JP2003171745A - Austenitic stainless steel sheet for heat exchanger - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a stainless steel sheet which has excellent heat resistance even if it is the thin one of ≤1.0 mm thickness, and particularly has properties that burning loss is hard to occur in a high temperature region, and has excellent cost effectiveness. <P>SOLUTION: The austenitic stainless steel sheet has a composition containing, by mass, 0.01 to 0.10% C, 0.01 to 1.0% Si, 19 to 26% Cr, 10 to 35% Ni, and 0.005 to 0.10% of one or more kinds selected from rare earth metals, and containing Mn in ≥0.01% also so as to satisfy the following relational inequality, and the balance Fe with impurities, and has a thickness of ≤1.0 mm: Mn(%)≤2.8×REM(%)-0.025×Ni(%)+0.95; wherein, Mn(%), REM(%), and Ni(%) denote the content (mass%) of each element contained in the steel. <P>COPYRIGHT: (C)2003,JPO

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a thin stainless steel plate having a thickness of 1.0 mm or less suitable for use in a heat exchanger.

[0002]

2. Description of the Related Art A micro gas turbine, which is drawing attention as a distributed power source, is equipped with a heat exchanger that heats combustion air by utilizing heat of combustion exhaust gas from the viewpoint of improving thermal efficiency. This heat exchanger is composed of a corrugated fin made of a stainless steel plate and a plate. In order to improve the thermal efficiency, it is necessary to raise the combustion exhaust gas temperature, but due to the heat resistance of the corrugated fin, the upper limit temperature is currently kept low at around 700 ° C, and improvement of the combustion exhaust gas temperature has been an issue. . Therefore, there is a demand for a stainless steel plate that can withstand severe machining of corrugated fins, has excellent heat resistance, and has good weldability. Various heat-resistant Fe-Cr-Al ferritic stainless steels have been proposed for catalyst carriers of automobile exhaust gas purification apparatuses. However, these ferritic stainless steels have problems that they are generally inferior in workability and difficult to weld. It is difficult to apply these stainless steel plates to a part requiring severe processing such as a corrugated fin of a heat exchanger which is one of the applications of the present invention.

Conventionally, SUS30 has been generally used for high temperature applications.
Austenitic stainless steel represented by No. 4 and SUS310 is often used.

For example, Japanese Patent Laid-Open No. 7-188869 has added rare earth elements (hereinafter abbreviated as REM) such as Al and B and La and Ce, and has a component in consideration of Ni balance, weldability and high temperature. Discloses an austenitic stainless steel excellent in oxidation resistance.

Japanese Unexamined Patent Publication No. 2000-303150 discloses a ferritic and austenitic stainless steel sheet which is for direct diffusion bonding but does not contain much Al. In particular, austenitic stainless steel is said to be easily rolled and excellent in workability.

However, in a heat exchanger using a stainless steel plate having a thickness of 1.0 mm or less, there is no description in any of the above publications regarding a phenomenon called burnout, which is a particular problem, and the usefulness is not considered. Unknown. Here, burnout refers to a phenomenon in which a steel sheet cannot retain its original shape due to high temperature oxidation.

[0007]

An object of the present invention is to provide an austenitic stainless steel sheet which can withstand severe working and can be easily rolled, and has excellent heat resistance even in a thickness of 1.0 mm or less, particularly in a high temperature region. The purpose of the present invention is to provide a stainless steel sheet which has the property of being less likely to be burnt out and is excellent in economic efficiency.

[0008]

Means for Solving the Problems The present inventors have paid attention to Ni—Cr—Fe type austenitic stainless steel which is excellent in workability, and studied the high temperature oxidation resistance of the steel sheet in a combustion exhaust gas atmosphere.

In the steel having such a composition, C is generally formed on the steel surface.
r_TwoO_ThreeOxide is uniformly generated, but the steel plate burns out.
The reason is that Cr, which is an alloying element in the stainless steel plate, is Cr.
_TwoO _ThreeMigrate into the oxide layer as the oxide grows,
Depletion of Cr in the steel plate causes abnormal oxidation,
The present invention has been completed by finding a phenomenon that a plate burns out.

Regarding abnormal oxidation that causes burnout, SU
Further details will be described based on the experiment using S310S.

