Analysis of roll-stamping process for rib grooves of ultra-high-strength beam pa

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: Roll-stamping technology,a new process with both roll-forming and stamping characteristics,is suitable for manufacturing ultra-high-strength beam parts,especially variable cross-section beam parts.The rib groove is a local shape often used in automotive parts for positioning,avoidance,and stiffness strengthening on longitudinal and stiffening beams.In this study,two typical rib grooves,the flat- and round-bottom rib grooves,were selected to investigate the characteristics of the roll-stamping process of rib grooves.Using the ABAQUS software platform,a simulation analysis model of rib-groove roll stamping was established and different size rib grooves produced by the roll-stamping process were compared and analyzed.The results show that when the fillet radius of the rib groove increases,the maximum Mises stress,the maximum strain,and the maximum thinning rate decrease.For roll stamping,the minimum safe fillet radii of the two types of rib grooves are 8 and 4 mm,respectively.

Key words: ultra-high-strength steel; roll stamping; beam parts; rib groove

1 Introduction

The roll-stamping process,a new process that integrates the characteristics of the roll-forming and stamping processes,is suitable for the production of ultra-high-strength steel parts,especially variable cross-section beam parts[1-2].As shown in Fig.1,the roll-stamping process involves the progressive bending or stretching of a sheet using a super-large roller in continuous rotation (the equivalent radius of the roller is greater than 10 m) by block dies designed according to the shape of the desired product to realize the formation of variable-section beam parts.

The rib groove is a common shape used in automotive parts,especially in beam parts.In the traditional stamping process,the main forming of a rib groove begins in the last stage,and the rib groove is formed when the upper and lower dies are completely closed.The existence of a rib groove significantly increases the forming force and has higher requirements for material plasticity.It is difficult to form a deep rib groove using the ultra-high-strength steel because of the cracking that tends to occur during cold stamping or when the forming force experiences a sharp increase.This leads to inadequate closing of the die and insuf-ficient shape forming,which then affect the accuracy of the parts.When a rib groove is formed using the new roll-stamping process,the require-ments for material plasticity and the forming force are greatly reduced due to the progressive deform-ation that characterizes the roll-stamping process,which facilitates the forming of a rib groove in the ultra-high-strength steel.

Using an ABAQUS software platform,a simulation analysis model of the rib-groove roll stamping was established.For two typical rib grooves,i.e.,the flat-and round-bottom rib grooves,the roll-stamping results of rib grooves of different sizes were compared,and the minimum fillet sizes of two types of rib grooves were obtained.

2 Dimensions and materials for the test parts

In this study,the material used for the rib-groove part was Baosteel’s ultra-high-strength steel HC820/1180QP (hereinafter referred to as QP1180).QP steel,a third-generation advanced high-strength steel that has both ultra-high strength and good formabil-ity[3-5],has good overall properties[6]and its typical mechanical properties are shown in Table 1.

Table 1 Mechanical properties of QP1180

In actual parts,the rib groove is often located on the bottom,which is especially true of beam parts.Therefore,the rib groove on the bottom of a beam part with equal cross-sections was used as the research object.As shown in Fig.2(a),the length of the cap beam part was 100 mm,with the rib groove located in its center.The shapes of the rib grooves of actual parts can be quite different.The two most representative types of rib grooves were selected as research objects: the round-bottom rib groove (Fig.2) and the flat-bottom rib groove (Fig.3).The dimensions of the round-bottom rib groove are shown in Table 2,and those of the flat-bottom rib groove are shown in Table 3.

Table 2 Dimensions of the round-bottom rib groove mm

Table 3 Dimensions of the flat-bottom rib groove mm

3 Simulation modeling of roll stamping

The discrete rigid body was adopted for the die,which can converge easily,and the element was C3D4.The thickness of the parts was 1.2 mm,with three layers of elements in the thickness direction.The blank element type was C3D8R.Fig.4 shows the mesh of the round-bottom rib groove,and Fig.5 shows that of the flat-bottom rib groove.Because of the symmetry of the model,the 1/2 model was adopted to save computing time and cost.

Fig.6 shows the assembled model.In the initial state,the die makes no contact with the sheet metal.The upper die moves downward,clamps onto the sheet metal,and then rotates circumferentially until the end of the sheet metal is formed.

