Specification
Title of invention: Ferritic stainless steel with improved pipe expansion processability and manufacturing method thereof
Technical field
[One]
The present invention relates to a ferritic stainless steel with improved pipe expansion workability, and more particularly, to a ferritic stainless steel for automobile exhaust system with improved pipe expansion workability by controlling the texture conditions for each thickness position of a cold rolled annealed material.
Background
[2]
Among stainless steels, especially ferritic stainless steel cold-rolled products have excellent high-temperature properties such as thermal expansion coefficient and thermal fatigue properties, and are resistant to stress corrosion cracking. Accordingly, ferritic stainless steel is widely used in automobile exhaust system parts, household appliances, structures, home appliances, elevators, and the like.
[3]
In general, automobile exhaust system members are classified into a hot part and a cold part according to the temperature of the exhaust gas. Automotive parts of high-temperature members include manifolds, converters, and bellows, and the operating temperature of these parts is mainly 600 or higher, and has excellent high-temperature strength, high-temperature heat fatigue, and high-temperature salt corrosion characteristics. Should be. On the other hand, the cold part is a member such as a muffler that reduces the noise of automobile exhaust gas with a use temperature of 400 or less.
[4]
These automotive exhaust system materials mainly use stainless steel with high resistance to external corrosion and internal condensate corrosion, and ferritic stainless steel without Ni is more widely used than austenitic stainless steel containing expensive Ni because of material cost reduction. Is being used. For example, there are materials such as stainless steel (or STS) 409, 409L, 439, 436L, or Al plated stainless steel 409.
[5]
The recent trend of automobile exhaust system parts is that the shape of each part is becoming very complicated in order to increase the space efficiency of the lower part of the car as the number of parts of the exhaust system under the car increases. .
[6]
Conventionally, in relation to deep drawing or pipe bending workability, there has been an approach to the overall thickness average texture perspective and R value (Plastic-strain ratio) perspective, but a technical method for improving pipe expansion processability has not yet been clearly established. .
[7]
In the present invention, by dividing the surface layer portion in the thickness direction and the center portion for increasing the pipe expansion processability, the conditions of each texture and the range of components for satisfying the conditions are clearly presented.
Detailed description of the invention
Technical challenge
[8]
Embodiments of the present invention are ferritic stainless steel for automobile exhaust system with improved pipe expansion processability by controlling the size, distribution density, and rolling process conditions of publications to satisfy the texture conditions and target texture conditions for each thickness position of the steel, and a manufacturing method thereof Want to provide.
Means of solving the task
[9]
According to the ferritic stainless steel with improved pipe expansion processability according to an embodiment of the present invention, by weight %, Cr: 10 to 25%, N: 0.015% or less (excluding 0), Al: 0.005 to 0.04%, Nb: 0.1 to 0.6%, Ti: 0.1 to 0.5%, the balance Fe and other inevitable impurities are included, and the following formula (1) is satisfied.
[10]
Equation (1): Z = X*Y ≥ 17
[11]
Here, based on the thickness T of ferritic stainless steel, X means [(111)//ND texture fraction]/[(100)//ND texture fraction] in the region from T/3 to 2T/3, Y means 10*[(100)//ND texture fraction]/[(111) //ND texture fraction] of the area from the surface layer to T/3.
[12]
In addition, according to an embodiment of the present invention, the ferritic stainless steel having improved pipe expansion workability has a maximum diameter of 0.05 to 5 μm, and an Al-Ca-Ti-Mg-O system having a distribution density of 9 pieces/mm 2 or more. It may contain an oxide.
[13]
In addition, according to an embodiment of the present invention, the ferritic stainless steel with improved pipe expansion processability may further include Ca: 0.0004 to 0.002%, Mg: 0.0002 to 0.001%.
[14]
In addition, according to an embodiment of the present invention, the ferritic stainless steel with improved pipe expansion processability may satisfy the following equation (2).
[15]
Equation (2): (D f -D 0 )/D 0 *100 ≥ 160
[16]
Here, D f denotes the hole length of the machined part after molding, and D 0 denotes the length of the initial machined hole.
