JP7482666B2 - Ferritic stainless steel and method for producing same - Google Patents

Description

The present invention relates to ferritic stainless steel and a method for producing ferritic stainless steel.

Stainless steel is widely used for exterior materials, home appliances, kitchenware, etc., due to its excellent corrosion resistance, workability, and beauty. Ferritic stainless steel is often used as a building material. However, stainless steel exposed to outdoor conditions, especially in coastal environments, can suffer from chloride-induced corrosion, even in highly corrosion-resistant steel types.

As a technique for improving corrosion resistance, for example, Patent Document 1 discloses a highly corrosion-resistant stainless steel sheet for exterior building materials that is excellent in preventing the occurrence of band-like appearance irregularities and is a bright annealed steel sheet or an annealed and pickled steel sheet of ferritic stainless steel containing, by mass%, 16-35% Cr, 0.05-0.5% Ti, 0-6% Mo (including no additives), 0-1.0% Nb (including no additives), and 0.005-0.025% N, with the C content limited to 0.015% or less, and in which the lightness difference ΔL within the sheet width of the steel sheet surface in the direction perpendicular to the rolling direction is adjusted to 5 or less.

For example, Patent Document 2 discloses a highly corrosion-resistant stainless steel sheet for building materials with good workability, which contains 16 to 35 mass% Cr, 0.05 to (0.5-10 x N) mass% Ti, 0.005 to 0.3 mass% Al, 0 to 6 mass% Mo (including no additives), 0 to 1.0 mass% Nb (including no additives), and N is 0.02 mass% or less, and has an uneven surface with a ten-point average roughness Rz of 1 to 50 μm to impart anti-glare properties and designability, and has a passive film formed on the surface through bright annealing, in which a Ti-enriched layer and an Al-enriched layer are present, and no nitride layer is present in the surface layer up to a depth of 50 μm.

JP 2000-129405 A Japanese Patent Application Laid-Open No. 11-61350

However, the techniques disclosed in Patent Documents 1 and 2 still have room for improvement in terms of corrosion resistance caused by chlorides.

The present invention has been made in consideration of the above problems, and the object of the present invention is to provide a ferritic stainless steel having excellent corrosion resistance against chloride-induced corrosion and a method for manufacturing the ferritic stainless steel.

The inventors discovered that the protective performance of an oxide film formed on the surface of stainless steel varies depending on the cationic components contained in the oxide film, and that the protective performance differs depending on the element. The inventors then controlled the cationic composition in the oxide film, leading to the invention.

The gist of the present invention, which has been completed based on the above findings, is as follows.
[1]
In mass percent,
C: 0.150% or less,
Si: 2.0% or less,
Mn: 1.0% or less,
P: 0.04% or less,
S: 0.05% or less,
Ni: 0.6% or less,
Cr: 10.0 to 35.0%,
Ti: 0.50% or less,
Nb: 0.50% or less,
Mo: 6.0% or less,
Cu: 2.0% or less,
N: 0.005 to 0.035%,
The balance is Fe and impurities,
A ferritic stainless steel having an FV value represented by the following formula (1) of 0.25 or more or -0.25 or less.
FV = CCr + _CSi + _{CTi -} CFe - _{CNi - CCu - CNb} - _{CMo ...} (1)
In the formula (1), CCr, CSi, CTi, CFe, CNi, CCu, _CNb and CMo respectively represent the cation fractions of _Cr , _Si , _Ti , _Fe , _Ni , _Cu , Nb and _Mo when the total amount of cations (atomic %) of all metal elements in the oxide film is taken as 1.
[2]
Instead of a part of Fe,
Al: 0.001 to 0.3%,
Ca: 0.0001 to 0.3%,
B: 0.0001 to 0.1%,
Co: 0.001 to 0.5%,
Mg: 0.001 to 0.5%, and REM: 0.001 to 0.5%
and the FV value is 0.25 or more or −0.32 to −0.25.
[3]
Instead of a part of Fe,
Sn: 0.001 to 0.5%,
V: 0.001 to 0.5%,
W: 0.001 to 0.5%,
Zr: 0.001 to 0.5%,
Hf: 0.001 to 0.5%,
Ta: 0.001 to 0.5%,
The ferritic stainless steel according to [1] or [2], containing one or more selected from the group consisting of Ga: 0.001 to 0.5% and Sb: 0.001 to 0.5% , and having an FV value of 0.25 or more or -0.32 to -0.25.
[4]
In mass percent,
C: 0.150% or less,
Si: 2.0% or less,
Mn: 1.0% or less,
P: 0.04% or less,
S: 0.05% or less,
Ni: 0.6% or less,
Cr: 10.0 to 35.0%,
Ti: 0.50% or less,
Nb: 0.50% or less,
Mo: 6.0% or less,
Cu: 2.0% or less,
N: 0.005 to 0.035%,
A first immersion step of immersing a stainless steel material consisting of the remainder Fe and impurities in a solution having a pH of 1.0 or less and an electrode potential of 0.35 V or more based on a silver-silver chloride electrode for 60 to 120 seconds;
a second immersion step of immersing the stainless steel material after the first immersion step in a solution having a pH of 4.0 to 8.0 and an electrode potential of 0.45 V or more based on a silver-silver chloride electrode for 5 to 60 seconds;
A method for producing ferritic stainless steel, comprising the steps of:
[5]
In mass percent,
C: 0.150% or less,
Si: 2.0% or less,
Mn: 1.0% or less,
P: 0.04% or less,
S: 0.05% or less,
Ni: 0.6% or less,
Cr: 10.0 to 35.0%,
Ti: 0.50% or less,
Nb: 0.50% or less,
Mo: 6.0% or less,
Cu: 2.0% or less,
N: 0.005 to 0.035%,
a third immersion step of immersing the stainless steel material consisting of the remaining Fe and impurities in a solution having a pH of 4.0 to 8.0 and an electrode potential of 0.45 V or more based on a silver-silver chloride electrode for 60 to 120 seconds;
and a fourth immersion step of immersing the stainless steel material after the third immersion step in a solution having a pH of 1.0 or less and an electrode potential of 0.35 V or more based on a silver-silver chloride electrode for 5 to 30 seconds.
[6]
The method for producing a ferritic stainless steel according to [4] or [5], wherein the stainless steel material is a stainless steel after hot working or a stainless steel after cold working.
[7]
In mass percent,
C: 0.150% or less,
Si: 2.0% or less,
Mn: 1.0% or less,
P: 0.04% or less,
S: 0.05% or less,
Ni: 0.6% or less,
Cr: 10.0 to 35.0%,
Ti: 0.50% or less,
Nb: 0.50% or less,
Mo: 6.0% or less,
Cu: 2.0% or less,
N: 0.005 to 0.035%,
A bright annealing process for bright annealing a stainless steel material consisting of the remainder Fe and impurities;
and a fifth immersion step of immersing the stainless steel material after the bright annealing step in a solution having a pH of 4.0 to 8.0 and an electrode potential of 0.45 V or higher based on a silver-silver chloride electrode for 5 to 60 seconds.

