JPH02274354A - Method for continuously casting steel - Google Patents

Abstract

PURPOSE:To prevent inner cracking in a cast slab from occurring in advance by comparing the total strain calculated with the specific equation from a contributing effect factor for strain difference set according to operational conditions with the limited strain and changing the casting conditions. CONSTITUTION:The widths at upper and lower faces are calculated with a computing element 11 from the signal of a cast slab width measuring instrument 10 at inlet side and outlet side in straightening zone, and an neutral shifting quantity eta and the strain difference epsilon are calculated in every moment from the equation I with a strain computing element 12. Further, information on condition from a casting condition instruction device and measured result from roll gap and arrangement measuring instrument 9 are transmitted to a strain computing element 12, and bulging strain epsilonB and straightened strain epsilonU are obtd. to operate a roll irregular strain epsilonM. Then, each strain is calculated in every moment with the strain computing element 12, and the total strain epsilonT in the cast slab is calculated with the strain computing element 12 referring to the pre-obtd. contributing effect factor alpha and limited strain epsilonC from the casting condition, and compared with the limited strain epsilonC. Then, when the total strain epsilonT becomes more than the limited strain epsilonC, this is outputted to a working instruction device 15 and the development of the inner cracking is prevented.

Description

[Detailed Description of the Invention] (Industrial Application Field) The present invention is used in continuous steel casting to predict the occurrence of internal cracks in slabs, and to quickly cause cracks to occur when it is predicted that cracks will occur. The present invention relates to a method for stably supplying defect-free slabs to the rolling process without operational troubles by changing the operating method to prevent such problems and not supplying defective slabs to the rolling process, which is a subsequent process.

[Prior Art] FIG. 9 is a schematic side view showing a general curved continuous casting equipment, in which 1 is a tundish that stores molten steel, 2 is an immersion nozzle for injecting molten steel, and 3 is a mold. Mold 3
The molten steel injected into the mold forms a solidified shell 5 from the surface in contact with the mold wall, and becomes a slab 6 having a predetermined cross-sectional shape. This slab 6 is continuously pulled out by a drawing roll 72 while being supported by a guide support roll 71. Further, the slab 6 is processed by a group of straightening rolls 73 with unsolidified molten steel 4 remaining in the core.
In the case of a vertical continuous casting machine, it is straightened from the vertical direction to the horizontal direction, and in the case of a curved continuous casting machine, it is straightened from a curved state to a horizontal state. Hereinafter referred to as the correction area, the correction operation in this single correction area is hereinafter referred to as straight correction),
Gas cutting is performed after the completely solidified machine end 8.

Now, when the slab is straightened in the horizontal direction by the group of straightening rolls 73, a straightening strain εC is added to the inside of the slab in addition to bulging strain ε1 due to bulging between the rolls and roll misalignment strain ε4 due to irregular roll arrangement. It is generally known that internal cracks in the pieces are likely to occur.

Particularly in recent years, the direct connection of the continuous casting process and the rolling process has been promoted, and in order to efficiently carry out this process, it is necessary to produce slabs at as high a temperature as possible in the continuous casting process. It is a well-known fact that in order to obtain a high-temperature slab, it is necessary to carry out the above-mentioned straightening in a high temperature state where the solidified shell 5 has not grown much, and this results in a state in which internal cracks are likely to occur.

Therefore, various methods have been studied to predict and prevent the occurrence of internal cracks by estimating the interfacial strain of the slab that occurs during the straightening process. For example, Japanese Patent Application Laid-Open No. 51-55730 discloses a method of measuring the temperature distribution of the slab to calculate the internal strain of the slab, and controlling the drawing speed to a safe range where no internal cracks occur. has been done. Also, JP-A-6
No. 0-24449 discloses a method of measuring the average width of the upper and lower surfaces of a slab or the amount of short side trapezoidization and estimating the internal quality of the slab based on this measurement value. Furthermore, in JP-A No. 61-159254, a plurality of slab transfer speed detectors are installed in the straightening area of a continuous casting machine, and the internal strain of the slab is calculated from the detected slab transfer speed. , a method for estimating internal cracks is disclosed.

(Problem to be Solved by the Invention) As mentioned above, when straightening with unsolidified molten steel remaining,
Interfacial strain increases and internal cracks are more likely to occur, but in the above-mentioned conventional technology, the occurrence of internal cracks is estimated simply from the temperature distribution of the slab or the correlation between changes in surface strain and the internal quality of the slab. Only such methods have been considered. The biggest problem with these methods is that the estimation accuracy itself is extremely poor. The reason for this is that a detection error in the feed rate of the slab occurs due to roll slip, which directly reduces the accuracy of estimating internal cracks, or simply because the surface strain and internal quality of the slab are correlated. If the operating conditions change, for example by slow cooling, the bulging strain εb and roll irregularity strain ε will change,
Since the original interfacial flux was not determined in consideration of this change, the accuracy of the estimation itself deteriorated.

