WO2012029631A1 - 冷延鋼板の製造方法、冷延鋼板および自動車部材 - Google Patents
冷延鋼板の製造方法、冷延鋼板および自動車部材 Download PDFInfo
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- WO2012029631A1 WO2012029631A1 PCT/JP2011/069192 JP2011069192W WO2012029631A1 WO 2012029631 A1 WO2012029631 A1 WO 2012029631A1 JP 2011069192 W JP2011069192 W JP 2011069192W WO 2012029631 A1 WO2012029631 A1 WO 2012029631A1
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Images
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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
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- C23G1/085—Iron or steel solutions containing HNO3
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- 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
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- C—CHEMISTRY; METALLURGY
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- 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
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- 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
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- C—CHEMISTRY; METALLURGY
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- 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
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- 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
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
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- C22C1/02—Making non-ferrous alloys by melting
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- C22C—ALLOYS
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- C—CHEMISTRY; METALLURGY
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- C22C—ALLOYS
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- C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
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- C22C—ALLOYS
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- C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
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- C—CHEMISTRY; METALLURGY
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- C22C—ALLOYS
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- C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
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- C—CHEMISTRY; METALLURGY
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- 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
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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
- C22C38/14—Ferrous alloys, e.g. steel alloys containing titanium or zirconium
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- C—CHEMISTRY; METALLURGY
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- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/16—Ferrous alloys, e.g. steel alloys containing copper
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- C—CHEMISTRY; METALLURGY
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- 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
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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
- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/32—Ferrous alloys, e.g. steel alloys containing chromium with boron
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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
- C22C38/18—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
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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
- C22C38/18—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
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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
- C22C38/18—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
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- C—CHEMISTRY; METALLURGY
- C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
- C23G—CLEANING OR DE-GREASING OF METALLIC MATERIAL BY CHEMICAL METHODS OTHER THAN ELECTROLYSIS
- C23G1/00—Cleaning or pickling metallic material with solutions or molten salts
-
- C—CHEMISTRY; METALLURGY
- 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
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- 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
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- 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
- the present invention relates to a method for producing a cold-rolled steel sheet, a cold-rolled steel sheet, and an automobile member. Specifically, it has excellent chemical conversion treatment properties and excellent post-coating corrosion resistance evaluated by a salt warm water immersion test or a combined cycle corrosion test.
- the present invention relates to a method for producing a cold rolled steel sheet, a cold rolled steel sheet produced by the method, and an automobile member using the cold rolled steel sheet.
- the cold-rolled steel sheet of the present invention can be suitably used for a high-strength cold-rolled steel sheet having a Si-containing tensile strength TS of 590 MPa or more.
- Patent Document 1 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 discloses that the Si-containing oxide concentrated on the surface of the steel sheet in the annealing process or the like is removed by pickling and further an S-based compound is added to the surface.
- Patent Document 4 discloses a technique for providing a P-based compound instead of an S-based compound in the above technique.
- JP 2004-204350 A JP 2004-244698 A JP 2007-217743 A JP 2007-246951 A
- Patent Documents 3 and 4 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.
- the present invention has been made in view of the above-mentioned problems of cold-rolled steel sheets containing a large amount of Si, and its purpose is to provide chemical conversion treatment even when a low-temperature chemical conversion treatment liquid is used.
- the inventors conducted detailed analysis on the steel sheet surface characteristics after annealing, and conducted intensive 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 produced 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 includes a steel sheet containing 0.5 to 3.0 mass% of Si, continuously annealed after cold rolling, and the ratio of the hydrochloric acid concentration to the nitric acid concentration when the nitric acid concentration exceeds 100 g / L and is 200 g / L or less.
- a method for producing a cold-rolled steel sheet characterized by pickling using an acid mixture of nitric acid and hydrochloric acid having R (HCl / HNO 3 ) of 0.01 to 0.25, is proposed.
- the acid obtained by mixing nitric acid and hydrochloric acid in the production method of the present invention has a nitric acid concentration of 110 g / L and 140 g / L or less, and a ratio R (HCl / HNO 3 ) of the hydrochloric acid concentration to the nitric acid concentration is 0.03 to 0.00. 25.
- the production method of the present invention is characterized in that the pickling is performed for 3 to 30 seconds at a temperature of the pickling solution of 20 to 70 ° C.
- the steel sheet in the production method of the present invention includes C: 0.01 to 0.30 mass%, Mn: 1.0 to 7.5 mass%, P: 0.05 mass% or less, and S: 0. .01 mass% or less and Al: 0.06 mass% or less, with the balance being a component composition of Fe and inevitable impurities.
- the cold-rolled steel sheet in the production method of the present invention further includes Nb: 0.3 mass% or less, Ti: 0.3 mass% or less, V: 0.3 mass% or less, Mo: 0.00. 1 type or 2 or more types chosen from 3 mass% or less, Cr: 0.5 mass% or less, B: 0.006 mass% or less, and N: 0.008 mass% or less are characterized by the above-mentioned.
- the cold-rolled steel sheet in the production method of the present invention further includes Ni: 2.0 mass% or less, Cu: 2.0 mass% or less, Ca: 0.1 mass% or less, and REM: 0.00. 1 type or 2 or more types chosen from 1 mass% or less are contained, It is characterized by the above-mentioned.
- the present invention is a cold-rolled steel sheet produced by any of the methods described above, wherein the Si-containing oxide layer on the steel sheet surface layer is removed by pickling after continuous annealing, and the acid A cold-rolled steel sheet characterized in that the surface coverage of the iron-based oxide on the surface of the steel sheet produced by washing is 85% or less.
- the cold-rolled steel sheet according to the present invention is characterized in that the maximum thickness of the iron-based oxide existing on the steel sheet surface is 200 nm or less.
- the present invention is an automobile member characterized by using any one of the cold-rolled steel sheets described above.
