JPH05239605A - Galvanizing method for high tensile strength steel sheet - Google Patents

Abstract

PURPOSE:To suppress the generation of a nonplating phemenon by forming an oxidized film of iron on the surface of a high tensile strength steel having high. oxidizability, annealing it in a hydrogen-contg. atmosphere and executing galvanizing. CONSTITUTION:A high tensile strength steel sheet contg. elements (such as Si and Mn) having high oxidizability is subjected to anodic oxidation in an electrolyte contg., by weight, about >=0.1% organic acid (such as oxalic acid and phthalic acid) to form an oxidized film of iron and/or a hydrous oxide film on the surface of the steel sheet by about 0.01 to 20g/m<2>. At this time, the electrolyte is preferably mixed with hydrogen peroxide, potassium dichromate or the like, and the pH of the electrolyte is regulated about <=3. Next, this steel sheet is annealed in an atmosphere of an Ar gas contg. about 3 to 25% hydrogen, is sufficiently reduced and is thereafter galvanized. In this way, the hot dip coating of the high tensile strength steel sheet free from nonplating can be obtd.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION The present invention relates to a method for producing hot dip galvanized and alloyed hot dip galvanized steel sheets used for automobiles, home appliances and building materials, particularly hot dip galvanized high alloy steel sheets and hot galvannealed galvanized sheets. The present invention relates to a manufacturing method for plating.

[0002]

2. Description of the Related Art In recent years, surface-treated steel sheets having high corrosion resistance have been required in fields such as automobiles and home appliances, and hot-dip galvanized steel sheets, alloyed hot-dip galvanized steel sheets, galvanized galvanized steel sheets, and Zn-Ni plated steel sheets. Alloy electroplated steel sheets such as are being developed and put into practical use. Among them, hot-dip galvanized steel sheets (hereinafter abbreviated as GI), alloyed hot-dip galvanized steel sheets (hereinafter abbreviated as GA), and the like have a lower manufacturing cost than that of galvanized galvanized steel sheets and have good manufacturing costs. Since it has corrosion resistance, it is currently used as an anti-rust steel plate for automobiles not only as an inner plate but also as an outer plate.

Recently, reduction of vehicle exhaust gas amount has been taken up as an important issue in view of global environmental problems, and vehicle manufacturers have been obliged to reduce the weight of vehicle bodies.
Against this background, since gauge reduction of steel sheets is effective for reducing the weight of automobile bodies, material manufacturers are strongly required to supply high-tensile steel sheets. Of Si, P, Mn, Ti, Nb, and A as elements that enhance the strength of the steel sheet without impairing the formability of
Research and development of high-tensile steel plates to which 1, Cr, Ni, Cu, Mo, V, B and the like are added have been conducted. In addition, since steel sheets have been conventionally required to have rust-preventive properties, development of high-strength steel sheets plated with zinc-based plating, especially hot-dip zinc-based plating with low manufacturing cost, is strongly desired by automobile manufacturers. ing.

However, the reinforcing element Si in the above steel,
P, Mn, Ti, Nb, Al, Cr, Ni, Cu, M
Since o, V, and B are easily oxidized and difficult to be reduced, in a Sendzimir type production line, which is currently a typical continuous production line for hot dip plating, the essence is that these strengthening elements are selectively oxidized during annealing and the surface is concentrated. Problems arise. In this case, Si concentrated on the steel plate surface during annealing,
Oxidation of strengthening elements such as Mn significantly reduces the wettability between the steel sheet and hot dip zinc, so the adhesion of hot dip galvanically decreases, and in extreme cases, hot dip zinc does not adhere to the steel sheet at all. Such a phenomenon occurs. Also, G produced by hot dip coating followed by alloying treatment
In the case of A, the alloying is remarkably delayed by the oxide of the strengthening element generated during annealing, and the alloying process cannot be performed unless the alloying temperature is extremely raised.

When hot-dip galvanizing or alloying hot-dip galvanizing is applied to such a hard-to-plate material, it is attempted to solve the above-mentioned problems by pre-treating the surface of the steel sheet in advance in order to prevent non-plating and proper alloying. A method of doing so is disclosed. For example, JP-A-55-131165 discloses a method of applying Ni plating before hot dip galvanizing, and JP-A-57-70268 and 57-79160 discloses Fe plating. Is disclosed.