The present invention makes it possible to provide a ferritic stainless steel having excellent corrosion resistance against chloride-induced corrosion and a method for producing the same.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Preferred embodiments of the present invention will now be described in detail with reference to the accompanying drawings in the following order:
<Ferritic stainless steel>
<Method of manufacturing ferritic stainless steel>
(First manufacturing method)
(Second manufacturing method)
(Modifications of the first and second manufacturing methods)
(Third manufacturing method)
(Modification of the third manufacturing method)

<Ferritic stainless steel>
The ferritic stainless steel according to this embodiment is composed, by mass%, of C: 0.150% or less, Si: 2.0% or less, Mn: 1.0% or less, P: 0.04% or less, S: 0.05% or less, Ni: 0.6% or less, Cr: 10.0 to 35.0%, Ti: 0.50% or less, Nb: 0.50% or less, Mo: 6.0% or less, Cu: 2.0% or less, N: 0.005 to 0.035%, with the balance being Fe and impurities, and has an FV value represented by the following formula (1) of 0.25 or more or -0.25 or less.
FV = CCr + _CSi + _{CTi -} CFe - _{CNi - CCu - CNb} - _{CMo ...} (1)
In the above formula (1), CCr, CSi, CTi, CFe, CNi, CCu, _{CNb and CMo respectively represent the cation fractions of Cr, Si, Ti, Fe, Ni, Cu, Nb and Mo when the total amount of} cations (atomic %) of all metal elements in the outermost layer of the coating is taken as 1.
The ferritic stainless steel according to this embodiment will be described in detail below.

[Chemical composition of steel]
First, the chemical components of the ferritic stainless steel according to this embodiment will be described. Note that % indicating the components means mass %.

C: 0.150% or less C reduces intergranular corrosion resistance and workability, so its content needs to be kept low. Therefore, the C content is set to 0.150% or less. The C content is preferably 0.100% or less, and more preferably 0.025% or less. There is no particular lower limit for the C content. However, from the viewpoint of cost, the C content is preferably 0.001% or more. The C content is more preferably 0.003% or more.

Si: 2.0% or less Si acts as a deoxidizer and a desulfurizer. If the Si content exceeds 2.0%, the toughness decreases, so the Si content is set to 2.0% or less. The Si content is preferably 1.00% or less, and more preferably 0.80% or less. There is no particular lower limit for the C content. However, in order for Si to act sufficiently as a deoxidizer and a desulfurizer, the Si content is preferably 0.01% or more, and more preferably 0.05% or more.

Mn: 1.0% or less Mn is useful as a deoxidizing element, but if Mn is contained in excess, oxidation resistance and corrosion resistance decrease. Therefore, the Mn content is set to 1.0% or less. The Mn content is preferably 0.8% or less, and more preferably 0.5% or less. On the other hand, the lower limit of the Mn content is not particularly limited. However, from the viewpoint of cost, the Mn content is preferably 0.01% or more, and more preferably 0.05% or more.

P: 0.04% or less P is an element that is inevitably contained in stainless steel. Since P deteriorates workability and weldability, and also deteriorates manufacturability, it is necessary to limit its content. Therefore, the P content is set to 0.04% or less. The P content is preferably 0.035% or less, and more preferably 0.030% or less. There is no particular lower limit, but from the viewpoint of cost, the P content is preferably 0.005% or more.

S: 0.05% or less S deteriorates hot workability, toughness and corrosion resistance, so the S content is limited to 0.05% or less. The S content is preferably 0.01% or less, and more preferably 0.001% or less. On the other hand, there is no particular lower limit for the S content. However, if the S content is excessively reduced, the raw material cost and refining cost will increase, so the S content is preferably 0.0003% or more.

Ni: 0.6% or less Excessive Ni content leads to increased costs. Therefore, the Ni content is set to 0.6% or less. The Ni content is preferably 0.5% or less, and more preferably 0.30% or less. Although there is no particular lower limit, since Ni improves corrosion resistance, the Ni content is preferably 0.01% or more. The Ni content is more preferably 0.05% or more.

Cr: 10.0 to 35.0%
Cr improves corrosion resistance and oxidation resistance against chlorides. Therefore, the Cr content is set to 10.0% or more. The Cr content is preferably 12.0% or more, and more preferably 14.0% or more. The higher the Cr content, the better the oxidation resistance and corrosion resistance, but the lower the workability and manufacturability. Therefore, the Cr content is set to 35.0% or less. The Cr content is preferably 30.00% or less, and more preferably 25.00% or less.

Ti: 0.50% or less Ti is an element that fixes C and N to prevent sensitization due to Cr carbide precipitation and improves corrosion resistance. On the other hand, excessive Ti content leads to increased alloy cost, decreased toughness, decreased corrosion resistance due to increased inclusions in the steel, and decreased manufacturability. Therefore, the Ti content is set to 0.50% or less. The Ti content is preferably 0.30% or less, and more preferably 0.20% or less. There is no particular lower limit, but if the Ti content is 0.005% or more, the corrosion resistance is improved. Therefore, the Ti content is preferably 0.005% or more, and more preferably 0.010% or more.

