Thin-walled cast slab of Cu-containing steel and its manufacturing method

Abstract

To provide a thin cast slab of a Cu-containing steel having a thickness of 6 mm or less in which Cu-cracking on a surface is reduced to a non-problematic level.SOLUTION: A thin cast slab of a Cu-containing steel comprising: Cu: 0.10 to 3.00% and Sn: 1.00% or less and with a thickness of 6 mm or less, wherein on a surface perpendicular to a thickness direction of the slab within a distance of 2 mm from the surface, 100 or more inclusions / mm2 having an equivalent circle diameter of 1 μm or more and 5 μm or less and a composition satisfying a formula (1) are distributed. (Cr2O3) / {(Cr2O3)+(MnO)+(SiO2)+(Al2O3)}×100≥1 (1). In the formula (1), description of (oxide name) means a content (mass%) of the oxide in the inclusions.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a thin-walled cast slab of Cu-containing steel and a method for producing the same, and more particularly to a thin-walled cast slab of Cu-containing steel that can be produced by a twin-drum continuous casting method and a method for producing the same. [Background technology]

[0002] The main raw materials for steel are pig iron, produced by reducing iron ore in a blast furnace, and scrap. The use of scrap is increasing due to rising domestic iron stockpiles and the need to conserve and recycle resources. One of the challenges with using scrap is the adverse effects of tramp elements, which are difficult to remove during the molten iron refining process. Cu and Sn are typical tramp elements, and when Cu is present alone or in combination with Sn, surface cracks on the slab can occur, known as Cu cracking or Cu embrittlement (hereafter simply referred to as Cu cracking).

[0003] The mechanism of Cu cracking is thought to be as follows: During continuous casting, when a slab descends from the mold and cools, or in the heating furnace before hot rolling, the slab is exposed to a high-temperature atmosphere and its surface oxidizes. Before the slab oxidizes, Cu is in solid solution within the steel. As oxidation progresses from the slab surface, Cu cannot dissolve in the scale, which is an oxide, so Cu is expelled to the scale / base metal interface beneath the scale coating and gradually accumulates at the interface. The melting point of Cu is low, at 1083°C. When the steel temperature rises above this melting point, liquid Cu distributes at the scale / base metal interface.

[0004] During continuous casting, the slab is exposed to a high-temperature atmosphere until it descends from the mold and cools. Generally, the temperature to which the slab is heated before hot rolling is around 1250°C, and hot rolling is carried out in the temperature range from around 1200°C to 900°C. In this temperature range, the steel structure is mainly austenite.

[0005] Liquid Cu accumulated at the scale / base metal interface penetrates into the grain boundaries (austenite grain boundaries) due to energy balance. However, because liquid Cu itself has no bonding strength, even if it fills the space, it is mechanically in a "cracked" state. Therefore, when stress during casting or rolling is applied to the grain boundaries into which liquid Cu has penetrated, the grain boundaries cannot withstand the stress, and the cracks propagate into the interior of the cast slab. If liquid Cu is distributed at the scale / base metal interface, it is supplied to the cracks, and eventually the liquid Cu accumulated at the interface penetrates into the grain boundaries. Thus, the more liquid Cu accumulates at the scale / base metal interface, the deeper the Cu cracks become. After the supply of liquid Cu from the interface stops, the opening width expands rather than the depth increases. Furthermore, when Sn is present together with Cu, it has the effect of promoting Cu cracks.

[0006] As can be understood from the above explanation, the greater the amount of oxidation on the slab surface, the greater the amount of liquid Cu that accumulates at the interface, and therefore the deeper the Cu cracks become. Therefore, reducing the amount of oxidation on the slab surface is one approach to reducing Cu cracks.

[0007] In the current mainstream slab continuous casting method, which is designed for mass production, the casting thickness is generally 200 mm or more, so the cooling rate is slow and the surface remains at high temperatures for a long time. Furthermore, after casting, the casting must be significantly reduced to approach the product thickness, so it must be heated to a high temperature, for example, around 1250°C, in a heating furnace before rolling. This results in a large amount of surface oxidation.

[0008] On the other hand, in the twin-belt or twin-drum continuous casting processes, where the slab is relatively thin, the cooling rate of the slab is high. Furthermore, since the thickness of the slab is close to the product thickness, the amount of reduction required is small. Therefore, the twin-belt continuous casting process can reduce the pre-rolling heating temperature, and the twin-drum continuous casting process can even omit the heating step. For these reasons, the amount of surface oxidation can be reduced in the rapid cooling casting process, where the slab is thin.

[0009] Therefore, thin-walled cast slabs of Cu-containing steel free from Cu cracks have been proposed, which are characterized by being cast by twin-belt continuous casting or twin-drum continuous casting, which are rapid cooling casting processes.

[0010] Patent Document 1 is directed to thin slab casting or twin-belt continuous casting for slabs with a thickness of 75 mm or less. It discloses a continuous casting method in which the residence time in the temperature range of 1300 to 1050°C during continuous casting is limited to less than one minute to prevent Cu cracking in the slab, and Cu cracking during rolling is prevented by not heating the slab to 1050°C or higher in the pre-hot rolling heating furnace.

[0011] Patent Document 2 is directed to a twin-drum continuous casting method and discloses a continuous casting method characterized by cooling to 550°C in a non-oxidizing state in molten salt.

