JP2006181583A - Method for producing continuously cast slab - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for producing a continuously cast slab excellent in surface characteristics by which an oxidized scale layer is frmed on the surface of the cast slab inexpensively without adding any new process to perform a HCR (Hot Charge Rolling) and a DHCR (Direct Hot Charge Rolling). <P>SOLUTION: When the cast slab 18 is secondarily cooled in a continuous caster 1, saturated steam or excessive heated steam as cooling medium injected toward the cast slab from spray nozzles 16, is used. Since the cast slab is cooled with the steam, steam partial pressure effected to the generation of the scale, can be made to high and the thickness of the scale generated on the surface of the cast slab becomes thicker in comparison with the case of the water-spray, and defect, such as crack, non-metallic inclusion. slag, etc., existing the surface layer of the cast slab and just below the surface layer of the cast slab, is removed by exfoliating together with the scale layer. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a method for producing a continuous cast slab capable of obtaining a cast slab having good surface properties.

  Recently, in continuous casting of steel, an attempt has been made to reduce the amount of heating furnace energy used during hot rolling by holding the hot slab drawn from the mold as high as possible. Yes. Specifically, it is a method called hot charge rolling (HCR) or direct rolling (DHCR), which is an effective process for energy saving.

  Compared with the usual method, that is, the method in which hot rolling is carried out by heating to a predetermined temperature in a heating furnace after allowing the high-temperature slab to cool to about room temperature, is a matter that becomes a problem in performing this HCR or DHCR. Thus, since the time for holding the slab at high temperature is shortened, the amount of oxide scale generated on the surface of the slab before hot rolling is reduced. For this reason, the probability that the cracks, non-metallic inclusions, slag, and other defects existing on the surface of the slab or directly under the surface layer will be peeled off and removed with the generation of oxide scale, and as a result, the steel after hot rolling. In this case, the manifestation of surface defects increases, leading to a decrease in yield. In order to suppress this, measures such as regulation of the upper limit of casting speed, optimization of mold flux, optimization of molten steel flow in the mold have been conventionally taken.

  However, even when these countermeasures are implemented, the surface defects of the slab cannot be completely prevented, and when surface defects are predicted during hot rolling, the application of HCR or DHCR is avoided and the slab is once cooled. Alternatively, it is necessary to incorporate a process of removing the surface defects of the slab before hot rolling by scarfing the slab surface.

  As a method for solving such a problem, for example, Patent Document 1 discloses a mold powder adhering to the surface of a slab by injecting high-pressure air onto a high-temperature slab in a secondary cooling zone of a continuous casting machine. There has been proposed a method for producing a slab having excellent surface properties by peeling off the slab.

However, in the method of Patent Document 1, since air is simply blown, it is a matter of course that not all the above-described defects existing on the slab surface layer or immediately below the surface layer can be removed.
JP-A 63-248549

  The present invention has been made in view of the above circumstances, and its object is to generate an oxide scale layer on the surface of a slab without adding a new process, thereby implementing HCR or DHCR. An object of the present invention is to provide a method for producing a continuous cast slab having excellent surface properties.

The present inventors have promoted the generation of oxide scale on the slab surface in the secondary cooling zone of a continuous casting machine for the purpose of establishing a method for producing a slab having excellent surface properties without adding a new process. Various investigations were added. As a result, among the influencing factors such as oxygen partial pressure (P _O2 ) and water vapor partial pressure (P _H2O ) on the production rate of oxide scale, the influence of water vapor partial pressure (P _H2O ) is particularly large. From knowledge (for example, JP-A-9-271917 filed by the present inventors), it has been found that steam is effective as a cooling medium in secondary cooling of a continuous casting machine. That is, by using water vapor as a cooling medium, it has been found that cooling of the slab, that is, promotion of solidification, and improvement in the production rate of oxide scale on the surface of the slab can be achieved.

  Normally, when water is sprayed on a high-temperature slab whose surface temperature exceeds 800 ° C., a vapor film is formed between the slab and water or a water film, and heat transfer, that is, cooling of the slab is performed via this vapor film. Will be done. As a result, the use of latent heat of vaporization of water becomes insufficient. Therefore, in order to increase the cooling capacity, the collision pressure of water on the slab, that is, the flow velocity is increased to reduce the vapor film thickness. It is necessary to break and bring the slab surface and water into direct contact.

Although the use of such high-pressure water can be expected to improve the cooling rate of the slab and increase the effect of removing the oxide scale on the slab surface, We cannot expect improvement. This is because, as described above, the use of high-pressure water reduces the vapor film, and the slab surface is not covered with the vapor film or the area of the vapor film is reduced, which is effective for the generation of oxide scale. This is because the water vapor partial pressure (P _H2O ) at the slab surface is lowered.

