Method for producing continuously cast steel slab

Abstract

To disclose a technique capable of reducing the occurrence of transverse cracks in continuously cast steel slabs.SOLUTION: In a cooling process of a continuously cast steel slab having a predetermined chemical composition and cross-sectional area, when a representative temperature on a surface along a width direction and longitudinal direction of the continuously cast steel slab falls within a range of (A1-320)°C or higher and (A1-30)°C or lower, the temperature is raised to fall within a range of (T1+50)°C or higher and (A1+30)°C or lower, and thereafter, the continuously cast steel slab is air-cooled, where A1 is defined by the following formula: A1=723+29.1(%Si)-10.7(%Mn)+16.9(%Cr)-16.9(%Ni)+6.38(%W).SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] This application discloses a method for producing a continuous cast steel billet. [Background technology]

[0002] High-carbon steel, which is used as a material for products such as bearings, blades, tools, springs, ropes, and steel cords, has high strength but low toughness. It is known that cracks easily occur during the cooling process of a slab to room temperature after continuous casting. This type of cracking is called "delayed cracking." Generally, during the cooling and heating processes of a slab, thermal strain due to the temperature difference between the surface and the interior of the slab and transformation strain associated with transformation occur, resulting in stress. High stress in the slab can lead to cracks during the cooling process. For these reasons, conventional techniques control the cooling rate of the slab after continuous casting to cool it slowly and minimize the temperature difference between the surface and the interior of the slab, thereby suppressing cracks during the cooling process (e.g., Patent Documents 1 to 6). However, due to equipment constraints, such temperature control can be difficult, and there is also the risk of process congestion due to long cooling times. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Publication No. 2019-167560 [Patent Document 2] Japanese Patent Application Publication No. 2019-167559 [Patent Document 3] Japanese Patent Application Laid-Open No. 2007-083274 [Patent Document 4] Japanese Patent Publication No. 2023-047054 [Patent Document 5] Japanese Patent Application Publication No. 2020-139209 [Patent Document 6] Japanese Patent Application Publication No. 2020-139210 Summary of the Invention [Problem to be solved by the invention]

[0004] The present application discloses a new method for producing a continuous cast steel slab, which is capable of reducing the occurrence of placement cracks in the slab. [Means for solving the problem]

[0005] The present application discloses the following aspects as means for solving the above problems. <Aspect 1> A method for producing a steel continuous casting slab, comprising: The chemical composition of the continuous cast slab is, in mass%, C: 0.50~1.50%, Si: 0.10 to 2.50% Mn: 0.30~3.50% P: 0.200% or less, S: 0.020% or less, Al: 0.0003 to 0.8000%, N: 0.0100% or less, and O: 0.0100% or less, Contains The cross-sectional area of ​​the surface along the width direction and thickness direction of the continuous cast slab is 90,000 mm 2 More than 300000mm 2 is as follows: In the cooling process of the continuously cast slab, when the representative temperature on the surface along the width direction and length direction of the continuously cast slab reaches a temperature T1 (°C) represented by the following formula 1, the temperature is raised to a temperature T2 (°C) represented by the following formula 2, and then the continuously cast slab is allowed to cool. A method for producing continuous cast steel slabs. A1-320≦T1≦A1-30...Formula 1 T1+50≦T2≦A1+30...Formula 2 A1=723+29.1(%Si)-10.7(%Mn)+16.9(%Cr)-16.9(%Ni)+6.38(%W)...Equation 3 Here, (%Si) (%Mn) (%Cr) (%Ni) and (%W) are the respective contents (mass%) of Si, Mn, Cr, Ni and W in the chemical composition. <Aspect 2> A method for producing a continuous cast slab of the steel of aspect 1, comprising: The representative temperature on the surface along the width direction and the length direction of the slab 60 seconds after the completion of the temperature increase to the temperature T2 is the temperature T3 (°C) defined by the following formula 4: A method for producing continuous cast steel slabs. T2-50≦T3≦T2...Formula 4 <Aspect 3> A method for producing a continuous cast slab of the steel of aspect 1 or 2, After the temperature increase to the temperature T2 is completed, when the representative temperature on the surface along the width direction and the longitudinal direction of the continuous cast slab is 400 ° C. or higher, the continuous cast slab is stacked and allowed to cool. A method for producing continuous cast steel slabs. [Effects of the Invention]

