JPH0459061B2 - - Google Patents

Description

[Detailed description of the invention]

TECHNICAL FIELD The present invention relates to molds used in horizontal continuous casting equipment. BACKGROUND ART A typical prior art is shown in FIG. Molten steel is stored in the tundish 1, and this molten steel flows from the tundish nozzle 2 to the intermediate ring 3.
It is then guided from the break ring 4 to the mold 5. The mold 5 includes a mold body 6 made of a material such as copper, and a jacket 7 for cooling the mold body 6 with water. The slab is designated by reference numeral 8. In such horizontal continuous casting equipment, the slab 8 is moved in the drawing direction 9 in order to prevent the slab 8 from seizing on the inner peripheral surface of the mold body 6, and to repair the solidified shell if it is torn off. It is being pulled out intermittently. That is, the slab 8 undergoes a repeated cycle of (a) drawing and stopping, or (b) drawing and stopping.
The slab 8 is drawn out and cast while repeating a cycle consisting of stopping, pushing back, and stopping. Each of the above-mentioned cycles is repeated several tens to hundreds of times per minute. The break ring 4 is used to connect the tundish 1 and the mold 5 together with the intermediate ring 3 to prevent leakage of molten steel and to keep the solidification start point of the molten steel constant in the mold body 6. Made of refractory material. The mold body 6 of the mold 5 is made of copper or a copper alloy and has a high cooling capacity, so that the solidification of the molten steel significantly progresses during the stop or push-back period during the above-mentioned intermittent drawing.
At this time, since the break ring 4 is also cooled by the mold body 6, the solidified shell indicated by reference numeral 10 in FIG. It becomes extremely thick at the portion 10a. Therefore, a seam developed in the direction perpendicular to the axis of the slab 8 is generated at the corner 10a of the solidified shell 10, and at this seam, not only the solidified structure becomes discontinuous, but also the solidified structure becomes discontinuous.
Due to poor welding, gaps are created and cracks occur on the surface of the slab 8. FIG. 8 shows a solidified shell 10 of such a slab 8.
FIG. 8 is an enlarged cross-sectional view of the solidified structure of the portion indicated by the section in FIG. 7 near the seam. The surface of the slab 8 is indicated by reference numeral 12. The surface cracks occurring at the seams of the solidified shell 10 are called witness cracks 13, and such discontinuous solidified structures are called witness marks 14. Reference numeral 15 indicates a dendrite structure. Witness cracks 13 are unlikely to disappear in subsequent processes such as rolling, and therefore it is necessary to scrape off the witness cracks 13 formed on the surface of slab 8. As a result, the yield of the slab 8 is poor and the manufacturing cost is high.If you try to eliminate the witness cracks 13 after rolling without scraping them off,
This means that the rolling process is time-consuming. An object of the present invention is to provide a horizontal continuous casting mold that can prevent the occurrence of witness cracks formed on the surface of slabs. Means for Solving the Problems The present invention provides a cylindrical insert made of a material having a lower thermal conductivity than the mold body, at least on the inner peripheral surface of the mold body on the upstream side in the drawing direction for cooling the slab. and the thickness of this interpolation material is t (cm),
The thermal conductivity at the operating temperature is k (cal/cm・
sec・℃), 2<t/k<15 (1) This is a horizontal continuous casting mold. Effect According to the present invention, a cylindrical insert is provided on the inner peripheral surface of the mold main body at least on the upstream side in the drawing direction, and the insert is made of a material having a lower thermal conductivity than the mold main body. Therefore, when the molten metal is prevented from being excessively cooled by the insert, and a break ring is provided on the upstream side in the drawing direction in connection with the insert, ,
This prevents the break ring from cooling excessively and thus prevents the solidified shell from becoming extremely thick in the area of molten metal that contacts the inner circumferential surface of the insert and the end surface of the break ring. It can be prevented. Therefore, at such joints of the solidified shells, poor welding between the solidified shells and the molten metal is suppressed, and the occurrence of width cracks is suppressed. Even when a break ring is not provided, the occurrence of a witness crack can be suppressed. In particular, according to the present invention, the thickness of the interpolation material is t (cm)
If the thermal conductivity is k (cal/cm・sec・℃), and if the insert material is considered to be a flat plate and is in perfect contact with the mold body, its thermal resistance is expressed as t/k. This value is selected to be greater than 2 and less than 15 as described above. When t/k becomes 2 or less, there are cases where the witness crack cannot be resolved.
If t/k exceeds 15, the cooling ability of the intercalant will be too low, and a solidified shell will not be formed that is sufficient to withstand friction with the inner surface of the mold body during withdrawal, making it difficult to cast at a practical speed. Become. Embodiment FIG. 1 is a sectional view of an embodiment of the present invention, and FIG. 2 is a sectional view taken along the section line - in FIG. 1. Molten steel 17 in the tundish 16 is guided from the tundish nozzle 18 to the mold 20 via the intermediate ring 19. The mold 20 includes a mold body 21 made of copper and a mold body 21 made of copper.
1 and a jacket 22 for water cooling. Slab 23
In the drawing direction 24, a cycle consisting of pulling out and stopping is repeated, or a cycle consisting of pulling out, stopping, pushing back and stopping is repeated, and such a cycle is performed, for example, several tens to hundreds of times per minute. , the slab 23 is intermittently drawn and cast. According to the present invention, a cylindrical insert member 25 is provided on the inner circumferential surface of the mold body 21 at least on the upstream side (left side in FIG. 1) in the drawing direction 24. The insert 25 has a first portion 25a that abuts the intermediate ring 19, and a second portion 25b having a larger inner diameter than the first portion 25a. 2nd part 2
The inner circumferential surface of 5b is flush with the inner circumferential surface of the mold body 21 that contacts the slab 23. First part 25a
As in the case where a break ring is provided upstream of the insert member 25, the solidification start point of the molten steel is kept constant,
Furthermore, since it acts as a stopper for the coagulated shell during pushback during intermittent drawing, it has the effect of suppressing tearing of the coagulated shell. The intermediate ring 19 is made of zirconia ceramic or the like. The insert 25 is made of a refractory material such as boron nitride BN or zirconia ZrO ₂ . The length l1 of the insert material 25 in the axial direction is such that even if there is a minute difference in level at the joint between the insert material 25b and the mold 21, the solidified shell formed in the slab 23 will remain as indicated by the arrow 24. The length is selected so that it can grow to a thickness that will not tear when it is pulled out in the direction, for example, a length of about 3 cm or more. The axial length l2 of the second portion 25b of the insert 25 is selected to have the following value. The slab 23 is
It is pulled out in the pulling direction 24 while repeating the cycle consisting of pulling out and stopping as described above, or repeating the cycle consisting of pulling out - stopping - pushing back - stopping, and the above-mentioned length l2 is determined during each cycle: The length by which the slab 23 is drawn out is selected to be longer than the one stroke. This length l
2 is, for example, about 2.5 cm or more. The first portion 25a and the second portion 2 of the insert 25
The shape of each inner circumferential surface of the mold body 5b and the inner circumferential surface of the mold body 21 following them are uniformly formed in the axial direction, and in this embodiment, it is a right cylindrical shape. The experimental results of the present inventor are shown in Table 1, and the experimental results conducted for comparison are shown in Table 2. where t is the second portion 2 of the interpolation material 25
5b is the thickness, and k is the thermal conductivity of the insert material at around 500°C, which is considered to be the temperature of the insert material during use.
In each of these experiments, one stroke in each cycle of intermittent drawing was 7 to 25 cm, and the number of cycles was 60 to 150 cycles per minute.
According to experiments conducted by the inventor of the present invention, it has been confirmed that the depth of the width mark formed on the slab 23 can generally be reduced by increasing the number of cycles per minute of intermittent drawing. ing.