FIG. 1 is a diagram showing the relationship between abnormal oxidation of a stainless steel plate and the amount of Cr transferred into the oxide.

A SUS310S stainless steel plate having a thickness of 0.1 mm is used, and at a temperature of 900 to 1050 ° C.
Heated for 500 hours. Here, the amount of Cr transferred from the stainless steel plate into the oxide layer was calculated by determining the Cr concentrations of the oxide and the base material by EPMA (wavelength dispersive X-ray analyzer) and comparing them. The thickness of the Cr ₂ O ₃ oxide layer was measured by observing the cross section of the sample with an optical microscope.

The following findings were obtained from the results shown in FIG.

A) Abnormal oxidation (black mark in FIG. 1) of a SUS310S stainless steel sheet having a thickness of 0.1 mm occurs when the thickness of the Cr ₂ O ₃ oxide layer is about 25 μm, and then burns out.

B) The amount of Cr transferred from the stainless steel sheet to the oxide layer, that is, the Cr consumption of the stainless steel sheet due to high-temperature oxidation, is roughly proportional to the thickness of the Cr ₂ O ₃ oxide layer, and the amount of Cr consumed from the stainless steel sheet to the oxide layer. When the amount of Cr transferred to is 0.02 g / cm ² , abnormal oxidation occurs, and then burnout occurs.

C) On the other hand, when the thickness of the oxide layer becomes 25 μm, a Cr amount of 0.02 g / cm ² is transferred to the oxide layer, and this value is originally contained in the SUS310S stainless steel sheet having a thickness of 0.1 mm. Equivalent to quantity. Therefore, the abnormal oxidation of the stainless steel plate occurs only under the condition that the amount of Cr originally contained in the base material is completely exhausted.

D) On the contrary, as long as Cr remains in the stainless steel plate, the steel plate maintains excellent heat resistance, and the life of the steel plate leading to burning is determined by the amount of Cr contained in the stainless steel plate before use,
It is determined by the growth rate of Cr ₂ O ₃ oxide formed on the steel surface, that is, the Cr consumption of the stainless steel sheet due to high temperature oxidation.

E) Burnout phenomenon is caused by the thickness of the steel plate being 1.0 mm.
This is a phenomenon that is less likely to occur in a steel sheet exceeding 1.0 mm due to the sufficient amount of Cr in the steel, and is likely to occur in a steel sheet of 1.0 mm or less.

Based on the above knowledge, the following method can be considered to delay the depletion of Cr in the stainless steel sheet in order to prevent the burnout phenomenon.

1) Increase the Cr content in the stainless steel plate.

2) Increase the thickness of the stainless steel plate.

3) Slow the growth rate of Cr ₂ O ₃ oxide.

In the above method 1), generally, when the Cr content is increased in the austenitic stainless steel, the steel quality is remarkably changed, and the workability, high temperature strength, aging embrittlement resistance and the like of the steel are remarkably deteriorated. Not preferable.

In the method 2), when the thickness of the steel sheet is increased, the pressure loss of the combustion exhaust gas and the combustion air of the heat exchanger is increased, and the overall efficiency of the system is decreased.
Have difficulty.

For this reason, the present inventors have conducted various studies on measures for suppressing the growth rate of Cr ₂ O ₃ oxide formed on the surface of the steel sheet. As a result, by adding a small amount of REM to the stainless steel and additionally defining the upper limit of the Mn content according to the Ni content and the REM content in the steel, Cr ₂ O ₃
We have found that the growth rate of oxide can be suppressed.

FIG. 2 is a diagram showing a comparison of the oxidation rate constants of the present invention steel and the conventional steel.

As the conventional steel, the steel plate with the reference numeral 42 shown in Table 1 in the examples described later and the steel plate with the reference numeral 5 in the same table as the steel of the present invention were used. The oxidation rate constant (kp) was calculated based on the thickness of the oxide layer obtained from the oxidation experiment of both steel sheets. For both steel types, oxidation proceeds according to the parabolic law, but k
A new fact has been found that p of the steel of the present invention is about an order of magnitude smaller than that of conventional steel.

The present invention made based on the above findings,
The gist is the following stainless steel plates for heat exchangers (1) to (7).

