Formability
A Forming Limit Curve (FLC) is a map of strains indicating the onset of critical through-thickness necking for different linear strain paths. The FLC is dependent on the metal grade and the specific methods used in its creation. When paired with the strains generated during forming of an engineered part, the associated Forming Limit Diagram (FLD) provides guidance on which areas of the part might be prone to necking failures during production stamping conditions that replicate those used in the analysis.
Several methods are available to measure the strains on formed parts. The earliest method is known as Circle Grid Strain Analysis (CGSA), with Dr. Stuart Keeler as its primary evangelist for nearly 50 years. Dr. Keeler was the Technical Editor of these AHSS Guidelines through Version 6.0, released in 2017.
As the name suggests, a flat blank is covered with a grid of circles of precisely known diameter, typically applied by electrochemical etching. Forming turns the circles into ellipses, with the dimensions related to the major and minor strains. Conventional measurement occurs after forming, and involves a calibrated Mylar™ strip marked with gradations indicating the expansion or contraction relative to the initial circle diameter. Typically, these are viewed through magnifiers, making it easier to discern the critical dimensional differences. Techniques and caveats are highlighted in Citations S-59 and S-60.
Instead of circles, most camera-based measurement techniques for analysis after forming use a regular grid pattern of squares or dots. Forming turns the squares into rectangles, and the camera/computer measures the expansion or contraction of the nodes at the corners of the squares to determine the strains. Similarly, forming changes the regular dot pattern, allowing for calculation of the strains.
These approaches determine only the strains after forming, and are constrained to assume linear strain paths. An alternate approach based on Digital Image Correlation (DIC), where a camera tracks the movement of a random speckle pattern applied prior to forming,W-26, H-22, M-21 follows the strain evolution which occurs during forming and is not affected by non-linear strain paths.
Although DIC strain analysis is more accurate and informative, it is a higher-cost approach best suited for laboratory environments. Circle-grid, square-grid, and dot-grid strain analysis are all lower cost options and readily applied on the shop floor. Each of these in-plant techniques have different merits and challenges, including ease of use, accuracy, and cost.
![Forming and Formability of AHSS]()
Forming
Introduction
Approaches for forming higher strength steels evolved with the commercialization of increased strength levels of High Strength Low Alloy (HSLA) steels. Demands for greater crash performance while simultaneously reducing mass and cost have spawned the development of new groups of steels that improve on the properties of these HSLA steels. Forming of Advanced High-Strength Steel (AHSS) is not a radical change from forming conventional HSLA steels, providing some of the key differences are understood and accounted for in die design, die process, and equipment selection.
AHSS grades solve two distinct automotive needs by two different groups of steels. The first group as a class has higher strength levels with improved formability and crash-energy absorption compared to HSLA grades. DP, TRIP, FB, and TWIP steels, which have increased values of the work hardening exponent (n-value), fulfill this requirement. The second group, including CP and MS steels, extends the availability of steel in strength ranges above what is available with HSLA grades. Originally targeted for chassis, suspension, and body-in-white components, AHSS grades are now being applied to doors and other body panels. New variations in microstructure help meet specific process requirements, including increased edge stretch, bendability, strengthening after forming, or tighter property tolerances.
The progressive increases in yield and tensile strength with these new AHSS grades magnifies existing forming issues with conventional HSLA grades and creates new challenges. Concerns include higher loads on processing equipment including presses, levelers, straighteners, blanking lines, coil slitting lines and roll forming equipment. Additionally, there are material and surface treatment considerations required for tooling in the stamping plants: draw dies, trim steels, and flange steels. Compared to conventional HSLA steels, greater energy requirements result from higher AHSS yield strengths, tensile strengths and significantly higher work hardening rates. This places new requirements on press capacity, leveler, straightener and slitting capabilities, tool construction/protection, lubricant capabilities, part and process design, and maintenance. Springback management becomes more critical as yield strengths continue to increase. Conventional and press hardened (hot formed) AHSS parts have very high strength after forming, so re- forming operations should be avoided. Trimming, cutting, and piercing equipment must be constructed and maintained to overcome the extreme high strength of the final stamping. Laser cutting of press hardened parts produces a finished part that avoids pushing the limits of trim and pierce tools and dies utilized for conventional HSLA steel.
