JP2023530911A - A method for mitigating surface cracks in cast slabs by ferritic phases - Google Patents

Abstract

A method is provided for mitigating surface cracks in cast slabs through the use of ferritic phases. Cooling water is used to control the surface layer temperature of the cast slab prior to the straightening section of the caster so that the cast slab remains constant around the austenite-ferrite phase transition temperature of the steel for an extended period of time. . Due to the constant temperature for a long time, a ferrite layer is formed on the surface layer of the cast slab, which increases the ratio of ferrite in the surface layer structure of the cast slab. It enhances plasticity and thus reduces cracks that form on the inner surface of the cast slab under tensile stress in the straightening area. This method can not only control the surface layer structure of the cast slab, but also enhance the plasticity of the surface of the cast slab, and reduce the surface cracks of the cast slab.

Description

The present invention relates to the technical field of continuous casting, and in particular to a method for mitigating surface cracks in cast slabs with ferrite phases.

In the production process of continuous metallurgical casting, metallurgists pay close attention to the internal and external quality of the cast slab. Internal quality problems of cast slabs are mainly manifested in inhomogeneous composition, looseness, shrinkage cavities, cracks and other defects, whereas external quality problems are characterized by slag entrainment, cracks and other defects. including. Cast slabs with good internal and external structure are desirable because these defects are carried over into the product that is subsequently rolled.

Crack formation 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 steel's structural performance, metallurgical solidification behavior, caster process parameters, and equipment operating conditions. Controlling the structure of cast slabs is one of the effective ways to improve the comprehensive properties of materials. In continuous casting processes, the as-cast structure of the cast slab is typically controlled by controlling the degree of subcooling of the molten steel, electromagnetic stirring, or the addition of nucleating agents such as aluminum, titanium, or rare earth elements. In the continuous casting process, the surface layer structure of the cast slab can also be controlled by varying the cooling rate and heating process of the cast slab. Since the cast slab is still in the austenitic single phase region when it exits the mold, the cooling strength of the mold is extremely important for the structure of the surface layer. A common surface crack in a cast slab occurs in the straightening section of a casting machine where the cast slab is subjected to a straightening action to generate tensile stress on the inner surface of the cast slab, thus creating a low plastic structure of the cast slab under the action of tensile stress. cause cracks in Therefore, subjecting the cast slab to controlled cooling in the casting machine prior to entering the straightening section can effectively control the structure of the cast slab to improve the mechanical properties of the cast slab.

In Patent Document 1, in order to avoid surface cracks of the cast slab caused by ferrite, a row of cooling water nozzles with a small injection angle is added before the correction part, and proeutectoid ferrite on the surface is avoided. Narrow area intensive cooling is applied to control surface cracking of the No. 2004/0000003 focuses on avoiding the precipitation of ferrite and carbonitrides on the surface layer of the cast slab so as to reduce the surface cracking of the cast slab by avoiding embrittlement of the cast slab and increasing its plasticity. Cold quenching is also used. In Patent Document 3, a lifting device for the secondary cooling water injection frame is installed to achieve dynamic control of the secondary cooling water cooling area and automatic adjustment of the water flow rate. Thus, the secondary cooling water does not directly cool the corners of the cast slab, avoiding corner cracks caused by extreme cold temperatures or temperature changes.

It can be seen from the above patent that there are two main technological routes to solving surface cracks in cast slabs. One route is to operate at elevated temperatures to increase the surface layer temperature of the cast slab to prevent phase transition and precipitation of the ferrite film and precipitation carbonitrides at grain boundaries. Thus, the continuous caster will be kept hot, which will have a great effect on the accuracy and service life of the equipment. Another route is to operate in cold conditions to avoid the third brittle area of the steel. Such a method requires a large amount of cooling water, which has a large impact on energy and the environment. Therefore, it is necessary to discover the structural state that can enhance the plasticity of the steel itself.

