OBRABOTKAMETALLOV MATERIAL SCIENCE Том 23 № 3 2021 EQUIPMEN . INSTRUM TS Vol. 6 No. 4 2024 The possibility of increasing the lifetime of operated working rolls of hot rolling mills is provided in some studies [2–9] related to direct infl uence of hot rolling process temperature on the stresses occurring in rolls. However, the materials of the above-presented works practically do not take into account specifi c features of stress-strain state during strip contact with working rolls [10–12].At the same time, application of calculation methods for such a stress-strain state based on the elastic-plastic model of the deformation zone [10–12] demonstrated that the calculations require some clarifi cation. The materials of the publication [13] reveal the infl uence of diff erent ranges of hot rolling temperatures and actual steel chemical compositions with carbon content less than 0.25 % on elastic and plastic properties of strips deformed in the mill. A conclusion is made in the same paper [13] that the length of elastic sections can reach 32 to 40 % of the total length of the deformation zone; this feature was not previously taken into account. The described changes in the structure of deformation zones when producing rolled products from structural low-alloyed steels with the minimal thickness range of 5.5 to 2.0 mm result in the problem of shortening lifetime of working rolls in last stands of continuous wide-strip hot rolling mills because of increased normal contact stresses in the deformation zone, as further calculations showed, to the dangerous level of 1,068 to 1,245 MPa, typical for cold rolling mills [10]. An eff ective solution to the problem of increasing the lifetime of operated working rolls at the stage of development hot rolling process modes at modern steel plants should begin with reliable methods for calculating the energy-force parameters and stress-strain state of the strip in the contact with working rolls [11–13]. The purpose of the work is to study the distribution of normal contact stresses in deformation zones during hot rolling of strips from low-alloyed structural steels to achieve high lifetime of working rolls. The research objectives are to supplement the method of calculation of normal contact stresses in production of low-alloyed structural carbon steels; to construct a linear regression within the elasticity modulus calculation; to study distribution of normal contact stresses in deformation zones during hot rolling considering special features of its stress-strain state on the basis of the existing process mode; to improve the technology of hot rolled strips production from low-alloyed structural steels in the fi nishing train of the wide-strip mill to achieve high lifetime of work rolls; to evaluate the effi ciency of the developed procedure and new hot rolling modes. Research methods Based on the modelling of the stress-strain state of the strip during hot rolling [11–13], Table 1 provides formulas for calculating рх(hх) for elastic and plastic sections of the deformation zone. The lengths of such sections are indicated as х1, х4 and х2, х3 respectively. The formulas help to study and reveal the regularities of changes in the maximum normal contact stresses р1max, p4max and pxmax, distributed along the length of the deformation zone lc in Fig. 1. Table 1 shows that the calculation of normal contact stresses px(hx) of strip hot rolling, with known values of reduction in thickness Δhi = hi−1 − hi and specifi c interstand tensions σi−1 and σi, directly depends on the correct defi nition of the elasticity modulus ЕS, the friction factor in the deformation zone μi and the actual plastic resistance σpl. The specifi cs of μi and σpl values determination depending on deformation-velocity parameters, working rolls material and chemical composition of hot rolled steel are given in [12–14]. Varying strip elasticity modulus ЕS at the temperature of 1,050 to 750 °С, which is typical for fi nishing stands of hot rolling mills, is of particular interest for the study of structural low-carbon steel rolling. For steels 0.14 C-2 Mn-N-V, 0.18 C-Cr-Mn-Ti and 0.1 C-Cr-Si-Ni-Cu, selected for further calculations of normal contact stresses during rolling and for evaluation of lifetime of working rolls in the fi nishing train of continuous wide-strip Mill 2000 at PAO Severstal, the dependence of ЕS on temperature, according to reference data [15], has the form shown in Fig. 2. Standard calculation methods intended for all machine parts were used to evaluate the rolls contact strength during rolling. The correction was performed using the formula of allowable stresses for the compression scheme from [16] to apply these conventional methods to cast iron rolls of rolling mills: [ ] 1.5 , u σ = ⋅ σ (1) where σu is the ultimate compressive strength of the working roll material, MPa.
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