[0012] The technologies described in Patent Documents 1 and 2 are characterized by casting using a twin-belt / twin-drum continuous casting method, limiting the residence time within a predetermined temperature range, and creating an oxidation-free state by cooling in molten salt. In other words, it is shown that simply casting using a twin-belt / twin-drum continuous casting method is not sufficient, and additional measures are required to reduce or prevent Cu cracking to below the target level. [Prior art documents] [Patent documents]

[0013] [Patent Document 1] Japanese Patent Application Publication No. 6-292949 [Patent Document 2] Japanese Patent Application Publication No. 7-100593 Summary of the Invention [Problem to be solved by the invention]

[0014] However, when the techniques of Patent Documents 1 and 2 are applied to continuous casting of thin-walled cast strips having a thickness of 6 mm or less, the following problems arise.

[0015] To satisfy the cooling conditions described in Patent Document 1, the slab must be intensely cooled from immediately below the mold. As described in the examples, Patent Document 1 assumes twin-belt continuous casting, which produces slabs with thicknesses of several tens to 75 mm and a casting speed of at most 10 m / min. On the other hand, even if this technology is applied to twin-drum continuous casting, which produces thin slabs with thicknesses of only a few millimeters, it is difficult to achieve intense cooling to satisfy the conditions described in Patent Document 1 using water cooling, a common cooling method. Because twin-drum continuous casting slabs are thin and brittle at high temperatures immediately below the casting drum, significantly increasing the water pressure or water flow can cause the thin slabs to fracture or break. Furthermore, the casting speed of twin-drum continuous casting is fast, at several tens of meters per minute, which requires a long water-cooling zone, resulting in high equipment costs.

[0016] Patent Document 2 assumes a twin-drum continuous casting method, in which cooling is performed using molten salt, which has high cooling efficiency, instead of water cooling. However, this requires a chamber to store the molten salt, and in reality, it is very difficult to construct and operate equipment with an entrance and exit that allows the slabs transported at high speed to pass through.

[0017] The object of the present invention is to provide a thin-walled cast slab of Cu-containing steel having a thickness of 6 mm or less, in which surface Cu cracking has been reduced to a level that is practically negligible, and to provide a method for producing the thin-walled cast slab of Cu-containing steel that can be achieved by ordinary water cooling rather than by special techniques such as molten salt cooling. [Means for solving the problem]

[0018] That is, the gist of the present invention is as follows. [1] In mass%, Contains C: 0.20% or less, Si: 0.001 to 0.25%, Mn: 0.15 to 2.00%, Al: 0.001 to 0.010%, Cr: 0.01 to 1.00%, and Cu: 0.10 to 3.00%; P: 0.020% or less, S: 0.015% or less, Sn: 1.00% or less, and the remainder consisting of Fe and impurities; A thin-walled cast piece with a thickness of 6 mm or less, The number of inclusions with a circle equivalent diameter of 1 μm or more and 5 μm or less and whose composition satisfies formula (1) is 100 / mm on a plane perpendicular to the thickness direction of the slab within 2 mm from the surface. 2 A thin-walled cast slab of Cu-containing steel, characterized in that the Cu content is distributed as above. (Cr2O3) / {(Cr2O3)+(MnO)+(SiO2)+(Al2O3)}×100≧1 ···(1) In formula (1), the description of (oxide name) means the content (mass %) of the oxide in the inclusion. [2] In place of a portion of the Fe, further in mass% A thin-walled cast slab of Cu-containing steel according to [1], characterized in that it contains one or more of Ca: 0.0040% or less, Ti: 0.050% or less, Nb: 0.050% or less, V: 0.050% or less, Mo: 0.050% or less, Ni: 1.00% or less, and B: 0.0050% or less.

[0019] [3] A method for producing a thin-walled cast slab of Cu-containing steel according to [1] or [2], characterized in that Cr is added to molten steel having a dissolved oxygen content of 0.0300% by mass or more, and the molten steel is cast. [Effects of the Invention]

[0020] According to the present invention, the depth of Cu cracks can be reduced and Cu cracks can be mitigated in thin-walled cast slabs of Cu-containing steel. DETAILED DESCRIPTION OF THE INVENTION

[0021] As mentioned above, liquid Cu accumulated at the scale / base metal interface penetrates into the grain boundaries (austenite grain boundaries). It was thought that by refining the austenite grains (hereafter also referred to as γ grains) and increasing the grain boundary area, the amount of liquid Cu penetrating per unit grain boundary area could be reduced, thereby reducing the penetration depth of liquid Cu and the extent of Cu cracking. Furthermore, it was thought that if a large number of fine inclusions could be dispersed in the steel, these inclusions would suppress the growth of austenite grains, leading to their refinement and an increase in the area of the austenite grain boundaries. As a result, the penetration depth of liquid Cu into the γ grain boundaries adjacent to the scale / base metal interface would be reduced, thereby reducing the depth of Cu cracking.

[0022] The inclusions present in steel are mainly oxide-based inclusions that remain in the steel as deoxidation products. In steel that has undergone strong Al deoxidation, oxides mainly consisting of Al2O3 remain in the steel. However, while the present invention aims to suppress the growth of austenite grains and refine them using finely dispersed inclusions, it is difficult to finely disperse Al2O3-based inclusions in steel.

