WO2015133077A1 - 冷延鋼板およびその製造方法、ならびに自動車部材 - Google Patents

冷延鋼板およびその製造方法、ならびに自動車部材 Download PDF

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WO2015133077A1
WO2015133077A1 PCT/JP2015/000777 JP2015000777W WO2015133077A1 WO 2015133077 A1 WO2015133077 A1 WO 2015133077A1 JP 2015000777 W JP2015000777 W JP 2015000777W WO 2015133077 A1 WO2015133077 A1 WO 2015133077A1
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Prior art keywords
acid
steel sheet
cold
rolled steel
pickling
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PCT/JP2015/000777
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English (en)
French (fr)
Japanese (ja)
Inventor
弘之 増岡
平　章一郎
雄太 寺崎
宏胤 近藤
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Ｊｆｅスチール株式会社
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Priority to CN201580011519.2A priority Critical patent/CN106062253B/zh
Priority to EP15759246.0A priority patent/EP3115482B1/en
Priority to MX2016011278A priority patent/MX2016011278A/es
Priority to JP2016506113A priority patent/JP6108028B2/ja
Publication of WO2015133077A1 publication Critical patent/WO2015133077A1/ja

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    • 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
    • C22C38/02—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
    • 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
    • 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
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/04—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing
    • C21D8/0478—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing involving a particular surface treatment
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
    • C21D9/46—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals

Definitions

  • the present invention relates to a cold-rolled steel sheet, its manufacture, and an automobile member.
  • the present invention relates to a cold-rolled steel sheet having excellent chemical conversion properties and excellent post-coating corrosion resistance evaluated by a salt warm water immersion test or a combined cycle corrosion test, a manufacturing method thereof, and an automobile member.
  • SiO 2 or SiO 2 is present at the interface between the steel sheet surface and the oxide scale during slab heating, hot rolling, or subsequent annealing. It is known that Si-containing oxides such as Si—Mn complex oxides are formed in large quantities. This Si-containing oxide significantly reduces the chemical conversion processability.
  • Patent Document 1 the slab is heated at a temperature of 1200 ° C. or higher during hot rolling, descaled at a high pressure, and hot-rolled steel plate before pickling.
  • a high-strength cold-rolled steel sheet is proposed in which the surface is ground with a nylon brush containing abrasive grains, dipped twice in a 9% hydrochloric acid bath and pickled to reduce the Si concentration on the steel sheet surface.
  • Patent Document 2 proposes a high-strength cold-rolled steel sheet in which the corrosion resistance is improved by setting the line width of a linear oxide containing Si observed at 1 to 10 ⁇ m from the steel sheet surface to 300 nm or less. .
  • Patent Document 3 proposes a technique for improving the oxide removal ability by setting the iron ion concentration (divalent) in hydrochloric acid to 0.5 to 18%.
  • the corrosion resistance does not become a problem in a corrosive environment such as the salt spray test specified in JIS Z2371, but unlike the salt warm water immersion test and the combined cycle corrosion test. In a severe corrosive environment, sufficient post-coating corrosion resistance cannot be obtained.
  • a high-strength cold-rolled steel sheet excellent in corrosion resistance after coating cannot be obtained simply by reducing the Si concentration on the surface of the steel sheet after hot rolling or reducing the linear oxide containing Si.
  • Patent Document 4 discloses that the Si-containing oxide concentrated on the surface of the steel sheet in an annealing process or the like is removed by pickling and further an S-based compound is added to the surface.
  • a technique for improving the chemical conversion treatment performance by increasing the reactivity with the chemical conversion treatment liquid is disclosed.
  • Patent Document 5 discloses a technique for providing a P-based compound instead of the S-based compound in Patent Document 4.
  • Patent Document 6 as a technique for solving the above problems, pickling with an oxidizing acid is performed at the first stage to remove SiO 2, and pickling with a non-oxidizing acid is performed at the second stage.
  • a technique for improving the chemical conversion treatment performance by increasing the reactivity with the chemical conversion treatment liquid by removing the Fe-based oxide formed by the first stage pickling is disclosed.
