JP3234284B2 - Austenitic stainless steel for flexible tubes with excellent resistance to high-temperature salt damage and high-temperature fatigue - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a flexible tube excellent in high-temperature fatigue characteristics and high-temperature salt damage corrosion resistance used in applications where it is subjected to repeated heating, cooling and vibration in a corrosive environment caused by high-temperature salt or molten salt. about use austenite <br/> site stainless steel.

[0002]

2. Description of the Related Art Materials used in exhaust gas purification systems for automobiles are exposed to high-temperature exhaust gas, so that materials having excellent high-temperature strength, high-temperature oxidation resistance, and thermal fatigue characteristics are used. You. Among these, austenitic stainless steel such as SUS304 is used from the viewpoint of workability for the flexible tube intended to absorb engine vibration, but the mesh covering the flexible tube is mainly made of NaCl on the road surface. It has been found that anti-freezing agent adheres and causes high-temperature salt damage corrosion, leading to accidents such as leakage of unpurified gas.

[0003] SUS304 for such high temperature salt corrosion
Sufficient properties cannot be obtained with existing general-purpose austenitic stainless steels such as On the other hand, it has been found that an austenitic stainless steel containing Si and Mo exhibits excellent characteristics in a high-temperature salt-corrosion environment. For example, JP-A-63-213643, JP-A-2-54741, JP-B-1-8695, and JP-A-3-91039 all describe the effects of Si and Mo.

On the other hand, when these alloys were processed into flexible tubes and subjected to a durability test in an actual vehicle or bench test, cracks occurred in some vehicle models. As a result of examining the cause, it became clear that the cracks were caused by high temperature fatigue. The temperature of a flexible tube mounted on a vehicle rises due to high-temperature exhaust gas, and also causes high-temperature fatigue due to vibration caused by the engine and road surface running. Even if it has excellent resistance to high-temperature salt damage and corrosion, even if it is inferior in high-temperature fatigue characteristics, it is clear that it is important to design and crack under high stress and induce gas leakage. Was. Conversely, even if it has excellent high-temperature fatigue properties, if it is inferior to high-temperature salt damage corrosion resistance, the corrosion will reduce the thickness quickly,
As a result, a high fatigue stress is received, and cracks tend to occur at an accelerated rate. Therefore, a steel excellent in both high-temperature salt corrosion resistance and high-temperature fatigue properties has been strongly demanded.

[0005] In the prior art described above, from the viewpoint of high temperature strength, N
It is described that b, Ti and the like are added, but no consideration is given to the ability to withstand high frequency vibrations at high temperatures. Therefore, an austenitic stainless steel excellent in high-temperature fatigue resistance as well as high-temperature salt damage corrosion resistance is desired.

SUMMARY OF THE INVENTION In view of the above circumstances, an object of the present invention is to provide an austenitic stainless steel having excellent high-temperature fatigue corrosion characteristics in addition to excellent high-temperature salt corrosion resistance, which is not available in conventional stainless steel for flexible tubes. I do.

[0007]

SUMMARY OF THE INVENTION It is an object of the present invention to improve the high-temperature fatigue resistance as well as the high-temperature salt corrosion resistance of austenitic stainless steel. It has been found that, in order to provide, it can be obtained by limited addition of Si and Nb.

[0008]

The above object is as follows: C: more than 0.03% and 0.06% or less Si: 1 to 4% Mn: 0.5 to 4% P: 0.04% or less S: 0.005% or less Ni: 10 to 17% Cr: 14 to 20% N: 0.035% or less Mo: 1.5 to 4% Nb: 0.05 to 0.5%, the balance being Fe and unavoidable impurities, and the total content of Si and Nb is expressed by the formula 6 ≧ Si% + 1
Austenitic stainless steel for flexible tubes excellent in high-temperature fatigue properties and high-temperature salt corrosion resistance satisfying 0Mo% ≧ 4, or one or two or more kinds of Y or REM (rare earth element) in addition to the above composition 0.01
Austena for flexible tubes containing up to 0.15%
Achieved by kit-based stainless steel.

Next, the reasons for limiting the steel composition in the present invention will be described below. C: An unavoidable component, which is effective for improving high temperature fatigue properties by solid solution strengthening as an interstitial element. However, excessive addition of C causes embrittlement and lowers workability, so the upper limit is made 0.06%.
On the other hand, the reduction of C is not preferable because it increases the refining time and increases the production cost, and it is not preferable to reduce the amount more than necessary to improve the high temperature fatigue strength. Therefore,
It shall be contained in excess of 0.03%.

