US20130149526A1 - Method of producing cold-rolled steel sheet as well as cold-rolled steel sheet and members for automobile - Google Patents

Method of producing cold-rolled steel sheet as well as cold-rolled steel sheet and members for automobile Download PDF

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US20130149526A1
US20130149526A1 US13/812,774 US201113812774A US2013149526A1 US 20130149526 A1 US20130149526 A1 US 20130149526A1 US 201113812774 A US201113812774 A US 201113812774A US 2013149526 A1 US2013149526 A1 US 2013149526A1
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acid
steel sheet
cold
rolled steel
mass
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Hiroyuki Masuoka
Satoru Ando
Shunsuke Yamamoto
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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
    • C23G1/02—Cleaning or pickling metallic material with solutions or molten salts with acid solutions
    • C23G1/08—Iron or steel
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B21—MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
    • B21B—ROLLING OF METAL
    • B21B45/00—Devices for surface or other treatment of work, specially combined with or arranged in, or specially adapted for use in connection with, metal-rolling mills
    • B21B45/02—Devices for surface or other treatment of work, specially combined with or arranged in, or specially adapted for use in connection with, metal-rolling mills for lubricating, cooling, or cleaning
    • B21B45/0269—Cleaning
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
    • C21D8/0236—Cold rolling
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • 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
    • C21D8/0278—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips 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
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/001—Ferrous alloys, e.g. steel alloys containing N
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/002—Ferrous alloys, e.g. steel alloys containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/005—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
    • C22C38/22—Ferrous alloys, e.g. steel alloys containing chromium 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
    • C22C38/32—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
    • C22C38/38—Ferrous alloys, e.g. steel alloys containing chromium 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
    • 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
    • C23G1/02—Cleaning or pickling metallic material with solutions or molten salts with acid solutions
    • C23G1/08—Iron or steel
    • C23G1/088—Iron or steel solutions containing organic acids
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B21—MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
    • B21B—ROLLING OF METAL
    • B21B3/00—Rolling materials of special alloys so far as the composition of the alloy requires or permits special rolling methods or sequences ; Rolling of aluminium, copper, zinc or other non-ferrous metals
    • B21B3/02—Rolling special iron alloys, e.g. stainless steel
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y10—TECHNICAL SUBJECTS COVERED BY FORMER USPC
    • Y10T—TECHNICAL SUBJECTS COVERED BY FORMER US CLASSIFICATION
    • Y10T428/00—Stock material or miscellaneous articles
    • Y10T428/26—Web or sheet containing structurally defined element or component, the element or component having a specified physical dimension

Definitions

  • This invention relates to a method of producing a cold-rolled steel sheet as well as a cold-rolled steel sheet and a member for automobile, and more particularly to a method of producing a cold-rolled steel sheet being excellent in not only the phosphate treatability but also the corrosion resistance after coating as evaluated by a hot salt water immersion test or a composite cycle corrosion test, a cold-rolled steel sheet produced by this method as well as a member for automobile using the cold-rolled steel sheet.
  • the cold-rolled steel sheet according to the invention can be preferably used in a high-strength cold-rolled steel sheet containing Si and having a tensile strength TS of not less than 590 MPa.
  • the Si-containing oxide considerably deteriorates the phosphate treatability
  • the high-strength cold-rolled steel sheets containing a great amount of Si have problems that the phosphate treatability is poor but the coating peeling is easily caused to deteriorate the corrosion resistance after the coating as compared with the commonly used steel sheets when the steels sheet after electrodeposition coating is subjected to severer corrosion environment as in a hot salt water immersion test or a composite cycle corrosion test repeating cycle of wetting-drying.
  • Patent Document 1 proposes a high-strength cold-rolled steel sheet obtained by heating a slab at a temperature of higher than 1200° C. in hot rolling, descaling under high pressure, polishing the surface of the hot-rolled steel sheet with a nylon brush containing abrasion grains prior to pickling and then immersing in a bath of 9% hydrochloric acid twice to perform pickling to lower the Si concentration on the surface of the steel sheet.
  • Patent Document 2 proposes a high-strength cold-rolled steel sheet wherein the corrosion resistance is improved by rendering line width of Si-containing linear oxide observed in 1-10 pm from the surface of the steel sheet into not more than 300 nm.
  • the Si-containing oxide is formed on the surface of the steel sheet by annealing after cold rolling, so that the improvement of the corrosion resistance after coating is not desired.
  • the high-strength cold-rolled steel sheet disclosed in Patent Document 2 there is no problem in the corrosion resistance under corrosion environment as in a salt spray test defined according to JIS Z2371, but sufficient corrosion resistance after coating is not obtained under severer corrosion environment as in a hot salt water immersion test or a composite cycle corrosion test. That is, the high-strength cold-rolled steel sheet having an excellent corrosion resistance after coating can not be obtained only by reducing the Si concentration on the surface of the steel sheet after hot rolling or by reducing the Si-containing linear oxide.
