JP2880839B2 - Steel for automotive exhaust manifolds - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high-temperature exhaust manifold steel for an automobile exhaust system, and more particularly to a steel excellent in formability and thermal fatigue characteristics.

[0002]

2. Description of the Related Art An exhaust manifold, which is one of the components of an automobile exhaust system, is located at a position where it comes into contact with high-temperature combustion gas discharged from an engine, and its constituent materials include oxidation resistance, high-temperature strength, and the like. Various characteristics such as thermal fatigue resistance are required.

Heretofore, cast iron has been mainly used as a material for an exhaust manifold. Recently, however, stainless steel welded pipes have been used as materials for exhaust manifolds in order to respond to demands for improved engine performance and improved fuel efficiency by reducing vehicle weight. Exhaust manifolds made of stainless steel can be expected to be 30% to 40% lighter than those made of cast iron.

However, conventional 430 (16-18% Cr) ferritic stainless steels do not have sufficient oxidation resistance and high-temperature strength, and their performance is still insufficient for high-temperature exhaust manifolds exceeding 900 ° C. Was. Also 304 series (18
% Cr-8% Ni) Austenitic stainless steel has a large coefficient of thermal expansion, and when repeatedly subjected to heating and cooling, it is easily broken by thermal fatigue caused by thermal strain.

[0005] From the viewpoint of thermal fatigue resistance and material cost, ferritic stainless steel is more suitable as an exhaust manifold material than austenitic stainless steel.

For example, JP-A-64-8254 discloses a ferrite containing 17 to 20% of Cr and 1.0% or less of Mo as an exhaust manifold material having excellent high-temperature oxidation resistance and high-temperature strength. A series stainless steel is shown. (In the present specification,% with respect to the alloy component means% by weight.) However, in the stainless steel disclosed in the above-mentioned publication, performance is most problematic particularly in an exhaust manifold exposed to a high temperature of 900 ° C. or more. No consideration is given to the thermal fatigue properties to be used.

[0007]

SUMMARY OF THE INVENTION An object of the present invention is to develop a stainless steel which can be used as a material for a so-called 1000 ° C. exhaust manifold having a maximum heating temperature of 900 to 50 ° C. Among the properties required for such materials, the most important ones are (1) oxidation resistance at 1000 ° C, no abnormal oxidation after 100 hours of atmospheric heating, and (2) high-temperature strength at 1000 ° C. 1.3 kgf in tensile strength
/ mm ² or more, (3) Thermal fatigue characteristics at 1000 ° C
(4) The elongation is 30% or more as the formability of the base steel sheet before forming a welded pipe.

[0008] The formability of the steel sheet of (4) is because when the exhaust manifold is manufactured, the welded pipe is subjected to severe bending and elongation processing, so that the ductility of the material steel sheet is required. is there.

It is an object of the present invention to provide an exhaust manifold steel having all of the above characteristics (1) to (4).

[0010]

The gist of the present invention resides in an exhaust manifold steel having the following chemical composition and particularly excellent in formability and thermal fatigue properties.

C: 0.02% or less, Si: 1.0% or less,
Mn: 1.0% or less, P: 0.04% or less, S: 0.005% or less, Cu: 0.1 to 1.0%, Cr: 19.0 to 25.0%, Mo: 1.
Over 0% to 3.0%, Nb: 0.1-1.0%, Al: 0.
20% or less, N: 0.02% or less, balance: Fe and unavoidable impurities, but the content of Cr, Mo and Nb is determined by the following formula:
The contents of C and N must satisfy the following equations, respectively.

21% ≦ Cr + Mo + Nb ≦ 25% C + N ≦ 0.03%

[0013]

The steel according to the present invention has excellent characteristics to achieve the above-mentioned object by the combination of the appropriate amounts of the respective alloy components and the strict regulation of the impurity elements. In the following points.

