TW202210642A - Austenitic stainless steel sheet and method for producing same - Google Patents

Abstract

In the present invention, a cold-rolled stainless steel sheet that exhibits excellent pitting corrosion resistance is provided. According to the present invention, a cold-rolled stainless steel sheet is produced by performing one or more rounds of cold rolling on a hot-rolled steel sheet having a compositional makeup that includes, in mass %, not more than 0.40% of C, not more than 1.00% of Si, not more than 2.00% of Mn, not more than 0.045% of P, not more than 0.030% of S, 3.5-36.0% of Ni, 15.00-30.00% of Cr, 0-7.0% of Mo, and not more than 0.25% of N. In the cold rolling, heat treatment is performed after the final round of cold rolling among the rounds of cold rolling or after a round of cold rolling except the final round of cold rolling among the rounds of cold rolling, and then dilute nitric acid electrolytic treatment is lastly performed. As the heat treatment, a heat treatment of maintaining the steel sheet at a temperature in a range of 150-600 DEG C for 30 s to 10 min or a heat treatment of maintaining the steel sheet at a temperature in a range of 150-700 DEG C for 15 min to 48 hr is preferable. Accordingly, a pitting initiation potential on a surface of the steel sheet becomes high and pitting corrosion resistance thereof is improved.

Description

Wostian iron-based stainless steel plate and method for producing the same

The present invention relates to a Vostian iron-based stainless steel sheet that is very suitable for use as automobile parts, electronic equipment, springs, and other industrial parts, and a manufacturing method thereof, and relates to improving its corrosion resistance, especially improving its pitting corrosion resistance. .

For example, the "manufacturing method of ferrite-based stainless steel for electrical materials excellent in ductility, wear resistance, and rust resistance" disclosed in Patent Document 1 is a method for improving the corrosion resistance of ferrite-based stainless steel sheets. In the technique disclosed in Patent Document 1, after final finish annealing, cold rolling is performed at a rate of 10% or more, and after reheating, for example, nitric acid electrolysis is performed in nitric acid (20° C.) with a concentration of 10%, and the surface is removed. In this way, it is possible to manufacture a ferrite-based stainless steel that is excellent in ductility, wear resistance, and rust resistance without deteriorating corrosion resistance.

Moreover, the patent document 2 discloses "the manufacturing method of the bright-annealed material of a ferrite-type stainless steel excellent in rust resistance". In the technique disclosed in Patent Document 2, after annealing the ferrite-based stainless steel sheet, performing descaling using a neutral salt electrolysis method and a nitric acid electrolysis method, and then performing cold rolling and bright annealing, it is possible to We have produced a bright annealed ferrite-based stainless steel with superior rust resistance compared to the conventional bright annealed ferrite-based stainless steel. In addition, the nitric acid electrolysis method used is alternate electrolysis in nitric acid (50°C) with a concentration of 15%.

Moreover, patent document 3 discloses "the manufacturing method of the stainless steel with good corrosion resistance". The technique disclosed in Patent Document 3 is directed to stainless steel containing Cr: 11 mass % or more and 35 mass % or less, and the contents of oxygen and sulfur have been reduced to O: 0.01 mass % or less and S: 0.01 mass % or less, Mechanical polishing is performed using an acidic aqueous solution containing an oxidizing agent as a polishing liquid to produce stainless steel with improved corrosion resistance. The technique described in Patent Document 3 employs buff grinding and belt grinding as mechanical grinding.

On the other hand, the Wastian iron-based stainless steel sheet is usually cold-rolled after heat treatment, thereby improving its mechanical properties and having a certain degree of pitting corrosion resistance. However, pitting corrosion is likely to occur in an environment containing chloride ions, in a gap structure, or in an environment of high temperature and high humidity. Therefore, in such an environment, steel types with increased Cr and Mo contents (SUS316L stainless steel, etc.) are often used. However, this type of steel is expensive, and based on cost considerations, it may not be available in all environments.

In general, when manufacturing a Vostian iron-based stainless steel sheet, heat treatment is performed for the purpose of eliminating internal stress, solid solution, and improving other mechanical properties. However, even if the heat treatment is performed in a mixed gas of nitrogen and hydrogen or in a reducing gas atmosphere such as a hydrogen atmosphere, oxidation cannot be completely prevented, and an oxide film is formed on the surface layer, so that Cr is formed directly under the surface layer. A poor layer results in deterioration of corrosion resistance. Therefore, in order to restore the corrosion resistance, conventionally, after heat treatment in a reducing gas atmosphere, immersion in an acid solution is performed, or electrolytic polishing is performed to restore the corrosion resistance. [Prior Art Literature] [Patent Literature]

Patent Document 1: Japanese Patent Application Laid-Open No. 04-371518 Patent Document 2: Japanese Patent Application Laid-Open No. 11-50202 Patent Document 3: Japanese Patent Application Laid-Open No. 03-193885

[Problems to be Solved by Invention]

The techniques disclosed in Patent Document 1 and Patent Document 2 are all related to improving the corrosion resistance of the iron-based stainless steel sheet. to reveal.

In addition, although the technique disclosed in Patent Document 3 can be applied to a Vostian iron-based stainless steel sheet, in order to improve corrosion resistance, mechanical polishing such as buffing using an acidic aqueous solution is required as a requirement. When the surface layer is ground, there is a possibility that the mechanical properties of the Vostian iron-based stainless steel sheet may be changed, and the method of grinding the surface layer may lead to deterioration of corrosion resistance. In addition, there is a problem that for products required to have surface thickness, they cannot meet the requirements if they are polished and ground.

The present invention has been developed in view of the problems of such a conventional technique, and an object of the present invention is to provide a Vostian iron-based stainless steel sheet excellent in corrosion resistance, especially pitting corrosion resistance, and a method for producing the same. [Technical means to solve problems]

In order to achieve the above-mentioned object, the inventors of the present invention have been diligently examining various factors that may affect the pitting corrosion resistance of the Vostian iron-based stainless steel sheet.

As a result, a creative idea was found, that is, when a hot-rolled steel sheet is subjected to cold rolling to produce a cold-rolled steel sheet, and when a cold-rolled steel sheet is produced by performing one or more times of cold-rolling, at the end of the cold-rolling process. After cold rolling, or after cold rolling other than final cold rolling in the aforementioned cold rolling process, heat treatment under specific conditions is performed, and then dilute nitric acid electrolytic treatment under appropriate conditions is performed. As a result, pitting corrosion on the surface of the steel sheet occurs. The potential will rise, and pitting corrosion resistance can be improved even in an environment that conventional cold-rolled steel sheets cannot cope with.

First, the experimental result which is the basis of this invention is demonstrated.