There are an ever-increasing number of AHSS multiphase microstructure grades available, each designed to resist various forming failure modes while achieving final part performance requirements. Sharing of information regarding the planned part geometry, die and stamping processing, and final part application between steel suppliers, product and die process engineering, and end users helps ensure selection of the right steel grade for the application. This becomes especially relevant since multiphase microstructures experience additional forming failure modes compared with conventional high strength products.
Tool Design Considerations
The characteristics associated with different AHSS grades influence die design and die processing decisions. Not only are these steels typically higher in strength, but they also undergo substantial work hardening during forming. These lead to increased local loads, and changes in friction, die wear, and press requirements. The multiphase microstructures increase cut edge and bending fracture sensitivity. As such, extending the life and performance of tooling in press shops requires a rethinking of tool and part design.
Part Design
Successful application of any material requires close coordination of part design and the manufacturing process. Consult product and manufacturing process engineers when designing AHSS parts to understand both the limitations and advantages of the grade and the proper forming process to be employed. Start in the concept and feasibility stage to ensure sufficient time for corrective actions and optimization.
Soft tool materials like kirksite may be used for manufacturing prototype parts and the inserts used to eliminate local wrinkles or buckles. However, wear resistant coatings are typically not applied to these tool surfaces, so the metal flow seen in these prototype parts may not match the metal flow seen under production conditions. The results from soft tool tryouts should not be used to assess manufacturability and springback of AHSS parts.
Design structural frames (such as rails, sills, cross members, and roof bows) as open-ended channels to permit forming operations rather than draw die processes. AHSS stampings requiring closed-end draw operations are limited by a reduced depth of draw, Figure 1. Less complex, open-ended stamped channels are less limited in depth. A rule of thumb is that DP 350Y600T can be formed to only half the draw depth of a mild steel.
Figure 1: Schematic of an open end part design (left) and a closed end part design (right). The open-ended design allows for greater depths when utilizing AHSS versus the closed ended design historically used with mild steel.A-5
Where possible, avoid closed-end developments to make more complex geometries with AHSS grades. Wrapping ends of “hat” sections increases forming loads, increases the chances of circumferential compression wrinkling on the binder, specifically in the corners, and increases wrinkling on the draw wall if the blank edge runs through the draw bead. Draw die developments that include a closed (or wrapped) end development usually also require a larger blank size. During draw die development, it is best to identify parts that have a “hat” section geometry in certain locations and develop the draw die accordingly to maximize the positive formability attributes of AHSS while minimizing the limitations of AHSS.
For example, the left image in Figure 2 shows a draw die development on a DP600 cowl side with a closed (wrapped) end, with the right image showing a similar part developed with an open end. Although both final part geometries are similar, the closed-end development led to significant global formability failures due to the excessive stretch. In contrast, the open-ended development had virtually no global formability related failures. Other design and die development differences in the part on the right include the use of stake beads to control springback and embossments to eliminate wavy metal. In addition, an open-ended development has the potential to reduce the blank size for material utilization savings.
Figure 2: Draw die development for a cowl side formed from DP600. Left image: closed-end development with global formability failures, waviness, and springback. Right image: open-ended development with no splits, waves, or dimensional concerns.U-6
The automotive industry has adopted a strategy for “lighter dies and fewer dies”, to reduce cost. One key element is “part consolidation”, such as one-piece body side outers and inners. High strength steels challenge the part consolidation mantra. When encountering extreme formability challenges, parts previously made with one set of dies when stamped from lower strength steels may benefit from transitioning to a laser welded blank with a lower strength grade in the challenging region and higher strength steels in the remainder of the part. Alternatively, splitting the consolidated part into two or more separate parts subsequently welded together may improve stamping success at the expense of another operation. In the past, one-piece rocker panels were stamped from conventional mild or HSLA steel. However, this component requires higher strength and reduced thickness to meet weight and crash requirements, so now DP980 is often considered as the grade of choice for this application. Figure 3 shows a rocker panel where insufficient formability of DP980 prevented a one-piece stamping. The OEM solved this by dividing the part into two stampings, putting a more formable grade where needed on the wrapped (or closed) end.
