Introduction to Advanced High Strength Steels
By Mr Daniel J Schaeffler, PhD
President of Engineering Quality Solutions Inc
Advanced high-strength steels (AHSS) offer enhanced formability. This article discusses the properties and performance of various grades.
Several new commercialized and near-commercialized advanced high-strength steels (AHSS) that exhibit high strength and enhanced formability are being offered around the world. These steels have the potential to effect cost and weight savings while improving performance.
The increased formability allows for greater part complexity, which leads to fewer individual parts (cost savings) and more manufacturing flexibility. Fewer parts mean less welding (cost and cycle-time savings) and weld flanges (mass and weight savings). Depending on design, the higher strength can translate into better fatigue and crash performance, while maintaining or even reducing thickness.
| Steel Grade | YS | UTS | Total EL | n Value | r Bar | K Value |
| BH 210/340 | 210 | 340 | 34-39 | 0.18 | 1.8 | 582 |
| BH 260/370 | 260 | 370 | 29-34 | 0.13 | 1.6 | 550 |
| DP 280/600 | 280 | 600 | 30-34 | 0.21 | 1 | 1,082 |
| IF 300/420 | 300 | 420 | 29-36 | 0.2 | 1.6 | 759 |
| DP 300/500 | 300 | 500 | 30-34 | 0.16 | 1 | 762 |
| HSLA 350/450 | 350 | 450 | 23-27 | 0.14 | 1.1 | 807 |
| DP 350/600 | 350 | 600 | 24-30 | 0.14 | 1 | 976 |
| DP 400/700 | 400 | 700 | 19-25 | 0.14 | 1 | 1,028 |
| TRIP 450/800 | 450 | 800 | 26-32 | 0.24 | 0.9 | 1,690 |
| DP 500/800 | 500 | 800 | 14-20 | 0.14 | 1 | 1,303 |
| CP 700/800 | 700 | 800 | 10-15 | 0.13 | 1 | 1,380 |
| DP 700/1000 | 700 | 1,000 | 12-17 | 0.09 | 0.9 | 1,521 |
| Mart 950/1200 | 950 | 1,200 | 5-7 | 0.07 | 0.9 | 1,678 |
| Mart 1250/1520 | 1,250 | 1,520 | 4-6 | 0.065 | 0.9 | 2,021 |
Figure 1
Steel Properties
YS and UTS are minimum values, other values are typical
Source: ULSAB-AVC TTD#6 (Technology Transfer Dispatch #6) available from www.ULSAB-AVC.org or www.autosteel.org
The first of this two-part series examines the similarities and differences between conventional HSS and the various AHSS grades. Figure 1 lists some mechanical properties of the grades discussed in this article. The values shown in the table are for comparison only, with the specific properties and ranges likely varying somewhat among steel companies. In addition, n value is a function calculated over a specific strain range and is more prone to vary as a function of the chosen strain range in the AHSS grades compared with the conventional HSS grades. As a result, it is important to consider data from the appropriate strain range pertinent to the specific forming operation.
Conventional High-strength Steels
It is generally accepted that the transition from mild steel to HSS occurs at a yield strength of about 210 megapascals (MPa) [30 kilopounds per square inch (KSI)]. For yield strength levels below 280 to 350 MPa (40 to 50 KSI), a simple carbon-manganese (CMn) steel typically is used. The composition of these steels is similar to low carbon mild steels, except they have more carbon and manganese to increase the strength to the desired level. This approach usually is not practical for yield strengths greater than 350 MPa (50 KSI) because of a drop-off in elongation and weldability.
One approach to achieving yield strengths between 280 and 550 MPa (40 and 80 KSI) is to use high-strength, low-alloy (HSLA) steels, also known as microalloyed (MA) steels. This family of steels usually has a microstructure of fine-grained ferrite that has been strengthened with carbon and/or nitrogen precipitates of titanium, vanadium, or niobium (columbium). Adding manganese, phosphorus, or silicon further increases the strength. These steels can be formed successfully when users know the limitations of the higher-strength, lower-formability trade-off.
Another approach to achieving these yield strength levels is to use AHSS grades. Dual-phase (DP), transformation-induced plasticity (TRIP), high hole expansion (HHE), complex-phase (CP), and martensitic steels are some of the grades collectively referred to as AHSS.
