Classifications

• • C—CHEMISTRY; METALLURGY
  • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
  • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
  • C23C8/00—Solid state diffusion of only non-metal elements into metallic material surfaces; Chemical surface treatment of metallic material by reaction of the surface with a reactive gas, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals
  • C23C8/06—Solid state diffusion of only non-metal elements into metallic material surfaces; Chemical surface treatment of metallic material by reaction of the surface with a reactive gas, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals using gases
  • C23C8/08—Solid state diffusion of only non-metal elements into metallic material surfaces; Chemical surface treatment of metallic material by reaction of the surface with a reactive gas, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals using gases only one element being applied
  • C23C8/10—Oxidising
  • C23C8/12—Oxidising using elemental oxygen or ozone
  • C23C8/14—Oxidising of ferrous surfaces
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
  • C21D8/04—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing
  • C21D8/0447—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing characterised by the heat treatment
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
  • C21D8/04—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing
  • C21D8/0478—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing involving a particular surface treatment
• • 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
  • C21D9/561—Continuous furnaces for strip or wire with a controlled atmosphere or vacuum
• • 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
  • C22C38/06—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
  • C22C38/08—Ferrous alloys, e.g. steel alloys containing nickel
• • 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
  • C22C38/14—Ferrous alloys, e.g. steel alloys containing titanium or zirconium
• • 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
  • C22C38/16—Ferrous alloys, e.g. steel alloys containing copper
• • 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
• • 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
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/42—Ferrous alloys, e.g. steel alloys containing chromium with nickel with copper
• • 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
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/44—Ferrous alloys, e.g. steel alloys containing chromium with nickel with molybdenum or tungsten
• • 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
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/48—Ferrous alloys, e.g. steel alloys containing chromium with nickel with niobium or tantalum
• • 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
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/50—Ferrous alloys, e.g. steel alloys containing chromium with nickel with titanium or zirconium
• • 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
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/54—Ferrous alloys, e.g. steel alloys containing chromium with nickel with boron
• • 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
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/58—Ferrous alloys, e.g. steel alloys containing chromium with nickel with more than 1.5% by weight of manganese
• • C—CHEMISTRY; METALLURGY
  • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
  • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
  • C23C8/00—Solid state diffusion of only non-metal elements into metallic material surfaces; Chemical surface treatment of metallic material by reaction of the surface with a reactive gas, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals
  • C23C8/06—Solid state diffusion of only non-metal elements into metallic material surfaces; Chemical surface treatment of metallic material by reaction of the surface with a reactive gas, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals using gases
  • C23C8/08—Solid state diffusion of only non-metal elements into metallic material surfaces; Chemical surface treatment of metallic material by reaction of the surface with a reactive gas, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals using gases only one element being applied
  • C23C8/10—Oxidising
• • 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
  • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
  • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
  • C23C22/00—Chemical surface treatment of metallic material by reaction of the surface with a reactive liquid, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals
  • C23C22/05—Chemical surface treatment of metallic material by reaction of the surface with a reactive liquid, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals using aqueous solutions
  • C23C22/06—Chemical surface treatment of metallic material by reaction of the surface with a reactive liquid, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals using aqueous solutions using aqueous acidic solutions with pH less than 6
  • C23C22/07—Chemical surface treatment of metallic material by reaction of the surface with a reactive liquid, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals using aqueous solutions using aqueous acidic solutions with pH less than 6 containing phosphates
  • C23C22/08—Orthophosphates
  • C23C22/18—Orthophosphates containing manganese cations
  • C23C22/182—Orthophosphates containing manganese cations containing also zinc cations
  • C23C22/184—Orthophosphates containing manganese cations containing also zinc cations containing also nickel cations

Definitions

• the present invention relates to a method of the manufacturing a high strength steel sheet having excellent chemical convertibility and corrosion resistance after electrodeposition coating even in the case where the steel sheet has a high Si content, as well as to a method for manufacturing such steel sheets.
• silicon is effective for increasing the strength and the ductility of steel sheets.
• silicon is oxidized even if the annealing is performed in a reductive N 2 + H 2 gas atmosphere which does not induce the oxidation of Fe (which reduces Fe oxides).