[17]
In addition, according to an embodiment of the present invention, the thickness of the ferritic stainless steel with improved pipe expansion processability may be 0.5 to 3 mm.
[18]
The manufacturing method of ferritic stainless steel with improved pipe expansion processability according to an embodiment of the present invention is, in weight%, Cr: 10 to 25%, N: 0.015% or less (excluding 0), Al: 0.005 to 0.04%, Nb : Hot rolling a slab containing 0.1 to 0.6%, Ti: 0.1 to 0.5%, the balance Fe and other inevitable impurities; Cold rolling the hot rolled material; And cold rolling annealing the cold-rolled material, wherein the cold-rolled annealing material may satisfy the following equation (1).
[19]
Equation (1): Z = X*Y ≥ 17
[20]
Here, based on the thickness T of ferritic stainless steel, X means [(111)//ND texture fraction]/[(100)//ND texture fraction] in the region from T/3 to 2T/3, Y means 10*[(100)//ND texture fraction]/[(111) //ND texture fraction] of the area from the surface layer to T/3.
[21]
In addition, according to an embodiment of the present invention, the cold-rolled annealed material may include an Al-Ca-Ti-Mg-O-based oxide having a maximum diameter of 0.05 to 5 μm and a distribution density of 9 pieces/mm 2 or more. .
[22]
Further, according to an embodiment of the present invention, the roll diameter in the cold rolling step may be 100 mm or less.
Effects of the Invention
[23]
The ferritic stainless steel according to the disclosed embodiment exhibits a sandwich effect due to the development of different structures of the core and the surface layer, thereby increasing the HER value and suppressing the occurrence of cracks during pipe expansion.
Brief description of the drawing
[24]
1 is a photograph taken of a part for an automobile exhaust system to which a pipe expansion process is applied and a crack generated during expansion processing.
[25]
2 is a cross-sectional view for explaining a texture parameter according to an embodiment of the present invention.
[26]
3 is a graph showing a correlation between a tissue parameter and HER according to an embodiment of the present invention.
[27]
4 is a graph showing X and Y values ​​of an embodiment and a comparative example of the present invention.
Best mode for carrying out the invention
[28]
According to the ferritic stainless steel with improved pipe expansion processability according to an embodiment of the present invention, by weight %, Cr: 10 to 25%, N: 0.015% or less (excluding 0), Al: 0.005 to 0.04%, Nb: 0.1 to 0.6%, Ti: 0.1 to 0.5%, the balance Fe and other inevitable impurities are included, and the following formula (1) is satisfied.
[29]
Equation (1): Z = X*Y ≥ 17
[30]
SCI = -[Cr] + 4[Ni] + 5[Mo] + 12[Cu]------ Equation (1)
[31]
Here, based on the thickness T of ferritic stainless steel, X means [(111)//ND texture fraction]/[(100)//ND texture fraction] in the region from T/3 to 2T/3, Y means 10*[(100)//ND texture fraction]/[(111) //ND texture fraction] of the area from the surface layer to T/3.
Mode for carrying out the invention
[32]
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 only to the embodiments presented herein, but may be embodied in other forms. In the drawings, in order to clarify the present invention, portions not related to the description may be omitted, and the size of components may be slightly exaggerated to aid understanding.
[33]
Throughout the specification, when a part "includes" a certain component, it means that other components may be further included rather than excluding other components unless otherwise stated.
[34]
Singular expressions include plural expressions, unless the context clearly has exceptions.
[35]
Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings. First, the ferritic stainless steel will be described, and then the manufacturing method of the ferritic stainless steel will be described.
[36]
The present inventors have conducted various studies to improve pipe expansion workability when ferritic stainless steel is used for an exhaust system heat exchanger, and as a result, the following knowledge can be obtained.
[37]
An arrangement having a certain surface and orientation created inside a crystal is called a texture, and a pattern in which these textures develop in a certain direction is called a texture fiber. The aggregated structure, which represents the collectiveness of crystals, has a close relationship with the expansion processability. Among them, the group of azimuths created in a direction perpendicular to the (111) plane of the aggregates is called gamma (γ)-fiber, and (100) A group of azimuth tissues created in a direction perpendicular to the) plane is called a cube-fiber.
