CA2215110C - Zinc and zinc-alloy hot-dip-coated steel sheet having decreased bare spots and excellent coating adhesion and a method for manufacturing the same - Google Patents
Zinc and zinc-alloy hot-dip-coated steel sheet having decreased bare spots and excellent coating adhesion and a method for manufacturing the same Download PDFInfo
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- CA2215110C CA2215110C CA002215110A CA2215110A CA2215110C CA 2215110 C CA2215110 C CA 2215110C CA 002215110 A CA002215110 A CA 002215110A CA 2215110 A CA2215110 A CA 2215110A CA 2215110 C CA2215110 C CA 2215110C
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- steel sheet
- zinc
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- hot
- coated
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- 229910000831 Steel Inorganic materials 0.000 title claims abstract description 176
- 239000010959 steel Substances 0.000 title claims abstract description 176
- 239000011701 zinc Substances 0.000 title claims abstract description 72
- HCHKCACWOHOZIP-UHFFFAOYSA-N Zinc Chemical compound [Zn] HCHKCACWOHOZIP-UHFFFAOYSA-N 0.000 title claims abstract description 70
- 238000000034 method Methods 0.000 title claims abstract description 50
- 229910001297 Zn alloy Inorganic materials 0.000 title claims abstract description 40
- 238000004519 manufacturing process Methods 0.000 title claims abstract description 8
- 239000011248 coating agent Substances 0.000 title abstract description 33
- 238000000576 coating method Methods 0.000 title abstract description 33
- 230000003247 decreasing effect Effects 0.000 title abstract description 8
- 229910052725 zinc Inorganic materials 0.000 claims abstract description 68
- 238000005098 hot rolling Methods 0.000 claims abstract description 17
- 238000001816 cooling Methods 0.000 claims abstract description 10
- 238000010583 slow cooling Methods 0.000 claims abstract description 5
- 239000010410 layer Substances 0.000 claims description 34
- XEEYBQQBJWHFJM-UHFFFAOYSA-N Iron Chemical compound [Fe] XEEYBQQBJWHFJM-UHFFFAOYSA-N 0.000 claims description 33
- 229910052760 oxygen Inorganic materials 0.000 claims description 21
- QVGXLLKOCUKJST-UHFFFAOYSA-N atomic oxygen Chemical compound [O] QVGXLLKOCUKJST-UHFFFAOYSA-N 0.000 claims description 19
- 239000001301 oxygen Substances 0.000 claims description 19
- 239000000758 substrate Substances 0.000 claims description 19
- 238000000137 annealing Methods 0.000 claims description 18
- 238000003618 dip coating Methods 0.000 claims description 15
- 229910052748 manganese Inorganic materials 0.000 claims description 15
- 229910052698 phosphorus Inorganic materials 0.000 claims description 15
- 239000002344 surface layer Substances 0.000 claims description 15
- 229910052710 silicon Inorganic materials 0.000 claims description 14
- 229910052742 iron Inorganic materials 0.000 claims description 11
- 238000005554 pickling Methods 0.000 claims description 9
- 238000005275 alloying Methods 0.000 claims description 8
- 229910052739 hydrogen Inorganic materials 0.000 claims description 5
- 238000001953 recrystallisation Methods 0.000 claims description 5
- 238000005246 galvanizing Methods 0.000 claims description 4
- 238000002203 pretreatment Methods 0.000 claims description 4
- 238000005097 cold rolling Methods 0.000 claims description 3
- 238000000227 grinding Methods 0.000 claims description 3
- UFHFLCQGNIYNRP-UHFFFAOYSA-N Hydrogen Chemical compound [H][H] UFHFLCQGNIYNRP-UHFFFAOYSA-N 0.000 claims description 2
- 239000001257 hydrogen Substances 0.000 claims description 2
- 239000012299 nitrogen atmosphere Substances 0.000 claims description 2
- VEXZGXHMUGYJMC-UHFFFAOYSA-N Hydrochloric acid Chemical compound Cl VEXZGXHMUGYJMC-UHFFFAOYSA-N 0.000 description 12
- 238000004458 analytical method Methods 0.000 description 8
- 239000010960 cold rolled steel Substances 0.000 description 8
- 238000007747 plating Methods 0.000 description 7
- 238000000921 elemental analysis Methods 0.000 description 6
- 229910052787 antimony Inorganic materials 0.000 description 5
- WATWJIUSRGPENY-UHFFFAOYSA-N antimony atom Chemical compound [Sb] WATWJIUSRGPENY-UHFFFAOYSA-N 0.000 description 5
- 230000000694 effects Effects 0.000 description 5
- 238000000879 optical micrograph Methods 0.000 description 5
- UQSXHKLRYXJYBZ-UHFFFAOYSA-N Iron oxide Chemical compound [Fe]=O UQSXHKLRYXJYBZ-UHFFFAOYSA-N 0.000 description 4
- 230000007797 corrosion Effects 0.000 description 4
- 238000005260 corrosion Methods 0.000 description 4
- 239000000463 material Substances 0.000 description 4
- 239000000203 mixture Substances 0.000 description 4
- OKKJLVBELUTLKV-UHFFFAOYSA-N Methanol Chemical compound OC OKKJLVBELUTLKV-UHFFFAOYSA-N 0.000 description 3
- HEMHJVSKTPXQMS-UHFFFAOYSA-M Sodium hydroxide Chemical compound [OH-].[Na+] HEMHJVSKTPXQMS-UHFFFAOYSA-M 0.000 description 3
- 239000012298 atmosphere Substances 0.000 description 3
- 238000005238 degreasing Methods 0.000 description 3
- 238000011156 evaluation Methods 0.000 description 3
- 230000009545 invasion Effects 0.000 description 3
- NLKNQRATVPKPDG-UHFFFAOYSA-M potassium iodide Chemical compound [K+].[I-] NLKNQRATVPKPDG-UHFFFAOYSA-M 0.000 description 3
- 239000000523 sample Substances 0.000 description 3
- 239000000126 substance Substances 0.000 description 3
- 239000004246 zinc acetate Substances 0.000 description 3
- 239000002253 acid Substances 0.000 description 2
- 229910045601 alloy Inorganic materials 0.000 description 2
- 239000000956 alloy Substances 0.000 description 2
- 230000001680 brushing effect Effects 0.000 description 2
- 230000007423 decrease Effects 0.000 description 2
- 230000007547 defect Effects 0.000 description 2
- 238000009792 diffusion process Methods 0.000 description 2
- 238000004453 electron probe microanalysis Methods 0.000 description 2
- 239000012634 fragment Substances 0.000 description 2
- OSWPMRLSEDHDFF-UHFFFAOYSA-N methyl salicylate Chemical compound COC(=O)C1=CC=CC=C1O OSWPMRLSEDHDFF-UHFFFAOYSA-N 0.000 description 2
- 229910052759 nickel Inorganic materials 0.000 description 2
- 238000011158 quantitative evaluation Methods 0.000 description 2
- YGSDEFSMJLZEOE-UHFFFAOYSA-N salicylic acid Chemical compound OC(=O)C1=CC=CC=C1O YGSDEFSMJLZEOE-UHFFFAOYSA-N 0.000 description 2
- 238000005728 strengthening Methods 0.000 description 2
- 238000003887 surface segregation Methods 0.000 description 2
- 238000005406 washing Methods 0.000 description 2
- 229910052684 Cerium Inorganic materials 0.000 description 1
- LFQSCWFLJHTTHZ-UHFFFAOYSA-N EtOH Substances CCO LFQSCWFLJHTTHZ-UHFFFAOYSA-N 0.000 description 1
- 102100040160 Rabankyrin-5 Human genes 0.000 description 1
- 101710086049 Rabankyrin-5 Proteins 0.000 description 1
- 239000006061 abrasive grain Substances 0.000 description 1
- 230000002411 adverse Effects 0.000 description 1
- 230000033228 biological regulation Effects 0.000 description 1
- 230000015572 biosynthetic process Effects 0.000 description 1
- 229910052791 calcium Inorganic materials 0.000 description 1
- 238000006243 chemical reaction Methods 0.000 description 1
- 229910052804 chromium Inorganic materials 0.000 description 1
- 238000009749 continuous casting Methods 0.000 description 1
- 238000007796 conventional method Methods 0.000 description 1
- 239000013078 crystal Substances 0.000 description 1
- 238000005868 electrolysis reaction Methods 0.000 description 1
- 238000005530 etching Methods 0.000 description 1
- 238000002474 experimental method Methods 0.000 description 1
- 239000007789 gas Substances 0.000 description 1
- 238000010438 heat treatment Methods 0.000 description 1
- 238000009863 impact test Methods 0.000 description 1
- 238000011835 investigation Methods 0.000 description 1
- 229910052746 lanthanum Inorganic materials 0.000 description 1
- 229910052745 lead Inorganic materials 0.000 description 1
- 229910052749 magnesium Inorganic materials 0.000 description 1
- 229960001047 methyl salicylate Drugs 0.000 description 1
- 230000003287 optical effect Effects 0.000 description 1
- 238000000399 optical microscopy Methods 0.000 description 1
- 230000033116 oxidation-reduction process Effects 0.000 description 1
- 230000001590 oxidative effect Effects 0.000 description 1
- FJKROLUGYXJWQN-UHFFFAOYSA-N papa-hydroxy-benzoic acid Natural products OC(=O)C1=CC=C(O)C=C1 FJKROLUGYXJWQN-UHFFFAOYSA-N 0.000 description 1
- 239000000843 powder Substances 0.000 description 1
- 239000002244 precipitate Substances 0.000 description 1
- 238000012545 processing Methods 0.000 description 1
- 239000000047 product Substances 0.000 description 1
- 238000005096 rolling process Methods 0.000 description 1
- 229960004889 salicylic acid Drugs 0.000 description 1
- 238000004611 spectroscopical analysis Methods 0.000 description 1
- 238000012360 testing method Methods 0.000 description 1
- 238000012546 transfer Methods 0.000 description 1
- 229910052727 yttrium Inorganic materials 0.000 description 1
- 229910052726 zirconium Inorganic materials 0.000 description 1
Classifications
-
- 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
- C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
- C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
- C23C2/04—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor characterised by the coating material
- C23C2/06—Zinc or cadmium or alloys based thereon
-
- 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
- C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
- C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
- C23C2/02—Pretreatment of the material to be coated, e.g. for coating on selected surface areas
-
- B—PERFORMING OPERATIONS; TRANSPORTING
- B05—SPRAYING OR ATOMISING IN GENERAL; APPLYING FLUENT MATERIALS TO SURFACES, IN GENERAL
- B05D—PROCESSES FOR APPLYING FLUENT MATERIALS TO SURFACES, IN GENERAL
- B05D3/00—Pretreatment of surfaces to which liquids or other fluent materials are to be applied; After-treatment of applied coatings, e.g. intermediate treating of an applied coating preparatory to subsequent applications of liquids or other fluent materials