Figure 3: When a one-piece rocker panel could not be successfully formed from DP980, the OEM stamped a DP980 rocker panel section with an open-ended design and spot welded it to a mild steel end cap.U-6
Trim and Pierce Tool Design
- Trim and pierce tools need to withstand higher loads since AHSS grades have higher tensile strengths than conventional high-strength steels.
- Edge cracking is minimized with proper support of the trim stock during trimming.
- Modify timing of the trim/pierce operation to minimize snap-through reverse loading.
- Scrap shedding may be an issue, since AHSS springback can cause scrap to stick in the tool.
Flange Design
- Design more formable flanges to reduce need for extra re-strike operations.
- Areas to be flanged should have a “break-line” or initial bend radius drawn in the first die to reduce springback.
- Adapt die radii for material strength and blank thickness.
Draw Bead Design
- Metal flow across draw beads generates strain and minimizes the elastic recovery which causes springback.
- Metal flow across draw beads generates large amounts of work hardening, leading to increased press loads.
- Optimizing blank size and shape reduces the reliance on draw beads, which can excessively work harden the material before entering the die opening.
Guidelines to Avoid Edge Cracking During Stretch Flanging
- Flange length transition should be gradual – abrupt changes in flange length cause local stress raisers leading to edge cracks.
- Use good cutting practices to achieve a high-quality edge.
- Avoid the use of sharp notch features in curved flanges.
- Avoid putting bypass notches in stretch or compression edges of blanks or progressive die carrier strips. These bypass notches can act as stress risers and lead to edge fractures in the draw or flange operation. In addition, bypass notches in blanks and progressive dies are difficult to maintain, which can increase the potential for edge fracture.
- Metal gainers in the draw die or in the die prior to the stretch flange operation compensates for change in length of line that occurs during flanging, helping to avoid edge cracking. In the example shown in Figure 4, edge fractures moved from the draw panel to flanged panel after grinding on the draw die to eliminate edge fractures in the draw operation. The draw panel underneath the flanged part in Figure 4 did not have edge fractures. The reduction in the length of line in the draw operation moved the problem to the flanged part where the stamping transitioned from bending and straightening in the flange operation to a stretch flange operation. A better practice is to add metal gainers to the draw panel to provide the feedstock which expands during stretch flanging.
Figure 4: Flanged panel fractures, with the draw panel underneath. Adding metal gainers to the draw panel would help minimize these fractures.U-6
- The higher strength of AHSS makes it more difficult to pull out loose metal or achieve a minimum stretch in flat sections of stampings. Addendum, metal gainers (Figures 5 and 6), and other tool features balance lengths of line and locally increase stretch.
Figure 5: Metal gainers help avoid insufficient stretched areas and eliminate buckles.T-3
Figure 6: Metal gainers and depressions balance stresses and minimizes wrinkled metal.A-41
Joining, Laser Welding, Press Hardened Steels
This studyR-25, conducted by the Centre for Advanced Materials Joining, Department of Mechanical & Mechatronics Engineering, University of Waterloo, and ArcelorMittal Global Research, utilized 2mm thick 22MnB5 steel with three different coating thicknesses, given in Table 1. The fiber laser welder used 0.3mm core diameter, 0.6mm spot size, and 200mm beam focal length. The trials were done with a 25° head angle with no shielding gas but high pressure air was applied to protect optics. Welding passes were performed using 3-6kW power increasing by 1 kW and 8-22m/min welding speed increasing by 4m/min. Compared to the base metal composition of mostly ferrite with colonies of pearlite, laser welding created complete martensitic composition in the FZ and fully austenized HAZ while the ICHAZ contained martensite in the intergranular regions where austenization occurred.
