TWI696710B - Stainless steel pipe with excellent corrosion resistance - Google Patents

Abstract

The problem to be solved by the present invention is to provide a stainless steel pipe excellent in corrosion resistance and a method for manufacturing the same. Even if the stainless steel pipe is in a sea environment affected by sea salt particles, it will not rust early. In order to solve this problem, the present invention provides a stainless steel tube with excellent corrosion resistance. The surface of the ferrite stainless steel tube has abrasive marks in the longitudinal direction, the pitting potential is 0.6V or more, and the 60-degree gloss is 75 or less. The composition includes 0.020 mass% or less carbon, 0.40 mass% or less silicon, 0.40 mass% or less manganese, 25.00-32.00 mass% chromium, 1.00-4.00 mass% molybdenum, 0.030 mass% or less phosphorus, 0.020 mass% The following sulfur, 0.50% by mass or less nickel, 0.020% by mass or less nitrogen, the rest is composed of iron and inevitable impurities, and the pitting corrosion resistance index (PI = Cr mass% + 3Mo mass%) is 30 or more .

Description

Stainless steel pipe with excellent corrosion resistance

The present invention relates to a stainless steel pipe excellent in corrosion resistance.

Stainless steel is excellent in weather resistance, workability, weldability, etc., so it is widely used in building materials such as roofing materials, wall materials, and building components. In addition, since the stainless steel pipe is also excellent in design, its surface is polished and used for handrails, fences, lattice doors, etc.

In general industrial polishing of this stainless steel, first, in order to remove traces on the material pipe (original pipe) before polishing, etc., the traces are removed for polishing, followed by final polishing and bright polishing (Bright Polishing) Wait. In this grinding operation, rough grinding and fine grinding use dry grinding using a flap wheel or a grinding belt. Further, after the above steps, wet polishing is performed by buff polishing in order to obtain a desired surface.

Previously, it has been known that stainless steel as a material has excellent weather resistance, but depending on the state of its grinding process, it may not be able to exhibit the original weather resistance of the material and may rust significantly, which may become the stability of losing the weather resistance of the stainless steel. (Reliability) One of the main reasons. For example, after the construction of the handrails outside the house, it may rust in a short period of about a month.

Regarding rust, it is considered that the starting point of rust is the oxide film or polishing marks remaining on the polished surface of the stainless steel pipe. The residual oxide film refers to a film generated due to heat generated during polishing, and a Cr (chromium) depleted layer is formed directly under the oxide film. Therefore, if an oxide film remains, rust will start from the oxide film and the Cr depletion layer directly below it, and corrosion resistance will easily deteriorate. In addition, as for the marks carved on the surface of the stainless steel tube due to polishing, that is, polishing marks, the deeper the concave portions of the polishing marks, the larger the concave portion of the polishing marks, the polishing film is used to remove the oxide film caused by the grinding of the impeller. The harder it becomes, the higher the possibility of remaining, and the recessed part of this polishing mark will become the starting point of rust and begin to rust, so that the corrosion resistance is easily deteriorated.

Patent Document 1 proposes a stainless steel tube whose surface is polished so that it will not rust in a short period of time even in an outdoor environment, and can maintain gloss and weather resistance for a long time.

The invention described in Patent Document 1 is a stainless steel tube whose surface roughness Ry after final polishing is 0.6 μm or less, and the area ratio of the remaining oxide film is 7.0% or less. That is, the surface roughness Ry after the final polishing is 0.6 μm or less, thereby reducing the oxide film remaining in the concave portion of the polishing mark. In addition, the area ratio of the remaining oxide film is 7.0% or less, thereby suppressing the occurrence of rust from the oxide film and the Cr depletion layer immediately below it as a starting point, and suppressing the deterioration of corrosion resistance.

However, referring to the example of Patent Document 1, the area ratio of the remaining oxide film of the weather resistance qualified product is 3.1 to 6.8%, and the oxide film remains. Therefore, the problem of deterioration of corrosion resistance caused by rusting starting with the remaining oxide film and the Cr depleted layer directly below it as a starting point still exists.

