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  • C21D8/0421—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing characterised by the working steps
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  • C22C38/002—Ferrous alloys, e.g. steel alloys containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60
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  • 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
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Definitions

• the present invention relates to a ferritic stainless steel excellent in formability and a method for producing the same.
• SUS430 (16-18mass% Cr) specified in Japanese Industrial Standard JIS G4305 is inexpensive and has excellent corrosion resistance, so building materials, transportation equipment, home appliances, kitchen appliances, automobile parts, etc.
• the application range has been further expanded.
• not only corrosion resistance but also sufficient formability that can be processed into a predetermined shape large elongation (hereinafter, high elongation may be referred to as ductility), average rankford
• the value (hereinafter sometimes referred to as the average r value) is large, and the absolute value of the in-plane anisotropy of the r value (hereinafter sometimes referred to as
• ) is determined.
• Patent Document 1 in 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: 11-30%, Ni: 0.7% or less, and 0.06 ⁇ (C + N) ⁇ 0.12, 1 ⁇ N / C and 1.5 ⁇ 10 ⁇ 3 ⁇ (V ⁇ N) ⁇ 1.5
• Patent Document 1 does not mention any anisotropy.
• box annealing for example, annealing at 860 ° C. for 8 hours
• box annealing takes about one week when heating and cooling processes are included, and productivity is low.
• Patent Document 2 C: 0.01 to 0.10%, Si: 0.05 to 0.50%, Mn: 0.05 to 1.00%, Ni: 0.01 to 0.50%, Cr: 10 to 20%, Mo: 0.005 to 0.50%, Cu: 0.01 to 0.50%, V: 0.001 to 0.50%, Ti: 0.001 to 0.50%, Al: 0.01 to 0.20%, Nb: 0.001 to 0.50%, N: 0.005 to 0.050%, and B: 0.00010 to 0.00500 % Hot-rolled steel, hot-rolled sheet annealing in the ferrite single-phase temperature range using a box furnace or AP line (annealing and pickling line) continuous furnace, followed by cold rolling and finish annealing A ferritic stainless steel excellent in workability and surface properties characterized by being performed is disclosed.
• AP line annealing and pickling line
• Patent Document 1 when a box furnace is used, there is a problem that productivity is low as in Patent Document 1 described above. Also, although there is no mention of elongation, when hot-rolled sheet annealing is performed in a continuous annealing furnace in the ferrite single-phase temperature range, recrystallization becomes insufficient due to the low annealing temperature, and in the ferrite single-phase temperature range. Elongation is lower than when box annealing is performed. In general, ferritic stainless steel as in Patent Document 2 has a problem that a crystal grain group (colony) having a similar crystal orientation is formed during casting or hot rolling, and
• Japanese Patent No. 3588281 (Republication WO00 / 60134)
• Japanese Patent No. 3582001 Japanese Patent Laid-Open No. 2001-3134
• An object of the present invention is to solve this problem and to provide a ferritic stainless steel having sufficient corrosion resistance and excellent formability and a method for producing the same.
• sufficient corrosion resistance refers to a salt spray cycle test ((Salt spray (35 ° C, 5 ° C (Mass% NaCl, spray 2h) ⁇ Drying (60 ° C, relative humidity 40%, 4h) ⁇ Wet (50 ° C, relative humidity ⁇ 95%, 2h))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))
• the rusting area ratio on the surface is 25% or less.
• Excellent formability means that the elongation at break (El) in a tensile test according to JIS Z2241 is 25% or more, and the following formula (1) when a strain of 15% is applied in a tensile test according to JIS Z2241
• the average rankford value calculated (hereinafter referred to as the average r value) is 0.70 or more, and the absolute value (
• r L is an r value when a tensile test is performed in a direction parallel to the rolling direction
• r D is an r value when a tensile test is performed in a direction of 45 ° with respect to the rolling direction
• r C is a direction perpendicular to the rolling direction. The r value when a tensile test is performed.
• the composition further includes one or more selected from Cu: 0.1 to 1.0%, Ni: 0.1 to 1.0%, Mo: 0.1 to 0.5%, Co: 0.01 to 0.5%
• a method for producing a ferritic stainless steel in which a hot-rolled annealed sheet is formed, followed by cold rolling, followed by cold-rolled sheet annealing in a temperature range of 800 to 950 ° C. for 5 seconds to 5 minutes.
• all% which shows the component of steel is the mass%.
• a ferritic stainless steel having sufficient corrosion resistance and excellent formability can be obtained.
