WO2022049796A1

WO2022049796A1 - Austenitic stainless steel sheet and method for producing same

Info

Publication number: WO2022049796A1
Application number: PCT/JP2021/000471
Authority: WO
Inventors: 栄司土屋; 雄太松村; 遼介小川; 修平蛭田; 裕樹太田; 悠太児玉; 正太廣瀬; 愛ダイアナ内野; 浩志和田; 邦彦小久保
Original assignee: 株式会社特殊金属エクセル
Priority date: 2020-09-01
Filing date: 2021-01-08
Publication date: 2022-03-10
Also published as: TWI763436B; JP7210780B2; TW202210642A; CN114901851A; JPWO2022049796A1; CN114901851B

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°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°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

Austenitic stainless steel sheet and its manufacturing method

The present invention relates to an austenitic stainless steel sheet suitable for automobile parts, electronic devices, springs, and other industrial parts and a manufacturing method thereof, and relates to improvement of corrosion resistance, particularly pitting corrosion resistance.

As a method for improving the corrosion resistance of a ferritic stainless steel sheet, for example, Patent Document 1 describes "a method for producing a ferritic stainless steel for electrical materials having excellent ductility, wear resistance and rust resistance". In the technique described in Patent Document 1, after finish annealing, cold rolling of 10% or more is performed, reheat treatment is performed, and then nitrate electrolysis in, for example, 10% nitric acid (20 ° C.) is performed on the surface. By removing the scale, it is possible to produce ferritic stainless steel having excellent ductility, wear resistance, and rust resistance without deteriorating corrosion resistance.

Further, Patent Document 2 describes "a method for producing a ferritic stainless steel brightly annealed material having excellent rust resistance". In the technique described in Patent Document 2, a ferritic stainless steel sheet is annealed, descaled using a neutral salt electrolysis method and a nitrate electrolysis method, then cold-rolled, and further annealed by brilliance. Also, it is possible to manufacture ferritic stainless steel brightly annealed material with excellent corrosion resistance. In addition, there is a description that the nitric acid electrolysis method was alternately electrolyzed in 15% nitric acid (50 ° C).

Further, Patent Document 3 describes "a method for manufacturing stainless steel having good corrosion resistance". In the technique described in Patent Document 3, an acidic aqueous solution containing an oxidizing agent is used for stainless steel containing Cr: 11 wt% or more and 35 wt% or less, O: 0.01 wt% or less, and S: 0.01 wt% or less. It is said that stainless steel with improved corrosion resistance can be obtained by mechanically polishing using the above as a polishing liquid. In the technique described in Patent Document 3, lapping polishing and belt polishing are used as mechanical polishing.

On the other hand, austenitic stainless steel sheets usually have improved mechanical properties by cold rolling after heat treatment, and have some degree of pitting corrosion resistance. However, pitting corrosion is likely to occur in an environment containing chloride ions, a gap structure, and an environment such as high temperature and high humidity. Therefore, in such an environment, steel grades with increased Cr and Mo (SUS316L, etc.) are often used. However, such steel grades are expensive and cannot be used in all environments in terms of cost.

Generally, when manufacturing an austenitic stainless steel sheet, heat treatment is performed to remove internal stress, solidify, and improve other mechanical properties. However, even if the heat treatment is performed in a reducing atmosphere such as in a mixed gas of nitrogen and hydrogen or in a hydrogen gas atmosphere, oxidation cannot be completely prevented and an oxide film may be formed on the surface layer, which is directly under the surface layer. A Cr-deficient layer may be formed and the corrosion resistance may deteriorate. Therefore, in order to restore the corrosion resistance, conventionally, after heat treatment in a reducing atmosphere, a treatment of immersing in an acidic liquid or electrolytic polishing is performed to restore the corrosion resistance.

Japanese Unexamined Patent Publication No. 04-371518 Japanese Unexamined Patent Publication No. 11-50202 Japanese Unexamined Patent Publication No. 03-193885

Both the techniques described in Patent Document 1 and Patent Document 2 relate to improving the corrosion resistance of ferritic stainless steel sheets, and Patent Documents 1 and 2 do not describe austenitic stainless steel sheets.

The technique described in Patent Document 3 is also applied to austenitic stainless steel plates, but in order to improve corrosion resistance, it is a requirement to perform mechanical polishing such as wrapping polishing using an acidic aqueous solution. In austenitic stainless steel sheets, the mechanical properties may change when the surface layer is polished, and there is a concern that the corrosion resistance may deteriorate due to the polishing of the surface layer. Further, there is a problem that wrapping polishing cannot deal with products that regulate the roughness of the surface layer.

The present invention has been made in view of the problems of the prior art, and an object of the present invention is to provide an austenitic stainless steel sheet having excellent corrosion resistance, particularly pitting corrosion resistance, and a method for producing the same.

The present inventors have diligently studied various factors affecting the pitting corrosion resistance of austenitic stainless steel sheets in order to achieve the above-mentioned object.

As a result, when the hot-rolled steel sheet is cold-rolled to obtain a cold-rolled steel sheet, the final cold-rolled cold-rolled steel sheet is manufactured by cold-rolling once or a plurality of times. After hot rolling under specific conditions or after cold rolling other than the final of the cold rolling, pore erosion occurs on the surface of the steel sheet by subjecting it to dilute nitrate electrolytic treatment under appropriate conditions. It was found that the rolling-rolling resistance is improved even in an environment where the potential becomes high and cannot be dealt with in the past.

First, the experimental results that form the basis of the present invention will be described.

