【正文】 he Rolling Process1 WORKPIECE TEMPERATURE CHANGE IN HOT STRIP MILLAfter reheating a slab to a desired temperature, it is subjected to rolling. A rolling cycle in a typical hot strip mill includes the following main steps: Descaling of the slab prior to flat rolling by using highpressure water descaling system in bination, in some cases, with edging. Rough rolling to a transfer bar thickness which may vary from 19 to 40 mm. The rough rolling is usually acpanied by edging and inter pass descaling. Transfer of the transfer bar from roughing mill to a flying shear installed ahesd of finishing mill. The shear is usually designed to cut both head and tail ends of the bar. Descaling of the transfer bar prior to entering the finishing mill. Finish rolling to a desired thickness with a possible use of interstand descaling and strip cooling. Air and water cooling of the rolled product on runout table. Cliling of the rolled product.Various types of heat transfer from the rolled workpiece to its surrounding matter occur during the rolling cycle. Some of the lost heat is recovered by generating heat inside the workpiece during its deformation.The main ponents of the workpiece temperature loss and gain in hot strip mill are usually identified as follows: loss due to heat radiation, loss due to heat convection, loss due to water cooling, loss due to heat conduction to the work rolls and table rolls, gain due to mechanical work and friction.The analytical aspects of these ponents are briefly described below.2 TEMPERATURE LOSS DUE TO TADIATION遼寧科技學(xué)院本科生畢業(yè)設(shè)計(jì)（論文） 第 34 頁(yè)Two methods have been employed to derive equations for temperature loss due to radiation.In the first method, the temperature gradient within the material is assumed to be negligible. The amount of heat radiated to the environment is then calculated using the StefanBoltzmann law:q =Sd39。rdtTAa)(4??Where —surface area of body subjected to radiation, m2。]rAq —amount of heat radiated by a body,J。d39。rS—StefanBoltzmann constant。T—temperature of rolled material at time,K。Ta—ambient temperature,K。t—time,s?！猠missivity.?The amount of heat lost by a body q is give by:d39。rq =39。rTcVr?Where c—specific heart of rolled material, J/（kgK ）。Vr—volume of body subjected to radiation, m3—density of rolled material, kg/m3。?The rate of temperature loss ar can be calculated by considering the heat balance condition q = q , and and 12:d39。r39。ar= )(4arTcVASdtT????Equations for the rate of temperature loss due to radiation which have been obtained by reducing some of the known equations to a patible form with an assumption that TaT are summarized in Table 11. In the derivation of these equations, the dependency of the parameters S、 、 and c on temperature is not taken into account. However, the ??遼寧科技學(xué)院本科生畢業(yè)設(shè)計(jì)（論文） 第 35 頁(yè)variations of these constants with temperature may be significant and,therefore, the final from of 13 will depend on the average values selected for these constants.The temperature loss during radiation time tr can be calculate by intergrating the cT?differential equation:=r?trda0The second method of calculating temperature loss due to radiation takes into account the heat transfer along the thickness of the material. If z is the distance from the center of the body toward its surface, then from a Fourier equation we obtain:2dzTt?Where a—thermal diffusivity of rolled material ,m2/sThe differential equation 15 can be solved numerically by the method of finite differences.The goal of these calculations is to establish a relationship between the average temperature of the material Tave which would affect the rolling deformation process and the material surface temperature Tsurface which could be measured.3 TEMPERTURE LOSS DUE TO CONVECTIONIn the hot strip mill, heat transfer by convection is related to the motion of air surrounding a workpiece. This motion continuously brings new particles of air into contact with the workpiece. Depending upon whether this internal motion is forced, or free, the heat transfer is referred to as either forced or free convection. The latter is a usual case in the hot strip mills.A key factor in the calculation of temperature losses due to convection is to determine the heat transfer coefficient, which depends on the material temperature, ambient temperature, material specific heat and density, and the dynamic viscosity of the air flow and its characteristic, ., free, enforced laminar, turbulent, etc. The known mathematical interpretations of this relationship are too controversial to be remended for practical calculation. A consensus among some research workers is that the temperature loss due to 遼寧科技學(xué)院本科生畢業(yè)設(shè)計(jì)（論文） 第 36 頁(yè)convection should be expressed as a certain percentage of the temperature loss due to cvT?radiation:= （ ）cvT?krHere is the ratio between the temperature loss due to convection and radiation and cvT?varies between and according to different studies.4 TEMPERATURE LOSS DUE TO WTER COOLINGThe temperature loss due to water cooling can be calculated by assuming that conduction plays a major role in heat transfer from a workpiece to water. Therefore, when water contacts one side of the workpiece continuously across its width, the amount of heat passing through the outer surface of the workpiece may be expressed by the formula:atTkbqww??)(239。 ?? Where k—thermal conductivity of the surface layer, W/（mK） ；—amount of heat passing through outer surface of the workpiece,J。39。wqb—water contact length, m。w—workpiece width, m。Tw—water temperature, K。tw—water contact time,s.The amount of heat released by a workpiece is given by:)(39。 dwTcVq???Where v—volume of workpiece cooled by the water,m3?！猼emperature loss due to wate