Title of Invention: Ferritic stainless steel with improved high temperature creep resistance and manufacturing method therefor
technical field
[One]
The present invention relates to a ferritic stainless steel, and more particularly, to a ferritic stainless steel having improved high temperature creep resistance and a method for manufacturing the same.
background
[2]
In general, stainless steel is classified according to its chemical composition or metal structure. According to the metal structure, stainless steel can be classified into austenite, ferrite, martensite, and dual phase.
[3]
Ferritic stainless steel has excellent corrosion resistance while adding a small amount of expensive alloying elements, so its price competitiveness is higher than that of austenitic stainless steel. In particular, high-Cr ferritic stainless steel containing 14 to 19% of chromium (Cr) is used in the exhaust gas temperature range of room temperature to 900° C. for automobile exhaust system parts (Muffler, Ex-manifold, Collector cone). , SCR (Selective Catalyst Reduction), etc.).
[4]
Materials used in a high-temperature environment where a constant load is applied must have excellent creep resistance as well as heat resistance. Creep refers to a phenomenon in which an object deforms with time due to constant stress under a high-temperature environment, and it is known that the deformation due to creep is affected by temperature, time, grain size and stress. Therefore, when deformation due to creep occurs along with high-temperature corrosion under a high-temperature environment, stress cannot be supported or the original function of the material cannot be performed by causing numerical deformation.
[5]
Creep resistance generally depends on precipitate formation and grain size. In the control of the creep resistance using the formation of precipitates, the creep resistance is improved by the precipitates, but the crystal grains are rather small due to the fine precipitates. Although the creep resistance is deteriorated due to such a small grain size, since the effect of improving the creep resistance due to the presence of precipitates is relatively greater, the creep resistance is improved when the precipitates are formed as a whole.
[6]
Recently, as the exhaust gas temperature increases due to the improvement of engine power, it is necessary to improve the high temperature creep characteristics of materials applied to exhaust system parts. Therefore, it is required to develop a ferritic stainless steel with improved high temperature creep resistance that can support stress in a high temperature environment.
DETAILED DESCRIPTION OF THE INVENTION
technical challenge
[7]
Embodiments of the present invention are to provide a ferritic stainless steel with improved high temperature creep resistance and a method for manufacturing the same.
means of solving the problem
[8]
Ferritic stainless steel with improved strength and improved high temperature creep resistance according to an embodiment of the present invention, by weight, C: 0.005 to 0.03%, N: 0.005 to 0.03%, Si: 0.05 to 0.9%, Mn: 0.05 to 0.9%, Cr: 14.0 to 19.0%, Ti: 0.1 to 0.6%, Nb: 0.1 to 0.6%, Cu: 0.1 to 0.6%, P: 0.01 to 0.04%, S: 0.01% or less (excluding 0) contains, and the remainder contains Fe and unavoidable impurities, and satisfies the following formulas (1) and (2).
[9]
Equation (1): 0.5 ≤ Nb/Cu ≤ 3
[10]
Equation (2): 20≤ [2Nb+Ti]/[C+N]
[11]
Here, Nb, Cu, Ti, C, and N mean the content (% by weight) of each element.
[12]
In addition, according to an embodiment of the present invention, based on the thickness T of the ferritic stainless steel, in the region from T/3 to T/5, the precipitates including Nb, Fe and Cr are 7*10 5 /mm 2 or more can be distributed.
[13]
In addition, according to an embodiment of the present invention, the size of the precipitate may be 5 to 500 nm.
[14]
Also, according to an embodiment of the present invention, the creep strain at 900° C. for 100 hours may be 50% or less.
[15]
In addition, according to an embodiment of the present invention, Al: 0.001 to 0.1%, and Ni: 0.001 to 0.6% may be further included.
[16]
A method of manufacturing a ferritic stainless steel with improved high temperature creep resistance according to another embodiment of the present invention, in weight %, C: 0.005 to 0.03%, N: 0.005 to 0.03%, Si: 0.05 to 0.9%, Mn: 0.05 to 0.9%, Cr: 14.0 to 19.0%, Ti: 0.1 to 0.6%, Nb: 0.1 to 0.6%, Cu: 0.1 to 0.6%, P: 0.01 to 0.04%, S: 0.01% or less (excluding 0) including, and the remainder including Fe and unavoidable impurities, manufacturing a cold-rolled steel sheet satisfying the following formulas (1) and (2); cold rolling annealing the cold rolled steel sheet; A cooling step of rapidly cooling the cold rolled annealing steel sheet to 650 to 750 ℃; and, after quenching, a cracking step of maintaining for 5 minutes or more.