• a silicon oxide (SiO 2 ) is formed on the outermost surface of a steel sheet. This SiO 2 inhibits a reaction for forming a chemical conversion film during a chemical conversion treatment, thereby resulting in formation of a microscopical region where any chemical conversion film is not generated. (Hereinafter, such a region will be sometimes referred to as "non-covered region"). That is, chemical convertibility is lowered.
• patent document 1 discloses a method in which an iron coating layer is electroplated at 20 to 1500 mg/m 2 onto a steel sheet.
• this method entails the provision of a separate electroplating facility and increases costs correspondingly to an increase in the number of steps.
• patent documents 2 and 3 provide an improvement in phosphatability by specifying the Mn/Si ratio and by adding nickel, respectively. However, the effects are dependent on the Si content in a steel sheet, and a further improvement will be necessary for steel sheets having a high Si content.
• Patent document 4 discloses a method in which the dew-point temperature during annealing is controlled to be -25 to 0°C so as to form an internal oxide layer which includes a Si-containing oxide within a depth of 1 ⁇ m from the surface of a steel sheet base as well as to control the proportion of the Si-containing oxide to be not more than 80% over a length of 10 ⁇ m of the surface of the steel sheet.
• the method described in patent document 4 is predicated on the idea that the dew-point temperature is controlled with respect to the entire area inside a furnace. Thus, difficulties are encountered in controlling the dew-point temperature and ensuring stable operation.
• patent document 5 describes a method in which the steel sheet temperature is brought to 350 to 650°C in an oxidative atmosphere so as to form an oxide film on the surface of the steel sheet, and thereafter the steel sheet is heated to a recrystallization temperature in a reductive atmosphere and subsequently cooled.
• this method it is often the case that the thickness of the oxide film formed on the surface of the steel sheet is variable depending on the oxidation method and that the oxidation does not take place sufficiently or the oxide film becomes excessively thick with the result that the oxide film leaves residue or is exfoliated during the subsequent annealing in a reductive atmosphere, thus resulting in a deterioration in surface quality.
• this patent document describes an embodiment in which oxidation is carried out in air.
• oxidation in air causes problems such as giving a thick oxide which is hardly reduced in subsequent reduction or requiring a reductive atmosphere with a high hydrogen concentration.
• patent document 6 describes a method in which a cold rolled steel sheet containing, in terms of mass%, Si at not less than 0.1% and/or Mn at not less than 1.0% is heated at a steel sheet temperature of not less than 400°C in an iron-oxidizing atmosphere to form an oxide film on the surface of the steel sheet, and thereafter the oxide film on the surface of the steel sheet is reduced in an iron-reducing atmosphere.
• iron on the surface of the steel sheet is oxidized at not less than 400°C using a direct flame burner with an air ratio of not less than 0.93 and not more than 1.10, and thereafter the steel sheet is annealed in a N 2 + H 2 gas atmosphere which reduces the iron oxide, thereby forming an iron oxide layer on the outermost surface while suppressing the oxidation of SiO 2 which lowers chemical convertibility from occurring on the outermost surface.
• Patent document 6 does not specifically describe the heating temperature with the direct flame burner.
• the oxidation amount of silicon which is more easily oxidized than iron, becomes large so as to suppress the oxidation of Fe or limit the oxidation of Fe itself to a too low level.
• the formation of a superficial reduced Fe layer by the reduction becomes insufficient and SiO 2 comes to be present on the surface of the steel sheet after the reduction, thus possibly resulting in a region which may not be covered with a chemical film.
• EP 1 936 000 A1 discloses a method of manufacturing a steel sheet by continuous annealing and hot dip plating, the annealing atmosphere being of the hydrogen reduction type.
• the present invention has been made in view of the circumstances described above. It is therefore an object of the invention to provide a method of manufacturing high strength steel sheet which exhibits excellent chemical convertibility and corrosion resistance after electrodeposition coating even in the case of a high Si content.
