KR20200025503A - Cold rolled steel sheet for exhaust system and manufacturing method of the same - Google Patents

Abstract

Introduced is a cold rolled steel sheet for an exhaust system, which includes: 0.015 to 0.05 wt% of C; equal to or less than 0.2 wt% (excluding 0 wt%) of Si; 0.1 to 0.3 wt% of Mn; equal to or less than 0.1 wt% (excluding 0 wt%) of Al; 2.0 to 4.0 wt% of Cr; one or more of equal to or less than 0.5 wt% (excluding 0 wt%) of Mo and equal to or less than 0.3 wt% (excluding 0 wt%) of W; and the balance of Fe and unavoidable impurities. In addition, the cold rolled steel sheet for the exhaust system satisfies formula 1 below and includes equal to or less than 5 area% of one or more of a pearlite phase and a ferrite phase. In formula 1: 0.05 <= 0.6 * [Mo] + [W] (unit: wt%), [Mo] and [W] mean contents (wt%) of Mo and W, respectively.

Description

Cold rolled steel sheet for exhaust system and its manufacturing method {COLD ROLLED STEEL SHEET FOR EXHAUST SYSTEM AND MANUFACTURING METHOD OF THE SAME}

The present invention relates to a cold rolled steel sheet for exhaust systems and a method of manufacturing the same. More specifically, the present invention relates to a cold rolled steel sheet for exhaust systems and a method of manufacturing the same, which can ensure corrosion resistance and processability at the same time by controlling the content of component composition and manufacturing conditions.

During operation of the car, fossil fuel is burned in the engine of the car, and water vapor is generated along with toxic exhaust gas such as sulfuric acid gas and nitrate gas and is discharged to the exhaust system. . In the process of cooling from a high temperature to a low temperature below the dew point, condensate is generated in the exhaust system, and the condensate thus generated includes ions dissolved from the exhaust gas.

The condensate containing these ions changes the concentration and pH of the ions as they are produced and dried, creating an environment in which complex corrosion by strong and weak acids proceeds actively. As a result, corrosion by condensate causes the steel sheet to penetrate from the inside of the exhaust system over time, resulting in the malfunction of the exhaust system. Therefore, the steel sheet used in the exhaust system for automobiles should be excellent in corrosion resistance to such a complex corrosion environment in order to extend the life. In addition, a certain amount or more of mechanical properties must be satisfied to form a desired type of exhaust system.

Typically, steel sheets used as exhaust system materials include aluminum plated steel sheets and stainless steel sheets. Among them, aluminum plated steel sheet is plated with aluminum on general carbon steel, but has strong corrosion resistance by Al ₂ O ₃ passivation film. In particular, the corrosion resistance against corrosion due to salt that may occur on the outer surface of the exhaust system has a very strong advantage, and also exhibits the corrosion resistance in a certain range of pH range even on the inner surface of the exhaust system. However, at low pH, once eluted and removed, there is a limit that can no longer exhibit corrosion resistance. In order to overcome such a problem, a method of suppressing corrosion in a low pH environment by adding Cu to a steel sheet has been attempted, but the addition of Cu has a disadvantage in that it promotes corrosion in a high pH region.

Stainless steel sheets also have corrosion resistance by Cr ₂ O ₃ passivation film at a range of pH against corrosion on the exhaust system inner surface. When the pH is low, the passivation film is activated to lose corrosion resistance, but when the pH rises, Cr in solid solution is oxidized again to passivate to restore corrosion resistance. However, compared with the case of aluminum plating, there is a disadvantage in that rust is easily generated by the salt at the outer surface of the exhaust system. In addition, there is a limit that the economical efficiency is reduced by adding a large amount of expensive alloying elements.

The present invention provides a cold rolled steel sheet for exhaust system and a method of manufacturing the same which can ensure corrosion resistance and processability at the same time by controlling the content of the ingredient composition and the manufacturing conditions.

Cold rolled steel sheet for exhaust system according to an embodiment of the present invention by weight% C: 0.015 to 0.05%, Si: 0.2% or less (excluding 0%), Mn: 0.1 to 0.3%, Al: 0.1% or less (0 %) And Cr: 2.0 to 4.0%, Mo: 0.5% or less (except 0%) and W: 0.3% or less (except 0%), balance It contains Fe and unavoidable impurities, satisfies Equation 1 below, and contains at least 5 area% of at least one of a pearlite phase and a ferrite phase.

0.05 ≦ 0.6 * [Mo] + [W] (unit: wt%)

(In Formula 1, [Mo] and [W] means the content of Mo and W (% by weight), respectively.)

P: 0.01% or less (except 0%), S: 0.01% or less (except 0%) and N: 0.007% or less (except 0%).

