Temperature and speed modes of rolling

Traditionally, it was believed that in order to obtain the required structure of the metal, and, consequently, its mechanical properties, three parameters should be strictly observed during rolling at the SHSHP: the temperature of the end of rolling t kp , the coiling temperature t cm and the value of the relative reduction of the metal in the last stand of the finishing group e.

The classical diagram of the microstructure of hot-rolled sheet steel depending on t kp and t cm is widely known. It is shown in Fig.36.

Fig.36. Microstructure Diagram of Hot Rolled Steel Sheet

It can be seen from the figure that in order to obtain a favorable fine-grained uniform metal structure of low-carbon steel, its rolling must be completed at t kp > 850°C, and wound into a roll at t cm < 680°C. To this it should be added that in order to ensure the required structure and mechanical properties of the metal, two more restrictions must be observed: t kp should not exceed 920 ° C, and the relative reduction in the last stand of the SHSHP should be within e ³ 15%. Taking into account that 90-95% of products manufactured at the ShSGP are not subjected to heat treatment from separate heating (unlike TLS), then keeping t kp , t cm and e within the specified limits determines the required structure and mechanical properties of finished hot-rolled sheets and strips, and not only in the case when hot-rolled products are marketable, but also when they serve as a rolling stock for cold rolling mills of sheet and tin.

Experimental studies of the temperature conditions of rolling carried out at the beginning of the 1970s at all rolling mills operating in the USSR at that time, including combined mills, showed that the existing differences in the temperature conditions of rolling at rolling mills are explained by the layout of their main equipment, the length technological line, technological process parameters: slab heating temperature, roll thickness, mill assortment, rolling speed. Studies have found that the temperature at the end of strip rolling does not always correspond to the optimal range. Moreover, for strips with a thickness of 2-2.5 mm, it is lower than optimal, and for thick ones – 4 mm or more – higher.

To increase the end-of-rolling temperature at the 1st generation SHSHP, we used a possible permissible increase in the heating temperature of slabs, a redistribution of reductions in the stands of the roughing and finishing groups (a larger load of the last and a smaller one of the first stands), an increase in the thickness of the rolling, and a possible increase in the rolling speed in the finishing group of stands.

Traditionally, strip temperature control is carried out in areas:

– behind the last stand of the roughing group (it is used to evaluate the metal heating mode and the need for its adjustment);

– in front of the finishing group of stands (it is used to evaluate the possibility of rolling the roll, and in some cases, adjusting the operation of automation systems);

– behind the last stand of the finishing group;

– in front of the winders.

According to the last two readings, the optimality of the temperature-speed regime of rolling is estimated.

Numerous studies have shown that the redistribution of reduction modes in the roughing and finishing groups of stands is ineffective, increasing the rolling speed in the roughing group of stands gives only a slightly greater effect.

The main heat losses at the SHSHP occur on the intermediate roller table (up to 60% of the total heat loss of the strip during rolling). One way to reduce them is to use different types of screens.

The ENKOPANEL thermal insulation system, developed by Enconiech Engineering Services (Great Britain), has become widely known. It is a tunnel consisting of top, bottom and side panels. The general view of the system is shown in Fig.37.

Fig.37. General view of the shielding device ENKOPANEL

Each panel is double layered. The first layer is a thin heat-resistant steel plate with high absorption and emissivity, the second layer is thermal insulation. With a stable rolling process, the working surface of the panels after passing 3-5 rolls acquires a temperature close to the temperature of the rolls. Thermal equilibrium is established between the roll and the panel, and heat loss by the roll is significantly reduced. The top panels consist of sections (in Fig. 9.40 some of the sections are raised, some are lowered), which are lifted independently of each other using special mechanisms. The installation height of the upper panels above the level of the roller table is 250-300 mm (the lower the panels are installed, the higher their efficiency). With an intermediate roller table length of 90 m, the length of the tunnel is 60 m. With a roll thickness of 26 mm, its temperature at the entrance to the finishing group of stands increases by 60-70 ° C compared to the temperature of the strips that entered the finishing group of stands without the use of ENKOPANELS.

The second technical solution that allows to reduce the heat loss of the roll on the intermediate roller table is an intermediate rewinding device (PPU or Coilbox). The PPU scheme is shown in Fig. 38.

Fig.38. Scheme of the intermediate rewinder: 1 – the last draft stand; 2 – floor rollers; 3 – guide rollers; 4 – bending rollers; 5 – roll end bender; 6 – winding stand; 7 – mandrel; 8 – transmission mechanism; 9 – unwind stand; 10 – pulling rollers; 11 – flying scissors; 12 – the first finishing stand; 13 – rollers of roller tables

The main part of the PPU is a coiler without a drum, which consists of floor, guide and bending rollers, winding and unwinding stands, a roll end deflector, a mandrel and a transmission mechanism. The sequence of operations is as follows. After leaving the last stand of the roughing group, the front end of the strip goes to the floor, guide and bending rollers, where the first coil of the coil is formed, and the rest of the strip is superimposed around it. After the end of the winding of the strip, a mandrel is inserted into the hole in the roll, which later holds the roll during its transfer to the winding and unwinding stand. When the roll is still on the winding stand, the bender is lowered onto it and the rollers of the winding stand begin to rotate, transferring the front end of the roll together with the bender to the pulling rollers. After these rollers capture the front (former back) end of the roll, they transfer it to the flying shears and to the first finishing stand. At the start of strip rolling in the finishing group of stands, the roll located on the mandrel is moved from stand 6 to stand 9 by a transmission mechanism. The unwinding of the roll continues. As soon as the roll has reached stand 9 , stand 6 is ready to receive the next strip.