- the present invention even when Si is contained in a large amount of 0.5 to 3.0 mass%, even when a low-temperature chemical conversion treatment liquid is used, it is excellent in chemical conversion treatment, and the salt warm water A cold-rolled steel sheet having excellent post-coating corrosion resistance can be provided even in a severe corrosive environment such as an immersion test or a combined cycle corrosion test. Therefore, according to the present invention, it becomes possible to greatly improve the chemical conversion property and the corrosion resistance after painting 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. It can be suitably used for strength members and the like.
- 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 coating after pickling, 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 of Comparative Example (No. 1) and Example (No. 18) of Example 1 by GDS.
- EDX energy dispersive X-ray
- 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 varies depending on the steel plate components and the annealing atmosphere, but generally both are often mixed.
- 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 as a pickling solution, and the Si-containing oxide layer on the steel sheet surface layer formed by continuous annealing after cold rolling can be 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.
- the thickness of the layer in which these Si-containing oxides are present varies depending on the steel plate components and annealing conditions (temperature, time, atmosphere), but is usually about 1 ⁇ m from the steel plate surface.
- 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.
- nitric acid as the pickling solution is that, among Si-containing oxides, Si—Mn composite oxide is easily dissolved in acid, but SiO 2 exhibits poor solubility, so that it can be removed. This is because it is necessary to remove the Si-containing oxide on the steel sheet surface together with the base iron using nitric acid which is a strong oxidizing acid.
- the chemical conversion processability is significantly improved by removing the Si-containing oxide layer existing on the steel sheet surface layer by performing strong acid washing with nitric acid after continuous annealing.
- the chemical conversion processability was inferior.
- the Si-based oxide layer was removed by the strong pickling with nitric acid, but separately, Fe dissolved from the steel plate surface by pickling produced iron-based oxide, which was the steel plate surface. It was newly discovered that the chemical conversion processability deteriorates by precipitating and covering the steel sheet surface.
- the nitric acid concentration is controlled within an appropriate range to suppress oxidation by nitric acid, and hydrochloric acid having an oxide film destruction effect is added to a predetermined level. It has been found that it is important to pickle acid using a mixture of nitric acid and hydrochloric acid mixed in a ratio for the pickling solution.
- the inventors set the coverage of the iron-based oxide generated on the steel plate surface by pickling to 85% or less, and when the maximum thickness of the iron-based oxide is further set to 200 nm or less, The chemical conversion processability is further improved, and the corrosion resistance is further improved.
- pickling is performed by controlling the concentration of hydrochloric acid, which is used for part of the pickling solution, to destroy the oxide film within an appropriate range. Found that it was effective.
- the iron-based oxide in the present invention means 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
- diffraction patterns diffraction patterns
- Si 0.5 to 3.0 mass%
- Si is an effective element for achieving high strength of steel because it has a great effect on enhancing the strength of steel without significantly reducing workability (solid solution strengthening ability). It is also an element that adversely affects When Si is added as a means for achieving high strength, it is necessary to add 0.5 mass% or more. Moreover, if Si is less than 0.5 mass%, there is little influence by the deterioration of chemical conversion treatment conditions. On the other hand, when the Si content exceeds 3.0 mass%, the hot rollability and the cold rollability are greatly lowered, which adversely affects the productivity and causes the ductility of the steel sheet itself to be lowered. Therefore, Si is added in the range of 0.5 to 3.0 mass%. Preferably, it is in the range of 0.8 to 2.5 mass%.
- the cold-rolled steel sheet of the present invention must contain Si in the above range, but the other components can be allowed as long as they are in the composition range of a normal cold-rolled steel sheet, and are particularly limited. Is not to be done. However, when the cold-rolled steel sheet of the present invention is applied to a high-strength cold-rolled steel sheet having a tensile strength TS of 590 MPa or more used for an automobile body or the like, it preferably has the following component composition.
- C 0.01-0.30 mass%
- 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 mass% or more, the above effect can be obtained. On the other hand, if C is 0.30 mass% or less, the weldability does not deteriorate. Therefore, C is preferably added in the range of 0.01 to 0.30 mass%, and more preferably in the range of 0.10 to 0.20 mass%.
- Mn 1.0 to 7.5 mass%
- 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 by addition of 1.0 mass% or more.
- Mn is preferably added in the range of 1.0 to 7.5 mass%, and more preferably in the range of 2.0 to 5.0 mass%.
- P 0.05 mass% 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 mass% or more. Is preferred. However, although P is an element which impairs spot weldability, if it is 0.05 mass% or less, a problem will not arise. Therefore, P is preferably 0.05 mass% or less, and more preferably 0.02 mass% or less.
- S 0.01 mass% or less
- S is an impurity element that is inevitably mixed in, and is a harmful component that precipitates as MnS in steel and lowers the stretch flangeability of the steel sheet.
- S is preferably 0.01 mass% or less. More preferably, it is 0.005 mass% or less, More preferably, it is 0.003 mass% or less.
- Al 0.06 mass% or less
- Al is an element added as a deoxidizer in the steelmaking process, and is an element effective for separating non-metallic inclusions that reduce stretch flangeability as slag. It is preferable to contain 0.01 mass% or more. If Al is 0.06 mass% or less, the above effects can be obtained without increasing the raw material cost. Therefore, Al is preferably 0.06 mass% or less. More preferably, it is in the range of 0.02 to 0.06 mass%.
- the cold-rolled steel sheet of the present invention further includes Nb: 0.3 mass% or less, Ti: 0.3 mass% or less, V: 0.3 mass% or less, Mo: 0.3 mass% or less, One or more selected from Cr: 0.5 mass% or less, B: 0.006 mass% or less, and N: 0.008 mass% or less can be contained.
- Nb, Ti and V are elements that form carbides and nitrides, suppress the growth of ferrite in the heating stage during annealing, refine the structure, and improve formability, particularly stretch flangeability.
- Mo, Cr and B are elements that improve the hardenability of the steel and promote the formation of bainite and martensite, and therefore can be added in the above range.
- N is an element that forms nitrides with Nb, Ti, and V or contributes to increasing the strength of the steel by forming a solid solution in the steel. If it is 0.008 mass% or less, a large amount of nitride is present. Since it is not formed, breakage due to void formation during press molding is suppressed, and the above effect can be obtained.