Patent Document 2 describes that a stainless steel tube is immersed in a mixed solution of hydrofluoric acid and nitric acid to dissolve in an oxide scale on the surface of the stainless steel or a Cr depleted layer directly below the skin layer.

[Prior Art Literature] (Patent Literature) Patent Literature 1: Japanese Patent Application Publication No. 2003-56755 Patent Literature 2: Japanese Patent Application Publication No. 2000-17469

[Problems to be Solved by the Invention] With the recent increase in construction needs such as urban redevelopment, the need for construction in a waterfront environment has also increased. In a seaside environment, there are problems in which building components are susceptible to the influence of aerosol particles contained in the atmosphere, that is, the influence of fine particles composed of salt from seawater, that is, sea salt particles. Therefore, the demand for highly corrosion-resistant building components has been further increased. In Patent Document 1, SUS304 is exemplified as a steel type of stainless steel pipes excellent in weather resistance. However, in the coastal environment that will be affected by the sea salt particles, SUS304 will still rust at an early stage and will need to be repaired.

In addition to high corrosion resistance, there is also a demand for building components with excellent anti-glare properties. As a means for imparting anti-glare properties, although there are polishing means for polishing the surface of the stainless steel tube in the longitudinal direction, if wet polishing is performed to impart polishing marks in the longitudinal direction, the stainless steel tube may be too slippery to be stably transported and unable The problem of even grinding. Therefore, dry grinding has to be performed, and there is a problem that an oxide film or the like easily occurs on the surface of the stainless steel pipe after dry grinding. Patent Document 2 describes that the stainless steel pipe is pickled to remove the oxide film on the surface of the steel pipe. However, the process of pickling the polished stainless steel pipe will increase the manufacturing cost. Therefore, a stainless steel pipe is sought, which can suppress rust at an early stage even without pickling.

An object of the present invention is to solve the above-mentioned problems and provide a stainless steel pipe excellent in corrosion resistance, which does not rust early even in a sea environment that is affected by sea salt particles.

[Technical Means for Solving the Problems] The inventors reviewed the stainless steel pipe described in Patent Document 1. In the example of Patent Document 1, dry grinding is performed by a blade wheel. Therefore, it has been found that the following reasons can cause surface defects: when dry grinding, that is, impeller grinding, the surface of the stainless steel tube becomes high temperature and an oxide film is generated; and, due to the high grinding force of dry grinding The traces are also grinding marks. The surface defect here refers to the "burr" or "overlaps" where the abrasive material or abrasive paper continuously contacts the surface of the steel pipe when grinding the surface of the steel pipe, so that part of the metal on the surface is peeled off and covers the substrate part )". Surface defects, including strip-shaped or bamboo leaf-shaped metal curled parts, and the maximum length from one end of the part connected to the base material to the other end of the peeled front end is Defects above 5μm. Since this surface defect forms a tiny gap with the surface base material portion of the stainless steel pipe, it is likely to cause corrosion of the void and become the main cause of the decrease in corrosion resistance of the steel pipe.

As mentioned above, if there is a colored oxide film or surface defect, the stainless steel pipe will be easily rusted, and it is common to have general knowledge in the technical field to which the invention belongs in order to remove the oxide film or surface defect by pickling treatment The resolution method carried out by the author. However, the present inventors explored a stainless steel tube excellent in corrosion resistance, which was originally a stainless steel tube that had a colored oxide film or a surface defect and was easily corroded. Even if the stainless steel tube was not pickled after grinding, it was subject to The sea environment affected by the sea salt particles will not rust at an early stage, and the present invention is further completed.

That is, the present invention provides the following stainless steel pipes excellent in corrosion resistance (1) to (2). (1) A stainless steel tube with excellent corrosion resistance. The surface of the ferrite stainless steel tube has grinding marks in the longitudinal direction, the pitting potential is 0.6V or more, and the 60-degree gloss is 75 or less. Its composition Contains 0.020 mass% or less of carbon (C), 0.40 mass% or less of silicon (Si), 0.40 mass% or less of manganese (Mn), 25.00 to 32.00 mass% of chromium (Cr), and 1.00 to 4.00 mass% of molybdenum (Mo), 0.030% by mass or less of phosphorus (P), 0.020% by mass or less of sulfur (S), 0.50% by mass or less of nickel (Ni), 0.020% by mass or less of nitrogen (N), the rest is made of iron ( Fe) and unavoidable impurities, the pitting corrosion resistance index (PI = Cr mass% + 3Mo mass%) is more than 30.

