Method for reducing surface cracks in cast slabs due to ferrite phase.

Abstract

A method for reducing surface cracks in cast slabs is provided by using ferrite phases. Cooling water is used to control the surface layer temperature of the cast slab before the straightening section of the casting machine so that the cast slab remains constant at a temperature near the austenite-ferrite phase transition temperature of the steel for a long period of time. Due to the constant temperature for a long period of time, a ferrite layer is formed in the surface layer of the cast slab, increasing the proportion of ferrite in the surface layer structure of the cast slab. The high plasticity properties of ferrite increase the plasticity of the surface layer structure of the cast slab, thereby reducing cracks that form on the inner surface of the cast slab under tensile stress in the straightening area. This method not only controls the surface layer structure of the cast slab but also increases the plasticity of the surface of the cast slab, thereby reducing surface cracks in the cast slab.

Description

[Technical field]

[0001] The present invention relates to the technical field of continuous casting, and more particularly to a method for reducing surface cracks in cast slabs with ferrite phase. [Background technology]

[0002] In the production process of continuous metallurgical casting, smiths pay close attention to the internal and external quality of the cast slab. The internal quality problems of the cast slab are mainly manifested in inhomogeneous composition, looseness, shrinkage cavities, cracks, and other defects, while the external quality problems include slag inclusions, cracks, and other defects. These defects will be carried over to the product that will be subsequently rolled, so cast slabs with excellent internal and external structure are desirable.

[0003] The formation of cracks in cast slabs is the result of the combined action of metallurgical and mechanical factors in the continuous casting process. Whether cracks appear in cast slabs depends on the structural performance of steel, metallurgical solidification behavior, process parameters of the caster, and the operation status of the equipment. Controlling the structure of cast slabs is one of the effective methods to improve the comprehensive properties of materials. In the continuous casting process, the as-cast structure of cast slabs is generally controlled by controlling the undercooling degree of molten steel, electromagnetic stirring, or adding nucleating agents such as aluminum, titanium, or rare earth elements. In the continuous casting process, the surface layer structure of cast slabs can also be controlled by changing the cooling rate and heating process of cast slabs. Since cast slabs are still in the austenite single phase region when they leave the mold, the cooling strength of the mold is crucial to the structure of the surface layer. Common surface cracks in cast slabs occur in the straightening section of the caster where cast slabs undergo straightening action, which generates tensile stress on the inner surface of cast slabs, thus leading to cracks in the low plastic structure of cast slabs under the action of tensile stress. Therefore, carrying out controlled cooling on the cast slab in the casting machine before entering the straightening section can effectively control the structure of the cast slab and improve the mechanical properties of the cast slab.

[0004] In Patent Document 1, in order to avoid the surface cracks of the cast slab caused by ferrite, a row of cooling water nozzles with a small injection angle is added before the straightening section, and narrow-area strong cooling is performed to avoid the proeutectoid ferrite on the surface and control the surface cracks of the cast slab. In Patent Document 2, concentrated cooling quenching is also used to avoid the precipitation of ferrite and carbonitride in the surface layer of the cast slab, so as to avoid the embrittlement of the cast slab and increase the plasticity, thereby reducing the surface cracks of the cast slab. In Patent Document 3, a lifting device of the secondary cooling water injection frame is installed, which can achieve the dynamic control of the secondary cooling water cooling area and the automatic adjustment of the water flow rate. In this way, the secondary cooling water does not directly cool the corners of the cast slab, and the corner cracks caused by the extremely low temperature or temperature change are avoided.

