JPH0479743B2 - - Google Patents

Description

[Detailed description of the invention]

[Industrial Application Field] The present invention relates to a continuous casting method for steel, in which a high-temperature slab is produced by continuous casting, and the hot slab is directly rolled and pickled. [Prior Art] As is well known, in continuous casting, molten steel is injected into a continuous casting mold (hereinafter referred to as the mold) to form a predetermined cross-sectional shape, and then continuously drawn by a group of guide rolls to produce slabs. is being carried out. In the mold, heat radiation of the molten steel in the mold;
Alternatively, a powder or granular coating agent is used to prevent oxidation of the molten steel surface, capture floating inclusions, and provide lubrication between the mold and the slab. By the way, in recent years, as the demand for energy and resource conservation increases, direct rolling, in which slabs manufactured by continuous casting are directly fed to the rolling process without cooling, has been actively adopted. . In order to carry out the direct rolling, it is necessary to maintain the slabs discharged from continuous casting at as high a temperature as possible. Therefore, in the cooling zone following the mold, after a solidified layer of a predetermined thickness has been formed on the surface of the slab, slow cooling is performed with a reduced cooling intensity to prevent the temperature of the slab from dropping. However, when hot slabs obtained under the above operating conditions were directly rolled, black band-like stains often appeared on the surface of the steel plate after rolling and pickling. The stains have a width of several mm to several centimeters and occur intermittently over several meters in the rolling direction, and are surface-colored defects without depth. Therefore, the inventors of the present invention conducted various experiments in order to remove the dirt, and as a result, they sprayed high-pressure water or high-pressure air water of 50 kg/mm ² or more onto the slab in a secondary cooling device following the mold. How to scale, and 2
Invented a method for altering and removing the scale structure by injecting impact water onto the surface of the slab in any part of the slow cooling zone in the secondary cooling device, and each patent application
No. 58-102497 and Japanese Patent Application No. 58-239527. On the other hand, for example, Japanese Patent Publication No. 11283/1983 proposes a technique for reducing the content of free carbon in the coating material used for molds to 2% by weight or less. [Problems to be solved by the invention] The above-mentioned Japanese Patent Application No. 102497/1982
No. 239527 is effective against dirt that has low adhesive strength to the slab surface, but there is a problem that the removal effect decreases as the adhesive strength increases. Since it was necessary to install nozzles, etc., there were difficulties in equipment and maintainability. In addition, the technology disclosed in Japanese Patent Publication No. 11283/1983 prevents the occurrence of hard spots with abnormally high carbon dioxide concentration and high hardness on the surface of slabs manufactured by continuous casting. The target was continuous casting at an extremely low casting speed of about 0.73 to 0.78 m/min, and the temperature of the slab at the end of the machine was also extremely low. The present invention maintains the slab obtained by the continuous casting at a high temperature at the end of the machine, and makes it possible to directly roll the hot slab. Efficiently solve problems that arise.
The purpose is to ensure that the problem is resolved. [Means for Solving the Problems] The present invention is a continuous casting method in which low-carbon ordinary steel is poured into a continuous casting mold, drawn out at high speed, cooled in a secondary cooling zone, and the slab is directly rolled and pickled. In the casting method, the inside of the mold has a C content of 0.7 to 1.5%, and
While covering with a coating material with a viscosity of 0.5 to 2.0 poise at 1300°C, the slab was pulled out at a speed of 1.4 m/min.
This is a continuous steel casting method characterized by slow cooling in a secondary cooling zone. [Effect] Carbon concentration C in steel is 0.10% or less (substantially C;
In order to directly roll low carbon ordinary steel (0.01-0.10, Mn: 0.15-0.35%), it is necessary to ensure that the cross-sectional average temperature of the slab at the machine end of continuous casting equipment is 1300°C or higher. For this reason, in the secondary cooling zone following the mold, after a solidified layer of a predetermined thickness has been formed on the surface of the slab, the cooling intensity is reduced, for example, by slow cooling with a specific water content of about 0.6/Kg. The drawing speed is 1.45 m/min or higher to prevent the temperature of the slab from dropping. However, when such a hot slab is directly rolled and pickled, black band-like stains occur as described above. The inventors of the present invention conducted various investigations and studies regarding the black band-like stain. As a result, this stain occurs only when the high-temperature slab is directly rolled and pickled, and does not occur at all when the slab is reheated in a heating furnace or the like and then rolled. I understand. As a result of further research, it was discovered that the stain was caused by the carbon in the coating agent in the mold adhering to and carburizing the surface of the slab, which was then formed as cementite on the surface of the steel plate after rolling. It was found that the cementite portion had turned black due to washing. The depth of carburization of the slab is about 1 mm, and when reheated in a heating furnace, all the carburized parts are oxidized and removed as scale, but when directly rolled, the surface thickness is oxidized and scale is removed. Since the diameter was about 0.5 mm or less, it was also found that some of the carburized parts remained and this turned black by pickling as described above. Based on the above-mentioned knowledge, the present invention has succeeded in making direct rolling possible by preventing carbon adhesion and carburization of the slab in the mold, and also in preventing the occurrence of the black band-like stain. be. That is, in the present invention, the C content of the coating material is
0.7~1.5% and viscosity at 1300℃ 0.5~
By setting the molten steel to 2.0 poise and performing continuous casting while covering the surface of the molten steel with the coating material, it was possible to completely solve the above problem. Now, as mentioned above, the coating material has the functions of dissipating heat from the molten steel in the mold, preventing oxidation of the molten steel surface, capturing floating inclusions, and lubricating between the mold and the slab. Carbon content is added to the coating material in order to control the melting rate of the coating material introduced into the mold and to stably exhibit the above-mentioned functions. The carbon concentration in this coating material varies depending on the type of steel being manufactured, casting conditions, etc. For example, if the carbon concentration is too low, the melting rate of the coating material will increase and the thickness of the molten layer in the mold will become excessive. , the slag flows unevenly, causing cracks in the slab and slag getting stuck. The present inventors directly rolled the hot slab, acid-washed it, varied the composition and physical properties of the coating material under the manufacturing conditions, and investigated the occurrence of black band stains, surface properties, etc. As a result, the C content is almost unaffected by other components of the coating material.
It is clear that when it exceeds 1.5%, the incidence of black band staining increases rapidly, and on the other hand, when it is below 0.7%, the coating material becomes red hot all the way to the surface, making it difficult to monitor the inside of the mold, causing operational problems. Summer. Further, even if the C content was 0.7 to 1.5%, if the viscosity of the coating material at 1300°C exceeded 2.0 poise, slag encroachment occurred frequently and cracks also occurred in some parts. Similarly, the occurrence of cracks increased when the temperature decreased below 0.5 poise. [Example] Example 1 Low-carbon ordinary steel having the composition shown in Table 1 was used at a concentration of 1.5 to 1.8
It is drawn out at a casting speed of m/min and slowly cooled in the secondary cooling zone (cooling intensity of 0.6 to 0.7/Kg is 1300
A high-temperature slab was produced at a temperature of 0.degree. C., and then the slab was directly rolled and pickled.

