JP4762104B2 - Austenitic steel - Google Patents

Description

  The present invention relates to an austenitic stainless steel suitable for an environment in which an automobile underfloor member such as a fuel tank or a fuel supply pipe of an automobile is used.

  In recent years, there has been a movement to apply austenitic stainless steel to automobile oil supply system members represented by automobile fuel tanks and oil supply pipes. This is because the warranty period associated with compliance with exhaust gas regulations of automobiles has been extended, and the conventional turn-plated steel sheets and Sn-Zn-plated steel sheets may have insufficient durability, and the materials need to be reviewed. This is a major factor. Although automobile members are often made of resin, there is a problem of gasoline permeation in fuel tanks and oil supply pipes, making it difficult to make resin. Therefore, austenitic stainless steel is promising as a material having corrosion resistance and no gasoline permeation.

Fuel tanks and oil supply pipes are often manufactured by being processed into complex shapes, and austenitic steel types are more advantageous than ferritic steel types from the viewpoint of workability. However, stainless steel is slight Cl ^- prone to pitting at a relatively high temperature environment even in the presence ions, that particularly prone to austenitic stainless steels to stress corrosion cracking the residual stress due to machining and welding is present Has drawbacks.

  Sea salt particles are likely to adhere to automobiles traveling near the coast, and chlorides such as road anti-freezing agents are likely to adhere to the fuel supply members often installed under the floor of automobiles in winter. In such a case, it will be exposed to the environment which repeats moistening and drying (hereinafter referred to as “salt-wet-and-moisture repetitive environment”) with chloride attached. Moreover, it may become high temperature of 50 degreeC or more with the heat from an exhaust gas member, or the radiant heat from a road surface. Moreover, in order to attach the oil supply system member to the vehicle body, it is inevitable that some gap structure is formed between the bolt, the washer, or a cushioning material such as rubber. These environments are harsh for stainless steel, and in order to use austenitic stainless steel as a vehicle oil system component, steel grades with improved pitting corrosion resistance and stress corrosion cracking resistance at the same time in salty and dry repeated environments are applied. It is desirable.

So far, regarding the stress corrosion cracking resistance of austenitic stainless steel, many studies have been conducted on the “water environment”, that is, the environment immersed in water. For example, Patent Documents 1 and 2 add Si and Cu in combination. In addition, Patent Document 3 discloses an austenitic stainless steel in which the stress corrosion cracking resistance in hot water is remarkably improved by adding Cu.
On the other hand, Patent Document 4 describes a Cu-containing austenitic stainless steel for automobile oil supply pipes and fuel tanks having improved pitting corrosion resistance and stress corrosion cracking resistance.

JP-A-1-159351 Japanese Patent Laid-Open No. 2-190451 JP-A 64-62443 JP 2004-277767 A

  By using austenitic stainless steel with improved resistance to stress corrosion cracking as described in Patent Documents 1 to 3, it has good corrosion resistance even in members used under the floor of automobiles (hereinafter referred to as “automotive under-floor members”). It was expected. However, according to the study by the inventors, it has been found that these steels do not necessarily exhibit stable and excellent corrosion resistance as automobile under-floor members. Although the austenitic stainless steel disclosed in Patent Documents 1 to 3 shows excellent corrosion resistance in a warm water environment, it is considered that corrosion progresses due to the action of a different corrosion mechanism from the warm water environment in a salt dry and wet environment. .

  Since the austenitic stainless steel of Patent Document 4 was developed for automobile oil supply system members, it was expected to exhibit excellent corrosion resistance in severe salty and dry repeated environments. However, the demand for durability of automobile oil supply system members has become stricter than ever. As a result of various studies, it has been clarified that the steel of Patent Document 4 may not be able to sufficiently meet the recent severe requirements particularly in terms of pitting corrosion resistance in the gaps.

  Thus, in the austenitic stainless steel material, a method for stably exhibiting the durability required for an automobile oil supply system member has not yet been found. In view of such a situation, the present invention stably exhibits excellent pitting corrosion resistance and stress corrosion cracking resistance in a salty and wet repeated environment, and in particular, a fuel supply system member such as a fuel tank and a fuel supply pipe, and a secondary battery case member. It is intended to develop and provide a stainless steel material suitable for “automotive under-floor members”.

