JP4752571B2 - Ferritic stainless steel sheet for bellows tube and bellows tube - Google Patents

Description

TECHNICAL FIELD The present invention relates to a ferritic stainless steel sheet used for a bellows base pipe, and in particular, a ferritic stainless steel sheet for a bellows base pipe excellent in workability and excellent in high temperature fatigue characteristics and high temperature salt corrosion characteristics, and a bellows manufactured using the steel sheet. It relates to the raw tube .

  In mechanical devices that handle liquids, gases, powders, and the like, it is generally performed to transport them through metal piping. In the middle of the pipe, a bellows-like metal pipe with parallel-wave walls is used to absorb strain and stress due to vibration of the machine and thermal expansion of the pipe, and to mitigate those effects. A certain bellows (also called a flexible tube) is usually installed.

  Conventionally, as a material used for the bellows, FCC metal typified by copper, SUS304 or the like, an austenitic stainless steel plate, or the like has been mainly used. The reason is that it was difficult to process the bellows with other metal materials. In other words, copper and austenitic stainless steel sheets have a larger elongation and n value near room temperature than other metal materials, and are therefore suitable for bulge forming that requires elongation characteristics.

  However, austenitic stainless steel sheets are easy to process into bellows, but oxidation, high temperature corrosion or stress corrosion cracking occurs when the gas or liquid passing through the bellows is highly corrosive or at high temperatures. There was a problem that it was likely to occur. In particular, the bellows made of austenitic stainless steel plate used in the connecting part of the automobile exhaust pipe rises to about 500 to 750 ° C. during use, and therefore the salt dispersed on the road due to snow melting adhered. In some cases, the material itself becomes sensitized, and high temperature salt damage corrosion is likely to occur.

Against this background, various ferritic stainless steel sheets for bellows pipes have been invented. For example, in Patent Document 1, by specifying the amounts of C, Si, Mn, S, Cr, Al, Ti, N, and O within a specific range, an elongation of 35% or more and an r value of 1.5 or more are set. The achieved ferritic stainless steel plate with excellent bellows workability is disclosed in Patent Document 2 in which the amounts of C, Cr, Al, N, Si, Mn, Ti, Nb, Mo, Cu, and Ni are specified in a specific range. Furthermore, a ferritic stainless steel sheet excellent in bellows workability by limiting the crystal grain size to an optimum range is disclosed. In Patent Document 3, the amount of C, Cr, Al, N, Si, Mn, Ti, Nb, Mo, Cu, and Ni is regulated to a specific range, thereby providing excellent bellows workability and high temperature fatigue characteristics. Ferritic stainless steel sheet is disclosed in Patent Document 4, in which the amount of C, Cr, N, Ti, and Mo is specified in a specific range, and the surface roughness of the material is limited to an optimal range, so that the bellows workability and high temperature are reduced. A ferritic stainless steel sheet having excellent salt corrosion resistance is disclosed.
JP-A-7-268560 JP-A-8-176750 JP-A-8-188854 JP-A-9-125208

  However, even with the above technology, the bellows workability of the base tube made of ferritic stainless steel plate is still insufficient compared to the base tube made of austenitic stainless steel plate, and can be applied to severe shapes. In fact, ferritic stainless steel sheets are not provided. Furthermore, from the viewpoint of improving the fuel efficiency of automobiles, the high-temperature characteristics (high-temperature salt corrosion resistance and high-temperature fatigue characteristics) required for automobile exhaust system components are becoming increasingly severe and excellent while satisfying these characteristics. Development of a ferritic stainless steel sheet for bellows pipes having excellent workability is strongly desired.

An object of the present invention is to provide a ferritic stainless steel plate for a bellows tube that has both excellent workability and excellent high-temperature properties (high temperature salt damage corrosion resistance and high temperature fatigue properties), and a bellows tube manufactured using the steel plate. It is to provide.

