Contributed by David W. Anderson, Senior Director, Automotive Market and Long Products Program, Steel Market Development Institute, USA
The introduction of Advanced High-Strength Steel (AHSS) to light vehicle body structure applications poses a significant challenge to organizations involved in vehicle repair. AHSS grades are typically produced by non-traditional thermal cycles and contain microstructural constituents whose mechanical properties can be altered by exposure to elevated temperatures. This temperature sensitivity can alter the mechanical behavior during repair welding or flame straightening, thus seriously affecting the structural performance of the AHSS components after the repair.
The Steel Market Development Institute (SMDI), with our automotive partners – FCA US LLC, Ford Motor Company, General Motors Company – and I-CAR, have completed studies examining the mechanical behavior of various AHSS products after exposure to typical repair arc welding and flame straightening temperature cycles. Recommended practices for repairing components made of these materials were also developed. The studies evaluated many of the AHSS grades being applied and built into vehicle structures today.
AHSS Thermal Evaluation
Several steel grades were evaluated for their sensitivity to thermal exposure taking place during heating to soften the material for straightening, typically by flame. The test results, conclusions and recommendations contained herein are the consensus views of the team members.
A time-temperature test matrix was developed to represent the various thermal conditions encountered during repair welding and flame straightening as shown in Table 1. Individual steel performance result discussions are based on this test matrix and discussed by grade category and specific type.
Table 1: Time-Temperature Test Matrix.
Interstitial free (IF) and high strength low alloy (HSLA) steels are conventional steel products which are essentially a single phase ferrite microstructure and obtain their strength by the addition of chemical elements. These steels have been used in body structures and closures for many years and are well-known to be repairable without substantial performance degradation by arc welding and flame straightening. For repair processes, this means conventional steels can be subjected to heat during repair with the finished repaired component having mechanical properties greater than the properties of the as-received steel. However, it is recommended heating is kept below 750 degrees Celsius to ensure no degradation in properties (Figure 1).
Figure 1: Ultimate tensile strength of Grade 4 IF and HSLA 340 steel after exposure to simulated repair thermal cycles.
Advanced High-Strength Steels
Dual Phase (DP) Steel
DP steels range in strength from 500 MPa to 1200 MPa and obtain their properties from the introduction of a martensitic phase into the ferrite microstructure. The ferrite phase provides formability, while the martensitic phase provides the improved strength. This category of steel grades obtains its microstructure, and thus its mechanical properties through a combination of alloying elements and thermomechanical processing. The processing involves some holding time at elevated temperatures and cooling at specific rates.
Two grades of DP steel were tested, DP 600 and DP 780. The number indicates the ultimate tensile strength (UTS) level of the material in MPa and is the common way to name these grades. UTS for both grades decreases with elevated temperature at a much faster rate than for conventional steels and is illustrated by DP 600 in Figure 2. At temperatures above 650 degrees Celsius the strength suddenly increases then upon additional heating decreases. This behavior is a result of changing the microstructure created during the original thermomechanical processing of the material. Once the microstructure is changed, it is very difficult to return it to its original state in a repair shop environment. Therefore, it is not recommended to subject DP steels to any kind of elevated temperature process for straightening or removing dents. The recommended repair procedure is to remove and replace the DP component. OEM repair guidelines and procedures should be referenced for approved cut and weld lines for replacement.
Figure 2: Ultimate tensile strength of advanced high-strength steels after exposure to simulated repair thermal cycles.
Transformation Induced Plasticity (TRIP) Steels
TRIP steels have a similar range of strength as DP steels, 500 MPa to 1200 MPa, while providing improved formability. The improved formability is obtained with the introduction of additional phases of austenite and bainite into the microstructure. These phases improve the work hardening properties of steel and provide additional energy absorption characteristics. TRIP steel microstructures are obtained in a similar manner as DP steels, and therefore have similar behaviors when heating.
TRIP 600 and 780 were evaluated in the studies and confirmed the expected results as demonstrated by TRIP 600 in Figure 2. Heating during repair of TRIP steel will also adversely affect their mechanical properties and thus the performance of the as repaired component may be compromised. OEM recommended repair procedures are similar to DP steels.
