Continuous casting method for steel

Abstract

To suppress the surface crack generated when a slab is corrected in continuous casting of a steel having high Si concentration and Mn concentration.SOLUTION: A continuous casting method for a steel that is a method for continuously casting a slab of a steel having high Si concentration and Mn concentration and a predetermined chemical composition using a continuous casting machine having a correction point includes: cooling the slab so that TA(s) defined by the following expression (1) is 30 or more, when a time during which the surface temperature of the slab is 350 to 450°C is represented by TL(s), and a time during which the surface temperature thereof is 550 to 650°C is represented by TH(s), before the slab reaches the bend correction point from the direct below of the slab; and heating the surface temperature of the slab to a temperature range of Ac3 temperature or higher again before the slab reaches the correction point. Expression (1): TA=TL×[Mn]-0.6×[Si]-1.1+TH×10-1×[Mn]-1.1. In the expression (1), [Si] and [Mn] are concentrations (mass%) of Si 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 plates, steels containing large amounts of alloying elements such as Si and Mn have been widely produced in order to improve the 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 pose problems in terms of operation and product quality. Surface cracks refer to a general term for crack forms such as transverse cracks, horizontal cracks, and corner cracks.

[0004] It is known that surface cracks occurring after the secondary cooling zone of continuous casting are cracks along the prior austenite grain boundaries on the surface of the slab. These cracks can occur when stress is concentrated on the austenite grain boundaries embrittled by the precipitation of AlN, NbC, etc., or on the film-like ferrite formed along the prior austenite grain boundaries. The form of the cracks differs depending on the direction of the stress. Transverse cracks are caused by tensile stress in the casting direction, and 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 phase transformation region from austenite to ferrite. Therefore, a method is usually adopted to prevent the occurrence of cracks by avoiding the surface temperature in the bending or straightening zone, where mechanical stress is applied to the slab surface, from the temperature range where ductility decreases (embrittlement temperature range). However, in recent years, as the number of steel types to which various elements are added to improve mechanical properties increases, the number of steel types with high cracking sensitivity is also increasing, and the above-mentioned continuous casting method alone cannot necessarily 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 or more and 50°C / s or less, or 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 less.

[0006] Patent Document 2 proposes a continuous casting method characterized in that the surface layer of a cast slab is cooled directly below the 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 maintained in the temperature range below the ferrite-pearlite transformation finish temperature and above the bainite transformation start temperature until it intersects with a constant-rate cooling curve that passes through the nose of the ferrite-pearlite transformation in the continuous cooling transformation diagram.

[0007] In Patent Document 3, 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 is L and time T at 600-675°C H The time T obtained from A A method for continuous casting of Ni-containing low alloy steel has been proposed, which is characterized in that the slab is cooled so that its melting point becomes 60 or more, and then reheated 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] JP 2020-131203 A Summary of the Invention [Problem to be solved by the invention]

[0009] All of the above-mentioned conventional techniques are intended to refine the crystal grains by utilizing the phase transformation of steel and improve the ductility at the straightening point. However, depending on the composition of the steel, there are steel types in which Ar3 cannot be defined at the cooling rate corresponding to the secondary cooling zone, and steel types in which surface cracks cannot be suppressed even after the temperature history is passed through, and these steel types are actually produced. For example, it is difficult to apply the above-mentioned conventional techniques to steels with high Si and Mn concentrations. In order to stably suppress surface cracks that occur when straightening a slab in the continuous casting of steels with high Si and Mn concentrations, a new technical guideline is 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 the surface layer of a slab with a temperature history of rapid cooling and reheating in the secondary cooling zone immediately below the mold, refine the surface structure, and avoid cracking, it is necessary to fully consider the effects of alloying elements on the transformation behavior. In particular, since the concentrations of Si and Mn have a significant effect on the transformation behavior of steel, the inventors fundamentally investigated the effects of these elements. Specifically, they focused on two temperature ranges, 350 to 450°C, which is near the nose of the bainite transformation, and 550 to 650°C, which is near the nose of the ferrite-pearlite transformation, and investigated the residence time T in these temperature ranges. L , T H The relationship between the Si concentration, Mn concentration, and residence time T L , T H Using the above, T can be calculated according to the following formula (1): A It was found that by cooling the slab just below the mold so that the Ac point is equal to or higher than a specified 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 ×[Mn] -0.6 ×[Si] -1.1 +T H ×10 -1 ×[Mn] -1.1 (1)

