JPH01321049A - Method for casting cast slab for producing thick steel plate - Google Patents

Abstract

PURPOSE:To obtain a cast slab for producing a thick steel plate having uniform fine structure and good surface characteristic by pouring dripped molten steel flows arranging three streams as spreading to flat plane state into a mold. CONSTITUTION:The molten steel 3 supplied into a tundish 2 from a ladle 1, is dropped as poured molten steel flows 4 from >=3 pieces of the nozzles 5 arranged at the bottom part thereof. Then, the molten steel flow 4 is made to drips with gas flow from circumference of each nozzle 5. Further, by arranging the plural streams of the poured molten steel flow 3 and the dripped molten steel flows 4 to the flat plate state, the drip is uniformly dispersed on the mold 7 as much as possible. By adopting such constitution, the cast slab having fine structure and uniformity in inner quality without any surface defect can be obtd. Then, by using this cast slab, rolling reduction ratio for hot rolling and forging in the following process can be made small and improvement of the quality of the thick steel plate as the product and reduction of the production cost can be obtd.

Description

Detailed Description of the Invention (Field of Industrial Application) This invention is a casting method that has no surface defects and is internally homogeneous, in order to produce thick steel plates free of internal defects such as component segregation and scratches at low cost. This invention relates to a method for casting ingots.

(Prior Art) Currently, except for special steel plates, thick steel plates are manufactured by hot rolling continuous cast slabs (hereinafter referred to as "CC slabs").

Normally, the thickness of the CC slab is about 150 to 300III.

However, for example, when trying to manufacture a thick steel plate with a thickness of 100 mm, it takes 500 mm to completely compress the component segregation, crack holes, etc. that appear at the center of the CC slab, and to completely eliminate the columnar crystals. It is said that it is necessary to have a rolling reduction ratio (slab thickness/product thickness) of 150 to 30 mm.
There was a problem in that such a high pressure reduction ratio could not be achieved with a CC slab of about 0III.

Therefore, in the past, in order to obtain thick-walled steel plates of 100 m5 or more, it was not possible to use the continuous casting method, and it was necessary to use the ingot method to create an ingot, bloom it, and then roll it. Ta. However, in the ingot method, the solidification rate is slow, so the occurrence of macro segregation, pitting, etc. becomes noticeable, and hot working with a reduction ratio of 5 or more is essential.
Cost increases cannot be avoided. Furthermore, even when considering the metal structure of the product, it was difficult to obtain one that was microscopically homogeneous, which caused quality problems.

On the other hand, when manufacturing a thick steel plate equivalent to a thickness of 10+ms, for example, the reduction ratio is sufficient even if the ingot is hot rolled using the continuous casting method, but on the other hand, an extra reduction ratio and rolling are required, which increases costs. connected to. Currently, thick steel plates of about 10 m are manufactured from CC slabs by hot rolling.
For the reasons mentioned above, this method does not take full advantage of the advantages of the continuous casting method.

Note that the lower limit of the slab thickness that can be cast is 50 m5 in the continuous thin slab casting method, but this has not yet been put to practical use.

By the way, as a measure to reduce the rolling reduction ratio, the steel ingot (hereinafter referred to as r
In this case, it takes a long time to solidify, so it is not suitable for mass production, and it is not possible to avoid segregation on the top surface of the steel ingot, so the yield is not good at all. . In addition, in this LH steel ingot, the development of columnar crystals was large, so a reduction ratio of 3 or more was required.

(Problem B to be Solved by the Invention) Recently, a method of casting in a semi-solidified state has been proposed for the purpose of eliminating the above-mentioned disadvantages. Specifically, inert gas is injected into the molten steel stream to form droplets, which are cooled while falling and then injected into the mold in a semi-solidified state, which is effective for manufacturing small steel ingots. When it comes to large steel ingots, the heat removal rate is different between the surface and the center of the steel ingot. In other words, the surface is cooled too much and becomes porous, but in the center, the semi-solidified droplets are reheated due to recuperation after injection, resulting in a solidification process similar to that of a normally cast steel ingot, resulting in macro-segregation. , Zaku nests, etc. will appear. See JP-A-58-86969 and JP-A-59-33056.

Therefore, when subjecting this steel ingot to hot rolling, the yield decreases due to surface care, component segregation in the center, and strong reduction rolling (reduction ratio of 5 or more) is required to compress the crack holes, which increases costs. This is a contributing factor.

