KR20190077874A - Cold-rolled steel sheet for exhaust system having excellent corrosion resistance and formability ad manufacturing method thereof - Google Patents

Abstract

The present invention relates to a cold-rolled steel sheet for an exhaust system, having excellent corrosion resistance and formability, and a manufacturing method for the same. According to an aspect of the present invention, the cold-rolled steel sheet comprises: 0.01 wt% or less of carbon (C); 0.5 wt% or less of silicon (Si); 0.05-0.5 wt% of manganese (Mn); 0.1 wt% or less of aluminum (Al); 0.01 wt% or less of phosphorous (P); 0.01 wt% or less of sulfur (S); 0.005 wt% or less of nitrogen (N); 2.0-4.0 wt% of chromium (Cr); 0.02-0.5 wt% of niobium (Nb); 0.5 wt% or less of tungsten (W); and residues containing inevitable impurities. The cold-rolled steel sheet has a composition in which the corrosion resistance index indicated by the following interaction formula 1 is 0.08 or greater. The interaction formula 1 is represented as follows: corrosion resistance index = Nb + W - (N/14 + C/12)X50.

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a cold rolled steel sheet for an exhaust system having excellent corrosion resistance and workability,

The present invention relates to a cold rolled steel sheet for an exhaust system excellent in corrosion resistance and workability and a method of manufacturing the same.

In operation of automobiles, fossil fuel is burned in the engine, and steam is generated along with toxic exhaust gas such as sulfuric acid gas and nitric acid gas, and is discharged to the exhaust system. The internal temperature of the exhaust system changes repeatedly at high and low temperatures depending on the running state. In the process of cooling to low temperature below the dew point at high temperature, condensate is generated inside the exhaust system. The condensed water thus generated is condensed with SO ₃ ^2- , SO ₄ ^2- , NO ₂ ^- , NO ₃ ^- , Cl ^- , NH ₄ ⁺ And includes ions dissolved from the exhaust gas.

The condensed water containing these ions has a wide pH range of 1 to 8 depending on the degree of drying after the formation. Accordingly, the condensed water can create an environment in which complex corrosion due to strong acid and weak acid actively progresses with respect to the exhaust system in contact with the condensed water. As a result, through corrosion of the steel sheet occurs in the exhaust system, thereby causing malfunction of the exhaust system. Therefore, the steel sheet used in the exhaust system should have excellent corrosion resistance to such a complex corrosion environment in order to prolong its service life.

In addition, since a certain amount of mechanical shaping is required to produce a desired type of exhaust system separately, a steel sheet for an exhaust system must have sufficient mechanical workability.

As the steel sheet which is usually used as a material of the exhaust system, there are an aluminum-plated steel sheet and a stainless steel sheet.

In Patent Document 1, a method of improving the corrosion resistance by hot-dip coating the steel sheet with Al to improve the corrosion resistance of the cold-rolled steel sheet has been described. The aluminum-plated steel sheet has a surface of carbon steel plated with aluminum and has strong corrosion resistance due to the Al ₂ O ₃ passivation film derived from the aluminum plating layer. Especially, it is very strong against the corrosion resistance by the salt which may occur on the outer surface of the exhaust system, and exhibits the corrosion resistance within a certain range of pH range even on the inner surface of the exhaust system. However, at low pH, the aluminum plating layer containing the passivation film is eluted and removed, and once it is removed, the corrosion resistance can no longer be exerted.

In order to overcome such a problem, Patent Document 2 and Patent Document 3 describe a method of suppressing corrosion in a low pH environment by adding Cu to a steel sheet. However, addition of Cu has a limitation because it accelerates corrosion in a high pH range.

As a method for greatly improving the corrosion resistance of a steel sheet, Patent Document 4 describes a stainless steel sheet for an exhaust system in which a large amount of various alloying elements including Cr is added. The stainless steel sheet also has corrosion resistance due to the Cr ₂ O ₃ passivation film at a certain range of pH for corrosion inside the exhaust system. When the pH is low, the passivation film is activated to lose its corrosion resistance. However, when the pH rises, the Cr employed in the steel sheet is oxidized again to passivate and recover the corrosion resistance. However, there is a disadvantage that rust easily occurs on the outer surface of the exhaust system due to salt as compared with the case of aluminum plating. In addition, expensive alloying elements are added in large quantities, resulting in low economic efficiency.

Patent Literature 5 describes a technique for manufacturing an Al-plated stainless steel sheet in order to reduce the external surface smear of the exhaust system, but this is also a technique using an expensive stainless steel sheet, which has a limit in economical efficiency.

In addition, even if sufficient corrosion resistance is ensured, the hot rolling property is poor and it may be difficult to produce a normal steel sheet.

Patent Document 1: Korean Patent Publication No. 0833050 Patent Document 2: Korean Patent Publication No. 0694697 Patent Document 3: Korean Patent Registration No. 1197955 Patent Document 4: Korean Patent Laid-Open Publication No. 2015-0140423 Patent Document 5: Korean Patent Registration No. 1485643

According to one aspect of the present invention, there can be provided a cold-rolled steel sheet which is economical and which is excellent in corrosion resistance and workability and is suitable for use as a material for an exhaust system of automobiles and the like, and a method for producing such a cold-rolled steel sheet.

According to another aspect of the present invention, there is provided a cold rolled steel sheet suitable for use as a material for an exhaust system of an automobile or the like, which is economical and excellent in corrosion resistance and workability without deteriorating the hot rolling property described above and a method for manufacturing such a cold rolled steel sheet .

