JP2964888B2 - Continuous casting method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a continuous casting method for preventing the center segregation of a slab by applying light pressure to the slab before it is completely solidified in continuous casting of steel.

[0002]

2. Description of the Related Art When producing a slab by a continuous casting method,
Often, internal defects called center segregation are problematic.
The center segregation is a phenomenon in which molten steel components such as C, S, P, Si, and Mn are positively segregated in a thickness center portion (final solidified portion) of a slab. The center segregation causes a decrease in toughness and hydrogen-induced cracking, and causes a serious problem particularly in a material for a thick plate.

The central segregation is caused by the fact that molten steel remaining between dendrite trees at the end of solidification moves macroscopically due to bulging or solidification shrinkage, and that the molten steel in which the above-mentioned components have been concentrated is locally accumulated. Is known to occur. As a countermeasure to prevent this center segregation, a method of suppressing the flow of concentrated molten steel by compensating for the solidification shrinkage of the terminal solidification part by reducing the vicinity of the solidification tip by some method has been considered effective. Have been proposed.

[0004] The degree of improvement of the center segregation due to the above-described reduction,
There is a clear correlation between the amount of reduction per unit time or the length in the unit casting direction (hereinafter simply referred to as "reduction amount") and the reduction timing. Quantitative studies have been made on the reduction amount and the reduction timing. I have.

For example, Japanese Patent Publication No. 59-39225 discloses that the degree of superheat of molten steel in a tundish is adjusted to 30 to 70 ° C.
A continuous casting method with a 2.0 mm / m reduction is shown.

[0006] Japanese Patent Publication No. 5-30548 discloses a reduction in the solidification time from the time when the center of the slab reaches the liquidus temperature to the time when the flow limit solid phase ratio is reached. Adjustment methods have been proposed.

However, in these methods, since the amount of reduction in the period near the crater end or in the center of the slab until the flow limit solid phase ratio is constant is constant, insufficient reduction or excessive reduction is likely to occur. There is. It is for the following reasons.

As will be described in detail later, the rate at which the set reduction amount is transmitted to the solid-liquid interface (hereinafter referred to as “reduction efficiency (α)”) decreases toward the downstream side of casting. Therefore, even if the same amount of reduction is applied from the surface, the amount of reduction transmitted to the solid-liquid interface changes at the time of solidification.

That is, when the coagulation contraction amount at an arbitrary position is used as a reference, the pressure is excessively increased on the upstream side and insufficiently reduced on the downstream side. As in the methods disclosed in the above-mentioned publications and the like, when a reduction is applied with a constant reduction amount over a long range, it is easy to induce overpressure or insufficient reduction in total. If the reduction is insufficient, the effect of preventing the center segregation is of course small. On the other hand, if the reduction is excessive, reverse V segregation occurs.

To solve the above problems, Japanese Patent Laid-Open No.
No. 90263, Japanese Patent Publication No. 5-73506 and Japanese Patent Publication No. 5
No. 73507 discloses a continuous casting method in which the rolling speed is increased toward the downstream side of casting. However, these methods still have the following problems. That is, the method disclosed in JP-A-3-90263 is
The range of the condition for increasing the rolling speed includes the condition of insufficient rolling to the condition of excessive rolling, and it is considered that the effect of improving center segregation cannot be stably obtained.

In the method disclosed in Japanese Patent Publication No. 5-73506, specific conditions regarding the amount of reduction for stably improving the center segregation are not clear.

In the method disclosed in Japanese Patent Publication No. 5-73507, the numerical value is limited by the roll reaction force. However, the relationship between the roll reaction force and the appropriate rolling reduction is not described, and in order to prevent flow due to solidification shrinkage. The idea of is not disclosed. Furthermore, the methods disclosed in the above three publications have a common problem in that the influence of the slab temperature distribution on the rolling conditions is not taken into account.

[0013]

It is known that light reduction at the end of solidification is effective in reducing the center segregation of a continuous cast slab, but it is difficult to select an appropriate reduction amount by the conventional method, and the reduction is difficult. Insufficient or excessive pressure has not improved the center segregation sufficiently, or even increased the center segregation.