Nb: 0.50% or less Nb is an element that fixes C and N to prevent sensitization due to Cr carbide precipitation and improves corrosion resistance. On the other hand, excessive Nb content leads to increased alloy cost, decreased toughness, decreased corrosion resistance due to increased inclusions in the steel, and decreased manufacturability. Therefore, the Nb content is set to 0.50% or less. The Nb content is preferably 0.40% or less, and more preferably 0.30% or less. There is no particular lower limit, but if the Nb content is 0.005% or more, the corrosion resistance is improved. Therefore, the Nb content is preferably 0.005% or more, and more preferably 0.010% or more.

Mo: 6.0% or less Mo has a high effect of improving corrosion resistance, but is a very expensive element, and if the Mo content is excessive, the manufacturing cost increases. In addition, excessive Mo content hardens the stainless steel and deteriorates the workability. For this reason, the Mo content is set to 6.0% or less. The Mo content is preferably 5.0% or less, and more preferably 4.0% or less. Although there is no particular lower limit, if the Mo content is 0.01% or more, the corrosion resistance is improved. For this reason, the Mo content is preferably 0.01% or more, and more preferably 0.05% or more.

Cu: 2.0% or less Cu improves corrosion resistance. On the other hand, excessive Cu content leads to increased costs. Therefore, the Cu content is set to 2.0% or less. The Cu content is preferably 1.0% or less, and more preferably 0.8% or less. There is no particular lower limit, but if the Cu content is 0.01% or more, the corrosion resistance is improved. Therefore, the Cu content is preferably 0.01% or more, and more preferably 0.20% or more.

N: 0.005 to 0.035%
N is a useful element for pitting corrosion resistance, but it reduces intergranular corrosion resistance and workability, and also reduces oxidation resistance and corrosion resistance. Therefore, it is necessary to suppress the N content low. Therefore, the N content is set to 0.035% or less. The N content is preferably 0.0300% or less, and more preferably 0.0150% or less. On the other hand, excessively reducing the N content increases refining costs. Therefore, the N content is set to 0.005% or more. The N content is preferably 0.0050% or more, and more preferably 0.0080% or more.

In the ferritic stainless steel according to the present invention, the remainder other than the above-mentioned elements is Fe and impurities. However, elements other than the above-mentioned elements can also be included within a range that does not impair the effects of this embodiment. Note that the impurities referred to here are components that are mixed in due to various factors in the raw materials such as ores and scraps and the manufacturing process when the ferritic stainless steel according to the present invention is industrially manufactured, and are acceptable within a range that does not adversely affect the present invention.

The basic components of the ferritic stainless steel according to the present invention have been described above, but the ferritic stainless steel according to the present invention preferably contains, in place of a portion of Fe, one or more elements selected from the group consisting of Al: 0.001-0.3%, Ca: 0.001-0.3%, B: 0.0001-0.1%, Co: 0.001-0.5%, Mg: 0.001-0.5%, and REM: 0.001-0.5%. The above elements improve manufacturability. These elements do not necessarily need to be contained, so the lower limit of the content of these elements is 0%.

Al: 0.001 to 0.3%
Al is an element having a deoxidizing effect. In order to stably obtain the deoxidizing effect by Al, the Al content is preferably 0.001% or more. The Al content is more preferably 0.005% or more. On the other hand, Al is likely to form nitrides together with N, and when nitrides are formed, the toughness is greatly reduced. Therefore, the Al content is preferably 0.3% or less. The Al content is more preferably 0.2% or less.

Ca: 0.0001 to 0.3%
Ca is an element having a desulfurization effect. In order to stably obtain the desulfurization effect by Ca, the Ca content is preferably 0.0001% or more. The Ca content is more preferably 0.001% or more. On the other hand, if Ca is contained excessively, water-soluble inclusions CaS are generated and the corrosion resistance is reduced. Therefore, the Ca content is preferably 0.3% or less. The Ca content is more preferably 0.2% or less.

B: 0.0001 to 0.1%
B is an element that has the effect of suppressing secondary work embrittlement and deterioration of hot workability. In order to stably obtain the above effect due to B, the B content is preferably 0.0001% or more. The B content is more preferably 0.0005% or more. On the other hand, if the B content exceeds 0.1%, the hot workability may deteriorate. Therefore, the B content is preferably 0.1% or less. The B content is more preferably 0.01% or less.

Co: 0.001 to 0.5%
Co is an element that improves secondary workability and toughness. In order to stably obtain the above effects due to Co, the Co content is preferably 0.001% or more. The Co content is more preferably 0.01% or more. On the other hand, since Co is a rare element and expensive, the Co content is preferably 0.5% or less. The Co content is more preferably 0.2% or less.

Mg: 0.0001 to 0.5%
Mg is an element that refines the structure and improves workability and toughness. In order to stably obtain the above effects of Mg, the Mg content is preferably 0.0001% or more. The Mg content is more preferably 0.001% or more. On the other hand, if Mg is excessively contained, manufacturability may decrease. Therefore, the Mg content is preferably 0.5% or less. The Mg content is more preferably 0.2% or less.

REM: 0.001 to 0.5%
REM is an element having a deoxidizing effect and is useful in refining. In order to stably obtain the deoxidizing effect and the like due to REM, the REM content is preferably 0.001% or more. The REM content is more preferably 0.05% or more. On the other hand, if REM is contained in excess, the workability may decrease. Therefore, the REM content is preferably 0.5% or less. The REM content is more preferably 0.2% or less.
In addition, REM (rare earth element) is Sc, Y, and 15 elements from La to Lu (lanthanoids), and REM is one or more selected from the above elements. When two or more elements are contained as REM, the REM content refers to the total amount of the contained elements.

Furthermore, the ferritic stainless steel according to the present invention preferably contains, in mass %, one or more elements selected from the group consisting of Sn: 0.001-0.5%, V: 0.001-0.5%, W: 0.001-0.5%, Zr: 0.001-0.5%, Hf: 0.001-0.5%, Ta: 0.001-0.5%, Ga: 0.001-0.5%, and Sb: 0.001-0.5%, in place of a portion of Fe. These elements improve corrosion resistance. Note that these elements do not necessarily have to be contained, so the lower limit of the content of these elements is 0%.

