JPS61195746A - Continuous casting mold - Google Patents

Abstract

PURPOSE:To prevent the surface cracking of an ingot by spacing passages for conducting a cooling medium from the inside wall surface of a casting mold near the meniscus of a molten metal and disposing the passages near the inside wall surface of the mold. CONSTITUTION:The passages 9 for conducting the cooling medium is provided to the inside of the mold wall of the mold 1 for continuous casting which are open at both ends. The distance t1 from he inside wall surface of the mold near the menisucs of the molten metal in the mold is made larger than the distance t2 in the lower position. The cooling capacity of mold 1 near the meniscus decreases and the molten metal is slowly cooled as the passages 9 are parted from the wall surface. The passages 9 are disposed nearer the inside wall surface of the mold 1 after the meniscus part and therefore the molten steel 3 is quickly cooled. The formation of the coarse austenite grains in the molten steel 3 is suppressed and the density of deposit and segregation is decreased by the above-mentioned method. The surface cracking of ingot is thus prevented.

Description

[Detailed Description of the Invention] <Industrial Application Field> This invention is applicable to the occurrence of surface flaws such as vertical cracks and horizontal cracks in slabs during continuous casting, and heat treatment including hot direct rolling and hot charge rolling. To stably produce slabs with low hot cracking susceptibility that do not generate surface flaws even during continuous casting of steel types that tend to generate surface flaws similar to the tank cracks during inter-rolling. This invention relates to a continuous casting mold that has a simple structure and is inexpensive.

<Background technology> In recent years, continuous casting processes that make use of vertical or curved continuous casting machines have become unreliable in the production of steel. When attempting to manufacture slabs such as these, surface flaws (surface cracks) may occur during the casting process due to bending stress applied to the slabs and thermal stress caused by cooling.
Furthermore, problems such as the occurrence of similar surface defects during hot rolling of the obtained slabs (particularly significant during hot direct rolling or hot charge rolling) are noticeable, and these problems lead to improvements in product yield and This has been a major obstacle in promoting labor-saving and energy-saving of the iron fII4 manufacturing process by adopting hot direct rolling or hot charge rolling.

By the way, when we investigated the occurrence of the above-mentioned surface flaws, it was observed that they occur together with cracking of the austenite (r) grain boundaries. As one example, carbides and nitrides (NbC, A/-N, etc.), (Mn, Fe) S that precipitate or segregate at austenite (γ) grain boundaries during solidification and cooling of slabs.
Sulfides such as sulfides and impurity elements such as P and S weaken grain boundaries, and the frequency of surface flaws (cracks) increases due to the presence of precipitates and segregation such as those mentioned above. It has come to be known that the content of the elements to be produced is greatly affected.

Therefore, attempts have been made to prevent surface defects in slabs by controlling the content of these elements, but in this case, there are limitations in terms of ensuring product quality (characteristics) and cost, and chemical The standards for adjusting ingredients are not very clear, so
It was not possible to achieve a sufficiently satisfactory effect by adjusting the V4 chemical components alone.

On the other hand, there is a fact that the frequency of occurrence of such surface defects in slabs is largely dependent on the C content of slabs, as shown in Figure 2, but the cause is still unknown and no measures have been taken to address this problem. In some cases, they were not found, and in the end, operations were carried out avoiding this C content range. However, as shown in Figure 2, the area where the frequency of surface flaw occurrence increases rapidly is It is not necessarily constant and varies depending on the steel type, and especially in the case of low-alloy steel, the frequency of surface flaw occurrence often becomes extremely high in unexpected compositional regions that cannot be estimated from the C content. This often led to extremely inconvenient operational results.

Therefore, the surface flaw prevention measures commonly implemented in the past are as follows:
The oscillation marks were made shallower, and the cooling rate of the slab was reduced to reduce the thermal stress acting on the solidified shell, which was insufficient.

For this reason, it is necessary to reliably prevent cracks from occurring on the surface of the slab during continuous casting of steel and subsequent hot rolling, and to produce hot-processed steel materials with good surface properties for industrial use. At present, there is a strong desire for the emergence of a means for mass production.

From the above-mentioned viewpoints, the present inventors have investigated the occurrence of surface flaws during the casting of f/R slabs produced by continuous casting, and the occurrence of surface flaws that tend to occur when continuously cast slabs are hot worked. In order to find easy-to-implement measures to reliably prevent
To this end, we have carried out various research on the actual situation based on the idea that it is essential to clarify the cause of the rapid increase in the frequency of surface flaws occurring near specific C-containing containers as shown in Figure 2. We obtained the knowledge shown below. That is.