  In order to achieve the above-mentioned object, the inventors have made extensive studies by paying attention to the component composition of a ferritic stainless steel sheet as a raw material for a raw tube. As a result, the workability of the bellows tube is improved by adding a small amount of B to the ferritic stainless steel plate as a raw material, while the high temperature salt corrosion resistance and the high temperature fatigue properties are the appropriate amount of Nb. The present invention has been completed by finding that it can be improved by the combined addition of Mo and Mo.

That is, the present invention includes C: 0.015 mass% or less, Si: 1.0 mass% or less, Mn: 1.0 mass% or less, P: 0.04 mass% or less, S: 0.010 mass% or less, Cr: 11-11 19 mass%, N: 0.015 mass% or less, Al: 0.15 mass% or less, Mo: 1.25 to 2.5 mass%, Nb: 0.3 to 0.7 mass%, B: 0.0003 to 0.003 mass %, And the balance is made of Fe and inevitable impurities.
Moreover, this invention is a bellows element | tube which was manufactured using the said ferritic stainless steel plate.
In addition, the bellows element tube of the present invention is a single or double tube having a plate thickness of 0.5 mm or less and an outer diameter of 28 to 80 mmφ.

  The ferritic stainless steel sheet of the present invention has high uniform elongation and excellent properties such as high temperature salt damage corrosion properties and high temperature fatigue properties. Therefore, the bellows tube made of the ferritic stainless steel sheet of the present invention is excellent in workability and high temperature characteristics, and therefore can be suitably used for a bellows tube used in an automobile exhaust system.

  A bellows is generally manufactured by forming a pipe-shaped raw tube by hydraulic bulge forming. Therefore, it is considered that the elongation characteristics, particularly the uniform elongation (uniform elongation) characteristics are important as the characteristics required for the raw material of the raw tube. This is because if uneven deformation occurs even at one location, the hydraulic pressure increases locally and becomes the starting point of cracking, and even if it does not lead to cracking, the life as a bellows is significantly reduced. . The inventors conducted a bellows forming test by hydraulic bulge forming on various ferritic stainless steel plates and a tensile test using a JIS No. 13 B test piece, and found that the maximum peak height that can be formed by bellows and the uniform elongation in the tensile test. The relationship was investigated. As a result, no clear relationship was observed between the two characteristics.

  Accordingly, the uniform elongation was measured by changing the shape of the tensile test piece in various ways, and the relationship between this and the maximum peak height that could be formed was examined. As a result, it was found that there is a correlation between the uniform elongation measured using a special tensile test piece having a parallel part width of 150 mm and a parallel part length of 25 mm and the maximum peak height that can be formed by bellows. It was. This shows that if even non-uniform deformation occurs in one place in a wide processing area, it is compatible with the processing of the bellows that causes cracking, and the bellows workability can be evaluated by devising a method for measuring uniform elongation. Is.

Therefore, the inventors investigated the influence of various additive elements on the uniform elongation of the ferritic stainless steel sheet. Among these, the B addition experiment in which the improvement effect was recognized will be described.
<Experiment 1>
C: 0.008 mass% -Si: 0.41 mass% -Mn: 0.31 mass% -P: 0.03 mass% -S: 0.003 mass% -Cr: 14.5 mass% -Al: 0.03 mass% -N : 0.01 mass%-Nb: 0.47 mass%-Mo: 1.35 mass% Basic component composition of steel, B content changed in the range of 0.0001 to 0.0040 mass% in the laboratory The steel ingot is heated to 1170 ° C. for 1 hour, and then hot-rolled to form a hot-rolled sheet having a thickness of 3.5 mm, and then annealed at 1070 ° C. for 30 seconds. Then, it was cold-rolled to obtain a cold-rolled sheet having a thickness of 0.4 mm. The cold-rolled sheet was annealed at 1030 ° C. for 30 seconds to form a cold-rolled annealed sheet, and from this cold-rolled annealed sheet, the above-mentioned special shape tensile test piece (parallel part width: 150 mm, parallel part length: 25 mm) was taken in parallel with the rolling direction and subjected to a tensile test to measure the uniform elongation.