[17]
Equation (1): 0.5 ≤ Nb/Cu ≤ 3
[18]
Equation (2): 20≤ [2Nb+Ti]/[C+N]
[19]
Here, Nb, Cu, Ti, C, and N mean the content (% by weight) of each element.
[20]
In addition, according to an embodiment of the present invention, in the cooling step, the cooling rate may be 10 ℃ / sec or more.
[21]
In addition, according to an embodiment of the present invention, the cracking step may be performed for 5 to 20 minutes.
[22]
In addition, according to an embodiment of the present invention, after the cracking step, 7 * 10 5 /mm 2 or more precipitates containing Nb, Fe and Cr may be distributed in the microstructure.
[23]
In addition, according to an embodiment of the present invention, Al: 0.001 to 0.1%, and Ni: 0.001 to 0.6% may be further included.
Effects of the Invention
[24]
According to an embodiment of the present invention, it is possible to provide a ferritic stainless steel having improved high temperature creep resistance and a method for manufacturing the same.
Brief description of the drawing
[25]
1 is a cross-sectional view for explaining the distribution of fine precipitates in a specific region based on the thickness direction and a photograph of the precipitate microstructure in a specific region.
[26]
2 is a view for explaining a creep sag amount measurement method introduced to show high-temperature creep characteristics.
Best mode for carrying out the invention
[27]
Ferritic stainless steel with improved strength and improved high temperature creep resistance according to an embodiment of the present invention, by weight, C: 0.005 to 0.03%, N: 0.005 to 0.03%, Si: 0.05 to 0.9%, Mn: 0.05 to 0.9%, Cr: 14.0 to 19.0%, Ti: 0.1 to 0.6%, Nb: 0.1 to 0.6%, Cu: 0.1 to 0.6%, P: 0.01 to 0.04%, S: 0.01% or less (excluding 0) contains, and the remainder contains Fe and unavoidable impurities, and satisfies the following formulas (1) and (2).
[28]
Equation (1): 0.5 ≤ Nb/Cu ≤ 3
[29]
Equation (2): 20≤ [2Nb+Ti]/[C+N]
[30]
Here, Nb, Cu, Ti, C, and N mean the content (% by weight) of each element.
Modes for carrying out the invention
[31]
Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings. The following examples are presented in order 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.
[32]
Throughout the specification, when a part "includes" a certain element, it means that other elements may be further included, rather than excluding other elements, unless otherwise stated.
[33]
The singular expression includes the plural expression unless the context clearly dictates otherwise.
[34]
Hereinafter, an embodiment according to the present invention will be described in detail with reference to the accompanying drawings.
[35]
The inventors of the present invention have obtained the following findings as a result of conducting various studies to improve the high-temperature creep characteristics of ferritic stainless steels.
[36]
In general, Nb is added to ferritic stainless steel for exhaust system in order to secure high-temperature strength. It is to improve the high-temperature strength by preventing the movement of dislocations at room temperature and high temperature through the solid solution strengthening effect of Nb.
[37]
However, creep deformation occurs at the same time in an exhaust gas environment of up to 900°C. Creep deformation is caused by the sliding of grain boundaries and diffusion of alloying elements, and in order to suppress such creep deformation, it is effective to form fine precipitates rather than to secure solid solution Nb that prevents the movement of dislocations.
[38]
In addition, focusing on securing strength by introducing a tire cord core into a panel of a sandwich structure with different strength or inside a tire rubber, when forming a fine precipitated phase only in a specific area based on the thickness direction of the ferritic stainless steel, high temperature It was found that the creep behavior can be effectively controlled in
[39]
In general, the initial solidification structure of Nb-added ferritic stainless steel can be classified into an equiaxed crystal and a columnar crystal. In particular, if it is possible to form a fine precipitated phase containing Nb by densely concentrating Nb at the interface between the equiaxed crystal and the columnar crystal (region from T/3 to T/5 based on the thickness T), the high temperature creep resistance is improved. was found to be possible, and accordingly, the distribution of precipitates by thickness location was derived.
