JP2011121063A - Continuous casting method with soft reduction - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for subjecting a slab of high internal quality having reduced center segregation and center porosities to soft reduction continuous casting only by the addition of a simple apparatus. <P>SOLUTION: In the continuous casting method with soft reduction for steel in which drawing with soft reduction is performed in such a manner that the opening degree of rolls in the middle period of solidification is made wider as it goes to the downstream side to introduce thickness expansion of a slab, and next, the opening degree of rolls in the last stage of the solidification is made narrower as it goes to the downstream side, the thickness of the slab after the soft reduction is measured at least in two parts of the edge part in the width direction of the slab and the part other than the edge part, a soft reduction evaluation value (ΔR) is obtained from the thickness Te of the slab at the edge part in the width direction and the thickness Tc of the slab other than the edge part in the width direction, and the opening degree S<SB>0</SB>of the rolls, the opening degree S<SB>1</SB>and S<SB>2</SB>of the rolls at the initial stage of the solidification, and on the basis of the soft reduction evaluation value (ΔR), the control of the opening degree of rolls is performed. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a light pressure continuous casting method, and in particular, prevents segregation of impurity elements such as S, P, Mn, etc. found in the thickness center part of a continuous cast slab, and has good internal quality with little center porosity. The present invention relates to a light rolling continuous casting method capable of casting a slab.

  The so-called continuous casting method, in which molten steel is drawn while being solidified and continuously cast, is characterized by good product (steel) yield and high productivity. Slabs, blooms, billets, and other cast pieces directly from the molten steel Is widely known as a casting method capable of continuously producing. However, this continuous casting technique has a problem that elements such as C, P, and S tend to concentrate (center segregation) in the center of the thickness of the slab, particularly in the case of steel.

  Therefore, in order to reduce the center segregation that occurs during continuous casting of steel, conventionally, the roll interval at the end of solidification is adjusted for the bending that occurs with the molten steel flow during shrinkage when the molten steel solidifies to complete solidification. A technique for lightly reducing the previous slab has been proposed.

  For example, in Patent Document 1, when steel is continuously cast, a bulging force is positively induced in a portion of a solidified shell between a mold and a liquid phase crater end of a slab (mid-solidification stage). By increasing the thickness of the solidified layer (thickness expansion) and then applying light pressure to the slab between the liquidus crater end and the solidus crater end (end of solidification), central segregation is achieved. A technique for reducing the above is disclosed.

  Moreover, in Patent Document 2, after installing a load cell on each roll bearing portion of a segment, applying a predetermined load to the upper segment offline, measuring strain of each roll with respect to the load, and creating a load-strain amount curve The roll reaction force is measured on-line with a load cell, the amount of strain is obtained from the obtained roll reaction force and the load-strain amount curve, and the roll is moved by the amount of the strain to roll distance (roll opening degree). ) Is kept constant.

  And in patent document 3, in the light reduction continuous casting method which leads the slab containing the unsolidified molten steel in continuous casting to a completely solidified state while lightly reducing by a plurality of pairs of rolls, the slab is completely reduced by light reduction. A light rolling continuous casting method that controls the strain rate of a slab by measuring the thickness of the slab after solidification and the roll thickness is adjusted when the measured thickness exceeds a predetermined allowable range. Is disclosed.

JP-A-60-6254 JP-A-5-131253 JP-A 63-278654

  The technique for reducing the center deflection described in Patent Document 1 is a method of setting the initial roll opening, and how much the slab is actually bulged and how much light rolling force is applied. It does not consider the dynamic control of whether or not it is necessary. However, the load required for light reduction changes every moment depending on casting conditions (casting speed, molten steel heating degree, secondary cooling conditions) and chemical composition of the slab, and the light pressure reaction force acting on the roll also changes every moment. For this reason, the thickness of the slab usually does not match the default roll gap. Therefore, it has not been possible to determine whether or not a predetermined light reduction has been performed, and as a result, there have been many cases where the center deflection is not reduced so much.

  With respect to the continuous casting method described in Patent Document 2, it is necessary to install a load cell in each roll that performs light reduction, and therefore, this method has a problem that the cost is increased because it requires further capital investment. there were.

  The casting technique disclosed in Patent Document 3 is a method of adjusting the roll reduction amount by measuring the thickness of a cast slab after casting and determining whether or not the thickness is within a predetermined range. It is. However, what is important in the technology for reducing center segregation by light reduction is to control the reduction gradient (the amount of reduction per unit time) until the center is finally solidified. It cannot be judged whether the reduction is good or bad.

