JPH03126879A - Production of cold rolled steel sheet excellent in chemical conversion treating property - Google Patents

Abstract

PURPOSE:To easily obtain a steel sheet excellent in chemical conversion treating property by carrying out cooling by means of an aqueous solution of boric acid under the prescribed conditions and successively forming prescribed amounts of metallic film of Ni and/or Mn in an aqueous alkaline solution at the time of applying continuous annealing to a cold rolled steel sheet. CONSTITUTION:In the course of cooling at the time of applying continuous annealing to a cold rolled steel sheet, cooling is carried out by means of an aqueous solution of boric acid. In the above aqueous solution of boric acid, concentration and the concentration of dissolved oxygen are regulated to 0.1-20% by weight and <=4ppm, respectively. Subsequently, in an aqueous alkaline solution, a metallic film of Ni and/or Mn is formed by 1-50mg/m<2>.

Description

[Detailed Description of the Invention] (Industrial Application Field) The present invention relates to a method for producing cold rolled steel sheets by continuous annealing.
In particular, the present invention relates to a method for producing cold-rolled *Fi with excellent chemical conversion treatment properties.

(Prior Art) Steel plates are prone to rust in the natural world, and in their final use, they are generally used after being subjected to some kind of surface treatment such as painting or plating. When painted, the steel plate is usually treated with phosphate prior to painting to improve paintability and corrosion resistance, and the phosphate treatment formed by this phosphate treatment It is well known that the quality of the coating greatly controls the durability of the coating after painting.

By the way, the strip obtained by cold rolling was conventionally annealed by soaking at 650 to 750"C for several hours to 10 hours by box annealing method, but in recent years, with the development of continuous annealing method, many of them are annealed continuously. With the advent of annealing, the soaking time has been shortened to about a few tens of seconds.However, the development of such short-time annealing methods has significantly changed the surface properties of steel sheets, as shown in the following. .

(2) A significant shortening of the annealing time hindered the formation of a surface enriched layer, which is effective for phosphate treatment and is formed by the concentration of components in the steel on the surface.

■The direct-fire heating method, which is increasingly being adopted as a continuous annealing method to improve annealing efficiency, results in the formation of an angstrom-thick oxide film on the steel strip surface no matter how the atmosphere is controlled. The presence of leads to a decrease in phosphatability.

(2) Ti and l/b, which are added to impart moldability, and Mn and Si, which are added to impart high tensile strength, exhibit particularly remarkable expansion in the above ((ii)).

Therefore, with the spread of continuous annealing, there is an urgent need to improve the chemical conversion properties of continuously annealed cold rolled steel sheets.

On the other hand, as methods for cooling steel plates during continuous annealing, methods such as gas die/7F cooling, roll cooling, air/water cooling, and water quenching cooling are adopted for boat fishing. With this method, it is not possible to modify the surface of a steel sheet to make it suitable for chemical conversion treatment.

On the contrary, air-water cooling and water quenching cooling produce several thousand oxide films (Fed, Pez02, Fezes) on the steel plate surface.
etc.), and on the contrary, it significantly impeded chemical conversion treatment properties.

Therefore, as a measure to improve chemical conversion properties, a method of forming a film of metal or metal compound on the surface of the steel sheet using continuous annealing post-treatment equipment is proposed in (1) JP-A-56-43392 and (2) JP-A-58-37391. It is proposed in the official gazette.

On the other hand, several methods have been proposed to try to improve the chemical conversion treatment properties of M plates without installing special post-treatment equipment as described above. That is, (3) Japanese Patent Publication No. 15132/1983 describes a cooling method using water containing formic acid, (4) Japanese Patent Publication No. 59/38285 describes a cooling method using water containing 10 to 90% alcohol, (5) ) JP-A-61-201737 discloses a cooling method using an α-amino acid-containing aqueous solution; (6) JP-A-49-122435 discloses a method of contacting with salts that generate hydrochloric acid; (7) ) Japanese Patent Publication No. 55-22534 and Japanese Unexamined Patent Publication No. 1983
Publication No. 156778 proposes a cooling method using water containing phosphate compounds, and (8) Japanese Patent Application Laid-Open No. 11623-1987 proposes a cooling method using cooling water with reduced dissolved oxygen concentration. .