- the cold-rolled steel sheet of the present invention further includes Ni: 2.0 mass% or less, Cu: 2.0 mass% or less, Ca: 0.1 mass% or less, and REM: 0.1 mass% or less.
- Ni and Cu are effective in promoting the formation of a low temperature transformation phase and increasing the strength of the steel, and therefore can be added in the above range.
- Ca and REM are elements that control the form of sulfide inclusions and improve the stretch flangeability of the steel sheet, and therefore can be added in the above range.
- the balance other than the above components is Fe and inevitable impurities. However, 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 composite oxide formed on the steel sheet surface layer during annealing is removed. is necessary. For that purpose, it must be pickled strongly using a pickling solution in which nitric acid and hydrochloric acid are mixed, and the Si-containing oxide formed on the steel plate surface and the grain boundary portion in the vicinity of the surface is dissolved and removed together with the base iron. is necessary.
- a pickling solution in which nitric acid and hydrochloric acid are mixed
- the cold-rolled steel sheet of the present invention has an area ratio of the steel sheet surface coverage by the iron-based oxide that is generated on the steel sheet surface by the strong pickling with nitric acid in addition to removing the Si-containing oxide layer. It is necessary to reduce it to 85% or less. This is because if the surface coverage of the iron-based oxide exceeds 85%, the dissolution reaction of iron in the chemical conversion treatment is inhibited, and the growth of chemical crystals such as zinc phosphate is suppressed. Preferably it is 80% 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 of view. 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.
- the ultra-low acceleration voltage scanning electron microscope (ULV-SEM) is, for example, manufactured by SEISS; ULTRA 55, and the energy dispersive X-ray spectrometer (EDX) is, for example, manufactured by Thermo Fisher. NSS 312E may be mentioned.
- the steel slabs of steel code G shown in Table 3 of Examples described later are the same as No. 1 of Table 4 of Examples described later. 7 was hot-rolled, cold-rolled and continuously annealed to obtain a cold-rolled steel sheet having a thickness of 1.8 mm.
- Table 1 shows the cold-rolled steel sheet after the continuous annealing. Under the conditions, pickling, re- pickling, water washing, drying, 0.7% temper rolling, and the amount of iron-based oxide on the steel sheet surface is different. 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.
- a gray value (Y point) corresponding to the intersection (X point) of the histograms a and b was determined as a threshold value.
- No. When the surface coverage of the iron-based oxide of the steel sheets a and b was determined, No.
- the steel sheet of a is 85.3%
- No. As for the steel plate of b 25.8% was obtained.
- the cold-rolled steel sheet of the present invention has an iron oxide coverage of 85% or less generated on the steel sheet surface by pickling in order to further improve the chemical conversion properties and corrosion resistance.
- the maximum thickness of the iron-based oxide is preferably 200 nm or less. If the maximum thickness of the iron-based oxide is 200 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 180 nm or less.
- the maximum thickness of the iron-based oxide is determined as follows. First, ten extracted replicas capable of observing a cross section of about 8 ⁇ m in the width direction of the steel sheet are produced from the steel sheet surface after pickling by focused ion beam (FIB) processing. Next, using a transmission electron microscope (TEM) equipped with an energy dispersive X-ray spectrometer (EDX) capable of examining local information of the cross section, the cross section of each replica at an acceleration voltage of 200 kV and a magnification of 100,000 times Take 8 ⁇ m continuously. As an example, FIG. 3 shows a photograph of a cross-section of the coating layer formed by pickling existing on the steel plate surface, and FIG. 4 shows an EDX analysis result of the coating layer.
- TEM transmission electron microscope
- EDX energy dispersive X-ray spectrometer
- the coating layer is an iron-based iron-based oxide
- a line A indicating the steel plate iron shown in the cross-sectional photograph of FIG. 3
- a line B indicating the thickest part of the oxide layer Is measured for 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 method for producing a cold-rolled steel sheet according to the present invention comprises heating a steel material (slab) containing 0.5 to 3.0 mass% of Si, followed by hot rolling, cold rolling, continuous annealing, and then adding nitric acid and By pickling using a pickling solution mixed with hydrochloric acid, the Si-containing oxide layer of the steel sheet surface layer portion is removed, and the surface coverage of the iron-based oxide generated on the steel plate surface by the pickling is It is necessary that the method can be 85% or less, and it is preferable that the maximum thickness of the iron-based oxide be 200 nm or less. Therefore, although it can manufacture in accordance with a conventional method from the steelmaking process to the continuous annealing process after cold rolling, it is preferable that the pickling after continuous annealing is made into the following conditions.
- the Si—Mn composite oxide is easily dissolved in an acid, but SiO 2 is hardly soluble in an acid. Therefore, in order to remove the Si-containing oxide including SiO 2 by pickling, it is necessary to remove the whole iron of the steel sheet using nitric acid, which is a strong acid. And in order to perform the strong pickling which removes an oxide layer together with the above-mentioned base iron, it is necessary to make nitric acid concentration over 100 g / L.
- the nitric acid concentration needs to be suppressed to 200 g / L or less. Therefore, the nitric acid concentration is in the range of more than 100 g / L and not more than 200 g / L. The range is preferably 110 to 150 g / L.
- the oxide film destruction effect In this case, the chloride ions, that is, hydrochloric acid, were pickled using an acid mixed so that the ratio R (HCl / HNO 3 ) of the hydrochloric acid concentration to the nitric acid concentration was in the range of 0.01 to 0.25.
- the ratio R is less than 0.01, the effect of suppressing the formation of the iron-based oxide is small.
- the ratio R exceeds 0.25, the dissolution amount of the steel sheet is reduced, and the Si-containing oxide layer is removed. It is because it becomes impossible.
- the maximum thickness of the iron-based oxide formed on the surface of the steel sheet by pickling is 200 nm or less.
- nitric acid and hydrochloric acid used for the above pickling The pickling solution mixed with the nitric acid concentration is in the range of 110 g / L to 140 g / L or less, and the ratio R (HCl / HNO 3 ) of the hydrochloric acid concentration to the nitric acid concentration is in the range of 0.03 to 0.25. Is preferred. If it is within the above range, the thickness of the iron-based oxide can be stably reduced to 200 nm or less, and the chemical conversion treatment property and the corrosion resistance after coating do not deteriorate.
