Stainless steel substrate for solar cell having superior insulating properties and low thermal expansion coefficient and method of producing the same

Abstract

Provided is a stainless steel substrate for a solar cell, the stainless steel substrate including, by mass %, Cr: 9% to 25%, C: 0.03% or less, Mn: 2% or less, P: 0.05% or less, S: 0.01% or less, N: 0.03% or less, Al: 0.005% to 5.0%, Si: 0.05% to 4.0%, and a remainder including Fe and unavoidable impurities, in which an oxide film containing (i) Al.sub.2O.sub.3 in an amount of 50% or more or containing (i) Al.sub.2O.sub.3 and (ii) SiO.sub.2 in a total amount of 50% or more is formed on a surface of stainless steel having a composition which contains Al: 0.5% or more and/or Si: 0.4% or more and satisfies the following expression (1).
Cr+10Si+Mn+Al>24.5  (1)

Claims

1. A stainless steel substrate for a solar cell having superior insulating properties and a low thermal expansion coefficient, the stainless steel substrate comprising: stainless steel which contains, by mass %, Cr: 9% to 25%, C: 0.03% or less, Mn: 2% or less, P: 0.05% or less, S: 0.01% or less, N: 0.03% or less, Al: 0.005% to 5.0%, Si: 0.05% to 4.0%, and a remainder including Fe and unavoidable impurities, satisfies one or both of Al content is 0.5% or more and Si content is 0.4% or more, and satisfies the following expression (1); and an oxide film formed on a surface of the stainless steel, the oxide film containing (i) Al.sub.2O.sub.3 in an amount of 50% or more or containing (i) Al.sub.2O.sub.3 and (ii) SiO.sub.2 in a total amount of 50% or more,
Cr+10Si+Mn+Al>24.5  (1) where a symbol of each element in the expression (1) represents the amount (mass %) of the element in the steel, wherein the oxide film contains (iii) Mg Al.sub.2O.sub.4.

2. The stainless steel substrate for a solar cell having superior insulating properties and a low thermal expansion coefficient according to claim 1, wherein the stainless steel contains Al: 2.0 to 5.0% and Si: 0.3 to 4.0%.

3. The stainless steel substrate for a solar cell having superior insulating properties and a low thermal expansion coefficient according to claim 1, wherein the stainless steel further contains one element or two or more elements selected from the group consisting of, by mass %, Sn: 1% or less, Zr: 0.5% or less, Mg: 0.0001 to 0.005%, Ni: 1% or less, Cu: 1% or less, Co: 0.5% or less, Mo: 2% or less, V: 0.5% or less, B: 0.005% or less, Ca: 0.005% or less, La: 0.1% or less, Y: 0.1% or less, Hf: 0.1% or less, REM: 0.1% or less, Nb: 1% or less, and Ti: 1% or less.

4. The stainless steel substrate for a solar cell having superior insulating properties and a low thermal expansion coefficient according to claim 3, wherein an amount of (iii) MgAl.sub.2O.sub.4 in the oxide film is 5% or more.

5. A method of producing a stainless steel substrate for a solar cell having superior insulating properties and a low thermal expansion coefficient, the method comprising: a film forming step of subjecting the stainless steel having the composition according to claim 1 to a heat treatment in an atmosphere containing water vapor in a temperature range of 300° C. to 1000° C. to form an oxide film on a surface of the stainless steel.

6. The method of producing a stainless steel substrate for a solar cell having superior insulating properties and a low thermal expansion coefficient according to claim 5, wherein in the film forming step, the heat treatment is performed in an atmosphere containing water vapor having a dew point of 40° C. or higher.

7. The stainless steel substrate for a solar cell having superior insulating properties and a low thermal expansion coefficient according to claim 2, wherein the stainless steel further contains one element or two or more elements selected from the group consisting of, by mass %, Sn: 1% or less, Zr: 0.5% or less, Mg: 0.005% or less, Ni: 1% or less, Cu: 1% or less, Co: 0.5% or less, Mo: 2% or less, V: 0.5% or less, B: 0.005% or less, Ca: 0.005% or less, La: 0.1% or less, Y: 0.1% or less, Hf: 0.1% or less, REM: 0.1% or less, Nb: 1% or less, and Ti: 1% or less.

8. The stainless steel substrate for a solar cell having superior insulating properties and a low thermal expansion coefficient according to claim 7, wherein the oxide film contains (iii) MgAl.sub.2O.sub.4.

9. The stainless steel substrate for a solar cell having superior insulating properties and a low thermal expansion coefficient according to claim 8, wherein an amount of (iii) MgAl.sub.2O.sub.4 in the oxide film is 5% or more.