Thus, from the above-mentioned viewpoint, the object of the present invention is to
A method for producing high-quality thick-walled steel plates by low-cost hot rolling with small rolling reduction ratios by producing ingots of arbitrary thickness with a microstructure free of surface and internal defects, i.e. An object of the present invention is to provide a method for casting an ingot.

(Means for Solving Problem 1) The inventors of the present invention have conducted various studies to achieve the above object, and have found that it is particularly effective to inject the molten steel flow as uniformly as possible, and that it is necessary to Injection into the mold can be carried out using multiple injected molten steel streams spread out over the area, and in that case, surface and internal defects can be eliminated by changing the injection conditions for each injected molten steel stream and providing optimal injection conditions. The present invention was completed based on the knowledge that improvements can be made more effectively.

Here, the gist of the present invention is to provide a casting method in which a molten steel stream is made into droplets and allowed to fall and solidify into an endless mold consisting of a bottom surface and a side wall surface, in which three or more of the above-mentioned injected molten steel streams are formed. This is a method for casting an ingot for producing thick-walled steel plates, which is characterized by comprising a molten steel flow of.

According to a preferred embodiment of the present invention, the injected molten steel flows are set to different dropletization conditions, or some of the molten steel flows are completely molten without being made into droplets. It may be injected into the mold as is.

As described above, according to the present invention, by injecting three or more injected molten steel streams spread out in a plane into a mold, preferably by changing the dropletization conditions of each injected molten steel stream, respectively. An ingot with a homogeneous microstructure and good surface quality can be obtained.

The ingot cast according to the present invention is then processed into a thick steel plate through processes such as hot rolling using conventional methods.

The ingot obtained in this way has no defects such as component segregation or hollow spots that are found in ordinary solidified materials, not only in surface quality but also in internal quality, so casting or rolling is only required to adjust the shape (dimensions). If the reduction ratio is small and is 1.3 or more, a thick steel plate of stable quality can be obtained.

Here, the thick steel plate obtained from the ingot according to the present invention is 100
This includes those with a thickness of approximately 10 mm and those with a thickness of approximately 10 mm, and generally has a wall thickness in the range of 10 to 10 mm, and refers to steel used for marine structures, for example. .

Note that the surface of the ingot will be improved if the molten steel flow corresponding to the ingot surface is made into a normal pouring flow without being turned into droplets. In particular, the effect becomes more pronounced as the casting speed increases.

(Operation) Next, the present invention will be described in more detail with reference to the accompanying drawings.

FIGS. 1(a) and 1(b) are a schematic side view and a plan view, respectively, illustrating the casting method according to the present invention, and FIGS. 2, 3(a), and 3(b) are FIG. 3 is a schematic explanatory diagram showing how molten steel is poured.

As shown in FIG. 1(a), the molten steel 3 supplied from the ladle l to the tundish 2 falls from a plurality of nozzles provided at the bottom as an injected molten steel flow 4, but at this time, the molten steel flows around each nozzle. The particles are individually formed into droplets by the gas flow from the droplet-forming gas outlet 5 provided in the . Note that FIG. 1(a) shows an example in which the outer molten steel 3 is not converted into droplets. - If the molten steel is formed into small droplets and cooled while falling, there is almost no flow of the molten steel after being poured into the mold, so there is a limit to the range in which a single molten steel flow can be uniformly dispersed. In particular, this is difficult to do with plate-shaped ingots, which is the subject of the present invention, so by arranging a plurality of large and small injected molten steel flows 3.4 in a planar manner as shown in Fig. 1 (b). , it is essential to distribute the droplets on the mold 7 as uniformly as possible.

The reason for setting the dropletization conditions for the plurality of injected molten steel flows is as follows.

Injected molten steel flow in the form of semi-solid droplets cooled during falling 4
The subsequent solidification process also differs depending on the cooling conditions within the mold 7 after injection. In other words, the surface of the ingot 6 (the area indicated by reference numeral 13 in FIG. 3(a) or the area indicated by reference numeral 12 in FIG. 2)
In the region shown by ), the droplet 4° cooled during falling is cooled as it is due to heat radiation toward the atmosphere, so there is no flow within the mold, and naturally no component segregation occurs.

However, the surface of the ingot 6 in contact with the mold 7 (for example, the third
In the region indicated by reference numeral 9 in Figure (A), the injected semi-solidified droplet 4° is rapidly cooled at the same time as the injection, so the droplet shape remains unchanged and the particles are piled up, unlike the conventional method. This leads to surface defects similar to splash defects normally seen in castings.