- the pickling using the pickling solution in which nitric acid and hydrochloric acid are mixed is preferably performed at a temperature of the pickling solution of 20 to 70 ° C. and a pickling time of 3 to 30 seconds. If the temperature of the pickling solution is 20 ° C. or more and the pickling time is 3 seconds or more, the Si-containing oxide layer on the surface layer of the steel sheet formed during annealing can be sufficiently removed, and after chemical conversion treatment and coating Corrosion resistance is not reduced. On the other hand, if the temperature of the pickling solution is 70 ° C.
- the steel sheet surface becomes rough due to excessive pickling, and the chemical conversion treatment film becomes non-uniform, or the surface coating with an iron-based oxide This is because the rate does not increase and the chemical conversion property and the corrosion resistance after coating are not lowered.
- the rolled steel sheet After continuous annealing as described above, it is pickled to cold-rolled steel sheet having a iron oxide coverage of 85% or less on the surface of the steel sheet, or a cold steel sheet having a maximum thickness of 200 nm or less. Thereafter, the rolled steel sheet is made into a product through a normal processing step such as temper rolling.
- these hot-rolled steel plates were pickled, removed the scale, and then cold-rolled to obtain cold-rolled steel plates having a thickness of 1.8 mm.
- 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 above-mentioned cooling stop temperature range, and then the steel sheet surface was pickled, washed with water and dried under the conditions shown in Table 2, and the elongation was 0.7%. No. 1 shown in Table 2 was applied. 1-25 cold-rolled steel sheets were obtained.
- Specimens were collected from each of the above cold-rolled steel sheets, and the surface of the steel sheet was accelerated using an ultra-low acceleration voltage scanning electron microscope (ULV-SEM; manufactured by SEISS; ULTRA55) at an acceleration voltage of 2 kV, a working distance of 3.0 mm, and a magnification.
- UUV-SEM ultra-low acceleration voltage scanning electron microscope
- Five fields of view were observed at 1000 times, and a reflection electron image was obtained by spectroscopic analysis using an energy dispersive X-ray spectrometer (EDX; manufactured by Thermo Fisher, Inc .; NSS312E). This reflected electron image was obtained by using the image analysis software (Image J) and the standard sample No. described above.
- 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 portion is measured, the average value of the five fields of view 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.
- the depth direction distribution of O, Si, Mn, and Fe on the surface of the test piece collected from each cold-rolled steel sheet was measured using GDS.
- Chemical conversion treatment conditions The test pieces collected from each of the above cold-rolled steel sheets were degreased by Nippon Parkerizing Co., Ltd .: FC-E2011, surface conditioner: PL-X, and chemical conversion treatment agent: Palbond PB-L3065.
- Chemical conversion treatment was performed so that the amount of chemical conversion coating film deposited was 1.7 to 3.0 g / m 2 under the following two conditions: standard conditions and comparative conditions in which the temperature of the chemical conversion liquid was lowered to lower the temperature.
- standard conditions standard conditions and comparative conditions in which the temperature of the chemical conversion liquid was lowered to lower the temperature.
- ⁇ Standard conditions> ⁇ Degreasing process: treatment temperature 40 ° C., treatment time 120 seconds ⁇ Spray degreasing, surface adjustment step: pH 9.5, treatment temperature room temperature, treatment time 20 seconds ⁇ Chemical conversion treatment process: temperature of chemical treatment liquid 35 ° C., treatment time 120 seconds ⁇ temperature reduction conditions> Conditions under which the temperature of the chemical conversion treatment liquid was lowered to 33 ° C.
- Corrosion test A film was formed on the surface of the test piece subjected to the chemical conversion treatment using an electrodeposition paint: V-50 made by Nippon Paint Co., Ltd. Electrodeposition coating was applied so that the thickness was 25 ⁇ m, and it was subjected to the following three types of corrosion tests.
- FIG. 5 shows the profile in the depth direction of O, Si, Mn, and Fe when 18 specimens were subjected to surface analysis by GDS.
- Steels A to X having the component composition shown in Table 3 were melted by a normal refining 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 4 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 plate A cold-rolled steel sheet having a thickness of 1.8 mm was used. Next, these cold-rolled steel sheets were continuously annealed under the conditions shown in Table 4 and then pickled under the conditions shown in Table 5, then washed with water, dried, and subjected to temper rolling with an elongation of 0.7%. No. 1-30 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
- the high-strength cold-rolled steel sheet of the example of the present invention containing 0.5 mass% or more of Si and pickled under conditions suitable for the present invention is not only excellent in corrosion resistance after coating, but also has a tensile strength. It can be seen that TS has a high strength of 590 MPa or more.
- the steel plate pickled under conditions suitable for the present invention does not show any Si or O peak, 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 surface was subjected to continuous annealing for 100 to 120 seconds within the above-mentioned cooling stop temperature range, and then the steel sheet surface was pickled, washed with water and dried under the conditions shown in Table 6, and the elongation percentage was 0.7%. No. 1 shown in Table 6 was applied. 1 to 12 cold-rolled steel sheets were obtained.
- Specimens were collected from each of the above cold-rolled steel plates, and the surface coverage and the maximum thickness of the iron-based oxide produced on the steel plate surface by pickling were measured using the method described above.
- Chemical conversion treatment was performed so that the amount of chemical conversion coating film deposited was 1.7 to 3.0 g / m 2 under the following two conditions: standard conditions and comparative conditions in which the temperature of the chemical conversion liquid was lowered to lower the temperature.
- standard conditions standard conditions and comparative conditions in which the temperature of the chemical conversion liquid was lowered to lower the temperature.
- ⁇ Standard conditions> ⁇ Degreasing process: treatment temperature 40 ° C., treatment time 120 seconds ⁇ Spray degreasing, surface adjustment step: pH 9.5, treatment temperature room temperature, treatment time 20 seconds ⁇ Chemical conversion treatment process: temperature of chemical treatment liquid 35 ° C., treatment time 120 seconds ⁇ temperature reduction conditions> Conditions under which the temperature of the chemical conversion treatment liquid was lowered to 33 ° C.
