JP2024127105A - Ferritic Stainless Steel Sheet - Google Patents

Abstract

[Problem] To provide a ferritic stainless steel sheet having excellent toughness and corrosion resistance after long-term use. [Solution] The ferritic stainless steel sheet contains, by mass%, C: 0.001-0.100%, Si: 0.01-1.00%, Mn: 0.01-1.00%, P: ≦0.050%, S: ≦0.0100%, Cr: 10.5-30.0%, Ni: 0.01-3.00%, Al: 0.01-1.00%, Sn: 0.01-0.50%, N: 0.001-0.050%, B: 0.0001-0.0050%, and further contains Ti and N. b in the range of 0.01 to 1.00%, with the remainder consisting of iron and unavoidable impurities, the average crystal grain size is 25 μm or less, the average Charpy impact value at a material temperature of 25°C is 10 J/cm2 or more, and when the peak intensity of the spectrum of each contained element is measured by Auger electron spectroscopy from the steel sheet surface to the measurement depth where the O concentration is 1/2 of the maximum value, the Cr content is 20.0 atomic% or more in cationic fraction. [Selected figure] None

Description

The present invention relates to ferritic stainless steel sheets that are used for containers, etc.

Containers are vessels used to transport, store, and handle a variety of items, including chemicals, acids, and food (hereafter referred to as "various items"), and are strong enough to withstand the transport, storage, and handling of a variety of items. Containers are often used for long periods of time, such as 10 years or more, and because they are used to transport a variety of items over long distances both domestically and internationally, the components that make up the container must have a high level of corrosion resistance.

Container components include steel and aggregates for exterior lining, but these components are intended to be painted. For example, Patent Document 1 discloses a structural Cr steel containing, by mass%, C: 0.002-0.02%, N: 0.002-0.02%, Si: 0.05-1.0%, Mn: 0.05-1.0%, P: 0.04% or less, S: 0.02% or less, Al: 0.001-0.1%, Cr: 6.0-10.0%, with the balance being Fe and unavoidable impurities, and the Cr concentration on the steel plate surface is (Cr concentration inside the steel plate - 1)% or more. This structural Cr steel achieves both corrosion resistance, toughness, and impact resistance by adjusting the amounts of Cr, C, and N, and corrosion resistance after painting is guaranteed by controlling the amount of the steel plate surface layer removed during descaling of the hot-rolled plate.

In addition, stainless steel has also been used as a component of containers because it exhibits excellent corrosion resistance without painting. For example, Patent Document 2 describes a material with the following composition by weight: C: 0.03% or less, Si: 0.30-0.60%, Mn: 0.70-1.00%, P: 0.040% or less, S: 0.010% or less, Ni: 0.15-0.45%, Cr: 11.50-12.50%, Mo: 0.50% or less, Cu: 0.30-0.50%, N: 0.060% or less, and γ = 420C + 470N + 23Ni + 9Cu + A steel for marine containers is disclosed in which the content of each component element is mutually adjusted so that the calculated value γ according to the formula 7Mn-11.5Cr-11.5Si-12Mo+189 (wt%)H=218C-2.6Si+2.9Mn+9.2Ni-4.6Cr+3.6Cu+138N+75.2 (wt%) is in the range of 80 or more and the calculated value H is in the range of 38 or less, with the remainder being Fe and unavoidable impurities. This steel for marine containers combines toughness, weldability, and moderate corrosion resistance through the adjustment of the components.

JP 2004-115909 A Japanese Patent Application Publication No. 63-436

However, steel materials that are intended to be painted, such as the structural Cr steel in Patent Document 1, have a problem in that their corrosion resistance drops significantly after the paint is peeled off. In addition, the steel for marine containers in Patent Document 2 maintains its toughness after long-term use, but its corrosion resistance is insufficient.

One aspect of the present invention has been made to solve the above-mentioned problems, and aims to provide a ferritic stainless steel sheet that has excellent toughness and corrosion resistance after long-term use.

In order to solve the above-mentioned problems, the inventors of the present invention have investigated whether it is possible to improve the toughness and corrosion resistance of ferritic stainless steel sheets after long-term use by producing and testing ferritic stainless steel sheets containing various elements. As a result, they have found that adding a very small amount of Sn is effective. They have also found that adjusting the Cr content in the passive film is effective. Furthermore, they have found that in the manufacturing method of ferritic stainless steel sheets, by setting the temperature rise rate of the cold-rolled sheet in the cold-rolled sheet annealing process to 100°C/s or more, the average crystal grain size of the ferritic stainless steel sheet becomes 25 μm or less, and the toughness is dramatically improved.

