STEEL CONTINUOUS CASTING METHOD

Abstract

Provided is a steel continuous casting method capable of improving the quality of a cast piece cast in the period after the casting completion. A steel continuous casting method of performing continuous casting while a cast piece (3) is subjected to rolling reduction includes subjecting a cast piece (3) being continuously cast to rolling reduction and, after the casting completion, changing a rolling reduction gradient in a segment (14) of applying the rolling reduction, to control a rolling reduction speed within a predetermined range.

Claims

1. A steel continuous casting method of performing continuous casting while a cast piece is subjected to rolling reduction, the method comprising: subjecting the cast piece being continuously cast to rolling reduction; and after casting completion, changing a rolling reduction gradient in a segment of applying the rolling reduction, to control a rolling reduction speed within a predetermined range.

2. The steel continuous casting method according to claim 1, the method comprising: after the casting completion, a deceleration step of decreasing a casting speed of the cast piece from a first casting speed that is a casting speed before the casting completion to a second casting speed that is a casting speed for head solidification of the cast piece; after the deceleration step, a head solidification step of maintaining the casting speed of the cast piece at the second casting speed for a time period for the head solidification; and after the head solidification step, an acceleration step of increasing the casting speed of the cast piece from the second casting speed to a third casting speed that is a casting speed of redrawing the cast piece.

3. The steel continuous casting method according to claim 2, wherein the rolling reduction gradient is controlled to satisfy Expression (4) in the deceleration step, the rolling reduction gradient is controlled to satisfy Expression (5) in the head solidification step, and the rolling reduction gradient is controlled to satisfy Expression (6) in the acceleration step: $\begin{array}{cc}0.5<VZ<3.& \left(4\right)\end{array}$ $\begin{array}{cc}0.5<VZ<1.5& \left(5\right)\end{array}$ $\begin{array}{cc}0.5<VZ<3.& \left(6\right)\end{array}$ where V is a casting speed (m/min), and Z is a rolling reduction gradient (mm/m).

Description

BRIEF DESCRIPTION OF DRAWINGS

[0023] FIG. 1 is a schematic side view illustrating a continuous casting machine pertaining to an embodiment of the present invention;

[0024] FIG. 2 is a schematic side view illustrating an example of the segment constituting a soft reduction zone;

[0025] FIG. 3 is a front view of the segment illustrated in FIG. 2 and viewed in the casting direction;

[0026] FIG. 4 is a graph illustrating an example of the casting speed after the casting completion; and

[0027] FIG. 5 is a graph illustrating an example of the rolling reduction gradient and the rolling reduction speed after the casting completion.

DESCRIPTION OF EMBODIMENTS

[0028] In the following detailed description, embodiments of the present invention will be described with reference to drawings. In drawings, identical or similar components are indicated by an identical or similar sign and are not described. Each drawing is schematic and may differ from reality. The embodiments described below are illustrative examples of apparatuses and methods for embodying the technical idea of the present invention, and the technical idea of the present invention is not limited to the following in terms of the materials, the structures, the configurations, and the like of components. The technical idea of the present invention may be variously modified within the technical scope defined by the claims.

[0029] FIG. 1 is a schematic view illustrating a continuous casting machine 1 to which a continuous casting method pertaining to an embodiment of the present invention is applied. The continuous casting machine 1 is a slab continuous casting machine and is, as an example, a vertical bending continuous casting machine. The continuous casting machine 1 may be a curved bending slab continuous casting machine. In the present embodiment, the longitudinal direction of a cast piece 3 and the moving direction of the cast piece 3 in the continuous casting machine 1 is called a casting direction, and the short direction of the rectangle on a cross section (a section orthogonal to the longitudinal direction) of the cast piece 3 (the direction orthogonal to the casting direction in a cross section of the cast piece 3 in FIG. 1) is called a thickness direction. The longitudinal direction of the rectangle on a cross section of the cast piece 3 (the front-to-rear direction in FIG. 1) is called a width direction. The dimension of the cast piece 3 in the thickness direction is called thickness, and the dimension in the width direction is called width.

[0030] As illustrated in FIG. 1, the continuous casting machine 1 includes a tundish 10 for pouring a molten steel 2 from a molten-steel ladle, a copper mold 12 for primarily cooling the molten steel 2 poured from the tundish 10 through an immersion nozzle 11, and multiple pairs of cast piece support rolls 13 for conveying a semi-solidified cast piece 3 drawn from the mold 12.

