JPH07252621A - Production of hot-dip galvanized type high tensile strength steel sheet - Google Patents

Abstract

PURPOSE:To stably produce a hot-dip galvanized steel sheet free from the occurrence of uncoated part and excellent in adhesion by previously forming an iron oxide film on a steel sheet at the time of galvanizing a high tensile strength steel sheet containing highly oxidizing elements. CONSTITUTION:Hot-dip galvanizing is applied to a high tensile strength steel sheet having a composition containing, by weight, >=0.1%, in total, of at least one or more kinds among Si, Ti, Ni, Cu, Mo, V, and Cr as highly oxidizing elements, >=0.5% Mn, and >=0.001%, in total, of at least one or more elements among P, Al, and Nb. At this time, the steel sheet is first brought into contact with a solution in which an iron ozide sol is dispersed, by which an oxide film is formed on the surface by >=(3 to 10)g/m<2> by dry weight. Then, the steel sheet is annealed in a hydrogen-containing atmosphere and subjected to hot-dip galvanizing. The concentration of the highly oxidizing elements into the surface of the steel sheet can be effectively inhibited by the previous formation of the iron oxide film, and hot-dip galvanizing and galvannealing, excellent in adhesion, can be done.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing hot dip galvanized steel sheets and alloyed hot dip galvanized steel sheets used as materials for automobiles, home electric appliances, building materials, etc. This is a proposal on an advantageous method of producing a hot-dip galvanized high-strength steel sheet by subjecting a hot-dip galvanizing treatment to it.

[0002]

2. Description of the Related Art In recent years, in the fields of automobiles and home appliances, there is an increasing demand for surface-treated steel sheets with high corrosion resistance. Research and development and practical application of steel sheets and Zn-Ni plated steel sheets are progressing. Among them, in the case of hot-dip galvanized steel sheets such as hot-dip galvanized steel sheet (GI) and alloyed hot-dip galvanized steel sheet (GA), the manufacturing cost is lower than that of electrogalvanized steel sheet and good corrosion resistance is exhibited. Therefore, it has been widely put to practical use not only for inner plates but also for outer plates for automobiles.

In recent years, 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, efforts have recently been made to reduce the weight of automobile bodies, and for that purpose, gauge reduction of steel sheets is effective. For this, some results have already been obtained by using high-tensile steel plates. That is, Si, P, Mn, Ti, Nb, Al, Cr, N as elements that enhance the strength of the low carbon steel plate or the ultra low carbon steel plate without impairing the formability of the steel plate.
This is the development of high-strength steel sheets containing i, Cu, B, Mo or V. Furthermore, it has been attempted to impart rust prevention properties to such high-tensile steel sheets, and plated high-tensile steel sheets have already been developed.

However, such a conventional high-strength steel sheet contains Si, P, Mn, Ti, Nb, Al, Cr, Ni, and
Although Cu, B, Mo or V is contained in the steel, these are easily oxidized and are not easily reduced, so in the Sendzimir type production line, which is a typical continuous production line for hot dip galvanizing, during annealing. There is a fatal problem that these strengthening elements are selectively oxidized and the surface is concentrated. That is, the concentration of the strengthening element on the surface of the steel sheet during annealing is such that the oxides thereof significantly reduce the wettability between the steel sheet and the hot dip galvanized, so that the adhesion of the hot dip galvanized significantly decreases, and In this case, the molten zinc does not adhere to the steel sheet at all, which leads to a so-called unplated state. On the other hand, GA manufactured by hot dip galvanizing and then alloying
In the above case, the alloying is significantly delayed by the strengthening element oxides generated during annealing, so that there is a new problem that the alloying process cannot be performed unless the alloying temperature is raised.