- Corrosion test A film was formed on the surface of the test piece subjected to the chemical conversion treatment using an electrodeposition paint: V-50 made by Nippon Paint Co., Ltd. Electrodeposition coating was applied so that the thickness was 25 ⁇ m, and the samples were subjected to the following three types of corrosion tests under more severe conditions as compared with Example 1.
- ⁇ Salt warm water immersion test> The surface of the above test piece (n 1) subjected to chemical conversion treatment and electrodeposition coating was applied with a 45 mm long crosscut wrinkle with a cutter, and then the test piece was transferred to a 5 mass% NaCl solution (60 ° C.) 360.
- the results of the above test are shown in Table 6. From this result, the surface of the steel sheet after pickling is pickled on the surface of the steel sheet after annealing under the condition that the surface coverage of the iron-based oxide is 85% or less and the maximum thickness of the iron-based oxide is 200 nm or less.
- the steel sheet according to the present invention has a long maximum peel width in all of the salt warm water immersion test, the salt spray test and the combined cycle corrosion test conducted under harsh conditions with a long test time compared to Example 1, and is extremely good. It can be seen that it shows corrosion resistance after painting.
- the steel plate pickled under conditions suitable for the present invention does not show any Si or O peak, 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 not only has excellent post-painting corrosion resistance, but also has high strength and excellent workability, so that it is not only used as a material for automobile body parts, It can also be suitably used as a material for applications that require similar characteristics in the field of building materials and the like.
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Abstract
Description
冷間圧延した冷延鋼板を再結晶させ、所望の組織と強度、加工性を付与するために行われる連続焼鈍炉を用いた焼鈍工程では、通常、雰囲気ガスとして非酸化性または還元性のガスが用いられており、露点も厳格に管理されている。そのため、合金添加量の少ない普通の一般冷延鋼板では、鋼板表面の酸化は抑制されている。しかし、0.5mass%以上のSiや、Mnを含有する鋼板では、焼鈍時の雰囲気ガスの成分や露点を厳格に管理しても、Feと比較して易酸化性であるSiやMn等が酸化して、鋼板表面にSi酸化物(SiO2)やSi-Mn系複合酸化物などのSi含有酸化物を形成することが避けられない。これら酸化物の構成は、鋼板成分や焼鈍雰囲気などによっても変化するが、一般的には、両者が混在していることが多い。そして、上記Si含有酸化物は、鋼板表面だけでなく、地鉄内部にまで形成されるため、電着塗装の下地処理としてなされる化成処理(リン酸亜鉛処理)における鋼板表面のエッチング性を阻害し、健全な化成処理皮膜の形成に悪影響を及ぼすことが知られている。
なお、本発明における鉄系酸化物とは、酸化物を構成する酸素以外の元素のうちで鉄の原子濃度比が30%以上である鉄主体の酸化物のことをいう。この鉄系酸化物は、鋼板表面上に不均一な厚さで存在しており、数nmの厚さで均一かつ層状に存在する自然酸化皮膜とは異なる酸化物である。なお、この冷延鋼板の表面に生成した鉄系酸化物は、透過型電子顕微鏡(TEM)による観察や電子線回折によるディフラクションパターン(回折図形)の解析結果から非晶質であることがわかっている。
本発明は、上記新規な知見に、さらに検討を加えて完成したものである。
Si:0.5~3.0mass%
Siは、加工性を大きく損なうことなく鋼の強度を高める効果(固溶強化能)が大きいため、鋼の高強度化を達成するのに有効な元素であるが、化成処理性や塗装後耐食性に悪影響を及ぼす元素でもある。Siを高強度達成手段として添加する場合には、0.5mass%以上の添加が必要である。また、Siが0.5mass%未満では、化成処理条件の悪化による影響は少ない。一方、Siの含有量が3.0mass%を超えると、熱間圧延性や冷間圧延性が大きく低下し、生産性に悪影響を及ぼしたり、鋼板自体の延性の低下を招いたりする。よって、Siは0.5~3.0mass%の範囲で添加する。好ましくは0.8~2.5mass%の範囲である。
