JP2009102728A - Ferritic stainless steel excellent in toughness and its manufacturing method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a ferritic stainless steel which realizes both high-temperature resistance and toughness that are required for automotive exhaust system components, and its manufacturing method. <P>SOLUTION: The ferritic stainless steel excellent in toughness contains deposits at an occupancy ratio of ≤0.4 on a crystal grain boundary, provided that the occupancy ratio of deposits on the crystal grain boundary is defined as a ratio of the length of the crystal grain boundary occupied by individual deposits to the length of the crystal grain boundary, which is determined through electron microscopic observation of crystal grains and deposits that appear on a given cross-section of a cold-rolled steel sheet and calculation by formula (1): occupancy ratio = Σ(the length of the crystal grain boundary occupied by individual deposits) / Σ(the length of the crystal grain boundary). <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention is a ferritic material having both heat resistance and formability suitable for use in a high-temperature environment such as an exhaust pipe of a car or a motorcycle, an exhaust duct of a plant, a heat exchanger, or a fuel cell. Regarding stainless steel.

  Automotive exhaust members such as exhaust manifolds, exhaust pipes, converter cases, and mufflers are required to have heat resistance to maintain their characteristics in high-temperature environments and formability to be placed in a limited space. . For such applications, there are many ferritic stainless steels containing Nb and Si, such as Type 429Nb steel (14Cr-0.9Si-0.4Nb steel), which are soft at room temperature, have excellent formability, and have relatively high high-temperature proof stress. in use.

  However, there is a problem that the high temperature proof stress is insufficient in the above-described Type 429Nb steel when the exhaust gas is heated to a high temperature due to the strengthening of automobile exhaust gas regulations. For such a problem, SUS444 steel (18Cr-2Mo-Nb steel) in which the addition amount of the alloy element is increased as compared with Type 429Nb steel is used.

  However, SUS444 steel has a problem that it has a lower workability at room temperature than Type 429Nb steel and has a tendency to cause brittle fracture during processing due to its poor toughness. As these countermeasures, there is a technique that ensures toughness by suppressing the amount of precipitates.

  For example, Patent Document 1 discloses a technique for suppressing a decrease in toughness by setting a cooling rate to 600 ° C. after heating to 2 ° C./s or more in final finish annealing. However, when the Nb content is high, the amount of precipitation increases, and even if cooling is performed at 2 ° C./s or more, precipitation may not be sufficiently suppressed and toughness may be reduced.

Patent Document 2 is a technique for improving toughness by changing the cooling rate after the final annealing according to the Nb content. However, there are cases where precipitation suppression is not sufficient only by increasing the cooling rate after the final annealing.
Japanese Patent No. 2923825 Japanese Patent Application No. 2006-18331

  There is a problem that even if the technique as described above is used, brittle fracture is not sufficiently suppressed depending on the degree of processing. SUMMARY OF THE INVENTION In view of the above circumstances, an object of the present invention is to propose a ferritic stainless steel that is compatible with high-temperature proof stress and toughness required for automobile exhaust system parts and a method for producing the same.

  The inventors have conducted intensive research on the relationship between precipitates affecting the toughness of ferritic stainless steel, and can appropriately obtain the ferritic stainless steel having excellent toughness by appropriately controlling the amount of precipitates precipitated at grain boundaries. I found out. The summary is as follows.

  In the first invention, the crystal grains and precipitates appearing on an arbitrary cut surface of the cold-rolled steel sheet are observed with an electron microscope, and the occupancy ratio of the precipitates on the crystal grain boundaries is determined on the crystal grain boundaries. It is a ferritic stainless steel excellent in toughness characterized in that the ratio between the occupied length and the grain boundary length is calculated by the formula (1) and the occupation ratio is set to 0.4 or less.

  The second invention is, by mass%, C <0.020%, Si ≦ 0.25%, Mn <2.00%, P <0.060%, S <0.008%, Cr: 12.0 % Or more, less than 20.0%, Ni <1.00%, Nb: 10 × (C + N)% or more, less than 0.80%, N <0.020%, B: 0.0005% or more, 0.0100 %, And the balance is a ferritic stainless steel having excellent toughness as set forth in the first invention, characterized in that it has a composition comprising Fe and inevitable impurities.

  The third invention is further in terms of mass%, Mo <3.00%, W <5.00%, Ti <0.5%, Zr <0.5%, Co <3%, Cu <2.00%. The ferritic stainless steel having excellent toughness according to the second aspect of the invention, containing one or more of V <0.5%.

