JPH02170925A - Manufacture of continuously annealed cold rolled steel sheet - Google Patents

Abstract

PURPOSE:To improve chemical conversion treating property on the surface of a cold rolled steel sheet by annealing a cold rolled steel sheet in a continuous annealing furnace and then applying cooling to the above sheet by means of an aqueous solution reduced in dissolved oxygen content and containing specific inorganic acid or organic acid in a nonoxidizing atmosphere. CONSTITUTION:At the time of heating and annealing a cold rolled steel sheet in a continuous annealing furnace and then cooling the steel sheet down to ordinary temp., cooling is carried out by using cooling water in an atmosphere of inert gas, such as N2, or reducing gas, such as N2+H2. As the cooling water to be used, water in which dissolved oxygen concentration is reduced to <=4ppm and inorganic acid, such as boric acid, or salt, such as sodium salt thereof, or organic acid, such as aliphatic monocarboxylic acid, aliphatic polyvalent carboxylic acid, and hydroxy carboxylic acid, or ammonium salt thereof, etc., are dissolved and contained by 0.1-20wt.% is used. By this method, the formation of an iron-oxide film on the steel sheet surface at the time of cooling can be prevented and the treatability of chemical conversion coating, such as phosphate film, can be improved and, as a result, the corrosion resistance and coating suitability of the steel sheet can be remarkably improved.

Description

[Detailed description of the invention] (Industrial application field) The present invention relates to a method for manufacturing a continuously annealed cold rolled steel plate.

More specifically, the present invention relates to a method for producing a cold rolled steel sheet having excellent chemical conversion treatability, particularly a continuously annealed cold rolled steel sheet.

(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 disassembly to improve paintability and corrosion resistance, and the phosphates formed by this phosphate treatment It is well known that the quality of the treated film greatly controls the durability of the coating after O coating.

By the way, copper strips obtained by cold rolling were 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 by continuous annealing method. As a result, the soaking time has been shortened to several tens of seconds. However, the development of such short-time annealing methods has improved the surface properties of steel sheets as shown below.
It has changed significantly as shown in () and (■).

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

■The direct heating method, which is increasingly being adopted as a continuous annealing method to improve annealing efficiency, produces oxidation on the steel strip surface as thick as angstroms, no matter how controlled the atmosphere.■9
The presence of this oxide film results in a decrease in phosphating properties.

(2) Ti and Nb, which are added to impart moldability, and Mn and Si, which are added to impart high tensile strength, are particularly susceptible to the above-mentioned tendency ((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, methods such as gas jet cooling, roll cooling, air/water cooling, and water quenching cooling have been adopted as methods for cooling steel sheets during continuous annealing, but this type of general cooling method In this case, it is not possible to modify the surface of the steel plate to make it suitable for chemical conversion treatment.

On the contrary, air-water cooling or water quenching cooling produces several thousand oxide films (FeO1Fez03, Fe5Oa, etc.) on the surface of the steel sheet, which significantly impairs chemical conversion properties.

Therefore, as a measure to improve chemical conversion treatment properties, equipment for performing post-treatment such as pickling equipment and Ni flash plating equipment is installed following the iz-continuous annealing equipment.

On the other hand, several methods have been proposed to try to improve the chemical conversion treatability of steel sheets without installing special post-treatment equipment as described above. That is, (1) Japanese Patent Publication No. 61-15132 describes a cooling method using water containing formic acid, (2) Japanese Patent Publication No. 59-38285 describes a cooling method using water containing 10 to 90% by weight of alcohol, (3) JP-A-31-201737 discloses a cooling method using an α-amino acid-containing aqueous solution, (4) JP-A-49-122435 discloses a method of contacting with salts that generate hydrochloric acid, and (5) JP-A-Sho. Publication No. 55-22534 and Japanese Unexamined Patent Publication No. 1983
-156778 proposes a cooling method using water containing phosphate compounds, and (6) JP-A-63-11623 proposes a cooling method using cooling water with reduced dissolved oxygen concentration. .

