JPS642464B2 - - Google Patents

Description

[Detailed description of the invention]

(Industrial Application Field) The present invention relates to the casting of extra-thick slabs, and more particularly to two-way solidification casting from the top and bottom surfaces of extra-thick slabs. (Prior art) In general, when casting extra-thick slabs using molten steel, the ingot making method, which has been used for a long time and involves rolling after obtaining an ingot, is continuously used for reasons such as low manufacturing cost and high productivity. It is well known that casting methods are widely used. However, with these casting methods, for example, in the ingot forming method, significant component segregation and shrinkage pores are formed at the head of the ingot, and a dense segregation zone also exists inside the ingot, resulting in lower yields. This will impede the decline in quality and the upgrading of quality.
On the other hand, the continuous casting method, although not as extreme as the ingot-forming method, similarly has the drawback of segregation of concentrated components in the center of the slab. Therefore, methods have been proposed that solve the problems of the ingot forming method or the continuous casting method and directly cast extremely thick slabs. For example, the so-called one-way solidification method, in which the top and side surfaces of the mold are kept warm and molten steel solidifies from the bottom surface of the mold, as in Japanese Patent Publication No. 53-19290, or the method of pouring molten steel as in Japanese Patent Publication No. 59-178152, A so-called two-way solidification method has been proposed in which the lower part of the molten steel is solidified while the surface of the molten steel is kept warm, and then the molten steel is solidified from the upper surface as well, and has been shown to be quite effective. (Problems to be solved by the invention) However, even in these new casting methods, for example, in the unidirectional solidification method, a large component segregation layer is formed in the upper layer of the final solidification part, and the cooling becomes slower in the upper layer. As a result, interparticle segregation increases, and both yield and quality are not sufficient. A two-way solidification method has been proposed as a means to solve the problems of this one-way solidification method.In this two-way solidification method, by suppressing the cooling from the top surface of the cast molten steel, the growth of the top surface solidification shell is delayed, and the top surface The shell strength becomes relatively low, which causes the upper shell to descend following the solidification shrinkage, thereby suppressing the formation of solidification shrinkage holes. However, if a heat insulating agent is added to the top surface of the molten steel and slow cooling is performed to maintain it in a molten state for an appropriate amount of time, uniform cooling cannot be achieved over the entire top surface due to uneven distribution and the generation of latent heat of solidification during the solidification process. This results in partial unidirectional solidification, which prolongs the solidification time, increases internal segregation, and causes defects in the solidified surface layer. In addition, by adding a heat insulating agent, a carburized layer of about 20 mm is formed on the surface of the slab, for example, in the case of a 300 mm thick slab, which causes drawbacks such as a significant drop in yield and impairing the quality of the slab. have. (Objective of the Invention) The present invention solves the drawbacks of the conventional method as described above, and is aimed at eliminating the large component uneven layer formed on the surface of the slab when molten steel is cast, or the top surface solidification such as two-way solidification. Defects in shape due to uneven cooling of the shell;
The purpose of the present invention is to provide a method for casting high-quality extra-thick slabs that does not prolong solidification time, and does not impede yield and quality due to formation of a carburized layer or increase in internal segregation due to heat retention. (Means for Solving the Problems) The gist of the present invention for achieving the above object is to solidify the molten steel being poured from the bottom of the mold, and then spray a gaseous refrigerant onto the surface of the molten steel to rapidly cool it and form a solidified shell. This is a casting method for high-quality, extra-thick slabs, which is characterized by forming a solidified shell, and then slowly cooling the solidified shell by adjusting the coolant. The method for casting high-quality extra-thick slabs according to the present invention will be described below. The present inventors based on the knowledge that when casting an extremely thick slab, it is difficult to obtain a slab with uniform composition and no shrinkage holes using the conventional method of solidifying the surface of the cast molten steel by keeping it warm. Various experiments were conducted. As a result, it was discovered that it was possible to cast an extremely thick slab with uniform composition and non-shrinkage pores only by using a method completely contrary to conventional wisdom. That is, in the present invention, molten steel is poured into a mold, and the bottom surface is cooled by cooling on the bottom surface of the mold (surface plate), or by air.
While the molten steel is solidified by cooling by active heat removal through a refrigerant such as gas or water, the surface of the molten steel is also cooled by spraying a gas fluid such as Ar or N ₂ to quickly form a solidified shell on the upper surface. form. The upper surface solidified shell formed by spraying this gas fluid has a uniform solidified composition in both the surface layer parts and the bottom surface, and has an extremely smooth surface shape. Therefore, the present invention solidifies the cast molten steel from the bottom and also from the top to an appropriate thickness, and then reduces or interrupts the blowing of the gas fluid so that the solidified shell on the top is absorbed mainly from the internal heat of the ingot. It is completely solidified from the bottom and top while reheating (raising the temperature). By reheating the upper surface solidified shell of this extremely thick slab-shaped ingot, the upper surface solidified shell easily follows and descends according to the contraction of the molten steel formed inside.
