High Strength Steels, also called as High Carbon Steels, have higher carbon content than mild steels. Thus, these steels are less ductile. Unlike mild steel, there is no proper yield point in high strength steels. Also, the necking of the steel does not occur as it is more brittle and less ductile. The typical stress strain curve for high carbon steel is shown in Figure 1 below.

For a mild steel, if the load is released below elastic limit, the material will return to its original shape and size. Beyond the elastic limit, the member experiences increase in strain with constant stress which causes a permanent deformation of the material. This permanent deformation is called a residual strain. The slope of the line after unloading will be slightly off the zero stress. The slope of that line will equal to the modulus of the elasticity, E of the material. 

In a high carbon steel, there will not be a well-defined yield point so it is arbitrarily defined using a method called 0.2% offset method. In this method, a line parallel to the initial linear stress-strain curve is constructed, offset by 0.2% (i.e., 0.002 units) from the zero stress/strain. The 0.2% Offset Yield Strength is the stress at which the parallel line/arbitrary line intersects the stress-strain curve (See Figure 1). In some cases, 0.01, 0.1 and 0.5% offset yield strength are also used.

References:

• ASTM A370 Standard Test Methods and Definitions for Mechanical Testing of Steel Products 

• Geschwindner, L. F. (2011). Unified design of steel structures. John Wiley & Sons.