- B05D3/002—Pretreatement
-
- B—PERFORMING OPERATIONS; TRANSPORTING
- B05—SPRAYING OR ATOMISING IN GENERAL; APPLYING FLUENT MATERIALS TO SURFACES, IN GENERAL
- B05D—PROCESSES FOR APPLYING FLUENT MATERIALS TO SURFACES, IN GENERAL
- B05D3/00—Pretreatment of surfaces to which liquids or other fluent materials are to be applied; After-treatment of applied coatings, e.g. intermediate treating of an applied coating preparatory to subsequent applications of liquids or other fluent materials
- B05D3/007—After-treatment
-
- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C18/00—Alloys based on zinc
-
- 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
- C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
- C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
- C23C2/02—Pretreatment of the material to be coated, e.g. for coating on selected surface areas
- C23C2/022—Pretreatment of the material to be coated, e.g. for coating on selected surface areas by heating
- C23C2/0224—Two or more thermal pretreatments
-
- 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
- C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
- C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
- C23C2/02—Pretreatment of the material to be coated, e.g. for coating on selected surface areas
- C23C2/024—Pretreatment of the material to be coated, e.g. for coating on selected surface areas by cleaning or etching
-
- 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
- C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
- C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
- C23C2/26—After-treatment
-
- 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
- C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
- C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
- C23C2/34—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor characterised by the shape of the material to be treated
- C23C2/36—Elongated material
- C23C2/40—Plates; Strips
-
- 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
- C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
- C25F—PROCESSES FOR THE ELECTROLYTIC REMOVAL OF MATERIALS FROM OBJECTS; APPARATUS THEREFOR
- C25F1/00—Electrolytic cleaning, degreasing, pickling or descaling
Abstract
A zinc and zinc-alloy hot-dip-coated steel sheet and an alloyed zinc and zinc-alloy hot-dip-coated steel sheet, and a method for manufacturing these steel sheets, which steel sheets are used for automobiles. A zinc and zinc-alloy hot-dip-coated steel sheet which has oxides of readily oxidizable elements just under a coated layer and which shows a decreased number of bare spots and excellent coating adhesion. According to the method, the temperature for coiling during hot rolling is set to not less than 600°C and the cooling is conducted at a slow-cooling rate so that the oxides remain after the following steps.
Description
,° ~ ~,~ ~,'~
Description ZINC AND ZINC-ALLOY HOT-DIP-COATED STEEL SHEET HAVING
DECREASED BARE SPOTS AND EXCELLENT COATING ADHESION AND A
METHOD FOR MANUFACTURING THE SAME
Technical Field The present invention relates to a zinc and zinc-alloy hot-dip-coated steel sheet having a decreased number of bare spots and excellent coating adhesion, and a method for manufacturing the same.
Backqround Art Zinc and zinc-alloy hot-dip-coated steel sheets are mainly used for automobile bodies because of low cost and excellent corrosion resistance, and in addition to the corrosion resistance due to coating, coating adhesion - during press working is required for applying the steel sheets to automobile bodies. When coating adhesion deteriorates, coated layers peel as a powder or blocks, which phenomenon sometimes causes galling in press forming or deteriorates corrosion resistance of the portions from which the coated layer peels; and also, peeled fragments disadvantageously inflict the steel sheet.
As a conventional technique for improving coating adhesion, Japanese Patent Laid-Open No. 61-276961 discloses a technique in which alloying Fe with Zn at a high temperature ranging from 700 to 850°C is performed after zinc hot-dip-coating. However, alloying at a high temperature leads to not only higher costs but also increased expenses for equipment such as rolls.
Additionally, in Japanese Patent Laid-Open No. 3-232926, steel containsiat least one of Zr, La, Ce, Y, and Ca, and the cooling rate from recrystallization annealing to coating is set to not less than 50°C/sec. The cost is 10' raised due to the addition of Zr or the like to steel and productivity deteriorates because the sheet-feeding rate has to be lowered due to the cooling capacity.
Furthermore, in Japanese Patent Laid-Open No. 2-163356, the O, A1, and N contents in steel are set to not more than 0.0045 wt~, (25 x N wt$) to 0.15 wt~, and not more than 0.0030, respectively. Moreover, restrictions on the Ti, Si, and P contents, and Si (wt~) + p (wt~) >- Ti (wt~) must be satisfied according to Japanese Patent Laid-Open No. 6-81101. Anyway, the desired steel-sheet 20 properties such as strength and drawing cannot be always achieved by such content restrictions, and there is a possibility that coating adhesion will deteriorate because of deviations from a predetermined composition range.
In Japanese Patent Laid-Open No. 4-333552, coating adhesion is improved by carrying out Ni pre-plating before galvanizing. However, in general, a continuous galvanizing line (hereinafter referred to as "CGL") does not have such equipment, and a large investment is required for improving equipment or the like.
Meanwhile, automobile bodies are required to be lighter because of recent regulations for exhaust gas.
Thinning the steel sheets is a method for lightening the automobile bodies. According to this method, it is necessary for ensuring safety to increase steel-sheet strength corresponding to the decreased thickness. Thus, high tensile-strength steel sheets have been developed for strengthening the steel sheets by increasing the steel contents of elements such as Si, Mn, and P. Since steel sheets for automobiles are subjected to press forming, excellent material characteristics with a high r-value (high Lankford value) are required, and in particular, the addition of these elements is essential for high-tensile - strength steel sheets.
In the case of zinc hot-dip-coating such steel sheets, recrystallization annealing at a high temperature ranging from approximately 700 to 900°C is necessary to attain excellent material characteristics. In the CGL, recrystallization annealing is generally carried out under a nitrogen atmosphere in the presence of hydrogen (hereinafter referred to as reduction annealing), and although this atmosphere is a reducing atmosphere for Fe, it is an oxidizing atmosphere for some elements such as Si, Mn, and P. Thus, elements such as Si, Mn, and P (referred to as readily oxidizable elements) which are more oxidizable than Fe externally diffuse during reduction annealing and bond to oxygen on the surface of steel sheets to form oxides (called as "surface segregated layer"). Since these oxides significantly impede wettability between molten zinc and the steel sheets, so-called bare spots, i.e., defects occurring when zinc does not adhere to the steel sheets, are seen.
For overcoming such problems, Japanese Patent Examined Publication (Kokoku) No. 61-9386 published on March 22, 1986 (corresponding to Japanese Patent Laid-Open (Kokai) No. 57-76176; Inventors, S. Higuchi et al; Applicant Nippon Steel Corporation) prc>poses a method of pre-plating the surface of steel sheets with Ni before the zinc hot-dip-coating process. However, according to this method, when steel contains at least Si and one more element among 0.2 to 2.0 wt%
of Si, 0.5 to 2.0 wto of Mn, and 0.1 to 20 wt% of Cr, Ni plating of not less than 10 g/mz is necessary, resulting in an increased cost. In addition, although such a large quantity of Ni plating improves the wettability between the zinc hot-dip coating and the steel sheet, disadvantageously, defects caused by Si and Ni on the coated surface frequently appear during the alloying process.
Furthermore, for example, Japanese Patent Laid-Open No. 57-70268 proposes a method of pre-plating the surface of steel sheets with Fe before the zinc hot-dip-coating process. According to this method, bare spots in Si-containing steel are preventable by pre-plating, however, not less than 5 g/mZ of Fe plating is required, which fact is extremely uneconomical.
Additionally, other methods are disclosed in Japanese Patent Laid-Open Nos. 55-122865 and 4-254531. In these methods, steel sheets are oxidized beforehand to form a Fe oxide film on their surface and then subjected to reduction annealing. However, according to these methods, alloy elements such as Si are segregated on the surface to form an oxide film because of excess reducing during reduction annealing, causing a problem of inferior coating. For preventing such excess reducing, a large amount of Fe oxide is necessary. However, if the amount of Fe oxide film is exceedingly large, the Fe oxide film peels due to rolling or the like, thus on the contrary, a surface segregated layer is produced and results in impeded coating or adverse effects on operation because the fragments of the peeled Fe oxide-film are scattered inside a furnace.
In addition, concerning known proposals for the steel composition and hot-rolling conditions for zinc hot-dip-coating the high-tensile steel sheets, Japanese Patent Laid-Open No. 6-158172 discloses a method in which a steel containing Si<_0.2 and Mn<_1.5 by wt~ is wound at a temperature not less than 650°C followed by acid washing, cold-rolling, annealing,, and zinc hot-dip-coating; and Japanese Patent Laid-Open No. 6-1'79943 discloses a method in which a steel containing 0.10 to 1.5 wto of Si and 1.00 to 3.5 wt% of Mn is wound at a temperature ranging from 500°C to 680°C, both inclusive, followed by acid washing, cold-rolling, annealing, and zinc hot-dip-coating.
