JP5960951B2 - Ferritic stainless steel sheet for automobile fuel tank with excellent fatigue characteristics and method for producing the same - Google Patents

Description

The present invention relates to a ferritic stainless steel sheet for automobile fuel tanks having excellent fatigue characteristics and a method for producing the same. In particular, in superior regarding ferritic stainless steel sheet and a manufacturing method thereof fatigue characteristics used for an automobile fuel tank with a pressure variation in use.

  Ferritic stainless steel sheets are widely used in kitchen equipment, home appliances, automobiles and the like. In particular, the high purity ferritic stainless steel sheet in which the contents of C and N are reduced as much as possible, trace amounts of Ti and Nb are added, and C and N are fixed as Ti and Nb carbonitrides, has its corrosion resistance and workability. However, since it has reached a level comparable to that of an austenitic stainless steel plate represented by SUS304, its application is greatly expanded. In recent years, it has been used for various containers due to its excellent corrosion resistance and workability, and its use has begun to be examined as a fuel tank for automobiles.

High purity ferritic stainless steel sheets are often used mainly as thin steel sheets of about 0.2 mm to 2.5 mm. In this case, there are almost no applications in which fatigue characteristics are a problem, and until now, fatigue characteristics have not been regarded as important.
However, when such a high-purity ferritic stainless steel plate is applied to a container such as an automobile fuel tank, it is assumed that excellent fatigue characteristics are required because the tank is repeatedly stressed during use.

On the other hand, a method for improving secondary workability by adding B to a high purity ferritic stainless steel sheet has been proposed.
For example, Patent Document 1 discloses a method of containing B in an amount of 0.0003 to 0.0050 mass%, and Patent Document 2 discloses a method of containing B in an amount of 0.0002 to 0.005 mass%.
Further, in Patent Document 3, while adding B, when manufacturing a high purity ferritic stainless steel sheet, by optimizing each process condition, particularly the annealing process of the cold-rolled sheet, secondary workability is improved. Techniques for improving are disclosed.

JP-A-8-333639 Japanese Patent Laid-Open No. 11-236650 JP 2004-323957 A

  When the present inventors applied a high-purity ferritic stainless steel sheet for containers, it was found that although the workability was good, the fatigue characteristics were not better than expected. Improvement of fatigue characteristics was recognized as a problem to be solved. However, all of Patent Documents 1 to 3 are intended to improve secondary workability by addition of B, and are not mentioned for improvement of fatigue properties. In the ferritic stainless steel plate manufactured by the prior art, It was difficult to ensure good fatigue properties. Also, there has been no clarification of the relationship between the precipitation behavior of added B and the fatigue characteristics.

  Therefore, the present invention has been made in view of the above circumstances, and in a high purity ferritic stainless steel sheet having good workability, the amount of B is adjusted and the precipitation of boride on the grain boundaries is suppressed. It aims at providing the ferritic stainless steel plate for containers which improved the fatigue characteristics, and its manufacturing method.

  The inventors of the present invention have been studying application of high purity ferritic stainless steel sheets to container materials such as automobile fuel tanks. As described above, in a container represented by an automobile fuel tank or the like, repeated fatigue stress is applied over the positive pressure to negative pressure region during its use, so the fatigue characteristics of the high purity ferritic stainless steel sheet are important. is there. However, the present inventors have found that the fatigue properties of the high-purity ferritic stainless steel sheet are lower than expected, and have worked on the improvement. As a result of various studies, it was clarified that the cause of low fatigue characteristics of high-purity ferritic stainless steel sheet is the occurrence of intergranular cracking on the fracture surface due to fatigue. In a high purity ferritic stainless steel sheet, since there are few impurities, a grain boundary is easy to clean, Therefore It is thought that the grain boundary strength fell and the grain boundary crack occurred. As a countermeasure, the grain boundary strengthening by adding a small amount of B, Nb, and V contributing to the improvement of the grain boundary strength by segregating at the grain boundary was examined.