In this regard, when steam is used, unlike the case of water, the cooling effect due to latent heat of vaporization cannot be expected. Therefore, in order to increase the cooling capacity, it is necessary to increase the impact pressure (= flow velocity) of steam on the slab. However, since it is a gas, it is possible to easily obtain a flow rate several times to several tens of times that of water. Moreover, in this case, the water vapor partial pressure (P _H2O ) on the surface of the slab does not decrease. That is, by using water vapor as the secondary cooling medium, it is possible to increase the secondary cooling strength without reducing the scale generation rate, and water vapor is more suitable than water sprays that are normally used. The knowledge that it is superior as a cooling medium of the next cooling was obtained.

  In order to confirm this phenomenon, a basic experiment was conducted to investigate the rate of oxide scale formation. In a basic experiment for cooling a high-temperature slab of about 1,000 ° C., as a result of comparing the generated scale thickness when water is used as the cooling medium and when saturated steam at about 160 ° C. is used, water vapor is used as the cooling medium. As a result, it was confirmed that the rate of scale formation reached about 1.2 times that when water was used.

Judging from the fact that film boiling heat transfer is generally performed at high temperatures as described above, it is considered that the slab surface is covered with water vapor even when water is used. Therefore, it is unlikely that there is a difference in the water vapor partial pressure (P _H2O ) on the slab surface. On the other hand, when the same spray nozzle is used, the collision flow velocity of the cooling medium to the slab surface is several to several tens times faster than that of water as described above. In such a case, it is considered that the removal and generation of the oxide scale occurred repeatedly, thereby improving the generation rate of the oxide scale.

  The present invention has been made on the basis of the above examination results, and the continuous casting slab manufacturing method according to the present invention is directed from the spray nozzle to the slab when the slab is secondarily cooled by a continuous casting machine. Saturated steam or superheated steam is used as a cooling medium to be injected.

  According to the present invention, the thickness of the scale generated on the surface of the slab cast by the continuous casting machine is thicker than in the case of the water spray, and the crack, non-metal present in the slab surface layer and immediately below the slab surface layer Since defects such as inclusions and slag are peeled and removed together with the scale layer, the surface defects of the continuous cast slab can be greatly reduced without adding a new process. As a result, even when HCR and DHCR are applied, it is possible to reduce the surface defects of the steel material after hot rolling, and it is possible not only to improve the product yield but also to enjoy the advantages of HCR and DHCR. Industrially beneficial effects are brought about.

  Hereinafter, the present invention will be described in detail with reference to the accompanying drawings. FIG. 1 is a schematic sectional view of a continuous casting machine suitable for carrying out the present invention.

  As shown in FIG. 1, a continuous casting machine 1 for casting slab slabs is provided with a mold 4 for injecting molten steel 17 to solidify, and above this mold 4 is a ladle. A tundish 2 that receives the molten steel 17 from (not shown) and supplies the received molten steel 17 to the mold 4 via the immersion nozzle 3 is disposed. On the other hand, a pair of opposing rolls 1 is provided below the mold 4. A plurality of slab support rolls 5 are installed as a set. And on the downstream side of the slab support roll 5, a plurality of transport rolls 6 and a gas cutter 7 positioned above the transport roll 6 and synchronized with the casting speed of the slab 18 are installed. Further, the slab support roll 5 has first cooling zones 8, 8, second cooling zones 9, 9, third cooling zones 10, 10, and fourth cooling zone 11 from directly under the mold 4 toward the downstream side. , 11, fifth cooling zones 12, 12, and sixth cooling zones 13, 13 are provided with a secondary cooling zone comprising cooling zones divided into a total of 12 locations.

  As illustrated in the first cooling zone 8 of FIG. 1, each cooling zone of the secondary cooling zone is provided with a cooling medium pipe 14 for supplying a cooling medium for secondary cooling, and the cooling medium pipe 14 is branched. A spray nozzle 16 is installed at each tip so as to face the surface of the slab 18, and the cooling medium supply side of the cooling medium pipe 14 supplies water with a supply pipe 14 a for supplying water. The flow control valve 15a is arranged in the supply pipe 14a for supplying water, and the flow control valve 15b is arranged in the supply pipe 14b for supplying water vapor. Has been. That is, by adjusting the flow rate adjusting valve 15a and the flow rate adjusting valve 15b, either water or water vapor or a mixture of water and water vapor can be supplied as a cooling medium. Water or water vapor or a mixture of water and water vapor as a cooling medium is sprayed from the spray nozzle 16 toward the surface of the slab 18. Although not shown in FIG. 1, all the cooling zones from the second cooling zone 9 to the sixth cooling zone 13 have such a structure.

  In addition, from the viewpoint of controlling the slab surface temperature and oxide scale generation amount, it is desirable to be able to control the type and flow rate of the cooling medium in each cooling zone as described above. It is not necessary to have such a structure, and a conventional water spray may be used in some cooling zones. Moreover, it is good also as a structure which supplies only water vapor | steam as a cooling medium. Furthermore, although the total number of cooling zones is 12 in FIG. 1, it may be divided into several according to the length of the continuous casting machine 1.

