JP4525690B2 - Steel continuous casting method - Google Patents

Description

The present invention relates to a continuous casting how the steel, in particular, relates to a continuous casting how the steel can effectively prevent the cast piece surface layer defect due to nonuniform cooling in the secondary cooling zone top.

When steel slabs are manufactured by continuous casting using a water-cooled mold, the molten steel is pulled out while solidifying in the mold. As a result, the solidified shell grows more unevenly. In the worst case, vertical cracks occur on the surface of the slab in the mold or in the secondary cooling zone.
Such vertical cracks on the surface of the slab occur frequently in medium carbon steel mainly having a carbon content of 0.10% by mass. When vertical cracks occur, the defect and defect removal work (hereinafter referred to as “care”) is required prior to sending the slab to the rolling process. Since the tendency of defect generation tends to increase significantly with the increase in casting speed, the casting speed in today's general slab continuous casting machine has increased by about 1.5 to 3 times compared to 10 years ago, for example. This is also a factor in increasing maintenance work.

Therefore, non-uniformity of solidified shell growth due to non-uniform cooling is an obstacle to the application of direct feed heating (hot charge) or direct feed rolling (direct charge), which has been increasingly applied today due to its outstanding economic efficiency. At the same time, it is an obstacle to high-speed casting, which is a requirement for improving productivity.
In order to prevent the occurrence of surface vertical cracks in such continuous cast slabs, uniform and slow cooling is performed in the initial stage of solidification (hereinafter referred to as initial solidification), and the growth of solidified shells is made uniform. It is thought that it is important to suppress the generation of “coagulation of the solidified shell”, and many techniques relating to initial solidification control have been disclosed.

  On the other hand, uniform cooling in the secondary cooling zone is also an extremely important factor for obtaining a sound slab, but there is only a cooling method such as mist spray that uniformly sprays spray water onto the slab.

  By the way, according to the research conducted by the present inventors, even when uniform cooling is achieved in the mold and in the secondary cooling zone, in the high speed casting in which the casting speed exceeds 2 m / min, the slab has the following reason. Often, the uniform cooling of the air is hindered. That is, due to the mold flux discharged from the mold and adhering unevenly to the slab, 1) Increase in the scale generation speed of the slab surface, 2) Adhesion accompanying the decrease in the scale of the slab surface and the melting point of the mold flux mixed layer This is non-uniform cooling of the slab due to non-uniformity of thin and thick oxide scale layers on the surface of the slab.

  The oxide scale has a thermal conductivity approximately one order of magnitude smaller than that of the solidified shell, and causes non-uniform cooling as a heat transfer resistance layer in the secondary cooling zone after the mold. This non-uniform cooling is one of the causes of surface cracks in the slab and fluctuations in the mold surface due to unsteady bulging, and acts as an obstacle to maintaining high quality and stable operation in the continuous casting process of steel. . FIG. 3 is a graph showing the correlation between the amount of slab adhesion mold flux and the slab surface quality in such a case. Here, for the data on the horizontal axis in FIG. 3, a value calculated from the concentration of CaO contained by collecting the slab short side surface scale before the hot rolling furnace was used.

The present invention aims to provide a continuous casting how the steel can effectively remove the mixed layer of mold flux and oxidation scale causative slab surface uneven cooling in such a secondary cooling zone .

The present invention relates to a continuous casting method of steel, using a nozzle that forms a linear spray pattern between the mold and the first slab support roll , on the slab surface at a casting speed of 2 m / min or more. A continuous casting method of steel , wherein a mold jet and an oxide film adhering to the surface of the slab are removed by causing a water jet to collide at a collision pressure of / cm ² or more and 22 N / cm ² or less .

  According to the present invention, in continuous casting of steel, the non-cooling in the secondary cooling zone that adheres unevenly to the slab surface between the mold and the first slab support roll that becomes the inlet of the secondary cooling zone. Since the flux and oxide film, which are a uniform facilitating factor, are removed, spray cooling in the subsequent secondary cooling zone is made uniform and strengthened, eliminating uneven solidification of the slab or local solidification delay. The present invention has an excellent effect that it can reduce defects on the surface of the slab such as vertical cracks and unstable factors such as unsteady bulging.

  In the present invention, a flux (non-uniform cooling promoting factor in the secondary cooling zone, which is unevenly adhered to the slab surface between the mold and the first slab support roll that becomes the entrance of the secondary cooling zone, (Mold flux) and oxide film (oxide scale) are removed, and spray cooling in the subsequent secondary cooling zone is made uniform and strengthened to eliminate uneven solidification of the slab or local solidification delay. In addition, defects on the surface of the slab such as vertical cracks and operational instability factors can be reduced.

