KR20160076792A

KR20160076792A - Ferritic stainless steel and manufacturing method thereof

Info

Publication number: KR20160076792A
Application number: KR1020140187305A
Authority: KR
Inventors: 김상석; 정일찬
Original assignee: 주식회사 포스코
Priority date: 2014-12-23
Filing date: 2014-12-23
Publication date: 2016-07-01
Also published as: WO2016105065A1

Abstract

Ferritic stainless steels excellent in high temperature strength and press formability are introduced.
The ferritic stainless steel of the present invention contains 0.007 wt% or less of C (excluding 0), 0.3 wt% or less of Si (excluding 0), 0.5 to 1.5 wt% of Mn and 0.02 wt% or less of P , S: not more than 0.02 wt% (excluding 0), Cr: 14 to 17 wt%, Mo: not more than 1.0 wt% (not more than 0), W: 1 to 4 wt% Nb: 0.6 wt% or less (excluding 0), N: 0.01 wt% or less (0 excluded), Al: 0.1 and the wt% or less (0 or less), the balance Fe and other inevitable contain impurities, and precipitated phase (Fe ₂ W ) Is less than 1 mu m, and the following formula is satisfied.
Α = [(Fe + W) precipitation amount / (Nb) precipitation amount] <1.0

Description

Technical Field The present invention relates to a ferritic stainless steel and a manufacturing method thereof,

The present invention relates to a ferritic stainless steel used for an automotive exhaust manifold and a method of manufacturing the ferritic stainless steel.

Ferritic stainless steels are less expensive than austenitic stainless steels, have lower thermal expansion rates, and have excellent surface gloss, formability, and oxidation resistance. They are widely used in heaters, sink tops, exterior materials, home appliances, electronic parts, .

Due to the seriousness of environmental problems caused by automobile exhaust gas, regulations on emission of exhaust gas are being implemented in each country. In response to this trend, a technique for improving the purifying ability of the exhaust gas by using a catalyst has been attracting attention.

Flue-gas shows a tendency to increase the purifying reaction of NOx, HC, and CO as the temperature increases. Therefore, in order to reduce pollutant emissions, there is a tendency to continuously increase the temperature of the exhaust gas, and accordingly, improvement of the high temperature characteristics of each component constituting the exhaust system for controlling the exhaust gas is required.

Specifically, researches have been made on weight reduction of automobile materials, increase of flue gas temperature by turbo engine, and improvement of fuel efficiency of automobile. For this purpose, ferrite stainless steels having higher temperature strength, heat resistance and corrosion resistance than conventional ferrite stainless steels Demand for stainless steel is increasing.

BACKGROUND OF THE INVENTION [0002] Spheroidal graphite cast iron or Si-spheroidal graphite cast iron having excellent heat resistance and high temperature characteristics are used in exhaust manifold materials in recent years, and stainless steel is used in recent years in accordance with demands for increasing temperature and weight of exhaust gas.

Exhaust system manifold collects the exhaust gas discharged from each cylinder of the engine and discharges it to the manifold pipe. Because exhaust gas temperature reaches 900 ℃, it is a part that requires excellent high temperature strength and excellent deterioration resistance property at high temperature exposure

In recent years, the exhaust gas temperature is expected to rise by more than 30 ^° C to 50 ^° C compared to conventional vehicles due to the turbo mounting and downsizing of the engine in order to improve the fuel efficiency of the vehicle.

Therefore, STS 429EM (14Cr-1Si steel), STS 441 (18Cr steel), and STS444 (19Cr-2Mo steel), which are ferritic stainless steel grade grades used for conventional exhaust manifolds, Various studies have been made on ferritic stainless steels having improved performance at high temperatures.

It should be understood that the foregoing description of the background art is merely for the purpose of promoting an understanding of the background of the present invention and is not to be construed as adhering to the prior art already known to those skilled in the art.

KR 10-2006-0007441 A (2006.01.24)

An object of the present invention is to provide a stainless steel excellent in high-temperature strength and free from cracks during press forming to solve such a conventional problem and its manufacturing method.

