JPH08260107A - Ferritic stainless steel excellent in oxidation resistance and high temperature strength - Google Patents

Abstract

PURPOSE: To improve oxidation resistance and high temp. strength by incorporating specific amounts of C, N, Si, Mn, Cr, Al, Ti, Nb, Cu, Ni, W, S, and Fe and precipitating Ti-C-S inclusions. CONSTITUTION: This steel has a composition consisting of, by weight, <=0.015% C, >=0.015% N, 0.8-1.5% Si, >=0.8% Mn, 11-21% Cr, 0.01-0.1% Al, 0.05-0.3% Ti, 0.1-0.4% Nb, 0-0.8% Cu, 0-0.8% Ni, 0-2% W, <=0.02% S, and the balance essentially Fe and satisfying Ti/S>15 and C/S>0.5. Ti-C-S inclusions, stable at high temp., can be formed, and S can be fixed and the amount of solid- solution C can be reduced and, as a result, workability, toughness, and corrosion resistance in weld zone can be improved. As to the amount of Si, an oxidation resistance improving effect can be obtained when it is >=0.8%, but elongation at ordinary temp. is decreased and workability is deteriorated when it exceeds 1.5%. By this method, the stainless steel, excellent in oxidation resistance and high temp. strength in a high temp. combustion gas atmosphere of 600-900 deg.C, can be produced.

Description

Detailed Description of the Invention

[0001]

The present invention is applicable to, for example, 600 ° C. or higher, 95
The present invention relates to a ferritic stainless steel excellent in oxidation resistance and high temperature strength in a high temperature combustion exhaust gas atmosphere of 0 ° C. or less. In particular, the present invention is characterized in that the degree of embrittlement and high temperature strength reduction due to Laves phase precipitation, which are problems when used for a long time at 600 ° C or higher and 900 ° C or lower, are significantly small. Regarding ferritic stainless steel.

Further, the present invention is excellent in the adhesion of scale produced in high temperature combustion exhaust gas having a high water content of 10 to 20%, and specifically, to a gas turbine for combined power generation using liquefied natural gas as a fuel. The present invention relates to a ferritic stainless steel excellent in oxidation resistance and high temperature strength, which is suitable for constituting a combustion exhaust gas duct such as an outlet side exhaust gas duct and an exhaust heat recovery boiler duct, an automobile exhaust system member such as an automobile exhaust manifold and a front pipe.

[0003]

2. Description of the Related Art Ferritic stainless steel contains a large amount of Ni.
Since it does not contain Al, it has a characteristic that it is cheaper than austenitic stainless steel, but it has drawbacks such as low toughness and ductility, and deterioration of performance during welding tends to be a problem.

For example, as ferritic stainless steels for heat resistance, JIS JIS SUH21, SUH409, SUH409L,
Although SUH446 has been standardized, it is easy to handle including welding and assembly, and austenitic stainless steel and
Although it is not a comparison with Ni-based alloys, at present the material cost is low, so SUH409 and 409L series are only used as general purpose steels for automobile exhaust systems, low temperature duct linings, etc.

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

By the way, the recent development of refining technology for ferritic stainless steel is remarkable.
It is possible to reduce C and N in steel (hereinafter referred to as high purification), which has been difficult in the past, and to control extremely low sulfurization and the addition amount of trace elements in steel. For example, SUS 430L series, SUS 436L
, SUS 444 series, SUS 447J1 series, etc., so-called high-purity ferritic stainless steels have been standardized. Therefore, for heat-resistant ferritic stainless steels, the development of high-performance steels backed by advanced refining technology is becoming active. For example, for supporting metal catalysts for automobile exhaust gas purification, through which combustion exhaust gas at about 950 ° C passes. There are practical applications such as foil. It is a high-purity ferritic stainless steel containing around 5% Al and around 20% Cr, and the decrease in base metal toughness due to high Al content can be achieved by minimizing the C and N in the 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 high temperature. The adhesion of scale is improved by REM of a small amount of Y, La, etc.
Is performed by adding.

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

As an additive element for improving the oxidation resistance of ferritic stainless steel, there is Si along with Al. However, when Si is added in a large amount, not only the toughness of the base material is significantly deteriorated, but also the productivity during hot work is significantly impaired. It didn't happen.

