JP2007217775A - Stainless steel member having crevice structure - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a technique capable of stably evading trouble caused by stress corrosion cracking in a worked member used in an air atmosphere where chloride is easy to be stuck to a crevice part by using an austenitic stainless steel to which large quantities of Si and Mo are not added. <P>SOLUTION: The stainless steel member has, as a component, an austenitic stainless steel having a composition containing, by mass, ≤0.08% C, ≤1.5% Si, ≤2.5% Mn, 14.0 to 23.0% Ni, 23.0 to 30.0% Cr and ≤0.08% N, and, if required, one or more selected from ≤1.0% Cu and ≤1.0% Mo, and the balance substantially Fe, and having a crevice structure formed by welding or plastic working joining in a part of the surface. The crevice structure has a crevice spacing of ≤0.5 mm, and is used so as to be exposed to an air atmosphere. The stainless steel member is suitable for an automobile oil supply system member. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a stainless steel member having improved resistance to stress corrosion cracking in a gap portion in an atmospheric environment, which is suitable for a fuel tank or a fuel supply pipe of an automobile.

  If gasoline tanks and refueling pipes installed in automobiles and motorcycles are not airtight, vaporized gasoline will dissipate into the atmosphere. Dissipation of gasoline is a major factor that adversely affects global environmental protection, which has been especially emphasized recently.

  In general, the above-mentioned oil supply system members are made of metal materials from the viewpoint of strength and safety, and maintaining their airtightness greatly depends on the characteristics of the materials (pitting corrosion resistance, crevice corrosion resistance, stress corrosion cracking resistance, etc.). Affected. Therefore, in order to maintain good hermeticity over a long period of time, studies are being made to apply a stainless steel material, which is a corrosion-resistant material, to an oil supply system member in place of a conventional surface-treated steel sheet.

  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, austenitic stainless steel inherently has the disadvantage of being prone to stress corrosion cracking. In particular, in the oil supply system member, residual stress tends to exist in a portion having a high degree of processing, a welded portion, a crimped portion, or the like. In addition, sea salt particles, snowmelt salt, rainwater, etc. intrude into gaps formed in seam welds, spot welds, or caulking parts of fuel pipes and fuel tanks. It tends to be the starting point of stress corrosion cracking.

  So far, regarding the stress corrosion cracking resistance of austenitic stainless steel, much research has been conducted on the “water environment”, that is, the environment immersed in water. For example, Patent Documents 1 and 2 include a large amount of Si and Cu. An austenitic stainless steel having significantly improved resistance to stress corrosion cracking in warm water by addition is disclosed.

JP-A-1-159351 Japanese Patent Laid-Open No. 3-104842 JP-A-6-128699

  The environment where the oil supply system member of the automobile is used is an “atmosphere environment”, and the corrosive environment is different from the above “water environment”. For this reason, when austenitic stainless steel with improved resistance to stress corrosion cracking in water environments such as Patent Documents 1 and 2 is used in an atmospheric environment where chloride adheres, it is not necessarily excellent in stress corrosion cracking resistance. It is not always possible to exert its stability stably. In fact, when the steel of Patent Documents 1 and 2 was processed into a member having a welded part and a caulking part and used in an atmospheric environment where chlorides adhered, the Cr content level and the Si content level were compared. In a relatively low composition range, crevice corrosion may occur in the gaps around the welded part and the caulking part, and in some cases, stress corrosion cracking occurs as a base point. From this, it was found that, for applications such as automobile oil supply system members, simply selecting austenitic steel with improved characteristics in a water environment does not provide a means for obtaining stable high durability.

  On the other hand, it is considered that the stress corrosion cracking resistance in the atmospheric environment can be raised to a problem-free level by adopting a steel type in which the Cr and Si content levels are considerably increased among the steels of Patent Documents 1 and 2. However, since Cr and Si are ferrite forming elements, it is necessary to add a large amount of an austenite forming element such as Ni in order to obtain an austenitic steel in which these elements are simultaneously increased, resulting in an increase in material cost. Moreover, manufacturability also deteriorates.

