JP4517687B2 - Method for producing galvanized steel sheet - Google Patents

Description

  The present invention relates to a method for producing a galvanized steel sheet having reduced color tone variation and excellent surface appearance.

  In steel products, various performances are imparted to products by applying a film to the surface. For example, galvanized steel sheets are increasingly demanding due to their excellent corrosion resistance, but their press formability is inferior to cold-rolled steel sheets. It is known that performance such as press formability is improved by applying a film containing an oxide (hereinafter referred to as oxide film) to the surface of a zinc-based plated steel sheet.

  For example, Patent Document 1 and Patent Document 2 describe weldability by forming an oxide film mainly composed of ZnO by performing electrolytic treatment, immersion treatment, coating oxidation treatment, or heat treatment on the surface of a zinc-based plated steel sheet. Or the technique which improves workability is disclosed.

  Patent Document 3 discloses that a plated steel sheet is immersed in an aqueous solution containing 5 to 60 g / l of sodium phosphate and having a pH of 2 to 6 on the surface of a zinc-based plated steel sheet, or by electrolytic treatment or spraying the aqueous solution. A technique for improving the press moldability and chemical conversion processability by forming an oxide film mainly composed of an oxide is disclosed.

  Patent Document 4 discloses a technique for improving press formability and chemical conversion treatment by forming Ni oxide on the surface of a zinc-based plated steel sheet by electrolytic treatment, immersion treatment, coating treatment, coating oxidation treatment, or heat treatment. Is disclosed.

In Patent Document 5, the adhesion amount of the Fe—Ni—O-based film on the surface of the zinc-based plated steel sheet is in the range of 10 to 1500 mg / m ^{2 in} terms of the total amount of metal elements in the film, By forming an Fe-Ni-O-based film in which the oxygen content of the Fe-Ni-O-based film is in the range of 0.5 to less than 30 wt%, the press formability is excellent, and further if necessary The technique which can improve spot-weldability, adhesiveness, and chemical conversion property suitably is disclosed.

The prior art document information is described below.
JP-A-53-60332 Japanese Patent Laid-Open No. 2-190483 JP-A-4-88196 Japanese Patent Laid-Open No. 3-191093 JP-A-8-158066

  The above-described zinc-based plated steel sheet having an oxide-based film on the plating surface exhibits a black-gray appearance, and the color tone varies because the film thickness, the processing conditions for forming the film, and the like vary. For example, when the oxide film becomes thick, the color tone becomes black, and when it becomes thin, it becomes white. When the color variation is large, there are the following problems. That is, when used for automobile parts, if the color tone is bright, spatters at the time of welding tend to be conspicuous, and if it is dark, it is difficult to find a press flaw. Therefore, it is necessary to be able to reduce color tone fluctuations.

  In addition, the design appearance may be recognized in the black-gray appearance itself, and it is advantageous from the aspect of the design nature that the color tone fluctuation is small.

  The color tone can be adjusted by adjusting processing conditions such as the concentration and pH of the processing liquid. However, since the adjustment of the processing conditions is inferior in responsiveness, there is no immediate effect of the color tone adjustment. In addition, since the change in the processing conditions may change the film performance, the allowable adjustment range of the processing conditions is limited to a narrow range, and the effect of adjusting the color tone becomes insufficient.

  An object of the present invention is to provide a method for producing a galvanized steel sheet which improves such problems, reduces color tone fluctuations in appearance, has excellent color tone stability, and has excellent surface appearance.

  The present inventor has made various studies to reduce the color tone variation of the appearance of a zinc-based plated steel sheet having an oxide-based film on the surface. As a result, temper rolling is performed before forming the oxide film, and at that time, by adjusting the rolling force (milling force) of the rolling roll, the color tone of the appearance after the oxide film is formed (lightness) ) Can be easily adjusted. The present invention is based on this finding.

  The gist of the present invention for solving the above problems is as follows.

1st invention includes the rolling process which temper-rolls the plated steel plate which has a zinc-type plating layer on the surface, and the film formation process which forms a Fe-Ni-O type film on the surface of the rolled said plated steel plate In the method for producing a zinc-based plated steel sheet,
In advance, the rolling process is performed so that the relationship between the mill rolling force of the rolling process and the color tone of the galvanized steel sheet on which the Fe—Ni—O-based film is formed is obtained, and the color tone is set in advance using the relationship. It is a manufacturing method of the galvanized steel sheet characterized by adjusting the mill rolling force.