Sn: 0.001 to 0.5%,
Sn is an element that improves corrosion resistance. In order to stably obtain the effect of improving corrosion resistance by Sn, the Sn content is preferably 0.001% or more. The Sn content is more preferably 0.01% or more. On the other hand, excessive Sn content leads to an increase in cost. Therefore, the Sn content is preferably 0.5% or less. The Sn content is more preferably 0.2% or less.

V: 0.001 to 0.5%
V is a carbide forming element. When V is contained, V carbide is preferentially formed, and the formation of Cr carbide is suppressed. This makes it possible to suppress the deterioration of corrosion resistance due to Cr carbide. V is also an element that improves corrosion resistance. In order to obtain the above effects, the V content is preferably 0.001% or more. The V content is more preferably 0.005% or more. On the other hand, since a high V content leads to hardening, the V content is preferably 0.5% or less. The V content is more preferably 0.2% or less.

W: 0.001 to 0.5%
W is an element that improves corrosion resistance. In order to stably obtain the effect of improving corrosion resistance by W, the W content is preferably 0.001% or more. The W content is more preferably 0.01% or more. On the other hand, if W is contained excessively, the workability may deteriorate. Therefore, the W content is preferably 0.5% or less. The W content is more preferably 0.2% or less.

Zr: 0.001 to 0.5%
Zr is an element that improves corrosion resistance. In order to stably obtain the effect of improving corrosion resistance by Zr, the Zr content is preferably 0.001% or more. The Zr content is more preferably 0.01% or more. On the other hand, if Zr is contained in excess, the workability may deteriorate. Therefore, the Zr content is preferably 0.5% or less. The Zr content is more preferably 0.2% or less.

Hf: 0.001 to 0.5%
Hf is an element that improves corrosion resistance. In order to stably obtain the above-mentioned effects due to Hf, the Hf content is preferably 0.001% or more. The Hf content is more preferably 0.05% or more. On the other hand, if Hf is contained excessively, the workability may decrease. Therefore, the Hf content is preferably 0.5% or less. The Hf content is more preferably 0.2% or less.

Ta: 0.001 to 0.5%
Ta is an element that improves corrosion resistance by modifying inclusions. In order to stably obtain the above-mentioned effect of Ta, the Ta content is preferably 0.001% or more. The Ta content is more preferably 0.05% or more. On the other hand, excessive Ta content may lead to a decrease in ductility and toughness at room temperature. Therefore, the Ta content is preferably 0.5% or less. The Ta content is more preferably 0.2% or less.

Ga: 0.001 to 0.5%
Ga is an element that improves corrosion resistance and hydrogen embrittlement resistance. In order to stably obtain the above-mentioned effects due to Ga, the Ga content is preferably 0.001% or more. The Ga content is more preferably 0.05% or more. On the other hand, if the Ga content exceeds 0.5%, the above-mentioned effects are saturated and only lead to an increase in costs. Therefore, the Ga content is preferably 0.5% or less. The Ga content is more preferably 0.2% or less.

Sb: 0.001 to 0.5%
Sb is an element that improves corrosion resistance. In order to stably obtain the effect of improving corrosion resistance by Sb, the Sb content is preferably 0.001% or more. The Sb content is more preferably 0.05% or more. On the other hand, excessive Sb content leads to an increase in cost. Therefore, the Sb content is preferably 0.5% or less. The Sb content is more preferably 0.2% or less.

[Oxide film]
The ferritic stainless steel according to this embodiment has an oxide film on its surface. The oxide film has an FV value, expressed by the following formula (1), of 0.25 or more or -0.25 or less.
FV = CCr + _CSi + _{CTi -} CFe - _{CNi - CCu - CNb} - _{CMo ...} (1)
In the above formula (1), CCr, CSi, CTi, CFe, CNi, CCu, _CNb and CMo are the cation fractions of _Cr , _Si , _Ti , _Fe , _Ni , _Cu , Nb and _Mo , respectively, when the total amount of cations of all metal elements in the outermost layer of the coating is taken as 1.

The work function of the oxide film changes depending on the content of Cr, Si, Ti, Fe, Ni, Cu, Nb, and Mo in the oxide film. Therefore, when the cationic fraction of the above elements changes, the corrosion resistance against corrosion caused by chloride ions changes. If the FV value represented by the above formula (1) is 0.25 or more or -0.25 or less, the corrosion resistance against corrosion caused by chloride ions is improved. The FV value is preferably 0.28 or more or -0.28 or less.

The outermost layer of the film here refers to the surface (a few nm deep) before the oxide film is destroyed by sputtering or other methods, and the metal element components used to calculate the FV value are those obtained by analyzing this outermost layer of the film using Auger electron spectroscopy.

The state of Cr, Si, Ti, Fe, Ni, Cu, Nb and Mo in the oxide film can be examined by X-ray photoelectron spectroscopy. For example, when the acceleration pressure is 10.0 keV and the irradiation current is 10.0 nA, the state of these elements can be confirmed by the presence or absence of peaks at the following binding energies:

Cr: 525 to 535 eV
Si: 1615 to 1625 eV
Ti: 415 to 425 eV
Fe: 700 to 710 eV
Ni: 845 to 855 eV
Cu: 915 to 925 eV
Nb: 165 to 175 eV
Mo: 185 to 195 eV

The cationic fraction of the above elements relative to the cationic amount of all metal elements can be determined by Auger electron spectroscopy using the following method. The spectrum of cationic elements is measured using an AES (Auger electron spectroscopy) without processing or chemical treatment of the surface, and without sputtering, and the intensity value of the differential peak of the spectrum is calculated. The intensity value is the "peak maximum intensity value - spectrum background value". The intensity value is then corrected by the relative sensitivity factor (intensity value ÷ relative sensitivity factor) to determine the cationic fraction of Cr, Si, Ti, Fe, Ni, Cu, Nb, and Mo when the total amount of the measured cationic elements is set to 1. Note that foreign matter may be adsorbed on the surface of the oxide film, which may be an impediment to determining the cationic amount of elements that contribute to corrosion resistance. For this reason, it is necessary to thoroughly remove foreign matter from the surface of the sample before analysis, for example by ultrasonic cleaning with an organic solvent such as acetone. In addition, since cleaning with organic solvents may not be able to fully remove foreign matter such as C, if C is detected at 50 atomic % or more, this value is excluded, and sputtering is performed at intervals of 0.5 nm or less, and the FV value is calculated using the analysis value from the depth where C becomes less than 50 atomic %.

