Title of Invention: Low CR ferritic stainless steel with excellent formability and high temperature characteristics and manufacturing method thereof
technical field
[One]
The present invention relates to a low-Cr ferritic stainless steel, and more particularly, to a low-Cr ferritic stainless steel capable of securing formability while having excellent high-temperature strength and high-temperature oxidation resistance, and a method for manufacturing the same.
background
[2]
Ferritic stainless steels have excellent corrosion resistance while adding a small amount of expensive alloying elements, and thus have high price competitiveness compared to austenitic stainless steels. In particular, 9~14% low Cr ferritic stainless steel has more excellent cost competitiveness and is used for exhaust system parts (Muffler, Ex-manifold, Collector cone, etc.) corresponding to the exhaust gas temperature range of room temperature to 800 °C.
[3]
However, the application expansion was limited because the high-temperature strength and high-temperature oxidation resistance were inferior to those of high-Cr and Nb-added steel. Increasing the Cr content or adding Nb to improve high-temperature strength and high-temperature oxidation resistance causes an increase in manufacturing cost, so there is a development direction that can improve high-temperature characteristics without adding Nb to low-Cr ferritic stainless steel. necessary.
DETAILED DESCRIPTION OF THE INVENTION
technical challenge
[4]
The embodiments of the present invention utilize solid solution strengthening and precipitation strengthening by optimizing the contents of Ci, Si, and Sn, so that high-temperature strength and high-temperature oxidation resistance corresponding to high-Cr ferritic stainless steel are excellent without increasing the Cr content or adding Nb. An object of the present invention is to provide a low-Cr ferritic stainless steel with secure formability and a method for manufacturing the same.
means of solving the problem
[5]
Low-Cr ferritic stainless steel having excellent formability and high-temperature characteristics according to an embodiment of the present invention, by weight, C: 0.005 to 0.015%, N: 0.005 to 0.015%, Si: 0.5 to 1.5%, Mn: 0.1 to 0.5%, Cr: 9 to 14%, Ti: 0.1 to 0.3%, Cu: 0.3 to 0.8%, Al: 0.01 to 0.05%, Sn: 0.005 to 0.15%, remaining Fe and unavoidable impurities, and the following Equations (1) and (2) are satisfied.
[6]
(1) Cu + Si = 1.3
[7]
(2) Si + Cu + 10*Sn ≤= 3.0
[8]
Here, Si, Cu, and Sn mean the content (wt%) of each element.
[9]
In addition, according to an embodiment of the present invention, Ni: 0.3% or less, P: 0.04% or less, and S: 0.002% or less may be further included.
[10]
In addition, according to an embodiment of the present invention, it may contain 0.03 wt% or more of the Cu precipitated phase having a size of 1 to 500 nm in the matrix structure.
[11]
In addition, according to an embodiment of the present invention, 900 ℃ high temperature strength may be 12 MPa or more.
[12]
In addition, according to an embodiment of the present invention, the elongation may be 30% or more.
[13]
In addition, according to an embodiment of the present invention, the following formula (3) may be satisfied.
[14]
(3) (Si + 5*Sn)/Ti = 5.0
[15]
The method for producing a low-Cr ferritic stainless steel having excellent formability and high-temperature characteristics according to an embodiment of the present invention, by weight, C: 0.005 to 0.015%, N: 0.005 to 0.015%, Si: 0.5 to 1.5%, Mn: 0.1 to 0.5%, Cr: 9 to 14%, Ti: 0.1 to 0.3%, Cu: 0.3 to 0.8%, Al: 0.01 to 0.05%, Sn: 0.005 to 0.15%, remaining Fe and unavoidable impurities, and Cold rolling annealing heat treatment of a ferritic stainless steel cold rolled steel sheet satisfying the following formulas (1) and (2); and rapidly cooling to a temperature range of 450 to 550° C. and maintaining for 5 minutes or more.
