JPS6326239A - Mold for continuous casting of steel - Google Patents

Abstract

PURPOSE:To prevent the generation of crack defects at the time of continuous casting and hot rolling by providing cooling medium blowing nozzles and temp. measuring sensors to the lower inside wall surface of a casting mold and disposing cooling medium sucking conduit holes to the inside wall surface upper than said wall surface. CONSTITUTION:The temp. measuring sensors 3 for measuring the surface layer temp. of an ingot and the cooling medium blowing nozzle holes 4 are disposed to the lower inside wall surface 2 of the casting mold 1. The cooling medium sucking conduit holes 6 are installed to the position upper than the lower inside wall surface 2. The temp. in the surface layer part of the ingot transferring at the lower inside wall surface 2 is measured by the sensors 3 according to the above-mentioned mechanism. The surface layer part of the ingot is cooled at a prescribed cooling rate after the existence of the measured temp. within the adequate range of the austenite single phase formation temp. Tgamma or above is confirmed. The generation of the crack flaws during the continuous casting and hot working is, therefore, prevented by the above-mentioned method.

Description

[Detailed Description of the Invention] <Industrial Application Field> The present invention suppresses surface flaws that propagate along austenite grain boundaries, which tend to occur when hot rolling slabs manufactured by continuous casting. , relates to a method for stably producing hot-worked steel materials with good surface properties.

Background technology In recent years, continuous casting processes using vertical or curved continuous casting machines have become indispensable in the production of steel. When trying to manufacture slabs, surface flaws (surface cracks) occur due to thermal stress and bending stress applied to the slab during casting, and these can be removed by direct hot rolling (heating the slab obtained by continuous casting). When attempting to perform real charge rolling (rolling performed immediately without cooling the slab obtained by continuous casting to room temperature), the surface flaws were carried over into the rolling process. There is also the disadvantage that continuous casting slabs are subjected to normal hot working (heat 1??1 forging, which is performed by reheating slabs that have been cooled to room temperature). Even when rolling), the problem of easy occurrence of surface defects is noticeable, and this has been a major obstacle in producing hot-worked steel materials with good surface properties.

By the way, when we investigate the occurrence of the above-mentioned surface flaws, we find that they occur together with cracking of auftinite (γ) grain boundaries. ``Carbides and nitrides that precipitate or segregate to austenite (γ) grain boundaries during solidification and cooling of slabs (
NbC1AIN, etc.), sulfides such as (Mn, Fe)S,
In addition, impurity elements such as P and S cause weakening of grain boundaries.''The degree of purity of surface defects (cracks) is determined by the presence of elements that cause the above-mentioned precipitates and segregation. It has come to be known that the amount 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 the ingredients were not very clear, and therefore, adjusting the chemical ingredients alone was not enough to achieve a satisfactory effect.

On the other hand, as shown in Figure 3, the frequency of occurrence of surface defects on slabs is largely dependent on the C content of slabs, but the cause is still unknown and no countermeasures have been taken. In some cases, they were unable to find it, and in the end, operations were even carried out avoiding such C content regions.

However, as shown in Figure 3, the region where the frequency of surface flaws increases rapidly is not necessarily constant, and varies depending on the steel type, and especially in the case of low-alloy steel, it cannot be estimated from the C content. Surface flaws often occur at an extremely high frequency in unexpected component composition regions that are unlikely to occur.
This often led to extremely inconvenient operational results.

Therefore, the surface flaw prevention measures commonly implemented in the past are as follows:
In order to make the oscillation mark shallower and to reduce the thermal stress acting on the solidified shell, the cooling rate of the slab was reduced, 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 casting of steel slabs manufactured by continuous casting,
In order to find an easy-to-implement means to reliably prevent the occurrence of surface flaws that tend to occur when hot working continuously cast slabs, it is necessary to use specific C-containing it 9 as shown in Figure 3.
Based on the idea that it is essential to elucidate the cause of the rapid increase in the frequency of surface flaws occurring near area i, we have conducted various experiments and research.
The following findings were obtained. That is, (a) Grain boundary cracking in continuously cast slabs, as previously said, affects the content of elements such as carbides, nitrides, sulfides, or impurities that precipitate or segregate at grain boundaries. In addition to this, the grain size of austenite (y) grains, which influences the precipitation and segregation density, is greatly affected, and the coarsening of austenite (γ) grains during solidification and cooling can cause intergranular cracking in slabs. To significantly facilitate.