[0023] The inventors noticed that the Cr2O3-MnO oxides produced by combined deoxidation with the weak deoxidizing elements Cr and Mn are small in size and easily dispersed in large numbers throughout the steel. Therefore, they attempted a method in which molten steel produced in a converter was tapped into a ladle, and weakly deoxidized by adding Cr and Mn before deoxidation with Al and Si, followed by finish deoxidation with Al and Si. Even in the Al finish deoxidation, if too much Al was added, the Cr and Mn deoxidation products produced would be reduced, so the Al content of the slab was kept low.

[0024] In the cast slabs produced as described above, attention was focused on inclusions with a circle equivalent diameter of 1 to 5 μm remaining near the surface of the cast slab, and the relationship between the number density of the inclusions in the cast slab and the occurrence of Cu cracks in the cast slab was evaluated. As a result, the number density of inclusions with a circle equivalent diameter of 1 to 5 μm was 100 pieces / mm 2 It was found that if the above conditions are met, it is possible to reduce Cu cracks that occur in the slab and also Cu cracks that occur when the slab is hot-rolled.

[0025] The present invention has been made based on the above findings, and will be described in detail below. The present invention is directed to thin-walled cast slabs of Cu-containing steel having a thickness of 6 mm or less and a method for producing the same. The reason for limiting the invention to Cu-containing steel is that the invention is directed to solving problems specific to Cu-containing steel. The reason for limiting the invention to thin-walled cast slabs having a thickness of 6 mm or less is that the effects of the invention are reliably achieved for thicknesses of 6 mm or less. A twin-drum continuous casting method is preferably used as a continuous casting method for casting thin-walled cast slabs having a thickness of 6 mm or less.

[0026] <Composition, shape, and density of inclusions> As described above, the present invention enables a large number of small inclusions to be dispersed in steel by forming Cr2O3-MnO-based oxides that are generated by complex deoxidation with Cr and Mn, which are weak deoxidizing elements.

[0027] The oxides formed in the steel according to the present invention, which have the chemical composition and are produced by the manufacturing method, are primarily composed of Cr2O3, MnO, SiO2, and Al2O3. The present invention focuses on the fact that the Cr2O3-MnO-based oxides formed by complex deoxidation with Cr and Mn, which are weak deoxidizing elements, are small in size and easily dispersed in large quantities throughout the steel. Specifically, the Cr2O3-MnO-SiO2-Al2O3-based oxides ultimately formed from the Cr2O3-MnO-based oxides are finely dispersed in large quantities to suppress the growth of γ grains. Among the Cr, Mn, Si, and Al components of these oxides, Cr has particularly weak deoxidizing strength. Therefore, excessive addition of Si or Al would completely reduce the existing Cr2O3. Therefore, the upper limits of the Si and Al contents in the steel are set forth below. It is important to control the composition to ensure that Cr2O3 remains in the inclusions.

[0028] Therefore, the composition of the oxides contained in inclusions is defined by formula (1). In formula (1), the (oxide name) denotes the content (mass%) of that oxide in the inclusion. Formula (1) indicates that it is sufficient for the Cr2O3 concentration in the Cr2O3-MnO-SiO2-Al2O3 oxide to be 1% or higher, i.e., it is sufficient for Cr2O3 to remain in the inclusion. If Cr oxides, which have the weakest deoxidizing strength, remain in the inclusion, this indicates evidence of Cr deoxidation, and indicates that the final inclusion was formed through the formation of Cr2O3-MnO oxides, which are easily finely dispersed.

[0029] As long as formula (1) is satisfied, the inclusions may contain other oxides, sulfides, carbonitrides, etc. For example, MnS may adhere to part of the periphery of the oxide.

[0030] Inclusions whose composition satisfies formula (1) are finely dispersed in steel, and therefore satisfy the conditions described in claim 1, that is, the inclusions have a circle equivalent diameter (1 μm or more and 5 μm or less) and a number density (100 pieces / mm 2 Since a large number of γ grains can be dispersed so as to satisfy the above condition, the growth of γ grains can be suppressed.

[0031] The specified circle equivalent diameter of the inclusion is 1 μm or more and 5 μm or less. Inclusions with a circle equivalent diameter of 1 μm or more are easy to observe, including their composition. Inclusions with a circle equivalent diameter of more than 5 μm do not contribute much to suppressing the growth of γ grain size. Then, in a thin cast slab, the number density of inclusions with a circle equivalent diameter of 1 μm or more and 5 μm or less and whose composition satisfies formula (1) was evaluated on a plane perpendicular to the thickness direction of the slab at a distance of 2 mm from the surface. Furthermore, when the occurrence of Cu cracks in the thin cast slab and the steel plate rolled from the thin cast slab was confirmed, the number density of oxides satisfying the above requirement was found to be 100 / mm 2 It was found that if the above conditions are met, the occurrence of Cu cracks in the thin-walled cast slab and the steel plate obtained by rolling the thin-walled cast slab can be suppressed.