Chinese Patent Application Publication No. 110653352 Chinese Patent Application Publication No. 107695313 China Patent No. 105478704

It is an object of the present invention to provide a method for mitigating surface cracking of cast slabs by ferritic phases, which forms a high proportion of ferrite structure in the surface layer of the cast slab to mitigate surface cracking of the cast slab. . This method can not only control the surface layer structure of the cast slab, but also enhance the plasticity of the surface layer of the cast slab to reduce the surface cracks of the cast slab.

The technical solution of the present invention is as follows.

A method for mitigating surface cracking of a cast slab by ferrite phase, wherein in the process of continuous casting, the surface layer temperature of the cast slab is controlled to maintain the austenite-ferrite transition temperature for a long period of time, thereby A high proportion of ferrite phase is obtained such that the proportion of ferrite phase in the surface layer of the cast slab reaches 35% or more before the straightening point.

Specifically, the method of the present invention for mitigating surface cracks in cast slabs with ferritic phases involves casting slabs with a steel having a carbon content of 0<C≤0.25% in the production of continuous metallurgical casting. is formed, and the temperature is within the austenite-ferrite transition temperature by controlling the surface layer temperature of the cast slab so that the ratio of ferrite phase in the surface layer of the cast slab reaches 35% or more before the straightening point of the casting machine keep

In the process of continuous casting, the smelting steel solidifies in the mold to form a solid casting slab with a certain thickness, at which point the temperature of the casting slab is 1000-1250°C. The cast slab is then cooled and transported by support rollers while cooling. After bending, the cast slab enters the bend and is further transported to the straightening point of the casting machine for straightening. The cast slab then passes through the horizontal section of the caster and is removed from the caster for complete solidification.

In the present invention, control of the surface temperature of the cast slab is performed before the cast slab reaches the straightening point of the caster. Initially, the solidified cast slab 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 judged by the steel's continuous cooling characteristic CCT curve. The present invention is centered on controlling the surface layer temperature of the cast slab to maintain the temperature within the austenite-ferrite transition temperature so as to achieve the austenite to ferrite transformation. It was found through research that the proportion of ferritic phase in the surface layer of the cast slab should reach 35% or more before the straightening point of the caster. After satisfying this condition, the cast slab with the desired phase ratio enters the straightening point of the caster so that the slab can be straightened into a horizontal cast slab.

In the present invention, the exact time to cool the cast slab or keep the temperature within the austenite-ferrite transition temperature range is 35% ferrite fraction in the surface layer of the cast slab before the straightening point of the caster. % or more, it can be adjusted according to the production conditions. The surface layer of the cast slab refers to the part with a cast slab thickness of 10 mm or less. Controlling the ratio of the ferrite phase in the surface layer of the cast slab to 35% or more and controlling the temperature of the surface layer of the cast slab are both performed for a portion having a cast slab thickness of 10 mm or less. be done.

According to the method of the present invention for mitigating surface cracking of cast slabs by ferritic phase, the range of cooling rates of cast slabs to ferritic phase is determined by the continuous cooling characteristic CCT curve of the steel. The cast slab is cooled within a range of cooling rates such that the surface layer temperature of the cast slab is lowered to the range of the austenite-ferrite transition temperature and maintained within the above temperature range. Specifically, the surface layer temperature of the cast slab is controlled in the bend of the caster to keep it within the austenite-ferrite transition temperature.

Preferably, according to the continuous cooling temperature characteristic CCT curve of steel, the cooling rate range of the ferrite phase of steel is 3-0.05° C./sec. 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 to help keep the temperature of the cast slab near the phase transition temperature of ferrite to promote ferrite formation.

Preferably, the surface layer temperature of the cast slab is controlled so that the proportion of ferrite phase in the surface layer of the cast slab reaches 35% to 100%, preferably 35% to 75%, before the straightening point of the casting machine. be done. The proportion 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.