• the ferritic stainless steel of the present invention is intended to be used in various applications such as building material parts, home appliance parts, kitchen appliances, and automobile parts by press working. In order to be applied to these uses, sufficient formability (elongation and average r value are large and
• the elongation characteristics are insufficient, it cannot be molded due to necking or breakage in the direction of the worst elongation during molding.
• the appearance of the product may be deteriorated due to the fact that the thickness of the overhang portion after forming varies greatly depending on the direction of the steel plate before forming.
• large pans manufactured by drawing or the like are necked or broken when the average r value is low, and cannot be formed into a predetermined product shape.
• the plate thickness of the body of the pan varies greatly depending on the location, which may cause problems in heat transfer characteristics.
• SUS430 (16 mass%), which is most produced among ferritic stainless steels, is less expensive than SUS430LX and SUS436L because it does not contain a large amount of Ti and Nb, but its formability is lower than that of SUS430LX and SUS436L. Inferior. Therefore, SUS430 having improved formability has been demanded.
• the inventors have obtained a method of obtaining ferritic stainless steel satisfying El ⁇ 25%, average r value ⁇ 0.70, and
• box annealing batch annealing
• continuous annealing as methods for annealing (hereinafter referred to as hot rolled sheet annealing) before cold rolling the ferritic stainless steel sheet after hot rolling.
• box annealing batch annealing
• continuous annealing as methods for annealing (hereinafter referred to as hot rolled sheet annealing) before cold rolling the ferritic stainless steel sheet after hot rolling.
• box annealing instead of box annealing with low productivity, it was studied to obtain a predetermined formability by continuous annealing with high productivity.
• the problem with the prior art using a continuous annealing furnace is that the hot-rolled sheet annealing is performed in the ferrite single-phase temperature range, so that sufficient recrystallization does not occur, sufficient elongation cannot be obtained, and the colony is cold-rolled.
• was large. Therefore, after performing the hot-rolled sheet annealing in the two-phase region of the ferrite phase and the austenite phase, the inventors shall perform cold rolling and cold-rolled sheet annealing by a conventional method, and finally make a ferrite single-phase structure again. Devised.
• a ⁇ -fiber texture that improves the r value develops in the metal structure of the cold-rolled annealed sheet after cold rolling and cold-rolled sheet annealing. Moreover, the colony is divided, the anisotropy of the metal structure is relaxed, and an excellent characteristic that
• the martensite phase is harder than the ferrite phase, so the ferrite phase near the martensite phase is preferentially deformed and the rolling strain is concentrated. And the recrystallization site at the time of cold-rolled sheet annealing further increases. Thereby, recrystallization at the time of cold-rolled sheet annealing is further promoted, and the anisotropy of the metal structure after the cold-rolled sheet annealing is further relaxed.
• C 0.005-0.05%
• C promotes the formation of the austenite phase and has the effect of expanding the two-phase temperature range where the ferrite phase and the austenite phase appear during hot-rolled sheet annealing. In order to acquire this effect, 0.005% or more needs to be contained. However, if the C content exceeds 0.05%, the steel sheet becomes hard and the ductility decreases. Therefore, the C content is in the range of 0.005 to 0.05%.
• the lower limit is preferably 0.01%, more preferably 0.015%.
• the upper limit is preferably 0.035%, more preferably 0.03%, and even more preferably 0.025%.
• Si 0.02-0.50% Si is an element that acts as a deoxidizer during steel melting. In order to obtain this effect, a content of 0.02% or more is necessary. However, if the Si content exceeds 0.50%, the steel sheet becomes hard and the rolling load during hot rolling increases. Moreover, the ductility after cold-rolled sheet annealing decreases. Therefore, the Si content is in the range of 0.02 to 0.50%. Preferably it is 0.10 to 0.50% of range. More preferably, it is in the range of 0.25 to 0.35%.
• Mn 0.05-1.0% Mn, like C, promotes the formation of an austenite phase and has the effect of expanding the two-phase temperature range in which the ferrite phase and austenite phase appear during hot-rolled sheet annealing. In order to acquire this effect, 0.05% or more needs to be contained. However, if the amount of Mn exceeds 1.0%, the amount of MnS produced increases and the corrosion resistance decreases. Therefore, the Mn content is in the range of 0.05 to 1.0%.
• the lower limit is preferably 0.1%, more preferably 0.2%.
• the upper limit is preferably 0.8%, more preferably 0.35%, and still more preferably 0.3%.
• P 0.04% or less
• P is an element that promotes grain boundary fracture due to grain boundary segregation, so a lower value is desirable, and the upper limit is made 0.04%.
