Press Hardening Steel (PHS / 22MnB5 / Usibor)

Press Hardening Steel (PHS), also called hot stamping steel or die quenching steel, is a boron-alloyed steel (standardly 22MnB5 chemistry) that achieves ultra-high tensile strength of 1,500–2,000 MPa through a combined forming-and-hardening operation called hot stamping (press hardening), rather than by cold forming already-hardened steel. The hot stamping process works in the following sequence: Step 1 — Blank preparation: A flat blank is cut from AlSi-coated PHS coil (in the soft, cold-rolled condition with tensile strength ~400–600 MPa, fully formable). The AlSi coating protects against oxidation and decarburisation in the subsequent furnace heating step. Step 2 — Furnace heating: The blank is loaded into a roller-hearth furnace maintained at 900–950°C and soaked for 4–7 minutes per mm of blank thickness (typically 3–6 minutes total for 1.2–1.8mm PHS), achieving complete austenitisation of the steel and transformation of the AlSi metallic coating into a hard, bonded Fe-Al intermetallic compound. In the austenitic state at 900°C, 22MnB5 has tensile strength of only ~30–50 MPa — extremely soft and fully formable, comparable to soft aluminium. Step 3 — Transfer: The austenitised blank (glowing orange at 900°C) is rapidly transferred by robotic arm to the water-cooled forming die within a maximum transfer time of 5–10 seconds, ensuring the blank temperature does not fall below ~750°C (the minimum temperature for complete austenitisation throughout the blank thickness before die closure). Step 4 — Simultaneous forming and quenching: The water-cooled forming die closes, simultaneously pressing the hot blank into the complex three-dimensional component geometry (A-pillar, B-pillar, door ring) using conventional stamping press force while the water-cooled die surface quenches the formed part at cooling rates of 27–50°C/s — well above the critical cooling rate for 22MnB5 (approximately 25–30°C/s) that ensures complete transformation from austenite to martensite, producing the 1,500 MPa martensitic microstructure in the finished component. The part is held in the closed die for 5–20 seconds until cooled below the martensite finish temperature (~200°C), then ejected fully hardened. Step 5 — Laser trimming: The fully hardened PHS component (1,500 MPa) is trimmed to final geometry by high-power laser cutting, which cleanly cuts through the martensitic steel without the extreme mechanical cutting forces that would be required for mechanical punching of 1,500 MPa material, and without the edge damage and microcracking that mechanical shearing would create in the brittle martensitic edge zone.

The aluminium-silicon (AlSi) metallic coating on press hardening steel (designated AS60, AS100, or AS150 per the approximate coating weight in g/m²) performs multiple critical functions that are collectively essential for commercially viable hot stamping production — making it the overwhelmingly dominant surface treatment for PHS worldwide, used on over 95% of all press-hardened automotive components. Protection against oxidation during furnace heating: When bare steel is heated to 900–950°C in a conventional roller-hearth furnace (the standard hot stamping furnace type), the steel surface rapidly forms a thick iron oxide (Fe3O4 and Fe2O3) scale layer within 3–5 minutes of furnace exposure. This scale layer: (1) physically prevents the die contact surface from achieving the required die-to-blank thermal contact for efficient quenching, reducing local cooling rates below the minimum required for complete martensite transformation and creating under-hardened zones with tensile strength below 1,500 MPa specification; (2) detaches from the component surface during die contact, depositing hard abrasive scale particles on the die face and water-cooling channels, causing die wear, die channel blockage, and production stoppages; (3) must be removed by shot blasting before painting, adding a secondary operation that damages the precision trimmed component geometry and creates dimensional variation unacceptable for automotive body assembly. The AlSi coating prevents all oxidation by forming a continuous, oxide-free Fe-Al intermetallic compound barrier during furnace heating. Protection against decarburisation: At 900°C furnace temperature, carbon diffuses from the steel surface into the furnace atmosphere (CO2, CO, H2O from combustion products), creating a surface decarburisation zone of 0.02–0.10mm depth in bare steel within the 5–7 minute furnace soak time. This decarburised surface zone has significantly lower martensite hardness than the subsurface (decarburised martensite hardness proportional to C content), reducing surface fatigue strength by 15–30% below the design requirement for cyclic-loaded structural components. The AlSi coating completely prevents decarburisation by blocking carbon diffusion from the steel surface. Cathodic corrosion protection in service: The Fe-Al intermetallic compound layer formed from the AlSi coating during hot stamping provides cathodic protection to the steel substrate at areas where the coating is damaged (scratches, cut edges, drilled holes) because aluminium is anodically active versus steel in the galvanic series, sacrificially oxidising to protect the steel from corrosion — a similar mechanism to zinc galvanic protection but operating through the Fe-Al intermetallic compound's aluminium-rich composition. Paint adhesion: The Fe-Al intermetallic surface provides excellent adhesion for the automotive electrocoating (e-coat) primer applied in the vehicle paint shop, enabling consistent paint film formation and long-term paint adhesion over the vehicle's design life.