The object of the present invention is not limited to the above description. Those skilled in the art will appreciate that there is no difficulty in identifying additional problems of the present invention based on the description of the present invention.

According to one aspect of the present invention, there is provided a cold rolled steel sheet comprising 0.01 to 5.0% by weight of carbon (C), 0.5 to 0.5% of silicon (Si), 0.05 to 0.5% of manganese (Mn) 0.01% or less of sulfur (P), 0.01% or less of sulfur (S), 0.005% or less of nitrogen (N), 2.0 to 4.0% of chromium (Cr), 0.015 to 0.5% of niobium (Nb) (W): 0.5% or less, the balance being inevitable impurities, and a corrosion resistance index expressed by the following relational expression 1 being 0.08 or more.

[Relation 1]

Corrosion resistance index = Nb + W - (N / 14 + C / 12) x 50

(Provided that Nb, W, N and C in the formula are the contents of the respective elements in weight% units)

At this time, the cold-rolled steel sheet may further include an Al-based plating layer on its surface.

According to one embodiment of the present invention, the cold-rolled steel sheet may have an average value (r _m ) of plastic anisotropy coefficients (r _m ) expressed by the following relational formula 2: 1.5 or more.

[Relation 2]

r _m = (r ₀ + 2 r ₄₅ + r ₉₀ ) / 4

(Where r ₀ , r ₄₅ and r ₉₀ are the plastic anisotropy coefficients in the rolling direction and 0 °, 45 ° and 90 °, respectively)

A method for manufacturing a cold-rolled steel sheet according to still another aspect of the present invention includes a step of preparing a cold-rolled steel sheet having a composition containing 0.01% or less of carbon (C), 0.5% or less of silicon (Si) 0.01% or less of sulfur (S), 0.005% or less of nitrogen (N), 2.0 to 4.0% of chromium (Cr), 0.015% of niobium (Nb) To 0.5% of tungsten (W): 0.5% or less, the balance being inevitable impurities, and having a corrosion resistance index of 0.08 or more expressed by the following relational expression 1; Hot-rolling the reheated steel slab at a temperature higher than Ar ₃ to produce a hot-rolled steel sheet; Winding the hot-rolled steel sheet at 550 to 750 ° C; Rolling the rolled hot-rolled steel sheet at a reduction ratio of 50 to 95% to produce a cold-rolled steel sheet; And annealing the cold-rolled steel sheet.

[Relation 1]

Corrosion resistance index = Nb + W - (N / 14 + C / 12) x 50

(Provided that Nb, W, N and C in the formula are the contents of the respective elements in weight% units)

At this time, the annealing step may be continuous annealing at a temperature of 600 to 900 ° C, or box annealing at a temperature of 600 to 800 ° C.

Further, the cold-rolled steel sheet manufacturing method of the present invention may further include an Al plating step.

As described above, since the cold-rolled steel sheet according to one aspect of the present invention has a composition that does not contain an expensive alloy component, it can provide a cold-rolled steel sheet that is economical and excellent in corrosion resistance and workability.

According to one aspect of the present invention, the cold-rolled steel sheet is plated and used to have excellent corrosion resistance. Even if the aluminum-plated layer is removed during use, it can still have excellent corrosion resistance in the internal corrosion environment of the exhaust system, Can be effectively overcome.

Hereinafter, the present invention will be described in detail.

The cold-rolled steel sheet for an exhaust system needs to have excellent corrosion resistance so as to have a long life, and it is required to be excellent in machinability to facilitate molding. In order to improve the corrosion resistance, a large amount of alloying elements may be added. In such a case, the cost of the material increases due to expensive alloying elements, which may result in not only lowering the economical efficiency but also lowering the workability of the steel sheet. Therefore, it is necessary to invent a method capable of simultaneously securing corrosion resistance and workability without adding a large amount of alloying elements.

The present inventors have found that a cold rolled steel sheet having the above-mentioned target properties can be produced through optimization of kinds and contents of alloying elements in order to attain the above-mentioned object, leading to the present invention.

Hereinafter, the composition of the cold-rolled steel sheet provided in the present invention will be described in detail. Unless otherwise specified, the content of each component means weight percent.

Carbon (C): Not more than 0.01%

When C is 0.01% or more, the carbon content is high, so that the ductility is lowered and the workability is greatly lowered, and the resistance to corrosion on the inner surface of the exhaust system is also lowered.

Silicon (Si): not more than 0.5%

Si is an element used as a deoxidizing agent and contributes to enhancement of strength by solid solution strengthening. The SiO ₂ oxide produced on the surface can also act to retard the corrosion of the condensate. However, if it is excessive, an Si-based oxide may be formed on the surface of the steel during annealing, which may cause defects in plating and deteriorate the plating ability. Therefore, the Si content is preferably 0.5% or less. However, even if no Si is added at all, the effect of deoxidation, strength improvement, or corrosion resistance can be obtained by other elements, so the lower limit of the Si content is not particularly limited.

Manganese (Mn): 0.05 to 0.5%

Mn is an element which prevents the hot shortness caused by the solid solution S by precipitating into MnS in combination with the solid solution solid S in the steel. In order to obtain such an effect, it is preferable that Mn is contained by 0.05% or more. However, when the Mn content exceeds 0.5%, the material is hardened and the ductility is significantly lowered, so that the upper limit of the content can be set at 0.5%.

