JPH0688168A - Heat resistant ferritic stainless steel sheet excellent in high temperature strength and weldability - Google Patents

Abstract

PURPOSE:To provide ferritic stainless steel for the exhaust manifold of an automotive engine excellent in high temp. strength and weldability. CONSTITUTION:The compsn. of the steel is constituted of a one contg., by mass, <=0.02% C, <0.6% Si, 0.6 to 2.0% Mn, <=0.006% S, <=0.04% P, 17.0 to 22.0S Cr, >0.6 to 1.5% Nb, 1.0 to 3.0% Mo, 0.01 to 0.5% V, 0.1 to <0.3% Cu, <=0.02% N, 0.005 to 0.05% Al and <=0.012% O. Furthermore, these elements are incorporated so as to satisfy the relationship of C+N<=O.03%, MW/S>=200 as well as 16.8<=0.6Cr+1.1Mo+1.5W+8.2Nb<=24.0, and the balance Fe with inevitable impurities on the producing process.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a ferrite heat-resistant stainless steel sheet having excellent high-temperature strength and weldability, which is used in exhaust gas system members of various internal combustion engines, such as exhaust manifolds, exhaust gas purification materials, and various combustion equipment. It is about.

[0002]

2. Description of the Related Art In recent years, air pollution caused by gas emitted from automobiles or factories has become a serious problem. For example, the exhaust gas of automobiles is NO _X , H from the viewpoint of pollution prevention.
Although the amount of C, CO, etc. has been regulated, recently the regulation tends to be more strict in view of acid rain and the like, and it is necessary to improve the exhaust gas purification efficiency.

On the other hand, in addition to improving purification efficiency in automobiles,
The demand for higher engine output or improved performance has increased,
Exhaust gas temperature tends to rise. Against this background, the exhaust gas system members, especially the exhaust manifold (exhaust gas pipe) directly connected to the engine, become extremely hot during operation. In addition, it is exposed to extremely severe conditions such as mechanical stress fluctuations due to machine vibrations or external vibrations, or thermal stress fluctuations due to cooling / heating cycles depending on operating patterns.

When heat-resistant steel such as stainless steel is used in these applications, various stresses are applied during use as described above, so it is important to have excellent high-temperature properties that can withstand this, that is, high-temperature strength properties. . Moreover, the exhaust gas system members are used by press-forming steel plates and then welding, or after welding and pipe forming, after processing. Therefore, in this application, it is important that the high-temperature strength properties are remarkably excellent, the weldability on the steel sheet is excellent, and the workability is also good.

Austenitic stainless steel represented by SUS304 has excellent workability and weldability. In addition, its high-temperature strength is higher than that of ferritic stainless steel, so it is considered to be a promising material for applications requiring heat resistance. However, since austenitic stainless steel has a large coefficient of thermal expansion, it is feared that thermal fatigue fracture will occur due to the thermal stress generated during use in applications where it is repeatedly heated and cooled. Also, since austenitic stainless steel has a large coefficient of thermal expansion, surface oxides are easily peeled off by repeated heating and cooling.

As a result, in some applications, Incone
A Ni-based alloy represented by l 600 is used. This alloy material is a promising material because it has a lower thermal expansion coefficient than austenitic stainless steel, good adhesion of surface oxides, excellent high-temperature oxidation resistance, and high high-temperature strength. Since it is an expensive material, it has not been widely used.

On the other hand, ferritic stainless steel is less expensive than austenitic stainless steel. In addition, since it has a small coefficient of thermal expansion, it has excellent thermal fatigue properties, and it is considered that this is an excellent feature in applications where repeated heating and cooling are used. So for some uses
Ferrite-based stainless steels such as Tepe409 and SUS430 are beginning to be used. However, these materials are
Since the high temperature strength at temperatures above ℃ is extremely low, there is a possibility of high temperature fatigue fracture and thermal fatigue fracture due to insufficient strength in this temperature range. Furthermore, at temperatures above 900 ° C, the ferrite phase may transform into the austenite phase, and the martensite formed during subsequent cooling may significantly reduce the toughness at room temperature.