[40]
1 is a cross-sectional view for explaining the distribution of fine precipitates in a specific region based on the thickness direction and a photograph of the precipitate microstructure in a specific region.
[41]
In order to distribute the precipitates in a specific thickness direction, it can be achieved by controlling the alloy composition and the compositional relationship of the ferritic stainless steel and introducing a specific heat treatment pattern after cold rolling annealing.
[42]
Ferritic stainless steel with improved high-temperature creep resistance according to an aspect of the present invention, by weight, C: 0.005 to 0.03%, N: 0.005 to 0.03%, Si: 0.05 to 0.9%, Mn: 0.05 to 0.9% , Cr: 14.0 to 19.0%, Ti: 0.1 to 0.6%, Nb: 0.1 to 0.6%, Cu: 0.1 to 0.6%, P: 0.01 to 0.04%, S: 0.01% or less (excluding 0), including, The remainder contains Fe and unavoidable impurities.
[43]
Hereinafter, the reason for numerical limitation of the content of the alloy component in the embodiment of the present invention will be described. Hereinafter, unless otherwise specified, the unit is % by weight.
[44]
The content of C is 0.005 to 0.03%.
[45]
Carbon (C) serves to improve the strength of ferritic stainless steel as an interstitial solid solution strengthening element. On the other hand, in order to control the C content to an extremely low level, the lower limit of C may be limited to 0.005% in consideration of the increase in the steelmaking VOD process cost. However, when the content is excessive, it induces grain boundary precipitation of Cr carbides such as Cr 23 C 6 by bonding with Cr , and there is a problem that local Cr depletion causes a decrease in high-temperature oxidation property, so the upper limit can be limited to 0.03%. .
[46]
The content of N is 0.005 to 0.03%.
[47]
Nitrogen (N) is an interstitial solid solution strengthening element like carbon and serves to improve the strength of ferritic stainless steel. On the other hand, in order to control the content to an extremely low level, the lower limit of N may be limited to 0.005% in consideration of an increase in the steelmaking VOD process cost. However, when the content is excessive, Cr 2 N precipitates are generated by combining with Cr, and high temperature oxidation property is reduced due to local depletion of Cr. In ferritic stainless steel, when the N content exceeds 0.015%, solid solution N In consideration of the saturation point, the upper limit of N may be limited to 0.03%.
[48]
The content of Si is 0.05 to 0.9%.
[49]
Silicon (Si) is an element that acts as a deoxidizer during the steelmaking process and stabilizes the ferrite phase. In the present invention, it is preferable to add 0.05% or more of Si in order to secure the strength and corrosion resistance of the ferritic stainless steel. However, when the content is excessive, there is a problem in that ductility and moldability are lowered, and the upper limit thereof is limited to 0.9% in the present invention.
[50]
The content of Mn is 0.05 to 0.9%.
[51]
Manganese (Mn) is an element for stabilizing austenite, and in the present invention, 0.05% or more may be added to secure corrosion resistance. However, if the content is excessive, austenite reverse transformation occurs in the annealing heat treatment process after hot rolling or cold rolling, and there is a problem of lowering elongation, and the upper limit may be limited to 0.9%.
[52]
The content of Cr is 14.0 to 19.0%.
[53]
Chromium (Cr) is an element that forms a passivation film that inhibits oxidation and stabilizes ferrite. In the present invention, 14.0% or more may be added in order to secure corrosion resistance and suppress high-temperature oxidation. However, when the content is excessive, there are problems in that the manufacturing cost increases and the moldability is inferior, so the upper limit may be limited to 19.0%.
[54]
The content of Ti is 0.1 to 0.6%.
[55]
Titanium (Ti) preferentially combines with interstitial elements such as carbon (C) and nitrogen (N) to form precipitates (carbonitrides), thereby reducing the amount of solid solution C and solid solution N in steel and forming a Cr depleted region. It is an element effective in securing corrosion resistance of steel by suppressing it, and in the present invention, 0.1% or more may be added. However, when the content is excessive, there is a problem in manufacturing that Ti-based inclusions are formed and there is a problem in manufacturing, and there is a problem in that the Ti component of the surface layer reacts with oxygen to cause a surface defect that turns yellow. can
[56]
The content of Nb is 0.1 to 0.6%.