(bl) The degree of austenite (γ) grain coarsening in a carbon steel slab during solidification and cooling varies greatly depending on changes in its C content, and it also changes while maintaining a simple proportional relationship with the C content. Rather, as shown in Fig. 4, the above-mentioned behavior shows a behavior that suddenly becomes more pronounced in the C-containing ISM region where surface flaws are likely to occur. During cooling, set the cooling rate to 5℃/sec.
This is a curve showing the relationship between C content and austenite grain size when

(C) These results and experimental confirmation that ``the coarsening of austenite (γ) grains during solidification and cooling occurs rapidly after becoming austenite single phase, and the higher the temperature, the more remarkable this tendency is.'' In view of the above, coagulation and
Under the same cooling conditions, a carbon steel slab being cooled will inevitably have a composition with the highest austenite single-phase temperature, which is clear from the Fe-C system equilibrium phase diagram shown in Figure 5. ”, that is, “those with a peritectic point composition (0.18 wt. (This shows the austenite grain coarsening behavior shown in the figure.) Therefore, it can be concluded that the hot cracking susceptibility increases rapidly near this area.

(dl) By the way, the austenite (γ) grain size coarsening behavior shown in FIG. 4 does not necessarily match the frequency trend of occurrence of defects on the slab surface shown in FIG. 3.

However, this is due to the difference that Figure 4 shows the experimental results for pure Fe-C system, while Figure 3 shows the data for practical steel. This is simply because the peritectic point is shifted due to the influence of the contained elements (alloy elements, etc.).

(e) Depending on the type of elements other than C contained in the steel, the hot cracking sensitivity of the steel may become more sensitive to -N, which may lead to an increase in flaws on the surface of the slab.

(f) Therefore, when evaluating the hot cracking susceptibility of a slab, it is necessary to use not only the C content but also the C equivalent (Cp), which includes the influence of alloying elements, as an index.

fg) From a phase diagram study, elements that are thought to affect the peritectic point of steel include C5Mn5 Ni, Cu
and N, and the C equivalent (Cp) is expressed by the following formula (hereinafter, % representing the component ratio is expressed as weight %). That is, (h) the above formula obtained by phase diagram examination agrees well with reality, and based on this, the hot cracking susceptibility of the slab can be evaluated very accurately.

Figure 6 shows the results of an experiment carried out by the inventors to confirm this, in which small pieces collected from a total of 50 types of steel within the composition shown in Table 1 were made into aluminum pots. This is a graph in which the austenite grain size was measured after remelting at a cooling rate of 5° C./sec and organized by the Cp value calculated by the above formula.

As is clear from Fig. 6, austenite l-(
r) It can be seen that the particle size is well organized by Cp value, and takes the maximum value at Cp value of 0.18.

(1) As mentioned above, the austenite (γ) grain size is C
The reason why it depends on the p value is the austenite single phase temperature (
This is because TT) changes with the same tendency as the Cp value changes and is determined by this value. For example, when the Cp value is a specific value (0, 18, peritectic point), austenite (
γ) The grain size reaches its maximum value because at the Cp value, the austenite single-phase temperature (TT) is the highest and the austenite (γ) grain growth period during the cooling process is also the longest, creating an opportunity for coarsening. This is because they are given enough.

Although this can be inferred from FIG. 5 shown above, please refer to FIG. 7 shown below.

Figure 7 shows that small pieces collected from a total of 50 kinds of ropes with the component composition shown in Table 1 were remelted in an aluminum crucible, and then the cooling rate was 0.1°C/sec and 2.0°C/sec. se<
This is a graph organized by the Cp value calculated by the above formula by measuring the austenite single phase temperature (TT) after cooling at It can be seen that in the range of ~2.0° C./sec, the austenite single phase temperature (TT) is well-organized by the Cp value and expressed as the following equation, and takes the maximum value when the Cp value is 0.18.

Tγ=ACp+B Furthermore, from Figure 8, which investigated the "influence of cooling rate on austenite single phase temperature (Tr)" for steels with the compositions shown in Table 2, it is clear that "the cooling rate is 2.0℃/ s
Austenite single phase temperature (TT) even if it is above ec
It was confirmed that "there is almost no change in the austenite single phase temperature (T
It is clear that 'γ) is calculated by the above formula as a value that substantially depends only on the Cp value.

