The Correlation Between Bending, Tensile and Charpy Impact Properties of Ultra-high-Strength Strip Steels

Two different ultra-high-strength B-containing steel strips, one treated with Ti and the other treated with Al, processed by controlled rolling, accelerated cooling, and coiling in two different temperatures ranges (450 °C to 460 °C and 360 °C to 380 °C) were subjected to tensile testing, bend testing, and Charpy impact testing. The relatively softer and homogeneous microstructure containing mostly granular bainite and upper bainite at the subsurface region (300 to 400 μ m depth below the surface), with a low intensity of BCC shear texture components ( e.g ., {112}〈111〉), generated at the higher coiling temperatures (450 °C to 460 °C), is preferred for bendability (more resistant to shear cracking during bending) and also for the impact upper shelf energy (USE). The hard surface layer dominated by martensite developed at lower coiling temperatures (360 °C to 380 °C) promoted cleavage cracking and is therefore, undesired for bendability and impact toughness. The impact toughness at − 40 °C improved with the intensification of high angle boundaries, refinement of effective grain size, and the reduction of detrimental ‘rotated cube’ texture component. Finally, as the different properties are correlated, a decrease in yield strength, increase in ductility (particularly post-uniform elongation) and tensile toughness are found to be beneficial for bendability and USE. It can be concluded that a higher coiling temperature is preferred to achieve a softer bainitic microstructure if improved bendability and toughness are required rather than higher tensile strength.