JP2803538B2 - Ferritic stainless steel for automotive exhaust manifold - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a ferritic stainless steel for an automobile exhaust manifold.

[0002]

2. Description of the Related Art Exhaust system parts such as exhaust manifolds, center pipes, front pipes and the like for automobiles are located at portions which come into contact with high-temperature combustion gas discharged from an engine. Various characteristics such as heat resistance and thermal fatigue resistance are required.

Heretofore, cast iron has generally been used as a material for automobile exhaust manifolds. But,
In order to respond to the recent demands for stricter exhaust gas regulations, improved engine performance, and improved fuel efficiency by reducing vehicle weight,
Stainless steel welded tubes have been used as exhaust manifold materials. In particular, the temperature of exhaust gas has recently exceeded 900 ° C., and a material having excellent oxidation resistance, high-temperature strength, and thermal fatigue resistance at 900 ° C. or higher has been required.

[0004] Austenitic stainless steel has excellent heat resistance and workability. Typical steel types are SUS304 (18Cr-8Ni) and SUS310S (25Cr-20N).
i). However, austenitic stainless steel has a large coefficient of thermal expansion, and is likely to be broken by thermal fatigue caused by thermal strain in applications such as an exhaust manifold that is subjected to repeated heating and cooling.

On the other hand, ferritic stainless steel generally has a smaller coefficient of thermal expansion than austenitic stainless steel, and is therefore advantageous for thermal fatigue characteristics. Therefore, from the viewpoint of thermal fatigue resistance and material cost, it can be said that ferritic stainless steel is suitable as a material for an exhaust manifold. For this reason, SUH409L and SUS410L have been used as exhaust manifold materials, but there has been a problem that the high-temperature strength and the oxidation resistance are inferior as the exhaust gas temperature increases.

[0006] Exhaust manifolds, front pipes, and the like exposed to high temperatures have a problem of high-temperature salt corrosion on the outer surface due to rock salt sprayed as a measure against snow melting on roads in winter. However, ferritic stainless steels used for exhaust manifolds, front pipes, etc., did not have sufficient measures against high-temperature salt corrosion.

By the way, even in the past, the exhaust gas temperature was 90
As a steel type that can cope with 0 ° C. or higher, Japanese Patent Application Laid-Open No. 64-8254 discloses a material containing 17% or more of Cr and having a high temperature and high strength by Nb and Mo. Japanese Patent Application Laid-Open No. 4-280947 discloses, as a steel type that can cope with an exhaust temperature of 1000 ° C., a material for an exhaust manifold having a high heat-resistant fatigue resistance in which the range of Nb is further increased to increase the temperature and strength. . However, since these steels contain 17% or more of Cr, which is essential for oxidation resistance, they must be expensive.

[0008] An example of a ferritic stainless steel with Cr: 6 to 25% is disclosed in Japanese Patent Application Laid-Open No. 60-145359.
The idea here is to replace some of the Cr with Si, but fix virtually all of the carbon and nitrogen with Ti, leaving a small amount of Nb. As a matter of fact, since C and N are relatively large and the amount of Nb is small, there is a disadvantage that high-temperature characteristics are not sufficient as described below.

That is, in JP-A-60-145359, C: 0.05% or less, Si: 1.00 to 2.00%, Mn: 2.0% or less, Cr: 6.0 to 25.0%, Mo: 5.0% or less (however, Cr + Mo ≧
8%), N: 0.05% or less, Al: 0.50% or less, Ti, Zr, T
a, Nb or more (but Ti, Zr, Ta, and Nb amounts are stoichiometric amounts necessary to convert all C and N into carbides and nitrides), and Nb: 0.30% or less A high-temperature ferritic steel comprising 0.10% or more (preferably 0.20% or more) of unbonded (solid solution) Nb having periodic oxidation resistance and creep strength is disclosed. Is effective, and the creep strength is 0.10% or more (preferably 0.1%).
20% or more) unbonded (solid solution) Nb presence and Si-rich Laves
It is stated that phase formation is important. However, if the Nb content is 0.3% or less, there is a problem that the reinforcement by unbonded (solid solution) Nb and Nb carbide, which greatly contribute to the high-temperature strength, is insufficient, and the high-temperature strength and the thermal fatigue properties are inferior.

[0010]

SUMMARY OF THE INVENTION An object of the present invention is to provide a material for an automobile exhaust manifold which can handle an exhaust temperature of around 900 ° C., particularly, having excellent oxidation resistance, high-temperature strength and thermal fatigue characteristics at 900 ° C. or higher. Another object of the present invention is to provide a ferritic stainless steel having excellent resistance to high-temperature salt damage and corrosion on the outer surface.

