JP3536568B2 - Ferritic stainless steel for engine exhaust parts with excellent heat resistance and muffler corrosion resistance at welds - 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 suitable for use as an exhaust member of an automobile, and in particular, a part close to an engine requiring high heat resistance is required to have a corrosion resistance like a muffler. The present invention proposes a ferritic stainless steel for an engine exhaust member, which is excellent in heat resistance and muffler corrosion resistance of a welded portion, which is assumed to be applied up to a certain portion, and is also excellent in economic efficiency.

[0002]

2. Description of the Related Art Ferritic stainless steels are inexpensive compared to austenitic stainless steels, and are therefore frequently used in exhaust system members of automobiles.

[0003] In the exhaust system environment of an automobile, a portion where the temperature rises to about 900 ° C, such as an exhaust manifold, needs to have excellent heat resistance.
L (19Cr-0.4Si-0.4Nb-0.5Cu) has been mainly used.
On the other hand, as represented by a muffler, condensate of exhaust gas tends to accumulate in a region where the exhaust gas temperature is relatively low, and muffler corrosion due to this condensate has become a problem.
8Cr-0.1Si-0.2Ti) is beginning to be mainly used.

[0004] As described above, the materials used for the exhaust system of automobiles are required to have completely different characteristics between the high temperature side and the low temperature side, and there is a problem that the materials must be used in distinction, and both materials are used. However, there is a problem that the cost is increased due to the high Cr steel.

[0005] In recent years, the exhaust gas temperature has risen in order to improve fuel efficiency. To improve quietness, a sub-muffler is used.
Since its position is located more upstream of the exhaust system than before,
The temperature of these mufflers and other members rises to about 600 ° C. Such a temperature promotes the sensitization of stainless steel (the appearance of a Cr-depleted layer due to the precipitation of Cr carbonitride), which makes the muffler corrosion environment more severe than the conventional one (temperature of about 400 ° C) ( The condensate of the exhaust gas accumulates during cooling and corrodes). In some cases, corrosion resistance is insufficient even with the 18Cr material, and it has been found that corrosion particularly at the welded portion is remarkable. Therefore, there has been a strong demand for the development of a material that is applicable from a high-temperature part to a low-temperature part, is inexpensive, and has more excellent properties than before.

[0006] Also, for the material supply side, unification of materials is a great merit in terms of management and production efficiency, and despite the necessity of developing unified materials for both material suppliers and users, At present, such materials did not exist.

Since the exhaust member is formed by forming a steel plate, it is one of the required characteristics that the exhaust member has excellent workability in order to facilitate the processing.

Next, among the prior art, a disclosure example assuming a high-temperature side member will be described below.

For example, Japanese Patent Application Laid-Open No. Hei 8-60306 (Ferritic stainless steel for automobile exhaust system members) discloses a ferritic stainless steel for the purpose of integrating exhaust system members into one steel type. . However,
According to the description (page 3, [0017]), it is specified that the exhaust manifold, the front pipe, and the center pipe can be used in common, and this is based on the assumption that the material of the exhaust system high-temperature portion is unified. Obviously, it is not intended to cover the muffler at the low-temperature part.

[0010] Therefore, the disclosed example shows that Si: 0.6 to 1.
It suggests that even relatively high Si and Cr levels such as 5 mass% (hereinafter simply referred to as%) and Cr: 16-22% cannot withstand corrosive environments such as muffler materials. It is conceivable, and in fact, experiments by the inventors have confirmed that application to a muffler material is unsuitable when the Cr content is increased.

JP-A-57-85960 (ferritic stainless steel having improved toughness and weldability)
A composite additive steel of Cb (Nb) and Al is disclosed. But,
This steel keeps Ti at an unavoidable residual level and is not intended for mufflers, so
There is no perception of the effect. Therefore, the steels of both the inventive examples and the comparative examples are inferior in muffler corrosion resistance at the welded portions, and have insufficient properties as a material applied from the high temperature side to the low temperature side of the exhaust system.

JP-A-6-248394 (Ferritic stainless steel for automobile exhaust system equipment excellent in high-temperature salt damage corrosion resistance)
Discloses a ferritic stainless steel excellent in high-temperature salt damage resistance in consideration of use in a front pipe and a center pipe.

However, as specified in the specification (page 3, 0020), in order to improve the corrosion resistance of the welded portion.
It is said that Ti and Nb are added, and it is recognized that their effects are equivalent to each other, and the effect of the composite addition of Ti-Nb which greatly affects the muffler corrosion resistance of the weld is not recognized. Therefore, there is only an example in which Nb alone is added to both the inventive steel and the comparative steel in the examples. In addition, since the effect of Mn on the muffler corrosion resistance of the welded portion was not recognized, the invention steel in the examples and the comparative steel both contained 0.28%
This is the higher Mn level. Therefore, at such a high Mn level by adding Nb alone, sufficient muffler corrosion resistance of the welded portion cannot be obtained, and it is unsuitable for low-temperature side members.