Table 1: Galvanneal Coatings.R-25
Figure 1: Base metal microstructure(P=pearlite, F=ferrite, Γ=Fe3Zn10, Γ1=Fe5Zn21 and δ=FeZn10).R-25
Figure 2: Welded microstructure — (a) overall view, (b) HAZ, (c) ICHAZ at low and (d) high magnifications, (e) UCHAZ (f) FZ, and (g) coarse-lath martensitic structure (where M; martensite, P: pearlite, F: ferrite).R-25
Given the lower boiling temperature of Zn at 900 °C as compared to Fe, the interaction of the laser with the Zn plasma that forms upon welding affects energy deliverance and depth of penetration. Lower coating weight of (100 g/m2) resulted in a larger process window as compared to (140 g/m2). Increased coating weight will reduce process window and need higher power and lower speeds in order to achieved proper penetration as shown in Figure 3 and Figure 4. Depth of penetration due to varying welding parameters was developed:
d=(H-8.6+0.08C)/(0.09C-4.8)
[d= depth of penetration(mm), H= heat input per unit thickness(J/mm2), C= coating weight(g/m2)]
Given the reduction in power deliverance, with an increase in coating weight there will be an expected drop in FZ and HAZ width. Regardless of the coating thickness, the HAZ maintained its hardness between BM and FZ. No direct correlation between coating thickness and YS, UTS, and elongation to fracture levels were observed. This is mainly due to the failure location being in the BM.
Figure 3: Process map of the welding window at coating weight of (a) 100 g/m2, (b) 120 g/m2, and (c) 140 g/m2.R-25
Figure 4: Heat input per unit thickness vs depth of penetration.R-25
Forming Modes
Stretching is the sheet metal forming process where the punch which creates the part shape forces the sheet metal to thin since lock beads prevent metal flow inward from the flange area. In contrast with drawing, significant metal thinning occurs in stretching, especially in the biaxial tension mode. The biaxial increase in surface area reduces the metal thickness, maintaining the constancy of volume. The thinning soon reaches the onset of the local neck and failure as defined by the appropriate forming limit curve. The steel property that improves stretching is the strain hardening exponent, or n-value.
Stretchability, or the ability for a sheet metal to be stretched with no metal flowing from the flange or binder, often is assessed by the hemispherical dome test. Here, a hemispherical punch (usually with a 100 mm diameter) deforms a fully clamped blank. This ensures pure biaxial stretch without metal flowing from the blank into the deformation zone (Figure 1).
Figure 1: Stretch forming generated by a hemispherical punch stretching a locked circular blank.
Comparing the ratio of maximum dome height to punch diameter (H/d) is one way to view the results. Figure 2 illustrates a typical test output. Note the maximum dome height (H/d) at failure decreased as the yield strength increased and the n-value decreased.
Figure 2: Dome stretch tests of 1mm thick steel using a 100 mm hemispherical punch and a clamped blank.C-9
Additional stretch tests are possible with the hemispherical dome tester other than the dome height at failure shown in Figure 2. The limiting dome height (LDH) test stretches a rectangular steel strip which is locked in the longitudinal direction (Figure 3). Typically, a conventional rust preventive oil coats the blanks. Strips of different width are tested, with a circular lock bead preventing metal flow from the binder in the regions where the blank dimensions are large enough. The output of this test is the maximum dome height at failure. Figure 3 shows the achievable hemispherical dome height is substantially higher for the TRIP steel compared to the HSLA steel grade of equivalent tensile strength.
Figure 3: Limiting Dome Height is greater for TRIP than HSLA at the same tensile strength.T-2
The same tooling, steels, and lubricant from Figure 3 generated the thinning strains in Figure 4. Instead of forming to failure, the 50 mm radius hemispherical punch stretched the dome height to only 25 mm for both steels. The high n-value of TRIP steels minimizes strain gradients and reduces localizes thinning, helping to delay necking and form more complex geometries.
Figure 4: TRIP steel experiences less thinning than HSLA steel of the same tensile strength when formed to a constant dome height.T-2
The Limiting Dome Height test results for EDDS (vacuum-degassed interstitial-free) steel and three Advanced High Strength Steel grades are in Figure 5. Instead of plotting the various dome heights (as in Figure 3) to find the minimum value, Figure 5 simply shows the minimum value for each steel. TWIP (Twinning Induced Plasticity) steel has unique properties for stretchability and total elongation. Stretchability exceeds even that of EDDS IF steel.