  The fourth invention is mass%, C <0.020%, Si ≦ 0.25%, Mn <2.00%, P <0.060%, S <0.008%, Cr: 12.0 % Or more, less than 20.0%, Ni <1.00%, Nb: 10 × (C + N%) or more, less than 0.80%, N <0.020%, B: 0.0005% or more, 0.0100 % Of the total time exposed to 900 to 700 ° C. in the hot rolling, hot rolled sheet annealing and cold rolled sheet annealing processes using a steel material having a composition comprising Fe and inevitable impurities. Of ferritic stainless steel having excellent toughness, characterized in that the total cooling time from 900 ° C. to 700 ° C. after hot-rolled sheet annealing and cold-rolled sheet annealing is 20 seconds or less Is the method.

  According to the fifth aspect of the present invention, Mo <3.00%, W <5.00%, Ti <0.5%, Zr <0.5%, Co <3%, Cu <2.00%. V <0.5% of one type or two or more types. The method for producing a ferritic stainless steel excellent in toughness according to the fourth invention.

  In the sixth invention, the total of the cooling time from 700 ° C. to 500 ° C. after hot-rolled sheet annealing and cold-rolled sheet annealing of a steel material containing Cu: 0.8 to 2.0% by mass% is further provided. The method for producing a ferritic stainless steel having excellent toughness according to the fourth invention, characterized in that the time is 20 seconds or less.

  The seventh invention further includes, in mass%, Cu: 0.8 to 2.0%, and Mo <3.00%, W <5.00%, Ti <0.5%, Zr <0. Total cooling time from 700 ° C. to 500 ° C. after hot-rolled sheet annealing and cold-rolled sheet annealing of steel materials containing 5%, Co <3%, V <0.5% The method for producing a ferritic stainless steel excellent in toughness according to the fourth invention, characterized in that is made 20 seconds or less.

  According to the present invention, ferritic stainless steel excellent in toughness and high-temperature strength can be obtained by controlling the proportion of precipitates on the grain boundaries of steel. And since the ferritic stainless steel of this invention is excellent in toughness and high temperature strength, for example, a suitable member as an exhaust system member for automobiles can be obtained at a low cost, and a great effect can be brought about industrially. Further, the present invention is required to have the same characteristics as those of an automobile exhaust system member, and is also suitable as various members used mainly in a high temperature environment, and has an extremely high industrial value.

  Hereinafter, the present invention will be described in detail.

1. About Experimental Results First, basic experimental results conducted by the present inventors will be described. In addition, in this specification, all the% which shows the component of steel represents mass%.

  After various melting of 100 kg steel ingots based on 0.01% C-0.01% N-0.1% Si-15% Cr-0.5% Nb-0.0015% B steel, Under the conditions, hot rolling, hot rolled sheet annealing, pickling, cold rolling, and cold rolled sheet annealing were sequentially performed to produce a cold rolled annealed sheet having a thickness of 1.5 to 2.5 mm. The obtained cold-rolled annealed sheet was evaluated by the following method.

(A) Occupancy ratio of precipitates on grain boundaries A sample was taken from an arbitrary cut surface perpendicular to the rolling direction of the obtained cold-rolled annealed plate, and the distribution of precipitates 3 was observed with an electron microscope. FIG. 3 shows an example of observation results of precipitates on the grain boundaries by a scanning electron microscope. The field of view corresponding to the crystal grain boundary length of 3 mm was observed in detail, and the length of the crystal grain boundary 2 and the length of the precipitate 3 deposited on the crystal grain boundary were measured.

  Here, the length of the crystal grain boundary 2 is a total value (A) + (B) + (C) of all the crystal grain boundary lengths appearing in one field of view of the electron microscope, as shown in FIG. And the occupation rate of the precipitate on a crystal grain boundary was computed by Formula (1).

(B) Toughness A V-notched Charpy test piece having the V-notch direction as the sheet width direction was collected from a cold-rolled annealed plate in accordance with JISZ2242, and a Charpy test was conducted in the range of -80 to + 20 ° C. The ductile brittle fracture surface transition temperature was investigated. Here, in the present invention, “excellent toughness” means that the ductile brittle fracture surface transition temperature is less than −30 ° C. In addition, particularly excellent toughness means that the ductile brittle transition temperature is less than −60 ° C.

(C) High temperature strength Two pieces of each No. 13 B test piece specified in JISZ2201 were sampled from each cold-rolled annealed plate, and the test temperature was 900 ° C. and the strain rate was 0.3% / min in accordance with the provisions of JISG0567. The high temperature tensile test was carried out under the following conditions. The 0.2% yield strength at 900 ° C. was measured, and the average value of the two was determined.