  Therefore, the object of the present invention is to solve the above-mentioned problems of these conventional technologies, and in particular, by adding a simple device without using expensive equipment required for load measurement or the like. The purpose is to propose a method for continuously casting a slab of good internal quality with little segregation and center porosity under light pressure.

The inventors have applied a positive bulging force that gradually increases the roll opening in the middle of solidification from the roll opening S _{0 in} the initial stage of solidification (directly under the mold) during continuous casting of steel slabs. The thickness of the inner non-solidified layer was increased, and then, at the end of solidification, an experiment was conducted in which the roll opening was gradually narrowed to lightly reduce the pressure (hereinafter, such a light reduction pattern is abbreviated as “IBSR”). . And the thickness distribution in the width direction about the slab after performing such casting was investigated. as a result,
a. When IBSR is performed, the thickness distribution often occurs in the width direction of the slab, and the slab thickness Tc at the center in the width direction is often thicker than the slab thickness Te at the width direction end. ,
b. Even under the same IBSR conditions, when the secondary cooling of the slab is strengthened, or in the case of a slab having a hard component, the thickness difference (Tc-Te) becomes large.
c. There is a correlation between the difference in roll opening (S ₀ -S ₁ ) and the slab thickness difference (Tc-Te),
We obtained knowledge such as.

The present invention is a method developed by further study based on these findings, and performs continuous casting of steel using a continuous casting apparatus in which a plurality of rolls are arranged from directly under the mold to the machine end. In the continuous casting method, the roll opening in the middle solidification phase is expanded as it goes downstream, leading to slab thickness expansion, and then the roll opening at the end of solidification is reduced as it goes downstream The thickness of the slab after the light reduction is measured at at least two locations, ie, the width direction end and other than the end of the slab, and the width direction end slab thickness Te and the slab thickness Tc other than the width direction end, From the information on the roll opening S _{0 at the} initial stage of solidification, the roll opening S ₁ at the time of thickness expansion, and the roll opening S ₂ at the time of light reduction, a light reduction evaluation value (ΔR) is obtained, and this light reduction evaluation value (ΔR) The roll opening is controlled based on It is soft reduction continuous casting method of steel according to claim.

In the light rolling continuous casting method according to the present invention,
(1) The light pressure reduction evaluation value (ΔR) is calculated based on the following formula (1), and at least one of the roll openings S ₁ and S ₂ is corrected based on the ΔR,
ΔRmm = (S ₁ −S ₀ ) mm− (Tc−Te) mm (1)
(2) The middle solidification means between the liquid phase crater end of the slab except directly under the mold, and the last solidification means between the liquid phase crater end and the solid phase crater end. To do,
Is a more preferable solution.

  In the present invention, the thickness of the slab after casting is measured, and the “light reduction evaluation value ΔR”, which is a control index peculiar to the present invention, is calculated only by a simple method, with an appropriate light reduction pattern (IBSR). Since continuous slab casting of light slabs can be performed, deterioration of center segregation due to excessive or insufficient light squeezing amount and increase in center porosity can be prevented in advance, and stable slabs with good internal quality can be prevented. Can be manufactured.

It is a basic diagram of a continuous casting equipment. (A), (b) is a graph which shows the light reduction patterns I and II, respectively. (A), (b), (c) is explanatory drawing which respectively shows the form of the bulging of a slab, and the pattern which lightly reduces it. It is a graph which shows the relationship between (DELTA) R and segregation degree C / Co (Mn).

FIG. 1 is a schematic view of a continuous casting facility used for carrying out the method of the present invention. In continuous casting performed using this equipment, a solidified shell on the surface of the slab is generated by rapid cooling in the mold M, and then the solidified shell gradually increases in the solidified thickness through secondary cooling (water spray cooling). Eventually, slabs were formed.
In the case of such a continuous casting facility, generally, the center portion of the slab is finally solidified in the horizontal belt portion. In the horizontal band, a plurality of devices called a segment, each having a plurality of pairs of rolls supported by one frame, are installed facing the cast piece (only one segment is shown in FIG. 1). Then, before and after the final solidification time, the slab is lightly reduced by the light reduction roll group of the corresponding segment. In this example, the reduction amount and the reduction gradient of a plurality of pairs of roll groups can be set on a segment basis.

