JPH01289542A - Casting mold for continuous casting of steel - Google Patents

Abstract

PURPOSE:To prevent nonuniform solidification and to improve productivity by providing grating grooves having a specific shape and spacing on a casting mold surface near meniscus and packing different metals or ceramics into the grooves down to the depth of the prescribed conditions. CONSTITUTION:The grating-shaped grooves 2 having 0.5-1.0mm depth and 0.5-1.00mm width are provided to the casting mold surface near the meniscus of a molten steel in the casting mold and the spacing between the grooves is specified to a 5-10mm range. The different metals consisting of Ni and Cr or the ceramics such as BN, AlN and ZrO2 are packed at need into the grooves down to the depth at which the thermal resistance ratio (h) expressed by equation I attains >=1.5. Since the grooves 2 are disposed near the meniscus part, the formation of hexagonal rugged patterns on the immersion side of the solidified shell in the initial period of solidification and the formation of the ruggedness on the molten steel side is prevented from the initial period of solidification. The nonuniform solidification of the molten steel is thereby prevented and, therefore, the high-speed casting is enabled and the productivity is improved.

Description

Detailed Description of the Invention [Industrial Application Field] The present invention is directed to a method for preventing longitudinal cracking of the solidified shell in the initial stage of subperitectic solidifying steel with a carbon content of 0.10 to 0.15%. Concerning continuous casting molds.

[Prior Art] In recent years, a continuous casting process using a vertical or curved continuous casting machine has become essential for producing steel. When attempting to manufacture slabs such as blooms and billets using such a continuous casting method, vertical cracks and horizontal cracks may occur on the surface of the slab. FIG. 17 is a graph showing the relationship between the carbon content of a slab and the surface crack index when the slab is cast using a conventional copper plate mold. As is clear from this figure, many surface cracks occur in steel types that undergo subperitectic solidification with a carbon content of 0.10-0.15%. The reason for this is that when a steel type with the above carbon content solidifies, it undergoes a transformation process of - δ + peritectic reaction (δ + L → γ) → δ + γ → γ. Among these, the δ phase is body-centered cubic (bc
c), the γ phase has a face-centered cubic (fee) crystal structure, and the δ
During the −γ transformation, volumetric contraction occurs due to this difference in crystal structure, generating large transformation stress.

In addition, during this peritectic reaction of δ→γ, the liquid phase disappears, so there is nothing to absorb the strain caused by contraction, and the solidified shell itself assumes a non-uniform solidification form, and the above stress is applied to the thin solidified shell. It is thought that cracks may occur in some parts. Conventionally, in order to prevent surface cracking of the steel types mentioned above, the mold powder was changed to one with lower cracking susceptibility through trial and error before casting, or the heat removed from the mold was reduced to perform low-speed casting. Ta.

[Problems to be solved by the invention] However, in order to prevent surface cracking, it is difficult to select a mold powder that satisfies all of the many casting conditions, and it takes a huge amount of time and money. . In addition, when low-speed casting is performed by reducing heat removal from the mold, it becomes difficult to synchronize with the rolling mill for direct rolling, making it impossible to perform hot direct rolling or hot charge rolling, which impedes labor and energy conservation in the steel manufacturing process. At the same time, there was a problem in that the yield of the product also decreased.

This invention has been made in view of the above circumstances, and is intended to prevent vertical cracking of the solidified shell in the initial stage of a subperitectic solidifying steel with a carbon content of 0.10 to 0.15%, and to prevent surface defects in the slab. The purpose is to provide a continuous casting mold for.

[Means for Solving the Problems] The mold for continuous casting of steel of the present invention has a depth of 0.5 to 1.0 mm and a width of O on the mold surface near the meniscus of molten steel in the mold.
It is characterized in that a lattice-like groove of 15 to 1.0 mm is provided, and the interval between the lattice-like grooves is 5 to 10 mm.

