KR20140117686A - Ferritic stainless steel plate having excellent heat resistance and excellent workability - Google Patents

Abstract

A ferritic stainless steel sheet which is excellent in heat resistance and workability and contains 0.02% or less of C, 0.02% or less of N, 2% or less of Si, 2% or less of Mn, 10 to 20% 0.4 to 3%, Ti: 0.01 to 0.5%, B: 0.0002 to 0.0030%, and the balance of Fe and unavoidable impurities.

Description

TECHNICAL FIELD [0001] The present invention relates to a ferritic stainless steel sheet having excellent heat resistance and workability,

The present invention relates to a ferritic stainless steel sheet excellent in heat resistance, which is most suitable for use in an exhaust system member requiring high temperature strength and oxidation resistance.

An exhaust system member such as an exhaust manifold of a vehicle, a front pipe, and a center pipe passes high temperature exhaust gas discharged from the engine. Therefore, various characteristics such as oxidation resistance, high temperature strength, and thermal fatigue characteristics are required for the material constituting the exhaust system member.

Conventionally, cast iron has been generally used for an automobile exhaust member, but an exhaust manifold made of stainless steel has been used from the viewpoints of enhancement of exhaust gas regulation, improvement of engine performance, and weight reduction of a vehicle body.

Although the exhaust gas temperature differs depending on the type of the vehicle and the engine structure, it is usually about 600 to 800 DEG C, and a material having high high-temperature strength and oxidation resistance in an environment where it is used for a long time in such a temperature range is desired.

Among the stainless steels, austenitic stainless steels are excellent in heat resistance and workability. However, since the austenitic stainless steel has a large thermal expansion coefficient, when it is applied to a member which is repeatedly heated and cooled like an exhaust manifold, thermal fatigue failure tends to occur.

Ferritic stainless steels are superior in heat fatigue characteristics and scaling resistance because they have a smaller coefficient of thermal expansion than those of austenitic stainless steels. Since it does not contain Ni, the material cost is lower than that of the austenitic stainless steel and is used for general purpose.

Ferritic stainless steels have lower high temperature strength than austenitic stainless steels. Therefore, techniques for improving the high-temperature strength have been developed. For example, SUS430J1 (Nb-added steel), Nb-Si-added steel, and SUS444 (Nb-Mo added steel). These are to strengthen the solid solution by Nb, or to increase the high temperature strength by precipitation strengthening.

When Nb is added, the recrystallization temperature of the stainless steel becomes high, so it is necessary to increase the annealing temperature in the production of the steel sheet.

Since stainless steel is hardened by the addition of Nb, it is necessary to anneal the hot-rolled sheet after hot-rolling, to soften it, and then to cold-anneal it.

The toughness is lowered due to the Nb precipitates precipitated in the hot rolling process, and cracks or breakage may occur in the manufacturing process.

In the case of Nb-added steel, the product plate is liable to be hardened and the elongation tends to deteriorate. Also, the r value which is an index of deep drawability is low. This is because the presence of solid Nb or precipitated Nb suppresses the hardening at room temperature or the development of recrystallized texture. Therefore, the pressability at the time of molding the exhaust part is lowered, and the degree of freedom in shape is reduced.

As described above, the Nb-added steel is inferior in productivity, manufacturability and workability of the steel sheet. Since Nb has a high alloy cost, the production cost is increased by the addition of Nb.

Further, since Mo added to SUS444 is expensive in alloy cost, the cost of components remarkably increases.

If the high-temperature strength can be increased by an additional element other than Nb, the amount of Nb added can be suppressed, so that it is possible to provide a heat-resistant ferritic stainless steel sheet excellent in workability at low cost.

And Cu as an additive element contributing to enhancement of high temperature strength other than Nb and Mo.

In Patent Document 1, Cu addition of not more than 0.5% is disclosed for the purpose of improving the low temperature toughness. This is not Cu addition from the viewpoint of heat resistance.

Patent Document 2 discloses a technique using an action of enhancing corrosion resistance and weather resistance of steel. This is not Cu addition from the viewpoint of heat resistance.

Patent Documents 3 to 6 disclose techniques for improving the high temperature strength at 600 占 폚 or 700 to 800 占 폚 by using precipitation hardening by Cu precipitates. However, they are all added in combination with Nb, resulting in poor productivity and processability, and high cost.

