JP2896077B2 - Ferrite stainless steel with excellent high-temperature oxidation resistance and scale adhesion - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a low-cost ferritic stainless steel excellent in high-temperature oxidation resistance and scale adhesion, which is particularly suitable for exhaust gas pipe members such as various internal combustion engines and gas turbines.

[0002]

2. Description of the Related Art In recent years, interest in environmental issues has increased, and there has been a demand for a thermal power generation system and an engine having good combustion efficiency and an automobile engine capable of meeting exhaust gas regulations. If measures are taken to satisfy these requirements, the temperature of the combustion gas will increase, and the temperature of peripheral members such as the exhaust gas purification system will increase. As a result, these members are required to have better heat resistance. For heat resistance, in addition to high temperature strength, high temperature oxidation resistance that can be used in a high temperature gas environment is required.

[0003] The high-temperature oxidation resistance is that the oxidation increase is small without causing abnormal oxidation and that the adhesion of the oxide scale (oxide film) is good. Internal combustion engines such as automobile engines are repeatedly operated and stopped, and even in thermal power generation systems, DSS (Daily Start and Stop) operations are performed, so that heat-resistant members are repeatedly subjected to overheating and cooling. Therefore, the material with poor adhesion of the oxide film peels off the oxide film, which causes clogging of the piping and a decrease in the thickness of the member itself, which causes problems such as breakage from the starting point. .

[0004] Austenitic stainless steel has higher high-temperature strength than ferritic stainless steel. However, since thermal expansion is large, thermal strain is large, and cracking due to thermal fatigue is likely to occur when subjected to repeated heating and cooling. Further, austenitic stainless steel has a large difference in thermal expansion between the steel base and the oxide scale, so that the oxide film often peels off.

[0005] For these reasons, ferrite stainless steel is used as an exhaust gas material for automobiles. For example, SUS430J1L ferrite stainless steel is used for the exhaust manifold of automobiles. However, it is considered that the oxide film is often peeled off and the cost of the material is high.

US Pat. No. 4,640,722 discloses that a ferrite stainless steel suitable for a material for automobile exhaust gas contains Si in the range of 6 to 25% of Cr instead of Al used in conventional heat-resistant steel. (Si: 1.0-2.0
Wt%), and Ti (or Zr, Ta) sufficient to fix carbon and nitrogen is added (Ti: 4C + 3.5 N-0.5
%), 0.1% by weight of non-bonded Nb not forming carbonitride
By the above, Nb-S by heating at 1010 ~ 1120 ℃
Disclosed is a steel that forms an i-rich Laves phase to improve high temperature oxidation resistance and creep properties. This steel further contains 5% or less of Mo, and specifies that Cr + Mo ≧ 8% by weight. However, this USP '722 does not teach how to prevent oxide film peeling. There is no teaching about improving low-temperature toughness and workability. Exhaust manifold applications in automobiles require not only high-temperature oxidation resistance, but also excellent adhesion of oxide films, low-temperature toughness and workability.

US Pat. No. 4,461,811 discloses that C.ltoreq.0.03%, N.ltoreq.0.05%, Cr: 10.5-13.5%, Al by weight.
≦ 0.10%, Ti ≦ 0.12%, Al + Ti ≦ 0.12%, Nb and / or Ta: an amount sufficient to fix C and N, the balance being Fe
Ferrite stainless steel consisting of
It teaches that the steel has good wettability with brazing fillers such as Cu and Ni. For this reason, it is said that it is suitable for brazing applications such as heat exchangers and exhaust gas systems that require oxidation resistance and corrosion resistance at high temperatures inherent to ferrite stainless steel. However, it is unclear whether the stabilized steel described in this USP'811 specification simultaneously satisfies the adhesion, low-temperature toughness and workability of the oxide film.
In addition, there is no suggestion or recognition about the treatment.

US Pat. No. 4,417,921 discloses that C ≦ 0.03%, N ≦ 0.03%, C + N ≦ 0.04%, Cr: 1
1.5-13.5%, Mn ≦ 1.0%, Si ≦ 1.0%, Ni ≦ 0.5
%, Cu ≦ 0.15%, Ni + 3Cu ≦ 0.80%, Ti and / or Nb: 0.1% or more and 4 (C + N) or more to 0.75%
%, With the balance being Fe. The steel in which C and N are fixed by Ti or Nb and Cu is added is excellent in weldability, ductility, workability, and stress corrosion cracking resistance, and is considered to be suitable for heat exchanger applications in which fins are integrally formed. . However, USP'921 does not teach the high-temperature properties of this type of ferritic stainless steel, especially the effect of each element on the oxidation resistance at high temperatures and the adhesion of the oxide film, and is not necessary for exhaust manifold applications in automobiles. There is no suggestion about the properties.

