EP1498507B1 - Kaltgewalztes Stahlblech und Zinkblech mit Reckalterungseigenschaften und Verfahren zur dessen Herstellung - Google Patents

Kaltgewalztes Stahlblech und Zinkblech mit Reckalterungseigenschaften und Verfahren zur dessen Herstellung Download PDF

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EP1498507B1
EP1498507B1 EP04023101A EP04023101A EP1498507B1 EP 1498507 B1 EP1498507 B1 EP 1498507B1 EP 04023101 A EP04023101 A EP 04023101A EP 04023101 A EP04023101 A EP 04023101A EP 1498507 B1 EP1498507 B1 EP 1498507B1
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steel sheet
cold
rolled
hot
temperature
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EP1498507A1 (de
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Chikara c/o Tech. Res. Lab. Kawasaki St. C Kami
Akio c/o Chiba Works Kawasaki St. Cor. Tosaka
Takuya c/o Tech. Res. Lab. Kawasaki Yamazaki
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JFE Steel Corp
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JFE Steel Corp
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Priority claimed from JP2000156274A external-priority patent/JP4524859B2/ja
Priority claimed from JP2000335803A external-priority patent/JP4665302B2/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
    • C21D8/04—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
    • 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
    • C21D8/0426—Hot rolling
    • 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
    • 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
    • C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
    • C21D8/0236—Cold 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
    • 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
    • C21D8/0273—Final recrystallisation annealing
    • 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
    • C21D9/48—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals deep-drawing sheets
    • 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
    • C22C38/001—Ferrous alloys, e.g. steel alloys containing N
    • 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
    • C22C38/004—Very low carbon steels, i.e. having a carbon content of less than 0,01%
    • 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
    • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
    • 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
    • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
    • 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
    • C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
    • 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
    • C22C38/12—Ferrous alloys, e.g. steel alloys containing tungsten, tantalum, molybdenum, vanadium, or niobium
    • 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
    • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
    • C21D1/18—Hardening; Quenching with or without subsequent tempering
    • C21D1/185—Hardening; Quenching with or without subsequent tempering from an intercritical temperature
    • 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
    • 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
    • C21D2211/008—Martensite
    • 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
    • C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
    • C21D8/0226—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
    • 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
    • C21D8/04—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
    • 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
    • C21D8/0436—Cold 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
    • 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
    • C21D8/04—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
    • C21D8/0447—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 heat treatment
    • C21D8/0473—Final recrystallisation annealing

Definitions

  • the present invention relates to a cold-rolled steel sheet, an electro-galvanized steel sheet, a hot-dip galvanized steel sheet, and an alloyed hot-dip galvanized steel sheet, which are suitable as raw material steel sheets for molded products such as building members, mechanical structural parts, automobile structural parts, etc., which are used at positions required to have structural strength, particularly, strength and/or stiffness in deformation, and which are subjected to heat treatment for increasing strength after processing such as pressing or the like, and a method of producing these steel sheets.
  • a process of coating and baking at lower than 200°C is used as a method in which a material having low deformation stress before press forming to facilitate press forming, and then hardened after press forming to increase the strength of a part.
  • a steel sheet for such coating and baking a BH steel sheet has been developed.
  • Japanese Unexamined Patent Application Publication No. 55-141526 discloses a method in which Nb is added according to the contents of C, N and Al of steel, Nb/(dissolved C + dissolved N) by at% is limited in a specified range, and the cooling rate after annealing is controlled to adjust dissolved C and dissolved N in a steel sheet.
  • Japanese Examined Patent Application Publication No. 61-45689 discloses a method in which baking hardenability is improved by adding Ti and Nb.
  • Japanese Unexamined Patent Application Publication No. 5-25549 discloses a method in which baking hardenability is improved by adding W, Cr and Mo to steel singly or in a combination.
  • Japanese Unexamined Patent Application Publication No. 10-310847 discloses an alloying ho-dip galvanized steel sheet having tensile strength increased by 60 MPa or more by heat treatment in the temperature region of 200 to 450°C.
  • This steel sheet contains, by mass%, 0.01 to 0.08% of C, and 0.01 to 3.0% of Mn, and at least one of W, Cr, and Mo in a total of 0.05 to 3.0%, and further contains at least one of 0.005 to 0.1% of Ti, 0.005 to 0.1% of Nb and 0.005 to 0.1% of V according to demand, and the microstructure of the steel is composed of ferrite or mainly composed of ferrite.
  • this technique comprises forming a fine carbide in the steel sheet by heat treatment after forming to effectively propagate a dislocation of stress applied during pressing, thereby increasing the amount of strain. Therefore, heat treatment must be performed in the temperature range of 220 to 370°C. There is thus the problem of a necessary heat treatment temperature higher than general bake-hardening temperatures.
  • An automobile part using a high-tensile-strength thin steel sheet must exhibit a sufficient property according to its function.
  • the property depends upon the part, and examples of the property include dent resistance, static strength against deformation such as bending, twisting, or the like, fatigue resistance, impact resistance, etc.
  • the high-tensile-strength steel sheet used for an automobile part is required to be excellent in such a property after forming.
  • the properties are related to the strength of a steel sheet after forming, and thus the lower limit of strength of the high-tensile-strength steel sheet used must be set for achieving thinning.
  • a steel sheet is press-molded. If the steel sheet has excessively high strength in press forming, the steel sheet causes the following problems: (1) deteriorating shape fixability; (2) deteriorating ductility to cause cracking, necking, or the like during forming; and (3) deteriorating dent resistance (resistance to a dent produced by a local compressive load) when the sheet thickness is decreased. These problems thus inhibit the extension of application of the high-tensile-strength steel sheet to automobile bodies.
