JPS6363559A - Prevention for hot cracking in continuously cast slab - Google Patents

Abstract

PURPOSE:To restrain surface defect and to stably obtain hog working steel having good surface characteristic by cooling the surface layer part in cast slab at the specific cooling speed after forming the surface layer part in the cast slab from molten steel in a mold having both end opening. CONSTITUTION:At the time of continuously casting the steel, the both end opening type mold providing plural sensors 3 measuring temps. of the surface layer part of cast slab and plural injection nozzle 4 for coolant at the inner wall part is used. After forming the surface layer part in the cast slab from the molten steel, it is cooled at >=10 deg.C/sec cooling speed from the austenitizing single phase temp. or more. In this way, the surface defect is eliminated and the continuously cast slab having low sensibility for surface cracking without effect to any steel kind is stably mass-produced.

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 cast iron produced 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 using straight or curved continuous casting machines has become indispensable in the production of steel. When attempting to manufacture slabs such as steel slabs, surface flaws (
Surface cracks) occur, which can be removed by hot direct rolling (rolling performed immediately without heating the slab obtained by continuous casting) or hot charge rolling (rolling the slab obtained by continuous casting to be cooled to room temperature). If the rolling process is carried out without reheating (rolling without reheating), there is the inconvenience that the surface flaws will be carried over into the rolling process and the cracks will be further exacerbated. Even when processing (hot forging or hot rolling performed by reheating a slab once cooled to room temperature), there is a noticeable problem that surface h↑E is easily generated.
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 austenite (γ) grain boundaries. ``Carbides and nitrides that precipitate or segregate to austenite (T) grain boundaries during solidification and cooling of slabs (
Sulfides such as NbC1^nN), (Mn, Fe)S, and impurity elements such as P and S cause weakening of grain boundaries.
It has become known that the frequency of occurrence of surface flaws (cracks) is greatly influenced by the content of the elements that cause the above-mentioned precipitates and segregation.

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, it is a fact that the frequency of occurrence of such surface defects on slabs is largely dependent on the C content of slabs, as shown in Figure 1, but the cause is still unknown and no countermeasures have been taken. In some cases, they were not found, and in the end, operations were even carried out avoiding such C-containing areas.

However, as shown in Figure 1, the region where the frequency of surface flaw occurrence increases rapidly is not necessarily constant and varies depending on the steel type, and especially in the case of low alloy steel, it is difficult to infer 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 situations that had extremely inconvenient operational consequences.

Therefore, the surface flaw prevention measures commonly implemented in the past are as follows:
In order to make the oscillation marks 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 defects that tend to occur when continuously cast slabs are hot-heated, it is necessary to Based on the belief that it is essential to elucidate the cause of the rapid increase in the frequency of surface defects near the surface area, we have conducted various experiments and research.
The following findings were obtained. That is, (a) Grain boundary cracking in continuously cast slabs is caused by the effect on the content of elements such as carbides, nitrides, sulfides, or impurities that precipitate or segregate at grain boundaries, as has been said in the past. In addition to this, the grain size of austenite (γ) grains, which influences the precipitation and segregation density, is greatly affected, and the coarsening of austenite (γ) grains during solidification and cooling can lead to intergranular cracking in slabs. To significantly facilitate.

(bl) The degree of austenite (T) 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 moderately proportional relationship with the C content. Rather, as shown in Fig. 2, the behavior becomes suddenly more pronounced in the C content region where surface flaws are likely to occur (in addition, Fig. 2 shows the behavior of F
This is a curve showing the relationship between C content and austenite grain size when the cooling rate is 5° C./sec during solidification and cooling of e-C steel.

fc) These results and the 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 this, 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 3. ”, that is, “those with a peritectic point composition (0.18% C by weight for Fe-C systems)” exhibit the coarsest austenite (T) grains (by the way, the broken line in (This shows the austenite grain coarsening behavior shown in Figure 2), and it can therefore be concluded that the hot cracking susceptibility also increases rapidly near this area.

fd) By the way, the austenite (T) grain size coarsening behavior shown in FIG. 2 does not necessarily match the tendency of frequency of occurrence of defects on the slab surface shown in FIG. 1.

