JPH0359780B2 - - Google Patents

Description

[Detailed description of the invention]

(Field of Industrial Application) The present invention relates to a continuous steel casting method for industrially stably producing slabs of excellent quality with few internal cracks. (Prior Art) In continuous casting of steel, the distance between the rolls of slab guide rolls (simply referred to as rolls in the present invention) (the distance between the rolls is, for example, n-1 and n-1' in Fig. 2). It is well known that there is a close relationship between the distance between facing rolls (hereinafter abbreviated as roll distance) and internal quality, such as the distance between rolls. In other words, if there is an irregularity (misalignment) in the spacing between the rolls in the area where the unsolidified part of the slab exists, as shown in Fig. 7a and b, the distortion due to bending straightening and bulging between the rolls will combine, and the slab will deteriorate. A large tensile strain occurs in the steel, making it easy for defects such as internal cracks to occur. Therefore, at present, in order to strictly control the roll spacing, the roll spacing is measured mechanically or manually when continuous casting operations are stopped, such as during periodic inspections, and rolls that are outside the control limit are measured. By returning it to within the supervision limit,
We are working to prevent the quality defects mentioned above. However, in reality, the frequency of regular inspections is generally
Since the casting time is 10 to 20 days, the roll interval may fluctuate from the initial setting value at the time of periodic inspection due to thermal load, mechanical load, and roll wear as casting is repeated, resulting in quality problems such as internal cracks. There is a problem that defects occur. Therefore, in order to solve this problem, as shown in Japanese Patent Application Laid-Open No. 57-88958, during continuous casting of steel, the distance between each pair of rolls is measured using a roll distance meter installed in advance on a dummy bar. By controlling the casting speed and water injection ratio (cooling water amount per unit weight), or both, determined from the roll spacing irregularity rate set in advance according to the measured value, internal cracks and center cracks can be prevented. Continuous casting methods have been proposed to prevent defects such as segregation and pipes. In addition, as a method of applying compressive force in the direction of slab casting to prevent internal cracks, for example,
As shown in the publication, compressive force in the slab casting direction is applied near the slab straightening point by a drive roll group that applies a driving force in the drawing direction to the slab and a drive roll group that applies a braking force to the slab. Therefore, a method has been proposed to prevent internal cracking during bending straightening of slabs. Here, the drive roll is a general term for rolls connected to a device that applies rotational force, such as an electric motor. (Problems to be Solved by the Invention) In the method disclosed in the above-mentioned Japanese Patent Application Laid-Open No. 57-88958, the average value of the roll intervals of the most recent 5 to 10 casts is taken as the standard roll interval, and the deviation from that The roll spacing irregularity rate (%) is defined as shown in equation (1). Actual roll spacing (mm) - Standard roll spacing (mm) / Actual slab thickness at standard roll spacing (mm) × 100...(1) However, the strain caused in the slab due to irregular roll spacing is as shown in Figure 2a. As shown in ,b, this is caused by the relative deviation between the front and rear roll spacing.