The stainless steel pipe of the present invention is excellent in design because it has abrasive marks in the longitudinal direction on the surface of the ferrite iron-based stainless steel pipe, and the 60-degree gloss is 75 or less, so it is excellent in anti-glare properties. In addition, it is a stainless steel tube with excellent corrosion resistance. Even if there is a colored oxide film on the surface of the stainless steel tube and there are surface defects, it has a predetermined composition and the pitting corrosion resistance index (PI) is as high as 30 or more, so it is suppressed When the oxide film and the Cr depleted layer directly below it are used as a starting point for rusting, the pitting potential is 0.6 V or more.

(2) The stainless steel tube according to (1), further including 0.1 to 1.0% by mass of niobium (Nb), 0.05 to 0.3% by mass of titanium (Ti), and 0.01 to 0.5% by mass of aluminum (Al) One or more than two of them. [Efficacy of invention]

According to the present invention, it is possible to provide a stainless steel pipe excellent in corrosion resistance, which does not rust early even in a seaside environment affected by sea salt particles.

Hereinafter, an embodiment for implementing the present invention will be described. In addition, this invention is not limitedly interpreted by this embodiment.

(Stainless steel pipe) The stainless steel pipe of the present invention is a stainless steel pipe with excellent corrosion resistance. It has excellent corrosion resistance. It has abrasive marks in the longitudinal direction on the surface of the ferrite stainless steel pipe, and the pitting potential is 0.6 V or more, 60 degrees gloss is 75 or less, and its composition contains 0.020 mass% or less of carbon (C), 0.40 mass% or less of silicon (Si), 0.40 mass% or less of manganese (Mn), 25.00 to 32.00 mass% Chromium (Cr), 1.00 to 4.00 mass% molybdenum (Mo), 0.030 mass% or less phosphorus (P), 0.020 mass% or less sulfur (S), 0.50 mass% or less nickel (Ni), 0.020 mass% The following nitrogen (N), the remaining part is composed of iron (Fe) and inevitable impurities, the pitting corrosion resistance index (PI = Cr mass% + 3Mo mass%) is 30 or more.

In the present invention, the surface of the stainless steel pipe is finely polished in order to have irregularities and gloss. By this, the stainless steel tube is provided with grinding marks, and a stainless steel tube excellent in designability and anti-glare property is made. Grinding marks refer to the marks carved on the surface of the stainless steel tube due to grinding. In the present invention, the polishing marks include longitudinal polishing marks. The stainless steel tube with longitudinal grinding marks has excellent anti-glare properties. As the fine polishing in the longitudinal direction, it is difficult to perform wet polishing, so it has been conventionally carried out by dry grinding performed by the impeller, etc. However, if the dry grinding is performed, the surface of the stainless steel tube will become high temperature, forming a colored Oxide film. Moreover, the deeper the polishing marks on the polished surface, the deeper the concave portions of the polishing marks are, the higher the possibility that the oxide film generated by the impeller grinding or the like will remain, and the concave portions of the polishing marks become the starting point of rust and rust , Corrosion resistance will easily deteriorate. In the present invention, the existence of a colored oxide film means that when an optical microscope is used to observe any 10 places on the surface of a stainless steel tube at a magnification of 400 times, the colored stain-like substance in a square with a side length of 50 μm is The oxide film may be more than 10% in terms of area ratio. Here, the color of the colored oxide film is not particularly limited as long as the color can be visually distinguished from the metal base material or metallic luster of the stainless steel tube. The representative color of this oxide film is dark brown.