Dual-phase (DP) Steels
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| Figure 2 |
DP steels have a microstructure of mainly soft ferrite, with islands of hard martensite dispersed throughout. The strength level of these grades is related to the amount of martensite in the microstructure.
As the product arrives from the steel mill, its yield strength typically is much lower than its tensile strength, with a YS-to-TS ratio of about 0.6. (For comparison, the YS-to-TS ratio for HSLA steels is closer to 0.75.) The lower yield strength at a given tensile strength translates to higher elongation values and better formability.
In addition, the work-hardening response to deformation is different between DP and HSLA steels. HSLA steels begin to lose formability as soon as deformation starts. As a result of the soft ferrite matrix of DP steels, they can maintain their formability further into the press stroke and can better distribute the strains across the part.
DP steels usually are bake-hardenable (strengthening occurs after the steel goes through a paint-bake cycle), whereas HSLA steels do not exhibit this characteristic (Figure 2). Between this bake hardenability and the higher level of work hardenability, it is not unusual to see an increase in yield strength of about 140 MPa (20 KSI) after forming and baking. In comparison, HSLA steels may have an increase of about 20 MPa (3 KSI).
Enhanced energy absorption is another DP steel characteristic. For a given yield strength, the DP steel tensile strength is higher than that of HSLA steels, which enhances crash performance. If crash performance equivalent to that of an HSLA steel is desired, using a DP steel may allow for downgauging of about 10 percent. 1
DP steel weldability usually is similar to that of HSLA steels, although different parameters may be required. The welding current range is almost the same (about 3 kiloamps), but the actual currents may be somewhat shifted.
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| Figure 3 |
Transformation-induced Plasticity (TRIP) Steels
Like DP steels, the microstructure of TRIP steels is comprised of mainly soft ferrite. While DP steels have martensite as the only other phase, TRIP steels have a combination of martensite, bainite, and retained austenite. The various levels of these phases give TRIP steels their unique balance of properties. Figure 3 shows the stress-strain curves for a HSLA steel, a DP steel, and a TRIP steel, each with a yield strength of about 350 MPa (50 KSI).
As forming continues, the retained austenite in TRIP progressively transforms to martensite with increasing strain. This leads to a volume and shape change within the microstructure, which accommodates the strain and increases the ductility. In TRIP steels, the high work-hardening rate persists at higher strains, while that of DP begins to diminish. This work-hardening difference is one of the primary reasons for the enhanced formability of DP steels over HSLA steels, and what gives TRIP steels a further advantage over DP steels (Figure 4).
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| Figure 4 |
The strain level of retained austenite-to-martensite transformation can be engineered through carbon content adjustment. If lower carbon levels are used, transformation starts at the beginning of forming, leading to excellent formability and strain distribution at the strength levels produced. At higher carbon levels, retained austenite is more stable and persists into the final part. The transformation occurs at strain levels beyond those produced during stamping and forming. Transformation to martensite occurs during subsequent deformation, such as a crash event, and provides greater crash energy absorption.
The additional alloying required to get the TRIP effect makes spot welding more challenging compared with DP steels. This can be addressed with modified welding cycles.
High Hole Expansion (HHE) Steels also known as Stretch Flanging High-strength (SFHS) Steels
For applications that require a high degree of sheared edge elongation (hole flanging), HHE steels are increasingly being used. The microstructure is primarily ferrite and bainite, with some retained austenite. These steels exhibit high strength, high formability (although less than some other AHSS grades), and high sheared edge extension (hole flanging) capability. The ferrite-bainite microstructure is associated with high hole expansion values. Parts stamped from these grades are replacing cast and forged parts made from other materials.
Complex-phase (CP) Steels
CP steels are characterized by a very fine microstructure of ferrite and a higher volume fraction of hard phases (martensite and bainite), strengthened further by fine carbon or nitrogen precipitates of niobium, titanium, or vanadium.
These steel grades have been used for parts that require high energy-absorption capacity, such as bumpers and B-pillar reinforcements.
