Mold powder for continuously casting free-cutting steel and method for continuously casting free-cutting steel

Abstract

To provide a mold powder for continuously casting a free-cutting steel, capable of suppressing the occurrence of surface defects in a casting piece when manufacturing the free-cutting steel by continuous casting, and to provide a method for continuously casting the free-cutting steel.SOLUTION: The present invention relates to a mold powder for continuous casting of free-cutting steel having a specific chemical composition, wherein, as components when carbon in the mold powder is completely combusted, a mass ratio of a content of CaO to a content of SiO2 is 0.50 or more and 0.59 or less, the total content of CaO and SiO2 is 70.0 mass% or more, a content of Al2O3 is 13.0 to 20.0 mass%, the total content of alkali metal oxides is 2.0 to 5.0 mass%, the balance comprises alkaline earth metal oxides other than F and CaO and inevitably contained components, and a viscosity at 1,300°C is 4.0 Pa_s or more.SELECTED DRAWING: None

Description

Technical Field

[0001] The present disclosure relates to a mold powder for continuous casting of free-cutting steel and a method for continuous casting of free-cutting steel.

Background Art

[0002] When continuously casting molten steel by a continuous casting machine to produce a slab, molten steel whose components have been adjusted in advance according to the purpose of the slab (steel slab) is poured into a mold. Further, in order to perform continuous casting stably, a mold powder mainly composed of CaO and SiO2 (which may be referred to as "mold powder" or simply "powder" in this specification) is put on the molten steel in the mold. The molten steel poured into the mold is cooled on the outside by the mold to form a solidified shell, and the slab with the solidified shell formed is solidified to the core of the slab while being withdrawn from the mold. The mold powder put into the mold is melted by the heat of the molten steel and becomes a liquid molten powder (which may also be referred to as "powder slag" etc. in this specification). The molten powder flows between the mold and the solidified shell and contributes to lubrication between the mold and the solidified shell (slab).

[0003] In such continuous casting, the composition of the powder may change due to decomposition, oxidation of the components contained in the mold powder, mixing of the components contained in the molten steel, etc., and the operability may decrease due to a decrease in viscosity. For example, in Patent Document 1, as a mold powder that suppresses compositional fluctuations and viscosity reduction of powder slag in the continuous casting of high-Mn steel with an Mn content of 10 to 30% by mass, it contains SiO2 and CaO as main components, and the mass ratio of CaO to SiO2 (CaO / SiO2) is 0.60 or more and less than 1.0, the content of Al2O3 is 12.0 to 25.0% by mass, the total content of Li2O, Na2O, MgO, B2O3 and F is 3.0 to 15.0% by mass, the content of MnO is 0.5% by mass or less, and the viscosity at 1300 °C is 1.0 to 10 Pa·s. A mold powder is disclosed.

Prior Art Documents

Patent Documents

[0004] [Patent Document 1] Japanese Unexamined Patent Application Publication No. 2020-121320 [Summary of the Invention] [Problems to be Solved by the Invention]

[0005] Steel bar wires used for mechanical parts such as springs, gears, and shafts are subjected to a cutting process in the machining finish stage in the manufacture of high-precision parts, so machinability (cutting efficiency) and a flat surface roughness are required. In order to meet these requirements, free-cutting steel that particularly considers machinability is manufactured by continuous casting. For example, low-carbon free-cutting steel is a non-deoxidized steel with improved machinability, and has a characteristic composition in which the C (carbon) content is suppressed to 0.18% by mass or less, and embrittling elements typified by P and S and T-O (total amount of oxygen in the components) are very high.

[0006] When continuously casting free-cutting steel, elements such as Mn contained in the molten steel react with SiO2 contained in the mold powder to form oxides such as MnO. During continuous casting, these oxides are mixed (picked up) into the powder slag, changing the composition of the powder and also changing physical properties such as viscosity and solidification temperature. Due to such deterioration of the powder, a problem arises in the operability of continuous casting. Specifically, there is a high risk of cracking during the solidification process of the molten steel and so-called breakout (BO) in which the solidification shell breaks and the unfrozen molten steel flows out. In the continuous casting of free-cutting steel, there are problems such as low yield and a decrease in productivity due to BO detection.

[0007] In view of the above circumstances, an object of the present disclosure is to provide a mold powder for continuous casting of free-cutting steel and a method for continuous casting of free-cutting steel that can suppress the occurrence of surface defects in the cast slab when manufacturing free-cutting steel by continuous casting. [Means for Solving the Problems]

[0008] Means for solving the above problems include the following aspects. <1> A mold powder used for continuous casting of free-cutting steel, wherein the free-cutting steel has a chemical composition containing C: 0.06 to 0.18% by mass, Si: 0.01 to 0.05% by mass, Mn: 0.20 to 1.50% by mass, P: 0.040 to 0.110% by mass, S: 0.180 to 0.450% by mass, and O: 70 to 300 ppm, as components when carbon in the mold powder is completely burned, the mass ratio of the content of CaO to the content of SiO2 is 0.50 or more and 0.59 or less, and the total content of CaO and SiO2 is 70.0% by mass or more, the content of Al2O3 is 13.0 to 20.0% by mass, the total content of alkali metal oxides is 2.0 to 5.0% by mass, the balance consists of F, alkaline earth metal oxides excluding CaO, and unavoidably contained components, A mold powder for continuous casting of free-cutting steel having a viscosity at 1300 °C of 4.0 Pa·s or more. <2> When continuous casting of the free-cutting steel is performed using the mold powder for continuous casting of free-cutting steel, differential thermal analysis is performed in which the temperature is raised to 1300 °C at 20 °C / min and cooled at 20 °C / min with respect to a powder film in which powder slag having a composition different from that of the mold powder for continuous casting of free-cutting steel is solidified. The mold powder for continuous casting of free-cutting steel according to <1>, in which no crystallization peak exists during cooling. <3> When continuous casting of the free-cutting steel is performed using the mold powder for continuous casting of free-cutting steel, the viscosity at 1300 °C of the powder slag having a composition different from that of the mold powder for continuous casting of free-cutting steel is 2.0 Pa·s or more. The mold powder for continuous casting of free-cutting steel according to <1>. <4> A step of injecting molten steel for casting free-cutting steel having a chemical composition containing C: 0.06 to 0.18% by mass, Si: 0.01 to 0.05% by mass, Mn: 0.20 to 1.50% by mass, P: 0.040 to 0.110% by mass, S: 0.180 to 0.450% by mass, and O: 70 to 300 ppm into a mold for continuous casting, A step of charging the mold powder for continuous casting of free-cutting steel according to any one of <1> to <3> into the molten steel poured into the mold; A step of continuously drawing out and cooling the cast slab solidified from the molten steel from the mold; A method for continuous casting of free-cutting steel, including the above steps.