  The Charpy test results are organized in relation to the results of investigating the occupancy rate of precipitates on grain boundaries, and are shown in FIG. FIG. 2 shows an example of the influence of the occupancy ratio of the precipitates on the grain boundaries on the toughness of the obtained cold-rolled annealed sheet.

  It can be seen that the ductile brittle wavefront transition temperature (abbreviated as DBTT in the figure) is less than −30 ° C. by controlling the occupancy ratio of the precipitates on the grain boundaries to 0.4 or less, and good toughness is obtained. Accordingly, the inventors have found that the ferritic stainless steel having excellent toughness, which is the subject of the present invention, is obtained by setting the occupancy ratio of the precipitates on the grain boundaries of the ferritic stainless steel to 0.4 or less. It was.

  Therefore, based on the above knowledge, the inventors controlled the ferritic stainless steel to the component composition and production method shown below, thereby setting the occupancy ratio of the precipitates on the grain boundaries to 0.4 or less. It is intended to provide a ferritic stainless steel excellent in the above.

2. About component composition The reason for limitation of the component composition of the ferritic stainless steel excellent in toughness according to the present invention is as follows. In addition, all component% means the mass%.

C: Less than 0.020% C is an element that increases the strength of the steel. However, if it is contained in an amount of 0.020% or more, the toughness and formability deteriorate significantly, so the content was made less than 0.020%. In consideration of moldability, the C content is preferably as low as possible, and more preferably 0.008% or less.

Si: 0.25% or less Si is an important element in the present invention. Si is an element that promotes precipitation of alloy elements, and by reducing Si, the occupancy ratio of precipitates on grain boundaries decreases. The effect is greater as Si is reduced, but if it is reduced too much, the oxidation resistance is lowered. Preferably it is 0.01% or more and less than 0.1%.

Mn: less than 2.00% Mn is an element that acts as a deoxidizer and improves the adhesion of the oxide film. However, when added excessively, coarse MnS is formed, and moldability and corrosion resistance are lowered. For this reason, in this invention, it limited to less than 2.00%. More preferably, it is less than 1.00%.

P: Less than 0.060% P is an element that lowers formability and toughness, and it is desirable to reduce it as much as possible. However, from the viewpoint of de-P cost, it is limited to less than 0.060%. More preferably, it is less than 0.030%.

S: Less than 0.008% S is an element that lowers corrosion resistance, and it is desirable to reduce it as much as possible. However, from the viewpoint of removing S cost, it is limited to less than 0.008%. More preferably, it is less than 0.005%.

Cr: 12.0% or more and less than 20.0% Cr is an element that improves corrosion resistance and oxidation resistance, and such an effect is recognized when 12.0% or more is added. However, if added in excess, the toughness is reduced, so it was limited to less than 20.0%. More preferably, it is 13.0% or more and less than 16.0%.

Ni: Less than 1.00% Ni is an element that improves toughness, but excessive addition causes an increase in raw material cost, so it was limited to less than 1.00%. More preferably, it is 0.01% or more and less than 0.80%.

Nb: 10 × (C + N)% or more and less than 0.80% Nb improves the formability and corrosion resistance of steel by fixing C and N, and increases the high temperature strength by dissolving in steel. Has an effect. Such an effect is recognized when the content is 10 × (C + N)% or more. However, excessive addition causes a reduction in toughness, so it was limited to less than 0.80%. More preferably, it is 0.20% or more and less than 0.70%.

N: Less than 0.020% N is an element that lowers the toughness and formability of steel. When 0.020% or more is contained, the toughness and formability are significantly reduced. For this reason, it is limited to less than 0.020%, more preferably less than 0.010%.

B: 0.0005% or more and less than 0.0100% B is an important element in the present invention. B is known as an element that improves workability, particularly secondary workability, and has the effect of reducing the precipitate occupancy ratio on grain boundaries as defined in this specification. Such an effect becomes remarkable when the content is 0.0005% or more. However, when 0.0100% or more is added, BN precipitates and the workability decreases, so the content is limited to less than 0.0100%. More preferably, it is 0.0005% or more and less than 0.0050%.