  It is considered possible to avoid troubles due to corrosion in the atmospheric environment by using austenitic steel with high Cr and high Mo as disclosed in Patent Document 3. However, since Mo is an expensive element, the increase in Mo greatly increases the material cost, and application to automobile oil supply system members is not realistic.

  In view of such a current situation, the present invention uses austenitic stainless steel to which a large amount of Si or Mo is not added, and causes trouble due to stress corrosion cracking of a processed member used in an atmospheric environment in which chloride easily adheres to a gap. This is to provide a technology that can stably avoid this problem.

  As a result of various studies, the inventors have found that the stress corrosion cracking resistance in the gap portion of the austenitic stainless steel member is greatly influenced not only by the characteristics of the steel material but also by the shape of the gap structure of the member. In the atmospheric environment, the stress corrosion cracking resistance is remarkably improved by forming a tight gap, as opposed to the case of the water environment. However, it is necessary to apply steel having a certain range of composition to the member.

  That is, in the present invention, austenite containing Ni: 14.0 to 23.0% by mass, Cr: 23.0 to 30.0% by mass, and having a gap structure formed by welding or plastic work joining on a part of the surface. There is provided a member having a stainless steel as a constituent element, wherein the gap structure has a gap gap of 0.5 mm or less, and the gap structure is used by being exposed to an atmospheric environment. An automobile oil supply system member is a suitable target. The preferred composition of the austenitic stainless steel is, by mass, C: 0.08% or less, Si: 1.5% or less, Mn: 2.5% or less, Ni: 14.0 to 23.0%, Cr: 23.0 to 30.0%, N: 0.08% or less, if necessary, further including one or more of Cu: 1.0% or less, Mo: 1.0% or less, with the balance being The composition which is substantially Fe is mentioned. “The balance is substantially Fe” means that mixing of elements other than the above is allowed within a range that does not impair the effects of the present invention, and the balance includes Fe and inevitable impurities.

  Plastic working joining is a working method in which two or more materials are joined together by applying deformation accompanied by plastic working, and a typical example is “caulking”. An example of welding is “resistance welding” in which two materials are partially overlapped and joined as a joining method involving a gap.

The gap structure is a portion in which a planar space or a contact portion is formed between opposing surfaces, and the “planar shape” includes a curved surface shape. However, when the average distance δ (mm) between the opposing surfaces and the area s (mm ² ) of the planar space or contact portion does not satisfy s ≧ 10δ, the gap structure in the present invention is not applicable. The gap interval of 0.5 mm or less means that the distance between the opposing surfaces in the gap structure is within 0.5 mm.

  The atmospheric environment is basically an environment that is used in a dry state, although the member may get wet by rainwater or moisture in the air, etc., and the member of the present invention is particularly excellent in an environment with chloride adhesion. Demonstrate durability. It can be said that the atmospheric environment in the present invention is an outdoor environment because it involves the adhesion of moisture and chloride existing outdoors.

  According to the present invention, it is possible to stably avoid troubles caused by stress corrosion cracking in a member that has a processed part with a gap structure and is exposed to an atmospheric environment where the processed part easily adheres to chloride. A highly durable member was provided. Since austenitic stainless steel is used as the material, a member processed into a complicated shape can be constructed. In addition, since austenitic stainless steel that is not particularly high in Si or Mo is used, the cost is reduced compared to a steel type that is excessively imparted with high corrosion resistance. Therefore, the present invention is suitable as a vehicle oil supply system member such as a gasoline tank or a fuel supply pipe that is required to maintain an excellent hermeticity for a long period of time, and contributes to improvement of durability of these members. Is.

  The stainless steel member of the present invention is used by exposing the gap to the atmospheric environment. In the atmospheric environment, the corrosive environment is greatly different from the water environment in that many corrosion products remain in the vicinity of the corroded portion. When corrosion progresses in the gap, Fe, Ni, Cr and the like contained in the austenitic stainless steel become ions and are eluted in the gap, and are present in the vicinity of the corrosion portion as precipitates of ions or oxides.

