Stands Of Mill Research Articles

Simulation of thermal fatigue in hot strip mill work rolls

The surface deterioration of work rolls constitutes a significant constraint on the operation of hot mills, as the duration of rolling campaigns are greatly restricted by the surface condition of the rolls. Work roll changes account for a large proportion of downtime in the operation of hot strip mills, resulting in financial penalties for the mill operators in terms of lost production and roll maintenance.It is generally accepted that thermal fatigue represents one of the main factors influencing work roll wear in the finishing stands of a hot strip mill. In order to assess the thermal fatigue behaviour of typical work roll alloy grades, a series of tests were performed on high‐speed steel (HSS) and high‐carbon high‐chromium steel samples using a stand alone twin‐disc simulation machine under conditions that are comparable to those of hot rolling of steel. The progressive changes in surface condition of the roll samples were monitored and analysed by means of stylus profilometry, optical and scanning electron microscopy. The experimental results were compared with fragments of scrapped work rolls taken from the mill. Tests performed on the HSS samples did not result in appreciable surface degradation of the test sample. On the other hand, examination of the high‐carbon high‐chromium samples after testing revealed thermal crack morphologies and surface defects comparable to those observed in mill roll samples.

Modeling the Thermal Regime of a Cold-Rolling Mill with Allowance for Differences in the Cooling Conditions for the Top and Bottom Rolls

A mathematical model is constructed to describe the thermal regime in cold rolling. The model links the perturbing and control variables together in a single algorithm based on a system of heat-balance equations solved in sucesssion for all of the stands of a continuous mill. Design parameters of the mill, parameters of the rolling regime, and thermophysical constants are used as the initial data. Heat-transfer coefficients that characterize conductive and convective heat exchange during the rolling operation are among those constants. Some numerical values for these quantities are given in the article, while other values are refined for each specific mill as part of the model's adaptation to the given situation. The reliability of the model was checked on a five-stand 1700 mill by measuring the temperature of strip at the inlet of the first stand and the outlet of the fifth and final stand. The temperatures of the work rolls in all of the stands were also measured immediately after roll changes. Testing of the proposed model showed that it can be used to improve the thermal regime and cooling systems of continuous cold-rolling mills.

Analysis and Active Rejection of Chatter in Rolling Mills

AbstractThis contribution is concerned with a vibration phenomenon in multistand rolling mills called (third octave) chatter. In order to analyse the dynamical system, which is nonlinear in nature, a mathematical lumped parameter model of the plant is presented. This model comprises the mill stands, the strip as a coupling element of neighbouring mill stands and the hydraulic actuators. By means of the mathematical model of the coupled system the third octave chatter can be explained by an instability of the dynamic system. For the purpose of an active vibration rejection a nonlinear controller based on the PCH structure of the hydraulic system is designed.

The rolling sliding wear response of conventionally processed and spray formed high speed steel at ambient and elevated temperature

The effect of spray forming on the wear properties of a 1.2C–3.4W–8.9Cr–4.3V–2.7Mo high speed steel is described. The microstructure, fracture behaviour and wear response of the spray cast materials are compared with those of conventionally cast material of the same composition. Spray forming resulted in a substantial reduction in microstructural scale for both grain size and carbide size. In addition, large primary and interconnected eutectic carbides observed in the conventionally cast material were replaced by a more uniform distribution of discrete globular carbides in the spray cast material. This refinement is shown to give rise to an increase in the resistance to fracture. Wear testing was undertaken in the rolling/sliding configuration (8% slippage) at 300 N against an M2 tool steel counterface in the temperature range 20–650 °C. The wear rate of the conventional material was significantly higher than that of the spray cast material at all temperatures. Both adhesive and oxidative wear mechanisms were observed, with the latter becoming more prominent as the test temperature increased. Surface degradation occurred preferentially at the carbide/matrix interface for both materials, but was particularly prominent in the Fe/M 2C–M 6C eutectic regions and at large primary VC carbides in the conventionally cast material, resulting in higher rates of material loss in these regions. The improvement in wear resistance observed for the spray formed material can be directly attributed to the refinement of both primary and eutectic carbide phases and the elimination of microsegregation. The laboratory observations are compared to a worn surface of a spun cast hot mill work roll of nominally the same composition following a normal campaign in the roughing stands of a hot rolling mill. This showed that the wear mechanisms in the field were reproduced in the laboratory. The relationship between process parameters microstructure and wear mechanisms is discussed.

