EP1704261B1 - Acier lamine a froid durci au four presentant une excellente formabilite, et procede de fabrication dudit acier - Google Patents

Acier lamine a froid durci au four presentant une excellente formabilite, et procede de fabrication dudit acier Download PDF

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Publication number
EP1704261B1
EP1704261B1 EP04808506A EP04808506A EP1704261B1 EP 1704261 B1 EP1704261 B1 EP 1704261B1 EP 04808506 A EP04808506 A EP 04808506A EP 04808506 A EP04808506 A EP 04808506A EP 1704261 B1 EP1704261 B1 EP 1704261B1
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Prior art keywords
steel sheet
less
steel
set forth
precipitates
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EP04808506A
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German (de)
English (en)
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EP1704261A4 (fr
EP1704261A1 (fr
Inventor
Jeong-Bong Yoon
Won-Ho Son
Ki-Bong Kang
Noi-Ha Cho
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Posco Holdings Inc
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Posco Co Ltd
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Priority claimed from KR1020030095394A external-priority patent/KR101105132B1/ko
Priority claimed from KR1020030095395A external-priority patent/KR101104981B1/ko
Priority claimed from KR1020030095393A external-priority patent/KR101105007B1/ko
Priority claimed from KR1020030099350A external-priority patent/KR101105098B1/ko
Priority claimed from KR1020030099351A external-priority patent/KR101105025B1/ko
Priority claimed from KR1020040071395A external-priority patent/KR101115709B1/ko
Priority claimed from KR1020040071705A external-priority patent/KR101115763B1/ko
Priority claimed from KR1020040084297A external-priority patent/KR101115842B1/ko
Application filed by Posco Co Ltd filed Critical Posco Co Ltd
Priority claimed from PCT/KR2004/003375 external-priority patent/WO2005061748A1/fr
Publication of EP1704261A1 publication Critical patent/EP1704261A1/fr
Publication of EP1704261A4 publication Critical patent/EP1704261A4/fr
Publication of EP1704261B1 publication Critical patent/EP1704261B1/fr
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    • 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
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
    • C21D9/46—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/002—Ferrous alloys, e.g. steel alloys containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60
    • 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
    • C22C38/12—Ferrous alloys, e.g. steel alloys containing tungsten, tantalum, molybdenum, vanadium, or niobium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium

Definitions

  • the present invention relates to cold rolled steel sheets for automobile bodies, and the like. More particularly, the present invention relates to bake-hardenable cold rolled steel sheets, improved in bake hardenability and formability by controlling a content of solid solution carbon in crystal grains with fine precipitates, and a method of manufacturing the same.
  • bake-hardening cold rolled steel sheets are generally used in order to improve dent resistance.
  • the bake-hardening cold rolled steel sheets have excellent ductility through press forming, and increased yield strength through paint baking or coating treatment after press forming. That is, as carbon or nitrogen is in solid solution in the steel as interstitial elements, and fixes dislocations created upon press forming, the yield point of the bake-hardening cold rolled steel sheets is increased.
  • the bake-hardening cold rolled steel sheets include aluminum-killed steels, which are batch-annealed materials, and interstitial free steels (IF steels).
  • IF steels As solid solution carbon or nitrogen in the steel is completely precipitated by adding titanium or niobium to the steel, the formability of the steel is enhanced.
  • Bake hardening IF steels are manufactured by imparting the bake hardenability to the IF steels.
  • the bake hardenability can be ensured by allowing an appropriate amount of carbon to remain in the steel through control of an added amount of titanium or niobium and an added amount of carbon.
  • the present invention has been made in view of the above problems, and it is an object of the present invention to provide bake-hardenable cold rolled steel sheets, which are improved in bake hardenability and formability due to a higher plasticity-anisotropy index and a lower in-plane anisotropy index without adding Ti and Nb, and a method of manufacturing the same.
  • a bake-hardenable cold rolled steel sheet comprising: 0.003 - 0.005 % of C; 0.003 - 0.03 % S; 0.01 ⁇ 0.1 % of Al; 0.02 % or less ofN; 0.2 % or less of P; at least one of 0.03 - 0.2 % of Mn and 0.005 - 0.2 % of Cu; and the balance of Fe and other unavoidable impurities, in terms of weight%, wherein, when the steel sheet comprises one of Mn and Cu, a composition of Mn, Cu, and S satisfies one of the relationships: 0.58*Mn/S ⁇ 10 and 1 ⁇ 0.5*Cu/S ⁇ 10 in terms of weight, and when the steel sheet comprises both Mn and Cu, a composition of Mn, Cu, and S satisfies the relationships: Mn+Cu ⁇ 0.3 and 2 ⁇ 0.5*(Mn+Cu)/S ⁇ 20 in
  • the cold rolled steel sheet of the present invention can be classified into three types in accordance with added elements selected from the group consisting of Mn and Cu. That is, (1) Mn solely-added steel (Cu excluded, which will also be referred to as “MnS-precipitated steel”), (2) Cu solely-added steel (Mn excluded, which will also be referred to as “CuS-precipitated steel”), and (3) Mn and Cu added steel (which will also be referred to as "MnCu-precipitated steel”), which will be described in detail as follows.
  • the above bake-hardenable cold rolled steel sheet of the present invention may be applied to ductile cold rolled steel sheets having a 240 MPa-grade tensile strength or to high strength cold rolled steel sheets having a 340 MPa-grade or more tensile strength.
