JP2000087180A - Steel sheet excellent in organic matter degradability and its production - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve organic matter degradability, such as antifouling property, deodorization characteristic, antibacterial characteristic, and germicidal characteristic, by allowing a surface oxide layer to contain specific amounts of TiO2 and making the X-ray diffraction intensity from anatase-type TiO2 and the X-ray diffraction intensity from rutile-type TiO2 satisfy a specific relationship. SOLUTION: This steel sheet has an oxide layer of >=0.01 μm in thickness on the surface, and this oxide layer contains >=5 atomic % TiO2. TiO2 has a catalytic action at the time of photoirradiation, and particularly, the catalytic action of anatase-type TiO2 is stronger than that of rutile-type TiO2. By the photocatalytic action, organic matter such as oil is decomposed into carbon dioxide gas and water. Therefore as the TiO2 to be contained in the oxide layer, TiO2 mainly of anatase type, satisfying IA/(IA+IR)}>=0.6 by X-ray diffraction, is used. In the inequality, IA is X-ray diffraction intensity from anatase-type TiO2, and IR is X-ray diffraction intensity from rutile-type TiO2.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

TECHNICAL FIELD The present invention relates to an antifouling property, a deodorizing property,
The present invention relates to a steel sheet excellent in decomposability of organic substances such as antibacterial property and bactericidal property and a method for producing the same.

[0002]

2. Description of the Related Art Conventionally, steel sheets have been developed with emphasis on materials such as workability, weldability, and rust resistance. However, recently, interest in human health and hygiene issues and global environmental issues has increased, and in the fields of building interior and exterior walls, medical and food-related facilities, and home appliances, for example, antifouling building materials , antibacterial kitchen equipment, deodorizing filter, NO _X decomposition building materials,
Steel sheets used in wastewater purification tanks, etc., are required to have characteristics that can prevent the growth of germs, odor, generation of dirt, generation of harmful substances, etc., in addition to the characteristics of the above materials. Have been.

In response to such a demand, for example, Japanese Patent Laid-Open
No. 170053 has an antibacterial effect in the matrix
A ferritic stainless steel excellent in antibacterial property in which a Cu-rich phase is precipitated in an amount of 0.2% by volume or more has been proposed. further,
JP-A-8-193218, a mixed gas of a + 10~-65 ℃ H ₂ or 90% by volume or more of H ₂ and N ₂ controlled to a dew point, stainless steel containing 0.1 to 1 wt% Ti 850 ° C steel material
Heat-treated at a temperature of ~ 1150 ° C, and Ti as TiO ₂
Discloses a method for producing a stainless steel having an antibacterial film formed by forming a film containing at least 20 at%. According to this method, since the antibacterial property of the photocatalytic action of TiO ₂ is used, the steel sheet is semipermanently exhibiting the antibacterial action.

Japanese Patent Application Laid-Open No. 9-310185 discloses that after forming a base layer made of a SiO ₂ precursor or SiO _{2 on the} surface of a metal plate, an organic titanium compound or a titania sol is applied and heat treated to form a TiO ₂ A method for producing a photocatalyst-coated metal plate in which a layer is baked on the metal plate is disclosed.

[0005]

SUMMARY OF THE INVENTION
In a ferritic stainless steel in which a Cu-rich phase described in 9-170053 is precipitated in a matrix, most of the Cu-rich phase exhibiting antibacterial properties will be buried in the steel, and will be in contact with various bacteria and bacteria. There are only sparsely present in the, low antibacterial expression, in order to express high antibacterial,
It is necessary to precipitate the Cu-rich phase densely. However, in order to precipitate the Cu-rich phase densely, it is necessary to add a large amount of Cu to the steel, and there has been a problem that the material cost is increased and it is economically disadvantageous.

Further, the steel sheet described in Japanese Patent Application Laid-Open No. 8-193218 has excellent adhesion of TiO ₂ , excellent workability, elution resistance and peeling resistance, can be mass-produced, is inexpensive and versatile. A steel plate that can be used for applications. However, the photocatalytic action of TiO _2, while indicating particularly strong effect TiO ₂ is with anatase crystalline form, TiO ₂ formed by this technique are mainly those of the rutile-type crystal form, photocatalysis effect Could not be obtained.
Also, in a mixed atmosphere of H ₂ + N ₂ , there is a problem that TiN is easily formed in addition to TiO ₂ , and the photocatalytic action is reduced.