(1) C: 0.01 to 0.10.
%, Si: 0.01 to 1.0%, Cr: 19 to 26%,
Ni: 10 to 35%, and one or more types of REM are contained in a total amount of 0.005 to 0.10%, and Mn is 0.01
% Or more and satisfying the following relational expression, the balance is Fe and impurities, and the thickness is 1.0 mm or less, an austenitic stainless steel sheet.

Mn (%) ≦ 2.8 × REM (%)-0.
025 × Ni (%) + 0.95 where Mn (%), REM (%) and Ni (%)
Is the content (% by mass) of each element contained in the steel.
Indicates.

(2) In place of a part of Fe in the composition of the stainless steel sheet described in (1) above, one or more selected from Mo, W, Cu and Co are respectively used in the range of 0. An austenitic stainless steel sheet containing 1 to 3% and having a thickness of 1.0 mm or less.

(3) Nb, T in place of a part of Fe in the composition of the stainless steel sheet described in (1) or (2) above.
Contains 0.01 to 1.0% of one or more selected from i, V and Zr, and has a thickness of 1.0 m.
An austenitic stainless steel sheet having m or less.

(4) In place of a part of Fe in the composition of the stainless steel sheet according to any one of the above (1) to (3), A
An austenitic stainless steel sheet containing 1 or less 0.60% and having a thickness of 1.0 mm or less.

(5) N in place of a part of Fe in the composition of the stainless steel sheet according to any one of (1) to (4) above.
Of 0.01 to 0.4% and having a thickness of 1.0 mm or less.

(6) B in place of a part of Fe in the composition of the stainless steel sheet according to any one of (1) to (5) above.
Contains 0.001 to 0.010% and has a thickness of 1.0 mm
The following are austenitic stainless steel sheets.

(7) C in place of a part of Fe in the composition of the stainless steel sheet according to any one of (1) to (6) above.
a and Mg, one or more selected from Mg.
Austenitic stainless steel sheet containing 001 to 0.010% and having a thickness of 1.0 mm or less.

[0037]

BEST MODE FOR CARRYING OUT THE INVENTION Next, the reasons for limitation of the present invention will be described. In addition, all the units expressing the composition are shown by mass%.

C: 0.01 to 0.10% C has the effect of suppressing the formation of δ ferrite, stabilizing the austenite structure and ensuring high temperature strength. In order to exert this effect, the C content needs to be 0.01% or more, but if the C content exceeds 0.10%, massive Cr ₂₃ C ₆ precipitates at the crystal grain boundaries of the steel. However, the toughness of the steel decreases and the resistance to thermal fatigue during the heating / cooling cycle deteriorates. Therefore, the content of C is set to 0.01 to 0.10%.

Si: 0.01 to 1.0% Si is added as a deoxidizer at the time of melting, and the content is 0.0.
The effect is demonstrated at 1% or more. However, 1.0
%, The precipitation of brittle intermetallic compounds is promoted and the structural stability of the alloy is impaired, that is, brittleness is accelerated, so the upper limit was made 1.0%.

Mn: 0.01% or more Mn has an effect of forming an austenite structure, and also acts as a deoxidizing agent at the time of dissolution, so it is added. The effect is achieved when the Mn content is 0.01% or more. The upper limit of Mn content is determined by the relationship between REM and Ni content. If Mn is contained in the steel in excess of this upper limit, the effect of suppressing the oxidation rate will not be exhibited.

FIG. 3 is a graph showing the effect of the amount of Mn in steel on the oxide thickness.

Using a 19% Ni austenitic stainless steel sheet (SUS310S equivalent material), the Mn content in the steel was changed, and the thickness of the Cr ₂ O ₃ oxide layer formed at 1000 ° C. Sorted by amount. Conventional steel SUS
By including 0.09% of REM in the 310S steel plate,
The thickness of the oxide layer is reduced from 25 μm to 20 μm by about 20%. Further, as a result of conducting a test while changing the Mn content, the oxide layer thickness is about 5 μm at a certain Mn content or less.
It turned out to be drastically reduced to about m.

The above oxidation experiment was carried out using Mn, Ni and RE.
Steels of various compositions produced by changing the M content were melted, and regression analysis was carried out to find the relational expression defining the upper limit of the Mn content shown below.

Mn (%) ≦ 2.8 × REM (%) − 0.
025 x Ni (%) + 0.95.

This mechanism is presumed as follows.