• S 0.01% or less
• S is an element that exists as sulfide inclusions such as MnS and reduces ductility, corrosion resistance, and the like.
• the S amount it is desirable that the S amount be as low as possible.
• the upper limit of the S amount is 0.01%. Preferably it is 0.007% or less. More preferably, it is 0.005% or less.
• Cr 15.5-18.0% Cr is an element having an effect of improving the corrosion resistance by forming a passive film on the steel sheet surface. In order to obtain this effect, the Cr content needs to be 15.5% or more. However, if the Cr content exceeds 18.0%, the austenite phase is not sufficiently generated during hot-rolled sheet annealing, and predetermined material characteristics cannot be obtained. Therefore, the Cr content is in the range of 15.5 to 18.0%. Preferably it is 16.0 to 18.0% of range. More preferably, it is in the range of 16.0 to 17.25%.
• Al 0.001 to 0.10%
• Al is an element that acts as a deoxidizer. In order to acquire this effect, 0.001% or more needs to be contained. However, when the Al content exceeds 0.10%, Al-based inclusions such as Al 2 O 3 increase, and the surface properties tend to deteriorate. Therefore, the Al content is set in the range of 0.001 to 0.10%. Preferably it is 0.001 to 0.05% of range. More preferably, it is in the range of 0.001 to 0.03%.
• N 0.01-0.06% N, like C and Mn, promotes the formation of the austenite phase and has the effect of expanding the two-phase temperature range in which the ferrite phase and austenite phase appear during hot-rolled sheet annealing.
• the N content needs to be 0.01% or more.
• the N content is in the range of 0.01 to 0.06%.
• it is 0.01 to 0.05% of range. More preferably, it is in the range of 0.02 to 0.04%.
• the balance is Fe and inevitable impurities.
• Cu and Ni are elements that improve corrosion resistance. In particular, it is effective to contain it when high corrosion resistance is required. Further, Cu and Ni have an effect of promoting the formation of the austenite phase and expanding the two-phase temperature range in which the ferrite phase and the austenite phase appear during hot-rolled sheet annealing. These effects become significant when the content is 0.1% or more. However, if the Cu content exceeds 1.0%, the hot workability may decrease, which is not preferable. Therefore, when Cu is contained, the content is set to 0.1 to 1.0%. Preferably it is 0.2 to 0.8% of range.
• the content is made 0.1 to 1.0%.
• it is 0.1 to 0.6% of range. More preferably, it is in the range of 0.1 to 0.3%.
• Mo is an element that improves corrosion resistance, and it is effective to contain it particularly when high corrosion resistance is required. This effect becomes significant when the content is 0.1% or more. However, if the Mo content exceeds 0.5%, the austenite phase is not sufficiently generated during hot-rolled sheet annealing, and predetermined material characteristics cannot be obtained. Therefore, if it contains Mo, the content is made 0.1 to 0.5%. Preferably it is 0.1 to 0.3% of range.
• Co is an element that improves toughness. This effect is obtained when the content is 0.01% or more. On the other hand, if the content exceeds 0.5%, the productivity is lowered. Therefore, if Co is contained, the content is made 0.01 to 0.5%.
• V 0.01-0.25%
• Ti 0.001-0.10%
• Nb 0.001-0.10%
• Mg 0.0002-0.0050%
• B 0.0002-0.0050%
• REM 0.01-0.10%
• Ca 0.0002-0.0020%
• V 0.01-0.25%
• V combines with C and N in the steel to reduce solute C and N. This improves the average r value.
• the surface property is improved by controlling the carbonitride precipitation behavior on the hot-rolled sheet to suppress the occurrence of linear flaws caused by hot-rolling and annealing.
• the V content needs to be 0.01% or more.
• V amount exceeds 0.25%, the workability is lowered and the manufacturing cost is increased. Therefore, when V is contained, the content is made 0.01 to 0.25%. Preferably it is 0.03 to 0.20% of range. More preferably, it is 0.05 to 0.15% of range.
• Ti and Nb are elements with a high affinity with C and N, and precipitate as carbides or nitrides during hot rolling, reducing the solid solution C and N in the matrix, and cold-rolled sheet annealing There is an effect of improving the later workability.
• it is necessary to contain 0.001% or more of Ti and 0.001% or more of Nb.
• the Ti content is 0.10% or the Nb content exceeds 0.10%, good surface properties cannot be obtained due to the precipitation of excess TiN and NbC.