However, this is due to the difference that Figure 2 shows the experimental results for pure Fe-C system, while Figure 1 shows the data for practical steel 1. This is simply because the peritectic point is shifted due to the influence of the contained elements (alloy elements, etc.).

(e) Also, depending on the type of elements other than C contained in the steel, the hot cracking sensitivity of the steel may become even more sensitive, leading to an increase in flaws on the surface of the slab.

(f) Therefore, when estimating the hot cracking susceptibility of a slab to i+F, 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.

(gl From the phase diagram study, elements that are thought to affect the peritectic point of steel include C, Mn, Ni, and Co.
and N, and the C equivalent (Cp) will be organized in the next generation (in the following, the component ratio will not be expressed and % will be the vehicle volume %). 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 4 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 this Figure 4, austenite (γ
) 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) Austenite as described above) (T) Grain size is Cp
(The reason it depends on a is the austenite single phase temperature (
This is because Tγ) changes with the same tendency as the Cp value changes, and is determined by this value. For example, when Cp4Fi is a specific 4Ft (0, 18, i.e., peritectic point), austenite (γ) The reason why the grain size reaches its maximum value is that when the Cp value is reached, the austenite single phase temperature (TT) is the highest and the austenite (γ) grain growth period during the cooling process is also the longest. This is because they are given enough.

This can also be seen from Figure 3 shown above. Please refer to FIG. 5 shown below.

Figure 5 shows that small pieces collected from a total of 70 types of steel with the composition shown in Table 1 were remelted in an aluminum crucible, and then cooled at a cooling rate of 20.1°C/sec, 2.0°C. C/s
cp calculated by the above formula by cooling at ec and 10°C/sec and measuring the austenite single phase temperature (TT).
This is a graph organized by values, but as is clear from this Figure 5, when the cooling rate is less than 0.1'C/sec, the austenite 1il-phase ratio temperature (TT) is well organized by Cp value, and is expressed by the following formula: It is expressed as follows, and the Cp value is 0.1
It can be seen that the maximum value is reached at 8.

T7=AC+10B (Jl On the other hand, the austenite (γ) grain size of the slab when steel with the same composition is solidified and cooled is the Cp value (i.e. T
γ") is greatly influenced by the cooling rate in the high temperature range, especially the cooling rate in the temperature range from the austenite single phase temperature ('"γ) to approximately 1000℃. −.

It has almost been decided.

Figure 6 shows the water quenching temperature and temperature of carbon steels with various C contents that were melted and then cooled at a cooling rate of 0.28°C/sec, and then water quenched midway through to fix the structure. It is a graph plotting the relationship between austenite I(r) and particle size. Moreover, FIG. 7 is an example of the structure before and after the Tr plane, and shows 0.15% C-1, 48% Mn steel.
Fig. 7(a) shows the microscopic structure when cooled at 1°C/sec at 1470'C (Tγ→-5°C
) and water quenched from ), and Fig. 7(b) is 146
The graphs show the results of water quenching from 0°C (TT-5°C), and it can be seen from this figure that one grain begins to rapidly coarsen immediately after the Tr surface. Furthermore, from Figures 6 and 7, it is clear that quenching has a large effect on austenite (r) grain growth only in extremely high temperature ranges, especially in the temperature range from the austenite single phase temperature (TT) to 1000°C. , it is clear that the effect of rapid cooling becomes less pronounced in the lower temperature range.

In addition, please refer to FIG. Figure 8 shows a total of 30 types of steel within the composition shown in Table 1, with various cooling rates after the austenite single phase temperature (Tr).
2 is a graph showing the austenite (r) grain size of a sample whose structure is fixed by rapid cooling after reaching 000° C., organized by the cooling rate. From this Figure 8, even if the steel has a peritectic composition (ccp = 0.11+) where the austenite single phase temperature (TT) is the highest and the austenite grains tend to coarsen, the austenite single phase temperature (TT) It can be seen that if the subsequent cooling rate is increased, coarsening of the austenite 1-(r) grains can be prevented.

(kl) By the way, the surface cracking tendency 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 3 vans, at most 10
It is determined by the cracking susceptibility (mW).