That is, REM segregates at the crystal grain boundaries of the Cr ₂ O ₃ oxide, whereby Cr ³⁺ in the oxide layer is formed.
And suppresses the diffusion of O ²⁻ ions, which suppresses the oxidation rate. On the other hand, Mn and Ni are considered to have the effect of inhibiting the segregation of REM into the oxide layer. This inhibitory action will be described by taking Mn as an example. Mn content is R
Above a certain value determined by the relationship with the EM and Ni contents, the MnO oxide partially forms a solid solution in the Cr ₂ O ₃ oxide layer during high temperature oxidation of steel, and the REM Cr ₂ O ₃ grain boundaries Segregation of the oxidization rate is disturbed and the effect of suppressing the oxidation rate is not exerted.

Cr: 19 to 26% Cr is an element having a function of uniformly forming a protective Cr ₂ O ₃ oxide film on the steel surface, preventing abnormal oxidation, and protecting the steel from burnout. When the content of Cr is less than 19%, Cr ₂ O ₃ oxide is not uniformly formed on the surface of the steel sheet at high temperature and the high temperature oxidation resistance is deteriorated. Therefore, it is necessary to contain 19% or more. If the Cr content exceeds 26%, the austenite structure cannot be stably formed, and in addition, the α-Cr phase, which is a brittle intermetallic compound, precipitates during long-term use at high temperature, and Embrittle. Therefore, the content of Cr is set to 19 to 26%. The preferable Cr content is 2
It is 1-26%.

Ni: 10 to 35% Ni has the effect of forming an austenite structure and the effect of increasing the heat resistance of the steel sheet. In order to obtain an austenite structure, it is necessary to contain at least 10% or more. However, even if Ni is contained in an amount of more than 35%, the above effect is saturated and the cost is increased. Therefore, the Ni content is set to 10 to 35%. The Ni content is preferably 10 to 29%, more preferably 10 to 24%.

REM: 0.005 to 0.10% When REM is oxidized, it segregates at the crystal grain boundaries of Cr ₂ O ₃ oxide as ions, and Cr is accompanied by the growth of Cr ₂ O ₃ oxide.
_It has the effect of suppressing the grain boundary diffusion of Cr ³⁺ and O ²⁻ ions through the ₂ O ₃ crystal grain boundaries and, as a result, slowing down the growth rate of Cr ₂ O ₃ oxide. The effect is achieved when the total content of REM is 0.005% or more. On the other hand, if the total content of REM exceeds 0.10%, a brittle intermetallic compound precipitates during use at high temperature, and the steel becomes brittle. Therefore, RE
The total content of M was set to 0.005 to 0.10%. The REM in the present invention refers to a total of 17 elements of Sc, Y and lanthanoid, and in the case of lanthanoid, it may be industrially added in the form of misch metal.

The steel sheet according to the present invention described in (1) above is an austenitic stainless steel sheet containing the above chemical components and the balance being Fe and impurities.

In addition to the above-mentioned components, if necessary, an element selected from at least one of the following (a) to (f) is selectively contained, whereby the above (2) to (7) are described. The austenitic stainless steel sheet according to the present invention can be obtained.

(A) one or more selected from Mo, W, Cu and Co, (b) one or more selected from Nb, Ti, V and Zr, ( c)
One or more selected from Al, (d) N, (e) B, (f) Ca and Mg. Hereinafter, the optional additional elements of the groups (a) to (f) will be described.

(A) Mo, W, Cu and Co: 0.1 to 3% Each of these elements may or may not be added, but if added, it has the effect of increasing the high temperature strength. In order to reliably obtain this effect, Mo, W, Cu and Co are each 0.
The content is preferably 1% or more. But M
If the contents of o, W, and Cu exceed 3%, brittle intermetallic compounds are precipitated during use, resulting in a decrease in toughness of the steel. Further, when the content of Co exceeds 3%, the high temperature strength is remarkably increased and the hot workability is deteriorated. Therefore, M
When adding o, W, Cu and Co, the content of each is preferably 0.1 to 3%, and more preferably 0.1 to 1.5%.