Advantages of the Invention

[0009] According to the present disclosure, there are provided a mold powder for continuous casting of free-cutting steel and a method for continuous casting of free-cutting steel, which can suppress the occurrence of surface defects on the cast slab when manufacturing free-cutting steel by continuous casting.

Brief Description of the Drawings

[0010]

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Mode for Carrying Out the Invention

[0011] Hereinafter, an embodiment which is an example of the present disclosure will be described with reference to the accompanying drawings. In this specification, the “%” indication of the content or concentration of each component means “mass %” unless otherwise specified. Also, in this specification, the numerical range represented by “~” means a range including the numerical values described before and after “~” as the lower limit value and the upper limit value. However, the numerical range in the case where “exceeding” or “less than” is attached to the numerical values described before and after “~” means a range not including these numerical values as the lower limit value or the upper limit value. In the numerical ranges described step by step in this specification, the upper limit value of a certain stepwise numerical range may be replaced with the upper limit value of the numerical range of other stepwise descriptions or the numerical value of the example, and the lower limit value of a certain stepwise numerical range may be replaced with the lower limit value of the numerical range of other stepwise descriptions or the numerical value of the example. The term “step” includes not only an independent step but also this term if the intended purpose of the step is achieved even when it cannot be clearly distinguished from other steps.

[0012] First, the process until the completion of the mold powder for continuous casting of free-cutting steel (hereinafter, may be referred to as “mold powder” or simply “powder”) according to the present disclosure will be described.

[0013] The low-carbon free-cutting steel has a chemical composition with high embrittlement elements typified by P and S and high T-O as shown in Table 1 below, for example.

[0014] [Table 1]

[0015] When such low-carbon free-cutting steel is produced by continuous casting, the risk of cracking during the solidification process and breakout (BO) is high. In continuous casting of low-carbon sulfur free-cutting steel, the inventors of the present disclosure have conducted repeated studies on the relationship between the mold powder and the defects occurring on the surface of the slab, and obtained the following findings.

[0016] (1) Since the low-carbon free-cutting steel contains a large amount of P and S, it has high crack sensitivity and easily forms a subsurface microsegregation band that serves as a crack initiation point. Fig. 1 is a diagram showing an example of the appearance of surface defects of free-cutting steel produced by continuous casting. Fig. 2 is a diagram showing a cross-section after metal flow corrosion of the peripheral part (region S) of the surface defect shown in Fig. 1. As a result of microscopic observation of the steel slab defects generated in the low-carbon free-cutting steel (Fig. 1), microsegregation (concentration of P) was confirmed around the defects with a metal flow corrosion solution (Fig. 2). The black spotted part in Fig. 2 is the microsegregation part. That is, it was confirmed that it is a defect in the initial solidification process in the mold. The microsegregation part opens during rolling after casting, causing a decrease in yield.

[0017] (2) Since the low-carbon free-cutting steel has a high concentration of free O (oxygen), the MnO content in the powder increases due to Mn, resulting in a significant decrease in viscosity, and it is likely to induce poor lubrication between the mold and the solidification shell and poor solidification of the molten steel. In particular, the non-uniform inflow of the powder between the mold and the solidification shell is an event that causes steel slab defects and BO detection. The inventors of the present disclosure collected slab samples of low-carbon free-cutting steel and conducted appearance monitoring. Fig. 3 is a diagram showing an example of the appearance of a slab sample of low-carbon free-cutting steel. The local inflow part (region A) and the insufficient inflow part (region B) of the powder are clear. This is considered to be due to the influence of the viscosity decrease caused by MnO pickup. Figure 4 is a diagram showing an enlarged view of a part of the boundary between region A and region B (the boundary between the inflow part and the insufficient inflow part) in Figure 3. Figure 5 is a diagram showing a cross-section after metal flow corrosion of the cracked part of the boundary shown in Figure 4. Vertical cracks were confirmed at the interface between the powder inflow part (region A in Figure 3) and the insufficient inflow part (region B in Figure 3) (Figure 4). The cracked part is a part with delayed solidification, and if it is severe, BO detection will occur. And solidification delay and microsegregation (poor solidification) were confirmed at the boundary part (Figure 5). The direction and growth rate of dendrites are different between the powder lubricated part and the insufficient part, tensile stress is generated, and a segregation band of concentrated molten steel is formed between the torn dendrite trees, and it is presumed that pores will form during rolling.

