METHOD FOR CONTINUOUSLY CASTING Cu-CONTAINING STEEL, AND METHOD FOR MANUFACTURING ROLLED STEEL PRODUCT

Abstract

To provide a method for continuously casting a Cu-containing steel, capable of preventing surface cracking of a Cu-containing casting piece in a simple and inexpensive manner, and to provide a method for manufacturing a rolled steel product.SOLUTION: In a method for continuously casting Cu-containing steel, the continuous casting is performed such that a product of an average crystal grain size index R determined according to the C, Si, and Mn contents of the casting piece, time tb (seconds) required for a maximum temperature of a surface of the casting piece to reach a temperature Tb (°C) or more determined according to the Cu and Sn contents from when the casting piece is removed from the mold to when the casting piece reaches the correction point, and a value LCu determined according to Cu, Sn, Ni, and Si contents of the casting piece is 9 or less.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a method for continuous casting of Cu-containing steel suitable for preventing surface cracks, and a method for producing a rolled steel material. [Background technology]

[0002] In recent years, from the perspective of preventing global warming, CO 2 Efforts to reduce emissions are being actively undertaken. In the steel industry, the ironmaking process that uses a large amount of waste scrap as a raw material for steelmaking has attracted attention, and technological development is underway. 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 normal continuous casting conditions, cracks may occur on the surface of the slab. This is believed to be because, when the steel is exposed to oxygen in the atmosphere during continuous casting and oxidized, liquid Cu is generated between the scale (iron oxide) and the base steel, which penetrates into the grain boundaries of the steel and reduces the strength of the interface (see Non-Patent Document 1). In addition, Sn reduces the solubility of Cu in steel, thereby promoting the phenomenon of cracking caused by Cu, so that even steels in which Sn and Cu coexist are prone to have problems with surface cracks on the slab (see Non-Patent Document 2).

[0004] This phenomenon is called surface embrittlement, and is believed to be caused by the fact that Cu and Sn are less easily oxidized than Fe, so that Cu and Sn concentrate in the metallic state during the scale growth process, and by the low solid solubility of Cu in Fe. On the other hand, Cu and Sn are difficult to remove during the steel refining process. In order to solve the problem of surface cracking of cast slabs due to embrittlement, it is effective to prevent Cu and Sn from being mixed into steel, or to add Ni, an element that increases the solubility of Cu in steel. In particular, in the current recycling-based society where a large amount of scrap containing a lot of Cu is used, there is an increasing need to make Cu harmless by adding Ni. However, Ni is a rare and expensive element, and it can significantly change the mechanical properties and hardenability of steel, so there are high expectations for a Cu and Sn harmless technology that does not rely on Ni addition or can keep the amount of Ni added to a very small amount.

[0005] In view of this, Patent Document 1 discloses a continuous casting method as a technique for preventing the red embrittlement of the surface of a slab, which uses a mold in which the shape of the inner surface of the mold near the molten steel surface is an inverse taper shape with a reverse taper value of 2 to 10% widening downward in the slab drawing direction and the shape of the inner surface of the mold below the inverse taper part is a forward taper shape narrowing in the slab drawing direction, the forward taper value being in the range of 0 to 1%, and a mold flux having a crystallization temperature of 900°C or less or a non-crystallizing property is used, and the contact angle between the mold flux and the steel is 70° or less. Patent Document 2 discloses a continuous casting method characterized by forming a coating layer of nickel oxide on the surface of the slab while supplying a mold flux containing Ni oxide.

[0006] Also, a technique for preventing cracks by controlling the surface temperature has been proposed. Patent Document 3 discloses a method for producing a Cu-containing high-strength steel material, characterized in that the steel slab is heated at an average heating rate of 50°C / h or more between 1000 and 1100°C, and then held at 1200 to 1350°C and in this temperature range for 1 hour or more, and then hot-rolled at a cumulative rolling reduction rate of 50% or more in a temperature range of 1000°C or more and a rolling finishing temperature of 700°C or more, and then air-cooled or accelerated-cooled at an average cooling rate of 1 to 80°C / s to a temperature range of 500 to 650°C. Furthermore, Patent Document 4 discloses a method for continuously casting an electromagnetic steel slab using a curved or vertical curved continuous casting machine, characterized in that the surface temperature of the slab passing through the straightening point of the cooling zone of the continuous casting machine is controlled to be outside the temperature range of 800 to 900°C. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] JP 2004-202523 A [Patent Document 2] Special Publication No. 2018-520004 [Patent Document 3] JP 2011-168843 A [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 are both intended to prevent oxidation of the slab surface layer by mold flux, and depending on the type of continuous casting machine and the secondary cooling method, the mold flux adhesion state to the slab surface is not stable, so the effects cannot be fully enjoyed. In addition, the method described in Patent Document 3 is related to hot rolling of the slab, and cannot be adopted in continuous casting, since the process is completely different in the continuous casting process in which the molten steel is solidified and pulled out of the mold. Furthermore, the method described in Patent Document 4 is a technology for preventing hook cracks (transverse cracks) that extend from the corners of the slab toward the inside with a length of several mm to several cm, which occur even if the steel does not contain Cu. The above-mentioned red embrittlement cracks are known to occur in the range of approximately 1050°C to 1200°C, and are cracks with a different generation mechanism from hook cracks. Therefore, this method is insufficient to prevent red embrittlement cracks.

