Modeling and finite element analysis of rod and wire steel r(3)

416 tion region exists near the interface of the rolls and billet, and the deformation gradually penetrates into the billet center. As shown in Fig. 8, the total strain of node a, which contacts each roll during rolling, is

higher than those of node b and node c.

Fig. 7. Temperature profiles at different positions of the

billet during rolling.

Fig. 8. History of total equivalent plastic strain at various positions during the rough rolling.

5.3. Strain rate distribution

Fig. 9 shows the equivalent plastic strain rate dis-tribution of the billet during the firstpass. From the figure, it can be seen that the nonuniformity of the strain rate in various directions is rather strong. As a result of friction, the surface of the billet experiences a higher strain rate than the center. The high equivalent plastic strain rate zone exists close to the entry of the roll gap owing to the drastic deformation there. It is very clear that the strain rate correlates positively with the speed of the billet, that is, as the rolling speed in-creases, the strain rate also increases. 5.4. Contact friction force

Fig. 10 shows the distribution of the contact friction component force in the x direction during the first and the seventh pass rolling. As shown in Fig. 10, the component of contact friction force is positive within

J. Univ. Sci. Technol. Beijing, Vol.15, No.4, Aug 2008

a large area at the entry of the roll gap, whilst the component of contact friction force is negative within a small region at the exit of the rolling pass. The dif-ference shows that the friction is complex within the contact area and has large variations along the x direc-tion. The friction force in the x direction drives the surface node into a roll gap, and then changes into a reverse direction when the node exits from the roll

gap.

Fig. 9. Equivalent strain rate distribution during the first

pass.

Fig. 10. Distribution of the contact friction component force: (a) the first pass; (b) the seventh pass.

5.5. Width spread

The bar with a radius of 16 mm would be formed after 12 passes rolling. The simulated grid shapes of the billet after the eleventh pass and twelfth pass are

S.L. Liao et al., Modeling and finite element analysis of rod and wire steel rolling process 417

illustrated in Fig. 11. Fig. 11(a) shows an excessive width spread in the vertical direction after the twelfth pass, which is often observed in practical production. The billet with the desired round shape of cross sec-tion will be obtained only by slightly increasing the roll gap in the twelfth pass compared with Fig. 11(a), as shown in Fig. 11(b). However, it is worth noting that when the roll gap is suitable in the twelfth pass, but is a bit larger in the eleventh pass, the excessive width spread will also be observed by simulation. In this case, a desired shape can be obtained through slightly shortening the roll gap in the eleventh pass. 5.6. Effect of roll diameter change on the rolling parameters

It can be observed that the roll groove shape at the stage of rough rolling has a notable change compared with the first use after a specific amount of production. To save costs and keep the dimension accuracy of bil-lets, the roll will be machined into the one with the original roll groove shape and be used again.Afterseveral repetitions, the newly generated roll will have a great decrease in diameter when employed again. To estimate the influence of roll diameter change on the process variables, the models adopting the standard roll and the roll after reduction in roll diameter are es-tablished according to the data in Table 2. In model b, the roll groove shape and rolling speed at each pass remain unchanged, but the roll diameters adopt the

actual values after reduction.

Fig. 11. Simulated shape of the billet after the eleventh pass and the twelfth pass.

Table 2. Roll diameter employed in model a and model b mm

Pass 1 2 3 4 5 6 Model a 580 520 580 520 580520Model b

525

470

520

495

540

440

Fig. 12 shows a comparison of temperature profiles calculated by model a and model b. At the end of rough rolling, the temperatures in model b are slightly lower than those in model a at the depth of one-fourth thickness and at the center of the billet, whilst the sur-face temperature in model b is slightly higher than that in model a.The difference between the two, however, is relatively little at the end of rough rolling, which is about 6-7qC. The heat of deformation is less because of the decrease in deformation power. Therefore, the inner temperature rises at the roll gap in model b is slightly lower than that in model a. The drop in the surface temperature in model b is less. This might be attributed to the reduction of the contact area between billet and rolls. Further, it should be noted that the temperature difference between the two also increases as the rolling speed increases.

The rolling force is directly affected by friction co-efficient, contact area, temperature, material properties, flow stress, and so on. The reduction and roll diameter affect the rolling force indirectly by changing the contact area. The rolling force increases with time at the bite stage of rolling, and then has fluctuations within a small range at the stable stages of rolling, af-ter which it drops at the exit stage of the billet. As only a quarter billet was modeled, the actual rolling force should be twice the average values at the stable state stages of rolling. Fig. 13 shows a comparison of the average rolling force of various passes at the stable stages of rolling in model a and model b. The reduc-tion under various passes is 19.4%, 21.6%, 28.0%, 22.7%, 23.5%, and 24.0%, respectively. As seen in Fig. 13, the reduced roll diameter causes the rollin …… 此处隐藏：5954字，全部文档内容请下载后查看。喜欢就下载吧 ……