JPS6192756A - Continuous casting method of preventing surface cracking of ingot and casting mold - Google Patents

Abstract

PURPOSE:To cast continuously an ingot without surface cracking on the ingot by casting a molten middle-carbon steel into a casting mold formed with plural vertical grooves each having an adequate shape and size to the surface in the upper part in the casting mold and cooling slowly only the solidified shell in the upper part of the casting mold. CONSTITUTION:The plural vertical grooves 5 each having 250-750mum width, 60-300mum depth D and 20-90% area rate are provided only on the inside surface 2 of the casting mold made of copper within 300mm from the top end of the mold 1 in parallel with the casting direction in a method for executing continuous casting by casting a molten metal cong. 0.09-0.16% C via an immer sion nozzle 3 into the casting mold 1 and floating a flux consisting of molding powder 4 on the surface of the molten steel in the casting mold. Then air stagnates in the grooves 5 to suppress the inflow of the powder 4 and the molten steel. The molten steel contacts with the mold 1 only at projecting parts 6 and the solidified shell is slowly cooled only in the upper part of the mold 1, by which the surface cracking of the ingot is prevented.

Description

Detailed Description of the Invention (Industrial Application Field) The present invention relates to a method for producing continuously cast slabs and a mold therefor, and particularly a method for producing continuously cast slabs free from surface cracks and a mold used therefor. Regarding structure.

(Prior art) When producing steel slabs by a continuous casting method, so-called medium carbon steel with a carbon content in the range of 0.09 to 0.16% has a high carbon content in the slab during casting. Vertical cracks are likely to occur. If such cracks are observed on the surface of a cast slab, it is necessary to remove the surface flaws before sending it to the rolling process. hot charging) or direct rolling (direct rolling)
charge) is an impediment to its application.

Various studies have been conducted on the generation mechanism of surface cracks, that is, vertical cracks, and in particular
The range of 9% to 0.16% is the peritectic reaction region, and as a result of the solidification reaction proceeding non-uniformly, the non-uniformity of the solidified shell thickness is large, which is likely to cause vertical cracks etc. Are known.

Therefore, although the mechanism has not yet been completely clarified, the mechanism by which such vertical cracks occur is thought to be as follows. In other words, in continuous casting, /8
Steel is injected into a water-cooled 8RL mold and a solidified shell is produced, but in this case, the amount of heat removed by the mold is large, so if there is even a slight unevenness in the amount of heat removed, the thickness of the solidified shell will be uneven, and therefore the solidified shell will Vertical cracks occur as a result of thermal stress due to shrinkage acting on thinner parts.

Therefore, in order to prevent longitudinal cracks in LJ in such continuously cast slabs, it is sufficient to make the thickness of the so-called initially solidified shell produced at the upper part of the mold uniform.

In this regard, r 1 (a
ndbuch des 5tranBicssens
J Kano, written by Erhard Ilermann ^1umi
nium-Verlag Gmbll (Dtfss
eldorf) describes that when AQ is continuously cast, irregularities are provided on the inner surface of the mold in order to improve the surface properties of the slab. Furthermore, P, Permino
v et, alJsteel in EnglishJ
7, 1968, pp. 560-562, 280
Similar statements can be found regarding 280 mm billet casting. These reduce the area ratio of the contact area with the molten steel on the inner surface of the mold, allowing for slow cooling as a whole, while increasing the amount of heat removed per unit area of the contact area, eliminating unevenness in the amount of heat removal and achieving uniform heat removal. The purpose is to perform cooling.

This idea is also seen in Japanese Patent Publication No. 57-11735, in which the dimensions of the recess are 2.5 mm or less in diameter or width, and the total area ratio of the recess is 20% or more, 90%. It is stipulated as follows. Regarding the depth of the recesses, it is desirable that the depth is 0.3 to 1.0 mm, and that the distance between the recesses is preferably about 1 to 3 mm.
Each is clearly stated.

Note that mechanical cutting methods and shot blasting methods are listed as methods for providing such uneven portions.