[57]
Niobium (Nb) is an element that improves high-temperature creep characteristics by a fine Nb precipitation phase, and in the present invention, 0.1% or more may be added to secure high-temperature strength through solid solution Nb. However, when the content is excessive, rather coarse Nb precipitated phase is formed to cause brittle fracture, and there is a problem in that high temperature characteristics are deteriorated, so the upper limit can be limited to 0.6%.
[58]
The content of Cu is 0.1 to 0.6%.
[59]
Copper (Cu) is an element that forms a fine Nb precipitation phase like niobium (Nb) to improve high-temperature creep characteristics and contributes to high-temperature strength, and may be added in an amount of 0.1% or more. However, when the content is excessive, there is a problem that not only increases the material cost but also reduces the hot workability, so the upper limit may be limited to 0.6%.
[60]
The content of P is 0.01 to 0.04%.
[61]
Phosphorus (P) is an impurity that is unavoidably contained in steel and is an element that causes intergranular corrosion during pickling or inhibits hot workability, so it is desirable to control its content as low as possible. In the present invention, the P content is managed to 0.01 to 0.04%.
[62]
The content of S is 0.01% or less (excluding 0).
[63]
Sulfur (S) is an impurity that is unavoidably contained in steel, and is an element that segregates at grain boundaries and is a major cause of impairing hot workability, so it is desirable to control its content as low as possible. In the present invention, the upper limit of the S content is managed as 0.01%.
[64]
In addition, the ferritic stainless steel with improved high temperature creep resistance according to the disclosed embodiment may further include Al: 0.001 to 0.1%, and Ni: 0.001 to 0.6%.
[65]
The content of Al is 0.001 to 0.1%.
[66]
Aluminum (Al) serves to lower the oxygen content in molten steel as a strong deoxidizer, and is added in an amount of 0.001% or more in the present invention. However, when the content is excessive, Al in the surface layer reacts with oxygen to form a non-uniform oxide layer, thereby deteriorating the high-temperature oxidation resistance, and may be limited to 0.1% or less.
[67]
The content of Ni is 0.001 to 0.6%.
[68]
Nickel (Ni) is an austenite stabilizing element that is unavoidably brought in from scrap iron in the steelmaking process, and is managed as an impurity in the present invention. Ni is an element that stabilizes the austenite phase, such as C and N, and improves corrosion resistance by slowing the corrosion rate. When the content of Ni is excessive, after hot rolling or cold rolling, reverse austenite transformation occurs in the annealing heat treatment process, and there is a problem of lowering elongation, and the upper limit thereof may be limited to 0.6%.
[69]
The remaining component of the present invention is iron (Fe). However, since unintended impurities from raw materials or the surrounding environment may inevitably be mixed in the normal manufacturing process, this cannot be excluded. Since these impurities are known to any person skilled in the art of manufacturing processes, all details thereof are not specifically mentioned in the present specification.
[70]
Meanwhile, the ferritic stainless steel having improved creep resistance according to an embodiment of the present invention may satisfy the following formulas (1) and (2).
[71]
Equation (1): 0.5 ≤ Nb/Cu ≤ 3.0
[72]
As described above, if a fine precipitated phase containing Nb can be formed only in a specific region (region from T/3 to T/5 based on the thickness T) based on the thickness direction of the ferritic stainless steel, high temperature resistance It was confirmed that the creep property could be improved.
[73]
By adding Cu to lower the solid solubility of Nb, it is possible to maximize Nb concentration at the interface between the columnar crystal and the equiaxed crystal during the initial casting of the slab. Accordingly, it is possible to form fine precipitates containing Nb during the final cold rolling annealing.
[74]
When the Nb/Cu value is less than 0.5, it is difficult to express high temperature creep resistance characteristics because Cu precipitates are generated instead of Nb precipitates. and the density of the precipitates is reduced, which also makes it difficult to express high temperature creep resistance due to the precipitates containing Nb. Therefore, in the present invention, it is intended to limit the Nb/Cu value to 0.5 to 3.0.
[75]
Equation (2): 20≤ (2Nb + Ti)/(C+N)
[76]
In general, Nb and Ti contained in ferritic stainless steel combine with C and N to form Nb(C,N) and Ti(C,N) carbonitrides. If the content of C and N is relatively large compared to that of Nb, there is a tendency to form coarse Nb (C, N) precipitates. Therefore, as described above, fine Nb 2 (Fe,Cr) precipitates can be formed in a specific region (region from T/3 to T/5 based on thickness T) based on the thickness direction of the ferritic stainless steel. none.