10. A method of producing a stainless steel substrate for a solar cell having superior insulating properties and a low thermal expansion coefficient, the method comprising: a film forming step of subjecting the stainless steel having the composition according to claim 2 to a heat treatment in an atmosphere containing water vapor in a temperature range of 300° C. to 1000° C. to form an oxide film on a surface of the stainless steel.

11. A method of producing a stainless steel substrate for a solar cell having superior insulating properties and a low thermal expansion coefficient, the method comprising: a film forming step of subjecting the stainless steel having the composition according to claim 3 to a heat treatment in an atmosphere containing water vapor in a temperature range of 300° C. to 1000° C. to form an oxide film on a surface of the stainless steel.

Description

DESCRIPTION OF EMBODIMENTS

(1) Hereinafter, each requirement according to an embodiment of the present invention will be described in detail. “%” indicating the amount of each element represents “mass %”.

(2) In a stainless steel substrate for a solar cell according to the embodiment, the oxide film containing (i) Al.sub.2O.sub.3 or the oxide film containing (i) Al.sub.2O.sub.3 and (ii) SiO.sub.2 and/or (iii) MgAl.sub.2O.sub.4 is formed on a surface of a body made of the stainless steel.

(3) The stainless steel contained in the stainless steel substrate for a solar cell according to the embodiment has a composition described below. Therefore, by performing a heat treatment on the stainless steel, the oxide film containing (i) Al.sub.2O.sub.3 or the oxide film containing (i) Al.sub.2O.sub.3 and (ii) SiO.sub.2 and/or (iii) MgAl.sub.2O.sub.4 can be formed on the surface of the stainless steel.

(4) (I) The reason for limiting components of the stainless steel will be described below.

(5) The stainless steel included in the stainless steel substrate for a solar cell according to the embodiment is ferritic stainless steel. Cr is a major constituent element of the ferritic stainless steel used in the embodiment. Cr is an essential element which promotes the formation of the oxide film containing (i) Al.sub.2O.sub.3 or the insulating oxide film containing (i) Al.sub.2O.sub.3 and (ii) SiO.sub.2 and/or (iii) MgAl.sub.2O.sub.4 and decreases a thermal expansion coefficient when being added in combination with Si and Al. In order to obtain the above-described effects, the lower limit of the Cr content is 9%, preferably 10%, and more preferably 11%. The upper limit of the Cr content is 25%, preferably 20%, and more preferably 18% from the viewpoint of suppressing a decrease in steel toughness and workability caused by addition of Si and Al.

(6) C inhibits the improvement of corrosion resistance and the formation of the insulating oxide film. Therefore, the less the C content there is, the better. The upper limit of the C content is 0.03% and preferably 0.02%. However, since excessive reduction in the amount of C leads to an increase in refining cost, the lower limit of the C content is preferably 0.001% and more preferably 0.002%.

(7) Mn suppresses the oxidation of Fe and promotes the formation of the insulating oxide film during the heat treatment of the stainless steel. In order to obtain the effect of promoting the formation of the insulating oxide film, the Mn content is preferably 0.06% or more, more preferably 0.3% or more, and still more preferably 0.4% or more. On the other hand, since excessive addition of Mn causes a decrease in corrosion resistance and oxidation resistance and an increase in thermal expansion coefficient, the upper limit of the Mn content is 2%, preferably 1.5%, and more preferably 1.0%.

(8) P is an element which inhibits producibility and weldability. Thus, the less the P content there is, the better. In order to suppress a decrease in producibility and weldability, the upper limit of the P content is 0.05% and preferably 0.04%. However, since excessive reduction in the amount of P leads to an increase in refining cost, the lower limit of the P content is preferably 0.005% and more preferably 0.01%.

(9) S inhibits the formation of the insulating oxide film. Thus, the less the S content there is, the better. Therefore, the upper limit of the S content is 0.01% and preferably 0.002%. However, since excessive reduction in the amount of S leads to an increase in refining cost, the lower limit of the S content is preferably 0.0001% and more preferably 0.0002%.

(10) Like C, N inhibits the formation of the insulating oxide film. Thus, the less the N content there is, the better. Therefore, the upper limit of the N content is 0.03% and preferably 0.015%. However, since excessive reduction in the amount of N leads to an increase in refining cost, the lower limit of the N content is preferably 0.001% and more preferably 0.005%.

(11) In order to obtain an effect as a deoxidizing element, the Si content is 0.05% or more and preferably 0.10% or more.

(12) On the other hand, excessive addition of Si causes a decrease in steel toughness and workability. Therefore, the upper limit of the Si content is 4.0%, preferably 3.5%, and more preferably 2.0%.

