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 0.005% or more and 0.050% or less of Sn by using a continuous casting machine which uses a bending-type or vertical bending-type mold, which method includes a step of cooling and recuperating the cast slab until the maximum temperature of the cast slab surface becomes an Ar1 point or lower during the duration from coming out of the mold until reaching the straightening point, and in which method, 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 becomes 180 seconds or less during the duration from the start of the recuperation until reaching the straightening point.SELECTED DRAWING: Figure 3

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]

[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: 0.03% 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: 0.005% or more and 0.050% or less, Ni: 0.100% 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 method includes a step of cooling and recuperating the slab 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, and the maximum temperature of the slab surface is equal to or lower than the Ar1 point during the period from when the slab starts to be recuperated until when the slab reaches the straightening point. 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 Cu_eq<0.250%, T b =1130℃, If 0.250%≦Cu_eq<0.300%, T b =1100℃, If 0.300%≦Cu_eq<0.350%, T b =1070℃, If 0.350%≦Cu_eq<0.400%, T b =1040℃, If 0.400%≦Cu_eq<0.450%, T b =1010℃, If 0.450%≦Cu_eq<0.500%, T b =980℃, When Cu_eq≧0.500%, 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 0.20%, 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 0.0100%, 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 showing the temperature history of a hot tensile test using a round bar test piece. [Figure 2] FIG. 1 shows the temperature history of a hot tensile test using a round bar test piece including 0.5 mm tension processing. [Figure 3] 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] Hereinafter, embodiments of the present invention will be described in detail. 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, Cu-based molten alloys are generated and concentrated at the interface between the scale and the base steel, some of which penetrate into the grain boundaries. Grain boundaries penetrated by Cu-based molten alloys are prone to opening even with 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 cracks are subjected to tensile strain at the straightening point, propagating and becoming surface cracks of 1 mm or more in depth, primarily on the upper surface of the slab. It is known that continuous casting of molten steel containing high concentrations of Cu and Sn tends to produce larger amounts of Cu-based molten alloys, which tend to penetrate deeper into the grain boundaries.

[0017] The inventors have found that the occurrence of such microcracks is related to the temperature history up to the correction point, and have attempted to make the surface cracks of the final slab harmless by refining the surface structure of the slab during continuous casting, thereby increasing the volume fraction of grain boundaries, which are penetration paths for the Cu-based molten alloy, and thereby minimizing the size of each crack. Furthermore, by appropriately controlling the surface temperature of the slab in the continuous casting machine, the inventors have attempted to neutralize the surface cracks of the final slab. To investigate the conditions under which this mechanism is established, microtensile tests were carried out on steel materials using the method described below, and the effect of temperature history on the occurrence of microcracks in the slab was investigated.

[0018] First, round bar specimens measuring 10 mm in diameter and 120 mm in length were prepared from steel with the chemical composition shown in Table 1 below. Tensile tests were performed using a hot tensile testing machine capable of atmospheric control. The temperature history is shown in Figure 1. First, the specimens were heated to 1400°C in a non-oxidizing atmosphere under reduced pressure and held there for 120 seconds to allow sufficient growth of the crystal structure. After 60 seconds had elapsed since reaching that temperature, the pressure was returned to air, allowing scale to form on the specimen surface. From this state, the specimens were rapidly cooled to the specified temperature at a rate of 15°C / s, then immediately heated to 920-1130°C and held there for 180 seconds. The round bars were then stretched 0.5 mm and cooled to room temperature, and the occurrence of cracks on the specimen surface was examined. In addition, "Cu_eq" in Table 1 represents the Cu equivalent (mass%), which is the sum of Cu and four times Sn. If the Cu concentration (mass%) is [Cu] and the Sn concentration (mass%) is [Sn], then Cu_eq = [Cu] + 4 × [Sn].

[0019] [Table 1]

[0020] To conduct a more detailed investigation, the inventors selected two types of samples (HT-D and ET-E) from the samples shown in Table 1 and investigated the effect of the temperature reached after quenching ("T1" in Figure 1) on the occurrence of microcracks. The results are shown in Table 2. In Table 2, "○" indicates an example in which no microcracks occurred, "×" indicates an example in which microcracks occurred, and "-" indicates an example in which the experiment was not performed. 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. Table 2 also shows the Ar1 temperature at a cooling rate of 15°C / s measured for each level using a transformation temperature recording and measuring device (Formaster testing machine).

[0021] [Table 2]

[0022] In this experiment, the holding temperature ("T2" in Figure 1) for 180 seconds after reheating was set in 30°C increments. Comparing the maximum temperature at which microcracks appeared, we found that when the temperature reached after quenching (T1) was below Ar1, the maximum temperature at which microcracks appeared tended to be 60–90°C higher than when it was not. Since the behavior of scale formation and growth is unlikely to differ significantly for the same sample at the same holding temperature (T2), this tendency is likely due to the steel structure—specifically, the grain size of the sample's surface—when held at temperature (T2) for 180 seconds. In other words, we found that setting the temperature reached after quenching (T1) below Ar1 can reduce grain boundary penetration of Cu-based molten alloys after reheating. The reason for setting the holding time to 180 seconds is that it is based on the time it takes for Cu to infiltrate into the steel, as previously reported.