Although these methods give specified processing conditions, such as the steel composition and hot-rolling conditions, for a serie~~ of manufacturing steps, they cannot suppress the surface segregated layer formed during reduction annealing or improve bare spots or coating adhesion.
Disclosure of the Invention As a result of. detailed experiments, the inventors of the present invention have found that bare spots and coating adhesion are remarkably improved by providing oxides of readily oxidizable elements just: under a coated layer of a zinc and zinc-alloy hot-dip-coated steel sheet. Although the expression "a zinc and zinc-alloy hot-dip-coated steel sheet" is employed in this specification, "a zinc or zinc-alloy hot-dip-coated steel sheet" would be more accurate.
In one aspect, the present invention provides a zinc or zinc-alloy hot-dip-coated steel sheet having oxides of readily oxidizable elements just under a coated layer, i.e.
inside of and near a surf=ace of a steel sheet substrate.
Moreover, in t:he zinc or zinc-alloy hot-dip-coated steel sheet, the oxygen concentration is preferably not less than 1 ppm, more preferably, 2 to 200 ppm, and further more preferably, 3 to 100 ppm, in a region from the surface layer of a steel-sheet substrate just under the coated layer to 3 E.cm deep in the sheet-thickness direction.
In addition, such hot-dip-coated steel sheets are preferably further subjected to heat-alloying after zinc hot-dip-coating, and excellent alloyed zinc and zinc-alloy hot-dip-coated steel sheets are thereby obtained. Also in the alloyed zinc and z_i.nc-alloy hot-dip-coated steel sheets, the oxygen concentration is preferably not less than 1 ppm, more preferably, 2 to 200 ppm, and further more preferably, 3 to :L00 ppm, in a region of from the surface layer of a steel-sheet substrate just under the coated layer to 3 um dE~ep in the sheet-thickness direction.
Furthermore, each of the zinc and zinc-alloy hot-dip-coated steel sheets and alloyed zinc and zinc-alloy hot-dip-coated steel sheets preferably contains at least one element selected from the group consisting of Si, Mn, and P as a steel component in the following ranges:
0.001 <_ Si <_ 3.0 wt~, 0.05 <_ Mn _< 2.0 Wt~, and 0.005 <_ P _< 0.2 Wt~, Additionally, the present invention provides a method for producing the above-mentioned zinc and zinc-alloy hot-dip-coated steel sheet; or the alloyed zinc-alloy hot-dip-coated steel sheets both of which show a decreased number of bare spots and excellent coating adhesion. In other words, the present invention provides a method having:
a step A for forming oxides just under the scale, which oxides are formed from elements more oxidizable than iron, by setting a temperature of a steel strip to not less than 600°C and setting the mean slow-cooling rate up to 540°C to not more than (CT - 540)°'9 . 40 (°C/min) during coiling the steel strip hot-rolled; and a step B for zinc and zinc-alloy hot-dip-coating the steel strip. Accordincr to this method, the step B follows the step A, and other steps may be also employed between the steps A and B. In general, steps of pickling, degreasing, cold roll.illg, annealing,. and the like may appropriately be used as such intermediate steps.
In addition, according to a method of the present invention, the oxides formed in the step A preferably remain after a pre-treatment step carried out after the step A until treatment conducted in an annealing furnace immediately before the step B.
Furthermore, according to these~methods, a slab subjected to hot rolling preferably contains at least one element selected from t:he group consisting of Si, Mn, and P as a steel component. in the following ranges:
0.001 <_ Si <_ 3.0 Wt~
0.05 <_ Mn <_ 2.0 Wt~
0.005 <_ P <_ 0.2 Wt~
Description ZINC AND ZINC-ALLOY HOT-DIP-COATED STEEL SHEET HAVING
DECREASED BARE SPOTS AND EXCELLENT COATING ADHESION AND A
METHOD FOR MANUFACTURING THE SAME
Technical Field The present invention relates to a zinc and zinc-alloy hot-dip-coated steel sheet having a decreased number of bare spots and excellent coating adhesion, and a method for manufacturing the same.
Backqround Art Zinc and zinc-alloy hot-dip-coated steel sheets are mainly used for automobile bodies because of low cost and excellent corrosion resistance, and in addition to the corrosion resistance due to coating, coating adhesion - during press working is required for applying the steel sheets to automobile bodies. When coating adhesion deteriorates, coated layers peel as a powder or blocks, which phenomenon sometimes causes galling in press forming or deteriorates corrosion resistance of the portions from which the coated layer peels; and also, peeled fragments disadvantageously inflict the steel sheet.
As a conventional technique for improving coating adhesion, Japanese Patent Laid-Open No. 61-276961 discloses a technique in which alloying Fe with Zn at a high temperature ranging from 700 to 850°C is performed after zinc hot-dip-coating. However, alloying at a high temperature leads to not only higher costs but also increased expenses for equipment such as rolls.
Additionally, in Japanese Patent Laid-Open No. 3-232926, steel containsiat least one of Zr, La, Ce, Y, and Ca, and the cooling rate from recrystallization annealing to coating is set to not less than 50°C/sec. The cost is 10' raised due to the addition of Zr or the like to steel and productivity deteriorates because the sheet-feeding rate has to be lowered due to the cooling capacity.
Furthermore, in Japanese Patent Laid-Open No. 2-163356, the O, A1, and N contents in steel are set to not more than 0.0045 wt~, (25 x N wt$) to 0.15 wt~, and not more than 0.0030, respectively. Moreover, restrictions on the Ti, Si, and P contents, and Si (wt~) + p (wt~) >- Ti (wt~) must be satisfied according to Japanese Patent Laid-Open No. 6-81101. Anyway, the desired steel-sheet 20 properties such as strength and drawing cannot be always achieved by such content restrictions, and there is a possibility that coating adhesion will deteriorate because of deviations from a predetermined composition range.
In Japanese Patent Laid-Open No. 4-333552, coating adhesion is improved by carrying out Ni pre-plating before galvanizing. However, in general, a continuous galvanizing line (hereinafter referred to as "CGL") does not have such equipment, and a large investment is required for improving equipment or the like.
Meanwhile, automobile bodies are required to be lighter because of recent regulations for exhaust gas.
Thinning the steel sheets is a method for lightening the automobile bodies. According to this method, it is necessary for ensuring safety to increase steel-sheet strength corresponding to the decreased thickness. Thus, high tensile-strength steel sheets have been developed for strengthening the steel sheets by increasing the steel contents of elements such as Si, Mn, and P. Since steel sheets for automobiles are subjected to press forming, excellent material characteristics with a high r-value (high Lankford value) are required, and in particular, the addition of these elements is essential for high-tensile - strength steel sheets.
In the case of zinc hot-dip-coating such steel sheets, recrystallization annealing at a high temperature ranging from approximately 700 to 900°C is necessary to attain excellent material characteristics. In the CGL, recrystallization annealing is generally carried out under a nitrogen atmosphere in the presence of hydrogen (hereinafter referred to as reduction annealing), and although this atmosphere is a reducing atmosphere for Fe, it is an oxidizing atmosphere for some elements such as Si, Mn, and P. Thus, elements such as Si, Mn, and P (referred to as readily oxidizable elements) which are more oxidizable than Fe externally diffuse during reduction annealing and bond to oxygen on the surface of steel sheets to form oxides (called as "surface segregated layer"). Since these oxides significantly impede wettability between molten zinc and the steel sheets, so-called bare spots, i.e., defects occurring when zinc does not adhere to the steel sheets, are seen.
For overcoming such problems, Japanese Patent Examined Publication (Kokoku) No. 61-9386 published on March 22, 1986 (corresponding to Japanese Patent Laid-Open (Kokai) No. 57-76176; Inventors, S. Higuchi et al; Applicant Nippon Steel Corporation) prc>poses a method of pre-plating the surface of steel sheets with Ni before the zinc hot-dip-coating process. However, according to this method, when steel contains at least Si and one more element among 0.2 to 2.0 wt%
of Si, 0.5 to 2.0 wto of Mn, and 0.1 to 20 wt% of Cr, Ni plating of not less than 10 g/mz is necessary, resulting in an increased cost. In addition, although such a large quantity of Ni plating improves the wettability between the zinc hot-dip coating and the steel sheet, disadvantageously, defects caused by Si and Ni on the coated surface frequently appear during the alloying process.
Furthermore, for example, Japanese Patent Laid-Open No. 57-70268 proposes a method of pre-plating the surface of steel sheets with Fe before the zinc hot-dip-coating process. According to this method, bare spots in Si-containing steel are preventable by pre-plating, however, not less than 5 g/mZ of Fe plating is required, which fact is extremely uneconomical.
Additionally, other methods are disclosed in Japanese Patent Laid-Open Nos. 55-122865 and 4-254531. In these methods, steel sheets are oxidized beforehand to form a Fe oxide film on their surface and then subjected to reduction annealing. However, according to these methods, alloy elements such as Si are segregated on the surface to form an oxide film because of excess reducing during reduction annealing, causing a problem of inferior coating. For preventing such excess reducing, a large amount of Fe oxide is necessary. However, if the amount of Fe oxide film is exceedingly large, the Fe oxide film peels due to rolling or the like, thus on the contrary, a surface segregated layer is produced and results in impeded coating or adverse effects on operation because the fragments of the peeled Fe oxide-film are scattered inside a furnace.
