JP2004115911A - Ferritic stainless steel for automobile fuel tank and for peripheral member of fuel tank - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide low Cr, i.e. inexpensive ferritic stainless steel which solves the problem as for external corrosion resistance in a salt damage environment in the conventional technique on using ferritic stainless steel after working and welding into an automobile fuel tank, fuel pipe or the like. <P>SOLUTION: The Zn-containing paint coated ferritic stainless steel is obtained by coating ferritic stainless steel comprising alloy elements having the following specified composition with a Zn-containing paint. The specified composition comprises, by mass, &le;0.1% C, &le;1.0% Si, &le;1.5% Mn, &le;0.06% P, &le;0.03% S, &le;1.0% Al, 11 to 20% Cr and &le;0.04% N, also comprises 0.002 to 0.8% Nb and/or 0.01 to 1.0% Ti, and the balance Fe with inevitable impurities. <P>COPYRIGHT: (C)2004,JPO

Description

  The present invention relates to a ferrite stainless steel suitable for use as a container or piping member for organic fuels such as gasoline and methanol, and particularly to a stainless steel for a fuel tank peripheral member such as a fuel tank or a fuel pipe of an automobile or a tank band. The present invention relates to a Zn-containing paint-coated automobile fuel tank and a ferritic stainless steel for a fuel tank peripheral member, in which a Zn-containing paint is partially applied for the purpose of improving the corrosion resistance of the entire member or mainly the gap. Here, the gap means a gap formed by assembling a ferritic stainless steel fuel tank and its peripheral members, a gap formed by welding the fuel tank and its peripheral members, and a ferrite in the fuel tank and its peripheral members. It refers to those that include gaps and the like that occur when joining stainless steel and dissimilar metals. The term “ferritic stainless steel for fuel tanks and fuel tank peripheral members” refers to ferrite stainless steel used for fuel tanks and fuel tank peripheral members, and should always be used simultaneously as fuel tanks and fuel tank peripheral members. It is not limited to.

  Conventionally, as a fuel tank for automobiles and its peripheral members, those obtained by forming and welding a turn sheet (Ternes steel sheet (Pb-Sn)) plated with lead on the surface of a mild steel sheet have been widely used. Was. However, the use of lead-containing materials has been severely restricted due to the recent rise in environmental problems. For this reason, the development of alternative materials to replace turn sheets is being sought.

For example, in order to improve the corrosion resistance to salt damage, a steel sheet which has been subjected to an Al—Si alloy plating as a lead-free plating material and further subjected to a chemical conversion treatment has been proposed (for example, Patent Document 1). However, there are concerns about its weldability and long-term corrosion resistance, and it has not been widely applied. In addition, if the equipment for obtaining the steel sheet is enlarged, the cost is increased and the productivity is unavoidably inferior, so that it cannot sufficiently meet the demand for mass supply.

Further, a stainless steel for a fuel tank has been proposed which secures resistance weldability, press formability by lubrication hardening, and corrosion resistance by applying Zn or a lubricating film containing Zn to a steel sheet before processing (for example, Patent Reference 2).
However, since the steel sheet with the Zn-containing lubricating film is subjected to resistance welding, carbon may be mixed into the welded portion from the resin component of the film and the corrosion resistance may be reduced due to sensitization. Further, when press forming is performed with a Zn-containing lubricating film, there is a problem that release powder is significantly generated at the time of pressing as compared with a lubricating film containing no Zn, making it difficult to maintain a mold.

Further, an attempt has been made to use an austenitic stainless steel typified by SUS304 as a steel that can be used without lining or the like. However, there is a fear of stress corrosion cracking (SCC) for a fuel tank. It has not been put to practical use.

試 み Although attempts have been made to use a synthetic resin fuel tank having a multilayer structure, it is unavoidable that the fuel slightly penetrates the walls of the fuel tank and the like, and there is an essential problem of fuel evaporation. In addition, the use of synthetic resins has naturally been limited in practical use due to the regulation of fuel evaporation and the regulation of synthetic resin recycling.