Cは、鋼を高強度化するのに有効な元素であり、さらに、TRIP(変態誘起塑性:Transformation Induced Plasticity)効果を有する残留オーステナイトや、ベイナイト、マルテンサイトを生成させるのにも有効な元素である。Cが0.01mass%以上であれば上記効果が得られ、一方、Cが0.30mass%以下であれば、溶接性の低下が生じない。よって、Cは0.01~0.30mass%の範囲で添加するのが好ましく、0.10~0.20mass%の範囲で添加するのがより好ましい。
Mnは、鋼を固溶強化して高強度化するとともに、焼入性を高め、残留オーステナイトやベイナイト、マルテンサイトの生成を促進する作用を有する元素である。このような効果は、1.0mass%以上の添加で発現する。一方、Mnが7.5mass%以下であれば、コストの上昇を招かずに上記効果が得られる。よって、Mnは1.0~7.5mass%の範囲で添加するのが好ましく、2.0~5.0mass%の範囲で添加するのがより好ましい。
Pは、固溶強化能の大きい割に絞り性を害さない元素であり、高強度化を達成するのに有効な元素であるため、0.005mass%以上含有させることが好ましい。ただし、Pは、スポット溶接性を害する元素であるが、0.05mass%以下であれば問題は生じない。よって、Pは0.05mass%以下が好ましく、0.02mass%以下とするのがより好ましい。
Sは、不可避的に混入してくる不純物元素であり、鋼中にMnSとして析出し、鋼板の伸びフランジ性を低下させる有害な成分である。伸びフランジ性を低下させないためには、Sは0.01mass%以下が好ましい。より好ましくは0.005mass%以下、さらに好ましくは0.003mass%以下である。
Alは、製鋼工程で脱酸剤として添加される元素であり、また、伸びフランジ性を低下させる非金属介在物をスラグとして分離するのに有効な元素であるので、0.01mass%以上含有させるのが好ましい。Alが0.06mass%以下であれば、原料コストの上昇を招かず、上記効果を得ることができる。よって、Alは0.06mass%以下とするのが好ましい。より好ましくは0.02~0.06mass%の範囲である。
Nb,TiおよびVは、炭化物や窒化物を形成し、焼鈍時の加熱段階でフェライトの成長を抑制して組織を微細化させ、成形性、特に伸びフランジ性を向上させる元素であるため、また、Mo,CrおよびBは、鋼の焼入性を向上し、ベイナイトやマルテンサイトの生成を促進する元素であるため、上記範囲で添加することができる。また、Nは、Nb,TiおよびVと窒化物を形成しあるいは鋼中に固溶して鋼の高強度化に寄与する元素であり、0.008mass%以下であれば、窒化物が多量に形成されないので、プレス成形時のボイド形成による破断が抑制され、上記効果を得ることができる。
NiおよびCuは、低温変態相の生成を促進し、鋼を高強度化する効果があるので、上記範囲で添加することができる。また、CaおよびREMは、硫化物系介在物の形態を制御し、鋼板の伸びフランジ性を向上させる元素であるので、上記範囲で添加することができる。
本発明の冷延鋼板は、上記成分以外の残部はFeおよび不可避的不純物である。ただし、本発明の作用効果を害しない範囲であれば、その他の成分の添加を拒むものではない。
前述したように、本発明の冷延鋼板は、焼鈍時に鋼板表層に形成されるSiO2やSi-Mn系複合酸化物等のSi含有酸化物層を除去した鋼板表面を有するものであることが必要である。そのためには、硝酸と塩酸を混合した酸洗液を用いて強酸洗し、鋼板表面や表面近傍の粒界部分に形成されたSi含有酸化物を地鉄ごと溶解、除去したものであることが必要である。
極表層情報を検出できる極低加速電圧の走査型電子顕微鏡(ULV-SEM)を用いて酸洗後の鋼板表面を加速電圧2kV、作動距離3.0mm、倍率1000倍程度で5視野程度を観察し、エネルギー分散型X線分光器(EDX)を用いて分光分析し、反射電子像を得る。この反射電子像を画像解析ソフト、例えば、Image Jを用いて2値化処理して黒色部の面積率を測定し、各視野の測定値を平均化することで鉄系酸化物の表面被覆率を得ることができる。なお、上記極低加速電圧の走査型電子顕微鏡(ULV-SEM)としては、例えば、SEISS社製;ULTRA55を、また、エネルギー分散型X線分光器(EDX)としては、例えば、Thermo Fisher社製;NSS312Eを挙げることができる。
後述する実施例の表3に示した鋼符号Gの鋼スラブを、同じく後述する実施例の表4のNo.7に示した条件で、熱間圧延し、冷間圧延し、連続焼鈍して板厚が1.8mmの冷延鋼板とし、次いで、上記連続焼鈍後の冷延鋼板を、表1に示した条件で、酸洗と再酸洗し、水洗し、乾燥した後、0.7%の調質圧延を施して、鋼板表面の鉄系酸化物量が異なるNo.aおよびbの2種類の冷延鋼板を得た。次いで、上記No.aの冷延鋼板を鉄系酸化物の多い標準サンプル、No.bの冷延鋼板を鉄系酸化物の少ない標準サンプルとし、それぞれの鋼板について、走査型電子顕微鏡を用いて前述した条件で反射電子像を得た。図1は、No.a,bの鋼板の反射電子像写真を、また、図2は、No.a,bの鋼板の上記反射電子像写真のグレー値に対するピクセル数のヒストグラムを示す。本発明では、上記図2に示したNo.a,bのヒストグラムの交点(X点)に対応するグレー値(Y点)を閾値として定めた。因みに、上記閾値を用いて、No.a,bの鋼板の鉄系酸化物の表面被覆率を求めたところ、No.aの鋼板は85.3%、No.bの鋼板は25.8%が得られた。
まず、酸洗後の鋼板表面から、集束イオンビーム(FIB)加工により、鋼板の幅方向に対して8μm程度の断面を観察できる抽出レプリカを10個作製する。次いで、断面の局所情報を調べることのできるエネルギー分散型X線分光器(EDX)を備えた透過型電子顕微鏡(TEM)を用いて、加速電圧200kV、倍率10万倍にて、各レプリカの断面8μmを連続して撮影する。一例として、図3には、鋼板表面に存在する酸洗で生成した被覆層の断面をTEMで観察した写真を、図4には、その被覆層のEDX分析結果を示した。図4から、上記被覆層は鉄主体の鉄系酸化物であることがわかるので、図3の断面写真に示した鋼板地鉄を示す線Aと酸化物層の最も厚い部分を示す線Bとの間隔を10個のレプリカについて測定し、それらの中の最大厚さを鉄系酸化物の最大厚さとする。なお、上記レプリカのサイズや個数、TEMによる測定条件等は一つの例示であり、適宜変更してよいことは勿論である。
