JP2011190523A - Ferrite single phase stainless steel sheet having excellent corrosion resistance in weld heat affected zone - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a stainless steel which has excellent corrosion resistance in the weld heat affected zone. <P>SOLUTION: The stainless steel sheet having improved corrosion resistance in the weld heat affected zone is characterized in that the thickness of an oxide film is ≥15 nm, the concentration of Al in the oxide film is ≥50 atomic% in the atomic ratio of Al, Si, Mn, Cr and Fe, and further the defect rate of the oxide film is ≤3.0%. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention improves the corrosion resistance of the oxide film formed at a site heated again in the atmosphere, such as a weld heat affected zone, by using a previously applied oxide film.

  Stainless steel is used for various applications due to its excellent corrosion resistance, but it is generally known that the corrosion resistance of the weld heat affected zone deteriorates for products manufactured by welding. This is considered to be because the passive film responsible for the corrosion resistance of stainless steel is destroyed by oxidation of the weld heat affected zone. As countermeasures, a method of reducing the oxidation of the welding heat-affected zone using a sealing gas such as Ar gas during welding, or removing the oxide film formed on the welding heat-affected zone after welding by polishing, pickling, etc. Corrosion resistance is improved by restoring the film.

  A method for suppressing oxidation at the time of reheating with an oxide film previously formed on a steel sheet has been proposed. For example, in Patent Document 1, Si is concentrated in the surface layer and Al is concentrated in the lower layer. Structured oxide films have been proposed. In addition, Patent Document 2 proposes an oxide film having a composition of (Al + Si) / (Mn + Cr + Fe)> 0.1 and Patent Document 3 has a composition of (Al + Si + Cr) /Fe≧0.6. According to Patent Document 4, the weather resistance is improved by setting Al ≧ 35%, Ti ≧ 8%, and Si ≦ 8% in the atomic ratio. Patent Documents 5 to 7 propose a film in which Al is concentrated.

JP2003-213378 JP2002-275592 JP-A-8-295999 JP-A-7-180001 JP-A-6-041695 JP-A-4-002075 JP-A-5-271880

  Patent Documents 1 to 3 have an oxide film containing Al and Si and further Cr, and Patent Document 4 is characterized by having an oxide film containing Al and Ti and reduced Si. It is intended for thickening and also refers to film thickness and defects. The reason is that Patent Documents 1 and 3 are intended to improve the oxidation resistance in a heating environment of about 250 to 350 ° C. and do not mention the corrosion resistance. The purpose is to modify the surface in a higher temperature environment exceeding 500 ° C. and improve the corrosion resistance thereafter. Patent Document 2 aims to suppress excessive oxidation during bright annealing, that is, in a mixed gas of hydrogen and nitrogen, and the present invention aims at surface modification assuming heating in the atmosphere such as welding. There is. Patent Document 4 evaluates the corrosion resistance in an environment where the generated coating is not reheated, whereas the present invention is intended to improve the corrosion resistance after being placed in the heating environment again.

  Patent Documents 5 and 7 are the same as the present invention in that Al is concentrated in the oxide film. However, unlike the present invention, the film thickness is different from that of the present invention in that Ti is used for the concentration of Al in the film. Also not mentioned. The reason for this is considered that Patent Document 5 evaluates the corrosion resistance in an environment where the use environment is up to 500 ° C. and Patent Document 7 does not reheat.

  Patent Document 6 is the same as the present invention in that Al is concentrated in the film, but it is said that the Al content of the material needs to be 0.5 mass% or more in order to concentrate Al in the film. Further, the obtained film thickness is supposed to be about 2 nm, which is thinner than the oxide film of the present invention. The reason for this is that Patent Document 7 measures the acid increase when re-heating the film-modified steel material at a high temperature of 1200 ° C., and evaluates only the oxidation resistance. It is assumed that the film composition, film thickness, and film defects after reheating are controlled for the purpose of improving the corrosion resistance. As a result, the present invention generates and modifies a film thicker than Patent Document 6 based on the new finding that reducing not only the film composition but also the film thickness and film defects are essential conditions for achieving the object. The point is different. Furthermore, Patent Document 6 does not mention the composition of the atmospheric gas in the bright annealing for carrying out the film modification, and the present invention differs from the manufacturing method in that the gas composition and the dew point are more strictly controlled. The thickness of the film and the film component, etc. are considered to be different in parts not disclosed in Patent Document 6.