The prior art for improving high-temperature strength by Cu addition is Cu precipitate. When Cu precipitates are exposed to a high temperature for a long time, coarsening rapidly occurs due to aggregation and coalescence of the precipitates, and the precipitation strengthening ability is remarkably lowered.

In the case of receiving a heat cycle accompanied by starting and stopping of the engine as in the case of an exhaust manifold, the high temperature strength is remarkably lowered by use for a long time, and there is a risk of causing thermal fatigue failure.

Particularly, in the case of the component composition in which Nb is added in a large amount, Cu precipitates are precipitated at the interface between the coarse precipitate (Fe ₂ Nb) called the Laves phase and the parent phase at the time of heating at high temperature, Precipitation strengthening effect can not be obtained.

Patent Document 6 discloses a technique of precipitating fine Cu by addition of Nb-Cu-B composite. However, this method can not avoid complex precipitation with the Laveth phase. Further, since a small amount of Mo is added, the workability is deteriorated and the cost is high.

As described above, there is an example in which Cu is precipitated in order to improve the high-temperature strength. However, in the prior art, it is not enough to precipitate Cu fine, and it is insufficient from the viewpoint of processability and cost.

Further, as a steel containing B, Patent Documents 7 to 9 disclose ferritic stainless steels excellent in high-temperature characteristics. All of them are added with B for improvement of workability and are not added in terms of heat resistance.

Patent Document 1: JP-A-2006-37176 Patent Document 2 Japanese Patent Publication No. 3446667 Patent Document 3: International Publication No. WO2003 / 004714 Patent Document 4: Japanese Patent Publication No. 3468156 Patent Document 5: Japanese Patent Publication No. 3397167 Patent Document 6: JP-A-2008-240143 Patent Document 7: JP-A-9-279312 Patent Document 8: Japanese Patent Application Laid-Open No. 2000-169943 Patent Document 9: JP-A-10-204590

The object of the present invention is to provide a ferritic stainless steel which is excellent in heat resistance and workability, which is used under a heat environment in which the maximum temperature of the exhaust gas is 600 to 800 占 폚 at low cost.

The present invention aims to improve the high-temperature characteristics of a ferritic stainless steel sheet by finely dispersing Cu precipitates by adding Cu in a steel component to which Nb is not added.

Therefore, the present inventors paid attention to utilizing precipitate refinement by addition of Ti-Cu-B composite.

The inventors of the present invention investigated in detail the strength and the room temperature ductility at about 500 DEG C to 800 DEG C of a steel to which Nb was not added to obtain the following findings.

In the case of a Cu-added steel, since Cu precipitates are precipitated in a large amount in the range of about 500 ° C to 800 ° C, a method of controlling the form of the precipitate is effective for improving the high-temperature strength.

Concretely, by adding Ti and Cu in combination and further adding B, the Cu precipitates are uniformly and finely precipitated to utilize the precipitation strengthening, and at the same time, it is possible to suppress the strength reduction due to the aging heat treatment. This is effective for the durability stability of parts used over a long period of time under repeated thermal cycling like an exhaust member.

When Cu is added to the Nb-added steel, Cu precipitates precipitate to act on the strengthening, while at the same time precipitates of Fe and Nb (Fe ₂ Nb) called Laves phase are produced.

In this case, since Cu precipitates at the interface between the coarse-grained Laveth phase and the parent phase, fine precipitation does not occur. Also, depending on the temperature condition, Cu precipitates sharply with time.

In the case of such a precipitation form, the effect of precipitation strengthening is lowered, so that sufficient high temperature strength can not be obtained and durability is lowered.

The present inventors have found that a fine precipitation technique in which the precipitation strengthening effect is obtained by separately precipitating a fine Cu precipitate based on the above knowledge and the coarsening of the Cu precipitate is suppressed, It is possible to provide a ferritic stainless steel sheet which exhibits heat resistance at low cost.

The gist of the present invention is as follows.