Figure 5: Limiting Dome Height values reflect relative stretchability of three AHSS compared with a low strength IF steel.P-2
![Forming and Formability of AHSS]()
Press Requirements
For a stamping operation, knowing the press tonnage required to produce a part is essential. Running a part in a press without enough capacity can cause press fatigue, damage and significant downtime. Also, an operation that must run a part in a much larger press than anticipated in order to get enough tonnage will see decreases in throughput and efficiency, resulting in increased cost. Therefore, it is critical to be able to predict a part’s stamping tonnage requirements early in the design phase of the component and the stamping process. This is even more important with the continual developments in Advanced High-Strength Steels (AHSS) that the steel industry has made. These steels offer similar formability of traditional high-strength steels with twice the strength or more, as shown in Figure 1. In this article, I will explain why typical rules of thumb that appeared to work in the past are no longer applicable and touch on how the automotive and steel industries are addressing the changes.
Figure 1: AHSS add to the spectrum of options available for automotive body construction.A-69
Tonnage Requirements and Press Capacity
When discussing the press capacity requirements, it is typical to mainly refer to ‘tonnage’ as the measurable needed. Tonnage is the peak load required during a stamping operation. The tonnage rating for a press refers to the peak load that the press can safely deliver without causing damage to the press frame, ram, bushings, etc. For a mechanical press, the peak tonnage is only available at the bottom of the stroke.
Besides peak load requirements, it is also important to understand the total energy required to form a part. It is typical for the force available from a mechanical press just a few inches off bottom to be 50% of the press tonnage rating. For drawing operations that may start several inches off bottom, it is important to predict the part tonnage requirements through the entire stroke and to compare that with the force curve from the press manufacturer. Integrating this press force curve over the entire stroke is a measure of the total energy capacity of the press. Many times, when a press stalls at bottom dead center, the issue may not be that the peak load of the press was exceeded, rather the full amount of energy that was stored in the flywheel had been expended. More information about press force and press energy are found on our page highlighting Press Requirements.
A few years ago, I saw an example of energy vs. tonnage requirements. The part was a relatively thick gage AHSS frame bracket. The design was a simple flanged hat section. In plan view, the part was only about 6-inch x 6-inch with the depth of the hat being about 4-inch. Based on the tonnage require to flange, trim and pierce a hole, the part should have easily been able to fit in a small 600-ton press. However, in order to have enough energy to produce the part, this small die was placed in the center of the 180-inch bed of a 1200-ton press. Not only was it ergonomically difficult for the operator, but the large press burden rate was much higher, and the speed was much slower than that of the smaller press, causing a significant cost increase.
Furthermore, it is good to know how off-center the load required to form a part is. Obviously, the die setter will center the die under the ram. However, most parts are not symmetric, causing the load going into the ram, bushings and press frame to be biased to one side or the other. This becomes more exaggerated in a large transfer press that may have a major draw operation at the entry end and a minor trim operation at the exit end.
Putting this all together, having the ability to predict not only the peak load required to form a part, but also the total energy through the stroke and the off-center bias gives us the ability to design a stamping process that is safe and efficient. Also, in lean manufacturing terms, a predicted through-stoke force curve provides a good baseline defining the basic condition of the die. Comparing this to actual force curves and evaluating any changes over time will help to develop preventive maintenance schedules and will offer many new capabilities with servo presses as they become more prevalent.
Conventional Rule-of-Thumb Calculations Lead to Inaccurate Press Tonnage Predictions, Especially in AHSS
In 1989, I worked for an automotive stamping supplier as a college intern. One of my tasks was to do basic calculations of part size and tonnage requirements that were used by the process engineers to design the die process and quote the manufacturing and die costs to produce the part. The rule of thumb estimates used at that time were simple calculations for the peak load. Tonnage for trim and pierce operations were dependent on the length of line of trim, material thickness and the shear strength of the material. Tonnage for forming operations were dependent on the size of the form, material thickness and material tensile strength. These calculations typically over-predicted the tonnage requirement, but because of the relative low strength steels used, as compared to today’s AHSS, most times, the limiting factor was the overall part size that dictated the size of the press to be used rather than the tonnage requirement.