  • Patent Document 3 discloses a technique wherein the Si-containing oxide enriched on the surface of the steel sheet by annealing step or the like is removed by pickling and further an S-based compound is applied to the surface to enhance the reactivity with a phosphate treating solution to thereby improve the phosphate treatability.
  • Patent Document 4 discloses a technique wherein a P-based compound is applied instead of the S-based compound of the above technique.
  • the invention is made in view of considering the above problems inherent to the cold-rolled steel sheet containing a great amount of Si and is to provide a method of producing a cold-rolled steel sheet being excellent in not only the phosphate treatability even when using a phosphate treating solution at a lower temperature but also in the corrosion resistance after coating as evaluated by a hot salt water immersion test or a composite cycle corrosion test, a cold-rolled steel sheet produced by this method as well as a member for automobile using the cold-rolled steel sheet.
  • the inventors have made detailed analysis on surface properties of steel sheets after annealing in order to solve the above problems and various studies on a method of enhancing the reactivity between the surface of the steel sheet and the phosphate treating solution.
  • it has been found that it is very important to subject the continuously annealed steel sheet surface to strong pickling after the cold rolling to thereby remove Si-containing oxide layer formed on the surface of the steel sheet during the annealing but also reduce a ratio of covering the surface of the steel sheet with an iron-based oxide formed on the steel sheet surface by the strong pickling, and consequently the invention has been accomplished.
  • the invention proposes a method of producing a cold-rolled steel sheet, comprising steps of cold rolling a steel sheet, continuously annealing, pickling and further repickling it.
  • the repickling in the production method of the invention is characterized in that a non-oxidizable acid is used instead of an acid used in the pickling prior to the repickling.
  • the non-oxidizable acid in the production method of the invention is characterized to be any of hydrochloric acid, sulfuric acid, phosphoric acid, pyrophosphoric acid, formic acid, acetic acid, citric acid, hydrofluoric acid, oxalic acid and a mixed acid of two or more thereof.
  • the non-oxidizable acid in the production method of the invention is characterized to be any of hydrochloric acid with a concentration of 0.1-50 g/L, sulfuric acid with a concentration of 0.1-150 g/L and a mixed acid of 0.1-20 g/L of hydrochloric acid and 0.1-60 g/L of sulfuric acid.
  • the production method of the invention is characterized in that the repickling is carried out at a temperature of a repickling solution of 20-70° C. for 1-30 seconds.
  • the production method of the invention is characterized in that the pickling is carried out with any of nitric acid, hydrochloric acid, hydrofluoric acid, sulfuric acid and a mixed acid of two or more thereof.
  • the production method of the invention is characterized in that the pickling is carried out with any of a mixed acid of nitric acid and hydrochloric acid wherein a concentration of nitric acid is more than 50 g/L but not more than 200 g/L and a ratio (HCl/HNO 3 ) of hydrochloric acid concentration to nitric acid concentration is 0.01-1.0, or a mixed acid of nitric acid and hydrofluoric acid wherein a concentration of nitric acid is more than 50 g/L but not more than 200 g/L and a ratio (HF/HNO 3 ) of hydrofluoric acid concentration to nitric acid concentration is 0.01-1.0.
  • a mixed acid of nitric acid and hydrochloric acid wherein a concentration of nitric acid is more than 50 g/L but not more than 200 g/L and a ratio (HF/HNO 3 ) of hydrofluoric acid concentration to nitric acid concentration is 0.01-1.0.
  • the steel sheet in the production method of the invention is characterized by comprising 0.5-3.0 mass % of Si.
  • the steel sheet in the production method of the invention is characterized by having a chemical composition comprising, in addition to Si, C: 0.01-0.30 mass %, Mn: 1.0-7.5 mass %, P: not more than 0.05 mass %, S: not more than 0.01 mass %, Al: not more than 0.06 mass % and the remainder being Fe and inevitable impurities.
  • the steel sheet in the production method of the invention is characterized by containing, in addition to the above chemical composition, one or more selected from Nb: not more than 0.3 mass %, Ti: not more than 0.3 mass %, V: not more than 0.3 mass %, Mo: not more than 0.3 mass %, Cr: not more than 0.5 mass %, B: not more than 0.006 mass % and N: not more than 0.008 mass %.
  • the steel sheet in the production method of the invention is characterized by containing, in addition to the aforementioned chemical composition, one or more selected from Ni: not more than 2.0 mass %, Cu: not more than 2.0 mass %, Ca: not more than 0.1 mass % and REM: not more than 0.1 mass %.
  • the invention is a cold-rolled steel sheet produced by any one of the aforementioned methods, characterized in that a Si-containing oxide layer is removed from a surface layer of the steel sheet by pickling after continuous annealing and a surface covering ratio of an iron-based oxide existing on the surface of the steel sheet after repickling is not more than 40%.
  • the cold-rolled steel sheet of the invention is characterized in that a maximum thickness of the iron-based oxide existing on the steel sheet surface after repickling is not more than 150 nm.
  • the invention is a member for automobiles, characterized by using a cold-rolled steel sheet as described in any one of the above.