In terms of mass %, it contains Cr: 10.5-23.2%, Ni: 0-35.1%, Mo: 0-7.00%, N: 0.02-0.07%, C: 0.01-0.10%, Si: 0.34- The annealed and pickled hot-rolled steel sheet (thickness: 2.5 mm) of 0.67% and Mn: 0.65 to 1.10% was cold-rolled three times to obtain a cold-rolled steel sheet (thickness: 0.1 mm). When producing a cold-rolled steel sheet, the heat treatment performed after the final cold rolling, or after the second cold rolling in the process of performing dilute nitric acid electrolysis after the third cold rolling, is heating at 145 to 720° C. The heat treatment in which the temperature is maintained for 20 seconds to 49 hours, and the heat treatment other than the above, is the heat treatment in which the temperature is maintained at a heating temperature of 850 to 1050° C. for 3 to 5 minutes. The obtained cold-rolled steel sheets were electrolytically treated with dilute nitric acid, and then subjected to surface grinding, and the pitting corrosion occurrence potential Vc on the surface of each steel sheet was measured in accordance with the Japanese Industrial Standard JIS G 0577. In addition, when the pitting corrosion generation potential was measured, the test solution (aqueous sodium chloride solution) was not subjected to degassing treatment. In addition, the control electrode is an Ag/AgCl (silver chloride) electrode. In addition, dilute nitric acid electrolytic treatment was not performed with respect to some steel sheets.

The condition of dilute nitric acid electrolysis treatment adopts: in the dilute nitric acid aqueous solution (liquid temperature: 60 ℃) that the nitric acid concentration is 3%, with the current density of ± 30 mA/cm ² , carry out anode and cathode electrolysis treatment, and the total treatment time is 20 seconds. The relationship between the measured pitting corrosion occurrence potential Vc and the pitting corrosion index X (=Cr+3.3Mo) is shown in the statistical graph of FIG. 1 .

From the statistical results in Fig. 1, it can be seen that the pitting corrosion occurrence potential in the case where the dilute nitric acid electrolytic treatment and the heat treatment after cold rolling are combined together (the black circle mark in Fig. 1) is higher than the The pitting corrosion generation potential in the case where only the heat treatment was performed without the dilute nitric acid electrolytic treatment (marked by the white circles ○ in FIG. 1 ). In other words, the combination of dilute nitric acid electrolytic treatment and heat treatment can effectively improve the pitting corrosion resistance. In addition, the pitting corrosion index X (=Cr+3.3Mo) is an index indicating the difficulty of pitting corrosion of stainless steel. The higher the pitting corrosion index, the higher the tendency of pitting corrosion resistance.

From this experimental result, according to its relationship with the pitting corrosion index X (=Cr+3.3Mo), the threshold value of the pitting corrosion occurrence potential increased by performing the dilute nitric acid electrolytic treatment is defined as the following formula A=0.039X ³ -5.2X ² +232X-2311 relationship. (X=Cr+3.3Mo in this formula) This formula is based on the numerical value of the pitting corrosion occurrence potential after dilute nitric acid electrolysis, which is higher than the pitting corrosion potential before dilute nitric acid electrolysis A trend line (approximate curve) made of values near the boundary of the lower limit of these points. In addition, when Mo is not contained, the numerical value of X is calculated as 0% of this element.

In addition, when the pitting corrosion occurrence potential Vc on the surface of the stainless steel plate was higher than the above-mentioned A value, it was considered that the pitting corrosion resistance was improved. In addition, in the steel sheet with a pitting corrosion index lower than 15.0, even if the dilute nitric acid electrolytic treatment and the heat treatment were combined, it was not found that the pitting corrosion occurrence potential increased more than the above-mentioned A value. Therefore, the range of X is limited to 15.0 to 50.0.

Therefore, a creative idea was found from such experimental results, that is, when a cold-rolled steel sheet is manufactured by performing one or more cold-rolling times on a hot-rolled steel sheet, after the final cold-rolling in the aforementioned cold-rolling process, or after the aforementioned cold-rolling process. After cold rolling other than final cold rolling in the rolling process, heat treatment under specific conditions and dilute nitric acid electrolytic treatment under suitable conditions can increase the pitting corrosion potential on the surface of the steel sheet, making it possible to achieve an environment that could not be dealt with in the past. The stainless steel sheet (stainless steel cold-rolled steel sheet) with excellent pitting corrosion resistance can also be applied below. In addition, another creative idea was found, that is, there is no problem in performing cold rolling either before or after the dilute nitric acid electrolytic treatment.

數式(2)中的Cr和Mo是表示該元素的含量(質量%)， 並且表面的孔蝕發生電位Vc是符合下列數式(1)的關係，

[2] 如前述[1]所述之沃斯田鐵系不鏽鋼板，其中，除了上述組成分之外，以質量%計，又含有從Ti：0.01～1.00%、Nb：0.01～1.00%、Cu：0.01～3.00%、Al：0.0001～1.50%、Ca：0.001～0.01%、Mg：0.001～0.01%、V：0.01～1.00%、Co：0.01～0.5%、W：0.01～1.0%、以及B:0.001～0.01%之中所選出的一種或兩種以上。 [3] 如前述[1]或[2]所述之沃斯田鐵系不鏽鋼板，其中，鋼板的表面粗細度Sa，是依據日本工業規格 ISO 25178所規定的Sa為0.80μm以下。 [4] 一種沃斯田鐵系不鏽鋼板之製造方法，其特徵為：當針對於具有前述[1]或[2]所述的組成分的熱軋鋼板實施一次或複數次冷軋來製造冷軋鋼板時，係在前述冷軋工序之中的最終冷軋之後，或者係在前述冷軋工序之中的最終冷軋以外的冷軋之後，實施在150～600℃之範圍的溫度下保持30秒鐘～10分鐘的熱處理，最後再實施稀硝酸電解處理。 [5] 一種沃斯田鐵系不鏽鋼板之製造方法，其特徵為：當針對於具有前述[1]或[2]所述的組成分的熱軋鋼板實施一次或複數次冷軋來製造冷軋鋼板時，係在前述冷軋工序之中的最終冷軋之後，或者係在前述冷軋工序之中的最終冷軋以外的冷軋之後，實施在150～700℃之範圍的溫度下保持15分鐘～48小時的熱處理，最後再實施稀硝酸電解處理。 [6] 如前述[4]或[5]所述的沃斯田鐵系不鏽鋼板之製造方法，其中，前述稀硝酸電解處理，是在硝酸濃度為3～10%及溫度為40～80℃的稀硝酸水溶液中，以±10～80 mA/cm² 的電流密度，進行陰極以及陽極電解處理，合計處理時間為10～60秒鐘。 [發明之效果]The present invention is completed by further review based on the above-mentioned innovations. In other words, the gist of the present invention is as follows. [1] A Vostian iron-based stainless steel sheet, characterized in that the composition, in terms of mass %, contains C: 0.40% or less, Si: 1.00% or less, Mn: 2.00% or less, P: 0.045% or less, S: 0.030% or less, Ni: 3.5 to 36.0%, Cr: 15.00 to 30.00%, Mo: 0 to 7.0%, and N: 0.25% or less, the rest is Fe and inevitable impurities, and the contents of Cr and Mo is a relationship in which the numerical value of X defined by the following equation (2) is 15.0 to 50.0,