In order to solve such a problem peculiar to ferritic stainless steel, it is conceivable to improve the high temperature strength by adding Mo and W which are ferrite forming elements and high temperature strengthening elements. However, a sufficient increase in high temperature strength cannot be expected by simply adding these elements. In addition, when the high temperature strength is increased, it becomes hard even at room temperature and the workability is deteriorated, and the weldability is deteriorated due to excessive addition.

[0009]

[Problems to be Solved by the Invention] As described above, with the progress of future improvement of exhaust gas purification efficiency, higher output and higher performance of internal combustion engine, etc., high temperature that can cope with increasingly severe operating conditions and environment Materials having excellent strength characteristics and weldability have been demanded, but at present, the above problems have not been completely solved. If a steel with excellent high-temperature strength properties, which is regarded as the most important of the heat-resistant ferrite-type stainless steels, can be obtained, it is considered that a very promising material can be obtained for the applications under the severe usage conditions as described above. To be This is the purpose of the present invention.

That is, according to the present invention, the heat resistance, that is, the high-temperature strength property most required for the heat-resistant structural material is more than twice that of the conventional SUS430LX steel, which is a heat-resistant ferrite stainless steel, which is an exhaust manifold material for automobiles.
The purpose is to develop a ferritic stainless steel sheet with a 0.2% proof stress of 15 N / mm ² or higher.

[0011]

According to the present invention, in mass%, C: 0.02% or less, Si: less than 0.6%, Mn: 0.6% or more.
2.0% or less, S: 0.006% or less, P: 0.04% or less, Cr: 1
7.0% to 22.0%, Nb: 0.6% to 1.5%, M
o: 1.0% to 3.0%, V: 0.01% to 0.5%, C
u: 0.1% or more and less than 0.3%, N: 0.02% or less, Al: 0.005
% To 0.05% or less, O: 0.012% or less, and in some cases, Ti: 1.5% or less, Zr: 2.0% or less, W:
5.0% or less, B: 0.01% or less, Y: 0.1% or less, REM: 0.1
% Or less, and in the above range, C + N ≦ 0.03% Mn / S ≧ 200 16.8 ≦ 0.6 Cr + 1.1Mo + 8.2Nb ≦ 24.0 However, if W is included, 16.8 ≦ 0.6 Provided ferritic stainless steel sheet containing these elements so as to satisfy the relation of Cr + 1.1Mo + 1.5W + 8.2Nb ≤ 24.0, with the balance of Fe and inevitable impurities in manufacturing, and excellent in high temperature strength and weldability. To do.

In particular, the steel sheet of the present invention is subjected to final finishing annealing in the manufacturing process thereof at a temperature of 1050 ° C. or higher and 1200 ° C. or lower for 10 minutes or less.
By cooling up to 0 ° C at a rate of 2 ° C / sec or more, excellent high temperature strength can be provided. This steel sheet material is suitable for producing an exhaust manifold for an automobile engine by welding (high-frequency welding) a pipe of a desired diameter and applying various processing and welding from this pipe. Accordingly, the present invention also provides a steel for an automobile engine manifold having the above-mentioned composition.

[0013]

[Function] Figure 1 shows low C, N-18% Cr-1.0% Mn-2.0% Mo-0.25%
Cu steel as a basic component, high temperature strength at 900 ℃ and 1000 ℃
The results of examining the effect of the amount of Nb in steel on (tensile strength, 0.2% proof stress) are shown. The test piece was made by annealing 4 mmt hot-rolled steel sheet at 1000 ° C and then cold rolling to 2 mmt.
The one annealed at 100 ° C. was used.

From the results shown in FIG. 1, the Nb content up to 0.6% is Nb.
The high-temperature strength (tensile strength and 0.2% proof stress) slightly increased with the increase of the amount, but when the Nb content exceeded 0.6%, the high-temperature strength rapidly increased at this point and the It can be seen that the% yield strength shows a very high value of 15 N / mm ² or more.

