JP2001303204A - Heat resistant ferritic stainless steel and steel sheet thereof - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an inexpensive heat resistance material laving an excellent high temperature characteristics and excellent cold temperature workability and suitable as the one for an exhaust manifold or the like. SOLUTION: The heat resistant ferritic stainless steel and steel sheet are composed so as to have a chemical composition containing <=0.02% C, <=1.5% Si, <=1.5% MN, <=0.04% P, <=0.01% S, <=1.5% Cu, 17 to 25% Cr, <=1.0% Ni, 0.5 to 2.5% Mo, 0.1 to 1.0% Nb, <=0.2% Al and <=0.02% N also so as to satisfy Cu-0.05(Mo+Nb) >=0.48 and Mo+Nb<=3 or, in addition to the same, in the case of Al/27-N/14>=0 and also Cu<0.55, satisfying 25(Cu-0.45)2-(Mo+Nb)>=-2.0. Moreover, the anisotropy of elongation and the average elongation in the plane of the steel sheet are prescribed to specified ranges.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION The present invention is excellent in workability, for example, exhibits excellent workability even in a state of being formed into a welded pipe, and has excellent high temperature even in a severe high temperature environment exceeding 900 ° C. The present invention relates to a relatively inexpensive heat-resistant ferritic stainless steel and a ferritic stainless steel sheet which exhibit characteristics and are suitable as a high-temperature member for a high-temperature exhaust manifold, a front pipe or a power plant of an automobile exhaust system. .

[0002]

2. Description of the Related Art For example, an exhaust manifold, which is one of the components of an automobile exhaust system, is located at a position where it comes in contact with high-temperature combustion gas discharged from an engine. Various properties such as heat resistance, high temperature strength, and thermal fatigue resistance are required. Conventionally, SUH409 and SUS430J1L specified by JIS have been used as such materials for the exhaust manifold.
However, in recent years, fuel efficiency and high output of automobiles have been remarkably improved, so that the maximum temperature of exhaust gas has risen to about 950 ° C., and in some cases up to 1000 ° C. When used below, it has become clear that conventional SUH409 and SUS430J1L have significantly deteriorated oxidation resistance and high-temperature strength and cannot be used.

As a material capable of maintaining excellent high-temperature strength and oxidation resistance even in a severe high-temperature environment, for example, Japanese Patent No. 2888039 discloses an exhaust manifold containing [Cr + Mo + Nb] in a specific range. -Steel for steels is disclosed. Japanese Patent No. 2959934 discloses that effective Nb (eff. Nb) is adjusted to a specific range and M
A heat-resistant ferritic stainless steel has been disclosed that attempts to ensure excellent high-temperature properties by adding o, V, and W. Further, Japanese Patent No. 2923825 discloses "Low C, N-18% Cr-1.0% Mn-2.0% Mo-0.25% Cu"
A ferritic stainless steel sheet containing more than 0.6% of Nb in steel containing aluminum as a basic component has been disclosed (hereinafter,% representing the component amount is mass% unless otherwise noted). It shows that it shows excellent high-temperature strength even in an environment of 1000 ° C. Since these high-temperature steels exhibit excellent high-temperature properties due to the solid solution strengthening action of Mo and Nb, they are currently used in exhaust
There are also many cases where it is applied to members for manifolds and the like.

[0004]

The main characteristics required for an exhaust manifold member and the like are, for example, as shown in Japanese Patent No. 2959934. High temperature strength, thermal fatigue properties, high temperature fatigue properties, 2) oxidation resistance in the region over 900 ° C, 3) workability at room temperature, etc. However, as described above, although heat-resistant materials whose properties have been improved in high-temperature environments have recently been produced one after another, these materials have been further improved in terms of "workability at room temperature". It was desired.

[0005] For example, ferrite stainless steel used for exhaust manifold members of automobiles is generally processed in the following manners: A) when the steel sheet is formed in a plate shape by pressing or the like; B) when the steel sheet is welded. Pipes are roughly classified into two types: pipes that are subjected to bending or the like. However, in recent years, in response to demands for further improvement in fuel efficiency and weight reduction of automobiles, it is expected that space saving around automobile engines will proceed more than ever before, and as a result, exhaust manifolds It is anticipated that the demands on the processed shape of the members will be extremely severe, and great attention has been paid to the room-temperature processing characteristics of these materials.

