WO2020080015A1 - フェライト系ステンレス鋼板およびその製造方法 - Google Patents

フェライト系ステンレス鋼板およびその製造方法 Download PDF

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WO2020080015A1
WO2020080015A1 PCT/JP2019/036345 JP2019036345W WO2020080015A1 WO 2020080015 A1 WO2020080015 A1 WO 2020080015A1 JP 2019036345 W JP2019036345 W JP 2019036345W WO 2020080015 A1 WO2020080015 A1 WO 2020080015A1
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steel sheet
cold
carbonitrides
less
rolled
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PCT/JP2019/036345
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English (en)
French (fr)
Japanese (ja)
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正崇 吉野
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Ｊｆｅスチール株式会社
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Priority to KR1020217000769A priority Critical patent/KR102517499B1/ko
Priority to JP2020501406A priority patent/JP6881666B2/ja
Publication of WO2020080015A1 publication Critical patent/WO2020080015A1/ja

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    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0247—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
    • C21D8/0263—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment following hot rolling
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
    • C21D9/46—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/005—Ferrite

Definitions

  • the present invention relates to a ferritic stainless steel sheet having sufficient corrosion resistance and excellent formability, particularly stretch formability, and a method for producing the same.
  • SUS430 (16-18 mass% Cr) based ferritic stainless steel sheet is economical and has excellent corrosion resistance, so it has been applied to various applications such as building materials, transportation equipment, home appliances, kitchen equipment and automobile parts. , Its scope of application has expanded further in recent years.
  • Patent Document 1 Mass%, C: 0.02 to 0.06%, Si: 1.0% or less, Mn: 1.0% or less, P: 0.05% or less, S: 0.01% or less, Al: 0.005% or less, Ti: 0.005% or less, Cr: 11 to 30% or less, Ni: 0.7% or less, and N so as to satisfy 0.06 ⁇ (C + N) ⁇ 0.12 and 1 ⁇ N / C in relation to the C content, and further V It is contained so as to satisfy 1.5 ⁇ 10 ⁇ 3 ⁇ (V ⁇ N) ⁇ 1.5 ⁇ 10 ⁇ 2 in relation to the N content, and is excellent in moldability characterized by being composed of the balance Fe and inevitable impurities. Ferritic stainless steel plate. " Is disclosed.
  • Patent Document 2 Mass%, C: 0.010 to 0.045%, N: 0.01 to 0.05%, Mn: 1% or less, Cr: 13 to 20%, Al: 0.01% or less, and C and N of Cr carbonitride It contains such that the volume ratio v is 0.09% or less, further contains Si: 0.4% or less, P: 0.05% or less, S: 0.010% or less, and has a composition consisting of the balance Fe and inevitable impurities, and further contains ferrite.
  • press forming is roughly classified into four types of forming modes such as overhang forming, deep drawing forming, stretch flange forming and bending forming.
  • members whose molding mode in press molding is mainly overhang molding for example, exterior members such as exhaust hoods, outdoor hoods of round louvers used for ventilation openings, etc., and design or functionality by embossing
  • the application of ferritic stainless steel to interior panel members, etc. which is aimed at improving Therefore, it is desired to develop a ferritic stainless steel sheet having excellent stretch formability that can be processed into such a member shape.
  • Patent Document 3 % By mass, C: 0.005-0.025%, Si: 0.02-0.50%, Mn: 0.55-1.00%, P: 0.04% or less, S: 0.0. 01% or less, Al: 0.001 to 0.10%, Cr: 15.5 to 18.0%, Ni: 0.1 to 1.0%, N: 0.005 to 0.025% ,
  • the balance consists of Fe and unavoidable impurities, the breaking elongation is 28% or more, the average r value is 0.75 or more, and the minimum value of the maximum logarithmic strain of the forming limit based on FLD (forming limit diagram) is 0.
  • a ferritic stainless steel plate that is 15 or more. Was developed. As a result, compared with the ferritic stainless steel sheets disclosed in Patent Documents 1 and 2, it is possible to obtain a ferritic stainless steel sheet having significantly improved stretch formability.
  • the present invention was developed in view of the above situation, and an object of the present invention is to provide a ferritic stainless steel sheet having sufficient corrosion resistance and excellent stretch formability together with its advantageous manufacturing method.
