EP2612956A1 - Procédé de production d'une tôle d'acier laminée à froid, tôle d'acier laminée à froid, et élément véhicule - Google Patents

Procédé de production d'une tôle d'acier laminée à froid, tôle d'acier laminée à froid, et élément véhicule Download PDF

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EP2612956A1
EP2612956A1 EP11821648.0A EP11821648A EP2612956A1 EP 2612956 A1 EP2612956 A1 EP 2612956A1 EP 11821648 A EP11821648 A EP 11821648A EP 2612956 A1 EP2612956 A1 EP 2612956A1
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Prior art keywords
steel sheet
cold
mass
rolled steel
pickling
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German (de)
English (en)
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EP2612956A4 (fr
EP2612956B1 (fr
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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
    • C23G1/085—Iron or steel solutions containing HNO3
    • 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
    • C21D8/0205—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips of ferrous alloys
    • 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
    • C21D8/0226—Hot 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
    • C21D8/0221—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
    • C21D8/0247—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
    • C21D8/0273—Final recrystallisation annealing
    • 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
    • C22C1/00—Making non-ferrous alloys
    • C22C1/02—Making non-ferrous alloys by melting
    • 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
    • C22C38/12—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
    • 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
    • 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
    • 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
    • Y10T428/263—Coating layer not in excess of 5 mils thick or equivalent
    • Y10T428/264—Up to 3 mils
    • Y10T428/265—1 mil or less

