JP2009174040A - Ferritic stainless steel for egr cooler, and egr cooler - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a ferritic stainless steel suitable as an EGR (Exhaust Gas Recirculation) cooler member to be subjected to Ni brazing. <P>SOLUTION: Disclosed is a ferritic stainless steel for an EGR cooler member having a composition comprising, by mass, ≤0.03% C, >0.1 to 3% Si, 0.1 to 2% Mn, 10 to 25% Cr, 0.3 to 0.8% Nb and ≤0.03% N, and, if required, further selectively comprising (a) one or more selected from Mo, Cu, V and W in the range of ≤4% in total, (b) one or more selected from Ti, Al and Zr in the range of ≤0.3% in total, (c) one or more selected from Ni and Co in the range of ≤5% in total and (d) one or more selected from REM (rare earth elements) and Ca in the range of ≤0.2% in total, respectively, and the balance Fe with inevitable impurities. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a ferritic stainless steel used as a member constituting an EGR cooler, and an EGR cooler configured using the member.

In an internal combustion engine mounted vehicle such as an automobile, for the purpose of NO _X reduction and fuel efficiency in exhaust gas, EGR; sometimes approach (Exhaust Gas Recirculation exhaust gas recirculation) is employed. This is a technique for extracting a part of the exhaust gas discharged from the internal combustion engine and sucking it again from the intake side of the internal combustion engine, which has been mainly used in diesel engines, but in recent years it has also been applied to gasoline engines. I came.

  In the EGR system, a device for cooling the exhaust gas to a temperature at which the exhaust gas can be circulated is required. This is the EGR cooler.

  1 and 2 schematically illustrate the structure of a general EGR cooler. A section partitioned by two partition plates is provided in a part of the exhaust gas flow path formed by the outer cylinder, and a heat exchanger that releases heat to the cooling water is configured in the section. This section partitioned by the partition plate is referred to herein as a “heat exchange section”. A vent pipe is joined to the partition plate at a location where a hole is provided, and exhaust gas flows through the vent pipe in the heat exchange section. Cooling water flows around the vent pipe. There are two types of vent pipes: a type composed of a simple pipe made of metal (FIG. 1) and a type provided with fins inside the pipe (FIG. 2).

  The EGR cooler is composed of a metal member such as an outer cylinder, a partition plate, a vent pipe, or a fin in the vent pipe, and these members are joined by brazing. However, the exhaust gas temperature on the side where the heat exchange section is entered may reach a maximum of about 800 ° C., and the temperature on the outlet side may reach a maximum of about 200 ° C. Therefore, Cu brazing used for assembling a general heat exchanger has durability. Run short. Therefore, Ni brazing (BNi-5, BNi-6, etc. of JIS Z3265) excellent in high temperature oxidation resistance and high temperature strength is applied to the EGR cooler.

The metal member constituting the EGR cooler is required to have the following characteristics.
(1) Ni brazing property is good.
(2) Good corrosion resistance against snow melting salt. This is because the EGR cooler is installed in the engine room and is in an environment where snow melting salt spread on the road surface is likely to adhere.
(3) Corrosion resistance to LLC (long life coolant; for example, ethylene glycol) is good. This is because LLC is usually added to the cooling water of EGR.
(4) High temperature strength and high temperature oxidation resistance are good. This is because the EGR cooler is exposed to high-temperature exhaust gas.
(5) Good corrosion resistance against condensation of condensed water. This is because in the EGR cooler, condensation is likely to occur near the exhaust gas outlet side during operation, and condensation is likely to occur at the exhaust gas contact location after operation.

JP 7-292446 A JP 2003-193205 A

  From the above required characteristics, at present, austenitic stainless steel represented by SUS304 and SUS316 is mainly used as the metal member constituting the EGR cooler. However, since austenitic stainless steel has a large coefficient of thermal expansion, the oxide scale generated at high temperature is liable to peel off during cooling and flow into the engine, or thermal fatigue failure due to repeated heating and cooling is likely to occur. Further improvement in high temperature strength is also desired. In addition, the material cost is high because a large amount of expensive Ni is contained.

