JPH05279804A - Steel for automobile exhaust manifold - Google Patents

Abstract

PURPOSE:To produce a steel for automobile exhaust manifold excellent in strength, workability, etc., at a low cost by preparing a steel having a specific composition where respective contents of Cr and Mo are specified. CONSTITUTION:This steel has a composition which consists of, by weight, 0.01-<1.2% Si, 0.02-1.0% Mn, 5.5-9.5% Cr, 0.9-4.0% Mo, 0.01-1.0% Al, <=0.03% C, <=0.03% P, <=0.03% S, <=0.02% N, and the balance Fe with inevitable impurities and where the value of M represented by equation (M value)=Cr+3S+4Mo+10Al+1.5W+5Nb+10Ti+10V -40C-30N-4Ni-Cu is regulated to =13.0. Further, W and Nb are regulated to 0.05 to 4.0% and 0.01 to <1.5%, respectively. By this method, the steel excellent in strength at high temp. and oxidation resistance at high temp. as steel for automobile exhaust manifold can be obtained at a low cost.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a steel for an automobile exhaust manifold, and more particularly to a steel for an exhaust manifold attached to an automobile engine, which is excellent in strength and workability and is low in cost.

[0002]

2. Description of the Related Art In recent years, an increasing number of exhaust manifolds (hereinafter referred to as "exhaust manifolds") for automobiles are replaced with exhaust manifolds made of stainless steel sheet instead of the conventional exhaust manifolds made of casting. This is because the exhaust manifold made of stainless steel plate can make the wall thickness thinner, which greatly contributes to the weight reduction of the automobile. Austenitic stainless steels such as JIS SUS310 have excellent heat resistance and workability, and are therefore widely used for general heat resistance applications. However, this type of steel is generally expensive, and has a large thermal expansion coefficient, and thus has a drawback of being inferior in thermal fatigue properties.

On the other hand, ferritic stainless steels generally have a smaller coefficient of thermal expansion than austenitic stainless steels and are therefore excellent in thermal fatigue characteristics, and are therefore suitable for applications such as exhaust manifolds that undergo a temperature cycle of heating and cooling. Can be said. Therefore, for example, JP-A-6
Japanese Patent Laid-Open No. 4-8254 and Japanese Patent Laid-Open No. 2-175843 propose ferritic stainless steel containing a large amount of Cr and having improved high temperature strength. However, these ferritic stainless steels are very expensive, and when used in an exhaust manifold, there is a drawback that the cost is significantly higher than in a cast exhaust manifold. Further, since it contains a large amount of Cr, it cannot be said that the workability is good, and in order to form a complicated shape such as exhaust manifold, there is a problem that the manufacturing process becomes complicated and the processing cost becomes high. ..

[0004]

SUMMARY OF THE INVENTION The present invention solves the above problems in the present situation, and is a steel for an exhaust manifold which is excellent in properties such as strength and workability and is attached to a low-cost automobile engine. Is intended to provide.

[0005]

As a result of various investigations to achieve the above object, the present inventors compared Cr with 5.5% or more and 9.5% or less, which is the reverse of the conventional stainless steel for exhaust manifold. The high temperature strength and the oxidation resistance at high temperature can be greatly improved by adding Mo to the steel which has been reduced in quantity, and the high temperature strength is further improved by strictly controlling the range of the amount of alloying elements so as to satisfy the specific conditions. I found that. Furthermore, the present inventors have found that the addition of W or Nb to such steel further improves the high temperature strength, while the addition of rare earth elements or Ca improves the oxidation resistance at high temperatures.

The present invention has been made mainly on the basis of the above findings, and the gist of the present invention is that, in% by weight, S
i: 0.01% or more and less than 1.2%, Mn: 0.02
1.0%, Cr: 5.5 to 9.5%, Mo: 0.9 to
4.0%, Al: 0.01 to 1.0%, C is 0.03% or less, P is 0.03% or less, and S is 0.03%.
Hereinafter, N is reduced to 0.02% or less, the balance is Fe and unavoidable impurities, and the M value is given by the following formula: M value = Cr + 3Si + 4Mo + 10Al + 1.5W + 5
Nb + 10Ti + 10V-40C-30N-2Mn-4
Ni-Cu is 13.0 or more, which is steel for automobile exhaust manifolds. In the above-mentioned steel of the present invention, as an additional component, W: 0.05% to 4.0%, N by weight%
b: 0.01% to 1.5% of 1 type or 2 types can be contained, and in the present invention, each of the above component steels is further added as an additional component by weight% with a rare earth element: 0.
005-0.1%, Ca: 0.001-0.03% of 1
One or two species can be contained.