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 ⁇ m 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, characterized in that a continuously annealed steel sheet after cold rolling is pickled with a mixture of nitric acid and hydrochloric acid having a nitric acid concentration of more than 100 g/L but not more than 200 g/L and a ratio R (HCl/HNO 3 ) of hydrochloric acid concentration to nitric acid concentration of 0.01-0.25.
  • the mixture of nitric acid and hydrochloric acid in the production method of the invention is characterized to have a nitric acid concentration of more than 110 g/L but not more than 140 g/L and a ratio R (HCl/HNO 3 ) of hydrochloric acid concentration to nitric acid concentration of 0.03-0.25.
  • the production method of the invention is characterized in that the pickling is carried out at a temperature of a pickling solution of 20-70°C for 3-30 seconds.
  • the steel sheet in the production method of the invention is characterized by 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% and Al: not more than 0.06 mass% and the remainder being Fe and inevitable impurities.
  • the cold-rolled steel sheet in the production method of the invention is characterized by comprising, in addition to the above chemical composition, one or more selected from the group consisting of 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 cold-rolled steel sheet in the production method of the invention is characterized by comprising, in addition to the above chemical composition, 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 aforementioned method, which is a cold-rolled steel sheet characterized in that a Si-containing oxide layer is removed from the surface of the steel sheet by pickling after continuous annealing and a ratio of covering the surface of the steel sheet with an iron-based oxide formed by the pickling is not more than 85%.
  • the cold-rolled steel sheet according to the invention is characterized in that a maximum thickness of the iron-based oxide existing on the surface of the steel sheet is not more than 200 nm.
  • the invention is a member for automobile characterized by using any aforementioned cold-rolled steel sheet.
  • 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.
  • 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 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).
  • nitric acid is used as the pickling solution is due to the fact that the Si-Mn based composite oxide among the Si-containing oxides is easily dissolved in the acid, but SiO 2 is hardly soluble, and in order to remove the latter, nitric acid as a strong-oxidizing acid must be used to remove the Si-containing oxide on the surface of the steel sheet together with the steel matrix.
  • the phosphate treatability is largely improved by conducting strong pickling with nitric acid after the continuous annealing to remove the Si-containing oxide layer existing on the surface of the steel sheet 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, 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 phosphate treatability is more improved and the corrosion resistance is further improved when the covering ratio of the iron-based oxide generated on the surface of the steel sheet by pickling is not more than 85% and further the maximum thickness of the iron-based oxide is not more than 200 nm and that it is effective as means for attaining the above to control the concentration of hydrochloric acid having the effect of breaking an oxide film, which is used in a part of the pickling solution, to an adequate range for pickling.
  • 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
  • analysis results of diffraction pattern analytical diagram
  • 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: not more than 0.5 mass%, B: not more than 0.006 mass% and N: not more than 0.008 mass%.
  • 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. However, 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 layer formed on the surface layer of the steel sheet during annealing. For this end, it is necessary to conduct strong pickling with a pickling solution of mixed nitric acid and hydrochloric acid to dissolve and remove the Si-containing oxide formed on the surface of the steel sheet and in the grain boundary portion in the vicinity of the surface together with the steel matrix.
  • Si-containing oxide layer such as SiO 2 or Si-Mn based composite oxide layer formed on the surface layer of the steel sheet during annealing.
  • a pickling solution of mixed nitric acid and hydrochloric acid to dissolve and remove the Si-containing oxide formed on the surface of the steel sheet and in the grain boundary portion in the vicinity of the surface together with the steel matrix.
  • the cold-rolled steel sheet of the invention it is necessary to reduce 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 to not more than 85% as an area ratio in addition to the removal of the Si-containing oxide layer.
  • the surface covering ratio of the iron-based oxide 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. It is preferably not more than 80%.
  • the surface covering ratio of the iron-based oxide is determined as follows:
  • 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. 7 in Table 4 of the following example to obtain a cold-rolled steel sheet of 1.8 mm in thickness, and then the cod-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 200 nm in addition that the covering ratio of the iron-based oxide produced on the steel sheet surface by pickling is not more than 85%.
  • the maximum thickness of the iron-based oxide is not more than 200 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 180 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 ⁇ m 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 according to the invention will be described below.
  • 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 pickled with a pickling solution of mixed nitric acid and hydrochloric acid, whereby a Si-containing oxide layer is removed from a surface layer portion of the steel sheet and a surface covering ratio of an iron-based oxide generated on the surface of the steel sheet by pickling can be made to not more than 85%.
  • the cold-rolled steel sheet after the annealing is strongly pickled with a pickling solution of mixed nitric acid and hydrochloric acid, whereby the Si-containing oxide layer on the surface of the steel sheet is removed with the steel matrix but also the formation of the iron-based oxide settled and precipitated on the surface of the steel sheet by pickling is suppressed.
  • 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 by pickling, the steel matrix is required to be removed with nitric acid as a strong acid. In order to conduct the strong pickling for removing the oxide layer including the steel matrix, it is necessary that the concentration of nitric acid is more than 100 g/L. However, since nitric acid is a strongly oxidizing acid, Fe eluted is oxidized to form an iron-based oxide, which is precipitated on the surface of the steel sheet and adversely affects the phosphate treatability and the corrosion resistance after coating.
  • the concentration of nitric acid is more than 100 g/L but not more than 200 g/L. Preferably, it is a range of 110-150 g/L.
  • the concentration of nitric acid is merely limited to the above range, it is difficult to stably control the surface covering ratio of the iron-based oxide generated on the surface of the steel sheet by pickling with nitric acid to not more than 85%.
  • the pickling is carried out with such a mixed acid that the concentration of nitric acid is limited to the above range but also a ratio R (HCl/HNO 3 ) of a concentration of a chloride ion having an effect of breaking an oxide film, i.e.
  • hydrochloric acid to the concentration of nitric acid is a range of 0.01-0.25.
  • the ratio R is less than 0.01, the effect of suppressing the formation of the iron-based oxide is small, while when it exceeds 0.25, the amount of the steel sheet dissolved is reduced and hence the Si-containing oxide layer cannot be removed.
  • the mixed acid pickling solution of nitric acid and hydrochloric acid is preferable to have a concentration of nitric acid within a range of more than 110 g/L but not more than 140 g/L and a ratio R (HCl/HNO 3 ) of hydrochloric acid concentration to nitric acid concentration within a range of 0.03-0.25.
  • concentrations are satisfied within the above ranges, it is possible to stably make the thickness of the iron-based oxide to not more than 200 nm, and hence the phosphate treatability and the corrosion resistance after coating are not deteriorated.
  • the pickling with the mixed pickling solution of nitric acid and hydrochloric acid is carried out at a temperature of the pickling solution of 20-70°C for a pickling time of 3-30 seconds.
  • the temperature of the pickling solution is not lower than 20°C and the pickling time is not less than 3 seconds, the Si-containing oxide layer formed in the surface layer of the steel sheet in the annealing can be removed sufficiently, and the phosphate treatability and the corrosion resistance after coating are never deteriorated.
  • 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 85% by pickling 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 200 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).
  • 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 ⁇ 4mm.
  • 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 surface is pickled 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. 1-25 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 microscope 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: PALBON 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
  • PALBON 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.
  • 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.
  • 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. 18 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 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. 1-30.
  • 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 pickling 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 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. 1-12 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.
  • 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 with more strict condition 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.
  • 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.
  • 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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