The inventors have focused on corrosion products and have conducted detailed studies on the resistance to stress corrosion cracking in gaps in the atmospheric environment. As a result, it has been found that when Ni ions are present as a corrosion product in the gap, the stress corrosion cracking resistance of the austenitic stainless steel in the gap is significantly improved. Even for general-purpose steel such as SUS304, which does not have particularly improved stress corrosion cracking resistance, an aqueous solution containing Ni ²⁺ is dropped into the gap and Ni ions are supplied into the gap. When subjected to the test, stress corrosion cracking is remarkably suppressed. Although the mechanism is not clear at present, the effect of Ni ions can be grasped by, for example, the following experiment.

That is, a 30 mm × 30 mm large piece and a 15 mm × 15 mm small piece are cut out from a SUS304 2D finish (plate thickness: 1 mm), the surface is finished by # 600 wet polishing, and then an electrode having a diameter of 5 mm is used at the center of the large piece. Then, joining was performed under the condition that a nugget was formed by spot welding. A gap structure close to a close contact state is formed in the vicinity of the weld. The weld gap test piece produced in this way (test piece in which a gap structure was formed by welding) was subjected to a salt-dry wet cycle test (CCT). The test piece was placed almost horizontally with the small piece side up. For CCT, “85% RH, 50 ° C. × 15 h hold → 30% RH. Forced drying by holding 50 ° C. × 3 h → 50% RH, 20 ° C. × 6 h hold” is one cycle. This was done up to 100 cycles. However, before the start of the cycle and every 10 cycles in the middle, either “5% NaCl aqueous solution” or “5% NaCl added with 2000 ppm Ni ²⁺ ions” is placed in the center of the small piece. 1 mL was added dropwise.

The large and small pieces of the weld gap test piece after 100 cycles were separated, and the presence or absence of stress corrosion cracking in the vicinity of the weld was examined. As a result, stress corrosion cracking occurred in the sample dropped with “5% NaCl aqueous solution”, whereas stress corrosion occurred in the sample dropped with “aqueous solution obtained by adding 2000 ppm Ni ²⁺ ions to 5% NaCl”. No cracks were observed. That is, it was revealed that even in the case of SUS304, the stress corrosion cracking resistance is remarkably improved in the weld gap where Ni ²⁺ ions exist. In addition, this weld gap test piece using SUS304 causes remarkable stress corrosion in a water environment.

In the actual atmospheric environment, Ni ²⁺ ions are not supplied from the outside. Therefore, the stainless steel member of the present invention is
i) Ni ²⁺ ions are supplied into the gap from the austenitic stainless steel of the material in an atmospheric environment where chloride adheres,
ii) the Ni ²⁺ ions stay in the gap;
Thus, the problem of stress corrosion cracking resistance is solved.
The above i) is realized by defining the composition of the austenitic stainless steel as a raw material. The above ii) is realized by making the gap portion a structure in which Ni ²⁺ ions hardly flow out. Hereinafter, the steel composition and the definition of the gap structure will be described.

[Steel composition]
As a result of various investigations, a sufficient amount of Ni ²⁺ ions are supplied from the material itself to the gap when used in an atmospheric environment where chlorides are likely to adhere, such as automobile refueling parts and underbody parts. It is effective to secure a large Ni content. In addition, since the pH is likely to decrease in the gap and become an acid environment, increasing the Ni content and reducing the critical passivation current is effective in suppressing the growth of corrosion, which is the starting point of stress corrosion cracking. Is. From these things, Ni content of a raw material needs to be 14.0 mass% or more. However, excessive Ni content tends to cause surface defects such as baldness and increases costs, so the Ni content is limited to 23.0% by mass or less.

