JP2010121208A - Ferritic stainless steel sheet for egr cooler - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a ferritic stainless steel sheet for EGR (Exhaust Gas Recirculation) coolers which combines excellent brazability and corrosion resistance to the condensed water of exhaust gas. <P>SOLUTION: The ferritic stainless steel sheet for EGR coolers is characterized by containing at least, by mass%, ≤0.03% C, ≤0.05% N, ≥0.1 to ≤1% Si, ≥0.02 to ≤2% Mn, ≥0.2 to ≤1.5% Cu, ≥15 to ≤25% Cr, ≥8(C+N) to ≤1% Nb, and ≤0.5% Al, with the balance Fe and unavoidable impurities. The ferritic stainless steel sheet is also characterized in that, by mass%, Ti is further contained therein in an amount satisfying formulae (1): Ti-3N≤0.03 and (2): 10(Ti-3N)+Al≤0.5, and that the amounts of Cr and Cu are within the ranges satisfying formula (3): Cr+2.3Cu≥18. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention is used in an EGR cooler that cools exhaust gas with engine cooling water or air in an exhaust gas recirculation (hereinafter referred to as EGR) system used in, for example, a diesel engine or a gasoline engine of an automobile. The present invention relates to a suitable ferritic stainless steel sheet for an EGR cooler.

  In recent years, in the automotive field, exhaust gas regulations have been further strengthened due to increasing awareness of environmental issues, and efforts are being made to reduce carbon dioxide emissions. In addition to efforts from the fuel side, such as bioethanol and biodiesel fuel, it is possible to improve fuel efficiency by reducing the weight and installing a heat exchanger that recovers exhaust heat. EGR, DPF (Diesel Particulate Filter), urea SCR ( Efforts are being made to install exhaust gas treatment equipment such as the Selective Catalytic Reduction system.

  Among these, the EGR system aims to lower the combustion temperature and lower NOx, which is a harmful gas, by cooling the exhaust gas of the engine and then returning it to the intake side to re-combust it. The EGR cooler is a device that cools the exhaust gas with engine cooling water or air, and it is desired that the heat exchange portion requires good thermal efficiency and good thermal conductivity.

  Conventionally, austenitic stainless steels such as SUS304 and SUS316 are generally used for EGR coolers as disclosed in the following Patent Document 1 and Patent Document 2 below. However, recently, in order to further reduce NOx, there is a demand for lowering the outlet side temperature of the EGR cooler, and in such austenitic stainless steel, there is a concern about thermal fatigue characteristic deterioration due to an enlarged temperature difference on the inlet / outlet side. It's getting on. Accordingly, ferritic stainless steels that are superior in thermal conductivity, have a smaller thermal expansion coefficient, and are less expensive than austenitic stainless steels have attracted attention.

  In addition, EGR coolers have been generally installed for diesel engines, but application to gasoline engines is also being studied in order to achieve both improved fuel consumption and NOx reduction through direct injection. In general, it is said that a gasoline engine has a higher exhaust gas temperature than a diesel engine, and an inlet temperature of an EGR cooler reaches 500 to 600 ° C. This temperature range is a region where intergranular corrosion due to sensitization is a concern for austenitic stainless steels such as SUS304 and SUS316, and from this point, ferritic stainless steel is attracting attention.

  In the production of an EGR cooler, in particular its heat exchange part, it is generally assembled by brazing joints. Further, on the exhaust gas side of the EGR cooler, exhaust gas components can be condensed during cooling. For this reason, brazing properties and corrosion resistance against exhaust gas condensed water are required.

  Patent Document 3 listed below discloses a pre-coated brazing metal sheet prepared by suspending a Ni-based brazing material together with an organic binder, spraying and heating the surface of a stainless steel plate.

  The following Patent Document 4 discloses a method for producing a nickel brazing coated stainless steel sheet excellent in self-brazing property, in which a Ni-based brazing material is coated by plasma spraying on a stainless steel sheet having an adjusted surface roughness. Yes. In any case, the stainless steel that is the subject of the examples is austenitic stainless steel.

  In the following Patent Document 5, C: 0.5% or less, Si: 2% or less, Mn: 3% or less, S: 0.2% or less, Ni: 8-18%, Cr: 12-25%, Mo : 0-4%, W: 0-2%, (Ni / Cu): 2 or more, Nb: 0-2.5%, and exhaust gas recirculation substantially made of austenitic stainless cast steel Parts are disclosed.