[16]
(1) Cu + Si = 1.3
[17]
(2) Si + Cu + 10*Sn ≤= 3.0
[18]
Here, Si, Cu, and Sn mean the content (wt%) of each element.
[19]
In addition, according to an embodiment of the present invention, the cold-rolled annealed steel sheet may include 0.09 wt% or more of Cu precipitated phases having a size of 1 to 500 nm in the matrix structure.
[20]
In addition, according to an embodiment of the present invention, the 900 ℃ high-temperature strength of the cold-rolled annealed steel sheet may be 14.5 MPa or more.
[21]
In addition, according to an embodiment of the present invention, the cold-rolled steel sheet may satisfy the following equation (3).
[22]
(3) (Si + 5*Sn)/Ti = 5.0
Effects of the Invention
[23]
The low-Cr ferritic stainless steel according to the embodiment of the present invention can increase the high-temperature strength by 30% or more compared to the conventional one by distributing a fine Cu precipitated phase at the same time as the solid-solution strengthening effect of Si and Cu. High-temperature oxidation resistance can also be improved by this.
[24]
In addition, it is possible to prevent inferiority in formability due to an increase in the content of alloying elements, and when the manufacturing method according to the present invention is applied, high-temperature strength characteristics can be improved.
Brief description of the drawing
[25]
1 is a graph showing the correlation between high-temperature characteristics according to Equations (1) and (3) of the present invention.
Best mode for carrying out the invention
[26]
Low-Cr ferritic stainless steel having excellent formability and high-temperature characteristics according to an embodiment of the present invention, by weight, C: 0.005 to 0.015%, N: 0.005 to 0.015%, Si: 0.5 to 1.5%, Mn: 0.1 to 0.5%, Cr: 9 to 14%, Ti: 0.1 to 0.3%, Cu: 0.3 to 0.8%, Al: 0.01 to 0.05%, Sn: 0.005 to 0.15%, remaining Fe and unavoidable impurities, and the following Equations (1) and (2) are satisfied.
[27]
(1) Cu + Si = 1.3
[28]
(2) Si + Cu + 10*Sn ≤= 3.0
[29]
Here, Si, Cu, and Sn mean the content (wt%) of each element.
Modes for carrying out the invention
[30]
Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings. The following examples are presented to sufficiently convey the spirit of the present invention to those of ordinary skill in the art to which the present invention pertains. The present invention is not limited to the embodiments presented herein, and may be embodied in other forms. The drawings may omit the illustration of parts irrelevant to the description in order to clarify the present invention, and may slightly exaggerate the size of the components to help understanding.
[31]
Also, when a part "includes" a certain component, it means that other components may be further included, rather than excluding other components, unless otherwise stated.
[32]
The singular expression includes the plural expression unless the context clearly dictates otherwise.
[33]
The present inventors were able to obtain the following findings as a result of various studies in order to improve the high-temperature strength and high-temperature oxidation resistance of low-cost, low-Cr ferritic stainless steels.
[34]
In general, Nb is added to ferritic stainless steel for exhaust system for high-temperature strength, but Nb is not a desirable development direction because it causes a relatively high raw material cost and increases the manufacturing cost. It is widely known that substitutional solid solution strengthening elements are effective in increasing high-temperature strength. In particular, when a substitutional solid solution strengthening element is added, the greater the difference in weight and atomic radius compared to Fe and Cr, the greater the solid solution strengthening effect. In the periodic table of elements, alloying elements such as Si, Cu, Sn, etc. are far apart from Fe and Cr and have a difference in weight and atomic radius. .
[35]
On the other hand, for high-temperature oxidation resistance, the content of Cr is generally increased, but since Cr is also expensive and causes an increase in manufacturing cost, it is not a desirable development direction. For high-temperature oxidation resistance, certain elements are densely concentrated on the surface when exposed to high temperatures for a long time to suppress the formation of an Fe-oxide film. In the present invention, Si, Cu, and Sn candidates were selected as elements that can be concentrated on the surface, and component optimization was performed for high-temperature oxidation resistance.