  • Patent Documents 4 and 5 are effective for conventional plain steel sheets, but for high-strength cold-rolled steel sheets containing a large amount of Si, the low temperature of the chemical conversion treatment liquid It is not possible to expect a sufficient improvement effect that can cope with the development.
  • Patent Document 6 it is known that the high-temperature cold-rolled steel sheet containing a large amount of Si can cope with the temperature reduction of the chemical conversion treatment liquid. .
  • the present invention has been made in view of such circumstances, and an object of the present invention is to provide a cold-rolled steel sheet having excellent chemical conversion properties and excellent corrosion resistance after coating, a manufacturing method thereof, and an automobile member.
  • the inventors conducted a detailed analysis on the steel sheet surface characteristics after annealing, and conducted extensive studies on a method for increasing the reactivity between the steel sheet surface and the chemical conversion treatment liquid.
  • the steel sheet surface that has been continuously annealed is strongly pickled, the Si-containing oxide layer formed on the steel sheet surface layer is removed during annealing, and the iron-based oxide that is generated on the steel sheet surface by the strong pickling.
  • the present inventors have found that it is extremely important to reduce the steel sheet surface coverage by the present invention.
  • the present invention has been made based on the above findings, and the gist thereof is as follows.
  • the steel sheet that has been continuously annealed after cold rolling is subjected to the first pickling with the acid solution of either a) or b) below, and then the second time with the acid solution comprising a non-oxidizing acid.
  • the manufacturing method of the cold-rolled steel plate which performs the pickling.
  • the non-oxidizing acid is one or more of hydrochloric acid, sulfuric acid, phosphoric acid, pyrophosphoric acid, formic acid, acetic acid, citric acid, hydrofluoric acid, and oxalic acid.
  • the non-oxidizing acid includes hydrochloric acid having a concentration of 0.1 to 50 g / L, 0.1 to 150 g / L sulfuric acid, 0.1 to 20 g / L hydrochloric acid and 0.1 to 60 g / L.
  • the second pickling is performed on the cold-rolled steel sheet according to any one of [1] to [4] above, wherein the temperature of the acid solution is 20 to 70 ° C. and the pickling time is 1 to 30 seconds.
  • Production method. [6] The method for producing a cold-rolled steel sheet according to any one of the above [1] to [5], wherein the cold-rolled steel sheet contains 0.5 to 3.0 mass% of Si.
  • a cold-rolled steel sheet produced by the production method according to any one of [1] to [7] above, wherein the Si-containing oxide layer on the surface layer of the steel sheet is removed, and the iron present on the steel sheet surface A cold-rolled steel sheet having a surface coverage of a system oxide of 40% or less.
  • the cold-rolled steel sheet according to [8], wherein the maximum thickness of the iron-based oxide existing on the steel sheet surface is 150 nm or less.
  • a cold-rolled steel sheet having excellent chemical conversion properties and excellent corrosion resistance after coating can be obtained.
  • a cold-rolled steel sheet having good chemical conversion property and good corrosion resistance after coating can be easily obtained through ordinary cold rolling and pickling processes only by adjusting the pickling conditions. And it can manufacture stably.
  • a cold-rolled steel sheet having excellent post-coating corrosion resistance even in a corrosive environment can be provided. Therefore, according to the present invention, it becomes possible to greatly improve the chemical conversion treatment property and post-coating corrosion resistance of a high-strength cold-rolled steel sheet containing a large amount of Si and having a tensile strength TS of 590 MPa or more.
  • a cold-rolled steel sheet can be suitably used for a strength member of an automobile body.
  • FIG. 1 shows a cold rolled steel sheet standard sample No. 1 for determining the surface coverage of the iron-based oxide.
  • the reflected-electron image of the steel plate surface of a and b is shown.
  • 2 shows a cold rolled steel sheet standard sample No.
  • Fig. 5 shows a histogram of the number of pixels against the gray values of the reflected electron image photographs of a and b.
