Title of invention: Ferritic steel sheet for exhaust system with excellent corrosion resistance
technical field
[One]
The present invention relates to a ferritic steel sheet for an exhaust system, and more particularly, to a ferritic steel sheet having excellent corrosion resistance and oxidation resistance suitable for an exhaust system.
background
[2]
Exhaust systems of automobiles and two-wheeled vehicles are exposed to the outside and are in an environment that can be easily corroded from contamination by snow removal salts in winter, and it is also an environment that can be easily corroded by acidic condensate generated from exhaust of fossil fuels.
[3]
In an environment where the exhaust gas temperature is increasing, the materials used in the exhaust system to prevent corrosion have been cast with high heat capacity and stainless steel with low heat capacity. In particular, ferritic stainless steels, which contain less expensive alloying elements compared to austenitic stainless steels, have excellent corrosion resistance and high price competitiveness. It has been mainly used for manifolds, collector cones, etc.).
[4]
The most common method for securing corrosion resistance and oxidation resistance is to use stainless steel with an increased Cr content, but ferritic stainless steel containing at least 11% by weight of Cr is expensive. In addition, stainless steel has high Cr content, so it is difficult to pickling and pickling cost is high, and since it contains a large amount of Nb, etc., the cold rolling annealing temperature must also be increased. Accordingly, there is a need for a steel sheet for an exhaust system that reduces Cr that causes a price increase and secures excellent corrosion resistance.
DETAILED DESCRIPTION OF THE INVENTION
technical challenge
[5]
Embodiments of the present invention are intended to provide a ferritic steel sheet having excellent corrosion resistance for an exhaust system while reducing Cr, which is an expensive element.
means of solving the problem
[6]
The ferritic steel sheet for an exhaust system having excellent corrosion resistance according to an embodiment of the present invention is, by weight, C: 0.02% or less, N: 0.02% or less, Si: 2.0% or less, Mn: 0.5% or less, Cr: 3.0 to 5.5%, Ti: 0.001 to 0.3%, Al: 1.0 to 4.0%, remaining Fe and unavoidable impurities, having a surface scale layer, and having an Al film index of 15.0 or more and a Si film index of 3.0 or less, which are defined as follows Satisfies.
[7]
[Al film index]: the maximum value of Al content (wt%) in the 0.2㎛ depth range from the surface
[8]
[Si film index]: the maximum value of the Si content (wt%) in the 0.2㎛ depth range from the surface
[9]
In addition, according to an embodiment of the present invention, the content of each element Al, Cr, Si may satisfy the following formula (1).
[10]
(1) 5*Al - (Cr+Si) > 0
[11]
In addition, according to an embodiment of the present invention, the corrosion wear rate represented by the following formula (2) may be less than 20%.
[12]
(2) Corrosion wear rate (%) = [(weight before corrosion test) - (weight after corrosion test)]/(weight before corrosion test) X 100
[13]
Here, the weight after the corrosion test is the weight (g) after removing the corrosion products generated after the corrosion test.
[14]
In addition, according to an embodiment of the present invention, the L* value of the L*a*b* color system of the surface may be 50 or more.
[15]
In addition, according to an embodiment of the present invention, the a* value of the L*a*b* color system of the surface may be in the range of -10 to +10 and the b* value may be in the range of -10 to +10.
Effects of the Invention
[16]
The ferritic steel sheet according to the embodiment of the present invention can significantly reduce the raw material cost and process cost, as well as exhibit excellent corrosion resistance, compared to the conventional stainless steel used for the exhaust system.
[17]
In addition, even without the final pickling process, the L* value of the L*a*b* color system is 50 or more, and the a* and b* values ​​show bright achromatic metallic properties in the range of -10 to +10, resulting in excellent surface properties.
Brief description of the drawing
[18]
1 is a color space (COLOR SPACE) representing the L*a*b* color space.
[19]
2 is an alloy component distribution analyzed by glow discharge spectroscopy for a range of 0.2 μm in the depth direction from the surface of Inventive Steel 2 according to the present invention.
