JP2003166039A - Austenitic heat-resistant steel excellent in sensitization characteristics, high-temperature strength and corrosion resistance and method for producing the same - Google Patents

Abstract

(57) [Summary] [PROBLEMS] To have excellent sensitizing properties in heat treatment of a heat affected zone of welding when used as a dissimilar material welding joint with ferritic heat-resistant steel, and excellent in severe use environment of boiler. An austenitic heat-resistant steel for boilers having excellent high-temperature strength and corrosion resistance is provided economically. SOLUTION: As a chemical component, in mass%, C: 0.005 or more.
Less than 0.03%, Si: 0.05-0.4%, Mn: 0.5-2%, P: 0.01-0.04
%, S: 0.0005 to 0.005%, Cr: 18 to 20%, Ni: 7 to 11%, Nb: 0.
2 to 0.5%, V: 0.2 to 0.5%, Cu: 2 to 4%, N: 0.10 to 0.30%, B:
0.0005 to 0.0080%, the balance being Fe and unavoidable impurities, and the total content of Nb and V
0.6% or more, the amount of Nb solid solution in steel is 0.15% or more,
An austenitic heat-resistant steel excellent in sensitization properties, high-temperature strength and corrosion resistance satisfying the relational expressions of / 14 ≧ Nb / 93 + V / 51 and Cr-16C-0.5Nb-V ≧ 17.5, and a method for producing the same.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an austenitic heat-resisting steel for boilers, and in particular, it has excellent sensitizing properties during heat treatment of a weld heat-affected zone when used as a dissimilar material welding joint with a ferritic heat-resisting steel. At the same time, the present invention relates to an austenitic heat-resistant steel for a boiler, which has good high-temperature strength and corrosion resistance under a severe usage environment of the boiler, and also has excellent weldability during construction, and a method for producing the same.

[0002]

2. Description of the Related Art In recent years, in a thermal power plant, an ultra-supercritical boiler whose steam condition is high temperature and high pressure has been planned from the viewpoint of improving economical efficiency and suppressing carbon dioxide gas emission.

In such a harsh environment of a boiler, conventional austenitic heat-resistant steels such as SUS347HTB have insufficient creep rupture strength and cannot be used. For example, as shown in "Iron and Steel" 70th Year S1409 page, or "Thermal Nuclear Power" Vol. 38 page 75, precipitation strengthening by carbonitrides such as Nb and Ti in steel, 25Cr-2 utilizing solid solution strengthening with Mo
A new high creep strength austenitic heat resistant steel of 0Ni (SUS310) system has been developed.

However, these new 25Cr-20N
i (SUS310) -based high-strength austenitic heat-resisting steel is expensive because it contains expensive Ni at a high concentration, and in addition to the conventional austenitic heat-resisting steels, welding is considered to be relatively difficult. Like SUS310TB, the weldability was not always good.

On the other hand, in recent years when the implementation of ultra-supercritical pressure boilers has been delayed, SUS347 is used from the viewpoint of cost reduction by thinning structural members even in the present steam condition boilers.
There is a demand for a material having a higher unit strength at a high temperature and a weight unit price similar to that of a conventional austenitic heat resistant steel such as HTB. Therefore, for example, 1 described in the technical standard of thermal power plant for power generation
A material such as fire SUS304J1HTB, which is an austenitic heat-resistant steel of 8Cr-8Ni (SUS304) system, at a relatively low price and having a higher strength than conventional materials has been developed and is being widely used.

However, these new 18-8 austenitic heat-resistant steels have the following problems when heat-treated when they are welded to a ferritic heat-resistant steel and used as a dissimilar material welding joint.

That is, normally, when ferritic heat-resistant steel is welded to form a weld joint, it is necessary to heat-treat at least the weld heat-affected zone at 600 to 750 ° C. after welding, but as in the case of fire SUS304J1HTB. The new 18-8 austenitic heat resistant steel has a C of 0.
About 1% is contained and C-fixing element such as Nb is C
Since the amount corresponding to the amount is not sufficiently added, there is a problem that the austenitic heat-resistant steel becomes sensitive during the heat treatment after welding and grain boundary cracking easily occurs.