In other words, the present invention was completed based on the above findings, and the gist of the present invention, which aims to solve the above-mentioned problems, is as follows.

[1] In mass%,
C: 0.001-0.100%,
Si: 0.01-1.00%,
Mn: 0.01-1.00%,
P:≦0.050%,
S:≦0.0100%,
Cr: 10.5-30.0%,
Ni: 0.01 to 3.00%,
Al: 0.01-1.00%,
Sn: 0.01-0.50%,
N: 0.001-0.050%,
B: 0.0001 to 0.0050%;
Further, at least one of Ti and Nb is contained in the range of 0.01 to 1.00%,
the balance being iron and unavoidable impurities;
The average crystal grain size is 25 μm or less,
The average Charpy impact value at a material temperature of 25°C is 10 J/ ^cm2 or more,
A ferritic stainless steel sheet characterized in that, when the peak intensity of the spectrum of each contained element is measured by Auger electron spectroscopy from the steel sheet surface to a measurement depth at which the O concentration is half of its maximum value, the ferritic stainless steel sheet contains Cr at a cation fraction of 20.0 atomic% or more.

[2] Further, in mass%,
Mo: 0.01-3.00%,
Cu: 0.01 to 0.50%,
W: 0.001-1.000%,
V: 0.001-1.000%,
Sb: 0.001 to 0.100%,
Co: 0.001 to 0.500%,
Ca: 0.0001-0.0050%,
Mg: 0.0001 to 0.0050%,
Zr: 0.0001 to 0.0300%,
Ga: 0.0001-0.0100%,
Ta: 0.001 to 0.050%,
REM: 0.001~0.100%
The ferritic stainless steel sheet according to [1], characterized in that it contains one or more of the following:

[3] A ferritic stainless steel sheet according to [1] or [2] used for container applications.

According to one aspect of the present invention, a ferritic stainless steel sheet having excellent toughness and corrosion resistance after long-term use can be provided.

One embodiment of the present invention will be described in detail below. Note that the present invention is not limited to the following embodiment or example, and various modifications are possible within the scope of the claims. The technical scope of the present invention also includes embodiments obtained by appropriately combining the technical means disclosed in the different embodiments and examples. In this specification, "A to B" indicates greater than or equal to A and less than or equal to B.

〔overview〕
The present inventors produced ferritic stainless steel sheets containing various concentrations of Sn at various heating rates in order to improve toughness and corrosion resistance after long-term use. Then, by comparing the properties of the sheets, the effects of the Sn concentration and the average crystal grain size (described in detail later) on the corrosion resistance and toughness of the ferritic stainless steel sheets were investigated. As a result, it was found that by making the ferritic stainless steel sheets into the following (I), (II), and (III), a ferritic stainless steel sheet having excellent toughness and corrosion resistance after long-term use, for example, for container applications, can be obtained.
(I) The Cr content in the passive film is set to 20.0 atomic % or more in terms of cation fraction.
(II) The Sn content in the base metal is in the range of 0.01 to 0.50%.
(III) The average crystal grain size is 25 μm or less.

First, the passive film formed on the surface of a ferritic stainless steel sheet is one element that ensures the corrosion resistance of the ferritic stainless steel sheet, but the passive film is subject to localized destruction over long-term use. Therefore, in a ferritic stainless steel sheet according to one embodiment of the present invention, the Cr content in the passive film is set to 20.0 atomic% or more in terms of cation fraction, thereby primarily improving the corrosion resistance of the ferritic stainless steel sheet.

In addition, in the ferritic stainless steel sheet according to one embodiment of the present invention, the Sn content is set to 0.01 to 0.50%. As a result, even if the passive film is destroyed by long-term use, an appropriate amount of Sn dissolves into the moisture adhering to the steel sheet surface from the destroyed part of the passive film. This dissolved Sn secondarily improves the corrosion resistance of the ferritic stainless steel sheet according to one embodiment of the present invention after long-term use.