Preferably, for the steel of the present invention, research has 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 0.44 to 35 minutes. there is

According to the method for mitigating surface cracking of cast slabs by ferrite phase, the surface layer temperature of cast slabs is controlled by cooling water. Unlike the cooling described above, which reduces temperature, the cooling described herein is cooling that maintains temperature. To distinguish from the above-described cooling that lowers the temperature, the cooling that cools the surface layer temperature of the cast slab to the austenite-ferrite transition temperature is called primary cooling, and the cooling that maintains the temperature at the austenite-ferrite transition temperature is called primary cooling. 2 called cooling. It is also necessary to control the surface layer temperature of the cast slab so that it is within a predetermined temperature range after the surface layer of the cast slab is cooled to a predetermined temperature range. In the present invention, the surface temperature of the cast slab can be maintained by injecting cooling water (ie, the second cooling described above). This is because the center temperature of the cast slab is relatively high, and the heat at the center is transferred to the surface layer of the cast slab to raise the surface layer temperature of the cast slab if no cooling is provided. Therefore, the surface layer temperature of the cast slab can be controlled by cooling water.

The cooling water flow rate required to keep the surface layer temperature of the cast slab within the range of the 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, and the surface temperature of the casting slab is the molten steel temperature, the casting slab casting It is calculated according to the heat transfer principle in combination with continuous casting process conditions such as speed and cooling water. The model can calculate the required cooling water flow rates for various areas of the cast slab according to target temperatures set by the process to control the temperature of the cast slab. In use, simply set the temperature that needs to be maintained for the surface layer of the casting slab, the continuous casting and secondary cooling online model will calculate the flow rate of the water injection, and cool the surface layer temperature of the casting slab. Water is jetted so as to control within a predetermined range at .

After research, generally, the water injection flow rate of the cooling water for secondary cooling is properly controlled within the range of 0 to 0.87 L/kg (including 0 and 0.87), thus the casting The surface layer temperature of the slab can be controlled within the range of the austenite-ferrite transition temperature. The unit of water injection flow rate, L/kg, refers to the water injection volume required by a unit mass of steel.

According to the method for reducing the surface cracks of the cast slab by the ferrite phase, the surface layer of the cast slab is preferably maintained at the austenite-ferrite transition temperature for a long time by cooling water, and the surface layer temperature of the cast slab is reduced. In a controlled manner, secondary cooling nozzles are used to inject water onto the surface layer of the cast slab for cooling. Specifically, with the present invention, uniform cooling can be achieved with a small water volume (0-0.62 L/kg).

Alternatively, a low primary cooling rate is sometimes required to keep the surface layer of the cast slab at the austenite-ferrite transition temperature for an extended period of time, so injection cooling other than water may be employed, i.e. Air cooling may be maintained, also called dry cooling. Similarly, dry cooling can be used where a low secondary cooling rate is required to keep the surface layer temperature of the cast slab within the austenite-ferrite transition temperature. Specifically, during continuous casting and application of the secondary cooling online model, when a low secondary cooling rate is required, the model reduces the water injection flow rate to reduce the surface layer temperature of the cast slab. To avoid. The model also uses air cooling to maintain the surface temperature of the cast slab when the temperature drops too quickly. When the surface layer temperature of the cast slab is controlled by air cooling, the corresponding relatively high temperatures readily occur due to the fact that no cooling water is used to cool the support rollers of the bends of the caster. Therefore, there is a need to cool the support rollers in the bend of the casting machine. In the present disclosure, the support rollers of the bend of the caster are internally cooled. Specifically, cooling water may be supplied to the inside of the support roller to control the surface temperature of the support roller to 550° C. or less so as to prevent breakage of the flexure. Here the bend is also referred to as bend.

According to the method of the present invention, cooling water is used to cool the cast slab prior to the straightening section of the caster so that the surface temperature of the cast slab remains constant around the austenite-ferrite transition temperature for an extended period of time. Control surface layer temperature. In order to better control the surface layer temperature of the casting slab with cooling water, a secondary cooling nozzle with good atomization and uniform jetting is required. There is a particular need for a nozzle capable of uniform jet cooling under low water volume conditions. In some practical cases, for dry cooling, i.e. continuous casting, injection other than secondary cooling water is required to keep the casting slab at a constant temperature. In this case, the flexure support rollers require good internal cooling so as to avoid damage to the flexure support rollers and bearings 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 on-line and in real time. Many casters are now equipped with continuous casting online control models. With this model, the amount of cooling water required for the surface layer temperature of the cast slab can be constructed. By keeping the temperature constant for a long period of time, a ferrite layer is formed on the surface layer of the cast slab. The plasticity of the surface layer is enhanced, which reduces cracks that occur on the inner surface of the cast slab due to tensile stresses in the straightening.

