JP4417899B2 - Continuous casting method - Google Patents

Description

  The present invention relates to a continuous casting method of steel, and particularly to medium carbon steel having a carbon content of 0.08 w% to 0.55 w%.

  In a general continuous casting method, the slab excessively bulges due to equipment failure such as roll gap increase due to roll wear, roll alignment abnormality, secondary cooling water spray equipment failure, etc., and internal cracks occur at the solidification interface. It is known to occur. Therefore, as the main means of suppressing bulging, which is the cause of internal cracks, narrowing the roll pitch, uniform cooling of the slab by optimizing the spray amount of the secondary cooling water, strengthening the management of roll alignment, reducing the casting speed, etc. Is listed.

  However, in view of needs such as higher casting speed to improve productivity and application of slabs to ultra-thick steel plates with low pressure ratio (for example, more than 100 mm thick), the actual damage caused by internal cracks is significant. Further technology for prevention is expected.

Patent Document 1 discloses a continuous casting method in which each part in a slab is managed so that the total amount of strain, which is the total amount of strain received by the reduction of the driving roll, becomes a predetermined value or less. According to this continuous casting method, it is possible to efficiently produce a slab having good internal quality without internal cracks at the maximum casting speed according to the steel type to be cast.
JP-A-4-75754

However, the continuous casting method described in Patent Document 1 does not disclose specific calculation conditions (such as input data).
The present invention has been made in view of such various points, and its main purpose is to grasp the inner cracking mechanism of the slab in more detail and appropriately deal with it, thereby casting a slab with very few inner cracks. It is to provide an objective and quantitative continuous casting method.

Means and effects for solving the problems

  The problems to be solved by the present invention are as described above. Next, means for solving the problems and the effects thereof will be described.

Slab,
While guiding and supporting with a group of rolls arranged at intervals of 200 mm to 400 mm in the casting direction,
The casting speed is 1.2 m / min or more and 1.9 m / min or less and is continuously cast.
In the continuous casting method of medium carbon steel,
Within a section of 5 m or more and 20 m or less from the meniscus toward the casting direction, the strain accumulation distance (arbitrarily selected from the section determined by the formula (1) based on the casting speed (Vc [m / min]) ( In Lst [m]), the roll gap of each roll pair is adjusted so as to satisfy the condition of the expression (2).
Lst = 2.1 × Vc (1)

However,
G _i [mm] is the i-th roll gap counted from the mold side among the n pairs of rolls simultaneously existing within the strain accumulation distance,
f (x) = x in the range of x> 0,
f (x) = 0 in the range of x ≦ 0,
age,
In the range where the carbon content of the slab is 0.08 w% or more and less than 0.10 w%, A = 2.0 [mm],
In the range where the carbon content of the slab is 0.10 w% or more and less than 0.20 w%, A = 1.5 [mm],
A = 1.0 [mm] in the range where the carbon content of the slab is 0.20 w% or more and 0.55 w% or less,
And
It should be noted that the condition of Expression (2) is satisfied at any arbitrarily selected strain accumulation distance Lst.

  Thereby, a slab with very few internal cracks can be cast. This continuous casting method is particularly useful in high speed casting in which the casting speed exceeds 1.2 m / min. Moreover, since this continuous casting method is extremely objective and quantitative, it can be easily introduced into the casting field.

  Embodiments of the invention will be described below.

First, based on FIG.1 and FIG.2, the structure and operation | movement of a continuous casting installation are demonstrated. FIG. 1 is a side sectional view of a continuous casting facility. FIG. 2 is a partially enlarged view of FIG. 1, schematically showing two pairs of rolls adjacent in the casting direction. For convenience of explanation, the roll interval in FIG. 2 does not correspond to that in FIG.
As shown in FIG. 1, a continuous casting machine 100 according to this embodiment includes a mold 1 for solidifying molten steel into a predetermined shape, a tundish 2 for pouring molten steel into the mold 1 at a predetermined flow rate, and a downstream side of the mold 1. It comprises a plurality of roll segments 3, 3, 3... Arranged side by side and a gas type cutter 4 that cuts the slab into a predetermined length.