The purpose is mass%, C: 0.05% or less, Si: less than 1.0%, Mn: 0.4 to 1.8%, P: 0.045% or less, S: 0.005% or less Ni: 8.5 to 12.0%, Cr: 17.5 to 19.5%, N: 0.05% or less, Al: 0.3% or less as required, B: 0.005 In steel containing Cu and Mo as essential components and the balance Fe and unavoidable impurities, the content of Cu and Mo is Cu: less than 0.5 to 1.5% , M o : Achieved by an austenitic stainless steel excellent in durability in salty and dry repeated environments characterized by being adjusted to a range satisfying 1.5 ≦ Cu + Mo ≦ 2.5 with 0.5 to 1.2% Is done.

In here, the content of Cu and Cu in place of, expressed in each mass% Mo and Mo in the formula above is substituted. About Si content, what is adjusted to the range of 0.5 +/- 0.25% becomes a suitable object.

  ADVANTAGE OF THE INVENTION According to this invention, the austenitic stainless steel which exhibits the outstanding durability in the member used under the floor of a motor vehicle is provided. Since this is an austenite type, it can be easily processed into a complex-shaped automobile member such as a fuel tank. In addition, it has higher basic corrosion resistance than various types of conventional plated steel sheets, and meets the needs for long-term guarantee of automobile parts. There is no problem of gasoline permeation, which is a drawback of resin materials. Furthermore, the austenitic stainless steel of the present invention can be applied to any member used in all salt-wet-and-humidity repeated environments in addition to general automotive under-floor members such as an oil supply member.

  It is known that the addition of Cu or the combined addition of Cu and Si is effective for improving the stress corrosion cracking resistance of austenitic stainless steel in a warm water environment (Patent Documents 1 to 3). However, as a result of detailed studies by the inventors, it has been found that the influence of the alloy component on the stress corrosion cracking property is different between a hot water environment and a salty and dry repeated environment. An example of the data is as follows.

  That is, the steel shown in Table 1 was melted, and the sheet thickness was 3.0 mm by hot rolling, followed by annealing at 1150 ° C. × 30 minutes, pickling, cold rolling, 1050 ° C. × annealing for 1 minute soaking. A material steel plate having a thickness of 1.0 mm was obtained. After cutting out 30 mm x 30 mm large pieces and 15 mm x 15 mm small pieces from each material steel plate and finishing the surface by # 600 wet polishing, the small pieces are placed in the center of the large pieces and spot welding is performed using an electrode with a diameter of 5 mm. Large pieces and small pieces were joined under the condition that a nugget was formed. In this joined body, a gap structure close to a close contact state is formed in the vicinity of the welded portion, and residual stress is generated in the vicinity of the weld nugget. Therefore, this joined body can be used for evaluation of pitting corrosion resistance and stress corrosion cracking resistance in the gap. This joined body is referred to as a “weld gap test piece” in the present specification.

  The weld gap test piece produced in this manner was subjected to a salt-wet combined cycle test (CCT). The test piece was placed almost horizontally with the small piece side up. CCT is “85% RH, 50 ° C. × 15 h hold → 30% RH, forced drying by holding 50 ° C. × 3 h → 50% RH, 20 ° C. × 3 h hold → 5% NaCl salt water. Spraying 0.5 h, 50% RH, 20 ° C. × 2.5 h holding ”is one cycle, and this is performed up to 200 cycles. After 50 cycles, 100 cycles, and 200 cycles, each test piece (each n The following investigation was conducted.

  The large piece and the small piece of the test piece were separated by applying a mechanical external force, the cross section in the vicinity of the nugget portion of the large piece and the small piece was observed with an optical microscope, and the depth from the surface of the crack caused by the stress corrosion cracking was measured. n = 3 With the depth of the deepest stress corrosion cracking crack in all large pieces and small pieces (this is called “maximum stress corrosion cracking (SCC) depth”), the stress corrosion cracking resistance of the steel type was evaluated. The result is shown in FIG.