  • sufficient corrosion resistance means that a salt spray cycle test specified in JIS H8502 is performed by salt spray (35 ° C, 5 mass% NaCl, spray time: 2 hours) ⁇ dry (60 ° C, relative humidity 40). %, Retention time: 4 hours) ⁇ wetting (50 ° C., relative humidity ⁇ 95%, retention time: 2 hours) as one cycle, and the rust area ratio on the steel plate surface (rust on the steel plate surface) (Area / total area of steel plate surface) ⁇ 100 (%)) is 25% or less.
  • excellent overhang formability means a forming limit determined based on a forming limit diagram (Forming Limit Di * gram, hereinafter also referred to as FLD) measured according to ISO12004-2: 2008. It means that the minimum value of the maximum logarithmic distortion is 0.20 or more.
  • FLD Forming Limit Di * gram
  • the inventors have made various studies in order to solve the above problems.
  • the inventors prepared various ferritic stainless steel sheets having different component compositions and manufacturing methods, and using these steel sheets, to a member including a part to be equibiaxially projecting and unequally biaxially projecting.
  • a press working test was conducted. Generally, it is considered that stretchability is higher when stretchability is higher, but in this press working test, cracks may occur even in steel sheets with high elongation at break. It was found that the superiority or inferiority of the sex is not necessarily determined only by the size of the elongation at break.
  • the inventors separately prepared a steel plate in which a crack had occurred in the previous test, performed a press working test again under the same conditions using the steel plate, and the upper die of which the crack had occurred in the previous test.
  • press working was stopped, a test piece was sampled from the steel sheet, and the metal structure thereof was observed in detail.
  • the steel plate is extracted from the mold, the cross section of the steel plate is mirror-polished, and then corrosion treatment is performed with a saturated picric acid-5 mass% hydrochloric acid aqueous solution to obtain a metal structure.
  • the inventors conducted experiments and studies focusing on the relationship between the overhang formability and the distance between Cr-based carbonitrides. As a result, it was confirmed that there is a correlation between the overhang formability and the average distance between Cr-based carbonitrides having a certain size or more. In particular, by setting the average distance between the Cr-based carbonitrides having a circle equivalent diameter of 0.05 ⁇ m or more to 3.0 ⁇ m or more, excellent overhang formability was obtained. Specifically, excellent stretch formability was obtained in which the minimum value of the maximum logarithmic strain of the forming limit determined based on the forming limit diagram (FLD) was 0.20 or more. As a result, it has been found that it becomes possible to press-mold a member such as an exhaust duct that requires particularly high stretch formability without cracking.
  • FLD forming limit diagram
  • the inventors consider the reason why excellent stretch formability can be obtained by increasing the average distance between Cr-based carbonitrides having a circle equivalent diameter of 0.05 ⁇ m or more as follows. That is, when processing a steel sheet, voids are generated at the interface between the ferrite matrix phase and the Cr-based carbonitride in the metallographic structure as the amount of strain increases. This void increases and grows with an increase in the amount of strain and / or stress concentration, and becomes a crack by connecting with other voids in the vicinity, eventually leading to the fracture of the steel sheet. In this way, the voids grow by receiving stress concentration as the amount of strain increases, and grow into microcracks by connecting with the neighboring voids.
  • the inventors further conducted studies and found that in order to set the average distance between the Cr-based carbonitrides having a circle equivalent diameter of 0.05 ⁇ m or more to 3.0 ⁇ m or more, a certain time or more in a predetermined temperature range.
  • the metal structure after hot-rolled sheet annealing is once made into a ferrite single-phase structure in which Cr-based carbonitride is precipitated, and then in cold-rolled sheet annealing after cold rolling. , (1) To slow down the heating rate from 500 ° C.
  • the solid solution of a Cr-based carbonitride in the ferrite phase is a phenomenon in which the Cr-based carbonitride decomposes into Cr, carbon and nitrogen in atomic units, and each element is contained in the ferrite phase), (2) Properly control the heating temperature and the holding time to further promote solid solution of the Cr-based carbonitride in the ferrite phase, and (3) To increase the cooling rate from the heating temperature to 500 ° C.
  • the gist of the present invention is as follows. 1. In mass%, C: 0.025 to 0.050%, Si: 0.10 to 0.40%, Mn: 0.45 to 1.00%, P: 0.04% or less, S: 0.010% or less, Cr: 16.0 to 18.0%, Al: 0.001 to 0.010%, N: 0.025 to 0.060% and Ni: 0.05 to 0.60%
  • the average distance between Cr-based carbonitrides having a circle equivalent diameter of 0.05 ⁇ m or more is 3.0 ⁇ m or more
  • a ferritic stainless steel sheet in which the minimum value of the maximum logarithmic strain at the forming limit based on the forming limit diagram is 0.20 or more.