A second invention is the method for producing a zinc-based plated steel sheet according to the first invention, wherein the zinc-based plated steel sheet is a hot-dip galvanized steel sheet.

A third invention is the method for producing a zinc-based plated steel sheet according to the second invention, wherein the hot-dip galvanized steel sheet is an alloyed hot-dip galvanized steel sheet.

  According to the present invention, when producing a zinc-based plated steel sheet having an oxide-based film on the surface, the color tone of the appearance is reduced by quickly adjusting the color tone, and the zinc-based plated steel sheet having an excellent surface appearance is obtained. can get.

  The embodiment of the present invention will be described by taking as an example the case of forming an oxide-based film made of an Fe—Ni—O-based film described in Patent Document 5 on the surface of an alloyed hot-dip galvanized steel sheet.

  The alloyed hot-dip galvanized steel sheet having an oxide-based film made of the Fe-Ni-O-based film is manufactured as follows. An original plating plate (a cold-rolled steel plate or a steel sheet descaled after hot rolling) manufactured by a conventional method is placed in a continuous hot-dip galvanizing line, annealed by a conventional method, and then immersed in a plating bath. Passing and plating, lifting from the plating bath to adjust the coating amount, and then heating in an alloying furnace to make the galvanized layer a zinc-iron alloy layer. Next, temper rolling is performed (rolling process), and then a required Fe—Ni—O-based film is formed on the plating surface (film forming process). The Fe—Ni—O-based film can be formed by the method described in Patent Document 5. For example, while performing cathodic electrolysis in a mixed solution of iron sulfate and nickel sulfate containing an oxidizer, the concentration of the oxidizer is adjusted to adjust the oxygen content of the film, and the required Fe-Ni-O system A film is formed. Alternatively, an aqueous solution containing nickel chloride and iron chloride is sprayed and dried while adjusting the oxygen content of the film in a mixed atmosphere of air and ozone to form a required Fe—Ni—O-based film.

  It is well known to perform temper rolling when producing an galvannealed steel sheet. In the conventional temper rolling method, the longitudinal tension of the mill is controlled so as to obtain a predetermined elongation rate with a constant mill rolling force. On the other hand, in the present invention, the mill rolling force is adjusted according to the color tone, and the longitudinal tension of the mill is controlled so as to obtain a predetermined elongation rate. Thereby, the color tone fluctuation of the appearance of the galvannealed steel sheet having the Fe—Ni—O-based film is reduced.

  The effect | action which reduces the color tone fluctuation | variation of the external appearance of the galvannealed steel plate which has an Fe-Ni-O type film in this invention is demonstrated.

  FIG. 1 shows a process of temper rolling in a manufacturing process of an galvannealed steel sheet that forms a Fe—Ni—O-based film after sequentially performing hot dip galvanization, alloying treatment of a plating layer, and temper rolling. It is a figure which shows the relationship between the variation | change_quantity of mill reduction amount, and the color change (brightness change) of the galvannealed steel plate surface after Fe-Ni-O type | system | group film formation when mill rolling force is changed.

The horizontal axis represents the amount of increase in the mill rolling force relative to the reference mill rolling force (Ps). Since ΔP = 0 corresponds to the reference mill reduction force (Ps), the mill reduction force is the reference mill reduction force Ps + ΔP. Therefore, it can be considered that the horizontal axis indicates the mill reduction force.
The vertical axis represents the lightness change (Ls) on the surface of the alloyed hot-dip galvanized steel sheet after the formation of the Fe—Ni—O-based film at the standard mill rolling force (Ps), and the lightness change relative thereto. However, lightness change (ΔL) = lightness L (Ps) at the reference mill rolling force (Ps) −lightness L (Ps + ΔP) at the mill rolling force (Ps + ΔP). Since ΔL = 0 corresponds to the standard brightness, the brightness is the standard brightness Ls + ΔL. Therefore, it can be considered that the vertical axis indicates lightness.