Aluminum (Al): 0.01 to 0.1%

Al is an element having a very large deoxidizing effect and reacts with N in the steel to precipitate AlN, thereby preventing degradation of moldability due to solid solution N. Therefore, it is preferable to add it by 0.01% or more. However, when added in large amounts, the ductility is rapidly lowered, so the content is limited to 0.1% or less.

Phosphorus (P): not more than 0.01%

P is an impurity, and if it exceeds 0.01%, it is segregated in the grain boundaries to harden the steel, so that it is preferable to limit it to 0.01% or less.

Sulfur (S): not more than 0.01%

Since S is an element which induces embrittlement of embrittlement upon employment, it is not preferable that S exists in an element state. In order to alleviate the problem caused by the solid solution S, Mn may be added to precipitate S into MnS. However, excessive MnS may cause an undesirable problem such as hardening of the steel. . Thus, in one embodiment of the present invention, the upper limit of S may be set at 0.01%.

Nitrogen (N): not more than 0.005%

N is often contained as an inevitable element in steel. N which is not precipitated and exists in a solid state deteriorates ductility, deteriorates endurance and deteriorates workability. In addition, when precipitates are formed by binding with elements such as Ti and Nb, the corrosion resistance is deteriorated, so the upper limit is limited to 0.005%.

Cr (Cr): 2.0 to 4.0%

Cr is dissolved in the steel to easily form a Cr ₂ O ₃ passivation film to improve the corrosion resistance and is used as a typical corrosion resistance improving element because it does not significantly decrease the workability of the steel sheet even when a large amount of Cr is added up to 4%. In general, Cr is not passivated in a strong acidic environment of pH 5 or lower, so its corrosion resistance effect is limited. However, the inventors of the present invention have found that when Nb or W, which can be passivated even in a strong acid environment, is added in combination with a small amount of Cr, Respectively. In order to obtain visible corrosion resistance improvement effect by interacting with Nb and W, it is required to add 2.0% or more. In the case of adding a large amount of Nb and W, the upper limit is limited to 4.0% because the effect of improving corrosion resistance is not great and cost is increased. In one embodiment of the present invention, the content of Cr may be limited to 2.5 to 3.5% or 2.7 to 3.3%.

Niobium (Nb): 0.015-0.5%

Nb is an element that binds with N at high temperature and is easily precipitated as NbN, and is effective in reducing the content of dissolved N. To obtain such an effect, it is necessary to add at least 0.015%. In addition, the solid solution Nb remaining after bonding with N forms an Nb ₂ O ₅ oxide film on the surface, thereby significantly improving the corrosion resistance in a corrosive environment. However, when the content exceeds 0.5%, the effect of improving the corrosion resistance against the addition amount is insignificant, and the content is limited to 0.015-0.5% because the hot rolling property is lowered. According to one embodiment of the present invention, the content of Nb may be limited to 0.02 to 0.4%.

Tungsten (W): not more than 0.5%

W is an element which forms oxides with excellent corrosion resistance by oxidizing in air, and it is effective when added in small amount. However, when it is added in a large amount, the effect of improving the corrosion resistance is small and the workability is decreased. However, since W is an arbitrary element, it is possible to achieve the object of the present invention even if W is not added. Therefore, in order to enjoy the effect of W in one embodiment of the present invention, %. In yet another embodiment of the present invention, the upper limit of the content of W may be limited to 0.4%.

Greater than or equal to 0.08

The inventors of the present invention have adopted a method of adding Nb and W to steel so as to effectively work in a wide pH range, taking into account that it is difficult to effectively prevent corrosion in a low pH range even when a large amount of Cr effective for corrosion prevention is added. However, since these elements are controlled only by their contents, effective corrosion resistance can not be obtained in the exhaust system environment, but the components must be controlled in a complex manner so that the corrosion resistance index expressed by the following relational expression 1 is 0.08 or more.

[Relation 1]

Corrosion resistance index = Nb + W - (N / 14 + C / 12) x 50

(Provided that Nb, W, N and C in the formula are the contents of the respective elements in weight% units)

That is, Nb and W are elements having an effect in a low pH range, but when these elements are present in the form of nitride or carbide, it is difficult to form a passivation film, so that the effect of corrosion resistance is difficult to expect. Therefore, in view of such a problem, it is necessary to secure a large amount of Nb and W that do not form carbide or nitride as much as possible, and the degree of corrosion can be represented by the above-described corrosion resistance index. According to the research results of the present inventors, the corrosion resistance index should be 0.08 or more so that corrosion resistance can be improved in the exhaust system environment.

In addition to the above composition, the remainder may contain Fe and unavoidable impurities.

The cold-rolled steel sheet of the present invention may be used as it is (including the case of resin coating on one side or both sides), but it may have an Al-based plating layer on its surface. When the Al-based plating layer is provided on the surface, the corrosion resistance of the steel sheet can be further improved, and the advantageous effect can be obtained. The Al-based plating used in the present invention means a plating containing Al as a main component, and it is not particularly limited whether it is a pure Al-based plating layer or an alloy-based plating layer. As an example, it may be plated with a so-called Type I plating solution containing 5 to 15% or 5 to 10% of Si, or may be plated using a plating solution of Type II. Particularly, if Al is the main component, there is no particular limitation on the additional component.

The steel sheet according to one embodiment of the present invention has excellent corrosion resistance and excellent workability. In particular, the steel sheet of the present invention is suitable for molding into an exhaust system such as an automobile, such as an average value (r _m ) of plastic anisotropy coefficient (r _m ) expressed by the following relational expression 2 as a measure of workability.

