WO2021259376A1

WO2021259376A1 - Method for reducing surface cracks of casting blank by using ferrite phase

Info

Publication number: WO2021259376A1
Application number: PCT/CN2021/102405
Authority: WO
Inventors: 王迎春; 徐国栋; 徐荣军; 范正洁; 阮晓明; 王妍
Original assignee: 宝山钢铁股份有限公司
Priority date: 2020-06-25
Filing date: 2021-06-25
Publication date: 2021-12-30
Also published as: CN113843403B; KR20230015949A; EP4144459A4; CN113843403A; JP2023530911A; EP4144459A1

Abstract

A method for reducing surface cracks of a casting blank by using a ferrite phase. Cooling water is used to control the surface layer temperature of the casting blank before a straightening section of a casting machine, so that the surface layer of the casting blank is kept constant for a long time in the vicinity of the phase transition temperature of transition of a steel austenite to a ferrite; due to the constant temperature for a long time, a ferrite layer is formed on the surface layer of the casting blank, thereby improving the proportion of ferrite in the surface layer structure of the casting blank, improving the plasticity of the surface layer structure of the casting blank by means of high plasticity of the ferrite, and thus reducing cracks generated on an inner arc surface of the casting blank under the tensile stress of a straightening area. The method can control the surface layer structure of the casting blank, improve the plasticity of the surface layer of the casting blank, and reduce the surface cracks of the casting blank.

Description

Method for improving surface cracks of casting slab by using ferrite phase

Technical field

The invention belongs to the technical field of continuous casting, and in particular relates to a method for improving surface cracks of a cast slab by using a ferrite phase.

Background technique

In the production process of metallurgical continuous casting, the internal and external quality of the slab is a matter of great concern to metallurgical workers. The internal quality problems of the cast slab are mainly manifested in uneven composition, such as looseness, shrinkage, and cracks. The external quality problems are defects such as slag inclusion and cracks. These defects will be inherited into the subsequent rolled products. Therefore, it is hoped to produce products with excellent internal and external structure of the billet.

The formation of slab cracks is the result of the combined effect of metallurgical and mechanical factors in the continuous casting process. Whether the slab cracks depends on the structure and properties of the steel, solidification metallurgical behavior, casting machine process parameters and equipment operating conditions. Controlling the slab structure is one of the effective ways to improve the overall performance of the material. In the continuous casting process, the method of controlling the subcooling of the molten steel, electromagnetic stirring, adding aluminum titanium or rare earth elements and adding nucleating agents is usually used to control the casting state of the slab organization. In the continuous casting process, the surface layer structure of the cast slab can also be controlled by changing the cooling rate and the heating history of the cast slab. The cast slab is still in the austenite single-phase zone when it exits the mold, so the influence of the mold and cooling strength on the surface structure is very important. The surface cracks of the cast slab generally occur in the straightening section of the casting machine, because the continuous casting slab is subjected to straightening in this area, thereby generating tensile stress on the upper surface of the casting slab, and the low plastic structure of the casting slab is under the action of tensile stress. Cracks are generated, so by controlling the cooling of the billet in the casting machine before the straightening zone of the caster, the structure of the billet can be effectively controlled and the mechanical properties of the billet can be improved.

Patent CN 110653352 A In order to eliminate the surface cracks of the cast slab caused by ferrite, a row of cooling water nozzles with a small spray angle is added before the straightening section to provide a narrow area strong cooling belt to eliminate the surface proeutectoid ferrite to control the cast slab Surface cracks. Patent CN107695313 A also uses rapid quenching to eliminate the precipitation of ferrite and carbonitride on the surface of the cast slab, avoid embrittlement, and reduce the surface cracks of the cast slab by improving the plasticity of the cast slab. Patent CN 105478704 B establishes a lifting device for the secondary cold water spray frame, which can realize the dynamic control of the secondary cooling water cooling area and the automatic adjustment of the water volume. This method avoids the direct spray of the secondary cold water on the corners of the casting slab that causes the temperature to be too low or temperature changes Initiated corner cracks.

