Continuous casting method and method for manufacturing steel products

Abstract

To provide a production method of a continuous cast strip for cooling a steel cast strip comprising a large amount of Mn without generating cracks on its surface or inside.SOLUTION: In a method for producing a continuous cast strip, the continuous cast strip comprising C: 0.40% or less, Mn: 2.5% or more and 13% or less by mass% and satisfying a component composition in which bainite is a main phase is cooled with a surface temperature of 700°C or more and a residence time t1 of less than 10h after starting cooling. A residence time t2 (h) at a surface temperature of less than 700°C and 500°C or more is cooled below a predetermined time in relation to a ferrite-pearlite carbon equivalent. It is cooled so that a residence time t3 (h) at a surface temperature of less than 500°C and 300°C or more is within a predetermined time range in relation to the bainite carbon equivalent. A surface temperature in the center of the continuous cast strip in a width direction is less than 300°C and 150°C or more, or an average cooling rate vc4 is 30°C / h or more until charging into a heating furnace.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a method for producing a continuously cast slab that prevents cracks during cooling, and more particularly to a method for producing a high-strength steel slab with a high Mn content, and a method for producing a steel product using the steel slab as a raw material. [Background technology]

[0002] In recent years, in response to growing global demands for reducing CO2 emissions, there has been a strong demand for structural steel products with higher strength in order to reduce the amount of steel used and make products lighter. On the other hand, properties such as high formability and high toughness are required to maintain formability and design in the manufacturing process of the final product and to improve manufacturing efficiency. Some of these properties are contrary to the strength of steel, so there is an increasing demand for the development of high-performance steel that combines multiple properties.

[0003] To achieve these unique properties, it is inevitable to add elements such as Si, Mn, Cr, and Mo to the steel, making it highly alloyed. This results in complex microstructures being created during the manufacturing process of steel products. The higher the process, the more difficult the steel becomes to manufacture, or the more unstable the manufacturing process becomes, leading to various quality problems.

[0004] For example, when alloying elements such as Si, Mn, Cr, and Mo are added to steel's composition, cracks are more likely to occur on the surface or inside the slab during the cooling process during the continuous casting process, in which molten steel solidifies and cools into a slab. Internal cracks are a major problem. Cracks must be removed by grinding or scarfing. This inspection and removal process requires significant manpower and costs, increasing production costs. Deep cracks within the slab cannot be completely removed, resulting in the disposal of the slab itself. Furthermore, slabs containing such defects can cause scavenging defects in the product when they are transported to downstream processes such as rolling, resulting in reduced production yields. Furthermore, this can lead to serious equipment problems due to slab or coil fractures. Such cracks in slabs caused by high-alloy steels are due to the decrease in ductility and toughness of steel as its strength increases. It is generally believed that cracks easily occur due to thermal stresses generated in the slab. As a countermeasure, various methods have been proposed to suppress the excessive stress generated during the cooling process of the cast slab.

[0005] For example, Patent Document 1 discloses a method for preventing cracks during continuous casting of high-tensile steel slabs by sandwiching the slab between two other slabs, thereby reducing the cooling rate of the slab surface during the cooling process to a certain level or less, and performing slow cooling. This slow cooling measure is said to suppress the occurrence of cracks in the embrittlement region of the slab.

[0006] Furthermore, Patent Document 2 discloses a method of reducing the cooling rate of a slab to a predetermined value or less depending on the size of the initial crack inside the slab, which is said to avoid bainite transformation and martensitic transformation, suppress the increase in stress caused by the transformation of the slab, and prevent the occurrence of cracks.

[0007] Furthermore, Patent Document 3 discloses a method for suppressing the generation of excessive stress by controlling the cooling rate between 700 and 500°C when cooling a steel slab containing Si. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] Japanese Patent Application Laid-Open No. 2007-083274 [Patent Document 2] Japanese Patent Application Publication No. 2020-139210 [Patent Document 3] Japanese Patent Application Publication No. 2019-167560 Summary of the Invention [Problem to be solved by the invention]

[0009] However, the conventional technology has the following problems. The techniques disclosed in Patent Documents 1 to 3 were all generally effective for general steels with medium alloy contents. However, when their application to crack suppression in higher alloy steels was investigated, it was found that not only could they not suppress cracking in cast slabs, but they could actually promote cracking.

[0010] In low-Mn steels, such as those with a Mn content of less than 2.5% by mass, as primarily shown in the examples of Patent Documents 1 and 3, a ferrite-pearlite structure containing a large proportion of soft ferrite can be obtained, even when various cooling rates are used, such as by cooling a single slab or by stacking the slab and then cooling it using a cooling cover. Therefore, the difference in structure due to the cooling method is small, and slow cooling is advantageous in terms of thermal and transformation stress. However, in high-Mn steels with a Mn content of 2.5% by mass or more, the hardenability is so high that the precipitation of soft ferrite phase is suppressed within the above-mentioned slab cooling range, and low-temperature transformation phases precipitate instead. In other words, slow cooling results in a structure containing a small amount of soft ferrite and high-strength low-temperature transformation phases, which actually increases the risk of cracking. While extremely slow cooling using a heating furnace or other device can produce a structure with a large proportion of soft phases with almost no low-temperature transformation phases, this is not practical in terms of heating costs and production lead time.