[77]
Therefore, in the present invention, the component relationship between C and N and Nb and Ti is derived so that the Nb element does not form Nb(C,N) precipitates and forms an Nb2(Fe,Cr) precipitated phase as much as possible.
[78]
When (2Nb + Ti)/(C + N) is less than 20, it was experimentally confirmed that Nb(C,N) precipitates of 500 nm or more were formed, thereby suppressing the formation of relatively fine Nb 2 (Fe,Cr) precipitates.
[79]
In addition, the size of the Nb 2 (Fe,Cr) precipitate may be 5 to 500 nm. When the size of the precipitates containing Nb is coarse, the density of the precipitates is reduced, so that it is difficult to express high temperature creep resistance due to the precipitates containing Nb. Therefore, it is preferable to control the size of the precipitate to be 500 nm or less in order to maximize the high temperature creep resistance.
[80]
In the ferritic stainless steel according to the present invention that satisfies the above alloy element composition range and component relational formulas, based on the thickness T, in the region from T/3 to T/5, precipitates containing Nb, Fe, and Cr are 7* 10 5 pieces /mm 2 or more can be distributed.
[81]
For example, the size of the precipitate including Nb, Fe and Cr may be 5 to 500 nm.
[82]
The ferritic stainless steel of the present invention containing 5 to 500 nm of Nb, Fe, and Cr precipitates containing 7 * 10 5 /mm 2 or more shows a creep deformation amount of 5 mm or less at 800 ° C. for 100 hours. , it is possible to secure creep resistance at high temperatures. Accordingly, it is possible to minimize the high-temperature strength decrease due to creep deformation of the ferritic stainless steel.
[83]
[84]
Next, a method for manufacturing a ferritic stainless steel having improved high temperature creep resistance according to another aspect of the present invention will be described.
[85]
The method for manufacturing a ferritic stainless steel with improved high temperature creep resistance according to an embodiment of the present invention can be manufactured as a cold rolled steel sheet through a conventional manufacturing process, and includes the above-described alloy composition, and Equations (1) and Equations ( 2) manufacturing a cold-rolled steel sheet that satisfies; cold rolling annealing the cold rolled steel sheet; A cooling step of rapidly cooling the cold rolled annealing steel sheet to 650 to 750 ℃; and, after quenching, a cracking step of maintaining for 5 minutes or more.
[86]
For example, a slab containing 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.
[87]
Cold-rolled steel sheet is subjected to a conventional recrystallization heat treatment process in the cold-rolled annealing process.
[88]
For example, the cold-rolled steel sheet may be cold-rolled annealed in a temperature range of 10° C. or less lower than the austenite-ferrite transformation temperature (Ac1). In the Cr content range of the present invention, since some austenite phase is present, the annealing temperature is limited to (Ac1-10)°C or less to prevent reverse transformation. Annealing is performed so that C and N can be sufficiently dissolved in the matrix in the above temperature range.
[89]
The cold-rolled steel sheet undergoes a cooling step of rapidly cooling to a temperature range of 650 to 750° C. after normal recrystallization heat treatment in the cold rolling annealing process.
[90]
In the present invention, in order to secure a fine precipitate containing Nb, it is preferable to control the cooling end temperature to 750 °C or less. However, when the heat treatment is performed at an excessively low temperature, there is a problem that residual stress is generated.
[91]
In this case, the cooling rate may be 10° C./sec or more. When the cooling rate is less than 10 ° C./sec, the time passing through the temperature range in which the size of the precipitates containing Nb is coarsened increases, and the size of the precipitates containing Nb becomes coarse and the distribution density accordingly decreases, so the high temperature There is a problem in that it is difficult to secure creep resistance.
[92]
After quenching, the cracking step, which is maintained for more than 5 minutes, is a process to evenly distribute Nb precipitates in an appropriate size. When the cracking time is less than 5 minutes, it is advantageous for the formation of fine Nb-containing precipitates, but may be clustered and distributed. There are disadvantages in that process efficiency is reduced due to the increase, and manufacturing cost is increased.