Mo: less than 3.00%, W: less than 5.00% Mo improves the high-temperature strength and corrosion resistance by dissolving in steel. Such an effect is recognized when 0.80% or more is added, but if it is contained 3.00% or more, the moldability is lowered and the raw material cost is increased, so the content is limited to less than 3.00%. More preferably, it is 1.00% or more and less than 2.50%. W also has the same effect as Mo, but the preferred range is different. When 5% or more is added, moldability is lowered. The preferred range is 1.00% or more and less than 3.00%.

Ti: Less than 0.5%, Zr: Less than 0.5% Ti and Zr are elements that improve the formability and have a stronger affinity for C and N than Nb, and therefore increase the solid solution amount of Nb. Such an effect becomes remarkable at 0.02% or more, but if added at 0.50% or more, coarse Ti (C, N) precipitates and deteriorates the surface properties, so it is limited to less than 0.50%. To do. More preferably, it is 0.02% or more and less than 0.40%.

Co: Less than 3% Co is an element that improves the high-temperature strength, and can be contained as necessary. Such an effect becomes remarkable when 0.50% or more is added, but if 3.00% or more is added, the steel becomes brittle, so the content is limited to less than 3.00%. More preferably, it is 0.80% or more and less than 2.00%.

Cu: Less than 2.00% Cu is an element that improves formability and corrosion resistance. Such an effect becomes remarkable when 0.05% or more is added, but it becomes brittle when excessive addition is 2.00% or more, so it is limited to less than 2.00%. More preferably, it is 0.05% or more and less than 1.5%.

  Further, Cu has an effect of improving the fatigue life over a long period of time by repeating solid solution and precipitation in a thermal cycle given to automobile exhaust system parts and the like. If less than 0.8% is added to the steel material, the effect is not likely to appear, and if added over 2.0%, embrittlement occurs, so a range of 0.8 to 2.0% is preferable.

V: Less than 0.5% V is an element that improves moldability. Such an effect becomes remarkable at 0.05% or more, but when 0.50% or more is added, coarse V (C, N) precipitates and deteriorates the surface properties, so that it is less than 0.50%. limit. More preferably, it is 0.05% or more and less than 0.40%.

3. About the manufacturing method The manufacturing method of the ferritic stainless steel excellent in toughness according to the present invention is limited to the range of the component composition described above, the hot rolled sheet, the annealing condition of the cold rolled sheet, the cold rolling reduction ratio, the heat Except for controlling the temperature range in the intermediate finish rolling, there is no particular limitation, and all known methods are generally applicable (hereinafter, the devices used in each manufacturing process are not shown because they are known).

  For example, in the steelmaking process, molten steel adjusted to the above-described proper composition range in a converter, electric furnace or the like is melted and subjected to secondary refining by strong stirring and vacuum oxygen decarburization treatment (referred to as SS-VOD method), It is preferable to melt molten steel. The casting method is preferably continuous casting in terms of productivity and quality. The rolling process of the steel slab (slab etc.) obtained by casting is good to re-heat if necessary, hot-roll, hot-rolled sheet annealing at 900-1100 ° C, and then pickled. The hot-rolled sheet that has been pickled is preferably made into a cold-rolled annealed sheet through the steps of cold rolling, finish annealing, and pickling. The following are preferred conditions for carrying out the present invention.

  (A) In the hot rolling, hot-rolled sheet annealing and cold-rolled sheet annealing steps after extraction from the heating furnace, the total time exposed to 900 to 700 ° C. is 150 seconds or less, and from 900 ° C. after annealing to 700 ° C. It is an essential matter of the present invention that the total cooling time is 20 seconds or less.

  In the present invention, it is important to reduce as much as possible the time during which the Nb-containing intermetallic compound and the like are easily deposited, and the exposure time is between 900 to 700 ° C. In particular, it is important to shorten the cooling time in the cooling process, but it is also important to shorten the temperature raising time in the annealing process.

  In addition, although the detailed cause is under study, the precipitation site and the precipitation amount are easily affected by the previous process, so the occupancy ratio of the precipitates on the grain boundaries in the cold-rolled annealed sheet is 0.4 or less. In order to make it within the range, it is necessary to control the total time of the temperature range in which precipitates are likely to precipitate.

  FIG. 1 shows the total time (seconds) of exposure to 900 to 700 ° C. and 900 to 700 ° C. after hot rolling and cold rolling annealing in hot rolling, hot rolling annealing and cold rolling annealing. The relationship between the total cooling time to ° C. (seconds) and the occupancy ratio of precipitates on grain boundaries is shown. Note that the cold rolling reduction ratio was 60%. According to this result, in order to reduce the occupancy ratio of the precipitates on the grain boundaries to 0.4 or less, it is exposed to 900 to 700 ° C. in the hot rolling, hot rolled sheet annealing and cold rolled sheet annealing processes. It can be seen that the total time needs to be 150 seconds or less and the total cooling time from 900 ° C. to 700 ° C. after hot-rolled sheet annealing and cold-rolled sheet annealing needs to be 20 seconds or less.