On the other hand, ferritic stainless steel is less costly because it is less susceptible to stress corrosion cracking and has less expensive Ni content than the austenitic stainless steel. However, when applied to a fuel tank or a fuel pipe, there is a drawback that the corrosion resistance mainly due to salt damage corrosion on the outer surface is insufficient. Therefore, it was necessary to contain a large amount of alloy elements such as Cr and Mo. However, since the workability is reduced with the increase in the alloy of steel, for example, the steel pipe cannot withstand severe pipe expansion or bending as a fuel pipe, and the processing shape is limited.

JP-A-2002-146553 JP-A-2002-146557

Accordingly, the present invention provides a ferritic stainless steel having more excellent corrosion resistance and workability than conventional high Cr ferritic stainless steel, and suitable for low Cr automobile fuel system members. It is the purpose.
That is, an object of the present invention is to solve the problem related to the external corrosion resistance due to salt damage that the conventional technology has at once, when processing and welding ferritic stainless steel to a fuel tank or a fuel pipe of an automobile, and the like. It is to provide Cr, that is, inexpensive ferritic stainless steel. The standard of corrosion resistance of the ferritic stainless steel for a fuel system member of an automobile according to the present invention is based on a salt-dry / wet combined cycle test (CCT; American Society of Automobile Engineers (SAEJ 2334)). It is assumed that it does not occur.

The present inventor paid attention to the sacrificial anticorrosion effect of Zn, focused on the use of Zn-containing paints, and partially or completely formed in the gaps (including welds and dissimilar metal joints) of ferritic stainless steel processed products. A Zn-containing paint was applied to the steel, and by the sacrificial anticorrosion action of Zn, the corrosion of the portion having the lowest corrosion resistance was prevented, and the corrosion of other portions was also successfully prevented, thereby completing the present invention. That is, the present invention is a Zn-containing paint-applied ferritic stainless steel obtained by applying a Zn-containing paint to a ferritic stainless steel containing an alloy element having a specific composition. For use, it can ensure sufficient external corrosion resistance even in a salt damage environment.

  In the present invention, C: 0.1% or less, Si: 1.0% or less, Mn: 1.5% or less, P: 0.06% or less, S: 0.03% or less, Al: 1 by mass%. 2.0% or less, Cr: 11-20% and N: 0.04% or less, and Nb: 0.002-0.8% and / or Ti: 0.01-1.0%. The remainder is ferritic stainless steel for a Zn-containing paint-coated automobile fuel tank and a fuel tank peripheral member, wherein a Zn-containing paint is applied to steel consisting of Fe and unavoidable impurities.

  It is preferable that the stainless steel further contains at least one selected from the group consisting of Mo: 3.0% or less, Cu: 2.0% or less, and Ni: 2.0% or less by mass%.

The stainless steel preferably contains B: 0.0003 to 0.005% by mass.

The stainless steel preferably contains Co: 0.3% or less by mass%.

The stainless steel preferably contains Mg: 0.0032% or less by mass%.

The Zn content (X) in the coating film of the Zn-containing coating material of the ferrite stainless steel for the automobile fuel tank and the fuel tank peripheral member is an amount defined by the following formula (1), and the thickness of the coating film is Is preferably 5 to 50 μm.
70 ≧ X ≧ 70− {2.7 × (Cr + 3.3Mo)} (1)
Where X is the Zn content (% by mass) in the coating film,
Cr is the Cr content (% by mass) in stainless steel,
Mo is the Mo content (% by mass) in stainless steel.

The average particle diameter of Zn in the Zn-containing paint of the ferrite stainless steel for the automobile fuel tank and the fuel tank peripheral member is preferably 3 μm or less.

According to the present invention, the corrosion resistance of the gap is compensated for by sacrificial corrosion prevention of Zn in the paint, thereby replacing expensive stainless steel containing a large amount of Cr, Ni, etc., and lowering the Cr and Ni content to a low price. A ferritic stainless steel can be obtained. As a result, the stainless steel is excellent in outer surface corrosion resistance and gasoline corrosion resistance under salt damage environment, high strength and good workability, and used as a fuel tank for automobiles and the like and its peripheral members that require a good balance between them. It became possible to do.

