Hot-Rolled Steel Sheet Cooling for SSC-Resistant ERW Pipe

The invention applies different cooling rates to different regions of the steel sheet: the surface region is cooled at a lower rate (5-30°C/s) to maintain lower hardness and better SSC resistance, while the interior region is cooled at a higher rate (10-60°C/s) to achieve higher strength. This spatial differentiation of cooling conditions resolves the contradiction between strength and SSC resistance by optimizing each region's properties for its specific function.

For thick-walled pipes, the invention implements region-specific cooling where the surface layer (0-5mm depth) receives reduced cooling intensity to maintain lower hardness and excellent SSC resistance, while the interior bulk material undergoes more aggressive cooling to achieve the required structural strength. This allows thick-walled pipes to simultaneously achieve both high structural strength and high SSC resistance.

The invention replaces conventional mechanical hardness testing with X-ray diffraction analysis to detect low-angle grain boundaries. This substitution enables the detection of localized high-stress regions that serve as SSC origins, providing much higher measurement precision while still maintaining ease of manufacture through automated non-destructive testing procedures.

The invention segments the single-stage cooling process into two distinct stages: a first cooling stage with lower cooling rate (5-30°C/s) that minimizes low-angle grain boundary formation and maintains SSC resistance, followed by a second cooling stage with higher cooling rate (10-60°C/s) that achieves the required strength. This segmentation resolves the contradiction by optimizing each stage for its specific purpose while maintaining overall process feasibility.