  Summarizing the differences between these known findings and the present invention, the present invention newly proposes that it is necessary not only to control the composition of the oxide film, but also to increase the thickness of the oxide film and further reduce defects in the oxide film. There is to do. Therefore, the heat treatment conditions for obtaining the target oxide film were also strictly controlled.

In order to achieve the object, the stainless steel of the present invention is a ferritic stainless steel having an oxide film formed by a heat treatment in a mixed gas atmosphere of nitrogen and hydrogen, and the oxide film has a thickness of 15 nm or more. The concentration of Al in the film is 50 atomic% or more in the atomic ratio of Al, Si, Mn, Cr, and Fe, and the defect rate of the film defined by the formula (1) is 3.0% or less. And
Stainless steel is characterized by containing 0.05% by mass or more of Al.
Stainless steel having such surface characteristics has a temperature of 1000 to 1050 ° C., a soaking time of 5 min or more, or a soaking time of less than 5 min in a mixed gas atmosphere of hydrogen and nitrogen having a hydrogen ratio of 60 to 90% by volume. In this case, the temperature rise rate from 600 ° C. to the ultimate temperature satisfies the formula (2), and the finish bright annealing is performed under the condition that the dew point of the atmospheric gas is −60 to −50 ° C.

The invention described in claim 1
A ferritic stainless steel having an oxide film formed by heat treatment in a mixed gas atmosphere of nitrogen and hydrogen, the oxide film having a thickness of 15 nm or more, and the concentration of Al in the oxide film is Al, Si, Mn, Cr, Stainless steel with improved corrosion resistance in the heat affected zone of the weld zone, wherein the atomic ratio of Fe is 50 atomic% or more and the defect rate of the oxide film defined by the formula (1) is 3.0% or less. It is a steel plate.
Sulfuric acid: 1 mol / L, potassium thiocyanate: 0.01 mol / L, anodic polarization in an aqueous solution at a liquid temperature of 30 ° C. under the condition of 10 mV / min, and a value calculated from the anodic current density at 250 mV by the formula (1) Is the film defect rate.
Defect rate of film = A / B × 100 (%) (1)
A: Anode current density of evaluation material, B: Anode current density of pickling finish

The invention described in claim 2
C: 0.02 mass% or less, Si: 1.0 mass% or less, Mn: 0.5 mass% or less, P: 0.05 mass% or less, S: 0.005 mass% or less, Ni: 0.5 % By mass, Cr: 16-24% by mass, Al: 0.05-1.0% by mass, Nb: 0.1-0.5% by mass or less, Ti: 0.1% by mass or less, N: 0.00%. The ferritic single-phase stainless steel sheet according to claim 1, comprising 02 mass% or less, Mo: 2.0 mass% or less, and the balance being Fe and inevitable impurities.

The invention according to claim 3
A heat treatment is performed in a mixed gas atmosphere of hydrogen and nitrogen having a hydrogen ratio of 60 to 90% by volume under conditions of a temperature of 1000 to 1050 ° C. and a soaking time of 5 min or more under a condition of −60 to −50 ° C. dew point of the atmosphere gas. It is a manufacturing method of the ferrite single phase type stainless steel plate of Claim 1 or Claim 2.

