Martensitic stainless steel for disc brakes

Abstract

A martensitic stainless steel for disc brakes, having high temper softening resistance. The martensitic stainless steel is not seriously softened by maintaining the steel at more than 600° C. The martensitic stainless steel contains less than 0.050 mass % carbon, 1.0 mass % or less silicon, 2.0 mass % or less manganese, 0.04 mass % or less phosphorus, 0.010 mass % or less sulfur, 0.2 mass % or less aluminum, more than 11.5 mass % to 15.0 mass % chromium, 0.5 mass % to 2.0 mass % nickel, more than 0.50 mass % to 4.0 mass % copper, more than 0.08 mass % to 0.6 mass % niobium, and less than 0.09 mass % nitrogen, the remainder being iron and unavoidable impurities.

1 . A brake disc for a disc brake system, the brake disc being in a disc shape and made of a martensitic stainless steel, the steel comprising:
 less than 0.050 mass % carbon, 1.0 mass % or less silicon, 2.0 mass % or less manganese, 0.04 mass % or less phosphorus, 0.010 mass % or less sulfur, 0.2 mass % or less aluminum, chromium in a range of from more than 11.5 mass % to 15.0 mass %, nickel in a range of from 0.5 mass % to 2.0 mass %, copper in a range of from more than 0.50 mass % to 4.0 mass %, niobium in a range of from more than 0.08 mass % to 0.6 mass %, less than 0.09 mass % nitrogen, and 0.0005 mass % to 0.0050 mass % calcium,   the remainder being iron and unavoidable impurities, wherein the carbon content, the nitrogen content, the niobium content, the chromium content, the silicon content, the nickel content, the manganese content, and the copper content satisfy the following relationships (1) and (2):
   0.03≦[C]+[N]−13/93×[Nb]≦0.09   (1)
 
   5×[Cr]+10×[Si]+30×[Nb]−9×[Ni]−5×[Mn]−3×[Cu]−225×[N]−270×[C]≦40   (2),
 
   the steel sheet being 90 percent or more martensite on a volume basis, and the remainder of the volume of the steel being ferrite.   
     
     
         2 . The brake disc according to  claim 1 , further containing 0.02 mass % to 0.3 mass % vanadium. 
     
     
         3 . The brake disc according to  claim 1 , further containing one or both of 0.02 mass % to 2.0 mass % molybdenum and 0.02 mass % to 2.0 mass % cobalt. 
     
     
         4 . The brake disc according to  claim 1 , further containing one or more of 0.02 mass % to 0.3 mass % titanium, 0.02 mass % to 0.3 mass % zirconium, and 0.02 mass % to 0.3 mass % tantalum. 
     
     
         5 . The brake disc according to  claim 1 , further containing 0.0005 mass % to 0.0050 mass % boron. 
     
     
         6 . The brake disc according to  claim 2 , further containing one or both of 0.02 mass % to 2.0 mass % molybdenum and 0.02 mass % to 2.0 mass % cobalt. 
     
     
         7 . The brake disc according to  claim 2 , further containing one or more of 0.02 mass % to 0.3 mass % titanium, 0.02 mass % to 0.3 mass % zirconium, and 0.02 mass % to 0.3 mass % tantalum. 
     
     
         8 . The brake disc according to  claim 3 , further containing one or more of 0.02 mass % to 0.3 mass % titanium, 0.02 mass % to 0.3 mass % zirconium, and 0.02 mass % to 0.3 mass % tantalum. 
     
     
         9 . The brake disc according to  claim 6 , further containing one or more of 0.02 mass % to 0.3 mass % titanium, 0.02 mass % to 0.3 mass % zirconium, and 0.02 mass % to 0.3 mass % tantalum. 
     
     
         10 . The brake disc according to  claim 2 , further containing 0.0005 mass % to 0.0050 mass % boron. 
     
     
         11 . The brake disc according to  claim 3 , further containing 0.0005 mass % to 0.0050 mass % boron. 
     
     
         12 . The brake disc according to  claim 4 , further containing 0.0005 mass % to 0.0050 mass % boron. 
     
     
         13 . The brake disc according to  claim 6 , further containing 0.0005 mass % to 0.0050 mass % boron. 
     
     
         14 . The brake disc according to  claim 7 , further containing 0.0005 mass % to 0.0050 mass % boron. 
     
     
         15 . The brake disc according to  claim 8 , further containing 0.0005 mass % to 0.0050 mass % boron. 
     
     
         16 . The brake disc according to  claim 9 , further containing 0.0005 mass %.to 0.0050 mass % boron. 
     
     
         17 . The brake disc according to  claim 1 , wherein the martensitic stainless steel is hot-rolled. 
     
     
         18 . The brake disc according to  claim 1 , wherein the martensitic stainless steel is cold-rolled. 
     
     
         19 . The brake disc according to  claim 1 , wherein the martensitic stainless steel has a hardness of 32 to 38 in HRC. 
     
     
         20 . The brake disc according to  claim 1 , wherein the brake disc comprises a circular opening in a center portion of the brake disc, the circular opening being surrounded by a first circular portion having a plurality of fixing holes, the first circular portion being surrounded by a second circular portion having a plurality of cooling holes, the second circular portion forming an outer circumference of the brake disc. 
     
     
         21 . The brake disc according to  claim 20 , wherein the second circular portion having the plurality of cooling holes is a friction section.