• a chemical conversion treatment is performed after annealing is carried out in such a manner that the dew-point temperature of the atmosphere is controlled to become not less than -10°C when the heating furnace inside temperature is in a limited range of not less than A°C and not more than B°C during the course of heating (A: 600 ⁇ A ⁇ 780, B: 800 ⁇ B ⁇ 900).
• the term "excellent chemical convertibility" means that a steel sheet having undergone a chemical conversion treatment has an appearance without any non-covered regions or uneven results of the chemical conversion treatment.
• a high strength steel sheet obtained in the above manner comes to have a microstructure and configuration in which a surface portion of the steel sheet extending from the steel sheet surface within a depth of 100 ⁇ m contains an oxide of at least one or more selected from Fe, Si, Mn, Al and P, as well as from B, Nb, Ti, Cr, Mo, Cu and Ni at 0.010 to 0.50 g/m 2 per single side surface, and in which a region extending from the steel sheet surface to a depth of 10 ⁇ m is such that a crystalline Si/Mn oxide has been precipitated in base iron grains that are within 1 ⁇ m from grain boundaries. Because of this configuration, deterioration in corrosion resistance after electrodeposition coating is realized and excellent chemical convertibility is obtained.
• the term "high strength" means that the tensile strength TS is not less than 340 MPa.
• the high strength steel sheets in the invention include both cold rolled steel sheets and hot rolled steel sheets.
• a high strength steel sheet is obtained which exhibits excellent chemical convertibility and corrosion resistance after electrodeposition coating even in the case where the steel sheet has a high Si content.
• a chemical conversion treatment is performed after a steel sheet is continuously annealed in such a manner that the dew-point temperature of the atmosphere is controlled to become not less than -10°C when the heating furnace inside temperature is in a limited range of not less than A°C and not more than B°C during the course of heating in an annealing furnace (A: 600 ⁇ A ⁇ 780, B: 800 ⁇ B ⁇ 900).
• oxides of easily oxidized elements such as Si and Mn
• internal oxides oxides of easily oxidized elements
• surface segregation selective surface oxidation of such elements as Si and Mn on the steel sheet surface that deteriorate the chemical convertibility of the steel after annealing
• the lower limit temperature A is limited to be 600 ⁇ A ⁇ 780 for the following reasons.
• the temperature is in the range of less than 600°C, the amount of surface segregation is inherently small. Thus, a deterioration in chemical convertibility is not caused in such a temperature range even if the dew-point temperature is not controlled and internal oxides are not formed.
• the temperature is raised to above 780°C without controlling of the dew-point temperature, the amount of surface segregation is so increased that the inward diffusion of oxygen is inhibited and internal oxidation is unlikely to occur. It is therefore necessary to control the dew-point temperature to become not less than -10°C at least from when the temperature is in the range of not more than 780°C.
• the acceptable range of A is 600 ⁇ A ⁇ 780. For the above reason, it is preferable that A be a temperature as low as possible within this range.
• the upper limit temperature B is limited to be 800 ⁇ B ⁇ 900 for the following reasons.
• the formation of internal oxides decreases the amount of easily oxidized elements (such as Si and Mn) present as solutes inside a surface portion of the steel sheet extending from the surface within a depth of 10 ⁇ m (hereinafter, such a portion will be referred to as "deficient layer"), and thereby the easily oxidized elements are suppressed from diffusing from the inside of steel toward the surface.
• the temperature B needs to satisfy 800 ⁇ B ⁇ 900. If the temperature is less than 800°C, internal oxides are not formed sufficiently. If the temperature exceeds 900°C, internal oxides are formed in excessively large amounts and serve as starting points of a deterioration in corrosion resistance after electrodeposition coating.
• the dew-point temperature is controlled to become not less than -10°C when the temperature is in the range of not less than A°C and not more than B°C for the following reasons.
• Increasing the dew-point temperature increases the potential of O 2 generated by the decomposition of H 2 O, and therefore internal oxidation can be promoted.
• the amount of formed internal oxides is small if the dew-point temperature is in the range of below -10°C.
• the upper limit of the dew-point temperature is not particularly limited. However, the amount of oxidation of iron increases if the dew-point temperature is in excess of 90°C, causing a risk that annealing furnace walls or rollers may be degraded.