(1) in mass%

C: 0.02% or less,

N: 0.02% or less,

Si: 0.22 to 2%

Mn: 2% or less,

Cr: 10 to 20%

Cu: 0.52 to 3%

Ti: 0.01 to 0.5%

B: 0.0002 to 0.0030%

Sn: 1% or less

And a balance of Fe and unavoidable impurities, wherein the 600 ° C internal strength is 150 MPa or higher, the 800 ° C internal strength is 30 MPa or higher, and the average r value is 1.3 or higher. A ferritic stainless steel sheet excellent in heat resistance and workability .

(2) Further, in terms of mass%

Mo: 0.01 to 0.3%

Al: 2.5% or less,

V: 1% or less,

Zr: 1% or less,

(1), wherein the ferritic stainless steel sheet has excellent heat resistance and processability.

(3) A ferritic stainless steel having the composition of the above item (1) or (2) was hot-rolled to form a hot-rolled sheet, and the hot-rolled sheet was subjected to pickling by omitting the annealing. Or more, and then performing final annealing at 850 to 970 占 폚. The method of producing a ferritic stainless steel sheet excellent in heat resistance and processability.

According to the present invention, it is possible to obtain a ferritic stainless steel sheet excellent in high-temperature strength and workability without addition of Nb. The ferritic stainless steel sheet of the present invention is particularly applied to an exhaust system member such as an automobile, so that it is possible to obtain a great effect in environmental measures and in cost reduction of parts.

BRIEF DESCRIPTION OF DRAWINGS FIG. 1 is a graph showing 0.2% proof stress in a high temperature tensile test of a steel according to the present invention and a comparative steel; FIG.

The reason for limiting the composition of the ferritic stainless steel sheet of the present invention will be described below. It is within the scope of the present invention that unlimited impurity levels are not provided for the lower limit.

C deteriorates the moldability and the corrosion resistance, resulting in deterioration of high-temperature strength. Therefore, the smaller the content of C is, the better. Therefore, the content of C is 0.02% or less, preferably 0.009% or less. The lower limit of the content of C is not particularly specified, but an excessive reduction leads to an increase in refining costs, so it is preferable to set the content to 0.001% or more.

N, like C, deteriorates in moldability and corrosion resistance, resulting in deterioration of high-temperature strength. Therefore, the content of N is preferably as small as possible. Therefore, the content of N is 0.02% or less, preferably 0.015% or less. The lower limit of the content of N is not particularly specified, but excessive reduction leads to an increase in refining costs, so it is preferable that the N content is 0.003% or more.

Si is an element that is useful as a deoxidizing agent and also improves high temperature strength and oxidation resistance. The high temperature strength up to about 800 ° C improves with the increase of Si amount. In order to obtain the effect, the content of Si is preferably 0.22% or more. The excessive addition of Si lowers the ductility at room temperature, so the upper limit of the Si content is 2%. Considering the oxidation resistance, 0.22 to 1.0% is preferable.

Mn is an element to be added as a deoxidizing agent and contributes to an increase in high temperature strength at a temperature range of about 600 to 800 ° C. Further, Mn-based oxides are formed on the surface layer during prolonged use, contributing to improvement of scale adhesion and suppression of abnormal oxidation. When the content of Mn exceeds 2%, the ductility at room temperature lowers and MnS is formed and the corrosion resistance is lowered. Therefore, the upper limit of the content of Mn is 2%. Taking into account high temperature ductility and scale adhesion, the content of Mn is preferably 0.1 to 1.0%.

In the present invention, Cr is an essential element for ensuring oxidation resistance and corrosion resistance. If the Cr content is less than 10%, the effect can not be obtained. If the Cr content exceeds 20%, the workability is lowered and the toughness is deteriorated. Therefore, the content of Cr is set to 10 to 20%. Considering the composition and high temperature ductility, 10 to 18% is preferable.

Cu is an element effective for improving high-temperature strength particularly at an intermediate temperature range of about 600 to 800 ° C. This is due to precipitation strengthening by the formation of Cu precipitates at the intermediate temperature region.

Fig. 1 shows the 0.2% proof stress of the steel (steel A, steel B, steel C) and comparative steel (SUH409L, Nb-Si steel) in the high temperature tensile test of the present invention.