In addition, concerning known proposals for the steel composition and hot-rolling conditions for zinc hot-dip-coating the high-tensile steel sheets, Japanese Patent Laid-Open No. 6-158172 discloses a method in which a steel containing Si<_0.2 and Mn<_1.5 by wt~ is wound at a temperature not less than 650°C followed by acid washing, cold-rolling, annealing,, and zinc hot-dip-coating; and Japanese Patent Laid-Open No. 6-1'79943 discloses a method in which a steel containing 0.10 to 1.5 wto of Si and 1.00 to 3.5 wt% of Mn is wound at a temperature ranging from 500°C to 680°C, both inclusive, followed by acid washing, cold-rolling, annealing, and zinc hot-dip-coating.
Although these methods give specified processing conditions, such as the steel composition and hot-rolling conditions, for a serie~~ of manufacturing steps, they cannot suppress the surface segregated layer formed during reduction annealing or improve bare spots or coating adhesion.
Disclosure of the Invention As a result of. detailed experiments, the inventors of the present invention have found that bare spots and coating adhesion are remarkably improved by providing oxides of readily oxidizable elements just: under a coated layer of a zinc and zinc-alloy hot-dip-coated steel sheet. Although the expression "a zinc and zinc-alloy hot-dip-coated steel sheet" is employed in this specification, "a zinc or zinc-alloy hot-dip-coated steel sheet" would be more accurate.
In one aspect, the present invention provides a zinc or zinc-alloy hot-dip-coated steel sheet having oxides of readily oxidizable elements just under a coated layer, i.e.
inside of and near a surf=ace of a steel sheet substrate.
Moreover, in t:he zinc or zinc-alloy hot-dip-coated steel sheet, the oxygen concentration is preferably not less than 1 ppm, more preferably, 2 to 200 ppm, and further more preferably, 3 to 100 ppm, in a region from the surface layer of a steel-sheet substrate just under the coated layer to 3 E.cm deep in the sheet-thickness direction.
In addition, such hot-dip-coated steel sheets are preferably further subjected to heat-alloying after zinc hot-dip-coating, and excellent alloyed zinc and zinc-alloy hot-dip-coated steel sheets are thereby obtained. Also in the alloyed zinc and z_i.nc-alloy hot-dip-coated steel sheets, the oxygen concentration is preferably not less than 1 ppm, more preferably, 2 to 200 ppm, and further more preferably, 3 to :L00 ppm, in a region of from the surface layer of a steel-sheet substrate just under the coated layer to 3 um dE~ep in the sheet-thickness direction.
Furthermore, each of the zinc and zinc-alloy hot-dip-coated steel sheets and alloyed zinc and zinc-alloy hot-dip-coated steel sheets preferably contains at least one element selected from the group consisting of Si, Mn, and P as a steel component in the following ranges:
0.001 <_ Si <_ 3.0 wt~, 0.05 <_ Mn _< 2.0 Wt~, and 0.005 <_ P _< 0.2 Wt~, Additionally, the present invention provides a method for producing the above-mentioned zinc and zinc-alloy hot-dip-coated steel sheet; or the alloyed zinc-alloy hot-dip-coated steel sheets both of which show a decreased number of bare spots and excellent coating adhesion. In other words, the present invention provides a method having:
a step A for forming oxides just under the scale, which oxides are formed from elements more oxidizable than iron, by setting a temperature of a steel strip to not less than 600°C and setting the mean slow-cooling rate up to 540°C to not more than (CT - 540)°'9 . 40 (°C/min) during coiling the steel strip hot-rolled; and a step B for zinc and zinc-alloy hot-dip-coating the steel strip. Accordincr to this method, the step B follows the step A, and other steps may be also employed between the steps A and B. In general, steps of pickling, degreasing, cold roll.illg, annealing,. and the like may appropriately be used as such intermediate steps.
In addition, according to a method of the present invention, the oxides formed in the step A preferably remain after a pre-treatment step carried out after the step A until treatment conducted in an annealing furnace immediately before the step B.
Furthermore, according to these~methods, a slab subjected to hot rolling preferably contains at least one element selected from t:he group consisting of Si, Mn, and P as a steel component. in the following ranges:
0.001 <_ Si <_ 3.0 Wt~
0.05 <_ Mn <_ 2.0 Wt~
0.005 <_ P <_ 0.2 Wt~
Moreover, according to any of the above-mentioned methods, an alloyed zinc and zinc-alloy hot-dip-coated steel sheet can be produced by employing heat-alloying treatment after the step B.
Next, oxides of readily oxidizable elements positioned just under a coated layer will be explained.
These oxides of readily oxidizable elements are formed during hot-rolling, in particular, the oxides are grown when the temperature (hereinafter referred to as "CT") during coiling is high and the cooling rate after coiling is low.
The oxides formed during hot-rolling are observed just under the scale, as is shown in figure 6. Meanwhile, in a conventional hot-rolled sheet, no oxide is observed just under the scale, as is shown in figure 7. The oxides observed during hot-rolling are analyzed by using an ' electron probe microanalyzer (hereinafter referred to as "EPMA") and the results are shown in figure 1. Since Mn, P, A1, and 0 show peaks, it is understood that oxides of these elements are formed. Steel sheets shown in figures 6 and 1 contain 0.1 wt~ of Mn, 0.006 wt~ of P, and 0.03 wto of A1, and they do not contain a particularly large amount of Mn, P, or A1.
The oxides positioned just under a coated layer of a zinc hot-dip-coated steel sheet or an alloyed zinc hot-dip-coated steel sheet of the present invention are produced such that oxides formed just under the scale during the hot-rolling process remain even after post-treatment steps such as pickling and coating.
The mechanism of producing oxides just under the scale is as follows: oxygen in a scale layer essentially consisting of iron oxide which has been formed during hot-rolling internally diffuses into steel during or after the coiling process, and then, forms an oxide of a readily oxidizable element in the steel. Therefore, oxides are produced even when only a trace amount of readily oxidizable elements is contained in the steel.
Although oxides of elements more oxidizable than iron exist just under the zinc and zinc-alloy hot-dip-coating according to the present invention, an oxide of an element less oxidizable than iron oxide or iron may also be contained. In addition, such an oxide is preferably ' formed in grain boundaries of a hot-rolled steel sheet.
As a result of studies and investigations conducted on various types of steel sheets, the inventors of the present invention have found oxides of Si-0, Mn-0, A1-0, P-0, and Fe-Si-0 in the steel sheets.
Figure 2 shows the result of elemental analysis of a conventional steel sheet and figure 3 shows that of an unannealed cold-rolled steel sheet wherein oxides were observed, which analysis was carried out in a region of from the surface of each steel sheet to approximately 10 ~m in the depth direction by glow-discharge spectroscopy (hereinafter referred to as "GDS"). The peaks of Mn, A1, P, and 0 observed at the depth of approximately 0.3 to 4 ~m from the surface layer correspond to the oxides.
Figure 4 shows the result of elemental analysis of a conventional steel sheet and figure 5 shows that of an annealed cold-rolled steel sheet wherein oxides were observed, which analysis was carried out by GDS in a region of from the surface of each steel sheet to approximately 10 um in the depth direction. A large amount of surface segregated substances generated by reduction annealing is observed in the conventional steel sheet of figure 4, meanwhile the generation of surface segregation products is suppressed and hardly observed in the steel sheet with oxides produced during hot-rolling.
Next, oxides of the present invention which exist in a surface layer of a steel sheet (surface layer of a steel-sheet substrate)'just under a coated layer can optical-microscopically be observed by etching the steel sheet with a 1~ nital solution for several to several dozen of seconds.
Figure 8 (photograph) and figure 9 (photograph) show a conventional alloyed zinc hot-dip-coated steel sheet not containing oxide and an alloyed zinc hot-dip-coated steel sheet containing oxides incorporated in the present invention, respectively. Figures 8 and 9 are cross-sectional optical micrographs of alloyed zinc hot-dip-coated steel sheets taken at a magnification of x1,000.
Black ribbon-like materials observed just under the coated layer are oxides (shown by arrows).
In addition, the formation of oxides can also be confirmed by analyzing oxygen contained in steel.
Concerning technique, oxygen in steel is analyzed in the total sheet-thickness direction using a hot-rolled steel sheet whose scale layer has been removed by pickling after coiling, a steel sheet obtained by dissolving only a coated layer of a zinc and zinc-alloy hot-dip-coated steel sheet, an unannealed cold-rolled steel sheet, or an annealed steel sheet, and the resulting values are compared with those of steel sheets obtained by grinding the surface layer in which oxides are formed. The steel sheets in which oxides are formed have larger oxygen values analyzed in the total sheet-thickness direction as compared with those of',the ground sheets.
Next, the mechanism of improving bare spots and coating adhesion by forming oxides just under a coated layer will be investigated.
First, concerning improvement in bare spots, it was found that surface segregation of readily oxidizable elements is suppressed during reduction annealing in the CGL when oxides are produced just under the scale by internal oxygen diffusion during or after coiling, as is above-mentioned.
This phenomenon is assumed to be due to following:
the amount of readily oxidizable elements in the surface layer decreases because the readily oxidizable elements already precipitate as oxides during or after coiling; the formed oxides impede transfer (external diffusion) of the readily oxidizable elements from bulk steel to the steel sheet surface; and oxidation-reduction occurs inside the steel sheet, in other words, a Fe-containing oxide produced during or after coiling changes to an oxide of readily oxidizable element during reduction annealing.
Therefore, the surface segregated substances of the readily oxidizable elements, which substances impede wettability between molten zinc and the steel sheet, extremely decrease, thereby remarkably improving bare spots.
Next, coating adhesion will be explained.
It has been known that coating peels mainly due to compressive stress during press forming.
Since a steel sheet having oxides just under a coated layer, i. e. a steel sheet of the present invention, has spaces between oxide crystals, zinc more readily penetrates into the steel sheet as compared with conventional steel sheets not containing oxides. As a result, the interface between the coated layer and the steel sheet is significantly roughened so that the coated layer can tightly adhere to the steel sheet. As a result, a zinc hot-dip-coated steel sheet and an alloyed zinc hot-dip-coated steel sheet both incorporated in the present invention acquire excellent coating adhesion during press forming.