According to experiments by the inventors, when the amount of Si is at a relatively low level (0.4% or less), when the amount of Mn is at a relatively high level, or when Ti or Nb is added alone, it is not sufficient. It has been found that it is difficult to obtain muffler corrosion resistance of a welded part.

US Pat. No. 4,479,921 (Welded feri
tic stainless steel article) discloses a welded ferritic stainless steel material equivalent to the effect of Cb (Nb) and Ti, but looking at the component composition of the steel of the examples and comparative examples, Since there is no description, Ti is the sole addition, and the Mn content is at a high level of 0.29% or more, sufficient muffler corrosion resistance of a welded portion can be obtained with a ferritic stainless steel having such a composition. No, it is not suitable for low-temperature side members. According to the experiments by the inventors,
It has been confirmed that it is difficult to improve the muffler corrosion resistance of the welded portion by using Ti or Nb solely added steel.

On the other hand, as a technology assuming a low-temperature side member,
Japanese Patent Application Laid-Open No. HEI 5-112848 (Motor exhaust system stainless steel excellent in overall corrosion resistance and local corrosion resistance) containing 1% or more of Mo is proposed and disclosed. This is intended to improve the corrosion resistance of the base material, particularly with muffler corrosion in mind.

Here, the portion which becomes a problem due to muffler corrosion is a welded portion inevitably generated in the forming process. In other words, the biggest problem is that cracks are propagated by external stress, leading to breakage of the weld, starting from significant corrosion in the weld, and the corrosion level of the base metal is less than that of the weld. However, the corrosion resistance of the base material does not significantly affect the muffler life.

As described above, since the welded portion is the most severely corroded, it is only necessary to reduce the occurrence of corrosion pits in the welded portion to prevent the occurrence of a fracture starting point.
The addition of Mo as disclosed in Japanese Patent Application Laid-Open No. 5-112848 has almost no effect on the corrosion of the welded portion and is disadvantageous in terms of cost.

In order to improve the corrosion resistance of the weld,
It is known to add stabilizing components such as Ti and Nb, and the above-mentioned JP-A-5-112848 describes the reason for adding Ti and Nb from the viewpoint of improving the intergranular corrosion resistance of the welded portion. ing. However, the problem of muffler corrosion that still occurs even in recent years has become apparent that the weld is broken starting from significant corrosion of the weld, and simply by adding Mo, Ti and Nb, It has also been found that the muffler corrosion resistance of the weld is not sufficient.

The reason is that, as described in Japanese Patent Application Laid-Open No. 5-112848, the pH of the exhaust gas condensate changes extremely from about 9 to about 1, and therefore, in such an environment, only Mo, It is considered that the addition of Ti and Nb alone was not sufficient for the muffler corrosion resistance of the weld.

Further, since the exhaust gas condensate contains nitrous acid and formic acid, and no material has been developed in consideration of such ions, an inexpensive material having excellent muffler corrosion resistance at the welded portion even at present. There was no.
In such a corrosive environment, high Cr, Ti or Nb
Surprisingly, the effect of adding a single agent is small.

In addition, in the above-mentioned Japanese Patent Application Laid-Open No. 5-112848, at present, an increase in cost due to the addition of 1% or more of Mo also poses a problem.

As described above, since no material has been developed in consideration of the muffler corrosion resistance of the welded portion, no inexpensive material having excellent muffler corrosion resistance has appeared at present. Therefore, there is a strong demand for cost reduction by reducing Cr.

Japanese Patent Application Laid-Open No. Hei 4-228547 (ferritic stainless steel excellent in intergranular corrosion resistance, tube formability and high-temperature strength) is characterized in that Nb and Ti are added to fix C and N. There has been proposed and disclosed a ferritic stainless steel for the purpose of improving the intergranular corrosion resistance and the high-temperature strength of the steel.

Although this is intended to improve the intergranular corrosion resistance, the intergranular corrosion resistance is merely evaluated by the Strauss test. In this test method, as in the case of the conventional knowledge, Ti is used. It is merely evaluated that the addition of components that fix C and N, such as Nb and Nb, will improve the intergranular corrosion resistance, and it is difficult to evaluate the muffler corrosion resistance using such a test method. This is because muffler corrosion is in a unique corrosive environment, such as when the pH of the exhaust gas condensate extremely changes from 9 to 1 as described above, and the condensate contains nitrous acid and formic acid. This is because the muffler corrosion resistance was not simulated in the evaluation in.

Therefore, in Japanese Patent Application Laid-Open No. 4-228547,
Although the low Si Ti-Nb composite additive shows excellent characteristics in the Strauss test, it is difficult to improve the actual muffler corrosion resistance with such a low Si material.