[93]
After cold rolling annealing, rapid cooling is required to a temperature range of 650 to 750 ° C. after cold rolling annealing to suppress the generation of Cr carbonitride, and through heat treatment maintained in the temperature range for 5 minutes or more, the precipitates containing Nb, Fe and Cr By increasing the number, creep resistance at high temperature can be maximized.
[94]
As such, it is possible to increase the formation of fine precipitates containing Nb in the same component system by introducing a heat treatment pattern after cold rolling annealing, along with alloy component control, and also secure high-temperature strength.
[95]
Accordingly, in the cold-rolled annealing steel sheet, in the region from T/3 to T/5 based on the thickness T, 7*10 5 pieces/mm 2 or more of precipitates including Nb, Fe, and Cr may be distributed.
[96]
[97]
Hereinafter, the present invention will be described in more detail through examples.
[98]
For the various alloy component ranges shown in Table 1 below, a slab was prepared through ingot melting, reheated at 1,200 ° C, hot rolled to 6 mm, and then hot rolled annealing was performed at 1,100 ° C. After cold rolling to 2.0 mm Cold-rolled annealed steel sheet was manufactured by annealing heat treatment at 1,100°C.
[99]
Table 1 below shows the alloy composition (% by weight) and the values ​​of Equation (1) and Equation (2) for each experimental steel type.
[100]
[Table 1]
steel grade Ingredients (wt%) Formula (1) Equation (2)
C N Si Mn Cr Ti Nb Cu P S
One 0.0083 0.0065 0.41 0.21 17.5 0.23 0.21 0.26 0.024 0.003 0.81 43.9
2 0.007 0.008 0.23 0.35 15.6 0.19 0.35 0.24 0.023 0.002 1.46 59.3
3 0.015 0.008 0.51 0.18 18.5 0.19 0.15 0.29 0.032 0.003 0.52 21.3
4 0.011 0.019 0.33 0.43 16.3 0.13 0.47 0.16 0.028 0.002 2.94 35.7
5 0.0083 0.0065 0.41 0.21 17.5 0.23 0.21 0.26 0.024 0.003 0.81 43.9
6 0.007 0.008 0.23 0.35 15.6 0.19 0.35 0.24 0.023 0.002 1.46 59.3
7 0.015 0.008 0.51 0.18 18.5 0.19 0.15 0.29 0.032 0.003 0.52 21.3
8 0.011 0.019 0.33 0.43 16.3 0.13 0.47 0.16 0.028 0.002 2.94 35.7
9 0.015 0.021 0.11 0.38 14.8 0.33 0.35 0.06 0.021 0.004 5.83 28.6
10 0.021 0.024 0.37 0.29 17.1 0.14 0.31 0.23 0.025 0.003 1.35 16.9
11 0.009 0.013 0.22 0.18 16.3 0.24 0.15 0.37 0.031 0.003 0.41 24.5
12 0.007 0.008 0.56 0.34 18.2 0.17 0.24 0.07 0.032 0.002 3.43 43.3
13 0.029 0.028 0.63 0.39 15.3 0.41 0.11 0.35 0.028 0.003 0.31 11.1
14 0.019 0.008 0.43 0.12 17.5 0.12 0.12 0.15 0.023 0.002 0.80 13.3
[101]
For some examples only, after heat treatment, it was rapidly cooled to 700° C. at a rate of 15° C./sec and maintained for about 10 minutes, followed by air cooling to prepare a cold-rolled annealed steel sheet, and the remaining Examples and Comparative Examples were air-cooled after annealing heat treatment.
[102]
The number of precipitates was collected by using a replica extraction method, and then the number of precipitates was measured through a transmission electron microscope (TEM). The number of precipitates was measured by dividing the precipitates observed per 1 mm 2 into a region from T/3 to T/5 and an average region of the base material, based on the thickness T, and described in Table 2.
[103]
2 is a view for explaining a creep sag amount measurement method introduced to show high-temperature creep characteristics.
[104]
The high temperature creep characteristics were expressed as creep sag (Creepage, mm) measured after maintaining a 2.0 mm thick cold-rolled annealed steel sheet at 800° C. for 100 hours. Referring to FIG. 2, the creep sag is the difference in the degree of sagging of the steel sheet before and after heat treatment at 800° C./100 hours by placing the cold-rolled annealing steel sheet according to Examples and Comparative Examples having a thickness of 2.0 mm on a pedestal at a distance of 250 mm. indicated.
