JP5786491B2 - Ferritic stainless steel for EGR cooler - Google Patents

Description

  The present invention relates to a ferritic stainless steel suitable for use in an EGR cooler installed in a gasoline engine, a diesel engine, or the like.

  EGR (Exhaust Gas Recirculation) refers to a system in which a part of exhaust gas discharged from an engine is returned to the intake system of the engine, mixed with air, and supplied to the combustion chamber of the engine. For example, in a diesel engine, this EGR can lower the maximum temperature during combustion by reducing the oxygen concentration in the intake air, thereby reducing the amount of nitrogen oxide NOx produced. In gasoline engines, the decrease in oxygen concentration in the intake air is the same as when the throttle is throttled, so reducing the pumping loss (resistance when the engine inhales air) by opening the throttle reduces fuel consumption. Can be improved. Therefore, in recent years, EGR has been installed in engines of various automobiles.

  However, since the decrease in oxygen concentration in the intake air due to the EGR installation tends to cause unstable combustion, it is necessary to precisely control the air fuel consumption, and it is important to control the amount of exhaust gas sent into the engine. Therefore, in order to control the volume of the exhaust gas, an EGR cooler that cools the exhaust gas to a constant temperature has been adopted.

  This EGR cooler lowers the exhaust gas temperature by heat exchange. The heat exchange member of the EGR cooler has a high temperature exhaust gas of 800 ° C. or higher on one side and a refrigerant (coolant) on the other side. Always in contact. Therefore, the heat exchange member of the EGR cooler is placed in a severe usage environment in which it is repeatedly subjected to oxidation at a high temperature and thermal stress distortion accompanying heating and cooling.

Conventionally, austenitic stainless steel such as SUS304 has been mainly used as a material for an EGR cooler used in such an environment. However, austenitic stainless steel
(1) It is expensive because it contains a large amount of expensive Ni.
(2) The coefficient of thermal expansion is high, and during manufacturing, particularly during welding or brazing, thermal distortion is generated, and the assembled product is deformed or the dimensional accuracy of the parts is deteriorated.
(3) Although the coolant flows on the cooling side in the EGR cooler, the side directly in contact with the exhaust gas is exposed to high temperature, so that an oxide scale is generated on the surface of the steel sheet, and the scale is easily peeled off.
There is a problem.

  Among the above problems, the problem (3) is that when the oxide scale generated in the exhaust gas path peels off and accumulates in the EGR cooler, the function of the cooler deteriorates, and further, the peeled scale reaches the engine. When it reaches, it causes an unexpected engine trouble, and has become an important solution. In recent years, in particular, in order to meet demands for improving fuel consumption and reducing emissions, the temperature of exhaust gas is becoming higher, and it is an urgent task to solve this scale peeling problem. Therefore, development of an inexpensive ferritic stainless steel that is excellent in oxide scale peeling resistance and does not contain Ni is desired.

  Several proposals have been made for ferritic stainless steel used in EGR coolers. For example, Patent Document 1 includes Cr: 23.0 to 33.0 wt%, Mo: 0.55 to 4.0 wt%, C: 0.01 wt% or less, N: 0.015 wt% or less, Si: 0.00. 4 wt% or less, Mn: 0.4 wt% or less, P: 0.03 wt% or less, S: 0.02 wt% or less as a basic component is a superferritic stainless steel. : Ferritic stainless steel whose basic components are 0.03% or less, Mn: 0.1 to 2%, Cr: 10 to 25%, Nb: 0.3 to 0.8%, In mass%, C: 0.03% or less, N: 0.05% or less, C + N: 0.015% or more, Si: 0.02-1.5%, Mn: 0.02-2%, Cr: 10 to 22%, Nb: 0.03 to 1%, Al: 0.5% or less, and further Ti added within the proper range Ferritic stainless steel is also disclosed in Patent Document 4 in terms of mass%, C: 0.03% or less, Si: 3% or less, Mn: 2% or less, P: 0.05% or less, S: 0 0.03% or less, Cr: 11 to 30%, Nb: 0.15 to 0.8%, and N: 0.03% or less are disclosed as ferritic stainless steels. However, these technologies are all aimed at improving the thermal fatigue characteristics and improving the Ni brazing property, and sufficient studies have been made to improve the peeling resistance of the oxide scale. Absent.

  Further, Non-Patent Document 1 discloses a ferritic stainless steel used for an automobile exhaust manifold that has significantly improved scale peel resistance by increasing Mn in a heat resistant ferritic stainless steel.