[0005] It can be seen from the above patents that there are mainly two technical paths to solve the surface cracks of cast slabs. One path is to operate at high temperature to increase the surface layer temperature of the cast slab, thereby preventing the phase transformation and precipitation of ferrite film and precipitation carbonitride at grain boundaries. In this way, the continuous caster will be kept at high temperature, which will have a great effect on the precision and service life of the equipment. The other path is to operate at low temperature to avoid the third brittle area of ​​the steel. Such a method requires a large amount of cooling water, which has a great impact on energy and environment. Therefore, it is necessary to find a structural state in the steel itself that can increase the plasticity of the steel. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] China Patent Application Publication No. 110653352 [Patent Document 2] China Patent Publication No. 107695313 [Patent Document 3] Chinese Patent No. 105478704 Summary of the Invention [Problem to be solved by the invention]

[0007] The present invention aims to provide a method for reducing surface cracks in a cast slab by forming a high ratio of ferrite structure in the surface layer of the cast slab. This method can not only control the surface layer structure of the cast slab, but also increase the plasticity of the surface layer of the cast slab to reduce surface cracks in the cast slab.

Means for Solving the Problems

[0008] The technical solution of the present invention is as follows.

[0009] A method for reducing surface cracks in a cast slab by a ferrite phase, in which, in the continuous casting process, the surface layer temperature of the cast slab is controlled to maintain the austenite-ferrite transformation temperature for a long time, so that a high ratio of ferrite phase is obtained such that the ratio of the ferrite phase in the surface layer of the cast slab reaches 35% or more before the straightening point of the casting machine.

[0010] Specifically, in the method of the present invention for reducing surface cracks in a cast slab by a ferrite phase, in the production of continuous metallurgical casting, a cast slab is formed of steel having a carbon content of 0 < C ≤ 0.25%, and the surface layer temperature of the cast slab is controlled to maintain the temperature within the austenite-ferrite transformation temperature such that the ratio of the ferrite phase in the surface layer of the cast slab reaches 35% or more before the straightening point of the casting machine.

[0011] In the continuous casting process, the molten steel solidifies in the mold to form a solid cast slab having a certain thickness. At this point, the temperature of the cast slab is 1000 - 1250°C. Then, the cast slab is cooled and conveyed by support rollers while being cooled. After bending, the cast slab enters the curved portion and is further conveyed to the straightening point of the casting machine for straightening. And the cast slab passes through the horizontal portion of the casting machine for complete solidification and is discharged from the casting machine.

[0012] In the present invention, the control of the surface temperature of the cast slab is carried out before the cast slab reaches the straightening point of the casting machine. Initially, the cast slab after solidification passing through the mold is relatively hot, and is cooled by cooling water to keep the surface layer temperature of the cast slab within the austenite-ferrite transition temperature. The cooling rate can be determined by the continuous cooling characteristic CCT curve of the steel. The present invention focuses on controlling the surface layer temperature of the cast slab to maintain the temperature within the range of the austenite-ferrite transition temperature so as to achieve the transformation from austenite to ferrite. Through research, it has been found that the ratio of ferrite phase in the surface layer of the cast slab needs to reach 35% or more before the straightening point of the casting machine. After this condition is met, the cast slab with the desired phase ratio can enter the straightening point of the casting machine to straighten the slab into a horizontal cast slab.

[0013] In the present invention, the exact time for cooling the cast slab or keeping the temperature within the range of the austenite-ferrite transition temperature can be adjusted according to the production conditions, as long as the ratio of ferrite phase in the surface layer of the cast slab before the straightening point of the caster is not less than 35%. The surface layer of the cast slab refers to the part with a cast slab thickness of not more than 10 mm. Both controlling the ratio of ferrite phase in the surface layer of the cast slab to not less than 35% and controlling the surface layer temperature of the cast slab are performed for the part with a cast slab thickness of not more than 10 mm.

[0014] According to the method of the present invention for reducing surface cracks in cast slabs due to ferrite phase, the range of cooling rates of the cast slab to the ferrite phase is determined by the continuous cooling characteristic CCT curve of the steel. The cast slab is cooled within the range of cooling rates so that the surface layer temperature of the cast slab is lowered to the austenite-ferrite transition temperature range and maintained within the above temperature range. Specifically, the surface layer temperature of the cast slab is controlled at the bend of the caster to be maintained within the austenite-ferrite transition temperature range.