In order to solve this problem, the applicant has developed a roll displacement meter that measures the distance between the guide rolls, a bulging meter that measures the bulging between the rolls, a surface thermometer that measures the surface temperature of the slab, and a solidified film thickness meter. Measuring coagulation R/! [Invented a method to install a pressure meter, calculate the strain of the slab during casting based on the measured values and casting conditions, and apply compressive force to the slab so as not to exceed the limit value, and filed a patent application earlier. Showa 62-5121
No. 8 (Japanese Unexamined Patent Publication No. 63-220960).

This is the true interfacial flux of the slab that also takes into account the variation in strain due to the operating conditions mentioned above (this interfacial flux is hereinafter referred to as the total strain ε↑).
In this method, the compressive force on the slab is controlled so that the total strain εT does not exceed the limit value.

However, even in this method, the strain in the casting direction caused by the compressive force (hereinafter referred to as compressive strain εcpc), which has a large effect on whether or not internal cracks occur, is determined by model calculation or offline testing, and the total strain is calculated. Ta. However, in reality, changes in the temperature distribution of the slab,
The compressive force fluctuates due to slips and rotational defects of the drawing roll 72, the adhesion of scale on the surface of the slab, etc., and the compressive strain εcpc generated in the slab due to these fluctuations changes from time to time, but this compressive strain εC, Since C was not determined from moment to moment, the accuracy of predicting the occurrence of internal cracks would deteriorate as a result.

The present invention aims to fundamentally solve the problems in the prior art, and further improves the technology of Japanese Patent Application No. 62-512111 mentioned above. The purpose of the present invention is to provide a continuous steel casting method in which the compressive strain εcpc is accurately determined from the state of width change, etc., the occurrence of internal cracks is accurately estimated based on the compressive strain εcpc, and the occurrence of internal cracks is prevented.

(Means for Solving the Problems) The present invention for solving the above-mentioned problems is based on the roll misalignment strain ε2 found from the amount of roll misalignment and the casting conditions, and the bulging strain ε3 and correction strain εC between the rolls found from the casting conditions.
In addition, the total strain ε7 of the solidification interface of the slab in the straightening zone is calculated from the compressive strain εcpc, and this value is combined with the critical strain 6c for internal cracking of the slab previously determined according to the casting conditions.
In a continuous casting method for steel in which internal cracking is prevented by changing the casting conditions when the comprehensive type εA exceeds the critical strain εC, A slab width measuring device is installed at the site, and the width of the upper and lower surfaces of the slab during continuous casting is measured by the width measuring device, and the slab thickness D, which is set based on this width measurement value, casting conditions, and equipment conditions, is determined by continuous casting. The radius of curvature R of the machine is calculated based on the following formula (1) to obtain the neutral axis movement amount η, and then the corrective strain εC and the compressive strain εc are calculated.
The strain difference Δε with respect to PC is determined based on the following equation (4), and the strain difference Δε is set according to the operating conditions based on the predetermined correlation between this strain difference Δε and the occurrence of internal cracks in the slab. From the contribution effect coefficient α to the overall strain ε7 reduction,
The total strain εT is calculated from time to time using the following formula (5) or (6), and when the total strain εT exceeds the critical strain εC, the casting conditions are changed to prevent internal cracking. It is.

However, ΔB= (B□+BL, -BF, -BL,)
/2・・・・・・・・・(2) Bo −(Brx+ BLX)/2””・・・(3)
・・・・・・・・・(4) However, η: Neutral axis movement amount εus: Straightening strain on the slab surface (%) Δε: Strain difference between the straightening strain εC and the compressive strain εCPC (%) D: Casting Thickness S: @ Solid shell thickness ΔB: Difference in the average width of the upper and lower surfaces of the slab on the entry side of the straightening side Bo: Average width of the upper and lower surfaces of the slab on the entrance side of the straightening area BFx: Width of the slab on the lower surface of the straightening area B Lll : Width of the slab on the upper surface on the entrance side of the straightening area Bp, : Width of the slab on the lower surface of the straightening area B, , : Width of the slab on the upper surface on the exit side of the straightening area Δ When the thickness is 0, εT = ε〪 +ε8 +α×Δε -...(5
) When Δε≧0, εT=ε−+ε♂+Δε ・・・・・・(6) However, εT: Total strain (%) ε2 ; Roll irregularity strain (%) εM: Bulging strain (%) α : of Δε Contribution effect coefficient R to overall strain ε7 reduction: radius of curvature of the i-th roll (effect) The present inventors believed that it was necessary to improve the conventional estimation accuracy of internal crack occurrence in continuous casting of steel. conducted extensive research on a method for accurately estimating the strain that occurs in slabs.