By mass%, Cr: 10.5 to 23.2%, Ni: 0 to 35.1%, Mo: 0 to 7.00%, N: 0.02 to 0.07%, C: 0.01 to 0.10%, Si: 0.34 to 0.67%, Mn: 0.65 to An annealed and pickled hot-rolled steel sheet (plate thickness: 2.5 mm) containing 1.10% was cold-rolled three times to obtain a cold-rolled steel sheet (plate thickness: 0.1 mm). In manufacturing the cold-rolled steel sheet, the heat treatment performed after the final cold rolling or after the second cold rolling when dilute nitrate electrolysis is applied after the third cold rolling is performed at a heating temperature of 145 to 720 ° C. , 20s to 49hr, and heat treatment other than the above was performed at 850 to 1050 ° C and 3 to 5min. Then, the obtained cold-rolled steel sheet was further subjected to dilute nitrate electrolysis treatment, and then the pitting corrosion potential Vc on the surface of each steel sheet was measured in accordance with JIS G0577 without polishing the surface layer. In the measurement of the pitting corrosion potential, the test solution (sodium chloride aqueous solution) was not degassed. The reference electrode was an Ag / AgCl (silver chloride) electrode. In addition, dilute nitric acid electrolysis treatment was not performed on some steel sheets.

The conditions for the dilute nitric acid electrolysis treatment were a dilute nitric acid concentration of 3% (liquid temperature: 60 ° C.), a current density of ± 30 mA / cm ² , and anode / cathode electrolysis for a total of 20 seconds. FIG. 1 shows the relationship between the obtained pitting corrosion potential Vc and the pitting corrosion index X (= Cr + 3.3Mo).

From FIG. 1, when the dilute nitric acid electrolysis treatment was performed in combination with the heat treatment after cold rolling (● mark), compared with the case where only the heat treatment was performed without the dilute nitric acid electrolysis treatment (○ mark). It can be seen that the potential for pitting corrosion increases. That is, the combination of the dilute nitric acid electrolysis treatment and the heat treatment is effective in improving the pitting corrosion resistance. 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 pitting corrosion resistance tends to be.

From this experimental result, the following formula A = 0.039X3 ^-5.2X2 ⁺ 232X-2311 in relation to the pitting corrosion index X (= Cr + 3.3Mo) as the threshold value for the increase in the pitting corrosion potential due to the dilute nitric acid electrolysis treatment.
(Here, X = Cr + 3.3Mo)
Was defined. This equation is an approximate curve created with values near the boundary smaller than the lower limit of those points when plotting the values of the pitting corrosion potential after dilute nitric acid electrolysis while exceeding the pitting potential before dilute nitric acid electrolysis. be. If Mo is not contained, X is calculated assuming that the element is 0%.

Then, when the pitting corrosion potential Vc on the surface of the stainless steel sheet becomes higher than the above A value, it is considered that the pitting corrosion resistance is improved. In the steel sheet having a pitting corrosion index of less than 15.0, no increase in the pitting corrosion potential was observed beyond the above A value even when the dilute nitric acid electrolysis treatment and the heat treatment were combined. Therefore, the range of X is limited to 15.0 to 50.0.

For this reason, when a hot-rolled steel sheet is cold-rolled once or a plurality of times to produce a cold-rolled steel sheet, after the final cold rolling of the cold rolling or the cold rolling. After cold rolling other than the final rolling, heat treatment under specific conditions is performed, and dilute nitric acid electrolytic treatment under appropriate conditions is applied to increase the potential for pitting on the surface of the steel sheet, which is an environment that could not be dealt with in the past. It was found that a stainless steel sheet (stainless cold-rolled steel sheet) having excellent pore corrosion resistance can be used as well. It has been found that there is no problem even if cold rolling is performed before or after the dilute nitric acid electrolysis treatment.

The present invention has been completed with further studies based on such findings. That is, the gist of the present invention is as follows.
[1] By mass%,
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: 3.5-36.0%,
Cr: 15.00 to 30.00%, Mo: 0 to 7.0%,
N: Contains 0.25% or less, and Cr and Mo are as follows (2) Equation X = Cr + 3.3Mo …… (2)
Here, Cr, Mo: content of each element (mass%)
X defined in the above is contained so as to satisfy 15.0 to 50.0, has a composition consisting of the balance Fe and unavoidable impurities, and has a pitting corrosion potential Vc on the surface of the following equation (1) Vc> 0.039X ³ −. 5.2X ² + 232X-2311 …… (1)
Austenitic stainless steel sheet characterized by satisfying.
[2] In addition to the above composition, 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%, B: 0.001 to 0.01%% The austenitic stainless steel sheet according to [1].
[3] The austenitic stainless steel sheet according to [1] or [2], wherein the surface roughness of the steel sheet is 0.80 μm or less in Sa conforming to the regulation of ISO 25178.
[4] In manufacturing a cold-rolled steel sheet by subjecting a hot-rolled steel sheet having the composition according to [1] or [2] to cold rolling once or a plurality of times.
After the final cold rolling of the cold rolling, or after the non-final cold rolling of the cold rolling, heat treatment is performed to maintain the temperature in the range of 150 to 600 ° C. for 30 s to 10 min. Finally, a method for producing an austenitic stainless steel sheet, which comprises subjecting it to a dilute nitrate electrolytic treatment.
[5] In manufacturing a cold-rolled steel sheet by subjecting a hot-rolled steel sheet having the composition according to [1] or [2] to cold rolling once or a plurality of times.
After the final cold rolling of the cold rolling, or after the non-final cold rolling of the cold rolling, heat treatment is performed at a temperature in the range of 150 to 700 ° C. for 15 min to 48 hours. Finally, a method for producing an austenitic stainless steel sheet, which comprises subjecting it to a dilute nitrate electrolytic treatment.
[6] The dilute nitric acid electrolysis treatment is performed in a dilute nitric acid aqueous solution having a nitric acid concentration of 3 to 10% and a temperature of 40 to 80 ° C., a current density of ± 10 to 80 mA / cm ² , and a total of cathode and anode electrolysis. The method for producing an austenitic stainless steel sheet according to [4] or [5], wherein the process is performed for 10 to 60 s.

