EP1937853B1 - Feuille d'acier laminee a froid, durcissable a la cuisson dotee d'une resistance superieure et d'une resistance au vieillissement, et procede de fabrication de la feuille d'acier laminee a froid - Google Patents

Feuille d'acier laminee a froid, durcissable a la cuisson dotee d'une resistance superieure et d'une resistance au vieillissement, et procede de fabrication de la feuille d'acier laminee a froid Download PDF

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EP1937853B1
EP1937853B1 EP06798860.0A EP06798860A EP1937853B1 EP 1937853 B1 EP1937853 B1 EP 1937853B1 EP 06798860 A EP06798860 A EP 06798860A EP 1937853 B1 EP1937853 B1 EP 1937853B1
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steel
steel sheet
bake
rolled steel
amount
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EP1937853A1 (fr
EP1937853A4 (fr
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Seong-Ho Han
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Posco Holdings Inc
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Posco Co Ltd
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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
    • 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
    • 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
    • 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
    • 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
    • C21D9/52—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for wires; for strips ; for rods of unlimited length
    • C21D9/54—Furnaces for treating strips or wire
    • C21D9/56—Continuous furnaces for strip or wire
    • 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

Definitions

  • the present invention relates to a cold-rolled steel sheet for outer panels and the like of an automobile body, and a method for manufacturing the cold-rolled steel sheet. More particularly, the present invention relates to a high strength cold-rolled steel sheet with superior bake hardenability, aging resistance at room temperature and secondary work embrittlement resistance, and a method for manufacturing the cold-rolled steel sheet.
  • a cold-rolled steel sheet As used for the outer panel of the automobile body, a cold-rolled steel sheet is required to have good properties in terms of tensile strength, yield strength, press formability, spot weldability, fatigue resistance, corrosion resistance, etc.
  • the corrosion resistance has been recently required for extension of lifetime of components for the automobile.
  • Steel sheets for improvement of the corrosion resistance can be generally classified into two types, i.e. a electroplated steel sheet and a galvannealed steel sheet.
  • the galvannealed steel sheet In comparison to the galvannealed steel sheet, although the electroplated steel sheet has better plating characteristics and superior corrosion resistance, it is rarely used due to its high price. Therefore, the galvannealed steel sheet is generally used in the art, and required to have improved corrosion resistance.
  • the steel sheet exhibits opposite characteristics in terms of strength and formability.
  • steel sheets capable of satisfying both characteristics there are multi-phase structure cold-rolled steel sheets and bake-hardenable cold-rolled steel sheets.
  • the multi-phase structure cold-rolled steel can be easily manufactured, and has a tensile strength in the level of 390 MPa or more. Regardless of its high tensile strength as materials for the automobiles, the multi-phase structure cold-rolled steel has a high elongation as a factor indicating stretchability. However, it has a low average r-value as a factor indicating press formability of the automobiles, and comprises excessive amounts of expensive alloying elements such as Mn, Cr and the like, which results in high manufacturing costs.
  • the bake-hardenable cold-rolled steel acts like mild steel in terms of yield strength upon press forming of the steel which has a tensile strength of 390 MPa or less.
  • the bake-hardenable cold-rolled steel has excellent ductility, and spontaneously increases in yield strength upon paint baking after press forming.
  • This steel is considered ideal in comparison to previous steel, which is generally deteriorated in formability as the strength of the steel increases.
  • Bake hardening is a process which employs a kind of strain aging occurring as interstitial elements dissolved in a solid solution state in the steel, such as solute nitrogen or solute carbon, fix dislocations created during deformation.
  • a kind of strain aging occurring as interstitial elements dissolved in a solid solution state in the steel such as solute nitrogen or solute carbon
  • fix dislocations created during deformation When the steel has high amounts of solute carbon and nitrogen, the bake hardenability value (BH value) advantageously increases, but the natural aging property also increases due to the high amounts of solid solution elements, deteriorating the formability. Thus, it is very important to optimize the amounts of solid solution elements in the steel.
  • a batch annealing method and a continuous annealing method are generally used.
  • the bake-hardenable cold-rolled steel sheet is produced by batch annealing a low carbon, P-added, Al-killed steel through coiling of a hot-rolled steel sheet at a lower temperature.
  • the hot-rolled steel sheet is coiled at the low temperature of 400 - 500 °C, followed by batch annealing to have BH value of about 40 to 50 MPa.
  • the batch annealing enables both of the formability and the bake-hardenability to be obtained more easily at the same time.