[0009]

From the above background, an inexpensive material that has the same high-temperature strength as SUS430J1L, but also has better high-temperature oxidation resistance, especially excellent properties of oxide film adhesion. Ferritic stainless steels that are excellent in low-temperature toughness and workability have been required for exhaust gas applications, especially for exhaust manifolds for automobiles. This requirement has become more severe with recent improvements in exhaust gas purification and higher internal combustion engine efficiency. An object of the present invention is to provide a ferritic stainless steel satisfying this requirement.

[0010]

According to the present invention, in mass%, C: 0.03% or less, Si: 0.80% to 1.20%,
Mn: 0.60% to 1.50%, Cr: 11.0% to 15.5%, Nb: 0.2
0% to 0.80%, Ti: 0.1% or less (including no addition), Cu:
0.02% to less than 0.30%, N: 0.03% or less, Al: 0.05% or less (including no addition), O: 0.012% or less, provided that 0.7 ≦ Mn / Si ≦ 1.5 (1) ) 1221.6 (C + N) -55.1Si + 65.7Mn-8.7Cr-99.5Ti
-40.4Nb + 1.1Cu + 54 ≦ 0 ··· (3) Relationship (1) containing these elements so as to satisfy our and the (3) At the same time, the balance being Fe and unavoidable impurities, 900 under an air atmosphere After continuous heating at 100 ° C for 100 hours, the increase in oxidation is 0.02 kg / m ² or less, and the amount of scale peeling is 0.01 kg / m ² or less.
1000 oxidation weight gain after 100 hours of continuous heating at ℃ provides a ferritic stainless steel scale peeling amount with excellent high-temperature oxidation resistance and scale adhesion is 0.02 kg / m ² or less at 0.4 kg / m ² or less.

[0011]

[0012]

[0013]

[Action] For ferrite stainless steel,
As described in JP-A-59-15976, it is well known that when a rare earth element such as La, Ce, or Y is contained, good high-temperature oxidation characteristics are exhibited. Also, as described in JP-B-57-2267, it is known that oxidation resistance, formability and weldability can be improved by reducing C, N and Mn and increasing the Si content. 4,640,7
As described in the specification of JP-A No. 22 and JP-A-60-145359, it is known that Al which is effective for oxidation resistance is replaced with Si to maintain oxidation resistance. The present inventors have found that the high-temperature oxidation characteristics (suppression of oxidation increase and scale adhesion) of ferritic stainless steel can be improved by a completely different treatment method. That is to strictly adjust the mutual content of Mn and Si to a certain specific range.

That is, the present inventors have proposed a low-cost 13Cr
After conducting extensive research on alloy components to suppress abnormal oxidation and to improve the adhesion of excellent oxide films, focusing on ferritic stainless steels, it was found that in order to suppress abnormal oxidation Was found to be effective. However, it was found that the addition of Si can suppress abnormal oxidation and reduce the amount of oxidation increase, but the generated oxide has the property of easily peeling off during the cooling process, as in the case of SUS430J1L.

However, it has been found that the addition of an appropriate amount of Mn significantly improves the adhesion of the oxide film. This is a completely new finding that overturns the common belief that Mn has an adverse effect on high-temperature oxidation in high Cr ferritic stainless steels.

However, it has also been found that when a large amount of Mn is added, an austenite phase is formed in the present component system to deteriorate the high-temperature oxidation resistance, and that abnormal oxidation occurs starting therefrom.

FIG. 1 shows that a continuous oxidation test was performed at 1000 ° C. for 100 hours on a ferrite stainless steel having the chemical composition values specified in the present invention except that the Mn / Si ratio was changed. In this case, the amount of increase in oxidation and the amount of scale exfoliation in the above case are arranged and shown by the Si / Mn ratio.

As can be seen from FIG. 1, the Mn / Si ratio is 0.7
When the above is 1.5 or less, both the increase in oxidation and the amount of scale peeling are extremely reduced. If this ratio is less than 0.7, the amount of scale peeling increases sharply, and if it exceeds 1.5, the oxidation increase rapidly increases.