[0011] In particular, Mn is added in large quantities to recent high-alloy steels because it inexpensively increases the strength of steel. However, it is an element that requires careful consideration regarding its effect on toughness. This is because an increase in Mn content reduces the Charpy impact value and increases the ductile-brittle transition temperature, increasing the risk of so-called slab-rest cracking. Furthermore, an increase in Mn content lowers the bainite transformation temperature and shifts the ferrite-pearlite nose to the long-term side. This increases the likelihood of low-temperature transformation phases appearing in the steel structure, which can lead to increased stress during transformation. None of the inventions disclosed in Patent Documents 1 to 3 suppress slab-rest cracking in compositions containing a large amount of Mn.

[0012] The present invention has been made in view of the above circumstances, and aims to provide a method for producing a continuously cast slab of high alloy steel, particularly a steel containing a large amount of Mn, which can be cooled without generating cracks on the surface or inside the slab, and a method for producing a steel product, such as a thin coil, a thick plate, or a steel bar, made from such a slab, without generating fractures or scab defects in the final product. [Means for solving the problem]

[0013] The inventors have conducted extensive research into the causes and prevention of cracking in cast slabs of high-alloy steels, particularly those containing a large amount of manganese. As a result, they have discovered the following: They observed the microstructure around cracks in slowly cooled slabs of high-alloy steel. They found that even under extremely slow cooling conditions, such as in slowly cooled slabs, high-alloy steels, especially those containing a large amount of manganese, exhibit a significantly higher amount of low-temperature transformation phases. They also found that the transformation fraction of soft phases, such as ferrite and pearlite, which are the main phases in general steels, is significantly reduced or even eliminated. When a slab of such a steel is slowly cooled as in the conventional technique, very small amounts of soft phases are formed only near prior austenite grain boundaries. This results in a significant difference in strength between the soft phase and the surrounding hard phase. Under these conditions, when thermal stresses and transformation stresses act on the steel slab, deformation strain is concentrated only in the soft phases, increasing the likelihood of cracking. Furthermore, in the conventional technique, slow cooling minimizes thermal stresses and transformation stresses to prevent cracking, even when the slab matrix is ​​highly susceptible to cracking. However, in the case of high-alloy steels such as those described above, the phase fraction of low-temperature transformation phases is very high, and therefore the generation of excessive transformation stress is unavoidable compared to general steels. Therefore, it was found that relying solely on slow cooling reduces the effect of reducing thermal cracking. It was found that for such high-alloy steels, it is preferable to intentionally control the cooling rate of the slab and accelerate it. In other words, the formation of soft phases is suppressed so that a fine, strong bainite structure becomes the main phase. It was then discovered that it is preferable to appropriately control the cooling history depending on the composition, and this led to the invention.

[0014] The present invention has been completed based on the above findings and further investigations. That is, the gist and configuration of the present invention are as follows. 1. By mass%, C: 0.40% or less, Mn: 2.5% or more and 13% or less, Optionally, one or more selected from Si: 0.10% or more and 2.50% or less, P: 0.100% or less, S: 0.0200% or less, N: 0.0100% or less, sol.Al: 0.100% or less, and O: 0.0100% or less, Further, optionally, Ti: 0.200% or less, Nb: 0.200% or less, V: 0.200% or less, Ta: 0.10% or less, W: 0.10% or less, B: 0.0100% or less, Cr: 1.00% or less, Mo: 1.00% or less, Ni: 1.00% or less, Co: 1.00% or less, Cu: 1.00% or less, Sn: 0.200% or less, Sb: 0.200% or less, A continuously cast slab containing one or more elements selected from Ca: 0.0100% or less, Mg: 0.0100% or less, REM: 0.0100% or less, Zr: 0.100% or less, Te: 0.100% or less, Hf: 0.10% or less, and Bi: 0.200% or less, the composition of which satisfies the following formulas (1) to (4), with the balance being Fe and unavoidable impurities: a first cooling step in which the continuous cast slab is cooled for a residence time t1 of less than 10 hours after the start of cooling, at which the surface temperature of the continuous cast slab in the width direction at the center of the slab is 700°C or higher; a second cooling step of cooling the continuous cast slab so that the residence time t2 (h) at a surface temperature of 500°C or higher but lower than 700°C at the width direction center of the slab satisfies the following formula (5); a third cooling step of cooling the continuous cast slab so that the residence time t3 (h) at a surface temperature of 300°C or higher but lower than 500°C at the width direction center of the slab satisfies the following formula (6): a fourth cooling step in which the surface temperature at the center of the width direction of the continuous cast slab is cooled to 150°C or higher but lower than 300°C, or the average cooling rate vc4 is 30°C / h or less until the slab is charged into a heating furnace; A method for producing a continuous cast slab, comprising: Ce,fp=C+Si / 24+Mn / 24+P-Al / 50+(Cu+Ni) / 40+Cr / 6+Mo+V / 14+Nb / 2+10B (1) Ce,b=C+Si / 24+Mn / 6+P-Al / 50+(Cu+Ni) / 40+Cr / 6+Mo+V / 14+1.5Nb+10B (2) Ce,b≧0.60 (3) Ce,b≧5×Ce,fp / 3+0.1 (4) t2≦120×(ln(Ce,fp)+1.25) (5) 10×(ln(Ce,b)+0.5)≦t3<200×(ln(Ce,b)+0.5) (6) Here, the element symbols in formulas (1) and (2) represent the content of each element in mass %. Using the continuous cast slab produced by the method described in 2.1 as a raw material, Through hot rolling, Optionally, by cold rolling Further, optionally by heat treatment, including annealing, Furthermore, optionally, by performing a surface treatment, A steel manufacturing method for producing steel products. [Effects of the Invention]

