Advanced High Strength Steels for Automotive Application

Advanced High Strength Steels for Automotive Application

The main difference between conventional high strength steel (HSS) and advanced high strength steel (AHSS) is in their microstructures. Conventional HSS are single-phase ferritic steels with a potential for some pearlite in carbon- manganese (C-Mn steels. AHSS are mainly steels with a microstructure containing a phase other than ferrite, pearlite, or cementite, as an example, martensite, bainite, austenite, 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.

AHSS is complex, sophisticated material. These steels are with carefully selected chemical compositions and multiphase microstructures resulting from precisely controlled heating and cooling processes. Different strengthening mechanisms are used for achieving a range of strength, ductility, toughness, and fatigue properties.  These steels are different from the mild steels. Rather, these steels are distinctively lightweight and engineered to meet the challenges of the present day automobiles for stringent safety regulations, emissions reduction, solid performance, at affordable costs.

Several key considerations drive material selection for automotive applications, including safety, fuel efficiency, environmentalism, manufacturability, durability, and quality. These factors manifest themselves differently in each component of the automobile, and materials are selected to match each set of performance requirements in the most efficient means possible. In the highly competitive automotive industry, cost is also extremely important in material selection. As the motivation to reduce the mass of automobiles continues to intensify, automobile producers seek to maximize the efficiency of their materials selection.

Conventional mild steel has a relatively simple ferritic microstructure. It typically has low C content and minimal alloying elements, is readily formed, and is especially sought for its ductility. Widely produced and used, mild steel frequently serves as a baseline for comparison of other materials. Conventional low-strength steels to high-strength steels include IF (interstitial free), BH (bake hardened), and HSLA (high-strength low-alloy) steels. These steels normally have yield strength (YS) of less than 550 MPa and ductility which decreases with increased strength.

AHSS are more complex, particularly through their microstructures, which are normally multi-phase for an improved combination of strength and ductility. This balance is carefully constructed to meet performance requirements while maintaining excellent formability. AHSS frequently has other advantageous mechanical properties, such as high strain-hardening capacity.

The terms ‘high-strength’ and ‘advanced high-strength’ refer generally to steels which share a family of behaviours. Normally, AHSS are different from the early HSS (IF, BH, HSLA steels) since they have been developed later for increased strength and ductility for enhanced formability. In reality, the steels fall into a continuum of strengths. So, the distinction between HSS and AHSS is defined somewhat arbitrarily. A typical definition calls steels with YS of 210 MPa to 550 MPa ‘high-strength’ and anything stronger ‘advanced high-strength’.  AHSS are also sometimes called ‘ultra high-strength steels’ (UHSS) for tensile strengths (TS) which are exceeding 780 MPa. AHSS with TS of at least 1,000 MPa are frequently called ‘Giga Pascal steel’ (1000 MPa = 1GPa).

The family of AHSS 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) steels. These first and second generation AHSS grades are distinctively qualified to meet the functional performance demands of certain parts.  For example, DP and TRIP steels are excellent in the crash zones of the car for their high energy absorption. For structural elements of the passenger compartment, extremely high-strength steels, such as Martensitic and boron (B) based ‘Press hardened steels’ (PHS) result in improved safety performance. Recently there has been increased funding and research for the development of the ‘third generation’ of AHSS. These are steels with improved strength-ductility combinations compared to present grades, with potential for more efficient joining capabilities, at lower costs. These grades reflect distinct alloys and microstructures to achieve the desired properties.

Automotive steels can be classified in many different ways. One is a metallurgical designation providing some process information. Common designations include (i) low-strength steels (IF and mild steels), (ii) conventional high strength steels (C-Mn, BH, and high-strength low-alloy steels), and (iii) the new AHSS (DP, TRIP, TWIP, FB, CP and MS steels). Additional higher strength steels for the automotive application include HF, post-forming heat-treated (PFHT) steels, and steels designed for distinct applications which include improved edge stretch and stretch bending.

A second classification method important to the automotive part designers is the strength of the steel. Hence, the general terms HSS and AHSS are used to designate all higher strength steels. This classification system has a problem with the on-going development of the many new grades for each type of steel. Hence, a DP steel or TRIP steel can have strength grades which encompass 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, or hole expansion ratio. As an example, Fig 1 compares total elongations – a steel property related to formability – to the tensile strength for the current types of steel. These properties are important for press shop operations and virtual forming analyses.