Furthermore, when adding B, it discovered that suppressing the precipitation of the boride on a grain boundary was effective for a fatigue characteristic improvement, and examined the control method. Among these, when Mo was not added or when the amount of Mo added was small, it became clear that precipitation of boride at the grain boundaries was particularly promoted, and a method for suppressing the precipitation was also examined.
As a result, it has been found that the factors that greatly affect the precipitation of boride at the grain boundaries are the annealing temperature of the cold-rolled sheet and the average cooling rate from the annealing temperature to 600 ° C. When the annealing temperature is relatively low as 850 ° C. or more and 900 ° C. or less, if the average cooling rate from the annealing temperature to 600 ° C. is 10 ° C./s or more, the time for passing through the temperature zone up to 600 ° C. where boride precipitates. Is short and precipitation of boride can be suppressed. However, when the annealing temperature exceeds 900 ° C., the time for passing through the temperature zone up to 600 ° C. is relatively longer than in the case where the annealing temperature is relatively low, so a faster cooling rate is required. And when the annealing temperature exceeded 900 degreeC, the present inventors clarified that boride precipitation can be suppressed by making an average cooling rate into 15 degree-C / s or more. It has also been clarified that the average cooling rate at 600 ° C. or less has little influence on the precipitation of boride.

Here, the influence of the B addition amount or the average cooling rate on the fatigue characteristics as described above will be described in detail.
FIG. 1 shows the endurance life (breaking life) in the pressurization endurance test. The used material is a ferritic stainless steel plate having a thickness of 0.8 mm, which is manufactured through a general steelmaking-hot rolling-pickling-cold rolling-annealing / pickling process. Moreover, the composition was 17Cr-0.2Ti steel, and only the addition amount of B was changed and it tested. The annealing temperature, which is an important parameter in the present invention, was 920 ° C., and the average cooling rate up to 600 ° C. was mainly 25 ° C./s and partly 10 ° C./s.
As is clear from FIG. 1, it can be seen that the durability life of the steel sheet to which B is added at about 5 ppm and 12 ppm is improved by 30% or more compared to the steel sheet to which B is not added. However, even in the same steel added with 12 ppm of B, when the average cooling rate of annealing is as low as 10 ° C./s, the durable life is a low value. This is considered to be because the annealing temperature was set to 920 ° C. and the average cooling rate was slow, so that precipitation of boride at the grain boundaries progressed, resulting in a decrease in the durability life and deterioration of the fatigue characteristics.
Further, when the average cooling rate is 25 ° C./s, the endurance life is increased until the amount of addition of B is about 12 ppm, but is decreased thereafter. This is probably because the addition amount of B was too large, so that the precipitation of boride could not be sufficiently suppressed, and as a result, the durable life was reduced.

  From the above examination results and knowledge, the inventors have invented a ferritic stainless steel for containers excellent in fatigue characteristics and a method for producing the same. The gist of the invention is as follows.

[1] By mass%
C: 0.01% or less,
Si: 1% or less,
Mn: 1% or less,
P: 0.04% or less,
S: 0.005% or less,
Mo: 0.05% or less,
Cr: 11% to 19%,
Ti: 10 × (C + N) or more and 0.3% or less,
Al: 0.02 to 0.2%,
N: 0.015% or less,
B: 0.0004% to 0.0015%,
Each containing and 0.0003% or more of solid solution B,
The balance has a steel composition consisting of Fe and inevitable impurities,
JIS G0571 when subjected to oxalic acid electrolytic etching provisions, boride due etch pits, 2 × ^{10 -5} cells on the grain boundary / [mu] m Ri der less positive pressure 40 kPa, pressurization repeating negative pressure -40kPa excellent fuel tank ferritic stainless steel sheet for automobiles in fatigue breakage life in durability test, wherein the fatigue properties der Rukoto over 20,000 times.
[2] Furthermore, in mass%,
V: 0.005 to 0.2%,
Nb: 0.005 to 0.2%,
The ferritic stainless steel sheet for fuel tanks for automobiles having excellent fatigue properties as described in [1] above, comprising at least one of the above.
[3] Furthermore, in mass%,
Ni: 0.005 to 0.5%,
Cu: 0.005 to 0.5%,
Sn: 0.005-0.3%,
The ferritic stainless steel sheet for fuel tanks for automobiles having excellent fatigue properties according to the above [1] or [2], comprising at least one of the above.
[4] When producing the ferritic stainless steel having the steel composition according to any one of [1] to [3] above, the annealing temperature of the cold-rolled sheet is set to 850 ° C. or more and 950 ° C. or less, and 600 ° C. The average cooling rate is 10 ° C./s or more when the annealing temperature is 850 ° C. or more and 900 ° C. or less, and 15 ° C./s or more when the annealing temperature is more than 900 ° C. and 950 ° C. or less. A method for producing a ferritic stainless steel sheet for an automobile fuel tank having excellent fatigue characteristics .