[0015] The continuous cast slab manufacturing method and steel manufacturing method of the present invention can provide a continuously cast slab that does not crack during the cooling process, even for high alloy steel, particularly steel containing a large amount of Mn. Using such a slab as a raw material, it is possible to reduce breakage and equipment damage during rolling and other processes. In addition, it is possible to suppress the occurrence of scabs during the steel product manufacturing process and produce high alloy steel products with excellent surface quality, which is industrially useful. DETAILED DESCRIPTION OF THE INVENTION

[0016] Hereinafter, embodiments of the present invention will be described in detail. The following embodiments are merely examples of steel compositions and methods for realizing the technical concept of the present invention, and are not intended to limit the configuration to those described below. In other words, the technical concept of the present invention can be modified in various ways within the technical scope described in the claims.

[0017] <Composition of steel slabs> First, the appropriate range of the elemental composition suitable for the steel slab (continuously cast slab) of this embodiment and the reasons for limiting it will be described. In the following description, "%" representing the content of the elemental elements in the steel means "% by mass" unless otherwise specified.

[0018] [C: 0.40% or less] C is an important element that increases the strength of high-alloy steel. However, in high-alloy steel, it not only reduces ductility and increases the risk of cracking, but also promotes the formation of pearlite near prior austenite grain boundaries, potentially increasing the possibility of thermal cracking. Therefore, adding large amounts of C is undesirable. If the C content exceeds 0.40%, the formation of pearlite and carbides at prior austenite grain boundaries cannot be fully suppressed. Therefore, the C content is set to 0.40% or less. There is no lower limit for the C content. However, a C content of less than 0.0005% will increase refining costs and alloy costs due to the use of low-carbon raw materials. Preferably, it is set to 0.0005% or more. Preferably, it is set to 0.35% or less. More preferably, it is set to 0.10% or more. More preferably, it is set to 0.30% or less.

[0019] [Mn:2.5% or more and 13% or less] Mn is necessary to inexpensively increase the strength of high-alloy steels, improve hardenability, and stabilize retained austenite in the microstructure of final products to improve ductility. To achieve high strength, improved hardenability, and improved ductility, a Mn content of 2.5% or more is required. Note that cast slabs with Mn contents less than 2.5% produce a ferrite-pearlite structure with a high concentration of soft ferrite over a wide range of cooling rates. Therefore, the cooling method has little effect on the microstructure. High-Mn steel cast slabs with Mn contents of 2.5% or more by mass exhibit excellent hardenability, requiring appropriate microstructural control through the cooling conditions described below. On the other hand, if the Mn content exceeds 13%, the steel becomes austenite single-phase steel, known as TWIP steel, even at room temperature. While TWIP steels offer excellent strength and ductility, they are expensive and suffer from significantly poor galvanic and weldable properties. Therefore, TWIP steels are undesirable due to their limited applicability in a wide range of applications. Furthermore, TWIP steels have a single-phase austenite structure with coarse grains. As a result, segregation of P, S, etc. to the grain boundaries becomes extreme. This embrittles the grain boundaries, making the slab surface more susceptible to cracking, which may significantly increase the production yield and processing costs. Therefore, the Mn content is set to a range of 2.5% to 13%. Preferably, it is set to 3.0% or more. Preferably, it is set to 11% or less. More preferably, it is set to 3.5% or more. More preferably, it is set to 9.0% or less. Furthermore, it is set to 4.0% or more.

[0020] The continuously cast slab of this embodiment has the above-mentioned chemical composition, with the remainder consisting of Fe and unavoidable impurities. It is produced under specified cooling conditions, taking into consideration the influence of each component on the microstructure, as described below. Insofar as this is the case, the slab may contain one or more elements selected from the group consisting of 0.10% to 2.50% Si, 0.100% or less P, 0.0200% or less S, 0.0100% or less N, 0.100% or less sol. Al, and 0.0100% or less O, taking other properties into consideration. Examples of unavoidable impurities include Zn, Pb, and As. A total content of 0.100% or less of these impurities is permitted.

[0021] [Si: 0.10% to 2.50%] Si is preferably added to ensure retained austenite in the annealing process. In addition, it is a preferred additive element because it contributes to high strength through solid solution strengthening. For this reason, it is preferable to add 0.10% or more. On the other hand, if added in excess of 2.50%, not only will the effect saturate, but there is also the risk of strong scale forming on the surface of the hot-rolled sheet. This may deteriorate the appearance and pickling properties, so the upper limit is preferably set to 2.50% or less. Therefore, the Si content is preferably set in the range of 0.10 to 2.50%, more preferably 0.50% or more, more preferably 2.0% or less, even more preferably 1.00% or more, and even more preferably 1.80% or less.

