KR910004116B1 - A testing method for the purity of slab - Google Patents

Abstract

A method for estimating a cleanliness of the cast slab comprises (a) grinding and cleaning a specimentaken from the cast slab, (b) electrolyzing the specimen for 20-30 minutes in an electrolytic soluton consisting of tetramethy ammonium chloride under the condition of 3-4 pH, 750-1300mA electric current, and (c) observing the specimen with the stereomicroscope and monitor.

Description

Cleanliness Evaluation Method of Continuous Casting

1 is a schematic diagram of an electrolytic apparatus for carrying out the electrolytic treatment of the present invention.

2 is a micrograph showing the shape of the oxidized inclusions in a large specimen.

3 is a graph comparing the cleanliness evaluation time of the present invention and the conventional method.

4 is a distribution chart showing the distribution of inclusion sizes in a specimen.

The present invention relates to a method for evaluating the cleanliness of continuously cast steel castings, and more particularly, to a method for evaluating the cleanliness of non-metal oxide inclusions in large specimens collected from continuously cast steel castings.

In general, non-metal oxide inclusions (hereinafter referred to as inclusions) present in continuous cast slabs (slab, bloom billet) are mixed during casting, in the form of deoxidation products and various slag refractory materials during the steelmaking process. However, the inclusions in these castings are not only a cause of the combination of slivers and black lines in hot and cold rolling, but also cause various defects such as fresh defects in final product processing. Therefore, only the precise identification of the cleanliness level, such as the shape, size, and number of inclusions in the cast can clearly identify its origin, and it is possible to establish effective measures to reduce such inclusions. In addition, a clear evaluation of the various mitigation measures is important.

As a method for evaluating inclusions in steel, electrolytic extraction methods disclosed in Image Analyzer and Japanese Patent Publication No. 54-12240 are known. In the image analysis method, after polishing a steel specimen, an optical microscope can be used to obtain an area ratio value of inclusions in a unit area, or to obtain size distribution information. However, the size of the specimen (typically, 10 mm width × 10 mm length) ), It is difficult to evaluate macro specimens (250mm thickness × 100mm width) such as cast pieces, and it is difficult to observe the actual size and shape of inclusions because the specimens are polished and observed in two dimensions. There is this.

In addition, the electrolytic extraction method presented in Japanese Patent Application Publication No. 54-12240 shows the actual size and shape of the substrate with respect to the size of the large specimen, but it is difficult to locate the inclusions because it dissolves the specimen. The separation process of elutriation, magnetic separation, hydrogen reduction, magnetic separation, hydrochloric acid treatment can be easily expected, and it is easy to anticipate the loss of many inclusions. It is impossible to apply the results to the in-process quality classification of hot rolling-cold rolling.

In particular, the hydraulic separation process of residues is easy to recover only inclusions of 50 µm or more (generally called large inclusions) due to the characteristics of Elutriation using the Stokes law and evaluation of fine inclusions of 50 µm or less. It is difficult and difficult to pinpoint the inclusions in the specimen because it dissolves all of the steel specimens by electrolysis. Therefore, the present invention uses the monitor attached to the stereo microscope and microscope after weakly electrolyzing the large specimens collected from the playing cast and the ingot, and then the three-dimensional observation of the oxidized inclusions in the specimen is smaller than in a short time. It is intended to provide a method for evaluating the cleanliness of continuous cast steels that can accurately assess inclusions of size.

Hereinafter, the present invention will be described in detail. According to the present invention, the specimens are taken from the continuously cast slab, polished, and ultrasonically cleaned, and then the pH is 3.4- and the current is 750 mA-1300 mA in an electrolytic solution consisting of 10-14 wt% tetramethylammonium chloride and an alcohol solution. This method relates to a method of evaluating the cleanliness of continuously cast steel castings after electrolysis for 20-30 minutes, and then observing the specimens with a stereo microscope and a monitor.

The electrolytic treatment method will be described in detail with reference to FIG. 1, in which the positive electrode is a steel piece 2 and the negative electrode is a stainless plate 3, respectively. The inclusions are separated from the specimen by the above electrolytic conditions. Instead of falling to the bottom, part of the inclusion is projected on the specimen by dissolving only Fe around the inclusion boundary on the specimen.

After drying the weakly electrolyzed specimens, put them on the specimen stand of the stereo microscope and move them from side to side at a constant magnification. Observe the inclusions through the monitor attached to the microscope. The actual size will be determined by using the inclusion size standard table.

The electrolyte solution consists of an organic solution, tetramethyl ammonium chloride (Tetramethyl Ammonium Chloride) + alcohol solution, when using this solution, an oxide layer is formed on the surface of the anode to prevent the passivation phenomenon does not occur easily.

The composition, pH, and current of the electrolyte are limited so that the inclusions to be analyzed on the specimen plane are properly exposed, so that the inclusions in the specimen are not separated during electrolysis, and there is a difference in localized current density and uneven electrolysis on the specimen plane. Unevenness due to and the like, and inclusions should not be dissolved by the electrolyte solution.

Hereinafter, the present invention will be described in detail through examples.

Example 1

After cutting, processing and polishing a 100 mm × 2500 mm thick specimen from a slab, which is a slab of a low carbon Al-Killed steel, electrolytic treatment was performed under the electrolytic conditions shown in Table 1 using the electrolytic apparatus of FIG. 1 after ultrasonic cleaning.

The electrolytic state and the inclusion protrusion state are shown in Table 1 by microscopic observation of the electrolytically treated specimen as described above. Inventive materials 5-6 and 8-9 electrolytically treated under the electrolytic conditions according to the present invention as shown in Table 1, unlike Comparative material 1-4, Comparative material 7, and Comparative material 10-12, subsequent stereo It can be seen that the electrolytic specimen is suitable for the cleanliness evaluation process using a microscope and a monitor.

Example 2

The specimen was taken from the slab, a low carbon Al-Killed steel slab, 100mm wide x 250mm long and 5mm thick, and surface treated, and then ultrasonically cleaned to obtain 800mA current and 800mV current in an electrolyte consisting of 10% tetramethyl ammonium chloride + alcohol. Electrolysis was carried out for about 30 minutes at the voltage and electrolysis conditions of pH 4.

After the electrolytic treatment is completed, the specimen is washed with alcohol, dried and placed on the microscope specimen stage. The magnification is 50x while moving left, right, up, and down, and is primarily observed by the microscope objective lens and the alternative lens. The oxidation inclusions shown were identified in FIG. 2 and the distribution state of the oxidation inclusions shown on the large specimens as shown in FIG. 2 is shown in FIG. 4 along with the conventional method. As shown in Figure 4, it can be seen that the evaluation method according to the present invention can also evaluate the oxidation inclusions of a very fine size compared to the conventional method. In addition, the time required for the evaluation method according to the present invention is shown in FIG. 3 in comparison with the conventional method. As can be seen from this, it can be seen that the present invention takes less time for evaluation than the conventional method. have.

As described above, the present invention not only significantly reduces the cleanliness evaluation time by electrolyzing under appropriate conditions and observing with a stereo microscope and monitor, but also has the effect of observing the positional shape of the inclusions and even fine oxidation inclusions. It is.

Claims (1)

The specimens were taken from the continuously cast slabs, polished and washed, and then in an electrolyte solution consisting of 10-14 wt% tetramethylammonium chloride and an alcohol solution for 20-30 minutes under the conditions of pH 3-4 and current 750 mA-1300 mA. Cleanliness evaluation method of the continuously cast cast, characterized in that after the electrolytic treatment, the specimen is observed with a stereo microscope and a monitor.

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