JP3021656B2 - Ferritic stainless steel with excellent high-temperature salt damage resistance and high-temperature strength - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to ferrite having high high-temperature strength and particularly excellent high-temperature salt damage resistance, which is used as a high-temperature member such as an exhaust pipe of an automobile, a catalyst outer casing material, and an exhaust duct of a power plant. Related to stainless steel.

BACKGROUND ART In recent years, it has been desired to improve the fuel efficiency and output of automobiles, and to strongly reduce the weight of automobile materials. In addition, due to the strengthening of pollution regulations, there is a strong demand for purification of exhaust gas. From such a background, SUS430LX and AISI409, which are existing ferritic stainless steels, are currently used as automotive exhaust system materials from the viewpoint of weight reduction and low heat capacity as components.
On the other hand, the fuel efficiency has been further improved and the output has been increased, and the maximum temperature of the exhaust gas has been rising to around 900 ° C., and the temperature rises to around 900 ° C. near the exhaust manifold and around 600 ° C. near the front pipe. Therefore, the heat-resistant material used for these thin plate structures needs the following material characteristics.

(1) Ensuring high high-temperature strength and high-temperature strength during use (2) Thermal fatigue properties and high-temperature fatigue properties (3) Workability at normal temperature (4) High-temperature salt damage resistance (5) Oxidation resistance (6) Stress concentration It is a weld bead shape that does not easily rise.

Here, it is expected that the high-temperature strength is improved and the high-temperature strength during use is ensured, thereby improving the high-temperature fatigue and the heat-resistant fatigue resistance. By achieving the characteristics (1), (2)
Is also expected to improve. In addition, since the above-mentioned structure has a thin plate welded structure, heat resistance and fatigue resistance including weldability are required. Therefore, the item (6) is also an important item.

Further, the high-temperature salt damage resistance property (4) has been particularly noted in automobile exhaust system parts which have recently been thinned.
When a car travels on a road surface sprayed with anti-freezing material (mainly salt) in severe winter, the anti-freezing material scatters on the exhaust manifold and front pipe heated to around 600 ° C by automobile exhaust system parts, especially exhaust gas. If adhered, the adhered surface of the part will be corroded, causing a thinning phenomenon, and eventually a broken state, causing a serious accident.

Therefore, improvement of high-temperature tensile strength and improvement of high-temperature salt damage resistance of automobile exhaust system materials by increasing the temperature of exhaust gas are important research subjects.

The following are disclosed as techniques for improving the above various characteristics.

Examples of exhaust manifold applications include JP-A-64-8254, JP-A-3-274245, and JP-A-4-74852. JP-A-64-8254 discloses Cr from the viewpoint of oxidation resistance.
Is increased to 17% or more, and Nb is indispensable and Mo is selected as an element, but an element (eg, Ti) having a higher affinity for C and N than Nb or Mo is not added. Because of this,
Nb is in a state where carbonitride is easily formed during use, and no consideration is given to ensuring high-temperature strength during use. JP Hei 3
With respect to JP-274245A, Nb and Mo are essential and Ti is a selective element on the high Cr side. Also in this case, it is not considered how to secure the strength during high temperature use, Ni and Cu are essential, and the present invention is in a different chemical component range including Cr. With respect to JP-A-4-74852, Cr is lower than that of JP-A-3-274245, and Nb and Ti are essential.
No addition of Al and 0.5% or less of Si, and no consideration is given to high-temperature salt damage resistance and oxidation resistance when low Cr is used.

Further, as a muffler application, JP-A-3-264652 can be mentioned. This has a wide Cr content of 11 to 30%, with Ti and Nb as essential elements and Mo as a selective element. In addition, a salt spray test at 35 ° C. was performed as salt resistance at room temperature. For salt damage corrosion, Cr was 18% or more and Mo was 1.0 to 1.0%.
It is desirable to add 4.0%. The normal temperature salt damage corrosion here is similar to the rust resistance (rust resistance) of stainless steel, and it is a general opinion that the addition of Cr and Mo improves the corrosion (for example, N. Suutala). et.al.:Sta
inless Steels '84 Gotebrog, Sweden, P. 240 (1984)).
On the other hand, high temperature salt damage resistance refers to salt damage corrosion resistance at high temperatures, and unlike rust resistance, a phenomenon in which corrosion in the order of 100μ proceeds in the form of temperature-dependent general corrosion, and As shown in FIG. 9, Cr has almost no effect on high-temperature salt damage resistance at 700 ° C. Therefore, it can be said that the salt damage corrosion phenomenon at normal temperature and the salt damage corrosion phenomenon at high temperature have completely different characteristics. As described above, JP-A-3-264652 does not consider any high-temperature salt damage resistance.