  According to the present invention, by adjusting the B addition amount and suppressing the precipitation of boride, it is possible to increase the grain boundary strength, maintain good workability, and obtain a ferritic stainless steel sheet having excellent fatigue characteristics. it can. In particular, by applying the ferritic stainless steel sheet according to the present invention to a container such as a fuel tank for automobiles that is subject to pressure fluctuation during use, it is possible to extend the life of parts and reduce costs by reducing the thickness. It is very useful in industry.

It is a graph which shows the influence of the addition amount of B or the average cooling rate on the fatigue characteristics in 17Cr-0.2Ti steel.

  Below, the ferritic stainless steel plate of this embodiment is demonstrated in detail.

The ferritic stainless steel sheet of the present embodiment is, in mass%, C: 0.01% or less, Si: 1% or less, Mn: 1% or less, P: 0.04% or less, S: 0.005% or less, Mo: 0.1% or less, Cr: 11% to 19%, Ti: 10 × (C + N) or more and 0.3% or less, Al: 0.02 to 0.2%, N: 0.015% or less, B : 0.0004% to 0.0015%, solute B is contained at 0.0003% or more, and the balance is a steel composition composed of Fe and inevitable impurities, and oxalic acid specified in JIS G0571 When electrolytic etching is performed, the number of etch pits due to boride is 2 × 10 ⁻⁵ / μm or less on the grain boundary.
Below, the reason which limited the steel composition of the ferritic stainless steel plate of this embodiment is demonstrated in detail. It should be noted that, here, those having no lower limit are included up to the inevitable impurity level. Further,% means mass% unless otherwise specified.

C: 0.01% or less Since C deteriorates workability and corrosion resistance, the lower the content thereof, the better. Therefore, the upper limit is made 0.01%. However, excessive reduction of C increases the refining cost, so the lower limit is preferably 0.003%.

Si: 1% or less Si is an element useful as a deoxidizer, but if added in a large amount, the workability is lowered, so the smaller the content, the better. Therefore, the upper limit of Si is 1%. In addition, when emphasizing workability, 0.2% or less is preferable.

Mn: 1% or less Mn is also an element useful as a deoxidizing material like Si, but if added in a large amount, the workability decreases, so the smaller the content, the better. Therefore, the upper limit of Mn is 1%. In addition, when emphasizing workability, 0.2% or less is preferable.

P: 0.04% or less P is a component that is inevitably contained in steel, but if it exceeds 0.04%, toughness and workability deteriorate, so its content is as low as possible. Is preferred. Therefore, the upper limit for the P content is 0.04%.

S: 0.005% or less S is a component inevitably contained in the steel. However, if it exceeds 0.005% in the present invention, not only the workability is lowered but also CaS is easily generated. In order to deteriorate the corrosion resistance, the content is preferably as small as possible. Therefore, the upper limit of the S content is 0.005%. Moreover, since making S less than 0.0005% causes a great increase in steelmaking cost, it is preferable to make 0.0005% the lower limit.

Cr: 11-19%
Cr is an essential element for developing corrosion resistance. In order to exhibit the effect, addition of 11% or more is necessary. However, excessive addition reduces workability, so the upper limit is 19%. Considering the balance between corrosion resistance and workability, it is preferably 16 to 18.5%.

