JPH0641693A - Steel for automobile exhaust manifold - Google Patents

Abstract

PURPOSE:To obtain the steel excellent in characteristics such as strength and workability, at a low cost by combinedly adding A1 and W to a steel relatively reduced in Cr content to improve oxidation resistance as well as strength at a high temp. and forming a steel with a specific composition. CONSTITUTION:This steel for automobile exhaust manifold has a composition which consists of, by weight, 0.01 to less than 1.2% Si, 0.02-2.0% Mn, 5.5-9.9% Cr, 0.6-4.0% W, more than 1.0 to 2.5% A1, C, P, S, N reduced to 0.03% or less, 0.03% or less, 0.02% or less, and 0.02% or less, respectively, and the balance Fe and where the M value represented by equation is regulated to 13.0 or higher. In the equation, [%X] means the content of an element X shown by weight percentage. Further, 0.05 to less than 0.5% Mo and/or 0.01-1.5% Nb, or further one or more kinds among 0.01-1.0% Ti, 0.01-1.5% V, and 0.01-2.0% Cu, or further one or more kinds among 0.001-0.1% rare earth elements and 0.001-0.03% Ca can be incorporated into the steel.

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 has excellent properties such as 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 being replaced with exhaust manifolds made of stainless steel plates 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 is very useful for reducing the weight of the automobile. Since austenitic stainless steel such as JIS SUS310 steel has excellent heat resistance and workability, it is 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 characteristics.

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 properties, so that they are suitable for applications such as exhaust manifolds that undergo a temperature cycle of heating and cooling. I can say. Therefore, for example, Japanese Patent Laid-Open No. 64-8254 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, they have a drawback that the cost is significantly higher than that in a cast exhaust manifold. In addition, 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, it is extremely difficult to produce, and in order to produce exhaust manifold, There is a drawback that the manufacturing process becomes extremely complicated and the processing cost becomes high.

[0005]

SUMMARY OF THE INVENTION In view of the above circumstances, the present invention has an object to provide a steel having excellent properties such as strength and workability for an exhaust manifold attached to an automobile engine and having a low cost. There is.

[0006]

As a result of various studies to achieve the above object, the present inventors have compared Cr with 5.5% or more and 9.9% 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 the combined addition of Al and W to the steel which has been reduced in quantity. Has been found to be improved. Furthermore, the present inventors have found that such steel has Mo
Alternatively, it has been found that the addition of Nb, Ti, V, and Cu 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. The gist of the first invention of the present invention is that, in weight%, Si: 0.01% or more and less than 1.2%, M
n: 0.02-2.0%, Cr: 5.5-9.9%,
W: 0.6 to 4.0%, Al: more than 1.0% and 2.5% or less are contained, C is 0.03% or less, P is 0.03% or less, and S is 0.02%. Below, N is 0.02%
It is characterized in that the M value ([% X] is the content of the element X expressed in% by weight) given by the following formula is 13.0 or more. It is in steel for automobile exhaust manifolds. M value = [% Cr] +3 [% Si] +4 [% Mo] +10
[% Al] +1.5 [% W] +5 [% Nb] +10 [%
Ti] +10 [% V] -40 [% C] -30 [% N]-
2 [% Mn] -4 [% Ni]-[% Cu]

The gist of the second invention is that, in addition to the steel of the first invention, as an additional component, by weight, Mo: 0.05% or more and less than 0.5%, Nb: 0.01% to 1.5. % Of the steel containing one or two of them, and the gist of the third invention is that, in addition to the steel of the first invention or the second invention, as an additional component, by weight%, Ti: 0.01 to 1 0.0%, V: 0.01 to 1.5%, Cu: 0.01 to 2.0% in steel containing one or more kinds, and the fourth gist of the invention is the first. In addition to the steel of the invention, the second invention or the third invention, as an additional component, by weight%, rare earth elements: 0.001 to 0.1%, Ca: 0.001 to 0.0
In steels containing 3% of one or two.

