JP6451437B2 - Continuous casting method - Google Patents

Description

  The present invention relates to a continuous casting method for rolling down a slab after completion of solidification during continuous casting.

  When casting slabs such as slabs and blooms by the continuous casting method, so-called center segregation in which components such as phosphorus and manganese segregate in the center of the slab may occur. In addition, a hole called center porosity is generated in the center of the slab.

  At the end of solidification during continuous casting, the unsolidified molten steel flows toward the solidification completion point of the final solidified portion as the solidification shrinkage of the molten steel. During the molten steel flow, the impurity-enriched molten steel at the solid-liquid interface accumulates in the final solidified part. This causes central segregation. Therefore, in order to reduce the center segregation, it is effective to reduce the molten steel flow in the vicinity of the final solidified portion by reducing the solidified shell in the vicinity of the final solidified portion by an amount corresponding to the solidification shrinkage of the molten steel. Based on such a concept, a light reduction technique is used in which a slab is reduced by a support roll before completion of solidification at the end of continuous casting (for example, Patent Document 1). However, if the amount of light reduction in the unsolidified region is increased, there is a problem of internal cracking and the occurrence of reverse V segregation, so the light reduction amount must be reduced, and is insufficient for reducing the center porosity. It has become a result.

  In Patent Document 2, strong reduction is performed at a reduction rate of 10 to 50% in an unsolidified region at the end of solidification, and both center segregation and center porosity are reduced. In order to reliably squeeze out the concentrated molten steel under strong pressure and prevent reverse V segregation, strong pressure is applied using a roll (hereinafter referred to as a “convex roll”) in which the center of the roll is thicker than the width end. In order to prevent the occurrence of internal cracks due to high pressure, the reduction roll diameter is set to 500 mm or more.

  Patent Document 3 describes a continuous casting method in which a slab is lightly reduced at the end of solidification and further greatly reduced immediately after solidification. The diameter of the center porosity before complete solidification is reduced under light pressure in the unsolidified region, and the center porosity is pressed under high pressure after complete solidification.

  In Patent Document 4, as a roll reduction method for reducing the center porosity, the surface temperature of the slab is 700 ° C. or more and 1000 ° C. or less after the slab is completely solidified and before cutting, and the inner center of the slab A continuous casting method is disclosed in which a region where the temperature difference between the surface and the surface is 250 ° C. or more is sandwiched between rotating upper and lower rolls to reduce the temperature. In the reduction part, the inner side is relatively soft because of the high temperature with respect to the surface layer side, and the reduction force applied to the surface of the slab can be transmitted to the inside of the slab. A convex roll is used as the reduction roll, and the width (reduction width) of the large diameter portion at the center of the roll width is preferably 40% or less of the slab width. The amount of reduction is preferably 2% or more of the thickness of the slab.

  In Patent Document 5, in order to crimp a gap interposed in the cast steel slab, the cast steel slab is lowered at a temperature at the center of the thickness of the cast steel slab at 1400 ° C. or more and below the freezing point by a reduction device disposed in the continuous casting line. A method for manufacturing a thick steel plate is disclosed.

Japanese Patent Publication No.62-34460 JP-A-6-126406 JP-A-8-164460 JP 2009-279651 A JP-A-7-227658

  During continuous casting, if rolling is performed at a large rolling reduction ratio before complete solidification as a measure for center porosity, the occurrence of reverse V segregation cannot be prevented. Therefore, as described in Patent Documents 3 to 5, as a countermeasure for center porosity, a method of performing reduction after complete solidification during casting is employed. However, even after complete solidification, it was found that when the slab is greatly reduced during casting, reverse V segregation occurs in the slab after casting.

  When a convex roll is used as the reduction roll, it is not necessary to reduce the vicinity of the short side where the reduction reaction force is strong, so that a large reduction can be performed with a small reduction force. However, if large reduction is performed using a convex roll, the slab indentation generated by the reduction may be folded during partial rolling, and may remain as surface defects in the final product. The reduction will be restricted.

  In order to effectively reduce the center porosity, the greater the difference between the slab surface temperature and the center temperature in the thickness direction during rolling, the more easily the center of the thickness is deformed due to the high temperature, and the center is concentrated. It is preferable because it can be squeezed and the borocity can be crimped. However, if the slab being cast is cooled from the surface, the thickness center portion is inevitably cooled, so the temperature difference between the slab surface and the center cannot be increased as desired.

