JP2019123895A - Austenitic stainless steel and manufacturing method therefor - Google Patents

Abstract

To provide an austenitic stainless steel having no deterioration of anticorrosion and manufacturability as the austenitic stainless steel, and no reduction of anticorrosion even after annealing and acid cleaning.SOLUTION: There is provided an austenitic stainless steel having steel and a coating on a surface of the steel, in which the steel contains, by mass%, C:0.100% or less, Si:3.00% or less, Mn:0.01 to 5.00%, P:0.100% or less, S:0.0050% or less, Ni:7.00 to 40.00%, Cr:17.00 to 28.00%, V:0.010 to 5.000%, and the balance Fe with impurities, the coating has a film with a peak value of V concentration in a depth direction from an outermost surface layer of 5.00 atomic% or more when total amount of a cation element in the coating is 100%.SELECTED DRAWING: None

Description

  The present invention relates to an austenitic stainless steel and a method of manufacturing the same, and more particularly, to an austenitic stainless steel used in a corrosive environment such as a marine environment or a chemical plant and a method of manufacturing the same.

  Since austenitic stainless steel has excellent corrosion resistance, it is applied as a member of marine environment and chemical plant. In recent years, the price of rare metals has risen, and the need for alloy-saving and thin-walled stainless steel excellent in corrosion resistance is increasing.

  Moreover, the corrosion resistance of stainless steel is affected not only by the amount of alloy but also by the surface film. In an austenitic stainless steel containing a large amount of Cr, a Cr-deficient layer is formed directly below the oxide scale produced by hot rolling or annealing. When a Cr deficient layer is formed, the Cr concentration of the surface of the austenitic stainless steel exposed after pickling is lower than the average Cr concentration of the steel material. Furthermore, after pickling, a surface film with low Cr concentration is formed on the austenitic stainless steel, and the inherent corrosion resistance can not be exhibited.

  In Patent Document 1, stainless steel which does not reduce the Cr concentration on the surface by cooling at a cooling rate of 25 ° C./sec or more and setting the rolling reduction at 30 ° C. or less to 30%, and further cooling it at 650 ° C. or less The manufacturing method of is described. However, controlling the hot rolling conditions leads to an increase in manufacturing costs.

  Patent Document 2 describes an acid detergent capable of dissolving a Cr-deficient layer by increasing the dissolution rate by adding an additive whose complex formation constant with Fe ions is larger than hydrofluoric acid. However, adding an additive to the pickling solution leads to an increase in the manufacturing cost, and dissolving the Cr-deficient layer leads to a decrease in the yield of steel products.

  Patent Document 3 describes a method of dissolving a Cr-deficient layer by pickling with a high concentration of hydrochloric acid to enable rapid pickling. However, using a high concentration of hydrochloric acid leads to an increase in manufacturing cost, and dissolving the Cr-deficient layer leads to a decrease in the yield of steel products.

Patent No. 3369570 gazette Patent 2981417 gazette Patent 2991829 gazette

  As described above, in the prior art, when austenitic stainless steel is manufactured, it is difficult to ensure appropriate corrosion resistance and manufacturability.

  The present invention has been made to solve such problems, and provides an austenitic stainless steel as an austenitic stainless steel which does not deteriorate in corrosion resistance and manufacturability and which does not deteriorate in corrosion resistance even after annealing and pickling. The purpose is

  In order to solve the said subject, the austenitic stainless steel of this invention and its manufacturing method have the following structures.