  • the P-added Al-killed steel is cooled at a relatively high speed, it is easy to secure the bake-hardenability, but there are problems in that the formability is deteriorated due to a high heating speed and a short annealing process.
  • the steel sheet produced by batch annealing is restricted in use for the outer panels of the automobile body, which do not require workability.
  • Japanese Patent Publication No. (Sho) 61-026757 discloses an ultra low carbon cold-rolled steel sheet, which comprises: 0.0005 ⁇ 0.01.5% of C; 0.05% or less of S+N; and Ti and Nb or a compound thereof.
  • Japanese Patent Publication No. (Sho) 57-089437 discloses a method for manufacturing a bake-hardenable cold-rolled steel sheet, which uses Ti-added steel comprising 0.010% or less of C, and has BH value of about 40 MPa or more.
  • the methods of the disclosures are to impart the bake hardenability to the steel sheet while preventing deterioration in other properties of the steel sheet by appropriately controlling the amount of solid solution elements in the steel through control of the added amount of Ti and Nb or the cooling rate during annealing.
  • the Nb-added steel described above has problems in that workability is degraded due to hot annealing, and in that manufacturing costs are increased due to addition of the specific elements.
  • US Patent Nos. 5,556,485 and 5,656,102 (Bethlehem Steel ) disclose methods of manufacturing a bake-hardenable cold-rolled steel sheet using a Ti-V based ultra low carbon steel, which comprises 0.0005 - 0.1% of C; 0 - 2.5% of Mn; 0 - 0.5% of Al; 0 - 0.04% of N; 0 - 0.5% of Ti; and 0.005 - 0.6% of V.
  • V is more stable than the carbon nitride formation elements such as Ti and Nb, it can reduce an annealing temperature.
  • carbide, such as VC and the like, created during high temperature annealing can impart the bake hardenability via re-melting even with a lower annealing temperature than that for the Nb-based steel.
  • V can create the carbide such as VC, since it does not sufficiently improve the formability due to its significantly low re-melting temperature, Ti is added at an amount of about 0.02% or more in order to enhance the formability, as disclosed in the publications.
  • Japanese Patent Laid-open Publication No. (Hei) 5-93502 discloses a method for enhancing the bake hardenability by addition of Sn
  • Japanese Patent Laid-open Publication No. (Hei) 9-249936 discloses a method for enhancing the ductility of steel by relieving stress concentration on grain boundaries through addition of V and Nb.
  • Japanese Patent Laid-open Publication No. (Hei) 8-49038 discloses a method for enhancing the formability through addition of Zr
  • Japanese Patent Laid-open Publication No. (Hei) 7-278654 discloses a method for enhancing the formability by increasing the strength while minimizing deterioration of work hardening index (N-value) through addition of Cr.
  • KR 2005 0070305 which discloses a bake-hardening cold rolled steel sheet having excellent ageing resistance at room temperature and resistance to secondary work brittleness has a composition comprising 0.0018 to 0.0025 wt% of C, 0.02 wt% or less of Si, 0.2 to 0.7 wt% of Mn, 0.05 to 0.08 wt% of P, 0.01 wt% or less of S, 0.02 to 0.06 wt% of acid soluble Al, 0.0025 wt% or less of N, 0.007 to 0.011 wt% of Nb, 0.01 to 0.05 wt% of Mo, 0.0005 to 0.0015 wt% of B and the balance of Fe and inevitable impurities, wherein the Nb and Mo are added to the composition to satisfy the following expressions: 50-(1589XNb)>30 and 44-(2119XNb)-(125XMo) ⁇ 30, and grain size of microstructure of the steel sheet is ASTM No.
  • a manufacturing method of the bake-hardening cold rolled steel sheet having excellent ageing resistance at room temperature and resistance to secondary work brittleness comprises: a step of homogenizing the slab at a temperature of 1200° C or more after making a slab using Al-killed steel having a composition comprising 0.0018 to 0.0025 wt% of C, 0.02 wt% or less of Si, 0.2 to 0.7 wt% of Mn, 0.05 to 0.08 wt% of P, 0.01 wt% or less of S, 0.02 to 0.06 wt% of acid soluble Al, 0.0025 wt% or less of N, 0.007 to 0.011 wt% of Nb, 0.01 to 0.05 wt% of Mo, 0.0005 to 0.0015 wt% of B and the balance of Fe and inevitable impurities; a step of finish hot rolling the homogenized slab in a temperature range of 900 to 950°C; a step of coiling the finish hot rolled steel
  • the increase in bake hardenability causes the deterioration in aging resistance at room temperature.