The technique described in Japanese Patent Application Laid-Open No. 9-310185 discloses a method of applying an organic titanium compound or a titania sol,
The baking process is complicated and is not suitable for mass production on an industrial scale. The present invention advantageously solves the above-mentioned problems of the prior art, is excellent in antifouling property, deodorizing property, antibacterial property, decomposability of organic substances such as bactericidal property and the like, and is inexpensive steel sheet suitable for mass production and its manufacturing method The purpose is to provide.

[0008]

Means for Solving the Problems The inventors of the present invention have conducted various experiments to achieve the above-mentioned object. As a result, the present inventors have found that an anatase-type crystal form having TiO ₂ of 5 atomic% or more and exhibiting the strongest photocatalytic action is obtained. An oxide layer mainly containing TiO ₂ on the steel sheet surface
By forming it with a thickness of 0.10 μm or more, antifouling property,
It has been found that the steel sheet is excellent in decomposability of organic substances such as deodorizing property, antibacterial property and bactericidal property, and is further inexpensive.

First, the results of experiments conducted by the present inventors will be described. When a 0.1 μm thick oxide layer containing 1% or 5% atomic% of TiO ₂ is formed on the surface of a steel sheet and the ratio of rutile type to anatase type of TiO ₂ in the oxide layer is variously changed. The oil degradability was investigated. Oil decomposability is 50mm x 50
After the vegetable oil edible was about 0.1 mg / cm ² applied to the mm of the test piece,
Irradiate black light with irradiation light intensity 2mW / cm ² for 8 hours,
The amount of oil decomposed was measured and evaluated by the oil decomposition rate (%) = (decomposition amount / oil application amount) × 100. The ratio between the rutile type and the anatase type in TiO ₂ was determined from the X-ray diffraction intensity, and IA / (IA +
IR). Here, IA is d of anatase type TiO ₂
= 3.52 ° X-ray diffraction intensity, and IR is the X-ray diffraction intensity of rutile TiO ₂ at d = 3.24 °. FIG. 1 shows the relationship between the ratio of anatase TiO ₂ in the oxide layer, IA / (IA + IR), and the oil decomposition rate.

From FIG. 1, it can be seen that when 5 atomic% of TiO ₂ is contained in the oxide layer, if IA / (IA + IR) is 0.6 or more, an oil decomposition rate of 50% or more can be obtained. Understand.
On the other hand, when the TiO ₂ content in the oxide layer is 1%, IA / (IA + IR) becomes close to 1 and almost the entire amount
Even if TiO ₂ is in anatase type, the oil decomposition rate is only about 25%.

[0011] Accordingly, the present inventors have found that if an oxide layer mainly containing 5 atomic% or more of anatase type TiO ₂ with a thickness of 0.1 μm or more is formed on the surface of a steel sheet, light irradiation It has been newly found that organic substances can be decomposed by the catalytic action of the compound. Further, the present inventors have found that the anatase type crystal form having a thickness of 0.1 μm or more is mainly used.
In order to form an oxide layer containing TiO ₂ on the surface of the steel sheet with good adhesion, it has been found that it is important to add Ti to the steel in an appropriate amount and to perform an appropriate heat treatment.

The present invention has been completed based on the above findings and further studies. That is, the present invention relates to a steel sheet having an oxide layer having a thickness of 0.10 μm or more on its surface, wherein the oxide layer contains 5% or more of TiO ₂ in atomic% and X from anatase type TiO ₂ Organic matter decomposition characterized in that the X-ray diffraction intensity IA and the X-ray diffraction intensity IR from rutile type TiO ₂ satisfy the following expression (1): {IA / (IA + IR)} ≧ 0.6 (1) It is a steel sheet with excellent properties. Further, the steel sheet is expressed by weight%, C: 1% or less,
N: 0.01% or less, Si: 1% or less, Mn: 1% or less, Ti: 0.
1 to 2.0% and C, N and Ti are represented by the following equation (2) {Ti (wt%) / (2.0 × C (wt%) + 1.7 × N (wt%))}> 1 (2) Is preferably contained in a range satisfying the following, and the balance is Fe and inevitable impurities.

