JP3560032B2 - Ferritic stainless steel for automobile exhaust system equipment - Google Patents

Description

【００３２】
【表２】

【００３３】
【発明の効果】
以上説明したように、この発明によれば、自動車が使用される環境において優れた耐食性を示す上、自動車排気系機器用鋼管に要求されるその他の特性をも十分に満足することができ、大幅なコストアップを伴うことなく自動車の更なる耐久性改善に寄与し得るなど、産業上有用な効果がもたらされるのである。
【図面の簡単な説明】
【図１】本発明におけるＣ、ＮならびにＴｉ、Ｎｂの含有量を図示するグラフである。
【図２】図２（ａ） は、再現熱サイクル装置による溶接熱影響部再現方法を、図２（ｂ） はそのときの加熱冷却処理条件をそれぞれ示す説明図である。
【図３】再現熱サイクル装置により鋭敏化処理した後、ストラウス試験を実施した鋼板の鋼中Ｃ 、Ｎ とＴｉ、Ｎｂの関係を粒界腐食有無で整理したグラフである。
【図４】ＭＡＧ溶接試験片の採取方法を示した説明図である。
【図５】ＭＡＧ溶接した後、ストラウス試験を実施した鋼板の鋼中Ｃ 、Ｎ とＴｉ、Ｎｂの関係を粒界腐食有無で整理したグラフである。
【図６】ＭＡＧ溶接した後、半浸漬試験を実施した鋼板の鋼中Ｃ 、Ｎ とＴｉ、Ｎｂの関係を粒界腐食有無で整理したグラフである。[0001]
[Industrial applications]
The present invention relates to a ferritic stainless steel exhibiting excellent corrosion resistance when used as a material for automobile exhaust system equipment, particularly as an exhaust pipe or a front center pipe.
[0002]
[Prior art]
In addition to salt damage caused by spraying rocks on the outer surface, which is used as a countermeasure against snow melting on the roads in winter, automobile exhaust system components include high-temperature oxidation in the exhaust manifold and front center pipe, and internal condensation due to exhaust gas cooling in the muffler. There are four types of corrosion actions that can be roughly classified into four types: water corrosion, and intergranular corrosion of welded joints.
[0003]
Conventionally, the SUS304 system used as a flexible pipe o The automotive exhaust system equipment material - austenitic stainless steel except the aluminized carbon steel or SUS410L steel ferritic stainless steel have been used. They are all configured as welded tubes and are connected to other equipment by welding.
[0004]
Certainly, although aluminum-plated carbon steel is excellent in salt damage corrosion resistance, its corrosion resistance itself in the exhaust gas environment such as condensation water corrosion is insufficient, and in SUS410L steel, intergranular corrosion in the weld heat affected zone is a major problem. Met. As a material for preventing grain boundary corrosion of the heat-affected zone of SUS410L steel, 409L steel containing Ti can be cited. However, even with this steel type, the intergranular corrosion resistance of the weld heat-affected zone is still insufficient, and the life of automobiles has been extended. However, conventional materials are not satisfactory in corrosion resistance, and the appearance of ferritic stainless steel exhibiting more excellent corrosion resistance is required.
[0005]
[Problems to be solved by the invention]
An object of the present invention is to use in an automobile exhaust system environment, particularly an exhaust manifold, a front pipe, a center pipe, and the like, and have excellent intergranular corrosion resistance in a heat affected zone of the welding and at the same time excellent mechanical properties. An object of the present invention is to provide an inexpensive ferritic stainless steel having the same function.
[0006]
[Means for Solving the Problems]
The present inventors consider that the corrosion environment of automobile exhaust system equipment described above is considered, and to improve the corrosion resistance of the welding heat affected zone of ferritic stainless steel used for automobile exhaust system equipment, the steel material itself is adjusted by adjusting the components. Considering that it is indispensable to improve intergranular corrosion, it improves corrosion resistance without adversely affecting the mechanical properties of steel used in automobile exhaust system equipment and each of the important factors, cost. We have been considering countermeasures.
[0007]
As a result, in order to improve the corrosion resistance of the weld heat affected zone to the environment of the automobile exhaust system, from the viewpoint of the material, in addition to containing Cr in a specific range, C and N are limited to a specific range. In comparison, by adding Ti and Nb specified in a much narrower addition region in an amount within a specified range, the intergranular corrosion resistance of the weld heat affected zone of the material is significantly improved as compared with the conventional steel, and the vehicle exhaust The inventor has found that the characteristics as a material for system equipment are further improved, and completed the present invention.
[0008]
Here, the gist of the present invention is as follows:
Si: 0.01 to 0.50%, Mn: 0.01 to 0.50%, Cr: 11.0 to 14.0%,
P: 0.030% or less, S: 0.0050% or less,
Further, C and N 2 and Ti and Nb are contained in amounts satisfying the following relational expressions (1) to (3);
0.010% ≦ C + N ≦ 0.020% (1)
0.4% ≦ Ti + Nb ≦ 0.6% (2)
(Ti + Nb) / (C + N) ≧ 30 (3)
A ferritic stainless steel for automobile exhaust system equipment, the balance of which has a steel composition comprising iron and unavoidable impurities.