  In Patent Document 6 below, in a heat exchanger in which a high heat fluid passage is formed by inserting fins in a pipe or between a plurality of pipes and a low heat fluid is formed adjacent to the high heat fluid, the fins are made of austenitic stainless steel. A heat exchanger in which steel and pipes are made of ferritic stainless steel is disclosed, and SUS304 is exemplified as an austenitic stainless steel, and SUS430 is exemplified as a ferritic stainless steel. It has a structure utilizing the difference in thermal expansion coefficient between austenitic stainless steel and ferritic stainless steel, and is characterized in that it can be manufactured at low cost and in a short time by omitting brazing joint. For this reason, no description regarding brazing is found, and no mention is made of condensed water corrosion resistance.

  In Patent Document 7 below, a large number of elongated exhaust gas flows are provided by dividing the wide direction of the exhaust gas flow path formed by the flat tube into small sections, which are built into the flat tube incorporated in the exhaust gas heat exchanger. An inner fin of a heat exchanger for exhaust gas in which the material of the inner fin is ferritic stainless steel is disclosed as the inner fin forming the path. SUS405 and SUS446 are exemplified as a feature that heat resistance is improved by adopting a shape that takes the formability of ferritic stainless steel into consideration. It only mentions that it has good heat resistance and that it can be bent, and does not mention brazing or condensate corrosion resistance.

  In the following Patent Document 8, C: 0.025% or less, Si: 0.10% or less, Mn: 1.0% or less, Cr: 17.0 to 25.0%, Ni: 0.50% or less, Mo: 0.50 to 2.00% or less, Al: 0.025% or less, N: 0.025% or less, and any one or two of Nb and Ti are 10 (C + N) to 1.0%. The ferritic stainless steel for heat exchangers contained in the range is disclosed. In addition to limiting the amount of Si and Al from the viewpoint of brazing, the amount of Cr and Mo added is high from the viewpoint of corrosion resistance and oxidation resistance. Of these, Mo is an element that is extremely effective for corrosion resistance against exhaust gas condensed water. It is said. When the corrosive environment becomes more severe, it is necessary to increase the amount of Mo added. However, since Mo is an expensive element, there is a problem that the cost performance is inferior.

  In the following Patent Document 9, C: 0.08% or less, Si: 0.01 to 2.0%, Mn: 0.05 to 1.5%, P: 0.05% or less, S: 0.01 %: Cr: 13-32%, Mo: 3.0% or less, Al: 0.005-0.1%, Ni: 1.0% or less, Cu: 1.0% or less, Ti: 0.05 A ferritic stainless steel for ammonia-water absorption cycle heat exchangers having excellent brazing properties contained in a range of not more than% is disclosed. From the viewpoint of brazing (brazing), Ti is limited to 0.05% or less, and from the viewpoint of corrosion resistance in a high-temperature and high-pressure ammonia water environment, Cr is 13% or more. Mo, Ni, and Cu are also described as elements effective for corrosion resistance, but their required amounts are not described.

  In the following Patent Document 10, Cr: 18.0 to 27.0%, Cu: 0.8 to 3.5%, Si: 0.5 to 2.0%, Mo: 0.5 to 1.5% Nb: 2.5% or less, Ni: 0.6% or less, C: 0.12% or less, Mn: 1.0% or less, Al: 0.10% or less, P: 0.15% or less, S : Ferritic stainless cast steel excellent in acid resistance, characterized in that: 0.15% or less, N: 0.10% or less, and (Cu + Si) exceeds 2.0%. From the viewpoint of machinability, the ferrite type is used, and from the viewpoint of acid resistance, the amounts of Cr, Cu, Si and (Cu + Si) are defined. Since a large amount of Cu and Si is required from the viewpoint of acid resistance, it becomes hard and has a problem in formability when used as a steel sheet.

JP 2007-64515 A JP 2007-224786 A JP-A-1-249294 JP 2001-26855 A JP 2003-193205 A JP 2005-55153 A JP 2008-96048 A JP 7-292446 A Japanese Patent Laid-Open No. 11-236654 JP 2008-195985 A

  This invention is proposed in view of such a conventional situation, and it aims at providing the ferritic stainless steel plate for EGR coolers which has the outstanding brazing property and the corrosion resistance with respect to exhaust gas condensate.