• the dew-point temperature is preferably not more than 90°C.
• Silicon increases the strength and the elongation of steel and is therefore an effective element for achieving a good quality.
• silicon needs to be contained at not less than 0.4%.
• Steel sheets having a Si content of less than 0.4% cannot achieve a strength of interest in the invention and are substantially free of problems in terms of chemical convertibility.
• containing silicon in excess of 2.0% results in the saturation of steel strengthening effects as well as the saturation of elongation enhancement, and achieving an improvement of chemical convertibility becomes difficult.
• the Si content is limited to be not less than 0.4% and not more than 2.0%.
• Manganese is an effective element for increasing the strength of steel. In order to ensure mechanical characteristics and strength, the Mn content needs to be not less than 1.0%. On the other hand, containing manganese in excess of 3.0% causes difficulties in ensuring weldability as well as in ensuring the balance between strength and ductility. Thus, the Mn content is limited to be not less than 1.0% and not more than 3.0%.
• Aluminum is added for the purpose of deoxidation of molten steel.
• the deoxidation effect for molten steel is obtained by adding aluminum at not less than 0.001%.
• adding aluminum in excess of 1.0% increases costs and further results in an increase in the amount of surface segregation of aluminum, thereby making it difficult to improve chemical convertibility.
• the Al content is limited to be not less than 0.001% and not more than 1.0%.
• Phosphorus is one of elements that are inevitably present in steel. An increase in cost is expected if the P content is reduced to below 0.005%. Thus, the P content is specified to be not less than 0.005%. On the other hand, any P content exceeding 0.060% leads to a decrease in weldability and causes a marked deterioration in chemical convertibility to such an extent that it becomes difficult to improve chemical convertibility even by the present invention. Thus, the P content is limited to be not less than 0.005% and not more than 0.060%.
• Sulfur is one of inevitable elements.
• the lower limit is not particularly limited. However, the presence of this element in a large amount causes decreases in weldability and corrosion resistance. Thus, the S content is limited to be not more than 0.01%.
• one or more elements selected from 0.001 to 0.005% of B, 0.005 to 0.05% of Nb, 0.005 to 0.05% of Ti, 0.001 to 1.0% of Cr, 0.05 to 1.0% of Mo, 0.05 to 1.0% of Cu and 0.05 to 1.0% of Ni may be added as required.
• the B content is less than 0.001%.
• adding boron in excess of 0.005% results in a decrease in chemical convertibility.
• the B content is limited to be not less than 0.001% and not more than 0.005%.
• the Nb content is less than 0.005%.
• containing niobium in excess of 0.05% results in an increase in cost.
• the Nb content is limited to be not less than 0.005% and not more than 0.05%.
• the Ti content is less than 0.005%.
• containing titanium in excess of 0.05% results in a decrease in chemical convertibility.
• the Ti content is limited to be not less than 0.005% and not more than 0.05%.
• the Cr content is less than 0.001%.
• containing chromium in excess of 1.0% results in the surface segregation of chromium and a consequent decrease in weldability.
• the Cr content is limited to be not less than 0.001% and not more than 1.0%.
• the Mo content is less than 0.05%.
• containing molybdenum in excess of 1.0% results in an increase in cost.
• the Mo content is limited to be not less than 0.05% and not more than 1.0%.
• the Cu content is less than 0.05%.
• containing copper in excess of 1.0% results in an increase in cost.
• the Cu content is limited to be not less than 0.05% and not more than 1.0%.
• Ni content is less than 0.05%.
• containing nickel in excess of 1.0% results in an increase in cost.
• the Ni content is limited to be not less than 0.05% and not more than 1.0%.
• the balance after the deduction of the aforementioned elements is represented by Fe and inevitable impurities.
• a steel having the above-described chemical composition is hot rolled and is thereafter cold rolled, and subsequently the steel sheet is annealed in a continuous annealing facility and is subjected to a chemical conversion treatment.
• the annealing is carried out in such a manner that the dew-point temperature of the atmosphere is controlled to become not less than -10°C when the heating furnace inside temperature is in the range of not less than A°C and not more than B°C during the course of heating (A: 600 ⁇ A ⁇ 780, B: 800 ⁇ B ⁇ 900).