As described above, none of the known documents discloses or suggests securing high-temperature strength of automobile exhaust system materials, particularly, strength during high-temperature use and improvement of high-temperature salt damage resistance.

The present invention is a heat-resistant ferrite-based material that achieves both material properties before use equal to or higher than conventional SUS430LX and low production cost, and ensures material properties such as high-temperature strength during use, and high-temperature salt damage resistance at the same time. The purpose is to provide stainless steel.

A further object of the present invention is to provide a heat-resistant ferritic stainless steel capable of achieving material properties such as oxidation resistance, workability, and a good weld bead shape in the above materials.

Configuration of the Invention In order to achieve the above object, the present invention provides a cold rolled annealed sheet
The required amount (the range of eff.Nb) of the solid solution of each component of Mo, W and Nb was secured.

According to the study of the present inventors, the improvement of high-temperature strength and high-temperature salt damage resistance of heat-resistant ferritic stainless steel is mainly caused by solid solution N
b, Mo and W, and to improve high temperature strength, solute Nb, M
For o and W, solid solution Mo and W are effective for high-temperature salt damage resistance. The present inventors have focused on securing solid solution of Mo, W, and Nb in a cold-rolled annealed sheet, and have sought to efficiently dissolve these elements. That is, by lowering C + N and further adding an appropriate amount of Ti to fix them, lowering the amount of solid solution C + N and improving the workability,
High temperature precipitation strengthening by Ti (C, N) also worked.

Also, the most important role of Ti addition is that C, N rather than Mo, W or Nb.
Utilizing its strong affinity with C, N
Is to fix. By fixing C and N, Mo, W and N
Prevention of the precipitation of carbonitride b) enables the solid solution amount of these elements to be secured not only before use but also for a long time at high temperature. This has the effect of reducing the amount of Nb added by adding Ti as compared with the case of adding Nb alone to obtain the same amount of solid solution Nb, so the raw material cost is also reduced especially when Ti is used. It is possible (in general, Nb has a higher raw material cost per unit weight than Ti). Therefore, it is inexpensive, and the high-temperature strengthening ability and high-temperature salt damage resistance of solid-solution Nb, W, and Mo can be effectively functioned before use, and the effects of these elements can be secured during use. Made it possible. In addition, in order to secure high high-temperature strength especially during driving for a long period of time, that is, to ensure high-temperature strength of members during actual vehicle running, the amount of eff. Stipulated.

Conventionally, the concept of eff.Nb is to consider MC type (Nb · C) as a precipitated carbide of Nb as disclosed in JP-A-1-41694, and to add the amount of Nb used in the MC-type carbide. The value subtracted from the amount of Nb was the amount of solute Nb, which was assumed to be eff.Nb.

This eff.Nb is (1) that only the so-called solid solution Nb amount before use is defined and taken into consideration. (2) When Ti is contained,
No reduction in strength during use was considered at all, such as the condition that preferential precipitation of Ti carbonitride occurred.

However, according to the present invention, (1) the precipitation of Nb during use (assuming that the vehicle is running: 600 to 900 ° C.) changes from MC type to M ₆ C type (Fe ₃ Nb ₃ C); 2) N is clarified that when Ti is added, N is fixed by Ti and 1/2 of N is precipitated as a nitride, and (3) precipitation of intermetallic compound (Laves phase) with Fe is to some extent. Eff.Nb was specified as an index for maintaining high-temperature strength before and during use, taking into account that it is effective for strengthening. The tensile strength at 850 ° C before use is 3
It is necessary that the tensile strength at 4 MPa or more and the tensile strength at 900 ° C. be 25 MPa or more in order to ensure high temperature fatigue properties and thermal fatigue properties. In addition, ensuring high-temperature strength during use dramatically improves thermal fatigue properties, which is the most important required property as a material for an exhaust manifold. This is because solid solution Nb, M
Since the amount of o and W is secured over a long period of time at a high temperature, a decrease in strength that occurs during thermal fatigue is unlikely to occur, and the thermal fatigue life is dramatically extended.