Here, "long-term use" in this specification specifically refers to using the ferritic stainless steel sheet according to one embodiment of the present invention in an environment at room temperature or below (about -20 to about 30°C) for 10 years or more. Examples of environments at room temperature or below include storage of agricultural products, frozen tuna, etc. In the case of such a usage mode, during the course of use, Sn dissolves from the destruction of the passive film formed on the surface layer of the ferritic stainless steel sheet according to one embodiment of the present invention and becomes an ionized state in water. Furthermore, if the steel sheet is temporarily placed in an environment at 0°C or below during the course of long-term use, the water containing Sn freezes and remains temporarily on the steel sheet surface. Then, when the steel sheet is placed again in an environment at room temperature, the frozen water melts and the Sn in the water exerts the effect of improving corrosion resistance.

Next, while Sn ensures the corrosion resistance of ferritic stainless steel sheets, especially in saltwater environments, it also reduces toughness, which has been an issue. Therefore, in the manufacturing method of ferritic stainless steel sheets according to one embodiment of the present invention, as described above, the heating rate in the cold-rolled sheet annealing process is set to 100°C/s or more. This allows the cold-rolled annealed sheet to have an extremely fine-grained structure due to both the rapid heating and the strain accumulated in the steel sheet from the time of hot rolling. Finally, a ferritic stainless steel sheet having sufficient toughness for use in containers, for example, is obtained.

As such, the cold-rolled sheet annealing conditions are extremely important in producing a ferritic stainless steel sheet according to one embodiment of the present invention. Specifically, by setting the heating rate to 100°C/s or more, the cold-rolled sheet annealing temperature to 850 to 1050°C, and the cold-rolled sheet annealing time to 30 seconds or less, the average crystal grain size of the stainless steel sheet (details described below) can be set to 25 μm or less.

[Component Composition]
The chemical composition of the ferritic stainless steel sheet according to one embodiment of the present invention (hereinafter, abbreviated as "the present stainless steel sheet") will be described in more detail below. In addition, unless otherwise noted, in this specification, % of element content means % by mass.

<C: 0.001-0.100%>
C reduces intergranular corrosion resistance and workability, so its content must be kept low. Therefore, the upper limit of the C content is set at 0.100%. On the other hand, if the C content is excessively high, Therefore, the lower limit of the C content is set to 0.001%. The preferred range of the C content is 0.002 to 0.010%.

<Si: 0.01-1.00%>
Silicon improves oxidation resistance, so the lower limit of the silicon content is set to 0.01%. On the other hand, excessive silicon content reduces the elongation of the stainless steel sheet, lowering its workability. In addition, excessive Si content also reduces toughness. Therefore, the upper limit of the Si content is set to 1.00%. The preferred range of the Si content is 0.10 to 0.70%. A more preferable range is 0.20 to 0.50%.

<Mn: 0.01 to 1.00%>
Mn is useful as a deoxidizing element, but if it is contained in excess, the corrosion resistance decreases. Therefore, the Mn content range is set to 0.01 to 1.00%. The preferred range of the Mn content is , 0.05 to 0.70%, and a more preferable range is 0.10 to 0.50%.

<P: 0.050% or less>
Since P is an element that reduces workability, weldability, and corrosion resistance, its content must be limited. Therefore, the upper limit of the P content is set to 0.050%. The range is 0.030% or less.

<S: 0.0100% or less>
Since S is an element that reduces corrosion resistance, its content must be limited. Therefore, the upper limit of the S content is set to 0.0100%. The preferred range of the S content is 0.0070%. % or less.

<Cr:10.5-30.0%>
The Cr content must be 10.5% or more in order to ensure corrosion resistance in salt damage environments. Here, the higher the Cr content, the better the corrosion resistance, but on the other hand, the worse the workability and toughness. Therefore, the upper limit of the Cr content is set to 30.0%. The preferred range of the Cr content is 12.0 to 20.0%, and the more preferred range is 13.5 to 18. The probability is 0%.

<Ni: 0.01-3.00%>
Ni improves corrosion resistance and toughness, so the lower limit of the Ni content is set to 0.01%. On the other hand, an excessive Ni content increases the alloy cost, so the upper limit of the Ni content is set to 3. The Ni content is preferably in the range of 0.02 to 2.50%, more preferably in the range of 0.05 to 2.00%, and even more preferably in the range of 0.10 to 1.50%. .

