JP5817665B2 - Continuous casting method for slabs - Google Patents

Description

  The present invention relates to a continuous casting method of a slab, and more particularly, to a method of obtaining a slab having a good internal quality and a circular cross section by rolling the slab in a state having an unsolidified portion inside.

  Currently, in continuous casting of steel, in order to improve the internal quality of the slab as it is cast (hereinafter also referred to as “inner quality”), electromagnetic stirring of the molten steel in the continuous casting machine or reduction of the slab is performed. Has been done.

  In the continuous casting machine, the slab whose center portion is unsolidified (hereinafter referred to as “unsolidified slab”) is reduced (hereinafter, the reduction of the unsolidified slab is also referred to as “unsolidified reduction”). By forcibly discharging the molten steel enriched in the solute at the center and increasing the solid phase rate of the slab, the center segregation and V-type segregation occurring at the end of solidification of the slab can be greatly reduced. .

  Moreover, the generation of porosity can be efficiently suppressed by physically pressing the shell of the slab at the time of the generation of porosity accompanying solidification shrinkage of steel.

  As described above, the unsolidified rolling method of rolling down the unsolidified slab is a very effective technique for improving the quality of the slab as it is cast. An example of the uncoagulated reduction method is described in Patent Document 1.

  By the way, in recent years, since it is required to increase the size of the cross section of the final product in the steel product, it is also required to increase the cross sectional size of the slab.

  Usually, when the size of the cross section of the slab is increased, the size of the solid-liquid coexistence area in the slab is also increased accordingly, and segregation, porosity, etc. are likely to occur. Further, when continuous casting is performed using a continuous casting machine having the same length, the casting speed must be reduced as the cross section of the slab increases. Also, the lower the casting speed, the lower the surface temperature of the slab. Therefore, it is considered that the profile in the thickness direction of the deformation resistance of the slab is different between the case where the cross section of the slab is large and the case where it is small, and the deformation behavior at the time of rolling is different.

  For these reasons, it has been difficult to obtain an equivalent quality in a slab having a large cross-sectional size even when the unsolidified reduction method conditions for a slab having a small cross-sectional size are applied.

  For example, although Patent Document 1 describes that the quality of a cross-sectional shape having a width of 430 mm and a thickness of 300 mm has been improved, the slab having a cross-sectional size exceeding this is described. The effect of improving internal quality is unknown.

  Patent Document 2 describes a method for improving center segregation by performing press working in the thickness direction of an unsolidified slab by a forging device during continuous casting as an unsolidified reduction method. A continuous casting of a 215 mm thick slab is shown. However, the method described in the document has a problem that continuous reduction cannot be performed because the slab is reduced by a reciprocating forging die that holds the slab instead of a roll.

  On the other hand, Patent Document 3 describes a method for improving the quality of a slab having a large cross-sectional size exceeding 300 mm in thickness. In this method, in a method of continuously casting a slab having a width of 1500 mm or more and a thickness exceeding 300 mm using a vertical continuous casting machine, a magnetic field is applied to the discharge flow of the immersion nozzle to control the flow, and the slab Describes a method for producing a slab with less central segregation and porosity by controlling the equiaxed crystal area ratio. However, in the method described in the same document, since the maximum porosity thickness is 1.8 mm or less, a large porosity exceeding 1 mm in thickness may remain.

Japanese Patent Laid-Open No. 3-124352 JP 60-82257 A JP-A-11-285788

  The present invention has been made in view of the above-described problems, and an object of the present invention is to provide a continuous casting method for a slab having a large cross-sectional size with little segregation and porosity and good internal quality.

  The present inventors examined a method for improving the quality of a slab having a large cross-sectional size.

  As a result, firstly, it has been found that a slab having a circular cross section can be easily improved in quality at the center of the slab as compared with a slab having a rectangular cross section. This is because when a slab having the same cross-sectional area is squeezed at the same reduction rate using a squeezing roll having the same diameter, a slab having a circular cross section is reduced more than a slab having a rectangular cross section. This is because the contact area with the roll is small and the rolling force is small.