Each of the plurality of roll segments 3, 3, 3... Supports and guides a cast piece to be cast, and a pair of rolls 3 a, 3 a provided so as to sandwich both wide surfaces of the cast piece. 3 pairs.
In addition, each of the roll segments 3, 3, 3... Is individually provided with an unillustrated cooling water spray device that can cool the slab by spraying water. 3.3 is also called a secondary cooling zone. The primary cooling means a cooling jacket (not shown) embedded in the inner wall of the mold 1.

The continuous casting machine 100 is configured as a so-called vertical sequential bending die. The vertical sequential bending die means an arrangement mode of the plurality of rolls 3a, and a vertical region in which the plurality of rolls 3a are arranged in the vertical direction directly under the mold 1, and the vertical region An arc region that is arranged downstream in a circular arc shape, a horizontal region that is downstream of the arc region and arranged in a horizontal direction, and a correction that smoothly connects the arc region and the horizontal region And a region.
In the present embodiment, the plurality of rolls 3a, 3a, 3a,... (Roll group) are arranged at intervals of 200 mm or more and 400 mm or less in the casting direction.

  As shown in the figure, the gas cutter 4 is provided in the horizontal region.

  The molten steel poured into the mold 1 forms a shell (solidified shell) from the portion in contact with the mold 1, and the shell grows as it proceeds in the casting direction, and eventually completely solidifies to the inside. Is to be formed. In the present embodiment, the casting speed of the slab is set to 1.2 m / min or more and 1.9 m / min or less, which is higher than a general casting speed, in order to improve productivity.

Moreover, the continuous casting method in this embodiment is intended for medium carbon steel having a carbon content of 0.08 w% or more and 0.55 w% or less, and from the meniscus as a molten steel surface in the mold 1 toward the casting direction. Strain accumulation distance (Lst [m]) arbitrarily selected from the above section within the interval of 5 m or more and 20 m or less, which is obtained by the formula (1) based on the casting speed (Vc [m / min]). In Fig. 2, the roll gap G between the roll pairs 3a and 3a is adjusted so as to satisfy the condition of the expression (2) (see also Fig. 2).
Lst = 2.1 × Vc (1)

However,
G _i [mm] is the i-th roll gap G counted from the mold 1 side among the n pairs of rolls 3a and 3a simultaneously existing within the strain accumulation distance Lst,
F (x) = x in the range of X> 0,
F (x) = 0 in the range of X ≦ 0,
age,
In the range where the carbon content of the slab is 0.08 w% or more and less than 0.10 w%, A = 2.0 [mm],
Similarly, A = 1.5 [mm] in the range of 0.10 w% or more and less than 0.20 w%,
Similarly, A = 1.0 [mm] in the range of 0.20 w% to 0.55 w%.
It should be noted that the condition of Expression (2) is satisfied at any arbitrarily selected strain accumulation distance Lst.

Also, as shown in FIG. 2, the roll gap G is the shortest distance between the roll surfaces of a pair of roll pairs 3a and 3a facing each other across the slab, whereas the roll interval is the roll 3a. The center-to-center distance between 3a.
Further, the strain accumulation distance Lst is a distance in the casting direction obtained by the equation (1).

  Hereinafter, the derivation process of Formula (1) and Formula (2) is demonstrated.

The inventor of the present invention, as a result of repeated studies to grasp the internal crack mechanism of the slab in more detail and quantitatively, as a result, the distortion of the solidification interface, which is the cause of the internal crack, is two pairs ( It was ascertained that they are closely related to the roll gaps G _i and G _{i + 1 in the} two pairs of roll pairs 3a and 3a (see FIG. 2).
In particular, the roll gap G _i in the roll-to-3a _i · 3a _i casting direction upstream side (hereinafter, also referred to as upper roll gap G _i.) And compared, in the same casting direction downstream roller pairs 3a i _{+ 1} · 3a _{i + 1} roll gap G _{i + 1} in (hereinafter, also referred to as a lower roll gap G _{i + 1.)} is narrow, coagulation occurs when the slab is pressed by a roll-to-3a i _{+ 1} · 3a i + ₁ on the downstream side It was found that the interfacial strain was significantly larger than other operating factors (for example, the surface temperature of the slab and roll alignment).
Conversely, when the lower roll gap G _{i + 1} is wider than the upper roll gap G _i , it has been found that almost no distortion occurs at the solidification interface.

