Materials & Steel Grades

Modern BIWs use a carefully engineered mix of materials—each chosen for specific performance requirements. This lesson covers the steel grades that make up 60-80% of most vehicles.

Steel Classification Overview

Automotive steels are classified by yield strength:

Mild Steel

Where Mild Steel is Used

• Outer body panels — doors, hood, fenders, roof
• Non-structural brackets
• Areas requiring deep draws

Mild steel is easy to stamp into complex shapes but provides minimal crash protection.

High-Strength Steel (HSS)

HSS Types

Advanced High-Strength Steel (AHSS)

AHSS Types (1st Generation)

AHSS Types (2nd & 3rd Generation)

Ultra-High-Strength Steel (UHSS)

Hot Stamping (Press Hardening)

Most UHSS parts are made via hot stamping:

Hot Stamping Process:
┌─────────────────────────────────────────────────┐
│                                                 │
│  1. Heat blank to 900°C+ (austenite phase)     │
│            ↓                                    │
│  2. Transfer to cooled die (< 10 sec)          │
│            ↓                                    │
│  3. Form and quench simultaneously             │
│            ↓                                    │
│  4. Part hardens to martensite (1500+ MPa)     │
│                                                 │
└─────────────────────────────────────────────────┘

Hot-Stamped Components

Boron Steel (22MnB5)

The most common hot-stamping steel is 22MnB5:

• 22 — 0.22% carbon
• Mn — manganese alloyed
• B5 — boron microalloyed
• Pre-heat strength: ~600 MPa
• Post-quench strength: 1500 MPa

Tailor-Welded Blanks (TWB)

Tailor-Welded B-Pillar Blank:
┌──────────┬──────────┬──────────┐
│ 1.0mm    │ 1.8mm    │ 1.2mm    │
│ PHS1500  │ PHS1500  │ PHS1000  │
│ (roof    │ (impact  │ (lower   │
│  area)   │  zone)   │  attach) │
└──────────┴──────────┴──────────┘
        Laser welds

Benefits:

• Optimized strength where needed
• Weight savings in low-stress areas
• Reduced part count — one stamping vs. multiple

Tailor-Rolled Blanks (TRB)

Tailor-Rolled Blank:
┌─────────────────────────────────┐
│ 1.0mm → 1.5mm → 2.0mm → 1.5mm  │
│         Continuous roll         │
└─────────────────────────────────┘

Advantages over TWB:

• No weld seams (potential weak points)
• Smoother thickness transitions
• Better fatigue performance

Aluminum in BIW

While this course focuses on steel, aluminum is increasingly used:

Aluminum vs. Steel

Material Selection by Zone

Modern BIWs use a zone-based approach:

Key Takeaways

• Steel grades range from mild (< 210 MPa) to UHSS (> 1000 MPa)
• AHSS (DP, TRIP, CP) balances strength and formability
• UHSS requires hot stamping for forming
• 22MnB5 boron steel reaches 1500 MPa after quenching
• Tailor-welded/rolled blanks optimize thickness distribution
• Zone-based approach: strongest materials in safety cage, energy-absorbing materials in crush zones

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