[0018] Figure 6 is a diagram showing an enlarged view of a part of the oscillation mark (OSM) part near the corner (region C) of the slab sample in Figure 3. Figure 7 is a diagram showing a cross-section after metal flow corrosion of the OSM part shown in Figure 6. A large slag bear was collected from the corner part of the mold. The corner part is remarkable for insufficient powder inflow due to the generation of a large slag bear, and deep OSM is formed (Figure 6), and microsegregation was confirmed in the valley part (Figure 7). Since the free-cutting steel contains a large amount of P and S, microsegregation is likely to become apparent. The deep OSM valley part has different dendrite directions and growth rates like the above-mentioned solidification delay, so tensile stress is generated, a segregation band is formed between the dendrite trees, and it is presumed that pores will form during rolling.

[0019] Furthermore, when the composition and physical properties of the used mold powder were measured before use (initial) and during 200t continuous casting, the results shown in Table 2 below were obtained. T.C (Total Carbon) is the total carbon, and C / S is the mass ratio of CaO / SiO2 content. Due to the continuous casting of free-cutting steel, the composition and physical properties have changed greatly compared to the initial stage, especially the MnO content has increased to nearly 10%, and the viscosity at 1300 °C has decreased significantly.

[0020]

Table 2

[0021] From the above investigation results, it was confirmed that both the surface defects and BO detection of free-cutting low-carbon steel were caused by solidification defects of molten steel originating from the deterioration of powder and the poor inflow of powder due to slag carryover during continuous casting. From these findings, the following is speculated. Due to the decrease in viscosity caused by MnO pickup in the mold powder, the uniform inflow of powder between the mold and the solidification shell is inhibited. Therefore, a difference in the growth rate of the solidification shell occurs at the boundary between the local excessive inflow part and the insufficient inflow part of the powder between the mold and the solidification shell, resulting in distortion being added to the shell. When the dendrite arms are torn, embrittling elements (P, S) concentrate, forming subsurface defects. In addition, the powder with decreased viscosity is likely to generate slag carryover in which molten powder (molten slag) solidifies by crystallization and high melting point. Particularly, huge slag carryovers grow at the mold corner parts, chronically lacking powder lubrication, and the above-mentioned subsurface defects are likely to occur.

[0022] Therefore, the inventors of the present disclosure considered that by pre-designing a high viscosity so that the powder can flow uniformly between the mold and the solidification shell even when MnO is picked up by the powder and the viscosity decreases, and by performing a component design that maintains the liquefaction (i.e., suppresses crystallization) even when MnO is picked up, the growth of slag carryover can be suppressed, and by improving the uniform inflow property of the powder between the mold and the solidification shell, the problems in the continuous casting of free-cutting steel as described above can be solved.

[0023] Then, in order to determine the optimal powder viscosity for uniform inflow into the mold, an experimental device that simulated mold oscillation was used to measure the powder inflow rate in the circumferential direction of the mold by varying the viscosity of silicone oil that simulated the powder. As a result, it was found that in order to achieve uniform inflow of the powder in the mold, the viscosity during casting is preferably 2.0 Pa·s or more.

[0024] Furthermore, in the powder with a CaO / SiO2 mass ratio (C / S) of 0.50 to 0.59, 5 to 10% by mass of MnO was added to the powder designed to have viscosities of 2.5 Pa·s and 4.0 Pa·s at 1300 °C, and the powder viscosity at 1300 °C was measured. As a result of the experiment, it was found that if the initial viscosity is 4.0 Pa·s or more, the condition of uniform flow can be satisfied during the casting of free-cutting steel.

[0025] Based on these findings, the inventors of the present disclosure have found the following improvement measures. [1] In the casting of low-carbon free-cutting steel, allow MnO pickup in the powder, maintain a viscosity that allows uniform flow, and perform casting using a powder in which the growth of slag bearers is suppressed, thereby achieving stable casting and improved surface quality. [2] It is preferable that the powder viscosity at 1300 °C, which enables uniform flow into the mold, be 2.0 Pa·s or more. In order to maintain a viscosity of 2.0 Pa·s or more during the casting of free-cutting steel, it is effective to set the initial viscosity of the powder to 4.0 Pa·s or more. [3] In order to prevent the crystallization of molten slag that leads to the growth of slag bearers during the casting of free-cutting steel, it is necessary to design the powder components to be amorphous, and it is effective to set the mass ratio of the CaO content to the SiO2 content (which may be referred to as "CaO / SiO2" or "C / S" in this specification) to 0.50 to 0.59.

[0026] As a result of applying the powder that satisfies the above conditions to the continuous casting of free-cutting steel, steel slab defects and BO detection were significantly improved, and it became possible to relax the operation regulations. In addition, the powder that satisfies the above conditions was sampled during the continuous casting of free-cutting steel, and composition and physical property measurements were carried out. As a result, as expected, the viscosity was 2.0 Pa·s or more. In addition, as a result of checking the slag bearer after casting, no significant growth of the slag bearer was observed. From the above results, it was confirmed that uniform flow of the powder was achieved and the solidification delay of the molten steel was improved, leading to the relaxation of operation regulations. The mold powder for continuous casting of free-cutting steel according to the present disclosure was completed based on the above findings.

[0027] [Mold powder for continuous casting of free-cutting steel] The mold powder for continuous casting of free-cutting steel according to the present disclosure is a mold powder used for continuous casting of free-cutting steel having a chemical composition containing C: 0.06 to 0.18% by mass, Si: 0.01 to 0.05% by mass, Mn: 0.20 to 1.50% by mass, P: 0.040 to 0.110% by mass, S: 0.180 to 0.450% by mass, and O: 70 to 300 ppm. And the mold powder for continuous casting of free-cutting steel according to the present disclosure, as components when the carbon in the mold powder is completely burned, the mass ratio of the content of CaO to the content of SiO2 is 0.55 to 0.59, and the total content of CaO and SiO2 is 70.0% by mass or more, the content of Al2O3 is 13.0 to 20.0% by mass, the total content of alkali metal oxides is 2.0 to 5.0% by mass, the balance consists of F, alkaline earth metal oxides excluding CaO, and unavoidably contained components, and the viscosity at 1300 °C is 4.0 Pa·s or more.

