JP3067577B2 - Ferritic stainless steel with excellent oxidation resistance and high-temperature strength - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION
The present invention relates to a ferritic stainless steel having excellent oxidation resistance and high-temperature strength in a high-temperature combustion exhaust gas atmosphere at 0 ° C or lower. In particular, the present invention is characterized in that the degree of embrittlement due to precipitation of Laves phase and decrease in high-temperature strength, which are problematic when used at a temperature of 600 ° C. or more and 900 ° C. or less for a long time, are remarkably small. Related to ferritic stainless steel.

Further, the present invention has excellent adhesion of scales generated in high-temperature combustion exhaust gas having a high water concentration of 10 to 20%. The present invention relates to a ferritic stainless steel excellent in oxidation resistance and high-temperature strength, which is suitable for constituting exhaust gas ducts such as an exhaust gas exhaust duct and an exhaust heat recovery boiler duct, and automobile exhaust system members such as an automobile exhaust manifold and a front pipe.

[0003]

2. Description of the Related Art Ferritic stainless steels contain a large amount of Ni.
Is less expensive than austenitic stainless steel because it does not contain, but has drawbacks such as low toughness and ductility, and deterioration of performance during welding tends to be a problem.

For example, ferritic stainless steels for heat resistance include SUH21, SUH409, SUH409L, and JIS in JIS.
SUH446 has been standardized, but in terms of ease of handling including welding and assembly, austenitic stainless steel and
Although this is not a comparison with Ni-based alloys, at present the SUH409 and 409L series are only used as general-purpose steels in automobile exhaust systems, low-temperature duct lining, etc. due to low material costs.

On the other hand, ferritic stainless steel is advantageous in that it has a low thermal expansion, but has a problem in that its high-temperature strength is low. In particular, the decrease in high-temperature strength in the temperature range exceeding 650 ° C. is one of the major factors limiting the applicable range of this steel type. Further, oxidation resistance in a temperature range exceeding 850 ° C. is one of the properties that are desired to be improved.

[0006] By the way, there has been remarkable progress in refining technology of ferritic stainless steel in recent years.
It has been possible to reduce C and N in steel (hereinafter, referred to as high purification), to achieve extremely low sulfuration, and to control the addition amount of trace elements in steel, which were difficult in the past. For example, SUS 430L, SUS 436L
So-called high-purity ferritic stainless steels such as stainless steel, SUS444 and SUS447J1 have been standardized. Therefore, in heat-resistant ferritic stainless steels, the development of high-performance steels backed by advanced refining technology has been actively pursued, for example, for supporting metal catalysts for purifying automobile exhaust gas through which combustion exhaust gas of about 950 ° C passes. There are practical applications such as foil. A high-purity ferritic stainless steel containing about 5% Al and about 20% Cr. The reduction in base metal toughness due to the increase in Al to a level that can be manufactured by minimizing C and N in steel. Has improved.

The high Al-containing ferritic stainless steel of this system exhibits excellent oxidation resistance at around 1000 ° C. by forming an Al ₂ O ₃ film on the steel surface with oxidation at a high temperature. To improve the adhesion of the scale, use a small amount of REM of Y, La, etc.
Is carried out.

[0008] As a high Al-containing ferritic stainless steel for lower temperature applications, 18Cr-3Al series has been commercialized and used as a member of household heating appliances and the like.

As an additive element for improving the oxidation resistance of ferritic stainless steel, there is Si in addition to Al. However, when Si is added in a large amount, it not only significantly lowers the toughness of the base material but also significantly impairs the hot workability. It has not been done.

In this respect, however, Japanese Patent Publication No. 62-14626
Discloses a ferritic stainless steel excellent in high-temperature oxidation resistance to which Si is added in excess of 1%, and contains S, O in steel.
Is defined as O: less than 0.005% and S + O: less than 0.008% to improve oxidation resistance at high temperatures.