  • a steel sheet composed of ultra-low-carbon steel is known as a raw material, for example, for a cold-rolled steel sheet for an external sheet panel, in which the content of C finally remaining in a solid solution state is controlled to an appropriate range.
  • This type of steel sheet is kept soft during press forming to ensure shape fixability and ductility, and its yield stress is increased by utilizing the strain aging phenomenon which occurs in the step of coating and baking at 170°C for about 20 minutes after press forming, to ensure dent resistance.
  • This steel sheet is soft during press forming because C is dissolved in steel, while dissolved C is fixed to a dislocation introduced in press forming in the coating and baking step after press forming to increase the yield stress.
  • Japanese Unexamined Patent Application Publication No. 60-52528 discloses a method of producing a high-strength steel thin sheet having good ductility and spot weldability, in which steel containing 0.02 to 0.15% of C, 0.8 to 3.5% of Mn, 0.02 to 0.15% of P, 0.10% or less of Al, and 0.005 to 0.025% of N is hot-rolled by coiling at a temperature of 550°C or less, cold-rolled, and then annealed by controlled cooling and heat treatment.
  • a steel sheet produced by the technique disclosed in Japanese Unexamined Patent Application Publication No. 60-52528 has a mixed structure comprising a low-temperature transformation product phase mainly composed of ferrite and martensite, and having excellent ductility, and high strength is achieved by utilizing strain aging due to positively added N during coating baking.
  • Japanese Examined Patent Application Publication No. 5-24979 discloses a high-tensile-strength cold-rolled steel thin sheet having baking hardenability which has a composition comprising 0.08 to 0.20% of C, 1.5 to 3.5% of Mn, and the balance composed of Fe and inevitable impurities, and a structure composed of homogeneous bainite containing 5% or less of ferrite, or bainite partially containing martensite.
  • 5-24979 is produced by quenching in the temperature range of 200 to 400°C in the cooling process after continuous annealing, and then slowly cooling to obtain a structure mainly composed of bainite and having a large amount of bake-hardening which is not obtained by a conventional method.
  • Japanese Examined Patent Application Publication No. 61-12008 discloses a method of producing a high-tensile-strength steel sheet having a high r value.
  • This method is characterized by annealing ultra-low-C steel used as a raw material in a ferrite-austenite coexistence region after cold rolling.
  • the resultant steel sheet has a high r value and a high degree of baking hardenability (BH property), but the obtained BH amount is about 60 MPa at most.
  • the yield point of the steel sheet is increased after strain aging, but TS is not increased, thereby causing the problem of limiting application to parts.
  • the above-described steel sheet exhibits excellent strength after coating and baking in a simple tensile test, but produces large variations in strength during plastic deformation under actual pressing conditions. Therefore, it cannot be said that the steel sheet is sufficiently applied to parts required to have reliability.
  • Japanese Examined Patent Application Publication No. 8-23048 discloses a method of producing a hot-rolled steel sheet which is soft during processing, and has tensile strength increased by coating and baking after processing to be effective to improve fatigue resistance.
  • steel contains 0.02 to 0.13 mass % of C, and 0.0080 to 0.0250 mass % of N, and the finisher deliver temperature and the coiling temperature are controlled to leave a large amount of dissolved N in the steel, thereby forming a composite structure as a metal structure mainly composed of ferrite and martensite. Therefore, an increase of 100 MPa or more in tensile strength is achieved at the heat treatment temperature of 170°C after forming.
  • Japanese Unexamined Patent Application Publication No. 10-183301 discloses a hot-rolled steel sheet having excellent baking hardenability and natural aging resistance, in which the C and N contents are limited to 0.01 to 0.12 mass % and 0.0001 to 0.01 mass %, respectively, and the average crystal grain diameter is controlled to 8 ⁇ m or less to ensure a BH amount of as high as 80 MPa or more, and suppress the AI amount to 45 MPa or less.
  • this steel sheet is a hot-rolled sheet, and is thus difficult to obtain a high r value because the ferrite aggregation texture is made random due to auste141-ferrite transformation. Therefore, the steel, sheet cannot be said to have sufficient deep drawability.
  • the hot-rolled steel sheet obtained by this technique is used as a starting material for cold rolling and recrystallization annealing, the increase in tensile strength obtained after forming and heat treatment is not always equivalent to a hot-rolled steel sheet, and a BH amount of as high as 80 MPa or more cannot be always obtained.
  • the microstructure of the cold -rolled steel becomes different from that of hot-rolled one due to cold rolling and recrystallization annealing, and strain greatly accumulates during cold rolling to easily form a carbide, a nitride or a carbonitride, thereby changing the states of dissolved C and dissolved N.
  • an object of the present invention is to provide a cold-rolled steel sheet and a hot-dip galvanized steel sheet (including an alloyed steel sheet) for deep drawing, which have excellent deep drawability, TS x r value ⁇ 750 MPa, and excellent strain aging hardenability (BH ⁇ 80 MPa and ⁇ TS ⁇ 40 MPa), and an effective method of producing these steel sheets.
  • the inventors produced various steel sheets having different compositions under various production conditions, and experimentally evaluated various material properties. As a result, it was found that both moldability and hardenability after forming can be improved by using as a strengthening element N, which has not be positively used before in a field requiring high processability, and effectively using the great strain age hardening phenomenon manifested by the action of the strengthening element.
  • the strain age hardening phenomenon due to N in order to advantageously use the strain age hardening phenomenon due to N, the strain age hardening phenomenon due to N must be advantageously combined with a condition for coating and baking an automobile, or further positively combined with a heat treatment condition after forming. It was thus found to be effective to appropriately control the hot rolling condition, the cold rolling and the cold rolling annealing condition to control the microstructure of a steel sheet and the amount of dissolved N in certain ranges. It was.also found that in order to stably manifest the strain age hardening phenomenon due to N, it is important to control the Al content of the composition according to the N content.
