Continuous casting method of steel

Abstract

To suppress the surface crack of a slab while suppressing addition of Ni to a steel containing Cu and Sn in continuous casting of the steel.SOLUTION: A continuous casting method for a steel that is a method for continuously casting a slab of a steel having a predetermined chemical composition using a continuous casting machine having a correction point includes: cooling a temperature of a surface of a slab drawn out from a mold to an Ar1 temperature or lower; raising the temperature of the surface of the slab cooled to the Ar1 temperature or lower, to an Ac3 temperature or higher; and setting the temperature of the surface of the slab raised to the Ac3 temperature or higher, to a temperature Tb°C, represented by the following conditions, or lower at the correction point. When [Cu]+4[Sn]≤0.15, Tb=1050. When 0.15<[Cu]+4[Sn]≤0.25, Tb=1000. When 0.25<[Cu]+4[Sn]≤0.35, Tb=950. When 0.35<[Cu]+4[Sn], Tb=900.SELECTED DRAWING: None

Description

Technical Field

[0001] This application discloses a method for continuous casting of steel.

[0002] In recent years, from the perspective of preventing global warming and other aspects, efforts to reduce CO2 have been actively carried out in various fields. In the steel industry, a steelmaking method that uses a large amount of scrap as a raw material for ironmaking has attracted attention, and technological development has been progressing. On the other hand, many scraps contain high concentrations of tramp elements such as Cu and Sn, and it is known that these elements are difficult to remove from molten steel.

[0003] Steel containing Cu tends to be inferior in hot workability. Therefore, when normal continuous casting conditions for steel are adopted during the continuous casting of steel containing Cu, cracks may occur on the surface of the slab. This is considered to be because when the steel is oxidized by oxygen in the atmosphere during continuous casting, liquid Cu is generated between the scale (iron oxide) and the base iron, penetrates into the grain boundaries of the steel, and reduces the interfacial strength (Non-Patent Document 1). In addition, since Sn lowers the solubility of Cu in steel and promotes the cracking phenomenon caused by Cu, the problem of surface cracking of the slab is likely to occur also for steel in which Sn and Cu coexist (Non-Patent Document 2).

[0004] This phenomenon is called surface hot shortness, and it is caused by the fact that Cu and Sn are difficult to be oxidized compared with Fe, so Cu and Sn are concentrated in the metallic state during the scale growth process, and the solid solubility of Cu in Fe is low. On the other hand, Cu and Sn are difficult to remove in the steel refining process. In order to solve the problem of surface cracking of the slab due to hot shortness, it is effective to prevent Cu and Sn from being mixed into the steel or to add Ni, which is an element that increases the solubility of Cu in the steel. In particular, in the current circular society where a large amount of scrap containing a large amount of Cu is used, the need to detoxify Cu by adding Ni is increasing. However, Ni is a rare and expensive element, and it can greatly change steel properties such as mechanical properties and hardenability. Therefore, there is great expectation for a detoxification technology for Cu and Sn that does not rely on Ni addition or can suppress the addition amount to an extremely small amount.

[0005] Under such circumstances, Patent Document 1 discloses a continuous casting method for preventing surface hot shortness of a slab. As a technique for preventing surface hot shortness of a slab, the inner surface shape of the mold near the molten steel surface has an inverse taper shape with an inverse taper value of 2 to 10% that widens downward in the slab withdrawal direction, and the inner surface shape of the mold below the inverse taper portion has a forward taper shape that narrows in the slab withdrawal direction, and the forward taper value is in the range of 0 to 1%. A mold is used, and a mold flux having a crystallization temperature of 900°C or lower or a property of not crystallizing is used, and the contact angle between the mold flux and the steel is 70 degrees or less.

[0006] Further, Patent Document 2 discloses a continuous casting method characterized by forming a coating layer of nickel oxide on the surface of a slab while supplying a mold flux containing Ni oxide.

[0007] However, all of these methods attempt to prevent oxidation of the slab surface layer by means of a mold flux. Depending on the type of continuous casting machine and the secondary cooling method, the adhesion situation of the mold flux to the slab surface is not stable, so the effects cannot be fully enjoyed.

[0008] Patent Document 3 proposes a method for preventing surface cracking of a continuously cast slab, characterized in that the surface of the continuously cast slab is cooled at a cooling rate of 300°C / s or more from a temperature range above the Ar3 transformation point to a temperature range below the Ar1 transformation point, and then the surface of the continuously cast slab is reheated to a temperature range above the Ar3 transformation point again.

[0009] In addition, Patent Document 4 discloses a method for continuously casting steel containing 0.1 to 2.0% of Cu or the like, the method including a series of steps: a step of rapidly cooling a slab withdrawn from a mold at a cooling rate of 3 to 20°C / s until the surface temperature of the slab reaches a rapid cooling temperature T1 (°C) that is equal to or lower than the Ar3 point; a step of raising the surface temperature of the slab at the rapid cooling temperature T1 to a reheating temperature T2 (°C) that is equal to or higher than the Ar3 point; a step of cooling the slab at the reheating temperature T2 until its surface temperature reaches a straightening temperature T3 (°C); and a step of performing straightening by applying a straightening strain amount ε (%) to the slab at the straightening temperature T3, wherein the temperatures T1 to T3 and the straightening strain amount ε satisfy a predetermined formula.