• the oxygen potential is increased with the result that easily oxidized elements such as Si and Mn are internally oxidized beforehand immediately before a chemical conversion treatment and the activities of Si and Mn in the surface portion of the steel sheet are lowered. Consequently, the external oxidation of these elements is suppressed, resulting in an improvement in chemical convertibility.
• Hot rolling may be performed under usual conditions.
• pickling conditions are not particularly limited.
• Cold rolling is preferably carried out with a draft of not less than 40% and not more than 80%. If the draft is less than 40%, the recrystallization temperature becomes lower and the steel sheet tends to be deteriorated in mechanical characteristics. On the other hand, because the steel sheet of the invention is a high strength steel sheet, cold rolling the steel sheet with a draft exceeding 80% increases not only the rolling costs but also the amount of surface segregation during annealing, possibly resulting in a decrease in chemical convertibility.
• the steel sheet that has been cold rolled or hot rolled is annealed and then subjected to a chemical conversion treatment.
• the steel sheet undergoes a heating step in which the steel sheet is heated to a predetermined temperature in an upstream heating zone and a soaking step in which the steel sheet is held in a downstream soaking zone at a predetermined temperature for a prescribed time.
• a cooling step is performed.
• the annealing is carried out in such a manner that the dew-point temperature of the atmosphere is controlled to become not less than -10°C when the heating furnace inside temperature is in the range of not less than A°C and not more than B°C (A: 600 ⁇ A ⁇ 780, B: 800 ⁇ B ⁇ 900). Except when the temperature is in the range of not less than A°C and not more than B°C, the dew-point temperature of the atmosphere in the annealing furnace is limited and is in the range of -50°C to -10°C.
• the gas components in the annealing furnace include nitrogen, hydrogen and inevitable impurities. Other gas components may be present as long as they are not detrimental in achieving the advantageous effects of the invention. If the hydrogen concentration in the annealing furnace atmosphere is less than 1 vol%, the activation effect by reduction cannot be obtained and chemical convertibility is deteriorated. Although the upper limit is not particularly limited, costs are increased and the effect is saturated if the hydrogen concentration exceeds 50 vol%. Thus, the hydrogen concentration is not less than 1 vol% and not more than 50 vol%.
• the gas components in the annealing furnace except hydrogen gas are nitrogen gas and inevitable impurity gases. Other gas components may be present as long as they are not detrimental in achieving the advantageous effects of the invention.
• tempering be performed at a temperature of 150 to 400°C. The reasons are because elongation tends to be deteriorated if the temperature is less than 150°C as well as because hardness tends to be decreased if the temperature is in excess of 400°C.
• electrolytic pickling be performed in order to remove trace amounts of oxides that have been inevitably generated by surface segregation during annealing and thereby to ensure better chemical convertibility.
• the electrolytic pickling conditions are not particularly limited. However, in order to efficiently remove the inevitably formed surface segregation of silicon and manganese oxides formed during the annealing, alternating electrolysis at a current density of not less than 1 A/dm 2 is desirable. The reasons why alternating electrolysis is selected are because the pickling effects are low if the steel sheet is fixed to a cathode as well as because if the steel sheet is fixed to an anode, iron that is dissolved during electrolysis is accumulated in the pickling solution and the Fe concentration in the pickling solution is increased with the result that the attachment of iron to the surface of the steel sheet causes problems such as dry contamination.
• the pickling solution used in the electrolytic pickling is not particularly limited.
• nitric acid or hydrofluoric acid is not preferable because they are highly corrosive to a facility and require careful handling.
• Hydrochloric acid is not preferable because chlorine gas can be generated from the cathode.
• the use of sulfuric acid is preferable.
• the sulfuric acid concentration is preferably not less than 5 mass% and not more than 20 mass%. If the sulfuric acid concentration is less than 5 mass%, the conductivity is so lowered that the bath voltage is raised during electrolysis possibly to increase the power load. On the other hand, any sulfuric acid concentration exceeding 20 mass% leads to a cost problem because a large loss is caused due to drag-out.