  • a steel sheet of the present invention exhibits higher strength after coating and baking in a simple tensile test, as compared with a conventional steel sheet, and exhibits small variations in strength in plastic deformation under actual pressing conditions and stable part strength, thereby enabling application to parts required to have reliability.
  • a portion where large strain is applied to decrease the thickness has higher hardenability than other portions, and is considered homogeneous when being evaluated based on a surcharge load ability of (thickness) x (strength), thereby stabilizing strength as a part.
  • the present invention has been achieved based on the above findings.
  • the findings were obtained from the experiment described below.
  • a sheet bar (thickness: 30 mm) having a composition containing, by mass %, 0.0015% of C, 0.0010% of B, 0.015 of Si, 0.5% of Mn, 0.03% of P, 0.08% of S and 0.011% of N, 0.005 to 0.05% of Nb and 0.005 to 0.03% of Al, and the balance composed of Fe and inevitable impurities was uniformly heated at 1150°C, hot-rolled by three passes so that the temperature of the final pass was 900°C higher than the Ar 3 transformation point, and then cooled with water for 0.1 second. Then, the sheet bar was subjected to heat treatment corresponding to coiling at 500°C for 1 hour.
  • the thus-obtained hot-rolled sheet having a thickness of 4 mm was cold-rolled with a rolling reduction ratio of 82,5%, recrystallized and annealed at 800°C for 40 seconds, and then temper-rolled with a rolling reduction ratio of 0.8%. Then, a tensile test specimen of JIS No. 5 was obtained from the resultant cold-rolled sheet in the rolling direction, and tensile strength was measured with a strain rate of 0.02/s by using a general tensile testing machine. Also, tensile strain of 10% was applied to a tensile test specimen of JIS No.
  • Fig. 1 shows the results of measurement of the relation between the steel compositions (N% - 14/93•Nb% - 14/27•Al% - 14/11•B%) and ⁇ TS.
  • ⁇ TS becomes 60 MPa or more when the value of (N% - 14/93•Nb% - 14/27•Al% - 14/11•B%) satisfies 0.0015 mass %.
  • a sheet bar (thickness: 30 mm) having a composition containing, by mass %, 0.0010% of C, 0.02 of Si, 0.6% of Mn, 0.01% of P, 0.009% of S and 0.012% of N, 0.01% of Al, 0.015% of Nb, 0.00005 to 0.0025% of B, and the balance composed of Fe and inevitable impurities was uniformly heated at 1100°C, hot-rolled by three passes so that the temperature of the final pass was 920°C higher than the Ar 3 transformation point, and then cooled with water for 0.1 second. Then, the sheet bar was subjected to heat treatment corresponding to coiling at 450°C for 1 hour.
  • the thus-obtained hot-rolled sheet having a thickness of 4 mm was cold-rolled with a rolling reduction ratio of 82,5%, recrystallized and annealed at 820°C for 40 seconds, and then temper-rolled with a rolling reduction ratio of 0.8%. Then, a tensile test specimen of JIS No. 5 was obtained from the resultant cold-rolled sheet in the rolling direction, and tensile strength was measured with a strain rate of 0.02/s by using a general tensile testing machine. Also, tensile strain of 10% was applied to a tensile test specimen of JIS No. 5 separately obtained from the cold-rolled sheet in the rolling direction, and then the specimen was subjected to a normal tensile test after heat treatment at 120°C for 20 minutes.
  • Fig. 2 shows the results of measurement of the relation between the B content of steel and ⁇ TS. This figure indicates that with a B content of 0.0005 to 0.0015 mass %, a high ⁇ TS of 60 MPa or more can be obtained.
  • steel B having a composition containing, by mass %, 0.010% of C, 0.0012% of N, 0.0010% of B, 0.01% of Si, 0.5% of Mn, 0.03% of P, 0.008% of S, 0.014% of Nb, 0.01% of Al, and the balance composed of Fe and inevitable impurities was uniformly heated at 1150°C, hot-rolled by three passes so that the temperature of the final pass was 910°C higher than the Ar 3 transformation point, and then cooled with a gas for 0.1 second. Then, each of the sheet bars was subjected to heat treatment corresponding to coiling at 600°C for
  • Each of the thus-obtained hot-rolled sheets having a thickness of 4 mm was cold-rolled with a rolling reduction ratio of 82,5%, recrystallized and annealed at 880°C for 40 seconds, and then temper-rolled with a rolling reduction ratio of 0.8%.
  • a tensile test specimen of JIS No. 5 was obtained from each of the resultant cold-rolled sheets in the rolling direction, and tensile strength was measured with a strain rate of 0.02/s by using a general tensile testing machine. Also, tensile strain of 10% was applied to a tensile test specimen of JIS No. 5 separately obtained from each of the cold-rolled sheets in the rolling direction, and then the specimen was subjected to a normal tensile test after heat treatment at various temperatures for 20 minutes.
  • Fig. 3 shows the results of measurement of the influence of the heat treatment temperature after forming on ⁇ TS. This figure indicates that in the relatively low temperature region of heat treatment temperatures of 200°C or less after forming, the ultra-low carbon steel A having a high N content exhibits higher ⁇ TS than the semi-ultra low carbon steel B having a low N content, and while in the high temperature region, both steel materials exhibit substantially the same ⁇ TS. There experimental results reveal that in order to ensure ⁇ TS in the low temperature region, it is effective to use dissolved N.