[0015] Preferably, according to the continuous cooling temperature characteristic (CCT) curve of the steel, the cooling rate of the ferrite phase of the steel is in the range of 3-0.05°C / s. Preferably, the cooling temperature range is 3-0.1°C / s, more preferably 1.5-0.08°C / s. The cooling rate is controlled by configuring parameters in the program model, which helps to keep the temperature of the cast slab near the phase transition temperature of ferrite to promote the formation of ferrite.

[0016] Preferably, before the straightening point of the casting machine, the temperature of the surface layer of the cast slab is controlled so that the ratio of ferrite phase in the surface layer of the cast slab reaches 35%-100%, preferably 35%-75%. The ratio of ferrite allows the surface layer of the cast slab to obtain good plasticity. Moreover, the time to form this ratio of ferrite is easy to achieve and control.

[0017] Preferably, for the steel of the present invention, studies have shown that the surface layer of the cast slab is maintained within the austenite-ferrite transition temperature range of 900°C to 600°C for a period of 0.44 to 35 minutes.

[0018] According to the method for reducing surface cracks in a cast slab due to ferrite phase, the surface layer temperature of the cast slab is controlled by cooling water. Unlike the above-mentioned cooling for decreasing the temperature, the cooling described herein is a cooling for maintaining the temperature. To distinguish from the above-mentioned cooling for decreasing the temperature, the cooling for cooling the surface layer temperature of the cast slab to the austenite-ferrite transition temperature is called the first cooling, and the cooling for maintaining the temperature at the austenite-ferrite transition temperature is called the second cooling. After the surface layer of the cast slab is cooled to a predetermined temperature range, it is also necessary to control the surface layer temperature of the cast slab to be within the predetermined temperature range. In the present invention, the surface temperature of the cast slab can be maintained by spraying cooling water (i.e., the above-mentioned second cooling). This is because the central temperature of the cast slab is relatively high, and if no cooling is performed, the heat from the center is transferred to the surface layer of the cast slab, causing the surface layer temperature of the cast slab to increase. Therefore, the surface layer temperature of the cast slab can be controlled by cooling water.

[0019] The flow rate of cooling water required to keep the surface layer temperature of the cast slab within the range of austenite-ferrite transition temperature is calculated by the continuous casting online model. Specifically, the model is a common model in the art, and the continuous casting online model of the present invention is a continuous casting and secondary cooling online model, in which the surface temperature of the cast slab is calculated according to the heat transfer principle in combination with the continuous casting process conditions such as the molten steel temperature, the casting speed of the cast slab, and the cooling water. The model can calculate the flow rate of cooling water required for various areas of the cast slab according to the target temperature set by the process so as to control the temperature of the cast slab. In use, one only needs to set the temperature that needs to be maintained for the surface layer of the cast slab, and the flow rate of the injected water is calculated by the continuous casting and secondary cooling online model, and the water is injected to control the surface layer temperature of the cast slab within the predetermined range by cooling.

[0020] After research, it is generally found that the water jet flow rate of the cooling water for secondary cooling can be properly controlled within the range of 0~0.87L / kg (including 0 and 0.87), thus controlling the surface layer temperature of the cast slab within the range of the austenite-ferrite transition temperature. The unit of water jet flow rate, L / kg, refers to the water jet volume required by unit mass of steel.

[0021] According to the method for reducing surface cracks in the cast slab due to ferrite phase, preferably, the surface layer of the cast slab is maintained at the austenite-ferrite transition temperature for an extended period of time by cooling water, and a secondary cooling nozzle is used to inject water onto the surface layer of the cast slab for cooling, so as to control the surface layer temperature of the cast slab. Specifically, with the present invention, uniform cooling can be achieved with a small water volume (0-0.62 L / kg).