[0011] According to the above invention, S, which is usually contained in steel at about 0.01%, combines with Mn also present in steel.
When steel is exposed to high temperatures, MnS decomposes and decomposes to Cr-Mn-O oxides, the free S concentrates on the steel surface and grain boundaries, and Around inclusions
It is stated that formation of a Cr-deficient layer deteriorates oxidation resistance. Although it is desirable to completely remove S and O, it is difficult to completely remove S and O because they are mixed as unavoidable impurities. In the invention disclosed in the above publication, the amounts of S and O in the steel are set to O: 0.
When the content is less than 005% and the content of S + O is less than 0.008%, a remarkable improvement effect is obtained. In addition, the improvement effect by adding a sulfide that has a stronger binding force with S than Mn such as Ca, Mg, a rare earth element, and Y, and is more stable even at a high temperature is also clarified.

Japanese Patent Application Laid-Open No. Sho 60-145359 discloses that a high-temperature oxidation and cooling cycle is repeated for parts, powder metallurgy products, and welding products of automobile exhaust systems.
A high temperature ferritic steel containing 0.1% Si and 6.0 to 20.0% Cr is disclosed.

According to the present invention, the addition of Si to steel achieves excellent oxidation resistance and strength at high temperatures and excellent weldability. As C and N stabilizing elements in steel, Ti,
Contains at least one of Zr, Ta, and Nb, and has at least C%
+ N% is present in an amount equal to the stoichiometric amount and for Nb the upper limit is 0.3%, at least 0.1% of unbound Nb
Is contained. Also, Al is added in an amount of 0.5% or less, which is required to ensure high-temperature oxidation resistance at 816 ° C. or higher.

By the way, the combustion temperature of a power generating turbine tends to increase in accordance with recent technological innovation. In particular, since liquefied natural gas is used as fuel, the combustion temperature is 1300 ° C. higher than the conventional 1100 ° C. class.
℃ class and 1400 ℃ class are actually being planned and installed. Under such circumstances, the heat recovery boiler for recovering the heat of the high-temperature exhaust gas on the turbine outlet side also tends to be heated to a high temperature. For example, in the 1100 ° C class, the exhaust gas temperature, which was conventionally around 550 ° C, now exceeds 600 ° C, and the exhaust gas duct surface temperature also increases.
The temperature rose to around 650 ° C.

In such an environment, the conventional 409L-based material may not have sufficient performance in the following points. That is, in commercial power generation of the gas turbine system, it is extremely rare to operate continuously all day, and it is common to start the turbine only during the day when power consumption increases and to stop it at night. When such an operation mode is adopted, the combustion exhaust gas is condensed in the duct when the vehicle is stopped at night, which promotes the generation of red rust. Also,
Even if there is no condensate corrosion, the conventional 409L system generates red scale with poor adhesion when used in a flue gas atmosphere containing a large amount of water, so the flue gas is reheated and returned to the power generation turbine. In the power generation system, there is a possibility that problems such as turbine damage caused by the peeling scale may occur. Also, in terms of high-temperature strength, there is a problem with the use of Ti-added 409L-based materials for prolonged use in a temperature range exceeding 600 ° C.

[0016]

SUMMARY OF THE INVENTION It is a general object of the present invention to solve the problems of the prior art and to improve the oxidation resistance at high temperature, the high temperature strength, the thermal fatigue properties and the condensate corrosion. An object of the present invention is to provide a ferritic stainless steel having sufficient performance.

A more specific object of the present invention is to provide, for example,
Ferritic stainless steel with excellent oxidation resistance and high-temperature strength in a high temperature combustion exhaust gas atmosphere at a temperature of ℃ to 950 ° C, especially Laves which becomes a problem when used at a temperature of 600 ° C to 900 ° C for a long time The degree of embrittlement and decrease in high-temperature strength due to phase precipitation is extremely small, and even when used in high-temperature flue gas with a high water concentration of 10 to 20%,
It has excellent adhesion of the generated scale.For example, combustion exhaust gas ducts such as exit gas exhaust ducts and exhaust heat recovery boiler ducts of gas turbines for combined power generation using liquefied natural gas as fuel, automobiles such as automobile exhaust manifolds and front pipes An object of the present invention is to provide a ferritic stainless steel excellent in oxidation resistance and high-temperature strength suitable for exhaust system members.

