US20170306507A1 - Cold-rolled steel sheet, method of manufacturing cold-rolled steel sheet, automobile member and facility for manufacturing cold-rolled steel sheet - Google Patents

Cold-rolled steel sheet, method of manufacturing cold-rolled steel sheet, automobile member and facility for manufacturing cold-rolled steel sheet Download PDF

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US20170306507A1
US20170306507A1 US15/507,601 US201515507601A US2017306507A1 US 20170306507 A1 US20170306507 A1 US 20170306507A1 US 201515507601 A US201515507601 A US 201515507601A US 2017306507 A1 US2017306507 A1 US 2017306507A1
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steel sheet
acid
pickling
cold
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Hiroyuki Masuoka
Shoichiro Taira
Shinichi Furuya
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JFE Steel Corp
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JFE Steel Corp
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    • 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
    • C23G—CLEANING OR DE-GREASING OF METALLIC MATERIAL BY CHEMICAL METHODS OTHER THAN ELECTROLYSIS
    • C23G1/00—Cleaning or pickling metallic material with solutions or molten salts
    • 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
    • C22C38/005—Ferrous alloys, e.g. steel alloys containing rare earths, i.e. Sc, Y, Lanthanides
    • 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
    • C22C38/32—Ferrous alloys, e.g. steel alloys containing chromium 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
    • C22C38/34—Ferrous alloys, e.g. steel alloys containing chromium with more than 1.5% by weight of 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
    • C22C38/18—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
    • C23G—CLEANING OR DE-GREASING OF METALLIC MATERIAL BY CHEMICAL METHODS OTHER THAN ELECTROLYSIS
    • C23G1/00—Cleaning or pickling metallic material with solutions or molten salts
    • C23G1/02—Cleaning or pickling metallic material with solutions or molten salts with acid solutions
    • C23G1/08—Iron or steel
    • 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
    • C23G—CLEANING OR DE-GREASING OF METALLIC MATERIAL BY CHEMICAL METHODS OTHER THAN ELECTROLYSIS
    • C23G1/00—Cleaning or pickling metallic material with solutions or molten salts
    • C23G1/02—Cleaning or pickling metallic material with solutions or molten salts with acid solutions
    • C23G1/08—Iron or steel
    • C23G1/081—Iron or steel solutions containing H2SO4
    • 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
    • C23G—CLEANING OR DE-GREASING OF METALLIC MATERIAL BY CHEMICAL METHODS OTHER THAN ELECTROLYSIS
    • C23G1/00—Cleaning or pickling metallic material with solutions or molten salts
    • C23G1/02—Cleaning or pickling metallic material with solutions or molten salts with acid solutions
    • C23G1/08—Iron or steel
    • C23G1/085—Iron or steel solutions containing HNO3
    • 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
    • C23G—CLEANING OR DE-GREASING OF METALLIC MATERIAL BY CHEMICAL METHODS OTHER THAN ELECTROLYSIS
    • C23G1/00—Cleaning or pickling metallic material with solutions or molten salts
    • C23G1/02—Cleaning or pickling metallic material with solutions or molten salts with acid solutions
    • C23G1/08—Iron or steel
    • C23G1/086—Iron or steel solutions containing HF
    • 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
    • C23G—CLEANING OR DE-GREASING OF METALLIC MATERIAL BY CHEMICAL METHODS OTHER THAN ELECTROLYSIS
    • C23G1/00—Cleaning or pickling metallic material with solutions or molten salts
    • C23G1/14—Cleaning or pickling metallic material with solutions or molten salts with alkaline solutions
    • C23G1/19—Iron or steel
    • 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
    • C23G—CLEANING OR DE-GREASING OF METALLIC MATERIAL BY CHEMICAL METHODS OTHER THAN ELECTROLYSIS
    • C23G3/00—Apparatus for cleaning or pickling metallic material
    • C23G3/02—Apparatus for cleaning or pickling metallic material for cleaning wires, strips, filaments continuously

Definitions

  • This disclosure relates to a cold-rolled steel sheet, and a method of manufacturing a cold-rolled steel sheet.
  • the disclosure also relates to a facility for manufacturing the cold-rolled steel sheet.
  • the disclosure relates to a cold-rolled steel sheet having excellent chemical convertibility and, at the same time, corrosion resistance after coating which is evaluated by a hot brine dipping test and a composite cycle corrosion test, a method of manufacturing the cold-rolled steel sheet, and an automobile member.
  • the cold-rolled steel sheet can be preferably used as a high-strength cold-rolled steel sheet containing Si and having a tensile strength TS of 590 MPa or more.
  • Japanese Unexamined Patent Application Publication No. 2004-204350 proposes a high strength cold-rolled steel sheet where a slab is heated at a temperature of 1200° c. or above at the time of hot rolling, descaling is performed at a high pressure, a surface of the hot-rolled steel sheet is ground by nylon brush containing abrasive grains before pickling, the sheet is dipped into a 9% hydrochloric acid tank twice to perform pickling to lower Si concentration on a surface of the steel sheet.