<Ti, Nb: 0.01 to 1.00%>
Both Ti and Nb prevent sensitization of the present stainless steel sheet, so the content of at least one of Ti and Nb must be 0.01% or more. If this content is less than 0.01%, The stainless steel sheet becomes sensitized and its corrosion resistance is reduced. On the other hand, if the content of at least one of Ti and Nb is excessively increased, it will lead to an increase in alloy cost, a decrease in toughness, and a decrease in corrosion resistance due to an increase in inclusions in the steel. Therefore, the upper limit of the content of at least one of Ti and Nb is set to 1.00%. The preferred range of this content is 0.03 to 0.50%, and the more preferred range is 0.10 to 0.25%. %.

<Nb: 0.01 to 1.00%>
Nb is an element useful for preventing sensitization of the present stainless steel sheet. Nb is also an element useful for improving both high-temperature strength and intergranular corrosion resistance of welds. However, if Nb is contained in an excessive amount, the workability and toughness are reduced. Therefore, the Nb content is set to a range of 0.01 to 1.00%. The preferred range of the Nb content is 0.05 to 0. . 50%.

<Al: 0.10-1.00%>
The lower limit of the content of Al is set to 0.10% in order to improve the oxidation resistance. On the other hand, excessive Al content reduces the elongation of the stainless steel sheet, thereby deteriorating the workability. At the same time, Al also increases the hardness, which causes surface defects. Surface defects can lead to rusting. Therefore, the upper limit of the Al content is set at 1.00%. Preferred range of Al content is 0.15 to 0.80%, and a more preferable range is 0.20 to 0.50%.

<Sn: 0.01 to 0.50%>
Sn improves corrosion resistance, so the lower limit of the Sn content is set to 0.01%. On the other hand, excessive Sn content leads to a decrease in toughness, so the upper limit of the Sn content is set to 0.50%. The preferred range of the Sn content is 0.05 to 0.40%, more preferably 0.07 to 0.30%, and even more preferably 0.10 to 0.20%.

<N: 0.001-0.050%>
N is a useful element for improving pitting corrosion resistance, but it also has the disadvantage of reducing intergranular corrosion resistance and workability. Therefore, it is necessary to keep the N content low. The upper limit of the N content is set to 0.050%. On the other hand, if the N content is excessively reduced, the refining cost increases, so the lower limit of the N content is set to 0.001%. The preferred range is 0.002 to 0.020%.

<B: 0.0001 to 0.0050%>
B is an element useful for improving secondary (reinforcement) workability, and may be contained up to an upper limit of 0.0050%. On the other hand, in order for the present stainless steel sheet to stably exhibit secondary workability, Furthermore, the lower limit of the B content is set to 0.0001%. The preferred range of the B content is 0.0005 to 0.0040%.

The above are the basic components of this stainless steel sheet. If necessary, this stainless steel sheet may contain one or more of the following elements (hereinafter sometimes collectively referred to as "optional components"): Mo, Cu, W, V, Sb, Co, Ca, Mg, Zr, Ga, Ta, and REM.

<Mo: 0.01-3.00%>
Mo improves corrosion resistance, so the lower limit of the Mo content is set to 0.01%. On the other hand, if Mo is contained excessively, not only does it reduce workability, but it is also an expensive element, so it is within the allowable range. Therefore, the upper limit of the Mo content is set to 3.00%. The preferred range of the Mo content is 0.05 to 1.00%.

<Cu: 0.01 to 0.50%>
Cu improves corrosion resistance, so the lower limit of the Cu content is set to 0.01%. On the other hand, excessive Cu content leads to an increase in costs that exceeds the allowable range. Therefore, the upper limit of the Cu content is set to 0.01%. The Cu content is preferably in the range of 0.02 to 0.40%, and more preferably in the range of 0.05 to 0.30%.

<W: 0.001-1.000%>
W is an element useful for improving corrosion resistance, and may be contained up to an upper limit of 1.000%. On the other hand, in order for the present stainless steel sheet to stably exhibit corrosion resistance, the lower limit of the W content is set to 1.000%. The W content is preferably in the range of 0.005 to 0.800%.

<V: 0.001-1.000%>
V is an element useful for improving corrosion resistance, and may be contained up to an upper limit of 1.000%. On the other hand, in order for the present stainless steel sheet to stably exhibit corrosion resistance, the lower limit of the V content is set to 1.000%. The V content is preferably in the range of 0.005 to 0.500%.

<Sb: 0.001 to 0.100%>
Sb is an element useful for improving general corrosion resistance, and may be contained up to an upper limit of 0.100%. On the other hand, in order for the present stainless steel sheet to stably exhibit general corrosion resistance, Sb The lower limit of the Sb content is 0.001%. The preferred range of the Sb content is 0.010 to 0.080%.

