CONTINUOUS CASTING METHOD OF Cu-CONTAINING STEEL

Abstract

To provide a continuous casting method of Cu-containing steel which is capable of preventing a surface of a casting piece containing Cu from being cracked simply and inexpensively.SOLUTION: The continuous casting method includes a step of cooling a casting piece until the maximum temperature of the casting piece surface becomes Ar1 point or less and reheating it during a period from coming out of a mold to reaching a correction point. Continuous casting is performed so that the product of time tb (sec) for which the maximum temperature of the casting piece surface becomes a temperature Tb (°C) determined according to content of Cu and Sn or more during a period from starting reheating to reaching the correction point and a value LCu determined according to content of Cu, Sn, Ni, Si of the casting pieces is 30 or less.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present invention relates to a continuous casting method for Cu-containing steel that is suitable for preventing surface cracks. [Background technology]

[0002] In recent years, efforts to reduce CO2 emissions have been actively undertaken in various fields, including from the perspective of preventing global warming. In the steel industry, the steelmaking process that uses a large amount of waste scrap as a raw material for steelmaking has attracted attention, and technological development is progressing. However, much of the scrap contains high concentrations of tramp elements such as Cu and Sn, which are known to be difficult to remove from molten steel.

[0003] In particular, steels containing Cu tend to have poor hot workability, and therefore, when steels containing Cu are continuously cast under the conditions used for normal steel, cracks may occur on the surface of the slab. This is thought to be because, when the steel is exposed to oxygen in the atmosphere and oxidizes during continuous casting, liquid Cu forms between the scale (iron oxide) and the base steel, penetrates into the grain boundaries of the steel, and reduces the interfacial strength (see Non-Patent Document 1). Furthermore, Sn reduces the solubility of Cu in steel, thereby promoting the phenomenon of Cu-induced cracking. Therefore, even steels containing both Sn and Cu are prone to surface cracking (see Non-Patent Document 2).

[0004] This phenomenon, called surface embrittlement, is believed to be caused by Cu and Sn being concentrated in their metallic state during scale growth due to their resistance to oxidation compared to Fe, and by the low solubility of Cu in Fe. However, Cu and Sn are difficult to remove during the steel refining process. To resolve the issue of surface cracking due to embrittlement, it is effective to either avoid Cu and Sn inclusion in steel or to add Ni, an element that increases Cu's solubility in steel. In particular, with the advent of a recycling-oriented society and the resulting increased use of Cu-rich scrap, there is an increasing need to neutralize Cu by adding Ni. However, Ni is a rare and expensive element, and it can significantly alter steel properties such as mechanical properties and hardenability. Therefore, there is great hope for a Cu and Sn neutralization technology that does not rely on Ni addition or can minimize its amount.

[0005] Patent Document 1 discloses a continuous casting method for preventing red embrittlement on the surface of a slab, in which the mold inner surface near the molten steel surface has a reverse taper value of 2 to 10% widening downward in the slab drawing direction, and the mold inner surface below the reverse taper portion has a forward taper value of 0 to 1%, and the mold flux used has a crystallization temperature of 900°C or less or does not crystallize, and the contact angle between the mold flux and the steel is 70° or less. Patent Document 2 discloses a continuous casting method in which a nickel oxide coating layer is formed on the surface of a slab while a mold flux containing Ni oxide is supplied.

[0006]

[0003] Patent Literature 3 discloses a method for producing a Cu-containing high-strength steel material, which comprises heating a slab between 1000 and 1100°C at an average heating rate of 50°C / h or more, holding the slab at 1200 to 1350°C for 1 hour or more, hot-rolling the slab at a cumulative reduction of 50% or more in the temperature range of 1000°C or higher and a finishing temperature of 700°C or higher, followed by air-cooling or accelerated cooling at an average cooling rate of 1 to 80°C / s to a temperature range of 500 to 650°C.

[0004] Patent Literature 4 further discloses a method for preventing surface cracking in a continuously cast slab, which comprises cooling the surface of the slab at a cooling rate of 300°C / s or more from a temperature range above the Ar3 transformation point until the surface temperature reaches a temperature range above the Ar1 transformation point, and then reheating the surface temperature of the slab to a temperature range above the Ar3 transformation point. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Japanese Patent Application Laid-Open No. 2004-202523 [Patent Document 2] Special Publication No. 2018-520004 [Patent Document 3] Japanese Patent Application Laid-Open No. 2011-168843 [Patent Document 4] Japanese Patent Application Laid-Open No. 2007-245232 [Non-patent literature]

[0008] [Non-Patent Document 1] "Materials Transactions" vol.43, (2002), No.3, pp.292-300 [Non-patent document 2] "Feram" vol.7, (2002), No.4, pp.18-22 [Non-patent document 3] Tatsuro Kunitake: Heat Treatment, 43, p. 99(2003) Summary of the Invention [Problem to be solved by the invention]

[0009] However, the methods described in Patent Documents 1 and 2 both attempt to prevent oxidation of the slab surface using mold flux, but depending on the type of continuous casting machine and the secondary cooling method, the mold flux may not adhere consistently to the slab surface, preventing the full benefit of these methods. Furthermore, the method described in Patent Document 3 relates to hot rolling of slabs, and cannot be used in continuous casting, where molten steel is withdrawn from the mold while solidifying, because this process is completely different. Furthermore, the method described in Patent Document 4 is primarily a technology for preventing transverse cracks, which have a different mechanism, and is unable to fully prevent red embrittlement cracking caused by elements such as Cu and Sn.