The invention according to claim 4
In a mixed gas atmosphere of hydrogen and nitrogen having a hydrogen ratio of 60 to 90% by volume, the average rate of temperature increase from 600 ° C. to the ultimate temperature is satisfied when the temperature is 1000 to 1050 ° C. and the soaking time is less than 5 minutes. The method for producing a ferritic single-phase stainless steel sheet according to claim 1 or 2, wherein the dew point of the atmospheric gas is heat-treated at a temperature of -60 to -50 ° C.
V ≦ 0.2 × T (2)
V: heating rate (° C / min), T: ultimate temperature (° C)

  According to the present invention, the corrosion resistance of the portion reheated at a high temperature exceeding 500 ° C. in the atmosphere such as the weld heat affected zone is remarkably improved. As a result, the Ar gas seal at the time of welding and the acid cleaning of the oxide film at the weld heat affected zone after welding are not required. Furthermore, in the past, in order to ensure the corrosion resistance of the weld heat-affected zone, when steel grades containing more alloy elements that improve corrosion resistance such as Cr and Mo have been selected, the cheaper steel grades with reduced such elements Selection is also possible, which can contribute to reducing the manufacturing cost of products that have welds and that require corrosion resistance.

Relationship between the thickness of the oxide film before welding, the defect rate of the oxide film, the Al atomic ratio in the oxide film, and the corrosion resistance of the heat affected zone An example of the change in the depth direction of the atomic ratio of each element contained in the film Relationship between thickness of oxide film after heat treatment and Al content in steel Relationship between Al atomic ratio in oxide film after heat treatment and Al content in steel Relationship between film thickness after annealing and annealing temperature Relationship between oxide film thickness after heat treatment and heating rate Corrosion resistance evaluation results after welding of steel sheets obtained by changing the temperature reached during heat treatment and the heating rate Relationship between oxide film thickness after heat treatment and soaking time in heat treatment Relationship between oxide film thickness and atmospheric gas dew point Relationship between defect rate of oxide film and dew point of atmospheric gas Relationship between oxide film thickness and atmospheric gas composition during heat treatment Relationship between defect rate of coating and atmospheric gas composition during heat treatment Relationship between oxide film composition and atmospheric gas composition during heat treatment

As a result of detailed investigation of the film of the ferritic stainless steel sheet at the site heated in the atmosphere such as the weld heat affected zone, the present inventors destroyed the passive film responsible for the corrosion resistance of the stainless steel by heating, particularly 500 to 800. An oxide film mainly composed of Fe ₂ O ₃ is formed in a region heated to 0 ° C., and as a result of the corrosion resistance test in salt water, the pitting corrosion is concentrated on the region where the oxide film mainly composed of Fe ₂ O ₃ is formed. Found that occurs.
Then, as a result of examining in detail the method of suppressing the formation of Fe ₂ O ₃ in the oxide film of the reheated part, the mixing ratio of nitrogen and hydrogen is 60 to 90% by volume before being reheated in the atmosphere during welding. In an atmosphere composed of gas, the thickness of the oxide film is 15 nm or more, and the concentration of Al in the oxide film is 50 atomic% or more in the atomic ratio of Al, Si, Mn, Cr, Fe, and is defined by the formula (1) If an oxide film with a defect rate of 3.0% or less is applied in advance, the oxide film in the reheated part in the atmosphere is modified to a film composition mainly composed of Al ₂ O ₃ and Cr ₂ O ₃ And the generation of Fe ₂ O ₃ was found to be suppressed.

  The reason for determining the scope of the present invention will be described below. Table 1 shows materials used for data collection in FIGS. For data collection in FIGS. 5 was used.

FIG. 1 shows the relationship between the thickness of the oxide film before welding, the defect rate of the oxide film, the Al atomic ratio in the oxide film, and the corrosion resistance of the weld heat affected zone. For welding, a TIG pulse welder was used, and a bead on the back surface at a distance of 1 mm between the steel plate and the electrode, a welding speed of 300 mm / min, and a frequency of 10 Hz in the width center of the steel plate having a thickness of 1.0 mm × width 75 mm × length 175 mm The welding current was adjusted so that the width was 4 mm, and the back surface was used as the observation surface without performing Ar back sealing. After welding, two test pieces each having a width of 50 mm and a length of 55 mm were cut out so that the weld bead fits in the center of the width of the test piece, immersed in 750 cc of 1000 ppm Cl ⁻ , 10 ppm Cu ²⁺ aqueous solution kept at 80 ° C. The portion was visually observed to evaluate the corrosion resistance based on the presence or absence of pitting corrosion. Referring to FIG. 1, it can be seen that if an oxide scale having a composition mainly composed of Al having a thickness of 15 nm or more and a defect rate of 3.0% or less is provided, the occurrence of welding heat affected zone is suppressed.