The reason for this is not always clear, but is considered as follows. Although the high-temperature oxidation resistance when the Si amount is large is improved, which is Cr ₂ O ₃ by increasing the Si
It is considered that an oxide mainly composed of is formed in the surface layer. However, simply adding Si causes scale peeling. This is considered to be due to the difference in the coefficient of thermal expansion between the oxide mainly composed of Cr ₂ O ₃ and the base material of the lower layer.

However, when Mn is present so that the Mn / Si ratio becomes 0.7 or more, a spinel-based Mn-containing spinel material having an intermediate thermal expansion coefficient between an oxide mainly composed of Cr ₂ O ₃ and a steel base material is used. Oxide forms. As a result, even if the amount of oxidation increases due to the increase in Mn, the generated oxide has a reduced thermal expansion difference with the steel base, so that the adhesion is improved. However, when the Mn content is such that the Mn / Si ratio is higher than 1.5, even if the adhesion of the scale is good, abnormal oxidation occurs and a problem occurs in heat resistance. For this reason, in the ferritic stainless steel of this type, if the Mn / Si ratio is strictly adjusted to be in the range of 0.7 to 1.5, the suppression of the increase in oxidation and the improvement of the scale adhesion can be achieved at the same time. It becomes oxidative.

In other words, it is necessary to increase the amount of Mn with the amount of Si in order to improve the adhesiveness of the scale by forming a large amount of Mn-based oxides. Therefore, it is necessary to reduce the amount of Mn. In a steel with a small Si content, when the Mn content is large, a γ phase is likely to be generated, and becomes a starting point of abnormal oxidation. In addition, the amount of Mn-based spinel oxide itself increases, leading to abnormal oxidation.

The actions of each component in the steel of the present invention and the reasons for limiting their content (% by mass) will be individually outlined below.

C and N: C and N are generally important elements for increasing the high-temperature strength, but on the other hand, when the content is large, the oxidation resistance, workability and toughness are reduced. In addition, C and N form a compound of Nb and reduce the effective Nb content in the ferrite phase, which acts to improve the high-temperature strength.
For these reasons, each of C and N is set to 0.03% or less.

Si: Si is an element indispensable for improving high-temperature oxidation resistance as described above. Even a steel having a relatively small amount of Cr, such as the steel of the present invention, is very effective in imparting excellent high-temperature oxidation resistance. However, if added excessively, it becomes hard and causes deterioration of workability and toughness. Therefore, the content is set in the range of 0.8% to 1.2%. The optimum content of Si is about
It is around 1.0%.

Mn: Mn is also an important element of the steel of the present invention.
By adding Si as in the steel of the present invention, the increase in oxidation is suppressed, but the generated oxide is easily separated during cooling after heating. When Mn is added, a spinel oxide is formed as described above, and the adhesion of the surface oxide is remarkably improved. However, excessive addition induces abnormal oxidation rather than precipitation of austenite phase. Therefore, the range is set to 0.60% to 1.50%. The optimum content of Mn is around 1.0%.

Cr: Cr is a very effective element for imparting high-temperature oxidation resistance. To maintain high-temperature oxidation resistance, addition of 11% or more is required. On the other hand, if it is added excessively, the steel becomes brittle, becomes hard and deteriorates workability, and the raw material price increases. Therefore, the range of Cr is 11.0% to 15.0%, preferably more than 13.5% and 15.5% or less. In particular, in exhaust manifold applications, the oxidation weight gain after continuous heating at 950 ° C for 200 hours is 0.2 kg / m ^2.
In order to satisfy the requirement of not more than 0.01 kg / m ² or less, the Mn / Si ratio becomes almost 1, and both Mn and Si are contained at about 1.0%, and the total content of Si + Mn + Cr is It is desirable that the content be 15.5 or more. In this case, however, it is necessary that the Cr content inevitably exceeds 13.5%. The optimum Cr content is around 14%.

Nb: Nb is an important element of the steel of the present invention because it works effectively to maintain high-temperature strength. In order to maintain high-temperature strength, it is necessary to add at least 0.20% or more. On the other hand, if Nb is added excessively, the susceptibility to welding hot cracking increases. The upper limit of Nb is set to 0.80% so as to maintain sufficient high-temperature strength and not significantly affect the susceptibility to welding hot cracking. The preferred lower limit of the Nb content is 8 ×
(C + N) +0.30, the upper limit of which is 0.60%.
The optimum value of the Nb content is around 0.50% when both C and N are as low as 0.015% or less.