Si: One of the important elements for improving high-temperature salt damage corrosion resistance. In addition, it is a very effective element for improving high temperature fatigue properties. To obtain this effect, addition of 1% or more is required. However, Si promotes the precipitation of the σ phase and lowers the toughness of the steel, and also lowers the hot workability, weldability and formability, so the upper limit was made 4%.

Mn: Mn has an effect of improving hot workability, and also removes and reduces S which is harmful to welding hot cracking. To obtain this effect, 0.5% or more is required. However, if Mn is too high, it adversely affects high-temperature oxidation characteristics,
Since it has a bad influence on the high temperature oxidation resistance of the steel of the present application, the maximum content is set to 4%.

P: The content of P is preferably low because it has an adverse effect on the high-temperature oxidation resistance, and is 0.04% or less because it has an adverse effect on the toughness of the hot-rolled sheet.

S: Since it is harmful to hot cracking of the weld, the lower the better, the better. However, an excessively low temperature causes an increase in the production cost, so the upper limit is made 0.005%.

Ni: One of the basic elements of austenitic stainless steel. It is effective in improving high-temperature salt damage corrosion resistance. In addition, since Si and Mo are added to the steel of the present invention, δ ferrite is easily formed and hot workability is deteriorated. And the lower limit
10%. However, since adding a large amount increases the cost, the upper limit is set to 17%.

Cr: The most basic element for maintaining the oxidation resistance and corrosion resistance of stainless steel. If it is less than 14%, a sufficient effect cannot be obtained in a high-temperature corrosive environment or a mere high-temperature oxidation resistance. On the other hand, if it exceeds 20%, it becomes difficult to adjust the composition balance, delta ferrite increases, and the workability deteriorates. Therefore, the upper limit was made 20% or less.

N: Like C, is an effective component for improving the high-temperature strength, but if added excessively, the workability is reduced. Therefore, the content is made 0.035% or less.

Mo: Mo is an element that should be positively added because it is an element effective in a high-temperature corrosion environment, high-temperature salt corrosion resistance, and high-temperature strength. If it is less than 1.5%, the effect of addition is small, so the lower limit is made 1.5% or more. On the other hand, Mo is expensive and promotes the precipitation of the σ phase, leading to a decrease in toughness.
Also, if added in excess of 4%, the hot workability will decrease, so the upper limit is 4%.

Nb: a very important additive element in the present invention. Nb combines with C and N to form fine precipitates and has the effect of increasing the high-temperature strength of steel. In particular, it is a very effective element for improving high-temperature fatigue strength as in this application. When a small amount of Nb is added to austenitic stainless steel to which Si and Mo are added in combination, extremely excellent high-temperature fatigue characteristics can be obtained. Can be To obtain this effect, 0.05% or more must be added. But,
If added in excess, the workability is degraded.

Y, REM: Y, REM (rare earth element) is effective in increasing the peel resistance of the oxide scale when subjected to a heating / cooling cycle. To obtain these effects, the total of Y and REM must be 0.01% or more.
When a large amount of REM is added, a large amount of oxides of Y and REM precipitate at the grain boundaries, lowering the grain boundary strength at high temperatures and causing embrittlement. Therefore, the upper limit is set to 0.15%.

Further, in order to obtain excellent high-temperature fatigue characteristics, the value of Si% + 10Nb% is set to 4 or more. However, excessive addition of Si and Nb deteriorates the workability, and it is not possible to endure the work with a material requiring severe bulging such as a flexible tube,
Cracks occur. Therefore, the value of Si% + 10Nb% is set to 6 or less.

[0021]

DETAILED DESCRIPTION OF THE INVENTION The present invention will be specifically described below.