Cr and Mo in the formula (2) represent the content (mass %) of the element, and the pitting corrosion occurrence potential Vc on the surface is in accordance with the following formula (1):

[2] The Vostian iron-based stainless steel sheet according to the above [1], which, in addition to the above-mentioned components, further contains, in mass %, Ti: 0.01 to 1.00%, Nb: 0.01 to 1.00%, Cu: 0.01 to 3.00%, Al: 0.0001 to 1.50%, Ca: 0.001 to 0.01%, Mg: 0.001 to 0.01%, V: 0.01 to 1.00%, Co: 0.01 to 0.5%, W: 0.01 to 1.0%, and B: One or more selected from among 0.001 to 0.01%. [3] The Vostian iron-based stainless steel sheet according to the above [1] or [2], wherein the surface roughness Sa of the steel sheet is 0.80 μm or less as defined by Japanese Industrial Standard ISO 25178. [4] A method for producing a Vostian iron-based stainless steel sheet, wherein the cold rolling is produced by subjecting a hot-rolled steel sheet having the composition described in [1] or [2] to one or more times of cold rolling. When rolling the steel sheet, after the final cold rolling in the cold rolling process, or after the cold rolling other than the final cold rolling in the cold rolling process, it is carried out at a temperature in the range of 150 to 600°C and maintained at 30°C. Heat treatment for seconds to 10 minutes, and finally, electrolytic treatment with dilute nitric acid. [5] A method for producing a Vostian iron-based stainless steel sheet, wherein the cold rolling is produced by subjecting a hot-rolled steel sheet having the composition described in [1] or [2] to one or more times of cold rolling. When rolling the steel sheet, after the final cold rolling in the cold rolling process, or after the cold rolling other than the final cold rolling in the cold rolling process, it is maintained at a temperature in the range of 150 to 700°C for 15 Heat treatment for minutes to 48 hours, and finally, electrolytic treatment with dilute nitric acid. [6] The method for producing a Vostian iron-based stainless steel sheet according to the above [4] or [5], wherein the dilute nitric acid electrolytic treatment is performed at a nitric acid concentration of 3 to 10% and a temperature of 40 to 80° C. In the dilute nitric acid aqueous solution of ±10-80 mA/cm ² , the cathodic and anodic electrolytic treatments are performed, and the total treatment time is 10-60 seconds. [Effect of invention]

According to the present invention, it is possible to provide a stainless steel sheet excellent in pitting corrosion resistance in which the pitting corrosion occurrence potential of the surface is increased, and the effect of industrially required usability can be achieved. In addition, according to the present invention, for example, it can be applied in a corrosive environment that the steel plate with a lower pitting corrosion index in the prior art cannot correspond to, and also has a steel plate with a higher pitting corrosion index in the prior art, that is, pitting corrosion. Corrosion resistance equivalent to nickel-based superalloys such as HASTELLY with a potential of over 1000 mV. In addition, according to the present invention, the same effect can be achieved even when applied to a precipitation hardening-based stainless steel sheet and a duplex stainless steel sheet specified in JIS G 4305, not limited to the Vostian iron-based stainless steel sheet.

The Vostian iron-based stainless steel sheet of the present invention contains, in mass %, C: 0.40% or less, Si: 1.00% or less, Mn: 2.00% or less, P: 0.045% or less, and S: 0.030% or less , Ni: 3.5 to 36.0%, Cr: 15.00 to 30.00%, Mo: 0 to 7.0%, and N: 0.25% or less, the rest is Fe and inevitable impurities, and the content of Cr and Mo is consistent with X=Cr +3.3Mo is a relationship of 15.0 to 50.0. In the following description, the mass % related to the composition is simply indicated as %.

First, the reason for limiting the composition will be described.

C: 0.40% or less C is an element which improves mechanical properties such as strength and abrasion resistance when contained in a small amount. In order to obtain such an effect, it is preferable to set the C content to 0.001% or more. On the other hand, when the C content exceeds 0.40%, Cr carbides are likely to be formed at the crystal grain boundaries, and grain boundary corrosion is likely to occur. In addition, when the C content exceeds 0.40%, the ductility is deteriorated and press workability is hindered. Therefore, the C content is limited to 0.40% or less. More preferably, the C content is set to 0.01 to 0.20%.

Si: 1.00% or less Si functions as a deoxidizer for molten steel, and is an element that contributes to increasing strength such as elastic limit and tensile strength. In order to obtain such an effect, it is preferable to set the Si content to 0.10% or more. On the other hand, when the Si content exceeds 1.00%, edge cracking occurs during hot rolling, resulting in poor product yield. Therefore, the Si content is limited to 1.00% or less.

Mn: 2.00% or less Mn is an element which contributes to increasing the strength such as tensile strength and improving toughness, and is an element that can effectively act on the deoxidation of molten steel. In order to obtain such an effect, it is preferable to set the Mn content to 0.10% or more. On the other hand, when the Mn content exceeds 2.00%, inclusions such as MnS in the steel increase, which adversely affects workability. Therefore, the Mn content is limited to 2.00% or less.

P: 0.045% or less, S: 0.030% or less P and S are elements that are inevitably present in steel and adversely affect mechanical properties. Therefore, although it is desirable to reduce the contents of P and S as much as possible, as long as the P content does not exceed 0.045% and the S content does not exceed 0.030%, there will be no problem with the practicality of the steel, and it is acceptable. Therefore, the P content is limited to 0.045% or less, and the S content is limited to 0.030% or less. More preferably, the P content is limited to 0.030% or less, and the S content is limited to 0.010% or less.

Ni: 3.5 to 36.0% Ni is an element that improves corrosion resistance and contributes to improving toughness, strength, and heat resistance. To obtain this effect, the Ni content must be set to 3.5% or more. If the Ni content is less than 3.5%, the metal structure of the steel at room temperature will become a fertile iron phase. On the other hand, when the Ni content exceeds 36.0%, the workability is deteriorated, and the weldability is also deteriorated. Therefore, the Ni content is limited to the range of 3.5 to 36.0%.

Cr: 15.00～30.00% Both Cr and Ni contribute to the improvement of corrosion resistance, and both are elements that can change the metal structure of steel at room temperature into a Worcesterian iron phase. To obtain this effect, the Cr content must be set to 15.00% or more. On the other hand, when the Cr content exceeds 30.00%, the ductility is deteriorated and the material cost is increased. Therefore, the Cr content is limited to the range of 15.00 to 30.00%. More preferably, it is in the range of 16.00 to 30.00%.

