JPS60128216A - Cooling method in continuous annealing of cold rolled steel sheet - Google Patents

Abstract

PURPOSE:To obtain a cold-rolled steel sheet having a clean surface and to eliminate the need for post treatment such as pickling by using an aq. soln. contg. an org. chelate agent as a coolant and using cyclically said soln. while adjusting the concn. thereof. CONSTITUTION:A cold-rolled steel sheet is held for specified time at the temp. above the recrystallization temp. in an annelaing gas consisting of hydrogen and nitrogen and is then cooled at a cooling rate of 50-500 deg.C/sec from the prescribed temp. in the primary cooling process in continuous annealing of the cold rolled steel sheet. Or the steel sheet is further subjected to a warm aging treatment for specified time in a 200-500 deg.C range and is then cooled down to 30- 100 deg.C at a cooling rate of 50-500 deg.C/sec as the secondary cooling stage. An aq. soln. contg. an org. chelating agent, for example, an aq. soln. of 2-sodium salt of ethylenediaminetetraacetic acid as a coolant is cyclically used while the concn. thereof is adjusted to 0.2-5wt% in the primary and secondary cooling stages thereof.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a cooling method for manufacturing cold-rolled steel sheets by continuous annealing. More specifically, a specific aqueous solution with a simple composition is circulated as a coolant to cool the surface. This invention relates to a cooling method for manufacturing excellent cold rolled steel sheets.

(Prior Art) Conventionally, in continuous annealing of cold rolled steel sheets, water is often used as a coolant to perform rapid cooling. However, when water is used, the surface of the cold-rolled steel plate becomes rough, forming a thick oxide film and becoming discolored. As a countermeasure for this, in order to remove the colored oxide film, after continuous annealing, inorganic acids such as hydrochloric acid, or
As shown in No. 156710, when pickling treatment is performed with a lower organic acid, and when an aqueous d solution containing multiple components such as a water-soluble organic carboxylic acid and an organic amine is used instead of water, the oxide film is removed during cooling. Method for suppressing the formation of
No. 923) is being carried out.

However, in the case of performing all the pickling after water cooling in the former case, a neutralization step is also required, which means that an extra pickling and neutralization step is required.
The manufacturing process becomes complicated. On the other hand, the latter, JP-A-57-85
As seen in 923, the formation of an oxide film can be suppressed by cooling using an aqueous solution containing multiple organic components (organic acid and organic amine, or polyalkylene glycol added), or the generated oxide can be There are also problems with the method of melting the film to obtain a cold-rolled steel sheet with a clean surface. That is,
When an aqueous solution of an organic compound is sprayed onto a surface at a high temperature of 650 to 850°C, thermal decomposition of the organic compound occurs on the surface of the steel plate. At this time, when an aqueous solution consisting of a plurality of organic compounds is used, the degree of thermal decomposition differs depending on the type of organic compound. Therefore, if such an aqueous solution is circulated and used for a long period of time, the composition of the organic compounds in the aqueous solution will be different, and there is a concern that the surface cleaning effect will be reduced.

In addition, if the effectiveness of the coolant decreases due to cyclic use, it is necessary to replenish the organic compound. Proper supply of organic compounds is difficult because it is difficult to easily understand.

When an aqueous solution containing a plurality of organic compounds is used as a coolant, there is a problem in controlling the bath composition. Furthermore, in the methods using organic compounds as coolants, including the prior art described above, the idea of circulating Jd while adjusting the concentration has never been proposed.

Generally, organic compounds are expensive and cannot be applied industrially from an economic point of view unless they are recycled, so recycling is essential. Therefore, in order to use an organic compound as a coolant, it is necessary to select the composition of the compound and apply it on the premise that it will be recycled.

(Object of the invention) The present invention solves these conventional problems and also provides a new method of applying a coolant.The purpose of the present invention is to obtain a cold-rolled steel sheet with an excellent appearance after cooling with a water-based coolant. The objective is to provide a cooling method. Another object of the present invention is to provide a simplified method for manufacturing a cold rolled steel sheet in which a pickling or alkali neutralization treatment step after cooling is omitted in a continuous annealing method in which cooling is performed using an aqueous refrigerant. Still another object is to provide a cooling method that uses an organic compound as a coolant and has low running costs.

(Structure of the Invention) In order to achieve the above object, the present invention uses a specific organic compound alone in the primary cooling and/or secondary cooling in continuous annealing of cold rolled steel sheets, and uses the organic compound cyclically while adjusting the concentration. The gist is that in continuous annealing of cold-rolled steel sheets, after holding the cold-rolled steel sheets at a temperature equal to or higher than the recrystallization temperature for a certain period of time in an annealing gas consisting of hydrogen and nitrogen, 50 to 5000 s after a cooling process or further thermal aging treatment for a certain period of time in a temperature range of 200 to 500°C.