The installation of PPU on existing mills makes it possible to increase the mass of rolls rolled (if this was prevented by the short length of the intermediate roller table), reduces heat loss by rolling on the intermediate roller table, and eliminates the temperature drop along the length of the roll from the front end to the rear (the so-called “temperature wedge”) when entering the finishing group of stands (allows you to refuse the use of acceleration in the finishing group of stands), in case of emergency in the finishing group of stands or on winders, it allows you to save the roll in the PPU for up to 15 s. When constructing new SHSHP, the PPU allows to reduce the length of the intermediate roller table (reduction of capital costs) and all other mentioned advantages remain in force.

At the first-generation SHSHP, the rolling temperature conditions were optimized primarily by choosing the general level of rolling speed – higher for thin strips (up to 8-10 m/s), for thick strips – lower (5-6 m/s) . Moreover, in the finishing group of the first generation SHSHP, when rolling the strips, there was no change in speed along their length (Fig. 39 a ).

Fig.39. Schemes of the high-speed mode of rolling on the ShSGP: V 1 – the speed of capturing the front end of the strip by the winder; V 2 is the maximum speed of strip rolling; a 1 , a 2 – slow and fast acceleration of the finishing group of stands

An increase in the mass of slabs used in second and subsequent generations of rolling mills, as well as the expansion of the assortment of rolled strips towards a decrease in their thickness, required a significant complication of the speed regimes of rolling strips in the finishing group of stands. The matter was complicated by the fact that the existing coilers could not ensure a reliable grip of the front end of the strip at a speed of more than 10-12 m/s. In addition, transporting the front end of thin strips on the discharge roller table at a speed higher than indicated is also problematic.

Therefore, they began to apply the speed mode shown in Fig. 39 b : rolling the strip at a speed of not more than 11 m / s until the strip is captured by the winder (point A) and after that the acceleration of the finishing group of stands with the acceleration value a 1 = 0.02-0, 08 m/s 2 , depending on the thickness of the strip. On the third generation SHSHP, they began to use fast acceleration a 2 to 1 m / s 2 (Fig. 39 c ) for an intensive and rapid increase in t kp , and then rolling followed with slow acceleration to eliminate the “temperature wedge”.

The temperature of the metal during rolling changes due to the cooling of the slab by radiation (Dt rad ) and convection (Dt k ) through the scale layer during transportation from the furnace to the mill, cooling with water for hydrodescaling the scale (Dt g.o. ), and the temperature of the rolled product – due to heat transfer to the work rolls (Dt in ), cooling by radiation and convection during pauses between passes and on roller tables, increasing the heat content of the roll due to the energy of plastic deformation (Dt d ), the work of friction forces in the contact zone of the strip-roll and exothermic oxidation reactions. In addition, the roll loses heat under the action of water falling on its surface, cooling rolls and fittings, during the passage of hydraulic knocks, due to contact heat exchange with the rollers of the roller tables and parts of the working stands.

You can write: Dt u003d Dt d – (Dt izl + Dt to + Dt in + Dt g.o. ).

It is assumed that Dt includes heat losses not only from contact with the rolls, but also from contact with other parts of the mill and cooling water.

The methodology for calculating all these components is presented in the methodological manual “Preparation of commodity sheets and rozrahunok modes in rolling”, Donetsk – 2006.

High rolling speeds, typical for modern wide-band mills, make it possible to achieve almost continuous deformation in the last two or three stands, when metal recrystallization in the interstand gaps does not have time to occur, in this case the effect of partial reductions is summed up. Then the amount of reduction in the last stand should not be of decisive importance, and restrictions on the amount of partial reductions are removed.

To ensure the required temperature of strip winding on SHSGP of all generations on the discharge roller table, installations for accelerated cooling of strips of various designs are used.

Figure 40 shows the layout of the collectors of the strip cooling system on the outlet roller table of mill 2000 of OAO Severstal.

Fig.40. Layout of the collectors of the strip cooling system: 1 – roller of the discharge roller table; 2 – lower collector; 3 – collector for cooling the rollers of the roller table; 4 – upper collector with a siphon; 5 – hydraulic cylinder of the upper manifold; 6 – water supply pipelines

Cooling of the strip from above is carried out from round collector-tanks located above the roller table. Each collector-tank has 79 siphons, of which water is supplied to the strip by laminar jets. Cooling from below is carried out through the openings of the collectors located between the rollers of the roller table (see Fig. 40).

The operation of the accelerated cooling unit is controlled by a computer.

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