• the range when Ti is contained, the range is 0.001 to 0.10%, and when Nb is contained, the range is 0.001 to 0.10%.
• the amount of Ti is preferably in the range of 0.001 to 0.015%. More preferably, it is in the range of 0.003 to 0.010%.
• the amount of Nb is preferably in the range of 0.001 to 0.025%. More preferably, it is in the range of 0.005 to 0.020%.
• Mg 0.0002-0.0050%
• Mg is an element that has the effect of improving hot workability. In order to acquire this effect, 0.0002% or more needs to be contained. However, when the Mg content exceeds 0.0050%, the surface quality decreases. Therefore, when Mg is contained, the content is made 0.0002 to 0.0050%. Preferably it is 0.0005 to 0.0035% of range. More preferably, it is in the range of 0.0005 to 0.0020%.
• B 0.0002-0.0050%
• B is an effective element for preventing low temperature secondary work embrittlement. In order to acquire this effect, 0.0002% or more needs to be contained. However, when the amount of B exceeds 0.0050%, the hot workability decreases. Therefore, when B is contained, the content is made 0.0002 to 0.0050%. Preferably it is 0.0005 to 0.0035% of range. More preferably, it is in the range of 0.0005 to 0.0020%.
• REM 0.01-0.10% REM is an element that improves the oxidation resistance, and in particular has the effect of suppressing the formation of an oxide film at the weld and improving the corrosion resistance of the weld. In order to obtain this effect, a content of 0.01% or more is necessary. However, if the content exceeds 0.10%, productivity such as pickling at the time of cold-rolled sheet annealing is lowered. Moreover, since REM is an expensive element, excessive inclusion causes an increase in manufacturing cost, which is not preferable. Therefore, when REM is contained, the content is made 0.01 to 0.10%.
• Ca 0.0002-0.0020%
• Ca is an effective component for preventing nozzle clogging due to crystallization of Ti-based inclusions that are likely to occur during continuous casting. In order to acquire this effect, 0.0002% or more needs to be contained. However, when the Ca content exceeds 0.0020%, CaS is generated and the corrosion resistance is lowered. Therefore, when Ca is contained, the content is made 0.0002 to 0.0020%. Preferably it is 0.0005 to 0.0015% of range. More preferably, it is 0.0005 to 0.0010% of range.
• the ferritic stainless steel of the present invention is hot-rolled annealed by subjecting a steel slab having the above composition to hot rolling, followed by hot-rolled sheet annealing in a temperature range of 900 to 1000 ° C. for 5 seconds to 15 minutes. It is obtained by subjecting it to a sheet, followed by cold rolling, followed by cold-rolled sheet annealing in a temperature range of 800 to 950 ° C. for 5 seconds to 5 minutes.
• the molten steel having the above component composition is melted by a known method such as a converter, electric furnace, vacuum melting furnace or the like, and is made into a steel material (slab) by a continuous casting method or an ingot-bundling method.
• This slab is heated at 1100 to 1250 ° C. for 1 to 24 hours, or directly hot-rolled as cast without heating to form a hot-rolled sheet.
• the winding temperature is preferably 500 ° C. or higher and 850 ° C. or lower. If it is less than 500 ° C., recrystallization after winding is insufficient, and ductility after cold-rolled sheet annealing may be lowered, which is not preferable. When it winds up above 850 degreeC, a particle size will become large and rough skin may generate
• hot-rolled sheet annealing is performed for 5 seconds to 15 minutes at a temperature of 900 to 1000 ° C., which is a two-phase temperature range of a ferrite phase and an austenite phase.
• pickling is performed as necessary, and cold rolling and cold rolled sheet annealing are performed. Furthermore, pickling is performed as necessary to obtain a product.
• Cold rolling is preferably performed at a rolling reduction of 50% or more from the viewpoints of extensibility, bendability, press formability, and shape correction.
• cold rolling and annealing may be repeated twice or more.
• the cold rolled sheet is annealed at a temperature of 800 to 950 ° C for 5 seconds to 5 minutes in order to obtain good formability. Further, BA annealing (bright annealing) may be performed to obtain more gloss.
• grinding or polishing may be performed.
• Hot-rolled sheet annealing which is held at a temperature of 900 to 1000 ° C. for 5 seconds to 15 minutes, is an extremely important process for the present invention to obtain excellent formability.
• the hot-rolled sheet annealing temperature is less than 900 ° C., sufficient recrystallization does not occur and the ferrite single-phase region is formed, so that the effect of the present invention expressed by the two-phase region annealing cannot be obtained.