(Problems to be Solved by the Invention) However, these conventional techniques simply provide unevenness to the inner surface of the mold, and considering the function of the mold as a slow cooling mold that reduces the amount of heat removed, the following points are considered: (1) The mechanism for reducing the amount of heat removed from the solidified shell has been conventionally thought to be as follows:
The purpose is to reduce the area ratio of the contact portion. Therefore, even in Japanese Patent Publication No. 57-11735, the width of the recess is limited to 2.5 mm or less, generally 1.0 mm, and this would cause the /8 molten powder to flow into the recess.
Almost no effect on reducing the amount of heat removed is expected.

(2) The range to which the uneven portion is provided is interpreted to cover almost the entire inner surface of the mold in conventional known literature, but if the uneven portion is provided over the inner metal surface of the mold in this way, the area near the mold outlet However, due to the slow cooling effect, the solidified thickness of the shell decreases, and the probability of breakout occurrence increases, especially in high-speed molds.

As described above, it is difficult to achieve slow cooling simply by forming uneven portions on the inner surface of the mold, and therefore the above-mentioned technology has not been put to practical use until now.

(Means for Solving the Problems) In order to solve the above-mentioned problems, the inventors of the present invention have conducted extensive studies and found that the objective can be achieved by utilizing the heat insulation effect of air found in the four parts. As a result of conducting various experiments with a focus on achieving this, it was found that if powder flows into the vertical grooves, the insulation effect of air cannot be expected. Therefore, in the present invention, the area where the vertical grooves are provided is Within 300mm from the top edge, the vertical groove shape is (i) width is 25mm
0-750 μm, (ii) depth 60-300 μm
1 and (iii) By limiting the area ratio of the vertical grooves to 20 to 90%, preferably 40 to 90%, the inflow of powder is prevented, while the heat insulating effect of air is maximized to achieve the desired effect. The present invention was completed by discovering that a drooping effect can be obtained.

Here, the gist of the present invention is (C):Q.

In a method of continuously casting slabs from molten steel containing 0.09 to 0.16%, a groove width of 250 to 750 μm parallel to the casting direction is formed only on the inner surface of the mold within 300 mm from the upper end of the mold.
The roughness is 60-300 μm, and the vertical groove area ratio is 20-9.
0%, the molten steel is poured into a mold with a plurality of longitudinal grooves, and flux is suspended on the surface of the self-molten steel in the mold, so that the solidified shell is slowly cooled only in the upper part of the mold. There is no continuous casting method.

Note that the vertical grooves were provided within 300 mm from the top of the mold because if the grooves were processed further than that, the thickness of the solidified shell would become thinner due to the slow cooling effect, resulting in unstable operation such as breakouts as mentioned above. This is because it becomes

In addition, 0.09 to 0616 is highly susceptible to surface cracking.
% carbon steel is cast because, as already mentioned, in this carbon region, the peritectic reaction occurs, and the occurrence of vertical cracks is particularly pronounced. Therefore, it goes without saying that the present invention can also be applied to other types of steel with similar excellent effects.

(Function) Next, the reason why the mold shape is limited as described above in the present invention will be explained.

FIG. 1 is a perspective view showing a mold 1 according to the present invention, in which vertical grooves (not shown) are machined on the inner surface 2 of the copper mold. The upper part of the molten steel injected into the immersion nozzle 3 mold 1 is covered with mold powder 4, and the molten steel is slowly cooled. Figure 2 shows a cross-sectional view of the inner surface of the mold. The longitudinal lengths tT15 are preferably provided regularly in the elongated direction on the inner surface of the mold 2, and their cross-sectional shape is such that, as described above, the inflow of mold powder is prevented while the inflow of air is allowed. In the figure, the convex portion 6 is /8
It forms the contact area with the steel and is heated through this area.

In the present invention, the range where the vertical grooves are provided is the inner surface area of the mold within 300 mm from the upper end of the mold, and the width W of the vertical grooves is 250 to 750 μm, and the depth of the grooves is 60 to 30 μm.
0μm, and the area ratio occupied by these vertical grooves is 20~
It is adjusted to 90%, preferably 40 to 90%. Note that this area ratio is for the area where the 11N groove is provided, and is not for the entire area of the inner surface of the mold.

(1) First, the reason why the vertical groove machining range was limited to within 300 mm from the upper end of the mold will be explained.

