JPH0571336B2 - - 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 bottom and top 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 a decrease in yield. This impedes quality improvement.
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, as in Japanese Patent Publication No. 53-19290, there is a so-called one-way solidification method in which the top and side surfaces of the mold are kept warm and molten steel is solidified from the bottom surface of the mold.
59-178152, a so-called two-way solidification method has been proposed in which the lower part of the cast molten steel is solidified, while the surface of the molten steel is kept warm and then solidified from the upper surface as well, and considerable effects have been obtained. . (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, prolonging the solidification time, increasing internal segregation, and causing 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 eliminates the large component segregation layer formed on the surface of the slab when molten steel is cast, or the upper surface solidification shell such as in two-way solidification. Shape defects due to uneven cooling,
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 cause yield or quality damage 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 to achieve the above object is to provide a heat removal lid that uses a refrigerant placed on the mold while solidifying the molten steel being poured from the bottom of the mold. A gas fluid is filled in the space between the lid and the top surface of the molten steel, and the surface of the molten steel is rapidly cooled to form a solidified shell on the surface layer, and then the solidified shell is slowly cooled by adjusting the refrigerant in the heat removal lid. This is a two-way solidification casting method for extremely thick slabs. The method for casting high-quality extra-thick slabs according to the present invention will be described below. The present inventors have developed various methods based on the knowledge that it is difficult to obtain a slab with uniform composition and no shrinkage holes when casting an extremely thick slab using the conventional method of keeping the surface of the cast molten steel warm and solidifying. conducted an experiment.
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 the bottom surface of the mold (surface plate), or by cooling the bottom surface with air.
The molten steel is solidified by either cooling by active heat extraction via a refrigerant such as gas or water, and the surface of the molten steel is cooled by a heat extraction lid installed on the upper surface of the mold to quickly form a solidified shell on the upper surface. Form. In this case, if a gas fluid such as Ar or N ₂ is filled in the cavity of the mold, heat removal proceeds by radiation and convective heat transfer of the gas in a non-oxidizing state. This top solidified shell has a homogeneous solidified composition in each part of the surface layer and the bottom surface,
Furthermore, the surface shape is extremely smooth. Therefore, in the present invention, the cast molten steel is solidified from the bottom and also from the top to an appropriate thickness, and then,
By reducing or discontinuing the supply of refrigerants such as water and gas into the heat extraction lid, the solidified shell on the upper surface is completely solidified from the bottom and top surfaces while recuperating (raising the temperature) mainly from the internal heat of the ingot. let In addition to the solidified shell on the upper surface formed at the initial stage of this extremely thick slab-shaped ingot, by reheating the solidified upper surface shell that has progressed to solidify, the upper surface can be easily reheated according to the contraction of the molten steel formed inside. Since the solidified shell follows and descends, no shrinkage pores occur. For this purpose and the uniformity of the solidified composition, the solidified thickness from above is 50
It is necessary to set it to ~200mm. 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 upper 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, as shown in FIG. 1.0 x 10 ⁴ by removing heat and radiant heat with heat removal lid 10
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 diagram showing a method of 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. can be supplied through a groove (not shown) in the bottom of the surface plate 1. Cooling is performed 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 cast molten steel 4 is placed in the upper part of the mold 2 using a material such as Ar for preventing oxidation (for convection heat transfer).
A supply pipe 5 and a sealing hole 6 are provided which communicate with a pressure source (not shown) of a gaseous fluid such as _N2 . Further, the upper surface of the mold 2 is filled with water, for example, through a jacket 7 or a cooling pipe (not shown).
A heat extraction system made of iron, steel, copper plate, etc. with good heat conductivity, with a supply pipe 8 and a discharge pipe 9 for refrigerant such as liquid such as oil or gas fluid such as air, N ₂ , Ar, _CO 2 etc. A lid 10 is provided. There are no particular restrictions on the heat removal lid 10 as long as the heat removal rate described above can be obtained by supplying a refrigerant. After this configuration, molten steel 4 is poured into the mold 2 through the pouring port 11 and Ar gas (room temperature) is filled in, and water is introduced as a refrigerant through the supply pipe 8 of the heat removal lid 10 to remove heat. The surface of the molten steel is cooled to form a solidified shell 12 on the top surface together with a solidified shell 13 on the bottom surface. Thereafter, the amount of refrigerant flowing through the heat removal lid 10 is reduced or interrupted to allow slow cooling, and the upper surface solidified shell 12 is reheated. Due to this slow cooling, the initial solidified shell 13 on the bottom surface and the solidified shell 12 on the top surface of the molten steel 4 further progress to form solidification 14 from above and below, but this solidification 14 is accompanied by contraction of the molten steel 4. cormorant. However, due to the recuperation (temperature increase) of the upper surface solidified shell 12, it can easily follow the shrinkage and descend, thereby preventing the generation of shrinkage holes (not shown) and segregation around the shrinkage holes. (Example) An example 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. Molten steel at 1570℃ was poured into mold 2. Heat removal lid 10
is made of cast iron and has a thickness of 200mm with a diameter of 50mm inside.
The experiments were carried out using the same structure in which water cooling grooves were drilled on the top and bottom sides so that the water circulated from the bottom to the top, and another case in which air was pressurized at 2 kg/cm ² using the same structure. Other conditions are shown in Table 1.

[Table] Table 2 compares the extremely thick slab obtained in this way with the conventional method. It is clear that there are carburization, hot water wrinkles caused by the surface shape, hollow defects, shrinkage pores, component segregation, etc. This method is superior in this respect, and the yield of good pieces is also extremely high. The experiment was also carried out when air was injected as a refrigerant, but almost the same results were obtained. In particular, this method has many advantages, such as reducing the amount of gas enclosed in Ar, _N2, etc., and easily preventing oxidation of the molten steel surface.

[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 in that it is possible to obtain the following properties.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing a cross-sectional view of the method of casting an extra-thick slab according to the present invention, and FIG. 2 shows the solidification rate and amount of heat removed from the top surface after completion of casting. In FIG. 2, the conventional method shows the heat removal in two-way solidification when heat retention 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... Insulation frame, 4...
... Molten steel, 5 ... Supply pipe, 6 ... Blow hole, 7 ... Jacket, 8 ... Refrigerant supply pipe, 9 ... Discharge pipe,
10...Heat removal lid, 11...Pouring port, 12...Top solidified shell (initial stage), 13...Bottom solidified shell.

Claims (1)

[Claims] 1. While solidifying the cast molten steel from the bottom of the mold, a heat extraction lid placed on the mold using a refrigerant and a gas fluid are sealed in the space between the heat extraction lid and the top surface of the molten steel, and the surface of the molten steel is rapidly cooled to remove the surface layer. A two-way solidification casting method for extremely thick slabs, characterized in that after forming a solidified shell, the solidified shell is slowly cooled by adjusting the refrigerant in the heat extraction lid.