Galvanized Steel Decarburization for Spot Welding

The patent applies local quality by creating a decarburized layer specifically at the surface region (0-35μm depth) of the steel sheet, while maintaining high carbon content in the bulk material. This localized modification of carbon concentration (0.05-0.30% Ceq in surface layer, 0.20-0.50% Ceq in bulk) allows the surface to have improved weldability by reducing microcrack susceptibility, while the core retains high strength properties. The differential carbon distribution resolves the contradiction between overall strength and local weldability.

The patent changes the carbon concentration parameter across different depths of the steel sheet. By controlling the carbon equivalent (Ceq) to be 0.05-0.30% in the surface layer and 0.20-0.50% in the bulk, the material properties are modified to achieve both high strength and good weldability. This parameter gradient approach allows the surface to resist liquid metal embrittlement while the core maintains high tensile strength.

The patent creates a localized decarburized layer at the surface (0-35μm depth) with lower carbon equivalent (0.05-0.30% Ceq) compared to the bulk (0.20-0.50% Ceq). This local quality modification reduces the susceptibility to microcrack formation and liquid metal embrittlement in the surface region where welding occurs, while the core material maintains high strength through higher carbon content.

The steel sheet is segmented into two distinct regions based on carbon concentration: a surface layer (0-35μm) with low carbon equivalent for weldability, and a bulk region with high carbon equivalent for strength. This segmentation of the material structure allows each region to optimize its properties independently, resolving the contradiction between strength and microcrack resistance.

The patent applies local quality by creating a decarburized surface layer with controlled composition that is less prone to oxide formation. The surface layer (0-35μm) has lower carbon equivalent (0.05-0.30% Ceq) which reduces the reactivity with atmosphere during handling and processing, thereby improving surface quality and plating adhesion. The bulk material maintains high alloy content for strength.

The decarburization process is performed as a preliminary action during hot rolling (coiling temperature 550-750°C) to create the low-carbon surface layer before subsequent cold rolling and galvanizing. This preliminary modification of the surface composition prevents oxide formation during later processing steps by reducing the carbon content that would otherwise react with oxygen and form surface defects.