In this sense, the conventional method of calculating the roll irregularity rate as in equation (1) from only the statistical deviation over time of each pair of rolls cannot accurately estimate the strain occurring in the slab, and it is difficult to accurately estimate the roll irregularity rate. Internal cracking cannot be prevented. That is, as shown in FIG. 3a, if the standard roll spacing between the irregular roll 1 and the front and rear rolls 2 and 3 is equal, and the distance between the front and rear rolls 2 and 3 does not vary,
The strain occurring in the slab can be estimated from the irregularity ratio of the irregular roll 1. However, as shown in Fig. 3b, if the distance between the front and rear rolls 2 and 3 moves in the same manner as the irregular roll 1, roll irregularities will be detected in the irregular roll 1, but in reality, the irregular rolls will be detected. This means that no distortion will occur. Further, as shown in FIG. 3c, if there is an irregularity in the standard roll spacing itself between the irregular roll 1 and the front and rear rolls 2 and 3, even though no roll irregularity is apparently detected in the irregular roll 1, In reality, distortion occurs in the slab, which poses a problem. Furthermore, in this method, as a measure to prevent internal cracking according to the roll spacing irregularity ratio, the casting speed is increased with the aim of reducing the strain occurring in the slab or easing the conditions for internal cracking (increasing the critical strain for internal cracking). In order to control operating conditions such as cooling water and increasing the amount of cooling water, a major problem that cannot be ignored from an industrial perspective has arisen: fluctuations in slab quality, a decrease in productivity, and energy loss cannot be avoided. As a method for solving such problems and preventing internal cracks, the method disclosed in Japanese Patent Publication No. 51664/1983 has been proposed. This method was invented with the aim of preventing straightening cracks in the straightening part of a cast slab, and applies a driving force to the upstream side of the straightening point and a braking force to the downstream side. The casting speed is increased by applying compressive force (compressive strain) intensively to the straightening point and its vicinity, and alleviating the strain (tensile strain) that occurs in the slab. This is a method to prevent internal cracking without changing operating conditions such as the amount of water or cooling water. However, in this method, the control system for the distribution of compressive force applied to the slab in the continuous casting machine is not subdivided;
As mentioned above, compressive force acts intensively at the straightening point and its vicinity, so there was some incidental effect in preventing internal cracking due to roll irregularities in that area unless the roll irregularities were extremely large. For internal cracks caused by roll irregularities in other parts, it is not possible to predict the location of internal crack occurrence or finely control the compressive force according to the magnitude of strain occurring in the slab, so it is difficult to accurately prevent internal cracks. That was difficult. (Means for Solving the Problems) The present invention solves all the above-mentioned problems. Namely, in continuous casting of steel, two or more adjacent pairs of rolls are measured using a roll spacing meter installed in advance on a dummy bar. The amount of roll irregularity is determined from the value of the roll spacing of the pair of rolls, and this value is combined with operating conditions such as casting speed, surface temperature, solidified shell thickness, slab size (thickness, width), and radius of curvature during bend straightening. From the characteristics of the continuous casting machine such as roll pitch, static pressure of molten steel, etc., we successively estimate the strain ε _T that occurs in the slab due to roll irregularities during casting, and determine the limit at which internal cracks will occur at all positions in the continuous casting machine. For strain ε _C , always ε _C > ε _T
By applying compressive strain ε _cpc in the slab casting direction by the rotational force of the drive roll so that the relationship +ε _cpc is satisfied, slabs with fewer internal cracks caused by roll irregularities etc. are obtained. This is a continuous casting method for steel. However, ε _T = ε _u + ε _b + ε _n ε _u : Straightening strain (%) generated in the slab during bending straightening ε _b : Bulging strain (%) generated in the slab due to bulging between rolls ε _n : Straightening strain caused in the slab due to roll irregularity Misalignment strain (%) that occurs in the slab due to the above causes ε _T : Total strain (%) that occurs in the slab due to the above reasons ε _C : Critical strain at which internal cracks occur in the slab (%) ε _cpc : Strain that occurs in the slab due to the rotational force of the drive roll Compressive strain (%) (effect) The effect brought about by the means of the present invention will be explained in detail below. Generally, the strain that occurs in continuously cast slabs during casting can be expressed as in equation (2). ε _T = ε _u + ε _b + ε _n …(2) Here, ε _T : Total strain (%), ε _u : Correction strain (%),
ε _b : Bulging strain (%), ε _n : Misalignment strain (strain caused by roll misalignment) The sign of strain is defined as positive for tensile strain and negative for compressive strain. Regarding the straightening strain ε _u and bulging strain ε _b in the first and second terms on the right side of equation (2), the characteristics of the continuous caster (radius of curvature during bend straightening, roll pitch, static pressure of molten steel, etc.) and operating conditions ( casting speed, surface temperature,
(solidified shell thickness, etc.), equations (3) and (4) are obtained, respectively.