(1) A steel and a film on the surface of the steel,
Said steel is in mass%,
C: 0. 100% or less,
Si: 3.00% or less,
Mn: 0.01 to 5.00%,
P: 0. 100% or less,
S: less than 0.0050%,
Ni: 7.00 to 40.00%,
Cr: 17.00-28.00%,
V: 0.010 to 5.000%
And the balance consists of Fe and impurities,
An austenitic stainless steel characterized in that the film is a film having a peak value of V concentration in a depth direction from an outermost layer to a depth direction of 5.00 atomic% or more when the total amount of cationic elements in the film is 100%.
(2) mass%,
Mo: 10.00% or less,
Cu: 3.00% or less,
W: 2.000% or less,
N: The austenitic stainless steel according to the above (1), which contains one or more selected from 0.400% or less.
(3) mass%,
Ca: 0.0002 to 0.0050%,
B: 0.0002 to 0.0050%,
Mg: 0.0002 to 0.0050%,
REM: 0.0010 to 0.1000%
The austenitic stainless steel according to (1) or (2) above, which contains one or more selected from the group consisting of
(4) mass%,
Al: 3.000% or less is contained, The austenitic stainless steel in any one of said (1)-(3) characterized by the above-mentioned.
(5) mass%,
Ti: 0.001 to 0.400%,
Nb: 0.001 to 0.400%,
Ta: 0.001 to 0.500%,
Zr: 0.001 to 0.500%,
Co: 0.001 to 0.500%,
Sn: 0.001 to 0.500%,
Sb: 0.001 to 0.500%,
Ga: 0.001 to 0.500%
The austenitic stainless steel according to any one of the above (1) to (4), which contains one or more selected from the above.
(6) The austenitic stainless steel according to any one of the above (1) to (5) characterized in that it is used in a chemical plant, a salt production facility, a flue gas desulfurization apparatus, an EGR cooler, a marine structure, and a water treatment facility. .
(7) A method for producing an austenitic stainless steel according to any one of (1) to (6) above,
The pickled steel has a pH of 6.0 to 7.5, and is electrolytically treated at −0.35 to −0.15 V vs SHE in an aqueous solution of 0.05 mol / L or more of Na ₂ SO _{4 for} 10 seconds or more A method of producing an austenitic stainless steel, wherein a peak value of V concentration in a film of the steel in a depth direction from the outermost layer is made 5.00 atomic% or more.

  The austenitic stainless steel of the present invention solves the problem that it was difficult to simultaneously achieve both productivity and corrosion resistance, because the corrosion resistance does not decrease even if a Cr-deficient layer exists on the surface exposed after annealing and pickling It is possible to provide an austenitic stainless steel excellent in corrosion resistance.

It is a figure which shows the relationship between V density | concentration peak value in the film in the depth direction from outermost layer, and (DELTA) CPT.

  The present invention will be described in detail below. In the present specification,% of the element content means mass% unless otherwise noted.

The inventors of the present invention obtained the following findings as a result of intensive investigations on the corrosion resistance and surface film of austenitic stainless steel.
(1) By pickling the austenitic stainless steel to which V is added and performing electrolysis in a neutral solution, V can be concentrated in the surface film.
(2) Even if a Cr-depleted layer exists on the surface of austenitic stainless steel, if V is concentrated in the surface film, the reduction in corrosion resistance can be suppressed.

  The limited range of each component element in the present embodiment and the reason thereof will be described.

C: not more than 0.100% C limits the content of C to a content of not more than 0.100% in order to secure the corrosion resistance of stainless steel. When C is contained in excess of 0.100%, Cr carbides are formed to deteriorate the corrosion resistance. From the viewpoint of suppressing the formation of Cr carbides, the preferable range of the amount of C is 0.05% or less, and the more preferable range is 0.03% or less.

Si: 3.00% or less Si is contained for deoxidation, but containing Si in excess of 3.00% promotes the precipitation of the σ phase. Therefore, the upper limit of the amount of Si is limited to 3.00% or less. An amount of Si of 1.50% or less is effective. The preferable range of the amount of Si is 1.50% or less, and the more preferable range is 0.80% or less.

Mn: 0.01 to 5.00%
Mn is contained as 0.01% or more as a deoxidizer. The lower limit of the amount of Mn is preferably 0.10% or more, more preferably 0.25% or more. However, when the content of Mn exceeds 5.00%, the corrosion resistance is deteriorated. Therefore, the upper limit of the amount of Mn is limited to 5.00% or less. The preferable range of the upper limit of the amount of Mn is 1.00% or less, and the more preferable range is 0.80% or less.

P: 0. 100% or less P degrades hot workability and toughness, so the amount of P is limited to 0.100% or less. The preferable range of P amount is 0.050% or less, and the more preferable range is 0.030% or less.