0) On the other hand, when steel of the same composition is solidified and cooled, the austenite (γ) grain size of the slab is determined by the Cp value (i.e., “T
In addition to being influenced by T"), it is also greatly influenced by the cooling rate in the high A range, and in particular, it is almost determined by the cooling rate in the temperature range from the austenite single phase temperature (Tr) to about 1000°C. To be left behind.

Figure 9 shows the water quenching temperature and austenite of carbon steels with various C contents that were melted and then cooled at a cooling rate of 70.28°C/sec, and then water quenched midway through to fix the structure. (γ) The relationship with particle size is
This is a rough graph. Figure 10 is an example of the structure after TT@, and 0.15%C-1,48%Mn@
．． Figure 10(a) shows the microscopic structure when cooled at 1°C/sec at 1470°C (TT+5°C).
) and water quenched from ), and Figure 10(b) is 14
The graphs show the results of water quenching from 60°C (T r -5°C), and it can be seen that the γ grains begin to coarsen rapidly immediately after Tγ. Furthermore, Figures 9 and 10
The figure shows that quenching has a large effect on austenite (T) grain growth only in extremely high temperature ranges, especially in the temperature range from the austenite single phase temperature (TT) to 1000°C, and in lower temperature ranges. It is clear that the effect of rapid cooling becomes less pronounced.

In addition, please refer to FIG. 11. Figure 11 shows
For a total of 30 types of steel with the composition shown in Table 1, the cooling rate after the austenite single-phase temperature (TT) was varied, and after reaching 1000°C, the structure was rapidly cooled to fix the austenite (γ ) is a graph illustrating particle diameters organized by the cooling rate. From this Figure 11, the peritectic composition (Cp = 0.18) where the austenite single phase temperature (Tr) is the highest and the austenite grains tend to become coarser is found.
Even if the steel is austenite single phase temperature (T
It can be seen that if the cooling rate after T) is increased, coarsening of austenite (γ) grains can be prevented regardless of the cooling rate in a temperature range higher than the austenite single-phase temperature (TT).

(k) By the way, the tendency of surface cracking of a slab during continuous casting, or the tendency of surface cracking of a slab during hot direct rolling or hot charge rolling that is performed subsequent to continuous casting, is determined by the surface cracking tendency of a slab during continuous casting. (about 3 mm, at most 10 dragons)
Determined by the cracking sensitivity of

(1) Therefore, even for slabs with a peritectic composition, if the cooling rate of the surface layer after the austenite single phase temperature (Tr) is increased, the coarsening of austenite grains in the surface layer is suppressed and A fine grain structure with a large grain boundary area can be obtained, which lowers the density of precipitates and segregation that gather at grain boundaries, reducing cracking susceptibility and increasing toughness. The fear of cracking is eliminated.

(m) For this reason, surface flaws (cracks) may occur during casting of slabs manufactured by continuous casting,
It is possible to easily and reliably predict steel types with a strong tendency to generate surface flaws (cracks) when continuously cast slabs are hot-rolled using the above formula (formula for calculating Cp value), and also to predict these steel types. Also, while the surface layer of the slab is in a specific high temperature range (actually, it is sufficient to aim for "Tr" or higher), rapid cooling treatment (cooling rate of the surface layer: 10℃/sec or higher) is performed. ), it is possible to stably suppress the occurrence of surface defects.

Therefore, based on the findings shown in (al~+m), the present inventors attempted to manufacture slabs with low surface crack susceptibility by increasing the cooling rate in the high temperature range of molten steel injected into the mold. However, in actual continuous steel casting operations, solidification progresses near the molten steel meniscus with the solidified shell and mold wall in close contact with each other via the molten powder, but below that point, the molten steel Due to the solidification shrinkage of the slab and the shrinkage caused by the temperature drop of the slab, the slab separates from the mold wall, creating an air gap that impairs the heat removal effect of the mold. In conventional molds (700 to 900 fl or more in length), there is a significant cooling delay that causes the austenite grains to coarsen to the extent that subsequent austenite grain boundary fracture occurs and surface defects are more likely to occur. We have run into a problem that cannot be avoided.

Therefore, by shortening the length of the mold, the solidification of the molten steel in the mold is limited to the formation of an extremely thin solidified layer on the surface of the slab, and a cooling medium is sprayed onto the slab that is pulled out from the bottom of the mold early. Attempts were also made to increase the cooling rate in the high temperature range, but in this case there was an extremely high risk of causing breakout of the slab, and this was not a desirable measure in practical towing operations.