Figures 10 and 11'show the observation results obtained from a steel sheet using a SEM, a coated layer of which steel sheet has been forcibly dissolved to the iron potential according to a galvanostatic process (4% methyl salicylate, 1% salicylic acid, and 10% potassium iodide/methanol solution; 5 mA/cmZ) so as to expose. the steel sheet. It is understood that the interface between the coated layer and the steel sheet is apparently more roughened as compared with the conventional steel sheet - not containing oxides.
In addition, the technique disclosed by the present invention exhibits more excellent effects when a steel sheet contains at least one component selected from the group consisting of Si, Mn, and P as a steel component in the following ranges:
0.001 <_ Si <_ 3.0 Wt%
0.05 <_ Mn _< 2.0 Wt%
0.005 <_ P <_ 0.2 Wt%
Problems such as bare spots and decreased coating adhesion hardly occur in steel sheets not containing the above elements, thus the lower limits for these elements are preferably 0.001 wt% for Si, 0.05 wt% for Mn, and 0.005 wt% for P. Meanwhile, the upper limit for each element is determined considering the preferable ranges for both the maximum effect for strengthening and cost.
Furthermore, the technique disclosed by the present invention exhibits sufficient effects on both bare spots and coating adhesion when even a small amount of oxides is observed by an optical microscope in a cross-section of a zinc and zinc-alloy hot-dip-coated steel sheet etched by 1% nital.
In addition, according to an oxygen analysis of steel, sufficient effects are shown, particularly on bare spots and coating adhesion when the value of the following formula is not less than 1 ppm:
(oxygen in a steel sheet whose coating has been removed by a hydrochloric acid~antimony method) - (oxygen in a steel sheet whose coating has been removed by a hydrochloric acid~antimony method and whose surface layer is then ground to remove 3 um thereof) Next, a technique for manufacturing the above-described coated steel sheets will be disclosed. It is required that the temperature for coiling after hot-rolling be high and cooling after the coiling be slow, and a detailed explanation will be given below.
The temperature for coiling after hot-rolling must be 600°C or more to produce oxides and the cooling rate up to 540°C after coiling must be not more than the following:
(CT - 540)°'9 . 40 (°C/min) Oxides are not formed at not more than 540°C even when slow-cooling is further carried out.
In addition, although pickling and/or grinding is generally carried out to remove the scale before coating, and sometimes, equipment for electrolytic degreasing or pickling is also provided for the CGL inlet side, the oxides produced in the surface layer of a steel sheet during or after coiling in the hot-rolling process must remain after the above treatment.
- Zinc and zinc-alloy hot-dip-coating of the present invention is a general term for molten zinc containing zinc and may include not only zinc hot-dip-coating but also galfan and galvalume, in both of which Si is contained in zinc. Moreover, Pb, Mg, Mn, etc. may be further contained. Therefore, conditions for a zinc bath are not particularly restricted.
Other conditions for the coated layer are not particularly limited, however, considering corrosion resistance and the like, the preferred amount of zinc and zinc-alloy coating is approximately 25 to 90 g/mZ and the preferred iron content in a coated layer in an alloyed zinc hot-dip-coated steel sheet is 8 to 13 wt~.
Furthermore, both hot-rolled steel sheets and cold-rolled steel sheets can be used as a material for coating.
Brief Description of Drawings [Figure 1] An EPMA analysis chart of oxides observed just under the scale during!hot-rolling.
[Figure 2] A graph showing the result of elemental analysis of a conventional unannealed cold-rolled steel sheet, which analysis was carried out by GDS from the surface to approximately 10 um in the depth direction.
[Figure 3] A graph showing the result of elemental analysis of an unannealed cold-rolled steel sheet of the present invention, which analysis was carried out by GDS
from the surface to approximately 10 ~m in the depth direction.
[Figure 4] A graph showing the result of elemental analysis of a conventional annealed cold-rolled steel sheet, which analysis was carried out by GDS from the surface to approximately 10 um in the depth direction.
[Figure 5] A graph showing the result of elemental analysis of an annealed cold-rolled steel sheet of the present invention, which analysis was carried out by GDS
from the surface to approximately 10 ~m in the depth direction.
[Figure 6] A cross-sectional optical micrograph, taken at a magnification of x1,000, showing oxides positioned just under the scale of a hot-rolled sheet of an example.
[Figure 7] A cross-sectional optical micrograph, taken at a magnification of x1,000, showing oxides positioned just under the scale of a conventional hot-rolled sheet.
[Figure 8] A cross-sectional optical micrograph, taken at a magnification of x1,000, showing an example alloyed zinc hot-dip-coated steel sheet containing oxides.
[Figure 9] A cross-sectional optical micrograph, taken at a magnification of x1,000, showing a conventional alloyed zinc hot-dip-coated steel sheet not containing oxides.
[Figure 10] A SEM photograph, taken at a magnification of x1,500, showing an example steel sheet whose coated layer has been dissolved.
[Figure 11] A SEM photograph, taken at a magnification of x1,500, showing a conventional steel sheet whose coated layer has been dissolved.
[Reference Numerals]
1, untreated steel-sheet portion; 2, scale; 3, oxides; 4, a coated layer; and 5, oxides.
Best Mode for Carrying Out the Invention An example of the present invention will be shown below:
Each sample shown in Table 1 was melted by a converter and formed into a slab by continuous casting.
Each of the resulting slabs was hot-rolled to 1.2 to 3.5 mm thick at a slab-heating temperature of 1150 to 1200°C, and with a finishing temperature of 900 to 920°C, and a coiling temperature and a cooling rate which are shown in Table 2. After that, the resulting sheets were pickled for 5 to 15 seconds at'80°C in an aqueous 5~ HC1 solution to remove scale layers, and then, divided into two groups one of which was directly subjected to the CGL and the other was cold-rolled into 0.7 mm thick. Furthermore, in the CGL inlet side, the following methods were also used in combination as a pre-treatment for removing the surface layer of a steel sheet, if required.
Electrolytic degreasing: electrolysis at 60°C in an aqueous 3~ NaOH solution for approximately 10 seconds.
Pickling: pickling at 60°C in an aqueous 5~ HCl solution for approximately 3 seconds.
Brushing roll: a'brushing roll with abrasive grains.
In the CGL, both the hot-rolled sheet and the cold-rolled sheet were zinc hot-dip-coated at 470°C after annealing at 800 to 850°C. In addition, alloyed zinc hot-dip-coated steel sheets were obtained by successively subjecting the annealed sheets to an alloying process conducted at 480 to 530°C for 15 to 30 seconds.
o Evaluation method for oxide Observation method for oxides in hot-rolled sheets A cross section of each hot-rolled sheet with the scale was ground and, without being etched, subjected to optical-microscopic observation so as to measure the depth of oxide invasion. The preferred magnification of the optical microscopy was 1,000.
Quantitative evaluation of oxides in hot-rolled sheets The following value was obtained:
(oxygen in steel analyzed in the total sheet-thickness direction of a hot-rolled sheet whose scale has been removed by pickling) - (oxygen in steel of a steel sheet which has been ground to reach the depth of oxide invention in the sheet-thickness direction after removing the scale) - Quantitative evaluation of oxides in coated sheets Each of the coated sheets was immersed in the solutions shown below until the end of the dissolving reaction of coating and then the concentration of the oxide-derived oxygen in a region of from the surface of the steel sheet to 3 ~m in the sheet-thickness direction was calculated according to the following formula:
(oxygen in a steel sheet whose coating was peeled by a hydrochloric acid~antimony method) - (oxygen in a steel sheet whose coating was peeled by a hydrochloric acid~antimony method and whose surface layer was then ground to remove 3 ~m thereof) 1% nital solution 1 vol% HN03-ethanol solution hydrochloric acid~antimony method Sb203 (20 g) + 35%
HCl (1 1) o Evaluation method for bare-spots Each of the coated sheets was evaluated by macroscopic observation.
Bare spots not observed: rank 1 a few were observed: rank 2 a small number were observed:
rank 3 observed: rank 4 o Test for coating-adhesion evaluation Each of the coated sheets was subjected to a Dupont impact test using a 1/2-inch punch and occurrence of peeling was confirmed by macroscopic observation.
Peeling was notobserved: o Peeling was observed: x Each alloyed zinc alloy hot-dip-coated steel sheet was bent to 90°, bent back, and then the compressed side of the steel sheet was peeled by a tape so as to measure the peeled amount of zinc by fluorescent X ray.
Count number of:
less than 500: rank 1 ( good not less than 500 to less than 1,000: rank 2 not less than 1,000 to less than 2,000: rank 3 not less than 2,000 to less than 3,000: rank 4 not less than 3,000 . rank 5 Table 3 shows the results of the zinc hot-dip-coated steel sheet and Table 4 shows those of the alloyed zinc hot-dip-coated steel sheets.
Table 1 Sample-Steel Composition Symbol C S i M n P
wt~ wt~ wt~ wt~
A 0.105 0.010 0.08 0.008 B 0.070 0.10 0.10 0.01 C 0.070 0.50 2.0 0.07 D 0.010 1..50 0.10 0.05 E 0.003 0.003 0.05 0.005 F 0.003 0.01 0.20 0.01 G 0.003 0.30 0.50 0.04 H 0.003 0.05 1.95 0.20 Table 2 Coiling conditions, depth of oxide invasion into hot-rolled sheet, and oxide amount in hot-rolled sheet SampleCT Mean cooling Depth of Oxide amount Sample steel C rate oxide in steel to invasion hot-rolled No.
