US Patent 7686897 Martensitic stainless steel seamless pipe and a manufacturing method thereof

The present invention provides a martensitic stainless steel seamless pipe and a method of making, wherein the pipe has minimal inner surface defects and prevents delayed fracture generation in the impact-worked portions. The pipe comprises C: 0.15 to 0.22%, Si: 0.1 to 1.0%, Mn: 0.10 to 1.00%, Cr: 12.00 to 14.00%, P: 0.020% or less, S: 0.010% or less, N: 0.05% or less, O(Oxygen): 0.0060% or less, at least one alloying element selected from V, Nb and Ti of 0.005 to 0.200% and B of 0.0005 to 0.0100%, and the balance Fe and impurities, wherein either the following inequalities (1), (2), (4) and (5) or the following inequalities (1), (3), (4) and (5) are satisfied:C*+10N*≦0.45,  (1)H1≦−0.003(C*+10N*)+0.0016,  (2)H2≦−0.0018(C*+10N*)+0.00096,  (3)Cr*≦9.0,  (4)S≦0.088N*+0.00056,  (5).