[Function] In the mold for continuous casting of steel according to the present invention, by providing a lattice-like groove on the surface of the mold, the strength of cooling is varied between the grooved portion and the other portion, and the initial cooling rate is lowered in the grooved portion which is the weakly cooled portion. The solidification of the solidified shell is slightly delayed. For this reason, a liquid phase remains at regular intervals, and this liquid phase absorbs the strain caused by contraction, suppressing the initial bending of the solidified shell, and preventing the mold from separating from the solidified shell locally.Therefore, heat is removed. becomes uniform,
Solidified shell thickness grows uniformly 0 By using the mold of the present invention, the initial solidified shell thickness is extremely uniform, so even if solidification shrinkage or transformation stress occurs during δ→γ transformation, localized Since there is no thin part of the solidified shell, −
The grid-like grooves are 0.5 to 1.0 mm deep and 0.5 to 1.0 mm wide, and the grooves are arranged in a grid pattern at intervals of 5 to 10 mm, so that no stress is applied to the points. The reason is that in other cases, the non-uniformity of the solidified shell thickness becomes large.

[Example] An embodiment of the present invention will be described below.

In steel types that undergo subperitectic solidification, when an initial solidified shell is formed, the solidified shell is bent by thermal strain and transformation stress due to δ→γ transformation, and a void is locally formed between the solidified shell and the mold wall, which causes local As a result, the solidified shell thickness grows non-uniformly. In the process leading up to the present invention, the present inventors have found that surface cracks occur where the thickness of the solidified shell is thin, and that surface cracks can be prevented by preventing uneven solidification.

Based on this knowledge, in order to investigate the cause of the non-uniformity of the solidified shell thickness, a water-cooled flat plate of 10100 mm x 360 mm = cooling water of the immersed body was 90 J/min.
) is immersed in molten steel using an air cylinder directly above a melting furnace of l 00 kg, and held for a certain period of time, and the unevenness of the solidified shell (expressed as solidified shell non-uniformity Δd/II, Δd
: Check the thickness difference between adjacent asperities, d protrusion - d concavity, (: distance between adjacent asperities). FIG. 6 is a diagram showing a method for measuring solidified shell non-uniformity. That is, immersed in molten steel, -
Solidified shell formed on the surface of the immersed body for a certain period of time]]
The solidified shell 11 is peeled off from the immersion body and placed on a flat plate F, and the thickness of the solidified shell 11 between adjacent asperities is d2.
) and the distance <1> between adjacent asperities, and calculate the difference in the thickness of the solidified shell 11 between adjacent asperities (for example, Δd=d2−c
The ratio (
The value obtained by dividing the integral value of Δd/i) by the number of pieces measured was taken as the average solidification shell heterogeneity.9 Average solidification shell heterogeneity - The experimental conditions were the carbon content in the molten steel and the surface texture of the immersed body. changed. The carbon content in molten steel is 0.01 to 0.
It was varied within a range of 50%.

At this time, Si: 0.20%, Mn: 0.60%.

P: 0.015%, S: 0.010%.

5OQA, +2: was kept almost constant at 0.10-0.30%.

FIG. 7 is a graph showing the relationship between the carbon content in molten steel and the average degree of solidification shell heterogeneity. Using a flat copper immersion body (thickness: 10 mm), after immersing it for 8 to 9 seconds, the copper immersion body was pulled up and the solidified shell formed on the surface of the copper immersion body was average ′a, (l111 shell The degree of non-uniformity was measured. The straight line portion indicates the variation in the average solidified shell non-uniformity, and the Φ symbol indicates the average value.

As is clear from this figure, at the same solidification time, the carbon content in molten steel is 0.10-0.15% (7) E 1ffl
When , the average degree of nonuniformity of the solidified shell is large, indicating that a solidified shell with severe irregularities is formed. In the above-mentioned steel types with a carbon content in the range of 0.10-0.15%, the initial solidification shell surface (surface on the immersion body side) is characteristically
A tortoiseshell-like uneven pattern is observed. This tortoise-shell-like uneven pattern has a high center and groove-like depressions around the periphery. In addition, for steel types that undergo hyperperitectic solidification with a carbon content of 0.15 or more, 0.1
Similar to steel types that undergo subperitectic solidification of 0 to 0.15%, no tortoiseshell-like uneven pattern is observed on the surface of the solidified shell on the side of the immersed body despite the δ→γ transformation.