  The present invention reduces the center porosity without reducing the reverse V segregation of the slab when rolling down the slab after completion of solidification during continuous casting, and without generating surface defects during rolling after casting. An object of the present invention is to provide a continuous casting method.

  When rolling down the slab after completion of solidification during continuous casting, if a large reduction is performed immediately after complete solidification during casting, reverse V segregation occurs in the cast slab. On the other hand, if the slab thickness center temperature at the rolling start position after completion of solidification is lower than the solidus temperature of −60 ° C., reverse V segregation of the slab will not occur.

  Further, the reduction amount is 10 to 50%, the slab thickness center temperature at the reduction end position is 800 ° C. or more, and the difference between the center temperature of the slab and the surface temperature at the reduction position is 200 ° C. or more. Furthermore, by performing descaling on the slab surface by liquid jet between the reduction roll and the adjacent upstream side roll, the slab surface temperature during reduction is reduced, and the temperature difference from the thickness center part is expanded, thereby the thickness center. The center porosity can be reduced by reducing the portion.

  Further, by using a flat roll as the reduction roll, it is possible to prevent a product surface defect that becomes a problem when the convex roll is used even when the reduction amount is a large reduction of 10% or more.

This invention is made | formed based on the above knowledge, and the place made into the summary is as follows.
(1) When the slab after completion of solidification is reduced during continuous casting, a flat roll is used as the reduction roll, and the reduction amount is 10 to 50%.
The slab thickness center temperature at the reduction start position after completion of solidification is a temperature lower than the solidus temperature -60 ° C, and the slab thickness center temperature at the reduction end position is 800 ° C or higher. The difference between the center temperature of the slab and the surface temperature is 200 ° C. or more,
Descaling the surface of the slab by liquid jet between the roll of the rolling roll and the adjacent upstream roll,
A continuous casting method, wherein the initial slab thickness in the mold part is 100 mm or more.

  The continuous casting method of the present invention can prevent the occurrence of surface defects during rolling by using a flat roll as a rolling roll when rolling down a slab after completion of solidification during continuous casting. The amount of reduction is 10 to 50%, and the slab surface is descaled by liquid injection between the reduction roll and the adjacent upstream roll, thereby sufficiently pressing the center porosity and pressing the scale with the reduction roll. It is possible to prevent surface flaws caused by By setting the slab thickness center temperature at the rolling start position after completion of solidification to a temperature lower than the solidus temperature of −60 ° C., the occurrence of reverse V segregation can be prevented. By setting the slab thickness center temperature at the reduction end position to 800 ° C. or higher, sufficient reduction can be performed. As a result, as shown in Table 1, if the rolling reduction is the same, the internal quality can be improved by reducing the maximum porosity diameter. Moreover, the temperature at which reverse V segregation does not occur by applying descaling to the same porosity diameter as when the reduction starts at a temperature at which reverse V segregation occurs without applying descaling (for example, the solidus temperature) ( For example, when the reduction is started at a solidus temperature of −60 ° C., the same reduction amount as when the reduction is started at a temperature at which reverse V segregation occurs can be obtained without adding descaling. Furthermore, if the porosity diameter is to be less than the predetermined value, it can be achieved with a small reduction ratio and the equipment capacity becomes small (if the same equipment is used, the quality target can be achieved with a smaller reduction ratio and the equipment load is reduced. Is reduced).

It is a partial side view explaining the continuous casting method of this invention. It is a figure which shows the relationship between the reduction amount according to slab center temperature in a reduction position, and the maximum borocity diameter. It is a figure which shows the relationship between slab temperature and rolling resistance. It is a figure which shows the descaling nozzle arrangement position in a comparative example.

  It has been known that it is effective to greatly reduce the slab after completion of solidification in order to reduce the center porosity of the continuously cast slab. Even after the completion of solidification, the higher the central temperature, the more advantageous for pressure bonding of the center porosity. Since the temperature of the center portion decreases with the lapse of time after completion of solidification, large pressure reduction was performed immediately after completion of solidification. When a large reduction is performed before the completion of solidification, reverse V segregation occurs. However, after completion of the solidification, it is considered that no reverse V segregation occurs even if the pressure is reduced.