  FIG. 1 shows the influence of the B content on the uniform elongation of the tensile test. From FIG. 1, it is possible to obtain a high uniform elongation by containing B in an amount of 0.0003 mass% or more, that is, the bellows processability is improved. However, when B is contained in an amount exceeding 0.0030 mass%, conversely, It can be seen that the uniform elongation decreases. The cause of the effect of B is not sufficiently clear, but since B is an element that segregates at the grain boundary and increases the grain boundary strength, when B is added in an amount of 0.0003 mass% or more. This is considered to suppress the generation of minute cracks at the grain boundaries during deformation. In addition, when B is added in excess of 0.0030 mass%, the decrease in uniform elongation is that a large amount of B compound precipitates at the grain boundary due to the addition of excess B, and micro cracks are formed at the interface between the B compound and the matrix. This is thought to be due to the generation and reduction in uniform elongation.

Next, in order to improve the high temperature characteristics (high temperature salt damage corrosion characteristics, high temperature fatigue characteristics) of ferritic stainless steel sheets, the effects of various additive elements were investigated. Of these, we will explain the experiments that were effective.
<Experiment 2>
C: 0.01 mass% -Si: 0.31 mass% -Mn: 0.34 mass% -P: 0.03 mass% -S: 0.005 mass% -Cr: 16 mass% -Al: 0.018 mass% -N: 0 .01 mass% -Mo: 1.45 mass% -B: 0.0006 mass% of the basic component composition, and Nb content varied in the range of 0.19 to 0.74 mass% in the laboratory. After making into a steel ingot, this steel ingot is heated at 1170 ° C. for 1 hour, and then hot rolled to form a hot-rolled sheet having a thickness of 3.5 mm, after annealing at 1070 ° C. for 30 seconds, and then cold rolled. Thus, a cold-rolled plate having a thickness of 0.4 mm was obtained. The cold-rolled sheet was annealed at 1030 ° C. for 30 seconds to obtain a cold-rolled annealed sheet, which was subjected to the following high-temperature salt damage corrosion test and high-temperature fatigue test.
[High temperature salt corrosion test]
After polishing the surface and end face of the cold-rolled annealed plate with No. 400 emery paper, it was immersed in a normal saline solution (26% NaCl aqueous solution) for 5 minutes, heated in the atmosphere at 750 ° C. for 2 hours, After 25 cycles of corrosion test with 1 cycle of cooling to room temperature, the corrosion product formed on the surface of the test piece was removed with ammonium citrate aqueous solution, and then the thickness reduction (corrosion depth) was measured. The high temperature salt corrosion resistance was evaluated.
[High temperature fatigue test]
Using a Schenk type high-temperature fatigue testing machine, a double-sided high-temperature fatigue test was conducted under various conditions of bending stress applied to the test piece under the conditions of 750 ° C. and 22 Hz. The bending stress that does not break even after one million bendings was determined as the fatigue limit.

<Experiment 3>
C: 0.01 mass% -Si: 0.31 mass% -Mn: 0.34 mass% -P: 0.03 mass% -S: 0.005 mass% -Cr: 16 mass% -Al: 0.018 mass% -N: 0 .01 mass% -Nb: 0.47 mass% -B: 0.0006 mass% The composition of the basic component and the Mo content varied from 0.65 to 2.41 mass% in the laboratory. After making into a steel ingot, it was made into a cold-rolled annealed plate having a thickness of 0.4 mm under the same conditions as in Experiment 2, and subjected to a high temperature salt corrosion test and a high temperature fatigue test.