JP 2003-222498 A JP 2009-174040 A JP 2009-174046 A JP 2009-299182 A

Oku, Nakamura, Uematsu, “Development of heat resistant ferritic stainless steel NSSEM-2”, Nisshin Steel Engineering Reports, No. 71 (1995), p. 70, FIG.

  In the said nonpatent literature 1, peeling resistance of an oxide scale is evaluated by naturally cooling (air cooling) from the high temperature of 900 degreeC or 1000 degreeC. However, it is considered that the heat exchange member of the EGR cooler is subjected to rapid cooling because one surface is in contact with the coolant. Then, when the inventors evaluated the peeling resistance of the oxide scale when rapidly cooling from a high temperature for steel to which Mn was added, it became clear that the amount of scale peeling increased on the contrary.

In addition, the EGR cooler is generally assembled by Ni brazing, which has excellent resistance to high-temperature oxidation and high-temperature strength. Therefore, the EGR cooler material contains Ni brazing (wetting), brazing strength and toughness. In addition to being superior to the above, it is required that the brazed portion is excellent in corrosion resistance against condensed water containing a large amount of SO ₂ and NOx in the exhaust gas.

  Accordingly, an object of the present invention is to provide a ferritic stainless steel for an EGR cooler in which oxide scale generated at a high temperature does not peel even when subjected to rapid cooling from a high temperature and is excellent in corrosion resistance of a brazed portion. There is.

  In order to solve the above-mentioned problems, the inventors have made extensive studies by paying attention to the effect of the composition of steel on the peelability of oxide scale. As a result, in order to improve the scale peel resistance, it is basically necessary to suppress the amount of oxide scale generated itself, and for that purpose, the content of Cr and Si is reduced after lowering Mn. In order to improve the corrosion resistance of the brazed part, it is effective to control the content of Mn and S within the proper range after sufficiently fixing C and N with Nb. The inventors have found that there is a need to control, and completed the present invention.

The present invention based on the above findings is: C: 0.02 mass% or less, Si: 0.3-2 mass%, Mn: less than 0.45 mass%, P: 0.040 mass% or less, S: 0.001 mass% or more. 005 mass% or less, Al: 0.01 to 0.1 mass%, N: 0.02 mass% or less, Cr: 14 to 30 mass%, Nb: 0.3 mass% or more and 15 (C (mass%) + N (mass%) ) 1.0 mass% or less, Ti: 0.01 mass% or less, and Cr and Si, and Mn and S satisfy the following formulas (1) and (2), and the balance is Fe And a ferritic stainless steel for an EGR cooler comprising inevitable impurities.
Cr + 6Si: 18-30 mass% (1)
Mn × S ≦ 0.0005 (2)
(However, the element symbol in the above formula indicates the content (mass%) of the element.)

Ferritic stainless steels of the present invention, in addition to the above chemical composition, N i: characterized by containing a 0.6 mass% hereinafter.

  Further, the ferritic stainless steel of the present invention is characterized by further containing Mo: 2 mass% or less in addition to the above component composition.

  According to the present invention, it is possible to provide a ferritic stainless steel having excellent oxide scale peel resistance even when rapidly cooled from a high temperature. In addition, the ferritic stainless steel of the present invention is not only excellent in scale peel resistance but also inexpensive because it does not contain a large amount of Ni, and is also excellent in brazeability and corrosion resistance of the brazed part. Therefore, it can be suitably used for an EGR cooler.

First, an experiment that triggered the development of the present invention will be described.
As described above, in the heat exchange portion of the EGR cooler, the surface in contact with the exhaust gas is heated to a high temperature, but the opposite surface is cooled by the coolant. Therefore, for example, when the engine is stopped, the surface in contact with the exhaust gas is considered to be rapidly cooled from a high temperature state. And the oxide scale produced | generated on the steel surface produces a big thermal distortion from the difference in a thermal expansion coefficient between raw materials, and causes peeling.

Therefore, the technique of Non-Patent Document 1 is to solve the above-mentioned problem of scale peeling by adding Mn 0.8 mass% or more and generating MnCr ₂ O ₄ at the interface between the material and the oxide scale Cr ₂ O _3. While mitigating, by increasing the unevenness of the steel substrate surface, the peeling of the scale is suppressed. However, Mn is an element that promotes the generation of oxide scale, and the oxide scale that is thickly formed by the addition of Mn may not necessarily have sufficient peeling resistance under the environment where the EGR cooler is used.

