**The function of the tension leveler**

The arc continuous casting machine is the main force in the production of cast billets in the iron and steel industry. Square billets, rectangular billets and round billets are generally produced by arc continuous casting machines, occupying a dominant position, even if the continuous casting machine adopts a straight arc arrangement, It is also necessary to use a circular arc section for transition, and straightening of the cast slab is still required after the arc section. The stretching and straightening machines of the billet and round billet arc continuous casters are centrally arranged near the horizontal line. The stretching and straightening machines in the continuous casting production process need more than two. The conventional continuous casting machines for billet production will be arranged with two stretching and straightening machines. In one frame, a five-roller tension leveler is formed. The function of the tension leveler is:

1）Balancing the sliding force of the cast slab: Use the upper and lower knuckles of the tension leveler to hug the slab in the sky to prevent the slab from falling and maintain continuous production. This is also the basic parameter of the hydraulic design of the tension leveler, such as the use of a 16-meter radius casting machine For o600mm round billets, 35 tons of sliding force are generated due to the high potential energy, which requires multiple tension leveling machines to work together to generate a friction force that can balance 35 tons. Figure 1 shows the drawing and leveling machine group of Wuxi Xuefeng Great Circle Continuous Casting Machine, where 5 drawing and leveling machines are used. Using a 9-meter radius casting machine to produce 150mm billets also has a sliding force of 1.5 tons. This only requires a pair of friction forces to balance this sliding force. Figure 2 shows a typical five-roller tension leveler. For billet production, the continuous casting machine with an arc radius of 6~10m is generally used, and most of them are in the form of the fifth series of stretching and straightening machine. The weight of the spindle to maintain normal production.

2）Constant casting billet drawing speed: According to the operation requirements, the drive roller of the tensile straightening machine is controlled by the frequency converter to control the speed of the drive motor and the reducer, and the power is transmitted to the roller of the tensile straightening machine. Under the action of the hydraulic pressure of the hydraulic cylinder, the roller and the casting billet generate friction In order to stabilize the declining billet, the rollers are driven according to the requirements of the drawing speed, so that the slab slides in a continuous and stable manner in accordance with the requirements of the drawing speed. In fact, from the force analysis, the tension leveler is actually a slab descending speed. Stabilizer.

3)Feeding the ingot bar: The casting and drainage of continuous casting is carried out through the ingot bar. At the beginning of the operation, the tension leveler is used for transmission, and it also relies on friction to feed the ingot bar. The ingot head is placed on the copper through the second cold room. Tube lumen, see Figure 3. After the starter head is ready, the starter is ready for pouring. When the continuous casting starts, the tension leveler friction drives the starter rod downwards, the solidified slab head is connected with the starter head, and the cast slab descends with the starter rod. After the slab head is separated from the starter bar, the rigid starter bar continues to rise by the friction of the clamping roller of the starter storage device and enters the starter bar storage device; the flexible starter bar passes through The roller table before and after cutting enters into the idler bar storage roller table, and the flexible idler bar storage device lifts it out of the roller table and puts it aside.

4) Straightening the slab: The slab with a radius of curvature of R is transformed into a straight slab with an infinite radius of curvature by the straightening action of the tension leveler. Even if a vertical bending continuous caster is used, the slab needs to be changed from the radius of curvature. The curved shape is transformed into a cast slab of straight length by the action of the straightener.

Therefore, the design of the tension leveler needs to consider the size of the cast billet section, the radius of the casting machine, the motor power, the speed ratio and strength of the reducer, and the number of the tension leveler’s clamping rollers and the size of the hydraulic cylinder should be reasonably configured.

**Calculation of output pressure of tension leveler**

2.1 The output pressure of the hydraulic cylinder of the tension leveler

Take an example to illustrate the force analysis of the tension leveler. See Figure 4 for the actual object of the tension leveler.

The force analysis of the tension leveler is shown in Figure 5. This is an independent tension leveler, and its force analysis is consistent with the five-roller tension leveler.

The force output by the hydraulic cylinder can be decomposed into horizontal force and vertical force, the angle is α=25.8298°, the horizontal force of the hydraulic pressure at the ball head is Fh=fcosa, the vertical force is Fv=fsina, and the hydraulic pressure is transmitted by the hydraulic cylinder The resulting dynamic torque is equal to the resistance torque for pressing the upper wheel, and N is the vertical pressure exerted on the billet, so:

Fcosa*700+Fsina*(170+900)=900N

900N=1096F N=1.2177F

It can be seen that through the structure design of the upper roller of the tension leveler, the effect of amplifying the hydraulic pressure is achieved, which increases the positive pressure of the tension leveler. This structure is widely used in the tension leveler of square billets and round billets.