Prior Art Documents

Non-Patent Documents

[0010]

Non-Patent Document 1

Non-Patent Document 2

Patent Documents

[0011]

Patent Document 1

Patent Document 2

Patent Document 3

Patent Document 4

Summary of the Invention

Problems to be Solved by the Invention

[0012] In the prior art, there is room for improvement in suppressing surface cracking of a slab while suppressing the addition of Ni to steel during continuous casting of steel containing Cu or Sn.

Means for Solving the Problems

[0013] As one of the means for solving the above problems, the present application provides by mass percentage, C: 0.03% or more and 0.20% or less, Si: 0.01% or more and 1.00% or less, Mn: 0.10% or more and 2.50% or less, P: 0.040% or less, S: 0.030% or less, Cu: 0.05% or more and 0.50% or less, Sn: 0% or more and 0.025% or less, Ni: 0% or more and 0.05% or less, Al: 0.005% or more and 0.100% or less, Cr: 0% or more and 0.50% or less, Mo: 0% or more and 0.05% or less, and N: 0.0150% or less, A method for continuously casting a steel slab containing the above components and having the balance composed of Fe and impurities, using a continuous casting machine having a straightening point, comprising cooling the surface temperature of the slab withdrawn from the mold to a temperature below the Ar1 temperature, raising the surface temperature of the slab cooled to a temperature below the Ar1 temperature to a temperature above the Ac3 temperature, and at the straightening point, setting the surface temperature of the slab raised to a temperature above the Ac3 temperature to a temperature Tb °C or lower represented by the following conditions: A continuous casting method for steel, including is disclosed.

[0014] When [Cu] + 4[Sn] ≤ 0.15: Tb = 1050 When 0.15 < [Cu] + 4[Sn] ≤ 0.25: Tb = 1000 When 0.25 < [Cu] + 4[Sn] ≤ 0.35: Tb = 950 When 0.35 < [Cu] + 4[Sn]: Tb = 900 Here, [Cu] and [Sn] are the contents (mass %) of Cu and Sn in the steel.

[0015] In the method of the present disclosure, the steel contains, by mass%, Ti: 0% or more and 0.020% or less, V: 0% or more and 0.20% or less, Nb: 0% or more and 0.030% or less, Zr: 0% or more and 0.010% or less, Ca: 0% or more and 0.010% or less, Mg: 0% or more and 0.010% or less, REM: 0% or more and 0.010% or less, and B: 0% or more and 0.0040% or less, and may contain one or more of these.

Advantages of the Invention

[0016] According to the technology of the present disclosure, during continuous casting of steel containing Cu and Sn, it is easy to suppress surface cracking of the slab while suppressing the addition of Ni to the steel.

Brief Description of the Drawings

[0017]

Figure 1

Figure 2

Figure 3

Figure 4

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Figure 8

Mode for Carrying Out the Invention

[0018] The continuous casting method of the steel of the present disclosure will be described with reference to FIG. 1. In FIG. 1, for clarity, cooling spray nozzles and the like are omitted. The cooling spray nozzle is provided, for example, between support rolls from directly below the mold 10 to before reaching the straightening point 20, and can spray cooling water from both sides of the slab 1. In FIG. 1, a vertical bending type continuous casting machine 100 is illustrated, but the continuous casting method of the present disclosure is applicable to any continuous casting machine having a straightening point. For example, a curved type continuous casting machine may be used. Note that the “straightening point” refers to a point where strain is applied to correct the casting direction of the slab 1 from curved to horizontal. The straightening may be performed at a plurality of locations. Since the configuration of the continuous casting machine 100 including the mold 10, the straightening point 20, etc. may be the same as the conventionally known configuration, detailed description thereof is omitted here.

[0019] As shown in FIG. 1, the continuous casting method of the steel according to the present embodiment is, by mass%, C: 0.03% or more and 0.20% or less, Si: 0.01% or more and 1.00% or less, Mn: 0.10% or more and 2.50% or less, P: 0.040% or less, S: 0.030% or less, Cu: 0.05% or more and 0.50% or less, Sn: 0% or more and 0.025% or less, Ni: 0% or more and 0.05% or less, Al: 0.005% or more and 0.100% or less, Cr: 0% or more and 0.50% or less, Mo: 0% or more and 0.05% or less, and, N: less than 0.0150%, A method of continuously casting a steel slab 1 containing the above and having the balance composed of Fe and impurities using a continuous casting machine 100 having a straightening point 20, comprising: Cooling the surface temperature of the slab 1 withdrawn from the mold 10 to a temperature below the Ar1 temperature; Raising the surface temperature of the slab 1 cooled to a temperature below the Ar1 temperature to a temperature above the Ac3 temperature; and At the straightening point 20, making the surface temperature of the slab 1 raised to a temperature above the Ac3 temperature not higher than a temperature Tb °C represented by the following conditions: including:

[0020] When [Cu] + 4[Sn] ≤ 0.15: Tb = 1050 When 0.15 < [Cu] + 4[Sn] ≤ 0.25: Tb = 1000 When 0.25 < [Cu] + 4[Sn] ≤ 0.35: Tb = 950 When 0.35 < [Cu] + 4[Sn]: Tb = 900 Here, [Cu] and [Sn] are the contents (mass%) of Cu and Sn in the steel.

