JP3269799B2 - Ferritic stainless steel for engine exhaust parts with excellent workability, intergranular corrosion resistance and high-temperature strength - 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 engine exhaust member which is suitable for use as an exhaust member of an automobile, and which is particularly excellent in workability, intergranular corrosion resistance and high-temperature strength.

[0002]

2. Description of the Related Art Ferritic stainless steels are inexpensive as compared with austenitic stainless steels, and are therefore frequently used as exhaust system members for automobiles. Now, materials used for automobile exhaust system members are required to have workability (workability in primary processing and secondary processing), high-temperature strength, and intergranular corrosion resistance. Here, secondary processing refers to primary processing (for example,
(Drawing or bending) followed by bulging. Recently, a major problem has been that stainless steel that has undergone such secondary processing is susceptible to brittle fracture, particularly in winter when temperatures are low. Regarding high-temperature strength and intergranular corrosion resistance, measures have been taken to increase the purity of steel components and to add a large amount of Nb and / or Ti.
At relatively low temperatures where 0J1L is likely to cause erosion, SUH 409L is used. All of these steel grades are meeting the demands for improved high-temperature strength and intergranular corrosion resistance.However, as recent automobile exhaust system members that have been subjected to increasingly severe processing, both steels are processed. Not enough, especially 1
There is a strong demand for improved secondary and secondary workability. Furthermore, regarding the corrosion resistance, for example, in the case of an automobile muffler, the corrosive environment is becoming increasingly severe, and the SUH
In 409L, further improvement in its intergranular corrosion resistance is required.

Incidentally, as a proposal of a ferritic stainless steel aiming at such a use, Japanese Patent Application Laid-Open No. Hei 4-22847 has been proposed.
Japanese Patent Application Laid-Open Publication No. H11-157, discloses a highly-purified Nb-V- (Ti, Zr) composite-added steel for the purpose of improving high-temperature strength, intergranular corrosion resistance and tube formability.

[0004]

However, in the conventional steel exemplified in the above-mentioned Japanese Patent Application Laid-Open No. 4-228547, although the normal primary working is possible, the secondary workability is not sufficient. In addition, the primary processing was not always sufficient. This is because conventionally, as an index for evaluating the primary workability, an average r obtained from r values in each direction in a plane has been conventionally used.
The value or the rolling direction r value is used, and material development has been promoted while keeping this index in mind. However, in the actual cold-rolled sheet, due to the presence of anisotropy in the plane, processing failure occurs in the direction showing the lowest r value, and does not always correspond to the evaluation of the material so far. Absent.
Rather, the actual workability is determined by the minimum value of the r value (hereinafter simply referred to as r
_min ). From this point of view, the reason why the technique disclosed in the above-mentioned publication does not provide sufficient primary workability may be that development using the concept of r _min has not been conventionally performed.

Accordingly, an object of the present invention is to solve the problems of the above-mentioned known technology in a ferritic stainless steel used for an exhaust system member of an automobile engine.
It is another object of the present invention to provide a material which simultaneously satisfies all of the secondary workability, high temperature strength, and intergranular corrosion resistance. here,
The specific target value of primary workability is r _min ≧ 0.9, and the target value of high temperature strength is 0.2% proof stress at 900 ° C. ≧ 15 MPa.
It is.

[0006]

Means for Solving the Problems The inventors of the present invention have studied the means for improving primary and secondary workability, high temperature strength, and intergranular corrosion resistance in order to realize the above-mentioned objects. V,
Furthermore, it has been found that it is effective to add B in an appropriate range. At that time, it was also found that it was important to limit the contents as much as possible so as not to include Ti and Al as much as possible. The present invention has been made based on such knowledge, and the gist configuration thereof is as follows.

(1) C: 0.02 wt% or less, Si: 3.0 wt% or less, Mn: 0.6 wt% or less, P: 0.06 wt% or less, S: 0.01 wt% or less, Cr: 2.0 to 20.0 wt%, Ni : 1.0 wt% or less, Nb: 0.1 to 0.6 wt%, V: 0.05 to 1.0 wt%, Ti: 0.020 wt% or less, B: 0.0002 to 0.005 wt%, Al: 0.05 wt% or less, N: 0.020 wt% or less And the Nb content and V
The amount is V (wt%) ≧ −4.5 Nb (wt%) + 1.05, and the balance is Fe and unavoidable impurities. The workability, intergranular corrosion resistance and Ferritic stainless steel for engine exhaust parts with excellent high-temperature strength.