The design of Wuxi Xuefeng large round billet stretch-leveling machine, when producing Q330mm round billet, α is equal to 56.35°, as shown in Figure 6. According to the above analysis of the structure, N=1.33F.

2.2 The tension and straightening machine pressure required to send the starter rod

The hydraulic pressure output by the tension leveler is a function of the cylinder diameter and the pressure. For example, the cylinder diameter is φ140mm, the hydraulic pressure is 20bar, and the output pressure of the hydraulic cylinder is 3.785 tons. Take the Wuxi Great Circle Continuous Casting Machine as an example, under the action of this hydraulic pressure , The pressure exerted on the casting billet is 4.09 tons, the friction force generated is 1.23 tons, and the friction force generated by the five tension levelers is 6.14 tons, which can hold the φ280mm round billet produced by the continuous casting machine with a radius of 11 meters. It is in stable production.

The coefficient of friction between the high-temperature cast slab and the roller of the tension leveler is calculated according to 0.3. I have done this test many times to verify the coefficient of friction, which is roughly the data. The starter bar is in a cold steel state, and its friction coefficient is much smaller. It is also through many tests of its own. This friction coefficient is about 0.1~0.15, so the cold billet pressure required to send the starter bar is much larger, such as The Wuxi Xuefeng large circle continuous casting machine used a pressure of more than 120bar to send the flexible starter bar. It should be noted that the flexible starter bar sends the starter head into the copper pipe. Generally, the tension leveler below only has 2~3 racks. If the dead weight of the ingot bar is large, a higher hydraulic pressure is needed to balance the sliding force of the ingot bar, as shown in Figures 7-9.

The process of sending the spindle rod requires the maximum hydraulic pressure output by the tension leveler, mainly:

- The friction coefficient between the straightening machine roller and the starter bar is small.
- The influence of the weight of the starter rod, for large-section billets, the starter rod needs to have a certain weight, so as to maintain the weight of the starter rod to pull the casting billet to overcome the resistance of the mold, so the pull leveler must pay enough friction Force to balance the sliding force caused by the rising of the inciting bar. Tang Gong has always insisted that overcoming the mold pulling resistance is the sliding force generated by the weight of the ingot bar and the cast slab, so it must have a certain height and self-weight potential energy to overcome the mold resistance, which is the basic condition for stable continuous casting production. Can not expect the tension leveler to generate tension to overcome resistance.
- Flexible starter bars are generally used for large-section cast billets. The basic radius of the casting machine is large, and the cast billet and the starter bars have large potential energy, so a large hydraulic pressure is required to generate enough friction to balance the sliding force.

At the end of casting of the continuous caster, the moment the billet leaves the copper tube, due to the absence of the mold drawing resistance, if the billet size is large, the weight per unit length is greater than the weight of the ingot bar, which is caused by the weight of the billet. The sliding force is the largest. At this time, the tension leveler needs the greatest hydraulic pressure to hold the cast slab in the sky. Therefore, the design of the hydraulic station of the continuous casting machine stretching and leveling machine requires a high pressure of more than 160 bar, which is to send the ingot bar and start to use the continuous casting. Once the stretching and leveling machine most of the rollers are pressed against the high temperature casting billet, it can be converted to The pressure of the hot billet is much lower than the hydraulic pressure of the delivery spindle.

**Continuous straightening curve**

3.1 Determination of the straightening curve equation

Figure 10 is the tension leveler design of ASIMCO for the Iran project. I simplified it here. The cast billet is 200mm square billet, the basic radius of the caster is R9 meters, and the rigid starter bar. The angle inside is selected as 3.542°: Tanα=0.061898433

This angle is the angle between the tangent line of the lower roll of the draw roll and the horizontal line. If this is the starting point of the curve between the draw roll and the intermediate roll, the corresponding slope of the curve is tan(α).

The derivative of the curve is the slope of the aforementioned great circle at the tangent point. Assuming that the center of the coordinate is on the center of the basic radius of the continuous casting machine, then the abscissa of the tangent point is x = -9000*sin3.542=-556.02171. : The derivative is:

Knowing the abscissa of point x, you can find the slope of this point by bringing it in. This method of finding is consistent with the above results.

Let’s put the coordinate origin on the intersection of the middle roller surface of the five-roller tension leveler and the vertical centerline. According to consensus, the cubic parabola equation is generally used to determine the straightening curve. The cubic parabola equation is: Derivation of the above formula:

The value of k| can be given according to the initial conditions, namely:

For the cubic curve, the slope of the tangent point, the distance from the tangent point to the vertical roller surface below the pulling roller a=175*sin3.542=10.81153397mm, so for the cubic curve the abscissa x=10.81153397=1114.188466mm , Into the formula, you can find k = -1.6621×10-8, so that the equation of the cubic curve can be determined.