As a method other than electroplating, there is a method of forming an oxide film having a film thickness of 400 to 10,000Å in an oxidation-free furnace system and then annealing in an atmosphere containing hydrogen.
It is disclosed in JP-A-55-122865.

[0007]

However, the above electroplating method requires a large-scale facility because the amount of pre-plating required to prevent non-plating is at least 7 to 10 g / m ² . In addition, there is a problem that the manufacturing cost becomes high, so it is hard to say that it is a preferable method for application to a hot dip plating line.

A method for forming an oxide film in an oxidation-free furnace system before annealing in a hydrogen-containing atmosphere is as follows.
When processing in-line, line speed, atmosphere,
Since the amount of oxide film produced varies depending on the steel plate temperature, the type and amount of constituent elements in the steel plate, it is difficult to obtain a predetermined amount of film in a stable manner, and the non-plating suppression effect in the actual production line is also unstable. Tends to become.

The object of the present invention is to contain one or more elements such as Si, P, Mn, Ti, Nb, Al, Cr and B, which are more oxidizable than iron, and, if necessary, Ni, Cu,
It is an object of the present invention to provide a method for stably suppressing non-plating at low cost when hot dip galvanizing or alloying hot dip galvanizing is performed on a steel sheet containing Mo, V and other elements in a continuous line.

[0010]

The present invention provides a method for hot-dip galvanizing a high-strength steel sheet containing an element that is highly oxidizable, in which the steel sheet surface is formed by anodizing in an electrolytic solution containing an organic acid. The present invention provides a hot dip galvanizing method for a high-strength steel sheet, which comprises forming an iron oxide film and / or a hydrated oxide film, annealing in an atmosphere containing hydrogen, and then hot dip galvanizing. There is also provided a method of performing an alloying treatment after this and performing galvannealing. At this time, the electrolytic solution containing the organic acid contains 0.1 wt% of the organic acid.
% Or more, and at least one oxidizing agent selected from hydrogen peroxide, potassium dichromate, and potassium permanganate is preferable.

The present invention will be described in more detail below. The high-strength steel sheet described in the present invention is a strengthening element Si, P, Mn, Ti, which can increase the steel sheet strength without impairing the formability of low carbon steel or ultra low carbon steel used in automobiles.
A steel sheet containing at least one of alloying elements such as Nb, Al, Cr and B, and if necessary, Ni,
It may be one in which components such as Cu, Mo and V are appropriately contained. It should be noted that the inclusion in the present invention means that Si, T
i, Ni, Cu, Mo, Cr and V are 0.1 wt% or more,
Mn is 0.5 wt% or more, P, Al and Nb are 0.05 w
t% or more and B means 0.001 wt% or more.

Of these elements, Si is the element that most affects non-plating, and its content in steel is 0.1 wt.
%, The deterioration of the plating property becomes remarkable. Therefore,
The method for suppressing non-plating by the anodizing method in the electrolytic solution containing the organic acid disclosed in the present invention is particularly preferably 0.1 w as described above.
It acts extremely effectively on a steel sheet containing at least t% Si.

When hot dip galvanizing is applied to a steel sheet containing the above-mentioned elements, non-plating occurs due to the oxides of the elements concentrated on the surface. In the present invention, as a result of various studies of hot dipping pretreatment methods for suppressing the surface concentration of each of these highly oxidizable elements, an electrochemically treated oxide film and / or hydrated oxide film, particularly an organic acid It was found that the film anodized in the contained electrolyte has the effect of suppressing the surface enrichment of elements in the steel during annealing.

The organic acid in the electrolytic solution in the present invention is not particularly limited, but oxalic acid, phthalic acid, citric acid and the like are practical because they are inexpensive. In this case, as long as the total concentration of the organic acids in the electrolytic solution is 0.1 wt% or more, a sufficient non-plating suppressing effect is recognized regardless of which organic acid is used. In addition to the organic acid, the electrolytic solution contains one or more oxidizing agents such as hydrogen peroxide, potassium permanganate, and potassium dichromate as an oxidizing agent, whereby the effect of suppressing non-plating is enhanced. In addition, the method disclosed in the present invention is particularly effective when the electrolytic solution is acidic. Therefore, it is desirable to add sulfuric acid, hydrochloric acid or the like to the electrolytic solution so that the pH of the electrolytic solution is 3 or less. .. The liquid temperature of the electrolytic solution is preferably from room temperature to 80 ° C, and industrially preferably 40 to 60 ° C.