(2) The steel according to the above (1), wherein R
EM: A ferritic stainless steel for engine exhaust parts having excellent workability, intergranular corrosion resistance and high-temperature strength characterized by containing 0.001 to 0.10 wt%, with the balance being Fe and unavoidable impurities.

(3) The steel according to the above (1) or (2), further containing one or two selected from Cu: 0.05 to 1.0 wt% and Mo: 0.05 to 3.0 wt%. And
A ferritic stainless steel for engine exhaust members having excellent workability, intergranular corrosion resistance and high-temperature strength, characterized in that the balance consists of Fe and unavoidable impurities.

(4) The steel according to any one of the above (1) to (3), further comprising 0.02 to 0.3% by weight of Co, with the balance being Fe and unavoidable impurities. Ferritic stainless steel for engine exhaust components that excels in workability, intergranular corrosion resistance and high-temperature strength.

(5) The steel according to any one of the above (1) to (4), further comprising 0.001 to 0.03 wt% of Ca, with the balance being Fe and unavoidable impurities. Ferritic stainless steel for engine exhaust components that excels in workability, intergranular corrosion resistance and high-temperature strength.

[0012]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below in detail.
The inventors first need to add a large amount of Nb to Nb alone-added steel in order to improve intergranular corrosion resistance. In such a component system, primary workability is deteriorated, and secondary It has been found that a large amount of B needs to be added because the processability is also deteriorated. Then, it was determined that the addition of such a large amount of Nb and B further deteriorates the primary workability, and that the addition of a large amount of Nb also causes an increase in cost and is not practical. Next, Nb−
We focused on Ti-added steel. In this case, relatively less
With Nb content, high temperature strength, intergranular corrosion resistance and primary workability are improved, but secondary workability is not improved unless a large amount of B is added, resulting in lower primary workability and toughness. In addition, there is a problem that the manufacturability deteriorates, and all the characteristics (high-temperature strength, intergranular corrosion, primary
It was thought that it was difficult to combine the following workability).

However, in the case of the Nb-VB system, relatively low Nb improves high-temperature strength, intergranular corrosion resistance, and primary workability, and further, secondary addition is improved with a small addition of B. I understand. Here, it has been obtained as a particularly important finding that the improvement of the primary workability by the addition of V is manifested when the amount of Al is not limited to an extremely small amount (see FIG. 1). The improvement of the properties is that it is not exhibited unless Ti is limited to an extremely small amount (see Comparative Steel B in Tables 1 and 2 described later). For the reasons described above, the present invention particularly
The upper limits of the contents of Ti and Al are strictly limited. The component system of Nb-V-B, in which the upper limits of the Ti and Al contents are limited to a very small amount, meets the intended purpose, high temperature strength, intergranular corrosion resistance, primary and secondary corrosion resistance. It was found that the material had all of the following workability. In addition to the above basic characteristics required for the exhaust member, REM is further added for improving oxidation resistance, and further for improving corrosion resistance.
The addition of Cu and Mo further increases Co to improve the toughness of the weld.
Was found to be effective. These additional elements are added according to the properties required by the application in addition to the basic properties.

Next, the reason why the composition of the ferrite steel is limited in the present invention will be described. C: 0.02 wt% or less C is an element that degrades toughness and workability.
If the content exceeds wt%, the toughness and workability deteriorate significantly, so that the content is set to 0.02 wt% or less. In addition, from the viewpoint of toughness and workability, the lower the C content, the better, and it is desirable to suppress the content to 0.01 wt% or less.

Si: 3.0 wt% or less Si is an element effective for improving oxidation resistance and high-temperature salt damage. However, if it exceeds 3.0 wt%, primary and secondary workability is significantly deteriorated. Therefore, the upper limit is 3.0 wt%. Note that the Si content is desirably 0.8 wt% or less.

Mn: not more than 0.6 wt% Mn is added for deoxidation, but excessive addition results in coarse MnS
To reduce workability and increase anisotropy. Therefore, the amount of Mn is set to 0.6 wt% or less, preferably 0.2 wt% or less, and more preferably 0.1 wt% or less.