  On the other hand, ferritic stainless steel has a thermal expansion coefficient smaller than that of austenitic steel, and the material cost is generally lower than that of austenitic steel. Ferritic stainless steel is frequently used for exhaust manifolds and mufflers that constitute the exhaust gas path. However, when performing Ni brazing, it is necessary to expose the material to a high temperature such as 1100 ° C. or higher. At such high temperatures, ferritic stainless steel usually causes coarsening of crystal grains and tends to cause a decrease in toughness.

  Patent Document 1 discloses a ferritic stainless steel for heat exchangers with good brazing properties. However, as for brazing, Cu brazing is assumed, and improvement of Ni brazing and suppression of crystal grain coarsening at that time are not intended.

  The present invention is to provide a ferritic stainless steel suitable as an EGR cooler member used for Ni brazing, and an EGR cooler using the same.

The purpose is mass%, C: 0.03% or less, Si: more than 0.1 to 3%, Mn: 0.1 to 2%, Cr: 10 to 25%, Nb: 0.3 to 0.3. 8%, N: 0.03% or less, preferably the total content of C and N is 0.01% or more, and other if necessary,
(A) A range of 4% or less in total of at least one of Mo, Cu, V and W;
(B) a range of one or more of Ti, Al and Zr in a total range of 0.3% or less,
(C) at least one of Ni and Co in a total range of 5% or less,
(D) a range in which at least one of REM (rare earth element) and Ca is 0.2% or less in total;
And is achieved by a ferritic stainless steel for an EGR cooler member, which is selectively contained in each of the remaining Fe and unavoidable impurities.

  Moreover, in this invention, the EGR cooler which has the structure which hold | maintained the steel material which consists of said steel to a structural member, and joined at least the member to the other member by Ni brazing is provided. Examples of the member include an outer cylinder, a partition plate, a vent pipe, and a fin attached to the vent pipe.

  Furthermore, the EGR cooler which has the structure which has the steel material which consists of said steel in the member which contacts both a motor vehicle exhaust gas and cooling water, and joined at least the member to other members by Ni brazing is provided. Examples of the member that contacts both the automobile exhaust gas and the cooling water include an outer cylinder, a partition plate, and a vent pipe.

  According to the present invention, a ferritic stainless steel with good Ni brazing and toughness has been provided. By using this steel, an EGR cooler having higher high-temperature strength, less oxide scale peeling, and lower material cost than a conventional EGR cooler using austenitic stainless steel as a member is realized.

  As described above, in order to improve the durability of the EGR cooler, it is advantageous to employ a ferritic steel type having a smaller thermal expansion coefficient. However, in the environment where the EGR cooler is used, the components may be exposed to a high temperature of 700 ° C. or higher and a maximum of about 800 ° C. In this case, especially for ferritic steel types, a measure for preventing a decrease in high-temperature strength is important. Further, since the Ni brazing is held at a high temperature of about 1050 to 1150 ° C., the component design for suppressing the coarsening of the crystal grains is important.

  In general, ferritic stainless steel exhibits a higher strength level than austenitic stainless steel in an intermediate temperature range from room temperature to about 600 ° C. However, the strength level decreases greatly at high temperatures such as 700 ° C. or higher. It is known that Nb addition is effective as a technique for preventing such a decrease in high-temperature strength. That is, the high temperature strength is remarkably improved by adding about 0.2% by mass of Nb to ferritic stainless steel. The improvement of the high temperature strength by Nb is mainly due to “solid solution Nb”. Therefore, in ferritic stainless steel used for high temperature applications, in order to ensure a sufficient amount of solute Nb, component design is usually made to reduce the C and N contents as much as possible.