The composition of the steel A is 0.005% C-0.007% N-0.41% Si-0.45% Mn-10.5% Cr-1.25% Cu-0.15% Ti-0.0009%

The composition of the steel B is 0.006% C-0.009% N-0.88% Si-0.31% Mn-13.9% Cr-1.42% Cu- 0.11% Ti-0.0005%

The composition of the steel C is 0.008% C-0.015% N-0.22% Si-0.16% Mn-11.1% Cr-1.56% Cu-0.12% Ti-0.0005% B-0.05% Al.

The comparative steel is a general-purpose steel.

The composition of SUH409L is 0.005% C-0.007% N-0.35% Si-0.50% Mn-10.5 Cr-0.15% Ti.

The composition of Nb-Si steel is 0.006% C-0.009% N-0.90% Si-0.35% Mn-13.8% Cr-0.45% Nb.

The high-temperature tensile test was conducted in accordance with JIS G0567 in a tensile test in the rolling direction, and a 0.2% proof stress was measured.

From the test results, it can be seen that the strength of the steel A, the steel B and the steel C is higher than that of the SUH409L and the Nb-Si steel even in the absence of Nb.

The steel of the present invention is particularly effective when used in an environment having a high strength at a temperature range of about 600 DEG C and a low exhaust gas temperature. The steel of the present invention is also applicable in an environment of less than 60 占 폚.

In the present invention, considering the high-temperature strength of Nb-Si steel used for general purpose, those having a 600 ° C proof stress of 150 MPa or more and a 800 ° C proof stress of 30 MPa or more are required properties at high temperature strength.

The reason why the high-temperature strength is increased as described above is the precipitation strengthening by the formation of the Cu precipitate.

In order to obtain this effect, the Cu content needs to be 0.4% or more.

In the present invention, the coarsening of the Cu precipitate due to the complex precipitation with the Lavess phase is suppressed, and fine Cu precipitates are produced by the combined addition of Ti and B.

When the content of Cu exceeds 3%, the ductility at room temperature and the oxidation resistance deteriorate. In addition, the edge cracks in the hot rolling process become remarkable, and the composition is deteriorated. Therefore, the upper limit of the content of Cu is set to 3%. Taking into account the composition, scale adhesion and weldability, the content of Cu is preferably 0.5 to 2.5%.

Ti is an element which combines with C, N and S to improve corrosion resistance, intercalation corrosion resistance, room temperature ductility and deep drawability. Further, in the composite addition with Cu, an appropriate amount is added to achieve uniform precipitation of Cu precipitates, thereby improving high-temperature strength and thermal fatigue characteristics.

This action is presumably because the Ti clusters in the crystal grains, or the Ti precipitates become sites for the formation of the Cu precipitates, thereby suppressing Cu from being generated in the grain boundaries to a great extent.

Further, when Ti is added, the recrystallized texture easily develops during the recrystallization annealing after the cold rolling, so that the r value is improved and the press formability is remarkably improved.

In order to obtain such an effect, the content of Ti should be 0.01% or more. If the content of Ti exceeds 0.5%, the amount of solute Ti increases, the ductility at room temperature decreases, and coarse Ti-based precipitates are formed, which becomes a starting point of cracking at the time of hole expanding, and the press formability is deteriorated. Further, oxidation resistance is deteriorated. Therefore, the Ti content should be 0.5% or less. Considering the occurrence of surface defects and toughness, the content of Ti is preferably 0.05 to 0.3%.

B is an element that improves the secondary workability at the time of press working of a product. In the present invention, the Cu precipitate is finely precipitated by the addition of Ti-Cu and the high-temperature strength is improved.

Generally, B is likely to form (Fe, Cr) ₂₃ (C, B) ₆ or Cr ₂ B at high temperature. However, it has been found that the precipitate does not precipitate in the Ti-Cu composite-added steel, but has an effect of fine precipitation of the Cu precipitate.

The Cu precipitate usually precipitates very finely at the initial stage of precipitation and has a large effect of improving the strength. However, the Cu precipitates are coarsened by the aging heat treatment, and the strength after aging is greatly decreased. However, the addition of B suppresses the coarsening of the Cu precipitate, thereby enhancing the stability of the strength at the time of use.

It is presumed that the mechanism of the effect of the refinement of Cu precipitates and the suppression of coarsening due to the addition of B is unclear but it is presumed that B segregates the intermetallic phases of the Cu precipitates by grain boundary segregation so as to precipitate Cu in the particles .