Further, the present invention provides a method of the present invention in which, by weight%, C: 1% or less;
N: 0.01% or less, Ti: 0.1 to 2.0%, and C,
Carbon steel sheet containing N and Ti in a range satisfying the following equation (2) {Ti (wt%) / (2.0 × C (wt%) + 1.7 × N (wt%))}> 1 (2) With a purity of 99.9% ~
The present invention provides a method for producing a steel sheet having excellent organic substance decomposability, characterized by performing a heat treatment at a temperature of 600 to 900 ° C. for 0.5 to 8 hours in an inert gas atmosphere of 99.999%.
H ₂ gas or 90% by volume in place of the inert gas atmosphere
The above mixed gas of H ₂ gas and inert gas is mixed with dew point: 0
The atmosphere may be set at −60 ° C.

[0014]

DESCRIPTION OF THE PREFERRED EMBODIMENTS First, the reasons for limiting the steel sheet of the present invention will be described. The steel sheet of the present invention is a steel sheet having an oxide layer having a thickness of 0.10 μm or more on its surface.
An oxide layer containing 5% or more of O ₂ in atomic%. Although TiO ₂ has a catalytic action at the time of light irradiation, anatase-type (anatase-type crystal form) TiO ₂ has a stronger catalytic action at the time of light irradiation than rutile-type (rutile-type crystal form) TiO _2. .
Organic matter such as oil is decomposed into carbon dioxide (CO ₂ ) and water (H ₂ O) by the photocatalytic action of TiO ₂ . Therefore, TiO ₂ contained in the oxide is mainly anatase-type TiO ₂ that satisfies the following formula (1) {IA / (IA + IR)} ≧ 0.6 (1) by X-ray diffraction. Here, IA is an X-ray diffraction intensity of the anatase type TiO _2, IR is the X-ray diffraction intensity IR from rutile TiO _2.

As a result, the catalytic action at the time of light irradiation is strong, and the manifestation of the organic matter decomposability becomes remarkable. IA / (IA + IR)
If it is less than 0.6, the amount of anatase-type TiO ₂ is small, and the expression of organic matter decomposability is small. If the content of TiO ₂ in the oxide layer is less than 5 atomic%, the decomposition of organic substances is not remarkably exhibited even if the expression (1) is satisfied. The thickness of the oxide layer is 0.10
μm or more. When the thickness is less than 0.10 μm, the catalytic action upon light irradiation is not effectively exhibited, and no decomposability of organic substances such as antibacterial property, bactericidal property, antifouling property and deodorizing property is exhibited.

Next, a method for producing the steel sheet of the present invention will be described. By weight%, C is 1% or less, N is 0.01% or less, Ti
0.1 to 2.0% and satisfying the following expression (2) {Ti (wt%) / (2.0 × C (wt%) + 1.7 × N (wt%)) >> 1 (2) Preferably, the steel sheet is heat treated.

Next, a preferred composition range of the carbon steel sheet used will be described. C: 1% or less C is an element that increases the strength of the steel sheet, and can be added according to the required strength level. However, 1%
If it is contained in excess, the Ti is fixed as TiC and the Ti required to form TiO ₂ on the steel sheet surface during heat treatment is required.
And the workability and weldability are further reduced. For this reason, C is preferably set to 1% or less. Preferably, 0.001% to 0.5%
It is.

N: 0.01% or less N bonds to Ti in steel and fixes Ti in steel as TiN. For this reason, an excessive content lowers the diffusion amount of Ti to the steel sheet surface, suppresses the production of TiO _{2 on} the steel sheet surface, and lowers the weldability. Because of this, N
Is preferably 0.01% or less.

Ti: 0.1 to 2.0% Ti is an important element for forming an oxide layer containing TiO ₂ on the surface of the steel sheet, and for this purpose, it is necessary to contain 0.1% or more. If the Ti content in the steel sheet is less than 0.1%,
An oxide layer containing 5 at% or more of TiO ₂
It is difficult to form a layer having a thickness of at least μm. on the other hand,
The content of Ti exceeding 2.0% can increase the amount of TiO ₂ in the oxide layer, but generates a large amount of intermetallic compounds such as FeTi and Fe ₂ Ti, thereby reducing the workability of the steel sheet. For this reason,
Ti is desirably in the range of 0.1 to 2.0%.