  • a cold-rolled steel sheet which is excellent in the phosphate treatability even when Si is contained as large as 0.5-3.0 mass % and when using a phosphate treating solution at a lower temperature but also is excellent in the corrosion resistance after coating under severer corrosion environment as in a hot salt water immersion test or a composite cycle corrosion test. According to the invention, therefore, it is possible to largely improve the phosphate treatability and corrosion resistance after coating in the high-strength cold-rolled steel sheets containing a greater amount of Si and having a tensile strength TS of not less than 590 MPa, so that it can be preferably used in strong members and the like in a vehicle body of an automobile.
  • FIG. 1 shows reflection electron microphotographs of steel sheet surfaces of standard cold-rolled steel sheet sample Nos. a and b for determining a surface covering ratio with an iron-based oxide.
  • FIG. 2 shows a histogram of pixel number to gray value in the reflection electron microphotographs of the standard cold-rolled steel sheet sample Nos. a and b.
  • FIG. 3 is a photograph of a section of a coating on a surface of a steel sheet after repickling observed by means of a transmission electron microscope.
  • FIG. 4 is a graph showing energy dispersion type X-ray (EDX) analytical results of an iron-based oxide observed in FIG. 3 .
  • EDX energy dispersion type X-ray
  • FIG. 5 is a graph of depth distribution of O, Si, Mn and Fe on a surface of a test specimen in Comparative Example (No. 1) and Invention Example (No. 9) of Example 1 as measured by GDS.
  • a non-oxidizing or reducing gas is usually used as an atmosphere gas, and also a dew point is strictly controlled.
  • the oxidation of the steel sheet surface is controlled.
  • the Si-containing oxide is formed not only the surface of the steel sheet but also in the interior of the steel matrix, it is known that the oxide obstructs the etching property on the surface of the steel sheet in the phosphate treatment (treatment with zinc phosphate) made as an underlaying treatment for electrodeposition coating and badly affects the formation of sound phosphate treated coating.
  • the lowering of the temperature of the phosphate treating solution is proceeding for the purpose of reducing the sludge amount generated in the phosphate treatment and the running cost, and hence the phosphate treatment is carried out under a condition that the reactivity of the phosphate treating solution to the steel sheet is considerably low as compared with the conventional technique.
  • the change of the phosphate treating condition is not particularly questioned by the improvement of the surface adjusting technique or the like in the conventionally used common steel sheets having a less addition amount of an alloy.
  • the steel sheet having a greater addition amount of alloying component particularly a high-strength cold-rolled steel sheet attempted to increase the strength by adding a greater amount of Si, however, the influence of changing the phosphate treating condition as mentioned above is very large.
  • the cold-rolled steel sheet having a greater amount of Si therefore, it is required that the surface of the steel sheet itself is activated in correspondence with the deterioration of the phosphate treating condition to enhance the reactivity with the phosphate treating solution.
  • the inventors have made various investigations on a method of improving the phosphate treatability for corresponding to the deterioration of the phosphate treating condition as mentioned above. As a result, it has been found out that it is effective to conduct strong pickling of the surface of the cold-rolled steel sheet after continuous annealing with nitric acid or the like as a pickling solution to remove a Si-containing oxide layer formed on the surface of the steel sheet by continuous annealing and the like after cold rolling.
  • Si-containing oxide used herein means SiO 2 or Si—Mn base composite oxide formed on the surface of the steel sheet or along crystal grain boundary inside the steel sheet in the slab heating or after hot rolling or in annealing after cold rolling.
  • the thickness of the layer containing these Si-containing oxides varied depending upon components of the steel sheet or the annealing condition (temperature, time, atmosphere), but is usually about 1 ⁇ m from the surface of the steel sheet.
  • the term “removal of the Si-containing oxide layer” according to the invention means that the pickling is carried out to remove the Si-containing oxide layer to a level that peaks of Si, O do not appear when the surface of the steel sheet is analyzed in depth direction by means of GDS (glow discharge optical emission spectroscopy).
  • the reason why a strong acid such as nitric acid or the like is used as the pickling solution is due to the fact that among the Si-containing oxides, Si—Mn based composite oxide is easily dissolved in an acid, but SiO 2 is hardly soluble, and in order to remove the latter, the Si-containing oxide on the surface of the steel sheet should be removed together with the steel matrix.
  • the phosphate treatability is largely improved by removing the Si-containing oxide layer existing on the steel sheet surface through strong pickling with nitric acid or the like after the continuous annealing but the phosphate treatability may be deteriorated at moments.
  • the cause is further investigated, it is newly found that although the Si-containing oxide layer is removed by the strong pickling with nitric acid or the like, Fe dissolved from the surface of the steel sheet by the pickling separately produces an iron-based oxide, which is settled and precipitated on the surface of the steel sheet so as to cover the steel sheet surface to thereby deteriorate the phosphate treatability.
  • the inventors have found that the phosphate treatability is more improved and the corrosion resistance is further improved when the maximum thickness of the iron-based oxide is not more than 150 nm in addition to the fact that the covering ratio of the iron-based oxide generated on the surface of the steel sheet by pickling is not more than 40% and that it is effective as means for attaining the above to conduct the repickling by properly increasing the concentration of the acid used in the repickling.