An object of the present invention is to change casting conditions such as casting speed and secondary cooling conditions (slab slab surface temperature, and thus temperature gradient of slab slabs) and the progress of solidification inside the slab (change in unsolidified thickness). Regardless of the present invention, it is an object of the present invention to provide a continuous casting method capable of performing reduction under predetermined conditions and effectively preventing shrinkage flow which causes center segregation as compared with the conventional method.

[0015]

The gist of the present invention is a continuous casting method of the following (1) and (2).

(1) In a continuous casting method in which a light reduction is applied at the final solidification part of a continuous cast slab, the rolling start point is set to 0.0 ≦ fs ≦
0.2, and the rolling end point is set to 0.8 ≦ fs ≦ 1.0, during which continuous rolling is performed, and the total amount of rolling at the solid-liquid interface based on the flow limit solid fraction is 0.6 to 1.4 m.
m or less.

(2) In a continuous casting method in which a light reduction is applied at the final solidification part of a continuous cast slab, the rolling start point is set to 0.0 ≦ fs ≦
0.2, the rolling end point is set to 0.8 ≦ fs ≦ 1.0, during which continuous rolling is performed, and the rolling speed at the solid-liquid interface based on the flow limit solid fraction is 0.15 to 0.3 mm / mi.
A continuous casting method characterized by being not more than n.

In the above methods (1) and (2), fs is the thickness center solid fraction of the slab. The above-mentioned "solid-liquid interface" means the position inside the slab which has reached the flow limit solid fraction (about 0.7 to 0.8).

FIG. 2 shows that the unsolidified portion 3 is formed inside the solidified shell 4.
FIG. 4 is a schematic cross-sectional view of a slab for explaining definitions of parameters such as a rolling speed and a rolling efficiency of a solid-liquid interface 5 when a slab having slabs is reduced. When a reduction by a pinch roll is applied at a position 10 shown in the figure, each symbol shown in the figure is defined as follows.

Δ _S : Surface reduction amount (mm) L: Distance subjected to reduction (m) δ _I : Solid-liquid interface reduction amount (mm) V _C : Casting speed (m / min) If the reduction speed is R _I (mm / min), R _I = (δ _I / L) × V _C. In other words, reduction speed R _I of the solid-liquid interface is determined as reduction amount per unit time in the solid-liquid interface. The rolling efficiency α is expressed by α = (δ _I / δ _S ). Incidentally, [delta] _I, for example, Fe meniscus
It can be measured by adding a tracer such as S or Pb which has a large specific gravity and deposits on the solid-liquid interface, and tracks its movement.

[0021]

It has been known that there is an optimum light reduction amount for improving the center segregation, and the reduction amount depends on the slab surface temperature (in other words, the temperature distribution inside the slab), It was also empirically found to vary with the degree of coagulation. Therefore, when rolling down a slab having an unsolidified portion, the degree of solidification of the slab, secondary cooling conditions (temperature gradient in the slab)
It was thought that the rolling conditions had to be changed according to the conditions.

The inventor of the present invention has provided an index of light reduction conditions capable of coping with various casting conditions, and has been required to reduce the solid-liquid interface in order to more effectively perform the final solidification light reduction for improving center segregation. Focusing on behavior, he studied.

FIG. 5 shows the slab surface reduction speed R _S (mm / min) when the slab including the unsolidified portion is subjected to reduction in continuous casting of steel.
FIG. 4 is a diagram showing the result of examining the relationship between and center segregation. Here, the case where the slab surface temperature during rolling is 700-900 ° C and 900-900 ° C
It is divided into two cases for 1000 ° C. The center segregation was evaluated based on the degree of carbon segregation (C / C _O ) shown in Examples described later.

As shown in FIG. 5, as the surface temperature of the slab becomes higher, it is necessary to increase the surface rolling speed (R _S ) for lowering the degree of segregation (C / C _O ).

FIG. 6 (b) shows the results of an investigation of the state of occurrence of center segregation when light reduction was performed in three types of reduction patterns as shown in FIG. 6 (a). In the pattern 1 shown in FIG. 6A, that is, in the rolling mode in which the rolling gradient is increased with the progress of solidification, the degree of improvement of the center segregation is greatest. These results indicate that it is necessary to select a reduction amount according to the temperature distribution inside the slab and the degree of progress of solidification.

The reason why the results shown in FIGS. 5 and 6 are generated is considered as follows.

