JP2014152368A - High-purity ferrite-based stainless steel sheet having excellent temper color resistance and workability and production method thereof - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a high-purity ferrite-based stainless steel plate having excellent temper color resistance and workability.SOLUTION: A high-purity ferrite-based steel sheet comprises 0.001-0.03% C, 0.01-0.7% Si, 0.01-1% Mn, 0.005-0.05% P, 0.0001-0.01% S, 12-20% Cr, 0.001-0.03% N, 0.005-0.5% Al, 0.05-1% Sn and remaining Fe and unavoidable impurities and has a surface oxide film of a thickness of smaller than 500Å. In the surface oxide film, Sn is present as a tetravalent oxide. The ratio of the Cr concentration in the range from the surface to the depth of 20Å to the amount of Cr contained in steel, (Cr concentration/Cr amount in steel) is 1.1 or higher, based on the ratio of cations alone, other than C, O and N.

Description

  The present invention relates to a high-purity ferritic stainless steel plate having a metallic luster inherent to stainless steel and excellent in temper color resistance and workability, which is difficult to cause coloration when heated, and a method for producing the same. This steel plate is suitable for use in cooking equipment such as pots, rice cookers, pans, ranges, and heaters, and heating equipment.

  In recent years, high purity ferritic stainless steel sheet, which has been made extremely low carbon and nitrogen reduced by refinement technology and further reduced impurity elements such as P and S, has a large amount of Ni, which has recently increased in price. As an alternative to the stainless steel plate, it is being applied to a wide range of applications.

  The high-purity ferritic stainless steel plate described above is also used for daily necessities that are exposed to higher temperatures (about 200 to 400 ° C.) than room temperature, such as cooking appliances and heating appliances. In general, when the temperature exceeds 200 ° C. in the atmosphere, stainless steel forms an oxide layer with a thickness of less than 1 μm, called a temper color, and the appearance and weather resistance of the product are significantly impaired due to discoloration and coloring of the surface. It has been known.

As a method for preventing discoloration or coloring due to a temper color, a method of applying a coating having excellent oxidation resistance such as SiO ₂ or Al ₂ O ₃ has been known for a long time. For example, Patent Documents 1 and 2 disclose techniques for coating a stainless steel surface with a compound mainly composed of silica. Such a coating treatment is extremely effective for preventing temper color, but it is carried out after the production of the steel sheet and after the processing of the product, and the reduction in productivity and the increase in cost are unavoidable problems.

  The oxidation resistance of stainless steel is improved by increasing Cr and adding a large amount of Si or Al. It is effective to use stainless steel having excellent oxidation resistance as a means for preventing discoloration and coloring due to the temper color. However, high Cr content exceeding 20% and addition of Si and Al exceeding 1% harden the steel sheet, lower the ductility, impair the workability of the steel sheet, and reduce the toughness of the steel ingot in industrial production. There is also a problem that deteriorates manufacturability.

  Examining overcoming the above-mentioned problems of stainless steel itself, Patent Document 3 includes C: 0.02% or less, Cr: 10-25%, Si: 0.1-1.5%, Mn: 1. 0% or less, P: 0.05% or less, S: 0.01% or less, Nb: 0.002 to 0.02%, B: 0.0003 to 0.0025%, Ti: 6 × (C + N) to Ferritic stainless steel containing 0.5% and having a concentrated layer of Cr, Si, B on the surface oxide film by bright annealing is disclosed, and this stainless steel is in a high temperature oxidizing environment of 200-400 ° C. It is said to be excellent in corrosion resistance. In Patent Document 4, the atomic concentration ratio of Cr, Si, Al, and Fe (Cr + Si + Al) / Fe having a surface oxide film with a depth of 50 or more from the surface is excellent in temper color resistance. A polished finish stainless steel sheet and a method for producing the same are disclosed. In these Patent Documents 3 and 4, the intended surface oxide film is obtained by defining the conditions of bright annealing. Patent Document 3 is characterized by the surface concentration of B added with a small amount of low Nb, and Patent Document 4 is characterized by mechanically polishing the steel sheet prior to cold rolling and bright annealing.