PHS1500 and PHS2000 are differentiated by carbon content, resulting martensite hardness and tensile strength after hot stamping, elongation, and application suitability — with each grade addressing a distinct position in the automotive structural performance-versus-weight optimisation design space. PHS1500 (22MnB5 chemistry, C 0.20–0.25%, EN designation per EN 10083-3 as 22MnB5): The dominant PHS grade representing approximately 95% of global press hardening steel production, providing tensile strength 1,500–1,700 MPa after hot stamping with elongation A80 of 5–8%. PHS1500 is the standard specification for: A-pillar and B-pillar reinforcements in all vehicle classes from economy through luxury; door ring structural stampings; longitudinal crash rail anti-intrusion sections; roof and header reinforcements; and EV battery floor cross-members. The 0.22% carbon content provides the martensite hardness for 1,500 MPa tensile strength while maintaining the minimum elongation (5–8%) required for crash energy absorption in bending (without catastrophic fracture of the structural member during crash deformation — a critical failure mode where the structural member breaks in two rather than folding progressively, providing no crash energy absorption after fracture). PHS2000 (modified chemistry, C 0.30–0.40%, designated 37MnB4 or proprietary compositions by different steel producers, commercial name Usibor 2000 by ArcelorMittal, MBW 2000 by ThyssenKrupp, B2000HS by Baosteel): Provides tensile strength 1,900–2,200 MPa after hot stamping with elongation A80 of 4–6%. PHS2000's 33% higher tensile strength versus PHS1500 enables a further 15–20% gauge reduction versus PHS1500 in strength-governed designs, or allows maintaining PHS1500 gauge while providing additional intrusion resistance safety margin. PHS2000 is specified for: the most critical anti-intrusion zones of A-pillar and B-pillar in premium and ultra-premium vehicles where absolute minimum intrusion depth is the overriding design requirement; high-performance EV battery protection structures where the combination of maximum intrusion resistance and minimum structural mass directly maximises battery capacity within the vehicle; future vehicle architectures targeting 30% body weight reduction requiring maximum lightweighting of each structural member; and components replacing what were previously thick-gauge multiple-reinforcement layer assemblies with a single-layer PHS2000 hot stamping. The primary limitation of PHS2000 versus PHS1500 is its slightly reduced elongation (4–6% versus 5–8% for PHS1500) which limits crash deformation capacity before fracture — component design for PHS2000 must ensure that the maximum deformation in the most severe design crash scenario does not locally exceed the fracture strain of the hot stamped component, requiring more careful crash simulation validation versus PHS1500 designs. Material cost of PHS2000 is approximately 20–35% higher than PHS1500 due to higher alloying cost (higher C, Mn, and potentially Cr/Mo) and more stringent process control requirements.

A tailor-rolled blank (TRB) is a cold-rolled AlSi-coated PHS coil or blank produced with continuously varying thickness across the coil/blank width or length — achieved by dynamically adjusting the roll gap of the cold rolling mill during rolling to produce a strip with defined thick zones, thin zones, and continuous transition ramps between them in a single integrated piece of steel. For press hardening steel, TRB technology is the principal manufacturing method for producing 'gradient property' hot stamped structural components — components where the ideal structural performance requires different thicknesses (and corresponding different section properties) at different positions along the component length or across its width, to match the local structural requirement (maximum bending moment, shear force, crash energy absorption zone) with the optimal material volume at each position. The B-pillar is the prototypical TRB application: the upper B-pillar zone (from the belt line to the roof rail) must resist side impact door intrusion with maximum stiffness — requiring maximum thickness for maximum section modulus and bending resistance; the lower B-pillar zone (from the sill to the belt line) must absorb crash energy by progressive folding — optimally requiring less thickness (to reduce crush resistance and allow controlled folding in the energy absorption zone) while still providing adequate structural integrity; and the end attachment flanges at the sill connection and roof rail connection require specific thicknesses for spot weld joint strength. A TRB B-pillar blank has thickness varying from (for example) 1.8mm at the upper zone through a continuous ramp to 1.2mm at the lower zone through another ramp back to 1.5mm at the lower attachment flange — replacing the conventional approach of three separate stampings (upper reinforcement, lower reinforcement, attachment flange) spot welded together, which requires three separate dies, three separate hot stamping operations, and three spot welded joint areas within the B-pillar assembly. The TRB approach produces the entire variable-thickness B-pillar structure in a single hot stamping from a single blank — eliminating the assembly joints, reducing the part count from 3 to 1, reducing assembly labor and tooling investment, and improving the structural integrity by eliminating the lap-shear joint between upper and lower reinforcements (which is a preferential fracture location in B-pillar crash loading). Weight savings from TRB technology versus constant-thickness equivalent assemblies are typically 10–20% for the individual component, contributing directly to vehicle weight reduction targets. Manufacturing TRB PHS coil requires specialised cold rolling mill control systems that can dynamically change the roll gap during rolling with high dimensional accuracy (thickness tolerance ±0.05mm across the transition ramp) while maintaining consistent AlSi coating weight and mechanical properties throughout the variable-thickness coil. The blanks cut from TRB coil are loaded into the hot stamping furnace with orientation markers to ensure correct positioning relative to the die geometry, as the thick and thin zones must align precisely with the die areas designed for those thicknesses.