According to the present invention, the potential for pitting corrosion on the surface is increased, and a stainless steel sheet having excellent pitting corrosion resistance can be obtained, which is extremely effective in industry. Further, according to the present invention, for example, a steel sheet having a low pitting corrosion index can be applied even in a corrosive environment that could not be dealt with in the past, and a steel sheet having a high pitting corrosion index causes pitting corrosion. It also has the effect that the potential exceeds 1000 mV and corrosion resistance comparable to that of nickel-based superalloys such as Hastelloy can be obtained. According to the present invention, the same effect can be obtained not only with austenitic stainless steel sheets but also with precipitation hardening stainless steel sheets and duplex stainless steel sheets specified in JIS G4305.

It is a graph which shows the relationship between the pitting corrosion occurrence potential and the pitting corrosion index.

The present invention is an austenitic stainless steel sheet, in mass%.
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: 3.5-36.0%,
Cr: 15.00 to 30.00%, Mo: 0 to 7.0%,
N: Contains 0.25% or less, contains Cr and Mo so that X = Cr + 3.3Mo satisfies 15.0 to 50.0, and has a composition consisting of the balance Fe and unavoidable impurities. Hereinafter, the mass% related to the composition is simply expressed as%.

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

C: 0.40% or less C is an element that improves mechanical properties such as strength and wear resistance by containing a small amount. In order to obtain such an effect, it is preferably contained in an amount of 0.001% or more. On the other hand, if the content exceeds 0.40%, Cr carbides are likely to be generated at the grain boundaries, which tends to cause intergranular corrosion. Further, if it is contained in excess of 0.40%, the ductility is lowered and the press workability is impaired. Therefore, C was limited to 0.40% or less. It is preferably 0.01 to 0.20%.

Si: 1.00% or less Si is an element that acts as a deoxidizing agent for molten steel and contributes to an increase in strength such as elastic limit and tensile strength. In order to obtain such an effect, it is preferable that Si is contained in an amount of 0.10% or more. On the other hand, if it is contained in excess of 1.00%, ear cracks occur during hot rolling and the product yield is lowered. Therefore, Si was limited to 1.00% or less.

Mn: 2.00% or less Mn is an element that contributes to increasing strength such as tensile strength and improving toughness, and also effectively acts on deoxidation of molten steel. In order to obtain such an effect, it is preferably contained in an amount of 0.10% or more. On the other hand, if the content exceeds 2.00%, inclusions such as MnS increase in the steel and adversely affect workability, so Mn 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 the mechanical properties. Therefore, it is desirable to reduce P and S as much as possible, but if P is contained up to 0.045% and S is contained up to 0.030%, there is no practical problem and it is acceptable. Therefore, it was limited to P: 0.045% or less and S: 0.030% or less. It should be noted that P: 0.030% or less and S: 0.010% or less are preferable.

Ni: 3.5-36.0%
Ni is an element that contributes to the improvement of corrosion resistance, toughness, strength, and heat resistance. In order to obtain such an effect, the content of 3.5% or more is required. If the content is less than 3.5%, the structure at room temperature becomes a ferrite phase. On the other hand, if it is contained in excess of 36.0%, the workability is lowered and the weldability is also lowered. Therefore, Ni was limited to the range of 3.5 to 36.0%.

Cr: 15.00-30.00%
Cr, together with Ni, contributes to the improvement of corrosion resistance, and together with Ni, makes the structure at room temperature an austenite phase. In order to obtain such an effect, Cr must be contained in an amount of 15.00% or more. On the other hand, if it is contained in excess of 30.00%, the ductility is lowered and the material cost is increased. Therefore, Cr was limited to the range of 15.00 to 30.00%. It is preferably 16.00 to 30.00%.

Mo: 0-7.0%
Mo is an element that contributes to the improvement of pitting corrosion resistance and also to the improvement of mechanical properties, and contains 0%, and can be contained as needed. When it is contained in order to obtain such an effect, it is preferably contained in an amount of 0.001% or more. If the Mo content is less than 0.001%, the mechanical properties will be slightly reduced. On the other hand, if the content exceeds 7.0%, the precipitation of the σ phase is promoted and the toughness is lowered during the heat treatment. In addition, the inclusion of a large amount causes an increase in material cost. Therefore, when it is contained, Mo is limited to 7.0% or less. It is preferably 0.5 to 3.0%.

N: 0.25% or less N is an element that stabilizes the austenite phase and at the same time, dissolves in an intrusive form and contributes to an increase in strength by strengthening the solid solution. In order to obtain such an effect, it is preferably contained in an amount of 0.01% or more. On the other hand, if it is contained in excess of 0.25%, it has adverse effects such as promotion of high temperature cracking, deterioration of secondary workability, and promotion of intergranular corrosion. Therefore, N was limited to 0.25% or less. It is preferably 0.20% or less, and more preferably 0.01 to 0.10%.