  • the steel sheet comprises 0.003 ⁇ 0.005 % of C, 0.003 ⁇ 0.03 % of S; 0.01 ⁇ 0.1 % of Al; 0.004 % or less of N; 0.015 % or less of P; at least one of 0.03 ⁇ 0.2 % of Mn and 0.005 ⁇ 0.2 % of Cu; and the balance of Fe and other unavoidable impurities, in terms of weight%, wherein, when the steel sheet comprises one of Mn and Cu, the composition of Mn, Cu, and S satisfies one of the relationships: 0.58*Mn/S ⁇ 10 and 1 ⁇ 0.5*Cu/S ⁇ 10 in terms of weight, and when the steel sheet comprises both Mn and Cu, the composition of Mn, Cu, and S satisfies the relationship: Mn+Cu ⁇ 0.3 and 2 ⁇ 0.5*(Mn+Cu)/S ⁇ 20 in terms of weight, and wherein the precipitates of M
  • the high strength cold rolled steel sheets in a 340 MPa-grade or more it can be classified into steel, which contains at least one of P, Si, and Cr, as a solid solution-intensifier, and steel which contains a higher content of N, as a precipitation-intensifier. That is, it is preferred that at least one of 0.2 % or less of P, 0.1 ⁇ 0.8 % of Si, and 0.2 - 1.2 % of Cr be contained in the ductile cold rolled steel sheet. If P is solely added to the ductile cold rolled steel sheet, the content of P is preferably in the range of 0.03 ⁇ 0.2 %. Alternatively, high strength characteristics can be ensured by means of AlN precipitates by increasing the content of N to 0.005 - 0.02 %, and adding 0.03 ⁇ 0.06 % of P.
  • the steel sheet may further comprise 0.01 ⁇ 0.2 % of Mo.
  • the bake-hardenable cold rolled steel sheets allows the content of solid solution in the crystal grains to be controlled by means of fine MnS, CuS, (Mn, Cu)S precipitates, thereby providing improved bake hardenability, formability, yield strength, and yield strength-ductility balance.
  • the inventors of the present invention have discovered new facts, as will be described below, through investigations into enhancement of bake hardenability without adding Ti and Nb. That is, the content of solid solution carbon in crystal grains can be appropriately controlled with fine precipitates of MnS, CuS, or (Mn, Cu) S, thereby increasing the yield strength, specifically the post-bake yield strength. These precipitates may have positive effects not only on an increase of the yield strength caused by precipitation strengthening, but also on the plasticity-anisotropy index and the in-plane anisotropy index.
  • the finer the distribution of MnS, CuS, and (Mn, Cu)S precipitates the greater the reduction in content of the solid solution carbon in the crystal grains.
  • This is attributed to the relatively free movement of the solid solution carbon remaining in the crystal grain.
  • the solid solution carbon can move freely in the crystal grains, and be then easily coupled to movable dislocations, thereby influencing aging characteristics at room temperature.
  • the solid solution carbon can also be moved to more stable regions, such as grain boundaries or around the precipitations, segregated in these regions, and activated therein at a high temperature, for example, during paint baking treatment, thereby influencing the bake hardenability.
  • reduction in content of the solid solution carbon in the crystal grains means that carbon exists in the more stable region, such as the grain boundaries or around the fine precipitates, and influences the bake hardenability.
  • Figs. 1a to 1c are graphical representations showing the relationship between the content of solid solution carbon in crystal grains and the size of precipitates, in which Fig. 1a shows the case of MnS-precipitated steel, Fig. 1b shows the case of CuS-precipitated steel, and Fig. 1c shows the case of MnCu-precipitated steel.
  • the finer the size of the precipitates the greater the reduction in the content of the solid solution carbon in the crystal grains, and for the carbon, which does not exist in the crystal grains among the total content of the carbon, it effectively influences the bake hardenability. Referring to in Figs.
  • the content of the solid solution carbon in the crystal grains is reduced to approximately 20 ppm or less, when the MnS precipitates have a size of about 0.2 ⁇ m or less ( Fig. 1a ), when the CuS precipitates have a size of about 0.1 ⁇ m or less ( Fig. 1b ), and when the MnCu-precipitates have a size of about 0.2 ⁇ m or less ( Fig. 1c ).
  • the MnS precipitates have a size of 0.2 ⁇ m or less by controlling the cooling rate of the steel sheet under the condition wherein the combination of Mn and S satisfies the relationship: 0.58*Mn/S ⁇ 10.
  • the bake-hardenable cold rolled steel sheet in accordance with the invention has high yield strength, and thus allows a reduction in thickness of the steel sheet.
  • the cold rolled steel sheet in accordance with the invention has an effect of weight reduction for the products thereof.
  • the low in-plane anisotropy of the cold rolled steel sheet of the invention minimizes formation of wrinkles or ears during or after processing of the steel sheet.
  • the cold rolled steel sheet of the invention also has grain boundaries reinforced due to an appropriate content of the carbon remaining in the grain boundaries by the fine precipitates, thereby preventing the brittleness fracture caused by grain boundaries weakened after processing.
  • the bake hardenability is increased without damaging aging characteristics at room temperature as a higher content of carbon is segregated in the grain boundaries or around the fine precipitates. That is, as the carbon content in the steel sheet increases, the increased carbon content is segregated in the grain boundaries or around the fine precipitates, which is an effective way to increase the bake hardenability of the steel sheet.
  • the carbon content must be 0.003 % or more in the steel sheet in order to ensure the bake hardenability. That is, in order to enhance the bake hardenability, the carbon content exceeds 0.0030 % and 0.0031 % or more. However, if the carbon content exceeds 0.005 %, the formability can be rapidly reduced. Accordingly, the carbon content is preferably in the range of 0.003 ⁇ 0.005 %.
  • a sulfur content less than 0.003 % can lead to not only decrease in amount of MnS, CuS and (Mn, Cu) precipitates, but also creation of excessively coarse precipitates, thereby lowering the bake hardenability of the steel sheet.
  • a content of sulfur more than 0.03 % can lead to a large amount of solid solution sulfur, thereby remarkably decreasing the ductility and the formability of the steel sheet, and increasing the possibility of hot shortness.