  • FIG. 3 is a photograph of a cross section of the steel sheet surface covering after pickling with a non-oxidizing acid, observed with a transmission electron microscope.
  • FIG. 4 is a graph showing the result of energy dispersive X-ray (EDX) analysis of the iron-based oxide observed in FIG.
  • FIG. 5 is a graph obtained by measuring the depth direction distribution of O, Si, Mn, and Fe on the surface of the test piece in Table 4-1, by GDS.
  • EDX energy dispersive X-ray
  • a non-oxidizing or reducing gas is usually used as an atmospheric gas.
  • the dew point is strictly controlled. Therefore, the oxidation of the steel plate surface is suppressed in a general cold-rolled steel plate with a small alloy addition amount.
  • steel sheets containing 0.5% or more of Si and Mn, Si and Mn, etc. which are more easily oxidized than Fe, even if the atmosphere gas components and dew point during annealing are strictly controlled.
  • Si-containing oxides such as Si oxide (SiO 2 ) and Si—Mn based complex oxide on the steel sheet surface.
  • the structure of these oxides also changes depending on the steel plate components and the annealing atmosphere.
  • the steel plate components and the annealing atmosphere are often mixed and changed.
  • the Si-containing oxide is formed not only on the steel sheet surface but also inside the base iron, it inhibits the etching property of the steel sheet surface in the chemical conversion treatment (zinc phosphate treatment) that is performed as a base treatment for electrodeposition coating.
  • the chemical conversion treatment zinc phosphate treatment
  • the cold-rolled steel sheet surface after continuous annealing is strongly pickled using nitric acid or the like as the pickling solution, and the Si-containing oxide layer on the steel sheet surface layer formed by continuous annealing after cold rolling is removed.
  • the Si-containing oxide is a SiO 2 or Si—Mn based composite oxide formed along the grain boundaries inside the steel sheet surface or inside the steel sheet during annealing after slab heating, hot rolling, or cold rolling. It refers to things.
  • the thickness of the layer in which these Si containing oxide exists changes with steel plate components and annealing conditions (temperature, time, atmosphere), it is about 1 micrometer from the steel plate surface normally.
  • the removal of the Si-containing oxide layer in the present invention means that pickling is performed to a level at which Si and O peaks do not appear when the steel sheet surface is analyzed in the depth direction by GDS (glow discharge emission spectroscopy). Removing the Si-containing oxide layer.
  • the Fe ion concentration in the present invention means that the divalent iron ions dissolved in nitric acid containing strong oxidizing power change relatively quickly to trivalent iron ions by nitric acid. The sum of the values is the iron ion concentration.
  • strong pickling is performed as the first pickling to suppress the formation of iron-based oxides on the steel sheet surface and to remove the Si-containing oxide layer present on the steel sheet surface.
  • pickling is performed using a non-oxidizing acid as the second pickling so that the surface coverage of the iron-based oxide existing on the steel sheet surface is 40% or less.
  • the inventors set the coverage of the iron-based oxide formed on the steel plate surface by pickling to 40% or less, and when the maximum thickness of the iron-based oxide is 150 nm or less, It was found that the processability was further improved and the corrosion resistance was further improved.
  • pickling conditions concentration, temperature, time
  • non-oxidizing pickling conditions acid concentration, temperature, time
  • the iron-based oxide in the present invention refers to an iron-based oxide having an atomic concentration ratio of iron of 30% or more among elements other than oxygen constituting the oxide.
  • This iron-based oxide is present in a non-uniform thickness on the surface of the steel sheet, and is an oxide different from a natural oxide film that is uniform and layered with a thickness of several nm.
  • the iron-based oxides formed on the surface of this cold-rolled steel sheet are found to be amorphous from observations with a transmission electron microscope (TEM) and analysis of diffraction patterns (diffraction patterns) by electron beam diffraction. ing.
  • TEM transmission electron microscope
  • a steel material (slab) containing 0.5 to 3.0% of Si is heated, hot-rolled, cold-rolled, and continuously annealed, and the following a) or b) The first pickling is performed with any one of the acid solutions, and then the second pickling is performed with an acid solution made of a non-oxidizing acid.
