In-situ laser ultrasonics measurements of ferrite formation during stepped cooling of a 0.1C-2Mn dual phase steel

Abstract

Dual-phase (DP) steels are advanced high-strength steels used in automotive design. To achieve optimal mechanical properties the control of phase transformations during processing is paramount, e.g. for hot-rolled DP steels a desired ferrite fraction is required to form during run-out table cooling. Thus, sensor technologies such as laser ultrasonics (LUS) are of considerable interest that can in-situ monitor ferrite formation. In this study, the ferrite formation kinetics in a laboratory DP steel were measured by LUS during stepped cooling treatments which were designed to simulate the cooling paths on the run-out table in hot strip mills. LUS measurements were first validated with well-established dilatometry measurements during continuous cooling. For the stepped cooling tests, the fractions transformed obtained from the ultrasonic velocity changes agree with the ferrite phase fractions as characterized by ex-situ metallography. Further, the velocity changes are described by the JMAK approach using parameters that are consistent for the austenite-to-ferrite transformation in low-carbon steels.