[105]
Specifically, referring to FIG. 2 , the initial deflection value is [Hi(a)+Hi(b)-2*Hi(c)]/2, and the deflection value after heat treatment is [Hf(a)+Hf(b)-2 *Hf(c)]/2, and the creep deflection amount (Creepage) was calculated as the difference between the initial deflection value and the deflection value after heat treatment. Here, Hi(a,b,c) and Hf(a,b,c) denote length data before/after heat treatment, respectively. Since the creep deflection amount represents the degree of deformation by a constant load at 800° C., the lower the value, the better the creep resistance.
[106]
In the present invention, the creep strain rates of Examples and Comparative Examples are shown in Table 2 with the creep deflection amount of 10 mm of the steel sheet as the standard (creep strain rate of 100%).
[107]
[Table 2]
steel grade Quick cooling and crack application Number of precipitates in the 1/3T~1/5T area (x10 5 pieces/mm 2 ) Average number of precipitates in the base material (x10 5 pieces/mm 2 ) Creep deflection (mm) Creep strain (%)
Example 1 One O 14.7 2.4 2.3 23
Example 2 2 O 19.5 3.2 1.9 19
Example 3 3 O 10.4 2.1 3.5 35
Example 4 4 O 12.9 3.7 3 30
Example 5 5 X 12.7 2.4 3.2 32
Example 6 6 X 15.6 3.2 2.7 27
Example 7 7 X 7.5 2.1 4.4 44
Example 8 8 X 9.6 3.7 3.9 39
Comparative Example 1 9 X 3.4 2.9 10.5 105
Comparative Example 2 10 X 3.6 3.1 11.5 115
Comparative Example 3 11 X 4.8 3.3 8.9 89
Comparative Example 4 12 X 4.3 3.2 9.6 96
Comparative Example 5 13 X 2.1 2 15.6 156
Comparative Example 6 14 X 2.7 2.2 13.2 132
[108]
Referring to Table 2, in the case of Examples 1 to 8 that satisfy the alloy composition of the present invention, the value of Equation (2), and the range of the value of Equation (2), based on the thickness T of the ferritic stainless steel, In the region from T/3 to T/5, precipitates containing Nb, Fe and Cr are distributed over 7*10 5 /mm 2 , and the creep strain at 900°C and 100 hours is 5 mm or less, and the creep strain rate This is 50% or less, and it can be confirmed that about 40 to 80% of the creep resistance is excellent compared to the comparative example.
[109]
In Comparative Examples 1 and 4, in the case of using steel grades 9 and 12 containing less than 0.1% Cu and thus exceeding the range of Formula (1), Nb, Fe in the region from T/3 to T/5 and Cr precipitates were distributed at a density of 3.4*10 5 /mm 2 and 4.3*10 5 /mm 2 , respectively . This is considered to be due to the insufficient Cu content for forming precipitates containing Nb, Fe and Cr.
[110]
In Comparative Examples 2 and 6, steel grades 10 and 14 having (2Nb + Ti)/(C+N) values ​​less than 20 in Formula (2) were used, and Nb, Fe in the region from T/3 to T/5 and distribution of precipitates containing Cr 7*10 5 /mm 2 did not reach. Comparative Examples 2 and 6 satisfy Equation (1), but a relatively high content of Ti and Nb reacts with C and N to form Nb(C,N) precipitates to lower solid solution C, N, and fine Nb 2 ( Fe, Cr) is considered to inhibit the formation of precipitates.
[111]
Comparative Examples 3 and 5 satisfy the component system range of the present invention including Ti, Nb, and Cu and Equation (2), but use steel grades 11 and 13 with a high Cu content compared to Nb and the value of Equation (1) is less than 0.5. In some cases, even if Nb 2 (Fe,Cr) precipitates are formed, the Nb 2 (Fe,Cr) precipitates are coarsened. When the precipitates are coarsened, there is a problem that the density of the precipitates proposed in the present invention cannot be secured, and Comparative Example 5 satisfies the compositional range of the present invention, but not only the region from T/3 to T/5, but also the average of the base material. It can be seen that the density of the Nb 2 (Fe,Cr) precipitates of Nb 2 (Fe,Cr) is the lowest, and thus the high-temperature creep characteristics are the most inferior.