(13) In order to obtain an effect as a deoxidizing element as with Si, the Al content is 0.005% or more and preferably 0.010% or more.

(14) On the other hand, excessive addition of Al increases the thermal expansion coefficient of the steel and inhibits the durability of the oxide film which is obtained through the heat treatment. Therefore, the upper limit of the Al content is 5.0%, preferably 3.5%, and more preferably 2.5%. When the Al content is more than 5.0%, the thermal expansion coefficient is high, which is not preferable in a substrate for a solar cell.

(15) Si and Al are elements which promote the formation of the insulating oxide film and improve the insulating properties of the oxide film obtained through the heat treatment. Therefore, the stainless steel used in the embodiment contains 0.4% or more of Si and/or 0.5% or more of Al. By using stainless steel which satisfies one of the conditions including Si: 0.4% or more and Al: 0.5% or more, an oxide film having insulating properties which can be used in a substrate for a solar cell can be obtained through the heat treatment.

(16) In addition, by using stainless steel which contains 0.4% or more of Si and 0.5% or more of Al, the formation of Al.sub.2O.sub.3 or an Al-containing spinel oxide can be significantly effectively promoted during the heat treatment.

(17) When the Si content is 0.4% or more, an effect of promoting the formation of the insulating oxide film and an effect of decreasing the thermal expansion coefficient of the stainless steel can be obtained. In order to obtain the effect of promoting the formation of the insulating oxide film, the Si content is preferably 0.5% or more and more preferably 1.0% or more.

(18) When the Al content is 0.5% or more, an effect of promoting the formation of the insulating oxide film can be obtained. In order to obtain the effect of promoting the formation of the insulating oxide film, the Al content is preferably 1.0% or more and more preferably 1.5% or more.

(19) In this embodiment, in order to promote the formation of the insulating oxide film by performing the heat treatment while maintaining the desired low thermal expansion coefficient, the amounts of Cr, Mn, Si, and Al are limited as described above, and Cr+10Si+Al+Mn>24.5 (wherein a symbol of each element represents the amount (mass %) of the element in the steel) is satisfied. In ferritic stainless steel containing Cr as a major constituent element, in order to form the insulating oxide film and to decrease the thermal expansion coefficient, Si addition acts effectively and addition of a combination of Si and Al is preferable. Further, Mn addition promotes the formation of the oxide film without an increase in thermal expansion coefficient. From the viewpoint of promoting the formation of the insulating oxide film, the value of “Cr+10Si+Al+Mn” is preferably 27 or more. The upper limit of the value of “Cr+10Si+Al+Mn” is not particularly limited but is preferably 40 and more preferably 35 in consideration of the effects of Si and Al addition on the producibility of the steel.

(20) In addition, the stainless steel used in the embodiment may contain Al: 2.0% or more and Si: 0.3% or more.

(21) When the Al content is 2.0% or more, the insulating properties of the oxide film obtained through the heat treatment are further improved. However, as the Al content increases, the thermal expansion coefficient also increases. Therefore, when the Al content is 2.0% or more, the Si content is preferably 0.3% or more. By adding 0.3% or more of Si, an increase in thermal expansion coefficient caused by addition of 2.0% or more of Al can be suppressed. When the Al content is 2.0% or more, the Si content is more preferably 0.4% or more in order to effectively suppress an increase in thermal expansion coefficient. When stainless steel having a sufficiently low thermal expansion coefficient is used as a substrate for a solar cell, adhesion between a substrate, an Mo electrode, and a CIS light-absorbing layer is high, and superior durability can be obtained.

(22) By adding 2.0% or more of Al and 0.3% or more of Si, a synergistic effect of promoting the formation of the insulating oxide film with Al and Si can be obtained. As a result, in this stainless steel, an oxide film having far superior insulating properties can be obtained through the heat treatment.

(23) In a case where the Al content is less than 2.0%, even when the Si content is less than 0.3%, stainless steel having a sufficiently low thermal expansion coefficient can be obtained. In addition, in a case where the Al content is more than 5.0%, even when an increase in thermal expansion coefficient is suppressed by Si addition, stainless steel having a sufficiently low thermal expansion coefficient cannot be obtained.

(24) In addition, optionally, the stainless steel used in the embodiment further contains one or more elements selected from the group consisting of Sn: 1% or less, Zr: 0.5% or less, Mg: 0.005% or less, Ni: 1% or less, Cu: 1% or less, Co: 0.5% or less, Mo: 2% or less, V: 0.5% or less, B: 0.005% or less, Ca: 0.005% or less, La: 0.1% or less, Y: 0.1% or less, Hf: 0.1% or less, REM: 0.1% or less, Nb: 1% or less, and Ti: 1% or less.