[36]
In the present invention including the above, the component system conditions and equations must be satisfied as follows.
[37]
Low-Cr ferritic stainless steel having excellent formability and high-temperature characteristics according to an embodiment of the present invention, by weight, C: 0.005 to 0.015%, N: 0.005 to 0.015%, Si: 0.5 to 1.5%, Mn: 0.1 to 0.5%, Cr: 9 to 14%, Ti: 0.1 to 0.3%, Cu: 0.3 to 0.8%, Al: 0.01 to 0.05%, Sn: 0.005 to 0.15%, remaining Fe and unavoidable impurities.
[38]
Hereinafter, the reason for numerical limitation of the content of alloying elements in an embodiment of the present invention will be described. Hereinafter, unless otherwise specified, the unit is % by weight.
[39]
The content of C is 0.005 to 0.015%.
[40]
When the C content exceeds 0.015%, Cr 23 C 6 precipitates are generated by combining with Cr, and the high temperature oxidation resistance is lowered due to local depletion of Cr in the matrix. In addition, in order to control the C content to less than 0.005%, the steelmaking VOD process ratio is increased, which is not preferable. Therefore, the content of C is limited in the range of 0.005 to 0.015%.
[41]
The content of N is 0.005 to 0.015%.
[42]
When N in steel exceeds 0.015%, the concentration of dissolved N reaches a limit, combines with Cr, and Cr 2 N precipitates are generated. In addition, in order to control the N content to less than 0.005%, the steelmaking VOD process ratio is increased, which is not preferable. Therefore, the content of N is limited in the range of 0.005 to 0.015%.
[43]
The content of Si is 0.5 to 1.5%.
[44]
Si is a solid solution strengthening element for increasing high-temperature strength, and at the same time forms a Si-enriched oxide film on the surface layer to increase high-temperature oxidation resistance. For the above two effects, a Si content of at least 0.5% is required, and if it exceeds 1.5%, the machinability of the material is greatly deteriorated, so the Si content is limited as above.
[45]
The content of Mn is 0.1 to 0.5%.
[46]
Mn is an impurity that is unavoidably included in steel and serves to stabilize austenite. When the Mn content exceeds 0.5% in low-Cr ferritic stainless steel, reverse austenite transformation occurs during annealing heat treatment after hot rolling or cold rolling, which adversely affects elongation. Therefore, the content of Mn is limited as above.
[47]
The content of Cr is 9 to 14%.
[48]
Cr is an essential element added to form a passivation film that inhibits oxidation in stainless steel. Cr content of 9% or more should be added to form a stable passivation film. However, since the present invention aims to develop a low-cost steel with reduced Cr, the upper limit is limited to 14%. More preferably, it may be in the range of 10.5 to 12.5%.
[49]
The content of Ti is 0.1 to 0.3%.
[50]
Ti must be added at least 0.1% in order to increase the corrosion resistance of the weld. Ti combines with C and N to form Ti(C,N) precipitates, thereby lowering the amount of solid solution C and N, and inhibiting the formation of a Cr depleted layer. However, when the Ti content exceeds 0.3%, the Ti component in the surface layer reacts with oxygen to cause yellow discoloration. Therefore, the Ti content is limited as above.
[51]
The content of Cu is 0.3 to 0.8%.
[52]
Cu is an element contributing to high-temperature strength by replacing Nb as a solid solution strengthening element. In addition, when Cu generates fine precipitates through appropriate heat treatment, additional high-temperature strength increase can be expected due to the precipitation strengthening effect. Therefore, 0.5% or more is added. However, if too much Cu is added, high-temperature hot workability may be inhibited, so the amount is limited to 0.8% or less.
[53]
The content of Al is 0.01 to 0.05%.