[20]
3 is an alloy component distribution analyzed by glow discharge spectroscopy for a range of 0.2 μm in the depth direction from the surface of Comparative Steel 5 according to the present invention.
[21]
4 is an alloy component distribution analyzed by glow discharge spectroscopy for a range of 0.2 μm in the depth direction from the surface after pickling of Inventive Steel 2 according to the present invention.
[22]
5 is a photograph showing the surface of a cold rolled annealed steel sheet specimen of comparative steel 10 according to the present invention.
[23]
6 is a photograph showing the surface of the cold rolled annealed steel sheet specimen of Inventive Steel 2 according to the present invention.
Best mode for carrying out the invention
[24]
The ferritic steel sheet for an exhaust system having excellent corrosion resistance according to an embodiment of the present invention is, by weight, C: 0.02% or less, N: 0.02% or less, Si: 2.0% or less, Mn: 0.5% or less, Cr: 3.0 to 5.5%, Ti: 0.001 to 0.3%, Al: 1.0 to 4.0%, remaining Fe and unavoidable impurities, having a surface scale layer, and having an Al film index of 15.0 or more and a Si film index of 3.0 or less, which are defined as follows Satisfies.
[25]
[Al film index]: the maximum value of Al content (wt%) in the 0.2㎛ depth range from the surface
[26]
[Si film index]: the maximum value of the Si content (wt%) in the 0.2㎛ depth range from the surface
Modes for carrying out the invention
[27]
Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings. The following examples are presented to sufficiently convey the spirit of the present invention to those of ordinary skill in the art to which the present invention pertains. The present invention is not limited to the embodiments presented herein, and may be embodied in other forms. The drawings may omit the illustration of parts not related to the description in order to clarify the present invention, and slightly exaggerate the size of the components to help understanding.
[28]
Also, when a part "includes" a component, it means that other components may be further included, rather than excluding other components, unless otherwise stated.
[29]
The singular expression includes the plural expression unless the context clearly dictates otherwise.
[30]
To stainless steel for exhaust systems, especially ferritic stainless steel, Nb is added or replaced with Sn to improve high-temperature strength, and it is common to increase Cr content to improve oxidation resistance. However, the addition of solid-solution strengthening elements such as Nb and Sn and an increase in the Cr content cause an increase in manufacturing cost, which is not a desirable development direction.
[31]
In order to reduce the raw material cost of ferritic stainless steel for exhaust system, it is essential to reduce the content of Cr, which is an expensive element with a relatively high content. However, since Cr is a key element for securing corrosion resistance in ferritic stainless steel for exhaust system, another method for securing corrosion resistance is required to reduce Cr. An object of the present invention is to provide a ferritic steel sheet having a Cr content lower than 11% by weight, which is the minimum Cr content as stainless steel, and reducing the raw material cost by securing corrosion resistance equivalent to or higher than that of conventional stainless steel.
[32]
The ferritic steel sheet for an exhaust system having excellent corrosion resistance according to an embodiment of the present invention is, by weight, C: 0.02% or less, N: 0.02% or less, Si: 2.0% or less, Mn: 0.5% or less, Cr: 3.0 to 5.5%, Ti: 0.001 to 0.3%, Al: 1.0 to 4.0%, remaining Fe and unavoidable impurities.
[33]
Hereinafter, the reason for numerical limitation of the alloying element content in the embodiment of the present invention will be described. Hereinafter, unless otherwise specified, the unit is % by weight.
[34]
The content of C is greater than 0 and not more than 0.02%.
[35]
If the C content exceeds 0.02%, the toughness of the weld joint may be reduced, and Cr 23 C 6 precipitates are generated by combining with Cr, thereby reducing corrosion resistance and oxidation resistance due to local depletion of Cr in the matrix. On the other hand, C is included in excess of 0 as an unavoidable impurity, and may preferably be included in an amount of 0.005% or more as the steelmaking VOD process ratio increases in order to control the extremely low content.
[36]
The content of N is greater than 0 and not more than 0.02%.