【table】

[Table] As a result of the experiment shown in Table 1, the cross section of the slab 23 is as shown in FIG.
The witness mark 27 extending from the slab 23
It is inclined with respect to the axial direction (horizontal direction in FIG. 3), and its radial length h is relatively shallow. Moreover, there are no cracks in the slab 23. This indicates that the initially solidified shell formed in the insert 25 does not have deep end faces in the direction perpendicular to the axis, and that it has not been excessively cooled.
Reference numeral 28 indicates a dendrite tissue. Thus, according to the invention, the Witness Mark 2
7 was shallow, and no cracks occurred. On the other hand, in Table 2 above, cracks occur in the first comparative example and the second comparative example, and the depth h is on average 0.7 mm in the first comparative example, and 0.5 mm on average in the second comparative example. be. In the third and fourth comparative examples shown in Table 2, the solidified shell was torn off within the mold body 21, making it impossible to stably pull out the slab 23. Therefore, from Tables 1 and 2, interpolation material 25
Let the thickness of the second portion 25b be t (cm), and the thermal conductivity of the material of the insert 25 be k (cal/cm・sec・
℃), if t/k is less than 2, cracks will occur on the surface of the slab 23, and if t/k is more than 15, the solidified shell will be torn off, making it impossible to pull out stably. FIG. 4 is a sectional view of another embodiment of the invention. Parts corresponding to those in the previous embodiment are given the same reference numerals. It should be noted that in this embodiment, the inner peripheral surfaces of the tundish nozzle 18, the intermediate ring 19, and the mold body 21 have uniform inner diameters in the axial direction, and a right cylindrical inner insert is formed across them. A material 29 is formed. Since the insert 29 extends between the tundish nozzle 18 and the intermediate ring 19, the solidification shell is prevented from entering the abutment surface 30 of the tundish nozzle 18 and the intermediate ring 19. , whereby the tandate nozzle 18
and the intermediate ring 19 can be prevented from being damaged. Furthermore, in this embodiment, since there is no step on the inner surface of the insert 29, the seam of the coagulation shell becomes shallower, making it possible to more effectively prevent a witness crack. Experimental results for the example shown in FIG. 4 are shown in Table 3, and for comparison, experimental results are shown in Table 4. Here, t indicates the thickness of the interpolation material 29, and k
is the thermal conductivity of the insert material at around 500℃, which is considered to be the temperature of the insert material during use. In each of these experiments, one stroke in each cycle of intermittent drawing was 15 to 60 mm, and the number of cycles was 1 min.
35-130 cycles.