Mo: 0 to 7.0% Mo is an element that contributes to the improvement of pitting corrosion resistance and mechanical properties, and Mo may be contained or not (0%) according to needs. In order to obtain such an effect and Mo is contained, it is preferable to set the Mo content to 0.001% or more. When the Mo content is less than 0.001%, the mechanical properties are slightly deteriorated. On the other hand, when the Mo content exceeds 7.0%, the precipitation of the σ phase is accelerated, and the toughness is deteriorated during heat treatment. Furthermore, if the Mo content is too large, the material cost will increase. Therefore, in order to contain Mo, the Mo content is limited to 7.0% or less. More preferably, it is in the range of 0.5 to 3.0%.

N: 0.25% or less N is an element which can stabilize the Worcester iron phase and contribute to increase in strength by solid solution strengthening by intrusive solid solution. In order to obtain such an effect, it is preferable to set the N content to 0.01% or more. On the other hand, when the N content exceeds 0.25%, there will be adverse effects such as promotion of cracking at high temperature, deterioration of secondary workability, promotion of grain boundary corrosion, and the like. Therefore, the N content is limited to 0.25% or less. It is preferably limited to 0.20% or less, more preferably in the range of 0.01 to 0.10%.

此外，如果未含有Mo的話，在進行計算數式(2)中的X的值時，係將Mo含量視為0%。另一方面，X的值超過50.0的話，合金元素量太多，延性將會變差，並且會導致材料成本的上昇。因此，是以使得孔蝕指數X的值落在15.0～50.0的範圍內的方式，來限定上述Cr和Mo的含量。Pitting index X: 15.0 to 50.0 If the value of the pitting index X defined by the content (mass %) of each element of Cr and Mo in the following formula (2) is less than 15.0, even if dilute nitric acid electrolysis is carried out in combination No increase in pitting corrosion occurrence potential was observed in the heat treatment after treatment and cold rolling.

In addition, when Mo is not contained, when calculating the value of X in Formula (2), the Mo content is regarded as 0%. On the other hand, if the value of X exceeds 50.0, the amount of alloying elements will be too large, the ductility will be deteriorated, and the material cost will be increased. Therefore, the contents of the above-mentioned Cr and Mo are limited so that the value of the pitting index X falls within the range of 15.0 to 50.0.

The above-mentioned components are the basic components of the present invention, but in the present invention, in addition to the above-mentioned basic components, Ti: 0.01-1.00%, Nb: 0.01-1.00%, Cu: 0.01- 3.00%, Al: 0.0001-1.50%, Ca: 0.001-0.01%, Mg: 0.001-0.01%, V: 0.01-1.00%, Co: 0.01-0.5%, W: 0.01-1.0%, and B: 0.001- One or two or more elements are selected from 0.01% as the selectively added element.

From Ti: 0.01 to 1.00%, Nb: 0.01 to 1.00%, Cu: 0.01 to 3.00%, Al: 0.0001 to 1.50%, Ca: 0.001 to 0.01%, Mg: 0.001 to 0.01%, V: 0.01 to 1.00%, Co: 0.01 to 0.5%, W: 0.01 to 1.0%, and B: 0.001 to 0.01% of one or more elements selected Ti, Nb, Cu, Al, Ca, Mg, V, Co, W, and B are all elements that are dispersed in the steel as fine precipitates and contribute to improving the strength and corrosion resistance of the steel sheet, and B is a To help improve the effect of high temperature characteristics, one or two or more elements can be selected and added according to requirements. To obtain this effect, its content must be Ti: 0.01% or more, Nb: 0.01% or more, Cu: 0.01% or more, Al: 0.0001% or more, Ca: 0.001% or more, Mg: 0.001% or more, V: 0.01% or more, Co: 0.01% or more, W: 0.01% or more, B: 0.001% or more. On the other hand, if the content of each element is Ti: more than 1.00%, Nb: more than 1.00%, Cu: more than 3.00%, Al: more than 1.50%, Ca: more than 0.01%, Mg: more than 0.01%, V: When it exceeds 1.00%, Co: more than 0.5%, W: more than 1.0%, and B: more than 0.01%, the amount of precipitates produced increases, and corrosion resistance and elongation are likely to deteriorate. Therefore, if these elements are to be contained, their contents are limited to Ti: 0.01 to 1.00%, Nb: 0.01 to 1.00%, Cu: 0.01 to 3.00%, Al: 0.0001 to 1.50%, Ca: 0.001 to 0.01% , Mg: 0.001 to 0.01%, V: 0.01 to 1.00%, Co: 0.01 to 0.5%, W: 0.01 to 1.0%, and B: 0.001 to 0.01%.

The remainder other than the above components is Fe and inevitable impurities.

In addition, O (oxygen) element is unavoidably present in steel in the form of oxide, and adversely affects ductility, toughness, and the like. Therefore, O (oxygen) element is regarded as an impurity, and it is advisable to reduce the content as much as possible, but the O content can be tolerated not to be higher than 0.010%. Further, if the O content is excessively reduced to less than 0.001%, the refining cost will increase, so the O (oxygen) content is preferably set to 0.001% or more.

在數式(1)中的X=Cr+3.3Mo……數式(2) X：15.0～50.0。在數式(2)中的Cr、Mo表示是Cr和Mo各元素的含量(質量%)。 如果所測定到的鋼板表面的孔蝕發生電位Vc太低，而不符合數式(1)的關係的話，將會無法確保所期望的耐孔蝕性。此外，鋼板表面的孔蝕發生電位Vc，係使用表層未經過研磨的測定樣本，並且依照日本工業規格JIS G 0577的規定來進行測定後所獲得的數值。此外，在進行測定孔蝕發生電位的時候，並未對於試驗溶液(氯化鈉水溶液)進行脫氣處理。對照電極是採用Ag/AgCl(氯化銀)電極。In addition to having the above-mentioned composition, the Vostian iron-based stainless steel sheet of the present invention has a surface pitting corrosion occurrence potential Vc that satisfies the relationship of the following formula (1).

X=Cr+3.3Mo in Formula (1)... Formula (2) X: 15.0 to 50.0. Cr and Mo in the formula (2) represent the contents (mass %) of each element of Cr and Mo. If the measured pitting corrosion occurrence potential Vc on the surface of the steel sheet is too low to satisfy the relationship of the formula (1), the desired pitting corrosion resistance cannot be secured. In addition, the pitting corrosion occurrence potential Vc on the surface of the steel sheet is a value obtained by measuring in accordance with the provisions of JIS G 0577 using a measurement sample whose surface layer has not been ground. In addition, when the pitting corrosion generation potential was measured, the test solution (aqueous sodium chloride solution) was not subjected to degassing treatment. The control electrode was an Ag/AgCl (silver chloride) electrode.

Next, the preferred manufacturing method of the Vostian iron-based stainless steel sheet of the present invention will be described.