[0006] According to the method of the present disclosure, it is possible to reduce the occurrence of placement cracks in continuously cast strands. [Brief explanation of the drawings]

[0007] [Figure 1] This shows the relationship between the "temperature difference from point A1 at T1" and the "temperature difference from point T1 at T2" and the stress state of the slab. DETAILED DESCRIPTION OF THE INVENTION

[0008] In conventional techniques, when continuously casting a slab having low toughness, such as high-carbon steel, the cooling rate of the slab after continuous casting is controlled to cool it slowly, minimizing the temperature difference between the surface and interior of the slab, thereby suppressing the occurrence of slab cracks. However, such temperature control can be difficult due to equipment limitations. The present inventors utilized thermal stress analysis (numerical calculations) to explore cooling conditions for the slab that can reduce slab cracks. As a result, they discovered that by increasing the temperature by a predetermined temperature during the cooling process of the slab after continuous casting when the surface temperatures of the slab's width and length directions reach a predetermined temperature range, a stress distribution that is less likely to cause slab cracks can be obtained, even if the slab is subsequently cooled to room temperature using equipment such as an annealing furnace without significantly slowing the cooling rate. Hereinafter, one embodiment of the method for producing a continuously cast slab of steel according to the present disclosure will be described.

[0009] In the method for producing a continuous cast steel slab disclosed herein, The chemical composition of the continuous cast slab is, in mass%, C: 0.50~1.50%, Si: 0.10 to 2.50% Mn: 0.30~3.50% P: 0.200% or less, S: 0.020% or less, Al: 0.0003 to 0.8000%, N: 0.0100% or less, and O: 0.0100% or less, Contains The cross-sectional area of ​​the surface along the width direction and thickness direction of the continuous cast slab is 90,000 mm 2 More than 300000mm 2 is as follows: During the cooling process of the continuously cast slab, when the representative temperature on the surface along the width direction and length direction of the continuously cast slab reaches temperature T1 (°C) as shown in the following formula 1, the temperature is raised to temperature T2 (°C) as shown in the following formula 2, and then the continuously cast slab is allowed to cool. A1-320≦T1≦A1-30...Formula 1 T1+50≦T2≦A1+30...Formula 2 A1=723+29.1(%Si)-10.7(%Mn)+16.9(%Cr)-16.9(%Ni)+6.38(%W)...Equation 3 Here, (%Si) (%Mn) (%Cr) (%Ni) and (%W) are the respective contents (mass%) of Si, Mn, Cr, Ni and W in the chemical composition.

[0010] 1. Shape of continuous cast slab In the method of the present disclosure, the continuously cast slab may be either a slab or a bloom. A slab is particularly preferred. The slab may have a width and a thickness in a cross-sectional shape perpendicular to the continuous casting direction, and a length in the continuous casting direction. When the slab is a slab, its width corresponds to the long side (corresponding to the long side of the mold) in the cross-sectional shape perpendicular to the continuous casting direction, and its thickness corresponds to the short side (corresponding to the short side of the mold) in the cross-sectional shape. Furthermore, when the slab is a bloom, its aspect ratio (length of long side / length of short side) is generally smaller than that of a slab. When the slab is a slab, its width may be, for example, 800 mm or more and 1500 mm or less, its thickness may be, for example, 100 mm or more and 300 mm or less, and its length may be, for example, 5 m or more and 10 m or less. When the cast piece is a bloom, its width may be, for example, 300 mm or more and 600 mm or less, its thickness may be, for example, 200 mm or more and 400 mm or less, and its length may be, for example, 5 m or more and 10 m or less.