In addition, as fine grinding, if dry grinding is performed by a blade wheel or the like, the grinding material or the grinding paper continuously contacts the surface of the stainless steel tube to produce surface defects. The surface defects are partially peeled off of the surface metal to cover the substrate Burrs or overlaps on the timber parts. This surface defect forms a tiny void with the surface base material part of the stainless steel pipe, and becomes the main cause of void corrosion. Fig. 1 is an enlarged photograph of the surface of the stainless steel tube using an optical microscope. Among them, Fig. 1(a) is a surface with suppressed surface defects, and Fig. 1(b) is a surface with surface defects. Fig. 1(a) is the surface of the stainless steel pipe of the present invention. Although there are grinding marks, surface defects are suppressed. On the other hand, Figure 1(b) is the surface of the stainless steel tube after dry grinding. The circled parts 1 to 9 indicate surface defects. This surface defect is that part of the surface metal is peeled off and covered. Surface defects on the substrate part. In the present invention, the surface defect is a defect in which the maximum length from one end of the portion connected to the base material to the other end of the peeling tip is 5 μm or more. In addition, when the optical microscope is used to magnify at 200 times and observe the range of 100 μm×100 μm (0.01 mm ² ) at any 10 locations on the surface of the polished stainless steel tube, the average number of measured surface defects is 6 or more Is considered to be a state where the surface defects of the present invention are not suppressed. In addition, there is no upper limit to the maximum length of surface defects, but as a reference during measurement, 50 μm may be used as an upper limit.

In view of the technical common sense of those with ordinary knowledge in the technical field to which the present invention belongs, in order to suppress the progress of rust or the deterioration of corrosion resistance, it has always been considered that it is preferable to polish the surface of the stainless steel pipe without the above-mentioned colored oxidation The film does not have burrs or overlaps as shown in Fig. 1 (a), that is, surface defects, and is subjected to pickling treatment to remove the oxide film and the like. However, the stainless steel pipe of the present invention is characterized in that the steel pipe may have a colored oxide film, or it can suppress the progress of rust and corrosion resistance without pickling treatment without suppressing surface defects. deterioration. In addition, as a means for suppressing surface defects and oxide films, although steel sheets processed with fine lines (Hairline Finish) are considered as steel pipes, the strength of the produced steel pipes may not be sufficient.

The stainless steel pipe of the present invention has a composition of 0.020 mass% or less carbon, 0.40 mass% or less silicon, 0.40 mass% or less manganese, 25.00 to 32.00 mass% chromium, 1.00 to 4.00 mass% molybdenum, and 0.030 mass% or less Phosphorus, sulfur below 0.020% by mass, nickel below 0.50% by mass, nitrogen below 0.020% by mass, the remainder is composed of iron and inevitable impurities, and the pitting corrosion resistance index (PI = Cr% by mass + 3Mo mass %) is more than 30. With this composition and the pitting corrosion resistance index (PI) of 30 or more, the ferrite grain stainless steel pipe of the present invention has a pitting potential as high as 0.6 V or more and is excellent in corrosion resistance. The SUS304 with a pitting corrosion resistance index as low as 19, which will rust early, can suppress rust. In addition, even if there is a colored oxide film or surface defect due to polishing, rust can be suppressed.

The stainless steel pipe of the present invention preferably further contains one or more of 0.1 to 1.0% by mass of niobium, 0.05 to 0.3% by mass of titanium, and 0.01 to 0.5% by mass of aluminum. By containing a predetermined amount of niobium (Nb), titanium (Ti), and/or aluminum (Al), there is a tendency to further improve corrosion resistance.

The reason for limiting the composition of the stainless steel pipe will be described below. C (carbon) is a useful element to obtain the strength of steel, but too much content tends to reduce the corrosion resistance. Therefore, the content of carbon is preferably 0.015% by mass or less, and more preferably 0.010% by mass or less.

Si (silicon) is a useful element as a deoxidizer and heat source in the steel making process, but too much content will tend to harden the steel. Therefore, the content of silicon is preferably 0.35 mass% or less, and more preferably 0.30 mass% or less.

Mn (manganese) is a useful element as a deoxidizer in the steel-making step, but too much content tends to form an austenite phase. Therefore, the content of manganese is preferably 0.35 mass% or less, and more preferably 0.30 mass% or less.