That is, in the future image of automobiles, if a member used for an exhaust manifold or the like is a "plate-shaped press-formed product", the processing shape becomes more complicated than before.
In addition, in the case of a "tubular member processed into a welded pipe", it is expected that the pipe material after welding will have to be subjected to more severe bending and the like to produce a product. Under such a severe processing environment, the aforementioned Japanese Patent No. 2880
No. 839, Japanese Patent No. 2959934 or Japanese Patent No. 2923825.
Manifold materials suffer from "breaking of the material plate during press forming" and "bending cracks caused by insufficient ductility of the tube after forming a welded tube", clearly not being able to withstand complex processing. It has become to.

Japanese Patent Application Laid-Open No. 9-279212 discloses that ferrite stainless steel containing Mo and Nb is
It has been proposed to improve the high-temperature properties, corrosion resistance and workability by adding chromium. However, this ferritic stainless steel has not been studied for its high-temperature properties exceeding 900 ° C., and although the workability in expanding the welded pipe has been evaluated, “examination on pressability of steel sheet” and “ No study has been made on the later processability (bendability, etc.) of the tube, and the results of the studies by the present inventors did not indicate that the tube had sufficiently satisfactory high-temperature properties and workability.

As described above, ferritic stainless steels that are subjected to complicated forming processes such as materials for exhaust manifolds and used in high-temperature environments are expected to have better processing performance in the future. It is considered that "formability of plate-like material" and "formability of pipe after being formed into a welded pipe (ductility)" are particularly important. Specific knowledge of formability (such as press formability of steel sheets and ductility of pipes after welded pipes) for “heat-resistant ferritic stainless steel that exhibits excellent high-temperature properties in high-temperature environments exceeding 900 ° C” Has not been obtained at present. Therefore, a heat-resistant steel that has high-temperature oxidizing property and high-temperature strength superior to or higher than that of conventional steel, and has excellent room-temperature processing characteristics that can sufficiently withstand complicated processing under stricter requirements than before. It has not been realized yet.

In view of the above, an object of the present invention is not only to have excellent high-temperature properties (high-temperature strength, thermal fatigue properties, high-temperature fatigue properties, and oxidation resistance) but also to obtain a “plate-like” and “ An object of the present invention is to provide a heat-resistant material which exhibits excellent processing characteristics in any form such as a "welded tube" and is relatively inexpensive and suitable for exhaust manifolds.

[0010]

Means for Solving the Problems In order to achieve the above object, the present inventors focused on the excellent high-temperature characteristics of heat-resistant ferritic stainless steel to which Mo and Nb were added. In order to develop a heat-resistant material that exhibits excellent processing characteristics, it is necessary to understand the processing form applied to the material and the performance of the processed material (base material) in more detail. " Assuming the case of processing into an exhaust manifold that requires complex forming processing, various plate-like and tubular materials are subjected to processing tests, and various studies are made on the correlation between the processing form and material properties. As a result, the following conclusions were obtained.

(A) Generally used iron and steel materials have anisotropy in the plane of the sheet in the state of the sheet (a property in which the material properties differ in the rolling direction, the direction perpendicular to the rolling direction, and the 45 ° direction of the rolling). Although it is unavoidable to be recognized, the conventionally known exo-
In the case of ferrite stainless steel for strike manifold, the elongation in the 45 ° direction of rolling is remarkably lower than that of general ferritic stainless steel, and the in-plane anisotropy of elongation is large. (b) Under a processing environment that assumes an exhaust manifold, etc., the smaller the anisotropy of elongation (elongation direction difference) within the plate surface is, the larger the processing limit area of the plate shape becomes. This is preferable because the processing characteristics are improved. In addition, the anisotropy of in-plane elongation also affects the ductility of the tube after forming the plate into a welded tube, and as with the plate-like processing characteristics, the smaller the anisotropy of in-plane elongation, the better. The ductility of the welded pipe is also improved. (c) In the case of heat-resistant ferritic stainless steel, the in-plane anisotropy of elongation is affected by the added element, and by adjusting the content of the added element comprehensively, the anisotropy is reduced and the formability is improved. It is possible to

That is, the present inventors have reported that the processing characteristics of heat-resistant ferritic stainless steel used for exhaust manifolds and the like are based on the anisotropic elongation in the plate surface when the plate is formed into a plate shape. By focusing on the properties, it was confirmed that they were arranged relatively well, and they found that if the in-plane anisotropy of elongation was within a certain range, it was possible to secure extremely excellent processing characteristics in the state of the plate. In addition, it was clarified that the anisotropy of in-plane elongation of the steel sheet used as the base metal had a large effect on the ductility of the pipe after it was made into a welded pipe. It was confirmed that when the in-plane anisotropy was within a certain range, the ductility of the pipe was greatly improved.