[0011] Based on the above findings, the present application provides, as one of the means for solving the above problems, A method for continuously casting a slab of steel having a composition, in mass%, of C: 0.10-0.40%, Si: 1.0-3.0%, Mn: 1.0-3.0%, Cr: 0-0.60%, Mo: 0-0.600%, Ni: 0-0.50%, and N: 0-0.0250%, 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), T is the time at 550-650℃ H (s) is defined by the following formula (1) A The slab is cooled so that (s) is 30 or more. Next, before the straightening point is reached, 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 ×[Mn] -0.6 ×[Si] -1.1 +T H ×10 -1 ×[Mn] -1.1 (1) (In formula (1), [Si] and [Mn] are the concentrations (mass%) of Si and Mn in the steel.)

[0013] In the method of the present disclosure, the cast piece may have a composition, in mass%, of Al: 0-0.10%, 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%. Effect of the Invention

[0014] According to the method of the present disclosure, in the continuous casting of steel having high Si and Mn concentrations, surface cracks that occur when the slab is straightened can be stably suppressed. By hot rolling the slab produced by the method of the present disclosure, it is possible to obtain a steel plate or slab in which the occurrence of surface cracks and the like is suppressed. [Brief description of the drawings]

[0015] [Figure 1] FIG. 1 is a schematic diagram for explaining an example of a continuous casting machine employed in the continuous steel casting method of the present disclosure. [Diagram 2] FIG. 13 is a diagram for providing a supplementary explanation of TL and TH. [Diagram 3] FIG. 2 is a photograph showing an example of a state of a 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 by a Formaster device and the obtained structure for steel type A. [Diagram 5] FIG. 1 is a diagram showing the relationship between the heat treatment pattern by a Formaster device and the obtained structure for steel type B. [Figure 6] FIG. 1 is a diagram showing the relationship between the heat treatment pattern by a Formaster device and the obtained structure for steel type C. [Figure 7] FIG. 1 is a diagram showing the relationship between the heat treatment pattern by a Formaster device and the obtained structure for steel type D. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0016] The continuous casting method of steel according to the present disclosure will be described with reference to FIG. 1. In FIG. 1, cooling spray nozzles and the like are omitted for clarity. The cooling spray nozzles are provided, for example, between support rolls between just below the mold 10 and before the straightening point 20, and can spray cooling water from both sides of the slab 1. Although a vertical bending type continuous casting machine 100 is illustrated in FIG. 1, the continuous casting method of the present disclosure can be applied to any continuous casting machine having a straightening point. For example, a curved type continuous casting machine may be used. The "straightening point" refers to a point where distortion is applied to correct the casting direction of the slab 1 from a curved direction to a horizontal direction. The straightening may be performed at a plurality of points. The configuration of the continuous casting machine 100 including the mold 10, the straightening point 20, and the like may be the same as 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 having a composition, in mass%, of C: 0.10-0.40%, Si: 1.0-3.0%, Mn: 1.0-3.0%, Cr: 0-0.60%, Mo: 0-0.600%, Ni: 0-0.50%, and N: 0-0.0250% by mass 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 until it reaches the straightening point 20 is defined as T L (s), T is the time at 550-650℃ H (s) is defined by the following formula (1) A The method is characterized in that 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 ×[Mn] -0.6 ×[Si] -1.1 +T H ×10 -1 ×[Mn] -1.1 (1) (In formula (1), [Si] and [Mn] are the concentrations (mass%) of Si 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, Si, and Mn in addition to Fe. In addition, as an optional element, for example, at least one selected from Cr, Mo, Ni, N, Al, Ti, V, Ca, Mg, REM, Nb, and B may be contained. In addition, as an impurity, for example, P or S may be contained.

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

[0021] 1.2 Si: 1.0-3.0% Silicon is an important element with the second highest solid solution strengthening ability after carbon. To obtain high strength steel, the concentration of silicon is set to a high concentration. Specifically, the lower limit of the silicon content may be 1.0 mass% or more, or 1.1 mass% or more. On the other hand, if the silicon content is too high, there is a risk of the toughness and workability being deteriorated. In this regard, the upper limit of the silicon content may be 3.0 mass% or less, or 2.5 mass% or less.

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

[0023] 1.4Cr:0~0.60% Cr is an element that is optionally added, for example, to improve the hardenability of steel. However, if the Cr content is too high, even if the slab is cooled just 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 lower limit of the Cr content may be 0 mass%, and the upper limit may be 0.60 mass% or less, 0.40 mass% or less, 0.20 mass% or less, or 0.10 mass% or less.