FIG. 2 shows the influence of the Nb content on the high temperature salt damage corrosion characteristics and the high temperature fatigue characteristics. From FIG. 2, it can be seen that a steel plate having excellent high temperature salt corrosion resistance and high temperature fatigue characteristics can be obtained by containing 0.3 mass% or more of Nb. Fig. 3 shows the effect of Mo content on high temperature salt corrosion and high temperature fatigue properties. By containing Mo in an amount of 1.25 mass% or more, high temperature salt corrosion resistance and high temperature fatigue properties are shown. It turns out that the steel plate which is excellent in is obtained.
The present invention has been developed based on the results of Experiments 1 to 3 described above.

Next, the component composition of the ferritic stainless steel sheet of the present invention will be described.
C: 0.015 mass% or less C is an element that lowers the ductility and toughness of the steel sheet, and particularly adversely affects the bellows workability that is the main subject of the present invention. Is 0.015 mass%. In the present invention, since there is no adverse effect even if the C content is extremely small, it is not necessary to define a lower limit.

Si: 1.0 mass% or less Si is an element effective for improving oxidation resistance and high-temperature salt damage corrosion resistance, but also an element that hardens steel and causes a decrease in ductility. In particular, when Si is added in excess of 1.0 mass%, the bellows workability, which is the main focus of the present invention, starts to deteriorate significantly, so the upper limit of Si is 1.0 mass%. Although the minimum of Si is not specifically limited, From a viewpoint of ensuring oxidation resistance and high temperature salt damage corrosion resistance, it is preferable to contain 0.2 mass% or more.

Mn: 1.0 mass% or less Mn is an effective element that reduces the susceptibility to weld solidification cracking in Nb-added ferritic stainless steel. However, since Mn is an austenite stabilizing element, a large amount of addition reduces the stability of the ferrite phase at high temperatures, and in some cases, an austenite phase is generated, leading to a reduction in oxidation resistance. Further, Mn hardens the steel by solid solution hardening, and particularly when it exceeds 1.0 mass%, the workability deteriorates significantly, so the upper limit is 1.0 mass%. In addition, although the minimum of Mn is not especially provided, it is preferable to contain 0.3 mass% or more from a viewpoint of suppressing weld solidification cracking sensitivity low.

P: 0.04 mass% or less P is an element that decreases toughness and ductility. In particular, if the content exceeds 0.04 mass%, the toughness and ductility are significantly lowered, so the upper limit is 0.04 mass%. Note that even if the content of P is low, the effect of the present invention is not adversely affected, so there is no need to limit the lower limit.

S: 0.010 mass% or less S is an element that reduces corrosion resistance and oxidation resistance. In particular, if it exceeds 0.010 mass%, the corrosion resistance and oxidation resistance are significantly lowered, so the upper limit is made 0.010 mass%. Note that even if the S content is low, the effect of the present invention is not adversely affected, so there is no need to limit the lower limit.

Cr: 11-19 mass%
Cr is a main alloy element constituting a ferritic stainless steel sheet, and is an essential element for improving corrosion resistance and oxidation resistance. In order to obtain the effect, it is necessary to add 11 mass% or more of Cr. On the other hand, the addition of a large amount of Cr not only leads to hardening of the steel, but also reduces the bellows workability, which is the main point of the present invention, so the upper limit of Cr is 19 mass%. Preferably, it is the range of 13-19 mass%.

N: 0.015 mass% or less N is an element that lowers toughness and ductility, and the content is preferably reduced as much as possible. In particular, N is limited to 0.015 mass% or less because it adversely affects the bellows processability, which is the main focus of the present invention. Note that even if the amount of N is small, there is no adverse effect on the characteristics of the bellows, so the lower limit is not limited.

Al: 0.15 mass% or less Al is a strong deoxidizing element and is added to reduce oxygen in steel during refining. It is also an element effective for improving oxidation resistance. However, when Al is added excessively, the toughness and ductility are lowered. In particular, when the Al content exceeds 0.15 mass%, the tendency becomes remarkable, so the content is limited to 0.15 mass% or less. In addition, although a minimum is not specifically limited, In order to perform deoxidation at the time of refining completely, adding 0.003 mass% or more is preferable.