  Secondly, it has been found that when a slab having a circular cross section is unsolidified, segregation and porosity can be reduced if the reduction ratio of the cross-sectional area and the area ratio of the unsolidified portion in the cross section satisfy a predetermined relationship. . Details of this examination will be described later.

  This invention is made | formed based on these knowledge, The summary exists in the continuous casting method of the following slab.

This is a continuous casting method of a slab having a circular cross section, the slab has a cross section diameter of 300 mm or more, and the inside of the slab is unsolidified and subjected to reduction to force unsolidified molten steel. In the step of discharging to the upstream side in the casting direction, the following formulas (1) to (3) are satisfied:
S ≦ 20 (1)
ΔS / S ≧ 5 (2)
ΔS × S _Liq / 100 ≧ 100 (3)
Where, S: ratio of the cross-sectional area of the unsolidified part to the cross-sectional area of the slab immediately before reduction (%), ΔS: rate of reduction of the cross-sectional area of the unsolidified part of the slab due to reduction (%), S _Liq : immediately before reduction Is the cross-sectional area (mm ² ) of the unsolidified part of the slab, and ΔS is expressed by the following equation (4).
ΔS = (D _R / D _I ) ^A × k ^B × R ^C × {exp (S)} ^D × E (4)
Here, D _R : diameter of the rolling roll (mm), D _I : diameter of the slab of the slab just before the reduction (mm), k: ratio of deformation resistance near the surface to deformation resistance near the center of the slab, R: Reduction ratio of cast slab (%), A = 0.047, B = 0.37, C = 1.8, D = −0.037, E = exp (−2.0).

  According to the continuous casting method of a slab of the present invention, it is possible to obtain a slab having a large cross-sectional size and a circular cross-section with a good segregation and porosity, a good internal quality, and a large cross-sectional size.

It is a figure which shows the calculated value of the parameter using (DELTA) S of the non-coagulation pressure preliminary test, (a) shows (DELTA) S / S, (b) shows ( _DELTA ) S * _SLiq / 100. It is a figure which shows the outline of the continuous casting machine which can apply the continuous casting method of this invention.

  Hereinafter, the contents of the study for completing the present invention and the mode for carrying out the present invention will be described.

1. Contents of examination 1-1. Outline of examination content It is the deformation behavior of the unsolidified part inside the slab during unsolidified reduction that is important for reducing the segregation and porosity inside the slab and improving the quality of the slab by unsolidified reduction. If the rate of reduction of the cross-sectional area of the unsolidified portion during rolling is insufficient, it is considered that segregation and porosity remain even after rolling.

  On the other hand, if the rate of reduction in the cross-sectional area of the unsolidified portion during rolling is sufficient, the unsolidified molten steel is sufficiently discharged upstream in the casting direction, and segregation inside the slab is reduced. At this time, the porosity is physically reduced at the generation stage, so that the porosity is greatly reduced. Therefore, the quality of the slab is improved.

  If the rate of reduction of the cross-sectional area of the unsolidified portion of the slab due to reduction can be estimated, it is considered that the rate of reduction of the cross-sectional area of the unsolidified portion of the slab required for improving the quality of the slab can be determined.

  Accordingly, as described below, the present inventors first performed deformation analysis of the slab by elasto-plastic FEM, and investigated the deformation behavior of the slab during unsolidified reduction. Next, several unsolidified reduction tests were conducted to investigate the quality of the slab. And the result of these deformation | transformation analysis and the internal quality of slab were compared, and the unsolidified reduction conditions which can obtain the slab with a favorable internal quality were determined.

1-2. Unsolidified reduction analysis model The present inventors used a three-dimensional elasto-plastic finite element method (FEM) and considered the size factor of the slab, and the unsolidified of the slab having a circular cross section during unsolidified reduction. The reduction rate of the cross section of the part was estimated.

Here, “size factor of slab”
(1) Ratio of slab size (diameter) to size (diameter) of the rolling roll,
(2) slab surface temperature dependence of the ratio of the deformation resistance near the surface to the deformation resistance near the center of the slab (hereinafter also referred to as “slab-internal / external deformation resistance ratio” or simply “deformation resistance ratio”);
(3) the cross-sectional area of the unsolidified portion during rolling,
It is.

