JP2921305B2 - Steel continuous casting machine - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION The present invention relates to a vertical bending and bending type continuous casting machine, in which carbon steel or low alloy steel is formed into a thick slab or bloom having a slab thickness in which slab internal cracking easily occurs at a bent portion. The present invention relates to a continuous casting machine that can reliably prevent the occurrence of internal cracks in a slab at a bent portion even in continuous casting.

[0002]

2. Description of the Related Art Continuous casting machines are roughly classified into three types: a vertical type, a curved type, and a vertical bending type. With a vertical continuous casting machine, the casting height is high for high speed casting and the construction cost of the continuous casting machine is enormous, and it is almost impossible to extend the length of the caster after installation because the height is increased. There is a lack of flexibility such that the casting speed cannot be increased above the casting speed.

[0003] Further, the curved type has a horizontal portion as a mechanism, so that the machine length can be easily increased, and the productivity is improved by extending the machine length. However, since the unsolidified slab is already curved in the mold, There is a problem that inclusions in the unsolidified metal do not float to the meniscus (fluid surface) but adhere to a solidified shell (hereinafter, referred to as a shell) on the upper surface side and remain in the product as it is, thus adversely affecting product quality.

[0004] In these vertical molds and curved molds, since the unsolidified slab is not bent at a stage where the shell thickness is small, internal cracking at a shell thickness equivalent to that of a vertical bending and curved continuous caster is common. Less likely to occur.

On the other hand, in the vertical bending and bending type continuous casting machine, the problem of inclusions does not occur as in the case of the vertical mold because of the vertical portion, and the flexibility such as extension of the machine length is excellent because of the horizontal portion. There is an advantage. However, it is necessary to bend the unsolidified slab from a vertical to a constant radius (hereinafter, the machine height for performing this bending is referred to as a bent portion).

In this bent portion, in addition to tensile strain due to bulging, misalignment, and the like that also occur in the other two types of continuous casting machines, tensile strain due to bending that does not exist in the other two types of continuous casting machines acts. I do. Therefore, the vertical bending and bending type continuous casting machine has a structure in which internal cracks are more likely to occur at the bent portion.

The tensile strain ε _Be due to this bending acts on the outer shell with respect to the radius of curvature drawn by the thickness center of the slab, and is geometrically expressed by the following equation. Equation 1 ε _Be = (t / 2−d) (1 / R _i −1 / R _i −1) where t: slab thickness, d: shell thickness, R _i :
The radius of curvature between the i-th and i-1 rolls.

As can be seen from the above equation (1), the greater the distance from the center of the thickness, that is, when the shell thickness is constant, the greater the thickness of the slab, the greater the ε _Be . For this reason, the distortion due to bending is a so-called bloom (the slab thickness is generally 30%).
0 mm to 400 mm and the length ratio of long side to short side is 1.2 to
When casting a thick slab referred to as (1, 6), it is a problem as a main cause of internal cracks. In addition, bending a cast slab at one bending point greatly changes the radius of curvature and increases the tensile strain due to bending, which may cause internal cracks. Therefore, the bending of the slab is usually performed stepwise at a plurality of bending points.

Techniques for reducing tensile strain and preventing the occurrence of internal cracks have been proposed as follows. Japanese Patent Application Laid-Open No. 60-6257 discloses a method for straightening continuously cast slabs. In the process of straightening continuously cast slabs having an unsolidified layer, the temperature of the short-side shell of the slab is changed to the temperature of the long-side shell. By maintaining the temperature higher than that of the lower side shell and making the short side shell more susceptible to shear deformation, the strain acting on the longer side shell by bending correction is reduced, and the occurrence of internal cracks is prevented. This method is considered to be effective to some extent for slabs with small slab thickness, but for slabs and blooms with large slab thickness,
Since the short side shell is high in temperature and easily deformed, bulging deformation caused by the molten steel static pressure from the inside of the slab is increased, and there is a disadvantage that the short side shell is more likely to break out.

