Forming Strategies and Solutions

• Notch-free Flanges on Channel Section Ends
• Balancing Splits and Wrinkles in Curved Sections
• Lightweighting with 3rd Generation AHSS

The information provided on this page represents generalizations and may not be directly applicable to every forming situation. OEMs factor in many considerations when choosing grades for each application, including but not limited to the cost and availability of products with the targeted strength level and coating consistent with the manufacturing and engineering strategy of the specific OEM.

Notch-free Flanges on Channel Section Ends

Joining longitudinal cross members to other parts typically is facilitated by an end flange joint, as shown on the left side of Figure 1. The corners of these flanges are highly strained, and are therefore at risk for edge cracking when higher strength sheet steels are used.  Notching the section corners (Figure 1, center) avoids these high strains and associated edge cracks, but also reduces the stiffness provided by a continuous flange in these sections. A notch-less end flange, such as shown in the right image in Figure 1, retains the desired structural benefits.

Figure 1: Notch-free flanges are preferred for structural benefits, but must avoid edge cracks from forming.^N-32

One potential forming approach that minimizes the edge strains^N-32 incorporates a developed blank edge where the edges of the sheet steel are cut to the precise dimensions to achieve the targeted contour of the formed part with no additional post-forming trimming. Additionally, the sheet is supported using a lower pad throughout the forming process, as indicated in Figure 2.^N-32

Figure 2: Comparison of conventional techniques and the developed approach to minimize the edge strains at the ends of longitudinal sections.^N-32

At the beginning of the forming process, the lower pad is held at a position higher by its vertical stroke (S_lp) than the upper surface of the punch, and during the forming process, the width center of the blank is maintained at that height.

After the upper pad bottoms to the die surface of the die, the upper pad and the die move as one unit, and the lower pad moves down. At bottom dead center of the press stroke, the level at which the blank was held becomes equal to the punch surface, and the forming of the first process ends here.  

The referenced study^N-32  tested steels with 980 MPa and 590 MPa tensile strength.  With a testing setup similar to what is shown in Figure 2, conventional approach resulted in edge cracks, but the developed method successfully formed crack-free parts.  A lower pad vertical stroke S_lp was optimized to minimize the thickness decrease seen in the flange. At this value, the thinning strain using the 980 MPa steel was essentially the same as that of the 590 MPa steel. 

Balancing Splits and Wrinkles in Curved Sections

When a high-strength steel sheet is formed into a curved shape, like what might be seen at the top and bottom of a full B-pillar reinforcement or in the L-shaped corner section of an A-pillar reinforcement where the dash transitions to the rocker, wrinkles can appear at the top flat portion. To suppress these wrinkles, a corrective approach of applying greater back tension, either by beads or binder force, risks fracture in these higher strength grades. Figure 3 shows these typical at-risk parts.

Figure 3: Examples of at-risk parts.^N-32

The developed method includes a pad to restrict the blank at the top flat portion to prevent the wrinkles and use of bending rather than drawing to prevent fracture. Material flow control accelerates the sheet metal flow into the regions at greatest risk of fracture.

Figure 4 shows a generic T-shape part, and Figure 5 presents a schematic of the developed forming method.

Figure 4: Generic “T”-shape section targeted by this technique. With conventional approaches, the top flat section is prone to wrinkles, while the curved edge leading to the door-opening is at risk of edge fracture.^N-32

Figure 5: Comparison between developed and conventional forming method.^N-32

Metal forming simulation of a typical conventional forming approach showed that the strain map in the top section has a strain ratio (minor true strain/ major true strain, ϵ₂/ϵ₁) of much less than -1, and is therefore in the compression zone of the Forming Limit Diagram (the black points in Figure 6b).

The developed approach changes the metal flow path to one that is much closer to a strain ratio of -1, and thus transitions away from the wrinkling risk (the black points in Figure 6a).

In the area of edge fracture risk adjacent to the door opening section, the developed process minimizes the strains, keeping them below the forming limit and removing the fracture risk (the gray points in Figure 6).