(25) In the ferritic stainless steel used in the embodiment, Sn is optionally added because it suppresses the oxidation of Fe and promotes the formation of the insulating oxide film rich in Si and/or Al. When Sn is added, the Sn content is preferably 0.01% or more, more preferably 0.05% or more, and still more preferably 0.1% or more to exhibit the effects of addition. However, since excessive addition of Sn causes a decrease in the producibility of the steel and an increase in alloy cost, the upper limit of the Sn content is 1%, preferably 0.5%, and more preferably 0.3%.

(26) Zr is optionally added because it promotes the formation of the insulating oxide film due to a synergistic effect with Si and Al. When Zr is added, the Zr content is preferably 0.005% or more, more preferably 0.01% or more, and still more preferably 0.05% or more to exhibit the effects of addition. However, since excessive addition of Zr causes a decrease in the producibility of the steel and an increase in alloy cost, the upper limit of the Zr content is 0.5%, preferably 0.3%, and more preferably 0.15%.

(27) Mg is an element which is effective for hot workability and solidification structure refinement and has an effect of promoting the formation of an Al spinel oxide (MgAl.sub.2O.sub.4) when the heat treatment is performed. When Mg is added, the Mg content is preferably 0.0001% or more and more preferably 0.0003% or more to exhibit the effects of addition. However, since excessive addition of Mg inhibits producibility, the upper limit of the Mg content is 0.005% and preferably 0.0015%.

(28) Ni, Cu, Co, Mo, and V are elements which are effective to promote the formation of the insulating oxide film or to improve corrosion resistance due to a synergistic effect with Si and Al, and are optionally added. When Ni, Cu, and Mo are added, the amount of each element is preferably 0.1% or more to exhibit the effect of addition. When V and Co are added, the amount of each element is preferably 0.01% or more to exhibit the effect of addition. However, since excessive addition of the above elements causes an increase in alloy cost and an increase in thermal expansion coefficient, the upper limit of the amount of each of Ni and Cu is 1%, and the upper limit of the amount of each of V and Co is 0.5%. Since Mo is an element which is effective to decrease the thermal expansion coefficient, the upper limit of the Mo content is 2%. The lower limit of the amount of each element is more preferably 0.1% and, the upper limit thereof is 0.5%.

(29) B and Ca are elements which improve hot workability and secondary workability, and addition of B and Ca to ferritic stainless steel is effective. When B and/or Ca is added, the lower limit of the amount of each element is preferably 0.0003% and more preferably 0.0005% to exhibit the effects of addition. However, since excessive addition of B and/or Ca causes a decrease in elongation, the upper limit of the amount of each of B and Ca is 0.005% and preferably 0.0015%.

(30) La, Y, Hf, and REM are elements which are effective to improve hot workability and the cleanliness of the steel and to improve the adhesion of the oxide film obtained through the heat treatment, and are optionally added. When La, Y, Hf, and REM are added, the amount of each element is preferably 0.001% or more to exhibit the effect of addition. However, since excessive addition of the above elements causes an increase in alloy cost and a decrease in producibility, the upper limit of the amount of each of La, Y, Hf, and REM is 0.1% and preferably 0.05%. Here, REM are elements whose atomic numbers range from 57 to 71, for example, Ce, Pr, or Nd.

(31) Nb is optionally added because it promotes the formation of the insulating oxide film through the purification of the steel caused by an effect of a stabilizing element which fixes C and N. When Nb is added, the Nb content is preferably 0.03% or more, more preferably 0.05% or more, and still more preferably 0.1% or more to exhibit the effects of addition. However, since excessive addition of Nb causes an increase in alloy cost and a decrease in producibility caused by an increase in recrystallization temperature, the upper limit of the Nb content is 1%, preferably 0.5%, and more preferably 0.3%.

(32) Ti is optionally added because it purifies the steel due to an effect of a stabilizing element which fixes C and N and promotes the formation of the insulating oxide film. When Ti is added, the Ti content is preferably 0.01% or more, more preferably 0.02% or more, and still more preferably 0.05% or more to exhibit the effects of addition. However, since excessive addition of Ti increases the alloy cost and inhibits the formation of aluminum oxide and SiO.sub.2, the upper limit of the Ti content is 1%, preferably 0.35%, and more preferably 0.2%.

(33) (II) The oxide film formed on the surface of the stainless steel will be described below.

(34) In the stainless steel substrate for a solar cell according to the embodiment, the following oxide film is formed on the surface of the stainless steel containing the components described above in (I) in order to provide a suitable insulating surface to the desired substrate for a solar cell according to the embodiment.