  As can be seen from FIG. 1, the addition of Cu and Mo is effective for improving the stress corrosion cracking resistance in a salty and dry repeated environment. In particular, when Cu and Mo were added in combination, the maximum stress corrosion cracking depth did not grow even in a long-term test, and it was revealed that the stress corrosion cracking resistance was good.

  Moreover, the result of investigating the relationship between the Cu content and the pitting corrosion resistance when the 19Cr-11Ni-0.5Si-0.8Mn-0.8Mo steel was subjected to the same salt dry-wet combined cycle test (CCT) as described above. Is illustrated in FIG. This is because the depth of pitting corrosion occurring on the surface of the large piece and small piece after the end of 200 cycles was measured with a depth-of-focus optical microscope, and n = 3, the deepest pitting depth in all large pieces and small pieces (this is expressed as “ The maximum pitting corrosion depth ”is plotted.

As can be seen from FIG. 2, when the Cu content was 1.5% by mass or more, the tendency of the pitting corrosion resistance to start decreasing was recognized. This tendency is not recognized in the water environment, and is a phenomenon peculiar to the salt dry and wet repeated environment. Although Cu ²⁺ is produced by corrosion of stainless steel containing Cu, this Cu ²⁺ is diluted with a large amount of water in an aqueous environment, whereas it is concentrated to a high concentration in a salt dry and wet environment. It is presumed that Cu ²⁺ acted as an oxidizing agent and accelerated corrosion. This is considered to be one reason why the stainless steels disclosed in Patent Documents 1 to 3 did not show good corrosion resistance in a stable salty and wet repeated environment. The steel of FIG. 2 has the Mo content at the 0.8% level, but in the range of the Mo content (described later) defined in the present invention, the Cu content is good in the range of less than 1.5% by mass. The tendency to obtain pitting corrosion resistance is maintained.

  Moreover, the result of investigating the relationship between the Mo content and the stress corrosion cracking resistance when 19Cr-11Ni-0.5Si-0.8Mn-1Cu was subjected to the same salt-wet combined cycle test (CCT) as described above. This is illustrated in FIG. This plots the result of having calculated | required the maximum stress corrosion cracking depth by the method similar to the case of FIG. 1 about the large piece and small piece after completion | finish of 200 cycles.

  As can be seen from FIG. 3, when the Mo content was 0.5% by mass or more, a phenomenon in which the stress corrosion cracking resistance was suddenly improved was observed. This phenomenon was also not observed in the water environment, and is a phenomenon peculiar to the salt dry and wet repeated environment. That is, it is important to ensure that the Mo content is 0.5% by mass or more, and it is not necessary to contain expensive Mo excessively. The steel of FIG. 3 has a Cu content of 1.0% level, but in the range of the Cu content (described later) defined in the present invention, the Mo content is good in the range of 0.5% by mass or more. The tendency to obtain stress corrosion cracking resistance is maintained.

  The present invention has been completed based on such new findings. Hereinafter, matters for specifying the present invention will be described.

  C is an element that enhances the austenite stability, and does not significantly affect the stress corrosion cracking resistance and pitting corrosion resistance. However, in order to increase the intergranular corrosion susceptibility at the welded portion, it is preferable that the C content is low. In the present invention, the C content is restricted to 0.05 mass% or less.

  It has been found that Si is an important element that raises the limit temperature of the occurrence of stress corrosion cracking in a hot water environment, but does not show a significant effect in a salty and wet repeated environment. Increasing the Si content necessitates uneconomical measures such as increasing the Ni content in order to maintain the austenite balance. Further, a large amount of Si is not preferable because it causes hardening of the steel by solid solution strengthening. However, a certain amount of Si is necessary as a deoxidizer. From the above, the Si content is restricted to a range of less than 1.0% by mass. For example, the Si content can be adjusted in the range of 0.5 ± 0.25 mass%, preferably in the range of 0.5 ± 0.2 mass%. Even if the Si content is regulated to less than 0.5% by mass, the effect of the present invention can be obtained.

  Mn contributes to the stabilization of the austenite phase and can also be used in place of Ni. However, since Mn forms a sulfide that tends to be a starting point of corrosion and lowers the corrosion resistance, its content is set to 0.4 to 1.8% by mass.