[0010] The object of the present invention is to provide a method for continuous casting of Cu-containing steel, which can simply and inexpensively prevent surface cracks in Cu-containing slabs, and a method for producing rolled steel material. [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: 0.10% or more and 0.50% 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 value R determined by the following formula (1) according to the contents of C, Si, and Mn of the slab; The maximum temperature of the slab surface from the time it leaves the mold to the time it is straightened is the temperature T b (℃) or more t b (seconds), The value L determined by the following formula (3) according to the contents of Cu, Sn, Ni, and Si in the slab Cu and, A method for continuous casting of a Cu-containing steel, comprising the steps of: If C_eq<0.061, R=0.4 If 0.061≦C_eq<0.081, R=-1.43+30×C_eq If 0.081≦C_eq, R=1.0 (1) [Sn] < 0.005%, T b = 1100°C, [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%, T b = 1040°C, [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℃, (2) L Cu=[Cu]+4×[Sn]-3×[Ni]-0.07×[Si] ···(3) t b ×L Cu ×R≦9.0 (4) Here, Cu_eq = [Cu] + 4 × [Sn], C_eq = [C] - 0.027 × [Si] + 0.006 × [Mn] - 0.001 × [Si] × [Mn], where [Cu] represents the Cu concentration (mass%) in the slab, [Sn] represents the Sn concentration (mass%) in the slab, [C] represents the C concentration (mass%) in the slab, [Si] represents the Si concentration (mass%) in the slab, [Mn] represents the Mn concentration (mass%) in the slab, and [Ni] represents the Ni concentration (mass%) in the slab. [2] The cast piece further comprises: In mass%, Al: more than 0.000% and less than 0.100%, Cr: more than 0.00% and less than 1.50%, Mo: more than 0.00% and less than 0.20%, Ti: more than 0.00% and less than 0.050%, V: more than 0.00% and less than 0.20%, Nb: more than 0.000% and less than 0.030%, Zr: more than 0.000% and less than 0.030%, Ca: more than 0.0000% and less than 0.0100%, Mg: more than 0.0000% and less than 0.0100%, REM: more than 0.0000% and less than 0.0100%, B: More than 0.0000% and 0.0040% or less, and Bi: more than 0.000% and less than 0.200%, The method for continuous casting of a Cu-containing steel according to the above [1], characterized in that the steel contains one or more selected from the group consisting of: [3] A method for producing rolled steel, comprising hot rolling a slab produced by the method for continuous casting of Cu-containing steel according to the above-mentioned [1] or [2] without surface treatment. Effect of the Invention

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

[0013] [Figure 1] FIG. 1 is a graph showing the relationship between C_eq and the average crystal grain size index R. [Diagram 2] FIG. 1 is a diagram for explaining an overview of the temperature history from inside the mold to the straightening point. [Diagram 3] FIG. 1 is a diagram showing the relationship between the three parameters R, tb, and LCu. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0014] Hereinafter, the embodiments of the present invention will be described in detail. First, the metallurgical effect will be described below. 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. For example, in the case of a composition in the δ solidification region, where the C concentration is low, the structure of the γ grains in the surface layer of the slab tends to be relatively fine, and the space for penetration into the grain boundaries is large, so the Cu penetration depth tends to be suppressed. On the other hand, in the case of a composition in the hypoperitectic, peritectic, or hyperperitectic solidification region, the structure of the γ grains in the surface layer of the slab tends to be coarse, and the space for penetration into the grain boundaries is limited, so the Cu penetration depth tends to be large. Hereafter, the hypoperitectic, peritectic, and hyperperitectic solidification regions will be referred to as the vicinity of the peritectic solidification region.