[0022] [P:0.100% or less] P segregates at prior austenite grain boundaries, embrittling the grain boundaries and potentially causing cracks in the cast slab. Therefore, the P content is preferably 0.100% or less. There is no particular lower limit for the P content, but since P is a solid solution strengthening element and can increase the strength of steel, it is preferably 0.001% or more. Therefore, the P content is preferably 0.100% or less, more preferably 0.001% or more, and even more preferably 0.070% or less.

[0023] [S:0.0200% or less] S exists as sulfide and is an element that causes embrittlement of cast slabs. Therefore, the S content is preferably 0.0200% or less. There is no particular lower limit for the S content, but due to constraints on production technology, it is preferably 0.0001% or more. Therefore, the S content is preferably 0.0200% or less, more preferably 0.0001% or more, and even more preferably 0.0050% or less.

[0024] [N:0.0100% or less] N exists as a nitride and is an element that causes embrittlement of the cast slab. Therefore, the N content is preferably 0.0100% or less. Although there is no particular lower limit for the N content, due to constraints on production technology, the N content is preferably 0.0001% or more. Therefore, the N content is preferably 0.0100% or less, more preferably 0.0001% or more, and even more preferably 0.0050% or less.

[0025] [sol.Al: 0.100% or less] Al is an element that inhibits the formation of carbides during cooling of a slab and promotes the formation of retained austenite, thereby affecting the fraction of retained austenite in the slab. If the sol. Al content exceeds 0.100%, the slab may become embrittled. Therefore, the sol. Al content is preferably 0.100% or less. For deoxidation, it is more preferable to add 0.005% or more of sol. Al. It is even more preferable to add 0.080% or less of sol. Al. It is even more preferable to add 0.010% or more of sol. Al.

[0026] [O:0.0100% or less] O exists as an oxide and is an element that causes embrittlement of the cast slab. Therefore, the O content is preferably 0.0100% or less. Although there is no particular lower limit for the O content, due to constraints on production technology, the O content is preferably 0.0001% or more. Therefore, the O content is preferably 0.0100% or less, more preferably 0.0001% or more, and even more preferably 0.0050% or less.

[0027] The continuously cast slab of this embodiment is for use as a high-strength steel and, in addition to the above-mentioned chemical composition, further contains Ti: 0.200% or less, Nb: 0.200% or less, V: 0.200% or less, Ta: 0.10% or less, W: 0.10% or less, B: 0.0100% or less, Cr: 1.00% or less, Mo: 1.00% or less, Ni: 1.00% or less, Co: 1.00% or less, and Cu: 1.00% or less. In addition, at least one element selected from Sn: 0.200% or less, Sb: 0.200% or less, Ca: 0.0100% or less, Mg: 0.0100% or less, REM: 0.0100% or less, Zr: 0.100% or less, Te: 0.100% or less, Hf: 0.10% or less, and Bi: 0.200% or less may be contained alone or in combination of two or more thereof.

[0028] [Ti: 0.200% or less, Nb: 0.200% or less, and V: 0.200% or less] If the Ti, Nb, and V contents are each 0.200% or less, large amounts of coarse precipitates and inclusions are not formed, and the toughness of the cast slab is not reduced. Therefore, the Ti, Nb, and V contents are preferably each 0.200% or less. While there are no particular lower limits for the Ti, Nb, and V contents, the Ti, Nb, and V contents are more preferably 0.001% or more because they increase the strength of the steel by forming fine carbides, nitrides, or carbonitrides during hot rolling or continuous annealing. Therefore, when Ti, Nb, and V are contained, their contents should each be 0.200% or less, more preferably 0.001% or more, and even more preferably 0.100% or less.

[0029] [Ta: 0.10% or less and W: 0.10% or less] If the Ta and W contents are each 0.10% or less, large amounts of coarse precipitates and inclusions are not formed, and the toughness of the cast slab is not reduced. Therefore, the Ta and W contents are preferably each 0.10% or less. Although there are no particular lower limits for the Ta and W contents, the Ta and W contents are more preferably 0.01% or more because they increase the strength of the steel by forming fine carbides, nitrides, or carbonitrides during hot rolling or continuous annealing. Therefore, when Ta and W are contained, their contents should each be 0.10% or less, more preferably 0.01% or more, and even more preferably 0.08% or less.

[0030] [B:0.0100% or less] If B is 0.0100% or less, it does not affect the toughness of the cast slab. Therefore, the B content is preferably 0.0100% or less. Although there is no particular lower limit for the B content, since B is an element that segregates to austenite grain boundaries during hot rolling and annealing and improves hardenability, the B content is more preferably 0.0003% or more. Therefore, if B is contained, its content is 0.0100% or less, more preferably 0.0003% or more, and even more preferably 0.0080% or less.