This is because in steel types that undergo hyperperitectic solidification, the liquid phase remains even during the δ-γ transformation, and the large transformation stress during the δ→γ transformation can be absorbed by the liquid phase.

Figure 8 shows the solidification time, the size of the unevenness on the molten steel side of the initial solidified shell (distance between adjacent depressions and openings M = mm), and the size of the unevenness (circular equivalent diameter) It is a graph figure showing the relationship of mrn). The same immersion body as in FIG. 7 was used.

・The size of the unevenness on the side of the shell immersed body (marked) (distance between opening and opening of the tortoiseshell pattern on the side of the solidified shell immersed body - pP) remains formed at the early stage of solidification and does not change over time, but for the solidified shell marked 0 The size of the unevenness on the molten steel side (distance between the protrusions -5 on the solidified shell molten steel side = iv) increases as solidification progresses.

Figure 9 shows the solidification time when fine vertical grooves are made in the immersed body and the size of the tortoise-concave pattern on the solidified shell side, equivalent circle diameter = m
It is a graph figure showing the relationship of (m).

There are three types of grooves: copper flat plate, copper vertical groove 1A, and fI vertical groove B. The vertical groove A has an anchor groove 13 on the surface of the immersion body 12,
The depth of Ko 13 is 0.5 mm, the width Lt 0.5 mm, and the width of 13
The distance between the two is 0.7mm.3 vertically? In 11B, the depth of clear 13 is 0.5 mrn, the width is 05 mm, and the distance between western 13 is 1.0
It is mm. As can be seen from this figure, the size of the hexagonal pattern on the side of the solidified shell immersed body is 7I, which is dense on the surface of the immersed body 12.
When I13 was inserted, the size was about 10 to 15 mm, the same as when the flat plate was not grooved. From these findings, in steel types that undergo subperitectic solidification with a carbon content of 0.10 to 0.15% (51), the initial coagulation shell undergoes thermal strain and δ→
Due to the transformation stress caused by γ deformation, the solidification ji and r ru are bent,
A void is formed locally between the solidified shell and the mold wall. This becomes a tortoise-like uneven pattern that is observed on the surface of the solidified shell immersed body, and once this uneven pattern is formed, it remains for a long time. These voids cause a decrease in heat removal from the solidified shell and non-uniform growth of the solidified shell. Therefore, in order to suppress the non-uniformity of the solidified shell of the above-mentioned steel types, the tortoise-shell-like uneven pattern on the surface of the solidified shell on the immersed body side during initial solidification should not be formed, or be minimized, and the surface of the immersed body 12 and the solidified shell should be formed as small as possible. It is sufficient to avoid forming a void between them. However, as shown in FIG. 9, even if dense vertical grooves with grooves 13 of 0.7 mm or 1.0 mm are formed on the immersed body 12, the size of the tortoiseshell pattern on the surface of the solidified shell immersed body will not change. do not have. Therefore, the present inventors attempted an experiment in which grooves were formed on the surface of the copper immersion body in a lattice pattern in order to cause non-uniform heat removal in a range smaller than the tortoiseshell pattern. FIG. 10 is a graph showing the relationship between immersion time and average solidified shell non-uniformity. In this figure, the mark indicates that the thickness is 8mm and the amount of cooling water is 90il/
The O mark is a dipping body of a copper flat plate of min. be. The straight line indicates the variation in the average solidified shell heterogeneity.

As is clear from this figure, the average solidified shell non-uniformity of the immersed body with grid-like grooves formed on the surface of the copper plate is smaller than that of the immersed body of the flat copper plate, and the variation is also smaller.
FIG. 1 is a graph showing the relationship between the solidified shell thickness and the immersion time of the immersed body. The ○ symbol is a dipped copper flat plate, and the ・ symbol is a dipped body with grid-like grooves on the surface of the copper plate, and the depth of the grooves is 0.5.
mm, the width is 0.5 mm, and the grid-like interval is 5 mm.