(What the invention seeks to solve! l! The chemical conversion treatment property of steel sheets is improved by methods (1) and (2), which include post-treatment equipment and form a film of metal or metal compound on the surface of the steel sheet. The present inventors have confirmed that.

However, these methods are usually combined with a pickling process to remove the oxide film on the surface of the steel sheet. Hydrochloric acid is used as the pickling solution, but the hydrochloric acid pickling itself deteriorates the chemical conversion treatment properties of the steel sheet, so there is room for improvement.

Furthermore, if the electrolytic bath is acidic, there are problems such as etching of the surface of the steel plate and corrosion of equipment.

On the other hand, method (3), which has been proposed for improving chemical conversion treatment properties without having post-treatment equipment, does not necessarily have a sufficient effect of improving chemical conversion treatment properties.

Although method (4) is effective for non-oxidation cooling, it has no etching effect when using acid and cannot modify the surface of the steel sheet.Also, as mentioned earlier, it does not produce high tensile strength. Materials such as Mn5S, Si.

In steel sheets containing a large amount of component elements such as P, a surface concentration layer is formed during annealing, resulting in deterioration of appearance and chemical conversion treatment properties, but method (4) does not have the effect of etching the surface of the steel sheet. The layer cannot be removed.

The present inventors have confirmed that in methods (5) and (6), even if sufficient rinsing is performed after cooling treatment, yellow-brown rust forms on the surface of the steel sheet, deteriorating the chemical conversion properties. There is.

In method (7), the effect of improving chemical conversion treatment properties is insufficient when using only a phosphate compound.

Furthermore, in the method (8), it is necessary to control the dissolved oxygen level in the cooling water to a considerably low level, and it cannot be applied to the actual manufacturing process.

The main object of the present invention is to provide a method that can produce a continuously annealed cold rolled steel sheet with excellent chemical conversion treatability using inexpensive equipment.

(Means for Solving the Problems) As a result of various studies to solve the above problems, the present inventors found that when cooling a high-temperature cold-rolled steel plate in the continuous annealing process, Using a 20% aqueous solution of boric acid, preferably at a steel plate temperature of 100°C to room temperature,
The present invention was completed based on the discovery that chemical conversion treatment properties are significantly improved by forming a Ni and/or Mn metal film of 1 to 50 B/- in an alkaline aqueous solution.

Furthermore, since the N1 and/or Mn metal film is formed using an alkaline bath, there is no etching of the steel sheet surface, which is advantageous in terms of rust prevention of equipment.

In an embodiment of the present invention, it is desirable that the dissolved oxygen concentration in the boric acid aqueous solution is 4 ppm or less.

(effect) Hereinafter, the present invention will be explained in detail along with its effects.

In this specification, "%" means "% by weight" unless otherwise specified.

In the present invention, cooling is performed using a 0.1 to 20% boric acid aqueous solution in the cooling process of the continuous annealing process of cold rolled steel sheets. This process is called "boric acid aqueous solution cooling."

This boric acid aqueous solution cooling cools the steel plate and at the same time
It removes oxide films and concentrated layers on the surface of steel sheets, and can also serve as a pickling process.

Moreover, whereas the commonly carried out hydrochloric acid pickling deteriorates the chemical conversion treatment properties of the steel sheet surface, cooling with a boric acid aqueous solution can improve the chemical conversion treatment properties.

The clear reason for the improvement in the chemical conversion properties of cold-rolled steel sheets due to contact with an aqueous boric acid solution has not been elucidated, but the oxidation film and At the same time as the surface concentrated layer is removed,
It is presumed that a very small amount of boron or boron compounds adheres to the surface of the steel sheet, and this works effectively to activate the surface of the steel sheet. That is, when the steel plate that has been brought into contact with the boric acid aqueous solution is treated with phosphate, the phosphate crystals are densified using boron or boron compounds attached to the surface of the tg4 plate as nuclei, and a uniform phosphate film is formed. It is thought that it is possible to do so.