That is, the present inventors have found that the addition of Nb in an amount of more than 0.6% in this component system dramatically increases the high temperature strength. It is considered that the reason why the high temperature strength is increased is that the amount of Nb that effectively acts as a strengthening element among the added amounts of Nb increases, although it is also affected by the amount of (C + N) and the annealing temperature as described later.

FIG. 2 shows 18% Cr-1.0% Mn-2.0% Mo-1.0% Nb-
Using 0.25% Cu steel as a basic component, the 1100 ° C annealed material similar to that shown in Fig. 1 has an effect on (C +
N) The influence of the amount was investigated.

From the results of FIG. 2, the 0.2% proof stress shows a high value of 15 N / mm ² or more in the region where the (C + N) amount is 0.03% or less, but decreases when the (C + N) amount exceeds 0.03%. I understand. This suggests that a dramatic increase in high temperature strength cannot be secured by simply adding a large amount of Nb to the ferrite stainless steel. In other words, in order to obtain ferritic stainless steel with excellent high temperature strength, N
It indicates that in addition to the b amount, the (C + N) amount must be strictly specified. The decrease in high-temperature strength with an increase in the amount of (C + N) is because Nb precipitates as carbonitrides when the amount of (C + N) increases, and the amount of solid solution Nb effective for strengthening decreases. Conceivable.

FIG. 3 shows low C, N-18% Cr-1.0% Mn-1.0% Mo-
With 0.6% Nb-0.25% Cu steel as the basic component, 110% as in the previous example
Effect of Cr on the high temperature strength at 1000 ℃ for 0 ℃ annealed material,
This is an examination of the effects of Mo and W. The results of FIG. 1 for the amount of Nb are also shown in the figure.

From the results shown in FIG. 3, it can be seen that the high temperature strength of this component system increases relatively monotonically with the addition amount of each element in this range. Where σ _0.2 : 0.2% at 1000 ℃
Proofing power (N / mm ² ) and Cr, Mo, W, Nb: each additive element amount (mass%), σ _0.2 = -1.8 + 0.6Cr + 1.1Mo + 1.5W + 8.2Nb ・ ・ ・ (1) obtain. Also from this equation (1), the contribution rate of each element to σ _0.2 (coefficient of each element) increases in the order of Cr, Mo, W, Nb, especially Nb is large, and the addition of Nb is larger than that of Mo or W. It can be seen that it is very effective in increasing the high temperature strength.

FIG. 4 shows 18% Cr-1.0% Mn-2.0% Mo-0.25% Cu.
-0.45% Nb steel (hereinafter referred to as 0.45% Nb steel) and 18% Cr-
For 1.0% Mn-2.0% Mo-0.25% Cu-1.0% Nb steel (hereinafter referred to as 1.0% Nb steel), the effect of the annealing temperature of the final finish annealing on the high temperature tensile properties at 1000 ° C was investigated.

From the results shown in FIG. 4, the 0.45% Nb steel is almost unaffected by the final annealing temperature and shows a substantially constant strength value, while the 1.0% Nb steel has a strength of 0.
No substantial difference with 45% Nb steel, but 1050
When the annealing temperature is above ℃, the high temperature strength rises rapidly.
It can be seen that with 00 ° C annealing, 0.2% proof stress, which is about twice that of 0.45% Nb steel, can be obtained. It is considered that such an increase in high temperature strength is due to the increase in the amount of solid solution Nb effective for strengthening as described above due to the increase in annealing temperature. The above facts indicate that the proper selection of Nb and (C + N)
In addition to this, it has been shown that performing final finishing annealing at 1050 ° C or higher is important for obtaining ferrite stainless steel sheets with excellent high temperature strength.