[38]
In the center of ferritic stainless steel, gamma-fibers are mainly developed, and cube-fibers are developed in the surface layer. Among these textures, it is known that the higher the fraction of gamma-fibers, the better the overall workability. In the conventional ferritic stainless steel, gamma-fibers are increased and cube-fibers are reduced.
[39]
On the other hand, during hole expansion processing, plane deformation occurs in the center, so only the (111)//ND texture needs to be strongly developed, but complex deformation behavior in three axes as well as simple plane deformation occurs in the surface layer around the hole. In this case, when only the (111)//ND texture is developed, cracks are generated as shown in FIG. 1, and thus there is a problem in that it is impossible to secure workability for various deformation behaviors. Accordingly, there is a need for research on the orientation of the texture that can secure the expansion processability of a certain level or more.
[40]
In the present invention, as a result of studying the texture orientation in order to improve the pipe expansion workability in ferritic stainless steel, the surface layer rather develops the (100)//ND texture to secure workability under deformation behavior conditions other than plane deformation. I found I could. In particular, it was found that if the cube-fiber in the surface layer and the gamma-fiber in the center were strongly developed, the hole expandability could be improved, and accordingly, the texture parameters for each thickness location were derived.
[41]
In order to develop different characteristics of the texture of the surface layer and the center in the thickness direction, it can be achieved by securing the roll diameter of 100 mm or less during cold rolling together with the alloy components and the size and distribution density of the inclusions.
[42]
Hereinafter, a ferritic stainless steel that exhibits excellent pipe expansion workability only by controlling the alloying element component system and the texture of each thickness position, even without an additional heat treatment process, will be described.
[43]
Ferritic stainless steel with improved pipe expansion processability according to an aspect of the present invention, by weight, Cr: 10 to 25%, N: 0.015% or less, Al: 0.005 to 0.04%, Nb: 0.1 to 0.6%, Ti: 0.1 to 0.5%, balance Fe and other unavoidable impurities.
[44]
Hereinafter, the reason for limiting the numerical value of the content of the alloying component in the examples of the present invention will be described. Hereinafter, unless otherwise specified, the unit is% by weight.
[45]
The content of Cr is 10 to 25%.
[46]
Chromium (Cr) is a basic element that is most often contained among the elements for improving corrosion resistance of stainless steel, and it is preferable to add 10% or more in order to develop corrosion resistance. However, if the content is excessive, there is a concern that intergranular corrosion may occur in ferritic stainless steel containing carbon and nitrogen, and there is a problem that the manufacturing cost increases, and the upper limit may be limited to 25%.
[47]
The content of N is not more than 0.015%.
[48]
Nitrogen (N) is an interstitial element, and if the content is excessive, the strength is excessively increased and the ductility is lowered, and the upper limit may be limited to 0.015%.
[49]
The content of Al is 0.005 to 0.05%.
[50]
Aluminum (Al) is an element added as a deoxidizing agent during steel making, and it is preferable to add 0.005% or more since it can lower the content of oxygen in molten steel. However, when the content is excessive, there is a concern that a sleeve defect of the cold-rolled strip may occur due to the presence of non-metallic inclusions, and there is a problem that the weldability is deteriorated, and the upper limit thereof may be limited to 0.05%.
[51]
The content of Nb is 0.1 to 0.6%.
[52]
Niobium (Nb) is an element that combines with solid solution C to precipitate NbC, and it is preferable to add 0.1% or more since it can improve corrosion resistance and high temperature strength by lowering the solid solution C content. However, if the content is excessive, there is a problem that the moldability is deteriorated by suppressing recrystallization, and the upper limit thereof may be limited to 0.6%.
[53]
The content of Ti is 0.1 to 0.5%.
[54]
Titanium (Ti) is an element that fixes carbon and nitrogen, and it is preferable to add 0.1% or more because it can improve the corrosion resistance of steel by lowering the content of solid solution C and solid solution N by forming precipitates. However, if the content is excessive, there is a concern that surface defects may occur due to coarse Ti inclusions, and there is a problem that the manufacturing cost increases, and the upper limit thereof may be limited to 0.5%.