[0011] In this embodiment, the cross-sectional area of ​​the surface along the width direction and thickness direction of the continuously cast slab is 90000 mm 2 More than 300000mm 2 If the cross-sectional area is too small, the problem of cracking of the cast slab in the first place is difficult to solve. If the cross-sectional area is too large, productivity decreases and costs increase. The cross-sectional area is 100,000 mm 2 or more than 110,000 mm 2 It is good to hit more than 290,000 mm 2 or less than 280,000 mm 2 It may be the following:

[0012] 2. Chemical composition of continuous cast slabs The method disclosed herein relates to a method for producing a continuously cast steel slab containing predetermined amounts of essential elements such as C and optionally containing other elements. The continuously cast slab may be used as a material for, for example, bearings, blades, tools, springs, ropes, and steel cords.

[0013] A continuously cast slab having a chemical composition according to an embodiment described below is made of high-carbon steel with a C content of 0.50% or more, which has low toughness and is prone to developing thermal cracks. However, according to the method disclosed herein, even in a continuously cast slab having such low toughness, the occurrence of thermal cracks can be reduced. Note that, in this specification, "%" in relation to a chemical composition means mass %. Furthermore, in this specification, unless otherwise specified, the symbol "to" indicating a numerical range is used to mean that the numerical values ​​before and after it are included as the lower and upper limits.

[0014] In this embodiment, the chemical composition of the continuously cast slab contains, in mass %, C: 0.50 to 1.50%, Si: 0.10 to 2.50%, Mn: 0.30 to 3.50%, P: 0.200% or less, S: 0.020% or less, Al: 0.0003 to 0.8000%, N: 0.0100% or less, and O: 0.0100% or less.

[0015] (C: 0.50 to 1.50%) C is the most fundamental element that affects not only the static strength of steel but also its fatigue strength, toughness, and ductility. If the C content is too low, the static strength and fatigue strength of the steel may be insufficient. In this embodiment, as described above, high-carbon steel, which is prone to staging cracking in cast slabs, is cast. In this regard, the C content is 0.50% or more. The C content may be 0.55% or more or 0.61% or more. On the other hand, if the C content is too high, the toughness of the steel will be excessively deteriorated, and staging cracking will not be prevented even if the temperature conditions of the present invention are met. In this regard, the C content is 1.50% or less. The C content may be 1.40% or less or 1.30% or less.

[0016] (Si: 0.10 to 2.50%) Si is an important element with a high strengthening ability. To obtain higher strength, the Si concentration should be increased. In this embodiment, the Si content is 0.10% or more. The Si content may be 0.15% or more or 0.20% or more. On the other hand, if the Si content is too high, there is a risk of deterioration in toughness and workability. In this regard, the Si content is 2.50% or less. The Si content may be 2.30% or less or 2.10% or less.

[0017] (Mn: 0.30 to 3.50%) Mn is an important element for improving hardenability and ensuring hardness deep into the steel material even when the cooling rate is insufficient. To obtain higher strength, the Mn concentration should be increased. Specifically, the Mn content is 0.30% or more. The Mn content may be 0.40% or more or 0.50% or more. On the other hand, if the Mn content is too high, there is a risk of deteriorating toughness and workability. In this regard, the Mn content is 3.50% or less. The Mn content may be 3.00% or less or 2.50% or less.

[0018] (P:0.200% or less) P is an element that reduces the ductility of steel, so the less P, the better. Excessive P content can lead to brittle fracture of the steel. In this regard, the P content is 0.200% or less. The P content may be 0.100% or less, 0.050% or less, or 0.020% or less. Meanwhile, the lower limit of the P content is not particularly limited. The P content may be 0% or more, or 0.001% or more.

[0019] (S:0.020% or less) S is an element that generates non-metallic inclusions such as MnS in steel, which reduces the ductility of the steel, and therefore the lower the S content, the better. In this regard, the S content is 0.020% or less. The S content may be 0.010% or less or 0.005% or less. On the other hand, the lower limit of the S content is not particularly limited. The S content may be 0% or more or 0.001% or more.