Furthermore, the addition of Ti and Nb also prevents coarsening of the grain size in the welded part and the weld affected part, so that the weldability is good.

On the other hand, Mo, W and Nb are easy to form an intermetallic compound with Fe,
If this precipitates in large quantities and becomes coarse, it deteriorates toughness during use and high-temperature strength. Mo and W are effective for high-temperature salt damage resistance, but the addition of a large amount degrades high-temperature salt damage resistance. Also, Nb and M
The addition of o and W has manufacturing problems such as raising the recrystallization temperature and lowering the toughness of the steel sheet. For these reasons, the upper limits of Mo, W, Nb and eff. Nb are determined.

Further, the steel of the present invention has a lower Cr content than SUS430LX from the viewpoint of reducing raw material costs. Therefore, in order to ensure oxidation resistance, Si and, if necessary, Al were added to such an extent that workability and weldability were not impaired. Si and Al are elements that improve not only oxidation resistance but also high-temperature salt damage resistance. In addition, a rare earth element was added in a range that does not impair hot workability in order to adapt particularly to a member requiring oxidation resistance.

As described above, the steel of the present invention fully considers the optimal addition balance of C, N, Si, Cr, Ti, Mo, W, Al and Nb, and can simultaneously achieve the required characteristics, and At the same time, material costs can be reduced.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a diagram showing the relationship between the Mo content in 17Cr alloy (without Si and Al) and the amount of high-temperature salt corrosion thinning.

FIG. 2 is a diagram showing the relationship between the content of Mo and / or W in a 17Cr, low Si alloy and the amount of high-temperature salt corrosion thinning.

FIG. 3 is a diagram showing the relationship between the Mo content and the high-temperature salt damage corrosion thinning amount in a 17Cr high Si alloy.

FIG. 4 is a diagram showing the relationship between temperature and tensile strength in various alloys.

FIG. 5 is a graph showing the relationship between the aging time associated with high-temperature aging and the amount of dissolved Nb.

FIG. 6 is a diagram showing the relationship between eff.Nb and tensile strength at 900 ° C. after aging at 900 ° C. for 500 hours.

FIG. 7 is a diagram showing the results of a 200 ° C / 900 ° C thermal fatigue test of Al and SUS430LX.

 FIG. 8 is a diagram showing the relationship between the temperature and the corrosion thinning amount.

FIG. 9 is a graph showing the effect of the amount of Cr on high-temperature salt damage resistance.

BEST MODE FOR CARRYING OUT THE INVENTION The present inventors have conducted various studies on high-temperature salt damage of automobile exhaust system materials related to ferritic stainless steel, which had not been sufficiently studied so far. It has been determined that the element is an extremely effective element.

FIG. 1 shows the relationship between the amount of Mo added to a 17Cr alloy (manufactured by vacuum melting) to which Si, Al, and the like are not added, and the amount of addition of Mo to high-temperature salt damage, that is, the amount of high-temperature salt corrosion thinning.

High-temperature salt damage thinning is an index of high-temperature salt damage, and refers to the thickness of thinning under the following conditions that simulates that a car actually runs on a road surface on which antifreeze has been sprayed. (Mm / 40 cycles).

(1) Immerse the material in saturated saline for 5 minutes.

(2) Heat to 700 ° C and hold for 120 minutes.

(3) Air-cool for 5 minutes.

(4) Repeat the above 40 times.

As shown in FIG. 1, the present inventor can reduce the wall thickness by about half by adding 0.4% by weight of Mo as compared with the case where Mo is not added, and further, by adding 1.5% by weight. By doing so, it was confirmed that the thickness could be reduced to about 1/5. In ferritic stainless steel,
This property of Mo was a completely new finding for the inventors.

The present inventors have further studied and found that W, which is the same substance as Mo, is Mo
It was confirmed that the same effect as described above was obtained, and that the addition of Si (Al) sign further reduced the above-mentioned wall thinning amount.