• the temperature of the electrolytic solution is preferably not less than 40°C and not more than 70°C. Because the bath temperature is raised by the generation of heat by continuous electrolysis, the pickling effect may be lowered if the temperature is less than 40°C. Further, maintaining the temperature below 40°C is sometimes difficult. Furthermore, a temperature exceeding 70°C is not preferable in view of the durability of the lining of the electrolytic cell.
• the high strength steel sheets of the present invention are obtained in the above manner.
• the inventive steel sheet has a characteristic structure of the surface described below.
• a surface portion of the steel sheet extending from the steel sheet surface within a depth of 100 ⁇ m contains an oxide of one or more selected from Fe, Si, Mn, Al and P, as well as from B, Nb, Ti, Cr, Mo, Cu and Ni at a total amount of 0.010 to 0.50 g/m 2 per single side surface. Further, with respect to a region extending from the steel sheet surface to a depth of 10 ⁇ m, a crystalline Si/Mn complex oxide is present in base iron grains that are within 1 ⁇ m from grain boundaries.
• the present invention first provides that the dew-point temperature is controlled as described hereinabove in order to increase the oxygen potential in the annealing step.
• the oxygen potential being increased, easily oxidized elements such as Si and Mn are internally oxidized beforehand immediately before a chemical conversion treatment and the activities of Si and Mn in the surface portion of the steel sheet are lowered.
• the present invention provides that internal oxidation is caused to take place not only at grain boundaries but also in grains by controlling the dew-point temperature of the atmosphere to become not less than -10°C when the heating furnace inside temperature is in the range of not less than A°C and not more than B°C (A: 600 ⁇ A ⁇ 780, B: 800 ⁇ B ⁇ 900).
• a crystalline Si/Mn complex oxide is caused to be present in base iron grains that are within 1 ⁇ m from grain boundaries in a region extending from the steel sheet surface to a depth of 10 ⁇ m. Because of the oxide being present in base iron grains, the amount of solute silicon and manganese in base iron grains in the vicinity of the oxide is decreased. As a result, the surface segregation of Si and Mn due to intragranular diffusion can be suppressed.
• the structure of the surface of the high strength steel sheet obtained by the manufacturing method according to the present invention is as described above. There is no problem even when the oxides have been grown so as to extend to a region that is more than 100 ⁇ m away from the steel sheet surface. Further, no problems are caused even when the crystalline Si/Mn complex oxide is caused to be present in base iron grains that are more than 1 ⁇ m away from grain boundaries in a region extending from the steel sheet surface to a depth in excess of 10 ⁇ m.
• Hot rolled steel sheets with a steel composition described in Table 1 were pickled to remove black scales and were thereafter cold rolled to give cold rolled steel sheets with a thickness of 1.0 mm. Cold rolling was omitted for some of the steel sheets. That is, as-descaled hot rolled steel sheets (thickness: 2.0 mm) were also provided.
• the cold rolled steel sheets and the hot rolled steel sheets obtained above were introduced into a continuous annealing facility.
• the steel sheet was passed through the annealing facility while controlling the heating furnace inside temperature and the dew-point temperature as described in Table 2.
• the annealed steel sheet was thereafter subjected to water hardening and then to tempering at 300°C for 140 seconds.
• electrolytic pickling was performed by alternating electrolysis in a 5 mass% aqueous sulfuric acid solution at 40°C under current density conditions described in Table 2 while switching the polarity of the sample sheet between anodic and cathodic alternately each after 3 seconds.
• sample sheets were prepared.
• the dew-point temperature in the annealing furnace was basically set at -35°C except when the dew-point temperature was controlled as described above.
• the gas components in the atmosphere included nitrogen gas, hydrogen gas and inevitable impurity gases.
• the dew-point temperature was controlled by dehumidifying the atmosphere or by removing water in the atmosphere by absorption.
• the hydrogen concentration in the atmosphere was basically set at 10 vol%.
• TS and El were measured in accordance with a tensile testing method for metallic materials described in JIS Z 2241. Further, the sample sheets were tested to examine chemical convertibility and corrosion resistance, as well as the amount of oxides present in a surface portion of the steel sheet extending immediately from the surface of the steel sheet to a depth of 100 ⁇ m (the internal oxidation amount). The measurement methods and the evaluation criteria are described below.
