FERRITIC STAINLESS STEEL FOR EGR SYSTEM

- KIA MOTORS CORPORATION

Disclosed is a ferritic stainless steel for an EGR system, more particularly, a ferritic stainless steel for an EGR system which may suppress the ferritic stainless steel from being discolored and which improves the moldability, oxidation resistance, and corrosion resistance by including iron (Fe) as a base material, about 18 to 20% by weight of chromium (Cr) based on a total weight of an alloy, molybdenum (Mo), carbon (C) and niobium (Nb).

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Description
CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority under 35 U.S.C. §119 to Korean Patent Application No. 10-2012-0158666, filed on Dec. 31, 2012, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein by reference in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a ferritic stainless steel for an exhaust gas recirculation (EGR) system, particularly a ferritic stainless steel for an EGR system, which resists a color change during the brazing thereof. More particularly, the present invention provides a ferritic stainless steel that resists color change by controlling the composition ratio of the ferritic stainless steel, and that further improves the moldability, oxidation resistance, and corrosion resistance.

2. Description of the Related Art

While air pollution caused by factories, heating, power generation facilities, and the like is decreasing, air pollution caused by exhaust gas from automobiles is increasing. Exhaust gas from automobiles is exhausted at high concentrations in densely populated areas and housing areas near roads, thereby causing great harm to people's health. Accordingly, a significant amount research and development has been directed towards treating exhaust gas from automobiles.

In general, an automobile is driven by an internal combustion engine, and the engine is operated by the combustion of fossil fuel. During the combustion of fossil fuel, hazardous materials that cause environmental pollution are discharged. Such hazardous materials include, for example, carbon monoxide (CO), nitrous oxide (NOx), carbon dioxide (CO2), sulfur oxide (Sox) and the like.

In particular, the nitrous oxide (NOx) included in the exhaust gas is responsible for acid rain and smog. Nitrous oxide (NOx) further irritates the eyes and the respiratory organs, thereby causing symptoms such as the release of saliva, sore throat, dizziness, headache, and vomiting, and further leads to plant death. Accordingly, nitrous oxide (NOx) is regulated as a main air pollutant, and numerous devices for reducing the discharge of nitrous oxide (NOx) have been developed.

Among these devices, an EGR system-related technology has been actively developed. The EGR system consists of an EGR cooler, an EGR pipe, an EGR valve, and the like. When a fuel-air mixture (i.e., a gas in which fuel and air are mixed) is combusted in a cylinder during the explosion stroke of an engine, the combustion temperature may be decreased by recirculating a part of the exhaust gas into an intake system of the engine. This suppresses nitrous oxide (NOx) from being produced by the reaction of nitrogen (N2) and oxygen (O2) in the combustion air at high temperature. That is, the EGR system is a system that decreases the amount of nitrous oxide (NOx) produced by lowering the combustion temperature when the fuel-air mixture is combusted after a part of the exhaust gas is returned to the intake system.

Austenitic stainless steel is one material that has been commonly used for the constituent parts of the EGR system. However, nickel (Ni), which is expensive, is included therein in a large amount. Thus, the use of austenitic stainless steel is problematic due to the steady increase in cost caused by the high cost of nickel and the price instability of nickel. For popularization of the EGR system, it is essential to reduce costs associated therewith, and it is necessary to use a reasonably priced and appropriate materials.

In order to reduce the cost as described above, there has been an attempt to replace a portion to which austenitic stainless steel is applied with ferritic stainless steel, which is relatively inexpensive. However, the EGR system is subjected to a brazing process of melting a metal insert to be bonded to a base metal in a batch furnace or continuous furnace. This disadvantageous because titanium (Ti) which is included in a conventional ferritic stainless steel reacts with nitrogen (N2) during the brazing thereof to produce titanium nitride (Ti—N) on the surface thereof. This results in a change in the color of the material.

Further, the conventional ferritic stainless steel has insufficient moldability, and thus is disadvantageous in that an orange peel phenomenon is generated (i.e., modified lines or wrinkles are generated on a surface that does not include cracks).

