Novel Hot-Rolled Structural Plate Steels with Yield Strength of 700 Mpa and Excellent Usability

Abstract

Hot rolled structural plate steels with yield strength of 700 MPa are an excellent choice for a variety of demanding applications that require excellent toughness and welding properties. SSAB has developed novel hot-rolled plate steels that are produced using precision controlled rolling and an innovative cooling and tempering strategy that ensures invariable mechanical properties in the width and the length directions of the plate. The recently developed steel meets or exceeds the requirements of EN 10025-6 for the S690QL grade. The minimum yield strength (ReH) is between 630 MPa and 690 MPa depending on plate the thickness, and the interval of tensile strength is 760 - 940 MPa, while the minimum elongation at fracture is 14 %. Further, an impact energy of 69 J at-40 °C on transverse V-notch specimen is guaranteed. The novel grades represent superior bendability and surface quality, weldability with excellent HAZ strength and toughness with very low CET and CEV values, exceptional consistency of properties within a plate guaranteed by close tolerances. In addition to the excellent formability, the novel hot rolled steels exhibit greatly improved toughness properties which provides for good resistance to fracture. These outstanding properties are achieved through carefully controlled manufacturing processes. In the present study, a sophisticated recrystallization based hot rolling process optimization method is presented. With the final aim to improve the impact toughness of the novel steel, recrystallization analyses and modelling of austenite grain size development through the rolling pass schedule is performed.