  • a steel material having the component composition described in 1 above is hot-rolled into a hot-rolled steel sheet, and the hot-rolled steel sheet is subjected to hot-rolled sheet annealing at a heating temperature of 800 to 900 ° C. and a holding time of 1 hour or more. Then, it is cold-rolled into a cold-rolled steel sheet, and then the cold-rolled steel sheet is annealed at a heating temperature of 800 to 900 ° C.
  • the present invention it is possible to obtain a ferritic stainless steel sheet having sufficient corrosion resistance and excellent stretch formability. Further, if the ferritic stainless steel sheet of the present invention is used, a member such as an exhaust duct which is required to have particularly high stretch formability can be produced by press forming, which is extremely useful in industry.
  • No. of the embodiment It is a metallographic photograph of No. 1.
  • No. of the embodiment. 12 is a photograph of 12 metallographic structures.
  • C 0.025 to 0.050%
  • C is an element effective in promoting the generation of an austenite phase during hot rolling and suppressing the occurrence of ridging. From the viewpoint of obtaining such effects, the C content is 0.025% or more.
  • the precipitation amount of Cr-based carbonitrides during hot rolling and hot-rolled sheet annealing becomes excessively large, and the average distance between Cr-based carbonitrides is lengthened. Becomes difficult. Therefore, it is not possible to prevent cracking due to the formation and development of cracks due to void coupling during bulging, and desired bulging formability cannot be obtained. Further, the steel becomes excessively hardened and the ductility decreases. Therefore, the C content is set to the range of 0.025 to 0.050%.
  • the lower limit of the C content is preferably 0.030%, more preferably 0.035%.
  • the upper limit of the C content is preferably 0.045%.
  • Si 0.10-0.40%
  • Si is an element that acts as a deoxidizer during steel melting. From the viewpoint of obtaining such effects, the Si content is set to 0.10% or more. However, if the Si content exceeds 0.40%, the steel becomes excessively hardened and the rolling load during hot rolling increases. In addition, the ductility of the steel sheet obtained after annealing the cold rolled sheet decreases. Therefore, the Si content is set to the range of 0.10 to 0.40%.
  • the lower limit of the Si content is preferably 0.20%.
  • the upper limit of the Si content is preferably 0.30%.
  • Mn 0.45 to 1.00% Similar to C, Mn is an element that promotes the formation of an austenite phase and is effective in suppressing the occurrence of ridging. From the viewpoint of obtaining such effects, the Mn content is 0.45% or more. However, if the Mn content exceeds 1.00%, the steel becomes excessively hardened and the rolling load during hot rolling increases. In addition, the ductility of the steel sheet obtained after annealing the cold rolled sheet decreases. Therefore, the Mn content is set to the range of 0.45 to 1.00%. The lower limit of the Mn content is preferably 0.60%. The upper limit of the Mn content is preferably 0.75%, more preferably 0.70%.
  • P 0.04% or less
  • P is an element that promotes grain boundary fracture due to grain boundary segregation. Therefore, it is preferable that the P content is small, and the upper limit is 0.04%. It is preferably 0.03% or less. It is more preferably 0.01% or less.
  • the lower limit of the P content is not particularly limited, but excessive dephosphorization causes an increase in cost. Therefore, the lower limit of the P content is preferably 0.005%.
  • S 0.010% or less
  • S is an element that exists in steel as a sulfide-based inclusion such as MnS and reduces ductility, corrosion resistance and the like, and particularly when the S content exceeds 0.010%. In addition, the adverse effect is remarkable. Therefore, the S content is preferably as low as possible, and the upper limit of the S content is 0.010%. It is preferably 0.007% or less. It is more preferably 0.005% or less.
  • the lower limit of the S content is not particularly limited, but excessive S removal causes an increase in cost. Therefore, the lower limit of the S content is preferably 0.001%.
