JPH05239606A - Galvanizing method for high tensile strength steel sheet - Google Patents

Abstract

PURPOSE:To provide the method for stably suppressing nonplating at a low cost at the time of applying galvanizing or galvannealing to a steel sheet contg. one or more kinds of elements among Si, P, Mn, Ti, Nb, Al, Cr, B or the like having high oxidizability and furthermore contg., at need, Ni, Cu, Mo, V or the other elements in a continuous line. CONSTITUTION:This is the galvanizing method for a high tensile strength steel sheet in which, at the time of galvanizing a high tensile strength steel sheet contg. elements having high oxidizability, a steel sheet is brought into contact with an acidic soln. contg. one or more kinds of oxidizing agents selected from hydrogen peroxide, potassium dichromate and potassium permanganate, is thereafter dried, is annealed in an atmosphere contg. hydrogen and is subsequently galvanized. After that, allaying treatment is executed. At this time, the concn. of the oxidizing agent in the soln. to be brought into contact with the steel sheet is regulated to >=0.1wt.%, and its pH is regulated to <=3.

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, B, Mo, V 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, B,
Since Mo and V are easily oxidized and difficult to be reduced, in the Sendzimir type production line, which is a typical continuous production line for hot dip plating, these strengthening elements are selectively oxidized during annealing and the surface is concentrated. The problem arises. In this case, Si, M concentrated on the steel plate surface during annealing
Since the wettability between the steel sheet and the hot-dip zinc is significantly reduced by the oxide of a strengthening element such as n, the adhesion of the hot-dip galvanization is significantly reduced, and in an extreme case, the hot-dip zinc does not adhere to the steel sheet at all. Such a phenomenon occurs. Also,
In the case of GA, which is manufactured by hot dip galvanizing and subsequent alloying treatment, there is a problem that the alloying treatment is significantly delayed by the oxide of the strengthening element generated during annealing and the alloying treatment cannot be performed unless the alloying temperature is extremely raised. Occurring in an emergency.

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, when hot dip galvanizing a high-strength steel sheet containing an element that is highly oxidizable, is selected from hydrogen peroxide, potassium dichromate, and potassium permanganate. Hot-dip galvanizing method for high-strength steel sheet, which comprises contacting the steel sheet with an acidic solution containing one or more oxidants, drying it, annealing it in an atmosphere containing hydrogen, and then hot-dip galvanizing. Is provided. There is also provided a method of performing an alloying treatment after this and performing galvannealing. At this time, the concentration of the oxidant in the solution to be brought into contact with the steel sheet is 0.1 wt% or more and the pH of the solution is
Is preferably 3 or less.

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 inclusion in the present invention means Si, Ti,
Ni, Cu, Mo, V, Cr is 0.1 wt% or more, Mn is 0.5 wt% or more, P, Al, Nb is 0.05 wt% or more,
B refers to the case of containing 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 of suppressing non-plating by dipping in an acidic solution containing an oxidizing agent disclosed in the present invention works extremely effectively particularly on a steel sheet containing 0.1 wt% or more of Si as described above.

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 on the hot dipping pretreatment method for suppressing the surface concentration of each of these highly oxidizable elements, when the steel sheet was dried by contacting it with an acidic solution containing an oxidizing agent such as hydrogen peroxide. It has been found that a film such as an iron oxide film and / or a hydrated oxide film formed on a steel sheet has an effect of suppressing surface enrichment of elements in the steel during annealing.

Examples of the oxidizing agent in the treatment liquid in the present invention include hydrogen peroxide, potassium dichromate, potassium permanganate, etc., which are cost-effective and industrially practicable. In this case, it is desirable that the total concentration of the oxidizing agent in the treatment liquid is 0.1 wt% or more. The present invention is
A simple method of soaking the steel sheet in a treatment solution before annealing, such as dipping or spraying, and drying it makes it possible to significantly improve the plating properties and adhesion of high-strength steel sheets. This is because if the concentration is less than this value, the oxide film and / or the hydrated oxide film formed on the steel sheet at the time of contact with the treatment liquid will not have a sufficient amount and the effect of improving the plating property will be insufficient.

Further, the method disclosed in the present invention can particularly effectively form an oxide film and / or a hydrated oxide film when the treatment liquid is acidic to improve the plating property. It is necessary to add sulfuric acid, hydrochloric acid, nitric acid, etc. to the treatment liquid. In the present invention, the type of acid is not particularly specified, and organic acids and the like are also effective in addition to the above acids. In order to effectively form an oxide film or a hydrated oxide film and suppress non-plating by immersing the steel sheet in the treatment liquid for a short time, the pH of the treatment liquid is preferably 3 or less.

The liquid temperature of the treatment liquid is preferably from room temperature to 80 ° C., and industrially preferably room temperature to 60 ° C.

The method of bringing the steel sheet into contact with the treatment liquid of the present invention before annealing in the hot dip production line is not particularly limited. As a method that can be industrially carried out, dipping treatment, spraying treatment, or the like can be performed. After performing the above treatment, the steel sheet is dried, but Mn in the oxidizer on the surface of the steel sheet during the drying,
If there is a residue such as a Cr compound, the plating properties during subsequent hot-dip plating will deteriorate, so if the solution to be used contains such a metal, the steel plate should be contacted with the treatment liquid and then rinsed with water. , Then it is desirable to dry.

The oxide film or hydrated oxide film thus formed on the steel sheet needs to be reduced in the annealing step in the continuous hot dip galvanizing line. The annealing conditions in this case are oxide film and / or water. It is only necessary that the Japanese oxide film is sufficiently reduced. 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 differs depending on the steel type, but in the case of cold rolled steel sheet, it is 700 ° C or higher, and the annealing time is 10 seconds.
The above is desirable.

By the method disclosed in the present invention, hot-dip galvanizing of high-tensile steel sheet material without unplating can be obtained. However, 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,
A steel sheet containing a component element such as P or Mn is difficult to alloy in a temperature range of 550 ° C. or lower, and the adhesion is deteriorated because the alloying occurs near 600 ° C. Since the alloying temperature can be reduced by the treatment with the acidic solution containing the agent, 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.

[0020]

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 the cold-rolled steel sheet, it was dipped or sprayed with each treatment solution shown in Table 1 to form an oxide film or a hydrated oxide film on the steel sheet. Then, it was washed with water and dried to obtain the following test materials. 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 were performed by a hot dip simulator, and the alloying treatment was performed by an infrared heating furnace in a laboratory.

As a comparative example, a steel sheet having the same composition as that used in the above-mentioned invention example but not subjected to an oxidation treatment was annealed, hot-dip galvanized or alloyed (Comparative Example 1) and more oxidizable than iron. Low content of strong elements C: 0.002 wt
%, Si: 0.002 wt%, Mn: 0.2 wt%, P: 0.
Steel sheet with chemical composition of 01 wt% is annealed, hot dip galvanized,
An example (Comparative Example 2) subjected to alloying treatment is 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 (a diameter of 1/4 inch, a weight of 1 kg 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.

[0027]

[Table 1]

[0028]

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. At least one oxidizing agent selected from hydrogen peroxide, potassium dichromate, and potassium permanganate for hot dip galvanizing a high-strength steel sheet containing an element that is highly oxidizable. A hot-dip galvanizing method for a high-strength steel sheet, comprising the steps of bringing a steel sheet into contact with an acidic solution containing, drying, 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.