  • inventors of the present invention have found that, as the content of P added for high strength of the steel increases, the steel is degraded in the secondary work embrittlement resistance even for the bake hardenable steel which comprises solute carbon dissolved in the solid solution state in the steel, and the more the severe the degradation.
  • the ductility-brittleness transition temperature (DBTT) of the steel as a reference to judge the secondary work embrittlement was -20 °C at a draw ratio of 1.9.
  • the DBTT of the steel was 0 ⁇ 10 °C, which can be considered to be a significantly deteriorated value.
  • an object of the present invention is to provide a high strength cold-rolled steel sheet with superior bake hardenability, aging resistance at room temperature and secondary work embrittlement resistance.
  • a bake-hardenable cold-rolled steel sheet with high strength and superior aging resistance comprising, by weight%,: C: 0.0016 ⁇ 0.0025%; Si: 0.02% or less; Mn: 0.2 - 1.2%; P: 0.05 - 0.11%; S: 0.01% or less; Sol. Al: 0.08 - 0.12%; N: 0.0025% or less; Ti: 0 ⁇ 0.003%; Nb: 0.003 ⁇ 0.011%; Mo: 0.01 - 0.1%; B: 0.0005 - 0.0015%; and the balance of Fe and other unavoidable impurities, wherein the steel sheet satisfies Equation 1:
  • Equation 1 GB-C (that is, the amount of solute carbon in the grain boundaries) is 5 ⁇ 10 ppm, and G-C (that is, the amount of solute carbon in the crystal grains) is 3 - 7 ppm]
  • ASTM No. ASTM grain size
  • BH bake hardening degree
  • AI aging index
  • DBTT DBTT
  • a method for manufacturing a bake-hardenable cold-rolled steel sheet with high strength and superior aging resistance comprising: performing homogenization heat treatment for an Al-killed steel slab at 1200 °C or more, the steel slab comprising, by weight%,: C: 0.0016 ⁇ 0.0025%, Si: 0.02% or less, Mn: 0.2 ⁇ 1.2%, P: 0.05 - 0.11%, S: 0.01% or less, Sol.
  • Fig. 1 is a graph decribing influence of a grain size on bake hardenability and aging index
  • Fig. 2 is a graph decribing influence of an amount of solute carbon in steel on the bake hardenability
  • Fig. 3 is a graph decribing a result of an internal friction test to Inventive steel No. 6.
  • Carbon or nitrogen in steel generally combines with precipitate formation elements such as Al, Ti, Nb, etc. in the steel during hot rolling, forming carbides and nitrides such as TiN, AlN, TiC, Ti 4 C 2 S 2 , NbC, etc.
  • precipitate formation elements such as Al, Ti, Nb, etc.
  • carbides and nitrides such as TiN, AlN, TiC, Ti 4 C 2 S 2 , NbC, etc.
  • solute carbon or solute nitrogen solute carbon or solute nitrogen
  • carbon is an essential element for the steel, and determines the characteristics of the steel depending on the carbon content in the steel.
  • carbon has a very important role, and only a small amount of solute carbon is allowed to remain in the steel as an attempt to improve the bake hardenability and the aging resistance.
  • influence of the solute carbon on the bake hardenability and the aging resistance can be changed according to locations of the solute carbon in the steel, that is, whether the solute carbon resides in grain boundaries or in crystal grains.
  • solute carbon With an internal friction test to detect the solute carbon, it can be found that the solute carbon generally exists in the crystal grains and moves relatively freely. Thus, the solute carbon can combine with movable dislocations, and affect aging properties.
  • an aging index (AI) is generally used in the art.
  • the steel has an AI of 30 MPa or more, aging can occur within six months after maintaining the steel at room temperature, and causes severe defects upon press forming.
  • solute carbon resides in the grain boundaries which are a relatively stable region, it is difficult to detect such solute carbon via a vibration test such as the internal friction test.
  • the solute carbon exists in a relatively stable state. Therefore, although the solute carbon of the grain boundaries rarely affects aging at low temperatures such as AI test, it is activated in a baking condition of high temperatures, and affects the bake hardenability.
  • solute carbon in the crystal grains can affect both of the aging properties and the bake hardenability at the same time, whereas the solute carbon the grain boundaries affects only the bake hardenability.