(Problems to be Solved by the Invention) However, the conventional pickling + N + flash plating method requires post-processing equipment, which has the problem of increasing equipment costs.

Further, the above-mentioned method (1) proposed for improving chemical conversion treatment properties does not necessarily have a sufficient effect of improving chemical conversion treatment properties.

Although method (2) is effective for non-oxidation cooling, it has no etching effect when using acid and cannot modify the surface of the steel sheet. In addition, as mentioned earlier, steel sheets containing high component elements such as Mn, S, Si, and P, such as high tensile strength materials, lead to the formation of a surface concentration layer during annealing, which leads to deterioration of appearance and chemical conversion treatment properties. However, (2
Since the method of ) has no etching effect on the surface of the steel sheet, it is not possible to remove the hardened layer on the surface.

The present inventors have confirmed that in methods (3) and (4), even if sufficient rinsing is performed after cooling treatment, yellow-brown rust occurs on the surface of the steel sheet, deteriorating chemical conversion treatability. .

In method (5), the effect of improving chemical conversion treatment properties is insufficient when using only a phosphate compound.

Furthermore, method (6) requires controlling the dissolved oxygen concentration in the cooling water to be quite low, and cannot be applied to actual manufacturing processes.

Up to now, there has been no method for manufacturing continuously annealed cold rolled steel sheets that can improve the chemical conversion treatment properties of steel sheets using any of the above methods.

The object of the present invention is that it can be carried out with inexpensive equipment,
Another object of the present invention is to provide a method for manufacturing a continuously annealed cold-rolled steel sheet that can provide a steel sheet with excellent chemical conversion treatment properties.

(Means for Solving the Problems) As a result of various studies to solve the above problems, the inventors of the present invention have found that dissolved oxygen ff. 0 for water with a specified range of degrees
．． i9! is obtained by containing 1 to 20% by weight of an organic acid, an aqueous acid, and/or a salt thereof to form an aqueous solution, and cooling the steel plate by bringing the steel plate into contact with the aqueous solution.
The present invention was completed based on the knowledge that the chemical conversion properties of subsequent annealing cold rolled steel sheets can be improved.

The gist of the present invention is that when a high-temperature cold-rolled steel sheet is cooled during continuous Vt annealing, dissolved oxygen? Water whose seismic intensity has been reduced to 4 ppm or less contains 0.1 to 20% by weight of a Ja acid, an inorganic acid and/or a salt thereof to form a water i8 solution, and the steel plate is brought into contact with the aqueous solution. This is a method for manufacturing continuous annealing cold rolling tJffl.

In an embodiment of the present invention, when the aqueous solution and the steel plate are brought into contact, it is desirable to do so in an inert atmosphere or a reducing atmosphere, and it is desirable to use boric acid as the acid or acid salt contained in the aqueous solution.

(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, since an aqueous solution containing acids or salts is used in water with a dissolved oxygen concentration reduced to 4ρρ- or less, when it comes into contact with a steel plate, the acid or salt solution is added to water with a normal dissolved oxygen concentration. It is possible to carry out the 9% 1ljB treatment while removing the oxide film and the surface concentration layer more effectively than an aqueous solution containing , and it is possible to improve the chemical conversion treatment properties of continuously annealed cold rolled steel sheets.

For example, Figure 1 shows the relationship between the dissolved oxygen concentration in the aqueous solution and the oxide film thickness on the surface of the steel plate when the steel plate is heated to 450°C in an atmosphere of (NZ95% + Ht5%) and then cooled with pure water. As is clear from Figure 1, the amount of dissolved oxygen is 4pp-
By doing the following, the thickness of the oxide film of 400 people, which is the same as that of the original film, can be maintained at 61%.

In the actual manufacturing process, the dissolved oxygen concentration in cooling water is
To make it less than ρρ■ ■Method of adding drugs N1H4 (hydrazine) + (h=Nz”211zO
■Method of reacting with dissolved hydrogen gas using deoxidizing resin as a catalyst 2H! +0□=2HtO, etc., but there are no particular restrictions.