好 適 The preferred components and their contents (% by mass) of the ferritic stainless steel for automobile fuel tanks and fuel tank peripheral members of the present invention are as follows.

Cr: Content of 11 to 20%
Cr is an element effective for improving oxidation resistance and corrosion resistance. When the Cr content is less than 11%, when used without painting, red rust is remarkably generated. It is difficult to secure sufficient corrosion resistance in steel. To obtain sufficient oxidation resistance and corrosion resistance, 11% or more is required. On the other hand, when the content exceeds 20%, the corrosion resistance of the steel itself is improved, no generation of red rust is observed, and the necessity of applying a paint is reduced. And, even when the r value is high, the workability is reduced due to an increase in strength and a decrease in ductility. For this reason, the amount of Cr is specified in the range of 11 to 20%. It is preferably 12% to 18% in consideration of workability, and more preferably 14 to 18% in consideration of corrosion resistance of a welded portion.

C: Content of 0.1% or less C is preferably contained in an amount of 0.0005% or more because it strengthens the grain boundaries and improves the resistance to secondary working brittleness. Is an element that has an adverse effect on secondary work brittleness resistance and intergranular corrosion. In particular, when the content of C exceeds 0.1%, this adverse effect appears remarkably, so the content is limited to 0.1% or less. In addition, from the viewpoint of improving the secondary work brittleness resistance, the content is preferably more than 0.002% and 0.008% or less.

Si: Content 1.0% or less Si is an element effective for improving oxidation resistance and corrosion resistance, and is preferably contained 0.2% or more. On the other hand, if the content exceeds 1.0%, the steel becomes brittle, and the secondary working brittleness resistance of the welded portion also deteriorates. Therefore, the content is made 1.0% or less. More preferred is 0.75% or less.

Mn: Content 1.5% or less Mn is an element effective for improving oxidation resistance. The content of Mn is preferably 0.5% or more. It also deteriorates the secondary work brittleness resistance of the part. Therefore, the content is 1.5% or less. More preferred is 1.3% or less.

P: content 0.06% or less P is an element that easily segregates at the grain boundary and reduces the strength of the grain boundary after performing strong working such as deep drawing of a fuel tank. Therefore, in order to improve the secondary work brittleness resistance (the phenomenon of breaking due to a slight impact after strong working), it is desirable to reduce the strength as much as possible. However, if it is too low, the steelmaking cost will increase. Therefore, the P content is set to 0.06% or less. More preferably, it is 0.03% or less.

S: content of 0.03% or less S is an element harmful to the corrosion resistance of stainless steel, but in consideration of desulfurization costs during steelmaking, 0.03% can be allowed as an upper limit. More preferable is 0.01% or less which can be fixed with Mn or Ti.

Al: Content 1.0% or less Al is an element necessary as a deoxidizing agent on steelmaking. In order to obtain the effect, the content is preferably 0.01% or more. However, if it is excessively contained, the surface appearance and the corrosion resistance are deteriorated due to the inclusions. More preferably, it is 0.50% or less.

N: 0.04% or less N enhances the secondary working brittleness when strengthening the grain boundaries and working into a tank or the like. In order to obtain the effect, the content is preferably 0.0005% or more. However, if the content is excessive, the element is converted into a nitride, which precipitates at the grain boundary and adversely affects the corrosion resistance. Therefore, the content of N is set to 0.04% or less. More preferable is 0.02% or less.

Nb: content 0.002 to 0.8%,
Ti: Content 0.01 to 1.0%
Nb and Ti are elements that improve the r value by fixing C and N in a solid solution state as a compound. These effects are exhibited when the content of Nb is set to 0.002% or more and the content of Ti is set to 0.01% or more, and contained alone or in combination. On the other hand, when Nb exceeds 0.8%, the toughness deteriorates remarkably, and when Ti exceeds 1.0%, the surface appearance and the toughness deteriorate. Therefore, these values are set as upper limits. More preferably, Nb is 0.003-0.4% and Ti is 0.05-0.4%.