It can be seen from the above patent analysis that there are two main current technical routes to solve the surface cracks of the cast slab. One is to use high-temperature operation to increase the surface temperature of the cast slab, avoid phase transformation and precipitate ferrite film at the grain boundary. With carbon and nitrogen precipitates, the continuous caster equipment has been kept at a high temperature in this way, which will have a great impact on the accuracy and life of the equipment. The other method uses a low temperature route to avoid the third brittle zone of steel grades. This method requires a large amount of cooling water and has a great impact on energy and the environment. Therefore, it is necessary to find a state of organization that improves the plasticity of the steel from the perspective of the steel itself.

Summary of the invention

The purpose of the present invention is to provide a method for improving the surface cracks of a cast slab by using the ferrite phase, and to reduce the surface cracks of the cast slab by forming a high proportion of ferrite structure on the surface of the cast slab. The method can control the surface layer structure of the cast slab, improve the surface plasticity of the cast slab, and reduce the surface cracks of the cast slab.

The technical scheme of the present invention is:

A method for improving the surface cracks of cast slabs by using the ferrite phase. In the production of metallurgical continuous casting, the surface temperature of the cast slabs is controlled to keep the temperature at the austenite-to-ferrite transformation temperature for a long time to obtain a high proportion The ferrite phase of the cast slab must reach more than 35% before the straightening point of the caster.

Specifically, a method for improving the surface cracks of a cast slab by using the ferrite phase of the present invention. In the production of metallurgical continuous casting, a steel grade with a carbon content of 0<C≦0.25% is formed into a cast slab, and the surface layer of the cast slab is controlled. Temperature, keep the surface temperature of the cast slab within the austenite to ferrite transformation temperature range, so that the ferrite ratio of the cast slab surface reaches 35% or more before the straightening point of the caster, and then enter the caster for straightening point.

In the production of metallurgical continuous casting, the smelted steel will be solidified in the mold to form a solid cast slab with a certain thickness. At this time, the cast slab temperature will be 1000-1250°C. Afterwards, the cast slab will be cooled down. The cast slab will be transported by the support rollers while cooling down. After the bending process, the cast slab will enter the arc-shaped section area, and the continuous transportation will reach the straightening point of the caster. The arc-shaped cast slab will be straightened. The horizontal section of the casting machine is completely solidified and transported out of the casting machine.

The present invention controls the surface temperature of the casting slab before the casting slab reaches the straightening point of the casting machine. The temperature of the solid casting slab formed after passing through the crystallizer is relatively high. The temperature of the casting slab can be lowered by cooling water. The surface temperature of the cast slab is within the transformation temperature range of austenite to ferrite, and the cooling rate for cooling can be determined according to the continuous cooling characteristic CCT curve of the steel grade. The present invention focuses on controlling the temperature of the surface layer of the cast slab after cooling to keep it within the austenite-to-ferrite transformation temperature range, so as to realize the transformation from austenite to ferrite. Research has found that the ferrite phase ratio of the cast slab surface layer needs to be more than 35% before the straightening point of the casting machine. After this condition is met, the casting slab under the phase ratio can enter the straightening point of the casting machine to reduce the arc The shaped cast billet is straightened into a horizontal cast billet.

According to the present invention, when the cast slab starts to cool down, and when to keep the surface temperature of the cast slab within the austenite-to-ferrite transformation temperature range, it can be adjusted according to the production situation, as long as the iron on the surface of the cast slab is straightened before the straightening point of the casting machine. The ratio of the element body should be more than 35%. Among them, the surface layer of the cast slab refers to the part whose thickness is within 10 mm. The ferrite phase ratio of the cast slab surface layer is controlled above 35%, and the control of the surface temperature of the cast slab is aimed at the control of the thickness of the cast slab within 10mm.

According to the method of the present invention for improving the surface cracks of the cast slab by using the ferrite phase, the continuous cooling characteristic CCT curve of the steel grade determines the cooling rate range at which the cast slab of the steel grade can be transformed into the ferrite phase. The cooling rate range cools the cast slab so that the surface layer temperature of the cast slab falls within the austenite-to-ferrite transformation temperature range, and then remains within the above-mentioned temperature range. Specifically, the present invention controls the surface temperature of the cast slab in the arc section of the caster and keeps it within the austenite to ferrite transformation temperature range.