It can be easily calculated as shown in the formula. ε _u = (D/2-S)・(1/Ri-1/R _i+1 )×100...
(3) Here, D: Thickness of slab (mm), S: Thickness of solidified shell of slab (mm), R _i : Radius of curvature of i-th roll (mm), R _i+1 : i+1-th Radius of curvature of the roll (mm) ε _b = 1600・δ _B・S/l ² …(4) where, l: Roll pitch (mm), δ _B : Amount of bulging (mm) δ _B = 11a・α ₀・p・l ⁴ /S ² √2 a=1.45×10 ³・exp (74000/1.986T _M ) T _M =T _S +1490/2+273 Here, T _s : Surface temperature of slab (℃), P : Molten steel Static pressure (Kg/mm ² ), V: casting speed (m/min), α ₀ : shape factor (correction coefficient depending on slab width W (mm) and roll pitch l (mm)). Further, although the static thickness P of molten steel is slightly influenced by the molten steel density, it is almost uniquely determined by the characteristics of the continuous casting machine, that is, the position from the meniscus in the mold. Next, the misalignment strain ε _n in the third term on the right side of equation (2) can be calculated as shown in equation (5) using the measured value of the amount of misalignment δ _n (mm). ε _n =C _n δ _n・S/l ² ...(5) Here, C _n is the misalignment coefficient, which varies slightly depending on the characteristics of the continuous casting machine, operating conditions, and type of misalignment, and is usually as shown in Figure 2 a. It is 200 to 400 in the case of a reduction type as shown in Figure 2b, and 100 to 300 in the case of a bulge type as shown in Fig. 2b. The present inventors investigated an accurate method for estimating the amount of misalignment _δn , measured the roll spacing of two or more adjacent roll pairs using a roll spacing meter attached to a dummy bar, and calculated the equation (6) by measuring the roll spacing of two or more adjacent roll pairs. It has been found that by determining the misalignment amount δ _n as follows, the misalignment amount δ _n corresponding to the behavior of the misalignment strain ε _n during casting can be easily obtained. δ _n =P _(o) −P _(o-1) +P _(o+1) /2 ...(6) Here, P _(o) , P _(o-1) , P _(o+1) : n, n-1, n
+ Roll spacing of the first pair of rolls (mm) However,
If the n-1 or n+1 roll does not exist, P _(o-1) = P _(o+1) or P _(o+1) = P _(o-1), respectively.
shall be. At this time, as shown in Figure 2 a and b, when δ _n is a positive value {Figure b}, misalignment distortion ε _n occurs at the n-1 and n+1 roll positions,
When δ _n is a negative value, misalignment strain ε _n occurs at the n-th roll position in {figure a}. In addition, for the portion where the two overlap, consider the one with larger ε _n . Using the method described above, the strain ε _T generated in the slab is successively determined from the measured values of the roll spacing meter installed on the dummy bar, the characteristics of the continuous casting machine, and the operating conditions during casting, and the critical strain at which internal cracks occur is determined. By comparing the size relationship with ε _c , it became possible to almost accurately estimate the location of internal crack occurrence. That is,
The internal crack generation condition is ε _T > ε _c . According to the experimental results of the present inventors, this internal cracking critical strain ε _c is
It varies with the distance from the meniscus and varies depending on the chemical composition of the material and operating conditions, but is usually 0.5%.