The yield strength may be 200 to 260 MPa, and the elongation may be 35% or more.

Cold rolled steel sheet manufacturing method for an exhaust system according to an embodiment of the present invention by weight% C: 0.015 to 0.05%, Si: 0.2% or less (excluding 0%), Mn: 0.1 to 0.3%, Al: 0.1% or less (Excluding 0%) and Cr: 2.0 to 4.0%, Mo: 0.5% or less (excluding 0%) and W: 0.3% or less (excluding 0%) Heating the slab comprising the balance Fe and unavoidable impurities and satisfying the following Equation 1; Hot rolling the slab to produce a hot rolled steel sheet; Winding the hot rolled steel sheet; Cold rolling the wound hot rolled steel sheet to produce a cold rolled steel sheet; Annealing the cold rolled steel sheet; And cooling the annealed cold rolled steel sheet to satisfy Equation 2 below.

0.05 ≦ 0.6 * [Mo] + [W] (unit: wt%)

(In Formula 1, [Mo] and [W] means the content of Mo and W (% by weight), respectively.)

[Cr] * [Cooling rate] ≤ 25

(In Formula 2, [Cr] means the content of Cr (% by weight), and [cooling rate] means the rate of cooling the cold rolled steel sheet (° C./sec.).)

After cooling the cold rolled steel sheet, rolling the cooled cold rolled steel sheet at a reduction ratio of 0.6 to 1.4%; may be further included.

In the winding of the hot rolled steel sheet, the hot rolled steel sheet may be wound at a winding temperature of 550 to 750 ° C.

In the annealing of the cold rolled steel sheet, the cold rolled steel sheet may be annealed at an annealing temperature of 550 to 900 ℃.

In the step of cooling the cold rolled steel sheet, the annealed cold rolled steel sheet may be cooled at a cooling rate of 5 to 9 ℃ / sec.

The cold rolled steel sheet for exhaust system according to the embodiment of the present invention may provide a cold rolled steel sheet having excellent economical resistance and workability by optimizing the addition amount without adding a large amount of expensive alloy components.

When the plating of aluminum or the like on the cold rolled steel sheet for exhaust system according to an embodiment of the present invention, even if the aluminum plating layer is removed can have excellent corrosion resistance in the corrosion environment of the exhaust system can effectively overcome the disadvantages of the aluminum plated steel sheet.

The terminology used herein is for reference only to specific embodiments and is not intended to limit the invention. As used herein, the singular forms “a,” “an,” and “the” include plural forms as well, unless the phrases clearly indicate the opposite. As used herein, the meaning of “comprising” embodies a particular property, region, integer, step, operation, element, and / or component, and the presence of another property, region, integer, step, operation, element, and / or component, It does not exclude the addition.

Although not defined otherwise, all terms including technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art. Commonly defined terms used are additionally interpreted as having a meaning consistent with the related technical literature and the presently disclosed contents, and are not interpreted as ideal or very formal meaning unless defined.

In addition, unless otherwise indicated,% means weight% and 1 ppm is 0.0001 weight%.

In the embodiment of the present invention, the meaning of further including an additional element means to include a residual amount of iron (Fe) by an additional amount of the additional element.

Hereinafter, embodiments of the present invention will be described in detail so that those skilled in the art can easily practice. As those skilled in the art would realize, the described embodiments may be modified in various different ways, all without departing from the spirit or scope of the present invention.

For exhaust system  Cold rolled steel sheet

Cold rolled steel sheet for exhaust system according to an embodiment of the present invention by weight% C: 0.015 to 0.05%, Si: 0.2% or less (excluding 0%), Mn: 0.1 to 0.3%, Al: 0.1% or less (0 %) And Cr: 2.0 to 4.0%, Mo: 0.5% or less (except 0%) and W: 0.3% or less (except 0%), balance Fe and unavoidable impurities.

Specifically, the cold rolled steel sheet for the exhaust system according to an embodiment of the present invention P: 0.01% or less (excluding 0%), S: 0.01% or less (excluding 0%) and N: 0.007% or less (0% Except for) may be further included.

First, the reason for limitation of the content of the components will be described below.

Carbon (C): 0.015 to 0.05%

If the content of C is too low, it is difficult to be used as a structural material due to its low strength, and in order to lower the content, an additional refining process may be required, which may reduce productivity. On the other hand, if the content of C is too high, if the cooling rate is fast due to the influence of Cr added to improve the corrosion resistance, it may cause excessive formation of the pearlite phase and ferrite phase may degrade the workability. The content of C is limited to 0.015 to 0.05% to ensure proper processability.

