method of manufacturing a steel product having a fine ferrite grain boundary precipitation type martensitic structure

摘要

The present invention addresses the problem of providing a steel product having a martensite structure of the fine ferrite grain boundary precipitation type as a structure having excellent heat treatment reliability, high toughness, and high strength and hardness, and of providing a method for manufacturing the same. The method for manufacturing a steel product having a martensite structure in which fine ferrite, cementite, or other carbide precipitates along grain boundaries so as to form a network comprises: heating steel stock or part of steel stock to or above a transformation point (A₃) or (A₁) by ohmic heat generated by the primary application of an electric current; allowing the steel to cool for 2 sec or less; reheating the steel by the secondary application of an electric current to raise or maintain the temperature at or above the temperature of the pearlite precipitation line on a continuous cooling diagram; and cooling the steel so as to pass over the pearlite precipitation line on a CCT diagram related to a continuous cooling transformation, and to pass through the martensite transformation point.