Mo: 0.05 % or less Mo is an element that improves the corrosion resistance. In the present embodiment, Mo is an element that has an effect of suppressing precipitation of boride. On the other hand, Mo is very expensive and may deteriorate the workability. Therefore, in the present invention, the Mo content is suppressed and the upper limit is set to 0.05 % .

Al: 0.002 to 0.2%
Since Al is very useful as a deoxidizing element, 0.002% or more is added. However, if it is added in a large amount, the workability decreases, so the upper limit is made 0.2%.

Ti: 10 × (C + N) or more and 0.3% or less Ti is an element that fixes C and N as carbonitrides and improves corrosion resistance and workability. Therefore, 10 × (C + N) or more is added. However, excessive addition of Ti increases surface defects due to coarse precipitates and decreases workability, so the upper limit is made 0.3%.

N: 0.015% or less N, like C, deteriorates workability and corrosion resistance. The lower the content, the better. Therefore, the upper limit is made 0.015%. However, excessive reduction increases the refining cost, so the lower limit is preferably 0.002 %%.

B: 0.0004% to 0.0015%
Solid solution B: 0.0003% or more B is an extremely important element in the present invention in order to improve fatigue characteristics. The grain boundary can be strengthened by segregating B as a solid solution B to the grain boundary. As a result, fatigue characteristics are improved. In order to express the effect, 0.0004% or more of addition is necessary. However, if added over 0.0015%, the precipitation of boride on the grain boundary cannot be suppressed and the effect of improving fatigue characteristics disappears, so the upper limit is made 0.0015%.
Moreover, the effect expression by such B addition depends on the amount of dissolved B. Therefore, it is necessary to contain 0.0003% or more of solid solution B. In consideration of the balance between improvement of fatigue characteristics and suppression of boride precipitation, the B content is preferably 0.0008 to 0.0012%.

Further, as described above, in order to strengthen the grain boundary and improve the fatigue characteristics, it is necessary to segregate B to the grain boundary and suppress precipitation as a boride. At this time, there is a method of observing etch pits caused by boride as a method for confirming suppression of precipitation of boride.
First, the steel sheet is subjected to oxalic acid electrolytic etching specified in JIS G0571, and then observed with an optical microscope to measure etch pits caused by boride. As a result, it was found that if the number of etch pits observed on the grain boundary is 2 × 10 ⁻⁵ pieces / μm or less, the fatigue characteristics are not adversely affected. Incidentally, if the etch pits on the grain boundaries is 2 × 10 ^-5 cells / [mu] m ultrasonic observation, that is, if the boride is 2 × 10 ^-5 cells / [mu] m ultra precipitated on the grain boundaries, the fatigue characteristics Deteriorated. This indicates the possibility that boride is the starting point of fatigue failure. Of course, it is most desirable that no boride is deposited on the grain boundaries, that is, no etch pits are generated. However, according to the present inventors, it has also been found that when the boride is present in the grains rather than at the grain boundaries, the fatigue characteristics are hardly affected. This suggests that the boride precipitated in the grains does not become the starting point of fracture.

  In the present embodiment, in addition to the above elements, one or more of the following elements can be added.

V: 0.005-0.2%
V has the effect of improving fatigue characteristics by segregating at the grain boundary as a solid solution V. The effect is manifested when 0.005% or more is added. However, since processability will fall when it adds excessively, it is preferable to make content of V 0.005 to 0.2%.

Nb: 0.005 to 0.2%
Nb, like V, has the effect of improving fatigue characteristics by forming a solid solution Nb and segregating at the grain boundary. The effect is manifested when 0.005% or more is added. However, since processability will fall when it adds excessively, it is preferable to make content of Nb 0.005-0.2%.

In this embodiment, in addition to the above elements, one or more of Ni: 0.005 to 0.5%, Cu: 0.005 to 0.5%, and Sn: 0.005 to 0.3% are added. May be.
Ni, Cu, and Sn are elements that improve corrosion resistance, and can be added when improvement in corrosion resistance is required. In order to exhibit the effect, addition of 0.005% or more is desirable. However, when Ni, Cu, and Sn are added in a large amount, the workability is remarkably lowered. Therefore, it is desirable that the upper limit is 0.5% for Ni and Cu and 0.3% for Sn.