Silicon (Si): 0.2% or less

Si is an element that can be used as a decarburizing agent and can contribute to the improvement of strength by solid solution strengthening. In addition, SiO ₂ oxides formed on the surface may also serve to delay condensate corrosion. However, if the content of Si is too high, Si-based oxide is generated on the surface during annealing, which may cause a defect in plating, thereby degrading the plating property. Therefore, the content of Si is limited to 0.2% or less.

Manganese (Mn): 0.1-0.3%

Mn is combined with solid solution S in steel to precipitate as MnS, thereby preventing hot shortness due to solid solution S. However, if the Mn content is too high, the material may be hardened to reduce ductility. Therefore, the content of Mn is limited to 0.1 to 0.3%.

Aluminum (Al): 0.1% or less

Al is an element having a very high deoxidation effect, and it is possible to prevent the formability due to the solid solution N from decreasing by reacting with N in steel to precipitate AlN. However, if the Al content is too high, the ductility can be drastically reduced. Therefore, the content of Al is limited to 0.1% or less.

Phosphorus (P): 0.01% or less

The addition of P below a certain amount does not significantly reduce the ductility of the steel and can increase the strength. However, if the content of P is too high, it may segregate at the grain boundary to cure the steel. Therefore, the content of P may be limited to 0.01% or less.

Sulfur (S): 0.01% or less

Since S is an element causing red brittleness in solid solution, precipitation of MnS should be induced through the addition of Mn. However, if the S content is too high, the steel may harden due to excessive precipitation of MnS. Therefore, the content of S may be limited to 0.01% or less.

Nitrogen (N): 0.007% or less

N may be contained as an unavoidable element in steel. If the content of N is too high, N, which does not precipitate and remains in solid solution, may degrade ductility, worsen aging resistance, and may degrade processability, and form precipitates by combining with elements such as Ti and Nb. If you do, the corrosion resistance can be greatly deteriorated. Therefore, the content of N can be limited to 0.007% or less.

Chromium (Cr): 2.0-4.0%

Cr not only improves the corrosion resistance of the steel sheet by easily forming a Cr ₂ O ₃ passivation film, but may be used as a typical corrosion resistance improving element because it does not significantly reduce the workability of the steel sheet even when a large amount is added. If the content of Cr is too low, the effect of improving the corrosion resistance may not be sufficient. On the other hand, if the Cr content is too high, the oxygen affinity in the molten steel is high, making it very difficult to control the C content in the refining process, which reduces productivity, and induces the formation of at least one of a pearlite phase and a ferrite phase in the cooling process. Can drop. Therefore, the content of Cr is limited to 2.0 to 4.0%.

Molybdenum (Mo): 0.5% or less

Mo is an element which combines with oxygen to form a passivation oxide having excellent corrosion resistance, and has an effect of improving corrosion resistance even when a small amount is added. Especially, when the pH is low, Cr oxide is ionized, whereas Mo oxide is maintained as a passivation oxide. It may be effective in improving corrosion resistance in a strong acid environment. However, when the amount of Mo is too high, the effect of improving the corrosion resistance is small compared to the added amount, and because it is an expensive element, the content of Mo is limited to 0.5% or less in consideration of economical efficiency.

Tungsten (W): 0.3% or less

W, similar to Mo, is an element that combines with oxygen to form a passivation oxide having excellent corrosion resistance in a strong acid having a low pH, and may be effective in improving corrosion resistance even when a small amount is added. However, if the content of W is too high, the effect of improving the corrosion resistance is small compared to the added amount, and the melting point is very high element to reduce the productivity in the steelmaking process, so the content of W is limited to 0.3% or less.

In addition to the above composition, the remainder may include Fe and unavoidable impurities. The steel of the present invention does not exclude the addition of other compositions. Unavoidable impurities may be unintentionally incorporated from raw materials or the surrounding environment in the course of ordinary steel manufacturing, and cannot be excluded. Unavoidable impurities can be understood by those skilled in the art of ordinary steel manufacturing.

Cold rolled steel sheet for exhaust system according to an embodiment of the present invention satisfies the following equation 1.

0.05 ≦ 0.6 * [Mo] + [W] (unit: wt%)

In Formula 1, [Mo] and [W] mean Mo and W content (% by weight), respectively.

Even when the pH is lower than 1, Mo and W added for the sake of excellent corrosion resistance effect are controlled through Equation 1 to control the content of Mo and W.

Considering the degree of contribution to the improvement of corrosion resistance at low pH due to the formation of the passivation oxide, the Mo content is multiplied by 0.6, and the value combined with the W content is controlled to 0.05 or more.

If the value of 0.6 * [Mo] + [W] is less than 0.05, the formation of the passivation oxide may not be sufficient, and thus the effect of improving corrosion resistance may not be sufficient.