  P is an element that reduces the stress corrosion cracking resistance, so is limited to 0.045% by mass or less.

  S forms a starting point of corrosion by forming sulfide with Mn in the steel and lowers the corrosion resistance. Therefore, S is limited to 0.005% by mass or less.

  Ni is a main element for maintaining the austenite phase. However, since a large amount causes an increase in cost, in the present invention, the Ni content is regulated to 8.5 to 12.0% by mass.

  Cr is an indispensable element for imparting corrosion resistance. It is desirable that the Cr content is 17.5% by mass or more in the use of automobile under floor members such as automobile oil supply system members. Corrosion resistance tends to improve as the Cr content increases, but if a large amount of Cr is contained, it is necessary to increase Ni and the like necessary for maintaining the austenite structure, and further, productivity and hot workability It becomes a factor that damages. Therefore, in the present invention, the Cr content is specified to be 17.5 to 19.5 mass%.

  Cu is an important element in the present invention, and is an element that improves stress corrosion cracking resistance. For this purpose, a Cu content of 0.5% by mass or more is necessary. However, in a salty and wet repeated environment, excessive Cu content is a factor that impairs pitting corrosion resistance, as seen in FIG. Therefore, the Cu content is restricted to a range of less than 1.5% by mass.

  N is effective for improving the pitting corrosion resistance, but is a factor for reducing the stress corrosion cracking resistance. Therefore, in the present invention, the N content is regulated to 0.05 mass% or less.

  Mo is an important element in the present invention, and is an element that improves both stress corrosion cracking resistance and pitting corrosion resistance. In particular, in order to impart stress corrosion cracking resistance in a salt dry and wet repeated environment, it is necessary to secure a Mo content of 0.5% by mass or more as shown in FIG. However, even if the Mo content is increased too much, the corrosion resistance improvement effect commensurate with the Mo increase cannot be expected so much in a salt dry and wet environment. Since excessive Mo content causes an increase in cost, the Mo content is set to a range of 1.2 mass% or less.

  Considering the synergistic effect with Cu on the corrosion resistance, the balance is good when the contents of Cu and Mo are adjusted to a range satisfying 1.5 ≦ Cu + Mo ≦ 2.5.

  Al has an action of improving the resistance to stress corrosion cracking, and particularly raises the limit temperature for the occurrence of stress corrosion cracking. Therefore, Al can be contained as necessary in the present invention. However, since the hot workability and workability deteriorate when the Al content increases, it is desirable that the Al content be in the range of 0.3% by mass or less.

  B is effective in improving hot workability in production, and can be added as necessary. However, excessive B addition may reduce corrosion resistance by forming borides with Cr in the steel. For this reason, when adding B, it carries out in the range of 0.005 mass% or less.

  The austenitic stainless steel whose components are adjusted in this way can be melted in accordance with a general method for melting stainless steel. The obtained slab (for example, continuous cast slab) is made into a cold-rolled annealed plate by the processes of hot rolling, annealing / pickling, cold rolling, annealing / pickling, and various automobile members including fuel tanks. Or, it is used for molding to other members.

  The steel shown in Table 2 was melted and hot rolled to a plate thickness of 3.0 mm. After annealing at 1150 ° C. × 30, pickling, cold rolling, 1050 ° C. and annealing for 1 minute, the plate thickness was 1 A steel plate of 0.0 mm was obtained. After cutting out 30 mm x 30 mm large pieces and 15 mm x 15 mm small pieces from each material steel plate and finishing the surface by # 600 wet polishing, the small pieces are placed in the center of the large pieces and spot welding is performed using an electrode with a diameter of 5 mm. Large pieces and small pieces were joined under the condition that a nugget was formed. In this joined body, a gap structure close to a close contact state is formed in the vicinity of the welded portion, and residual stress is generated in the vicinity of the weld nugget. As described above, this is called a “weld gap test piece”.