本発明の冷延鋼板の製造方法は、Siを0.5~3.0mass%含有した鋼素材(スラブ)を加熱後、熱間圧延し、冷間圧延し、連続焼鈍し、その後、硝酸と塩酸とを混合した酸洗液を用いて酸洗することにより、鋼板表層部分のSi含有酸化物層を除去し、かつ、上記酸洗により鋼板表面に生成する鉄系酸化物の表面被覆率を85%以下にできる方法であることが必要であり、さらに、上記鉄系酸化物の最大厚さが200nm以下にできる方法であることが好ましい。したがって、製鋼工程から冷間圧延後の連続焼鈍工程までは、常法に従って製造することができるが、連続焼鈍後の酸洗は、以下の条件とするのが好ましい。
上記連続焼鈍後の鋼板表層には、SiO2やSi-Mn系複合酸化物等のSi含有酸化物が多量に生成されており、このままでは化成処理性や塗装後耐食性が著しく低下する。そこで、本発明の製造方法では、焼鈍後の冷延鋼板を、硝酸と塩酸を混合した酸を酸洗液に用いて強酸洗し、鋼板表層のSi含有酸化物層を地鉄ごと除去してやるとともに、上記酸洗によって鋼板表面に沈殿析出してくる鉄系酸化物の生成を抑制してやることが必要である。
(1)化成処理条件
上記各冷延鋼板から採取した試験片に、日本パーカライジング社製の脱脂剤:FC-E2011、表面調整剤:PL-Xおよび化成処理剤:パルボンドPB-L3065を用いて、下記の標準条件および化成処理液の温度を下げて低温度化した比較条件の2条件で、化成処理皮膜付着量が1.7~3.0g/m2となるよう化成処理を施した。
<標準条件>
・脱脂工程:処理温度 40°C、処理時間 120秒
・スプレー脱脂、表面調整工程:pH 9.5、処理温度室温、処理時間 20秒
・化成処理工程:化成処理液の温度 35℃、処理時間 120秒
<低温度化条件>
上記標準条件における化成処理液の温度を33℃に低下した条件
(2)腐食試験
上記化成処理を施した試験片の表面に、日本ペイント社製の電着塗料:V-50を用いて、膜厚が25μmとなるように電着塗装を施し、下記3種類の腐食試験に供した。
<塩温水浸漬試験>
化成処理および電着塗装を施した上記試験片(n=1)の表面に、カッターで長さ45mmのクロスカット疵を付与した後、この試験片を、5mass%NaCl溶液(60℃)に240時間浸漬し、その後、水洗し、乾燥し、カット疵部に粘着テープを貼り付けた後、引き剥がすテープ剥離試験を行い、カット疵部左右を合わせた最大剥離全幅を測定した。この最大剥離全幅が5.0mm以下であれば、耐塩温水浸漬試験における耐食性は良好と評価することができる。
<塩水噴霧試験(SST)>
化成処理、電着塗装を施した上記試験片(n=1)の表面に、カッターで長さ45mmのクロスカット疵を付与した後、この試験片を、5mass%NaCl水溶液を使用して、JIS Z2371:2000に規定される中性塩水噴霧試験に準拠して1000時間の塩水噴霧試験を行った後、クロスカット疵部についてテープ剥離試験し、カット疵部左右を合わせた最大剥離全幅を測定した。この最大剥離全幅が4.0mm以下であれば、塩水噴霧試験における耐食性は良好と評価することができる。
<複合サイクル腐食試験(CCT)>
化成処理、電着塗装を施した上記試験片(n=1)の表面に、カッターで長さ45mmのクロスカット疵を付与した後、この試験片を、塩水噴霧(5mass%NaCl水溶液:35℃、相対湿度:98%)×2時間→乾燥(60℃、相対湿度:30%)×2時間→湿潤(50℃、相対湿度:95%)×2時間、を1サイクルとして、これを90サイクル繰り返す腐食試験後、水洗し、乾燥した後、カット疵部についてテープ剥離試験し、カット疵部左右を合わせた最大剥離全幅を測定した。この最大剥離全幅が6.0mm以下であれば、複合サイクル腐食試験での耐食性は良好と評価できる。
圧延方向に直角方向(C方向)から採取したJIS Z2201:1998に規定のJIS5号引張試験片(n=1)を用いて、JIS Z2241:1998の規定に準拠して引張試験を行い、引張強さTSを測定した。
(2)塗装後耐食性
各冷延鋼板から採取した試験片に、実施例1と同じ条件で、化成処理し、電着塗装を施した試験片を作製し、実施例1と同様にして、塩温水浸漬試験、塩水噴霧試験(SST)および複合サイクル腐食試験(CCT)の3種類の腐食試験に供して、塗装後耐食性を評価した。
(1)化成処理条件
上記各冷延鋼板から採取した試験片に、日本パーカライジング社製の脱脂剤:FC-E2011、表面調整剤:PL-Xおよび化成処理剤:パルボンドPB-L3065を用いて、下記の標準条件および化成処理液の温度を下げて低温度化した比較条件の2条件で、化成処理皮膜付着量が1.7~3.0g/m2となるよう化成処理を施した。
<標準条件>
・脱脂工程:処理温度 40°C、処理時間 120秒
・スプレー脱脂、表面調整工程:pH 9.5、処理温度室温、処理時間 20秒
・化成処理工程:化成処理液の温度 35℃、処理時間 120秒
<低温度化条件>
上記標準条件における化成処理液の温度を33℃に低下した条件
(2)腐食試験
上記化成処理を施した試験片の表面に、日本ペイント社製の電着塗料:V-50を用いて、膜厚が25μmとなるように電着塗装を施し、実施例1と比較してより厳しい条件の下記3種類の腐食試験に供した。
<塩温水浸漬試験>
化成処理および電着塗装を施した上記試験片(n=1)の表面に、カッターで長さ45mmのクロスカット疵を付与した後、この試験片を、5mass%NaCl溶液(60℃)に360時間浸漬し、その後、水洗し、乾燥し、カット疵部に粘着テープを貼り付けた後、引き剥がすテープ剥離試験を行い、カット疵部左右を合わせた最大剥離全幅を測定した。この最大剥離全幅が5.0mm以下であれば、耐塩温水浸漬試験における耐食性は良好と評価することができる。
<塩水噴霧試験(SST)>
化成処理、電着塗装を施した上記試験片(n=1)の表面に、カッターで長さ45mmのクロスカット疵を付与した後、この試験片を、5mass%NaCl水溶液を使用して、JIS Z2371:2000に規定される中性塩水噴霧試験に準拠して1200時間の塩水噴霧試験を行った後、クロスカット疵部についてテープ剥離試験し、カット疵部左右を合わせた最大剥離全幅を測定した。この最大剥離全幅が4.0mm以下であれば、塩水噴霧試験における耐食性は良好と評価することができる。
<複合サイクル腐食試験(CCT)>
化成処理、電着塗装を施した上記試験片(n=1)の表面に、カッターで長さ45mmのクロスカット疵を付与した後、この試験片を、塩水噴霧(5mass%NaCl水溶液:35℃、相対湿度:98%)×2時間→乾燥(60℃、相対湿度:30%)×2時間→湿潤(50℃、相対湿度:95%)×2時間、を1サイクルとして、これを120サイクル繰り返す腐食試験後、水洗し、乾燥した後、カット疵部についてテープ剥離試験し、カット疵部左右を合わせた最大剥離全幅を測定した。この最大剥離全幅が6.0mm以下であれば、複合サイクル腐食試験での耐食性は良好と評価できる。
Claims (9)