Here, a value obtained by subtracting the lower roll gap G _{i + 1} from the upper roll gap G _i is defined as a "roll gap reduction ΔGd". That is, ΔGd = G _i −G _{i + 1} . Accordingly, positive ΔGd means that the roll gap G is narrowed, while negative ΔGd means that the roll gap G is widened.
In other words, it can be said that it is necessary to appropriately manage the roll gap reduction amount ΔGd (especially positive ΔGd) in order to suppress the distortion of the solidification interface which is the cause of the internal crack.

  Therefore, the inventor of the present invention investigated the relationship between the roll gap reduction amount ΔGd and the amount of strain at the solidification interface in more detail. Specifically, the solidification interface strain amount in three cases of zero, positive, and negative when the roll gap reduction amount ΔGd is zero was obtained by numerical calculation for one roll interval. The calculation results are shown in FIG. As calculation conditions, the roll interval was 355 mm, the shell thickness (see FIG. 2) was 100 mm, and the roll diameter was 145 mm.

[FIG. 3 Symbol A ΔGd = 0 [mm]]
When the upper roll gap G _i is equal to the lower roll gap G _{i + 1} , that is, when the roll gap reduction amount ΔGd is zero, the calculation result of the solidification interface strain amount for one roll interval is indicated by a symbol A in FIG. (See also FIG. 2).
From this result, when the upper roll gap G _i is equal to the lower roll gap G _{i + 1} , the amount of solidification interface strain is substantially equal to zero for a while after passing through the upstream roll pair 3 a _i , 3 a _i , After that, it was found that the bulging slab was pressurized by the lower roll pair 3a _{i + 1} · 3a _{i + 1 to} cause some tensile strain at the solidification interface.
[Fig. 3 Symbol B ΔGd = 1 [mm]]
When the lower roll gap G _{i + 1} is 1 mm narrower than the upper roll gap G _i , that is, when the roll gap reduction amount ΔGd is 1 mm, the result is indicated by B (FIGS. 2 and 4A are also shown). See).
From this result, when the lower roll gap G _{i + 1} is narrower than the upper roll gap G _i , the solidification interface strain amount of the slab is remarkably increased when passing through the lower roll pair 3a _{i + 1} · 3a _{i + 1.} In addition, it was found that the amount of distortion reaches about three times as compared with the case where ΔGd = 0 [mm] (reference A).
[Fig. 3 Symbol C ΔGd = −1 [mm]]
When the lower roll gap G _{i + 1} is 1 mm wider than the upper roll gap G _i , that is, when the roll gap reduction amount ΔGd is −1 mm, the result is indicated by the symbol C (FIGS. 2 and 4B are also shown). See also).
From this result, it was found that when the lower roll gap G _{i + 1} is wider than the upper roll gap G _i , almost no distortion occurs at the solidification interface of the slab.

  As can be seen from the above calculation results, in order to suppress the distortion of the solidification interface, which is the cause of the internal crack, the spread of the roll gap G (negative ΔGd) does not need to be considered much, but the roll gap G It can be said that it is important to strictly suppress the narrowing (positive ΔGd).

FIG. 5 is a diagram showing the relationship between the maximum value of the solidification interface strain amount and the roll gap reduction amount ΔGd for one roll interval in a general medium carbon steel having a carbon content of 0.10 w% to 0.20 w%. is there. The solidified interface strain maximum value means the peak value of the solidified interface strain amount appearing in FIG.
As can be seen from FIG. 5, it can be said that the roll gap reduction amount ΔGd and the maximum value of the solidification interface strain amount are in a substantially proportional relationship. In addition, the broken line in this figure shows the minimum amount of strain (internal crack occurrence minimum strain amount) in which the medium carbon steel can generate an internal crack. From this figure, it can be seen that if the roll gap reduction amount ΔGd in one roll interval exceeds a certain value (about 1.68 mm in this figure), internal cracks may occur.
Therefore, in order to prevent internal cracks, the roll gap reduction amount ΔGd for one roll interval may be set to a certain value or less so that the solidification interface strain amount does not exceed the above-mentioned minimum internal crack generation strain amount. I understand.

