JPH09165655A - Austenitic stainless steel for high temperature equipment and method for producing the same - Google Patents

Abstract

(57) Abstract: [PROBLEMS] To provide an austenitic stainless steel for high temperature equipment, which has excellent strength as well as high temperature corrosion resistance and high temperature oxidation resistance at high temperatures, and a method for producing the same. SOLUTION: C: 0.05 to 0.12% in weight fraction,
Si: 2.5% or less, Mn: 2.0% or less, Cr: 1
6.0-20.0%, Ni: 6.0-14.0%, M
o: 0.01 to 4%, N: 0.005 to 0.25%, M
g: 0.05% or less (including 0%), Cu: 0.01 to
4%, B: 0.001 to 0.005%, Zr: 0.1%
The following (including 0%), Al: 0.5% or less (including 0%), and Nb: 0.45% or less, Ti:
Austenite for high-temperature equipment, which contains at least one of 0.25% or less and V: 0.25% or less, satisfies the following formula, and the balance is Fe and unavoidable impurities and has excellent high-temperature strength. Stainless steel. 1 ≦ (C / 12 + N / 14) / (Nb / 93 + Ti / 4
8 + V / 51) ≦ 2

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to high temperature corrosion resistance,
The present invention relates to an austenitic stainless steel for high-temperature equipment, which has both high-temperature oxidation resistance and high-temperature strength, and a method for producing the same.

[0002]

2. Description of the Related Art The maximum operating temperature of heating furnace materials, boiler tubes and exhaust gas ducts of thermal power plants is about 60.
Materials for high temperature and heat resistant equipment exceeding 0 ° C have high temperature corrosion resistance,
High temperature oxidation resistance and high temperature strength are required at the same time.

Conventionally, Cr-Mo has been used for these applications.
Steel, ferritic heat-resistant steel, austenitic stainless steel, etc. are used. However, when the operating temperature exceeds 600 ° C., ferritic heat-resistant stainless steel lacks high-temperature strength. % Or more of Cr
Almost limited to the content of austenitic stainless steel.

Among them, Japanese Patent Laid-Open No. 59-70752,
The Nb, Ti or V-containing stainless steel as disclosed in JP-A-56-169755 is intended to obtain high high temperature strength due to the strengthening action by fine dispersion of carbonitride. Therefore, Cr not only has good high-temperature corrosion resistance and high-temperature oxidation resistance, but also has a certain degree of reliability as a device structural material.

[0005]

However, in the carbonitride precipitation type stainless steel as described above, C
In order to sufficiently dissolve N, Nb, Ti, and V in the matrix,
It is necessary to carry out the solution heat treatment at a high temperature. A solution treatment temperature of 18% Cr or higher usually requires a solution treatment temperature of 1100 ° C. or higher. If the temperature is lower than this, the added C, N,
Not only high-temperature strength commensurate with the amounts of Nb, Ti, and V cannot be obtained, but also the undissolved contents of these exist in the structure in the form of coarse carbonitrides or intermetallic compounds, resulting in ductility and toughness of the base metal. Is often significantly impaired. For this reason, a solution treatment temperature of 1150 ° C. or higher may be required depending on the amount of these added.

Thus, carbonitride precipitation type stainless steel is usually required to undergo solution treatment at a relatively high temperature. On the other hand, the heat treatment usually performed on a production scale is often an atmospheric furnace or a gas combustion type industrial furnace, and is not necessarily optimum for high temperature heat treatment. Further, in the above-described heating furnace in the presence of oxygen, surface oxidation of steel during heat treatment progresses faster at higher temperatures, and particularly at high temperatures exceeding 1150 ° C, excessive production of scale becomes a serious problem. Taking a plate material as an example, the weight loss due to oxidation becomes excessively large, and it becomes very difficult to control the plate thickness tolerance, and at the same time, the production cost increases.
In addition, grain boundary oxidation is likely to occur, and the scale rooted in the metal base material is also likely to cause scratches during hot working.If internal oxidation progresses, pickling, shot descaling Even with such means, a situation occurs in which the scale cannot be removed.