Japanese Patent Application Laid-Open No. Sho 59-118254, “Method for Preventing Bending and Straightening Cracks in Continuous Casting Machine” This is a method of supporting a shell on the side where a tensile strain acts on the bending portion or the upstream and / or downstream side of the straightening portion. Are provided with a driving mechanism and a braking mechanism, respectively, and apply a mechanical compressive force to the shell on which the tensile strain is acting, thereby relaxing the tensile strain.

[0011] Iron and steel, 73 (1987), S909
"Development of Online Internal Crack Diagnosis Prevention System" This is a method of determining the amount of strain for each roll and comparing it with cracks to diagnose the occurrence of cracks. As a method for preventing cracking, a compressive strain is applied to the shell of the slab corresponding to the roll position where the occurrence of cracking was diagnosed by the same device as described above to reduce the strain and at the same time reduce the casting speed.

The above strains and bending strains and correction strains, mainly focusing on misalignment strains, show measures to prevent internal cracks caused by these strains. In each case, the strain applied to each roll is determined. It only discusses the size. For example, the bending strain defined by Equation 1 is a strain generated at each bending point.

[0013] On the other hand, despite the above measures, the present inventors have considered that the cause of the failure to sufficiently eliminate internal cracks is the distortion (hereinafter referred to as "incremental distortion") that these internal crack measures are applied to each roll. It was considered that this was because it was merely considering the magnitude relation of the above, and it was not a global measure considering the effect of the history of distortion, so the following method was proposed.

Japanese Patent Application Laid-Open No. 3-174962, "Continuous casting method of steel" This is performed while each part of the slab is in a temperature range between at least the tensile strength appearance temperature (ZST) and the ductility appearance temperature (ZDT) during casting. A steel casting method characterized in that casting is performed under a condition that the sum of strain amounts (hereinafter referred to as cumulative strain) does not exceed a critical strain of a steel type to be cast.

However, since the above-mentioned invention basically deals with slabs, internal cracks frequently occur in continuous casting of slabs. However, even in the case of slabs, especially when the thickness of the slab is large or in bloom, measures for internal cracking at the bent portion which is the most problematic are not disclosed, and these thick slabs and bloom continuous casters are not disclosed. At present, internal cracks in the bent portion have not been eliminated.

[0016]

As described in the above prior art, there are still many problems when increasing the casting speed in continuous casting despite various measures for preventing internal cracks. . This is because the conventional measures against internal cracking do not consider that various strains received until the slab leaves the mold and completely solidifies remain as a history, and that the strain received earlier affects later. In addition, it is considered that the measures against internal cracks are limited to local portions where the cracks occur.

In the vertical bending and bending type continuous casting machine, since the above-mentioned strain history is not taken into consideration at the design stage, the design value of the incremental strain received by each roll is less than the limit value of the crack. Even if it is designed, the cumulative value of strain due to bending actually exceeds the limit value, so internal cracks may occur regardless of how to control misalignment distortion or bulging distortion in operation .

Further, in the conventional vertical bending and bending type continuous casting machine, in order to make the machine height as low as possible, the bending point rolls are arranged continuously, and have a structure most susceptible to the influence of strain history. In such a vertical bending type continuous casting machine, in a continuous casting of a slab or a bloom slab having a slab thickness of 300 mm or more, a vertical bending bending type continuous casting machine capable of reliably preventing internal cracks from being generated is provided. is there.

[0019]

In the above-mentioned prior art, the present inventors take (1) the strain (incremental strain) of each part of the slab individually, and in the conventional idea of discussing the magnitude thereof, the internal cracking is considered. That we cannot always predict the occurrence of
(2) Tensile appearance temperature (ZST) of each part of the slab during casting
And the ductility appearance temperature (ZDT), the strain received while in the temperature range is not relaxed every time the strain is received, but is accumulated as it is, and the sum of the accumulated strains, that is, the cumulative strain or

[0020]

[Equation 1] 2 equations

It has been shown that when the sum represented by the above two formulas exceeds the critical strain of the steel type to be cast, internal cracking occurs, and if it is less than that, no cracking occurs. Based on this finding, the state of occurrence of internal cracks in the existing vertical bending type continuous casting machine and its cause were investigated, and the following new finding was obtained.