In addition to the above main components, the following components are preferably further contained alone or in combination.
Mo: Mo content of 3.0% or less Mo is an element effective for improving corrosion resistance and improves external salt damage corrosion resistance. For this purpose, the Mo content is preferably set to 0.5% or more, but if it exceeds 3.0%, workability is deteriorated. Therefore, the Mo content is set to 3.0% or less. A preferred range is 0.5 to 1.6% from the viewpoint of workability and corrosion resistance.

Ni: content 2.0% or less Ni is an element that improves the corrosion resistance of stainless steel, and is preferably contained at 0.2% or more. If the content exceeds 2.0%, the steel is hardened, and stress corrosion cracking tends to occur due to the formation of an austenite phase. Therefore, the content of Ni is 2.0% or less. More preferred is 0.2-0.8%.

Cu: 2.0% or less Cu is an element effective for improving corrosion resistance. In order to obtain the effect, the content is preferably 0.05% or more. However, since the steel is hardened and the productivity is lowered, the upper limit is 2.0%. The preferred range is less than 0.5% from the viewpoint of workability and corrosion resistance.

B: Content 0.0003-0.005%
B is an element effective for improving the brittleness in secondary working. In particular, fuel tank peripheral members are subjected to complicated forming processes, and are often used in cold regions below freezing. B is also effective in increasing the grain boundary strength. However, in order to obtain the effect, the content needs to be 0.0003% or more. On the other hand, if the content exceeds 0.005%, the workability and toughness of steel are impaired, so the range is 0.0003 to 0.005%. Preferred is 0.0005 to 0.0010%.

Co: Content of 0.3% or less Co is an element effective for improving the brittleness in secondary working and the toughness of a steel sheet. In particular, fuel-related peripheral members such as fuel tanks, filler pipes, and fuel tank bands are subjected to complicated forming and are often used in cold regions below the freezing point. However, the effect is that if the content is 0.3% or more, the steel becomes hard, and particularly the workability is lowered, so the upper limit was made 0.3%. Preferred is 0.05-0.2%.

Mg: Content 0.0032% or less Mg becomes oxide-based inclusions and functions as crystallization nuclei of TiN, and TiN functions as crystallization nuclei of δ-ferrite, thus effectively acting on the equiaxed crystal ratio of the slab. As a result, it is an effective element for improving the workability, toughness, and ridging of a steel sheet. However, if the content exceeds 0.0032%, excess Mg is dispersed in the steel as oxide-based inclusions, impairing corrosion resistance and workability, so the upper limit was made 0.0032%. Preferred is 0.0015-0.0030%.

In the stainless steel of the present invention, in addition to the above components, unavoidable impurities include Zr of 0.5% or less, Ca of 0.1% or less, Ta of 0.3% or less, and W of 0% or less. The effect of the present invention is not particularly reduced even if Sn is contained in the range of 0.3% or less and Sn is contained in the range of 0.3% or less.

The automotive fuel tank and the ferrite stainless steel for fuel tank peripheral members of the present invention can be manufactured by directly applying a general method for manufacturing a ferritic stainless steel, but the hot rolling process and the cold rolling process can be performed. It is preferable to set some conditions as specific conditions. In steelmaking, it is preferable that steel containing the essential components and components added as necessary is melted in a converter or an electric furnace, and the secondary refining is performed by a VOD method. The molten steel can be made into a steel material according to a known production method, but from the viewpoint of productivity and quality, it is preferable to use a continuous casting method. The steel material obtained by continuous casting is heated to, for example, 1000 to 1250 ° C., and is hot-rolled into a hot-rolled sheet having a desired thickness. Of course, it can also be processed as a material other than a plate material.