  On the other hand, solute Nb is considered to act effectively in order to suppress the grain coarsening of ferritic stainless steel. However, according to studies by the inventors, it has been found that adding Nb of about 0.2% by mass is insufficient to suppress the grain coarsening at the Ni brazing temperature. As a result of detailed studies using various Ni brazing filler metals, it has become clear that it is important to secure an Nb content of 0.3% by mass or more.

Suppression of grain coarsening due to the addition of Nb is due to Nb carbide, Nb nitride, or Nb carbonitride (collectively referred to as “Nb carbide / nitride”) in addition to the action of suppressing coarsening due to solid solution Nb. The pinning effect is also considered to contribute greatly. Therefore, in the component design in the steel of the present invention, it is rather advantageous to secure the C and N contents to some extent. Specifically, it is more effective to set the total content of C and N to 0.01% by mass or more. In addition, by securing a sufficient Nb content, the pinning effect by precipitates such as Fe ₂ Nb (Laves) and Fe ₃ NbC (M6X) is also considered to act effectively on suppressing the grain coarsening.
Suppressing the coarsening of crystal grains is particularly effective in preventing a decrease in low temperature toughness.

  As for alloy components other than Nb, it has been found that each element of Mo, Cu, V, and W also contributes to the improvement of high-temperature strength as the amount of solid solution increases. Ni and Co were also found to be extremely effective in suppressing toughness degradation when the crystal grains were slightly coarsened. On the other hand, it has been clarified that Ti, Al, Zr, REM, and Ca have a factor that deteriorates the flow (wetting property) of the Ni brazing material on the surface of the steel material when Ni brazing is performed. This is thought to be due to the fact that oxides of these elements are likely to be formed on the steel surface during heating of the Ni brazing. However, there is no problem if the content of these elements is regulated to an appropriate range as described later.

  The present invention has been completed based on such findings. Below, each alloy component is demonstrated.

  C and N are elements that form Nb carbide / nitride when combined with Nb. When Nb is consumed by these precipitates and the solid solution Nb is reduced, the effect of improving the high temperature strength and the effect of suppressing the coarsening of the crystal grains due to the solid solution Nb are hindered. Therefore, in the present invention, the C content needs to be limited to 0.03% by mass or less, and is preferably 0.025% by mass or less. Moreover, it is necessary to restrict N content to 0.03 mass% or less, and it is preferable that it is 0.025 mass% or less.

  However, as described above, the pinning effect by Nb carbide / nitride can also contribute to the suppression of grain coarsening during Ni brazing. Therefore, it is advantageous to secure a certain amount of C and N. As a result of various studies, it is desirable that the total content of C and N is 0.01% by mass or more. About each element, it is more preferable to ensure C: 0.005 mass% or more and N: 0.005 mass% or more.

  Si is an element that improves high-temperature oxidation characteristics. However, excessive Si content hardens the ferrite phase and causes deterioration of workability. Moreover, Ni brazing property (wetting property with Ni brazing material) is deteriorated. As a result of various studies, the Si content is limited to the range of more than 0.1 to 3% by mass, and more preferably in the range of 0.3 to 2.5% by mass. The upper limit can be regulated to 1.5% by mass.

  Mn is an element that improves high-temperature oxidation characteristics, particularly scale peel resistance. However, excessive addition promotes the formation of an austenite phase at high temperatures. In the present invention, it is desirable to have a ferrite single-phase component composition that does not generate an austenite phase at the Ni brazing temperature. As a result of various studies, the Mn content is specified in the range of 0.1 to 2% by mass.

  Cr has the effect of stabilizing the oxidation resistance at high temperatures. For this purpose, it is necessary to secure a Cr content of 10% by mass or more. However, excessive Cr content impairs manufacturability and workability of the steel material. Therefore, the Cr content is limited to a range of 25% by mass or less.