[37]
When N in the steel exceeds 0.02%, the concentration of dissolved N reaches a limit, combines with Cr to form Cr 2 N precipitates, and corrosion resistance and oxidation resistance are lowered due to local depletion of Cr in the matrix. On the other hand, N is included in excess of 0 as an unavoidable impurity, and may preferably be included in an amount of 0.005% or more as the steelmaking VOD process ratio increases in order to control the extremely low content.
[38]
The content of Si is 2.0% or less.
[39]
Si is a solid solution strengthening element and at the same time forms a Si-enriched oxide film on the surface layer to increase oxidation resistance. However, in the present invention, the Si film index after annealing heat treatment should be limited to 3.0 or less in order to implement 'omission of pickling', which will be described later, and for this purpose, the total content is limited to 2.0% or less. However, from the viewpoint of more easily controlling discoloration prevention, it may be included in an amount of 1.5% or less, and may be included in an amount of 1.0% or less.
[40]
The content of Mn is 0.5% or less.
[41]
Mn is an impurity that is unavoidably included in steel and serves to stabilize austenite. When the Mn content exceeds 0.5%, reverse austenite transformation occurs during annealing heat treatment after hot rolling or cold rolling, which adversely affects elongation. Therefore, the content of Mn is limited as above.
[42]
The content of Cr is 3.0 to 5.5%.
[43]
Cr is an element that improves corrosion resistance, but is limited to 5.5% or less according to the purpose of the present invention for reducing raw material cost. However, 3.0% or more is added to ensure minimal corrosion resistance.
[44]
The content of Ti is 0.001 to 0.3%.
[45]
Ti is combined with C and N to form Ti(C,N) precipitates to reduce the amount of solid solution C and N and suppress the formation of a Cr depleted layer. should be Ti combines with C and N to form Ti(C,N) precipitates, thereby lowering the amount of solid solution C and N, and suppressing the formation of a Cr depleted layer. However, if the Ti content is excessive, it adversely affects the casting, so it is limited to 0.3% or less.
[46]
The content of Al is 1.0 to 4.0%.
[47]
In the present invention, 1.0% or more of Al is sufficiently added to form an oxide film during annealing heat treatment. However, if an excessive amount is added, casting and rolling may become difficult, so the upper limit is limited to 4.0% or less.
[48]
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 the impurities are known to any person skilled in the art of a conventional manufacturing process, all details thereof are not specifically mentioned in the present specification.
[49]
However, the above-described alloy component system alone is insufficient to ensure corrosion resistance. According to the review by the present inventors, if the Cr content is reduced to reduce the raw material cost, there is a problem in that corrosion resistance is extremely weak, such as corrosion occurs when exposed to the outside. Therefore, in the present invention, a special method was introduced to secure corrosion resistance.
[50]
Stainless steel cold-rolled steel sheet for exhaust system is generally shipped after cold rolling, annealing heat treatment for softening, and then pickling treatment to remove scale from the surface. In the present invention, in the annealing heat treatment of the cold-rolled steel sheet having the above alloy composition, after annealing to have a surface satisfying the Al film index and Si film index ranges defined as follows, the final product is manufactured without pickling treatment. That is, the ferritic steel sheet according to the present invention is a cold rolled annealed steel sheet, and has a scale layer on the surface.
[51]
Conventionally, the scale layer has been an object of avoidance due to the high content of Fe, which is unfavorable to corrosion resistance, but in the present invention, it has the meaning of intentionally including an Al-enriched oxide film advantageous for corrosion resistance. By controlling the content of Al and Si that are concentrated and oxidized to the surface layer through annealing heat treatment, corrosion resistance and oxidation resistance equivalent to or higher than that of stainless steel can be secured even in a ferritic steel sheet containing 3.0 to 5.5% Cr.
[52]
The ferritic steel sheet for exhaust system according to an embodiment of the present invention satisfies an Al film index of 15.0 or more and a Si film index of 3.0 or less in a depth range of 0.2 μm including the scale layer in the depth direction from the surface. Al film index and Si film index are defined as follows.