FIG. 5 shows low C, N-18% Cr-1.0% Mn-2.0% Mo-
For 1.0% Nb-0.25% Cu steel, when the amount of solute Nb was changed by changing the heat treatment conditions,
It shows the relationship with 0.2% proof stress. The amount of dissolved Nb is the amount of Nb added minus the amount of Nb as a precipitate. When the precipitate was identified by X-ray diffraction, Fe ₃
It was revealed that Nb ₃ C and Fe ₂ Nb are the main components.
The amount of Nb as the precipitate was obtained by electrolytically extracting each heat-treated material with a nonaqueous solvent-based extract. The grain size number of each heat-treated material is approximately constant at about 5.

From the results of FIG. 5, the high temperature strength of this steel is 0.5
It can be seen that the amount of solid solution Nb is rapidly increasing in the range of over%.

A method for increasing creep strength by combining an Nb-added ferrite type stainless steel with an annealing step is described in, for example, Japanese Patent Application Laid-Open No. 60-145359 and Japanese Patent Publication No. 1-41695. In both publications, the amount of solid solution Nb effective for strengthening is obtained by subtracting the amount of Nb precipitated as NbC or NbN from the amount of added Nb, and in order to increase the creep strength, a certain amount of carbonitride It is said that existence is necessary. However, when the component system of the present invention, that is, when Nb exceeding 0.6% is added, the presence of precipitates such as NbC and NbN is not recognized, and Fe ₃ Nb ₃ C and Fe ₂ Nb are precipitated. Therefore, the formula for calculating the amount of solute Nb published in the above publication cannot be applied to the component system of the present invention. Also, unlike the teaching of the publication, the inclusion of C or N to allow the presence of an appropriate amount of carbonitride is not necessary in the component system of the present invention, rather as seen in the results of FIG. 2 above. In order to secure the Nb amount effective for strengthening, it is necessary to strictly limit the upper limit of the (C + N) amount.

Based on the above facts, the present invention provides a heat-resistant ferrite-type stainless steel sheet excellent in high-temperature strength and weldability, and particularly when directly connected to a high-power automobile engine in the future. It provides a material that can be used as a reliable stainless steel exhaust manifold.

The action of each component of the steel of the present invention and the reason for defining the content range of the chemical component value will be individually described below.

C and N: C and N are generally considered to be the most effective elements for high-temperature strength such as creep strength and creep rupture strength, but on the other hand, when the content is high, oxidation resistance is high. , Workability and toughness decrease. Therefore, it is desirable that C and N are low in this component system, and each is made 0.02% or less. Also, C
Since N and N easily form a compound with Nb, the amount of Nb effective for solid solution strengthening of the ferrite phase, more specifically, the amount of effective Nb for increasing 0.2% proof stress and tensile strength at high temperature is reduced. As shown in Fig. 2, the amount of (C + N) is 0.03%.
By setting the following, it is possible to secure a 0.2% proof stress of 15 N / mm ² or more at 1000 ° C.

Si: Si is an element effective for improving the oxidation resistance. However, if Si is added excessively, the hardness increases,
Si is less than 0.6% because workability and toughness decrease.

Mn: Mn is S which is harmful to hot cracking in welding.
Fix in the form of S to remove and reduce S in the weld metal.
In order to prevent hot cracking in the weld, it is necessary to add Mn corresponding to S contained in the steel, and when Mn / S is 200 or more, a crack preventing effect appears. The addition of Mn also improves scale peeling resistance, but excessive addition causes problems in workability and weldability. Therefore, the range of Mn is 0.6% or more and 2.0% or more so that it has sufficient scale peeling resistance and does not affect workability and weldability.
% Or less.

S: S is harmful to high temperature welding cracks as described above, so it is desirable that it be as low as possible, but the lower it is, the higher the manufacturing cost. In the steel of the present invention, since S has a sufficient weld-resistant hot cracking even if S is allowed up to 0.006%, the range of S is made 0.006% or less.

P: P has no particular adverse effect on the improvement in high temperature strength aimed at by the present invention, but if it is contained in excess of 0.04%, the toughness of the steel sheet is significantly impaired. Therefore, the range of P is 0.04% or less.