  • Japanese Unexamined Patent Application Publication No. 2004-244698 proposes a high strength cold-rolled steel sheet where corrosion resistance is enhanced by setting a line width of a filamentous oxide containing Si which is observed at a depth of 1 to 10 ⁇ m from a surface of the steel sheet to 300 nm or less.
  • Japanese Unexamined Patent Application Publication No. 64-62485 proposes a technique for enhancing an oxide removing ability of a steel sheet by setting an iron ion concentration (divalent) in a hydrochloric acid to 0.5 to 18%.
  • SiO 2 is insoluble in a hydrochloric acid and hence, even when iron ion concentration is set to 0.5 to 18%, SiO 2 cannot be removed.
  • Japanese Unexamined Patent Application Publication No. 2007-217743 discloses a technique which can enhance chemical convertibility by increasing reactivity of a steel sheet with a chemical conversion treatment solution by removing an Si containing oxide concentrated on a surface of the steel sheet in an annealing step or the like by pickling and by further imparting an S-based compound to such a surface.
  • Japanese Unexamined Patent Application Publication No. 2007-246951 discloses a technique where a P-based compound is imparted in place of an S-based compound described in Japanese Unexamined Patent Application Publication No. 2007-217743.
  • Japanese Unexamined Patent Application Publication No. 2012-132092 discloses, as a technique which can overcome the above-mentioned drawbacks, a technique which enhances chemical convertibility by increasing reactivity with chemical conversion treatment solution.
  • SiO 2 is removed by performing pickling using an oxidizing acid in a first stage and a Fe-based oxide formed in the first-stage pickling is removed by performing pickling using a non-oxidizing acid in a subsequent second stage.
  • a method of manufacturing a cold-rolled steel sheet where first pickling is applied to a steel sheet which is continuously annealed after cold rolling, second pickling is applied to the steel sheet subsequently and, thereafter, neutralizing treatment is applied to the steel sheet using an alkaline solution.
  • [5] The method of manufacturing a cold-rolled steel sheet described in any one of [1] to [4] where the first pickling is performed using either one of the following acidic solutions (a) and (b).
  • (a) The acidic solution containing a nitric acid and a hydrochloric acid, wherein concentration of nitric acid is more than 50 g/L and 200 g/L or less, a ratio R1 of the concentration of hydrochloric acid to the concentration of nitric acid (hydrochloric acid/nitric acid) is set to a value which falls within a range of 0.01 to 0.25, and the concentration of Fe ion is set to a value which falls within a range of 3 to 50 g/L.
  • non-oxidizing acid is any one of a hydrochloric acid, a sulfuric acid, a phosphoric acid, a pyrophosphoric acid, a formic acid, an acetic acid, a citric acid, a hydrofluoric acid, an oxalic acid, and an acid which is a mixture of two or more of these acids.
  • a cold-rolled steel sheet being excellent in chemical convertibility as well as corrosion resistance after coating. Further, according to the manufacturing method, a cold-rolled steel sheet having favorable chemical convertibility and favorable corrosion resistance after coating can be manufactured easily and in a stable manner through usual cold-rolling step and pickling step by merely adjusting a pickling condition.
  • a cold-rolled steel sheet being excellent in chemical convertibility even when chemical conversion treatment solution having a low temperature is used and also being excellent in corrosion resistance after coating even in a severe corrosion environment such as a hot brine dipping test or a composite cycle corrosion test even when the cold-rolled steel sheet contains 0.5 to 3.0 mass % Si.
  • chemical convertibility and corrosion resistance after coating of a high strength cold-rolled steel sheet containing a large amount of Si thus having a tensile strength TS of 590 MPa or more can be largely improved and hence, the high strength cold-rolled steel sheet can be preferably used as a reinforcing member of a vehicle body of an automobile or the like.
  • FIGS. 1A and 1B are views showing reflection electron images of surfaces of steel sheets which are cold-rolled steel sheet standard samples No. a and No. b prepared to obtain a surface coverage of an iron-based oxide.
  • FIG. 2 is a histogram of the number of pixels with respect to gray values of reflection electron image photographs of the cold-rolled steel sheet standard samples No. a and No. b.
  • FIG. 3 is a view showing a result of observation of a cross section of a steel sheet surface covering material after a surface of a steel sheet is pickled using a non-oxidizing acid by a transmission-type electron microscope.
  • FIG. 4 is a graph showing a result of an energy distribution type X-ray (EDX) analysis of an iron-based oxide observed in FIG. 3 .
  • EDX energy distribution type X-ray
  • FIGS. 5A and 5B are graphs showing results obtained by measuring the distribution of O, Si, Mn and Fe in the depth direction on a surface of specimens shown in Table 2 by GDS.
  • a non-oxidizing gas or a reducing gas is usually used as an atmospheric gas, and a dew point is strictly controlled. Accordingly, in an ordinary general-use cold-rolled steel sheet having a low alloy content, oxidation of a surface of the steel sheet is suppressed.
  • Si, Mn or the like which is easily oxidized compared to Fe is oxidized so that the formation of an Si containing oxide such as an Si oxide (SiO 2 ) or an Si—Mn based composite oxide on a surface of a steel sheet cannot be avoided.
