The microstructure and technological properties of ultra high strength 1100MPa grade strip steel

Abstract

The article describes the microstructure and the technological properties of a direct quenched ultrahigh
strength strip steel with the minimum specific yield strength of 1100MPa. The microstructure of this
low carbon, Mn-Cr-Mo-Cu-Ni alloyed steel consists mainly of auto-tempered lath martensite. Due to the
sophisticated thermo-mechanical controlled processing schedule, the martensite transformation takes
place from a fine and uniform austenite grain structure. State-of-the-art steelmaking and continuous casting
operations guarantee a good inclusion cleanness and low level of segregation. The steel has excellent impact
and fracture toughness properties with respect to its ultra-high strength level. The determined transition
temperature for 28J in Charpy-V test and fracture toughness characteristic temperature, T0, were below
-100°C. The weldability tests indicated that the impact toughness of the heat affected zone (HAZ) is excellent
and there is no significant softening in the HAZ or in the welded joint in the wide range of t8/5 cooling times.
The steel allows crack-free bending with a minimum inside bending radius equal to 3 times material thickness
irrespective of the bending direction. In addition, the steel has a good resistance to atmospheric corrosion.