  If the horizontal axis represents the mill rolling force and the vertical axis represents the lightness, FIG. 1 is also a diagram showing the correspondence between the mill rolling force and the lightness. The corresponding mill rolling force and corresponding brightness are shown on the upper side (horizontal axis) and the right side (vertical axis) of FIG.

  In the vicinity of the reference mill rolling force, that is, in the vicinity of ΔP = 0, there is almost no change in brightness even when the mill rolling force is increased, but the increase amount of the mill rolling force is greater than a certain amount (that is, there is a mill rolling force). When the mill rolling force is increased, the lightness is significantly reduced correspondingly. In a region where the lightness decreases in response to an increase in the mill rolling force, for example, the region A in FIG. 1, the lightness can be adjusted by adjusting the mill rolling force. The present invention utilizes this action.

  In the present invention, the mill rolling force in the temper rolling process is set to a mill rolling force selected from a region having a large change in brightness corresponding to the mill rolling force change, for example, within the range of region A in FIG. When the temper rolling is performed with the mill rolling force set as described above, the color tone (brightness) after forming the Fe-Ni-O-based film is a value within a predetermined brightness target range, for example, an intermediate value of the brightness target range and The standard conditions for the film formation process are set so that

  Conditioning rolling and film formation treatment are performed under the above conditions to form an Fe—Zn—O-based film, and the color tone (lightness) is measured after the film is formed. The measured value of the color tone (lightness) is L1 with respect to the intermediate value L0 of the target range of the color tone (lightness), and the color tone difference (lightness difference) between them is ΔLm (= L0−L1). Further, it is assumed that the set value of the mill rolling force for temper rolling at that time is P1.

  Using the relationship shown in FIG. 1, a change amount ΔPm of the mill rolling force that can be zeroed by eliminating the color difference ΔLm is obtained, and based on the obtained ΔPm, the setting value of the mill rolling force is changed from P1 to P1−ΔPm. Change to By doing in this way, mill rolling force can be adjusted so that the color tone after Fe-Zn-O type | system | group film formation may approach the intermediate value L0 of a target range range, and color tone fluctuation | variation can be reduced as a result. Since the mill rolling force adjustment has good responsiveness, color tone fluctuation can be effectively reduced.

  In region A in FIG. 1, there is a good correspondence between the amount of increase in mill rolling force and the amount of decrease in brightness. The correspondence between the two may be approximated by a straight line. When approximated by a straight line, the slope α of the straight line representing the relationship between the two is about −0.065. Based on the color tone difference (lightness difference) ΔLm determined above and the gradient α, the amount of change ΔPm in the mill rolling force that can be eliminated by eliminating the color tone difference ΔLm is obtained by calculating ΔPm = ΔLm / α. Based on ΔPm, the set value of the mill rolling force may be changed from P1 to P1−ΔPm.

  The reason why the appearance color tone (brightness) after the oxide film formation is changed by changing the mill rolling force in the temper rolling process is estimated as follows.

  The alloyed hot-dip galvanized steel sheet is a film having irregularities as a result of the plating film being formed into an Fe—Zn alloy. When temper rolling is performed, the convex portion of the plating film is flattened by the rolling roll, and the crushed portion is activated compared to the uncrushed portion. An oxide-based film is formed on the plating surface in the film forming step performed after temper rolling. In the film formation step, the amount of film formation is greater in the flat portion where the surface is activated. The color tone becomes black at the portion where the coating amount is large.

  In the temper rolling process, when the mill rolling force is high, as shown in FIG. 2 (a), the area of the smooth portion of the plated layer that is flattened by the rolling roll increases, and the smooth portion is oxide-based. The amount of coating increases. That is, it is estimated that the surface color tone becomes black as the area of the black color tone portion increases. On the other hand, when the mill rolling force is lowered, as shown in FIG. 2 (b), the area of the smooth portion of the plating layer that is flattened by the rolling roll is high in the mill rolling force. Less than the case. That is, when the mill rolling force is lowered, it is estimated that the amount of oxide-based film adhesion is larger than in the case of (a) where the mill rolling force is high, and the surface color tone becomes white by reducing the area of the black tone portion. To do.