The obtained hot rolled sheet is box-annealed at a temperature range of 600 to 900 ° C or continuous annealing at a temperature range of 800 to 1100 ° C, preferably 900 to 1100 ° C (hot rolled sheet annealing) depending on the required strength level. After that, a pickling and a cold rolling process are performed to obtain a cold rolled sheet. In this cold rolling step, two or more times of cold rolling including intermediate annealing may be performed as necessary according to production reasons. In this case, in order to obtain a steel plate having a high r-value, the linear pressure in the final pass of the hot rolling described above is ensured, and the total rolling reduction in the cold rolling process including one or two or more cold rollings is reduced to 75%. % Or more, preferably 82% or more.
The cold-rolled sheet is subjected to continuous annealing (cold-rolled sheet annealing) at 700 to 1050 ° C., more preferably 850 to 1000 ° C., and then pickled to form a cold-rolled annealed sheet. In addition, depending on the use, the shape and quality of the steel sheet can be adjusted by applying light rolling after cold rolling annealing. The cold-rolled annealed plate or the quality-controlled cold-rolled annealed plate is subjected to the application of the Zn-containing paint.

Ｚｎ The Zn-containing paint used in the present invention usually comprises a binder, an additive and a solvent or diluent, but the composition and preparation method are not particularly limited. When a Zn-containing paint is applied and left at room temperature or, if necessary, heated (baked) and dried, a cured coating film composed of a binder, an additive and Zn is formed. The additives are added for the purpose of dispersing a paint or drying, curing, and improving various physical properties of a coating film, and include a drying agent, a curing agent, a plasticizer, and an emulsifier.

The Zn-containing paints include a room temperature curing type and a heat curing type depending on the type of curing agent.
Examples of the binder include an acrylic resin, a vinyl chloride resin, a vinyl acetate resin, a silicone resin, a vinyl acetal resin, a polyurethane resin, a polyarylate resin, a phenol resin, an epoxy resin, an alkyd resin, a polyamide resin, and a polyimide resin. Matching or the like is used. As the inorganic binder, calcium fluoride, barium fluoride, sodium silicate or the like is used.

Zn particles: Average particle diameter of 3 μm or less Zn is a particularly important element for securing the corrosion resistance of Fe—Cr alloys such as stainless steel by sacrificial corrosion protection. Its average particle size is 3 μm or less. If the average particle size exceeds 3 μm, the adhesion of the coating film to stainless steel will be poor if the coating film is thin. In addition, when the Zn particles are finely dispersed in the coating film, the sacrificial corrosion protection performance of Zn tends to be improved, and from this viewpoint, the average particle diameter is preferably 3 μm or less. A more preferred average particle size is 0.5 to 2.5 μm, and a still more preferred average particle size is 1.0 to 2.0 μm.
The particle diameter of the Zn particles is a value obtained by measuring the maximum particle diameter and the minimum particle diameter of one Zn particle, adding the measured values, and dividing by two. The average particle diameter was determined by observing the cross section of the dried coating film after applying the Zn-containing paint in five visual fields by microscopic observation (magnification: 400 times), and measuring the particle diameters of all Zn particles in the visual field as described above. It is a numerical value obtained by arithmetically averaging the diameter.

It is known that the corrosion resistance of stainless steel has a positive correlation with the pitting corrosion index (Cr + 3.3Mo). Then, the present inventor investigated the relationship between the Zn content in the coating film and the pitting corrosion index of stainless steel, and as shown in FIG. 1, the Zn content in the coating film was 70-７０2. It has been found that when it is not less than 7 × (Cr + 3.3Mo)｝, the corrosion resistance is sufficiently exhibited, and the salt-corrosion outer surface corrosion resistance required for the clearance of the stainless steel processed product can be sufficiently satisfied.

On the other hand, if the Zn content exceeds 70% by mass of the whole coating film, the primary adhesion to the stainless steel surface becomes poor. In particular, when a stepping stone or the like is received, the coating film itself is easily peeled, and the adhesion is poor, so that it is difficult to secure an effective Zn amount. In addition, when the Zn content is large, Zn precipitates under the paint at the time of preparing the paint, and the paint becomes non-uniform if not constantly stirred, so that the efficiency of the coating operation deteriorates. Therefore, in order to use Zn efficiently, the upper limit of the Zn content is determined to be 70% from the viewpoint of corrosion resistance and adhesion, and the total is determined to satisfy the range of 70− {2.7 × (Cr + 3.3Mo)} or more. did.