[0031] The cast piece support rolls 13 include support rolls, guide rolls, and driving rolls arranged in sequence below the mold 12. Between the cast piece support rolls 13 adjacent in the casting direction, spray nozzles such as water spray nozzles and air mist spray nozzles (not illustrated) are arranged, and a secondary cooling zone is provided from just below the mold to the cast piece support rolls 13 at the end of the machine. While being drawn, a cast piece 3 is cooled with a secondary cooling water sprayed from the spray nozzles in the secondary cooling zone.

[0032] The continuous casting machine 1 also includes a plurality of segments in which multiple pairs of cast piece support rolls 13 are arranged. Of the plurality of segments, the segment in a soft reduction zone 15 is called a soft reduction segment 14. The soft reduction zone 15 is a region (cast piece support rolls 13) in which a cast piece 3 in the late solidification stage is subjected to rolling reduction in the casting direction of the continuous casting machine 1 and is a region in which the solid phase ratio at the thickness center of a cast piece 3, or the center solid phase ratio, is at least not less than 0.2 and less than 1.0. In the embodiment, the thickness center of a cast piece 3 means the center in the thickness direction at a position in the width direction where the solid phase ratio of a thickness center portion in a cross section of the cast piece 3 is lowest.

[0033] FIG. 2 and FIG. 3 are schematic views illustrating the soft reduction segment 14. As the segment in the soft reduction zone 15, a single soft reduction segment 14 may be provided or a plurality of soft reduction segments 14 may be provided. In the present embodiment, an example including a single soft reduction segment 14 will be described. In FIG. 1, only the soft reduction segment 14 is illustrated as the segment, but segments other than the soft reduction zone 15 are provided.

[0034] As illustrated in FIG. 2 and FIG. 3, the soft reduction segment 14 includes six pairs of cast piece support rolls 13 arranged in the casting direction. Of the six pairs of cast piece support rolls 13, a pair of rolls that are rotary driven while applying a pressing force to a cast piece 3 in the thickness direction to draw out the cast piece 3 are called driving rolls 140, and other rolls that rotate in response to a ferrostatic pressure are called guide rolls 141. The driving rolls 140 may be provided at any position in the casting direction in the soft reduction segment 14, and multiple pairs of driving rolls may be provided in the soft reduction segment 14.

[0035] The soft reduction segment 14 includes an upper frame 142, a lower frame 143, upstream struts 144, and downstream struts 145. The upper frame 142 and the lower frame 143 are provided to face each other in the thickness direction while a cast piece 3 being cast is interposed therebetween and are connected by the upstream struts 144 and the downstream struts 145. To the upper frame 142 and the lower frame 143, a plurality of cast piece support rolls 13 are rotatably fixed through bearings 146.

[0036] The upstream struts 144 and the downstream struts 145 are capable of expansion and contraction by means of hydraulic pressure or the like and are expanded or contracted to adjust the distance between the upper frame 142 and the lower frame 143. Accordingly, the roll gap that is the distance in the thickness direction between pairs of cast piece support rolls 13 facing each other in the thickness direction is adjusted.

[0037] A steel continuous casting method pertaining to the present embodiment will next be described. In the present embodiment, the continuous casting machine 1 is used to perform steel continuous casting. In the continuous casting, the period in which casting is performed while a molten steel 2 is continuously supplied to the mold 12 is called an ordinary period, and the period after the casting completion is called a casting completion period. After the casting completion is when the supply of a molten steel 2 to a mold 12 is completed. Specifically, after the casting completion is when pouring the molten steel 2 in a ladle into the tundish 10 is completed and pouring the molten steel 2 remaining in the tundish 10 into the mold 12 is completed or is when a sliding nozzle 16 connected the immersion nozzle 11 is finally closed.

[0038] In the ordinary period, a cast piece 3 is preferably subjected to soft reduction in the soft reduction zone 15. In a region upstream from the soft reduction zone 15, the roll opening degree may be increased, and the long side faces of the cast piece 3 may be intentionally subjected to bulging by a molten steel static pressure. The intentional bulging is preferably started when the center portion of the cast piece 3 has a solid phase ratio of 0 and is preferably completed when the long side faces of the cast piece 3 have a total bulging amount of 3 mm or more and 10 mm or less. In the soft reduction in the soft reduction zone 15, the cast piece 3 is preferably subjected to rolling reduction at a rolling reduction speed U of 0.3 mm/min or more and 2. 0 mm/min or less when having a center solid phase ratio of at least not less than 0.2 and less than 1.0. If a cast piece having a center solid phase ratio within the above range is subjected to soft reduction at a rolling reduction speed U of less than 0.3 mm/min, V segregation highly probably occurs. If a cast piece having a center solid phase ratio within the above range is subjected to soft reduction at a rolling reduction speed U of more than 2. 0 mm/min, reverse V segregation highly probably occurs.