The analysis of the oxide film composition and the measurement of the film thickness were performed by X-ray photoelectron spectroscopy (XPS). FIG. 2 shows an example of the change in the depth direction of the atomic ratio of each element contained in the film. The atomic ratio of each element is a value calculated by a semi-quantitative analysis value based on the integral area of the spectrum of each element by XPS. The film thickness was measured using the sputtering function by Ar ions of the XPS apparatus. Specifically, the position where the atomic ratio of oxygen is 1/2 compared to the surface by sputtering is defined as the interface between the oxide film and the metal, and a standard sample having a SiO ₂ film with a known film thickness on the steel sheet is prepared. Using this, the relationship between the sputtering time and the depth was calculated, and the sputtering time was converted into the film thickness. Furthermore, the Al concentration in the oxide film was high on the surface of the film, and it was observed that the Al concentration in the oxide film tended to decrease as it was closer to the interface with the metal. It was set as the average value with the density | concentration of the position of the depth of 2.5 nm from the interface with the metal calculated | required from the film side. At this time, the Al concentration was determined as an atomic ratio of Al, Si, Mn, Cr, and Fe excluding oxygen and other trace detection elements.

The reason why the Al-based oxide film according to the present invention suppresses the formation of Fe ₂ O ₃ during heating in the atmosphere is that oxygen supplied to the steel sheet surface during heating is blocked by the Al-based oxide film and reaches the steel sheet surface. It is considered that the amount of oxygen is extremely low, the progress of oxidation is suppressed, and part of the oxygen reaching the steel plate surface also grows as an oxide of Al or Cr that is more stable than Fe in the low oxygen potential region. Further, it is considered that the oxygen barrier effect is insufficient when the film thickness is less than 15 nm. Further, even if a sufficient film thickness is obtained, it is considered that the effect of blocking oxygen is insufficient even when the film has many defects.

  FIG. 3 shows the relationship between the thickness of the oxide film after heat treatment and the Al content in the steel, and FIG. 4 shows the relationship between the Al atomic ratio in the oxide film after heat treatment and the Al content in the steel. As the Al content in the steel increases, the thickness of the oxide film and the Al atomic ratio in the oxide film increase, and it is clear that a higher Al content in the steel is desirable. The effect is small. On the other hand, when an excessive amount of Al is added, the steel material is hardened, and there is a concern about deterioration of workability and an increase in manufacturing cost, so the upper limit was made 1.0%.

Next, the reasons for limiting the components of materials other than Al will be described.
C: 0.02 mass% or less Since C is an element that forms carbides and deteriorates corrosion resistance, the upper limit is set to 0.02 mass%.

Si: 0.2-1.0 mass%
Si stabilizes the ferrite structure. In order to maintain the ferrite single phase structure in the heat treatment of the present invention, 0.2% by mass or more may be added. However, among the components of the steel of the present invention, Si is an element that is easily oxidized next to Al. Therefore, excessive addition prevents the formation of an Al-based oxide film proposed in the present invention and hardens the material to improve workability. In order to deteriorate, the upper limit was made 1.0 mass%.

Mn: 0.5% by mass or less Mn inhibits the stabilization of the ferrite structure and forms sulfides and deteriorates the corrosion resistance, so the upper limit was made 0.5% by mass.

P: 0.05% by mass or less P is an element that forms a phosphide and deteriorates the corrosion resistance, so the upper limit was made 0.05% by mass.
S: 0.005% by mass or less Since S is an element that forms sulfides and deteriorates corrosion resistance, the upper limit is set to 0.005% by mass.

