| 摘要 |
A tool for a piercing mill with excellent wear resistance and a method for producing the tool for a piercing mill are provided. A scale layer is formed in a surface layer of a substrate steel having a composition containing, on a mass% basis, C: 0.05% to 0.5%, Si: 0.1% to 1.5%, Mn: 0.1% to 1.5%, Cr: 0.1% to 1.5%, Mo: 0.6% to 3.5%, W: 0.5% to 3.5%, and Nb: 0.1% to 1.0% and further containing Co: 0.5% to 3.5% and Ni: 0.5% to 4.0% so as to satisfy 1.0 < Ni + Co < 4.0. The scale layer includes a net structure scale layer that is formed on a substrate steel side, has a thickness of 10 to 200 µm in a depth direction, and is complicatedly intertwined with a metal. A microstructure on the substrate steel side in a range of at least 300 µm in the depth direction from an interface between the net structure scale layer and the substrate steel contains a ferrite phase at an area fraction of 50% or more, the ferrite phase containing 400 /mm 2 or more of ferrite grains having a maximum length of 1 to 60 µm. Such a microstructure can be formed by performing a scale-forming heat treatment in which, after heating, cooling to at least 700°C is conducted with first rapid cooling and second slow cooling. Thus, the adhesiveness of the scale layer is improved and the lifetime of the tool for a piercing mill is increased. |