The Mu mark is an immersion body in which lattice grooves are formed on the surface of a copper plate, and the depth of the grooves is 0.5 mm, the width is 0.5 mm, and the interval between the lattice grooves is 10 mm. As is clear from this figure, the presence of lattice grooves causes slow cooling and does not cause the solidified shell thickness to become thinner. Therefore, by using a mold with lattice grooves, the non-uniformity of the solidified shell thickness is reduced. Therefore, surface cracking of the above-mentioned steel types can be reduced, and there is no need to lower the casting speed since slow cooling is not performed, so hot direct rolling can be performed.

Next, we investigated the optimal conditions for lattice grooves to reduce surface cracks.

(1) Effect of lattice groove spacing FIG. 12 is a graph showing the relationship between lattice groove spacing and average solidified shell non-uniformity. The immersion time of the immersed body is 8-9 seconds, the depth of the groove is 0.5 mm, the width is 0.5 mm, and the interval of the lattice groove is 0-30 mm (0, 5, 10, 15, 30 mm).
), as is clear from this figure, by making the spacing between the lattice grooves smaller than the unevenness of the tortoise-shell pattern made of flat copper plates shown in Fig. 9, there is a large effect on improving the average solidified shell non-uniformity. demonstrate. On the other hand, if it is too small, the processing will be complicated, the overall heat removal will be reduced, and the cooling will be slow, making it impossible to secure the casting speed necessary for hot direct rolling.
A groove spacing of 10 mm is optimal.

(2) Effect of the shape of the lattice grooves FIG. 13 is a graph showing the relationship between the shape of the lattice grooves and the average solidified shell non-uniformity. The immersion time of the immersion body was 8 to 9 seconds, and the depth of the groove was 0.5 mm.

1.0mm, 1.5mm, width 0.5mm.

1.0 mm and 1.5 mm, and the interval between the grating grooves is 5 mm. The cross-sectional shape of the grating groove is V-shaped as shown in FIG.
There are three types: U-shaped and square-shaped.

As is clear from this figure, when the groove depth is 1.5 mm and the width is 1.5 mm, the average solidified shell nonuniformity is 0.1
In addition, insertion of molten steel was confirmed. When the depth of the groove is 1.0 mm or less and the width is 1.0 mm or less, the average solidified shell non-uniformity is improved regardless of the cross-sectional shape of the lattice groove.

(3) Effect of embedding foreign materials inside 711 Next, we investigated the average solidification shell non-uniformity when materials with different thermal conductivities were embedded in the grooves.

Here, the ratio of local thermal resistance values in the body part and the groove part is h
It was evaluated as the degree of unripeness of the immersed body. FIG. 14 is an explanatory diagram showing the degree of unrestricted ripening of the immersed body.

The thermal resistance R3 of the copper flat plate portion of the immersed body 12 is Rc, = d
,,/λ6u d cu: Thickness (m) of the copper flat plate part of the immersed body λc,: Thermal conductivity of the copper flat plate part of the immersed body (Kcal/m-Hr-'C) On the other hand, The thermal resistance R0 of the groove portion filled with the substance 6 is R,=d0. '/λeu+dc/λ.

dcu': Thickness from the bottom of the groove to the cooling water surface (m) do: depth of groove (m> λC: thermal conductivity of embedded material (Kcal/+-Hr-'C) From this, the thermal resistance ratio was set as h=R,/Rcu.

FIG. 15 is a graph showing the relationship between various embedded materials having different thermal conductivities and the average solidified shell non-uniformity. The experimental conditions were as follows: depth of grid was 0.5 mm, width was 0.5 mm, width of grid was 5 mm, shape was V-shaped, thermal resistance ratio was 1.5, embedded material 6 was metal (Ni, Cr) - Ceramic (BN, Z
r02), the immersion time of the immersed body was 8 to 9 seconds. As is clear from this figure, the buried material 6 is metal (Ni.