<Co:0.001-0.500%>
Co is an element useful for improving secondary workability and toughness, and may be contained up to an upper limit of 0.500%. In order to stably exert the effect, the lower limit of the Co content is set to 0.001%, and the preferred range of the Co content is 0.010 to 0.300%.

<Ca: 0.0001-0.0050%>
Ca is an element that is mainly contained for the purpose of desulfurization, and if it is contained in excess, water-soluble inclusions called CaS are generated, which reduces corrosion resistance. Therefore, the Ca content range is set to 0.0001 to 0.0050. The preferred range of the Ca content is 0.0005 to 0.0030%.

<Mg: 0.0001 to 0.0050%>
Mg is an element that refines the structure and improves workability and toughness, and may be contained up to an upper limit of 0.0050%. On the other hand, in order for the present stainless steel sheet to stably exhibit both workability and toughness, Furthermore, the lower limit of the Mg content is set to 0.0001%. The preferred range of the Mg content is 0.0005 to 0.0030%.

<Zr: 0.0001 to 0.0300%>
Zr is an element useful for improving corrosion resistance, and may be contained up to an upper limit of 0.0300%. On the other hand, in order for the present stainless steel sheet to stably exhibit corrosion resistance, the lower limit of the Zr content is set to 0.0300%. The Zr content is preferably in the range of 0.0010 to 0.0100%.

<Ga: 0.0001 to 0.0100%>
Ga is an element useful for improving corrosion resistance and hydrogen embrittlement resistance, and may be contained up to an upper limit of 0.0100%. In order to achieve this, the lower limit of the Ga content is set to 0.0001%, and the preferred range of the Ga content is 0.0005 to 0.0050%.

<Ta: 0.001 to 0.050%>
Ta is an element useful for improving corrosion resistance, and may be contained up to an upper limit of 0.050%. On the other hand, in order for the present stainless steel sheet to stably exhibit corrosion resistance, the lower limit of the Ta content is set to 0.050%. The Ta content is preferably in the range of 0.005 to 0.030%.

<REM (rare earth element): 0.001 to 0.100%>
REM is an element that has a deoxidizing effect and is useful in refining, and can be contained up to an upper limit of 0.100%. On the other hand, in order to stably obtain the deoxidizing effect, the content of REM is The lower limit is 0.001%. The preferred range of the REM content is 0.003 to 0.050%.

Here, REM is a general term for two elements, scandium (Sc) and yttrium (Y), and one or more elements selected from the 15 elements (lanthanoids) from lanthanum (La) to lutetium (Lu), according to the general definition. Therefore, the REM content is the total content of one or more elements selected from the 17 elements mentioned above.

<Other ingredients>
The balance of this stainless steel sheet other than the above-mentioned basic components and optional components is Fe and unavoidable impurities.

In the general production of ferritic stainless steel sheets, scrap raw materials are often used, and therefore various impurity elements inevitably become mixed into the ferritic stainless steel sheets. Here, it is difficult to determine the content of the inevitably mixed-in impurity elements in a univocal manner. Therefore, the unavoidable impurities contained in the present stainless steel sheet refer to impurity elements contained in an amount that does not impair the effects of the present stainless steel sheet.

[Charpy impact value]
The method for measuring the Charpy impact value is, for example, as follows: A test piece for the Charpy test is cut out from the stainless steel plate, and the Charpy test is carried out in accordance with JIS Z 2242 at a material temperature of 25° C. It is preferable that the average Charpy impact value of the stainless steel plate is 10 J/cm ² or more.

[Average crystal grain size]
As described above, the average crystal grain size of the present stainless steel sheet is preferably 25 μm or less. The more preferred range of the average crystal grain size is 20 μm or less. By making the average crystal grain size 25 μm or less, the present stainless steel sheet has sufficient toughness for use as a structural material for containers and the like.

The average crystal grain size is measured, for example, as follows. First, a test piece 30 mm long and 20 mm wide is cut out from the stainless steel plate. Next, the test piece is embedded in resin so that the cross-sectional structure in the rolling direction of the test piece can be observed with a metallurgical microscope. Here, "rolling direction" refers to the rolling direction of the stainless steel plate. Next, the resin in which the test piece is embedded is mirror polished and etched, and the average crystal grain size is measured in accordance with the "straight test line cutting method" of JIS G 0551 "Steel - Microscopic test method for crystal grain size."