In addition, the conventional ferritic stainless steel is disadvantageous in that the ferritic stainless steel has lower oxidation resistance and corrosion resistance than austenitic stainless steel.

Thus, what is needed is a ferritic stainless steel for an ERG system, which is inexpensive, has improved moldability, oxidation resistance and corrosion resistance, and which further avoids discoloration thereof during brazing.

SUMMARY OF THE INVENTION

The present invention provides a ferritic stainless steel for an EGR system, which is inexpensive and has improved moldability, oxidation resistance, and corrosion resistance, and which is not discolored during brazing. More particularly, the present invention provides a ferritic stainless steel that provides these benefits by adjusting the constituent components of the ferritic stainless steel. As such, the present ferritic stainless steel can suitably replace a portion to which an austenitic stainless steel is applied in a conventional EGR system.

According to one aspect, the present invention provides a ferritic stainless steel for an EGR system comprising iron (Fe) as a base material, about 18 to 20% by weight of chromium (Cr) based on a total weight of an alloy (as used herein, “an alloy” refers to the components forming the ferritic stainless steel), and which can further include one or more of molybdenum (Mo), carbon (C) and niobium (Nb).

According to various embodiments, a % by weight of the niobium (Nb) is about 5 times or more % by weight as compared to the % by weight of carbon (C).

According to an exemplary embodiment of the present invention, the niobium (Nb) is present in an amount of about 0.2 to 0.7% by weight based on the total weight of the alloy.

According to various embodiments, the ferritic stainless steel for an EGR system further comprises one or more of nitrogen (N) and carbon (C). According to an exemplary embodiment, the ferritic stainless steel further comprises more than 0 and up to about 0.01% by weight nitrogen (N), and more than 0 and up to about 0.02% by weight carbon (C) based on the total weight of the alloy.

According to various embodiments, the ferritic stainless steel further includes aluminum (Al). According to an exemplary embodiment, the molybdenum is present in an amount of about 0.75 to 1.5% by weight and aluminum (Al) is present in an amount of about 0.3 to 0.8% by weight based on the total weight of the alloy.

According to further aspects, the present invention provides an EGR system comprising the ferritic stainless steel, particularly an EGR cooler, an EGR pipe, an EGR valve or an EGR bracket comprising the ferritic stainless steel.

According the present invention, a ferritic stainless steel is provided that reduces costs compared that of an austenitic stainless steel, particularly by including carbon (C), nitrogen (N), chromium (Cr), molybdenum (Mo), aluminum (Al) and niobium (Nb).

According to embodiments of the present invention, by excluding titanium from the present composition, titanium nitride (Ti—N) is not produced on the surface of the ferritic stainless steel during brazing, thus avoiding discoloration.

Further, according to embodiments of the present invention, moldability is secured by controlling the content of carbon (C), nitrogen (N) and niobium (Nb) in the ferritic stainless steel composition.

In addition, according to embodiments of the present invention, oxidation resistance and corrosion resistance are improved by controlling the content of molybdenum (Mo) and aluminum (Al) in the ferritic stainless steel composition.

Other features and aspects of the present invention will be apparent from the following detailed description, drawings and claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a photograph of an EGR cooler which is made of the ferritic stainless steel in the related art and, thus, is discolored.

FIG. 2 is a photograph of an EGR cooler which is manufactured of a ferritic stainless steel for an EGR system according to an embodiment of the present invention and, thus, is not discolored.

FIG. 3 is an electron micrograph before the brazing of a ferritic stainless steel according to an embodiment of the present invention.

FIG. 4 illustrates the constituent components on the surface of a ferritic stainless steel according to an embodiment of the present invention.

FIGS. 5 to 7 are photographs of a discolored site of the ferritic stainless steel in the related art, which are magnified 100 times, 200 times, and 500 times, respectively through an electronic microscope.

FIG. 8 illustrates the constituent components of the discolored site of the ferritic stainless steel in the related art.

FIGS. 9 to 11 are photographs observed after brazing the ferritic stainless steel for an EGR system according to embodiments of the present invention, a conventional ferritic stainless steel and an austenitic stainless steel, respectively.