This is shown in FIGS. 2 and 3. FIG. 2 is a plot of the data of Example 1 described below.
The addition of Si in the range of 0.1 to 0.5% by weight to the composite addition of Mo and W, and the addition of 0.2 to 2.7% of Mo and W results in 0.2 mm /
It shows that the amount of thinning around 40 cycles can be obtained.

FIG. 3 is a plot of the data of Example 2 to be described later, showing the relationship between the amount of Mo added and the amount of wall thinning when Si is added in excess of 0.5. Quantity
It shows that it falls within the range of 0.1 to 0.2 mm / 40 cycles, which indicates that the salt damage resistance is superior to the case of FIG.

That is, it has been found that when Mo or W is added within the range of 0.1 to 2.0%, the amount of high-temperature salt-corrosion thinning rapidly decreases.

(1) Soak the material in 5% saline for 10 minutes.

(2) Heat to 720 ° C and hold for 90 minutes.

(3) Air-cool for 5 minutes.

(4) Repeat the above 20 times.

Thus, Mo or W has a very remarkable effect on high-temperature salt damage.

Further, as described above, since the present invention can sufficiently secure the intrinsic Nb amount, the tensile strength at 850 ° C. as a high-temperature strength before use can be 34 MPa or more as a high-temperature strength before use, and the tensile strength at 900 ° C. can be 25 MPa or more. As a result, excellent high-temperature fatigue characteristics and thermal fatigue characteristics can be ensured, and FIG. 4 shows these. The figure shows the relationship between tensile strength and temperature,
It shows that the steel of the present invention (the Al steel in Table 3 in Table 1) has a desired high strength at high temperatures of 850 ° C. and 900 ° C. (the dotted line in the figure is the lower limit of the present invention). In addition, the □ marks indicate steel (SUS430LX (19Cr 0.5N
b)), the tensile strength decreased at the high temperature side of 900 ° C, and the symbol △ indicates steel with only Ti added (AISI409 (11Cr 0.3T
i)), which shows lower strength than the steel of the present invention.

 Hereinafter, the reasons for limiting the components of the present invention will be described.

C: The steel of the present invention mainly supports high-temperature strength before and during use by solid solution strengthening of Nb, Mo, and W, and also contains C from the viewpoint of improving workability and toughness of hot-rolled sheet. Want to keep low. However, the extremely low temperature is disadvantageous to the economy, and a part of the high temperature strength before use is supported by carbonitride of Ti and Nb. C + N ≦
0.03%.

Si: an element that is effective as a deoxidizing material and improves oxidation resistance and high-temperature salt damage resistance. The steel of the present invention is SUS430LX
Since the Cr content is lower than that of, the addition amount is required to be 0.1% or more from the viewpoint of improving oxidation resistance, particularly from the viewpoint of improving oxidation resistance in exhaust gas. In addition, since it is an effective element for high-temperature salt damage resistance,
More than 0.5% is preferred. On the other hand, the lower limit was set to 1% in order to reduce workability and weldability.

Mn: At least 0.1% is necessary because it is a deoxidizing element. Further, the upper limit is set to 1% in order to prevent martensitic transformation as an austenite forming element.

P: Useful for high temperature and high strength (solid solution strengthening), but causes deterioration of weldability, so it was made 0.01 to 0.1%.

S: an element forming MnS, the content is set to 0.01% or less in order to reduce the corrosion resistance which is a basic property of stainless steel.

Cr: effective at improving oxidation resistance and high-temperature salt damage resistance, 900 ° C
It is 13% or more to secure oxidation resistance in the vicinity. or,
Assuming that the maximum temperature of the steel of the present invention is around 900 ° C, the addition of 17% or more is not very effective.
Less than 17%.

Nb: an additional element for preventing grain growth and ensuring high-temperature strength in the weld and the weld affected zone. However, C, N and Fe
To form precipitates during use and to make the effect of solid solution strengthening of Nb work more effectively, 0.1 to less than 0.5%, and preferably 0.1 to 0.4% as eff.Nb. did.