(35) The oxide film containing (i) Al.sub.2O.sub.3 in an amount of 50% or more or containing (i) Al.sub.2O.sub.3 and (ii) SiO.sub.2 in a total amount of 50% or more is formed on the surface of the stainless steel substrate for a solar cell according to the embodiment. In the stainless steel substrate for a solar cell according to the embodiment, the above-described oxide film is formed on the surface of the stainless steel. Therefore, the stainless steel substrate for a solar cell has a suitable insulating surface.

(36) In addition, the oxide film may contain (i) Al.sub.2O.sub.3 and (iii) MgAl.sub.2O.sub.4 (Al-containing spinel oxide), or may further contain (iii) MgAl.sub.2O.sub.4 (Al-containing spinel oxide) in addition to (i) Al.sub.2O.sub.3 and (ii) SiO.sub.2. When the oxide film contains (iii) MgAl.sub.2O.sub.4, insulating properties can be improved irrespective of the characteristics of (i) Al.sub.2O.sub.3, which is preferable.

(37) When the oxide film does not contain (ii) SiO.sub.2 or (iii) MgAl.sub.2O.sub.4, the amount of (i) Al.sub.2O.sub.3 is preferably 55% or more and more preferably 60% or more in order to obtain far superior insulating properties. The upper limit of the amount of (i) Al.sub.2O.sub.3 in the oxide film is not particularly limited. When the oxide film contains (i) Al.sub.2O.sub.3 and (iii) MgAl.sub.2O.sub.4, the upper limit of the amount of (i) Al.sub.2O.sub.3 in the oxide film is preferably 95% and more preferably 80% such that far superior insulating properties can be obtained by addition of (iii) MgAl.sub.2O.sub.4.

(38) In addition, when the oxide film contains (ii) SiO.sub.2, the amount of (ii) SiO.sub.2 in the oxide film is preferably 5% or more and more preferably 15% or more.

(39) When the amount of (ii) SiO.sub.2 in the oxide film is 5% or more, (ii) SiO.sub.2 is formed, and thus the formation of (i) Al.sub.2O.sub.3 is sufficiently promoted. In addition, when the oxide film contains (ii) SiO.sub.2, the amount of (ii) SiO.sub.2 in the oxide film is preferably 30% or less and more preferably 25% or less in order to secure insulating properties by securing the amount of (i) Al.sub.2O.sub.3 in the oxide film.

(40) When the oxide film contains (iii) MgAl.sub.2O.sub.4, the amount of (iii) MgAl.sub.2O.sub.4 in the oxide film is preferably 5% or more and more preferably 10% or more such that the effect of improving insulating properties can be sufficiently obtained by (iii) MgAl.sub.2O.sub.4. The upper limit of the amount of (iii) MgAl.sub.2O.sub.4 in the oxide film is not particularly limited. However, in order to obtain superior insulating properties by securing the amount of (i) Al.sub.2O.sub.3 in the oxide film, the amount of (iii) MgAl.sub.2O.sub.4 is preferably 50% or less and more preferably 30% or less.

(41) In order to maintain the insulating surface, the thickness of the oxide film is preferably 0.01 μm or more. The upper limit of the film thickness is not particularly limited but is preferably 5 μm in consideration of the efficiency of the heat treatment described below. In addition, in the embodiment, the oxide film containing (iii) MgAl.sub.2O.sub.4 is formed. As a result, even when the thickness thereof is 1 μm or less, the insulating properties of the surface can be secured.

(42) (III) A method of producing a stainless steel substrate for a solar cell will be described.

(43) In the producing method according to the embodiment, it is preferable to form the oxide film described in (II) on a surface of the stainless steel containing the components described in (I) by performing a heat treatment on the stainless steel in an atmosphere containing water vapor in a temperature range of 300° C. to 1000° C. (film forming step).

(44) The stainless steel on which the heat treatment is performed contains the components described in (I) and can be obtained using a well-known producing method of the related art. The stainless steel subjected to the heat treatment may have any shape that can be used as a stainless steel substrate for a solar cell. The surface texture of the stainless steel subjected to the heat treatment is not particularly limited. For example, the surface of the stainless steel can be polished according to BA, 2B, 2D, No. 4 defined in JIS G4304:2012 and JIS G4305:2012 (corresponding to ISO 16143-1:2004).

(45) In order to form the oxide film described in (II) which is effective for insulating properties, the heat treatment is performed preferably at 300° C. or higher and more preferably 400° C. or higher. When the heat treatment temperature is excessively high, the Al concentration and the Si concentration decreases and the Fe concentration increases in the oxide film, which inhibits the insulating properties and adhesion of the oxide film. Therefore, the upper limit of the heat treatment temperature is preferably 1000° C. and more preferably 900° C.