The “unsolidified portion” of the slab refers to a portion having a solid phase ratio of less than 0.8. The solidification interface is a solid-liquid interface defined by an isotherm having a solid phase ratio of 0.8. The reduction rate of the cross-sectional area of the unsolidified portion of the slab due to the reduction is expressed by the following equation (5).
ΔS = (S _Liq −S ₂ ) / S _Liq × 100 (5)
Here, ΔS: Reduction rate of cross-sectional area of unsolidified portion of cast slab by reduction (%), S _Liq : Cross-sectional area of unsolidified portion of cast slab immediately before reduction (mm ² ), S ₂ : Casting after reduction The cross-sectional area (mm ² ) of the unsolidified portion of the piece.

  The temperature of the node given to the FEM model and the definition of the shell were separately calculated by axisymmetric unsteady solidification analysis. The temperature in the longitudinal direction of the slab is assumed to be small, and the temperature in the longitudinal direction of the slab is constant in the calculation range. A portion having a solid phase ratio of less than 0.8 was treated as a void, and only a solidified shell having a solid phase ratio of 0.8 or more was defined as a mesh. Mechanical and thermal properties gave the properties of medium carbon steel. The rolling roll was defined as a rigid body and the shape was flat. The slab was subjected to a slab reduction analysis by applying displacement and rotational displacement to the slab. At this time, the bending of the rolling roll due to the rolling reaction force is not considered. Further, heat exchange at the time of contact between the rolling roll and the slab was not included in the calculation because it had a small effect on the internal deformation of the slab.

  It was confirmed that the deformation behavior of the slab in this analytical model and the deformation behavior of the slab in the unsolidified reduction test described later substantially coincide.

  Furthermore, the reduction calculation was performed within the parameter range shown in Table 1.

  Of the parameters shown in Table 1, the slab inner / outer deformation resistance ratio was the ratio of the yield stress at the slab surface temperature to the yield stress at 1400 ° C.

In this reduction calculation, the deformation resistance ratio was calculated from data on the relationship between the yield stress and temperature of the medium carbon steel, and the relationship between the deformation resistance ratio and the surface temperature was obtained as a regression equation of the following equation (6).
k = 5.6 × 10 ¹³ × T ^−4.37 (6)
Here, k is the slab inner / outer deformation resistance ratio (−), and T is the surface temperature (° C.) of the slab.

  And the surface temperature of the slab at the time of unsolidification reduction calculated | required by unsteady solidification analysis was substituted for (6) Formula, and the value of the deformation resistance ratio shown in Table 1 was obtained.

  Although the yield stress differs depending on the steel type, the formula (6) can be used in the rough calculation of the deformation resistance ratio even if the steel type is different. When calculating a more precise deformation resistance ratio, the yield stress may be used for each steel type obtained by a tensile test or the like.

  In the main reduction calculation, the yield stress ratio is used as the deformation resistance ratio, but the ratio of the tensile strength at the temperature may be used.

The rolling reduction was defined by the following formula (7).
R = r1 / D _I × 100 (7)
Here, R: reduction ratio (%), r1: reduction amount, D _I : diameter of the slab immediately before reduction.

  The ratio of the cross-sectional area of the unsolidified portion to the cross-sectional area of the slab immediately before rolling was obtained from the thickness of the solidified shell obtained by unsteady solidification analysis.

  The position immediately before the reduction was defined as the contact start position (biting start position) between the reduction roll and the slab.

As a result of the reduction calculation, the relationship between the unsolidified reduction condition and the reduction rate of the cross-sectional area of the unsolidified portion of the slab due to reduction was obtained as the following equation (4).
ΔS = (D _R / D _I ) ^A × k ^B × R ^C × {exp (S)} ^D × E (4)
Here, ΔS: Reduction rate (%) of cross-sectional area of unsolidified portion of slab by reduction, D _R : Diameter of reduction roll (mm), D _I : Diameter of slab of slab immediately before reduction (mm) , K: slab internal / external deformation resistance ratio (−), R: slab reduction ratio (%), S: ratio of the cross-sectional area of the unsolidified portion to the cross-sectional area of the slab immediately before reduction, A , B, C, D and E are coefficients.