Martensitic Steel (MS)
MS steels typically have a microstructure of 100 percent martensite and have tensile properties greater than 980 MPa. Martensite is the strongest microstructural phase in steel and is obtained by alloying and rapid controlled cooling. This grade is used in areas where exceptional strength and anti-intrusion are needed, including such applications as the A-pillars, B-pillars, rockers and rails.
The effect of heat on MS 1300 is shown in Figure 2. Like other AHSS, it is adversely affected by heat and the performance of the as repaired component may be compromised. Thus, heat should be used only as outlined in OEM repair procedures.
The steel industry, working closely with automotive OEMs and the repair community, have developed and validated repair procedures applicable to the new AHSS used in today’s vehicles. Each OEM has taken the results from the AHSS repairability studies and developed their own repair guidelines.
The steel industry continues to develop AHSS grades with strength levels at and above 1000 MPa. These microstructures contain martensite and will be affected by heat exposure during repair as shown in previous studies. Collaborative studies will continue to update repair procedures for higher strength AHSS, including DP, MS and Press Hardened grades, and new 3rd Gen. AHSS as they are introduced.
David W. Anderson, Senior Director
Automotive Market and
Long Products Program
Steel Market Development Institute, USA
With more than 30 years of experience, David Anderson serves as senior director, Automotive Technical Panel and Long Products Program for the Steel Market Development Institute (SMDI), a position he has held since January 2012. In this position, he is responsible for leveraging his expertise to support SMDI’s Automotive Applications Council’s (AAC) – a consortium of major North American steel producers – expansion into chassis and powertrain programs, as well as serving as the liaison between the AAC and its partner organizations. In addition, he is responsible for coordinating efforts to grow steel long products in the North American automotive /
Vehicle LCA encompasses all phases of the product cycle, from raw material extraction to end of life recycling and disposal.
Life Cycle Assessment (LCA) continues to gain traction as the preferred method for assessing the environmental impacts of a product, and we are seeing more and more automotive LCA studies, and claims or conclusions based on LCA, being published. Here are five things to look for in an automotive LCA study. This is not an exhaustive list – there are many more things that need to be considered when conducting an LCA (the international standard that lays out requirements and guidelines for LCA, ISO 14044, lists 14). For a detailed look at LCA, I recommend Environmental Life Cycle Assessment, an excellent textbook published by the American Center for Life Cycle Assessment (ACLCA).
1. Is the study comparing “apples to apples”?
This can sometimes be a bit of a challenge in an automotive LCA, as cars are complex products made up of many systems and sub-systems, with supply chains that reach around the world. It is easy to lose sight of apparently subtle differences.
One such difference arises in material comparisons of an existing product and one at the design stage. Before making choices about which material has the lower impact, it is important to decide if this is a fair comparison. Have both designs been fully optimized? Does the theoretical design meet all the requirements (e.g. crash, NVH) of an existing design that has gone through all the additional steps necessary to get to production?
Functional equivalence is another important factor to consider. For example, it would be inappropriate to compare the “body” of a vehicle with a body-on-frame design with the “body” of a vehicle with a unibody design. Though both systems are casually referred to as the “body”, their functional requirements are very different. Similarly, when comparing an assembly of stamped parts to a single casting, it is important to include all the stamped parts that are required to meet the same functional performance as the cast part.
2. Has the best and most appropriate data been used?
As with any assessment tool, data is key, and there are many factors that can help determine which is the most appropriate to be used in a particular study. There are ten of these factors listed in ISO 14044, but I will only talk about three of them here. If you want more detail, Guidance on Data Quality Assessment for Life Cycle Inventory Data, published by USEPA’s National Risk Management Research Laboratory is a great resource.
Temporal scope – the temporal scope of the data describes when the data was collected, as well as the time period to which the data applies. An often-used rule of thumb is that the data used for the study should be no more than five years old. The goal is to use either data that reflects the time period when the product under study was actually produced or the most recent data that captures the current state of the art (these may of course be the same thing).