  • Si containing oxide such as an Si oxide (SiO 2 ) or an Si—Mn based composite oxide on a surface of a steel sheet.
  • the Si containing oxide is formed not only on a surface of a steel sheet but also in the inside of a base steel. Hence, an etching property of the surface of the steel sheet in chemical conversion treatment (zinc phosphate treatment) which is performed as a surface treatment for electrodeposition coating is impaired thus adversely affecting the formation of a sound chemical conversion treatment film.
  • Si containing oxide means SiO 2 or an Si—Mn based composite oxide formed along a surface of the steel sheet or a grain boundary in the inside of the steel sheet at heating of a slab, after hot rolling or at annealing after cold rolling.
  • a thickness of a layer where the Si containing oxide is present changes depending on the composition of the steel sheet or an annealing condition (temperature, time, atmosphere), the thickness is usually approximately 1 ⁇ m from a surface of the steel sheet.
  • “removing an Si containing oxide layer” means that the Si containing oxide layer is removed by pickling to a level that a peak of Si and a peak of O do not appear when the surface of the steel sheet is analyzed in a depth direction by GDS (glow discharge atomic emission spectrochemical analysis).
  • strong pickling is performed as first pickling to suppress the formation of an iron-based oxide on a surface of a steel sheet and to remove an Si containing oxide layer present on a surface of the steel sheet.
  • pickling is performed using a non-oxidizing acid as second pickling to set a surface coverage of the iron-based oxide present on the surface of the steel sheet to 40% or less.
  • neutralizing treatment is applied to the steel sheet using an alkaline solution.
  • Iron-based oxide means an oxide which contains iron as a main component where an atomic percentage of iron among elements other than oxygen which constitute oxides is set to 30% or more.
  • the iron-based oxide is an oxide which is present on a surface of a steel sheet with a non-uniform thickness and differs from a natural oxide film having a uniform thickness of several nm and being present as a layer. Further, an iron-based oxide formed on a surface of a cold-rolled steel sheet is amorphous based on the observation using a transmission type electron microscope (TEM) or a result of analysis of a diffraction pattern obtained by an electron beam diffraction.
  • TEM transmission type electron microscope
  • pickling is applied to a steel sheet which is produced by applying heating, hot rolling, cold rolling and continuous annealing to a steel material (slab) containing 0.5 to 3.0% Si, for example, and second pickling is applied to the steel sheet subsequently and, thereafter, neutralizing treatment is applied to the steel sheet using an alkaline solution.
  • an Si containing oxide such as SiO 2 or an Si—Mn based composite oxide is formed on a surface layer of a steel sheet. If this state is maintained as it is, chemical convertibility and corrosion resistance after coating are extremely lowered.
  • an Si containing oxide layer formed on a surface of the steel sheet is removed together with a base steel.
  • an Si—Mn based composite oxide is easily dissolved by an acid among Si containing oxides, SiO 2 exhibits insolubility against an acid. Accordingly, to remove an Si containing oxide including SiO 2 , it is necessary to remove an oxide layer together with a base steel of a steel sheet by strong pickling. Accordingly, as an acid which can be used as an acidic solution, a nitric acid which is a strong oxidizing acid can be favorably used. Further, provided that an acid can remove an Si containing oxide layer, the acid may be a hydrofluoric acid, a hydrochloric acid, a sulfuric acid or the like. That is, a kind of acid is not particularly specified. Further, an acid prepared by mixing these two or more of acids may be used. It is also effective to accelerate dissolving of a base steel by adding a pickling accelerating agent to an acidic solution or by using electrolytic treatment in combination with the use of an acid.
  • Fe which is dissolved from a surface of a steel sheet by pickling forms an iron-based oxide, and this iron-based oxide is deposited and precipitates on the surface of the steel sheet and covers the surface of the steel sheet thus giving rise to a possibility that chemical convertibility is lowered.
  • the acidic solution contain the nitric acid and the hydrochloric acid such that the concentration of nitric acid be set to a value which is more than 50 g/L to 200 g/L or less, and a ratio R1 (hydrochloric acid/nitric acid) of the concentration of hydrochloric acid having an oxide film breaking effect to the concentration of nitric acid be set to 0.01 to 0.25, and the concentration of Fe ion (sum of bivalence and trivalence) be set to 3 to 50 g/L.
  • the concentration of nitric acid be set to 100 g/L to 200 g/L. It is more preferable that the above-mentioned R1 be set to 0.02 to 0.15. It is more preferable that the concentration of Fe ion be set to 3 to 25 g/L.
  • the acidic solution contains the nitric acid and the hydrofluoric acid such that the concentration of nitric acid be set to more than 50 g/L to 200 g/L or less, and a ratio R2 (hydrofluoric acid/nitric acid) of the concentration of hydrofluoric acid having an oxide film breaking effect to the concentration of nitric acid be set to 0.01 to 0.25, and the concentration of Fe ion (sum of bivalence and trivalence) be set to 3 to 50 g/L. It is more preferable that the concentration of nitric acid be set to 100 g/L to 200 g/L.