In the continuous casting process, the surface layer temperature of the cast slab after exiting 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. Steel has relatively high plasticity in the single phase case and will not initiate cracks. However, since the cast slab is cooled by the cooling water at the curved portion, the temperature of the surface layer structure of the cast slab is continuously lowered. When the temperature reaches the phase transition temperature, the transformation of austenite to proeutectoid ferrite in the cast slab belongs to diffusion type phase transition. At relatively low cooling rates, proeutectoid ferrite first nucleates at the primary austenite grain boundaries and grows at the grain boundaries. After continuous cooling, particulate ferrite begins to nucleate. At that moment, relatively coarse proeutectoid ferrite films are formed at the primary austenite grain boundaries. As 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, with gradual propagation of the cracks at a later stage. During straightening of cast slabs, if the cast slab has a low proportion of ferrite in its structure, i.e. less than 35%, the proeutectoid ferrite film will easily develop stress concentrations and cracks, whereas ferrite is greater than 35%, no stress concentration occurs, thus avoiding cracks. Both thermodynamic and mechanical factors, namely temperature and duration, affect the rate of ferrite precipitation. The phase transition temperatures at various cooling rates are determined according to the continuous cooling characteristic CCT curve of the steel. Generally, ferrite can be formed in steel within a cooling rate of 3-0.05°C/sec and a temperature range of 900°C to 600°C. If the surface layer of the cast slab is controlled near the phase transition temperature for a long time, a large amount of ferrite will form in the surface layer of the cast slab. The retention time of the present invention is 0.44-35 minutes. When the ferrite proportion exceeds 35%, the structural plasticity of the cast slab becomes significantly higher, thus avoiding cracks. Therefore, after the cast slab has passed through the bend, i.e. the curve, weak cooling is performed in a cooling regime with a cooling rate of less than 3°C/s to reduce the surface layer temperature of the cast slab to the austenite-ferrite transition temperature. Keep constant within the range and maintain this temperature until the correction. Thus, a high-fraction ferrite phase is formed in the surface layer of the cast slab instead of a low-fraction ferrite film only at the austenite grain boundaries. Cast slabs with a high percentage ferrite layer do not face cracks caused by stress concentrations 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 around the austenite-ferrite transition temperature for an extended period of time. There are two main points. One is to keep the temperature for a long time and the other is to control the temperature around the phase transition temperature.

The present invention has the following beneficial technical effects.

The method of the present invention uses a caster curvature such that the surface layer temperature of the cast slab remains constant around the phase transition temperature of the steel over time and numerous ferritic phases form on the surface of the cast slab throughout the process. Control the surface layer temperature of the cast slab at the part. When a cast slab with a high proportion of ferrite phase passes through the straightening section of the casting machine, a large number of ferrite phases are found in the structure, so that the tensile stress on the inner surface of the cast slab is not concentrated on the grain boundary, and the grain boundary is It does not split, thus avoiding surface cracking of the cast slab. This technique is very useful in enhancing the surface plasticity of cast slabs, while mitigating surface cracks in cast slabs and enhancing the surface quality of the product.

4 is a schematic diagram showing the microstructure in the surface layer of the cast slab of Comparative Example 1. FIG. FIG. 4 is a schematic diagram showing the microstructure in the surface layer of the cast slab of Embodiment 3; 4 is a continuous casting cooling characteristic CCT curve of the steel of Embodiment 1. FIG.

In the following the invention is further described in combination with embodiments. Those skilled in the art should understand that the embodiments are examples only and are not intended to be limitations on the present invention.

The execution process of the method of the invention is described in detail below.

First, to obtain a range of primary cooling rates for ferrite-forming steels, ie, 3-0.05° C./sec, and austenite/ferrite transition temperatures, ie, 900° C. to 600° C., at various cooling rates. A continuous cooling characteristic CCT curve for the steel is tried or calculated. After cooling, the temperature is held between 900° C. and 600° C. for 0.44-35 minutes to complete the process before the correction point.