[0010] In view of the above-mentioned problems, an object of the present invention is to provide a method for continuous casting of Cu-containing steel that can easily and inexpensively prevent surface cracks in Cu-containing slabs. [Means for solving the problem]

[0011] The present invention has been made to solve the above-mentioned problems, and has the following configuration. [1] In mass%, C: 0.002% or more and 0.250% or less, Si: 0.01% or more and 2.00% or less, Mn: 0.10% or more and 2.50% or less, P: 0.040% or less, S: 0.030% or less, Cu: 0.100% or more and 0.500% or less, Sn: 0.050% or less, Ni: more than 0.050% and not more than 0.300%, and N: 0.0040% or more and 0.0150% or less, and the remainder being Fe and impurities, using a curved or vertical bending type continuous casting machine, a step of cooling and reheating the slab surface until the maximum temperature of the slab surface is equal to or lower than the Ar1 point during the period from when the slab leaves the mold to when the slab reaches the straightening point; From the start of the reheating to the straightening point, the maximum temperature of the slab surface is a temperature T b (℃) or more time t b (seconds) and The value L determined by the following formula (2) according to the contents of Cu, Sn, Ni, and Si in the cast slab Cu and, A method for continuous casting of Cu-containing steel, characterized in that continuous casting is carried out so as to satisfy the condition of the following formula (3) in relation to the above. When [Sn]<0.005%, T b =1100℃, [Sn] ≥ 0.005% and Cu_eq < 0.150%, T b =1100℃, [Sn]≧0.005% and 0.150%≦Cu_eq<0.200%, T b =1070℃, [Sn]≧0.005% and 0.200%≦Cu_eq<0.250%, T b =1040℃, [Sn]≧0.005% and 0.250%≦Cu_eq<0.300%, T b =1010℃, [Sn]≧0.005% and 0.300%≦Cu_eq<0.350%, T b =980℃, [Sn] ≥ 0.005% and Cu_eq ≥ 0.350%, T b =950℃ ···(1) L Cu =[Cu]+4×[Sn]-3×[Ni]-0.07×[Si] ···(2) t b ×L Cu ≦30 (3) Here, Cu_eq = [Cu] + 4 × [Sn], [Cu] represents the Cu concentration (mass%) in the slab, [Sn] represents the Sn concentration (mass%) in the slab, [Ni] represents the Ni concentration (mass%) in the slab, and [Si] represents the Si concentration (mass%) in the slab. [2] The cast piece further comprises: In mass%, Al: more than 0% and less than 0.100%, Cr: more than 0% and less than 1.50%, Mo: more than 0% and less than 0.20%, Ti: more than 0% and less than 0.050%, V: more than 0% and less than 0.20%, Nb: more than 0% and less than 0.030%, Zr: more than 0% and less than 0.030%, Ca: more than 0% and less than 0.0100%, Mg: more than 0% and less than 0.0100%, REM: Over 0% and 0.0100% or less, and B: More than 0% and less than 0.0040%, The method for continuous casting of Cu-containing steel according to the above [1], characterized in that the steel contains one or more selected from the group consisting of: [Effects of the Invention]

[0012] According to the present invention, surface cracks in Cu-containing slabs can be prevented simply and inexpensively. [Brief explanation of the drawings]

[0013] [Figure 1] FIG. 1 is a diagram for explaining an outline of the temperature history from inside the mold to the correction point. [Figure 2] FIG. 10 is a diagram showing the relationship between two parameters, tb and LCu. DETAILED DESCRIPTION OF THE INVENTION

[0014] The embodiments of the present invention will be described in detail below. First, the metallurgical effect will be described below. Generally, red embrittlement in Cu-containing steels occurs between 1050°C and 1200°C, and it is known that the lower limit of red embrittlement temperature is lowered by increasing Cu concentration and the coexistence of Sn. To prevent red embrittlement cracking during continuous casting, it is effective to maintain the surface temperature of the slab as low as possible below this temperature range. However, it is important to understand the effect of Sn on this temperature range. Furthermore, the amount of Cu-containing liquid phase produced per unit time varies depending on the components in Cu-containing steel. Therefore, the content of elements such as Sn, which reduces the solubility of Cu in steel, and Ni, which increases the solubility of Cu in steel, is important.

[0015] Generally, red embrittlement cracking is more likely to occur as the Cu penetration depth increases, and the Cu penetration depth varies depending on the structure of the austenite grains (γ grains) in the surface layer of the slab. In particular, in steel with a C concentration of 0.08 to 0.25% by mass, the structure of the γ grains in the surface layer of the slab tends to be coarse, and the grain boundary volume fraction in the surface layer decreases, which makes the Cu penetration depth more likely to increase.