[0021] 1. Chemical composition of steel First, the chemical composition of the steel in the present embodiment will be described. In the present embodiment, the steel contains, by mass%, C: 0.03% or more and 0.20% or less, Si: 0.01% or more and 1.00% or less, Mn: 0.10% or more and 2.50% or less, P: 0.040% or less, S: 0.030% or less, Cu: 0.05% or more and 0.50% or less, Sn: 0% or more and 0.025% or less, Ni: 0% or more and 0.05% or less, Al: 0.005% or more and 0.100% or less, Cr: 0% or more and 0.50% or less, Mo: 0% or more and 0.05% or less, and N: 0.0150% or less.

[0022] (C: 0.03% or more and 0.20% or less) 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, no significant improvement in these properties will be seen, and it will lead to an increase in the cost of decarburization. In this regard, the C content may be 0.03% or more, or 0.05% or more. Also, if the C content is too high, the toughness of the steel may deteriorate. In this regard, the C content may be 0.20% or less, or 0.17% or less.

[0023] (Si: 0.01% or more and 1.00% or less) Si is an element that can increase the strength of steel by appropriate addition. When the Si content is 0.01% or more, such an effect is easily obtained. The Si content may be 0.05% or more, or 0.10% or more. On the other hand, if the Si content is too high, the toughness and workability of the steel may deteriorate. In this regard, the Si content may be 1.00% or less, 0.80% or less, 0.60% or less, 0.40% or less, 0.30% or less, or 0.20% or less.

[0024] (Mn: 0.10% or more and 2.50% or less) Mn is an element that can increase the strength of steel by appropriate addition, similar to Si. When the Mn content is 0.10% or more, such an effect is easily obtained. The Mn content may be 0.25% or more, 0.50% or more, or 0.75% or more. On the other hand, if the Mn content is too high, the toughness and workability of the steel may deteriorate. In this regard, the Mn content may be 2.50% or less, 2.00% or less, 1.75% or less, or 1.50% or less.

[0025] (P: 0.040% or less) P is an element that promotes the occurrence of surface cracks in the slab during continuous casting. That is, when the P content is too high, it becomes difficult to suppress slab cracks. The P content may be 0.040% or less, may be 0.030% or less, may be 0.020% or less, or may be 0.015% or less. The lower limit of the P content is not particularly limited and may be 0%, may be more than 0%, may be 0.001% or more, or may be 0.002% or more.

[0026] (S: 0.030% or less) Similar to P, S is also an element that promotes the occurrence of surface cracks in the slab during continuous casting. In addition, S is an element that deteriorates the bend formability of steel, and a smaller amount is preferred. The S content may be 0.030% or less, may be 0.020% or less, or may be 0.010% or less. The lower limit of the S content is not particularly limited and may be 0%, may be more than 0%, may be 0.001% or more, or may be 0.002% or more.

[0027] (Cu: 0.05% or more and 0.50% or less) If the Cu content is too low, the amount of the liquid phase generated by the oxidation of the steel material becomes sufficiently small, and cracks due to embrittlement are less likely to occur, that is, the problems to be solved are less likely to occur. The method according to this embodiment is a technique for suppressing surface cracks in the slab during continuous casting while suppressing the addition of Ni to steel containing a certain amount of Cu. In this regard, the Cu content may be 0.05% or more, may be 0.07% or more, or may be 0.09% or more. On the other hand, if the Cu content is too high, it will have an adverse effect on the material properties of the steel. In this regard, the Cu content may be 0.50% or less, may be 0.45% or less, or may be 0.40% or less.

[0028] (Sn: 0% or more and 0.025% or less) It is desirable not to mix Sn as much as possible in order to significantly lower the liquid-phase stabilization temperature and widen the embrittlement temperature range. If the Sn content is too high, hot shortness cracking is likely to occur during continuous casting, and a large amount of Ni is required to suppress this. In this regard, the Sn content may be 0.025% or less, 0.022% or less, or 0.020% or less. The lower limit of the Sn content is not particularly limited and may be 0%, more than 0%, 0.001% or more, or 0.002% or more.

[0029] (Ni: 0% or more and 0.05% or less) Although Ni is known to have the effect of suppressing hot shortness cracking caused by Cu or Sn, it is an expensive element, and it is desirable to keep its addition amount as small as possible. If the Cu content and Sn content are within the above ranges, it is possible to sufficiently suppress surface cracking of the slab by controlling the surface temperature of the slab described later. Therefore, in this embodiment, the addition of Ni is not particularly required. The Ni content is sufficient at a concentration mixed from scrap, for example, 0.05% or less, and may be 0.04% or less, or 0.03% or less. The lower limit of the Ni content is not particularly limited and may be 0%, more than 0%, 0.01% or more.