[0018]

Means for Solving the Problems The inventors of the present invention have conducted various studies in order to achieve the above object, and have obtained a steel composition which can exhibit the desired effect even when used under the above-mentioned severe conditions. And completed the present invention.

That is, the findings in the present invention are as follows. (i) The corrosion resistance to condensed liquid is sufficient,
In order to produce a scale with good adhesion even in a high-temperature environment exceeding 600 ° C. containing 20% to 20%, Si: 0.80 to 1.50%, Cr: 11.0 to 21.0%, Al: 0.01 to 0.10%
% Should be effective.

(Ii) In order to impart excellent high-temperature strength and thermal fatigue properties, Nb and Ti, which are stabilizing elements, are added in a combined manner within the following range to suppress Nb-based Laves phase precipitation. High temperature strength can be secured by solid solution strengthening with Nb. Nb: 0.10 to 0.40%, Ti: 0.05 to 0.30%.

(Iii) By setting Ti / S> 15 and C / S> 0.5, it is possible to form a Ti—CS—based inclusion that is stable at a high temperature. C is reduced, workability, toughness, corrosion resistance of welds can be improved,
The decrease in solid solution C facilitates recrystallization, and increases the amount of solid solution Nb that is not fixed as Nb carbide, resulting in high-temperature strength,
Improved thermal fatigue properties.

(Iv) By adding 0.1 to 2.0% of W, which has a smaller tendency to form a Laves phase than Nb, as required,
It must be able to ensure higher strength at high temperatures.

Here, the gist of the present invention is that
%: C: 0.015% or less, N: 0.015% or less, Si: 0.80 to 1.50
%, Mn:0.6% or less, Cr: 11.0-21.0%, Al: 0.01-0.10
%, Ti: 0.05-0.30%, Nb: 0.10-0.40%, S: 0.0033-0.
020%, Fe and inevitable impurities: Remaining composition, satisfying Ti / S> 15 and C / S> 0.5
And into steelTi _Four C _Two S _Two Characterized by precipitation of system inclusions
Ferritic stainless steel with excellent oxidation resistance and high-temperature strength
Less steel.From yet another aspect, the present invention provides C: 0.015% or less, N: 0.015% or less, Si: 0.80 to 1.50
%, Mn: 0.8% or less, Cr: 11.0 to 21.0%, Al: 0.01 to 0.10
%, Ti: 0.05 ~ 0.30%, Nb: 0.10 ~ 0.40%, Cu: 0.80% or less and / or Ni: 0.80% or less, S: 0.0033-0.020%, Fe and unavoidable impurities: balance With composition of Ti / S> 15 and C / S> 0.5
And Ti in steel _Four C _Two S _Two Characterized by precipitation of system inclusions
Ferritic stainless steel with excellent oxidation resistance and high-temperature strength
Less steel. In the present invention, the content of W is 2.00% or less.
You may.

The present invention is also directed to gas turbine outlet ducts and exhaust heat recovery boiler ducts for combined power generation using liquefied natural gas as fuel, having the above steel composition and having an energy efficiency of at least 48%. This is a representative flue gas duct.

Further, the present invention is a vehicle exhaust system member represented by a vehicle exhaust manifold, a front pipe, and the like having excellent high-temperature strength, oxidation resistance and thermal fatigue characteristics. Furthermore, the present invention is an exhaust gas system device used in a similar environment, and is a material for a welded steel pipe.

[0026]

Next, the reason why the steel composition is limited in the present invention as described above, including the specific examples, and the operation and effect thereof will be described more specifically. In this specification, “%” is “% by weight” unless otherwise specified.

