JP2959934B2 - Heat-resistant ferritic stainless steel - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a ferritic stainless steel used as a high-temperature member for an exhaust pipe of an automobile, an outer casing of a catalyst, an exhaust duct of a power plant, a casing of a denitration equipment, and the like.

[0002]

2. Description of the Related Art In recent years, it has been desired to improve the fuel efficiency and output of automobiles, and also to reduce the weight of automobile materials. In addition, due to the strengthening of pollution regulations, purification of exhaust gas is also strongly demanded. Against this background, SUS430J1L, an existing ferritic stainless steel, is used in automobile exhaust system materials from the viewpoint of weight reduction and low heat capacity as components.
And AISI 409 are currently used.

[0003] On the other hand, further improvement in fuel efficiency and higher output have been promoted, and accordingly, the maximum temperature of exhaust gas has been increased to 900 ° C or more.
It is said that it will rise soon. It is important for the heat-resistant material used for these thin plate structures to achieve both (1) high-temperature strength during use, thermal fatigue characteristics and high-temperature fatigue characteristics, and (2) workability at room temperature. Further, the front pipe and the like suffer from the problem of (3) high-temperature salt damage caused by spraying snow-melting material (salt) performed in some areas. Here, it is expected that the high-temperature fatigue is improved and the thermal fatigue resistance is also improved by improving the high-temperature strength and securing the high-temperature strength during use.

[0004] At present, SUS430 containing only Nb is added.
J1L is used. With the addition of Nb, high-temperature strength is improved by solid solution strengthening, and workability is also improved by the fixation of C and N. However, since Nb has a strong affinity with C and N and easily forms a carbonitride, the carbonitride precipitates during use and further agglomerates and coarsens, so the amount of solid solution strengthening of Nb decreases with use. I do.

[0005] Therefore, it is difficult to maintain high high-temperature strength during use only by adding Nb alone. On the other hand, since AISI409 has low Cr, abnormal oxidation occurs around 900 ° C. Also, since it does not contain high-temperature strengthening elements such as Nb and Mo, the high-temperature strength is low.
Related known examples are as follows.

[0006] For heat-resistant applications, see JP-A-64-8254.
JP-A-2-175843, JP-A-4-74852
And other publications. JP-A-64-82543 discloses that Cr is increased to 16% or more from the viewpoint of oxidation resistance, Nb is essential, and Mo is a selective element.
An element (eg, Ti) having a higher affinity for C and N than for Nb or Mo is not added. For this reason, Nb is particularly in a state where carbonitride is easily formed during use, and no consideration is given to securing high-temperature strength during use. JP-A-2-1
With regard to the publication of No. 75843, on the high Cr side, Nb and Mo are essential, and Ti is a selective element. Again,
No consideration is given to securing the strength during use at high temperatures, and Ni and Cu are essential. In JP-A-4-74852, Nb and Ti are essential, but Mo and Al
No consideration is given to oxidation resistance and high-temperature salt damage characteristics in a temperature range exceeding 900 ° C. when low Cr is added without adding. The present inventors have also filed Japanese Patent Application No. 4-270.
No. 102 invents a ferritic stainless steel for a temperature range up to around 900 ° C.
Therefore, the oxidation resistance in a region exceeding 900 ° C. is not sufficient.

As a general use, see Japanese Patent Application Laid-Open No. 3-264.
652, which has a Cr content of 11 to 30%.
And a wide range, including the low Cr side. Also,
Ti and Nb are essential, and Mo is a selective element. However, from the viewpoint of moldability, the amount of Si is 0.5% or less,
Since Al is not added, when low Cr is used, 9
Oxidation resistance in the region exceeding 00 ° C. cannot be sufficiently ensured.
In addition, no consideration has been given to high-temperature salt damage characteristics.