[0009]
According to a preferred aspect of the present invention, the steel composition further satisfies 0.3 ≦ (% Ti /% Nb) ≦ 1.0 to further improve the effect of preventing crystal grain coarsening.
The ranges of the above-mentioned relational expressions (1) to (3) are shown as shaded areas in FIG. 1, and it can be seen that the object of the present invention is achieved only in a very limited area.
[0010]
[Action]
The reason why the steel composition of the ferritic stainless steel according to the present invention is numerically limited as described above will be described below. In this specification, “%” indicates “% by weight” unless otherwise specified.
[0011]
(I) C, N
When the content of C and N in the steel is increased, not only the mechanical properties and the corrosion resistance are deteriorated, but also the corrosion resistance of the heat-affected zone formed during welding is adversely affected, but the upper limit is (% C +% N) ≦ By limiting to 0.020%, such adverse effects are practically acceptable. Further, if the (C + N) content is extremely reduced, the production cost is increased. Therefore, (% C +% N) ≧ 0.010%, which can be reduced by ordinary refining technology. See equation (1) above.
[0012]
(Ii) Si
Si in steel is an effective deoxidizing element, and is effective in improving oxidation resistance when added in an amount of 0.01% or more. However, if its content exceeds 0.50%, the steel quality becomes harder than necessary. Therefore, the content of Si is limited to 0.50% or less in order to lower the cold workability.
[0013]
(Iii) Mn
Mn in steel is an effective deoxidizing element. Addition of 0.01% or more has the effect of improving hot workability. However, if its content is high, MnS is formed and corrosion resistance is degraded. Is preferred. Therefore, the content of Mn is limited to 0.50% or less.
[0014]
(Iv) P
P in steel may be detrimental to corrosion resistance and is generally an impurity element, so that a lower one is preferable. Therefore, the content of P was limited to 0.030% or less.
[0015]
(V) S
S in steel is also an impurity element like P, and the presence of an excessive amount thereof tends to significantly deteriorate the corrosion resistance and hot workability, and the lower the content, the better. Therefore, the content of S 2 is limited to 0.0050% or less.
[0016]
(Vi) Cr
Cr is an important element that determines the basic corrosion resistance of the ferritic stainless steel according to the present invention, and the corrosion resistance improves as its content increases. For example, considering a corrosive environment such as high-temperature oxidation, exhaust gas dew condensation corrosion, and salt damage corrosion when used as a front center pipe, a Cr content of 11% or more is required. The cost of the material is more noticeable than the improvement of the corrosion resistance. Therefore, the content of Cr is limited to 11% or more and 14% or less.
[0017]
(Vii) Ti, Nb
Ti and Nb in steel are added in order to prevent deterioration of corrosion resistance and toughness in a weld due to C and N contamination due to external factors at the time of welding. In addition, Ti and Nb have the effect of suppressing the coarsening of the crystal grains of the base material and the coarsening of the weld heat affected zone, respectively. To achieve such effects, (% Ti +% Nb) ≧ 30 (C + N)% and (% Ti +% Nb) ≧ 0.4% are required. However, when a large amount of Ti and Nb are present, the precipitation of the Laves phase becomes remarkable and the deterioration of toughness becomes conspicuous, so the upper limit of the content is limited to (% Ti +% Nb) ≦ 0.6%. See equations (2) and (3) above.
[0018]
Further, in order to more effectively exhibit the above-described crystal grain coarsening prevention performance, the Ti / Nb concentration ratio (% Ti /% Nb) in the steel is set to approximately 0.50, generally from 0.3 to 1.0. It is preferable that
[0019]
Thus, the ferritic stainless steel according to the present invention has the following corrosion resistance and mechanical properties, and is manufactured as a steel plate and then processed into a so-called welded steel pipe to produce an exhaust system for automobiles such as an exhaust manifold, a center pipe, a front pipe, and the like. Used for equipment. Such processing means and the like are not particularly limited in the present invention, and may be conventional means.
[0020]
Corrosion resistance: Excellent intergranular corrosion resistance in the heat affected zone of welding in the internal and external environment of exhaust system equipment.
Mechanical properties: It has the same mechanical properties as 12% Cr steel (SUS410L, SUH409) conventionally used, and can be easily formed.