  In Patent Documents 5 and 6, as a high-purity ferritic stainless steel having excellent weather resistance and workability by adding Sn, from the viewpoint of resource saving and economical efficiency, regardless of the alloying of Cr and Mo, Cr : 13-22%, Sn: 0.001-1%, C, N, Si, Mn, P is reduced, Nb and Ti stabilizing elements are added if necessary, the surface by pickling and bright annealing A high-purity ferritic stainless steel is disclosed in which Cr and Sn are concentrated in the passive state film formed on the surface.

Japanese Patent No. 4786576 JP 2006-63427 A Japanese Patent No. 3477957 JP-A-8-295999 Japanese Patent No. 4651682 Japanese Patent No. 4624473

  As described above, the method of imparting temper color resistance to stainless steel without relying on high Cr and excessive addition of Si and Al is limited to controlling the surface film composition by defining bright annealing conditions. Yes. On the other hand, resource-saving high-purity ferritic stainless steel characterized by the addition of Sn is also disclosed, but no consideration is given to temper color resistance.

  Therefore, the present invention does not rely on excessive alloy addition, and is not limited to the formation of a surface film by bright annealing, but a high-purity ferritic stainless steel sheet excellent in temper color resistance and workability utilizing Sn addition. The issue is to provide.

  In order to solve the above-mentioned problems, the present inventors have focused on the effect of the surface oxide film that bears the temper color resistance on the effect of Sn addition on the temper color resistance in high purity ferritic stainless steel. The present invention was completed through extensive experiments and studies. The knowledge obtained by the present invention will be described below.

(A) Sn is an element effective in improving the weather resistance of high-purity ferritic stainless steel, and by adding Sn, it does not rely on the addition of high-concentration Cr or Mo, and has excellent workability due to alloy saving. Steel plate can be obtained. In the present invention, when Sn-added stainless steel is heated to 100 ° C. or higher in the atmosphere or in an inert gas, the temper color resistance can be improved regardless of the high Cr and Si / Al addition due to the modification effect of the surface oxide film. It turned out that it can improve remarkably. Although there are still many unclear points regarding such an effect of improving the temper color resistance, the mechanism of its action is presumed based on experimental facts as described below.

(B) 16Cr steel added with 0.3% Sn (hereinafter referred to as Sn-added steel) and high workability SUS430LX (17Cr-0.3Ti) and heat-resistant stainless steel (18Cr-2.5Si) as comparative materials. Using. The shape of the test piece was 40 mm square, and the surface state was a normal cold-rolled and pickled finish (hereinafter referred to as 2B finish). For these Sn-added steels and comparative materials, 2B-finished test materials and test materials that were heat-treated by heating them at 150 ° C. in air or nitrogen gas for 30 seconds were used for evaluation. About these test materials, it heated at 200-400 degreeC in air | atmosphere for 1 hour, and performed the salt spray test while performing the color difference measurement (b value) based on JIS. Compared with SUS430LX, the Sn-added steel subjected to the heat treatment did not increase the b value, and the occurrence of wrinkling by the salt spray test was also suppressed. In addition, Sn addition steel which did not heat-process and SUS430LX had surface coloring (metallic luster-yellow) by heating at 200-400 degreeC in air | atmosphere, and b value raised.
Moreover, the fogging by salt water spray was suppressed significantly in the heat-treated Sn-added steel.

  The heat treatment is presumed to affect the soundness of the surface oxide film, and the heat-treated Sn-added steel is presumed to have a surface oxide film that is not colored and has Cr concentrated on the surface. Thus, it was found that the heat-treated Sn-added steel can exhibit good temper color resistance comparable to heat-resistant stainless steel without impairing good workability.

(C) From the detailed surface analysis of Sn-added steel, it was confirmed that (i) the thickness of the surface oxide film grows from 20 to 50 to about 100 to 100 by applying the heat treatment after 2B finishing. Further, as the film thickness of the surface oxide film grows, (ii) the Cr concentration in the surface oxide film increases. (Iii) Sn changes from a mixed state of metal and oxide to a tetravalent oxide. (Iv) Concentration of Ti in the surface oxide film is suppressed in accordance with the results of (ii) and (iii). New knowledge was obtained. That is, the present inventors have the effect that Sn addition and heat treatment have the effect of modifying the conventional surface film to the surface film characterized by the above-mentioned items (i) to (iv). It has been found that there is an effect of suppressing the generation of a temper color by atmospheric heating. It was found that the effect of film modification by such addition of Sn and heat treatment is manifested when the Sn amount is 0.05%, and the effect is superimposed particularly when combined with Nb.