[0032] (Method for measuring equivalent circle diameter) It is preferable to use a scanning electron microscope (SEM) to measure the equivalent circle diameter of the inclusions and analyze their composition to evaluate whether they satisfy formula (1). The specified equivalent circle diameter of the inclusions is 1 μm or more. Inclusions can be easily identified at a magnification of 400x or more, and observing at even higher magnification, such as 2000x, can improve the accuracy of size measurement. Since the number of observation fields increases as the magnification increases, the conditions can be set by considering the balance between workload and accuracy. At 400x or more, inclusions with a maximum length of 1 μm or more are selected, and their size is measured by observing at a higher magnification as necessary. The shape is approximated as an ellipse, with the maximum length taken as the major axis and the length perpendicular to the maximum length as the minor axis, and the equivalent circle diameter Dave. = √(major axis × minor axis) is calculated.

[0033] (Method for calculating the average composition of inclusions) EDS analysis (energy dispersive characteristic X-ray analysis) attached to an SEM is widely used to analyze the composition of inclusions. Inclusions with an equivalent circle diameter of 1 μm to 5 μm within 2 mm from the surface on a plane perpendicular to the thickness direction, i.e., parallel to the surface, are observed, and the cross-sectional average composition of each inclusion can be determined using the method described below.

[0034] Inclusions can be either a homogeneous phase or comprised of multiple phases, which can be determined from the SEM image. If the interior of the inclusion is a homogeneous phase, it is sufficient to measure one representative point, usually the center. If the inclusion is comprised of multiple phases, the cross-sectional average composition is determined. In this case, the SEM electron beam irradiation range can be expanded to measure the entire cross section at once, or point analysis can be performed for each phase that can be considered to be homogeneous internally, and the cross-sectional average composition can be calculated by multiplying the area ratio.

[0035] The main composition of the inclusions observed in the thin cast slabs of the present invention is Cr2O3-MnO-SiO2-Al2O3-based oxides, or these oxides with MnS attached. Depending on the form of Mn present, the Mn detected in the inclusions is classified into those that form MnS and those that form MnO, and formula (1) is calculated using the MnO content. (Mn)_MnS=55 / 32×(S) ···(2-a) (Mn)_MnO=T.(Mn)-(Mn)_MnS ···(2-b) Here, the parentheses ( ) indicate the concentration in the inclusions (units are mass%), and the _MnS and _MnO following the parentheses indicate that they form MnS and MnO. T. stands for Total and indicates the total concentration regardless of the form (MnS, MnO) present in the inclusions. 55 and 32 in formula (2-a) are the atomic weights of Mn and S, respectively. If 55 / 32 × (S) ≥ T.(Mn), all of the Mn in the inclusions is considered to have formed MnS. (Mn)_MnS=T.(Mn) (2-c) (Mn)_MnO=0 (2-d)

[0036] Using formulas (2-a) to (2-d), the MnS concentration and MnO concentration in the inclusions are calculated using the following formula using atomic weights. (MnS)=87 / 55×(Mn)_MnS ···(3-a) (MnO)=71 / 55×(Mn)_MnO ···(3-b)

[0037] If N is detected, it is believed that AlN is being produced. Therefore, Al is distinguished between those that produce AlN and those that form Al2O3. The Al2O3 content thus distinguished is used in formula (1). (Al)_AlN=27 / 14×T.(N) ···(4-a) (Al)_Al2O3=T.(Al)-(Al)_AlN ···(4-b) (AlN)=41 / 27×(Al)_AlN ···(4-c)

[0038] The oxides, Cr2O3, SiO2, and Al2O3 are calculated using the following formula. (Cr2O3)=152 / 104×T.(Cr) ···(5) (SiO2)=60 / 28×T.(Si) ···(6) (Al2O3)=102 / 54×(Al)_Al2O3···(7) Equation (1) can be calculated using the oxide concentrations (Cr2O3), (MnO), (SiO2), and (Al2O3) in the inclusions thus determined.

[0039] (Method for calculating the average composition of inclusions in Ca-added samples) When Ca is added, the formation of CaS is expected. While MnS is mainly formed after solidification, CaS is formed in the molten steel, so CaS and MnS can be considered to be distributed separately. Therefore, if Ca is detected during inclusion composition analysis using an EDS attached to an SEM, an elemental map showing the elemental distribution of Mn, Ca, and S is measured, and the composition of each is analyzed by dividing the Ca-detected area into the Ca-undetected area and the Ca-undetected area.

[0040] (Calculation method for composition of Ca detection part) All Mn in the Ca detection area is considered to have formed MnO. (MnO) = 71 / 55 × T.(Mn) (8)

[0041] (Calculation method for composition of Ca undetected parts) Since the S detected from the Ca undetected portion is MnS, the MnS concentration and MnO concentration can be found using the above-mentioned formulas (2-a) to (3-b).

[0042] (Method for calculating the average composition of all inclusions in which Ca is detected) The area ratio of the Ca-detected area to the Ca-undetected area can be obtained from the elemental map, and the average composition of the entire inclusion can be calculated by multiplying each composition by the area ratio. Then, equation (1) can be evaluated. If the Ca-detected area and the Ca-undetected area can be distinguished from each other by the contrast difference in the SEM image, the area ratio can also be obtained from the SEM image.

[0043] As described above, rather than converting the total amount of elements detected during inclusion analysis into oxides, the composition can be calculated based on thermodynamic knowledge and evaluated to see if it satisfies formula (1).