Geographical scope – the geographical scope of the data describes the location to which the data applies. This is critical, as the same process may have a very different impact profile in different locations. For example, primary aluminium made largely using electricity from hydropower in Canada will have a much different impact from primary aluminium made in China, where the electricity used comes almost entirely from coal. With automakers increasingly developing global platforms (i.e. using the same parts all over the world, as often as possible), it is important to consider how the results of an LCA might change if a product is produced in a region other than the one studied. The same is true when considering where a product is used. For example, the differences in electricity generation in Canada and China will greatly affect the impact of charging an electric vehicle.
Technological scope – the technological scope of the data describes the technology to which the data applies. Often the same product can be made from a variety of production pathways. Much like production in different regions, production via different pathways can result in much different impacts. An example of this is primary magnesium production. Magnesium produced via the Pidgeon process (a declining, but still-used method) can have a global warming impact more than 1.5 times higher than magnesium produced via the electrolytic method.
3. How sensitive are the results to changes in the choices made?
Because LCA involves choices about things like data and functional requirements (as well as a host of other parameter choices), it is important to understand what happens to the results if different choices are made or underlying conditions change. Inclusion of a sensitivity analysis of this kind allows us to evaluate the results of an LCA and look for areas that might require further study, perhaps by looking for more precise data. It could also affect decisions we might make based on the results. If the results of a study are very sensitive to the location in which the materials are made, we want to evaluate this if we are considering making the product in many different places.
4. What is the fundamental approach taken?
ISO 14040, which lays out the principles and framework for LCA, describes two fundamental approaches to LCA:
- one which assigns elementary flows and potential environmental impacts to a specific product system typically as an account of the history of the product, and
- one which studies the environmental consequences of possible (future) changes between alternative product systems.
It is important to consider which approach was used, and what the chosen approach means in terms of interpreting the results. In a very basic sense, the key lies in the words “history” and “future”. A study that considers the historical impact of a product (i.e. the impact of any product that has already been made) tells us about the impact of that particular product, but may be of limited use in making decisions about what the impact of that same product may be in the future. Studies of the potential future impact of a product, while necessary for decision making, are fraught with all the perils of predicting the future. Both approaches are valuable, and it is important to understand which approach was taken in order to make decisions about how to use the results of the study. The ILCD Handbook: General guide for Life Cycle Assessment, published by the Joint Research Center of the European Commission, provides guidance on when to use each approach in assessing the technological representativeness of LCI data.
5. How broadly can I apply the results?
It is often tempting to apply the results of a study more broadly than is justified. Given all the factors listed above (as well as the many others not included), it is critical that careful consideration be given when making decisions based on the results of automotive LCA studies in order to make sure that the scope of the study is in alignment with the scope of the decision. It is clearly inappropriate to make global decisions based on local studies, or to make future decisions based solely on past conditions. LCA is a valuable, well-developed tool for assessing environmental impacts, but we must be careful to use it appropriately.
The UCSB Automotive Energy and GHG Model, developed on behalf of WorldAutoSteel, is a publicly available, peer-reviewed tool for the assessment of automotive emissions on a life cycle basis. Version 5 of the UCSB Model can be downloaded for free at www.worldautosteel.org. At the UCSB Model page, you’ll find a video workshop at the end featuring Russ Balzer explaining the contents of the Model. A user guide is also available for download.
Does the subject of Life Cycle Assessment confuse you or would you like to learn more? There is a great free online course at our sister organization steeluniversity, “Introduction to LCA”, taught by Dr. Matthias Finkbeiner, head of the department of Sustainability Engineering at Technical University of Berlin, who introduces LCA methodology to analyse the environmental burdens of product and service systems. The course is fun and interactive, and you can finish it at your own pace.
Russ Balzer, LCACP, Technical Director, WorldAutoSteel and Phoenix Group
Russ Balzer is the LCA Technical Director at WorldAutoSteel and Phoenix Group. As Technical Director, Russ manages a variety of engineering projects, and has tactical and strategic responsibilities in WorldAutoSteel’s efforts to use and promote Life Cycle Assessment (LCA). Russ recently achieved LCACP certification and was recognized for his work in the field of LCA with the ACLCA’s Rising Star Award.