As shown in FIG. 2, the CCT curves of the steel in Embodiment 1 below show the evolution of the steel structure at several cooling rates and the relationship between temperature and time. The austenite-ferrite transition temperature at a given cooling rate and the time to form a given proportion of ferrite can be obtained from the figure. In FIG. 2, the X-axis of the diagram represents cooling time and the Y-axis represents temperature, while the multiple curves in the diagram, which resemble a parabola, represent cooling rates. The area marked F in the figure is the area where ferrite is formed. The area marked P in the figure represents perlite. The area marked B in the figure represents bainite. The cooling rate included in area F is the range of cooling rates that can form ferrite. According to the CCT curve of FIG. 2, the austenite-ferrite transition temperature of 560-620° C. is also determined. Regarding the holding time of the austenite-ferrite transition temperature in Embodiment 1, research has determined that a holding time of 0.44 to 35 minutes ensures that the ratio of ferrite phase on the surface of the cast slab reaches 35% or more. obtained in the present invention by

Embodiment 1: Steel 1 contains 0.08% C, 0.14% Si, 1.69% Mn, 0.41% Cr, 0.02% Mo and optionally iron and Contains impurities. According to continuous cooling characteristics, the cooling rate range in which ferrite can be formed is less than 0.1° C./sec. This embodiment employs a cooling rate of 0.1° C./sec. 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 over 35% ferrite can be formed in the surface layer of the cast slab. This process is completed before the correction point. After production, it can be determined that more than 35% ferrite was formed by observation and calculation of the cast slab specimens with a metallurgical microscope.

Embodiment 2: Steel 2 contains 0.16% C, 0.07% Si, 1.04% Mn, 0.88% Cr, 0.02% Ti and optionally iron and Contains impurities. According to 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, 3° C./s. , the cooling rate is 630°C. In Embodiment 2, cooling is performed at a cooling rate of 0.2° C./sec, and then the surface layer temperature of the cast slab is maintained at 720° C. for 10 minutes. This process is completed before the correction point. After production, it can be determined that more than 35% ferrite was formed by observation and calculation of the cast slab specimens with a metallurgical microscope.

Embodiment 3: Steel 3 contains 0.077% C, 0.09% Si, 1.45% Mn, 0.03% Cr, 0.01% Mo and optionally iron and Contains impurities. According to continuous cooling characteristics, the cooling rate range in which ferrite can be formed is 0.1-3° C./sec. 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./sec and then the surface layer temperature of the cast slab is held at 780° C. for 7.22 minutes. This process is completed before the correction point. After production, it can be determined that more than 35% ferrite was formed by observation and calculation of cast slab specimens viewed with a metallurgical microscope.

Embodiment 4: Steel 4 contains 0.09% C, 0.17% Si, 0.83% Mn, 0.02% Cr, as well as iron and unavoidable impurities. According to continuous cooling characteristics, the cooling rate range in which ferrite can be formed is 0.1-3° C./sec. 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./sec and then the surface layer temperature of the cast slab is held at 820° C. for 5.67 minutes. This process is completed before the correction point. After production, it can be determined that over 35% ferrite was formed by observation and calculation of cast slab specimens.

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, among other things, iron and inevitable impurities. Casting is carried out by conventional processes. The cast slab is cooled after exiting the mold. The surface temperature of the cast slab at the exit of the mold is 1200°C. The surface temperature of the cast slab is gradually lowered 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 precipitates ferrite 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 locations where ferrite precipitates at the austenite grain boundaries.

Although only combinations of cooling rates, phase transition temperatures and times for four steel grades are listed above, combinations of other process parameters for other steel grades are not excluded.