  • the above-mentioned R2 be set to 0.02 to 0.15. It is more preferable that the concentration of Fe ion be set to 3 to 25 g/L. When R1 and R2 are larger than 0.25 or when the concentration of Fe ion (the sum of bivalence and trivalance) is less than 3 g/L, a desired pickling speed cannot be acquired and hence, an Si containing oxide cannot be efficiently removed.
  • R1 and R2 are smaller than 0.01 or when the concentration of Fe ion is larger than 50 g/L, although a desired pickling speed can be acquired, an amount of Fe ion in an acidic solution is large and hence, a large amount of Fe-based oxide is formed on a surface of the steel sheet whereby an Fe-based oxide cannot be completely removed by the second pickling. Accordingly, chemical convertibility and corrosion resistance cannot be improved.
  • methods are considered including a method where when the concentration of Fe ion exceeds 50 g/L, an acidic solution is diluted, a method where a nitric acid or a hydrochloric acid is additionally charged, and a method where an iron component in an acid is lowered by an iron removing device.
  • a maximum thickness of an iron-based oxide can be set to 150 nm or less by properly setting a pickling condition (concentration, temperature, time).
  • a pickling condition concentration, temperature, time
  • the maximum thickness of the iron based oxide becomes 150 nm or less and hence, chemical convertibility is further improved and the corrosion resistance is also further improved.
  • second pickling is performed.
  • the second pickling is preferably performed using an acidic solution made of a non-oxidizing acid, and an iron-based oxide is removed by dissolving by second pickling.
  • non-oxidizing acid one kind or two or more kinds selected from a group consisting of a hydrochloric acid, a sulfuric acid, a phosphoric acid, a pyrophosphoric acid, a formic acid, an acetic acid, a citric acid, a hydrofluoric acid, and an oxalic acid are preferably used.
  • a hydrochloric acid and a sulfuric acid which are used commonly in a steel making industry may be preferably used.
  • a hydrochloric acid can be preferably used since a hydrochloric acid is a volatile acid so that a residue such as a sulfate group minimally remains on a surface of a steel sheet after water cleaning unlike a sulfuric acid, and an oxide breaking effect by chloride ion is large and the like. Further, a mixed acid prepared by mixing a hydrochloride acid and a sulfuric acid may be used.
  • a hydrochloric acid having the concentration of 0.1 to 50 g/L a sulfuric acid having the concentration of 0.1 to 150 g/L
  • the second pickling be performed in a state where a temperature of an acidic solution is set to 20 to 70° C. and a pickling time is 1 to 30 seconds.
  • a temperature of an acidic solution is set to 20° C. or above and a treatment time is 1 second or more, it is sufficient to remove an iron-based oxide remaining on a surface of a steel sheet.
  • a treatment time is 30 seconds or less, a surface of a steel sheet is not excessively dissolved so that there is no possibility that a new surface oxide film will be formed.
  • a temperature of an acidic solution of 30 to 50° C.
  • it is more preferable to set a pickling time to 2 to 20 seconds.
  • the concentration of an acidic solution consisting of a non-oxidizing acid is preferably set to 3 to 50 g/L.
  • the concentration of sulfuric acid is preferably set to 8 to 150 g/L.
  • a pickling solution prepared by mixing a hydrochloric acid and a sulfuric acid it is preferable to use an acid prepared by mixing a hydrochloric acid having the concentration of 3 to 20 g/L and a sulfuric acid having the concentration of 8 to 60 g/L.
  • a thickness of an iron-based oxide can be surely decreased to 150 nm or below and, hence, chemical convertibility and corrosion resistance after coating can be enhanced.
  • a surface of a steel sheet is not excessively resolved so that there is no possibility that a new surface oxide film is formed.
  • Our method is characterized in that neutralizing treatment is further performed using an alkaline solution after second pickling is performed.
  • a residue of a pickling solution cannot be completely neutralized.
  • condensed phosphate for example, sodium pyrophosphate, sodium polyphosphate and the like are named.
  • the above-mentioned pH is more preferably set to 10.0 to 12.0.
  • a temperature of the alkaline solution be set to 20 to 70° C., and a treatment time be set to 1 to 30 seconds.
  • a solution temperature of the alkaline solution is set to 20° C. or above and the treatment time is set to 1 second or more, a residue of a pickling solution is sufficiently neutralized.
  • a temperature of a pickling solution exceeds 70° C.
  • an alkaline fume is generated.
  • a treatment time exceeds 30 seconds, a length of a facility is elongated so that a huge facility cost becomes necessary.
  • a temperature of the alkaline solution is more preferably set to 30 to 50° C. It is more preferable to set a treatment time of 2 to 20 seconds.
  • a steel sheet is subjected to first pickling and second pickling and, then, the steel sheet is subjected to neutralizing treatment using an alkaline solution. Thereafter, the steel sheet is formed into a product sheet (cold-rolled steel sheet) through usual treatment steps such as temper rolling.
  • a pickling method that is, a method of bringing a steel sheet into contact with an acidic solution is not particularly limited.
  • a method in which an acidic solution is sprayed to a steel sheet, a method in which a steel sheet is dipped into an acidic solution and the like are named.
  • first pickling and second pickling are continuously performed.