  • Fig. 1 shows the BH value and the aging index (AI) in relation to variation in grain size, which is a result of investigation by the inventors of the present invention.
  • the inventors of the invention try to decrease the grain size of an annealed sheet to a suitable level or less to make the solute carbon distributed as much as possible into the grain boundaries of the steel sheet.
  • the inventors of the present invention have found that the grain size is desirably controlled to ASTM No. 9 or more to maximize the aging resistance while minimizing deterioration of the bake hardenability.
  • the total amount of carbon is set to 16 ⁇ 25 ppm to satisfy the above conditions.
  • Nb when added to the steel, it combines with C to precipitate a carbide such as NbC, thereby reducing the amount of solute carbon in the steel.
  • Nb-added steel a precipitation ratio of Nb/C is determined depending on the content of Nb and C in the steel. Furthermore, while some of the solute carbon in the steel combines with Nb to precipitate NbC, remaining carbon exist in the solid solution state, and affects the bake hardenability and the aging resistance.
  • control of the solute carbon in the steel is more important than control of the added amount of Nb or the carbon content.
  • a total amount of solute carbon is about 8 - 15 ppm, wherein the total amount of solute carbon is obtained by excluding the NbC precipitates in consideration of the added amount of Nb and the carbon content.
  • Equation 1 Equation 1:
  • GB-C that is, the amount of solute carbon in the grain boundaries
  • G-C that is, the amount of solute carbon in the crystal grains
  • Equation 1 the term "C in NbC” means an amount of carbon precipitated in the form of the NbC precipitate.
  • an effect of AlN precipitates through addition of Al is also considered as well as the addition of Nb in order to more stably secure the bake hardenability and the aging resistance.
  • AlN precipitates have an effect of removing the solute nitrogen in the steel.
  • the bake hardenable steel of the present invention since the carbon content of the inventive steel is restricted narrowly in the range of 16 - 25 ppm, the bake hardenable steel of the present invention has the bake hardenability and the aging resistance in a narrow range.
  • the steel has a grain size of ASTM No. 9 or less upon the recrystallization annealing, so that the AI exceeds 30 MPa, which is the upper limit of the present invention.
  • secondary work embrittlement it can be considered that components of automobiles are generally formed to desired shapes through several repetitions of press forming by automobile manufacturers.
  • the secondary work embrittlement means that cracks occur during a process performed after primary press working.
  • P is not added to the steel in order to prevent the secondary work embrittlement.
  • P has merits in that it serves to increase the strength while suppressing reduction of elongation, and in that it is very low in price.
  • the present invention suggests addition of Mo to improve the secondary work embrittlement resistance more stably. From the results of the investigation, since Mo improves the bonding force between the grain boundaries, it is very advantageous to improve the secondary work embrittlement resistance.
  • Mo has an affinity to the solute carbon in the steel, it suppresses diffusion of the solute carbon into the dislocations when being maintained for a long period of time at room temperature, thereby providing an advantageous effect in terms of aging resistance.
  • Equation 3 shows an effect of improving the aging resistance by Mo in a statistical manner.
  • the inventors of the present invention deduce an optimal composition for the steel by suitably using the characteristics of Mo without deterioration in properties of the steel possibly caused by excessive addition of Mo.
  • the inventors try to maximize the effect of improving the secondary work embrittlement resistance through addition of a suitable amount of B and selection of a suitable coiling temperature at the same time among various methods conventionally used to improve the secondary work embrittlement resistance.
  • Carbon (C) is an element used for solid solution strengthening and bake hardening.
  • the carbon content is less than 0.0016%, the tensile strength of the steel is lowered due to such a low content of carbon, and a sufficient bake hardenabilitycannot be obtained due to a low absolute amount of carbon in the steel even with an attempt to achieve the grain refining effect through addition of Nb.
  • the secondary work embrittlement resistance is significantly deteriorated since the site competition effect between solute carbon and P is removed.
  • the carbon content exceeds 0.0025%, the aging resistance at room temperature cannot be obtained irrespective of the BH value due to excessive quantities of solute carbon above 3 ⁇ 7 ppm of the present invention in the crystal grains of the steel, and thus stretcher strain occurs during press working, causing deterioration in formability and ductility of the steel.
  • Silicon (Si) is an element for increasing the strength of the steel. However, as the silicon content is increased, the ductility is significantly deteriorated. In addition, since silicon deteriorates galvannealing capability, it is advantageous to minimize the added amount of silicon.
  • the added amount of silicon is preferably 0.02% or less.