[0020] (Al: 0.0003 to 2.5000%) Al is an element that acts as a deoxidizer for steel. This effect is easily achieved when the Al content is 0.0003% or more. The Al content may be 0.0010% or more, or 0.0020% or more. On the other hand, excessive Al content may reduce the strength of the final product. In this regard, the Al content is 2.5000% or less. The Al content may be 2.0000% or less, 1.5000% or less, or 1.0000% or less.

[0021] (N:0.0100% or less) N is an element that forms coarse nitrides and reduces the workability of steel, so the lower the content, the better. The N content is 0.0100% or less, and may be 0.0070% or less, or 0.0050% or less, or may be 0% or more, 0.0001% or more, or 0.0010% or more.

[0022] (O:0.0100% or less) O is an element that can be mixed in during the manufacturing process, and the less O present, the better. However, reducing the O content to the minimum required refining time, resulting in reduced productivity. On the other hand, excessive O content can lead to the formation of coarse inclusions, which can reduce the toughness of the steel. In this regard, the O content is 0.0100% or less. The O content may be 0.0070% or less, 0.0050% or less, or 0.0030% or less. The O content may be 0% or more, 0.0005% or more, or 0.0010% or more.

[0023] In this embodiment, the continuously cast slab may contain, in addition to the above-mentioned basic elements, optional elements other than those mentioned above. The optional elements do not substantially affect the problem-solving mechanism of the method of the present disclosure. Since the optional elements may not be included, the lower limit is 0%. The chemical composition of the continuously cast slab may include, for example, in mass %, one or more elements selected from Ni: 0-1.00%, Cr: 0-2.00%, and W: 0-0.10%, and in addition thereto, may further include Mo: 0-1.500%, V: 0-0.500%, Ti: 0-0.500%, Nb: 0-0.500%, Co: 0-0.500%, B: 0-0.0100%, and Cu: 0-0.50 The cast slab may contain one or more elements selected from the group consisting of 0%, Te: 0-0.500%, Ta: 0-0.1000%, Hf: 0-0.0500%, Sn: 0-0.0500%, Sb: 0-0.0500%, As: 0-0.0500%, Mg: 0-0.0500%, Ca: 0-0.0500%, Zr: 0-0.0500%, Bi: 0-0.0500%, and REM: 0-0.0500%. Note that the types and contents of the optional elements listed above are merely examples, and the types and amounts of the optional elements that can be contained in the slab are not limited to those listed above. "REM" is a collective term for 17 elements: scandium (Sc), atomic number 21; yttrium (Y), atomic number 39; and the lanthanides lanthanum (La), atomic number 57, through lutetium (Lu), atomic number 71. The "REM content" is the total content of these elements.

[0024] 3. Cooling process of continuously cast slabs In the method of the present disclosure, the "surface along the width direction and longitudinal direction of the continuously cast slab" refers to the broad surface of the continuously cast slab (broad surface of the slab). In the method of the present disclosure, the "representative temperature" refers to the temperature at the center of the surface in the length direction of the slab and in the width direction of the slab. In the method of the present disclosure, during the cooling process of a continuously cast slab having the above-described cross-sectional area and chemical composition, it is important that, when the representative temperature of the continuously cast slab reaches temperature T1 (°C) as shown in Equation 1 above, the temperature is raised to temperature T2 (°C) as shown in Equation 2 above, and then the continuously cast slab is allowed to cool naturally. That is, during the cooling process, the temperature of the continuously cast slab is raised and then allowed to cool naturally. In the method of the present disclosure, by controlling the "representative temperature" during the cooling process of a continuously cast slab having the above-described cross-sectional area, a stress distribution that is less likely to cause ruptures can be obtained throughout the slab. In the method of the present disclosure, it is not necessary to specifically control the "surface temperature of the slab wide face other than the widthwise center" or the "internal temperature of the slab."

