Introduction

Introduction

With escalating concerns about human-induced greenhouse gases, global legislators have passed more stringent vehicle emissions regulations through 2020, while considering further, aggressive targets for the next ten years. Automakers are searching for new materials and engineering capabilities to meet requirements that often conflict. As an example, structural applications require materials characterized by high strength and stiffness, often achieved with greater thickness.  But fuel economy and emissions are positively impacted when component thickness is reduced.  New vehicle designs with complex geometries are aesthetically pleasing, but difficult to form and join, compromised further by thickness reduction to achieve mass reduction targets. The global steel industry continues to develop new grades of steel, defined by ever-increasing strength and formability capabilities, continually reinventing this diverse material to address these opposing demands. These Advanced High-Strength Steels (AHSS) are characterized by unique microstructures and metallurgical properties that allow OEM’s to meet the diverse functional requirements of today’s vehicles.

AHSS are complex, sophisticated materials, with carefully selected chemical compositions and multiphase microstructures resulting from precisely controlled heating and cooling processes. Various strengthening mechanisms are employed to achieve a range of strength, ductility, toughness, and fatigue properties.

The AHSS family includes 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). These 1st and 2nd Generation AHSS grades are uniquely qualified to meet the functional performance demands of certain parts.  For example, DP and TRIP steels are excellent in the crash zones of the car for their high energy absorption. For structural elements of the passenger compartment, extremely high-strength steels, such as Martensitic and boron-based Press Hardened Steels (PHS) result in improved safety performance. Recently there has been increased funding and research for the development of the “3rd Generation” of AHSS. These are steels with special alloying and thermo-mechanical processing to achieve improved strength-ductility combinations compared to present grades, with potential for more efficient joining capabilities, at lower costs. The broad range of properties is best illustrated by the famous Steel Strength Ductility Diagram, captured in the figure.

steel strength and ductility diagram

Steel Strength Ductility Diagram

AHSS grades contain significant alloying and two or more phases. The multiple phases provide increased strength and ductility not attainable with single phase steels, such as high strength, low alloy (HSLA) grades. HSLA materials achieve their strength through alloying and solid solution hardening, whereas AHSS are produced by using specific alloys and precise thermomechanical processing.

In the past, steels with tensile strength (UTS) levels in excess of 550 MPa were generally categorized as AHSS, and the name “ultra high-strength steels” was reserved for tensile strengths exceeding 780 MPa. However, today there are multiple phase AHSS with tensile strengths as low as 440 MPa, and so using strength as the threshold for whether a steel qualifies as “AHSS” is no longer suitable. AHSS with tensile strengths of at least 1000 MPa are often called “GigaPascal steel” (1000 MPa = 1GPa).  Third Generation AHSS seeks to offer comparable or improved capabilities at significantly lower cost.