On this point, however, Japanese Patent Publication No. 62-14626
Japanese Patent Publication discloses a ferritic stainless steel containing Si in an amount of more than 1% and having excellent high temperature oxidation resistance.
O is less than 0.005% and S + O is less than 0.008%, the oxidation resistance at high temperature is said to be improved.

According to the above invention, S which is usually contained in the steel at about 0.01% is combined with Mn which is also present in the steel.
MnS is formed, but when the steel is exposed to high temperatures, MnS decomposes and decomposes into an oxide of Cr-Mn-O, free S is concentrated on the steel surface and grain boundaries, and Around inclusions
It is said that the formation of the Cr-deficient layer deteriorates the 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 O: 0.
It is said that if it is less than 005% and S + O is less than 0.008%, a remarkable improvement effect can be obtained. Moreover, the improvement effect by adding a sulfide that has a stronger bonding force with S than Mn such as Ca, Mg, rare earth elements, and Y and is more stable even at high temperature is clarified.

Further, Japanese Patent Laid-Open No. 60-145359 discloses 1.0-2.0 for parts of an automobile exhaust system in which cyclic high temperature oxidation and cooling are repeated, powder metallurgy products and welding products.
% High-temperature ferritic steels containing 6.0% Si and 6.0-20.0% Cr.

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

By the way, the combustion temperature of the turbine for power generation tends to increase with the recent technological innovation, and in particular, since liquefied natural gas is used as a fuel, the combustion temperature of the conventional 1100 ° C class is 1300 ° C.
The ℃ class and 1400 ℃ class are actually planned and installed. Under such circumstances, the heat recovery boiler that recovers the heat of the high-temperature exhaust gas at the turbine exit side also tends to become hot. For example, in the 1100 ° C class, the exhaust gas temperature, which used to be around 550 ° C, now exceeds 600 ° C, and the exhaust gas duct surface temperature also increases.
The temperature rose to around 650 ℃.

In such an environment, the conventional 409L-based material has a point that the performance is not sufficient in the following points. That is, in gas turbine type business power generation, continuous operation is extremely rare throughout the day, and it is common to start the turbine only during the daytime when power consumption increases and stop it at night. When such an operation type is adopted, 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 produces a red scale with poor adhesion when used in a combustion exhaust gas atmosphere containing a large amount of water, so the combustion exhaust gas is reheated and returned to the power generation turbine. In the power generation method, problems such as turbine damage due to peeling scale may occur. Also, in terms of high temperature strength, the Ti-containing 409L-based material had a problem for long-term use in a temperature range exceeding 600 ° C.

[0016]

SUMMARY OF THE INVENTION The general object of the present invention is to solve the problems of the prior art by improving the oxidation resistance at high temperature, the high temperature strength, the thermal fatigue property, and the condensation liquid corrosion resistance. It is to provide a ferritic stainless steel having sufficient performance.

A more specific object of the present invention is, for example, 600
Ferritic stainless steel with excellent oxidation resistance and high temperature strength in the high temperature combustion exhaust gas atmosphere of ℃ to 950 ℃, especially Laves that becomes a problem when used for a long time at 600 ℃ to 900 ℃ The degree of embrittlement due to phase precipitation and the decrease in high temperature strength are extremely small, and when used in high temperature combustion exhaust gas with a high water concentration of 10 to 20%,
The generated scale has excellent adhesion. For example, a combustion exhaust gas duct such as an exhaust side exhaust gas duct and an exhaust heat recovery boiler duct of a gas turbine for combined power generation that uses liquefied natural gas as a fuel, an automobile exhaust manifold and a front pipe. An object of the present invention is to provide a ferritic stainless steel which is suitable for an exhaust system member and has excellent oxidation resistance and high-temperature strength.

[0018]

[Means for Solving the Problems] The inventors of the present invention have conducted various studies in order to achieve such an object, and have a steel composition capable of exhibiting a desired effect even when used under the severe conditions as described above. The present invention has been found out and the present invention has been completed.

That is, the findings in the present invention are as follows. (i) It has sufficient corrosion resistance to condensate and
In order to produce a scale with good adhesion even in a high temperature environment of more than 600 ° C containing 20% of Si, Si: 0.80 to 1.50%, Cr: 11.0 to 21.0%, Al: 0.01 to 0.10
% Is effective.