It is a value of degree. Next, the present inventors conducted repeated research on methods for preventing the internal cracks mentioned above, subdivided the control system for the compressive force distribution generated by the combination of drive rolls in the continuous casting machine, and succeeded in casting at all positions in the continuous casting machine. The sum of the strain ε _T generated in the piece and the compressive strain ε _cpc caused by the compressive force ε _T +
An attempt was made to prevent the internal cracking by appropriately applying compressive force in the casting direction so that ε _cpc always satisfies the relationship of equation (7) with respect to the internal cracking critical strain ε _c . ε _c > ε _T + ε _cpc (7) As a result, it was found that slabs with few internal cracks could be stably obtained without changing operating conditions such as casting speed and amount of cooling water. At this time, the compressive force is applied by the front and rear drive rolls that include at least the part that applies the compressive force, so in order to prevent the above internal cracks at all positions in the continuous casting machine, at least the first part on the mold side It is desirable that the roll and any roll after the final solidification part be used as drive rolls. Furthermore, as described above, strain calculation depends on operating conditions such as casting speed, surface temperature, solidified shell thickness, and slab size. Among these, the surface temperature and solidified shell thickness are usually determined by the chemical composition of the steel, physical properties, casting speed, water injection ratio (amount of cooling water per unit weight), initial molten steel temperature (usually the molten steel temperature in tundishing), and slab size. It is obtained by calculating the heat transfer difference temperature using the following as input data. As for the calculation cycle, there is no need for calculation as these are stable during steady operation, and at least the casting speed, water injection ratio, and molten steel temperature in the tundish are subject to large fluctuations, taking into account the increase and decrease in strain caused by these fluctuations. It is necessary to calculate at predetermined intervals during unsteady operation such as when changing the pot during continuous casting. (Example) Next, an example of the present invention will be described. FIG. 1 is a simplified block diagram of a continuous casting apparatus used to carry out the present invention. The device of this embodiment is
A curved continuous casting machine with a bending radius of 10.5 m is equipped with a roll interval meter attached to a dummy bar, a system that calculates equations (2) to (6) to predict the position of internal cracks, and a system that predicts internal crack positions based on the results. It consists of a system that controls the rotational force of the drive rolls in the continuous casting machine to prevent this. In other words, the rotational force of each drive roll is controlled so as to satisfy equation (7) at all roll positions in the continuous caster according to the strain distribution generated in the slab. Here, the arrangement of drive rolls was 1 to 2 per 5 rolls. Also, roll pitches start from 200
635mm and varies depending on the position within the continuous casting machine. Table 1 is
Effects of the present invention and conventional example a (Japanese Patent Application Laid-Open No. 57-88958
In order to compare the results of conventional example b (Japanese Patent Publication No. 55-51664), as an example, the apparatus shown in Fig.
These are the results of an investigation into the occurrence of internal cracks when killed steel was cast. The slab size was 280 mm thick x 1800 mm wide in all cases. Casting speed is 1.1
～1.5m/min, water injection ratio is based on 0.6～1.2/Kg

[Table] As a quasi-condition. In addition, internal cracking was determined using a sulfur print, and 1 to 5 cross-sectional samples were taken per charge to investigate the incidence. In Table 1, the roll irregularity position (*1) is the distance (m) from the meniscus of the irregular roll, and the casting speed index (*
2) indicates the ratio when the reference speed is 100,
The standard speed is 1.1 to 1.5 m/min. The water injection ratio index (*3) indicates the ratio when the standard water injection ratio is 100, and the standard water injection ratio is 0.6 to 1.2/Kg. Compressive force level (*4) indicates the force acting on the roll misalignment position,
The maximum value was set to 10, and the time when no compressive force was applied was set to 0. Furthermore, in conventional example b, the maximum compressive force was applied to the correction point. From Table 1, in the case of conventional example b, the internal crack occurrence rate is
0.3 to 6.4%, and applying compressive force only to the straightening point and its vicinity is effective in preventing internal cracks during straightening of slab bends, but internal cracks caused by roll irregularities in other parts are effective. It can be seen that it is not very effective in preventing. Furthermore, in comparison with conventional example b, the incidence of internal cracks in conventional example a is reduced to 0.1 to 3.0%;
It cannot be said that internal cracking has been sufficiently prevented. Furthermore, in order to prevent internal cracks, the casting speed is lowered and the water injection ratio is increased to control the surface temperature and solidified shell thickness of the slab, and in order to change the operating conditions, fluctuations in slab quality such as center segregation. Side effects such as decreased productivity and energy loss also arose. On the other hand, in the case of the present invention, all such problems have been completely resolved, and the internal crack occurrence rate has been drastically reduced to 0.3% or less, and internal cracks can be prevented accurately according to the amount of misalignment δ _n . I can see what is being done. (Effects of the Invention) As described above, according to the present invention, internal cracks caused by roll irregularities can be prevented from occurring without reducing productivity or energy loss due to a decrease in casting speed or an increase in the amount of cooling water. Slabs with few internal cracks can be industrially produced stably and economically. Therefore, it has a great industrial effect, such as being able to be used as a useful means of quality assurance in the current or future casting process, which aims to directly connect continuous casting and rolling processes, that is, direct rolling. Furthermore, it is clear from what has been described above that the example of the present invention works effectively as a method for managing roll irregularities.

