Effect of coiling temperature on impact toughness of hot rolled ultra-high-strength multiphase steel strips

Abstract

The tensile and Charpy impact properties of four strip samples from two different B-added low-C ultra-high-strength steel strips (Al-treated and Ti-treated), coiled at two different temperature ranges (360–380 °C and 450–460 °C), have been evaluated and correlated to the microstructural parameters, dislocation density, and the intensity of high-angle boundaries. The effects of coiling temperatures on the microstructural evolution and mechanical properties have been discussed. The volume fraction of the individual phase constituents (namely, granular bainite, upper bainite, lower bainite and tempered martensite) and their hardness, local deformation response and the strain-hardening ability, as determined from nanoindentation testing, influenced the bulk properties such as hardness, tensile properties (strength and ductility), Charpy impact properties (upper shelf energy, USE, and ductile-to-brittle transition temperature, DBTT) and strain-hardening abilities under both quasi-static and dynamic loading conditions. The dominance of granular bainite and upper bainite (75–90 %) reduced the strength (670–722 MPa yield strength), improved ductility (16.7–19.5 % elongation to failure) and USE (35–42 J) in the samples coiled at the higher temperatures. In contrast, a higher fraction of tempered martensite and lower bainite (78–82 %) significantly increased the strength (808–814 MPa), reduced ductility (13.0–14.5 %) and USE (19–29 J) in the lower temperature coiled samples. The DBTT showed a complex trend with the microstructural parameters. It depended on the USE level, as well as on the ‘effective grain size’ of the matrix.