Therefore, the inventors investigated the peeling resistance of the oxide scale by changing the cooling condition (cooling rate) after the oxide scale was generated.
In the experiment, test pieces of plate thickness: 1.5 mm × width: 20 mm × length: 40 mm were sampled from three types of steel plates having different component compositions shown in Table 1, and each test piece was 950 in an air atmosphere. After heating at 100 ° C. for 100 hours to generate an oxide scale on the specimen surface, the cooling rate was
(I) natural cooling (air cooling);
(Ii) When quenching by blowing argon gas at 10 L / min,
It was divided into two levels and cooled.
Then, visually observe the surface of each test piece, confirm the presence or absence of traces of scale peeling, collect the oxide scale peeled during cooling, measure its mass, determine the amount of peel per unit surface area, No trace and scale peel-off amount of 1.0 mg / cm ² or less, good scale peel resistance (◯), trace of peel is visually recognized and / or scale peel-off amount exceeds 1.0 mg / cm ² The thing was evaluated as inferior in scale peel resistance (x). In each level, the number of n was set to 2, and even one corresponding to the above x was evaluated as x.

,
The results of the above test are also shown in Table 1. From these results, the conventional SUS304 steel and SUS430LX steel have poor scale peel resistance in both cases of air cooling and rapid cooling conditions (x), and the steel in which 1.0 mass% of Mn is added to SUS430LX is air cooled. Although the scale peel resistance was good (◯) under the conditions, the results showed that the scale peel resistance was poor (×) under the rapid cooling conditions.

  The above results show that the addition of Mn, which increases the amount of oxide scale produced, has a small effect of improving the scale peeling resistance in a use environment where a large thermal stress is generated due to rapid cooling like an EGR cooler. In order to improve the scale peeling resistance, it is shown that it is most effective to suppress the formation of oxide scale itself. Therefore, as a result of repeated studies to reduce the amount of oxide scale generated in a high-temperature oxidizing atmosphere, the inventors have to include the Cr and Si contents so as to satisfy the following relational expression. I found it.

In addition, the inventors investigated the cause of a decrease in the corrosion resistance of the brazed part. As a result, the reduction in the corrosion resistance of the brazed part is caused by the fact that C and N are combined with Cr in the steel by heating during brazing, resulting in a Cr-deficient layer and intergranular corrosion. In order to improve the corrosion resistance of the brazed portion due to the formation of MnS and become a starting point of pitting corrosion, etc., after adding a predetermined amount of Nb and sufficiently fixing C and N, It has been found that the product (Mn × S) of the content (mass%) of Mn and S needs to be controlled to a predetermined value or less.
Hereinafter, the component composition that the ferritic stainless steel of the present invention should have will be specifically described.

C: 0.02 mass% or less C combines with Cr during cooling after brazing to form a Cr-deficient layer, impairs intergranular corrosion resistance, or combines Nb added to steel with Nb. Since it is a component that consumes or adversely affects the weldability and workability of steel, the lower the content, the better. Therefore, in the present invention, C is set to 0.02 mass% or less. Preferably, it is 0.005 mass% or less.

Si: 0.3-2 mass%
Si is added as a deoxidizer for steel, but it is also an extremely effective component for improving the high-temperature oxidation characteristics and improving the peeling resistance of the generated oxide scale. As 0.3 mass% or more. In order to further improve the peeling resistance of the scale, 0.5 mass% or more is preferable, and 0.8 mass% or more is more preferable. However, addition exceeding 2 mass% hardens the steel and significantly impairs the workability, so the upper limit is made 2 mass%.

Mn: Less than 0.45 mass% Mn is an element that generates an intermediate oxide such as MnCr ₂ O _{4 in} a high-temperature oxidizing atmosphere and increases the amount of oxidation, and at a cooling rate of about air cooling that does not generate large thermal distortion, This has the effect of improving the peel resistance of the scale. However, when subjected to rapid cooling in which a large thermal strain is generated between the oxide scale and the material, the addition of Mn adversely affects the scale peeling resistance. Therefore, in the present invention, Mn is limited to less than 0.45 mass%. Preferably it is 0.30 mass% or less, More preferably, it is 0.25 mass% or less.