In the present invention, a thickness meter 4 for measuring a thickness distribution in the width direction of the slab is installed at a position on the most downstream side of the continuous casting facility. The thickness Te of the end portion in the direction and the thickness of the other portion (usually the thickness of the central portion) Tc can be measured online. Then, the measurement result is sent to the arithmetic unit 5, and the arithmetic unit 5 uses a measured value of the slab thickness (Te, Tc) and the roll opening degree (S ₀ , S ₁ , S ₂ ) to explain in detail later. A roll-down evaluation value ΔR is calculated, and based on the light roll-down evaluation value ΔR, a roll opening correction amount is output to each segment for control.

Hereinafter, a method for calculating the correction amount of the roll opening will be described.
First, an example of the light pressure pattern IBSR is shown in FIG. First, FIG. 2A shows that the position from the roll opening degree S _{0 at the} initial stage of solidification directly under the mold to the roll opening degree S ₁ during the thickness expansion is widened to cause the thickness expansion (bulging) of the slab. and so, the subsequent solidification end, shows a reduction pattern I to reduce the soft reduction at the roll angle S ₂ to make soft reduction is carried out by reducing the roll gap. On the other hand, FIG. 2 (b) showing the rolling pattern II, to reduce the amount of reduction after to promote bulging, the soft reduction at the roll angle S ₂ in the coagulation end, than the freezing initial roll opening S ₀ It is a rolling reduction pattern.

As shown in FIG. 3 (a), the cross-sectional shape when both bulging patterns I and II promote bulging is such that the slab thickness Tc at the center in the width direction is equal to the slab thickness Te at the width direction end. It has a bulging cross-sectional shape. Slab thickness Tc of the width center portion of this time is approximately equal to the roll angle S ₁ of the coagulation medium term. On the other hand, the end of the slab is shell-like when it comes out of the mold, but is already a solidified part. Therefore, if heat shrinkage or solidification shrinkage is ignored, the end slab at this time the thickness Te is approximately equal to the solidification initial roll opening S _0. Hereinafter, in the present invention, Te refers to the thickness of the end portion in the width direction of the completely solidified metal piece, slab width: about 2100 mm, slab thickness: about 250 mm, a portion within about 100 mm from the end portion, Tc refers to the thickness in the portion near the center excluding the end portion in the slab width direction.

In this regard, the soft reduction pattern I, since the narrower than the roll angle S ₀ of the roll angle S ₂ coagulation end solidification initial, final cross-sectional shape after soft reduction as considering bulging As shown in FIG. 3 (b), it should be almost rectangular. However, in general, the roll opening degree is opened by the reaction force of the reduction, so if the load for light reduction is too large, the slab thickness Tc of the width direction center part is larger than the slab thickness Te of the width direction end part. It is often thicker.

Meanwhile, in the example of soft reduction pattern II, since wider than the roll angle S ₀ of the roll angle S ₂ coagulation end solidification initial, final cross-sectional shape after soft reduction Fig 3 (c) As shown, the slab thickness Tc at the center in the width direction is inevitably thicker than the slab thickness Te at the width direction end. That is, the slab thickness theoretically satisfies the relationship of Tc-Te = S ₀ -S ₂ , but in reality, the roll opening is opened by the reaction force when light pressure is applied. In addition, it usually changes after light pressure under the influence of coagulation shrinkage or heat shrinkage. For these reasons, the above relational expression is rather often Tc−Te> S ₀ −S ₂ .

Hereinafter, this will be described with reference to a method of correcting (feedback control) the light reduction amount by using the light reduction evaluation value ΔR defined as the following equation (1) in the case of T _c > Te.

ΔRmm = (S ₁ −S ₀ ) mm− (Tc−Te) mm (1)

For example, from the expression of the light rolling evaluation value ΔR, when Tc> Te,
a. If ΔR is smaller than the reference, the light reduction amount is relatively increased by increasing (widening) the roll opening S ₁ or decreasing (narrowing) the roll opening S ₂ .
b. On the contrary, if ΔR is larger than the reference, the roll opening S ₁ is decreased or the roll opening S ₂ is increased so that the light reduction is relatively reduced.
By performing this control, the light reduction amount can be corrected (controlled) within an appropriate range.