Further, in the present invention, an organic acid, an inorganic acid and/or a salt thereof is used as a content, and the content thereof is 0.1 to 20%. The reason for limiting the pH to this range is to maintain the pH of the aqueous solution used as cooling water within an appropriate range. In other words, if the content of organic acids, inorganic acids, and/or their salts exceeds 20%, it becomes water? $ ρ of liquid
II is less than 2, which may corrode the steel plate surface, and if the content of organic acids, inorganic acids, and/or their salts is less than 0.1%, the water-soluble: night ρ11 is greater than 5. This is because the removal effect of the surface concentrated layer becomes smaller due to oxidation. For example, FIG. 2 shows the relationship between boric acid concentration and pH of an aqueous solution using boric acid as an inorganic acid, that is, an aqueous boric acid solution. If pl+ is greater than 5, oxide film, surface? The effect of removing the layer is small, and if it is less than 2, the acid TL is too strong and may corrode the steel plate surface.

Here, from the viewpoint of solubility in water, the organic acid in the present invention preferably has 6 or less carbon atoms, and examples thereof include aliphatic acids such as formic acid, acetic acid, propionic acid, pentanoic acid, and hexanoic acid. Aliphatic polycarboxylic acids such as carboxylic acid, oxalic acid, malonic acid, maleic acid, succinic acid, glutaric acid, adipic acid, 1.2.4-bucuntricarboxylic acid-2-phosphonate, ethylenediaminetetraacetic acid, nitrilotriacetic acid, etc. , oxyacetic acid, gluconic acid,
Examples include oxycarboxylic acids such as citric acid, lactic acid, tartaric acid, malic acid, gallic acid, and tannic acid, and carbolic acid.

The formic acid in the above example is excellent because it becomes boiling and generates CO gas when it comes into contact with a cold-rolled steel plate at a high temperature of about 100 to 900°C, and therefore has a large reduction effect on the steel plate. Ammonium salts of the organic acids mentioned above can also be used. This ammonium salt of an organic acid is suitable because it can suppress corrosion of metals caused by acids.

Examples of the inorganic acid include water-soluble acids such as boric acid, hydrochloric acid, nitric acid, and phosphoric acid. Of course, these acids may be used in combination.

In the case of boric acid in the above example, at the same time as removing the oxide film and the surface layer, extremely high amounts of boron or boron compounds adhere to the steel sheet surface, and the number of crystal nuclei in the phosphate film increases using this as a nucleus. Therefore, a dense chemical conversion coating is formed, which is particularly effective in improving chemical conversion treatment properties.

Ammonium and sodium salts of inorganic acids, such as sodium borate, sodium phosphate, and ammonium chloride, can also be used. Sodium borate causes boron or a boron compound to adhere to the surface of the steel sheet, and is effective in improving chemical conversion treatment properties in the same way as when boric acid is used. In addition, ammonium chloride generates 11□ gas when it comes into contact with a metal material in a high temperature state, and has a large reducing effect on the metal material, so it is excellent in terms of performance.

In the present invention, methods for bringing the steel plate into contact with each aqueous solution include spraying the aqueous solution onto the steel plate,
Alternatively, there is a method of placing a steel plate in a tank containing an aqueous solution.

Note that N2, N2+lI□, Ar gas atmosphere, etc. are effective as the atmosphere, and in practice, from the viewpoint of oxidation prevention, 0! It is effective to keep the concentration below 0.5%.

By subjecting the steel plate that has been cooled in this manner as it is or by subjecting it to skin pass rolling as necessary, it is possible to obtain a continuously annealed cold-rolled steel plate that is excellent in chemical conversion treatability.

Further, the present invention will be described in detail based on Examples, but these are illustrative of the present invention and the present invention is not unduly limited thereby.

(Example) Strip running speed of a cold-rolled ultra-low carbon steel plate (CiO, 0O 15%) with plate T ¥0.8 dragon and plate width 10001 fi! Heating and soaking tests were conducted on a 50 mpm continuous annealing line.