  The weld gap test piece was subjected to a salt dry / wet combined cycle test (CCT). The test piece was placed almost horizontally with the small piece side up. CCT is the same as described above, that is, “85% RH, 50 ° C. × 15 h hold → 30% RH, forced drying by holding 50 ° C. × 3 h → 50% RH, 20 ° C. × 3 h hold “5% NaCl salt spray 0.5 h, 50% RH, 20 ° C. × 2.5 h hold” was defined as one cycle, and this was repeated up to 200 cycles. The following investigation was conducted on the test pieces after 200 cycles (implemented at each n = 3).

  The large piece and the small piece of the test piece were separated by applying a mechanical external force, the cross section in the vicinity of the nugget portion of the large piece and the small piece was observed with an optical microscope, and the depth from the surface of the crack caused by the stress corrosion cracking was measured. n = 3 With the depth of the deepest stress corrosion cracking crack in all large pieces and small pieces (this is called “maximum stress corrosion cracking (SCC) depth”), the stress corrosion cracking resistance of the steel type was evaluated. For steel grades with a maximum stress corrosion cracking depth of 0.2 mm or less after the end of 200 cycles, even if an initial stage of stress corrosion cracking occurs, the progress of the crack is prevented and no further progress is expected. Therefore, it was set as a pass (○ evaluation). Steel types with a maximum stress corrosion cracking depth of more than 0.2 mm are considered to be in a process in which stress corrosion cracking progresses, so they were rejected (x evaluation) as well as those having penetrated the plate thickness. The results are shown in Table 3.

  Moreover, the depth of the pitting corrosion produced on the surface of the large piece and the small piece was measured by a focal depth method using an optical microscope. Also in this case, the pitting corrosion resistance of the steel type was evaluated with the deepest pitting corrosion depth (this is called “maximum pitting corrosion depth”) in all the large pieces and small pieces of n = 3. Since the steel type with a maximum erosion depth of 0.2 mm or less after the end of 200 cycles is judged to have been inhibited from proceeding by repassivation, the steel type exceeding 0.2 mm was rejected. It was set as a pass (x evaluation). The results are shown in Table 3.

  As can be seen from Table 3, all of the examples of the present invention in which the content of each component was appropriate exhibited stable and excellent stress corrosion cracking resistance and pitting corrosion resistance in a salt dry and wet repeated environment. On the other hand, Nos. 31 and 32 with a Cu content less than specified are inferior in stress corrosion cracking resistance, and Nos. 33 to 36 with a Mo content less than specified in pitting corrosion resistance or further stress corrosion cracking resistance. inferior.

The graph which illustrated the relationship between the cycle number of a salt dry-wet combined cycle test (CCT), and the maximum stress corrosion cracking depth about the austenitic stainless steel of Table 1. The graph which illustrated the relationship between the Cu content and the maximum pitting corrosion depth after a salt dry-wet combined cycle test (CCT) test about 19Cr-11Ni-0.5Si-0.8Mn-0.8Mo steel. The graph which illustrated the relationship between the Mo content and the maximum stress corrosion cracking depth after a salt dry-wet combined cycle test (CCT) test about 19Cr-11Ni-0.5Si-0.8Mn-1Cu steel.

Claims (5)

In mass%, C: 0.05% or less, Si: less than 1.0%, Mn: 0.4 to 1.8%, P: 0.045% or less, S: 0.005% or less, Ni: 8 In a steel comprising 1.5 to 12.0%, Cr: 17.5 to 19.5%, N: 0.05% or less, and containing Cu and Mo as essential components, the balance being Fe and inevitable impurities , the content of Cu and Mo, Cu: less than 0.5 to 1.5% M o: and 0.5 to 1.2%, and was adjusted to a range satisfying 1.5 ≦ Cu + Mo ≦ 2.5 An austenitic stainless steel with excellent durability in repeated salt and wet environments   The austenitic stainless steel according to claim 1, further comprising Al: 0.3% or less.   The austenitic stainless steel according to claim 1 or 2, further comprising B: 0.005% or less. The austenitic stainless steel according to any one of claims 1 to 3 , wherein the Si content is adjusted to a range of 0.5 ± 0.25%. The austenitic stainless steel according to any one of claims 1 to 4 , which is used for an automobile underfloor member.

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