Figure 6: Formability improvement associated with the developed process.^N-32

Application of this method is shown to keep the edge strains essentially independent of material strength, in tests ranging from 590 MPa to 1470 MPa tensile strength.

More details about this process can be found in Citations N-32  and T-55.

Lightweighting with 3^rd Generation AHSS

Material for this section comes from Citation H-75. 

DH steels, or High Ductility Dual Phase steels, are a type of 3^rd Generation advanced high strength steel that has approximately 5% better elongation compared to conventional dual phase steels of the same strength level.  This extra ductility may help justify a substitution of the current approach with a higher strength grade at reduced thickness to achieve comparable performance with lighter weight and a minimal cost impact since less metal will need to be purchased.

Two front structure parts are evaluated in Citation H-75.  For front longitudinal components, the incumbent base materials are HSLA260 and HSLA300, at thicknesses ranging from 1.35 mm to 2.6 mm, as indicated in Figure 7. 

Figure 7: Front longitudinal components currently made using 260LA or 300LA grades that are candidates for replacement with DH800, a 3rd Generation AHSS with 800 MPa minimum tensile strength.^H-75

A contender substitute grade is DH800, a 3^rd Generation AHSS with an 800 MPa minimum specified tensile strength.   DH 800 has 140 MPa and 180 MPa higher yield strength than the 260LA and 300LA baseline grades it might replace, as indicated in Table 1.

Crash performance of the baseline HSLA approach was assessed. By switching to DH800, comparable crash performance was achieved with a 23% gauge reduction in each component, representing a 5 kg savings.  At that stage, stamping feasibility was assessed with forming simulations which indicated similar performance between all grades. One of the sections is shown in Figure 8.

Figure 8: Forming simulation results comparison between 260LA and DH800 on one of the longitudinal rail components.^H-75

In another front-end structure, DP800 base material was compared to DH1000. The property requirements are reproduced in Table 1, with the parts shown in Figure 9.

Figure 9: Front-end components currently made using DP800 that are candidates for replacement with DH1000, a 3rd Generation AHSS with 1000 MPa minimum tensile strength.^H-75

As before, the targeted thickness was determined as that which gives equal crash performance as the incumbent grade.  The analysis considers the strengthening associated with work hardening.  Replacing the baseline DP800 with DH1000 showed a downgauging potential of 17% per part, representing a 3 kg weight savings.  Figure 10 presents a comparison of forming simulation results.

Figure 10: Forming simulation results between the baseline DP800 and the replacement candidate DH1000.^H-75

The extra formability that the 3^rd Generation AHSS grades offer results in an improved deep drawing capability which is necessary to obtain good parts at higher strength levels. Incorporating these higher strength grades may allow for thickness reduction while maintaining crash performance and potentially being cost-neutral.

Formability Evaluations of Advanced High Strength Steels

This content is based on Citation L-77.

All metals strengthen with increasing strain in a process referred to as work hardening, which is characterized by the n-value.  As described in Equation 1, n-value is referred to as the strain hardening exponent in the Holloman equation:

Equation 1

where
σ is the true flow stress (the strength at the current level of strain),
K is a constant known as the Strength Coefficient, defined as the true strength at a true strain of 1,
ε is the applied strain in true strain units, and
n is the work hardening exponent

Depending on the grade, n-value may change with strain. Therefore, the range over which n-value is calculated must be reported.

For conventional steel grades, n-value remains basically constant after the strain exceeds the initial yield point.  In grades like mild steels where uniform elongation is greater than 20%, n-value is usually determined between engineering strain in the range of 10 to 20%.  In higher strength steels that have uniform elongation (A_g) less than 20%, n-value is calculated between 10% and uniform elongation.