The amount of iron oxide film and / or hydrated oxide film formed on the steel sheet by the above method is as follows.
As an oxide film and / or a hydrated oxide film, 0.01 to
It is preferably 20 g / m ² . If the amount of the oxide film or hydrated oxide film deposited is less than 0.01 g / m ² , sufficient non-plating suppression effect cannot be obtained, and if it exceeds 20 g / m ^2, the annealing process in the hot dip coating production line Even if the reducing power of the atmosphere is increased, the oxide film or the hydrated oxide film cannot be fully reduced, rather causing non-plating. Therefore, it is desirable to control the oxide film and / or the hydrated oxide film in the present invention within the above range.

Further, since the oxide film and / or the hydrated oxide film in the present invention is formed in an electrolytic solution containing an organic acid, carbon is necessarily contained in the film. It was clarified that has a great influence on the surface thickening and suppression of non-plating of the strengthening elements in steel during annealing. Even when the same amount of iron oxide film is produced by anodizing treatment, the electrolytic solution contains an organic acid and the oxide film contains carbon, and the electrolytic solution contains an organic acid. There is a large difference in the effect of suppressing surface concentration and the effect of suppressing non-plating when the oxide film does not contain carbon without it, and the former is more advantageous in suppressing non-plating. The carbon content in the oxide film or the hydrated oxide film is preferably 0.01 wt% or more, but is obtained by electrolysis in an electrolytic solution containing 0.1 wt% or more of the organic acid as described above. The oxide film and / or the hydrated oxide film necessarily contains 0.01 wt% or more of carbon.

The amount of electricity supplied during anodic oxidation is advantageously 1 C / dm ² or more. If the amount of electricity is less than this value, in order to obtain the amount of oxide film or hydrated oxide film deposited as described above, if the upper limit of the electrolytic current density is limited by the rectifier capacity, the line speed will be reduced and productivity will be reduced. To do.

The present invention does not particularly limit the current density during the electrolytic treatment, but from the viewpoint of productivity and electrolytic treatment equipment, the anodizing treatment is performed at a current density in the range of 1 to 100 A / dm ^2. Is desirable.

The oxide film and / or the hydrated oxide film thus formed on the steel sheet need to be reduced in the annealing step in the continuous hot dip coating line. Any condition that can be reduced may be used. As the atmosphere gas, hydrogen alone or a mixed gas of hydrogen and nitrogen, argon or the like can be used, but 3 to 25% hydrogen gas is industrially practical.
The annealing temperature varies depending on the steel type, but in the case of cold rolled steel sheet, it is 70
Desirably, the annealing time is 0 ° C. or higher and the annealing time is 10 sec or longer.

The method disclosed in the present invention makes it possible to obtain hot-dip galvanizing of a high-strength steel sheet material without unplating, but the hot-dip galvanized steel sheet subjected to this pretreatment is 450 to 550.
It becomes possible to easily perform alloying treatment in a temperature range of about ℃, and it is possible to obtain an alloyed hot-dip galvanized steel sheet of a high-strength steel sheet material. Si, which significantly delays the alloying rate,
Steel sheets containing component elements such as P and Mn are difficult to alloy in the temperature range of 550 ° C. or lower, and the alloying occurs near 600 ° C., so that the adhesiveness deteriorates. Since the alloying temperature can be reduced by anodizing in an electrolytic solution containing an acid, it becomes possible to obtain an alloyed hot-dip galvanized steel sheet of a high-strength steel sheet material having good adhesion. The alloying temperature varies depending on the coating amount, line speed, etc., but it is desirable that the alloying temperature be as low as possible in order to obtain a GA having good adhesion.