Life Cycle Assessment (LCA), and particularly vehicle product life cycle assessment, is a topic we are very passionate about here at WorldAutoSteel. So much so that we focus on LCA intensively for the entire month of October across all of our communications channels. Though it’s not an AHSS forming or joining topic, it is one that is critical to truly reducing vehicle emissions for future generations. Russ Balzer, Technical Director at WorldAutoSteel and our resident LCA professional, in this blog and the next, will talk about LCA, its importance, and the tools WorldAutoSteel has developed to provide environmental insight to design decision tradeoffs.
All over the world there are continuing and growing efforts to address transportation greenhouse gas (GHG) emissions, which remain a major unresolved issue. These efforts are intended to help the transportation sector make its contribution to global emissions reduction goals. Unfortunately, much of this effort is focused on reducing emissions only from the vehicle tailpipe, with no consideration of the other sources of emissions in that vehicle’s life. This is not an effective way to meet these goals. In fact, this approach could lead to the unintended consequence of increasing GHG emissions in some cases. Fortunately, there is a better way – life cycle assessment (LCA), a tool for looking at the environmental impact of a product across its entire life cycle (Figure 1).
Figure 1: Vehicle LCA encompasses all phases of the product cycle, from raw material extraction to end of life recycling and disposal.
Focusing solely on the tailpipe emissions means ignoring other significant sources of GHG emissions, such as vehicle production and emissions generated (or avoided) at the end of the vehicle’s useful life (see Figure 2 on Page 2). An example of this is that tailpipe-only thinking can put too much emphasis on lightweighting. Don’t get me wrong, lightweighting can be an important part of the solution. Three of the four main drivers of fuel consumption (and therefore tailpipe emissions) – rolling resistance, acceleration and gravity (as in climbing a hill) – are dependent on the vehicle’s mass. So we can see why vehicle lightweighting is an obvious choice. It is a direct way to reduce these power demands and achieve better fuel consumption and fewer tailpipe emissions. The problem with lightweighting arises when we are so focused on reducing a vehicle’s mass that we fail to consider the consequences to the vehicle’s overall emissions.
One of the potential consequences arises from the use of lower-density materials like aluminium, magnesium and even carbon fibre to replace steel in a vehicle. From a tailpipe perspective, this can seem like a good (if expensive) solution. Vehicle mass may be reduced, resulting in improved fuel consumption and fewer tailpipe emissions. Sadly, it is not that simple. These low-density materials come with an environmental cost in addition to their higher financial cost. This cost comes in the form of higher GHG emissions in the production of the material itself. On a global average basis, GHG emissions from aluminium production can be as much as eight times as high per kilogram of material as the GHG emissions from steel production. For carbon fibre and magnesium the difference in production GHG emissions is even greater. This means that, even though you may save tailpipe emissions with some applications of these low-density materials, there is always a trade-off of higher production emissions.
Figure 2: The difference between a regulatory focus that includes LCA and current tailpipe emissions.
In the best case, the reduction of emissions in the use phase does result in overall lower emissions, though, because of the trade-off between the tailpipe and production emissions, not as low as predicted by a tailpipe-only metric. Also possible is an intermediate case in which the use phase savings and the production phase increase are approximately equal, resulting in no net savings at all. In the worst case, the production emissions outweigh the use phase savings, resulting in the unintended consequence—higher overall emissions, the very opposite of what the regulation intends.
All three of these cases have two things in common. First, under a tailpipe-only regulation, we don’t know what the actual emissions are, because production emissions impacts are not being monitored. Second, because the low-density materials we are talking about are more expensive, all three of these cases come at a higher cost. So, we must ask ourselves: do we want to force automakers and consumers to pay more money without knowing the outcome? It’s time to consider another route to reducing emissions, and we believe that taking a life cycle approach is the correct route.
LCA assesses all the stages of a product’s life, from raw material extraction through production, use, and end of life processing. Though awareness of LCA has grown rapidly over the last 10-15 years, LCA methodology and practice have been developing since the early 1970s. Today, it is a mature assessment tool with global standards. Many car manufacturers are already using life cycle thinking and LCA, recognizing its importance and effectiveness in product and process design. LCA is equally accepted and used by material producers. In fact, together with many of their member companies, the trade associations of the steel, aluminium, and plastic industries are among the most active members of the global LCA community.