Simulation of the deformation of strip between stands during the process of rolling with a loop

Increased requirements for the dimensional accuracy of rolled products demand the analysis of the causes of product dimensional deviations to be carried out in greater detail and accuracy. For a continuous rolling mill, the inter-stand tension in the strip is one of the main reasons for these deviations. Usually, this tension is not constant during rolling due to changing rolling process parameters. For this reason, the rolling process is accomplished with a loop. This is especially important for the finishing group of stands of a continuous rolling mill. In practice, however, even rolling with a loop does not always guarantee the required dimensional tolerances of the finished product. The results of the theoretical analysis of forces, strengths and deformation resulting in the loop of the strip as it passes between the stands is discussed in the paper. The computer simulation of this process using the FORGE 3D program is also presented. It has been shown that alternate plastic bending of the strip in the inter-stand distance leads to a change in the strip dimensions.

Thermomechanical treatment of reinforcing steel

The work deals with application of interstand and after deformation cooling of coiled ( .10, 12 mm) and reinforcing ( .14,16 mm) bar steel in the accelerated cooling installations behind the prefinishing and finishing stands of the light-section mill which provides the formation of the even fine-grained ferrite-perlite structure and reduces grain growth in the mid section of the coil. The application of the roll stock interstand cooling behind the 11th stand together with the afterdeformation accelerated roll stock cooling to 800-8500C enabled us getting reinforcing bar steel diameter .14mm with high impact strength..

Neutral angle and forward slip during high-speed rolling in finishing blocks of wire mill stands

Neutral angle and forward slip during high-speed rolling in finishing blocks of wire mill stands

The hot strip mill as an experimental tool

The characteristics of work hardening and of strain accumulation at the high temperatures (850–1050°C) involved in strip rolling are described. A method is outlined for quantifying the kinetics of softening (by static or by metadynamic recrystallisation) during the interpass intervals in various grades of steel. Using these kinetics in an appropriate mill model, the mean flow stresses pertaining to various mill stands are predicted. These values are compared with the measured ones derived directly from mill logs. It is shown how discrepancies between the predicted and measured values can be used to improve the accuracy of the expressions for the kinetics. It is in this way that a hot strip mill can be used as an ‘experimental tool’.

Hot strip width control method by using looper tension measuring system in finishing mill

A high precision hot strip width control method has been developed in this study by applying an interstand looper tension measuring system at the finishing mill stands. As the width deviation of hot rolled steel strip is closely related to abnormal increase in the interstrip tension, an on-line measuring device of looper tension and a data analyzing system were developed in this study. To determine dominant factors that will cause local width shortage, the logged data sets of bar width, bar thickness, looper tension, and strip thickness along the strip length were correlated with the data set of strip width change. With the result of the correlation analysis, existing sequence control logic and parameters for looper actuator were modified for strip width quality and the gains of the looper control were refined for stable operation during the full passage of rolled strip. The on-line tension measurement and tension feedback control for the looper system, improved the strip travelling stability and reduced strip width deviation in the strip top end region.

Regenerative Effect in Rolling Chatter

The regenerative effect has been extensively studied in the field of metal cutting and recognized as one of the major causes of self-excited vibrations commonly known as chatter. In the presence of interactions between rolling mill stands, affecting the overall system dynamics, it will be shown that a tandem rolling mill may also become unstable due to a similar regenerative mechanism even though the individual stands are stable. In this paper, tandem mill stability will be investigated, in terms of the regenerative effect, using a multistand chatter model that incorporates a homogeneous process model and a mill structural dynamics model. Finally, simulation results will be presented to demonstrate the effects of regeneration on a three-stand tandem mill.

A three-dimensional finite element simulation of the vertical–horizontal rolling process in the width reduction of slab

A thermal coupling analysis is carried out by the full three-dimensional rigid–plastic finite element method to simulate vertical–horizontal rolling process during width reduction in the roughing stands of a hot strip mill. The slab shape, the spread and temperature as calculated are in good agreement with the experimental results in production mills, and the sectional distribution of temperature is given.