  • R2 hydrofluoric acid concentration ratio
  • Si-containing oxides such as SiO 2 and Si—Mn complex oxides are formed on the steel sheet surface layer after the first annealing under the pickling conditions, and the chemical treatment and post-coating corrosion resistance are remarkable. descend. Therefore, in the production method of the present invention, as the first pickling, the cold-rolled steel sheet after annealing is strongly pickled using an acid solution containing nitric acid and hydrochloric acid or an acid solution containing nitric acid and hydrofluoric acid, The Si-containing oxide layer is removed together with the base iron.
  • Si-containing oxides Si—Mn-based composite oxides are easily dissolved in acid, but SiO 2 is hardly soluble in acid.
  • nitric acid which is a strong oxidizing acid
  • an acid solution containing nitric acid and hydrochloric acid or an acid solution containing nitric acid and hydrofluoric acid is used.
  • Fe dissolved from the steel sheet surface by pickling may produce iron-based oxides, which may precipitate on the steel sheet surface and cover the steel sheet surface, thereby reducing chemical conversion properties.
  • the nitric acid concentration is in the range of 50 g / L to 200 g / L, and further, hydrochloric acid having an oxide film destruction effect is added. It contains nitric acid and hydrochloric acid so that the ratio of hydrochloric acid concentration to nitric acid concentration R1 (hydrochloric acid / nitric acid) is in the range of 0.01 to 0.25, and the Fe ion concentration (sum of divalent and trivalent) is 3 to 50 g. The range is / L.
  • the nitric acid concentration is set in the range of 50 g / L to 200 g / L, and the hydrofluoric acid having an effect of destroying the oxide film is converted into a ratio R2 ( Nitric acid and hydrofluoric acid are contained so that (hydrofluoric acid / nitric acid) is in the range of 0.01 to 0.25, and the Fe ion concentration (sum of divalent and trivalent) is in the range of 3 to 50 g / L.
  • the nitric acid concentration is 50 g / L or less, pickling takes a long time, so the equipment length becomes long and the equipment cost increases.
  • the nitric acid concentration exceeds 200 g / L
  • the eluted Fe is oxidized to produce an iron-based oxide and deposited on the surface of the steel sheet, which adversely affects chemical conversion treatment properties and post-coating corrosion resistance.
  • R1 and R2 are greater than 0.25 or the Fe ion concentration (the sum of divalent and trivalent) is less than 3 g / L, the desired pickling speed cannot be obtained and the Si-containing oxide is efficiently removed. I can't.
  • the maximum thickness of the iron-based oxide can be made 150 nm or less.
  • the maximum thickness of the iron-based oxide becomes 150 nm or less, and the chemical conversion treatment property is further improved. Corrosion resistance is also improved.
  • Second pickling conditions It is difficult to stably control the surface coverage of the iron-based oxide formed on the steel sheet surface to 40% or less simply by performing strong pickling as the first pickling. So, in this invention, in order to reduce more reliably the iron-type oxide produced
  • hydrochloric acid As the non-oxidizing acid, one or more of hydrochloric acid, sulfuric acid, phosphoric acid, pyrophosphoric acid, formic acid, acetic acid, citric acid, hydrofluoric acid, and oxalic acid are used. Any of them may be used, but any hydrochloric acid or sulfuric acid generally used in the steel industry can be preferably used.
  • hydrochloric acid is a volatile acid, so that it is difficult for residues such as sulfate radicals to remain on the surface of the steel sheet after washing like sulfuric acid, and because the oxide destruction effect by chloride ions is large, It can be used suitably.
  • the concentration is 0.1 to 50 g / L hydrochloric acid, 0.1 to 150 g / L sulfuric acid, 0.1 It is preferable to use any acid obtained by mixing ⁇ 20 g / L hydrochloric acid and 0.1 to 60 g / L sulfuric acid.