(JL) Grain boundary cracking in continuously cast slabs is caused by carbides precipitated or segregated at grain boundaries, as previously said.
Nitride: Not only is it affected by the content of elements related to sulfides or impurities, but it is also greatly affected by the particle size of austenite (γ) grains, which influences the precipitation and segregation density, and the Coarsening of austenite (r) grains significantly promotes intergranular cracking in slabs; ('b) austenite (r) grains in carbon steel slabs during solidification and cooling;
r) The degree of grain coarsening changes greatly depending on changes in the C content, and there is no way that it will change while maintaining a mere proportional relationship with the C content, as shown in Figure 3. The above-mentioned surface flaws occur and exhibit a behavior that rapidly becomes noticeable in the C-containing region (by the way, Fig. 3 shows the behavior of the Fe
- This is a curve showing the relationship between C content and austenite grain size when the cooling rate is 5° C./sea during solidification and cooling of C-based steel.

(c) These results and the experimental confirmation that ``the coarsening of austenite grains during solidification and cooling occurs rapidly after the austenite becomes a single phase, and the higher the temperature, the more remarkable this tendency is.'' Considering this, under the same cooling conditions, the carbon t#4 slab during solidification and cooling will inevitably change to austenite single phase, which is clear from the Fe-C system equilibrium phase diagram shown in Fig. 4. Compositions with high temperatures, that is, peritectic point compositions (0.18 f
lc) exhibits the coarsest austenite (r) grains (the broken line in Fig. 4 represents the austenite grain coarsening behavior shown in Fig. 3), and therefore It can be concluded that the cracking susceptibility is significantly higher in this area.

By the way, austenite (r) shown in Figure 3
The grain size coarsening behavior does not necessarily match the tendency of frequency of occurrence of defects on the slab surface shown in FIG. However, this is due to the difference that Figure 3 shows the experimental results for pure Fe-C system, while Figure 2 shows the data for practical steel. This is because the peritectic point is shifted due to the influence of the contained elements (alloy elements, etc.); 1.(f)
Therefore, when evaluating the hot cracking susceptibility of slabs, it is important to consider not only the C content but also the influence of alloying elements.
It is necessary to use the equivalent weight (Cp) as an index. From a phase diagram study, C and Mn + Ni are elements that are considered to affect the peritectic point of steel.

Examples include Cu and N, and the C equivalent (Cp) is expressed by the following formula (hereinafter, the component ratios are expressed as parts by weight). That is.

(Company) Phase diagram study (=The above equation obtained from the equation is in good agreement with the actual situation, and based on this, the hot cracking susceptibility of slabs can be evaluated very accurately.

Figure 5 shows the results of an experiment conducted by the inventors in order to confirm this. After being redissolved in the water, it was cooled at a cooling rate of 5° C./sea, and the austenite grain size was measured, and the Cp value was calculated using the above formula (this is a graph organized from two points).

As is clear from this Figure 5, Austenai) (
r) Grain size is well organized by Cp value, and it can be seen that the maximum value is reached at Cp value of 0.180 (i) On the other hand, when the same composition is solidified and cooled, the austenite of the slab is The particle size is greatly influenced by the cooling rate in the high temperature region, and in particular, it is almost determined by the cooling rate of 1,450 to 1,200 and the cooling rate of 1,450 to 1,200 culms.
What to do.

As a result, the austenite () ffi is high and the austenite) flQ becomes coarse and the peritectic mGW (Cp=
1118), if the cooling rate in the above temperature range is increased, the coarsening of austenite grains will be suppressed and fine-grained crystals with large grain interfaces per unit volume will be obtained. Therefore, the density of precipitates and segregation that collect in the grain layer becomes lower, and hot cracking susceptibility is alleviated.

Figure @6 shows the austenite grain sizes of steels with the chemical composition shown in Table 2, which are arranged at various cooling rates following solidification, and after reaching 1000°C, are rapidly cooled to fix the structure. This is a graph showing test specimens (diameter lowφ) taken from these slabs, with the middle and bottom portions partially remelted by electrical heating (1580°C).
After that, the temperature was lowered to 1000 ° C. at each of the cooling rates described above, and the cross-sectional shrinkage ratios [R people] obtained by tensile fracture at a strain rate of 7.2.05 ec-' were arranged by the cooling rates and listed together. be. Also, from this Figure 6, even in steels with a fine peritectic composition where the most austenitic (fff) growth occurs, coarsening of austenite grains can be prevented by increasing the cooling rate following solidification. As a result, it can be seen that the ductility also shows a sufficiently good value.