  • Cr 16.0 to 18.0% Cr is an element having the effect of forming a passivation film on the surface of the steel sheet and improving the corrosion resistance. From the viewpoint of obtaining such effects, the Cr content is 16.0% or more. However, if the Cr content exceeds 18.0%, the amount of austenite phase generated during hot rolling decreases, and the ridging resistance may decrease. Therefore, the Cr content is in the range of 16.0 to 18.0%. The upper limit of the Cr content is preferably 17.0%, more preferably 16.5%.
  • Al 0.001 to 0.010%
  • Al is an element that acts as a deoxidizing agent. From the viewpoint of obtaining such effects, the Al content is set to 0.001% or more. However, if the Al content exceeds 0.010%, the amount of Al-based inclusions such as Al 2 O 3 increases and the surface properties are likely to deteriorate. Therefore, the Al content is set to the range of 0.001 to 0.010%.
  • the lower limit of the Al content is preferably 0.002%.
  • the upper limit of the Al content is preferably 0.007%, more preferably 0.005%.
  • N 0.025 to 0.060% Similar to C and Mn, N is an element effective in promoting the generation of an austenite phase during hot rolling and suppressing the occurrence of ridging. From the viewpoint of obtaining such effects, the N content is 0.025% or more. However, if the N content exceeds 0.060%, the ductility of the steel sheet obtained after cold-rolled sheet annealing is significantly reduced. Further, the amount of precipitation of Cr-based carbonitrides during hot rolling and hot-rolled sheet annealing becomes excessively large, which makes it difficult to increase the average distance between Cr-based carbonitrides.
  • the N content is set to the range of 0.025 to 0.060%.
  • the lower limit of the N content is preferably 0.030%, more preferably 0.040%.
  • the upper limit of the N content is preferably 0.055%, more preferably 0.050%.
  • Ni 0.05-0.60%
  • Ni is an element that has the effect of promoting the generation of the austenite phase, increasing the amount of the austenite phase generated during hot rolling, and improving the ridging resistance.
  • Ni is an element effective in improving corrosion resistance. From the viewpoint of obtaining such effects, the Ni content is set to 0.05% or more. However, when the Ni content exceeds 0.60%, the steel is excessively hardened and the formability is deteriorated. Therefore, the Ni content is set to the range of 0.05 to 0.60%.
  • the lower limit of the Ni content is preferably 0.10%.
  • the upper limit of the Ni content is preferably 0.50%, more preferably 0.30%.
  • the components other than the above are Fe and inevitable impurities.
  • the metal structure of the ferritic stainless steel sheet of the present invention has a structure mainly composed of a ferrite phase, specifically, has a volume ratio of 90% or more of the ferrite phase to the whole structure, and the remaining structure other than the ferrite phase is the whole structure.
  • the volume ratio is 10% or less. Further, it may be a single ferrite phase.
  • the residual structure mainly includes the martensite phase, and does not include the volume ratio of precipitates and inclusions.
  • the volume ratio of the ferrite phase was measured at a 1/4 position of the plate thickness after preparing a test piece for observing a cross section from a stainless steel plate, mirror-polishing the same, and performing etching treatment with a saturated picric acid-5% by mass hydrochloric acid aqueous solution. After observing with an optical microscope at a magnification of 100 for arbitrary 10 fields of view, the martensite phase and the ferrite phase are distinguished from each other based on the morphology of the metal structure, the volume ratio of the ferrite phase is obtained by image processing, and the average value thereof is calculated. To ask.
  • the average distance between Cr-based carbonitrides having a circle equivalent diameter of 0.05 ⁇ m or more precipitated in the steel may be 3.0 ⁇ m or more. It is essential.
  • the upper limit is not particularly limited and is usually about 6.0 ⁇ m. It should be noted that the reason why the Cr-based carbonitride having a circle equivalent diameter of less than 0.05 ⁇ m is not targeted is that an extremely fine Cr-based carbonitride having a circle equivalent diameter of less than 0.05 ⁇ m is in contact with the ferrite phase which is the parent phase. Since the area is small, even if plastic deformation such as press working is applied, almost no voids are generated at the interface between the ferrite phase and the Cr-based carbonitride, and therefore, the formability, particularly the overhang formability, is affected. Is almost negligible.
  • Cr-based carbonitride used herein is a general term for Cr carbide and Cr nitride.
  • the Cr carbide include Cr 23 C 6
  • examples of the Cr nitride include Cr 2 N.
  • a portion of Cr in the Cr carbide and Cr nitride substituted with an element such as Fe or Mn is also included in the Cr-based carbonitride.