Cr: Cr is an element which stabilizes the ferrite phase and is indispensable for improving the oxidation resistance, which is important in high temperature materials. However, if it is less than 17.0%, the above effect is not sufficiently exhibited. From the viewpoint of oxidation resistance, the higher Cr is, the more preferable it is, but if added in excess of 22.0%, the steel becomes brittle, and the workability deteriorates due to the increase in hardness. Therefore, the range of Cr is 17.0% or more and 22.0% or less.

Nb: Nb is an element necessary for maintaining high temperature strength as shown in the high temperature tensile test result of FIG. Moreover, since it is a strong carbonitride-forming element, in order to achieve a marked increase in high-temperature strength by solid solution strengthening, in addition to the reduction of C and N, it is necessary to add Nb in excess of 0.6%. Also, the addition of Nb has a favorable effect on the improvement of workability and oxidation resistance. However, if Nb is added excessively, the hot cracking susceptibility of the weld becomes high. Maintain sufficient high temperature strength,
In addition, the range of Nb is set to more than 0.6% and 1.5% or less so as not to affect the welding hot cracking sensitivity.

Mo: Mo is an element that increases high temperature strength by solid solution strengthening. It is also effective in improving high temperature oxidation resistance and corrosion resistance. On the other hand, if added excessively, the toughness at low temperature is remarkably deteriorated, and the weldability and workability are deteriorated. Therefore, the range of its content is 1.0% or more and 3.0% or less, preferably more than 1.0% and 3.0% or less.

V: V is a carbonitride forming element, which increases high temperature strength and creep rupture strength and improves workability. Also, by combining with C and N, the amount of solute Nb effective for increasing the high temperature strength is increased. Further, V improves the intergranular corrosion resistance and the toughness of the weld heat affected zone by coexisting with Nb. These effects have a V amount of 0.01
Less than% is not enough. On the other hand, if V is added in excess of 0.5%, the workability will be adversely affected.
% Or more and 0.5% or less.

Al: Oxygen is blown during decarburization in the steelmaking process. At this time, the residual oxygen in the steel is
Significantly adversely affects weldability. Therefore, Al is an essential element as a deoxidizing material. However, excessive addition causes a decrease in weldability. The range of Al is sufficient so that sufficient deoxidation can be performed and the weldability is not affected.
It should be 0.005% or more and 0.05% or less.

O: As described above, O has an adverse effect on weldability, so it is desirable that it be as low as possible, but the lower it is, the higher the manufacturing cost will be. In this component system,
O can be easily reduced to 0.012% or less by adding deoxidized Al, and at this time, since it has sufficient weldability, the range of O is 0.012% or less.

Cu: Cu acts very effectively in terms of toughness,
0.1% or more is necessary to obtain the toughness improving effect at room temperature. On the other hand, if added excessively, it becomes hard and the workability is impaired, and the weldability also deteriorates. For this reason, the range of Cu is
0.1% or more and less than 0.3%.

Ti: Ti is a strong carbonitride forming element similar to Nb, increases the high temperature strength and the creep rupture strength, and improves the workability. However, like Al, if added too much, it causes problems in manufacturability and weldability, so it is made 1.5% or less.

Zr: Zr increases high temperature strength and improves high temperature oxidation characteristics. However, if added excessively, the workability and weldability are deteriorated, so the content is made 2.0% or less.

W: W, like Ti and V, also increases high temperature strength and improves workability. However, if added excessively, the workability and weldability are deteriorated, so the content is made 5.0% or less.

B: B improves hot workability, raises high temperature strength, and also improves workability. However, if added excessively, the hot workability deteriorates, so the content is made 0.01% or less.

Y and REM: Y and REM (rare earth elements) improve the hot workability by adding a trace amount and improve the oxidation resistance, especially the adhesion of scale. However, if it is added excessively, the hot workability is deteriorated.