[0011]

Heretofore, in order to have excellent oxidation resistance at 900 ° C. or higher, a Cr content exceeding 16% has been required. In that case, the cost of the material becomes a problem as described above. Therefore, the present inventors have conducted various studies in order to achieve the object and obtained the following findings to complete the present invention.

(I) The amount of Cr is set to 11 to 14%, and the oxidation resistance deteriorated due to the decrease of Cr can be secured by positively adding Si to the oxidation resistance at 900 ° C. or more, and C + N ≦ Workability and toughness are ensured by setting the content to 0.025 %, and further high-temperature strength can be obtained by setting 0.3% <Nb.

(Ii) Under such circumstances, 0.8-1.
Addition of an appropriate amount of 5% Si can further improve high-temperature strength and contribute to improvement of thermal fatigue resistance. In the Laves phase (mainly Fe ₂ Nb) precipitated at a high temperature, a part of Nb is replaced by Si, thereby suppressing a decrease in solid solution Nb and maintaining high-temperature strength. However, an excessive addition exceeding 1.50%, on the contrary, promotes the precipitation of the Laves phase and reduces the high-temperature strength. (iii) Increasing the amount of Si in steel is very effective for improving the high-temperature salt damage corrosion resistance.

Here, in the broadest sense, the present invention relates to
, C: 0.015% or less, Si: 0.80 to 1.50%, Mn: 0.20 to
0.60%, P: 0.030% or less, Cr: 11.0 to 14.0%, Ni: 0.50
% Or less, Nb: more than 0.30% and 0.60% or less, N: 0.015% or less, provided that C + N ≦ 0.025%, Ti: 0 to 0.20%, S: 0 to
0.010%, Mo: 0 to 1.50%, Al: 0 to 0.20%, Ca, Y, La,
One or more of Ce, in total: 0 to 0.10%, provided that S: 0.003 to 0.
When it is 010%, Ti: 0.05 to 0.20%.

A ferritic stainless steel for automobile exhaust manifolds, the balance of which has a chemical composition consisting of Fe and inevitable impurities in production.

Some preferred embodiments of the present invention are as follows. (1) By weight%, C: 0.015 % or less, Si: 0.80 to 1.50%,
Mn: 0.20 to 0.60%, Cr: 11.0 to 14.0%, P: 0.03% or less,
S: 0.002% or less, Ni: 0.50% or less, Nb: more than 0.30% and 0.60% or less, N: 0.01
Ferritic stainless steel for automobile exhaust manifolds having a chemical composition of 5% or less, where C + N ≦ 0.025%, and the balance is Fe and unavoidable impurities in production.

(2) In weight%, C: 0.015% or less, Si: 0.80 to 1.50%,
Mn: 0.20 to 0.60%, P: 0.03% or less, S: 0.002% or less, Cr: 11.0 to 14.0%,
Ni: 0.50% or less, Nb: more than 0.30% and 0.6% or less, Ti: 0.05
0.25%, N: 0.015 % or less, where C + N ≦ 0.025%, the balance being Fe and a chemical composition consisting of unavoidable impurities in production, a ferritic stainless steel for an automobile exhaust manifold.

(3) In weight%, C: 0.015% or less, Si: 0.80 to 1.50%,
Mn: 0.20 to 0.60%, P: 0.03% or less, S: 0.002% or less, Cr: 11.0 to 14.0%,
Ni: 0.50% or less, Nb: more than 0.30% and 0.6% or less, Ti: 0.05
0.25%, N: 0.015% or less, where C + N ≦ 0.025%, the balance being Fe and a chemical composition consisting of unavoidable impurities in production, a ferritic stainless steel for an automobile exhaust manifold.

(4) The automobile exhaust manifold according to any one of (1) to (3), wherein the ferritic stainless steel further contains Mo in a range of 0.03 to 1.50%. Ferritic stainless steel.

(5) The automobile exhaust manifold according to any one of (1) to (4), wherein the ferritic stainless steel further contains Al in a range of 0.02 to 0.20%. Ferritic stainless steel.

(6) The ferritic stainless steel further comprises at least one of Ca, Y, La, and Ce, in total of 0.003
The ferritic stainless steel for an automobile exhaust manifold according to any one of the above (1) to (5), which contains 0.1 to 0.10%.

[0022]

The reason why the steel composition is limited as described above in the present invention will be described in detail together with the function of each alloy element. In this specification, "%" means "% by weight" unless otherwise specified.

