R&D Update: Forming high-strength steels, Properties and applications, Part I

Because of higher saftey standards, the automotive industry needs lightweight, high-strength, and high-energy-absorbent parts. These requirements led to the development of high-strength steels (HSSs), advanced high-strength steels (AHSSs), and ultrahigh-strength steels (UHSSs).

According to the 2001 UltraLight Steel Auto Body (ULSAB)— Advanced Vehicle Concepts (AVC) Consortium, high-strength steels are classified as:

1.Dent-resistant

• Bake-hardenable
• Non-bake-hardenable

2. High-strength solution-strengthened

3. High-strength, low-alloy (HSLA)

4. High-strength, recovery annealed

5. AHSS

• Low-strength steels
• UHSS
• Complete-phase (CP) alloys
• Martensitic steels
• Extra-formability high-strength steels
• Dual-phase (DP) alloys
• Transformation-induced-plasticity (TRIP) steels
• Postform-strengthening steels AHSS is designated by its steel grade, such as DP or CP, followed by its yield-to-tensile-strength ratio (YS/TS). For example, DP 500/800 denotes a DP alloy with a yield strength of 500 MPa and an ultimate tensile strength of 800 MPa. Figure 1 compares HSSs recommended for use in body structures by the ULSAB-AVC Consortium.

HSS’s high strength is obtained by changing its chemical composition; alloying; or special thermomechanical processing techniques such as cold rolling, temperature control during hot rolling, and time/temperature control in the annealing process. HSS grades, however, exhibit lower formability (see Figure 2 ) than conventional steel grades, so special care must be taken when designing parts and tooling for them.

AHSS Characteristics

The most common HSS grades used in automotive bodies are AHSSs, specifically, DP and TRIP. These AHSS grades exhibit high yield strengths—higher than 690 MPa/100 KSI—and low elongation (see Figure 3 ).

AHSSs can be further classified based on formability:

• Low-strength steel

• UHSS

• Extra-formability high-strength steel

• Postform-strengthening steel

Extra-formability High-strength Steels. These steel grades have higher formability than standard HSSs. The commonly used steels of this grade include DP alloys and TRIP steels.

DP Steels. These grades have a high initial work-hardening coefficient (n value), which distributes plastic strain and improves uniform elongation (see Figure 3). This work-hardening coefficient produces a higher ultimate tensile strength than conventional high-strength steels have with similar initial yield strengths.

DP steels also exhibit a higher uniform and total elongation and a lower yield-to-tensile-strength ratio when compared HSSs. These characteristics improve formability and structural performance in automotive components. DP steels are available in two types: a high-yield-ratio product designated by the grade DH and a low-yield-ratio product designated by the grade DL.

TRIP Steels . Unlike DP alloy steels, the initial n value in TRIP steels is lower and it increases and maintains itself in higher strain ranges (see Figure 3). This gives the material an advantage in server stretch applications. TRIP steels exhibit excellent deep drawability, and automotive components that cannot be manufactured from DP steels often are made from TRIP steels.

Postform-strengthening Steels. These HSSs require strengthening operations after forming, such as induction heating, hot stamping, or by diffusion strengthening, such as carburizing or nitriding by salt bath or gaseous treatments.

Structural Performance

AHHSs’ formability compared to conventional HSSs’ with similar yield strengths offer more flexibility to optimize part geometry, reduce weight, and improve strength. Among the other component performance criteria that affect vehicle performance are stiffness, strength, durability, and crash energy management.

A component’s stiffness is controlled by the material’s modulus of elasticity (E) and component geometry (including thickness). The enhanced formability of AHSS offers improved design flexibility, which results in more component stiffness without increasing mass or sacrificing strength.

Component strength is a function of its geometry and yield or tensile strength. A combination of work-hardening and bake-hardening enhances the strength of AHSS components.

Structural component fatigue involves complicated relationships among several factors, such as geometry, thickness, applied loads,, and material endurance limit. The combination of work— and bake-hardening significantly increases the as-manufactured strength of AHSS components, which results in better fatigue performance.

Steel exhibits high sensitivity to strain under dynamic loading conditions such as a crash. Higher energy absorption is attributed to a high work-hardening rate and flow stress that distribute strain more evenly during a crash. Work-hardening and bake-hardening improve energy absorption because the formed and baked component has a higher flow stress than the as-rolled material from which the component was manufactured.