  • Fig. 4 shows the results of measurement of the influences of the crystal grain diameter d and steel compositions (N% - 14/93•Nb% - 14/27•A1% - 14/11•B%) on a decrease ( ⁇ El) in elongation by natural aging and an increase in tensile strength ( ⁇ TS) after forming.
  • the decrease ( ⁇ El) in elongation was evaluated by the difference between the total elongation measured with the test specimen of JIS NO. 5 obtained from each of the cold-rolled sheets in the rolling direction, and the total elongation measured with the separately obtained test specimen after holding at 100°C for 8 hours for accelerating natural aging.
  • Fig. 4 indicates that when the value of (N% - 14/93•Nb% - 14/27•Al% - 14/11•B%) is 0.0015 mass % or more, and the crystal grain diameter d is 20 ⁇ m or less, both high ⁇ TS and low ⁇ El can be achieved.
  • a sheet bar of steel containing 0.0015% of C, 0.30 of Si, 0.8% of Mn, 0.03% of P, 0.005% of S and 0.012% of N, and 0.02 to 0.08% of Al was uniformly heated at 1050°C, hot-rolled by seven passes so that the temperature of the final pass was 670°C, and then recrystallized and annealed at 700°C for 5 hours.
  • the thus-obtained hot-rolled sheet having a thickness of 4 mm was cold-rolled with a rolling reduction ratio of 82,5%, recrystallized and annealed at 875°C for 40 seconds, and then temper-rolled with a rolling reduction of 0.8%. Then, a tensile test specimen of JIS No.
  • a sheet bar of steel containing 0.0015% of C, 0.0010% of B, 0.01 of Si, 0.5% of Mn, 0.03% of P, 0.008% of S and 0.011% of N, 0.005 to 0.05% of Nb, and 0.005 to 0.03% of Al was uniformly heated at 1000°C, hot-rolled by seven passes so that the temperature of the final pass was 650°C, and then recrystallized and annealed at 800°C for 60 seconds.
  • the thus-obtained hot-rolled sheet having a thickness of 4 mm was cold-rolled with a rolling reduction ratio of 82,5%, recrystallized and annealing at 880°C for 40 seconds, and then temper-rolled with a rolling reduction ratio of 0.8%.
  • a tensile test specimen of JIS No. 5 was obtained from the resultant cold-rolled sheet in the rolling direction, and TS x r value, BH and ⁇ TS were measured with a strain rate of 3 x 10 -3 /s by using a general tensile testing machine.
  • the relations between the measured values and N/(Al+Nb+B) are shown in Fig. 5.
  • steel containing 0.005 to 0.05% of Nb and 0.0010% of B was used. This figure indicates that in the range of N/(Al+Nb+B) ⁇ 0.30, BH ⁇ 80 MPa, ⁇ TS ⁇ 60 MPa, and TS x r value ⁇ 850 are achieved.
  • a sheet bar of steel containing 0.0010% of C, 0.02 of Si, 0.6% of Mn, 0.01% of P, 0.009% of S and 0.015% of N, 0.01% of Al, 0.015% of Nb and 0.0001 to 0.0025% of B was uniformly heated at 1050°C, hot-rolled by seven passes so that the temperature of the final pass was 680°C, and then recrystallized and annealed at 850°C for 5 hours.
  • the thus-obtained hot-rolled sheet having a thickness of 4 mm was cold-rolled with a rolling reduction ratio of 82,5%, recrystallized and annealed at 880°C for 40 seconds, and then temper-rolled with a rolling reduction ratio of 0.8%.
  • a tensile test specimen of JIS No. 5 was obtained from the resultant cold-rolled sheet in the rolling direction, and TS x r value, BH and ⁇ TS were measured with a strain rate of 3 x 10 -3 /s by using a general tensile testing machine.
  • the relations between the measured values and the B content are shown in Fig. 6.
  • a cold-rolled steel sheet having excellent strain age hardenability comprises the compositions in the following ranges. Namely, a cold-rolled steel sheet having excellent strain age hardenability comprises a composition, by mass %, comprising:
  • the composition by mass %, further comprises:
  • the above composition by mass %, optionally further comprises at least one of Cu, Ni and Mo in a total of 1.0% or less according to demand.
  • the steel sheet preferably has a crystal grain diameter of 20 ⁇ m or less.
  • strength after forming is preferably increased by 60 MPa or more by heat treatment in the low temperature region of 120 to 200°C.
  • the surface of the cold-rolled steel sheet may be coated by electro-galvanization, hot-dip galvanization, or alloying hot-dip galvanization.
  • a method of producing a cold-rolled steel sheet having excellent strain age hardenability comprises hot-rolling a steel slab under conditions in which the slab is cooled immediately after the end of finish rolling and coiled at a coiling temperature of 400 to 800°C, cold-rolling the hot-rolled sheet with a rolling reduction ratio of 60 to 95%, and then performing recrystallization annealing at a temperature of 650 to 900°C, wherein the steel slab has a composition, by mass %, comprising:
  • the composition by mass %, further comprises:
  • the temperature in the heating-up step in recrystallization annealing, is preferably increased at a rate of 1 to 20°C/s in the temperature region from 500°C to the recrystallization temperature.
  • hot-dip galvanization and heat alloying may be performed after the recrystallization annealing.
  • C is advantageously as small as possible. Also, redissolution of NbC proceeds in the annealing step after cold rolling to increase the amount of dissolved C in crystal grains, thereby easily causing deterioration in natural aging resistance. Therefore, the C amount is preferably suppressed to less than 0.01 mass %, more preferably 0.0050 mass % or less, and most preferably 0.0030 mass % or less.
  • Si is a useful composition for suppressing a decrease in elongation, and improving strength.
  • the Si content is limited to the range of 0.005 to 1.0 mass %, and preferably the range of 0.01 to 0.75 mass %.