  • the sulfur content is preferably in the range of 0.005 ⁇ 0.03 %
  • the sulfur content is preferably in the range of 0.003 ⁇ 0.025 %.
  • the sulfur content is preferably in the range of 0.003 ⁇ 0.025 %.
  • Aluminum is an alloying element generally used as a deoxidizing agent.
  • aluminum is added to the steel for precipitating nitrogen in the steel, and thus prevents the formability from being lowered due to solid solution nitrogen.
  • An aluminum content of less than 0.01 % can lead to an increase in content of solid solution nitrogen, thereby lowering the formability, whereas an aluminum content of more than 0.1 % can lead to an increase of solid solution aluminum, thereby lowering the ductility of the steel.
  • the aluminum content is preferably in the range of 0.01 ⁇ 0.08 %. If the nitrogen content is increased to 0.005 ⁇ 0.02%, a high strength steel sheet can be obtained by virtue of strengthening effects of AlN precipitates.
  • Nitrogen is an unavoidable element introduced into the steel during the steel manufacturing process, and in order to obtain strengthening effects, it is preferably added to the steel in an amount not exceeding 0.02 %.
  • the nitrogen content is preferably 0.004 % or less.
  • the nitrogen content is preferably 0.005 ⁇ 0.02 %.
  • a nitrogen content of more than 0.02 % leads to deterioration in formability of the steel sheet.
  • a phosphorus content is preferably 0.03 ⁇ 0.06 %.
  • the combination of Al and N that is, 0.52*Al/N (where Al and N are denoted in terms of wt%) is preferably in the range of 1 ⁇ 5.
  • the combination of Al and N (0.52*Al/N) of less than 1 can lead to lowering the formability due to the solid solution nitrogen, whereas the combination of Al and N (0.52*Al/N) exceeding 5 leads to negligible strengthening effects.
  • the P content is an alloying element, which can increase solid solution strengthening effects while allowing a slight reduction in r-value (plasticity-anisotropy index), and can ensure the high strength of the steel in which the precipitates are controlled. Accordingly, in order to ensure the high strength by controlling the content of P, the P content is preferably 0.2 % or less. A phosphorus content of more than 0.2 % can lead to a reduction in ductility of the steel sheet. When phosphorous alone is added to the steel in order to ensure the high strength of the steel sheet, the P content is preferably 0.03 ⁇ 0.2 %. For the ductile steel sheet, the P content is preferably 0.015 % or less.
  • the P content is preferably 0.03 ⁇ 0.06 %. This is attributed to the fact that, although a phosphorus content of 0.03 % or more enables a target strength to be ensured, a phosphorus content exceeding 0.06 % can lower the ductility and formability of the steel.
  • the P content in the case where the high strength of the steel sheet is ensured by addition of Si and Cr, the P content can be appropriately controlled within 0.2 wt% or less in order to obtain the target strength. In this case, even if the P content is 0.015 % or less, the high strength can be ensured.
  • At least one of manganese (Mn) and copper (Cu) is preferably added to the steel. These elements are combined with sulfur (S), and create the MnS, CuS or (Mn, Cu)S precipitates.
  • Manganese is an alloying element, which precipitates the solid solution sulfur in the steel as the MnS precipitates, thereby preventing the hot shortness caused by the solid solution sulfur.
  • Mn is precipitated as the fine MnS and/or (Mn, Cu)S precipitates under appropriate conditions for the combination of S and/or Cu with Mn and for the cooling rate.
  • the fine precipitates can impart the bake hardenability to the steel sheet during the paint baking treatment by causing carbon to be segregated in the grain boundaries or around the precipitates rather than the crystal grains.
  • the Mn content must be 0.03 % or more.
  • the manganese content is preferably 0.05 ⁇ 0.2 %.
  • Copper is an alloying element, which creates fine precipitates under appropriate conditions for the combination of S and/or Mn with Cu, and the cooling rate before a winding process in a hot rolling process.
  • the fine precipitates can impart the bake hardenability to the steel sheet during the paint baking treatment by causing carbon to be segregated in the grain boundaries or around the precipitates rather than the crystal grains.
  • the Cu content must be 0.005 % or more. Meanwhile, a copper content greater than 0.2 % causes coarse precipitates due to a higher content of copper, thereby deteriorating the bake hardenability of the steel sheet. If Cu alone is added to the steel (excluding Mn), the copper content is preferably 0.01 ⁇ 0.2 %.
  • the contents and the combination of Mn, Cu and S are controlled so as to create fine precipitates, and these are varied according to the added amount of Mn and/or Cu.
  • the combination of Mn and S preferably satisfies the relationship: 0.58*Mn/S ⁇ 10 (where Mn and S are denoted in terms of wt%).
  • Mn combines with S to create the MnS precipitates.
  • the MnS precipitates can be varied in a precipitated state according to the added amount of Mn and S, and thereby influence the bake hardenability, the yield strength, and the in-plane anisotropy index of the steel sheet.
  • a value of 0.58*Mn/S greater than 10 creates coarse MnS precipitates, resulting in deterioration of the bake hardenability and the in-plane anisotropy index.
  • the combination of Cu and S preferably satisfies the relationship: 1 ⁇ 0.5*Cu/S ⁇ 10 (where Cu and S are denoted in terms of wt%).
  • Cu combines with S to create the CuS precipitates, which can be varied in a precipitated state according to the added amount of Cu and S, and thereby influence the bake hardenability, the plasticity-anisotropy index, and the in-plane anisotropy index.
  • a value of 0.5*Cu/S of 1 or more enables effective CuS precipitates to be created, and a value of 0.5*Cu/S greater than 10 creates coarse CuS precipitates, resulting in deterioration of the bake hardenability, the plasticity-anisotropy index, and of the in-plane anisotropy index.
  • the value of 0.5*Cu/S is preferably 1 ⁇ 3.