Therefore, when an austenitic heat-resistant steel having inferior sensitization characteristics is welded to a ferritic heat-resistant steel to form a dissimilar material welding joint, an austenitic dissimilar material weld excellent in sensitization characteristics is formed in the groove of the ferritic heat-resistant steel. Very troublesome measures such as welding by overlay welding with a welding material and then heat-treating the target austenitic heat-resisting steel are taken, which increases the construction cost. There was a problem.

On the other hand, the present inventors have already disclosed in Japanese Patent Laid-Open No. 9-2.
In the publication No. 28003, using the 18-8 system as a basic component,
The amount of C is reduced to improve the weldability and W, N
By adding b, V, and N at the same time in a limited specific component range, a heat-resistant steel with high weld strength and high weldability that compensates for the decrease in high temperature strength due to the reduction in the amount of C is proposed. .

However, since this heat-resistant steel uses an expensive alloying element such as W in order to improve the high temperature strength, it cannot sufficiently meet the demand for further cost reduction of the austenitic heat-resistant steel. It was Also,
Since the C content is reduced, as a result, the sensitizing property is superior to the new high-strength 18-8 austenitic heat-resistant steel such as Fire SUS304J1HTB, but the sensitizing property is not sufficiently improved. It was

[0011]

In view of the above problems of the prior art, the present invention provides excellent sensitization during heat treatment of a weld heat affected zone when used as a dissimilar material welding joint with a ferritic heat resistant steel. An object of the present invention is to economically provide an austenitic heat-resistant steel for boilers, which has properties and has good high-temperature strength and corrosion resistance under the severe usage environment of the boiler.

[0012]

SUMMARY OF THE INVENTION The present inventors have defined the relationship between the reduction of C and the relationship between Cr and C, Nb and V in order to improve the sensitization property while ensuring high temperature corrosion resistance. Welding heat when used as a dissimilar material welding joint with ferritic heat resistant steel by adding Cu, Nb and V together without using expensive alloy elements such as conventional W to secure high temperature strength. We have succeeded in economically obtaining an austenitic heat-resistant steel that has excellent sensitization properties during heat treatment of the affected zone, as well as high-temperature strength and high-temperature corrosiveness.

The present invention was made based on this finding, and the gist of the invention is as follows.

(1) As a chemical component, in mass%, C:
0.005 to less than 0.03%, Si: 0.05 to 0.4
%, Mn: 0.5 to 2%, P: 0.01 to 0.04%,
S: 0.0005 to 0.005%, Cr: 18 to 20
%, Ni: 7 to 11%, Nb: 0.2 to 0.5%, V:
0.2 to 0.5%, Cu: 2 to 4%, N: 0.10
0.30%, B: 0.0005 to 0.0080%, the balance consisting of Fe and inevitable impurities, and
The total content of Nb and V is 0.6% or more, the solid solution amount of Nb in steel is 0.15% or more, and further, the following expressions (1) and (2) are satisfied. Austenitic heat resistant steel with excellent sensitizing properties, high temperature strength and corrosion resistance.

[0015]         N / 14 ≧ Nb / 93 + V / 51 (1)         Cr-16C-0.5Nb-V ≧ 17.5 (2)

(2) Further, the austenite system excellent in sensitizing property, high temperature strength and corrosion resistance according to the above (1) is characterized in that the average crystal grain size of the austenite structure is 7 or more in grain size number. Heat resistant steel.

(3) As a chemical component, in mass%, C:
0.005 to less than 0.03%, Si: 0.05 to 0.4
%, Mn: 0.5 to 2%, P: 0.01 to 0.04%,
S: 0.0005 to 0.005%, Cr: 18 to 20
%, Ni: 7 to 11%, Nb: 0.2 to 0.5%, V:
0.2 to 0.5%, Cu: 2 to 4%, N: 0.10
0.30%, B: 0.0005 to 0.0080%, the balance consisting of Fe and inevitable impurities, and
A steel material having a total content of Nb and V of 0.6% or more and further satisfying the following expressions (1) and (2) is cold-worked at a cross-sectional area reduction rate of 10% or more, and then ,
A method for producing an austenitic heat-resistant steel having excellent sensitization characteristics, high-temperature strength, and corrosion resistance, which is characterized by performing solution heat treatment in a temperature range of 1160 to 1200 ° C for 2 to 10 minutes.