The present invention is a cold-rolled steel sheet having a predetermined thickness by performing one or more cold-rolling on a hot-rolled steel sheet having the above-mentioned composition and subjected to annealing and pickling treatment. In this production process, in the present invention, heat treatment is performed after final cold rolling in a plurality of cold rolling steps, or after cold rolling other than final cold rolling in a plurality of cold rolling steps.

The purpose of the above-mentioned heat treatment is to recover and improve the mechanical properties. Therefore, a heat treatment is performed at a temperature in the range of 150 to 600° C. for 30 seconds to 10 minutes (hereinafter, this heat treatment is also referred to as heat treatment.) A) is appropriate. If the heat treatment temperature is lower than 150°C, the recovery of mechanical properties is insufficient. On the other hand, if the heat treatment temperature exceeds 600°C, the growth of the nitride layer and the Cr-depleted layer is too large. In order to remove these by subsequent electrolytic treatment For the nitrided layer and the Cr-depleted layer, the acid concentration of the electrolytic treatment solution must be increased, and the electricity consumption during electrolysis must be increased. If such electrolytic treatment is performed, the surface skin of the steel sheet will be greatly changed. Therefore, when performing the heat treatment A, it is preferable to limit the holding time in the above temperature range to a range of 30 seconds to 10 minutes.

In addition, for the purpose of recrystallization or inversion, a heat treatment (hereinafter, this heat treatment is also referred to as heat treatment B) at a temperature in the range of 150 to 700° C. for 15 minutes to 48 hours may be used instead of the above-mentioned heat treatment. . If the temperature of the heat treatment is lower than 150°C, the recrystallization will be insufficient. On the other hand, if the temperature of the heat treatment exceeds 700°C, the Cr-depleted layer will grow significantly. Therefore, even if the electrolytic treatment with dilute nitric acid is performed later, it cannot be ensured. The expected pitting corrosion occurrence potential. Therefore, it is preferable to set the heat treatment temperature to a temperature in the range of 150 to 700°C. In addition, if the holding time in the above-mentioned temperature range is less than 15 minutes, the recrystallization will be insufficient. On the other hand, if the holding time is too long exceeding 48 hours, the Cr-depleted layer will grow significantly. Therefore, when performing the heat treatment B, it is preferable to limit the holding time in the above temperature range to a range of 15 minutes to 48 hours.

In addition, in the present invention, the gas atmosphere when performing the annealing treatment is not particularly limited, and the annealing may be performed in, for example, a passive gas atmosphere or a gas atmosphere containing a combustible gas, oxygen, etc., in addition to the atmospheric atmosphere deal with. In addition, annealing treatment called bright annealing (sometimes also called BA annealing) may be performed in a reducing gas atmosphere containing hydrogen.

In the present invention, after the above-mentioned heat treatment, dilute nitric acid electrolytic treatment is performed as the final step.

The method of dilute nitric acid electrolysis treatment is to carry out cathodic electrolysis and anodic electrolysis at a current density of ±10 to 80 mA/cm ² in a dilute nitric acid aqueous solution with a nitric acid concentration of 3 to 10% and a temperature of 40 to 80 ° C. The electrolytic treatment time is preferably 10 to 60 seconds.

If the nitric acid concentration is less than 3%, the effect of the dilute nitric acid electrolytic treatment is insufficient. On the other hand, if the nitric acid concentration exceeds 10%, the dissolution of the surface layer of the steel sheet tends to be remarkable, and the accuracy of the thickness of the steel sheet will be deteriorated. Therefore, the nitric acid concentration of the dilute nitric acid aqueous solution is limited to 3 to 10%. In addition, under the same current density and the same electrolysis time, when the nitric acid concentration is in the range of 3 to 10%, the dissolved amount of the steel sheet surface layer changes little, and the surface roughness almost does not change, but with the nitric acid As the concentration increases, the solid-state coating formed on the surface layer becomes firm, and the pitting corrosion potential will rise.

In addition, if the temperature of the dilute nitric acid aqueous solution is lower than 40°C, the effect of the dilute nitric acid electrolysis is insufficient even if the heat treatment conditions in the present invention are combined with the dilute nitric acid electrolysis. On the other hand, the temperature of the dilute nitric acid aqueous solution exceeds 80°C In this case, the dissolution of the surface layer of the steel sheet tends to be significant. Therefore, the temperature of the dilute nitric acid aqueous solution is limited to a range of 40 to 80°C. In addition, if the current density is lower than 10 mA/cm ² , the effect of dilute nitric acid electrolysis is insufficient, and on the other hand, if the current density is too large and exceeds 80 mA/cm ² , the dissolved amount of the surface layer becomes too large. Therefore, the current density is limited to the range of 10 to 80 mA/cm ² . In addition, when the total electrolysis time is less than 10 seconds, the effect of dilute nitric acid electrolysis is insufficient, and on the other hand, when the total time exceeds 60 seconds, the dissolved amount of the surface layer becomes too large. Therefore, the total time of the cathodic electrolysis and the anodic electrolysis of the electrolysis time is limited to the range of 10 to 60 seconds. In addition, when carrying out the dilute nitric acid electrolysis treatment, from the viewpoint of removing the surface layer, either cathodic electrolysis may be performed first, and then anodic electrolysis may be performed, or cathodic electrolysis and anodic electrolysis may be performed repeatedly, and the effects of the two methods are the same.

Moreover, if it is the said dilute nitric acid electrolysis process, the surface skin which has a glossy feeling can be obtained. In this case, the surface roughness Sa is 0.80 μm or less. If the surface roughness Sa is too coarse and exceeds 0.80 μm, it will not be possible to obtain glossy surface skin. In the present invention, the surface thickness Sa of the steel sheet is set to 0.80 μm or less. In addition, the surface roughness is regarded as the surface roughness by using the arithmetic mean height Sa after measuring according to the method prescribed|regulated by Japanese Industrial Standard ISO 25178.

Surface roughness is an important item for products as an index of glossiness, and also strongly affects corrosion resistance. When the surface roughness Sa exceeds 0.80 μm, the corrosion resistance tends to become unstable. In addition, from the viewpoint of stabilizing corrosion resistance, it is preferable to set the surface roughness Sa to 0.40 μm or less. The more preferable surface roughness Sa is 0.35 μm or less, and the more preferable surface roughness Sa is 0.30 μm or less. Furthermore, when it is required to have particularly stable corrosion resistance, it is effective to set the surface roughness Sa to 0.25 μm or less, more preferably to set the surface roughness Sa to 0.20 μm or less, and more preferably to set the surface roughness Sa to 0.25 μm or less. The surface roughness Sa is set to 0.15 μm or less.

After performing the above-mentioned heat treatment, as the final step, dilute nitric acid electrolytic treatment is performed, as shown in an example in FIG. , the pitting corrosion occurrence potential Vc (black circles ●) after the electrolytic treatment of dilute nitric acid is further improved, which can further improve the pitting corrosion resistance.