WorldAutoSteel has been directly involved with LCA since 2007, when we partnered with Dr. Roland Geyer of the University of California at Santa Barbara to develop an LCA tool for the assessment of material choices in passenger vehicles. The UCSB Automotive Materials Energy and Green House Gas (GHG) Comparison Model that Dr. Geyer developed on behalf of WorldAutoSteel is now in its fifth version and continues to be one of the most comprehensive publicly available vehicle LCA tools in the world.
The UCSB model is a full vehicle model assessing both GHG and energy effects of automotive material substitution over the entire life cycle of the vehicle.
Computation and parameter values are kept separate for maximum transparency and flexibility. This allowed the computational structure to be peer reviewed by members of the LCA community. The model calculates 27 main result values: three environmental indicators x three life cycle stages x three vehicles, as shown in Figure 3.
Figure 3: UCSB Model calculates 27 main result values.
The model has the flexibility to allow a multitude of different scenario evaluations, offering 14 structural material categories, 24 total material categories, adjustable material recycling methodology, a variety of biofuel and electricity pathways, as well as the powertrains, driving cycles and vehicle classes noted in Figure 4.
Figure 4: UCSB Model analysis options.
To maximize flexibility and transparency, all calculations are shown, and no parameter values are locked or hidden. This makes the UCSB Model an excellent tool for teaching LCA, particularly automotive LCA.
GHG emissions in the transport sector must be reduced to meet global emissions reduction goals. Lightweighting of passenger vehicles can be an important part of the emissions reduction solution in the transport sector, but only if lightweighting scenarios are viewed in the context of the overall vehicle emissions. Many companies inside and outside of the transport sector use Life Cycle Assessment, which considers environmental impacts from the whole of a vehicle’s life cycle, as their primary method to develop this overall view. The UCSB Automotive Energy and GHG Model, developed on behalf of WorldAutoSteel, is a publicly available, peer-reviewed tool for the assessment of automotive emissions on a life cycle basis. Version 5 of the UCSB Model can be downloaded for free at the WorldAutoSteel website here.
At the UCSB Model download page, you’ll find a video workshop featuring Russ Balzer explaining the contents of the Model. A user guide is also available for download.
Russ Balzer, Technical Director, WorldAutoSteel
Russ Balzer is the LCA Technical Director at WorldAutoSteel and Phoenix Group. As Technical Director, Russ manages a variety of engineering projects, and has tactical and strategic responsibilities in WorldAutoSteel’s efforts to use and promote Life Cycle Assessment (LCA). Russ recently achieved ACLCA LCACP certification and was recognized for his work in the field of LCA with the ACLCA’s Rising Star Award.
As promised, in this blog we’ll review the Direct and Indirect Hot Forming processes. The goal of hot-forming is to achieve complex geometries and part consolidation at strength levels previously unimaginable, while minimizing springback and distortion. While several steels are applicable, the most common hot forming steels are Boron-based (between 0.001% and 0.005% boron); the industrial workhorse is 22MnB5. Hot Forming with these steels has been in use since the 1990s for various body structure components and two types of press-hardening or hot forming applications, Direct Hot Forming and Indirect Hot Forming, are currently available.
Direct Hot Forming Process
Figure 1 shows the process flow for the Direct Hot Forming process. During Direct Hot Forming, all deformation of the blank is done in the high temperature austenitic range (minimum temperature of 850 °C), followed by quenching with a cooling rate greater than 50 °C/s. The initial HF steel microstructure is composed of ferrite and pearlite, with the following room temperature properties: yield strength of 350-400 MPa, tensile strength of 550-600 MPa, and total elongation around 25%.
Figure 1: Graphic showing steps in the Direct Hot Forming process.1
In Step 1 (Marker 1 in Figure 2), the steel coil is cut into blanks that will be loaded into a high temperature oven with multiple zones to achieve uniform temperatures across the material.
Figure 2: Steel properties during the forming process.