  Nb is an important element in the present invention and, as described above, effectively acts to increase the high-temperature strength and suppress the grain coarsening during Ni brazing. Regarding the improvement of the high temperature strength, the solid solution strengthening of Nb largely contributes, but the precipitation strengthening by Nb carbide / nitride finely dispersed in the ferrite matrix is also effective for improving the high temperature strength. Regarding the suppression of the coarsening of crystal grains, the pinning effect by Nb carbide / nitride works effectively together with the solid solution strengthening of Nb. In order to fully exhibit these actions, it is important to secure the Nb content at 0.3 mass% or more after restricting the C and N contents to the above ranges. In particular, in order to suppress the coarsening of crystal grains during Ni brazing, it is effective to increase the Nb content, and it is preferable to set the Nb content to 0.4% or more or even 0.5% or more. However, when the Nb content is increased, the hot workability and the surface quality characteristics of the steel material are adversely affected. Therefore, the Nb content is limited to a range of 0.8% by mass or less.

  Mo, Cu, V, and W also contribute to the improvement of high temperature strength mainly by solid solution strengthening. Therefore, one or more of these elements can be contained as necessary. In particular, it is more effective to secure a total content of these elements of 0.05% by mass or more. However, when these elements are added excessively, the hot workability is adversely affected. Moreover, it becomes a factor which inhibits low temperature toughness. As a result of various studies, when adding one or more of Mo, Cu, V, and W, the total content needs to be suppressed to 4% by mass or less.

  Of Ti, Al, and Zr, Ti and Zr combine with C and N to form fine precipitates, which are dispersed in the steel, thereby exhibiting an effect of improving high temperature strength. Al exhibits the effect of improving the high temperature oxidation characteristics. Therefore, one or more of these elements can be contained as necessary. However, if any of these elements is contained in a large amount, it causes a decrease in hot workability and surface quality characteristics. Moreover, since it is an element which forms a strong oxide film on the steel material surface, the flow of Ni brazing may be deteriorated by the oxide film. As a result of investigation, when adding one or more of Ti, Al, and Zr, it is necessary to suppress the total content to 0.3% by mass or less. In particular, it is effective to set the total content in the range of 0.03 to 0.3% by mass, and more preferably 0.03 to 0.25% by mass.

  Ni and Co are remarkably effective in suppressing a decrease in toughness when crystal grains are slightly coarsened by Ni brazing. These elements are also advantageous for improving the high temperature strength. Therefore, it is possible to contain one or more of these elements as required, and it is more effective to secure a total content of Ni and Co of 0.5% by mass or more. However, excessive addition of Ni and Co is not preferable because it causes formation of an austenite phase in a high temperature range. When adding 1 or more types of Ni and Co, it is necessary to suppress the total content of Ni and Co to the range of 5 mass% or less.

  REM (rare earth element) and Ca are elements that improve high-temperature oxidation characteristics like Al, and in the present invention, one or more of them can be added as necessary. In particular, it is more effective to secure a total content of REM and Ca of 0.01% by mass or more. However, if it is added in a large amount, the manufacturability is lowered due to a decrease in toughness. Therefore, when adding 1 or more types of REM and Ca, it is necessary to suppress the total content to the range of 0.2 mass% or less.

  The ferritic stainless steel having the above composition has no problem with respect to the corrosion resistance against snow melting salt, the corrosion resistance against LLC, and the corrosion resistance against condensed water compared to the austenitic steel types used in conventional EGR coolers. Was confirmed. The high temperature strength (0.2% proof stress) and scale peel resistance in the exhaust gas environment are improved over the austenitic steel grade.

Ferritic stainless steel having the above composition is made into a steel plate by a normal method, and is processed into members such as an outer cylinder, a partition plate, a vent pipe, and a fin attached to the vent pipe constituting the EGR cooler. These members are joined by Ni brazing, and an EGR cooler is constructed. It is not necessary that all the constituent members of the EGR cooler are made of the steel of the present invention. However, the steel member of the present invention has sufficiently secured corrosion resistance to LLC, and at the same time, improved high temperature oxidation characteristics in exhaust gas environment, resistance to crystal grain coarsening during Ni brazing, and wettability. It is a thing. For this reason, it is particularly effective to use the steel of the present invention for a member that contacts both automobile exhaust gas and cooling water and is subjected to Ni brazing.
Examples of such a member include an outer cylinder, a partition plate, and a vent pipe.