  In the continuous casting machine 1 having such a configuration, a continuous cast slab is cast as follows. That is, molten steel 17 is cast from the tundish 2 to the mold 4 through the immersion nozzle 3. The molten steel 17 cast in the mold 4 is cooled by the mold 4 to form a solidified shell 19, and is continuously drawn downward as a slab 18 having an unsolidified layer 20 therein while being supported by the slab support roll 5. It is. While the slab 18 passes through the slab support roll 5, it is cooled in a secondary cooling zone using saturated steam or superheated steam as a cooling medium to increase the thickness of the solidified shell 19 and eventually solidify to the center. Complete. In this case, when carrying out HCR or DHCR, it is desirable to raise the temperature of the slab 18a. Therefore, in order to obtain a high-temperature slab 18a, the solidification completion position 21 should be located on the exit side of the continuous casting machine 1. preferable. The slab 18 thus cast is cut by the gas cutter 7 to obtain a slab 18a. The slab 18a is hot-rolled in the next step by HCR or DHCR, or a normal method, that is, a method in which the slab 18a is allowed to cool to room temperature and then heated in a heating furnace and hot rolled. Here, the saturated water vapor is water vapor in a state where the steam is saturated, and the superheated water vapor is water vapor heated to a temperature equal to or higher than the saturation temperature of the vapor.

Thus, in the present invention, since steam is used as the secondary cooling medium in the secondary cooling zone of the continuous casting machine 1, the steam partial pressure (P _H2O ) is increased on the surface of the slab 18 cooled in the secondary cooling zone. The thickness of the scale produced on the surface of the slab 18 is maintained higher than that in the case of the water spray, and defects such as cracks, non-metallic inclusions, slag, etc. existing on the surface layer of the slab 18 and the surface layer are scaled. Since it is peeled and removed together with the layers, it is possible to greatly reduce the surface defects of the slab 18a discharged from the continuous casting machine 1 without adding a new process. As a result, even when HCR and DHCR are applied, it is possible to reduce the surface defects of the steel material after hot rolling, and it is possible not only to improve the product yield but also to enjoy the advantages of HCR and DHCR. In addition, the change width of the supply pressure of water vapor can be made larger than that of water, and a predetermined cooling strength can be obtained by arbitrarily changing the pressure of water vapor sprayed from the spray nozzle 16 toward the slab 18. Therefore, it is possible to cast without setting an upper limit of the casting speed.

  In addition, although the said description was performed regarding the slab slab, even if it is a bloom slab and a billet slab, this invention can be applied along the above.

  A cast obtained by performing continuous casting of the low carbon steel shown in Table 1 using a slab continuous casting machine shown in FIG. 1 having a cast piece thickness of 260 mm and a cast piece width of 1,200 mm to 1,500 mm. The piece was DHCRed to finally produce a cold rolled steel sheet having a thickness of 0.8 mm to 1.5 mm. And in the obtained cold-rolled steel sheet, the occurrence state of surface defects such as hege and sliver was investigated.

  The casting speed was 1.8 m / min, the average charging temperature of the slab into the heating furnace was 880 ° C. to 920 ° C., the in-furnace time was 80 minutes to 90 minutes, and the furnace temperature was 1,150 ° C. In this embodiment, a two-strand continuous slab caster is used, and the method of the present invention is applied to one strand and saturated water vapor (temperature 170 ° C.) is used as a cooling medium for secondary cooling in all cooling zones (the present invention). Example), the other strand was subjected to secondary cooling by a normal water spray (comparative example). Table 2 shows the casting conditions and the incidence of surface defects in the cold-rolled steel sheet.

  As is clear from Table 2, in the present invention example, a significant reduction in the surface defect rate in the cold-rolled steel sheet was recognized compared to the comparative example. This is because the use of water vapor as the cooling medium promotes the generation of oxide scale on the surface of the slab, and the oxide scale thickness can be secured to such an extent that all surface defects can be removed.

It is a schematic sectional drawing of the continuous casting machine suitable for implementation of this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Continuous casting machine 2 Tundish 3 Immersion nozzle 4 Mold 5 Casting piece support roll 6 Transport roll 7 Gas cutting machine 8 1st cooling zone 9 2nd cooling zone 10 3rd cooling zone 11 4th cooling zone 12 5th cooling zone 13 Sixth cooling zone 14 Cooling medium piping 15a, 15b Flow rate adjusting valve 16 Spray nozzle 17 Molten steel 18 Cast slab 19 Solidified shell 20 Unsolidified layer 21 Solidification completion position

Claims (1)

  A method for producing a continuous cast slab, wherein saturated steam or superheated steam is used as a cooling medium sprayed from a spray nozzle toward the slab when the slab is secondarily cooled by a continuous casting machine.

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