JPH0472913B2 - - Google Patents

Description

[Detailed description of the invention]

<Industrial Application Field> The present invention relates to a pickling method for the purpose of removing a surface oxide film of a hot rolled steel sheet having poor descaling properties. <Prior art and its problems> In the steel sheet manufacturing process, pickling is usually performed for the purpose of removing a surface oxide film after hot rolling. Generally, an aqueous solution of hydrochloric acid or sulfuric acid is used to pickle a steel plate, but when these acids are used, a certain amount of time is required to completely remove the oxide film. In actual operation, this pickling time is rate-limiting. In particular, steel sheets that have been hot-rolled at 630° C. or higher or ultra-low carbon steels with a C content of 0.01% or less have poor descaling properties and require a long time for pickling. If we simply try to increase the sheet threading speed, the equipment will become longer and the equipment construction costs and operating costs will be enormous. Therefore, various methods for improving oxide film removal have already been proposed. For example, there is a method of mechanically removing oxide films by spraying fine grains onto a steel plate at high speed (Japanese Patent Application Laid-Open No. 1983-69524),
Alternatively, there are many known methods, such as applying electric potential and applying electrolysis (Japanese Patent Publication No. 55-9079), and processing the surface layer by rolling or bending and then pickling (Japanese Patent Publication No. 54-58632). There is. However, all of these methods have been difficult to put into practical use due to excessive equipment investment costs and poor surface properties. At least, there were problems when it was applied to the mass production process of steel plates. Also, it was difficult to apply to existing equipment. A further major problem is that, in any of the known improvement methods, the improvement effect on steel sheets with poor descaling properties, such as high-temperature coiling, is inferior to the improvement effect on ordinary steel sheets. Therefore, even if it is known that the material quality is greatly improved by applying high temperature winding,
In practice, high-temperature winding was very rarely adopted because pickling properties were a problem. Further, in recent years, the production of ultra-low carbon steel for cold-rolled steel sheets for deep drawing has increased, but this also has inferior pickling properties compared to ordinary low carbon steel, which has become a major problem. Therefore, it has been desired to develop a pickling method suitable for these steel plates with poor pickling properties. <Purpose of the invention> Therefore, the present invention has been made to solve the above-mentioned drawbacks of the conventional technology.It has a surface quality that is completely the same as that of a steel plate that has gone through a conventional pickling process, and can also be applied to existing pickling equipment. The present invention aims to provide a new pickling method that is both possible and particularly effective for steel plates with poor pickling properties, such as high-temperature webs. <Structure of the Invention> The present inventors have conducted various studies and found that in the case of steel plates that have particularly poor descaling properties in a normal pickling process, the pickling speed can be significantly increased by adding ultrasonic waves, and the present invention has shown that the pickling speed can be significantly increased by applying ultrasonic waves. It has been found that the objectives of the term can be advantageously achieved. That is, the present invention provides a pickling method for removing a surface oxide film from a hot rolled steel sheet with a hot rolling coiling temperature of 630°C or higher or an ultra-low carbon hot rolled steel sheet with a carbon content of 0.01% or less. The present invention provides a method for pickling a steel plate, which is characterized by adding ultrasonic waves to the solution. Further, by dispersing cementite particles with a particle size of 0.5 to 2000 μm in the pickling solution, the above effects can be further enhanced. The present invention will be explained in more detail below. In particular, the process that led to the present invention will be explained based on experimental results. Effect of ultrasonic application during pickling using commercial low carbon (C = 0.02-0.07%) and Al-killed steel and ultra-low carbon steel (C = 0.002%) rolled at various temperatures after hot rolling. I looked into it. These steel plates are said to have particularly poor pickling properties. The pickling solution used was a hydrochloric acid solution with a concentration of 10% heated to 50°C. Ultrasonic oscillation frequency 28KHz, output
It was added to the liquid using a 100W device. The time required for 99% of the scale on the steel plate to be removed was measured by visual judgment and was defined as the descaling time. At the same time, the descaling time was also confirmed by potentiometry, but it was not much different from the visual method. Figure 1 shows the results. Low carbon, cold rolled
Although the effect of ultrasonic application was observed in Al-killed steel, the effect was small. This effect was of a level known from the past. However, for high-temperature coiled materials, the pickling time was significantly shortened by adding ultrasonic waves. 1st
As shown in the figure, the higher the winding temperature, the greater the effect of applying ultrasonic waves. This effect became particularly noticeable when the winding temperature was 630°C or higher. As a result, it was found that when ultrasonic waves were applied, the descaling time of high-temperature webs was shorter than that of low-temperature webs, which is contrary to conventional wisdom. In other words, while conventional pickling methods and mechanical descaling methods were not very effective for high-temperature webs, only the ultrasonic pickling method was very effective for high-temperature webs. The present inventors made a new discovery. A similar phenomenon was also observed in ultra-low carbon steel, which is known to be difficult to scale.
An example is shown in FIG. It can be seen from Figure 1 that the application of ultrasonic waves becomes most effective when the winding temperature reaches 630°C or higher. In addition, 10% hydrochloric acid, liquid temperature
As a result of investigating the effect of applying ultrasonic waves in a 50°C liquid, it was found that the effect is significant in steels with carbon content of 0.01% or less, as shown in Figure 2. Therefore, the range in which the effect of the pickling method of the present invention is significant is steel plate with a coiling temperature of 630°C or higher or a C of 0.01
% or less. Figure 3 shows the results of a similar experiment conducted with low-carbon Al-killed steel and varying the concentration of hydrochloric acid. At concentrations above 5%, the effect of the main application of ultrasound became more pronounced. As described above, the effects of the present invention become more pronounced as the acid concentration becomes lower and the liquid temperature becomes lower. Appropriately, the frequency of the high frequency is about 1 to 100 KHz and the output is about 0.5 to 10 KW, but it is not limited to these. The higher the intensity of the high frequency, the greater the effect. Although the effects of the present invention are fully exhibited in a static bath, the effects are not lost even in a stirred bath. Furthermore, in order to enhance the effect of applying ultrasonic waves, it is very effective to disperse solid particles such as cementite with a particle size of about 0.5 to 2000 μm in the pickling solution. In these cases, the location where the ultrasonic waves were applied was clearly recognized whether it was the steel plate or the tank. The reason why the addition of ultrasonic waves is particularly effective in increasing the pickling speed of high-temperature web material is not yet clear.It is not only the simple liquid stirring effect, but also the hydrogen bubbles generated when the oxide film is dissolved and the scales that are difficult to remove with ultrasonic waves. In other words, it is presumed that some kind of interaction occurs with the dense scale, which is very effective in removing the oxide film. In particular, the fact that the effect of adding ultrasonic waves becomes noticeable only in the case of dense scales such as high-temperature web material indicates that not only the already known cleaning effect of ultrasonic waves but also the existence of other effects exist. Possibilities are also suggested. <Example> Continuous casting, hot rolling finishing temperature 860℃, coiling temperature 550℃
Alternatively, a 3.2 mm thick low carbon and ultra low Al killed steel hot rolled at 680°C was pickled under various conditions and the oxide film removal time was investigated. The added ultrasound has a frequency of 28K
Hz, output 2KW. The results are shown in Table 1.