(D) There are still many unclear points regarding the above-described film modification mechanism of Sn addition. In Patent Document 6, the concentration of Cr and Sn in the passive film by addition of Sn and the effect of improving the weather resistance associated therewith are clarified. In the present invention, in addition to the thickening action of Cr and Sn in the surface oxide film, the presence of Sn is controlled, so that the concentration of Ti in the film that inhibits the temper color resistance is suppressed. It is estimated to be.

(E) In order to enhance the effect of improving the above-described temper color resistance, it is preferable to increase the purity of stainless steel by reducing C, N, P, and S. It is also effective to add Nb as a stabilizing element.

(F) For the film modification described in (c) above, it is effective to raise the temperature of a steel sheet obtained by known annealing and pickling to 100 to 300 ° C. in the air or in an inert gas. . Further, in the case of a bright annealed steel sheet, it is preferable that the steel sheet is retained in an inert gas not containing hydrogen gas or in the air during cooling.

  The gist of the present invention based on the above findings (a) to (f) is as follows.

(1) In mass%, C: 0.001 to 0.03%, Si: 0.01 to 0.7%, Mn: 0.01 to 1%, P: 0.005 to 0.05%, S: 0.0001-0.01%, Cr: 12-20%, N: 0.001-0.03%, Al: 0.005-0.5%, Sn: 0.05-1%, balance Is a steel plate composed of Fe and inevitable impurities, having a surface oxide film of less than 500 mm, Sn present in the state of tetravalent oxide in the surface oxide film, and C, O and N The ratio of the Cr concentration in the range from the surface to a depth of 20 mm to the Cr content in the steel (Cr concentration / steel content Cr content) is 1.1 or more in the proportion of only cations removed. High purity ferritic stainless steel sheet with excellent temper color and workability.

(2) The steel further includes, in mass%, Nb: 1% or less, Ti: 0.3% or less, Ni: 1% or less, Cu: 1% or less, Mo: 1% or less, V: 0.00. 5% or less, Zr: 0.5% or less, Co: 0.5% or less, Mg: 0.005% or less, B: 0.005% or less, Ca: 0.005% or less The high-purity ferritic stainless steel sheet excellent in temper color resistance and workability as described in (1), characterized in that it contains at least seeds.

(3) In the steel, any one of La: 0.1% or less, Y: 0.1% or less, Hf: 0.1% or less, REM: 0.1% or less in mass% Alternatively, the high purity ferritic stainless steel sheet having excellent temper color resistance and workability according to (1) or (2), wherein two or more types are contained.

(4) Finish annealing and pickling at an annealing temperature of 800 to 1000 ° C. are continuously performed on the cold-rolled steel sheet having the steel component according to any one of (1) to (3). Then, a method for producing a high-purity ferritic stainless steel sheet excellent in temper color resistance and workability, characterized by performing a heat treatment that stays at 100 to 300 ° C. for 10 seconds or more in air or an inert gas.

(5) Bright annealing is performed at an annealing temperature of 800 to 1000 ° C. on the steel sheet after cold rolling having the steel component according to any one of (1) to (3), and cooling is performed after the bright annealing. Alternatively, a method for producing a high-purity ferritic stainless steel sheet excellent in temper color resistance and workability, characterized in that after cooling, a heat treatment is carried out at 100 to 300 ° C. for 10 seconds or more in air or an inert gas.

  According to the present invention, it does not depend on high Cr, excessive addition of Si or Al, and is not limited to the formation of a surface film by bright annealing. There is a remarkable effect that an excellent high purity ferritic stainless steel sheet can be obtained.

  Hereinafter, each requirement of the present invention will be described in detail. In addition, "%" display of the content of each element means "mass%".