[0022] Alternatively, a low first cooling rate is sometimes required to keep the surface layer of the cast slab at the austenite-ferrite transition temperature for a long time, and therefore, a jet cooling other than water may be adopted, i.e., air cooling may be kept, which is also called dry cooling. Similarly, dry cooling may be used when a low second cooling rate is required to keep the surface layer temperature of the cast slab within the austenite-ferrite transition temperature. Specifically, during the application of the continuous casting and secondary cooling online model, when a low second cooling rate is required, the model reduces the water jet flow rate to avoid a drop in the surface layer temperature of the cast slab. Also, when the temperature drops too quickly, the model uses air cooling to keep the surface temperature of the cast slab. When the surface layer temperature of the cast slab is controlled by air cooling, due to the fact that no cooling water is used to cool the support rollers of the curved part of the casting machine, a relatively high temperature is easily generated accordingly. Therefore, it is necessary to cool the support rollers of the curved part of the casting machine. In the present disclosure, the support rollers of the curved part of the casting machine are cooled from the inside. Specifically, cooling water can be supplied to the inside of the support roller to control the surface temperature of the support roller to below 550° C., so as to prevent the curved portion from being damaged. Here, the curved portion is also called the curved portion.

[0023] According to the method of the present invention, cooling water is used to control the surface layer temperature of the cast slab before the straightening section of the caster so that the surface temperature of the cast slab is kept constant around the austenite-ferrite transition temperature for a long time. In order to better control the surface layer temperature of the cast slab with cooling water, a secondary cooling nozzle with good atomization and uniform injection is required. Nozzles that can perform uniform injection cooling under conditions of low water volume are particularly required. In some practical cases, dry cooling, i.e., injection of secondary cooling water other than that for continuous casting, is required to keep the cast slab at a constant temperature. In this case, the support rollers of the bends require good internal cooling to avoid damage to the support rollers and bearings of the bends caused by the high temperature of the cast slab. In order to control the surface layer temperature of the cast slab to a certain temperature, a continuous casting model is required to control the surface layer temperature of the cast slab online and in real time. Currently, many casters are equipped with a continuous casting online control model. With this model, the amount of cooling water required for the surface layer temperature of the cast slab can be configured. By maintaining a constant temperature for a long period of time, a ferrite layer is formed in the surface layer of the cast slab, whereby as the proportion of ferrite in the surface layer structure of the cast slab increases, the high plasticity of ferrite enhances the plasticity of the surface layer of the cast slab, thereby reducing cracks that occur on the inner surface of the cast slab due to tensile stress in the straightening section.

[0024] In the continuous casting process, the surface layer temperature of the cast slab after leaving the mold is generally in the high temperature range of 1000-1250°C. The surface layer structure of the cast slab at that moment is in the austenite region. In the case of single phase, steel has relatively high plasticity and will not generate cracks. However, as the cast slab is cooled by cooling water at the bend, the temperature of the surface layer structure of the cast slab decreases continuously. When the temperature reaches the phase transition temperature, the transformation of austenite to proeutectoid ferrite in the cast slab belongs to the diffusion type phase transition. At a relatively low cooling rate, proeutectoid ferrite first nucleates at the initial austenite grain boundaries and grows at the grain boundaries. After continuous cooling, the granular ferrite starts to nucleate. At that moment, a relatively coarse proeutectoid ferrite film is formed at the initial austenite grain boundaries. When the cast slab passes through the straightening area, the structure is subjected to straightening tensile stresses that lead to cracks on the ferrite film at the austenite grain boundaries, and the cracks gradually spread at the later stage. During the straightening of the cast slab, when the proportion of ferrite in the structure of the cast slab is low, i.e., less than 35%, the proeutectoid ferrite film easily causes stress concentration and forms cracks, but when the proportion of ferrite is more than 35%, no stress concentration occurs, thus cracks can be avoided. Thermodynamic and mechanical factors, i.e., temperature and duration, together affect the precipitation proportion of ferrite. According to the continuous cooling characteristic CCT curve of the steel, the phase transition temperature at various cooling rates is obtained. Generally, ferrite can be formed in the steel at a cooling rate of 3-0.05°C / s and within the temperature range of 900°C to 600°C. If the surface layer of the cast slab is controlled around the phase transition temperature for a long time, a large amount of ferrite will be formed in the surface layer of the cast slab. The holding time of the present invention is 0.44-35 minutes. When the proportion of ferrite exceeds 35%, the plasticity of the structure of the cast slab is significantly increased, thus cracks can be avoided. Therefore, after the cast slab has passed the bend, i.e. the curved section, a weak cooling is carried out with a cooling rate of less than 3°C / s to keep the surface layer temperature of the cast slab constant within the range of the austenite-ferrite transformation temperature and maintain this temperature until the straightening section, thus forming a high ferrite phase in the surface layer of the cast slab instead of a low ferrite film only at the austenite grain boundaries.Cast slabs with high ferrite ratio do not face cracks caused by stress concentration when passing through the straightening section. One innovation of the present invention is that the method keeps the surface temperature of the cast slab constant near the austenite-ferrite transformation temperature for a long time. There are two main points: one is to keep the temperature for a long time and the other is to control the temperature near the phase transformation temperature. Effect of the Invention