  • Manganese (Mn) is an element used for preventing hot embrittlement caused by formation of FeS by completely precipitating sulfur in the steel into MnS while refining the crystal grains without deteriorating the ductility, and for strengthening the steel. According to the present invention, if the Mn content is less than 0.2%, a suitable tensile strength cannot be obtained, whereas if the Mn content exceeds 1.2%, the formability is deteriorated with a rapid increase in strength due to solid solution strengthening.
  • the Mn content is preferably in the range of 0.2 - 1.2%.
  • Phosphorus (P) is a substitutional alloying element which provides the highest solid solution strengthening effect, and serves to enhance the anisotropy while increasing the strength of the steel.
  • the P content is less than 0.05%, the secondary work embrittlement resistance can be improved due to such a low content of P in the grain boundaries, but it is difficult to sufficiently obtain the effect of improving the other properties of the steel through the grain refining effect of P.
  • the P content is above 0.11 %, it occurs a rapid increase of the strength compared with an improved degree of the formability.
  • the P content is preferably in the range of 0.05 - 0.11 %.
  • S Sulfur
  • MnS sulfide
  • FeS Fe-sulfide
  • S is preferably in the range of 0.01% or less.
  • Aluminum (Al) is an element which is generally added for deoxidization of the steel. However, according to the present invention, aluminum is used for an effect of improving the grain refining effect and the bake hardenability via precipitation of AlN.
  • the grain refining effect is generally obtained using NbC precipitates by addition of Nb
  • aluminum serves to further improve the grain refining effect by the AlN precipitates, thereby improving the bake hardenability without deteriorating the aging resistance.
  • Equation 2 increase of the Al content is advantageous in view of the bake hardenability.
  • Al must be added at an amount of at least 0.08% or more in order to achieve advantageous effects of the present invention.
  • the Al content is above 0.12%, oxide inclusions are increased during steel making process and causes degradation of surface quality along with deterioration of the formability. Furthermore, the excessive content of Al results in high manufacturing costs.
  • the Al content is preferably in the range of 0.08 - 0.12%.
  • Nitrogen (N) exists in the solid solution state before or after annealing, and deteriorates the formability of the steel. Furthermore, since nitrogen has a higher aging ability than other interstitial solid solution elements, it is necessary to fix nitrogen by use of Ti or Al.
  • the yield strength and the r-value of the steel are lowered due to the solute nitrogen, it is preferable to have a nitrogen content of 0.0025% or less.
  • Titanium (Ti) is added to the steel as a carbide and nitride forming element, and forms nitrides such as TiN, sulfides such as TiS or Ti C S , and carbides such as TiC, in the steel.
  • Ti is added at an amount of 0.003% or less so as to fix a small amount of nitrogen.
  • the reason of adding such a small amount of Ti is that, generally, when manufacturing the steel in practice, an ultra low amount of Ti is contained with other components to the steel for the purpose of satisfying the properties of the steel, and that, when it is simultaneously added to steel slabs several times for the purpose of continuous casting of the steel, Ti of a previously added steel slab can be transferred into a subsequently added steel slab of the present invention.
  • Ti is not necessarily added to the steel, and, if any, Ti is added at an ultra low amount of 0.003% or less under consideration of practical manufacturing conditions irrespective of reduction in bake hardenability thereby.
  • Niobium (Nb) is a very important element together with Al and Mo in the present invention.
  • Nb is an intensive carbide and nitride former, and serves to control an amount of solute carbon in the steel by pinning carbon of the steel into NbC precipitates.
  • NbC precipitates are very fine compared with other precipitates, it acts as an intensive barrier to impede the grain growth during recrystallization annealing.
  • the grain refining effect of Nb is obtained by use of such effect of the NbC precipitates.
  • the present invention is suggested in an attempt to improve the bake hardenability using the solute carbon, which is allowed to remain in the steel.
  • the Nb content is preferably in the range of 0.003 - 0.011 % under consideration of the carbon content of 16 - 25 ppm according to the present invention to obtain both of the bake hardenability and the aging resistance by allowing an amount of solute carbon of about 3 ⁇ 7 ppm to remain in the crystal grains of the steel while providing the grain refining effect by the NbC precipitates.
  • Molybdenum (Mo) is another very important element of the present invention.
  • Mo exists in the solid solution state in the steel, and serves to enhance the strength of the steel or to form an Mo-based carbide.
  • Mo serves to increase the bonding force of the grain boundaries while existing as a solid solution element in the steel, so that fracture of the grain boundaries due to phosphorus is prevented, that is, the secondary work embrittlement resistance is improved.