Both Cr) and ceramics (BN, Zr02) had improved effects on the average solidified shell non-uniformity, and there was no difference depending on the embedding material 6. Furthermore, there was no difference in the average solidified shell nonuniformity between the grooves alone and the grooves filled with a different material. FIG. 16 is a graph showing the relationship between the thermal resistance ratio and the average solidified shell non-uniformity. The experimental conditions were: the depth of the grid was 0.5 mm, the width was 0.5 mm, and the grid spacing was 5 mm.
mm, the shape was V-shaped, the embedded material was Ni metal, and the immersion time of the immersion body was 8 to 9 seconds. As is clear from this figure, when the thermal resistance ratio is 1.5 or more, the average solidified shell non-uniformity is improved. In order to maintain the thermal resistance ratio at 1.5 or more, when N1 is embedded in a 10 mm copper plate, it is necessary to ensure a depth of 1.8 mm or more.

(4) Range of lattice grooves As mentioned above, in order to prevent uneven solidification, it is necessary to prevent the formation of the tortoise-concave pattern that occurs on the surface of the immersed solidified shell. This is because, as shown in Figure 8, a hexagonal pattern is formed on the side of the solidified shell immersed body during the initial stage of solidification.
This size does not change as the solidified shell grows. On the other hand, the unevenness on the molten steel side has a size corresponding to the hexagonal uneven pattern on the surface of the solidified shell on the immersed body side at the initial stage of solidification, and the interval between them increases as the solidified shell grows. Therefore, the unevenness on the molten steel side will not be generated from the initial stage of solidification unless the uneven pattern on the immersed body side is formed, resulting in uniform solidified shell growth. In other words, if the formation of uneven patterns on the immersed body side is prevented at the initial stage of solidification, non-uniform growth can be completely prevented thereafter.

Therefore, in order to suppress unevenness, the range of the lattice grooves needs to be only directly below the meniscus at the initial stage of solidification, and the range is 60 m from the meniscus.
The range may be up to 300 mm, but considering fluctuations in the molten steel level, the range is actually up to about 300 mm from the upper surface of the mold.

Fig. 1 is a schematic diagram of the upper part of a mold according to an embodiment of the present invention, <a> is a front view, and (b) is a sectional view taken along line A-A in (a>. 1 is a mold, 2 is a groove, 3 is the slit for cooling water, and the slits 2 are arranged in a grid pattern. 4 is the molten steel surface of the mold, and 5 is the slit for cooling water.
is the cooling surface of the mold, and since the cooling water slit is arranged in this part, the mold 1 is cooled.

(Example 1) Fig. 2 is a schematic diagram of the upper part of a mold according to an embodiment of the present invention, (a) is a front view, (b) is a sectional view taken along line AA in (a), and (c) is a It is an enlarged view of the groove part of (b). As shown in Figure 2, 50 to 300 m from the upper end of the molten steel surface 4 side of the mold 1.
m, 2000m in the width direction from the center of the width by 1000mm
V with a depth of 0.5 mm and a width of 0.5 mm in a length range of m
A mold 1 was used in which grooves 2 of the mold were arranged in a lattice shape at intervals of 10 mm.

A steel type with a carbon content of 0.10 to 0.15% was actually cast using this mold. FIG. 3 is a graph showing the relationship between slab surface cracking index and casting speed according to an embodiment of the present invention.・The symbol indicates the conventional method, and the symbol 0 indicates an embodiment of the present invention. As is clear from this figure, in this example, the slab surface cracking index is improved compared to the conventional method, and the slab surface cracking index is improved even during high-speed casting (1.5 m/min or more).

(Example 2) Fig. 4 is a schematic diagram of the upper part of the mold according to another example of the present invention, where (a> is a front view and (b) is A- of <a, ).
A sectional view, (c) is an enlarged view of the groove part in (b). Fourth
As shown in the figure, from the upper end of the molten steel surface 4 side of the mold 1,
300mm, 1. Square grooves 2 with a depth of 3.5 mm and a width of 0.5 mm are arranged in a lattice shape with an interval of 10 mm in a length range of 2000 mm in the width direction by m each, and dissimilar metals 6 are placed in the grooves 2. Nt gold metal was used. The depth of the Ni metal was set to 3.5 mm from the surface of the 19 mm copper mold 1 so that the thermal resistance ratio was 1.5.