When hot-dip galvanizing or alloying hot-dip galvanizing is applied to such a high-strength steel sheet that is difficult to plate, in order to prevent the formation of an unplated portion and properly alloy it, at least the front surface of the steel sheet should be covered. Treatment was essential. As such a technique, conventionally, Japanese Patent Laid-Open No. 55-1311
Japanese Patent Laid-Open No. 57-70268 proposes a method of performing Ni electroplating before hot dip galvanizing.
Japanese Unexamined Patent Publication No. 57-79160 discloses a method of performing Fe electroplating in advance. Furthermore, JP-A-55-12
No. 2865 discloses a non-oxidizing furnace method (N
In OF), an oxide film having a film thickness of 400 to 10000 A is formed in advance, and then annealed in an atmosphere containing hydrogen.

[0006]

[Problems to be Solved by the Invention]
Method by electroplating Ni or Fe (Japanese Patent Laid-Open No. 55-131165,
No. 57-70208, No. 57-79160), there is a problem in that large-scale equipment is required to prevent the occurrence of non-plated parts, and the manufacturing cost increases.
It is not the preferred method for application to hot dip galvanizing lines. Further, in the conventional technique disclosed in Japanese Patent Laid-Open No. 55-122865, the amount of oxide film produced varies depending on the line speed, the atmosphere, the temperature of the steel sheet, the type and amount of the constituent elements in the steel, etc. Since it is easy, it is difficult to stably obtain a predetermined coating amount, and it was expected that the effect of suppressing the non-plated portion in the actual production line would be unstable.

The main object of the present invention is to contain an element (Si, P, Mn, Ti, Nb, Al, Cr, B, etc., which is more oxidizable than iron, and at least one of Ni, if necessary. C
It is an object of the present invention to provide a method of stably suppressing the generation of an unplated portion at low cost when performing hot dip plating on a steel sheet containing u, Mo, V and other elements) in a continuous line. That is, it is an object of the present invention to propose an advantageous method for producing a high-tensile steel sheet which exhibits the same level of plating property as that of a high-tensile steel sheet containing only a small amount of these elements for a steel containing a large amount of strongly oxidizable elements. Another object of the present invention is to effectively suppress the concentration of an element that is highly oxidizable on the surface of a steel sheet by preliminarily forming a suitable iron oxide film. Still another object of the present invention is to perform hot dip galvanizing and alloying hot dip galvanizing with good adhesion by preforming a suitable iron oxide film.

[0008]

[Means for Solving the Problems] As a means for achieving the above-mentioned object, the present invention provides a method for producing Si, Ti, Ni, Cu, Mo,
0.1 in total for at least one of V and Cr
wt% or more, Mn is 0.5 wt% or more, and at least one of P, Al and Nb is 0.001 wt in total.
%, The high-strength steel sheet containing at least 3% is first contacted with the iron oxide sol dispersion solution to form an iron oxide film having a dry weight of 3 to 10 g / m ² or more on the surface thereof. , A method of manufacturing a hot-dip galvanized high-strength steel sheet characterized by performing hot-dip galvanizing, and then annealing in an atmosphere containing hydrogen. Among elements with strong oxidizability, Si, Ti, Ni, Cu , Mo,
0.1 in total for at least one of V and Cr
wt% or more, Mn is 0.5 wt% or more, and at least one of P, Al and Nb is 0.001 wt in total.
%, The high-strength steel sheet containing at least 3% is first contacted with the iron oxide sol dispersion solution to form an iron oxide film having a dry weight of 3 to 10 g / m ² or more on the surface thereof. Then, a method for producing an alloyed hot-dip galvanized high-strength steel sheet, characterized in that it is annealed in an atmosphere containing hydrogen, then hot-dip galvanized, and then heated and alloyed To do. And each of the above inventions, the iron oxide sol alloy solution to be brought into contact with the high-strength steel plate, 10% or more, preferably
A sol containing 20% or more of Fe ₂ O ₃ is used, and in each of the above inventions, the pH of the iron oxide sol dispersion solution is 5 or less, more preferably 4 or less.