[0025] The present invention has the following beneficial technical effects.

[0026] The method of the present invention controls the surface layer temperature of the cast slab at the bend of the casting machine so that the surface layer temperature of the cast slab is constant around the phase transition temperature of steel for a long time, and a large amount of ferrite phase is formed on the surface of the cast slab throughout the process. When the cast slab with a high proportion of ferrite phase passes through the straightening section of the casting machine, a large amount of ferrite phase is found in the structure, so that the tensile stress on the inner surface of the cast slab will not concentrate on the grain boundary and will not break the grain boundary, thus avoiding the surface crack of the cast slab. This technology is very helpful to increase the surface plasticity of the cast slab, while reducing the surface crack of the cast slab and improving the surface quality of the product. [Brief description of the drawings]

[0027] [Figure 1A] FIG. 2 is a schematic diagram showing the microstructure in the surface layer of the cast slab of Comparative Example 1. [Figure 1B] FIG. 11 is a schematic diagram showing the microstructure in the surface layer of the cast slab of embodiment 3. [Diagram 2] 1 is a continuous casting cooling characteristic CCT curve of the steel of embodiment 1. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0028] The present invention will be further described below in combination with the embodiments. Those skilled in the art should understand that the embodiments are merely examples, and are not intended to be limitations on the present invention.

[0029] The implementation process of the method of the present invention is described in detail below.

[0030] First, the continuous cooling characteristic CCT curve of the steel is tried or calculated to obtain the range of first cooling rate of the steel to form ferrite i.e. 3-0.05°C / sec and austenite / ferrite transition temperature at various cooling rates i.e. 900°C to 600°C. After cooling, the temperature is kept at 900°C to 600°C for 0.44-35 minutes to complete the process before the straightening point.

[0031] As shown in FIG. 2, the CCT curve of the steel in the following embodiment 1 shows the relationship between the transition of the steel structure at several cooling rates and the temperature and time. The austenite-ferrite transition temperature at a certain cooling rate and the time to form a certain proportion of ferrite can be obtained from the diagram. In FIG. 2, the X-axis of the diagram represents the cooling time and the Y-axis represents the temperature, while the many curves in the diagram, which resemble parabolas, represent the cooling rates. The area marked F in the diagram is the area where ferrite is formed. The area marked P in the diagram represents pearlite. The area marked B in the diagram represents bainite. The cooling rate included in the area F is the range of cooling rates that can form ferrite. According to the CCT curve in FIG. 2, the austenite-ferrite transition temperature, that is, 560-620°C, can also be obtained. Regarding the holding time of the austenite-ferrite transition temperature in embodiment 1, it is obtained in the present invention by research judgment that a holding time of 0.44-35 minutes ensures that the proportion of ferrite phase on the surface of the cast slab reaches 35% or more.