In the present invention, the concentration of the boric acid aqueous solution that is brought into contact with the steel plate is 0.1 to 20% by weight. The relationship between the concentration of boric acid aqueous solution and po is shown in Fig. 1. When the boric acid concentration is 0. IM
If it is less than 20% by weight, the pH value will be greater than 5, and the effect of removing the oxide film and surface concentration layer on the surface of the steel sheet as described above will be small, and if it exceeds 20% by weight, the pH value will be less than 2. The acidity is too strong and there is a risk of corroding the steel plate surface. The preferred concentration of boric acid aqueous solution is 5 to 15% by weight.
It is.

Note that methods for cooling using a boric acid aqueous solution include methods such as spraying and dipping.

The cooling of the boric acid aqueous solution may be started at any time during the cooling process of the continuous annealing process, that is, after the aging treatment, but it is usually started at a temperature of 450°C or less after the aging process. is preferable. The end of cooling is when the steel plate is cooled to a temperature at which oxidation in the atmosphere no longer progresses. Generally it is 100°C or less.

Following cooling of the boric acid aqueous solution, Ni and/or Mn
A metal film is formed on the surface of a steel sheet by, for example, electrolytic treatment in an alkaline aqueous solution.

The weight of the film is 1 to 50 mg/M'', and the improvement in chemical conversion treatment is most effective within this range.This weight standard is based on the amount of electrolytically deposited metal.

When a steel sheet is subjected to phosphate treatment for the purpose of improving the chemical conversion treatment property of a cold rolled steel sheet by forming such a metal film, Ni or Mn on the surface of the steel sheet serves as a nucleus to densify the phosphate crystals,
This is because a uniform phosphate film can be formed.

Further, the temperature of the steel sheet when forming the metal film is preferably in the range of 100° C. to room temperature, at which the steel sheet does not oxidize even in the atmosphere after cooling with the boric acid aqueous solution.

The effect of dissolved oxygen concentration in a boric acid aqueous solution is shown graphically in Figure 2. As is clear from Figure 2, which shows the relationship between the dissolved oxygen concentration in the steel plate and the oxide film thickness on the steel plate surface, the amount of dissolved oxygen was
400, which is the same as the original plate, by setting ppII or less
A large oxide film thickness can be secured.

Therefore, in the embodiment of the present invention, it is desirable that the dissolved oxygen concentration of the boric acid aqueous solution is 4119-.

In addition, in the actual manufacturing process, in order to reduce the dissolved oxygen concentration in the cooling water to 4 ppph or less, ■Method of adding chemicals N! +14 (hydrazine) +Ch=Nz +21
There are methods such as 2Hz + - OR = 2H20 in which a 1□O■ oxygen-reducing resin is used as a catalyst to react with dissolved hydrogen gas, but these methods are not particularly limited.

(Example) A cold-rolled ultra-low carbon steel Fi (C: 0.0O15%) with a temporary r￥0.8 wa and a plate width of 1000 mg was subjected to a normal annealing process on a continuous annealing line with a strip running speed of 150 mpm. A heating and soaking treatment was performed.

That is, first, the steel plate was heated to 850° C. and then kept soaked, then cooled to 450 ″C by gas jet cooling, and then subjected to an overaging treatment for 3 minutes.

During the subsequent cooling process, samples A to F were cooled to 100°C using a 1.0% boric acid aqueous solution by applying the method according to the present invention. Among these, samples B, D, and F contain 0 hydrazine.
．． 1% was added to make the dissolved oxygen concentration in the liquid 4 ppm.

Subsequently, both samples A to D were cooled with water to room temperature, which also served as rinsing.

Subsequently, Na0II 10 g/l was added to samples Δ and B.
, NiSO415g/l (NH,) 2S042
5g/1. 10 mg/m2 of N was applied to the surface of the I plate at a current density of I A/d- in an alkaline electrolytic bath diluted with water.
i A metal film was deposited.