[0028] Hereinafter, the components (chemical composition) and viscosity of the mold powder according to the present disclosure will be described. Note that the free-cutting steel produced by continuous casting using the mold powder according to the present disclosure will be described later in the free-cutting steel continuous casting method. When the mold powder is heated to the molten steel temperature, chemical reactions such as decomposition and oxidation occur, and the chemical composition varies before and after heating. However, the chemical composition (components) of the mold powder according to the present disclosure, unless otherwise specified, is the components when the carbon in the mold powder is completely burned, in other words, the composition excluding the carbon mass contained in the carbon powder, carbonate, and organic binder that are oxidized and decomposed and disappear by the time of reaching the molten steel temperature. Also, during continuous casting, the chemical composition of the mold powder also changes mainly due to components derived from the molten steel, and physical property values such as viscosity also fluctuate. However, the physical property values etc. of the mold powder according to the present disclosure are the initial physical property values before being used for continuous casting, unless otherwise specified.

[0029] <Chemical composition when carbon in the mold powder is completely burned> (CaO and SiO2: 70.0 mass% or more in total) For the mold powder according to the present disclosure, the total content of CaO and SiO2 is 70.0 mass% or more with respect to the total mass of the powder. CaO and SiO2 are the main components necessary for adjusting physical properties such as the melting temperature and viscosity of the mold powder. When the total content of CaO and SiO2 is less than 70.0 mass%, the change in physical properties due to MnO pickup into the powder becomes large, and it becomes difficult to maintain the viscosity. The total content of CaO and SiO2 is preferably 72 mass% or more, and more preferably 74 mass% or more. The upper limit of the total content of CaO and SiO2 is less than 85 mass% in relation to the respective contents of Al2O3 and alkali oxides. Considering the melting point of the powder and from the viewpoint of lubricity, the upper limit of the total content of CaO and SiO2 is preferably 83 mass% or less, and more preferably 80 mass% or less.

[0030] (CaO / SiO2: 0.50 to 0.59) For the mold powder according to the present disclosure, the mass ratio of the content of CaO to the content of SiO2 is 0.50 to 0.59. If the CaO / SiO2 of the mold powder is too low, the reactivity between the powder and the molten steel increases, the viscosity of the powder greatly decreases, non-uniform inflow of the powder is likely to occur between the mold and the solidified shell, and the quality of the slab deteriorates. Therefore, the CaO / SiO2 of the mold powder is 0.50 or more, and preferably 0.55 or more. On the other hand, when CaO / SiO2 increases, crystals crystallize in the adhering layer of the powder, and it becomes easy for the slag bearer to grow. In order to suppress the growth of the slag bearer, CaO / SiO2 is 0.59 or less, and preferably 0.58 or less.

[0031] From the viewpoints of meltability and viscosity, the content of CaO in the mold powder is preferably 26 to 34%, and more preferably 26 to 32 mass%. Also, from the viewpoints of crystallization and viscosity, the content of SiO2 in the mold powder is preferably 42 to 53% by mass, more preferably 44 to 51% by mass.

[0032] (Content of Al2O3: 13.0 to 20.0% by mass) When the content of Al2O3 is low, the contents of CaO and SiO2 increase, and calcium silicate crystals are likely to crystallize and slag bearers are likely to grow. On the other hand, when the content of Al2O3 is too high, calcium aluminum silicate crystals are likely to crystallize and slag bearers are likely to grow. From such viewpoints, the mold powder according to the present disclosure has a content of Al2O3 of 13.0 to 20.0% by mass, preferably 15.0 to 19.0% by mass, more preferably 15.5 to 18.0% by mass.

[0033] Total content of alkali metal oxides: 2.0 to 5.0% by mass The mold powder according to the present disclosure has a total content of alkali metal oxides (R2O, where R represents an alkali metal) of 2.0 to 5.0% by mass. Examples of the alkali metal oxides include Li2O and Na2O. When the content of the alkali metal oxides is too low, the amount of flux components for melting the powder is small, so the melting point of the powder increases and the lubricating function of the powder becomes insufficient. On the other hand, when the content of the alkali metal oxides is too high, crystals containing alkali metals are likely to crystallize and slag bearers are likely to grow. Also, solidification disturbance is likely to occur. From such viewpoints, the content of the alkali metal oxides is preferably 2.5 to 4.9% by mass.

[0034] (Balance: Alkaline earth metal oxides excluding F and CaO, and impurities) The balance of the mold powder according to the present disclosure is alkaline earth metal oxides excluding F and CaO, and components inevitably contained. The total content of the balance is 15% by mass or less in relation to the respective contents of the essential components described above, but from the viewpoint of suppressing a decrease in viscosity, the total content of the balance is preferably 10.0% by mass or less, and preferably 5.0% by mass or less.

[0035] The mold powder according to the present disclosure may contain F for viscosity adjustment. The content of F only needs to be within a predetermined range for the contents of other components and the viscosity at 1300 °C, respectively, and is not particularly limited. For example, it is 0.5 to 5.0% by mass.

[0036] The mold powder according to the present disclosure may contain an alkaline earth metal oxide excluding CaO for viscosity adjustment. Examples of the alkaline earth metal oxide excluding CaO include MgO and SrO. The content of the alkaline earth metal oxide excluding CaO is not particularly limited as long as the contents of other components and the viscosity at 1300 °C are within predetermined ranges, respectively. For example, it is 0.3 to 2.0% by mass.