(I) Steel component The reason for limiting the steel component will be described below.
C deteriorates the temper color resistance which is the object of the present invention in addition to the corrosion resistance. Therefore, the lower the content of C, the better. The upper limit is made 0.03%. However, excessive reduction of C leads to an increase in refining costs, so the lower limit is made 0.001%. Preferably, considering the characteristics and manufacturing cost, 0.002 to 0.02%.

  Si is a deoxidizing element and an element effective for temper color resistance. However, the addition of Si leads to a decrease in the workability that is the object of the present invention, so the less Si, the better. Therefore, the upper limit is made 0.7%. However, excessive reduction of Si leads to a decrease in deoxidizing ability and an increase in refining cost, so the lower limit is made 0.01%. Preferably, considering the characteristics and manufacturability, the content is made 0.05 to 0.5%. A more preferable range is 0.1 to 0.3%.

  Since Mn is an element that forms sulfides and hinders weather resistance, the smaller the content, the better. The upper limit of Mn is made 1% in order to suppress the decrease in corrosion resistance. However, excessive reduction of Mn leads to an increase in refining costs, so the lower limit is made 0.01%. Preferably, considering the corrosion resistance and manufacturability, the content is made 0.05 to 0.5%.

  Since P is an element that hinders manufacturability and weldability, the smaller the content, the better. The upper limit of P is set to 0.05% in order to suppress a decrease in manufacturability and weldability. However, excessive reduction of P leads to an increase in refining costs, so the lower limit is made 0.005%. Preferably, considering the manufacturing cost, the content is made 0.01 to 0.04%.

  Since S deteriorates temper color resistance and hot workability, the smaller the content, the better. Therefore, the upper limit of S is set to 0.01%. However, excessive reduction of S leads to an increase in refining costs, so the lower limit is made 0.0001%. Preferably, considering the temper color resistance and the manufacturing cost, 0.0002 to 0.002%.

  Cr is a constituent element of the high purity ferritic stainless steel of the present invention, and is an essential element that improves the temper color resistance targeted by the present invention by adding Sn. In order to obtain the effect of improving the temper color resistance, the lower limit of Cr is made 12%. The upper limit of Cr is set to 20% from the viewpoint of manufacturability in addition to the workability targeted by the present invention. Preferably, considering the characteristics, manufacturability and economics of the alloy cost, it is made 13 to 18%.

  N, like C, deteriorates the temper color resistance, so the content is preferably as small as possible, and the upper limit is made 0.03%. However, excessive reduction of N leads to an increase in refining costs, so the lower limit is made 0.001%. Preferably, considering the characteristics and manufacturing cost, 0.005 to 0.015%.

  In addition to being an effective element as a deoxidizing element, Al is an element that improves the temper color resistance targeted by the present invention. The lower limit of Al is set to 0.005% in order to obtain a deoxidation effect when Si is reduced. The upper limit of Al is 0.5% from the viewpoint of workability and manufacturability. Preferably, considering the characteristics and manufacturability, the content is made 0.02 to 0.2%. More preferably, it is 0.03 to 0.1%.

  Sn is an essential element for exhibiting the temper color resistance aimed at by the present invention without depending on the workability degradation due to alloying of Cr, Si or Al or the film control by bright annealing. In order to obtain the effect of improving the temper color resistance which is the target of the present invention, the lower limit of Sn is set to 0.05%. The upper limit of Sn is 1% from the viewpoints of workability and manufacturability. Preferably, considering the characteristics and manufacturability and the economics of the alloy cost, the content is made 0.1 to 0.5%. More preferably, it is 0.1 to 0.3%.

  Moreover, in the high purity ferritic stainless steel sheet of the present invention, in addition to the above components, any one or more of Nb, Ti, Ni, Cu, Mo, V, Zr, Co, Mg, B, and Ca are used. It may contain. Furthermore, the high purity ferritic stainless steel sheet of the present invention may contain one or more of La, Y, Hf, and REM.

  Nb is an element that overlaps with Sn to improve the temper color resistance in addition to the action of a stabilizing element that fixes C and N, and is added as necessary. When adding Nb, it is made into 0.03% or more which the effect expresses. However, excessive addition leads to a decrease in manufacturability accompanying an increase in alloy cost and a recrystallization temperature, so the upper limit is made 1%. A preferable range of Nb is 0.05 to 0.5% in consideration of the effect, the alloy cost, and the manufacturability. A more preferable range is 0.1 to 0.3%.

