Continuous casting method for Cu-containing steel

Abstract

To provide a continuous casting method of Cu-containing steel capable of simply and inexpensively preventing the surface crack of a cast slab containing Cu.SOLUTION: Provided is a method for producing a cast slab containing at least 0.10% or more and 0.50% or less of Cu and 0.050% or less of Sn by using a continuous casting machine which uses a bending-type or vertical bending-type mold, wherein the cooling is controlled such that the time tb (seconds) required for the maximum temperature of the surface of the cast slab becoming the temperature Tb (°C) or more determined according to the component of the molten steel becomes 180 seconds or less during the duration from coming out of the mold until reaching the straightening point.SELECTED DRAWING: Figure 1

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, and it is known that these elements are 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] Furthermore, a technique for preventing cracking by controlling the surface temperature has also been proposed. Patent Document 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 it at 1200 to 1350°C within this temperature range for 1 hour or more, hot rolling it 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, and then air-cooling or accelerated cooling to a temperature range of 500 to 650°C at an average cooling rate of 1 to 80°C / s. Furthermore, Patent Document 4 discloses a method for continuously casting an electrical steel slab using a curved or vertical curved continuous casting machine, which comprises controlling the surface temperature of the slab passing through the straightening point in the cooling zone of the continuous casting machine to outside the temperature range of 800 to 900°C. [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] Patent No. 4016843 [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 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. However, depending on the type of continuous casting machine and the secondary cooling method, the mold flux adhesion to the slab surface may be unstable, preventing the full benefit of these methods. Furthermore, the method described in Patent Document 3 relates to hot rolling of slabs, but cannot be used in continuous casting, where the molten steel is withdrawn from the mold while solidifying. This process is completely different. Furthermore, the method described in Patent Document 4 is a technology for preventing hook cracks (transverse cracks) that extend from the corners of a slab toward the interior, measuring several millimeters to several centimeters, even when the steel does not contain Cu. The aforementioned red embrittlement cracking is known to occur between 1050°C and 1200°C, and its mechanism of occurrence is different from that of hook cracks. Therefore, this method is insufficient to prevent red embrittlement cracking.

[0010] In view of the above-mentioned problems, an object of the present invention is to provide a continuous casting method for 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: 0.03% or more and 0.40% 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: 0.10% or more and 0.50% or less, Sn: 0.050% or less, Ni: 0.050% or less, and N: 0.0040% or more and 0.0150% or less, and the remainder being Fe and unavoidable 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 is straightened is the following temperature T b (℃) or more time t b A continuous casting method for Cu-containing steel, characterized by controlling cooling so that (seconds) is 180 seconds or less. 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℃, Here, Cu_eq=[Cu]+4×[Sn], [Cu] represents the Cu concentration (mass%) in the slab, and [Sn] represents the Sn 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 0.020%, V: more than 0% and less than 0.200%, Nb: more than 0% and less than 0.030%, Zr: more than 0% and less than 0.010%, Ca: more than 0% and less than 0.010%, Mg: more than 0% and less than 0.010%, 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 (1) above, 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, it is possible to prevent surface cracks in Cu-containing slabs simply and inexpensively without providing any special device in a continuous casting facility. [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. 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. It is known that red embrittlement of Cu-containing steel generally occurs in the range of 1050 to 1200°C, and that the lower limit of red embrittlement temperature is extended downward by increasing the Cu concentration and the coexistence of Sn. To suppress 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, but in doing so, it is important to understand the effect of the Sn content on this temperature range.

[0015] The present inventors have conducted extensive research into the mechanism of slab cracking caused by Cu and Sn, and as a result have found that slab cracking is not affected by whether the surface temperature of the slab at the straightening point is in the red embrittlement temperature range or not, but is affected by the surface temperature history of the slab up to the straightening point. More specifically, this is due to the following mechanism.

[0016] After the slab is removed from the mold, selective oxidation occurs as scale forms, forming a layer enriched in tramp elements such as Cu and Sn. When the temperature of this enriched layer exceeds the solidus temperature, a Cu-based molten alloy is generated and concentrates at the interface between the scale and the base steel, some of which penetrates into the grain boundaries. Grain boundaries into which the Cu-based molten alloy has penetrated are easily opened by even very small strains caused by contact with the rolls of the continuous casting machine, and microcracks (hereinafter simply referred to as microcracks) with a depth of 0.2 mm or more form before reaching the straightening point. These cracks are subjected to tensile strain at the straightening point and propagate, becoming surface cracks with a depth of 1 mm or more, mainly on the upper surface of the slab.