  The method for determining the appropriate value of the light reduction evaluation value ΔR can be considered as follows. FIG. 4 shows the light rolling evaluation value ΔR and the degree of central segregation of Mn C / Co (Mn) for carbon steel (slab thickness 250 mm) having a carbon content of 0.05 mass% and a manganese content of 1.3 mass%. It shows the relationship. The segregation degree of the slab is defined as a manganese analysis value at a 1/4 position in the thickness direction of the slab as a reference value (Co) without segregation, and a ½ position (1/2 W) in the width direction of the slab slab. A longitudinal section sample in the casting direction is cut out from the 1/4 position (1/4 W), sliced by 1 mm from the section position in the thickness direction of the slab, and an analysis sample is taken. The highest manganese analysis value in this analysis sample The ratio (C / Co) between (C) and the reference value (Co) was evaluated as the degree of segregation. A segregation degree closer to 1.0 means less segregation.

  As shown in FIG. 4, as the light rolling evaluation value ΔR increases, the segregation degree C / Co decreases. Therefore, if the target value of the segregation degree C / Co in this steel type is 1.05 or less, it is understood that the light rolling evaluation value ΔR needs to be 4 mm or more. In this way, such a database is created in advance for each casting condition such as the steel type and the thickness of the cast slab, and the appropriate range of the light reduction evaluation value ΔR at that time is determined from the target center segregation range. do it.

When Tc≈Te, the above equation (1) is substantially
ΔRmm = (S ₁ −S ₀ ) mm (2)
It becomes the expression. That is, when Tc≈Te, a control method is conceivable in which S ₁ -S ₀ is set to a predetermined light pressure evaluation value ΔR and it is always confirmed that Tc≈Te. Alternatively, the light pressure reduction condition may be changed so that Tc> Te, and light pressure reduction control may be performed so that the light pressure evaluation value ΔR becomes a predetermined value in this state (Tc> Te). In this case, if the amount of light reduction before correction is excessive, if it is reduced to an appropriate amount, internal cracks and negative deflection can be avoided.

  According to the present invention, the reason why the light rolling evaluation value ΔR can be calculated from the thickness distribution in the width direction of the slab is that an appropriate “IBSR” that first performs casting that promotes bulging of the slab, and then performs light rolling. This technique is an effective light pressure control method particularly when IBSR is adopted.

  By the above method, it becomes possible to control the amount of light reduction from the mid-coagulation phase to the end of coagulation within an appropriate range, and as a result, prevent deterioration of center segregation due to insufficient light reduction force and increase of center porosity. Thus, a continuous cast slab having good internal quality can be obtained.

Example 1
Carbon steel with a carbon content of 0.05 mass% and a manganese content of 1.3 mass% (slab thickness 250 mm, slab) using the continuous casting equipment with the lightly pressed segment (segment length 2 m) shown in FIG. Continuous casting with a width of 2100 mm was performed, and the occurrence of center segregation was investigated.
In this example, the standard casting speed is 1.4 m / min, and the standard light reduction pattern is S ₀ = 255 mm, S ₁ = 260 mm, S ₂ = 256 mm as shown in FIG. It was. The crater end position under the standard casting and cooling conditions is calculated as a meniscus distance of 25 m from solidification / heat transfer calculation. In the vicinity of the end of solidification (final solidification), a setting was made to lightly reduce 0.7 m per m in the casting direction, and the set light reduction speed per time was set to 0.98 mm / min.

  In this example, continuous casting for 2 tons and 500 tons was performed. The thickness gauge 4 at the end of the machine measured the thickness distribution in the width direction of the cast slab (slab) as needed. As a result, in the initial stage of casting (initial stage of solidification), the difference in thickness (Tc−Te) between the central part and the end part was 2.5 mm, and the central part was thick. From this result, the light reduction evaluation value ΔR calculated by the above formula (1) was (5 mm) − (2.5 mm) = 2.5 mm.

In this embodiment, since the light reduction evaluation value ΔR is set to increase the light reduction amount when the evaluation value ΔR is 4.0 mm or less (standard) based on the situation shown in FIG. It has changed the degree _{S 2} to 252mm. As a result, the thickness difference (Tc−Te) between the central part and the end part measured at the machine end can be reduced to about 0.5 mm, and the light rolling evaluation value ΔR = 4.5 mm, and the segregation degree C / Co is small. It was found that (applicable example). Meanwhile, for comparison, 2 charge th, was returned to the roll angle _{S 2} to 256mm origin, soft reduction evaluation value [Delta] R = 2.5 mm, and the segregation ratio is increased (Comparative Example).
That is, in the above-mentioned conformity example, a good slab of internal quality with very little center segregation was obtained, whereas in the above comparative example, the amount of light reduction was insufficient, and center segregation of C, Mn, etc. No good slab was obtained.