C, N C and N in the steel are elements that deteriorate the toughness, workability and weldability of the ferritic stainless steel. However, the presence of an appropriate amount of C has an effect of improving high-temperature strength. In particular, in the steel of the present invention, Ti-CS-based inclusions are formed together with Ti,
It also has the effect of fixing S as a thermally stable sulfide.
The amount of C that can sufficiently exhibit such an effect is at least 1/2 times, preferably more than 1/2 times the amount of S. The upper limit of C is set to 0.015%, and the upper limit of N is set to 0.015%. Preferably, both C and N
0.010% or less.

Si: Si tends to increase in strength at room temperature and high temperature and decrease in elongation as the amount of Si added increases. However, the oxidation resistance is significantly improved, and the water content is 10 to 20%.
It is effective in improving red rust resistance in a combustion exhaust gas environment.

FIG. 1 shows the change in the amount of oxidation increase when the Si content of the 13Cr ferritic stainless steel in Examples described later was changed at 900 ° C. The effect of improving oxidation resistance is more remarkable than about 1%. In the present invention, the amount of Si in steel
0.80% or more and 1.50% or less. This is because the amount of Si in steel is 0.
This is because at 80% or more, the effect of improving the oxidation resistance by increasing the Si content is clearly recognized, and when added in excess of 1.50%, the room temperature elongation is remarkably reduced and the workability is required. This is because it is not suitable for the use of the invention steel. Preferably it is 0.80-1.30%.

Mn: Mn has an upper limit of 0.8%. Mn in steel
When the amount is increased, abnormal oxidation is easily caused. Preferably, it is 0.6% or less.

Cr: Cr is a very important basic element,
It is an alloying element that basically determines the corrosion resistance and oxidation resistance of the steel of the present invention. If it is less than 11.0%, red rust-like scale tends to be generated at a high temperature, and the corrosion resistance at room temperature is greatly deteriorated. Therefore, the lower limit is set to 11.0%. The higher the Cr content, the better the oxidation resistance at high temperatures and the better the corrosion resistance, but the lower the cost performance. In addition, the toughness of both the base metal and the welded part deteriorates, and the ease of handling during production and the ease of handling as general-purpose steel by the user are reduced.
%. A preferred range is 12.0 to 19.0%.

Nb: Nb is one of the additional elements having an effect of stabilizing C in steel, similar to Ti and Zr, and Nb carbide is superior to Ti in thermal stability at high temperatures. Because 900
It suppresses crystal grain coarsening at a temperature of not less than ℃ and suppresses deterioration of oxidation resistance and toughness due to the coarsening. It also has the effect of improving high-temperature strength by solid solution strengthening as a substitution element. The steel of the present invention is also a very important element, and it is necessary to add at least 0.10% or more in order to exert those effects.

However, as already known, the presence of Nb forms a Laves phase (Fe ₂ Nb) when used in a temperature range of 900 ° C. or less and 600 ° C. or more, and reduces the toughness at 150 ° C. or less. May deteriorate significantly. The present invention seeks to provide a material suitable for use in an outlet exhaust gas duct and an exhaust heat recovery boiler duct of a combined cycle gas turbine using liquefied natural gas as a fuel, or an automobile exhaust manifold and a front pipe. Under use conditions in which cooling and heating to near room temperature are repeated, as in these applications, embrittlement in a state of cooling to near room temperature is a very serious problem.

The present inventors have proposed 0.01C-13Cr-1Si-0.1Ti-0.0
The embrittlement behavior due to Laves phase precipitation was examined in detail by changing the amount of Nb in the steel in the 1N system, but when the Nb amount exceeds 0.40%, the embrittlement to a practically problematic level occurs. It was confirmed.

FIG. 2 shows that the aging material at 600 ° C. for 1000 hours was used.
4 shows the results of a Charpy impact test at ℃. Beyond 0.40%
It is confirmed that there is a practical problem when Nb is present. Here, the “impact absorption energy value that is problematic in practical use” shown in the figure is an allowable level obtained by the inventors through trial and error over many years of experience, and is an index used in evaluating the present invention. one of.

FIG. 3 shows that the above-mentioned Nb changing material is 600 ° C. ×
The tensile strength at 600 ° C after aging for 1000 hours is shown. The effect of the amount of Nb on the high-temperature strength after aging was small, and almost the same tensile strength was exhibited. When the Nb content is less than 0.40%, which is less affected by the embrittlement due to the Laves phase, the decrease in high-temperature strength is small.