[Table] Figures 1 and 2 show the carbon concentration in the coating material, the incidence of black band staining, the carbon concentration in the coating material, and the thickness of the molten layer of the coating material on the surface of the molten steel in the mold in this example. This is a chart showing an example of the results of investigating the relationship between As can be seen from FIG. 1, if the carbon concentration in the coating material is 1.5% or less, the stain hardly occurs, but if it exceeds 1.5%, the rate of stain generation increases rapidly. On the other hand, as shown in Figure 2, when the carbon concentration is less than 0.7%, the melting rate increases and even the surface of the coating material inside the mold becomes red hot, making it impossible to monitor the inside of the mold, causing a major hindrance to operations. It became like that. Example 2 A low carbon ordinary steel having a thickness of 250 mm and a width of 930 nm and having the composition shown in Table 1 above was drawn at a casting speed of 1.45 m/min or more and slowly cooled in a secondary cooling zone (cooling intensity 0.6 to 0.7 /Kg) to produce a high-temperature slab at 1300°C, and then the slab was directly rolled and pickled. Table 2 shows the results of investigating the incidence of black band stains, surface properties of the slab, etc. in this example by varying the composition of the coating material, physical properties, and drawing speed of the slab. In Table 2, the incidence of black band contamination is expressed as the ratio of the length of black band contamination to the total length of the coil, and the surface quality of the slab is determined by counting the number of slag bites and cracks per 1 ^m2 of slab. The results are expressed as slab observation results. As can be seen from Table 2, examples based on the present invention (No. 1 to 4) Then,
In all cases, the occurrence of black band-like stains was extremely low, and no surface defects occurred. In contrast, the comparative example
Nos. 6 and 7 have a low carbon content, so black band stains are less likely to occur, but surface defects are more common. Further, Comparative Example No. 5 had a high carbon content, so the occurrence of black band stains was significantly increased.

【table】

〔Effect of the invention〕

By implementing the present invention, it has become possible to prevent the occurrence of black band-like stains without reducing the drawing speed or increasing the cooling intensity in the secondary cooling zone, and without causing surface defects on the slab. As a result, the rate of implementation of direct rolling has increased significantly, and has had a significant effect on energy and resource conservation.

[Brief explanation of the drawing]

Figure 1 is a chart showing an example of the results of investigating the relationship between the carbon concentration in the coating material and the incidence of black band staining, and Figure 2 is the graph showing the carbon concentration in the coating material and the coating on the surface of the molten steel in the mold. 2 is a chart showing an example of the results of investigating the relationship between the agent and the thickness of the melted layer.

Claims (1)

[Claims] 1. In the continuous casting method for steel, low carbon ordinary steel is poured into a continuous casting mold, pulled out at high speed, cooled in a secondary cooling zone, and the slab is directly rolled and pickled. While covering with a coating material of 1.5% and a viscosity of 0.5 to 2.0 poise at 1300°C,
The drawing speed of this slab is 1.4 m/min or more,
A continuous casting method for steel characterized by slow cooling in a secondary cooling zone.