However, it is conventionally known that the strain at the solidification interface has a property of accumulating.
Therefore, even if the roll gap reduction amount ΔGd in one roll interval is equal to or less than the predetermined value and the corresponding solidification interface strain amount is less than the minimum internal crack generation strain amount, the strain accumulates. Due to the nature, it is considered that internal cracks will occur as a result.

Therefore, the inventor of the present invention investigated in detail the property of accumulating solidification interface strain.
The outline is as follows: (a) the average growth rate Vs of the shell is defined, (b) the internal crack length D of the slab is measured, and (c) the strain is determined based on the average growth rate Vs and the internal crack length D of the shell. The accumulation distance Lst is obtained, and (d) the strain accumulation time T is obtained from the casting speed Vc and the strain accumulation distance Lst.
(A) [Definition of average growth rate Vs of shell]
First, the relationship between the shell thickness Ds and the elapsed time t will be described with reference to FIG. In general, it is known that the shell thickness Ds is proportional to the 0.5th power of the elapsed time t from the start of solidification as in the following formula (3a).
(Shell thickness Ds) = (coagulation constant K) × (elapsed time t) ^0.5 (3a)
The elapsed time t in equation (3a) can be replaced as in equation (3b).
(Elapsed time t) = (meniscus distance Lme) / (casting speed Vc) (3b)
The meniscus distance Lme means a length in the casting direction with the meniscus as a base point.
Here, the average growth rate Vs is defined as the average value of the gradient of the shell thickness Ds with respect to the meniscus distance Lme. Specifically, the average growth rate Vs is obtained by linearly approximating the shell thickness Ds gradient in the range of the meniscus distance of 5 to 20 m obtained from the equations (3a) and (3b) as shown in FIG. Table 1 shows the average growth rate Vs at each casting rate Vc.

(I) [Measurement of internal crack length D of slab]
Incidentally, FIGS. 7A to 7C are diagrams showing the internal crack length D at each casting speed in a histogram. This internal crack length D is measured in the following manner.
As shown in FIG. 8, the cross-sectional sulfur print test of the slab was performed a plurality of times, and the length of the detected internal crack (however, limited to the growth direction of the shell) was measured. The measurement was performed about 35 to 55 times for each casting speed.
As can be seen from FIGS. 7A to 7C, the distribution of the internal crack length D of the slab at each casting speed Vc was substantially the same. Moreover, each average value Dav of the said internal crack length D calculated | required from Fig.7 (a)-(c) became all about 10 mm. This result is also shown in Table 1 above.

(C) [Calculation of strain accumulation distance Lst]
By the way, the strain accumulation distance Lst (the maximum value of the distance at which the slab moves in the casting direction while accumulating strain) is expressed by the equation (4) based on the average internal crack length Dav and the average growth rate Vs (dimensionless number). It can be estimated as follows. This is because the nature of strain accumulation and internal cracks are interrelated.
(Strain accumulation distance Lst) = (Average value of internal crack length Dav) / (Average growth rate Vs) (4)

(D) [Calculation of strain accumulation time T]
Similarly, the strain accumulation time T (maximum time during which strain accumulation can be continued) can be obtained as shown in Expression (5) based on the strain accumulation distance Lst and the casting speed Vc. Table 1 shows the strain accumulation distance Lst and the strain accumulation time T at each casting speed Vc.
(Strain accumulation time T) = (Strain accumulation distance Lst) ÷ (Casting speed Vc) (5)

It can be seen from Table 1 that the strain accumulation time T is always constant at about 2.1 min regardless of the casting speed Vc. Therefore, it can be said that the strain accumulation distance Lst can be approximately estimated by the following formula (1) using the casting speed Vc.
(Strain accumulation distance Lst) = 2.1 × (Casting speed Vc) (1)

  Therefore, in consideration of the above-described property of accumulation of strain, it is not sufficient to set the roll gap reduction amount ΔGd for one roll interval to a certain value or less in order to prevent internal cracks. It can be said that it is necessary to consider all the roll gap reduction amounts ΔGd within the expressed casting direction length. Therefore, it is considered that the total sum P of the roll gap reduction amount ΔGd at each roll interval needs to be always equal to or less than a certain value A as in the above formula (2). As described above, the negative ΔGd is not taken into consideration when determining the total sum P of the roll gap reduction amount ΔGd.