In order to obtain such an effect, the content of B is 0.0002% or more. When the content of B exceeds 0.0030%, the steel is hardened, the intergranular corrosion resistance and oxidation resistance are deteriorated, or weld cracks are likely to occur. Therefore, the content of B is 0.0002 to 0.0030%. Considering the corrosion resistance and the manufacturing cost, 0.0003 to 0.0015% is preferable.

In addition to the above elements, Mo, Al, V, and Zr may be added as necessary.

Mo is an element that further improves high temperature strength and thermal fatigue characteristics. If the Mo content is less than 0.01%, the effect of addition can not be obtained. When Mo is added, a Lavess phase is generated, the effect of precipitation strengthening by Cu precipitation is suppressed, and the ductility at room temperature is lowered, so that addition of a large amount is not preferable. Therefore, the content of Mo is set to 0.3% or less. The more preferable content of Mo is 0.01% or more and 0.2% or less.

Al is an element to be added, if necessary, because it is a deoxidizing element and further improves oxidation resistance. Further, it is useful for enhancing the strength of 600 to 700 DEG C as the solid solution strengthening element. In order to obtain this effect stably, the content of Al is preferably 0.01% or more. When Al is excessively added, the steel becomes hard, the uniform elongation remarkably decreases, and the toughness remarkably decreases. Therefore, the upper limit of the content of Al is set to 2.5%. Considering the occurrence of surface defects, weldability and manufacturability, the content of Al is preferably 0.01 to 2.0%.

V is an element to be added as needed because it forms fine carbonitride and contributes to improvement of high-temperature strength by precipitation strengthening action. In order to obtain this effect stably, the content of V is preferably 0.01% or more. When the content of V exceeds 1%, the precipitates coarsen and the high-temperature strength is lowered, and the thermal fatigue life is lowered. Therefore, the upper limit of the content of V is set to 1%. Considering the production cost and the composition, the content of V is preferably 0.08 to 0.5%.

Zr is a carbonitride-forming element and contributes to improvement of high-temperature strength and oxidation resistance due to an increase in the amount of solid solution Ti. In order to obtain this effect stably, the content of Zr is preferably 0.2% or more. When the content of Zr exceeds 1%, the deterioration of the productivity is remarkable. Therefore, the upper limit of the content of Zr is 1%. Considering the cost and surface quality, 0.2 to 0.6% is preferable.

Sn is an element which has a large atomic radius and is effective for solid solution strengthening and does not significantly deteriorate the mechanical properties at room temperature and is added as needed. In order to obtain this effect stably, the content of Sn is preferably 0.1% or more. If the Sn content exceeds 1%, the composition and weldability are remarkably deteriorated. Therefore, the upper limit of the Sn content is 1%. Considering oxidation resistance and the like, the content of Sn is preferably 0.2 to 0.5%.

The steel of the present invention contains Mo in a non-added or low concentration and secures high temperature strength. As a result, improvement in room temperature stretching can be realized.

Next, a method of manufacturing the steel sheet of the present invention will be described. The manufacturing process of the steel sheet according to the present invention comprises the steps of steelmaking, hot rolling, pickling, cold rolling, annealing and pickling.

In steelmaking, a method of converting a steel containing the above-mentioned essential components and a component to be added, if necessary, into a converter solvent and then performing secondary refining is very suitable. The molten steel to be melted is made into a slab according to a known casting method such as continuous casting.

The slab is heated to a predetermined temperature and hot-rolled to a predetermined thickness in continuous rolling. The cold-rolling of the stainless steel sheet is generally performed by a reverse milling method using a Zenjimer mill having a roll diameter of about 60 to 100 mm or a one-direction rolling with a tandem mill having a roll diameter of 400 mm or more. Both cases are rolled in multiple passes.

In the present invention, in order to increase the r value as the index of workability, it is preferable to carry out cold rolling with a tandem type rolling mill having a roll diameter of 400 mm or more. When the roll diameter is 100 mm or less, shear deformation is introduced much in the vicinity of the surface layer during cold rolling, and the development of the <111> and <554> crystal orientations is suppressed during the recrystallization annealing, making it difficult to improve the r value. By the cold rolling with large diameter rolls, the crystal orientation remarkably develops by suppressing shear deformation, and contributes to the improvement of the r value.