  Moreover, when containing Cu 0.8 to 2.0%, the total of the cooling time from 700 degreeC to 500 degreeC after hot-rolled sheet annealing and cold-rolled sheet annealing shall be 20 seconds or less, and a cold-rolled sheet of It becomes possible to make the toughness more excellent. The smaller the time maintained at 700 ° C. or lower, the better.

(B) About cold rolling If a too large strain is introduced in the cold rolling, the precipitation amount in the final annealing increases, or a too high rolling reduction increases the anisotropy of the steel sheet. If the rolling reduction is low, a sufficient recrystallized structure cannot be obtained. Therefore, a preferable cold rolling reduction is 40 to 75%.

(C) About annealing conditions In hot-rolled sheet annealing and cold-rolled sheet annealing, if the time exposed to high temperature is long, the crystal grains become coarse and the toughness may be reduced. Therefore, the shorter the time of exposure to 1000 ° C. or higher, the better, preferably 300 seconds or less.

  Various ferritic stainless steels having the component compositions shown in Tables 1 and 2 were melted. The obtained steel ingot was heated to 1200 ° C., and then hot-rolled to a hot-rolled sheet having a thickness of 5 to 6 mm, and subjected to hot-rolled sheet annealing and pickling treatment at 900 to 1200 ° C. Next, cold rolling was performed to obtain a cold-rolled sheet having a thickness of 1.5 to 2.5 mm, and finish annealing was performed at 900 ° C to 1200 ° C.

  A test sample was collected from the ferritic stainless steel cold-rolled annealed plate thus obtained, and a Charpy test, a high-temperature tensile test, and an electron microscope observation were performed based on the evaluation methods described below.

(A) Toughness evaluation In accordance with JISZ2242, the V-notch Charpy test piece with the V-notch direction as the plate width direction is collected from the cold-rolled annealed plate, and the Charpy test is performed in the range of -80 to + 20 ° C. The ductile brittle fracture surface transition temperature was determined. The ductile brittle wavefront transition temperature was determined to be less than −30 ° C. within the scope of the present invention.

(B) High-temperature strength evaluation Two pieces of each No. 13 B test piece specified in JISZ2201 were sampled from the cold-rolled annealed plate, and the test temperature was 900 ° C and the strain rate was 0.3% / in accordance with the provisions of JISG0567. A high temperature tensile test was performed under the condition of min. The 0.2% yield strength at 900 ° C. was measured, and the average value of the two was determined. Regarding the high-temperature strength, a strength of 25 MPa or more was judged as the range of the present invention.

(C) Evaluation of Occupancy Ratio of Precipitates on Grain Boundary A sample was taken from an arbitrary cut surface perpendicular to the rolling direction of the cold-rolled annealed plate, and the precipitate distribution was observed with an electron microscope. Similar to the experimental results shown in FIG. 3, the field of view corresponding to the grain boundary length of 3 mm was observed in detail, and the length of the crystal grain boundary and the length of precipitates deposited on the grain boundary were measured. .

  The length of the crystal grain boundary was represented by the total value (A) + (B) + (C) of all the grain boundary lengths appearing in one field of view of the electron microscope, as in the case shown in FIG. And the occupation rate of the precipitate on a crystal grain boundary was computed by Formula (1). The calculated value is 0.4 or less as the range of the present invention.

Table 1 shows the production conditions, the ductile brittle fracture surface transition temperature of the obtained steel sheet, the occupancy ratio of precipitates on the grain boundaries, and the high-temperature strength at 900 ° C.
In the classification categories in Tables 1 and 2, HR + HA + CA is the time (seconds) exposed to 900 to 700 ° C. in hot rolling, hot rolled sheet annealing, and cold rolled sheet annealing. CA ↓ is the cooling time from 900 ° C to 700 ° C (seconds) and 700 ° C to 500 ° C (seconds) in hot-rolled sheet annealing and cold-rolled sheet annealing, and HA + CA is hot-rolled sheet annealing. In addition, the time (seconds) exposed to 1000 ° C. or more in the cold-rolled sheet annealing, Σ / Σ means the occupancy ratio of precipitates on the grain boundaries, and DBTT means the ductile brittle fracture surface transition temperature.

