Bendability and Microstructure of Direct Quenched Optim^® 960QC

Abstract

Use of ultra-high-strength steels (UHSS) in weight critical constructions is an effective way to save energy and minimize carbon footprint in the end use. On the other hand, the demands for reducing manufacturing costs and energy consumption of the steelmaker are increasing. This has led to development of energy efficient direct quenching (DQ) steelmaking process as an alternative to the conventional quenched and tempered or thermomechanical rolling and accelerate cooled processes. Ruukki has employed thermomechanical rolling and direct quenching process (TM + DQ) for a novel type of ultra-high-strength strip and plate steels since 2001. Advantages of the ultra-high-strength level (>900MPa) can be fully utilized only if fabricated properties are on a sufficient level. Bending is one of the most important workshop processes and a good bendability is essential for a structural steel. Hence, the metallurgy and bendability of Ruukki ́s TM + DQ strip steel Optim® 960QC have been investigated closely. It was found that by optimizing process parameters and chemical composition, a good combination of strength and ductility can be achieved by a modification of martensitic-bainitic microstructure. Despite of smaller total elongation, the bendability of Optim® 960QC is at least on the same level as on conventionally manufactured 960MPa steels. However, it is important to pay special attention to bending process (tool parameters, springback, bending force, material handling) when bending UHSS. It was also found that the bendability of Optim® 960QC can be significantly enhanced by local laser heat treatments or roll forming.

Investigation of microstructure, and mechanical properties of dissimilar high and ultra-high steel welded joints: application for extreme climate conditions

“…When different heat inputs were used, there were not many differences in the longitudinal direction. Similar behavior was determined in the hotrolled ultra-high-strength steel by Kesti et al [28], Deole et al [29], Anna-Maija et al [30], and Kaijalainen et al [31].…”

Total strain on the outer surface of steel sheets in air bending

“…Various terms have been used referring to this phenomenon in previous studies: multi-breakage, gap formation, punch-sheet separation, punch detachment, punch-sheet-liftoff, loss of contact with the punch/sheet etc. [2][3][4][5][6][7][8]. The phenomenon has been known for a long time.…”

Strain distribution during air bending of ultra-high strength steels

“…In this phenomenon, also known as "multibreakage", the inner radius of the sheet becomes smaller than the punch radius and the loading scheme changes from 3-point bending to something akin to 4-point bending [1,2]. Multi-breakage and severe punch-sheet separation has been previously associated with a flat-top (or "nut-like") bend geometry [3][4][5][6]. Multi-breakage also affects springback and bend allowance, and the decrease of the inner radius may lead to higher strains than expected.…”