  • the iron-based oxide in the invention means an oxide composed mainly of iron wherein an atomic concentration ratio of iron is not less than 30% as an element other than oxygen constituting the oxide.
  • the iron-based oxide is existent on the surface of the steel sheet at an uneven thickness, which is different from a natural oxide film existing uniformly and in layer at a thickness of few nm.
  • the iron-based oxide generated on the surface of the cold-rolled steel sheet is confirmed to be amorphous from the observation by means of a transmission electron microscope (TEM) and analysis results of diffraction pattern (analytical diagram) through an electron diffractometry.
  • TEM transmission electron microscope
  • the invention is accomplished by conducting further examinations on the above new knowledge.
  • Si is an element effective for attaining the increase of the strength of the steel because the effect of enhancing the strength of steel (solid-solution strengthening ability) is large without largely damaging the workability, but is also an element adversely exerting on the phosphate treatability and the corrosion resistance after coating.
  • Si is added as means for attaining a high strength, the addition of not less than 0.5 mass % is necessary. If the Si content is less than 0.5 mass %, the influence due to the deterioration of the phosphate treating conditions is less. On the other hand, when the Si content exceeds 3.0 mass %, the hot rolling property and cold rolling property are largely deteriorated, which is adversely influenced on the productivity and leads to the deterioration of ductility of the steel sheet itself. Therefore, Si is added within a range of 0.5-3.0 mass %. Preferably, it is a range of 0.8-2.5 mass %.
  • the cold-rolled steel sheet of the invention is an essential feature to include Si in the above range.
  • the other components are acceptable as far as they are included within composition ranges in the common cold-rolled steel sheet, and are not particularly limited.
  • the cold-rolled steel sheet of the invention is preferable to have the following component composition when it is applied to a high-strength cold-rolled steel sheet having a tensile strength of not less than 590 MPa for use in vehicle bodies for automobiles and so on.
  • C is an element effective for enhancing the strength of steel and further is an element effective for producing residual austenite having an effect of TRIP (Transformation Induced Plasticity), bainite and martensite.
  • TRIP Transformation Induced Plasticity
  • bainite and martensite.
  • C content is not less than 0.01 mass %, the above effect is obtained, while when C content is not more than 0.30 mass %, the deterioration of the weldability is not caused. Therefore, C is added preferably within a range of 0.01-0.3 mass %, more preferably within a range of 0.10-0.20 mass %.
  • Mn is an element having an action for solid-solution strengthening steel to increase the strength and enhance the hardenability and promoting the formation of residual austenite, bainite and martensite. Such effects are developed by the addition of not less than 1.0 mass %. On the other hand, when Mn content is not more than 7.5 mass %, the above effect is obtained without the increase of the cost. Therefore, Mn is added preferably within a range of 1.0-7.5 mass %, more preferably within a range of 2.0-5.0 mass %.
  • P is an element damaging no drawability though the solid-solution strengthening ability is large and is also an element effective for attaining a high strength, so that it is preferable to be included in an amount of not less than 0.005 mass %.
  • P is an element damaging the spot weldability, but there is no problem when it is not more than 0.05 mass %. Therefore, P is preferably not more than 0.05 mass %, more preferably not more than 0.02 mass %.
  • S is an impurity element inevitably incorporated, and is a harmful element which is precipitated in steel as MnS to deteriorate the stretch-flanging property.
  • S is preferably not more than 0.01 mass %, more preferably not more than 0.005 mass %, further preferably not more than 0.003 mass %.
  • Al is an element added as a deoxidizer at steel-making step, and is also an element effective for separating non-metallic inclusion, which deteriorates the stretch-flanging property, as a slug, so that it is preferable to be included in an amount of not less than 0.01 mass %.
  • Al content is not more than 0.06 mass %, the above effect is obtained without the increase of cost for material. Therefore, Al is preferable to be not more than 0.06 mass %, More preferably, it is a range of 0.02-0.06 mass %.
  • the cold-rolled steel sheet of the invention may contain one or more selected from Nb: not more than 0.3 mass %, Ti: not more than 0.3 mass %, V: not more than 0.3 mass %, Mo: not more than 0.3 mass %, Cr:
  • Nb, Ti and V are elements forming carbide and nitride to suppress ferrite growth at a heating stage in the annealing and finely divide the structure to improve the formability, particularly stretch-flanging property, and also Mo, Cr and B are elements improving the hardenability of steel and promoting the formation of bainite and martensite, so that they can be added within the above ranges.
  • N is an element forming nitrides with Nb, Ti and V or solid-soluting in steel to contribute to the increase of the strength of steel, so that when it is not more than 0.008 mass %, a greater amount of the nitride is not formed, and hence the breakage due to the formation of voids in the press forming can be suppressed to obtain the above effect.
  • the cold-rolled steel sheet of the invention may contain one or more selected from Ni: not more than 2.0 mass %, Cu: not more than 2.0 mass %, Ca: not more than 0.1 mass % and REM: not more than 0.1 mass %.