In addition to the above component composition, in order to achieve the object of the present invention, that is, 0.2% proof stress at 1000 ° C.
In order to achieve 5 N / mm ² or more, it is necessary to set the right side of Eq. (1) to 15 or more. That is, it is necessary to regulate the contents of these elements so as to satisfy the relationship of 15.0≤-1.8 + 0.6Cr + 1.1Mo + 1.5W + 8.2Nb. However, if these elements are added excessively, workability and weldability may decrease,
In particular, when the right side of Eq. (1) exceeds 22.2, weldability deterioration was observed. For this reason, the addition amount of these elements is 15.0≤-1.8 + 0.6Cr + 1.1Mo + 1.5W + 8.2Nb≤22.2, that is, 16.8≤0.6Cr + 1.1Mo + 1.5W + 8.2Nb≤24.0. Is good.

Next, in the production of the steel sheet of the present invention, the annealing temperature is preferably as high as possible in order to sufficiently dissolve the solid solution strengthening element in the final finish annealing as described above. That is, as shown in FIG. 4, when the annealing temperature is lower than 1050 ° C, the reason is that the solid solution of Nb is not sufficient, but the remarkable improvement in high temperature strength cannot be expected. Further, FIG. 4 shows that the increase in high temperature strength saturates when the annealing temperature exceeds 1200 ° C. In addition, if the annealing temperature is too high, the crystal grains become coarse and the toughness decreases and other disadvantages occur. From the above, the range of final finish annealing temperature is set to 1050 ° C or higher and 1200 ° C or lower.

The cooling rate after the final finish annealing is set to 6 at the annealing temperature so that Nb dissolved in the solid solution during the annealing does not precipitate during cooling.
It is recommended that the temperature up to 00 ℃ be 2 ℃ / sec or more.

Regarding the final finish annealing time, crystal grains and precipitates become coarse when annealed at a temperature of 1050 ° C. or higher and 1200 ° C. or lower for a long time. Since it decreases, it is practical to set it to 10 minutes or less.

The final finish annealing means finishing annealing of a steel strip or a steel sheet after being rolled to a target thickness through hot rolling and cold rolling in the manufacturing process by a material manufacturer. In some cases, a light skin pass rolling may be performed after this final finish annealing.

[0049]

[Examples] Tables 1 and 2 (continuation of Table 1) show the chemical composition values and final finish annealing temperatures of the test materials. A in the table
01 to A20 are inventive steels, and A21 to A30 are comparative steels.
Each of the steels was melted in a vacuum melting furnace for 500 kg, forged and hot rolled into a hot rolled steel strip of 4.0 mmt. This was annealed at 950 to 1250 ° C and cold rolled to 2.0 mmt. Then, this was cut and annealed at the final finishing annealing temperature shown in the table to obtain a steel sheet. This was subjected to a test after cutting into a high temperature tensile test piece. Table 3
Shows the high temperature strength characteristics of the steel sheet of the present invention and the comparative steel sheet.
The high temperature strength characteristics were evaluated by 0.2% proof stress and tensile strength in a high temperature tensile test according to JIS G 0567.

The weld hot cracking property was evaluated by the critical strain amount. That is, except that each steel was cold-rolled to a thickness of 1.2 mmt, cold-rolled annealed sheets similar to the above were prepared, processed into 40 mm × 200 mm test pieces, and both ends of the test pieces were held to apply tensile stress in the longitudinal direction. TIG welding was performed in the applied state, and the minimum strain amount at which cracking started to occur was defined as the critical strain amount, which was used as an index of weld cracking susceptibility. The weld hot cracking characteristics are also shown in Table 3.

[0051]

[Table 1]

[0052]

[Table 2]

[0053]

[Table 3]

As can be seen from the results in Table 3, all the steel sheets of the present invention have very high high temperature strength. In other words, the steel sheet with Nb content of 0.6% or more, the (C + N) content of 0.03% or less, and the final finish annealing at a high temperature of 1050 ° C or higher could not be obtained with conventional ferrite stainless steel. It shows high strength characteristics and is 0 at 1000 ℃.
A strength value of 15 N / mm ² or more can be obtained with a 2% proof stress. This is a conventional SU used in automobile exhaust manifold materials.
It has more than twice the high temperature strength characteristics of S430LX series steel (corresponding to A21 steel in Table 2).