P: 0.06% by weight or less P is bad as it has a bad influence on corrosion resistance.
The de-P treatment involves an increase in cost. For this reason, the upper limit is set to 0.06 wt% which is acceptable in actual production.

S: 0.01 wt% or less S has a bad effect on corrosion resistance, so the smaller the better, the better.
The processing for lowering the S involves an increase in cost. For this reason, the content is set to 0.01 wt% or less, which is acceptable in actual production, and the cost is 0.002 to 0.002%.
0.01 wt% is desirable.

Cr: 2.0 to 20.0 wt% Cr is an element that contributes to the improvement of corrosion resistance, and it is necessary to add 2.0 wt% or more for use in automobiles. But 20.0
If it is added in excess of wt%, the processability decreases and the cost increases, so the upper limit is 20.0 wt%, preferably 16.0 wt%.
Less than wt%.

Ni: 1.0 wt% or less Since Ni is an element that lowers workability, it is set to 1.0 wt% or less.

Nb: 0.1-0.6 wt% Nb is an element that enhances high-temperature strength and intergranular corrosion resistance.
It is necessary to add 1 wt% or more. In particular, by adding Nb-V composite, excellent primary workability can be obtained. However, 0.6 wt
% Tends to saturate the effect, and the cost is unnecessarily high.
wt%, preferably 0.45 wt%. In addition, when adding Nb-V composite, it is necessary to satisfy V (wt%) ≧ −4.5 Nb (wt%) + 1.05 as shown in FIG. 2 from the viewpoint of intergranular corrosion resistance.

V: 0.05-1.0 wt% V greatly contributes to the improvement of the primary workability by the complex addition with Nb. FIG. 1 shows r _min as an index of primary workability.
The effect of V on the value is shown. According to the investigations of the inventors, even if the average r value is high, if r _min is low, processing may not be performed. This is because the actual breaking occurs in the direction of the lowest r value (r _min ) in the plane of the plate. Although this r _min value is important in considering the primary workability, it has conventionally been considered little. As a result of further studies by the inventors, it has been found that the combined addition of Nb-IV is effective in improving both the average r value and the r _min value. The effect becomes remarkable at 0.05 wt% or more in the case of composite addition. However, even if added over 1.0 wt%, the effect is saturated and the cost is increased. I do. As described above, it is necessary to satisfy V (wt%) ≧ −4.5 Nb (wt%) + 1.05 from the viewpoint of intergranular corrosion resistance.

Ti: 0.020 wt% or less Ti has conventionally generally been considered to have an effect equivalent to Nb, V, etc., but for the purpose of the present invention, the secondary workability and the surface properties are reduced. It is a harmful element that degrades and must be actively eliminated. The effect becomes remarkable when it exceeds 0.020 wt%, so it is limited to 0.020 wt% or less.

B: 0.0002 to 0.005 wt% In the Nb-V component system, high temperature strength, intergranular corrosion resistance and primary workability are improved, but secondary workability is deteriorated. B is an element effective for improving the secondary workability. The effect of improvement is apparent with the addition of 0.0002 wt% or more,
If it is added in excess of 005 wt%, the deterioration of the primary workability becomes significant. Therefore, the amount of B is added in the range of 0.0002 to 0.005 wt%.

Al: 0.05 wt% or less Al is usually used as a deoxidizing material, but is a harmful element that deteriorates the primary workability and must be positively eliminated. The effect becomes remarkable when it exceeds 0.05 wt%, so it is limited to 0.05 wt% or less.

N: 0.020 wt% or less N is an element that deteriorates the toughness and workability of steel.
If the content exceeds 0.020 wt%, the degree becomes remarkable.
020 wt% or less, preferably 0.010 wt% or less.

REM: 0.001 to 0.10 wt% REM means a rare earth element, that is, Y and a group of lanthanoid elements such as La and Ce. At 0.001% by weight or more, it has an effect of improving the oxidation resistance. However, if it exceeds 0.10% by weight, the workability deteriorates remarkably and it is expensive, so it is limited to 0.001 to 0.10% by weight. What is called misch metal containing La and Ce is also a kind of REM. Any elemental group of lanthanoids, such as Y, La and Ce, alone or in combination, contributes to the improvement of oxidation resistance.

Cu: 0.05-1.0 wt% Cu is added as necessary to improve corrosion resistance. The effect is exhibited by addition of 0.05 wt% or more, but since addition of a large amount deteriorates workability, it is limited to 1.0 wt% or less. A desirable addition range is 0.15 to 0.30 wt%.