No shrinkage pores occur. For this purpose and the uniformity of the solidified composition, it is necessary that the solidified thickness from above is 50 to 200 mm. If the solidification thickness on the top surface is less than 50 mm, pockmark defects will occur in the surface layer due to insufficient cooling of the top surface of the ingot, and solidification will approximate unidirectional solidification, resulting in an increase in internal component segregation. In addition, if the solidification thickness is larger than 200 mm, the solidified shell on the upper surface becomes thicker, and even if it is recuperated by internal heat, the solidified shell on the upper surface cannot follow the downward movement due to internal solidification, resulting in component segregation in the shrinkage hole and its surroundings. Occur. Here, the solidified thickness on the upper surface indicates the so-called total solidified amount from above, including the initial solidified shell on the upper surface and the subsequent solidified amount from above. In addition, the thickness of the solidified shell on the top surface formed initially by gas fluid spraying differs slightly depending on the thickness, length x width, etc. of the extra-thick slab-shaped ingot, according to the results of embedded measurements using thermocouples, etc. 3 mm to 50 mm is preferred. This is due to the reason mentioned above regarding the total solidification amount on the upper surface. Therefore, in order to realize the solidification form of the cast molten steel in the form of an extremely thick slab according to the present invention, the heat removal from the upper surface of the cast molten steel must be 1 m/sec to
By blowing gas at 50Nm ³ /hr to 3000Nm ³ /hr at 6m/sec, 1.0×10 ⁴ Kcal/m ² hr to 8.0×10 ⁴ kcal/m ²
hr is required. Note that an example of the top surface solidification rate (%) at this time is also shown in FIG. As shown in the figure, with the heat retention type (conventional method) or the closed lid (not shown), the initial purpose cannot be achieved due to the initial formation of a solidified shell on the upper surface and poor progress of recuperative resolidification. Next, details of casting an extra-thick slab according to the present invention will be described. FIG. 1 is a drawing showing a method for casting an extra-thick slab according to the present invention, and shows a cross-sectional view thereof. In the figure, reference numeral 1 denotes a surface plate made of cast iron, cast steel, copper, etc., and the surface plate 1 may be used for simple heat dissipation or, if necessary, air, N2 _, etc., through a groove (not shown) in the bottom of the surface plate 1. Cooling is achieved by passing a refrigerant such as cold gas or water, or by simply spraying it onto the bottom surface. A mold 2 made of the same material and having a desired slab shape is placed on the surface plate 1, and a heat insulating frame 3 is provided on the inner surface of the mold 2 by means such as hanging or nailing. The upper part of the mold 2 is provided with a plurality of supply pipes 5 and blow holes 6 that communicate with a pressure source (not shown) for gas fluids such as Ar and N ₂ to be sprayed onto the surface of the molten steel 4. An upper lid 8 made of steel or refractory material and having gas vent holes 7 is provided on the upper part of the mold 2. Note that the blow hole 6 and/or the gas vent hole 7 can be provided at desired positions of the upper lid 8 and/or the mold 2. 9 is a pouring spout. (Example) The case of casting an extremely thick slab with a length of 3.7 m x width of 1.2 m and a thickness of 0.4 m will be described below. Within 5 minutes of pouring 1570℃ molten steel 4 into mold 2, spray Ar gas (room temperature 30℃) as a gas fluid for spraying.
was blown at an initial rate of 400 Nm ³ /hr as shown in FIG. 2, and this was continued for 30 minutes. During this time, the molten steel 4 in the mold 1 solidified 10 from the bottom and formed a solidified shell 11 on the top of about 30 mm. Thereafter, as shown by the dotted line in the figure, the amount of blown gas was reduced to about 20 Nm ³ /hr to maintain only the internal Ar atmosphere, and the amount was reduced to about 150 Nm ³ /hr for slow cooling. As a result, the initial solidified shell on the upper surface of the molten steel further thickens, resulting in a thickness of approximately 20Nm ³ /hr.
In the case of 100 mm and about 150 Nm ³ /hr, a 140 mm piece was obtained, and in this way the molten steel was solidified from the top and bottom by solidifying from the bottom 10 and from the top. Solidification is accompanied by contraction of the molten steel 4. However, the upper solidified shell 11 easily follows the shrinkage and descends due to reheating (temperature rise), thereby preventing the formation of shrinkage holes (not shown) and segregation around the shrinkage holes. The results were almost the same for about 20Nm ³ , although there was a slight difference from about 150Nm ³ /hr. Table 1 shows a comparison of the extremely thick slab obtained in this manner with that of the conventional method. The same diagram is also shown for cases in which Ar spraying is performed only at the initial stage and then discontinued. As is clear from Table 1, hot water wrinkles due to carburization and surface shape,
This method is superior in terms of grain defects, shrinkage pores, component segregation, etc., and is also extremely superior in yield of good pieces.