Although there has been a strong demand for inexpensive materials having excellent heat resistance and workability and excellent muffler corrosion resistance at welds, there has been no material that can meet the requirements. There was no.

[0028]

SUMMARY OF THE INVENTION The present invention advantageously solves the above-mentioned problems, and aims at unifying the materials of the high-temperature side member and the low-temperature side member of the engine exhaust system, thereby achieving excellent heat resistance. It has excellent weldability, corrosion resistance of welded parts compared to 18Cr-0.2Ti ferritic stainless steel conventionally used for mufflers, and is inexpensive and high temperature based on Mo-saving, low Cr steel. It is an object of the present invention to propose a ferritic stainless steel for an engine exhaust member that can be used on both the low-temperature side and the low-temperature side.

[0029]

[Means for Solving the Problems] It is known that the heat resistance (high-temperature strength) required for an engine exhaust member is improved by adding Nb, and the workability is also improved by adding an appropriate amount of Nb. From the above, based on the Nb material having a content of 0.3% or more, the inventors conducted detailed experiments and studies mainly on the improvement of the muffler corrosion resistance at the welded portion, and achieved the present invention. That is, the gist of the present invention is as follows.

[0030]

(1) C: less than 0.02 mass%, Si: 0.5 mass% or more, less than 1.5 mass%, Mn: less than 0.2 mass%, Cr: 10 mass% or more, less than 14 mass%, Ni: 0.05 mass% or more, 1.0 mass% or less, Ti: 0.05 mass% or more, less than 0.3 mass%, Nb: 0.3 mass% or more, less than 0.6 mass%, P: less than 0.06 mass%, S: less than 0.01 mass%, and N: less than 0.020 mass% V: 0.05 mass% or more, less than 1.0 mass%, Cu: 0.02 mass% or more, less than 1.0 mass%, Zr: 0.05 mass% or more, less than 0.5 mass%, and Al: 0.01 mass% or more, less than 0.5 mass% Contains one or more selected from
And wherein Rukoto such a component composition of Fe and unavoidable impurities
A ferritic stainless steel for an engine exhaust member having excellent heat resistance and excellent muffler corrosion resistance at a weld ( first invention).

[0032]

(2) C: less than 0.02 mass%, Si: 0.5 mass% or more, less than 1.5 mass%, Mn: less than 0.2 mass%, Cr: 10 mass% or more, less than 14 mass%, Ni: 0.05 mass% or more, 1.0 mass% or less, Ti: 0.05 mass% or more, less than 0.3 mass%, Nb: 0.3 mass% or more, less than 0.6 mass%, P: less than 0.06 mass%, S: less than 0.01 mass%, and N: less than 0.020 mass% V: 0.05 mass% or more, less than 1.0 mass%, Cu: 0.02 mass% or more, less than 1.0 mass%, Zr: 0.05 mass% or more, less than 0.5 mass%, and Al: 0.01 mass% or more, less than 0.5 mass% one or more selected from among the Mo: 0.01 mass% or more, containing a less than 0.8 mass%, the balance being refractory characterized by Rukoto such a component composition of Fe and unavoidable impurities and welding Ferritic stainless steel for engine exhaust members having excellent muffler corrosion resistance in the part ( second invention).

[0034]

(3) C: less than 0.02 mass%, Si: 0.5 mass% or more, less than 1.5 mass%, Mn: less than 0.2 mass%, Cr: 10 mass% or more, less than 14 mass%, Ni: 0.05 mass% or more, Less than 1.0 mass% Ti: 0.05 mass% or more, less than 0.3 mass% Nb: 0.3 mass% or more, less than 0.6 mass% P: less than 0.06 mass%, S: less than 0.01 mass%, and N: less than 0.020 mass%, and V: 0.05 mass% or more, less than 1.0 mass%, Cu: 0.02 mass% or more, less than 1.0 mass%, Zr: 0.05 mass% or more, less than 0.5 mass%, and Al: 0.01 mass% or more, less than 0.5 mass% selected one or more and B: 0.0002mass% to less than 0.005 mass% and Ca: 0.001% or more, contains a less than 0.03 mass%, the balance that Do the component composition of Fe and unavoidable impurities A ferritic stainless steel for an engine exhaust member having excellent heat resistance and muffler corrosion resistance at a welded portion ( third invention).

