Continuous casting method for steel

Abstract

To stably suppress surface cracking which occurs when correcting a cast piece, in continuously casting steel whose Al density and Mn density are high.SOLUTION: In a continuously casting method for steel, which is a method for continuously casting a cast piece of steel having a predetermined composition whose Al density and Mn density are high, using a continuously casting machine having a straightening point, the cast piece is cooled so that TA(s) determined by the following formula (1) is equal to 30 or higher, where the duration when the surface temperature of the cast piece is maintained in a range of 350-450°C is defined as TL(s) and the duration when the surface temperature is maintained in a range of 550-650°C is defined as TH(s), in a stage where casting is not yet progressed from a portion just below a casing mold to the straightening point. Then, the surface temperature of the cast piece is returned to a temperature range of Ac3 or higher before reaching the straightening point. TA=TL×[Al]-1.1×[Mn]-0.6+TH×10-1×[Al]-1.5×[Mn]-1.1 (1). In the formula (1), [Al] and [Mn] are densities (mass%) of Al and Mn in the steel.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present application discloses a method for continuous casting of steel. [Background technology]

[0002] In recent years, in the field of high-strength steel materials such as thin steel sheets, steels containing large amounts of alloying elements such as Mn and Al have been widely produced in order to improve mechanical properties.

[0003] However, the addition of these alloying elements causes defects in the form of surface cracks in the slabs produced by continuous casting, which poses problems in terms of both operation and product quality. Surface cracks are a general term for crack forms such as transverse cracks, horizontal cracks, and corner cracks.

[0004] Surface cracks occurring after the secondary cooling zone of continuous casting are known to occur along prior austenite grain boundaries on the slab surface. These cracks can occur due to stress concentration at austenite grain boundaries embrittled by precipitation of AlN, NbC, etc., or at film-like ferrite formed along prior austenite grain boundaries. The crack morphology varies depending on the direction of the applied stress; transverse cracks are caused by tensile stress in the casting direction, while longitudinal cracks are caused by tensile stress in the width direction of the slab. These cracks are particularly likely to occur in the temperature range near the austenite-to-ferrite phase transformation region. Therefore, cracking is typically prevented by avoiding the surface temperature in the bending and straightening zones, where mechanical stresses are applied to the slab surface, above the temperature range where ductility decreases (the embrittlement temperature range). However, as the number of steels containing various elements to improve mechanical properties has increased, the number of steels with high cracking susceptibility has also increased. Therefore, the continuous casting method alone is not necessarily sufficient to prevent cracking.

[0005] Patent Document 1 proposes a method for continuous casting of steel, characterized in that, in the cooling process from just below the mold in continuous casting to just before the straightening point, the surface layer of the slab is cooled to a temperature below the bainite, ferrite, or pearlite transformation start temperature in the continuous cooling transformation diagram of steel, and then reheated to a temperature of Ac3 or higher at a heating rate of 3°C / s to 50°C / s, or alternatively, the surface layer of the slab is cooled to a temperature below Ar3-100°C, and then reheated to a temperature of Ac3 or higher at a heating rate of 1.4°C / s or lower.

[0006] Patent Document 2 proposes a continuous casting method characterized by cooling the surface layer of a slab directly below a continuous casting mold to a temperature range below the ferrite-pearlite transformation finish temperature and above the bainite transformation start temperature in a continuous cooling transformation diagram of steel, and then maintaining the temperature in the temperature range below the ferrite-pearlite transformation finish temperature and above the bainite transformation start temperature until the temperature intersects with a constant-rate cooling curve that passes through the nose of ferrite-pearlite transformation in the continuous cooling transformation diagram.