[53]
[Al film index]: the maximum value of Al content (wt%) in the 0.2㎛ depth range from the surface
[54]
[Si film index]: the maximum value of the Si content (wt%) in the 0.2㎛ depth range from the surface
[55]
In general, it is known that Si forms a Si-enriched oxide film on the surface layer to increase high-temperature oxidation resistance. However, in the present invention that does not carry out pickling, when the Si film index exceeds 3.0, a dark brown scale layer is formed on the surface to deteriorate the surface properties, and thus the Si film index must be limited to 3.0 or less.
[56]
Al also reacts with oxygen in the surface layer to form a non-uniform oxide layer. When annealing heat treatment is performed after adding Al content of 1.0 to 4.0% according to the present invention, it prevents movement and reaction of Si to the surface layer and preferentially Al A thickened oxide film is formed. When the Al film index is 15.0 or more because the Al oxide film is densely formed, a bright metallic color can be exhibited.
[57]
The metallic color of the material surface can be expressed by the L*a*b* color system established by the International Lighting Commission. The L*a*b* color space system is currently the most popular color space system used in all fields to express the color of an object, and FIG. 1 shows a color space representing the L*a*b* color space system. In this case, L* represents black when 0 and white strongly when 100, a* represents red when positive and green when negative, and b* represents yellow when positive. Direction, when negative, indicates the blue direction. If a* and b* are both 0, the color becomes achromatic.
[58]
According to an embodiment of the present invention, a bright metallic surface having an L* value of 50 or more in the L*a*b* color system can be obtained by forming an Al-enriched oxide film. It is also possible to obtain a metallic surface with a bright achromatic color in the range of -10 to +10 at the same time as the a* value and the b* value with an L* value of 50 or higher.
[59]
2 to 4 are alloy component distributions analyzed by glow discharge spectroscopy from the surface to 0.2 μm in the depth direction in the embodiments according to the present invention.
[60]
Figure 2 shows the distribution of alloy components of a specimen that is not subjected to pickling after annealing heat treatment of a cold-rolled steel sheet according to an embodiment of the present invention. Among the measured values ​​in the depth direction, the Al film index, which is the maximum value of the Al content, is 15.0 or more.
[61]
3 shows the alloy composition distribution of a cold-rolled steel sheet having the same content range of Si and Al as that of ferritic stainless steel for general exhaust system and lowering only the Cr content in order to reduce raw material cost, and annealing heat treatment followed by no pickling. That is, the content range of Cr and Al corresponds to the cold rolled annealing steel sheet out of the composition range of the present invention. Referring to FIG. 3, the Al film index is low and the Si film index is close to 5.0 in the outermost layer. In this case, corrosion resistance and oxidation resistance were insufficient, and surface discoloration also occurred due to the Si oxide film.
[62]
4 shows the alloy composition distribution after annealing the cold-rolled steel sheet according to the embodiment of the present invention on the same alloy composition specimen as in FIG. 2, and then performing up to pickling. Even if the same content of Cr, Al, and Si are included, the Al film index is low when the pickling is not omitted after the annealing heat treatment provided in the present invention.
[63]
Further, according to an embodiment of the present invention. In order to simultaneously satisfy the Al film index and the Si film index, the ferritic steel sheet may satisfy the following formula (1).
[64]
(1) 5*Al - (Cr+Si) > 0
[65]
When Al is sufficiently contained as in Formula (1), a sufficient Al-enriched oxide film can be formed during annealing. On the other hand, if this is not the case, oxygen to form an Al-enriched oxide film may become insufficient due to oxidation of Cr and Si, or movement of oxygen required to form some Al-enriched oxide film may be restricted due to the formation of Cr or Si oxide film. do.
[66]
On the other hand, the thickness of the scale layer may appear differently depending on the annealing heat treatment temperature and time, but in the present invention, it may be defined as the thickness at the point where the Al film index becomes half. For example, the scale layer thickness in FIG. 1 may be about 0.1 μm, which corresponds to the median value of the Al film index, which is the maximum value of the Al content.
[67]
The annealing heat treatment to satisfy the Al film index and the Si film index according to the present invention does not require the use of an expensive bright annealing (BAL) process that uses 75% or more of high-purity hydrogen in the atmosphere gas, and continuous use of inexpensive gas It is possible by going through an annealing process. For example, it is possible to achieve the object of the present invention by using a fuel gas as a heat source and limiting the excess oxygen of the waste gas to 0.1 to 10%.