First of all, in general, the strain that occurs in slabs during casting is as follows (7)
It can be expressed as follows.

εT=ε舖+εB+εU−εcpc=...(7
) However, εT: 18The total strain (%) generated in the slab during construction
) ε - Two-roll irregular strain (%) εa = Bulging strain (%) εC: Corrective strain (%) εcpc: Compressive strain (%) The sign of strain is defined as tensile strain as positive and compressive strain as negative. Using. The bulging strain ε5 and straightening strain εC in the second and third terms on the right-hand side of equation (7) are determined by the casting conditions such as the amount of secondary cooling water, casting speed, slab width, slab thickness, and the radius of curvature during bend straightening. It is well known that it changes depending on the type of continuous casting machine such as roll pitch and roll pitch, and can be calculated, for example, as shown in equations (8) and (9) below.

RP: Roll pitch vC: Casting speed R: Gas constant T: Solidified shell average temperature... (9) However, D = slab thickness R,: radius of curvature of the i-th roll In this equation (9) S is the solidified shell thickness, and when this solidified shell thickness S is zero (S = Omm = slab surface), (
Equation 9) is as follows: (8) where, al: Cross-sectional shape factor of slab (= 0.13) a; Creep constant = 0.56 x 106x axp (-5
8000/(RX(T+273>))a,:
Constant (= (0,23x Rp/S) -0,4
5) P: Static pressure of molten steel (9-1) This is the well-known formula for determining the correction strain εus on the surface of the slab. In other words, the correction strain εus on the surface of the slab can be determined from the thickness D of the slab determined from the casting conditions and the radius of curvature R of the roll determined from the equipment conditions. Slab thickness D
Needless to say, the value actually measured at the time of casting may be used more accurately.

Next, the roll irregularity strain ε2 of the first term on the right side of the above equation (7)
Regarding the calculation method, as shown in Japanese Patent Application No. 63-157929 previously filed by the present applicant, three consecutive pairs of guide rolls are used as the unit measurement object, and the surfaces of both end rolls are calculated. Measure the amount of movement in and out of the center roll with respect to the connecting tangent line for each inner and outer roll, calculate the amount of roll irregularity δ4 from the preset reference amount of movement, and calculate the amount of roll irregularity δ4 as shown below (lO
) The roll misalignment strain εM can be calculated using the equation.

However, ：Coefficient based on the deformation form of the slab (=50~
1200) δM: Roll irregularity amount Now, in recent years, in order to produce high-temperature and defect-free slabs, a casting method has been developed in which compressive force is applied to the slab to straighten the slab while reducing the straightening strain ε. has been adopted. The amount of strain in the slab that is reduced by this compressive force, that is, the compressive strain εCPC, has only been proposed in the past, as described above, by determining it in advance through experiments or estimating it by calculation. In reality, it has not yet been possible to accurately determine this from the change in the state of the slab. As a result, as described above, a problem arises in that the accuracy of estimating the occurrence of internal cracks is low.

In order to solve this problem, the present inventors have developed a compressive strain εcp
We researched a method to accurately determine c from changes in the condition of the slab during casting, and found that it is possible to do so by actually measuring the width of the slab at the entrance and exit sides of the straightening zone.

The method will be explained in detail below.

FIG. 2 is a diagram schematically showing changes in the cross-sectional shape of the slab 6. FIG. 2(a) shows the cross-sectional shape of the slab 6a before it is subjected to the above-mentioned compressive force and straightening, and the cross-sectional shape of the slab 6b after the slab 5a has been subjected to the above-mentioned straightening. Figure 2 (
Shown in b). When a rectangular slab undergoes straightening, the width of the slab decreases on the upper side of the slab because it receives tensile force, while the width of the slab expands on the bottom side of the slab because it receives compression. Further, if only compressive force is applied without straightening, the width of the slab 6C will expand on both the upper and lower surfaces as shown in FIG. 2(c).

The cross-sectional shape of the slab when subjected to straightening and compression at the same time is
As shown in Figure (d), the width of the slab 6d changes in a complex manner depending on the degree of strain generated on the upper and lower surfaces of the slab 6d. By grasping the changes in the slab width from time to time from this Figure 2,
It can be seen that it is possible to determine the state of strain occurring in the slab.