Ni: 0.5% by mass or less Ni has an upper limit of 0.5% by mass because it inhibits the stabilization of the ferrite single-phase structure.
Cr: 16 to 24% by mass
Cr is an essential element for improving corrosion resistance and has an effect of stabilizing the ferrite structure. In order to acquire the effect, it was set as addition of 16 mass% or more. On the other hand, the upper limit was set to 24% by mass in order to cause an excessive increase in cost.

Ti: 0.1% by mass or less Ti is an element that improves corrosion resistance and is effective for stabilizing the ferrite structure, but among the additive elements normally found in stainless steel, it is an element that easily oxidizes with Al. This prevents the formation of the oxide film proposed in the invention. Therefore, it was regulated to 0.1 mass% or less.

N: 0.02% by mass or less N has an upper limit of 0.02% by mass because N forms nitrides and deteriorates corrosion resistance, or hardens steel and deteriorates workability.

Mo: 2.0% by mass or less Mo is an element added as necessary to improve the corrosion resistance. The upper limit is set to 2.0% by mass in order to cause excessive workability reduction and cost increase.

  The method for producing an Al-based oxide film, which is a feature of the present invention, is characterized by finishing a steel plate produced through normal melting, hot rolling, hot-rolled sheet annealing, and cold rolling under the following heat treatment conditions. And FIG. 5 shows the relationship between the film thickness after the heat treatment and the annealing temperature. The soaking time was 0 min, and air cooling was performed immediately after reaching the target temperature. The cooling rate was about 90 s to 600 ° C. and about 400 s from 600 ° C. to 200 ° C. The temperature in the furnace reached the ultimate temperature, and the furnace atmosphere was maintained up to 200 ° C. after cooling.

  According to FIG. 5, the higher the heat treatment temperature, the greater the thickness of the oxide film, but it is also affected by the rate of temperature rise during overheating. Although the effect of the heating rate will be described later, when the temperature reached during the heat treatment is less than 1000 ° C., the heat treatment furnace with a short soaking time such as a continuous bright annealing furnace used industrially has a film thickness exceeding 10.0 nm. It is considered difficult to obtain. On the other hand, at a high temperature exceeding 1050 ° C., the cost for the heat treatment increases, and excessive coarsening of crystal grains is caused. FIG. 5 also shows the corrosion resistance after welding, but it can be seen that all of the films with rusting are thin. Moreover, what was annealed at 900 degreeC has confirmed that the defect rate of an oxide film is also large.

  FIG. 6 shows the relationship between the thickness of the oxide film after heat treatment and the rate of temperature rise. When the inventors intend to control an oxide film having a thickness of nanometer level as in the present invention even if the heat treatment temperature is the same, it is necessary to consider the temperature rising process until the target temperature is reached. It was found that the influence of the average heating rate up to the ultimate temperature is large. According to FIG. 6, it can be seen that the smaller the temperature increase rate, the greater the thickness of the oxide film, and there is an appropriate temperature increase rate range for each heat treatment temperature. FIG. 7 shows the corrosion resistance evaluation results after welding of the steel sheets obtained by changing the ultimate temperature and the heating rate during the heat treatment. The claims indicated by hatching in FIG. 7 are the conditions indicated by equation (2).

  FIG. 8 shows the relationship between the oxide film thickness after the heat treatment and the soaking time in the heat treatment. It can be seen that the longer the soaking time, the greater the thickness of the oxide film. When the soaking time reaches 5 minutes, the effect of the heating rate is small, and an oxide film thickness of around 15 nm can be obtained. Therefore, in the case where the soaking is 5 min or more, the temperature rise rate is not limited.

FIG. 9 shows the relationship between the thickness of the oxide film and the dew point of the atmospheric gas. The higher the dew point, the greater the thickness of the oxide film. At a dew point of −40 ° C., a target film thickness of 15 nm was obtained even if the rate of temperature increase was faster than the range of the present invention. However, rusting was observed in the corrosion resistance test after welding.
In order to clarify this reason, the defect amount of the oxide film was evaluated by the following method. That is, a current density at a potential of 250 mV was obtained from an anodic polarization curve obtained in an aqueous solution of sulfuric acid: 1 mol / L, potassium thiocyanate: 0.01 mol / L, and a liquid temperature of 30 ° C. under the condition of 10 mV / min. Since the film of the present invention is mainly composed of Al ₂ O ₃ having insulating properties, the anode current is lowered when the film is free of defects.

