JPS58192662A - Method for recovering heat from continuous casting mold for steel - Google Patents

Abstract

PURPOSE:To prevent the generation of any cracking defect in an ingot in the stage of cooling a continuous casting mold for steel with liquid alkali metal by specifying the temp. of the liquid metal flowing out from the casting mold. CONSTITUTION:A NaK alloy (about 56% Na, about 44% K) is used as liquid alkali metal, and is introduced through a heat exchanger 5, a tank 4, a pump 3, a filter 2 and a flowmeter 6 into a casting mold 1 to cool the mold, whereafter the metal is circulated again to the heat exchanger 5. Cold water or hot water is supplied by a circulation pump 11 to the water side of the heat exchanger 5 to control the temp. of the liquid metal. The temp. of the liquid alkali metal flowing out through the outlet of the mold 1 is controlled by such cooling mechanism so that said temp. is kept always in a specified range of <=330 deg.C, and particularly in the case of the liquid metal in upward current to<=250 deg.C. The generation of any defect such as breakout in the ingot is thus prevented.

Description

DETAILED DESCRIPTION OF THE INVENTION This invention relates to a method for recovering heat from continuous steel casting molds.

Continuous casting of steel is a method that has been replacing the ingot-blooming rolling method in recent years, and involves placing molten steel in a mold to form the surface layer.
This is then completely solidified by pouring water and cooling while supported by guide rolls in a spray machine, pulled out with cap pinch rolls, and then cut to a certain length to form finished products.

At this time, the mold is continuously supplied with high-temperature bath water, which places a large heat load on it. For this reason, continuous casting candy J1") is generally made of pure copper or copper alloy, which has good thermal conductivity.
High-quality fresh water is allowed to flow into the surrounding area or inside at a high flow rate to achieve a high degree of cooling.

Therefore, the temperature of the cooling water that passes through the mold usually increases by 4~
The temperature is kept at 5℃.

Continuous casting molds are classified into tube molds, block molds, and single-strand molds, based on the shape of the copper member that cools in direct contact with the steel billet. The first two types are used to manufacture small-sized steel pieces, and currently assembled @types are often used. Although the method of the present invention can of course be used without mold limitations, the following explanation will be based on an assembled mold.

A combination mold consists of a thick copper plate supported by a steel plate or steel structure on each side, with grooves cut on the outside of the anti-icing material or conduits formed inside, which are then closed. Cooling water is introduced as a water channel to cool the copper plate. Gather the four sides of each section constructed in this way.

The assembled box shape is constructed to have a square cross section or a rectangular cross section t.

The mold is mounted on a mold vibrator stand, and the casting operation is performed while moving it up and down with a vibration width of several to several tens of millimeters and a frequency of several tens to hundreds of cycles per minute. Figure 1 shows the configuration of one side of the assembled mold.

FIG. 1 is a conceptual diagram of a longitudinal section of an assembled mold for continuous casting, with the wide surfaces facing each other. 21 is the copper plate in contact with the slab;
22 is a steel support plate, and 23 is a frame.

Reference numeral 24 denotes a cooling water road groove, which has a cross section of, for example, a number n times a number n as shown in the main text, but is a road extending over the entire length of the mold in the longitudinal direction. 25 is a cooling water pillow pipe, 26 is a cooling water inlet, 27
indicates the cooling water outlet.

The present inventor constructed an experimental apparatus as shown in FIG. 2 as a method for cooling such a continuous casting mold with a liquid alkali metal.

Figure 2-1 is a flow sheet of the experimental apparatus when the molten metal in the mold is in an upward flow similar to the water-cooled mold in Figure 1, and Figure 2-2 is a flow sheet of the experimental apparatus when a downward flow is formed in the mold. . The symbols are the same. 1 is a mold copper plate, 2 is a filter, 3 is a bath molten metal circulation pump, and 4 is a bath molten metal reservoir.

5 is a heat exchanger, 6 is a flow meter, 7 is a hot water reservoir, 7-2 is open to the atmosphere, 8 is a hot water supply valve, 9 is a cold fresh water supply valve, 1
0 is the current water adjustment valve, 11 is the cooling water circulation pump, 12
13 is a thermometer, and 13 is an expansion tank, which absorbs excess or deficiency of molten metal in the alkali metal line.

2-3-1 shows a flow sheet in an example of steam recovery. 1 to 6 are the same as those in Figures 2-1 and 2-2. The cooler 7 is cooled with cold fresh water (not shown).

