| 摘要 |
PURPOSE:To prevent the decrease in surface hardness of a steel material due to decarburization in the stage of annealing a hard steel material having rolled scale to improve cold processability by using an atmosphere of the specific compsn. having a small moisture content for the annealing atmosphere. CONSTITUTION:A steel material which is hard and has poor formability, such as middle carbon steel and low carbon steel is subjected to an annealing treatment for spheroidizing the laminar carbide in the texture in order to improve cold processability such as cold forging. Inert gas, such as nitrogen, which has <=0.1vol% moisture and is incorporated therein with 0.05-1vol% gaseous lower hydrocarbon, such as methane, ethane, propane or butane, is used as the annealing atmosphere in this case. Since the scale on the surface of the steel material to be annealed does not produce any decarburizing component, such as CO2 and H2O, in the annealing atmosphere, there is no possibility of a decrease in the surface hardness of the steel material to be annealed due to decarburization and annealing. |
