Abstract: The dual-phase structure of the low-carbon Mn-Si steel for seamless pipe can be obtained by applying the process of step-quenching and boundary zone heat-treatment. The test result shows that the mechanical properties of the dual-phase structure are much higher than those of ferritic plus pearlitic structures, that the dual-phase structure is of better corrosion resistance in some cases for Ca（OH）₂ has corrosion-moderating effects, and that hot-rolling dual-phase structure of the low-carbon Mn-Si steel without Cr and Mo can be easily obtained with the step-quenching process.