[0023] Next, the influence of the holding temperature (T2) on the occurrence of microcracks was investigated using the 20 types of samples shown in Table 1. The quenching temperature reached before holding at temperature T2 for 180 seconds was set to be below the Ar1 point in all cases. The results are shown in Table 3.

[0024] [Table 3]

[0025] As shown in Table 3, it was found that the presence or absence of microcracks is correlated with the Cu equivalent (Cu_eq). Furthermore, experiments were also conducted on some samples with holding times of 300 seconds and 600 seconds, but no significant effect on the presence or absence of microcracks was observed.

[0026] Furthermore, under the conditions indicated by an × in Table 3, as shown in Figure 2, the specimens were stretched 0.5 mm, then the specimen temperature was lowered to 780-900°C, and a further 5 mm stretch (strain rate: 1 / s) was performed under conditions similar to those used for straightening at the straightening point of a continuous casting machine. As a result, regardless of the steel type or specimen temperature, clear surface cracks were visually observed in the necked portion of the round bar specimens in all specimens, with depths ranging from 0.80 to 3.6 mm.

[0027] From the above experimental results, it was estimated that if 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. Furthermore, it was also found that by setting the temperature reached after quenching to Ar1 point or below, the structure can be refined and the penetration of Cu-based molten alloy into grain boundaries after reheating can be reduced.

[0028] Furthermore, the above experimental results confirmed that even with a holding time of 180 seconds, microcracks sometimes occurred and sometimes did not. It was also confirmed that microcracks occurred when the slab surface temperature remained above the lower limit temperature correlated with Cu equivalent (Cu_eq) for more than 180 seconds. For example, comparing sample HT-A with holding temperatures of 1130°C and 1160°C, the time spent above the lower limit temperature was longer for the 1160°C holding temperature, due to the time required to heat the slab from 1130°C to 1160°C and the time required to cool it from 1160°C to 1130°C. In other words, the lower limit temperature can be estimated to be a temperature between 1130°C and 1160°C. In the 1130°C holding temperature, microcracks did not occur because the holding time remained above the lower limit temperature for less than 180 seconds. In contrast, in the 1160°C holding temperature, microcracks likely occurred because the holding time remained above the lower limit temperature for more than 180 seconds.

[0029] 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.

[0030] It is also 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 operational cost, and therefore such conditions are not covered in this embodiment.

[0031] As described above, in this embodiment, in continuous casting, after the slab leaves the mold, it is cooled and reheated until the maximum temperature of the slab surface becomes equal to or lower than the Ar1 point until it reaches the straightening point. Furthermore, from the start of reheating until it reaches the straightening point, the maximum temperature of the slab surface is maintained at the following temperature T b (℃) or more time t b Cooling shall be controlled so that (seconds) is 180 seconds or less.

[0032] In mass%, When Cu_eq<0.250%, T b =1130℃ If 0.250%≦Cu_eq<0.300%, T b =1100℃ If 0.300%≦Cu_eq<0.350%, T b =1070℃ If 0.350%≦Cu_eq<0.400%, T b =1040℃ If 0.400%≦Cu_eq<0.450%, T b =1010℃ If 0.450%≦Cu_eq<0.500%, T b =980℃ When Cu_eq≧0.500%, T b =950℃

[0033] Here, the temperature T b and time t bThis will be explained with reference to Figure 3. Figure 3 is a diagram for explaining an overview of the temperature history from inside the mold to the straightening point. The horizontal axis of Figure 3 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).

[0034] As shown in Figure 3, immediately after leaving the mold, the slab is rapidly cooled using a spray cooling device to a temperature T1, which is the maximum temperature of the slab surface and is below the Ar1 point. If the temperature T1 reached after rapid cooling is higher than the Ar1 point, the transformation from austenite to ferrite does not occur sufficiently, and the surface structure cannot be sufficiently refined. This reduces the volume fraction of the grain boundaries, which are the penetration path for the Cu-based molten alloy. Subsequent reheating allows the Cu-based molten alloy to penetrate deeper into the grain boundaries, making microcracks more likely to occur.

[0035] 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. 3, in this embodiment, after reheating, the surface temperature T2 is b The time t b By controlling the time to be 180 seconds or less, micro-cracks are prevented and red embrittlement cracks at the correction point are prevented.

[0036] 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. This is because if the temperature of that point becomes Ar1 or lower during rapid cooling, the other points will also satisfy this condition, and structural modification of the entire circumferential direction of the slab can be expected. Furthermore, if the temperature of that point becomes T 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.

[0037] 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.

[0038] 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 caused by the Cu-based molten alloy. In other words, the temperature T2 shown in FIG. 3 is preferably (Ac3 point + 60°C) or higher. If the temperature T2 shown in FIG. 3 is (Ac3 point + 60°C) or higher, even the coldest part of the mold surface can be heated to a temperature of (Ac3 point + 60°C). If the temperature T2 shown in FIG. 3 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 structure with poor ductility, such as tempered bainite. Depending on the conditions, this structure may cause transverse cracks or other cracks due to factors other than tramp elements. Therefore, it is preferable to reheat the slab so that the maximum temperature of the slab surface is (Ac3 point + 60°C) or higher.