Next, FIG. 3 is a drawing for explaining the effect of reducing the straightening strain εU of the slab when a compressive force is applied to the slab that is undergoing straightening. The distribution of the correction strain εC in the case of only correction is shown in FIG. 3 by a solid line a. As mentioned above, tensile strain is generated on the upper surface of the slab undergoing straightening, and compressive strain is generated on the lower surface. The corrective strain εC herein refers to the tensile strain εM occurring at the solidification interface S in FIG. The large solid line in FIG. 3 shows the distribution of correction strain within the slab when compressive force is applied to the slab in this state. It is found that the distribution of the straightening strain εU of the slab is displaced in the compression direction due to this compressive force, and the tensile strain at the solidification interface S is also reduced from 61 to εb (Fig. 3 shows that due to the compressive force (This shows the state in which the corrective strain εU at the solidification interface has become zero). The amount of strain reduction due to compressive force, that is, compressive strain εCPe, is the magnitude (εCPC) shown in FIG. In general, the effect of reducing strain at the solidification interface due to this compressive force is expressed by the neutral axis B!, indicated by η in Figure 3. Express as a quantity. In the present invention, this neutral axis movement amount η is used to calculate the strain difference Δε, etc., which will be described later.

The inventors of the present invention have conducted various studies regarding the method of determining this compressive strain εcpc, and have focused on the fact that the width of the upper and lower surfaces of the slab changes when the compressive force and straightening described above are applied. The following equation (11) was created based on the upper and lower surface width ratio, the casting direction of the slab generally referred to as Poisson's ratio, and the strain ratio νcpc in the width direction, and its suitability for actual operation was investigated.

In this equation (11), ΔB is the difference between the average width of the upper and lower surfaces of the straightened adult side of the slab and the average width of the upper and lower surfaces of the outlet side, and is expressed as follows:
2) It can be obtained from the formula.

ΔB” (Bry+ BLyBFII BLX)
/ 2... (2) However, B□: Width of the slab on the lower surface of the entry side BLII: Width of the slab on the upper surface of the entry side BWy: Width of the slab on the lower surface of the exit side BLy: Top surface of the exit side The slab width or B is the average width of the upper and lower surfaces of the slab, which serves as a reference for determining the above-mentioned ΔB, and was determined from the slab width on the entrance side of the straightening area using the following formula (3).

Bo” (Brx+BLIll) / 2
"・"・(3) The present inventors have determined that ν in the above formula (11)
In order to experimentally determine cpc, casting was performed by forming protrusions on the surfaces of a plurality of guide rolls 7 disposed at the straightening end position before compressive force was applied. The protrusions cause scratches on the surface of the slab each time the roll rotates, but the spacing between the scratches is measured after the slab is turned into a cold slab, and the change in this value at each measurement position indicates the casting caused by the compressive force of the slab. Strain in the direction, that is, compressive strain εcpc, was calculated using the following equation (12).

Δ1 εCPC=X 1GG (%) ・-
-----(12)! However, Δn=J2o−n.

X,: Roll mark interval on the exit side 10: Roll mark interval on the input side 1; The roll circumference of the input roll and the width of the slab at the position where the roll with protrusions is installed at the same time are shown in Figure 5, which will be described later. The strain in the width direction (ΔB/B
ox 1oo) was calculated. FIG. 4 shows an example of the results of an investigation into the relationship between the two. As is clear from FIG. 4, although there are variations in the relationship, the νcpc
is almost any casting condition! It could be considered as

Since νcpc is approximately 1 in this way, the above (11
) can be expressed as the following equation (13), and by actually measuring ΔB and 80 during casting, the compressive strain εcpc during casting can be determined.

εcpc ”ΔB/BoX100(%) ・Old...(
13) Now, as shown in Figure 3, this compressive strain εcpc
Although the corrective strain εU is reduced by the compressive strain εcpc, a method for determining the strain (ε1 in FIG. 3) after being reduced by the compressive strain εcpc will be described below.

First, the compressive strain εCPC obtained from equation (13) is equivalent to the strain in the casting direction that the slab undergoes between the entrance and exit sides where the width of the slab is measured, and as shown in Figure 3. When the width of the slab on the outlet side is measured at the position where the slab is straightened, the amount of reduction in the straightening strain εC when the slab is straightened is a value determined by the above equation (13). Therefore, the strain after being reduced by the compressive strain εcpc is calculated as (1
3) The compressive strain εcpc determined by equation 3) may be reduced.

However, when straightening is performed between the entrance and exit side measurement positions, for example in a multi-point straightening continuous casting machine, the width of the slab is measured at the entrance side before straightening and the exit side after final straightening. In case 1. The amount of correction distortion reduction at each correction position is as described in (13) above.
It cannot be evaluated as a value determined by an expression. In other words, the value determined by the above equation (13) is a value determined based on the strain that the slab has undergone up to the exit side slab width measuring device. Therefore, at the straightening position between the entry side and the exit side, This is because the strain that the slab undergoes up to the location of the slab width measuring device is, of course, a factor. The relationship between the strain experienced by the slab and the compressive strain εC, C determined by equation (13) is expressed as follows (1) using the neutral axis movement amount η shown in Fig.
4) It can be expressed as follows.

Therefore, the strain after the correction strain εC at each correction position is reduced by the compressive strain εCPC determined by the equation (14) can be obtained from the equations (9) to (14), and the following (4)
) can be expressed as the formula.