In order to achieve the above object, the ferritic stainless steel according to the present invention contains 0.007 wt% or less of C (excluding 0), 0.3 wt% or less of Si (excluding 0), 0.5 to 1.5 wt% of Mn, (excluding 0), S: not more than 0.02 wt% (excluding 0), Cr: 14 to 17 wt%, Mo: not more than 1.0 wt% (not more than 0), W: 1 to 4 wt%, Ti: , N: not more than 0.01 wt% (excluding 0), Al: not more than 0.1 wt% (not more than 0), the balance Fe and other unavoidable impurities, , The average size of the precipitated phase (Fe ₂ W) is less than 1 탆, and the following formula is satisfied.

Α = [(Fe + W) precipitation amount / (Nb) precipitation amount] <1.0

In order to achieve the above object, the ferritic stainless steel according to the present invention contains 0.007 wt% or less of C (excluding 0), 0.3 wt% or less of Si (excluding 0), 0.5 to 1.5 wt% of Mn, (excluding 0), S: not more than 0.02 wt% (excluding 0), Cr: 14 to 17 wt%, Mo: not more than 1.0 wt% (not more than 0), W: 1 to 4 wt%, Ti: , N: not more than 0.01 wt% (excluding 0), Al: not more than 0.1 wt% (not more than 0), the balance Fe and other unavoidable impurities, , The following expression is satisfied.

19.5? [(Ti + 1 / 2Nb) / (C + N)]? 32

The ferrite-based stainless steel of the present invention has a crystal grain size of 5.0 or more (based on ASTM No.).

The ferritic stainless steel of the present invention is characterized by a Charpy impact energy of 10 J or more at 0 占 폚 impact test using a 2.0 to 3.0 mm V-notch impact specimen.

In the ferritic stainless steel of the present invention,? Satisfying the following expression is 0.3 or more and 0.5 or less.

Γ = (mean grain size average size of cold rolled products) / (mean grain size average size of hot rolled products)

The ferritic stainless steel of the present invention has an average r-bar (r) value of plastic anisotropy (r) values in directions of 0 °, 45 ° and 90 ° after 15% stretching through a crosshead speed of 20 mm / Value is 1.0 or more.

The ferrite stainless steel of the present invention is characterized in that the surface roughness (Rt) of the gage portion of the specimen stretched at 15% is 20 m or less.

The ferritic stainless steel of the present invention is characterized by having a tensile strength of at least 45 MPa at 900 ° C.

In order to attain the above object, the present invention provides a ferritic stainless steel manufacturing method, comprising the steps of: C: not more than 0.007 wt% (excluding 0), Si: not more than 0.3 wt% (excluding 0) Cr: 14-17 wt%, Mo: 1.0 wt% or less (0 or less), W: 1 to 4 wt%, Ti: not more than 0.02 wt% 0.3 wt% or less (not more than 0), Nb: not more than 0.6 wt% (excluding 0), N: not more than 0.01 wt% (excluding 0), Al: not more than 0.1 wt% (not more than 0), and the balance Fe and other unavoidable impurities And cold-rolled and annealed at 1040 to 1080 ° C within 200 seconds, wherein the cold-rolled annealing temperature / hot-rolled annealing temperature is 1.05 to 1.15.

The present invention has the advantage that the high-temperature strength is improved and the press formability is improved owing to the technical structure described above.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a graph showing a high temperature strength according to an average size of a precipitation phase (Fe ₂ W)
2 is a graph showing the results of processing V-notch impact specimens of 2.0 to 3.0 mm thickness cold rolled products according to [(Ti% + 1 / 2Nb%) / (C + N)] and the grain size A chart showing the impact energy value at 0 占 폚 impact test,
3 is a graph showing an average value of the molded anisotropy r-bar in accordance with the ratio (the average crystal grain size of the cold-rolled products to the average grain size) / (the average crystal grain size average size of the hot-

Hereinafter, a ferritic stainless steel excellent in high-temperature strength and press formability according to a preferred embodiment of the present invention will be described with reference to the accompanying drawings.