[0016] The inventors have conducted extensive research into the mechanism of slab cracking caused by Cu and Sn, and as a result have found that red embrittlement cracking occurs due to a combination of factors including the surface temperature history of the slab from the time it leaves the mold until it reaches the straightening point, and the amount of Cu-containing liquid phase produced per unit time. The amount of Cu liquid phase produced is determined by the surface temperature history of the slab during reheating and the compositions of Cu, Sn, Ni, etc., but by controlling these factors and suppressing the amount of Cu liquid phase produced, the Cu penetration depth can be reduced, and by optimizing the cooling of the slab during casting, steels with various compositions including the above-mentioned C concentration range can be cast without red embrittlement cracking.

[0017] Hereinafter, an embodiment of the present invention will be described with reference to the drawings. The continuous casting machine used in the continuous casting method for Cu-containing steel according to this embodiment may be a curved type or a vertical bending type. Furthermore, when there are multiple straightening points, cooling is controlled under the conditions described below after the steel leaves the mold until it reaches the first straightening point closest to the meniscus. Cooling control is mainly performed using a spray cooling device, where the amount, time, and timing of spraying water or mist are controlled.

[0018] Next, the components of the steel (bill) specified in the present invention will be explained. Note that "%" in the following explanation means "% by mass."

[0019] [C: 0.002% or more and 0.250% or less] Carbon is the most fundamental element that affects not only the static strength of steel, but also its fatigue strength, toughness, and ductility. A carbon concentration of less than 0.002% does not significantly improve these properties and only increases the cost of decarburization, making it undesirable. Therefore, the lower limit is set at 0.002%. Furthermore, if the carbon concentration exceeds 0.250%, toughness deteriorates. Therefore, the upper limit is set at 0.250%.

[0020] [Si:0.01% or more and 2.00% or less] Si is an element that can increase the strength of steel when added appropriately. To achieve this effect, a Si content of 0.01% or more is necessary. Therefore, the lower limit is set to 0.01%. Since Si has the effect of suppressing red embrittlement cracking caused by Cu, the lower limit is preferably set to 0.10%. On the other hand, if the Si concentration exceeds 2.00%, toughness and workability will be significantly deteriorated. Therefore, the upper limit is set to 2.00%. However, from the viewpoint of toughness and workability, the upper limit is preferably set to 1.00%.

[0021] [Mn: 0.10% or more and 2.50% or less] Like Si, Mn can increase the strength of steel when added appropriately. If the Mn concentration is less than 0.10%, the necessary strength cannot be ensured. Therefore, the lower limit is set to 0.10%. Furthermore, if the Mn concentration exceeds 2.50%, toughness and workability deteriorate. Therefore, the upper limit is set to 2.50%.

[0022] [P:0.040% or less] P is an element that promotes cracking during casting, and if the P concentration exceeds 0.040%, it becomes difficult to suppress cracking of the cast slab. Therefore, the upper limit is set to 0.040%. However, since the lower the P content, the better, so 0% is also acceptable.

[0023] [S:0.030% or less] Like P, S also promotes the suppression of cracking during casting and deteriorates the bending workability of steel sheets. If the S concentration exceeds 0.030%, the above adverse effects become significant. Therefore, the upper limit is set to 0.030%. Similarly, the lower the S content, the better, so 0% is also acceptable.

[0024] [Cu:0.100% or more and 0.500% or less] If the Cu content is less than 0.100%, the amount of liquid phase generated by oxidation of the steel is sufficiently small, preventing cracking due to embrittlement or causing no harm. On the other hand, when scrap is used as an environmental measure, low-quality scrap containing relatively high concentrations of Cu may be used. If the Cu content is less than 0.100%, the iron source composition must be changed to use high-grade scrap or reduced iron to dilute the Cu, resulting in increased costs. Therefore, the lower limit is set to 0.100%. On the other hand, if the Cu content exceeds 0.500%, it will adversely affect the steel's properties. Therefore, the upper limit is set to 0.500%. Furthermore, from the viewpoint of preventing cracking during hot rolling and forging, the upper limit is preferably set to 0.350%.

[0025] [Sn:0.050% or less] Sn significantly lowers the Cu liquid phase stabilization temperature and widens the embrittlement temperature range, so it is desirable to avoid its inclusion as much as possible. If the Sn concentration exceeds 0.050%, red embrittlement cracking occurs even at lower temperatures, which is undesirable because a large amount of Ni is required to suppress red embrittlement cracking. Therefore, the upper limit is set to 0.050%. On the other hand, from the viewpoint of preventing cracking during hot rolling and forging, the upper limit is preferably set to 0.030%.

[0026] [Ni: more than 0.050% and less than 0.300%] Ni increases the static strength of steel but reduces elongation. Therefore, when manufacturing steel materials such as low-carbon steel, where elongation is more important than strength, reduced iron or other additives are added to reduce the Ni concentration. Setting the Ni concentration below 0.050% requires a large amount of reduced iron, which is more expensive than scrap, while the effect of improving elongation is small, resulting in increased costs and being undesirable. Therefore, the Ni content is set to exceed 0.050%. Ni is known to suppress red embrittlement cracking caused by Cu and Sn, but since it is an expensive element, intentionally increasing its content would incur significant costs. Therefore, the amount typically mixed in from scrap is sufficient. Therefore, the upper limit is set to 0.300%, preferably 0.200%.

