HIGH CHROMIUM MARTENSITIC HEAT-RESISTANT STEEL WITH COMBINED HIGH CREEP RUPTURE STRENGTH AND OXIDATION RESISTANCE
Provided is martensitic heat-resistant steel for boiler applications with a unique combination of enhanced creep strength and excellent oxidation resistance upon high temperature exposure in steam containing environments contacts (in wt.-%): C: 0.10 to 0.16%, Si: 0.20 to 0.60%, Mn: 0.30 to 0.80%, P≤0.020%, S≤0.010%, Al≤0.020%, Cr: 10.5 to 12.00%, Mo: 0.10 to 0.60%, V: 0.15 to 0.30%, Ni: 0.10 to 0.40%, B: 0.008 to 0.015%, N:0.002 to 0.020%, Co: 1.50 to 3.00%, W: 1.50 to 2.50%, Nb: 0.02 to 0.07%, Ti: 0.001-0.020%, iron and unavoidable impurities. The steel is normalized for about 10 to about 120 minutes at a temperature of 1050-1170° C. and cooled down in air or water to room temperature, and then tempered for at least one hour at a temperature of 750-820° C. The steel has a martensitic microstructure with average δ-ferrite content of less than 5 vol.-%.
1 . A seamless tubular product, made of a steel comprising, in weight percent:
C: 0.10 to 0.16%,
Si: 0.20 to 0.60%,
Mn: 0.30 to 0.80%,
P≤0.020%,
S≤0.010%,
Al≤0.020%,
Cr: 10.50 to 12.00%,
Mo: 0.10 to 0.60%,
V: 0.15 to 0.30%,
Ni: 0.10 to 0.40%,
B: 0.008 to 0.015%,
N: 0.002 to 0.020%,
Co: 1.50 to 3.00%,
W: 1.50 to 2.50%,
Nb: 0.02 to 0.07%,
Ti: 0.001 to 0.020%,
iron and unavoidable impurities.
2 . The seamless tubular product according to claim 1 , wherein: B/N≤1.5.
3 . The seamless tubular product according to claim 1 , wherein, the contents of Mo and W in wt % satisfy:
1.00%≤Mo+0.5W≤1.50%.
4 . The seamless tubular product according to claim 1 , wherein the contents of B, N, and Ti in wt % satisfy:
B−(11/14)(N−10 −(1/2.45)·(log B+6.81) −(14/48)·Ti)≥0.007.
5 . The seamless tubular product according to claim 1 , wherein, the contents of Ni, Co, Mn, C, and N in wt.-% satisfy:
2.6≤4·(Ni+Co+0.5·Mn)−20·(C+N)≤11.2.
6 . The seamless tubular product according to claim 1 , wherein the carbon content is between 0.13 and 0.16%.
7 . The A seamless tubular product according to claim 1 , wherein the Mo content is between 0.30 and 0.60%.
8 . The seamless tubular product according to claim 1 , wherein the B content is between 0.0095 and 0.013%.
9 . The seamless tubular product according to claim 1 , wherein the Ti content is between 0.001 and 0.005%.
10 . The seamless tubular product according to claim 1 , wherein the product has a microstructure comprising at least 95% of tempered martensite, the balance being delta ferrite.
11 . The seamless tubular product according to claim 10 , wherein the microstructure comprises at least 98% of tempered martensite.
12 . The seamless tubular product according to claim 10 , wherein the microstructure is martensitic and free of delta ferrite.
13 . The seamless tubular product according to claim 1 , which is a seamless tube.
14 . A method of producing the seamless tubular product according to claim 1 , the method comprising:
i) casting said steel,
ii) hot forming said steel,
iii) heating and holding said steel for a time between 10 and 120 minutes at a temperature ranging between 1050° C. and 1170° C.,
iv) cooling said steel down to room temperature,
v) reheating and holding said steel up to a tempering temperature TT that is between 750° C. and 820° C. for at least one hour, and
vi) cooling said steel down to room temperature.
15 . The method according to claim 14 , wherein the cooling iv) and vi) are done using air cooling or water cooling.