Issue 44

G. G. Bordonaro et alii, Frattura ed Integrità Strutturale, 44 (2018) 1-15; DOI: 10.3221/IGF-ESIS.44.01

Other parameters are defined as follows

n

       0.2

       0.2

f = 1.3 

 - 0.3 

n = 0.41 - 0.07 C T+273.15 T=  1000

T = temperature in °C, 700°C < T < 1200°C C = carbon content in weight percent, C < 1.2%   -1 ε = strain rate, 0.1 < ε < 100 s ε = strain < 70%

Shida's material formulation allows to define thermal properties as a function of temperature T  for low-carbon steels, as follows:

 thermal conductivity (k):

       -2.03 T W k = 23.16 + 51.96  e m 

 material density (ρ):   2 7850 ρ =  1+0.004  T  specific heat capacity (C p ):   3.78 T p        3 3 kg m

C = 689.2 + 46.2 e , T < 700°C

  

  

 p J C = 207.9 + 294.4 e , T  700°C  kgK  1.41 T 

Shida's equation is applied to compute flow curves for the low-carbon steel S235JR (0.17 %C) at different working temperatures in the range of 800°C ≤ T ≤ 1200°C and at different strain rates in the range of 0.1 ≤  ε  ≤ 10 s 1 . These flow curves are used in this study as an input to the material module of the software MSC Simufact Forming. It is assumed a temperature independent modulus of elasticity of 210 GPa and Poisson's ratio of 0.3.

FEM RESULTS AND VALIDATION

D FE models of multi-pass hot rolling processes are developed for a total of six profiles with a complex cross-section. Simulations are carried out with the commercial software Simufact Forming. This software is a sophisticated simulation tool which considers the triaxiality of the stress state with dedicated meshing and remeshing techniques for metal forming processes. To achieve the required precision in the simulations an adequately fine mesh is required, which must be carried out due to the three-dimensional stress state with continuous elements (full 3D elements). Due to the large plastic deformations the simulation mesh requires continuous remeshing to prevent the simulation from failing due to extremely distorted elements. Simufact Forming provides for this purpose a powerful algorithm that execute an automatic hexahedral remeshing when it is required. Thermo-mechanical boundary conditions and material model flow curves are implemented based on assumptions described in the previous sections. Process parameters at each deformation stand are defined from real plant data: billet cross-section geometry, workpiece temperature, rolls angular velocity, rolling mill set-up equipment either idle or motorized, horizontal or vertical axes. Table 1 shows ranges for the workpiece temperatures and rolls angular velocities performed in the multi-pass rolling. The entry billet diameter is also shown in the same table for each analyzed profile type. 3

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