(1) In order to increase the oxidation resistance at 1000 ° C., Cr
Increased content. FIG. 1 shows C: 0.01%, Si: 0.4
%, Mn: 0.4%, Cu: 0.5%, Mo: 2%, Nb: 0.6%, N: 0.01
5 shows the results obtained by changing only the Cr content in the range of 0 to 24% based on% as a base composition and determining the Cr content necessary for preventing abnormal oxidation by the test method used in Examples described later. The definition of abnormal oxidation was set to 5 mg / cm ² or more in terms of increased oxidation when heated in air for 100 hours. As shown, the temperature at which abnormal oxidation occurs increases as the Cr content increases.
In other words, the higher the operating temperature, the higher the required amount of Cr for preventing abnormal oxidation. To prevent abnormal oxidation at 1000 ° C., Cr must be 19% or more.

(2) To increase the high-temperature strength at 1000 ° C., the Mo content must be increased.

FIG. 2 shows that C: 0.01%, Si: 0.4%, Mn: 0.4
%, Cu: 0.5%, Cr: 20%, Nb: 0.6%, N: 0.01%, based on the tensile strength of steel at 1000 ° C with only Mo content changed in the range of 0-4% It is a survey result. As shown in the figure, the high temperature strength is significantly increased by increasing the amount of Mo. It can be seen that in order to achieve the above-mentioned target of a tensile strength of 1.3 kgf / mm ² or more at 1000 ° C., the Mo content should be more than 1.0%.

(3) In order to improve the thermal fatigue resistance, the lower limit of the above equation is determined.

Material properties that affect the thermal fatigue life include:
In addition to the aforementioned coefficient of thermal expansion, there are oxidation resistance, high temperature strength and high temperature ductility. When ferritic stainless steel having an essentially small coefficient of thermal expansion is used, Cr described in (1) and (2) above
By considering Mo and Nb, which is effective for improving the high-temperature strength, if Cr + Mo + Nb ≧ 21%, the thermal fatigue properties are greatly improved.

(4) In order to improve the formability of the steel sheet, the above formula and the upper limit of the formula have been determined.

In order to improve the formability of a steel sheet, it is necessary to increase its softness and ductility. FIG. 3 shows Si: 0.4%, Mn: 0.4%, Cu: 0.5
%, Cr: 20%, Mo: 2%, and Nb: 0.6% are the results of examining the relationship between the C and N contents and elongation based on the base composition.
The lower each of C and N, the greater the elongation.
If + N is 0.03% or less, the target growth of 30% or more can be secured. Since Cr, Mo, and Nb are components that reduce elongation, it is necessary to suppress Cr + Mo + Nb to 25% or less. Further, Cu of 0.1 to 1.0% is effective for improving ductility.

The reasons for limiting the action and content of the alloy components and impurities of the steel of the present invention will be described below.

C and N: C and N are impurities that harden the steel and lower the formability. As described above, in order to secure an elongation of 30% or more in the steel sheet, C needs to be 0.02% or less, N also needs to be 0.02% or less, and C + N needs to be 0.03% or less.

Si and Mn: Since the steel hardens when their contents increase, the content of Si must be 1.0% or less and the content of Mn must be 1.0% or less.

P and S: These are impurities inevitably associated with the steel, and adversely affect various properties including the mechanical properties of the steel. In the present invention, P is regulated to not more than 0.04% and S is restricted to not more than 0.005% in order to prevent particularly hot cracking (solidification cracking) of the welded portion.

Cu: Cu has an effect of improving the deep drawability of a steel sheet at a content of 0.1% or more. However, if the content exceeds 1.0%, the yield strength becomes too high and the formability is impaired. Therefore, it is preferable that the content is in the range of 0.1 to 1.0%.

Cr: As described in detail above, Cr is effective in improving the oxidation resistance of steel.
Abnormal oxidation does not occur even at 1000 ° C. However,
If the Cr content exceeds 25%, the steel becomes hard and the formability of the steel sheet decreases, so the upper limit is 25%.

Mo: Mo is important as an element for increasing the high-temperature strength of steel. As shown in FIG.
To obtain a tensile strength of 1.3 kgf / mm ² or more at ° C., more than 1% of Mo is required. On the other hand, if the Mo content exceeds 3.0%, hardening of the steel becomes conspicuous, the formability is reduced, and the toughness of the hot-rolled sheet is reduced, which makes production difficult. Therefore, the proper content of Mo ranges from more than 1.0% to 3.0%.

Nb: Nb has the effect of suppressing the precipitation of carbides and nitrides at the crystal grain boundaries and improving the oxidation resistance. Nb also has a great effect of improving high-temperature strength in a solid solution state.
These effects become apparent from 0.1%,
If it exceeds 1.0%, the steel becomes hardened, so the upper limit is made 1.0%.