The high-temperature salt damage corrosion, which is the most important in the above applications, was examined. As a basic experiment, the steel shown in Table 1 was vacuum melted, forged into a plate having a thickness of 30 mm, heated at 1200 ° C. for 2 hours, and then subjected to hot rolling to a plate thickness of 5 mm. By performing cold rolling, the sheet thickness was reduced to 2 mm. This was processed into a 25 mm × 35 mm test piece, and the entire surface was polished to # 400 and used for the test. The test material was immersed in a saturated saline solution at 20 ° C for 5 minutes, then heated at 650 ° C for 2 hours and air-cooled for 5 minutes as one cycle. Was done. After the test, the scale was removed by spraying glass beads, the weight of the test piece was measured, and the weight loss per unit area (corrosion loss) with respect to the weight before the test was evaluated for high-temperature salt damage corrosion resistance. The results are shown in FIGS. FIG. 1 shows the effect of the amount of Si on the high-temperature salt corrosion resistance. From this figure, it can be seen that the addition of Si reduces corrosion weight loss,
It can be seen that the high-temperature salt corrosion resistance is improved. But,
Even if 2.5% or more of Si is added, no remarkable improvement effect is observed. FIG. 2 shows the effect of Mo on the corrosion weight loss of steel containing 2.5% Si. According to this figure, M
It was found that by adding o, the high-temperature salt corrosion resistance was remarkably improved, and excellent high-temperature salt corrosion resistance was obtained. Based on the above results, a high-temperature fatigue test was performed on E steel having excellent resistance to high-temperature salt damage corrosion. The high temperature fatigue test is performed by fixing one end of a strip-shaped test piece whose longitudinal direction is the rolling direction, at a test temperature of 600 ° C., and at the other end at a rate of ± 2 at 2500 times per minute.
This was performed by a method giving an amplitude of 0 mm. The result is shown in FIG. As a comparison material, SUSXM15J1, which is widely used as a flexible tube for automobile exhaust gas purification systems and is said to have good resistance to high-temperature salt damage corrosion, but is said to have good high-temperature fatigue properties, is used. .
As shown in this figure, when the stress at which the number of fracture cycles is 106 is defined as the fatigue limit stress, SUSXM15J1 is 27.5 kgf / m
The steel E was 22 kgf / mm ² with respect to m ² , indicating that the high temperature fatigue properties were inferior. When high-temperature fatigue characteristics are important depending on the system design such as a flexible tube, even if the steel is excellent in high-temperature salt damage corrosion resistance like E steel, poor high-temperature fatigue characteristics may cause a problem in use. . Then, the high temperature fatigue properties of the austenitic stainless steel containing Si and Mo were examined. Test pieces were prepared for the steels shown in Table 2 in the same manner as described above and subjected to a high-temperature fatigue test at 600 ° C. to measure the fatigue limit stress. Si and Nb
FIG. 4 shows the results obtained by arranging the high temperature fatigue characteristics by the amount. SUSXM1
Those exceeding 27.5kgf / mm2 is the fatigue limit stress 5J1 ○, shows what was 27.5kgf / mm ² or less ●.
From this figure, it is understood that the fatigue limit stress exceeds 27.5 kgf / mm ² when Si% + 10Nb% ≧ 4 is satisfied. Si is effective for improving high-temperature salt damage corrosion, but is also considered to contribute to the increase in strength of steel by solid solution strengthening,
In particular, it provides an excellent improvement effect under high strain rate conditions such as high temperature fatigue. Nb also has the effect of improving strength by precipitation and solid solution. Therefore, S
By controlling the amounts of i and Nb and adding them, excellent high-temperature fatigue properties can be obtained.

[0023]

[Table 1]

[0024]

[Table 2]

Since the flexible tube is subjected to bulging, good workability is required. Therefore, a tensile test was performed on these steels, and the elongation was measured. The results are shown in FIG. From this result, when Si and Nb are excessively added, elongation is deteriorated. In order to perform bulge processing like a flexible tube, elongation of 50% or more is required, so it is necessary to suppress the value of Si% + 10Nb% to 6 or less.

In order to examine the high-temperature salt-corrosion characteristics of 17Cr-13Ni-2.5Si-2.5Mo-0.2Nb-added steel, a high-temperature salt-corrosion test was performed at 650 ° C for 10 cycles. SUSXM15J1 and 17Cr-13
FIG. 5 shows the results obtained by using Ni-2.5Si-2.5Mo steel as a comparative material. From this figure, 17Cr-13Ni-2.5Si-2.5Mo-0.2Nb steel is 17C
Although the corrosion weight loss is slightly larger than that of r-13Ni-2.5Si-2.5Mo steel, the corrosion weight loss is about 1/2 that of SUSXM15J1, indicating excellent resistance to high-temperature salt damage corrosion.

From the above results, Nb was added to the austenitic stainless steel containing Si and Mo, and
It has been found that by regulating the amount of Nb and the amount of Nb, a steel having excellent high-temperature salt damage corrosion resistance and high-temperature fatigue characteristics and excellent workability can be obtained.