[0031] [Cr: 1.00% or less, Mo: 1.00% or less, and Ni: 1.00% or less] If Cr, Mo, and Ni are each 1.00% or less, the amount of coarse precipitates and inclusions will not increase, and the toughness of the cast slab will not decrease. Therefore, the Cr, Mo, and Ni contents are preferably each 1.00% or less. Although there are no particular lower limits for the Cr, Mo, and Ni contents, since these elements improve hardenability, it is more preferable that the Cr, Mo, and Ni contents be 0.01% or more. Therefore, when Cr, Mo, and Ni are contained, their contents should each be 1.00% or less, more preferably 0.01% or more, and even more preferably 0.80% or less.

[0032] [Co:1.00% or less] If Co is 1.00% or less, the amount of coarse precipitates and inclusions does not increase, and the toughness of the cast slab does not decrease. Therefore, the Co content is preferably 1.00% or less. There is no particular lower limit for the Co content, but since Co is an element that improves hardenability, the Co content is more preferably 0.001% or more. Therefore, if Co is contained, its content should be 1.00% or less, more preferably 0.001% or more, and even more preferably 0.80% or less.

[0033] [Cu:1.00% or less] If Cu is 1.00% or less, the amount of coarse precipitates and inclusions will not increase, and the toughness of the cast slab will not decrease. Therefore, the Cu content is preferably 1.00% or less. Although there is no particular lower limit for the Cu content, since Cu is an element that improves hardenability, the Cu content is more preferably 0.01% or more. Therefore, if Cu is contained, its content should be 1.00% or less. More preferably, it should be 0.01% or more. Even more preferably, it should be 0.80% or less.

[0034] [Sn:0.200% or less] Sn does not affect the toughness of the cast slab if its content is 0.200% or less. Therefore, the Sn content is preferably 0.200% or less. Although there is no particular lower limit for the Sn content, since Sn is an element that improves hardenability and generally improves corrosion resistance, the Sn content is more preferably 0.001% or more. Therefore, if Sn is contained, its content should be 0.200% or less, more preferably 0.001% or more, and even more preferably 0.100% or less.

[0035] [Sb:0.200% or less] If Sb is 0.200% or less, the amount of coarse precipitates and inclusions does not increase, and the toughness of the cast slab does not decrease. Therefore, the Sb content is preferably 0.200% or less. Although there is no particular lower limit for the Sb content, since Sb is an element that suppresses decarburization and enables the strength of the steel to be adjusted, the Sb content is more preferably 0.001% or more. Therefore, if Sb is contained, its content should be 0.200% or less. More preferably, it should be 0.001% or more. Even more preferably, it should be 0.100% or less.

[0036] [Ca: 0.0100% or less, Mg: 0.0100% or less, and REM: 0.0100% or less] If the content of Ca, Mg, and REM is 0.0100% or less, the amount of coarse precipitates and inclusions will not increase and the toughness of the cast slab will not decrease. Therefore, the Ca, Mg, and REM contents are preferably 0.0100% or less. Although there are no particular lower limits for the Ca, Mg, and REM contents, since these elements spheroidize the shape of nitrides and sulfides and improve the toughness of the cast slab, the Ca, Mg, and REM contents are more preferably 0.0005% or more. Therefore, when Ca, Mg, and REM are contained, their contents should each be 0.0100% or less, more preferably 0.0005% or more, and even more preferably 0.0050% or less.

[0037] [Zr: 0.100% or less and Te: 0.100% or less] If Zr and Te are each 0.100% or less, the amount of coarse precipitates and inclusions does not increase, and the toughness of the cast slab does not decrease. Therefore, the Zr and Te contents are preferably 0.100% or less. Although there are no particular lower limits for the Zr and Te contents, since these elements spheroidize the shape of nitrides and sulfides and improve the toughness of the cast slab, it is more preferable that the Zr and Te contents be 0.001% or more. Therefore, if Zr and Te are contained, their contents should be 0.100% or less, more preferably 0.001% or more, and even more preferably 0.080% or less.

[0038] [Hf:0.10% or less] If Hf is 0.10% or less, the amount of coarse precipitates and inclusions does not increase, and the toughness of the cast slab does not decrease. Therefore, the Hf content is preferably 0.10% or less. Although there is no particular lower limit for the Hf content, since Hf is an element that spheroidizes the shape of nitrides and sulfides and improves the ultimate deformability of the steel, the Hf content is more preferably 0.01% or more. Therefore, if Hf is contained, its content should be 0.10% or less. More preferably, it should be 0.01% or more. Even more preferably, it should be 0.08% or less.

[0039] [Bi:0.200% or less] If Bi is 0.200% or less, the amount of coarse precipitates and inclusions does not increase, and the toughness of the cast slab does not decrease. Therefore, the Bi content is preferably 0.200% or less. Although there is no particular lower limit for the Bi content, since Bi is an element that reduces segregation, the Bi content is more preferably 0.001% or more. Therefore, if Bi is contained, its content should be 0.200% or less, more preferably 0.001% or more, and even more preferably 0.100% or less.

[0040] In addition, when the content of each of the above-mentioned Ti, Nb, V, Ta, W, B, Cr, Mo, Ni, Co, Cu, Sn, Sb, Ca, Mg, REM, Zr, Te, Hf, and Bi is less than the preferable lower limit value, the effect of the present invention is not impaired, and therefore these elements are included as unavoidable impurities.

