Development of BethStar 50-A New 50 ksi Yield Strength HSLA Plate

This paper describes the development and mechanical properties of Bethlehem's new 50 ksi (345 MPa) minimum yield strength plate grade, BethStar 50. This material is available in thicknesses from 0.25 to 2.5 inches (6.4 to 64 mm) and widths up to 150 inches (3, 810 mm), with lengths up to 1, 440 inches (36, 580 mm) depending on thickness and width. Each BethStar 50 plate is thermomechanically processed using a special computerized rolling practice that is a function of the plate thickness, thereby producing optimum mechanical properties. A low-carbon, low-sulfur, microalloyed steel composition, combined with controlled rolling or controlled finish temperature (CFT) processing, provides toughness, weld-ability, and formability superior to A572 Grade 50. Severe low temperature toughness requirements such as ASTM A808 (45 ft-lb longitudinal at −50 F or 61 Joules at −46 C) are exceeded by large margins. The key microstructural features that produce these properties are precipitation hardening and a uniform, fine ferrite grain size. A low temperature slab heating practice and CFT rolling reduce coarse grained ferrite regions and the degree of duplex microstructure. Microstructural evaluation shows that the ferrite grain size in these plates is less than 10 microns (ASTM number >10.3). The excellent toughness, formability, and weldability of BethStar 50 are useful in large off-highway truck frames and other structural applications.