FIGS. 12 to 14 are photographs of the corrosion resistance test results under a heat treatment condition of 400° C., which is similar to an actuation environment of an EGR cooler of the ferritic stainless steel for an EGR system according to embodiments of the present invention, after 10 days, 80 days and 160 days have passed.

FIGS. 15 to 17 are photographs of the corrosion resistance test results of the ferritic stainless steel in the related art under a heat treatment condition of 400° C. after 10 days, 80 days and 160 days have passed.

FIGS. 18 to 20 are photographs of the corrosion resistance test result of the austenitic stainless steel in the related art under a heat treatment condition of 400° C. after 10 days, 80 days and 160 days have passed.

It should be understood that the appended drawings are not necessarily to scale, presenting a somewhat simplified representation of various preferred features illustrative of the basic principles of the invention. The specific design features of the present invention as disclosed herein, including, for example, specific dimensions, orientations, locations, and shapes will be determined in part by the particular intended application and use environment.

In the figures, reference numbers refer to the same or equivalent parts of the present invention throughout the several figures of the drawing.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Terms or words used in the present specification and claims should not be interpreted as being limited to typical or dictionary meanings, but should be interpreted as having meanings and concepts, which comply with the technical spirit of the present invention, based on the principle that an inventor can appropriately define the concept of the term to describe his/her own invention in the best manner.

It is understood that the term “vehicle” or “vehicular” or other similar term as used herein is inclusive of motor vehicles in general such as passenger automobiles including sports utility vehicles (SUV), buses, trucks, various commercial vehicles, watercraft including a variety of boats and ships, aircraft, and the like, and includes hybrid vehicles, electric vehicles, plug-in hybrid electric vehicles, hydrogen-powered vehicles and other alternative fuel vehicles (e.g. fuels derived from resources other than petroleum). As referred to herein, a hybrid vehicle is a vehicle that has two or more sources of power, for example both gasoline-powered and electric-powered vehicles.

The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. As used herein, the singular forms “a,” “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises” and/or “comprising,” when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof. As used herein, the term “and/or” includes any and all combinations of one or more of the associated listed items.

Unless specifically stated or obvious from context, as used herein, the term “about” is understood as within a range of normal tolerance in the art, for example within 2 standard deviations of the mean. “About” can be understood as within 10%, 9%, 8%, 7%, 6%, 5%, 4%, 3%, 2%, 1%, 0.5%, 0.1%, 0.05%, or 0.01% of the stated value. Unless otherwise clear from the context, all numerical values provided herein are modified by the term “about”.

Hereinafter, the present invention will be described in detail based on Tables and drawings.

FIG. 1 is a photograph of an EGR cooler which is manufactured of the ferritic stainless steel in the related art, and FIG. 2 is a photograph of an EGR cooler which is manufactured of the ferritic stainless steel for an EGR system according to an embodiment of the present invention. As clearly shown, the EGR cooler manufactured of the conventional ferritic stainless steel is discolored, while the EGR cooler manufactured of the present invention ferritic stainless steel is not discolored.

According to a preferred embodiment of the present invention, the constituent components of the ferritic stainless steel include iron (Fe) as a base material (in other words, iron is a main component and makes up the balance of the composition), carbon (C), nitrogen (N), chromium (Cr), molybdenum (Mo), aluminum (Al) and niobium (Nb). The constituent components and contents of an embodiment of the present invention will be investigated in detail through the Table 1.

TABLE 1 Physical properties Constituent components (% by weight based on total weight of the composition) YP TS EL Classification C N Cr Ni Ti Mo Al Nb Fe (MPa) (MPa) (%) Hv Comparative 0.08 or — 18 to 20 8 to — — — — Remainder 205 450 59 or — Example 1 less 10.5 more Comparative 0.025 — 16 to 19 — 0.30 0.75 to — — Remainder 300 470 32 or 155 Example 2 or less or less 1.5 more Present invention about about about — — about about about Remainder 235 435 46 or — 0.02 or 0.01 or 18 to 20 0.75 to 0.3 to 0.2 to more less less 1.5 0.8 0.7

Table 1 is a table that compares the constituent components, contents, and physical properties of Comparative Example 1, Comparative Example 2, and the present invention. YP means the yield point, TS means the tensile strength, EL means the elongation, and Hv means the hardness in vickers.