Specifically, a straight line is drawn in the thickness direction of the test piece shown in the photograph of the cross-sectional structure (from the top to the bottom of the photograph of the cross-sectional structure), and the number of trapped grains and the number of intersections between the grain boundaries and the line are determined. This measurement is performed in five fields of view. The average number of trapped grains and the average number of intersections are then determined, and the average grain size is roughly calculated based on the "relationship between the number of grains and each variable" shown in JIS G 0551. Alternatively, five of the aforementioned test pieces may be cut from this stainless steel plate, and the number of trapped grains and the number of intersections between the grain boundaries and the line are determined once for each test piece.

[Passive film]
As described above, the present stainless steel sheet preferably has a Cr content of 20.0 atomic% or more in terms of cationic fraction in the passive film. The passive film is formed on the surface layer of the present stainless steel sheet and is composed of oxides of Fe, Cr, Mn, Ti, Nb, Al, Si and Sn, as well as the remainder (inclusions, etc.). The cationic fraction is the ratio (%) of the atomic weight of each element to the total atomic weight of Fe, Cr, Mn, Ti, Nb, Al, Si, Sn and the remainder other than these elements in the passive film.

Here, the Cr content in the passive film of the present stainless steel sheet is specifically the Cr content obtained by measuring the peak intensity of the spectrum of each contained element from the steel sheet surface to a specified measurement depth by Auger electron spectroscopy. Auger electron spectroscopy is a method for analyzing elements localized on the surface of a sample by measuring the energy of Auger electrons excited by an electron beam and emitted from the sample. In addition, Auger electron spectroscopy can also analyze elements localized in the depth direction of the sample by using Ar ion sputtering in combination. In this embodiment, Ar ion sputtering is used in combination, and the component composition at each measurement depth is identified using a relative sensitivity factor from the peak intensity of the spectrum of each contained element measured at each measurement depth from the steel sheet surface to the specified measurement depth.

In this embodiment, the measurement depth at which the O concentration is 1/2 of the maximum value is regarded as the thickness of the passive film. Furthermore, for each measurement depth up to the measurement depth at which the O concentration is 1/2 of the maximum value, the average value of the cation fraction of each contained element measured at each measurement depth is regarded as the cation fraction of each contained element in the passive film.

[Production method]
The method for producing the present stainless steel sheet in this embodiment is as follows. First, molten steel containing the above-mentioned basic components and optional components is prepared in a converter or electric furnace, and then refined in an AOD furnace or VOD furnace. Next, a steel slab is produced by a continuous casting method or an ingot casting method. The steel slab is then subjected to predetermined treatments in each of the steps of a hot rolling process - a pickling process - a cold rolling process - a cold-rolled sheet annealing process - and a pickling process to produce the present stainless steel sheet.

In this embodiment, the hot-rolled sheet annealing process is omitted from the viewpoint of improving the toughness of the hot-rolled sheet and refining the average crystal grain size in the cold-rolled annealed sheet, but the hot-rolled sheet annealing process may be included.

In this method for manufacturing stainless steel sheets, the cold-rolled sheet annealing conditions are strictly controlled to keep the average crystal grain size at 25 μm or less. Specifically, as mentioned above, the heating rate is 100°C/s or more, the cold-rolled sheet annealing temperature is 850 to 1050°C (preferably 900 to 1000°C), and the cold-rolled sheet annealing time is 30 seconds or less (preferably 20 seconds or less).

In the pickling process, pickling is performed using a solution containing nitric acid at 50 g/L or more. The nitric acid content is preferably 60 g/L or more, more preferably 70 g/L or more. The total pickling time is 5 seconds or more.

The solution may contain sodium nitrate, sulfuric acid, sodium sulfate, hydrochloric acid, hydrofluoric acid, etc. The above-mentioned acids may be contained in the same solution, or the cold-rolled annealed sheet may be pickled in succession in multiple separate steps with different types of acid stored in each of the multiple tanks. When pickling in succession in multiple separate steps, the order in which the acids are used is not particularly limited and may be any order. Furthermore, the pickling method may be electrolytic pickling or pickling by immersion only.