4 Simulation results of rib-groove roll stamping

The roll-stamping processes of the round-bottom rib groove with a rounded corner radius(R) of 5,6,8,and 14 mm were analyzed and compared.The principal strain distributions of the round-bottom grooves with different fillet radii are shown in Fig.7.

From Fig.7,it can be seen that the maximum principal strain position in the groove occurs in the bottom corner region.Table 4 shows the maximum principal strain and the maximum Mises stress in the groove area,in which it can be seen that both of them increase with a decrease in the radius of the fillet.

Table 5 shows the minimum thickness and the maximum thinning rates of the round-bottom rib grooves after roll stamping,in which it can be seen that the maximum thinning rate decreased with an increase in the radius of the round corner.When the radius of the fillet increased from 5 to 14 mm,the maximum thinning rate decreased from 25.58% to 7.56%.This indicates that the radius of the fillet has a significant effect on the maximum thinning rate.Fig.8 shows the FLD distributions of the groove areas.From these distribution results,it can be seen that when the radius of the fillet was 5 mm,the strain on some local areas of the round-bottom rib groove exceeded the forming limit.When the radius of the fillet increased to 8 mm,however,all areas of the groove could be formed safely.

Table 4 The maximum principal strain and the maximum Mises stress when roll stamping the round-bottom rib grooves

R/mmThe maximum principal strainThe maximumMises stress/MPa50.2301 53060.1951 50680.1501 460140.1501 356

Table 5 The minimum thickness and the maximum thinning rates of the round-bottom rib grooves after roll-stamping

R/mmThe minimum thickness/mmThe maximum thinning rate/ %50.8925.5860.9520.8481.0214.80141.117.56

A comparative analysis was conducted of the roll-stamping process for six flat-bottom rib grooves of different sizes,with a rounded radius(R) of 1,2,3,4,5,and 6 mm.Fig.9 shows the principal strain distributions of flat-bottom rib grooves after roll stamping.

As can be seen from Fig.9,the maximum prin-cipal strain position of the flat-bottom rib groove remained in the bottom rounded area.Table 6 shows the maximum principal strain and the maximum Mises stress in the groove area,which show that both of them decrease with an increase in the radius of the fillet.

Table 6 The maximum principal strain and the maximum Mises stress when roll stamping flat-bottom rib grooves

R/mmThe maximum principal strainThe maximum Mises stress/MPa10.421 55720.251 50230.181 45340.151 41550.131 38860.121 366

Table 7 shows the minimum thickness and the maximum thinning rates of flat-bottom rib grooves after roll stamping,from which it can be seen that the maximum thinning rate decreased with an increase in the fillet radius.When the radius of the fillet increased from 1 to 6 mm,the maximum thinning rate decreased from 17.88% to 4.64%.This indicates that the radius of the fillet has a significant effect on the maximum thinning rate.

Table 7 The minimum thickness and the maximum thin-ning rates of flat-bottom rib grooves after roll stamping

R/mmThe minimum thickness/mmThe maximum thinning rate/ %10.9917.8821.0611.2731.108.3541.126.5851.135.5061.144.64

Fig.10 shows the FLD distributions in a charac-teristic area of flat-bottom rib grooves after roll stamping.The results show that when the radius of the fillet was less than 3 mm,the strain on some local areas of the flat-bottom rib groove exceeded the forming limit.When the radius of the fillet was larger than 4 mm,however,all areas of the flat-bottom rib groove could be formed safely.

5 Conclusions

Finite element modeling and simulation analysis was performed for the roll stamping of flat- and round-bottom rib grooves.Based on the analyzed simulation results,the following conclusions can be drawn:

(1) Ultra-high-strength-steel beam parts with rib grooves can be formed using the roll-stamping process.

(2) When the fillet radius of the rib groove increases,the maximum Mises stress,the maximum strain,and the maximum thinning rate in the rib groove area will decrease after roll stamping.

(3) The minimum radius of a round corner that can be formed safely is 8 mm when forming a round-bottom rib groove by the roll-stamping process.

(4) The minimum radius of a round corner that can be formed safely is 4 mm when forming a flat-bottom rib groove by the roll-stamping process.

Acknowledgement

This paper is funded by National Key R & D Program Funding Project (No.2017YFB0304404).