[68]
By providing oxygen with an excess oxygen of 0.1% or more, Al according to the content range of the present invention reacts with oxygen during annealing heat treatment to form a film providing high corrosion resistance. If excess oxygen is insufficient, a sufficient Al-enriched oxide film may not be formed. On the other hand, if the excess oxygen exceeds 10%, Fe, Cr, or Si and oxygen react with the material to form an oxide film of Fe, Cr, and Si in addition to an Al-enriched oxide film, which is inappropriate because discoloration may occur in this case.
[69]
On the other hand, if you want to limit oxygen to 0.1% or less for easy production, when hydrogen in the atmosphere gas is mixed in the range of 0.1% to 10%, oxidation with Fe, Cr, and Si is suppressed, so that a small amount of 0.1% or less Al-enriched oxide film can be formed even with oxygen. Mixing at 10% or more is unnecessary because it causes an increase in cost as described above, and hydrogen of less than 0.1% has insufficient ability to suppress oxidation of Fe, Cr, and Si, resulting in insufficient formation of an Al-enriched oxide film. .
[70]
After annealing heat treatment, pickling should be omitted. By not performing pickling, it is possible to obtain an outermost layer that satisfies the Al and Si film index and does not remove the scale layer, and the manufacturing cost can also be reduced by omitting the pickling process using a mixed acid solution of nitric acid and/or hydrofluoric acid.
[71]
Cold-rolled steel sheet before annealing heat treatment and pickling is omitted can be manufactured through a conventional manufacturing process, for example, hot-rolling a slab containing the above-described alloy component composition, annealing the hot-rolled hot-rolled steel sheet, and pickling after It can be manufactured into cold rolled steel sheet by cold rolling.
[72]
The ferritic steel sheet for an exhaust system having excellent corrosion resistance according to an embodiment of the present invention may have a corrosion wear rate of less than 20%, which is expressed by the following formula (2).
[73]
(2) Corrosion wear rate (%) = [(weight before corrosion test) - (weight after corrosion test)]/(weight before corrosion test) X 100
[74]
Here, the weight after the corrosion test is the weight (g) after removing the corrosion products generated after the corrosion test.
[75]
Corrosion resistance, that is, resistance to corrosion, can be known by exposure to an arbitrarily created corrosive environment. For example, a solution containing NaCl in water is sprayed on the material so that it is 5% by volume, maintained for the next 4 hours, and the process of drying by heating at about 60° C. for 4 hours is repeated 30 times in total to corrode by the method described later degree can be assessed. Since the evaluation environment can be configured in various ways, the present invention is not limited thereto.
[76]
In the present invention, '[(weight before corrosion test) - (weight after corrosion test)]/(weight before corrosion test)' is defined as the rate of wear loss, multiplied by 100 and expressed in % units. In this way, the weight after removing the corrosion products generated after the corrosion test, that is, the 'weight after the corrosion test' is measured and compared with the 'weight before the corrosion test' to measure the wear loss. If the wear rate is not easy in the sense that the removal of corrosion products is required, thickness can be substituted for weight. In this case, there is no need to remove corrosion products, and the thickness of the base metal part excluding corrosion products can be compared by observing the cross section with an optical microscope.
[77]
In the steel containing 11% Cr to be replaced in the present invention, if Al is added at the same time by 1.0% or more, workability may deteriorate. The same is true of Si, and this phenomenon is because not only Al and Si, but also Cr substitutes Fe and atomic positions to inhibit elongation, which is a representative index of workability. On the other hand, when the formula (1) presented by the present invention is satisfied, it is possible to secure an elongation of 28% or more even when Al is contained in an amount of 1.0% or more. This point is an effect that can be obtained incidentally through the present invention along with the effect for the Al-enriched oxide film.
[78]
[79]
Hereinafter, it will be described in more detail through preferred embodiments of the present invention.