[0027] [N: 0.0040% or more and 0.0150% or less] N is an element that affects the mechanical properties of steel, reduces hot ductility, and is also an element that causes surface defects during casting or rolling. N is mainly removed in the degassing process of secondary refining, but an N concentration of less than 0.0040% is undesirable because it requires a long degassing process and increases costs. Therefore, the lower limit is set to 0.0040%. On the other hand, an N concentration exceeding 0.0150% is undesirable because it leads to coarsening of nitride inclusions and reduces fatigue strength. Therefore, the upper limit is set to 0.0150%, but from the perspective of steel cleanliness, it is preferable to set the upper limit to 0.0080%.

[0028] In the present invention, in order to achieve the desired properties of the product, the cast slab may further contain one or more of the following elements: Note that the elements described below are optionally contained, and the content of each element may be 0%, but in order to achieve the properties described below, the lower limit of the content of each element is a concentration exceeding 0%.

[0029] [Al: more than 0% and less than 0.100%] Al is an element that is widely used for deoxidation purposes, but if the Al concentration exceeds 0.100%, problems such as nozzle clogging during casting and oxide-based inclusions remaining in the steel that degrade performance are likely to occur. Therefore, the upper limit is set to 0.100%, and preferably 0.030%.

[0030] [Cr: more than 0% and less than 1.50%] Cr is a useful element for increasing the strength of steel, but if the Cr concentration exceeds 1.50%, the effect is almost saturated and costs increase, which is undesirable. Therefore, the upper limit is set to 1.50%, and preferably 1.20%.

[0031] [Mo: more than 0% and less than 0.20%] Mo, like Cr, is an element that increases the strength of steel, but the effect saturates when the Mo concentration exceeds 0.20%, so the upper limit is set to 0.20%, and preferably 0.15%.

[0032] [Ti: more than 0% and less than 0.050%] Ti not only has a deoxidizing effect similar to Al, but also forms nitrides with high thermal stability, which can refine the structure in the heating furnace. On the other hand, if the Ti concentration exceeds 0.050%, the amount of nitride precipitates increases, increasing the cracking susceptibility due to embrittlement around 700°C (region III). Furthermore, nozzle clogging due to oxides frequently occurs during casting, which is undesirable. Therefore, the upper limit is set to 0.050%, and preferably 0.030%.

[0033] [V: more than 0% and less than 0.20%] V, like Ti, is an element that forms nitrides and is used to improve strength. However, if the V concentration exceeds 0.20%, VN tends to grow coarsely, causing a decrease in fatigue strength. Therefore, the upper limit is set to 0.20%, and preferably 0.15%.

[0034] [Nb: more than 0% and less than 0.030%] Nb, like Ti, is an element that forms nitrides, etc. Furthermore, even a small amount of Nb has the effect of significantly increasing the strength of steel. However, if the Nb concentration exceeds 0.030%, not only does the effect saturate, but it also causes frequent cracking during casting. Therefore, the upper limit is set to 0.030%, and preferably 0.025%.

[0035] [Zr: more than 0% and less than 0.030%] Zr, like Ti, is an element that forms nitrides and other compounds, and is effective in suppressing the coarsening of oxide-based inclusions. However, if the Zr concentration exceeds 0.010%, not only does this effect saturate, but it also causes clogging of the submerged entry nozzle used to pour molten steel into the mold. Therefore, the upper limit is set to 0.030%, and preferably 0.025%.

[0036] [Ca: more than 0% and less than 0.0100%] 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, coarse oxide-based inclusions composed mainly of CaO-Al2O3 may form, which may become the starting point for fatigue fracture. Therefore, the Ca concentration is set to 0.0100% or less, preferably 0.0050% or less. There is no particular lower limit for the Ca concentration, and it may be 0%, but in order to obtain the effect of suppressing the coarsening of oxide-based inclusions, it is preferably more than 0%, and more preferably 0.0010% or more.

[0037] [Mg: more than 0% and less than 0.0100%] Like Ca, Mg modifies Al2O3 and has the effect of suppressing the coarsening of oxide-based inclusions. It also acts on sulfide-based inclusions, reducing their aspect ratio after rolling. On the other hand, if the Mg concentration is too high, coarse cluster-like oxide-based inclusions composed primarily of MgO may form, which may become the starting point for fatigue fracture. Therefore, the Mg concentration is set to 0.0100% or less, preferably 0.0050% or less. There is no particular lower limit for the Mg concentration, and it may be 0%, but in order to obtain the effect of suppressing the coarsening of oxide-based inclusions, it is preferably more than 0%, more preferably 0.0010% or more.

[0038] [REM: More than 0% and less than 0.0100%] REM also modifies Al2O3 and has the effect of suppressing the coarsening of oxide-based inclusions. On the other hand, if the REM content is too high, the cleanliness of the steel may be reduced, and the toughness of the steel may be deteriorated. Therefore, the REM concentration is set to 0.0100% or less, preferably 0.0050% or less. There is no particular limitation on the lower limit of the REM concentration, and it may be 0%, but in order to obtain the effect of suppressing the coarsening of oxide-based inclusions, it is preferable that the REM concentration be more than 0%, and more preferably 0.0003% or more. Note that REM refers to rare earth elements such as La and Ce, and any one or more of these REMs can be used.