[0039] In the casting completion period, as illustrated in FIG. 4, the casting speed V is decreased after the casting completion, and a low casting speed is maintained for a predetermined period of time. Accordingly, the top portion of the cast piece 3 (the last end portion of the cast piece 3) is cooled and solidified for head solidification. The head solidification is a process of solidifying the top portion of a cast piece 3 by feeding a cold charge to the molten steel 2 remaining in the mold 12 after the completion of casting because the molten steel 2 leaks out of the top portion when the top portion of the cast piece 3 drawn out of the mold 12 is unsolidified. At the time of head solidification, the casting speed is typically decreased to certainly solidify the top portion. After the head solidification, the casting speed V (m/min) is increased, and the cast piece 3 in the machine is drawn out. In other words, the casting speed V decreases when the elapsed time t is 0 or more and less than t.sub.1 (time period T.sub.a) where t is the elapsed time from the casting completion. The casting speed (drawing speed) V of the cast piece 3 before the casting completion (just before the casting completion) is a first casting speed V.sub.a (m/min), and the casting speed V of the cast piece 3 changes to a second casting speed V.sub.0 (m/min) that is the casting speed (drawing speed) for head solidification. The second casting speed V.sub.0 is the casting speed required for head solidification and is appropriately set according to the specification of a continuous casting machine 1. Next, when the elapsed time t is not less than t.sub.1 and less than t.sub.2 (time period T.sub.0 ), the casting speed V is constant at the second casting speed V.sub.0, and head solidification is performed. Then, when the elapsed time t is t.sub.2 or more and t.sub.3 or less (time period T.sub.b), the casting speed V increases. The casting speed V of the cast piece 3 changes from the second casting speed V.sub.0 to a third casting speed V.sub.b (m/min) that is the casting speed for redrawing after the casting completion. Redrawing means that a cast piece 3 is redrawn by increasing the drawing speed after the completion of head solidification. When the elapsed time t exceeds t.sub.3, the cast piece 3 is drawn at the third casting speed V.sub.b until the drawing is completed. In the period after the casting completion, the period of time T.sub.a is also called a deceleration step, the period of time T.sub.0 is also called a head solidification step, the period of time T.sub.b is also called an acceleration step, and the period after the time period T.sub.b(t>t.sub.3) is also called a redrawing step.

[0040] In the casting completion period, as illustrated in FIG. 5, the rolling reduction gradient Z (mm/m) and the rolling reduction speed U (mm/min) change with time. The rolling reduction speed U is the value calculated by multiplying the drawing speed (casting speed) V by the rolling reduction gradient Z (U=VZ). In the present embodiment, control is performed such that the rolling reduction speed U is within a predetermined range. Specifically, in the casting completion period, as the drawing speed V changes, the rolling reduction gradient Z, that is, the roll opening degree in the soft reduction segment 14 is dynamically changed, and accordingly, such control that the rolling reduction speed U is within a predetermined range is performed. In the deceleration step, the head solidification step, and the acceleration step in the time periods, T.sub.a, T.sub.0 , and T.sub.b, the rolling reduction speed U preferably satisfies Expression (1) to Expression (3). The rolling reduction speed U in the ordinary period of t<0 before the casting completion is also called first rolling reduction speed U.sub.a, the rolling reduction speed U in the head solidification of t.sub.1t<t.sub.2 is also called second rolling reduction speed U.sub.0, and the rolling reduction speed U in the redrawing of t<t.sub.3 is also called third rolling reduction speed U.sub.b. The rolling reduction gradient Z satisfies Expression (4) to Expression (6). The rolling reduction gradient Z when t<0 is also called first rolling reduction gradient Z.sub.a, the rolling reduction gradient Z when t.sub.1t<t.sub.2 is also called second rolling reduction gradient Z.sub.0, and the rolling reduction gradient Z when t<t.sub.3 is also called third rolling reduction gradient Z.sub.b.