In Step 2, the blank is fed into the furnace. To enable the material to harden, it must first be heated above 900 ºC to change the microstructure to austenite. Generally, this is accomplished in continuous furnaces to ensure a continuous heating process. Typical furnace time is 5-8 minutes. The exposure of the tool steel to the high temperatures necessary for hot-forming can result in large variations in friction because of changes in the surface topography, removal of oxide layers, and excessive wear of the tool. One approach to overcome the issues of friction is to apply suitable coatings or various surface treatments to the tool steel.
Currently, an aluminium-silicon (AS) coating is the most common coating applied to blanks to prevent the formation of this surface oxide. Other coatings include hot-dipped galvanized (GI), galvannealed (GA), zinc-nickel (Zn-Ni), and organic substances. Inert gasses can be used for special applications. The coatings also help prevent in-service corrosion in part areas difficult to shot blast or otherwise remove the surface oxide prior to application of additional corrosion protection treatments.
In Step 3, the “hot” blank is transferred into a forming die. Robots or linear transfer systems (feeders) can transfer the blank to the water-cooled die in about three seconds. To protect the transfer system from overheating and minimize the heat loss of the blank, insulation should be used – an example is the placement of heat shields between the blanks and the transfer system. Once transferred, positioning aids ensure that the blank is located precisely in the die.
In Step 4, the hot blank is formed into the part geometry. Forming temperature typically starts at 850 ºC and ends at 650 ºC. While in the austenitic range, the true yield stress is relatively constant at 40 MPa with high elongations greater than 50%. This enables stampings with complex geometries and part consolidation to form successfully with limited springback issues.
Step 5 reflects In-Die Quenching (Marker 3 in Figure 2), where the die is liquid-chilled. When forming is completed, the stamping now contacts both the punch and die for both side quenching. The minimum quench rate is 50 ºC/sec. Some actual cooling rates are two or three times the minimum rate. Quenching the formed part leads to a significant increase in the strength of the material and a greater precision in its final dimensions. The quench process transforms the austenite to martensite throughout the entire stamping, which accounts for the increase in strength. The room temperature properties of the final stamping are 1000-1250 MPa yield strength, 1400 -1700 tensile strength, and 4-8% elongations (See the true stress-strain curve in Figure 3, lower graph). Total time for robot transfer, forming, and quenching is about 20-30 seconds and depends heavily on the quench rate and quenching system. With smaller stampings, forming and quenching of multiple stampings in the die reduces per stamping processing time.
Figure 3: True stress-strain curves for different sheet thickness of as-received boron-based HF steel tested at room temperature (left curve) and tested after heat treatment and quenching (right curve).2
Step 6 is the post-forming operations. The very high strength and low elongations of the final stamping restrict these final operations. The room temperature stamping should not undergo additional forming. Any special cutting, trimming, and piercing equipment must utilize appropriate materials and equipment capacities to withstand the high loads generated during these operations. Production speeds range between 2 to 4 parts per minute, slowed to allow sufficient quenching.
Indirect Hot-Forming Process
The Indirect Hot Forming process (Figure 4) accomplishes initial forming and trimming cold (prior to hot forming), shown as preform Step 1A. Here, 90-95% of the stamping geometry is pre-formed in conventional dies at room temperature, based on incoming steel properties. The stamping is trimmed (2A) and then subjected to the usual heating cycle in Step 3A. Additional hot-forming (4A) is now possible for areas of the stamping too severe to form at room temperature. However, the in-direct forming process has a cost increase over the Direct Hot Forming process since two forming dies are required instead of one.
Figure 4: Graphic showing steps in the In-Direct Hot-Forming process.1
Indirect Hot Forming was developed to reduce wear on the tool when dealing with uncoated steel. The added cold forming stage reduced movement between the steel and the tool, thus leading to less wear on the tool. On the contrary, with Indirect Hot Forming rapid cooling of the finished stamping takes place via the surfaces of the tool. The Indirect Hot Forming process is introduced to develop stampings with more complex form features. Since the stamping cavity depth is formed during cold stamping and the detail features are formed thereafter in the Hot Forming press, more complex geometry can be achieved and distortion is minimized.
Figure 5: Post forming heat treating. Lower strength, more formable steel is formed to final shape (Marker 1), heated, and quenched to achieve the final high strength (Marker 2).
