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
AHSS applications are growing, where the high strength body components help meet crash requirements, and thinner sections achieve weight reduction for improved fuel economy and lower overall emissions. Both higher strength and thickness reduction contribute to lower overall formability, higher forces, greater temperatures and accelerated die wear, and each of these outcomes reduce the size of the manufacturing window. To manage all of these challenges, the pressroom must implement advanced process control measures in order to minimize normal process variation, as the process is now less tolerant of this variation. The same holds true for die and recipe maintenance.
Understanding this reality, it’s important to make process decisions that enhance the forming window. This includes better die materials, surface treatments, surface coatings and data-based lubricant specification. Cross-functional communication is required to create awareness among employees in the level of process discipline critical to success. Controlling the inputs of the stamping process become vital to achieving predictable outputs, therefore process recipes should be defined and fixed across all shifts. This is the spirit behind process control, which defines the relationship between key inputs and outputs of the stamping process. Figure 1 shows critical stamping inputs, such as the steel, lubricant, press and die conditions. All of these contribute to final part outputs, and thus need to well-documented, maintained and understood for process reliability and repeatability.
Figure 1: Stamping pressroom critical inputs including the steel, lubricant, press and die.
Reference Panels are ideal for AHSS processes, and effectively establish process control of the forming operation. A reference panel is a draw shell that documents input settings and process outputs (forming strains, draw-in lines, trim scrap) at the time the panel was formed. It thus serves as a useful reference for each die-set, to compare to the current operational panel and process, ensuring that all inputs are stable. Visual changes in panel appearance or measured changes in strains or draw-in lines allow rapid detection of input variation, and process adjustments to be incorporated prior to incurring unacceptable panels (characterized by scoring, distortion, buckles or splits).
The use of reference panels should be part of a Formability System, characterized by systematic panel reviews, recipe confirmations, and die improvement / die maintenance planning. As an example, production realities are such that reference panels won’t be used if they are not stored in a protective rack, near the floor (so access is easy). Figure 2 shows an example reference panel rack system in a North American stamping plant.
Figure 2: Door Outer Reference Panel and Storage Rack.
All affected employees need to be trained on the importance of this tool, so panels are preserved and not discarded during housekeeping events. And a disciplined panel review process aligns production and trade personnel to ensure that the reference panels are used during each part run. Draw-in or thickness templates allow for formability measures on panels in very precise and consistent locations. Measuring these outputs on a regular basis (every part run) enables a determination of process stability and trends, which becomes important towards scheduling PM. Examples of draw-in templates are shown below in Figure 3.
Figure 3: Draw-in template for draw panel; cutout allows for accurate, repeatable measurement.
Because AHSS products are susceptible to edge shearing from trim and pierce operations, these operations require advanced tooling and more frequent PM intervals. All die maintenance will be critical, requiring more frequent polishing, insert replacement (or reconditioning), surface treatment assessments to plan repair or reapplication, etc. Finally, tool and die personnel should attempt to define the size of the forming window with split-buckle analysis, varying shut height while maintaining all other variables constant to determine when unacceptable buckles or necking initiate. This information is then used to develop input (recipe) settings that achieve production stability and robustness.
To understand the difference between localized and global fractures, you must first understand strain gradients (see the article in our blog, AHSS Strain Hardening and Gradients). Gradients can result in highly concentrated strains (peak strain condition) that typically occurs in an embossment or character line where the deformation mode is in plane strain. Peak strains can develop rapidly in a very localized area (Figure 1). Under additional loads, this can result in the onset of localized necking, which means the material has reached its tensile strength and will fail at its weakest point or highest strain. When a slight increase in strain is applied, the material will fracture, sometimes at deformation levels less than predicted. This condition can be found in AHSS products, where multiple phases exist within the steel’s microstructure, each with different properties. A global fracture also typically occurs in plane strain, but more commonly down a sidewall or other area with more moderate geometry complexity.
Figure 1: Peak strain in the localized area or embossment
Peak (concentrated) strains are susceptible to localized fractures when even slight variation exists in the forming process. Examples of variation include lubrication pattern and volume, die recipe including blank position, press conditions, and material characteristics.
A localized neck and/or fracture (Figure 2) reduces the sheet metal’s thickness, reducing part strength, and compromising functional performance such as fatigue life, crash worthiness, and stamping stiffness. There are a number of formability analysis tools that can differentiate localized and global fractures and enable die makers to implement die and process improvements that minimize fracture susceptibility. The result is a more robust stamping process.
Figure 2: Schematic of Localized Necking and Fracture
Process control is critical; die recipe discipline is needed to minimize tinkering with die recipe, press settings, and lubrication settings. Mechanical properties of the sheet metal should be tracked to identify trends or variations in the material, and establish the material forming window. Typical mechanical properties that are available from the steel supplier are yield strength, tensile strength, n-value, total and uniformed elongation, and sheet thickness. Additional properties that should be determined include hole expansion and deep cup draw ratios. Failure to identify strain levels, process variables and variation will lead to a reactionary approach to controlling the output. This will lead to an increase in scrap, die-related downtime, and of course, costs.
Contributions made by Phoenix Group.