3.2 Calculation of the descending height of the middle roller and the straightening roller in the straightening section

We find the descending distance between the intersection of the middle roll surface and the vertical centerline to the tangent point of the continuous casting reference circle on the drawing roll, set x=-1114.188466mm, which can be calculated as 22.98884mm, plus the vertical point of the drawing roll The amount of descent h=175-175*cos3.542=0.334288616mm, the descent amount of the centerline of the lower roller and the middle roller of the pulling roller=22.98884+0.334288616=23.3212862mm. In this way, all the geometric parameters of the so-called continuous straightening of the tensile straightening machine are determined.

The drop amount on the drawing is 23.53mm, which is a little different from my calculation. I estimate that some designers use empirical formulas in the calculation of the K value, so there is a little difference. This difference does not affect the production and use of the equipment, but from three times From the perspective of the square curve, the deduced formula is accurate, but I think this error will not have any negative impact on straightening.

The drop amount on the drawing is 23.53mm, which is a little different from my calculation. I estimate that some designers use empirical formulas in the calculation of the K value, so there is a little difference. This difference does not affect the production and use of the equipment, but from three times From the perspective of the square curve, the deduced formula is accurate, but I think this error will not have any negative impact on straightening.

A lower roll is added between the pull roll and the straightening roll. The purpose is to convert single-point straightening into two-point or continuous straightening, but I always think it should be two-point straightening, and it should be based on the principle of continuous straightening. The calculated descent of the intermediate roller and the straightening roller takes into account the elastic recovery of the object after straightening, that is, exceeding the required straightening amount, giving the casting billet space for recovery. Therefore, the design of the roller for straightening is lower.

I was observing the Xixing company’s R9-meter and R8-meter billet continuous casters, and saw that the leveling rolls were lifted during normal production, relying on the weight of the cast slab to generate the straightening force, but the middle roll was separated from the bottom of the cast slab. On the surface, this amount is about 2mm. Of course, it also leaves the bottom roller surface of the straightening roll. Observe that the two bottom rolls of the straightening roll and the middle roll are not rotating. This is the single-point straightening mode, as shown in Figure 11. From the long-term production practice, no impact on product quality has been seen. Therefore, there is no need to believe in the so-called continuous straightening, that is, two-point straightening and single-point straightening are no problem.

I used to be a steel rolling engineer. The rolled rebars need to undergo bending and reverse bending tests. Sometimes, in order to observe the bending of the steel bar under extreme conditions, I bend the steel bar to 135°, and then consciously bend it to 90~180°. The deformation conditions are very harsh, and the strain has reached more than 15%, but basically no cracks on the surface of the steel bar can be seen, which shows that the steel can adapt to a large strain and deformation. Although the slab straightening is the as-cast structure, the slab under high temperature still has a certain degree of plasticity, and straightening cracks will not appear at a small strain rate.

Once the pouring was stopped due to power failure, the φ280mm cast slab has been cooled to below 500°C. In order to reduce the workload, continue to turn on the straightening machine to pull it out after the power is turned on, carefully observe the surface of the cast slab, and then cut the low-magnification sample. Nothing is found Any crack defect, according to the normal billet delivery, everything is normal, indicating that as long as it is not straightened in the dual phase zone, it will not affect the billet. The rolling straightening process is everywhere, and the straightening deformation is very large. The workmanship is normal, and generally no straightening defects will occur.

I think the continuous straightening roll system layout idea should be the same as the roll system layout idea in the secondary cooling chamber of the vertical bending continuous casting machine from a straight section to an arc section, and it can also be designed with reference to the above calculation. Vertical bending continuous casters are rarely used on billets and round billets. I always think that the cleanliness of molten steel must be completed in the refining process. You cannot expect to remove inclusions in the tundish and mold, and in the mold The downward scouring action of the inner molten steel creates agitation of the molten steel in the cavity. It is difficult to have the conditions for the inclusions to float up. It does not require the straightness provided by the vertical bending continuous casting machine to float the inclusions. The vertical bending continuous casting machine is complicated in equipment. Maintenance is troublesome and there are many spare parts, so there is no need to increase the trouble. The arc continuous caster is fully capable of doing it.

The problems that arise in the production process can always be solved through on-site efforts, continuous adjustment and continuous improvement. As long as the on-site engineering and technical personnel, management managers and on-site employees continue to learn and practice, they can use the existing equipment Produce higher-level steel grades under horizontal conditions.

The purpose of writing this article is to solve the problem, expounding the ingot process and the straightening process, hoping to solve the existing problems, and at the same time expounding some theories on the casting slab straightening, hoping to give you a clear concept. Understand this starter and straightening process. At the end of the article, the gradual straightening parameters are deduced. Everyone is engaged in the work related to the continuous casting machine. It may be useful to understand this design.