[0008]

As described above, some conventional steels and known examples include high-temperature members that are used at high temperatures for a long time, particularly automotive exhaust system materials used in a temperature range exceeding 900 ° C. The required properties of (1) high temperature strength during use, thermal fatigue properties and high temperature fatigue properties, (2) compatibility of workability at room temperature, and (3) high temperature salt damage resistance (600 ° C to 80 ° C)
0 ° C.), and (4) there is no example in which oxidation resistance in a region exceeding 900 ° C. is sufficiently satisfied.

The present invention can ensure high-temperature strength and oxidation resistance even when the maximum temperature exceeds 900 ° C.
An object of the present invention is to provide a high heat-resistant ferritic stainless steel capable of simultaneously achieving material properties excellent in cold workability and high-temperature salt damage resistance.

[0010]

Means for Solving the Problems To improve the high-temperature strength of heat-resistant ferritic stainless steel, there are a method using solid solution Nb, Mo and W, and a method of finely dispersing a precipitated phase stable at high temperature. The present inventors first set forth Mo,
Focusing on securing solid solution of W and Nb in the cold-rolled annealed sheet, these elements were efficiently dissolved. That is, the amount of C + N in steel is reduced, and by adding an appropriate amount of Ti, these are fixed and the amount of solute C + N is reduced to improve workability, and at the same time, precipitation strengthening at high temperature by Ti (C, N) is performed. Is also functioning.

The most important role of the addition of Ti is Mo or Nb.
It is to fix C and N with Ti by utilizing the fact that the affinity with C and N is stronger than that. By fixing C and N by Ti, the precipitation of carbonitrides of Mo, W and Nb can be suppressed, and the solid solution amount of these elements can be secured not only before use but also for a long time at high temperature. ing. In order to secure a particularly high high-temperature strength for a long time, eff. The amount of Nb is specified, and the amount of solid solution Nb during long-time use at high temperature is ensured, whereby the tensile strength at 950 ° C. is set to 20 MPa or more.

Here, eff. The concept of Nb is the same as the concept in the specification of Japanese Patent Application No. 4-270102, and in the present invention, in particular, in the temperature range exceeding 900 ° C. (9
(50 ° C.). eff. Nb is (1) Nb precipitation during use is changed from MC type to M ₆ C type (Fe
₃ Nb ₃ C) change things, (2) half of N is fixed by Ti, the other half N due to high temperature use, such that the Ti remaining precipitate as nitrides of Nb to form carbides Nb Changes in the precipitation morphology of Ti and Ti, and a report on refractory metal materials (v.
ol. 33, 1992, p. As described in 1), (3) the high-temperature strength shown in claim 1 of the present application is taken into consideration, considering that the precipitation of an intermetallic compound (Laves phase) with Fe is effective to some extent in strengthening. As an index for holding eff. Nb was specified. This makes it possible to estimate the high-temperature strength level during use.

[0013] Ensuring high-temperature strength during use dramatically improves the thermal fatigue property, which is the most important required property as a material for an exhaust manifold. In addition, Ti and N
Addition of b also prevents coarsening of the grain size in the welded zone and the weld-affected zone, and has good weldability. On the other hand, Mo, W and Nb easily form an intermetallic compound with Fe, and when a large amount thereof precipitates and becomes coarse, the toughness during use and the high-temperature strength deteriorate. Further, Mo and W improve the high-temperature salt damage resistance, but when added in excess, the effect of improving the high-temperature salt damage resistance is lost.

Next, in the invention according to claim 3 of the present application,
Utilizing precipitation strengthening of precipitates of V (estimated as VN),
This is intended to enhance the high temperature in a temperature range exceeding 900 ° C. The effect of the precipitation strengthening of V does not decrease significantly even after high-temperature aging. Also, the amount of reinforcement is larger than that of solid solution strengthening. However, when 1> (V / 51) / (N / 14), if N becomes excessive with respect to V, other nitrides are precipitated in a complex manner and the precipitates are coarsened, so that the amount of precipitation strengthening decreases. Therefore, it is necessary to keep the number of V atoms in excess of the number of N atoms ((V / 51 ) / (N / 14) ≧ 1).
Further, the effect of improving the high-temperature strength and the high-temperature salt damage resistance of Mo and W also acts on the V-added steel.