Cr (chromium) is a useful element for ensuring corrosion resistance, but too much content is not only costly but also tends to reduce processability. Therefore, the content of chromium is preferably 25.00 to 31.50 mass%, more preferably 25.00 to 31.00 mass%.

Mo (molybdenum) is a useful element for improving the corrosion resistance of stainless steel when chromium is present, but too much content is not only costly but also tends to reduce workability. Therefore, the content of molybdenum is preferably 1.50 to 4.00 mass%, and more preferably 1.80 to 3.80 mass%.

P (phosphorus) tends to reduce corrosion resistance. Therefore, the content of phosphorus is preferably 0.025% by mass or less, and more preferably 0.020% by mass or less.

S (sulfur) tends to reduce corrosion resistance. Therefore, the sulfur content is preferably 0.015% by mass or less, and more preferably 0.010% by mass or less.

Ni (nickel) has the effect of suppressing the progress of corrosion and can improve the toughness of the ferrite iron-based stainless steel pipe, but too much content will cause the generation of Vostian iron phase and high cost. Therefore, the content of nickel is preferably 0.45% by mass or less, and more preferably 0.40% by mass or less.

N (nitrogen), like carbon, has too much content and tends to reduce corrosion resistance. Therefore, the nitrogen content is preferably 0.015% by mass or less, and more preferably 0.010% by mass or less.

Nb (niobium) has a strong affinity for carbon and nitrogen and can suppress the grain boundary corrosion of ferrite stainless steel pipes, but too much niobium tends to hinder toughness. Therefore, the content of niobium is preferably 0.1 to 0.9% by mass, and more preferably 0.1 to 0.8% by mass.

Ti (titanium) has a strong affinity for carbon and nitrogen and can suppress grain boundary corrosion (intergranular corrosion) of fat-grained iron-based stainless steel tubes, but too much titanium content tends to reduce the surface quality of steel. Therefore, the content of titanium is preferably 0.05 to 0.25% by mass, and more preferably 0.05 to 0.2% by mass.

Al (aluminum) is an effective element as a deoxidizer in refining and casting, but if added excessively, the surface quality is deteriorated, and the weldability and low-temperature toughness of steel are reduced. Therefore, the content of aluminum is preferably 0.01 to 0.45% by mass, and more preferably 0.01 to 0.4% by mass.

FIGS. 2 and 3 are diagrams showing surface defects and pitting potential, FIG. 2(a) is an enlarged photograph showing surface defects of a stainless steel tube, and FIG. 2(b) is a measurement result showing pitting potential The graph, Figure 3(a) is an enlarged photograph showing the surface of the stainless steel tube where the surface defects are suppressed, and Figures 2(b) and 3(b) are the figures 2(a) and 3(a). The graph of the measurement result of the pitting potential of the stainless steel pipe of the figure. The stainless steel pipes in Figures 2 and 3 have a pitting corrosion resistance index (PI) of about 24, which is lower than the stainless steel pipe of the present invention. In Fig. 2(a), there is a surface defect, and the pitting potential is as low as about 0.3V as shown in Fig. 2(b). In addition, in Fig. 3(a), although surface defects are suppressed, the pitting potential is as low as about 0.5V as shown in Fig. 3(b). On the other hand, the stainless steel pipe of the present invention has a pitting potential as high as 0.6 V or more and is excellent in corrosion resistance. Therefore, even if there is a colored oxide film or a surface defect, the progress of rust and the deterioration of corrosion resistance can be suppressed. The pitting potential is preferably 0.65V or more, and more preferably 0.7V or more.

The method for measuring the pitting potential of stainless steel is based on JIS G 0577 and uses the "Method B". This "method B" is a pitting potential measurement method based on the potentiodynamic method in a 3.5% by mass sodium chloride aqueous solution. The Ph value of this sodium chloride aqueous solution was set to 7, and the temperature was set to 30°C. In addition, the potential scanning speed was set to 20 mV/minute.

The surface roughness Ra of the surface of the stainless steel tube in the present invention is preferably 0.1 to 1.0 μm, and more preferably 0.2 to 0.5 μm. If the surface roughness Ra is less than 0.1 μm, the anti-glare property is poor, it is difficult to maintain the polishing marks, and it tends to be difficult to ensure the designability.