Resistance spot welding of hot stamped PHS1500 components (in the hardened 1,500 MPa martensitic condition) presents significant challenges compared to welding mild steel or conventional high-strength steel, because the high carbon content (0.22% for 22MnB5) and fully martensitic microstructure of the hot stamped parts create extremely hard, brittle weld nuggets and heat-affected zones (HAZ) at conventional mild steel welding parameters — making specially developed and validated PHS-specific RSW schedules essential for achieving the minimum joint strength requirements of automotive body assembly quality standards. The fundamental metallurgical challenges for PHS RSW are: (1) Weld nugget brittleness — the weld nugget in a spot welded PHS assembly solidifies and cools through the martensitic transformation range during the welding cycle, forming untempered martensite of hardness HV 550–650 (even harder than the surrounding hot stamped base metal HV 450–520) that is extremely brittle and prone to cold cracking if any diffusible hydrogen is present from moisture contamination of the AlSi-Fe intermetallic coating surface or electrode surface. (2) HAZ softening — immediately adjacent to the weld nugget, the heat input of welding reheats the hot stamped martensite to temperatures sufficient to temper the martensite (Ac1 ≈ 700°C defines the upper softening boundary), producing a locally softened HAZ ring of HV 280–380 that has yield strength 40–55% of the surrounding hot stamped base metal — creating a preferential fracture path in coach-peel and cross-tension weld testing. RSW parameter requirements for PHS components: (a) Multi-pulse current schedule — PHS RSW typically requires 2–3 current pulses with intermediate cooling periods (impulse welding schedule) rather than a single trapezoidal pulse used for mild steel. The multi-pulse approach allows partial tempering of the weld nugget HAZ during the intermediate cooling periods and reduces peak weld nugget temperature — reducing HAZ brittleness and hydrogen cold cracking risk. (b) Post-weld temper pulse — many PHS RSW schedules include a final low-current temper pulse applied 1–3 seconds after the main welding pulse sequence, at a current level below the expulsion threshold that heats the weld HAZ to 200–350°C for partial in-situ tempering — significantly improving cross-tension strength and reducing hydrogen-induced cold cracking susceptibility. (c) Minimum nugget diameter — PHS RSW specifications typically require larger minimum nugget diameters than mild steel (5.5√t to 6√t for PHS1500 versus 4√t for mild steel) to ensure that weld shear failure occurs through the nugget centre (plug fracture mode, full-strength joint) rather than through the softened HAZ ring (interfacial fracture mode, reduced-strength joint). (d) Electrode type — dome-radius CuCrZr electrodes of RWMA Class 2 material with 6–8mm tip diameter provide the required current density for PHS nugget formation while the dome geometry provides self-normalising contact angle correction as the electrode tip contacts the curved hot stamped component surface. (e) Electrode force — PHS requires higher electrode force than mild steel (typically 3.5–5.0 kN for 1.5mm PHS) to achieve adequate contact pressure against the harder base metal and prevent expulsion at the higher currents required for PHS nugget formation. (f) AlSi coating welding compatibility — the Fe-Al intermetallic compound surface of hot stamped AlSi-coated PHS has significantly different electrical resistivity than the zinc coating on galvanized steel, requiring specific current calibration. The Fe-Al coating has higher electrical resistance than Zn coating, generating more interface heating at lower current levels — PHS RSW currents must be calibrated specifically for the AlSi coated condition versus zinc coated PHS where this becomes available.