On the other hand, samples C and D contained 10 g of NaOH/j2, M
n5O< ], 5g/1 (N114) zSO-2
In an electrolytic bath diluted with 5g/l in water, the density of 2It is 2.
A 10 mg/m'' Mn metal film was deposited on the surface of the steel plate at a rate of A/dm''.

Sample G-1 was water cooled from 450'C to room temperature.

Thereafter, samples G and H were pickled and rinsed with a 2% aqueous hydrochloric acid solution. Thereafter, for Sample G, a Ni metal film was deposited on the sample under the same electrolytic conditions as above.

Also, sample H is N15O, Log/jL NiC2z 2
g#! , 113BOs1g//! was applied to the steel plate surface at a current density of 10^/da'' in an acidic electrolytic bath diluted with water.
A Ni metal film of mg/m'' was deposited.

Both samples A-1 were subjected to temper rolling at a rolling reduction of 1.0% to obtain final products, which were sampled to investigate chemical conversion treatment properties. The results are shown in Table 1.

However, the chemical conversion treatment conditions were as follows.

(1) Degreasing: “Fine Cleaner 4826T1 (
(trade name) at 50°C for 2 minutes.

(2) Surface conditioning: Processed for 20 seconds using Vercolene Ztl (trade name).

(3) Phosphate treatment: “Bonderite 3030J (
(trade name) for 2 minutes at 50°C.

Moreover, the chemical conversion treatment property was evaluated by the following (a) to c).

(a) The ratio of phos-phophyllite (P) to hovite (■) in the chemical conversion film, P/(P+H), was determined from X-ray diffraction.

(b) The size of the crystal was determined using a scanning electron microscope.

(c) The amount of attached chemical conversion film was determined by gravimetric method.

Here, the larger the value of P/(P+11), the smaller the size of the chemical crystal, and the larger the amount of chemical conversion film attached,
Chemical conversion treatment properties are judged to be good. In addition, when specifically judging the chemical conversion treatment properties of the samples in Table 1, the cases where P/(P+H) ≧95, crystal size≦5-1, adhesion amount≧2.5 g/ld were considered good. .

(Hereinafter, blank space) Table 1 As is clear from the results in Table 1, Sample A according to the present invention
It can be seen that -D satisfies all of the above criteria regarding good chemical conversion treatment properties.

On the other hand, Samples E and F, which were only cooled with boric acid, and Samples G and H, in which Ni was precipitated in an acidic bath and an alkaline bath, respectively, after pickling with hydrochloric acid, also showed good chemical conversion properties, but all standards It doesn't taste good because I'm satisfied with it.

(Effects of the Invention) According to the method for manufacturing a continuously annealed cold-rolled steel sheet according to the present invention, which has been described in detail above, an m-plate with excellent chemical conversion properties can be provided at low cost and reliably.

[Brief explanation of the drawing]

Figure 2 is a graph showing the relationship between boric acid concentration and pl+; Figure 2 is a graph showing the relationship between boric acid concentration and pl+; and Figure 2 is a graph showing the relationship between boric acid concentration and pl+.
It is a graph showing the relationship between the dissolved oxygen concentration in pure water and the oxide film thickness on the surface of a steel plate when the steel sheet is heated to 0°C and then cooled with pure water.

Claims (2)

[Claims] (1) In the cooling process when continuously annealing cold rolled steel sheets,
A method with excellent chemical conversion properties characterized by cooling an aqueous solution of 0.1 to 20% by weight of boric acid and subsequently forming a metal film of Ni and/or Mn at a rate of 1 to 50 mg/m^2 in an alkaline aqueous solution. A method for producing cold-rolled steel sheets. (2) The method for producing a cold-rolled steel sheet according to claim 1, wherein the boric acid aqueous solution used in the boric acid aqueous solution cooling has a dissolved oxygen concentration of 4 ppm or less.