Method of manufacturing welded structure of ferritic heat-resistant steel and welded structure of ferritic heat-resistant steel

1. A method of manufacturing a welded structure of a ferritic heat-resistant steel including a base material, a weld heat-affected zone and a weld metal, comprising:

the step of preparing the base material, the base material having a chemical composition including, in mass %:

0.08 to 0.15% C;

0.02 to 0.45% Si;

0.40 to 0.80% Mn;

1.0 to 7.0% Cr;

0.003 to 0.020% N;

0.20 to 1.10% Mo;

0.005 to 0.08% Nb;

0.005 to 0.40% V;

not less than 0% and less than 1.5% W;

0 to 0.12% Ti;

0 to 0.0050% Ca;

0 to 0.0050% Mg;

0 to 0.0500% Y;

0 to 0.0500% Ce; and

0 to 0.0500% La,

B being limited to below 0.005%,

Al below 0.025%,

P below 0.020%,

S below 0.010%, and

O below 0.010%,

the balance being Fe and impurities;

the step of forming an edge on the base material;

a pre-weld heat treatment step in which a region located between a surface of the edge and a position distant from the surface of the edge by a pre-weld heat treatment depth of 10 to 50 mm is heated to a temperature of 950 to 1050° C. and is held at this temperature for 10 to 30 minutes;

a welding step in which, after the pre-weld heat treatment step, the edge is welded to form the weld metal; and

a post-weld heat treatment step in which, after the welding step, a region located between the surface of the edge and a position distant from the surface of the edge by a distance not smaller than the pre-weld heat treatment depth and not greater than 100 mm is heated to a temperature of 680 to 750° C. and is held at this temperature for a time period not shorter than 30 minutes and satisfying the following formula, (1):

(Log( t )+10)·( T+ 273)<10539  (1),

where t is the holding time and T is the temperature; the unit of t is hour and the unit of T is ° C.; and Log is the common logarithm.

2. The method of manufacturing a welded structure of a ferritic heat-resistant steel according to claim 1 , wherein the pre-weld heat treatment step is performed in two or more divided sub-steps.

3. The method of manufacturing a welded structure of a ferritic heat-resistant steel according to claim 1 , wherein an average grain size of M 23 C 6 -type carbides precipitated on large-angle grain boundaries of the weld heat-affected zone is not greater than 200 nm, each of the large-angle grain boundaries being a grain boundary with an angle relative to an adjacent crystal grain of 15° or larger,

an average inter-particle-surface distance of the M 23 C 6 -type carbides on the large-grain boundaries is not greater than 150 nm, and

a coverage of the large-angle grain boundaries with the M 23 C 6 -type carbides is not smaller than 50%, and

wherein M in the M 23 C 6 -type carbides is one or more of Cr, Fe, Mo and W in a total of 70 atom % or more.

4. The method of manufacturing a welded structure of a ferritic heat-resistant steel according to claim 1 , wherein the base material has a chemical composition including one or two selected from the group consisting of, in mass %:

not less than 0.5% and less than 1.5% W; and

0.05 to 0.12% Ti.

5. The method of manufacturing a welded structure of a ferritic heat-resistant steel according claim 1 , wherein the base material has a chemical composition including one or more selected from the group consisting of, in mass %:

0.0003 to 0.0050% Ca;

0.0003 to 0.0050% Mg;

0.0100 to 0.0500% Y;

0.0100 to 0.0500% Ce; and

0.0100 to 0.0500% La.

6. The method of manufacturing a welded structure of a ferritic heat-resistant steel according to claim 1 , wherein the temperature of the post-weld heat treatment step is 680 to 715° C.