  • Mo since Mo has an affinity to carbon, it serves to suppress the diffusion of carbon, improving the aging resistance. Equation 3 represents the effect of improving the aging resistance by Mo in a quantitative manner. For this purpose, it is necessary to add a suitable amount of Mo.
  • the Mo content is preferably in the range of 0.01 ⁇ 0.1%.
  • B Boron
  • B is an interstitial element residing in the steel. B is dissolved as a solid solution element in the grain boundaries, or combines with nitrogen to form the nitride such as BN.
  • B has a highly significant influence on the properties of the steel compared with an added amount, it is necessary to precisely control the amount added. That is, when even a small amount of B is added, B is segregated in the grain boundaries and improves the secondary work embrittlement resistance.
  • the content of B is preferably in the range of 0.0005 ⁇ 0.0015%.
  • the steel slab After manufacturing a steel slab having the composition as described above, the steel slab is reheated at a temperature of 1,200 °C or more, where austenite structure before hot rolling can be sufficiently homogenized.
  • the reheated steel slab is then subjected to hot-rolling with finish rolling at a finish rolling temperature of 900 ⁇ 950 °C, which is just above the Ar 3 temperature, thereby providing a hot rolled steel sheet.
  • the structure of the steel is likely to become multi-phase structure, and cannot have homogeneous austenite crystal grains, causing deterioration in properties of the steel.
  • finish hot rolling temperature is less than 900°C, a top portion, a tail portion, and edges of a hot-rolled coil become single-phase regions, thereby increasing the anisotropy while deteriorating the formability of the steel. If the finish hot rolling temperature is above 950°C, crystal grains of the steel become remarkably coarsened, causing defects such as orange peel to be formed on the surface of the steel sheet after machining.
  • the steel sheet has refined crystal grains, which improve the aging resistance and the secondary work embrittlement resistance, but it suffers from an excessive increase in yield strength along with deterioration of the formability due to the excessively refined grain size.
  • the coiling temperature is preferably controlled to 630°C or less.
  • the coiling temperature is controlled to be in the range of 580 ⁇ 630 °C as an exemplary means to make the amount of the solute carbon in the grains, and the amount of the solute carbon in the grain boundaries satisfy Equation 1.
  • cold rolling of the hot rolled steel sheet is performed at a reduction rate of 75 - 80%.
  • Such a high reduction rate of 75% or more is set for the purpose of enhancing the formability of the steel sheet, in particular, the r-value, together with the aging resistance through the grain refining effect.
  • the steel sheet has a high grain refining effect.
  • such an excessive reduction rate results in gradual decrease of the r-value, and excessive reduction in grain size, which makes the steel sheet hard.
  • the steel sheet After cold rolling, the steel sheet is subjected to continuous annealing by a typical method at a temperature of 770 - 830 °C.
  • annealing of the steel sheet is preferably performed at a temperature of 770 °C or more. That is, when the annealing is performed at a temperature less than 770 °C, non-recrystallized crystal grains exist in the steel sheet, causing an increase of the yield strength while reducing the elongation and the r-value.
  • the formability can be enhanced.
  • ASTM No. 9 which is the desired ASTM grain size in the present invention
  • the steel sheet has the AI of 30 MPa or less, and thus is deteriorated in aging resistance.
  • the cold-rolled steel sheet is subjected to temper rolling at a reduction ratio of 1.2 ⁇ 1.5%, which is more or less higher than a typical temper rolling reduction ratio.
  • the reason of such a higher reduction ratio of 1.2 or more for the temper rolling is to prevent the aging resistance from being deteriorated due to the solute carbon in the steel.
  • the reduction ratio of the temper rolling is set to an excessively high value above 1.5%, work hardening occurs and deteriorates the properties of the steel sheet irrespective of improved aging resistance.
  • the temper rolling is preferably performed at the reduction ratio of 1.2 ⁇ 1.5%, which is a suitable condition to solve the above problems.
  • the hot rolled steel sheets were coiled at a coiling temperature of 610 ⁇ 630 °C. Then, the hot rolled steel sheets were subjected to cold rolling at a reduction rate of 75 - 78%, followed by continuous annealing at an annealing temperature of 800 - 820 °C. Then, the annealed cold-rolled steel sheets were subjected to hot-dipping at a temperature of 450 - 470 °C and galvannealing at a temperature of 500 - 530 °C, followed by temper rolling at a temper rolling reduction ratio of about 1.5%.