Comparative Example 1 in Table 1 shows the constituent components, contents, and physical properties of the austenitic stainless steel (SUS304 and 300 series), and Comparative Example 2 shows the constituent components, contents, and physical properties of the ferritic stainless steel (SUS436L and 400 series).

The present invention has been developed in order to solve the discoloration problem during brazing of the ferritic stainless steel. In particular, according to the present invention, the constituent components and contents of ferritic stainless steel, which is more expensive than the austenitic stainless steel, are controlled within a particular range. As shown in Table 1, the ferritic stainless steel of the present invention does not include titanium (Ti), which is responsible for the discoloration during brazing, which is in contrast with the conventional ferritic stainless steel of Comparative Example 2.

FIG. 3 is an electron micrograph before the brazing of the conventional ferritic stainless steel, and FIG. 4 illustrates the constituent components on the surface of the ferritic stainless steel of FIG. 3 with a graph and a Table. Iron (Fe) and chromium (Cr) are usually detected in the components on the surface thereof because the ferritic stainless steel includes iron (Fe) as a base material and chromium (Cr) in the largest amount.

FIGS. 5 to 7 are photographs of a discolored site of the conventional ferritic stainless steel of FIG. 3 after the brazing thereof, which are magnified 100 times, 200 times, and 500 times, respectively, through an electronic microscope. FIG. 8 illustrates the constituent components of the discolored sites of the ferritic stainless steel with a graph and a Table. It can be confirmed that in FIG. 8, which shows the constituent components of the discolored site, that titanium (Ti) has been considerably increased in comparison with FIG. 4 which shows the original constituent components which are not discolored (i.e. prior to brazing). Accordingly, it was demonstrated that titanium (Ti) is the main factor of the discoloration of the ferritic stainless steel.

Hereinafter, the principle through which titanium (Ti) changes the color of the conventional ferritic stainless steel will be investigated. An EGR system is manufactured through brazing, which is one of the methods of bonding a metal in a batch furnace or continuous furnace. Titanium (Ti), which is one of the constituent components of the conventional ferritic stainless steel, reacts with nitrogen (N2) during brazing to form dark titanium nitride (Ti—N) on the surface of the stainless steel. As a result, the ferritic stainless steel is discolored.

According to an aspect of the present invention, a ferritic stainless steel composition is provided which does not include titanium (Ti). As such, titanium nitride, which is responsible for discoloration, is not produced during the brazing of the ferritic stainless steel.

However, the use of titanium in stainless steel provides a benefit in that the titanium (Ti) bonds with carbon (C), thus stabilizing the carbon (C) and strengthening the stainless steel. By elimination of the titanium (Ti), titanium (Ti) and carbon (C) do not bind, the carbon (C) becomes unstable, and the strength of the stainless steel becomes weaker.

According to the present invention, in order to stabilize the carbon (C), niobium (Nb) is added. According to preferred embodiments, the amount of niobium (Nb) added is about 5 times or more % by weight the amount of carbon (C) added. Such amounts facilitate the stabilization of the alloy (i.e. the ferritic stainless steel composition).

Hereinafter, in the present invention, the role of each constituent component and their preferred amounts will be described in further detail.

The carbon (C) reacts with niobium (Nb) to form carbon nitride, which increases the strength of the stainless steel. However, when carbon (C) is added in a content of more than about 0.02% by weight based on the total weight of the ferritic stainless steel composition, moldability and corrosion resistance deteriorate and the precipitation of carbon nitride including niobium (Nb) results to reduce the high temperature strength. As such, the content of carbon (C) is preferably more than 0 and up to about 0.02% by weight based on the total weight of the ferritic stainless steel composition.

Further, the nitrogen (N) serves to increase the strength of the stainless steel by forming carbon nitride. However, but when the nitrogen (N) is added in an amount of more than about 0.01% by weight based on the total weight of the ferritic stainless steel composition, the moldability and corrosion resistance deteriorate. As such, the content of nitrogen (N) is preferably more than 0 and up to about 0.01% by weight based on the total weight of the ferritic stainless steel composition.