[0039] [B: More than 0% and less than 0.0040%] A small amount of B has the effect of improving the mechanical properties of steel. On the other hand, if the B content is too high, the effect saturates and cracks are more likely to occur during casting. Therefore, the B concentration is set to 0.0040% or less, and preferably 0.0030% or less. There is no particular lower limit for the B concentration, and it may be 0%, but in order to obtain the effect of improving the mechanical properties, it is preferably more than 0%, and more preferably 0.0001% or more.

[0040] The balance of the chemical composition of the steel slab according to this embodiment may be Fe and impurities. Examples of impurities include elements that are inevitably mixed into molten steel from scrap, which is a steel raw material, or the atmosphere, and are permissible within a range that does not impair the properties of the steel slab according to this embodiment. Examples of such impurities include O, Sb, and As.

[0041] Next, detailed conditions for preventing red embrittlement cracking will be described. As mentioned above, important parameters for preventing red embrittlement cracking include the surface temperature history of the slab from leaving the mold to the straightening point, and the amount of Cu-containing liquid phase generated per unit time. Also, considering the average grain size in the surface layer of the slab is important for preventing red embrittlement cracking, and the average grain size varies depending on the composition of the slab and the surface temperature history during cooling before reheating. In this embodiment, red embrittlement cracking is prevented by comprehensively evaluating these parameters. These parameters will be described in detail below.

[0042] [Surface temperature history of cast slab] The Cu liquid phase penetrates into the grain boundaries, but if the grain boundary volume fraction in the slab surface is large, the Cu liquid phase remains there, so the Cu liquid phase does not penetrate deep into the slab surface. gb is the average crystal grain size (equivalent to a sphere) R c and the grain boundary thickness d, it is expressed by the following equation (4). f gb = {(surface area of ​​crystal grain) × d / 2} / (volume of crystal grain) = 3d / R c (4)

[0043] Here, when the same amount of Cu liquid phase is generated, the Cu infiltration depth is the grain boundary volume fraction f gb Since the average grain size R c In other words, in order to reduce the Cu infiltration depth, it is also important to control the average grain size of γ grains to be small.

[0044] The inventors considered it important to accurately estimate the diameter (equivalent spherical diameter) of γ grains in compositions near the peritectic solidification region as one of the parameters that serves as an indicator for preventing red embrittlement cracking. Therefore, they conducted experiments using a directional solidification experimental apparatus. Specifically, steels with the compositions shown in Table 1 were melted and solidified in a directional solidification experimental apparatus consisting of a water-cooled copper plate and a refractory tube. The steel samples were then cooled until the temperature at the point in contact with the water-cooled copper plate was below the Ar1 point. The steel samples were then immediately placed in a heating furnace controlled at 1250°C and reheated to 1110–1140°C. After reaching the predetermined temperature, they were quenched. The microstructure of the steel samples was then observed from the surface that had been in contact with the water-cooled copper plate to a depth of 1 mm, and the average grain size of the prior γ grains was measured. For comparison, the average grain size of the prior γ grains at the same location was also measured for a steel sample obtained by cooling to below the Ar1 point and then quenching without reheating. The results are shown in Table 2.

[0045] [Table 1]

[0046] [Table 2]

[0047] In Table 2, the average grain size index (AGR) is a numerical value representing the average grain size (equivalent spherical diameter) of prior γ grains. It was normalized so that the average grain size of prior γ grains measured by microstructural observation for five samples with a thermal history of cooling, reheating, and cooling was 1.0. As shown in Table 2, the AGR in the samples with reheating had a smaller AGR than the samples without reheating because the AGR transformed into ferrite and then retransformed back to AGR upon reheating. Furthermore, the AGR at the contact point with the water-cooled copper plate tended to vary significantly with C concentration in the samples without reheating, whereas the AGR in the samples with reheating showed almost no difference with C concentration. These results indicate that in the actual continuous casting process of molten steel, the AGR can be reduced by applying a thermal history of reducing the surface temperature of the slab below Ar1 and then reheating, thereby reducing the Cu infiltration depth and suppressing Cu-induced surface cracking. This effect is not significantly affected by C concentration.

[0048] As described above, the Cu infiltration depth can be reduced by cooling the surface temperature of the slab to below the Ar1 point and then reheating. However, to prevent red embrittlement cracking, it is necessary to further reduce the amount of Cu liquid phase produced. The amount of Cu liquid phase produced varies depending on the surface temperature history of the slab during reheating. Next, the surface temperature history of the slab during reheating will be explained.

[0049] After the slab is removed from the mold, selective oxidation occurs as scale forms, forming a layer enriched with tramp elements such as Cu and Sn. When the temperature of this enriched layer exceeds the solidus temperature, a Cu liquid phase is generated and concentrates at the interface between the scale and the base steel, and some of the Cu liquid phase penetrates into the grain boundaries. Grain boundaries that have been penetrated by the Cu liquid phase are easily opened by even very small strains caused by contact with the rolls of the continuous casting machine, resulting in microcracks of 0.2 mm or more in depth (hereinafter simply referred to as microcracks) before reaching the straightening point. These microcracks are subjected to tensile strain at the straightening point and propagate, becoming harmful surface cracks of 1 mm or more in depth, mainly on the upper surface of the slab, due to red embrittlement.