Accordingly, when the corrosion test is performed in a corrosion solution of pH 1.5 to 2.5 including SO ₄ ^2- and SO ₃ ^2- at 70 ° C., the average corrosion rate may be 0.53 mg / cm ² / hr or less. In addition, when the corrosion experiments in a corrosion solution of pH 1.0 or less including 70 ℃ SO ₄ ^2- and SO ₃ ^2- and the like, the average corrosion rate may be 6.58mg / cm ² / hr or less.

That is, it can have excellent corrosion resistance in both a range of pH range and low pH.

On the other hand, the cold rolled steel sheet for exhaust system according to an embodiment of the present invention includes at least one of the pearlite phase and the ferrite phase of 5 area% or less. The pearlite phase and the bainite phase are related to the amount of Cr and closely related to the cooling rate when the cold rolled steel sheet is annealed during the manufacturing of the cold rolled steel sheet for exhaust system according to the embodiment of the present invention. Details thereof will be described later.

When the pearlite phase alone or the bainite phase alone or the sum of the pearlite phase and the bainite phase exceeds 5 area%, the workability is inferior, and molding to a desired shape may be difficult. The balance can be made of ferrite phase.

Accordingly, the cold rolled steel sheet for exhaust system according to the embodiment of the present invention may have a yield strength of 200 to 260 MPa and an elongation of 35% or more.

For exhaust system  Cold rolled steel sheet manufacturing method

Cold rolled steel sheet manufacturing method for an exhaust system according to an embodiment of the present invention by weight% C: 0.015 to 0.05%, Si: 0.2% or less (excluding 0%), Mn: 0.1 to 0.3%, Al: 0.1% or less (Excluding 0%) and Cr: 2.0 to 4.0%, Mo: 0.5% or less (excluding 0%) and W: 0.3% or less (excluding 0%) , The remaining Fe and unavoidable impurities, heating the slab satisfying the following formula 1, hot rolling the slab to produce a hot rolled steel sheet, winding the hot rolled steel sheet, cold rolled wound hot rolled steel sheet Manufacturing a steel sheet, annealing the cold rolled steel sheet, and cooling the annealed cold rolled steel sheet.

Specifically, the method for manufacturing a cold rolled steel sheet for exhaust system according to an embodiment of the present invention further comprises the steps of rolling the cooled cold rolled steel sheet at a rolling reduction rate and plating Al (aluminum) or Zn (zinc) on the surface of the cold rolled steel sheet. It may include.

Hereinafter, each step will be described in detail.

First, the slab is heated. Specifically, it may be reheated at a temperature of 1200 ° C or higher. This is to reclaim most of the precipitates in the river. More specifically, to re-use the precipitate, it may be reheated at a temperature of 1250 ° C or more.

On the other hand, the reason for limiting the addition ratio of each composition in the slab and the effect of satisfying the equation 1 is the same as the reason for limiting the composition of the cold-rolled steel sheet for exhaust system described above and will not be repeated description. Since the composition of the slab is not substantially changed in the manufacturing process of hot rolling, winding, cold rolling, annealing, cooling, etc., which will be described later, the composition of the slab and the composition of the cold rolled steel sheet for exhaust system are substantially the same.

Next, the slab is hot rolled to produce a hot rolled steel sheet. Specifically, hot-rolled slabs may be manufactured by hot finishing rolling at a temperature of Ar ₃ or higher. Thereby, the slab can be hot rolled in the austenite single phase region.

Next, the hot rolled steel sheet is wound up. Specifically, it can be wound up at a coiling temperature of 550 to 750 ° C. By winding the hot-rolled steel sheet at a temperature of 550 ° C. or higher, it is possible to further precipitate N still in the state with AlN, thereby ensuring excellent aging resistance. When winding up at the temperature below 550 degreeC, there exists a danger that workability will fall by solid solution N which does not precipitate as AlN and remains. On the other hand, when winding up at a temperature exceeding 750 ℃, grains may be coarsened, which may be a factor in reducing cold rolling.

Next, the cold rolled hot rolled steel sheet is cold rolled to produce a cold rolled steel sheet. Specifically, it can be cold rolled at a reduction ratio of 50 to 95%. The final thickness of the cold rolled steel sheet is determined according to the reduction ratio. When the reduction ratio is less than 50%, it may be difficult to secure the final target thickness. On the other hand, when the reduction ratio exceeds 95%, the rolling load may be large and cold rolling may be difficult.