P: 0.040 mass% or less P is a harmful component that concentrates at the grain boundary during Ni brazing cooling at a temperature of about 1100 ° C. and lowers the corrosion resistance. 0.040 mass% or less. Preferably it is 0.030 mass% or less.

S: More than 0.001 mass% and not more than 0.005 mass% S is a component that forms Mn and MnS and adversely affects the corrosion resistance, and is desirably as low as possible. In the present invention, it is 0.005 mass% or less. However, excessive reduction of S causes an increase in steelmaking costs, so the lower limit is made to exceed 0.001 mass%.

Al: 0.01-0.1 mass%
Al is a component added as a deoxidizer. Further, since the weld bead surface is covered with a strong Al oxide film and has the effect of blocking the entry of oxygen from the outside, it is necessary to add 0.01 mass% or more. However, addition exceeding 0.1 mass% lowers the wettability of Ni brazing by the oxide film, so the upper limit is 0.1 mass%.

N: 0.02 mass% or less N, like C, forms a Cr-deficient layer by bonding with Cr during cooling after brazing, impairs intergranular corrosion resistance, or combines with Nb and added to steel The lower the content, the more desirable it is because it is a component that consumes the produced Nb or adversely affects the weldability and workability of steel. Therefore, in the present invention, N is set to 0.02 mass% or less.

Cr: 14-30 mass%
Cr needs to be added in an amount of 14 mass% or more in order to ensure the corrosion resistance as stainless steel. However, the addition exceeding 30 mass% impairs workability. Therefore, Cr is set to a range of 14 to 30 mass%. Preferably it is the range of 15-23 mass%.

Nb: 0.3 to 1.0 mass% and Nb ≧ 15 (C + N)
Nb is a useful element for forming carbonitrides and suppressing the coarsening of crystal grains caused by heating during brazing to ensure the strength and toughness of the steel and to increase the high-temperature strength of the steel. Addition of 0.3 mass% or more is required. However, addition exceeding 1 mass% is not preferable because the steel becomes brittle. Therefore, Nb is set to a range of 0.3 to 1.0 mass%.
In addition to the effects described above, Nb fixes C and N and combines with Cr to form a Cr-deficient layer, thereby preventing sensitization and preventing intergranular corrosion after brazing. There is an effect to. In order to obtain such an effect, it is necessary to add Nb of 15 (C + N) or more (however, C and N are mass%). Preferably it is 25 (C + N) or more, more preferably 50 (C + N) or more.

Ti: 0.01 mass% or less Ti is an element that has the effect of fixing C and N, like Nb, but is also an easily oxidizable component, so it forms a strong oxide film during brazing, In particular, the wettability of Ni brazing is significantly reduced. Therefore, Ti is desirably as low as possible. In the present invention, the upper limit is set to 0.01 mass%. Preferably it is 0.005 mass% or less.

In addition to satisfying the above component composition, the ferritic stainless steel of the present invention must contain Cr, Si, Mn, and S satisfying the following formulas (1) and (2): It is.
Cr + 6Si: 18-30 mass% (1)
In order to further improve the peeling resistance of the oxide scale when subjected to rapid cooling from a high temperature of about 950 ° C. as in the use environment of the EGR cooler, Cr and Si are changed to (Cr + 6Si) ≧ 18 mass% (however, Cr, Si must be contained so as to satisfy mass%). This is because if (Cr + 6Si) is less than 18 mass%, the scale thickness increases due to insufficient oxidation resistance. However, when (Cr + 6Si) exceeds 30 mass%, the embrittlement of the steel becomes significant. Therefore, in this invention, Cr + 6Si is taken as the range of 18-30 mass%. In the case of an EGR cooler that is rapidly cooled from 1000 ° C., Cr + 6Si is preferably 20% by mass or more, and more preferably 22% by mass or more.

Mn × S ≦ 0.0005 (2)
Mn and S are components that form MnS and adversely affect the corrosion resistance, and it is desirable to reduce both elements as much as possible. Therefore, in the present invention, the product of Mn and S contents (mass%) is limited to 0.0005 or less. Preferably, it is 0.0003 or less.

Moreover, in addition to the said component composition, the ferritic stainless steel of this invention can further add 1 type or 2 types chosen from Cu and Ni in the following range.
Cu: 0.6 mass% or less, Ni: 0.6 mass% or less Cu and Ni can be added because they have an effect of improving the corrosion resistance of the steel. However, if both Cu and Ni are added in excess of 0.6 mass%, the effect of improving corrosion resistance is saturated. Therefore, it is preferable to add Cu and Ni at 0.6 mass% respectively.