  • first pickling and the second pickling are continuously performed.
  • water cleaning treatment may be performed after first pickling, after second pickling and after neutralizing treatment respectively. Further, in performing first pickling, second pickling, neutralizing treatment and water cleaning treatment respectively, additional water cleaning may be further performed on an inlet side and/or on an outlet side of the respective treatments using a water cleaning spray. It is preferable that drying treatment is performed using a dryer or the like after water cleaning treatment.
  • the steel sheet has the composition which allows the steel sheet to have a high strength such that the steel sheet can be used for forming a suspension member of an automobile and also has favorable chemical convertibility.
  • the content of Si is preferably set to 0.5 to 3.0%.
  • Si is an element highly effective in increasing strength of steel (solid solution strengthening ability) without largely deteriorating workability of steel and hence, Si is an effective element in achieving high strengthening of steel.
  • Si is also an element which adversely affects chemical convertibility and corrosion resistance after coating. Due to such reasons, while it is preferable to add 0.5% or more of Si, when the content of Si exceeds 3.0%, hot rolling property and cold rolling property are largely lowered thus giving rise to a possibility that productivity is adversely affected or ductility of a steel sheet per se is lowered.
  • the content of Si is preferably set to 0.5 to 3.0%.
  • the content of Si is more preferably set to 0.8 to 2.5%.
  • the cold-rolled steel sheet contains components other than the above-mentioned components within the component range of the ordinary cold-rolled steel sheet.
  • the cold-rolled steel sheet in applying the cold-rolled steel sheet to a high strength cold-rolled steel sheet having a tensile strength TS of 590 MPa or more which is used to form a vehicle body of an automobile or the like, it is preferable to set the contents of desired components other than the above-mentioned components as follows.
  • C is an element effective in increasing strength of steel.
  • C is also an element effective in forming residual austenite having TRIP (Transformation Induced Plasticity) effect, bainite or martensite.
  • TRIP Transformation Induced Plasticity
  • the content of C is set to 0.01% or more, the above-mentioned effects can be obtained.
  • the content of C is 0.30% or less, lowering of weldability does not occur. Accordingly, the content of C to be added is preferably set to 0.01 to 0.30%, and the content of C is more preferably set to 0.10 to 0.20%.
  • Mn is an element having a function of increasing strength of steel by solid solution strengthening of steel, a function of enhancing hardenability and a function of accelerating formation of residual austenite, bainite or martensite. Such an effect can be realized by adding 1.0% or more Mn. On the other hand, when the content of Mn is 7.5% or less, the above-mentioned advantageous effect can be obtained without the increase of cost. Accordingly, the content of Mn to be added is preferably set to 1.0 to 7.5%, and the content of Mn is more preferably set to 2.0 to 5.0%.
  • P is an element which does not deteriorate a drawability although P has a large solid solution strengthening ability and is also an element effective for acquiring a high strength. Accordingly, the content of P is preferably set to 0.005% or more. Although P is an element which deteriorates a spot weldability, there arises no problem provided that the content of P is set to 0.05% or less. Accordingly, the content of P is preferably set to 0.05% or less, and the content of P is more preferably set to 0.02% or less.
  • S is an impurity element which is unavoidably mixed into steel.
  • S is a harmful component which precipitates as MnS in steel and lowers the stretch-flangeability of the steel sheet.
  • the content of S is preferably set to 0.01% or less.
  • the content of S is more preferably set to 0.005% or less, and the content of S is still further preferably set to 0.003% or less.
  • Al is an element to be added as a deoxidizing agent in a steel making step. Further, Al is an element effective in separating non-metallic inclusion which lowers the stretch-flangeability as a slag. Accordingly, the content of Al is preferably set to 0.01% or more. When the content of Al is 0.06% or less, the above-mentioned effects can be obtained without increasing a cost of raw material. Accordingly, the content of Al is preferably set to 0.06% or less. The content of Al is more preferably set to 0.02 to 0.06%.
  • the cold-rolled steel sheet may contain one or two or more of elements selected from a group consisting of 0.3% or less Nb, 0.3% or less Ti, 0.3% or less V, 1.0% or less Mo, 1.0% or less Cr, 0.006% or less B and 0.008% or less N in addition to the above-mentioned components.
  • Nb, Ti and V are elements which form carbide and nitride, make the microstructure fine by suppressing the growth of ferrite in a heating step during annealing, and enhance formability, particularly the stretch-flangeability.
  • Mo, Cr and B are elements which enhance hardenability of steel and accelerate formation of bainite or martensite. Accordingly, Nb, Ti, V, Mo, Cr, and B may be added to the steel within the above-mentioned ranges.
  • N is an element which forms nitride with Nb, Ti and V or is dissolved in steel in a solid solution state and hence, N contributes to increasing of strength of steel.
  • the cold-rolled steel sheet may also contain one or two or more selected from a group consisting of 2.0% or less Ni, 2.0% or less Cu, 0.1% or less Ca and 0.1% or less REM in addition to the above-mentioned composition of components.
  • Ni and Cu are elements effective in accelerating formation of a low temperature transformation phase and increasing a strength of steel. Accordingly, Ni and Cu which fall within the above-mentioned ranges may be added to the steel sheet.