[0037] The components inevitably contained in the mold powder according to the present disclosure are components inevitably contained derived from the raw materials for manufacturing the mold powder. For example, Fe2O3, MnO, TiO2, P2O5, S, etc. are included. The content of the inevitably contained components is not particularly limited as long as the contents of other components and the viscosity at 1300 °C are within predetermined ranges, respectively. For example, it is 1.5% by mass or less.

[0038] <Physical properties> (Viscosity at 1300 °C: 4.0 Pa·s or more) When continuously casting free-cutting steel, during casting, the MnO component in the mold powder increases and the viscosity decreases, making uneven inflow likely to occur. From the perspective of maintaining a high viscosity even when the viscosity of the powder slag decreases due to the mixing of MnO during casting, the mold powder according to the present disclosure has a viscosity at 1300 °C of 4.0 Pa·s or more. If the viscosity of the mold powder at 1300 °C is 4.0 Pa·s or more, even if the viscosity decreases due to the mixing of MnO during casting, the powder slag is likely to flow between the mold and the solidified shell. The viscosity of the mold powder according to the present disclosure at 1300 °C is preferably 4.5 Pa·s or more, more preferably 5.0 Pa·s or more, and even more preferably 5.5 Pa·s or more.

[0039] The upper limit of the viscosity of the mold powder according to the present disclosure is not particularly limited as long as the above-described composition is satisfied. However, if the viscosity is too high, it may cause poor inflow between the mold and the solidified shell. From this perspective, the viscosity of the mold powder according to the present disclosure at 1300 °C is preferably 8.0 Pa·s or less, more preferably 7.0 Pa·s or less.

[0040] The viscosity of the mold powder according to the present disclosure at 1300 °C can be adjusted by the components constituting the powder. For example, SiO2 and Al2O3 are components that increase the viscosity of the powder slag, and Li2O, Na2O, F, CaO, alkaline earth metal oxides excluding CaO, etc. are components that lower the viscosity of the powder slag.

[0041] The viscosity of the mold powder according to the present disclosure at 1300 °C is measured by the rotating cylinder method. The mold powder to be measured is inserted into a crucible and preliminarily melted at 1400 °C for 10 to 15 minutes, then placed in a vertical tubular furnace. The rotor of a B-type viscometer is immersed in the molten powder, stabilized at 1300 °C for 30 minutes, and then the rotor is rotated to measure the torque due to viscous resistance to obtain the viscosity. The B-type viscometer is calibrated in advance with a standard viscosity liquid.

[0042] The form of the mold powder according to the present disclosure is not particularly limited, and examples include powder and granules.

[0043] (Mold powder after modification during continuous casting of free-cutting steel) During the casting of free-cutting steel, Mn contained in the molten steel diffuses into the mold powder as MnO, thereby reducing the viscosity of the powder slag in the mold. When the viscosity of the powder slag decreases, the inflow uniformity between the mold and the solidified shell decreases, and solidification disturbance of the slab is likely to occur. The mold powder according to the present disclosure has a viscosity of 4.0 Pa·s or more at 1300°C in the initial stage before being used for continuous casting. Therefore, even if the viscosity decreases due to the mixing of MnO, the decrease in the inflow uniformity is suppressed, and the solidification disturbance of the slab can be suppressed. For example, when continuous casting of free-cutting steel is performed using the mold powder for continuous casting of free-cutting steel according to the present disclosure, MnO caused by Mn contained in the free-cutting steel is mixed in, and the MnO content of the powder slag changes to 5 to 12% by mass. However, even such a powder slag that has been altered during continuous casting can maintain a viscosity of 2.0 Pa·s or more at 1300°C. In order to suppress non-uniform inflow even if the viscosity of the powder slag decreases due to the mixing of MnO during casting, it is more preferable that the viscosity of the powder slag at 1300°C during casting is 2.1 Pa·s or more.

[0044] In addition, when the mold powder according to the present disclosure is used in the continuous casting of free-cutting steel, crystals are difficult to crystallize, and the growth of slag bearers is suppressed. When differential thermal analysis (DTA) is performed on the altered powder slag in which MnO or the like is mixed in the mold powder according to the present disclosure during the continuous casting of free-cutting steel, heating it to 1300°C at a rate of 20°C / min and then cooling it at a rate of 20°C / min, it is preferable that there is no crystallization peak during cooling. If there is no crystallization peak in such DTA, it can be determined that the altered mold powder is amorphous. The cooling rate of 20°C / min is slower than that of the actual continuous casting machine, and it is an environment in which crystals are more likely to form than in the actual continuous casting. Therefore, if no crystallization peak is observed in the DTA under the above conditions, even if the composition of the mold powder according to the present disclosure changes during the continuous casting of free-cutting steel in the actual machine, crystals will not crystallize in the slag film, and the growth of slag bearers is also effectively suppressed.

[0045] [Continuous casting method for free-cutting steel] Next, the continuous casting method for free-cutting steel according to the present disclosure will be described. The continuous casting method for free-cutting steel according to the present disclosure includes a step of injecting molten steel having a chemical composition for producing free-cutting steel into a mold for continuous casting, A step of introducing the mold powder for continuous casting of free-cutting steel according to the present disclosure into the molten steel poured into the mold; A step of continuously withdrawing and cooling the slab solidified from the molten steel from the mold; including.