In the step of secondary cooling at a cooling rate of 30 to 100°C, the concentration is adjusted using an aqueous solution containing one type of organic chelating agent as a coolant in the first cooling step and/or the second cooling step. The present invention relates to a cooling method for continuous annealing of cold-rolled steel sheets, which is characterized by cyclic use.

The present invention will be explained in more detail below.

When water is sprayed onto a heated steel plate in a gas consisting of nitrogen and hydrogen at a high temperature of 250 to 850°C, an oxide film is formed due to the rise in the dew point of the atmospheric gas, and the composition of this oxide film is mainly composed of Fe304. In addition, there is Fe2O3.

The inventors heat-treated a cold-rolled steel sheet by applying a normal thermal cycle for continuous annealing of a cold-rolled steel sheet, and used water as a coolant during rapid cooling. On the surface of the cold-rolled steel sheet obtained by this method, a brown to purple oxide film called temper color was formed, and the thickness of this oxide film was approximately 300 to 500X'. Removing such colored cold-rolled steel sheets with an organic compound
When it was immersed in an aqueous solution of an organic chelating agent and heated, the oxide film was dissolved and the surface was cleaned.
I put it out. Therefore, as a result of logging the applicability of various 41 chelating agents as coolants for continuous annealing of cold-rolled steel sheets, it was found that most aqueous solutions of organic chelating agents have this effect. Sarak has found that even if these aqueous solutions are injected during rapid cooling, a steel plate with a clean surface can be obtained after cooling. Among these chelating agents, 2-sodium salt of ethylenediaminetetraacetic acid and triethanolamine have been found to be particularly excellent.

It has been found that these chelating agents are generally preferably used in an aqueous solution of 01 to 20 wt %, the lower limit concentration is the concentration at which the effect is recognized, and the upper limit is determined from the surface properties of the steel sheet and from an economic point of view.

The present invention will be explained below mainly using ethylenediamine 17cI acetic acid 2-sodium salt and ethanolamine.

The reason why the concentration of the specific organic chelating agent is limited in the second and third claims of the fourth set of claims of the present invention will be explained. First, in the case of the 2-sodium salt of ethylenediaminetetraacetic acid, if an aqueous solution with a concentration exceeding 5% by weight is sprayed, the surface of the cold rolled steel sheet will be colored, and the sodium remaining on the surface of the steel sheet will color the surface.

There is a concern that the chemical conversion properties of cold-rolled steel sheets may deteriorate, so the upper limit is set at 5% by weight3.
If it is less than the weight ratio, the effect of removing the oxide film will be small. In particular, when only water is used as a coolant in the first stage cooling process, if the concentration of disodium ethylenediaminetetraacetic acid is less than 02% by weight in the second stage cooling process, the oxide film can be removed sufficiently. Hard to get. From these things,
The concentration of 2-storium salt of ethylenediaminetetraacetic acid is
A range of 02 to 50 weight ratio is optimal.

In addition, the appropriate concentration of triethanolamine is 05 to 20
He is in charge of weight. In this case, the lower limit is 0.5 multiplier because the effect of removing the oxide film is small below 0.5 multiplicity. Further, even if an aqueous solution with a weight factor of 20 weight molecules or more is used, it will not have any negative effect on the surface of the steel plate, but the maximum concentration is 20 weight molecules in view of ease of handling and economical efficiency of the aqueous solution.

Using these aqueous solutions, 4! Any known method may be used to cool the d-plate, but a method of spraying an aqueous solution onto the steel plate surface is preferred since the cooling rate can be controlled over a wide range.

Therefore, we will explain how to spray an aqueous solution of these organic chelating agents onto a steel plate.When rapidly cooling a steel plate, it is necessary to uniformly spray the above-mentioned aqueous solution of the organic chelating agent onto the surface of the steel plate. The aqueous solution of the agent may be sprayed alone, or air may be used to spray the agent, but the most appropriate method is to use air and water spray using a gas with the same composition as the blunt gas. . The water flow density when spraying is 50
In the case of a cooling rate of ~500''C/sec, it may vary slightly depending on the thickness of the cold-rolled steel sheet, but the cooling rate may be about 50 to 5004 C/sec per 1 mi' of the surface of the steel sheet.

Note that the starting temperature for secondary cooling is preferably 350°C or lower, taking into account the bending properties of cold-rolled steel sheets.If the aqueous solution of the organic chelating agent mentioned above is used, the oxide film can be formed even if the secondary cooling starting temperature is as low as 200°C. A cold-rolled steel sheet with a clean surface is obtained by molten S.