[0025] 3.1 Cooling to temperature T1 In the method disclosed herein, during the cooling process after continuous casting, the representative temperature at the surface of the broad face of the slab is cooled to temperature T1. Here, it is important that temperature T1 be (A1-320)°C or higher and (A1-30)°C or lower. If temperature T1 is too low or too high, it will be difficult to impart a predetermined stress distribution to the slab even if the slab is subsequently heated. Temperature T1 may be (A1-280)°C or higher, or (A1-240)°C or higher, or may be (A1-70)°C or lower, or (A1-110)°C or lower. Note that there are no particular limitations on the temperature history during the cooling process of the continuously cast slab until the representative temperature at the surface of the broad face of the slab reaches temperature T1.

[0026] 3.2 Heating to temperature T2 In the method disclosed herein, during the cooling process after continuous casting, the representative temperature at the surface of the broad face of the slab reaches temperature T1, and is then raised to temperature T2. It is important that temperature T2 be between (T1 + 50)°C and (A1 + 30)°C. By ensuring that temperature T2 after heating is within this temperature range, a desired stress distribution can be obtained in the slab after cooling is complete. Temperature T2 may be (T1 + 70)°C or higher, or (T1 + 90)°C or higher, or may be (A1 + 20)°C or lower, or (A1 + 10)°C or lower. During the cooling process of a continuously cast slab, there are no particular limitations on the rate at which the representative temperature at the surface of the broad face of the slab rises from temperature T1 to temperature T2.

[0027] In the method of the present disclosure, the method for raising the representative temperature at the surface of the broad face of the slab from temperature T1 to temperature T2 is not particularly limited. For example, a slab whose representative temperature at the surface of the broad face of the slab has reached temperature T1 may be subjected to heat treatment in a heating device such as a heating furnace to raise the representative temperature at the surface of the broad face of the slab to temperature T2. Alternatively, the surface layer of the broad face of the slab may be forcedly cooled to bring the representative temperature at the surface to temperature T1, and then the representative temperature at the surface of the broad face of the slab may be restored to temperature T2 using the internal heat of the slab.

[0028] In the present application, "A1" is calculated based on the following formula 3. In the formula 3, (%Si) (%Mn) (%Cr) (%Ni) and (%W) are the respective contents (mass%) of Si, Mn, Cr, Ni and W in the above-mentioned chemical composition. The formula 3 is based on the content described in "Masashi Maki, Steel and Microstructure Control: Its Principles and Methods, Uchida Rokakuho Co., Ltd., Publication Date: December 10, 2015." A1=723+29.1(%Si)-10.7(%Mn)+16.9(%Cr)-16.9(%Ni)+6.38(%W)...Equation 3

[0029] 3.3 Holding time after heating In the method of the present disclosure, sufficient effects can be obtained even if the slab is allowed to cool immediately after the temperature increase. However, according to the inventor's findings, if the temperature drop 60 seconds after the completion of the temperature increase is 50°C or less, the tensile stress generated in the center of the slab after cooling is completed can be further reduced, making it even less likely that slab cracks will occur. That is, in the method of the present disclosure, the representative temperature at the surface of the broad face of the slab 60 seconds after the completion of the temperature increase to temperature T2 may be set to temperature T3 (°C) defined by the following equation 4. T2-50≦T3≦T2...Formula 4

[0030] The temperature maintenance after the temperature increase can be performed, for example, by a heating device such as a heating furnace. That is, in the method disclosed herein, after the representative temperature on the surface of the broad face of the slab has reached temperature T1, the temperature of the slab may be increased by a heating device such as a heating furnace, and then the temperature of the slab may be maintained within the heating device. Note that in the method disclosed herein, the temperature may be further increased after the temperature increase. However, in such a case, the effect of preventing temperature cracks may be saturated, and costs may increase.

[0031] 3.4 Cooling According to the findings of the present inventors, if the continuously cast slab after the temperature increase is subjected to forced cooling (e.g., water cooling) at an extremely high cooling rate, the effect of the temperature increase is canceled. In this regard, in the method disclosed herein, it is important that the continuously cast slab is naturally cooled after the temperature increase. "Natural cooling" means that forced cooling such as water cooling is not performed.