  As a preliminary experiment, after the coating of the present invention was once applied, a part of the coating was peeled off by polishing (SiC polishing paper # 400), and as a result of correlating the material exposed area and the anode current value, a part of the coating was obtained. The anode current value of the peeled sample rides on the straight line connecting the current value of the sample (pickling finish material) of the material exposed area rate 100% with no origin and coating originally applied, and the coating exposed area and anode current value Have a first-order correlation. FIG. 10 shows the relationship between the defect rate of the oxide film and the dew point of the atmospheric gas. According to FIG. 10, it was found that the film obtained at a dew point of −40 ° C. had many defects. On the other hand, the dew point on the wet and dry side from −60 ° C. is difficult to maintain industrially and only increases the production cost. Therefore, the appropriate range of the dew point of the atmospheric gas during the heat treatment is set to −60 to −50 ° C.

  FIG. 11 shows the relationship between the thickness of the oxide film and the atmospheric gas composition during heat treatment. Although the influence of the atmospheric gas composition on the thickness of the oxide film is small, even if an oxide film thickness of 15 nm or more can be obtained in a 100% hydrogen gas atmosphere, igniting may be observed in the corrosion resistance test after welding. Looking at the relationship between the defect rate of the film shown in FIG. 12 and the atmosphere gas composition during the heat treatment, it was found that the oxide film has many defects when a 100% hydrogen gas atmosphere is used. In addition, when the film thickness was sufficiently increased by setting the soaking time to 10 nm, the defect rate of the oxide film also decreased, and no igniting was observed in the corrosion resistance test after welding. On the other hand, in a 100% nitrogen gas atmosphere, as can be seen from the relationship between the composition of the oxide film shown in FIG. 13 and the atmosphere gas composition during the heat treatment, the Al concentration in the film after the heat treatment decreases, and the oxidation targeted by the present invention is achieved. The film composition cannot be obtained.

  It has been confirmed that the Cr concentration increases instead of the Al concentration decreasing. In the present invention, by providing an oxide film mainly composed of easily oxidizable Al in advance, the oxide film is maintained in the atmosphere when exposed to a high temperature in the atmosphere like a weld heat affected zone. It is considered that the oxygen can be prevented from touching the surface of the steel sheet, and the oxidation of iron, which causes corrosion resistance deterioration, can be suppressed. Therefore, in order to obtain an Al-based oxide film with few defects, the proper atmosphere gas composition is a mixed gas atmosphere of hydrogen and nitrogen having a hydrogen ratio of 60 to 90% by volume.

The ferritic stainless steel plate previously shown in Table 1 was manufactured through normal melting, hot rolling, hot rolled sheet annealing, and cold rolling, and annealed under the conditions shown in Table 2 in the final step. Using a TIG pulse welding machine, the steel plate was placed at the center of the width of the steel plate having a thickness of 1.0 mm × width of 75 mm × length of 175 mm. The welding current was adjusted so that the bead width was 4 mm, welding was performed without performing the Ar back seal, and the sample was subjected to a corrosion resistance test.
In the corrosion resistance test, two test pieces each having a width of 50 mm and a length of 55 mm were cut out so that the weld bead fits in the center of the test piece, immersed in 750 cc of 1000 ppm Cl ⁻ , 10 ppm Cu ²⁺ aqueous solution kept at 80 ° C. The heat-affected zone was visually observed and evaluated by the presence or absence of pitting corrosion.