In the present invention, when an aqueous solution of an organic chelating agent is circulated and used as a coolant, the water may evaporate at high temperatures, or the organic chelating agent may thermally decompose at high temperatures. Since the concentration will differ from the initial concentration due to reaction with existing iron-based metals or metal oxides, it is necessary to quantify the organic chelating agent in the coolant and adjust the concentration. In the case of a coolant using the 2-sodium salt of ethylenediamine-4-acetic acid, this coolant is added with a pl-110 buffer of ammonia-ammonia chloride to make it an eriochrome black indicator, and it is used as an indicator for zinc chloride. It can be easily determined by titration with an aqueous solution. In addition, triethanolamine can be easily determined by hydrochloric acid titration.

In this way, chelating agents are easy to quantitatively analyze;
By using only seeds, the concentration of organic chelating agent in the coolant can be easily controlled. That is, if the concentration is out of the control range as a result of quantitative analysis, it may be diluted by adding an organic chelating agent or, in some cases, water. In this way, the cooling liquid management in the present invention,
Controlling the concentration of the organic chelating agent is easier than in the conventional method of using multiple chelating compounds described above.

Further, when a coolant containing the above-mentioned organic chelating agent is circulated and used, metal ions, mainly iron ions, accumulate in the coolant, reducing the ability of the coolant to remove oxide films. In such cases, discarding the coolant increases costs and is also undesirable from the standpoint of environmental regulations. Therefore, in order to use the coolant for a full period of time, it is necessary to remove the accumulated metals, mainly iron. Insoluble metals, mainly iron, or their oxides in the coolant can be easily removed by sand filtration or the like. However, the iron-based metal that has reacted with the aforementioned disintegrator chelating agent, that is, the iron-based metal that has been dissolved as a metal chelate compound, cannot be removed by this method. The simplest removal method is to pass a coolant from which insoluble substances have been removed through a chelate resin column, and the soluble metals, mainly iron, in the coolant are adsorbed by the chelate resin and removed from the coolant. It's good. Therefore, when applying the method of the present invention in the process of continuous annealing of cold-rolled steel sheets, the following method is preferable, as an example. A 0.20 to 5% aqueous solution of the 2-sodium salt of ethylenediamine/4-acetic acid, or a 0.5 to 5% aqueous solution of triethanolamine, is placed in a tank of about 20%.
Store the 20% by weight aqueous solution.

Using this, water flow density 50-5
Air and water are sprayed at a rate of about 0.001/ni'·min, and the collected cooling water is about 50 to 80°C, so it is cooled as necessary, and first passed through a sand filter to remove insoluble matter. removes substances. Next, this cooling water is passed through the column of all chelate resin, and the metals, mainly iron, which have reacted with the above-mentioned organic chelating agent are adsorbed onto the chelate resin and removed.

Next, the concentration of the chelating agent in the cooling water after passing through the chelating resin is quantitatively analyzed by the method described above, and if necessary, an organic chelating agent is added or water is added to obtain a predetermined concentration. to be managed.

By circulating the cooling water using this method, the performance of the cooling water hardly deteriorates and it can be used for a long period of time.

On the other hand, cold-rolled steel sheets after continuous annealing that have been cooled with an aqueous solution of an organic chelating agent have a clean surface and do not require post-treatment such as pickling, so they can be completely dried and prepared as they are. After rolling, a trace amount of the above-mentioned organic chelating agent remains on the surface of this cold-rolled steel sheet. ,
Since this has little effect on surface treatment properties such as chemical conversion treatment and plating treatment, there is no particular need to remove the organic chelating agent by washing.

(Example) Next, the present invention will be explained by examples.

Example 1 An aqueous solution of 2-sodium salt of ethylenediamine/tetraacetic acid was used for continuous annealing and cooling, and the relationship between the additive concentration and the oxide film thickness was investigated. 'The annealing heat treatment cycle for a cold-rolled steel plate involves heating the steel plate in an annealing gas consisting of 3 volumes of dihydrogen and the balance being nitrogen, holding the temperature at 750°C for 2 minutes, and then using the cooling water at a water flow density of 2001/rr?・Spray with air and water in minutes,
Cooling was performed to 350°C at a cooling rate of 200°C/sec, and after holding at 350°C for 3 minutes, secondary cooling was performed by air/water spray to 50°C under the same conditions as the primary cooling.

After drying this cold-rolled steel sheet, the oxide film on the surface was analyzed by AFiS analysis. Table 1 shows the relationship between the concentration of an aqueous solution of 2+1um salt of ethylenediamine/tetraacetic acid and the oxide film thickness.