[0024] 1.5 Mo: 0~0.600% Mo is an element that is optionally added for the purpose of, for example, secondary hardening during tempering of steel, improving fatigue strength, and improving hardenability. However, if there is too much Mo, 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 lower limit of the Mo content may be 0 mass%, and the upper limit may be 0.600 mass% or less, 0.400 mass% or less, 0.200 mass% or less, or 0.100 mass% or less.

[0025] 1.6 Ni: 0-0.50% Ni is an element that is optionally added to ensure the strength and toughness of steel, improve hardenability, etc. However, if there is too much Ni, even if the slab is cooled just 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 lower limit of the Ni content may be 0 mass%, and the upper limit may be 0.50 mass% or less, 0.30 mass% or less, or 0.10 mass% or less.

[0026] 1.7 N: 0~0.0250% N can form nitrides in steel, for example, and exert the effect of suppressing grain coarsening. However, if there is too much N, it may cause the nitrides to become coarse, which may reduce the fatigue strength of the steel. It may also reduce hot ductility and cause surface defects during casting or rolling. In this regard, the upper limit of the N content may be 0.0250 mass% or less, and from the viewpoint of steel cleanliness, it may be 0.0200 mass% or less. The lower limit of the N content may be 0 mass% or 0.0020 mass% or more.

[0027] In the continuous casting method of the present disclosure, the steel to be cast may further contain the following elements in addition to the above elements. That is, the slab 1 may have a composition of Al: 0-0.10%, 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% in mass %. According to the continuous casting method of the present disclosure, by cooling the slab just below the mold so as to satisfy the above formula (1), surface cracks that occur when straightening the slab can be stably suppressed regardless of the contents of optional elements such as Al, Ti, V, Ca, Mg, REM, Nb, and B contained in the slab. That is, the elements described below do not substantially affect the validity of the above formula (1).

[0028] 1.8 Al: 0-0.10% Al is the most widely used element for deoxidation, and also has the effect of suppressing the coarsening of crystal grains by generating AlN. However, if there is too much Al, there is a risk of problems such as nozzle clogging during casting due to the aggregation of Al2O3, and Al2O3 remaining in the steel deteriorating performance. In this regard, the upper limit of the Al content may be 0.10 mass% or less or 0.05 mass% or less. The lower limit of the Al content may be 0 mass% or 0.01 mass% or more.

[0029] 1.9 Ti: 0~0.100% Ti is an element that can form nitrides like Al, has excellent thermal stability, and maintains the effect of suppressing grain coarsening up to higher temperatures. However, if there is too much Ti, TiN tends to grow coarsely, which may reduce fatigue strength. In this regard, the upper limit of the Ti content may be 0.100 mass% or less. The lower limit of the Ti content may be 0 mass%, 0.001 mass% or more, or 0.002 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 upper limit of the V content may be 0.400 mass% or less. The lower limit of the V content may be 0 mass%, or may be 0.001 mass% or more, or 0.002 mass% or more.

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

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

[0033] 1.13 REM: 0~0.0100% REM also has the effect of modifying Al2O3 and suppressing the coarsening of oxide-based inclusions. However, if there is too much REM, the cleanliness of the steel may be reduced and the toughness of the base material may be deteriorated. In this regard, the upper limit of the REM content may be 0.0100 mass% or less. The lower limit of the REM content may be 0 mass%, or may be 0.0001 mass% or more, or 0.0002 mass% or more. REM refers to rare earth elements such as La and Ce, and any one or two 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 is saturated. In this respect, the upper limit of the Nb content may be 0.050 mass% or less. The lower limit of the Nb content may be 0 mass%, 0.001 mass% or more, or 0.002 mass% or more.

[0035] 1.15 B: 0~0.0040% A small amount of B has a significant effect of improving hardenability. However, if the amount of B is too large, the effect becomes saturated. In this regard, the upper limit of the B content may be 0.0040 mass% or less. The lower limit of the B content may be 0 mass%, or may be 0.0001 mass% or more, or 0.0002 mass% or more.

[0036] 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 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 before it moves from just below the mold 10 to the straightening point 20 is defined as T L (s), T is the time at 550-650℃ H (s) is T defined by the above formula (1) A It is important to cool the slab 1 so that (s) is 30 or more.

[0037] In the continuous casting method of the present disclosure, T L However, depending on the steel type, this temperature may be below the martensite 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 the desired effect can be obtained.