[0041] Next, we will explain the effect of the above-mentioned steel composition on cracking of slabs. The higher the alloy content of the steel, the more likely it is to produce low-temperature transformation phases, increasing the risk of thermal cracking. The effect of alloying elements on transformation behavior varies for each composition. Therefore, we investigated the conditions under which the phase fraction of low-temperature transformation phases exceeds that of soft phases, and the risk of thermal cracking increases as the steel is cooled more slowly. As a result, we found that this can be predicted in the form of the carbon equivalent equation below. Equation (1) below represents the carbon equivalent Ce,fp when the microstructure becomes ferrite and pearlite. Equation (2) below represents the carbon equivalent Ce,b when the microstructure becomes bainite. The element symbols in equations (1) and (2) represent the content of each element in mass%. Ce,fp=C+Si / 24+Mn / 24+P-Al / 50+(Cu+Ni) / 40+Cr / 6+Mo+V / 14+Nb / 2+10B (1) Ce,b=C+Si / 24+Mn / 6+P-Al / 50+(Cu+Ni) / 40+Cr / 6+Mo+V / 14+1.5Nb+10B (2)

[0042] It has been found that in a composition system in which the carbon equivalent of bainite, Ce,b, is 0.60 or more, simply slow cooling the slab after continuous casting increases the risk of thermal cracking. Therefore, it is necessary to control the cooling conditions appropriately. On the other hand, if the cooling conditions of this embodiment are applied to a steel with a composition in which the carbon equivalent of bainite, Ce,b, is less than 0.60, there is a risk of thermal cracking. Therefore, the carbon equivalent of bainite, Ce,b, is set to 0.60 or more, as shown in the following formula (3). Ce,b≧0.60 (3)

[0043] In addition, the balance between the rate of bainite formation and the rate of ferrite and pearlite formation must also be considered when determining the condition for the low-temperature transformation phase to exceed the soft phase. If the following formula (4) is not satisfied, that is, if the carbon equivalent of bainite, Ce,b, is smaller than (5 × Ce,fp / 3 + 0.1), the amounts of ferrite and pearlite formed will be dominant. In this case, increasing the cooling rate of the slab after continuous casting may result in thermal cracking of the slab. Therefore, the chemical composition of the steel slab must be within a range that satisfies the following formula (4). Ce,b≧5×Ce,fp / 3+0.1 (4)

[0044] <Method of manufacturing steel slabs> Next, the manufacturing conditions for the steel slab (continuously cast slab) will be described. Steel having the above-mentioned composition range can be refined and cast in accordance with conventional methods. It is sufficient to cast a slab having a rectangular cross section that is free from significant temperature unevenness, vertical cracks, horizontal cracks, and internal cracks, which have not been tolerated in the past, at the outlet of a continuous casting machine. Examples of steel slabs include slabs for producing thin steel plates and thick steel plates, and blooms and billets for producing steel pipes, shaped steel, and steel bars.

[0045] If the high-temperature slab discharged from the outlet of the continuous casting machine is, for example, a slab, cooling in the slab yard begins when one or more slabs are stacked and placed in a stationary state, as is the case with conventional methods. The temperature history of the slab from that point onward is defined in this embodiment as it cools down to room temperature or until it is charged into a hot strip heating furnace. It is assumed that temperature variations occurring on the slab surface due to temporary slab relocation and re-stacking during the cooling process will be immediately averaged out within the slab once the slab is re-stacking completed and placed in a stationary state again. The instantaneous temperature variations are treated as a temperature history leveled out by taking a moving average over time. Furthermore, because the internal temperature of a slab is difficult to measure, the surface temperature of the slab is represented by the surface temperature at the center of the slab's long side (width center), which can be measured using a thermocouple or thermal camera. While the above description is based on an example in which the slab is a slab, the same applies to blooms and billets.

[0046] [First cooling process] If the slab is held at 700°C or higher for a long period of time after cooling begins, the slab may experience a temperature history similar to isothermal holding, resulting in the formation of ferrite-pearlite at the prior austenite grain boundaries. Therefore, even if the subsequent temperature history satisfies the criteria of this embodiment, thermal cracking may occur in the slab. Furthermore, when precipitation elements such as Nb, V, S, or Al are added, the precipitation of fine carbonitrides at the prior austenite grain boundaries may also cause a deterioration in toughness. Therefore, the residence time t1 from the start of cooling of the slab to 700°C is set to less than 10 hours. It is more preferably less than 8 hours, and even more preferably less than 6 hours. On the other hand, setting t1 to less than 1 hour requires rapid cooling equipment. Furthermore, because uniform cooling throughout the slab is difficult, uneven cooling may make it difficult to control the slab structure. Therefore, t1 is preferably set to 1 hour or more.