[0030] (Al: 0.005% or more and 0.100% or less) Al is an element used for deoxidation purposes. If the Al content is too low, it is difficult to obtain the effect of deoxidation. In this regard, the Al content may be 0.005% or more, 0.007% or more, or 0.010% or more. On the other hand, if the Al content is too high, problems such as nozzle clogging during casting or deterioration of the steel performance due to oxide inclusions remaining in the steel are likely to occur. In this regard, the Al content may be 0.100% or less, 0.075% or less, or 0.050% or less.

[0031] (Cr: 0% or more and 0.50% or less) Cr is an element useful for increasing the strength of steel. However, if the Cr content is too high, not only does the effect almost saturate, leading to an increase in cost, but also the Ar1 temperature decreases, making it difficult to refine the surface structure of the slab. In this regard, the Cr content may be 0.50% or less, 0.40% or less, 0.30% or less, or 0.20% or less. The lower limit of the Cr content is not particularly limited and may be 0%, more than 0%, or 0.01% or more.

[0032] (Mo: 0% or more and 0.05% or less) Mo is an element that enhances hardenability. If the Mo content is too high, similar to Cr, the Ar1 temperature decreases, making it difficult to refine the surface structure of the slab. In this regard, the Mo content may be 0.05% or less, 0.04% or less, 0.03% or less, or 0.02% or less. The lower limit of the Mo content is not particularly limited and may be 0%, more than 0%, or 0.01% or more.

[0033] (N: 0.0150% or less) N is an element that affects the mechanical properties of steel, reduces hot ductility, and is also a factor causing surface defects during continuous casting or hot rolling. N is mainly removed in the degassing process of secondary refining. The N content may be 0%, but from the perspective of suppressing refining costs, it may also be more than 0%, 0.0001% or more, 0.0010% or more, or 0.0040% or more. On the other hand, if the N content is too high, it causes coarsening of nitride-based inclusions and reduces the fatigue strength of the steel. In this regard, the N content may be 0.0150% or less. Also, from the perspective of the cleanliness of the steel, the N content may be 0.0080% or less.

[0034] In this embodiment, in order to exhibit the characteristics required for the product, the steel may further contain one or more of the following elements. Since all of these elements sensitize embrittlement in the III region, a particularly large effect can be obtained by the method according to this embodiment during the casting of the steel containing these elements. That is, in this embodiment, the steel may contain one or more of Ti: 0% or more and 0.020% or less, V: 0% or more and 0.20% or less, and Nb: 0% or more and 0.030% or less, by mass%.

[0035] Further, in this embodiment, as optional elements other than the above, the above steel may further contain one or more of Zr: 0% or more and 0.010% or less, Ca: 0% or more and 0.010% or less, Mg: 0% or more and 0.010% or less, REM: 0% or more and 0.010% or less, and B: 0% or more and 0.0040% or less.

[0036] (Ti: 0% or more and 0.020% or less) Ti not only has the effect of deoxidation similar to Al, but also forms nitrides with high thermal stability, and can refine the structure of the steel in the heating furnace. On the other hand, if the Ti content is too high, the amount of nitride-based precipitates increases, and the crack sensitivity due to embrittlement in the III region increases. Also, there is a risk of frequent nozzle clogging due to oxides during casting. In this regard, the Ti content may be 0.020% or less, and may be 0.015% or less. The lower limit of the Ti content is not particularly limited, and may be 0%, may be more than 0%, may be 0.001% or more, and may be 0.005% or more.

[0037] (V: 0% or more and 0.20% or less) V is an element that forms nitrides like Ti and is used to improve strength. On the other hand, if the V content is too high, the nitrides tend to grow coarsely, which causes a decrease in fatigue strength. In this regard, the V content may be 0.20% or less, and may be 0.10% or less. The lower limit of the V content is not particularly limited, and may be 0%, may be more than 0%, may be 0.01% or more.

[0038] (Nb: 0% or more and 0.030% or less) Nb is an element that forms nitrides and the like, similar to Ti. Also, even in small amounts, it has the effect of significantly increasing the strength of steel. On the other hand, if the Nb content is too high, not only does the above effect saturate, but it also causes frequent cracking during casting. In this regard, the Nb content may be 0.030% or less, or may be 0.020% or less. The lower limit of the Nb content is not particularly limited and may be 0%, may be more than 0%, or may be 0.001% or more.

[0039] (Zr: 0% or more and 0.010% or less) Zr, when properly added, forms oxides that serve as inoculation nuclei for the solidification structure and has the effect of increasing the equiaxed crystal ratio. On the other hand, if the Zr content is too high, it forms coarse oxide-based inclusions, which not only become the starting points of fatigue fracture but also may cause clogging of the nozzles used for supplying molten steel to the mold. In this regard, the Zr content may be 0.010% or less, or may be 0.005% or less. The lower limit of the Zr content is not particularly limited and may be 0%, may be more than 0%, or may be 0.001% or more.

[0040] (Ca: 0% or more and 0.010% or less) Ca has the effect of modifying Al2O3 and suppressing the coarsening of oxide-based inclusions. On the other hand, if the Ca content is too high, it forms rather coarse oxide-based inclusions mainly composed of CaO - Al2O3, which may become the basis of fatigue fracture. In this regard, the Ca content may be 0.010% or less, or may be 0.005% or less. The lower limit of the Ca content is not particularly limited and may be 0%, may be more than 0%, or may be 0.001% or more.