[Relation 2]

r _m = (r ₀ + 2 r ₄₅ + r ₉₀ ) / 4

(Where r ₀ , r ₄₅ and r ₉₀ are the plastic anisotropy coefficients in the rolling direction and 0 °, 45 ° and 90 °, respectively)

The above-described steel sheet of the present invention can be used in the form of an Al-plated steel sheet on which the surfaces of cold-rolled steel sheets or cold-rolled steel sheets are plated with Al. The method for producing the cold-rolled steel sheet or the coated steel sheet of the present invention is not particularly limited as far as the characteristic conditions of the steel sheet of the present invention can be satisfied. One method suggested by the present inventors is as follows.

The method for manufacturing the cold-rolled steel sheet according to the present invention is not necessarily limited to the above, but includes the steps of reheating the steel slab satisfying the above-mentioned composition of the material at a temperature of 1200 ° C or higher; Hot-rolling the reheated steel slab at a temperature higher than Ar ₃ to produce a hot-rolled steel sheet; Winding the hot-rolled steel sheet at 550 to 750 ° C; Rolling the rolled hot-rolled steel sheet at a reduction ratio of 50 to 95% to produce a cold-rolled steel sheet; And annealing the cold-rolled steel sheet. Hereinafter, each step will be described in detail.

(Reheating slab)

First, in the present invention, the steel slab having the above composition is reheated to a temperature of 1200 캜 or higher. Since most of the precipitates present in the steel are to be recycled, a temperature of 1200 ° C or higher is required, and more preferably, the precipitate is heated to 1250 ° C or higher in order to solidify the precipitate well. In terms of dissolution of the precipitate, the higher the heating temperature is, the more advantageous it is, so that the upper limit of the heating temperature is not particularly limited. However, if the temperature is excessively high, there may be a problem with the equipment, so in one embodiment of the present invention, the upper limit of the heating temperature may be set at 1400 캜.

(Hot rolling)

The reheated slab is subjected to hot rolling at a temperature higher than Ar ₃ to produce a hot-rolled steel sheet. The reason for limiting the hot rolling finishing temperature to Ar ₃ or more is for rolling in the austenite single phase region.

(Hot rolled steel sheet winding)

The hot-rolled steel sheet is rolled at 550 to 750 ° C. It is possible to additionally precipitate N, which is still remaining in the solid state by winding it at 550 DEG C or higher, to AlN, thereby ensuring excellent endurance. There is a risk that the workability is deteriorated by the solute N remaining without being precipitated as AlN. It may be wound at 750 DEG C or higher to prevent grain boundary coarsening and ensure cold rolling property.

(Cold rolling)

The rolled hot-rolled steel sheet is cold-rolled at a reduction ratio of 50 to 95% to produce a cold-rolled steel sheet. The reduction rate determines the final thickness of the cold-rolled steel sheet. The rolling reduction rate can be set to 50% or more in order to secure the final target thickness, and can be cold-rolled at a reduction ratio of 95% or less to prevent the rolling load from becoming too large have.

(Annealing)

A step of annealing the cold-rolled steel sheet may be performed. The crystal grains to be stretched upon cold rolling through annealing can be recrystallized. As the annealing method, both continuous annealing and box annealing can be used. In the case of continuous annealing, the continuous annealing temperature may be set to 600 ° C or more in order to eliminate dislocations generated during cold rolling through sufficient recrystallization. In order to prevent coarsening of crystal grains and ensure strength and workability, the continuous annealing temperature may be set to 900 占 폚 or less. According to one aspect of the present invention, recrystallization through continuous annealing is also possible through box annealing at about 600 to 800 ° C.

(Al plating - selective process)

If the steel sheet is used for plating and used, a step of plating the surface of the steel sheet can be used. There is no particular limitation on the specific method of plating, and all conventional plating methods can be applied

Hereinafter, the present invention will be described in detail by way of examples. It should be noted, however, that the following examples are intended to illustrate and explain the present invention in detail but not to limit the scope of the present invention. The scope of the present invention is determined by the matters set forth in the claims and the matters reasonably inferred therefrom.

(Example)

A steel slab having a composition of the following Table 1 having a thickness of 250 mm was reheated to 1250 占 폚 and then hot-rolled. During hot rolling, the hot finish rolling was terminated at a temperature of 890 캜 higher than the Ar 3 of all the steel materials, and rolled at a temperature of 620 캜 to obtain a hot-rolled steel sheet having a thickness of 4 mm.

The obtained hot-rolled steel sheet was cold-rolled at a reduction ratio of 70%, and then continuously annealed at a temperature of 830 ° C to finally obtain a cold-rolled steel sheet having a thickness of 1.2 mm.

Simulation was performed for the test (in accordance with B method of JASO M 611-92) to simulate the environment of the vehicle exhaust system for each of the prepared cold-rolled steel sheet, and average value of the plastic anisotropy factor (Lankford value) index for evaluating the workability r _m Room temperature tensile test was performed.