Further, Fig. 7 shows that a small piece taken from steel having the composition shown in Table 2 above was remelted in an aluminum crucible, and then cooled at a cooling rate of 5°C/see and 12°C/see. This is a graph plotting the relationship between the water φ addition temperature and the austenite grain size for those whose structure was fixed by water quenching from the middle of the process, and from this Figure 7,
It is clear that the cooling rate has a significant effect on austenite grain growth only at extremely high temperatures.

For this reason, surface flaws (cracks) may occur during the casting of continuous cast slabs, and surface flaws (cracks) may occur during hot rolling of continuously cast slabs. It is possible to easily and reliably predict the steel type using the above formula (formula for calculating Cp), and also for these steel types, it is possible to predict the surface solidified iI during continuous casting.
To be able to suppress the occurrence of surface flaws by cooling the next piece at a high cooling rate as early as possible.

Therefore, based on these findings, the present inventors determined that the cooling rate = 10℃/
Attempts were made to produce continuous casting fishing rods with good surface properties by rapid cooling at a temperature of 14 sec or more.
This is because it has been found that if a slab is rapidly cooled (to a temperature of 00°C), the solidified shell may become non-uniform, and conversely, surface cracking of the slab may be promoted.

For this reason, the present inventors have conducted further research and found that in order to suppress the coarsening of austenite grains in the surface layer of continuously cast slabs and prevent the occurrence of surface defects, it is necessary to It is sufficient to quench the temperature until the temperature reaches about 1200℃, but in this case, in order to make the solidified shell in the area above 1400℃ uniform both in structure and strength, Unless the solidified shell portion with 3 stomachs or less is not slowly cooled, it is not possible to stably and reliably secure the effect of preventing the slab surface cracking caused by the rapid cooling. It was concluded that a continuous casting mold with slow cooling was essential.

By the way, the following means of slow cooling of slabs have been known so far, which were created from the viewpoint of making the oscillation marks of slabs shallower and reducing the influence of thermal stress on solidified shells. It was getting worse. (1) As shown in FIG. 8, the thickness and material of the plating layer 2 on the inner wall surface of the mold 1 are partially changed to vary the amount of heat removed. In the figure, numeral 3 represents molten steel, numeral 4 represents a solidified shell, and numeral 5 represents a cooling water spray nozzle.

■ As shown in FIG. 9, means for partially pasting dissimilar metals 6 having different thermal conductivities on the inner wall surface of the mold 1.

■ As shown in Figure 10, there is a heater etc. in the molten steel 3;
+7. Boo method of inserting +o attractive means 7.

(2) As shown in FIG. 11, a wall 8 is attached to the inner wall surface of the mold to change the weld contact surface between the molten steel 3 and the inner wall surface of the mold.

However, the method described in Section ① has the disadvantages of making the plating too thick and causing peeling, and the cost of the plating material is low.However, with the method described above, the cost of pressing dissimilar metals is high, and The method (2) above tends to cause premature destruction of the metal, the heater itself is easily contaminated with molten H and molten steel, and the method (2) above allows the casting powder to flow into the grooves provided on the inner wall of the mold to achieve slow cooling. Each method has its own problems, such as not being able to be used, and all of them are unsuitable for manufacturing slabs with good surface quality as described above at low cost.

Means for Solving the Problems> The present invention is based on the above-described problems and is not influenced by the component composition of fl/4.

A series with no surface flaws and low susceptibility to surface cracks [v5
This was done in an attempt to provide a means to produce cast slabs stably and at low cost, as shown in Part 1.

continuous? ′! A mold with both ends open for immersion.The cooling medium (usually water is used) conduit 9 provided inside the mold wall is located far away from the inner wall surface of the mold at a position near the meniscus of the hot water inside the mold. By arranging them so that they are spaced apart from each other, and closer to the inner wall surface of the mold below the position near the meniscus, the cooling ability of the mold near the meniscus is lowered to achieve slow cooling, and the subsequent To achieve rapid cooling up to the exit side of the mold,
This feature makes it possible to stably produce continuously cast slabs with low susceptibility to surface cracks.

In addition, what is indicated by the symbol 10 in Figure @1 is the 8th
This is a single layer of fA powder, which is omitted in Figures 1 to 11.

Now, in Figure @1, the molten metal M3 poured into the mold through the tundish forms a solidified shell 4 by removing heat from the mold wall, but the cooling medium conduit 9 inside the mold wall forms a molten steel meniscus. The solidified shell r! It is extremely easy to obtain a continuously cast slab that is slowly cooled up to about 13 y and rapidly cooled thereafter.