[0028]

EXAMPLES The present invention will be specifically described with reference to Examples and Comparative Examples. Table 3 shows the composition of the steel used. These steels were smelted by vacuum melting, forged, and then reduced to a thickness of 30 mm, hot-rolled, annealed and cold-rolled to a thickness of 0.4 mm. This test material was pipe-formed by TIG welding to make a pipe with a diameter of 50 mm, and the length was bulged.
It was processed into a 400mm flexible tube shape. At this time, No. 25 and No. 26 steels were cracked and could not be processed into flexible tubes. The flexible tube that could be processed was immersed in a saturated saline solution, heated at 600 ° C. for 24 hours, and then subjected to a vibration durability test. Heating temperature is 600 ° C in vibration endurance test
The test was performed by fixing one end of the tube and applying an amplitude of ± 20 mm to the other end at a rate of 2500 times per minute until a break occurred. The durability was evaluated by using SUS304 as a basic comparative material and dividing the number of fracture cycles of the test steel by the number of fracture cycles of SUS304. The results are shown in Table 3. From this evaluation, it is understood that the steel of the present invention has good durability and has a breaking life of 6 to 8 times that of SUS304. On the contrary,
The comparative steel has a shorter life. For example, it is considered that, because No. 28 steel does not have good resistance to high-temperature salt damage, the reduction in sheet thickness due to high-temperature salt damage is large, and as a result, the load increased in the vibration durability test and the life was shortened.
In addition, No. 23 steel has excellent high-temperature salt damage corrosion resistance, and although the reduction in sheet thickness due to high-temperature salt damage corrosion is small, the fatigue life is shortened due to poor high-temperature fatigue properties. Conceivable.

[0029]

[Table 3]

According to the present invention, by limiting the composition of the steel, an austenitic stainless steel exhibiting excellent high-temperature salt corrosion resistance and excellent high-temperature fatigue characteristics and having unprecedented excellent heat resistance can be obtained. can get. Thus, for example, a steel that can be sufficiently used even in an environment that is subjected to high-temperature salt damage and high-temperature fatigue such as a flexible tube of an automobile exhaust gas purification system is provided.

[0030]

[Brief description of the drawings]

FIG. 1 is a diagram showing the effect of Si on high-temperature salt damage corrosion resistance by performing ten cycles of high-temperature salt damage corrosion tests at 650 ° C.

FIG. 2 is a diagram showing the effect of Mo on high-temperature salt damage corrosion resistance by performing 10 cycles of high-temperature salt damage corrosion tests at 650 ° C. on steel containing 2.5% Si.

FIG. 3 is a diagram showing the results of a high-temperature fatigue test performed at 600 ° C. on SUSXM15J1 and a composite steel containing Si and Mo.

FIG. 4 shows the case where the fatigue limit stress due to high temperature fatigue exceeds 27.5 kgf / mm ^2, and the case where the fatigue limit stress does not exceed 27.5 kgf / mm ² , and shows the influence of Si and Nb on the high temperature fatigue characteristics. The numbers indicate the values of elongation by a tensile test.

Fig.5 High temperature salt corrosion test of 17Cr-13Ni-2.5Si-2.5Mo-0.2Nb steel at 10 cycles of 650 ° C and 10 cycles of high temperature salt corrosion test for SUSXM15J1 and 17Cr-13Ni-2.5Si-2.5Mo steel The result of having investigated as a comparative material is shown.

──────────────────────────────────────────────────続 き Continuation of front page (56) References JP-A-2-544741 (JP, A) JP-A-54-56018 (JP, A) JP-A-60-230966 (JP, A) JP-A 64-64 73057 (JP, A) JP-A-58-71360 (JP, A) JP-A-4-272132 (JP, A) JP-A-57-35667 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) C22C 38/00-38/60

Claims (2)

(57) [Claims] 1. C: more than 0.03% and 0.06% or less Si: 1 to 4% Mn: 0.5 to 4% P: 0.04% or less S: 0.005% or less Ni: 10 to 17% Cr: 14 to 20% N: 0.035 % Or less Mo: 1.5 to 4% Nb: 0.05 to 0.5%, the balance being Fe and unavoidable impurities,
And, the total content of Si and Nb is excellent in high-temperature fatigue characteristics and high-temperature salt corrosion resistance satisfies the following formula Flexi
Austenitic stainless steel for bull tube 6 ≧ Si% + 10Nb% ≧ 4 2. C: more than 0.03% and 0.06% or less Si: 1 to 4% Mn: 0.5 to 4% P: 0.04% or less S: 0.005% or less Ni: 10 to 17% Cr: 14 to 20% N: 0.035 % Or less Mo: 1.5 to 4% Nb: 0.05 to 0.5% One or more of Y or REM (rare earth element) contains 0.01 to 0.15% in total, the balance being Fe and unavoidable impurities, and the content of Si and Nb sum, the high-temperature fatigue characteristics and high-temperature salt corrosion property superior flexible sheet for Blu tube austenitic stainless steel satisfies the following equation. 6 ≧ Si% + 10Nb% ≧ 4