  In the present invention, even after completion of solidification, if a large reduction is performed with a reduction rate of 10% or more immediately after the completion of solidification, the influence of the large reduction affects the solid-liquid coexistence region before the completion of solidification, and uncoagulated It was discovered for the first time that molten steel flows backward and reverse V segregation occurs. If the slab thickness center temperature at the rolling start position after completion of solidification is lower than the solidus temperature of −60 ° C., even if the rolling rate is 10% or higher, It became clear that reverse V segregation did not occur.

  Generally, the higher the temperature of the steel, the lower the hot strength, so the higher the temperature, the easier the reduction. When carrying out large pressure reduction after solidification under a condition where the temperature difference between the slab surface and the thickness center portion is 200 ° C. or more with a pair of opposed flat rolls described in the present invention, the thickness center position (1/2 thickness portion) ) Is efficiently applied, the higher the temperature of the slab, in particular, the higher the temperature of the slab (in the center of the width and half the thickness) is. However, as a result of investigations by the present inventors, after solidification at a temperature (Ts; solidus temperature) at which the solid phase ratio = 1 at which the temperature of the half-thickness portion (solid width) is completely solidified is 1 When large reduction starts, the 1/2 thick part is pushed up to the upstream side, and as a result, the concentrated molten steel is pushed out to the upstream side, forming reverse V segregation and inferior slab quality. I found out that As a result of further investigations by the present inventors, the influence of the ½ thickness portion being pushed down by the large pressure after solidification is pushed up to the upstream side. It was found that the temperature disappears when Ts is set to 60 ° C. or lower.

  Further, in order to make the reduction effect for center porosity compression effective, it is important that the difference between the slab center temperature and the surface temperature is large at the reduction position. The deformation resistance decreases as the temperature of the slab increases, and the deformation resistance increases as the temperature decreases. Therefore, the lower the slab surface temperature is, the more effective it is to contribute as much as possible to the deformation of the center part of the slab. In the present invention, the difference between the center temperature and the surface temperature of the slab is 200 ° C. or more at the reduction position, and the slab surface is descaled by liquid jet between the reduction roll and the adjacent upstream roll. The center porosity crimping was further improved by lowering the surface temperature of the slab at the reduction position.

  First, large reduction was performed on the slab after completion of solidification, and the effect of the reduction amount on the center porosity condition was investigated. About the maximum porosity diameter, it evaluated by the maximum diameter of the porosity which evaluated the slab 1/2 thickness part measured using the high frequency ultrasonic wave (frequency 100MHz) by ultrasonic flaw detection. The slab thickness center position was set as a center line, and evaluation was performed using a slab of 70 mm in the upper and lower sides of 70 mm × the total width of the slab × 10 mm in thickness. The results are shown in FIG. There are two reduction positions: the position where the center temperature is the solidus temperature −100 ° C. (“●” in the figure) and the position where the center temperature is the solidus temperature −60 ° C. (“□” in the figure). Either way, it was reduced using a flat roll. When no reduction was performed (reduction amount = 0%), the maximum volatility diameter was 1000 μm. However, when the reduction amount was 10%, the maximum boronity diameter was greatly reduced, and as the reduction amount was increased, the maximum porosity diameter was reduced. It became clear to do. Based on this result, in the present invention, the lower limit of the amount of reduction was set to 10%. On the other hand, as shown in FIG. 2, when the reduction amount is increased, the maximum porosity diameter at the thickness center position is reduced, but when it exceeds 50%, the improvement effect tends to be saturated, while the total reduction amount increases, As the capability of the reduction device increases and the diameter of the reduction roll 1 increases, the upper limit of the reduction amount is set to 50%. In addition, the amount of reduction means the% notation of the numerical value which remove | divided the difference of the slab thickness before and behind reduction by thickness of the slab before reduction.