  • Mn not only is useful as a strengthening composition for steel, but also has the function to suppress embrittlement with S due to the formation of MnS.
  • Mn content is limited to the range of 0.01 to 1.5 mass %, and preferably the range of 1.10 to 0.75 mass %.
  • P is a solid solution strengthening element which effectively contributes to reinforcement of steel.
  • P content of over 0.01 mass %, deep drawability deteriorates due to the formation of phosphide such as (FeNb) x P or the like. Therefore, P is limited to 0.10 mass % or less.
  • Al is added as a deoxidizer for improving the yield of carbonitride forming components.
  • Al content is less than 0.005 mass %, the effect is insufficient, while with an Al content of over 0.030 mass %, the amount of N to be added to steel is increased to easily cause slab defects during steel making. Therefore, Al is contained in the range of 0.005 to 0.030 mass %.
  • N is an important element which plays the role of imparting strain age hardenability to a steel sheet.
  • N content is contained in the range of 0.005 to 0.040 mass %, and preferably in the range of 0.008 to 0.015 mass %.
  • B is added in a combination with Nb to exhibit the function to effectively make fine the hot-rolled structure and the cold-rolled recrystallized annealed structure and to improve cold-work embrittlement resistance.
  • B content is contained in the range of 0.0001 to 0.003 mass %, preferably in the range of 0.0001 to 0.0015 mass %, and more preferably in the range of 0.0007 to 0.0012 mass %.
  • Nb is added in a combination with B to contribute to refinement of the hot-rolled structure and the cold-rolled recrystallized annealed structure, and have the function to fix dissolved C as NbC. Furthermore, Nb forms a nitride NbN to contribute to refinement of the cold-rolled recrystallized annealed structure.
  • Nb is contained in the range of 0.005 to 0.050 mass %, and preferably 0.010 to 0.030 mass %.
  • Nb has the function to fix dissolved C as NbC, and forms a nitride NbN.
  • Al and B form AIN and BN, respectively. Therefore, in order to ensure the sufficient amount of dissolved N and sufficiently decrease the amount of dissolved C, it is important to satisfy the following relations (1) and (2): N % ⁇ 0.0015 + 14 / 93 ⁇ Nb % + 14 / 27 ⁇ Al % + 14 / 11 ⁇ B % C % ⁇ 0.5 ⁇ ( 12 / 93 ) ⁇ Nb %
  • the crystal grain diameter is preferably decreased.
  • ⁇ El can be suppressed to 2.0% or less by decreasing the crystal grain diameter d to 20 ⁇ m or less.
  • the crystal grain diameter d is more preferably decreased to 15 ⁇ m or less. This is because, as shown in Fig. 4, ⁇ El can be suppressed to 2.0% or less by decreasing the crystal grain diameter d to 15 ⁇ m or less.
  • Steel having the above-described suitable composition is melted by a known melting method such as a converter or the like, and a steel slab is formed by an ingot making method or a continuous casting method.
  • the heating temperature of hot rolling is not specified, but the heating temperature of hot rolling is preferably set to 1300°C or less. This is because it is advantageous to fix and precipitate dissolved C as a carbide in order to improve deep drawability.
  • the heating temperature is preferably set to 1150°C or less.
  • the lower limit of the heating temperature is preferably 900°C.
  • the total rolling reduction ratio of hot rolling is preferably 70% or more. This is because with a toal rolling reduction ratio of less than 70%, the crystal grains of the hot-rolled sheet are not sufficiently made fine.
  • finish rolling is preferably finished in the temperature region of 650 to 960°C, and the finishing temperature of hot-rolling may be in the ⁇ region above the Ar 3 transformation point, or the ⁇ region below the Ar 3 transformation pint.
  • the finishing temperature in hot-rolling process over 960°C, the crystal grains of the hot-rolled sheet are coarsened to deteriorate deep drawability after cold rolling and annealing.
  • a temperature of less than 650°C deformation resistance is increased to increase the hot-rolling load, causing difficulties in rolling.
  • cooling is started immediately after the end of final rolling in hot-rolling process to prevent normal grain growth and suppress AlN precipitation in the cooling step.
  • the starting time of the cooling step is preferably within 1.5 seconds, more preferably 1.0 second, and most preferably 0.5 second, after the end of finish rolling. This is because when cooling is performed immediately after the end of rolling, a large amount of ferrite nuclei is produced due to an increase in the degree of over cooling with accumulated strain to promote ferrite transformation and suppress the diffusion of dissolved N in the ⁇ phase into the ferrite grains, thereby increasing the amount of dissolved N present in the ferrite grain boundaries.
  • the cooling rate is preferably 10°C/s or more in order to ensure dissolved N.
  • the cooling rate is preferably 50°C/s or more in order to ensure dissolved N.
  • the hot-rolled sheet is coiled.
  • the coiling temperature is advantageously as high as possible.
  • the scale formed on the surface of the hot-rolled sheet is thickened to increase the load of the work of removing the scale, and progress the formation of a nitride, causing a change in the amount of dissolved N in the coil length direction.
  • the coiling temperature of the hot-rolled sheet must be in the range of 400 to 800°C.
  • the hot-rolled sheet is cold-rolled, but the rolling reduction ratio of cold rolling must be 60 to 95%. This is because with a rolling reduction ratio of cold rolling of less than 60%, a high r value cannot be expected, while with a rolling reduction ratio of over 95%, the r value is decreased.
  • the cold-rolled sheet subjected to cold rolling is then recrystallized and annealed.
  • the annealing method may be either continuous annealing or batch annealing, continuous annealing is advantageous.
  • the continuous annealing may be performed either in a normal continuous annealing line or in a continuous hot-dip galvanization line.