Further, in order to evaluate the aging characteristics, it was held at 100 ° C for 1 hour and then subjected to a room temperature tensile test to determine whether or not aging occurred due to the occurrence of yield point elongation. In addition, in the hot rolling process, the hot rolling property was determined according to the shape of the steel sheet. When there is no problem in the shape of the steel sheet, there is no problem in the overall shape, but in some cases, , And when the overall shape is uneven and difficult to use, it is judged to be 'poor'.

division Component (% by weight) C Si Mn Al P S N Cr Nb W Comparative River 1 0.003 0.08 0.19 0.04 0.008 0.009 0.003 - - - Comparative River 2 0.003 0.07 0.21 0.04 0.008 0.009 0.003 - 0.020 - Comparative Steel 3 0.003 0.06 0.21 0.03 0.008 0.009 0.003 0.98 0.021 - Comparative Steel 4 0.003 0.08 0.20 0.04 0.008 0.009 0.003 2.02 0.020 - Comparative Steel 5 0.003 0.04 0.19 0.03 0.009 0.008 0.003 3.08 0.021 - Comparative Steel 6 0.003 0.05 0.20 0.03 0.008 0.008 0.003 3.95 0.021 - Comparative Steel 7 0.003 0.05 0.21 0.04 0.008 0.008 0.003 5.01 0.021 - Comparative Steel 8 0.003 0.04 0.19 0.04 0.009 0.009 0.003 - 0.110 - Comparative Steel 9 0.003 0.06 0.19 0.03 0.009 0.009 0.003 - 0.201 - Comparative Steel 10 0.003 0.04 0.21 0.03 0.008 0.008 0.003 - 0.300 - Comparative Steel 11 0.003 0.04 0.20 0.03 0.009 0.008 0.003 - 0.021 0.10 Comparative Steel 12 0.003 0.06 0.19 0.03 0.009 0.008 0.003 - 0.021 0.21 Comparative Steel 13 0.003 0.06 0.20 0.03 0.009 0.008 0.003 - 0.020 0.29 Comparative Steel 14 0.003 0.08 0.19 0.04 0.008 0.008 0.003 1.01 0.100 - Comparative Steel 15 0.003 0.07 0.21 0.04 0.008 0.008 0.003 1.00 0.201 - Comparative Steel 16 0.003 0.08 0.19 0.03 0.008 0.009 0.003 0.98 0.310 - Comparative Steel 17 0.003 0.04 0.20 0.04 0.009 0.009 0.003 1.02 0.021 0.09 Comparative Steel 18 0.003 0.05 0.19 0.04 0.009 0.008 0.003 1.03 0.021 0.20 Comparative Steel 19 0.003 0.06 0.19 0.03 0.009 0.009 0.003 0.98 0.020 0.30 Inventive Steel 1 0.003 0.08 0.20 0.04 0.008 0.009 0.003 2.00 0.101 - Invention river 2 0.003 0.07 0.20 0.03 0.009 0.009 0.003 2.03 0.201 - Invention steel 3 0.003 0.04 0.20 0.04 0.009 0.008 0.003 2.00 0.499 - Inventive Steel 4 0.003 0.06 0.20 0.04 0.009 0.009 0.003 2.01 0.020 0.10 Invention steel 5 0.003 0.07 0.21 0.04 0.008 0.008 0.003 2.00 0.020 0.20 Invention steel 6 0.003 0.05 0.21 0.03 0.009 0.009 0.003 2.00 0.021 0.50 Invention steel 7 0.003 0.08 0.21 0.03 0.009 0.008 0.003 2.02 0.101 0.10 Inventive Steel 8 0.003 0.04 0.19 0.03 0.008 0.009 0.003 2.00 0.201 0.20 Invention river 9 0.003 0.07 0.20 0.03 0.008 0.008 0.003 2.00 0.300 0.30 Invented Steel 10 0.003 0.03 0.20 0.04 0.009 0.009 0.003 2.97 0.100 - Invention steel 11 0.003 0.07 0.19 0.03 0.009 0.008 0.003 3.04 0.211 - Invention steel 12 0.003 0.04 0.21 0.03 0.008 0.009 0.003 3.00 0.498 - Invention steel 13 0.003 0.07 0.20 0.04 0.009 0.009 0.003 3.03 0.020 0.10 Invented Steel 14 0.003 0.04 0.19 0.03 0.009 0.008 0.003 3.00 0.020 0.20 Invented Steel 15 0.003 0.04 0.20 0.03 0.009 0.008 0.003 3.04 0.020 0.48 Invented Steel 16 0.003 0.06 0.19 0.04 0.008 0.008 0.003 3.03 0.100 0.10 Inventive Steel 17 0.003 0.08 0.19 0.03 0.008 0.009 0.003 3.02 0.221 0.20 Inventive Steel 18 0.003 0.07 0.20 0.04 0.008 0.008 0.003 3.03 0.301 0.31 Invented Steel 19 0.003 0.05 0.20 0.04 0.009 0.008 0.003 4.00 0.111 - Invented Steel 20 0.003 0.07 0.19 0.04 0.009 0.008 0.003 4.00 0.200 - Inventive Steel 21 0.003 0.08 0.20 0.04 0.009 0.009 0.003 3.97 0.491 - Invented Steel 22 0.003 0.04 0.21 0.04 0.009 0.009 0.003 4.00 0.021 0.10 Invented steel 23 0.003 0.08 0.20 0.04 0.009 0.008 0.003 3.96 0.020 0.20 Invented Steel 24 0.003 0.06 0.20 0.04 0.009 0.008 0.003 4.00 0.021 0.50 Invented steel 25 0.003 0.04 0.21 0.04 0.009 0.009 0.003 3.97 0.101 0.09 Inventive Steel 26 0.003 0.03 0.20 0.04 0.008 0.008 0.003 4.00 0.210 0.20 Inventive Steel 27 0.003 0.04 0.20 0.03 0.008 0.009 0.003 3.99 0.300 0.29 Comparative Steel 20 0.003 0.03 0.19 0.03 0.009 0.008 0.003 5.02 0.101 - Comparative Steel 21 0.003 0.05 0.19 0.04 0.008 0.009 0.003 5.00 0.201 - Comparative Steel 22 0.003 0.03 0.20 0.03 0.009 0.008 0.003 5.03 0.491 - Comparative Steel 23 0.003 0.07 0.20 0.04 0.009 0.008 0.003 5.02 0.021 0.10 Comparative Steel 24 0.003 0.04 0.20 0.04 0.008 0.008 0.003 4.95 0.020 0.20 Comparative Steel 25 0.003 0.03 0.19 0.04 0.008 0.009 0.003 4.97 0.020 0.50 Comparative Steel 26 0.003 0.08 0.21 0.04 0.008 0.008 0.003 5.03 0.101 0.09 Comparative Steel 27 0.003 0.05 0.20 0.04 0.008 0.008 0.003 5.04 0.211 0.20 Comparative Steel 28 0.003 0.06 0.20 0.04 0.008 0.009 0.003 5.01 0.301 0.30 Comparative River 29 0.003 0.07 0.19 0.04 0.009 0.008 0.003 2.15 0.030 0.05 Comparative Steel 30 0.003 0.04 0.20 0.04 0.009 0.009 0.003 2.35 0.030 0.06 Comparative Steel 31 0.003 0.04 0.21 0.04 0.008 0.008 0.003 2.56 0.020 0.06 Comparative Steel 32 0.003 0.04 0.21 0.03 0.009 0.008 0.003 2.75 0.021 0.01 Comparative Steel 33 0.003 0.07 0.19 0.04 0.008 0.009 0.003 3.22 0.081 0.01 Comparative River 34 0.003 0.05 0.19 0.04 0.009 0.008 0.003 3.52 0.050 0.00 Comparative River 35 0.003 0.07 0.21 0.03 0.008 0.009 0.003 3.62 0.041 0.03 Comparative River 36 0.003 0.06 0.19 0.00 0.008 0.009 0.003 3.70 0.021 0.06 Comparative Steel 37 0.003 0.05 0.20 0.04 0.009 0.009 0.003 3.88 0.071 0.02 Comparative Steel 38 0.003 0.06 0.20 0.04 0.008 0.009 0.003 3.95 0.020 0.03 Comparative River 39 0.003 0.03 0.20 0.03 0.008 0.008 0.003 3.00 0.013 0.10 Invention river 28 0.003 0.05 0.20 0.04 0.008 0.008 0.003 3.00 0.015 0.10 Comparative Steel 40 0.003 0.04 0.20 0.04 0.009 0.009 0.003 2.01 0.550 0.10 Comparative Steel 41 0.003 0.04 0.20 0.04 0.009 0.009 0.003 3.02 0.612 0.10 Comparative Steel 42 0.003 0.04 0.20 0.04 0.009 0.009 0.003 3.95 0.572 0.10