So why do we hear today that these same rules of thumb do not work, or worse, that they now under predict the tonnage requirements? To understand this, let us look at a few guidelines I used over 30 years ago.
For piercing a hole: Tonnage = d * t * 80 Equation 1
In this equation d is the punch diameter in inches, t is the material thickness in inches, and it gives the tonnage in tons. This was a very simple and effective way to estimate the tonnage of all the holes pierced. Let us look at how this rule of thumb was derived. The equation is a simplification of the fact that the actual calculation is the length of line doing the work, in this case the circumference of a circle, multiplied by the material thickness and the material’s shear strength (ꚍ). The generic equation for any type of piercing or trimming is Tonnage = P * t * ꚍ where P is the perimeter or length of line of the trim, t is the material thickness and ꚍ is the shear strength of the material. A typical estimate for the shear strength (ꚍ) is 60% of the tensile strength (T) for the material. Therefore, the equation development for a simple hole piercing looks like:
| Generic trim equation: |
Tonnage = P * t * ꚍ |
Equation 2 |
| Specific for a round hole: |
Tonnage = πd * t * 0.6T |
|
| Simplifying: |
Tonnage = d * t * 0.6Tπ |
|
| Mild steel T = 300 MPa = 43.5 ksi: |
0.6 * 43.5 * 3.14 = 82 |
|
| Pierce a round hole: |
Tonnage = d * t * 80 |
|
Now that we know how the rule of thumb was derived, we can point out some possible sources of error. First, the equation uses the full thickness of the material. In reality, a typical trim operation for steel consists of 20% to 50% trimming and the remainder is breakage. The press only needs to produce load for the trimming portion. Second, the shear strength of the material is not a fixed percentage of the tensile strength. The actual shear strength should be measured for each specific grade as the microstructure differences of the AHSS will affect the material strength in shear. Lastly any of these errors are multiplied since today’s AHSS material has an overall tensile strength of three, four or even five times that of mild steel – taking any error in this estimate and magnifying it. To see this, we can take a look at a simple example of piercing a 1-inch hole in 1.5 mm thick mild steel. The steel tensile strength has a range of 40 ksi to 55 ksi (280 MPa to 380 MPa). If we look at Equation 1 with the high- and low-end assumptions, we see:
| Equation 1 estimate |
Tonnage = 1 * 0.06 * 80 = 4.8 tons |
| Equation 2 minimum |
Tonnage = 3.14 * 0.06(20%) * 0.6(40) = 0.9 tons |
| Equation 2 maximum |
Tonnage = 3.14 * 0.06(50%) * 0.6(55) = 3.1 tons |
In this very simple example, we see sources of error that could lead to an estimate of 0.9 to 4.8 tons to pierce a single hole. A similar exercise could be taken on a drawing operation. In this situation, most rules of thumb attempt to use the perimeter or surface area of the part, the material thickness and the material tensile strength to predict the tonnage needed. Sources for error in this type of calculation include: 1) Using the perimeter of the draw area, tending to under predict; 2) Using the surface area of the part, tending to overpredict; and 3) Using the tensile strength of the material, also tending to over predict as it assumes the material is stretched right to the level of splitting. Correction factors have been developed overtime, but it is still easy to see there are many possible sources of error in these types of calculations.
AHSS Magnifies Press Tonnage Prediction Challenges
We can see that there are inherent challenges with the old school rules of thumb, but why are they so exaggerated with today’s AHSS? There are a number of reasons.
- Strength: The strength of today’s cold stamped steels is quite incredible. Where a mild steel may have a tensile strength of 280 MPa, it is now common to cold stamp dual phase (DP) steels and 3rd Generation steels with up to 1180 MPa. In addition, new materials having a tensile strength of 1500 MPa with enough elongation to allow for cold stamping are starting to enter the market. This five-fold increase in strength acts as a multiplying factor for any errors in traditional predictions.