eff.Nb: An index for securing high-temperature strength during use. The steel of the present invention supports high-temperature strength with solid-solution Nb, Mo, and W. Among them, solid-solution Nb has the highest-temperature strengthening effect. However, Nb easily forms precipitates with C, N and Fe, and it is thought that some of these also contribute to high-temperature strengthening like solid-solution Nb,
During use at high temperatures (during actual vehicle operation), the precipitated phase aggregates and becomes coarse and solid solution
Nb decreases. FIG. 5 (based on each sample of Example 2) shows the change in the amount of solute Nb due to simple aging at 900 ° C. in consideration of continuous use at 900 ° C. With aging, the solute Nb decreases. However, the degree of the decrease was due to SUS430 with only Nb added.
Compared with LX and B2, Al, which is one of the steels of the present invention to which Ti and Nb are added in combination, is less. This means that solid solution Nb, which is a high-temperature strengthening factor, is secured with high temperature use, and the high temperature strength of the member during high temperature use is secured. This is because the effect of adding Ti prevents the bonding of C and Nb, so that the amount of solid-dissolved Nb during high-temperature use can be secured. Focusing on this point, eff.Nb based on the following equation in the case of the composite addition of Ti and Nb, that is, a factor relating to securing high-temperature strength during high-temperature use was defined. As shown in FIG. 6 (based on the sample of Mo: 0.4 to 0.6% in Example 2), in consideration of high temperature use, a high temperature strength (tensile strength at 900 ° C.) of 18 MPa after aging at 900 ° C. for 500 hours can be secured. The lower limit of the amount of eff.Nb was 0.1. On the other hand, if it exceeds 0.4, the increase in the high-temperature strength accompanying the increase in eff and Nb is saturated, so the upper limit was made 0.4%.

 Note that eff.Nb is obtained by the following equation.

eff.Nb (%) = Nb (%)-3 × 93 × fc / 12−93 × fn / 14 (1) where (1) Ti (%) − 48 · (N (%) / 2) / When 14> 0, C (%)-12 · {Ti (%)-48 · (N (%) / 2) / 14} /
When 48> 0, fc = C (%)-12 ｛Ti (%)-48 · (N (%) / 2) / 1
4｝ / 48 fn = N (%) / 2 C (%)-12 ｛{Ti (%)-4848 (N (%) / 2) / 14｝ /
When 48 ≦ 0, fc = 0 fn = N (%) / 2 (2) When Ti (%) − 48 · (N (%) / 2) / 14 ≦ 0, fc = C (%) fn = N ( %)-14. (Ti (%) / 2) / 48 That is, (1) When the number of Ti atoms is more than 1/2 of the number of N atoms, half of N is NbN and the other half is TiN.

i) When the number of Ti atoms remaining from TiN is smaller than the number of C atoms, C becomes TiC first and the rest becomes Fe ₃ Nb ₃ C.

ii) When the number of Ti atoms remaining from TiN is equal to or greater than the number of C atoms,
C becomes TiC.

(2) When the number of Ti atoms is less than 1/2 of the number of N atoms, N is first T
iN and the rest NbN.

Ti: C + N is an element necessary for fixing the workability and ensuring long-term stability of the metal phase structure. Since Ti has a higher affinity for C and N than Mo, W and Nb, it has a function of suppressing the precipitation of carbonitrides of Nb, Mo and W during use. Thereby, Mo, W, and Nb can be secured during use, and high-temperature strength during use can be secured. In order to fix C and N which do not form a solid solution in the mother phase, the minimum addition amount was 0.01%. Also, since part of the high temperature strength before use is supported by Ti carbonitride,
Addition of more than 0.5% of Ti reduces the high-temperature strength before use to coarsen the carbonitride. Further, the addition exceeding 0.5% deteriorates the weld bead shape.

Therefore, the upper limit is set to 0.5%.

W: an additive element that enhances high-temperature strength and high-temperature salt damage resistance,
It is necessary to add 0.1% or more. Further, since it is harder to precipitate than Nb, the amount of solid solution can be secured even during use, which is effective for maintaining high-temperature strength during use. However, the upper limit is set to 2% because the high-temperature salt damage resistance is deteriorated. Where W is Mo
In addition, the upper limit of the total addition amount was 3%. Note that W is one of the elements that may increase the recrystallization temperature and precipitate a large amount of intermetallic compounds and carbonitrides with Fe, so the above range is determined in consideration of these.

