JPH0678589B2 - Method for manufacturing cold-rolled steel sheet having excellent phosphatability - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a method for manufacturing a cold rolled steel sheet, and particularly to a method for manufacturing a cold rolled steel sheet excellent in chemical conversion treatment.

(Prior Art) A steel sheet is prone to rust in the natural world, and in its final use state, it is used after being subjected to some surface treatment such as painting or plating. Prior to coating, the steel sheet is usually subjected to a phosphate treatment in order to improve so-called paintability and corrosion resistance. It is well known that the quality of the coating largely controls the durability of the coating after coating.

By the way, the steel strip obtained by cold rolling has been conventionally annealed by soaking at 650 to 750 ° C. for several hours to 10 hours by the box-type annealing method, but in recent years, most of it has been developed due to the development of the continuous annealing method. The continuous annealing process has become possible, and the soaking time has been shortened to several tens of seconds. However, the development of such a short-time annealing method has greatly changed the surface characteristics of the steel sheet as described below.

The significant reduction of the annealing time has hindered the formation of a surface-enriched layer effective for phosphating, which is formed by the concentration of the components in the steel on the surface.

The direct-fired heating method, which is being increasingly used in continuous annealing to increase the annealing efficiency, results in the formation of an oxide film with a thickness of angstrom units on the surface of the steel sheet, no matter how the atmosphere is controlled. The presence results in reduced phosphatability.

The above tendency is particularly remarkable in Ti and Nb added for imparting moldability, and Mn and Si added for imparting high tensile strength.

Therefore, with the spread of the continuous annealing process, there is an urgent need to improve the phosphate treatment property (also simply referred to as chemical conversion processability) of the continuously annealed cold rolled steel sheet.

On the other hand, as a method for cooling a steel sheet in continuous annealing, methods such as gas jet cooling, roll cooling, steam cooling, and water quenching cooling are generally adopted, but this type of general cooling method is used. Then, surface modification cannot be performed so as to be suitable for chemical conversion treatment. Rather, in the steam cooling or water quenching cooling method, an oxide film (FeO, Fe ₂ O ₃ , Fe
₃ O ₄ etc.) and, on the contrary, markedly impairs the chemical conversion treatability.

Therefore, a method of trying to improve the chemical conversion treatability has been actively proposed.

(1) JP-B-61-15132 discloses a method of cooling with formic acid-containing water, (2) JP-B-59-38285 discloses a method of cooling with 10-90% by weight of alcohol-containing water, and (3) In JP 61-201737 A, a cooling method using an aqueous solution containing an α-amino acid is described. (4) In JP 49-122435 A, a method of contacting salts such as hydrochloric acid is used. In JP-A-22534 and JP-A-56-156778, there are proposed methods for cooling with water containing a phosphate compound.

(Problems to be Solved by the Invention) Among the methods proposed for improving the above-described chemical conversion treatment method, the method (1) is not always effective enough.

Even though the method (2) is effective for non-oxidative cooling, it has no etching effect as in the case of using an acid, and surface modification cannot be performed. Especially, Mn such as high tensile strength material,
In a steel sheet containing a large amount of component elements such as S, Si and P, a surface concentrated layer is formed by these components during annealing, which causes deterioration of appearance and chemical conversion treatability, but this method has no etching effect. Therefore, the surface concentrated layer cannot be removed.

Regarding the methods of (3) and (4), as a result of the study by the present inventors, even after sufficient rinsing after cooling treatment, yellowish brown rust is generated on the steel plate surface, and chemical conversion treatability is deteriorated. Have confirmed.

Further, in the method using only the phosphate compound as in the method (5), the effect of improving the chemical conversion treatability is insufficient.

On the other hand, the present inventors previously applied a method of cooling by directly contacting the cold rolled steel sheet with a boric acid aqueous solution from a high temperature region in the continuous annealing final cooling step (Japanese Patent Application No. 63-323056),
In this case, the high-temperature steel plate will be continuously cooled with the boric acid solution, and the boric acid solution will boil due to the amount of heat removed from the steel plate, and the corrosion of the solution tank will be accompanied by such equipment and operation management. There was a problem above.

Therefore, a main object of the present invention is to provide a method capable of producing a cold rolled steel sheet having excellent chemical conversion treatability with inexpensive equipment.

(Means for Solving the Problem) The inventors of the present invention have conducted extensive studies in order to achieve the above-mentioned object, and as a result, after the continuous annealing cooling was completed, a boric acid aqueous solution was applied to a steel sheet that had a temperature near room temperature. By bringing them into contact with each other, the above-mentioned problems do not occur, and the facility and operational management become very simple, and the effect on chemical conversion treatability, that is, phosphate treatability is the effect of contact between boric acid aqueous solution and steel sheet. It was confirmed that the same was performed in both the high temperature region and the low temperature region, and the present invention was completed.

The gist of the present invention is, as post-treatment after completion of continuous annealing final cooling, characterized by bringing into contact with a cold-rolled steel sheet 0.1 to 20% by weight boric acid aqueous solution, of a cold-rolled steel sheet having excellent phosphatability. It is a manufacturing method.

(Operation) Hereinafter, the present invention will be described in detail.

In the present invention, for the cold-rolled steel sheet that has reached the temperature near room temperature after the final cooling in the continuous annealing treatment step,
A boric acid aqueous solution is contacted. It was confirmed that this significantly improved the phosphate treatment of the steel sheet.