Nb: 0.3-0.7 mass%, Mo: 1.25-2.5 mass%
Mo and Nb are elements that improve high-temperature characteristics, and are important additive elements in the present invention. As apparent from Experiment 2 described above, Nb has an effect of improving high temperature salt corrosion resistance and high temperature fatigue characteristics by containing 0.3 mass% or more. However, when the Nb content exceeds 0.7 mass%, the toughness and ductility are significantly reduced. Therefore, Nb content shall be the range of 0.3-0.7 mass%. Further, as apparent from Experiment 3 described above, Mo is effective to improve the high temperature salt corrosion resistance and high temperature fatigue characteristics by containing 1.25 mass% or more. However, when Mo is contained exceeding 2.5 mass%, ductility and toughness are reduced. Therefore, the Mo content is in the range of 1.25 to 2.5 mass%.

B: 0.0003 to 0.003 mass%
B is an important element for improving the bellows processability, and is an essential element in the present invention. As is clear from the results of Experiment 1 described above (FIG. 1), by containing B in an amount of 0.0003 mass% or more, uniform elongation is improved and excellent bellows workability is obtained. On the other hand, when B is contained exceeding 0.0030 mass%, conversely, the uniform elongation is reduced. Therefore, in the present invention, the B content is in the range of 0.0003 to 0.0030 mass%.

In addition to the above essential components, the ferritic stainless steel plate of the present invention can contain the following components according to required characteristics.
One or more selected from Cu: 0.1 to 0.6 mass%, Ni: 0.1 to 0.6 mass% and Co: 0.03 to 0.6 mass% Cu, Ni and Co are: It is an element effective in improving toughness and corrosion resistance. In order to obtain these effects, it is preferable to add one or more selected from Cu: 0.1 mass% or more, Ni: 0.1 mass% or more, and Co: 0.03 mass% or more. However, since excessive addition hardens the steel, the upper limit of each is preferably Cu: 0.6 mass%, Ni: 0.6 mass%, and Co: 0.6 mass%.

V: 0.04 to 1.0 mass%, W: 0.04 to 5.0 mass%, Ta: 0.04 to 1.0 mass%, and Ti: 0.02 to 1.0 mass% Or, two or more kinds of steel sheets containing a large amount of Nb, such as the ferritic stainless steel sheet of the present invention, are produced by adding one or more kinds selected from V, W, Ta and Ti. Carbonitride can be finely dispersed to refine crystal grains and improve toughness. In addition, when there is C or N contamination during welding, this is fixed as carbonitride such as V and the formation of Cr carbonitride can be suppressed, thereby preventing formation of a Cr-deficient layer. This also contributes to improving the corrosion resistance of the parts. In order to obtain these effects, it is preferable to add V of 0.04 mass% or more, W of 0.04 mass% or more, Ta of 0.04 mass% or more, and Ti of 0.02 mass% or more. However, an excessive amount of addition lowers the toughness of the steel sheet itself and lowers the manufacturability, so that the upper limits are V: 1.0 mass%, W: 5.0 mass%, Ta: 1.0 mass%, and Ti: It is preferable to set it as 1.0 mass%.

  The ferritic stainless steel sheet of the present invention preferably has a thickness of 0.5 mm or less. As described above, the bellows is formed in a parallel wave bellows shape, and has a function of absorbing displacement due to thermal expansion and vibration. The displacement is loaded on the bellows as a bend. At this time, if the plate thickness is large, the distortion of the material increases. Strain exceeding the elastic limit of the material becomes plastic strain and causes fatigue fracture of the bellows tube. Therefore, from the viewpoint of preventing fatigue failure, the thinner the plate thickness, the better. In the present invention, considering the manufacturability and manufacturing cost of the stainless steel plate as the material, it is preferable that the plate thickness is 0.5 mm or less as a range that does not substantially cause a problem.