  And about the cross section of the slab cast in the unsolidified reduction preliminary test described below, the area of the unsolidified portion is obtained by image analysis, and the reduction rate ΔS of the cross-sectional area of the unsolidified portion of the slab due to reduction is calculated as described above. Calculated by equation (5). Multiple regression analysis was performed by applying the calculated ΔS and casting conditions to the above equation (4), and values of coefficients A, B, C, D, and E were obtained. As a result, A = 0.047, B = 0.37, C = 1.8, D = −0.037, and E = exp (−2.0) were obtained. Hereinafter, the equation (4) in which the values of A, B, C, D, and E are substituted is referred to as “regression equation (4)”.

1-3. Pre-solidification reduction preliminary test The non-solidification reduction preliminary test was performed by casting slabs having diameters of 300 mm and 420 mm. The rolling roll used for the unsolidified rolling was a flat roll having a diameter of 450 mm. The target steel types were medium carbon low alloy steel and high chromium steel with chemical compositions shown in Table 2. The test conditions are Test Nos. 1 to 13 shown in Table 3. The table shows the diameter D _I of the slab immediately before the reduction, the amount of reduction, the diameter of the unsolidified part at the time of reduction, and the deformation resistance near the center of the slab. The ratio k of the deformation resistance near the surface, the ratio S of the cross-sectional area of the unsolidified portion to the cross-sectional area of the slab immediately before the reduction, and the reduction ratio R are shown.

  The time of reduction was determined by calculating the thickness of the solidified shell (isothermal line with a solid phase ratio of 0.8), temperature, and the like by unsteady heat transfer analysis. The accuracy of the unsteady heat transfer analysis was confirmed in advance based on the surface temperature of the slab, the measurement result of the thermocouple disposed inside the slab, and the diameter of the unsolidified portion measured by adding the tracer.

  The occurrence of segregation, internal cracks and porosity was observed for the samples of the transverse and longitudinal sections taken from the cast slab. Segregation and internal cracks were observed by sulfur printing. The presence or absence of these internal defects was determined visually, and Table 3 shows the evaluation of the samples together with the test conditions. The evaluation was made in three stages: ○ (no defect or minute, no practical problem), Δ (small defect, practical problem) and x (large defect, practical problem).

  In the test Nos. 1 to 3, the rating was “◯”, and no slab segregation, porosity, and internal cracks were obtained, and a slab having a good internal quality was obtained. In the test numbers 4 to 11, the evaluation was Δ or ×, and the target slab of the internal quality was not obtained. Among these, internal cracks occurred in Test Nos. 4-6. In Test Nos. 7 to 9 and 11, segregation and porosity remained, and in Test No. 10, segregation remained.

1-4. Comparison of regression formula obtained with uncoagulated reduction analysis model and uncoagulated reduction preliminary test results The conditions of the uncoagulated reduction preliminary test shown in Table 3 above are the regression equations (4 ) And ΔS values of Test Nos. 1 to 13 were calculated. As a result of examining the value of ΔS and the result of the pre-coagulation preliminary test from various angles, as shown in Table 4, both ΔS / S ≧ 5 and ΔS × S _Liq / 100 ≧ 100 (mm ² ) are satisfied. In some cases, it was found that a slab having a good internal quality could be obtained. Among these indexes, “ΔS × S _Liq / 100” is equivalent to “S _Liq −S ₂ ”, that is, the difference in the cross-sectional area of the unsolidified portion of the slab immediately before and after the reduction from the above equation (5). To do.

FIG. 1 is a diagram showing calculated values of parameters using ΔS in a pre-coagulation preliminary test. FIG. 1A shows ΔS / S, and FIG. 1B shows ΔS × S _Liq / 100. . As shown in Table 4 and FIGS. 3A and 3B, in Test Nos. 1 to 3, both ΔS / S ≧ 5 and ΔS × S _Liq / 100 ≧ 100 (mm ² ) were satisfied. As described above, in these tests, neither segregation nor porosity was present or minimal. This is presumably because sufficient uncoagulated reduction was performed. Moreover, since no internal crack was observed, it is considered that the unsolidified reduction was not excessive.