  • Ni and Cu promote the formation of the low-temperature transformation phase to develop the effect of increasing the strength of steel, so that they can be added within the above ranges.
  • Ca and REM are elements controlling the form of the sulfide base inclusion to improve the stretch-flanging property of the steel sheet, so that they can be added within the above ranges.
  • the remainder other than the above components is Fe and inevitable impurities.
  • other components may be optionally added within a scope of not damaging the action and effect of the invention.
  • the cold-rolled steel sheet of the invention is necessary to have a steel sheet surface obtained after the removal of Si-containing oxide layer such as SiO 2 or Si—Mn based composite oxide formed on the surface layer of the steel sheet during annealing.
  • Si-containing oxide layer such as SiO 2 or Si—Mn based composite oxide formed on the surface layer of the steel sheet during annealing.
  • the ratio of covering the surface of the steel sheet with iron-based oxide generated on the steel sheet surface by the strong pickling with nitric acid or the like is not more than 85% as an area ratio in addition to the removal of the Si-containing oxide layer.
  • the surface covering ratio exceeds 85%, the dissolving reaction of iron in the phosphate treatment is inhibited to suppress the crystal growth of phosphate such as zinc phosphate or the like.
  • the covering ratio of not more than 85% is insufficient in cold-rolled steel sheets used in applications requiring an extremely severe corrosion resistance after coating such as leg members for vehicle bodies particularly subjected to severe corrosion, so that it should be further reduced to not more than 40%, preferably not more than 35%.
  • the surface covering ratio of the iron-based oxide is determined as follows:
  • the surface of the steel sheet after the pickling is observed at about 5 fields with a ultra-low acceleration voltage scanning type electron microscope (ULV-SEM) capable of detecting information of an extremely surface layer under conditions of acceleration voltage: 2 kV, operating distance: 3.0 mm and magnification: about 1000 times and spectroscopy is conducted with an energy dispersion type X-ray spectrometer (EDX) to obtain a reflection electron image.
  • the reflection electron image is binarized with an image analysis software, e.g. Image J to measure an area ratio of a black portion.
  • the measured results on the fields can be averaged to obtain a surface covering ratio of the iron-based oxide.
  • ultra-low acceleration voltage scanning type electron microscope (ULV-SEM) may be mentioned, for example, ULTRA 55 made by SEISS, and as the energy dispersion type X-ray spectrometer (EDX) may be mentioned, for example, NSS 312E made by Thermo Fisher.
  • UUV-SEM ultra-low acceleration voltage scanning type electron microscope
  • EDX energy dispersion type X-ray spectrometer
  • threshold value in the binarization will be described.
  • a steel slab of Steel symbol G shown in Table 3 of the following example is subjected to hot rolling, cold rolling and continuous annealing under conditions of No. 8 in Table 4 of the following example to obtain a cold-rolled steel sheet of 1.8 mm in thickness, and then the cold-rolled steel sheet after the continuous annealing is subjected to pickling and repickling under conditions shown in Table 1, washed with water, dried and subjected to 0.7% temper rolling to obtain two cold-rolled steel sheets of Nos. a and b having different iron-based oxide amounts on their steel sheet surfaces. Then, the cold-rolled steel sheet of No. a is a standard sample having a large amount of iron-based oxide and the cold-rolled steel sheet of No.
  • FIG. 1 shows photographs of reflection electron images of steel sheets Nos. a and b
  • FIG. 2 shows a histogram of pixel number to a gray value in the photographs of the reflection electron images of the steel sheets Nos. a and b.
  • a gray value (Y point) corresponding to an intersecting point (X point) of the histograms of Nos. a and b shown in FIG. 2 is defined as a threshold value.
  • the surface covering ratio of the iron-based oxide in the steel sheets Nos. a and b is determined with the above threshold value, it is 85.3% in the steel sheet No. a and 25.8% in the steel sheet No. b.
  • the maximum thickness of the iron-based oxide is not more than 150 nm in addition that the covering ratio of the iron-based oxide produced on the steel sheet surface by repickling is not more than 40%.
  • the maximum thickness of the iron-based oxide is not more than 150 nm, the dissolving reaction of iron through the phosphate treatment is not inhibited locally and also the precipitation of crystal of phosphate such as zinc phosphate or the like is not inhibited locally. More preferably, it is not more than 130 nm.
  • the maximum thickness of the iron-based oxide is measured as follows. First, 10 extraction replicas are prepared from the surface of the steel sheet after the pickling by a focused ion beam (FIB) work for observing a section of about 8 pm relative to the widthwise direction of the steel sheet. Then, the section of 8 ⁇ m in the each replica is continuously shot by means of a transmission electron microscope (TEM) provided with an energy dispersion type X-ray spectrometer (EDX) capable of checking local information of the section at an acceleration voltage of 200 kV and a magnification of 100000 times.