Next, the cold rolled steel sheet is annealed. Specifically, annealing may be performed at an annealing temperature of 550 to 900 ° C. Due to the annealing of the cold rolled steel sheet, the drawn grains may be recrystallized during cold rolling. When annealing at a temperature below 550 ° C., recrystallization does not occur sufficiently, so the dislocations generated during cold rolling may not be sufficiently removed, resulting in poor ductility. On the other hand, when annealing at a temperature exceeding 900 ℃, the grains are coarsened, the strength may be lowered and workability may be inferior. The annealing process is possible through ordinary annealing or continuous annealing.

Next, the cold rolled steel sheet is cooled. Specifically, cooling is performed to satisfy the following formula (2).

[Cr] * [Cooling rate] ≤ 25

In Equation 2, [Cr] means the content (wt%) of Cr, and [cooling rate] means the rate (℃ / sec) for cooling the cold rolled steel sheet. Cooling can be performed at a cooling rate of 5 to 9 ° C / sec.

As mentioned above, the formation of the pearlite phase and the bainite phase is closely related to the amount of Cr and the cooling rate during the annealing of the cold rolled steel sheet. Therefore, by controlling the value obtained by multiplying the Cr content and the cooling rate to 25 or less, one or more of the pearlite phase and the ferrite phase may be controlled to 5 area% or less.

The higher the amount of Cr and the cooling rate, the more active one or more of the pearlite and ferrite phases are formed. Therefore, when [Cr] * [cooling rate] exceeds 25, one or more of the pearlite and ferrite phases is excessively excessive. Because it is formed, the workability is poor and the elongation may be low.

Next, the cooled cold rolled steel sheet is rolled at a reduction ratio. Specifically, the rolling can be carried out at a reduction ratio of 0.6 to 1.4%. If the reduction ratio is less than 0.6%, the yield point stretching generated in the annealing process may not be controlled. On the other hand, if the reduction ratio exceeds 1.4%, it may cause a decrease in workability due to work hardening.

Next, Al or Zn may be plated on the surface of the cold rolled steel sheet. Cold rolled steel sheet for exhaust system according to an embodiment of the present invention is excellent in corrosion resistance and workability, when Al plating is performed, even if Al plating layer is removed can have excellent corrosion resistance in the corrosion environment of the exhaust system is a disadvantage of the conventional aluminum plated steel sheet Can be effectively overcome.

Hereinafter, specific examples of the present invention will be described. However, the following examples are only specific examples of the present invention, and the present invention is not limited to the following examples.

Example

(Manufacture of cold rolled steel sheet)

To prepare a steel having a composition of Table 1, the component is the performance value. The steel slab having the composition of Table 1 was reheated to 1250 ° C. and hot rolled at 900 ° C. or higher. Thereafter, it was wound up at 620 ° C. and cold rolled at a reduction ratio of 70%. Then, after annealing at 700 ° C. and cooling at the cooling rate shown in Table 2 below, correction rolling was performed at a rolling reduction of 1.2% to finally obtain a cold rolled steel sheet having a thickness of 1.2 mm.