  On the other hand, in test Nos. 4 to 9 and 11, ΔS / S ≧ 5 was not satisfied. Among these, the reason why the internal cracks occurred as described above in Test Nos. 4 to 6 is considered that the diameter of the unsolidified portion at the time of the reduction was too large. The reason why segregation and porosity remained in Test Nos. 7 to 9 and 11 is considered to be due to insufficient rolling reduction.

In Test Nos. 10 and 11, ΔS × S _Liq / 100 ≧ 100 (mm ² ) was not satisfied. The reason why segregation remained in Test No. 10 is considered to be because the diameter of the unsolidified portion at the time of reduction was too small to squeeze out the unsolidified molten steel.

Thus, it is possible to predict the quality of the slab from the values of ΔS / S and ΔS × S _Liq / 100 obtained by substituting the unsolidified reduction conditions during continuous casting into the regression equation (4). .

  However, as in test No. 4, when the ratio S of the cross-sectional area of the unsolidified portion to the cross-sectional area of the slab immediately before reduction exceeds 20%, ΔS / S ≧ To satisfy 5, ΔS must exceed 100%. However, when the actual unsolidified pressure is reduced, when the ΔS is 100%, the unsolidified interface (interface with a solid phase ratio of 0.8) is completely pressure-bonded, so the cross-sectional area of the unsolidified portion of the slab is reduced by the reduction. The rate ΔS cannot exceed 100%. Therefore, when S exceeds 20%, it can be determined that improvement of the internal quality by reduction is impossible.

2. Basic Configuration of Continuous Casting Device FIG. 2 is a diagram showing an outline of a continuous casting machine to which the continuous casting method of the present invention can be applied. The tundish 1 accommodates molten steel 6 supplied from a ladle (not shown). The molten steel 6 is injected from the tundish 1 through the immersion nozzle 2 so as to form a meniscus 6a in the mold 3, and is cooled by cooling water sprayed from the mold 3 and a group of secondary cooling spray nozzles (not shown) below the mold 3. Then, a solidified shell 7 is formed to become a slab.

  While the slab is supported by a foot roll 4 disposed immediately below the mold 3 and a plurality of roller aprons 5 disposed downstream in the casting direction of the foot roll 4 while holding the unsolidified molten steel 6 therein, It is pulled out by a pinch roll 8 arranged on the downstream side of the roller apron 5 and unsolidified and reduced by a reduction roll 9. The above-mentioned unsolidified reduction test was performed by the continuous casting machine shown in the figure.

3. Example of Continuous Casting Machine and Continuous Casting Method An example of the continuous casting method of the present invention using the continuous casting machine shown in FIG. 2 will be described. As the mold 3, a length of 800 mm, a copper water-cooled type, and a circular cross section is used. The casting speed is controlled to 0.10 m / min to 0.60 m / min according to the diameter of the slab and the rolling time. The reduction roll 9 has a diameter of 650 mm and is disposed at a position of 20.0 m to 30.0 m downstream from the meniscus 6a in the casting direction. The slab is unsolidified and reduced using this reduction roll 9. That is, the slab holding the unsolidified molten steel 6 inside is squeezed.

  The shape of the rolling roll 9 is not limited, but a flat shape or a caliber shape is preferable. Moreover, it becomes easier to obtain a slab having a good inner quality as the diameter of the rolling roll 9 is larger. However, the partial pressure reduction force increases.

  The assumed rolling force varies depending on the diameter of the slab and the steel type, and the larger the slab diameter, the larger the rolling force is required. However, a rolling force of about 1000 t is sufficient.

  The pinch force necessary for drawing the slab increases with an increase in the rolling force. Therefore, the pinch roll 8 needs to be provided with a motor having an appropriate capacity in order to set a pinch force corresponding to the reduction force. For example, when the friction coefficient between the reduction roll 9 and the slab is 0.4 and the maximum reduction force is 1000 t, the pinch force increases due to an increase in friction force corresponding to 400 t (1000 t × 0.4). An increase can be predicted.