The tandem rolling is one-way rolling, and since the rolling pass number is smaller than that of Zen zmir rolling, the productivity is also excellent. If the reduction rate in the cold rolling process is low, a recrystallized structure can not be obtained after annealing, or excessive agglomeration is caused to deteriorate mechanical properties. Therefore, the reduction ratio in the cold rolling process is preferably 30% or more.

The hot-rolled sheet annealing usually carried out in the production of the ferritic stainless steel sheet may be carried out, but from the viewpoint of productivity improvement, it is preferable not to anneal the hot-rolled sheet.

Since the normal Nb-added steel is hard in the hot-rolled steel sheet, annealing is performed before cold-rolling. However, since Nb is not added to the steel of the present invention, it is possible to omit annealing of the hot-rolled steel sheet, and as a result, manufacturing cost can be reduced. Further, by omitting the annealing of the hot rolled sheet, the aggregate structure after cold rolling and annealing develops, and the press formability is improved by the improvement of the r-value and the reduction of the anisotropy.

The manufacturing method of other processes is not particularly specified. The hot rolling conditions, the hot rolled sheet thickness, the cold rolled sheet annealing temperature, and the atmosphere may be selected as appropriate. After cold rolling and annealing, temper rolling or tension leveler may be applied. Further, the product plate thickness may be selected according to the required member thickness.

Since the steel of the present invention is free of Nb, the annealing temperature after cold rolling can be set to a low temperature of 850 to 970 캜. As a result, compared with the case where the annealing temperature exceeds 970 占 폚, the high temperature resistant property is improved.

<Examples>

Steels having the constituent compositions shown in Tables 1 and 2 were melted and cast into slabs, and the slabs were hot-rolled to obtain hot-rolled coils having a thickness of 5 mm. Thereafter, the hot-rolled coil was pickled without annealing, cold-rolled to a thickness of 2 mm, annealed and pickled to obtain a product plate.

For cold rolling, one-directional multi-pass rolling was carried out with a rolling machine having a large diameter roll (diameter: 450 mm), and a reverse type multi-pass rolling was performed by a rolling machine having a small diameter roll (diameter of 100 mm) as a comparative example.

The annealing temperature of the cold-rolled sheet was set to 850 to 970 캜 so as to make the grain size number 6 to 8 or so. For the comparative example containing Nb, the annealing temperature of the cold-rolled sheet was set at 1000 to 1050 ° C.

Nos. 1, 2, 4 to 7, 9, 10 and 15 in the tables are the inventive steels, and Nos. 18 to 36 are comparative steels. No. 18 of comparative steel is SUH409L, and Nos. 19 and 20 are steels with Nb-Si addition.

From the product plate thus obtained, a high-temperature tensile test piece was taken, subjected to a tensile test at 600 캜 and 800 캜, and a 0.2% proof stress was measured (in accordance with JIS G0567). The tensile test was carried out at a temperature of 600 ° C above 150 MPa and a temperature of 800 ° C above 30 MPa.

As a test for oxidation resistance, a continuous oxidation test was conducted in air at 900 DEG C for 200 hours to evaluate the occurrence of abnormal oxidation (in accordance with JIS Z2281). As a result of the test, it was determined that there was no abnormal oxidation.

A JIS No. 13 B test piece was prepared as a workability at room temperature and subjected to a tensile test in a direction parallel to the rolling direction, and fracture elongation was measured. If the breaking elongation at room temperature is 35% or more, it is possible to process into a complicated part, so that the elongation at break of 35% or more is acceptable.

The average r value was calculated by using the equations (1) and (2) after taking the JIS No. 13 B tensile test specimen and imparting 15% strain in the rolling direction, the 45 ° direction in the rolling direction and the 90 ° direction in the rolling direction.

_{r = ln (W 0 / W} ) / ln (t 0 / t) (1)

In this case, W ₀ is plate width before stretching, W is plate width after stretching, t ₀ is plate thickness before stretching, and t is plate thickness after stretching.