[00002]$\begin{array}{cc}\mathrm{When}0t<t_1\left(\mathrm{in}\mathrm{the}\mathrm{time}\mathrm{period}{T}_a\right),0.5<U<3.& \left(1\right)\end{array}$$\begin{array}{cc}\mathrm{When}{t}_{1}t<t_2\left(\mathrm{in}\mathrm{the}\mathrm{time}\mathrm{period}{T}_0\right),0.5<U<1.5& \left(2\right)\end{array}$$\begin{array}{cc}\mathrm{When}{t}_{2}t{t}_3\left(\mathrm{in}\mathrm{the}\mathrm{time}\mathrm{period}{T}_b\right),0.5<U<3.& \left(3\right)\end{array}$$\begin{array}{cc}\mathrm{When}0t<t_1\left(\mathrm{in}\mathrm{the}\mathrm{time}\mathrm{period}{T}_a\right),0.5<VZ<3.& \left(4\right)\end{array}$$\begin{array}{cc}\mathrm{When}{t}_{1}t<t_2\left(\mathrm{in}\mathrm{the}\mathrm{time}\mathrm{period}{T}_0\right),0.5<VZ<1.5& \left(5\right)\end{array}$$\begin{array}{cc}\mathrm{When}{t}_{2}t{t}_3\left(\mathrm{in}\mathrm{the}\mathrm{time}\mathrm{period}{T}_b\right),0.5<VZ<3.& \left(6\right)\end{array}$

[0041] FIG. 5 illustrates an example in which the rolling reduction gradient Z in the time period T.sub.a, T.sub.0, T.sub.b satisfies Expression (4) to Expression (6). In FIG. 5, in the time period T.sub.a, the rolling reduction speed U decreases from the first rolling reduction speed U.sub.a and reaches the second rolling reduction speed U.sub.0 when the elapsed time t reaches t.sub.1. Next, in the time period T.sub.0, the second rolling reduction speed U.sub.0 is maintained. In the time period T.sub.0, the rolling reduction speed U increases from the second rolling reduction speed U.sub.0 and reaches the third rolling reduction speed Up when the elapsed time t reaches t.sub.3. By dynamically changing the rolling reduction gradient Z as above, the rolling reduction speed U can be dynamically changed to be in a predetermined range.

[0042] In FIG. 4 and FIG. 5, the numerical values of time t on the horizontal axis are an example, and the numerical values of elapsed time t.sub.1, t.sub.2, t.sub.3 and time period T.sub.a, T.sub.0, T.sub.b are appropriately set according to the specification of a continuous casting machine 1 or casting conditions. The rolling reduction gradient or the casting speed in the continuous casting machine 1 are controlled by a controller (not illustrated) including a computer and the like.

[0043] According to the steel continuous casting method pertaining to the present embodiment, by controlling the rolling reduction gradient Z in the casting completion period in which the casting speed V changes for head solidification, the rolling reduction speed U is set within a predetermined range. This can prevent the center segregation of a cast piece due to an insufficient rolling reduction amount or prevent internal cracking of a cast piece due to an excess rolling reduction amount. This enables quick response to the request to produce steel products with various specifications and industrially provides beneficial effects.

[0044] In the present embodiment, in the deceleration step, the head solidification step, and the acceleration step, the rolling reduction speed U is controlled as shown in Expression (1) to Expression (3), or the rolling reduction gradient Z is controlled as shown in Expression (4) to Expression (6). In particular, the rolling reduction speed U in the deceleration step (time period T.sub.a) satisfies Expression (1), and accordingly the rolling reduction gradient Z increases when the casting speed V decreases. This can prevent a solidification shrinkage flow in a cast piece 3 and can prevent sucking of a concentrated molten steel. Accordingly, the generation of the center segregation of a cast piece can be suppressed. In the head solidification step (time period T.sub.0), the second rolling reduction speed U.sub.0 is set at more than 0.5 mm/min and less than 1.5 mm/min. This can prevent the rolling reduction gradient from excessively increasing and can prevent internal cracking of a cast piece 3. In the acceleration step (time period T.sub.b), the rolling reduction speed U satisfies Expression (3). This enables efficient acceleration while internal cracking of a cast piece 3 is prevented and can increase the production efficiency.

[0045] At the casting completion, the casting speed decreases, and accordingly the rolling reduction speed U decreases. If the rolling reduction speed U decreases, and the speed of a flowing molten steel 2 caused by solidification shrinkage exceeds the rolling reduction speed U, the concentrated molten steel is sucked, and the segregation of the cast piece 3 deteriorates. In the present embodiment, however, the rolling reduction speed U is set within a predetermined range at the casting completion. This can prevent sucking of a concentrated molten steel and can suppress cast piece segregation and microporosity.

[0046] Hereinabove, the present invention has been described with reference to specific embodiments, but the above description is not intended to limit the invention. By referring to the description of the present invention, other embodiments of the invention, including various variations, as well as the disclosed embodiments are also apparent to a person skilled in the art. It should therefore be understood that the scope of the invention described in the claims also encompasses embodiments that include variations of those described herein, alone or in combination.