- Formability: The formability of AHSS has also increased dramatically. Today a DP 590 steel and even a 980 3rd Generation steel can have nearly the same elongation as a high-strength low alloy (HSLA) steel of 30 years ago. This affords the part designers the ability to incorporate more complex forms into a part including using darts and beads to increase a part’s stiffness, tight radii and deeper draws. All of these add to the tonnage used and are generally not part of the old school rule of thumb calculations.
- Springback Corrections: Springback is linearly related to the yield strength of a material. Therefore, stamping AHSS grades require more features to be added to the die process to control springback. These may include draw beads (used to control material flow early in the press stroke), stake beads (used at the bottom of the stroke to minimize springback) and tighter radii (Figure 2). These features are typically off product, in the addendum, and are easily ignored by typical rule of thumb calculations.
Figure 2: Draw and Stake Bead PlacementA-6
- Hardening Curves: The complex microstructure of AHSS offers many advantages to increase formability. All AHSS grades produce microstructural phase transformations during the stamping process. This allows the lower yield strength in the as-rolled material, which aids in formability, to increase during the stamping operation. This yield strength increase can be as much as 100 MPa. Models that estimate these hardening curves of the material are ignored when doing hand calculations.
- Other Considerations: Lastly the typical rule of thumb calculations, as we have discussed, only consider the part characteristics. They generally do not include the other sources that consume energy during the stamping process including off-product feature (beads, pilot holes, etc.), spring stripper pressure, pad pressure from nitrogen springs or air cushions, driven cams and part lifters. Many of these could be ignored 30 years ago with mild steels, but they become more significant with the strength of today’s AHSS.
Accurate Tonnage Predictions Require Accurate and Complete Inputs
The answer in recent years is to rely more on simulations using finite element analysis (FEA) software. Care must be taken when using these sophisticated software packages. Many times, the software is blamed when inaccurate press tonnage predictions are given. However, we know that if any of the characteristics mentioned above are ignored, the user will develop a very precise but very inaccurate tonnage prediction.
Next Steps
The issue with accurately predicting press requirements is industry-wide and has been around for decades. The recent advances in AHSS have multiplied the sources of errors that exist in the past rules of thumb and from the incorrect usage of advanced software packages. Many people within the steel and automotive industries are working on improving the reliability of these predictions including the Auto/Steel Partnership (A/SP). A/SP, founded in 1987, is a partnership between automotive OEMs, steel mills and affiliate suppliers. A/SP has teamed up with formability software suppliers to collaborate on this subject. A/SP’s project will work to measure sources of error and develop guidelines to address these in the areas of material characterization, modeling techniques and numerical analysis techniques. A/SP’s efforts, including this project, looks to bridge the gap between research laboratories and the shop floor.
What can stamping manufacturers do? First, prepare for the digital manufacturing era (often call the 4th Industrial Revolution) by keeping press tonnage monitors in good working order. Also, consider upgrading to systems that can capture full through-stroke force curves. Secondly, do not go it alone. Engage with organizations like A/SP, OEMs and steel mills. When evaluating new parts using AHSS, get the steel mill involved early, even in the die design phase. All steel mills have teams of application engineers to help OEMs and their suppliers to transition into using the newest grades of steel – they want stampers to succeed and have the tools and data to help.
Back To Top
structural performance
These Guidelines focus on the forming and joining of Advanced High-Strength Steels, but those aspects are only important in the context of how and why steels are used in automotive body structures or other engineered products.
Normal vehicle use involves repeated loading of components and joints, potentially resulting in fatigue failures occurring at stresses lower than otherwise expected. Conventional High-Strength Steel fatigue behavior correlates with their tensile strength. However, in multiphase AHSS grades, the strain distribution between phases within the steel microstructure affects the fatigue response, leading to a fatigue response which may vary depending on the grade and the microstructural approach.
Although lightweight and low cost are certainly important, crash safety is the most critical aspect of structural designs. Steels, especially AHSS grades, offer automakers new options to cost-effectively improve crash performance and achieve lightweighting targets promoting improved fuel economy and a greener life cycle.
The articles in this section address these topics.