(4) That is, by determining the neutral axis movement amount η, it is possible to grasp the strain state of the slab when undergoing straightening. Further, the neutral axis movement amount can be expressed as the following equation (1) from equations (13) and (14).

The slab thickness D and correction strain εus on the slab surface in equation (1) are:
As mentioned above, it is a value specific to the continuous casting machine determined by casting conditions, equipment conditions, etc., and there is no problem in assuming that it does not change during casting. Further, the solidified shell thickness S in equations (17) to (20) described later may be determined as Oma+. In order to understand the strain state of the slab when undergoing straightening, it is sufficient to find the change in the width of the slab, that is, ΔB and Bo in equation (1), and the solidified shell thickness S from time to time. The solidified shell thickness S is estimated by a well-known method by model calculation based on casting conditions such as secondary cooling conditions, casting speed, and steel type, or is actually measured with a shell thickness measuring device using an electromagnetic ultrasonic meter or the like. You can find it by doing something like this.

Next, the inventors found that the radius of curvature is 10.5.12° If
f, In actual operation of a 30 m four-point bending type (curved type) continuous casting machine, the relationship between the neutral axis movement amount η determined by the method described above and the compressive force was investigated. The slab width for determining the neutral axis movement amount η is determined by installing an optical slab width measuring device io at two locations, one on the straightening adult side and the other on the exit side, as shown in Fig. 5. The slab width was measured every moment during continuous casting using the device IO. The detection signal of the slab width measuring device 10 is input to the slab width calculation device 11, which calculates the width of the upper and lower surfaces of the slab, and
The calculated value was processed based on the above equation (1) to determine the neutral axis movement amount η. The casting conditions are as shown in Table 1, and the compression force is controlled within the range of 40 tons to 100 tons by adjusting the braking force of the horizontal drawing roll 72.
Table 6 is a drawing showing an example of the investigation results of the relationship between the determined neutral axis movement amount η and the compressive force. In addition, the correction strain εU at the final correction position under the casting conditions shown in Table 1 is as described above (9).
When calculated based on the formula, it is approximately O1!3%. On the other hand, under the casting conditions shown in Table 1, the compression force is often controlled to about 75 tons, and the neutral axis movement amount η obtained from the change in slab width under the condition where this 75 tons compression force is applied was about 801111 from FIG. When the compressive strain εcpc at this time is calculated using equation (14), it is approximately 0.28%, and in this state, the compressive strain (-CP
C is thick, and as shown in FIG. 3, the tensile strain caused by straightening becomes zero due to the compressive strain εcpc. By the way, it has been known for a long time that the compressive strain εCPC has the effect of reducing the orthodontic strain εU, but conventionally the above-mentioned (7)
As shown in the formula, the comprehensive type ε↑ was simply obtained by subtracting the compressive strain εcpc from (ε−+66+εu). However, the compressive strain εcpc is the corrective strain ε
When the difference between the compressive strain εcpc and the corrective strain εC when it is larger than U, that is, Δε in the above case, is calculated using equation (4), it reaches about -0.15%. The influence was not considered at all in the prior art.

Therefore, the present inventors investigated the influence of the difference Δe between the compressive strain εcpc and the corrective strain εC on the total strain εT.

When investigating the relationship between compressive force and compressive strain 6 CPC under the casting conditions shown in Table 1, it was found that when the compressive force was extremely reduced to approximately 40 tons, the internal Cracks had occurred, that is, the comprehensive type εT exceeded the critical strain εC. When this compressive force is about 40 tons, the neutral axis movement ff1n is about 40u+m (the above Δε is -0.02%)
, which is larger than the above-mentioned corrective strain εC. However, when the compressive force was set to about 75 tons and the neutral axis movement amount η was set to about 75 ■ (the above Δε is -0.14%), no internal cracks occurred, that is, the overall type εT became less than the critical strain εC. . From this, it was estimated that Δε contributes to reducing the roll misalignment strain ε2 and the bulging strain εB, that is, to reducing the overall type εT. Based on each of the measured data, the influence of Δε was investigated based on the following assumptions.

In other words, assuming that the contribution rate of Δε to the reduction of the total strain εT is the contribution effect coefficient α, we devised a calculation formula for the total type εT that incorporates this contribution effect coefficient α into the above equation (7), and conducted research and study. Ta.

As mentioned above, the contribution effect coefficient α is considered to be incorporated into the comprehensive type εT only when the compressive strain εcpc is larger than the corrective strain εU, that is, when Δε is negative, so the inventors express the total strain εT. Two types of distortion calculation formulas were considered, one in which Δε is positive and one in which Δε is negative, and expressed as the following equations (5) and (6).

In the case of ΔSakuO, 6 pieces = ε−+εB +α×Δε ・・・−・・1(
5) When Δε≧0, εT8ε第+εS+Δε (6) Here, if α=I, the formula is the same as the conventional concept (7), and the formulas (5) and (6) Of course, they are also equal.