(11 Therefore, even for slabs with a peritectic composition, if the cooling rate of the surface layer after the austenite single phase temperature (TT) is increased, the coarsening of austenite grains in the surface layer is suppressed, and the 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.

(ml) For these reasons, steel types with a strong tendency to generate surface TM (cracks) during the casting process of slabs produced by continuous casting, and surface flaws (cracks) when continuously cast slabs are hot rolled. It is possible to easily and reliably predict using the above formula (formula for calculating the Cp value), and also for these steel types, it is possible to calculate the cooling rate of the surface layer while the surface layer of the slab is at a temperature of Tr or higher. : It is possible to stably suppress the occurrence of surface flaws by rapid cooling treatment at 10° C./sec or more.

(nl) By the way, in a normal mold (length: 700 to 900 mm or more) used in a vertical or curved continuous casting machine, the solidified shell and mold wall melt only near the molten steel meniscus. Although solidification progresses in close contact with the powder and a sufficient cooling rate is ensured, below this point the slab separates from the mold wall due to the solidification shrinkage of the molten steel and the shrinkage of the slab due to the temperature drop. The problem is that it is impossible to secure a high cooling rate at an early stage as described in 1. A method of increasing the cooling rate in high temperature ranges by using a short mold to form only an extremely thin solidified layer on the surface of the slab inside the mold, and by spraying a cooling medium onto the slab that is pulled out early from the bottom of the mold. However, as a mold with both ends open for continuous casting of steel, a mold was installed on the upper inner wall surface 2 of the mold 1 for measuring the temperature of the surface layer of the slab, as shown in Figure 9. By providing a temperature measuring sensor 3 and a cooling medium injection nozzle hole 4, and providing a cooling medium suction passage hole 6 above the upper inner wall surface 2, the temperature of the surface layer of the slab reaching the lower inner wall surface is reduced. 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 of 1-1 or less can be expected), and the surface layer of the slab is detected in the appropriate temperature range. The cooling medium blowing from the cooling medium blowing nozzle hole 4 can be adjusted through the temperature measurement sensor 3 so that the mold can be cooled at a cooling rate of 10° C./sec or more, and the cooling medium blown into the mold l
A gap is created between the upper inner wall surface 5 of the molten steel 7 and the meniscus 8 of the molten steel 7, and the l-force blows through from there, destabilizing the cooling in the vicinity of the meniscus 8. If a device that can be reliably prevented by discharge is used (in Fig. 9, numeral 9 indicates the solidified shell, numeral 10 indicates the cooling water passage, and numeral 11 indicates the cooling water spray nozzle), the slab break. It is now possible to easily maintain a high cooling rate of the cast molten steel at high temperatures while reliably avoiding the risk of outflow, and the surface layer of the slab can be cooled at a rate of 10°C/sec or more from a temperature of T1 or higher. To be able to stably achieve the conditions of rapid cooling.

That is, in FIG. 9, when molten steel 7 is poured into the mold 1, an extremely thin solidified shell is first formed by the heat removal action of the upper inner wall surface 5 of the mold, but the length of this upper inner wall surface 5 is For example, about 500 fl (length below the meniscus: 300 m)
The part of the slab where the solidified shell has just been formed is immediately lowered to the lower inner wall surface 2, and is cooled by being blown from the cooling medium injection nozzle hole 4. Since the cooling medium (for example, He gas, etc.) flowing back into the medium suction passage hole 6 efficiently cools the mold, it is possible to
“The solidified shell surface separates from the mold inner wall surface due to contraction due to solidification and cooling, forming an air layer between both surfaces, which causes cooling delays.
Furthermore, the temperature sensor 3 allows the surface temperature of the slab to be confirmed as a positive value at the start of rapid cooling, and the cooling rate can be easily adjusted, thus stably ensuring a high cooling rate in the high temperature range of the slab surface. It will be done. Furthermore, with such a mold, the slab can be kept in the mold until a solidified shell of the desired thickness is formed, so there is no risk of breakage. Note that the cooling medium blown in from the cooling medium injection nozzle hole 4 may be a cooling gas such as 11(.gas) or a mixture gas of these and water.