S: 0.0050% or less S degrades the hot workability, toughness and corrosion resistance, so the S content is limited to 0.0050% or less. The preferable range of S amount is 0.0050% or less, and the more preferable range is 0.0010% or less.

Ni: 7.00 to 40.00%
Ni has the effect of suppressing the growth of corrosion when corrosion occurs and also suppressing the precipitation of the σ phase, but if the amount of Ni is less than 7.00%, sufficient corrosion resistance can not be obtained. When the amount of Ni exceeds 40.00%, the corrosion resistance effect is saturated. In addition, the amount of use of Ni increases and the steel plate becomes expensive. Therefore, it is necessary to make Ni amount into the range of 7.00-40.00%. If the amount of Ni is small, sufficient corrosion resistance can not be secured. Therefore, the preferable lower limit of the amount of Ni is 10.00% or more, and the preferable upper limit of the amount of Ni is 30.00% or less. Moreover, in order to hold down material cost while securing necessary and sufficient corrosion resistance, a more preferable lower limit of the amount of Ni is 17.00% or more, and a more preferable upper limit of the amount of Ni is 26.00% or less.

Cr: 17.00 to 28.00%
If the amount of Cr is less than 17.00%, sufficient corrosion resistance can not be obtained, and if the amount of Cr exceeds 28.00%, the precipitation of the sigma phase increases and the corrosion resistance deteriorates. Therefore, it is necessary to make Cr amount into the range of 17.00-28.00%. If the amount of Cr is small, sufficient corrosion resistance can not be secured. Therefore, the preferable lower limit of the amount of Cr is 20.00% or more, and the preferable upper limit of the amount of Cr is 27.00% or less. Moreover, in order to hold down material cost while securing necessary and sufficient corrosion resistance, a more preferable lower limit of the amount of Cr is 23.00% or more, and a more preferable upper limit of the amount of Cr is 26.00% or less.

V: 0.010 to 5.000%
V has the effect of improving corrosion resistance, particularly pitting corrosion resistance and crevice corrosion resistance in a chloride environment, and by concentrating in the oxide film on the surface, the corrosion resistance reduction due to the Cr-deficient layer generated by annealing and pickling is It has a compensating effect and is an important element constituting the present invention. However, when it is contained in an excessive amount, the formability is lowered and the corrosion resistance improving effect is also saturated, so the lower limit of the V amount is made 0.010% or more and the upper limit is made 5.00% or less. The preferable lower limit of the amount of V is 0.040% or more, and the preferable upper limit of the amount of V is 3.000% or less. Moreover, the more preferable lower limit of the amount of V is 0.070% or more, and the more preferable upper limit of the amount of V is 2.000% or less.

  In the present embodiment, in addition to the above-described elements, the following alloying elements may be contained in order to adjust various properties of the steel.

  Mo, Cu, W, N are elements that improve the corrosion resistance, and for that purpose, one or more of these elements may be contained.

Mo: 10.00% or less Since the effect of Mo is expressed when the amount of Mo is 0.10% or more, the lower limit of the amount of Mo is made 0.10% or more. However, when Mo is excessively contained, the precipitation of the σ phase increases and the reaction force at the time of hot rolling becomes high, and the productivity deteriorates. Therefore, the Mo content needs to be 0.10 to 10.00%. The lower limit of the amount of Mo is preferably 1.50% or more, the upper limit is preferably 8.50% or less, the more preferable lower limit is 5.00% or more, and the more preferable upper limit is 7.00% or less.

Cu: 3.00% or less The effect of Cu is expressed from the amount of Cu of 0.10%, so the lower limit of the amount of Cu is 0.10% or more, but if it is contained excessively, cracking is likely to occur during casting Become. For this reason, it is necessary to make Cu amount into 0.10 to 3.00%. The preferable lower limit of the amount of Cu is 0.30% or more. A more preferable lower limit is 0.60% or more.

W: 2.000% or less Since the effect of W is expressed when the amount of W is 0.010% or more, the lower limit of the amount of W is made 0.010% or more. However, when W is contained excessively, the processability is reduced. For this reason, it is necessary to make W amount into 0.010 to 2.000%. The lower limit of the W content is preferably 0.030% or more, more preferably 1.000% or less, more preferably 0.050% or more, and still more preferably 0.500% or less.