[0044] <Steel composition> The thin-walled cast slab of the present invention contains, by mass%, C: 0.20% or less, Si: 0.001 to 0.25%, Mn: 0.15 to 2.00%, Al: 0.001 to 0.010%, Cr: 0.01 to 1.00%, Cu: 0.10 to 3.00%, and is limited to P: 0.020% or less, S: 0.015% or less, and Sn: 1.00% or less. Hereinafter, % in the steel composition means % by mass.

[0045] (Cu: 0.10 to 3.00%) The present invention is directed to thin-walled cast slabs of Cu-containing steel, which contain 0.10 to 3.00% Cu. Since Cu cracking becomes apparent when the Cu content is 0.10% or more, application of the present invention can effectively suppress Cu cracking. When the Cu content exceeds 3.00%, the harmful effects of Cu cracking cannot be suppressed by the present invention alone.

[0046] (Sn:1.00% or less) As mentioned above, the occurrence of Cu cracking becomes more pronounced when Sn is contained in addition to Cu. Therefore, the present invention specifies that the Sn content be 1.00% or less, with the aim of covering both cases where Sn is not contained and where Sn is contained. If the Sn content is 0.01% or more, the adverse effects of Sn become clearly apparent. On the other hand, if the Sn content is 1.00% or less, the effects of the present invention are reliably exhibited, so the upper limit is set to 1.00%.

[0047] (Cr: 0.01 to 1.00%) The inclusions targeted by the present invention are fine Cr2O3-MnO-SiO2-Al2O3-based oxides that are ultimately formed from fine Cr2O3-MnO-based oxides as a base material through the subsequent addition of Al and other elements, or inclusions in which these oxides are combined with sulfides or carbonitrides.

[0048] The concentration range of Cr in steel is set to a value where the number density of inclusions required to obtain the effect of the present invention is 100 pieces / mm 2On the other hand, even if the Cr concentration is increased beyond 1.00%, the number density saturates, so the upper limit is set to 1.00%.

[0049] The final number density of inclusions generally tends to increase as the steel composition ratio [Cr] / [Mn] increases. Here, the [element name] indicates the content of the element in the steel in mass%. When the [Cr] / [Mn] ratio is 0.05 or more, the number density is 300 / mm 2 A value of 0.12 or more is most preferable because the frequency of increase in the value of 0.10 or more is high, and a value of 0.12 or more is even more preferable. Although the effect tends to saturate, a value of 0.12 or more is the most preferable because the maximum effect can be obtained.

[0050] (Si:0.001~0.25%, Mn:0.15~2.00%, Al:0.001~0.010%) The present invention specifies the concentrations of Si, Mn, and Al in steel so that Cr2O3 remains in inclusions in thin-walled cast slabs and satisfies Equation (1). Cr is a weak deoxidizing element, and if the Si, Mn, and Al contents in steel exceed these upper limits, Cr2O3 is reduced, preventing the benefits of the present invention from being achieved. The upper limits of Si, Mn, and Al were determined based on inclusion observations and may not necessarily coincide with thermodynamic equilibrium calculations. While this is considered to indicate a kinetic non-equilibrium state in the reduction and reforming processes of inclusions in molten steel, it is sufficient that Cr2O3 remains. This remaining Cr2O3 serves as a trace of Cr deoxidation. On the other hand, reducing the Si, Mn, and Al contents in steel below the lower limits results in excess dissolved oxygen in the molten steel, which coarsens the inclusions and becomes a crack initiation point. For these reasons, the upper and lower limits of Si, Mn, and Al are specified.

[0051] (C: 0.20% or less) C (carbon) is an important element for ensuring the strength (hardness) of steel sheet. The required amount is added depending on the application of the product. If the C content exceeds 0.20%, the hardness increases and workability decreases, so even if Cu cracks are minor at the thin-walled cast slab stage, there is a risk that the cracks will expand during rolling. Therefore, the C content is controlled to 0.20% or less. Preferably, the lower limit of the C content is set to 0.0003%.

[0052] (P:0.020% or less) Since P is an impurity element contained in steel, there is no need to specifically set a lower limit. The lower limit of the P content may be 0%. Considering current general refining (including secondary refining), the lower limit of the P content may be 0.005%. Although P has the function of solid solution strengthening, excessive content impairs the workability of steel sheet. Therefore, the P content is limited to 0.020% or less.

[0053] (S:0.015% or less) S (sulfur) is an impurity contained in steel, so there is no need to specifically set a lower limit. The lower limit of the S content may be 0%. Furthermore, considering current general refining (including secondary refining), the lower limit of the S content may be 0.0003%. S is an impurity element that forms nonmetallic inclusions and impairs the workability of steel sheet. Therefore, the S content is limited to 0.015% or less. Preferably, it is limited to 0.007% or less.

[0054] As described above, the steel contains C, Si, Mn, Al, Cr, and Cu in the content ranges specified above, P, S, and Sn are within the above-mentioned component ranges, and the balance is Fe and impurities. Other than the above components, N is usually contained in the steel, mainly as an impurity. The N content range to which the present invention is applicable is 0.001 to 0.015%.

[0055] Furthermore, the following components may be selectively contained in place of a portion of the Fe.