[0032] Embodiment 1: Steel 1 contains 0.08% C, 0.14% Si, 1.69% Mn, 0.41% Cr, 0.02% Mo, and other iron and unavoidable impurities. According to the continuous cooling characteristics, the cooling rate range in which ferrite can be formed is less than 0.1°C / s. This embodiment adopts a cooling rate of 0.1°C / s. At this cooling rate, the austenite-ferrite transition temperature is 620°C and the holding time is 11.67 minutes, so that a proportion of ferrite of more than 35% can be formed in the surface layer of the cast slab. This process is completed before the straightening point. After production, it can be determined that more than 35% ferrite has been formed by observing the cast slab specimen with a metallurgical microscope and calculating.

[0033] Embodiment 2: Steel 2 contains 0.16% C, 0.07% Si, 1.04% Mn, 0.88% Cr, 0.02% Ti, and other iron and unavoidable impurities. According to the continuous cooling characteristics, the cooling rate range in which ferrite can be formed is 0.1-3°C / s, and the austenite-ferrite transition temperature is 750°C at a cooling rate of 0.1°C / s and 630°C at a cooling rate of 3°C / s. In embodiment 2, cooling is performed at a cooling rate of 0.2°C / s, and then the surface layer temperature of the cast slab is kept at 720°C for 10 minutes. This process is completed before the straightening point. After production, it can be determined that more than 35% ferrite has been formed by observing the cast slab specimen with a metallurgical microscope and calculating.

[0034] Embodiment 3: Steel 3 contains 0.077% C, 0.09% Si, 1.45% Mn, 0.03% Cr, 0.01% Mo, and other iron and unavoidable impurities. According to the continuous cooling characteristics, the cooling rate range in which ferrite can be formed is 0.1-3°C / s. The austenite-ferrite transition temperature is 790°C at a cooling rate of 0.1°C / s and 730°C at a cooling rate of 3°C / s. In embodiment 3, cooling is performed at a cooling rate of 0.3°C / s, and then the surface layer temperature of the cast slab is kept at 780°C for 7.22 minutes. This process is completed before the straightening point. After production, it can be determined that more than 35% ferrite has been formed by observing the cast slab specimen observed by metallurgical microscope and by calculation.

[0035] Embodiment 4: Steel 4 contains 0.09% C, 0.17% Si, 0.83% Mn, 0.02% Cr, and other iron and unavoidable impurities. According to the continuous cooling characteristics, the cooling rate range in which ferrite can be formed is 0.1-3°C / s. The austenite-ferrite transition temperature is 830°C at a cooling rate of 0.1°C / s and 780°C at a cooling rate of 3°C / s. In embodiment 4, cooling is performed at a cooling rate of 0.5°C / s, and then the surface layer temperature of the cast slab is kept at 820°C for 5.67 minutes. This process is completed before the straightening point. After production, it can be determined that more than 35% ferrite has been formed by observation and calculation of the cast slab specimen.

[0036] Comparative Example 1: The steel of Comparative Example 1 contains 0.077% C, 0.09% Si, 1.45% Mn, 0.03% Cr, 0.01% Mo, and other impurities such as iron and unavoidable impurities. Casting is carried out by a conventional process. After leaving the mold, the cast slab is cooled. The surface temperature of the cast slab at the outlet of the mold is 1200°C. The surface temperature of the cast slab is gradually reduced by the secondary cooling water at the bend and reaches 750°C when the cast slab enters the straightening point. At that moment, the surface structure of the cast slab has ferrite precipitated at the austenite grain boundaries, as shown in FIG. 1A. Due to the straightening, tensile stress is generated on the surface of the cast slab, and cracks are formed at the positions where ferrite precipitates at the austenite grain boundaries.