[0018]         N / 14 ≧ Nb / 93 + V / 51 (1)         Cr-16C-0.5Nb-V ≧ 17.5 (2)

[0019]

BEST MODE FOR CARRYING OUT THE INVENTION The inventors of the present invention have made it an economical component system to maintain a high temperature strength and a high temperature corrosion resistance while maintaining a fire SUS.
Steel that can improve the sensitization property during heat treatment of the weld heat affected zone when used as a dissimilar material welding joint with ferritic heat resistant steel, which was a problem with new high strength 18-8 austenitic heat resistant steel such as 304J1HTB We conducted a thorough study.

In order to suppress the sensitization during the heat treatment after welding and the generation of grain boundary cracks resulting therefrom, the amount of Cr carbide precipitated at the grain boundaries during the heat treatment is reduced, and the Cr deficient layer near the grain boundaries is reduced. It is necessary to suppress the formation of
For that purpose, needless to say, the amount of C added may be reduced. However, since the creep rupture strength is remarkably reduced due to the decrease in the amount of C, it is necessary to compensate for the decrease in the creep rupture strength by some method.

As a result of examination by various experiments by the present inventors, N was added at a high concentration as a substitute for C, and Nb was added.
And V in a certain relationship to the amount of N, in addition to solid solution strengthening by N, Cr, V and N
It was found that the precipitation strengthening by the composite nitride of b was effectively obtained, and that by additionally adding Cu thereto, the creep strength could be economically improved without using an expensive alloying element such as W. It was

Further, even if Cr is used to form a composite nitride with V and Nb, in order to ensure good high-temperature corrosion resistance, the lower limit of Cr is set from the relationship with the amounts of C, V and Nb. It was found out that The present invention is based on these basic ideas.

The composition of the steel of the present invention and the reasons for limiting the content range will be described below.

In addition, "%" shown below means "mass%" unless otherwise specified.

C: C is an element having an effect of improving high temperature strength, but it is necessary to reduce it as much as possible in order to prevent high temperature cracking and deterioration of ductility at the time of welding and to improve sensitizing property. In the present invention, while improving the weldability, precipitation of Cr carbide in the grain boundaries during post-weld heat treatment of dissimilar material welded joints, which is an important subject of the present invention, and the Cr deficient layer in the vicinity of the grain boundaries resulting therefrom In order to suppress the formation and improve the sensitizing property, the upper limit of the C content is specified to be less than 0.03%.

On the other hand, if the C content is less than 0.005%, the ferrite phase appears and the steelmaking cost increases due to decarburization, so the lower limit of the content was set to 0.005%.

Si: Si is an element which not only is effective as a deoxidizing agent, but also improves the oxidation resistance and high temperature corrosion resistance. However, Si is an element that remarkably promotes the formation of the σ phase, which is an embrittlement phase, so if it is too large, it not only lowers the creep rupture strength and toughness, but also adversely affects the weldability.

In order to suppress these problems, the upper limit of the Si content is 0.4% in the present invention. However, if the Si content is excessively reduced, the steam oxidation resistance is deteriorated, so the lower limit is set to 0.05%.

Mn: Mn is an element that forms austenite and has a deoxidizing effect, and is an element that fixes S in steel to improve weldability and hot workability. In order to obtain these effects. In the present invention, the lower limit of the content is set to 0.5%. However, if the Mn content is too large, the oxidation resistance is deteriorated, so the upper limit of the content is set to 2%.

P: P forms a phosphide and has the effect of improving the breaking strength for a long time. Therefore, it is contained in an amount of 0.01% or more. However, in the present invention, when the content exceeds 0.04%, In addition to significantly deteriorating the weldability, the hot workability is also impaired, so the upper limit of the content was made 0.04%.

S: S segregates at grain boundaries, deteriorates hot workability, and promotes grain boundary embrittlement during creep, so the upper limit of its content is set to 0.005% in the present invention. On the other hand, FIG. 1 shows the relationship between the steam oxidation scale thickness and the S content. As can be seen from FIG. 1, S has an action of improving steam oxidation characteristics by adding an appropriate amount. However, the effect is not sufficient if the S content is less than 0.0005%, the steam oxidation characteristics are significantly deteriorated, and in addition, if the S content is extremely reduced, it also leads to an increase in steelmaking cost. The lower limit of the content was set to 0.0005%.