A ferritic stainless steel sheet excellent in high-temperature strength and press formability according to the present invention is characterized in that C: not more than 0.007 wt% (excluding 0), Si: not more than 0.3 wt% (excluding 0), Mn: 0.5 to 1.5 wt% Cr: 14-17 wt%, Mo: 1.0 wt% or less (W or less), W: 1 to 4 wt%, Ti: 0.3 wt% or less (excluding 0) , N: not more than 0.01 wt% (excluding 0), Al: not more than 0.1 wt% (not more than 0), and the balance Fe and other unavoidable impurities (Fe ₂ W) precipitation amount / (Nb) precipitation amount] < 1.0 in the cold-rolled and annealed cold-rolled product, .

Hereinafter, the reason for limiting the component system will be described. In the following, there is no particular mention, and the content range of the component is the weight percent (wt%).

C is added in an amount of 0.007 wt% or less.

C is an element that increases the room temperature strength of ferritic stainless steels. When the addition amount exceeds 0.007 wt%, the room temperature strength of the ferritic stainless steels increases, but the ductility, workability, toughness and the like at relatively high temperature strength and room temperature decrease . Therefore, C should be added in an amount of 0.007 wt% or less, preferably 0.005 wt% or less.

Si is added in an amount of 0.3 wt% or less.

Si is an element that acts as a deoxidizing agent in the molten steel and is an essential element in the steelmaking process. In addition, Si can favorably improve the oxidation resistance of the ferritic stainless steel. On the other hand, when Si is added in an amount exceeding 0.3 wt%, the hardness of the ferritic stainless steel is increased due to the Si solid solution strengthening phenomenon, and the elongation and workability of the ferritic stainless steel are lowered. Therefore, Si is added in an amount of 0.3 wt% or less.

Mn is added in the range of 0.5 to 1.5 wt%.

When ferritic stainless steels are used as materials for exhaust manifolds for automobiles, scales and the like can be generated at high temperatures. At this time, the generated scale can easily be eliminated, and the eliminated scale can flow into the catalytic converter (converter) to block the catalytic converter passage.

Therefore, ferritic stainless steels must have resistance to scaling, and for this purpose Mn is added. On the other hand, when the Mn content exceeds 1.5 wt%, Mn reacts with S to form MnS. Since MnS may adversely affect the corrosion resistance of ferritic stainless steels, Mn should be added in a controlled manner within the range of 0.5 to 1.5 wt%.

Cr is added in a controlled manner within the range of 14 to 17 wt%.

Cr is an alloying element that must be added to improve the corrosion resistance and oxidation resistance of ferritic stainless steels. That is, in the ferritic stainless steel, it is difficult to obtain sufficient corrosion resistance when the amount of Cr added is low. Therefore, Cr should be added by 14 wt% or more, whereas when the amount of Cr added exceeds 17 wt%, corrosion resistance of the ferritic stainless steel is improved However, since the strength is too high and the elongation and impact properties are rapidly lowered, Cr should be added within a range of 14 to 17 wt%.

Ti is added in an amount of 0.3 wt% or less.

Ti is an alloy element added to improve the high-temperature strength and intercalation corrosion resistance of ferritic stainless steels. When the amount of Ti added in the ferritic stainless steel exceeds 0.3 wt%, the steel inclusions increase, surface defects such as scabs are frequently generated, and nozzle clogging occurs during the performance, thereby lowering the process efficiency. Further, the elongation of the ferritic stainless steel and the low-temperature impact resistance are deteriorated by the increase of the solid solution Ti.

N is added to 0.01 wt% or less.

N, like C, plays a role of increasing the strength of the ferritic stainless steel, but may deteriorate ductility and workability. Particularly, in order to secure sufficient toughness and workability of the ferritic stainless steel, N is added in an amount of 0.01 wt% or less, preferably 0.007 wt% or less.

