PHS in Vehicles: An Abridged History

The Beginnings of PHS Use

Press hardening, as we know it today, was developed in Luleå, Sweden, by Norrbottens Järnverks AB (abbreviated as NJA, translated as Norrbotten Iron Works). The first patent application was completed in 1973 and awarded in 1977.^N-23 The technology was first commercialized in agriculture components, where the high strength of Press Hardened Steels (PHS) was favored for wear resistance.^B-45

In 1984, automotive applications of PHS started with the Saab 9000 side impact door beams, as seen in Figure 1. A total of 4 parts were used in this car.^A-66 The uncoated blanks were almost half the thickness of a cold stamped beam.^T-26

Figure 1: Door beams of the Saab 9000 (1984-1998): (a) A see-through car in Saab Museum^S-82, (b) the hot stamped part.^L-42

The majority of the PHS parts were door beams through the mid-1990s, with approximately 6 million beams produced in 1996. By this time, the demand for bumper beams was also increasing.^F-31 By the end of 1996, the European New Car Assessment Program (EuroNCAP) was formed, which increased the pressure on the OEMs for improved crashworthiness.^T-26 In 1998, both the new Volvo S80^L-44 and Ford Focus5 were equipped with Press Hardened bumper beams.

The year 1998 saw the development of one of the most important breakthroughs in Press Hardening technology. French steel maker Usinor developed an aluminum-silicon (AlSi) pre-coated steel, commercialized as Usibor 1500 (indicating the typical tensile strength, 1500 MPa.^{C-24, L-39} In 2000, BMW rolled out its new 3 series convertible. In this vehicle, the A-pillar is made from 3 mm thick uncoated, PHS sheet. This was BMW’s first PHS application, and one of the first PHS A-pillar reinforcement.^{S-83, S-84} Accra started delivering roll formed PHS components for the Volvo V70, initially an optional 3rd row seating support. Approximately 10,000 parts/year were supplied.^G-28

AlSi coated steel was first hot stamped at a French Tier 1 supplier, Sofedit.^V-15 This grade was first used in the front bumper beam of the 2nd Generation Renault Laguna (2000-2007). Laguna 2 was the first car to receive a 5-star safety rating from Euro NCAP.^V-10 AlSi coated blanks were also used in PSA Group’s Citroën C5 (1st Gen: 2001-2007) in the front bumper beam, and the A-pillars. These three parts weighed a total of 4.5 kg, approximately 1% of the total BIW weight, Figure 2a. About one month later, PSA Group started production of the compact hatchback Peugeot 307, which had five hot stamped components (A- and B-pillars and rear bumper beam). Unlike the Citroën C5, these parts were uncoated. The total weight was 12 kg, corresponding to 3.4% of the BIW weight.^{R-17, P-27}

Figure 2: Increase in press hardened component usage: (a) 2001 Citroën C5^P-27, (b) 2002 Volvo XC90^L-29 and (c) 2005 VW Passat.^H-50

Volvo started producing the XC90 SUV in 2002. The body-in-white with doors and closures weighed 531 kg.^B-44 A total of 10 parts, weighing 37 kg are either roll formed or direct stamped PHS. This accounts for approximately 7% of the BIW weight.^L-43 During its time, this was the highest use of PHS in car bodies. In Figure 2b, the Press Hardened components other than the 2nd row seat frame, which is a load bearing body part, are shown.

Accelerated Use and Globalization

The use of press hardened parts increased rapidly after the introduction of the VW Passat in 2005. This car had approximately 19% of its BIW (by weight) made from press hardened steels, Figure 2c. Some parts in this car saw the first use of varnish coated blanks in a two-step hybrid process. Three parts were produced using either an indirect or hybrid process, including the transmission tunnel.^H-50

Following are a few highlights of PHS use in vehicle applications during this time period :

• In 2006, the Dodge Caliber^K-37 and BMW X5^P-28 were among the first cars to have tailor-rolled and Press Hardened components in their bodies (Figure 3).

Figure 3: (a) Tailor Rolling Process^Z-5, (b) B-pillar of BMW X5 (2nd Gen: 2006-2013).^P-28

• BMW 7 Series (5th Gen: 2008-2015) became the first car to have Zn-coated Press Hardened components in its body-in-white. The car also contained uncoated parts, as shown in Figure 4 (next page). The total PHS usage in this car was approximately 16%.^P-20

Figure 4: PHS usage in BMW 7 Series (5th Gen: 2008-2015) (re-created using P-20).