[80]
Example
[81]
After casting with the alloy composition system shown in Table 1 below, it was hot rolled to 3 mm. The hot rolling initiation temperature was adjusted to around 1,200° C., which is desirable in order to prevent excessive tissue growth and obtain sufficient hot workability. After surface pickling, cold rolling was performed to 1 mm, and then annealed at a temperature of 900° C. or higher for 10 seconds or longer in an atmosphere gas containing 5% of excess oxygen. Thereafter, specimens subjected to or not subjected to pickling were prepared for the invention steel and the comparative steel, respectively, and corrosion occurrence in the same environment as that of the external exposure was evaluated, and the results are shown in Table 2. The simulation of external exposure was judged by whether or not spot rust occurred on the surface after spraying a solution containing 5% NaCl in volume ratio in water and leaving it for 72 hours. Corrosion occurrence is indicated by ○, and corrosion not occurring is indicated by ×.
[82]
[Table 1]
Category (wt%) C Si Mn Al Cr Ti N Equation (1)
Comparative lecture 1 0.008 0.4 0.3 0.004 13.4 0.2 0.006 -13.8
Comparative lecture 2 0.006 0.4 0.3 0.003 11.5 0.2 0.006 -11.9
Comparative lecture 3 0.010 0.4 0.3 0.004 8.9 0.2 0.007 -9.3
Comparative lecture 4 0.008 0.4 0.3 0.002 7.1 0.2 0.007 -7.5
Comparative steel 5 0.008 0.4 0.3 0.003 5.2 0.2 0.006 -5.6
Comparative lecture 6 0.008 1.6 0.3 0.005 5.3 0.2 0.005 -6.9
Comparative lecture 7 0.009 2.6 0.3 0.007 5.6 0.2 0.006 -8.2
Comparative steel 8 0.009 3.5 0.3 0.007 4.9 0.2 0.006 -8.4
Comparative lecture 9 0.007 0.4 0.3 0.9 5.3 0.2 0.010 -1.2
Comparative Steel 10 0.007 1.6 0.3 1.3 5.2 0.2 0.007 -0.3
Invention lecture 1 0.008 0.4 0.3 1.4 5.4 0.2 0.007 +1.2
Invention lecture 2 0.008 0.5 0.3 1.8 5.2 0.2 0.006 +3.3
Invention lecture 3 0.007 0.5 0.3 3.5 5.5 0.2 0.006 +11.5
Invention lecture 4 0.007 0.3 0.3 2.4 3.2 0.1 0.006 +8.5
Invention River 5 0.007 1.6 0.3 1.8 3.2 0.1 0.006 +4.2
[83]
[Table 2]
division Whether external exposure corrosion occurs
pickling No pickling
Comparative lecture 1 × ○
Comparative lecture 2 × ○
Comparative lecture 3 ○ ○
Comparative lecture 4 ○ ○
Comparative steel 5 ○ ○
Comparative lecture 6 ○ ○
Comparative lecture 7 ○ ○
Comparative steel 8 ○ ○
Comparative lecture 9 ○ ○
Comparative Steel 10 ○ ○
Invention lecture 1 ○ ×
Invention lecture 2 ○ ×
Invention lecture 3 ○ ×
Invention lecture 4 ○ ×
Invention River 5 ○ ×
[84]
Table 2 shows that even if the alloy composition range according to the present invention is satisfied, corrosion occurs when the material subjected to pickling after annealing heat treatment is exposed to the outside. However, Comparative Steel 1 and Comparative Steel 2 were ferritic stainless steels containing a large amount of Cr, an expensive element, to reduce in the present invention, and corrosion did not occur even in an externally exposed environment. Although the invention steels according to the present invention were specimens of the same alloy composition, it was confirmed that corrosion did not occur as a result of not performing pickling after annealing heat treatment.
[85]
Table 3 below shows the Al film index and Si film index, the corrosion wear rate of the specimens not subjected to pickling, and the corrosion suitability judged based on the corrosion wear rate of 20%. A case suitable for corrosion suitability is marked with ○, and an unsuitable case is marked with ×.