{Ti (wt%) / (2.0 × C (wt%) + 1.7 × N (wt%))}> 1 (2) Usually, a steel sheet is subjected to a process of melting, casting, and hot rolling. The Ti in the steel is fixed as TiC and TiN by C and N in a cooling process after casting or in a hot rolling process. In order to form TiO ₂ on the surface of the steel sheet, it is necessary that solid solution Ti not fixed as TiC and TiN exist in the steel. Therefore, it is desirable to adjust the contents of Ti, C, and N so as to satisfy the expression (2). By adjusting the Ti, C, and N contents so as to satisfy the expression (2), solid solution Ti is secured in the steel, and TiO ₂ can be formed on the steel sheet surface by the subsequent heat treatment.

The steel sheet used in the present invention desirably contains Si: 1% or less, Mn: 1% or less, and Al: 0.2% or less, in addition to the above chemical components. Si and Mn are elements that increase the strength of steel, and are contained in appropriate amounts according to the required strength.
However, when the content exceeds 1%, ductility is reduced and workability is deteriorated. In addition, Al has a function to refine the crystal grains of the steel sheet to improve the ductility, and a function as a deoxidizing agent for the steel. Such an effect is recognized when the content is 0.01% or more. However, if the content exceeds 0.2%, the amount of nonmetallic inclusions increases, and the cleanliness of the steel deteriorates.

In addition, one or more of Nb: 0.08% or less, V: 0.01% or less, and Mo: 0.1% or less may be added. These elements are added for the purpose of improving the strength and workability of steel. However, Nb: 0.08%, V:
Addition exceeding 0.01% and Mo: 0.1% deteriorates cold workability. In addition, other than the above-mentioned chemical components, the balance is Fe and inevitable impurities. The unavoidable impurities can be P: 0.05% or less and S: 0.01% or less.

Next, the reasons for limiting the heat treatment conditions will be described. The carbon steel sheet having the above composition has a purity of 9
600 to 900 in an inert gas atmosphere of 9.9% to 99.999%
A heat treatment at a temperature of 0.5 ° C. for 0.5 to 8 hours. In order to selectively and efficiently oxidize only Ti in the steel, the atmosphere of the heat treatment (final annealing) was in an inert gas having a purity of 99.9% or more. Further, by performing heat treatment in such an atmosphere, oxidation of elements in steel such as Mn, Si, Fe, and P is suppressed, and the amount of TiO ₂ in the generated oxide layer can be increased to 5 atomic% or more. it can. If the purity of the inert gas is less than 99.9%, oxygen, water vapor, etc. are mixed into the inert gas, the oxygen potential in the atmosphere increases, and Mn, Si, Fe, etc. are oxidized and formed on the surface layer of the steel sheet. The concentration of TiO ₂ in the oxide decreases. For this reason, the photocatalytic action of TiO ₂ is significantly reduced, and the organic matter decomposability is reduced. On the other hand, when the purity of the inert gas is 99.999% or more, the oxygen potential is low and Ti is not oxidized at all, or the oxygen concentration such as TiO ₂ or Ti ₂ O ₃ is low and a Ti oxide which does not exhibit photocatalysis is formed. , Organic matter decomposability is reduced. For this reason, the purity of the inert gas (rare gas) forming the heat treatment atmosphere is 99.9% to 99.999%, preferably
99.99 to 99.999%.

Further, instead of the inert gas atmosphere, H ₂
An atmosphere in which a gas or a mixed gas of 90% by volume or more of an H ₂ gas and an inert gas is set to have a dew point of 0 to −60 ° C. When the dew point exceeds 0 ° C., the oxygen potential in the atmosphere becomes too high, Mn, Si, Fe, etc. are oxidized, and the concentration of TiO _{2 in} the oxide formed on the surface layer of the steel sheet decreases. Also,
Similarly, even when the mixing ratio of the H ₂ gas is less than 90% by volume, there is a problem that Mn, Si, and Fe are contained in the oxide,
The expression of catalytic action at the time of light irradiation of TiO ₂ is reduced. here,
The reason why N ₂ was not used as the atmosphere gas was that Ti nitride, such as TiNx, which does not exhibit photocatalysis, was not generated.

The heat treatment is performed in the above-mentioned atmosphere at 600 to 90
Hold at a heating temperature of 0 ° C for 0.5 to 8 hours. By performing such a heat treatment on the steel sheet, the generation of anatase-type crystalline TiO ₂ having the most photocatalytic action on the steel sheet surface is increased. When the heating temperature is lower than 600 ° C, oxides such as Mn and Si are easily generated, and the diffusion of Ti to the surface is slow. To obtain an oxide layer of 0.10 μm or more, heating and holding for an extremely long time is required. And the manufacturing cost increases. On the other hand, when the heating temperature exceeds 900 ° C, the crystal grains of the steel sheet become coarse and the steel
Al tends to be oxidized. In addition, rutile-type crystalline TiO ₂ that does not show strong photocatalytic action is easily generated,
IA / (IA + IR) cannot be greater than 0.6. Therefore, the heating temperature is 600 to 900 ° C, preferably 700 to 800 ° C.
° C.

