Types of High Strength Steels in Cars

Advanced high strength steel and ultra high strength steel are becoming a popular material in the world of auto manufacturing because their attributes and characteristics help automakers meet fuel efficiency, safety, and design standards. With increasingly stringent regulations driving the future of the industry, steel – particularly ultra-high strength steel and advanced high strength steel – plays an important role in meeting the demands for strength and safety. Read on to learn more about the benefits and potential of this exciting material!

Why Steel in the First Place?

According to worldsteel.org, steel is an alloy of iron and carbon, and also contains small amounts of manganese, silicon, phosphorus, sulfur, and oxygen. There are several benefits to using steel in automotive production. It is:

• Environmentally friendly, as it can be easily recycled multiple times and produces lower CO2 life cycle emissions than other materials
• Design friendly, offering flexibility and the ability to engineer crash-resistant structures
• Easy and affordable to repair and cost efficient compared with other materials
• A key part of constructing lightweight vehicles that are strong, safe, and fuel-efficient

Steel usually shows up in automobiles in the “bones” or chassis of the autobody, sometimes referred to as the “body in white.” Body in white is the stage when the car’s unpainted frame and foundation come together before various finishing components are added.

How is Advanced High Strength Steel Defined?

Advanced High Strength Steel (AHSS) is the result of carefully constructed chemical compositions and precisely controlled heating and cooling processes. 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). Among this family, differed strengthening techniques create a range of strength, ductility, toughness, and fatigue characteristics. AHSS has a yield strength of 550 MPa or greater and a minimum tensile strength of 500 to 800 MPa.

How is Ultra-High Strength Steel Defined?

Ultra-High Strength Steel (UHSS) is specially formulated steel with a tensile strength above 780 MPa. AHSS with a tensile strength of at least 1,000 MPa is often called “GigaPascal steel” (1000 MPa = 1GPa).

How Do UHSS and AHSS Compare?

Steels with yield strengths higher than 550 MPa are considered AHSS, and when the tensile levels exceed 780 MPa, they are referred to as UHSS. The key difference between UHSS and AHSS is in their microstructures. The microstructures of AHSS can be engineered to produce unique mechanical properties; some have a higher strain hardening capacity, which gives them a superior strength/ductility balance, while others have higher yield and tensile strengths and demonstrate bake-hardening behavior.

How Do UHSS and AHSS Benefit the Automotive Industry?

According to the American Iron and Steel Institute, AHSS help automakers reduce the mass of vehicles, which increases fuel economy and reduces tailpipe emissions. Current AHSS grades can reduce a vehicle’s structural weight by as much as 25% and cut total life cycle CO2 emissions by up to 15% than other automotive materials.

Using AHSS for automotive ‘light-weighting’ results in an immediate and sustained decrease in greenhouse gas emissions. The goal of light-weighting is to reduce overall vehicle weight to improve fuel economy; typically, a 10% reduction in vehicle weight can lead to an 8% improvement in fuel economy.

New grades of AHSS enable carmakers to reduce vehicle weight by 35-40% compared to conventional steel. For a five-passenger family car, AHSS can reduce the overall vehicle weight by almost 600 pounds, which corresponds to four tons of greenhouse gases saved over the vehicle’s life cycle.

Lastly, UHSS and AHSS are completely recyclable. Each year, the steel industry recycles more than 14 million tons of steel from junk vehicles, the equivalent to the amount of steel required for nearly 13 million new automobiles. In other words, the industry is self-sustaining. Steel also generates relatively low energy and emissions during the manufacturing phase; in fact, steel manufacturing is the lowest emissions producer of all automotive structural materials.

Safety of UHSS and AHSS

According to World Auto Steel, “steel is the easiest and best material for designing safe vehicles.” It has a unique ability to absorb an impact and diffuse crash energy. In fact, it can become harder when it’s crushed, meaning it becomes stronger on impact and can absorb more energy. For this reason, most cars today have a steel front end that can collapse like an accordion upon impact.

AHSS delivers high strength with good ductility especially for a car’s structural elements. It has become the material of choice for passenger safety cage components like sill reinforcements, A-pillars, B-pillars, side impact beams, waistline reinforcements, bumpers, and roof bows, seats. Some AHS steels are used to provide high energy absorption in frontal and rear crumple zones, while others are used to resist side and roof impacts.

Outstanding corrosion protection can be achieved with special metallic coatings, which also increases safety.

Steel has emerged as a winner in the automotive industry because of its unique characteristics. In particular, AHSS and UHSS are able to deliver strong, yet lightweight results which enable products that can meet both environmental standards and safety requirements.