[0056] (Ca:0.0040% or less) Ca is an effective selective element for controlling the morphology of inclusions such as MnS and improving the workability of steel sheets. If the Ca content exceeds 0.0040%, inclusions that are easily elongated during rolling, such as coarse low-melting-point oxides, are more likely to form, which may deteriorate the workability of the steel sheet. Therefore, the Ca content is controlled to 0.0040% or less. The above effects can be preferably obtained by setting the lower limit of the Ca content to 0.0003%. The preferred range is 0.0005 to 0.0035%.

[0057] (Ti:0.050% or less) Ti is an optional element that forms carbonitrides and is effective in preventing grain coarsening and improving workability. Therefore, Ti may be added in a range of 0.050% or less, as needed. On the other hand, if the Ti content exceeds 0.050%, coarse Ti carbonitrides may precipitate, which may lead to a decrease in workability of the steel sheet. When the lower limit of the Ti content is set to 0.005%, the above effects can be preferably obtained. A more preferable range is 0.010 to 0.040%.

[0058] (Nb:0.050% or less) Nb is an optional element that forms carbonitrides and is effective in preventing grain coarsening and improving workability. Therefore, Nb may be added in a range of 0.050% or less, as needed. Furthermore, the above effects can be preferably obtained by setting the lower limit of the Nb content to 0.010%. On the other hand, if the Nb content exceeds 0.050%, coarse Nb carbonitrides may precipitate, which may lead to a decrease in workability of the steel sheet. A more preferable range is 0.020 to 0.040%.

[0059] (V:0.050% or less) V, like Nb, forms carbonitrides and is an effective selective element for preventing grain coarsening and improving workability. Therefore, V may be added in a range of 0.050% or less, as needed. Furthermore, the above effects can be preferably obtained by setting the lower limit of the V content to 0.010%. On the other hand, if the V content exceeds 0.050%, coarse precipitates may be formed, which may lead to a decrease in the workability of the steel sheet. A more preferable range is 0.020 to 0.040%.

[0060] (Mo: 0.050% or less) Mo is an optional element that has the effect of improving the strength (hardness) of steel sheet by improving hardenability and temper softening resistance. Therefore, Mo may be added in a range of 0.050% or less, if necessary. Furthermore, the above effect can be preferably obtained by setting the lower limit of the Mo content to 0.010%. On the other hand, if the Mo content exceeds 0.050%, the cost of adding Mo increases while the effect of adding Mo saturates, so the upper limit is set to 0.050% or less. A more preferable range is 0.020 to 0.040%.

[0061] (Ni: 1.00% or less) Ni is an optional element that is effective in improving the strength (hardness) of steel sheets by improving hardenability and in improving workability. It is also an optional element that has the effect of preventing Cu cracking. Therefore, Ni may be added in a range of 1.00% or less, if necessary. Furthermore, the above effects can be preferably obtained by setting the lower limit of the Ni content to 0.01%. On the other hand, if the Ni content exceeds 1.00%, the cost of addition increases, while the effect of addition saturates, so the upper limit is set to 1.00%. A more preferable range is 0.02 to 0.10%.

[0062] (B:0.0050% or less) B (boron) is an optional element that has the effect of improving the hardenability and increasing the strength (hardness) of steel sheet. Therefore, B may be added in a range of 0.0050% or less, if necessary. Furthermore, the above effect can be preferably obtained by setting the lower limit of the B content to 0.0010%. On the other hand, if the B content exceeds 0.0050%, B-based compounds are generated, which reduces the workability of the steel sheet, so the upper limit is set to 0.0050% or less. A more preferable range is 0.0020 to 0.0040%.

[0063] <<Method for manufacturing thin-walled cast slabs>> The method for producing a thin-walled cast slab of Cu-containing steel of the present invention is characterized in that Cr is added to molten steel having a dissolved oxygen content of 0.0300% by mass or more at the stage of molten steel tapped from a primary refining furnace into a ladle, and the molten steel is cast.

[0064] The above-described manufacturing method shows a preferred order of element addition for dispersing a large number of fine Cr2O3-MnO-SiO2-Al2O3-based oxides, or inclusions in which these oxides are combined with sulfides or carbonitrides, as the objective of the present invention.

[0065] It is particularly important to add Cr and Mn before adding Al, or even after adding Al, when the dissolved oxygen content in the molten steel is 0.0300% by mass or more, to disperse a large number of fine Cr2O3-MnO-based oxides. If Al or Si is added thereafter, a sufficiently large number of Cr2O3-MnO-SiO2-Al2O3-based oxides will be finally formed. If the dissolved oxygen content before adding Cr is 0.0300% by mass or more, a large number of fine oxides containing Cr2O3 will be formed. It is preferable that the dissolved oxygen content be 0.0500% or more.

[0066] The main deoxidizing elements added in the present invention are Cr, Mn, Si, and Al, with Cr having the weakest deoxidizing strength, followed by the stronger elements in that order, with Al being the strongest. Therefore, Cr is generally added first, and they are added in that order. Al, which has the strongest deoxidizing power, is often added last. If Cr is added first, it is easy to ensure a dissolved oxygen concentration of 0.0300% or more, since it is before deoxidation by other deoxidizing elements. The same applies when Cr and Mn are added first at the same time.