Furthermore, as mentioned above, it is clear from the relationship between internal cracks in slabs and compressive force when compressive force and compressive strain εcpc are investigated that Δε in equation (5) reduces the overall type εT. , the contribution effect coefficient α in equation (5) is considered to be α>0.

Now, regarding the specific method for determining the contributing effect coefficient α, since α is the effect coefficient that contributes to the reduction of the total strain εT of Δε as mentioned above, we must consider the state of Δ×0, and therefore ( The following equation (15) can be derived from equation 5).

In this equation (15), since the comprehensive type ε1 on the right side is unknown, the contribution effect coefficient α cannot be determined by the equation (15). On the other hand, from equation (5) above, Δε is always negative and α>O, so when Δε is the minimum value, the comprehensive type εT also becomes the minimum value.

In addition, when internal cracks occur, the comprehensive type εT is naturally larger than the minimum critical strain εC at which internal cracks occur (ε7≧εM
) Therefore, if the minimum value of Δε when an internal crack occurs is found as Δε2, then it can be determined that the total type εT at that time is approximately equal to the minimum critical strain εC at which an internal crack occurs, so the following Equation (16) can be derived. This (1
The contribution effect coefficient α can be calculated by determining in advance the minimum critical strain εC at which internal cracks occur from the casting conditions and substituting it into equation 6).

However, Δε-: Minimum value of Δε when ε-side deviation occurs εC: Minimum critical strain at which internal cracking occurs (%) The relationship is shown in Figure 7. The minimum critical strain εC at which internal cracking occurs under the casting conditions of this example was determined to be 0.7 by off-line testing.
0%, so in order to calculate the contribution effect coefficient α using the method described above, for convenience, we assume that the comprehensive type εa at each Δε is equal to the critical strain g C (0,7Q%). It is shown in Figure 7. The opening in FIG. 7 indicates the case where no internal cracks occurred, and the symbol ■ indicates the case where internal cracks occurred.

There is a correlation between the occurrence of internal cracks and Δε, and under the casting conditions of this example, internal cracks will occur when Δε is approximately -0.11% or more. That is, Δε engineering = −0,11
%. Furthermore, it has been found that the critical strain εC is 0.70%, and the roll misalignment strain εM is the (lO
) was determined to be 0.55%, and the bulging strain εB was determined to be 0.20% from the above formula (8).

Therefore, the contribution effect coefficient α is approximately 0.4 from equation (16) above.
It can be calculated as 5. Therefore, under these casting conditions, the influence of Δε on the total strain εT is approximately 45% of that value.
%.

In this way, depending on the casting conditions, the contribution effect coefficient α to the overall strain εT reduction can be determined and set based on the predetermined correlation between the strain difference Δε and the occurrence of internal cracks in the slab. It becomes possible to accurately grasp the total strain εa.

Now, under the casting conditions shown in Table 1, the molds and slabs measured during casting in the straightening area when the casting conditions were changed using the method described above were cooled down and checked for the presence or absence of internal cracks. The results of further investigation are shown in Table 2.In this investigation,
As the critical strain εC, a limit value at which cracking occurs, that is, a value beyond which internal cracking occurs is used.

As shown in Table 2, for slab numbers 1 to 4, which had been in normal operation for m, the neutral axis movement η varied from 75 am to 85 am, which was sufficiently large, so the comprehensive type εT reached its limit. It was estimated that the strain was less than εC, that is, no internal cracks occurred. When the cast slab at that time was used as a cold slab and the occurrence of internal cracks was investigated, no cracks were found, and the estimation and reality matched. Furthermore, in slab numbers 5 to 8, the compressive force was extremely reduced, and the neutral axis movement amount η was set to 12a+m to 50■■. As a result, it was estimated that the total strain εT was greater than the critical strain εC, that is, internal cracking occurred. When the cast slab at that time was used as a cold slab and the occurrence of internal cracking was investigated, cracking was found, and the estimation and actual condition were in good agreement. In addition, for slab No. 9, by increasing the compressive force and changing the operating conditions to increase the neutral axis movement to about 9 ha, the overall type εT becomes less than the critical strain εC, which prevents the occurrence of internal cracks. It was assumed that there was no such thing. The estimated results of crack occurrence at that time also match the reality.

As described above, while measuring the neutral axis movement amount η from time to time, it is possible to estimate that an internal crack has occurred when the total strain εT obtained by the above-mentioned total strain formula exceeds the critical strain εC. It was confirmed that very good turbidity could be obtained.

If the occurrence of internal cracks is estimated, the occurrence of internal cracks can be effectively prevented by taking well-known operational actions such as increasing the compression force, decreasing the casting speed, and increasing the amount of secondary cooling water. It was confirmed that it is possible to prevent this.