  • the temperature of the acid solution is 20 to 70 ° C. and the pickling time is 1 to 30 seconds. If the temperature of the pickling solution is 20 ° C. or higher and the treatment time is 1 second or longer, it is sufficient to remove the iron-based oxide remaining on the steel sheet surface. On the other hand, when the temperature of the pickling solution is 70 ° C. or less and the treatment time is 30 seconds or less, the surface of the steel sheet is not excessively dissolved and a new surface oxide film is not generated.
  • the concentration of the acid solution made of an oxidizing acid is increased appropriately.
  • the hydrochloric acid concentration is preferably 3 to 50 g / L
  • sulfuric acid concentration is preferably 8 to 150 g / L.
  • a pickling solution in which hydrochloric acid and sulfuric acid are mixed it is preferable to use an acid in which hydrochloric acid having a concentration of 3 to 20 g / L and sulfuric acid having a concentration of 8 to 60 g / L are mixed. If it is the said density
  • the first pickling and the second pickling are performed, and then a product plate (cold rolled steel plate) is obtained through a normal processing step such as temper rolling.
  • the pickling method that is, the contact method with the acid solution described in the present invention is not particularly limited.
  • a method of spraying an acid solution or a method of immersing in an acid solution can be considered.
  • the first pickling and the second pickling are preferable to perform continuously. By performing continuously, it can prevent that a steel plate naturally oxidizes after the first pickling, and can be made into a final product at a time, and can be manufactured at low cost.
  • a component composition that has high strength that can be used for automobile underbody members and the like, and that also has good chemical conversion treatment properties.
  • Si is an effective element for achieving high strength of steel because it has a large effect on enhancing the strength of steel without significantly reducing the workability (solid solution strengthening ability). It is also an element that adversely affects For the above reasons, addition of 0.5% or more is preferable. On the other hand, if the Si content exceeds 3.0%, the hot rollability and the cold rollability are greatly lowered, which may adversely affect the productivity and may cause the ductility of the steel sheet itself to be lowered. . Therefore, when added, Si is preferably in the range of 0.5 to 3.0%. More preferably, it is in the range of 0.8 to 2.5%.
  • Components other than the above can be allowed as long as they are in the composition range of a normal cold-rolled steel sheet.
  • a suitable component composition other than the above as follows. .
  • C 0.01 to 0.30%
  • C is an element effective for increasing the strength of steel, and is also an element effective for generating retained austenite, bainite and martensite having a TRIP (Transformation Induced Plasticity) effect. is there. If C is 0.01% or more, the above-described effect can be obtained. On the other hand, if C is 0.30% or less, the weldability does not deteriorate. Therefore, C is preferably added in the range of 0.01 to 0.30%, and more preferably in the range of 0.10 to 0.20%.
  • Mn 1.0 to 7.5%
  • Mn is an element having an effect of enhancing the hardenability by solid solution strengthening of steel, enhancing hardenability, and promoting the formation of retained austenite, bainite, and martensite. Such an effect is manifested when 1.0% or more is added. On the other hand, if Mn is 7.5% or less, the above effect can be obtained without causing an increase in cost. Therefore, Mn is preferably added in the range of 1.0 to 7.5%, more preferably in the range of 2.0 to 5.0%.
  • P 0.05% or less
  • P is an element that does not impair the drawability for a large solid solution strengthening ability, and is an element effective for achieving high strength. Therefore, P should be contained in an amount of 0.005% or more. Is preferred. However, although P is an element which impairs spot weldability, if it is 0.05% or less, a problem will not arise. Therefore, P is preferably 0.05% or less, and more preferably 0.02% or less.
  • S 0.01% or less S is an impurity element inevitably mixed in, and is a harmful component that precipitates as MnS in the steel and lowers the stretch flangeability of the steel sheet.
  • S is preferably 0.01% or less. More preferably, it is 0.005% or less.
  • Al 0.01 to 0.06%
  • Al is an element added as a deoxidizer in the steelmaking process, and is an element effective for separating nonmetallic inclusions that lower stretch flangeability as slag, so it is contained in an amount of 0.01% or more. Is preferred. If Al is 0.06% or less, the above effects can be obtained without increasing the raw material cost. Therefore, Al is preferably 0.01 to 0.06%. More preferably, it is in the range of 0.02 to 0.06%.