So far, we have described the oxide film of the ferritic stainless steel according to this embodiment. The ferritic stainless steel according to this embodiment has improved corrosion resistance against corrosion caused by chloride ions.

The ferritic stainless steel according to this embodiment may be in various shapes, such as thin plate, thick plate, wire, and rod.

<Method of manufacturing ferritic stainless steel>
(First manufacturing method)
The method for producing ferritic stainless steel according to this embodiment includes a steelmaking process, a hot rolling process, a hot rolling annealing process, a hot rolled sheet pickling process, a cold rolling process, a cold rolling annealing process, a cold rolled sheet pickling process, a first immersion process, and a second immersion process. For the processes other than the first immersion process and the second immersion process, the production conditions are not particularly limited, and known methods can be applied. In the first production method, the first immersion process and the second immersion process are extremely important. The first immersion process and the second immersion process will be described in detail below.

[First immersion step]
In the first immersion step, the cold-rolled stainless steel material having the above-mentioned chemical components and having been subjected to the pickling step is immersed for 60 to 120 seconds in a solution having a pH of 1.0 or less and an electrode potential of 0.35 V or more based on a silver-silver chloride electrode. This causes Ti, Fe, Ni, Cu, and Nb present in the oxide film to dissolve, and Si, Cr, and Mo to concentrate.

A pH above 1.0 is not preferred because the dissolution of Ti, Fe, Ni, Cu and Nb in the oxide film is insufficient and the concentration of Si, Cr and Mo is insufficient.

If the electrode potential based on the silver-silver chloride electrode is less than 0.35 V, the dissolution of Ti, Fe, Ni, Cu and Nb in the oxide film will be insufficient, and the concentration of Si, Cr and Mo will be insufficient, which is undesirable.

Immersion times of less than 60 seconds are not preferred because Ti, Fe, Ni, Cu, and Nb are not sufficiently dissolved and Si, Cr, and Mo are not sufficiently concentrated. On the other hand, immersion times of more than 120 seconds are not preferred because Cr and Mo are excessively dissolved and Cr and Mo are not sufficiently concentrated.

The solution used in the first immersion step is not particularly limited as long as the pH and the electrode potential based on a silver-silver chloride electrode satisfy the above conditions. For example, a solution containing sulfuric acid or nitric acid is used as the solution.
If the electrode potential based on a silver-silver chloride electrode does not reach 0.35 V or more simply by immersing the cold-rolled sheet in the above solution, an electrode may be inserted into the solution and a voltage may be applied between the cold-rolled sheet and the electrode from an external power source to raise the electrode potential based on a silver-silver chloride electrode to 0.35 V or more.

[Second immersion step]
In the second immersion step, the stainless steel material after the first immersion step is immersed for 5 to 60 seconds in a solution having a pH of 4.0 to 8.0 and an electrode potential of 0.45 V or more based on a silver-silver chloride electrode. This causes Mo present in the oxide film to dissolve, and Cr, Si, and Ti to become concentrated.

At a pH of less than 4.0, Ti is excessively dissolved and Ti is not concentrated sufficiently, which is not preferred. At a pH of more than 8.0, Fe is not dissolved, Fe is excessively concentrated, and Cr, Si, and Ti are not concentrated sufficiently, which is also not preferred.

If the electrode potential based on the silver-silver chloride electrode is less than 0.45 V, excessive Cr dissolution will occur and Cr will not be sufficiently concentrated, which is undesirable.

If the immersion time is less than 5 seconds, the dissolution of Mo is insufficient, which is undesirable. On the other hand, if the immersion time is more than 60 seconds, Fe does not dissolve, resulting in excessive Fe concentration, and the concentration of Cr, Si, and Ti is insufficient, which is undesirable.

The solution used in the second immersion step is not particularly limited as long as the pH and the electrode potential based on a silver-silver chloride electrode satisfy the above conditions. For example, a 0.1 mol/L aqueous sodium sulfate solution is used as the solution.
If the electrode potential of the cold-rolled sheet does not reach 0.45 V or more with respect to a silver-silver chloride electrode by simply immersing the cold-rolled sheet in the above solution, the potential can be adjusted in the same manner as in the first immersion step.

The first manufacturing method produces a ferritic stainless steel sheet containing the above chemical components and having an FV value of 0.25 or more.

It is preferable to rinse the surface of the stainless steel material after the first immersion step with water between the first immersion step and the second immersion step. This prevents the solution used in the second immersion step from being contaminated by the solution that adheres to the stainless steel material in the first immersion step. As a result, the frequency of changing the solution used in the second immersion step is reduced, and manufacturing costs are reduced.

(Second manufacturing method)
The manufacturing method of the ferritic stainless steel according to the second embodiment includes a steelmaking process, a hot rolling process, a hot rolling annealing process, a hot rolled sheet pickling process, a cold rolling process, a cold rolling annealing process, a cold rolled sheet pickling process, a third immersion process, and a fourth immersion process. For the processes other than the third immersion process and the fourth immersion process, the manufacturing conditions are not particularly limited, and known methods can be applied. In the second manufacturing method, the third immersion process and the fourth immersion process are extremely important. The third immersion process and the fourth immersion process will be described in detail below.

[Third immersion step]
In the third immersion step, the stainless steel material after the cold-rolled sheet pickling step having the above-mentioned chemical components is immersed for 60 to 120 seconds in a solution having a pH of 4.0 to 8.0 and an electrode potential of 0.45 V or more based on a silver-silver chloride electrode. This causes Cr and Mo present in the oxide film to dissolve, and Si, Ti, Ni, Cu, Nb and Fe to concentrate.