  Steel having the chemical composition shown in Table 1 was melted, and the obtained steel ingot was hot-forged into a round bar and a plate to process a round bar having a diameter of 15 mm and a plate having a thickness of 30 mm. The round bar was subjected to a solution treatment at a holding temperature set within a range of 1000 to 1100 ° C. The sheet is hot rolled into a hot rolled sheet having a thickness of 4 mm, annealed, and then cold rolled to a sheet thickness of 1.5 mm, and then the holding temperature is set within a range of 1000 to 1100 ° C. The final annealing was performed. B4 and B5 are austenitic stainless steels.

The following characteristics were investigated using the obtained steel materials.
[Coefficient of thermal expansion]
A test piece of 5 mm square × 50 mm length is prepared from the round bar after solution treatment, set in a thermal expansion measuring device, and the average heat from room temperature (25 ° C.) to 700 ° C. at a temperature rising rate of 2 ° C./min. The expansion coefficient was determined.

[0.2% yield strength at 700 ° C]
A high-temperature tensile test piece having a parallel part diameter of 10 mm was prepared from the round bar after solution treatment, a high-temperature tensile test at 700 ° C. was performed in accordance with JIS G056, and a 0.2% yield strength was measured. Since those having a 0.2% proof stress at 700 ° C. of 100 N / mm ² or more exhibit characteristics superior to those of conventional austenitic steel types as EGR coolers, those having such characteristics were determined to be acceptable.

[High-temperature oxidation characteristics in repeated cycles]
A 25 mm × 35 mm × 1.5 mm sample was cut out from the cold-rolled annealed plate, and a high-temperature oxidation test piece with the entire surface # 400 wet polished was produced. For this test piece, 1000 cycles of a cycle of “900 ° C. × 25 minutes heating → cooling at room temperature for 10 minutes” were performed in an atmosphere of atmosphere + 60 ° C. saturated steam, simulating repeated use as an EGR cooler member. The mass change per unit area was determined by dividing the mass change before and after the test (plus is increase, minus is decrease) by the surface area of the test piece before the test. If the absolute value of this mass change is 10 mg / m ² or less, it is evaluated as having excellent high-temperature oxidation characteristics as an EGR cooler member, and those having a mass change of 5 mg / m ² or less are particularly excellent.

[Ni brazing (wetting)]
Two 10 mm × 20 mm × 1.5 mm brazing specimens were cut out from each cold rolled annealed plate. One of the test pieces was placed horizontally, and a paste-like Ni solder was applied to the entire surface of the test piece with a thickness of 0.5 mm. A test piece / Ni brazing / test piece three-layered laminate was formed by stacking another test piece thereon, and placed in a vacuum furnace while keeping it horizontal. Heated for minutes. Take out the laminate after cooling, observe the surface of the test piece that was superposed on the upper surface (the one that was not coated with Ni brazing), and divide the area wetted with Ni brazing by the total area of the test piece surface Thus, the Ni brazing coverage was determined. A Ni brazing rate of 50% or more was evaluated as A, 20% or more and less than 50% was evaluated as B, and less than 20% was evaluated as C. In addition, the Ni brazing filler used was a 19 mass% Cr-10 mass% Si-71 mass% Ni composition (equivalent to BNi-5 in JIS Z3265).

[Resistance to grain coarsening]
About the test piece which evaluated said Ni brazing property, the metal structure of the cross section (cross section parallel to a rolling direction and a plate | board thickness direction; L cross section) was observed with the optical microscope. Etching was performed with a mixed acid of hydrofluoric acid and nitric acid. An average crystal grain size of 200 μm or less was evaluated as A, 200 μm or more and 500 μm or less as B, 500 μm or more as C, and B evaluation or more as a pass.

