Martensitic stainless steel seamless pipe and a manufacturing method thereof

1. A martensitic stainless steel seamless pipe, characterized by consisting of, by mass %, 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, Al: 0 to 0.1%, Ni: 0 to 0.5%, Cu: 0 to 0.25%, Ca: 0 to 0.0050% and at least one element selected from at least one group mentioned below (in total 0.005 to 0.200 mass % in case of including two or more kinds of these elements), and the balance Fe and impurities:

the first group: V: 0.005 to 0.200%, Nb; 0.005 to 0.200%, and Ti: 0.005 to 0.200 mass %, respectively,

the second group: B: 0.0005 to 0.0100 mass %, and also characterized by satisfying either the following inequalities (1), (2), (4) and (5) or the following inequalities (1), (3), (4) and (5):

C*+10 N*≦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)

wherein C* is an effective solute carbon content (mass %) defined by the following equation (6), N* is an effective solute nitrogen content (mass %) defined by equation (7), and Cr* is a Cr equivalent defined by equation (8), H1 of inequality (2) is the amount (mass %) of residual hydrogen in an as-rolled steel pipe, and H2 of inequality (3) is the amount (mass %) of residual hydrogen in the steel pipe after a heat treatment, and a symbol of element in each equation or inequality is a content (mass %) of the respective element:

C*=C−[12{(Cr/52)×(6/23)}/10],  (6)

N*=N−[14{(V/51)+(Nb/93)}/10]−[14{(Ti/48)+(B/11)+(Al/27)}/2],  (7)

Cr*=Cr+4Si−(22C+0.5Mn+1.5Ni+30N)  (8).

2. A martensitic stainless steel seamless pipe according to claim 1 , wherein the C content is 0.18 to 0.21%, the Si content is 0.20 to 0.35%, the Cr content is 12.40 to 13.10%, the S content is 0.003% or less, and the N content is 0.035% or less, by mass.

3. A method for manufacturing a martensitic stainless steel seamless pipe, characterized by pierce-rolling a stainless steel with an inclined roller type piercing mill under conditions of satisfying the inequality (9) below, which consists of, by mass %, 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, Al: 0 to 0.1%, Ni: 0 to 0.5%, Cu: 0 to 0.25%, Ca: 0 to 0.0050% and at least one alloying element selected from at least one group mentioned below (in total 0.005 to 0.200 mass % in case of including two or more kinds of these alloying elements), and the balance Fe and impurities:

the first group: V: 0.005 to 0.200%, Nb: 0.005 to 0.200%, and Ti 0.005 to 0.200 mass %, respectively,

the second group: B; 0.0005 to 0.0100 mass %, and which also satisfies all the following inequalities (1), (2), (4) and (5)or the following inequalities (1), (3), (4) and (5):

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)

Cr*<0.00009(C.A.+F.A.) 3 −0.0035(C.A.+F.A.) 2 +0.0567(C.A.+F.A.)+8.0024  (9)

wherein C* is an effective solute carbon content (mass %) defined by the following equation (6), N* is an effective solute nitrogen content (mass %) defined by equation (7), and Cr* is a Cr equivalent defined by equation (8), C.A. (≧0°) and F.A. in inequality (9) express a toe angle and a feed angle, respectively, and a symbol of element in each equation or inequality represents a content (mass %) of the respective element:

C*=C−[12{(Cr/52)×(6/23)}/10],  (6)

N*=N−[14{(V/51)+(Nb/93)}/10]−[14{(Ti/48)+(B/11)+(Al/27)}/2],  (7)

Cr*=Cr+4Si−(22C+0.5Mn+1.5Ni+30N)  (8).

4. A method for manufacturing a martensitic stainless steel seamless pipe according to claim 3 , wherein the C content is 0.18 to 0.21%, the Si content is 0.20 to 0.35%, the Cr content is 12.40 to 13.10%, the S content is 0.003% or less, and the N content is 0.035% or less, by mass.

5. A method for manufacturing a martensitic stainless steel seamless pipe according to claim 3 , characterized by further comprising of the following steps (10) and (11) after pierce-rolling:

(10) soaking a pipe at a temperature 920° C. or more, and

(11) carrying out a hot rolling.

6. A method for manufacturing a martensitic stainless steel seamless pipe according to claim 4 , characterized by further comprising of the following steps (10) and (11) after pierce-rolling:

(10) soaking a pipe at a temperature 920° C. or more, and

(11) carrying out a hot rolling.