  • Ca and REM are elements which control a morphology of sulfide-based inclusion and enhance the stretch-flangeability of the steel sheet. Accordingly, Ca and REM which fall within the above-mentioned ranges may be added to the steel sheet.
  • the cold-rolled steel sheet a balance other than the above-mentioned components is Fe and unavoidable impurities.
  • the cold-rolled steel sheet may contain other components unless such components impair the desired effects.
  • a cold-rolled steel sheet has a steel sheet surface from which an Si containing oxide layer such as SiO 2 and an Si—Mn based composite oxide formed on a surface layer of the steel sheet during annealing is removed.
  • an Si containing oxide layer such as SiO 2
  • an Si—Mn based composite oxide formed on a surface layer of the steel sheet during annealing is removed.
  • the above-mentioned surface coverage of an iron-based oxide is obtained as follows.
  • a surface of a steel sheet after pickling is observed using a scanning electron microscope (ULV-SEM) of an extremely low acceleration voltage which can detect extreme surface layer information at approximately five fields of view with an acceleration voltage of 2 kV, a working distance of 3.0 mm and a magnification of approximately 1000 times, and a spectroscopic analysis is performed using an energy dispersion type X-ray spectrometer (EDX) thus obtaining a reflection electron image.
  • UUV-SEM scanning electron microscope
  • EDX energy dispersion type X-ray spectrometer
  • a binary coded processing is applied to the reflected electron image using an image analysis software, for example, Image J thus measuring an area ratio of a black-colored portion, and a surface coverage of an iron-based oxide can be obtained by averaging measured values of the respective fields of view.
  • an image analysis software for example, Image J
  • Image J image analysis software
  • UUV-SEM scanning electron microscope
  • ULTRA55 extremely low acceleration voltage
  • SEISS Inc. SEISS Inc.
  • EDX energy dispersion type X-ray spectrometer
  • NSS312E made by Thermo Fisher Inc.
  • a threshold value used in the above-mentioned binary coded processing is described.
  • a steel slab having a steel symbol G shown in Table 3 of an example described later was subjected to hot rolling, cold rolling and continuous annealing under a condition indicated at No. 93 of Table 4 in the example described later in the same manner so that the steel slab was formed into a cold-rolled steel sheet having a sheet thickness of 1.8 mm.
  • the cold-rolled steel sheet after continuous annealing was subjected to pickling, water cleaning and drying under a condition shown in Table 1 and, thereafter, the cold-rolled steel sheet was subjected to temper rolling with elongation of 0.7% thus manufacturing two kinds of cold-rolled steel sheets No. a and No.
  • FIGS. 1A and 1B show reflection electron image photographs of the steel sheets No. a and No. b
  • FIG. 2 shows a histogram of the number of pixels with respect to gray values of the reflection electron image photographs of the steel sheets No. a and No. b.
  • a gray value (Y point) corresponding to an intersection (X point) of the histograms of No. a and No. b shown in FIG. 2 was set as a threshold value.
  • the surface coverage of an iron-based oxide was obtained with respect to the steel sheets No. a and No. b using the above-mentioned threshold value, the surface coverage of the steel sheet No. a was 85.3% and the surface coverage of the steel sheet No. b was 25.8%.
  • a maximum thickness of the iron-based oxide be 150 nm or less. This is because when the maximum thickness of the iron-based oxide is 150 nm or less, there is no possibility that a dissolving reaction of iron by chemical conversion treatment is locally impaired, so that the precipitation of chemically converted crystals such as zinc phosphate cannot be locally suppressed.
  • the maximum thickness of an iron-based oxide may preferably set to 130 nm or less.
  • the maximum thickness of an iron-based oxide is obtained as follows.
  • FIG. 3 shows a photograph obtained by observing a cross section of a coating layer formed by first pickling present on a surface of a steel sheet by a transmission electron microscope (TEM), and FIG. 4 shows a result of an EDX analysis of the coating layer.
  • the coating layer is formed of an iron-based oxide and hence, a distance between a line A indicating a base steel of the steel sheet and a line B indicating the most largest portion of the iron-based oxide layer shown in the photograph of the cross section in FIG. 3 was measured with respect to all of ten replicas, and the maximum thickness among the measured maximum thicknesses is assumed as the maximum thickness of the iron-based oxide.
  • the sizes and the number of the above-mentioned replicas, measurement conditions by the TEM and the like are provided as only one example, and may be changed as desired.
  • the cold-rolled steel sheet obtained by the above-mentioned method exhibits excellent chemical convertibility and also exhibits excellent corrosion resistance after coating which is evaluated by a hot brine dipping test and a composite cycle corrosion test and, hence, the cold-rolled steel sheet can be preferably used to produce automobile members.
  • Steel having the composition containing 0.125% C, 1.5% Si, 2.6% Mn, 0.019% P, 0.008% S and 0.040% Al and comprising Fe and unavoidable impurities as a balance was manufactured such that molten steel was produced by an ordinary refining process including a converter treatment, a degassing treatment and the like and molten steel was formed into steel materials (slabs) by continuous casting.
