Steel Continuous Casting Method

The present invention relates to a steel continuous casting method.

In the final step of solidification in steel continuous casting, solidification shrinkage causes suction flow of an unsolidified molten steel (called “unsolidified layer”) in the withdrawal direction of a cast piece. The unsolidified layer contains concentrated solute elements such as carbon (C), phosphorus (P), sulfur(S), and manganese (Mn). If the concentrated molten steel flows to the center of a cast piece and is solidified, what is called center segregation is caused. Factors causing a concentrated molten steel to flow in the late solidification stage include, in addition to the above solidification shrinkage, cast piece bulging between rolls due to molten steel static pressure and roll misalignment of cast piece support rolls.

The center segregation decreases the quality of a steel product, especially, a steel plate. For example, in a line pipe material for petroleum transportation or natural gas transportation, sour gas causes hydrogen-induced cracking from a center segregation as the starting point. Similar problems arise also in marine structures, storage tanks, petroleum tanks, or the like. Moreover, recent steel materials are often required to be used in harsh environments such as lower temperature environments or more corrosive environments, and this has increased the importance of reducing the center segregation of cast pieces.

In such circumstances, from a continuous casting step to a rolling step, many countermeasures have been disclosed to reduce or eliminate the center segregation of cast pieces. Of them, a late solidification-stage soft reduction method in which a continuous cast piece having an unsolidified layer therein is subjected to rolling reduction in a continuous casting machine is known to be specifically effective in improving center segregation. The “late solidification-stage soft reduction method” is a method in which rolling reduction rolls are placed near the solidification completion position of a cast piece, and the rolling reduction rolls are used to subject a cast piece during continuous casting to gradual rolling reduction at a rolling reduction speed corresponding to the solidification shrinkage amount. In the late solidification-stage soft reduction method, rolling reduction with rolling reduction rolls prevents voids from generating at the center of a cast piece or a concentrated molten steel from flowing, and this suppresses the center segregation of the cast piece.

To effectively prevent the center segregation of a cast piece from generating by the late solidification-stage soft reduction method, it is important to appropriately set the start and end timings of applying soft reduction during the final solidification period of a cast piece and the applied rolling reduction amount during the soft reduction. Hence, for the rolling reduction amount during soft reduction in the late solidification-stage soft reduction method, many setting methods have been disclosed.

For example, PTL 1 discloses that in a continuous casting method of applying soft reduction to the late-stage solidification portion of a continuous cast piece, the rolling reduction amount per unit time of a cast piece in the section where soft reduction is applied is defined from the surface temperature of the cast piece at the start of the rolling reduction and from the thickness of the unsolidified layer of the cast piece at the rolling reduction position. PTL 1 focuses on the thickness of the unsolidified layer of a cast piece as an index for effective soft reduction. According to PTL 1, this is based on the finding that the rolling reduction amount set for rolling reduction rolls is transmitted to the solid-liquid interface of a cast piece at a smaller rate (hereinafter called “rolling reduction efficiency”) toward the casting downstream side or when the unsolidified layer of the cast piece has a smaller thickness.

PTL 2 and PTL 3 each disclose a continuous casting method in which the rolling reduction speed of a cast piece is set to be larger toward the downstream side in the casting direction where the center of the cast piece in the thickness direction has a larger solid phase ratio. In each continuous casting method in PTL 2 and PTL 3, continuous casting is performed while rolling reduction is applied with multiple pairs of rolls in a region from the temperature at which the center of a bloom cast piece in the thickness direction has a solid phase ratio of 0.1 to 0.3 to the temperature corresponding to the flow-limit solid phase ratio.

PTL 4 discloses a continuous casting method of performing continuous casting while a rolling reduction force is applied to a cast piece being drawn. In the method, rolling reduction conditions are set or adjusted on the basis of information on a cross-sectional shape perpendicular to the longitudinal direction of the cast piece and information on the unsolidified portion shape on the cross section.

PTL 5 discloses that at the completion of casting with a continuous casting machine, casting is completed while a normal casting speed is maintained without deceleration or stop of casting and without treatment of a bottom portion as the last end portion of a cast piece. PTL 5 is characterized in that a crater end at the last end of a cast piece is so controlled as to be positioned in a predetermined section, and a portion near the crater end is subjected to soft reduction with small diameter rolls provided in the section.

In continuous casting, in the period from the casting completion when the supply of a molten steel to a mold is completed until a cast piece is drawn from the inside of a continuous casting machine, the behavior of the speed of drawing the cast piece is different from the period when a molten steel is continuously supplied to the mold to perform continuous casting at a stable casting speed. Hence, when the late solidification-stage soft reduction method is applied, and rolling reduction conditions in the period after the casting completion are set to be the same as those in the ordinary period, the quality of the cast piece may deteriorate due to center segregation or internal cracking. In the description, the period from the casting completion until a cast piece is drawn from the inside of a continuous casting machine is called a period after the casting completion, and the period when a molten steel is continuously supplied to a mold to perform continuous casting at a stable casting speed is called an ordinary period.