Cr: Cr is an essential element for ensuring steam oxidation resistance and high temperature corrosion resistance by forming a stable protective oxide film on the surface during use in a high temperature environment. In the invention, the lower limit of the Cr content is set to 18% in order to secure the steam oxidation resistance and the high temperature corrosion resistance equivalent to or higher than those of the conventional SUS347H steel, which is a typical austenitic heat resistant steel.

However, an excessive increase in the Cr content hinders the formation of the austenite phase, and the content of expensive austenite-forming elements such as Ni for suppressing this must be increased. Therefore, the upper limit of the Cr content is set to 20%.

In the present invention, the Cr content is defined as described above, and Cr forms a carbide in a high temperature use environment, or forms one of the solid solution Cr contents by forming a composite nitride together with Nb and V. In order to secure the required steam oxidation resistance and high temperature corrosion resistance by forming a Cr oxide film on the surface under high temperature use environment in consideration of part consumption, the Cr content was further specified by the following conditional expression. . Cr-16C-0.5Nb-V ≧ 17.5 (2)

FIG. 2 shows the thickness of the steam oxidation scale produced when the sample was held at 650 ° C. for 3000 hours, which is the basis of the limitation of the equation (2), and “(Cr-16C-0.5Nb).
-V) -17.5 ".

From FIG. 2, it is defined in the present invention that the equation (2), which is the relational expression of the contents of Cr and C, Nb and V, for satisfying the steam oxide scale thickness of 60 μm or less is satisfied.

Ni: Ni is essential for enhancing the stability of the austenite phase and suppressing the formation of the σ phase (the intermetallic compound of Fe and Cr) which causes the creep rupture strength to decrease at high temperature for a long time. It is an element. In the present invention, the Ni content needs to be 7% or more in order to stabilize the austenite and suppress the formation of the σ phase with respect to the content of the ferrite-forming element such as Cr. on the other hand,
If the amount of Ni increases excessively, it is disadvantageous in terms of economy, so the upper limit was made 11%.

Nb, V: Nb and V all form fine nitrides in a high temperature environment and significantly improve the creep rupture strength on the long-term side. To obtain this effect, Nb, V
The lower limit of the content of each is set to 0.2%, and the lower limit of the total content of Nb and V is set to 0.6%.

On the other hand, addition of Nb far exceeding the solid solution limit at the time of solution heat treatment only increases the amount of undissolved coarse nitride, and the above effect cannot be obtained, rather the mechanical effect is rather increased. Reduce the characteristics. Therefore, the upper limit of the Nb content is set to 0.5% from the solid solution limit during solution heat treatment. Further, although the solid solubility limit of V is larger than that of Nb, if it is excessively added, δ-ferrite remains and mechanical properties are deteriorated. Therefore, from the viewpoint of suppressing δ-ferrite, the upper limit of V content is set to 0.5%. . The total content of Nb and V is preferably set to 0.9% as an upper limit from the viewpoint of suppressing δ ferrite.

The effect of improving the creep rupture strength on the high temperature and long time side by the addition of Nb and V becomes more remarkable as the amount of solid solution that can be dissolved in the solution heat treatment increases. In addition, among them, Nb also has an effect of preventing coarsening of crystal grains by keeping part of it as a nitride even during solution heat treatment and ensuring good steam oxidation characteristics.

FIG. 3 shows the relationship between the amount of solid solution of Nb dissolved in steel at room temperature after solution heat treatment and the creep rupture strength at high temperature for a long time.

From FIG. 3, in the present invention, in order to secure the creep rupture strength equal to or higher than that of the conventional steel, the Nb solid solution amount in the steel after the solution heat treatment is specified to be 0.15% or more. Since the larger the amount of solid solution Nb is, the more preferable it is. Therefore, the upper limit is not particularly defined, and 0.5%, which is the upper limit of the amount of added Nb, may be completely dissolved.