The present invention has been made in view of the above circumstances, and provides an austenitic stainless steel for high temperature equipment, which is excellent not only in high temperature corrosion resistance and high temperature oxidation resistance at high temperatures but also in strength, and a method for producing the same. The purpose is to do.

[0008]

DISCLOSURE OF THE INVENTION The present inventors have devised a drastic solution for suppressing excessive scale generation at high temperature by examining the composition of components, thereby making it possible to carry out high temperature solution treatment even by solution treatment at low temperature. The study was repeated based on the idea that it is to develop a steel that can obtain the same high-temperature strength as in the case of chemical treatment. The solid solution amount of the precipitation strengthening element in the matrix phase at a certain solution treatment temperature is generally larger as the temperature is higher.However, the solubility and solubility of the precipitation strengthening element are variously distributed. As a result of examining the product formula,
It has been found that when the precipitation strengthening element is added at a certain ratio, the high temperature strength may increase even if the heat treatment temperature is constant. In order to verify this, the relationship between dissolution and precipitation was grasped by many experimental dissolutions. As a result, for example, containing 18 to 20% Cr,
In the case of precipitation strengthened austenitic steel containing C, N, Nb, Ti and V, 6 when solution-treated at 1140 ° C.
Creep rupture strength at 105 ° C for 10 ⁵ hours is C, N, N
If the addition levels of b, Ti, and V do not change, 1180
Although it is reduced by about 20 MPa as compared with the case where the solution heat treatment is performed at 0 ° C., the creep strength is remarkably improved when the ratio of Nb, Ti, and V is increased even when the total amount of the additive elements is constant.
It became clear that there is a condition that the value is higher by about 5 to 10 MPa than the value lowered by about MPa. When this is mathematically expressed from the viewpoint of additional elements, it is always (C / 12 + N / 14) / (Nb / 93 + Ti / 48 +
It became clear that the relationship of V / 51) ≦ 2 was satisfied. That is,
It was concluded that the high temperature strength in the low temperature solution treatment is significantly improved when this equation is satisfied.

On the other hand, the present inventors have also clarified that in such a steel, element addition should be performed in a region where the atomic fraction of Ti + Nb + V content relative to C + N content is relatively low. When this is expressed by a mathematical expression, 1 ≦ (C / 12 + N / 14) / (Nb / 93 + Ti / 4
8 + V / 51). This is the excess Ti, N that does not become carbonitride.
It is considered that when b and V are present, the part that cannot be completely dissolved in the mother phase forms an unstable metal compound or composite compound, and coarsely precipitates at grain boundaries and within grains to deteriorate high temperature strength. To be

Based on these findings, Ti + Nb + V
The ratio of the atomic fraction of the amount of C + N to the amount is 1 ≦ (C / 12 + N / 14) / (Nb / 93 + Ti / 4
It was concluded that high temperature strength can be obtained even by low temperature solution heat treatment when 8 + V / 51) ≦ 2 is satisfied.

The present invention has been completed based on these findings. That is, the present invention is, firstly, by weight fraction, C: 0.05 to 0.12%, Si: 2.5% or less,
Mn: 2.0% or less, Cr: 16.0 to 20.0%, N
i: 6.0 to 14.0%, Mo: 0.01 to 4%, N:
0.005-0.25%, Mg: 0.05% or less (0%
), Cu: 0.01-4%, B: 0.001-
0.005%, Zr: 0.1% or less (including 0%), A
1: 0.5% or less (including 0%), and N
b: 0.45% or less, Ti: 0.25% or less, V: 0.
Austenitic stainless steel for high-temperature equipment, which contains at least one of 25% or less, satisfies the following formula (1), and the balance is Fe and unavoidable impurities, and has excellent high-temperature strength. I will provide a.

The present invention is secondly the weight fraction of C: 0.
05-0.12%, Si: 2.5% or less, Mn: 2.0
% Or less, Cr: 16.0 to 20.0%, Ni: 6.0 to
14.0%, Mo: 0.01-4%, N: 0.005-
0.25%, Mg: 0.05% or less (including 0%), C
u: 0.01 to 4%, B: 0.001 to 0.005%,
Zr: 0.1% or less (including 0%), Al: 0.5% or less (including 0%), Nb: 0.45% or less, T
i: 0.25% or less, V: 0.25% or less, and at least one of Ce, La, and Hf 0.1% or less, and the following ( Provided is an austenitic stainless steel for high-temperature equipment, which is characterized by satisfying the formula (1) and the balance being Fe and inevitable impurities and having excellent high-temperature strength.