Further, the present inventors have found that "anisotropy of elongation in the plane of the sheet" is affected by the added element of ferritic stainless steel as described above. In order to combine properties (oxidation resistance, high-temperature strength, etc.) and excellent processing characteristics, various studies were conducted on the effects of added elements on high-temperature characteristics and processing characteristics, and the following new findings were obtained. .

A) Conventional exhaust manifold steels contain a large amount of various alloying elements such as Mo, Nb, V, W, etc. in order to ensure high-temperature strength. Compared to stainless steel, it has high strength and low ductility even at room temperature. In particular, the elongation in the 45 ° direction of rolling is significantly deteriorated, and the anisotropy of the in-plane elongation is increased. However, Cu was added according to the amount of Mo and Nb added,
If the amounts of Cu, Mo, and Nb are specified according to specific conditions, it is possible to reduce the anisotropy of elongation in the plane of the sheet while maintaining excellent high-temperature characteristics, and to greatly improve the processing characteristics.

B) In addition to the above measures, when a small amount of Al is added according to specific conditions, the optimum range of the amount of Cu to be added becomes wider, and the necessary amount of Mo, Nb, and Cu can be suppressed. As a result, it is possible to dramatically improve processing characteristics while maintaining excellent high-temperature characteristics.

Although the above-mentioned Japanese Patent No. 2,880,839 describes an exhaust manifold steel to which Cu is added in order to improve the deep drawability, it does not contain other additive elements. In this connection, there is no report on a clear effect of the addition of Cu on the material properties (for example, the effect on the ductility of a plate-shaped or welded pipe and the effect on high-temperature properties). And such a mere Cu
The addition alone does not ensure the effect of remarkably improving the room-temperature formability (particularly the plate shape and ductility when it is used as a welded tube) without adversely affecting the high-temperature properties of the heat-resistant ferritic stainless steel. could not.

On the other hand, the present inventors further studied “the effect of Cu on the high-temperature properties of heat-resistant ferritic stainless steel” and found that the addition of Mo and Nb in a specific amount range as a composite. For the first time, it was confirmed that the anisotropy of elongation in the plane of the sheet could be reduced as much as possible, and that excellent high-temperature properties could be secured under the influence of solid solution Cu. In addition, the effect is higher than that of the conventional heat-resistant ferritic stainless steel with Mo,
It is effective even in the range where the amount of Nb added is low, so it is possible to reduce Mo and Nb, which are expensive alloying elements that have been actively added so far to ensure high-temperature characteristics, and the cost of the added element is lower than before. Can be suppressed. Furthermore, it was also found that the addition of a small amount of Al widens the optimum addition range of Cu, and that the effect of the compound addition with Cu can maintain extremely excellent high-temperature characteristics and processing characteristics even with a small amount of Cu added. We succeeded in producing a heat-resistant ferritic stainless steel with excellent high-temperature characteristics and processing characteristics, in which the Cu content was further reduced in addition to Mo and Nb.

