JPS5830390B2 - Rapid cooling surface treatment method for cold-rolled steel sheets - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for applying extremely excellent surface treatment to cold rolled steel sheets in an extremely short period of time.

Normally, to apply surface treatment to a cold rolled steel sheet, the strip is subjected to continuous annealing or box annealing, followed by skin pass rolling and then sent to a separately installed surface treatment facility.

s) When continuously annealing the IJ tube, as shown in Figure 4, the strip coil is unwound and first sent through a horizontal direct-fired non-oxidizing heating furnace to a vertical radiant reduction heating furnace for degreasing and annealing. Alternatively, it is wet-electrically degreased and cleaned and sent to a vertical radiant reduction heating furnace where it is heated to 700-800°C to anneal and reduce the oxides on the surface.
400-450℃ in a non-oxidizing quenching furnace with 2+N2 gas atmosphere
Perform gas cooling.

Next, this is sent to an overaging treatment furnace with an N2 + H2 gas atmosphere, where it is allowed to stay for about 3 minutes to perform an overaging treatment, and then it is sent to a non-oxidizing quenching furnace with an N2 + H2 gas atmosphere to bring it to room temperature. Since gas cooling is usually performed, the time required from exiting the vertical radiant reduction heating furnace to exiting the final gas cooling furnace is usually about 9 minutes.

This also applies to cooling in the 300-350°C temperature range from 700-800°C in a radiant reduction heating furnace to 400-450°C in an over-aging furnace, and from 400-450°C in an over-aging furnace to room temperature 5°C.
Cooling in the temperature range of 350 to 400 degrees Celsius at 0 degrees Celsius takes a very long time because N2 + H2 gas cooling is used to avoid oxidation. Although this is highly undesirable, it is unavoidable for non-oxidation.

It has also been proposed to use a water cooling method for this cooling, but
None of these can be said to be sufficient.

First, as shown in FIG. 1, water spray boxes 2 are placed on both sides of a heating steel strip 1 that runs vertically from above to below to form a curtain of water continuous in the horizontal direction with a width equal to or greater than the width of the steel strip. A multi-stage configuration is known.

However, in these cases, the water that reaches the surface of the heated steel strip covers the surface of the steel strip and flows upward or downward;
Since the steel strip is heated to a high temperature, the water covering the steel strip surface will evaporate and the generated atmospheric pressure will prevent the water spray or water curtain from reaching the heated steel strip surface. There are parts where the water reaches and is rapidly cooled, and there are parts where the water does not reach and the water cooling effect is diminished, so not only is the cooling rate uneven in each part, but the overall cooling rate itself is inevitably slow. In the case of Fig. 2, 2 is a pressure water box and 3 is an injection slit, and it is stated that the cooling rate is 300 to 300 kg.

In this case, bluing is a thin film of iron oxide that occurs on the surface of a bright steel plate and is visible to the naked eye.The thinnest layer is a bright pale yellow color, and as the layer becomes thicker, it changes to yellow, yellowish brown, grass color, etc.
It changes from green, dark green, sky blue, navy blue, dark blue, purple, dark purple, dark purple, dark color, and gray.

When bluing that can be seen with the naked eye occurs on the surface of a bright steel sheet like this, it can no longer be considered a bright steel sheet surface, but rather a blackened steel sheet covered with iron oxide scale, that is, a thin layer of black skin. Chemical conversion treatment cannot be performed unless this black scale is removed, that is, descaling treatment is performed by pickling or the like to restore the bright steel sheet portion.

However, pickling treatment is necessary for descaling black scale, and if this process is carried out in-line by incorporating it into the surface treatment line for bright steel strips, it will be carried out after the treatment of the used acid solution. Ancillary equipment such as processing of dilute acid solution generated from washing with water is required.

In other words, although it can be easily inferred that the water cooling method has the effect of shortening the processing line, the reason why the gas cooling method using non-oxidizing gas is adopted is to maintain the bright surface.
Even when using a gas cooling method, the formation of a very thin oxide film cannot be avoided, and when performing wet metal plating or other surface treatments, it is customary to perform a mild pickling followed by water washing prior to wet metal plating or other surface treatments.

However, in this case, since the oxide film is extremely thin, a light pickling is sufficient, so this process can be incorporated into the surface treatment line and carried out in-line, so there is no problem with the execution of the line, but it is desirable to increase the number of processes. That's not the point.

In the present invention, cold-rolled steel sheets heat-treated in a non-oxidizing or reducing atmosphere are heated from 350 to 800°C to 200°C or less.
At a temperature of 50°C or higher, water is used as a refrigerant to rapidly cool the steel plate at an ultra-high speed of 1170°C/sec or higher, where no bluing is observed, to obtain a steel plate with a bright surface, and at the same time generate a high degree of activity on the steel plate surface. A cold-rolled steel sheet characterized in that the surface of the steel sheet is subjected to phosphate treatment or metal plating without performing pickling in a subsequent step by utilizing the activity during a short period of time during which the activity remains. This is a rapid cooling surface treatment method.

The present invention will be explained in detail based on examples.