8 is a molten metal flow rate adjustment valve, and 9 is an expansion tank that absorbs excess or deficiency in the line tank. In case of a shortage in the system or inspection of the pipe, all the metal can be taken out into the storage tank 10 and given to someone. (
12 is the boiler circulation pump.

13 is a pure water device, 14 is a condensate water source, and 15 is a steam usage source.

16 is a boiler tank, 17.18 is an alkali metal thermometer, and 19 is a water thermometer. Details of each part of the boiler system have been omitted. ) The continuous pattern mold is based on the continuous casting mold of the actual machine mentioned above.
□I used it. The heat exchanger is also made of copper, and like the mold, many grooved pipes are formed on the outer wall.

In this example, a groove with a diameter of 4 mm and a depth of 16 grooves is cut with a 36W11 pinch. On the other hand, water conduits were formed in the same way as liquid metal channel grooves alternately within the copper plate.

The liquid alkali metal conduit is the heat exchanger mentioned above.

Consisting of a pump, filter, tank, and flow control piping, the control piping controls the flow rate in the cooling pipe in the mold.
It was possible to change from 1771/S to sm/s.

In addition, we also made it possible to experiment with a method in which the liquid metal conduit in the continuous casting mold flows from the top to the bottom as shown in Figure 2-2.

Cold water or hot water is passed through the water side of the heat exchanger, and the temperature of the liquid metal is controlled by adjusting the flow rate.

In this way, the temperature of the liquid metal exiting the heat exchanger and into the mold can be varied over a wide range from about 40°C to 230'C.

As the alkaline earth metal, we mainly used the so-called N2K (tic) alloy with Nα of 56% and 44%, but we also used pure J, which has good thermal conductivity especially under low flow rate conditions. I conducted a test.

In a vertical continuous casting machine that casts a medium 1n'L x 200" thick slab, CO, 12%, Mn O, 4
5% Si, 0.30% Si (target composition), 1m/
Casting was carried out at a casting speed of min, and the flow rate of the bath molten alkali metal in the pipe and the mold temperature were varied.

In addition to measuring the mold outlet temperature, we also looked at operational problems and the effect on the slab. After cooling, the slab was scarfed with an oxygen-propane flame to investigate surface defects.

Table 1 shows some of the test results.

Table 1 C-+I:) When N and i were not written in other conditions, all conditions were set to N.

As seen from Table 1, generally when the outlet liquid temperature is high, there are many defects in one piece. The slabs are said to be in good condition.

JI! 1' It is at least the same (or has fewer defects) than the usual water-cooled casting (with fewer defects).
Defects are surface defects that can be used by removing them with a scarf, but defects are so severe that they require removal with a scarf once, for example, twice. The surface defects are mainly vertical cracks and cracks, the latter being due to the copper scraped off from castings I to V being diffused onto the steel coating surface.

Interestingly, when the slab is defective, the slab sticks to the mold, increasing the pull-out resistance. In addition, although the experiment lasted about 10[] hours in total, there was one breakout accident (stta'J, +-' caused the surface of the piece to break, and the unsolidified bath inside (hot water leaks) -C
These factors are thought to correspond to an increase in defects.

,J,,To make the results in Table 1 easier to understand, Fig. 3 is used.
Figure 3 shows the upward flow and downward flow separately. In other words, Figure 3 shows the relationship between the molten metal flow velocity in the upward flow and the temperature of the outgoing metal. It is a diagram showing the quality status of copper pieces. In the figure, area a: The slab was good, area b: One piece was good, area C: defective, and d: the slab was defective.

FIG. 3-2 is also in the downward flow.

The data are from Table 1, where O indicates one piece is good, .

Including the data in Table 1 and data not listed, the boundary line in Figure 1 shows the range in which a good slab can be produced (1).

I tried to draw the boundaries of the properties of slabs, including data other than those shown in [Part 1]. The results can be summarized as follows. When the flow of the bath molten metal in the @-type cooling pipe is upward, the flow rate of the bath molten metal is "N jr'a", regardless of the type of molten metal, and the N jr'a of the bath molten metal, regardless of the internal flow velocity. Exit width is 2
When the temperature is 00°C or lower, the seal is 1-1, which is good. However, the temperature of the molten metal 'trJ' on January 1110 is 25 (1
When the temperature is □C or higher, cracking defects increase. This middle two
(1) Regardless of the temperature from 1 to 250'C, the slab has good shape and is usable.

