Effect of Alloying and Coiling Temperature on the Microstructure and Bending Performance of Ultra-High-Strength Strip Steel

ABSTRACT

Two different high-strength B-containing microalloyed steel strips produced in industrial processing conditions, one treated with Ti and the other treated with Al, processed by controlled rolling, accelerated cooling, and coiling in two different temperature ranges [723 K to 733 K (450 °C to 460 °C)] and [633 K to 653 K (360 °C to 380 °C)] were subjected to bend testing. The Ti-treated steel coiled at the higher temperature 733 K (460 °C) showed the best bending performance. The relatively softer (tensile strength of < 900 MPa) and homogeneous microstructure containing mostly granular bainite and upper bainite to ~ 300-400 μm depth below the surface, generated at the higher coiling temperature, is preferred for bendability. The lower temperature coiling resulted in the formation of a hard surface layer dominated by martensite which is undesired as the steel becomes prone to shear cracking and interphase separation due to strain localization. The combined effects of beneficial texture components such as γ-fiber, {332} <113> and even {112} <131> in the sub-surface region as well as uniformity of through thickness texture of the rolled sheet improve the bendability. In the presence of crack initiators, like coarse and brittle TiN particles found in the Ti-treated steel, a harder microstructure and the presence of Cube and Goss texture in the sub-surface layer, seen for the lower coiling temperature can cause local transgranular cleavage cracking. Finally, the post-uniform elongation obtained from tensile testing and bendability follow a good correlation.