[0046] <Free-cutting steel> The slab produced by continuous casting using the mold powder according to the present disclosure is free-cutting steel. The free-cutting steel (which may be referred to as "the free-cutting steel in the present disclosure" or simply "free-cutting steel" in this specification) produced by continuous casting using the mold powder according to the present disclosure has a chemical composition including C: 0.06 to 0.18% by mass, Si: 0.01 to 0.05% by mass, Mn: 0.20 to 1.50% by mass, P: 0.040 to 0.110% by mass, S: 0.180 to 0.450% by mass, and O: 70 to 300 ppm. Further, other components (optional elements) such as Al, Pb, Bi, etc. may be included according to the use, machinability, etc. of the slab (steel slab) to be produced. Hereinafter, each component that may be included in the free-cutting steel will be described.

[0047] C (carbon): 0.06 to 0.18% by mass C is an element that increases the strength of steel and affects machinability. When the C content in free-cutting steel is lower than 0.06%, the strength of the solidification shell becomes weak and breakouts are likely to occur. On the other hand, when the C content in free-cutting steel is higher than 0.18%, the strength and hardness are high and the machinability decreases. The C content in free-cutting steel is 0.06 to 0.18%, and preferably 0.10% or less.

[0048] Si (silicon): 0.01 to 0.05% by mass Si increases the strength of steel and acts as a deoxidizer in molten steel. When Si is higher than 0.05%, a large amount of SiO2 is generated and soft MnO is not generated, so the machinability decreases. On the other hand, when the Si content is lower than 0.01%, coarse MnO is generated, and coarse inclusions fall off during cutting, reducing the finish accuracy of the cutting surface. From these viewpoints, the Si content in free-cutting steel is 0.01 to 0.05%.

[0049] Mn (Manganese): 0.20 - 1.50 mass% Mn combines with oxygen in the molten steel to form soft MnO, suppressing the formation of hard inclusions and enhancing the machinability of the steel. On the other hand, excessive Mn combines with S, reducing the amount of solid - solution S and increasing the strength of the steel while decreasing the machinability. If the Mn content is too low, not only can't soft inclusions be formed, but it also causes a decrease in the strength of the steel, making breakout more likely to occur. Therefore, the Mn content in free - cutting steel is 0.20 - 1.50%, preferably 0.86 - 1.13%.

[0050] P (Phosphorus): 0.040 - 0.110 mass% P embrittles the steel and enhances the machinability. If P is too little, the machinability decreases, and if P is too much, it causes embrittlement of the shell, making breakout more likely to occur. Therefore, the P content in free - cutting steel is 0.040 - 0.110%, preferably 0.070 - 0.110%.

[0051] S (Sulfur): 0.180 - 0.450 mass% S combines with Mn to form MnS, enhancing the machinability of the steel. If the S content is too low, sufficient MnS cannot be formed, and the machinability decreases. On the other hand, if the S content is too high, FeS is formed, embrittling the steel and making breakout more likely to occur. Therefore, the S content in free - cutting steel is 0.180 - 0.450%, preferably 0.280 - 0.440%.

[0052] Al (Aluminum): 0.005 mass% or less In the free - cutting steel of the present disclosure, there is no need to add Al. On the other hand, Al acts as a deoxidizer and may be contained in the free - cutting steel. However, if the Al content is higher than 0.005%, a large amount of Al2O3 is generated, and soft MnO is not formed, resulting in a decrease in machinability. From this perspective, the upper limit of the Al content in free - cutting steel is 0.005%.

[0053] O (Oxygen): 70 - 300 ppm O affects the shape of MnS. If the O content is too low, the amount of oxygen in MnS decreases, and the ductility increases. As a result, MnS extends in the rolling direction, causing anisotropy in the steel and leading to rough steel surface due to irregular chip shedding during cutting. On the other hand, if the O content is too high, a large amount of hard oxides are generated, resulting in a decrease in machinability. Therefore, the O content is 70 - 300 ppm, preferably 70 - 250 ppm.

[0054] Other components (optional components) The free-cutting steel may contain elements other than those described above. For example, the addition of Pb, Bi, or N can significantly improve the machinability. From the perspective of reducing environmental impact, the Pb content is preferably 0.09% or less, and it is also possible not to contain Pb.

[0055] The balance: Fe and impurities The balance of the free-cutting steel in the present disclosure is Fe (iron) and impurities. Impurities refer to components that are unintentionally mixed during the industrial production of free-cutting steel due to raw materials such as ores and scraps, or other factors.

[0056] In the continuous casting method of the free-cutting steel according to the present disclosure, for example, molten steel adjusted to a predetermined chemical composition as free-cutting steel is supplied into a tundish in a ladle or the like, and the molten steel in the tundish is injected into a mold having a water-cooling function through a nozzle. Also, the mold powder for continuous casting of the free-cutting steel according to the present disclosure is put on the molten steel surface in the mold. The slab withdrawn from the mold is conveyed by a plurality of rolls and cooled by the cooling water sprayed from an injection spray, thereby performing continuous casting. The slab conveyed while being cooled is cut to a predetermined length by a slab cutting machine and conveyed as a steel slab such as a billet or a bloom to the equipment in the next process.

[0057] In such a continuous casting method for free-cutting steel, by using the mold powder for continuous casting of free-cutting steel described above, even if MnO caused by Mn contained in the molten steel is mixed into the powder slag, a high viscosity is maintained, and the powder flows into the mold-slab gap with high uniformity. Therefore, solidification disturbance is suppressed, surface defects in the flat part are particularly suppressed, and the occurrence of BO detection is also suppressed. In addition, since crystallization of crystals is not confirmed in the powder slag, the growth of slag bear is suppressed. Therefore, defects in the corner part of the slab are particularly suppressed. As a result, the operation stability is improved, and high-quality free-cutting steel can be produced with a high yield and high productivity.

Example

[0058] Hereinafter, the mold powder for continuous casting of free-cutting steel and the continuous casting method for free-cutting steel according to the present disclosure will be described more specifically with reference to examples. However, the present disclosure is not limited by the following examples.