In the present invention, the amount of Mo added is adjusted to 1.0 wt% or less and the amount of W is controlled to be 1 to 4 wt%. In the case of ferritic stainless steels, various studies and efforts have been made to add Mo to improve the high temperature strength. When Mo is added in an amount of 3 wt% or more, a sigma phase of a ferritic stainless steel is generated. The sigma phase not only can cause defects in the production of ferritic stainless steels, but can also cause durability problems when used for automotive exhaust manifolds. In the ferritic stainless steel according to the present invention, the Mo content can be reduced and the generation of the sigma phase can be suppressed. Mo is preferably added in an amount of 0.8 wt% or less. Since it is not easy to control the amount of the material to be added in the steel making process of the ferritic stainless steel to a very small amount, it may be inefficient to control it. However, since the elements such as Mo are expensive raw materials, It is possible to maintain the physical properties of a predetermined ferritic stainless steel and improve the process efficiency by controlling the addition amount of Mo to 0.8 wt% or less.

When the amount of W added is less than 1 wt%, the amount of nano-sized fine precipitates such as Fe ₂ W and the amount of W in the matrix are lowered, so that it is difficult to obtain sufficient high-temperature strength and thermal fatigue characteristics of the ferritic stainless steel. When the amount of W added exceeds 4 wt%, the raw material ratio of the ferritic stainless steel may be increased, and a large amount of Fe ₂ W is generated in the ferritic stainless steel, which adversely affects the line throughput, And moldability. The ferritic stainless steel can be applied to automotive exhaust manifolds that require high strength at high temperatures with a tensile strength of 40 MPa or more in a high temperature tensile test conducted at 900 ° C by further including W.

In the present invention, Ti is added in an amount of 0.3 wt% or less, Nb in an amount of 0.6 wt% or less, N in an amount of 0.01 wt% or less, and Al in an amount of 0.01 wt% or less.

19.5? [(Ti + 1 / 2Nb) / (C + N)]? 32

In order for the ferritic stainless steel to satisfy the high temperature strength and the thermal fatigue characteristics, predetermined Ti and Nb must be added. At this time, when the amount of addition of Ti and Nb is less than a predetermined value, grain boundary corrosion occurs in the weld heat affected zone of the ferritic stainless steel, and the high temperature strength and the thermal fatigue property are deteriorated. As a result, (Ti + 1 / 2Nb) / (Ti + 1 / 2Nb) / (C + N) exceeds 32, it may be advantageous for the high temperature characteristics of the ferritic stainless steel. However, when the added amount of solid solution Nb is excessively high Room temperature elongation, toughness and workability may be deteriorated. Therefore, (Ti + 1 / 2Nb) / (C + N) should be adjusted within the range of 19.5 to 32.

In the present invention, the grain size of the cold-rolled product was determined according to ASTM No. 1. 5 or more. When the number is less than 5, there is a problem that not only an orange peel phenomenon occurs but also a press formability is lowered during processing.

When the value of [(Ti% + 1 / 2Nb%) / (C + N)] is controlled in the range of 19.5 to 32 and the crystal grain size (ASTM No. standard) is controlled to 5.0 or more, Of cold-rolled products are processed into V-notch impact specimens, and at least 10 J of the Charpy impact test is obtained at 0 ° C impact test.

The average size of the precipitated phase (Fe ₂ W) composed of Fe-W is less than 1 μm and the precipitation amount / (Nb) precipitation amount of α = [(Fe + W) Amount] < 1.0. When the average size of the precipitate phase exceeds 1 탆, the effect of strengthening micro-precipitation deteriorates, so the high-temperature strength is lowered. When Nb-C and Nb-N precipitates are large in relation to the Fe- N) precipitation phase is formed and the high-temperature strength at 900 ° C is 40 MPa or less.

In the present invention, by adjusting the ratio of (cold rolling annealing temperature) / (hot rolling annealing temperature) in the range of 1.05 to 1.15 (average grain size of cold rolled products) / (determination of center portion of hot rolled products) Particle size average size) value satisfies 0.3 to 0.5. When the value of (cold rolling annealing temperature) / (hot rolling annealing temperature) is less than 1.05, the average anisotropy of plasticity anisotropy r-bar due to hot rolled annealing decreases, and when it exceeds 1.15, the graininess of cold rolled products becomes rough, Peel phenomenon and cracking during press forming.

Hereinafter, the present invention will be described in more detail based on examples and comparative examples.

Table 1 shows the alloying components of Examples and Comparative Examples.