N: 0.400% or less The effect of N is expressed when the amount of N is 0.100% or more, so the lower limit of the amount of N is 0.100% or more. However, when N is contained excessively, bubbles are generated during casting. Therefore, the N content needs to be 0.100 to 0.400%. The preferable lower limit of the amount of N is 0.150% or more. A more preferable lower limit is 0.200% or more. The preferred upper limit is 0.300% or less.

  Ca, B, Mg, and REM are elements for improving the hot workability, and for that purpose, one or more of these elements may be contained. These elements may not be contained, and the lower limit in the case of not containing is 0% or more.

Ca: 0.0002 to 0.0050%
Since the effect of Ca is expressed when the amount of Ca is 0.0002% or more, the lower limit of the amount of Ca is made 0.0002% or more. However, if the content of Ca is excessive, the hot workability is reduced, so the upper and lower limits of the amount of Ca may be determined as follows. The amount of Ca is 0.0002 to 0.0050%. The preferable lower limit of the amount of Ca is 0.0010% or more, and the preferable upper limit is 0.0030% or less.

B: 0.0002 to 0.0050%
Since the effect of B is expressed when the amount of B is 0.0002% or more, the lower limit of the amount of B is made 0.0002% or more. However, if the content of B is excessive, the hot workability is lowered, so the upper and lower limits of the amount of B may be determined as follows. The B content is 0.0002 to 0.0050%. The preferable lower limit of the amount of B is 0.0010% or more, and the preferable upper limit is 0.0030% or less.

Mg: 0.0002 to 0.0050%
Since the effect of Mg is expressed when the amount of Mg is 0.0002% or more, the lower limit of the amount of Mg is made 0.0002% or more. However, if the content of Mg is excessive, the hot workability is deteriorated, so the upper and lower limits of the amount of Mg may be set as follows. The amount of Mg is 0.0002 to 0.0050%. The preferable lower limit of the amount of Mg is 0.0010% or more, and the preferable upper limit is 0.0030% or less.

REM: 0.0010 to 0.1000%
The lower limit of the REM amount is 0.0010% or more. Here, the REM amount is the total amount of rare earth elements described later. However, if the content of REM is excessive, the hot workability is deteriorated, so the upper and lower limits of the amount of Mg may be determined as follows. The REM amount is 0.0010 to 0.1000%. The preferable lower limit of the REM amount is 0.0050% or more, and the preferable upper limit is 0.0300% or less.
Here, REM (rare earth element) is two elements of scandium (Sc) and yttrium (Y) according to a general definition, and 15 elements (lanthanoids) from lanthanum (La) to lutetium (Lu) in the periodic table. Refers to the generic term of These may be contained alone or as a mixture.

Al: 3.000% or less Al is useful as a deoxidizing element, and contains Al in an amount of 0.001% or more. However, Al should not be contained in a large amount to deteriorate the processability, and the upper limit of the Al amount is preferably limited to 3.000% or less. The preferable lower limit of the amount of Al is 0.005% or more, and the preferable upper limit of the amount of Al is 1.000% or less. Moreover, Al may not be contained, and the lower limit in the case of not containing is 0% or more.

Ti, Nb, Ta, Zr, Co, Sn, Sb, and Ga are elements for improving the corrosion resistance, and may be contained singly or in combination in the following range. These elements may not be contained, and the lower limit in the case of not containing is 0% or more.
Ti: 0.001 to 0.400%, Nb: 0.001 to 0.400%, Ta: 0.001 to 0.500%, Zr: 0.001 to 0.500%, Co: 0.001 to 0.500% 0.500%, Sn: 0.001 to 0.500%, Sb: 0.001 to 0.500%, Ga: 0.001 to 0.500%.

  Ti and Nb fix C, N as carbonitrides and have the effect of suppressing corrosion resistance, particularly intergranular corrosion. For this reason, one or both of Ti and Nb may be contained, but even if it contains excessive amounts of Ti and Nb, the effect is saturated, so the upper limit of each of the Ti amount and the Nb amount is 0.400% I assume. Here, if at least one of the Ti amount and the Nb amount is 0.001% or more, the effect can be exhibited. In addition, as an appropriate value of Ti amount and Nb amount, the total amount of Ti amount and Nb amount is 5 times or more and 30 times or less of the total amount of C amount and N amount. Preferably, the total amount of Ti and Nb amounts to 10 times or more and 25 times or less of the total amount of C and N amounts.