The present invention has been made based on the above findings and the like, and provides a ferritic stainless steel and a steel sheet which are excellent in heat resistance and workability as described in the following. C: 0.02% or less, Si: 1.5% or less, Mn: 1.5% or less, P: 0.04% or less, S: 0.01% or less, Cu: 1.5% or less, Cr: 17 to 25%, Ni: 1.0% or less, Mo : 0.5 to 2.5%, Nb: 0.1 to 1.0%, Al: 0.2% or less, N: 0.02% or less, with the balance being Fe and unavoidable impurities.
Regarding the contents of Cu, Mo and Nb, the following equation (1) and
Ferritic stainless steel with excellent heat resistance and workability, characterized by satisfying formula (2). Cu (%) − 0.05 ｛Mo (%) + Nb (%)｝ ≧ 0.48 (1) Mo (%) + Nb (%) ≦ 3 (2) The ferritic stainless steel according to the above item, further comprising Al , N, Cu, Mo and Nb contents are as follows (3) ~
A ferritic stainless steel with excellent heat resistance and workability characterized by satisfying the formula (5). Al (%) / 27−N (%) / 14 ≧ 0… (3) Cu (%) <0.55… (4) 25 ｛Cu (%) −0.45｝ ² − ｛Mo (%) + Nb (%)｝ ≧ −2.0 (5) The ferritic stainless steel according to the above item, wherein the content of Al, N, Cu, Mo and Nb is also determined by the following formulas (3) and (6) to (7). Ferritic stainless steel excellent in heat resistance and workability, characterized by satisfying. Al (%) / 27−N (%) / 14 ≧ 0… (3) Cu (%) ≧ 0.55… (6) Cu (%) −0.05 ｛Mo (%) + Nb (%)｝ ≧ 0.48… (7 The ferrite stainless steel sheet having excellent heat resistance and workability made of the steel according to any one of the above-mentioned items, characterized in that the value of ΔEL represented by the following formula (8) is 9% or less. . ΔEL (%) = (EL ₀ + EL ₉₀ ) / 2−EL ₄₅ (8) where EL ₀ : elongation in the rolling direction (%), EL ₉₀ : elongation in the direction perpendicular to the rolling (%), EL ₄₅ : rolling Elongation in 45 ° direction (%) The heat resistance and workability of the steel according to any of the above-mentioned items, wherein the value of EL _S represented by the following formula (9) is 27% or more. Excellent ferritic stainless steel sheet. EL _S (%) = (EL ₀ + EL ₉₀ + 2EL ₄₅ ) / 4 (9) where EL ₀ : elongation in the rolling direction (%), EL ₉₀ : elongation in the direction perpendicular to the rolling (%), EL ₄₅ : rolling 45 ° direction elongation (%) wherein the value of EL _S represented by the following equation (9) is 27% or more, the heat resistance of the above, wherein, excellent ferritic stainless steel in workability. EL _S (%) = (EL ₀ + EL ₉₀ + 2EL ₄₅ ) / 4 (9) where EL ₀ : elongation in the rolling direction (%), EL ₉₀ : elongation in the direction perpendicular to the rolling (%), EL ₄₅ : rolling 45 ° elongation (%)

[0019]

The reason why the structure of "steel" and "steel plate" in the present invention is limited as described above, together with the function of the constituent elements, will be described. [A] Composition of steel a) C C is an undesired element that hardens steel or a steel sheet to reduce workability, and is also an element that deteriorates corrosion resistance. Therefore, its content is preferably minimized. . In order to ensure the excellent workability expected of the present invention in steel or steel plate, it is necessary to regulate the C content to 0.02% or less.

B) Si Si is an effective component as a deoxidizing agent for steel, and is also an element for improving the oxidation resistance of steel or steel sheet. However, if the content exceeds 1.5%, the steel or steel sheet is hardened with the increase in the amount of addition and the workability is deteriorated, and the excellent workability expected by the present invention cannot be secured. The amount was set at 1.5% or less.

C) Mn Mn has a deoxidizing effect on steel and an effect of suppressing scale peeling at a high temperature. Therefore, Mn is contained in a range of 1.5% or less. If the Mn content exceeds 1.5%, not only does rusting and pitting occur, the corrosion resistance decreases,
The cost of steel increases, which is economically disadvantageous.

D) Since PP is an unavoidable impurity element that lowers the corrosion resistance and toughness of steel or steel sheet, it is desirable that the content thereof is as low as possible. In particular, when the P content exceeds 0.04%, the workability of the steel or the steel plate deteriorates remarkably, so the P content is set to 0.04% or less. e) S Since S is an unavoidable impurity element that becomes a starting point of rusting and pitting corrosion and deteriorates the corrosion resistance of steel or steel sheet, its content is preferably as low as possible. In particular, when the S content is 0.01
%, The corrosion resistance of the steel or steel sheet deteriorates remarkably. Therefore, the upper limit of the S content is set to 0.01%.

F) Cu Cu is an essential element for ensuring the desired workability and high-temperature characteristics of steel or steel sheet, and in order to obtain the effect stably, the amount of Mo or Nb depends on the added amount. It is necessary to add an appropriate amount (the details of this addition amount adjustment will be described later). In addition, when Cu is not added, the excellent workability and high-temperature characteristics expected by the present invention cannot be secured to steel or steel plate. On the other hand, if the content of Cu exceeds 1.5%, not only the effect is saturated, but also the hot workability of the steel is undesirably deteriorated. Therefore, the upper limit of the Cu content is set to 1.5%. Here, the preferable range of the Cu content for more stably obtaining the desired excellent processing characteristics and high-temperature characteristics of the steel or the steel sheet is 0.1 to 1.3%.

