Advanced High-Strength Steels (AHSS) A baseline understanding of their unique mechanical properties

Advanced High Strength Steel (AHSS) Microstructure, Mechanical Bahaviour, and Alloy Design

The fundamental metallurgy of conventional low- and high-strength steels is generally well understood by manufacturers and users of steel products. Since the metallurgy and processing of AHSS grades is, however, somewhat novel compared to conventional steels, they will be
described briefly to provide a baseline understanding of how their unique mechanical properties evolve from their unique processing and structure.

Dual Phase (DP) Steels

The microstructure of dual phase (DP) steels is comprised of soft ferrite and, depending on strength, between 20 and 70% volume fraction of hard phases, normally martensite*.

Figure 1 displays the micro-structure of a DP ferrite + martensite steel with 350 MPa yield strength and 600 MPa. The soft ferrite phase is generally continuous, giving these steels excellent ductility. When these steels deform, however, strain is concentrated in the lower strength ferrite phase, creating the unique high work hardening rate exhibited by these steels.

The work hardening rate along with excellent elongation combine to give DP steels much higher ultimate tensile strength than conventional steels of similar yield strength. Figure 2 illustrates this, where the quasi-static stress-strain behavior of high-strength, low alloy (HSLA) steel is compared with that of a DP steel of similar yield strength. The DP steel exhibits higher initial work hardening rate, uniform and total elongation, ultimate tensile strength, and lower YS/TS ratio than the similar yield strength HSLA. DP and other AHSS also have another important benefit compared with conventional steels. The bake hardening effect, which is the increase in yield strength resulting from prestraining (representing the work hardening due to stamping or other manufacturing process) and elevated temperature aging (representing the curing temperature of paint bake ovens) continues to increase with increasing strain. Conventional bake hardening effects, of BH steels for example, remain somewhat constant after prestrains of about 2%. The extent of the bake hardening effect in AHSS depends on the specific chemistry and thermal histories of the steels. DP steels are designed to provide ultimate tensile strengths of up to 1000 MPa.

In DP steels, carbon enables the formation of martensite at practical cooling rates. That is, it increases the hardenability of the steel. Manganese, chromium, molybdenum, vanadium and nickel added individually or in combination also increase hardenability. Carbon also strengthens the martensite as a ferrite solute strengthener, as do silicon and phosphorus. Silicon also strengthens the martensite since it helps to partition carbon to the austenite to increase its hardenability and the strength of the resultant martensite phase. These additions are carefully balanced, not only to produce unique mechanical properties, but also to minimize any difficulties with resistance spot welding, which is, in general good. However, when welding the highest strength grade (DP 700/1000) to itself, the spot weldability may require welding practice adjustments.

Transformation Induced Plasticity (TRIP) Steels

The microstructure of TRIP steels consists of a continuous ferrite matrix containing a dispersion of hard second phases--martensite and/or bainite. These steels also contain retained austenite in volume fractions greater than 5%. A typical TRIP steel microstructure is shown in  Figure 3.