  Any physical method can be applied as means for removing the flux and oxide film. For example, a method of spraying high pressure water on the surface of the slab (a method using high pressure water spray), a method of pressing the surface of the slab with a roll having unevenness, a method of rubbing the surface of the slab with a brush of high-temperature strength alloy wire, etc. Can be mentioned. However, considering the running cost, the certainty of the effect, the ease of maintenance, the ease of equipment construction, and the additional effect of improving the productivity of the continuous caster by strengthening the cooling with high impact force spray, A spray method is preferred.

In this high-pressure water spray method, as shown in FIG. 1, for example, high-pressure water is applied to the surface of the slab 5 between the mold 1 and the first slab support roll 4 (1st). It can be carried out by providing the spray nozzle (high pressure water spraying means) 2. However, if the impinging pressure of the water jet on the slab surface is less than 10 N / cm ² , the mixed layer 6 of the flux and oxide film adhering to the slab 5 discharged from the mold 1 cannot be destroyed or removed. Therefore, spray conditions such as the nozzle type of the high-pressure water spray nozzle 2, the spray distance, the nozzle back pressure, and the amount of water spray are determined so that the impinging pressure of the water jet on the slab surface is 10 N / cm ² or more. It is necessary to attach.

A nozzle type for obtaining a high collision pressure is preferably a descaling type that forms a substantially linear spray pattern as shown in FIG. On the other hand, the normal secondary cooling spray nozzle 3 as shown in FIG. 1 is a flat type that forms an elliptical spray pattern as shown in FIG. It is difficult to obtain a collision pressure of 10 N / cm ² or more with a flat type nozzle.

In the present invention, as to the upper limit of the collision pressure, applying a collision pressure more than necessary to a high-temperature solidified shell (solidified shell) having a thickness of about 10 to 20 mm is a steel leakage accident due to shell breakage, etc. , Damage to operations is also a concern and should be avoided as much as possible.
The present invention, in particular casting speed (cast strip withdrawing speed) faster casting work than 2m / min, because the effect is prominently expressed, performed at such a high speed casting campaign.

  In the continuous casting machine of the form shown in Fig. 1, high-pressure water spray is used to cast medium carbon steel containing 0.11% by mass of carbon with high cracking sensitivity under casting conditions of molten steel superheat 25K and casting speed 2.4 m / min. Two types of descaling type A and flat type B, which form the spray patterns shown in FIGS. 2 (A) and 2 (B), are used as nozzles 2, and water jets are sprayed onto the slab under various conditions shown in Table 1. It was. The amount of water jetted was 70 L / min per slab width for each condition.

Table 1 shows the quality primary failure index (based on condition C1) due to the occurrence of vertical cracks in the slab investigated on the outlet side of the secondary cooling zone for each condition. In the comparative example (conditions C1 to C3) in which the mixed layer 6 of the flux and oxide film could not be broken and removed because the slab impingement pressure of the water jet was less than 10 N / cm ² , a slight improvement was observed in the condition C3. However, sufficient quality has not been obtained. On the other hand, in the embodiment (conditions E1 to E3 ) of the present invention in which the mixed layer of the flux and the oxide film was destroyed and removed by setting the slab impact pressure of the water jet to 10 N / cm ² or more, a remarkable quality improvement effect was obtained. I was able to.

  In addition, when a general low carbon steel containing 0.04% by mass of carbon was cast under the same trial conditions as described above, in the comparative example, there were some fluctuations in the mold surface due to unsteady bulging in the secondary cooling zone. On the other hand, in the examples, uniform strong cooling in the secondary cooling zone was promoted, the shell thickness was increased, and unsteady bulging was suppressed.

It is a mimetic diagram showing one embodiment of the present invention. It is a spray pattern comparison figure of the high-pressure water spray (A) which concerns on this invention, and the normal secondary cooling spray (water spray or mist spray) (B). It is a graph which shows the correlation with the amount of residual flux of the slab surface and the quality rejection index by the vertical crack defect generation | occurrence | production based on this invention.

Explanation of symbols

1 Mold (Water-cooled copper plate mold)
2 High-pressure water spray nozzle (high-pressure water spraying means)
3 Normal secondary cooling spray (water spray or mist spray) nozzle 4 Casting support roll 5 Casting (continuous casting cast)
6 Mixed layer of flux (mold flux) and oxide film

Claims (1)

In the continuous casting method of steel, a nozzle that forms a linear spray pattern between the mold and the first support roll is used, and the surface of the slab with a casting speed of 2 m / min or more is 10 N / cm ² or more and 22 N / A continuous casting method of steel , wherein a mold flux and an oxide film adhering to the surface of the slab are removed by causing a water jet to collide with a collision pressure of cm ² or less .

Images