The configuration of the present invention will be specifically described below.
The high-strength steel sheet to which the method of the present invention is applied is a low-carbon steel or an ultra-low-carbon steel that has Si, P, Mn, Ti, Nb, Al as a strengthening element in order to increase the strength of the steel sheet without impairing the formability. , C
A steel sheet containing at least one element such as r or B. In addition, these alloying elements may further contain components such as Ni, Cu, Mo and V, if necessary. In particular, the high-strength steel sheet used in the present invention has at least 0.1 wt% or more of Si and 0.5 wt% or more of Mn.
And it must contain 0.001 wt% or more of P.

Among them, Si is the element that most affects non-plating, so if the Si content in the steel becomes 0.1 wt% or more, the deterioration of the plating property becomes remarkable. Therefore, in the present invention, the effect of suppressing the non-plating portion by the iron oxide film formed by the immersion treatment in the iron oxide sol-containing solution is 0.1% by weight or more of the steel sheet containing Si, particularly 0.2 to 2.0 w.
It works extremely effectively on high strength steel sheets containing t% Si. This tendency is also large for Mn and P.
In the case of, the content of 0.5 to 4.0 wt% and the content of P in the range of 0.01 to 0.2 wt% have a great influence on the non-plating.

As described above, the occurrence of the non-plated portion observed when the hot-dip galvanizing the above high-strength steel sheet containing an element which is highly oxidizable is caused by the above-mentioned element concentrated on the surface. It was found that this is caused by the oxide. Therefore, the inventors have studied various techniques for suppressing the surface concentration of these highly oxidizable elements, that is, a pretreatment technique prior to the hot dip galvanizing treatment, and as a result, formed an iron oxide sol on the steel sheet surface. It has been found that, for example, it is effective in suppressing the surface concentration of elements in steel during annealing and promoting alloying.

The present invention provides a method for obtaining such effects.
Then, the high-strength steel sheet to be treated is
Contact the iron oxide sol dispersion solution described above to give a predetermined thickness.
I decided to form the iron oxide film of. In this processing solution
As the iron oxide sol of Fe,₃O_Four, Fe₂O₃Use
It is possible, but above all, Fe₂O₃Is particularly preferred. this
The reason is, as described below, to improve the plating property,
This is because oxygen in iron oxide is effective. Ie Fe₂O
₃Is Fe₃O_FourOxygen content in the same amount of coating
Is high.

As a solution for dispersing the iron oxide sol, an aqueous solution or a non-aqueous solution (methanol, n-propyl cellosolve, butyl cellosolve) is used.

The mechanism by which the concentration of elements in the steel on the surface of the steel sheet can be suppressed by forming a film of iron oxide sol on the surface of the steel sheet is not clear, but it is considered as follows. That is, during annealing, oxygen in iron oxide existing near the steel sheet interface forms oxides with Si and Mn that have diffused from the steel, thereby suppressing the diffusion of Si and Mn to the steel sheet surface. On the other hand, the iron oxide on the outermost surface is reduced in the reducing atmosphere during annealing to become metallic iron, which accelerates the reaction with molten zinc,
Not only good plating property is obtained, but it also contributes to promotion of alloying.

The amount of the iron oxide film deposited is 3 on a dry weight basis.
The range of up to 10 g / m ² is preferred. If it is less than 3 g / m ² , the effect of suppressing the surface enrichment of elements in the steel during annealing becomes insufficient, and no improvement in hot-dip galvanizability is observed. On the other hand, if the dry weight exceeds 10 g / m ^2, iron oxide This is because the adhesion of the coating deteriorates and peeling occurs, so that the effect of suppressing the surface enrichment of elements in the steel during annealing becomes insufficient. More preferably 4 to
It is in the range of 8 g / m ² .