Steels with YS levels in excess of 550 MPa are normally referred to as AHSS. These steels are also sometimes called UHSS for tensile strengths exceeding 780 MPa. There is another category of steels known as Austenitic stainless steel.  These materials have excellent strength combined with excellent ductility, and thus meet many functional requirements of the automobiles.

Third generation AHSS seeks to offer comparable or improved capabilities at a significantly lower cost.  However, since they are so new, there is yet no clear definition of what they comprise. The main objectives in developing third generation steels are (i) to use steels with lower alloying content to reduce costs, and (ii) to target minimum strength and ductility levels of 1,200 MPa and 30 % elongation. At present, there is no pure formula, as there are several different processing routes available to the steel producers to achieve third generation grades and properties. Also, ‘Nano steels’ are being categorized as third generation. The third generation steels are under discussion in the industry. The steel strength ductility diagram for present day AHSS is at Fig 1.

Fig 1 Steel strength ductility diagram for present day AHSS

Metallurgy of AHSS

Producers and users of steel products know normally the fundamental metallurgy of conventional low- strength steel and high-strength steels. However, the metallurgy and processing of AHSS grades are somewhat different than the conventional steels. Hence, a baseline understanding is necessary to know how the remarkable mechanical properties of AHSS evolve from their unique processing and structure. All AHSS is produced by controlling the chemistry and cooling rate from the austenite or austenite plus ferrite phase, either on the run-out table of the hot rolling mill (for hot-rolled products) or in the cooling section of the continuous annealing furnace (continuously annealed or hot-dip coated products). Research has provided chemical and processing combinations which have created many additional grades and improved properties within each type of AHSS.

Important considerations during the materials selection process include (i) YS, (ii) ultimate tensile strength (UTS), (iii) ductility (sustaining plastic deformation before fracture), (iv) toughness (absorbing energy before fracture, indicated by the total area under the tensile stress-strain curve), and (v) hardness (resisting deformation on the surface). Steel is a versatile material since these properties can vary in a wide range. The chemistry and microstructure which determine these properties can be tailored to meet the broad range of requirements of the automotive industry.

Several methods are used to achieve the desired properties from steel. Strengthening and hardening mechanisms are frequently used in various combinations to meet specific requirements, such as fatigue strength or dent resistance. Strengthening mechanisms typically work by hindering or impeding the movement of dislocations through the steel. Typical strengthening mechanisms are described below.

Solid solution strengthening – When another species is added to form a solid solution, the interstitial or substitutional atoms form localized strain fields which can increase the strength and hardenability, although they can decrease ductility simultaneously .

Grain refinement – As dislocations travel through a material, they tend to pile up at grain boundaries, preventing further plastic deformation. As grain size decreases, the effective area of grain boundaries increases, increasing the strength of the material.

Work hardening or strain hardening – As a result of cold working (rolling, drawing, and bending etc.), dislocations in steel become more entangled, preventing their relative movement. Work hardening typically increases YS, UTS, and hardness, but frequently has an adverse effect on ductility and toughness.

Dispersion strengthening or precipitation hardening – The steel matrix, normally ferritic or austenitic, frequently contains other phases, which can range from fine particles (e.g. cementite particles, islands tempered martensite, or discreet carbide or nitride alloy precipitates) to lamellar sheets (e.g. the ferrite and cementite layers of pearlite). These micro-structural features can affect the overall properties of the material considerably and show some of the many ways to increase strength.

Transformation strengthening – In the production processing of steel, phase transformations can frequently occur which enable strengthening by creating microstructures with significant amounts of hard phases, such as martensite or bainite. Such transformations occur in operations like hot rolling, hot-dip galvanizing, or continuous annealing where steel can cool from high-temperature austenite and transform to these harder low-temperature phases. This mechanism is fundamental to the development of AHSS and enables DP steel, TRIP steel, and other AHSS to be manufactured.

As AHSS are developed for combinations of characteristics ideal for the final part, the feasibility of production is supreme to actual application and implementation in automobiles. Concerns about formability and weldability have prompted much research and development in the area of processing of steel. In some cases, traditional process methods are just as effective with AHSS as with mild steels, while in others, some modifications to equipment or methods are necessary. In some others, new processing technologies have even enabled the development of new steel grades.

Transformation strengthening is the main strengthening mechanism used in producing AHSS in steel plant processes. Heating cycles are especially important in the production of these grades. Temperature and cooling rates are to be carefully controlled within tight windows to develop the desired microstructures, as per the time-temperature-transformation (TTT) diagram. Automotive steel is typically produced as large coils, which can then be processed into blanks or tubular products.