(1) When slabs and blooms having a slab thickness of more than 300 mm are manufactured by a vertical bending type continuous casting machine, internal slab cracks occur frequently from the middle to the exit side of the bent portion.

(2) The structure of the bent portion of the continuous casting machine in which the internal cracks occurred was a general structure, that is, a continuous multipoint bending structure in which bending points were continuous.

(3) The multi-point bending in the continuous bending portion causes the value of each incremental bending strain at each bending point to be smaller than the limit value, but the internal strain because the cumulative strain exceeds the limit value. Cracks have occurred.

Therefore, in these continuous casting machines, in order to prevent the occurrence of internal cracks in the bent portion, it is necessary to dispose the slab support rolls so that the cumulative strain in the bent portion is less than the limit value. is there. The present invention has been completed based on this finding.

That is, the present invention relates to a vertical bending and bending type continuous casting machine for casting a slab or bloom of carbon steel or low alloy steel having a thickness of 300 mm or more, wherein the casting is stepwise from a vertical portion until a constant bending radius is obtained. In the bending part where the slab is bent, the internal cracks determine the total amount of tensile strain received while each part of the slab during solidification is in the temperature range between the tensile strength appearance temperature (ZST) and the ductility appearance temperature (ZDT). as less than generated limit distortion, the roll bending (bending point) group is changed before and after the curvature radius of the slab, select discontinuously from among the group of rolls for supporting the slab, the bending of each The bending roll and the support roll are arranged so as to include at least one or more support rolls on an arc drawn by a constant radius of curvature defined between the point and the bending point.

[0027]

FIG. 1 is an explanatory view schematically showing a solidification process of a slab in continuous casting. In the figure, the molten steel injected into the mold 1 via a tundish (not shown) cools down as it is drawn downward, and the solidified phase (shell) S gradually becomes thicker. This solidification phase S and liquid phase L in the center
There is a solid-liquid coexistence phase (S + L). It is known that each part of the slab during such continuous casting first undergoes a tensile strength appearance temperature (ZST) and a ductile appearance temperature (ZDT) from the state of the liquid phase to the complete solidification through the solid-liquid coexistence state. ing.

These ZST and ZDT are values determined by the type of steel, and the values are, for example, ArchEisen-hout
tenwessen, Vol. 54 (1983), p.
It can be measured by a known method shown in 367.
It is also known that the values of ZST and ZDT correspond well to the solid fraction (fs), and ZST almost matches the point of fs = 0.8, and ZDT almost matches the point of fs = 0.99 ( Iron and steel, 73 (1987), S896).

That is, at a certain point of the slab, fs <
If 0.8, the point is at a higher temperature than ZST,
No resistance to tensile force. Therefore, the point is not regarded as a solid that is continuous with the region of fs ≧ 0.8, and the strain generated by the deformation in the region of fs ≧ 0.8 is not transmitted, so that no internal crack occurs. If fs> 0.99 at a certain point on the slab, the point is at a temperature lower than ZDT, and since it has ductility, even if strain is applied, no internal crack occurs.

That is, the region where internal cracks may occur is a region where each part of the slab is in a temperature range of ZST or lower and ZDT or higher. For example, in FIG. 1, at a position P ₁ with a continuous casting machine, if fs is 0.8 at the point A of the inner slab, the temperature at point A is ZST, begin to appear tensile strength from here. Also, at the same point P ₁ ,
Assuming that the point B in the slab is a point where fs is 0.99, the temperature at this point is ZDT, from which ductility starts to appear.
Therefore, if the internal crack occurs in the position of the P ₁ point, between the point A point B in the cast piece, internal cracks will occur in the region where strain exceeds the occurrence limit of internal cracks.