  Ti has the effect of increasing the temper color resistance through the purification of steel by the action of a stabilizing element that fixes C and N. On the other hand, it selectively concentrates in the surface oxide film to cause coloring, and has an effect of reducing the temper color resistance. When adding Ti, the content of the former is 0.01% or more. However, excessive addition leads to the latter action, so the upper limit is made 0.3%. The preferable range of Ti is 0.02 to 0.2% in consideration of the effect and the alloy cost. A more preferable range is 0.05 to 0.15%.

  Ni, Cu, Mo, V, Zr, and Co are effective elements for enhancing the temper color resistance by a synergistic effect with Sn, and are added as necessary. When adding Ni, Cu, and Mo, it is set as 0.1% or more which the effect expresses, respectively. When V, Zr, and Co are added, the content is set to 0.01% or more where the effect is exhibited. However, excessive addition leads to an increase in alloy costs and a decrease in manufacturability, so the upper limit of Ni, Cu, and Mo is 1%, and the preferred range is 0.1 to 0.5%. Similarly, the upper limit of V, Zr, and Co is 0.5%, and the preferred range is 0.01 to 0.3.

  In addition to forming Mg oxide together with Al in molten steel and acting as a deoxidizer, Mg acts as a crystallization nucleus of TiN. TiN becomes a solidification nucleus of the ferrite phase in the solidification process, and by facilitating crystallization of TiN, the ferrite phase can be finely formed during solidification. By making the solidified structure fine, it is possible to prevent surface defects due to coarse solidified structure such as ridging and roping of the product, and to improve workability. When adding Mg, it is made 0.0001% which expresses these effects. However, if it exceeds 0.005%, manufacturability deteriorates, so the upper limit is made 0.005%. Preferably, considering the manufacturability, the content is made 0.0003 to 0.002%.

  B is an element that improves hot workability and secondary workability, and addition to high purity ferritic stainless steel is effective. When adding, it is made 0.0003% or more to express these effects. However, excessive addition of B causes a decrease in elongation, so the upper limit is made 0.005%. Preferably, considering the workability and manufacturability, the content is made 0.0005 to 0.002%.

  Ca is an element that improves hot workability and steel cleanliness, and is added as necessary. When adding Ca, it is made 0.0003% or more which expresses these effects. However, excessive addition of Ca leads to a decrease in manufacturability and a decrease in corrosion resistance due to water-soluble inclusions such as CaS, so the upper limit is made 0.005%. Preferably, considering the manufacturability and corrosion resistance, the content is made 0.0003 to 0.0015%.

  La, Y, Hf, and REM are effective elements for improving the hot workability and the cleanliness of steel and improving the temper color resistance of the present invention, and may be added as necessary. When added, the content is set to 0.001% or more where the effect is exhibited. However, excessive addition leads to an increase in alloy cost and a decrease in manufacturability, so the upper limit is made 0.1%. Preferably, considering the effect, economic efficiency, and manufacturability, one or two or more is 0.001 to 0.05%. REM is an element belonging to atomic numbers 57 to 71, and examples thereof include La, Ce, and Nd.

(II) Surface oxide film In the steel of the component described in the above item (I), a surface oxide film that exhibits temper color resistance will be described below.
The thickness of the surface oxide film in the high purity ferritic stainless steel sheet of the present invention is preferably 20 mm or more. The upper limit of the thickness of the surface oxide film is 500 mm from the viewpoint of coloring. A preferable film thickness is in the range of 30 to 300 mm in consideration of coloring and temper color resistance. A more preferable range is 50 to 100 mm.

  Moreover, Sn in the conventional Sn-added stainless steel exists in a mixed state of metal and oxide in the surface oxide film. On the other hand, in the high purity ferritic stainless steel sheet of the present invention, Sn is present as a tetravalent oxide. Since the presence state of Sn is a tetravalent stable Sn oxide, it is possible to suppress the surface enrichment of trace elements such as Ti which inhibits the temper color resistance.