(Ii) In order to impart excellent high-temperature strength and thermal fatigue properties, Nb and Ti which are stabilizing elements are added in combination within the following range to suppress precipitation of Nb-based Laves phase, It is possible to secure high temperature strength 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, stable Ti-C-S inclusions can be formed at high temperature, whereby S is fixed and solid solution is formed at the same time. C can be reduced, workability, toughness, and corrosion resistance of welded parts can be improved.
Recrystallization is facilitated by the decrease of solute C, and the amount of solute Nb that is not fixed as Nb carbide increases, so that high temperature strength,
Improved thermal fatigue properties.

(Iv) W, which has a smaller Laves phase formation tendency than Nb, is added in an amount of 0.1 to 2.0%, if necessary.
It is possible to secure higher temperature strength.

Here, the gist of the present invention is that, in 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 to 0.80%, Ni: 0 to 0.80%, W: 0 to 2.00%,
S: 0.020% or less, Fe and inevitable impurities: The composition is composed of the balance, Ti / S> 15 and C / S> 0.5 are satisfied, and Ti-C-S type inclusions are precipitated in the steel. It is a ferritic stainless steel with excellent oxidation resistance and high temperature strength.

Further, the present invention provides a gas turbine outlet duct, an exhaust heat recovery boiler duct, etc. for combined power generation using the liquefied natural gas as a fuel, which has an extremely high energy efficiency of 48% or more, having the above steel composition. This is a typical combustion exhaust gas duct.

Further, the present invention is an automobile exhaust system member represented by an exhaust manifold and a front pipe for automobiles which are excellent in high temperature strength, oxidation resistance and thermal fatigue characteristics. Furthermore, the present invention is an exhaust gas system device used in the same environment and is a material for welded steel pipe.

[0026]

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

C, N In steels, C and N are elements that deteriorate the toughness, workability and weldability of ferritic stainless steel. However, the presence of an appropriate amount of C has an effect of improving high temperature strength. Particularly, in the steel of the present invention, Ti-C-S 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 exert such an effect is 1/2 times or more, preferably more than 1/2 times the amount of S. The upper limit of C is 0.015% and the upper limit of N is 0.015%. Preferably, both C and N
It is 0.010% or less.

Si: Si tends to increase strength at normal temperature and high temperature and decrease elongation as the amount of addition 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 13Cr type ferritic stainless steel in Examples described later was changed at 900 ° C. The effect of improving the oxidation resistance is remarkable from around 1%. In the present invention, the amount of Si in steel is
It should be 0.80% or more and 1.50% or less. This is because the Si content in steel is 0.
This is because at 80% or more, the effect of improving the oxidation resistance due to the high Si content is clearly recognized, and when it is added in excess of 1.50%, the decrease in room temperature elongation becomes remarkable and the workability is required. This is because it is not suitable for the use of the invention steel. It is preferably 0.80 to 1.30%.

The upper limit of Mn: Mn is 0.8%. Mn in steel
Increasing the amount facilitates abnormal oxidation. It is preferably 0.6% or less.

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

Nb: Nb is one of the additive elements having the effect of stabilizing C in steel, as is the case with Ti and Zr. Nb carbide is superior to Ti in thermal stability at high temperatures. For 900
Suppresses crystal grain coarsening at or above ° C and suppresses deterioration of oxidation resistance and toughness due to coarsening. Further, it also has an effect of improving high temperature strength by solid solution strengthening as a substitutional element. Also in the steel of the present invention, it is an extremely important element, and at least 0.10% or more must be added in order to exert these effects.

However, as is already known, the presence of Nb produces a Laves phase (Fe ₂ Nb) when used in the temperature range of 900 ° C. or lower and 600 ° C. or higher, so that the toughness at 150 ° C. or lower is improved. May cause significant deterioration. The present invention is intended to provide a material suitable for use in an outlet side exhaust gas duct and an exhaust heat recovery boiler duct of a gas turbine for combined power generation using liquefied natural gas as a fuel, or an automobile exhaust manifold and a front pipe. Under the use conditions in which cooling to near room temperature and temperature increase are repeated as in these applications, embrittlement in a state of being cooled to around room temperature becomes an extremely serious problem.

