Press Hardened Steels (PHS): Advanced Materials for Automotive Lightweighting and Safety

Introduction to Press Hardened Steels in Automotive Manufacturing

Environmental considerations have become paramount in modern passenger car engineering. To reduce fuel consumption and toxic emissions, automotive manufacturers must decrease vehicle weight while maintaining performance standards. For car body structures, this translates to down-gauging sheet components and introducing advanced high-strength steels (AHSS) to preserve crashworthiness and durability performance.

Advanced high-strength steels help engineers meet comprehensive requirements for safety, efficiency, emissions, manufacturability, durability, and quality at competitive costs. AHSS represent a newer generation of steel grades that provide extremely high strength and advantageous properties while maintaining the formability required for manufacturing processes. Although AHSS have been utilized in automotive applications for many years, continued research and development enable automakers to implement these advanced grades in increasingly diverse applications.

The AHSS Family and PHS Classification

The AHSS family encompasses several distinct categories: Dual Phase (DP), Complex Phase (CP), Ferritic-Bainitic (FB), Martensitic (MS or MART), Transformation-Induced Plasticity (TRIP), Hot-Formed (HF), and Twinning-Induced Plasticity (TWIP) steels.

Among Advanced High Strength Steels, Press Hardening steels form a unique group consisting primarily of various boron-alloyed manganese steels. These materials have gained extensive application in producing high-strength structural body elements and are widely utilized in car body manufacturing through hot forming processes.

Figure 1: Automotive body structures. Press-hardened components are shown in red, otherwise indicated with arrows.

Understanding Press-Hardened Steel Composition and History

Press-Hardened Steels are boron-added (0.001%-0.005%) carbon/manganese steels that have been utilized since the mid-1980s for automotive body-in-white construction. These steels are also recognized as HF (Hot Formed) Steel in industry terminology.

The Hot Forming Process: Achieving Martensitic Transformation

Process Overview and Temperature Requirements

Hot forming of steels represents a complex forming and tempering operation, often termed hot press forming or press hardening. The process begins with full austenitization of the material as the initial step. During forming, the material is rapidly cooled in the tool at the critical cooling rate, resulting in a martensitic microstructure.

To enable boron-alloyed steel material to be formed and subsequently cooled to a fully martensitic structure, the material must first be heated to its austenitization temperature range of approximately 880-950°C. Achieving a fully martensitic structure requires cooling rates exceeding 25-30°C/s. The small amount of boron (approximately 0.002 weight-%) facilitates the quenching process, leading to the colloquial term "Boron steel" for these materials.

Figure 2: Temperature vs. process time for hot press forming of PHS

Direct and Indirect Hot Forming Approaches

The hot-forming process divides into two primary approaches: indirect and direct processes, with the direct process being most commonly utilized among automotive OEMs. These processes offer different advantages for final applications in terms of design flexibility versus cost considerations and available surface protection options.

The indirect process performs initial forming steps through conventional cold forming, offering possibilities for more complex geometries and undercut designs before shaped components are heated, transferred to the oven, and finally tempered. This approach requires two tool sets: one for pressing and one for cooling. However, the indirect process provides the advantage of enabling conventional zinc-based coating application for cathodic corrosion protection.

Figure 3: Schematic illustration of the indirect hot-forming process steps