Means for Solving the Problems> Based on the problems explained above, the present invention provides a method for stably producing continuously cast slabs with low surface crack susceptibility, without being affected by the chemical composition of steel. This was done in an attempt to provide a means for manufacturing with high efficiency.As shown in Fig. 1, a mold with both ends open for continuous casting of steel was equipped with a thermometer on the lower inner wall surface 2 of the mold 1 to measure the temperature of the surface layer of the slab. By arranging a sensor 3 and a nozzle hole 4 for blowing a cooling medium, and providing a passage hole 6 for sucking a cooling medium above the lower inner wall surface 2, the temperature of the surface layer of the slab that has reached the inner wall surface of the lower part 4 can be adjusted. The temperature sensor 3 detects whether or not the temperature is within the appropriate range mentioned above (the range in which a rapid cooling effect higher than "Tr" can be expected), and the surface layer of the slab in the appropriate temperature range is heated at 10°C/sec. The amount of cooling medium blown from the cooling medium injection nozzle hole 4 can be adjusted through the temperature measurement sensor 3 so that cooling can be performed at the above cooling rate, and
The coolant blown into the upper inner wall surface 5 of the mold 1 and the molten steel 7
By creating a gap between the coolant and the meniscus 8 and blowing upward from there, destabilizing the cooling in the vicinity of the meniscus 8, can be reliably prevented by smooth discharge from the coolant suction passage hole 6. Alternatively, as shown in FIG. 2, the lower inner wall surface 2 of the mold 1, which is provided with a temperature sensor 3 for measuring the temperature of the surface layer of the cast slab and a nozzle hole 4 for blowing a cooling medium, is removed from the upper surface. By setting it back from the inner wall surface 5, the cooling effect and uniformity of the cooling medium is further increased (it is reliably prevented that air or the cooling medium remains partially between the slab surface and the lower inner wall surface). The structure is designed to facilitate the suction of the cooling medium from the cooling medium suction passage hole 6 and increase the suction capacity, thereby achieving high cooling of the cast molten steel at high temperatures. Not only can the speed be easily ensured, but the danger of slab breakout can be reliably avoided, and continuous casting slabs with no surface flaws and low susceptibility to surface cracking can be stably produced without being affected by the steel type. It is characterized by the fact that it can be mass-produced.

In FIGS. 1 and 2, reference numeral 9 indicates a solidified shell, reference numeral 10 indicates a cooling water passage, and reference numeral 11 indicates a cooling water spray nozzle.

Now, in Fig. 1, when the molten @7 is poured into the mold 1, an extremely thin solidified shell is formed by the heat removal action of the upper inner wall surface 5 of the mold. For example, about 500 m+s (length below the meniscus: 3
If this is done, the part of the slab where the solidified shell has just been formed will be immediately lowered to the position of the lower inner wall surface 2, and the cooling medium injection nozzle hole 4 will be The cooling medium (e.g., e-gas, etc.) that is blown in and flows back into the cooling medium suction passage hole 6 efficiently cools the body (as mentioned earlier, the lower inner wall surface 2 recedes as shown in FIG. 2). (The cooling efficiency is further improved in the case of molds with open ends.) Unlike conventional molds with both ends open, the solidified shell surface separates from the inner wall surface of the mold due to shrinkage due to solidification and cooling, forming an air layer between both surfaces. In addition, the temperature sensor 3 allows the temperature of the surface layer of the slab to be rapidly cooled to be accurately confirmed, and the cooling rate can be easily adjusted. Since a high cooling rate is stably maintained in the high temperature range of the surface layer of the slab, it is possible to reliably prevent coarsening of austenite grains in the surface layer, resulting in no surface flaws.
And slabs with low surface crack susceptibility can be efficiently produced.

In addition, with such a mold, the slab remains in the mold until a solidified shell of the desired thickness is formed.

Since it can be safely stored, there is no risk of breakout.

As the cooling medium injected from the cooling medium injection nozzle hole 4, in addition to cooling gas such as He gas, a mixed gas of these and water can also be employed.

Next, the present invention will be explained using examples and comparing with comparative examples.

<Example> Example 1 The water-cooled copper according to the present invention as shown in FIG. The mold (the shape and dimensions are shown in Table 4. The mold shown in Figure 2 is the same as that in Figure 1 except that the lower inner wall surface is set back two inches from the upper inner wall surface) and the conventional mold. Practical curved continuous casting machine (bending radius: 12.5
m), a slab with cross-sectional dimensions of 250 m x 1200 mm is cast at a casting speed of about 150 m at 1.2 m/min.
Manufactured.