When the slab is rolled down by a roll, the amount of reduction applied from the surface does not propagate to the solid-liquid interface as it is, but the ratio of the amount of surface reduction to the amount of reduction transmitted to the solid-liquid interface (the aforementioned reduction efficiency α). Is less than or equal to 1.0. This is because the surface reduction amount is consumed for the expansion of the width of the slab, the advance of the slab, etc. due to deformation resistance, flow resistance, etc. of the unsolidified portion of the slab.

FIG. 7 shows an example of the relationship between the distance from the meniscus, that is, the distance in the casting direction and the rolling efficiency α. α
Was determined by the method shown in FIG. 2 as described above.

The inventor constructed a stress analysis model based on the experimental data or the finite element method and estimated the rolling behavior of the solid-liquid interface under various conditions.

As the solidification proceeds, the influence of the resistance of the unsolidified portion on the solid-liquid interface increases, so that α decreases.
Also, the higher the slab temperature (ie, the smaller the temperature gradient inside the slab), the lower the solidified shell stiffness, and the smaller the α because the surface reduction is more likely to be consumed in the wider and more advanced slab. . Therefore, even if the slab is reduced with a constant reduction gradient, as shown in FIG. 8, the reduction propagated to the solid-liquid interface due to the solidification time (distance from the meniscus) and the temperature gradient in the slab (slab surface temperature). The amount will be different.

Based on these results, the present inventors independently developed the rolling behavior at the solid-liquid interface in order to pursue parameters that can be universally arranged even if the degree of solidification and the temperature distribution in the slab differ. Investigation of the correlation between and the degree of center segregation revealed the phenomenon described below, and completed the present invention.

FIG. 3 shows the sum Σδ _{I of the} amount of reduction propagated to the solid-liquid interface based on the flow limit solid fraction and the degree of carbon segregation (C / C
_It shows the relationship with the center segregation evaluated in _O ).
This figure shows data over a wide range from 750 to 1100 ° C.

FIG. 3 shows the case where the rolling timing is changed to the following four types.

The slab thickness center solid phase ratio (the above fs) is
Continuous reduction from the point exceeding 0.0 to 0.8 (○ in Fig. 3) Continuous reduction in the range of 0.2 <fs <0.8 (（in Fig. 3) From 0 <fs to fs = 0.5 Continuous reduction (□ in FIG. 3) Continuous reduction from 0 <fs to fs = 1.0 (● in FIG. 3) In FIG. 3, the solid phase ratio that forms the solid-liquid interface is determined by the flow limit solid. The percentage is assumed.

As is apparent from FIG. 3, the center segregation is largely improved by controlling Σδ _I within the range of 0.6 to 1.4 mm regardless of the slab surface temperature.

[0036] From the above results, for the number of slab surface temperature is different over a wide range, it is clear that you can determine the range to reduce the center segregation in a factor called Sigma] [Delta] _I, the present invention using the Sigma] [Delta] _I It can be seen that the method has more universality than the conventional method of controlling the rolling reduction on the slab surface.

It is desirable that Σδ _I be distributed to substantially equal interfacial reduction speeds R _I (mm / min) during the reduction section. However,
It is distributed in the range of ± 20 to 30% of the variation that section significant improvement of center segregation is obtained by proper control of the Sigma] [Delta] _I if it is continuous pressure. In other words, even if it deviates from the appropriate range when viewed locally, it is only necessary to determine whether the rolling reduction will be insufficient or excessive under the rolling reduction range as a whole.

Next, from the viewpoint of the timing of applying the reduction (the range of the solid phase ratio fs at the center of the slab thickness) in FIG. 3, when comparing the center segregation with the same amount of Δδ _I , 0 <fs to fs = 0.8
In the case of continuous reduction in the range up to (upper circle in Fig. 3)
In the case of continuous reduction in the range of fs = 1.0 from 0 <fs to 0 <fs (the above-mentioned mark ● in FIG. 3), the center segregation is reduced most. Less than,
The segregation increases in the order of continuous reduction in the range of 0.2 <fs <0.8 (above, △ in FIG. 3) and continuous reduction from 0 <fs to fs = 0.5 (□ in FIG. 3). Has become.