X: 15.0-50.0
Next (2) Equation X = Cr + 3.3Mo …… (2)
Here, Cr, Mo: content of each element (mass%)
When the pitting corrosion index X defined in is less than 15.0, no increase in the pitting corrosion potential is observed even when the dilute nitric acid electrolysis treatment and the heat treatment after cold rolling are combined. If Mo is not contained, Mo is treated as 0% in the calculation of Eq. X in (2). On the other hand, when X exceeds 50.0, the amount of alloying elements becomes too large, the ductility decreases, and the material cost rises. Therefore, it contains the above-mentioned Cr and Mo, and X is limited to the range of 15.0 to 50.0.

The above-mentioned components are the basic components, but in the present invention, in addition to the above-mentioned basic components, as a selective element, Ti: 0.01 to 1.00%, Nb: 0.01 to 1.00%, Cu: as required. 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%, B: 0.001 It may contain one or more selected from ~ 0.01%.

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%, B: One or more selected from 0.001 to 0.01% Ti, Nb, Cu, Al, Ca, Mg, V, Co, W, B Are elements that contribute to increasing the strength and corrosion resistance of steel sheets by dispersing them as fine precipitates in steel, and B is effective in improving high temperature characteristics, so select as necessary. It can contain one kind or two or more kinds. In order to obtain such an effect, 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% As mentioned above, it is necessary to contain Co: 0.01% or more, W: 0.01% or more, and B: 0.001% or more, respectively. On the other hand, Ti: 1.00%, Nb: 1.00%, Cu: 3.00%, Al: 1.50%, Ca: 0.01%, Mg: 0.01%, V: 1.00%, Co: 0.5%, W: 1.0%, B: 0.01 If it is contained in excess of%, the amount of precipitates produced increases, which tends to cause a decrease in corrosion resistance and a decrease in elongation. Therefore, when it is contained, 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%, B: 0.001 to 0.01%, respectively.

The rest other than the above components consist of Fe and unavoidable impurities.

O (oxygen) is inevitably contained and exists as an oxide in steel, which adversely affects the ductility, toughness, etc. Therefore, it is preferable to reduce O (oxygen) as an impurity as much as possible, but up to 0.010% is acceptable. It is preferable that O (oxygen) is 0.001% or more because an excessive reduction of less than 0.001% raises the refining cost.

The austenitic stainless steel sheet of the present invention has the above-mentioned composition and has the following formula (1) Vc> ^{0.039X3-5.2X2} ⁺ 232X-2311 …… (1).
Here, X = Cr + 3.3Mo …… (2)
X: 15.0-50.0,
Cr, Mo: Content of each element (mass%)
It has a pitting corrosion potential Vc on the surface that satisfies the above. If the measured pitting corrosion potential Vc on the surface of the steel sheet is low and the equation (1) is not satisfied, the desired pitting corrosion resistance cannot be ensured. The pitting corrosion potential Vc on the surface of the steel sheet shall be a value measured in accordance with JIS G 0577 using a sample without polishing the surface layer. When measuring the pitting corrosion potential, the test solution (sodium chloride aqueous solution) shall not be degassed. The reference electrode is an Ag / AgCl (silver chloride) electrode.

Next, a preferable manufacturing method of the austenitic stainless steel sheet of the present invention will be described.

In the present invention, a hot-rolled steel sheet having the above-mentioned composition and having been annealed and pickled is subjected to cold rolling once or a plurality of times to obtain a cold-rolled steel sheet having a predetermined plate thickness. At that time, in the present invention, the heat treatment is performed after the final cold rolling in the plurality of cold rollings or after the cold rolling other than the final in the plurality of cold rollings.

The above-mentioned heat treatment is preferably a heat treatment (hereinafter, also referred to as heat treatment A) in which the temperature is maintained in the range of 150 to 600 ° C. for 30 s to 10 min for the purpose of recovering and improving the mechanical properties. If the heat treatment temperature is less than 150 ° C, the recovery of mechanical properties is insufficient, while if it exceeds 600 ° C, the growth of the nitrided layer and the Cr-deficient layer is large, and in order to remove them in the subsequent electrolytic treatment, electrolysis is performed. It is necessary to increase the acid concentration of the treatment liquid and increase the amount of electricity. When such an electrolytic treatment is performed, the surface skin is significantly changed. Therefore, in the heat treatment A, the holding time in the above temperature range is preferably limited to the range of 30 s to 10 min.

Further, instead of the above-mentioned heat treatment, a heat treatment (hereinafter, also referred to as heat treatment B) in which the temperature is maintained in the range of 150 to 700 ° C. for 15 min to 48 hours may be used for the purpose of recrystallization or reverse transformation. If the heat treatment temperature is less than 150 ° C, recrystallization is insufficient, while if it exceeds 700 ° C, the Cr-deficient layer grows large. Cannot be secured. Therefore, the heat treatment temperature is preferably limited to a temperature in the range of 150 to 700 ° C. Further, if the holding time in the above temperature range is less than 15 min, recrystallization is insufficient, while if it is longer than 48 hr, the Cr-deficient layer grows significantly. Therefore, in the heat treatment B, the holding time in the above temperature range is preferably limited to the range of 15 min to 48 hr.

The burning atmosphere is not particularly limited in the present invention, and may be performed in an atmosphere containing, for example, an inert gas atmosphere, combustion gas, oxygen, etc., in addition to the atmospheric atmosphere. Further, the annealing may be performed by bright annealing (sometimes referred to as BA annealing) in a reducing atmosphere containing hydrogen.