C, N: As in the present invention, nearly 1.0% of Si
In particular, when the content of C and N is high, the toughness is reduced and the workability is adversely affected. Therefore, it is desirable that C and N are as low as possible. Therefore, C: 0.015% or less, N: 0.015 or less, and C: N
+ N ≦ 0.025%. Preferably, C + N ≦ 0.020%. Preferably, C: 0.010% or less, N: 0.010% or less.

Si: Si is an important element in the present invention for improving oxidation resistance, thermal fatigue resistance, and high-temperature salt corrosion resistance. Oxidation resistance and high-temperature salt corrosion resistance improve with an increase in the amount of Si, but if it is less than 0.8%, the effect is not sufficient. Desirably, the effect is sufficiently obtained if the content is 1.0% or more. Addition of an appropriate amount of Si improves high-temperature strength and contributes to improvement of thermal fatigue resistance. This is because Si substitutes for a part of Nb in the Laves phase (mainly Fe ₂ Nb) precipitated at high temperature, thereby suppressing the decrease of solid solution Nb and maintaining high temperature strength. However, excessive addition, on the contrary, promotes precipitation of the Laves phase, and not only lowers the high-temperature strength, but also deteriorates the toughness and workability.
%. Preferably it is 1.0-1.5%.

Mn: Mn is known as a deoxidizing agent at the time of steel making and an element improving hot workability. However, since MnS is formed to be a starting point of oxidation or an austenite forming element, it is not preferable for oxidation resistance. Therefore,
0.2 to 0.6%. Preferably it is 0.2-0.5%.

Cr: In the present invention, Cr is an element essential for ensuring oxidation resistance. If it is less than 11%, the effect does not appear.If it exceeds 14%, toughness and workability will deteriorate,
The upper limit was set to 14%. Preferably, Cr is 12.0 to 14.0%.

Ni: The addition of Ni is effective for improving toughness and improving high-temperature salt damage corrosion resistance. However, since it is an austenite forming element and adversely affects oxidation resistance, and is more expensive, the content is particularly set to 0.50% or less.

Nb: Nb is an essential element for improving high-temperature strength. Since Nb acts as a carbonitride to fix C and N, the required amount of Nb has a correlation with the amounts of C and N.
In the present invention, Nb: more than 0.30%, 0.60% or less, (C +
N) ≤ 0.025%, so that% Nb / (% C +% N)
≧ 10, and secure the amount of solute Nb necessary to obtain sufficient high-temperature strength. The Nb content is preferably as large as possible for the purpose of securing the required amount of solute Nb from the viewpoint of high-temperature strength. However, if it is less than 0.3%, sufficient high-temperature strength cannot be obtained, while if it exceeds 0.6%, the toughness is adversely affected. Therefore, the content is set to more than 0.3% and 0.6% or less. Preferably it is 0.4-0.6%. More preferably, Nb% ≧ 15 (% N +% C).

S: S is one of the unavoidable impurities in production, but if the amount of S is large, it is not preferable from the viewpoint of oxidation resistance like Mn. It also has an adverse effect on weldability. Therefore, the content of S is set to 0.002% or less. However, if the removal of S is made insufficient to make S: 0.003% or more, Ti is reduced to 0.01%.
%, Stable at high temperature by adding Ti carbosulfide
By forming (Ti (C, S)), the generation of MnS, which is a starting point of oxidation, can be suppressed, and the adverse effect on oxidation resistance can be eliminated.
On the other hand, since the particle system of the precipitates becomes large and the precipitation density decreases, recrystallization becomes easy. Thereby, workability is improved. Therefore, if necessary, S may be positively present as an alloy element on the assumption that Ti is added. But,
S: If it exceeds 0.010%, MnS or TiS is formed and the oxidation resistance is impaired, so the upper limit was made 0.010%.

Ti: Ti is a desired additive element, and is effective as a fixing element of C and N like Nb, and can partially replace Nb. Further, the composite addition of Nb and Ti lowers the recrystallization temperature and contributes to the improvement of workability. However, since excessive addition causes surface flaws during rolling, the upper limit of Ti is set to 0.20%.
Furthermore, according to the preferred embodiment, it has been found that when the ratio of Nb / Ti is 2.50 to 5.0, the thermal fatigue characteristics are further improved. Therefore, since Nb: more than 0.30% and 0.60% or less, Ti:
0.05 to 0.20%. Preferably, Ti is 0.10 to 0.20%. More preferably, (% Nb +% Ti) ≧ 20 (% C +%
N).