If the pH is less than 4.0, excessive dissolution of Fe occurs, and Fe is not concentrated sufficiently, which is not preferred. If the pH is more than 8.0, Cr and Mo are not dissolved sufficiently, and Fe is not concentrated sufficiently, which is not preferred.

If the electrode potential based on the silver-silver chloride electrode is less than 0.45 V, the dissolution of Cr and Mo will be insufficient and the concentration of Fe will be insufficient, which is undesirable.

Immersion times of less than 60 seconds are not preferred because Cr and Mo are not sufficiently dissolved and Fe is not sufficiently concentrated. On the other hand, immersion times of more than 120 seconds are not preferred because Si and Ti are excessively concentrated and Fe is not sufficiently concentrated.

The solution used in the third immersion step is not particularly limited as long as the pH and the electrode potential based on a silver-silver chloride electrode satisfy the above conditions, and for example, the solution used in the second immersion step can be used.
If the electrode potential of the cold-rolled sheet does not reach 0.45 V or more with respect to a silver-silver chloride electrode by simply immersing the cold-rolled sheet in the above solution, the potential can be adjusted in the same manner as in the first immersion step.

[Fourth immersion step]
In the fourth immersion step, the stainless steel material after the third immersion step is immersed for 5 to 30 seconds in a solution having a pH of 1.0 or less and an electrode potential of 0.35 V or more based on a silver-silver chloride electrode. This causes Ti present in the oxide film to dissolve and Mo to become concentrated.

A pH above 1.0 is not preferred because Fe, Ni, Cu, and Nb dissolve excessively and the concentration of Fe, Ni, Cu, and Nb is insufficient.

If the electrode potential based on the silver-silver chloride electrode is less than 0.35 V, the dissolution of Mo will be excessive and the concentration of Mo will be insufficient, which is undesirable.

If the immersion time is less than 5 seconds, the dissolution of Ti is insufficient, and Ti is not sufficiently concentrated, which is not preferred. On the other hand, if the immersion time is more than 30 seconds, the dissolution of Fe, Ni, Cu, and Nb is excessive, and Fe, Ni, Cu, and Nb are not sufficiently concentrated, which is not preferred.

The solution used in the fourth immersion step is not particularly limited as long as the pH and the electrode potential based on a silver-silver chloride electrode satisfy the above conditions, and for example, the solution used in the first immersion step can be used.
If the electrode potential of the cold-rolled sheet does not reach 0.35 V or more with respect to a silver-silver chloride electrode by simply immersing the cold-rolled sheet in the above solution, the potential can be adjusted in the same manner as in the first immersion step.

The second manufacturing method produces a ferritic stainless steel sheet containing the above chemical components and having an FV value of -0.25 or less.

It is preferable to rinse the surface of the stainless steel material after the third immersion step with water between the third immersion step and the fourth immersion step. This prevents the solution used in the fourth immersion step from being contaminated by the solution that adheres to the stainless steel material in the third immersion step. As a result, the frequency of changing the solution used in the fourth immersion step is reduced, and manufacturing costs are reduced.

(Modifications of the first and second manufacturing methods)
In the above-mentioned first and second manufacturing methods, the first and third immersion steps are performed on the stainless steel material after the cold-rolled sheet pickling step, that is, the cold-rolled sheet or cold-rolled coil is produced. However, these steps may be performed on the stainless steel material after the hot-rolled sheet pickling step. In this case, a hot-rolled coil or hot-rolled sheet having excellent corrosion resistance against corrosion caused by chloride ions is obtained.

(Third manufacturing method)
The manufacturing method of the ferritic stainless steel according to the third embodiment includes a steelmaking process, a hot rolling process, a post-hot rolling annealing process, a hot-rolled sheet pickling process, a cold rolling process, a bright annealing process, and a fifth dipping process. The third manufacturing method differs from the first manufacturing method and the second manufacturing method in that it does not include the annealing process after cold rolling and the cold-rolled sheet pickling process, but performs the bright annealing process after the cold rolling process. In the third manufacturing method, the manufacturing conditions are not particularly limited for the processes other than the fifth dipping process, and known methods can be applied. In the third manufacturing method, the fifth dipping process is extremely important. The fifth dipping process will be described in detail below.

[Fifth immersion step]
In the fifth immersion step, the stainless steel material after the bright annealing step, which has the above-mentioned chemical components, is immersed for 5 to 60 seconds in a solution with H4.0 to 8.0 and an electrode potential of 0.45 V or more based on a silver-silver chloride electrode. The oxide film after the bright annealing step contains more Cr and Si than other elements. By carrying out the above treatment on such a stainless steel material after the bright annealing step, Mo is dissolved, Si is further concentrated, and Ti is also concentrated.

At a pH of less than 4.0, excessive dissolution of Cr occurs, and the concentration of Cr, Si, and Ti is insufficient, which is not preferred. At a pH of more than 8.0, Fe does not dissolve, and instead concentrates, which is not preferred, as Cr, Si, and Ti are not sufficiently concentrated.

If the electrode potential based on the silver-silver chloride electrode is less than 0.45 V, the dissolution of Mo will be insufficient, which is undesirable.

If the immersion time is less than 5 seconds, the dissolution of Mo will be insufficient, which is undesirable. On the other hand, if the immersion time is more than 60 seconds, the dissolution of Cr will be excessive, and the concentration of Cr, Si, and Ti will be insufficient, which is undesirable.

The solution used in the fifth immersion step is not particularly limited as long as the pH and the electrode potential based on a silver-silver chloride electrode satisfy the above-mentioned conditions, and for example, the solution used in the second immersion step can be used.
If the electrode potential of the cold-rolled sheet does not reach 0.45 V or more with respect to a silver-silver chloride electrode by simply immersing the cold-rolled sheet in the above solution, the potential can be adjusted in the same manner as in the first immersion step.

The third manufacturing method produces a ferritic stainless steel sheet containing the above chemical components and having an FV value of 0.25 or more.