The present inventors have found that 0.01C-13Cr-1Si-0.1Ti-0.0
The Nb content in the 1N system was varied and the embrittlement behavior due to Laves phase precipitation was studied in detail. When the Nb content exceeds 0.40%, embrittlement reaches a level that poses a practical problem. It was confirmed.

In FIG. 2, 25 using an aging material at 600 ° C. for 1000 hours is used.
The Charpy impact test result at ℃ is shown. Over 0.40%
It is confirmed that when Nb is present, it poses a practical problem. The “impact absorption energy value which is a problem in practical use” shown in the figure is an allowable level obtained by trial and error by the inventors through many years of experience, and is an index used for evaluating the present invention. one of.

FIG. 3 shows the Nb changing material at 600 ° C. ×
It shows the tensile strength at 600 ℃ after aging for 1000hr. The effect of Nb content on the high temperature strength after aging was small and showed almost the same tensile strength. It is shown that at a Nb content of 0.40% or less, which is less affected by embrittlement due to the Laves phase, the decrease in high temperature strength is small.

Ti: Ti stabilizes C and N in steel in the same manner as Nb, suppresses coarsening of crystal grains in welds, and improves intergranular corrosion resistance in a condensed water environment. This effect is 0.05%
It appears with the above additions. Addition of more than necessary is not preferable because rolling defects become remarkable and manufacturability is deteriorated. Therefore, the upper limit was made 0.30%.

In the present invention, Ti is used as a composite additive element with Nb. When Nb and Ti are added in combination,
Primarily Ti combines with N to form a nitride, and in the subsequent cooling process Nb and the remaining Ti combine with C, (Nb, Ti)
Form carbides. (Nb, Ti) Carbide is Nb or Ti
TiN that precipitates at a higher temperature than a single carbide and then precipitates
It is easy to deposit as a nucleus. Therefore, a region (interstitial free region) where solid solutions C and N do not exist is formed around it. As a result, the steel becomes soft and good workability and formability are obtained. When the Si content is as high as around 1% as in the present invention, the compound tends to become hard, and therefore this composite addition works effectively.

FIG. 4 shows a composition example showing a tensile strength of 150 N / mm ² at 600 ° C. after aging at 600 ° C. × 1000 hr for a series of steel compositions within and outside the range of the present invention, and after aging at 600 ° C. × 1000 hr. ² is a graph in which composition examples having an impact value of 20 J / cm ² or more at 25 ° C. and composition examples showing whether rolling flaws are remarkable, that is, good manufacturability are arranged by the Ti content and Nb content in steel. In the figure, the shaded area satisfies all of the above characteristics
This is the range of composition examples of Ti and Nb.

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

FIG. 5 shows the influence of the amount of Al in steel on the amount of scale delamination of 13Cr-1.2 Si type ferritic stainless steel in the atmosphere at 900 ° C. for 200 hours. The amount of scale peeling decreases remarkably as the amount of Al increases. This is Al
It has been confirmed that the addition of a small amount of the compound causes an internal oxidation tendency, and that the scale is "fixed" so that it is fixed.

In the present invention, the amount of Al in the steel is 0.01% or more and 0.10 or more.
% Or less. This is because even with a trace amount of Al of about 0.01%, the improvement effect is recognized, while on the other hand, even if added over 0.10%, no further improvement effect is recognized. Furthermore, it has the effect of improving the strength at high temperature due to the presence of a trace amount of Al in the steel.

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

Cu, Ni: Cu and Ni are desired additive elements which have the effect of increasing the room temperature tensile strength 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 in improving toughness even when added in a small amount. If it exceeds 0.80%, embrittlement may occur due to the formation of intermetallic compounds, so the upper limit is 0.80%.
And

W: W is also a desired additive element. In ferritic stainless steel, it is a substitutional solid solution element and is effective in improving high temperature strength by solid solution strengthening. Since W has a significantly slower diffusion rate than Nb, it is represented by the Laves phase.
The formation of intermetallic compounds with Fe is remarkably slow, the solid solution strengthening action is maintained for a long time, and the high temperature strength is maintained. However, addition of W reduces the workability at room temperature, and therefore a large amount of W is not preferable.