Further, in order to be adapted to a member requiring further oxidation resistance, in claim 5 of the present application, a rare earth element is added within a range that does not impair hot workability.

[0016]

The reasons for limiting the steel components of the present invention will be described below (all chemical components are% by weight). C: In order to support high-temperature strength by solid solution strengthening of Nb and Mo, it is desired to keep the strength low from the viewpoint of workability and toughness of hot rolled sheet. However, the extremely low temperature is disadvantageous to the economy, and a part of the high temperature strength before use is supported by the carbonitride of Ti and Nb. In case of, together with N; C + N ≦ 0.03
%.

Si: An element that is effective as a deoxidizing material and improves the oxidation resistance and high-temperature salt damage resistance, so that the content of Si is 0.1% or more. On the other hand, 2% is used to reduce workability and weldability. It was as follows. Mn: Since it is a deoxidizing element, at least 0.1% is required.
The upper limit is set to 2% in order to prevent martensitic transformation as an austenite forming element.

P: Useful for increasing the strength at high temperatures (solid solution strengthening), but causes deterioration in weldability, so it was made 0.01 to 0.1%. S: an element forming MnS, and is set to 0.01% or less in order to reduce corrosion resistance, which is a basic property of stainless steel. Cr: Effective for improving the oxidation resistance, and 17% or more to secure the oxidation resistance at 900 ° C. or more. When the maximum temperature is considered to be 900 ° C. or higher as the use environment of the steel of the present invention, the addition of 20% or more is not very effective.
0%.

Nb: an additional element for preventing grain growth and ensuring high-temperature strength in the weld and the weld affected zone.
However, in order to have a strong affinity with C, N and Fe, to form a precipitate during use, and to make the effect of solid solution strengthening of Nb work more effectively in the region of 900 ° C. or more, in the case where V is not added (ie, When mainly using solid solution strengthening of Nb)
0.3-0.8% and eff. 0.3 or more as Nb
0.6%. When using precipitation strengthening of V, the content is set to 0.2 to 0.6% in order to balance with hot workability.

Since Ti: has a strong affinity for C and N, it was added only when V was not added. C + N is an element necessary for fixing C, improving workability and ensuring long-term stability of the metal phase structure. Since Ti has a stronger affinity for C and N than Mo, W and Nb, it has a function of suppressing the precipitation of carbonitrides of Nb and Mo during use. Thereby, solid solution Mo, W and Nb during use can be secured, and high-temperature strength during use can be secured. In order to fix C and N which do not form a solid solution in the mother phase, the minimum addition amount was 0.01%. In addition, since a part of the high-temperature strength before use is supported by the carbonitride of Ti, the addition of Ti exceeding 0.5% lowers the high-temperature strength before use to coarsen the carbonitride. Therefore, the upper limit of Ti is set to 0.5%.

Mo: An additive element for increasing high-temperature strength and high-temperature salt damage resistance, it is necessary to add 0.1% or more to ensure corrosion resistance, which is a basic characteristic of stainless steel. Further, since it is harder to precipitate than Nb, the amount of solid solution can be ensured even during use, which is effective for maintaining high-temperature strength during use. However, when it exceeds 2%, ductility at normal temperature, hot workability and high-temperature salt damage resistance deteriorate. In consideration of the fact that a large amount of intermetallic compounds and carbonitrides with Fe do not precipitate, the upper limit is set to 2% for single addition and 3.0 ≧ M for composite addition with W.
o + W ≧ 0.1.