[0015] It is known that red embrittlement of Cu-containing steel generally occurs in the range of 1050°C to 1200°C, and the lower limit of red embrittlement temperature is expanded downward by increasing Cu concentration and coexistence of Sn. In order to suppress red embrittlement cracking during continuous casting, it is effective to maintain the surface temperature of the slab as low as possible below the temperature range, but in this case, it is important to understand the effect of the Sn component on the temperature range. Furthermore, the amount of liquid phase containing Cu generated per unit time varies depending on the components contained in Cu-containing steel, and the content of Sn, which reduces the solubility of Cu in steel, and Ni, which increases the solubility of Cu in steel, are important.

[0016] The inventors have thoroughly studied the mechanism of slab cracking caused by Cu and Sn, and have found that red embrittlement cracking occurs due to a combination of factors including the average crystal grain size of the slab surface, the surface temperature history of the slab from the time it leaves the mold to the straightening point, and the amount of liquid phase containing Cu generated per unit time. In the case of a composition near the peritectic solidification region, the Cu liquid phase is more likely to penetrate deep into the slab than in the case of a composition in the δ solidification region, but the amount of Cu liquid phase generated is determined by the surface temperature history of the slab and the compositions of Cu, Sn, Ni, etc., so by controlling these factors and suppressing the amount of Cu liquid phase generated, the Cu penetration depth can be reduced and red embrittlement cracking can be prevented.

[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. In addition, when there are multiple straightening points, cooling is controlled under the conditions described below until the first straightening point closest to the meniscus is reached after the steel is released from the mold. The cooling is mainly controlled using a spray cooling device, and the amount, time, and timing of water or mist sprayed by spraying are controlled.

[0018] In the method according to the present disclosure, the slab may be a slab, a bloom, or a billet. In particular, the slab or bloom is preferable, and the slab is more preferable. The slab may have a width and a thickness in a cross-sectional shape perpendicular to the continuous casting direction, and may have a length in the continuous casting direction. When the slab is a slab, its width corresponds to the long side (corresponding to the long side of the mold) in the cross-sectional shape perpendicular to the continuous casting direction, and its thickness corresponds to the short side (corresponding to the short side of the mold) in the cross-sectional shape. In addition, when the slab is a bloom or a billet, its aspect ratio (length of long side / length of short side) is generally smaller than that of a slab. When the slab is a slab, its width may be, for example, 800 mm or more and 2200 mm or less, and its thickness may be, for example, 100 mm or more and 400 mm or less. When the cast piece is a bloom, its width may be, for example, 300 mm or more and 800 mm or less, and its thickness may be, for example, 200 mm or more and 600 mm or less.

[0019] In the method according to the present disclosure, the rolled steel material may be a steel plate, a steel pipe, a steel bar, or a wire rod, and is particularly preferably a steel plate.

[0020] Next, the components of the steel (cast piece) defined in the present invention will be described. Note that "%" in the following description means "% by mass."

[0021] [C: 0.002% or more and 0.250% or less] C is the most fundamental element that affects not only the static strength of steel, but also its fatigue strength, toughness, and ductility. A C concentration of less than 0.002% does not significantly improve these properties and only leads to increased decarburization costs, which is undesirable. Therefore, the lower limit is set to 0.002%. Furthermore, if the C concentration exceeds 0.250%, toughness deteriorates. Therefore, the upper limit is set to 0.250%.

[0022] [Si: 0.01% or more and 2.00% or less] Silicon is an element that can increase the strength of steel when added in an appropriate amount. To obtain this effect, it is necessary to include 0.01% or more of silicon. The silicon content is preferably 0.10% or more. Therefore, the lower limit is set to 0.01%. On the other hand, if the silicon concentration exceeds 2.00%, toughness and workability are significantly deteriorated. Therefore, the upper limit is set to 2.00%. The silicon content is preferably 1.00% or less.

[0023] [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 secured. 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%.

[0024] [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%. Note that the lower the P content, the better, so 0% may be acceptable.

[0025] [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 concentration, the better, so 0% is acceptable.

[0026] [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 oxidation of the steel material is sufficiently small, and cracks due to embrittlement do not occur or are not harmful. On the other hand, when scrap is used as an environmental measure, inferior scrap containing relatively high concentrations of Cu may be used. If the Cu concentration is less than 0.10%, it is necessary to change the composition of the iron source, such as high-grade scrap or reduced iron, to dilute the Cu, which leads to an increase in costs. Therefore, the lower limit is set to 0.10%. On the other hand, if the Cu concentration exceeds 0.50%, it will have an adverse effect on the material properties of the steel. Therefore, the upper limit is set to 0.50%. The Cu content is preferably 0.35% or less.