[0007] Patent Document 3 describes the time T during which the surface temperature of the slab is between 350 and 475°C from just below the mold to the straightening point in continuous casting. L and time T at 600-675°C H The time T obtained from A A continuous casting method for Ni-containing low alloy steel has been proposed, which is characterized by cooling the slab so that the slab has a temperature of 60 or more, and then reheating it to a temperature range of Ac3 or more. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] Patent No. 5928413 [Patent Document 2] Patent No. 5884479 [Patent Document 3] Japanese Patent Publication No. 2020-131203 Summary of the Invention [Problem to be solved by the invention]

[0009] All of the above-mentioned conventional techniques utilize phase transformations in steel to refine grains and improve ductility at the straightening point. However, depending on the steel composition, some steels cannot be defined as Ar3 at cooling rates equivalent to the secondary cooling zone, and some steels cannot suppress surface cracking even after undergoing a specified temperature history. For example, it is difficult to apply these conventional techniques to steels with high Al and Mn concentrations. In particular, Al concentration significantly affects not only the precipitation of AlN, which causes high-temperature embrittlement of steel, but also the transformation from austenite to bainite or ferrite-pearlite during cooling. Therefore, in order to consistently suppress surface cracking during straightening of slabs in continuous casting of steels with high Al and Mn concentrations, new technical guidelines are needed for cooling the slab directly below the mold during continuous casting. [Means for solving the problem]

[0010] The inventors of the present invention came up with the idea that in order to provide a temperature history of rapid cooling and reheating to the surface layer of a cast slab in the secondary cooling zone immediately below the mold, refine the surface structure, and prevent cracking, it is necessary to fully consider the influence of alloying elements on the transformation behavior. In particular, the Al and Mn concentrations have a significant effect on the transformation behavior of steel, so the influence of these elements was fundamentally investigated. Specifically, they focused on two temperature ranges, 350 to 450°C, which is near the nose of bainite transformation, and 550 to 650°C, which is near the nose of ferrite-pearlite transformation, and investigated the residence time T in these temperature ranges. L , T H As a result, the relationship between the Al concentration and Mn concentration and the residence time T L , T H and T can be calculated using the following formula (1): A It was found that by cooling the slab just below the mold so that the Ac3 point is equal to or higher than a predetermined value and then reheating it to the Ac3 point or higher, the slab surface structure can be refined before the straightening point is reached. T A =T L ×[Al] -1.1 ×[Mn]-0.6 +T H x10 -1 ×[Al] -1.5 ×[Mn] -1.1 (1)

[0011] Based on the above findings, the present application proposes the following as one of the means for solving the above problems: A method for continuously casting a slab of steel containing, by mass%, C: 0.05 to 0.40%, Si: 0 to less than 1.0%, Mn: 1.0 to 3.0%, Al: 0.1 to 2.0%, Cr: 0 to 0.60%, Mo: 0 to 0.600%, Ni: 0 to 0.50%, and N: 0 to 0.0250%, using a continuous casting machine having a straightening point, The time during which the surface temperature of the slab is between 350 and 450°C from just below the mold before reaching the straightening point is defined as T L (s), the time at 550-650°C, T H (s) is defined by the following formula (1) A Cooling the slab so that (s) is 30 or more, Next, before reaching the straightening point, the surface temperature of the slab is reheated to a temperature range of Ac3 or higher. Continuous casting method for steel Disclose.

[0012] T A =T L ×[Al] -1.1 ×[Mn] -0.6 +T H x10 -1 ×[Al] -1.5 ×[Mn] -1.1 (1) (In formula (1), [Al] and [Mn] are the concentrations (mass%) of Al and Mn in the steel.)

[0013] In the method of the present disclosure, the cast slab may contain, by mass%, Ti: 0 to 0.100%, V: 0 to 0.400%, Ca: 0 to 0.0100%, Mg: 0 to 0.0100%, REM: 0 to 0.0100%, Nb: 0 to 0.050%, and B: 0 to 0.0040%. [Effects of the Invention]

[0014] According to the method of the present disclosure, it is possible to stably suppress surface cracks that occur when straightening a cast piece in continuous casting of steel with high Al and Mn concentrations. By hot rolling the cast piece produced by the method of the present disclosure, it is possible to obtain a steel plate or steel piece in which the occurrence of surface cracks and the like is suppressed. [Brief explanation of the drawings]

[0015] [Figure 1] 1 is a schematic diagram for explaining an example of a continuous casting machine employed in the method for continuous casting steel of the present disclosure. FIG. [Figure 2] FIG. 10 is a diagram for providing a supplementary explanation of TL and TH. [Figure 3] FIG. 2 is a photograph showing an example of the state of the surface layer structure of a cast slab obtained by a model experiment. [Figure 4] FIG. 1 is a diagram showing the relationship between the heat treatment pattern using a Formaster device and the resulting structure for steel type A1. [Figure 5] FIG. 1 is a diagram showing the relationship between the heat treatment pattern using a Formaster device and the resulting structure for steel type B1. [Figure 6] FIG. 1 is a diagram showing the relationship between the heat treatment pattern using a Formaster device and the resulting structure for steel type C1. [Figure 7] FIG. 1 is a diagram showing the relationship between the heat treatment pattern by a Formaster device and the resulting structure for steel type D1. DETAILED DESCRIPTION OF THE INVENTION