division Corrosion resistance Processability Hot rolling property
◎: Good
○: Normal
●: Poor Corrosion resistance index Maximum corrosion depth (mm) Remarks r _m Aging Properties Remarks Comparative River 1 -0.02 0.78 fail 1.45 Occur fail ◎ Comparative River 2 0.00 0.82 fail 1.81 Not occurring pass ◎ Comparative Steel 3 0.00 0.75 fail 1.78 Not occurring fail ◎ Comparative Steel 4 0.00 0.72 fail 1.74 Not occurring fail ◎ Comparative Steel 5 0.00 0.69 fail 1.70 Not occurring fail ◎ Comparative Steel 6 0.00 0.65 fail 1.65 Not occurring fail ◎ Comparative Steel 7 0.00 0.61 fail 1.51 Not occurring fail ◎ Comparative Steel 8 0.09 0.72 fail 2.12 Not occurring pass ○ Comparative Steel 9 0.18 0.70 fail 2.28 Not occurring pass ○ Comparative Steel 10 0.28 0.66 fail 2.31 Not occurring pass ○ Comparative Steel 11 0.10 0.75 fail 2.02 Not occurring pass ◎ Comparative Steel 12 0.21 0.71 fail 2.11 Not occurring pass ◎ Comparative Steel 13 0.29 0.65 fail 2.15 Not occurring pass ◎ Comparative Steel 14 0.08 0.65 fail 2.06 Not occurring pass ○ Comparative Steel 15 0.18 0.58 fail 2.21 Not occurring pass ○ Comparative Steel 16 0.29 0.53 fail 2.24 Not occurring pass ○ Comparative Steel 17 0.09 0.68 fail 1.96 Not occurring pass ◎ Comparative Steel 18 0.20 0.55 fail 2.05 Not occurring pass ◎ Comparative Steel 19 0.30 0.51 fail 2.09 Not occurring pass ◎ Inventive Steel 1 0.08 0.39 pass 1.99 Not occurring pass ○ Invention river 2 0.18 0.37 pass 2.15 Not occurring pass ○ Invention steel 3 0.48 0.37 pass 2.17 Not occurring pass ○ Inventive Steel 4 0.10 0.38 pass 1.90 Not occurring pass ◎ Invention steel 5 0.20 0.37 pass 1.99 Not occurring pass ◎ Invention steel 6 0.50 0.37 pass 2.02 Not occurring pass ◎ Invention steel 7 0.18 0.37 pass 2.05 Not occurring pass ○ Inventive Steel 8 0.38 0.37 pass 2.15 Not occurring pass ○ Invention river 9 0.58 0.37 pass 2.28 Not occurring pass ○ Invented Steel 10 0.08 0.35 pass 1.93 Not occurring pass ○ Invention steel 11 0.19 0.33 pass 2.08 Not occurring pass ○ Invention steel 12 0.48 0.33 pass 2.11 Not occurring pass ○ Invention steel 13 0.10 0.34 pass 1.84 Not occurring pass ◎ Invented Steel 14 0.20 0.33 pass 1.93 Not occurring pass ◎ Invented Steel 15 0.48 0.33 pass 1.96 Not occurring pass ◎ Invented Steel 16 0.18 0.33 pass 1.95 Not occurring pass ○ Inventive Steel 17 0.40 0.33 pass 2.12 Not occurring pass ○ Inventive Steel 18 0.59 0.33 pass 2.25 Not occurring pass ○ Invented Steel 19 0.09 0.32 pass 1.88 Not occurring pass ○ Invented Steel 20 0.18 0.31 pass 2.02 Not occurring pass ○ Inventive Steel 21 0.47 0.31 pass 2.05 Not occurring pass ○ Invented Steel 22 0.10 0.32 pass 1.81 Not occurring pass ◎ Invented steel 23 0.20 0.31 pass 1.87 Not occurring pass ◎ Invented Steel 24 0.50 0.31 pass 1.90 Not occurring pass ◎ Invented steel 25 0.17 0.31 pass 1.92 Not occurring pass ○ Inventive Steel 26 0.39 0.31 pass 2.08 Not occurring pass ○ Inventive Steel 27 0.57 0.31 pass 2.04 Not occurring pass ○ Comparative Steel 20 0.08 0.32 pass 1.69 Not occurring fail ○ Comparative Steel 21 0.18 0.30 pass 1.73 Not occurring fail ○ Comparative Steel 22 0.47 0.30 pass 1.79 Not occurring fail ○ Comparative Steel 23 0.10 0.31 pass 1.63 Not occurring fail ◎ Comparative Steel 24 0.20 0.30 pass 1.65 Not occurring fail ◎ Comparative Steel 25 0.50 0.30 pass 1.66 Not occurring fail ◎ Comparative Steel 26 0.17 0.30 pass 1.68 Not occurring fail ○ Comparative Steel 27 0.39 0.30 pass 1.72 Not occurring fail ○ Comparative Steel 28 0.58 0.30 pass 1.75 Not occurring fail ○ Comparative River 29 0.06 0.52 fail 2.15 Not occurring pass ◎ Comparative Steel 30 0.07 0.43 fail 2.05 Not occurring pass ◎ Comparative Steel 31 0.06 0.48 fail 2.03 Not occurring pass ◎ Comparative Steel 32 0.01 0.59 fail 1.95 Not occurring pass ◎ Comparative Steel 33 0.07 0.47 fail 2.11 Not occurring pass ○ Comparative River 34 0.03 0.55 fail 2.05 Not occurring pass ◎ Comparative River 35 0.05 0.51 fail 2.01 Not occurring pass ◎ Comparative River 36 0.06 0.45 fail 1.95 Not occurring pass ◎ Comparative Steel 37 0.07 0.42 fail 1.90 Not occurring pass ○ Comparative Steel 38 0.03 0.45 fail 1.81 Not occurring pass ◎ Comparative River 39 0.09 0.35 pass 1.80 Occur fail ◎ Invention river 28 0.09 0.35 pass 1.81 Not occurring pass ◎ Comparative Steel 40 0.63 0.37 pass 2.00 Not occurring pass ● Comparative Steel 41 0.69 0.33 pass 2.11 Not occurring pass ● Comparative Steel 42 0.65 0.31 pass 1.95 Not occurring pass ●