  • the preferable annealing conditions include 650°C or more for 5 seconds or more. This is because with an annealing temperature of less than 650°C, and an annealing condition of less than 5 seconds, recrystallization is not completed to decrease deep drawability. In order to improve deep drawability, annealing is preferably performed in the ferrite single phase region at 800°C or more for 5 seconds or more.
  • Annealing in the high-temperature ⁇ + ⁇ two-phase region partially produces ⁇ ⁇ ⁇ transformation to improve the r value due to the development of the ⁇ 111 ⁇ aggregation structure.
  • the aggregation structure is made random to decrease the r value, thereby deteriorating deep drawability.
  • the upper limit of the annealing temperature is preferably 900°C. This is because with an annealing temperature of over 900°C, redissolution of a carbide proceeds to excessively increase the amount of dissolved C, thereby deteriorating the natural aging property.
  • ⁇ ⁇ ⁇ transformation occurs, the aggregation structure is made random to decrease the r value, deteriorating deep drawability.
  • the temperature region in which controlled heating must be performed is 500°C, at which AlN or the like starts to precipitate, to the recrystallization temperature
  • the heating rate is preferably in the range of 1 to 20°C/s because with a heating rate of over 20°C/s, the sufficient amount of precipitates cannot be obtained, while with a heating rate of less than 1°C/s, precipitates are coarsened to weaken the effect of suppressing grain growth.
  • temper rolling of 10% or less may be performed for correcting the shape and controlling surface roughness.
  • the cooling rate after soaking in recrystallization annealing is preferably 10 to 50°C/s. This is because with a cooling rate of 10°C/s or less, grains are grown during cooling to coarsen the crystal grains, thereby deteriorating the strain aging property and natural aging property. While with a cooling rate of 50°C/s or more, dissolved N does not sufficiently diffuse into the grain boundaries, deteriorating the natural aging property.
  • the cooling rate is preferably 10 to 30°C/s.
  • the hot-dip galvanization and alloying are not limited, and may be performed according to a conventional known method.
  • a steel sheet subjected to surface treatment generally used for steel thin sheets such as a steel sheet (a dull-finished steel sheet, a bright-finished steel sheet, or a steel sheet having a specified roughness pattern formed on the surface thereof), which is produced by temper-rolling the alloyed hot-dip galvanized steel sheet, for improving processability and the appearance after processing, a steel sheet having an oil film layer of antirust oil or lubricating oil formed on the surface thereof, or the like, the effect of the present invention can be sufficiently exhibited in the composition range of the prevent invention.
  • a cold-rolled steel sheet and a galvannealed steel sheet can be obtained, which have excellent deep drawability and excellent strain age hardenability, that tensile strength increased by press forming and heat treatment.
  • the amount of pre-strain is an important factor.
  • the deformation stress in the above-described deformation system can be referred to as an amount of approximately uniaxial strain (tensile strain) except the case of excessive deep drawing, (2) the amount of uniaxial strain of an actual part exceeds 5%, and (3) the strength of a part sufficiently corresponds to the strength (YS and TS) obtained after strain aging with a pre-strain of 5%.
  • the pre-deformation of strain aging is defined to a tensile strain of 5%.
  • Conventional coating and baking conditions include 170°C and 20 min as standards.
  • a strain of 5% is applied to the steel sheet of the present invention, which contains a large amount of dissolved N, hardening can be achieved even by aging at low temperature. In other words, the range of aging conditions can be widened.
  • retention at a higher temperature for a longer time is advantageous as long as softening does not occurs by over aging.
  • the lower limit of the heating temperature at which hardening significantly takes place after pre-deformation is about 100°C.
  • the heating temperature of over 300°C hardening peaks, thereby causing the tendency to soften and significantly causing thermal strain and temper color.
  • the retention time is preferably 60 seconds or more.
  • retention for over 20 mines is practically disadvantageous because further hardening cannot be expected, and the production efficiency significantly deteriorates.
  • the conventional coating and baking conditions i.e., the heating temperature of 170°C and the retention time of 20 minutes
  • the heating temperature i.e., the heating temperature of 170°C and the retention time of 20 minutes
  • the heating method is not limited, and atmospheric heating with a furnace, which is generally used for coating and baking, and other methods such as induction heating, heating with a nonoxidation flame, a laser, plasma, or the like, etc. can be preferably used.
  • BH is 80 MPa or more
  • ⁇ TS is 40 MPa or more
  • BH is 100 MPa or more
  • ⁇ TS is 50 MPa or more.
  • the heating temperature in aging may be set to a higher temperature, and/or the retention time may be set to a longer time.
  • the steel sheet of the present invention has the advantage that when the steel sheet is allowed to stand at room temperature for about one week without heating after forming, an increase in strength of about 40% of that at the time of complete aging can be expected.
  • the steel sheet of the present invention also has the advantage that even when it is allowed in an unmolded state at room temperature for a long time, aging deterioration (an increase in YS and a decrease in El (elongation)) does not occurs, unlike a conventional aging steel sheet.
  • aging deterioration an increase in YS and a decrease in El (elongation)
  • an increase in YS is 30 MPa or less
  • a decrease in elongation is 2% or less
  • a recovery of yield point elongation is 0.2% or less.
  • the surface of the cold-rolled steel sheet may be coated by hot-dip galvanization or alloying hot-dip galvanization without any problem, and TS, BH and ⁇ TS are equivalent to those before plating.
  • TS, BH and ⁇ TS are equivalent to those before plating.
  • electro-galvanization, hot-dip galvanization, alloying hot-dip galvanization, electro-tinning, electric chromium plating, electro-nickeling, and the like may be preferably used.