  In rolling down the slab after completion of solidification in the present invention, a flat roll is used as the reduction roll. A flat roll means the roll which the roll diameter of the part which touches a slab does not change in the width direction of a reduction roll. When a large reduction is performed using a roll (convex roll) where the width center of the roll is thicker than the width end, the indentation of the slab generated by the reduction is broken during the batch rolling, and the surface of the final product Since it may remain as wrinkles, large reduction using a convex roll is subject to limitations. On the other hand, in the present invention, since large rolling is performed using a flat roll, the upper limit of the rolling amount is not limited, and the rolling amount can be increased to 50% as described above.

  Next, the effect of the difference between the slab center temperature and the solidus temperature at the reduction position on the slab quality was investigated. The slab center temperature at the reduction position was changed from the solidus temperature to the solidus temperature of −100 ° C. by keeping the reduction amount constant at 30% and changing the reduction position. When the reduction position coincides with the complete solidification position, the slab center temperature becomes equal to the solidus temperature, and as the reduction position moves further downstream from the complete solidification position, the slab center temperature and the solidus temperature And the difference becomes larger. The results are shown in Table 1. As is apparent from Table 1, even in the reduction after complete solidification, the center temperature at the reduction position is close to the solidus temperature, that is, the reduction position is downstream of the complete solidification position and close to the complete solidification position. Reverse V segregation occurs at the position, whereas if the rolling start temperature is equal to or lower than the solidus temperature −60 ° C. and the rolling position is sufficiently far from the complete solidification position downstream, the reverse V segregation occurs. It was found that segregation does not occur and the center porosity can be reduced.

  If the center temperature at the reduction position is too low, that is, if the reduction position is too far from the complete solidification position, the deformation resistance at the center becomes too large, and it becomes difficult to reduce the porosity. The rolling resistance was measured when the temperature of the sample was changed from 600 to 1200 ° C. and 0.5% rolling strain was applied to the sample. The results are shown in FIG. When the sample temperature is lowered, the rolling resistance rapidly increases. However, when the temperature is 800 ° C. or higher, the rolling resistance can be kept low. Therefore, in the present invention, the slab thickness center temperature at the end of reduction is defined as 800 ° C. or higher. In addition, if there is a pair of reduction rolls that perform the reduction, the slab center temperature at the reduction start position and the reduction end position can be set to the same temperature.

  This invention makes it a subject to manufacture the slab with few center porosity. The products with strong center porosity reduction needs are mainly thick plate materials. Thick plate materials, particularly those requiring high strength, have many alloys and have a low solidus temperature, specifically, a variety having a solidus temperature of 1450 ° C. or lower. In the varieties having a low solidus temperature, the slab center temperature (solidus temperature-60 ° C) in the reduction position is inevitably low. In the present invention, it is sufficient if the center temperature of the slab at the reduction position is 800 ° C. or more. Therefore, it is possible to sufficiently reduce the center porosity by reducing the solidus temperature, such as a thick plate material. it can.

  If the slab center temperature at the reduction position is the same, the lower the slab surface temperature at the reduction position, that is, the greater the difference between the center temperature and the surface temperature of the slab at the reduction position, the more effective the center porosity is. improves. In the present invention, it is first necessary that the difference between the center temperature of the slab and the surface temperature is 200 ° C. or more at the reduction position. If the initial slab thickness in the mold part is 100 mm or more, the difference between the center temperature and the surface temperature of the slab can be 200 ° C. or more at the reduction position in normal continuous casting. Then, as shown in FIG. 3, it becomes possible to increase the difference in strength between the slab surface and the inside (increase the strength of the surface layer and soften the vicinity of the thickness center). It is possible to reduce the pressure.

  Further, in the present invention, as shown in FIG. 1, a descaling nozzle 3 is provided between the roll 1 between the reduction roll 1 and the adjacent upstream roll (support roll 2a), and the surface of the slab 4 is descaled by liquid injection. I do. By descaling the slab surface by liquid jetting, the scale of the slab surface can be removed and the slab surface temperature can be rapidly reduced. As a result, it has become clear that the slab surface temperature at the reduction position can be further reduced, the difference between the center temperature of the slab and the surface temperature can be expanded, and the center porosity improvement effect by reduction can be improved.