It is necessary that the total amount of Cr and Mo, which are also effective for improving the high-temperature strength, that is, Cr + Mo + Nb be 21% or more.

Al: Al has the effect of reducing solid solution N, lowering the yield point and improving formability. The upper limit with such an effect is 0.2%, and if it exceeds this, the adverse effect that solid solution Al lowers toughness appears.

The method for producing the steel of the present invention is essentially the same as the method for producing ordinary ferritic stainless steel. It is smelted in an electric furnace or converter, refined in an AOD furnace, VOD furnace, etc., and converted into a slab by continuous casting or ingot-bulking method. To manufacture welded pipes. It is this welded tube that is the material for the exhaust manifold. 950-10 for heat treatment
Air-cooling after soaking at 50 ° C for 0.5 to 30 minutes is desirable.

[0032]

[Example 1] 15 types of steels shown in (1) and (2) in Table 1 were used.
After melting in a 100 kg vacuum melting furnace, forging and hot rolling, annealing at 950 ° C x 1 minute (air cooling), and further 6.0 mm
Cold rolling from thickness to 2.0mm thickness, 980 ℃ × 1 minute (air cooling)
Finish annealing. A welded tube was manufactured from the thus obtained hoop having a width of 400 mm, and a thermal fatigue test specimen was collected.

After the hot rolling, the coil was taken up, cooled, and developed at normal temperature. If the coil cracked at that time, the toughness of the hot rolled sheet was evaluated as poor.

As the thermal fatigue test piece, an actual pipe as a raw material of an exhaust manifold as shown in FIG. 4 was used. In FIG. 4, reference numeral 1 denotes a tube of a test material, and holes (2, 3) having a diameter of 8 mm were formed in two places to provide a supply port 2 and a discharge port 3 for cooling air. Reference numeral 4 denotes a holder (core metal) from the inner surface of the tube, and reference numeral 5 denotes a mounting portion of the tester to a holder. Tube 1 and holder 4
Are fixed by fixing pins 6 and welded portions 7 at the ends.

As the thermal fatigue test, a computer controlled electrohydraulic servo type high temperature thermal fatigue test apparatus was used, and the test was performed under the temperature cycle and the mechanical strain waveform shown in FIG.

A high-frequency heating device was used for heating the test pieces.
Cooling was performed by blowing air from the air supply port into the inner surface of the tube. The test temperature was 200 ° C for the lowest temperature and 1000 ° C for the highest temperature. The constraint is 50%
(The degree of constraint η = 0.50).

When processing into the shape of the exhaust manifold, severe processing such as expansion, bending, and expansion of the pipe is applied. In order to withstand such processing, it is necessary to improve the workability of the material plate. Workability has a strong correlation with the elongation of the plate, and many test results have revealed that the elongation of the plate is required to be 30% or more. Therefore, a tensile test piece was collected from the above-mentioned finish annealed plate, and the plate elongation was measured.

Separately, using a finish annealing plate,
A continuous oxidation test was conducted by continuously heating at 00 ° C. for 100 hours, and the oxidation resistance was investigated. Those with an increase in oxidation of 5 mg / cm ² or more were regarded as abnormal oxidations. A high-temperature tensile test was performed at 1000 ° C. to determine a high-temperature strength.

Table 2 summarizes the above test results.

[0040]

[Table 1 (1)]

[0041]

[Table 1 (2)]

[0042]

[Table 2]

Nos. 1 to 10 in Table 1 (1) are steels of the present invention.
No. 1 has a standard chemical composition within the range defined by the present invention, and all properties are good. No.2 is especially C
And steel with low N, excellent in sheet elongation. No.3 is Cr
+ Mo + Nb is 21.5%, which is close to the lower limit, and the high temperature strength is 1.4 kgf / mm ² , slightly exceeding the target value.
In No. 4, Cr + Mo + Nb is 24.7%, which is close to the upper limit, and is particularly excellent in high-temperature strength and thermal fatigue resistance, but the elongation of the plate is 30%, which is the lower limit of the target value.