Mold 1 has a length of 950 mm and a width of 2320 mm in the casting direction.
A steel type with a carbon content of 0.10 to 0.15% was actually cast using a mold with a thickness of 40 rrt m and a depth of the cooling water slit 3 of 21 mm.6 Figure 5 shows the mold of this invention. FIG. 7 is a graph showing the relationship between slab surface cracking index and casting speed according to another example. - The mark indicates the conventional method, and the mark ○ indicates another embodiment of the present invention. As is clear from this figure, the slab surface cracking index in this example is improved compared to the conventional method, and the slab surface cracking index is improved even during high-speed casting (1,"im/min or higher). As a result, direct rolling became possible and productivity improved. [Effects of the Invention] Since this invention is constructed as described above, 1) the carbon content in the molten steel is 0.10 to 0.15%; It is possible to improve uneven solidification of steel types that undergo subperitectic solidification.

2> High-speed casting of the above steel types has become possible, and slab surface defects have been improved.

3) As a result, direct rolling becomes possible, improving productivity.

[Brief explanation of the drawing]

FIG. 1 is a schematic diagram of the upper part of a mold according to an embodiment of the present invention;
Figure 2 is a schematic diagram of the upper part of the mold according to an embodiment of the present invention, Figure 3 is a graph showing the relationship between slab surface crack index and casting speed according to an embodiment of the present invention, and Figure 4 is a diagram of the invention. FIG. 5 is a graph showing the relationship between slab surface cracking index and casting speed according to another embodiment of the present invention. FIG. 6 is a graph showing the solidified shell non-uniformity. A diagram showing the measurement method, Figure 7 is a graph showing the relationship between the carbon content in molten steel and the average degree of solidification shell heterogeneity, and Figure 8 is a graph showing the relationship between solidification time and the size of hexagonal irregularities. , Figure 9 is a graph showing the relationship between groove type and hexagonal pattern size, Figure 10 is a graph showing the relationship between immersion time and average solidified shell non-uniformity, and Figure 11 is a graph showing the relationship between solidified shell thickness and immersion. Figure 12 is a graph showing the relationship between the immersion time of the body, Figure 12 is a graph showing the relationship between the spacing of the lattice grooves and the average solidified shell heterogeneity, and Figure 13 is the relationship between the shape of the lattice grooves and the average solidified shell heterogeneity. FIG. 14 is an explanatory diagram showing the degree of unrestricted heat removal of the immersed body. FIG. 15 is a graph diagram showing the relationship between various embedded materials with different thermal conductivities and the average solidified shell heterogeneity. FIG. 16 17 is a graph showing the relationship between the thermal resistance ratio and the average solidified shell non-uniformity, and FIG. 17 is a graph showing the relationship between the carbon content and surface cracking index of a conventional molded copper plate. DESCRIPTION OF SYMBOLS 1... Mold, 2... Groove, 3... Slit for cooling water, 4... Molten steel surface of the mold, 5... Cooling surface of the mold, 6...
...Dissimilar metals.

Claims (2)

[Claims] (1) In a continuous casting mold made of copper, a depth of 0.5 to 1.0 mm is placed on the mold surface near the meniscus of the molten steel in the mold.
A mold for continuous casting of steel, characterized in that lattice-shaped grooves with a width of 0.5 to 1.0 mm are provided, and the intervals between the lattice-shaped grooves are 5 to 10 mm. (2) A claim characterized in that the lattice-shaped grooves are filled with dissimilar metals (Ni, Cr) or ceramics (BN, AlN, ZrO_2) to a depth such that the thermal resistance ratio is 1.5 or more. A mold for continuous casting of steel according to item 1. However, thermal resistance ratio: h = R_O/R_C_U R_C_U: Thermal resistance of copper plate = D_C_U/λ_C_U
R_C: Thermal resistance of the dissimilar material embedded part = D_C_U'/λ_C_U+D_C/λ_C where D_C_U: Thickness of the copper plate of the mold (m) λ_C_U: Thermal conductivity of the copper plate (Kcal/m・Hr・℃)
) D_C_U: Thickness from the bottom of the dissimilar material embedded part to the cooling water surface (m) D_C: Embedded thickness of the dissimilar material embedded part (m) λ_C: Thermal conductivity of the dissimilar material (Kcal/m・Hr・℃)
)