  If a large reduction is performed with the scale attached to the slab surface, the attached scale is pushed into the slab, and the scale may not be sufficiently removed even if the descaling is performed after the large reduction. As a result, there is a possibility that scale defects will occur during rolling in the subsequent process. In the present invention, as described above, the slab surface immediately before the reduction is descaled to prevent the slab surface scale from being pushed into the slab and the reduction roll due to the large reduction.

  At the time of descaling, it is preferable that water is used as the liquid to be ejected and the discharge pressure from the descaling nozzle 3 is 5 to 15 MPa. In addition, by setting the liquid flow rate per nozzle to 130 liters / minute or more and the distance between the nozzle and the slab to 120 mm or less, it is possible to sufficiently descale and reduce the slab surface temperature to a sufficient level. The porosity crimping effect can be enhanced.

  As shown in FIGS. 4B and 4C, when descaling is performed by providing a descaling nozzle 3 further upstream of the upstream roll 2a adjacent to the reduction roll, the slab surface temperature during reduction Is recovered, the effect of improving the center porosity due to the decrease in surface temperature is diminished, and after the descaling, the scale grows again until the slab reaches the reduction roll, and the pressing of the scale due to large reduction occurs. In the present invention, as shown in FIG. 1, descaling is performed between rolls of the rolling roll 1 and the adjacent upstream roll 2 a.

  In continuous casting, a powder layer is formed by supplying continuous casting powder to the surface of molten steel in a mold. The powder layer is melted by heat from the molten steel and flows between the mold and the solidified shell to form a powder film layer. Therefore, the scale adhering to the surface of the continuous cast slab is impregnated with powder. When hot rolling is performed with the powder impregnated scale attached to the slab surface, the grain boundaries on the slab surface are oxidized when the slab is heated in a heating furnace, and the scale containing powder is placed inside the slab. It is captured. As a result, powdery quality defects may occur on the surface of the steel sheet after hot rolling. In particular, when large slab reduction is performed during continuous casting to reduce center porosity, a large indentation of the scale layer infiltrated with powder occurs and the scale is pushed into the slab, resulting in high pressure descaling in the subsequent process. There is a concern that the descaling effect does not work on the scale pushed into the back even if the is carried out, and the powder quality defect after hot rolling becomes conspicuous. On the other hand, according to the present invention, it is possible to effectively prevent powder defects after hot rolling by performing descaling on the surface of the slab immediately before rolling as described above.

  In the present invention, light reduction performed before the completion of solidification during continuous casting may be used in combination. By lightly reducing the slab by an amount commensurate with the solidification shrinkage before completion of solidification, the center segregation of the slab can be reduced and the degree of center porosity can be reduced. Furthermore, since the large reduction of the present invention is performed after completion of solidification, the effect of improving the center porosity can be further ensured.

  The present invention was applied when casting a slab using a mold having a casting width of 2000 mm and a casting thickness of 240 mm using a vertical bending continuous casting apparatus. The components of the casting varieties were C: 0.09%, Si: 0.15%, Mn: 1.6%, P: 0.005%, S: 0.004% by mass. Using the Hirai formula (Ts = 1538-476.5 [% C] -20.5 [% Si] -6.5 [% Mn] -500 [% P] -700 [% S]) The solidus temperature calculated from the components is 1476 ° C.

  One set of rolling rolls was provided at a position after solidification during continuous casting (position of 30 m in casting length). A flat roll having a diameter of 600 mm was used as the reduction roll. The solidification completion position was changed by changing the casting speed from 0.5 m / min to 1.5 m / min, and the slab center temperature at the reduction position was changed. Since there is one set of reduction rolls, the slab thickness center temperature at the reduction start position and the slab thickness center temperature at the reduction end position can be regarded as the same temperature.

  Descaling nozzles were provided before and after the rolling roll to descale the slab. The descaling nozzle arrangement position in the casting direction is as shown in FIGS. FIG. 1 shows a descaling nozzle 3 before the reduction, FIG. 4A shows a reduction after the reduction, (b) shows one reduction roll, and (c) shows a reduction scaling nozzle 3 before two reduction rolls. The distance between the descaling nozzle 3 and the slab 4 was 120 mm, the discharge pressure was changed, and the nozzle was changed to change the flow rate.