[0041] (Mg: 0% or more and 0.010% or less) Mg, like Ca, has the effect of modifying Al2O3 and suppressing the coarsening of oxide inclusions. It also acts on sulfide inclusions and has the effect of reducing the aspect ratio. On the other hand, if the Mg content is too high, it may form coarse cluster-shaped oxide inclusions mainly composed of MgO, which may become the starting point of fatigue fracture. In this regard, the Mg content may be 0.010% or less, or 0.005% or less. The lower limit of the Mg content is not particularly limited and may be 0%, more than 0%, or 0.001% or more.

[0042] (REM: 0% or more and 0.010% or less) REM also has the effect of modifying Al2O3 and suppressing the coarsening of oxide inclusions. On the other hand, if the REM content is too high, it may reduce the cleanliness of the steel and deteriorate the toughness of the steel. In this regard, the REM content may be 0.010% or less, or 0.005% or less. The lower limit of the REM content is not particularly limited and may be 0%, more than 0%, or 0.001% or more. Note that REM as used in this application represents rare earth elements such as La and Ce, and any one or two or more of them can be used.

[0043] (B: 0% or more and 0.0040% or less) A small amount of B has the effect of enhancing the mechanical properties of steel. On the other hand, if the B content is too high, the effect will saturate, and cracks are likely to occur during casting. In this regard, the B content may be 0.0040% or less, or 0.0030% or less. The lower limit of the B content is not particularly limited and may be 0%, more than 0%, or 0.0001% or more.

[0044] In this embodiment, the balance of the steel other than the components described above may be Fe and impurities. Impurities are components such as those mixed in due to various factors in the steel manufacturing process, starting from raw materials such as ores and scraps.

[0045] 2. Secondary cooling of the slab Red-hot embrittlement occurs when Cu-containing steel is oxidized, generally in the range of 1050°C to 1200°C, and the lower limit temperature expands downward due to an increase in Cu concentration and the coexistence of Sn. Therefore, during continuous casting using a continuous casting machine having a straightening point such as a curved type or a vertical bending type, by avoiding this temperature range at the straightening point where a large strain is applied to the slab surface, it is considered possible to prevent red-hot embrittlement cracks on the slab surface.

[0046] However, when the surface temperature of the slab at the straightening point decreases, it enters the embrittlement temperature range (so-called Type III embrittlement) caused by precipitates deposited along the γ grain boundaries and film-like ferrite during slab cooling, and the crack sensitivity increases. Although this embrittlement depends on the components of the steel material, it is generally in the range of 700°C to 950°C. That is, in the casting of Cu- and Sn-containing steel by the normal continuous casting method, in order to avoid cracks, it is necessary to control the surface temperature of the slab at the straightening point within a very narrow range. Depending on the components of the steel to be cast, there may be no appropriate straightening point temperature, and it may be difficult to avoid crack generation.

[0047] The present inventors have intensively studied the above problems, and by optimizing the secondary cooling conditions during continuous casting, a slab surface structure that is less likely to crack is obtained, and by appropriately controlling the straightening point temperature, even if liquid Cu is generated between the scale and the base metal, a method capable of fundamentally preventing both red-hot embrittlement cracks and Type III cracks has been found.

[0048] Specifically, in the secondary cooling zone, the structure of the slab surface layer is transformed into ferrite-perlite or bainite, and then reheated to above Ac3 for reverse transformation, thereby refining the austenite structure. This not only prevents cracking in Region III, but also increases the liquid Cu-based alloy infiltration sites and can significantly reduce the crack depth. Furthermore, by controlling the correction point temperature below a value determined according to the Cu and Sn concentrations in the steel, red-hot embrittlement cracking can be prevented without overcooling the slab. As described above, in the continuous casting method according to this embodiment, from directly below the mold 10 to the correction point 20, after cooling the temperature of the surface of the slab 1 below the A1 temperature, it is raised above the Ac3 temperature, and it is important that the temperature of the surface of the slab 1 at the correction point is below the following temperature Tb (°C) according to the Cu content [Cu] (mass%) and Sn content [Sn] (mass%) in the steel. In the continuous casting method according to this embodiment, since slab surface cracking can be suppressed by the above mechanism without increasing the solubility of Cu in the steel, addition of Ni to the steel is unnecessary, or the addition amount of Ni to the steel can be made extremely small.

[0049] When [Cu] + 4[Sn] ≤ 0.15: Tb = 1050 When 0.15 < [Cu] + 4[Sn] ≤ 0.25: Tb = 1000 When 0.25 < [Cu] + 4[Sn] ≤ 0.35: Tb = 950 When 0.35 < [Cu] + 4[Sn]: Tb = 900

[0050] Cooled below the Ar1 temperature and then raised above the Ac3 temperature As described above, in this embodiment, in the secondary cooling zone, the structure of the slab surface layer is transformed into ferrite-perlite or bainite, and then reheated to a temperature above Ac3 and reverse-transformed, thereby refining the austenite structure. As a result, not only can the cracking in Region III be prevented, but also the sites of liquid Cu-based alloy infiltration increase, and the crack depth can be significantly reduced. When one or both of the above conditions are not satisfied, a reverse-transformed structure with high ductility cannot be obtained throughout the slab surface layer. That is, it is necessary to suppress cracking to cool to a temperature below Ar1 in the cooling process (referred to as Step 1) after mold withdrawal and then reheat to a temperature above Ac3 before reaching the correction point in the subsequent reheating process (referred to as Step 2).