This means that the solidification time at which the center segregation is formed is in the range from the solid phase ratio fs exceeding 0 to 0.8, and this range is surely reduced by an appropriate interfacial pressure reduction amount. Indicates that it is important to

However, even when the rolling timing is set to 0.2 <fs <0.8, the effect of improving segregation is considerably improved as compared with the case where the rolling timing is set to 0.2 <fs <0.8, and it is considered that the minimum range in which the rolling is required is in this range.

Based on the above findings, the above
The invention of (1), that is, `` the rolling start point is in the range of 0 to 0.2 in the thickness center solid phase ratio of the slab, and the rolling end point is 0.8 to 1.0
And continuous rolling is performed in the entire region, and the total amount of rolling at the solid-liquid interface is set to 0.6 to 1.4 mm.

Next, the invention of the above (2), that is, "the rolling start point is in the range of 0 to 0.2 in the thickness center solid fraction of the slab, the rolling end point is in the range of 0.8 to 1.0, The invention of a continuous casting method characterized in that a continuous reduction at a solid-liquid interface with a reduction speed of 0.15 to 0.30 mm / min is performed ".

FIG. 4 shows the amount of reduction per unit time propagating at the solid-liquid interface based on the flow limit solid fraction, that is, the above-mentioned interfacial reduction speed R _I and the center segregation (the carbon segregation degree, C /
It is also the result of investigation on the relationship of C _O ).

The rolling timing starts from 0 <fs, which is the optimum rolling timing for improving center segregation determined from FIG.
Cases up to 0.8 are shown. As apparent from FIG. 4, the proper range for the solid-liquid pressure rate R _I to the improvement of the center segregation of the interface is present. That is, R _I
When it is in the range of 0.15 to 0.30 mm / min, a large improvement in center segregation is obtained.

The optimum range of R _I is from the same as the shrinkage per unit time of the slab to about twice. That is, R _I that balances the flow of molten steel due to volumetric shrinkage of the slab
It is important to select 2 compared to volume shrinkage
The center segregation is improved even under conditions of about twice the overpressure.Besides volume shrinkage, there are other phenomena that cause molten steel flow, such as bulging between rolls.Therefore, it is necessary to perform reduction only to offset this. Because there is.

When R _I is smaller than the optimum range, center segregation increases due to insufficient rolling. On the other hand, when R _I is larger than the optimum range, center segregation also increases due to over-pressing (reverse v segregation occurs).

It is desirable that the conditions of the invention (1) and the conditions of the invention (2) are compatible. That is, "the reduction rate at the solid-liquid interface is 0.15 to 0.30 mm / min in the entire range where the solid fraction at the center of the thickness of the slab is greater than 0 and 0.8 or less, and the sum of the reduction amounts at the solid-liquid interface Is 0.6 ~
It is most desirable to continuously reduce the pressure to 1.4 mm. "

The method of the present invention can be applied to casting of slabs of various shapes such as slabs, blooms and billets, and can be applied to any position in the width direction of slabs such as slabs having a large aspect ratio.

[0049]

EXAMPLE 1 Continuous casting of a steel slab was performed using a continuous casting machine having a schematic structure shown in FIG.

This continuous caster is an S-type continuous caster with a radius of curvature of 12.5 m, and the length of the rolling zone is 5 m.

In FIG. 1, the molten steel 3 cast into the mold 1 from the immersion nozzle 2 is solidified through a support roll group 6, a pressing roll group 7, and a pinch roll 8, and is drawn out. The rolling roll group is composed of a plurality of rolling rolls, and the amount of rolling (rolling speed) can be adjusted by controlling the hydraulic pressure applied to the rolls. Table 1 shows casting conditions (other than rolling down conditions)
Are shown in Tables 2 and 3 below.

Examples 1 to 4 are examples of the present invention (1), in which continuous reduction is applied over the entire range where the thickness center solid phase ratio fs at the center of the width of the slab is in the range of 0.0 to 0.8. is when the total rolling amount Sigma] [Delta] _I received in the liquid interface was adjusted set reduction amount so that 0.6 to 1.4 mm. However, the interface reduction rate R _I is not necessarily contains in the range of the present invention (2).