[0027] [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 mixing as much as possible. If the Sn concentration exceeds 0.050%, red embrittlement cracking occurs even at a lower temperature, so a large amount of Ni is required to suppress red embrittlement cracking, which is undesirable. Therefore, the upper limit is set to 0.050%. The Sn content is preferably 0.30% or less.

[0028] [Ni: more than 0.050% and less than 0.300%] Ni increases the static strength of steel, but on the other hand, it reduces elongation, so that when manufacturing steel materials such as low carbon steel, in which elongation is more important than strength, reduced iron and the like are added to reduce the Ni concentration. If the Ni concentration is 0.050% or less, a large amount of expensive reduced iron is required compared to scrap, but the effect of improving elongation is small, which leads to increased costs and is not preferable. Therefore, Ni is made to contain more than 0.050%. Also, Ni is known to have the effect of suppressing red embrittlement cracking caused by Cu and Sn, but it is an expensive element, and intentionally increasing the content will incur a lot of costs, so the amount generally mixed in from scrap is sufficient. Therefore, the upper limit is set to 0.300%. The Ni content is preferably 0.200% or less.

[0029] [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 it is not preferable to set the N concentration below 0.0040% because the degassing process requires a long time, which increases costs. Therefore, the lower limit is set to 0.0040%. On the other hand, if the N concentration exceeds 0.0150%, it is not preferable because it causes the nitride inclusions to become coarse and causes a decrease in fatigue strength. Therefore, the upper limit is set to 0.0150%, but from the viewpoint of steel cleanliness, it is preferable to set the upper limit to 0.0080%.

[0030] In the present invention, in order to realize the properties required for the product, the slab may further contain one or more of the following elements. Note that since the elements described below are optionally contained, the content of each element may be less than the lower limit value described for each element, but in order to realize the properties described below, the lower limit of the content of each element is the concentration of the lower limit value described for each element.

[0031] [Al: more than 0.000% and less than 0.100%] Al is an element that is widely used for deoxidization purposes, but if the Al concentration exceeds 0.100%, problems such as nozzle clogging during casting and oxide-based inclusions remaining in the steel deteriorating performance are likely to occur. Therefore, the upper limit is set to 0.100%. The Al content is preferably 0.030% or less.

[0032] [Cr: more than 0.00% 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 it leads to an increase in cost, which is not preferable. Therefore, the upper limit is set to 1.50%. The Al content is preferably 1.20% or less.

[0033] [Mo: more than 0.00% 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%. The Mo content is preferably 0.15% or less.

[0034] [Ti: more than 0.000% and less than 0.050%] 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 0.050%, the amount of nitride precipitates increases, and the cracking sensitivity due to embrittlement at around 700°C (region III) increases. In addition, nozzle clogging due to oxides frequently occurs during casting, which is not preferable. Therefore, the upper limit is set to 0.050%. The Ti content is preferably 0.030% or less.

[0035] [V: more than 0.00% and less than 0.20%] V, like Ti, is an element that generates nitrides and is used to improve strength. However, if the V concentration exceeds 0.20%, VN tends to grow coarsely, which causes a decrease in fatigue strength. Therefore, the upper limit is set to 0.20%. The V content is preferably 0.15% or less.

[0036] [Nb: more than 0.000% and less than 0.030%] Nb is an element that forms nitrides, just like Ti. A small amount of Nb has the effect of significantly increasing the strength of steel. On the other hand, 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%. The Nb content is preferably 0.025% or less.

[0037] [Zr: more than 0.000% and less than 0.030%] Zr, like Ti, is an element that forms nitrides and the like, and has the effect of suppressing the coarsening of oxide-based inclusions. On the other hand, if the Zr concentration exceeds 0.010%, not only does the effect saturate, but it also causes clogging of the submerged nozzle used to inject molten steel into the mold. Therefore, the upper limit is set to 0.030%. The Zr content is preferably 0.025% or less.

[0038] [Ca: more than 0.0000% and less than 0.0100%] Ca is Al 2 O 3On the other hand, if the Ca content is too high, the CaO-Al 2 O 3 However, Ca may form coarse oxide-based inclusions mainly composed of Ca, which may become the starting point of fatigue fracture. Therefore, the Ca concentration is set to 0.0100% or less, and preferably 0.0050% or less. The lower limit of the Ca concentration is not particularly limited and may be 0%, but in order to obtain the effect of suppressing the coarsening of oxide-based inclusions, it is preferably more than 0.0000%, and more preferably 0.0010% or more.