[0059] <Continuous casting> Mold powders having the compositions and physical properties shown in Tables 3 and 4 were prepared. The underlines mean that they are outside the scope of the present disclosure. Each component and content shown in Tables 3 and 4 are the contents of each component when the carbon in the mold powder is completely burned.

[0060]

Table 3

[0061]

Table 4

[0062] "Remainder" in each table is F, MgO (alkaline earth metal oxide excluding CaO), and components inevitably contained from raw materials. Each component of the mold powder was measured by X-ray fluorescence analysis (XRF). In addition, the viscosity of each powder at 1300°C was adjusted according to the content of each component and measured by the rotating cylinder method described above.

[0063] Using the mold powders shown in each table, continuous casting of free-cutting steel having the chemical composition shown in Table 5 was carried out. A mold with a size of 240 mm × 240 mm was used, and the casting speed was set to 0.80 to 1.40 m / min.

[0064]

Table 5

[0065] <Evaluation> For the produced cast slabs, the solidification disturbance on the flat part and the slag bear at the position corresponding to the corner of the cast slab were evaluated according to the following criteria.

[0066] (Solidification disturbance) A 150-mm cast slab sample was cut out, and the number of depressions (dents on the cast slab that occur during abnormal solidification) on the four sides of the sample was counted, and the solidification disturbance was evaluated according to the following evaluation criteria. 〇: No depression occurred ×: The number of depressions is 1 or more

[0067] (Slag bear) Among the slag bears existing directly above the molten steel surface, the thickest one was collected and its thickness was measured. Then, the slag bear was evaluated according to the following evaluation criteria. 〇: The slag bear thickness is 5 mm or less △: The slag bear thickness is more than 5 mm and 25 mm or less ×: The slag bear thickness is more than 25 mm

[0068] (Casting quality) The surface cracks of the cast slab were evaluated by the following method. Specifically, 120 test steel slabs of 165 mm × 165 mm (2 t per piece) were produced by hot rolling the cast slab under hot rolling conditions of 1200°C. Then, the presence or absence of defects on the surface of the test steel slab was visually inspected. Specifically, a water film was formed on the surface of the test steel sheet, and a fluorescent magnetic powder dispersion was uniformly applied to the surface. Subsequently, the test steel sheet was magnetized. When there are defects on the surface of the test steel sheet, magnetic poles are generated at the defect parts, and thus fluorescent magnetic powder concentrates there. Subsequently, by irradiating the surface of the test steel sheet with a black light, the fluorescent magnetic powder was made to emit light. Subsequently, the presence or absence of defects with a length of 2 mm or more was visually determined, and surface cracks were evaluated according to the following evaluation criteria. 〇: No test steel sheet with detected defects △: The number of test steel sheets with detected defects exceeds 0% and is 5% or less of the total number of steel sheets (= 120) ×: The number of test steel sheets with detected defects exceeds 5% of the total number of steel sheets Considering the quality required for the slab, ○ is the passing level.

[0069] When the mold powders of Examples 1 to 5 were used, both the solidification disturbance and the slag carryover were good, and the casting quality passed. On the other hand, when the mold powders of Comparative Examples 1, 2, 4 to 6 were used, at least one of the evaluations of the solidification disturbance or the slag carryover was poor, and the casting quality failed.

[0070] <Evaluation of the modified powder> Regarding the powders of the examples and comparative examples, mold powders modified by continuous casting of free-cutting steel and mold powders with MnO added assuming modification by continuous casting of free-cutting steel (Comparative Examples 1A, 2A, 4A to 6A) were prepared, and the viscosity at 1300 °C of these modified mold powders (hereinafter, may be referred to as "modified powders") was measured. Furthermore, differential thermal analysis (DTA) was performed on a part of the modified mold powder, and the presence or absence of crystallization peaks during cooling was confirmed. In DTA, after heating from room temperature to 1300 °C at a rate of 20 °C / min, it was cooled at a rate of 20 °C / min.

[0071] FIG. 8 is a diagram showing the results of DTA performed on a modified powder (slag film) collected after continuous casting of free-cutting steel using the powder of Comparative Example 1. FIG. 9 is a diagram showing the results of DTA performed on a modified powder (slag film) collected after continuous casting of free-cutting steel using the powder of Example 1. A crystallization peak was confirmed in the modified powder of Comparative Example 1 (FIG. 8). On the other hand, no crystallization peak was confirmed in the modified powder of Example 1 (FIG. 9).

[0072] The composition of the modified mold powder, the viscosity at 1300° C., and the DTA analysis results are shown in Tables 6 and 7. In "amorphization" in each table, ○ was used when no crystallization peak was confirmed by DTA during cooling, and × was used when a crystallization peak was confirmed. Note that the modified mold powder is denoted by adding A to each number of the examples and comparative examples. For example, Example 1A means a mold powder obtained by modifying the composition of the mold powder of Example 1. Since the modified mold powder has a composition changed by continuous casting of free-cutting steel or is assumed to have a composition change by continuous casting of free-cutting steel, even if it is outside the scope of the present disclosure, it is not underlined.

[0073]

Table 6

[0074]

Table 7

[0075] In the modified mold powder of each example, the viscosity at 1300° C. is maintained at 2.0 Pa·s or more. Therefore, it is considered that the inflow uniformity between the mold and the solidified shell is high even in the continuous casting of free-cutting steel, and solidification disturbance is suppressed. Also, no crystallization peak was present in DTA. Therefore, it is considered that the growth of slag bear is suppressed even in the continuous casting of free-cutting steel.