On the other hand, when there is a downward flow, the range in which one piece is good is increased by d. υ[j Also, if you use one beat for molten metal, meet.

When the M71i tapping temperature is 250°C or higher, the slab is in good condition. Furthermore, at temperatures above 330°C, defects increase and operational problems arise;
'C and above, the slower the molten metal flow rate, the wider the good range;
If the flow rate of the bath molten metal is high, y is good or X is bad.
1,000 surface quality is to low.

The conditions for cooling continuous casting molds using alkaline earth metals can be summarized very simply as a result of experiments. That is, for molds of bath molten metal [J temperature is 330'
It is an essential condition to keep the temperature below 250°C, and when cooling the mold with normal upward flow, it must be even lower, not below 250°C.

Although omitted in Table 1, 2
In the case of m/6, it was almost always necessary to raise the population temperature by 1-1 in trA-1 to keep it below 250 ''C with this variant. Not only can the temperature be lower than 250°C in countercurrent flow, but also 330°C in downward flow.
It was also difficult to set the following conditions. Therefore 1゜5
For m/S and F, almost all results were one defect or one defect, but these data have been omitted.

However, theoretically speaking, expanding the channel area and increasing the flow rate should lower the liquid temperature, and it is unlikely that the trend in Figure 3 would suddenly change at 2 m/s or more. Including the fact that the 2 m/S data with N.2 shows the same results, it is considered that the conclusion that the above-mentioned constraint can be limited only by the mold outlet liquid temperature can be sufficiently extrapolated.

It has been found that, in addition to allowing a wider range of exit molten metal temperature tolerances, there are three additional important advantages of having a downward flow of molten metal. One is a natural conclusion: when the heat of the molten metal in the bath is transferred to water to generate steam, the thermal efficiency increases regardless of whether the temperature of the molten metal on the heating side is 10°C or 5°C. Therefore, a downward flow that can increase the outlet temperature by 50°C to 80°C is of great advantage.

The second is that the surface of the slab is good. Compared to the case of pouring in a normal water-cooled mold, the surface of the slab is evaluated as good even when the molten metal is cooled at the upper H1l&
In particular, it was found that cracks, pinholes and inclusions in the extended surface skin were slightly superior. However, it has been found that when 10 - tN is used, the incidence of cracks is even lower.

The reason for these results is only speculation, but it is thought that when using a downward flow, the temperature on the inner surface becomes uniform due to the third advantage of the first order, and there is less distortion of the mold. Also, there are fewer pinholes and inclusions on the surface.
As can be seen from Table 1.

This is because the temperature entering the mold is usually over 100'Cτ.

Because the temperature of the mold 1 itself is higher than that of water cooling,
This is thought to be due to the effect of trapping inclusions and pinholes in the early stages of solidification.Thirdly, the life of the mold is increased (extended) by about 30% compared to water cooling.

Now, in this way, it is possible to recover heat from the molten metal that has been heated to about 200"C to 250C.
The advantages and objectives of the present invention are to be completed. 2-1
．． This becomes possible by using the cooling water pipes shown in Figures 2 and 2 as a boiler system. Although this is easily accomplished with existing technology, it will be explained by way of example.

Example: In the vertical continuous casting equipment for casting slabs of Iil 1 m x 200 mm in thickness, class M ordinary steel was
．． Casting was performed at a casting speed of 8 to 1.2 m/min.

The equipment including the boiler system shown in Figures 2-3 was used, and the molded copper plate had the dimensions described above.

N for the mold. The K alloy was supplied in a downward flow at a flow rate of 4 m/s while controlling the temperature of one population to be 120°C. The temperature of the molten metal exiting the mold was around 230"C.

This bath molten metal heats the boiler circulating water in a heat exchanger, and
It leaves at 64℃. At this temperature, the temperature is too high to feed it directly to the mold, so it is cooled down to a predetermined temperature in a cooler and then fed into a circulation pump.

The heat exchanger circulates hot water of 25"C supplied from a boiler circulation bong with a pressure of 20"/~200"
Raise the temperature to C.

In this way, it was possible to generate steam of 3 Kg/crI-G at a rate of 1100 Kg/h during the casting time.

As described above, the present invention is a method for recovering the latent heat of solidification of metal, which is extremely rare in conventional embodiments. Furthermore, in order to recover heat in the continuous casting mold J from commercial steel, which has a particularly difficult melting point, by using molten alkali metal gold instead of conventional water as a cooling medium for the mold, experimental results showed that it was possible to recover the heat in the continuous casting mold J. This is based on IQ's knowledge that the temperature must be kept within a narrow range of 330''C or higher, particularly when using an upward flow similar to a conventional water-cooled mold, to below zbOc.