[0047] [Second cooling process] The temperature range between 500°C and 700°C is where the ferrite-pearlite transformation nose protrudes. Because soft phases tend to form along prior austenite grain boundaries in this range, appropriate cooling conditions must be established. If the residence time t2 (h) between 500°C and 700°C is longer than {120 × (ln(Ce,fp) + 1.25)}(h), the amount of localized ferrite-pearlite formation increases, resulting in reduced toughness. Therefore, cooling is performed so that the residence time t2 satisfies the following formula (5). From the perspective of controlling the slab structure, the lower limit of the residence time t2 is not a problem. However, because slabs are austenite single phase with low thermal conductivity, uneven cooling can cause unnecessary thermal strain due to temperature inconsistencies, potentially resulting in the risk of thermal cracking. To prevent this, the lower limit of the residence time t2 is preferably {11 × (ln(Ce,fp) + 1.25)}(h). t2≦120×(ln(Ce,fp)+1.25) (5)

[0048] [Third cooling process] Because the bainite transformation nose exists in the range of 300°C or higher but below 500°C, appropriate conditions are required to ensure the formation of fine bainite, particularly lower bainite and tempered bainite. However, if the residence time t3 (h) at 300°C or higher but below 500°C is set to {200 × (ln(Ce,b) + 0.5)}(h) or greater, relatively oriented upper bainite forms, increasing the number of fracture surfaces. At the same time, carbon partitioning to the untransformed austenite phase adjacent to the bainite increases the hardness difference with the bainite. This results in a deterioration of toughness and hot cracking of the slab. On the other hand, if the residence time t3 in this temperature range is too short, the formation of sufficiently fine bainite is not possible, resulting in an increase in the harder, less ductile martensite phase, resulting in a deterioration of toughness. Therefore, cooling should be performed so that the residence time t3 (h) at 300°C or higher but below 500°C satisfies the following equation (6): 10×(ln(Ce,b)+0.5)≦t3<200×(ln(Ce,b)+0.5) (6)

[0049] [Fourth cooling process] Cooling is performed at an average cooling rate vc4 of 30°C / h or less from 150°C to less than 300°C or until the slab is charged into the heating furnace. If the cooling rate is faster than this, the transformed structure will not self-temper sufficiently, which may result in slab cracking.

[0050] <Steel manufacturing method> The steel slabs produced by the above method can be reheated in a heating furnace or the like and then subjected to hot rolling. If the steel slabs are slabs, hot-rolled steel strips (thin steel coils) or thick steel plates can be produced. If the steel slabs are blooms or billets, seamless steel pipes, shaped steel, steel bars, etc. can be produced. The hot-rolled steel strips can be cold-rolled, with or without annealing, to produce cold-rolled steel strips.

[0051] The temperature at which steel slabs are heated in a heating furnace is selected based on their chemical composition and size. For example, it can be selected from the range of 1000 to 1300°C. In the production of hot-rolled steel strips, after rough rolling, finish rolling is performed at a finish rolling temperature of 750°C to 1000°C, and coiling is performed at a coiling temperature of room temperature to 750°C. Rapid cooling, sheet temperature maintenance, or air cooling may be performed between the completion of finish rolling and coiling. After coiling, the strip may be rolled at an elongation of 0.05% to 1.00%. Pickling may also be performed. Pickling may be performed once or multiple times. Cold-rolled steel strips can be produced by cold-rolling hot-rolled steel strips in a single pass or multiple passes, including intermediate annealing. The cold-rolled steel strips can be used as is, or they can be subjected to heat treatments, including annealing, or surface treatments such as galvanizing, to form products. Steel plates are produced by heating slabs in a heating furnace and then hot-rolling them, for example, with a reversing rolling mill. They can then be used as products, or cut and heat-treated as necessary to produce products, or processed into large-diameter steel pipes, steel structures, and other products. Continuously cast slabs such as blooms and billets can be heated in a heating furnace and hot-rolled to produce round slabs, which can then be processed into seamless steel pipes. Furthermore, billets can be heated in a heating furnace and caliber-rolled to produce sectional steel. The continuously cast slabs of the above embodiments have reduced internal cracks and thermal cracks during the cooling process of the slabs, improving the surface quality of each product and enabling the stable production of steel products without manufacturing defects. [Example]

[0052] [Evaluation method for cast cracks] The evaluation method for cracks in the slab was based on the penetrant testing specified in JIS Z 2343:2017, and the presence or absence of cracks was evaluated on the wide surface (length x width) and narrow surface (length x thickness) excluding the cut surface of the slab. After applying a developer, the appearance of the penetrant was visually checked to check for cracks and defects on the surface.

[0053] Example 1 The chemical composition of the steel used in the study is shown in Table 1, and steel slabs were produced by continuous casting. In Table 1, steel symbols A to N represent steel slabs, and steel symbol O represents billets. A "○" is indicated in the "Compliance with formula 4" column when formula (4) above is satisfied. An "×" is indicated when formula (4) above is not satisfied. Table 2 shows the cooling conditions for the steel slabs and their relationship to slab cracking. The slab cooling conditions were organized based on the temperature history of a thermocouple in contact with the central surface of the slab's long side. The slab cooling conditions were determined by transferring the slabs and combining the following conditions arbitrarily. (1) Allow to cool in one piece (one stick). (2) Stacking multiple cards (multiple sticks) of the same charge, (3) Cold, warm and hot billets, i.e., billets of different temperatures, may be placed on or sandwiched between billets of different charges; and (4) Cover the slab with a cooling cover.