From the above situation, it is important that the Zn content (X) in the coating film is within the range defined by the following empirical formula (1).
70 ≧ X ≧ 70− {2.7 × (Cr + 3.3Mo)} (1)
Here, X is the content (% by mass) of Zn in the coating film,
Cr is the Cr content (% by mass) of the stainless steel,
Mo is the Mo content (% by mass) of the stainless steel.

As described above, the Zn content in the coating film depends on the pitting corrosion index, in other words, the corrosion resistance of the stainless steel (Fe—Cr alloy). Although the amount can be reduced, if the corrosion resistance is low, it is necessary to increase the Zn content in the coating film. Needless to say, if the Zn content in the coating film can be reduced, the specific gravity of the coating material becomes lighter, and workability and cost are also advantageous.
However, if the Cr content in the stainless steel exceeds 20% by mass, corrosion resistance in a neutral chloride environment becomes sufficient, and a coating film becomes unnecessary, so that the stainless steel targeted by the present invention is as described above. In addition, it is limited to the case where the Cr content is 20% by mass or less.

Incidentally, Zn content in the dried coating, after drying the coating film was measured mass W ₁ of the steel sheet in a state of adhering to the steel sheet, the coating release agent: Neoriba; using (R manufactured by Kansai Paint Co., Ltd.) Then, the coating film was peeled off from the steel sheet, and the steel sheet was dried. Thereafter, the mass W ₂ of the steel sheet was measured again. Then, to dissolve the retirement coating film in sulfuric acid or perchloric acid, and analyzed the solution with an atomic absorption method to determine the Zn content W _3. The Zn content in the dried coating film is calculated from W ₃ / (W ₁ −W ₂ ) × 100 (% by mass).

Coating: 5 to 50 μm thick
The thickness of the coating film is 5 to 50 μm as a dry film thickness. This is determined from the viewpoint of the corrosion resistance of the stainless steel and the adhesion of the coating film of the Zn-containing paint to the stainless steel. That is, if the film thickness is less than 5 μm, it becomes difficult to secure the adhesion as the Zn content increases. Further, the sacrificial anticorrosion ability of Zn depends on the Zn content per unit area of the coating film, but if the film thickness is 5 μm or less, a sufficient effective Zn content cannot be secured. On the other hand, when the film thickness exceeds 50 μm, the quality becomes excessive, and the drying time of the coating film becomes longer, thereby lowering the working efficiency. Note that an excessive thickness may adversely affect the adhesion. A preferred thickness is 10 to 30 μm, and a particularly preferred thickness is 15 to 25 μm.
In addition, the dry film thickness is obtained by observing the cross section of the dried coating film after application with a microscope (magnification: 400 times) in each of five visual fields, obtaining film thicknesses at three locations for each visual field, and averaging these values to obtain an average film thickness. Further, the average film thickness in five visual fields was averaged.

The method of applying the paint used in the present invention to stainless steel is not particularly limited, such as spray painting, brush painting, immersion in the paint, and the like. Specifically, it may be appropriately selected according to the production line of processed products such as filler pipes and fuel tanks.
For example, a steel plate is formed into a fuel tank, a fuel pipe, a fuel tank peripheral member such as a fuel band by a hydraulic press, opposed hydraulic forming, spinning, pipe processing, etc., followed by seam welding, laser welding, A Zn-containing paint is partially or entirely applied to a member assembled into a predetermined structure by spot welding or the like.
In the case of a room temperature curing type, the coating is applied and then left at room temperature. In the case of a heat-curing type, after applying a coating material, it is heated and dried (baked). As a result, a cured film composed of a binder, Zn particles, and an additive, that is, a coating film having excellent corrosion resistance is formed.

In the present invention, the coating film containing Zn particles is formed on the surface of the processed stainless steel product, and the range is a range including all the gaps formed in the processed product. Or the entire surface of the processed product. Since it is necessary to enhance the corrosion resistance by the coating film in the gap, if the portion is covered at least, the corrosion resistance of the entire stainless steel workpiece can be sufficiently maintained.