540C C/min into hot- sheet rolled sheetppm wm A 540 1.0 0 0 1 A 600 1.0 1 1 2 A 600 1.5 0 <1 3 A 700 2.0 7 5 4 B 650 1.5 8 8 5 C 650 1.5 ' 6 7 6 D 580 1.0 0 0 7 D 620 1.2 <1 <1 8 E 650 1.2 5 5 9 E 650 1.6 <1 1 10 E 650 1.8 0 <1 11 F 650 1.0 10 11 12 G 650 1.0 12 15 13 H 600 1.8 0 0 14 H 650 1.0 12 18 15 %
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Industrial availability The technique disclosed by the present invention relates to a zinc hot-dip-coated steel sheet and an alloyed zinc hot-dip-coated steel sheet showing a decreased number of bare spots and excellent coating adhesion, and are appropriately used mainly for steel sheets of automobile bodies.
' 27
Next, oxides of readily oxidizable elements positioned just under a coated layer will be explained.
These oxides of readily oxidizable elements are formed during hot-rolling, in particular, the oxides are grown when the temperature (hereinafter referred to as "CT") during coiling is high and the cooling rate after coiling is low.
The oxides formed during hot-rolling are observed just under the scale, as is shown in figure 6. Meanwhile, in a conventional hot-rolled sheet, no oxide is observed just under the scale, as is shown in figure 7. The oxides observed during hot-rolling are analyzed by using an ' electron probe microanalyzer (hereinafter referred to as "EPMA") and the results are shown in figure 1. Since Mn, P, A1, and 0 show peaks, it is understood that oxides of these elements are formed. Steel sheets shown in figures 6 and 1 contain 0.1 wt~ of Mn, 0.006 wt~ of P, and 0.03 wto of A1, and they do not contain a particularly large amount of Mn, P, or A1.
The oxides positioned just under a coated layer of a zinc hot-dip-coated steel sheet or an alloyed zinc hot-dip-coated steel sheet of the present invention are produced such that oxides formed just under the scale during the hot-rolling process remain even after post-treatment steps such as pickling and coating.
The mechanism of producing oxides just under the scale is as follows: oxygen in a scale layer essentially consisting of iron oxide which has been formed during hot-rolling internally diffuses into steel during or after the coiling process, and then, forms an oxide of a readily oxidizable element in the steel. Therefore, oxides are produced even when only a trace amount of readily oxidizable elements is contained in the steel.
Although oxides of elements more oxidizable than iron exist just under the zinc and zinc-alloy hot-dip-coating according to the present invention, an oxide of an element less oxidizable than iron oxide or iron may also be contained. In addition, such an oxide is preferably ' formed in grain boundaries of a hot-rolled steel sheet.
As a result of studies and investigations conducted on various types of steel sheets, the inventors of the present invention have found oxides of Si-0, Mn-0, A1-0, P-0, and Fe-Si-0 in the steel sheets.
Figure 2 shows the result of elemental analysis of a conventional steel sheet and figure 3 shows that of an unannealed cold-rolled steel sheet wherein oxides were observed, which analysis was carried out in a region of from the surface of each steel sheet to approximately 10 ~m in the depth direction by glow-discharge spectroscopy (hereinafter referred to as "GDS"). The peaks of Mn, A1, P, and 0 observed at the depth of approximately 0.3 to 4 ~m from the surface layer correspond to the oxides.
Figure 4 shows the result of elemental analysis of a conventional steel sheet and figure 5 shows that of an annealed cold-rolled steel sheet wherein oxides were observed, which analysis was carried out by GDS in a region of from the surface of each steel sheet to approximately 10 um in the depth direction. A large amount of surface segregated substances generated by reduction annealing is observed in the conventional steel sheet of figure 4, meanwhile the generation of surface segregation products is suppressed and hardly observed in the steel sheet with oxides produced during hot-rolling.
Next, oxides of the present invention which exist in a surface layer of a steel sheet (surface layer of a steel-sheet substrate)'just under a coated layer can optical-microscopically be observed by etching the steel sheet with a 1~ nital solution for several to several dozen of seconds.
Figure 8 (photograph) and figure 9 (photograph) show a conventional alloyed zinc hot-dip-coated steel sheet not containing oxide and an alloyed zinc hot-dip-coated steel sheet containing oxides incorporated in the present invention, respectively. Figures 8 and 9 are cross-sectional optical micrographs of alloyed zinc hot-dip-coated steel sheets taken at a magnification of x1,000.
Black ribbon-like materials observed just under the coated layer are oxides (shown by arrows).
In addition, the formation of oxides can also be confirmed by analyzing oxygen contained in steel.
Concerning technique, oxygen in steel is analyzed in the total sheet-thickness direction using a hot-rolled steel sheet whose scale layer has been removed by pickling after coiling, a steel sheet obtained by dissolving only a coated layer of a zinc and zinc-alloy hot-dip-coated steel sheet, an unannealed cold-rolled steel sheet, or an annealed steel sheet, and the resulting values are compared with those of steel sheets obtained by grinding the surface layer in which oxides are formed. The steel sheets in which oxides are formed have larger oxygen values analyzed in the total sheet-thickness direction as compared with those of',the ground sheets.
Next, the mechanism of improving bare spots and coating adhesion by forming oxides just under a coated layer will be investigated.
First, concerning improvement in bare spots, it was found that surface segregation of readily oxidizable elements is suppressed during reduction annealing in the CGL when oxides are produced just under the scale by internal oxygen diffusion during or after coiling, as is above-mentioned.
This phenomenon is assumed to be due to following:
the amount of readily oxidizable elements in the surface layer decreases because the readily oxidizable elements already precipitate as oxides during or after coiling; the formed oxides impede transfer (external diffusion) of the readily oxidizable elements from bulk steel to the steel sheet surface; and oxidation-reduction occurs inside the steel sheet, in other words, a Fe-containing oxide produced during or after coiling changes to an oxide of readily oxidizable element during reduction annealing.
Therefore, the surface segregated substances of the readily oxidizable elements, which substances impede wettability between molten zinc and the steel sheet, extremely decrease, thereby remarkably improving bare spots.
Next, coating adhesion will be explained.
It has been known that coating peels mainly due to compressive stress during press forming.
Since a steel sheet having oxides just under a coated layer, i. e. a steel sheet of the present invention, has spaces between oxide crystals, zinc more readily penetrates into the steel sheet as compared with conventional steel sheets not containing oxides. As a result, the interface between the coated layer and the steel sheet is significantly roughened so that the coated layer can tightly adhere to the steel sheet. As a result, a zinc hot-dip-coated steel sheet and an alloyed zinc hot-dip-coated steel sheet both incorporated in the present invention acquire excellent coating adhesion during press forming.
Figures 10 and 11'show the observation results obtained from a steel sheet using a SEM, a coated layer of which steel sheet has been forcibly dissolved to the iron potential according to a galvanostatic process (4% methyl salicylate, 1% salicylic acid, and 10% potassium iodide/methanol solution; 5 mA/cmZ) so as to expose. the steel sheet. It is understood that the interface between the coated layer and the steel sheet is apparently more roughened as compared with the conventional steel sheet - not containing oxides.
In addition, the technique disclosed by the present invention exhibits more excellent effects when a steel sheet contains at least one component selected from the group consisting of Si, Mn, and P as a steel component in the following ranges:
0.001 <_ Si <_ 3.0 Wt%
0.05 <_ Mn _< 2.0 Wt%
0.005 <_ P <_ 0.2 Wt%
Problems such as bare spots and decreased coating adhesion hardly occur in steel sheets not containing the above elements, thus the lower limits for these elements are preferably 0.001 wt% for Si, 0.05 wt% for Mn, and 0.005 wt% for P. Meanwhile, the upper limit for each element is determined considering the preferable ranges for both the maximum effect for strengthening and cost.
Furthermore, the technique disclosed by the present invention exhibits sufficient effects on both bare spots and coating adhesion when even a small amount of oxides is observed by an optical microscope in a cross-section of a zinc and zinc-alloy hot-dip-coated steel sheet etched by 1% nital.
In addition, according to an oxygen analysis of steel, sufficient effects are shown, particularly on bare spots and coating adhesion when the value of the following formula is not less than 1 ppm:
(oxygen in a steel sheet whose coating has been removed by a hydrochloric acid~antimony method) - (oxygen in a steel sheet whose coating has been removed by a hydrochloric acid~antimony method and whose surface layer is then ground to remove 3 um thereof) Next, a technique for manufacturing the above-described coated steel sheets will be disclosed. It is required that the temperature for coiling after hot-rolling be high and cooling after the coiling be slow, and a detailed explanation will be given below.
The temperature for coiling after hot-rolling must be 600°C or more to produce oxides and the cooling rate up to 540°C after coiling must be not more than the following:
(CT - 540)°'9 . 40 (°C/min) Oxides are not formed at not more than 540°C even when slow-cooling is further carried out.
In addition, although pickling and/or grinding is generally carried out to remove the scale before coating, and sometimes, equipment for electrolytic degreasing or pickling is also provided for the CGL inlet side, the oxides produced in the surface layer of a steel sheet during or after coiling in the hot-rolling process must remain after the above treatment.
- Zinc and zinc-alloy hot-dip-coating of the present invention is a general term for molten zinc containing zinc and may include not only zinc hot-dip-coating but also galfan and galvalume, in both of which Si is contained in zinc. Moreover, Pb, Mg, Mn, etc. may be further contained. Therefore, conditions for a zinc bath are not particularly restricted.
Other conditions for the coated layer are not particularly limited, however, considering corrosion resistance and the like, the preferred amount of zinc and zinc-alloy coating is approximately 25 to 90 g/mZ and the preferred iron content in a coated layer in an alloyed zinc hot-dip-coated steel sheet is 8 to 13 wt~.
