CROSS REFERENCE TO RELATED APPLICATIONS

This is the U.S. National Phase application of PCT/JP2017/030439, filed Aug. 25, 2017, which claims priority to Japanese Patent Application No. 2016-171520, filed Sep. 2, 2016 and Japanese Patent Application No. 2017-104059, filed May 26, 2017, the disclosures of each of these applications being incorporated herein by reference in their entireties for all purposes.

FIELD OF THE INVENTION

The present invention relates to ferritic stainless steel which is used in an environment of exhaust gas condensed water in an automobile. More specifically, the present invention relates to ferritic stainless steel which is used, for example, for an exhaust heat recovery device or an exhaust gas recirculation device such as an exhaust gas recirculation (EGR) cooler.

BACKGROUND OF THE INVENTION

Nowadays, in the field of automobiles, exhaust gas emission control is tightened, and there is a demand for a further improvement in fuel efficiency. Therefore, there is a growing trend toward applying heat exchangers such as an exhaust heat recovery device and an EGR cooler to automobiles.

An exhaust heat recovery device is a device with which exhaust gas heat is recovered and reused and which is mounted mainly on hybrid automobiles. In the case of a system utilizing an exhaust heat recovery device, fuel efficiency and room-heating performance are improved as a result of accelerating engine warming-up by transferring heat from exhaust gas to engine-cooling water through a heat exchanger.

In addition, an EGR cooler is a device with which exhaust gas is recirculated. In the case of a system utilizing an EGR cooler, the combustion temperature of an engine is decreased to inhibit the generation of NOx by cooling high-temperature exhaust gas on the exhaust side through a heat exchanger, and by returning the cooled exhaust gas into the engine.

The heat exchange unit of such an exhaust heat recovery device or an EGR cooler is exposed to a severe corrosive environment due to generated condensed water. In the heat exchange unit, since stainless steel is interposed between exhaust gas and cooling water, leakage of the cooling water occurs when pitting occurs due to corrosion. Therefore, there is a demand for corrosion resistance to condensed water.

Patent Literature 1 discloses austenitic stainless steel for an EGR cooler and exhaust heat recovery device in the case where a fuel having high S concentration due to insufficient refining is used. However, there is a problem of high cost as a result of austenitic stainless steel containing a large amount of Ni. And there is a problem of low thermal fatigue resistance at a high temperature, that is, fatigue resistance in an environment in which the steel is subjected to severe vibration at a high temperature as in the case where the steel is used as, for example, parts around an exhaust manifold.

Therefore, consideration is given to using steel other than austenitic stainless steel for heat exchange units of an exhaust heat recovery device and an EGR cooler.

For example, Patent Literature 2 discloses an exhaust heat recovery device for an automobile constituted of ferritic stainless steel as a material. In this case, sufficient pitting corrosion resistance and sufficient crevice corrosion resistance are achieved by adding Mo to 18 mass % or more of Cr contained stainless steel in an environment in which exhaust gas is condensed and vaporized.

In addition, since brazing is used for joining the heat exchange unit or the like in the EGR cooler described above, such members are desired to have not only improved corrosion resistance to condensed water but also excellent brazeability.

From such a viewpoint, for example, Patent Literature 3 discloses ferritic stainless steel for an EGR cooler. In this case, excellent brazeability and excellent corrosion resistance to condensed water in exhaust gas are achieved by adding Cr and Cu so that the relationship Cr+2.3Cu≥18 is satisfied.

Patent Literature 4 discloses ferritic stainless steel for an exhaust heat recovery device having corrosion resistance to condensed water in exhaust gas after brazing has been performed. In this case, the steel is characterized in that the cation fraction in a layer formed after brazing and heating is specified from the viewpoint of corrosion resistance after the brazing.