Cu: Cu works very effectively in the steel of the present invention to improve both low-temperature toughness and workability. This fact is shown below in the test results.

The tests were performed for 14% Cr, 1.0% Si, 1.0% M
The effect of Cu on the fracture surface transition temperature was examined by changing the content of Cu and using 0.5% Nb steel as the base steel. FIG. 2 shows the test results. The fracture surface transition temperature was measured using a 2 mm V-notch Charpy impact test specimen from -75 ° C to 50 ° C.
The impact test was performed in the range of ° C., and the temperature was defined as the temperature at which the ductile fracture ratio became 50%. The fracture surface transition temperature, which is an index of low-temperature toughness, is preferably −30 ° C. or lower. As can be seen from FIG. 2, it can be seen that the fracture surface transition temperature is −30 ° C. or lower when the Cu content is in the range of 0.02 to less than 0.30%. Note that C
When the content of u is set to 0.30% or more, the toughness is slightly improved as compared with the case where Cu is not added, but it is also clear that the fracture surface transition temperature tends to be increased.

The same 14% Cr, 1.0% Si, 1.0
% Mn and 0.5% Nb were used as basic steels, and the effect of Cu on the total elongation and uniform elongation was examined by changing the Cu content.
The result is shown in FIG. The total elongation and the uniform elongation were determined by taking a specimen from a cold-rolled annealed sheet having a thickness of 2 mm and performing a tensile test in a direction parallel to the cold rolling direction (L direction) at a strain rate of 3 mm / min. As can be seen in FIG.
It can be seen that the total elongation increases and the uniform elongation, which is an index of workability, increases when the content of is between 0.02% and less than 0.30%.

As described above, in the steel of the present invention, Cu
It was found that when contained in a range of not less than 0.3% and less than 0.30%, low-temperature toughness and workability were simultaneously excellent. With such a small amount of Cu, there is almost no adverse effect on the high-temperature characteristics due to the addition of Cu (for example, a decrease in hot workability).

O: O (oxygen) has a bad effect on weldability, so it is preferable that O (oxygen) is as low as possible. However, the lower the value, the higher the manufacturing cost. In the steel of the present invention,
O can be easily reduced by adding Al and Si. At this time, O is set to 0.012% or less as a range having sufficient weldability.

Ti and Al: Ti and Al can be allowed up to 0.10% each in the steel of the present invention, with or without addition. It is known that Ti improves the r value (Rankford value) of steel and improves steel formability. However, when Ti is added, steel sheet production due to generation of steel sheet surface flaws (scab flaws) due to generation of TiN. Yield decreases and weldability also decreases. In particular, when TiN is generated during welding during pipe making for manufacturing an exhaust manifold or during welding for assembly, adverse effects are exerted when severe processing is performed thereafter. Therefore, the Ti content in the steel of the present invention is preferably 0.10% or less, and more preferably 0.05% or less, and such a Ti content is acceptable as an impurity in the steel of the present invention.

Further, Al is useful as a deoxidizing agent for removing residual oxygen during melting of steel. That is, if oxygen remains in the steel, the weldability deteriorates, so that Al deoxidation is useful. However, since the steel of the present invention contains Si, this Si functions as a deoxidizing agent, and deoxidation by Al is not necessarily performed. do not need. If Al is excessively mixed into the steel, a large amount of Al-based oxide is generated during welding, which results in deterioration of weldability. Therefore, the content of Al is preferably set to 0.05% or less irrespective of the presence or absence of addition, and this amount of Al is acceptable in the steel of the present invention.

P, S, Ni, and the like are other impurities that are mixed in during production. Since none of these elements provides useful effects in the steel of the present invention, the smaller the better, the better. However, in the steel of the present invention, P is up to 0.040% and S is
Even if the content is up to 0.008% and Ni up to 0.50%, no particular adverse effect is exhibited. Therefore, the content of these elements to this extent is permissible.

It is an object of the present invention to regulate the amount of Mn and the amount of Si so that the relationship of 0.7 ≦ Mn / Si ≦ 1.5 (1) is satisfied in the content of each component as described above. Important to achieve
If the condition of equation (1) is satisfied, as shown in FIG.
100 hours of ferritic stainless steel oxidation weight gain after continuous heating and descaling amount 0.4 kg / m ² or less with excellent high-temperature oxidation resistance and scale adhesion which is a 0.02 kg / m ² or less can be obtained at ℃ . Note outcome of FIG. 1, when optimize the Mn / Si ratio, the high-temperature oxidation resistance and to a small value to the far than the upper limit value 0.02 kg / m ² of an upper limit value 0.4 kg / m ² and scale peeling of oxidation weight gain This shows that the scale adhesion can be improved.