G) Cr Cr is a main component for maintaining the expected corrosion resistance and oxidation resistance of the steel or the steel sheet. As the Cr content increases, the corrosion resistance and the oxidation resistance of the steel or the steel sheet improve. In order to ensure the expected corrosion resistance and oxidation resistance of steel or steel sheet, a Cr content of 17% or more is necessary, but if it exceeds 25%, the manufacturability deteriorates and the cost increases. Therefore, the upper limit of the Cr content is set to 25%.

H) Ni Ni is an element added as needed to improve the toughness of the steel or the steel sheet. However, Ni exceeds 1.0%
The upper limit of the Ni content is set at 1.0%, because the inclusion of Ni will increase the cost of steel or steel sheets.

H) Mo Mo is an element necessary for increasing the high-temperature strength of steel or a steel sheet.
% Or more Mo content is required. However, 2.5%
When Mo is contained in excess of the above, the strength of the steel or steel plate increases markedly and the workability deteriorates. Mo is an expensive alloy element, and the upper limit of the Mo content is set to 2.5% from the viewpoint of suppressing a rise in cost.

I) Nb Nb has the effect of suppressing the precipitation of carbides and nitrides at the crystal grain boundaries and improving the oxidation resistance of the steel or steel sheet. Also,
Nb has a great effect of improving the high-temperature strength of steel or steel sheet in a solid solution state. And these effects become remarkable
The content is from 0.1%, but if the content exceeds 1.0%, the steel or steel plate becomes hard, so the Nb content is
It was set to 0.1 to 1.0%.

J) Al Al Al has the effect of reducing the solute N in steel or steel sheet and improving the workability by lowering the yield point, and the effect of improving the toughness of steel or steel sheet. However, when Al is contained in excess of 0.2%, the solute Al lowers the toughness, resulting in an adverse effect that the productivity is deteriorated. Therefore, the upper limit of the Al content is 0.2
%. Further, in order to stably secure excellent processing characteristics to steel or steel plate, the Al content is set to “Al (%) / 27−N
(%) / 14 ≧ 0 ”. Because Al
This is because, if the content is adjusted to the above range, almost all of the solute N which adversely affects the processing characteristics is precipitated as AlN and rendered harmless. Further, when Al is contained in the range of “Al (%) / 27−N (%) / 14 ≧ 0”, the optimum addition range of Cu becomes wider. Therefore, the amount of Al added is preferably adjusted to “Al (%) / 27−N (%) / 14 ≧ 0” in the sense of suppressing the amount of Cu added.

K) NN Like N, N is an undesired element that hardens steel or a steel plate and lowers workability. Therefore, the content of N is preferably as small as possible. In the present invention, the upper limit of the N content is set to 0.02% from the viewpoint of suppressing the deterioration of workability and corrosion resistance of steel or steel sheet.

L) Other components V, W, Ca, B, Mg, La, Ce, Y may be added to the steel or steel sheet according to the present invention, if necessary, in order to enhance its high-temperature properties.
One or more elements can be added. The preferred content of each of these elements is V between 0.02% and 0.3%, W
0.02 to 1.0%, Ca, B or Mg each 0.0002 to
0.005%, La, Ce or Y each 0.002-0.05%
It is. Among them, V and W are particularly effective in ensuring high-temperature strength more stably. In addition, in the steel or the steel sheet according to the present invention, even if these elements are added within the above range, there is no particular adverse effect on the basic characteristics.

M) Addition balance of Mo, Nb, Cu and Al The present invention relates to a method of reducing the Cu content in steel or a steel sheet according to the content of Mo and Nb by “Cu (%) − 0.05 ° Mo (%) + Nb (%)｝ ≧ 0.48
It has a great feature that it is contained in a range satisfying the relational expression of "." By including Cu in this range, it becomes possible to ensure both high-temperature properties and excellent processing properties in a heat-resistant ferritic stainless steel or a steel sheet. The value of “Cu (%) −0.05 − Mo (%) + Nb (%)｝” is
If the ratio is less than 0.48, the processing characteristics of steel or steel plate deteriorate. Preferably, Mo, Nb, and Cu are “Cu (%) − 0.05 ｛Mo (%)
+ Nb (%)｝ ≧ 0.53 ”.

Further, “Cu (%) − 0.05 ｛Mo (%) + Nb (%)｝
≧ 0.48 ”, the expensive alloying elements Mo and Nb may be contained in the range of“ Mo (%) + Nb (%) ≦ 3 ”. Sufficient high-temperature characteristics are ensured. In this case, if Mo and Nb are added in an amount such that the value of “Mo (%) + Nb (%)” exceeds 3, not only will the cost be increased, but also the strength of the steel or steel plate will increase and the processing will be increased. It is not preferable because the characteristics are deteriorated. More preferred
The addition amounts of Mo and Nb are “Mo (%) + Nb (%) ≦ 2.7”.