Another process similar to the Indirect Hot Forming process is Post-Forming Heat-Treating (PFHT). Very high strength steels generally have greatly reduced stretchability. The PFHT goal is to create the stamping from lower strength, but more formable steels (Marker 1 in Figure 5) by traditional Cold Forming processes. The final processing heat and quench sequence creates a very high-strength stamping (Marker 2 in Figure 5). The major issue restricting widespread implementation of PFHT typically has been maintaining stamping geometry during and after the heat treatment process. Fixturing the stamping and then heating (furnace or induction) and immediate quenching appear to be the solution for production applications. Current quenching processes are water, air hardening, or water-cooled dies.
1 Hot forming Process photos courtesy of M. Peruzzi, voestalpine Stahl GmbH and R. Mohan Iyengar et al, “Implications of Hot-Stamped Boron Steel Components in Automotive Structures,” SAE Paper 2008-01-0857 (2008).
2 Figure 3 photos provided courtesy of C. Walch, voestalpine Stahl GmbH.
Today many product designs tend to combine geometry complexity and part consolidation with the highest possible final strength steel required for in-service applications. Maximum part complexity usually requires superior stretchability as evidenced by high work hardening capability and defined by the n-value. Part consolidation might take three non-severe stampings and make one very severe large part. Imagine three separate stampings formed with extensive metal flow from the binder to provide maximum part depth. Lay the three stampings side-by-side in a straight line and successfully connect them with welds to make the final part. Now attempt to make all three attached stampings from a single blank in one die. There is no binder area to feed the middle stamping, which now must form almost completely by excessive stretch forming. Making the problem worse, increasing the strength of the as-received steel reduces the stretching capacity of the steel because the work hardening exponent (n-value) decreases with increasing strength for each type of steel. Finally, springback problems increase as the yield strength increases.
The hot-forming process can minimize all the above problems. Hot forming, also called hot stamping or press hardening, is a process used to form very thin, but also very strong metals into automotive parts to reduce weight while also increasing strength and safety. By performing the stamping while the steel is nearly molten, the process eliminates springback and allows for the forming of complex geometries while eliminating springback often associated with stamping advanced high strength steels.
Hot forming once was a novice technology; first used in the 1984 Saab 9000, hot-forming has steadily increased in popularity. For instance, the number of production stampings from hot-forming went up from 8 million per year in 1997 to 107 million per year in 2007 and continues to rise in all regions of the globe. For some recent vehicles, the percentage of these steels is close to 25% of the body in white (BIW) weight. Today, hot-forming is used to stamp product geometries with relatively complex shapes such as A-pillars, B-pillars, front/rear side members, bumpers, door rings, and more.
Figure 1: Vehicles using hot-stamped components to achieve lightweighting and strength for crash requirements.1
At the Steel Market Development Institute’s (SMDI) 2018 Great Designs in Steel conference in Livonia, Mich., several automakers and Tier 1 suppliers revealed 2019 vehicles where they used hot-stamped components to achieve lightweighting and strength for crash requirements (Figure 1). (Click the vehicle-name links to review the event presentations on the SMDI website.) For example, the new Chevy Silverado pick-up truck uses 5.5 percent hot stampings in the cab and truck box, and the 2019 Dodge Ram truck uses a 6-piece laser-welding door ring that is hot stamped. The new Acura RDX uses 15 percent press-hardened steels in its body structure.
Figures 2 and 3 show the process flow for direct vs. indirect hot forming. Our next blog will go into more detail about these two processes.
Figure 2: Graphic showing steps in the Direct Hot-Forming process.2
Figure 3: Graphic showing steps in the In-Direct Hot-Forming process.2
Benefits of Hot Forming
- Hot-forming has the highest potential for weight reduction of crash components.
- Tailored welded blanks with different combinations of thickness, properties, and surface coatings can be hot-formed as a single stamping.
- Controlling the temperature in various locations of the forming die can create zones with different strength levels in the final stamping.
- Springback issues eliminated, which is remarkable considering the extremely high final part strength.
- Manufactured stampings have low distortion.
- Stamping consolidation has high feasibility for success.
- Stampings have low directionality of properties measured by r-value anisotropy.
- A 10% increase in yield strength (about 100 MPa) bake hardening effect can further increase in-service strength.