Further, the steel according to the present invention has the above-mentioned relational expression (1)
In addition to the above, adjusting the amounts of the respective components so as to satisfy the requirements of the relational expression ( 3 ) plays an important role in solving the above problem. Furthermore, to obtain excellent high temperature fatigue properties,
Equation (2), that is, 1.4 ≦ Nb + 1.2Si ≦ 2.0,
To provide high temperature oxidation resistance required for manifolds
Satisfies relational expression (4), that is, Cr + Mn + Si ≧ 14.7
is important. These points are clear from the examples described later, and the outline thereof will be described below in advance.

When Nb and Si are added in combination to satisfy the relational expression (2): 1.4 ≦ Nb + 1.2Si ≦ 2.0 (2), the steel of the present invention exhibits excellent high-temperature fatigue properties. . This effect is N
It is expressed when the amount of b + 1.2Si is 1.4 or more. However, if both Nb and Si are added in excess, they have the effect of reducing workability. For this reason, the amount of Nb + 1.2Si is preferably kept within 2.0%.

The relational expression (3), that is, 1221.6 (C + N) -55.1Si + 65.7Mn-8.7Cr-99.5Ti-40.4Nb + 1.1Cu + 54.ltoreq.0 (3). As a result, in the steel of the present invention, no austenite phase is formed in a temperature range up to 1000 ° C. In the case of the exhaust manifold, it is necessary to consider the temperature range up to 1000 ° C from the material side, but if the austenite phase is formed at this service temperature, abnormal oxidation starting from the austenite phase occurs. If the components are balanced so as to satisfy the relationship of the relational expression (3), this abnormal oxidation can be prevented.

It is required for the exhaust manifold to strictly adjust the total amount of Cr, Mn, and Si so as to satisfy the relational expression (4), that is, Cr + Mn + Si ≧ 14.7 (4). It was found to be important in providing high-temperature oxidation resistance. Hereinafter, this point will be described with reference to test results.

The test steel was composed of Cr: 11.0 to 15.5%, Si: 0.8
, 1.2%, Mn: 0.7-1.5%, and the Cr, Si, and Mn contents were varied, and Nb was 0.5% and Cu was 0.1%, and each of these test steels had a constant (Cr + Mn + Si). ) And the high-temperature oxidation resistance were examined. In the test, a 2 mm-thick plate-shaped specimen was continuously heated in an air atmosphere for 200 hours for each steel, and then the mass increase per unit area was measured. The results are shown in FIG. 4 and FIG. Fig. 4 shows the case where the continuous heating temperature = 930 ° C, and Fig. 5 shows the case where the continuous heating temperature = 9
At 50 ° C.

From the results shown in FIGS. 4 and 5, the oxidation increase as an index of the high-temperature oxidation resistance is (Cr + Mn + Si) in the steel.
It can be seen that the total amount can be well organized. Assuming that the target of the increase in oxidation that causes abnormal oxidation is 0.2 kg / m ² , FIG.
As shown in the figure, Cr, Si, M
The total amount of n is 14.7 or more by mass%, and 950 ° C as shown in Fig. 5.
It was found that the total amount was 15.5 or more in continuous heating for 200 hours, and that abnormal oxidation could be suppressed.

Therefore, from the test results, it can be seen from the test results that in the steel of the present invention, the formula (4) is used under the continuous heating condition at 930 ° C., and the formula (4) ′ is obtained under the continuous heating condition at 950 ° C. (4) It was found that if the relationship of Cr + Mn + Si ≧ 15.5 (4) ′ was satisfied, excellent high-temperature oxidation resistance could be obtained at each temperature.

The ferritic stainless steel of the present invention in which the respective components are balanced as described above has excellent high-temperature oxidation resistance and scale adhesion at the same time, has excellent low-temperature toughness, workability, high-temperature strength and High temperature fatigue properties are also good. Moreover, it can be manufactured at lower cost than 18Cr stainless steel. In general, exhaust gas pipe members have welds, but the steel of the present invention has good thermal fatigue characteristics of welds.