  • the difference ( ⁇ TS) between the tensile strength of the specimen after application of tensile strain and heat treatment and the tensile strength of a product is defined as the strength increasing ability of heat treatment.
  • the amount of strain introduced by forming, or the heat treatment temperature after processing is preferably as high as possible.
  • the strength can be sufficiently increased even by heat treatment at a temperature lower than conventional heat treatment, i.e., a temperature of 200°C or less, after forming.
  • a heat treatment temperature of less than 120°C the strength cannot be sufficiently increased with the low.train applied.
  • the heat treatment temperature of over 350°C after forming softening proceeds. Therefore, the temperature of heat treatment after forming is preferably about 120 to 350°C.
  • the heating method is not limited, and hot gas heating, infrared furnace heating, hot-bath heating, direct current heating, induction heating, and the like can be used. Alternatively, only a portion where strength is desired to be increased is selectively heated.
  • the amount of dissolved N, the microstructure, tensile properties, the r value, strain age hardenability, and aging property were examined.
  • the examination methods were as follows:
  • the amount of dissolved N was determined by subtracting the amount of precipitated N from the total N amount of steel determined by chemical analysis.
  • the amount of precipitated N was determined by an analysis method using a constant-potential electrolytic method.
  • a test specimen was obtained from each of cold-rolled annealed steel sheets, and the microstructure of a section (C section) perpendicular to the rolling direction was imaged with an optical microscope or a scanning electron microscope. Then, the fraction of the ferrite texture and the type and the structure fraction of a second phase were determined by an image analysis apparatus.
  • the value used as the average crystal grain diameter was a higher one of the value calculated from a photograph of a sectional structure by a quadrature method defined by ASTM, and the nominal value determined from a photograph of a sectional structure by an intercept method defined by ASTM (refer to, for example, Umemoto et al.: Heat Treatment, 24 (1984), p334).
  • test specimen of JIS No. 5 was obtained from each of cold-rolled annealed steel sheets in the rolling direction, and a tensile test was carried out with a strain rate of 3 x 10 -3 /s according to the regulations of JIS Z 2241 to determine yield stress YS, tensile strength TS, and elongation El.
  • YS5% represents deformation stress in 5% pre-deformation of the produced sheet
  • YSBH and TSBH represent yield stress and tensile strength, respectively, after pre-deformation and heat treatment
  • TS represents the tensile strength of the produced sheet.
  • a test specimen of JIS No. 5 was obtained from each of the cold-rolled annealed steel sheets in each of the rolling direction (L direction), the direction (D direction) at 45° with the rolling direction, and the direction (C direction) at 90° with the rolling direction.
  • r mean ( r L + 2 r D + r D ) / 4 wherein rL represents the r value in the rolling direction (L direction), rD represents the r value in the direction (D direction) at 45° with the rolling direction, and rL represents the r value in the direction (C direction) at 90° with the rolling direction.
  • test specimen of JIS No. 5 was obtained from each of cold-rolled annealed steel sheets in the rolling direction, and then subjected to aging at 50°C for 200 hours, followed by a tensile test.
  • the difference in yield elongation ⁇ Y-El between before and after aging was determined from the obtained results to evaluate aging properties at normal temperature. When ⁇ Y-El was zero, it was evaluated that the specimen has non-aging properties and excellent natural aging resistance.
  • test specimen of JIS No. 5 was obtained from each of produced sheets in the rolling direction, and then a pre-strain of 10% was applied thereto. Then, heat treatment was conducted for 20 minutes at a conventional heat treatment temperature of 120°C and a temperature of 170°C corresponding to coating and baking, and then tensile strength was determined.
  • the decrease ( ⁇ El) in total elongation by natural aging was determined as the difference between the total elongation measured with a specimen of JIS N0 5 obtained from the produced sheet in the rolling direction, and the total elongation measured with a specimen of JIS N0 5 separately obtained from the produced sheet in the rolling direction after accelerated aging (retention at 100°C for 8 hours) of natural aging.
  • a steel slab having each of the compositions shown in Table 1 was hot-rolled to a hot-rolled sheet having a thickness of 3.5 mm, and then cold-rolled to a cold-rolled sheet having a thickness of 0.7 mm under the conditions shown in Table 2. Then, the cold-rolled sheet was recrystallized, annealed and further galvannealed in a continuous annealing line or a continuous annealing and galvanizing line. Then, the annealed sheet was temper-rolled with a rolling reduction ratio of 1.0% to produce a cold-rolled steel sheet and a galvannealed steel sheet having both sides coated with a weight of 45 g/m 2 per side.
  • the finisher deliver temperatures in the hot rolling process of Steel Nos. 3 and 8 are less than the Ar 3 transformation point, and the finisher deliver temperatures of the others are the Ar 3 transformation point or more.
  • Table 3 indicates that with all the cold-rolled steel sheets and the galvannealed steel sheets obtained according to the present invention, a high r value and excellent strain age hardenability are obtained, as compared with comparative examples. Particularly, in the suitable examples in which the crystal grain diameter is 20 ⁇ m or less, the decrease in elongation due to natural aging is also as low as 2.0% or less.
  • a slab of steel symbol B shown in Table 1 was hot-rolled under the same production conditions as No. 2 shown in Table 2 in which the heating temperature was 1100°C, and the finisher deliver temperature of hot rolling was 900°C, and then coiled at coiling temperature of 550°C into a coil.
  • the thus-obtained coil was cold-rolled with a reduction ratio of 80%, and then recrystallized and annealed at 840°C.
  • tensile strength TS was 365 MPa, and the r value was 1.7.
  • a test specimen of JIS No. 5 was obtained from the cold-rolled steel sheet in the rolling direction, and a tensile strain of 10% was applied by a tensile test machine.