[54]
Al is an element added for deoxidation during the steelmaking operation. When the Al content exceeds 0.05%, Al in the surface layer reacts with oxygen to form a non-uniform oxide layer, which adversely affects high-temperature oxidation resistance. Therefore, the Al content is limited as above.
[55]
The content of Sn is 0.005 to 0.15%.
[56]
Sn is a solid solution strengthening element for increasing high-temperature strength, and at the same time forms a Sn-enriched oxide film on the surface layer to increase high-temperature oxidation resistance. For the above two effects, at least 0.005% of Sn must be added. However, when it exceeds 0.15%, Sn segregates at the grain interface during hot rolling, weakens the bonding force between grains, and causes microcracks in the surface layer. Therefore, the upper limit of the Sn content is limited to 0.15% or less.
[57]
In addition, according to an embodiment of the present invention, Ni: 0.3% or less, P: 0.04% or less, and S: 0.002% or less may be further included.
[58]
The content of Ni is 0.3% or less. Ni is an impurity that is unavoidably included in steel and may be contained in an amount of 0.01% or more, and serves to stabilize austenite. When the Ni content exceeds 0.3% in low-Cr ferritic stainless steel, reverse austenite transformation occurs during annealing heat treatment after hot rolling or cold rolling, which adversely affects elongation. Therefore, the content of Ni is limited as above.
[59]
The content of P is 0.04% or less. P is an unavoidable impurity contained in steel, and its content is adjusted to 0.04% or less because it causes intergranular corrosion during pickling or inhibits hot workability.
[60]
The content of S is 0.002% or less. S is an unavoidable impurity contained in steel and segregates at grain boundaries to inhibit hot workability, so its content is limited to 0.002% or less.
[61]
The remainder of the ferritic stainless steel except for the above-mentioned alloying elements consists of Fe and other unavoidable impurities.
[62]
Meanwhile, the low-Cr ferritic stainless steel having excellent formability and high-temperature characteristics according to an embodiment of the present invention may satisfy the following formulas (1) to (3).
[63]
(1) Cu + Si = 1.3
[64]
High-temperature strength is usually affected by solid solution strengthening and precipitation strengthening. Cu and Si are representative solid solution strengthening elements, so it is preferable to add them to increase strength at high temperature. When Cu is precipitated as Cu precipitation phase, the high temperature strength is more effectively increased due to the precipitation strengthening effect. In addition, when the content of Si is increased, since the solubility limit of Cu is lowered, the precipitation of the Cu precipitation phase becomes easier. Accordingly, it is possible to precipitate 0.03 wt% or more of the Cu precipitated phase having a size of 1 to 500 nm in the matrix structure. Therefore, the Cu+Si content is controlled in the range of 1.3% or more.
[65]
Through the above solid solution strengthening and precipitation strengthening effects, the low-Cr ferritic stainless steel according to the present invention may exhibit a high temperature strength of 12 MPa or more at 900°C.
[66]
(2) Si + Cu + 10*Sn = 3.0
[67]
Si, Cu, and Sn alloy elements have a positive effect on high-temperature strength or high-temperature oxidation resistance, respectively, but they harden the material too much, resulting in poor elongation and poor formability. In the present invention, when Si and Cu improve high-temperature strength and simultaneously satisfy Equation (3), an elongation of 30% or more can be secured to prevent inferiority in formability. Therefore, the relationship between Si, Cu, and Sn content is controlled within the above range in order to secure material processability.
[68]
(3) (Si + 5*Sn)/Ti = 5.0
[69]
In high-temperature oxidation, when Si and Sn are added to low-Cr ferritic stainless steel, a uniform oxide film of Si and Sn is first formed to suppress abnormal oxidation. However, in the case of adding Ti, the Ti oxide film is non-uniformly formed, and since the Ti oxide film itself is yellow, discoloration at high temperature appears. Therefore, it is possible to improve the high temperature oxidation resistance by controlling the Si, Sn, and Ti contents within the above range.
[70]
Next, a method for manufacturing a low-Cr ferritic stainless steel having excellent formability and high temperature characteristics according to an embodiment of the present invention will be described.