[0009]

The reason why the range of each component is limited in the present invention will be described below. Si: Si is effective as a deoxidizing agent and as a strengthening element in steel containing Cr in an amount of 5.5% or more and W in an amount of 0.6% or more. The acid effect is not sufficient, and when 1.2% or more is contained, the effect is already saturated and the workability is deteriorated.
The content range is limited to 0.01% or more and less than 1.2%.

Mn: Mn is necessary as a deoxidizing agent for steel,
It is necessary to contain 02% or more, but even if the content exceeds 2.0%, 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 2.0%.

Cr: Cr needs 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 if it is contained in excess of 9.9%, the cost is unnecessarily increased. Not only increases, but also the workability deteriorates, so the upper limit content is made 9.9%.

W: W must be added to steel containing 5.5% or more of Cr in an amount of 0.6% or more in order to secure high temperature strength required as an exhaust manifold.
Even if added in excess of%, the effect is already saturated, but the cost merely increases unnecessarily, so the upper limit content is made 4.0%.

Al: Al is effective as a deoxidizing agent for steel, and it is necessary to add more than 1.0% in order to secure the oxidation resistance which is an important characteristic for exhaust manifold.
If added in excess of 5%, not only the effect is saturated but also the workability is lowered, so the content is limited to more than 1.0% and 2.5% or less.

C, N: In addition to C and N decreasing the workability, C forms a compound with W to precipitate W,
Since it lowers the high temperature strength and N lowers the toughness, it is desirable that the amounts of C and N are small, and the upper limit content for ensuring the properties as exhaust manifold is 0.0.
3% and N are 0.02%.

P: If P is present in a large amount, it lowers the toughness, so it is desirable that the content be small, and the upper limit content is made 0.03%. S: If S is also present in a large amount, the hot workability and ductility are deteriorated, so it is preferable that the amount be small, and the upper limit content is 0.02.
%.

Further, in the present invention, the M value ([% X] is the content of the element X expressed in% by weight) given by the following formula as a combination of the content of each element is 13.0 or more. is necessary. M value = [% Cr] +3 [% Si] +4 [% Mo] +10
[% Al] +1.5 [% W] +5 [% Nb] +10 [%
Ti] +10 [% V] -40 [% C] -30 [% N]-
2 [% Mn] -4 [% Ni]-[% 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 M value is 1
If it is less than 3.0, it is difficult to make the structure a ferrite single phase regardless of how the heat treatment conditions are controlled, and the thermal fatigue property, which is an important property as an exhaust manifold, deteriorates. In order to manufacture it, it is important to set the M value to 13.0 or more.

The above is the basic composition of the steel for automobile exhaust manifolds, which is the subject of the present invention. However, in the present invention, the steel whose characteristics are further improved by further adding the following elements as required is also intended. There is. Mo: When Mo is added to steel containing Cr in an amount of 5.5% or more and W in an amount of 0.6% 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 0.5%, the effect is not sufficiently exerted, so in order to obtain the sufficient effect of adding Mo, it is necessary to add it by 0.05% or more, but even if it is added by 0.5% or more, the effect is no longer saturated. On the other hand, since the cost is unnecessarily increased, the content is set to 0.05% or more and less than 0.5%.

Nb: Nb also contains 5.5% or more of Cr and W
Is added to the steel containing 0.6% or more, it is effective in further increasing the high temperature strength required for the exhaust manifold. If the addition amount is less than 0.01%, the effect is not sufficiently exhibited, so it is necessary to add 0.01% or more in order to obtain the sufficient effect of adding Nb, but if the addition amount exceeds 1.5%, However, the effect is no longer saturated, but the cost is unnecessarily increased, so the content is 0.01-
1.5%.