By the way, in the present invention, in order to stably maintain the processing characteristics at room temperature, Al is changed to "Al (%) / 27-N
(%) / 14 ≧ 0 ”. In this case, excellent processing characteristics can be obtained even on the side where the Cu content range is lower, and the amount of Cu added can be reduced. That is, in the range of “Cu (%) <0.55”, Cu, Mo
And the content of Nb is “25 ｛Cu (%) −0.45｝ ² − ｛Mo (%) +
If the range of Nb (%)｝ ≧ −2.0 ”is satisfied, desired high-temperature characteristics and workability can be secured. Of course, if it is out of this range, processing characteristics deteriorate, which is not preferable.
More preferable Cu, Mo and Nb in the range of “Cu (%) <0.55”
Is in the range of “25 (Cu−0.45) ² − (Mo + Nb) ≧ −1.2
". Of course, if “Al (%) / 27−N (%) / 14 ≧ 0” is satisfied, even if “Cu (%) ≧ 0.55”, “Cu (%) −
0.05 ｛Mo (%) + Nb (%)｝ ≧ 0.48 ”
If the Nb content is within the range, the processing characteristics do not deteriorate at all. The more preferable range in this case is also `` Cu (%)-0.
05 ｛Mo (%) + Nb (%)｝ ≧ 0.53 ”.

[B] “Anisotropy of elongation of sheet surface” and “Average elongation of sheet surface” In the ferritic stainless steel sheet according to the present invention,
In order to obtain excellent processing characteristics in the “plate shape” and “tubular shape after forming it into a welded pipe”, anisotropic parameter of elongation in the plate surface {EL (%)} = (EL ₀ + EL ₉₀ ) / 2-EL
₄₅ % must be 9% or less. When ΔEL exceeds 9%, the processing characteristics in the form of a plate and the ductility in a tubular form after pipe formation are significantly deteriorated. The preferred range of ΔEL is 7% or less. In order to reduce ΔEL (%) to 9% or less, the content of the additive element may be comprehensively adjusted and optimized as described above.

Further, in order to secure excellent processing characteristics in the ferritic stainless steel sheet according to the present invention, the average elongation ELs (%) of the sheet surface is given by: ｛= (EL ₀ + EL ₉₀ + 2EL)
₄₅ ) / 4} is preferably as high as possible, and is preferably at least 27%. If the ELs is less than 27%, the processing characteristics in the form of a plate and the ductility of the tube after pipe formation (particularly the ductility of the welded pipe) are significantly reduced. More preferably, “ELs (%) ≧ 29
% ". ELs (%) is also greatly affected by the composition of the steel sheet. In order to make ELs (%) 27% or more,
It is effective to adjust the content of the additional element comprehensively as described above to make it appropriate.

[0036]

The present invention will be specifically described below with reference to examples. [Example 1] Steel having each composition shown in Table 1 was melted in a 30 kg vacuum melting furnace to form a flat steel ingot, and then a block having a thickness of 45 mm, a width of 140 mm, and a length of 85 mm was cut out and cut at 1150 ° C for 1 hour. And a hot-rolled steel sheet having a thickness of 6.0 mm was formed by hot rolling.

[0037]

[Table 1]

The hot-rolled steel sheet is annealed at 1000 ° C.
After performing descaling with alumina shot,
Cold rolled from 6.0mm to 2.0mm thickness, 1025 ℃
For recrystallization annealing.

From the ferritic stainless steel sheet obtained in this manner, the steel sheet is rolled in the rolling direction, the 45 ° rolling direction, and the direction perpendicular to the rolling direction, respectively, according to JIS Z 2201.
Sample No. 13B was sampled and subjected to a tensile test at room temperature by the method specified in JIS Z 2241 to measure EL ₀ , EL ₄₅ and EL ₉₀ . Then, based on this measured value, “the in-plane anisotropy parameter of elongation {EL} = (EL ₀ + EL ₉₀ ) / 2−2
EL ₄₅ ° ”and“ average elongation of the plate surface ELs ｛= (EL ₀ +
_{EL 90 + 2EL 45) / 4} } "was calculated.