Therefore, the inventors of the present invention set the above constant value A as follows according to the carbon content of the slab. This is because it has been clarified by the inventor's test research that the higher the carbon content, the easier the internal cracks are generated.
In the range where the carbon content of the slab is 0.08 w% or more and less than 0.10 w%, A = 2.0 [mm],
Similarly, A = 1.5 [mm] in the range of 0.10 w% or more and less than 0.20 w%,
Similarly, A = 1.0 [mm] in the range of 0.20 w% to 0.55 w%.

Hereinafter, the test for confirming the technical effect of the continuous casting method in this embodiment is demonstrated. The comparative example implemented without applying this continuous casting method first is demonstrated, and the Example implemented by applying this continuous casting method is described next.
In both of these comparative examples and examples, the mold size is width 2100 × thickness 280 mm, the steel type is medium carbon steel with a carbon content of 0.10 to 0.20 w%, and the length of each roll segment 3 in the casting direction. Was 2.5 m, and the casting speed was 1.2 m / min. The results of the casting test for one month are shown in FIGS. 9 and 10, respectively. 9 (a) and 10 (a) both show the relationship between the internal crack occurrence index and the shell thickness Ds at the time of internal crack occurrence, and FIGS. 9 (b) and 10 (b) both show the above formula (1). 3 shows the relationship between the total sum P of the roll gap reduction amount ΔGd calculated for each strain accumulation distance Lst obtained in (1) and the meniscus distance Lme.
In addition, the internal crack occurrence index in FIGS. 9A and 10A is “cracked” when one or more internal cracks are detected as a result of the cross-sectional sulfur print test (see FIG. 8). When no detection was made, “no crack” was assumed, and the occurrence rate was quantified as follows.
・ (Internal crack occurrence index) ∝ (number of tests for “cracking”) / (number of cross-sectional sulfur print tests)
Further, the roll gap reduction amount ΔGd in FIGS. 9B and 10B was measured by a roll gap measuring device attached to the dummy bar.

[Comparative Example]
According to FIG. 9A, it can be seen that an internal crack occurred when the shell thickness Ds was 80 to 110 mm. Referring to FIG. 6 (a), it can be seen that the shell thickness Ds range of 80 to 110 mm corresponds to a meniscus distance Lme of about 15 to 20 m. The roll segments 3 arranged in the meniscus distance range were the ninth to eleventh ones counted from the meniscus side. Therefore, it is considered that there was some problem in the ninth to eleventh roll segments 3.
According to FIG. 9B, when the meniscus distance Lme is in the range of approximately 17.5 to 20 m, the total P of the roll gap reduction amount ΔGd exceeds the fixed value A (1.5 mm) set as described above. You can see that
Here, when FIG. 9A is compared with FIG. 9B, the meniscus distance Lme at which the internal crack has started to occur substantially coincides with the meniscus distance Lme at which the total sum P has started to exceed a certain value A. It can be seen that internal cracks frequently occur depending on the degree of the increase.
Therefore, in order to prevent internal cracking, it can be said that it is necessary to adjust the roll gap G between the roll pairs 3a and 3a included in the ninth to eleventh roll segments 3.

〔Example〕
Therefore, in this embodiment, as shown in FIG. 10B, the roll pairs provided in the ninth to eleventh roll segments 3 so that the total sum P of the roll gap reduction amount ΔGd at the strain accumulation distance Lst does not exceed 1.5 mm. The roll gap G of 3a and 3a was adjusted in advance before the casting test.
According to FIG. 10A, it can be seen that after the above adjustment, internal cracking can be effectively prevented in any growth period of the shell thickness Ds, including the growth period when the shell thickness Ds is 80 to 110 mm. .

  FIG. 11 is a diagram showing the relationship between the maximum value of the total sum P of the roll gap reduction amount ΔGd (see FIG. 9B) and the occurrence of internal cracks in each steel type having a different carbon content. As shown in this figure, internal cracking can be substantially avoided in any steel type by appropriately setting the constant value A that defines the upper limit of the total sum P of the roll gap reduction amount ΔGd according to the carbon content. I was able to.