  In addition, in order to ensure basic corrosion resistance in the presence of chlorides and to cope with a decrease in corrosion resistance due to a decrease in pH in the gap, a Cr content of 23.0% by mass or more is ensured. However, if the Cr content is too high, an austenite single phase structure cannot be obtained, and workability is impaired. For this reason, Cr content is restrict | limited to 30.0 mass% or less.

The other elements usually contained in the austenitic stainless steel are not particularly limited as long as they do not impair manufacturability, workability, and weldability. For example, C: 0.08 mass% or less, Si: 1. Good results are obtained in a range of 5% by mass or less, Mn: 2.5% by mass or less, N: 0.08% by mass or less, Cu: 1.0% by mass or less, and Mo: 1.0% by mass or less.
In addition, for example, B is allowed to be contained in a range of 0.03 mass% or less, Ti and Nb are both 0.05 mass% or less, and REM (rare earth element), Y, and Ca are contained in a total range of 0.02 mass% or less. . It is desirable to keep S as low as possible. In the present invention, up to 0.007% by mass is acceptable.

(Gap structure)
When constructing automobile parts such as oil supply system members, parts are often joined by resistance welding or caulking. The resistance welding is most commonly seam welding or spot welding. Usually, two plate-like parts are overlapped, and they are sandwiched between electrodes and are energized while being pressed and joined. In the vicinity of the welded portion joined, a gap is formed between the two superimposed components. The gap may be in surface contact with both parts, or may be partially or wholly separated. In the caulking process, a gap having a form close to surface contact is formed in the processed portion, and a gap in which both parts are separated may be formed around the gap. In any case, the gap formed in the vicinity of the joint portion of resistance welding or caulking has a function of maintaining the strength of the joint portion by pressing both parts when a force is applied to the joint portion. If the area of the gap is too small, the function of maintaining the bonding strength is not exhibited. When the average distance δ (mm) between the opposing surfaces and the area s (mm ² ) of the planar space or contact portion does not satisfy s ≧ 10δ, the area of the overlapping portion is too small with respect to the gap interval, The function of maintaining the bonding strength tends to be insufficient.

If Ni ²⁺ generated inside the gap flows out of the gap, sufficient stress corrosion cracking resistance in the gap cannot be obtained. For this reason, it is very important in the present invention to form a gap structure with a gap gap as small as possible. As a result of detailed research, by constructing a tight gap structure in which the gap interval is 0.5 mm or less and using the steel material having the above composition for at least one of the facing parts, the gap portion is It was found that stress corrosion cracking, which causes trouble, does not occur when exposed to an atmospheric environment where chlorides are likely to adhere. In this respect, it can be said that the corrosion behavior is greatly different from the water environment in which tight gaps promote crevice corrosion and stress corrosion cracking. In the atmospheric environment, when the gap interval exceeds 0.5 mm, it becomes difficult to stably obtain a remarkable improvement effect of stress corrosion cracking resistance.

By forming either one of the parts sandwiching the gap with the austenitic stainless steel having the above composition as defined in the present invention, the generation of Ni ²⁺ ions inside the gap is realized, and the resistance in the gap is reduced. Stress corrosion cracking is significantly improved. If both parts having a gap are made of the austenitic stainless steel having the above composition defined in the present invention, the reliability is further improved, which is preferable.

An austenitic stainless steel having the composition shown in Table 1 is melted, and a 2D-finished steel plate (2 mm thick) is manufactured by a general stainless steel plate manufacturing process. As shown in FIG. A weld gap test piece having a simple structure was prepared. That is, a large piece 1 of 30 mm × 30 mm and a small piece 2 of 15 mm × 15 mm were cut out from the material steel plate. Of these, the small piece 2 was formed with a projection 3 having a diameter of 5 mm and a height of 10 μm to 1.0 mm at the center of one surface by precision cutting. As for the surface finish, the large piece 1 was subjected to # 600 wet polishing finish, and the small piece was subjected to precision cutting. The large piece 1 was joined so that the tip of the projection 3 was in contact with the center of the large piece 1 under the condition that a nugget was formed by spot welding using an electrode having a diameter of 5 mm. Spot welding was performed so that the axis of the electrode coincided with the axis of the protrusion 3 of the small piece 2.
FIG. 1 schematically shows the structure of the weld gap test piece thus produced. FIG. 1A is a plan view, and FIG. 1B is a cross-sectional view at the center. Weld gap test pieces in various stages with a gap interval δ of 10 μm to 1.0 mm were prepared.
In addition, all S content of each steel is reduced to 0.007 mass% or less.