The Hot Strip Mill as an Experimental Tool.

The occurrence of carbonitride precipitation during the short interpass times associated with hot strip rolling is considered, together with the interaction between precipitation and static recrystallization under these conditions. The characteristics of work hardening and of strain accumulation at the high temperatures (850 to 1050°C) involved in strip rolling are described. Expressions are introduced that specify the kinetics of softening (by static or metadynamic recrystallization) during the interpass interval in various grades of steel. Using these kinetics in an appropriate mill model, the mean flow stresses pertaining to various mill stands are predicted. These values are compared with the measured ones derived directly from mill logs. It is shown how discrepancies between the predicted and measured values can be used to improve the accuracy of the expressions for the kinetics. This is one way in which a hot strip mill can be used as an experimental tool.

Analysis of dynamic behaviors of tandem cold mills using generalized dynamic and control equations

A generalized equation set is proposed to investigate dynamic behaviors of tandem cold mills. The system is composed of the intrastands and the interstands equations and the boundary conditions of the first and the last mill stands. The solution provides detailed dynamic effects of disturbance variables such as anneal and entry gauges, hardness, and roll eccentricity. The transportation delay is considered as well. The mill dynamic equations and the control equations can be combined to form a linear system. The mill performance and the actuator response can be solved directly from the linear system.

Hot compression testing of mild steel industrial reheat furnace scale

The deformation of reheat furnace scale, formed on mild steel slabs in an industrial reheat furnace, was investigated using a high temperature compression test technique, over the temperature range of 650–850°C. Strain was nominally 30% for all tests, conducted at two strain rates, 0.1 and 1.0 s−1. The mill scale samples were 6mm thick, ground to obtain cylindrical compression test pieces. The test parameters were chosen to demonstrate the deformation behaviour of a porous scale at temperatures similar to the entry conditions to the finishing stands of a hot strip mill. Oxide scales are generally assumed to be brittle at these temperatures. In the present tests the scale deformed in a linear‐like elastic/plastic manner under these conditions. Although cracking occurred, the samples remained intact as the deformation proceeded. Below 650°C the scale was crushed to a powder immediately after the start of the compression process.

Mathematical modelling of mill set-up in hot strip rolling of high strength steels

Abstract This paper describes research work concerned with the development of finite-element simulations and constitutive equations to be used to idealise the hot strip rolling process and to aid in the improvement of online mill set-up for new grades of strip steels, i.e. carbon manganese, HSLA. Hot deformation test data were obtained using plane strain compression testing and were utilised to develop approximate constitutive equations to idealise the flow stress for a given material. These equations were based on the high stress law and were compared with the plane strain compression tests. The relationships were incorporated into finite-element models using the ABAQUS/Explicit software package. The finite-element models simulated all seven finishing stands of the hot mill and were verified and compared to the measured rolling loads and to predicted rolling loads from Ekelund and Sims rolling models.

Improvement of roll-gap set-up accuracy using a modified mill stiffness from gaugemeter diagrams

Mill-stiffness modification was performed to achieve a more precise roll-gap set-up in the finishing stands of a hot-strip mill. The exit strip-thickness error was assumed to be caused by incorrect mill stiffness. A modification equation which can eliminate thickness set-up error as well as roll-force prediction error was derived from the gaugemeter diagram on the assumption of constant strip stiffness. Measured mill-stiffness values were replaced successively by modified mill-stiffness values. The replacement of the mill stiffness was repeated until saturated mill-stiffness values were obtained at each stand. After mill-stiffness modification, the exit thickness accuracy was markedly increased and the operational stability also improved.

