Controlling of the Softened Region in Weld Heat Affected Zone of Ultra Fine Grained Steels

Abstract

When many additional alloying elements are used for high strength of steels, the weldability is usually poor. The development of high-strength steels of 800 MPa class with good weldability is preferred to proceed on the basis of a new concept for innovative materials design which may remarkably reduce the carbon equivalent of steels. The higher-strength steels of 700 to 800 MPa class with the same carbon equivalent as mild steels could be achieved by creating ultra fine ferrite grains of less than 1μm.Efficient and high speed joining processes with low heat input and narrow width of heat affected zone (HAZ) were developed to obtain high-performance welded structures while preserving ultra fine microstructure. The softened region in the HAZ of ultra fine-grained steels can be controlled with the newly developed welding process.