[0054] Three slabs produced under the same conditions were subjected to the above-mentioned evaluation method for slab cracks. A condition in which none of the three slabs had any thermal cracks on their surfaces was indicated by a "◎." A condition in which fine cracks occurred and could be removed by localized grinding 2-3 mm deep was indicated by a "○." A condition in which an average of one to fewer than five thermal cracks occurred per slab, or in which one of the three slabs had thermal cracks that were insurmountable by grinding and required cutting or disposal, was indicated by a "△." A condition in which an average of five or more thermal cracks occurred per slab, or in which two or more of the three slabs had deep thermal cracks that could not be removed by grinding, was indicated by an "×."

[0055] [Table 1]

[0056] [Table 2]

[0057] From the results in Table 2, the steel slabs under conditions Nos. 1 to 3 and 9 to 16, which conformed to the chemical composition and cooling conditions of the present invention, and the billet under condition No. 21 were evaluated as ⊚ or ◯ for cracking. On the other hand, the steel slabs under conditions Nos. 17 to 20, which had chemical compositions that did not satisfy the ranges of the present invention, and the steel slabs under conditions Nos. 4 to 8, which had cooling conditions that did not satisfy the ranges of the present invention, suffered from unacceptable thermal cracking.

[0058] Example 2 The steel slabs manufactured under conditions No. 1 to 20 were reheated in a heating furnace and then subjected to hot rolling. The steel slabs under conditions No. 1 to 3 and 9 to 16 were hot rolled without any manufacturing problems. Surface defects such as scabs were also within the acceptable range. On the other hand, the steel slabs under conditions No. 4 to 8 and 17 to 20, in which thermal cracking occurred, broke during hot rolling or developed scabs at an unacceptable frequency.

[0059] Example 3 Steel pipes, shaped steel, and steel bars were manufactured using the billets manufactured under condition No. 21. As a result, no problems occurred with the surface quality of the steel pipes, shaped steel, and steel bars obtained. [Industrial Applicability]

[0060] The component composition and cooling method according to the present invention can provide a continuously cast slab for high-alloy, high-strength steel that is free from thermal cracking after casting, even if the slab contains a large amount of Mn. This can improve surface quality and prevent manufacturing problems during rolling, thereby improving productivity and providing industrial utility.

Claims

1. In mass%, C: 0.40% or less, Mn: 2.5% or more and 13% or less, Si: 0.10% or more and 2.50% or less, P: 0.100% or less, S: 0.0200% or less, N: 0.0100% or less, and sol. Al: 0.100% or less, optionally containing O: 0.0100% or less; Further, optionally, Ti: 0.200% or less, Nb: 0.200% or less, V: 0.200% or less, Ta: 0.10% or less, W: 0.10% or less, B: 0.0100% or less, Cr: 1.00% or less, Mo: 1.00% or less, Ni: 1.00% or less, Co: 1.00% or less, Cu: 1.00% or less, Sn: 0.200% or less, Sb: 0.200% or less, A continuously cast slab containing one or more elements selected from Ca: 0.0100% or less, Mg: 0.0100% or less, REM: 0.0100% or less, Zr: 0.100% or less, Te: 0.100% or less, Hf: 0.10% or less, and Bi: 0.200% or less, the composition of which satisfies the following formulas (1) to (4), with the balance being Fe and unavoidable impurities: a first cooling step in which the continuous cast slab is cooled for a residence time t1 of less than 10 hours after the start of cooling, at which the surface temperature of the continuous cast slab in the width direction center is 700°C or higher; a second cooling step of cooling the continuous cast slab so that the residence time t2 (h) at a surface temperature of 500°C or higher but lower than 700°C at the width direction center of the slab satisfies the following formula (5): a third cooling step of cooling the continuous cast slab so that the residence time t3 (h) at a surface temperature of 300°C or higher but lower than 500°C at the width direction center of the slab satisfies the following formula (6): a fourth cooling step of cooling the continuous cast slab so that the surface temperature at the width direction center thereof is 150°C or higher but lower than 300°C, or the average cooling rate vc4 is 30°C / h or lower until the slab is charged into a heating furnace; A method for producing a continuous cast slab, comprising: Ce, fp=C+Si / 24+Mn / 24+P-Al / 50+(Cu+Ni) / 40+Cr / 6+Mo+V / 14+Nb / 2+10B (1) Ce,b=C+Si / 24+Mn / 6+P-Al / 50+(Cu+Ni) / 40+Cr / 6+Mo+V / 14+1.5Nb+10B (2) Ce,b≧0.60 (3) Ce,b≧5×Ce,fp / 3+0.1 (4) t2≦120×(ln(Ce, fp)+1.25) (5) 10×(ln(Ce,b)+0.5)≦t3<200×(ln(Ce,b)+0.5) （６） Here, the element symbols in formulas (1) and (2) represent the content of each element in mass %.

2. Using the continuous cast slab produced by the method of claim 1 as a raw material, Through hot rolling, Optionally, by cold rolling Further, optionally by heat treatment, including annealing, Furthermore, optionally, by performing a surface treatment, Manufacturing steel products, manufacturing methods of steel products.

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