weight% C Si Mn Al P S N Cr Mo W Corrosion resistance Development Steel 1 0.018 0.03 0.19 0.04 0.009 0.009 0.0028 3.01 0.05 0.06 0.09 Development Steel 2 0.028 0.08 0.2 0.03 0.009 0.009 0.0029 3.06 0.05 0.05 0.08 Development Steel 3 0.04 0.06 0.19 0.03 0.008 0.009 0.0029 3.07 0.06 0.05 0.086 Development Steel 4 0.048 0.07 0.19 0.04 0.008 0.008 0.0029 3.05 0.04 0.04 0.064 Development Steel 5 0.036 0.04 0.19 0.03 0.009 0.008 0.0029 2.05 0.04 0.06 0.084 Development Steel 6 0.034 0.08 0.21 0.03 0.009 0.009 0.0029 2.96 0.05 0.05 0.08 Development Steel 7 0.036 0.06 0.21 0.03 0.008 0.009 0.003 3.98 0.04 0.06 0.084 Development Steel 8 0.032 0.06 0.19 0.03 0.009 0.009 0.0029 2.95 0.09 0 0.054 Development Steel 9 0.034 0.04 0.19 0.04 0.008 0.008 0.003 2.94 0.15 0 0.09 Development Steel 10 0.037 0.05 0.2 0.04 0.008 0.008 0.003 2.92 0.2 0 0.12 Development Steel 11 0.037 0.03 0.2 0.03 0.009 0.009 0.003 2.91 0.45 0 0.27 Development Steel 12 0.037 0.04 0.21 0.03 0.009 0.009 0.0028 2.97 0 0.06 0.06 Development Steel 13 0.037 0.07 0.2 0.03 0.008 0.009 0.0027 2.91 0 0.15 0.15 Development Steel 14 0.036 0.08 0.21 0.03 0.008 0.009 0.0029 2.98 0 0.24 0.24 Development Steel 15 0.036 0.04 0.2 0.04 0.008 0.009 0.0029 3.05 0.02 0.04 0.052 Development Steel 16 0.038 0.05 0.21 0.04 0.008 0.009 0.0027 3.08 0.03 0.04 0.058 Development Steel 17 0.036 0.07 0.2 0.04 0.009 0.008 0.0029 2.95 0.04 0.03 0.054 Development Steel 18 0.031 0.03 0.19 0.03 0.009 0.008 0.003 3.06 0.1 0.05 0.11 Development Steel 19 0.034 0.07 0.19 0.03 0.008 0.009 0.0028 3 0.05 0.09 0.12 Development Steel 20 0.033 0.06 0.2 0.04 0.008 0.009 0.0028 2.98 0.04 0.04 0.064 Development Steel 21 0.039 0.03 0.19 0.04 0.008 0.008 0.0027 3.03 0.04 0.06 0.084 Development Steel 22 0.034 0.05 0.19 0.04 0.008 0.008 0.0029 2.95 0.05 0.06 0.09 Development Steel 23 0.033 0.06 0.2 0.03 0.009 0.009 0.0028 3.06 0.06 0.04 0.076 Comparative Steel 1 0.012 0.06 0.2 0.03 0.009 0.008 0.0029 2.93 0.06 0.05 0.086 Comparative Steel 2 0.062 0.06 0.2 0.03 0.009 0.008 0.0029 2.9 0.05 0.04 0.07 Comparative Steel 3 0.039 0.04 0.2 0.04 0.008 0.008 0.003 0 0.06 0.05 0.086 Comparative Steel 4 0.04 0.05 0.2 0.03 0.009 0.009 0.003 1.5 0.05 0.06 0.09 Comparative Steel 5 0.031 0.07 0.21 0.03 0.008 0.008 0.0028 4.8 0.05 0.05 0.08 Comparative Steel 6 0.03 0.05 0.2 0.03 0.008 0.008 0.003 2.95 0 0 0 Comparative Steel 7 0.033 0.03 0.2 0.03 0.008 0.009 0.0028 3.1 0.07 0 0.042 Comparative Steel 8 0.036 0.06 0.19 0.03 0.009 0.008 0.0028 3 0 0.04 0.04 Comparative Steel 9 0.037 0.08 0.2 0.03 0.009 0.008 0.0028 2.95 0.03 0.02 0.038 Comparative Steel 10 0.038 0.05 0.19 0.03 0.009 0.008 0.003 3 0.05 0.05 0.08 Comparative Steel 11 0.039 0.05 0.19 0.04 0.009 0.008 0.0028 3.05 0.05 0.04 0.07 Comparative Steel 12 0.039 0.05 0.19 0.04 0.009 0.008 0.0028 3.05 0.05 0.04 0.07

division Bainite + pearlite
(area%) Cooling rate (℃ / sec) Curing Index Development Steel 1 2.4 5.8 17.458 Development Steel 2 2.3 5.7 17.442 Development Steel 3 2.4 5.7 17.499 Development Steel 4 2.1 5.3 16.165 Development Steel 5 One 5.7 11.685 Development Steel 6 2.2 5.7 16.872 Development Steel 7 2.9 4.9 19.502 Development Steel 8 1.9 5.5 16.225 Development Steel 9 1.8 5.2 15.288 Development Steel 10 1.8 5.1 14.892 Development Steel 11 1.8 5.4 15.714 Development Steel 12 2.4 5.9 17.523 Development Steel 13 2.3 5.8 16.878 Development Steel 14 2.2 5.8 17.284 Development Steel 15 2.3 5.7 17.385 Development Steel 16 2.4 5.8 17.864 Development Steel 17 1.9 5.3 15.635 Development Steel 18 2.5 5.9 18.054 Development Steel 19 2.4 5.9 17.7 Development Steel 20 0.7 3.5 10.43 Development Steel 21 2.4 5.8 17.574 Development Steel 22 3.8 7.4 21.83 Development Steel 23 4.9 8 24.48 Comparative Steel 1 1.8 5.3 15.529 Comparative Steel 2 2.1 5.7 16.53 Comparative Steel 3 0 5.1 0 Comparative Steel 4 0.4 5.5 8.25 Comparative Steel 5 5.3 5.2 24.96 Comparative Steel 6 1.7 5.2 15.34 Comparative Steel 7 2 5.1 15.81 Comparative Steel 8 2.2 5.4 16.2 Comparative Steel 9 2.3 5.9 17.405 Comparative Steel 10 7.2 9.8 29.4 Comparative Steel 11 19.1 15.8 48.19 Comparative Steel 12 31.6 20.2 61.61

In Table 1, the corrosion index represents 0.6 * [Mo] + [W], and the unit is weight%, and the hardening index in Table 2 represents [Cr] * [cooling rate], and the unit is weight% * ° C. / sec.