As shown in Table 2, inventive steels 1 to 28 satisfying the composition of the present invention all have a maximum corrosion depth of 0.4 mm or less and a level similar to that of a commercial stainless steel, and r _m is 1.8 or more. Overall, the higher the content of Cr, Nb and W, the better the corrosion resistance. However, as the amount of Cr increases, the effect of improving the corrosion resistance against the addition amount tends to decrease. From the viewpoint of workability, the workability is slightly decreased when Cr is added, and the workability tends to increase when Nb and W are added.

The comparative steel 1 is a steel to which Cr, Nb and W are not added, and Nb is not added. As a result, the aging phenomenon occurs due to the presence of a large amount of solid N, and the _rm value is very low. It is confirmed that the addition of 0.02% of Nb commonly through comparative steels 2 to 7 prevents the occurrence of the aging phenomenon, increases the workability slightly, and gradually increases the corrosion resistance as the Cr content increases. However, even if Cr is added up to 5.01% (comparative steel 7), the effect of improving corrosion resistance is not large and the maximum corrosion thickness is higher than 0.6 mm. In order to further improve the corrosion resistance, it is possible to consider a method of increasing the Cr content. However, as shown in Comparative Steel 2 to 7, as the content of Cr increases, the workability is deteriorated. Particularly, when the Cr content is 4% or more, the workability is drastically reduced, and it is confirmed that the upper limit of Cr is preferably limited to 4%.