(Low temperature toughness)
After heat-treating a 1.5 mm thick cold-rolled annealed plate with a heat pattern equivalent to the Ni brazeability evaluation, a V-notch Charpy impact test piece was produced from the steel plate, and Charpy at 0 ° C. in accordance with JIS Z2242. An impact test was performed. The test specimens were collected so that the direction in which the hammer hits was perpendicular to the rolling direction (C direction). 0 ℃ in the Charpy impact value 100 J / cm ² or more of the A, 50J / cm ² or more 100 J / cm ² less than that of B, and evaluate those less than 50 J / cm ² and C, and B or more evaluation It was determined to pass.
These results are shown in Table 2.

  As can be seen from Table 2, the ferritic stainless steel of the present invention has a much smaller coefficient of thermal expansion than the austenitic stainless steels of Comparative Examples B4 and B5, 0.2% proof stress at 700 ° C. and repeated cycles. It was also superior in high-temperature oxidation characteristics. Further, it was confirmed that the Ni-brazing property (wetting property), the resistance against coarsening of crystal grains, and the low temperature toughness were sufficiently satisfactory as an EGR cooler member.

  In contrast, Comparative Example B1 has a high C content and a low Nb content, so that the amount of dissolved Nb is insufficient, high temperature strength (0.2% proof stress at 700 ° C.), and resistance to coarsening of crystal grains. Inferior to Moreover, since the Mn content was high, an austenite phase was generated at a high temperature, and it was considered that this was partly transformed into a martensite phase, and the low temperature toughness was inferior. Although B2 had a low Nb content, since the C and N contents were also relatively low, the solid solution Nb content was adequately secured and the high-temperature strength (0.2% yield strength at 700 ° C.) was good. However, the fine dispersion of Nb carbide / nitride is small, and the resistance performance against crystal grain coarsening is insufficient. Since B3 had an excessive Ti content, an oxide film was likely to be formed on the surface during Ni brazing, and the Ni brazing property was inferior. Moreover, it was inferior to low-temperature toughness because the total content of Mo, Cu, V, and W was too high. B4 and B5 are austenitic stainless steels and have a high coefficient of thermal expansion. High temperature strength (0.2% proof stress at 700 ° C.) was also lower than other ferritic steel types. In addition, due to the high coefficient of thermal expansion, the scale easily peeled off in repeated cycles, and the mass change became a large negative value. B6 had an excessively high total content of Ti, Al, and Zr, so that an oxide film was liable to be formed on the surface during Ni brazing, resulting in poor Ni brazing.

The figure which illustrated the structure of the EGR cooler typically. The figure which illustrated typically the structure of the type of EGR cooler which has a fin in a vent pipe.

Claims (8)

  In mass%, C: 0.03% or less, Si: more than 0.1 to 3%, Mn: 0.1 to 2%, Cr: 10 to 25%, Nb: 0.3 to 0.8%, N : Ferritic stainless steel for EGR cooler member consisting of 0.03% or less, balance Fe and inevitable impurities.   Furthermore, the ferritic stainless steel of Claim 1 which contains 1 or more types of Mo, Cu, V, and W in the range of a total of 4% or less.   Furthermore, the ferritic stainless steel of Claim 1 or 2 which contains 1 or more types of Ti, Al, and Zr in the range of a total of 0.3% or less.   Furthermore, the ferritic stainless steel in any one of Claims 1-3 which contains 1 or more types of Ni and Co in the range of 5% or less in total.   Furthermore, the ferritic stainless steel in any one of Claims 1-4 which contains 1 or more types of REM (rare earth element) and Ca in the range of 0.2% or less in total.   The ferritic stainless steel according to claim 1, wherein the total content of C and N is 0.01% by mass or more.   The EGR cooler which has the structure which has the steel material which consists of steel in any one of Claims 1-6 in a structural member, and joined the member at least to the other member by Ni brazing.   EGR which has the structure which has the steel material which consists of steel in any one of Claims 1-6 in the member which contacts both a motor vehicle exhaust gas and cooling water, and joined at least that member to other members by Ni brazing cooler.

Images