Ti: Ti, along with Nb, stabilizes C and N in steel, suppresses coarsening of grains in a weld, and improves intergranular corrosion resistance in a condensed water environment. This effect is 0.05%
Appears with the above additions. Addition in excess of necessity is not preferable because rolling flaws become remarkable and productivity is deteriorated. Therefore, the upper limit is set to 0.30%.

In the present invention, Ti is used as a composite additive element with Nb. When Nb and Ti are combined,
Mainly, Ti combines with N to form a nitride, and in the subsequent cooling process, Nb and the remaining Ti combine with C to form (Nb, Ti)
Form carbides. (Nb, Ti) Carbide is Nb or Ti
Precipitated TiN precipitates at a higher temperature than single carbides
Nuclei easily precipitate. Therefore, a region (Interstitial Free region) where no solid solution C and N exist is formed around the periphery. Thereby, the steel becomes soft and good workability and formability can be obtained. When the content of Si is as high as about 1% as in the present invention, the composition tends to be hard, so that this composite addition works effectively.

FIG. 4 shows, for a series of steel compositions within and outside the range of the present invention, a composition example in which the tensile strength at 600 ° C. after aged at 600 ° C. for 1000 hours shows 150 N / mm ^2, and the tensile strength after aged at 600 ° C. for 1000 hours. FIG. 2 is a graph in which the composition examples in which the impact value at 25 ° C. is 20 J / cm ² or more and whether the rolling flaws are remarkable, that is, the composition examples in which the manufacturability is good, are arranged by the amounts of Ti and Nb in steel. In the figure, the hatched area satisfies all of the above characteristics (1) to (4).
This is the range of composition examples of Ti and Nb.

Al: Next, the effect of adding Al in the steel of the present invention will be described. Al is added to a ferritic stainless steel having a Si concentration of 0.80% or more and 1.50% or less to improve the scale adhesion.

FIG. 5 shows the effect of the Al content of the 13Cr-1.2 Si ferritic stainless steel on the amount of scale peeling in the air at 900 ° C. for 200 hours. As the amount of Al increases, the amount of scale peeling decreases significantly. This is Al
It has been confirmed that, due to the addition of a small amount of, an internal oxidation tendency is observed, which is caused by the scale being fixed so as to “grow root”.

In the present invention, the content of Al in steel is set to 0.01% or more and 0.10% or more.
% Or less. This is because the improvement effect is recognized even with a small amount of Al of about 0.01%, whereas no further improvement effect is recognized even if the content exceeds 0.10%. Further, it has the effect of improving the strength at high temperatures due to the presence of a small amount of Al in the steel.

FIG. 6 shows the high-temperature tensile strength of 13Cr-1.2Si-Nb at 700 ° C. Al addition tends to increase high-temperature strength, and is effective in improving high-temperature fatigue properties and thermal fatigue properties. The addition amount determined to be desirable as the Al amount is 0.02 to 0.08%.

Cu, Ni: Cu and Ni are desired additive elements that have the effect of increasing the tensile strength at room temperature and improve the high-temperature salt damage resistance. Usually, it is effective to include trace amounts of Cu and Ni that are mixed as unavoidable impurities, but the lower limit of each is preferably 0.10%. In particular, Ni is effective for improving toughness even with a small amount of addition. If it exceeds 0.80%, embrittlement may occur due to the formation of intermetallic compounds, so the upper limit is 0.80% for each.
And

W: W is also a desired additive element. In ferritic stainless steel, it is a substitution type solid solution element and has an effect of improving high temperature strength by solid solution strengthening. W has a significantly lower diffusion rate than Nb, and is therefore represented by the Laves phase
The formation of an intermetallic compound with Fe is extremely slow, so that the solid solution strengthening action is maintained for a long time, and the high-temperature strength is maintained. However, addition of a large amount of W is not preferable because workability at room temperature is also reduced.