In the present invention, the gloss of the surface of the stainless steel tube is 60 degrees gloss, preferably 75 or less, and more preferably 60 or less. The gloss is measured based on JIS Z8741 as a reference, and can be measured with a gloss meter, for example. Specifically, when measuring glossiness, a light beam with a predetermined aperture angle is incident on the sample surface at a predetermined angle of incidence, and a light beam with a predetermined aperture angle reflected in the reflection direction is measured with a light receiver. 60 degree gloss refers to the gloss when the specified angle of incidence is 60 degrees. The gloss of 60 degrees is 75 or less, thereby making the surface of the stainless steel tube good anti-glare. [Example]

Pipe-making of stainless steel pipes, shape correction, and fine grinding for decoration. Use the following 2 types of stainless steel pipes. The composition (mass %) and dimensions are shown below.

The composition of steel type 1 (SUS447J1) is: Cr (30%), Mo (2%), Ti (0.15%), Nb (0.15%), Al (0.09%), and Fe (remaining part). The composition of steel type 2 (SUS445J1) is: Cr (22%), Mo (1.05%), Ti (0.2%), Nb (0.2%), Al (0.09%), and Fe (remaining part). The composition of steel type 3 (SUS304) is: Cr (18%), Ni (8%), Si (0.6%), Mn (0.8%), and Fe (remaining part). The dimensions are: diameter 34mm × thickness 1.5mm × length 4000mm.

Grinding is carried out by using a working line formed by arranging four chival wheels (#80, #80, #80, #150) side by side in the longitudinal direction of the surface of the steel pipe (given circumferential grinding marks), and The grinding carried out is dry grinding. In addition, "#80" and the like indicate the sieving particle size.

(Grinding conditions) 　　Operational line speed: 1.8m/min 　　Revolution of steel pipe: 380rpm 　　Turbine of revolution: 1500rpm 　　Diameter of rotor: 400mm

After grinding, only a part of the stainless steel tube was pickled as shown in Table 1 (Comparative Example 3, Reference Example 1). As for Examples 1 to 3 and Comparative Examples 1 and 2, no pickling treatment was performed.

(Surface defects) Using an optical microscope, the surface of the stainless steel tubes of Examples 1 to 3 and Comparative Examples 1 to 3 and Reference Example 1 was magnified by 200 times to observe the range of 100 μm×100 μm, and the number of surface defects was measured (see table 1).

(Oxide film) The surfaces of the stainless steel tubes of Examples 1 to 3, Comparative Examples 1 to 3, and Reference Example 1 were observed with an optical microscope at a magnification of 400 times, and it was calculated that in a square with a side length of 50 μm, a brown stained substance It is the extent to which the oxide film exists in terms of area ratio. When the area ratio of the residual oxide film is less than 10%, it is evaluated as "no" as if there is no colored oxide film, and when the area ratio of the residual oxide film is 10% or more, as if there is a colored oxide film The evaluation is "Yes" (refer to Table 1).

(Pit Corrosion Potential) The pitting potential of the stainless steel pipes of Examples 1 to 3, Comparative Examples 1 to 3, and Reference Example 1 were measured. Specifically, the measurement is based on JIS G 0577 and is measured using "Method B (Test Method for 3.5% Mass Aqueous Sodium Chloride Aqueous Solution)". The "Method B" uses chlorination at 3.5% by mass Potential method in sodium aqueous solution. The pH value of this sodium chloride was set to 7, and the temperature was set to 30°C. In addition, the potential scanning speed was set to 20 mV/minute (see Table 1).

(Remaining polishing marks) In order to evaluate the remaining polishing marks, the surface roughness Ra of the stainless steel pipes of Examples 1 to 3, Comparative Examples 1 to 3, and Reference Example 1 was measured. When Ra≧0.1 μm, there were residual polishing marks The design is excellent, so the evaluation is "○". On the other hand, when Ra<0.1 μm, since there are few remaining polishing marks and the design is not excellent, it is evaluated as “×”. The surface roughness Ra was measured based on JIS B 0601 as a reference, and a contact-type surface roughness meter was used (refer to Table 1).