  • the total content of Mn and Cu is preferably 0.3 % or less. This is attributed to the fact that a total Mn and Cu content exceeding 0.3 % is likely to create coarse precipitates, thereby detracting from the bake hardenability.
  • the value of 0.5*(Mn+Cu)/S (where Mn, Cu, and S are denoted in terms of wt%) is preferably 2 ⁇ 20.
  • Mn and Cu combine with S to create the MnS, CuS, and (Mn, Cu)S precipitates, which can be varied in a precipitated state according to the added amount of Mn, Cu, and S, and influence the bake hardenability, the plasticity-anisotropy index, and the in-plane anisotropy index.
  • a value of 0.5*(Mn+Cu)/S of 2 or more enables effective precipitates to be created, and a value of 0.5*(Mn+Cu)/S exceeding 20 creates coarse precipitates, resulting in deterioration of the bake hardenability, the plasticity-anisotropy index, and the in-plane anisotropy index.
  • the average size of the precipitates is reduced to 0.2 ⁇ m or less.
  • the kinds and the number of the precipitates are remarkably varied. Specifically, when the value of 0.5*(Mn+Cu)/S is 7 or less, lots of very fine MnS and CuS separate precipitates are uniformly distributed rather that the (Mn, Cu)S complex precipitates.
  • the number of precipitates distributed in the grain is decreased because of an increase in amount of the (Mn, Cu)S complex precipitates.
  • an increase in the number of precipitates can enhance the bake hardenability, the in-plane anisotropy index, the secondary work embrittlement resistance, and the like.
  • the MnS, CuS, and (Mn, Cu)S precipitates preferably have an average size of 0.2 ⁇ m or less.
  • the MnS, CuS, and (Mn, Cu)S precipitates can have different appropriate sizes according to an added amount of Mn and Cu.
  • the precipitates have a size of 0.2 ⁇ m or less for the MnS precipitates, a size of 0.1 ⁇ m or less for the CuS precipitates, and a size of 0.2 ⁇ m or less for the mixture of MnS, CuS, and (Mn, Cu)S precipitates.
  • the bake hardenability is particularly deteriorated, as well as deteriorating the plasticity-anisotropy index and the in-plane anisotropy index. As the size of the precipitates is reduced, it is preferred in terms of the bake hardenability.
  • At least one of the solid solution strengthening elements that is, at least one of P, Si, and Cr may be added to the steel sheet.
  • the effects obtained by adding phosphorus were previously described, the description thereof will be omitted.
  • Si is an alloying element, which can increase the solid solution strengthening effect while allowing a slight reduction in ductility, thus ensuring high strength of the steel sheet in which the precipitates are controlled according to the present invention.
  • the silicon content of 0.1 % or more can ensure the strength of the steel sheet, but a silicon content exceeding 0.8 % can cause a reduction in the ductility thereof.
  • Cr is an alloying element, which can increase solid solution strengthening effects while enhancing aging resistance at room temperature, thus ensuring the high strength of the steel sheet while reducing the in-plane anisotropy index of the steel sheet in which the precipitates are controlled according to the present invention.
  • the chrome content of 0.2 % or more can ensure the strength of the steel sheet, but a chrome content of more than 1.2 % can cause the reduction in the ductility thereof.
  • Molybdenum (Mo) may be added to the cold rolled steel sheet of the present invention.
  • Molybdenum 0.01 ⁇ 0.2 %.
  • Mo is an alloying element, which can increase the plasticity-anisotropy index of the steel sheet.
  • the molybdenum content of 0.01 % or more can increase the plasticity-anisotropy index, but a molybdenum content exceeding 0.2 % can cause hot shortness without any additional improvement in the plasticity-anisotropy index.
  • the present invention is characterized in that steel sheets satisfying the above-described compositions may be processed to have a finely reduced average size of precipitates through hot rolling and cold rolling.
  • the average size of the precipitates is influenced by the contents and composition of Mn, Cu, and S, and the manufacturing process, and in particular, is directly influenced by a cooling rate after hot rolling.
  • the steel satisfying the above-described compositions is reheated, followed by hot rolling.
  • the reheating temperature is preferably 1,100 °C or more. This is attributed to the fact that a reheating temperature lower than 1,100 °C causes coarse precipitates to be created during continuous casting and to remain in an incompletely dissolved state, whereby the coarse precipitates remain even after the hot rolling.
  • the hot rolling is performed under the condition that finish rolling is performed at an Ar 3 transformation temperature or more. If the finish rolling is performed below the Ar 3 transformation temperature, rolled grains are created, and remarkably lower the ductility as well as the formability of the steel sheet.
  • the cooling rate is preferably 200 °C/min or more after the hot rolling. More specifically, there is a slight difference between the cooling rates of (1) MnS-precipitated steel, (2) CuS-precipitated steel, and (3) MnCu-precipitated steel.
  • the cooling rate is preferably 200 °C/min or more. Even when the composition of Mn and S satisfies the relationship: 0.58*Mn/S ⁇ 10 according to the present invention, a cooling rate lower than 200 °C/min can create coarse MnS precipitates having a size greater than 0.2 ⁇ m. That is, as the cooling rate is increased, a number of nuclei are created, so that the MnS precipitates become finer.
  • the cooling rate is 1,000 °C/min or more, the MnS precipitates are not further reduced in size, and thus the cooling rate is more preferably in the range of 200 ⁇ 1,000 C /min.
  • the cooling rate is preferably 300 °C/min or more after the hot rolling. Even when the composition of Cu and S satisfies the relationship: 0.5*Cu/S ⁇ 10 according to the present invention, a cooling rate lower than 300 °C/min creates coarse CuS precipitates having a size greater than 0.1 ⁇ m. That is, as the cooling rate is increased, a number of nuclei are created, so that the CuS precipitates become finer.