In the present invention, when W is added, the lower limit is set to 0.10% at which improvement in high-temperature strength is confirmed, and the upper limit is set to 2.00% which does not cause the problem of productivity and scrap recycling. It is preferably lower than the viewpoint of scrap recycling, and desirably 0.30 to 1.50%. O (oxygen) is an unavoidable impurity, but has an adverse effect on oxidation resistance and toughness. Therefore, a desirable O content is 0.005% or less.

Ti and S: Next, the coexistence effect of Ti and S, which is a major feature of the steel of the present invention, will be described. In the steel of the present invention, Ti: 0.05 to 0.30%, and S satisfying Ti / S> 15 is 0.1%.
020% or less. The lower limit of the amount of S that satisfies this is about 0.
0033%.

Generally, as described in detail in the specification of Japanese Patent Publication No. 62-14626, S exists as sulfide in steel and is considered to decompose at high temperatures to promote oxidation. Have been. The present inventors have found that the addition of an appropriate amount of Ti and the reduction of S in steel, which can be extremely reduced in refining, to Ti / S> 15
By controlling the upper limit to be as high as 0.020% or less, S in the steel is fixed as a thermally stable Ti-CS-based Ti-based inclusion, thereby obtaining an extremely low S. It has been discovered that superior properties can be obtained over high-temperature properties. Ti-CS
The system nonmetallic inclusions are thermally stable even in a temperature range around 1000 ° C., and are unlikely to cause deterioration of oxidation resistance due to decomposition.

The Ti-CS-based inclusions finely dispersed in steel also have the effect of increasing the high-temperature strength and improve the thermal fatigue characteristics. This is for the following reason.

That is, when the solid solution C decreases due to the formation of Ti-CS-based inclusions, the solid solution Nb which is not solid-solved as Nb carbide is secured, the high temperature strength is maintained, and the steel is easily recrystallized. Improves thermal fatigue properties. In addition, solid solution C
The reduction of the steel makes the steel soft and improves workability and formability.

In order to precipitate Ti-CS-based inclusions including Ti-S-based inclusions, rolling may be performed after holding at a temperature range of 1050 ° C or more and 1180 ° C or less for one hour or more before rolling. ,
Precipitates during heating and rolling and in the rolling process.

FIGS. 7 (a) and 7 (b) show that Ti ₄ C ₂ precipitated around the TiN when the test steel having the composition of the test steel No. 5 (Example, Table 1) was heated at 1150 ° C. An example of the precipitation of S ₂ and (Nb, Ti) C is shown in a sketch.

FIG. 8 shows the relationship between the amount of S deposited and the heating temperature after the precipitate of FIG. 7 was chemically deposited and the amount of S in the precipitate was analyzed. In FIG. 8, the heating temperature is 1100 ° C., 1150 ° C.,
The material hot rolled at 1200 ° C. and 1230 ° C. was subjected to hot-rolled sheet annealing-cold rolling-finish annealing to produce a cold-rolled sheet having a thickness of 2 mm, and subjected to a normal temperature tensile test and a thermal fatigue test. The test method was the same as in the example.

The results are shown in FIG. 9 and FIG. 9 and 10
From this, it is understood that the larger the amount of precipitated S, that is, the more Ti-CS-based inclusions, the more the room temperature elongation and the thermal fatigue strength are improved. Furthermore, if added, Nb and Ti compound addition and Ti-
The decrease in solid solution C caused by the formation of CS improves the workability and formability by softening the steel, and also suppresses the sensitization at the welded portion to improve the intergranular corrosion resistance in a condensed water environment. It also has the effect of improving.