In the present invention, when W is added, the lower limit is 0.10% at which improvement in high temperature strength is confirmed, and the upper limit is 2.00% at which manufacturability and scrap recycling problems do not occur. It is preferably lower from the viewpoint of scrap reuse, and is preferably 0.30 to 1.50%. O (oxygen) is an unavoidable impurity, but since it adversely affects the oxidation resistance and toughness, the 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 that satisfies Ti / S> 15 is 0.
020% or less. The lower limit of the amount of S that satisfies this is about 0.
0033%.

Generally, as described in detail in Japanese Patent Publication No. 62-14626, S is considered to exist as sulfide in steel and decomposes at high temperature to promote oxidation. Has been. The inventors of the present invention added Ti in an appropriate amount and made S in steel that can be extremely lowered in refining Ti / S> 15
By controlling the upper limit to a high value of 0.020% or less, S in steel is fixed as a thermally stable Ti-C-S based Ti-based inclusion, which is obtained when the S is extremely low. It has been discovered that superior characteristics can be obtained over high temperature characteristics. Ti-C-S
The non-metallic inclusions are thermally stable even in the temperature range of over 1000 ° C, and are unlikely to cause deterioration of oxidation resistance due to decomposition.

Further, the Ti--C--S type inclusions finely dispersed in the steel also have the effect of increasing the high temperature strength and improve the thermal fatigue properties. This is for the following reason.

That is, when the solid solution C decreases due to the formation of Ti--C--S inclusions, the solid solution Nb which is not solid solution as Nb carbide is secured, the high temperature strength is maintained, and the steel quality becomes easy to recrystallize. Improves thermal fatigue properties. Also, solid solution C
Decrease the steel to improve the workability and formability.

In order to precipitate Ti—C—S inclusions including Ti—S inclusions, rolling may be performed after holding at a temperature range of 1050 ° C. or higher and 1180 ° C. or lower for 1 hour or more before rolling. ,
Precipitates during heating and rolling.

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

FIG. 8 shows the relationship between the amount of deposited S and the heating temperature by chemically depositing the deposit in the case of FIG. 7 and then analyzing the amount of S in the deposit. 8, heating temperature 1100 ℃, 1150 ℃,
The material hot-rolled at 1200 ° C and 1230 ° C was subjected to hot-rolled sheet annealing-cold rolling-finish annealing to prepare a cold-rolled sheet having a plate thickness of 2 mm, and subjected to a room temperature tensile test and a thermal fatigue test. The test method was the same as in the example.

The results are shown in FIGS. 9 and 10. 9 and 10
From the above, it can be seen that as the amount of precipitated S, that is, the amount of Ti—C—S inclusions, increases, the room temperature elongation and the thermal fatigue strength are improved. Furthermore, if added, Nb and Ti combined addition and Ti-
The reduction of the solid solution C caused by the formation of C-S softens the steel to improve the workability and formability, and also suppresses the sensitization in the weld zone to improve the intergranular corrosion resistance in the condensed water environment. It also has an improving effect.

[0055]

【Example】

[Example 1] Each of the test steels shown in Table 1 was used in a vacuum melting furnace.
Kg was melted and a round ingot was cast. Then, after hot forging to a thickness of 30 mm, hot rolling was performed to obtain a hot rolled steel sheet having a thickness of 6 mm. The heating conditions for hot rolling were 1140 ° C. × 1.5 hours in the atmosphere. The hot-rolled sheet was annealed at 980 ° C. for 3 minutes and then forced air cooling. After descaling with an aqueous solution of nitric hydrofluoric acid, a plate with a thickness of 2 mm was cold-rolled, and finish annealing was carried out at 980 ° C. for 1 minute. From this, JIS1 for normal temperature tension
A No. 3B test piece was taken in the T direction to obtain a test piece having a thickness of 2 mm, a width of 20 mm, and a length of 25 mm 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 like No. 11 was prepared.

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

The oxidation test conditions were 900 ° C. × 200 hours and continuous heating in the atmosphere. The high temperature salt damage test condition was the cycle shown in FIG. The thermal fatigue test uses the thermal fatigue test piece shown in FIG. 11, and the temperature cycle shown in FIG. 13 and the mechanical strain waveform history are taken by the computer-controlled electrohydraulic high temperature thermal fatigue tester under the conditions of 200 to 900. It was carried out at 50 ° C. and 50% restraint (η = 0.501).