(46) The heat treatment time is not particularly limited and may be, for example, 1 minutes to 72 hours.

(47) It is preferable that continuous annealing is performed for 10 minutes or less, or a batch type heat treatment is performed for 24 hours to 72 hours as the heat treatment.

(48) It is preferable that the heat treatment for forming the oxide film is performed in an atmosphere containing water vapor. By performing the heat treatment in the atmosphere containing water vapor, the oxidation of Al and Si on the surface of the stainless steel is promoted. Examples of the atmosphere containing water vapor include an atmosphere containing water vapor which is obtained by humidifying dry air (20% oxygen-80% nitrogen). In addition, it is more preferable that the heat treatment for forming the oxide film is performed in an atmosphere containing 5% or more of water vapor in pure oxygen gas. By performing the heat treatment in the above-described atmosphere, the desired oxide film according to the embodiment can be easily formed.

(49) It is preferable that the heat treatment for forming the oxide film is performed in an atmosphere containing water vapor having a dew point of 40° C. or higher. By performing the heat treatment in the above-described atmosphere, (iii) MgAl.sub.2O.sub.4 can be effectively formed. The upper limit of the dew point is not particularly limited but is 90° C. in consideration of the workability of the heat treatment.

(50) The amount of each of (i) to (iii) contained in the oxide film can be controlled by changing the composition in the above-described ranges and changing the heat treatment conditions in the above-described ranges of the heat treatment atmosphere and the heat treatment temperature.

EXAMPLES

(51) Hereinafter, examples of the embodiment will be described.

(52) Ferritic stainless steel containing components shown in Table 1 was melted and subjected to hot rolling, annealing, and then cold rolling. As a result, cold-rolled steel sheets having a thickness of 0.5 mm were obtained. Here, as the components of the steel, components which were in the range specified in the embodiment and components which were outside of the range specified in the embodiment were used. The cold-rolled steel sheets were subjected to finish annealing and pickling in a range of 800° C. to 1000° C. in which recrystallization was completed.

(53) A heat treatment was performed on the steel sheets under heat treatment conditions (temperature, holding time, and dew point) shown in Table 2 in an atmosphere containing water vapor obtained by humidifying dry air to a dew point shown in Table 2. The obtained steel sheets were provided for the evaluation of the insulating properties of the surface and the measurement of the thermal expansion coefficient. In addition, proportions (%) of the components of the following (i) to (iii) in the oxide films formed on the surfaces of the obtained steel sheets were calculated. The results are shown in Table 2.

(54) The presence of oxides constituting the oxide films formed on the surfaces were verified by measuring diffraction peaks shown below by X-ray diffraction (CuKα rays were used), and the proportions thereof were obtained.

(55) (i) Al.sub.2O.sub.3: (104) plane, 2θ=35.15°

(56) (ii) SiO.sub.2: (101) plane, 2θ=26.64°

(57) (iii) MgAl.sub.2O.sub.4: (311) plane, 2θ=36.85°

(58) (iv) Cr.sub.2O.sub.3: (110) plane, 2θ=36.16°/(104) plane, 2θ=33.6°

(59) First, the heights (cps) of the diffraction peaks of the (i) to (iii) measured using X-ray diffraction were measured. It was assumed that the oxide films were formed of (i) to (iv) described above, and the abundance ratios of the oxides (i) to (iii) were calculated according to the following method of calculating the abundance ratio of (i) Al.sub.2O.sub.3 described below.

(60) The abundance ratios of the oxides, for example, the abundance ratio of (i) Al.sub.2O.sub.3 was calculated from (i)/{(i)+(ii)+(iii)+(iv)}×100. In the expression, (i) to (iv) represent the diffraction peak heights (cps) which were measured by the X-ray diffraction from the (104) plane of (i) Al.sub.2O.sub.3, the (101) plane (ii) SiO.sub.2, the (311) plane of (iii) MgAl.sub.2O.sub.4, and the (110) plane of (iv) Cr.sub.2O.sub.3.

(61) As the value of (iv) in the expression, the diffraction peak height of the (110) plane which was the main diffraction peak was adopted. Regarding the presence of (iv) Cr.sub.2O.sub.3, the presence of the diffraction peak of the (104) plane was verified in order to distinguish the presence of (iv) Cr.sub.2O.sub.3 from the diffraction peak of (i) Al.sub.2O.sub.3.

(62) In Table 2, “ratio %” represents the sum of the abundance ratios of (i) to (iii).