The gist of the present invention aimed at solving the above problems is as follows.
[1] By mass%
C: 0.03% or less,
N: 0.05% or less,
Si: 0.1% or more, 1% or less,
Mn: 0.02% or more, 2% or less,
Cu: 0.2% or more, 1.5% or less,
Cr: 15% or more, 25% or less,
Nb: 8 (C + N)% or more, 1% or less,
Al: containing at least 0.5% or less, the balance consisting of Fe and inevitable impurities,
Furthermore, in mass%,
For Ti, the following formulas (1) and (2) are satisfied,
A ferritic stainless steel sheet for an EGR cooler characterized by containing C and Cu in a range satisfying the following formula (3).
Ti-3N ≦ 0.03 (1)
10 (Ti-3N) + Al ≦ 0.5 (2)
Cr + 2.3Cu ≧ 18 (3)
[2] Furthermore, in mass%,
Mo: 3% or less,
Ni: The ferrite system for EGR cooler according to [1] above, which contains any one or more of 3% or less and satisfies the following formula (4): Stainless steel sheet.
Cr + 1.9Mo + 1.6Ni + 2.3Cu ≧ 18 (4)
[3] Furthermore, in mass%,
V: 3% or less,
W: Ferritic stainless steel sheet for EGR cooler according to [1] or [2] above, containing either one or both of 5% or less.
[4] Furthermore, in mass%,
Ca: 0.002% or less,
Mg: 0.002% or less,
B: The ferritic stainless steel sheet for an EGR cooler according to any one of [1] to [3], wherein one or more of 0.005% or less are contained.
[5] The ferritic stainless steel sheet for an EGR cooler according to any one of [1] to [4], wherein C + N: 0.015% or more.

  As described above, according to the present invention, it is possible to provide a ferritic stainless steel plate having both excellent brazing property and corrosion resistance against exhaust gas condensed water. It is possible to use it suitably.

It is a characteristic view which shows the relationship between the wettability of wax and the amounts of Ti and Al. It is a characteristic view which shows the relationship (pH1.5) of the corrosion rate in exhaust gas simulation condensed water, and Cr + 2.3Cu> = 18. It is a characteristic view which shows the relationship (pH1.5) of the corrosion rate in exhaust gas simulation condensed water, and Cr + 1.9M + 1.6Ni + 2.3Cu. It is a characteristic view which shows the influence (pH1) of Cu which acts on the corrosion rate in exhaust gas simulation condensed water. It is a characteristic view which shows the influence (pH1) of Mo which acts on the corrosion rate in exhaust gas simulation condensed water. It is a characteristic view which shows the influence (pH0.5) of Ni which has on the corrosion rate in exhaust gas simulation condensed water.

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.
The EGR cooler is required to be brazed with Ni or Cu. For this reason, the present inventors have conducted intensive studies on the influence of alloy elements on brazeability. As a result, as shown in the following formulas (1) and (2), in ferritic stainless steel sheets, Ti is often added for the purpose of improving workability and intergranular corrosion, and for the purpose of deoxidation. It has been found that Al has an upper limit value that can ensure good brazing properties.
Ti-3N ≦ 0.03 (1)
10 (Ti-3N) + Al ≦ 0.5 (2)

  In order to obtain good brazing properties, it is necessary that the molten brazing wets and spreads on the surface of the stainless steel plate, but the wettability is affected by the surface film formed on the stainless steel plate in the brazing atmosphere. In the brazing atmosphere, even if the conditions under which the Fe and Cr oxides can be reduced can be maintained, Ti and Al, which are easier to oxidize than Fe and Cr, form oxides and spread the wetting of the solder. Inhibits and degrades brazing. Contributing to this film formation is Ti and Al in solid solution, and if they exist as relatively stable nitrides even at the brazing temperature, they do not contribute to film formation and spread the wetting of the brazing. Does not disturb.

  From these points, the relationship between the amounts of Ti and Al and the wettability of the wax was evaluated using the 16 to 21 Cr ferritic stainless steel sheets shown in Table 1 under the same test conditions as those in Examples described later. In Table 1, the balance is Fe and inevitable impurities. The result is shown in FIG.

  From the results shown in FIG. 1, it was found that the wettability of the wax is good within the range satisfying the above formulas (1) and (2). Further, when the surface film after brazing heat treatment was analyzed for steel whose Ti or Al content does not satisfy the above conditions, the Ti or Al concentrated oxide film was uniform with a thickness of several tens to several hundreds of nanometers. Formed. Therefore, it is considered that such film formation inhibits the wetting spread of the wax.