Further, in order to clarify the correlation between ΔEL and ELs and the processing characteristics of the steel plate in the form of a plate, a cylindrical blank having a diameter of 90 mm was prepared from the above ferritic stainless steel plate, and a deep drawing test apparatus was used. , Wrinkle holding force: 10k
N, punch diameter: 50 mm, lubricating oil: Nippon Machine Oil # 660
A cylindrical molding test was performed under the following conditions (trade name), and the presence or absence of necking near the punch and the shoulder of the punch was investigated.

Further, in order to evaluate the ductility of the welded pipe, the steel sheet was used as a TIG welded pipe having a diameter of 40 mm, and a JIS No. 11 test piece specified in JIS Z 2201 was sampled from the welded pipe and converted to JIS Z 2241. A tensile test was performed by a specified method, and ductility in a tube was evaluated.

In order to investigate the high temperature strength of a steel sheet,
A plate-like high-temperature tensile test specimen specified in JIS Z 2201 was sampled from the rolling direction of the steel sheet, and subjected to a high-temperature tensile test at 950 ° C. according to a method according to JIS G 0567 to determine a high-temperature strength at 950 ° C. Then, in order to evaluate the high-temperature oxidation resistance, a test piece having a thickness of 2.0 mm, a width of 20 mm, and a length of 30 mm was sampled from a steel plate, and the surface was polished and degreased with an emery paper up to 600, and then immersed in air. At 950 ° C. for 200 hours continuously, and the presence or absence of abnormal oxidation was visually evaluated.

As other general characteristics, evaluations on corrosion resistance were also performed. Corrosion resistance evaluation is JIS Z 2
Perform the salt spray test specified in 371 for 10 consecutive days.
After 0 days, the surface of the test piece was observed with a 50-fold visual field of an optical microscope to evaluate the presence or absence of rust. In addition, by observing the appearance of the steel sheet in each manufacturing process, the steel sheet was fractured,
The presence or absence of occurrence of ear cracks) was observed to evaluate the productivity.

Table 2 shows the results.

[0045]

[Table 2]

As is clear from the results shown in Table 2, each of the ferritic stainless steels according to the examples of the present invention has a ΔEL of 9% or less, and does not cause fracture and necking in cylindrical molding. 32% ductility in tubular form
The above can be ensured, and good processing characteristics are exhibited in either a plate-like or tubular state. Also, 950
The high-temperature strength at 18 ° C. is 18Mpa or more stably, and no abnormal oxidation is observed in the evaluation of oxidation resistance. In addition, there is no rust after the corrosion resistance test and no problems during production. .

On the other hand, “Cu (%) − 0.05 ｛Mo (%) +
Comparative example where the value of “Nb (%) 8” is less than 0.48 (No. 14 to 16)
In this case, ΔEL exceeds 9% and ELs is less than 27%, and workability in a plate or a welded pipe is inferior. Also,
In the comparative examples (Nos. 18 and 19) having small contents of Mo and Nb, 9
Comparative examples in which high-temperature strength at 50 ° C. could not be secured, while Mo and Nb were excessively contained beyond the scope of the present invention (Nos. 20 and 21)
) Is inferior in workability.

In the comparative example (No. 17) in which the Cu content is excessive beyond the range of the present invention, ear cracks are generated during hot rolling, which is not preferable because the productivity is deteriorated. In addition, Mo, Nb
In the comparative example (No. 22) containing more than 3, steel is inferior in processing characteristics and the amount of alloying elements added is excessive, so that steel is undesirably high in cost.

In Comparative Examples (Nos. 23 to 31) in which the other components were outside the specified range of the present invention, the present invention required the workability, high-temperature strength, oxidation resistance and heat resistance required for heat-resistant ferritic stainless steel. All of corrosion resistance and manufacturability are not satisfied.

Example 2 Table 3 in a 30 kg vacuum melting furnace
The steels having the respective component compositions shown in Table 1 were melted, and cold-rolled steel sheets having a thickness of 2.0 mm were obtained in the same manner as in Example 1.

[0051]

[Table 3]

The processing characteristics, high temperature characteristics, and other characteristics of these ferritic stainless steels were evaluated in the same manner as described in Example 1. Table 4 shows the results.

[0053]

[Table 4]

As is clear from the results shown in Table 4,
The ferritic stainless steel in the examples of the present invention shows good processing characteristics in either a plate-like or tubular state. In addition, other required performances such as high temperature strength, oxidation resistance, corrosion resistance, and manufacturability are good without any trouble.