Furthermore, both hot-rolled steel sheets and cold-rolled steel sheets can be used as a material for coating.
Brief Description of Drawings [Figure 1] An EPMA analysis chart of oxides observed just under the scale during!hot-rolling.
[Figure 2] A graph showing the result of elemental analysis of a conventional unannealed cold-rolled steel sheet, which analysis was carried out by GDS from the surface to approximately 10 um in the depth direction.
[Figure 3] A graph showing the result of elemental analysis of an unannealed cold-rolled steel sheet of the present invention, which analysis was carried out by GDS
from the surface to approximately 10 ~m in the depth direction.
[Figure 4] A graph showing the result of elemental analysis of a conventional annealed cold-rolled steel sheet, which analysis was carried out by GDS from the surface to approximately 10 um in the depth direction.
[Figure 5] A graph showing the result of elemental analysis of an annealed cold-rolled steel sheet of the present invention, which analysis was carried out by GDS
from the surface to approximately 10 ~m in the depth direction.
[Figure 6] A cross-sectional optical micrograph, taken at a magnification of x1,000, showing oxides positioned just under the scale of a hot-rolled sheet of an example.
[Figure 7] A cross-sectional optical micrograph, taken at a magnification of x1,000, showing oxides positioned just under the scale of a conventional hot-rolled sheet.
[Figure 8] A cross-sectional optical micrograph, taken at a magnification of x1,000, showing an example alloyed zinc hot-dip-coated steel sheet containing oxides.
[Figure 9] A cross-sectional optical micrograph, taken at a magnification of x1,000, showing a conventional alloyed zinc hot-dip-coated steel sheet not containing oxides.
[Figure 10] A SEM photograph, taken at a magnification of x1,500, showing an example steel sheet whose coated layer has been dissolved.
[Figure 11] A SEM photograph, taken at a magnification of x1,500, showing a conventional steel sheet whose coated layer has been dissolved.
[Reference Numerals]
1, untreated steel-sheet portion; 2, scale; 3, oxides; 4, a coated layer; and 5, oxides.
Best Mode for Carrying Out the Invention An example of the present invention will be shown below:
Each sample shown in Table 1 was melted by a converter and formed into a slab by continuous casting.
Each of the resulting slabs was hot-rolled to 1.2 to 3.5 mm thick at a slab-heating temperature of 1150 to 1200°C, and with a finishing temperature of 900 to 920°C, and a coiling temperature and a cooling rate which are shown in Table 2. After that, the resulting sheets were pickled for 5 to 15 seconds at'80°C in an aqueous 5~ HC1 solution to remove scale layers, and then, divided into two groups one of which was directly subjected to the CGL and the other was cold-rolled into 0.7 mm thick. Furthermore, in the CGL inlet side, the following methods were also used in combination as a pre-treatment for removing the surface layer of a steel sheet, if required.
Electrolytic degreasing: electrolysis at 60°C in an aqueous 3~ NaOH solution for approximately 10 seconds.
Pickling: pickling at 60°C in an aqueous 5~ HCl solution for approximately 3 seconds.
Brushing roll: a'brushing roll with abrasive grains.
In the CGL, both the hot-rolled sheet and the cold-rolled sheet were zinc hot-dip-coated at 470°C after annealing at 800 to 850°C. In addition, alloyed zinc hot-dip-coated steel sheets were obtained by successively subjecting the annealed sheets to an alloying process conducted at 480 to 530°C for 15 to 30 seconds.
o Evaluation method for oxide Observation method for oxides in hot-rolled sheets A cross section of each hot-rolled sheet with the scale was ground and, without being etched, subjected to optical-microscopic observation so as to measure the depth of oxide invasion. The preferred magnification of the optical microscopy was 1,000.
Quantitative evaluation of oxides in hot-rolled sheets The following value was obtained:
(oxygen in steel analyzed in the total sheet-thickness direction of a hot-rolled sheet whose scale has been removed by pickling) - (oxygen in steel of a steel sheet which has been ground to reach the depth of oxide invention in the sheet-thickness direction after removing the scale) - Quantitative evaluation of oxides in coated sheets Each of the coated sheets was immersed in the solutions shown below until the end of the dissolving reaction of coating and then the concentration of the oxide-derived oxygen in a region of from the surface of the steel sheet to 3 ~m in the sheet-thickness direction was calculated according to the following formula:
(oxygen in a steel sheet whose coating was peeled by a hydrochloric acid~antimony method) - (oxygen in a steel sheet whose coating was peeled by a hydrochloric acid~antimony method and whose surface layer was then ground to remove 3 ~m thereof) 1% nital solution 1 vol% HN03-ethanol solution hydrochloric acid~antimony method Sb203 (20 g) + 35%
HCl (1 1) o Evaluation method for bare-spots Each of the coated sheets was evaluated by macroscopic observation.
Bare spots not observed: rank 1 a few were observed: rank 2 a small number were observed:
rank 3 observed: rank 4 o Test for coating-adhesion evaluation Each of the coated sheets was subjected to a Dupont impact test using a 1/2-inch punch and occurrence of peeling was confirmed by macroscopic observation.
Peeling was notobserved: o Peeling was observed: x Each alloyed zinc alloy hot-dip-coated steel sheet was bent to 90°, bent back, and then the compressed side of the steel sheet was peeled by a tape so as to measure the peeled amount of zinc by fluorescent X ray.
Count number of:
less than 500: rank 1 ( good not less than 500 to less than 1,000: rank 2 not less than 1,000 to less than 2,000: rank 3 not less than 2,000 to less than 3,000: rank 4 not less than 3,000 . rank 5 Table 3 shows the results of the zinc hot-dip-coated steel sheet and Table 4 shows those of the alloyed zinc hot-dip-coated steel sheets.
Table 1 Sample-Steel Composition Symbol C S i M n P
wt~ wt~ wt~ wt~
A 0.105 0.010 0.08 0.008 B 0.070 0.10 0.10 0.01 C 0.070 0.50 2.0 0.07 D 0.010 1..50 0.10 0.05 E 0.003 0.003 0.05 0.005 F 0.003 0.01 0.20 0.01 G 0.003 0.30 0.50 0.04 H 0.003 0.05 1.95 0.20 Table 2 Coiling conditions, depth of oxide invasion into hot-rolled sheet, and oxide amount in hot-rolled sheet SampleCT Mean cooling Depth of Oxide amount Sample steel C rate oxide in steel to invasion hot-rolled No.
540C C/min into hot- sheet rolled sheetppm wm A 540 1.0 0 0 1 A 600 1.0 1 1 2 A 600 1.5 0 <1 3 A 700 2.0 7 5 4 B 650 1.5 8 8 5 C 650 1.5 ' 6 7 6 D 580 1.0 0 0 7 D 620 1.2 <1 <1 8 E 650 1.2 5 5 9 E 650 1.6 <1 1 10 E 650 1.8 0 <1 11 F 650 1.0 10 11 12 G 650 1.0 12 15 13 H 600 1.8 0 0 14 H 650 1.0 12 18 15 %
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Industrial availability The technique disclosed by the present invention relates to a zinc hot-dip-coated steel sheet and an alloyed zinc hot-dip-coated steel sheet showing a decreased number of bare spots and excellent coating adhesion, and are appropriately used mainly for steel sheets of automobile bodies.
' 27
Claims (18)
1. A zinc or zinc-alloy hot-dip-coated steel sheet comprising a steel sheet substrate and a zinc or zinc-alloy hot-dip-coated layer on a surface of the steel sheet substrate, in which the steel sheet substrate comprises oxides inside of and near the surface of the steel sheet substrate under the coated layer, the oxides being of elements more oxidizable than iron.
2. The zinc or zinc-alloy hot-dip-coated steel sheet as set forth in Claim 1, wherein the steel sheet substrate in a region from a surface layer of the steel sheet substrate just under the coated layer too 3 µm deep in the sheet-thickness direction has an oxygen concentration of not less than 1 ppm.
3. The zinc and zinc-alloy hot-dip-coated steel sheet as set forth in Claim 1, wherein the steel sheet has further been subjected to heat-alloying.
4. The zinc or zinc-alloy hot-dip-coated steel sheet as set forth in Claim 3, wherein the steel sheet substrate in a region from a surface layer of the steel sheet substrate just under the coated layer to 3 µm deep in the sheet-thickness direction has an oxygen concentration of not less than 1 ppm.
5. The zinc and zinc-alloy hot-dip-coated steel sheet as set forth in any one of Claims 1 to 4, wherein at least one element selected from the group consisting of Si, Mn, and P is contained as a steel component in the following ranges:
0.001 ~ Si ~ 3.0 Wt%, 0.05 ~ Mn ~ 2.0 Wt%, and 0.005 ~ P ~ 0.2 Wt%.
0.001 ~ Si ~ 3.0 Wt%, 0.05 ~ Mn ~ 2.0 Wt%, and 0.005 ~ P ~ 0.2 Wt%.
6. A method for producing a zinc or zinc-alloy hot-dip-coated steel sheet, comprising:
a step A for forming oxides just under a scale, which oxides are of elements more oxidizable than iron, by setting a temperature of a steel strip to not less than 600°C and setting a mean slow-cooling rate up to 540°C to not more than (CT-540)0.9 - 40 (°C/min) during coiling the steel strip hot-rolled; and a step B for zinc and zinc-alloy hot-dip-coating the steel strip.
a step A for forming oxides just under a scale, which oxides are of elements more oxidizable than iron, by setting a temperature of a steel strip to not less than 600°C and setting a mean slow-cooling rate up to 540°C to not more than (CT-540)0.9 - 40 (°C/min) during coiling the steel strip hot-rolled; and a step B for zinc and zinc-alloy hot-dip-coating the steel strip.