(Corrosion resistance and processability evaluation)

Each manufactured cold rolled steel sheet was evaluated for corrosion resistance through the corrosion test for the condensate corrosion environment simulation corrosion conditions of Table 3 below, and the mechanical properties were evaluated through the room temperature tensile test. Considering the increase of ion concentration and decrease of pH as evaporation of condensate for the evaluation of corrosion resistance, corrosion conditions with low corrosive environment and high pH and low pH and corrosion condition B The simulations were performed to represent a large corrosion environment and the average corrosion rate was measured for each corrosion condition. In addition, elongation and yield strength were measured through a tensile test at room temperature for workability evaluation.

division Ion concentration (ppm) pH Corrosion temperature Corrosion time SO ₄ ^2- SO ₃ ^2- NO ^3- Cl ^- CH ₃ COO ^- NH ⁴⁺ Corrosion condition A 1000 150 30 200 200 adding 2.0 70 ℃ 6 hours Corrosion condition B 5000 750 150 1000 1000 No addition 0.8

The measured average corrosion rate, elongation, and yield strength are shown in Table 4 below.

division Corrosion resistance Machinability Corrosion condition A Corrosion condition B Elongation (%) Yield strength (MPa) Development Steel 1 0.47 5.10 38.8 201.2 Development Steel 2 0.50 5.74 38.3 221.1 Development Steel 3 0.53 5.34 36.8 238.5 Development Steel 4 0.48 6.10 38.9 247.8 Development Steel 5 0.53 5.88 39.8 230.3 Development Steel 6 0.51 5.65 37.7 226.6 Development Steel 7 0.47 5.71 38.8 233.1 Development Steel 8 0.50 6.56 38.4 228.4 Development Steel 9 0.49 5.12 39.2 240.2 Development Steel 10 0.52 4.63 40.0 230.3 Development Steel 11 0.47 3.01 36.7 250.3 Development Steel 12 0.49 6.61 39.0 228.3 Development Steel 13 0.52 3.98 37.6 243.1 Development Steel 14 0.49 3.07 37.2 228.4 Development Steel 15 0.47 6.53 37.7 239.7 Development Steel 16 0.48 6.49 36.2 240.4 Development Steel 17 0.53 6.58 37.8 238.6 Development Steel 18 0.47 4.77 36.3 233.9 Development Steel 19 0.48 4.59 37.8 232.9 Development Steel 20 0.49 5.83 41.2 231.4 Development Steel 21 0.49 5.76 37.7 234.0 Development Steel 22 0.49 5.10 35.5 235.2 Development Steel 23 0.49 5.93 35.1 248.1 Comparative Steel 1 0.47 5.78 39.8 181.2 Comparative Steel 2 0.52 5.78 36.7 292.6 Comparative Steel 3 1.02 4.25 41.7 231.3 Comparative Steel 4 0.90 4.72 38.2 238.6 Comparative Steel 5 0.42 6.37 33.9 257.9 Comparative Steel 6 0.53 13.35 37.5 223.1 Comparative Steel 7 0.53 9.53 37.5 240.6 Comparative Steel 8 0.50 9.83 37.8 237.9 Comparative Steel 9 0.49 10.05 38.7 242.8 Comparative Steel 10 0.49 5.72 32.5 264.0 Comparative Steel 11 0.53 5.53 21.7 329.7 Comparative Steel 12 0.47 5.57 8.6 470.1

In Table 4, the corrosion conditions A and B show the average corrosion rates (mg / cm ² / hr).

As shown in Table 4, the development steel 1 to 23 that satisfies the composition and manufacturing conditions of the present invention, the corrosion rate under corrosion condition A is less than 0.6mg / cm ² / hr, corrosion rate under corrosion condition B Is 7.0 mg / cm ² / hr or less, the elongation is 35% or more, yield strength of 200 to 260MPa can be seen that it has excellent corrosion resistance and mechanical properties.

On the other hand, Comparative steel 1 has a low content of C, which is advantageous for securing workability, but has a low yield strength of 181.2 MPa, which is disadvantageous for use as a structural material. It was difficult.

Comparative steel 3 was a material without Cr, and the corrosion rate was 1.02 mg / cm ² / hr at the corrosion condition A, resulting in inferior corrosion resistance. This is because the Cr oxide layer improves the corrosion resistance for relatively low corrosive condensate such as corrosion condition A. Comparative steel 4 had a relatively high Cr content of 1.5% compared to comparison steel 3, but the corrosion rate under corrosion condition A was 0.90 mg / cm ² / hr.