The reason for this can be considered to be due to the following phenomenon.

When a cold-rolled steel sheet is subjected to heat treatment, the Cr element diffuses toward the surface of the steel sheet, and a part of it becomes a gaseous component and evaporates from the surface into the heat treatment furnace. On the other hand, in the outermost layer of the steel sheet, a nitride layer and an oxide layer (coating film) are formed during the heat treatment. These layers are all removed by electrolytic treatment with dilute nitric acid, so that a Cr-concentrated layer is present on the surface and the pitting corrosion resistance can be improved.

Cr has a strong affinity for gas components such as O (oxygen) and N (nitrogen). Therefore, it is considered that Cr is concentrated in the vicinity of the surface of the steel sheet by contacting with the atmospheric gas during the heat treatment. It is also considered that the concentrated Cr combines with O, N, and C that have entered from the atmosphere or O, N, and C that are originally present in the steel to form Cr precipitates. Once Cr precipitates are formed, the amount of Cr originally dissolved in the parent phase (solid solution Cr amount) decreases. The effect of Cr on improving corrosion resistance depends on the amount of solid solution Cr in the steel sheet, so the reduction in the amount of solid solution Cr is thought to lead to deterioration of the corrosion resistance of the steel sheet itself. In addition, once the Cr precipitates are formed, Cr diffuses to the surface layer, so that a Cr-depleted layer is formed in the inner side.

For example, if the heating is performed at more than 950°C in normal annealing (heat treatment), the thickness of the above-mentioned Cr-depleted layer will become thick, and the corrosion resistance near the surface of the steel sheet will be deteriorated. On the other hand, when annealing (heat treatment) is performed at a relatively low temperature of 950°C or lower, the amount of Cr depletion is smaller than that of normal annealing (heat treatment) heated to over 950°C. There is also less damage. It is conceivable to increase the effective Cr amount (solid solution Cr amount) by forming a C-depleted layer near the surface to suppress precipitation of Cr carbides and the like inside the outermost surface of the steel sheet where Cr precipitates have already occurred. Then, it can be considered that the layer containing Cr precipitates formed in the outermost layer is removed by electrolytic treatment with dilute nitric acid. In this way, the Cr precipitates existing in the inner side are reduced, and the effective Cr can be increased. The portion with excellent corrosion resistance of the amount (solid solution Cr amount) is exposed, thereby improving the corrosion resistance of the surface of the steel sheet. In particular, the annealing (heat treatment) performed at a low temperature of 700° C. or lower (150° C. or higher) in the present invention is considered to be more effective than the case of performing the annealing (heat treatment) at a high temperature range exceeding 700° C. The thickness of the Cr-depleted layer can be made thinner, the Cr-depleted amount can be reduced, and the precipitation of Cr carbides can also be reduced, so that the effective Cr amount can be further increased. Therefore, it is considered that, compared with the steel sheet having the same composition range, the steel sheet after heat treatment in the temperature range of 150°C or higher and 700°C or lower is higher than the steel sheet after heat treatment in the temperature range exceeding 700°C. For steel sheets, the pitting corrosion occurrence potential after dilute nitric acid electrolysis treatment becomes higher, which can significantly improve the pitting corrosion resistance.

Cr forms a Cr oxide layer on the outermost layer of the steel sheet, and near the surface, Cr combines with O (oxygen), C (carbon), etc. to form fine Cr oxides, Cr carbides, etc., which are deposited just below the surface. side of the steel plate. Due to the precipitation of Cr precipitates, the effective Cr amount (solid solution Cr amount) in this part is decreased, and the corrosion resistance is deteriorated. In addition, it is considered that a C-depleted layer in which the C concentration is reduced will be formed in the vicinity of the portion where the Cr carbides are formed.

For example, in the case of normal annealing (heat treatment) heated at more than 950°C, the decarburization phenomenon of the outermost layer of the steel sheet, the formation of a Cr oxide layer, and the formation of Cr precipitates in the vicinity of the surface are obvious. From the viewpoint of increasing the effective Cr content by reducing the C content due to decarburization or the like, it can be considered to contribute to the improvement of corrosion resistance, but the deCr layer derived from the formation of the Cr oxide layer The formation of Cr and the formation of Cr precipitates near the surface will lead to a decrease in the amount of effective Cr near the surface, resulting in poor corrosion resistance.

On the other hand, when heat treatment is performed at a low temperature of 950° C. or lower, it is considered that the same phenomenon occurs although the speed is slow. However, since the heat treatment is performed at a low temperature, it is considered that the diffusion rate of Cr is slow, and the generation of Cr oxides is small, and the influence on the deterioration of corrosion resistance due to the formation of the de-Cr layer is relatively small. few. On the other hand, it is considered that since Cr carbides are formed near the surface, the C concentration in the parent phase near the surface decreases. However, because the diffusion rate of C is not fast enough to supply the C needed to make up for the decrease in the C concentration, a C-depleted layer (C-depleted layer) after the C decrease is formed. Because of the formation of the C-depleted layer, the effective Cr content of this portion increases. Therefore, it can be considered that if the layer containing Cr precipitates formed on the outermost layer is removed by electrolytic treatment with dilute nitric acid, the C-depleted layer with improved corrosion resistance can be exposed on the surface, and as a result, the resistance of the steel sheet can be improved. Corrosive.

In particular, annealing (heat treatment) at a low temperature of 700° C. or lower (150° C. or higher) in the present invention is considered to be the reason why Cr is annealed (heat-treated) at a high temperature range exceeding 700° C. Since the diffusion rate is slower and the amount of Cr oxides produced is smaller, the pitting corrosion resistance (corrosion resistance) deterioration caused by the formation of the Cr-removed layer is also smaller. In addition, annealing (heat treatment) at a low temperature of 700° C. or lower (150° C. or higher) is considered to cause the diffusion of C to be slower than when annealing (heat treatment) is performed at a high temperature range exceeding 700° C. The formation of the C-depleted layer is also less, and the effective Cr content is correspondingly increased. Therefore, when the heat treatment is performed at a low temperature of 700°C or lower (150°C or higher), it is considered that the pitting corrosion after the dilute nitric acid electrolytic treatment undergoes a potential change compared with the case where the heat treatment is performed at a high temperature range exceeding 700°C. The higher the value, the greater the effect of improving the corrosion resistance.

As described above, C existing near the surface combines with Cr existing near the surface to precipitate in the form of Cr carbides, so the amount of C near the surface decreases and the effective Cr amount increases accordingly. It is considered that the amount of evaporation due to the reaction with the gas component in the gas atmosphere is greater in the steel sheet with a higher C content than in the steel sheet with a lower C content. Therefore, in the steel sheet with a high C content, the change (increase) of the effective Cr amount due to the reduction of the C amount due to the heat treatment is large. Based on the above phenomenon, in the present invention, it is considered that a steel sheet with a relatively high C content has a significant effect of improving corrosion resistance by performing electrolytic treatment with dilute nitric acid after heat treatment.