- Siを0.5~3.0mass%含有し、冷間圧延後、連続焼鈍した鋼板を、硝酸濃度が100g/L超え200g/L以下で、硝酸濃度に対する塩酸濃度の比R(HCl/HNO3)が0.01~0.25である硝酸と塩酸を混合した酸を用いて酸洗することを特徴とする冷延鋼板の製造方法。
- 上記硝酸と塩酸を混合した酸は、硝酸濃度が110g/L超え140g/L以下で、硝酸濃度に対する塩酸濃度の比R(HCl/HNO3)が0.03~0.25であることを特徴とする請求項1に記載の冷延鋼板の製造方法。
- 上記酸洗を、酸洗液の温度を20~70℃として3~30秒間行うことを特徴とする請求項1または2に記載の冷延鋼板の製造方法。
- 上記鋼板は、Siの他に、C:0.01~0.30mass%、Mn:1.0~7.5mass%、P:0.05mass%以下、S:0.01mass%以下およびAl:0.06mass%以下を含有し、残部がFeおよび不可避的不純物からなる成分組成を有することを特徴とする請求項1~3のいずれか1項に記載の冷延鋼板の製造方法。
- 上記鋼板は、上記成分組成に加えてさらに、Nb:0.3mass%以下、Ti:0.3mass%以下、V:0.3mass%以下、Mo:0.3mass%以下、Cr:0.5mass%以下、B:0.006mass%以下およびN:0.008mass%以下のうちから選ばれる1種または2種以上を含有することを特徴とする請求項1~4のいずれか1項に記載の冷延鋼板の製造方法。
- 上記鋼板は、上記成分組成に加えてさらに、Ni:2.0mass%以下、Cu:2.0mass%以下、Ca:0.1mass%以下およびREM:0.1mass%以下のうちから選ばれる1種または2種以上を含有することを特徴とする請求項1~5のいずれか1項に記載の冷延鋼板の製造方法。
- 請求項1~6のいずれか1項に記載の方法で製造された冷延鋼板であって、連続焼鈍後の酸洗により鋼板表層のSi含有酸化物層が除去されてなり、かつ、上記酸洗により生成した鋼板表面の鉄系酸化物の表面被覆率が85%以下であることを特徴とする冷延鋼板。
- 上記冷延鋼板は、鋼板表面に存在する鉄系酸化物の最大厚さが200nm以下であることを特徴とする請求項7に記載の冷延鋼板。
- 請求項7または8に記載の冷延鋼板を用いてなることを特徴とする自動車部材。
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US13/812,438 US20130149529A1 (en) | 2010-08-31 | 2011-08-25 | Method of producing cold-rolled steel sheet as well as cold-rolled steel sheet and members for automobile |
CN201180029545XA CN102959129A (zh) | 2010-08-31 | 2011-08-25 | 冷轧钢板的制造方法、冷轧钢板和汽车构件 |
KR1020127033101A KR20130031284A (ko) | 2010-08-31 | 2011-08-25 | 냉연 강판의 제조 방법, 냉연 강판 및 자동차 부재 |
EP11821648.0A EP2612956B1 (en) | 2010-08-31 | 2011-08-25 | Method of producing cold-rolled steel sheet as well as cold-rolled steel sheet and members for automobile |
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JP5919920B2 (ja) * | 2011-03-28 | 2016-05-18 | Jfeスチール株式会社 | Si含有冷延鋼板の製造方法及び装置 |
JP5818046B2 (ja) | 2012-02-28 | 2015-11-18 | Jfeスチール株式会社 | Si含有高強度冷延鋼板の製造方法 |
JP5821874B2 (ja) * | 2013-02-28 | 2015-11-24 | Jfeスチール株式会社 | 高Si冷延鋼板の製造方法 |
JP6137089B2 (ja) * | 2014-09-02 | 2017-05-31 | Jfeスチール株式会社 | 冷延鋼板の製造方法および冷延鋼板の製造設備 |
EP3276022B1 (en) * | 2015-03-25 | 2019-09-04 | JFE Steel Corporation | Cold-rolled steel sheet and manufacturing method therefor |
CN107709620B (zh) * | 2015-07-08 | 2020-04-14 | 杰富意钢铁株式会社 | 冷轧钢带的制造方法及制造设备 |
JP2016065319A (ja) * | 2015-11-30 | 2016-04-28 | Jfeスチール株式会社 | 高強度鋼板の表面性状の評価方法および高強度鋼板の製造方法 |
EP3399064B1 (en) * | 2016-02-18 | 2021-07-14 | JFE Steel Corporation | High-strength cold-rolled steel sheet |
US11008635B2 (en) | 2016-02-18 | 2021-05-18 | Jfe Steel Corporation | High-strength cold-rolled steel sheet |
KR101889193B1 (ko) * | 2016-12-22 | 2018-08-16 | 주식회사 포스코 | 내식성 및 가공성이 우수한 냉연강판 및 그 제조방법 |
JP6191810B1 (ja) * | 2017-03-24 | 2017-09-06 | 新日鐵住金株式会社 | 鋼板の製造方法 |
JP6806128B2 (ja) * | 2018-01-09 | 2021-01-06 | Jfeスチール株式会社 | 冷延鋼板の判定方法および冷延鋼板の製造方法 |
KR102178809B1 (ko) * | 2018-11-30 | 2020-11-13 | 주식회사 포스코 | 내산용 열연강판 및 그 제조방법 |
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EP2612956B1 (en) | 2016-06-01 |
JP2012132093A (ja) | 2012-07-12 |
CN102959129A (zh) | 2013-03-06 |
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