Furthermore, according to the continuous cooling characteristic CCT curve of the steel grade, the cooling rate range for obtaining the ferrite phase of the steel grade is 3 to 0.05° C./s. Preferably, the cooling rate ranges from 3 to 0.1°C/s, and further is 1.5 to 0.08°C/s. The cooling rate can be controlled by parameter settings in the cooling program. The cooling rate is more conducive to maintaining the temperature of the cast slab near the ferrite transformation temperature, which is conducive to the formation of ferrite.

Further, the temperature of the surface layer of the cast slab is controlled so that the ferrite ratio of the surface layer of the cast slab reaches 35%-100%, preferably 35%-75%, before the straightening point of the caster. This ferrite ratio can make the cast slab The surface layer is well shaped, and the time for forming the ferrite ratio is easier to satisfy and control.

Further, it has been found through research that, for the steel grade of the present invention, the austenite-to-ferrite transformation temperature range of the cast slab surface layer is 900°C to 600°C, and the holding time is 0.44 to 35 minutes.

According to the method for improving the surface cracks of the cast slab by using the ferrite phase, the surface temperature of the cast slab is controlled by cooling water. Different from the above-mentioned cooling type, what is mentioned here is a kind of cooling that maintains the temperature. In order to distinguish it from the above-mentioned cooling type, the above-mentioned cooling that lowers the surface temperature of the cast slab to the transformation temperature of austenite to ferrite can be called the first cooling, and the surface temperature of the cast slab is kept at austenite. The cooling to the ferrite transformation temperature is called the second cooling. After the surface layer of the cast slab is cooled to within the set temperature range, the temperature of the surface layer of the cast slab needs to be controlled within the set temperature range. The present invention can be sprayed with cooling water (that is, the second cooling mentioned above) The temperature of the surface layer of the cast slab is maintained. This is because the center temperature of the cast slab is relatively high. If cooling is not performed, the transfer of heat from the center to the surface of the cast slab will also increase the temperature of the surface of the cast slab. Therefore, the use of cooling water can control the surface temperature of the cast slab.

The continuous casting online model is used to calculate the amount of cooling water required to obtain the surface temperature of the cast slab, so that the surface temperature of the cast slab is maintained within the austenite to ferrite transformation temperature range. Specifically, the model belongs to a commonly used model in this field, and the continuous casting online model of the present invention is a continuous casting secondary cooling online model. According to the principle of heat transfer, combined with continuous casting process conditions, such as molten steel temperature, slab drawing speed and cooling water, the surface temperature of the cast slab is calculated. The model can calculate the amount of cooling water required in different areas of the slab according to the target temperature set by the process, so as to achieve the purpose of controlling the temperature of the slab. When in use, you only need to set the temperature of the surface layer of the cast slab, and use the continuous casting secondary cooling online model to calculate the amount of water that should be sprayed to the surface of the cast slab, and perform water spray cooling to control the surface temperature of the cast slab to be maintained at the set temperature. Within a given range.

After research, it is generally appropriate to control the spray amount of cooling water used in the second cooling within the range of 0-0.87L/kg (including 0 and 0.87), which can control the surface temperature of the cast slab from austenite to ferrite Within the transition temperature range. The unit of spray water L/kg refers to the amount of water sprayed per unit mass of steel.

According to the method for improving the surface cracks of the cast slab by using the ferrite phase, further, the surface of the cast slab is maintained at the austenite-to-ferrite transformation temperature for a long time by means of cooling water, and the secondary cooling nozzle is used to move the surface of the cast slab to the surface of the cast slab. Water spray cooling is used to control the surface temperature of the cast slab. Specifically, for the present invention, uniform cooling can be achieved under low water volume (0-0.62L/kg).