As described above, in this embodiment, the casting speed is 1.2 m / min or more and 1.9 m / min or less while the slab is guided and supported by a group of rolls arranged at intervals of 200 mm or more and 400 mm or less in the casting direction. In the continuous casting method of medium carbon steel, which is continuously drawn and drawn, the following effects are achieved by adjusting the roll gap G as follows.
Within a section of 5 m or more and 20 m or less from the meniscus toward the casting direction, the strain accumulation distance (arbitrarily selected from the section determined by the formula (1) based on the casting speed (Vc [m / min]) ( In Lst [m]), the roll gap G of each roll pair is adjusted so as to satisfy the condition of the expression (2).
Lst = 2.1 × Vc (1)

However,
G _i [mm] is an i-th roll gap G counted from the mold 1 side among the n pairs of roll pairs 3a and 3a existing within the strain accumulation distance Lst,
f (x) = x in the range of x> 0,
f (x) = 0 in the range of x ≦ 0,
age,
In the range where the carbon content of the slab is 0.08 w% or more and less than 0.10 w%, A = 2.0 [mm],
In the range where the carbon content of the slab is 0.10 w% or more and less than 0.20 w%, A = 1.5 [mm],
A = 1.0 [mm] in the range where the carbon content of the slab is 0.20 w% or more and 0.55 w% or less,
And

(effect)
Thereby, a slab with very few internal cracks can be cast. This continuous casting method is particularly useful in high speed casting in which the casting speed exceeds 1.2 m / min. Moreover, since this continuous casting method is extremely objective and quantitative, it can be easily introduced into the casting site.

Side surface sectional drawing of a continuous casting installation. The elements on larger scale of FIG. The figure which shows the relationship between roll gap reduction amount (DELTA) Gd for one roll space | interval, and the amount of solidification interface distortion. Schematic diagram of slab bulging considering creep. The figure which shows the relationship between the solidification interface distortion amount maximum value and roll clearance gap reduction amount (DELTA) Gd for 1 roll space | interval. The figure which shows the gradient of the shell thickness Ds with respect to the meniscus distance Lme. The figure which shows the internal crack length D in each casting speed with a histogram. Explanatory drawing of the cross-sectional sulfur print test of a slab. The figure which shows the result of the casting test in the comparative example The figure which shows the result of the casting test in an Example. The figure which shows the relationship between the maximum value of the sum total P of roll clearance gap reduction amount (DELTA) Gd, and the internal crack generation condition in each steel type from which carbon content differs.

Explanation of symbols

3a Roll G Roll gap ΔGd Roll gap reduction amount G _i Upper roll gap G _{i + 1} Lower roll gap Vc Casting speed Lme Meniscus distance 100 Continuous casting equipment

Claims (1)

Slab,
While guiding and supporting with a group of rolls arranged at intervals of 200 mm to 400 mm in the casting direction,
The casting speed is 1.2 m / min or more and 1.9 m / min or less and is continuously cast.
In the continuous casting method of medium carbon steel,
Within a section of 5 m or more and 20 m or less from the meniscus toward the casting direction, the strain accumulation distance (arbitrarily selected from the section determined by the formula (1) based on the casting speed (Vc [m / min]) ( Lst [m]), the roll gap of each roll pair is adjusted so as to satisfy the condition of the formula (2), and the continuous casting method of medium carbon steel,
Lst = 2.1 × Vc (1)
However,
Gi [mm] is the i-th roll gap counted from the mold side among the n pairs of rolls simultaneously existing within the strain accumulation distance,
f (x) = x in the range of x> 0,
f (x) = 0 in the range of x ≦ 0,
age,
In the range where the carbon content of the slab is 0.08 w% or more and less than 0.10 w%, A = 2.0 [mm],
In the range where the carbon content of the slab is 0.10 w% or more and less than 0.20 w%, A = 1.5 [mm],
A = 1.0 [mm] in the range where the carbon content of the slab is 0.20 w% or more and 0.55 w% or less,
And