  Moreover, it is preferable that the bellows pipe | tube made from the ferritic stainless steel plate of this invention is a single or double thing whose outer diameter is 28-80 mmphi. This is because the amount of displacement within the elastic limit range that can be absorbed by the bellows is determined by the height of the peak, but even at the same peak height, the narrower the outer diameter of the tube, the greater the expansion rate of the peak and the more difficult the molding becomes. . That is, when the outer diameter of the raw tube is small, the peak height that can be formed is small. Therefore, in order to be able to absorb a predetermined amount of displacement in automotive exhaust system bellows applications, it is preferable that the outer diameter of the raw pipe be 28 mmφ or more. On the other hand, in the case where the outer diameter of the raw tube is large and the wall is thin, the bellows itself is insufficient in rigidity, so that it is liable to be crushed or buckled, making it substantially difficult to use. Therefore, the outer diameter of the raw tube is preferably 80 mmφ or less.

  In designing, if the bellows tube needs to have rigidity, it is conceivable to increase the wall thickness. However, as described above, an increase in the wall thickness is not preferable because it causes fatigue failure. In order to avoid this, rigidity and strength can be ensured by stacking thin-walled tubes to form a multiple tube. Specifically, a double pipe is preferable, and a triple pipe or more is not preferable in terms of manufacturability and manufacturing cost. In addition, if the wall thickness is the same, the use of a double pipe also has the effect of improving the fatigue strength because the rigidity decreases and the flexibility increases. Furthermore, depending on the usage environment, the required characteristics may differ between the inner surface and the outer surface of the bellows. In that case, it is preferable to use a double tube using a material that meets the respective requirements.

  Various steels having the composition shown in Table 1 were melted in a high-frequency vacuum melting furnace to form a 50 kg steel ingot, heated at 1170 ° C. for 1 hour, and then hot rolled at a rolling end temperature of 800 ° C. A hot-rolled sheet having a thickness of 3.5 mm was used. Then, after subjecting this hot-rolled sheet to annealing at 1040 ° C. for 30 seconds, the oxidized scale produced on the steel sheet surface was removed by pickling, and cold-rolled to form a cold-rolled sheet having a thickness of 0.4 mm, After degreasing, annealing was performed at 1030 ° C. for 30 seconds to obtain a cold-rolled annealed plate. From this cold-rolled annealed plate, the above-mentioned specially-shaped tensile test piece (parallel part width: 150 mm, parallel part length: 25 mm) was sampled and subjected to a tensile test to measure uniform elongation. Moreover, each characteristic was evaluated also in the high temperature salt damage corrosion test and high temperature fatigue test which were mentioned above.

  The ferritic stainless steel sheet of the present invention can be used in applications and fields where high temperature characteristics are required, including the fields of heat exchangers and fuel cells.

It is a graph which shows the influence which B content has on uniform elongation. It is a graph which shows the influence which Nb content has on a high temperature salt damage corrosion depth and a high temperature fatigue limit. It is a graph which shows the influence which Mo content has on a high temperature salt damage corrosion depth and a high temperature fatigue limit.

Claims (3)

C: 0.015 mass% or less, Si: 1.0 mass% or less, Mn: 1.0 mass% or less, P: 0.04 mass% or less, S: 0.010 mass% or less, Cr: 11 to 19 mass%, N: 0 .015 mass% or less, Al: 0.15 mass% or less, Mo: 1.25 to 2.5 mass%, Nb: 0.3 to 0.7 mass%, B: 0.0003 to 0.003 mass%, the balance A ferritic stainless steel sheet for bellows pipes, characterized in that consists of Fe and inevitable impurities. A bellows element tube manufactured using the ferritic stainless steel sheet according to claim 1 . The bellows element tube according to claim 2, wherein the sheet thickness is 0.5 mm or less and the outer diameter is single or double of 28 to 80 mmφ.

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