  In addition, the ferritic stainless steel sheet in the present embodiment is substantially made of Fe, and the elements other than the above-described elements can be added in minute amounts, including inevitable impurities, which do not impair the effects of the present invention.

Next, the manufacturing method of the ferritic stainless steel sheet in this embodiment is demonstrated. The method for producing a stainless steel sheet according to the present invention comprises general steps of steelmaking, hot rolling, pickling, cold rolling, annealing and pickling.
Specifically, in steelmaking, a method obtained by melting ferritic stainless steel having the above steel composition in a converter and subsequently performing secondary refining is desirable.
Next, the molten steel is made into a slab by a known casting method such as continuous casting. The slab is rolled to a predetermined plate thickness by hot rolling. After that, hot-rolled sheet annealing may be performed as necessary. After hot rolling, pickling and then cold rolling to obtain the final thickness. And it anneals and pickles to make the final product. If necessary, cold rolling and annealing / pickling may be repeated.
In the present embodiment, in the annealing after the cold rolling, the annealing temperature is set to 850 ° C. or more and 950 ° C. or less. Further, the average cooling rate from such an annealing temperature to 600 ° C. is 10 ° C./s or more when the annealing temperature is 850 ° C. or more and 900 ° C. or less, and 15 ° C. when the annealing temperature is more than 900 ° C. and 950 ° C. or less. Cool at a cooling rate of at least / s.
Below, the reason for limitation of the annealing temperature and cooling rate in this embodiment is demonstrated.

As described above, in the present embodiment, the annealing temperature in the annealing after cold rolling is set to 850 ° C. or more and 950 ° C. or less. By setting the annealing temperature within such a range, precipitation of boride can be suppressed.
However, if the annealing temperature is less than 850 ° C., the recrystallization of the steel sheet becomes insufficient, and the workability may be reduced. On the other hand, when the annealing temperature exceeds 950 ° C., segregation of boride to the grain boundary becomes remarkable, and it may be impossible to suppress boride precipitation on the grain boundary. Therefore, in this embodiment, the annealing temperature is set to 850 ° C. or more and 950 ° C. or less.

Further, when cooling from the annealing temperature to 600 ° C., the average cooling rate is 10 ° C./s or more when the annealing temperature is 850 ° C. or more and 900 ° C. or less, and the annealing temperature is more than 900 ° C. and 950 ° C. or less, 15 ° C./s or more. Thus, precipitation of boride can be prevented by limiting the cooling rate.
However, when the average cooling rate from the annealing temperature to 600 ° C. is less than 10 ° C./s when the annealing temperature is 850 ° C. or more and 900 ° C. or less, and when the annealing temperature is more than 900 ° C. and less than 950 ° C., less than 15 ° C./s There is a possibility that the precipitation of boride at the grain boundary cannot be suppressed. Therefore, in this embodiment, the average cooling rate is 10 ° C./s or more when the annealing temperature is 850 ° C. or more and 900 ° C. or less, and 15 ° C./s or more when the annealing temperature is more than 900 ° C. and 950 ° C. or less. To do.
As described above, by controlling the annealing conditions when performing annealing after cold rolling to the optimum range, it becomes possible to suppress the precipitation of boride on the grain boundaries, and a ferritic stainless steel sheet having excellent fatigue properties Can be obtained.
Although the annealing atmosphere is not particularly limited, as a preferable condition, it is desirable to use a combustion gas atmosphere or a mixed atmosphere of hydrogen and nitrogen.

  Moreover, there is no restriction | limiting in particular in the final board thickness of the ferritic stainless steel plate of this invention, It can apply to 0.2 mm thickness to 3 mm thickness which can be manufactured with the general above-mentioned manufacturing method. However, in the case of a thick steel plate, it is highly possible that special equipment is required to obtain the cooling rate of the final annealing necessary for the present invention. Moreover, when applying this invention for containers, such as a fuel tank, as plate | board thickness, 0.3 mm-1.2 mm are desirable.