[0039] [Mg: more than 0.0000% and less than 0.0100%] Mg is similar to Ca in that it is Al. 2 O 3 It has the effect of modifying the surface roughness and suppressing the coarsening of oxide-based inclusions. It also acts on sulfide-based inclusions and has the effect of reducing the aspect ratio of the sulfide-based inclusions after rolling. On the other hand, if the Mg concentration is too high, coarse cluster-like oxide-based inclusions containing MgO as the main component may be formed, which may become the starting point of fatigue fracture. Therefore, the Mg concentration is set to 0.0100% or less, and preferably 0.0050% or less. There is no particular limit to the lower limit of the Mg concentration, and it may be 0.0000%, but in order to obtain the effect of suppressing the coarsening of oxide-based inclusions, it is preferably more than 0.0000%, and more preferably 0.0010% or more.

[0040] [REM: More than 0.0000% and less than 0.0100%] REM is also Al 2 O 3It has the effect of modifying the steel and suppressing the coarsening of oxide-based inclusions. On the other hand, if the REM content is too high, it may reduce the cleanliness of the steel and deteriorate the toughness of the steel. Therefore, the REM concentration is set to 0.0100% or less, and preferably 0.0050% or less. There is no particular limit to the lower limit of the REM concentration, and it may be 0.0000%, but in order to obtain the effect of suppressing the coarsening of oxide-based inclusions, it is preferable that the REM concentration is more than 0.0000%, and more preferably 0.0003% or more. Note that REM refers to rare earth elements such as La and Ce, and any one or two or more of these REMs can be used.

[0041] [B: More than 0.0000% 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 is saturated and cracks are likely to occur during casting. Therefore, the B concentration is 0.0040% or less, and preferably 0.0030% or less. There is no particular lower limit for the B concentration, and it may be 0.0000%, but in order to obtain the effect of improving the mechanical properties, it is preferably more than 0.0000%, and more preferably 0.0001% or more.

[0042] [Bi: more than 0.000% and less than 0.200%] A small amount of Bi has the effect of improving the mechanical properties of steel. On the other hand, if the Bi content is too high, the effect is saturated and cracks are likely to occur during casting. Therefore, the Bi concentration is set to 0.200% or less, and preferably 0.050% or less. There is no particular lower limit for the Bi concentration, and it may be 0.000%, but in order to obtain the effect of improving the mechanical properties, it is preferably more than 0.000%, and more preferably 0.003% or more.

[0043] The balance of the chemical composition of the steel slab according to the present embodiment may be Fe and impurities. Examples of impurities include elements that are inevitably mixed into molten steel from the scrap steel raw material or the atmosphere, and are permissible within a range that does not impair the properties of the steel slab according to the present embodiment. Examples of such impurities include O, Sb, and As.

[0044] Next, detailed conditions for preventing red embrittlement cracking will be described. As described above, important parameters for preventing red embrittlement cracking include the average crystal grain size in the surface layer of the slab, the surface temperature history of the slab from leaving the mold to the straightening point, and the amount of liquid phase containing Cu generated per unit time. In this embodiment, red embrittlement cracking is prevented by comprehensively evaluating these parameters. These parameters will be described in detail below.

[0045] [Average grain size in the surface layer of the 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, and therefore the Cu liquid phase does not penetrate deep into the slab surface. gb is the average crystal grain size (equivalent sphere diameter) R c and the grain boundary thickness d, it is expressed by the following equation (5). f gb = {(surface area of ​​crystal grain) × d / 2} / (volume of crystal grain) = 3d / R c (5)

[0046] Here, the Cu infiltration depth when the same amount of Cu liquid phase is generated is the grain boundary volume fraction f gb Since the average grain size R c In this embodiment, the average grain size index R in the surface layer of a slab is defined as one of the parameters that is an index for preventing red embrittlement cracking, based on the grain size (equivalent sphere diameter) of γ grains in a composition near the peritectic solidification region.