PTLs 1 to 4 did not disclose soft reduction conditions for a cast piece remaining in a machine after the casting completion, and thus in the casting methods disclosed in PTLs 1 to 4, it has been difficult to set appropriate rolling reduction conditions in the period after the casting completion. By the casting method disclosed in PTL 5, the quality of the bottom portion is improved, but in the period after the casting completion, soft reduction is not applied in conditions equivalent to those in the ordinary period. Hence, the quality of a cast piece cast in the period after the casting completion may deteriorate as compared to a cast piece cast in the ordinary period.

The present invention has been completed in consideration of the above problems and is intended to provide a steel continuous casting method capable of improving the quality of a cast piece cast in the period after the casting completion.

According to an aspect of the present invention, a steel continuous casting method capable of improving the quality of a cast piece cast in the period after the casting completion is provided.

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.

is a schematic view illustrating a continuous casting machineto which a continuous casting method pertaining to an embodiment of the present invention is applied. The continuous casting machineis a slab continuous casting machine and is, as an example, a vertical bending continuous casting machine. The continuous casting machinemay be a curved bending slab continuous casting machine. In the present embodiment, the longitudinal direction of a cast pieceand the moving direction of the cast piecein the continuous casting machineis 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(the direction orthogonal to the casting direction in a cross section of the cast piecein) is called a thickness direction. The longitudinal direction of the rectangle on a cross section of the cast piece(the front-to-rear direction in) is called a width direction. The dimension of the cast piecein the thickness direction is called thickness, and the dimension in the width direction is called width.

As illustrated in, the continuous casting machineincludes a tundishfor pouring a molten steelfrom a molten-steel ladle, a copper moldfor primarily cooling the molten steelpoured from the tundishthrough an immersion nozzle, and multiple pairs of cast piece support rollsfor conveying a semi-solidified cast piecedrawn from the mold.

The cast piece support rollsinclude support rolls, guide rolls, and driving rolls arranged in sequence below the mold. Between the cast piece support rollsadjacent 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 rollsat the end of the machine. While being drawn, a cast pieceis cooled with a secondary cooling water sprayed from the spray nozzles in the secondary cooling zone.

The continuous casting machinealso includes a plurality of segments in which multiple pairs of cast piece support rollsare arranged. Of the plurality of segments, the segment in a soft reduction zoneis called a soft reduction segment. The soft reduction zoneis a region (cast piece support rolls) in which a cast piecein the late solidification stage is subjected to rolling reduction in the casting direction of the continuous casting machineand is a region in which the solid phase ratio at the thickness center of a cast piece, 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 piecemeans 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 pieceis lowest.

andare schematic views illustrating the soft reduction segment. As the segment in the soft reduction zone, a single soft reduction segmentmay be provided or a plurality of soft reduction segmentsmay be provided. In the present embodiment, an example including a single soft reduction segmentwill be described. In, only the soft reduction segmentis illustrated as the segment, but segments other than the soft reduction zoneare provided.

As illustrated inand, the soft reduction segmentincludes six pairs of cast piece support rollsarranged in the casting direction. Of the six pairs of cast piece support rolls, a pair of rolls that are rotary driven while applying a pressing force to a cast piecein the thickness direction to draw out the cast pieceare called driving rolls, and other rolls that rotate in response to a ferrostatic pressure are called guide rolls. The driving rollsmay be provided at any position in the casting direction in the soft reduction segment, and multiple pairs of driving rolls may be provided in the soft reduction segment.

The soft reduction segmentincludes an upper frame, a lower frame, upstream struts, and downstream struts. The upper frameand the lower frameare provided to face each other in the thickness direction while a cast piecebeing cast is interposed therebetween and are connected by the upstream strutsand the downstream struts. To the upper frameand the lower frame, a plurality of cast piece support rollsare rotatably fixed through bearings.

The upstream strutsand the downstream strutsare 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 frameand the lower frame. Accordingly, the roll gap that is the distance in the thickness direction between pairs of cast piece support rollsfacing each other in the thickness direction is adjusted.

A steel continuous casting method pertaining to the present embodiment will next be described. In the present embodiment, the continuous casting machineis used to perform steel continuous casting. In the continuous casting, the period in which casting is performed while a molten steelis continuously supplied to the moldis 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 steelto a moldis completed. Specifically, after the casting completion is when pouring the molten steelin a ladle into the tundishis completed and pouring the molten steelremaining in the tundishinto the moldis completed or is when a sliding nozzleconnected the immersion nozzleis finally closed.