Next, the operation and effect of the present invention will be described more specifically with reference to examples.
[0021]
【Example】
(Example 1)
First, a ferritic stainless steel having a component composition shown in Table 1 was melted, cast into an ingot, and after casting, each ingot was forged, and then hot-rolled at 1200 ° C. Further, the hot-rolled sheet thus obtained was annealed at 920 ° C., heat-treated by air cooling, and then processed into a test piece.
[0022]
The specimen thus obtained was subjected to high-frequency heating and quenching by a reproducible heat cycler to reproduce the heat affected zone, and then subjected to a grain boundary using a Strauss test (JIS G-0575 sulfuric acid-copper sulfate corrosion test method). The corrosivity was investigated.
[0023]
FIGS. 2 (a) and 2 (b) show a method of reproducing the heat affected zone by the reproduction heat cycle device. According to this, as shown in FIG. 2 (a), the reproducible heat affected zone 24 indicated by the dotted line in the figure was locally heated using the high frequency heating coil 22 on the test piece 20 prepared as described above. At this time, as shown in FIG. 2 (b), the processing condition was a rapid cooling process in which the temperature was kept at 1350 ° C. for 1 second, and then the temperature was reduced from 800 to 500 ° C. in 10 seconds.
[0024]
The test results are summarized in a graph, organized by the (Ti + Nb) amount and the (C + N) amount, and are shown in FIG. The numbers and symbols in the figure indicate the numbers and symbols of the test steels, respectively. In the figure, the symbol "o" was regarded as a pass. The same applies hereinafter. As shown by the above formulas (2) and (3), excellent grain boundary when 0.4% ≦ (Ti + Nb) ≦ 0.6% and (Ti + Nb) / (C + N) ≧ 30. It turns out that it shows corrosiveness.
[0025]
From the results shown in FIG. 3, it was found that the steel according to the present invention had excellent intergranular corrosion resistance in the above-mentioned heat affected zone.
It should be noted that within the range shown in FIG. 1, tube making could be easily performed, and excellent mechanical properties were exhibited.
[0026]
(Example 2)
MAG welding was performed on ferritic stainless steel having the component composition shown in Table 1 in the same manner as in Example 1, and a steel plate on which a weld bead was placed as shown in FIG. After processing into test pieces for the semi-immersion test and the Strauss test, a Strauss test and a semi-immersion test simulating the internal corrosion environment of an automobile muffler were performed.
[0027]
The MAG welding and the half immersion test were performed under the following conditions.
[MAG welding conditions]
Atmosphere: Ar-20% CO ₂
Welding current: 40-60 [A]
Welding voltage: 18-20 [V]
Welding speed: 500 to 600 [mm / min]
Welded core wire: Y309, 1.2 φ
(Final adjustment of welding conditions to the extent that welding backwash appears)
[Semi-immersion test conditions]
Use a conical flask with a condenser. The test piece was erected at 80 ° C. for 500 hours. Simulated condensate [Cl ⁻ : 100 ppm, NO ₃ ⁻ : 20 ppm, SO ₄ ²⁻ : 600 ppm, SO ₃ ²⁻ : 600 ppm, CH ₃ COO ⁻ : 800 ppm, pH 4.0]
FIG. 5 shows the results of the Strauss test, and FIG. 6 shows the results of the semi-immersion test.
[0028]
From the results shown in FIG. 5, as compared with FIG. Although the degree of corrosion was slightly moderate, such as passing 7, the tendency was the same, and it was found that the steel according to the present invention exhibited excellent intergranular corrosion resistance. FIG. 6 also shows that the steel according to the present invention exhibits excellent intergranular corrosion resistance, and exhibits excellent corrosion resistance in a simulated environment of an automobile muffler.
From the above results, it is clear that the steel of the present invention exhibits sufficiently satisfactory performance as a material for automobile exhaust system equipment.
[0029]
(Example 3)
Examples 1 and 2 show that the addition of Ti and Nb in combination provides excellent intergranular corrosion resistance in the heat affected zone. Therefore, in this example, a test piece was prepared by changing the concentration ratio of Ti and Nb in the steel with respect to the ferritic stainless steel having the component composition shown in Table 2, and the same as Example 1, using a reproducible heat cycle apparatus. After sensitization, Strauss test was performed.
[0030]
The test results are also shown in Table 2. It can be seen that intergranular corrosion of the heat-affected zone is almost completely prevented by regulating Ti / Nb to 0.3 to 1.0.
From the results in Table 2, it was found that the steel according to the present invention exhibited excellent intergranular corrosion resistance.
[0031]
[Table 1]