  The slab surface temperature at the rolling position is calculated by performing slab heat transfer calculation during continuous casting using a finite element model, and by measuring the slab surface temperature during casting with a two-color thermometer. The results were verified. The temperature at the center of the slab thickness at the reduction position was also calculated by the same calculation method. The amount of decrease in the slab surface temperature due to descaling was evaluated by how many degrees Celsius the surface temperature at the reduction position was reduced by descaling, compared to the case where descaling was not performed before reduction.

  About the maximum porosity diameter, it evaluated by the maximum diameter of the porosity which evaluated the slab 1/2 thickness part measured using the high frequency ultrasonic wave (frequency 100MHz) by ultrasonic flaw detection. The slab thickness center position was set as a center line, and evaluation was performed using a slab of 70 mm in the upper and lower sides of 70 mm × the total width of the slab × 10 mm in thickness. If the maximum porosity diameter was 500 μm or less, it was judged as acceptable. More preferably, it is 250 μm or less. More preferably, it is 100 μm or less. Most preferably, it is 50 μm or less.

  The surface defects were evaluated by hot rolling the slab, cold rolling to a plate thickness of 0.8 mm, and inspecting the entire length of the surface. A surface defect index (%) was obtained by dividing one scale-type and powder-type surface defect by 1 m and dividing the total length of the coil by 1%. A surface defect index of 0.0015% or less was accepted.

  For reverse V segregation, after casting, a slab of 1/2 width portion is cut 20 cm in the casting length direction, and the segregation form existing in the upper and lower thickness 35 mm (thickness 70 mm) is evaluated. If V segregation was present, it was rejected as “occurrence”.

  The results are shown in Table 2. Numerical values that are outside the scope of the present invention are underlined.

  Invention Examples 1 to 15 are examples of the present invention. The maximum porosity diameter and surface defect index were both good, no reverse V segregation occurred, and the final pass / fail was acceptable.

  In general, when the temperature at the center of the slab is higher and closer to the solidus temperature, the strength at the center is lower and the slab can be efficiently reduced. In Examples 2 and 8, the amount of reduction is the same as in Comparative Examples 27 and 28, but the center temperature at the time of reduction is low, but the temperature of the slab surface is lowered by the application of the descaling nozzle immediately before the large reduction, and the center The temperature difference between the surface and the surface increases, and the maximum porosity diameter decreases. That is, it is efficiently reduced.

  In Comparative Examples 1 to 6, the amount of reduction was insufficient, the center porosity was not improved, and the maximum porosity diameter was unacceptable.

  In Comparative Examples 7, 11, 15, 19, 23, 27, and 28, the center temperature of the slab at the reduction position was higher than the solidus temperature of −60 ° C., and reverse V segregation occurred and was rejected.

  In Comparative Examples 8, 12, 16, 20, 24, 27, and 28, descaling is not performed before the large pressure reduction. Therefore, comparing the example of the present invention that is being reduced with the same amount of reduction and the maximum porosity diameter, it can be seen that the effect of improving the maximum porosity is lower than that of the example of the present invention. These comparative examples have a significantly increased surface defect index. Since descaling was not performed before the large pressure reduction, it seems that the powder adhering to the surface was pushed into the slab during the large pressure reduction and appeared as a powder defect at the steel plate stage.

  In Comparative Examples 9, 10, 13, 14, 17, 18, 21, 22, 25, and 26, descaling is performed before large pressure reduction, but not immediately before large pressure reduction. The surface defect index is rejected. It is considered that scale reattachment occurred after descaling and before large pressure, and the scale was pushed into the slab during large pressure and developed as a scale defect at the steel plate stage.

1 Rolling roll 2 Support roll 3 Descaling nozzle 4 Cast slab 5 Casting direction

Claims (1)

When rolling down the slab after completion of solidification during continuous casting, a flat roll is used as the rolling roll, and the rolling amount is 10 to 50%.
The slab thickness center temperature at the reduction start position after completion of solidification is a temperature lower than the solidus temperature -60 ° C, and the slab thickness center temperature at the reduction end position is 800 ° C or higher. The difference between the center temperature of the slab and the surface temperature is 200 ° C. or more,
Descaling the surface of the slab by liquid jet between the roll of the rolling roll and the adjacent upstream roll,
A continuous casting method, wherein the initial slab thickness in the mold part is 100 mm or more.

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