(63) The insulating properties of the steel sheet surface was evaluated by vapor-depositing an aluminum film (10 mm×10 mm×0.2 μm thickness) on the surface as an electrode and placing a probe of a tester on the electrode to measure the electrical resistance. The measurement was performed on a measurement area 10 times, and the average value thereof was obtained as a measured value. Regarding the desired insulating properties of the embodiment, an electrical resistance value which is desired for a substrate for a CIS solar cell is 1 kΩ or higher. A steel sheet having the desired electrical resistance value was evaluated as “B”, and a steel sheet stably exhibiting a higher electrical resistance value (10 kΩ or higher) was evaluated as “A”. In addition, a steel sheet having an electrical resistance value of lower than 1 kΩ was evaluated as “C”.

(64) A specimen having a size of 1 mm thickness×10 mm width×50 mm length was prepared, and the thermal expansion coefficient thereof was measured using a push-rod type thermal dilatometer. In an Ar atmosphere, the measurement was performed with a spring compression load of 50 g or lower. The thermal expansion coefficient was calculated by measuring the thermal expansion of the specimen when the temperature was increased from 50° C. to 600° C. on the assumption of the film formation on the CIS solar cell. Regarding the desired thermal expansion coefficient of the embodiment, an average linear expansion coefficient measured when the temperature is increased from 50° C. as a base point to 600° C., is preferably 12.5×10.sup.6/° C. or lower to maintain the durability of the film formed on the substrate for a CIS solar cell. A steel sheet having the desired thermal expansion coefficient was evaluated as “B”, and a steel sheet having a thermal expansion coefficient higher than 12.5×10.sup.−6/° C. was evaluated as “C”.

(65) Table 2 collectively shows the heat treatment conditions and the evaluation results.

(66) In Test Nos. 1 to 10, oxide films containing (i) Al.sub.2O.sub.3 in an amount of 50% or more or containing (i) Al.sub.2O.sub.3 and (ii) SiO.sub.2 in a total amount of 50% or more were formed on surfaces of stainless steel having the composition specified in the embodiment through the heat treatment.

(67) In the steel sheets of Test Nos. 1 to 10, the desired surface insulating properties and the desired thermal expansion coefficient of the embodiment were obtained.

(68) Among these, in Nos. 3, 4, 6, and 9 in which the heat treatment was performed on Steels C, D, F, G, H, and I under heat treatment conditions of a dew point of 40° C. or higher, an oxide film containing (iii) MgAl.sub.2O.sub.4 was formed, and the surface insulating properties were evaluated as “A”.

(69) In Test Nos. 11 to 13, 15, and 16, steel which did not satisfy either or both of the composition specified in the embodiment and the expression (1) was used.

(70) In the steel sheets of Test Nos. 12, 15, and 16, an oxide film was formed by performing the heat treatment under heat treatment conditions shown in Table 2, but this oxide film did not contain (i) Al.sub.2O.sub.3 in an amount of 50% or more or did not contain (i) Al.sub.2O.sub.3 and (ii) SiO.sub.2 in a total amount of 50% or more. Therefore, the desired surface insulating properties of the embodiment was not obtained.

(71) In addition, in Test Nos. 11 and 13, the insulating properties were superior, but the thermal expansion coefficient was significantly high. Therefore, the steel sheets were not preferable as a substrate for a solar cell.

(72) In Test No. 14, low Cr steel containing components which were outside of the range specified in the embodiment and did not satisfy the expression (1) specified in the embodiment was used. In Test No. 14, the desired thermal expansion coefficient of the embodiment was not obtained. In addition, an oxide film was formed by performing the heat treatment under heat treatment conditions shown in Table 2, but this oxide film did not contain (i) Al.sub.2O.sub.3 in an amount of 50% or more and did not contain (i) Al.sub.2O.sub.3 and (ii) SiO.sub.2 in a total amount of 50% or more. Therefore, the desired surface insulating properties of the embodiment were not obtained.

(73) The following was found from the above results: in order to impart surface insulating properties to a ferritic stainless steel sheet, it is necessary that an oxide film containing (i) Al.sub.2O.sub.3 in an amount of 50% or more or containing (i) Al.sub.2O.sub.3 and (ii) SiO.sub.2 in a total amount of 50% or more as specified in the embodiment is formed on the surface of the stainless steel sheet. Here, in order to improve surface insulating properties, it is effective to form an oxide film further containing (iii) MgAl.sub.2O.sub.4 in addition to (i) Al.sub.2O.sub.3 or in addition to (i) Al.sub.2O.sub.3 and (ii) SiO.sub.2 on the surface of the stainless steel sheet. Further, in order to simultaneously realize the desired thermal expansion coefficient and the desired surface insulating properties of the embodiment, the components specified in the embodiment and the component adjustment for satisfying the expression (1) are effective.