Martensitic Steels
Martensitic steels have a microstructure that is 100 percent martensite. Minimum tensile strengths of this family of steels are typically between 900 and 1,500 MPa (130 and 220 KSI). These grades can be made directly at the steel mill (quenching after annealing) or via postforming heat treatment. Because of its limited elongation, mill-produced martensite typically is roll-formed. More complex shapes can be fabricated by hot forming and quenching a lower carbon grade.
Based on the targeted strength level, these grades can have carbon content typical of low-carbon steel or levels greater than 0.20 percent. Manganese, silicon, chromium, molybdenum, boron, vanadium, and nickel also are used in various combinations to increase hardenability. As a result, adjustments in the welding procedure may be needed.
Typical applications for martensitic steels usually are those that require high strength and good fatigue resistance, with relatively simple cross sections (although profiles of hot stamped parts are becoming more complex). Good candidates for martensitic parts include door intrusion beams, bumper reinforcement beams, side sill reinforcements, and belt line reinforcements.
Increasing Applications
When combined with appropriate manufacturing techniques, advanced high-strength steels offer opportunities for reduced product weight, enhanced crash performance, manufacturing process consolidation, and cost reduction.
These engineered steels are being used in more applications throughout various manufacturing industries, and their use should continue to grow as die and process engineers become familiar with the different techniques that are required for manufacturability. The second article of this two-part series will highlight some of these techniques and issues that should be considered when processing these grades.
Processing Considerations
Understanding and compensating for the challenges associated with processing advanced high-strength steels (AHSS) can help you minimize springback, edge cracking, trimming, wrinkling, and die wear.
Using AHSS in appropriate applications offers opportunities for reduced product weight, enhanced crash performance, manufacturing process consolidation, and cost reduction. However, because these grades have different microstructures, chemistries, and properties, die processing must be optimized to take advantage of the differences. Proactively addressing the associated challenges can help minimize costly tryout.
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| Figure 6 |
As with all high-strength steels, springback is a concern with AHSS. Without appropriate compensation, the higher strength of the AHSS grades usually leads to more springback compared to conventional HSS. The degree of springback correlates with the yield strength level after forming (rather than the yield strength in the flat sheet) and the tensile strength. As shown in Figures 5 and 6, this correlation is independent of the microstructure or strengthening approach -high-strength, low-alloy (HSLA), dual-phase (DP), or transformation-induced plasticity (TRIP).
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| Figure 5 |
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| Figure 6 |
Comparing a DP steel with an HSLA steel, each with 350-MPa (50-KSI) yield strength and formed under the same conditions, the DP steel has much greater springback than the HSLA steel. This difference is because the parts made from DP steel have greater yield strength after forming (greater work hardening) compared with the HSLA steel parts. Also, the DP steel's tensile strength is about one-third greater than the HSLA steel's. Adjusting tooling parameters, such as the die radii and bead placement and severity, can control springback to manageable levels (Figure 7).
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| Figure 7 |
Die adjustments can be made to achieve even greater stretch forming with TRIP steels. As shown in Figure 8, TRIP steel with a tensile strength of about 800 MPa performs similarly to 600- and 440-MPa grades with punch shoulder radii of 2 or 6 mm. This advantage is lost at the largest tested punch shoulder radius of 25 mm, presumably because the sheet metal does not stretch as much, which reduces the work-hardening effect. A tight die shoulder radius helps with the dimensional stability of TRIP (and non-TRIP) steels ( Figure 6). Although a tight radius reduces springback, it also increases forming challenges and press load requirements.
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| Figure 8 |
Figure 9 compares the performance of several steels in various simulative tests. The TRIP grade's draw depth is comparable to that of deep-drawing steel (DDS) and superior to that of other high-strength grades, except in hole flanging. In this stretched edge test, the hard second phases act as stress risers and inhibit the degree of edge elongation.
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| Figure 9 |
Springback, side wall curl, and panel twist have their origins in unbalanced stresses in the formed part. These can be caused by part and die processing design (nonsymmetrical parts or cutouts, rapid changes in cross section, or unequal flange lengths) or forming process differences (uneven lubrication, die polishing, blank holder forces, or broken or worn draw beads).