[0016] The continuous casting method for steel according to the present disclosure will be described with reference to FIG. 1. For clarity, cooling spray nozzles and the like are omitted from FIG. 1. The cooling spray nozzles are provided, for example, between the support rolls located immediately below the mold 10 and before the straightening point 20, and can spray cooling water from both sides of the slab 1. While FIG. 1 illustrates a vertical bending type continuous casting machine 100, the continuous casting method according to the present disclosure can be applied to any continuous casting machine having a straightening point. For example, a curved type continuous casting machine may also be used. The "straightening point" refers to a point where distortion is applied to straighten the casting direction of the slab 1 from a curved direction to a horizontal direction. Straightening may be performed at multiple locations. The configuration of the continuous casting machine 100, including the mold 10, the straightening point 20, and the like, may be the same as that of a conventionally known configuration, and therefore a detailed description thereof will be omitted here.

[0017] As shown in FIG. 1 , the continuous casting method of steel of the present disclosure is a method for continuously casting a slab 1 of steel containing, by mass%, C: 0.05 to 0.40%, Si: 0 to less than 1.0%, Mn: 1.0 to 3.0%, Al: 0.1 to 2.0%, Cr: 0 to 0.60%, Mo: 0 to 0.600%, Ni: 0 to 0.50%, and N: 0 to 0.0250% using a continuous casting machine having a straightening point 20, and the time during which the surface temperature of the slab 1 is between 350 and 450°C from just below the mold 10 before reaching the straightening point 20 is defined as T L (s), the time at 550-650°C, T H (s) is defined by the following formula (1) A The slab 1 is cooled so that (s) is 30 or more, and then, before reaching the straightening point 20, the surface temperature of the slab 1 is reheated to a temperature range of Ac3 or more.

[0018] T A =T L ×[Al] -1.1 ×[Mn] -0.6 +T H x10 -1 ×[Al] -1.5 ×[Mn] -1.1 (1) (In formula (1), [Al] and [Mn] are the concentrations (mass%) of Al and Mn in the steel.)

[0019] 1.Steel type In the continuous casting method of the present disclosure, the steel to be cast essentially contains C, Mn, and Al in addition to Fe. It may also contain at least one optional element selected from the group consisting of Si, Cr, Mo, Ni, N, Ti, V, Ca, Mg, REM, Nb, and B.

[0020] 1.1 C: 0.05-0.40% Carbon is the most fundamental element that affects not only the static strength of steel but also its fatigue strength, toughness, and ductility. If the carbon content is too low, the static strength and fatigue strength of the steel may be insufficient. In this regard, the carbon content is 0.05% by mass or more, and may be 0.10% by mass or more, or 0.15% by mass or more. Furthermore, if the carbon content is too high, the toughness of the steel is likely to deteriorate. In this regard, the carbon content is 0.40% by mass or less, and may be 0.38% by mass or less, or 0.35% by mass or less.

[0021] 1.2 Si: 0-1.0% Si is an important element with the second highest solid solution strengthening ability after C. In the method of the present disclosure, the steel does not necessarily contain Si. In this regard, the Si content may be 0% by mass or more, or may be 0.1% by mass or more, or 0.2% by mass or more. On the other hand, if the Si content is too high, even if the slab is cooled directly below the mold so as to satisfy the above formula (1), it may be difficult to stably suppress surface cracks that occur when straightening the slab. In this regard, the Si content may be less than 1.0% by mass, or may be 0.8% by mass or less, 0.7% by mass or less, 0.6% by mass or less, or 0.5% by mass or less.

[0022] 1.3 Mn: 1.0-3.0% Mn is an important element for improving hardenability and ensuring hardness deep into parts even when the cooling rate is insufficient. To obtain high-strength steel, the Mn concentration is set to a high level. Specifically, the Mn content is 1.0 mass% or more, and may be 1.2 mass% or more or 1.5 mass% or more. On the other hand, if the Mn content is too high, there is a risk of deteriorating toughness and workability. In this regard, the Mn content is 3.0 mass% or less, and may be 2.5 mass% or less.