  Time to be exposed to 900-700 ° C in hot rolling, hot rolled sheet annealing and cold rolled sheet annealing, cooling time from 900 ° C to 700 ° C in hot rolled sheet annealing and cold rolled sheet annealing, exposed to 1000 ° C or more Steel types A to Q, which are examples of the invention in which the time and cold rolling reduction ratio are within the scope of the present invention, have an occupancy ratio of precipitates on grain boundaries of 0.4 or less and a ductile brittle wavefront transition temperature of less than −30 ° C. The high temperature strength at 900 ° C. was 25 MPa or more.

  On the other hand, steel types V to Z, which are comparative examples, are insufficient in toughness and high-temperature strength because the component composition and / or production conditions are outside the scope of the invention.

  Since the ferritic stainless steel of the present invention is excellent in toughness and high temperature strength, a member suitable as an exhaust system member for automobiles can be obtained. Moreover, the same characteristic as the exhaust system member for motor vehicles is requested | required, and it can apply also to various members which are mainly used in a high temperature environment.

It is a figure explaining the relationship of the total of the time exposed to 900-700 degreeC after hot rolling, the total of the cooling time from 900 degreeC to 700 degreeC, and the occupation rate of the precipitate on a grain boundary. It is a figure explaining the relationship between the occupation rate of the precipitate on a crystal grain boundary, and toughness (ductile brittle fracture surface transition temperature). It is a figure explaining the example of observation of the precipitate on a crystal grain boundary.

Explanation of symbols

1 Crystal grain 2 Grain boundary 3 Precipitate

Claims (7)

The crystal grains and precipitates appearing on an arbitrary cut surface of the cold-rolled steel sheet are observed with an electron microscope, and the occupancy ratio of the precipitates on the crystal grain boundaries is determined by the length of each precipitate on the crystal grain boundaries and the crystal grain boundaries. A ferritic stainless steel excellent in toughness, characterized by being calculated by the formula (1) as a ratio to the length and having an occupation ratio of 0.4 or less.

  % By mass, C <0.020%, Si ≦ 0.25%, Mn <2.00%, P <0.060%, S <0.008%, Cr: 12.0% or more, 20.0 %, Ni <1.00%, Nb: 10 × (C + N)% or more, less than 0.80%, N <0.020%, B: 0.0005% or more, less than 0.0100%, the balance The ferritic stainless steel having excellent toughness according to claim 1, which has a composition comprising Fe and inevitable impurities.   Further, by mass%, Mo <3.00%, W <5.00%, Ti <0.5%, Zr <0.5%, Co <3%, Cu <2.00%, V <0. The ferritic stainless steel having excellent toughness according to claim 2, wherein the ferritic stainless steel contains 5% or more.   % By mass, C <0.020%, Si ≦ 0.25%, Mn <2.00%, P <0.060%, S <0.008%, Cr: 12.0% or more, 20.0 %, Ni <1.00%, Nb: 10 × (C + N%) or more, less than 0.80%, N <0.020%, B: 0.0005% or more, less than 0.0100%, the balance In a hot rolling, hot-rolled sheet annealing and cold-rolled sheet annealing process using a steel material having a composition consisting of Fe and inevitable impurities, the total time exposed to 900 to 700 ° C. is 150 seconds or less, and A method for producing ferritic stainless steel having excellent toughness, characterized in that the total cooling time from 900 ° C. to 700 ° C. after hot-rolled sheet annealing and cold-rolled sheet annealing is 20 seconds or less.   Further, by mass%, Mo <3.00%, W <5.00%, Ti <0.5%, Zr <0.5%, Co <3%, Cu <2.00%, V <0. The method for producing ferritic stainless steel having excellent toughness according to claim 4, comprising 5% of one kind or two or more kinds.   Furthermore, the total cooling time from 700 ° C. to 500 ° C. after hot-rolled sheet annealing and cold-rolled sheet annealing of a steel material containing Cu: 0.8 to 2.0% by mass% should be 20 seconds or less. The method for producing a ferritic stainless steel excellent in toughness according to claim 4.   Further, in mass%, Cu: 0.8 to 2.0%, and Mo <3.00%, W <5.00%, Ti <0.5%, Zr <0.5%, Co < The total cooling time from 700 ° C. to 500 ° C. after hot-rolled sheet annealing and cold-rolled sheet annealing of steel materials containing 3%, V <0.5%, or two or more is 20 seconds or less. The method for producing a ferritic stainless steel excellent in toughness according to claim 4.

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