[0017] The present inventors have found that the occurrence of such microcracks is related to the temperature history up to the leveling point, and then, in order to investigate the influence of the temperature history on the occurrence of microcracks in a cast slab, they conducted a microtensile test on a steel material by the method described below.

[0018] Round bar test specimens, 10 mm in diameter and 120 mm in length, were prepared from steel with the chemical composition shown in Table 1 below. These test specimens were then heated to 1400°C in a reduced-pressure, non-oxidizing atmosphere. After allowing the crystal structure to fully grow, the temperature was lowered to 900°C and the pressure was restored to atmospheric air. The specimens were held in this state for 900 seconds to allow scale growth, and then the temperature was raised to 900-1100°C and held there for 180 seconds. The round bars were then stretched to a depth of 0.5 mm, similar to the conditions experienced when the specimens were in contact with the rolls of a continuous casting machine, and then cooled to room temperature. The surface cracking of the test specimens was investigated. The experimental results are shown in Table 2.

[0019] In Table 2, "○" indicates an example in which no microcracks occurred, and "×" indicates an example in which microcracks occurred. Furthermore, "-" indicates an example in which the experiment was not conducted. The presence or absence of microcracks was determined by whether or not cracks of 0.2 mm or more in depth were confirmed along the prior austenite grain boundaries when observing the surface layer of the sample with an optical microscope. Furthermore, "Cu_eq" in Table 1 represents the Cu equivalent (mass%), which is expressed as the linear sum of Cu and Sn. If the Cu concentration (mass%) is [Cu] and the Sn concentration (mass%) is [Sn], then Cu_eq = [Cu] + 4 × [Sn].

[0020] [Table 1]

[0021] [Table 2]

[0022] As shown in Table 2, it was found that the presence or absence of microcracks is correlated with the Sn concentration and Cu equivalent (Cu_eq). The reason for setting the holding time to 180 seconds is that it is based on past experience showing the time it takes for Cu to penetrate into the steel. Experiments were also conducted on some samples with holding times of 300 and 600 seconds, but no significant effect on the presence or absence of microcracks was observed.

[0023] In addition, among the conditions indicated by an × in Table 2, after 0.5 mm of tension processing was performed, the sample temperature was lowered to 780-900°C, and then a 5 mm of tension processing (strain rate: 1 / s) was performed, which is a condition similar to the straightening at the straightening point of a continuous casting machine.As a result, regardless of the steel type or sample temperature, clear surface cracks were confirmed visually in the necked portion of the round bar test specimen in all samples, and the depth was in the range of 0.84-1.36 mm.

[0024] From the above experimental results, it was estimated that when the above-mentioned microcracks occur, the subsequent application of corrective strain in the continuous casting machine will cause harmful surface cracks due to red embrittlement.

[0025] Furthermore, the above experimental results confirmed that even with a holding time of 180 seconds, microcracks sometimes occur and sometimes do not, and that microcracks occur when the slab surface temperature remains above the lower limit temperature correlated with the Sn concentration and Cu equivalent (Cu_eq) for more than 180 seconds. For example, when comparing sample LT-A when the holding temperature is 1100°C with that of 1130°C, the time spent above a certain lower limit temperature is longer in the example where the holding temperature is 1300°C, since this is the time required to heat the material from 1100°C to 1130°C and the time required to cool it from 1130°C to 1100°C. In other words, the certain lower limit temperature can be estimated to be a temperature equal to or higher than 1100°C and lower than 1130°C. In the example where the holding temperature was 1100°C, the temperature remained above the certain lower limit temperature for 180 seconds or less, so no microcracks occurred. On the other hand, in the example where the holding temperature was 1130°C, the temperature remained above the certain lower limit temperature for more than 180 seconds, so it is thought that microcracks occurred.

[0026] Therefore, the inventors have derived this lower limit temperature and found the temperature conditions under which microcracks do not occur. In finding the temperature conditions, the same temperature was maintained for 180 seconds in the above experiment, but in actual continuous casting, the degree of heat recovery varies depending on the cooling conditions, making it impossible to maintain the same temperature for 180 seconds. Therefore, the temperature conditions were found by taking such differences into consideration.