Examples 5 to 8 are examples of the present invention (2), in which a continuous reduction is applied over the entire range of the thickness center solid phase ratio fs of the slab from 0.0 to 0.8, and interfacial pressure rate R _I in the phase rate interval is adjusted for setting reduction ratio so that 0.15~0.30mm / min.

On the other hand, in Comparative Examples 1 to 4, as a conventional example, when the rolling speed R _S of the slab surface was controlled, and in Comparative Example 5, the interfacial rolling speed R _I was controlled to a substantially appropriate value. If there thickness center solid phase ratio fs of the slab is in the range of 0.1 to 0.7, Comparative example 6 is the interface total rolling amount Sigma] [Delta] _I is controlled to a proper value, that was reduction of the slab thickness center Solid phase ratio fs is 0.
It is the case where it was in the range of 3 to 0.7.

[0055]

[Table 1]

[0056]

[Table 2]

[0057]

[Table 3]

Table 4 shows the results of investigation of center segregation in the examples and comparative examples. The degree of carbon segregation, which indicates the degree of center segregation, is measured by measuring the carbon concentration (C) of the center segregation part at the center of the slab width at 10 points by emission spectrometry, and the peak value and the average concentration
It was evaluated as (C _O) and a ratio of (C / C _O).

Further, in order to estimate the rolling state of the slab,
The segregation form at the center of the slab width was investigated. It is considered that V segregation corresponds to the state of insufficient reduction and reverse V segregation corresponds to the state of overpressure. Therefore, the segregation form is preferably neither V segregation nor reverse V segregation.

Examples 1 to 4 show various slab surface temperatures (Tsu).
But is obtained by controlling the interfacial total rolling amount Sigma] [Delta] _I to an appropriate value in a range of, in this case carbon segregation ratio also (C / C _O) is very low and 1.10 or less, V segregation in the slab longitudinal section No reverse V segregation was observed.

[0061] Examples 5-8, which is obtained by controlling the interfacial pressure rate R _I to an appropriate value in a range of different pressure zones entry side of the slab surface temperature (Tsu), either case the carbon segregation ratio (C / C _O ) showed a low degree of segregation of 1.10 or less. In addition, neither V segregation nor reverse V segregation was observed in the vertical section of the slab, and it was inferred that a good rolling state was obtained, which was neither insufficient rolling reduction nor excessive pressure reduction.

In Examples 1 to 8, the reason why a low degree of center segregation was obtained over a wide range of slab surface temperature was that the rate of the solid-liquid interfacial pressure reduction or the total amount of the solid-liquid interfacial pressure reduction was within an appropriate range. This is because the surface reduction speed was set.

On the other hand, in Comparative Examples 1 and 2, the rolling speed R _S from the slab surface was kept constant, but the degree of carbon segregation was higher than that of the Examples. Comparative Example 1
fs is smaller than the appropriate value interfacial pressure rate R _I is at the position of 0.4 or more, because the interface total rolling amount Sigma] [Delta] _I also smaller than the proper value, the V segregation at a reduction shortage center segregation caused deteriorates.

[0064] Comparative Example 2 is large in surface pressure rate R _I in the range of fs = from .0 to 0.7, since larger than the interface total rolling amount Sigma] [Delta] _I also proper value, the center segregation was exacerbated by excessive pressure. As described above, when the reduction in the range of fs = 0.0 to 0.8 is performed under the condition of the constant R _S , the reduction is likely to be insufficient or the overpressure is reduced, and the improvement of the center segregation cannot be obtained.

In Comparative Examples 3 and 4, the surface reduction speed R _S was increased toward the downstream side of the casting in consideration of the fact that the rolling efficiency from the surface of the slab became smaller toward the downstream side of the casting. The degree of carbon segregation was higher than that of the examples.

In Comparative Example 3, since the interfacial rolling speed R _I did not enter the proper value over the entire rolling range and the interfacial total rolling amount Σδ _I was smaller than the proper value, the center segregation was caused by insufficient rolling (V segregation). It got worse.

In Comparative Example 4, the interface reduction speed R _I at a position where fs was 0.4 or more was larger than an appropriate value, and the total interface reduction amount Δδ
_{Since I} was also larger than the appropriate value, center segregation worsened due to overpressure. Thus, increasing the surface pressure rate R _S as will cast downstream, it is not set the reduction amount so interfacial pressure rate R _I or surfactants total rolling amount Sigma] [Delta] _I is a proper value, the center segregation No improvement is obtained.

