What is high grade-tensile steel?

Classification of high grade-tensile steel for Automotive: Automotive steels can be classified in several different ways. One is a metallurgical designation. Common designations include low-strength steels (interstitial-free and mild steels); conventional high-tensile or high-strength steels, otherwise called HSS (carbonmanganese, bake hardenable, high-strength interstitial-free, and high-strength, low-alloy steels); and the newer types of ultra-high tensile or Advanced High-Strength Steels, called AHSS (dual phase, transformation-induced plasticity, complex phase, and martensitic steels). Additional higher strength steels for the automotive market include ferritic-bainitic, twinning-induced plasticity, hot-formed, and post-forming heat-treated steels.

A second classification method important to part designers is strength of the steel. In this answer, the general terms HSS and AHSS is used to designate all higher strength steels. In contrast, much of the current literature in the world today uses narrowly defined ranges to categorize different steel strength levels. One such system defines High-Strength Steels (HSS) as yield strengths from 210 to 550 MPa and tensile strengths from 270--700 MPa, while Ultra-High-Strength Steels (UHSS) steels have yield strengths greater than 550 MPa and tensile strengths greater than 700 MPa. These arbitrary ranges suggest discontinuous changes in formability when moving from one category to another. However, data show property changes are a continuum across the entire span of steel strengths. In addition, many steel types have a wide range of grades covering two or more strength ranges.

A third classification method presents various mechanical properties or forming parameters of different steels, such as total elongation, work hardening exponent n, or hole expansion ratio .

The principal difference between conventional HSS and AHSS is their microstructure. Conventional HSS are single phase ferritic steels. AHSS are primarily multi-phase steels, which contain ferrite, martensite, bainite, and/or retained austenite in quantities sufficient to produce unique mechanical properties. Some types of AHSS have a higher strain hardening capacity resulting in a strength-ductility balance superior to conventional steels. Other types have ultra-high yield and tensile strengths and show a bake hardening behaviour.