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

For dilute nitric acid electrolysis treatment, in a dilute nitric acid aqueous solution with a nitric acid concentration of 3 to 10% and a temperature of 40 to 80 ° C, a current density of ± 10 to 80 mA / cm ² and a total of 10 to 60 s for cathode and anode electrolysis. It is preferable to carry out the treatment.

If the nitric acid concentration is less than 3%, the effect of the dilute nitric acid electrolysis treatment is insufficient, while if it exceeds 10%, the surface layer of the steel sheet is significantly melted and the plate thickness accuracy is lowered. Therefore, the nitric acid concentration of the dilute nitric acid aqueous solution was limited to 3 to 10%. In the case of the same current density and the same electrolysis time, when the nitric acid concentration is in the range of 3 to 10%, there is little change in the dissolution amount and there is almost no change in the surface roughness, but it is formed on the surface layer as the nitric acid concentration increases. The passivation film is strengthened and the pitting potential rises.

Further, when the temperature of the dilute nitric acid aqueous solution is less than 40 ° C., the effect of dilute nitric acid electrolysis is insufficient under the combination of the heat treatment conditions and the dilute nitric acid electrolysis in the present invention, while when the temperature exceeds 80 ° C., the melting of the surface layer of the steel sheet becomes remarkable. .. Therefore, the temperature of the dilute nitric acid aqueous solution was limited to the range of 40 to 80 ° C. Further, when the current density is less than 10 mA / cm ² , the effect of dilute nitric acid electrolysis is insufficient, while when it is larger than 80 mA / cm ² , the dissolution of the surface layer becomes too large. Therefore, the current density was limited to the range of 10 to 80 mA / cm ² . Further, if the total electrolysis time is less than 10 s, the effect of dilute nitric acid electrolysis is insufficient, while if it is longer than 60 s, the amount of dissolution becomes too large. Therefore, the electrolysis time was limited to the range of 10 to 60 s in total for cathode and anode electrolysis. In the dilute nitric acid electrolysis treatment, from the viewpoint of removing the surface layer, the order of cathode electrolysis and anodic electrolysis may be changed, and the effect is the same even if cathode electrolysis and anodic electrolysis are repeated.

Further, with the above-mentioned dilute nitric acid electrolytic treatment, a glossy surface skin can be obtained. In that case, the surface roughness Sa is 0.80 μm or less. If the surface roughness Sa exceeds 0.80 μm and becomes rough, it is not possible to obtain a glossy surface surface. In the present invention, the surface roughness is 0.80 μm or less in Sa. For the surface roughness, the arithmetic mean height Sa measured in accordance with the regulations of ISO 25178 shall be used.

Surface roughness is important for products as an index of glossiness, but it also strongly affects corrosion resistance. If the surface roughness exceeds 0.80 μm in Sa, the corrosion resistance tends to become unstable. From the viewpoint of stabilizing corrosion resistance, it is preferable that the surface roughness is 0.40 μm or less in terms of Sa. More preferably, Sa is 0.35 μm or less, and further preferably Sa is 0.30 μm or less. Further, when stable corrosion resistance is particularly required, it is effective to set the surface roughness to 0.25 μm or less for Sa, preferably 0.20 μm or less for Sa, and more preferably 0.15 μm or less for Sa. be.

After the above heat treatment, the dilute nitric acid electrolysis treatment is performed as the final step, and as shown in FIG. 1, after the dilute nitric acid electrolysis treatment as compared with the case before the dilute nitric acid electrolysis treatment (marked with ◯). The pitting corrosion potential Vc (marked with ●) is improved, and the pitting corrosion resistance is further improved.

It is thought that this is because the following phenomena are occurring.

When the cold-rolled steel sheet is heat-treated, Cr diffuses toward the surface of the steel sheet, and part of it evaporates from the surface into the furnace as a gas component. It is formed. On the other hand, a nitrided layer and an oxide layer (coating) are formed on the outermost layer during the heat treatment. By removing these layers by dilute nitric acid electrolysis treatment, a Cr-concentrated layer appears and pitting corrosion resistance is improved.

Cr has a strong affinity for gas components such as O (oxygen) and N. Therefore, it is considered that Cr is concentrated in the vicinity of the surface in contact with the atmospheric gas during the heat treatment. It is considered that the concentrated Cr binds to O, N, C invading from the atmosphere or O, N, C existing in the steel to form a Cr precipitate. When Cr precipitates are formed, the amount of Cr dissolved in the matrix (solid solution Cr amount) decreases. Since the improvement in corrosion resistance due to Cr is derived from the amount of solid solution Cr, it is considered that a decrease in the amount of solid solution Cr leads to a decrease in the corrosion resistance of the steel sheet itself. Further, when a Cr precipitate is formed, Cr diffuses into the surface layer, so that a Cr-deficient layer is formed inside the Cr precipitate.