[0032]
[Table 2]

[0033]
【The invention's effect】
As described above, according to the present invention, in addition to exhibiting excellent corrosion resistance in an environment in which an automobile is used, other characteristics required for a steel pipe for an automobile exhaust system device can be sufficiently satisfied, and Industrially useful effects, such as being able to contribute to a further improvement in the durability of an automobile without a significant increase in cost, are brought about.
[Brief description of the drawings]
FIG. 1 is a graph illustrating the contents of C and N and Ti and Nb in the present invention.
FIG. 2A is an explanatory view showing a welding heat affected zone reproducing method using a reproducing heat cycle device, and FIG. 2B is an explanatory view showing heating / cooling processing conditions at that time.
FIG. 3 is a graph showing the relationship between C 1, N 2, Ti, and Nb in steel of a steel plate subjected to a Strauss test after sensitization treatment by a reproducible heat cycle apparatus according to the presence or absence of grain boundary corrosion.
FIG. 4 is an explanatory view showing a method of collecting a MAG welding test piece.
FIG. 5 is a graph in which the relationship between C 1, N 2, Ti, and Nb in steel of a steel sheet subjected to a Strauss test after MAG welding is arranged according to the presence or absence of intergranular corrosion.
FIG. 6 is a graph in which the relationship between C 1, N 2, Ti, and Nb in steel sheets of a steel sheet subjected to a half immersion test after MAG welding is arranged according to the presence or absence of intergranular corrosion.

Claims (2)

Si: 0.01 to 0.50%, Mn: 0.01 to 0.50% by weight%,
Cr: 11.0 to 14.0%, P: 0.030% or less,
S: 0.0050% or less,
Further, C and N 2 and Ti and Nb are contained in amounts satisfying the following relational expression,
0.010% ≦ C + N ≦ 0.020%, 0.4% ≦ Ti + Nb ≦ 0.6%,
(Ti + Nb) / (C + N) ≧ 30,
A ferritic stainless steel for automobile exhaust system equipment, the balance having a steel composition comprising iron and unavoidable impurities. 2. The ferritic stainless steel according to claim 1, wherein 0.3 ≦ (% Ti /% Nb) ≦ 1.0.

Images