In addition, in the above explanation, the slab width measuring device 10 is installed at two locations before and after the straightening band, but by installing the slab width measuring device 10 also during straightening, it is possible to The comprehensive type ε1 of can also be calculated with high accuracy by using the method described above.

That is, as explained in FIG. 2, when the slab is straightened, the width of the slab changes, and by measuring this width, the straightening strain εC actually occurring in the slab can be calculated.

As shown in Fig. 5, when the slab width measuring device 10 is installed only on the entrance and exit sides of the straightening area, the straightening strain εC is calculated based on equation (9), and the total type εT is However, if, for example, one additional slab width measuring device 1O is installed during straightening, it is possible to grasp the actual straightening strain εU occurring in the slab during straightening. An example of a method of installing a slab width measuring device 10 during straightening in addition to the entrance and exit sides of the straightening area and calculating the straightening strain εC from the value of the slab width measured by this device will be described below.

δBI −(art-BLI)−(Brl−+ BL
I−+)・・・・・・(1'l) δBo = (Bry Bi, y) −(BP
, BLII) ... (20) where εuo: Total correction strain (%) on the slab surface between the slab width measuring devices on the exit side Brl: Bottom surface measured by the i-th slab width measuring device Slab width BLI: Top slab width B measured by the i-th slab width measuring device, Knee 1: (t-i) Bottom slab width BLI-1 measured by each side slab width measuring device: (i -1) By measuring the width of the slab at multiple locations, such as the width of the slab on the top surface or higher, using the second slab width measuring device, the correction strain εuB+ ε occurring in the actual slab can be determined.
By calculating this value from time to time and substituting it into ευ in the above-mentioned equations (1), (4), (5), or (6), a more accurate total strain εT can be obtained. can be understood.

Furthermore, the slab width measuring device 10 is not limited to the above-mentioned optical type; for example, it is also possible to use a contact type slab width measuring device, and the ease of maintenance of the device is taken into consideration depending on the atmosphere of the place where it is installed. However, the type of measuring device may be determined and used as appropriate.

In addition, by setting the critical strain εM to a value smaller than the strain at which internal cracks occur within a practically acceptable range, it is possible to completely eliminate the occurrence of internal cracks.

In addition, if excessive compressive force is applied to the slab in order to prevent internal cracks, it may lead to internal defects in the slab other than internal cracks, such as worsening of center segregation or center cracks in the slab. Furthermore, if the compressive force is increased too much, the drawing roll will be severely worn and the life of the roll will be shortened. Therefore, it is desirable that the compressive force be applied to the slab at the minimum necessary level without causing internal cracks, and this can also be achieved by the present invention.

〔Example〕

The casting method of the present invention was carried out in a continuous casting machine with continuous casting capability, monthly production of 160,000 tons, and machine length of 37 m. In the same manner as in the explanation of the effect described above, the effect coefficient α at which the Δε contributes to the reduction of strain in the slab was determined for each casting condition.

Table 3 shows the casting conditions and the contribution effect coefficient α determined in advance.

Table 3 FIG. 1 is an overall configuration diagram of the continuous casting equipment based on this example. The equipment configuration in Fig. 1 is basically the same as in Fig. 5 described above, and an optical slab width measuring device IO is installed at two locations, one on the entrance side and one on the exit side of the straightening area, and The width of the upper and lower surfaces of the slab is calculated by the slab width calculating device 11 based on the signal from the width measuring device 10, and the information is transmitted to the strain calculating device 12, and the neutral axis movement amount and the strain difference Δε are calculated. It is configured to calculate every moment.

Further, the casting condition information from the casting condition indicating device 13 and the measurement results from the roll spacing and roll arrangement measuring device 9 are also transmitted to the strain calculating device 12, and the bulging strain ε8 and corrective strain 6u are calculated from the casting conditions. , the roll misalignment strain ε- is calculated from the roll spacing and roll arrangement measurement results. Then, the strain calculating device 12 calculates each mold moment by moment according to the method described above, and also refers to the contribution effect coefficient α and critical strain εC determined in advance from the casting conditions. The total strain εT is calculated by the strain calculation unit W12 and compared with the limit strain εC. Then, when the total strain εT exceeds the critical strain εC, an alarm is issued to the alarm device 14, and the operator can take appropriate measures based on the information (in this embodiment, the compressive force is increased). It was decided to output work instructions to the work instruction device 15 to prevent internal cracks from occurring.

The results of operation using this device are shown in FIG. 8, and by adopting the method based on the present invention, the index of the number of cars with internal cracks was drastically reduced from about lθ% in the conventional method to about 2%.

(Effects of the invention) As described above, by applying the method of the present invention to actual operations, it becomes possible to predict the occurrence of internal cracks in slabs and prevent the occurrence of cracks by taking appropriate measures. It was confirmed that excellent effects could be exhibited.