  • TEM transmission electron microscope
  • EDX energy dispersion type X-ray spectrometer
  • FIG. 3 is a photograph showing a section of a covering layer existing on the surface of the steel sheet and generated by pickling as observed by TEM
  • the covering layer is an iron-based oxide composed mainly of iron. Therefore, the interval between a line A showing a surface of the steel sheet and a line B showing a thickest portion of an oxide layer shown by the photograph of the section in FIG. 3 is measured with respect to the 10 replicas, and a maximum thickness among them is a maximum thickness of the iron-based oxide. Moreover, the size and numbers of the replicas, measuring conditions by TEM and the like as mentioned above are merely exemplified, and may be properly modified as a matter of course.
  • the production method of the cold-rolled steel sheet of the invention is necessary to be a method wherein a steel material (slab) having Si: 0.5-3.0 mass % is heated, hot rolled, cold rolled, continuously annealed and then strong-pickled with nitric acid or the like to remove Si-containing oxide layer on a surface layer portion of the steel sheet and further repickled to render a surface covering ratio of an iron-based oxide not more than 40% generated on the steel sheet surface by the above strong pickling. Further, it is preferable to be a method wherein a maximum thickness of the iron-based oxide can be made to not more than 150 nm. Therefore, the procedure ranging from the steel-making step to the continuous annealing step after the cold rolling can be carried out according to the usual manner, but the pickling after the continuous annealing is preferable to be conducted under the following conditions.
  • the Si-containing oxide such as SiO 2 , Si—Mn based composite oxide or the like, so that the phosphate treatability and the corrosion resistance after coating are considerably deteriorated.
  • the cold-rolled steel sheet after the annealing is strongly pickled with nitric acid or the like, whereby the Si-containing oxide layer on the surface of the steel sheet is removed with the steel matrix.
  • Si—Mn based composite oxide among the Si-containing oxides is easily dissolved in an acid, but SiO 2 is insoluble in an acid. Therefore, in order to remove the Si-containing oxide including SiO 2 , it is necessary to remove the oxide layer together with the steel matrix of the steel sheet by the strong pickling.
  • the acid usable in the strong pickling can be preferably used nitric acid as a strong oxidizable acid, but hydrofluoric acid, hydrochloric acid, sulfuric acid or the like may be used as long as the Si-containing oxide layer can be removed, so that the kind of the acid is particularly no matter. Also, it is effective to add a pickling promoting agent to the acid, or to co-use an electrolytic treatment to promote the dissolution of the steel matrix.
  • a temperature of the pickling solution is 20-70° C. and a pickling time is 3-30 seconds.
  • the iron-based oxide is dissolved and removed by further repickling the steel sheet pickled after the continuous annealing with a non-oxidizable acid.
  • the non-oxidizable acid usable in the repickling includes hydrochloric acid, sulfuric acid, phosphoric acid, pyrophosphoric acid, formic acid, acetic acid, citric acid, hydrofluoric acid, oxalic acid and a mixed acid of two or more thereof. Any of these may be used, but hydrochloric acid or sulfuring acid commonly used in the iron-making industry may be preferably used. Among them, hydrochloric acid is preferable because it is a volatile acid and hardly remains a residue on the steel sheet surface after washing with water different from sulfuric acid retaining sulfuric acid root and is large in the effect of breaking the oxide by chloride ion. Also, a mixed acid of hydrochloric acid and sulfuric acid may be used.
  • hydrochloric acid When hydrochloric acid is used as the pickling solution in the repickling, it is preferable that a concentration of hydrochloric acid is 0.1-50 g/L, while in case of using sulfuric acid, it is preferable that a concentration of sulfuric acid is 0.1-150 g/L. Also, when the mixed acid of hydrochloric acid and sulfuric acid is used in the repickling, it is preferable to use a mixed acid having a hydrochloric acid concentration of 0.1-20 g/L and a sulfuric acid concentration of 0.1-60 g/L. Also, the repickling of the invention is preferable to be conducted at a temperature of a repickling solution of 20-70° C.
  • the concentration of the repickling solution is more than the above lower limit and the liquid temperature is not lower than 20° C. and the treating time is not less than 1 second, it is sufficient to remove the iron-based oxide existing on the steel sheet surface, while when the concentration of the repickling solution is not more than the above upper limit and the temperature is not higher than 70° C. and the treating time is not more than 30 seconds, the dissolution of the steel sheet surface becomes not excessive and a new surface oxide film is not formed.
  • the maximum thickness of the iron-based oxide existing on the steel sheet surface after the pickling is surely thinned to not more than 150 nm.
  • the concentration of the pickling solution used in the repickling it is preferable to properly increase the concentration of the pickling solution used in the repickling.
  • the concentration of hydrochloric acid is 3-50 g/L
  • sulfuric acid is used in the repickling
  • the concentration of sulfuric acid is 8-150 g/L.
  • a mixed acid having a hydrochloric acid concentration of 3-20 g/L and a sulfuric acid concentration of 8-60 g/L.
  • the iron-based oxide when the acid concentration is within the above range, the iron-based oxide can be surely thinned to not more than 150 nm, whereby the phosphate treatability and the corrosion resistance after coating are improved. Also, when the acid concentration is within the above range, the dissolution of the steel sheet surface becomes not excessive, and hence new surface oxide film is never formed.