Mo: Mo is a desired additive element and, like Nb, is known as an element for improving high-temperature strength. Also,
It also improves high-temperature salt damage corrosion resistance. In order to obtain a sufficient effect, it is preferable to add 0.03% or more. However, excessive addition lowers processability. To further increase the cost, the upper limit was set to 1.50%. Preferably, it is 0.1-1.0%.

Al: Al is a desired additive element and is known as a deoxidizing element. It is also known that toughness and oxidation resistance are improved by adding a small amount of Al. In particular, when the steel contains approximately 1.0% of Si as in the steel according to the present invention, the peeling resistance of the oxide scale can be improved without changing the oxidation weight increase. This suppresses the incorporation of the oxide scale into the exhaust gas. Furthermore, a small amount of Al addition
It also has a high temperature strength improving effect. However, an excessive addition causes a reduction in workability, so the content was set to 0.02 to 0.20%. Preferably, it is 0.02-0.1%.

Ca, Y, La, Ce: It is known that Ca and rare earth elements such as Y, La, and Ce improve oxidation resistance and improve adhesion of an oxide scale. In the present invention, it can be added as needed. In addition, it has a de-S action. In order to sufficiently exhibit the effect, it is preferable to add 0.003% or more, and if added in excess of 0.10%, the toughness is deteriorated. Therefore, the upper limit is set to 0.10%. Preferably, 0.01 to
0.1%.

In addition, P is one of the unavoidable impurities in production. P is generally 0.05% or less from the viewpoint of toughness and workability, but is preferably 0.03% or less.

The method for producing a ferritic stainless steel according to the present invention is essentially the same as the ordinary method for producing a ferritic stainless steel. Melted in an electric furnace or converter, refined in an AOD furnace, VOD furnace, etc., and continuously cast or ingot-
The slab may be formed by the slab method, and thereafter, may be formed into a plate through the steps of hot rolling and cold rolling. A welded pipe is manufactured using this as a material, and it is the welded pipe that is used as a material for the exhaust manifold. As the heat treatment, it is desirable to perform soaking at a temperature of 900 to 50 ° C. for 0.5 to 30 minutes and then air cooling. Next, the operation of the present invention will be described more specifically with reference to examples.

[0036]

EXAMPLES First, steels having the compositions shown in Tables 1 and 2 were melted and forged, and then hot-rolled at 1200 ° C.
After annealing, the hot-rolled sheet was subjected to cold rolling and finish annealing was performed at 980 ° C. to obtain a cold-rolled sheet having a thickness of 2 mm. Than this,
Room temperature and high temperature tensile test specimens with a thickness of 2 mm, thickness 2 mm x width 20
An oxidation test piece and a high-temperature salt damage corrosion test piece of mm × 25 mm in length were cut out.

Further, the cold-rolled sheet is formed by electric resistance welding,
A thermal fatigue test piece whose shape and dimensions are shown in FIG. 1 was prepared. In FIG. 1, reference numeral 1 denotes a tube of test material, and a diameter of 8
Holes (2, 3) having a diameter of 2 mm were drilled to provide a supply port 2 and a discharge port 3 for cooling air. 4 is a holder from the inside of the pipe (core metal)
Numerals 5 and 5 denote attachment portions to the holder of the testing machine. The tube 1 and the holder 4 are fixed by a fixing pin and an end weld 7.

The high temperature tensile test was performed at 900 ° C. The oxidation test was performed at 900 ° C. for 200 hours under continuous heating conditions in the atmosphere.
The high-temperature salt damage corrosion test was performed under the conditions shown in FIG. The thermal fatigue test was performed by using the test piece of FIG. 1 and performing a temperature cycle and a mechanical strain waveform history shown in FIG.
The test was conducted at 50 ° C. and 50% constraint (degree of constraint η = 0.501).

The results of these tests are summarized in Table 3 and FIGS. According to Table 3, the steels 1 to 25 of the present invention have a normal temperature elongation of 30% or more, a tensile strength at 900 ° C. of 15 N / mm ² or more,
The oxidation increase in the above-mentioned continuous air heating test at 0 ° C
1.5 mg / cm ² or less, decrease in sheet thickness after high-temperature salt corrosion test 450
μm or less, the thermal fatigue life in a thermal fatigue test is 780 cycles or more, and it is understood that the material has excellent characteristics as a material for an exhaust manifold in consideration of high-temperature salt damage corrosion resistance.