On the other hand, in Comparative Examples (Nos. 47 to 49) in which the value of “Al (%) / 27−N (%) / 14” is less than 0, Cu, Mo,
Even if the Nb content is appropriate, the average elongation of the steel sheet is inferior, so that fracture occurs in the cylindrical forming test, resulting in poor ductility of the welded pipe. In Comparative Examples (Nos. 50 to 54) in which the contents of Cu, Mo, and Nb were not appropriate, the ductility of the welded pipe was significantly poor.

Comparative Example in which "Mo + Nb" is excessively contained (N
o.55) is unfavorable because of inferior processing characteristics and high cost of steel. Further, in the comparative example (No. 56) in which Cu was not added, good processing characteristics were not obtained.

[0057]

[Summary of Effects] As described above, according to the present invention, it has excellent processing characteristics, high temperature strength, and oxidation resistance, both in the form of a plate and a welded pipe, compared to conventional materials. It is possible to stably provide heat-resistant ferritic stainless steel or steel sheet with excellent high-temperature properties such as high-temperature exhaust manifolds, front pipes, etc., and high-temperature components such as power plants. It can be expected to greatly contribute to the performance improvement. In addition, since it is possible to reduce the amount of Mo and Nb, which are expensive alloying elements, compared to conventional steel, it is necessary to provide a heat-resistant ferritic stainless steel material with excellent processing characteristics at a lower cost than before. Thus, the present invention has an extremely large industrial effect.

Claims (6)

[Claims] 1. In mass%, C: 0.02% or less, Si: 1.5% or less, Mn: 1.5% or less, P: 0.04% or less, S: 0.01% or less, Cu: 1.5% or less, Cr: 17 to 25%, Ni: 1.0% or less, Mo: 0.5 to 2.5%, Nb: 0.1 to 1.0%, Al: 0.2% or less, N: 0.02% or less, with the balance being Fe and unavoidable impurities.
Regarding the contents of Cu, Mo and Nb, the following equation (1) and
Ferritic stainless steel with excellent heat resistance and workability, characterized by satisfying formula (2). Cu (%) −0.05 ｛Mo (%) + Nb (%)｝ ≧ 0.48… (1) Mo (%) + Nb (%) ≦ 3… (2) 2. The ferritic stainless steel according to claim 1, wherein the contents of Al, N, Cu, Mo and Nb also satisfy the following expressions (3) to (5). Ferritic stainless steel with excellent heat resistance and workability. Al (%) / 27−N (%) / 14 ≧ 0… (3) Cu (%) <0.55… (4) 25 ｛Cu (%) −0.45｝ ² − ｛Mo (%) + Nb (%)｝ ≧ −2.0… (5) 3. The ferritic stainless steel according to claim 1, wherein the contents of Al, N, Cu, Mo and Nb satisfy the following expressions (3) and (6) to (7). Ferritic stainless steel with excellent heat resistance and workability. Al (%) / 27−N (%) / 14 ≧ 0… (3) Cu (%) ≧ 0.55… (6) Cu (%) −0.05 ｛Mo (%) + Nb (%)｝ ≧ 0.48… (7 ) 4. The value of ΔEL represented by the following equation (8) is 9%
A ferritic stainless steel sheet comprising the steel according to any one of claims 1 to 3 and having excellent heat resistance and workability, characterized in that: ΔEL (%) = (EL ₀ + EL ₉₀ ) / 2−EL ₄₅ (8) where EL ₀ : elongation in the rolling direction (%), EL ₉₀ : elongation in the direction perpendicular to the rolling (%), EL ₄₅ : rolling 45 ° elongation (%) 5. The value of EL _S represented by the following equation (9) is 27:
%. A ferritic stainless steel sheet made of the steel according to claim 1 and having excellent heat resistance and workability. EL _S (%) = (EL ₀ + EL ₉₀ + 2EL ₄₅ ) / 4 (9) where EL ₀ : elongation in the rolling direction (%), EL ₉₀ : elongation in the direction perpendicular to the rolling (%), EL ₄₅ : rolling 45 ° elongation (%) 6. The value of EL _S represented by the following equation (9) is 27:
%. The ferritic stainless steel sheet having excellent heat resistance and workability according to claim 4. EL _S (%) = (EL ₀ + EL ₉₀ + 2EL ₄₅ ) / 4 (9) where EL ₀ : elongation in the rolling direction (%), EL ₉₀ : elongation in the direction perpendicular to the rolling (%), EL ₄₅ : rolling 45 ° elongation (%)