PATENT LITERATURE

PTL 1: Japanese Unexamined Patent Application Publication No. 2013-199661

PTL 2: Japanese Unexamined Patent Application Publication No. 2009-228036

PTL 3: Japanese Unexamined Patent Application Publication No. 2010-121208

PTL 4: Japanese Unexamined Patent Application Publication No. 2012-214880

SUMMARY OF THE INVENTION

However, in the case of the stainless steels according to Patent Literature 2 through Patent Literature 4, there may be a case of insufficient corrosion resistance to condensed water when a test is conducted by simulating a practical environment in which generation, vaporization, and heating of condensed water are cyclically repeated. Like this, in the case of the conventional techniques, it may be said that desired corrosion resistance to condensed water has not yet been achieved with sufficient brazeability being achieved at the same time.

Therefore, an object according to aspects of the present invention is to provide ferritic stainless steel having excellent brazeability and excellent corrosion resistance to condensed water in an environment in which the steel is used for an exhaust heat recovery device or an EGR cooler.

Here, the expression “excellent brazeability” refers to a case where, when a brazing treatment is performed by applying 1.2 g of a brazing filler metal, that is, BNi-5(Ni-19Cr-10Si) to one end surface of overlapped two steel sheets and by heating the steel sheets at a temperature of 1170° C. for 600 seconds in a vacuum environment of 10−2Pa, the penetration distance of the brazing filler metal is 50% or more of the lapping length of the two steel sheets.

In addition, the expression “excellent corrosion resistance to condensed water” refers to a case where, when a unit test cycle in which a whole test piece is subjected to full immersion in a solution containing 200 ppm Cl−+600 ppm SO42−and having a pH of 8.0, holding at a temperature of 80° C., immersion for 24 hours while the solution is vaporized in 24 hours, and being held in a furnace at a temperature of 250° C. for 24 hours, is performed four times (hereinafter, may be referred to as “condensed water corrosion test”), the maximum corrosion depth is less than 100 μm.

The present inventors conducted the condensed water corrosion test described above and found that it is possible to achieve excellent corrosion resistance to condensed water by containing an appropriate amount of Si in addition to Cr, Mo, Ni, C, and N. Moreover, it was found that it is also possible to achieve sufficient brazeability by controlling the Al content.

The subject matter of aspects of the present invention, which are intended to solve the problems described above, is as follows.

[1] A ferritic stainless steel having a chemical composition containing, by mass %,

C: 0.025% or less,

Si: 0.40% to 2.0%,

Mn: 0.05% to 1.5%,

P: 0.05% or less,

S: 0.01% or less,

Cr: 16.0% to 30.0%,

Mo: 0.60% to 3.0%,

Ni: 0.10% to 2.5%,

Nb: 0.20% to 0.80%,

Al: 0.001% to 0.15%,

N: 0.025% or less, and

the balance being Fe and inevitable impurities,

in which relational expressions (1) and (2) below are satisfied.
C+N≤0.030%  (1),
2Si+Ni≥1.0%  (2),
(in relational expressions (1) and (2), C, N, Si, and Ni each denote the contents (mass %) of the corresponding elements).

[2] The ferritic stainless steel according to item [1] above, in which the chemical composition further contains, by mass %, one, two, or all selected from

Cu: 0.01% to 1.0%,

W: 0.01% to 1.0%,

Co: 0.01% to 1.0%.

[3] The ferritic stainless steel according to item [1] or [2] above, in which the chemical composition further contains, by mass %, one, two, or more selected from

Ti: 0.01% to 0.10%,

V: 0.01% to 0.50%,

Zr: 0.01% to 0.30%,

B: 0.0003% to 0.005%,

Ca: 0.0003% to 0.003%,

Mg: 0.0003% to 0.003%,

one or more REMs (rare earth metals): 0.001% to 0.10%.

[4] The ferritic stainless steel according to any one of items [1] to [3] above, in which the steel is steel for an exhaust heat recovery device or exhaust gas recirculation device of an automobile.

According to aspects of the present invention, it is possible to provide ferritic stainless steel having excellent brazeability and excellent corrosion resistance to condensed water in the case where the steel is used for an automobile part such as an exhaust heat recovery device or an EGR cooler which is exposed to a condensed water corrosive environment.