[0036]

(4) C: less than 0.02 mass%, Si: 0.5 mass% or more, less than 1.5 mass%, Mn: less than 0.2 mass%, Cr: 10 mass% or more, less than 14 mass%, Ni: 0.05 mass% or more, 1.0 mass% or less, Ti: 0.05 mass% or more, less than 0.3 mass%, Nb: 0.3 mass% or more, less than 0.6 mass%, P: less than 0.06 mass%, S: less than 0.01 mass%, and N: less than 0.020 mass% V: 0.05 mass% or more, less than 1.0 mass%, Cu: 0.02 mass% or more, less than 1.0 mass% Zr: 0.05 mass% or more, less than 0.5 mass% and Al: 0.01 mass% or more, less than 0.5 mass% Contains one or more selected from among them, and Mo: 0.01 mass% or more and less than 0.8 mass% and B: 0.0002 mass% or more and less than 0.005 mass% and Ca: 0.001 mass% or more and less than 0.03 mass% containing bets, the balance engine exhaust portion having excellent muffler corrosion resistance of heat resistance and weld, characterized in Rukoto such a component composition of Fe and unavoidable impurities Use ferritic stainless steel (4th invention).

[0038]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The results of experiments and studies which led to the achievement of the present invention will be described.

First, factors governing the life of the muffler will be described. The problem with the durability of the muffler is mainly the fracture of the weld. It is known that the welded part has lower corrosion resistance than the base metal part, and it is also clear that stress is most concentrated on the welded part. Therefore, even if only the base material shows excellent corrosion resistance, the most susceptible to corrosion is the welded portion, and the welded portion is subjected to the most concentrated stress, so that large stress in the corrosion pit portion induces rupture.

Particularly, in recent years, in some cases, the exhaust temperature has risen and the temperature of the welded portion has risen to about 600 ° C. In view of such deterioration of the corrosion resistance due to such reheating of the welded portion, the muffler corrosion resistance is improved. Need to be measured.

Therefore, as a result of various examinations of the corrosion test conditions together with the results of actual vehicle tests, the test pieces before the corrosion test were subjected to a heat treatment at 600 ° C. for 5 hours, and the synthesized exhaust gas condensate was used. With this, it became clear that it was possible to simulate actual corrosion well.

Based on the results of the investigation including the actual vehicle, the results of detailed experiments and examinations on the improvement of the muffler corrosion resistance of the welded parts resulted in extremely important findings as described below.

(1) Although it is clear that the base metal portion has better corrosion resistance than the welded portion, a material having good corrosion resistance in the base material portion is as follows.
The difference in corrosion resistance from the weld is large and corrosion concentrates on the weld,
Therefore, it was found that it was difficult to improve the corrosion resistance of the weld. As shown in FIG. 1, it became clear that a low Cr material (10 to 14% Cr), which has a lower corrosion resistance of the base material than the high Cr material, is advantageous for improving the muffler corrosion resistance of the welded portion.

(2) In the low Cr (11% Cr) Ti-Nb composite-added steel, as shown in FIG. 2, by containing 0.5% or more of Si and lowering the Mn content, the muffler corrosion resistance of the welded portion is improved. Is significantly improved, and surprisingly its corrosion resistance is better than 18Cr-0.2Ti steel.

In the above description, the corrosion resistance of the base material is 18
The Cr-0.2Ti steel is better, but the erosion depth of the weld is significantly deeper and worse than the base metal. Therefore, the durability of the muffler should be evaluated based on the erosion depth of the welded part, since the most corrosive part becomes a problem. Therefore, it can be said that the low-Cr Ti-Nb composite-added steel has better corrosion resistance than the 18Cr-0.2Ti steel.

This is because the scale of the welded portion of the low Cr-Ti-Nb composite-added steel was strengthened by adding a suitable amount of Si, Ti and Nb and reducing the amount of Mn, so that the corrosion resistance of the welded portion was reduced. It is highly probable that corrosion approached the material and no corrosion was concentrated on the weld. On the other hand, in the case of 18Cr-0.2Ti steel, the base material has extremely excellent corrosion resistance, so corrosion concentrates on the weld,
As a result, it can be estimated that the muffler corrosion resistance of the welded portion was inferior.

(3) Further, it was found that the inclusion of at least one of V, Cu, Zr and Al in a relatively small amount further improved the muffler corrosion resistance of the welded portion.

Here, FIGS. 1 and 2 show finish annealing at a temperature of hot rolling-cold rolling-1000 ° C., respectively, of steel materials prepared to various component compositions including a composite addition of Ti and Nb. Simulate muffler corrosion environment for each of the cold-rolled annealed sheets with a thickness of 2mm by performing the process of (1), then applying bead-on TIG welding to these cold-rolled annealed sheets and then heating at 600 ° C for 5 hours. The corrosion test was conducted using the exhaust gas condensate synthesized in this way and the corrosion resistance (erosion depth) of the weld was investigated. Figure 1 shows the relationship between the Cr content and the weld erosion depth. FIG. 2 is a graph showing the relationship between the Si content and the Mn content and the erosion depth of the weld.

As described above, the present invention has led to the development of a ferritic stainless steel for an engine exhaust member having excellent heat resistance, workability, and muffler corrosion resistance at a welded portion based on the above novel findings.

Next, the reasons for limiting the composition of the present invention will be described.