Mo: an additive element that enhances high-temperature strength and high-temperature salt damage resistance. Since the steel of the present invention is characterized by low Cr, it is 0.1% in the sense of improving corrosion resistance, which is a basic characteristic of stainless steel.
The above addition is necessary. Further, compared to Nb, since precipitation is less likely to occur, the amount of solid solution can be ensured even during use, which is effective for maintaining high-temperature strength during use. However, the addition of a large amount deteriorates the high-temperature salt damage resistance, so that the upper limit is 2%. However, when combined with W, the upper limit of the total amount was 3%. Since Mo is one of the elements that may increase the recrystallization temperature and precipitate a large amount of intermetallic compounds and carbonitrides with Fe, the above range is determined in consideration of these.

Al: An element that is effective as a deoxidizing material and improves oxidation resistance and high-temperature salt damage resistance. The steel of the present invention is SUS430LX
It is an additive element useful from the viewpoint of improving the oxidation resistance, particularly from the viewpoint of improving the oxidation resistance in exhaust gas, because it has a lower Cr than that of Cr.
Further, 0.02% or more is preferable because it is a useful element for high-temperature salt damage resistance. On the other hand, the content is set to 0.3% or less in order to deteriorate the workability and deteriorate the shape of the welding beat. When combined with Si, salt damage resistance can be further improved.

N: The steel of the present invention mainly supports high-temperature strength by solid solution strengthening of Nb, Mo, and W, and further, it is desired to suppress the workability and toughness of hot-rolled sheet as low as possible. However, since the extreme decrease is economically disadvantageous, the level at which the high-temperature strength is not decreased is set to 0.02% or less by itself, and combined with C; C + N ≦ 0.03%.

Rare earth elements (Milan metals containing lanthanoid elements or Y): Since the steel of the present invention has a low Cr content of 13 to 17%, Si and Al are added to improve oxidation resistance. This element is added when the oxidation resistance needs to be further improved. However, if the addition amount is less than 0.001%, a stable effect cannot be obtained, and if it exceeds 0.05%, hot workability is inferior.

High temperature strength: Exhaust gas temperature has risen to around 900 ° C due to improved fuel efficiency and high output of automobiles. Therefore, to withstand thermal fatigue and high temperature fatigue under these conditions, the tensile strength at 850 ° C is 34MPa. Above and 900 ℃ tensile strength 25MPa
It was above. During high temperature use, the solid solution
Since Nb decreases, high-temperature strength also decreases with high-temperature use. From the viewpoint of ensuring high-temperature strength during high-temperature use, as an example, the steel of the present invention containing Mo: 0.4 to 0.6% is 900 ° C. × 500 h.
FIG. 6 shows that the tensile strength at 900 ° C. after aging is secured to 18 MPa or more.

EXAMPLES Example 1 8 kg each of test steels having the chemical components shown in Table 1 were melted in a vacuum.
It was melted and then subjected to heating at 1250 ° C., hot rolling, pickling, cold rolling, annealing at 850 to 1000 ° C., and pickling to produce a 2 mm thin plate. Using this thin plate, a tensile test, a cycle aging test, a high-temperature salt damage test and a high-temperature tensile test were performed, and various characteristics are shown in Table 2.

Regarding high temperature salt damage resistance, Mo alone is 0.48%,
Inventive steels (NUS13, 21, and 22) to which 1.46 and 2.67% were added showed good values with little corrosion thinning. On the other hand, in NUS24, 26 and 27 to which Mo and W were excessively added, the amount of high-temperature salt corrosion thinning was large, and in addition, the room-temperature ductility was low. Also, NUS30 with high Si had good high-temperature salt damage resistance, but low room-temperature ductility.

Regarding the high temperature strength, NUS25 having a low eff.Nb amount had low high temperature strength after cycle aging, and NUS28 and 30 without Nb had low values of high temperature strength before and after cycle heating. Furthermore, even though the eff.Nb amount was within the range of the present invention, NUS29 to which Ti was excessively added had low initial high-temperature strength. In addition, the inventive steel having eff.Nb in the range of 0.187 to 0.343 was able to secure high high-temperature strength before and after cycle aging, and also had good room-temperature ductility.

Example 2 Test steels having the chemical components shown in Table 3 were melted in a slab shape by vacuum melting, and then slab heating at 1250 ° C.-hot rolling-
A 2 mm thin plate was prepared through pickling-cold rolling-annealing at 850 to 1000 ° C.-pickling. In each of the above steps, the amounts of dissolved Mo and Nb were measured, and finally, a tensile test, a high-temperature salt damage test and a high-temperature tensile test were performed using a cold-rolled annealed plate to measure various properties thereof.