In addition, the mold according to the present invention used at this time uses a temperature sensor to detect when the temperature of the surface layer of the slab has reached Tr calculated from "Cp value calculated using the Cp formula from the added amount of C and component elements". Table 4 (Note) The position in the casting direction is the distance from the upper end of the mold.

The amount of coolant ejected from the nozzle hole is 0.2A/mi only for the nozzle hole directly above the temperature sensor that senses TT.
The cooling rate was set to increase from n to Q, 4 f/min, thereby making the cooling rate of the surface layer of the slab whose temperature was Tr to 10"C/sec or more.

The surface defects of the slab thus obtained were visually evaluated, and the results are shown in FIG. In FIG. 12, the "crack index" is the total number of surface cracks occurring per 1 d of slab.

As is clear from FIG. 12, when the mold of the present invention is used, almost no surface flaws occur even in steel that is prone to surface cracking, and maintenance can be avoided.

Example 2 Steel B, which is shown in Table 3 and has a composition that does not easily cause surface defects in continuously cast slabs but is likely to crack during subsequent hot rolling, was melted and water-cooled copper mold (lower inner wall surface 2 is 20+
The cross-sectional dimension was 25011 under the same conditions as in Example 1 using
A slab with a diameter of 1200 mm was manufactured.

When the surface properties of the slabs that had passed through the slab straightening point at a surface temperature of 950°C were observed, no surface flaws were observed on both slabs when the mold according to the present invention was used and when the conventional mold was used. However, when the slab that had passed through the slab straightening point was cut and rolled in 5 baths at a temperature of about 900°C to a thickness of 125 fl, it was found that using the mold according to the present invention No surface flaws were observed at all, whereas many cracks were observed in the molds made using conventional molds.

Overall Effects> As explained above, according to the present invention, even if a steel type that is prone to cracking is used during continuous casting or during subsequent hot rolling, these problems will not occur. This brings about extremely useful effects industrially, such as making it possible to manufacture desired products.

[Brief explanation of the drawing]

Fig. 1 and Fig. 2 are both schematic diagrams showing the situation of continuous casting using the mold of the present invention, and Fig. 1 and Fig. 2 respectively show the progress of continuous casting using molds of different types. Fig. 3 is a graph showing the relationship between C content and the frequency of occurrence of defects on the slab surface; Fig. 4 is a graph showing the relationship between C content and austenite grain size of Fe-C steel; Figure 5 is a Fe-C system equilibrium phase diagram, Figure 6 is a graph showing the relationship between Cp value of steel and austenite grain size, and Figure 7 is a graph showing the relationship between Cp value of zeta steel and austenite single phase temperature (T
Figure 8 is a graph showing the relationship between the cooling rate of tears and the austenite single phase temperature (T).
Tr) is a graph showing the relationship between water quenching temperature and austenite grain system when carbon steels with various C contents are water quenched at various temperatures during cooling to fix the structure. The graph showing the relationship, Figure 10, is a micrograph comparing the structural changes before and after the austenite single phase temperature (Ty).
Figure 0 (a) shows the state after water quenching from 1470°C (T r + 5°C), and Figure 10 (b) shows the state after water quenching from 1460'C.
Figure 11 is a graph showing the relationship between the cooling rate and austenite grain system after the austenite single-phase temperature of the fence, and Figure 12 is a graph showing the relationship between the austenite grain system and the austenite grain system. It is a graph comparing the cracking index of the slab when using the mold according to the present invention. In the drawing, ■... Mold, 2... Lower inner wall surface, 3...
... Temperature sensor, 4... Nozzle hole for cooling medium injection, 5... Upper inner wall surface, 6... Conduction hole for cooling medium suction, 7...? Yonggang, 8...meniscus,
9... Solidified shell, 10... Cooling water passage, -
11... Cooling water spray nozzle.

Claims (2)

[Claims] (1) In a mold with both ends open for continuous casting, a temperature sensor and a nozzle hole for blowing a coolant are arranged on the inner wall surface of the lower part of the mold, and a conduction hole for sucking the coolant is provided above the inner wall surface of the lower part of the mold. A mold for continuous steel casting, which is characterized by: (2) In a mold with both ends open for continuous casting, the inner wall surface of the lower part of the mold is retracted, and a temperature sensor and a cooling medium injection nozzle hole are arranged on the retracted inner wall surface, and above the retracted inner wall surface. A mold for continuous casting of steel, characterized in that it is provided with a through hole for sucking a cooling medium.