  • the target for Cr-based carbonitrides having a circle equivalent diameter of 0.05 ⁇ m or more is that the voids that accompany the increase in the strain amount are mainly the ferrite matrix phase and the Cr-based diameters of 0.05 ⁇ m or more of a circle equivalent diameter. This is because the distance between the Cr-based carbonitrides, which are generated at the interface of carbonitrides and have a circle equivalent diameter of 0.05 ⁇ m or more, particularly affect void coupling, and by extension, overhang formability.
  • the size of the Cr-based carbonitride is usually about 0.5 ⁇ m in equivalent circle diameter.
  • the average distance between the Cr-based carbonitrides having a circle equivalent diameter of 0.05 ⁇ m or more is measured as follows. That is, a rolled parallel section of a steel plate is mirror-polished and then etched with a picric acid saturated hydrochloric acid solution to reveal a metal structure, and one metal structure at a plate thickness 1/4 position is photographed with an optical microscope at a magnification of 500 times. . It should be noted that the fact that the precipitate captured in the metallographic photograph is a Cr carbonitride can be confirmed by performing a component analysis of the precipitate by energy dispersive X-ray spectroscopy under a scanning electron microscope.
  • the Cr peak in the element spectrum obtained from the precipitate by energy dispersive X-ray spectroscopy is higher than the Cr peak in the element spectrum obtained from the parent phase by the same method, and the precipitate
  • the precipitate In the quantitative analysis value of each element calculated from the spectral intensity ratio of each element, when the main components of the precipitate are Cr, Fe, C and N, the precipitate is judged to be a Cr-based carbonitride. be able to.
  • any Cr-based carbonitride having a circle-equivalent diameter of 0.05 ⁇ m or more hereinafter, also referred to as reference carbonitride was selected, and the distance from the reference carbonitride was closer.
  • 10 Cr-based carbonitrides (also referred to as target carbonitrides) having a circle equivalent diameter of 0.05 ⁇ m or more are selected, and the distance between the reference carbonitrides and each target carbonitride (distance between centers) is selected. Measure on a metallographic photograph. This measurement is performed 20 times by arbitrarily changing the reference carbonitrides, and the average distance between the Cr-based carbonitrides is calculated by arithmetically averaging the distances between all the measured reference carbonitrides and the target carbonitrides. Ask. The above measurement is not limited to a single ferrite grain, and may extend across grain boundaries. In addition, in order to perform a representative measurement, the reference carbonitride and the target carbonitride selected in the previous measurement should not be the reference carbonitride or the target carbonitride in another measurement. Selects locations that are sufficiently distant from each other.
  • the ferritic stainless steel sheet of the present invention has the above-described composition and is formed by setting the average distance between Cr-based carbonitrides having a circle equivalent diameter of 0.05 ⁇ m or more to 3.0 ⁇ m or more.
  • the minimum value of the maximum logarithmic strain of the forming limit determined based on the limit diagram (FLD) to 0.20 or more, preferably 0.23 or more, excellent stretch formability can be obtained.
  • the plate thickness of the ferritic stainless steel plate according to one embodiment of the present invention is not particularly limited, but is, for example, 0.8 to 2.0 mm.
  • the ferritic stainless steel sheet of the present invention is obtained by hot rolling a steel material having the above component composition into a hot rolled steel sheet, and heating the hot rolled steel sheet at a heating temperature of 800 to 900 ° C. for a holding time of 1 hour or more. After hot-rolled sheet annealing, the hot-rolled steel sheet is cold-rolled into a cold-rolled steel sheet, and the cold-rolled steel sheet is annealed at a heating temperature of 800 to 900 ° C. and a holding time of 5 to 300 seconds. In the above cold-rolled sheet annealing, the average heating rate at 500 ° C.
  • the ferritic stainless steel sheet according to one embodiment of the present invention is a ferritic stainless cold rolled annealed steel sheet.
  • molten steel having the above-described composition is melted by a known method such as a converter, an electric furnace, a vacuum melting furnace, etc., and made into a steel material (slab) by a continuous casting method or an ingot-casting method. Then, the obtained steel material is preferably heated at 1100 to 1250 ° C. for 1 to 24 hours, or a high temperature slab is directly heated, and then this steel material is subjected to hot rolling to obtain a hot rolled steel sheet. And The hot rolling conditions may be in accordance with ordinary methods. Then, the obtained hot rolled steel sheet is annealed under the following conditions.