(B) Nb, Ti, V and Zr: 0.01 to 1.0% Each of these elements may or may not be added, but if they are added, carbonitrides are formed to form high temperatures. It has the effect of increasing strength. In order to reliably obtain this effect, it is preferable that the contents of Nb, Ti, V, and Zr are each 0.01% or more. However, even if Nb, Ti, V, and Zr are all contained in an amount of more than 1.0%, the above effect is saturated and the cost is increased. Therefore, Nb, T
When i, V and Zr are added, the content of each is preferably 0.01 to 1.0%. When added, the preferable contents of Nb, Ti, V, and Zr are each 0.1 to 1.0%, and more preferable contents are each 0.1 to 0.6%.

(C) Al: 0.60% or less Al may or may not be added, but if it is added, the deoxidizing effect of steel is enhanced. In order to reliably obtain this effect, the content of Al is preferably 0.005% or more. However, if Al is contained in an amount of more than 0.60%, Ni ₃ Al, which is an intermetallic compound that is brittle at high temperature, precipitates, the hot workability deteriorates significantly, and the creep rupture elongation decreases. Therefore, the Al content is set to 0.60% or less. When Al is added, its content should be 0.
005-0.60% is good, more preferable Al
The content is 0.02 to 0.30%, and 0.02 to 0.
20% is more preferable. Furthermore, 0.05-0.
20% is extremely preferable.

(D) N: 0.01 to 0.4% N is an element contained as an impurity in the steel, but it not only contributes to the stabilization of the austenite structure but also has the effect of increasing the high temperature strength. , These effects have a N content of 0.
It is surely obtained at 01% or more. Therefore, if it is desired to stabilize the austenite structure and enhance the high temperature strength, N may be added to make N content 0.01% or more. However, even when N is added, it is difficult to make the content exceed 0.4% by a usual melting technique. Therefore, when N is added, its content is 0.01 to
0.4% is preferable. The more preferable content when N is added is 0.1 to 0.4%.

(E) B: 0.001 to 0.010% B may or may not be added, but if it is added, it has the effect of strengthening the crystal grain boundaries and increasing the high temperature strength. Further, in order to surely obtain these effects, which also has the effect of improving hot workability, the content of B is preferably 0.001% or more. However, if its content exceeds 0.010%, the susceptibility to hot cracking during welding becomes high. Therefore, if B is added, its content should be 0.00
It is preferable to set it to 1 to 0.010%.

(F) Ca and Mg: 0.00 each
1 to 0.010% These elements may or may not be added, but if added, they have the effect of enhancing hot workability. To ensure this effect, Ca and Mg are each 0.001%.
The above content is preferable. However, if both Ca and Mg are contained in excess of 0.010%, Ni-Ca and Ni-Mg compounds, which are low melting point compounds, are formed and the hot workability is rather deteriorated. Therefore, when Ca and Mg are added, the content of each is preferably 0.001 to 0.010%.

[0059]

EXAMPLES Next, the effects of the present invention will be described in more detail with reference to examples.

Forty-five types of test steels (reference numbers 1 to 40 and reference numbers 44 to 48) having the chemical compositions shown in Tables 1 to 3 were melted in a vacuum induction heating furnace of 10 kg each.

[0061]

[Table 1]

[0062]

[Table 2]

[0063]

[Table 3]

After mechanically grinding the surface of the ingot, 125
Heated at 0 ° C for 3 hours, hot forged to a thickness of 25 mm, 90
It was formed into a plate having a width of mm. Then, after heating at 1250 ° C. for 3 hours, hot rolling was performed to obtain a steel plate having a thickness of 5 mm.

A 5 mm thick steel plate thus obtained was
1.2mm by cold rolling after softening and annealing at 100 ℃
The thickness of the steel sheet is further softened and annealed at 1100 ° C., and then cold rolling is repeated to obtain 0.1 mm.
A stainless steel plate having a thickness of

1 mm at 1100 ° C. on the above 0.1 mm thick steel plate
After the final heat treatment of water cooling after heating for 15 hours, width 15m
A test piece having a size of m and a length of 35 mm was cut out and used for a high temperature oxidation test.

As the alloy steels denoted by reference numerals 41 to 43 shown in Table 3, commercially available steel sheets were used. The test steel of reference numeral 42 is SUS310S steel described in JIS G 4305,
The test steel of reference numeral 43 is NC described in JIS G 4902.
Equivalent to F800 steel. All of these steel sheets are 1.2
Although it was obtained as a cold-rolled steel sheet having a thickness of mm, a 0.1 mm-thick stainless steel sheet was also obtained by repeating the step of performing cold rolling after softening annealing at 1100 ° C.