[0047] When defining the average grain size index R, the following C_eq, calculated based on a phase diagram, is used. Note that [C], [Si], and [Mn] respectively represent the C concentration, Si concentration, and Mn concentration (all in mass%) in the cast slab. C_eq=[C]-0.027×[Si]+0.006×[Mn]-0.001×[Si]×[Mn]

[0048] Next, the inventors conducted an experiment using a directional solidification experimental apparatus in order to find the relationship between C_eq expressed by the above formula and the average crystal grain size index R. Specifically, steels of various compositions were melted and solidified in a directional solidification experimental apparatus consisting of a water-cooled copper plate and a refractory tube, and then rapidly cooled from 1000°C. The structure was observed from the surface in contact with the water-cooled copper plate to a depth of 1 mm, and the average grain size of γ grains was measured.

[0049] FIG. 1 is a diagram showing the relationship between C_eq and the average grain size index R obtained from the experiment. The horizontal axis represents C_eq, and the vertical axis represents the average grain size index R of γ grains. As can be seen from FIG. 1, when C_eq≧0.081, the composition is in the hypoperitectic to hyperperitectic region, so the average grain size index R is fixed at 1. On the other hand, when C_eq<0.061, the composition is in the δ solidification region. Compared to steel with a composition in the hypoperitectic solidification region, the average grain size of γ grains is about 0.4 times, so when C_eq<0.061, the average grain size index R is fixed at 0.4. In addition, when 0.061≦C_eq<0.081, the composition is also in the δ solidification region, but the average grain size of γ grains tends to increase with increasing C_eq, so the average grain size index R is a value that corresponds to C_eq.

[0050] Based on the above, the average grain size index R is defined as follows in three stages according to the composition of the slab: If C_eq<0.061, R=0.4 If 0.061≦C_eq<0.081, R=-1.43+30×C_eq If 0.081≦C_eq, R=1.0 (1)

[0051] [Cast surface temperature history] After the slab is pulled out of the mold, a concentrated layer of tramp elements such as Cu and Sn is formed by selective oxidation in conjunction with the formation of scale. When the temperature of this concentrated layer exceeds the solidus temperature, a Cu liquid phase is generated and concentrated at the interface between the scale and the base steel, and some of the Cu liquid phase penetrates into the grain boundaries. The grain boundaries into which the Cu liquid phase penetrates 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 are formed before the straightening point is reached. These microcracks are subjected to tensile strain at the straightening point and extend, becoming harmful surface cracks with a depth of 1 mm or more, mainly on the upper surface of the slab, due to red embrittlement.

[0052] In particular, in the case of Cu-containing steel, the solidus temperature is significantly lowered by the coexistence of Sn, so 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 up to the straightening point, but also with the Sn concentration and Cu equivalent (Cu_eq). Here, the Cu equivalent represents the equivalent (mass%) expressed as the linear sum of Cu and Sn, and if the Cu concentration (mass%) is [Cu] and the Sn concentration (mass%) is [Sn], then Cu_eq = [Cu] + 4 × [Sn]. In other words, the longer the slab surface temperature stays above the lower limit temperature correlated with the Sn concentration and Cu equivalent (Cu_eq), the more likely microcracks are to occur.

[0053] 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 In this case, the temperature T b was calculated according to the Sn concentration and Cu equivalent as shown in the following formula (2). [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℃ (2)

[0054] It is known that when the slab surface temperature exceeds about 1250° C., the Cu liquid phase is 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, and since it is not possible to produce the slab at low cost in terms of operation, such conditions are not covered in this embodiment.

[0055] 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. 2. Fig. 2 is a diagram for explaining an outline of the temperature history from inside the mold to the straightening point. The horizontal axis of Fig. 2 represents the time from the meniscus in the mold, with the time when the slab leaves the mold being set as 0 seconds. Meanwhile, the vertical axis represents the maximum temperature of the slab surface (center).

[0056] As shown in Fig. 2, 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 recuperation, and the temperature increases by time t b2Only at temperature T b The time t immediately after removal from the mold is b1 Since the Cu liquid phase can be generated during the time t b is the time t b1 and time t b2 The total shall be the sum of the above.

[0057] [Amount of Cu liquid phase produced per unit time] As described 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 L represented by the following formula (3) 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. Note that [Ni] represents the Ni concentration (mass%) in the slab. L Cu =[Cu]+4×[Sn]-3×[Ni]-0.07×[Si] ···(3)

[0058] [R, t b , L Cu Comprehensive evaluation of the three parameters As mentioned above, L Cu is an index of the amount of Cu liquid phase generated per unit time, so the lower limit temperature T b The time t b and L Cu The product of R and t can be used as an index of the amount of Cu liquid phase generated. On the other hand, the average grain size index R is an index that represents the infiltration depth per unit amount of Cu liquid phase generated. b , L Cu The product of these three parameters can be used as an index of the penetration depth of the Cu liquid phase.