No. 5 has a high Mo content of 2.8%, and thus has good high-temperature strength and thermal fatigue resistance. No. 6 has low C, Si and N and has good elongation. No. 7 also has low C, Si, Mn and N, and has even better elongation. No.8 is 25.0% Cr + Mo + Nb
And the upper limit, and the high temperature strength and the thermal fatigue resistance are the most excellent.
No. 9 has an elongation of 30% because Cu is close to the lower limit of 0.15%.
And the goal is the lowest value. No. 10 is particularly excellent in high temperature strength and thermal fatigue resistance since Nb is as high as 0.97%.

Nos. 11 to 15 in Table 1 (2) are comparative steels in which one of the components is out of the range defined by the present invention.

No. 11 had 3.2% Mo and 25.8% Cr + Mo + Nb.
Therefore, the elongation is inferior to 28%.

Also, due to the high Mo, the toughness of the hot rolled sheet is inferior,
Cracks occurred during coil deployment. No.12 has C + N of 0.05
Since it is too high as 0%, the elongation is extremely poor at 26%. No.13
In the case of Cr, the excessively low Cr content of 17.5% caused abnormal oxidation, and as a result, the thermal fatigue resistance was also reduced. No.14 is Cr, Mo,
Although the respective contents of Nb are within the range defined in the present invention, the high temperature strength and the thermal fatigue resistance are inferior because Cr + Mo + Nb is as low as 20.3%. In No.15, Nb is too high at 1.3%, resulting in poor elongation.

Table 1 which is a typical example of the steel of the present invention
(1) No.1 steel ERW pipe (outer diameter 38.1mm, wall thickness 2.5m
m), an actual exhaust manifold was prototyped, and a thermal cycle test was performed on an automobile engine.
As a result, it was confirmed that the steel of the present invention was highly practical as a material for high-temperature exhaust manifolds, showing durability equal to or higher than that of conventional materials even in tests at high temperatures 100 to 200 ° C higher than the conventional test temperature. Was done.

[0049]

As demonstrated in the examples, the steel of the present invention has a
Excellent oxidation resistance at 00 ° C, high-temperature strength, and thermal fatigue resistance,
Furthermore, since the elongation of the steel sheet is large, it is easy to form various exhaust manifold shapes after forming the welded pipe. In particular, the steel of the present invention has remarkably excellent thermal fatigue resistance at high temperatures, so that the exhaust gas temperature is 1000 ° C.
It is suitable as a material for a high-temperature exhaust manifold used under high and severe conditions.

[Brief description of the drawings]

FIG. 1 is a diagram showing the relationship between the Cr content of steel and oxidation resistance.

FIG. 2 is a diagram showing a relationship between Mo content of steel and high-temperature strength.

FIG. 3 is a graph showing the relationship between the C and N contents of steel and elongation.

FIG. 4 is a view for explaining the shape of a test piece in a thermal fatigue test performed in an example and the procedure of the test.

FIG. 5 is a diagram showing a temperature and a load strain waveform of a displacement constraint type fatigue test.

──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Kenji Higuchi 1-3-1 Otemachi, Chiyoda-ku, Tokyo Sumitomo Metal Industries Co., Ltd. (72) Inventor Tomoyuki Sugino 1-Toyota Town, Toyota-cho, Toyota City, Aichi Prefecture (72) Inventor: Shinji Shibata 1st Toyota Town, Toyota City, Aichi Prefecture Toyota Motor Corporation (56) References JP-A 64-8254 (JP, A) JP-A-3-274245 (JP) , A) (58) Field surveyed (Int. Cl. ⁶ , DB name) C22C 38/00 302 C22C 38/26 F01N 7/10

Claims (1)

(57) [Claims] C .: 0.02% or less, Si: 1.0% or less, Mn: 1.0% or less, P: 0.04% or less, S: 0.005% or less, Cu: 0.1 to 1.0%, Cr: 19.0 to 1% by weight 25.0%, Mo: more than 1.0% to 3.0%, Nb: 0.1 to 1.0%, Al: 0.20% or less,
N: 0.02% or less, balance being Fe and unavoidable impurities, and having a chemical composition satisfying 21% ≦ Cr + Mo + Nb ≦ 25% and C + N ≦ 0.03%, and having excellent moldability and thermal fatigue properties for automobile exhaust manifolds. steel.