For example, in normal annealing (heat treatment) of heating to over 950 ° C., the thickness of the Cr-deficient layer described above may increase, and the corrosion resistance near the surface of the steel sheet may decrease. On the other hand, in annealing at a low temperature of 950 ° C. or lower (heat treatment), the amount of Cr deficiency is smaller and the corrosion resistance is less likely to be impaired as compared with normal annealing (heat treatment) in which heating is performed above 950 ° C. Rather, it is considered that the formation of the C-deficient layer in the vicinity of the surface suppresses the precipitation of Cr carbides and the like inside the outermost surface where the precipitation occurs, and the effective Cr amount (solid solution Cr amount) increases. Then, when the layer containing the Cr precipitates formed on the outermost layer is removed by dilute nitrate electrolysis treatment, the amount of Cr precipitates existing inside the layer is small, and the effective Cr amount (solid solution Cr amount) is increased, which is excellent in corrosion resistance. Is exposed, and it is considered that the corrosion resistance of the surface of the steel sheet is improved. In particular, in the annealing (heat treatment) at a low temperature of 700 ° C. or lower (150 ° C. or higher) as in the present invention, the thickness of the Cr-deficient layer becomes even thinner than in the high-temperature region of 700 ° C. or higher, and Cr deficiency occurs. Since the amount is small and the precipitation of Cr carbide is also small, it is considered that the effective Cr amount is further increased. Therefore, when comparing steel sheets with the same composition range, the steel sheet heat-treated in the temperature range of 150 ° C or higher and 700 ° C or lower is more dilute nitric acid electrolyzed than the steel sheet heat-treated in the temperature range of 700 ° C or higher. It is considered that the potential for pitting corrosion after the treatment becomes higher and the pitting corrosion resistance is significantly improved.

Cr forms a Cr oxide layer on the outermost layer of the steel sheet, and in the vicinity of the surface, Cr binds to O (oxygen), C, etc., and precipitates as fine Cr oxides, Cr carbides, etc. on the steel sheet side directly below the surface. When the Cr precipitate is deposited, the effective Cr amount (solid solution Cr amount) at that portion is reduced, and the corrosion resistance is lowered. Further, it is considered that a C-deficient layer in which the C concentration is reduced is formed in the vicinity of the portion where the Cr carbide is formed.

For example, in the normal annealing (heat treatment) of heating to over 950 ° C., the decarburization of the outermost surface layer of the steel sheet, the formation of the Cr oxide layer, and the formation of Cr precipitates in the vicinity of the surface become remarkable. A decrease in C due to decarburization or the like is considered to have an advantageous effect on improving corrosion resistance from the viewpoint of increasing the amount of effective Cr, but the formation of a deCr layer accompanying the formation of a Cr oxide layer and Cr in the vicinity of the surface The formation of precipitates reduces the amount of effective Cr in the vicinity of the surface and reduces corrosion resistance.

On the other hand, in the heat treatment at a low temperature of 950 ° C or less, the speed is slow, but it is considered that the same phenomenon occurs. However, since the heat treatment is performed at a low temperature, the diffusion rate of Cr is slow, the formation of Cr oxide is small, and it is considered that the influence of the deterioration of corrosion resistance due to the formation of the deCr oxide layer is small. On the other hand, the formation of Cr carbide near the surface reduces the C concentration in the matrix in the vicinity. However, since the diffusion rate of C is not sufficiently fast, it is conceivable that the diffusion supply of C required to supplement it is not in time, and a C-depleted layer (C-deficient layer) in which C is reduced is formed. The formation of the C-deficient layer increases the effective Cr content in that region. It is considered that if the layer containing the Cr precipitate formed on the outermost surface is removed by the dilute nitric acid electrolysis treatment, the C-deficient layer having improved corrosion resistance is exposed on the surface, and as a result, the corrosion resistance of the steel sheet is improved.

In particular, in the annealing (heat treatment) at a low temperature of 700 ° C. or lower (150 ° C. or higher) as in the present invention, the diffusion rate of Cr is further slower than in the high temperature region of 700 ° C. or higher, and the Cr oxide It is considered that the decrease in pitting corrosion resistance (corrosion resistance) due to the formation of the deCr layer is reduced because the formation is smaller. Furthermore, in annealing (heat treatment) at a low temperature of 700 ° C or lower (150 ° C or higher), it is considered that the diffusion of C is slower than in the high temperature region of 700 ° C or higher, and the formation of the C-deficient layer is small. Therefore, it is considered that the effective Cr amount will increase accordingly. Therefore, when heat treatment is performed at a low temperature of 700 ° C or lower (150 ° C or higher), the potential for pitting corrosion after dilute nitric acid electrolysis treatment is higher than when heat treatment is performed in a high temperature range of 700 ° C or higher. It is considered that the temperature increases and the effect of improving corrosion resistance increases.

As described above, C existing near the surface binds to Cr existing near the surface and precipitates as Cr carbide, so that the amount of C decreases near the surface and the effective Cr amount increases accordingly. It is considered that the evaporation accompanying the reaction with the gas component in the atmosphere is larger in the steel sheet having a high C content than in the steel sheet having a low C content. For this reason, the change (increase) in the effective Cr amount with the decrease in the C amount due to the heat treatment is larger in the steel sheet having a higher C amount. Therefore, in the present invention, it is considered that the effect of improving the corrosion resistance by the dilute nitric acid electrolysis treatment after the heat treatment becomes remarkable in the high C steel sheet.

Similarly to Cr, Mo also contributes to improving the corrosion resistance (pitting corrosion resistance) of the steel sheet by being in a solid solution state. That is, the corrosion resistance (pitting corrosion resistance) of the steel sheet is improved by increasing the effective Mo amount (solid solution Mo amount). In addition, Mo also easily binds to C like Cr, so if gasification of C in steel occurs near the surface during heat treatment, C will decrease and the amount of effective Mo near the surface will increase. Become. Since this increase in the effective Mo content increases as the Mo content increases, it is considered that the steel sheet having a higher Mo content has a greater effect of improving the pitting corrosion resistance.

When Mo is in the range of 3.0 to 7.0%, the effect of increasing Mo gradually decreases. Therefore, in the Mo content range exceeding 3.0%, the resistance per unit Mo amount is higher than in the Mo content range of 3.0% or less. The effect of improving pitting corrosion is reduced.

