Classifications

• • 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
  • C22C38/14—Ferrous alloys, e.g. steel alloys containing titanium or zirconium
• • 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
  • C21D6/00—Heat treatment of ferrous alloys
  • C21D6/02—Hardening by precipitation
• • 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
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
  • C21D9/46—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/001—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
  • C22C38/002—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
  • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese

Definitions

• the present invention relates to a high- strength cold-rolled steel sheet suitable for automotive outer panels. More particularly, the present invention relates to a high- strength cold-rolled steel sheet, which has an r of 1.3 or less, an average plastic strain ratio r near 1, a low in-plane anisotropy index Dr of 0.15 or less, thereby providing excellent shape-fixability so as to allow isotropic plastic deformation of the steel sheet during press forming, and a method for manufacturing the same.
• the automotive outer panels since the panels are generally subjected to deformation in a stretching mode, it is advantageous to provide cold-rolled steel sheets which are subjected to uniform deformation in the plane direction and have low biaxial yield strength. With such cold-rolled steel sheets having excellent shape- fixability so as to permit uniform plastic deformation in the plane direction and low biaxial yield strength, the automotive outer panels which have a complicated shape can be advantageously produced.
• Elongation is one of the mechanical properties of a material, and measures a percentage change in length of the material which elongates without fracture when tensile force is applied to the material.
• high elongation of a steel sheet permits large deformation of the steel sheet.
• Plastic strain ratio "r" is a value which can be defined by a ratio of strain in a width direction to strain in a thickness direction.
• a high plastic strain ratio means that, assuming that a steel sheet has a constant strain amount in the width direction, a steel sheet with a high plastic strain ratio has a low strain in the thickness direction when applying tensile force to the steel sheet by a predetermined deformation amount in a certain direction, and thus the steel sheet can be worked without necking even with a large deformation amount.
• the plastic strain ratio is caused by anisotropic properties of the steel sheet, and thus exhibits different values according to tensile directions.
• Dr (r- 2r + r )/2 — - (2)
• FIG. 1 shows theoretical results based on Taylor polycrystal modeling as to influence of plastic strain ratio on a locus of the yield strength of steel which comprises two different major textures.
• IF Interstitial Free
• the low Dr means that the distribution of strain is uniform in the plane direction of the steel sheet during the press forming, and is advantageous for forming of the steel sheet while leading to uniform deformation thereof in a stretching mode.
• steel having an r approaching 1 and a low Dr enhances the shape fixability during work for the automotive outer panels which will be subjected to major deformation in the stretching mode.
• Ti or Nb is added as a single component or a mixture thereof to an ultra- low carbon cold-rolled steel sheet, and solid-solutions C and N are precipitated as a carbide and nitride to improve the elongation and the plastic strain ratio, thereby enhancing formability.
• the in-plane anisotropy of the steel sheet is reduced to lower defects such as plane defects during the press forming.
• the in-plane anisotropy of the ultra-low carbon cold-rolled steel sheet is lowered through grain refinement of hot-rolled structures using a quenching apparatus immediately after finishing mill.
• Nb-added ultra-low carbon steel has relatively high r and Dr, it exhibits severe in-plane anisotropy and high biaxial yield strength for the deformation in the stretching mode, and thus is disadvantageous in terms of shape fixability even though it exhibits excellent formability in a deep drawing mode.
• Dr Nb-added ultra-low carbon steel
• DE 3843732, DE 3803064, and US Patent No. 5,139,580 disclose a method for manufacturing high strength cold-rolled steel sheets having isotropic plasticity by controlling a carbide and fine textures during hot-rolling and annealing through addition of Ti, Nb, V, and the like, which are carbide formation elements in the low carbon steel.
• Japanese Patent Laid-open Publication No.(Hei) 10-130780 discloses a technique for manufacturing high-strength isotropic steel from Ti or Nb added low- carbon steel using a continuous annealing apparatus. The purpose of this technique is to manufacture steel sheets of a low Dr using strong correlation between recrys- tallization total elongation and Dr of the Ti or Nb-added steel sheet.
• US Patent No.6,162,308 discloses a technique for manufacturing a high-strength isotropic steel sheet from Ti and/or Nb added low-carbon steel using a continuous annealing apparatus. Since the purpose of the conventional technique is to manufacture a non-aging low-carbon steel sheet which does not require overaging, it is necessary to add at least one of Cu, V and Ni up to an amount of 0.15 % in addition to Ti and Nb. In addition, since the conventional steel sheet has Dr in the range of 0.15 ⁇ 0.28, it is undesirable in view of isotropy.