[0021]

EXAMPLES The present invention will be specifically described below with reference to examples. (Example) C: 0.002 wt%, Si: 1.0 wt%, M
Steel containing n: 3.0 wt% and P: 0.15 wt% was melted and subjected to hot rolling and cold rolling according to a conventional method to produce a steel plate having a plate thickness of 0.7 mm. After degreasing and pickling treatment on this cold rolled steel sheet, an anodizing treatment using the steel sheet as an anode and Pb as a cathode in each electrolytic bath shown in Table 1 is performed to form an oxide film or a hydrated oxide film on the steel sheet. Let This steel sheet is analyzed for oxygen and carbon, and the difference from the untreated steel sheet and
The amount of the oxide film or the hydrated oxide film attached and the carbon content in the oxide film or the hydrated oxide film were determined from the form of the oxide or hydrated oxide identified by thin film X-ray diffraction. The above steel plates were treated under the following (A) annealing conditions, (B) hot dip plating conditions, and (C) alloying conditions, and these steel plates were designated as invention examples 1 to 5. The annealing and hot dip galvanizing are performed by a hot dip simulator, and the alloying treatment is performed by an infrared heating furnace.
Each was done like a laboratory.

As comparative examples, a steel sheet having the same composition as that used in the above-mentioned invention example but not anodized was annealed, hot-dip galvanized, alloyed (Comparative Example 1) and more oxidized than iron. Low content of strong elements C: 0.002 w
t%, Si: 0.002 wt%, Mn: 0.2 wt%, P:
An example (Comparative Example 2) in which a steel sheet having a chemical composition of 0.01 wt% was annealed, hot dip galvanized, and alloyed was shown.

The following evaluations were performed on these. (A) Annealing condition Temperature rising rate: 10 ° C / sec Holding temperature: 850 ° C Holding time: 30sec Temperature falling rate: 20 ° C / sec Atmosphere in annealing furnace: 5% H ₂ -N ₂ (dew point -20 ° C) (B) Hot-dip plating conditions Bath temperature: 470 ° C Infiltration plate temperature: 470 ° C Al content rate: 0.15wt% Adhesion amount: 60g / m ² (one side) Plating time: 1sec (C) Alloying treatment conditions Temperature rising rate: 20 ° C / sec Cooling rate: 15 ℃ / sec Alloying temperature: 490 ℃ Alloying time: 30sec

(Evaluation method of plating property) The plating property was evaluated according to the following criteria by visual inspection of appearance after hot dip galvanizing. ○ No non-plating × Non-plating occurred

(Evaluation of Plating Adhesion) DuPont impact test (diameter ¼ inch, weight of 1 kgf was dropped on a steel plate from a height of 50 cm, and evaluated by the following criteria: ○ No peeling of plating × Plating With peeling

(Evaluation of alloying rate) The alloying rate was evaluated based on whether the zinc η phase remained on the surface of the alloyed material treated under the above conditions. ○ Without zinc η phase × With zinc η phase

Table 1 shows the evaluation results of Invention Examples 1 to 5 and Comparative Examples 1 and 2. As is clear from this, by the method disclosed in the present invention, even in a steel sheet containing an element that is more oxidizable than iron, it becomes possible to produce a hot-dip galvanized steel sheet with excellent non-plating adhesion. Further, it was shown that the alloying rate was moderately promoted and the alloyed hot-dip galvanized steel sheet could be obtained by the same method as the conventional method.

[0028]

[Table 1]

[0029]

When hot-dip galvanizing a high-strength steel sheet, the present invention makes it possible to inexpensively and stably produce hot-dip galvanizing free from non-plating and having excellent adhesion. Moreover, when performing an alloying process, a galvannealed steel sheet can be obtained at a relatively low temperature. Due to the urgent need to reduce the weight of automobiles, the development of hot-dip galvanized steel sheets and alloyed hot-dip galvanized steel sheets, which are materials for high-strength steel sheets, has been desired in recent years.

Claims (2)

[Claims] 1. A method of hot dip galvanizing a high-strength steel sheet containing an element that is highly oxidizable, in which an iron oxide film and / or water is formed on the surface of the steel sheet by anodizing in an electrolytic solution containing an organic acid. A hot dip galvanizing method for a high-strength steel sheet, which comprises forming a Japanese oxide film, annealing in an atmosphere containing hydrogen, and then hot dip galvanizing. 2. A method for hot dip galvannealing of a high-strength steel sheet, which comprises subjecting a steel sheet to hot dip galvanizing according to the method of claim 1 and then subjecting it to heat alloying treatment.