[0050] In particular, in the case of Cu-containing steel, the presence of Sn significantly lowers its solidus temperature, and the temperature range in which the Cu liquid phase forms varies depending on the Sn concentration. Therefore, the presence or absence of microcracks is correlated not only with the temperature history leading up to the straightening point, but also with the Sn concentration and Cu equivalent (Cu_eq). Here, Cu equivalent refers to the equivalent (mass%) expressed as the linear sum of Cu and Sn. If Cu concentration (mass%) is [Cu] and Sn concentration (mass%) is [Sn], then Cu_eq = [Cu] + 4 × [Sn]. In other words, the longer the slab surface temperature remains above the lower limit temperature correlated with Sn concentration and Cu equivalent (Cu_eq), the more likely microcracks are to occur.

[0051] The inventors performed thermodynamic calculations to determine the temperature range in which the Cu-Sn liquid phase becomes stable in the Fe-Cu-Sn system, and divided the range into several stages according to the Sn concentration and Cu equivalent. As a result, the lower limit of the temperature range was determined as T b When the temperature T b was calculated according to the Sn concentration and Cu equivalent using the following formula (1). When [Sn]<0.005%, T b =1100℃ [Sn] ≥ 0.005% and Cu_eq < 0.150%, T b =1100℃ [Sn]≧0.005% and 0.150%≦Cu_eq<0.200%, T b =1070℃ [Sn]≧0.005% and 0.200%≦Cu_eq<0.250%, T b =1040℃ [Sn]≧0.005% and 0.250%≦Cu_eq<0.300%, T b =1010℃ [Sn]≧0.005% and 0.300%≦Cu_eq<0.350%, T b =980℃ [Sn] ≥ 0.005% and Cu_eq ≥ 0.350%, T b =950℃ ···(1)

[0052] In this embodiment, the maximum temperature of the slab surface is the lower limit temperature T b The time t b is defined as one of the parameters that is an index for preventing red embrittlement cracking. Here, the temperature T b and time t b This will be explained with reference to FIG.

[0053] Figure 1 is a diagram for explaining the outline of the temperature history from inside the mold to the straightening point. The horizontal axis of Figure 1 represents the time from the meniscus inside the mold, with the time when the slab leaves the mold being set as 0 seconds. On the other hand, the vertical axis represents the maximum temperature of the slab surface (center).

[0054] As shown in Figure 1, immediately after leaving the mold, the slab is rapidly cooled using a spray cooling device until it reaches a temperature T1, which is the maximum temperature of the slab surface and is below the Ar1 point. As can be seen from the experiments using the directional solidification experimental device mentioned above, if the temperature T1 reached after quenching is higher than the Ar1 point, the transformation from austenite to ferrite does not occur sufficiently, and the reverse transformation during reheating does not sufficiently refine the surface structure. As a result, the volume fraction of the grain boundaries, which are the penetration path for the Cu liquid phase, decreases, and subsequent reheating allows the Cu liquid phase to penetrate deeper into the grain boundaries, making microcracks more likely to occur.

[0055] After cooling to a temperature T1 below the Ar1 point, the cooling is stopped and the slab is reheated to raise the surface temperature to a temperature T2. At this time, as shown in FIG. 1, in this embodiment, after cooling, the temperature T2 defined above is b The time t b By controlling the time to be short, the amount of Cu liquid phase produced is reduced, the Cu penetration depth is reduced, and microcracks are prevented, thereby preventing red embrittlement cracking at the correction point.

[0056] The temperature of the slab surface can be determined by taking the temperature at the hottest point in the circumferential direction of the slab. This is because if the temperature of that point falls below Ar1 during rapid cooling, the other points will also meet this condition, and structural reform can be expected for the entire circumferential direction of the slab. Also, if the temperature of that point falls below T b More than t b If the Cu liquid phase does not exist for more than 100 seconds, the other portions also satisfy this condition, and the penetration of the Cu liquid phase into grain boundaries in the entire circumferential direction of the slab is suppressed to a low level.

[0057] Furthermore, after quenching the slab to below the Ar1 point, it is preferable to reheat the slab surface to a temperature of (Ac3 point + 60°C) or higher to refine the surface structure and suppress microcracks due to the Cu liquid phase. In other words, the temperature T2 shown in FIG. 1 is preferably (Ac3 point + 60°C) or higher. If the temperature T2 shown in FIG. 1 is (Ac3 point + 60°C) or higher, even the lowest temperature part of the mold surface can be heated to a temperature of (Ac3 point + 60°C). If the temperature T2 shown in FIG. 1 is lower than (Ac3 point + 60°C), a portion of the mold surface will not reach the Ac3 point upon reheating. If the reheating temperature is lower than the Ac3 point, a portion of the structure may remain as a poorly ductile structure, such as tempered bainite. Depending on the conditions, this structure may cause transverse cracks or other cracks due to elements other than tramp elements. Therefore, it is preferable to reheat the slab so that the maximum surface temperature is (Ac3 point + 60°C) or higher.