Table 2 shows the results of these tests. Room temperature elongation 30% or more, oxidation increase 1.5 mg / cm ² or less, scale peeling 0.5 mg / cm ² or less, plate thickness reduction after salt damage test 45
Passes of 0 μm or less and thermal fatigue life of 800 cycles or more were accepted.

It can be seen that the steel of the present invention has excellent properties such as room temperature elongation, oxidation resistance, high temperature salt corrosion resistance, and thermal fatigue properties. As is clear from comparison with Comparative Steels 24 and 25, the steels of the present invention have improved room temperature elongation and thermal fatigue properties by containing S satisfying Ti / S> 15 at S <0.020%. Further, as shown in Invention Steels 2, 9, 10, 12, and 14, the thermal fatigue properties are further improved by adding 0.1% or more and 0.2% or less of W.

Comparative steel 18 is SUH409L equivalent steel, but is inferior in oxidation resistance, high temperature salt damage resistance and thermal fatigue properties. Comparative steel 19
Has poor oxidation resistance because Si is less than 0.8%. Comparative Steel 20 has low room temperature elongation and poor workability because Si exceeds 1.5%. Although Comparative Steel 21 contains 0.65% of Nb, the effect of improving the thermal fatigue property is not recognized and the room temperature elongation is low and the workability is poor.
In Comparative Steel 22, since Al is less than 0.01%, oxide scale is almost peeled off and scale adhesion is poor. As described above, the results of this example are shown in the graphs of FIGS. 1 to 6, and the subscripts in each graph indicate the test steel No.

[Example 2] In this example, a mass production trial was performed using an 80 ton AOD refining furnace. Table 3 shows the compositional components of the two test steels. In the trial production method, VOD refining and continuous casting were carried out under the same conditions as in Example 1, a hot rolled steel sheet with a thickness of 6 mm was manufactured, and further cold rolled to a thickness of 2 mm. The cold-rolled steel sheet was subjected to finish annealing at 980 ° C. for 1 minute, and then descaled with an aqueous solution of nitric hydrofluoric acid.

The above-mentioned mass-produced trial materials are LN for all steel types.
It is applied to the high temperature side (near 600 ° C) in front of the denitrification equipment of the exhaust heat recovery boiler duct of the combined heat and power plant of the industrial complex consisting of the gas turbine using G as fuel, the exhaust heat recovery boiler and the steam turbine Was offered. Similarly, the welded high fin tube was produced and applied to the fin portion of the welded high fin tube without any problem due to its excellent cold workability.

[0063]

[Table 1]

[0064]

[Table 2]

[0065]

[Table 3]

[0066]

As described above, the steel of the present invention is 600
It has excellent oxidation resistance and high temperature strength in the high temperature combustion gas atmosphere of ℃ to 950 ℃ and below, and other characteristics can be fully satisfied as a material for exhaust heat recovery boiler duct and fins, and the material cost is not increased significantly. Durability can be improved. Further, the steel of the present invention can exhibit excellent properties as an automobile exhaust system material such as an automobile exhaust manifold and a front pipe.

[Brief description of drawings]

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

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

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

FIG. 4 is a graph showing the correlation between the Ti amount and the Nb amount 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 Al content in steel at 700 ° C. and high temperature tensile strength.

7 (a) and 7 (b) are sketches showing Ti ₄ C ₂ S ₂ and (Nb, Ti) C precipitation examples, respectively.

FIG. 8 is a graph showing the relationship between the quantitative analysis value of S by the extraction residue analysis and the 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 thermal fatigue strength.

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

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

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

Front page continuation (72) Inventor Shinji Tsuge 4-53-3 Kitahama, Chuo-ku, Osaka City Sumitomo Metal Industries, Ltd.

Claims (3)

[Claims] 1. 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 to 0.80%, Ni: 0 to 0.80%, W: 0 to 2.00%, S: 0.020% or less, Fe and inevitable impurities: from the balance Has the composition of Ti /
Ferritic stainless steel excellent in oxidation resistance and high temperature strength, characterized by satisfying S> 15 and C / S> 0.5 and having Ti-C-S inclusions precipitated in the steel. 2. A combustion exhaust gas duct made of the ferritic stainless steel according to claim 1. 3. An automobile exhaust system member made of the ferritic stainless steel according to claim 1.