[0051] When cooling the slab 1 withdrawn from the mold 10 to a temperature below Ar1, various methods such as cooling with water and cooling with a gas-liquid two-phase can be used. In this embodiment, there is no particular limitation on the cooling rate of the surface of the slab 1. For example, there is no problem even with a cooling rate of 1 °C / s or less. However, from the viewpoint of facilitating subsequent reheating, that is, causing the slab surface temperature to rise again without applying energy from the outside, the cooling rate may be 3 °C / s or more. On the other hand, if the cooling rate becomes excessive, unevenness occurs in the temperature distribution on the slab surface, and there is a risk of promoting the generation of cracks due to thermal stress. In this regard, the cooling rate may be 20 °C / s or less.

[0052] The reheating of the surface of the slab 1 cooled to a temperature below Ar1 occurs because the amount of heat transferred from the inside of the slab 1 exceeds the amount of heat released from the surface of the slab 1. The reheating of the surface of the slab 1 can be relatively easily performed by relaxing the cooling in the secondary cooling zone. Alternatively, a heat source or high-frequency induction heating equipment may be arranged around the casting line to heat the surface. The reheating rate (heating rate) of the slab 1 is not particularly limited, and any reheating rate can exhibit the desired effect. Also, the surface of the slab 1 may be reheated to Ac3 or higher, but from the viewpoint of further suppressing the variation in the surface temperature and structure of the slab 1, the maximum temperature after reheating may be set to Ac3 + 30°C or higher. Note that if the reheating temperature is too high, there is a risk that the austenite grain size will coarsen again. In this regard, the maximum temperature after reheating may be 1200°C or lower.

[0053] Note that Ar1 and Ac3 can be measured using a transformation point recording and measuring device (Formaster device) or the like. Alternatively, Ar1 and Ac3 can also be specified using the following formulas (a) and (b) proposed in a prior document (Takeru Kuniyoshi: Heat Treatment, 43, p. 99 (2003)).

[0054] Ar1 = (52C + 122Si + 66Cu + 6Cr) - (65Mn + 36Ni + 58Mo) - 73.2 / log((Ac3 - 500) / v) + 713 (a) Ac3 = (32Si + 17Mo) - (231C + 20Mn + 40Cu + 18Ni + 15Cr) + 912 (b) Here, v in formula (a) is the average cooling rate (°C / s) from Ac3 to the cooling arrival temperature.

[0055] 2.2 Temperature at the straightening point At the straightening point 20 of the continuous casting machine, when the temperature range is such that Cu and Sn are in a liquid state, when subjected to straightening strain, the liquid Cu and Sn can penetrate into the austenite grain boundaries. The greater the penetration depth, the greater the crack susceptibility. On the other hand, when the amount of the liquid of Cu and Sn is small, even during the deformation of the slab, it does not penetrate deeply into the grain boundaries, and cracks are less likely to occur. In this regard, in the present embodiment, by setting the surface temperature of the slab 1 at the straightening point 20 to be equal to or lower than the temperature Tb defined according to the amounts of Cu and Sn, it is possible to prevent the occurrence of cracks due to red hot embrittlement. Furthermore, due to the effects of the above steps 1 and 2, it is possible to simultaneously prevent the cracks in the III region, which are a concern when the straightening point temperature is lowered. Incidentally, when the continuous casting machine has a plurality of straightening points 20, at the straightening point 20 closest to the meniscus, the surface temperature of the slab 1 may be set to be equal to or lower than Tb.

[0056] The lower limit of the surface temperature of the slab 1 at the straightening point 20 is not particularly limited as long as it is a temperature that does not cause problems in the operation of continuous casting. For example, the surface temperature of the slab 1 at the straightening point 20 may be 500 °C or higher. The surface temperature of the slab 1 at the straightening point 20 may be controlled by the cooling conditions for the slab 1 until it reaches the straightening point 20 after reheating and the roll temperature at the straightening point 20, etc.

[0057] According to the findings of the present inventor, when focusing only on the amount of Cu contained in the steel, it is difficult to set a straightening point temperature that can appropriately prevent cracks due to red hot embrittlement. In contrast, in the present embodiment, as described above, not only Cu but also the amount of Sn is considered, and the surface temperature of the slab at the straightening point is defined. According to the findings of the present inventor, by defining the upper limit Tb of the temperature at the straightening point according to the value of the sum ([Cu] + 4[Sn]) of the amount of Cu and four times the amount of Sn, it is possible to appropriately prevent cracks due to red hot embrittlement.

[0058] 3. Supplementary The method according to this embodiment is mainly conceived for a method of continuously casting molten steel using scrap as a raw material. However, it can be applied without problems even when performing continuous casting using molten steel adjusted to a predetermined composition using iron ore or reduced iron instead of scrap. Further, the method according to this embodiment can be applied regardless of the shape of the cast slab, such as a slab, a rectangular bloom, or a round bloom, as long as it is continuous casting using continuous casting with a straightening point. In the method according to this embodiment, the continuous casting conditions (casting speed, etc.) other than the cooling conditions, reheating conditions, and straightening point temperature are not particularly limited.