[0067] On the other hand, as long as a dissolved oxygen content of 0.0300% or more can be ensured before adding Cr, it is not prohibited to add other elements in advance. It does not matter whether the amount added is a part or the entire amount of the final target concentration. For example, Mn can be added first, followed by Cr. Al can also be added to raise the temperature of the molten steel, or the Al temperature can be increased. In these cases, the dissolved oxygen content can be confirmed by measuring it with a commercially available oxygen probe based on the principle of an oxygen concentration cell before adding Cr after raising the temperature of Mn or Al.

[0068] On the other hand, if Al is added before Cr and Mn to reduce the dissolved oxygen content to less than 0.0300% by mass, Al2O3 will form in the molten steel. Al2O3 has a lower number density than Cr2O3-MnO oxides. Even if Cr or Mn is then added to deoxidize the molten steel, Cr2O3-MnO oxides will form starting from the Al2O3 already present in the molten steel, resulting in a lower final inclusion number density.

[0069] For the above reasons, Al is added after Cr and Mn are added. Although the order of adding Cr and Mn is not particularly specified, it is more preferable to add Cr first to generate a large amount of Cr2O3, and then add Mn to generate Cr2O3-MnO-based oxides.

[0070] It is preferable to add Si after adding Cr and Mn. Furthermore, it is more preferable to add Cr, Mn, Si, and Al in this order, according to the order of the strength of the deoxidizing power.

[0071] The addition of Cr, Mn, Si, and Al can be carried out in accordance with the usual steel refining method. Usually, they can be added during secondary refining, for example, during the composition adjustment process after decarburization in an RH unit.

[0072] After the composition adjustment, the molten steel is cast into a thin slab having a thickness of 6 mm or less. A twin-drum continuous casting method is preferably used as the casting method. The thin slab of the Cu-containing steel of the present invention has an inclusion density of 100 pieces / mm , which satisfies the predetermined conditions. 2For the above reasons, the occurrence of Cu cracks during continuous casting can be prevented.

[0073] The thin cast slab of Cu-containing steel produced as described above is hot rolled to produce a hot rolled steel sheet. The thin cast slab of Cu-containing steel of the present invention has an inclusion density of 100 pieces / mm 3 which satisfies the predetermined conditions. 2 For the above reasons, regardless of the level of the hot rolling heating temperature, the occurrence of Cu cracks during hot rolling heating and hot rolling can be prevented. [Example]

[0074] The results of experiments carried out to confirm the effects of the present invention will be described below. Thin-walled slabs were cast by twin-drum continuous casting under the following conditions. Cooling roll diameter: 600mm Casting width: 400mm Casting thickness: 2~6mm, casting speed can be changed to change the casting thickness. Casting speed: 7~70m / min. Casting atmosphere: Ar Casting volume: 100kg Steel composition: 0.1%C, 1%Cu-0.1%Sn as a base, modified as appropriate.

[0075] Addition order of Cr, Mn, Si, Al: Examples were carried out with appropriate changes based on the order of deoxidation strength: Cr → Mn → Si → Al.

[0076] Dissolved oxygen content before Cr addition: The dissolved oxygen content before Cr addition was measured with an oxygen probe and confirmed to be 0.0300% or more, except for Comparative Example No. 52, in which Al was added first. The measured values were in the range of 0.0300 to 0.0800%, with most examples being 0.0500 to 0.0700%. Comparative Example No. 52 was less than 0.0200%.

[0077] This molten steel was used to cast thin billets by the twin-drum continuous casting method described above, and the cast thin billets were heated to 1100°C and then hot-rolled to produce hot-rolled steel sheets with a thickness of 1 mm.

[0078] (1.Method of measuring the number density and average composition of inclusions in thin-walled cast slabs by SEM observation) A sample was taken from the 1 / 4 width section of the slab, and the surface was polished parallel to the surface to the 1 / 4 thickness section, and inclusions were observed using an SEM. 2 , total area 0.11mm 2 The target was inclusions contained in the steel.

[0079] Inclusions with a maximum length of 1 μm or more were selected at 1000x magnification and, if necessary, observed at higher magnifications to measure their size. The shape was approximated as an ellipse, with the maximum length defined as the major axis and the length perpendicular to the maximum length as the minor axis, and the equivalent circle diameter Dave. = √(major axis × minor axis) was calculated. Inclusions with an equivalent circle diameter of 1 μm or more and 5 μm or less were extracted.

[0080] The cross-sectional average composition of the extracted inclusions with a circle equivalent diameter of 1 to 5 μm was analyzed using an EDS (energy dispersive characteristic X-ray analysis) device attached to the SEM. The irradiation beam was spread to cover the entire inclusion, and the average composition was analyzed in a single measurement. The detected Fe was considered to have been detected in the matrix and was excluded from the calculation of the inclusion composition. Using the above-mentioned equations (2-a) to (8), the cross-sectional average composition of each inclusion was calculated from the detected element ratio, and the number of inclusions satisfying equation (1) was counted and the number density was measured.

[0081] (2.Method for measuring Cu crack depth in thin-walled cast slabs and hot-rolled steel sheets) Because copper cracks occur along γ grain boundaries and may not have a particular directionality like longitudinal or transverse cracks, 50 mm-thick cross sections of the slab and steel plate in the width direction and in the longitudinal direction of the casting and rolling process were observed with an optical microscope, and the crack depth was measured. Samples were taken from the quarter-width section of the slab and steel plate, polished, but not etched, and observed at magnifications of 250–1000x. Cu cracks were considered to be cracks that contained copper, rather than simple cracks. Cu crack depth was measured from the interface between the surface oxide scale and the matrix iron, and the crack termination was measured as the distance to the crack tip, not the Cu penetration depth. The crack depth was evaluated as the average of the three deepest cracks. If there were fewer than three cracks, the average of all measurements was used. The average value of three points for both the slab and the steel plate was evaluated as follows: ⊚: less than 30 μm, ○: 30 μm or more and less than 60 μm, △: 60 μm or more and less than 100 μm, ×: 100 μm or more.

