A correlation between the friction coefficient and the roughness of

rolls in a hot flat rolling

M. Durante, F. Memola Capece Minutolo, A. Langella

University of Naples Federico II, Italy

Hot flat rolling

Two of the most important parameters in flat rolling design are:

• The spread of the workpiece
• The power required for the process

The evaluation of these parameters by analytical formulae or FEM requires the knowledge of the friction coefficient.

Numerous parameters influence the friction coefficient: rolls roughness, process temperature, workpiece material.

AIM OF THE WORK

To find a correlation between
the rolls roughness and the friction coefficient 

In order to obtain this correlation, some experimental tests have been carried out to measure the spread of the workpiece as a function of roll roughness; then, the spread has been evaluated by FEM simulation as a function of the friction coefficient.

R_a ------> experimetal tests -----> workpiece spread                

                                                                                            ]---->   μ = f (R_a)

 μ ------> FEM          ----->       workpiece spread                       

In order to verify the FEM simulation accuracy, experimental test and simulation results have been compared in terms of maximum height reduction obtainable.

analytical model

In technical literature some models are quoted to calculate the friction coefficient as a function of rolls roughness and process temperature:

[1] for chilled and smooth steel rolls         μ=0,55 (1,05-0,0005 T)

[2] for ground steel rolls                           μ=0,8 (1,05-0,0005 T)

[3] for cast iron and rough steel rolls         μ=1.05-0,0005 T

with T rolling temperature in °C

experimental tests

A two high reversing mill has been used for experimental tests

Rolls

Materials : steel and cast iron

Diameter: 480 mm

Roughness: Measured as arithmetic mean deviation of the profile in parallel (R_al) and trasversal (R_at) direction with respect to the roll axis

experimental tests

Workpiece

Material :  steel

Initial dimension of the workpiece section: 125x125 mm²

Final height of the workpiece : 104 mm

Initial workpiece temperature: 1070°C-1080°C

rolls roughness

Four values of roughness have been tested for each roll type

experimental results

Spread as a function of the rolls roughness

fEM analysis

Elements of the simulation:

• the rolls are rigid tools the workpiece material is C22 set from the program library a rigid surface brings into contact the workpiece with the rolls

• 2 symmetry plane are presents

• the workpiece material is C22 set from the program library

• a rigid surface brings into contact the workpiece with the rolls

FEM simulation

Scheme of the simulation

fEM simulation

Flat rolling simulation

FEM simulation

Final width of the workpiece

fEM results

In order to obtain the same values of the final workpiece width measured in experimental tests, different values of the friction coefficient have been set in the FE analysis 

Ra - Friction Coefficient  Correlation 

By comparison between the spread resulting from experimental tests and from FE analysis, the correlation friction coefficient - rolls roughness is possible

Validity of FEM Method

By FEM analysis a value of  0.7 for the friction coefficient results for the steel rolls with Ra equal to 10.30 μm

In order to controll the FEM validity an experimental test has been conducted with the same rolls to evaluate the maximum height reduction Δh_max . Knowing maximum height reduction the friction angle value has been calculated by formula: 

Δh=2r(1-cosρ)             with r rolls radius and  ρ angle of friction

  Δh _max =64 mm  ----->   ρ= 34.5°----->  μ = tg ρ = 0.69

Conclusions