The amount of solid solution of Nb is obtained by subtracting the amount of precipitated Nb obtained by quantitatively analyzing Nb in the residue extracted by potentiostatic electrolysis from the total amount of Nb added to the steel. And

Cu: Cu is an element that improves the creep rupture strength by coherently depositing a Cu-rich phase in steel during use in a high temperature environment. In order to obtain this effect, the lower limit of this content is set to 2% in the present invention. But,
If the Cu content exceeds 4%, the ductility decreases and the hot workability also deteriorates. Therefore, the upper limit of the content was set to 4%.

N: N is an element for significantly improving creep rupture strength by solid solution strengthening and formation of a nitride. In the present invention, when the N content is less than 0.10%, low C
Since the strength reduction due to quantification cannot be compensated by the solid solution strengthening and the strength improvement due to the formation of nitrides, the lower limit of the content is made 0.10%.

On the other hand, excessive addition of N in excess of 0.30% is not only difficult in production because pressurization is required in the steel making process, but also the effect of improving creep rupture strength at high temperature and long time side. Not only does it decrease the toughness, but also reduces the toughness. Therefore, the upper limit of the N content is set to 0.30%.

Further, in the present invention, in order to effectively precipitate the nitrides of Nb and V for obtaining the above-mentioned effect of improving the creep rupture strength, the N content is further defined by the following formula (1). There is a need to. N / 14 ≧ Nb / 93 + V / 51 (1)

B: B enhances the creep rupture ductility by strengthening the crystal grain boundaries by segregating the grain boundaries, and at the same time improves the creep rupture strength. In order to obtain these effects, the lower limit of the B content is set to 0.0005% in the present invention. However, if added in excess of 0.0080%, the weldability is significantly reduced, so the upper limit was set to 0.0080%.

The reasons for limiting the structure of the steel of the present invention will be described below. In the steel of the present invention, in order to further improve steam oxidation characteristics, it is necessary to define the component composition as described above and to refine the grain size of the steel structure.

FIG. 4 shows the relationship between the average grain size of the austenite structure and the steam oxidation scale thickness in the steel of the present invention. In order to maximize the steam oxidation resistance of the steel of the present invention, as shown in FIG. 4, it is necessary to reduce the average grain size of the austenite structure to 7 or more in terms of grain size number. is there.

Therefore, in the present invention, the average grain size of the austenite structure is set to 7 or more in terms of grain size number. The grain size is measured according to JIS G0551 “Steel grain size test method”.

The reasons for limiting the production conditions for the steel of the present invention will be described below. In the method for producing the steel of the present invention, the composition of the steel slab is specified as described above, and a solid content for ensuring a sufficient amount of Nb solid solution in the steel for the purpose of improving the creep rupture strength at high temperature for a long time. It is necessary to specify the solution heat treatment conditions and optimize the cross-sectional area reduction rate during cold working and the solution heat treatment conditions in order to refine and uniformize the average grain size of the austenite structure for the purpose of improving steam oxidation characteristics. Is.

The cross-sectional area reduction rate during cold working is such that the working strain is introduced into the steel during cold working and the crystal grain size of the structure is sufficiently refined by recrystallization in the solution heat treatment that is performed next. Is important for. In the present invention, when the cross-sectional area reduction rate is less than 10%, recrystallization is not sufficiently performed within the range of heat treatment conditions described below, so the cross-sectional area reduction rate is 1
It was set at 0% or more.

The cross-sectional area reduction rate mentioned here means, for example, in the case of cold drawing of a pipe, for a cross section perpendicular to the cold drawing direction, the cross-sectional area before cold working and after cold working. Is the percentage obtained by dividing the difference in the cross-sectional area of the product by the cross-sectional area before cold working.

Next, the solution heat treatment conditions after cold working are as follows:
By sufficiently dissolving Nb in steel, the creep rupture strength of the steel of the present invention at high temperature for a long time in the use environment is improved, and the structure in which work strain is accumulated by cold working is recrystallized. This is important for uniformly refining the crystal grains of the structure.

FIG. 5 shows the relationship between the solution heat treatment temperature in the production of the steel of the present invention and the amount of Nb solid solution dissolved in the steel at room temperature after the solution heat treatment. Further, FIG. 5 shows the relationship between the solution heat treatment temperature and the average grain size of the austenite structure in the production of the steel of the present invention. The heat treatment time was 6 minutes in all cases.

