,

1)Research Institute,Baoshan Iron &Steel Co.,Ltd.,Shanghai 201999,China;

2)State Key Laboratory of Development and Application Technology of Automotive Steels(Baosteel),Shanghai 201999,China

Abstract：Increasing geometrical accuracy at open ends of the roll-formed part is difficult due to the release of residual stress after end cutting.In this work,a typical rail with a high requirement of geometry accuracy was selected to realize the behaviors of residual stress release.First,residual stress distribution after roll forming is discussed in detail by finite element analysis with ABAQUS.In addition,two different approaches are proposed to check their capabilities in reducing the residual stress level.The results indicate that both additional rolling passes and multiple bending processes are beneficial to reducing uniform residual stress.

Key words：roll forming; ultra-high-strength steel; residual stress

1 Introduction

The automotive industry has been widely applying advanced high-strength steels (AHSS) in body-in-white and structure components to achieve good performance in terms of safety,light weight,and cost savings[1].Roll forming,as one of the incremental forming processes,is capable of providing a fabri-cating solution to AHSS due to its high production efficiency and high material utilization rate.More-over,roll forming is more flexible for shaping com-ponents in each stage as compared to conventional stamping.

However,springback still remains a very challeng-ing issue for roll forming in processing AHSS.Many research groups have investigated the influence of component sharpness,including sheet thickness,blank width,forming angle,and flange length[2-4]and process parameters,including forming modes,roller gap,forming speed,and friction coefficient[5-9],on geometrical accuracy.Furthermore,a series of studies focusing on springback compensation have been per-formed by different strategies.ONA et al.[10]directly adjusted rolling force and over-bending to perform angle compensation.PARK et al.and ABEYRATHNA et al.reduced springback by controlling or optimiz-ing forming processes[11-13].These methods are effective in improving forming accuracy for com-ponents with simple shapes but do not work well for components with complex geometric features.

Another critical issue during AHSS roll forming is the release of residual stress after end cutting,which results in low geometrical accuracy at open ends of roll-formed components.This phenomenon is very difficult to avoid because it is one of the most important characteristics of roll forming.In this paper,finite element analysis was utilized to investigate the behaviors of residual stress release in roll forming a specific component.Then the effects of two strategies on residual stress are discussed in detail.

2 Simulation modeling of a typical roll-formed component

2.1 Specific roll-formed component

Seat rails are required to achieve much higher accuracy due to their functions as compared to conventional stamped or roll-formed automotive com-ponents and are usually produced by the bending process.The dimensional tolerance at the open ends of seat rails is up to ±0.2 mm.A typical com-ponent,including the main features of the seat rail,was designed in this study,as shown in Fig.1.

Fig.1 Cross section of the designed typical component

The cold-rolled ultra-high-strength steel HC700/980DP with a thickness of 1.8 mm was adopted for this component.The typical properties of this material,namely,yield strength,tensile strength,and elongation,are 700-920 MPa,above 980 MPa,and above 8%,respectively.Based on the commercial profile design code COPRA and engineering experience,the initial profile with 30 passes was generated for roll forming,as shown in Fig.2.

Fig.2 Initial profile passes for roll forming

2.2 Simulation modeling

Commercial software ABAQUS was used to perform numerical studies.Three layers of solid elements were utilized in the blank sheet through the thickness direction.All the rollers were set as rigid bodies.The master-slave mode and penalty method were used to deal with the contact between the blank and the roller.The friction coefficient was set constant at 0.1.Explicit and implicit algorithms were used to simulate the roll-forming process and predict springback behaviors,respectively.The plastic stress-strain curve of HC700/980DP was imported to describe its behaviors.

The residual stress distribution after 30 passes of roll forming but without end cutting is shown in Fig.3.To consider stress release after end cutting,a tie constraint was applied at the end of the roll-formed component.The tie constraint allows con-tinuous deformation at the connected nodes with the same degrees of freedom,as shown in Fig.4.Once finished,the tie constraint of the forming process was removed to perform the end-cutting process.Fig.5 shows the springback phenomenon after end cutting at the open end.

Fig.3 Residual stress distribution after roll forming

Fig.4 Sheet blank modeling with tie constraint

Fig.5 Springback phenomenon after end cutting

Fig.6 Residual stresses along the length direction at A1

Fig.7 Residual stresses along the length direction at A5

3 Results and discussion for FEA simulation

3.1 Distribution of residual stress at specified locations

In Figs.1 and 5,A1 and A5 are two critical locations that have a critical influence on the dimensional deviation.This is because A1 has a significant influence on the entire open-ended shape,and it is the closest fillet to the center line of the cross section.Additionally,the largest shape deviation occurs at A5.