As described above, a Zn-containing paint-coated stainless steel obtained by applying a Zn-containing paint to stainless steel is excellent in strength, weld properties, workability, and corrosion resistance, and has a good balance among them. And application to fuel tank peripheral members.

Hereinafter, the present invention will be described more specifically with reference to invention examples and comparative examples.
(Examples 1 to 69)
Thirteen kinds of stainless steels having the component compositions shown in Table 1 were continuously cast, and a hot-rolled sheet having a thickness of 5.0 mm was manufactured under ordinary hot rolling conditions. This hot-rolled sheet was continuously annealed at 980 ° C. for 1 hour, pickled, cold rolled to a sheet thickness of 2.3 mm, intermediately annealed at 900 ° C., pickled, cold rolled to 0.8 mm, and 920 ° C. Finish annealing-pickling to obtain a cold rolled annealed sheet.

This cold-rolled annealed plate is pressed into an L-shaped test piece 1 (width 80 mm, long side 150 mm, short side 50 mm) shown in FIG. 2 and a surface (width 80 mm, width 80 mm, A test piece 1 having a seam weld 7 was prepared by seam welding centered at 20 mm from the top of the short side (50 mm). On the entire surface of the test piece 1, a paint having a Zn content in the dried coating film in the amount shown in Table 2 was spray-coated so as to have a dry average film thickness shown in Table 2, and left for 1 hour to dry. The coating film was cured to obtain a test piece 1 of a stainless steel coated with a Zn-containing paint. The measurement of the thickness, the Zn content, and the average particle diameter of the dried coating film is as described above.
Next, a plastic clip 3 was put on a portion where two test pieces were butted to form a gap 2.

The test piece 1 was processed and processed for measuring corrosion resistance and the like in the following manner.
The corrosion resistance of the gap 2 was evaluated by placing a plastic clip 3 on the surface of the test piece 1 (with paint applied) to form a gap between plastic and metal.
In the Erichsen test, a dome-shaped overhang 4 was formed on the bottom surface of the test piece 1 using a punch having a diameter of 15 mm and a height of 8 mm on the bottom surface of the test piece 1 in accordance with the Erichsen tester and the test method specified in JIS B7729. Provided and evaluated.

In the cross-cut test, as shown in FIG. 2, a cross-cut test was performed on the bottom surface of the test piece 1 on a diagonal line having a length of 115 mm drawn in a rectangle having a length of 60 mm and a width of 80 mm. It was cut and provided with a cross-cut portion 5 for evaluation.
In the gravure test, 100 g of basalt (average crushed stone, average particle size 8 to 12 mm) was vertically placed on the surface (width 40 mm, length 100 mm) of the bottom surface of the test piece 1 at normal temperature (20 ° C.) and pressure 7 kgf / cm ² . And the surface of the bottom surface was scratched by a stepping stone, and a gravure test part 6 was provided for evaluation. The gravure test section 6 was formed using an apparatus according to ASTM D3170.

Next, the SAE was applied to six parts of the test piece 1, namely, a gravure test part 6 by a stepping stone test (normal temperature), a 15 mmφ overhang (Erichsen) 4, a cross cut part 5, a gap part 2 and an end face by a plastic clip 3. After 120 cycles of the cycle under the conditions shown in FIG. 3 were repeated by the salt dry / wet combined cycle test (CCT) according to J2334, the state of rust generation was visually observed, and the corrosion resistance (corrosion) was evaluated in the following five stages. evaluated. The results are shown in Table 2.
1: Red rust (point rust exceeding 2mm in diameter) ... failed 2: Minor red rust (point rust of 2mm or less in diameter) ... failed 3: Spot rust (point rust exceeding 2mm in diameter) ...・ ・ Pass 4: Minor spotted rust (point rust of 2mm or less in diameter) ・ ・ Pass 5: No rust ・ ・ ・ ・ ・ ・ ・ Pass