In the high temperature oxidation test, 3% O ₂ -16% H ₂ O-9% CO ₂ -ba simulating the combustion exhaust gas of city gas was used.
l. 900, 950, 10 in an air stream having a composition of N _2.
At each temperature of 00 and 1050 ° C., oxidation was carried out for 500 hours, and abnormal oxidation occurred and C formed on the surface of the stainless steel plate.
The thickness of the r ₂ O ₃ oxide was measured. The thickness of the oxide was measured by observing the cross section of the sample with an optical microscope.

The test results are summarized in Tables 4 and 5.

[0070]

[Table 4]

[0071]

[Table 5]

At the heating temperature of 1050 ° C., the conventional steels, reference numerals 42 (SUS310S) and 43 (NCF8) are used.
No. 00) was completely burned out. It was also found that in the conventional steel of reference numeral 41 and the comparative steels of reference numerals 44 to 48, a part of the steel sheet was abnormally oxidized and was about to burn.
However, all of the steel sheets of the present invention, numbered 1 to 40, had a good appearance without burning or abnormal oxidation.

Similarly, at test temperatures other than the above, the thickness of the Cr ₂ O ₃ oxide layer formed on the steels of the present invention (reference numbers 1 to 40) may be as much as 20 to 30% of that of conventional steels. It could be confirmed.

[Brief description of drawings]

1] Abnormal oxidation of stainless steel sheet and Cr in oxide
It is a figure which shows the relationship with the transfer amount of.

FIG. 2 is a diagram showing a comparison of the oxidation rate constants of the present invention steel and conventional steel.

FIG. 3 is a diagram showing the influence of the amount of Mn in steel on the oxide thickness.

   ─────────────────────────────────────────────────── ─── Continued front page    (72) Inventor Yoshio Kudo             4-53 Kitahama, Chuo-ku, Osaka City, Osaka Prefecture             Sumitomo Metal Industries, Ltd. (72) Inventor Kazuyoshi Kuramura             4-53 Kitahama, Chuo-ku, Osaka City, Osaka Prefecture             Sumitomo Metal Industries, Ltd.

Claims (7)

[Claims] 1. C: 0.01 to 0.10% by mass% and S
i: 0.01 to 1.0%, Cr: 19 to 26%, Ni:
10 to 35%, and one or more of REMs in total 0.0
0.05 to 0.10%, Mn is contained in 0.01% or more and satisfies the following relational expression, and the balance is Fe.
And an impurity, and an austenitic stainless steel sheet having a thickness of 1.0 mm or less. Mn (%) ≦ 2.8 × REM (%) − 0.025 × Ni
(%) + 0.95 where Mn (%), REM (%) and Ni (%)
Is the content (% by mass) of each element contained in the steel.
Indicates. 2. In place of a part of Fe in the composition of the stainless steel sheet according to claim 1, 0.1 or more selected from Mo, W, Cu and Co are used. Three
%, And an austenitic stainless steel sheet having a thickness of 1.0 mm or less. 3. In place of a part of Fe in the composition of the stainless steel sheet according to claim 1 or 2, one kind or two kinds or more selected from Nb, Ti, V and Zr is added to each. An austenitic stainless steel sheet containing 01 to 1.0% and having a thickness of 1.0 mm or less. 4. The composition of the stainless steel sheet according to any one of claims 1 to 3 contains Al in an amount of 0.60% or less in place of part of Fe and has a thickness of 1.0 mm or less. Austenitic stainless steel plate. 5. The N of the stainless steel sheet according to claim 1 is replaced by a part of Fe in the composition of N.
An austenitic stainless steel sheet containing 01 to 0.4% and having a thickness of 1.0 mm or less. 6. The B of the stainless steel sheet according to claim 1 is replaced with B.
Austenitic stainless steel sheet containing 001 to 0.010% and having a thickness of 1.0 mm or less. 7. One or more selected from Ca and Mg in place of a part of Fe in the composition of the stainless steel sheet according to any one of claims 1 to 0.001
An austenitic stainless steel sheet containing 0.010% to 0.010% and having a thickness of 1.0 mm or less.