[0032] The continuously cast slab after the temperature increase may be allowed to cool by itself. Alternatively, the continuously cast slab after the temperature increase may be allowed to cool after stacking multiple slabs, i.e., stacking. For example, after the temperature increase to temperature T2 is complete, the continuously cast slabs may be stacked and allowed to cool when the representative temperatures of the surfaces along the width and length of the slab are 400°C or higher. By stacking the continuously cast slabs and allowing them to cool in this way, the generation of finer defects in the slab can be suppressed. When stacking, it is preferable that the representative temperatures of the upper and lower slabs after the temperature increase be 300°C or higher. The upper limit of the number of continuously cast slabs to be stacked is not particularly limited, but from the viewpoints of productivity and safety, it is preferable to set the number to 10 or less. In this case, it is sufficient that at least one of the stacked slabs has been subjected to the temperature increase treatment.

[0033] 4.Effects As described above, according to the method of the present disclosure, by subjecting the slab to a predetermined temperature increase treatment and natural cooling at a predetermined stage in the cooling process of the continuously cast slab, it is possible to appropriately control the state of compressive plastic deformation and tensile plastic deformation occurring in the slab, thereby reducing the occurrence of slab-induced cracks. Furthermore, with the method of the present disclosure, there is no need to precisely control the cooling rate of the slab, as in the prior art. According to the method of the present disclosure, special equipment for cooling control is not required, and the degree of freedom in cooling conditions is increased compared to the prior art. [Example]

[0034] The following examples of the present invention are given, but the present invention is not limited to these examples. Various conditions can be adopted in the present invention as long as they do not deviate from the gist of the invention and the object is achieved.

[0035] 1. Experiment 1 (thermal stress analysis) A thermal stress analysis was performed on a slab (1.0%C-0.3%Si-0.3%Mn steel slab, 240 mm thick, 1000 mm wide, and 7500 mm long) after continuous casting to examine the effects of the starting temperature and temperature rise during cooling on the stress distribution in the slab after cooling to room temperature. The thermal stress analysis was performed using a conventional thermo-elastic-plastic analysis that takes phase transformation into account. The physical properties used in the analysis were measured using conventional testing methods using test specimens. The results are shown in Figure 1. In Figure 1, the starting temperature (T1) on the vertical axis is shown as the temperature difference from point A1, and the temperature after heating (T2) on the horizontal axis is shown as the temperature difference from the starting temperature (T1). Similar calculations were also performed on a slab of the same shape made of 0.2%C-0.3%Si-0.3%Mn steel, a steel type that does not develop cracks. Since slab cracking often originates in the center (both in thickness, width, and length) of the slab, and the tensile stress at the center is the largest in calculations, we focused on the stress at this location. In Figure 1, the tensile stress at the center of a 1.0%C-0.3%Si-0.3%Mn steel slab at room temperature is indicated by a "●" if it was less than 1.2 times the calculated value for a 0.2%C-0.3%Si-0.3%Mn steel slab without heating during cooling. Values ​​greater than 1.2 times the calculated value are indicated by an "×." The results in Figure 1 clearly demonstrate that increasing the temperature of slabs within a specified temperature range by a specified amount during the cooling process is effective in reducing the risk of slab cracking. Specifically, it was found that if the surface temperature of the center of the broad face of the slab during the cooling process is raised by 50°C or more when it reaches (A1-320)°C or higher and (A1-30)°C or lower, there is a high possibility that the occurrence of cracks in high-carbon steel slabs will be reduced. Furthermore, similar calculations were performed for steel types other than high-carbon steel, and it was found that the above effect is particularly noticeable with high-carbon steel.