In addition, the chromium (Cr) is an important element in improving the high temperature strength, oxidation resistance and corrosion resistance of the stainless steel. According to preferred embodiments, the chromium (Cr) is included in an amount of about 18% by weight or more in order to obtain sufficient high temperature strength, oxidation resistance, and corrosion resistance. However, when the content of chromium (Cr) exceeds about 20% by weight, the strength of the stainless steel is increased to reduce the elongation, thereby leading to a deterioration of moldability. As such, the content of chromium (Cr) is preferably about 18 to 20% by weight.

Furthermore, the molybdenum (Mo) is an important element in improving the high temperature strength and corrosion resistance of the stainless steel. According to preferred embodiments, the molybdenum (Mo) is included in an amount of about 0.75% by weight or more in order to obtain sufficient oxidation resistance, corrosion resistance, and high temperature strength. However, when the content of molybdenum (Mo) exceeds about 1.5% by weight, the tenacity thereof deteriorates. As such, the content of molybdenum (Mo) is preferably about 0.75 to 1.5% by weight.

Further, the aluminum (Al) is an element which is added as a deoxidizer of the stainless steel. According to preferred embodiments, the aluminum (Al) is included in a content of about 0.3% by weight or more in order to improve the oxidation resistance, corrosion resistance, and high temperature strength of the stainless steel. However, when the content thereof exceeds about 0.8% by weight, the stainless steel is hardened, thus resulting in cracks occur and reduction in tenacity. Accordingly, the content of aluminum (Al) is preferably about 0.3 to 0.8% by weight.

In addition, the niobium (Nb) serves to stabilize the stainless steel by fixing carbon (C) and nitrogen (N) as a carbon nitride, and is also an element effective in improving the high temperature strength. According to preferred embodiments, the niobium (Nb) is included in a content of about 0.2% by weight or more in order to obtain sufficient high temperature strength. However, when the content exceeds about 0.7% by weight, the tenacity deteriorates. As such, the content of niobium (Nb) is preferably about 0.2 to 0.7% by weight.

The ferritic stainless steel for an EGR system according to the present invention may be appropriately prepared by those skilled in the art using any known technology, and may be extensively applied as a material that requires corrosion resistance for an EGR cooler, an EGR pipe, an EGR valve, an EGR bracket and the like.

EXAMPLE 1

Hereinafter, the present invention will be described in more detail through the Examples. These Examples are only for illustrating the present invention, and it will be obvious to those skilled in the art that the scope of the present invention is not interpreted to be limited by these Examples.

FIGS. 9 to 11 are photographs taken after brazing the ferritic stainless steel for an EGR system according to an embodiment of the present invention, a conventional ferritic stainless steel, and a conventional austenitic stainless steel, respectively. As demonstrated in FIGS. 9 to 11, the ferritic stainless steel of the present invention and the austenitic stainless steel, both of which do not include titanium (Ti), are not discolored during the brazing thereof. This is in contrast with the conventional ferritic stainless steel in which titanium (Ti) is included and which shows discoloration.

Furthermore, FIGS. 12 to 14 are photographs of the corrosion resistance test results under a heat treatment condition of 400° C., which is similar to an actuation environment of an EGR cooler, of the ferritic stainless steel for an EGR system according to the present invention, after 10 days, 80 days and 160 days have passed.

FIGS. 15 to 17 are photographs of the corrosion resistance test results of the conventional ferritic stainless steel under a heat treatment condition of 400° C. after 10 days, 80 days and 160 days have passed.

FIGS. 18 to 20 are photographs of the corrosion resistance test results of the conventional austenitic stainless steel under a heat treatment condition of 400° C. after 10 days, 80 days and 160 days have passed.

As demonstrated by the tests, the ferritic stainless steel for an EGR system according to the present invention has corrosion resistance at least equivalent to that of the conventional austenitic and ferritic stainless steels. On particular, corrosion does not proceed in the ferritic stainless steel for an EGR system according to the present invention.