W: an additive element for increasing high-temperature strength and high-temperature salt damage resistance. In addition, compared to Nb, it is less likely to precipitate,
Effective for maintaining high-temperature strength during use. However, when it exceeds 2%, ductility at normal temperature, hot workability and high-temperature salt damage resistance deteriorate. In consideration of the fact that a large amount of intermetallic compounds and carbonitrides with Fe do not precipitate, the upper limit was set to 2% for single addition, and 3.0 ≧ Mo + W ≧ 0.1 for composite addition with Mo.

In the case where N: V is not added, high-temperature strength is mainly supported by solid solution strengthening of Nb and Mo, and further, it is desired to suppress the workability and toughness of hot-rolled sheet as low as possible.
However, since the extremely low temperature is disadvantageous to the economics, the level at which the high-temperature strength is not reduced is set to 0.02% or less by itself, and the sum of C and C + N ≦ 0.03%. Also,
In the case of addition of V, the content of (V / 51) / (N / 14) ≧ 1 is set to 0.005 to 0.3% in order to greatly contribute to high-temperature strengthening as a VN precipitation forming element.

V: Carbonitride forming element, necessary for precipitation strengthening. On the other hand, excessive addition saturates the precipitation strengthening ability of V and adversely affects the room temperature ductility.
(V / 51) / (N / 14) ≧ 1 with respect to 0.01 to 1.0%. Rare earth element (lanthanoid element or misch metal containing Y): Claim 5 of the present application
Are additional elements included in the invention described in (1). This element is added when further improvement in oxidation resistance is required. However, if the addition amount exceeds 0.05%, the hot workability is inferior, so this was made the upper limit.

High-temperature strength: The high-temperature strength at 950 ° C. was set to 20 MPa or more in order to secure heat-resistant fatigue properties and fatigue properties in a temperature range exceeding 900 ° C.

[0026]

EXAMPLES Test steels having the chemical components shown in Table 1 were melted in a slab shape by vacuum melting, and then slab heating-hot rolling-
A 2 mm thin plate was produced through cold rolling and annealing pickling. A tensile test, a high-temperature salt damage test and a high-temperature tensile test were performed on this thin plate.

[0027]

[Table 1]

Table 2 shows a comparison of the material properties between the steel of the present invention and the comparative steel. The steels of the present invention of Sample Nos. A3 to 5 and A7 to 16 each had a high temperature strength of 20 MPa or more at 950 ° C. before and after aging (assuming before use) and after aging (assuming during use). Has strength. Further, the respective properties of high-temperature salt damage resistance and ordinary temperature ductility are also good.

In the sample No. B3 in which the amount of Ti added is large, the eff. Sample No. having Nb and Mo + W amounts
The high temperature strength before aging is lower than that of A7. (V
The sample No. B5 having a small ratio of (/ 51) / (N / 14) has a low high-temperature strength before and after aging (particularly after aging). Mo
In Sample Nos. B3, B4, B7 and B8 in which N and W are excessively added, the resistance to high-temperature salt damage is poor. Excessive addition of Mo and W, that hot workability also lowers, Sample No. B7
And B8. Sample No. B6 with excessive addition of V
Has low ductility at room temperature and also has a problem in hot workability. On the other hand, when the REM amount exceeds 0.05%, the hot workability is poor as in the examples of Sample Nos. B9 and B10.

[0030]

[Table 2]

[0031]

According to the present invention, there is provided a member which is used for a long time at a high temperature,
In particular, as a material for automobile exhaust systems, we have found a component system that can cope with a future increase in exhaust gas temperature due to improvements in fuel efficiency and air purification, and a high heat-resistant ferritic stainless steel that can be used even in a temperature range exceeding 900 ° C. It can provide steel.