Figure 3 shows the drawing speed of 0.10% C steel at 0.8 m/min.
The results of an investigation of the shell solidification thickness at the mold outlet when casting is shown below. This shell thickness was measured from S prints with S addition. As is clear from the results shown in Figure 3, when the machining length exceeds 300 mm, the shell solidification thickness decreases, becoming thinner by 1 to 2 mm on average. The probability of this occurring increases, making operations unstable.

Also, the meniscus position is usually 100mm from the top of the mold.
Therefore, if the machining length is short, the effect of making the thickness of the solidified shell uniform is not noticeable, and the variation becomes large. Therefore, the range of vertical groove machining is preferably 200 mm or more and 300 mm or less from the upper end of the mold. In other words, the thickness of the solidified shell can be made uniform by slow cooling in the range where the solidified shell and the inner surface of the mold are in contact (approximately 150 to 200 mm from the meniscus position, that is, 250 to 300 mm from the upper end of the mold). Therefore, making the shell more uneven is counterproductive in terms of shell growth.

(2) The reason why the groove width was limited to 750 to 250 μm is as follows.

Normally, the mold powder is in a molten state within a length of 150 mm from the meniscus position of the mold, and therefore, the surface tension of the mold powder is less than 115 compared to molten steel, so the mold powder easily flows into the groove.

The above-mentioned Japanese Patent Publication No. 57-11735, which is a conventionally known example, states that the groove width is 2.51 mm or less (generally 1 mm), but as is clear from these examples, conventionally the vertical groove The main focus is on suppressing the inflow of molten steel. However, the present invention focuses on suppressing the inflow of mold powder from a completely different viewpoint than the conventional one, and experimentally and theoretically, if the groove width is 750 μm or less, mold powder will not flow in. This is based on the fact that we discovered this for the first time.

That is, FIG. 4 shows the relationship between the mold powder pressure near the meniscus position in the mold and the groove width at the mold powder inflow limit. Normally, pressure is applied to one layer of molten powder by mold oscillation, and lo
A pressure on the order of g/Cnt acts between the mold and the solidified shell. From the results shown in Figure 4, when there is a mold powder pressure of Log/cnl, the groove width without mold powder inflow is:
It becomes 750 μm or less.

Therefore, it can be seen that mold powder tends to flow in when the groove width is several mm or less as in the conventional known example, and when the groove is filled with mold powder, almost no decrease in heat flux can be expected.

The reason why the groove width is set to 250 μm or more is that processing becomes difficult if the groove width is smaller than this.

(3) The appropriate depth of the groove is 60 to 300 μm.

The reason why the lower limit of m depth is set to 60 μm in the present invention is that if the groove depth is less than 60 μm, the heat insulating effect of air intended in the present invention is not sufficient.

Here, 20 no. from the meniscus position in the mold is shown in Figure 5.
The results of measuring the heat flux at a position below n are shown.

Normally, in order to make the thickness of the solidified shell uniform by slow cooling, it is necessary to reduce the heat flux by 20% or more.

From the results shown in FIG. 5, it can be seen that the depth of the groove is required to be 60 μm or more. On the other hand, the reason why the groove depth is set to 300 μm or less is that the effect hardly changes even if the groove depth exceeds 300 μm.

As mentioned above, in the present invention, the width of the vertical groove is 250 to 7
By using a mold with a depth of 50 μm, a depth of 60 to 300 μm, and a processing range within 300 mm from the upper end of the mold, the effect of slow cooling aimed at by the present invention can be obtained. Furthermore, the area ratio of the recesses formed by these longitudinal grooves is generally 20 to 90%, preferably 40 to 90%. The larger the area ratio, the greater the gradual cooling effect.

Next, details of the method of machining such a vertical groove will be explained.

Chemical etching This chemical etching process either creates corrosion grooves in the copper plate that makes up the mold before plating, or removes the plating layer (usually N) after plating.
This can be done either by etching the plating layer) or by etching the plating layer.