(74) TABLE-US-00001 TABLE 1 Steel C Si Mn P S Cr Nb Ti Al N Others Index A 0.028 0.45 0.80 0.032 0.008 24.0 0 0 1.20 0.025 30.5 B 0.005 2.70 0.30 0.025 0.001 18.2 0.45 0 0.30 0.010 Sn: 0.05, V: 0.2, Co: 0.1 45.8 C 0.006 0.48 0.21 0.030 0.001 17.9 0 0.19 2.10 0.012 La: 0.01, REM: 0.01, Mg: 0.001 25.0 D 0.025 1.55 0.10 0.035 0.002 10.9 0 0.15 1.20 0.010 Sn: 0.2, Mg: 0.0008 27.7 E 0.010 3.80 1.70 0.030 0.001 11.5 0.25 0.20 0.40 0.010 51.6 F 0.005 2.55 0.30 0.030 0.002 12.5 0.15 0.15 0.60 0.012 Mg: 0.0005 38.9 G 0.015 0.55 0.50 0.030 0.001 14.7 0.30 0.01 4.60 0.015 Mg: 0.005, B: 0.005, Ca: 0.003 25.3 H 0.008 0.95 0.30 0.028 0.001 13.2 0.42 0.03 1.60 0.015 Ni: 0.15, Cu: 0.15, Mg: 0.001 24.6 I 0.005 0.30 0.65 0.030 0.001 18.2 0.10 0.18 3.00 0.013 Zr: 0.01, Mg: 0.005, Hf: 0.02 24.9 J 0.012 1.50 0.06 0.035 0.001 12.7 0 0.15 1.10 0.015 Zr: 0.1, Mo: 0.3, Y: 0.01 28.9 K 0.004 0.10 0.14 0.022 0.001 18.2 0 0.11 2.99 0.010 22.3 L 0.005 0.32 0.15 0.020 0.001 18.0 0 0.13 1.20 0.010 22.6 M 0.005 0.36 0.20 0.020 0.001 18.2 0 0.11 5.78 0.012 27.8 N 0.010 1.50 0.50 0.030 0.001 8.8 0.15 0.20 2.10 0.010 26.4 O 0.010 0.30 0.50 0.030 0.001 18.0 0.15 0.20 0.45 0.010 22.0 P 0.010 0.30 0.35 0.030 0.001 22.0 0 0.25 0.40 0.012 25.8 (Note) Index: Cr + 10Si + Mn + Al

(75) TABLE-US-00002 TABLE 2 Heat Treatment Conditions Dew Surface Oxide Film Surface Thermal Temperature, Time, Point, Al.sub.2O.sub.3 SiO.sub.2 MgAl.sub.2O.sub.4 Insulating Expansion No. Steel ° C. min ° C. (%) (%) (%) Ratio % Properties Coefficient Remark 1 A 980 10 35 55 0 0 55 B B Example 2 B 900 1 45 25 25 0 50 B B Example 3 C 850 10 50 80 0 10 90 A B Example 4 D 380 55 55 60 5 5 70 A B Example 5 E 450 10 45 20 35 0 55 B B Example 6 F 500 10 55 35 15 15 65 A B Example 7 G 600 50 40 55 0 35 90 A B Example 8 H 550 30 45 60 5 10 75 A B Example 9 I 920 55 70 75 0 20 95 A B Example 10 J 500 30 35 60 10 0 70 B B Example 11 K 930 50 50 80 0 0 80 B C Comparative Example 12 L 600 50 45 45 0 0 45 C B Comparative Example 13 M 980 10 45 90 0 0 90 B C Comparative Example 14 N 380 55 55 20 5 0 25 C C Comparative Example 15 O 900 1 45 20 0 0 20 C B Comparative Example 16 P 930 1 45 25 0 0 25 C B Comparative Example (Note 1) Surface insulating properties: 1 kΩ or higher which is desired in the present invention was evaluated as “B”, and a value of less than 1 kΩ was evaluated as “C”. (Note 2) Thermal Expansion Coefficient: 600° C., 12.5 × 10.sup.−6/° C. or lower which is desired in the present invention was evaluated as “B”, and a value of higher than 12.5 × 10.sup.−6/° C. was evaluated as “C”.

INDUSTRIAL APPLICABILITY

(76) According to the present invention, it is possible to obtain a stainless steel substrate for a solar cell which have a low thermal expansion coefficient and is preferable as a substrate for a solar cell having, and in which an insulating surface capable of maintaining the conversion efficiency of a solar cell at a high level can be formed without using a coating or plating method. In particular, the present invention is suitable for a substrate for a CIS solar cell in which an electrode and a light-absorbing layer are formed on an insulating substrate.