[55]
In addition, the ferritic stainless steel with improved pipe expansion processability according to an embodiment of the present invention may further include Ca: 0.0004 to 0.002% and Mg: 0.0002 to 0.001%.
[56]
The content of Ca is 0.0004 to 0.002%.
[57]
Ca is an element introduced for deoxidation in the steel making process and remains as an impurity after the deoxidation process. However, if the content is excessive, corrosion resistance is deteriorated. Therefore, it is limited to less than 0.002%?c, and since it is impossible to completely remove it, it is desirable to manage it at 0.0004% or more.
[58]
The content of Mg is 0.0002 to 0.001%.
[59]
Mg is an element introduced for deoxidation in the steel making process and remains as an impurity after the deoxidation process. However, if the content is excessive, the moldability is deteriorated. Therefore, the content is limited to 0.001% or less, and it is impossible to completely remove it, so it is preferable to manage it to 0.0002% or more.
[60]
The remaining component of the present invention is iron (Fe). However, since unintended impurities from the raw material or the surrounding environment may inevitably be mixed in the normal manufacturing process, this cannot be excluded. Since these impurities are known to anyone of ordinary skill in the manufacturing process, all the contents are not specifically mentioned in this specification.
[61]
2 is a cross-sectional view for explaining a texture parameter according to an embodiment of the present invention.
[62]
According to an embodiment of the present invention, a ferritic stainless steel having improved pipe expansion workability that satisfies the alloy composition described above may satisfy Equation (1) below.
[63]
Equation (1): Z = X*Y ≥ 17
[64]
Here, based on the thickness T of ferritic stainless steel, X means [(111)//ND texture fraction]/[(100)//ND texture fraction] in the region from T/3 to 2T/3, Y means 10*[(100)//ND texture fraction]/[(111) //ND texture fraction] of the area from the surface layer to T/3.
[65]
As described above, in the surface layer of ferritic stainless steel, the fraction of crystal grains having a cube-fiber texture is increased while suppressing the gamma-fiber texture as much as possible, and crystal grains having a gamma-fiber texture in the center while suppressing the cube-fiber texture as much as possible. It was confirmed that the expansion processability in the deformation behavior condition can be improved by increasing the fraction of the particles.
[66]
The Z value is a parameter derived in consideration of the thickness position and the texture fraction of other properties, and 10 in Y is a weight considering that the cube fiber is less developed than the gamma fiber.
[67]
At this time, the (111)//ND texture fraction in the center of the cold-rolled annealed ferritic stainless steel sheet may be 70% or less, and the (100)//ND texture fraction may be 2% or more. In addition, the (100)//ND texture fraction in the surface layer may be 30% or less, and the (111)//ND texture fraction may be 10% or more. Accordingly, X may satisfy a range of 35 or less and Y may satisfy a range of 30 or less.
[68]
According to an embodiment of the present invention, a ferritic stainless steel having improved pipe expansion workability that satisfies the alloy composition described above may satisfy Equation (2) below.
[69]
Equation (2): (D f -D 0 )/D 0 *100 ≥ 160
[70]
Here, D f denotes the hole length of the machined part after molding, and D 0 denotes the length of the initial machined hole.
[71]
3 is a graph showing the correlation between the texture parameter Z and the HER (Hole Expansion Ratio).
[72]
Hole expandability is a material characteristic of how wide a hole processed through various processing methods in a steel plate can be expanded without defects such as cracks or necking.(The hole length of the processed part after molding)-( It is defined as length)*100/(length of initial processing hole).
[73]
When Equation (1) is satisfied, the HER value increases due to the similar cladding (sandwich) effect due to the formation of different textures in the surface layer and the center, and cracks can be suppressed during the expansion processing of real parts.
[74]
3, the ferritic stainless steel with improved pipe expansion processability according to an embodiment of the present invention has a Z value of 17 or higher.
[75]
Accordingly, the ferritic stainless steel according to an embodiment of the present invention may have a HER value of 160 or more. As the HER value increases, it is easier to expand the pipe, and the larger the value is, the more advantageous it is.
[76]
According to an embodiment of the present invention, when developing from a deformed texture to a recrystallized texture as a method for implementing different characteristics of the recrystallized texture of the surface layer and the center, the recrystallized texture is reduced by suppressing randomization of the texture. It contains Al-Ca-Ti-Mg-O-based oxide to be constrained to the deformed texture developed before annealing. In addition, it was confirmed that the size and distribution density of these oxides should be secured in order to suppress the randomization of the texture of the weld.
[77]
For example, the Al-Ca-Ti-Mg-O-based oxide may include TiO 2 , CaO, Al 2 O 3 , MgO, and the like.
[78]
In the present invention, the Al-Ca-Ti-Mg-O-based oxide having a maximum diameter of 0.05 to 5 μm can be defined as an effective oxide, and when the effective oxide has a distribution density of 9 pieces/mm 2 or more, pipe expansion processability It can work effectively for improvement.
[79]
When the maximum diameter of the Al-Ca-Ti-Mg-O-based oxide is less than 0.05 μm, the oxide is too small to constrain the strained texture during recrystallization, so it cannot play a role in improving workability, and exceeds 5 μm. In the case of, there is a problem that causes surface defects such as Scab.
[80]
In addition, even if the distribution density of the Al-Ca-Ti-Mg-O-based oxide is less than 9/mm2, the present invention does not implement the desired recrystallized texture characteristics because the role of restraining the strained texture during recrystallization behavior is insufficient. There is a problem that cannot be done.
[81]
Next, a method of manufacturing a ferritic stainless steel with improved pipe expansion processability according to another aspect of the present invention will be described.
[82]
The manufacturing method of ferritic stainless steel with improved pipe expansion processability according to an embodiment of the present invention is, in weight%, Cr: 10 to 25%, N: 0.015% or less (excluding 0), Al: 0.005 to 0.04%, Nb : Hot rolling a slab containing 0.1 to 0.6%, Ti: 0.1 to 0.5%, the balance Fe and other inevitable impurities; Cold rolling the hot rolled material; And cold rolling annealing the cold rolled material.
[83]
The explanation of the reason for the numerical limitation of the content of the alloying element is as described above.
[84]
The stainless steel containing the above composition may be subjected to conventional hot rolling and hot rolling annealing, followed by cold rolling and cold rolling annealing below to form a final product.
[85]
In order to develop different characteristics of the texture of the surface layer and the center in the thickness direction, the roll diameter must be small during cold rolling. This is because the smaller the roll diameter, the more severe the difference between the deformation modes of the surface layer and the center (surface shear deformation, center plane deformation), and the deformation texture also greatly differs. Specifically, as the roll diameter decreases, the cube-fiber fraction in the surface layer may increase.
[86]
In this way, when the final cold-rolled annealed material is manufactured through cold-rolling and cold-rolling annealing by controlling the roll diameter during cold-rolling together with the alloy composition and inclusion conditions, the characteristics of the required texture of the surface layer in the thickness direction and the central part are different. It can be developed to maximize the texture sandwich effect. The cold rolling may be performed under conditions of a roll diameter of 100 mm or less.
[87]
The cold-rolled annealed material thus produced satisfies the following formula (1).
[88]
Equation (1): Z = X*Y ≥ 17
[89]
Here, based on the thickness T of ferritic stainless steel, X means [(111)//ND texture fraction]/[(100)//ND texture fraction] in the region from T/3 to 2T/3, Y means 10*[(100)//ND texture fraction]/[(111) //ND texture fraction] of the area from the surface layer to T/3.
[90]
Hereinafter, it will be described in more detail through a preferred embodiment of the present invention.
[91]
Example
[92]
An experiment was conducted to produce the final product according to the production conditions of commercially produced ferritic stainless steel. Hot-rolled annealing was performed to prepare a hot-rolled annealed steel sheet.
[93]
Thereafter, cold rolling was performed with different cold rolling roll diameters, and cold rolled annealing treatment was performed to prepare a cold rolled annealed steel sheet having a thickness of 0.5 to 3 mm.
[94]
[Table 1]
Cr N Al Nb Ti Ca Mg
Invention Lecture 1 18.3 0.009 0.007 0.33 0.21 0.0008 0.0005
Invention Lecture 2 17.2 0.008 0.021 0.43 0.18 0.0009 0.0006
Invention Lecture 3 18.9 0.009 0.034 0.38 0.28 0.0007 0.0004
Comparative lecture 1 16.5 0.007 0.009 0.47 0.22 0.0010 0.0008
Comparative lecture 2 19.3 0.008 0.021 0.26 0.26 0.0014 0.0009
Comparative lecture 3 17.5 0.009 0.015 0.32 0.14 0.0007 0.0007
Comparative lecture 4 18.2 0.010 0.038 0.45 0.35 0.0005 0.0008
[95]
Inventive steel and comparative steel according to Table 1 were used in the experiment.
[96]
The texture fraction was measured using EBSD (Electron Backscatter Diffraction) with respect to the transverse direction cross section of the final cold-rolled annealed material, and accordingly, the texture parameters for each thickness location were calculated and shown in Table 2 below.
[97]
In addition, the distribution density of effective oxides was measured by SEM (Scanning Electron Microscope) for the transverse direction of the final cold-rolled annealed material, and the roll diameter, HER value, thickness during cold rolling, and whether cracks occurred when expanding actual parts. It is shown in Table 3 below.
[98]
[Table 2]
Remark Center Surface X Y Z
111//ND 100//ND 111//ND 100//ND
Example 1 36.9% 8.4% 23.8% 10.0% 4.4 4.2 18.5
Example 2 35.1% 6.9% 27.4% 10.7% 5.1 3.9 19.9
Example 3 46.2% 7.3% 38.2% 14.8% 6.3 3.9 24.5
Comparative Example 1 28.2% 10.8% 19.8% 10.4% 2.6 5.3 13.7
Comparative Example 2 27.5% 9.5% 18.7% 10.6% 2.9 5.7 16.4
Comparative Example 3 37.9% 6.8% 32.0% 8.3% 5.6 2.6 14.5
Comparative Example 4 36.4% 7.5% 33.2% 8.5% 4.9 2.6 12.4
[99]
[Table 3]
Remark Number of effective oxides/mm 2 Rolling roll diameter (mm) HER value Whether cracks occur Thickness(mm)
Example 1 13 90 164.3 X 2.5
Example 2 10 90 166.8 X 2
Example 3 18 90 177 X 1.2
Comparative Example 1 8 150 143.3 O 2.5
Comparative Example 2 14 300 154.6 O 2
Comparative Example 3 7 150 140.2 O 1.2
Comparative Example 4 6 300 135.3 O 2
[100]
4 is a graph showing the tissue parameters according to the disclosed Example 2 and Comparative Example 3.
[101]
As described above, the texture capable of securing the workability under the condition of the plane deformation occurring in the center is gamma-fiber, and the texture capable of securing the workability under the conditions of deformation behavior other than the plane deformation occurring in the surface part is Since it is a cube-fiber, in order to maximize the texture sandwich effect of the final cold rolled annealed steel sheet, the recrystallization texture characteristics of the surface layer and the center should be different.
[102]
In the case of the above examples, compared to the comparative examples, in the surface portion, the fraction of the cube-fiber texture is higher than that of the gamma-fiber, and in the center portion, the fraction of the gamma-fiber texture is higher than that of the cube-fiber, and the texture parameter Z value is It can be confirmed that it is 17 or higher.
[103]
In contrast, in Comparative Examples 1 and 2, the gamma-fiber texture fraction was lower than that of the central cube-fiber, and the Z value was less than 17.
[104]
In addition, in Comparative Examples 3 and 4, the fraction of the cube-fiber texture compared to the gamma-fiber in the surface layer was low, and the Z value was less than 17.
[105]
Specifically, referring to Tables 2 and 3, in the case of Comparative Example 1, the roll diameter was large as 150 mm during cold rolling, and the distribution density of effective oxides was measured as 8 pieces/mm 2, and the texture of the final cold rolled annealed material The parameter Z was 13.7, which did not reach 17, and accordingly, cracks occurred during the expansion processing of the actual part.
[106]
Referring to Tables 2 and 3, in the case of Comparative Example 2, the distribution density of effective oxides was satisfied, but the roll diameter was 300mm during cold rolling, so the texture parameter Z of the final cold-rolled annealed material was 16.4, which did not reach 17. As a result, cracks occurred during expansion processing of actual parts.
[107]
Referring to Tables 2, 3, and 4, in the case of Comparative Example 3, when cold rolling, the roll diameter was large as 150 mm, and the distribution density of effective oxides was measured as 7 pieces/mm 2, and the texture of the final cold-rolled annealed material The parameter Z was 14.5, which did not reach 17, and accordingly, cracks occurred during the expansion processing of the actual part.
[108]
Referring to Tables 2 and 3, in the case of Comparative Example 4, the roll diameter was large as 300 mm during cold rolling, and the distribution density of effective oxides was measured as 6 pieces/mm 2, and the texture parameter Z of the final cold rolled annealed material was It did not reach 17 at 12.4, and as a result, cracks occurred during the expansion and processing of actual parts.
[109]
Ferritic stainless steel manufactured according to an embodiment of the present invention can minimize cracking while increasing pipe expansion processability by maximizing the HER value of the final cold-rolled annealed material to 160 or more by controlling the texture condition for each thickness position.
[110]
As described above, although exemplary embodiments of the present invention have been described, 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 following claims. It will be appreciated that various changes and modifications are possible in.
Industrial availability
[111]
The ferritic stainless steel according to the present invention has improved pipe expansion processability and can be used as a component of an automobile exhaust system.
Claims
[Claim 1]
By weight%, Cr: 10 to 25%, N: 0.015% or less (excluding 0), Al: 0.005 to 0.04%, Nb: 0.1 to 0.6%, Ti: 0.1 to 0.5%, the balance Fe and other inevitable impurities Ferritic stainless steel containing improved pipe expansion workability satisfying the following formula (1). Equation (1): Z = X*Y ≥ 17 (here, based on the thickness T of ferritic stainless steel, X is [(111)//ND texture fraction]/[( 100)//ND texture fraction], Y means 10*[(100)//ND texture fraction]/[(111) //ND texture fraction] in the area from the surface to T/3 do.)
[Claim 2]
The ferritic stainless steel according to claim 1, wherein the maximum diameter is 0.05 to 5 µm and includes an Al-Ca-Ti-Mg-O-based oxide having a distribution density of 9 pieces/mm 2 or more.
[Claim 3]
According to claim 1, Ca: 0.0004 to 0.002%, Mg: ferritic stainless steel with improved pipe expansion processability further comprising 0.0002 to 0.001%.
[Claim 4]
The ferritic stainless steel according to claim 1, wherein the pipe expansion processability is improved satisfying the following formula (2). Equation (2): (D f -D 0 )/D 0 *100 ≥ 160 (here, D f is the hole length of the machined part after molding, and D 0 is the length of the initial machined hole.)
[Claim 5]
The ferritic stainless steel according to claim 1, wherein the thickness is 0.5 to 3 mm.
[Claim 6]
By weight%, Cr: 10 to 25%, N: 0.015% or less (excluding 0), Al: 0.005 to 0.04%, Nb: 0.1 to 0.6%, Ti: 0.1 to 0.5%, the balance Fe and other inevitable impurities Hot rolling the slab containing; Cold rolling the hot rolled material; And cold-rolling annealing the cold-rolled material, wherein the cold-rolled annealing material satisfies the following formula (1). Equation (1): Z = X*Y ≥ 17 (here, based on the thickness T of ferritic stainless steel, X is [(111)//ND texture fraction]/[( 100)//ND texture fraction], Y means 10*[(100)//ND texture fraction]/[(111) //ND texture fraction] in the area from the surface to T/3 do.)
[Claim 7]
The method of claim 6, wherein the cold-rolled annealed material has a maximum diameter of 0.05 to 5 µm, and an Al-Ca-Ti-Mg-O-based oxide having a distribution density of 9 pieces/mm 2 or more. Manufacturing method.
[Claim 8]
The method of claim 6, wherein the diameter of the roll in the cold rolling step is controlled to 100 mm or less.