1A and 1B show the cast slabs after cooling after the cast slabs of steel in Comparative Example 1 and Embodiment 3 have passed the correction point, and the percentage of surface microstructure of the cast slabs is measured. It can be seen from the figure that the surface microstructure of the cast slab in Comparative Example 1 is predominantly austenitic with a ferrite content of only 8%. Therefore, stress concentration occurs easily in ferrite, and cracks are formed under the action of external force. In embodiment 3, the cast slab is held at 780°C with a cooling rate of 0.3°C/sec and held for 7.22 minutes. When the cast slab enters the correction point, 95% of the ferrite forms on the surface of the cast slab. In such a structure, stress concentrations due to external forces do not occur, and since ferrite has good plasticity, cracks do not form.

Of course, those skilled in the art should recognize that the above embodiments are only used to illustrate the invention and do not limit the invention. All modifications and variations made to the above-described embodiments within the scope of the essential spirit of the present invention shall be included in the scope of the claims of the present invention.

Claims (10)

A method for mitigating surface cracks in a cast slab by means of a ferrite phase in the production of continuous metallurgical casting, wherein the temperature of the surface layer of the cast slab is controlled to maintain the austenite-ferrite transition temperature for a long period of time in a casting machine. obtaining a high proportion of ferrite phase such that the proportion of said ferrite phase in the surface layer of said cast slab reaches 35% or more before the correction point of . The cast slab is formed by a steel with a carbon content of 0<C≦0.25% and the proportion of the ferrite phase in the surface layer of the cast slab is 35% before the straightening point of the caster. The surface layer temperature of the cast slab is controlled to keep the temperature within the range of the austenite-ferrite transition temperature so that the cast slab enters the correction point of the casting machine after reaching the above. 2. The method of claim 1 for mitigating surface cracks with a ferrite phase. The range of cooling rates for transforming the cast slab of steel to the ferrite phase is determined by the continuous cooling characteristic CCT curve of the steel, and the surface layer temperature of the cast slab is up to the range of the austenite-ferrite transition temperature. Cooling the cast slab within the cooling rate range such that the surface layer temperature of the cast slab is controlled at the bend of the caster after being lowered, mitigating surface cracking of the cast slab by ferrite phases. 3. The method of claim 1 for doing. The surface layer temperature of the cast slab is controlled by the bending section of the casting machine, and the cooling rate of the ferrite phase of the steel determined by the continuous cooling characteristic CCT curve of the steel is 3 to 0.05° C./sec. 2. A method according to claim 1, for mitigating surface cracks in cast slabs with ferritic phases. The surface layer temperature of the cast slab is controlled by the bending part of the casting machine, and the cooling rate of the ferrite phase of the steel determined by the continuous cooling characteristic CCT curve of the steel is 3 to 0.1 ° C./sec. 2. A method according to claim 1, for mitigating surface cracks in cast slabs with ferritic phases. The cast slab, wherein the surface layer temperature of the cast slab is controlled such that the ratio of the ferrite phase in the surface layer of the cast slab reaches 35% to 75% before the straightening point of the caster. 2. A method according to claim 1 for mitigating surface cracks in a ferrite phase. 2. The method of claim 1 for mitigating surface cracking of cast slabs by ferrite phase, wherein said surface layer of said cast slab is maintained at an austenite-ferrite transition temperature of 900° C. to 600° C. for 0.44 to 35 minutes. Method. wherein the surface layer temperature of the cast slab is controlled by cooling water, and the amount of the cooling water required for the surface layer temperature of the cast slab is calculated by a continuous casting online model, the surface cracks of the cast slab are controlled by the ferrite phase. 8. A method as claimed in any one of claims 1 to 7 for mitigating. of the cast slab, wherein the surface layer of the cast slab is maintained at the austenite-ferrite transition temperature for an extended period of time, and secondary cooling nozzles with good injection characteristics are used to achieve uniform cooling with low water flow rates. 9. A method according to claim 8 for mitigating surface cracks with a ferrite phase. The surface layer of the cast slab is maintained at the austenite-ferrite transition temperature for a long period of time by jet cooling other than water, and the support rollers of the curved portion of the casting machine are cooled from the inside to reduce the surface temperature of the support rollers. 8. A method according to any one of claims 1 to 7 for mitigating surface cracks in cast slabs by means of ferritic phases, controlled below 550<0>C to prevent breakage of said bends.

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