Embodiment 1 Figure 3 shows an example of a device for water-cooling a high-temperature steel strip according to the present invention, in which a large number of spots are ejected on both sides of a heated steel strip 1 running vertically from above to below. This is a device in which a water inlet 3 and a number of spot intake water inlets 6 are arranged in adjacent rows in multiple stages across the course of the steel strip, and this device is used to jet high-pressure water onto the surface of the high-temperature steel strip. When water is injected from the spot spout water port 3 by the pump 4, it collides with the steel strip surface and creates a turbulent flow, and at the same time, the water is sucked and removed from the suction water port 6 by the suction pump 5 before it reaches a high temperature to vaporize.
Since the surface of the copper strip is subsequently cooled by the turbulent flow of liquid cooling water, each part is uniformly cooled with water at an extremely high speed.

Therefore, in the present invention, as shown in FIG.
To 450℃@, when cooling, increase the surface temperature to 350-400℃
'C4 water cools a temperature range of 350℃ in about 0.3 seconds or less,
This is reheated to bring the steel strip temperature to 400-450°C, and when rapidly cooling from 400-450°C in the overaging furnace to a cooling water temperature of 50°C, the temperature range of 350-400°C is also about 0. Cool in water for less than 3 seconds.

The cooling rate for water cooling this temperature range of 600 to 350°C in 0.3 seconds or less is 1170°C to 2000°C/sec or more.

When the steel strip is water-cooled in this way, a sufficient effect can be obtained with the overaging treatment I/'i at 400 to 450°C for 1 minute. The total time required is approximately 3 minutes, which has the effect of shortening the time to 3 minutes compared to the 9 minutes shown in FIG.

In the case of the present invention, the surface of the water-cooled steel strip has a brightness and no bluing can be observed.

However, this does not mean that no oxides were generated on the surface of the water-cooled steel strip; in this case as well, oxides are generated by water vapor, but the oxide layer formed is extremely thin. This is because the thickness has not yet reached a level where bluing can be seen.

In general, the amount of iron oxide generated during water cooling of high-temperature steel sheets is proportional to the integral value of the cooling start temperature, end temperature, and cooling time, so to simplify calculations, we can connect them with straight lines and schematically display them as shown in Figure 6. become that way.

That is, 300℃/5ec500℃/sec, 117
The amount of iron oxide generated at each cooling rate of 0°C/sec and 2000°C/sec is proportional to the required cooling time, and at 500°C/sec, it is about 6 times that of the present invention, 300°C.
/Sec, it is known that approximately 10 times the amount of iron oxide is generated.

The inventor further found that the ultrafine structure on the surface of the steel sheet that has been rapidly cooled with water is a mixture of many ultrafine iron oxide particles and metallic iron that is not covered by the oxidized iron particles. It was discovered that the mixture of iron and exposed iron gives an extremely high level of surface activity.

However, during the short time period during which the activity due to the fine mixture of the iron oxide and exposed bare iron remains, the activity is utilized to perform wet surface treatments such as phosphate treatment, chromate treatment, wet metal plating, wet painting, etc. on the surface of the steel sheet. When coating, the extremely fine oxide protrusions become the core of the surface treatment particles that adhere to the copper strip surface, and become the legs of the coating.The ultrafine oxide protrusions densely cover the steel strip surface and strengthen it. It has been found that the wet surface treatment film applied to the copper strip has the effect of tightly and firmly attaching it to the surface of the copper strip.

According to the method of the present invention, surface treatment is performed continuously immediately after ultra-high-speed water cooling, so not only can the two steps of pickling and water washing that are normally carried out be omitted, but also the steel strip surface formed by pickling can be Compared to irregularities, the oxide convexities of the present invention are much finer and have a much stronger gripping force on the steel sheet surface.

If the film thickness of the oxide protrusions generated in the present invention is observed, it is an ultra-thin layer of one to two orders of magnitude in A units.

If oxides are further accumulated on this ultra-thin layer, the surface activity due to the presence of oxides will be lost, and if oxides are further accumulated, bluing will eventually become visible to the naked eye.

Example 2 This example relates to a method for producing superior phosphate-treated steel sheets.

Forming a phosphate treatment film on the surface of a steel sheet significantly improves the adhesion and corrosion resistance of the paint film when it is painted on top of it. Because it shows excellent results in properties tests, etc., it is widely used as a base treatment for painting steel plates.

However, as a phosphate treatment method, a spraying method in which an aqueous phosphate solution such as zinc phosphate, iron phosphate, manganese phosphate, or aluminum phosphate is sprayed onto the steel plate surface, or a dipping method in which the steel plate is immersed in the aqueous solution are commonly used. In these cases, the amount of phosphate deposited is 1.
0. A reaction time of several tens of seconds to several minutes is required to reduce the reaction time to about ＠/m2, and electrolytic methods are known to accelerate the reaction and shorten the reaction time. It is also said that several tens of seconds or more is required to achieve a coating amount of about 1.027 m2.