<Example 2>
Carbon steel with a carbon content of 0.1 mass% and a manganese content of 1.5 mass% (slab thickness 250 mm, slab) using the continuous casting equipment with the lightly pressed segment (segment length 2 m) shown in FIG. Continuous casting with a width of 2100 mm was performed, and the occurrence of center segregation was investigated in the same manner as in Example 1.

In this example, the standard casting speed is 1.6 m / min, the standard light reduction pattern is as shown in FIG. 2A, and the roll opening is S ₀ = 255 mm, S ₁ = 258 mm and S ₂ = 252 mm. The crater end position under the standard casting and cooling conditions is calculated to be 27 m in meniscus distance from solidification / heat transfer calculations. In the vicinity of the end of solidification (final solidification), a setting was made to lightly reduce 0.7 mm per meter in the casting direction, and the set light reduction speed per hour was set to 1.12 mm / min.

Also in this example, continuous charging for 2 charges and 500 tons was performed. The thickness gauge 4 at the end of the machine measured the thickness distribution in the width direction of the cast slab (slab) as needed. As a result, in the initial stage of casting (initial stage of solidification), there is no difference in thickness (Tc−Te) between the central part and the end part, which is approximately 0.0 mm, and the light rolling evaluation value ΔR is obtained from the above formula (1). It was 0 mm. In this embodiment, since the soft reduction evaluation value ΔR at least 2.5mm was set of reducing the soft reduction amount (reference), which in accordance with, and changing the opening S ₂ of the roll in the soft reduction to 254 mm. As a result, the thickness difference (Tc−Te) between the central portion and the end portion measured at the machine end can be set to 1.0 mm, the light rolling evaluation value ΔR can be set to 2.0 mm, and the segregation degree is reduced. (Conformity example). For comparison, 2 charge th, was returned to the roll angle S ₂ to the original setting is 252 mm, soft reduction evaluation value ΔR3mm next, it was found that segregation ratio increases (Comparative Example).

  That is, in the above-mentioned conformity example, a slab of good internal quality with little center segregation and no internal cracks was obtained, whereas in the above comparative example, the amount of light reduction was too large, the occurrence of internal cracks and the occurrence of C, Mn Negative segregation was observed, and a slab having good internal quality could not be obtained.

  The light rolling continuous casting technique of the present invention is not only applied to light rolling pattern (IBSR) casting under the continuous casting method of steel, but the same concept is applied to other continuous casting techniques. be able to.

DESCRIPTION OF SYMBOLS 1 Continuous casting equipment 2 Light reduction segment 3 Light reduction roll 4 Thickness meter 5 Calculation device

Claims (3)

When performing continuous casting of steel using a continuous casting apparatus in which a plurality of rolls are arranged from directly under the mold to the machine end,
Expand the roll opening in the middle of solidification as it goes downstream, leading to slab thickness expansion,
Next, in a continuous casting method in which light pressure drawing is performed to narrow the roll opening at the end of solidification as it goes downstream,
Measure the thickness of the slab after the light reduction at least at two locations other than the end in the width direction of the slab,
Information on the width direction end slab thickness Te, the slab thickness Tc other than the width direction end, the roll opening S _{0 at the} initial stage of solidification, the roll opening S ₁ during thickness expansion, and the roll opening S ₂ during light pressure reduction From this, a light reduction evaluation value (ΔR) is obtained, and the roll opening degree is controlled based on the light reduction evaluation value (ΔR).
This is a continuous light casting method for steel. The light reduction evaluation value (ΔR) is calculated based on the following formula (1), and at least one of the roll openings S ₁ and S ₂ is corrected based on the ΔR. Continuous casting method under light pressure.
ΔRmm = (S ₁ −S ₀ ) mm− (Tc−Te) mm (1) The middle solidification means between the liquid phase crater end of the slab excluding directly under the mold, and the end of solidification means between the liquid phase crater end and the solid phase crater end. The light-casting continuous casting method according to claim 1, wherein

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