[Table] (Effects of the invention) As described above, by using the casting method of the present invention, there is no dense segregation zone in the surface layer that occurs due to unidirectional solidification, and the composition of the carburized layer etc. due to heat retention in two directions is eliminated. There are no defective parts, the time from casting to complete solidification is shortened, and segregation zones between particles and in the thickness direction can be improved by increasing the solidification rate.
In addition, since it is easy to form a solidified shell on the top surface, it is possible to produce extremely thick slabs with a good surface shape and no accompanying defects, easy casting and solidification work, and no quality problems at a high yield and in a stable manner. The casting method according to the present invention is extremely superior.

[Brief explanation of the drawing]

Fig. 1 is a drawing showing a cross-sectional view of the method of casting an extra-thick slab according to the present invention, Fig. 2 shows an example of the spray pattern of gas fluid, and Fig. 3 shows the top surface after completion of casting. Shows the amount of gas sprayed from the top, the solidification rate of the entire top surface, and the amount of heat removed. In FIG. 3, the conventional method shows the heat removal in two-way solidification when heat insulation is strengthened, and the top surface solidification rate (%) shows the solidification rate from the top surface during complete solidification. 1...surface plate, 2...mold, 3...thermal frame, 4...molten steel,
5... Supply pipe, 6... Blow hole, 7... Gas vent hole, 8... Top lid, 9... Pouring port, 10... Solidification from the bottom, 11...
Top solidified shell (initial stage).

Claims (1)

[Claims] 1. While solidifying the cast molten steel from the bottom of the mold, a gaseous refrigerant is sprayed onto the surface of the molten steel to rapidly cool it and form a solidified shell, and then the solidified shell is slowly cooled by adjusting the refrigerant. A casting method for high-quality, extra-thick slabs.