  • the composition of Cu and S has the relationship: 0.5*Cu/S > 10, the number of coarse precipitates in an incompletely dissolved state during the reheating process is increased, so that an increase of the cooling rate does not result in an increase of the number of nuclei, and thus the CuS precipitates do not become any finer ( Fig. 3c , 00039 % C; 0.01 % P; 0.005 % S; 0.03 % Al; 0.0015 % N; and 0.28 % Cu).
  • Figs. 3a to 3c since an increase of the cooling rate leads to creation of finer CuS precipitates, it is not necessary to provide an upper limit of the cooling rate. However, even when the cooling rate is 1,000 °C/min or more, the CuS precipitates are not further reduced in size, and thus the cooling rate is more preferably in the range of 300 ⁇ 1,000 °C/min.
  • Figs. 3a and 3b (0.0043 % C; 0.01 % P; 0.005 % S; 0.03 % Al; 0.0024 % N; and 0.081 % Cu) show the cases of 0.5*Cu/S ⁇ 3, and of 0.5*Cu/S > 3, respectively. As shown in the drawings, it can be seen that, when the value of 0.5*Cu/S is 3 or less, the CuS precipitates having a size of 0.1 ⁇ m or less can be more stably obtained.
  • the cooling rate is preferably 300 °C/min or more. Even when the composition of Mn, Cu and S satisfies the relationship: 2 ⁇ 0.5*(Mn+Cu)/S ⁇ 20 according to the present invention, a cooling rate lower than 300 °C/min creates coarse precipitates having an average size greater than 0.2 ⁇ m. That is, as the cooling rate is increased, a number of nuclei are created, so that the precipitates become finer.
  • the cooling rate is 1,000 °C/min or more, the precipitates are not further reduced in size, and thus the cooling rate is more preferably in the range of 300 ⁇ 1,000 °C/min.
  • the winding process is preferably performed at a temperature of 700 °C or less.
  • the winding process is performed at a temperature higher than 700 °C, the precipitates are grown too coarsely, thereby reducing the bake hardenability of the steel.
  • the steel is cold rolled to a desired thickness, preferably at a reduction rate of 50 ⁇ 90 %. Since a reduction rate lower than 50 % leads to creation of a small amount of nuclei upon recrystallization annealing, the crystal grains are grown excessively upon annealing, thereby coarsening of the crystal grains recrystallized through annealing, which results in reduction of the strength and formability. A cold reduction rate more than 90 % leads to enhanced formability, while creating an excessive number of nuclei, so that the crystal grains recrystallized through annealing become excessively fine, thereby reducing the ductility of the steel.
  • the continuous annealing temperature plays an important role in determining the mechanical properties of the products.
  • the continuous annealing is preferably performed at a temperature of 500 - 900 °C.
  • Continuous annealing at a temperature lower than 500 °C creates excessively fine recrystallized crystal grains, so that a desired ductility cannot be ensured.
  • Continuous annealing at a temperature higher than 900 °C creates coarse recrystallized crystal grains, so that the strength of the steel is reduced.
  • Holding time upon the continuous annealing is provided so as to complete the recrystallization of the steel, and the recrystallization of the steel can be completed for about 10 seconds or more upon the continuous annealing.
  • cold rolled steel sheets were machined to standard samples according to ASTM standards (ASTM E-8 Standard), and the mechanical properties thereof were measured.
  • the mechanical properties were measured by use of a tensile strength tester (available from INSTRON Company, Model No. 6025).
  • the post-bake yield strength was measured, after the samples were subjected to 2 % strain, followed by the heat treatment at 120 °C for twenty minutes.
  • samples A1 ⁇ A4 have excellent yield strength, elongation ratio, and yield strength-ductility balance as well as the bake hardenability. Additionally, these samples have a high plasticity-anisotropy index and a low in-plane anisotropy index, thereby providing excellent formability.
  • sample A5 due to its low carbon content, sample A5 provides low post-bake yield strength. Due to its large size of the precipitates, sample A6 also has low post-bake yield strength. Due to its high carbon content, sample A7 has low elongation ratio and plasticity anisotropy index, thereby providing a high possibility of fracture during the forming process. Due to its low post-bake yield strength and high secondary work embrittlement temperature, sample A8, which is a conventional IF steel sheet, provides a high possibility of fracture upon impact.
  • Samples A9 to A12 have excellent formability together with the bake hardenability. On the contrary, due to its high amount of Mo added, sample A13 has poor formability.
  • YS Yield strength
  • TS Tensile strength
  • EI Elongation
  • r-value Plasticity-anisotropy index
  • ⁇ r-value In-plane anisotropy index
  • PBYS Post-bake yield strength
  • DBTT Ductility-brittleness transition temperature for investigating secondary work embrittlement
  • AS Average size of precipitates
  • IS Steel of the invention
  • CS Comparative steel
  • the finish rolling was performed at 910 °C, which is above the Ar 3 transformation temperature, and the continuous annealing was performed by heating the steel sheets to 750 °C at a velocity of 10 °C/second for 40 seconds. Exceptionally, for samples F8 to F10 of Table 11, after reheating to a temperature of 1,200 °C, followed by finish rolling, these samples were cooled at a cooling rate of 550 °C/minute, and then coiled at 650 °C. Table 11 Sample No.