In Table 1, 0% organic chelating agent in 81-R< is the case where water was used as cooling water for primary cooling and secondary cooling.

From the results in Table 1, when water was used for primary and secondary cooling, an oxide film of 309X was formed on the surface of the cold rolled steel sheet, giving it a brown temper color. On the other hand, according to the method of the present invention,
When the concentration of the aqueous solution was adjusted to a range of 0.20 to 5.0% by weight using ethylenediamine/tetraacetic acid (+) lamb salt and the solution was cooled, the thickness of the oxide film was 42 to 73%. We were able to obtain a beautiful cold-rolled steel plate with a metallic luster on the surface. Furthermore, in addition to filtering out insoluble matter in these aqueous solutions, the filtered water was passed through a water treatment facility equipped with a chelate exchange resin, and then ethylenediamine and sodium tetraacetate were analyzed in situ using the method described above. After using it continuously for the first month while adjusting the concentration to the target concentration by adding chemicals or diluting it with water, the results were almost the same as Saitoyu, and a steel plate with a beautiful surface was obtained.

Example-2 In the same annealing heat treatment cycle as in Example-1, water was used for cooling one sheet, and ethanolamine or ethylenediamine/tetraacetic acid diamine was used as an aqueous solution of an organic chelating agent for secondary cooling.
An aqueous solution of sodium salt was used. The results are shown in Table-2. Note that EDTA in Table C is an aqueous solution of ethylenediamine/4-acetic acid and 2-sodium, and TEA is an aqueous solution of triethanolamine. Table 2 shows the relationship between the concentration of the organic chelate compound and the oxide film thickness on the surface of the steel plate after cooling.

Table 2 According to Table 2, the oxide film of the cold rolled steel sheet of Example 2 is slightly thicker than that of Example 1 shown in Table 1, but no temper color occurs at all, and both have metallic luster. . In addition, after treating the secondary cooling liquid in the same manner as in Example 1, it was used continuously for the first month while analyzing and adjusting the concentration of ethylenediamine/tetraacetic acid disodium salt or triethanolamine. A steel plate with a beautiful surface, almost the same as in Example 2, was obtained.

As explained above, by using an aqueous solution containing an organic chelating agent for rapid cooling during continuous annealing of a cold-rolled steel sheet and recycling it, a cold-rolled steel sheet with a beautiful surface could be obtained.

(Effect of the invention) As described above, in the present invention, ethylenediamine/tetraacetic acid
By using an aqueous solution of a chelating agent such as sodium salt or triethanolamine as a coolant for continuous annealing, a cold-rolled steel sheet with a clean surface can be obtained, and unlike the conventional water cooling method, pickling etc. can be performed in the post-process. Cold-rolled steel sheets can be produced without post-treatment.

In addition, the concentration of organic chelating agents can be easily controlled by simple analysis, so by adjusting the concentration using one type of chelating agent, it can be circulated and used for a long time without reducing its effectiveness. It is also more cost effective than the conventional method.

Claims (5)

[Claims] (1) In continuous annealing of cold-rolled steel sheets, after holding the cold-rolled steel sheet at a temperature equal to or higher than the recrystallization temperature for a certain period of time in an annealing gas consisting of hydrogen and nitrogen, the temperature is lowered by 50 to 50 b cess, or further, After temperature aging treatment for a certain period of time in the quality range of 200 to 50.0°C, the process of secondary cooling to 30 to 100°C at a cooling rate of 50 to 500°C/sec includes the first cooling step and Alternatively, a cooling method for continuous annealing of cold-rolled steel sheets, characterized in that in the second cooling step, an aqueous solution containing one type of organic chelating agent is used as a coolant and circulated while adjusting the concentration. (2) The method according to claim 1, wherein the aqueous solution of the organic chelating agent is a 0.2 to 5 weight aqueous solution of 2-sodium salt of ethylenedianetetraacetic acid. (3) The method according to claim 1, wherein the aqueous solution of the organic chelating agent is a 0.5 to 20 weight percent aqueous solution of triethanolamine. (4) Water is used as a coolant in the primary cooling process, and an aqueous solution of a 411 machine chelating agent is used as a coolant in the secondary cooling process. The method described in Section 3. (5) In order to circulate and reuse the aqueous solution of the organic chelating agent, water treatment equipment using an E-filtration device and a chelate exchange resin is installed in the circulation process of the aqueous solution of the organic chelating agent, and the organic chelating agent is Claims 1 and 2, characterized in that after removing insoluble substances and chelate compounds of metals mainly containing iron from the aqueous solution, the concentration of the chelating agent is adjusted and recycled. The method described in Sections 3 and 4.