From the above, in the heat treatment at a low temperature of 700 ° C. or lower (150 ° C. or higher) as in the present invention, the Cr-deficient layer is reduced and the effective Cr amount is increased as compared with the case of the high temperature region of 700 ° C. or higher. For Mo-containing steel sheets that have been heat-treated at a low temperature of ℃ or less, especially Mo-containing steel sheets up to 3.0%, the pitting corrosion potential after dilute nitrate electrolysis will be higher than that of heat treatment in the high temperature range of over 700 ℃. Conceivable.

After the above-mentioned dilute nitric acid electrolysis treatment, a healthy passivation film having excellent corrosion resistance is formed by performing a post-heat treatment at 150 ° C. or lower in an oxygen-enriched atmosphere, and corrosion resistance and pitting corrosion resistance are formed. Eating habits can be improved. Further, the formation of a passivation film can be promoted by immersing in a nitric acid solution. It is also effective to immerse in an oxidizing acid for the purpose of promoting the formation and growth of a passivation film.

In addition, in order to form a sound passivation film, the corrosion resistance of the matrix should be improved, specifically, the precipitation of carbides should be suppressed, the amount of Cr that works effectively for corrosion resistance should be increased, and the film should be formed during heat treatment. It is effective to remove the coarse oxide film and the dechromium layer that may be formed immediately under the oxide film, and to make the metal surface on which the passivation film is formed smooth.

In addition to the above-mentioned dilute nitric acid electrolytic treatment, for the removal of the surface layer including the oxide layer and the Cr-deficient layer formed after the heat treatment, for example, electrolytic treatment using an alkali, sputtering, mechanical polishing, etc. are performed. It is also possible to apply any industrial surface removal method used. Further, in the electrolytic treatment, the electrolytic solution is not limited to dilute nitric acid, and may be a treatment using non-oxidizing sulfuric acid, hydrochloric acid or the like.

Hereinafter, the present invention will be further described based on Examples.

(Example 1)
The annealed and pickled hot-rolled steel sheet (plate thickness: 2.5 mm) having the composition shown in Table 1 was cold-rolled three times to obtain a cold-rolled steel sheet having a plate thickness of 0.1 mm. After the final cold rolling, heat treatment A was performed mainly for the purpose of recovering and improving the mechanical properties shown in Table 2. After the cold rolling other than the final one, the heat treatments shown in Table 2 (heat treatment for the purpose of softening) were performed respectively. Some steel sheets were not heat-treated after the final cold rolling, but were subjected to the heat treatment shown in Table 2 (heat treatment for the purpose of restoring and improving mechanical properties) after the non-final cold rolling.

Then, the obtained cold-rolled steel sheet was further subjected to dilute nitrate electrolysis treatment, and then the pitting corrosion potential Vc on the surface of each steel sheet was measured using a sample not polished, in accordance with the provisions of JIS G 0577. In the measurement of the pitting corrosion potential, the test solution (sodium chloride aqueous solution) was not degassed. The reference electrode was an Ag / AgCl (silver chloride) electrode. In addition, dilute nitric acid electrolysis treatment was not performed on some steel sheets. The conditions for the dilute nitric acid electrolysis treatment were a dilute nitric acid concentration of 3% (liquid temperature: 60 ° C.), a current density of ± 30 mA / cm ² , and anode / cathode electrolysis for a total of 20 seconds. The electrolysis was performed in the order of the anode and the cathode on the steel plate side. In addition, the arithmetic mean height Sa was measured for the steel sheet after the dilute nitric acid electrolysis treatment in accordance with the regulations of ISO 25178. The measurement field of view was 1.0 μm × 1.0 μm, and the measurement interval was 25 μm.

The results obtained are shown in Table 2.

In each of the examples of the present invention, the pitting corrosion potential Vc satisfies the equation (1), and the stainless steel sheet has a high pitting corrosion potential, and it is presumed that the stainless steel sheet has excellent pitting corrosion resistance. On the other hand, in the comparative example outside the present invention, it is presumed that the pitting corrosion generation potential Vc does not satisfy the equation (1) and the pitting corrosion resistance is low. The steel sheets No. A5 and No. A6 and No. A7 and No. A8 have the same pitting corrosion index X, but the pitting corrosion potential Vc is the steel sheets No. A7 and No. A8 having a high C content. Shows a higher value. Further, the steel sheets No. A18 and No. A19 whose pitting corrosion index X is out of the range of the present invention cannot be applied to the use in which the pitting corrosion resistance is required because the pitting corrosion generation potential Vc is 0 or less.
(Example 2)
An annealed and pickled hot-rolled steel sheet (plate thickness: 2.5 mm) having the composition shown in Table 1 is cold-rolled three times, and cold-rolled with a plate thickness of 0.1 mm in the same manner as in Example 1. It was made of steel plate. After the final cold rolling, the heat treatment B for the purpose of recrystallization or reverse transformation shown in Table 3 is performed, and after the cold rolling other than the final, the heat treatment for the purpose of softening shown in Table 3 is performed. provided. Some steel sheets were not heat-treated after the final cold rolling, but were subjected to the heat treatment shown in Table 3 (heat treatment for the purpose of recrystallization or reverse transformation) after the cold rolling other than the final.

Then, the obtained cold-rolled steel sheet was further subjected to dilute nitric acid electrolysis treatment, and then the pitting corrosion potential Vc on the surface of each steel sheet was measured using a sample not polished. In the measurement of the pitting corrosion potential, the test solution (sodium chloride aqueous solution) was not degassed as in Example 1. In addition, dilute nitric acid electrolysis treatment was not performed on some steel sheets. The conditions for the dilute nitric acid electrolysis treatment were the same as in Example 1. The results obtained are shown in Table 3.