  • the balance other than the above components is Fe and inevitable impurities.
  • addition of other components is not rejected as long as the effects of the present invention are not impaired.
  • the cold-rolled steel sheet of the present invention has a steel sheet surface from which a Si-containing oxide layer such as SiO 2 or Si—Mn-based composite oxide formed on the steel sheet surface layer during annealing is removed.
  • a Si-containing oxide layer such as SiO 2 or Si—Mn-based composite oxide formed on the steel sheet surface layer during annealing is removed.
  • the cold-rolled steel sheet according to the present invention needs to reduce the surface coverage of the iron-based oxide existing on the steel sheet surface and reduce it to 40% or less. . If it exceeds 40%, the dissolution reaction of iron in the chemical conversion treatment is inhibited, and the growth of chemical crystals such as zinc phosphate is suppressed.
  • a coating of 40% or less is required in the case of using a chemical conversion treatment liquid whose temperature has been lowered. The rate is not sufficient and needs to be reduced to 35% or less, which is even lower. Preferably it is 35% or less.
  • the surface coverage of the iron-based oxide is determined as follows. Using a scanning electron microscope (ULV-SEM) with ultra-low acceleration voltage that can detect extremely surface layer information, the steel plate surface after pickling is observed at an acceleration voltage of 2 kV, an operating distance of 3.0 mm, and a magnification of about 1000 times to observe about 5 fields Then, spectral analysis is performed using an energy dispersive X-ray spectrometer (EDX) to obtain a reflected electron image. This reflected electron image is binarized using image analysis software, for example, Image J, the area ratio of the black portion is measured, and the surface coverage of the iron-based oxide is obtained by averaging the measured values of each field of view. Can be obtained.
  • image analysis software for example, Image J
  • Image J the area ratio of the black portion is measured
  • the surface coverage of the iron-based oxide is obtained by averaging the measured values of each field of view. Can be obtained.
  • Examples of the ultra-low acceleration voltage scanning electron microscope include, for example, ULTRA55 manufactured by SEISS.
  • Examples of the energy dispersive X-ray spectrometer (EDX) include NSS312E manufactured by Thermo Fisher.
  • the steel slab shown in Table 1 is hot-rolled under the conditions shown in Table 2, cold-rolled, continuously annealed to obtain a cold-rolled steel sheet having a thickness of 1.8 mm, and then cold-rolled after the above-described continuous annealing.
  • the steel sheets were pickled, washed with water and dried under the conditions shown in Table 3, and then subjected to temper rolling with an elongation of 0.7%, resulting in different numbers of iron-based oxides on the steel sheet surface.
  • Two types of cold-rolled steel sheets a and b were obtained. Then, the above No.
  • the cold rolled steel sheet a is a standard sample with a lot of iron-based oxides, No.
  • the cold rolled steel sheet b was used as a standard sample with a small amount of iron-based oxides, and a reflected electron image was obtained for each steel sheet using the scanning electron microscope under the conditions described above.
  • FIG. The reflection electron image photograph of the steel plates a and b is shown in FIG.
  • the histogram of the pixel number with respect to the gray value of the said reflection electron image photograph of the steel plate of a and b is shown.
  • the gray value (Y point) corresponding to the intersection (X point) of the histograms a and b was determined as the threshold value.
  • No. When the surface coverage of the iron-based oxide of the steel plates a and b was determined, No.
  • the steel sheet of a is 85.3%
  • the steel sheet for b was 25.8%.
  • the surface coverage of the iron-based oxide on the steel sheet surface after the second pickling is 40% or less, and further, an iron-based oxide.
  • the maximum thickness is preferably 150 nm or less. If the maximum thickness of the iron-based oxide is 150 nm or less, the dissolution reaction of iron in the chemical conversion treatment is not locally inhibited, and precipitation of chemical crystals such as zinc phosphate is not locally suppressed. is there. More preferably, it is 130 nm or less.