FIG. 3(B) is a partially enlarged view of the area indicated by the reference numeral 9 in FIG. 3(A), and shows such a situation in an enlarged manner.

On the other hand, in the center of the ingot 6, the heat is removed slowly in the mold 7 after pouring, and especially in large ingots (thickness), the droplets 4°, which are cooled during falling and are once in a semi-solidified state, are Since the heat is removed slowly in the mold 7, double heat is generated, and the semi-solidified grains are remelted. That is, in the center of the ingot 6, a solidification phenomenon similar to that in normal casting occurs. So-called completely molten molten steel gradually solidifies from the surface of the ingot.

Therefore, as shown in FIG. 3(a), cavities 1) due to solidification shrinkage appear at the same time as the structure becomes coarse due to columnar crystals 1O from the ingot surface and component segregation appears.

In the present invention, in order to solve the above problem, three or more molten steel flows 4 are arranged with two-dimensional expansion, and if necessary, the flow rate of the molten steel flows 4 is further limited. In this way, arbitrary droplet formation conditions are set for each of these conditions.

Thus, according to the present invention, by adopting such a configuration, it is possible to produce an ingot that has no surface defects and is internally homogeneous with a fine structure, and with this ingot,
It is possible to reduce the rolling reduction ratio in hot rolling and forging in the post-process, and it is possible to improve the quality of the thick steel plate as a product and reduce the manufacturing cost.

Here, an example of droplet-forming conditions for obtaining an ingot free of surface defects and homogeneous inside in the present invention is as follows.

After pouring, the injected molten steel 3 corresponding to the surface of the ingot 6 in contact with the mold 7 (the area indicated by reference numeral 8 in FIG. 2 and the area indicated by reference numeral 9 in FIG. Inject droplets. That is, the surface 8 of the ingot 6 is made smooth by injecting droplets with a low solidification rate. In other words, a smooth ingot surface is formed using highly fluid molten steel.

In particular, if the molten steel corresponding to the surface of the ingot (area indicated by reference numeral 8 in Fig. 2 and numeral 9 in Fig. 3) is made into a normal casting flow without being made into droplets, it is possible to It goes without saying that the ingot surface texture equivalent to that of an ingot (slab, etc.) can be obtained, see surface 8 in FIG. 2.

In this case, since the solidification proceeds from the mold 7 side, the structure has directionality, but since it is rapidly cooled, the structure is fine.

In addition, since cooled droplets are deposited inside the molten steel (the area indicated by 8° in FIG. 2), the flow of the molten steel is suppressed, no component segregation occurs, and the structure becomes finer. Naturally, it is also effective to make the inside of the particles finer. However, if the grain size is too fine (solidification rate of 10% or more), cooling after injection into the mold is slow and the ingot becomes porous.

On the other hand, on the ingot surface that does not come into contact with the mold 7 (the area indicated by the reference numeral 12 in FIG. 2), the cooling rate after the diameter is increased is lower than that on the ingot surface 8 that comes into contact with the mold 7.
As a condition for smoothing this, it is preferable to inject droplets with a low solidification rate, that is, coarse droplets or normally cast molten steel (not turned into droplets).

Here, when talking about the arrangement of the injected molten steel flow, as is clear from what has already been said, there are "planar undulations",
In other words, it is necessary to arrange them with two-dimensional waves, but this consists of at least three injected molten steel flows, and generally, as shown in Figure 1 (a), the surrounding area is A small amount of injected molten steel flow is arranged in the molten steel, and this is coarse droplets or usually cast molten steel, and inside it, as much flow as possible is arranged evenly and flatly. Preferably, in the arrangement example shown in FIG. 1 (B), the conditions for forming the injected molten steel flow corresponding to the periphery of the ingot are coarse droplets, and a part of the injected flow is made into normal cast molten steel. It is also possible to combine them as

Next, there is a method for changing the injection conditions for each injected molten steel flow, but this can be easily done by changing the droplet-forming gas spray conditions when the molten steel flow from the corresponding tump push nozzle is turned into droplets. Can be changed to 0. For example, by reducing the amount of dropletizing gas sprayed, the droplets can be made coarser and the coagulation rate can be reduced (on the contrary, by increasing the amount of dropletizing gas sprayed, the coagulation rate can be increased). Smaller droplets can be produced by increasing the dropletization gas spray velocity, and these can be varied for each injected steel stream.Also, the diameter of the nozzle hole at the bottom of the tundish can itself be changed. The flow rate can be restricted by changing the .

Next, the present invention will be explained in more detail with reference to Examples.