[0037] Although only the combinations of cooling rate, phase transformation temperature and time for four steel types are listed above, other process parameter combinations for other types of steels are not excluded.

[0038] 1A and 1B show the cast slabs of steel in Comparative Example 1 and in Example 3 after cooling after passing through the straightening point, and the percentage of microstructure on the surface of the cast slab is measured. It can be seen from the figure that the microstructure on the surface of the cast slab in Comparative Example 1 is mainly austenite, with only 8% ferrite content. Therefore, stress concentration easily occurs in ferrite and cracks are formed under the action of external force. In Example 3, the cast slab is kept at 780°C with a cooling rate of 0.3°C / s and held for 7.22 minutes. When the cast slab enters the straightening point, 95% of ferrite is formed on the surface of the cast slab. In such a structure, no stress concentration occurs due to external force, and no cracks are formed because ferrite has good plasticity.

[0039] Needless to say, those skilled in the art should recognize that the above embodiments are only used to illustrate the present invention, but do not limit the present invention. Any modifications and variations made to the above embodiments within the essential spirit of the present invention shall be included in the scope of the claims of the present invention.

Claims

1. A method for reducing surface cracks of cast slabs with ferrite phase, in which in the production of continuous metallurgical casting, the surface layer temperature of the cast slab is controlled to maintain an austenite-ferrite transformation temperature of 900°C-600°C for 0.44-35 minutes to obtain a high proportion of ferrite phase in the surface layer of the cast slab before the straightening point of the caster, so that the proportion of the ferrite phase in the surface layer of the cast slab reaches 35%-75%.

2. 2. The method for reducing surface cracks in a cast slab by a ferrite phase according to claim 1, wherein the cast slab is formed from a steel having a carbon content of 0<C≦0.25%, and the temperature of the surface layer of the cast slab is controlled to keep the temperature within the range of the austenite-ferrite transition temperature so that the proportion of the ferrite phase in the surface layer of the cast slab reaches 35% or more before the straightening point of the casting machine before the cast slab enters the straightening point of the casting machine.

3. 2. The method of claim 1 for reducing surface cracks in a cast slab by the ferrite phase, wherein a range of cooling rates for transforming the cast slab of steel to the ferrite phase is determined by a continuous cooling characteristic (CCT) curve of the steel, and the cast slab is cooled within the range of cooling rates such that the surface layer temperature of the cast slab is lowered to the austenite-ferrite transition temperature range and then the surface layer temperature of the cast slab is controlled at a bend in the casting machine.

4. 2. The method of claim 1 for reducing surface cracks in a cast slab by ferrite phase, wherein the surface layer temperature of the cast slab is controlled at a bend in the casting machine, and the cooling rate of the ferrite phase of the steel is 3-0.05°C / s as determined by a continuous cooling characteristic (CCT) curve of the steel.

5. 2. The method of claim 1 for reducing surface cracks in a cast slab by ferrite phase, wherein the surface layer temperature of the cast slab is controlled at a bend in the casting machine, and the cooling rate of the ferrite phase of the steel is 3-0.1°C / s as determined by a continuous cooling characteristic (CCT) curve of the steel.

6. 6. The method according to any one of claims 1 to 5 for reducing surface cracks in a cast slab by ferrite phase, wherein the surface layer of the cast slab is maintained at the austenite-ferrite transition temperature of 900°C to 600°C for 0.44 to 35 minutes, and a secondary cooling nozzle is used to achieve uniform cooling with a water flow rate of 0 to 0.62 L / kg.

7. 6. A method according to any one of claims 1 to 5 for mitigating surface cracks in cast slabs with ferrite phase, wherein the surface layer of the cast slab is maintained at the austenite-ferrite transition temperature of 900°C to 600°C for 0.44 to 35 minutes by non-water jet cooling, and support rollers in the bends of the casting machine are cooled from the inside to control the surface temperature of the support rollers below 550°C to prevent breakage of the bends.

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