C Si Mn Cr Mo W Ti Nb N Al Inventory 1 0.0050 0.28 0.98 14.9 0.51 3.10 0.10 0.43 0.0050 0.06 Inventory 2 0.0043 0.13 0.98 14.9 0.74 3.80 0.09 0.44 0.0063 0.07 Inventory 3 0.0060 0.15 0.97 14.9 0.52 2.70 0.10 0.38 0.0077 0.07 Honorable 4 0.0050 0.28 0.92 14.7 0.50 3.30 0.10 0.42 0.0050 0.06 Inventory 5 0.0066 0.19 0.97 14.8 0.70 3.90 0.09 0.38 0.0076 0.05 Inventory 6 0.0050 0.21 1.01 15.1 0.76 3.50 0 0.50 0.0064 0.07 Honorable 7 0.0046 0.29 0.96 14.9 0.70 3.60 0.11 0.41 0.0068 0.08 Comparative Example 1 0.0100 0.28 0.98 16.7 0.51 3.40 0.10 0.43 0.020 0.08 Comparative Example 2 0.0200 0.13 0.98 14.2 0.74 3.50 0.10 0.44 0.0063 0.09 Comparative Example 3 0.0050 0.15 0.97 16.5 0.52 1.00 0.10 0.87 0.0077 0.07 Comparative Example 4 0.0060 0.28 0.92 15.5 0.50 0.75 0.10 0.67 0.0050 0.06 Comparative Example 5 0.0050 0.19 0.97 14.3 0.70 3.70 0.10 0.38 0.0076 0.07 Comparative Example 6 0.0060 0.21 1.01 15.5 0.76 3.60 0.10 0.50 0.0064 0.06 Comparative Example 7 0.0070 0.29 0.96 16.9 0.70 3.50 0.10 0.41 0.0068 0.05 Comparative Example 8 0.0050 0.20 1.00 15.5 0.70 3.70 0.10 0.40 0.0070 0.09

Table 2 shows various parameter measurements for the steel of the examples and comparative examples shown in Table 1.

division

Hot-rolled annealing temperature
(° C)

Cold annealing temperature
(° C) Cold grain center average crystal grain size
(ASTM No.)

Fe ₂ W
Average size
(탆)

A

(Ti% + 1 / 2Nb%) / (C + N)

Γ

Charpy impact energy (J) @ 0 ℃

High temperature strength
(MPa)
@ 900 ℃

Average R-bar

Whether orange peel phenomenon Inventory 1 980 1040 6.0 0.9 0.95 31.5 0.42 21 47 1.2 Not occurring Inventory 2 1020 1080 5.0 0.1 0.40 29.2 0.50 17 53 1.3 Not occurring Inventory 3 1000 1050 5.5 0.6 0.70 21.2 0.45 20 45 1.1 Not occurring Honorable 4 1000 1055 5.5 0.6 0.70 31.0 0.48 23 47 1.1 Not occurring Inventory 5 990 1045 6.0 0.8 0.75 19.7 0.45 22 51 1.1 Not occurring Inventory 6 980 1050 5.5 0.6 0.75 21.9 0.40 17 48 1.1 Not occurring Honorable 7 1010 1070 5.5 0.4 0.50 27.6 0.50 18 49 1.2 Not occurring Comparative Example 1 980 1045 6.0 0.9 0.75 10.5 0.25 9 42 0.7 Not occurring Comparative Example 2 1000 1055 5.5 0.7 0.70 12.1 0.28 5 39 0.8 Not occurring Comparative Example 3 980 1045 6.0 0.9 2.01 48.6 0.35 9 35 1.3 Not occurring Comparative Example 4 990 1040 6.0 0.9 1.50 34.2 0.38 8 33 1.3 Not occurring Comparative Example 5 980 1000 7.0 2.0 1.30 23.0 0.23 27 42 0.9 Not occurring Comparative Example 6 980 950 8.0 4.0 1.70 28.2 0.15 32 39 0.8 Not occurring Comparative Example 7 1010 1100 4.0 0.05 0.20 22.1 0.62 5 46 1.2 Occur Comparative Example 8 1020 1120 3.0 0.03 0.10 25.0 0.75 2 49 1.3 Occur

As shown in Table 2, in Inventive Examples 1 to 7, the average size of the precipitated phase (Fe ₂ W) was controlled to be 1 탆 or less and defined as [(Fe + W) precipitation amount / (Nb) Is less than or equal to 1.0. Further, the value of [(Ti% + 1 / 2Nb%) / (C + N)] is in the range of 19.5 to 32, the crystal grain size (ASTM No. standard) is 5.0, (Average grain size average size of the hot-rolled product)) is 0.3 to 0.5.