C: less than 0.02% C is a component that deteriorates toughness and workability. When the content is 0.02% or more, the toughness and workability deteriorate remarkably, so the content is less than 0.02%. I do. In order to further improve toughness and workability, the lower the content, the better, and preferably 0.01% or less.

Si: 0.5 to less than 1.5% Si is one of the important components for the present invention. In addition to improving heat resistance, as shown in Fig. 2 above, 0.5
%, The muffler corrosion resistance of the weld is significantly improved, and the corrosion resistance reaches the level of conventional steel (18Cr-0.2Ti) or more. This is due to the fact that the scale formed in the weld becomes strong due to the combined addition of Si and Ti-Nb and the reduction of Mn, and the difference between the corrosion resistance level of the base metal and the weld is compared. It is thought that corrosion resistance does not concentrate on the welded portion due to the small component system, and so it is considered to exhibit good muffler corrosion resistance, but details are unknown. on the other hand,
When the content is 1.5% or more, the workability is greatly deteriorated. Therefore, the content is 0.5% or more and less than 1.5%, preferably 0.6% or more and 1.3% or less,
More preferably, it is 0.8% or more and 1.0% or less.

Mn: less than 0.2% Mn is a component whose content is particularly important for the present invention. Although generally known as a steel deoxidizer, excessive addition forms MnS and reduces workability. Fig. 2
As shown in the above, particularly, the effect on the muffler corrosion resistance of the welded portion is great, and when the content is less than 0.2%, the corrosion resistance is improved. Therefore, the content is preferably as low as 0.2%.
, But preferably less than 0.15%.

Cr: less than 10 to 14% Cr is a component that improves the corrosion resistance, which is a basic characteristic of stainless steel. However, as shown in FIG. In addition, the content is limited to less than 14% because of high cost. On the other hand, if the content is less than 10%, the muffler corrosion resistance of the welded portion is significantly deteriorated. Therefore, the content should be 10% or more and less than 14%, but from the viewpoint of the muffler corrosion resistance of the welded portion, in particular, 10%
It is preferable that it is at least 12%.

Ni: 0.05 to less than 1.0% Ni has an effect of improving the muffler corrosion resistance of the welded portion, and is contained in an amount of 0.05% or more to exhibit the effect. On the other hand, when the content is 1.0% or more, the ferrite structure becomes unstable and the effect on corrosion resistance is saturated. Therefore, the content is 0.05% or more and 1.0% or less, preferably 0.1% or more and 0.8% or less, more preferably 0.5% or less.
% Or more and 0.8% or less.

Ti: 0.05 to less than 0.3% Ti is one of the important components for the present invention. By adding 0.05% or more to the Nb-added steel, the muffler corrosion resistance of the weld is significantly improved. Although the reason is not clear, it is thought that the composition of the scale generated at the time of welding is changed by the trace amount of Ti when Nb and the composite are added. However, if the content is more than 0.3%, the muffler corrosion resistance of the welded portion is deteriorated. For this reason, Ti
It is easier to combine with N than Nb and Al and absorbs N in the atmosphere. Excessive Ti may absorb N and even C from the atmosphere during welding and reduce the muffler corrosion resistance of the weld. Conceivable. Furthermore, if the content is 0.3% or more,
Deterioration of surface properties due to TiN is also significant. Therefore,
Its content is not less than 0.05% and less than 0.3%.

Nb: 0.3 to less than 0.6% Nb is an important component having an effect of enhancing high-temperature strength, workability, and muffler corrosion resistance of a welded portion. To exhibit the effect, it is necessary to contain 0.3% or more. is necessary. However, when the content is 0.6% or more, a large amount of Laves phase precipitates, and the strength at room temperature is significantly increased.
Deterioration of workability and muffler corrosion resistance of welds. Therefore, the content is 0.3% or more and less than 0.6%, preferably 0.45% or more and less than 0.6%, and more preferably 0.5% or more and less than 0.6%.

P: less than 0.06% P is a component contained in steel as an unavoidable impurity. The smaller the content, the more preferable. However, the dephosphorization treatment increases the cost and, in this sense, does not reduce the P. Is advantageous. In the case of the present invention, a content of less than 0.06% is acceptable. Therefore, its content is less than 0.06%.

S: less than 0.01% S, like P, is a component contained in steel as an unavoidable impurity, and desulfurization increases the cost.
From the viewpoint of cost, it is advantageous not to lower the S. In the case of the present invention, a content of less than 0.01% is acceptable. Therefore, its content is less than 0.01%.

N: less than 0.020% N is a component that deteriorates the toughness and workability of steel.
The lower the content, the better, the content is 0.02
If it is 0% or more, the deterioration of toughness and workability becomes remarkable. Therefore, its content should be less than 0.020%.