Alternatively, the cold rolling process - cold rolled sheet annealing process - pickling process may be repeated as necessary. In this case, surface grinding may be performed between each process. Furthermore, only the conditions of the final cold rolled sheet annealing process and the final pickling process need to be strictly controlled as described above, and there is no need to particularly restrict the conditions of the other cold rolled sheet annealing processes and pickling processes.

Here, the cold-rolled sheet annealing performed in the final cold-rolled sheet annealing process corresponds to the finish annealing. Therefore, when the cold-rolled sheet annealing process is performed only once, that cold-rolled sheet annealing process corresponds to the finish annealing process.

The following tests were conducted to confirm in detail the effects of this stainless steel sheet. Note that the example shown below is merely one example among many examples of this stainless steel sheet. In other words, this stainless steel sheet is not limited to the configuration of the example shown below.

[Production method]
Stainless steel having the composition shown in Table 1 below was melted, hot rolled to a thickness of 4 mm, and then pickled. Thereafter, cold rolling was performed to a thickness of 2 mm, and cold-rolled sheet annealing was performed under the conditions of a cold-rolled sheet annealing temperature of 800 to 1100°C x cold-rolled sheet annealing time of 20 s at various heating rates within a range of 10 to 1000°C/s. In the cold-rolled sheet annealing, the cooling rate was 20°C/s.

Finally, the cold-rolled annealed sheet was subjected to salt immersion and electrolytic pickling to produce a stainless steel sheet according to this example (a stainless steel sheet according to an example of the present invention shown in Table 1 below). The salt used in the salt immersion was molten NaOH salt, and the temperature was 480° C. The electrolytic pickling was performed at a temperature of 60° C., with a nitric acid concentration of 50 g/L, and electrolysis conditions of 30 A/dm ² anode and 60 A/dm ² cathode, with alternating electrolysis for a total of 6.0 s.

[Test Method]
First, a test piece with a width of 70 mm and a length of 150 mm was cut out from the prepared stainless steel plate to prepare a test piece for the CCT test. In the CCT test (Cyclic Corrosion Test), the VDA (Verband der Automobilindustrie; German Association of the Automotive Industry) 233-102 test was performed for 12 cycles. The time required for the 12 cycles of the test was regarded as the "long period" which is the reference period for the corrosion resistance of the test piece for the CCT test.

In addition, test pieces for Auger electron spectroscopy were cut out from the prepared stainless steel plate and Ar ion sputtering was performed at a pitch of 0.5 nm from the surface of the steel plate to a measurement depth of 10 nm to 20 nm inclusive. In this Ar ion sputtering, the acceleration voltage of the ion gun was 1 kV, the current value of the ion current was about 650 nA, and the sputtering rate was 0.09 nm/s in terms of the sputtering rate of _SiO2 .

Then, the component composition at each measurement depth for each pitch was identified using a relative sensitivity factor from the peak intensity of the spectrum of each contained element measured at each measurement depth for each pitch. Specifically, the component composition was identified using a relative sensitivity factor from the intensity of the LMM peak of the spectrum for Fe and Cr, and the intensity of the KLL peak of the spectrum for Al, Si, O, S, C, and N, measured at each measurement depth for each pitch. In this Auger electron spectroscopy, the accelerating voltage of the electron gun was set to 10 kV, the current value of the current flowing through the test piece was set to 10 nA, and the degree of vacuum in the apparatus was set to 5×10 ⁻⁷ Pa or less.

In this example, a known Auger electron spectroscopy device (JAMP-9510F, serial number: AP1610000220022, manufactured by JEOL Ltd.) was used to perform the above-mentioned Auger electron spectroscopy and Ar ion sputtering.

Furthermore, in this embodiment, the O concentration was measured using the Auger electron spectroscopy device described above, and the measurement depth at which the O concentration was 1/2 of the maximum value was regarded as the thickness of the passive film. Furthermore, for the measurement depth for each pitch up to the measurement depth at which the O concentration was 1/2 of the maximum value, the average value of the cation fraction of each contained element measured at the measurement depth for each pitch was regarded as the cation fraction of each contained element in the passive film.

The test specimen for the CCT test was also placed in the device so that the rolling direction of the test specimen was at an angle of 75° with respect to the horizontal plane.

After the test was completed, the degree of rust on the CCT test specimens was evaluated using the rating number (S.A.R.N.) evaluation established by the Stainless Steel Association. In this evaluation, specimens with a rating number of 3 points or more were rated as passing (○), and specimens with a rating number of 2 points or less were rated as failing (×).