  Ta, Zr, Co, Sn, and Sb are elements which are useful for improving the corrosion resistance even in a slight amount, and may be contained in a range which does not impair the cost price. When the amount of each of Ta, Zr, Co, Sn and Sb is less than 0.001%, the effect of improving the corrosion resistance is not exhibited. When the amount of each of Ta, Zr, Co, Sn, and Sb exceeds 0.500%, the increase in cost becomes apparent and the processability also decreases. Therefore, the respective amounts of Ta, Zr, Co, Sn, and Sb fall within the appropriate range of 0.001 to 0.500%. With respect to the respective amounts of Ta, Zr, Co, Sn and Sb, the preferable lower limit is 0.010% or more, and the preferable upper limit is 0.300% or less.

  Ga is an element which contributes to corrosion resistance and processability improvement, and can be made to contain Ga in 0.001 to 0.500% of range. The preferable lower limit of the amount of Ga is 0.015% or more, and the preferable upper limit of the amount of Ga is 0.300% or less.

  In the steel plate of the present embodiment, the balance other than the above-described elements is Fe and unavoidable impurities, but can be contained in addition to the above-described elements without impairing the effects of the present embodiment.

Next, surface components according to the present embodiment will be described.
The surface component of the austenitic stainless steel satisfies the following requirement (1).
(1) The peak value of V concentration in the depth direction from the outermost layer is 5.00 atomic% or more, where the total amount of cationic elements in the film is 100%

  The decrease in pitting resistance after annealing and pickling means that a Cr-depleted layer is formed on the substrate under the oxide scale due to annealing, and this Cr-depleted layer can not be removed by pickling, the surface Cr concentration after pickling Is lower than the Cr concentration of the average composition of the steel, and only an oxide film having a Cr concentration corresponding to the surface Cr concentration is formed. The present inventors diligently investigated the relationship between the oxide film composition on the surface and the corrosion resistance, and revealed that the corrosion resistance of stainless steel is improved by the concentration of V in the oxide film on the surface. This is considered to be because V oxide in the oxide film is dense and has the same environmental barrier properties as Cr oxide. In order to reduce the decrease in pitting resistance due to Cr deficiency caused after annealing and pickling, the film is made to have a V concentration peak value in the depth direction of 5 from the outermost layer when the total amount of cationic elements in the film is 100%. The film needs to be .00 atomic% or more. The preferable lower limit of the V concentration peak value in the depth direction from the outermost layer when the total amount of cationic elements in the film is 100% is 7.00 atomic% or more, and the more preferable lower limit is 10.00 atomic% or more.

The method of measuring the V concentration peak value in the film is as follows.
The steel after pickling is used as a sample, and the sample is cut into a shape that enters the analyzer without processing and chemical treatment on the surface, and analyzed using AES (Auger electron spectroscopy). The untreated sample surface is sputtered with Ar gas from the outermost layer and profile analysis in the depth direction is performed to analyze the Cr concentration in the Cr-deficient region and the V concentration in the film. Here, the Cr deficient area is the position from the outermost layer to the value of the base material from the outermost layer, and the lowest Cr concentration in the Cr deficient area is the Cr deficient area of the concentration of Cr when the total of the cation elements is 100%. And the lowest value. In addition, the film is positioned from the outermost layer to the half of the peak value of O (oxygen), and the peak value of V concentration is the highest value in the film of V concentration when the total of cationic elements is 100%. Do. Here, the concentration of each element is calculated by atomic%.