[0058] It is known that when the surface temperature of a slab exceeds approximately 1250°C, the Cu liquid phase is more likely to be incorporated into the scale, and surface cracking tends to be suppressed, but reheating rarely causes the surface temperature of the slab to exceed 1250°C. In order to perform an operation in which the surface temperature of the slab exceeds 1250°C, a special device must be installed in the continuous casting equipment to raise the temperature, which makes it difficult to produce the slab at low operational cost, and therefore such conditions are not covered in this embodiment.

[0059] The Ar1 point and Ac3 point can be values ​​measured using a transformation point recording and measuring device (Formaster testing machine). The Ar1 point and Ac3 point may also be calculated using the following formulas (5) and (6) proposed in Non-Patent Document 3. Ar1=(52[C]+122[Si]+66[Cu]+6[Cr])-(65[Mn]+36[Ni]+58[Mo])-228.5 / log((Ac3-500) / v)+713 ···(5) Ac3=(32[Si]+17[Mo])-(231[C]+20[Mn]+40[Cu]+18[Ni]+15[Cr])+912...(6) Here, v in the formula represents the average cooling rate (°C / sec) from the Ac3 point to the cooling temperature, and [C], [Si], [Cr], [Mn], [Ni], and [Mo] represent the concentrations of C, Si, Cr, Mn, Ni, and Mo in the slab, respectively.

[0060] Furthermore, in the example shown in Figure 1, the slab is quenched immediately after coming out of the mold and cooled only once to a temperature T1 below the Ar1 point at the maximum temperature of the slab surface, but the number of cooling passes may be two or more. By increasing the number of cooling passes, the transformation from austenite to ferrite occurs multiple times, making it possible to further refine the surface structure. Also, increasing the number of cooling passes increases the number of reheating passes, but the temperature T b The time t b is the temperature T b The total time is the sum of the above times.

[0061] [Amount of Cu liquid phase produced per unit time] As mentioned above, Sn is an element that reduces the solubility of Cu in steel, while Ni and Si are elements that increase the solubility of Cu in steel. Based on laboratory experiments, the present inventors have used the value L expressed by the following formula (2) as an index of the amount of Cu liquid phase generated per unit time. Cu is defined as one of the parameters that is an index for preventing red embrittlement cracking. L Cu=[Cu]+4×[Sn]-3×[Ni]-0.07×[Si] ···(2)

[0062] [t b , L Cu Comprehensive evaluation of the two parameters As mentioned above, the value L Cu is an index of the amount of Cu liquid phase generated per unit time, the lower limit temperature T b The time t b and the value L Cu The product of t and t can be used as an index of the amount of Cu liquid phase generated. On the other hand, the average grain size after the reverse transformation from ferrite to austenite by reheating after cooling does not depend greatly on the reheating temperature or steel composition. b , L Cu The product of these two parameters can be used as an indicator of the penetration depth of the Cu liquid phase.

[0063] The inventors conducted experiments to investigate in detail the relationship between these two parameters and red embrittlement cracking. As a result, the tendency shown in Figure 2 was found. In Figure 2, the circle marks indicate cases where red embrittlement cracking did not occur, and the cross marks indicate cases where red embrittlement cracking occurred. Sets 21 and 22 show the results of the experiment in which the amount of cooling water during casting was changed to reduce the time t b Set 23 shows the results of a test group in which the concentrations of Cu and Sn in the molten steel were changed during casting. b ×L Cu It can be considered that red embrittlement cracking occurs when the value of exceeds a certain value. As a result of experiments conducted by the present inventors, it has been found that the following formula (3) must be satisfied in order to prevent red embrittlement cracking from occurring. t b ×L Cu ≦30 (3)

[0064] Depending on the composition of the slab, the value L Cu In such cases, the left side of equation (3) also becomes 0 or a negative value, regardless of the surface temperature history of the slab.Cu In the case of a composition where L is a negative value, it can be assumed that red embrittlement cracking will not occur regardless of the surface temperature history of the slab. Cu When the value L exceeds 0, the left side of the formula (3) also exceeds 0, and the composition is such that red embrittlement cracking may occur. Cu When is greater than 0, satisfying equation (3) can be said to produce a significant effect.

[0065] Also, the temperature T b The time t b The shorter the temperature T b Time to stay longer than t b In this case, the left side of the formula (3) is also 0 regardless of the composition of the slab. In this embodiment, the slab is cooled to the Ar1 point or below by rapid cooling, so the temperature T b The above time is short. Furthermore, time t b If the value of L is 0 seconds, it means that the temperature range where the Cu liquid phase becomes stable has not been reached by reheating. Cu It can be assumed that no red embrittlement cracking occurs regardless of the temperature. [Example]

[0066] Next, examples of the present invention will be described. Note that the data shown in these examples are merely examples of cases in which the present invention is applied, and the scope of application of the present invention is not limited by these examples.