Minimizing springback requires forming process consideration. Steel will work-harden as it is drawn over a radius, and this higher strength will cause both increased springback and side wall curl and make restrike operations difficult. As such, metal movement across these radii should be limited. Furthermore, a conventional closed-end draw process might have a reduced draw-depth capability compared with an open channel approach. To achieve the desired channel height while limiting movement across radii, a hat section can be formed using a different approach, like a two-stage gull-wing process. Although the general guidance is to minimize the number of forming operations, the gull-wing process eliminates having to rework the same areas (Figure 10).
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| Figure 10 |
Another method for improving dimensional accuracy is to stretch the side walls at the bottom of the press stroke.2 Also, by employing variable binder force control, it is possible to reduce springback and obtain a more uniform strain distribution.3 Controlling metal flow of these higher-strength steels can increase the binder tonnage requirements by 20 percent compared with conventional HSLA steel grades,4 and double that needed for mild steels.5 In the right press, these are not overwhelming issues. However, a single-acting press with a nitrogen cushion is probably insufficient for processing AHSS grades.
Edge Cracking and Trimming
Martensite and other hard second-phases of the AHSS grades reduce stretch flangeability of these steels, with the issue becoming more severe as the strength increases. The part and process should be designed to minimize features that lead to the reduced stretch flangeability. For example, stress risers can be induced by abrupt changes in flange length. A gradual transition, free of sharp notches, will minimize edge cracking potential. In addition, length of line increases occurring during stretch flanging can be compensated for by using metal gainers.
A clean edge cut is especially important for blanked or sheared edges and punched holes on AHSS material. A consistent and appropriate clearance is vital to minimize burr height. Cutting tools also need to be sufficiently sharp. Compared to sharp tools, worn tools result in a 20 percent reduction in hole expansion (stretch flangeability) in mild steels, but a reduction of 50 percent or more in DP and TRIP grades. 6
During trimming, it is important to support the panel and trim stock. This reduces the tendency for a bad burr, which in turn minimizes the tendency for edge cracking. Trim steels also need to be engineered with a higher strength in mind-the tensile strength of the AHSS grades can be substantially higher than that of conventional HSS.
Furthermore, during tool development, the blank die sometimes is the last one to be completed, necessitating the use of laser-cut blanks during prototype and even at the beginning of hard tool tryout. Laser-cut edges have much higher stretch flangeability than the sheared edge obtained from a conventional blank die, and as a result, tryout performance may be different.
Press and Tooling
One benefit of the AHSS grades is the ability to reduce the sheet steel thickness. However, the decreased thickness has the potential to increase wrinkling if die clearances are not adjusted to reflect the reduced gauge. Controlling the wrinkling requires higher press forces, which may lead to the need for higher-tonnage presses. The wrinkling, combined with the sheet steel's overall strength, increases the potential for die wear. Upgraded die materials, premium wear-resistant coatings, and strategically placed inserts all may be needed to address die wear. Also, the high press forces cause higher temperatures, which can cause lubricants to break down or burn, resulting in extreme wear conditions. Appropriate lubricants designed for these grades should be considered.
Important to Remember
Compared to conventional HSS, the AHSS grades typically have higher tensile strength and higher formability for a given yield strength. As such, the forming system needs to change accordingly. Higher-tonnage presses are needed, appropriate die materials with inserts should be used, and optimized lubricants and tool coatings should be considered. Springback control may necessitate using variable binder force control with the ability to achieve at least 25 percent higher total binder force. The great work-hardening benefits useful for kicking up formed panel strength (discussed in Part I of this series) make restrike operations something to avoid. Minimizing stretch flanging requires attention to the trim dies, trim steels, and clearance (burrs). Metal gainers should be used to balance length-of-line. All of these issues are magnified when a process tuned for conventional HSS is re-engineered for AHSS, rather than starting with an optimized AHSS design.
With these challenges in mind, you may be overwhelmed by the requirements to process AHSS. However, with a systematic and thorough understanding of metal movement and interaction with the forming and processing system, it is possible to realize the performance, weight, and cost benefits that these grades offer.
Mr Danny Schaeffler is the president of Engineering Quality Solutions Inc, a provider of practical solutions for sheet metal forming challenges, PO Box 187, Southfield, MI 48037, 248-539-0162, DS@EQSgroup.com, www.EQSgroup.com
Other information related to steel and sheet metal formability can be found at www.EQSgroup.com.