Therefore, the determination of the presence or absence of internal cracks should be made based on the strain received while each part of the slab is in the temperature range of ZST to ZDT. Further, as described above, whether or not an internal crack has occurred is determined not by the magnitude of the strain that is incrementally received by each roll, but by the fact that each part of the slab is ZST to ZDT.
Must be performed based on the magnitude of the sum of the strains received while in the temperature range.

[0032] For example, in the position of P ₂ points in FIG. 1, it is assumed that the temperature point A was ZST at the position of the P ₁ points to the ZDT is reduced. Even if the value of the incremental strain newly added at the position of the point P ₂ is less than the internal crack limit between the point C as the ZST and the point A as the ZDT, the position of the P ₁ point ( for history incremental strain ε receiving while extending from ZST) to the position of P ₂ points (ZDT) remain, the sum of the incremental distortion (cumulative strain) is when the limit is exceeded, the internal cracks are generated . The limit value is determined by adding various tensile strains to the unsolidified ingot, and after the ingot has completely solidified, examine the cross section of the ingot and determine whether or not internal cracks have occurred. Can be measured by examining

Next, a description will be given of the reasons for limiting the constituent elements of the present invention. Each part of the slab during solidification is ZST and Z
The reason why the total tensile strain received during the temperature range between DT and DT is less than the limit strain for internal cracking is that, as described above, this is a necessary and sufficient condition for preventing internal cracking. by.

The reason for disposing the bending points discontinuously (intermittently) in the support roll group is that the incremental bending strain received at each bending point is a zone where internal cracks occur in the slab, that is, ZS.
This is to prevent accumulation in a certain region of the temperature range from T to ZDT, and prevent the occurrence of internal cracking due to the accumulated strain exceeding the limit value.

As shown in FIG. 6 , the bending point is the first bending point (10
No. 8 roll) from the eighth bending point (No. 17 roll) distortion when casting a bloom slab with a conventional vertical bending continuous casting machine that is arranged continuously (in the slab at each position of the continuous casting machine) Z
(Maximum value of incremental strain and maximum value of cumulative strain in ST to ZDT temperature range)
Is shown in FIG. Here, it was assumed that bulging, bending, and straightening were the only causes of distortion. Although that not fall below the limit strain in the concept of the traditional incremental strain for cumulative distortion exceeds the limit strain in the bending portion, in this continuous multi-point bending caster, internal cracks occur at bent portions. In addition, the bulging strain generated immediately below the rolls due to the slab swelling between the rolls due to the molten steel static pressure is very small compared to the bending strain. Is large.

Next, a method of intermittently arranging the bending points for reducing the accumulated strain will be described. For example, in FIG. 1, it is assumed that the point P1 is _one of the bending points. In P ₁ point is a region between of ZST~ZDT the A point B point in the slab. A
Point temperature cools to ZDT in time of the moving cast strip to _two points P. Therefore, in order to incremental bending strain received at _one point P does not overlap the incremental bending strain receive in the next bending point P ₁ points, may be arranged next bending point lower than P ₂ points.

Whether the intervals between the bending points are set apart so that the incremental bending strains at different bending points do not completely accumulate as described above, or whether a certain degree of accumulation is recognized depends on the magnitude of the accumulated strain and the steel to be cast. Of the internal crack limit strain of If the cumulative bending strain at a plurality of bending points is less than the critical strain even if the cumulative bending strain is accumulated, it is not necessary to further increase the interval between the bending points. Because the larger the interval between the bending points, the longer the slab needs to be bent to bend to the target radius of curvature,
This is because the height of the machine and the length of the machine are increased, and the production cost of the equipment is increased.

When determining the location of the bending point, the bending strain and bulging when the steel type having the smallest critical strain among the steel types to be manufactured in the continuous casting machine is cast at the maximum casting speed of the continuous casting machine are determined. The distortion and the misalignment distortion may be calculated, and the interval between the necessary bending points may be determined so that the cumulative distortion becomes less than the critical distortion.