Or, in order to keep the surface layer of the cast slab at the austenite-to-ferrite transformation temperature for a long time, sometimes the required first cooling rate is lower, and non-spray cooling can be used, that is, to maintain air cooling, also called dry cooling. . Similarly, in order to keep the surface temperature of the cast slab within the temperature range of the transformation from austenite to ferrite, dry cooling can also be used if the second cooling rate is lower. In the specific application of the continuous casting secondary cooling online model, when the second cooling required speed is low, the model will reduce the amount of water sprayed to prevent the surface temperature of the slab from falling, and even when the temperature drops too fast, it will control the use of air cooling to maintain the casting The surface temperature of the blank. Air cooling is used to control the surface temperature of the cast slab. Correspondingly, the support roll of the arc section of the caster is not cooled by the cooling water, which will easily lead to a higher temperature. Therefore, it is necessary to cool the support roll of the arc section of the casting machine. The present invention performs internal cooling on the support roll of the arc section of the casting machine. Specifically, cooling water can be passed into the support roll to control the surface temperature of the support roll below 550°C to prevent The sector is damaged. The fan-shaped section here can also be called an arc-shaped section.

The method of the invention uses cooling water to control the surface temperature of the cast slab before the straightening section of the caster, so that the surface of the cast slab is kept constant for a long time near the phase transition temperature of the steel grade austenite to ferrite. In order to better control the surface temperature of the cast slab through the cooling water, a secondary cooling nozzle with good atomization effect and uniform spraying is required. In particular, the nozzle can perform uniform spray cooling with low water volume. In some practical situations, the cast slab needs to be dry-cooled for a constant temperature, that is, continuous casting operations are performed without spraying secondary cold water. In this case, good internal cooling of the fan-shaped support roller is required to avoid damage to the fan-shaped support roller and bearings caused by the high temperature of the casting billet. In order to control the surface temperature of the cast slab at a certain temperature, it is necessary to use a continuous casting model to control the surface temperature of the cast slab on-line in real time. At present, many casters have been equipped with continuous casting online control models. This model can be used to set the amount of cooling water required for the surface temperature of the cast slab. A ferrite layer is formed on the surface of the cast slab through a long-term temperature constant, which increases the proportion of ferrite in the surface structure of the cast slab, and uses the high plasticity of ferrite to improve the plasticity of the surface structure of the cast slab, thereby reducing the straightening of the inner arc of the cast slab. Cracks generated under tensile stress in the zone.

In the continuous casting process, the surface temperature of the cast slab after exiting the mold is in the high temperature zone, generally between 1000 and 1250 ℃. At this time, the surface structure of the cast slab is in the austenite zone. In the case of a single phase, the steel grade has higher plasticity and is not easy to Cracks occur. However, as the arc zone is cooled by spray water, the surface layer structure temperature of the cast slab continues to drop. When the temperature reaches the phase transition temperature, the transformation from austenite to pro-eutectoid ferrite in the cast slab is a diffusion-type phase transition. At a lower cooling rate, the pro-eutectoid ferrite first occupies the original austenite grain boundary. It nucleates and grows along the grain boundary. After cooling continues, the ferrite within the grain begins to nucleate. At this time, a relatively coarse proeutectoid ferrite film has been formed on the original austenite grain boundary. The structure is affected by the straightening tensile stress when the cast slab passes through the straightening zone. The stress will crack at the ferrite film at the austenite grain boundary and will gradually expand in the later stage. When the slab is straightened, if the ferrite ratio in the slab structure is low, less than 35%, the film-like proeutectoid ferrite is likely to cause stress concentration and cracks, but if the ferrite ratio exceeds 35%, it will not Strain concentration will occur and cracks can be avoided. There are two factors that affect the ratio of ferrite precipitation: thermodynamics and kinetics, that is, one is temperature and the other is duration. According to the continuous cooling characteristic CCT curve of the steel grade, the phase transition temperature at different cooling rates is obtained. Generally, steel grades can form ferrite in the cooling rate range of 3～0.05℃/s and the temperature range of 900℃～600℃. If the surface temperature of the cast slab is controlled near the phase transition temperature for a long time, a large amount of ferrite will be formed on the surface of the cast slab. The time range of the present invention is 0.44 to 35 min. When the proportion of ferrite exceeds 35%, the ductility of the cast slab structure will be significantly improved, which can avoid the occurrence of cracks. Therefore, when the cast slab passes through the curved section, that is, the arc section adopts a weak cooling method. The cooling method with a cooling strength of less than 3°C/s keeps the surface temperature of the cast slab constant within the temperature range of austenite to ferrite transformation. And keep it until the straightening section. In this way, a high-proportion ferrite layer is formed on the surface of the cast slab, instead of a low-proportion ferrite film with only austenite grain boundaries. When the cast slab with a high proportion of ferrite layer is over-straightened, the surface cracks of the cast slab will not occur due to the concentration of stress on the grain boundary. One of the innovations of the present invention, that is, the method is that the surface of the cast slab is kept constant for a long time near the phase transition temperature of the steel austenite to ferrite. The key points are: one is constant for a long time, and the other is that the temperature is controlled at the phase transition temperature. nearby.