Particularly, 0.020 as shown in the steels 11 to 15 of the present invention.
It was confirmed that when the Al content was 0.20%, the oxidation resistance was not changed and the peeling resistance of the oxide scale was improved. See FIG. 6 described below.

Further, as shown in Steels 16 to 25 of the present invention,
It was confirmed that the addition of 003 to 0.10% of Ca, Y, La, and Ce improved the oxidation resistance. Comparative steel 1 contains no Nb and Ti
Although it is a material equivalent to SUH409L containing a large amount of, it has poor tensile strength at 900 ° C, oxidation resistance and thermal fatigue properties.

Comparative steel 2 corresponds to the steel composition disclosed in the above-mentioned Japanese Patent Application Laid-Open No. 60-145359. Since Nb is not more than 0.30%, both tensile strength at 900 ° C. and thermal fatigue properties are high. Inferior. Comparative steel 3 contains less than 0.80% Si, comparative steel 5 contains 11.0% Cr
Therefore, the oxidation resistance and the high-temperature salt corrosion resistance are not sufficient.

In Comparative Steel 4, Si was 1.50%, and in Comparative Steel 6, Mo was
Since it exceeded 1.50%, the elongation at room temperature was less than 30%, and the workability was poor. Comparative steel 7 has an Mn content of more than 0.60% and an S content of more than 0.002%, and thus has insufficient oxidation resistance.

In Comparative Steel 8, C + N exceeded 0.025%, and the amount of solute Nb required for improving the high-temperature strength was insufficient.
Poor tensile strength at 0 ° C and poor thermal fatigue properties. Comparative steel 9 is a steel obtained by adding Mo to comparative steel 8, but improvement in high-temperature strength and thermal fatigue properties was hardly recognized.

FIG. 4 is a graph showing the increase in oxidation by a continuous test in the atmosphere when the amount of Si is changed based on the steel 1 of the present invention as a basic composition. It can be seen that high-temperature oxidation resistance is significantly improved at Si: 0.80% or more. FIG. 5 is a graph showing a decrease in sheet thickness in a high-temperature salt damage corrosion test.

FIG. 6 is a graph showing the increase in oxidation and the amount of scale exfoliation in a continuous heating test in the atmosphere when the amount of Al is changed with the steel 14 of the present invention as a basic composition. FIG. 7 shows the steel 6 of the present invention.
4 is a graph showing the effect of Nb / Ti on the thermal fatigue life when the base composition is a basic composition.

[0047]

[Table 1]

[0048]

[Table 2]

[0049]

[Table 3]

[0050]

According to the present invention, it has excellent oxidation resistance, high-temperature strength, and thermal fatigue characteristics at an exhaust temperature of around 900 ° C.
Further, a ferritic stainless steel having excellent resistance to high-temperature salt damage and corrosion on the outer surface and applicable to not only the exhaust manifold but also the center pipe and the front pipe can be obtained.

[Brief description of the drawings]

FIG. 1 is an explanatory diagram of a shape (gauge length: 12 mm) of a thermal fatigue test specimen.

FIG. 2 is an explanatory diagram of conditions for a high-temperature salt damage corrosion test.

FIG. 3 is an explanatory diagram of temperature and strain waveforms during a thermal fatigue test.

FIG. 4 is a graph showing the effect of the amount of Si in steel on oxidation resistance when continuously operated at 900 ° C. for 200 hours in the atmosphere.

FIG. 5 is a graph showing the effect of Si in steel on high-temperature salt damage corrosion resistance.

FIG. 6 is a graph showing the effect of the amount of Al in steel on the peeling resistance of an oxide scale when continuously heated in air at 900 ° C. for 200 hours.

FIG. 7 is a graph showing the effect of Nb / Ti on the thermal fatigue life.

Claims (1)

(57) [Claims] C .: 0.015% or less, Si: 0.80 to 1.50%, Mn: 0.20 to 0.60% by weight.
%, P: 0.030% or less, Cr: 11.0 to 14.0%, Ni: 0.50% or less, Nb: 0.30% to 0.60% or less, N: 0.015% or less, provided that C + N ≦ 0.025%, Ti: 0 to 0.20% , S: 0 ~ 0.010%, Mo: 0 ~ 1.50%, Al: 0 ~
0.20%, one or more of Ca, Y, La, Ce, 0 to 0.10% in total
When S: 0.003 to 0.010%, Ti: 0.05 to 0.20%.
Ferritic stainless steel for automobile exhaust manifolds, the balance of which has a chemical composition consisting of Fe and unavoidable impurities in production.