• Press hardening also allowed car makers to create unconventional cars. In 2011, Hyundai rolled out the 1st generation Veloster, a 3-door coupé (also known as 2+1, with one door on the driver side and 2 doors on the passenger side), and as such containing axisymmetric front doors. Thus, the car could not have a full B-ring, as illustrated in Figure 5a.^{B-14, R-19} Another unconventional design was the Ford B-Max subcompact MPV sold in Europe between 2012 and 2017. The car had conventional swing doors in the front and two sliding rear doors. A PHS B-pillar was integrated in the doors, providing ease of ingress. Its PHS components (integrated B-pillar in front and rear doors, door beams and cantrail) are shown with blue color in Figure 5b.^{B-14, L-45}

In 2013, the Acura MDX (3rd Gen: 2013-2020) became the first car to have a Hot Stamped door ring. The part was a tailor welded blank comprised of two sub-blanks, as shown in Figure 6a. The design saved about 6.2 kg weight per car and had high material utilization ratio thanks to sub-blank nesting optimization.^{A-67, M-46}  One of the most recent PHS applications was in 2017 Chrysler Pacifica with 5 sub-blanks, as shown in Figure 6b. This car also has a PQS550 sub-blank at the lower B-pillar region.^D-28

Figure 6: Hot stamped door rings: (a) First application in 2013 Acura MDX had 2 sub-blanks, (b) a more recent application in 2017 Chrysler Pacifica has five sub-blanks with PQS550 at the lower B-pillar (re-created after Citations B-14, A-67, D-28).

• Tubular hardened steels have been long used in car bodies, with minimal forming. Since 2013, a special 3-D hot bending and quenching (3DQ) process has been employed. One of the earliest uses of this technology was Mazda Premacy (known as Mazda 5 in some markets). The same process was also used in making the A-pillars of the Acura NSX (Honda NSX in some markets, 2016-present), as seen in Figure 7a.^H-29 Since 2018, tubular parts formed with internal pressure, called form blow hardened parts, are used in the Ford Focus (4th Generation) (Figure 7b) and Jeep Wrangler (4th Generation).^{B-16, B-17}
  

  Figure 7: Tubular hardened steel usage in A-pillars of: (a) 2015 Acura NSX^H-29, (b) 2018 Ford Focus.^B-16

PHS Use in xEVs: Hybrid Electric, Battery Electric,

Plug-in Hybrid Electric & Fuel Cell Electric Vehicles

The first commercially available Hybrid Electric Vehicle (HEV) was the Toyota Prius (1st Gen: 1997-2003). The second-generation Prius (2003-2009) had very few Press Hardened components, as shown with red color in Figure 8a. This was the first time Toyota used hot stamped components.^M-47 The third generation Prius (2009-2015) had approximately 3% of its BIW Press Hardened. In the 4th generation Prius released in 2015, the share of >980 MPa steels has risen to 19%.^U-10 Figure 8b shows the Press Hardened parts in this latest Prius.^K-38

Figure 8: PHS usage in Toyota Prius: (a) 2nd generation (2003-2009) and (b) 4th generation (2015-present) (re-created after Citations M-47 and K-38)

The 2012 Tesla Model S and Model X launched using aluminium bodies, with PHS reinforcements in the pillars and the bumpers. Model S is known to have a roll-formed PHS bumper beam. High volume Model 3 and Model Y have a significant amount of press hardened components in their bodies.^T-35

In 2011, General Motors started production of its first Plug-in Hybrid Electric Vehicle (PHEV), the Chevrolet Volt (known as Opel Ampera in EU and Vauxhall Ampera in the UK). This car had six Hot Stamped components, including A and B pillars, accounting for slightly over 5% of the BIW mass.^P-29

The smaller BEV Chevrolet Bolt, launched in 2017, had aluminum closures, but a steel-intensive BIW that is 80% steel, 44% of which is Advanced High-Strength Steels including 11.8% PHS. Figure 9.^A-69

Figure 9: Chevrolet Bolt Body Structure and Steel Content.^A-69

In December 2020, Hyundai announced their new electric platform, E-GMP. The platform will utilize Press Hardened steel components to secure the batteries.^H-52

Automakers have turned to PHS to manage the extra load of Fuel Cell powertrains as well. The first-generation Toyota Mirai had only Press Hardened B-pillars, cantrails and lateral floor members.^T-38 The second generation has a number of parts with PHS in its under body as well.^T-39

In 2018, Hyundai Nexo became the first fuel-cell car to be tested by EuroNCAP, achieving a 5-Star rating. The car has PHS A- and B-pillars, rocker reinforcements, and several under body components, as seen in Figure 10.^H-53

Figure 10: Press hardened steel usage in Hyundai Nexo Fuel Cell vehicle: (a) side view and (b) top view (re-created after Citation H-53).

Did you enjoy this story?  You can find a much more detailed article here in the Guidelines with many more vehicle examples and history data.

Thanks are given to Eren Billur, Ph.D., Billur MetalForm, who contributed this article.