[Table] ○ mark Invention example
*Marked: Cementite particles with a particle size of 1 to 100μ were dispersed in the bath.
Under all pickling conditions in the invention examples, the effect of applying ultrasonic waves was clearly observed, and the time for removing the oxide film was significantly shortened for high-temperature rolled material and ultra-low carbon steel.
Incidentally, in No. 8, cementite was dispersed in the bath, but the effect of applying ultrasonic waves was further promoted. <Effects of the Invention> According to the present invention, even low carbon steel or ultra-low carbon steel, which has particularly poor descaling properties by pickling, can be descaled in less time than before by applying ultrasonic waves during pickling. Can be effectively pickled.

[Brief explanation of drawings]

FIG. 1 is a graph showing the effects of winding temperature and ultrasonic application on oxide film removal time. FIG. 2 is a graph showing the influence of C content on the effect of ultrasonic application. FIG. 3 is a graph showing the influence of hydrochloric acid concentration on the effect of ultrasound application.

Claims (1)

[Scope of Claims] 1. In a pickling method for removing a surface oxide film of a hot-rolled steel sheet with a hot-rolling temperature of 630°C or higher or an ultra-low carbon hot-rolled steel sheet with a C content of 0.01% or less, pickling is carried out during pickling. A method for pickling steel sheets characterized by adding ultrasonic waves to the solution. 2. In a pickling method for removing the surface oxide film of a hot-rolled steel sheet with a hot-rolling temperature of 630°C or higher or an ultra-low carbon hot-rolled steel sheet with a C content of 0.01% or less, a particle size of 0.5% is added to the pickling solution during pickling. A method for pickling steel sheets characterized by applying ultrasonic waves to dispersed cementite particles of ~2000 μm.