Further, regarding the weldability, it can be seen that all the steels of the examples of the present invention show a critical strain amount of 3.5% or more, and the weldability is good. Therefore, it is understood that a pipe-made product (for example, an exhaust manifold for automobiles) having excellent welding hot crack resistance can be obtained.

On the other hand, Comparative Examples A21, A22 and A28
~ A30 are all elements (C + N,
Although the range of Nb, Mo) is out of the range specified in the present invention, even if these are annealed at a high temperature,
The high temperature strength is inferior to that of the steel of the present invention, and the 0.2% proof stress at 1000 ° C is all less than 15 N / mm ² .

Even if the composition range is within the range of the present invention, as in Comparative Example A26, sufficient high temperature strength characteristics cannot be obtained if the annealing temperature range is outside the range of the present invention. Furthermore A
When the contents of Mn, S, Al, O, and Cu such as 23 to A25 and A31 are out of the range of the present invention, the high temperature strength is high but the weldability of the steel sheet is poor. In this respect, the same applies to A27, which is out of the range of the above formula (1), and it is considered that excessive addition of the high temperature strengthening element deteriorates the weldability.

Further, an actual exhaust manifold was prototyped using the electric resistance welded pipe of the steel of the present invention as described below, and a cooling / heating cycle was carried out in an automobile engine.
As a result, it was confirmed that the steel of the present invention showed higher durability than conventional materials even in a high temperature test that was 100 to 200 ° C higher than the conventional test temperature, and that the steel of the present invention was extremely useful as a material for exhaust manifolds. .

FIG. 6 shows an outline of a prototype exhaust manifold. Reference numeral 1 is a main pipe, and branch pipes 2a to 2e are connected to the main pipe 1. Main pipe 1 and branch pipe 2
Both have a thickness equivalent to A5 of the above embodiment of 2.0 as a material steel plate.
A cold rolled annealed plate of mm was used. From this steel plate, a pipe with the diameter of the main pipe 1 and a pipe with the diameter of the branch pipe 2 are made by high-frequency welding, cut into the required length, drilling and reducing the diameter of the main pipe 1, bending the branch pipe 2 and expanding and After flaring the flange mounting end, each branch pipe 2 was connected to the hole of the main pipe 1 by MAG welding, and the flange was attached to the end by MAG welding.

In the test, a cooling / heating cycle of heating with engine exhaust gas having a temperature of 900 ° C. or 940 ° C. and then air cooling was carried out. For comparison, the same exhaust manifold was prototyped except that the steel corresponding to A21 of the comparative example was used, and the same test was conducted. As a result, the example of the present invention had durability about twice as many cycles as that of the comparative example at any test temperature.

[0061]

As described above, the ferrite-type stainless steel sheet of the present invention has extremely high high-temperature strength of 600 ° C. or higher, particularly 900 ° C. to 1000 ° C., as compared with other ferrite-type stainless steels, and has a good weldability. Are better. In addition, since this steel sheet is subjected to final finishing annealing at a temperature above recrystallization, it has excellent workability as well as high temperature and high strength.

Therefore, according to the present invention, it is manufactured through many processes such as welded pipe forming, bending work, diameter reduction pipe expansion work, and welding joining, and has strength even when used in a high temperature range of 600 ° C. or higher. It provides automotive engine manifold materials with improved material properties such as high temperature fatigue fracture and thermal fatigue fracture due to shortage, and can greatly contribute to higher output and performance improvement of automobiles.

[Brief description of drawings]

FIG. 1 is a diagram showing the influence of the amount of Nb added on 0.2% proof stress and tensile strength in 900 ° C. and 1000 ° C. short-time tensile tests in the indicated component system.