  In the method of the present invention, the thickness profile of the solidified shell 7 is obtained in advance using an unsteady axisymmetric heat transfer analysis or the like, and the casting speed is adjusted, so that the unsolidified portion at the position of the rolling roll 9 is determined. Control the diameter.

  In the case of expanding the application range to a slab having a larger diameter, the casting speed is reduced according to the diameter of the slab, or the position of the rolling roll 9 from the meniscus 6a is moved downstream in the casting direction. To do.

  Below, the test done in order to confirm the effect of this invention is demonstrated.

1. Test Method Using the continuous casting machine shown in FIG. 2, a test of unsolidified reduction during continuous casting was performed. The dimensions and configuration of each part of the continuous casting machine were as described in “3. Examples of continuous casting machine and continuous casting method”, and the shape of the rolling roll was flat.

The test conditions are Nos. 1 to 14 shown in Table 5. The table shows the diameter D _I of the slab immediately before the reduction, the diameter D _R of the reduction roll, the casting speed, the distance d from the meniscus of the reduction roll, the reduction amount, and the reduction time. The ratio of the deformation resistance near the surface to the deformation resistance near the center of the slab, the surface temperature of the slab, the ratio S of the cross-sectional area of the unsolidified portion to the cross-sectional area of the slab immediately before reduction, and The rolling reduction R was shown.

2. Test results The occurrence of segregation, internal cracks, and porosity was observed for samples of the transverse and longitudinal sections taken from the cast slab. Segregation and internal cracks were observed by sulfur printing. The presence or absence of these internal defects was determined visually, and Table 6 shows the calculated values of ΔS / S and ΔS × S _Liq / 100 and the evaluation of the samples. The evaluation was made into two stages: ○ (good, good internal quality) and x (impossible, defective).

As shown in Table 6, the inner quality of the slab was good in Nos. 1 to 10 satisfying both ΔS / S ≧ 5 and ΔS × S _Liq / 100 ≧ 100 (mm ² ). On the other hand, in No11-14 which did not satisfy | fill one of (DELTA) S / S> = 5 and (DELTA) S * _SLiq / 100> = 100 (mm < ² >), the slab had a defect and the internal quality was not favorable.

  According to the continuous casting method of a slab of the present invention, it is possible to obtain a slab having a large cross-sectional size and a circular cross-section with a good segregation and porosity, a good internal quality, and a large cross-sectional size.

1: tundish, 2: immersion nozzle, 3: mold, 4: foot roll,
5: Roller apron 6: Molten steel 6a: Meniscus 7: Solidified shell
8: Pinch roll, 9: Rolling roll

Claims (1)

A slab continuous casting method in which a cross section of a slab including an unsolidified portion before rolling is circular,
The diameter of the cross section of the slab immediately before reduction is 300 mm or more,
Casting characterized in that the following formulas (1) to (3) are satisfied in the step of applying a reduction in an unsolidified state of the slab and forcibly discharging the unsolidified molten steel to the upstream side in the casting direction. Method for continuous casting of pieces.
S ≦ 20 (1)
ΔS / S ≧ 5 (2)
ΔS × S _Liq / 100 ≧ 100 (3)
Where, S: ratio of the cross-sectional area of the unsolidified part to the cross-sectional area of the slab immediately before reduction (%), ΔS: rate of reduction of the cross-sectional area of the unsolidified part of the slab due to reduction (%), S _Liq : immediately before reduction Is the cross-sectional area (mm ² ) of the unsolidified part of the slab, and ΔS is expressed by the following equation (4).
ΔS = (D _R / D _I ) ^A × k ^B × R ^C × {exp (S)} ^D × E (4)
Here, D _R: the reduction roll diameter (mm), D _I: diameter of the slab in the pressure immediately before (mm), k: relative yield stress at 1400 ° C. of the slab at a ratio of yield stress at the billet surface temperature A certain slab inner / outer deformation resistance ratio , R: slab reduction ratio (%), A = 0.047, B = 0.37, C = 1.8, D = −0.037, E = exp ( -2.0).

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