[71]
The manufacturing method of low-Cr ferritic stainless steel having excellent formability and high-temperature characteristics of the present invention can be manufactured into a cold-rolled steel sheet through a conventional manufacturing process, and includes the above-described alloy component composition and satisfies Formulas (1) to (3) Cold-rolled annealing heat-treating the ferritic stainless steel cold-rolled steel sheet; and rapidly cooling to a temperature range of 450 to 550° C. and maintaining for 5 minutes or more.
[72]
For example, a slab including the above-described alloy component composition may be hot-rolled, the hot-rolled hot-rolled steel sheet may be annealed, and cold-rolled to manufacture a cold-rolled steel sheet.
[73]
The cold-rolled steel sheet can be rapidly cooled to a temperature range of 450 to 550° C. and maintained for 5 minutes or more after the normal recrystallization heat treatment in the cold-rolled annealing process. Through the cooling and maintenance, it is possible to increase the precipitation of the Cu precipitation phase in the same component system and further improve the high temperature strength.
[74]
Accordingly, the cold-rolled annealing steel sheet may include 0.09 wt% or more of Cu precipitated phases having a size of 1 to 500 nm in the matrix structure, and 900° C. high temperature strength may be 14.5 MPa or more.
[75]
[76]
Hereinafter, it will be described in more detail through preferred embodiments of the present invention.
[77]
Example
[78]
Using the stainless steel lab scale melting and ingot production equipment, 20 mm bar samples were prepared with the alloy composition system shown in Table 1 below. After reheating at 1,200 °C, hot rolling to 6 mm, hot rolling annealing at 1,100 °C, cold rolling to 2.0 mm, and annealing at 1,100 °C were performed. For some invention examples only, after heat treatment, it was rapidly cooled to 500° C., maintained for about 7 minutes, and then air-cooled to prepare a cold-rolled annealed steel sheet, and the remaining invention examples and comparative examples were air-cooled after annealing heat treatment.
[79]
[Table 1]
division C N Si Mn Cr Ti Cu Al Sn
Comparative Example 1 0.005 0.010 0.41 0.21 11.4 0.21 0.05 0.02 0
Comparative Example 2 0.006 0.008 0.6 0.21 12.1 0.19 0.15 0.02 0.05
Comparative Example 3 0.007 0.007 0.2 0.21 11.1 0.18 0.24 0.03 0.06
Comparative Example 4 0.006 0.008 1.1 0.20 11.7 0.20 0.08 0.02 0.1
Comparative Example 5 0.006 0.007 1.31 0.20 11.9 0.21 0.41 0.02 0.18
Comparative Example 6 0.005 0.009 0.6 0.19 11.3 0.15 0.76 0.02 0.21
Invention Example 1 0.006 0.009 0.64 0.21 11.5 0.22 0.73 0.02 0.03
Invention Example 2 0.006 0.008 1.1 0.16 11.8 0.24 0.65 0.02 0.11
Invention example 3 0.005 0.010 0.86 0.21 12.2 0.22 0.75 0.02 0.08
Invention Example 4 0.007 0.008 0.96 0.21 12.0 0.16 0.49 0.03 0.14
Invention Example 5 0.006 0.008 1.1 0.21 11.8 0.24 0.65 0.02 0.11
Invention example 6 0.005 0.009 0.86 0.23 12.2 0.22 0.75 0.02 0.08
Invention Example 7 0.007 0.008 0.96 0.20 12.0 0.16 0.49 0.02 0.14
[80]
The fraction of Cu precipitated phase was measured for each cold rolled annealed steel sheet, and it was confirmed whether discoloration occurred after 1 hour at 500°C. In addition, the high-temperature strength at 900°C and the elongation at room temperature were measured and shown in Table 2.
