JPH01156429A - Method for cooling continuously annealed and cold-rolled steel sheet - Google Patents

Abstract

PURPOSE:To enhance the susceptibility of a cold-rolled steel sheet to chemical conversion treatment by bringing a high-temp. cold-rolled steel sheet into contact with the aq. org. acid soln. having a specified concn. and then with the aq. soln. added with a soluble reducing agent in the continuous annealing process. CONSTITUTION:When the high-temp. cold-rolled steel sheet is cooled in the continuous annealing process, a primary coolant contg. 1-30wt.% aq. soln. of an org. acid (formic acid, oxalic acid, etc.) or an inorg. acid (hydrochloric acid, nitric acid, etc.) or their salts (ammonium salts, etc.) is brought into contact with the steel sheet to cool the steel sheet. A secondary coolant obtained by adding a soluble reducing agent (hypophosphorous acid, phosphorous acid, etc.) to the primary coolant is then brought into contact with the steel sheet to cool the steel sheet. By this method, the steel sheet can be quenched without being oxidized, the concd. layer on the surface can be removed, and further the residue detrimental to the chemical conversion treatment can be removed.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to a cooling treatment method for continuously annealed cold rolled steel sheets, and particularly for cooling the surface of continuously annealed cold rolled steel sheets which is inferior in phosphate chemical treatment as a base treatment for painting. The present invention relates to a cooling treatment method for improving chemical conversion treatment properties by modifying in a cooling treatment process.

[Conventional technology]

Steel plates are prone to rust in the natural world, and in their final use, they are 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, steel strips obtained by cold rolling have traditionally been annealed by soaking at 650 to 750°C for several hours to over 10 hours using a box annealing method, but in recent years, with the development of continuous annealing methods, Many products are now subjected to continuous annealing, and the soaking time has been shortened to several tens of seconds. However, the development of such short-time annealing methods has significantly changed the surface properties of steel sheets as described below.

(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 Nb, which are added to provide bottom shape properties, and Mn and St, which are added to provide high tensile strength, exhibit the above-mentioned tendency (2).

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

On the other hand, as a cooling method for continuous annealing, - for boat fishing,
- Although methods such as gas jet cooling, roll cooling, air/water cooling, and water quenching cooling have been adopted, these types of general cooling methods cannot modify the surface to make it suitable for chemical conversion treatment. . Rather, air-water cooling and water quenching cooling methods create several thousand oxide films (Fed, Fe, etc.) on the surface of the steel sheet.
zOz, FezOa, etc.), and conversely, the chemical conversion treatment properties were significantly impaired.

Therefore, methods that appear to be effective in improving chemical conversion treatment properties have been proposed.

(11 Japanese Patent Publication No. 61-15132 discloses a method of cooling with formic acid-containing water, (2) Japanese Patent Publication No. 59-38285
(3) In JP-A-61-201737, a cooling method using an aqueous solution containing α-amino acids is described (4).
) JP-A-49-122435 discloses a method of contacting salts that generate hydrochloric acid, (5) JP-A-55-2
No. 2534 and Japanese Unexamined Patent Publication No. 156778/1987 each propose a cooling method using water containing a phosphate compound.

[Problem that the invention seeks to solve]

However, the present inventors have confirmed that in the method (1) above, even if sufficient rinsing is performed after cooling treatment, yellow-brown rust occurs on the surface of the steel sheet, deteriorating chemical conversion treatability. .

Although method (2) is effective for non-oxidation cooling, it has no etching effect when using acid and cannot perform surface modification. In addition, as mentioned earlier, in steel types containing high component elements such as Mn+Sr SLP, which are high tensile strength materials, a surface concentration layer is formed during annealing, leading to deterioration in appearance and chemical conversion treatment properties. ,(2
) method does not have an etching effect, so the surface concentrated layer cannot be removed.

Methods (3) and (4) lead to the same results as method (1), resulting in poor chemical conversion properties.