Thirdly, the present invention has a weight fraction of C: 0.
05-0.12%, Si: 2.5% or less, Mn: 2.0
% Or less, Cr: 16.0 to 20.0%, Ni: 6.0 to
14.0%, Mo: 0.01-4%, N: 0.005-
0.25%, Mg: 0.05% or less (including 0%), C
u: 0.01 to 4%, B: 0.001 to 0.005%,
Zr: 0.1% or less (including 0%), Al: 0.5% or less (including 0%), Nb: 0.45% or less, Ti: 0.25% or less, V: After the steel containing at least one of 0.25% or less and satisfying the following formula (1) is formed into a predetermined product shape or a semi-finished product through a casting process and a hot working process, 1150 Provided is a method for producing austenitic stainless steel for high-temperature equipment, which is excellent in high-temperature strength, characterized by performing solution treatment at a temperature of ℃ or less.

Fourthly, the present invention provides a weight fraction of C: 0.
05-0.12%, Si: 2.5% or less, Mn: 2.0
% Or less, Cr: 16.0 to 20.0%, Ni: 6.0 to
14.0%, Mo: 0.01-4%, N: 0.005-
0.25%, Mg: 0.05% or less (including 0%), C
u: 0.01 to 4%, B: 0.001 to 0.005%,
Zr: 0.1% or less (including 0%), Al: 0.5% or less (including 0%), Nb: 0.45% or less, T
i: 0.25% or less, V: 0.25% or less, and at least one of Ce, La, and Hf 0.1% or less, and the following ( Excellent high temperature strength, characterized by subjecting steel satisfying the formula (1) to a predetermined product shape or semi-finished product through a casting process and a hot working process, and then subjecting it to solution treatment at a temperature of 1150 ° C or lower. A method for producing austenitic stainless steel for high temperature equipment is provided. 1 ≦ (C / 12 + N / 14) / (Nb / 93 + Ti / 48 + V / 51) ≦ 2 (1)

[0015]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, the present invention will be described specifically. The austenitic stainless steel according to the present invention,
Weight fraction C: 0.05 to 0.12%, Si: 2.5
% Or less, Mn: 2.0% or less, Cr: 16.0 to 20.
0%, Ni: 6.0 to 14.0%, Mo: 0.01 to 4
%, N: 0.005 to 0.25%, Mg: 0.05% or less (including 0%), Cu: 0.01 to 4%, B: 0.0
01-0.005%, Zr: 0.1% or less (including 0%), Al: 0.5% or less (including 0%), and Nb: 0.45% or less, Ti: 0. 0.25% or less,
V: Contains at least one of 0.25% or less. Furthermore, at least one of Ce, La, and Hf is set to 0.
You may contain 1% or less.

The reasons for limiting each of these elements will be described in detail below. C is an element effective for obtaining the high temperature strength of heat resistant steel, but its effect is not sufficient unless it is 0.05% or more, while if it exceeds 0.12%, it becomes harmful to weldability. Therefore, the C content is 0.05 to 0.12.
%.

Si is effective for deoxidizing steel, but 2.5%
If it is added in excess of 1, the phase stability will be significantly impaired, so the content is made 2.5% or less. Mn is an element effective for stabilizing austenite in steel, but if it is added in an amount exceeding 2.0%, the high temperature oxidation resistance is impaired, so its content is 2.
It is 0% or less.

Cr is the most effective element for imparting general corrosion resistance to steel, and as described above, the maximum operating temperature exceeds about 600 ° C. for heating furnace materials, boiler tubes of thermal power plants, exhaust gas ducts, etc. For high-temperature / heat-resistant equipment, unless the content is 16.0% or more, the high-temperature corrosion resistance and high-temperature oxidation resistance are insufficient. However, if the content exceeds 20.0%, it becomes difficult to secure phase stability. Therefore, C
The r content is in the range of 16.0 to 20.0%.