The heating time is preferably in the range of 0.5 to 8 hours. If the time is less than 0.5 h, an oxide layer of 0.10 μm or more required for obtaining a photocatalytic action cannot be formed on the surface, and if the heating time exceeds 8 h, the time required for the annealing step becomes longer and the productivity decreases. I do. For this reason, the heating time is set to 0.5 to 8 hours, preferably 1 to 3 hours. The heat treatment described above, by subjecting the carbon steel having the composition described above containing Ti, has a TiO ₂ 5 atomic% or more and mainly anatase above 0.10μm comprises TiO ₂ of thickness oxide layer It can be formed on the surface of a steel sheet.

[0027]

EXAMPLE A steel having the composition shown in Table 1 was melted by vacuum melting to obtain a small steel ingot (100 kg). These ingots were formed into sheet bars by slab rolling, then heated to 1200 ° C, and hot-rolled at a finish-rolling end temperature of 900 ° C to obtain a hot-rolled steel sheet having a thickness of 7.0 mm. The surfaces of these hot-rolled steel sheets were mechanically polished with # 600 to # 80 emery paper, and annealed under the heat treatment conditions (atmosphere gas, heating temperature, heating time) shown in Table 2.
Some steel sheets were subjected to electrolytic pickling after annealing.

The thickness of the oxide layer formed on the surface of the steel sheet, the analysis of the oxide layer, and the oil decomposition test by light irradiation and the antibacterial test under light irradiation using Escherichia coli using these steel sheets. went. The thickness of the oxide layer is determined from the surface of the rare gas ion oxide layer by glow discharge emission spectroscopy (GDS).
The element concentration profile in the depth direction in the plate thickness direction was measured, and the profile was obtained from the distance between the oxide layer surface and the iron base interface.

The oxide layer was analyzed by diffraction intensity IA (d from TiO ₂ anatase type TiO ₂ measured by thin film X-ray diffraction.
= 3.52 °) and the diffraction intensity IR from the rutile type TiO ₂ (d =
3.24Å), IA / (IA + IR)
), The volume% of anatase TiO ₂ in the Ti oxide was estimated. Note that the oxide layer formed on the surface, the TiO _2, anatase TiO _2, rutile TiO _2, as the other oxides, Mn, Si, oxides or Ti containing one or more of Fe And an oxide containing at least one of Mn, Si, and Fe. When IA / (IA + IR) is 0.6 or more, it is defined as A,
When it was less than 0.6, it was indicated as R, and the formed layer in the oxide layer formed on the surface was evaluated. It should be noted that when IA = 0 and IR = 0, it was set to 0. The TiO ₂ content (atomic%) in the oxide layer was measured by XPS (X-ray Photoelectron Spectroscopy).

The oil decomposability test was carried out by cutting 50 mm
After applying about 0.1 mg / cm ² of edible vegetable oil to a test piece of × 50 mm, the amount of oil decomposition when irradiated with black light having an irradiation light intensity of ² mW / cm ² for 8 hours was measured, and the oil decomposition rate (% ) = (Decomposition amount / oil application amount) × 100. For comparison, the oil decomposition rates of the steel sheets No. 1, No. 11 and No. 13 before the heat treatment were all 5% or less.

The antibacterial test was carried out in a normal broth medium at 30 ± 1.
The cultured cells cultured at 16 ° C. for 16 to 20 hours were diluted with physiological saline to adjust the number of cells per ml to 1.0 × 10 ^{5 to} 5.0 × 10 ⁵ . After washing this bacterial solution with a neutral detergent / ion-exchanged water, a heat-sterilized test piece (50 mm x 50 mm) was placed in a 0.1 m
l The solution was dropped and irradiated with 1 mW / cm ² black light at room temperature for 2 hours. After irradiation, the viable bacteria are washed out of the sample and the viable count is
The presence or absence of the number of remaining bacteria was evaluated by an agar plate culture method using a medium. For comparison, before heat treatment No.1, No.11,
As a result of conducting an antibacterial test on No. 13, bacteria remained in all cases.