Next, a method of manufacturing the austenitic stainless steel according to the present embodiment will be described.
The austenitic stainless steel of the present embodiment is basically manufactured by applying a general process of manufacturing stainless steel. For example, an electric furnace is used as a molten steel having the above-described chemical composition, and is refined by an AOD (Argon Oxygen Decarburization) furnace or a VOD (Vacuum Arc Degassing) furnace. Then, it is made into a steel piece by a continuous casting method or an ingot formation method, then, it hot-rolls and performs annealing (solution heat treatment) of a hot-rolled sheet. In the case of manufacturing a thin plate (for example, a steel plate having a thickness of about 3 mm), cold rolling is performed after the solution heat treatment described above, and then annealing (solution heat treatment) and pickling are performed again. Thereby, a thin plate is manufactured. The steel to which the present invention can be applied may be any steel material that has been subjected to pickling after annealing, and there is no limitation on plate-like steel materials, linear steel materials, tubular steel materials, and the like. In the case of a plate-like steel material, any of a hot-rolled sheet, a hot-rolled and annealed sheet, a cold-rolled sheet and a cold-rolled and annealed sheet may be used.

Next, neutral electrolytic treatment of austenitic stainless steel will be described.
In order to produce an austenitic stainless steel the surface component of which satisfies requirement (1), electrolytic treatment must be performed at a specific pH and potential at which V forms an oxide. For example, even when the pH range of 6.0 to 7.5, which is the pH range of neutral electrolytic treatment in the method for producing austenitic stainless steel according to the present embodiment, when electrolysis is performed by 0.2 V vs SHE (Standard Hydrogen Electrode), Since V oxide dissolves in this potential-pH region, V does not concentrate in the oxide film. Moreover, even if it is -0.35--0.15V vs SHE which is a potential range of neutral electrolysis processing in the manufacturing method of austenitic stainless steel of this embodiment, when electrolysis is carried out at pH 5.0, this potential Since V oxide dissolves in the pH range, V does not concentrate in the oxide film. Therefore, it is necessary to carry out the electrolytic treatment at pH 6.0 to 7.5 at which V oxide is formed on the oxide film for 10 seconds or more at -0.35 to -0.15 V vs SHE at which neutral electrolytic treatment is performed. In addition, if the concentration of sodium sulfate (Na ₂ SO ₄ ) added as an electrolyte is low, the conductivity can not be achieved due to insufficient electric conductivity, so the concentration of Na ₂ SO ₄ in the electrolytic solution used for neutral electrolytic treatment is 0.05 mol / It is necessary to be more than L.

  The austenitic stainless steel of the present embodiment is suitably used for a chemical plant, a salt production facility, a flue gas desulfurization apparatus, an EGR cooler, an offshore structure, and a water treatment facility.

  The following experiments were conducted to confirm the effects of the present invention in detail. In addition, a present Example shows one Example of this invention, and this invention is not limited to the following structures.

Stainless steel having the chemical components shown in Tables 1 to 5 was melted and cast in a vacuum induction melting furnace.
Next, after soaking at 1200 ° C., it was hot forged, hot rolled to a plate thickness of 6 mm, and annealed and pickled.
Subsequently, it cold-rolled until plate | board thickness was set to 1 mm, and also it annealed and pickled. The obtained austenitic stainless steel was subjected to neutral electrolytic treatment by the following method in order to concentrate V into the surface film.

Neutral electrolytic treatment of austenitic stainless steel was performed by the following method.
As an electrolytic solution, with aqueous _Na 2 SO ₄ of 0.02~0.50mol / L was prepared by mixing pure water and _Na 2 SO _4. The electrolytic solution used here only needs to have electrical conductivity, so other aqueous solutions may be used within the pH range described later. The types of aqueous solutions that can be used as the electrolyte solution, for _example, aqueous solution _{NaNO 4, KNO 3, K 2} SO 4 and the like as a solute. If a solute containing Cl such as NaCl is used, it can not be used because pitting occurs during electrolysis. The pH of the electrolyte was adjusted to 4.0 to 8.0 using H ₂ SO ₄ , NaOH or the like. The electrolysis potential was -0.40 to -0.10 V vs SHE, and electrolysis was carried out at normal temperature for 5 to 60 seconds.
The steel plate was manufactured by the above.

  Next, the characteristic test was performed by the following method.