[81]
[Table 2]
division Formula (1) Equation (2) Equation (3) Quenching and holding after heat treatment Cu precipitated phase (wt%) Whether high temperature discoloration occurs High-temperature strength (MPa) Elongation (%)
Comparative Example 1 0.46 0.46 1.95 × 0.01 generation 9.5 35.5
Comparative Example 2 0.75 1.25 4.47 × 0.03 generation 9.7 33.6
Comparative Example 3 0.44 1.04 2.78 × 0.02 generation 9.6 34.1
Comparative Example 4 1.18 2.18 8.00 × 0.04 non-occurring 11.9 33.1
Comparative Example 5 1.72 3.52 10.52 × 0.06 non-occurring 14.3 27.5
Comparative Example 6 1.36 3.46 11.00 × 0.05 non-occurring 13.3 26.5
Invention Example 1 1.37 1.67 3.59 × 0.05 generation 12.4 33.3
Invention Example 2 1.75 2.85 6.88 × 0.07 non-occurring 14.7 31.5
Invention example 3 1.61 2.41 5.73 × 0.06 non-occurring 14.1 32.7
Invention Example 4 1.45 2.85 10.38 × 0.06 non-occurring 13.5 30.8
Invention Example 5 1.75 2.85 6.88 ○ 0.11 non-occurring 15.8 31.2
Invention example 6 1.61 2.41 5.73 ○ 0.10 non-occurring 15.2 31.2
Invention Example 7 1.45 2.85 10.38 ○ 0.09 non-occurring 14.6 30.3
[82]
In Comparative Examples and Inventive Examples described in Table 1, alloy elements such as C, N, Cr, and Ti were controlled within the content range of the component system of the present invention, except for different contents of Cu, Si, and Sn.
[83]
In Comparative Examples 1 to 4, the Cu content was less than 0.3%, so the value of Equation (1) was less than 1.3, and accordingly, the amount of fine Cu precipitated phase was low. It was confirmed that the high-temperature strength was lower than 12 MPa due to insufficient solid solution strengthening and precipitation strengthening effects.
[84]
In Comparative Examples 1 to 3, the content of Si and Sn was less than that of Ti, so Equation (3) was not satisfied, and the Si and Sn enriched oxide film of the surface layer was not sufficiently formed, resulting in discoloration at high temperature. Comparative Example 4 had a low Cu content but a high Si content, which satisfies Equation (3), so discoloration did not occur, and high-temperature oxidation resistance according to Equation (3) was confirmed.
[85]
Comparative Examples 5 and 6 had a high Sn content, so that the value of Equation (2) exceeded 3.0, and thus, it was confirmed that the elongation was reduced by 5.0% compared to other Comparative Examples.
[86]
Invention Example 1 satisfies the component system composition of the present invention and Formulas (1) and (2). Although discoloration occurred at high temperature, 0.05 wt% of Cu precipitates were precipitated by satisfying Equation (1), and the high temperature strength was 12 MPa or more. In addition, it was confirmed that the formability was excellent as the elongation was measured to be 33.3% while ensuring high-temperature strength by satisfying Equation (2).
[87]
Inventive Examples 2 to 4 satisfies all of Formulas (1) to (3) by optimizing the Si, Cu, and Sn content, and thus exhibited high-temperature strength of 13.5 MPa or more and elongation of 30.8% or more, and high-temperature discoloration did not occur.
[88]
Inventive Examples 5 to 7 show that the cooling schedule after heat treatment according to the present invention is applied as well as satisfying all of Equations (1) to (3) by optimizing the Si, Cu, and Sn contents. The elongation was secured to 30.3% or more, and as a result of satisfying the quenching and holding time after heat treatment, 0.09 wt% or more of the fine Cu precipitated phase was precipitated, and the high-temperature strength was higher than 14.6 MPa. In particular, Inventive Examples 5 and 6 showed high-temperature strength of 15 MPa or more.
[89]
1 is a graph showing the values ​​of Equations (1) and (3) in Examples according to the present invention. 1, it can be confirmed that the correlation of formulas (1) and (3) regarding high-temperature strength and high-temperature oxidation resistance.