The atomic ratio of each element is a value calculated by a semi-quantitative analysis value based on the integral area of the spectrum of each element by XPS. The film thickness was measured using the sputtering function by Ar ions of the XPS apparatus. Specifically, the position where the atomic ratio of oxygen is 1/2 compared to the surface by sputtering is defined as the interface between the oxide film and the metal, and a standard sample having a SiO ₂ film with a known film thickness on the steel sheet is prepared. Using this, the relationship between the sputtering time and the depth was calculated, and the sputtering time was converted into the film thickness. Furthermore, the Al concentration in the oxide film was high on the surface of the film, and it was observed that the Al concentration in the oxide film tended to decrease as it was closer to the interface with the metal. It was set as the average value with the density | concentration of the position of the depth of 2.5 nm from the interface with the metal calculated | required from the film side. At this time, the Al concentration was determined as an atomic ratio of Al, Si, Mn, Cr, and Fe excluding oxygen and other trace detection elements.

Film defects were determined by determining the current density at an electric potential of 250 mV from an anodic polarization curve obtained in an aqueous solution of sulfuric acid: 1 mol, potassium thiocyanate: 0.01 mol, and liquid temperature of 30 ° C. under the condition of 10 mV / min. Based on the current density of 2D), the film defect rate was determined by the following equation.
Defect rate of film = A / B × 100 (%) (1)
A: Anode current density of evaluation material, B: Anode current density of polished finish material Table 2 also shows the oxide film composition, oxide film thickness, oxide film defect rate on the steel sheet surface, and corrosion resistance test results after welding.
In the corrosion resistance test results, ◯ indicates that there was wrinkling, and X indicates that there was no wrinkling. From this result, it can be seen that the film defects greatly influence the corrosion resistance after welding.

  ADVANTAGE OF THE INVENTION According to this invention, the stainless steel excellent in the corrosion resistance of a welding heat affected zone can be provided.

Claims (4)

A ferritic stainless steel having an oxide film formed by heat treatment in a mixed gas atmosphere of nitrogen and hydrogen, the oxide film having a thickness of 15 nm or more, and the concentration of Al in the oxide film is Al, Si, Mn, Cr, Stainless steel with improved corrosion resistance in the heat affected zone of the weld zone, wherein the atomic ratio of Fe is 50 atomic% or more and the defect rate of the oxide film defined by the formula (1) is 3.0% or less. steel sheet.
Sulfuric acid: 1 mol / L, potassium thiocyanate: 0.01 mol / L, anodic polarization in an aqueous solution at a liquid temperature of 30 ° C. under the condition of 10 mV / min, and a value calculated from the anodic current density at 250 mV by the formula (1) Is the film defect rate.
Defect rate of film = A / B × 100 (%) (1)
A: Anode current density of evaluation material, B: Anode current density of pickling finish   C: 0.02 mass% or less, Si: 1.0 mass% or less, Mn: 0.5 mass% or less, P: 0.05 mass% or less, S: 0.005 mass% or less, Ni: 0.5 % By mass, Cr: 16-24% by mass, Al: 0.05-1.0% by mass, Nb: 0.1-0.5% by mass or less, Ti: 0.1% by mass or less, N: 0.00%. The ferritic single-phase stainless steel sheet according to claim 1, which has a composition containing not more than 02% by mass, Mo: not more than 2.0% by mass, and the balance consisting of Fe and inevitable impurities.   A heat treatment is performed in a mixed gas atmosphere of hydrogen and nitrogen having a hydrogen ratio of 60 to 90% by volume under conditions of a temperature of 1000 to 1050 ° C. and a soaking time of 5 min or more under a condition of −60 to −50 ° C. dew point of the atmosphere gas. A method for producing a ferritic single-phase stainless steel sheet according to claim 1 or 2. In a mixed gas atmosphere of hydrogen and nitrogen having a hydrogen ratio of 60 to 90% by volume, the average rate of temperature increase from 600 ° C. to the ultimate temperature is satisfied when the temperature is 1000 to 1050 ° C. and the soaking time is less than 5 minutes. The method for producing a ferritic single-phase stainless steel sheet according to claim 1, wherein the dew point of the atmospheric gas is heat-treated at −60 to −50 ° C.
V ≦ 0.2 × T (2)
V: heating rate (° C / min), T: ultimate temperature (° C)

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