(CPT measurement)
In order to evaluate the corrosion resistance on the steel plate surface, the critical temperature (CPT) of pitting corrosion occurrence was measured.
The test solution was prepared by adding 1% hydrochloric acid to a 6% aqueous solution of ferric chloride.
In the test, two kinds of surface finish materials were provided: an abrasive material from which all the Cr-deficient area was removed by # 600 emery wet polishing, and a pickling material finished with # 600 emery wet polishing only on the cut surface. This steel plate was immersed in the test solution for 72 h, and the lowest temperature at which pitting occurred was defined as CPT (pitting critical temperature). The test temperature was set in 2 ° C. units. The CPT of the abrasive obtained in this manner is the abrasive CPT, the CPT of the pickling material is the pickling material CPT, and the value obtained by arithmetically subtracting the abrasive CPT from the pickling material CPT is ΔCPT. When ΔCPT was 4 ° C. or less, it was judged that the corrosion resistance was sufficient, and when ΔCPT exceeded 4 ° C., it was judged that the corrosion resistance was insufficient.

(Film analysis)
In order to investigate the properties of the surface film after pickling, analysis of the surface film was performed.
The steel plate after pickling was not subjected to processing and chemical treatment on the surface, and was cut into a shape that enters the analyzer, and was analyzed using AES (Auger electron spectroscopy). The untreated sample surface was sputtered with Ar gas from the outermost layer and profile analysis in the direction of depth was performed to analyze the Cr concentration in the Cr-depleted region and the V concentration in the film. Here, the Cr deficient area is the position from the outermost layer to the value of the base material from the outermost layer, and the lowest Cr concentration in the Cr deficient area is the Cr deficient area of the concentration of Cr when the total of the cation elements is 100%. And the lowest value. In addition, the film is positioned from the outermost layer to the half of the peak value of O (oxygen), and the peak value of V concentration is the highest value in the film of V concentration when the total of cationic elements is 100%. did. Here, the concentration of each element was calculated by atomic%.

  The results of the CPT test are shown in Tables 1 to 5 and FIG.

From Tables 1 to 4, according to the steels 1 to 64 of the present invention, since the peak value of V concentration in the film in the depth direction from the outermost layer is 5.00 atomic% or more, ΔCPT is 4 ° C. or less in all cases. Was found to exhibit good corrosion resistance.
On the other hand, according to Table 5, in Comparative Steels 4 to 12, the V concentration peak value in the film in the depth direction from the outermost layer is less than 5.00 atomic%, so ΔCPT exceeds 4 ° C. It turned out to be inadequate. Further, in Comparative Steels 1 to 3, even if the V concentration peak value in the film in the depth direction from the outermost layer is 5.00 atomic% or more, the amounts of Mn, S and Cr are out of the range of the present invention Therefore, it has been found that ΔCPT exceeds 4 ° C., and the corrosion resistance is insufficient.
In addition, PRE shown in Tables 1-5 is a general index which shows the pitting corrosion resistance of a stainless steel plate, and is a value calculated by a following formula from the average composition of steel.
PRE = {Cr} + 3.3 {Mo} + 16 {N}
Here, the elemental symbol enclosed by {} in the above-mentioned formula means the average content (mass%) in the whole steel plate.

  The austenitic stainless steel of the present embodiment can provide extremely excellent resistance to crevice corrosion in a high salt environment. Therefore, the austenitic stainless steel of the present embodiment is a material for marine steel structure, a material for marine structure lining material, a material for exhaust gas desulfurization apparatus, a material for food manufacturing plant, a material for building exterior material, for salt production plant Materials, materials for hot water storage, materials for chemical plants, materials for sewage treatment facilities, materials for ozone treatment facilities, materials for seawater desalination plants, materials for seawater pumps, materials for automotive EGR (Exhaust Gas Recirculation) coolers is there.