In the ordinary period, a cast pieceis preferably subjected to soft reduction in the soft reduction zone. In a region upstream from the soft reduction zone, the roll opening degree may be increased, and the long side faces of the cast piecemay be intentionally subjected to bulging by a molten steel static pressure. The intentional bulging is preferably started when the center portion of the cast piecehas a solid phase ratio of 0 and is preferably completed when the long side faces of the cast piecehave a total bulging amount of 3 mm or more and 10 mm or less. In the soft reduction in the soft reduction zone, the cast pieceis 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.

In the casting completion period, as illustrated in, 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(the last end portion of the cast piece) is cooled and solidified for head solidification. The head solidification is a process of solidifying the top portion of a cast pieceby feeding a cold charge to the molten steelremaining in the moldafter the completion of casting because the molten steelleaks out of the top portion when the top portion of the cast piecedrawn out of the moldis 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 piecein 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(time period T) where t is the elapsed time from the casting completion. The casting speed (drawing speed) V of the cast piecebefore the casting completion (just before the casting completion) is a first casting speed V(m/min), and the casting speed V of the cast piecechanges to a second casting speed V(m/min) that is the casting speed (drawing speed) for head solidification. The second casting speed Vis the casting speed required for head solidification and is appropriately set according to the specification of a continuous casting machine. Next, when the elapsed time t is not less than tand less than t(time period T), the casting speed V is constant at the second casting speed V, and head solidification is performed. Then, when the elapsed time t is tor more and tor less (time period T), the casting speed V increases. The casting speed V of the cast piecechanges from the second casting speed Vto a third casting speed V(m/min) that is the casting speed for redrawing after the casting completion. Redrawing means that a cast pieceis redrawn by increasing the drawing speed after the completion of head solidification. When the elapsed time t exceeds t, the cast pieceis drawn at the third casting speed Vuntil the drawing is completed. In the period after the casting completion, the period of time Tis also called a deceleration step, the period of time Tis also called a head solidification step, the period of time Tis also called an acceleration step, and the period after the time period T(t>t) is also called a redrawing step.

In the casting completion period, as illustrated in, 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=V×Z). 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 segmentis 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, T, and T, 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, the rolling reduction speed U in the head solidification of t≤t<tis also called second rolling reduction speed U, and the rolling reduction speed U in the redrawing of t<tis also called third rolling reduction speed U. 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, the rolling reduction gradient Z when t≤t<tis also called second rolling reduction gradient Z, and the rolling reduction gradient Z when t<tis also called third rolling reduction gradient Z.

illustrates an example in which the rolling reduction gradient Z in the time period T, T, Tsatisfies Expression (4) to Expression (6). In, in the time period T, the rolling reduction speed U decreases from the first rolling reduction speed Uand reaches the second rolling reduction speed Uwhen the elapsed time t reaches t. Next, in the time period T, the second rolling reduction speed Uis maintained. In the time period T, the rolling reduction speed U increases from the second rolling reduction speed Uand reaches the third rolling reduction speed Up when the elapsed time t reaches t. 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.

Inand, the numerical values of time t on the horizontal axis are an example, and the numerical values of elapsed time t, t, tand time period T, T, Tare appropriately set according to the specification of a continuous casting machineor casting conditions. The rolling reduction gradient or the casting speed in the continuous casting machineare controlled by a controller (not illustrated) including a computer and the like.

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.

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) 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 pieceand 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), the second rolling reduction speed Uis 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. In the acceleration step (time period T), the rolling reduction speed U satisfies Expression (3). This enables efficient acceleration while internal cracking of a cast pieceis prevented and can increase the production efficiency.

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 steelcaused by solidification shrinkage exceeds the rolling reduction speed U, the concentrated molten steel is sucked, and the segregation of the cast piecedeteriorates. 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.

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.

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 machineillustrated in. The continuous casting machinewas 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 piecethat had been cast (in conditionsand). 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 conditionsand) for each cast piece thickness. In all the tests, each cast piecehad a thickness of 250 mm and a width of 2,000 mm. The periods T, T, and Twere the deceleration step, the head solidification step, and the acceleration step, respectively, in the above embodiments.

The center segregation degree of the cast pieceevaluated 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. The C/Cwas then calculated as the center segregation degree where Cwas the maximum value in the thickness direction, and Cwas the carbon concentration determined from the molten steelsampled from the tundishduring casting. As the center segregation degree is closer to 1.0, the cast piecehad less center segregation and was better. In the examples, a cast piecehaving a center segregation degree of 1.10 or more was evaluated to have poor center segregation degree.

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

In the examples, the segregation degree of each cast piecethat 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.

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. 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.