[0023] 1.4 Al: 0.1-2.0% Al is the most widely used element for deoxidation and also has the effect of suppressing grain coarsening by generating AlN. On the other hand, excessive Al content can lead to problems such as nozzle clogging during casting due to Al2O3 agglomeration, and residual Al2O3 in the steel can degrade performance. Furthermore, Al not only contributes to the precipitation of AlN, but also significantly affects the transformation from austenite to bainite or ferrite-pearlite during the cooling process. According to the inventor's new findings, when the Al content of steel is 0.1% by mass or more and 2.0% by mass or less, cooling the slab immediately below the mold so as to satisfy the above formula (1) can significantly suppress surface cracking during slab straightening. The Al content may be 0.2% by mass or more, 0.3% by mass or more, or 1.8% by mass or less, 1.7% by mass or less, or 1.6% by mass or less.

[0024] 1.5 Cr: 0~0.60% Cr is an element that is optionally added to improve the hardenability of steel, for example. In this regard, the Cr content is 0% by mass or more, and may be 0.01% by mass or more. On the other hand, if the Cr content is too high, even if the slab is cooled directly below the mold so as to satisfy the above formula (1), it may be difficult to stably suppress surface cracks that occur when the slab is straightened. In this regard, the Cr content may be 0.60% by mass or less, 0.50% by mass or less, 0.40% by mass or less, 0.30% by mass or less, 0.20% by mass or less, or 0.10% by mass or less.

[0025] 1.6 Mo: 0-0.600% Mo is an element that is optionally added for the purposes of, for example, secondary hardening during tempering of steel, improving fatigue strength, and improving hardenability. The Mo content is 0% by mass or more, and may be 0.001% by mass or more. On the other hand, if the Mo content is too high, even if the slab is cooled directly below the mold so as to satisfy the above formula (1), it may be difficult to stably suppress surface cracks that occur when straightening the slab. In this regard, the Mo content may be 0.600% by mass or less, 0.500% by mass or less, 0.400% by mass or less, 0.300% by mass or less, 0.200% by mass or less, or 0.100% by mass or less.

[0026] 1.7 Ni: 0-0.50% Ni is an element that is optionally added to steel, for example, to ensure its strength and toughness and improve its hardenability. The Ni content is 0% by mass or more, and may be 0.01% by mass or more. On the other hand, if the Ni content is too high, even if the slab is cooled directly below the mold so as to satisfy the above formula (1), it may be difficult to stably suppress surface cracks that occur when the slab is straightened. In this regard, the Ni content may be 0.50% by mass or less, 0.40% by mass or less, 0.30% by mass or less, 0.20% by mass or less, or 0.10% by mass or less.

[0027] 1.8 N: 0~0.0250% N, for example, forms nitrides in steel and can exert the effect of suppressing grain coarsening. However, too much N can lead to coarsening of nitrides, which may reduce the fatigue strength of the steel. It can also reduce hot ductility and cause surface defects during casting or rolling. In this regard, the N content is 0.0250% by mass or less, and may be 0.0200% by mass or less from the viewpoint of steel cleanliness. The N content may be 0% by mass or more, or may be 0.0020% by mass or more.

[0028] In the continuous casting method of the present disclosure, the steel to be cast may further contain the following elements in addition to the elements described above. That is, the slab 1 may contain, by mass%, Ti: 0-0.100%, V: 0-0.400%, Ca: 0-0.0100%, Mg: 0-0.0100%, REM: 0-0.0100%, Nb: 0-0.050%, and B: 0-0.0040%. According to the continuous casting method of the present disclosure, by cooling the slab immediately below the mold so as to satisfy the above formula (1), surface cracking that occurs during slab straightening can be stably suppressed regardless of the contents of optional elements contained in the slab, such as Ti, V, Ca, Mg, REM, Nb, and B. That is, the elements described below do not substantially affect the validity of the above formula (1).