Production processes continue to stand out as a vital factor in the development of new materials. Much of the current research is focused on identifying new processes and technologies to improve the consistency, reduce cycle time and cost, and enable the production of parts using AHSS. In some cases, the processing of the part can be instrumental in developing the final strength in AHSS applications. The most notable example of this is with B-treated hot stamped parts. B is alloyed with these steels to provide sufficient hardenability so that, on quenching hot-stamped parts in water-cooled dies, the austenite-to-martensite transformation can occur.

Steel types

There are many types of steel, which include both conventional and AHSS. Mild steel, low- strength to high-strength steels have been widely used in automobiles for many years. These conventional steels are (i) mild steel, (ii) IF steels, (iii) BH steels, and (iv) HSLA (high-strength low-alloy) steels.

Mild steel – Conventional mild steel has a relatively simple ferritic microstructure. These steels with low carbon content and minimal alloying elements are soft and formable. Widely produced and used, mild steels are inexpensive and frequently serve as a baseline for comparison for other materials. Mild steels have relatively low strength, but excellent formability. Mild steels have long been used for many applications in automobiles, including the body structure, closures, and other ancillary parts.

IF steel – IF steel has ultra-low C content, achieved by removing carbon monoxide (CO), hydrogen (H2), nitrogen (N2), and other gasses during steelmaking through a vacuum degassing process. Interstitial elements like N2 or C also form nitrides and carbides with alloying elements such as niobium (Nb) or titanium (Ti) to stabilize the residual interstitials. Hence, IF steel is typically a non-aging steel. The lack of interstitial atoms in the atomic structure enables IF steel to have extremely high ductility, ideal for deep-drawn products. In fact, IF steels are sometimes called extra-deep drawing steels (EDDS). These steels have relatively low strength but high work hardening rates and excellent formability. Sometimes IF steel is strengthened by the reintroduction of N2 or other elements. Applications for IF steel include elements of the body structure and closures.

BH steel – Although these steels have a simple ferritic microstructure, solid solution strengthening gives BH steels a boost in strength. These steels have a more complicated chemistry than mild steels or IF steels. Special techniques are used to keep C in solution through processing until it is released during paint baking. The bake hardening procedure increases the YS of BH steels, while maintaining excellent formability. BH steels have high dent resistance and are frequently selected for closure panels like door outers, hoods, and deck lids.

HSLA steel – HSLA steels were among the first widely-used HSS in automotive applications. Finely dispersed alloy carbides and ferrite-pearlite aggregates sit in a ferrite matrix, with minimal alloying content. This complex structure combined with grain size refinement for increased strength give HSLA its name. HSLA is normally designed to meet mechanical specifications and is typically tough, corrosion resistant, formable, and weldable. Many automotive ancillary parts, body structure, suspension and chassis components, as well as wheels, are made of HSLA steel.

AHSS are newer steels which have enhanced strength and formability achieved through the development of more complex microstructures through controlled cooling processes. The first generation of these AHSS is ferrite based. This steels include (i) DP steel, (ii) FB steel, including stretch-flangeable (SF) steel, (iii) CP steel, (iv) MS steel, (v) TRIP steel, and (vi) HF steel. The second generation AHSS are more austenite-based, and include TWIP steels

DP steel – The microstructure of DP steel consists of a soft ferrite matrix and discreet hard martensitic islands I(Fig 2). The ferrite is continuous for many grades upto DP 780, but as volume fractions of martensite exceed 50 % (as can be found in DP 980 or higher strengths), the ferrite can become discontinuous. The combination of hard and soft phases results in an excellent strength-ductility balance, with strength increasing with increasing amount of martensite.

DP steels can be hot- formed or cold-formed and also have high bake hardening behaviour. If hot-rolled, cooling is carefully controlled to produce the ferritic-martensitic structure from austenite. If continuously annealed or hot-dipped, the final structure is produced from a dual phase ferritic-austenitic structure which is rapidly cooled to transform some of the austenite to martensite. The soft ferrite in the DP steel is exceptionally ductile and absorbs strain around the martensitic islands, enabling uniform elongation with high work hardening rate and fatigue strength. Additionally, DP steels can absorb a lot of strain energy. Unlike conventional steels (even the traditional BH steel), bake hardening does not decrease with increasing pre-strain for DP steels.