  In the EGR cooler targeted by the present invention, strength is also required, and it is desirable that the strength decrease after brazing is small. In the case of brazing at a high temperature of 1000 to 1150 ° C. like Ni brazing or Cu brazing, it is considered important to suppress a decrease in strength due to crystal grain coarsening. Pinning with precipitates is useful for suppressing coarsening of crystal grains. In the present invention, by utilizing Nb carbonitride as a precipitate and making C + N 0.015% or more, the precipitation amount of Nb carbonitride, which is useful for suppressing grain coarsening, and stability (See Japanese Patent Application No. 2007-339732).

  In the EGR cooler, acidic condensed water composed of sulfuric acid, nitric acid, and organic acid is generated due to SOx, NOx, and HC contained in the exhaust gas. In the EGR cooler, unlike a downstream member such as a muffler, the exhaust gas is directly under the engine and in front of the catalyst, and the exhaust gas before purification is targeted. Therefore, the acid concentration of the generated condensed water is also high.

  Recently, with globalization, low-grade fuel with high S concentration is sometimes used as fuel for automobiles. In this case, the sulfuric acid concentration in the condensed water also increases. Such an increase in acid concentration leads to a decrease in pH, and it is said that the pH of condensed water in the EGR cooler reaches about 1.5. In general, since the plate thickness of the heat exchanging part of the EGR cooler is as thin as 0.1 to 0.5 mm, excellent corrosion resistance is required in a condensate having a pH of about 1.5 composed of sulfuric acid, nitric acid, and organic acid. .

Therefore, the present inventors examined the influence of Cr and Cu on the condensed water corrosion resistance by using a ferritic stainless steel in the range of 16 to 19 Cr and 0 to 0.5 Cu by a corrosion test under the same conditions as in the examples. did. The result is shown in FIG. Since NO ³⁻ ions act as corrosion-inhibiting ionic species, they were evaluated as safety-free as no addition.

  FIG. 2 shows the test results in a solution having a pH of 1.5. It can be seen that excellent corrosion resistance is exhibited by satisfying Cr + 2.3Cu ≧ 18.

Next, the effect of Cr, Ni, Mo and Cu on the corrosion resistance of condensed water using ferritic stainless steel in the range of 13 to 21 Cr, 0 to 2 Mo, 0 to 3 Ni and 0 to 1 Cu by adding Ni and Mo Were examined by a corrosion test under the same conditions as in the examples. The result is shown in FIG. Here, NO ³⁻ ions were not added.

  FIG. 3 shows the test results in a solution having a pH of 1.5. Any element of Cr, Ni, Mo, and Cu is effective in improving the corrosion resistance, but Cu is the most effective in improving the corrosion resistance, and Cr + 1. It can be seen that excellent corrosion resistance is exhibited by satisfying 9Mo + 1.6Ni + 2.3Cu ≧ 18.

Here, the coefficient of each alloy element is obtained by multiple regression analysis of the degree of contribution of the alloy element to the critical pH. The critical pH is the upper limit pH at which the corrosion rate is 0.1 g · m ⁻² · h ⁻¹ or less. 4 shows the effect of Cu on the corrosion rate in the pH 1 solution, FIG. 5 shows the effect of Mo on the corrosion rate in the pH 1 solution, and FIG. 6 shows the effect in the pH 0.5 solution. The influence of Ni on the corrosion rate is shown respectively.

  From the results of FIGS. 4, 5 and 6, it can be seen that Cu is a very effective element for improving corrosion resistance because the corrosion rate is remarkably reduced with a smaller addition amount than Mo and Ni. In addition, Cu is known as an element that improves acid resistance. When the influence of Cu was examined by electrochemical measurement, a phenomenon that the corrosion potential became noble by addition of Cu was observed. This indicates that Cu has an action of promoting passivation in addition to the action of suppressing the active dissolution, and it is considered that the contribution to the improvement of corrosion resistance is increased by these two effects.

  The present invention has been made on the basis of the above findings, and provides a ferritic stainless steel sheet for EGR cooler that has both excellent brazing properties and corrosion resistance against exhaust gas condensed water. The contents are as described in the claims.