[0038] 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 the slab reaches the straightening point 20. L , T H is expressed as the sum of the time it takes to pass through each temperature region. For example, in the thermal history shown in Figure 2, T L , T H can be calculated using the following equations (2) and (3).

[0039] T L =t4-t3(2) T H =(t2-t1)+(t6-t5) (3)

[0040] 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 air flow, a method of cooling 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 any cooling rate will produce the desired effect.

[0041] 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 restored to a temperature of Ac3 or higher before reaching the straightening point 20 (for example, at the time t7 in FIG. 2). This reheating is essential for obtaining a so-called reverse transformation structure, which is a fine austenite structure of the surface layer structure of the slab 1. If the reheating temperature is lower than Ac3, there will remain areas where reverse transformation does not occur. Since such structures have the influence of the as-cast structure that is prone to cracking due to straightening distortion, 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 once reheated to Ac3 or higher before reaching the straightening point 20, the hot ductility of the surface of the slab 1 is maintained high thereafter, so that even if the temperature drops at the straightening point 20, surface cracking does not become a problem.

[0042] By reheating to Ac3, the surface structure of the slab 1 is modified, and in combination with appropriate secondary cooling, a slab with few surface cracks can be obtained. From the viewpoint of further suppressing the variation 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 become coarse again. In this regard, the maximum temperature after reheating may be 1200°C or lower.

[0043] Incidentally, Ac3 can be measured using a transformation point recorder (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)).

[0044] 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.)

[0045] Reheating of the surface of the slab 1 is a phenomenon that occurs when the amount of heat transferred from inside 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 performed relatively easily by reducing 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 speed (temperature rise speed) of the slab 1 is not particularly limited, and the desired effect can be achieved at any reheating speed.

[0046] As described above, according to the continuous casting method of the present disclosure, in the continuous casting of steel having high Si and Mn concentrations, surface cracks of a slab that occur when the slab is straightened can be stably suppressed. In addition, in the present application, the "surface of the slab" does not necessarily mean the entire surface of the slab. In other words, at least a portion of the surface of the slab where surface cracks are to be suppressed is cooled directly below the mold so that the above formula (1) is satisfied, and then the slab is reheated to a temperature of Ac3 or higher. EXAMPLES

[0047] The effects of the technology of the present disclosure will be described in more detail below with reference to examples; however, the technology of the present disclosure is not limited to the following examples.

[0048] 1. Model Experiment 1 In order to clarify the conditions for fully obtaining the effect of refining the surface structure of the slab by secondary cooling and reheating, a model experiment was carried out using a transformation point recording and measuring device (Formaster device).

[0049] Steel types A to D having the compositions shown in Table 1 below were each heated to 1400°C using a Formaster device, then quenched with helium gas to a specified temperature of 350 to 750°C, and the quenched 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 by SEM.

[0050] [Table 1]

[0051] Examples of observation are shown in Figures 3(A) and (B). The structures confirmed by cross-sectional observation were 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 µm was observed in structure A, and grain boundary ferrite was unclear, which is considered to be a structure with low cracking sensitivity. On the other hand, as is clear from Figure 3(B), granular ferrite was not observed in structure B, and some grain boundary ferrite was observed, which is considered to be a structure with high cracking sensitivity.

[0052] For each of steel types A to D, the relationship between the temperature pattern of heat treatment by the Formaster device and the obtained structure is shown in Figures 4 to 7. Figure 4 is for steel type A, Figure 5 is for steel type B, Figure 6 is for steel type C, and Figure 7 is for steel type D. In Figures 4 to 7, the case where the obtained structure was A over the entire surface is indicated by "○", the case where it was a mixed phase of A and B is indicated by "△", and the case where it was B over the entire surface is indicated by "×". As is clear from Figures 4 to 7, the temperature conditions under which structure A was obtained were significantly different for each steel type. In other words, it was found that appropriate temperature conditions for obtaining a structure with low cracking sensitivity cannot be determined unless the effects of Si and Mn concentrations are taken into consideration. From this model experiment, it can be seen that the residence time in two temperature ranges, where bainite transformation occurs at 350-450 °C and ferrite-pearlite transformation occurs at 550-650 °C, is important for the decomposition of austenite. From this, a model experiment was conducted to understand the transformation behavior when the surface layer of the cast slab crosses both of these two temperature ranges in actual continuous casting.

[0053] 2.Model Experiment 2 Steel types A, B, and D similar to those in model experiment 1 above were heated to 1400°C using a Formaster device, held at 400°C for a prescribed time, then held at 600°C for a prescribed 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 shown as "○", cases where it was a mixed phase of A and B are shown as "△", and cases where it was B over the entire surface are shown as "×".