  • ⁇ Heating temperature for hot-rolled sheet annealing 800 to 900 ° C, holding time: 1 hour or more>
  • the metallographic structure of the hot-rolled steel sheet when the winding temperature during hot rolling is high, a metallographic structure in which a ferrite phase and a ferrite phase generated by decomposition of the austenite phase generated at high temperature are layered,
  • the coiling temperature at the time of hot rolling is low, it has a metal structure in which a ferrite phase and a martensite phase generated by transformation of an austenite phase generated at a high temperature are layered.
  • the coiling temperature is not particularly limited, but when the coiling temperature is 450 ° C. to 500 ° C., the toughness of the hot-rolled steel sheet may be significantly reduced due to 475 ° C. embrittlement. Therefore, the winding temperature is preferably higher than 500 ° C or lower than 450 ° C.
  • the winding temperature is more preferably 600 ° C. or higher at which the decomposition of the austenite phase into the Cr-based carbonitride and the ferrite phase is further promoted.
  • the hot rolled steel sheet having such a metallographic structure is annealed in the temperature range of 800 to 900 ° C. for 1 hour or more, recrystallization and precipitation of Cr-based carbonitride occur in the metallographic structure.
  • a metal structure in which the Cr carbonitride is sufficiently and uniformly dispersed in the ferrite single phase structure is obtained.
  • the heating temperature of the hot-rolled sheet annealing is set to less than 800 ° C.
  • a predetermined metallographic structure cannot be obtained due to insufficient aggregation and coarsening of the Cr-based carbonitride and insufficient solid solution in the ferrite phase.
  • the recrystallization becomes insufficient, and the layered structure formed during hot rolling remains, particularly in the central portion of the plate thickness. Therefore, after cold-rolled sheet annealing, a non-uniform metallographic structure having remarkable expanded grains may be formed in the central portion of the sheet thickness, and ridging resistance may be reduced.
  • the austenite phase is regenerated during the holding of the hot-rolled sheet annealing, and the Cr-based carbonitride precipitated in the hot rolling step is solid-dissolved in the austenite phase.
  • Cr-based carbonitrides cannot be sufficiently precipitated in the metallographic structure of the steel sheet obtained after hot-rolled sheet annealing.
  • a decomposition reaction occurs in the austenite phase into a ferrite phase and a Cr-based carbonitride.
  • the metal structure after hot-rolled sheet annealing is a mixed grain of a ferrite phase and a ferrite phase generated by decomposition of the austenite phase, that is, a ferrite phase in which a large amount of Cr-based carbonitride is distributed around the ferrite phase. It becomes a structure and the distribution of Cr-based carbonitrides becomes non-uniform. Therefore, even if cold-rolled sheet annealing is performed under predetermined conditions in the subsequent steps, regions where the average distance between Cr-based carbonitrides is not sufficient are locally generated, and a predetermined overhang formability is obtained. I can't. Therefore, the heating temperature in the hot rolled sheet annealing is set in the range of 800 to 900 ° C. It is preferably in the range of 800 to 860 ° C.
  • the holding time in hot-rolled sheet annealing is set to 1 hour or more. It is preferably 3 hours or longer, more preferably 5 hours or longer. Although the upper limit of the holding time is not particularly limited, it is preferably 24 hours or less from the viewpoint of productivity.
  • the steel sheet (hot-rolled annealed steel sheet) obtained after hot-rolled sheet annealing is subjected to pickling, if necessary, and cold-rolled to obtain a cold-rolled steel sheet.
  • Cold rolling is preferably performed at a reduction rate of 50% or more from the viewpoints of extensibility, bendability, and shape correction.
  • the cold-rolled sheet annealing may be repeated twice or more as long as the cold-rolled sheet annealing conditions described later are satisfied.
  • the steel sheet obtained after annealing the hot rolled sheet may be subjected to grinding, polishing, or the like.
  • the cold rolled steel sheet thus obtained is subjected to cold rolled sheet annealing under the following conditions.
  • Cold-rolled sheet annealing is a process for recrystallizing a rolling structure formed by cold rolling and for sufficiently increasing the average distance between Cr-based carbonitrides. It is important that the average heating rate at temperature is 20 ° C./s or less. That is, when the average heating rate at 500 ° C. to the heating temperature is slowed down, the driving force for recrystallization becomes small, so the temperature at which recrystallization starts becomes high, and the dislocations or shear bands introduced by cold rolling become higher. Maintained up to.