This makes it possible to recover the latent heat of solidification of steel as steam, which could not be used in the past, and the quality of slabs also deteriorates.
Improvements were also seen in pinholes, inclusions, etc.

A commercially advantageous advantage in carrying out the present invention is that the flow of the cooling medium is made downward. This has the advantage of widening the allowable range of the outlet temperature, increasing the degree of freedom in operation, increasing the outlet temperature, increasing the efficiency of heat recovery, and increasing the steam pressure. Secondary benefits such as further improvements have been discovered.

In the continuous casting of steel, the use of downward flow as a cooling medium has almost never been done in water-cooled molds, so it is novel to discover the above-mentioned advantages and adopt downward flow. . On the other hand, the reason why downward flow is not used in water-cooled molds is that it is difficult to maintain uniformity of flow rate, and air pockets are likely to form in the upper part, which inhibits heat transfer. To remove these, the capacity of the inlet column pipe will be increased. The column pipe metal is made higher and longer than before, and the shape of the pipe is made smooth. Considerations were taken, such as installing a throttling fund inside the exit piping.

The molten metal released from the mold will heat the water in a heat exchanger.1
Little steam can be recovered at temperatures below 50''C.

In order to generate steam of several kg/cr1 or more, the temperature must preferably be 200° C. or higher.

Therefore, the temperature must be above 150"C or preferably above 200°C, and in some cases above -330"C and above 250°C, so it can be seen that the constraints of the present invention are severe. Ru.

In order to maintain such strict temperature conditions, control of the outlet temperature is an essential condition. This is accomplished by adjusting the mold temperature and flow rate of the molten metal, as described in the embodiment.

As shown in Table 1, the outlet temperature of the bath molten metal is determined by the inlet temperature and the flow rate in the pipe. In this case, if the flow velocity in the pipe is changed widely, the conditions will become complicated as shown in Fig. 3-2, and the capacity of the device will also be increased in order to be able to change the flow rate.

It should be controlled by the population temperature for renovation.

By the way, in the flow notes in Figure 2-3, the temperature of the molten metal leaving the heat exchanger is usually too high to return to the mold, so it is necessary to lower the temperature through the entire cooler before circulating. 1 more. The cooler adopted in this project is structurally the same cross-through type as the heat exchanger described above, and fresh water is circulated on the cooling side.

Although the present invention is clear from the above description, the following conditions are especially effective for exhibiting its effects.

(1) At least one of the mold population temperature of the alkali metal and the alkali metal flow rate so that the exit temperature from the mold or the temperature of the molten alkali metal in the heat exchanger 1] is always within a constant temperature range of 330°C or higher. The method according to the claims, characterized in that the method is controlled by:

(2) In order to maintain the temperature of the molten alkali metal in the mold at a predetermined temperature, the alkali metal flow path is provided with a cooler in addition to a heat exchanger for steam generation. The claimed method as recited in .

(3) The exit temperature from the mold is particularly preferably 250″C
(4) The method according to the claims, characterized in that the molten alkali metal flowing into the mold is caused to flow from the top to the top in the mold. (4) Flowing into the mold 10. A method as claimed in claim 1, characterized in that 10,000 ml of molten alkali gold is arranged to flow upwardly through the mold.

[Brief explanation of drawings]

FIG. 1 is an explanatory diagram of the configuration of the group) γ practice, and FIG. 2 is an explanatory diagram of the configuration of the apparatus of the present invention. FIG. 3 is a diagram explaining the effects of the device of the present invention. 1: Mold copper plate 2; Filter 4: l @ molten metal reservoir 5: Heat exchanger /: Hot water f?
1 9: Cold fresh water supply valve fan 2-2 Figure 2-3 Closing fan 3-1 Foot of eye 3-2 Leap 2 4 Otsu 8 "-8
Henban ／＋／＋＼

Claims (1)

[Claims] A steel continuous casting mold is cooled by a liquid alkali metal circulating in a closed circuit, and the liquid alkali metal is exchanged with hot water in a return path through a heat exchanger to recover steam, which then flows out of the mold. The temperature of the liquid metal is 33
A method for recovering heat from a steel continuous casting mold, which is characterized by operating at temperatures below 0°C.