【table】

[Table] According to the experimental results shown in Table 3, no witness cracks were generated in slab 23. On the other hand, the fourth
Witness cracks occur in the fifth and sixth comparative examples in the table, and the width of the width cracks in the fifth comparative example is 0.12 mm on average, and in the sixth comparative example, the average depth is 0.13 mm. . In the 7th to 10th comparative examples, the insert material 29 in the mold body 21 is
The coagulated shell was torn off inside the tube, making it impossible to draw it out stably. From the experimental results shown in Tables 3 and 4, it can be seen that t/k must be greater than 2 and less than 15. FIG. 5 is an axial cross-sectional view of an insert 34 according to another embodiment of the present invention. This insert material 34 has a plurality of (three in this embodiment) layers 31, 32,
33 is formed and configured. The inner layer 31 is made of zirconia ZrO ₂ with excellent wear resistance, and the outer layers 32 and 33 are made of boron nitride BN with high thermal conductivity and have a laminated structure. Each layer 31, 32, 3
According to the present invention, when the thickness of layers 3, etc. is ti, their thermal conductivity is ki, and the number of layers 31, 32, 33, etc. is n, the following equation 1 is established. . 2< _o 〓 ⁱ⁼¹ ti/ki<15...(2) If there is a void between each layer, the structure is configured so that the second equation holds, including the value related to air. As another embodiment of the present invention, depending on the seizure and wettability of the insert material to molten steel, the slab may be continuously pulled out instead of being pulled out intermittently to smoothly perform casting. It is also possible to cast other molten metals instead of molten steel, and it is also possible to apply not only horizontal continuous casting but also other casting methods such as vertical continuous casting. Effects As described above, according to the present invention, an insert material is provided on the inner peripheral surface of at least the upstream side in the drawing direction of the mold body that cools the slab, and the thickness of the insert material is set to t (cm).
Assuming that the thermal conductivity is k (cal/cm・sec・℃), t/k was chosen to be more than 2 and less than 15, so the cooling ability of the mold body was weakened.
Therefore, the width mark becomes shallower, and the occurrence of the width crack can be suppressed. As mentioned in connection with the prior art described above, this eliminates the need for surface preparation work to remove cracks from the slab, thus improving product yield, shortening the process, and reducing costs. It becomes possible to realize it. Furthermore, according to the present invention, it is also possible to cast steel types that are highly susceptible to cracking.

[Brief explanation of the drawing]

FIG. 1 is a cross-sectional view of one embodiment of the present invention, FIG. 2 is a cross-sectional view taken from the cutting plane line - in FIG. 5 is a cross-sectional view of an insert 34 according to another embodiment of the present invention, FIG. 6 is a cross-sectional view of the prior art, and FIG. 7 is a cross-sectional view of another embodiment of the present invention. FIG. 6 is a sectional view of the break ring 4 and the vicinity of the mold body 6 of the prior art, and FIG. 8 is an enlarged sectional view of the vicinity of the surface 12 of the slab 8 of the prior art. 16... Tandaitsuyu, 17... Molten steel, 18
... Tandate nozzle, 19 ... Intermediate ring, 20 ... Mold, 21 ... Mold body,
22... Jacket, 23... Slab, 24... Pulling direction, 25, 29, 34... Insertion material.

Claims (1)

[Claims] 1. A cylindrical insert made of a material having a lower thermal conductivity than the mold main body is provided on the inner peripheral surface of the mold main body that cools the slab at least on the upstream side in the drawing direction, and The thickness of the material is t (cm), and the thermal conductivity at the operating temperature is k (cal/cm・sec・℃)
A mold for horizontal continuous casting, characterized in that, when 2<t/k<15.