  • Table 4 indicates that the increase in strength increases as the heat treatment temperature increases, and the heat treatment time increases.
  • a sufficient increase in tensile strength of 82 MPa (85% or more of an increase in heat treatment for 20 minutes) can be obtained even by heat treatment at low temperature of 120°C for a short retention time of 2 minutes. It is thus found that with the steel sheet of the present invention, good strain age hardenability can be obtained even by heat treatment at a low temperature for a short time.
  • heat treatment at a normal temperature for a normal time causes no problem. It was confirmed that with the galvanized steel sheets and the galvannealed steel sheets obtained by hot-dip galvanizing and heat alloying the cold-rolled sheets, the same results as shown in Table 4 are obtained.
  • All the examples of the present invention have excellent strain age hardenability and a high r value, and exhibit extremely high stable BH amount, ⁇ TS and mean r value regardless of variations in production conditions. It was also recognized that in the examples of the present invention, by performing continuous rolling and controlling the temperature of the sheet bar in the long direction and the width direction, the thickness precision and the shape of the produced steel sheet are improved, and variations in material properties are decreased.
  • a cold rolled steel sheet can be obtained, in which TS is greatly increased by press forming and heat treatment while maintaining excellent deep drawability in press forming.
  • the cold-rolled steel sheet has the excellent effect of industrially producing coated steel sheets by electro-galvanization, hot-dip galvanization, alloying hot-dip galvanization. Table 2 No.
  • the cooling conditions after finish rolling include the cooling start time(s) and the cooling rate (°C/s).
  • Table 3 No. Steel Product characteristics Charge in tensile strength after forming-heat treatment Remarks Tensile strength T.S. (MPa) r value Crystal grain diameter ( ⁇ m) ⁇ E1 (%) T.S. after heat treatment at 120°C (MPa) ⁇ T.S. after heat treatment at 120°C (MPa) T.S. after heat treatment at 170°C (MPa) ⁇ T.S.

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Claims (7)

  1. Ein kaltgewalztes Stahlblech mit exzellenter Reckalterungseigenschaft, umfassend eine Zusammensetzung, in Gew.-%:
    C: weniger als 0,01 %;
    Si: 0,005 bis 1,0 %;
    Mn: 0,01 bis 1,5 %;
    P: 0,1 % oder weniger;
    S: 0,01 % oder weniger;
    Al: 0,005 bis 0,030 %;
    N: 0,005 bis 0,040 %;
    B: 0,0001 bis 0,0030 %;
    Nb: 0,005 bis 0,050 %; und
    wahlweise ferner umfassend wenigstens eine von Cu, Ni und Mo in einer Gesamtmenge von 1,0 % oder weniger, und der Rest besteht aus Fe und unvermeidbaren Verunreinigungen,
    wobei N/Al 0,30 oder mehr ist, die Menge an gelöstem N 0,0010 % oder mehr ist und die Bereiche von B und Nb die folgenden Gleichungen (1) und (2) erfüllen: N % ≥ 0 , 0015 + 14 / 93 · Nb % + 14 / 27 · Al % + 14 / 11 · B %
    Figure imgb0012
     C % ≤ 0 , 5 · ( 12 / 93 ) · Nb %
    Figure imgb0013
  2. Ein kaltgewalztes Stahlblech mit exzellenter Reckalterungseigenschaft nach Anspruch 1, wobei das Stahlblech einen Kristallkorndurchmesser von 20 µm oder weniger aufweist.
  3. Ein kaltgewalztes Stahlblech mit exzellenter Reckalterungseigenschaft nach Anspruch 1 oder 2, wobei die Festigkeit nach Umformen durch Wärmebehandlung in einem Niedrigtemperaturbereich von 120 bis 200°C mit 60 MPa oder mehr erhöht ist.
  4. Ein galvanisch verzinktes, feuerverzinktes oder legiertes, feuerverzinktes Stahlblech mit exzellenter Reckalterungseigenschaft, umfassend eine auf der Obertläche eines kaltgewalzten Stahlblechs gemäß Anspruch 1 bis 3 durch galvanisches Verzinken, Feuerverzinken oder legiertes Feuerverzinken beschichtete Schicht.
  5. Ein Verfahren zum Herstellen eines kaltgewalzten Stahlblechs mit exzellenter Reckalterungseigenschaft, umfassend Warmwalzen einer Stahlbramme unter Bedingungen, bei welchen die Stahlbramme unmittelbar nach dem Ende des Endwalzens abgekühlt wird und bei einer Aufwickeltemperatur von 400 bis 800°C aufgewickelt wird, Kaltwalzen des warmgewalzten Blechs bei einem Walzreduktionsverhältnis von 60 bis 95 % und dann Rekristallisieren und Glühen des kaltgewalzten Blechs bei einer Temperatur von 650 bis 900°C, wobei die Stahlbramme eine Zusammensetzung nach Anspruch 1 aufweist.
  6. Ein Verfahren zum Herstellen eines kaltgewalzten Stahlblechs mit exzellenter Reckalterungseigenschaft nach Anspruch 5, wobei in dem Erwärmungsschritt beim Rekristallisationsglühen in dem Temperaturbereich von 500°C bis zu der Rekristallisationstemperatur die Temperatur bei einer Rate von 1 bis 20°C/Sek. erhöht wird.
  7. Ein Verfahren zum Herstellen eines nach dem Verzinken wärmebehandelten Stahlblechs mit exzellenter Reckalterungseigenschaft, umfassend Feuerverzinken und dann Wärmelegieren nach Rekristallisation und Glühung gemäß Anspruch 5 oder 6.
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EP1498506B1 (de) 2006-06-28
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