The steel of the present invention having such good properties at the same time is a material suitable for an exhaust manifold application which is directly connected to an automobile engine and has a high temperature. Exhaust manifolds are manufactured by processing and welding pressed plates or pipes previously formed by high-frequency welding to the required shape and dimensions.In use, they are exposed to vibration and high-temperature exhaust gas. receive. The steel of the present invention has sufficient durability and is less expensive than conventional materials in such applications, as will be shown in Examples described later.

Not only the exhaust manifold, but also the low-cost ferritic stainless steel of the present invention is 700 ° C to 950 ° C.
Suitable for use at high temperatures of ℃ and where high-temperature oxidation resistance and scale peeling are considered important, such as the outer cylinder of a metallic converter in the exhaust pipe of an automobile engine, the exhaust pipe of a thermal power generation system, etc. Can be used.

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

[0048]

EXAMPLES Tables 1 to 3 show chemical component values (% by mass) in steel of test materials. In these tables, F01 to F10, E01 to E08, G01 to G07, and A1 to A7 are steels of the present invention. From F11 to F17, E09 and E1
0, and G08 are steels (comparative steels) outside the range specified in the present invention. All steels were melted in a vacuum melting furnace, and hot-rolled steel strips with a thickness of 4.5 mm were forged and hot-rolled. This is 105
Annealed at 0 ° C and made a 2.0 mm thick cold-rolled steel strip.
Annealed at 50 ° C. Each of the cold-rolled and annealed materials was processed into various test specimens, which were then tested. In addition, F01 and F14 were used for grasping the thermal fatigue characteristics using the high-frequency pipe making pipe.

900 ° C. of the steels of the present invention and comparative steels shown in Tables 1 to 3
Tables 4 and 5 show the results of a 100-hour continuous oxidation test at 1000 ° C. and 1000 ° C. The high-temperature oxidation resistance was evaluated by the amount of oxidation increase and the amount of scale peeling. That is, a test piece having a length of 35 mm, a width of 25 mm, and a thickness of 2.0 mm was used, and a continuous oxidation test was performed at each temperature for 100 hours. The scale peeling amount was measured by collecting and measuring the weight of the oxide scale that had naturally peeled off from the specimen surface during cooling after the oxidation test.
The amount of peeling per unit area was determined. In the case of abnormal oxidation indicated by the mark x in Table 2, the bump-shaped oxide covered the test piece, and it was judged that it was not appropriate to evaluate the oxidation resistance by the amount of scale peeling. .

Table 6 shows the test results of low-temperature toughness and workability, and the results of high-temperature tensile and high-temperature fatigue tests on typical steels of the present invention and comparative steels. These test conditions are as follows.

The low-temperature toughness was evaluated based on the fracture surface transition temperature. In other words, a V-notch test piece with a thickness of 2.0 mm conforming to “JIS Z 2202” was prepared, and the metal material impact test method (Charpy impact test) specified in “JIS Z 2241” was applied in the temperature range of -75 ° C to 50 ° C. The temperature at which the brittle fracture rate was 50% was defined as the fracture transition temperature.

The workability was evaluated by a tensile test and a bending test.
That is, a tensile test piece conforming to “JIS Z 2201 No. 13B” and a metal material bending test piece conforming to “JIS Z 2204 No. 1” were prepared, and the elongation (total elongation) in the tensile test specified in “JIS Z 2241” was performed. And uniform elongation) and the bending angle in the bending test of the bending test specified in "JIS Z 2248".

The high temperature tensile properties were evaluated by a 0.2% proof stress at 700 ° C. and 900 ° C. by a high temperature tensile test in accordance with “JIS G 0567”. The high-temperature fatigue properties were determined by conducting a plane bending fatigue test in accordance with “JIS Z 2275” at a maximum stress of 180 N / m at 600 ° C.
m ² , average stress 0 N / mm ² , repetition rate 40 Hz
Maximum stress 30 N / mm ² at 900 ° C., an average stress 0 N / mm ^2, carried out under the condition of repetition rate 60 Hz, breakage repetition number is determined as good ones 10 ⁷ or more.

Table 7 shows the results of the thermal fatigue test using the pipes of the invention steel and the comparative steel. Thermal fatigue test is φ42.7mm
For high-frequency tube-forming pipes under stress, the minimum temperature is 200
And a heating / cooling cycle having an upper limit temperature of 900 ° C. were repeatedly applied. The heating and cooling rates were 3 ° C./min, and the holding time at the upper and lower temperature limits was 0.5 min. The stress application is the constraint rate (the ratio of the applied strain to the free thermal expansion of the material)
50%. The test results were evaluated based on the number of failure cycles (the number of cycles when the maximum tensile stress during the test was reduced to 75% of the initial stress) and the scale adhesion state of the surface visually.