[0059] The inventors conducted experiments to investigate in detail the relationship between these three parameters and red embrittlement cracking. As a result, the trends shown in Figure 3 were found. In Figure 3, circles indicate cases where red embrittlement cracking did not occur, and crosses indicate cases where red embrittlement cracking occurred. Set 31 shows the results for steel types with a composition of C_eq<0.061, set 32 ​​shows the results for steel types with a composition of C_eq≧0.081, and set 33 shows the results for steel types with a composition of 0.061≦C_eq<0.081. In this way, R×t 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 was found that the following formula (4) needs to be satisfied as a condition for preventing red embrittlement cracking from occurring. R×t b ×L Cu ≦9.0 (4)

[0060] Depending on the composition of the slab, L Cu In some cases, L may be 0 or a negative value. In such cases, the left side of equation (4) will also be 0 or a negative value, regardless of the temperature history of the slab or the average grain size of the γ grains. Cu In the case of a composition where L is a negative value, it can be considered that red embrittlement cracking does not occur regardless of the temperature history of the slab. Cu When L exceeds 0, the left side of the formula (4) also exceeds 0, and the composition is such that red embrittlement cracking may occur. Cu It can be said that a significant effect can be obtained by satisfying formula (4) when is greater than 0.

[0061] [Manufacturing method of rolled steel] Since the slab produced by the method according to the present disclosure is prevented from having surface cracks, even if hot rolling is performed without surface treatment, surface cracks of the rolled steel material can be prevented. In the present invention, surface treatment of the slab includes melt cutting (sometimes called hot scarf) and mechanical grinding using a grinder or the like. The slab produced by the method according to the present disclosure may be subjected to hot rolling without heating or may be subjected to hot rolling after heating, depending on the surface temperature of the slab before hot rolling. The surface temperature of the slab immediately before hot rolling is preferably 1000°C or higher and 1150°C or lower. The surface temperature of the rolled steel material at the exit side of the final rolling mill is preferably 850°C or higher and 1000°C or lower. EXAMPLES

[0062] Next, an embodiment of the present invention will be described. Note that the data shown in the embodiment is merely an example of the application of the present invention, and is not intended to limit the scope of application of the present invention.

[0063] (First embodiment) Molten steel was produced in an electric furnace and then subjected to secondary refining to obtain the molten steel shown in Table 1. The steel composition shown in Table 1 is composed of Fe and impurities as the balance. The molten steel was then poured into a mold through a tundish, and the slab coming out of the mold was cooled by a spray cooling device. A slab with a width of 2000 mm, a thickness of 250 mm, and a length of 9000 mm was produced using a curved continuous casting machine (five-point correction type) with a curvature radius of 12.0 m, and immediately cut into a length of 5.0 ± 0.2 m and the remaining part by a gas cutting machine. The casting speed at this time was 0.8 to 1.7 m / min. The slab with a length of 5.0 ± 0.2 m was cooled to room temperature, and the surface was observed, and the surface cracks of the slab were evaluated by visual surface observation and magnetic particle inspection of a 500 mm sample taken from an arbitrary position after pickling the slab surface. If a crack of 5 mm or more was found by visual inspection, magnetic particle inspection, or both, it was determined that there was a crack.

[0064] 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 face 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 obtained by heat transfer calculation and the actual measurement value obtained from the radiation thermometer, and that the surface temperature of the slab did not exceed 1250°C at any position after the mold exit. From the calculation results, the time when a certain position of the slab left the mold was set to 0, and a temperature graph was drawn up to the straightening point, and the temperature T determined by the Cu and Sn contents in the steel was calculated. b Time t b It was read as follows.

[0065] [Table 1]

[0066] [Table 2]

[0067] The experimental results are shown in Table 2. In Table 2, those without cracks are marked with an ◯, and those with cracks are marked with an X. Levels 1, 3 to 7, 10, and 11 all satisfied the condition of formula (4), and all of the cast slabs had good surface quality and no cracks. On the other hand, levels 2, 8, 9, and 12, which are comparative examples, did not satisfy the condition of formula (4), and all of the cast slabs had cracks on the surface.

[0068] (Second Example) Molten steel was produced in an electric furnace and then subjected to secondary refining to obtain the molten steel shown in Table 3. The balance of the steel composition shown in Table 3 was Fe and impurities. Continuous casting was carried out in the same manner as in the first example, and the surface cracks of the cast slab were evaluated.