[0076] On the other hand, for the mold powders after deterioration in each comparative example other than Comparative Example 6A, the viscosity at 1300°C decreased to less than 2.0 Pa·s. Therefore, it is considered that in the continuous casting of free-cutting steel, the inflow uniformity between the mold and the solidified shell was low and the solidification disturbance became large. Although the viscosity of the mold powder after deterioration in Comparative Example 6A was 2.0 Pa·s or more at 1300°C, slag carryover was confirmed in comparative examples other than Comparative Example 2 in the continuous casting of free-cutting steel, and crystallization peaks were confirmed by DTA in comparative examples other than Comparative Example 2A even for the mold powders after deterioration.

[0077] On the other hand, in Comparative Example 2, no slag carryover was observed in the continuous casting of free-cutting steel, but no crystallization peak was present by DTA even for the mold powder after deterioration in Comparative Example 2A. However, the viscosity of the mold powder after deterioration in Comparative Example 2A decreased significantly to less than 2.0 Pa·s at 1300°C. Therefore, in the continuous casting of free-cutting steel using the mold powder of Comparative Example 2, it is considered that the powder inflow uniformity between the mold and the solidified shell decreased and solidification disturbance of the slab occurred.

[0078] From the results of the examples and comparative examples, it can be seen that the evaluation of slag carryover and the evaluation of crystallization by DTA are correlated. That is, when the evaluation of slag carryover is ○, the evaluation of non-crystallization by DTA of the powder after deterioration is also ○. Therefore, when no crystallization peak exists in the DTA of the powder after deterioration, it can be considered that the growth of slag carryover in the continuous casting of free-cutting steel is also suppressed.

[0079] Mold powders A and B having the compositions and physical properties shown in Table 8 below were prepared, and low-carbon free-cutting steel having the composition shown in Table 9 below was continuously cast using each powder.

[0080]

Table 8

[0081]

Table 9

[0082] Figure 10 is a diagram showing an example of the appearance of a free-cutting steel slab continuously cast using powder B of the comparative example. Figure 11 is a diagram showing an example of the appearance of a free-cutting steel slab continuously cast using powder A of the example. As a result of applying powder A to the continuous casting of low-carbon free-cutting steel, a remarkable inflow improvement effect was manifested as compared with the case of applying powder B. The insufficient inflow part of the powder and deep OSMs were significantly reduced.

[0083] Figure 12 is a diagram showing the number of boundaries between the lubricating inflow part and the insufficient inflow part of the powder on the appearance (four sides) of each slab. Figure 13 is a diagram showing the ratio of the depth of OSM at the corner part of the slabs manufactured in the example and the comparative example. When powder A was used, although shallow OSMs remained at the corner part, they were deep enough to be sufficiently removed by hot scarfing (1.5 mm).

[0084] Also, when powder A was used, the number of slabs with surface defects in slab finishing was reduced by about 80% as compared with the case of using powder B. Also, the frequency of breakout detection during casting was reduced by about 90%.

[0085] The results of measuring the composition and physical properties of powder A at the initial stage and during 200 t casting are shown in Table 10 below.

[0086]

Table 10

[0087] The viscosity of the modified powder during 200 t casting was 2.0 Pa·s or more. Also, as a result of checking the slag bear after casting, the generation of slag bear was not observed. From the above results, it was confirmed that uniform inflow of the powder was achieved and the solidification delay was improved, leading to relaxation of the operation regulations.

Claims

1. A mold powder used for continuous casting of free-cutting steel, wherein the free-cutting steel has a chemical composition containing C: 0.06 to 0.18% by mass, Si: 0.01 to 0.05% by mass, Mn: 0.20 to 1.50% by mass, P: 0.040 to 0.110% by mass, S: 0.180 to 0.450% by mass, and O: 70 to 300 ppm, as components when carbon in the mold powder is completely combusted, SiO ２ The mass ratio of the content of CaO to the content of SiO is 0.50 or more and 0.59 or less, and the total content of CaO and SiO ２ is 70.0% by mass or more, Al ２ O ３ The content of which is 13.0 to 20.0% by mass, the total content of alkali metal oxides is 2.0 to 5.0% by mass, the balance consists of alkaline earth metal oxides excluding F and CaO, and unavoidably contained components, and a mold powder for continuous casting of free-cutting steel having a viscosity at 1300 °C of 4.0 Pa·s or more.

2. When continuous casting of the free-cutting steel is carried out using the mold powder for continuous casting of free-cutting steel, differential thermal analysis is performed in which the temperature is raised to 1300 °C at 20 °C / min and cooled at 20 °C / min for a powder film in which a powder slag having a composition different from that of the mold powder for continuous casting of free-cutting steel is solidified. The mold powder for continuous casting of free-cutting steel according to claim 1, wherein no crystallization peak exists during cooling.

3. When continuous casting of the free-cutting steel is carried out using the mold powder for continuous casting of free-cutting steel, the viscosity at 1300 °C of a powder slag having a composition different from that of the mold powder for continuous casting of free-cutting steel is 2.0 Pa·s or more. The mold powder for continuous casting of free-cutting steel according to claim 1.

4. A step of injecting molten steel for casting free-cutting steel having a chemical composition containing C: 0.06 to 0.18% by mass, Si: 0.01 to 0.05% by mass, Mn: 0.20 to 1.50% by mass, P: 0.040 to 0.110% by mass, S: 0.180 to 0.450% by mass, and O: 70 to 300 ppm into a mold for continuous casting; a step of introducing the mold powder for continuous casting of free-cutting steel according to any one of claims 1 to 3 into the molten steel injected into the mold; and a step of continuously withdrawing and cooling a slab in which the molten steel has solidified from the mold. A method for continuous casting of free-cutting steel, comprising the above steps.

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