Comparative steel 5 had a high Cr content of 4.8% and a low corrosion rate of 0.42 mg / cm ² / hr under corrosion condition A. However, the elongation was low to 33.9%, inferior to workability. This is because the sum of the hard pearlite phase and the bainite phase was formed in a large amount of 5.2 area%. As a result, the pearlite and bainite phases are better formed with higher Cr content and faster cooling rate, so that at least one of the pearlite and ferrite phases has an area of 5 when the curing index defined by Equation 2 is 25 or less. It was found that the characteristic was less than% and the elongation was 35% or more.

In Comparative Steels 6 to 9, the corrosion rate for corrosion condition A is 0.49 to 0.53 mg / cm ² / hr, and the corrosion resistance is good, but the corrosion rate for corrosion condition B is 9.53 to 13.35 mg / cm ² / hr. Corrosion resistance was significantly inferior to that of developed steel. This is because the Cr content has a corrosion resistance of the corrosion condition A of about 3.0% but the content of Mo and W is small. Through this, it was found that the content of Mo and W is effectively improved in a low pH environment when added to the corrosion index is defined as 0.05 or more. However, since Mo and W are very expensive elements, it is not economical to add a large amount, and in particular, since W has a high melting point of 3000 ° C. or higher, it may be effective to use 0.5% or less of Mo and 0.3% or less of W.

The present invention is not limited to the above embodiments and / or embodiments, but may be manufactured in various forms, and a person of ordinary skill in the art to which the present invention pertains may change the technical spirit or essential features of the present invention. It will be understood that other specific forms may be practiced without doing so. Therefore, it is to be understood that the embodiments and / or embodiments described above are illustrative in all respects and not restrictive.

Claims (8)

C: 0.015 to 0.05%, Si: 0.2% or less (excluding 0%), Mn: 0.1 to 0.3%, Al: 0.1% or less (excluding 0%) and Cr: 2.0 to 4.0% by weight. At least one of Mo: not more than 0.5% (except 0%) and W: not more than 0.3% (except 0%), including the balance Fe and unavoidable impurities, satisfying the following formula ,
A cold rolled steel sheet for exhaust system comprising at least one of pearlite phase and ferrite phase in an area of 5% by area or less.
0.05 ≦ 0.6 * [Mo] + [W] (unit: wt%)
(In Formula 1, [Mo] and [W] means the content of Mo and W (% by weight), respectively.) The method of claim 1,
P: 0.01% or less (except 0%), S: 0.01% or less (except 0%) and N: 0.007% or less (except 0%). The method of claim 1,
A cold rolled steel sheet for exhaust system having a yield strength of 200 to 260 MPa and an elongation of 35% or more. C: 0.015 to 0.05%, Si: 0.2% or less (excluding 0%), Mn: 0.1 to 0.3%, Al: 0.1% or less (excluding 0%) and Cr: 2.0 to 4.0% by weight. At least one of Mo: not more than 0.5% (except 0%) and W: not more than 0.3% (except 0%), including the balance Fe and unavoidable impurities, satisfying the following formula Heating the slab;
Hot rolling the slab to produce a hot rolled steel sheet;
Winding the hot rolled steel sheet;
Cold rolling the wound hot rolled steel sheet to produce a cold rolled steel sheet;
Annealing the cold rolled steel sheet; And
And cooling the annealed cold rolled steel sheet to satisfy Equation 2 below.
0.05 ≦ 0.6 * [Mo] + [W] (unit: wt%)
(In Formula 1, [Mo] and [W] means the content of Mo and W (% by weight), respectively.)
[Cr] * [Cooling rate] ≤ 25
(In Formula 2, [Cr] means the content of Cr (% by weight), and [cooling rate] means the rate of cooling the cold rolled steel sheet (° C./sec.).) The method of claim 4, wherein
After the step of cooling the cold rolled steel sheet,
Rolling the cooled cold rolled steel sheet at a rolling reduction of 0.6 to 1.4%; Cold rolled steel sheet manufacturing method for an exhaust system further comprising. The method of claim 4, wherein
In the step of winding the hot rolled steel sheet,
A method for producing a cold rolled steel sheet for exhaust system, wherein the hot rolled steel sheet is wound at a winding temperature of 550 to 750 ° C. The method of claim 4, wherein
In the annealing of the cold rolled steel sheet,
The cold rolled steel sheet manufacturing method for an exhaust system for annealing at an annealing temperature of 550 to 900 ℃. The method of claim 4, wherein
In the step of cooling the cold rolled steel sheet,
A method for producing a cold rolled steel sheet for exhaust system, which cools the annealed cold rolled steel sheet at a cooling rate of 5 to 9 ° C / sec.