Influence of Strip Bite Time in Work Rolls Gap on Dynamic Loads in Strip Rolling Stands

Reasons of increase of dynamic coefficients of main drive lines are discussed. Methods of their calculation and technical solutions for their reduction are offered by means of increase of elastic torsion angle of main drive lines during unsteady rolling when metal bites happen. According to results of calculations of elastic shaft-energy accumulator it was recommended to install it in motor section of #4A rolling stand of hot rolling mill 1700 of PJSC 'ILYICH iron and steel works' (Mariupol city, Ukraine).


Introduction
Productivity of rolling stands depend on speeds of their acting parts and minimal technological time.Due to it there is a 'speed problem' in rolling stands equipment design which is in constant solve with durability and stability of equipment [1].To the point, reduction of dynamics in rolling stands is important for production of new flat products.
For characteristics of dynamic loads in rolling stands main drive lines dynamic coefficient is used [2,3]: where T d is rolling torque during unsteady rolling, N•m; T st is rolling torque during steady rolling, N•m.
Value of k d is not constant for single main drive line.It is proved that at bigger reduction of metal values of k d and T d are less compared to ones at smaller reductions.In addition, when width of strip is increased by 1.5 times k d increases by 10...20% .Also, it is important to consider: x difference between speed of metal strip entering work rolls (WRs) gap and circumferential speed of WRs; x values of radial gaps in elements of main drive lines; x forms of front parts of metal sheet; x time of metal strip bite t b .This paper is devoted to consideration of t b influence on increase of T d .For characteristics of work of metallurgical and rolling equipment a term 'Quality' Q is equal to technological load divided by maximum load.Q = 0.50…0.55during metal strip bite.It means that k d = const can be used for preliminary evaluation of dynamic loads acting in main drive lines during particular periods of time.For calculation of maximum values of dynamic coefficients ' d k at interval 0.01sec ≤ t b ≤ 0.03sec when radial gaps are closed it is reasonable to use method which was modified by authors of this paper by means of modern mathematical software and data given in particular scientific work reports : where λ is a coefficient that takes into account dynamics due to t b during hot strip rolling and defined from graph on Fig. 1; А is a coefficient that takes into account parametric characteristics of main drive line and its flexibility , sec.where m 1 = 0.3…0.35mm - is a coefficient that takes into account technological features of use of thick sheet rolling stands, m 1 = 0.3 is for roughing rolling stands, m 1 = 0.35 is for finishing rolling stands; e is gap in main drive line within interval 0.0mm ≤ е ≤ 6.0mm which opens and closes during metal strip bite by WRs, mm; m 2 = 10…20sec -1 is a coefficient that takes into account speed of metal strip bite by WRs; m 2 = 10 is for high-speed main drive lines with speeds of metal strip bite equal to 2m/s and more, m 2 = 20 is for low-speeds main drive lines (low speeds of metal strip bite by WRs).Based on proposed methods of dynamics coefficients calculations at hot sheet/strip rolling it is possible to theoretically define dynamics coefficients relations to times of bites by WRs for different types of rolling stands.Initial data for calculation and its results are given in Tab. 1, where k d theor. is theoretically calculated dynamics coefficient at particular time of bite as per methods ( 2) and ( 3), k d exper. is experimentally calculated dynamics coefficient at particular time of bite, Δ is calculation error of k d theor. .Analysis of graphs of theoretical relation of dynamics coefficient and time of strip metal bite for main drive lines of different sheet rolling stands (refer to Fig. 2 -Fig.6) and results of dynamics coefficients calculations bring us to conclusions: x on practice, for preliminary calculation of elements and details of hot strip stands it is possible to use methods ( 2) and (3) because calculation error of k d theor. is not more than 5%; x according to theoretical calculation results, depending on type of rolling stand increase of time of metal bite by 30% can reduce dynamics coefficient by 5…20%; moreover, reduction of dynamics coefficient results in reduction of amplitude of T d oscillations at unsteady rolling that decreases thickness deviation of finish strip  For reversing rolling stands where WRs are driven by DC motors there is a possibility to regulate speed of metal bite.For rolling stands where WRs are driven by synchronous AC motors there is possibility to regulate speed of metal bite by change of front form of metal sheet/strip.There are numerous proposals to make cuts of front form of metal sheets/strips by angular, chevron and trapezoidal contours.Experimental results of length evaluation of such contours within one rolling campaign on hot rolling mill 1680 of PJSC 'Zaporizhstal' (Zaporizhya city, Ukraine) after roughing group were 0.05…0.2mand after finishing group were 0.6…1.5m.Similar experiments were done on roughing group of hot rolling mill 2000 of PJSC 'NLMK' (Lipetsk city, Russia) where dynamics coefficients were reduced by 8...13% by usage of contours length equal to 0.22m.On practice method of use of angular, chevron and trapezoidal contours is not popular because of uniform further deformation (during reduction) of sheets/strips front parts.
Based on information given above authors of this paper propose new approach to increase time of metal bite by WRs and to reduce oscillations of T d by means of usage of flexibility of main drive line and increase of its energy capacity due to use of high-torque elastic shaft in main drive line and elastic shaft-energy accumulator which has elastic torsion angle φ oi (given to WRs) comparable to bite angle α o .WRs during metal bite will be rotating by angle (refer to Fig.  Theoretical calculation shows that during steady rolling elastic shaft-energy accumulator (installed instead of initial shaft in motor section) has elastic torsion angle which does not significantly affect productivity of rolling stand.
Experimental shaft, which can provide dynamic rolling torque up to 250 kN•m and torsion angle up to 180deg will be installed soon in the main drive line of hot strip rolling stand #4А of rolling mill 1700 of PJSC 'ILYICH iron and steel works'.It is planned to increase metal bite time by 2...The reported study was funded by RFBR according to the research project №16-08-00845a «Verification and development of models of inelastic deformation at the passive loading».The authors declare that there is no conflict of interest regarding the publication of this paper.

Conclusions
1. Almost all hot sheet/strip rolling mills have low 'Quality' of their main drive lines that results in low durability and stability of their equipment.

One of main parameters which provokes inertia forces to arise during metal bite by
WRs is time of metal bite which is reasonable to increase.

Theoretical methods of relation between dynamics coefficient and time of metal bite
for main drive line are given.4. Innovative approach to increase time of metal bite for main drive lines with synchronous AC motors is proposed.

Fig. 1 .
Fig. 1.Graph of coefficient definition which takes into account dynamics due to t b For approximate calculation of maximum value of dynamics coefficient ' ' d k during thick sheet rolling at intervals 0.006sec ≤ t b ≤ 0.5sec and 1.0 ≤ k d ≤ 3.0 it is reasonable to use method described as linear function, modified by authors of this paper:

Fig. 5 .Fig. 6 .Fig. 7 .
Fig. 5. Graph of relation between dynamics coefficient and time of metal bite for main drive line of roughing rolling stand of thick sheet rolling mill 3000

Table 1 .
Initial data and results of theoretical calculations of relation between dynamics coefficient and time of metal strip bite