(Modification of the third manufacturing method)
In the above third manufacturing method, the fifth dipping step is performed on the stainless steel material after the cold rolling step, that is, a cold-rolled sheet or a cold-rolled coil is produced. However, the fifth dipping step may be performed on the stainless steel material after the hot rolling step. In this case, a hot-rolled coil or a hot-rolled sheet having excellent corrosion resistance against corrosion caused by chloride ions is obtained.

So far, the method for producing ferritic stainless steel according to this embodiment has been described. In the first to third production methods, the ferritic stainless steel produced is a thin plate, but the method for producing ferritic stainless steel according to this embodiment can also be applied to thick plates, bars and wires. When producing a thick plate, for example, the first immersion step, the third immersion step or the fifth immersion step may be performed after rolling the thick plate, and when producing a bar or wire, for example, the first immersion step, the third immersion step or the fifth immersion step may be performed after rolling, extrusion or drawing. And when the first immersion step or the third immersion step is performed, the second immersion step or the fourth immersion step may be performed after these steps, respectively.

The following provides a detailed explanation of the embodiments of the present invention, with reference to examples. Note that the examples shown below are merely examples of the present invention, and the present invention is not limited to the examples below.

Example 1
The effect of the first manufacturing method was confirmed. First, stainless steel materials having the chemical compositions shown in Tables 1 to 8 were melted in a vacuum induction melting furnace and cast to obtain slabs. Then, each slab was soaked at 1200°C, hot forged and hot rolled to obtain 6 mm hot-rolled sheets. The hot-rolled sheets were annealed and pickled, and then cold rolled to obtain 1 mm cold-rolled sheets. The cold-rolled sheets were further annealed and pickled.

The cold-rolled sheet after pickling was subjected to a first immersion step and a second immersion step under the conditions shown in Tables 1 to 8 to obtain a stainless steel sheet. Between the first and second immersion steps, the surface of the cold-rolled sheet after the first immersion step was washed with water.

The cation fraction of the cold-rolled sheet after the pickling process and the cation fraction of the cold-rolled sheet after the second immersion process were obtained by the above-mentioned method, and the FV value was calculated from the obtained cation fraction. Tables 1 to 8 show the cation fraction of the cold-rolled sheet after the pickling process and the cation fraction of the cold-rolled sheet after the second immersion process. FV ₀ shown in Tables 1 to 8 is the FV value of the cold-rolled sheet after the pickling process, and FV is the FV value of the cold-rolled sheet after the second immersion process.

The corrosion resistance of the stainless steel sheet was evaluated by the following method. Samples cut into 20 cm squares from the cold-rolled sheet after the pickling process and the obtained stainless steel were placed on a horizontal surface, and a saturated NaCl solution was sprayed with a sprayer so that NaCl adhered to the surface at an area density of 1 mg/ ^dm2 . After spraying, the sample was immediately placed horizontally in a thermo-hygrostat chamber at a relative humidity of 30% and a temperature of 40°C and held for 6 hours. After holding for 6 hours, the sample was held in an environment of a relative humidity of 90% and a temperature of 40°C for 6 hours. This cycle was repeated six times, and the degree of rust in the appearance was given an RN (rating number) of 0 to 9 according to a method conforming to JIS G 0595:2004. When the difference ΔRN between the RN of the cold-rolled sheet after the second immersion process and the RN of the cold-rolled sheet after the pickling process (cold-rolled sheet as pickled) is 1 or more, the corrosion resistance against corrosion caused by chloride ions is higher than that of the cold-rolled sheet as pickled, and even in a practical environment, it exhibits higher corrosion resistance than the cold-rolled sheet as pickled. Table 1 shows the RN of the cold-rolled sheet after the pickling process and the RN of the cold-rolled sheet after the second immersion process, as well as the difference ΔRN (RN-RN ₀ ) between the RN of the cold-rolled sheet after the second immersion process and the RN of the cold-rolled sheet after the pickling process. RN ₀ shown in Tables 1 to 8 is the RN for the cold-rolled sheet after the pickling process, and RN is the RN for the cold-rolled sheet after the second immersion process.

As shown in Tables 1 to 8, the ferritic stainless steel produced by the first manufacturing method had better corrosion resistance to chloride ions than the cold-rolled sheet that had not been pickled.

Example 2
The effect of the second manufacturing method was confirmed. First, a stainless steel material having the chemical composition shown in Table 9 was melted in a vacuum induction melting furnace and cast to obtain a slab. Then, each slab was soaked at 1200°C, hot forged and hot rolled to obtain a 6 mm hot-rolled sheet. The hot-rolled sheet was annealed and pickled, and then cold rolled to obtain a 1 mm cold-rolled sheet. The cold-rolled sheet was further annealed and pickled.

The cold-rolled sheet after pickling was subjected to the third and fourth immersion steps under the conditions shown in Table 10 to obtain a stainless steel sheet. Between the third and fourth immersion steps, the surface of the cold-rolled sheet after the third immersion step was washed with water.

The cation fraction of the cold-rolled sheet after the pickling process and the cation fraction and FV value of the cold-rolled sheet after the fourth immersion process were calculated in the same manner as in Example 1. Table 10 shows the cation fraction of the cold-rolled sheet after the pickling process and the cation fraction of the cold-rolled sheet after the fourth immersion process. FV ₀ shown in Table 10 is the FV value of the cold-rolled sheet after the pickling process, and FV is the FV value of the cold-rolled sheet after the fourth immersion process.

The corrosion resistance of the stainless steel sheet was evaluated in the same manner as in Example 1. RN ₀ shown in Table 10 is the RN for the cold-rolled sheet after the pickling process, and RN is the RN for the cold-rolled sheet after the fourth immersion process.

As shown in Tables 9 and 10, the ferritic stainless steel produced by the second manufacturing method had excellent corrosion resistance against corrosion caused by chloride ions.

Example 3
The effect of the third manufacturing method was confirmed. First, a stainless steel material having the chemical composition shown in Table 11 was melted in a vacuum induction melting furnace and cast to obtain a slab. Then, each slab was soaked at 1200°C, hot forged and hot rolled to obtain a 6 mm hot-rolled sheet. The hot-rolled sheet was annealed, pickled, and then cold rolled to obtain a 1 mm cold-rolled sheet. The cold-rolled sheet was bright annealed.