  • the cold-rolled steel sheet wherein the covering ratio of the steel sheet surface with the iron-based oxide is made to not more than 40% by pickling and repickling after the continuous annealing as mentioned above, or alternately the cold-rolled steel sheet, wherein the maximum thickness of the iron-based oxide is made to not more than 150 nm, is subsequently subjected to usual treating steps such as temper rolling and the like to provide products.
  • a steel comprising C: 0.125 mass %, Si: 1.5 mass %, Mn: 2.6 mass %, P: 0.019 mass %, S: 0.008 mass %, Al: 0.040 mass % and the remainder being Fe and inevitable impurities is prepared according to common refining process such as melting in a converter, degassing treatment and the like and continuously cast into a steel material (slab). Then, the slab is reheated to a temperature of 1150-1170° C., hot rolled at a terminating temperature of finish rolling of 850-880° C. and coiled at a temperature of 500-550° C. to obtain a hot-rolled steel sheet having a thickness of 3-5 mm.
  • the hot-rolled steel sheet is pickled to remove scales and thereafter cold rolled to obtain a cold-rolled steel sheet having a thickness of 1.8 mm.
  • the cold-rolled steel sheet is subjected to such a continuous annealing that it is heated to a soaking temperature of 750-780° C. and held at this temperature for 40-50 seconds and then cooled at a rate of 20-30° C./second from the soaking temperature to a cooling stop temperature of 350-400° C. and held at the cooling stop temperature range for 100-120 seconds, and then the steel sheet is pickled and further repickled under conditions shown in Table 2, washed with water, dried and subjected to a temper rolling at a stretching ratio of 0.7% to obtain cold-rolled steel sheets Nos. 1-85 shown in Table 2.
  • a test specimen is sampled from each of the above cold-rolled steel sheets and observed at 5 fields of the steel sheet surface with a scanning type electron micrcope of ultra-low acceleration voltage (ULV-SEM; made by SEISS; ULTRA 55) at an acceleration voltage of 2 kV, an operating distance of 3.0 mm and a magnification of 1000 times. And analyzed with an energy dispersion X-ray spectrometer (EDX; made by Thermo Fisher; NSS 312E) to obtain a reflection electron image.
  • the reflection electron image is binarized with an image analyzing software (Image J) with respect to gray value (Y-point) corresponding to intersect point (X-point) and threshold value defined in histograms of the aforementioned standard samples Nos. a and b to measure an area ratio of a black portion.
  • the values measured at 5 fields are averaged as a surface covering ratio of iron-based oxide.
  • a test specimen is sampled from each of the above cold-rolled steel sheets and subjected to a phosphate treatment and a coating treatment under the following conditions and then subjected to three corrosion tests of hot salt water immersion test, salt water spray test and composite cycle corrosion test to evaluate a corrosion resistance after coating. Further, a distribution of O, Si, Mn and Fe in depth direction on the surface of the test specimen sampled from each cold-rolled steel sheet is measured with GDS.
  • test specimen sampled from each cold-rolled steel sheet is subjected to a phosphate treatment with a degreasing agent: FC-E2011, a surface regulator: PL-X and a phosphate treating agent: PALBOND PB-L3065, which are made by Nihon Parkerizing Co., Ltd., so as to provide a phosphate coating adhered amount of 1.7-3.0 g/m 2 under two conditions of the following standard condition and comparative condition of lowering the phosphate treating temperature to a low temperature.
  • a degreasing agent FC-E2011
  • PL-X a surface regulator
  • PALBOND PB-L3065 which are made by Nihon Parkerizing Co., Ltd.
  • the surface of the test specimen subjected to the phosphate treatment is electrodeposited with an electrodeposition paint : V-50 made by Nippon Paint Co., Ltd. so as to have a coating thickness of 25 ⁇ m and then subjected to the following three corrosion tests.
  • a test of peeling off the tape is carried out to measure a maximum peeled full width combining either side of the cut flaw portion. When the maximum peeled full width is not more than 5.0 mm, the corrosion resistance can be evaluated to be good in the hot slat water immersion test.
  • the maximum peeled full width is not more than 4.0 mm, the corrosion resistance can be evaluated to be good in the salt water spray test.
  • test results are also shown in Table 2.
  • the steel sheets of Invention Examples subjected to the pickling and repickling under the conditions adequate for the invention after the continuous annealing are small in the maximum peeled full width on all of the hot salt water immersion test, salt water spray test and composite cycle corrosion test and show the good corrosion resistance after coating.
  • all of the cold-rolled steel sheets having the surface covering ratio of the iron-based oxide of not more than 40% are excellent in the corrosion resistance after coating under severe corrosion environment.
  • FIG. 5 shows the profile in depth direction of O, Si, Mn and Fe as surface-analyzed with GDS with respect to the test specimens of Comparative Example No. 1 and Invention Example No. 9 in Table 2.
  • Each of steels A-X having a chemical composition shown in Table 3 is prepared according to common refining process such as melting in a converter, degassing treatment and the like and continuously cast into a steel slab.