[0029] 1.9 Ti: 0 to 0.100% Like Al, Ti is an element that can form nitrides, has excellent thermal stability, and maintains its effect of suppressing grain coarsening up to higher temperatures. However, if Ti is too much, TiN tends to grow coarsely, which may reduce fatigue strength. In this regard, the Ti content may be 0.100% by mass or less. The Ti content may be 0% by mass or more, 0.001% by mass or more, or 0.002% by mass or more.

[0030] 1.10 V: 0 to 0.400% V, like Ti and Al, is an element that can form nitrides and is used to improve strength. However, if there is too much V, VN tends to grow coarsely, which may reduce fatigue strength. In this regard, the V content may be 0.400% by mass or less. The V content may be 0% by mass or more, 0.001% by mass or more, or 0.002% by mass or more.

[0031] 1.11 Ca: 0 to 0.0100% Ca has the effect of modifying Al2O3 and suppressing the coarsening of oxide-based inclusions. However, if the Ca content is too high, coarse oxide-based inclusions composed mainly of CaO-Al2O3 may be formed, which may become the starting point for fatigue fracture. In this regard, the Ca content may be 0.0100% by mass or less. The Ca content may be 0% by mass or more, 0.0001% by mass or more, or 0.0002% by mass or more.

[0032] 1.12 Mg: 0~0.0100% Like Ca, Mg modifies Al2O3 and has the effect of suppressing the coarsening of oxide-based inclusions. It also acts on sulfide-based inclusions, reducing their aspect ratio. However, if there is too much Mg, coarse cluster-like oxide-based inclusions containing MgO as the main component may form, which may become the starting point for fatigue fracture. In this regard, the Mg content may be 0.0100% by mass or less. The Mg content may be 0% by mass or more, 0.0001% by mass or more, or 0.0002% by mass or more.

[0033] 1.13 REM: 0 to 0.0100% REM also modifies Al2O3 and has the effect of suppressing the coarsening of oxide inclusions. However, too much REM may reduce the cleanliness of the steel and the toughness of the base material. In this regard, the REM content may be 0.0100% by mass or less. The REM content may be 0% by mass or more, 0.0001% by mass or more, or 0.0002% by mass or more. REM refers to rare earth elements such as La and Ce, and any one or more of these REMs may be used.

[0034] 1.14 Nb: 0-0.050% Nb is effective in improving strength and toughness. However, if there is too much Nb, the effect saturates. In this regard, the Nb content may be 0.050% by mass or less. The Nb content may be 0% by mass or more, 0.001% by mass or more, or 0.002% by mass or more.

[0035] 1.15 B: 0 to 0.0040% A small amount of B has a significant effect of improving hardenability. However, if the amount of B is too much, the effect saturates. In this regard, the B content may be 0.0040% by mass or less. The B content may be 0% by mass or more, 0.0001% by mass or more, or 0.0002% by mass or more.

[0036] 1.16 Other elements In the continuous casting method of the present disclosure, the steel to be cast may contain impurities such as P and S. In addition, at least one selected from Cu, O, W, Ta, Co, Sn, Sb, and As may be contained as an optional element or impurity. These elements do not substantially affect the validity of formula (1) above. There are no particular limitations on the contents of these elements in the steel. In the continuous casting method of the present disclosure, the steel may contain, for example, by mass, P: 0.100% or less, S: 0.020% or less, Cu: 0-1.000%, O: 0-0.0200%, W: 0-0.100%, Ta: 0-0.100%, Co: 0-0.500%, Sn: 0-1.000%, Sb: 0-0.500%, and As: 0-0.050%. Furthermore, the steel may contain elements other than these.

[0037] 2. Secondary cooling of slab 1 As shown in Fig. 1, in the continuous casting method of the present disclosure, a slab 1 of steel having the above-mentioned composition is continuously withdrawn from a mold 10, and secondary cooling of the slab 1 is performed by, for example, spraying cooling water onto the surface of the slab 1 before it moves from just below the mold 10 to the straightening point 20. Here, in the continuous casting method of the present disclosure, the time during which the surface temperature of the slab 1 is between 350 and 450°C from just below the mold 10 to the straightening point 20 is defined as T L (s), the time at 550-650°C, T H (s) is T defined by the above formula (1) A It is important to cool the slab 1 so that T (s) is 30 or more. A The upper limit of (s) is not particularly limited. A (s) may be, for example, 100 or less, 80 or less, or 60 or less.