In each of the examples of the present invention, the pitting corrosion potential Vc satisfies the equation (1), and the stainless steel sheet has a high pitting corrosion potential, and it is presumed that the stainless steel sheet has excellent pitting corrosion resistance. On the other hand, in the comparative example outside the present invention, it is presumed that the pitting corrosion generation potential Vc does not satisfy the equation (1) and the pitting corrosion resistance is low. The steel sheets No. B5 and No. B6 and No. B7 and No. B8 have the same pitting corrosion index X, but the pitting corrosion potential Vc is the steel sheets No. B7 and No. B8 having a high C content. Shows a higher value. Further, the steel sheets No. B18 and No. B19 whose pitting corrosion index X is out of the range of the present invention cannot be applied to the uses for which pitting corrosion resistance is required because the pitting corrosion generation potential Vc is 0 or less.
(Example 3)
A hot-rolled steel sheet (plate thickness: 2.5 mm) having the composition of steel No. D shown in Table 1 is cold-rolled twice under the conditions shown in Table 4, and the cold-rolled steel sheet (plate thickness: 0.1 mm) is subjected to cold rolling. And said. Between the first and second cold rolling, heat treatment (1050 ° C × 5 min, 1000 ° C × 2 min) was performed for the purpose of softening. After the final cold rolling, heat treatment A (500 ° C. × 2 min) was performed for the purpose of recovering mechanical properties, and further, dilute nitric acid electrolysis treatment was performed under the conditions shown in Table 4.

Then, the pitting corrosion potential Vc on the surface of each steel sheet was measured in the same manner as in Example 1. In addition, the surface roughness (arithmetic mean height) Sa was measured for the surface roughness of the steel sheet in accordance with the ISO 25178 regulations.

The obtained results are shown in Table 4.

In each of the examples of the present invention, the pitting corrosion generation potential Vc satisfies the equation (1) and is a stainless steel plate having a high pitting corrosion generation potential, and it is presumed that the pitting corrosion resistance is excellent. Further, all of the examples of the present invention exhibit excellent surface texture with a surface roughness Sa of 0.80 μm or less. On the other hand, it is presumed that the comparative example in which the pitting corrosion potential Vc does not satisfy the equation (1) and deviates from the present invention has low pitting corrosion resistance. Further, in the comparative example in which the dilute nitric acid electrolysis treatment conditions are low outside the range of the present invention, it is presumed that the pitting corrosion potential Vc does not satisfy the equation (1) and the pitting corrosion resistance is low. On the other hand, in the comparative example in which the dilute nitric acid electrolysis treatment conditions are far outside the scope of the present invention, the pitting corrosion potential Vc satisfies the equation (1), but the surface roughness (arithmetic mean height) Sa exceeds 0.80 μm and becomes rough. It is the surface. In the comparative example in which the temperature of the dilute nitric acid electrolysis treatment deviates high within the range of the present invention, the pitting corrosion potential Vc does not satisfy the equation (1). Further, in the example of the present invention having a surface roughness Sa of 0.40 μm or less, the pitting potential Vc is stably 1000 mV or more.

Claims

By mass%,
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: 3.5-36.0%,
Cr: 15.00 to 30.00%, Mo: 0 to 7.0%,
N: Contains 0.25% or less, Cr and Mo are contained so that X defined by the following equation (2) satisfies 15.0 to 50.0, has a composition consisting of the balance Fe and unavoidable impurities, and has a surface. The austenitic stainless steel sheet is characterized in that the pitting corrosion potential Vc of the above satisfies the following equation (1).
Note Vc> 0.039X3 -5.2X2 + 232X-2311 …… (1)
Here, X = Cr + 3.3Mo …… (2)
Cr, Mo: Content of each element (mass%)
In addition to the above composition, 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 The composition should contain one or more selected from 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 austenitic stainless steel sheet according to claim 1.
The austenitic stainless steel sheet according to claim 1 or 2, wherein the surface roughness is 0.80 μm or less in Sa conforming to the regulation of ISO 25178.
In manufacturing a cold-rolled steel sheet by subjecting a hot-rolled steel sheet having the composition according to claim 1 or 2 to cold rolling once or a plurality of times.
After the final cold rolling of the cold rolling, or after the non-final cold rolling of the cold rolling, heat treatment is performed to maintain the temperature in the range of 150 to 600 ° C. for 30 s to 10 min. Finally, a method for producing an austenitic stainless steel sheet, which comprises subjecting it to a dilute nitrate electrolytic treatment.
In manufacturing a cold-rolled steel sheet by subjecting a hot-rolled steel sheet having the composition according to claim 1 or 2 to cold rolling once or a plurality of times.
After the final cold rolling of the cold rolling, or after the non-final cold rolling of the cold rolling, heat treatment is performed at a temperature in the range of 150 to 700 ° C. for 15 min to 48 hours. Finally, a method for producing an austenitic stainless steel sheet, which comprises subjecting it to a dilute nitrate electrolytic treatment.
The dilute nitric acid electrolysis treatment is performed in a dilute nitric acid aqueous solution having a nitric acid concentration of 3 to 10% and a temperature of 40 to 80 ° C., a current density of ± 10 to 80 mA / cm 2 , and a total of 10 to 60 s for cathode and anode electrolysis. The method for producing an austenitic stainless steel plate according to claim 4 or 5, wherein the treatment is performed.