Since the steel of the present invention has a high N content and is likely to precipitate Nb as a nitride, it is an essential condition for the steel of the present invention.
In order to make the solid solution amount of Nb in steel 0.15% or more,
As shown in, solid solution heat treatment at a temperature of 1160 ° C. or higher is required.

In the steel of the present invention, in order to suppress the progress of recrystallization of Nb nitride and to completely recrystallize it by solution heat treatment, as shown in FIG.
Solution heat treatment at a temperature of 60 ° C. or higher is required.

On the other hand, when the solution heat treatment temperature exceeds 1200 ° C., as shown in FIG. 6, the crystal grains of the austenite structure grow rapidly and become coarse, and the average crystal grain size of the austenite structure of the above-described steel of the present invention. Since it cannot be refined to a grain size number of 7 or more, it is necessary to set the upper limit of the solution heat treatment temperature to 1200 ° C. For the above reasons, in the present invention, the solution heat treatment temperature is set to 1160 to 120.
It was set to 0 ° C.

In order to completely dissolve the precipitates other than Nb nitride of the steel of the present invention within the above solution heat treatment temperature range, the solution heat treatment time must be at least 2 minutes or more. However, if the heat treatment time is too long, not only does the crystal grain become coarse, but it is also economically disadvantageous, so the upper limit must be 10 minutes. Therefore, in the present invention, the solution heat treatment time is set to 2 to 10 minutes.

[0061]

EXAMPLES The effects of the present invention will be specifically described below with reference to examples.

Table 1 shows the chemical composition of the test steel. In Table 1, the chemical compositions of the steel materials of steel numbers a to p are within the scope of the present invention, and the chemical compositions of the steel materials of steel number q to aa are outside the scope of the present invention. Using steel pipes obtained by hot-extruding these steel materials, cold drawing was performed at a cross-sectional area reduction rate shown in Table 2, and solution heat treatment was performed under the conditions shown in Table 2. For the obtained steel pipe, the amount of Nb solid solution in the steel was measured and the average crystal grain size of the austenite structure was measured. Further, a creep rupture test was conducted at 650 ° C, 700 and 750 ° C. Further, according to JIS G 0575 "Stainless Steel Sulfuric Acid / Copper Sulfate Corrosion Test Method", intergranular corrosion characteristics,
That is, the sensitizing property was evaluated. In addition, the steam oxidation characteristics were also evaluated.

[0063]

[Table 1]

[0064]

[Table 2]

For the creep rupture strength, data is L
Arson-Mi11er method arranges, 650 ° C x 10
Tensile strength at break was estimated. For intergranular corrosion characteristics,
A strip-shaped test piece which had been subjected to sensitization heat treatment under the condition of air cooling after heating at 650 ° C. for 2 hours was held in a boiling sulfuric acid / copper sulfate solution for 16 hours, and then bent at a bending angle of 180 ° and examined for cracks. Regarding steam oxidation characteristics, 300 pieces of test pieces obtained by removing the processed layer on the surface in steam at 650 ° C. and 250 atm were used.
After holding for 0 hour, the thickness of the produced oxide scale was measured.

The amount of Nb solid solution is quantitatively analyzed by chemical analysis of Nb in the residue extracted from the material by the potentiostatic electrolysis method,
The amount of precipitated Nb obtained from it is the total Nb added to the steel.
It was determined by subtracting from the amount.

The average grain size of the austenite structure was measured according to JIS G 0551 "Testing method for grain size of austenite of steel".

Table 3 also shows the amount of Nb solid solution in steel, the average grain size number of the austenite structure, and the test results.

[0069]

[Table 3]

Test No. shown in Table 3 1 to 19
It is an example of the present invention manufactured under the chemical composition and manufacturing conditions within the range of the present invention, and the chemical composition of the obtained steel pipe and the amount of Nb solid solution in the steel both satisfy the range of the present invention, and the creep rupture strength is The intergranular corrosion characteristics and steam oxidation characteristics were all good. The test No. In Nos. 8 and 9, the average grain size number of the austenite structure is smaller than the range specified in the invention of claim 2, but since the formula (2) in the invention of claim 1 is satisfied, the steam oxidation scale thickness 6
A thickness of 0 μm or less was secured.