DP steel is presently one of the most widely used AHSS. Automobile producers are increasingly using DP steel to increase strength and down gauge HSLA structural components. Important to consider when designing with DP steel, as with other AHSS, is the effect of strain and bake hardening. DP steels can be developed with low to high YS to UTS ratios, allowing for a broad range of applications from crumple zone to body structure. DP steel is sometimes selected for visible body parts and closures, such as doors, hoods, front and rear rails. Other popular applications include beams and cross members, rocker, sill, and pillar reinforcements, cowl inner and outer, crush cans, shock towers, fasteners, and wheels.

Fig 2 Microstructures of DP and FB steels

FB steel – FB steel is also DP steel, with soft ferrite and hard bainite. The microstructure, shown in Fig 2, is finer than the typical DP steel. However, it can be even more finely tuned to be SF steel. This characteristic can be measured by the hole expansion test and gives FB / SF steels the ability to resist stretching on blanked edge. The second hard phase bainite and grain refinement make FB steel a strong material with excellent formability.

FB steel performs well under dynamic loading conditions, making it well suited to carry vibration loads. The stretchability of FB steel (or SF steel) at sheared edges makes it an excellent choice for tailored blank applications. Frequently cold-drawn FB steel is used for profiles, mechanical parts, cross beams and reinforcements, and wheels. SF steels are also recommended for suspension and chassis. Since FB steel has good fatigue properties in dynamic load conditions, it is an outstanding choice for shock towers and control arms.

CP steel – CP steels have a mixed microstructure with a ferrite / bainite matrix containing bits of martensite, retained austenite, and pearlite (Fig 3). Grain refinement is necessary for achieving the desired properties from CP steel. Delayed recrystallization is frequently used to develop very small grains for a very fine microstructure. Micro-alloying elements such as Ti or Nb can also be precipitated. The fine, complex microstructure gives CP steel high YS and high elongation at TS similar to DP steels. CP steel can have good edge stretchability. Additionally, CP steels have good wear properties and fatigue strength. They can be bake hardened.

CP steel has several automotive applications, particularly in body structure, suspension, and chassis components. The high YS and elongation enables high energy absorption, also make it a good choice for crash safety components, such as fender beams, door impact beams, and reinforcements for B-pillar etc.

Fig 3 Microstructures of CP steel and MS steel

MS steel – In MS steels, nearly all austenite is converted to martensite. The resulting martensitic matrix contains a small amount of very fine ferrite and / or bainite phases. This structure typically forms during a swift quench following hot-rolling, annealing, or a post-forming heat treatment. Increasing the C content increases strength and hardness. The resulting structure is mostly lath (plate-like) martensite (Fig 3). Careful combinations of Mn, B, Si (silicon), Cr (chromium), Ni (nickel), Mo (molybdenum), and / or V (vanadium) can increase hardenability. The resulting martensitic steel is best known for its extremely high strength, with UTS ranging from 900 MPa to 1,700 MPa which have been achieved.

MS steel has relatively low elongation, but post-quench tempering can improve ductility, allowing for adequate formability considering its extreme strength. MS steel is frequently used where high strength is critical. It is typically roll formed and can be bake hardened and electro-galvanized for applications needing corrosion resistance, but heat-treatment of MS steel decreases its strength. Since MS steel has such high strength to weight ratio, it is weight and cost effective. MS steel is frequently selected for body structures, ancillary parts, and tubular structures. MS steels are recommended for bumper reinforcement and door intrusion beams, rocker panel inners and reinforcements, side sill and belt line reinforcements, springs, and clips.

TRIP steel – Like CP steel grades, TRIP steel benefits from a multi-phase microstructure with a soft ferrite matrix embedded with hard phases. The matrix contains a high amount of retained austenite (at least 5 %), plus some martensite and bainite (Fig 4). TRIP steel has a high C content to stabilize the meta-stable austenite below ambient temperatures. Si and / or Al (aluminum) are frequently included to accelerate the ferrite / bainite formation while suppressing carbide formation in this region. TRIP steel received its name for its unique behaviour during plastic strain. In addition to the dispersal of hard phases, the austenite transforms to martensite. This transformation allows the high hardening rate to endure at very high strain levels, hence ‘transformation-induced plasticity’.