[86]
Al film index and Si film index can be analyzed by glow discharge spectroscopy, which is a method widely known in the art and can be analyzed by a method similar to the glow discharge spectroscopy method commonly used in academia. However, when analyzing components according to the distance from the surface in the depth direction to sufficiently secure data, the resolution is required to be 10 nm or less.
[87]
[Table 3]
division Al film index Si film index Whether discoloration occurs corrosion wear rate Corrosion fitness
Comparative lecture 1 0 10.6 ○ 4% ○
Comparative lecture 2 0 6.9 ○ 10% ○
Comparative lecture 3 0 6.0 ○ 20% ×
Comparative lecture 4 0 3.3 ○ 28% ×
Comparative steel 5 0 4.6 ○ 41% ×
Comparative lecture 6 0.1 4.5 ○ 30% ×
Comparative lecture 7 0.2 6.4 ○ 33% ×
Comparative steel 8 0 11.3 ○ 31% ×
Comparative lecture 9 8.8 3.6 ○ 30% ×
Comparative Steel 10 23.1 3.3 ○ 12% ○
Invention lecture 1 16.2 1.1 × 18% ○
Invention lecture 2 24.2 0.9 × 17% ○
Invention lecture 3 24.4 0.8 × 12% ○
Invention lecture 4 22.1 0.2 × 13% ○
Invention River 5 23.0 1.9 × 12% ○
[88]
Comparative Steels 1 to 5 have similar contents of C, Si, Mn, Al, Ti, and N, and are only specimens in which the content of Cr is gradually reduced. Referring to Table 3, Comparative Steels 1 and 2 correspond to ferritic stainless steels having a Cr content of 11% or more, and because they have sufficient corrosion resistance, the corrosion wear rate is low and corrosion suitability is also suitable. However, as a result of not performing pickling according to the present invention, the Si film index was as high as 10.6, and surface discoloration occurred accordingly.
[89]
Comparative steels 3, 4, and 5 had a low Al content, so the Al film index was low even without pickling, and the corrosion suitability was unsuitable because of the high wear rate. In addition, although the Si content is an appropriate amount, it was found that the Si film index, which is the maximum value of Si in the oxide film including the scale layer, was high, and thus discoloration occurred. In particular, Comparative Steel 5, except for the Al content, satisfies the range of the present invention in the content of the remaining alloying elements, but when referring to Inventive Steels 1 to 3 below, it was confirmed that the Al content for securing the Al film index was insufficient when pickling was not carried out. It was found that when the Al content was included to satisfy Equation (1), it was possible to lower the Si film index and increase the Al film index as in Inventive Steels 1 to 3.
[90]
Comparative steels 6, 7, and 8 correspond to specimens with increased Si content. In general, even if the Si content, which is known to be effective for corrosion resistance and oxidation resistance, is high, in the case of not carrying out pickling, the Al content is not sufficient, so the corrosion evaluation is inappropriate, and surface discoloration has also occurred.
[91]
Comparative Steel 9 contained 0.9% Al, but did not satisfy the Al content range and Equation (1), so that the Al film index did not reach the target range, and thus the corrosion evaluation was unsuitable. It could be determined that the Al content was not sufficient to prevent the formation of the Si oxide film, and accordingly, the Si oxide film was predominantly formed and discoloration occurred.
[92]
Comparative Steel 10 contained sufficient Al content, resulting in an Al film index of 15 or more, and was suitable for corrosion evaluation, but did not satisfy Equation (1), resulting in an increase in the Si film index. Comparative steel 10 had surface discoloration, and it was found that even if the corrosion evaluation was suitable because the Al film index was satisfied, when the Si film index exceeded 3.0, it was found that the discoloration of the surface could not be suppressed.
[93]
Inventive steels 1, 2, and 3 satisfied the alloy composition range of the present invention and satisfies both the Al film index of 15.0 or more and the Si film index of 3.0 or less after not performing pickling, so that the corrosion evaluation was excellent and discoloration did not occur.
[94]
Inventive Steel 4 has a rather low Cr content within the composition range of the present invention, but by controlling the Si and Al content to satisfy Equation (1), the Al film index and the Si film index could be controlled within the target range. .
