Advances in high-strength steels for auto making

Over the past decade, advanced high-strength  steels  (AHSS) have become the fastest growing material  for  vehicle  use.  AHSS are stronger, lighter, and have low emissions, helping automakers decrease a vehicle’s life-long carbon footprint. In the Middle East, reports say that Iran’s automotive
industry is the second most active industry of the country, after its oil and gas industry, accounting for 10% of Iran’s GDP.

Several new commercialised and near-commercialised AHSS that exhibit high strength and enhanced formability are being offered around the world. These steels have the potential to affect cost and weight savings, while improving performance. The  increased  formability  allows  for greater part complexity, which leads to fewer individual parts (cost savings) and more manufacturing flexibility. Fewer parts mean less welding (cost and cycle-time savings) and weld flanges (mass and weight savings). Depending on design, the higher strength can translate into better fatigue and crash performance, while maintaining or even reducing thickness.

Conventional high-strength steels

It is generally accepted that the transition from mild steel to HSS occurs at yield strength of about 210 megapascals (MPa) [30 kilopounds per square inch (KSI)]. For yield strength levels below 280 to 350 MPa (40 to 50 KSI), a simple carbon-manganese (CMn) steel typically is used. The composition of these steels is similar to low carbon mild steels, except they have more carbon and manganese to increase the strength to the desired level. This approach usually is not practical for yield strengths greater than 350 MPa (50 KSI) because of a drop-off in elongation and weldability.

One approach to achieving yield strengths between 280 and 550 MPa (40 and 80 KSI) is to use high-strength, low-alloy (HSLA) steels, also known as microalloyed (MA) steels. This family of steels  usually has a microstructure of  fine-grained ferrite that has been strengthened with carbon and/or nitrogen precipitates of titanium, vanadium, or niobium (columbium). Adding manganese, phosphorus, or silicon further increases the strength. These steels can be formed successfully  when  users  know  the limitations of the higher-strength, lower-formability trade-off.

Another approach to achieving these yield strength levels is to use AHSS grades. Dual-phase (DP), transformation-induced plasticity (TRIP), high-hole expansion (HHE),  complex-phase  (CP),  and martensitic steels are some of the grades collectively referred to as AHSS.

Dual-phase (DP) steels

DP steels have a microstructure of mainly soft ferrite, with islands of hard martensite dispersed  throughout.  The strength level of these grades is related to the amount of martensite in the microstructure.

As the product arrives from the steel  mill,  its yield strength typically is much lower than its tensile strength, with a YS-to-TS ratio of  about  0.6. (For comparison,  the YS-to-TS ratio for HSLA steels is closer to 0.75.) The lower yield strength at a given tensile strength translates to higher elongation values and better formability.

In addition,  the work-hardening response to deformation is different between DP and HSLA steels.  HSLA steels begin to lose formability as soon as deformation starts. As a result of the soft ferrite matrix of DP steels, they can maintain their formability furthAir Jordan 30.5 Shoes