Continuation of the front page (56) References JP-A-5-125549 (JP, A) JP-A-3-264652 (JP, A) JP-A-61-117251 (JP, A) JP-A-55-110759 (JP, A) , A) JP-A-53-118218 (JP, A) (58) Fields investigated (Int. Cl. ⁶ , DB name) C22C 38/00 302 C22C 38/28

Claims (5)

(57) [Claims] 1. In% by weight, C: 0.003 to 0.015 N: 0.02 or less C + N: 0.03 or less Si: 0.1 to 2 Mn: 0.1 to 2 P: 0.01 to 0.1 S: 0.01 or less Cr: 17 to 20 Ti: 0.01 to 0.5 Nb: 0.3 to 0.8 Mo: 0.1 to 2.0 , balance Fe and unavoidable impurities Eff. Calculated from the following equation: Nb is characterized scope der Rukoto of 0.3 to 0.6, 20 MPa tensile strength at 950 ° C.
The above is a heat-resistant ferritic stainless steel excellent in high-temperature salt damage corrosion resistance and workability . eff. Nb (%) = Nb (%) − 3.93 · fc / 1
2-93 · fn / 14 where Ti (%)-48 · (N
(%) / 2) / 14> 0, C (%)-12 · (Ti (%) − 48 · (N (%) /
2) / 14) / 48> 0, fc = C (%) − 12 · ｛Ti (%) − 48 · (N
(%) / 2) / 14 ° / 48 fn = N (%) / 2 C (%)-12 · (Ti (%) − 48 · (N (%) /
2) / 14) / 48 ≦ 0, fc = 0 fn = N (%) / 2 Ti (%) − 48 · (N (%) / 2) / 14 ≦ 0, fc = C (%) fn = N (%)-14. (Ti (%) / 2) / 48 2. In % by weight, C: 0.003 to 0.015 N: 0.02 or less C + N: 0.03 or less Si: 0.1 to 2 Mn: 0.1 to 2 P: 0.01 to 0.1 S: 0.01 or less Cr: 17 to 20 Ti: 0.01 to 0.5 Nb: 0.3 to 0.8 Mo: 0.1 to 2.0 W: 0.1 to 2.0 (However, Mo + W: 0.1-3.
0) , the balance being Fe and unavoidable impurities, and eff. Nb is characterized scope der Rukoto of 0.3 to 0.6, 20 MPa tensile strength at 950 ° C.
The above is a heat-resistant ferritic stainless steel excellent in high-temperature salt damage corrosion resistance and workability . eff. Nb (%) = Nb (%) − 3.93 · fc / 1
2-93 · fn / 14 where Ti (%)-48 · (N
(%) / 2) / 14> 0, C (%)-12 · (Ti (%) − 48 · (N (%) /
2) / 14) / 48> 0, fc = C (%) − 12 · ｛Ti (%) − 48 · (N
(%) / 2) / 14 ° / 48 fn = N (%) / 2 C (%)-12 · (Ti (%) − 48 · (N (%) /
2) / 14) / 48 ≦ 0, fc = 0 fn = N (%) / 2 Ti (%) − 48 · (N (%) / 2) / 14 ≦ 0, fc = C (%) fn = N (%)-14. (Ti (%) / 2) / 48 3. In weight%, C: 0.003 to 0.015 Si: 0.1 to 2 Mn: 0.1 to 2 P: 0.01 to 0.1 S: 0.01 or less Cr: 17 Nb: 0.2 to 0.6 N: 0.005 to 0.3 V: 0.01 to 1.0, satisfying 1 ≦ (V / 51) / (N / 14), and the balance Fe and wherein the unavoidable impurities, heat ferritic stainless steel is the tensile strength at 950 ° C. 20 MPa or higher. (4) Mo: 0.1 to 2.0 W: 0.1 to 2.0 in weight%, and 0.1 to 2.0 in the case of composite addition.
The heat-resistant ferritic stainless steel according to claim 3, which is contained in a range of 3.0% by weight. 5. (The here rare earth element, indicating that the lanthanoid elements and Y) at least one rare earth element and claims comprises in the range of 0.05 wt% or less in total 1, 2, 3 Or a heat-resistant ferritic stainless steel according to item 4.