• the present invention has been made to solve the above problems, and it is an object of the present invention to provide a high-strength isotropic cold-rolled steel sheet, which is made using a low-carbon steel comprising a little amounts of Ti to have an r of 1.3 or less, an r approaching 1 , and a Dr of 0.15 or less, ensuring excellent
• a high-strength cold-rolled steel sheet having excellent shape-fixability comprising: 0.01 ⁇ 0.05 % of C; 0.005 ⁇ 0.06 % of Ti; 0.1 ⁇ 1 % of Mn; 0.1 % or less of Si; 0.03 % or less of P; 0.03 % or less of S; 0.08 % or less of Sol.
• the steel sheet comprises 0.015 ⁇ 0.035 % of C.
• the steel sheet comprises 0.01 ⁇ 0.04 % of Ti.
• a method for manufacturing a high-strength cold-rolled steel sheet having excellent shape-fixability comprising the steps of: finish rolling steel at an Ar temperature or more to provide a hot rolled steel sheet, the steel comprising: 0.01 ⁇ 0.05 % of C; 0.005 ⁇ 0.06 % of Ti; 0.1 ⁇ 1 % of Mn; 0.1 % or less of Si; 0.03 % or less of P; 0.03 % or less of S; 0.08 % or less of Sol.
• the steel comprises 0.015 ⁇ 0.035 % of C.
• the steel comprises 0.01 ⁇ 0.04 % of Ti.
• rapid quenching of the hot rolled steel sheet is performed within 1 second of completion of finish rolling.
• annealing of the steel sheet is performed at a temperature of 760 ⁇
• the steel sheet is heated at a rate of 3 °C/sec or more for annealing.
• the present invention can provide a high- strength isotropic cold-rolled steel sheet, which has an r of 1.3 or less, an r ap-
• Fig. 1 is a diagram illustrating a relationship between a plastic strain ratio and a locus of yield strength
• FIG. 2 is a diagram illustrating a continuous annealing process in accordance with one embodiment of the present invention, and change in microstructure by the continuous annealing process;
• FIG. 3 is a diagram illustrating components of major textures developing in steel
• Fig. 4 is a diagram illustrating influence of the textures on an r value
• Fig. 5 is a crystallographic orientation map of Inventive steel A obtained using an
• Fig. 6 is an optical micrograph obtained after continuous annealing of the Inventive steel A.
• the inventors of the present invention have theoretically found that as an r of steel m is lowered to near 1, the biaxial yield strength of the steel is also lowered, thereby providing excellent shape-fixability to the steel. Then, various investigations were continuously carried out by the inventors in order to provide a technique for manufacturing a cold-rolled steel sheet using low-carbon steel comprising a little amounts of Ti to have excellent shape-fixability, isotropic structure, and an aging index of 30 MPa or less such that the steel sheet can be suitably used for automotive outer panels.
• the high-strength cold-rolled steel sheet having an r of 1.3 or less, an r approaching 1, a low Dr of 0.15 or less, and an aging index of 30 MPa can be m manufactured through a continuous annealing apparatus.
• Carbon is an interstitial solid solution element in steel, and has a very significant influence on strength and texture of a steel sheet during cold rolling and annealing while existing in the form of cementite.
• the carbon content is preferably in the range of 0.01 ⁇ 0.05 %. When the carbon content is less than 0.01 %, the steel sheet is lowered in strength, and excessively increased in Dr.
• the carbon content be 0.01 % or more.
• C is coupled with Fe to form the cementite in the steel, C can be stably present in the steel.
• the present invention in order to avoid room temperature aging, it is necessary to have an appropriate amount of C such that C is precipitated to the cementite in the steel. Since an excessive amount of C causes a significant increase in strength, and reduction in ductility of the steel so that cold rolling properties of the steel is deteriorated, it is preferable that the upper limit of carbon content be 0.05 % or less.
• the carbon content is in the range of 0.015 ⁇ 0.035 %.
• C is coupled with Ti to precipitate TiC in the steel.
• the precipitated TiC provides precipitation hardening effect to the steel, which results in an increase in the strength of the steel.
• Normal Direction (ND) advantageous for reduction in Dr servers to extend recovery and recrystallization rates of crystal grains which have a crystallographic orientation in parallel to ⁇ l l l> ( ⁇ 111> / /ND), so that a fraction of the crystal grains having a crystallographic direction of ⁇ 111 >//ND is lowered.