Of course, by lowering the position of the molten steel meniscus in the mold when the casting speed is slow, and raising the position of the meniscus during high-speed casting, it is possible to freely control the thickness of the solidified shell that is slowly cooled. .

There is no particular restriction on the range of the part near the meniscus of the molten steel in the mold that requires slow cooling; for example, it may be between 10 and 30 degrees below the meniscus, but preferably from the meniscus to at least 501R. It is recommended that the cooling medium passages in the mold wall be designed accordingly.

Next, the present invention will be explained using Examples and in comparison with Comparative Examples.

<Example> First, after melting molten steel having the composition shown in Table 3 in a 250-ton converter, a conventional mold and a mold according to the present invention were installed in a curved continuous casting machine. (curvature radius: 12.5 m), and each cross-sectional dimension is 200 w-
Casting speed of x x 1200- slab: 1.0~1
．． Approximately 150 ml of C3 was produced at 2 m/min.

The molds used at this time were all water-cooled copper molds.
The cooling water conduit is located in a normal mold...at a certain position 40 FI away from the inner wall surface of the mold, and in the neat structure according to the present invention...which has a structure similar to that illustrated in Fig. 1, near the meniscus. [t, = 50% + 3 up to t, = 50m
Then, the part after [tyo=70-] is t,=
The dimensions were 20 West.

, -Q') The results of visual evaluation of the surface flaws of the slab obtained in the above manner are shown in Fig. 12.In Fig. 12,
The "cracking index" is the total wall length x number of surface cracks that occur per meter of cracks.

As is clear from Figure 2, by using the mold of the present invention, even steel with a strong tendency to surface cracking will hardly have any surface flaws, and maintenance-free work will be possible. Recognize.

Fourth, by adding FeS to the molten steel in the mold, the first IO! The state of the solidified shell was observed using a sulfur print, and as shown in FIG. 13, it is clear that the thickness of the solidified shell becomes much more uniform when the mold of the present invention is used. In addition, i! In Fig. 13, "nonuniformity of solidified shell thickness" is the ratio of the area where the thickness varies by 10% to the average value of the solidified shell thickness in the width direction of the slab, and the smaller this value is, the more solidified it is. It can be said that the shell thickness is uniform.

In this example, the occurrence of surface flaws on slabs during continuous casting was investigated, but hot direct rolling and hot charge rolling (
It was confirmed that the cast slab manufactured by the type I2 of the present invention has almost no surface flaws even when

<Overall Effects> As explained above, according to the present invention, even if a steel grade that is difficult to generate cracks during continuous casting or subsequent hot rolling is used, these problems will not occur. This brings about extremely useful effects industrially, such as making it possible to manufacture a desired product without any problems.

[Brief explanation of the drawing]

FIG. 1 is a schematic diagram of the main parts of an example of the continuous casting mold of the present invention. Figure 2 is a graph showing the relationship between the C content and the frequency of occurrence of defects on the slab surface. Figure 3 is a graph showing the relationship between the C content and austenite grain size of Fe-C steel. Figure 4 is a Fe-C system equilibrium phase diagram, Figure 5 is a graph showing the relationship between Cp value of steel and austenite grain size, Figure 6 is a graph showing the relationship between copper cooling rate and austenite grain size, and cooling rate. Figure 7 is a graph showing the relationship between water φ addition temperature and austenite grain size of steel during cooling at various cooling rates; Figures 8 to 11 are: 8, 9, 10Z and 11 are different examples, and FIG. 12 is a conventional mold and a mold according to the present invention. What is the cracking index of slabs obtained using each method? Comparative graph. FIG. 13 is a graph comparing the solidified shell thickness uniformity of slabs obtained using a conventional concave mold and a mold of the present invention. On the 1g13 side, l...mold, 2... plating layer. 3... Molten steel, 4... Solidified shell. 5... Cooling water spray nozzle. 6...Dissimilar metal, 7...Heating child actor. 8...Groove, 9...Cooling medium conduction path. 10... One layer of casting bulk. Applicant Sumitomo Metal Industries Co., Ltd. Agent Tomi 1) Kazuo Bakaka2 figure 2 c4 skin amount + Lf - person) Jie 3 figure C content (! amount y,) Article 4 figure 0 01 α2 0] α4
05cl weight (weight °8) Difference 7 figures Water 1 person 3; 2 meters (°C)

Claims (1)

[Claims] In a mold with both ends open for continuous casting, the cooling medium passage is
1. A slow cooling mold near the meniscus, characterized in that a position near the meniscus of the molten metal in the mold is spaced apart from the inner wall surface of the mold, and thereafter, it is provided close to the inner wall surface of the mold.