Examples Table 1 shows examples and comparative examples of the method of the present invention, as well as a casting example (comparative N1) when two injection streams are used in the current process, and a casting example N (comparative example 2) manufactured by the t, Hm block method. Table 2 shows the manufacturing conditions and results, and the components of the manufactured ingots.

Moreover, FIG. 4 shows an outline of the manufacturing process of the LH steel ingot used as a comparative example. Molten steel is injected from an injection pipe into a large surface plate and a flat mold placed on the surface plate using a bottom pouring method, and solidification is performed in one direction by providing an insulating sleeve inside the mold. A heat insulating plate and a heat insulating lid are placed to ensure one-way solidification.

Width 1000+u+, height 40
0ma+.

While manufacturing an ingot with a length of 3000 mm, Comparative Example 1
With the current process of two injection streams, a width of 100
An ingot with a height of 400m and a length of 3000m was manufactured. In addition, as Comparative Example 2, a steel ingot of 1000 x 1000 mm and a height of 920 m-
In the case of the present invention, in which an ingot was produced, eight nozzles were used to inject from the tundish into the mold. The injection rate was 0.39 ton/min for each nozzle, and liquid nitrogen was used to form droplets into the molten steel stream using a 5US304 annular nozzle. The amount of liquid nitrogen is 0.651 from each injection nozzle for the injection flow of 6 of the 8 injection nozzles shown in Figure 1.
/5teel kg・win. In addition, in the case of Comparative Example 1, two injection nozzles were used, and the injection speed was 1.
Liquid nitrogen was used for droplet formation at 45 tons/min, and the same annular nozzle as in the example of the present invention was used.

The amount of liquid nitrogen is 0.6j for each of the two injection streams! /5
I went with Teel kg-win. In addition, the degree of superheating of molten steel is ΔT=60°C in the case of the present invention method and Comparative Example 1, and Comparative Example 2.
In the case of ΔT-40°C.

Among the manufactured ingots, those manufactured by the method of the present invention were directly rolled into steel plates with a thickness of 200+ms by plate rolling.

In the steel ingot of Comparative Example 1, the surface defects (porous parts due to supercooling) on the bottom and side surfaces of the steel ingot were removed by 10+g-, the height was 390 mm, and the radius was 980+*-, and the top surface of the steel ingot was also cleaned. After that, it was rolled into a steel plate with a thickness of 2001 mm by plate rolling, similar to the ingot produced by the method of the present invention.

In addition, steel ingots manufactured using the current LH steel ingot process are manufactured by first cutting off the segregated portion (approximately 30%) on the top side of the steel ingot, and then
0+u+, and after rolling to a height of 40 (1 wm) by blooming, it was rolled to a steel plate with a thickness of 200 mm+ by plate rolling.

As a result of performing an ultrasonic inspection on each steel plate, as also summarized in Table 1, no defects caused by ultrasonic waves were observed in any of the steel plates iii.

As a result of the above, it was confirmed that the steel plates manufactured from the ingots obtained by the method of the present invention can have quality assurance equivalent to the current process, and also that it is possible to significantly improve the yield and reduce the rolling reduction ratio, which is directly linked to higher costs. Obviously 9.

Table 1 Manufacturing conditions and results for Examples and Comparative Examples Table 2
Ingot composition (%) (Effects of the invention) As detailed above, according to the present invention, there is no surface defect, no internal macroscopic defects such as cavities and component segregation, and fine ingots. An ingot with a texture can be obtained, and by using this ingot, a thick steel plate with excellent quality can be obtained even by rolling or forging with a small reduction ratio, so costs can be significantly reduced. can.

[Brief explanation of the drawing]

Figures 1 (A) and 1 (B) are schematic side views and plan views, respectively, showing an outline of the casting method according to the present invention; Figure 2 is a schematic diagram 3 (
A) and (B) are a schematic explanatory diagram and a partially enlarged view, respectively, schematically showing the solidification status of an ingot using the conventional method; and Figure 4 is a diagram of the conventional LH steel ingot process used in the comparative example. FIG.

Claims (2)

[Claims] (1) A casting method in which the molten steel stream is made into droplets and dropped into an endless mold consisting of a bottom surface and a side wall surface to solidify, characterized in that the above-mentioned injected molten steel stream is composed of three or more molten steel streams. A method for casting ingots for manufacturing thick-walled steel plates. (2) Setting the injected molten steel flows to different dropletization conditions, or placing some of the injected molten steel flows in the mold in a completely molten state without turning them into droplets. The casting method according to claim 1, further comprising injection.