Originally, the phosphate film used as a paint base does not require a large amount of adhesion and a large film thickness, but the particles are fine and dense, there are no voids, and it adheres firmly to the steel plate surface, so it does not peel off due to bending impact of the steel plate, etc. It is necessary to firmly hold the paint film as the legs of the paint, but since it is not possible to obtain the desired fine and dense phosphate film, it is possible to at least compensate for the defects by stacking the particles thickly. However, as mentioned above, it is not easy to create a thick film by short-time treatment.

This example utilizes Example 1 to create a phosphate coating that is extremely fine, has no gaps between particles, is dense, and has good adhesion to the steel plate surface, which was completely unknown in the past, in an extremely short period of time, for example, 2 to 3 seconds. The thickness is also 1.0. ? /C crown size can be created,
This is an epoch-making method that makes it possible for the first time to perform in-line phosphate treatment on a thin steel plate processing line.

In this example, it is used as a refrigerant for ultra-high speed water cooling in 0.3 seconds or less at a cooling water temperature of about 50°C from the N2 + H2 gas atmosphere of 400 to 450°C in the secondary water cooling, that is, overaging treatment furnace of Example 1. Instead of water, (1) an aqueous suspension of zinc phosphate, (2) an aqueous titanium colloid, or a sodium phosphate solution may be used as the phosphate pretreatment liquid to form the nuclei of phosphate particles.

However, electrolytic phosphate treatment is performed immediately after the secondary water cooling, but in this case, AC, 10A/
Three types of phosphate treatments were performed at a current density of dm2 for 1 second, 2 seconds, and 3 seconds.

As a reference example, a box annealed material was treated with the same phosphate as the secondary cooling liquid for 5 to 60 seconds.

All these runs are collected in Table 1 below.

As is clear from the table, the corrosion resistance of the phosphate-treated steel sheet of the present invention was significantly superior in the salt spray test, with a blistering width of 0.2 mm compared to the conventional xx units, and the adhesion was excellent. It was excellent in the cross-cut test, bending test, and impact test, achieving near perfect scores, and the processing time was extremely short at 1 to 3 seconds, making it suitable for in-line processing.

This is because the extremely small oxides generated on the surface of the steel sheet by ultra-high water cooling give extremely high surface activity, which captures phosphate colloid particles or titanium colloid particles in the pre-treatment liquid and forms nuclei. This is because phosphoric acid explosive powder is created on top of this core, and this is a unique effect unique to the present invention.

Example 3 This example relates to a method for manufacturing a plated steel sheet with excellent corrosion resistance.

In addition to electroplating, metal plating methods for the purpose of improving corrosion resistance on the surface of mild steel plates include Sn-plated tin steel plates, Zn-plated galvanized steel plates, and Cr-plated T, F, and S steel plates. AI plating, Zn plating, Pb
Widely used for plating, etc.

When plating these corrosion-resistant metals on the surface of a mild steel plate, in the case of wet plating, the oxide film on the surface is removed by pickling, and in the case of hot plating, the oxide film on the surface is reduced in a reducing atmosphere before plating. A process is being followed.

A feature of the present invention is that among these metal plating methods, wet metal plating is used to omit the pickling step, and plating is performed immediately after annealing and rapid cooling.

The steps for manufacturing a Zn-plated steel sheet using Example 1 are as follows.

<Conventional method for manufacturing Zn-plated steel sheet> Annealing → cooling → cleaning → pickling → water washing → electric Zn plating → water washing → post-treatment → drying - finished product Method for manufacturing Zn-plated steel sheet of the present invention> Annealing - quenching → electric Zn plating Plating → Washing → Post-treatment → Drying - Finished product The amount of coating and corrosion resistance of the Zn-plated steel sheet using the conventional plating bath produced in both these steps are shown in Table 2, compared to the conventional manufacturing method of Zn-plated steel sheet. It turns out that not only is it comparable to the previous model, but it actually tends to be better.

[Brief explanation of drawings]

FIGS. 1 and 2 are explanatory diagrams of a conventional vertical water cooling device equipped with a water curtain, and FIG. 3 is an explanatory diagram of a vertical water cooling device of the present invention equipped with a water curtain having a water outlet and an intake water port. Explanatory drawings, Fig. 4 is a schematic diagram of a conventional strip cooling method in continuous annealing, Fig. 5 is a schematic diagram of a strip cooling method in continuous annealing of the present invention, and Fig. 6 is a schematic diagram of oxides generated during water cooling of a high-temperature steel plate. A schematic diagram of cooling time and temperature. 1... Steel plate strip, 2... Spray box, 3... Ejection slit, 4...
...Ejection pump, 5...Suction pump, 6.
...Inhalation port.

Claims (1)

[Claims] 1. A cold-rolled steel sheet heat-treated in a non-oxidizing or reducing atmosphere is heated at a temperature of 350 to 800°C to 200°C or less for 150°C.
℃ or higher, using water as a refrigerant at an ultra-high speed of 1170℃/SeC or higher, where no bluing is observed, to obtain a steel plate with a bright surface, while at the same time generating a high degree of activity on the steel plate surface. Utilizing the activity during the short time that the activity lasts, without performing pickling in the next continuous process,
A rapid cooling surface treatment method for a cold-rolled steel sheet, characterized in that the surface of the steel sheet is subjected to phosphate treatment or metal plating.