[0055]

【Example】

[Example 1] Each of the test steels shown in Table 1
Kg was melted to form a round ingot. Next, after being formed into a thickness of 30 mm by hot forging, a hot-rolled steel sheet having a thickness of 6 mm was formed by hot rolling. The heating condition of the hot rolling was 1140 ° C. × 1.5 hours in the air. The hot-rolled sheet annealing was carried out by forced air cooling after holding at 980 ° C. for 3 minutes. After descaling treatment with an aqueous nitric hydrofluoric acid solution, a cold-rolled plate having a thickness of 2 mm was subjected to finish annealing at 980 ° C. for 1 minute. From this, JIS1 for room temperature tensile
A test piece having a thickness of 2 mm, a width of 20 mm and a length of 25 mm was sampled from the No. 3B test piece in the T direction for an oxidation test and a high-temperature salt damage test. Furthermore, pipes were made from the cold-rolled sheet by electric resistance welding,
A thermal fatigue test tube such as 11 was prepared.

In FIG. 11, reference numeral 1 denotes a test tube to be used for the test, and holes having a diameter of 8 mm were formed at two places, and the holes were used as a supply port 2 and a discharge port 3 for cooling air, respectively. Reference numeral 4 denotes a holder (core bar) from the inner surface of the tube, and reference numeral 5 denotes a portion for attaching the test piece to the holder. The tube 1 and the holder 4 are fixed by a fixing pin 6 and an end weld.

The oxidation test conditions were 900 ° C. × 200 hr and continuous heating in air. The conditions for the high-temperature salt damage test were the cycle shown in FIG. In the thermal fatigue test, the thermal fatigue test specimen shown in FIG. 11 was used, and the temperature cycle and mechanical strain waveform history shown in FIG. The test was performed at 50 ° C. and 50% constraint (η = 0.501).

Table 2 summarizes the results of these tests. Room-temperature elongation of 30% or more, oxidation weight gain 1.5 mg / cm ² or less, the scale peelability 0.5 mg / cm ² or less, the thickness after the salinity test decrease 45
Passes of 0 μm or less and a thermal fatigue life of 800 cycles or more were accepted.

It can be seen that the steel of the present invention has excellent properties in all of room temperature elongation, oxidation resistance, high temperature salt corrosion and thermal fatigue properties. As is clear from comparison with the comparative steels 24 and 25, the steel of the present invention contains S that satisfies S / 0.020% and Ti / S> 15, thereby improving the room temperature elongation and the thermal fatigue properties. Further, as shown in Invention Steels 2, 9, 10, 12, and 14, the addition of W of 0.1% or more and 0.2% or less further improves the thermal fatigue characteristics.

The comparative steel 18 is a steel equivalent to SUH409L, but is inferior in oxidation resistance, high-temperature salt damage resistance and thermal fatigue properties. Comparative steel 19
Is inferior in oxidation resistance because Si is less than 0.8%. The comparative steel 20 has a low room temperature elongation since Si exceeds 1.5%, and is inferior in workability. Comparative steel 21 contained 0.65% of Nb but did not show any effect of improving thermal fatigue properties, and had low room temperature elongation and poor workability.
Since the comparative steel 22 contains less than 0.01% of Al, almost all of the oxide scale is peeled off and the scale adhesion is poor. As described above, the results of this example are graphically shown in FIGS. 1 to 6, and in each graph, the subscript indicates the test steel No.

Example 2 In this example, a trial production of a mass production scale was performed using an 80-ton AOD refining furnace. Table 3 shows the composition of the two test steels. The prototype was manufactured under the same conditions as in Example 1 by performing VOD refining and continuous casting, then manufacturing a hot-rolled steel sheet having a thickness of 6 mm, and further cold-rolling the steel sheet to a thickness of 2 mm. The cold-rolled steel sheet was subjected to finish annealing at 980 ° C. × 1 minute, and then subjected to descaling treatment with an aqueous nitric hydrofluoric acid solution.

[0062] The above-mentioned mass-produced prototype materials are LN
It is applied to the high-temperature side (near 600 ° C) in front of the denitration device of the exhaust heat recovery boiler duct of a commercial combined cycle power plant consisting of a gas turbine using G as fuel, an exhaust heat recovery boiler, and a steam turbine. Provided. Similarly, the fin portion of the welded high fin tube was manufactured and applied without any problem due to excellent cold workability.