  Hereinafter, the reason which limited each composition of the ferritic stainless steel plate for EGR coolers is demonstrated. In the following description, unless otherwise specified,% of each component represents mass%.

(C: 0.03% or less)
Since C reduces intergranular corrosion resistance and workability, it is necessary to keep the content low. For this reason, C was made 0.03% or less. However, excessively lowering promotes crystal grain coarsening during brazing and increases the scouring cost, so C is preferably made 0.002% or more. More preferably, it is 0.005 to 0.025%.

(N: 0.05% or less)
N is an element useful for pitting corrosion resistance, but its content needs to be kept low in order to reduce intergranular corrosion resistance and workability. For this reason, N was made into 0.05% or less. However, excessively lowering promotes coarsening of the crystal grains during brazing and increases the scouring cost, so N is preferably made 0.002% or more. More preferably, it is 0.005 to 0.03%.

(Si: 0.1% or more, 1% or less)
Si is necessary as a deoxidizing element because it limits Ti and Al useful as a deoxidizing element. Further, since the Cr concentration on the surface is lowered by brazing heat treatment, Si becomes an effective element for improving the oxidation resistance after brazing. For this reason, it is necessary to contain Si at least 0.1% or more. However, since excessive addition reduces workability, it is preferable to make it 1% or less. More preferably, it is 0.1 to 0.5%.

(Mn: 0.02% to 2%)
Mn is an element useful as a deoxidizing element and needs to be at least 0.02% or more. However, since it will degrade corrosion resistance if it is contained excessively, it is preferably made 2% or less. More preferably, it is 0.1 to 1%.

(Cu: 0.2% or more, 1.5% or less)
Cu is an element as important as Cr in securing the corrosion resistance of the exhaust gas condensate and needs to be at least 0.2% or more. On the other hand, Cu can improve the corrosion resistance as the content thereof is increased. However, since excessive addition deteriorates workability, it is preferably made 1.5% or less. More preferably, it is 0.2 to 1.0%.

(Cr: 15% or more, 25% or less)
Cr is an element that is fundamental for ensuring corrosion resistance and oxidation resistance of the exhaust gas condensate, and at least 15% or more is necessary. On the other hand, Cr can improve the corrosion resistance and the oxidation resistance as the content thereof is increased. However, since excessive addition reduces workability and manufacturability, the Cr content is preferably 25% or less. More preferably, it is 17 to 23%.

(Nb: 8 (C + N)% or more, 1% or less)
Since Nb is an element useful for fixing C and N and improving the intergranular corrosion resistance of the welded portion, it is necessary to contain Nb at least eight times the (C + N) amount. Nb is also useful for improving the high-temperature strength, and is necessary for a member used at a high temperature such as an EGR cooler. Further, Nb carbonitride is useful for suppressing grain coarsening during brazing. However, excessive addition reduces workability and manufacturability, so 1.0% or less is preferable. More preferably, it is 10 (C + N) to 0.6%.

(C + N: 0.015% or more)
Furthermore, it is preferable that C + N is set to 0.015% or more from the viewpoint of suppressing the strength decrease accompanying the coarsening of crystal grains during brazing. C + N is desirably 0.02% or more. Since excessive addition with C reduces intergranular corrosion resistance and workability, C + N is preferably 0.04% or less.

(Al: 0.5% or less)
Al is an element useful for scouring because it has a deoxidizing effect and the like, and has an effect of improving moldability, but it is 0.5% or less in order to inhibit brazing, which is the most important characteristic in the present invention. did. Preferably it is 0.001-0.1%, More preferably, it is 0.001-0.05%.

(Ti: range satisfying the formulas (1) and (2))
In the present invention, it is necessary to satisfy the above formulas (1) and (2) at the same time in order to obtain good brazing spreadability in the brazability which is the most important characteristic. In order to satisfy this, on the basis of the above knowledge, Ti is set in a range that satisfies the above formulas (1) and (2). The value of Ti-3N is desirably 0.02% or less. However, if the Ti content is too low, the workability is deteriorated. Therefore, it is desirable to adjust the Ti content so that the value of Ti-3N is -0.08% or more. When workability is not particularly required, Ti need not be added.