In addition, Mo also becomes a solid solution state similarly to Cr, and contributes to the improvement of the corrosion resistance (pitting corrosion resistance) of a steel plate. In other words, the corrosion resistance (pitting corrosion resistance) of the steel sheet can be improved as the effective Mo amount (solid solution Mo amount) is increased. In addition, like Cr, Mo is also easily combined with C. Therefore, during the heat treatment process, if C vaporizes in the steel near the surface, the amount of C will decrease, so the effect near the surface is effective. The amount of Mo will increase. This increase in the effective Mo content is that as the Mo content increases, the effective Mo content increases more. Therefore, it is considered that the higher the Mo content, the greater the effect of improving the pitting corrosion resistance.

In addition, when the Mo content is in the range of 3.0 to 7.0%, the effect of increasing the Mo content gradually decreases. Therefore, the improvement of the pitting corrosion resistance per unit of the Mo content in the range of the Mo content exceeding 3.0% The effect of resistance is less than the effect of improving the pitting corrosion resistance per unit of the Mo content in the range of 3.0% or less of the Mo content.

For the above reasons, it is considered that the heat treatment performed at a low temperature of 700° C. or lower (150° C. or higher) of the present invention has a Cr-depleted layer compared to the heat treatment performed at a high temperature range exceeding 700° C. becomes less and the effective Cr content also increases, therefore, heat treatment at a low temperature of 700°C or lower is performed. Especially for steel sheets with a Mo content as high as 3.0%, the pitting corrosion occurrence potential after electrolysis with dilute nitric acid is higher than In the case of heat treatment performed in a high temperature range of 700°C.

In addition, after performing the above-mentioned dilute nitric acid electrolytic treatment, a post-heat treatment at 150° C. or lower is performed in an oxygen-enriched atmosphere, so that a sound solid-state coating with excellent corrosion resistance can be formed. , which can improve corrosion resistance and pitting corrosion resistance. In addition, by immersing the steel sheet in a nitric acid solution, the formation of a solid coating film can be promoted. For the purpose of promoting the formation and growth of the solid coating film, it is also effective to immerse the steel sheet in an oxidizing acid solution.

In addition, in order to form a sound solid-state coating, it is necessary to improve the corrosion resistance of the parent phase in advance. An effective method is to suppress the precipitation of carbides and increase the Cr that effectively acts on the corrosion resistance. The method of content; the method of removing the rough oxide coating film formed during heat treatment and the method of removing the Cr layer that may be formed directly under the oxide coating film; and then the metal surface of the substrate to be formed solid coating film be kept smooth, etc.

In addition, after the heat treatment, when removing the oxide layer and the surface layer containing the Cr-depleted layer produced during the heat treatment, in addition to the above-mentioned dilute nitric acid electrolytic treatment, for example, electrolytic treatment using alkaline solution; sputtering can also be applied. , mechanical grinding and all other industrial methods of removing the surface layer. In addition, when electrolytic treatment is performed, the electrolytic solution is not limited to dilute nitric acid, and the electrolytic treatment may be performed using non-oxidizing sulfuric acid, hydrochloric acid, or the like.

Hereinafter, the present invention will be further described based on the examples. [Example]

(Example 1) The hot-rolled steel sheet (thickness: 2.5 mm) having the composition shown in Table 1 and subjected to annealing and pickling treatment was subjected to cold rolling three times to prepare a cold-rolled steel sheet with a thickness of 0.1 mm. In addition, after the final cold rolling, the heat treatment A shown in Table 2 was performed for the main purpose of restoring and improving mechanical properties. In addition, after the cold rolling other than the final cold rolling, the heat treatment (heat treatment for softening) shown in Table 2 was carried out, respectively. Some of the steel sheets were not subjected to heat treatment after final cold rolling, but were subjected to heat treatment shown in Table 2 (heat treatment for the purpose of restoring and improving mechanical properties) after cold rolling other than final cold rolling.

Then, the obtained cold-rolled steel sheet was further electrolyzed with dilute nitric acid, and then the pitting corrosion occurrence potential Vc on the surface of each steel sheet was measured using unpolished samples according to the method specified in JIS G 0577. In addition, when measuring the pitting corrosion generation potential, the test solution (aqueous sodium chloride solution) was not subjected to degassing treatment. The control electrode was an Ag/AgCl (silver chloride) electrode. In addition, dilute nitric acid electrolytic treatment was not performed on some of the steel sheets. The conditions of the dilute nitric acid electrolysis treatment are to perform anodic electrolysis and cathodic electrolysis at a current density of ±30 mA/cm ² in a dilute nitric acid aqueous solution with a nitric acid concentration of 3% (the liquid temperature is 60 °C), and the total electrolysis time is 20 seconds. Electrolysis is performed in this order with the steel plate side serving as an anode and a cathode. In addition, about the steel sheet after the electrolytic treatment of dilute nitric acid, the arithmetic mean height Sa was measured according to the method prescribed by Japanese Industrial Standards ISO 25178. The size of the measurement field was 1.0 μm×1.0 μm, and the measurement interval was 25 μm.

The obtained results are shown in Table 2.

In the examples of the present invention, the pitting corrosion occurrence potential Vc conforms to the relationship of the formula (1), and it can be seen that all the stainless steel plates have a high pitting corrosion occurrence potential and have excellent pitting corrosion resistance. On the other hand, in the comparative examples falling outside the scope of the present invention, the pitting corrosion occurrence potential Vc did not satisfy the relationship of the formula (1), and it was found that the pitting corrosion resistance was low. In addition, although the steel plate numbers No.A5 and No.A6 have the same pitting corrosion index X as those of No.A7 and No.A8, the pitting corrosion occurrence potential Vc of the steel plate numbers No.A7 and No.A8 with more C content value is higher. In addition, the pitting corrosion occurrence potential Vc of the steel sheets No. A18 and No. A19 whose pitting corrosion index X falls outside the scope of the present invention is 0 or less, and cannot be used for applications requiring pitting corrosion resistance. (Example 2) The annealed and pickled hot-rolled steel sheet (thickness: 2.5 mm) having the compositions shown in Table 1 was cold-rolled three times, and a cold-rolled steel sheet with a thickness of 0.1 mm was produced in the same manner as in Example 1. In addition, after the final cold rolling, the heat treatment B for recrystallization or inversion shown in Table 3 was carried out, and after the cold rolling other than the final cold rolling, the heat treatment B for the purpose of softening shown in Table 3 was carried out, respectively. heat treatment. Some of the steel sheets were not subjected to heat treatment after the final cold rolling, but were subjected to the heat treatment shown in Table 3 (for the purpose of recrystallization or inversion transformation) after cold rolling other than the final cold rolling. heat treatment).