• trace amounts of C are precipitated to Ti C S at high temperature, which is coarser than TiC, and thus has substantially no influence on development in crystallographic orientation of recrystalline grains.
• Ti is one of the most important elements in addition to C.
• Ti is coupled with N as well as C to form TiN, and provides effects of suppressing formation of AlN.
• AlN precipitates formed during hot rolling cause elongation of a hot rolled structure, thereby increasing shape anisotropy of the steel sheet.
• Ti serves to lower a fraction of crystal grains having an orientation of strong anisotropy by suppressing formation of AlN while precipitating TiC, and thus has effect of lowering Dr and increasing the strength of the steel by virtue of precipitation hardening.
• Ti is an expensive element, it is advantageous in view of manufacturing costs to add as little Ti to the steel as possible.
• Ti is in the range of 0.005 ⁇ 0.06 % under consideration of manufacturing costs without deteriorating the effects obtained by addition of Ti. More preferably, Ti is in the range of 0.01 ⁇ 0.04 0 Ic.
• Ti in order to permit Ti to be precipitated to TiC during annealing while suppressing formation of AlN, Ti must be added to the steel such that a ratio of Ti to N (Ti/N) is more than 5, i.e. Ti/N > 5.
• Ti refers to an effective Ti content, which is an amount of Ti necessary to form TiC excluding an amount of Ti necessary to form TiN in order to suppress the formation of AlN during hot rolling. More preferably, a ratio of C to the effective Ti (Ti * ), i.e. (48/12)C - Ti * , is in the range of 0.06 ⁇ 0.l l %.
• Mn is an effective element for solid solution strengthening in steel, and precipitates
• Si serves as a solid solution strengthening element in steel, and is preferably added to an amount of 0.1 % or less in order to ensure proper elongation of the steel.
• N and S Nitrogen and sulfur are unavoidable elements introduced into steel during a steel manufacturing process, and thus it is important to keep contents of N and S as low as possible.
• S content is preferably restricted to 0.03 % or less.
• N content is preferably restricted to 0.01 % or less.
• Sol. Al effectively serves as a deoxidation element of molten steel. However, since an excessive amount of Sol. Al can have a negative influence on the formability of the steel, the content of Sol. Al is preferably restricted to 0.08 % or less.
• steel formed to have the above composition through continuous casting may be used without being formed into an ingot.
• a steel ingot having the above composition may be used after being reheated. At this time, when forming the steel sheet using the ingot, it is desirable that the ingot be reheated to 1,200 0 C or more such that Ti C S formed
• hot rolling is performed to form a hot rolled steel sheet according to a typical process, and it is desirable that a final pass of finish rolling be terminated at a temperature of Ar or more. If the final temperature of hot rolling is lowered, the surface and the edges of the hot rolled steel sheet are hot-rolled at a temperature of two-phase region so that crystal grains become coarse and non-uniform, causing surface defects of the steel sheet during press forming.
• the steel sheet After finish rolling, the steel sheet is rapid quenched at a rate of 50 °C/sec or more to a coiling temperature or more on a Run Out Table (ROT) so as to form fine crystal grains in the hot rolled steel sheet. If the steel sheet is quenched at a rate less than 50 °C/sec, crystal grains become coarse. [96]
• the steel sheet is quenched within 1 second of completion of finish rolling so as to form finer crystal grains. Rapid quenching of the steel sheet can be performed using a high density cooler equipped in front of the ROT. After quenching the steel sheet, the steel sheet is preferably coiled at a temperature of 650 0 C or less. The reason being that a coiling temperature exceeding 650 0 C causes coarsening of TiC precipitates, which weakens the function of delaying recovery and recrystallization rate of sub-grains which have an orientation of strong anisotropy during annealing, thereby increasing a fraction of crystal grains having the orientation of strong anisotropy.
• the coiled steel sheet is acid-pickled by a typical process, and is then preferably subjected to cold rolling at a reduction rate of 50 ⁇ 80 %. If the reduction rate of cold rolling is less than 50 %, recrystallization does not sufficiently occurred during annealing, thereby lowering ductility, and if the reduction rate of cold rolling is more than 80 0 Ic, the in-plane anisotropy of the steel sheet is increased.