Table 2 shows the results.

[0033]

[Table 1]

[0034]

[Table 2]

[0035]

[Table 3]

Table 2 shows that the steel sheet of the present invention (steel sheets No. 1 to No. 5,
No.11, No.15 to No.17, No.20 to 25, No.30, No.31
, No.38-No.40, No.43, No.44, No.45, No.46
) Had better oil-decomposability and antibacterial properties as compared with Comparative Examples outside the scope of the present invention. In particular, steel sheet No.1, No.2, N
o.4, No.15, No.17, No.20 to No.23, No.30, No.3
1, No.38, the heat treatment conditions are within the optimal range, the oxide layer is thick 0.3 [mu] m or more, which is mainly formed of TiO ₂ anatase showing a photocatalytic activity effectively, oil breakdown The rate was 80% or more, and no residual bacteria were detected in the antibacterial test. On the other hand, the steel sheet No.
In the case of 3, No. 5, No. 16, No. 24 and No. 25, no residual bacteria were detected in the antibacterial test, but the oil decomposition rate was 60 to
It was slightly low at 75%, and the organic matter decomposability due to the photocatalytic effect was slightly inferior.

The steel sheets No. 11, No. 30, and No. 31 had an oil decomposition rate of 70 to 80%, showed no residual bacteria in the antibacterial test, and showed good organic substance decomposability. However, it was confirmed that since a large amount of Ti was added, a large amount of intermetallic compounds such as FeTi and Fe ₂ Ti was generated in the steel, and the ductility of the steel sheet was deteriorated.

[0038]

According to the present invention, it is possible to obtain a steel sheet in which an oxide layer containing an anatase type TiO ₂ phase which effectively exhibits a photocatalytic action is formed on the surface of the steel sheet. It is excellent in decomposability of organic substances such as antifouling property and has a remarkable industrial effect. Since the steel sheet of the present invention has a TiO ₂ layer formed on the surface by heat treatment, it also has the effect of being industrially manufactured at low cost.

[Brief description of the drawings]

FIG. 1: Ratio of anatase TiO ₂ in oxide layer, IA /
It is a graph which shows the relationship between (IA + IR) and an oil decomposition rate.

──────────────────────────────────────────────────の Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat ゛ (Reference) C23C 8/10 C23C 8/10 30/00 30/00 C (72) Inventor Toshiyuki Hoshino Mizushima, Kurashiki City, Okayama Prefecture Kawasaki-dori 1-chome (without address) Kawasaki Steel Corporation Mizushima Works (72) Inventor Koichi Tozawa 1-chome, Mizushima-Kawasaki-dori (Kurashiki-shi, Okayama Prefecture) Kawasaki Steel Corporation Mizushima Works (72) Inventor Honda Atsushi 1-chome, Mizushima-Kawasaki-dori, Kurashiki City, Okayama Prefecture (without address) F-term in Kawasaki Steel Corporation Mizushima Works (reference) 4C058 AA02 AA12 AA23 AA24 BB07 JJ04 JJ05 JJ30 4H011 AA02 BA01 BB18 BC18 DG03 4K044 AA12 BA

Claims (3)

[Claims] 1. A steel sheet having an oxide layer having a thickness of 0.10 μm or more on its surface, wherein the oxide layer contains 5% or more of TiO ₂ in atomic% and X-rays from anatase type TiO ₂ A steel sheet excellent in organic matter decomposability, wherein the diffraction intensity IA and the X-ray diffraction intensity IR from rutile type TiO ₂ satisfy the following formula (1). {IA / (IA + IR)} ≥0.6 (1) 2. Carbon containing, by weight%, C: 1% or less, N: 0.01% or less, Ti: 0.1 to 2.0%, and containing C, N and Ti in a range satisfying the following formula (2). Steel sheet, purity
600 to 900 ° C in an inert gas atmosphere of 99.9% to 99.999%
A heat treatment at a temperature of 0.5 to 8 hours. {Ti (wt%) / (2.0 × C (wt%) + 1.7 × N (wt%)) >> 1 (2) 3. An atmosphere in which H ₂ gas or a mixed gas of H ₂ gas of 90% by volume or more and an inert gas is set to a dew point of 0 to −60 ° C. instead of the inert gas atmosphere. The method for producing a steel sheet according to claim 2, wherein: