Title of Invention: Low CR ferritic stainless steel with improved pipe expansion workability and manufacturing method therefor
technical field
[One]
The present invention relates to a ferritic stainless steel, and more particularly, to a low-Cr ferritic stainless steel for an automobile exhaust system with improved pipe expansion workability 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, ferritic stainless steels such as STS 409L, 439, and 436L are used as materials for automobile exhaust system members such as muffler cases, pipes, and plates applied in a temperature range of 400° C. or less.
[4]
For example, STS 409L steel uses about 11% Cr and stabilizes carbon (C) and nitrogen (N) with titanium (Ti) to prevent sensitization of welds and improve workability. It is mainly used in temperature and has been most widely used because it has some corrosion resistance to condensate components generated in automobile exhaust systems.
[5]
STS 439 steel is made by stabilizing carbon (C) and nitrogen (N) with titanium (Ti), and contains about 17% of chromium (Cr). In addition, STS 436L steel is a steel in which about 1% of molybdenum (Mo) is added to STS 439 steel, and has excellent condensate corrosion properties and anti-rust corrosion properties.
[6]
On the other hand, recent trends in automobile penetration in various countries such as China, Latin America, and India are rapidly increasing, and these countries contain a large amount of sulfur (S) in gasoline components compared to other developed countries. For example, Korea and Japan regulate the sulfur (S) component in gasoline components to 10 ppm or less, but China regulates it to 500 ppm or less, and it is estimated that more sulfur (S) components are actually contained. .
[7]
Sulfur (S) component of gasoline component is concentrated as SO4 2- ion in condensate component of automobile exhaust gas, and is changed to highly corrosive sulfuric acid (H 2 SO 4 ) of PH 2 or less.
[8]
As such, in regions where a large amount of sulfur (S) component in gasoline components is contained, STS 409L steel, which is used as a material for automobile mufflers, gradually contains 17% or more of chromium (Cr) components such as STS 439 steel and 436L steel. It is in a situation where it has no choice but to be replaced with a chrome-based stainless material. Therefore, with the increase in resource prices, it is required to develop a stainless material having condensate corrosion characteristics equal to or greater than that of STS 439 steel or 436L steel material without adding expensive elements such as molybdenum (Mo) or adding a small amount.
[9]
On the other hand, in the actual vehicle exhaust system environment, not only the internal condensate corrosion caused by condensate water but also the external corrosion caused by snow removal salt or sea salt occurs at the same time. The 439 round of substitution is impossible.
[10]
In addition, the recent trend of automobile exhaust system parts is that as the number of parts in the exhaust system under the automobile increases, the shape of each part is becoming very complicated in order to increase the space efficiency of the lower part of the automobile. the current situation.
[11]
Therefore, it is required to develop a ferritic stainless steel with improved pipe expansion workability, which has condensate corrosion characteristics equal to or greater than that of the existing STS 439 steel or 436L steel material in consideration of not only the inner surface condensate corrosion but also the outer surface corrosion.
DETAILED DESCRIPTION OF THE INVENTION
technical challenge
[12]
Embodiments of the present invention optimize the content of Sn, Si, and Cu to secure the resistance to external corrosion and internal condensate corrosion corresponding to the high Cr ferritic stainless steel without increasing the Cr content, while securing the ferritic stainless steel with improved pipe expansion workability and An object of the present invention is to provide a method for manufacturing the same.
means of solving the problem
[13]
Low-Cr ferritic stainless steel with improved pipe expansion workability according to an embodiment of the present invention, by weight, C: 0.01% or less (excluding 0), N: 0.01% or less (excluding 0), Si: 1.0 to 2.0%, Mn: 0.5% or less (excluding 0), Cr: 9.0 to 15.0%, Ti: 0.1 to 0.5%, Sn: 0.05 to 0.2%, Cu: 1.0% or less (excluding 0), P: 0.035% or less (excluding 0), S: 0.01% or less (excluding 0), the average grain size (Gs) of the region corresponding to a depth of 100 μm or less from the surface and the average of the central region, including the remaining Fe and unavoidable impurities The ratio (Gs/Gc) of the grain size (Gc) is 1.5 or less, and the following formula (1) is satisfied.
[14]
Formula (1): Cr+3Si+10Sn+2Cu ≥ 17
[15]
Here, Cr, Si, Sn, and Cu mean the content (wt%) of each element
[16]
In addition, the low-Cr ferritic stainless steel with improved pipe expansion workability according to an embodiment of the present invention may satisfy the following formula (2).
[17]
Formula (2): Cr+2Si+15Sn+5Cu ≥ 17
[18]
Here, Cr, Si, Sn, and Cu mean the content (% by weight) of each element.
[19]
In addition, according to an embodiment of the present invention, the expansion rate defined by the following formula (3) may be 25% or more.
[20]
Equation (3): (D f -D 0 )/D 0 *100
[21]
Here, D f is the hole length of the machining part after molding, and D 0 is the length of the initial machining hole.
[22]
In addition, according to an embodiment of the present invention, the elongation in the vertical direction of the rolling direction may be 30% or more.
[23]
Also, according to an embodiment of the present invention, the average grain size of a region corresponding to a depth of 100 μm or less from the surface may be 50 μm or less.
[24]
The manufacturing method of low-Cr ferritic stainless steel with improved pipe expansion workability according to another embodiment of the present invention, in weight%, C: 0.01% or less (excluding 0), N: 0.01% or less (excluding 0), Si : 1.0 to 2.0%, Mn: 0.5% or less (excluding 0), Cr: 9.0 to 15.0%, Ti: 0.1 to 0.5%, Sn: 0.05 to 0.2%, Cu: 1.0% or less (excluding 0), P : 0.035% or less (excluding 0), S: 0.01% or less (excluding 0), including the remaining Fe and unavoidable impurities, hot rolling a slab satisfying the following formula (1); cold rolling and cold rolling annealing; and cold rolling pickling through neutral salt electrolysis and sulfuric acid electrolysis.
[25]
Formula (1): Cr+3Si+10Sn+2Cu ≥ 17
[26]
Here, Cr, Si, Sn, and Cu mean the content (% by weight) of each element.
[27]
In addition, according to an embodiment of the present invention, the slab may satisfy the following formula (2).
[28]
Formula (2): Cr+2Si+15Sn+5Cu ≥ 17
[29]
Here, Cr, Si, Sn, and Cu mean the content (% by weight) of each element.
[30]
In addition, according to an embodiment of the present invention, the slab may be hot rolled at a temperature of 1,020 to 1,180 °C.
[31]
In addition, according to an embodiment of the present invention, cold rolling annealing may be performed in a temperature range of 900 to 1,100 °C.
Effects of the Invention
[32]
According to an embodiment of the present invention, it is possible to provide a low-Cr ferritic stainless steel capable of securing resistance to STS439 level of external corrosion and internal condensate corrosion while improving pipe expansion workability and a method for manufacturing the same.
Brief description of the drawing
[33]
1 is a graph showing the results of an external corrosion test generated by a snow removal salt for each steel type in an automobile exhaust system environment.
[34]
2 is a graph showing the corrosion resistance evaluation results according to the external corrosion index defined as Cr+3Si+10Sn+2Cu in an automobile exhaust system environment.
[35]
3 is a graph showing the corrosion resistance evaluation results according to the internal corrosion index defined as Cr+2Si+15Sn+5Cu in a condensed water environment of an automobile exhaust system.
[36]
4 is a view showing the scale structure after cold rolling annealing of Example 2, and FIG. 5 is a view showing the scale structure after cold rolling annealing of Comparative Example 12.
[37]
6 is a photograph showing the surface state of Example 2 after cold rolling pickling through neutral salt electrolysis and sulfuric acid electrolysis, and the surface state after evaluation of corrosion resistance.
[38]
7 is a photograph showing the surface state of Example 2 after cold rolling pickling through neutral salt electrolysis, sulfuric acid electrolysis, and mixed acid (nitric acid + hydrofluoric acid) immersion and after corrosion resistance evaluation.
[39]
8 is a photograph of observing the microstructure according to the change in the cold rolling annealing temperature of Example 2.
[40]
9 is a photograph of observing the microstructure according to the change in the cold rolling annealing temperature of Comparative Example 12.
Best mode for carrying out the invention
[41]
Low-Cr ferritic stainless steel with improved pipe expansion workability according to an embodiment of the present invention, by weight, C: 0.01% or less (excluding 0), N: 0.01% or less (excluding 0), Si: 1.0 to 2.0%, Mn: 0.5% or less (excluding 0), Cr: 9.0 to 15.0%, Ti: 0.1 to 0.5%, Sn: 0.05 to 0.2%, Cu: 1.0% or less (excluding 0), P: 0.035% or less (excluding 0), S: 0.01% or less (excluding 0), the average grain size (Gs) of the region corresponding to a depth of 100 μm or less from the surface and the average of the central region, including the remaining Fe and unavoidable impurities The ratio (Gs/Gc) of the grain size (Gc) is 1.5 or less, and the following formula (1) is satisfied.
[42]
Formula (1): Cr+3Si+10Sn+2Cu ≥ 17
[43]
Here, Cr, Si, Sn, and Cu mean the content (% by weight) of each element.
Modes for carrying out the invention
[44]
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.
[45]
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.
[46]
The singular expression includes the plural expression unless the context clearly dictates otherwise. Hereinafter, an embodiment according to the present invention will be described in detail with reference to the accompanying drawings.
[47]
The present inventors have obtained the following knowledge as a result of various studies in order to improve the external corrosion resistance and pipe expansion workability caused by snow removal or sea salt of low-cost low-Cr ferritic stainless steel.
[48]
In order to improve corrosion resistance, the content of Cr is generally increased, but Cr is also high in raw material cost and causes an increase in manufacturing cost, so it is not a desirable development direction.
[49]
In the present invention, Si, Sn, and Cu candidates were selected as alloying elements to improve the corrosion resistance of the ferritic stainless steel and the corrosion resistance of the inner surface of the ferritic stainless steel. On the other hand, Sn is known as an element that reduces hot workability. However, the present inventors have found that when the Sn content is controlled to 0.2% or less, the deterioration of hot workability can be effectively controlled.
[50]
In addition, it was found that when 0.5% or less of Cu and 1 to 2% of Si were added in combination with Sn, the corrosion resistance of the exterior surface of the automobile exhaust system was rapidly improved while ensuring hot workability.
[51]
On the other hand, Cu is an element that improves the external corrosion and internal condensate corrosion resistance, but as its content increases, the crystal grain size of the surface layer of ferritic stainless steel rapidly increases. .
[52]
Accordingly, the present inventors have found that, if the Si content is secured to 1.0% or more in a state where the Cu content is 0.5% or less, the growth of surface layer crystal grains is suppressed, and component optimization was performed in consideration of external corrosion resistance and pipe expansion workability.
[53]
1 is a graph showing the results of an external corrosion test generated by a snow removal salt for each steel type in an automobile exhaust system environment.
[54]
Referring to FIG. 1 , when 11% of Cr is contained in a state where no alloying element is added, the corrosion depth is about 0.6 mm, and when Sn, Cu and Si are added alone in a state containing 11% of Cr, corrosion is It can be seen that the depth is 0.4 to 0.5 mm, which is slightly reduced than that of 11Cr STS steel.
[55]
On the other hand, when the alloying elements Sn, Cu, and Si were simultaneously added in a composite containing 11% of Cr, the corrosion depth was sharply decreased to 0.1mm, and it was confirmed that the corrosion resistance of 18Cr STS steel could be secured. .
[56]
Low-Cr ferritic stainless steel with improved pipe expansion workability according to an aspect of the present invention, by weight, C: 0.01% or less (excluding 0), N: 0.01% or less (excluding 0), Si: 1.0 to 2.0 %, Mn: 0.5% or less (excluding 0), Cr: 9.0 to 15.0%, Ti: 0.1 to 0.5%, Sn: 0.05 to 0.2%, Cu: 1.0% or less (excluding 0), P: 0.035% or less (excluding 0), S: 0.01% or less (excluding 0), including remaining Fe and unavoidable impurities.
[57]
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.
[58]
The content of C and N is 0.01% or less (excluding 0).
[59]
Carbon (C) and nitrogen (N) are interstitial elements that form Ti(C,N) carbonitrides. When the C, N content increases, the solid solution C, N, which does not form Ti(C,N) carbonitride, reduces the elongation and low-temperature impact properties of the material. Since Cr carbides such as Cr 23 C 6 are generated and intergranular corrosion is generated, the upper limits of C and N can be limited to 0.01%.
[60]
In addition, when the C+N content is increased, the steel-making inclusions increase according to the increase in the Ti content, which causes surface defects such as scabs. In addition, there is a problem that the nozzle clogging occurs during playing, and the elongation and impact properties are lowered, so the total content of C + N may be limited to 0.02% or less.
[61]
The content of Si is 1.0 to 2.0%.
[62]
Silicon (Si) serves as a deoxidizer during the steelmaking process and is an element that stabilizes the ferrite phase. When the content of Si increases, a thickening phenomenon occurs around grain boundaries, and grain growth is suppressed by the concentrated Si. In the present invention, it is preferable to add 1.0% or more of Si in order to increase corrosion resistance and suppress the growth of surface crystal grains in a condensed water atmosphere. However, when the content is excessive, there is a problem in that ductility and moldability are reduced, and in the present invention, the upper limit may be limited to 2.0%.
[63]
The content of Mn is 0.5% or less (excluding 0).
[64]
Manganese (Mn) is an element that stabilizes austenite. When the Mn content increases, it forms precipitates such as MnS to reduce pitting resistance, but when excessively reduced, the refining cost increases, so the upper limit is limited to 0.5% can do.
[65]
The content of Cr is 9.0 to 15.0%.
[66]
Chromium (Cr) is an element that forms a passivation film that inhibits oxidation and stabilizes ferrite. In the present invention, 9.0% or more may be added to secure corrosion resistance in a condensed water atmosphere. However, when the content is excessive, there are problems in that the manufacturing cost increases and the workability and impact properties are inferior, so the upper limit may be limited to 15.0%.
[67]
The content of Ti is 0.1 to 0.5%.
[68]
Titanium (Ti) is an element that prevents intergranular corrosion by forming Ti(C,N) carbonitride. Ti preferentially combines with interstitial elements such as carbon (C) and nitrogen (N) to form precipitates (Ti(C,N) carbonitrides), thereby reducing the amount of solid solution C and solid solution N in steel and depleting Cr As an element effective in securing corrosion resistance of steel by suppressing region formation, 0.1% or more may be added in the present invention. However, if the content is excessive, a large amount of surface defects such as scabs occur due to the formation of Ti-based inclusions, and there is a problem that nozzle clogging occurs during playing, so the upper limit can be limited to 0.5%. .
[69]
The content of Sn is 0.05 to 0.2%.
[70]
Tin (Sn) is an essential element for securing corrosion resistance in a condensed water atmosphere targeted in the present invention, and may be added by 0.05% or more to secure corrosion resistance equivalent to or higher than that of STS439 steel containing 18Cr. However, when the content is excessive, there is a problem in that the hot workability is lowered and the manufacturing process efficiency is lowered, and the upper limit thereof may be limited to 0.2%.
[71]
The content of Cu is 1.0% or less (excluding 0).
[72]
Copper (Cu) is an essential element for securing corrosion resistance in a condensed water atmosphere targeted in the present invention, and is added to secure corrosion resistance equivalent to or higher than that of STS439 steel containing 18Cr. 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 1.0%.
[73]
The content of P is 0.035% or less (excluding 0).
[74]
Phosphorus (P) is an impurity that is unavoidably contained in steel, and is an element that forms grain boundary segregation and MnS precipitates to reduce hot workability, so it is desirable to control its content as low as possible. In the present invention, the P content is managed to 0.035% or less.
[75]
The content of S is 0.01% or less (excluding 0).
[76]
Sulfur (S) is an unavoidably contained impurity, and since it is an element that forms grain boundary segregation and MnS precipitates to reduce hot workability, it is desirable to control its content as low as possible. In the present invention, the S content is controlled to 0.01% or less.
[77]
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.
[78]
On the other hand, the low-Cr ferritic stainless steel with improved pipe expansion workability according to an embodiment of the present invention may satisfy the following formula (1).
[79]
Formula (1): Cr+3Si+10Sn+2Cu) ≥ 17
[80]
Here, Cr, Si, Sn, and Cu mean the content (% by weight) of each element.
[81]
In the present invention, as a result of evaluating the corrosion resistance in a solution simulating the external corrosion environment of ferritic stainless steel, the external corrosion index expressed by Equation (1) was derived.
[82]
2 is a graph showing the corrosion resistance evaluation results according to the external corrosion index defined as Cr+3Si+10Sn+2Cu in an automobile exhaust system environment. In FIG. 2 , the corrosion depth of the existing STS 439 steel was measured to be 1 mm, and the outer corrosion index was limited to 17 or more in order to secure the external corrosion resistance equivalent to or higher than that of STS 439 steel.
[83]
Referring to FIG. 2 , when the external corrosion index is less than 17, the corrosion depth exceeds 1 mm, and resistance to external corrosion caused by snow removal or sea salt at the level of STS 439 steel cannot be secured.
[84]
On the other hand, the low-Cr ferritic stainless steel with improved pipe expansion workability according to an embodiment of the present invention may satisfy the following formula (2).
[85]
Formula (2): Cr+2Si+15Sn+5Cu ≥ 17
[86]
Here, Cr, Si, Sn, and Cu mean the content (% by weight) of each element.
[87]
In the present invention, as a result of evaluating corrosion resistance in a solution simulating condensate as well as an external corrosion environment of ferritic stainless steel, an internal corrosion index expressed by Equation (2) was derived.
[88]
3 is a graph showing the corrosion resistance evaluation results according to the internal corrosion index defined as Cr+2Si+15Sn+5Cu in a condensed water environment of an automobile exhaust system. In FIG. 3 , the corrosion depth of the existing STS 439 steel was measured to be 2.5 mm, and the inner corrosion index was limited to 17 or more in order to secure the external corrosion resistance equivalent to or higher than the STS 439 steel.
[89]
Referring to FIG. 3 , when the inner surface corrosion index is less than 17, the corrosion depth exceeds 2.5 mm, and there is a problem in that corrosion resistance in a condensed water environment of the level of STS 439 steel cannot be secured.
[90]
As described above, when Cu and Si are compounded with Sn, the grain size of the ferritic stainless steel surface layer rapidly increases as the Cu content increases, so there is a problem in that workability cannot be secured during pipe expansion after pipe making. . In the present invention, in a state where the Cu content is 0.5% or less, the Si content is controlled to 1.0 to 2.0% to suppress the growth of crystal grains in the surface layer.
[91]
Low-Cr ferritic stainless steel with improved tube expansion workability according to an embodiment of the present invention, the ratio of the average grain size (Gs) of the region corresponding to a depth of 100 μm or less from the surface and the average grain size (Gc) of the central region (Gs/Gc) is 1.5 or less.
[92]
That is, compared to internal crystal grains in ferritic stainless steel, growth of surface crystal grains distributed in an area of ​​100 μm or less from the surface is suppressed, thereby securing pipe expansion workability during pipe making. For example, the average grain size (Gs) of the surface area may be 50 μm or less in consideration of the elongation of the tube.
[93]
Accordingly, the ferritic stainless steel according to the disclosed embodiment has an expansion rate of 25% or more defined by the following formula (3).
[94]
Equation (3): (D f -D 0 )/D 0 *100
[95]
(Here, D f means the hole length of the machining part after molding, and D 0 means the length of the initial machined hole.)
[96]
The expansion rate is a material property that determines how much the hole processed in the steel sheet through various processing methods can be expanded without defects such as cracks or necking. It is defined as length)*100/(length of initial machining hole).
[97]
Next, a method for manufacturing a low-Cr ferritic stainless steel with improved pipe expansion workability according to another aspect of the present invention will be described.
[98]
For example, a slab containing the above-described alloy component composition may be hot-rolled, the hot-rolled hot-rolled steel sheet is annealed, and cold-rolled and cold-rolled annealed to manufacture a cold-rolled annealed steel sheet.
[99]
In the case of hot rolling conditions, the higher the slab heating temperature is, the more advantageous it is for recrystallization during the hot rolling operation, but if the heating temperature is too high, a large amount of surface defects are generated.
[100]
The lower the finish rolling temperature during hot rolling, the higher the strain accumulation energy during mild steel rolling, which helps recrystallize during annealing, which is advantageous for elongation improvement. ), the lower limit of the hot rolling temperature can be limited to 1,020°C because defects are easy to occur.
[101]
On the other hand, if the cold reduction ratio of the material is too low, it is difficult to remove surface defects and secure surface properties, and if the cold reduction ratio is too high, the r-bar value increases and the formability is improved. can do.
[102]
Next, after the conventional cold-rolling annealing at a temperature range of 900 to 1,100 ℃, the cold-rolled annealed steel sheet may be cold-rolled and pickled through neutral salt electrolysis and sulfuric acid electrolysis.
[103]
The austenitic stainless steel of the present invention is formed uniformly in a thin layer without forming annular scales on the surface of the cold-rolled annealed steel sheet by adding Sn, Cu, and Si simultaneously.
[104]
That is, by including a certain amount of Sn, the formation of the SiO 2 scale layer after cold rolling annealing may be suppressed. Therefore, in the prior art, as the SiO 2 scale layer was formed to be thick annularly, a mixed acid immersion process was performed in which hydrofluoric acid and nitric acid were added in the cold rolling pickling process to remove such scale, but without adding such hydrofluoric acid and nitric acid, neutral salt electrolysis And sufficient cold rolling pickling effect can be obtained even if only the sulfuric acid electrolysis is performed, thereby reducing the process cost.
[105]
Accordingly, the cold-rolled annealing steel sheet may have a ratio (Gs/Gc) of an average grain size (Gs) of a region corresponding to a depth of 100 μm or less from the surface and an average grain size (Gc) of a central region of 1.5 or less.
[106]
That is, it is possible to secure the pipe expansion processability during pipe pipe making by suppressing the growth of surface crystal grains, and thus, it is possible to secure the pipe expansion rate of 25% or more when the pipe made of the ferritic stainless steel according to the disclosed embodiment is piped. .
[107]
[108]
Hereinafter, the present invention will be described in more detail through examples.
[109]
For the various alloy component ranges shown in Table 1 below, after casting an ingot having a thickness of 120 mm through ingot melting, hot rolling was performed at a temperature of 1,150° C. to prepare a hot-rolled steel sheet having a thickness of 3.0 mm. Thereafter, a cold rolled steel sheet having a thickness of 1.2 mm was manufactured through cold rolling, and then cold rolling annealing was performed at a temperature of 1,100° C. for 1 minute.
[110]
After that, the cold rolled annealed steel sheet was immersed at a molten salt temperature of 400°C for 5 seconds, then immersed in a nitric acid solution at 60°C for about 10 seconds, and then cold rolled and pickled to prepare a final cold-rolled pickling steel sheet. At this time, the concentration of the nitric acid solution was maintained at 110 g/L.
[111]
Table 1 below shows the alloy composition (weight %) and the values ​​of Equation (1) and Equation (2) for each experimental steel type.
[112]
[Table 1]
division Ingredients (wt%) Formula (1) Equation (2)
C Si Mn P S Cr Ti Cu N Sn
Example 1 0.007 1.020 0.200 0.022 <0.003 13.100 0.220 0.480 0.007 0.060 17.72 18.44
Example 2 0.005 1.100 0.200 0.021 <0.003 14.000 0.220 0.300 0.005 0.150 19.40 19.95
Example 3 0.006 1.190 0.200 0.020 <0.003 12.900 0.240 0.500 0.006 0.058 18.05 18.65
Example 4 0.006 1.220 0.200 0.017 <0.003 13.100 0.250 0.300 0.005 0.059 17.95 17.93
Example 5 0.005 1.190 0.200 0.018 <0.003 11.200 0.220 0.500 0.006 0.150 17.27 18.33
Example 6 0.005 1.220 0.200 0.020 <0.003 12.500 0.220 0.450 0.006 0.150 18.56 19.44
Example 7 0.006 1.200 0.210 0.020 <0.003 13.500 0.220 0.480 0.006 0.098 19.04 19.77
Comparative Example 1 0.008 0.500 0.280 0.018 <0.003 11.500 0.220 0.014 0.005 0.000 13.03 12.57
Comparative Example 2 0.007 0.180 0.190 0.019 <0.003 17.300 0.310 0.013 0.005 0.000 17.87 17.73
Comparative Example 3 0.003 0.000 0.480 0.022 <0.003 10.500 0.287 0.150 0.007 0.000 10.80 11.25
Comparative Example 4 0.003 0.000 0.300 0.021 <0.003 9.500 0.211 0.210 0.005 0.000 9.92 10.55
Comparative Example 5 0.003 0.000 0.480 0.022 <0.003 10.500 0.287 0.000 0.007 0.080 11.30 11.70
Comparative Example 6 0.003 0.000 0.300 0.021 <0.003 9.500 0.211 0.000 0.005 0.120 10.70 11.30
Comparative Example 7 0.003 1.200 0.480 0.022 <0.003 10.500 0.287 0.000 0.007 0.000 14.10 12.90
Comparative Example 8 0.003 0.950 0.300 0.021 <0.003 9.500 0.211 0.000 0.005 0.000 12.35 11.40
Comparative Example 9 0.003 0.350 0.480 0.022 <0.003 12.610 0.287 0.250 0.007 0.050 14.66 15.31
Comparative Example 10 0.003 0.340 0.300 0.021 <0.003 11.100 0.211 0.000 0.005 0.210 14.22 14.93
Comparative Example 11 0.002 0.850 0.310 0.020 <0.003 12.020 0.045 0.000 0.006 0.210 16.67 16.87
Comparative Example 12 0.003 0.210 0.300 0.017 <0.003 14.000 0.211 0.300 0.005 0.000 15.23 15.92
Comparative Example 13 0.003 0.150 0.150 0.018 <0.003 13.010 0.185 0.000 0.006 0.100 14.46 14.81
Comparative Example 14 0.003 0.000 0.260 0.020 <0.003 13.200 0.101 0.210 0.005 0.090 14.52 15.60
Comparative Example 15 0.003 0.000 0.450 0.020 <0.003 13.810 0.211 0.310 0.006 0.080 15.23 16.56
Comparative Example 16 0.002 0.000 0.320 0.021 <0.003 15.200 0.188 0.120 0.005 0.050 15.94 16.55
[113]
The corrosion depth of each was measured by simulating the outer surface corrosion caused by snow removal salt or sea salt and the inner surface corrosion environment caused by condensate water.
[114]
For the external corrosion test, each Example and Comparative Example specimen size was cut to a size of 150 * 70 mm, oil and the like existing on the surface were removed with caustic soda, and then heat treatment was performed in a heat treatment furnace maintained at 400 ° C. for about 24 hours.
[115]
Afterwards, a combined cycle corrosion test was performed. Specifically, after spraying each specimen with a 5% NaCl solution at 30 ° C. for 2 hours, drying for about 4 hours in an atmosphere of 25% relative humidity and 60 ° C., and 2 hours in an atmosphere of 90% relative humidity and 50 ° C. A corrosion test was performed by repeating 100 cycles as one cycle for maintaining time. Then, each specimen was immersed in a 60% nitric acid solution to remove the oxidized scale, and the corrosion depth was measured. The corrosion depth was measured by selecting the 10 deepest parts of each specimen with the naked eye, and then calculating the average value.
[116]
For the internal corrosion test, a pretreatment process was performed in which the specimen size of each Example and Comparative Example was cut to a size of 40 * 70 mm, and maintained in an electric furnace maintained at 400° C. for about 24 hours.
[117]
Then, Cl - concentration is 50ppm, SO4 2 - concentration is 100ppm, HCl, H 2 SO 4 solution of the condensed water simulation environment in which the pH is maintained at 8.0 was prepared. At this time, the pH was adjusted to 8.0 using an NH3 solution. Thereafter, 10 mL of the test solution was injected into each specimen every 6 hours, and a corrosion test repeated 100 cycles was performed.
[118]
On the other hand, the grain size of the central region corresponding to half the thickness and the region corresponding to a depth of 100 μm or less from the surface is etched and measured using an optical microscope, and the average grain size of the surface region and the average grain size of the central region The ratio (Gs/Gc) and the average grain size of the surface area are shown in Table 2 below.
[119]
[Table 2]
Formula (1) Depth of external corrosion Equation (2) inner corrosion depth Gs/Gc Gs (μm)
Example 1 17.72 0.99 18.44 2.06 0.95 35
Example 2 19.40 0.72 19.95 1.75 1.05 41
Example 3 18.05 0.82 18.65 2.1 1.04 32
Example 4 17.95 0.91 17.93 1.88 1.05 41
Example 5 17.27 1.00 18.33 2.39 1.13 29
Example 6 18.56 0.82 19.44 1.62 1.25 45
Example 7 19.04 0.69 19.77 1.8 1.23 29
Comparative Example 1 13.03 2.50 12.57 4.14 1.25 30
Comparative Example 2 17.87 1.00 17.73 2.5 1.15 25
Comparative Example 3 10.80 3.01 11.25 5.01 2.54 65
Comparative Example 4 9.92 3.47 10.55 5.13 3.45 51
Comparative Example 5 11.30 2.78 11.70 4.32 5.42 60
Comparative Example 6 10.70 3.40 11.30 5.11 9.56 51
Comparative Example 7 14.10 2.34 12.90 4.1 13.25 62
Comparative Example 8 12.35 3.09 11.40 4.43 3.45 68
Comparative Example 9 14.66 1.85 15.31 3.76 6.75 63
Comparative Example 10 14.22 1.65 14.93 3.13 10.25 52
Comparative Example 11 16.67 1.49 16.87 2.01 13.25 62
Comparative Example 12 15.23 1.45 15.92 2.86 18.96 72
Comparative Example 13 14.46 1.85 14.81 3.5 12.45 85
Comparative Example 14 14.52 1.95 15.60 3.05 9.85 62
Comparative Example 15 15.23 1.28 16.56 2.6 13.25 72
Comparative Example 16 15.94 1.42 16.55 2.83 16.25 100
[120]
In Tables 1 and 2, Comparative Examples 1 and 2 correspond to STS 409 steel with 11% Cr and STS 439 steel with 18% Cr, respectively, which are generally used as materials for exhaust system of automobiles.
[121]
2 is a graph showing the corrosion resistance evaluation results according to the external corrosion index defined as Cr+3Si+10Sn+2Cu in an automobile exhaust system environment.
[122]
Referring to FIG. 2 , it can be confirmed that the outer surface corrosion depth decreases linearly as the outer surface corrosion index increases, and in the case of Examples 1 to 7 in which the outer surface corrosion resistance index expressed by Equation (1) is 17 or more, corrosion With a depth of 1.0mm or less, it was possible to secure external corrosion resistance equivalent to or higher than that of STS 439 steel.
[123]
3 is a graph showing the corrosion resistance evaluation results according to the internal corrosion index defined as Cr+2Si+15Sn+5Cu in a condensed water environment of an automobile exhaust system.
[124]
Referring to FIG. 3 , it can be seen that the inner surface corrosion depth decreases linearly as the inner surface corrosion index increases, and in Examples 1 to 7, in which the inner surface corrosion resistance index expressed by Equation (2) is 17 or more, corrosion With a depth of 2.5 mm or less, it was possible to secure the corrosion resistance of the inner surface equivalent to or higher than that of STS 439 steel.
[125]
4 is a view showing the scale structure after cold rolling annealing of Example 2. 5 is a view showing the scale structure after cold rolling annealing of Comparative Example 12.
[126]
Referring to FIGS. 4 and 5 , in the case of Comparative Example 12 that does not include Sn, an SiO 2 annealing scale after cold rolling annealing is formed entirely on the surface in an annular shape. On the other hand, in the case of Example 2 containing Sn content of 0.05% or more, for example, 0.15%, the SiO 2 annealing scale is not formed in an annular shape on the surface, and is uniform as a very thin layer. is formed. Therefore, even if hydrofluoric acid is not added during cold rolling annealing pickling, sufficient cold rolling pickling effect can be obtained.
[127]
6 is a photograph showing the surface state of Example 2 after cold rolling pickling through neutral salt electrolysis and sulfuric acid electrolysis, and the surface state after evaluation of corrosion resistance. 7 is a photograph showing the surface state of Example 2 after cold rolling pickling through neutral salt electrolysis, sulfuric acid electrolysis, and mixed acid (nitric acid + hydrofluoric acid) immersion and after corrosion resistance evaluation.
[128]
Corrosion resistance was evaluated using a combined cycle corrosion tester. The combined cycle corrosion test conditions are salt spray (5% NaCl solution sprayed at 30°C for 2 hours), dry (4 hours drying at 25% relative humidity and temperature 60°C), and wet (90% relative humidity, 50°C wet for 2 hours). In this condition, the corrosion resistance was evaluated by observing a photograph of the surface of the specimen after repeating 5 cycles.
[129]
Referring to FIG. 7( a ), when cold rolling pickling of a nitrogen/hydrofluoric acid mixed immersion condition is introduced, it can be confirmed that a large number of pits in which the base material is dissolved on the surface are generated as hydrofluoric acid is used. In addition, referring to FIG. 7( b ), it can be confirmed that a large number of rusting occurs due to the influence of the pits formed on the surface.
[130]
On the other hand, referring to FIG. 6( a ), when cold rolling pickling of neutral salt electrolysis-sulfuric acid electrolysis conditions, omitting the mixed acid immersion process, was introduced, a uniform stainless steel surface was obtained without pits being observed. In addition, referring to FIG. 6(b), it can be seen that the occurrence of rusting is small and the timing of occurrence of rusting is also delayed.
[131]
That is, in the ferritic stainless steel cold-rolled annealing steel sheet according to an embodiment of the present invention, it is possible to completely remove the cold-rolled annealing scale through neutral salt electrolysis and sulfuric acid electrolysis. Even if the mixed acid process is not carried out at the time of pickling, which is late compared to the above, not only can a sufficient cold rolling pickling effect be obtained, but also surface properties can be secured, thereby reducing the process cost.
[132]
On the other hand, as the cold rolling annealing temperature of Example 2 and Comparative Example 12 changed from 900 to 1,030 ° C., the ratio of the average grain size of the surface area to the average grain size of the central area in the thickness direction in the rolling direction TD plane (Gs/ Gc), elongation, and whether cracks occurred during pipe expansion of 25% or more are shown in Table 3 below.
[133]
The elongation was measured according to JIS 2241 by processing the elongation value in the direction perpendicular to the rolling direction to JIS 13B size. When making a pipe, a 25% expansion rate was given to check whether cracks occurred.
[134]
[Table 3]
Example 2 Annealing temperature (℃) Gs/Gc Elongation (%) No cracks after 25% of pipe expansion
900 0.95 32.5 X
930 0.91 33.5 X
950 1.0 32.5 X
970 1.2 32.5 X
990 1.05 33.4 X
1030 1.5 33.5 X
Comparative Example 12 900 0.98 35.6 O
930 8.95 34.1 O
950 12.56 34.5 O
970 15.89 35.1 O
990 18.96 35.5 O
1030 20.56 35.4 O
[135]
8 is a photograph observing the microstructure according to the change in the cold rolling annealing temperature of Example 2, and FIG. 9 is a photograph observing the microstructure according to the change in the cold rolling annealing temperature of Comparative Example 12.
[136]
Referring to FIGS. 8 and 9 , in the case of Comparative Example 12, it can be seen that the crystal grain size of the surface layer rapidly increases from 930° C. or higher. On the other hand, in the case of Example 2, there is no abrupt change in the grain size of the surface layer up to 1,030° C., and uniform grain size distribution is shown in the surface layer part and the center part.
[137]
Referring to Table 3, in the case of Example 2, the elongation value was 32 to 33%, and was measured relatively 1 to 2% lower than that of Comparative Example 12. In the case of Example 2, it is determined that the Si content is higher than 1%, which is due to the occurrence of work hardening.
[138]
In general, if the elongation is excellent, the expansion rate is accordingly high.
[139]
However, when a cold-rolled annealed steel sheet is piped into a pipe and expanded by 25% or more, in the case of Comparative Example 12, the crystal grain sizes of the surface layer and the center are non-uniformly distributed, so it can be confirmed that cracks occur during pipe expansion. .
[140]
On the other hand, in Example 2, by adding 1.0% or more of Si, and controlling the ratio of the average grain size of the surface region to the average grain size of the central region to 1.5 or less, the occurrence of cracks was suppressed.
[141]
As described above, according to the disclosed embodiment, it is possible to manufacture a ferritic stainless steel with improved pipe expansion workability while securing not only condensate corrosion but also external corrosion resistance by controlling alloy components and component relational expressions.
[142]
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 fall within the scope and concept of the following claims. It will be understood that various changes and modifications are possible.
Industrial Applicability
[143]
The ferritic stainless steel according to the present invention can secure STS439-level resistance to external corrosion and internal condensate corrosion while improving pipe expansion processability, so it can be applied to materials for automobile exhaust systems.
Claims
[Claim 1]
By weight%, C: 0.01% or less (excluding 0), N: 0.01% or less (excluding 0), Si: 1.0 to 2.0%, Mn: 0.5% or less (excluding 0), Cr: 9.0 to 15.0% , Ti: 0.1 to 0.5%, Sn: 0.05 to 0.2%, Cu: 1.0% or less (excluding 0), P: 0.035% or less (excluding 0), S: 0.01% or less (excluding 0), remaining Fe and unavoidable impurities, and the ratio (Gs/Gc) of the average grain size (Gs) of the region corresponding to a depth of 100 μm or less from the surface and the average grain size (Gc) of the central region is 1.5 or less, and the following formula ( Low-Cr ferritic stainless steel with improved pipe expansion processability that satisfies 1). Formula (1): Cr+3Si+10Sn+2Cu ≥ 17 (here, Cr, Si, Sn, and Cu mean the content (wt%) of each element)
[Claim 2]
The low-Cr ferritic stainless steel according to claim 1, wherein the pipe expansion workability satisfying the following formula (2) is improved. Formula (2): Cr+2Si+15Sn+5Cu ≥ 17 (here, Cr, Si, Sn, and Cu mean the content (wt%) of each element)
[Claim 3]
The low-Cr ferritic stainless steel according to claim 1, wherein the pipe expansion workability is improved at a pipe expansion ratio of 25% or more defined by the following formula (3). Equation (3): (D f -D 0 )/D 0 *100 (here, D f is the hole length of the machined part after forming, and D 0 is the initial machined hole length.)
[Claim 4]
The low-Cr ferritic stainless steel according to claim 1, wherein the elongation in the vertical direction of the rolling direction is 30% or more and the pipe expansion workability is improved.
[Claim 5]
The low-Cr ferritic stainless steel with improved pipe-expanding workability according to claim 1, wherein the average grain size of a region corresponding to a depth of 100 μm or less from the surface is 50 μm or less.
[Claim 6]
By weight%, C: 0.01% or less (excluding 0), N: 0.01% or less (excluding 0), Si: 1.0 to 2.0%, Mn: 0.5% or less (excluding 0), Cr: 9.0 to 15.0% , Ti: 0.1 to 0.5%, Sn: 0.05 to 0.2%, Cu: 1.0% or less (excluding 0), P: 0.035% or less (excluding 0), S: 0.01% or less (excluding 0), remaining Fe and hot-rolling a slab that contains unavoidable impurities and satisfies the following formula (1); cold rolling and cold rolling annealing; and cold rolling pickling through neutral salt electrolysis and sulfuric acid electrolysis. Formula (1): Cr+3Si+10Sn+2Cu ≥ 17 (here, Cr, Si, Sn, and Cu mean the content (wt%) of each element)
[Claim 7]
[Claim 7] The method of claim 6, wherein the slab satisfies the following formula (2), wherein the pipe expansion workability is improved. Formula (2): Cr+2Si+15Sn+5Cu ≥ 17 (here, Cr, Si, Sn, and Cu mean the content (wt%) of each element)
[Claim 8]
[Claim 7] The method of claim 6, wherein the slab is hot-rolled at a temperature of 1,020 to 1,180 °C.
[Claim 9]
The method for producing a low-Cr ferritic stainless steel according to claim 6, wherein the pipe expansion workability is improved by cold rolling annealing at a temperature range of 900 to 1,100°C.