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  • Mechanical Engineering (AREA)
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  • Metallurgy (AREA)
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  • Physics & Mathematics (AREA)
  • Thermal Sciences (AREA)
  • Crystallography & Structural Chemistry (AREA)
  • Heat Treatment Of Sheet Steel (AREA)

Abstract

L'invention concerne une feuille d'acier laminée à froid durcie au four présentant une excellente formabilité, et un procédé de fabrication dudit acier. La feuille d'acier est mise au point de manière à présenter d'excellentes propriétés de durcissement au four et de formabilité appropriées pour la carrosserie d'automobile ou analogue. La feuille d'acier de l'invention comprend 0,003 à 0,005 % de C; 0,003 à 0,03 % de S; 0,01 à 0,1 % de Al; au plus 0,02 % de N; au plus 0,2 % de P; 0,03 à 0,2 % de Mn et/ou 0,005 à 0,2 % de Cu; ainsi que l'équilibre de Fe et d'autres impuretés inévitables en pourcentage en poids. Lorsque la feuille d'acier contient Mn ou Cu, la composition de Mn, Cu et S satisfait l'un de rapports: 0,58*Mn/S=10 et 1=0,5*Cu/S=10. Lorsqu'elle contient Mn et Cu, la composition satisfait les rapports: Mn+Cu=0,3 et 2=0,5*(Mn+Cu)/S=20. Les précipités de MnS, CuS et (Mn, Cu)S présentent une dimension moyenne d'au plus 0,2 µm. Les feuilles d'acier de l'invention permettent au précipités fins de MnS, CuS et (Mn, Cu)S de réguler la teneur de la solution solide, ce qui améliore les propriétés de durcissement, de formabilité, de limite d'élasticité, et l'équilibre entre la limite d'élasticité et la ductilité.

Claims (46)

  1. Tôle d'acier laminée à froid pouvant être durcie au four présentant une excellente aptitude au formage, comprenant : 0,003 ~ 0,005 % de C ; 0,003 ~ 0, 03 % de S ; 0,01 ~ 0, 1 % d'Al ; 0, 02 % ou moins de N ; 0,2 % ou moins de P ; au moins l'un parmi 0,03 ~ 0,2 % de Mn et 0,005 ~ 0,2 % de Cu ; et comprenant facultativement au moins l'un parmi 0,1 ~ 0,8 % de Si, 0,2 ~ 1,2 % de Cr et 0,01 ~ 0,2 % de Mo ; le reste étant constitué de Fe et d'autres impuretés inévitables, en termes de % en poids, dans laquelle, lorsque la tôle d'acier comprend l'un de Mn et de Cu, une composition de Mn, de Cu et S satisfait l'une des relations : 0,58*Mn/S ≤ 10 et 1 ≤ 0,5*Cu/S ≤ 10, et lorsque la tôle d'acier comprend à la fois du Mn et du Cu, une composition de Mn, Cu et S satisfait les relations : Mn+Cu ≤ 0,3 et 2 ≤ 0,5*(Mn+Cu)/S ≤ 20, et dans laquelle les précipités de MnS, de CuS, et de (Mn, Cu)S ont une taille moyenne de 0,2 µm ou moins.
  2. Tôle d'acier laminée à froid pouvant être durcie au four présentant une excellente aptitude au formage selon la revendication 1, comprenant : 0,003 ~ 0,005 % de C; 0,005 ~ 0,03 % de S; 0,01 ~ 0,1 % d'Al ; 0,02 % ou moins de N ; 0,2 % ou moins de P ; 0,05 ∼ 0,2 % de Mn ; le reste étant constitué de Fe et d'autres impuretés inévitables, en termes de % en poids, dans laquelle une composition de Mn et de S satisfait la relation : 0,58*Mn/S ≤ 10 en termes de poids, et dans laquelle les précipités de MnS ont une taille moyenne de 0,2 µm ou moins ; et la tôle d'acier comprend facultativement au moins l'un parmi 0,1 ~ 0,8 % de Si, 0,2 ~ 1,2 % de Cr et 0,01 ~ 0,2 % de Mo.
  3. Tôle d'acier selon la revendication 2, dans laquelle la tôle d'acier comprend 0,015 % ou moins de P.
  4. Tôle d'acier selon la revendication 2, dans laquelle la tôle d'acier comprend 0,004 % ou moins de N.
  5. Tôle d'acier selon la revendication 2, dans laquelle la tôle d'acier comprend 0,03 - 0,2 % de P.
  6. Tôle d'acier selon la revendication 2, dans laquelle la tôle d'acier comprend 0,005 ∼ 0,02 % de N et 0,03 ∼ 0,06 % de P.
  7. Tôle d'acier selon la revendication 6, dans laquelle une composition d'Al et de N satisfait la relation : 1 ≤ 0,52*Al/N ≤ 5.
  8. Tôle d'acier laminée à froid pouvant être durcie au four présentant une excellente aptitude au formage selon la revendication 1, comprenant : 0,003 - 0,005 % de C ; 0,003 - 0,025 % de S ; 0,01 ∼ 0,08 % d'Al ; 0,02 % ou moins de N ; 0,2 % ou moins de P ; 0,01 ∼ 0,2 % de Cu ; le reste étant constitué de Fe et d'autres impuretés inévitables, en termes de % en poids, dans laquelle une composition de Cu et de S satisfait la relation : 1 ≤ 0,5*Cu/S ≤ 10 en termes de poids, et dans laquelle les précipités de CuS ont une taille moyenne de 0,1 µm ou moins ; et la tôle d'acier comprend facultativement au moins l'un parmi 0,1 ~ 0,8 % de Si, 0,2 ~ 1,2 % de Cr et 0,01 ~ 0,2 % de Mo.
  9. Tôle d'acier selon la revendication 8, dans laquelle la tôle d'acier comprend 0,015 % ou moins de P.
  10. Tôle d'acier selon la revendication 8, dans laquelle la tôle d'acier comprend 0,004 % ou moins de N.
  11. Tôle d'acier selon la revendication 8, dans laquelle la composition de Cu et de S satisfait la relation : 1 ≤ 0,5*Cu/S ≤ 3.
  12. Tôle d'acier selon la revendication 8, dans laquelle la tôle d'acier comprend 0,03 ∼ 0,2 % de P.
  13. Tôle d'acier selon la revendication 8, dans laquelle la tôle d'acier comprend 0,005 ~ 0,02 % de N et 0,03 ∼ 0,06 % de P.