[0069] The remaining slabs that were not subjected to the evaluation of surface cracks were subjected to hot rolling immediately or after heating in a heating furnace without surface treatment. Table 4 shows whether the slabs were heated or not. The temperature of the slabs immediately before hot rolling was 1030 to 1140 ° C, and the temperature of the rolled steel plate at the final rolling mill outlet was 900 to 950 ° C to obtain a rolled steel plate with a thickness of 3.0 mm. The rolled steel plate was then cut into a length of 5.0 ± 0.2 m using a gas cutter, and the surface cracks were visually observed to investigate the presence or absence of cracks. If cracks were confirmed, samples for cross-sectional observation were taken from three locations in order of the length of the cracks, and the cross sections including the cracks were mirror-polished and observed with an optical microscope to measure the maximum crack depth for each sample, and the largest crack depth among the three samples was taken as the maximum crack depth for that level.

[0070] [Table 3]

[0071] [Table 4]

[0072] The experimental results are shown in Table 4. In Table 4, the evaluation of slab cracks is indicated by a circle for no cracks, a triangle for minor cracks of 10 or less per meter of slab length, and an x ​​for neither. Levels 21 to 37 all satisfied the condition of formula (4), and all of the resulting slabs had good surface quality and were free of cracks. On the other hand, levels 38 to 47, which are comparative examples, did not satisfy the condition of formula (4), and all of the slabs exhibited cracks on their surfaces.

[0073] In addition, in the evaluation of cracks in rolled steel sheets, cracks and flaws observed were judged as × if the maximum crack depth was 10 μm or more, and ◯ if it was less than 10 μm. All of levels 21 to 37 satisfied the condition of formula (4), and all had a maximum crack depth of less than 10 μm, satisfying the target of the present invention. On the other hand, all of levels 38 to 47, which are comparative examples, did not satisfy the condition of formula (4), and all had a maximum crack depth of 10 μm or more, not satisfying the target of the present invention.

Claims

1. In mass percent, 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: 0.10% or more and 0.50% 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 value R determined by the following formula (1) according to the contents of C, Si, and Mn of the cast slab; The maximum temperature of the slab surface from the time it leaves the mold to the time it is straightened is the temperature T b (℃) or more t b (seconds), The value L is determined by the following formula (3) according to the contents of Cu, Sn, Ni, and Si in the cast slab. Cu and, A method for continuous casting of a Cu-containing steel, comprising the steps of: If C_eq<0.061, R=0.4 If 0.061≦C_eq<0.081, R=−1.43+30×C_eq If 0.081≦C_eq, R=1.0 ・・・（１） When [Sn] is less than 0.005%, T b = 1100 ° C. [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%, T 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%, T b = 980°C, When [Sn] ≧ 0.005% and Cu_eq ≧ 0.350%, T b = 950°C, ・・・（２） L Cu = [Cu] + 4 × [Sn] - 3 × [Ni] - 0.07 × [Si] ・・・ (3) t b ×L Cu ×R≦9.0 ・・・(4) Here, Cu_eq = [Cu] + 4 × [Sn], C_eq = [C] - 0.027 × [Si] + 0.006 × [Mn] - 0.001 × [Si] × [Mn], [Cu] represents the Cu concentration (mass %) in the slab, [Sn] represents the Sn concentration (mass %) in the slab, [C] represents the C concentration (mass %) in the slab, [Si] represents the Si concentration (mass %) in the slab, [Mn] represents the Mn concentration (mass %) in the slab, and [Ni] represents the Ni concentration (mass %) in the slab.

2. The cast piece further comprises: In mass percent, Al: more than 0.000% and less than 0.100%, Cr: more than 0.00% but not more than 1.50%, Mo: more than 0.00% and less than 0.20%, Ti: more than 0.000% and less than 0.050%, V: more than 0.00% and less than 0.20%, Nb: more than 0.000% and less than 0.030%, Zr: more than 0.000% and less than 0.030%, Ca: more than 0.0000% and less than 0.0100%, Mg: more than 0.0000% and less than 0.0100%, REM: more than 0.0000% and less than 0.0100%, B: More than 0.0000% and not more than 0.0040%, and Bi: more than 0.000% and less than 0.200%, 2. The method for continuous casting of Cu-containing steel according to claim 1, further comprising the step of:

3. 3. A method for producing a rolled steel plate, comprising hot rolling a slab produced by the method for continuous casting of Cu-containing steel according to claim 1 or 2 without surface treatment.

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