Test pieces measuring 20 mm wide x 30 mm long were cut out from the prepared stainless steel plate and embedded in resin so that the cross-sectional structure in the rolling direction of the test pieces could be observed under a microscope. The resin in which the test pieces were embedded was then mirror polished and etched. The average crystal grain size of the test pieces was then measured in accordance with the "cutting method using a straight test line" of JIS G 0551 "Steel - Microscopic test method for crystal grain size". With regard to the measurement results, test pieces with an average crystal grain size of 25 μm or less were evaluated as passing "○", and test pieces with an average crystal grain size exceeding 25 μm were evaluated as failing "×".

Further, test pieces for Charpy tests were cut out from the prepared stainless steel plates and subjected to Charpy tests in accordance with JIS Z 2242 under the condition of a material temperature of 25°C. The number of tests for this Charpy test was 3. Regarding the test results, test pieces with an average Charpy impact value of 10 J/ ^cm2 or more were evaluated as passing "○", and test pieces with an average Charpy impact value of less than 10 J/ ^cm2 were evaluated as failing "×".

[Test results]
The test results are shown in Table 2 below. As shown in Table 2, in all of the stainless steel sheets according to the present invention, the Cr content in the passive film was 20.0 atomic % or more in terms of cation fraction. In addition, in the rating number (SARN) evaluation after the CCT test, all of the stainless steel sheets according to the present invention passed the test with a rating number of 3 points or more. This result demonstrated that all of the stainless steel sheets according to the present invention have excellent corrosion resistance even after long-term use.

In terms of average grain size, in both the present invention and comparative examples, the stainless steel sheets produced under conditions of a heating rate of 100°C or more and a cold-rolled sheet annealing temperature of 850 to 1050°C had an average grain size of 25 μm or less. On the other hand, for example, in comparative examples B5 and B6, which were produced at a cold-rolled sheet annealing temperature of less than 850°C, recrystallization was incomplete and the average grain size could not be measured. Also, for example, in comparative examples B7 and B8, which were produced at a cold-rolled sheet annealing temperature of more than 1050°C, the average grain size exceeded 25 μm.

As for the results of the Charpy test, all of the stainless steel sheets according to the present invention passed the test with an average Charpy impact value of 10 J/ ^cm2 or more. On the other hand, Comparative Examples B9 to B13, which have compositional components outside the range of the present stainless steel sheet, and Comparative Examples B3, B4, B7, and B8, which have average grain size outside the range of the present stainless steel sheet, failed the test with an average Charpy impact value of less than 10 J/ ^cm2 .

The ferritic stainless steel sheet according to one aspect of the present invention is suitable, for example, as a stainless steel sheet for container applications.

Claims (3)

In mass percent,
C: 0.001-0.100%,
Si: 0.01-1.00%,
Mn: 0.01-1.00%,
P:≦0.050%,
S:≦0.0100%,
Cr: 10.5-30.0%,
Ni: 0.01 to 3.00%,
Al: 0.01-1.00%,
Sn: 0.01-0.50%,
N: 0.001 to 0.050%,
B: 0.0001 to 0.0050%;
Further, at least one of Ti and Nb is contained in the range of 0.01 to 1.00%,
the balance being iron and unavoidable impurities;
The average crystal grain size is 25 μm or less,
The average Charpy impact value at a material temperature of 25°C is 10 J/ ^cm2 or more,
A ferritic stainless steel sheet characterized in that, when the peak intensity of the spectrum of each contained element is measured by Auger electron spectroscopy from the steel sheet surface to a measurement depth at which the O concentration is half of its maximum value, the ferritic stainless steel sheet contains Cr at a cation fraction of 20.0 atomic% or more. Further, in mass%,
Mo: 0.01-3.00%,
Cu: 0.01 to 0.50%,
W: 0.001-1.000%,
V: 0.001-1.000%,
Sb: 0.001 to 0.100%,
Co: 0.001 to 0.500%,
Ca: 0.0001-0.0050%,
Mg: 0.0001 to 0.0050%,
Zr: 0.0001 to 0.0300%,
Ga: 0.0001 to 0.0100%,
Ta: 0.001 to 0.050%,
REM: 0.001~0.100%
2. The ferritic stainless steel sheet according to claim 1, further comprising one or more of the following: The ferritic stainless steel sheet according to claim 1 or 2, which is used for container applications.