  Furthermore, in order to develop temper color resistance, it is effective to increase the Cr concentration from the surface to a depth of 20 mm to 1.1 times or more that of the base material. Preferably, the Cr concentration from the surface to 20% is 1.2 times or more that of the base material.

  The Cr concentration of the above-described oxide film and the presence state of Sn can be examined by X-ray photoelectron spectroscopy. In the case of X-ray source mono-AlKα ray and incident X-ray energy = 1486.6 eV, the states of Fe, Cr, and Sn can be confirmed by detecting peaks with the following binding energies. Other detection elements include Ti, Si, Mn, Al, and the like.

Fe2p: 705 to 714 eV, Cr2p: 570 to 580 eV,
Sn3d: 480 to 500 eV

Here, the Cr concentration is determined in terms of cation ions excluding C, O, and N light elements by measuring the integrated intensity of the oxide detection spectrum.

  The presence state of Sn can also be analyzed with high sensitivity by hard X-ray photoelectron spectroscopy with an incident energy of 7939 eV using synchrotron radiation. When such high energy X-rays are used, it is useful to analyze the inner core level electron orbits that are not subject to interference such as O, Fe, and Cr. For the Sn state analysis, it is effective to detect a peak with the following binding energy.

Sn2s: 4460-4470 eV, Sn2p3 / 2: 3925-3935 eV

  The thickness of the surface oxide film can be measured by the Ar sputtering method of Auger electron spectroscopy. The thickness of the oxide film can be obtained as a region corresponding to the half-value width in the depth profile of O (oxygen).

(III) Manufacturing method Next, the manufacturing method of the high purity ferritic stainless steel sheet of this invention is demonstrated.
The high purity ferritic stainless steel of the present invention is subjected to the following conditions for the cold-rolled steel sheet in order to form the surface oxide film described in the section (II) and to exhibit the temper color resistance. Manufacture steel sheets. In addition, the manufacturing method until obtaining a cold-rolled steel plate is not specifically limited.

  In the production method of the present invention, finish annealing and pickling are continuously performed on the cold-rolled steel sheet after cold rolling, and then heat treatment is performed for 10 seconds or more at 100 to 300 ° C. in air or an inert gas. Or performing a bright annealing on the cold-rolled steel sheet after the cold rolling, and performing a heat treatment that stays at 100 to 300 ° C. for 10 seconds or more in the air or an inert gas during or after the cooling after the bright annealing. Do.

(When finishing annealing and pickling are performed continuously)
The annealing temperature in the case of continuously performing finish annealing and pickling on a cold-rolled steel sheet is set to 800 ° C. or more for the purpose of recrystallization of the steel sheet. The upper limit of the annealing temperature of finish annealing is set to 1000 ° C. in order to suppress coarsening of crystal grains. A preferable temperature is in the range of 850 to 950 ° C. in order to form a fine grain recrystallized structure and obtain good processability.

  The pickling method after finish annealing is not particularly specified, and can be carried out by a method commonly used in industry. For example, a process of electrolytic pickling after immersion in an alkali salt bath and immersion in nitric hydrofluoric acid can be exemplified. In the electrolytic pickling, neutral salt electrolysis or nitric acid electrolysis is performed.

Next, in order to form the above-described surface oxide film on the steel plate finished by pickling, heat treatment is performed at 100 ° C. or higher and 300 ° C. or lower for 10 seconds or longer in the air or an inert gas. The inert gas contains nitrogen or argon gas as a main component and may contain O ₂ , H ₂ , CO ₂ or the like. In the case where O ₂ , H ₂ , CO _{2 and the} like are contained, the total content is preferably 3% or less. When the heat treatment temperature is less than 100 ° C., Cr concentration and Sn oxide formation in the surface oxide film become difficult. On the other hand, if the heat treatment temperature exceeds 300 ° C., coloring is feared due to excessive growth of the surface oxide film. The heat treatment holding time is preferably 10 seconds or longer in consideration of the stability of the heat treatment. The upper limit of the holding time is not particularly specified, but is preferably 24 hours or less in consideration of coloring due to the growth of the oxide film.