Each 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. For CCT, “85% RH, 50 ° C. × 15 h hold → 30% RH. Forced drying by holding 50 ° C. × 3 h → 50% RH, 20 ° C. × 6 h hold” is one cycle. This was done up to 100 cycles. However, 1 mL of 5% NaCl aqueous solution (with no Ni ²⁺ ions added) was dropped at the center of the small piece before the start of the first cycle and every 10 cycles in the middle. Each steel type was performed with n = 3.

  After 100 cycles, the large and small pieces of the test piece were separated, and the presence or absence of stress corrosion cracking in the vicinity of the weld was examined in detail with an optical microscope, and no stress corrosion cracking was observed for all the test pieces of n = 3. A thing (circle) (good) and the one where stress corrosion cracking was recognized were evaluated as x (bad). The results are shown in Table 2.

  As can be seen from Table 2, all of the examples of the present invention in which the test piece is composed of austenitic stainless steel having the composition defined in the present invention and a gap structure with a gap interval of 0.5 mm or less is formed are stress corrosion cracking. It was confirmed that the material exhibits stable stress corrosion cracking resistance stably in an atmospheric environment where chlorides are likely to adhere.

On the other hand, even in the austenitic stainless steel having the composition defined in the present invention, stress corrosion cracking occurred when the gap interval was 0.8 mm and 1.0 mm. This is probably because Ni ²⁺ ions easily flow out of the gap due to the wide gap interval, while the oxygen concentration remains lower in the gap and an oxygen concentration cell is formed.

  In addition, when the Cr content or the Ni content is made of steel (No. 21 to 26) that is less than the scope of the present invention, stress corrosion cracking occurs even in a tight gap structure with a gap interval of 0.5 mm or less. Could not be avoided.

The figure which showed typically the structure of the welding clearance gap test piece used in the Example.

Explanation of symbols

1 Large piece 2 Small piece 3 Protrusion

Claims (6)

  Constructs austenitic stainless steel containing Ni: 14.0 to 23.0 mass%, Cr: 23.0 to 30.0 mass%, and having a gap structure formed by welding or plastic work joining on a part of the surface A stainless steel member used as an element, wherein the gap structure has a gap distance of 0.5 mm or less and the gap structure is exposed to an atmospheric environment.   In mass%, C: 0.08% or less, Si: 1.5% or less, Mn: 2.5% or less, Ni: 14.0 to 23.0%, Cr: 23.0 to 30.0%, N: 0.08% or less, with the balance being substantially Fe, and a member having, as a constituent element, austenitic stainless steel having a gap structure formed by welding or plastic working joining on a part of the surface. The gap structure has a gap distance of 0.5 mm or less, and the gap structure is a stainless steel member used by being exposed to the atmospheric environment.   In mass%, C: 0.08% or less, Si: 1.5% or less, Mn: 2.5% or less, Ni: 14.0 to 23.0%, Cr: 23.0 to 30.0%, N: 0.08% or less, Cu: 1.0% or less, Mo: 1.0% or less, one or more of the following composition, the balance being substantially Fe, A member having an austenitic stainless steel having a gap structure formed by welding or plastic work joining at a part, the gap structure having a gap interval of 0.5 mm or less, and the gap structure being in an atmospheric environment Stainless steel member used by being exposed.   The stainless steel member according to claim 1, wherein the welding is resistance welding.   The stainless steel member according to any one of claims 1 to 3, wherein the plastic working joining is caulking.   The stainless steel member according to claim 1, wherein the member is an automobile oil supply system member.

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