[0054] [Table 2]

[0055] As is clear from the results shown in Table 2, the samples held at 400 ° C for 40 seconds showed structure A over the entire surface when held at 625 ° C for 40 seconds or less for steel type A, 40 to 120 seconds for steel type B, and 240 to 600 seconds for steel type D, whereas the samples held at 400 ° C for 20 seconds showed structure A over the entire surface when held at 600 ° C for 40 to 120 seconds for steel type A, 120 to 360 seconds for steel type B, and 240 to 600 seconds for steel type D. At this time, it is estimated that the coarse austenite is transformed into bainite during holding at 400 ° C, and the untransformed austenite at this point is transformed into ferrite-pearlite during holding at 600 ° C. That is, it is considered that the decomposition of austenite is completed when the sum of the amount of bainite transformation and the amount of ferrite-pearlite transformation is balanced with the amount of austenite before transformation, and after the subsequent reheating to 900 ° C, the structure A is exhibited during slow cooling.

[0056] For steels with high Si and Mn concentrations, the residence time T L (s), T H As a result, it was found that the relationship between the surface structure of the slab and the T AIt 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.

[0057] T A =T L ×[Mn] -0.6 ×[Si] -1.1 +T H ×10 -1 ×[Mn] -1.1 (1) (In formula (1), [Si] and [Mn] are the concentrations (mass%) of Si and Mn in the steel.)

[0058] It has been confirmed that the above formula (1) is also valid in experiments carried out using steel types other than the above steel types A to D that have high Si and Mn concentrations. That is, when a steel having a composition, in mass%, of C: 0.10-0.40%, Si: 1.0-3.0%, Mn: 1.0-3.0%, Cr: 0-0.60%, Mo: 0-0.600%, Ni: 0-0.50%, and N: 0-0.0250% is cast, the slab surface structure can be refined by performing secondary cooling of the slab so that the above formula (1) is 30 or more, and surface cracks of the slab at the straightening point can be suppressed.

[0059] 3. Actual machine testing To verify the validity of the above model experiment, an actual test was carried out using a continuous slab caster. Specifically, a slab measuring 240 mm x 1500 mm was cast using a curved continuous caster with a radius of curvature of 12.0 m for the molten steel with the composition shown in Table 3 below. The casting speed was 1.0 to 1.4 m / min. The slab pulled out of the mold was 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 in the normal secondary cooling spray was adjusted to control the reheating. The slab was cut into a length of 5.0 ± 0.2 m using a gas cutter, and the surface was then observed.

[0060] [Table 3]

[0061] The temperature of the slab surface 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 specified by the above formula (4).

[0062] The surface cracks of the resulting steel slab were visually observed. The time T during which the surface temperature of the slab was between 350 and 450°C in the cooling zone was L , time T between 550 and 650°C H The results of the investigation, together 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 represented as "○" for no cracks, "△" for all cracks less than 0.5 mm deep and 10 or fewer cracks per slab, and "×" for anything that did not fall into either category.

[0063] [Table 4]

[0064] As is clear from the results shown in Table 4, T A In the slabs (Examples 1 to 9) in which the surface of the slab was cooled just below the mold so that the temperature was 30 or more, and then the temperature was restored to Ac3 or more before 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 maximum temperature from the start of reheating to the straightening point was less than Ac3 (Comparative Examples 1, 2, 4, 5, 7 and 8), 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]

[0065] 1. Casting 10 Mold 20 correction points 100 Continuous Casting Machine

Claims

1. A method for continuously casting a slab of steel having a composition, in mass%, of C: 0.10-0.40%, Si: 1.0-3.0%, Mn: 1.0-3.0%, Cr: 0-0.60%, Mo: 0-0.600%, Ni: 0-0.50%, and N: 0-0.0250%, using a continuous casting machine having a straightening point, comprising: The time during which the surface temperature of the slab is between 350 and 450° C. from just below the mold until it reaches 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 adjusted to Ac ３ Reheat to the above temperature range. A method for continuous casting of steel. T Ａ =T Ｌ ×[Mn] －０．６ ×[Si] －１．１ +T Ｈ ×10 －１ ×[Mn] －１．１ (1) (In formula (1), [Si] and [Mn] are the concentrations (mass%) of Si and Mn in the steel.)

2. The cast slab has a composition, in mass%, of Al: 0 to 0.10%, 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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