(Cr, Cu: range satisfying the formula (3))
In the present invention, it is necessary to satisfy the following formula (3) for Cr and Cu in order to develop good corrosion resistance in an exhaust gas condensate having a pH of about 1.5 consisting of sulfuric acid, nitric acid, and organic acid.
Cr + 2.3Cu ≧ 18 (3)

In the present invention, either one or both of Mo and Ni may be further contained.
(Mo: 3% or less)
Mo can be contained in an amount of 3% or less as required in order to improve the corrosion resistance. It is 0.3% or more that a stable effect can be obtained. However, excessive addition deteriorates processability and increases the cost because it is expensive. Therefore, it is preferable to contain 0.3 to 3%.

(Ni: 3% or less)
Ni can be contained in an amount of 3% or less as required in order to improve the corrosion resistance. It is 0.2% or more that a stable effect is obtained. However, excessive addition deteriorates processability and increases the cost because it is expensive. Therefore, it is preferable to contain 0.2 to 3%.

Furthermore, in the case of adding either one or both of Mo and Ni, in order to develop good corrosion resistance in an exhaust gas condensate having a pH of about 1.5 consisting of sulfuric acid, nitric acid, and organic acid, the following formula (4 ) Must be satisfied.
Cr + 1.9Mo + 1.6Ni + 2.3Cu ≧ 18 (4)

In the present invention, either one or both of V and W may be further contained.
(V: 3% or less)
V can be contained in an amount of 3% or less as required for improving the corrosion resistance. It is 0.2% or more that a stable effect is obtained. However, excessive addition deteriorates processability and increases the cost because it is expensive. Therefore, it is preferable to contain 0.2 to 3%.

(W: 5% or less)
W can be contained in an amount of 3% or less as required for improving the corrosion resistance. It is 0.5% or more that a stable effect is obtained. However, excessive addition deteriorates processability and increases the cost because it is expensive. Therefore, it is preferable to contain 0.5 to 5%.

In the present invention, any one or both of Ca, Mg, and B may be further contained.
(Ca: 0.002% or less)
Ca is an element useful for scouring because it has a deoxidizing effect and the like, and can be contained in an amount of 0.002% or less as required. Moreover, when Ca is contained, the content is preferably 0.0002% or more so that a stable effect can be obtained.

(Mg: 0.002% or less)
Mg is a useful element for scouring because it has a deoxidizing effect and the like, and it is also useful for improving the workability and toughness by refining the structure. be able to. Further, when Mg is contained, the content is preferably 0.0002% or more so that a stable effect can be obtained.

(B: 0.005% or less)
B is an element useful for improving secondary workability, and can be contained in an amount of 0.005% or less as required. When B is contained, the content is preferably 0.0002% or more so that a stable effect can be obtained.

  Of the inevitable impurities, P is preferably 0.04% or less from the viewpoint of weldability. Moreover, about S, it is preferable to set it as 0.01% or less from a corrosion-resistant viewpoint.

The method for producing stainless steel of the present invention may be a general process for producing ferritic stainless steel. Generally, it is made into molten steel in a converter or electric furnace, scoured in an AOD furnace or VOD furnace, and made into a steel piece by a continuous casting method or an ingot-making method, and then hot-rolled-annealed hot-rolled sheet-pickled-cooled It is manufactured through a process of hot rolling, finish annealing and pickling. If necessary, annealing of the hot-rolled sheet may be omitted, or cold rolling-finish annealing-pickling may be repeated.
(Example)

  Hereinafter, the effects of the present invention will be made clearer by examples. In addition, this invention is not limited to a following example, In the range which does not change the summary, it can change suitably and can implement.

  In this example, a steel having the chemical composition shown in Table 2 below is melted and subjected to hot rolling, cold rolling, and annealing processes to produce a cold rolled steel sheet having a thickness of 0.4 mm. The corrosion resistance in the condensed water was evaluated.

(Brassability)
A test piece having a width of 50 mm and a length of 70 mm was cut out from the cold-rolled steel sheet, and then one side was wet-polished to # 400 with emery paper. Thereafter, 0.1 g of Ni solder was placed on the polished surface and heated in a vacuum atmosphere at 1100 ° C. and 5 × 10 ⁻³ torr for 10 minutes. And after cooling to normal temperature, the brazing area after a heating was measured. The measurement results are shown in Table 3.

  As for the brazing property shown in Table 3, when the brazing area after heating is more than twice the brazing area before heating, the wetting spread is “◯”, and when it is less than 2 times, the wetting spread is shown. Was evaluated as “×”. Thereafter, the cross-sectional microstructure was observed. Then, the number of crystal grains existing in the plate thickness direction is measured over a range of 20 mm in length in parallel with the rolling direction, and the microstructure is “◯” when two or more crystal grains exist in the plate thickness direction. In the case of only one, the microstructure was evaluated as “x”.