(1) A steel and an oxide film on the surface of the steel,
Said steel is in mass%,
C: 0. 100% or less,
Si: 3.00% or less,
Mn: 0.01 to 5.00%,
P: 0. 100% or less,
S: less than 0.0050%,
Ni: 7.00 to 40.00%,
Cr: 17.00-28.00%,
V: 0.010 to 5.000%
And the balance consists of Fe and impurities,
The oxide film, austenitic stainless steel, wherein the peak value of V concentrations in the depth direction from the outermost layer when the total amount of cationic element in the oxide film is 100% is 5.00Atomic% or more membrane steel.
(2) mass%,
Mo: 10.00% or less,
Cu: 3.00% or less,
W: 2.000% or less,
N: The austenitic stainless steel according to the above (1), which contains one or more selected from 0.400% or less.
(3) mass%,
Ca: 0.0002 to 0.0050%,
B: 0.0002 to 0.0050%,
Mg: 0.0002 to 0.0050%,
REM: 0.0010 to 0.1000%
The austenitic stainless steel according to (1) or (2) above, which contains one or more selected from the group consisting of
(4) mass%,
Al: 3.000% or less is contained, The austenitic stainless steel in any one of said (1)-(3) characterized by the above-mentioned.
(5) mass%,
Ti: 0.001 to 0.400%,
Nb: 0.001 to 0.400%,
Ta: 0.001 to 0.500%,
Zr: 0.001 to 0.500%,
Co: 0.001 to 0.500%,
Sn: 0.001 to 0.500%,
Sb: 0.001 to 0.500%,
Ga: 0.001 to 0.500%
The austenitic stainless steel according to any one of the above (1) to (4), which contains one or more selected from the above.
(6) The austenitic stainless steel according to any one of the above (1) to (5) characterized in that it is used in a chemical plant, a salt production facility, a flue gas desulfurization apparatus, an EGR cooler, a marine structure, and a water treatment facility. .
(7) A method for producing an austenitic stainless steel according to any one of (1) to (6) above,
The pickled steel has a pH of 6.0 to 7.5, and is electrolytically treated at −0.35 to −0.15 V vs SHE in an aqueous solution of 0.05 mol / L or more of Na ₂ SO _{4 for} 10 seconds or more A method of producing an austenitic stainless steel, wherein the peak value of V concentration in the oxide film of the steel in the depth direction from the outermost layer is 5.00 atomic% or more.

Claims (7)

A steel and a coating on the surface of the steel;
Said steel is in mass%,
C: 0. 100% or less,
Si: 3.00% or less,
Mn: 0.01 to 5.00%,
P: 0. 100% or less,
S: less than 0.0050%,
Ni: 7.00 to 40.00%,
Cr: 17.00-28.00%,
V: 0.010 to 5.000%
And the balance consists of Fe and impurities,
An austenitic stainless steel characterized in that the film is a film having a peak value of V concentration in a depth direction from an outermost layer to a depth direction of 5.00 atomic% or more when the total amount of cationic elements in the film is 100%. In mass%,
Mo: 10.00% or less,
Cu: 3.00% or less,
W: 2.000% or less,
The austenitic stainless steel according to claim 1, characterized in that it contains one or more selected from N: 0.400% or less. In mass%,
Ca: 0.0002 to 0.0050%,
B: 0.0002 to 0.0050%,
Mg: 0.0002 to 0.0050%,
REM: 0.0010 to 0.1000%
The austenitic stainless steel according to claim 1 or 2, characterized in that it contains one or more selected from the above. In mass%,
The austenitic stainless steel according to any one of claims 1 to 3, which contains Al: 3.000% or less. In mass%,
Ti: 0.001 to 0.400%,
Nb: 0.001 to 0.400%,
Ta: 0.001 to 0.500%,
Zr: 0.001 to 0.500%,
Co: 0.001 to 0.500%,
Sn: 0.001 to 0.500%,
Sb: 0.001 to 0.500%,
Ga: 0.001 to 0.500%
The austenitic stainless steel according to any one of claims 1 to 4, containing one or more selected from the group consisting of   The austenitic stainless steel according to any one of claims 1 to 5, which is used for a chemical plant, a salt production facility, a flue gas desulfurization apparatus, an EGR cooler, a marine structure, and a water treatment facility. It is a manufacturing method of the austenitic stainless steel of any one of Claims 1-6, Comprising:
The pickled steel has a pH of 6.0 to 7.5, and is electrolytically treated at −0.35 to −0.15 V vs SHE in an aqueous solution of 0.05 mol / L or more of Na ₂ SO _{4 for} 10 seconds or more A method of producing an austenitic stainless steel, wherein a peak value of V concentration in a film of the steel in a depth direction from the outermost layer is made 5.00 atomic% or more.

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