Patent ID: 8657971

Claim:
A brake disk excellent in temper softening resistance and toughness comprising, by mass, 0.1% or less of C, 1.0% or less of Si, 2.0% or less of Mn, 10.5% to 15.0% of Cr, 0.02% to 0.6% of Nb, 0.01 to 0.43% of Cu, 0.0005% to 0.0050% of B, 0.1% or less of N, and not containing Mo and Ni, the remainder being Fe and unavoidable impurities, such that the following inequalities are satisfied: 5Cr+10Si+30Nb−5Mn−3Cu−225N−270C<45 (1) 0.03≦{C+N−(13/92)Nb}≦0.09 (2) wherein a ratio of precipitated Nb to total Nb is greater than 0.5 and less than or equal to 0.7 and precipitated Nb and total Nb represent an amount of Nb precipitated in the form of precipitate and the amount of total Nb contained, respectively, on a mass percent basis, Cr, Si, Nb, Mn, Cu, N, and C each represent the content of the corresponding elements on a mass percent basis, and having a martensitic structure having prior-austenite gains with an average diameter of 8 to less than 15 μm, and the brake disk has a hardness of 27 HRC or more after tempering at 600° C. for one hour and a Charpy impact value of 50 J/cm 2 or more at a test temperature of 25° C. after tempering at 600° C. for one hour.