• the hot-rolled sheet annealing temperature exceeds 1000 ° C., the amount of austenite phase produced decreases.
• the amount of martensite phase generated after hot-rolled sheet annealing is reduced, and cold rolling of the metal structure containing the ferrite phase and martensite phase results in concentration of rolling strain on the ferrite phase near the martensite phase.
• the anisotropic relaxation effect of the metal structure cannot be sufficiently obtained, and the predetermined
• the annealing time is less than 5 seconds, even if annealing is performed at a predetermined temperature, generation of austenite phase and recrystallization of the ferrite phase do not occur sufficiently, so that desired formability cannot be obtained.
• the microstructure after cold-rolled sheet annealing was a ferrite phase during hot-rolled sheet annealing, so there were few carbides in the grains and on the grain boundaries, and the austenite phase during hot-rolled sheet annealing, so that the grains and grains It becomes a mixed grain structure of ferrite grains with excessive carbide on the boundary.
• hot-rolled sheet annealing is held at a temperature of 900 to 1000 ° C. for 5 seconds to 15 minutes.
• the holding is performed at a temperature of 910 to 960 ° C. for 15 seconds to 3 minutes.
• Cold-rolled sheet annealing which is held at a temperature of 800-950 ° C for 5 seconds to 5 minutes. It is a difficult process. If the cold-rolled sheet annealing temperature is less than 800 ° C., sufficient recrystallization does not occur and the predetermined ductility and average r value cannot be obtained. On the other hand, when the cold-rolled sheet annealing temperature exceeds 950 ° C, the steel component becomes hard because the martensite phase is formed after the cold-rolled sheet annealing in the steel component in which the temperature is a two-phase temperature range of the ferrite phase and the austenite phase. The predetermined ductility cannot be obtained.
• the glossiness of the steel sheet is lowered due to marked coarsening of crystal grains, which is not preferable from the viewpoint of surface quality.
• the annealing time is less than 5 seconds, even if annealing is performed at a predetermined temperature, the ferrite phase is not sufficiently recrystallized, so that the predetermined ductility and average r value cannot be obtained. If the annealing time exceeds 5 minutes, the crystal grains become extremely coarse and the glossiness of the steel sheet is lowered, which is not preferable from the viewpoint of surface quality. Therefore, cold-rolled sheet annealing is held at 800 to 950 ° C for 5 seconds to 5 minutes. The holding is preferably performed at 850 ° C. to 900 ° C. for 15 seconds to 3 minutes.
• Stainless steel having the chemical composition shown in Table 1 was melted in a 50 kg small vacuum melting furnace. These steel ingots were heated at 1150 ° C. for 1 h and then hot rolled to form 3.5 mm thick hot rolled sheets. Subsequently, these hot-rolled sheets were subjected to hot-rolled sheet annealing under the conditions shown in Table 2, and then the surfaces were descaled by shot blasting and pickling. Furthermore, after cold-rolling to a sheet thickness of 0.7 mm, after performing cold-rolled sheet annealing under the conditions shown in Table 2, descaling treatment by pickling was performed to obtain a cold-rolled pickled and annealed sheet.
• the cold roll pickling annealed plate thus obtained was evaluated as follows.
• r L , r D , and r C are r values in the L direction, the D direction, and the C direction, respectively.
• 0.70 or more was regarded as acceptable ( ⁇ ), and less than 0.70 was regarded as unacceptable (x).
• 0.20 or less was accepted ( ⁇ ), and more than 0.20 was rejected (x).
• the salt spray cycle test consists of 1 cycle of salt spray (5 mass% NaCl, 35 ° C, spray 2h) ⁇ dry (60 ° C, 4h, relative humidity 40%) ⁇ wet (50 ° C, 2h, relative humidity ⁇ 95%) As a result, 8 cycles were performed.
• No.4 corresponding to steel D and AC containing 0.4% Ni, steel F0.3 containing 0.3% Cu, steel AR containing 0.4% Cu, steel G containing 0.3% Mo and steel AI.
• the rust area ratio after the salt spray cycle test was 10% or less, and the corrosion resistance was further improved.
• the hot-rolled sheet annealing temperature becomes the ferrite single-phase temperature and becomes the austenite phase. It was not formed, and the effect of anisotropic relaxation of a predetermined metal structure obtained by cold rolling a steel sheet containing martensite was not obtained, and a predetermined
• the ferritic stainless steel obtained in the present invention is particularly suitable for press-formed products mainly composed of a drawing and applications requiring high corrosion resistance, such as building materials, transportation equipment, and automobile parts.

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