That is, first, the steel plate was heated to 850° C. and kept soaked, then cooled to 450° C. by gas jet cooling, and subjected to an overaging treatment for 3 minutes. The method according to the present invention was applied to the subsequent cooling step, and cooling was performed by infiltration to 100° C. using each polar liquid shown in FIG.

Subsequently, water cooling was performed to room temperature, which also served as a rinse. then 1
．． Temper rolling was performed at a reduction rate of 0% to obtain Q holly products, which were used as test pieces of sample ml to sample 7.

Sample 11kll to sample 1 obtained by this process
The chemical conversion treatment property of h7 was investigated. However, the chemical conversion treatment conditions are as follows.

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

(2) Surface conditioning: “Processed for 20 seconds using Vercolene 7, Tl (trade name).

(3) Phosphate treatment: [Bonderey l-30301 (
(trade name) for 2 minutes at 50°C.

Further, chemical conversion treatment properties were evaluated using the following (al to fcl).

fa) Phosphofluorite (P) as a chemical conversion film;
The ratio P/(P + I+) to hovite (H) was determined from X-ray analysis.

The crystal size was determined using a tb+ scanning electron microscope.

+ Increase the amount of Cl chemical conversion film attached! It was determined by the tt method.

In addition, since the present invention is intended to be applied to a continuous annealing process, this chemical conversion test was conducted (N195% (1) □5
%) in a reducing atmosphere.

Test conditions and results are shown in Table 1.

Table 1 (Note) The ratio is outside the scope of the present invention.The larger the value of P/ (P + I+), the smaller the size of the chemical crystal, and the greater the amount of chemical conversion film deposited, the higher the chemical conversion rate. Processability is judged to be good. Therefore, when judging the chemical conversion treatability of the samples in Table 1, the cases where P/(P + I+)≧95, crystal size≦5p, and deposit amount≧2.58/m'' were considered good. As is clear from Table 1, the samples according to the present invention (sample 1kl to sample size 1kan 4) satisfy all of the above criteria and are excellent in chemical conversion treatment properties.

On the other hand, sample magnet 5 is a sample that was subjected to gas jet cooling after annealing, but because the crystal size became too large, the values of P/(P+H) and adhesion amount became small, resulting in poor chemical conversion treatment properties. It turns out that it's not good.

Further, sample floor 6 is a test t1 in which the amount of dissolved oxygen was cooled with an aqueous solution outside the range of the present invention, and it can be seen that the chemical conversion treatment properties are still insufficient.

Furthermore, formula 1 ning 4 patient 7 is an organic acid, flil, mechanism and/or
Alternatively, the samples were cooled with an aqueous solution that does not contain these salts, but it is clear that the chemical conversion treatment properties are still insufficient.

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

The significance of the present invention having such effects is remarkable.

[Brief explanation of the drawing]

Figure 1 shows a steel plate being quarried in a 25% N295% (1) atmosphere.
Graph showing the relationship between dissolved oxygen concentration in pure water and oxide film thickness on the steel plate surface when heated to 50°C and then cooled with pure water:
And Figure 2 is boric acid? This is a graph showing the relationship between p) and l.

Claims (3)

[Claims] (1) When cooling a high-temperature cold-rolled steel sheet in the continuous annealing process, 0.1 to 20% by weight of organic acids, inorganic acids, and/or these are added to water whose dissolved oxygen concentration has been reduced to 4 ppm or less. 1. A method for manufacturing a continuously annealed cold rolled steel sheet, which comprises making an aqueous solution containing salt and bringing the steel sheet into contact with the aqueous solution. (2) Claim (1) characterized in that the steel plate is brought into contact with the aqueous solution in an inert atmosphere or a reducing atmosphere.
) The method for producing a continuously annealed cold rolled steel sheet. (3) The method for manufacturing a continuously annealed cold rolled steel sheet according to claim (1) or (2), wherein the inorganic acid is boric acid.