Moreover, the ferritic stainless steel of this invention can add Mo in the following range further in addition to the said component composition.
Mo: 2 mass% or less Mo has a great effect on improving the corrosion resistance of steel, and can be added as necessary. However, since addition exceeding 2 mass% significantly decreases the workability, it is preferable to add the upper limit of 2 mass%.

  In the ferritic stainless steel of the present invention, the balance other than the above components is Fe and inevitable impurities. However, as long as the effects of the present invention are not impaired, the inclusion of components other than those described above is not rejected.

  In addition, the manufacturing method of the ferritic stainless steel of this invention can be manufactured with the manufacturing method of a well-known ferritic stainless steel except controlling the component composition of steel to the said range, and there is no restriction | limiting in particular.

  Steel having the composition shown in Table 2 was melted to form a steel slab, which was then hot-rolled, cold-rolled to obtain a cold-rolled sheet having a thickness of 1.5 mm, and then subjected to finish annealing and cold-rolling annealing. A board was used. Thereafter, test pieces were collected from these cold-rolled annealed plates and subjected to the following high-temperature oxidation test and corrosion resistance test.

<High temperature oxidation test>
From each of the above-mentioned cold-rolled annealed plates, a test piece having a thickness x width: 20 mm x length: 40 mm was taken, and a high-temperature oxidation test was performed in which these test pieces were held in an atmosphere atmosphere furnace at 950 ° C. for 100 hours. After the test piece is taken out of the furnace, it is naturally cooled (air cooled), or cooled by blowing argon gas at 10 L / min (rapidly cooled), and then the presence of traces of scale peeling is confirmed by visual observation. The scale peeled during cooling was collected and the amount of scale peel was measured. As a result, when there is no trace of scale peeling and the scale peel amount is 1.0 mg / cm ² or less, the scale peel resistance is good (◯), there is a trace of scale peel and / or the scale peel amount is 1. Those exceeding 0 mg / cm ² were evaluated as poor (×) in scale peel resistance. The number of n tested was 2 for each steel plate, and even if there was one corresponding to x, it was evaluated as x.

<Corrosion resistance test>
From each of the above-mentioned cold-rolled annealed plates, specimens having a thickness x width: 20 mm x length: 40 mm were collected, and the thermal history at the time of brazing was simulated on these specimens at 1100 ° C. in a vacuum atmosphere. After heating for 10 minutes, a heat treatment for air cooling was performed. Next, these specimens were subjected to a neutral salt spray test (NSS) in accordance with JIS Z2371, and then the state of rust generated on the specimen surface was visually observed, and those without rusting were excellent in corrosion resistance (◎ ), Those having a rusting area ratio of 10% or less were evaluated as good corrosion resistance (◯), and those having a rusting area ratio exceeding 10% were evaluated as inferior corrosion resistance (×).

  The measurement results are also shown in Table 2. From these results, it was confirmed that all the steels satisfying the component composition of the present invention were excellent in scale peel resistance and corrosion resistance.

  Since the ferritic stainless steel of the present invention is excellent in scale peel resistance, brazing resistance and corrosion resistance after brazing, it can be suitably used for EGR pipes and heat exchanger parts as well as for EGR coolers. .

Claims (3)

C: 0.02 mass% or less,
Si: 0.3-2 mass%
Mn: less than 0.45 mass%,
P: 0.040 mass% or less,
S: more than 0.001 mass% and 0.005 mass% or less,
Al: 0.01-0.1 mass%,
N: 0.02 mass% or less,
Cr: 14-30 mass%,
Nb: 0.3 mass% or more and 15 (C (mass%) + N (mass%)) or more and 1.0 mass% or less,
EGR cooler containing Ti: 0.01 mass% or less, Cr and Si, Mn and S satisfying the following formulas (1) and (2), and the balance being Fe and inevitable impurities Ferritic stainless steel.
Cr + 6Si: 18-30 mass% (1)
Mn × S ≦ 0.0005
... (2)
(However, the element symbol in the above formula indicates the content (mass%) of the element.) In addition to the above chemical composition, N i: 0.6mass% EGR cooler for ferritic stainless steel according to claim 1, characterized in that it contains hereinafter. The ferritic stainless steel for an EGR cooler according to claim 1 or 2, further comprising Mo: 2 mass% or less in addition to the above component composition.