Ni is very important as an element that enhances the austenite stability of steel, and its content is 6.0%.
The effect is not sufficient unless it is above. However, 14.
If it exceeds 0%, not only the effect is saturated, but also it becomes extremely expensive. Therefore, the Ni content is 6.0 to 14.0.
% Range.

Mo is an element that contributes to the increase in high temperature strength by forming a solid solution in the matrix and strengthening the solution. However, 0.
If it is not contained more than 01%, its effect is not sufficient, and 4%
If it exceeds, the ductility will be extremely reduced. Therefore, the Mo content is set to the range of 0.01 to 4%.

N is an element effective for both austenite stabilization and high temperature strength of steel. However, the effect is not sufficient unless the amount is 0.005% or more,
If the content exceeds 0.25%, the weldability is impaired, so the content is made 0.005-0.25%.

Mg is an element effective for deoxidation, and is 0.0
If it is 5% or less, deoxidation can be performed without impairing hot workability. However, when it contains 0.02% or more of Al, it is not necessary to contain it. Therefore, the Mg content is set to 0.05% or less (including 0%).

Cu is an element that contributes to the increase in high temperature strength by precipitation strengthening. However, if it is not more than 0.01%, the effect is not sufficient, and if it exceeds 4%, the ductility is extremely lowered. Therefore, the Cu content is 0.01 to 4%.
And

B is an element effective in improving creep ductility, and it is necessary to contain 0.001% or more in order to exert its effect, but if it is added in excess of 0.005%, hot working is performed. Content of 0.001
To 0.005%.

Zr is not an essential component, but is an element effective for improving creep ductility, like B. However, 0.
If added in excess of 1%, hot workability is impaired, so the content is made 0.1% or less (including 0%).

Al is an element effective for deoxidation, and is 0.5
%, It is possible to deoxidize without impairing the hot workability. However, when 0.01% or more of Mg is contained, it is not necessary to contain Mg. Therefore, the Al content is set to 0.5% or less (including 0%).

Nb, Ti, and V are elements that form carbonitrides and are finely dispersed in the metal structure to contribute to the improvement of the creep rupture strength of steel. The main purpose of the present invention is to include one or more of these. one of.

However, if Nb is added in excess of 0.45%, coarse precipitation of intermetallic compounds and the like will occur and creep ductility will be impaired, and both Ti and V will be 0.25.
If it is added in an amount of more than 0.1%, coarse precipitation of intermetallic compounds and the like also occurs, and the creep ductility is impaired. In addition,
Since all of Nb, Ti, and V have the same function, at least one of them may be contained.

In addition to these, Ce, La, and Hf are all elements effective for high-temperature oxidation resistance, so at least one of them may be contained in the range of 0.1% or less. .

In the present invention, as described above, in addition to defining the range of each component as described above, 1 ≦ (C / 12 + N / 14) / (Nb / 93 + Ti / 4
8 + V / 51) ≦ 2 is required to be satisfied. By satisfying this formula, effective precipitation strengthening can be achieved even at high temperature, so that high temperature strength can be obtained, and further, the temperature of the solution treatment for obtaining a predetermined strength can be lowered.

Next, the manufacturing method of the present invention will be described. In the present invention, the steel having the composition range as described above is subjected to a solution treatment at a temperature of 1150 ° C. or lower after forming a predetermined product shape or a semi-finished product through a casting process and a hot working process.
That is, the steel having the above composition is excellent in strength at high temperature even in the solution treatment at a temperature of 1150 ° C. or lower.

[0032]

Embodiments of the present invention will be described below.
Steels of the present invention and comparative steels having the components / compositions shown in Tables 1 and 2 were melted in a laboratory 150kgw vacuum melting furnace, cast into 25kgw ingots, and then hot rolled to obtain a reduction ratio of 2.5. Rough rolling was performed and finish rolling with a reduction ratio of 3.5 was performed to obtain a plate thickness of 12 mm.