[0038] In the continuous casting method of the present disclosure, T L However, depending on the steel type, this temperature may fall below the martensitic transformation start temperature, and some or all of the structure may become martensite. However, even in this case, it is possible to refine the surface structure after reverse transformation, and obtain the desired effect.

[0039] In an actual continuous casting machine, the surface temperature of the slab 1 may pass through the ranges of 350 to 450°C and 550 to 650°C several times between just below the mold 10 and before reaching the straightening point 20. L , T H is expressed as the sum of the time it takes to pass through each temperature range. For example, in the thermal history shown in Figure 2, T L , T H can be calculated using the following equations (2) and (3).

[0040] T L =t4-t3(2) T H =(t2-t1)+(t6-t5) (3)

[0041] As a method for cooling the slab 1 in the secondary cooling zone, in addition to the above-mentioned method of spraying cooling water using a cooling spray nozzle, a method using an air flow, a method of cooling naturally without using special cooling equipment, etc. are all effective. Furthermore, a cooling method that combines these may also be used. The cooling rate of the slab 1 is not particularly limited, and the desired effect can be achieved at any cooling rate.

[0042] 3. Reheating of slab 1 In the continuous casting method of the present disclosure, after decomposing austenite in the secondary cooling zone, the surface temperature of the slab 1 is reheated to a temperature of Ac3 or higher before reaching the straightening point 20 (for example, at time t7 in Figure 2). This reheating is essential for forming a fine austenite structure in the surface layer of the slab 1, i.e., to obtain a so-called reverse transformation structure. If the reheating temperature is lower than Ac3, some areas will remain where reverse transformation does not occur. This structure is influenced by the as-cast structure, which is prone to cracking due to straightening strain. Therefore, reheating to Ac3 or higher to form an austenite single-phase structure is effective in suppressing cracking. Note that if the surface temperature of the slab 1 is reheated to Ac3 or higher before reaching the straightening point 20, the hot ductility of the surface of the slab 1 will be maintained high thereafter, and surface cracking will not be a problem even if the temperature drops at the straightening point 20.

[0043] By reheating to Ac3, the surface structure of the slab 1 is improved, and in combination with appropriate secondary cooling, a slab with fewer surface cracks can be obtained. From the viewpoint of further suppressing variations in the surface temperature and structure within the slab 1, the maximum temperature after reheating may be set to Ac3 + 30°C or higher. However, if the reheating temperature is too high, there is a risk that the austenite grains will coarsen again. In this regard, the maximum temperature after reheating may be 1200°C or lower.

[0044] Ac3 can be measured using a transformation point recording and measuring device (Formaster device), etc. Alternatively, Ac3 can be determined using the following formula (4) proposed in a prior document (Tatsuo Kunitake: Heat Treatment, 43, p. 100 (2003)).

[0045] Ac3=(32[Si]+17[Mo])-(231[C]+20[Mn]+40[Cu]+18[Ni]+15[Cr])+912 (4) (In formula (4), [Si], [Mo], [C], [Mn], [Cu], [Ni], and [Cr] represent the concentration (mass%) of each component.)

[0046] Reheating of the surface of the slab 1 is a phenomenon that occurs when the amount of heat transferred from the interior of the slab 1 exceeds the amount of heat released from the surface of the slab 1. Reheating of the surface of the slab 1 can be achieved relatively easily by slowing down the cooling in the secondary cooling zone. Alternatively, the surface may be heated by arranging a heat source or high-frequency induction heating equipment around the casting line. The reheating rate (temperature increase rate) of the slab 1 is not particularly limited, and the desired effect can be achieved at any reheating rate.

[0047] As described above, the continuous casting method of the present disclosure can stably suppress surface cracks at the straightening point of a slab during continuous casting of steel with high Al and Mn concentrations. In this application, the term "surface of the slab" does not necessarily refer to the entire surface of the slab. That is, at least a portion of the surface of the slab where surface cracks are desired to be suppressed can be cooled directly below the mold so that the above formula (1) is satisfied, and then the slab can be reheated to a temperature of Ac3 or higher. [Example]

[0048] The effects of the technology of the present disclosure will be described in more detail below with reference to examples, but the technology of the present disclosure is not limited to the following examples.