[95]
On the other hand, Inventive Steel 5 has a rather high Si content within the composition range of the present invention. could
[96]
[Table 4]
division Whether discoloration occurs L* a* b*
Comparative lecture 1 ○ 49 +12 +15
Comparative lecture 2 ○ 49 +13 +13
Comparative lecture 3 ○ 47 +14 +10
Comparative lecture 4 ○ 44 +15 +4
Comparative steel 5 ○ 40 +15 0
Comparative lecture 6 ○ 39 +14 -3
Comparative lecture 7 ○ 36 +13 -13
Comparative steel 8 ○ 34 +11 -20
Comparative lecture 9 ○ 35 0 -One
Comparative Steel 10 ○ 36 0 -15
Invention lecture 1 × 78 +5 +2
Invention lecture 2 × 79 0 +1
Invention lecture 3 × 78 0 +1
Invention lecture 4 × 78 0 +1
Invention River 5 × 78 +5 -8
[97]
Table 4 shows the L*a*b* color system values ​​of the comparative steel and the invention steel, and shows the discoloration in Table 3 in more detail. When pickling was not performed after annealing, Comparative Steels 1 to 5 exhibited a reddish scale layer due to oxidation of Cr. In addition, Comparative Steels 6 to 10 exhibited a purple or blue scale layer because the Si film could not be controlled. On the other hand, Inventive Steels 1 to 5 showed a scale layer having a bright metallic color through the manufacturing method presented by the present invention.
[98]
5 is a photograph showing the surface of a cold rolled annealed steel sheet specimen of comparative steel 10 according to the present invention. 5, it can be seen that dark brown scales are formed on the surface by annealing heat treatment like a conventional steel type.
[99]
6 is a photograph showing the surface of the cold rolled annealed steel sheet specimen of Inventive Steel 2 according to the present invention. 6, it can be seen that the specimen of Inventive Steel 2 exhibits bright metallic luster even without pickling, and the L*a*b* color system values ​​were L*: 79, a*: 0, b*: +1.
[100]
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 may not depart from the concept and scope of the claims described below. It will be appreciated that various modifications and variations are possible.
[101]
Industrial Applicability
[102]
The ferritic steel sheet for exhaust system according to the present invention can be applied as a material for exhaust system parts and the like (Muffler, Ex-manifold, Collector cone, etc.).
Claims
[Claim 1]
By weight %, C: 0.02% or less, N: 0.02% or less, Si: 2.0% or less, Mn: 0.5% or less, Cr: 3.0 to 5.5%, Ti: 0.001 to 0.3%, Al: 1.0 to 4.0%, remainder A ferritic steel sheet for exhaust system with excellent corrosion resistance that contains Fe and unavoidable impurities, has a surface scale layer, and satisfies Al film index of 15.0 or more and Si film index of 3.0 or less, which are defined as follows. [Al film index]: Maximum value of Al content in a range of 0.2 μm depth from the surface (% by weight) [Si film index]: Maximum value of Si content in a range of 0.2 μm depth from the surface (weight%)
[Claim 2]
The ferritic steel sheet for exhaust system excellent in corrosion resistance according to claim 1, which satisfies the following formula (1). (1) 5*Al - (Cr+Si) > 0 (here, Al, Cr, and Si mean the content (wt%) of each element)
[Claim 3]
The ferritic steel sheet for exhaust system excellent in corrosion resistance according to claim 1, wherein the corrosion wear loss rate represented by the following formula (2) is less than 20%. (2) Corrosion wear rate (%) = [(weight before corrosion test) - (weight after corrosion test)]/(weight before corrosion test) X 100 (here, the weight after corrosion test is calculated by removing corrosion products after corrosion test) is the weight of the back (g))
[Claim 4]
The ferritic steel sheet for exhaust system according to claim 1, wherein the L* value of the L*a*b* color system of the surface is 50 or more and the corrosion resistance is excellent.
[Claim 5]
The ferritic steel sheet for exhaust system excellent in corrosion resistance according to claim 4, wherein the a* value of the surface L*a*b* color system is in the range of -10 to +10 and the b* value is in the range of -10 to +10.