GRADIENT STEEL MATERIAL HAVING HIGH-PLASTICITY SURFACE LAYER AND HIGH-STRENGTH INNER LAYER, AND MANUFACTURING METHOD

摘要

Provided are a gradient steel material with a high plastic surface layer and a high strength inner layer, and a manufacturing method are provided. Weight percentages of the components of the gradient steel material are: C≤0.15%, Si≤1%, Mn≤1.5%, and the balance of Fe and inevitable impurities, the surface layer of the steel material being ferrite, and the inner layer being ferrite+bainite. The manufacturing method therefor comprises: smelting, casting, rolling, and a heat treatment, wherein in the heat treatment step, a steel material is heated to an austenite temperature Ac3 or more and kept at said temperature for more than 3 min; thereafter, the material is cooled to a temperature range between Ar3 and Ar1 in a two-phase zone at a cooling rate of less than 0.5°C/s, and is then cooled to room temperature at a cooling rate of greater than 5°C/s. The present steel material does not need to be obtained by means of the compound preparation of different materials as only a single material is processed. At the same time, the composition of the steel material is simple. Although the internal and external microstructures are different, the difference is a gradual process, and the strength at the interface is good.