V: 0.05 to less than 1.0% V is a useful component. When V is contained in an amount of 0.05% or more, the muffler corrosion resistance of the welded portion is improved. However, when the content is 1.0% or more, the processability is deteriorated.
05% or more, less than 1.0%, preferably 0.05% or more,
Less than 0.2% is good.

Cu: 0.02 to less than 1.0% Cu is a useful component having the same effect as V. When contained in an amount of 0.02% or more, the muffler corrosion resistance of the welded portion is improved. But 1.0
%, The workability deteriorates. Therefore, its content is preferably 0.02% or more and less than 1.0%,
Preferably it is more than 0.15% and less than 1.0%. More preferably, it is more than 0.3% and less than 1.0%.

Zr: 0.05 to less than 0.5% Zr is a useful component having the same effect as V and Cu. When contained in 0.05% or more, the muffler corrosion resistance of the welded portion is improved, but when the content is 0.5% or more. Workability deteriorates. Therefore, its content is preferably 0.05% or more and less than 0.5%.

Al: 0.01 to less than 0.5% Al is a useful component having the same effect as V, Cu, and Zr. When 0.01% or more is contained, the muffler corrosion resistance of the welded portion is further improved. The reason for this is not clear, but since Al is a component that is easily oxidized, including Al sufficiently is because the alumina coating at the time of welding has the effect of preventing absorption of N and C from the atmosphere. Conceivable. However, when the content is 0.5% or more, the workability is significantly deteriorated. Therefore, its content is preferably 0.01% or more and less than 0.5%. In addition, it is generally used as a deoxidizing agent for steel and may be inevitably contained. In this case, however, there is no adverse effect. By adding an appropriate amount as described above, the muffler corrosion resistance of the welded portion can be improved. Also, in the present invention, Si is used as a deoxidizing agent.
, The Al deoxidation is optional.

Mo: 0.01 to less than 0.8% Mo is a solid solution strengthening component and is effective for improving high temperature strength.
Particularly, it is preferable to add it when it is desired to increase the high temperature strength. The effect is exhibited when the content is 0.01% or more. However, when the content is excessively 0.8% or more, Mo is an expensive component, so that the cost is increased. Therefore, its content is more than 0.01%, 0.8%
% Is better.

B: 0.0002 to less than 0.005 B is effective for improving workability, and may be added when particularly high workability is required. The effect is 0.
Appears at 0002% or more, but when 0.005% or more, a large amount of BN
Conversely, the toughness deteriorates due to the formation of. Therefore, the content is preferably 0.0002% or more and less than 0.005%.

Ca: 0.001 to less than 0.03% Ca is a component that suppresses nozzle clogging due to Ti-based inclusions during slab casting.
Since it appears from 1% or more, adding 0.001% or more does not matter at all. However, even if the content is 0.03% or more, the effect is not only saturated, but also inclusions containing Ca become a starting point of pitting corrosion and deteriorate the corrosion resistance. Therefore, the upper limit of the content is preferably less than 0.03%.

The steel of the present invention adjusted to the above-mentioned composition has the following features.
It is important that the weld has a ferrite single phase structure .
This requires that the metallographic structure of the steel be a ferrite single phase structure over the entire temperature range of the welding heat cycle . When a mixed phase structure of ferrite-martensite is formed in the welded portion, the corrosion resistance is deteriorated.

[0069]

EXAMPLES After smelting steel having the various component compositions shown in Tables 1 and 2, a slab was formed and heated to a temperature of 1250 ° C.
Hot-rolled sheets each having a thickness of 5 mm were formed by hot rolling. Thereafter, these hot-rolled sheets were sequentially subjected to pickling, cold rolling, finish annealing at a temperature of 1000 ° C., and pickling to obtain cold-rolled annealed sheets each having a thickness of 2 mm.

[0070]

[Table 1]

[0071]

[Table 2]

With respect to each of the thus obtained cold rolled annealed sheets,
The high-temperature strength, the workability at room temperature, and the muffler corrosion resistance of the welded part were evaluated by the following methods.

High-Temperature Strength A plate (plate thickness: 2 mm) was pulled at a strain rate of 0.3% / min using a test piece, and the 0.2% proof stress at a temperature of 900 ° C. was measured. In Table 3 below, 18 MPa or more is shown as AA 15 MPa or more, less than 18 MPa is shown as A 10 MPa or more, and less than 15 MPa is shown as B is less than 10 MPa as C.

Workability at room temperature Elongation (El) was selected as an index of workability, and the tensile test pieces of JIS No. 13B shape at 0 °, 45 °, and 90 ° with respect to the rolling direction were each El. _0, El ₄₅ and El ₉₀ were measured, and El was calculated by the formula of El = (El ₀ + 2 × El ₄₅ + El ₉₀ ) / 4. In Table 3 below, A is 35% or more and B is less than 35%.