(In case of bright annealing)
In addition, the annealing temperature when performing bright annealing on a cold-rolled steel sheet is set to 800 ° C. or more for the purpose of recrystallization of the steel sheet, as in the case of performing continuous annealing and pickling. The upper limit of the annealing temperature for bright annealing is set to 1000 ° C. in order to suppress coarsening of crystal grains. A preferable temperature is in the range of 850 to 950 ° C. in order to form a fine grain recrystallized structure and obtain good processability.

The heat treatment after the bright annealing may be performed during the cooling after the bright annealing, or after the bright annealing and cooling are completed, the heat treatment may be performed by reheating. When the heat treatment is performed during the cooling after the bright annealing, the atmospheric gas is changed from the bright annealing atmospheric gas (for example, hydrogen gas) to an inert gas or air in a temperature range of 300 ° C. or lower. And while letting a steel plate pass in the changed atmosphere (inert gas or air), the steel plate temperature is made to stay for 10 seconds or more in the range of 100-300 degreeC. The inert gas in this case is mainly composed of nitrogen or argon gas, and may contain O ₂ , H ₂ , CO ₂ or the like. In the case where O ₂ , H ₂ , CO _{2 and the} like are contained, the total content is preferably 3% or less.

In addition, when the heat treatment is performed by reheating after completion of the bright annealing and cooling, it is preferable to perform the heat treatment at 100 ° C. or higher and 300 ° C. or lower for 10 seconds or longer in the air or an inert gas. The inert gas contains nitrogen or argon gas as a main component and may contain O ₂ , H ₂ , CO ₂ or the like. In the case where O ₂ , H ₂ , CO _{2 and the} like are contained, the total content is preferably 3% or less. When the heat treatment temperature is less than 100 ° C., it is difficult to concentrate Cr and form Sn oxide in the surface oxide film. On the other hand, if the heat treatment temperature exceeds 300 ° C., coloring is feared due to excessive growth of the surface oxide film. The heat treatment holding time is set to 10 seconds or more in consideration of the heat treatment stability. The upper limit of the holding time is not particularly specified, but is preferably 24 hours or less in consideration of coloring due to the growth of the oxide film.

  Further, the brightly annealed steel sheet may be subjected to nitric acid electrolysis or the like as necessary in order to increase the Cr concentration in the surface oxide film.

  Examples of the present invention will be described below.

After ferritic stainless steel having the components shown in Table 1 was melted and subjected to hot rolling and annealing, a cold rolled steel sheet having a thickness of 0.4 to 1.0 mm was obtained through cold rolling. All the cold-rolled steel sheets were finish-annealed in the range of 850 to 1000 ° C. where recrystallization was completed. As the final annealing, oxidizing atmosphere annealing or bright annealing was performed. The pickling conditions after oxidizing atmosphere annealing were alkali salt bath immersion + neutral salt electrolysis + nitric hydrofluoric acid immersion. The steel sheet obtained by finish annealing / pickling or bright annealing was subjected to heat treatment that stayed at 100 to 300 ° C. in the atmosphere or in an inert gas in a timely manner.
Moreover, the steel component was also carried out in the range specified in the present invention and other ranges. As the comparative steel, SUS430LX (17Cr-Ti), which is a high workability and high purity ferrite, was used. Further, heat resistant SUS (18Cr-2.5Si), which serves as a reference as a target characteristic of the present invention, was used as a reference example. In addition, La and Y of steel G are 0.1%, La and Hf of steel I are 0.05%, B and Mg of steel J are 7 ppm, Zr and Co of steel K, respectively. Is 0.05% respectively.

  The surface oxide film was evaluated for the Cr concentration, the presence state of Sn, and the film thickness by X-ray photoelectron spectroscopy. For the temper color resistance, b value was determined by color difference measurement on the surface of a steel sheet heated at 300 ° C. for 1 hour in air. Also, the surface texture (metallic luster, coloring) was determined by appearance observation. Furthermore, the mist generation (existence: x, absence: o) by the salt water spray test (24hr spraying) based on JIS was confirmed. The inventive example had a b value smaller than that of the comparative material 430LX in the above test, had a metallic luster texture similar to the heat-resistant SUS, and had no rust in the SST test.