On the other hand, the test No. Nos. 20 to 36 are comparative examples in which the manufacturing conditions are within the scope of the present invention, but the chemical composition of the steel material is out of the scope of the present invention, and at least any of creep rupture strength, intergranular corrosion characteristics or steam oxidation characteristics. The characteristics were poor.

The test No. The steels obtained in the comparative examples of 20 and 21 are SUS3, which has been often used conventionally.
47HTB equivalent steel and fire SUS304J1HTB
It is an equivalent steel and is not within the scope of the present invention.

Test No. SUS obtained in 20 comparative examples
The steel equivalent to 347HTB has excellent sensitization characteristics because it contains a large amount of C but contains Nb, which is a C-fixing element. However, V is not added and the creep rupture strength is low because V falls outside the range of the present invention. . On the other hand, the test No. 1-1
The example of the present invention of No. 9 is Test No. Compared to 20 comparative examples, 65
Estimated creep rupture strength at 0 ° C x 100,000 hours is 120MP
It has a very excellent creep rupture strength of a or more.

Further, the test No. The fire SUS304J1HTB equivalent steel obtained in Comparative Example No. 21 has a high creep rupture strength, but has a C content exceeding the range of the present invention, and a C-fixing element such as Nb is sufficiently added in an amount commensurate with the C content. In addition, since V is not added and the content is out of the range of the present invention, the sensitizing property is remarkably inferior. On the other hand, the test No. The invention examples of Nos. 1 to 19 are Test No. Two
Compared with Comparative Example 1, the creep rupture strength is equal to or higher than that of Comparative Example 1, and the sensitization property is also excellent.

Test No. among the comparative steels. No. 22 is No.
No. 21 fire SUS304J1HTB equivalent steel is a material in which only the amount of C is reduced for the purpose of improving the sensitization characteristics, but although the sensitization characteristics are certainly improved, V is not added, This is an example in which the creep rupture strength is significantly reduced due to the reduction of the C content because the content is out of the range.

Test No. Comparative Examples 23 to 26 are Test N
o. Compared to the examples of the present invention of Nos. 1 to 19, the materials have an excessively large amount of Si or insufficient amounts of Nb, Cu and B, respectively, but the creep rupture strength is determined by the test number. 1-1
9 of the present invention and the conventional steel, Test No. This is an example lower than 21.

Further, the test No. Comparative examples of 27 and 28 are
Test No. Although the specified ranges of the respective elements are satisfied with respect to Examples 1 to 19 of the present invention, the formula, Nb + V ≧
Since the condition of 0.6 and N / 14 ≧ Nb / 93 + V / 51 is not satisfied, the creep rupture strength of Test No.
Examples 1 to 19 of the present invention and the conventional steel, Test No.
This is an example lower than 21.

Test No. The comparative example of 29 is the formula, Cr-1.
Since the condition of 6C-0.5Nb-V ≧ 17.5 is not satisfied, the steam oxidation scale thickness is set to Test No. This is an example thicker than the examples of the present invention 1 to 19.

Test No. Nos. 30 to 35 are comparative examples in which the chemical composition of the steel material is within the scope of the present invention, but the manufacturing conditions are out of the scope of the present invention, and at least any of creep rupture strength, intergranular corrosion characteristics or steam oxidation characteristics. The characteristics were poor.

Test No. The comparative example of 30 is an example in which the fine grain structure was not obtained and the steam oxidation scale was thick because the recrystallization was not sufficiently performed due to the insufficient reduction of the cross-sectional area during cold working.

Test No. The comparative example 31 is an example in which the solution heat treatment temperature was too high, the recrystallized grains were coarsened, and the steam oxidation scale was thick.

Test No. Comparative Example 32 is an example in which the creep rupture strength was low because Nb could not be sufficiently dissolved in the steel because the solution heat treatment time was too short.

Test No. Comparative examples 33 and 35 are examples in which the creep rupture strength was low because Nb could not be sufficiently dissolved in the steel because the solution heat treatment temperature was too low.

Test No. Comparative Example 34 is an example in which the solution heat treatment time was too long, the recrystallized grains were coarsened, and the steam oxidation scale was thick. Test No. The comparative example of No. 36 is an example in which the steam oxidation scale became thick because the S content was excessively reduced.

