 mm, i.e. , 5 pct of the sheet thickness. Intense ~{112}〈111〉 α texture combined with upper bainite containing MA islands in the subsurface region is shown to be detrimental to bendability when the bend axis is perpendicular to the rolling direction probably as a result of geometrical softening combined with high hardness.