タングステン・カーバイドロールの腐食摩耗に及ぼす冷却水質の影響

Tungsten Carbide Cemented Rolls are widely used in hot-steel-products rolling because of good wear resistance. At the early period, small diameter ring rolls with tungsten carbide mounted on steel parts were used in finishing cantilever blocks of wire rod mills. Recently, medium diameter ring rolls and solid rolls with tungsten carbide have been used in modern intermediate stands of rod mills and stretch reducing stands of tube mills. Moreover, they have started to be applied to finishing stands in stainless section mills because of good galling resistance. But the tungsten carbide rolls is very expensive compared with the conventional cast iron rolls. To apply tungsten carbide rolls instead of cast iron rolls economically, it is necessary to get about five to ten times roll life of cast rolls. In this report, the corrosive characteristic of tungsten carbide alloy in high temperature water, which is one of main reasons for roll wear, is investigated. It was found that the optimization of the pH and calcium concentration of cooling water is effective to suppress corrosion of rolls. This effect was assured in the real production of three roll type stretch reducing stands of the tube mill in Kashima Steel Works.

Fatigue characteristics of work roll in roughing stands of hot strip mill: Lee, W. H., Kim, S.J., Son, B.H., Lee, Y.H., Yi, J.J. and Kim, J.K. Research Institute of Industrial Science and Technology (Pohang City) Technical Research Report5 4 (Dec 1991) 822–832 (in Korean)

This paper presents a comprehensive investigation into non-proportional multi-axial fatigue of welded components by introducing an equivalent structural stress parameter that takes into account of load-path non-proportionality in addition to plate thickness and stress state effects. This is accomplished by formulating a “moment of load path” or “MLP” based fatigue damage parameter that provides a consistent treatment of load-path non-proportionality under arbitrary multi-axial loading conditions for which cycle counting can be consistently performed by means of a previously developed path-dependent maximum range (PDMR) cycle counting procedure. To examine its broad applicability and effectiveness, non-proportional multi-axial test data obtained using different components, joint types, and loading conditions from various sources are analyzed using the newly developed equivalent stress parameter. The results show that the new equivalent stress parameter enables not only an effective consolidation of all multi-axial test data (up to about 300 tests) analyzed in this paper into a narrow band, but also the demonstrated transferability between the master S-N curve (dominated by test data under uniaxial cyclic loading conditions) adopted by the 2007 ASME Div 2 and API 579 RP/ASME FFS-1 Codes and the consolidated S-N curve dominated by severe non-proportional multi-axial cyclic loading conditions. As a result of the present development, a unified fatigue evaluation procedure based on the newly proposed effective stress parameter and a single master S-N curve can be implemented for arbitrary cyclic loading conditions regardless of stress multi-axiality or load path proportionality.

Evaluation of the Finishing Roll Surface Deterioration at Hot Strip Mill.

Effects of the service conditions on the roll surface deterioration in the front finishing stands of hot strip mill were analysed by observing the roll surface after each roll change and by calculating the roll surface temperature and the heat penetration depth. The roll surface temperature was varied with the rolling conditions and the steel grades rolled. The heat penetration depth was greatest at the F1 stand, and was gradually decreased in the latter stands. The roll surface deterioration was greatest at the F2 stand, which was thought to be due to the severe thermal fatigue of the F2 rolls. A parameter representing the thermal fatigue damage was developed by considering the effects fo roll surface temperature and heat penetration depth on the degradation of the roll surface. It was found that the parameter had a good correlation with the degree of the observed roll surface breakdown.

Roll pass evaluation for hot shape rolling processes

Since the complex shaped caliber rolling usually takes place in hot operation, three dimensional mechanical behavior coupled with the heat transfer in and around the billet must be taken into account for accurate evaluation of the roll pass system. In the present study, the authors propose a new simulation approach to deal with the above coupling problem: the deformation mode method for the three dimenional deformation in the steady-state rolling process, and the two dimensional boundary element method for radiation and convection heat transfer between mill stands. Furthermore, to deal with temperature history in plastic working, heat generation by plastic deformation and friction work and heat transfer between rolls and billet are both taken into account in every incremental reduction. As an example, the butterfly roll pass system is taken in the actual angle steel rolling. It is found that (1) the mechanical behavior and characteristics can be discussed by the calculated metal flow, strain and temperature distributions in the whole roll pass system, and (2) the numerically estimated cross-sectional shapes after rolling are in fairly good agreement with the experimental measurements in the actual processing. The above example demonstrates the validity and effectiveness of the present modelling even for the industrial shape rolling process.