Modelling of the rolling of ribbed bimetal bars

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Numerical modelling of bimetallic round ribbed bar rolling process

To develop a technology for the manufacture of ribbed bimetallic bars with a corrosion-resistant steel cladding layer, it is necessary to carry out theoretical studies. On the basis of the results of such studies, it will be possible to determine the rolling parameters and to work out the optimal shape of the pre-finished and finishing passes. The rolling parameters obtained from the theoretical studies will enable the shortening of the time and reducing of the costs of experimental tests and technological trials [8-11].

Fig. 1. Shape and dimensions of the pre-finished pass and dimension of initial band

Development of rollRib cross-section

Fig. 2. Shape and dimensions of the round ribbed finished pass

The paper presents the preliminary theoretical studies on the rolling of bimetallic bar in the oval pre-finished pass and the ribbed finished pass. Numerical modelling of the rolling process was carried out in two stages. In the first stage, the rolling of round bimetallic band in the flat oval pre-finished pass was modelled. The shape and dimensions of the oval pre-finished pass are shown in Figure 1. The starting band for rolling in the pre-finished band was 19.2 mm-diameter round bimetallic bar. The outer layer accounted for 20% of the whole cross-sectional area of the bimetallic bar. The core diameter with this outer layer proportion was equal to 17.2 mm. The cladding layer was made of corrosion- resistant steel 306L (according to ASTM). For the bar core, steel C45 was used. In the second stage of studies, numerical modelling of the rolling of oval bimetallic band in the ribbed finished pass was carried out.

One of a number of computer programs designed for the modelling of plastic processing processes based on the finite element method is the F0RgE2005® commercial software [12,13]. This software offers a possibility to model rolling processes in a threedimensional strain condition. For a description of the object being deformed, the Norton-Hoff law was used, which can be expressed with the following equation. bimetallic bar numerical

The following initial parameters were adopted for simulation: a roll diameter of 350 mm; the temperature of rolled bimetallic band was assumed to be uniform and equal to 1000°C; the rolling speed was taken equal to 3 m/s, friction coefficient 0.3, and friction factor 0.7.

It was assumed that the joint between the core and the cladding layer was closely adhering. The nodes of both meshes were not shared. The properly chosen size of the mesh on the contact surfaces does not cause any significant problems either during computation or during re-meshing. The thermal conductivity was chosen identical as for the heat exchange between the band and the rolls (20 k-W/(K-m²)).

Numerical modelling results

As a result of the performed computer simulation of the rolling of round bimetallic band in the flat pre-finished pass, an oval band was obtained, as shown in Figure 3.

Fig. 3. Shape and dimensions of the bimetallic flat oval band obtained in numerical modelling - cross-section

The dimensions of the oval bimetallic band after rolling in the pre-finished pass were 24.13 x 11.41 mm. The width-to-height ratio of the bimetallic oval amounted to 2.1 [1-6]. The crosssectional area of the oval bimetallic band was 251.7 mm², of which the cladding layer area was 49.5 mm². The initial share of the cladding layer was 19.8% and it did not change after rolling in the oval pass. The elongation factor was equal to 1.13. The initial cladding layer thickness on the perimeter was uniform and equal to 1 mm, whereas after rolling this thickness changed. The thickness of the layer in the locations of band contact with the rolls was 0.72 mm on the average, while on the lateral surfaces in the band widening direction it was equal to 0.9 mm.

On the basis of the obtained theoretical study results, no possibility of formation of surface defects, which could have been caused by high magnitudes of stress and strain, was found. When analyzing the computer simulation results, consideration was given to the Cocroft-Latham criterion that allows the determination of the conditions for the occurrence of a crack in the material based on the main stresses and strain intensities occurring during deformation [14]. The value of this criterion during rolling in the pre-finished pass did not exceed 0.2, which indicates that there are no conditions favouring the cracking of the cladding layer (the criterion value, at which cracks might occur is approx. 0.6, depending on the deformation conditions) [15].

The next stage of the studies was the numerical modelling of the obtained oval bimetallic band in the round ribbed pass (Fig. 2). This pass is used for rolling usual (not bimetallic) ribbed bars. As a result of the studies carried out, the model of the 16 mm- diameter ribbed bimetallic bar was obtained (Fig. 4). Figure 5 shows the dimensions of the cladding layer in the characteristic locations on the ribbed bimetallic bar.

Fig. 4. Bimetallic round ribbed bar model 16 mm: a) view of bar outer layer, b) view of bar longitudinal section

When analyzing the obtained results it can be found that the obtained bimetallic ribbed bar model is characterized by correct outer dimensions and a correct rib height of 1.1 mm. The bar width was equal to 16.4 mm and was within the negative dimensional tolerance. The thickness of the cladding layer between the ribs was equal to 0.61 mm on the average, with a deviation of ±0.07 mm. At the rib top, this layer reached the thickness ranging from 0.9 mm to 1.0 mm. On the cross-section A and B (Fig. 5) it is shown how the cladding layer thickness varies on the core surface. A smaller cladding layer thickness can be observed at the rib base, for both the cross-section and the longitudinal section. The thickness of this layer in these places depends on the rolling direction. At the rib base on the side opposite to the rolling direction, the cladding layer thickness is averagely 0.55 mm. On the other side of the rib, this thickness is much smaller, amounting to 0.22 mm on the average.

The cause of such a large difference in cladding layer thickness is the mode of rib formation in the rolls bite region. Figure 6 show the formation of ribs in the initial zone of the rolls bite region. The cause of such a large difference in cladding layer thickness is the mode of rib formation in the rolls bite region. Figure 6 show the formation of ribs in the initial zone of the rolls bite region.

Numerical modelling of the bimetallic reinforcement bar rolling process computer simulation

Fig.5. Forming of bimetallic bar ribs in ribbed finished pass

During rolling the oval bimetallic bar in the ribbed finished pass, ribs are formed as a result of filling of the pass grooves. Due to the fact that the plastic resistance of the core is high and a free space occurs in the pass groove, the metal (cladding layer) displaces in this particular direction to cause a reduction in cladding layer thickness at the rib base. In addition, a factor influencing the reduction of the cladding layer thickness is the lag phenomenon occurring at the beginning of the rolls bite region. This results in the occurrence of a difference in speed between the roll surface and the bar surface, which creates unfavourable conditions for the formation of a rib and decreases the cladding layer thickness. In the central part of the roll bite region those speeds equalize and filling of the rib groove follows. At exit from the rolls bite region, the speed of the cladding layer is higher than that of the rolls. The interaction of the rib grooves in this location increases the rib height and the complete filling of the rib grooves. The conditions prevailing in this part of the rolls bite region do not have any effect on increasing the height of the cladding layer at the rib base.

Conclusions

The theoretical studies carried out have proved that the computer program Forge2005 makes it possible to carry out the numerical modelling of the rolling of ribbed bimetal bars and to make the analysis of this process.

From the study results, a substantial reduction in cladding layer thickness at the rib base has been found, which is asymmetrical in relation to the rib axis and is dependent on the rolling direction.

Further studies will enable the determination of the rolling parameters that allow obtaining a bimetallic ribbed bar of the thinnest possible corrosion-resistant layer without cracks in it.

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