Average r value = (r ₀ + 2r ₄₅ + r ₉₀ ) / 4 (2)

In this case, r ₀ is the r value in the rolling direction, r ₄₅ is the r value in the rolling direction and 45 ° direction, and r ₉₀ is the r value in the direction perpendicular to the rolling direction. When the average r value is 1.3 or more, machining to a complicated part becomes possible, so it is preferable that the average r value is 1.3 or more.

The underlines of the component compositions in Table 1 and Table 2 are outside the scope of the present invention. The underline of the quality evaluation result means that the test has failed.

It can be seen from Tables 1 and 2 that the steels having the component compositions specified in the present inventions of Nos. 1, 2, 4 to 7, 9, 10, and 15 were produced at a temperature of 600 ° C , The high-temperature proof stress at 800 ° C is high, and there is no abnormal oxidation at 900 ° C, and the oxidation resistance is also excellent.

In addition, the steels of Nos. 1, 2, 4 to 7, 9, 10 and 15 have a high fracture ductility of 35% or more in mechanical properties at room temperature and are superior in workability to the comparative steels.

Nos. 18, 19 and 20 of the comparative steel are conventional steels, but the high temperature strength is lower than the required value. The comparative steels Nos. 19 and 20 in which Nb is excessively added have a low r value.

Nos. 21 and 22 have C and N exceeding the upper limit, respectively, and are inferior in high temperature strength, oxidation resistance and workability.

In No. 23, Si is excessively added, resulting in poor processability.

No.24 is excessively added with Mn, resulting in poor oxidation resistance and processability.

The No. 25 has a low amount of Cr, low in high-temperature strength, and low in oxidation resistance.

Since the amount of Cu is small in No. 26, the 0.2% proof stress at 600 ° C and 800 ° C is low.

In No. 27, since the amount of Ti exceeds the upper limit, oxidation resistance and workability are poor.

In No. 28, the amount of Ti is less than the lower limit, and Nb is excessively added, so that ductility is low.

In No. 29, since Nb is excessively added, ductility and r value are low.

No. 30 has a low oxidation resistance and processability because B exceeds the upper limit.

In No. 31, since the amount of B added is less than the lower limit of 0.0001%, the Cu precipitates coarsen at 800 ° C, the effect of precipitation strengthening is lowered, and the proof stress is low.

In Nos. 32 to 36, Mo, Al, V, Zr, and Sn exceed the upper limit, respectively, so that the ductility at room temperature is low, which hinders part processing.

Among the examples of the present invention, No. 1, No. 2, No. 4, No. 7, No. 9, No. 10, and No. 15 using the large-diameter roll for cold rolling had a good average r value of 1.3 or more.

According to the present invention, a stainless steel sheet excellent in high-temperature characteristics and processability can be provided without adding a large amount of expensive alloying elements such as Mo. Particularly, when applied to an exhaust member, social contribution such as reduction of parts cost and lightening of the environment is remarkably large.

Claims (3)

In terms of% by mass,
C: not more than 0.02% (not including 0%),
N: not more than 0.02% (not including 0%),
Si: 0.22 to 2%
Mn: 2% or less (not including 0%),
Cr: 10 to 20%
Cu: 0.52 to 3%
Ti: 0.01 to 0.5%
B: 0.0002 to 0.0030%
Sn: 1% or less (not including 0%)
And a balance of Fe and unavoidable impurities, wherein the 600 ° C internal strength is 150 MPa or higher, the 800 ° C internal strength is 30 MPa or higher, and the average r value is 1.3 or higher. A ferritic stainless steel sheet excellent in heat resistance and workability . 3. The composition according to claim 1, wherein, in mass%
Mo: 0.01 to 0.3%
Al: 2.5% or less (not including 0%),
V: 1% or less (not including 0%),
Zr: 1% or less (not including 0%)
By weight of at least one member selected from the group consisting of iron and iron. A ferritic stainless steel having the composition of claim 1 or 2 is hot-rolled to form a hot-rolled sheet, the hot-rolled sheet is subjected to pickling by omitting the hot-rolled sheet annealing and then cold rolled with a rolling roll having a diameter of 400 mm or more , And then performing final annealing at 850 to 970 占 폚. The method for producing a ferritic stainless steel sheet excellent in heat resistance and processability.

Images