In the hot dip galvanizing production line, the method of contacting the high-strength steel sheet with the iron oxide sol solution before annealing is not particularly limited, but dipping treatment, spray treatment, coating treatment, etc. are industrially used. It can be said that the method can be implemented in The iron oxide film formed on the steel sheet by such treatment needs to be reduced in the annealing process in the continuous hot-dip galvanizing line, but the annealing condition in this case is a condition that the iron oxide film is sufficiently reduced. I wish I had it. For example, as the atmosphere gas, hydrogen alone or hydrogen and nitrogen,
A mixed gas with argon or the like can be used, but 3 to 25% hydrogen gas is industrially practical. Further, the annealing temperature varies depending on the steel type, but in the case of hot rolled steel sheet,
It is preferable to perform the annealing at 700 ° C. or more and the annealing time of 10 seconds or more.

As described above, by forming a predetermined iron oxide film on the surface of a high-strength steel sheet, hot dip galvanizing without an unplated portion can be obtained on the high-strength steel sheet. The hot-dip galvanized high-strength steel sheet that has been treated can also be easily alloyed by heating it in the temperature range of approximately 450 to 550 ° C. By this treatment, the high-strength steel sheet material is alloyed and melted. It can be a galvanized high-strength steel sheet.

In this respect, it is generally difficult to alloy a high-strength steel sheet containing an element such as Si, Mn, or P, which significantly delays the alloying rate, in a temperature range of 550 ° C. or lower, and 600 ° C. Adhesion is significantly deteriorated due to alloying in the vicinity. However, according to the present invention, since the alloying temperature can be reduced by the above-mentioned treatment with the solution in which the iron oxide sol is dispersed, it is possible to obtain an alloyed hot-dip galvanized steel sheet of a high-strength steel sheet material having good adhesion. Becomes
In this treatment, the alloying temperature varies depending on the amount of plating deposited, the line speed, etc., but in order to obtain good adhesion, it is desirable that the alloying temperature be as low as possible, for example, 470-550. ℃, more preferably 480 ~ 520
℃.

[0019]

EXAMPLES Examples of the present invention will be specifically described below in comparison with comparative examples. (1) Inventive Example C: 0.002 wt%, Si: 1.0 wt%, Mn: 3.0 wt%, P: 0.
Steel containing 15 wt% was melted and hot-rolled and cold-rolled according to a conventional method to produce a cold-rolled steel sheet having a plate thickness of 0.7 mm. After degreasing and pickling this cold rolled steel sheet, it was immersed in the iron oxide sol dispersion solution shown in Table 1 to form an iron oxide film on the steel sheet. Then, it was washed with water and dried to obtain the following test materials. (2) Comparative Example An example in which a cold-rolled steel sheet having the same composition as that used in the above-mentioned present invention example was subjected to annealing, hot dip galvanizing, and alloying treatment (Comparative Example 1) and which had a stronger oxidizability than iron. Low element content C: 0.002 wt%, Si: 0.002 wt%, Mn: 0.2 wt
%, P: 0.01 wt% of a cold-rolled steel sheet having a chemical composition subjected to annealing, hot dip galvanizing, and alloying treatment (Comparative Example 2). Comparative Examples 3 and 4 are examples in which the weight of the iron oxide film is outside the range of the present invention. The annealing and hot dip galvanizing are
The galvanizing simulator was used, and the alloying treatment was carried out in the infrared heating furnace in the laboratory.

<Processing conditions> (A) Annealing conditions Temperature rising rate: 10 ° C / sec Holding temperature: 850 ° C / sec Holding time: 30 sec Cooling temperature: 20 ° C / sec Annealing furnace atmosphere: 5% H ₂ -N ₂ (Dew point -20 ° C) (B) Hot dip galvanizing conditions Bath temperature: 470 ° C Infiltration plate temperature: 470 ° C Al content rate: 0.15wt% Coating weight: 60g / m ² (one side) Plating time: 1sec (C ) Alloying conditions Temperature rising rate: 20 ℃ / sec Cooling temperature: 15 ℃ / sec Alloying temperature: 490 ℃ Alloying time: 30sec