For the investigation of the formability, a tensile test using a JIS No. 13B test piece was performed in accordance with JIS Z2254, and evaluated by the r value as an index of deep drawability and the elongation El as an index of overhang property.
Furthermore, cylindrical deep drawing was performed under the conditions of a punch diameter of 33 mmφ and a blank diameter of 70 mm, and the presence or absence of cracks was observed.
Further, a corrosion test was conducted in which the deep drawn product was immersed in a deteriorated gasoline containing 1200 ppm of formic acid and 450 ppm of acetic acid for 20 days, and from the surface appearance and the mass change after the test, the mass change was 0.05 g / m ² or less. , The case where there was no redness in the appearance was evaluated as “good”, and the others were evaluated as “fail”.
When there were three or more failures in the corrosion resistance test, or when the gasoline corrosion test failed, the overall evaluation was regarded as failure.

The obtained test results are also shown in Table 2. As a result, it can be seen that all of the invention examples are not only excellent in outer surface corrosion resistance especially in a salt damage environment, but also excellent in workability due to reduction of alloying elements, and are also sufficient in corrosion resistance in deteriorated gasoline.

The graph which shows the relationship between the pitting corrosion index (Cr + 3.3Mo) of a steel plate and the Zn content in a coating film which affect the pass / fail judgment of corrosion resistance. Explanatory drawing (b) and a side view (a) showing the pretreatment state performed for the evaluation of corrosion resistance etc. on the surface of a test piece. The figure which shows the flow and conditions of the salt dry-wet combined cycle test of a test piece.

Explanation of reference numerals

1: L-shaped test piece 2: gap 3: plastic clip 4: dome-shaped overhang by Erichsen 5: cross cut 6: gravero test 7: seam weld

Claims (7)

  C: 0.1% or less, Si: 1.0% or less, Mn: 1.5% or less, P: 0.06% or less, S: 0.03% or less, Al: 1.0% or less by mass% , Cr: 11 to 20% and N: 0.04% or less, and Nb: 0.002 to 0.8% and / or Ti: 0.01 to 1.0%, the balance being Fe A ferrite stainless steel for a Zn-containing paint-coated automobile fuel tank and a fuel tank peripheral member, wherein a Zn-containing paint is applied to steel comprising unavoidable impurities.   2. The stainless steel according to claim 1, further comprising at least one selected from the group consisting of Mo: 3.0% or less, Cu: 2.0% or less, and Ni: 2.0% or less by mass%. 2. A ferritic stainless steel for a Zn-containing paint-coated automotive fuel tank and fuel tank peripheral members according to 1. The ferrite for a Zn-containing paint-coated automobile fuel tank and a fuel tank peripheral member according to claim 1 or 2, wherein the stainless steel further contains B: 0.0003 to 0.005% by mass%. Series stainless steel. 4. The ferrite for a Zn-containing paint-coated automotive fuel tank and a fuel tank peripheral member according to claim 1, wherein the stainless steel further contains 0.3% or less of Co by mass%. Series stainless steel. 5. The ferrite for a Zn-containing paint-coated automotive fuel tank and a fuel tank peripheral member according to claim 1, wherein the stainless steel further contains Mg: 0.0032% or less by mass%. Series stainless steel. The Zn content (X) in the coating film of the Zn-containing paint is an amount defined by the following formula (1), and the thickness of the coating film is 5 to 50 μm. 7. A ferrite stainless steel for a Zn-containing paint-coated automotive fuel tank and a fuel tank peripheral member according to any one of claims 5 to 5.
70 ≧ X ≧ 70− {2.7 × (Cr + 3.3Mo)} (1)
Where X is the Zn content (% by mass) in the coating film,
Cr is the Cr content (% by mass) in stainless steel,
Mo is the Mo content (% by mass) in stainless steel. The ferrite stainless steel for a Zn-containing paint-coated automobile fuel tank and a fuel tank peripheral member according to any one of claims 1 to 6, wherein the Zn-containing paint has an average particle diameter of Zn of 3 µm or less.

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