[0059] As described above, according to the continuous casting method according to this embodiment, during the continuous casting of steel containing Cu or Sn, it is possible to suppress surface cracking of the cast slab caused by III-region cracking or hot shortness while suppressing the addition of Ni to the steel. In the present application, the "surface of the cast slab" does not necessarily mean the entire surface of the cast slab. That is, for at least the portion of the surface of the cast slab where surface cracking is desired to be suppressed, cooling to Ar1 or lower, reheating to Ac3 or higher, and control of the temperature at the straightening point may be performed.

Examples

[0060] Examples according to the present invention are shown below. The present invention is not limited to this one conditional example. In the present invention, various conditions can be adopted as long as the gist of the present invention is not deviated from and the object of the present invention is achieved.

[0061] 1. Model experiment In order to clarify the optimum conditions for sufficiently obtaining the cracking prevention effect by secondary cooling, reheating, and the strain application temperature at the straightening point for the cast slab pulled out from the mold, a model experiment using a high-temperature tensile apparatus was carried out.

[0062] A 100-mm long sample made of steel with the composition shown in Table 1 was heated in air to 1400 °C to obtain an austenite structure with an average grain size of 1.5 mm or more, and then rapidly cooled in a helium gas stream. The cooling rate was -10 °C / s, and the cooling end temperature was set to two values: the Ar1 temperature or 20 °C higher than the Ar1 temperature. Immediately after reaching the target temperature, the sample was reheated to the tensile temperature. After holding for 60 s at the predetermined temperature, the sample was pulled. The hot working pattern is shown in Figure 2. The cooling conditions, reheating conditions, and tensile conditions are as follows, respectively.

[0063]

Table 1

[0064] Cooling condition: Cooling to the Ar1 temperature at -10 °C / s

[0065] Reheating condition: Reheating at 5 °C / s to 825 - 1075 °C (in 25 °C increments)

[0066] Tensile condition: Tensile speed 0.01 / s, tensile length 5 mm

[0067] For the test piece after hot tension, the deformed part was observed, and the presence or absence of cracks was observed. The results are shown in Figures 3 and 4. Figure 3 has the Cu concentration (mass%) on the horizontal axis, and Figure 4 has the sum of the Cu concentration (mass%) and four times the Sn concentration (mass%) on the horizontal axis. As is clear from Figures 3 and 4, it can be seen that not only the Cu concentration but also the Sn concentration affects the cracks. As a result of intensive studies on the influence of Sn on red-hot embrittlement, as shown in Figure 4, it was found that it is best to organize by the sum of the Cu concentration and four times the Sn concentration ([Cu] + 4[Sn]), so this relationship was used hereafter.

[0068] From Figure 4, it was found that the crack sensitivity is high in the high-temperature region exceeding 900 - 1050 °C and the low-temperature region below 875 °C. These are considered to correspond to red-hot embrittlement due to selective oxidation and embrittlement in the III region, respectively.

[0069] In order to avoid embrittlement in the III region and prevent cracking, it is essential not only to transform austenite into structures such as ferrite - pearlite or bainite by cooling, but also to obtain a so - called reverse transformation structure with a fine austenite structure until reaching the recrystallization point. When the reheating temperature is less than Ac3, there will remain locations where reverse transformation does not occur. Regarding the level where cracking occurred despite cooling down to Ar1, since reheating to a temperature above Ac3 was not performed before reaching the tensile temperature and the influence of the coarse structure before cooling remained, it is presumed that it was more likely to crack with respect to the straightening strain. Therefore, it was assumed that reheating to a temperature above Ac3 to obtain a single - phase austenite structure was effective in suppressing crack generation.

[0070] To confirm the above, the hot - working pattern of the hot tensile test was changed to the pattern shown in Fig. 5, and the evaluation of cracking was carried out again. As a result, as shown in Fig. 6, the cracks that occurred at temperatures below 875 °C were not confirmed in this hot - working pattern.

[0071] Furthermore, for samples A - 5, A - 6, A - 7, and A - 8 in which cracks were observed in a wide tensile temperature range, tensile processing was carried out from 1400 °C according to the hot - working pattern shown in Fig. 7. As a result, as shown in Fig. 8, not only could the cracks considered to be caused by embrittlement in the III region not be suppressed, but also the cracks considered to be caused by hot shortness showed cracking at lower temperatures. This is presumably because the austenite grains remained coarse until tensile deformation, and the liquid Cu - based alloy infiltration sites penetrated deeper, increasing the crack susceptibility.

[0072] From the results of the above model experiments, it is considered that in order to suppress surface cracking of the slab caused by cracking in the III region and hot shortness during continuous casting of steel containing Cu and Sn, while suppressing the addition of Ni to the steel, it is necessary to meet the following requirements. (1) Cooling the surface temperature of the slab withdrawn from the mold to a temperature below Ar1 (2) Raising (reheating) the surface temperature of the slab cooled to a temperature below Ar1 to a temperature above Ac3 The temperature of the surface of the slab heated above the Ac3 temperature shall be made not higher than the temperature Tb °C represented by the following conditions at the straightening point.