- A-pillar upper
- B-pillar outer and reinforcement
- Front/rear side members
- BumperRoof rail
- Door ring
- Cross member
Some of these parts are often made with tailored welded blanks, mixing different thicknesses and different grades (500 MPa for the areas which must allow buckling during crash and 1500 MPa for areas where no deformation under crash is accepted).
1 2019 Chevy Silverado photo courtesy of General Motors Company
2019 Dodge Ram photo courtesy of FCA USA
2019 Acura RDX photo courtesy of Honda
2 Hot forming Process photos courtesy of M. Peruzzi, voestalpine Stahl GmbH
The first operation when unwinding a coil is straightening to ensure flatness before further processing. There are two main types of equipment used to create a flat coil – a straightener and a precision leveller. In some respects, these two types of equipment are similar, but a precision leveller has additional capabilities.
Straighteners – All coils must be wound under tension to avoid “soft” or collapsed coils. As a result, tension and compression stresses are induced into the strip during the coiling process, and these can contribute to blank or part distortion in subsequent processes. Straighteners are designed to eliminate these stresses and create flat material; the objective is to relieve the residual stresses in the outer 20% of the strip.
Straightening of coils is accomplished by bending the strip around a set of rolls to alternately stretch and compress the upper and lower surfaces of the steel to erase coil set or crossbow. Critical parameters include roll diameter, roll spacing, backup roll configuration, roll material type, gear design, overall system rigidity, and power requirements. The higher the material’s yield strength, the greater the force that’s required to relieve those residual stresses. Thus, equipment capability needs to be considered when processing AHSS products. Figure 1 shows a schematic of a straightener working the outer 20% of the steel strip. Figure 2 depicts coil set and crossbow conditions.
Figure 1: Schematic of a straightener roll working the outer 20% of the steel strip. 1
Figure 2: Schematic of a coil with a coil set and crossbow conditions, that can be removed with a straightener. 1
When utilizing a straightener, the two critical steel variables that impact the process are the yield strength and the thickness. Straighteners have a series of rolls that progressively flex the strip to remove the residual stresses. These rolls have an entry and exit gap that need to be adjusted when thickness and yield strengths vary.
Figure 3 shows a schematic of a typical set of straightener rolls required to obtain a flat strip. These gaps should be adjusted based on the yield strength and thickness of the material. Many equipment manufacturers have generated tables to guide the operator as to the best settings for various yield strength-thickness combinations.1
Figure 3: Schematic of a set of straightener rolls indicating the two main adjustments – entry and exit gaps. 1
Precision Levellers – Although precision levellers can perform all the functions of a straightener, they have additional capabilities. They can remove coil set and crossbow conditions and are capable of correcting other shape issues in coils that can create manufacturing problems in subsequent operations, for example, edge waves and center buckles.
Correcting edge waves and distortion requires exceeding the yield strength of the steel strip at levels of 50% or more of the cross section of the strip; this may increase the potential for work hardening the steel strip, negatively affecting formability. Figure 4 shows a schematic of a precision levelling roll working 50% of the cross section of the steel strip, which is the typical process for removing edge waves and center buckles.
Figure 4: Schematic of a precision leveller roll working the outer 50% portion of a steel strip. 1
Design and Processing Implications
The progressively higher yield strengths for AHSS are challenging the capabilities of straighteners and precision levellers that were not designed for flattening these high strength materials. Equipment manufacturers have been studying and developing solutions to address this issue. There are a series of components related to the design of straighteners and precision levellers that need to be considered:
Roll Diameter – Straightener rolls for AHSS generally may need to be smaller in diameter than those used for mild steel, providing a smaller radius around which to bend the material. This is because AHSS must be bent more severely to exceed its higher yield strength.
Roll Spacing – Work roll center-spacing may need to be closer for AHSS than for comparable mild steels. Closer spacing means that more force is required to back-bend the material, requiring greater power for processing.
Roll Support – Larger journal diameters with larger radii and bearing capacity may be needed to withstand the greater forces and higher power required to straighten AHSS.
1 Courtesy of Peter Mooney, 3S-Superior Stamping Solutions, LLC