  • Fig. 3 the amount of solute carbon was measured using an internal friction tester (Horizontal type, 10 KHz).
  • Inventive steel Nos. 1 to 6 have an amount of solute carbon of 3.1 - 6.6 ppm in crystal grains, and thus satisfy the condition for the amount of solute carbon in the grains according to the present invention, which is in the range of 3 - 7 ppm.
  • the inventive steels do not have solute nitrogen in the steel and has an amount of solute carbon of 3.1 ppm or more in the grains.
  • solute carbon in the crystal grains influences the bake hardenability.
  • the Inventive steels Nos. 1 to 6 have a grain size of ASTM No. 9.8 - 11.5 (average grain size of 6.7 - 12.0 ⁇ m), and thus satisfy the condition for the grain size of the present invention, which is ASTM No. 9 or more.
  • the Inventive steels Nos. 1 to 6 have fine crystal grains. That is, since the Inventive steel Nos. 1 to 6 have higher Al contents than a typical Al content, fine AlN precipitates are actively formed in the steel and obstruct the grain growth along with the NbC precipitates upon recrystallization annealing, which results in such fine crystal grains.
  • the inventive steels have a bake hardening degree of 38.1 ⁇ 47.9 MPa, and an AI of 9.3 - 28.3 MPa, which is used for indicating the aging resistance at room temperature. Accordingly, it can be appreciated that balance between the bake hardenability and the aging resistance is excellent.
  • the Inventive steels Nos. 1 to 6 have a relatively low AI in comparison to a relatively high bake hardening degree. It is considered that this phenomenon is based on a retarding effect of the solute carbon in the steel through addition of Mo in addition to the grain refining effect by the AlN precipitates.
  • the DBTT at the draw ratio of 2.0 is in the range of -40 - -60 °C.
  • Comparative steel No. 7 has a carbon content of 0.0054%, which is higher than the carbon content of the present invention in the range of 0.0016 ⁇ 0.0025%, but satisfies the conditions for the high coiling temperature and the annealing temperature of the present invention.
  • the Comparative steel No. 7 has a very fine grain size of ASTM No. 11.7 that satisfies the condition for the grain size of the present invention, and is excellent in view of DBTT and BH value.
  • the Comparative steel No. 7 has an AI of 30 MPa or more, which indicates a significantly low aging resistance.
  • Comparative steel No. 8 has a significantly low carbon content of 0.0011%, which forms the NbC precipitates without remaining in a solid solution state in the steel. Thus, it can be found that the Comparative steel No. 8 does not exhibit the bake hardenability at all, and has coarsened grains and a low DBTT due to the low carbon content.
  • Comparative steel No. 9 has a soluble Al content of 0.023%, which is lower than the Al content of the present invention in the range of 0.08 - 0.12%, and an Nb content of 0.035%, which is higher than the Nb content of the invention.
  • the Comparative steel No. 9 the grain refining effect and the BH value increasing effect by the AlN precipitates were not achieved. Furthermore, since the high Nb content causes all carbon in the steel to be precipitated into NbC, the Comparative steel No. 9 hardly exhibits the bake hardenability and has a low DBTT due to a low site competition effect between C and P caused by reduction in amount of solute carbon in the steel.
  • Comparative steel No. 10 does not comprise Mo, and thus cannot be expected to have improved secondary work embrittlement resistance by Mo.
  • the Comparative steel No. 10 was coiled at a temperature of 720 °C, which is higher than the coiling temperature of the present invention, causing a high possibility of activating movement of P.
  • the Comparative steel No. 10 is significantly deteriorated in DBTT irrespective of excellent bake hardenability and high AI value.
  • Comparative steel No. 11 has a very high Ti content of 0.035%, which causes no solute carbon to remain in the steel. Thus, the BH value and the AI of Comparative steel No. 11 are zero.
  • the Comparative steel No. 11 does not comprises the solute carbon, which is advantageous to improve the secondary work embrittlement resistance, and cannot be expected to have an increased bonding force of the grain boundaries by addition of B, so that the DBTT of the Comparative steel No. 11 is deteriorated.
  • Comparative steel No. 12 sufficiently satisfies the requirement for the composition suggested by the present invention except for a high content of nitrogen.
  • Nitrogen is the element which causes a detrimental problem in terms of bake hardenability and aging resistance.
  • the excessively high content of nitrogen results in a high amount of solute nitrogen of 11.3 ppm in the grains and high AI value, which is detrimental to the bake hardenability and the aging resistance.