These Examples 1 to 7 were subjected to a high temperature tensile test according to JIS G0567 (after holding for 15 minutes at a test temperature before tensile, deformation at 0.15 mm / min before yielding, and at 4.8 mm / min after yielding) ℃ High Tensile Strength was measured at 45 MPa or more.

In addition, V-notch impact specimens were manufactured from 2.0 to 3.0 mm thick cold rolled products corresponding to Inventive Examples 1 to 7, and the impact energy at the temperature of 0 ° C was confirmed to be higher than 10 J, and JIS 13B specimen was used , The plastic anisotropy for 0 °, 45 ° and 90 ° directions after 15% stretching through a tensile test according to JIS Z 2241 (temperature at the time of testing is specified as room temperature and the rate of crosshead speed is 20 mm / min) (r) value, the average r-bar value was 1.0 or more. Also, the orange peel was not generated because the surface roughness (Rt) of the gauge portion of the specimen stretched to 15% was 20 탆 or less.

On the other hand, in Comparative Example 1 and Comparative Example 2, the contents of C and N were out of the range of the present invention, and the values of [(Ti% + 1 / 2Nb%) / (C + N)] were as low as 10.5 and 12.1, The average r-bar value of the cold rolled products did not reach 1.0 even though the energy was less than 10J and due to the high C and N contents, the hot rolled and cold rolled annealed.

In Comparative Example 3 and Comparative Example 4, although the W content was less than 1 wt% and the Nb content exceeded 0.6 wt%, the [(Fe + W) precipitation amount / (Nb) precipitation amount ], The effect of improving the high-temperature strength by Fe ₂ W precipitates was not exhibited, and the Charpy impact energy of 10 J was not reached due to a large amount of Nb content.

Although the component systems of Comparative Example 5 and Comparative Example 6 fall within the scope of the present invention, although the Fe ₂ W is to be finely precipitated after being reused at high temperature during cold rolling annealing, the cold rolling annealing temperature is low, Coarse Fe ₂ W phase remained as it was, and high-temperature strength was not sufficiently developed. In addition, sufficient recrystallization does not occur during the annealing process after cold rolling, and the {111} texture is not sufficiently developed, resulting in a low average r-bar value.

The component system of Comparative Example 7 and Comparative Example 8 belonged to the scope of the present invention, and Fe ₂ W was finely precipitated after being reused at a high temperature during the cold rolling annealing, and sufficient high temperature strength was secured. However, the annealing temperature was excessively increased, When the crystal grain size is ASTM No. 5 or less. For this reason, the surface roughness of the gauge part of the specimen, which was pulled to 15% at a tensile test according to JIS Z 2241 (at a test temperature of room temperature and at a crosshead speed of 20 mm / min as a deformation rate) using JIS 13B specimen Rt) exceeding 20 占 퐉 and orange peel phenomenon occurred upon visual inspection.

Meanwhile, FIG. 1 is a diagram showing the high temperature strength according to the average size of the precipitated phase (Fe ₂ W) of the cold rolled product. The intensity of Fe ₂ W precipitates in the form of clusters in the form of clusters of less than 1 μm in the precipitation phase of Laves type at 900 ° C is more than 45 MPa in the case of precipitation of precipitates in the form of clusters. .

2 shows the impact energy values according to the grain size (based on ASTM No.) of [(Ti% + 1 / 2Nb%) / (C + N)] and cold rolled products, V-notch impact specimen, and subjected to a 0 占 폚 impact test.