In advanced high strength steels, however, the strain is distributed among several phases, especially for austenite-containing steel grades.  Here, the degree of work hardening changes as deformation progresses.  In these cases, instantaneous work hardening is used for analysis, Equation 2:

Equation 2

 
One method to compare the behavior of steel grades with different uniform elongation is to use the relative plastic strain, which is the ratio of the plastic strain to the plastic strain corresponding to uniform elongation.  Figure 1 shows how instantaneous n-value changes as a function of plastic strain for several steel grades.

Figure 1: N-Value as a function of Relative Plastic Strain.^L-77

For conventional steels shown in Figure 1a, n-value remains essentially constant after the strain exceeds the initial yield point.  These are the only grades that were included in the studies to develop simple correlations between n-value and Forming Limit Curves in the 1960s and 1970s.  Since other steel grades were not included in the correlation studies, it is not appropriate to use those equations to estimate the FLC in advanced grades.

Figure 1b compares the first generation of advanced high strength steels.  At lower strains, DP980 has an extremely high n-value. Yet after reaching approximately 30% of uniform elongation (0.3 on the horizontal axis), DP980, CP980 and MS1180 all show a strong decreasing trend of n value with increasing strain.

Unlike the 1^st Gen AHSS grades in Figure 1b, the 3^rd Generation advanced high strength steel grades of QP980, QP1180 and DH980 (Figure 1c) are all influenced by the TRIP effect.  The continuous decreasing trend of n value still exists, but the n-values are much higher.

Figure 1d highlights grades that have a large percentage of austenite in the microstructure.  A pronounced TRIP effect in the Mn-TRIP980 leads to a sharp increase followed by a sharp decrease in the n value, while the n value of TWIP steels increases slowly to a very high value.

In summary, ferrite-based steels (mild steels and conventional high strength steels) have stable n values, multiphase 1^st Generation AHSS grades show an overall decreasing trend in n-value, while 2^nd Gen and 3^rd Gen AHSS show complex n-value behavior from phase transformations and twinning mechanisms.

Experimental forming limit curves were generated for multiple grades of steel and are presented in Figure 2. FLC₀ of 780 MPa to 1180 MPa 1^st and 3^rd Gen AHSS grades are all in the range of approximately 0.1 to 0.2.

Figure 2: Forming Limit Curves for multiple grades of steel.^L-77

The forming limit curves of DP980 and QP980 are highlighted in Figure 3a, which shows very similar behavior between the two grades in terms of the FLC positioning.  On the other hand, Figure 3b shows measurable differences in the limiting dome height (LDH) values at specimen widths from 20mm (specimen number 1) to 180mm (specimen number 9). The higher n-value of QP980 is likely responsible for improved performance in the LDH tests.^Z-23

Figure 3: a) Forming limit curve and b) limiting dome height results of QP980 and DP980.^{Z-23 as reported in Citation L-77}

Under the same die and stamping process conditions, QP980 and DP980 were used to produce reinforcing beam components with a production stamping process. After the drawing operation, QP980 did not split when formed to the final position, while DP980 cracked at two locations, noted in Figure 4a.  Figure 4b shows the major strain levels at three locations are all lower on the stamping formed from QP980 than on that from DP980.  This reinforces that QP980 has better formability, and better ability to distribute strains over a wider region of the panel.

Figure 4: Stamping results using QP980 and DP980. a) DP980 shows splits in two locations. b) Strains in QP980 are lower.^L-77

For traditional grades, reviewing the forming limit curve to assess formability has been used successfully for decades. However, as shown above, the QP980 and DP980 grades studied have very similar FLCs (Figure 3a) but different stamping performance (Figure 4a).

This difference may be a result of the differences in the partitioning of strains between the multiple microstructural phases, the strain hardening evolution, and the impact of the TRIP effect present in different degrees in all advanced high strength steels. 

Along with the complexities in forming behavior, the way forming behavior is assessed must evolve as well.  One possible approach is described in Citation L-77, which proposes a new index describing the overall forming capability called the strain homogenization capability (SHC).  At this time, however, there is no industry consensus on the best approach.