The cold-rolled sheet after bright annealing was subjected to a fifth immersion process under the conditions shown in Table 12 to obtain a stainless steel sheet.

The cation fraction of the cold-rolled sheet after bright annealing and the cation fraction and FV value of the cold-rolled sheet after the fifth dipping step were calculated in the same manner as in Example 1. Table 12 shows the cation fraction of the cold-rolled sheet after the bright annealing step and the cation fraction of the cold-rolled sheet after the fifth dipping step. FV ₀ shown in Table 12 is the FV value of the cold-rolled sheet after the bright annealing step, and FV is the FV value of the cold-rolled sheet after the fifth dipping step.

The corrosion resistance of the stainless steel sheet was evaluated in the same manner as in Example 1. RN ₀ shown in Table 12 is the RN for the cold-rolled sheet after the bright annealing process, and RN is the RN for the cold-rolled sheet after the fifth immersion process.

As shown in Tables 11 and 12, the ferritic stainless steel produced by the third manufacturing method had excellent corrosion resistance against corrosion caused by chloride ions.

Although the preferred embodiment of the present invention has been described in detail above, the present invention is not limited to such examples. It is clear that a person with ordinary knowledge in the technical field to which the present invention pertains can conceive of various modified or revised examples within the scope of the technical ideas described in the claims, and it is understood that these also naturally fall within the technical scope of the present invention.

Claims (7)

In mass percent,
C: 0.150% or less,
Si: 2.0% or less,
Mn: 1.0% or less,
P: 0.04% or less,
S: 0.05% or less,
Ni: 0.6% or less,
Cr: 10.0 to 35.0%,
Ti: 0.50% or less,
Nb: 0.50% or less,
Mo: 6.0% or less,
Cu: 2.0% or less,
N: 0.005 to 0.035%,
The balance is Fe and impurities,
A ferritic stainless steel having an FV value represented by the following formula (1) of 0.25 or more or -0.25 or less.
FV = CCr + _CSi + _{CTi -} CFe - _{CNi - CCu - CNb} - _{CMo ...} (1)
In the formula (1), CCr, CSi, CTi, CFe, CNi, CCu, _CNb and CMo respectively represent the cation fractions of _Cr , _Si , _Ti , _Fe , _Ni , _Cu , Nb and _Mo when the total amount of cations (atomic %) of all metal elements in the oxide film is taken as 1. Instead of a part of Fe,
Al: 0.001 to 0.3%,
Ca: 0.0001 to 0.3%,
B: 0.0001 to 0.1%,
Co: 0.001 to 0.5%,
Mg: 0.001 to 0.5%,
REM: 0.001 to 0.5%
Contains one or more selected from the group consisting of
The ferritic stainless steel according to claim 1, wherein the FV value is 0.25 or more or −0.32 to −0.25. Instead of a part of Fe,
Sn: 0.001 to 0.5%,
V: 0.001 to 0.5%,
W: 0.001 to 0.5%,
Zr: 0.001 to 0.5%,
Hf: 0.001 to 0.5%,
Ta: 0.001 to 0.5%,
Contains one or more selected from the group consisting of Ga: 0.001 to 0.5% and Sb: 0.001 to 0.5%;
The ferritic stainless steel according to claim 1 or 2 , wherein the FV value is 0.25 or more or −0.32 to −0.25. In mass percent,
C: 0.150% or less,
Si: 2.0% or less,
Mn: 1.0% or less,
P: 0.04% or less,
S: 0.05% or less,
Ni: 0.6% or less,
Cr: 10.0 to 35.0%,
Ti: 0.50% or less,
Nb: 0.50% or less,
Mo: 6.0% or less,
Cu: 2.0% or less,
N: 0.005 to 0.035%,
A first immersion step of immersing a stainless steel material consisting of the remainder Fe and impurities in a solution having a pH of 1.0 or less and an electrode potential of 0.35 V or more based on a silver-silver chloride electrode for 60 to 120 seconds;
a second immersion step of immersing the stainless steel material after the first immersion step in a solution having a pH of 4.0 to 8.0 and an electrode potential of 0.45 V or more based on a silver-silver chloride electrode for 5 to 60 seconds;
A method for producing ferritic stainless steel, comprising the steps of: In mass percent,
C: 0.150% or less,
Si: 2.0% or less,
Mn: 1.0% or less,
P: 0.04% or less,
S: 0.05% or less,
Ni: 0.6% or less,
Cr: 10.0 to 35.0%,
Ti: 0.50% or less,
Nb: 0.50% or less,
Mo: 6.0% or less,
Cu: 2.0% or less,
N: 0.005 to 0.035%,
a third immersion step of immersing the stainless steel material consisting of the remaining Fe and impurities in a solution having a pH of 4.0 to 8.0 and an electrode potential of 0.45 V or more based on a silver-silver chloride electrode for 60 to 120 seconds;
and a fourth immersion step of immersing the stainless steel material after the third immersion step in a solution having a pH of 1.0 or less and an electrode potential of 0.35 V or more based on a silver-silver chloride electrode for 5 to 30 seconds. The method for producing ferritic stainless steel according to claim 4 or 5, wherein the stainless steel material is stainless steel after hot working or stainless steel after cold working. In mass percent,
C: 0.150% or less,
Si: 2.0% or less,
Mn: 1.0% or less,
P: 0.04% or less,
S: 0.05% or less,
Ni: 0.6% or less,
Cr: 10.0 to 35.0%,
Ti: 0.50% or less,
Nb: 0.50% or less,
Mo: 6.0% or less,
Cu: 2.0% or less,
N: 0.005 to 0.035%,
A bright annealing process for bright annealing a stainless steel material consisting of the remainder Fe and impurities;
and a fifth immersion step of immersing the stainless steel material after the bright annealing step in a solution having H4.0 to 8.0 and an electrode potential of 0.45 V or more based on a silver-silver chloride electrode for 5 to 60 seconds.