The beneficial technical effects of the present invention:

The method of the present invention controls the surface temperature of the cast slab in the arc section of the casting machine to keep the surface temperature of the cast slab constant near the phase transition temperature of the steel grade for a long time. Through this process, a large amount of ferrite phase can be formed on the surface of the cast slab. . When the high-proportion ferrite phase cast slab passes through the straightening area of the caster, because there is a large amount of ferrite phase in the structure, the tensile stress of the arc surface of the cast slab will not be concentrated at the grain boundary, so it is not It will tear the grain boundaries, so as to avoid the occurrence of cracks on the surface of the cast slab. This technology is very helpful to improve the surface plasticity of the cast slab, reduce the occurrence of surface cracks of the cast slab, and improve the surface quality of the product.

Description of the drawings

Figure 1A is a physical schematic diagram of the surface layer microstructure of the cast slab of Comparative Example 1;

FIG. 1B is a physical schematic diagram of the microstructure of the surface layer of the cast slab in Example 3. FIG.

Figure 2 Example 1 steel type continuous casting cooling characteristics CCT curve.

detailed description

The present invention will be further elaborated below in conjunction with the embodiments. Those skilled in the art should understand that the embodiments are only examples and do not constitute any limitation to the present invention.

The following describes the implementation process of the method of the present invention in detail:

First, test or calculate the continuous cooling characteristic CCT curve of the steel grade to obtain the first cooling rate range of the steel grade to obtain ferrite, which is 3～0.05℃/s, and obtain austenite at different cooling rates The phase transition temperature for ferrite transformation is in the range of 900°C to 600°C. After cooling, keep at 900℃～600℃, the holding time is 0.44～35min, the process is completed before the straightening point.

Fig. 2 shows the CCT curve of the steel grade of the following Example 1, and the figure shows the relationship between the evolution of the steel grade and the temperature and time at several cooling rates. From the figure, the austenite to ferrite transformation temperature of the steel grade at a certain cooling rate can be obtained, as well as the time to obtain a certain proportion of ferrite. In Figure 2, the abscissa in the figure represents the cooling time, the ordinate represents the temperature, and many parabolic-like curves in the figure represent the cooling rate. The F zone in the figure is the formation of ferrite, the P zone represents pearlite, and the B zone Represents bainite. It can be seen that the cooling rate contained in the F zone is the range of the cooling rate capable of forming ferrite. The CCT curve in Figure 2 can also be used to obtain the transformation temperature from austenite to ferrite, which is 560-620°C. Regarding the retention time of the austenite to ferrite transformation temperature of Example 1, the present invention obtained according to research and measurement. The retention time maintained at 0.44 to 35 min can ensure that the ferrite ratio on the surface of the cast slab reaches more than 35%.