These steels were subjected to solution heat treatment (ST) at 1140-1180 ° C. using a gas combustion type simulator furnace simulating actual production, and grain boundary oxidation and internal oxidation were observed by cross-section optical microscope observation. In addition to confirming the presence or absence, a 650 ° C. creep rupture test was performed, and the test results up to about 10,000 hours were extrapolated to obtain the 100,000 hour rupture strength. Tables 1 and 2 collectively show the chemical composition, the value of the equivalent ratio R shown below, and the creep rupture strength at 650 ° C. for 100,000 hours.

R = (C / 12 + N / 14) / (Nb / 9
3 + Ti / 48 + V / 51) Steel Nos. 1 to 16 in Table 1 are the present invention steels, and Steel Nos. 17 to 31 in Table 2 are comparative steels.

[0035]

[Table 1]

[0036]

[Table 2]

Further, Table 3 shows the degree of grain boundary oxidation in the cross-section observation after the solution treatment. 11 excluding some steel
No grain boundary oxidation occurred in the solution heat treated material at 40 ° C.
On the other hand, in the solution heat-treated material at 1180 ° C., grain boundary acid was found in almost all. Since these non-uniform scale formation had a remarkable correspondence with the occurrence of surface scratches after rolling, it is generally considered that 1180 ° C. is too high as a solution heat treatment condition for obtaining a healthy plate material. .

[0038]

[Table 3]

When the creep rupture strength is arranged by the R value, the relationship shown in FIG. 1 is obtained. 1140 ° C, 118
Both of the 0 ° C. solution heat-treated materials had 65 within the range of 1 ≦ R ≦ 2.
It can be seen that the creep rupture strength at 0 ° C. for 100,000 hours is particularly good.
The difference in creep rupture strength of 0 ° C. treated materials is also 1 ≦ R ≦ 2
It is also characterized by being relatively small in the range of. This means that in this range, even if the solution treatment is performed at a low temperature, the decrease in the high temperature strength can be suppressed to the minimum.

By examining the evaluation with such R value at the same time as optimizing the addition amount of each element, it is possible to grasp the range in which good creep rupture strength is exhibited. That is, in the steel of the present invention, since 1 ≦ R ≦ 2, 1140
Even at the solution treatment temperature of ℃, high temperature strength is high and no grain boundary oxidation occurs. On the other hand, in Comparative Steels 18 and 27,
Since the contents of C and N were smaller than the lower limits, respectively, sufficient high temperature strength could not be obtained. Comparative steels 20, 21, 2
In No. 2, the addition of Nb, Ti, and V was excessive, so that a coarse phase was precipitated in the metal structure, leading to a decrease in creep ductility and a decrease in rupture life. Also,
In Comparative Steel 23, the Cr content was too low, the atmospheric oxidation during the 650 ° C. creep rupture test was remarkable, and the rupture was accelerated due to the thinning. Comparative steels 24, 25, 26 are Cr, Si, N
When the content of i, etc. deviates from the appropriate value, it becomes difficult to adjust the phase stability, which leads to precipitation of brittle intermetallic compounds such as sigma phase, resulting in low long-time high-temperature strength. Comparative steel 2
7 also showed a similar tendency due to low N. Also,
Comparative steels 30 and 31 had low ductility and low creep rupture strength because Mo and Cu were out of the proper ranges, respectively. Further, the comparative steels 17, 19, 28 and 29 are R
Since the value was out of the proper value, the high temperature creep rupture strength after the solution treatment at 1140 ° C was low.

As described above, the steels 1 to 1 of the present invention were prepared by optimizing the contents of the respective elements and the R value.
6, a good high temperature strength can be obtained, for example, a creep rupture strength of 100,000 hours at 650 ° C. is 11
It was confirmed that even in the case of the solution heat treated at 40 ° C., the value was as high as about 85 MPa.

[0042]

As described above, according to the present invention, for the first time, it is possible to sufficiently meet the requirements for heat resistance strength at a high temperature of 600 ° C. or higher, as well as in the middle / low temperature range, and high temperature corrosion resistance. A material having excellent high temperature oxidation resistance can be obtained. Moreover, the characteristics as described above can be achieved in the solution treatment at a temperature of 1150 ° C. or lower, which avoids the high temperature solution treatment of the metal that causes the deterioration of the surface performance and the yield. It is possible to contribute to higher efficiency of high-performance high-temperature equipment, and improvement of economic efficiency in construction.

