Superfine grain steel having nitrided layer

Abstract

An ultra fine grain steel has a ferrite grain structure having an average grain size of 3 μm or less and has a nitride layer formed on a surface of the steel. The nitride layer is formed without adding an alloy element such as Cr or Mo that is expensive and detrimental to recycling. Fatigue strength is enhanced by the nitride layer.

1 . An ultra fine grain steel having a nitride layer, wherein the steel has a ferrite grain structure having an average grain size of 3 μm or less and the nitride layer is formed on a surface of the steel.  
   
   
       2 . The ultra fine grain steel having a nitride layer as claimed in  claim 1 , wherein grain growth at a time of nitrifying is suppressed by precipitation of carbide or addition of a solid solute element or both of them.  
   
   
       3 . The ultra fine grain steel having a nitride layer as claimed in  claim 1 , wherein the amount of C is 0.01 mass % or more.  
   
   
       4 . The ultra fine grain steel having a nitride layer as claimed in  claim 1 , wherein at least one element selected from a group consisting of Mn, Cr, Mo, Ti, Nb, V and P is added.  
   
   
       5 . The ultra fine grain steel having a nitride layer as claimed in  claim 4 , wherein the amount of Mn is 0.4 mass % or more.  
   
   
       6 . The ultra fine grain steel having a nitride layer as claimed in  claim 4 , wherein the amount of P is 0.035 mass % or more.  
   
   
       7 . The ultra fine grain steel having a nitride layer as claimed in  claim 4 , wherein the steel is a carbon steel and the total amount of Cr, Mo, Ti, Nb, and V is 0.1 mass % or less.  
   
   
       8 . The ultra fine grain steel having a nitride layer as claimed in  claim 1 , wherein a fatigue limit is 1.6 times larger than Vickers hardness of a base material.  
   
   
       9 . A molded part, a part, or a member which is formed from the ultra fine grain steel having a nitride layer as claimed in  claim 1.