In the muffler corrosion resistance welding of the welded portion, TIG welding was performed on the plate surface (plate thickness: 2 mm) by a bead-on method under the following conditions. Welding speed: 600 mm / min Welding current: 200 A Atmosphere: Ar: sprayed onto the weld surface at 15 l / min. After this welding, heating was performed at 600 ° C. for 5 hours in air.

The welding sample of 500 mm × 100 mm obtained in this manner was suspended in a beaker and repeatedly subjected to repeated immersion-evaporation in a corrosion solution, as shown in FIG. The erosion depth was measured for each of these corrosion test pieces, and the erosion depth was 5
The corrosion resistance was evaluated by the average value of the points.

[0077] Incidentally, the liquid composition of the corrosion solution, to simulate the muffler corrosive _{^{environment, Cl -: 250, NO 2}} -: 100,
^{_{NO 3 -: 20, CO 3}} 2-: 2000, SO 3 2-: 1250, SO 4 2-: 1250, CH 3 C
OO ^-: 400, HCHO: 250 and HCOO ^-: 100 and (in PPm), PH: it was 8.5.

Table 3 summarizes the evaluation results thus obtained.

[0079]

[Table 3]

As is evident from Table 3, each of the comparative examples is inferior in high-temperature strength, workability, or muffler corrosion resistance of a welded portion as listed below.

Steel symbol A has too little Si, and the muffler corrosiveness of the weld is as poor as conventional steel symbol J (SUH409L).

The steel symbol B has poor workability due to too much Si.・ Since the steel symbol C contains an excessive amount of Ti, the steel symbol D contains an excessive amount of Mn, so that the muffler corrosion resistance of the welded portion is the same as the conventional steel symbol K (18Cr-0.2Ti). even worse. -The steel symbol E is inferior in workability due to excessive Nb, and is expensive. -Steel symbol F has poor workability due to excessive Ni. -In the steel symbol G, Si and Nb are too small, and both the high-temperature strength and the muffler corrosion resistance of the welded part are poor.・ Steel symbol H is poor in muffler corrosion resistance of welded parts because Ti is not actively added and its content is very small. -The steel symbol I is a steel with Ti alone and not a composite addition with Nb, so that the high-temperature strength and the muffler corrosion resistance of the welded portion are extremely poor.

On the other hand, in the conventional example, the steel symbol J (SUH409
L) is inferior in high temperature strength, muffler corrosion resistance of the welded part and muffler corrosion resistance of the base metal, and the steel symbol K (18Cr-0.2T
In (i), both the workability and the muffler corrosion resistance of the welded part are poor, and the steel symbol L (SUS430J1L) is inferior to the muffler corrosion resistance of the welded part.

In contrast to the comparative example and the conventional example, steel codes 1 to 18 of the applicable examples of the present invention all show high-temperature strength values higher than those of the conventional material (SUS 430 J1L), and the muffler of the welded portion. Corrosion resistance is superior to the conventional example (18Cr-0.2Ti), and the workability is better than conventional materials (SUS 430 J1L, 18Cr-0.2T).
i) The above characteristics are shown. In addition, the applicable example of the present invention has a lower Cr than the conventional SUS 430 J1L and 18Cr-0.2Ti, and is therefore less expensive.

Incidentally, in the muffler corrosion resistance of the base material, the applicable examples of these steel symbols 1 to 18 were inferior to the conventional 18Cr-0.2Ti. However, what is practically problematic is the most corrosive part, that is, the welded part. Therefore, since these conforming examples are superior to the muffler corrosion resistance of the 18Cr-0.2Ti weld, the muffler manufactured in the conforming example is 18Cr-0.2Ti.
It can be said that it has better durability than 0.2Ti muffler.

As described above, the applicable example of the present invention is that the heat resistance of the automobile exhaust system is equal to or higher than that of the conventional high temperature member (SUS 430 J1L) and the conventional low temperature member (18Cr-0.2Ti). It has the above-mentioned muffler corrosion resistance of the welded portion, and also has the workability more than that of each of the above conventional materials.In addition, it is economically advantageous because it has a lower Cr than these conventional examples. It is.

[0087]

According to the present invention, Ti-Nb with reduced Cr can be applied to both high and low temperatures for engine exhaust members.
According to the present invention, it is possible to provide a steel material excellent in heat resistance, workability and muffler corrosion resistance of a welded portion more inexpensively than a conventional material. It is required to be applicable to any material such as exhaust manifold of automobile engine exhaust system, front pipe, outer cylinder of converter, center pipe and muffler, etc., as well as oxidation resistance, workability and corrosion resistance in special corrosive environment. It can exhibit more excellent properties than materials.

Further, since the exhaust path member of the thermal power generation system is required to have the same characteristics as the exhaust member of the automobile engine, it can be very advantageously applied to this use.

[Brief description of the drawings]

FIG. 1 is a graph showing the relationship between the Cr content and the erosion depth of a weld.