[Brief explanation of drawings]

FIG. 1 is a simplified block diagram of a continuous casting apparatus used in an embodiment of the present invention. FIG. 2 is a diagram illustrating roll misalignment and the location where tensile strain occurs in the slab as a result of roll misalignment. FIG. 3 is a diagram showing problems when determining roll irregularity using conventional example a (Japanese Unexamined Patent Publication No. 57-88958).

Claims (1)

[Scope of Claims] 1. During continuous casting of steel, the distance between the rolls of each upper and lower roll guiding the slab is measured using a roll distance measuring meter installed in advance on a dummy bar, and the distance between two or more adjacent pairs of the measured rolls is measured. The amount of misalignment is calculated from the value of the roll spacing of the body, and this value is combined with operating conditions such as casting speed, surface temperature, solidified shell thickness, and slab thickness during casting, radius of curvature during bend straightening, roll pitch, molten steel static pressure, etc. From the characteristics of the continuous casting machine, the strain ε _T that occurs in the slab due to roll irregularities during casting is estimated for each billet guide roll, and the limit strain ε _C at which internal cracks occur is always ε _C ＞ε _T +
By applying compressive strain ε _cpc in the slab casting direction by the rotational force of the drive roll so that the relationship ε _cpc is satisfied, slabs with fewer internal cracks caused by roll irregularities etc. are obtained. Continuous casting method for steel with fewer internal cracks. However, ε _T = ε _u + ε _b + ε _n ε _u : Straightening strain (%) generated in the slab during bending straightening ε _u = (D/2−S)・(1/Ri−1/R _i+1 )× 100 D: thickness of slab (mm), S: thickness of solidified shell of slab (mm), Ri: radius of curvature of i-th roll (mm),
R _i+1 : Radius of curvature of i+1st roll (mm) ε _b : Bulging strain (%) caused in slab due to bulging between rolls ε _b =1600・δ _B・S/l ² l: Roll pitch (mm), δ _B : Bulging amount (mm) δ _B = 11a・α ₀・p・l ⁴ /S ² √2 a=1.45×10 ³・exp (74000/1.986T _M ) T _M =T _S +1490/2+273 T _S : Surface temperature of slab (°C), P: Static pressure of molten steel (Kg/mm ² ), V: Casting speed (m/min), α ₀ : Shape factor {slab width W (mm) and roll pitch l (mm )} ε _n : Misalignment strain (%) caused in the slab due to roll misalignment ε _n = C _n・δ _n・S/l ² C _n : Misalignment coefficient, which depends on the characteristics of the continuous casting machine,
It varies slightly depending on the operating conditions and type of misalignment, 200 to 400 for the reduction type and 100 to 300 for the bulge type. ε _T : Total strain (%) caused in the slab due to the above causes ε _c > ε _T + ε _cpc ( Internal crack prevention conditions) ε _c : Critical strain at which internal cracks occur in the slab (%) ε _cpc : Compressive strain imparted to the slab by the rotational force of the drive roll (%)