The comparative steels 8 to 13 do not contain Cr and are added with Nb and W and have excellent _rm value of 2 or more and thus have excellent processability. On the other hand, the corrosion resistance is still insufficient and it is necessary to add an appropriate amount of Cr in order to improve the corrosion resistance. When 1% of Cr is contained in addition to Nb and W, the corrosion resistance is visibly increased and the change of the workability by 1% Cr is insignificant, as can be seen in comparative steels 14 to 19. However, it was found that the maximum corrosion thickness is still 0.5 mm or more, which means that additional Cr addition is necessary.

In the above-mentioned context, when the Cr content is increased to 2 to 4% along with the addition of Nb and W, corrosion resistance is 0.4 mm or less as in inventive steels 1 to 27, and a range having an r _m value of 1.8 or more is formed do. As described above, when Cr is added, the corrosion resistance is further increased as the content of Nb and W increases. However, as the content of Nb and W increases, the effect of improving the corrosion resistance is decreased, and the upper limit of the addition amount of Nb and W is preferably 0.5%, and further addition is not preferable from the economical point of view. In order to quantify the effect of improving the corrosion resistance by the content of Nb and W, the corrosion resistance index can be calculated as shown in the above-mentioned relational expression 1. When the corrosion resistance index is 0.08 or more in the state of Cr containing 2% or more, excellent corrosion resistance Can be obtained.

As can be seen from Comparative Steel Nos. 20 to 28, when the Cr content is more than 4%, it may be advantageous from the viewpoint of corrosion resistance, but since the workability is drastically reduced, addition of Cr by more than 4% is avoided.

Comparative Steel Nos. 29 to 38 show that although the composition of the steel satisfies the conditions of the present invention, the corrosion resistance index is less than 0.08, and as can be seen from the results of Table 2, the workability is excellent but the corrosion resistance is not satisfied. From this, it can be seen that it is very difficult to merely combine workability and corrosion resistance by simply controlling the steel composition, and it is necessary to control the corrosion resistance index to 0.08 or more in combination with steel composition.

As a result of comparison between the comparative steel 39 and the invention steel 28, it can be seen that when the Nb content is less than 0.015%, the solid solution C is not precipitated sufficiently in NbC, and as a result, the agglomeration occurs and the workability deteriorates. However, as can be seen from Comparative Steel Nos. 40 to 42, when Nb is added in excess of 0.5%, it is difficult to produce a normal steel sheet because the hot rolling property is excessively high. It can be seen that the range of the Nb content which can obtain the most excellent steel sheet shape in terms of hot rolling property is 0.05% or less.

Claims (7)

(C): not more than 0.01%, silicon (Si): not more than 0.5%, manganese (Mn): 0.05 to 0.5%, aluminum (Al): 0.01 to 0.1%, phosphorus (P): not more than 0.01% , Sulfur (S): not more than 0.01%, nitrogen (N): not more than 0.005%, chromium (Cr): 2.0 to 4.0%, niobium (Nb): 0.015 to 0.5%, tungsten (W) The balance being made of unavoidable impurities and having a corrosion resistance index of 0.08 or more expressed by the following relational expression (1).
[Relation 1]
Corrosion resistance index = Nb + W - (N / 14 + C / 12) x 50
(Provided that Nb, W, N and C in the formula are the contents of the respective elements in weight% units)
The cold rolled steel sheet for an exhaust system according to claim 1, further comprising an Al-based plating layer on the surface.
The cold rolled steel sheet for an exhaust system according to claim 1, wherein an average value (r _m ) of the plastic anisotropy coefficient (Lankford value) represented by the following relational expression 2 is 1.5 or more.

[Relation 2]
r _m = (r ₀ + 2 r ₄₅ + r ₉₀ ) / 4
(Where r ₀ , r ₄₅ and r ₉₀ are the plastic anisotropy coefficients in the rolling direction and 0 °, 45 ° and 90 °, respectively)
(C): not more than 0.01%, silicon (Si): not more than 0.5%, manganese (Mn): 0.05 to 0.5%, aluminum (Al): 0.01 to 0.1%, phosphorus (P): not more than 0.01% , Sulfur (S): not more than 0.01%, nitrogen (N): not more than 0.005%, chromium (Cr): 2.0 to 4.0%, niobium (Nb): 0.015 to 0.5%, tungsten (W) Reheating the steel slab having a composition in which the remainder is inevitable impurities and whose composition has a corrosion resistance index of 0.08 or more expressed by the following relational formula 1 at a temperature of 1200 ° C or higher;
Hot-rolling the reheated steel slab at a temperature higher than Ar ₃ to produce a hot-rolled steel sheet;
Winding the hot-rolled steel sheet at 550 to 750 ° C;
Rolling the rolled hot-rolled steel sheet at a reduction ratio of 50 to 95% to produce a cold-rolled steel sheet; And
And annealing the cold-rolled steel sheet.
[Relation 1]
Corrosion resistance index = Nb + W - (N / 14 + C / 12) x 50
(Provided that Nb, W, N and C in the formula are the contents of the respective elements in weight% units)
5. The method of manufacturing a cold-rolled steel sheet for an exhaust system according to claim 4, wherein said annealing is continuous annealing at a temperature of 600 to 900 占 폚.
5. The method of manufacturing a cold-rolled steel sheet for an exhaust system according to claim 4, wherein said annealing is box annealing at a temperature of 600 to 800 占 폚.
The method of manufacturing a cold-rolled steel sheet for an exhaust system according to any one of claims 4 to 6, further comprising a step of Al plating.