  In addition, the ferritic stainless steel sheet according to the present invention can be used bare, and can be subjected to various platings such as Sn-Zn plating and Al plating as required. Thus, when plating is performed, effects such as improvement in corrosion resistance can be obtained, and the present invention can be applied to various applications.

  As described above, according to the ferritic stainless steel sheet according to the present invention, it is possible to prevent intergranular cracking by segregating B as a solid solution B to the grain boundary and suppressing the precipitation of boride. As a result, the grain boundary strength is improved, good workability can be maintained, and excellent fatigue characteristics can be obtained.

In addition, according to the method for producing a ferritic stainless steel sheet according to the present invention, by optimizing the annealing temperature and cooling rate in the annealing after cold rolling, the precipitation of boride which is a cause of deteriorating fatigue characteristics is suppressed. can do. As a result, it becomes possible to produce a ferritic stainless steel sheet having good workability and excellent fatigue characteristics.
In addition, because excellent fatigue characteristics can be obtained, it can be applied to containers that are subject to pressure fluctuations during use, such as fuel tanks for automobiles. Is possible.

  Hereinafter, the effects of the present invention will be described with reference to examples, but the present invention is not limited to the conditions used in the following examples.

In this example, first, steels having the composition shown in Tables 1 and 2 were melted and cast into a slab, and the slab was hot-rolled to form a hot rolled coil having a thickness of 4 mm. Thereafter, the hot-rolled coil was pickled and cold-rolled to a thickness of 0.8 mm to obtain a cold-rolled sheet. Next, after annealing in a hydrogen-nitrogen mixed atmosphere, pickling was performed to obtain a product plate. In addition, as shown in Table 3, the annealing conditions of the cold-rolled plate at this time mainly include an annealing temperature of 920 ° C. and an average cooling rate up to 600 ° C. of 25 ° C./s. In Examples 34 to 38, The annealing temperature was changed to 820 to 970 ° C., and the average cooling rate was changed in the range of 5 to 25 ° C./s. In addition, in the component composition shown in Table 1 and Table 2, the numerical value which remove | deviates from the scope of the present invention is underlined.
Next, various test pieces were collected from the product plate thus obtained, and evaluated and measured.

First, the precipitation state of boride was measured.
Precipitation of boride was observed with an optical microscope after collecting a test piece from the obtained product plate with a cross section parallel to the rolling direction as an observation surface, performing oxalic acid electrolytic etching according to JIS G0571. From the observation results, the etch pits on the grain boundaries were measured, and the linear density was calculated.

Next, workability was evaluated.
For the evaluation of workability, a JIS 13B test piece defined in JIS Z2201 was used, a tensile test based on JIS Z2241 was performed, and the total elongation value was used as an index. Further, the tensile direction is parallel to the rolling direction. In addition, since this invention assumes the container use which performs press molding, workability is an important characteristic, and the total elongation value of 30% was evaluated as an acceptance criterion.

Fatigue properties were evaluated by a pressurization durability test using a simulated tank.
First, a method for producing a simulated tank will be described. Two square tubes having a side of 200 mm and a molding height of 30 mm were formed from a blank having a side of 400 mm by a press. At this time, one of the edges of one square tube was formed with R (curvature radius): 2 mm, the other was formed with R: 15 mm, and the other edge of the other square tube was formed with R: 15 mm. These two pieces were matched to the worship shape, and the seam welding of the flange portion was performed to obtain a tank shape. In addition, an air inlet / outlet was installed on the bottom of the tank so that the internal pressure could be varied. Next, the test was performed by repeating the conditions of the pressurization / decompression durability test with a positive pressure of 40 kPa and a negative pressure of −40 kPa, and the fracture life was evaluated. In addition, a fracture | rupture part becomes the edge formed by R: 2mm used as a stress concentration part. In this evaluation method, fatigue characteristics were evaluated, and a rupture life of 20,000 times or more was regarded as acceptable.
The above production conditions and evaluation results are shown in Table 3.