In the above embodiments, the shape of the final product is a steel pipe, but the shape of the final product is not particularly limited to the steel pipe, and any shape such as plate, bar and wire can be used. Needless to say, it can be applied to the case of manufacturing the product.

[0086]

By applying the present invention, when used as a dissimilar material welded joint with a ferritic heat-resistant steel, it has excellent sensitization characteristics during heat treatment of the heat-affected zone of welding, and also has excellent weldability during construction, and When used as a steel for boilers in a harsh environment, it becomes possible to inexpensively provide austenitic heat-resistant steel having excellent corrosion resistance and high-temperature strength. Therefore, in the present invention, there is an extremely great contribution to the industrial development.

[Brief description of drawings]

FIG. 1 is a diagram showing a relationship between an S content and a steam oxidation scale thickness (μm) generated by holding at 650 ° C. for 3000 hours.

FIG. 2 is a relational index (Cr-16C-0.5Nb-V-17.5) between the Cr content and the C, Nb and V contents, and
It is a figure which shows the relationship with the steam oxidation scale thickness (micrometer) produced | generated by hold | maintaining 650 degreeC x 3000 hours.

FIG. 3 is a diagram showing the relationship between the amount of Nb solid solution in steel and the estimated creep rupture strength (MPa) at 650 ° C. for 100,000 hours.

FIG. 4 is a diagram showing the relationship between the average grain size of an austenite structure and the thickness of steam oxidation scale.

FIG. 5 is a diagram showing the relationship between the solution heat treatment temperature and the amount of Nb solid solution dissolved in steel at room temperature after solution heat treatment.

FIG. 6 is a diagram showing a relationship between a solution heat treatment temperature and an average grain size of an austenite structure.

   ─────────────────────────────────────────────────── ─── Continued front page    F-term (reference) 4K032 AA02 AA04 AA13 AA15 AA16                       AA17 AA21 AA22 AA24 AA25                       AA27 AA29 AA31 AA36 CG01                       CG02 CH06

Claims (3)

[Claims] 1. As a chemical component, in mass%, C: 0.0
05-0.03%, Si: 0.05-0.4%, M
n: 0.5 to 2%, P: 0.01 to 0.04%, S:
0.0005-0.005%, Cr: 18-20%, N
i: 7 to 11%, Nb: 0.2 to 0.5%, V: 0.2
~ 0.5%, Cu: 2-4%, N: 0.10-0.30
%, B: 0.0005 to 0.0080%, the balance being Fe and unavoidable impurities, and the above Nb.
And the total content of V is 0.6% or more, the amount of Nb solid solution in the steel is 0.15% or more, and further, the following expressions (1) and (2) are satisfied. Austenitic heat resistant steel with excellent sensitizing properties, high temperature strength and corrosion resistance. N / 14 ≧ Nb / 93 + V / 51 (1) Cr-16C-0.5Nb-V ≧ 17.5 (2) 2. The austenitic heat-resistant steel excellent in sensitization characteristics, high temperature strength and corrosion resistance according to claim 1, characterized in that the average crystal grain size of the austenite structure is 7 or more in grain size number. . 3. As a chemical component, in mass%, C: 0.0
05-0.03%, Si: 0.05-0.4%, M
n: 0.5 to 2%, P: 0.01 to 0.04%, S:
0.0005-0.005%, Cr: 18-20%, N
i: 7 to 11%, Nb: 0.2 to 0.5%, V: 0.2
~ 0.5%, Cu: 2-4%, N: 0.10-0.30
%, B: 0.0005 to 0.0080%, the balance being Fe and unavoidable impurities, and the above Nb.
And the total content of V is 0.6% or more, and the steel materials satisfying the following expressions (1) and (2) are
Cold working with a cross-sectional area reduction rate of at least%, then 1160
A method for producing an austenitic heat-resistant steel having excellent sensitization characteristics, high-temperature strength, and corrosion resistance, which is characterized by performing solution heat treatment for 2 to 10 minutes in a temperature range of to 1200 ° C. N / 14 ≧ Nb / 93 + V / 51 (1) Cr-16C-0.5Nb-V ≧ 17.5 (2)