  • Cr-based carbonitrides formed during annealing of hot-rolled sheet are aggregated and coarsened (individual Cr-based carbonitrides grow while the volume ratio remains almost constant, and Cr-based carbonitrides increase.
  • This aggregation / coarsening is rate-determined by the diffusion of Cr, which is the main constituent element of Cr-based carbonitrides.
  • high-speed diffusion of Cr occurs through the dislocations or shear bands, which promotes aggregation and coarsening of Cr-based carbonitrides.
  • the solid solution of the Cr-based carbonitride occurs in the high temperature region close to the heating temperature because the upper limit (solid solution limit) of C and N that can be dissolved in the ferrite phase increases.
  • the accelerating effect of these Cr-based carbonitrides on the agglomeration / coarsening and the solid solution in the ferrite phase increase the average distance between the Cr-based carbonitrides. That is, it is possible to increase the average distance between Cr-based carbonitrides by slowing the heating rate, specifically, controlling the average heating rate at 500 ° C. to the heating temperature to 20 ° C./s or less. .
  • the average heating rate at 500 ° C to the heating temperature exceeds 20 ° C / s, the driving force for recrystallization of the ferrite phase becomes excessively large, and the recrystallization of the ferrite phase occurs from the relatively low temperature region of the heating process.
  • the work structure such as dislocations or shear bands introduced by cold rolling is replaced by recrystallized grains in the low temperature region.
  • the effect of promoting the agglomeration / coarsening of Cr-based carbonitrides becomes insufficient and the average distance between Cr-based carbonitrides in the steel sheet obtained after cold-rolled sheet annealing becomes short, so that the desired overhang formability is obtained. Can't get Therefore, the average heating rate from 500 ° C.
  • the heating temperature is 20 ° C./s or less. It is preferably 15 ° C / s or less, more preferably 12 ° C / s or less.
  • the lower limit of the average heating rate is not particularly limited, but if the heating rate is excessively slowed, the productivity is lowered, so that it is preferably 1 ° C./s or more.
  • the heating rate can be controlled by setting the furnace temperature or the strip passing speed of the continuous annealing line. Further, the temperature range to be controlled is set to 500 ° C. or higher because recovery or recrystallization does not occur in a temperature range lower than 500 ° C.
  • the upper limit (solid solution limit) of C and N that can be dissolved in the ferrite phase increases as the temperature increases.
  • the heating temperature: 800 to 900 ° C. and the holding time: 5 to 300 seconds allow some of the Cr-based carbonitrides generated during hot-rolled sheet annealing to form a solid solution in the ferrite phase. It is possible to reduce the number density of the system carbonitrides and increase the average distance between the Cr system carbonitrides. Therefore, the heating temperature in cold-rolled sheet annealing is 800 to 900 ° C., and the holding time is 5 to 300 seconds.
  • the heating temperature in cold-rolled sheet annealing is 800 to 860 ° C.
  • the holding time is 15 seconds to 180 seconds.
  • the holding time mentioned here is the residence time in the temperature range of the heating temperature ⁇ 10 ° C.
  • the heating temperature is lower than 800 ° C.
  • the solid solution limit of C and N in the ferrite phase does not become sufficiently large, and the amount of Cr-based carbonitrides that form a solid solution in the ferrite phase decreases, resulting in a Cr-based carbonitride.
  • the distance between things becomes shorter. Therefore, the desired stretchability cannot be obtained. Further, unrecrystallized grains remain and the ductility is greatly reduced.
  • the heating temperature exceeds 900 ° C., an austenite phase is generated during the holding, and the austenite phase is transformed into the martensite phase in the subsequent cooling, and the steel sheet is significantly hardened. Further, the metal structure of the final product plate becomes a two-phase structure of a ferrite phase and a martensite phase, the plastic deformability is remarkably reduced, and the desired stretch formability cannot be obtained.
  • the holding time is less than 5 seconds, the solid solution of the Cr-based carbonitride during the holding is incomplete, the distance between the Cr-based carbonitrides becomes short, and the desired overhang formability is obtained. Can't get Furthermore, since unrecrystallized grains remain, the ductility is greatly reduced. On the other hand, if the holding time exceeds 300 seconds, the crystal grains are remarkably coarsened and the glossiness of the steel sheet is lowered, which is not preferable from the viewpoint of surface quality.