Hereinafter, the test results of FIGS. 12 to 20 will be investigated in more detail through Tables 2 and 3.

TABLE 2 10 days 80 days Residual Residual 160 days thickness thickness Residual thickness Classification (%) (%) (%) Comparative Example 1 100 100 100 Comparative Example 2 100 100 100 Example 1 100 100 100

TABLE 3 10 days 80 days 160 days Residual Residual Residual weight Classification weight (%) weight (%) (%) Comparative Example 1 100 100 100 Comparative Example 2 100 100 100 Example 1 100 100 100

Tables 2 and 3 are tables that compare the residual thickness (%) and the residual weight (%) after an experiment is performed under a heat treatment condition of 400° C., which is similar to an actuation environment of an EGR cooler in order, to investigate the corrosion resistance of Comparative Example 1, Comparative Example 2, and Example 1. Comparative Example 1 means the conventional austenitic stainless steel , and Comparative Example 2 refers to the conventional ferritic stainless steel. Example 1 is the ferritic stainless steel according to an embodiment of the present invention, based on the constituent components and contents of the present invention of Table 1.

Each of the residual thickness (%) and residual weight (%) of Comparative Example 1, Comparative Example 2, and Example 1 was measured after 10 days, 80 days and 160 days of the start of the test had passed, and as a result of the test, it was shown that Example 1 secured corrosion resistance at least equivalent to those of Comparative Example 1 and Comparative Example 2 from the fact that Comparative Example 1, Comparative Example 2, and Example 1 all have a residual thickness and a residual weight of 100%, respectively.

As described above, the present invention has been described in relation to specific embodiments of the present invention, but this is only illustration and the present invention is not limited thereto. Embodiments described may be changed or modified by those skilled in the art to which the present invention pertains without departing from the scope of the present invention, and various alterations and modifications are possible within the technical spirit of the present invention and the equivalent scope of the claims which will be described below.

Claims

1. A ferritic stainless steel for an EGR system, comprising: iron (Fe) as a base material, 18 to 20% by weight of chromium (Cr) based on a total weight of an alloy forming the ferritic stainless steel, molybdenum (Mo), carbon (C) and niobium (Nb).

2. The ferritic stainless steel for an EGR system of claim 1, wherein a % by weight of the niobium (Nb) is about 5 times the % by weight of carbon (C) based on the total weight of the alloy.

3. The ferritic stainless steel for an EGR system of claim 1, comprising about 0.2 to 0.7% by weight niobium (Nb) based on the total weight of the alloy.

4. The ferritic stainless steel for an EGR system of claim 1, further comprising:

nitrogen (N), wherein the carbon (C) is present in an amount of more than 0 and up to about 0.02% by weight, and the nitrogen (N) is present in an amount of more than 0 and up to about 0.01% by weight based on the total weight of the alloy.

5. The ferritic stainless steel for an EGR system of claim 1, further comprising:

aluminum (Al), wherein the molybdenum (Mo) is present in an amount of about 0.75 to 1.5% by weight, and aluminum (Al) is present in an amount of about 0.3 to 0.8% by weight.

6. The ferritic stainless steel for an EGR system of claim 1, which does not include titanium (Ti).

7. The ferritic stainless steel for an EGR system of claim 1, wherein the ferritic stainless steel is applied to an EGR cooler, an EGR pipe, an EGR valve or an EGR bracket.

Patent History
Publication number: 20140186206
Type: Application
Filed: Jul 24, 2013
Publication Date: Jul 3, 2014
Applicants: KIA MOTORS CORPORATION (Seoul), HYUNDAI MOTOR COMPANY (Seoul)
Inventors: Joo-Sung Park (Yongin), Seung-Hyun Lee (Kimcheon), Ho-Chul Shin (Suwon)
Application Number: 13/949,658
Classifications
Current U.S. Class: Molybdenum Or Tungsten Conaining (420/63); Molybdenum Or Tungsten Containing (420/67)
International Classification: C22C 38/26 (20060101); C22C 38/06 (20060101); C22C 38/22 (20060101);