Furthermore, in the method (5), the effect of improving chemical conversion treatment properties is insufficient if only the phosphate compound is used.

SUMMARY OF THE INVENTION Therefore, the main object of the present invention is to provide a cooling treatment method that allows non-oxidation cooling, removes surface concentrated layers, and removes residues that are harmful to chemical conversion properties.

[Means for solving problems]

In order to solve the above problems, the present invention provides a method for cooling a high-temperature cold-rolled steel sheet during continuous annealing; is brought into contact with a steel plate and cooled, and then a secondary coolant containing a reducing agent soluble in the primary coolant is brought into contact with the steel plate and cooled.

In addition, the above-mentioned "soluble" means soluble in water or an organic solvent.

[For production]

In the present invention, since water containing an acid or an acid salt is used as the primary coolant, non-oxidation cooling can be performed while removing the oxide film and surface concentration layer. Since a liquid containing a soluble reducing agent is used as the primary coolant, acid remaining in the primary cooling process and corrosion products such as rust can be removed. Moreover, the oxidation products of the reducing agent are adsorbed on the surface, and the number of crystal nuclei in the phosphate coating increases using them as nuclei, resulting in the formation of a dense chemical conversion coating and improved chemical conversion treatment properties.

[Specific structure of the invention]

The present invention will be explained in further detail below.

In the present invention, an organic or inorganic acid or a salt thereof is used as the primary coolant, and the content thereof is 1
~30% by weight (all percentages below are by weight), preferably 3~
It is said to be 10%. This is because if the acid content is small, the effect of removing the oxide film and the surface concentration layer will be small, while if the acid content is too large, there is a risk of corroding the metal material.

The organic acid used in the present invention preferably has a carbon number of 6 or less from the viewpoint of solubility in water, and examples thereof include formic acid, acetic acid, allonic acid, pyropionic acid, butyric acid, pentanoic acid,
Aliphatic monocarboxylic acids such as hequinic acid, oxalic acid, maleic acid, succinic acid, glutaric acid, adipic acid, 1,2.
Aliphatic polycarboxylic acids such as 4-butanetricarboxylic acid-2-phosphonate, ethylenediaminetetraacetic acid, and nitrilotriacetic acid, oxycarboxylic acids such as oxyacetic acid, gluconic acid, citric acid, lactic acid, tartaric acid, malic acid, gallic acid, and tannic acid. Examples include 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 high-temperature cold-rolled steel sheet at a temperature of about 100 to 900'C, and has a large reduction effect on metal materials. Also, ammonium salts of the above-mentioned organic acids can be used. This ammonium salt of an organic acid is suitable because it can suppress the corrosion of metals caused by acids.

Inorganic acids include water-soluble acids such as hydrochloric acid, nitric acid,
Examples include sulfuric acid and phosphoric acid. Of course, these acids may be used in combination.

Ammonium and sodium salts of inorganic acids, such as sodium phosphate and ammonium chloride, can also be used.
NH4C1 generates H2 gas when it comes into contact with high-temperature metal materials, and has a large reduction effect on metal materials, so it is excellent in performance, like formic acid.

As mentioned above, when using acids that generate reducing gases,
The use of a surfactant, particularly a nonionic one, is effective because the vapor film cooling of the reducing gas can be performed evenly for a long time due to the adsorption effect of the surfactant.

All known surfactants can be used as the surfactant, but
The amount of O added is preferably 1 to 10%.

This is because if the amount of surfactant is small, the effect of retaining the vapor film of the reducing gas on the metal surface will be small, and even if it exceeds 10%, no increase in the effect can be expected, resulting in a cost disadvantage.

Surfactants that can be used include anionic surfactants such as carboxylates, sulfonates, and sulfuric acid ester salts; ether type, ether ester type, ester type, and nitrogen-containing nonionic surfactants; amine salts; Cationic surfactants such as quaternary ammonium salts, amino acid type, betaine type, and imidacilline type amphoteric surfactants can be used. Two or more surfactants may be used in combination.