Table 2 summarizes the manufacturing method and each test result.
From Table 2, test numbers 7 to 12 and 13 to 16 are high-purity ferritic stainless steel sheets that satisfy the preferred components and production methods defined in the present invention. In these steel sheets, the film thickness of the surface oxide film, the Cr concentration, and the presence of Sn as defined in the present invention were confirmed, and excellent temper color resistance was obtained in the same manner as heat resistant SUS.

  Test numbers 1, 2, and 4 are high-purity ferritic stainless steel plates that have been subjected to the components and manufacturing methods defined in the present invention. These steel sheets obtained excellent temper color resistance equivalent to the heat resistant SUS targeted by the present invention.

  Test Nos. 3, 5, and 6 have components specified by the present invention, but deviate from the manufacturing method specified by the present invention. These steel sheets did not have the temper color resistance targeted in the present invention.

  Test numbers 17 to 20 and 22 deviate from the components defined in the present invention. Although these steel plates were subjected to the preferred production method defined in the present invention, the temper color resistance targeted by the present invention was not obtained.

  Test number 21 deviates from the components defined in the present invention, and the preferred production method defined in the present invention was not carried out. These steel sheets did not have the temper color resistance targeted in the present invention.

  From this result, in addition to good workability in Sn-added high-purity ferritic stainless steel sheet, it is necessary to control the film thickness of the surface film, Cr concentration, and the presence of Sn to exhibit temper color resistance. It is effective to adjust to the components specified in the invention or preferable components and to carry out a preferable manufacturing method.

  According to the present invention, it does not depend on high Cr, excessive addition of Si or Al, and is not limited to the formation of a surface film by bright annealing, but the effect of Sn addition improves the temper color resistance and workability. An excellent alloy-saving high-purity ferritic stainless steel sheet can be obtained.

Claims (5)

  In mass%, C: 0.001 to 0.03%, Si: 0.01 to 0.7%, Mn: 0.01 to 1%, P: 0.005 to 0.05%, S: 0 0.0001-0.01%, Cr: 12-20%, N: 0.001-0.03%, Al: 0.005-0.5%, Sn: 0.05-1%, the balance being Fe and A steel plate comprising inevitable impurities, having a surface oxide film of less than 500 mm, Sn present in the state of a tetravalent oxide in the surface oxide film, and a cation excluding C, O and N The ratio of the Cr concentration to the amount of Cr in the steel in the range from the surface to a depth of 20 mm (Cr concentration / the amount of Cr contained in the steel) is 1.1 or more. And high purity ferritic stainless steel sheet with excellent workability.   The steel further includes, in mass%, Nb: 1% or less, Ti: 0.3% or less, Ni: 1% or less, Cu: 1% or less, Mo: 1% or less, V: 0.5% or less Zr: 0.5% or less, Co: 0.5% or less, Mg: 0.005% or less, B: 0.005% or less, Ca: 0.005% or less The high-purity ferritic stainless steel sheet excellent in temper color resistance and workability according to claim 1, which is contained.   The steel further includes one or two of La: 0.1% or less, Y: 0.1% or less, Hf: 0.1% or less, REM: 0.1% or less in mass%. The high purity ferritic stainless steel sheet excellent in temper color resistance and workability according to claim 1 or 2, wherein the above is contained.   Finished annealing and pickling at an annealing temperature of 800 to 1000 ° C are continuously performed on the cold-rolled steel sheet having the steel component according to any one of claims 1 to 3, and then the atmosphere or non-washing is performed. A method for producing a high-purity ferritic stainless steel sheet excellent in temper color resistance and workability, characterized by performing a heat treatment that retains at 100 to 300 ° C. for 10 seconds or longer in an active gas.   Bright annealing at an annealing temperature of 800 to 1000 ° C. is performed on the steel sheet after cold rolling having the steel component according to any one of claims 1 to 3, and during or after cooling after the bright annealing, the atmosphere Or the manufacturing method of the high purity ferritic stainless steel plate excellent in temper color resistance and workability characterized by performing the heat processing which retains for 10 second or more at 100-300 degreeC in inert gas.