[0039] 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 (1) and (2) 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 ···(1) Ac3=(32[Si]+17[Mo])-(231[C]+20[Mn]+40[Cu]+18[Ni]+15[Cr])+912...(2) Here, v in the formula represents the average cooling rate (°C / sec) from the Ac3 point to the cooling temperature, and [C], [Si], [Cu], [Cr], [Mn], [Ni], and [Mo] represent the concentrations of C, Si, Cu, Cr, Mn, Ni, and Mo, respectively, in the cast slab.

[0040] 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.

[0041] Furthermore, in the example shown in Figure 3, 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.

[0042] Next, the components of the steel (bill) specified in the present invention will be explained. Note that "%" in the following explanation means "% by mass."

[0043] [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%.

[0044] [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%.

[0045] [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%.

[0046] [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.

[0047] [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.

[0048] [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%.

[0049] [Sn: 0.005% or more and 0.050% or less] Sn significantly lowers the liquidus stabilization temperature of the tramp element-enriched layer that forms with scale formation, significantly increasing the susceptibility to red embrittlement cracking, so it is desirable to avoid its inclusion as much as possible. If the Sn concentration is less than 0.005%, the amount of liquid phase generated by the above mechanism is sufficiently reduced, thereby suppressing embrittlement cracking to some extent. On the other hand, when using scrap as an environmental measure, low-quality scrap containing relatively high concentrations of Sn may be used. If the Sn concentration is less than 0.005%, it is necessary to change the iron source composition, such as high-grade scrap or reduced iron, to dilute the Sn, which increases costs. Therefore, the lower limit is set to 0.005%. On the other hand, if the Sn concentration exceeds 0.050%, red embrittlement cracking occurs even at lower temperatures, requiring a large amount of Ni to suppress it, which is undesirable. Therefore, the upper limit is set to 0.050%.

[0050] [Ni:0.100% 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 as possible. Since intentionally increasing the content incurs a lot of costs, the amount generally mixed in from scrap is sufficient. Therefore, the upper limit is set to 0.100%. The lower limit of the Ni concentration is not particularly limited and may be 0%, but in order to obtain the above effect, the Ni concentration is preferably 0.050% or more. In addition, the temperature T b is determined, and therefore, when the Ni concentration is [Ni], in relation to the Cu equivalent (Cu_eq), it is preferable that [Ni] / Cu_eq>0.5.

[0051] [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%.

[0052] 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%:

[0053] [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%.

[0054] [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%.

[0055] [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%.

[0056] [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%.

[0057] [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 at 0.20%.

[0058] [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%.

[0059] [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%.

[0060] [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.

[0061] [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.

[0062] [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.

[0063] [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]

[0064] 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.

[0065] Molten steel was produced in an electric furnace and then subjected to secondary refining to obtain the molten steel shown in Table 4. The molten steel was then poured into a mold via a tundish. The cast slab was cooled and produced into a 2000 mm wide x 250 mm thick cast slab using a curved continuous casting machine (five-point correction type) with a 12.0 m radius of curvature. The water flow rate of the spray cooling device was adjusted to control the reheating temperature, thereby varying the cooling temperature T1 and the reheating temperature T2. 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 cutter and the surface was observed. The slab surface was pickled and evaluated for surface cracking using magnetic particle testing.

[0066] 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.

[0067] [Table 4]

[0068] [Table 5]

[0069] The experimental results are shown in Table 5. In Table 5, the absence of cracks is indicated by a circle, the number of minor cracks per meter of slab length is 10 or less is indicated by a triangle, and the number of cracks not falling into either category is indicated by an x. In all of the levels 1 to 16, the cooling temperature T1 is below the Ar1 point and the temperature T b Since the time required for the above to occur was within 180 seconds, the cast slabs obtained had good surface quality and no cracks.

[0070] On the other hand, in the comparative examples 17 to 22, the ultimate cooling temperature T1 was higher than the Ar1 point, and therefore cracks appeared on the slab surface. b Since the time when this occurred exceeded 180 seconds, cracks appeared on the surface of all the cast pieces.

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: 0.005% or more and 0.050% or less, Ni: 0.100% 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 reaches the straightening point is Ar. 1 a step of cooling the slab surface to a 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 Cu_eq<0.250%, T b = 1130 ° C., When 0.250%≦Cu_eq<0.300%, T b = 1100 ° C., When 0.300%≦Cu_eq<0.350%, T b = 1070 ° C., When 0.350%≦Cu_eq<0.400%, T b = 1040 ° C., When 0.400%≦Cu_eq<0.450%, T b = 1010 ° C., When 0.450%≦Cu_eq<0.500%, T b = 980°C, When Cu_eq≧0.500%, 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 0.20%, 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 0.0100%, 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:

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