  14. Tôle d'acier selon la revendication 13, dans laquelle une composition d'Al et de N satisfait la relation : 1 ≤ 0,52*Al/N ≤ 5.
  15. Tôle d'acier laminée à froid pouvant être durcie au four présentant une excellente aptitude au formage selon la revendication 1, comprenant : 0,003 ~ 0,005 % de C ; 0,003 ∼ 0,025 % de S; 0,01 ~ 0,08 % d'Al ; 0,02 % ou moins de N ; 0,2 % ou moins de P ; 0,03 - 0,2 % de Mn ; 0,005 - 0,2 % de Cu ; le reste étant constitué de Fe et d'autres impuretés inévitables, en termes de % en poids, dans laquelle une composition de Mn, de Cu et de S satisfait les relations : Mn+Cu ≤ 0,3 et 2 ≤ 0,5*(Mn+Cu)/S ≤ 20 en termes de poids, et dans laquelle les précipités de MnS, de CuS, et de (Mn, Cu)S ont une taille moyenne de 0, 2 µm ou moins; et la tôle d'acier comprend facultativement au moins l'un parmi 0,1 ~ 0,8 % de Si,0,2 ~ 1,2 % de Cr et 0,01 ~ 0,2 % de Mo.
  16. Tôle d'acier selon la revendication 15, dans laquelle la tôle d'acier comprend 0,015 % ou moins de P.
  17. Tôle d'acier selon la revendication 15, dans laquelle la tôle d'acier comprend 0,004 % ou moins de N.
  18. Tôle d'acier selon la revendication 15, dans laquelle le nombre de précipités est de 2 x 106 ou plus par unité d'aire (mm2).
  19. Tôle d'acier selon la revendication 15, dans laquelle la composition de Mn, de Cu et de S satisfait la relation : 2 ≤ 0,5*(Mn+Cu)/S ≤ 7.
  20. Tôle d'acier selon la revendication 19, dans laquelle le nombre de précipités est de 2 x 108 ou plus par unité d'aire (mm2) .
  21. Tôle d'acier selon la revendication 15, dans laquelle la tôle d'acier comprend 0,03 ∼ 0,2 % de P.
  22. Tôle d'acier selon la revendication 15, dans laquelle la tôle d'acier comprend 0,005 ∼ 0,2 % de N et 0,03 ~ 0,06 % de P.
  23. Tôle d'acier selon la revendication 22, dans laquelle une composition d'Al et de N satisfait la relation : 1 ~ 0,52*Al/N ≤ 5.
  24. Procédé de fabrication d'une tôle d'acier laminée à froid pouvant être durcie au four selon la revendication 1, comprenant les étapes suivantes :
    laminage à chaud d'une brame d'acier avec un laminage de finition à une température de transformation Ar3 ou plus pour fournir une tôle d'acier laminée à chaud, après le réchauffage de la brame d'acier à une température de 1 100 °C ou plus ;
    refroidissement de la tôle d'acier à une vitesse de 200 °C/min ou plus ;
    enroulage de la tôle d'acier refroidie à une température de 700 °C ou moins ;
    laminage à froid de la tôle d'acier ; et
    recuit continu de la tôle d'acier laminée à froid.
  25. Procédé de fabrication d'une tôle d'acier laminée à froid pouvant être durcie au four selon la revendication 24, dans lequel la brame d'acier comprend : 0,003 - 0,005 % de C ; 0,005 - 0,03 % de S ; 0,01 ∼ 0,1 % d'Al ; 0,02 % ou moins de N ; 0,2 % ou moins de P ; 0,05 ~ 0,2 % de Mn ; le reste étant constitué de Fe et d'autres impuretés inévitables, en termes de % en poids, dans lequel une composition de Mn et de S satisfait la relation : 0,58*Mn/S ≤ 10 en termes de poids, et la brame d'acier comprend facultativement au moins l'un parmi 0,1 - 0,8 % de Si, 0,2 ~ 1,2 % de Cr et 0,01 ~ 0,2 % de Mo.
  26. Procédé selon la revendication 25, dans lequel la brame d'acier comprend 0,015 % ou moins de P.
  27. Procédé selon la revendication 25, dans lequel la brame d'acier comprend 0,004 % ou moins de N.
  28. Procédé selon la revendication 25, dans lequel la brame d'acier comprend 0,03 ∼ 0,2 % de P.
  29. Procédé selon la revendication 25, dans lequel la brame d'acier comprend 0,005 ~ 0,2 % de N et 0,03 ~ 0,06 % de P.
  30. Procédé selon la revendication 25, dans lequel une composition d'Al et de N satisfait la relation : 1 ≤ 0,52*Al/N ≤ 5.
  31. Procédé de fabrication d'une tôle d'acier laminée à froid pouvant être durcie au four selon la revendication 24, dans lequel la brame d'acier comprend : 0,003 ∼ 0,005 % de C ; 0,003 ∼ 0,025 % de S; 0,01 ∼ 0,08 % d'Al ; 0,02 % ou moins de N ; 0,2 % ou moins de P ; 0,01 ~ 0,2 % de Cu ; le reste étant constitué de Fe et d'autres impuretés inévitables, en termes de % en poids, dans lequel une composition de Cu et de S satisfait la relation : 1 ≤ 0,5*Cu/S ≤ 10, en termes de poids, et la brame d'acier comprend facultativement au moins l'un parmi 0,1 ∼ 0,8 % de Si, 0,2 ∼ 1,2 % de Cr et 0,01 ∼ 0,2 % de Mo, et dans lequel l'étape de refroidissement de la tôle d'acier est mise en oeuvre à une vitesse de 300 °C/min ou plus.