  • the slabs were reheated to a temperature of 1150 to 1170° C. and, thereafter, were subjected to hot rolling where a finish rolling completion temperature is set to 850 to 880° C., and were wound up into coils at a temperature of 500 to 550° C.
  • hot-rolled steel sheets having a thickness of 3 to 4 mm.
  • scales were removed from the steel sheets by applying pickling to these hot-rolled steel sheets and, thereafter, cold rolling was applied to the steel sheets thus obtaining cold-rolled steel sheets having a thickness of 1.8 mm.
  • continuous annealing was performed where these cold-rolled steel sheets were heated to a soaking temperature of 750 to 780° C. and were held for 40 to 50 seconds and, thereafter, these steel sheets were cooled to a cooling stop temperature of 350 to 400° C. from the soaking temperature at a cooling rate of 20 to 30° C./second and were held for 100 to 120 seconds at a cooling stop temperature range.
  • Specimens were sampled from the above-mentioned respective cold-rolled steel sheets. Surfaces of the steel sheets were observed using a scanning electron microscope (ULV-SEM; made by SEISS Inc.; ULTRA55) at an extremely low acceleration voltage at five fields of view with an acceleration voltage of 2 kV, a working distance of 3.0 mm and a magnification of 1000 times, and spectroscopic analysis was performed using an energy dispersion type X-ray spectrometer (EDX; made by Thermo Fisher Inc.; NSS312E) thus obtaining reflected electron images.
  • UUV-SEM scanning electron microscope
  • EDX energy dispersion type X-ray spectrometer
  • a binary coded processing was applied to the reflected electron images using an image analysis software (Image J) while setting gray values (Y points) corresponding to intersection points (X points) of histogram of the above-mentioned standard samples No. a and No. b as threshold values thus measuring area ratios of black-colored portions, and an average value of the area ratios at five fields of view was obtained and the average value was set as a surface coverage of an iron-based oxide.
  • Image J image analysis software
  • specimens were sampled from the above-mentioned respective cold-rolled steel sheets, and spot rust generation evaluation of the cold-rolled steel sheets was carried out under the following conditions. After a chemical conversion treatment and a coating treatment were applied to the specimens under the following conditions, specimens were subjected to three kinds of corrosion tests consisting of a hot brine dipping test, a salt water spraying test and a composite cycle corrosion test and, then, the corrosion resistance after coating was evaluated. Further, the distribution of O, Si, Mn and Fe in the depth direction on surfaces of the specimens sampled from the respective cold-rolled steel sheets was measured using a GDS.
  • the cold-rolled steel sheets were left outdoors while preventing influences from external factors such as dusts. Presence or non-presence of generation of spot rust on the cold-rolled steel sheets was checked after approximately one month elapsed from starting the test. The evaluation “0” is given to cases where the specimens had no spot rust, and the evaluation “X” is given to cases where the specimens had spot rust.
  • a chemical conversion treatment was applied to specimens sampled from the above-mentioned respective cold-rolled steel sheets using a degreasing agent: FC-E2011, a surface conditioner: PL-X and a chemical conversion treatment agent: palbond PB-L3065 made by Nihon Parkerizing Co., Ltd. such that a coating weight of chemical conversion treatment coating was set to 1.7 to 3.0 g/m 2 under two conditions, that is, the standard condition and the comparison condition under a low temperature by lowering a temperature of a chemical conversion treatment solution.
  • Degreasing step treatment temperature 40° C., treatment time 120 seconds
  • Spray degreasing and surface adjustment steps pH 9.5, treatment temperature room temperature, treatment time 20 seconds
  • Chemical conversion treatment step temperature of chemical conversion treatment solution 35° C., treatment time 120 seconds
  • Electrodeposition coating was applied to surfaces of the specimens to which the above-mentioned chemical conversion treatment had been applied by electrodeposition paint: V-50 made by NIPPONPAINT Co., Ltd. such that a film thickness of electrodeposition coating is set to 25 ⁇ m, and the specimens were subjected to the following three kinds of corrosion tests. Hot brine dipping test
  • Steels A to O containing compositions shown in Table 3 were manufactured such that molten steel was produced by an ordinary refining process including a converter treatment, a degassing treatment and the like, and molten steel was formed into steel slabs by continuous casting. Hot rolling was applied to these steel slabs under hot rolling conditions shown in Table 4, and the steel slabs are formed into hot-rolled steel sheets having a thickness of 3 to 4 mm, scales were removed from surfaces of the steel sheets by applying pickling to these hot-rolled steel sheets and, thereafter, cold rolling was applied to the steel sheets thus obtaining cold-rolled steel sheets having a thickness of 1.8 mm.
  • Specimens were sampled from the above-mentioned respective cold rolled steel sheets obtained in the same manner as the example 1, and, after surface coverage of the iron-based oxide on the surface of the steel sheet after pickling was measured, the specimen is subjected to the following tensile test and the corrosion resistance test after coating. Further, the distribution of O, Si, Mn and Fe in the depth direction on surfaces of the specimens sampled from the respective cold-rolled steel sheets was measured using a GDS.