  • the bake-hardenable cold-rolled steel sheet has excellent bake hardenability, aging resistance, and secondary work embrittlement resistance, as well as high strength.

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Claims (3)

  1. Une tôle d'acier laminée à froid, durcissable par cuisson, présentant une résistance mécanique élevée et une résistance élevée au vieillissement, comprenant, en % en poids: C: 0,0016 à 0,0025%, Si: 0,02% ou moins; Mn: 0,2 à 1,2%, P: 0,05 à 0,11%, S: 0,01% ou moins; Sol. de Al: 0,08 à 0,12%, N: 0,0025% ou moins; Ti: 0 à 0,003%; Nb: 0,003 à 0,011%; Mo : 0,01 à 0,1%, B: 0,0005 à 0,0015%,
    le reste consistant en Fe et autres impuretés inévitables, dans laquelle
    la tôle d'acier satisfaisant à l'équation 1: C * quantité de soluté de carbone dans les joints des grains GB - C + quantité de soluté de carbone dans les grains des cristaux G - C = Total C ppm - C dans NbC = 8 à 15 ppm
    Figure imgb0013
    [dans l'équation 1, GB-C (c'est-à-dire, la quantité de carbone dissous dans les joints de grains) est de 5 à 10 ppm, et G-C (c'est-à-dire, la quantité de carbone dissous dans des grains des cristaux) est de 3 à 7 ppm], et
    dans laquelle la tôle d'acier a une granulométrie selon ASTM N° 9 ou plus, un valeur de la trempabilité par cuisson (BH) de 30 Mpa ou plus, un indice de vieillissement (AI) de 30 Mpa ou moins, et une DBTT de -30 °C ou moins à un taux d'étirage de 2,0, la valeur de la trempabilité par cuisson (BH) et l'indice de vieillissement (AI) satisfaisant aux Equations 2 et 3, respectivement: BH = 50 - 885 x Ti - 1589 x Nb + 62 x Al
    Figure imgb0014
     AI = 44 - 423 x Ti - 2119 x Nb - 125 x Mo
    Figure imgb0015
  2. Une tôle d'acier laminée à froid, durcissable par cuisson, présentant une résistance élevée et une résistance élevée au vieillissement selon la revendication 1, dans laquelle la tôle laminée à froid est galvanisée-recuite.
  3. Un procédé de fabrication d'une tôle d'acier laminée à froid, durcissable par cuisson, présentant une résistance élevée et une résistance élevée au vieillissement, comprenant,
    - le traitement thermique d'homogénéisation d'une brame d'acier calmé à l'Al à 1200 °C ou plus, la brame d'acier comprenant en % en poids: C: 0,0016 à 0,0025%, Si: 0,02% ou moins; Mn: 0,2 à 1,2%, P: 0,05 à 0,11%, S: 0,01% ou moins; Sol. de Al: 0,08 à 0,12%, N: 0,0025% ou moins; Ti: 0 à 0,003%; Nb: 0,003 à 0,011%; Mo: 0,01 à 0,1%, B: 0,0005 à 0,0015%,
    le reste consistant en Fe et autres impuretés inévitables,
    - le laminage à chaud de la brame d'acier avec un laminage de finition à une température de laminage de finition de 900 à 950° C, pour former une tôle d'acier laminée à chaud, suivi par le bobinage de la tôle d'acier laminée à chaud à une température de 580 à 630°C;
    - le laminage à froid de la tôle d'acier laminée à chaud à un taux de réduction de 75 à 80%,
    - le recuit en continu de la tôle d'acier laminée à froid à une température de 770 à 830 °C, et
    - l'écrouissage de la tôle d'acier recuit à un taux de réduction de 1,2 à 1,5%.
EP06798860.0A 2005-09-23 2006-09-22 Feuille d'acier laminee a froid, durcissable a la cuisson dotee d'une resistance superieure et d'une resistance au vieillissement, et procede de fabrication de la feuille d'acier laminee a froid Active EP1937853B1 (fr)

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PCT/KR2006/003777 WO2007035059A1 (fr) 2005-09-23 2006-09-22 Feuille d'acier laminee a froid, durcissable a la cuisson dotee d'une resistance superieure et d'une resistance au vieillissement, feuille d'acier recuite par galvanisation au moyen de ladite feuille d'acier laminee a froid, et procede de fabrication de la feuille d'acier laminee a froid

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