Example 1: The element content of steel type 1 is C: 0.08%, Si: 0.14%, Mn: 1.69%, Cr: 0.41%, Mo: 0.02%, and the balance is iron and unavoidable impurities. According to the continuous cooling characteristics, The cooling rate range for obtaining ferrite is less than 0.1°C/S. This embodiment uses a cooling rate of 0.1°C/s. At this cooling rate, the austenite-to-ferrite transformation temperature is 620°C, and the surface layer of the cast slab needs to be maintained for 11.67min A ferrite ratio higher than 35% can be formed, and the process is completed before the straightening point. After production, the casting billet sample was observed through a metallographic microscope and calculated to form a ferrite ratio of more than 35%.

Example 2: The element content of steel type 2 is C: 0.16%, Si: 0.07%, Mn: 1.04%, Cr: 0.88%, Ti: 0.02%, the balance is iron and inevitable impurities, which can be changed according to the continuous cooling characteristics The cooling rate to obtain ferrite is 0.1～3℃/s, the austenite-to-ferrite transformation temperature is 750℃ at a cooling rate of 0.1℃/s, and the austenite-to-ferrite transformation temperature is at a cooling rate of 3℃/s The ferrite transformation temperature is 630°C. In Example 2, a cooling rate of 0.2°C/s was used for cooling, and then the surface temperature of the cast slab was maintained at 720°C for 10 minutes, and the process was completed before the straightening point. After production, the casting billet sample was observed through a metallographic microscope and calculated to form a ferrite ratio of more than 35%.

Example 3: The element content of steel type 3 is C: 0.077%, Si: 0.09%, Mn: 1.45%, Cr: 0.03%, Mo: 0.01%, the balance is iron and inevitable impurities, which can be determined according to the continuous cooling characteristics. The cooling rate to obtain ferrite is 0.1～3℃/s, the transformation temperature of austenite to ferrite at a cooling rate of 0.1℃/S is 790℃, and the austenite to ferrite is transformed at a cooling rate of 3℃/s The ferrite transformation temperature is 730°C. In Example 3, a cooling rate of 0.3°C/s was used for cooling, and then the surface temperature of the cast slab was maintained at 780°C for a holding time of 7.22min, and the process was completed before the straightening point. After production, the casting billet sample was observed through a metallographic microscope and calculated to form a ferrite ratio of more than 35%.

Example 4: The element content of steel type 4 is C: 0.09%, Si: 0.17%, Mn: 0.83%, Cr: 0.02%, and the balance is iron and inevitable impurities. According to continuous cooling characteristics, ferrite can be obtained The cooling rate is 0.1～3℃/S, the transformation temperature of austenite to ferrite at a cooling rate of 0.1℃/S is 830℃, and the transformation temperature of austenite to ferrite at a cooling rate of 3℃/S The temperature is 780°C. In Example 3, a cooling rate of 0.5° C./s was used for cooling, and then the surface temperature of the cast slab was kept at 820° C. for a holding time of 5.67 min, and the process was completed before the straightening point. After production, the casting billet sample was observed through a metallographic microscope and calculated to form a ferrite ratio of more than 35%.

Comparative Example 1: The element content of the steel of Comparative Example 1 is C: 0.077%, Si: 0.09%, Mn: 1.45%, Cr: 0.03%, Mo: 0.01%, and the balance is iron and inevitable impurities. Conventional The process is for casting. The slab is cooled after it comes out of the mold. The surface of the slab at the exit of the mold is 1200℃. Under the action of the secondary cooling water in the arc zone, the surface temperature of the slab gradually decreases. The surface temperature of the slab is 750°C. At this time, in the structure of the surface of the cast slab, ferrite precipitation occurs at the austenite grain boundary, as shown in Figure 1A. The surface of the cast slab produces tensile stress under the action of straightening, so cracks occur at the precipitated ferrite at the austenite grain boundary.

The above only lists the cooling rate, phase transition temperature and time combination of the 4 steel grades, and does not exclude other process parameter combinations of other steel grades.