7. The method as set forth in Claim 6, wherein the oxides formed in the step A remain after a pre-treatment step carried out after the step A and until annealing treatment is conducted in a furnace immediately before the step B.
8. The method as set forth in Claim 6 or 7, wherein a slab subjected to hot rolling contains at least one element selected from the group consisting of Si, Mn, and P as a steel component in the following ranges:
0.001 ~ Si ~ 3.0 Wt%, 0.05 ~ Mn ~ 2.,0 Wt%, and 0.005 ~ P ~ 0.2 Wt%.
0.001 ~ Si ~ 3.0 Wt%, 0.05 ~ Mn ~ 2.,0 Wt%, and 0.005 ~ P ~ 0.2 Wt%.
9. The method as set forth in Claim 6 or 7, wherein the zinc or zinc-alloy hot-dip-coated steel sheet is treated with heat-alloying after the step B.
10. The method as set forth in Claim 9, wherein a slab subjected to hot rolling contains at least one element selected from the group consisting of Si, Mn, and P as a steel component in the following ranges:
0.001 ~ Si ~ :3.0 Wt%, 0.05 ~ Mn ~ 2.0 Wt%, and 0.005 ~ P ~ 0.2 Wt%.
0.001 ~ Si ~ :3.0 Wt%, 0.05 ~ Mn ~ 2.0 Wt%, and 0.005 ~ P ~ 0.2 Wt%.
11. A zinc or zinc-alloy hot-dip-coated steel sheet comprising:
a steel sheet substrate which contains at least one element that is more readily oxidizable than iron and is selected from the group consisting of Si, Mn and P in an amount effective to incease tensile-strength of the steel sheet, and a zinc or zinc-alloy hot-dip-coated layer formed on a a surface of the steel sheet substrate, wherein the steel sheet substrate comprises an oxide of the element more readily oxidizable than iron inside of the substrate and near the surface on which the coated layer is formed and wherein the steel sheet substrate has an oxygen concentration of 1 to 200 ppm in a region from the surface to 3 µm deep in the sheet-thickness direction.
a steel sheet substrate which contains at least one element that is more readily oxidizable than iron and is selected from the group consisting of Si, Mn and P in an amount effective to incease tensile-strength of the steel sheet, and a zinc or zinc-alloy hot-dip-coated layer formed on a a surface of the steel sheet substrate, wherein the steel sheet substrate comprises an oxide of the element more readily oxidizable than iron inside of the substrate and near the surface on which the coated layer is formed and wherein the steel sheet substrate has an oxygen concentration of 1 to 200 ppm in a region from the surface to 3 µm deep in the sheet-thickness direction.
12. The steel sheet as set forth in claim 11, wherein the steel sheet substrate contains at least one of Si, Mn and P in the following amounts:
0.001 ~ Si ~ 3.0 Wt%, 0.05 ~ Mn ~ 2.0 Wt%, and 0.005 ~ P ~ 0.2 Wt%.
0.001 ~ Si ~ 3.0 Wt%, 0.05 ~ Mn ~ 2.0 Wt%, and 0.005 ~ P ~ 0.2 Wt%.
13. The steel sheet as set forth in claim 12, wherein the steel sheet substrate contains all of Si, Mn and P in the amounts defined in claims 12.
14. A process for producing the steel sheet as defined in claim 11, 12 or 13, which comprises:
(a) subjecting a slab of steel containing the element more readily oxidizable than iron to a hot rolling, to obtain a hot rolled steel sheet;
(b) coiling the hot rolled steel sheet at a high coiling temperature (CT) that is 600°C or more;
(c) cooling the coiled steel sheet at a slow cooling rate to 540°C that is not more than a rate determined by the follwing formula:
(CT-540)0.9 ~ 40 (°C/min), thereby obtaining a steel sheet having a scale on its surface and the oxide of the element more readily oxidizable than iron in a surface layer under the scale;
(d) removing the scale from the cooled steel sheet by pickling or grinding the steel sheet; and (e) subjecting the steel sheet from which the scale has been removed, to a recrystallization annealing at a temperature of from 700 to 900°C under a nitrogen atmosphere in the presence of hydrogen; and (f) subjecting the recrystallization annealed steel sheet to a zinc or zinc-alloy hot-dip-coating, wherein the steps (e) and (f) are combined in a continuous galvanizing line (CGL).
(a) subjecting a slab of steel containing the element more readily oxidizable than iron to a hot rolling, to obtain a hot rolled steel sheet;
(b) coiling the hot rolled steel sheet at a high coiling temperature (CT) that is 600°C or more;
(c) cooling the coiled steel sheet at a slow cooling rate to 540°C that is not more than a rate determined by the follwing formula:
(CT-540)0.9 ~ 40 (°C/min), thereby obtaining a steel sheet having a scale on its surface and the oxide of the element more readily oxidizable than iron in a surface layer under the scale;
(d) removing the scale from the cooled steel sheet by pickling or grinding the steel sheet; and (e) subjecting the steel sheet from which the scale has been removed, to a recrystallization annealing at a temperature of from 700 to 900°C under a nitrogen atmosphere in the presence of hydrogen; and (f) subjecting the recrystallization annealed steel sheet to a zinc or zinc-alloy hot-dip-coating, wherein the steps (e) and (f) are combined in a continuous galvanizing line (CGL).
15. The process as set forth in claim 14, which further comprises:
(g) subjecting the steel sheet to a cold rolling between the steps (d) and (e).
(g) subjecting the steel sheet to a cold rolling between the steps (d) and (e).
16. The process as set forth in claim 14 or 15, wherein the coiling temperature (CT) of the step (b) is from 600 to 700°C.
17. The process as set forth in any one of claims 14 to 16, wherein the cooling rate of the step (c) is from 1.0 to 2.0 °C/min.
18. The process ass set forth in any one of claims 14 to 17, which further comprises:
removing partially the surface layer between the steps (d) and (e), as a pre-treatment at an inlet side of the continuous galvanizing line (CGL).
removing partially the surface layer between the steps (d) and (e), as a pre-treatment at an inlet side of the continuous galvanizing line (CGL).
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| PCT/JP1997/000045 WO1998030729A1 (en) | 1997-01-13 | 1997-01-13 | Hot dip galvanized steel sheet reduced in defects derived from failed plating and excellent in contact plating adhesion and process for producing the same |
Publications (2)
| Publication Number | Publication Date |
|---|---|
| CA2215110A1 CA2215110A1 (en) | 1998-07-13 |
| CA2215110C true CA2215110C (en) | 2001-08-14 |
Family
ID=14179915
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA002215110A Expired - Fee Related CA2215110C (en) | 1997-01-13 | 1997-01-13 | Zinc and zinc-alloy hot-dip-coated steel sheet having decreased bare spots and excellent coating adhesion and a method for manufacturing the same |
Country Status (5)
| Country | Link |
|---|---|
| EP (1) | EP0900857B1 (en) |
| KR (1) | KR100325755B1 (en) |
| CA (1) | CA2215110C (en) |
| DE (1) | DE69728389T2 (en) |
| WO (1) | WO1998030729A1 (en) |
Families Citing this family (6)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| BR0005133B1 (en) * | 1999-02-25 | 2014-11-04 | Jfe Steel Corp | Hot-dip steel sheets and alloy steel sheets and method for their production |
| JP4886118B2 (en) * | 2001-04-25 | 2012-02-29 | 株式会社神戸製鋼所 | Hot-dip galvanized steel sheet |
| CN100540718C (en) * | 2002-03-01 | 2009-09-16 | 杰富意钢铁株式会社 | Surface treated steel plate and manufacture method thereof |
| EP2520686B1 (en) * | 2009-12-29 | 2021-04-07 | Posco | Hot-pressed parts with zinc-plating and a production method for the same |
| JP6694511B2 (en) | 2015-12-23 | 2020-05-13 | ポスコＰｏｓｃｏ | High-strength cold-rolled steel sheet excellent in ductility, hole workability, and surface treatment characteristics, hot-dip galvanized steel sheet, and methods for producing them |
| KR102330604B1 (en) * | 2019-12-03 | 2021-11-24 | 주식회사 포스코 | Zinc plated steel sheet having excellent fatigue strength of electrical resistance spot welds and manufacturing method thereof |
Family Cites Families (2)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| JPH0790529A (en) * | 1993-09-24 | 1995-04-04 | Sumitomo Metal Ind Ltd | Production of galvanized silicon-containing steel sheet and galvannealed steel sheet |
| JP3277063B2 (en) * | 1994-01-25 | 2002-04-22 | 日新製鋼株式会社 | Hot-dip galvanizing method for high-strength hot-rolled steel sheet |
-
1997
- 1997-01-13 WO PCT/JP1997/000045 patent/WO1998030729A1/en active IP Right Grant
- 1997-01-13 EP EP97900126A patent/EP0900857B1/en not_active Expired - Lifetime
- 1997-01-13 CA CA002215110A patent/CA2215110C/en not_active Expired - Fee Related
- 1997-01-13 KR KR1019970706334A patent/KR100325755B1/en not_active IP Right Cessation
- 1997-01-13 DE DE69728389T patent/DE69728389T2/en not_active Expired - Lifetime
Also Published As
| Publication number | Publication date |
|---|---|
| EP0900857A1 (en) | 1999-03-10 |
| WO1998030729A1 (en) | 1998-07-16 |
| DE69728389T2 (en) | 2005-02-24 |
| KR19980702926A (en) | 1998-09-05 |
| DE69728389D1 (en) | 2004-05-06 |
| EP0900857A4 (en) | 2000-08-02 |
| EP0900857B1 (en) | 2004-03-31 |
| CA2215110A1 (en) | 1998-07-13 |
| KR100325755B1 (en) | 2002-07-18 |
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| EEER | Examination request | ||
| MKLA | Lapsed |
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