  • the average cooling rate at the heating temperature of cold-rolled sheet annealing up to 500 ° C is 10 ° C / s or more.
  • the average cooling rate is not particularly limited. However, when cooling is performed rapidly, the steel sheet may be distorted, so the average cooling rate is preferably 200 ° C./s or less.
  • the cooling method is not particularly limited, and gas jet cooling, mist cooling, roll cooling, or the like can be used.
  • gas jet cooling, mist cooling, roll cooling, or the like can be used.
  • BA annealing (bright annealing) may be performed in order to obtain higher gloss. Then, after the cold-rolled sheet is annealed, if necessary, pickling is performed to produce the ferritic stainless steel sheet.
  • Example 1 Molten steel having the components shown in Table 1 (the balance being Fe and unavoidable impurities) was melted by a converter with a capacity of 150 tons and refining using a vacuum oxygen decarburization (VOD) method, and then by continuous casting.
  • the slab had a width of 1000 mm and a thickness of 200 mm.
  • these hot-rolled steel sheets were subjected to hot-rolled sheet annealing using the box annealing method under the conditions shown in Table 2, and then the surface was subjected to shot blasting and descaling by pickling.
  • the steel sheet thus obtained was cold-rolled to a plate thickness of 1.0 mm, and then cold-rolled sheet was annealed under the conditions shown in Table 2.
  • the cooling after the holding was performed by gas jet cooling or mist cooling. After cooling, descaling treatment by pickling was performed.
  • the holding time in Table 2 is the residence time in the temperature range of heating temperature ⁇ 10 ° C.
  • the average heating rate shown in Table 2 is the average heating rate from 500 ° C. to the heating temperature. Further, the average cooling rate shown in Table 2 is the average cooling rate until the heating temperature reaches 500 ° C.
  • the metal structure was identified, the volume fraction of ferrite was measured, and the average distance between Cr-based carbonitrides having a circle equivalent diameter of 0.05 ⁇ m or more was measured.
  • the identification of the metal structure and the measurement of the volume ratio of ferrite were performed by the method described above. That is, a test piece for observing a cross section was prepared from the obtained steel plate, subjected to etching treatment with a picric acid saturated hydrochloric acid solution, and then observed with an optical microscope at a magnification of 100 for 10 fields of view at a 1 ⁇ 4 position of the plate thickness.
  • the volume ratio of the ferrite phase was obtained in each visual field by image processing, and the average value was taken as the volume ratio of the ferrite phase. The volume ratio of precipitates and inclusions is excluded.
  • the average distance between Cr-based carbonitrides having a circle equivalent diameter of 0.05 ⁇ m or more was also measured by the method described above. The results are also shown in Table 2.
  • FIG. 1 and FIG. 1 and No. 12 shows a photograph of the metal structure used for measuring the average distance between Cr-based carbonitrides.
  • the minimum value of the maximum logarithmic strain of the forming limit is obtained from the obtained forming limit diagram (FLD), and the case where the minimum value of the maximum logarithmic strain is 0.20 or more is acceptable ( ⁇ ), and the case of less than 0.20 is unacceptable. It was evaluated as passing (x).
  • No. 12 (Steel B1) and No. No. 13 (Steel B2) has appropriate production conditions, but the C content and the N content exceed the appropriate ranges, respectively, so that the precipitation amount of Cr-based carbonitrides becomes excessive and the Cr-based carbonitrides The average distance could not be sufficiently secured, and the desired stretchability could not be obtained.
  • No. 14 (Steel B3) the Si content exceeds the appropriate range, so that the steel plate is hardened and the plastic deformability is lowered, and desired stretch formability cannot be obtained.
  • the cooling rate of the cold-rolled sheet annealing is below the appropriate range, so a large amount of Cr-based carbonitrides are re-precipitated during the cooling, and a sufficient average distance between the Cr-based carbonitrides is secured.
  • the desired stretchability was not obtained. No. In No. 28, since the heating temperature of hot-rolled sheet annealing is below the proper range, Cr-based carbonitride in the hot-rolled sheet annealing does not agglomerate / coarse and the solid solution in the ferrite phase becomes insufficient, resulting in Cr-based carbonitriding. A non-uniform distribution of material occurred.
  • the ferritic stainless steel sheet of the present invention is particularly advantageous when applied to applications requiring high stretch formability during press forming, such as exterior members, kitchen appliances, and tableware.

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