• annealing refers to continuous annealing as shown in
• Fig. 2 is performed at a recrystallization temperature or more, and at a temperature less than Ac or less. If the annealing temperature exceeds Ac , the steel sheet is annealed in a two-phase coexistence region of ⁇ and ⁇ , so that recrystallization and grain growth of the crystal grains having the orientation of strong anisotropy are promoted, causing coarsening of the crystal grains. Since coarsened crystal grains cause deterioration in strength and ductility at the same time, the annealing temperature is preferably restricted to a temperature of Ac or less.
• the annealing temperature is significantly lowered below the recrystallization temperature, ductility is deteriorated.
• the annealing temperature is in the range of 760 ⁇ 820 0 C.
• the cold-rolled steel sheet is preferably heated to the annealing temperature at a rate of 3 °C/sec or more. The reason being that a heating rate less than 3 °C/sec causes an increase in annealing period, thereby possibly coarsening the crystal grains.
• skin pass rolling is preferably performed upon the steel sheet at a reduction rate of 0.5 % or more.
• the cold-rolled steel sheet was subjected to heat treatment on a continuous annealing line.
• the steel sheet was heated to a maximum temperature of 780 ⁇ 800 0 C.
• the steel sheet was primarily cooled to 700 0 C at a rate of 5 °C/sec, and then secondarily cooled to 100 0 C at a rate of 60 °C/sec.
• the steel sheet was reheated to a temperature of 300 ⁇ 350 0 C, and was subjected to overaging for 3 minutes, and skin pass rolling at a reduction rate of 1 ⁇ 1.3 %.
• Tensile test of the annealed sheet obtained by the above processes was performed after processing the annealed sheet to EN10002-1 test samples.
• Table 2 shows conditions of manufacturing a cold-rolled steel sheet having the composition shown in Table 1, and results of a uni-axial test.
• FDT indicates final temperature of finish rolling
• CT indicates a coiling temperature
• ST indicates an annealing temperature
• YP indicates a yield strength
• TS indicates a tensile strength
• El indicates total elongation
• r indicates a plastic strain ratio in a directions of 90 degrees with respect to a rolling direction of the steel sheet
• Dr indicates an in-plane anisotropy index
• AI indicates an aging index.
• AI was calculated using a difference between flow stress after application of 7.5 % pre-strain before heating the steel sheet and flow stress after heating the steel sheet at 100 0 C for 1 hour.
• Comparative steels I ⁇ L deviate from the range of the present invention, and have a low Ti content relative to an N content. In other words, since Ti/ N is lower than 5 which is in the range of the present invention, these comparative steels has a Dr of 0.15 or more.
• Comparative steels I and J have manufacturing conditions wherein quenching start time thereof after finish rolling is longer than that of the present invention.
• Comparative steel M it is considered that, since a coiling temperature is higher than that of the invention, allowing TiC to be precipitated by solid solution C and coarsened in the hot rolled sheet, precipitation of TiC is insufficient during annealing, so that development of crystallographic orientation (
• Comparative steel O for Comparative steel O, a quenching start time is not in the range of the present invention. In comparison to a shortened quenching start time, the structure of the hot rolled steel sheet is coarsened, reducing the number of nucleation sites for cementite during cooling after annealing. Thus, Comparative steel O has a high room temperature aging index, and a Dr of 0.15 or more.
• Fig. 4 shows results of theoretical calculation for influence of texture on anisotropy in plastic strain ratio of major components of the texture shown in Fig. 3 using Taylor polycrystal theory.
• Fig. 5 is a Crystallographic Orientation Map (COM) of Inventive steel A obtained using an Electron Back-Scattered Diffraction (EBSD) apparatus attached to a Field Emission Scanning Electron Microscope (FE-SEM).
• EBSD Electron Back-Scattered Diffraction
• FE-SEM Field Emission Scanning Electron Microscope
• Fig. 6 shows results of analyzing crystal grains and cementite via optical microscopy. It can be seen from Fig. 6 that the cementite is mainly formed in the grain boundaries.
• ODF Orientation Distribution Function

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• 2004
  • 2004-09-30 KR KR1020040077814A patent/KR20060028909A/ko not_active Application Discontinuation
• 2005
  • 2005-09-30 US US11/664,182 patent/US20070289679A1/en not_active Abandoned
  • 2005-09-30 EP EP05856408A patent/EP1805339A4/en not_active Withdrawn
  • 2005-09-30 JP JP2007534514A patent/JP2008514820A/ja not_active Withdrawn
  • 2005-09-30 WO PCT/KR2005/003239 patent/WO2006080670A1/en active Application Filing
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