This invention was made based on the above knowledge, and after forming the surface layer of a slab from molten steel in a mold with both ends open during continuous casting of steel, the surface layer of the slab is heated to a temperature higher than the austenite single-phase temperature. By cooling at a cooling rate of 10°C/sec or higher, continuous casting slabs with no surface flaws and low susceptibility to surface cracking can be stably mass-produced without being affected by the steel type. The present invention is characterized in that it is also possible to suppress hot cracking during hot direct rolling or real charge rolling of a piece, thereby preventing the occurrence of surface flaws.

Here, the "slab surface layer" refers to the area from the surface of the slab to about 311, at most about Ion.

It goes without saying that the method of this invention can be applied to any type of steel, but in terms of the effect obtained, C: 0
．． 7% or less, Mn: 3% or less, Ni: 3% or less, Cu:
2% or less and N: 0.5% or less, and further Cr if necessary.
: 3% or less, Nb: 0.5% or less, ■2 0.5% or less, Ti: 0.5% or less, hej! Low alloy steel that also contains one or more of the following: Q, 1% or less and Si: 3% or less, and whose CaO value calculated by the formula is less than 0.6 (contains normal unavoidable impurities) is not affected in any way)
This is especially noticeable when targeting

In addition, in this invention, the reason why the cooling conditions for the surface layer of the slab immediately after it is formed from molten steel is specifically limited to [cooling at a cooling rate of 10 ° C / sac or more from the austenite single phase temperature or higher] is as follows. Explain.

In other words, as is clear from Figures 6 and 7 mentioned earlier,
The temperature of the surface layer of the slab formed after solidification in the mold is T.
When the temperature is below γ, the tendency of γ grains to become coarse becomes remarkable, but at this time, if the surface layer of the slab is cooled at a specific high cooling rate, the coarsening of austerite (γ) grains is suppressed,
The occurrence of surface defects on the slab can be sufficiently suppressed. However, if the cooling rate at this time is less than 10° C./sec, the effect of suppressing the coarsening of austenite 1-D grains will be insufficient, making it impossible to stably obtain slabs with low cracking susceptibility. Therefore, the cooling conditions for the surface layer of the slab immediately after it has been solidified and formed from molten steel are determined as [cooling at a cooling rate of 10° C./sec or higher from the austenite single-phase temperature or higher]. As explained earlier, Figure 8 shows the relationship between the cooling rate and the austenite (
T) This is a diagram showing the relationship with grains, and from this FIG. 8, it is clear that if the cooling rate is 10°C/sec or more, a sufficient effect of suppressing austenite grain coarsening can be obtained; It is clear that when the cooling rate is less than 10° C./sec, the austenite grains in the surface layer of the slab tend to become coarser, leading to frequent occurrence of defects on the slab surface accompanied by austenite grain boundary fracture.

Furthermore, as mentioned earlier, with the molds of conventional vertical or curved continuous casting machines, it is impossible to achieve a cooling rate of 10°C/sec or higher for the surface layer of the slab in the high temperature range.
As shown in Figure 9, a mold with open ends is equipped with a plurality of sensors on the inner wall that can measure the temperature of the surface layer of the slab and a plurality of cooling medium jetting nozzles (placed between the sensors, allowing adjustment of the cooling medium jetting). Alternatively, shorten the length of the mold with both ends open, and adjust the drawing speed of the slab so that the surface temperature of the slab at the outlet is below Tr, and then pull the slab directly under the mold. By adopting a method of spraying a cooling medium on the cooling medium, etc., it is possible to sufficiently secure the cooling rate.

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

<Examples> Example 1 Steel A, which has a composition that is prone to cracking during hot rolling, as shown in Table 2, was melted and cast using a practical curved continuous casting machine (curving radius: 12.5 m). Cross-sectional dimension is 250
A slab of m×1200 μl was manufactured under the following two conditions,
The occurrence of surface defects was observed.