[0067] Molten steel was produced in an electric furnace and then subjected to secondary refining to obtain the molten steel shown in Table 3. The molten steel was then poured into a mold via a tundish. The slab that emerged from the mold was cooled using a spray cooling device, and a slab measuring 2000 mm wide and 250 mm thick was produced using a curved continuous casting machine (five-point correction type) with a curvature radius of 12.0 m. The casting speed was 0.8 to 1.5 m / min. The slab was then cut into lengths of 5.0 ± 0.2 m using a gas cutting machine, and the surface was inspected. Surface cracking of the slab was evaluated visually and by magnetic particle testing after pickling the slab surface.

[0068] The surface temperature of the slab was measured using multiple radiation thermometers installed on the outer periphery of the curved section inside the continuous casting machine. A heat transfer and solidification analysis was performed using these actual measurements, along with heat removal conditions using cooling water and rolls, to determine the surface temperature distribution of the slab, and the surface temperature at the center of the width direction of the long side surface on the inner periphery of the curved section was used as the representative temperature. It was confirmed that there was no deviation of 20°C or more between the surface temperature calculated by heat transfer calculation and the actual measurement obtained from the radiation thermometer, and that the surface temperature of the slab did not exceed 1200°C at any point after the mold exit. From these calculation results, the time when a certain point of the slab left the mold was set to 0, and a temperature graph was plotted up to the straightening point, and the temperature T determined by the Cu and Sn contents in the steel was calculated. b At time t b It was read as follows.

[0069] [Table 3]

[0070] [Table 4]

[0071] The experimental results are shown in Table 4. In Table 4, ○ indicates that there were no cracks, △ indicates that the number of minor cracks was 10 or less per meter of slab length, and × indicates that neither of these conditions was met. All of the levels 1 to 15 satisfied the condition of formula (3), and all of them produced slabs with good surface quality and no cracks. On the other hand, the comparative levels 21 to 25 were produced in which the slab was not rapidly cooled after leaving the mold, or was rapidly cooled but the surface temperature did not reach the Ar1 point or below. Although they satisfied the condition of formula (3), all of them exhibited cracks on the slab surface. Furthermore, all of the levels 25 to 30 did not satisfy the condition of formula (3), and all of them exhibited cracks on the slab surface. [Explanation of symbols]

[0072] 21, 22, 23 set

Claims

1. In mass%, C: 0.002% or more and 0.250% or less, Si: 0.01% or more and 2.00% or less, Mn: 0.10% or more and 2.50% or less, P: 0.040% or less, S: 0.030% or less, Cu: 0.100% or more and 0.500% or less, Sn: 0.050% or less, Ni: more than 0.050% and not more than 0.300%, and N: 0.0040% or more and 0.0150% or less, and the remainder being Fe and impurities, using a curved or vertical bending type continuous casting machine, The maximum temperature of the slab surface from the time it leaves the mold to the time it reaches the straightening point is Ar. 1 a step of cooling the temperature to a temperature below the temperature limit and recuperating the temperature; From the start of the reheating to the straightening point, the maximum temperature of the slab surface is a temperature T b (°C) or more. b (seconds) and The value L determined by the following formula (2) according to the contents of Cu, Sn, Ni, and Si in the cast slab Cu and, A method for continuously casting a Cu-containing steel, characterized in that continuous casting is carried out so as to satisfy the condition of the following formula (3) in relation to the above. When [Sn]<0.005%, T b = 1100 ° C., When [Sn] ≧ 0.005% and Cu_eq < 0.150%, T b = 1100 ° C., [Sn] ≧ 0.005% and 0.150% ≦ Cu_eq < 0.200%, T b = 1070 ° C., [Sn] ≧ 0.005% and 0.200% ≦ Cu_eq < 0.250%. b = 1040 ° C., [Sn] ≧ 0.005% and 0.250% ≦ Cu_eq < 0.300%, T b = 1010 ° C., [Sn] ≧ 0.005% and 0.300% ≦ Cu_eq < 0.350%. b = 980 ° C. When [Sn] ≧ 0.005% and Cu_eq ≧ 0.350%, T b = 950°C ・・・（１） L Cu = [Cu] + 4 × [Sn] - 3 × [Ni] - 0.07 × [Si] ・・・ (2) t b ×L Cu ≦30 ・・・(3) Here, Cu_eq=[Cu]+4×[Sn], [Cu] represents the Cu concentration (mass%) in the slab, [Sn] represents the Sn concentration (mass%) in the slab, [Ni] represents the Ni concentration (mass%) in the slab, and [Si] represents the Si concentration (mass%) in the slab.

2. The cast piece further comprises: In mass%, Al: more than 0% and less than 0.100%, Cr: more than 0% but not more than 1.50%, Mo: more than 0% and less than 0.20%, Ti: more than 0% and less than 0.050%, V: more than 0% and less than 0.20%, Nb: more than 0% and less than 0.030%, Zr: more than 0% and less than 0.030%, Ca: more than 0% and less than 0.0100%, Mg: more than 0% and less than 0.0100%, REM: more than 0% and 0.0100% or less, and B: more than 0% and less than 0.0040%, 2. The method for continuous casting of Cu-containing steel according to claim 1, wherein the casting contains at least one selected from the group consisting of:

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