The amount of strain needed to initiate this transformation can be managed by regulating the stability of the austenite by controlling its C content, size, morphology or alloy content. With less stability, the transformation begins almost as soon as deformation transpires. With more stability, the austenitic transformation to martensite is delayed until higher levels of strain are reached, typically beyond those of the forming process. In highly stabilized TRIP steel automotive parts, this delay can allow austenite to remain until a crash event transforms it to martensite. Other factors also affect the transformation, including the specific conditions of deformation, such as the strain rate, the mode of deformation, the temperature, and the object causing the deformation. When the austenite-martensite transformation occurs, the resulting structure is toughened by the hard martensite. Deformation can continue through very high strain levels.

TRIP steel, as a result of its high work hardening rates, has excellent formability and a high capacity for stretch. Complex shapes are possible since TRIP steel shows good bendability and resists the onset of necking. TRIP steel also has excellent bake-hardening capacity. High work hardening, total stretchability, and total elongation, however, limit local elongation and edge stretchability. Shear cracking at the interfaces between the ductile ferrite and the hard martensite phases also reduces the hole expansion limits for edges of TRIP steels. Careful design can minimize areas with stretch flange edges. Trimming and notching can also lessen problems with poor edge stretchability. Poor resistance spot-welding behaviour caused by alloying can be addressed by modifying welding cycles (for example, using dilution or pulsating welding).

TRIP steels are the new in development, but steel industry is quickly offering a large variety of TRIP steels for automotive applications. The steel has its wide applicability, especially in complicated parts, and its high potential for mass savings. It can now be obtained in a variety of grades. Automotive applications of TRIP steels include body structure and ancillary parts. With high energy absorption and strengthening under strain, it is frequently selected for components which need high crash energy management, such as cross members, longitudinal beams, A- pillar and B- pillar reinforcements, sills and bumper reinforcements.

Fig 4 Microstructure of TRIP steel and TWIP steel

 HF steel – HF steel is typically B-based, containing 0.002 % to 0.005 % B, and can even be called ‘boron steel’. The processes used to produce HF steel give a unique combination of properties. ‘Direct hot-forming’ can be used to deform the blank in the austenitic state (at high temperatures) or ‘indirect hot-forming’ can be used to heat and finish the piece after most forming is completed at room temperature. In either case, the steel undergoes a series of transitions in elongation and strength, finishing with a rapid cooling to achieve the final desired mechanical properties.

In direct hot-forming, the B-based steel is blanked at room temperature and then heated to high enough temperature for the austenization. The steel is then formed while hot and quenched in the forming tool, developing the martensitic microstructure. Some special post-forming work can be needed to finish the pieces, which are exceptionally high strength. For indirect-hot forming, the steel is blanked and pre-formed at room temperature. The part is then heated and forming is completed while the steel is in this low strength, high elongation state. A final quench in the die produces the final properties and shape.

Parts made from HF steel benefit from several material advantages, including high strength and improved (reduced) spring back. The part remains in the die through the cooling phase, and so the spring back is virtually non-existent. However, repairability is limited, since HF steel becomes brittle through the work hardening of a crash event and the heat needed to straighten the damage degrades the strength of the part. The use of HF boron steel, called UHSS by some automobile producers, has grown rapidly in Europe while other materials for hot forming are also being investigated, as well as new coatings to improve corrosion resistance.

Other areas of present research and development include improving the heating, forming, and tooling technologies. Applications for HF steels include reinforcements for and A- pillars and B-pillars, roof bows, side-wall members, and beams for crash management structures and other parts which carry severe loads. UHSS have been used in the A and B posts, as well as the floor sill reinforcements.

TWIP steel – TWIP steel is part of the ‘second generation’ of AHSS. Austenite based, it sits apart from conventional and first generation AHSS on the elongation tensile strength diagram. High Mn content enables this austenitic structure to exist at room temperature. In fact, the Mn content is so high that some argue that TWIP steel is not steel at all, but rather an advanced alloy. TWIP steel received its name for its particular mode of deformation, deformation twinning, where slip causes the formation of symmetric twin boundaries. These boundaries are much like grain boundaries in their functionality, restricting the movement of dislocations through the material. They strengthen TWIP steel and increase the work hardening rate. As part of the second generation of AHSS, TWIP steel is known for its combination of very high strength with extreme elongation capacity.

TWIP steel application is being explored and developed in the final design of the future steel automobiles. TWIP has been selected for the shock towers and apron reinforcements, although it is also considered and deemed a suitable option for other components in vehicle design, such as the front rail.

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