FIG. 2 is a graph showing the relationship between Si content and Mn content and the erosion depth of a weld.

FIG. 3 is an explanatory view showing a corrosion test method.

────────────────────────────────────────────────── ─── Continuation of front page (72) Inventor Susumu Sato 1 Kawasaki-cho, Chuo-ku, Chiba-shi, Chiba Kawasaki Steel Engineering Co., Ltd. (56) References JP-A-6-184705 (JP, A) JP-A-Hei 8-120417 (JP, A) JP-A-8-325672 (JP, A) JP-A-4-228547 (JP, A) JP-A-9-268350 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) C22C 1/00-49/14 F01N 7/16

Claims (4)

(57) [Claims] 1. C: less than 0.02 mass%, Si: 0.5 mass% or more, less than 1.5 mass%, Mn: less than 0.2 mass%, Cr: 10 mass% or more, less than 14 mass%, Ni: 0.05 mass% or more, 1.0 mass%, Ti: 0.05 mass% or more, less than 0.3 mass%, Nb: 0.3 mass% or more, less than 0.6 mass%, P: less than 0.06 mass%, S: less than 0.01 mass%, and N: less than 0.020 mass% V: 0.05 mass% or more, less than 1.0 mass%, Cu: 0.02 mass% or more, less than 1.0 mass%, Zr: 0.05 mass% or more, less than 0.5 mass% and Al: 0.01 mass% or more, less than 0.5 mass% Contains one or two or more selected from the rest, and the rest is
And wherein Rukoto such a component composition of Fe and unavoidable impurities
Heat resistance and engine exhaust member for ferritic stainless steel having excellent muffler corrosion resistance of welds. 2. C: less than 0.02 mass%, Si: 0.5 mass% or more, less than 1.5 mass%, Mn: less than 0.2 mass%, Cr: 10 mass% or more, less than 14 mass%, Ni: 0.05 mass% or more, 1.0 mass%, Ti: 0.05 mass% or more, less than 0.3 mass%, Nb: 0.3 mass% or more, less than 0.6 mass%, P: less than 0.06 mass%, S: less than 0.01 mass%, and N: less than 0.020 mass% V: 0.05 mass% or more, less than 1.0 mass%, Cu: 0.02 mass% or more, less than 1.0 mass%, Zr: 0.05 mass% or more, less than 0.5 mass%, and Al: 0.01 mass% or more, less than 0.5 mass% selected inner shell one or more and Mo: 0.01 mass% or more, containing a less than 0.8 mass%, the balance is characterized Rukoto such a component composition of Fe and unavoidable impurities heat resistance and weld Ferrite stainless steel for engine exhaust parts with excellent muffler corrosion resistance. 3. C: less than 0.02 mass%, Si: 0.5 mass% or more, less than 1.5 mass%, Mn: less than 0.2 mass%, Cr: 10 mass% or more, less than 14 mass%, Ni: 0.05 mass% or more, 1.0 less than mass% Ti: 0.05 mass% or more, less than 0.3 mass% Nb: 0.3 mass% or more, less than 0.6 mass% P: less than 0.06 mass%, S: less than 0.01 mass%, and N: less than 0.020 mass%, and V : 0.05 mass% or more, less than 1.0 mass%, Cu: 0.02 mass% or more, less than 1.0 mass%, Zr: 0.05 mass% or more, less than 0.5 mass% and Al: 0.01 mass% or more, less than 0.5 mass% one or more and the B's: 0.0002mass% to less than 0.005 mass% and Ca: 0.001% or more, contains a less than 0.03 mass%, the balance being such a component composition of Fe and unavoidable impurities Rukoto A ferritic stainless steel for engine exhaust parts that has excellent heat resistance and muffler corrosion resistance at welds. 4. C: less than 0.02 mass%, Si: 0.5 mass% or more, less than 1.5 mass%, Mn: less than 0.2 mass%, Cr: 10 mass% or more, less than 14 mass%, Ni: 0.05 mass% or more, 1.0 mass%, Ti: 0.05 mass% or more, less than 0.3 mass%, Nb: 0.3 mass% or more, less than 0.6 mass%, P: less than 0.06 mass%, S: less than 0.01 mass%, and N: less than 0.020 mass% V: 0.05 mass% or more, less than 1.0 mass%, Cu: 0.02 mass% or more, less than 1.0 mass% Zr: 0.05 mass% or more, less than 0.5 mass%, and Al: 0.01 mass% or more, less than 0.5 mass% Mo: 0.01 mass% or more and less than 0.8 mass%, B: 0.0002 mass% or more, less than 0.005 mass%, and Ca: 0.001 mass% or more and less than 0.03 mass%. containing the balance being an engine exhaust member excellent in muffler corrosion resistance of heat resistance and weld, characterized in Rukoto such a component composition of Fe and unavoidable impurities Ferritic stainless steel.