■ Conditions according to the method of the present invention (example of the present invention) As shown in Fig. 9, the inner wall is equipped with a plurality of sensors capable of measuring the temperature of the surface layer of the slab and a plurality of coolant jetting nozzles, and both ends are open. Using a mold (the shape and dimensions are shown in Table 3), increase the amount of cooling medium ejected from the nozzle located above the sensor that detects that the temperature of the surface layer of the slab is close to Tr. By this, the cooling rate of the surface layer of the slab is adjusted to 10'C/sec or more.

■Conventional method conditions (comparative example) A conventional mold with both ends open is used, and the slab is cooled by regular water spray after it leaves the mold.

The cooling rate of the slab surface at this time was confirmed by the arm spacing of the δ-dendrites, and the results are shown in FIG.

Visual evaluation of surface defects on the slab was conducted on the slab after passing through the slab straightening point at a surface temperature of 2,850°C, and no surface defects were observed in the slab obtained by the method of the present invention. In contrast, with the comparative method, multiple surface defects were observed.

Example 2 Steel No. 13, which is shown in Table 2 and whose composition does not easily cause surface flaws in continuously cast slabs but is likely to crack during subsequent hot rolling, was melted and Continuous casting was carried out using the method of the present invention and the comparative method under the same conditions as described above to produce a slab with a cross-sectional dimension of 250 m x 1200-1.

The cooling rate of the slab surface at this time was confirmed by the arm spacing of the δ-dendrites of the sample taken from the obtained slab, and the results are as shown in Figure 11. Also, the surface temperature: 950℃ The surface properties of the slabs that had passed through the slab straightening point were observed, but no surface flaws were observed on any of the slabs.

Subsequently, the slab that had passed through the slab straightening point was cut and rolled in 5 passes at a temperature of about 900°C to a thickness of 125 mm. As a result, the slab obtained by the method of the present invention had no defects. While no cracks were observed, it was observed that cracks occurred frequently in the slab prepared by the comparative method.

As explained above, according to the present invention, even if a steel type that is likely to generate cracks during continuous casting or during subsequent hot direct rolling or hot charge rolling is used, desired results can be achieved without causing these troubles. This brings about extremely useful effects industrially, such as making it possible to manufacture products.

[Brief explanation of drawings]

Fig. 1 is a graph showing the relationship between C content and slab surface flaw initiation W degree; Fig. 2 is a graph showing the relationship between C content and austenite grain size of Fe-C steel; Figure 3 is a Fe-C system equilibrium phase diagram, Figure 4 is a graph showing the relationship between Cp value of steel and austenite grain size, and Figure 5 is a graph showing the relationship between Cp value of steel and austenite l phase temperature (Tr
), Figure 6 shows the relationship between water quenching temperature and austenite grain size when carbon steels with various C contents are water quenched at various temperatures during cooling to fix the structure. Figure 7 is a micrograph comparing the structural changes before and after the austenite single-phase temperature ('l'r), and Figure 7 (a) is a graph showing the change in structure after water quenching from 1470°C (Tr + 5°C). Figure 7 (bl is 1460℃ (Tγ-5
8 is a graph showing the relationship between the cooling rate and austenite grain size after the austenite single-phase temperature of the steel, and FIG. 9 is a graph showing the relationship between the austenite grain size and the cooling rate after the austenite single-phase temperature of steel. Figures 10 and 11, which are schematic diagrams showing continuous casting using a mold suitable for casting, show the distance from the slab surface and the cooling rate of the slabs obtained in Examples 1 and 2, respectively. This is a graph showing the relationship between In the drawings: 1... Mold, 2... Lower inner wall surface, 3
... Temperature sensor, 4 ... Nozzle hole for blowing cooling medium, 5 ... Upper inner wall surface, 6 ... Conduction hole for cooling medium suction, 7 ... Molten steel, 8 ... Meniscus, 9
... Solidified shell, 10 ... Cooling water passage, 11
...cooling water spray nozzle.

Claims (1)

[Claims] After forming the surface layer of a slab from molten steel in a mold with both ends open, the surface layer of the slab is heated to a temperature higher than the austenite single-phase temperature by 10°C.
Cooling is performed at a cooling rate of /sec or more,
Method for preventing hot cracking in continuously cast steel billets.