[0055]

[Table 1]

[0056]

[Table 2]

[0057]

[Table 3]

[0058]

[Table 4]

[0059]

[Table 5]

[0060]

[Table 6]

[0061]

[Table 7]

As can be seen from the results in Tables 4 and 5, the steel of the present invention had an oxidation weight gain of not more than 0.02 kg / m ^{2 in} the continuous oxidation test at 900 ° C. and 0.4 kg / m ^{2 in} the continuous oxidation test at 1000 ° C. It shows very good high temperature oxidation resistance of m ² or less. At the same time, it excels in scale peeling resistance. At 900 ° C, it does not peel at all, and even at 1000 ° C, the amount of scale peeling is as small as 0.02 kg / m ² or less. As mentioned above, the addition of Si effectively suppresses the oxidation increase, and the addition of Mn effectively suppresses the scale exfoliation, as described above. Both of these characteristics depend on the Mn / Si ratio. Ruled.

Looking further at the results in Tables 4 and 5, it can be seen that Cr, Mn,
Steel with a total amount of Si of 14.7 or more has an oxidation weight of 0.2 kg / m ² or less even after continuous heating at 930 ° C for 200 hours.
No abnormal oxidation has occurred. Steel with a total amount of Cr, Mn, and Si of 15.5 or more has an oxidation increase of 0.2 kg / m ² or less even after continuous heating at 950 ° C. for 200 hours, and does not show abnormal oxidation. All of these steels that do not cause abnormal oxidation have good scale adhesion.

On the other hand, as shown in the comparative steel G08, when the amount of Si and the amount of Mn are almost the same as those of ordinary ferritic stainless steel, the Mn / Si ratio falls within the range specified in the present invention. Also, since the amounts of both elements are lower than the lower limits specified in the present invention, abnormal oxidation has already occurred at 900 ° C., and the amount of scale peeling is remarkable. Comparative steel F12 is S
Since the i content is less than the lower limit specified in the present invention, the other components have caused abnormal oxidation in the oxidation test at 1000 ° C. even in the range specified in the present invention. Although the comparative steel F14 contains the Si content within the range specified in the present invention, almost all of the oxides are separated because the Mn amount that suppresses scale separation is less than the lower limit specified in the present invention. .

This tendency becomes more remarkable when the correlation between Mn and Si is viewed. For example, a steel such as F11 in which Si is larger than the upper limit of the present invention, a steel such as F14 in which the Mn content is lower than specified in the present invention, and a Mn / Si ratio such as F16 in which the Mn / Si ratio is higher than the ratio specified in the present invention. In all small steels, the relative amount of Mn to the Si amount is smaller than an appropriate range, so that the amount of scale peeling is large, and at 1000 ° C., abnormal oxidation may be caused.

On the other hand, a steel having a higher Mn content, such as F13, as defined in the present invention and a steel, such as F15, having an Mn / Si ratio higher than the ratio specified in the present invention, have Mn added to Si added. Although the amount is large, the amount of scale peeling at 900 ° C is suppressed, but the amount of oxidation increase is large and abnormal oxidation occurs at 1000 ° C.

Further, F17 which does not satisfy the requirement of the above-mentioned equation (3) (the value of the equation (3) is indicated by G in Tables 1 to 3) is 900
An austenite phase (martensite phase when observed at room temperature) is formed in the temperature range of ℃ to 1000 ℃, and abnormal oxidation occurs starting from the austenite phase. For this reason, both the amount of oxidation increase and the amount of scale peeling are large, and the high-temperature oxidation characteristics are essentially inferior.

On the other hand, from the results of the low-temperature toughness and workability tests shown in Table 6, the steels of the present invention E01 to E08 and A1 to A7 all have very low fracture surface transition temperatures of -40 ° C. or less, and are excellent in low-temperature toughness. I understand. In contrast, comparative steels E09 and E10
The fracture surface transition temperature was as high as −20 ° C. and 0 ° C., and was inferior in low-temperature toughness as compared with the steel of the present invention.