[112]
On the other hand, referring to Table 2, Examples 1 to 4, in which a specific heat treatment pattern was introduced after cold rolling annealing, compared with Examples 5 to 8, Nb, Fe and Cr in the region from T/3 to T/5. It was possible to secure a large amount of precipitates containing In particular, in Example 2, in the region from T /3 to T/5, precipitates containing Nb, Fe and Cr were distributed at a density of 19.5*10 5 /mm 2 , and accordingly, at 900° C. for 100 hours. The creep strain was measured to be 1.9 mm.
[113]
As such, according to the disclosed embodiment, fine precipitated phases including Nb, Fe and Cr are formed in a specific region (region from T/3 to T/5 based on thickness T) by controlling alloy components and compositional relationships. Thus, it is possible to manufacture a ferritic stainless steel with improved high temperature creep resistance.
[114]
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 are within the scope not departing from the concept and scope of the claims described below. It will be understood that various changes and modifications are possible.
Industrial Applicability
[115]
According to the present invention, it is possible to improve the high temperature creep resistance of ferritic stainless steel, so that it can be applied even in a high temperature environment where a constant load is applied.
Claims
[Claim 1]
By weight%, C: 0.005 to 0.03%, N: 0.005 to 0.03%, Si: 0.05 to 0.9%, Mn: 0.05 to 0.9%, Cr: 14.0 to 19.0%, Ti: 0.1 to 0.6%, Nb: 0.1 to 0.6%, Cu: 0.1 to 0.6%, P: 0.01 to 0.04%, S: 0.01% or less (excluding 0), the remainder including Fe and unavoidable impurities, the following formula (1), formula (2) ), ferritic stainless steel with improved creep resistance at high temperatures. Formula (1): 0.5≤ Nb/Cu ≤ 3 Formula (2): 20≤ [2Nb+Ti]/[C+N] (where Nb, Cu, Ti, C, N are the content of each element (wt%) ) means.)
[Claim 2]
The high temperature creep resistance according to claim 1, wherein, based on the thickness T of the ferritic stainless steel, 7*10 5 /mm 2 or more of precipitates including Nb, Fe and Cr are distributed in the region from T/3 to T/5. Ferritic stainless steel with improved properties.
[Claim 3]
[Claim 3] The ferritic stainless steel of claim 2, wherein the size of the precipitate is 5 to 500 nm.
[Claim 4]
The ferritic stainless steel with improved high temperature creep resistance according to claim 1, wherein the creep strain at 900°C and 100 hours is 50% or less.
[Claim 5]
The ferritic stainless steel of claim 1, wherein the high temperature creep resistance is improved further comprising Al: 0.001 to 0.1%, and Ni: 0.001 to 0.6%.
[Claim 6]
By weight%, C: 0.005 to 0.03%, N: 0.005 to 0.03%, Si: 0.05 to 0.9%, Mn: 0.05 to 0.9%, Cr: 14.0 to 19.0%, Ti: 0.1 to 0.6%, Nb: 0.1 to 0.6%, Cu: 0.1 to 0.6%, P: 0.01 to 0.04%, S: 0.01% or less (excluding 0), the remainder including Fe and unavoidable impurities, the following formula (1), formula (2) ) to manufacture a cold-rolled steel sheet that satisfies; cold rolling annealing the cold rolled steel sheet; A cooling step of rapidly cooling the cold rolled annealing steel sheet to 650 to 750 ℃; and a cracking step of maintaining for 5 minutes or more after rapid cooling. Formula (1): 0.5≤ Nb/Cu ≤ 3 Formula (2): 20≤ [2Nb+Ti]/[C+N] (where Nb, Cu, Ti, C, N are the content of each element (wt%) ) means.)
[Claim 7]
The method of claim 6, wherein in the cooling step, the cooling rate is 10° C./sec or more, wherein the high temperature creep resistance is improved.
[Claim 8]
[Claim 7] The method of claim 6, wherein the cracking step is performed for 5 to 20 minutes.
[Claim 9]
The method of claim 6, wherein after the cracking step, 7*10 5 /mm 2 or more precipitates containing Nb, Fe and Cr are distributed in the microstructure and the high temperature creep resistance is improved.
[Claim 10]
According to claim 6, Al: 0.001 to 0.1%, and Ni: The method of manufacturing a ferritic stainless steel with improved high temperature creep resistance further comprising 0.001 to 0.6%.