Then, the obtained cold-rolled steel sheet was further electrolyzed with dilute nitric acid, and then the pitting corrosion occurrence potential Vc on the surface of each steel sheet was measured in the same manner as in Example 1 using an unpolished sample. In addition, when measuring the pitting corrosion generation potential, as in Example 1, the test solution (aqueous sodium chloride solution) was not subjected to deaeration treatment. In addition, dilute nitric acid electrolytic treatment was not performed on some of the steel sheets. The conditions of the dilute nitric acid electrolytic treatment were the same as those in Example 1. The obtained results are shown in Table 3.

In the examples of the present invention, the pitting corrosion occurrence potential Vc conforms to the relationship of the formula (1), and it can be seen that all the stainless steel plates have a high pitting corrosion occurrence potential and have excellent pitting corrosion resistance. On the other hand, in the comparative examples falling outside the scope of the present invention, the pitting corrosion occurrence potential Vc did not satisfy the relationship of the formula (1), and it was found that the pitting corrosion resistance was low. In addition, although the steel plate numbers No.B5, No.B6 and No.B7, No.B8 have the same pitting corrosion index X, the steel plate numbers No.B7 and No.B8 with a large C content have the pitting corrosion occurrence potential Vc value is higher. In addition, the pitting corrosion occurrence potential Vc of the steel sheets No. B18 and No. B19 in which the pitting corrosion index X falls outside the scope of the present invention is 0 or less, which is not suitable for applications requiring pitting corrosion resistance. (Example 3) With respect to the hot-rolled steel sheet (thickness: 2.5 mm) having the composition of Steel No. D shown in Table 1, cold-rolling under the conditions shown in Table 4 was performed twice to prepare a cold-rolled steel sheet (thickness: 0.1 mm). mm). Between the first and second cold rolling, heat treatment for softening was performed (1050° C.×5 minutes and 1000° C.×2 minutes, respectively). After the final cold rolling, heat treatment A (500° C.×2 minutes) was performed for the purpose of recovering mechanical properties, and dilute nitric acid electrolytic treatment was performed under the conditions shown in Table 4.

Then, in the same manner as in Example 1, the pitting corrosion occurrence potential Vc on the surface of each steel sheet was measured. In addition, the surface roughness of the steel sheet was measured in accordance with the method prescribed by Japanese Industrial Standards ISO 25178, and the surface roughness (arithmetic mean height) Sa was measured.

The obtained results are shown in Table 4.

In the examples of the present invention, the pitting corrosion occurrence potential Vc conforms to the relationship of the formula (1), and it can be seen that all the stainless steel plates have a high pitting corrosion occurrence potential and have excellent pitting corrosion resistance. In addition, all of the examples of the present invention exhibited excellent surface properties in which the surface roughness Sa was 0.80 μm or less. On the other hand, the pitting corrosion resistance was found to be low in the comparative examples in which the pitting corrosion occurrence potential Vc did not satisfy the relation of the formula (1) and was outside the scope of the present invention. In addition, in the comparative example whose dilute nitric acid electrolytic treatment conditions were low and fell outside the scope of the present invention, it was found that the pitting corrosion occurrence potential Vc did not satisfy the relationship of the formula (1), and the pitting corrosion resistance was low. On the other hand, in the comparative example in which the dilute nitric acid electrolytic treatment conditions are high and fall outside the scope of the present invention, it can be seen that although the pitting corrosion occurrence potential Vc conforms to the relationship of the formula (1), the surface roughness (arithmetic average) height) Sa exceeds 0.80 μm, and a rough surface appears. In addition, in the comparative example in which the temperature of the dilute nitric acid electrolysis treatment was high and fell outside the scope of the present invention, it was found that the pitting corrosion occurrence potential Vc did not satisfy the relationship of the formula (1). In addition, in the example of the present invention in which the surface roughness Sa is 0.40 μm or less, the pitting corrosion potential Vc is stably maintained at 1000 mV or more.

[Fig. 1] is a statistical graph showing the relationship between the pitting corrosion occurrence potential and the pitting corrosion index.

Claims (6)

A Vostian iron-based stainless steel plate, characterized in that its composition, in mass %, contains C: 0.40% or less, Si: 1.00% or less, Mn: 2.00% or less, P: 0.045% or less, S: 0.030 % or less, Ni: 3.5 to 36.0%, Cr: 15.00 to 30.00%, Mo: 0 to 7.0%, and N: 0.25% or less, the rest is Fe and inevitable impurities, and the contents of Cr and Mo are as follows The numerical value of X defined by the formula (2) is 15.0 to 50.0. ), and the pitting corrosion occurrence potential Vc of the surface is in accordance with the following equation (1),

The Vostian iron-based stainless steel sheet according to claim 1, wherein in addition to the above-mentioned components, in terms of mass %, Ti: 0.01-1.00%, Nb: 0.01-1.00%, Cu: 0.01- 3.00%, Al: 0.0001-1.50%, Ca: 0.001-0.01%, Mg: 0.001-0.01%, V: 0.01-1.00%, Co: 0.01-0.5%, W: 0.01-1.0%, and B: 0.001- One or more selected from 0.01%. The Vostian iron-based stainless steel sheet according to claim 1 or claim 2, wherein the surface roughness Sa of the steel sheet is 0.80 μm or less as defined by Japanese Industrial Standard ISO 25178. A method for producing a Vostian iron-based stainless steel sheet, characterized in that: when a hot-rolled steel sheet having the composition described in claim 1 or claim 2 is subjected to one or more times of cold rolling to produce a cold-rolled steel sheet, After the final cold rolling in the cold rolling process, or after the cold rolling other than the final cold rolling in the cold rolling process, it is maintained at a temperature in the range of 150 to 600°C for 30 seconds to 10 seconds. 10 minutes of heat treatment, and finally, electrolytic treatment with dilute nitric acid. A method for producing a Vostian iron-based stainless steel sheet, characterized in that: when a hot-rolled steel sheet having the composition described in claim 1 or claim 2 is subjected to one or more times of cold rolling to produce a cold-rolled steel sheet, After the final cold rolling in the cold rolling process, or after the cold rolling other than the final cold rolling in the cold rolling process, the temperature is kept in the range of 150 to 700°C for 15 minutes to 48 hours. heat treatment, and finally electrolytic treatment with dilute nitric acid. The method for producing a Vostian iron-based stainless steel plate according to claim 4 or claim 5, wherein the dilute nitric acid electrolytic treatment is performed in a dilute nitric acid aqueous solution with a nitric acid concentration of 3 to 10% and a temperature of 40 to 80°C Among them, the cathode and anode electrolysis treatments were performed at a current density of ±10 to 80 mA/cm ² , and the total treatment time was 10 to 60 seconds.

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