(Glossiness) The 60-degree glossiness of the surface of the stainless steel tube of Examples 1-3, Comparative Examples 1-3, and Reference Example 1 was measured using the gloss meter based on JIS Z 8741 as a reference. When the gloss at 60 degrees is 75 or less, it is evaluated as "○", and when it is greater than 75, it is evaluated as "×" (refer to Table 1).

(Corrosion resistance test) The stainless steel pipes of Examples 1 to 3, Comparative Examples 1 to 3, and Reference Example 1 were subjected to a corrosion resistance test (salt dry and wet combined cycle test (CCT test)) under the following conditions. Conditions: (1) Salt water spray (35°C, 5% sodium chloride (NaCl), 15 minutes) 　 (2) Drying (60°C, 30%RH (relative humidity), 60 minutes) 　 (3) Humidity (50°C, 95%RH, 3 hours) The above conditions (1) to (3) are regarded as 1 cycle, and 30 cycles are repeated. Evaluation: When the rust area after the test is within 5% of the entire surface of the steel pipe, it is evaluated as "○" as good corrosion resistance; when it is greater than 5%, it is evaluated as poor corrosion resistance It is "×" (refer to Table 1).

[Table 1]

As shown in Table 1, the stainless steel tube of Example 1 has abrasive marks on the surface of the stainless steel tube, as many as 15 surface defects (see Figure 4), and a colored oxide film also exists on the surface, but even if the potential is There is still no pitting corrosion at 1.0V (refer to Figure 5). In addition, even if the stainless steel pipe of Example 1 was subjected to the CCT test, rust was suppressed (see FIG. 6), and it was confirmed that it was excellent in corrosion resistance. In addition, as for the stainless steel tubes of Examples 2 and 3, as in Example 1, pitting corrosion did not occur even at a potential of 1.0 V, and rusting was suppressed even when subjected to the CCT test.

1～9‧‧‧Surface defect 20‧‧‧Stainless steel tube of Example 1

Fig. 1 is a photograph of an enlarged surface of a stainless steel tube using an optical microscope. Fig. 1 (a) is a surface with suppressed surface defects, and Fig. 1 (b) is a surface with surface defects. FIG. 2(a) is an enlarged photograph showing surface defects of the stainless steel tube, and FIG. 2(b) is a graph showing the measurement results of pitting potential. FIG. 3(a) is an enlarged photograph showing the surface on which the surface defects of the stainless steel tube are suppressed, and FIG. 3(b) is a graph showing the measurement result of the pitting potential. FIG. 4 is a photograph of an enlarged surface of the stainless steel tube of the present invention using an optical microscope. FIG. 5 is a graph showing the measurement results of the pitting potential of Example 1. FIG. FIG. 6 is a photograph showing the appearance after the salt dry-wet composite cycle test of Example 1. FIG.

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Claims (2)

A stainless steel tube with excellent corrosion resistance. The surface of the ferrite stainless steel tube has abrasive marks in the longitudinal direction, the pitting potential is 0.6V or more, and the 60-degree gloss is 75 or less. The aforementioned surface contains surface defects. The surface defect has a maximum length from one end of the part connected to the base material to the other end of the front end of the peeling of 5 μm or more. When magnified at 200 times and observe any of the above-mentioned surface 10 In the range of 100 μm ×100 μm , the average number of the aforementioned surface defects is 6 or more, and the composition of the stainless steel tube includes 0.020 mass% or less carbon, 0.40 mass% or less silicon, 0.40 mass% The following manganese, 25.00 to 32.00% by mass of chromium, 1.00 to 4.00% by mass of molybdenum, 0.030% by mass or less of phosphorus, 0.020% by mass or less of sulfur, 0.50% by mass or less of nickel, 0.020% by mass or less of nitrogen, the remainder It is composed of iron and inevitable impurities, and the pitting corrosion resistance index PI is more than 30, where PI = Cr mass% + 3Mo mass%. The stainless steel tube according to claim 1, further comprising one or more of 0.1 to 1.0% by mass of niobium, 0.05 to 0.3% by mass of titanium, and 0.01 to 0.5% by mass of aluminum.

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