  As described above, the case of forming the Fe—Ni—O-based film on the surface of the galvannealed steel sheet has been described as an example, but the present invention is not limited to the above. The plating type may be a hot dip galvanized steel sheet such as a galvanized steel sheet in which the galvanized film is not alloyed, or a galvanized steel sheet such as an electrogalvanized steel sheet. The oxide film type is not limited to the Fe—Ni—O film. Since the appearance color tone (brightness) may be different if the plating type and oxide film type are different, the correspondence shown in FIG. 1 is obtained for each plating type and oxide film type, and the obtained relationship is The mill rolling force may be adjusted according to the color tone.

(Example of the present invention)
Cold rolled steel sheets with various dimensions of thickness 0.6 to 1.6 mm x width 800 to 1880 mm are placed in a continuous hot dip galvanizing line, annealed, galvanized, adjusted for plating amount, heated and plated The layer is formed into an Fe—Zn alloy, subjected to temper rolling with a temper rolling mill after cooling, and then Fe—Ni—O having a target adhesion amount of 400 mg / m ² and a color tone target range of 40 to 45 in an electrolytic treatment facility. A system film was formed.

As the electrolytic treatment solution, the following treatment solution was used as a reference bath, and the current density was 40 A / dm ² .
Electrolytic treatment solution:
NiSO ₄ .6H ₂ O: 250 g / l,
FeSO ₄ .7H ₂ O / (NiSO ₄ .6H ₂ O + FeSO ₄ .7H ₂ O): 0.12
Zn ²⁺ ion / (Ni ²⁺ ion + Fe ²⁺ ion): 0.5 or less NaNO ₃ : 1.9 g / l
pH: 2.0.

  Measure the color tone of the alloyed hot-dip galvanized steel sheet after film formation treatment for each charging coil, and based on the measurement results, use the correspondence between mill rolling force and brightness shown in Fig. 1 to measure the color tone value and color tone. The mill rolling force of temper rolling was adjusted to eliminate the deviation from the intermediate value of the target range. As a result, the color tone fluctuation range was suppressed to 5 or less, and none of them exceeded the target range of 40 to 45.

(Comparative example)
In the same manner as in the present invention example, an Fe-Ni-O-based film is formed, and the color tone of the galvannealed steel sheet after film formation treatment is measured for each charging coil. Based on the measurement result, the color tone measurement value and The concentration of the treatment liquid was adjusted so as to eliminate the deviation from the intermediate value of the color tone target range. As a result, the color tone fluctuation range was 10, and it was difficult to suppress the color tone fluctuation within the target range of 40 to 45.

  INDUSTRIAL APPLICABILITY The present invention can be used as a method for producing a zinc-based plated steel sheet that has an excellent surface appearance and has an oxide film (a film containing an oxide) on the plating surface.

It is a figure which shows the relationship between the variation | change_quantity of the mill rolling force of a temper rolling process, and the color tone change of the galvannealed steel plate after oxide-type film formation on the surface. FIG. 4 is a schematic diagram for explaining the action of changing the color tone of the galvannealed steel sheet after the oxide film is formed when the mill rolling force in the temper rolling process is changed. FIG. b) is a schematic diagram illustrating a case where the mill rolling force is low.

Explanation of symbols

1 Plating layer 2 Oxide film

Claims (3)

Manufacture of a zinc-based plated steel sheet comprising: a rolling process for temper rolling a plated steel sheet having a zinc-based plating layer on the surface; and a film forming process for forming a Fe—Ni—O-based film on the surface of the rolled plated steel sheet In the method
In advance, the rolling process is performed so that the relationship between the mill rolling force of the rolling process and the color tone of the galvanized steel sheet on which the Fe—Ni—O-based film is formed is obtained, and the color tone is set in advance using the relationship. A method for producing a galvanized steel sheet, comprising adjusting a mill rolling force of the steel sheet. The method for producing a zinc-based plated steel sheet according to claim 1, wherein the zinc-based plated steel sheet is a hot-dip galvanized steel sheet. The said hot dip galvanized steel plate is an alloyed hot dip galvanized steel plate, The manufacturing method of the galvanized steel plate of Claim 2 characterized by the above-mentioned.

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