[0036] 2. Experiment 2 2.1 Continuous casting conditions Molten steel having a predetermined chemical composition was used for continuous casting to produce a slab. Table 1 below shows the chemical composition of the slab produced by continuous casting. In the chemical composition shown in Table 1, the balance is Fe and impurities. Note that underlined values ​​in Table 1 indicate values ​​outside the range of the present invention. Continuous casting was performed using a slab casting mold having a thickness of 240 to 280 mm and a width of 1000 mm, or a bloom casting mold having a thickness of 210 to 300 mm and a width of 410 mm, and the casting speed was 0.65 to 1.8 m / min. The slab was then cut into lengths of 6200 to 9600 mm, and cooling was initiated. The surface temperatures of the slab's surface along the width and longitudinal directions, as well as the center of the slab's length, were taken as the representative temperature of the slab. Hereinafter, the term "slab temperature" refers to this representative temperature.

[0037] [Table 1]

[0038] 2.2 Cooling conditions 2.2.1 Examples 1 to 9 and Comparative Examples 1 to 8 Each continuously cast slab was heated and cooled under the conditions shown in Table 2 below. A radiation thermometer was used to measure the slab temperatures T1, T2, and T3. The slab at temperature T1 was inserted into a heating furnace set at the temperature shown in Table 2 and removed from the heating furnace after the time shown in Table 2 had elapsed. The slab temperature was measured immediately after removal from the heating furnace and designated as temperature T2. Sixty seconds after measuring T2, the slab temperature was measured and designated as temperature T3. After measuring temperature T3, the slab was cooled either as a single slab or stacked in multiple slabs. Forced cooling was performed for Comparative Example 7 using the method shown in Table 2, except for the slabs that were allowed to cool to room temperature. Note that "-" for the number of slabs stacked in Table 2 indicates that cooling was performed on a single slab without stacking. For stacking conditions, the number of slabs stacked, the stacking position (the number counted from the bottom) and the temperature at the start of stacking are also listed. Furthermore, when stacking, the representative temperatures of the upper and lower continuously cast slabs of the examples and comparative examples were 300°C or higher. Note that the underlined values ​​in Table 2 indicate values ​​outside the scope of the present invention.

[0039] 2.2.2 Examples 10 to 12, Comparative Examples 9 and 10 Each continuously cast slab was spray-cooled under the conditions shown in Table 3 below. A radiation thermometer was used to measure the slab temperature just before the start of water cooling, as well as the slab temperatures of T1, T2, and T3. Based on the slab temperature just before the start of water cooling shown in Table 3, the water temperature was set at 15°C and the water flow rate was set at 550 L / min m. 2 Spray water cooling was performed under the conditions for the time shown in Table 3. The slab temperature immediately after water cooling was stopped was designated T1. Thereafter, the slab temperature was continuously measured. The temperature at which the slab temperature at the surface rose due to internal heat and then began to drop was designated T2. Sixty seconds after T2 measurement, the slab temperature was measured again, and this temperature was designated T3. Thereafter, the slab was allowed to cool to room temperature without forced cooling. The stacking method and conditions in Table 3 are the same as those in Table 2. Underlined values ​​in Table 3 indicate values ​​outside the range of the present invention.

[0040] 2.3 Evaluation of cracking due to placement After cooling to room temperature, each slab was visually inspected for cracks on the top and bottom surfaces of the broad side of the slab for a 3-m section in the center of its length to confirm the presence of cracks. If even one crack 10 mm or longer was found, the slab was judged to have cracks. Taking into account the risk of cracks occurring in the reheating furnace or hot rolling stages, hot rolling was not performed and the slab was judged to have cracks before rolling. Furthermore, 1000 mm wide slabs that did not find any cracks 10 mm or longer through visual inspection were hot rolled under standard conditions to produce 2.5 mm thick steel plates. If a fracture occurred during hot plate rolling, including the reheating process, or if any visual defects were found during rolling, rolling was stopped and the slab was judged to have cracks before rolling. 410 mm wide slabs that did not find any cracks 10 mm or longer were bloomed under standard conditions to produce slabs with cross sections of 150 mm square. This billet was hot rolled into a steel bar with a diameter of 25 mm under standard conditions. If a break occurred during blooming and bar rolling, which include the heating process, or if a defect was found visually during rolling, the rolling was stopped and the product was judged to have a "delayed crack."