Mo: 0.05 to 3.0 wt% Mo is added as necessary to improve corrosion resistance. The effect is exhibited by addition of 0.05 wt% or more, but since addition of a large amount deteriorates workability, it is limited to 1.0 wt% or less.

Co: 0.02 to 0.3 wt% Co improves the toughness of the weld. The effect is 0.02wt
%, The addition of a large amount leads to deterioration of workability and high cost, so the upper limit is 0.3 wt%.

Ca: 0.001 to 0.03 wt% Ca is an element having an effect of suppressing nozzle clogging due to inclusions during slab casting, and is added as necessary. The effect is manifested when 0.001 wt% or more is added. However, even if added over 0.03 wt%, the effect is not only saturated, but inclusions containing Ca serve as a starting point of pitting corrosion and deteriorate corrosion resistance. The upper limit is 0.003 wt%.

The method for producing the steel of the present invention may be a general process used in producing this type of steel. For example, a steel having a predetermined component composition is melted by a conventional steelmaking method such as a converter or an electric furnace, and is made into a slab by a continuous casting method or an ingot casting method, and then hot-rolled-(hot-rolled sheet annealing) -Pickling-Cold rolling-Finish annealing-Pickling, and depending on the application, cold rolling-Finish annealing-
A method of repeating pickling may be used. In particular, a process in which the hot-rolled sheet annealing is omitted is advantageous in terms of cost and desirable.

[0033]

EXAMPLES Steel having the composition shown in Table 1 was melted, heated to 1160 ° C., and then hot-rolled to a thickness of 5 mm to obtain a hot-rolled sheet. This hot rolled sheet is pickled, cold rolled to a sheet thickness of 2 mm,
Finish annealing and pickling were sequentially performed at a temperature of at least ℃ to obtain a cold-rolled annealed plate. The cold-rolled annealed sheet thus obtained is subjected to the following methods for high-temperature strength, intergranular corrosion resistance, primary and secondary corrosion resistance.
The following workability was evaluated. In addition, about the hot-rolled sheet, the surface was also observed, and it was judged by ○ (care required) and × (care required).

[0034]

[Table 1]

(1) High-Temperature Strength At 900 ° C., a 0.3% /
A tensile test was performed at a speed of one minute, and a 0.2% proof stress was measured. Among the current materials, SUS430 J1L used for high-temperature members such as an exhaust manifold has a value of 15 MPa. Therefore, 15 MPa or more was evaluated as 、, and less than 15 MPa was evaluated as ×.

(2) Intergranular Corrosion Resistance SUH 409L is mainly used in a relatively low temperature use environment such as a muffler and an exhaust pipe. The test conditions
For the current material SUH 409L, severe conditions were set so as to cause intergranular corrosion, and the state of corrosion was evaluated. The conditions are as follows. Test conditions After heating at 1250 ° C. for 10 minutes, air cooling was performed to make the grain boundaries easily corroded, and then an improved Strauss test was performed. Corrosion solution is 100 ml / l sulfuric acid, 100 g /
l and immersed in the solution during boiling for 16 hours. After that, bending was performed at 180 degrees, and evaluation was made with or without (x) and with (o) cracks having a depth of 1 mm or more.

(3) Primary workability From the 0 °, 45 °, and 90 ° directions with respect to the rolling direction, JIS 13
No. B tensile test pieces (plate thickness 2 mm) were sampled, and r
The values were measured and evaluated from the lowest value r _{min as} follows. ：: r _min ≧ 0.9 △: r _min = 0.8 to less than 0.9 ×: r _min <0.8 (4) Secondary workability A cup having a drawing ratio of 2 was prepared by the primary processing, and this cup is shown in FIG. Perform a drop test at 40 ° C
Evaluation was made with or without cracks (x) or with (o).