• a chemical conversion treatment liquid (PALBOND L3080 (registered trademark)) manufactured by Nihon Parkerizing Co., Ltd. was used. A chemical conversion treatment was carried out in the following manner.
• the sample sheet was degreased with degreasing liquid FINE CLEANER (registered trademark) manufactured by Nihon Parkerizing Co., Ltd., and was thereafter washed with water. Subsequently, the surface of the sample sheet was conditioned for 30 seconds with surface conditioning liquid PREPAREN Z (registered trademark) manufactured by Nihon Parkerizing Co., Ltd. The sample sheet was then soaked in the chemical conversion treatment liquid (PALBOND L3080) at 43°C for 120 seconds, washed with water and dried with hot air.
• FINE CLEANER registered trademark
• PREPAREN Z registered trademark
• the sample sheet after the chemical conversion treatment was observed with a scanning electron microscope (SEM) at 500x magnification with respect to randomly selected five fields of view.
• SEM scanning electron microscope
• the area ratio of the regions that had not been covered with the chemical conversion coating was measured by image processing. Chemical convertibility was evaluated based on the area ratio of the non-covered regions according to the following criteria.
• the symbol ⁇ indicates an acceptable level.
• test piece was cut out from the sample sheet that had been subjected to the above chemical conversion treatment.
• the test piece was cationically electrodeposition coated with PN-150G (registered trademark) manufactured by NIPPON PAINT Co., Ltd. (baking conditions: 170°C ⁇ 20 min, film thickness: 25 ⁇ m) . Thereafter, the edges and the non-test surface were sealed with an Al tape, and the test surface was cut deep into the base steel with a cutter knife to create a cross cut pattern (cross angle: 60°), thereby preparing a sample.
• the sample was soaked in a 5 mass% aqueous NaCl solution (55°C) for 240 hours, removed from the solution, washed with water and dried. Thereafter, an adhesive tape was applied to the cross cut pattern and was peeled therefrom.
• the exfoliation width was measured and was evaluated based on the following criteria. The symbol ⁇ indicates an acceptable level.
• a JIS No. 5 tensile test piece was sampled from the sample sheet in a direction that was 90° relative to the rolling direction.
• the test piece was subjected to a tensile test at a constant cross head speed of 10 mm/min in accordance with JIS Z 2241, thereby determining the tensile strength (TS/MPa) and the elongation (El %).
• TS/MPa tensile strength
• El % elongation
• workability was evaluated to be good when TS ⁇ El ⁇ 22000 and to be bad when TS ⁇ El ⁇ 22000.
• the internal oxidation amount was measured by an "impulse furnace fusion-infrared absorption method". It should be noted that the amount of oxygen present in the starting material (namely, the high strength steel sheet before annealing) should be subtracted. Thus, in the invention, surface portions on both sides of the continuously annealed high strength steel sheet were polished by at least 100 ⁇ m and thereafter the oxygen concentration in the steel was measured. The measured value was obtained as the oxygen amount OH of the starting material. Further, the oxygen concentration was measured across the entirety of the continuously annealed high strength steel sheet in the sheet thickness direction. The measured value was obtained as the oxygen amount OI after internal oxidation.
• the high strength steel sheets manufactured by the inventive method were shown to be excellent in chemical convertibility, corrosion resistance after electrodeposition coating and workability in spite of the fact that these high strength steel sheets contained large amounts of easily oxidized elements such as Si and Mn.
• the steel sheets obtained in COMPARATIVE EXAMPLES were poor in at least one of chemical convertibility, corrosion resistance after electrodeposition coating and workability.
• the high strength steel sheets according to the present invention are excellent in chemical convertibility, corrosion resistance and workability, and can be used as surface-treated steel sheets for reducing the weight and increasing the strength of bodies of automobiles. Besides automobiles, the inventive high strength steel sheets can be used as surface-treated steel sheets having corrosion resistance on the base steel sheet in a wide range of applications including home appliances and building materials.

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• 2010
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