<Method of evaluating hot dip galvanizing property> The appearance after hot dip galvanizing was subjected to image processing to obtain and evaluate the non-plating area ratio, and the judgment was made according to the following criteria. 5: Non-plating area ratio 0% 4: Non-plating area ratio 0 to 0.1% 3: Non-plating area ratio 0.1 to 0.3% 2: Non-plating area ratio 0.3 to 0.5% 1: Non-plating area ratio 0.5% or more <Plating adhesion Evaluation> DuPont impact test (weight of 1/4 inch, weight of 1 kg is dropped on the steel plate from a height of 50 cm)
The plating adhesion was evaluated by. The judgment criteria are shown below. ◯: No plating peeling X: Plating peeling <Evaluation of alloying rate> The alloying rate was evaluated by whether or not the zinc η phase remained on the surface of the alloyed material treated under the above conditions. ○: Zinc η phase not present ×: Zinc η phase present

[0022]

[Table 1]

Table 2 shows the evaluation results of Examples 1 to 6 of the present invention and Comparative Examples 1 and 2. Regarding the iron oxide sol A / B coating,
The relationship between the amount of adhesion after drying and the hot dip coating property is shown in FIG. As is clear from these results, by the method according to the present invention, even in a steel sheet having an element that is more oxidizable than iron, it is possible to produce a hot-dip galvanized steel sheet having excellent adhesion without an unplated portion. It is possible. Further, according to the present invention, since the alloying rate is moderately promoted, it is excellent in the treatment at a low temperature without changing from the conventional method, even though it contains an element that is strongly oxidizable. An alloyed hot-dip galvanized steel sheet can be obtained.

[0024]

[Table 2]

[0025]

As described above, according to the present invention, when hot dip galvanizing is performed on a high-strength steel sheet, the present invention makes it possible to inexpensively and stably produce a hot-dip galvanized steel sheet having no adhesion and excellent adhesion. Can be manufactured. Also,
When the alloying treatment is performed, the galvannealed steel sheet can be obtained at a relatively low temperature.

[Brief description of drawings]

FIG. 1 is a graph showing the relationship between the amount of iron oxide sol deposited and the hot dip coating property.

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification number Internal reference number FI Technical indication C23C 2/40 (72) Inventor Kazuo Mochizuki 1 Kawasaki-cho, Chuo-ku, Chiba-shi, Chiba Kawasaki Steel Stock Corporate Technology Research Headquarters (72) Inventor Masayoshi Kuwagata 1 Kawasaki-cho, Chuo-ku, Chiba, Chiba Prefecture Inside Kawasaki Steel Co., Ltd. (72) Inventor Takashi Ono 1 Kawasaki-cho, Chuo-ku, Chiba Prefecture Kawasaki Chiba Steel Works, Ltd.

Claims (3)

[Claims] 1. Among elements having strong oxidizability, Si, Ti, N
At least one of i, Cu, Mo, V and Cr is 0.1 wt% or more in total, Mn is 0.5 wt% or more, and at least one of P, Al and Nb is 0.001 wt in total. %, A high-strength steel sheet containing at least 3% is first brought into contact with an iron oxide sol dispersion solution to give a dry weight of 3 to 10 g /
A method for producing a high-strength hot-dip galvanized steel sheet, which comprises forming an iron oxide film of m ² or more, annealing in an atmosphere containing hydrogen, and then performing hot dip galvanizing. 2. Among elements having strong oxidizability, Si, Ti, N
At least one of i, Cu, Mo, V and Cr is 0.1 wt% or more in total, Mn is 0.5 wt% or more, and at least one of P, Al and Nb is 0.001 wt in total. %, A high-strength steel sheet containing at least 3% is first brought into contact with an iron oxide sol dispersion solution to give a dry weight of 3 to 10 g /
An alloyed hot-dip galvanized product characterized by forming an iron oxide film of m ² or more, followed by annealing in an atmosphere containing hydrogen, then hot-dip galvanizing, and then heating to perform an alloying treatment. Of high-strength high-strength steel. 3. The method according to claim 1, wherein Fe ₂ O ₃ sol is used as the iron oxide sol.