  • the maximum thickness of the iron-based oxide is obtained as follows.
  • FIG. 3 shows a photograph of a cross-section of the coating layer generated by the first pickling existing on the steel plate surface
  • FIG. 4 shows the EDX analysis result of the coating layer. Show. Since FIG.
  • the coating layer is an iron-based oxide
  • the line A indicating the steel plate steel shown in the cross-sectional photograph of FIG. 3
  • the line B indicating the thickest part of the iron-based oxide layer are shown.
  • the interval is measured for all 10 replicas, and the maximum thickness among them is taken as the maximum thickness of the iron-based oxide.
  • the size and number of replicas, the measurement conditions by TEM, and the like are merely examples, and it is needless to say that the replicas may be changed as appropriate.
  • the cold-rolled steel sheet obtained by the above is excellent in chemical conversion treatment property and excellent in post-painting corrosion resistance evaluated by a salt warm water immersion test or a combined cycle corrosion test, it can be suitably used as an automobile member.
  • these hot-rolled steel sheets were pickled, scales were removed, and then cold-rolled to obtain cold-rolled steel sheets having a thickness of 1.8 mm.
  • these cold-rolled steel sheets are heated to a soaking temperature of 750 to 780 ° C., held for 40 to 50 seconds, and then from a soaking temperature to a cooling stop temperature of 350 to 400 ° C. at 20 to 30 ° C./sec.
  • the steel sheet surface was pickled, washed with water and dried under the conditions shown in Tables 4-1 and 4-2, and then the elongation was increased.
  • the temper rolling of 0.7% was performed, and No. shown in Table 4-1 and Table 4-2. 2-80 cold-rolled steel sheets were obtained.
  • Specimens were taken from each of the above-mentioned cold-rolled steel sheets, and the surface of the steel sheet was accelerated at an acceleration voltage of 2 kV and a working distance of 3.0 mm using an ultra-low acceleration voltage scanning electron microscope (ULV-SEM; manufactured by SEISS; ULTRA55). Then, 5 fields of view were observed at a magnification of 1000 times, and a reflection electron image was obtained by spectroscopic analysis using an energy dispersive X-ray spectrometer (EDX; manufactured by Thermo Fisher Co., Ltd .; NSS312E). This reflected electron image was obtained by using the image analysis software (Image J) and the standard sample No. described above.
  • EDX energy dispersive X-ray spectrometer
  • the gray value (Y point) corresponding to the intersection (X point) of the histograms a and b is set as a threshold value, binarization processing is performed, the area ratio of the black part is measured, the average value of the five visual fields is obtained, and iron The surface coverage of the system oxide was used.
  • specimens are collected from each of the above cold-rolled steel sheets, subjected to chemical conversion treatment and coating treatment under the following conditions, and then subjected to three types of corrosion tests: a salt warm water immersion test, a salt spray test, and a combined cycle corrosion test.
  • the corrosion resistance after coating was evaluated.
  • collected from each cold-rolled steel plate was measured using GDS.
  • Chemical conversion treatment conditions The degreasing agent: FC-E2011, the surface conditioner: PL-X, and the chemical conversion treatment agent: Palbond PB-L3065 manufactured by Nihon Parkerizing Co., Ltd. were used for the test pieces collected from each of the cold-rolled steel sheets.
  • the steel sheets of the inventive examples that were pickled under conditions suitable for the present invention after continuous annealing showed maximum peeling in any of the salt warm water immersion test, salt spray test, and combined cycle corrosion test. It can be seen that the overall width is small, indicating good chemical conversion and post-coating corrosion resistance. In particular, it can be seen that all of the cold-rolled steel sheets having an iron-based oxide surface coverage of 40% or less are excellent in post-painting corrosion resistance in a harsh corrosive environment.
  • the steel sheets pickled under conditions suitable for the present invention include: It was confirmed that Si and O peaks did not appear and the Si-containing oxide layer was sufficiently removed.
  • FIG. 5 shows the profile in the depth direction of O, Si, Mn, and Fe when the surface analysis is performed by using GDS for 9 test pieces.