[0073] When [Cu] + 4[Sn] ≤ 0.15: Tb = 1050 When 0.15 < [Cu] + 4[Sn] ≤ 0.25: Tb = 1000 When 0.25 < [Cu] + 4[Sn] ≤ 0.35: Tb = 950 When 0.35 < [Cu] + 4[Sn]: Tb = 900 Here, [Cu] and [Sn] are the contents (mass%) of Cu and Sn in the said steel.

[0074] 2. Continuous casting test The results of the above model experiment were confirmed by a continuous casting test. Specifically, molten steel with the composition shown in Table 2 was melted in an electric furnace, and a continuous casting machine (5-point straightening type) with a radius of curvature of 12.0 m was used to produce a slab with a width of 2000 mm and a thickness of 250 mm. The casting speed was 0.8 to 1.5 m / min. The slab withdrawn from the mold was rapidly cooled by a spray cooling device with a zone length of 1.0 m installed immediately below the mold. After passing through the zone, the amount of water in the secondary cooling spray was adjusted and reheating was controlled to vary the rapid cooling temperature T1, the reheating temperature T2, and the temperature T3 at the straightening point (the one closest to the meniscus). The slab was cut to a length of 5.0 ± 0.2 m by a gas cutting machine and then used for surface observation.

[0075]

Table 2

[0076] The surface temperature of the slab was measured by a plurality of radiation thermometers installed on the outer peripheral side of the curved portion in the continuous casting machine. Heat transfer and solidification analysis was performed by applying the heat extraction conditions by cooling water and rolls together with this measured value, and the surface temperature distribution of the slab was obtained. Based on this surface temperature distribution of the slab, the minimum rapid cooling temperature T1, the maximum reheating temperature T2, and the straightening point temperature T3 were obtained. T1, T2, and T3 were all taken as the surface temperatures at the center in the width direction among the long side surfaces on the outer peripheral side of the curved portion. At that time, it was confirmed that there was no deviation of 20 °C or more between the surface temperature obtained by heat transfer calculation and the measured value obtained from the radiation thermometer.

[0077] For Ar1 and Ac3, calculations were performed based on the above equations (a) and (b). The cooling rate in Step 1 was estimated to be v = 15 °C / s from heat transfer calculation and calculated.

[0078] The test results are shown in Table 3. For the evaluation of surface cracks, those without cracks were marked with ○, those with 10 or fewer cracks per 1 m of slab length were marked with △, and those not meeting either were marked with ×.

[0079]

Table 3

[0080] As is clear from the results shown in Table 3, for levels 1 to 14 that satisfy the above requirements (1) to (3), sound slabs with no cracks were obtained in all cases. On the other hand, for levels 15 to 18, 19 to 22, and 23 to 36, the cooling temperature due to secondary cooling of the slab did not reach below Ar1, the temperature did not reach above Ac3 until reaching the straightening point after cooling, and the straightening point temperature was not below Tb, respectively, and all showed cracks on the slab surface.

Explanation of Symbols

[0081] 1 Slab 10 Mold 20 Straightening point 100 Continuous casting machine

Claims

1. By mass%, C: 0.03% or more and 0.20% or less, Si: 0.01% or more and 1.00% or less, Mn: 0.10% or more and 2.50% or less, P: 0.040% or less, S: 0.030% or less, Cu: 0.05% or more and 0.50% or less, Sn: 0.001% or more and 0.025% or less, Ni: 0% or more and 0.05% or less, Al: 0.005% or more and 0.100% or less, Cr: 0% or more and 0.50% or less, Mo: 0% or more and 0.05% or less, and N: 0.0150% or less, A method for continuously casting a steel slab containing the above and having the balance composed of Fe and impurities using a continuous casting machine having a straightening point, Cooling the temperature of the surface of the said slab drawn out from the mold to a temperature equal to or lower than the Ar １ temperature; Ar １ raising the temperature of the surface of the slab cooled below the Ar temperature to a temperature equal to or higher than the Ac temperature, and ３ ​ Ac ３ Making the temperature of the surface of the slab raised above the temperature be equal to or lower than a temperature Tb °C represented by the following conditions at the straightening point. A continuous casting method of steel, including the above. When [Cu] + 4[Sn] ≤ 0.15: Tb = 1050 When 0.15 < [Cu] + 4[Sn] ≤ 0.25: Tb = 1000 When 0.25 < [Cu] + 4[Sn] ≤ 0.35: Tb = 950 When 0.35 < [Cu] + 4[Sn]: Tb = 900 Here, [Cu] and [Sn] are the contents (mass%) of Cu and Sn in the above steel.

2. When the above steel is by mass%, Ti: 0% or more and 0.020% or less, V: 0% or more and 0.20% or less, Nb: 0% or more and 0.030% or less, Zr: 0% or more and 0.010% or less, Ca: 0% or more and 0.010% or less, Mg: 0% or more and 0.010% or less, REM: 0% or more and 0.010% or less, and B: 0% or more and 0.0040% or less, Containing one or more of the above, The method according to Claim 1.

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