When the present inventors conducted a tensile test on an unsolidified ingot and measured it, it was found that C: 0.4% by weight or more of carbon steel having a large cast amount in blooms and slabs with a slab thickness of 300 mm or more. In the alloy steel, the critical strain for internal cracking was 0.5%, which was the smallest value among the carbon steels and low-alloy steels targeted by the continuous casting machine. That is, the critical strain 0.
By suppressing the cumulative strain to less than 5%, the carbon content becomes 0.4%.
Even when casting a steel type of less than% by weight, the occurrence of internal cracks can be prevented.

Although misalignment distortion is not shown in FIG. 2, in the design of the continuous casting machine, misalignment in the cold caused by the limit of the control accuracy of the roll alignment, static pressure of molten steel during casting, and roll The maximum amount of hot misalignment caused by temperature rise etc. is estimated in advance, and bending is performed so that the cumulative strain of bending strain, bulging strain, and the above misalignment strain is less than the critical strain of cracking in consideration of this. Designing points is a desirable way to reliably prevent internal cracking.

As shown in the embodiment of the present invention in FIG. 3, at least one or more slab supporting rolls 4 are arranged between the bending points 3 for the following reasons. Installation is required.

That is, assuming that points P ₁ and P ₂ in FIG. 1 are adjacent bending points, respectively, the radius of curvature of an arc passing through these two points (Ri in equation 1) is determined by the spatial coordinates of the bending points. ) Is decided.

The reason why the slab supporting roll 4 is provided between the bending points 3 is that the radius of curvature of the slab at the time of casting is maintained at the above design value, and the value of the incremental strain at each bending point is reduced. This is to ensure that there is no difference between design and casting.

If there is no slab support roll on the arc between points P ₁ and P ₂ , the actual slab may deform arbitrarily through points P ₁ and P ₂ , By arranging at least one or more slab support rolls, the slab shape at the time of casting can be made into a circular arc in design. Also, in the case of a slab continuous casting machine, the interval between the bending point and the bending point is widened, and if the slab support roll is not arranged between them, the bulging distortion increases with an increase in the roll pitch, In order to prevent this, a slab support roll is required between the bending points. In this case, it is desirable to arrange the slab support rolls so as to narrow the roll pitch so that the incremental bulging strain is 1/10 or less of the incremental bending strain.

[0045]

An embodiment of the present invention will be described with reference to the drawings. FIG. 3 shows a roll layout of a continuous casting machine in which rolls of a bending portion are provided according to an embodiment of the present invention. This continuous casting machine is 10.1 m from 1.7 m from meniscus 2.
Is a vertical bending and bending type continuous casting machine that bends a slab to a bending radius of up to 15.5 m in the range described above. The slab is bent at five bending points from a first bending point to a fifth bending point, and these bending points are 1.7 m, 3.2 m, and 1.7 m from the meniscus 2.
They are located at 4.7m, 7.1m and 10.1m.

FIG. 4 shows that the continuous casting machine uses C: 0.
8%, Si: 0.2%, Mn: 0.5%, P: 0.01
%, S: carbon steel of 0.01%, slab thickness 310 mm, width 4
FIG. 9 shows the relationship between the position of ZST and ZDT and the distance from meniscus 2 when casting at a casting speed of 0.8 m / min into a 35 mm bloom.

As can be seen from FIG. 4, each bending point passes through the bending point up to twice while each part of the slab is in the ZST to ZDT temperature range, but does not bend more than three times. Are located. For example, the second bending point (No. 10 roll)
Part of the portion between the point A in the slab, which is ZST at the time of passing, and the point B, which is ZDT, causes bending distortion again when the slab passes the third bending point (No. 15 roll). Although it is allowed to accumulate bending strains, it completely solidifies between points A and B by the time of passing the fourth bending point, and even if it is bent at the fourth bending point, internal cracks occur. Danger zone (ZST to ZDT temperature zone)
Each bending point is arranged so that the accumulation of strain at the point does not occur.