  • Steels A to O having the composition shown in Table 5 were melted by a normal scouring process through a converter, degassing treatment, etc., and continuously cast into a steel slab. These steel slabs are hot-rolled under the hot rolling conditions shown in Table 6 to form hot-rolled steel sheets with a thickness of 3 to 4 mm, pickled to remove the scale on the steel sheet surface, and then cold-rolled to obtain the plate thickness. A 1.8 mm cold-rolled steel sheet was used. Subsequently, these cold-rolled steel sheets were continuously annealed and pickled under the conditions shown in Tables 6 and 7, then washed with water, dried, subjected to temper rolling with an elongation of 0.7%, and 81 to 111 cold-rolled steel sheets were obtained.
  • a specimen was collected from each of the cold-rolled steel sheets thus obtained, and after measuring the surface coverage of the iron-based oxide on the steel sheet surface after pickling in the same manner as in Example 1, the following tensile test was performed. And subjected to a corrosion resistance test after painting. Moreover, the depth direction distribution of O, Si, Mn, and Fe on the surface of the test piece extract
  • (1) Mechanical properties Using a JIS No. 5 tensile specimen (n 1) specified in JIS Z2201: 1998 taken from the direction perpendicular to the rolling direction (C direction), tension is applied in accordance with the provisions of JIS Z2241: 1998. A test was performed and the tensile strength TS was measured.
  • Example 2 Corrosion resistance after coating A test piece sampled from each cold-rolled steel sheet was subjected to chemical conversion treatment under the same conditions as in Example 1 to produce a test piece subjected to electrodeposition coating.
  • the corrosion resistance after coating was evaluated by three types of corrosion tests, a hot water immersion test, a salt spray test (SST), and a combined cycle corrosion test (CCT).
  • the present invention has a high surface coverage of 40% or less with iron-based oxides on the steel sheet surface containing 0.5% or more of Si and pickling under conditions suitable for the present invention. It can be seen that the high-strength cold-rolled steel sheet not only has excellent chemical conversion properties and post-coating corrosion resistance, but also has a high strength with a tensile strength TS of 590 MPa or more. In addition, as a result of measuring the depth direction distribution of O, Si, Mn and Fe by GDS, none of the steel plates pickled under conditions suitable for the present invention show Si or O peaks, and Si-containing oxidation It was confirmed that the material layer was sufficiently removed.
  • these cold-rolled steel sheets are heated to a soaking temperature of 750 to 780 ° C. and held for 40 to 50 seconds, and then, from the soaking temperature to a cooling stop temperature of 350 to 400 ° C., 20 to 30 ° C./sec.
  • the steel sheet was subjected to continuous annealing for 100 to 120 seconds within the cooling stop temperature range, and then the steel sheet surface was pickled, washed with water and dried under the conditions shown in Table 8, and the elongation ratio was 0.7%.
  • No. 1 shown in Table 8 was obtained.
  • Cold rolled steel sheets 112 to 149 were obtained. Test pieces were collected from each of the cold-rolled steel sheets, and the surface coverage and the maximum thickness of the iron-based oxide produced on the steel sheet surface by pickling were measured using the above-described method.
  • the steel plate surface after annealing was acidified under the condition that the surface coverage of the iron-based oxide on the steel plate surface after re-acid washing was 40% or less and the maximum thickness of the iron-based oxide was 150 nm or less.
  • the washed steel plate of the present invention has a long maximum test width compared to Example 1 and is extremely good in all of the salt warm water immersion test, salt spray test and combined cycle corrosion test conducted under severe conditions. It can be seen that it shows excellent corrosion resistance after painting.
  • none of the steel plates pickled under conditions suitable for the present invention show Si or O peaks, and Si-containing oxidation It was confirmed that the material layer was sufficiently removed.
  • the cold-rolled steel sheet produced according to the present invention is not only excellent in chemical conversion treatment and post-coating corrosion resistance, but can also have high strength, and is similar in fields such as home appliances and architecture in addition to materials for automobile parts. It can also be suitably used as a material for members that require characteristics.

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