Ti, V, Cu: Ti, V, Cu are also added to steel containing Cr in an amount of 5.5% or more and W in an amount of 0.6% or more, and are effective in further increasing the high temperature strength required as an exhaust manifold. There is. The addition amount of each element is 0.
If it is less than 01%, its effect is not sufficiently exerted, so in order to fully exert the effect of adding these elements, 0.0
It is necessary to add 1% or more, but 1.0% for Ti, V
However, even if added over 1.5% for Cu and over 2.0% for Cu, the effect is no longer saturated, while it only unnecessarily increases the cost, so the content of Ti is 0.01%.
~ 1.0%, V 0.01-1.5%, Cu 0.01
~ 2.0%.

Rare earth elements (REM), Ca: Rare earth elements and Ca are elements effective in improving hot workability and oxidation resistance at high temperatures, but the addition amount of each is 0.0
If it is less than 01%, the effect is not sufficient, and if it is added in excess of 0.1% in rare earth elements and in excess of 0.03% in Ca, coarse non-metallic inclusions are generated, respectively, and conversely hot working Therefore, the upper limit of the content is set to 0.1% for rare earth elements and 0.03% for Ca because it deteriorates the properties and oxidation resistance. In the present invention, the rare earth element has an atomic number of 57 to 7.
It refers to elements Nos. 1 and 89 to 103 and Y.

In addition to the above components, the steel of the present invention may contain Ni, Zr, Ta, B, etc. as impurities mixed from scraps or the like, or for the purpose of adjusting toughness, workability, etc. Needless to say, it is necessary that the M value specified in the claims is 13.0 or more even when Ni is added.

The steel proposed in the present invention may be manufactured as a steel plate after being manufactured as a steel plate for use as an exhaust manifold, and may be manufactured as an exhaust manifold by press forming the steel plate and further processing and welding the steel plate. 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.

[0024]

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 a normal 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 performed. 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 in 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 x is less than 20 N / mm ^2. Indicates that In addition, in the high temperature oxidation test result, ○ indicates 800 ° C.
The increase in oxidation was 0.01 g / m ² / hr or less, and the mark 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.

As is clear from Table 1, the steel of the present invention is
No. Nos. 1 to 12 are excellent in high temperature strength, oxidation resistance and thermal fatigue resistance, while No. 13-15
It is understood that is inferior in high temperature strength, oxidation resistance at high temperature or heat fatigue resistance.

[0026]

[Table 1]

[0027]

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

Claims (4)

[Claims] 1. By weight%, Si: 0.01% or more and less than 1.2%, Mn: 0.02-2.0%, Cr: 5.5-9.9%, W: 0.6- 4.0%, Al: more than 1.0% and 2.5% or less, C is 0.03% or less, P is 0.03% or less, S is 0.02% or less, and N is 0. To less than 02%, consisting of the balance Fe and unavoidable impurities, and having an M value ([% X] is the content of the element X expressed in% by weight) of 13.0 or more. Characteristic steel for automobile exhaust manifold. M value = [% Cr] +3 [% Si] +4 [% Mo] +10
[% Al] +1.5 [% W] +5 [% Nb] +10 [%
Ti] +10 [% V] -40 [% C] -30 [% N]-
2 [% Mn] -4 [% Ni]-[% Cu] 2. The steel according to claim 1, further comprising, as an additional component, in a weight percentage, Mo: 0.05% or more and less than 0.5%, Nb: 0.01% to 1.5%, one kind. Or steel for automobile exhaust manifolds, characterized by containing two kinds. 3. The steel according to claim 1 or 2, further comprising, as an additive component, by weight, Ti: 0.01 to 1.0%, V: 0.01 to 1.5%, Cu: 0.01. Steel for automobile exhaust manifolds, characterized by containing one or more of 2.0 to 2.0%. 4. The steel according to claim 1, 2 or 3, further comprising, as an additive component, by weight, a rare earth element: 0.001 to 0.1%, Ca: 0.001 to 0.03%, A steel for an automobile exhaust manifold, characterized by containing one or two of the above.