Regarding the improvement of the chemical conversion treatment property of the cold-rolled steel sheet by the contact with the boric acid aqueous solution, the obvious reason has not been clarified, but the oxide film and the surface concentration of the steel sheet surface which cause the reduction of the phosphoric acid treatment property as described above are not understood. It is presumed that at the same time as the removal of the oxide layer, a trace amount of boron or a boron compound adheres to the surface of the steel sheet, which effectively acts on the activation of the surface of the steel sheet. That is, when the steel sheet brought into contact with the aqueous boric acid solution is subjected to a phosphate treatment, the phosphate crystals are densified with boron or a boron compound attached to the steel sheet surface as a nucleus, and a uniform phosphate film is formed. It is considered possible.

The action of the aqueous solution of boric acid is that the temperature of the steel sheet to be contacted is in a high temperature range (for example, 450 → 10
(0 ° C.), even if it is the same in the low temperature region near room temperature where the final cooling is completed, in the present invention,
In order to make the boric acid aqueous solution act in the low temperature range, there is no concern about boiling of the boric acid aqueous solution or the corrosion of the aqueous solution layer accompanying it, and the facility and operational management are very simple and practically advantageous. Is the way.

In the present invention, the concentration of boric acid aqueous solution to be brought into contact with cooling is set to 0.1 to 20% by weight. The relationship between the concentration of boric acid aqueous solution and PH is shown in Fig. 1. When the concentration of boric acid is less than 0.1% by weight, the PH value becomes larger than 5, and the above-mentioned oxide film and surface concentration on the steel sheet surface are increased. The effect of removing the layer is small. Again 20
If it exceeds 5% by weight, the PH value becomes smaller than 2, the acidity is too strong, and there is a risk of corroding the cooling surface. The preferred concentration of the boric acid aqueous solution is 5 to 15% by weight.

In the present invention, as a method of bringing the aqueous solution of boric acid into contact with the steel sheet, the aqueous solution of boric acid may be sprayed onto the steel sheet after completion of continuous annealing final cooling by a spray or the like, or a storage tank for the aqueous solution of boric acid after the final cooling step. There is a method of providing a steel plate and passing it through a sufficiently cooled steel plate.

Then, by subjecting the steel sheet thus brought into contact with the boric acid aqueous solution to pickling and temper rolling as necessary, a cold rolled steel sheet having excellent chemical conversion treatability can be obtained.

Hereinafter, the present invention will be described in more detail with reference to Examples.

Example A cold rolled ultra low carbon steel plate (C: 0.0015%) having a plate thickness of 0.8 mm and a plate width of 1000 mm was heated and cooled as a normal annealing process in a continuous annealing line with a strip traveling speed of 50 mpm,
And a post-treatment according to the invention.

That is, first, the steel sheet was heated to 850 ° C., soaked and held, and then cooled to 450 ° C. by gas jet cooling and subjected to a temporary treatment for 3 minutes. Thereafter, the samples A to D were subjected to gas jet cooling to 100 ° C., the sample E to immersion cooling with water, and the sample F to 10% by weight boric acid aqueous solution for cooling. It was immersed in water and cooled to 30 ° C. After that, as a treatment process, sample A
The present invention was applied to C to C, immersed in a boric acid aqueous solution of 10 to 20% by weight, rinsed with water, and temper-rolled at a rolling reduction of 1.0% to obtain a final product.

The steel sheet obtained by this process was sampled to investigate the chemical conversion treatability. The results are shown in Table 1.

However, the chemical conversion treatment conditions are as follows.

(1) Degreasing: “Fine Cleaner 4826T” (trade name) was used and treated at 50 ° C. for 2 minutes.

(2) Surface adjustment: 20% with "Borcolen ZT" (trade name)
Processed for seconds.

(3) Phosphate treatment: “Bondelite 3030” (trade name) was used for immersion treatment at 50 ° C. for 2 minutes.

The chemical conversion treatability was evaluated by the following (a) to (c).

(B) The ratio of phosphophyllite (P) to hopite (H), P / (P + H), of the chemical conversion coating was determined by X-ray diffraction.

(B) The crystal size was determined by a scanning electron microscope.

(C) The adhered amount of the chemical conversion coating was determined by a gravimetric method.

Here, the larger the value of P (P + H), the smaller the size of the chemical conversion crystal, and the larger the amount of the chemical conversion film deposited, the better the chemical conversion treatability is judged to be. Moreover, when judging the chemical conversion treatability of the sample in Table 1 specifically, P / (P +
H) ≧ 95, crystal size ≦ 5 μm, and adhesion amount ≧ 2.5 g / m ² were considered good.

As is clear from the results shown in Table 1, it is understood that the samples (A to C) according to the present invention satisfy all the above-mentioned criteria for good chemical conversion treatment.

On the other hand, when the post-treatment with boric acid was not performed, the chemical conversion treatability was insufficient when the cooling method was gas jet or water. Regarding sample F, since the boric acid aqueous solution was brought into contact with the steel sheet in the high temperature region during cooling, the chemical conversion treatment was better than the other comparative examples and it was equivalent to the example of the present invention. Thus, there was a problem in the equipment of the boric acid aqueous solution tank, and its management was complicated.

(Effects of the Invention) As described in detail above, according to the present invention, a boric acid aqueous solution of 0.1 to 20% by weight with respect to the cold rolled steel sheet, which has reached a temperature around room temperature after completion of continuous annealing final cooling, is prepared. By bringing them into contact with each other, it is possible to provide a method capable of producing a cold-rolled steel sheet having excellent chemical conversion treatability with inexpensive equipment. Therefore, the method of the present invention is an industrially meaningful method.

[Brief description of drawings]

FIG. 1 is a graph showing the relationship between boric acid concentration in an aqueous boric acid solution and PH.

Claims (1)

[Claims] 1. A post-treatment after completion of final cooling of continuous annealing,
A method for producing a cold-rolled steel sheet having excellent phosphating property, which comprises contacting a cold-rolled steel sheet with a 0.1-20 wt% boric acid aqueous solution.