[0027] It is known that when the slab surface temperature exceeds approximately 1250°C, the Cu-based molten alloy tends to be easily incorporated into the scale, and surface cracking tends to be suppressed, but the slab surface temperature after leaving the mold rarely exceeds 1250°C. In order to perform an operation in which the slab surface temperature 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 cost from an operational standpoint, and therefore such conditions are not covered in this embodiment.

[0028] As described above, in this embodiment, in continuous casting, the maximum temperature of the slab surface after it leaves the mold until it reaches the straightening point is set to the following temperature T b(℃) or more time t b Cooling shall be controlled so that (seconds) is 180 seconds or less. In mass%, When [Sn]<0.005%, T b =1100℃ [Sn] ≥ 0.005% and Cu_eq < 0.15%, 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℃

[0029] Here, the temperature T b and time t b This will be explained with reference to Figure 1. Figure 1 is a diagram for explaining an overview 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 to 0 seconds. On the other hand, the vertical axis represents the maximum temperature of the slab surface (center).

[0030] As shown in Figure 1, the slab is cooled by the spray cooling device immediately after it leaves the mold. b1 Only at temperature T b After cooling, the mold surface temperature rises due to heat recovery, and the temperature rises by time t b2 Only at temperature T b The time t immediately after ejection from the mold is b1 Since a Cu-based molten alloy can be generated even during the time t b is the time t b1and time t b2 In this embodiment, the time t b By controlling the cooling time to 180 seconds or less, micro-cracks are prevented and red embrittlement cracks at the straightening point are prevented.

[0031] In addition, the temperature T b The reason for not allowing the steel to stay for more than 180 seconds is to prevent the Cu-based molten alloy from penetrating deep into the grain boundaries on the steel surface. b The time spent above this temperature may be 0 seconds. The temperature of the slab surface can be determined by taking the temperature at one point in the circumferential direction of the slab. b If the above condition does not exceed 180 seconds, other portions also satisfy this condition, and the penetration of the Cu-based molten alloy into grain boundaries in the entire circumferential direction of the slab is suppressed to a low level.

[0032] In this embodiment, the temperature T b The reason for this is as follows. First, during the time it takes for Cu to infiltrate into the steel, there are cases where microcracks occur and cases where they do not. Therefore, the lower limit temperature T b Here, the temperature T is set based on a time period significantly shorter than 180 seconds (e.g., 60 seconds) or significantly longer than 180 seconds (e.g., 300 seconds). b If the temperature history is set to stricter or significantly more relaxed, it becomes impossible to clearly define the conditions under which microcracks do not occur. As mentioned above, based on past performance, the time it takes for Cu to penetrate into steel is thought to be 180 seconds, so using this time as a standard makes it easier to determine whether microcracks will occur.

[0033] The continuous casting machine used in this embodiment may be a curved type or a vertical bending type. Furthermore, if there are multiple straightening points, cooling is controlled under the above-mentioned conditions after the steel sheet leaves the mold until it reaches the first straightening point closest to the meniscus. Cooling control is mainly performed using a spray cooling device, which sprays water or mist to control the amount, time, and timing.

[0034] Next, the components of the steel (bill) specified in the present invention will be explained. Note that "%" in the following explanation means "% by mass."

[0035] [C: 0.03% or more and 0.40% 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 0.03% does not significantly improve these properties and only increases the cost of decarburization, making it undesirable. Therefore, the lower limit is set at 0.03%. Furthermore, if the carbon concentration exceeds 0.40%, toughness deteriorates. Therefore, the upper limit is set at 0.40%.

[0036] [Si:0.01% or more and 1.00% or less] Silicon is an element that can increase the strength of steel when added appropriately. To achieve this effect, the silicon content must be 0.01% or more. Therefore, the lower limit is set at 0.01%. On the other hand, if the silicon concentration exceeds 1.00%, toughness and workability will deteriorate significantly. Therefore, the upper limit is set at 1.00%.

[0037] [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 2.5%, toughness and workability deteriorate. Therefore, the upper limit is set to 2.50%.

[0038] [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.

[0039] [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.