[0082] [Table 1]

[0083] [Table 2]

[0084] (result) Examples are shown in Tables 1 and 2. Nos. 1 to 31 are examples of the present invention and satisfy the conditions of the present invention. As a result, the inclusion density in all cases was 100 pieces / mm 2 As a result, the Cu crack depth in the thin-walled cast slab and the hot-rolled sheet was less than 100 μm. Since the pre-rolling heating temperature was set to 1100°C, Cu cracking was promoted during heating, but in the present invention, γ grain growth during heating could be suppressed, and promotion of Cu cracking during heating was sufficiently suppressed.

[0085] The order of addition of Cr, Mn, Si, and Al was basically Cr → Mn → Si → Al, but Nos. 10 and 24 are examples where Mn was added first in the order of Mn → Si → Cr → Al, and Nos. 11, 17, and 20 are examples where Cr and Mn were added simultaneously, followed by Si, and finally by Al. No. 4 is an example where Mn and Si were added simultaneously, and No. 5 is an example where Si and Al were added simultaneously.

[0086] Nos. 25 and 26 are examples in which Ca was added. In No. 26, the amount of Ca was so high that CaS was formed, and the average composition of the inclusions, including CaS, was less than 1% (Cr2O3), but the value on the left side of formula (1) was 1.2, which satisfies formula (1). In No. 27, the amount of N was so high that AlN was formed, and the average composition of the inclusions, including AlN, was less than 1% (Cr2O3), but the value on the left side of formula (1) was 1.2, which satisfies formula (1).

[0087] Nos. 51 to 57 are comparative examples. Nos. 51, 56, and 57 were free of Cr and did not produce Cr2O3-MnO-based oxides, which are easily finely dispersed, so the number of inclusions that were ultimately produced was small. As a result, the inclusion density was 100 pieces / mm 2 The Cu crack depths of the thin-walled cast slab and the hot-rolled sheet were both 100 μm or more.

[0088] No. 52 is an example in which Al was added first, followed by Cr and Mn. Because Al was added first and Al deoxidized, the dissolved oxygen concentration before Cr addition decreased to less than 0.0200%, and the number of inclusions formed decreased. As a result, the inclusion density was 100 pieces / mm 2 The Cu crack depth was 100 μm or more.

[0089] Nos. 53 to 55 are examples in which the concentrations of Mn, Si, and Al exceeded their respective upper limits. In No. 53, where the Mn concentration exceeded the upper limit, Mn reduced the Cr2O3 that had formed before the Mn addition, resulting in fewer Cr2O3-MnO oxides and a smaller number of inclusions ultimately formed. In Nos. 54 and 55, where the Si or Al concentration exceeded the upper limit, Si or Al reduced the Cr2O3-MnO oxides, resulting in a smaller number of inclusions ultimately formed. As a result, in Nos. 53 to 55, the inclusion density was 100 pieces / mm 2 The Cu crack depth was 100 μm or more.

[0090] The heating temperature before rolling was set to 1100°C, which promoted Cu cracking during heating. As a result, the Cu crack depth in the hot-rolled sheets of Comparative Examples 51 to 57 was deeper than that in the slab stage, and as a result, the difference in Cu crack depth between the invention examples and the hot-rolled sheets was increased.

Claims

1. In mass%, C: 0.20% or less, Si: 0.001 to 0.25%, Mn: 0.15-2.00%, Al: 0.001-0.010%, Cr: 0.01-1.00%, Cu: 0.10-3.00% Contains P: 0.020% or less, S: 0.015% or less, Sn: 1.00% or less and the balance being Fe and impurities. A thin-walled cast piece having a thickness of 6 mm or less, The number of inclusions having a circle equivalent diameter of 1 μm or more and 5 μm or less and a composition satisfying formula (1) is 100 / mm on a surface perpendicular to the thickness direction of the slab within 2 mm from the surface. ２ A thin-walled cast slab of Cu-containing steel, characterized in that the above distribution is (Cr ２ O ３ ) / {(Cr ２ O ３ )+(MnO)+(SiO ２ )+(Al ２ O ３ )}×100≧1 ・・・(1) In formula (1), the description of (oxide name) means the content (mass %) of the oxide in the inclusion.

2. In place of a part of the Fe, further in mass%: Ca: 0.0040% or less, Ti: 0.050% or less, Nb: 0.050% or less, V: 0.050% or less, Mo: 0.050% or less, Ni: 1.00% or less, B: 0.0050% or less 2. The thin-walled cast slab of Cu-containing steel according to claim 1, further comprising one or more of the following:

3. 3. A method for producing a thin-walled cast slab of Cu-containing steel according to claim 1, wherein Cr and Mn are added to molten steel having a dissolved oxygen content of 0.0300% by mass or more, and the molten steel is cast.

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