[0049] 1. Model Experiment 1 In order to clarify the conditions for fully obtaining the effect of refining the slab surface structure by secondary cooling and reheating, a model experiment was carried out using a transformation point recording and measuring device (Formaster device).

[0050] Steel types A1 to D1 having the compositions shown in Table 1 below were each heated to 1400°C using a Formaster apparatus, then rapidly cooled with helium gas to a predetermined temperature of 350 to 750°C. The rapidly cooled samples were held isothermally for 30 to 3000 seconds, then heated to 900°C at 20°C / s, and cooled to room temperature at 0.4°C / s. The cross sections of the obtained samples were etched with nital and observed with an SEM.

[0051] [Table 1]

[0052] Observation examples are shown in Figures 3(A) and (B). The structure confirmed by cross-sectional observation was composed of structure A shown in Figure 3(A) and / or structure B shown in Figure 3(B). As is clear from Figure 3(A), fine granular ferrite on the order of several tens of micrometers was observed in structure A, and grain boundary ferrite was unclear, suggesting that structure A has low cracking susceptibility. On the other hand, as is clear from Figure 3(B), no granular ferrite was observed in structure B, and some grain boundary ferrite was observed, suggesting that structure B has high cracking susceptibility.

[0053] Figures 4–7 show the relationship between the temperature patterns of heat treatment using the Formaster machine and the resulting microstructures for steel grades A1–D1. Figure 4 shows the results for steel grade A1, Figure 5 shows the results for steel grade B1, Figure 6 shows the results for steel grade C1, and Figure 7 shows the results for steel grade D1. In Figures 4–7, the resulting microstructures are indicated as "○" when they are entirely A, "△" when they are a mixed phase of A and B, and "×" when they are entirely B. As is clear from Figures 4–7, the temperature conditions under which microstructure A is obtained differed significantly for each steel grade. In other words, it was found that the appropriate temperature conditions for obtaining a microstructure with low cracking susceptibility cannot be determined without considering the effects of Al and Mn concentrations. This model experiment revealed that the residence time in two temperature ranges, 350–450°C (where bainite transformation occurs) and 550–650°C (where ferrite–pearlite transformation occurs), is important for the decomposition of austenite. Therefore, a model experiment was conducted to understand the transformation behavior of the slab surface when it crosses both of these temperature ranges during actual continuous casting.

[0054] 2. Model Experiment 2 Steel types A1 to D1, the same as those in Model Experiment 1 above, were heated to 1400°C using a Formaster apparatus, held at 400°C for a predetermined time, then held at 600°C for a predetermined time, reheated to 900°C at 20°C / s, and then cooled to room temperature at 0.4°C / s. The relationship between the holding time at 400°C and 600°C and the resulting structure is shown in Table 2 below. In Table 2 below, cases where the resulting structure was A over the entire surface are indicated by "○", cases where it was a mixed phase of A and B are indicated by "△", and cases where it was B over the entire surface are indicated by "×".

[0055] [Table 2]

[0056] It is estimated that during the cooling and heating processes of steel, coarse austenite transforms into bainite while held at 400°C, and untransformed austenite transforms into ferrite-pearlite while held at 600°C. In other words, for the samples showing Structure A throughout the steel in the results shown in Table 2, the decomposition of austenite is complete when the sum of the amount of bainite transformed and the amount of ferrite-pearlite transformed balances with the amount of austenite before transformation, and then, after reheating to 900°C, Structure A is assumed to have been exhibited during slow cooling.

[0057] For steels with high Al and Mn concentrations, the residence time T in the temperature range of 350 to 450°C and the temperature range of 550 to 650°C L (s), T H As a result, it was found that the relationship between the surface structure of the slab and the temperature difference between the slab and the surface structure of the slab was A It was discovered that when the total holding time in the transformation temperature range (s) is 30 or more, subsequent reheating and cooling to Ac3 or more results in the formation of a structure with low cracking sensitivity.

[0058] T A =T L ×[Al] -1.1 ×[Mn] -0.6 +T H x10-1 ×[Al] -1.5 ×[Mn] -1.1 (1) (In formula (1), [Al] and [Mn] are the concentrations (mass%) of Al and Mn in the steel.)