[0041] 2.4 Evaluation results The cooling conditions and the evaluation results of cracking during storage are shown in Tables 2 and 3 below. [Table 2] [Table 3]

[0042] As is clear from the results shown in Tables 2 and 3, in Comparative Examples 1 to 10, all of the cracks occurred during the aging process, whereas in Examples 1 to 12, no cracks occurred during the aging process. In Reference Example 1, the C content was less than 0.50%, and in Reference Example 2, the cross-sectional areas of the faces along the width direction and thickness direction of the continuously cast slab were 90,000 mm 2 Therefore, even if the temperature condition of the present invention was not satisfied, no cracking occurred.

[0043] 3. Summary From the above results, it can be said that the occurrence of placement cracks in the slab can be reduced by a method for cooling a continuously cast slab that satisfies the following requirements (I) to (III).

[0044] (I) The chemical composition of the continuously cast slab is, in mass%, C: 0.50~1.50%, Si: 0.10 to 2.50% Mn: 0.30~3.50% P: 0.200% or less, S: 0.020% or less, Al: 0.0003 to 0.8000%, N: 0.0100% or less, and O: 0.0100% or less, Contains:

[0045] (II) The cross-sectional area of ​​the surface along the width direction and thickness direction of the continuous cast slab is 90,000 mm 2 More than 300000mm 2 The following is the result.

[0046] (III) During the cooling process of a continuously cast slab, when the representative temperature of the surface along the width and length directions of the continuously cast slab reaches temperature T1 (°C) shown in Equation 1, the temperature is raised to temperature T2 (°C) shown in Equation 2, and then the continuously cast slab is allowed to cool. A1-320≦T1≦A1-30...Formula 1 T1+50≦T2≦A1+30...Formula 2 A1=723+29.1(%Si)-10.7(%Mn)+16.9(%Cr)-16.9(%Ni)+6.38(%W)...Equation 3

Claims

1. A method for producing a steel continuous casting slab, comprising: The chemical composition of the continuous cast slab is, in mass%, C: 0.50-1.50%, Si:0.10~2.50%, Mn: 0.30-3.50%, P: 0.200% or less, S: 0.020% or less, Al: 0.0003-0.8000%, N: 0.0100% or less, and O: 0.0100% or less, Contains The cross-sectional area of ​​the surface along the width direction and thickness direction of the continuous cast slab is 90,000 mm ２ More than 300000mm ２ is as follows: In the cooling process of the continuously cast slab, when the representative temperature on the surface along the width direction and the length direction of the continuously cast slab reaches a temperature T1 (°C) represented by the following formula 1, the temperature is raised to a temperature T2 (°C) represented by the following formula 2, and then the continuously cast slab is allowed to cool. A method for producing continuous cast steel slabs. A1-320≦T1≦A1-30...Formula 1 T1+50≦T2≦A1+30...Formula 2 A1=723+29.1(%Si)-10.7(%Mn)+16.9(%Cr)-16.9(%Ni)+6.38(%W)...Formula 3 Here, (%Si), (%Mn), (%Cr), (%Ni), and (%W) are the respective contents (mass%) of Si, Mn, Cr, Ni, and W in the chemical composition.

2. A method for producing a continuous cast steel slab according to claim 1, comprising: The representative temperature of the surface along the width direction and the length direction of the slab 60 seconds after the completion of the temperature increase to the temperature T2 is set to a temperature T3 (°C) defined by the following formula 4: A method for producing continuous cast steel slabs. T2-50≦T3≦T2...Formula 4

3. A method for producing a continuous cast slab of steel according to claim 1 or 2, After the temperature increase to the temperature T2 is completed, when the representative temperature of the surface of the continuously cast slab along the width direction and the longitudinal direction is 400°C or higher, the continuously cast slab is stacked and allowed to cool. A method for producing continuous cast steel slabs.

Images