  As is apparent from Table 3, Examples 1 to 4 are obtained by changing the B amount with respect to the 17Cr—Ti steel. Example 1 without B and Example 4 with B: 20 ppm are comparative steels, and the fracture life in the durability test is as low as 15000 times or less. Moreover, in Example 4, it can also confirm that there is much boride precipitation on a grain boundary. On the other hand, in Example 2 and Example 3 which are steels of the present invention, there is little precipitation of boride on the grain boundary, and the fracture life exceeds 20000 times, which is an excellent result. Also, the elongation value exceeds 30%, indicating good workability.

The inventive steels of Examples 5 to 15 have less boride precipitation and also show an excellent fracture life. Also, the elongation value is sufficient.
On the other hand, in the comparative steels of Example 16 to Example 33, the elongation value was not sufficient or the fracture life was short.

  In Examples 34 to 38, steels having the same components as in Example 3 were tested while changing the final annealing conditions. Example 34 is a steel of the present invention in which the annealing temperature is 880 ° C. and the cooling rate to 400 ° C. is 15 ° C./s, and there is little precipitation of boride and an excellent fracture life. On the other hand, Example 35 has the same annealing temperature of 880 ° C., but the cooling rate is as low as 5 ° C./s, so that there is much precipitation of boride and the fracture life is deteriorated. In Example 36, the steel was annealed at a temperature of 920 ° C. and a cooling rate of 15 ° C./s, and since the cooling rate was slow, there was much boride precipitation and the fracture life was deteriorated. Furthermore, since Example 37 has an annealing temperature as low as 830 ° C., recrystallization is insufficient and good workability cannot be obtained. On the contrary, in Example 38, since the annealing temperature is as high as 970 ° C., boride precipitation is large and the fracture life is deteriorated.

  From these results, the above-mentioned findings could be confirmed, and the grounds for limiting the above-described steel compositions and configurations could be supported.

  As is apparent from the above description, according to the present invention, a ferritic stainless steel plate having excellent fatigue characteristics can be provided at a relatively low cost, and in particular, by applying it to a container material such as a fuel tank member, Social contributions such as environmental measures through cost reduction and weight reduction are much greater. That is, the present invention has sufficient industrial applicability.

Claims (4)

% By mass
C: 0.01% or less,
Si: 1% or less,
Mn: 1% or less,
P: 0.04% or less,
S: 0.005% or less,
Mo: 0.05% or less,
Cr: 11% to 19%,
Ti: 10 × (C + N) or more and 0.3% or less,
Al: 0.02 to 0.2%,
N: 0.015% or less,
B: 0.0004% to 0.0015%,
Each containing and 0.0003% or more of solid solution B,
The balance has a steel composition consisting of Fe and inevitable impurities,
When subjected to oxalic acid electrolytic etching of JIS G0571 defined, etch pits originating from boride is, 2 × ^{10 -5} cells on the grain boundary / [mu] m Ri der less positive pressure 40 kPa, repeated vacuum -40kPa excellent fuel tank ferritic stainless steel sheet for automobiles in fatigue breakage life at pressurization durability test wherein the fatigue properties der Rukoto over 20,000 times. % By mass, further V: 0.005 to 0.2%,
Nb: 0.005 to 0.2%,
The ferritic stainless steel sheet for fuel tanks for automobiles having excellent fatigue characteristics according to claim 1, wherein the ferritic stainless steel sheet has excellent fatigue characteristics. Furthermore, in mass%,
Ni: 0.005 to 0.5%,
Cu: 0.005 to 0.5%,
Sn: 0.005-0.3%,
The ferritic stainless steel sheet for a fuel tank for automobiles having excellent fatigue characteristics according to claim 1 or 2, wherein the ferritic stainless steel sheet has excellent fatigue characteristics.   When manufacturing the ferritic stainless steel sheet according to any one of claims 1 to 3, the annealing temperature of the cold-rolled sheet is set to 850 ° C or more and 950 ° C or less, and the average cooling rate up to 600 ° C is set to the annealing temperature. When the temperature is 850 ° C. or higher and 900 ° C. or lower, cooling is performed at 10 ° C./s or higher, and when the annealing temperature is higher than 900 ° C. and lower than 950 ° C., cooling is performed at 15 ° C./s or higher. Manufacturing method of ferritic stainless steel sheet for tanks.

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