[0040] [Cu:0.10% or more and 0.50% or less] If the Cu content is less than 0.10%, the amount of liquid phase generated by the oxidation of the steel is sufficiently small that cracking due to embrittlement does not occur or is not harmful. 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 0.10%, it is necessary to change the iron source composition, such as using high-grade scrap or reduced iron, to dilute the Cu, which increases costs. Therefore, the lower limit is set to 0.10%. On the other hand, if the Cu content exceeds 0.50%, it will have a negative effect on the steel's properties. Therefore, the upper limit is set to 0.50%.

[0041] [Sn:0.050% or less] Sn significantly lowers the liquidus 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 it. Therefore, the upper limit is set at 0.050%.

[0042] [Ni:0.050% or less] Ni is known to have the effect of suppressing red embrittlement cracking caused by Cu and Sn, but it is an expensive element, so it is desirable to add as little Ni as possible. Since intentionally increasing the content would incur high costs, the amount generally mixed in from scrap is sufficient. Therefore, the upper limit is set at 0.050%.

[0043] [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%.

[0044] In the present invention, in order to achieve the desired properties of the product, the molten steel may further contain one or more of the following elements, the lower limits of which are all greater than 0%:

[0045] [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 at 0.100%.

[0046] [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%.

[0047] [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 at 0.20%.

[0048] [Ti: more than 0% and less than 0.020%] Ti not only has a deoxidizing effect like 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 0.020%, the amount of nitride precipitates increases, increasing the susceptibility to cracking 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 at 0.020%.

[0049] [V: more than 0% and less than 0.200%] V, like Ti, is an element that forms nitrides and is used to improve strength. However, if the V concentration exceeds 0.200%, VN tends to grow coarsely, causing a decrease in fatigue strength. Therefore, the upper limit is set at 0.200%.

[0050] [Nb: more than 0% and less than 0.030%] Nb, like Ti, is an element that forms nitrides. A small amount of Nb significantly increases the strength of steel. However, if the Nb concentration exceeds 0.030%, not only does the effect saturate, but it also frequently causes cracks during casting. Therefore, the upper limit is set at 0.030%.

[0051] [Zr: more than 0% and less than 0.010%] Zr, like Ti, is an element that forms nitrides and other compounds, and has the effect of suppressing the coarsening of oxide 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 at 0.010%.

[0052] [Ca: more than 0% and less than 0.010%] 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 0.010% or less, preferably 0.005% 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 0.001% or more.

[0053] [Mg: more than 0% and less than 0.010%] 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. On the other hand, if the Mg concentration is too high, coarse cluster-like oxide-based inclusions composed mainly of MgO may form, which may become the starting point for fatigue fracture. Therefore, the Mg concentration is set to 0.010% or less, preferably 0.005% 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%, and more preferably 0.001% or more.

[0054] [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.

[0055] [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. [Example]

[0056] 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.

[0057] 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.

[0058] 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. Using these actual measurements and heat removal conditions using cooling water and rolls, a heat transfer and solidification analysis was performed 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 1250°C at any point after the mold exit. From these calculation results, the time when a certain position of the slab left the mold was set to 0, and a temperature graph was plotted up to the correction 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.

[0059] [Table 3]

[0060] [Table 4]

[0061] The experimental results are shown in Table 4. In Table 4, the absence of cracks is indicated by a circle, the number of minor cracks per 1 m of slab length is indicated by a triangle, and the number of cracks not falling into either category is indicated by an x. b In contrast, in the comparative examples 16 to 30, the time taken to reach the temperature Tb or higher exceeded 180 seconds, and cracks appeared on the slab surface.

Claims

1. In mass%, C: 0.03% or more and 0.40% 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: 0.10% or more and 0.50% or less, Sn: 0.050% or less, Ni: 0.050% or less, and N: 0.0040% or more and 0.0150% or less, and the remainder being Fe and unavoidable 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 is straightened is the following temperature T b (°C) or more. b A method for continuous casting of Cu-containing steel, characterized in that cooling is controlled so that (seconds) is 180 seconds or less. 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. Here, Cu_eq=[Cu]+4×[Sn], [Cu] represents the Cu concentration (mass%) in the cast slab, and [Sn] represents the Sn concentration (mass%) in the cast 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 0.020%, V: more than 0% and less than 0.200%, Nb: more than 0% and less than 0.030%, Zr: more than 0% and less than 0.010%, Ca: more than 0% and less than 0.010%, Mg: more than 0% and less than 0.010%, 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:

Images