The workability of the steels E01 to E10 of the present invention was also improved.
E08 and A1 to A7 all show an abnormal total elongation of 35% and a uniform elongation of 25% or more, and very good results are obtained. On the other hand, the comparative steel E09 is good, but the comparative steel E10 has a total elongation of 30% and a uniform elongation of 20%, which is inferior to that of the steel of the present invention. Regarding the bending workability, it was found that all steels can be bent to close contact.

Further, from the results of the high-temperature property tests shown in Table 6, the steels of the present invention all exhibited 0.2% proof stress of 100 N / mm ² or more at 700 ° C., 13 N / mm ² or more at 900 ° C. Shows a value of 10 ⁷ cycles or more in both cases of 600 ° C (180 N / mm ² ) and 900 ° C (30 N / mm ² ), demonstrating excellent high temperature strength and high temperature fatigue properties.

The results in Table 7 show that the steel of the present invention was subjected to the repeated stress of heating / cooling and the repeated stress of tension / compression.
This shows that no peeling of the scale was observed in both the base material and the weld. The thermal fatigue characteristics of the steel of the present invention are SUS with high Cr content.
Shows the same level as 430J1L. However, scale peeling occurred during the test for SUS430J1L. Similarly, the comparative fatigue strength of the comparative steel F14 is slightly inferior to that of the steel of the present invention, however, scale detachment occurs under such severe test conditions because the amount of Mn added is out of the range of the present invention.

[0072]

As described above, according to the present invention,
An inexpensive ferritic stainless steel with a relatively low Cr content that can be used at high temperatures of 700 ° C to 950 ° C and that can be sufficiently durable as an exhaust gas pipe member where high-temperature oxidation characteristics and the amount of scale peeling are important. In particular, a material that is superior to conventional materials in terms of economy and characteristics as a material for an exhaust manifold of an automobile engine or a material for a high-temperature exhaust gas pipe member of a thermal power generation system is provided. Technology development.

[Brief description of the drawings]

FIG. 1 is a graph showing the relationship between the Si / Mn ratio in steel on the high-temperature oxidation resistance at 1000 ° C. and the scale adhesion.

FIG. 2 is a diagram showing the effect of the amount of Cu in steel on the fracture surface transition temperature.

FIG. 3 is a view showing the effect of the amount of Cu in steel on the total elongation and uniform elongation in a tensile test.

FIG. 4 is a view showing the effect of the total amount of (Cr + Mn + Si) in steel on the increase in oxidation after continuous heating at 930 ° C. for 200 hours in an air atmosphere.

FIG. 5 is a view showing the effect of the total amount of (Cr + Mn + Si) in steel on the increase in oxidation after continuous heating at 950 ° C. for 200 hours in an air atmosphere.

────────────────────────────────────────────────── ─── Continuation of the front page (56) References JP-A-5-125491 (JP, A) JP-A-4-228547 (JP, A) JP-A-60-13060 (JP, A) (58) Field (Int. Cl. ⁶ , DB name) C22C 38/00 302

Claims (3)

(57) [Claims] 1. In mass%, C: 0.03% or less, Si: 0.80% to 1.20%, Mn: 0.60% to 1.50%, Cr: 11.0% to 15.5%, Nb: 0.20% to 0.80%, Ti: 0.1 % Or less (including no addition), Cu: 0.02% to less than 0.30%, N: 0.03% or less, Al: 0.05% or less (including no addition), O: 0.012% or less. ≤Mn / Si≤1.5 (1) 1221.6 (C + N) -55.1Si + 65.7Mn-8.7Cr-99.5Ti
-40.4Nb + 1.1Cu + 54 ≦ 0 ··· (3) Relationship (1) containing these elements so as to satisfy our and the (3) At the same time, the balance being Fe and unavoidable impurities, 900 under an air atmosphere After continuous heating at 100 ° C for 100 hours, the oxidation gain is 0.02 kg / m ² or less, and the scale peeling amount is 0.01 kg / m ² or less.
1000 ° C. for 100 hours high-temperature oxidation resistance scale peeling amount of oxidation weight gain is 0.4 kg / m ² or less of after continuous heating is 0.02 kg / m ² or less and scale adhesion excellent ferritic stainless steel. 2. The ferrite stainless steel according to claim 1, wherein the steel is processed into a member constituting an exhaust gas pipe of an internal combustion engine. 3. The ferrite stainless steel according to claim 2, wherein the member constituting the exhaust gas line of the internal combustion engine is an exhaust manifold connected to an automobile engine.