FIG. 5A shows the relationship between the distance from the meniscus and the incremental strain when passing through the bending point, and FIG. 5B shows the relationship between the distance and the cumulative strain. As can be seen from FIG. 5B, the cumulative strain that occurs is less than the limit value of 0.5% at which internal cracking occurs even if distortion due to misalignment is included. Observation of the cross section of the slab cast by this continuous casting machine shows no occurrence of internal cracks, and according to the embodiment of the present invention, internal cracks of the slab can be reliably prevented.

On the other hand, as a comparative example, a conventional continuous multi-point bending type vertical bending and bending type continuous casting machine (8-point bending) shown in FIG. Steel was cast at the same casting speed. In this comparative example, the internal crack was 5 mm from 35 mm from the skin on the lower surface side of the slab.
It occurred in a range of 5 mm. The strains generated in the casting of this comparative example are shown in FIGS. 2A and 2B. FIG. 2A and FIG.
As can be seen from the comparison of A, although the value of the incremental bending strain generated is almost the same in both cases, in the comparative example, the cumulative bending strain after the fourth bending point (13th roll) is affected only by bulging and bending. The distortion exceeds the limit value of 0.5%. That is, in the case of the comparative example, it was found that even if the continuous casting machine was maintained to an ideal state in which no misalignment was present, internal cracks were generated.

From the results of the above-mentioned comparative test, it is impossible to prevent internal cracking of a bloom having a thick slab with a conventional continuous multipoint bending continuous casting machine. Indicates that a continuous casting machine in which bending points are distributed and arranged so that the cumulative strain according to the embodiment of the present invention is less than the limit value of internal cracks is required.

According to the present invention, the bending of the vertical bending type continuous casting machine is performed so that the strain generated inside the slab is less than the critical strain.
Discontinuous bending rolls in the section from support rolls
When selecting slabs and blooms with thick slabs without special equipment,
Can reliably prevent the occurrence of internal cracks in the slab.

[Brief description of the drawings]

[1] ZST continuous casting slab (P ₁ point) and ZDT (P ₂
FIG.

FIG. 2 shows ZST to ZDT inside a slab when cast by a conventional continuous bending type continuous casting machine of a continuous multipoint bending system.
It is a figure which shows the largest generation | occurrence | production distortion in a temperature range, FIG. 2A shows an incremental distortion, FIG. 2B shows cumulative distortion.

FIG. 3 is an explanatory view showing a roll layout of a vertical bending and bending type continuous casting machine according to an embodiment of the present invention.

FIG. 4 is a graph showing a relationship between a distance from a skin and a distance from a meniscus of ZST and ZDT inside a slab cast by a vertical bending and bending type continuous casting machine according to an embodiment of the present invention.

FIG. 5 is a diagram showing the maximum generated strain in a ZST to ZDT temperature range inside a slab when casting is performed by a vertical bending and bending type continuous casting machine according to an embodiment of the present invention. FIG. 5A shows incremental strain. 5B shows the cumulative strain.

FIG. 6 is an explanatory view showing a roll layout of a conventional continuous bending / bending type continuous casting machine of a continuous multipoint bending system.

[Description of Signs] 1 Mold 2 Meniscus 3 Bending point 4 Slab support roll

Claims (1)

(57) [Claims] 1. A carbon steel or low alloy steel having a thickness of 300 mm or more.
In a vertical bending and bending type continuous casting machine for casting a slab or bloom , in a bending portion that bends a slab stepwise from a vertical portion until a constant radius of curvature, each part of the slab during solidification has a tensile strength appearance temperature ( (ZST) and ductile appearance temperature (ZDT) Change the radius of curvature of the slab before and after so that the total amount of tensile strain received while in the temperature range between the two is less than the critical strain at which internal cracks occur the roll bending (bending point) group, select discontinuously from among the group of rolls for supporting the slab, at least one or more on an arc drawn by the constant curvature radius stipulated between the respective bending points and bending points A continuous casting machine for steel, wherein a bending roll and a supporting roll are arranged so as to include the supporting roll.