[90]
In the foregoing, exemplary embodiments of the present invention have been described, but the present invention is not limited thereto, and those of ordinary skill in the art will not depart from the concept and scope of the following claims. It will be appreciated that various modifications and variations are possible.
Industrial Applicability
[91]
The ferritic stainless steel according to the present invention can increase the high temperature characteristics of the existing steel by 30% or more without increasing the Cr content and adding Nb, thereby reducing the raw material cost.
Claims
[Claim 1]
In wt%, C: 0.005 to 0.015%, N: 0.005 to 0.015%, Si: 0.5 to 1.5%, Mn: 0.1 to 0.5%, Cr: 9 to 14%, Ti: 0.1 to 0.3%, Cu: 0.3 to 0.8%, Al: 0.01 to 0.05%, Sn: 0.005 to 0.15%, the remaining Fe and unavoidable impurities, and low-Cr ferritic stainless steel with excellent formability and high temperature characteristics satisfying the following formulas (1) and (2) River. (1) Cu + Si = 1.3 (2) Si + Cu + 10*Sn ≤= 3.0 (here, Si, Cu and Sn mean the content (wt%) of each element)
[Claim 2]
The low-Cr ferritic stainless steel having excellent formability and high temperature characteristics according to claim 1, further comprising: Ni: 0.3% or less, P: 0.04% or less, and S: 0.002% or less.
[Claim 3]
The low-Cr ferritic stainless steel having excellent formability and high-temperature characteristics according to claim 1, comprising 0.03 wt% or more of a Cu precipitated phase having a size of 1 to 500 nm in the matrix structure.
[Claim 4]
The low-Cr ferritic stainless steel according to claim 1, wherein the high-temperature strength at 900°C is 12 MPa or more and excellent in formability and high-temperature characteristics.
[Claim 5]
The low-Cr ferritic stainless steel according to claim 1, wherein the elongation is 30% or more and the formability and high temperature characteristics are excellent.
[Claim 6]
The low-Cr ferritic stainless steel according to claim 1, wherein the low-Cr ferritic stainless steel has excellent formability and high-temperature characteristics satisfying the following formula (3). (3) (Si + 5*Sn)/Ti = 5.0 (here, Si, Sn, and Ti mean the content (wt%) of each element)
[Claim 7]
In wt%, C: 0.005 to 0.015%, N: 0.005 to 0.015%, Si: 0.5 to 1.5%, Mn: 0.1 to 0.5%, Cr: 9 to 14%, Ti: 0.1 to 0.3%, Cu: 0.3 to 0.8%, Al: 0.01 to 0.05%, Sn: 0.005 to 0.15%, cold rolling annealing heat treatment of a ferritic stainless steel cold rolled steel sheet containing the remaining Fe and unavoidable impurities and satisfying the following formulas (1) and (2); and rapid cooling to a temperature range of 450 to 550° C. and maintaining it for 5 minutes or more. (1) Cu + Si = 1.3 (2) Si + Cu + 10Sn ≤= 3.0 (here, Si, Cu, and Sn mean the content (wt%) of each element)
[Claim 8]
[Claim 8] The method of claim 7, wherein the cold-rolled annealed steel sheet contains 0.09 wt% or more of a Cu precipitated phase having a size of 1 to 500 nm in a matrix structure, and has excellent formability and high-temperature characteristics.
[Claim 9]
The method according to claim 7, wherein the cold-rolled annealed steel sheet has a high-temperature strength at 900°C of 14.5 MPa or more, and has excellent formability and high-temperature characteristics.
[Claim 10]
The method according to claim 7, wherein the cold-rolled steel sheet has excellent formability and high-temperature characteristics satisfying the following formula (3). (3) (Si + 5*Sn)/Ti = 5.0 (here, Si, Sn, and Ti mean the content (wt%) of each element)