EXAMPLES

[0047] The present invention will next be described in more detail on the basis of examples. The continuous casting machine used in tests was substantially the same as the continuous casting machine 1 illustrated in FIG. 1. The continuous casting machine 1 was used to cast a low-carbon aluminum-killed steel. Table 1 shows casting conditions in a continuous casting method in the examples and the test results of the center segregation degree, the presence of porosity, and the presence of internal cracking of a cast piece 3 that had been cast (in conditions 1 and 2). In the examples, the test was performed in the casting conditions that the rolling reduction speed U was set within the range of Expression (1) to Expression (3). Table 1 also shows casting conditions and test results in comparative examples in which the test was performed in conditions that the rolling reduction speed U was set out of the range of Expression (1) to Expression (3) (in conditions 3 and 4) for each cast piece thickness. In all the tests, each cast piece 3 had a thickness of 250 mm and a width of 2,000 mm. The periods T.sub.a, T.sub.0, and T.sub.b were the deceleration step, the head solidification step, and the acceleration step, respectively, in the above embodiments.

TABLE-US-00001 TABLE 1 Maximum Minimum Maximum Minimum rolling rolling Center casting casting reduction reduction segregation speed speed gradient gradient degree Internal Conditions Period (m/min) (m/min) (mm/m) (mm/m) (C.sub.max/C.sub.0) Porosity cracking 1 Ex. T.sub.a 1.4 0.3 2.0 2.0 1.054 Not Not observed observed T.sub.0 0.3 0.3 2.0 2.5 1.061 Not Not observed observed T.sub.b 1.6 0.3 1.7 1.7 1.048 Not Not observed observed 2 Ex. T.sub.a 1.3 0.4 1.6 1.4 1.044 Not Not observed observed T.sub.0 0.4 0.4 3.5 3.0 1.054 Not Not observed observed T.sub.b 1.3 0.4 1.5 1.4 1.059 Not Not observed observed 3 Comp. T.sub.a 1.4 0.3 0.30 0.30 1.104 Observed Not Ex. observed T.sub.0 0.3 0.3 1.00 1.00 1.189 Observed Not observed T.sub.b 1.4 0.3 0.3 0.3 1.135 Observed Not observed 4 Comp. T.sub.a 1.3 0.2 2.5 2.5 1.105 Not Observed Ex. observed T.sub.0 0.2 0.2 1.2 1.2 1.150 Observed Not observed T.sub.b 1.2 0.2 2.2 2.2 1.102 Not Observed observed

[0048] The center segregation degree of the cast piece 3 evaluated in the test was determined by the following method. In other words, on a cross section orthogonal to the drawing direction of a cast piece, the carbon concentration was determined at equal intervals along the thickness direction of the cast piece 3. The C.sub.max/C.sub.0 was then calculated as the center segregation degree where C.sub.max was the maximum value in the thickness direction, and C.sub.0 was the carbon concentration determined from the molten steel 2 sampled from the tundish 10 during casting. As the center segregation degree is closer to 1.0, the cast piece 3 had less center segregation and was better. In the examples, a cast piece 3 having a center segregation degree of 1.10 or more was evaluated to have poor center segregation degree.

[0049] For the porosity and the internal cracking of a cast piece 3, on a cross section orthogonal to the drawing direction of the cast piece 3, microscopic observation was performed around the thickness center portion of the cast piece, and the porosity and the internal cracking were observed.

[0050] In the examples, the segregation degree of each cast piece 3 that had been produced in conditions that the rolling reduction speed U was within or out of the range shown by Expression (1) to Expression (3) was evaluated. As apparent from the center segregation degrees shown in Table 1, each center segregation degree was less than 1.10 and was good in the conditions that the rolling reduction speed U was within the range of Expression (1) to Expression (3). No porosity or internal cracking was observed in the cast piece 3.

[0051] In contrast, in the conditions in comparative examples where the rolling reduction gradient Z was less than the range in the above embodiments, the center segregation degree exceeded 1.10, and porosity was observed in the cast piece 3. In the conditions that the rolling reduction gradient Z exceeded the range in the above embodiments, the rolling reduction speed was excess. Accordingly, the center segregation degree exceeded 1.10, and internal cracking was observed in the cast piece 3.

REFERENCE SIGNS LIST

[0052] 1: continuous casting machine [0053] 10: tundish [0054] 11: immersion nozzle [0055] 12: mold [0056] 13: cast piece support roll [0057] 14: soft reduction segment (segment) [0058] 140: driving roll [0059] 141: guide roll [0060] 142: upper frame [0061] 143: lower frame [0062] 144: upstream strut [0063] 145: downstream strut [0064] 146: bearing [0065] 15: soft reduction zone [0066] 16: sliding nozzle [0067] 2: molten steel [0068] 3: cast piece