  • the steel slab is hot rolled under hot rolling conditions shown in Table 4 to obtain a hot-rolled steel sheet having a thickness of 3-4 mm, which is pickled to remove scales on the surface of the steel sheet and thereafter cold rolled to obtain a cold-rolled steel sheet having a thickness of 1.8 mm.
  • the cold-rolled steel sheet is continuously annealed under the conditions shown in Table 4, pickled and repickled under conditions shown in Table 5, washed with water, dried and subjected to a temper rolling at a stretching ratio of 0.7% to obtain cold-rolled steel sheets Nos. 1-39.
  • a test specimen is sampled from each of the cold-rolled steel sheets and subjected to the following tensile test and test for the corrosion resistance after coating after the surface covering ratio of iron-based oxide on the steel sheet surface after the repickling is measured in the same manner as in Example 1. Also, the distribution in depth direction of O, Si, Mn and Fe on the surface of the test specimen sampled from each of the cold-rolled steel sheets is measured with GDS.
  • test specimen is prepared by subjecting the test specimen sampled from each of the cold-rolled steel sheet to phosphate treatment and electrodeposition under the same conditions as in Example 1 and then subjected to three corrosion tests of hot salt water immersion test, salt water spray test (SST) and composite cycle corrosion test (CCT) likewise Example 1 to evaluate the corrosion resistance after coating.
  • SST salt water spray test
  • CCT composite cycle corrosion test
  • a steel comprising C: 0.125 mass %, Si: 1.5 mass %, Mn: 2.6 mass %, P: 0.019 mass %, S: 0.008 mass %, Al: 0.040 mass % and the remainder being Fe and inevitable impurities is melted and continuously cast into a steel material (slab).
  • the slab is reheated to a temperature of 1150-1170° C., hot rolled at a terminating temperature of finish rolling of 850-880° C. and coiled at a temperature of 500-550° C. to obtain a hot-rolled steel sheet having a thickness of 3-4 mm.
  • the hot-rolled steel sheet is pickled to remove scales and thereafter cold rolled to obtain a cold-rolled steel sheet having a thickness of 1.8 mm.
  • the cold-rolled steel sheet is subjected to such a continuous annealing that it is heated to a soaking temperature of 750-780° C. and held at this temperature for 40-50 seconds and then cooled at a rate of 20-30° C./second from the soaking temperature to a cooling stop temperature of 350-400° C. and held at the cooling stop temperature range for 100-120 seconds, and then the steel sheet is pickled and repickled under conditions shown in Table 6, washed with water, dried and subjected to a temper rolling at a stretching ratio of 0.7% to obtain cold-rolled steel sheets Nos. 1-61 shown in Table 6.
  • a test specimen is sampled from each of the above cold-rolled steel sheets to measure a surface covering ratio and maximum thickness of iron-based oxide generated on the surface of the steel sheet by pickling through the aforementioned methods.
  • test specimen is sampled from each of the above cold-rolled steel sheets and subjected to phosphate treatment and coating treatment under the following conditions and then subjected to three corrosion tests of hot salt water immersion test, salt water spray test and composite cycle corrosion test to evaluate the corrosion resistance after coating. Further, the distribution in depth direction of O, Si, Mn and Fe on the surface of the test specimen sampled from each of the cold-rolled steel sheets is measured with GDS.
  • test specimen sampled from each cold-rolled steel sheet is subjected to a phosphate treatment with a degreasing agent: FC-E2011, a surface regulator: PL-X and a phosphate treating agent: PALBOND PB-L3065, which are made by Nihon Parkerizing Co., Ltd., so as to provide a phosphate coating adhered amount of 1.7-3.0 g/m2 under two conditions of the following standard condition and comparative condition of lowering the phosphate treating temperature to a low temperature.
  • a degreasing agent FC-E2011
  • PL-X a surface regulator
  • PALBOND PB-L3065 which are made by Nihon Parkerizing Co., Ltd.
  • the surface of the test specimen subjected to the phosphate treatment is electrodeposited with an electrodeposition paint : V-50 made by Nippon Paint Co., Ltd. so as to have a coating thickness of 25 ⁇ m and then subjected to the following three corrosion tests under more strict conditions than the one with Example 1.
  • a test of peeling off the tape is carried out to measure a maximum peeled full width combining either side of the cut flaw portion. When the maximum peeled full width is not more than 5.0 mm, the corrosion resistance can be evaluated to be good in the hot slat water immersion test.
  • the maximum peeled full width is not more than 4.0 mm, the corrosion resistance can be evaluated to be good in the salt water spray test.
  • salt water spraying aqueous solution of 5 mass % NaCl: 35° C., relative humidity: 98%) for 2 hours drying (60° C., relative humidity: 30%) for 2 hours
  • the cold-rolled steel sheets produced according to the invention not only are excellent in the corrosion resistance after coating but also have a high strength and a good workability, so that they can be preferably used as not only a starting material used in members of the automotive vehicle body but also a starting material for applications requiring the same properties such as household electrical goods, building members and so on.

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