Table 2 shows the test results obtained. Comparative steel A
Is a single addition of Nb, but has relatively low Nb and thus has excellent secondary workability, but has a low r _min , and FIG.
, The grain boundary corrosion resistance is poor. Furthermore, the high temperature strength is insufficient. Since the comparative steel B contains Ti, the secondary workability is inferior even though 0.0003 wt% of B is added, and the surface properties of the hot-rolled steel sheet are also poor. Comparative steel C is an Nb-trace amount V material and satisfies the range of FIG. 2 and thus has excellent intergranular corrosion resistance, but a large amount of Nb results in low r _min representing primary workability. ,
It is also costly. In addition, since B is not added, 2
Secondary workability is also insufficient. Comparative steel D is inferior in primary workability because Al is added. Comparative steel E is Nb-V
-B material. 2, the intergranular corrosion resistance is excellent, but a large amount of Nb is added, so despite the addition of V, the primary workability is insufficiently improved and the cost is high. Also. Comparative steel F is a V-trace Nb material. Since the range of FIG. 2 is not satisfied, the intergranular corrosion resistance is insufficient and the high-temperature strength is also insufficient.
Comparative steel G is the current material (SUH 409L). Neither property is equal to invention steels 1-7. Comparative steel H is the current material (SU
S430 J1L). Since it satisfies the range shown in FIG. 2, it has excellent intergranular corrosion resistance, but has a small amount of V added, so that r _min representing primary workability is low. Further, since B is not added, the secondary workability is also insufficient. In contrast to these comparative examples, the inventive steels 1 to 7 all have all the properties such as high-temperature strength, intergranular corrosion resistance, primary workability, and secondary workability required by automobile exhaust system members. This can be achieved with a relatively small amount of added Nb.

[0039]

[Table 2]

[0040]

As described above, according to the present invention, it is possible to provide a ferritic stainless steel excellent in all of high temperature strength, grain boundary corrosion resistance, and workability (primary and secondary). It becomes possible to further improve the manufacturability of engine exhaust members and the quality of products that require these characteristics. In addition, this steel can be applied to the use of an exhaust path member of a power generation system in which characteristics similar to those of an exhaust member of an automobile engine are required.

[Brief description of the drawings]

FIG. 1 is a graph showing the influence of the amount of V on the r _min value of ferritic stainless steels having different amounts of Al and Ti.

FIG. 2 is a graph showing the influence of the amount of Nb and the amount of V on intergranular corrosion resistance.

FIG. 3 is a schematic diagram for explaining a test method of secondary workability.

Continuation of the front page (56) References JP-A-9-3606 (JP, A) JP-A-10-237600 (JP, A) JP-A-4-228547 (JP, A) JP-A-10-17999 (JP, A) JP-A-7-126812 (JP, A) JP-A-7-331389 (JP, A) JP-A-8-120417 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB Name) C22C 38/00-38/60

Claims (5)

(57) [Claims] C: 0.02% by weight or less, Si: 3.0% by weight or less, Mn: 0.6% by weight or less, P: 0.06% by weight or less, S: 0.01% by weight or less, Cr: 2.0 to 20.0% by weight, Ni: 1.0 wt% or less, Nb: 0.1 to 0.6 wt%, V: 0.05 to 1.0 wt%, Ti: 0.020 wt% or less, B: 0.0002 to 0.005 wt%, Al: 0.05 wt% or less, N: 0.020 wt% or less The workability and intergranular corrosion resistance, characterized in that the content is such that the content of Nb and the content of V satisfy the relation of V ≧ −4.5 Nb + 1.05 and the balance consists of Fe and unavoidable impurities. Ferrite stainless steel for engine exhaust parts with excellent high-temperature strength. 2. The steel according to claim 1, further comprising REM.
: A ferritic stainless steel for engine exhaust parts having excellent workability, intergranular corrosion resistance and high-temperature strength characterized by containing 0.001 to 0.10 wt%, with the balance being Fe and unavoidable impurities. 3. The steel according to claim 1 or 2, further comprising one or two selected from Cu: 0.05 to 1.0 wt% and Mo: 0.05 to 3.0 wt%. A ferritic stainless steel for engine exhaust members having excellent workability, intergranular corrosion resistance and high-temperature strength, characterized in that the balance consists of Fe and unavoidable impurities. 4. The steel according to claim 1, further comprising Co: 0.02 to 0.3 wt%, with the balance being Fe.
A ferritic stainless steel for engine exhaust members having excellent workability, intergranular corrosion resistance and high-temperature strength, characterized by being composed of unavoidable impurities. 5. The steel according to claim 1, further comprising 0.001 to 0.03 wt% of Ca, with the balance being Fe and unavoidable impurities.
Ferritic stainless steel for engine exhaust parts with excellent workability, intergranular corrosion resistance and high-temperature strength.