[Brief explanation of drawings]

Figure 1 is an overall configuration diagram of continuous casting equipment showing an embodiment of the present invention, and Figures 2 (a) to (d) are cross sections of slabs schematically showing changes in the cross-sectional shape of slabs. Figure 3 is a diagram schematically showing the strain distribution inside the slab when compressive force is applied to the slab undergoing straightening, and Figure 4 shows the relationship between strain in the casting direction and strain in the width direction. Figure 5 is a diagram of the test equipment for investigating the relationship between compressive force and compressive strain, Figure 6 is a diagram showing the relationship between compressive force and neutral axis movement, and Figure 7 is for determining the contribution effect coefficient α. FIG. 8 is a diagram showing the occurrence of internal cracks when the method of the present invention is applied to actual operation, and FIG. 9 is a schematic side view of continuous casting equipment illustrating the conventional method. DESCRIPTION OF SYMBOLS 1... Tanditetsu, 2... Immersion nozzle, 3... Mold, 4... Molten steel, 5... Solidified shell, 6...
... Slab, 7... Guide roll, 71... Guide support roll, 72... Drawing roll, 73... Straightening roll,
8... Cutter 9... Roll distance meter and roll arrangement measuring device, 10... Slab width measuring device, 11... Slab width calculating device, 12... Strain calculating device, 13... Casting condition indicating device, 14... Alarm device, 15... Work instruction device, Agent: Patent attorney Masamitsu Akizawa and one other name 2 Figure 7i 1 Figure 13 Weight making force 100 distortion (''/,) 25 (Fig. Compressive force 0 hair) Off Fig. 28 Fig.

Claims (1)

[Claims] In addition to the roll misalignment strain ε_M found from the roll misalignment amount and the casting conditions, and the bulging strain ε_B and corrective strain ε_U between the rolls found from the casting conditions, the compressive strain ε_C
From _P_C, calculate the total strain ε_T of the solidification interface of the slab in the straightening area, and compare this value with the critical strain ε_C for occurrence of internal cracks in the slab, which has been determined in advance according to the casting conditions,
In the continuous casting method for steel in which internal cracking is prevented by changing the casting conditions when the total strain ε_T exceeds the critical strain ε_C, slab width measuring devices are provided at least at two locations, one on the entry side and one on the exit side of the straightening area. The width of the upper and lower surfaces of the slab during continuous casting is measured using the width measuring device, and the thickness D of the slab set based on this width measurement value, casting conditions, and equipment conditions, and the radius of curvature R of the continuous casting machine are calculated. is calculated based on the following formula (1) to obtain the neutral axis movement amount η, and then the correction strain ε_U and the compressive strain ε
The strain difference Δε with respect to _C_P_C is determined based on the following equation (4), and the strain difference Δε is set according to the operating conditions based on the predetermined correlation between this strain difference Δε and the occurrence of internal cracks in the slab. The total strain ε_T is calculated from time to time by the following formula (5) or (6) from the contribution effect coefficient α to the total strain ε_T reduction, and this total strain ε_T is the critical strain ε
A continuous casting method for steel, characterized in that when the temperature exceeds _C, the casting conditions are changed to prevent internal cracks. ▲There are mathematical formulas, chemical formulas, tables, etc.▼・・・・・・・・・(1
) However, there are ▲mathematical formulas, chemical formulas, tables, etc.▼・・・・・・
・・・・・・(2) ▲There are mathematical formulas, chemical formulas, tables, etc.▼・・・・・・・・・
(3) ▲There are mathematical formulas, chemical formulas, tables, etc.▼・・・・・・・・・
(4) However, η: Neutral axis movement amount ε_U_S: Straightening strain on the slab surface (%) ▲ There are mathematical formulas, chemical formulas, tables, etc. ▼ Δε: Strain difference (%) between the straightening strain ε_U and the compressive strain ε_C_P_C D: Slab thickness S: Solidified shell thickness ΔB: Difference between the average width of the upper and lower surfaces of the slab on the entrance side and exit side of the straightening area B_O: Average width of the upper and lower surfaces of the slab on the entrance side of the straightening area B_P_x: Width of the slab on the lower surface on the entrance side of the straightening area B_L_x: Width of the slab on the upper surface of the straightening area entrance side B_F_y: Slab width of the lower surface of the straightening area exit side B_L_y: Slab width of the upper surface of the straightening area exit side When Δε<0 ε_T=ε_M+ε_B+α×Δε... ...(5)
When Δε≧0, ε_T=ε_M+ε_B+Δ_ε・・・・・・(6) However, ε_T: Total strain (%) ε_M: Roll irregularity strain (%) ε_B: Bulging strain (%) α: Total strain of Δε ε_T low range Contribution effect coefficient R_i: radius of curvature of the i-th roll