  32. Procédé selon la revendication 31, dans lequel la brame d'acier comprend 0,015 % ou moins de P.
  33. Procédé selon la revendication 31, dans lequel la brame d'acier comprend 0,004 % ou moins de N.
  34. Procédé selon la revendication 31, dans lequel la composition de Cu et de S satisfait la relation : 1≤ 0,5*Cu/S ≤ 3.
  35. Procédé selon la revendication 31, dans lequel la brame d'acier comprend 0,03 ∼ 0,2 % de P.
  36. Procédé selon la revendication 31, dans lequel la brame d'acier comprend 0,005 ~ 0,02 % de N et 0,03 ~ 0,06 % de P.
  37. Procédé selon la revendication 31, dans lequel la composition d'Al et de N satisfait la relation : 1 ≤ 0,52*Al/N ≤ 5.
  38. Procédé de fabrication d'une tôle d'acier laminée à froid pouvant être durcie au four selon la revendication 24, dans lequel la brame d'acier comprend : 0,003 ∼ 0,005 % de C ; 0,003 ∼ 0,025 % de S; 0,01 ∼ 0,08 % d'Al ; 0,02 % ou moins de N ; 0,2 % ou moins de P ; 0,03 ~ 0,2 % de Mn ; 0,005 ∼ 0,2 % de Cu ; le reste étant constitué de Fe et d'autres impuretés inévitables, en termes de % en poids, dans lequel une composition de Mn, de Cu et de S satisfait les relations : Mn+Cu ≤ 0.3 et 2 ≤ 0,5*(Mn+Cu)/S ≤ 20, en termes de poids, et la brame d'acier comprend facultativement au moins l'un parmi 0,1 ∼ 0,8 % de Si, 0,2 ∼ 1,2 % de Cr et 0,01 ∼ 0,2 % de Mo, et dans lequel l'étape de refroidissement de la tôle d'acier est mise en oeuvre à une vitesse de 300 °C/min ou plus.
  39. Procédé selon la revendication 38, dans lequel la brame d'acier comprend 0,015 % ou moins de P.
  40. Procédé selon la revendication 38, dans lequel la brame d'acier comprend 0,004 % ou moins de N.
  41. Procédé selon la revendication 38, dans lequel le nombre de précipités est de 2 x 106 ou plus par unité d'aire (mm2).
  42. Procédé selon la revendication 38, dans lequel la composition de Mn, de Cu et de S satisfait la relation : 2 ≤ 0,5*(Mn+Cu)/S ≤ 7.
  43. Procédé selon la revendication 42, dans lequel le nombre de précipités est de 2 x 108 ou plus par unité d'aire (mm2).
  44. Procédé selon la revendication 38, dans lequel la brame d'acier comprend 0,03 ∼ 0,2 % de P.
  45. Procédé selon la revendication 38, dans lequel la brame d'acier comprend 0,005 ~ 0,02 % de N et 0,03 ~ 0,06 % de P.
  46. Procédé selon la revendication 45, dans lequel une composition d'Al et de N satisfait la relation : 1 ≤ 0,52*Al/N ≤ 5.
EP04808506A 2003-12-23 2004-12-21 Acier lamine a froid durci au four presentant une excellente formabilite, et procede de fabrication dudit acier Not-in-force EP1704261B1 (fr)

Applications Claiming Priority (19)

Application Number Priority Date Filing Date Title
KR1020030095395A KR101104981B1 (ko) 2003-12-23 2003-12-23 내2차가공취성이 우수한 소부경화형 고강도 냉연강판과 그제조방법
KR1020030095393A KR101105007B1 (ko) 2003-12-23 2003-12-23 소부경화형 냉연강판과 그 제조방법
KR1020030095394A KR101105132B1 (ko) 2003-12-23 2003-12-23 소부경화형 고강도 냉연강판과 그 제조방법
KR20030099435 2003-12-29
KR1020030099351A KR101105025B1 (ko) 2003-12-29 2003-12-29 면내이방성이 작은 소부경화형 고강도 냉연강판과 그제조방법
KR20030098743 2003-12-29
KR20030099437 2003-12-29
KR1020030099350A KR101105098B1 (ko) 2003-12-29 2003-12-29 가공성이 우수한 소부경화형 고강도 냉연강판과 그 제조방법
KR20030098746 2003-12-29
KR20030098745 2003-12-29
KR20030098744 2003-12-29
KR20030099464 2003-12-30
KR20030099462 2003-12-30
KR20030099461 2003-12-30
KR20030099463 2003-12-30
KR1020040071395A KR101115709B1 (ko) 2004-09-07 2004-09-07 가공성이 우수한 소부경화형 냉연강판과 그 제조방법
KR1020040071705A KR101115763B1 (ko) 2004-09-08 2004-09-08 가공성이 우수한 소부경화형 고강도 냉연강판과 그 제조방법
KR1020040084297A KR101115842B1 (ko) 2004-10-21 2004-10-21 가공성이 우수한 소부경화형 고강도 냉연강판과 그 제조방법
PCT/KR2004/003375 WO2005061748A1 (fr) 2003-12-23 2004-12-21 Acier lamine a froid durci au four presentant une excellente formabilite, et procede de fabrication dudit acier

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JP4957829B2 (ja) * 2010-05-11 2012-06-20 Ｊｆｅスチール株式会社 冷延鋼板およびその製造方法
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US4410372A (en) * 1981-06-10 1983-10-18 Nippon Steel Corporation Process for producing deep-drawing, non-ageing, cold rolled steel strips having excellent paint bake-hardenability by continuous annealing
JPH05195060A (ja) * 1992-01-13 1993-08-03 Kobe Steel Ltd 耐時効性、プレス成形性の優れた焼付硬化型冷延鋼板の製造方法
JP3958921B2 (ja) * 2000-08-04 2007-08-15 新日本製鐵株式会社 塗装焼付硬化性能と耐常温時効性に優れた冷延鋼板及びその製造方法

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EP1704261A1 (fr) 2006-09-27
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