Residual stresses of A1 and A5 at different dire-ctions along the length of the formed component are summarized in Figs.6 and 7,respectively.The results show an increasing trend ofS22(transversal stress),S11(longitudinal stress),andS12(longi-tudinal shear stress) as compared to stresses in the rest of the directions.At location A1,S22changes from 200 to 300 MPa and slightly fluctuates at the two open ends.S11andS12vary between -100 to 100 MPa.The stress states at the two open ends are obviously different from the center area of the component.For location A5,S22,S11,andS12still have more influence than other stress components.However,the distribution of residual stress at A5 is significantly different from that observed at A1.This indicates that the residual stress distribution is strongly related to geometrical features.S22has opposite trends at the two open ends and almost linearly changes from -550 to 650 MPa.S11andS12range between -600 and 400 MPa,and between 100 and 340 MPa,respectively.Moreover,S11andS12are obviously different at two ends.

3.2 Influence of the multipass strategy on residual stress

In this study,the multipass strategy was investi-gated to verify its influence on residual stress.Here,the simulation results at A1 were taken as an example,as shown in Fig.8.Six-pass (20°,40°,60°,80°,105°,and 90°),eight-pass (15°,30°,45°,60°,75°,90°,105°,and 90°),and 10-pass (15°,30°,40°,50°,60°,70°,80°,90°,105°,and 90°) strategies were adopted.

Fig.8 Influence of additional passes on residual stresses S11,S22,and S12

Generally,residual stressesS11,S22,andS12tend to reduce with increasing pass numbers.Additional passes have a more obvious influence onS11andS12as compared toS22.Furthermore,another interesting phenomenon is noticed that the difference of residual stress at the open ends and center decreases by applying the multipass strategy.

3.3 Influence of the multiple bending process on residual stress

From the discussion in Section 3.1,it can be concluded that the existence of large residual stress and its nonuniform distribution result in severe springback.The multipass strategy is effective in reducing the residual stress level but has limitations because more rolling passes are required.The multiple bending process is another feasible way to restrict the negative effect of springback.

In Fig.9,the residual stresses after multiple bend-ing at the longitudinal direction are plotted.Moreover,the maximum deviations after multiple transversal bending at location A1 are described in Fig.10.Although all the main residual stress components can be slightly reduced,and their distributions are more uniform,this approach may lead to unexpected distortion or abnormal failure due to the formability limits of ultra-high-strength steels.

Fig.9 Influence of multiple longitudinal bending on residual stresses S11,S22,and S12

Fig.10 Influence of multiple transversal bending on residual stresses

4 Case study

The typical component in Fig.1 was physically fabricated by the initial profile with 30 passes mentioned in Section 2.1.Cross-sectional profiles at different passes are illustrated in Fig.11.The multi-bending strategy of two times was applied at the A1 location (Fig.1).

Fig.11 Cross-sectional profiles at different passes

The maximum dimensional deviations at ten different locations of 11 cross sections along the length direction of the formed component were measured using a three-coordinate measuring ma-chine (as shown in Fig.12).The results are listed in Table 1.The observed maximum deviations under roll forming with multi-bending at the A1 loca-tion are improved,but they still cannot totally meet the dimensional accuracy of ±0.2 mm.The num-bers of maximum deviation near the two open ends are much larger than those at other locations.Therefore,the off-line post forming also involves investigating its possibility of increasing dimen-sional accuracy.This indicates that the maximum deviations are significantly reduced to achieve an acceptable tolerance.

Fig.12 Maximum dimensional deviations at ten different locations of 11 cross sections along the length direction

5 Conclusions

In this paper,residual stress after roll forming a typical asymmetrical component was investigated by simulation in detail.Two online strategies were also involved in verifying their influence on residual stress.The main findings of this study are summarized as follows:

Table 1 Maximum dimensional deviations at different locations

(1)S22,S11,andS12have increasing values as com-pared to other stress components.The existence of large residual stress and its nonuniform distribution result in severe springback.

(2) The multipass strategy is effective in reduc-ing residual stress and slightly improves the nonuni-form distribution of residual stress.

(3) As compared to multiple longitudinal bend-ing,multiple transversal bending is more efficient in reducing springback.

(4) From an engineering point of view,online strategies may not be the most effective and cost-efficient way to increase the geometrical accuracy of roll-formed components.Off-line solutions are necessary to be developed in the near future.