Table 4 shows a comparison of the material properties of the steel of the present invention and the comparative steel.

As can be seen from Table 4, the B2,4
30LX, B4 with a high C + N amount and B7 with a low Ti addition amount had high high-temperature strength before use, but eff.
Since the content was 1% or less, the high-temperature strength after the aging treatment was reduced, and the tensile strength was reduced to 18 MPa or less, which was 70% or less of the high-temperature strength before use. On the other hand, Ti, Nb, Mo and C + N are appropriately balanced to secure eff.
A8 had high strength before use, and could secure high-temperature strength of about 23 MPa even after aging treatment.

As shown in FIG. 7, the thermal fatigue life of the steel A1 of the present invention is SU
It was found that the service life was about 1.6 times that of the S430LX. This is because, by fixing C + N by Ti, the function of strengthening the solute Nb and Mo at high temperatures was effectively functioned over a long period of time, thereby significantly extending the thermal fatigue life.

The thermal fatigue test was performed as follows. 2mm
Using a thick, thin plate-shaped test piece, with the distance between the grades constrained to prevent expansion and contraction by heating and cooling (referred to as complete constraint or 100% constraint), the area between the grades was 200% in advance.
At which point the starting point is 200 ° C to 900 ° C
Temperature rise in 60 seconds → Hold at 900 ° C for 30 seconds → Cooling to 200 ° C in 60 seconds as one cycle, repeat this cycle, and when the load falls to 10% of the initial load The number of repetitions (Nf) was determined.

B6 with a high eff.Nb also had good high-temperature strength after aging, but A3 with almost the same amount of Mo, C + N.
Also, compared with A5 and A7 which have higher eff.Nb, the high temperature strength after aging treatment is almost the same level, and 0.4% as eff.Nb
The above chemical composition distribution was not effective. This is eff.
If Nb exceeds 0.4, the intermetallic compound of Fe and Nb becomes coarse, meaning that precipitation strengthening does not work effectively.

On the other hand, if the amount of C + N exceeds 0.03%, it becomes difficult to secure the amount of solute Nb, so that the high temperature strength after aging treatment cannot be maintained as in B4. On the other hand, in the case of A4 having a lower C content, the initial strength was slightly lower. This is because strengthening of Ti (C, N) became difficult to work. As can be seen from the example of B3, when Ti is excessively added, the high-temperature strength before use decreases and the weld bead shape also deteriorates. The addition of Mo is effective in improving the high-temperature strength, and at the same time, improving the high-temperature salt damage resistance.
It can be seen from a comparison between A4 and A8 with a high addition amount and B7 with a low addition amount.

On the other hand, as can be seen from the example of B1, excessive addition of Mo deteriorated the high-temperature salt damage resistance. Si is also effective for high-temperature salt damage resistance, and this effect is clear from the examples of A3 and B2.
In addition, as can be seen from the comparison between A2 and A6 and B6, Si needs to be added in an amount of more than 0.5% to improve oxidation resistance, and particularly to prevent abnormal oxidation in exhaust gas at 900 ° C for 500 hours. there were. It was also evident from the examples of A3, A4, B2 and the like that the elongation at break at room temperature tension did not decrease when about 1% of Si was added. It was found from the examples of A2 and A6 that A1 also improved oxidation resistance and high-temperature salt damage resistance similarly to Si. On the other hand, it was found that the addition of a large amount of Al from B5 tends to lower the room-temperature ductility and deteriorate the weld bead shape.

Since B8 contains a large amount of rare earth elements, hot rolling was impossible.

Next, regarding the high-temperature salt damage of the steel A1, the comparative steel B2, and the SUS430LX of the present invention, the relationship between the temperature and the amount of corrosion thinning is shown in FIG. As shown in the figure, A1 and B2 had the same wall loss up to 700 ° C, but at 750 ° C, the wall loss of B2 was smaller than that of A1.
0.1mm / 40 cycles increased. SUS430LX is A1 at 700 ℃
Was reduced by nearly three times.

This shows that the steel of the present invention is very excellent in high-temperature salt damage.