[0007]

The reason why the range of each component is limited in the present invention will be described below. Si: Si is effective to be added as a deoxidizing agent and a strengthening element to steel containing 5.5% or more of Cr, but if the content is less than 0.01%, the deoxidizing effect is not sufficient.
Since the effect is already saturated when the content is 2% or more, the content range is limited to 0.01% or more and less than 1.2%.

Mn: Mn is required as a deoxidizing agent for steel,
It is necessary to contain 02% or more, but even if the content exceeds 1.0%, the effect is already saturated, and if Mn is excessively contained, it is difficult to secure a ferrite single-phase structure. Therefore, the upper limit content is 1.0%.

Cr: Cr is required to be contained in an amount of 5.5% or more in order to secure the corrosion resistance and the oxidation resistance required as exhaust manifold, but even if it exceeds 9.5%, the cost is unnecessarily increased. Not only increases, but also the workability decreases, so the upper limit content is made 9.5%.

Mo: Mo must be added to steel containing 5.5% or more of Cr so as to contain 0.9% or more in order to secure the high temperature strength required as an exhaust manifold.
Even if it is added in excess of 4.0%, its effect will be saturated, but the cost will increase unnecessarily, so the above content is set to 4.0%.

Al: Al is 0.01% as a deoxidizing agent for steel.
The above-mentioned addition is necessary, but if added in excess of 1.0%, coarse oxide-based inclusions are formed and the workability is reduced, so the content is limited to 0.01-1.0%. ..

C, N: C forms a compound with Mo to form Mo.
Since N precipitates to lower the high temperature strength and N lowers the toughness, it is preferable that the amount of C and N is small, and the upper limit content for securing the characteristics as exhaust manifold is 0.03% of C. , N is 0.02%. P: If P is present in a large amount, it lowers the toughness, so it is preferable that the content be small, and the upper limit content is made 0.03%. S: If S is present in a large amount, the hot workability and ductility are deteriorated, so it is preferable that the S content is small, and the upper limit content is 0.03.
%.

Further, in the present invention, it is necessary that the M value defined by the following formula is 13.0 or more as a combination of the contents of the respective elements.

M value = Cr + 3Si + 4Mo + 10Al +
1.5W + 5Nb + 10Ti + 10V-40C-30N
-2Mn-4Ni-Cu This is a finding based on the results of the present inventors' detailed examination of the optimum component composition and combination for obtaining the characteristics as exhaust manifold, and the heat treatment condition when the M value is less than 13.0. This is because it is difficult to make the structure a ferrite single phase regardless of the control such as, and the thermal fatigue property, which is an important property as an exhaust manifold, deteriorates. To produce a highly reliable exhaust manifold, the M value is Is important to be 13.0 or more.

The above is the basic composition of the steel for automobile exhaust manifold which is the subject of the present invention, but the present invention is also intended for the steel whose properties are further improved by further adding the following elements as required. .. W: When W is added to steel containing Cr in an amount of 5.5% or more and Mo in an amount of 0.9% or more, it is effective in further increasing the high temperature strength required as an exhaust manifold. Addition amount is 0.05%
If it is less than 4.0%, the effect is not sufficiently exerted, so in order to obtain the sufficient effect of adding W, it is necessary to add it in an amount of 0.05% or more, but even if added in excess of 4.0%, the effect is no longer saturated. On the other hand, since the cost is unnecessarily increased, the upper limit content is set to 4.0%.

Nb: Nb also contains 5.5% or more of Cr and is M
Adding o to steel containing 0.9% or more is effective in further increasing the high-temperature strength required as an exhaust manifold. If the addition amount is less than 0.01%, the effect is not sufficiently exhibited. Therefore, in order to obtain the sufficient Nb addition effect, 0.01% is required.
Although it is necessary to add more than 1.5%, even if adding 1.5% or more, the effect is no longer saturated, while it only unnecessarily increases the cost, so the content range is 0.01%.
It should be at least 1.5%.