[Brief description of the drawings]

FIG. 1 is a diagram showing the relationship between the R value of steel and creep rupture strength at 650 ° C. for 100,000 hours.

Claims (4)

[Claims] 1. A weight fraction of C: 0.05 to 0.12.
%, Si: 2.5% or less, Mn: 2.0% or less, Cr:
16.0 to 20.0%, Ni: 6.0 to 14.0%, M
o: 0.01 to 4%, N: 0.005 to 0.25%, M
g: 0.05% or less (including 0%), Cu: 0.01 to
4%, B: 0.001 to 0.005%, Zr: 0.1%
The following (including 0%), Al: 0.5% or less (including 0%), and Nb: 0.45% or less, Ti:
Excellent in high temperature strength, containing at least one of 0.25% or less and V: 0.25% or less, satisfying the following formula, and the balance being Fe and inevitable impurities. Austenitic stainless steel for high temperature equipment. 1 ≦ (C / 12 + N / 14) / (Nb / 93 + Ti / 4
8 + V / 51) ≦ 2 2. A weight fraction of C: 0.05 to 0.12.
%, Si: 2.5% or less, Mn: 2.0% or less, Cr:
16.0 to 20.0%, Ni: 6.0 to 14.0%, M
o: 0.01 to 4%, N: 0.005 to 0.25%, M
g: 0.05% or less (including 0%), Cu: 0.01 to
4%, B: 0.001 to 0.005%, Zr: 0.1%
The following (including 0%), Al: 0.5% or less (including 0%), Nb: 0.45% or less, Ti: 0.25%
V: 0.25% or less, at least one of Ce, La, and Hf is 0.1% or less, and the following formula is satisfied. An austenitic stainless steel for high-temperature equipment, which has excellent high-temperature strength and is Fe and inevitable impurities. 1 ≦ (C / 12 + N / 14) / (Nb / 93 + Ti / 4
8 + V / 51) ≦ 2 3. A weight fraction of C: 0.05 to 0.12.
%, Si: 2.5% or less, Mn: 2.0% or less, Cr:
16.0 to 20.0%, Ni: 6.0 to 14.0%, M
o: 0.01 to 4%, N: 0.005 to 0.25%, M
g: 0.05% or less (including 0%), Cu: 0.01 to
4%, B: 0.001 to 0.005%, Zr: 0.1%
The following (including 0%), Al: 0.5% or less (including 0%), and Nb: 0.45% or less, Ti:
Steel containing at least one of 0.25% or less and V: 0.25% or less and satisfying the following formula is formed into a predetermined product shape or a semi-finished product through a casting process and a hot working process. After that, a solution treatment is performed at a temperature of 1150 ° C. or less, and a method for producing an austenitic stainless steel for high temperature equipment, which is excellent in high temperature strength. 1 ≦ (C / 12 + N / 14) / (Nb / 93 + Ti / 4
8 + V / 51) ≦ 2 4. A weight fraction of C: 0.05 to 0.12.
%, Si: 2.5% or less, Mn: 2.0% or less, Cr:
16.0 to 20.0%, Ni: 6.0 to 14.0%, M
o: 0.01 to 4%, N: 0.005 to 0.25%, M
g: 0.05% or less (including 0%), Cu: 0.01 to
4%, B: 0.001 to 0.005%, Zr: 0.1%
The following (including 0%), Al: 0.5% or less (including 0%), Nb: 0.45% or less, Ti: 0.25%
Hereinafter, V: steel containing at least one of 0.25% or less, 0.1% or less of at least one of Ce, La, and Hf, and satisfying the following formula, Manufacture of austenitic stainless steel for high temperature equipment excellent in high temperature strength, which is characterized by performing solution treatment at a temperature of 1150 ° C. or less after forming a predetermined product shape or semi-finished product through a casting process and a hot working process. Method. 1 ≦ (C / 12 + N / 14) / (Nb / 93 + Ti / 4
8 + V / 51) ≦ 2