  • the cold-rolled steel sheets were left outdoors while preventing influences from external factors such as dusts. Presence or non-presence of generation of spot rust on the cold-rolled steel sheets after approximately one month was checked. The evaluation “0” is given to cases where the specimens had no spot rust, and the evaluation “X” is given to cases where the specimens had spot rust.
  • Specimens were prepared by applying chemical conversion treatment and electrodeposition coating to specimens sampled from the respective cold-rolled steel sheet under the same condition as the example 1.
  • the specimens were subjected to three kinds of corrosion tests consisting of a hot brine dipping test, a salt water spraying test (SST) and a composite cycle corrosion test (CCT) and, then, the corrosion resistance after coating was evaluated.
  • SST salt water spraying test
  • CCT composite cycle corrosion test
  • the high strength cold-rolled steel sheet of our example where the steel sheet contains 0.5% or more Si, and a surface coverage of iron-based oxide on the surface of the steel sheet to which neutralizing treatment is performed by applying pickling twice under the condition which conforms to our method is set to 40% or less not only is excellent in chemical convertibility and corrosion resistance after coating but also has a high strength of 590 MPa or more of tensile strength TS. It is ascertained from the result obtained by measuring the distribution of O, Si, Mn and Fe in the depth direction by a GDS that a peak of Si and a peak of O did not appear in any steel sheets which were subjected to pickling under the conditions which conform to our method so that an Si containing oxide layer was sufficiently removed.
  • Steel having the composition containing 0.125% C, 1.5% Si, 2.6% Mn, 0.019% P, 0.008% S and 0.040% Al and comprising Fe and unavoidable impurities as a balance was manufactured such that molten steel was formed into steel materials (slabs) by continuous casting.
  • the slabs were reheated to a temperature of 1150 to 1170° C. and, thereafter, were subjected to hot rolling where a finish rolling completion temperature is set to 850 to 880° C., and were wound up at a temperature of 500 to 550° C. thus forming hot-rolled steel sheets having a thickness of 3 to 4 mm.
  • first pickling and second pickling were applied to surfaces of the steel sheets under conditions shown in Table 6-1 to Table 6-2 (hereinafter, Table 6-1 and Table 6-2 being also collectively referred to as Table 6) and, then, the steel sheets were washed with water, neutralizing treatment was applied to the steel sheets, and the steel sheets were washed with water and were dried. Thereafter, temper rolling was applied to the steel sheets at a rate of elongation of 0.7% thus obtaining cold-rolled steel sheets No. 108 to No. 162 shown in Table 6.
  • Specimens were sampled from the above-mentioned respective cold-rolled steel sheets and, using the above-mentioned method, a surface coverage of iron-based oxide generated on the surface of the steel sheets by pickling and a maximum thickness were measured.
  • specimens were sampled from the above-mentioned respective cold-rolled steel sheets, and spot rust generation evaluation during storage of the cold-rolled steel sheets was carried out under the following conditions and, after a chemical conversion treatment and a coating treatment were applied to the specimens under the following conditions, specimens were subjected to three kinds of corrosion tests consisting of a hot brine dipping test, a salt water spraying test and a composite cycle corrosion test, and then, the corrosion resistance after coating was evaluated. Further, the distribution of O, Si, Mn and Fe in the depth direction on surfaces of the specimens sampled from the respective cold-rolled steel sheets was measured using a GDS.
  • the cold-rolled steel sheets were left outdoors while preventing influences from external factors such as dusts. Presence or non-presence of generation of spot rust on the cold-rolled steel sheets after approximately one month was checked. The evaluation “0” is given to cases where the specimens had no spot rust, and the evaluation “X” is given to cases where the specimens had spot rust.
  • a chemical conversion treatment was applied to specimens sampled from the above-mentioned respective cold-rolled steel sheets using a degreasing agent FC-E2011, a surface conditioner: PL-X and a chemical conversion treatment agent: palbond PB-L3065 made by Nihon Parkerizing Co., Ltd. such that a coating weight of chemical conversion treatment film was set to 1.7 to 3.0 g/m 2 under two conditions, that is, the standard condition and the comparison condition under a low temperature by lowering a temperature of a chemical conversion treatment solution.
  • Degreasing step treatment temperature 40° C., treatment time 120 seconds
  • Spray degreasing and surface adjustment steps pH 9.5, treatment temperature room temperature, treatment time 20 seconds
  • Chemical conversion treatment step temperature of chemical conversion treatment solution 35° C., treatment time 120 seconds
  • Electrodeposition coating was applied to surfaces of the specimens to which the above-mentioned chemical conversion treatment had been applied by electrodeposition paint: V-50 made by NIPPONPAINT Co., Ltd. such that a film thickness of electrodeposition coating is set to 25 ⁇ m, and the specimens were subjected to the following three kinds of corrosion tests under more severe condition than the example 1.
  • our cold-rolled steel sheet can possess not only excellent chemical convertibility and corrosion resistance after coating but also a high strength and hence, the cold-rolled steel sheet can be preferably used as a raw material for forming automobile members and also as a raw material for forming members which are required to possess the substantially similar property as the automobile member in other fields such as household electric appliances and architecture.

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