[0063]

[Table 1]

[0064]

[Table 2]

[0065]

[Table 3]

[0066]

As described above, the steel of the present invention has a
It has excellent oxidation resistance and high-temperature strength in high-temperature combustion gas atmospheres of 950 ° C or higher and 950 ° C or lower, and can fully satisfy other properties as a material for exhaust heat recovery boiler ducts and fins. Durability can be improved. Further, the steel of the present invention can exhibit excellent characteristics as an automobile exhaust system material such as an automobile exhaust manifold and a front pipe.

[Brief description of the drawings]

FIG. 1 is a graph showing the relationship between the amount of Si in steel and the increase in oxidation after continuous heating in air at 900 ° C. for 200 hours.

FIG. 2 is a graph showing the effect of the amount of Nb in steel on the impact value at 25 ° C. after aging at 600 ° C. for 1000 hours.

FIG. 3 is a graph showing the influence of the amount of Nb in steel on the tensile strength at 600 ° C. after aging at 600 ° C. for 1000 hours.

FIG. 4 is a graph showing a correlation between the amount of Ti and the amount of Nb in the ferritic stainless steel according to the present invention.

FIG. 5 is a graph showing the relationship between the amount of Al in steel and the amount of scale peeling after continuous heating in air at 900 ° C. for 200 hours.

FIG. 6 is a graph showing the relationship between the amount of Al in steel and the high-temperature tensile strength at 700 ° C.

FIGS. 7A and 7B are sketches showing examples of precipitation of Ti ₄ C ₂ S ₂ and (Nb, Ti) C, respectively.

FIG. 8 is a graph showing a relationship between a quantitative analysis value of S by an extraction residue analysis and a heating temperature.

FIG. 9 is a graph showing the relationship between the amount of precipitated S and room temperature elongation.

FIG. 10 is a graph showing the relationship between the amount of precipitated S and the thermal fatigue strength.

FIG. 11 shows the shape of a thermal fatigue test specimen.

FIG. 12 is a graph showing a high-temperature salt damage corrosion test cycle.

FIG. 13 shows temperature and strain waveforms during a thermal fatigue test.

Continuation of the front page (72) Inventor Shinji Tsuge 4-5-33 Kitahama, Chuo-ku, Osaka City Inside Sumitomo Metal Industries, Ltd. (56) References JP-A-7-70709 (JP, A) JP-A-5-239600 (JP, A) Japanese Patent Application Laid-Open No. 8-144012 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) C22C 38/00-38/60 JICST file (JOIS)

Claims (5)

(57) [Claims] 1. In weight%, C: 0.015% or less, N: 0.015% or less, Si: 0.80 to 1.50
%, Mn: 0.6% or less, Cr: 11.0 to 21.0%, Al: 0.01 to 0.10
%, Ti: 0.05-0.30%, Nb: 0.10-0.40%, S: 0.0033-0.
020%, Fe and incidental impurities: having the composition and the balance, satisfied the Ti / S> 15, C / S> 0.5, wherein the Ti ₄ C ₂ S ₂ inclusions are precipitated in the steel Ferritic stainless steel with excellent oxidation resistance and high-temperature strength. 2. In% by weight, C: 0.015% or less, N: 0.015% or less, Si: 0.80 to 1.50
%, Mn: 0.8% or less, Cr: 11.0 to 21.0%, Al: 0.01 to 0.10
%, Ti: 0.05 to 0.30%, Nb: 0.10 to 0.40%, Cu: 0.80% or less and / or Ni: 0.80% or less, S: 0.0033 to 0.020%, Fe and inevitable impurities: balance A ferritic stainless steel that satisfies Ti / S> 15 and C / S> 0.5 and is excellent in oxidation resistance and high-temperature strength, characterized in that Ti ₄ C ₂ S ₂ inclusions are precipitated in the steel. 3. The ferritic stainless steel having excellent oxidation resistance and high-temperature strength according to claim 1, further comprising W: 2.00% or less. 4. A flue gas duct made of the ferritic stainless steel according to claim 1. 5. An automobile exhaust system member made of the ferritic stainless steel according to claim 1.