[0059] It has been confirmed that the above formula (1) also holds true in experiments conducted using steel types other than the above steel types A1 to D1 that have high concentrations of Al and Mn. That is, when a steel containing, by mass%, C: 0.05 to 0.40%, Si: 0 to less than 1.0%, Mn: 1.0 to 3.0%, Al: 0.1 to 2.0%, Cr: 0 to 0.60%, Mo: 0 to 0.600%, Ni: 0 to 0.50%, and N: 0 to 0.0250% is cast, secondary cooling of the slab so that the above formula (1) is 30 or more can refine the slab surface structure and suppress surface cracking at the straightening point.

[0060] 3. Actual machine testing To verify the validity of the above model experiments, actual tests were conducted using a continuous slab caster. Specifically, for each of the steel types A2 to D2 listed in Table 3 below, a curved continuous caster with a curvature radius of 12.0 m was used to cast a slab measuring 240 mm x 1500 mm. The casting speed was 1.0 to 1.4 m / min. The slab was withdrawn from the mold and quenched in a spray quenching device with a zone length of 1 m installed directly below the mold. After passing through the zone, the amount of water used for the standard secondary cooling spray was adjusted to control reheating. The slab was cut into lengths of 5.0 ± 0.2 m using a gas cutting machine and then subjected to surface observation.

[0061] [Table 3]

[0062] The slab surface temperature is the temperature at the center of the L-face of the slab calculated by heat transfer solidification calculation. The temperature calculated by heat transfer solidification calculation was verified to have sufficiently high accuracy by comparison with data from a slab surface temperature system installed in a continuous casting machine. The value of Ac3, which is the target maximum temperature after reheating, was determined by the above formula (4).

[0063] The resulting slabs were visually inspected for surface cracks. The time T during which the slab surface temperature was between 350 and 450°C in the cooling zone was L , time T was between 550 and 650°C H The results of the investigation, along with the maximum temperature from the start of reheating to the straightening point, are shown in Table 4 below. In Table 4 below, the evaluation of surface cracks is given as "0" if there are no cracks, "1" if the depth of all cracks is less than 0.5 mm and the number of cracks is 10 or less per cast piece, and "2" if it falls into neither category.

[0064] [Table 4]

[0065] As is clear from the results shown in Table 4, T shown in the above formula (1) A In the slabs (Examples 1 to 9) in which the slab surface was cooled immediately below the mold so that the temperature was 30 or higher, and then reheated to Ac3 or higher before reaching the straightening point, no cracks were observed on the surface, and a fine structure with dispersed ferrite was observed in the slab surface structure. A When the temperature was less than 30 (Comparative Examples 1, 2, 4, 5, 7, 8 and 9) or when the maximum temperature from the start of reheating to the straightening point was less than Ac3 (Comparative Examples 3, 6 and 9), cracks were observed on the slab surface. [Explanation of symbols]

[0066] 1. Castings 10 Mold 20 correction points 100 Continuous Casting Machine

Claims

1. A method for continuously casting a slab of steel containing, by mass%, C: 0.05 to 0.40%, Si: 0 to less than 1.0%, Mn: 1.0 to 3.0%, Al: 0.1 to 2.0%, Cr: 0 to 0.60%, Mo: 0 to 0.600%, Ni: 0 to 0.50%, and N: 0 to 0.0250%, using a continuous casting machine having a straightening point, The time during which the surface temperature of the slab is between 350 and 450°C from just below the mold before reaching the straightening point is defined as T Ｌ (s), the time at 550 to 650°C is T Ｈ (s) is T defined by the following formula (1) Ａ Cooling the slab so that (s) is 30 or more, Next, before reaching the straightening point, the surface temperature of the slab is ３ Reheat to the above temperature range, A method for continuous casting of steel. T Ａ =T Ｌ ×[Al] －１．１ ×[Mn] －０．６ +T Ｈ ×10 －１ ×[Al] －１．５ ×[Mn] －１．１ (1) (In formula (1), [Al] and [Mn] are the concentrations (mass%) of Al and Mn in the steel.)

2. The cast slab contains, in mass%, Ti: 0 to 0.100%, V: 0 to 0.400%, Ca: 0 to 0.0100%, Mg: 0 to 0.0100%, REM: 0 to 0.0100%, Nb: 0 to 0.050%, and B: 0 to 0.0040%. The method of claim 1.

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