Steel product and its manufacturing method

Abstract

To provide a steel product excelling in both corrosion resistance and adhesion. A steel product having a nitride layer and a nitrogen diffused layer formed on the surface of a steel base material is provided. The nitride layer includes a first compound layer formed on a nitrogen diffused layer side and a second compound layer formed on a surface side of the first compound layer. The first compound layer has an ε-structure mainly made of Fe 3 N, and the second compound layer has a higher nitrogen concentration than the first compound layer and has a concavoconvex formed on the surface thereof. Thus, the nitride layer having excellent adhesion, an extremely small carbon containing amount, and high corrosion resistance can be formed.

1 . A steel product having a nitride layer and a nitrogen diffused layer formed on the surface of a steel base material, characterized in that:
 the nitride layer includes a first compound layer formed on a nitrogen diffused layer side and a second compound layer formed on a surface side of the first compound layer,   the first compound layer has an ε-structure mainly made of Fe 3 N,   the second compound layer is a ζ-structured layer containing Fe 2 N of which a nitrogen concentration in mass % is higher up to 11 mass % than the first compound layer, and   a concavoconvex is formed on the surface of the ζ-structured layer.   
     
     
         2 . The steel product of  claim 1 , wherein concaves of the surface concavoconvex of the second compound layer having depths of 0.5 μm or more are provided with a high density so that three or more concaves are located per at least 50 μm of nominal length. 
     
     
         3 . The steel product of  claim 1 , wherein a thickness of the second compound layer is 0.7 μm or more. 
     
     
         4 . The steel product of any one of  claim 1 , wherein a thickness of the first compound layer is 5 μm or more. 
     
     
         5 . The steel product of  claim 1 , wherein an actual surface area ratio of the surface per unit area is a value exceeding 1.8. 
     
     
         6 . The steel product of  claim 1 , wherein an oxide layer mainly made of ferrioxide having a thickness of 3 μm or less is formed on the second compound layer as an outermost layer. 
     
     
         7 . The steel product of  claim 1 , wherein penetrating defects existing in the second compound layer are filled by oxide mainly made of ferrioxide. 
     
     
         8 . A steel product manufacturing method of forming a nitride layer and a nitrogen diffused layer on the surface of a base material by applying a fluoride treatment for diffusing fluorine on the surface of a steel material and, then, applying a gas nitriding treatment for diffusing nitrogen, characterized in that:
 the fluoride treatment includes a fluoride process of reacting fluorine with the steel material while introducing fluorine source gas into a treatment furnace, and a diffusion process of heating and keeping the fluorine and the steel material in a state where the supply of fluorine source gas is stopped, and diffusing to at least a depth of 1 μm or more, the fluorine permeated into the steel material surface, and   in the gas nitriding treatment, the diffused fluorine component is reduced and vaporized by a gas atmosphere without a carbon source, and the nitride layer and the nitrogen diffused layer are formed by diffusing nitrogen to permeate.   
     
     
         9 . The steel product manufacturing method of  claim 8 , wherein the fluoride treatment includes repeating the fluoride process and the diffusion process twice or more. 
     
     
         10 . The steel product manufacturing method of  claim 8 , wherein after the gas nitriding treatment is applied, penetrating defects existing in the nitride layer are filled by forming an oxide layer mainly made of iron oxide having a thickness of 3 μm or less. 
     
     
         11 . The steel product manufacturing method of any one of  claim 8 , wherein after the gas nitriding treatment is applied, a surface concavoconvex with a depth of 0.5 μm or more is exposed by applying a fine particle peening treatment of which an average particle diameter is 100 μm or less so as to remove unnecessary nitride, oxide, oxynitride and the like covering the outermost surface.