(Corrosion test)
A 25 W × 40 L test piece was cut out from the cold rolled steel sheet, and the entire surface was wet-polished to # 320 with emery paper. Ammonium chloride reagent, with sulfuric acid, formic acid, acetic _{^{acid, 50ppmCl - + 5000ppmSO 4 2- +}} 5000ppmHCOO - + 3000ppmCH 3 COO - to prepare a solution. Then, it adjusted to pH1.5 and pH1.0 using sulfuric acid or ammonia water. The test piece was immersed in this solution heated to 60 ° C. for 3 h, and the corrosion rate was determined from the mass change before and after immersion. The measurement results are shown in Table 3.
For the corrosion tests shown in Table 3, those having a corrosion rate of 0.1 g · m ⁻² · h ⁻¹ or less are “◯” and those having a corrosion rate exceeding 0.1 g · m ⁻² · h ^−1. Evaluation was performed as “×”.

  From the test results shown in Table 3, Experimental Example No. within the scope of the present invention. Steel Nos. 1 to 13 have good wetting and spreading properties of the brazing, are suppressed in crystal grain coarsening after brazing, and have good corrosion resistance in exhaust gas simulated condensed water having a pH of 1.5. Among these, Experimental Example No. Steel of 2, 3, 4, 6, 7, 9, 10, 11 shows good corrosion resistance in exhaust gas simulated condensed water with a pH of 1.0, and can be used for EGR coolers that can cope with a more severe corrosive environment Suitable as a material.

  On the other hand, Experimental Example No. 1 in which Al deviates from the scope of the present invention. 14, Experimental Example No. that does not satisfy the above formula (2) 15 shows that it is inferior to the wettability of wax. In addition, all of the above formulas (1) to (3) deviate from the light range of the present invention. No. 16 is inferior in both the wettability of the wax and the corrosion resistance in the exhaust gas simulated condensed water. Moreover, while the amount of Cr and said Formula (3) remove | deviate from the range of this invention, (C + N) amount is less than 0.015% of experiment example No. It can be seen that No. 17 is inferior in corrosion resistance in exhaust gas simulated condensed water, and the crystal grains are markedly coarsened.

  The ferritic stainless steel sheet having both excellent brazing property and corrosion resistance against exhaust gas condensate of the present invention is suitable for an EGR cooler member, particularly an EGR cooler heat exchange member. In addition, it is also suitable for an exhaust gas path member that is exposed to exhaust gas condensed water and brazed.

Claims (5)

% By mass
C: 0.03% or less,
N: 0.05% or less,
Si: 0.1% or more, 1% or less,
Mn: 0.02% or more, 2% or less,
Cu: 0.2% or more, 1.5% or less,
Cr: 15% or more, 25% or less,
Nb: 8 (C + N)% or more, 1% or less,
Al: containing at least 0.5% or less, the balance consisting of Fe and inevitable impurities,
Furthermore, in mass%,
For Ti, the following formulas (1) and (2) are satisfied,
A ferritic stainless steel sheet for an EGR cooler characterized by containing Cr and Cu in a range satisfying the following formula (3).
Ti-3N ≦ 0.03 (1)
10 (Ti-3N) + Al ≦ 0.5 (2)
Cr + 2.3Cu ≧ 18 (3) Furthermore, in mass%,
Mo: 3% or less,
The ferritic stainless steel sheet for an EGR cooler according to claim 1, wherein Ni: One or both of Ni and 3% or less is contained, and is contained in a range satisfying the following formula (4).
Cr + 1.9Mo + 1.6Ni + 2.3Cu ≧ 18 (4) Furthermore, in mass%,
V: 3% or less,
The ferritic stainless steel sheet for an EGR cooler according to claim 1 or 2, wherein either or both of W: 5% or less are contained. Furthermore, in mass%,
Ca: 0.002% or less,
Mg: 0.002% or less,
B: Ferritic stainless steel sheet for EGR cooler according to any one of claims 1 to 3, which contains any one or more of 0.005% or less.   C + N: 0.015% or more, The ferritic stainless steel sheet for an EGR cooler according to any one of claims 1 to 4.

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