INDUSTRIAL APPLICABILITY The present invention finds a component system in which necessary properties are well-balanced as a member used at high temperature for a long time, in particular, as a material for an automobile exhaust system, in consideration of a use environment, and a future automobile system. It is possible to provide a ferritic stainless steel capable of coping with higher fuel efficiency, higher output, and improved exhaust gas purification performance.

──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-64-8254 (JP, A) JP-A-3-274245 (JP, A) JP-A-4-74852 (JP, A) JP-A-3-74845 264652 (JP, A) JP-A-62-112757 (JP, A) JP-A-5-125491 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) C22C 38/00 302 C22C 38/28

Claims (5)

(57) [Claims] C. 0.003 to 0.015, N: 0.02 or less, C + N: 0.03 or less, Si: 0.1% by weight.
~ 1.0, Mn: 0.1 ~ 1.0, P: 0.01 ~ 0.1, S: 0.01 or less, Cr: 13
~ 17, Ti: 0.01 ~ 0.5, Nb: 0.1 ~ 0.5, and Mo:
0.1 to 2.0, the balance consisting of Fe and unavoidable impurities,
And eff.Nb calculated by the following equation (1) is 0.1 to 0.4.
(%) Range, and the tensile strength at 900 ° C is 25MPa.
A ferritic stainless steel having a tensile strength at 850 ° C. of at least 34 MPa and a high temperature resistance to salt damage. eff.Nb (%) = Nb (%)-3 × 93 × fc / 12−93 × fn / 14 (1) where (1) Ti (%) − 48 · (N (%) / 2) / When 14> 0, C (%)-12 · {Ti (%)-48 · (N (%) / 2) / 14} /
When 48> 0, fc = C (%)-12 ｛Ti (%)-48 · (N (%) / 2) / 1
4｝ / 48 fn = N (%) / 2 C (%)-12 ｛{Ti (%)-4848 (N (%) / 2) / 14｝ /
When 48 ≦ 0, fc = 0 fn = N (%) / 2 (2) When Ti (%) − 48 · (N (%) / 2) / 14 ≦ 0, fc = C (%) fn = N ( %)-14 ・ (Ti (%) / 2) / 48 2. In% by weight, C: 0.003 to 0.015, N: 0.02 or less, C + N: 0.03 or less, Si: 0.1
~ 1.0, Mn: 0.1 ~ 1.0, P: 0.01 ~ 0.1, S: 0.01 or less, Cr: 13
~ 17, Ti: 0.01 ~ 0.5, Nb: 0.1 ~ 0.5, MO: 0.1 ~
2.0 and W: 0.1 to 2.0 (however, Mo + W: 0.2 to 3.0), the balance being Fe and unavoidable impurities, and eff.Nb calculated by the following formula (1) is 0.1 to 0.4 (% ), The tensile strength at 900 ° C is 25 MPa or more, and 850
A ferritic stainless steel having a high-temperature strength of 34 MPa or more at a tensile strength of 30 ° C. and excellent resistance to salt damage at high temperatures. eff.Nb (%) = Nb (%)-3 × 93 × fc / 12−93 × fn / 14 (1) where (1) Ti (%) − 48 · (N (%) / 2) / When 14> 0, C (%)-12 · {Ti (%)-48 · (N (%) / 2) / 14} /
When 48> 0, fc = C (%)-12 ｛Ti (%)-48 · (N (%) / 2) / 1
4｝ / 48 fn = N (%) / 2 C (%)-12 ｛{Ti (%)-4848 (N (%) / 2) / 14｝ /
When 48 ≦ 0, fc = 0 fn = N (%) / 2 (2) When Ti (%) − 48 · (N (%) / 2) / 14 ≦ 0, fc = C (%) fn = N ( %)-14 ・ (Ti (%) / 2) / 48 3. The ferritic stainless steel according to claim 1, wherein said chemical component contains more than 0.5 to 1.0% by weight of Si. 4. The ferritic stainless steel according to claim 1, wherein the chemical component contains 0.02 to 0.3% by weight of Al. 5. A method according to claim 1, wherein said chemical component comprises at least one element selected from the group consisting of rare earth elements (here, rare earth elements are lanthanoid elements and Y) in a total amount of 0.001 to 0.05% by weight. Claims 1, including
The ferritic stainless steel according to 2, 3, or 4.