Fig. 1A and Fig. 1B are the cast slabs formed by the steel types in Comparative Example 1 and Example 3 after passing through the straightening point and then cooled to form cast slabs. The surface microstructure percentages of the cast slabs are tested. It can be seen from the figure that the surface microstructure of the casting slab of Comparative Example 1 is mainly austenite, and the ferrite content is only 8%. Therefore, when subjected to external force, stress concentration is easily formed at the ferrite and cracks are generated. Example 3 The cast slab was kept at 780°C at a cooling rate of 0.3°C/s for 7.22 min. The surface of the cast slab will form 95% ferrite when it enters the straightening point. This structure is subject to external force. There will be no stress concentration, and ferrite has better shaping, so no cracks occur.

Of course, those of ordinary skill in the art should realize that the above-mentioned embodiments are only used to illustrate the present invention, not to limit the present invention. As long as the above-mentioned embodiments are changed within the essential spirit of the present invention, All the deformations will fall within the scope of the claims of the present invention.

Claims

A method for improving the surface cracks of cast slabs by using ferrite phase, which is characterized in that: in the production of metallurgical continuous casting, the surface temperature of the cast slabs is controlled to keep the temperature at the austenite-to-ferrite transformation temperature for a long time , To obtain a high proportion of ferrite phase, so that the ferrite phase ratio of the cast slab surface layer needs to reach 35% or more before the straightening point of the caster.
The method for improving the surface cracks of a cast slab by using the ferrite phase according to claim 1, characterized in that: a steel grade with a carbon content of 0<C≦0.25% is formed into a cast slab, and the surface temperature of the cast slab is controlled to allow The surface temperature of the cast slab is kept within the austenite to ferrite transformation temperature range, so that the ferrite phase ratio of the cast slab surface reaches more than 35% before the straightening point of the caster, and then enters the straightening point of the caster.
The method for improving the surface cracks of a cast slab by using the ferrite phase according to claim 1, characterized in that: according to the continuous cooling characteristic CCT curve of the steel grade, it is determined that the cast slab of the steel grade can be transformed into the ferrite phase for cooling Speed range, the casting slab is cooled in the cooling speed range, so that the surface temperature of the casting slab is reduced to the austenite to ferrite transformation temperature range, and the surface of the casting slab is controlled in the arc section of the caster temperature.
The method for improving the surface cracks of the cast slab by using the ferrite phase according to claim 1, characterized in that: the surface temperature of the cast slab is controlled in the arc section of the caster, and the continuous cooling characteristic CCT curve of the steel is obtained The cooling rate to obtain the ferrite phase of the steel grade is 3 to 0.05°C/s.
The method for improving the surface cracks of the cast slab by using the ferrite phase according to claim 1, characterized in that: the surface temperature of the cast slab is controlled in the arc section of the caster, and the continuous cooling characteristic CCT curve of the steel is obtained The cooling rate to obtain the ferrite phase of the steel grade is 3 to 0.1°C/s.
The method for improving the surface cracks of a cast slab by using the ferrite phase according to claim 1, characterized in that: the surface temperature of the cast slab is controlled so that the ferrite phase ratio of the cast slab surface reaches 35 before the straightening point of the caster. %～75%.
The method for improving the surface cracks of a cast slab by using the ferrite phase according to claim 1, wherein the surface layer of the cast slab has a transition temperature from austenite to ferrite in the range of 900°C to 600°C, and the holding time For 0.44～35min.
The method for improving the surface cracks of a cast slab by using the ferrite phase according to any one of claims 1-7, characterized in that: the surface temperature of the cast slab is controlled by cooling water, and the continuous casting online model is used to calculate the required surface temperature of the cast slab. The amount of cooling water.
The method for improving the surface cracks of a cast slab by using the ferrite phase according to claim 8, wherein the surface layer of the cast slab is maintained at the austenite-to-ferrite transformation temperature for a long time, and a secondary cooling nozzle with good spray performance is used, It can achieve uniform cooling under low water volume.
The method for improving the surface cracks of a cast slab by using ferrite phase according to any one of claims 1-7, characterized in that the surface layer of the cast slab is kept at the austenite-to-ferrite transformation temperature for a long time, and non-spray cooling is adopted. Therefore, the fan-shaped support roller of the casting machine needs good internal cooling. The surface temperature of the support roller should be controlled below 550℃ to prevent the fan-shaped segment from being damaged.