In this chemical etching process, the surface is first cleaned with an organic solvent or an alkaline solution as a pretreatment, and then areas other than the etched area are protected by masking. Masking methods include a screen printing method and a mechanical masking method. For etching, methods such as immersing the workpiece in a corrosive solution or spraying the workpiece are used. As a corrosive liquid for copper, FeCn3, (N
H4) 2s204, CuC/22, CrO3, Fe'
There is C#3+HNO3, and the corrosion rate is 0.01
~0.05mm/min is selected.

As for the mechanical processing of the Terutocho plate, it is difficult to use ordinary mechanical cutting, so grinding using a special hide or processing using a milling machine can be used. As is clear to those skilled in the art, cutting with a width of about 250 to 750 m (-60 to 300 μm in depth) is inevitably expensive, but it can be easily realized with today's precision cutting.

Laser Processing Laser processing is suitable for fine processing, and therefore, for example, a copper plate may be plated with Ni thickly and then laser processing may be performed. Conventional laser processing methods can be used.

In the present invention, ``machining/notching'', ``mechanical processing,'' and ``laser processing are mentioned, but in any case, a method for machining fine grooves is required, and it is not limited to specific means. It's not something you can do.

Thus, according to the present invention, unprecedentedly fine vertical grooves are machined on the inner surface of the mold to reduce the initial heat flux for solidification shell generation, thereby improving the uniformity of the solidification shell thickness. It becomes possible to reduce surface flaws such as vertical cracks.

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

Continuously cast slabs were manufactured using a 12.5 mR curved two-strand continuous casting machine under the conditions shown in Table 1. The length of the mold used was 700 mm, and the inner surface of the mold was coated with Ni plating and Ni+P plating.
1 mml! It is attached so that it becomes J.

Table 2 summarizes the conditions for forming grooves on the inner surface of the mold and the casting results. For comparison, an example using a mold without grooves is also shown.

Test 11hl was a normal CC mold and no grooves were formed. Identical Nos. 2 and 3 are grooved, but the shape of the vertical grooves is outside the scope of the present invention. Test N[L
No. 4 has proper vertical grooves, but the vertical grooves are machined over the entire length of the mold.

Table 2 also shows the heat flux reduction rate and vertical crack occurrence index of the copper plate at a position 20 mm below the meniscus position. In the case of test numbers 1 to 3, the heat flux reduction was insufficient at 10% or less, and almost no reduction in longitudinal cracking was observed. On the other hand, a breakout occurred in the 11h4 full-length vertically grooved mold. On the other hand, in test cases 5 and 6, which represent the cases of the present invention, no vertical cracks were observed at all, and no operational problems were observed.

[Brief explanation of drawings]

FIG. 1 is a schematic perspective view M of a mold according to the present invention; FIG.
A partial cross-sectional view of the inner wall of the mold; Figure 3 is a graph showing the relationship between solidified shell thickness and length of vertical grooves from the upper end of the mold; Figure 4 is a graph showing the relationship between vertical groove width and powder pressure. and FIG. 5 is a graph showing the relationship between heat flux reduction rate and groove depth. 1: Mold 2: Mold inner surface 3: Immersion nozzle 4: Mold powder applicant
Sumitomo Metal Industries Co., Ltd. Agent Patent Attorney Dou Hirose - (1 other person) Figure 1 Figure 2 Figure 3 The mold is made into a pigeon J゛Ss whip bottle processing needle (recommendation 20 100 20o, 3o)
.

Claims (2)

[Claims] (1) [C]: In a method of continuously casting slabs from molten steel containing 0.09 to 0.16%,
Groove width 2 parallel to the casting direction only on the inner surface of the mold within 0 mm
The molten steel is poured into a mold having a plurality of vertical grooves of 50 to 750 μm, depth of 60 to 300 μm, and a vertical groove area ratio of 20 to 90%, and the flux is suspended on the surface of the molten steel in the mold, so that the flux is suspended only in the upper part of the mold. A continuous casting method characterized by slow cooling of the solidified shell, which does not cause surface cracks in the slab. (2) Groove width 250-750 μm, depth 60-750 μm parallel to the casting direction only on the inner surface of the mold within 300 mm from the upper end of the mold
A mold for continuous casting that prevents surface cracking of slabs, characterized by having a plurality of vertical grooves of 300 μm and an area ratio of 20 to 90%.