In the case of the comparative example in which (Ti% + 1 / 2Nb%) / (C + N) is less than 19.5, the Charpy impact energy at 0 ° C did not reach 10J due to the curing of the material by the large amount of solid solution C and N, (ASTM No. standard) of the cold-rolled product does not reach 5, the Charpy impact energy at 0 ° C reaches 10 J, even if the (Ti + 1 / 2Nb%) / (C + N) Can not be confirmed.

Fig. 3 shows the molded anisotropy average r-bar value according to the ratio (average crystal grain size of cold-rolled products) / (crystal grain size average size of hot-rolled products). If the cold-rolled annealing temperature is lower than 1040 ° C, the average r-bar value is lowered due to the undeveloped structure of the {111} Although the r-bar value reached 1.2, the surface roughness (Rt) of the gauge part of the specimen was more than 20 μm at the time of 15% pulling, and orange peel phenomenon occurred during visual inspection.

The crystal grain size at the level at which the orange peel is formed reduces the Charpy impact energy value as observed in Fig.

While the present invention has been particularly shown and described with reference to specific embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the following claims It will be apparent to those of ordinary skill in the art.

Claims

C: not more than 0.007 wt% (excluding 0), Si: not more than 0.3 wt% (excluding 0), Mn: 0.5 to 1.5 wt%, P: not more than 0.02 wt% Mo: less than 0 wt%, W: 1 ~ 4 wt%, Ti: not more than 0.3 wt% (not more than 0), Nb: not more than 0.6 wt% (excluding 0) ), N: not more than 0.01 wt% (excluding 0), Al: not more than 0.1 wt% (not more than 0), the balance Fe and other unavoidable impurities, and the average size of the precipitated phase (Fe ₂ W) , Wherein the ferrite-based stainless steel satisfies the following formula.

Α = [(Fe + W) precipitation amount / (Nb) precipitation amount] <1.0

C: not more than 0.007 wt% (excluding 0), Si: not more than 0.3 wt% (excluding 0), Mn: 0.5 to 1.5 wt%, P: not more than 0.02 wt% Mo: less than 0 wt%, W: 1 ~ 4 wt%, Ti: not more than 0.3 wt% (not more than 0), Nb: not more than 0.6 wt% (excluding 0) ), N: not more than 0.01 wt% (excluding 0), Al: not more than 0.1 wt% (not more than 0), and the balance Fe and other unavoidable impurities.

19.5? [(Ti + 1 / 2Nb) / (C + N)]? 32

The method according to claim 1 or 2,
And a crystal grain size of 5.0 or more (based on ASTM No.).

The method according to claim 1 or 2,
And a Charpy impact energy of 10 J or more at 0 占 폚 impact test using 2.0 to 3.0 mm V-notch impact specimen.

The method according to claim 1 or 2,
Wherein? Satisfying the following expression is 0.3 or more and 0.5 or less.
Γ = (mean grain size average size of cold rolled products) / (mean grain size average size of hot rolled products)

The method according to claim 1 or 2,
Wherein the average r-bar value for the plastic anisotropy (r) value in each direction of 0 DEG, 45 DEG and 90 DEG after 15% stretching at a crosshead speed of 20 mm / min at room temperature is at least 1.0, Ferritic stainless steel.

The method according to claim 1 or 2,
And the surface roughness (Rt) of the gage portion of the specimen stretched at 15% is 20 占 퐉 or less.

The method according to claim 1 or 2,
A ferritic stainless steel characterized by having a tensile strength of at least 45 MPa at 900 占 폚.

C: not more than 0.007 wt% (excluding 0), Si: not more than 0.3 wt% (excluding 0), Mn: 0.5 to 1.5 wt%, P: not more than 0.02 wt% Mo: less than 0 wt%, W: 1 ~ 4 wt%, Ti: not more than 0.3 wt% (not more than 0), Nb: not more than 0.6 wt% (excluding 0) ), N: not more than 0.01 wt% (excluding 0), Al: not more than 0.1 wt% (not more than 0), the balance Fe and other unavoidable impurities,
Cold rolled annealing is performed within the range of 1040 ~ 1080 ℃ within 200 seconds,
And the cold-rolled annealing temperature / hot-rolled annealing temperature is 1.05 to 1.15.