For steam film cooling, instead of relying on the reducing gas generated from the acid used, it may be supplied externally, and this reducing gas may include carbon oxide, hydrogen, water gas, modified gas, generator gas, etc. , or DX known to those skilled in the art,
NX, HNX, RX.

AX, ASRX, SAX or a mixture of these gases is used. This reducing gas can be brought into contact with the metal material by, for example, spraying it from a nozzle toward the surface of the metal material during the process of passing the metal material into a tank containing the coolant.

On the other hand, the secondary coolant that is brought into contact with the steel plate to remove residues on the surface of the steel plate after contacting the above-mentioned primary coolant with the steel plate is as follows:
A soluble reducing agent is used as the primary coolant, and from the viewpoint of liquid concentration control and stability, it is preferable to use this at a relatively low temperature range where the steel plate temperature is 150° C. or lower. Examples of this reducing agent include hypophosphorous acid, phosphorous acid, sodium boron hydride (NaBH4), a boron hydride compound, and dimethylamine borane ((CH:l)
JHBH3) etc. It is also possible to add a metal salt to the secondary coolant, and in this case, the reducing agent causes a trace amount of metal to precipitate from the metal salt, promoting improvement in chemical conversion treatment properties.

In the present invention, methods for bringing the steel plate into contact with each coolant include methods such as spraying the coolant onto the steel plate or passing the steel plate through a coolant storage tank.

In addition, the timing of using each coolant may be sequentially performed in the first cooling zone of the continuous annealing equipment, or the first coolant is applied in the first cooling zone and the second coolant is used after overaging treatment.
A secondary coolant may be applied after the next cooling zone. Further, in the first cooling zone, after using a boat cooling method, the first and second coolants may be applied in the second cooling zone and thereafter.

〔Example〕

Next, examples will be shown together with comparative examples.

A cold-rolled ultra-low carbon steel plate (C = 0.0015%) with a plate thickness of 0.8 mm and a plate width of 1000 mm was subjected to heating and cooling treatment tests on a continuous annealing line with a strip running speed of 150 mρm. First, it was heated to 850°C and then kept soaked. Thereafter, it was cooled to 450° C. by gas jet cooling, and over-aged for 3 minutes. The present invention was applied to the subsequent cooling process, and as shown in Table 1, the primary cooling (coolant A) was performed by spraying 11
The secondary cooling (coolant B) was carried out by an immersion method from 110°C to room temperature. Thereafter, temper rolling was performed at a rolling reduction of 1.0% to obtain a final product.

Various steel sheets manufactured through this process were sampled and their chemical conversion properties were investigated. The results are shown in Table 1.

In addition, all cases of tests NIA to G are average values of N=10 coils (0.8 dragon thickness, 1000 dragon plate width, 23 tons per coil), and are highly reliable.

However, the chemical conversion treatment conditions are as follows.

(1) Degreasing = [Fine Cleaner 4826TJ (
(trade name) for 2 minutes at 50°C (2) Surface conditioning: "Percolene ZTj (trade name) for 20 seconds (3) Phosphate treatment = "Bonderite 3030" (
(trade name) at 50° C. for 2 minutes.The chemical conversion treatment properties were evaluated according to the following (a) to (c).

(a) The ratio P/(P + H) between phosphofluorite (P) and hopai (H) of the chemical conversion coating 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.

[Effects of the Invention] As described above, the present invention provides a cooling treatment method that can perform oxidation-free quenching, remove surface concentrated layers, and further remove residues harmful to chemical conversion properties. be able to.

Claims (1)

[Claims] (1) In a method of cooling a high-temperature cold-rolled steel sheet during a continuous annealing process; a primary coolant containing 1 to 30% by weight of an organic or inorganic acid or a salt thereof in water is brought into contact with the steel sheet, and then cooled. A method for cooling a continuously annealed cold-rolled steel sheet, comprising bringing a secondary coolant containing a reducing agent soluble in the primary coolant into contact with the steel sheet to cool the steel sheet.