[0068] Comparative Example In 5, fs = the range of 0.1 to 0.7 interfacial pressure rate was controlled R _I to a proper value fs = 0.0 to 0.1,
Since the range of fs = 0.7 to 0.8 was not reduced, the degree of carbon segregation was higher than that of the example, and the segregation form of the longitudinal section was V segregation indicating insufficient rolling reduction.

[0069] Comparative Example 6 were pressure only in the range of fs = 0.3 to 0.7, but is obtained by controlling the interfacial total rolling amount Sigma] [Delta] _I to a proper value, fs = 0.0~0.30, a range of fs = 0.7 to 0.8 Since no reduction was performed, the interfacial reduction speed at fs = 0.3 to 0.7 increased, and the degree of carbon segregation was high because the overpressure was reduced in this range. From the results of Comparative Examples 5 and 6, fs = 0.
It can be seen that improvement of center segregation cannot be obtained unless proper continuous reduction is performed in the entire range of 0 to 0.8.

From the above results, it is clear that the continuous casting method of the present invention is more effective in reducing center segregation than the conventional method.

[0071]

[Table 4]

[0072]

According to the method of the present invention, regardless of the casting conditions, the reduction control may be performed only in accordance with the progress of solidification of the slab. In addition, this makes it possible to perform ideal rolling with no rolling reduction or excessive rolling reduction, and a great effect is obtained in reducing the center segregation of the slab.

[Brief description of the drawings]

FIG. 1 is a schematic structural view of a continuous caster for implementing a method of the present invention.

FIG. 2 is a schematic view of a section of a slab for explaining definitions of various parameters.

FIG. 3 is a diagram showing an example of the relationship between the total reduction amount Δδ _{I at} the solid-liquid interface and the degree of carbon segregation.

FIG. 4 is a diagram showing an example of the relationship between the solid-liquid interfacial reduction velocity R _I and the degree of carbon segregation.

FIG. 5 is a diagram showing an example of the relationship between the surface rolling velocity R _S and the degree of carbon segregation.

FIG. 6 is a diagram illustrating an example of a relationship between a rolling pattern and center segregation.

FIG. 7 is a diagram showing an example of transition of the rolling efficiency in the casting direction.

FIG. 8 is a diagram showing an example of transition of the interfacial reduction speed R _{I in} the casting direction.

[Explanation of symbols]

1: water-cooled copper mold, 2: immersion nozzle, 3: molten steel, 4: solidified shell, 5: solid-liquid interface 6: support roll group, 7: rolling roll group, 8: pinch roll, 9: non-pressed lower part 10: Depression

────────────────────────────────────────────────── ─── Continuation of the front page (56) References JP-A-8-132205 (JP, A) JP-A 8-132203 (JP, A) JP-A-7-299550 (JP, A) JP-A-6-132 262324 (JP, A) JP-A-4-309446 (JP, A) JP-A-4-279265 (JP, A) JP-A-4-22549 (JP, A) JP-A-63-63561 (JP, A) JP-A-62-158554 (JP, A) (58) Fields investigated (Int. Cl. ⁶ , DB name) B22D 11/20 B22D 11/128 350 B22D 27/09

Claims (2)

(57) [Claims] In a continuous casting method in which a light reduction is performed in a late solidification part of a continuous casting slab, a rolling start point is set to 0.0 ≦ fs ≦ 0.2.
The rolling end point shall be 0.8 ≤ fs ≤ 1.0, during which continuous rolling is performed, and the total amount of rolling at the solid-liquid interface based on the flow limit solid fraction shall be 0.6 to 1.4 mm or less. A continuous casting method characterized by the following. Here, fs is the thickness center solid fraction of the slab. 2. A continuous casting method in which a light reduction is applied in a final solidification portion of a continuous cast slab, wherein a rolling start point is set to 0.0 ≦ fs ≦ 0.2.
The rolling end point is set to 0.8 ≦ fs ≦ 1.0, during which continuous rolling is performed, and the rolling speed at the solid-liquid interface based on the flow limit solid fraction is set to 0.15 to 0.30 mm / min or less. A continuous casting method characterized by the following. Here, fs is the thickness center solid fraction of the slab.