Rare earth elements (REM), Ca: Rare earth elements and Ca are elements which are effective in improving hot workability and oxidation resistance at high temperatures, but rare earth elements are 0.00
If the content is less than 5% or less than 0.001% in Ca, the effect is not sufficient. Since inclusions are formed and conversely deteriorate the hot workability and oxidation resistance, the upper limit content of the rare earth element is 0.
1% and Ca of 0.03%. In the present invention, the rare earth element has an atomic number of 57 to 71 and 89 to
It refers to element 103 and Y.

The steel of the present invention may contain Ti, V, Ni, Cu and the like in addition to the above components as impurities mixed from scraps or the like, or for the purpose of adjusting toughness, workability and the like. Needless to say, the M value specified in the claims should be 13.0 or more.

The steel proposed in the present invention may be manufactured as a steel sheet after being manufactured as a steel sheet for use as an exhaust manifold, and then press-formed and further processed / welded to produce an exhaust manifold. First, the steel pipe may be formed into a shape and then processed into an exhaust manifold by secondary processing, welding, etc., and any steel having a composition and a combination of elements limited by the present invention including other processes is the object of the present invention. However, the optimum exhaust manifold manufacturing process can be selected depending on the cost, constraints of the existing manufacturing equipment, and the like, and whichever exhaust manifold manufacturing process is selected does not depart from the scope of the present invention.

[0020]

EXAMPLES Examples of the present invention will be described below. Steels having the components shown in Table 1 were melted and made into a steel plate having a thickness of 2 mm by an ordinary hot rolling process, and annealed at 950 ° C. Next, test pieces are taken from these steel plates and subjected to high temperature (800 ° C)
The tensile test and the high temperature oxidation test were conducted. Further, a thermal fatigue test in which the maximum heating temperature was 800 ° C was also carried out. The test results are also shown in Table 1. Here, in the high temperature tensile test results of Table 1, ◯ indicates that the 0.2% proof stress at 800 ° C. was 20 N / mm ² or more, which is the strength required as an exhaust manifold, and × was less than 20 N / mm ^2. Indicates that In addition, in the high temperature oxidation test result, ○ indicates 800 ° C.
Shows that the increase in oxidation was less than or equal to 0.01 g / m ² / hr, and x indicates that the increase in oxidation was greater than 0.01 g / m ² / hr. Furthermore, in the thermal fatigue test results, ◯ indicates that the number of repetitions up to fracture was 500 or more, and x indicates that the number of repetitions was less than 500.

[0021]

[Table 1]

As is clear from Table 1, the steel of the present invention is
No. Nos. 1 to 7 which are comparative examples are excellent in high temperature strength, oxidation resistance and heat fatigue resistance. It can be seen that 8 to 10 are inferior in high temperature strength, high temperature oxidation resistance or heat fatigue resistance.

[0023]

INDUSTRIAL APPLICABILITY As described above, the present invention makes it possible to provide, at a low cost, steel having excellent high-temperature strength and oxidation resistance at high temperatures as steel for automobile exhaust manifolds. It is extremely large that it contributes to.

Claims (3)

[Claims] 1. By weight%, Si: 0.01% or more and less than 1.2%, Mn: 0.02-1.0%, Cr: 5.5-9.5%, Mo: 0.9- 4.0%, Al: 0.01 to 1.0%, C is 0.03% or less, P is 0.03% or less, S is 0.03% or less, N is 0.02%. The M value which is reduced to the following and consists of the balance Fe and unavoidable impurities, and is given by the following formula: M value = Cr + 3Si + 4Mo + 10Al + 1.5W + 5
Nb + 10Ti + 10V-40C-30N-2Mn-4
Ni-Cu is 13.0 or more, steel for automobile exhaust manifolds. 2. As an additive component to the component steel according to claim 1, W: 0.05% to 4.0%, Nb: 0.01% or more and less than 1.5%, or 1% by weight, or Steel for automobile exhaust manifolds, characterized by containing two kinds. 3. One of the constituent steels according to claims 1 and 2, further comprising, as an additional component, by weight, a rare earth element: 0.005-0.1%, Ca: 0.001-0.03%. Or steel for automobile exhaust manifolds, characterized by containing two kinds.