FIG. 2 is a diagram showing the influence of (C + N) content on 0.2% proof stress and tensile strength in a 1000 ° C. short-time tensile test in the indicated component system.

FIG. 3 is a diagram showing the influence of various alloy elements on 0.2% proof stress in a 1000 ° C. short-time tensile test in the indicated component system.

FIG. 4 is a diagram showing the effect of final finishing annealing temperature on the 0.2% proof stress in a 1000 ° C. short-time tensile test in the indicated component system.

FIG. 5 is a diagram showing the influence of the amount of solid solution Nb on the 0.2% proof stress in the 1000 ° C. short-time tensile test in the indicated component system.

FIG. 6 is a schematic view showing an example of an exhaust manifold of an automobile engine.

[Explanation of symbols]

 1 main pipe 2 branch pipes

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Sadayuki Nakamura 4976 Nomura-Minami-cho, Shinnanyo-shi, Yamaguchi Prefecture Steel Research Laboratory, Nisshin Steel Co., Ltd. (72) Oku Manabu 4976 Nomura-Minami-cho, Shinnanyo-shi, Yamaguchi Prefecture Nisshin Steel Manufacturing Co., Ltd. Steel Research Laboratory (72) Inventor Tomoyuki Sugino 1 Toyota Town, Toyota City, Aichi Prefecture Toyota Automobile Co., Ltd. (72) Inventor Shinji Shibata 1, Toyota Town, Aichi Prefecture Toyota Motor Co., Ltd.

Claims (4)

[Claims] 1. In mass%, C: 0.02% or less, Si:
Less than 0.6%, Mn: 0.6% to 2.0%, S: 0.006% or less,
P: 0.04% or less, Cr: 17.0% or more and 22.0% or less, Nb: 0.6
% To 1.5% or less, Mo: 1.0% to 3.0%, V: 0.01
% To 0.5%, Cu: 0.1% to less than 0.3%, N: 0.02%
Below, Al: 0.005% or more and 0.05% or less, O: 0.012% or less, but within the above range, C + N ≦ 0.03% Mn / S ≧ 200 16.8 ≦ 0.6Cr + 1.1Mo + 8.2Nb ≦ 24.0 A heat-resistant ferritic stainless steel sheet that contains the elements described above and has a balance of Fe and inevitable impurities in manufacturing, and has excellent high-temperature strength and weldability. 2. In mass%, C: 0.02% or less, Si:
Less than 0.6%, Mn: 0.6% to 2.0%, S: 0.006% or less,
P: 0.04% or less, Cr: 17.0% or more and 22.0% or less, Nb: 0.6
% To 1.5% or less, Mo: 1.0% to 3.0%, V: 0.01
% To 0.5%, Cu: 0.1% to less than 0.3%, N: 0.02%
The following are contained: Al: 0.005% or more and 0.05% or less, O: 0.012% or less, and one or more of the following elements, that is, Ti: 1.5% or less, Zr: 2.0% or less, W: 5.0% Less than,
B: 0.01% or less, Y: 0.1% or less, REM: 0.1% or less of one or more, and within the above range, C + N ≦ 0.03% Mn / S ≧ 200 16.8 ≦ 0.6Cr + 1.1Mo + 1. A heat-resistant ferrite-type stainless steel plate containing these elements so as to satisfy the relationship of 5W + 8.2Nb ≦ 24.0, and having a balance of Fe and inevitable impurities in manufacturing, which has excellent high-temperature strength and weldability. 3. The steel sheet is heated to a temperature of 1050 ° C. or higher and 1200 ° C. or lower for 10 minutes or less in the final finish annealing in the manufacturing process, and then cooled to 600 ° C. at a rate of 2 ° C./sec or higher. The ferrite stainless steel plate according to claim 1 or 2. 4. The ferritic stainless steel plate according to claim 1, wherein the steel plate is used as a material for forming an exhaust manifold of an automobile engine.