Advanced high strength steels for automotive application
Advanced high strength steels for automotive application
Advanced high strength steels (AHSS) are being developed for automotive applications. These automotive grades are different when compared with the conventional low and high strength steels. AHSS have superior mechanical properties which are developed in it due to the steel’s structure and due to its distinct processing. AHSS are manufactured by adopting control cooling from austenite or austenite plus ferrite phases on the run out roller table in a hot rolling mill or in the cooling section of a continuous annealing furnace in cold rolled product. A comparison of AHSS with other types of high strength steel is given in Fig 1. Some types of these steels are described below.
Fig 1 Comparison of high strength steel with AHSS
Dual Phase (DP) steel
This steel has two phases namely ferrite and martensite. The hard second phase of martensite is present in the form of islands in a matrix of ferrite. Higher is the volume fraction of second phase higher is the strength. During the production of this steel, a portion of the austenite phase is first converted into ferrite before rapid cooling to transform the remaining austenite to martensite. Some hot rolled steels can have a microstructure which contains considerable amount of bainite. The property of good ductility to this steel is imparted by soft ferrite phase which is generally continuous. During the working of this steel, lower strength ferrite phase gets strained giving the steel a distinctive high work hardening rate. The work hardening rate along with good elongation provides DP steel better ultimate tensile strength (UTS) values then conventional steel of similar yield strength (YS). Accordingly DP steel has low YS/TS ratios. DP steel also has a bake hardening effect which is an important advantage. The bake hardening effect is the increase in the yield strength resulting from the elevated temperature aging after pre-straining. The degree of the bake hardening effect will depend on the specific chemistry and thermal history of the DP steel. Carbon enables martensite formation at practical cooling rates in DP steel by increasing the steel hardenability. Chromium, manganese, molybdenum, nickel and vanadium when added individually or in combination, help also to increase the hardenability. Carbon, as a ferrite solute strengthener, also strengthens the martensite as done by silicon and phosphorus. These additions are carefully balanced in DP steel for producing distinct mechanical properties as well as for maintaining superior capability for resistance spot welding.
Transformation induced plasticity (TRIP) steel
TRIP steel has a microstructure consisting of retained austenite which is embedded in a ferrite matrix. Retained austenite amount is a minimum of 5 % by volume. There are other hard phases martensite and bainite which are present in varying amounts. This steel is to be kept at an isothermal hold at an intermediate temperature for bainite in the microstructure. The greater amount of carbon and silicon in this steel also help in getting considerable amounts of retained austenite in the microstructure. In TRIP steel the dispersion of a hard second phase in softer ferrite matrix causes a high rate of work hardening during deformation. However with increasing strain the retained austenite also progressively get transformed to martensite. This increases the work hardening rate at higher level of strain.
When compared with DP steel, TRIP steel has slower rate of initial work hardening, but the hardening rate continues at higher level of strains against the diminishing of the hardening rate in the DP steel. Since the rate of the work hardening is better in TRIP steel when compared to the conventional high strength steel, it provides significant stretch forming properties. This property is advantageous to the designers since they can take advantage of the high rate of work hardening rate for designing a part utilizing the as formed mechanical properties. Further in the severe stretch forming applications TRIP steel is advantageous over DP steel because of higher work hardening rate persisting at higher strains. TRIP steel has higher amounts of carbon than DP steel for obtaining required carbon content to stabilize the retained austenite phase to below ambient temperature. In this steel higher amount of silicon and/or aluminium is used for accelerating thr ferrite to bainite transformation. These elements help in maintenance of required amount of carbon within the retained austenite.
Suppression of the carbide precipitation during bainitic transformation is crucial for TRIP steel. In this steel, silicon and aluminum are used for avoiding carbide precipitation in the bainite region. By adjustment of the carbon content, the strain level can be designed at which retained austenite starts transforming to martensite. When carbon content is low, the retained austenite starts transforming almost immediately upon deformation. This increases the work hardening rate and formability during the process of stamping. On the other hand when the carbon content is higher, the retained austenite is more stable. It starts transforming only at strain levels beyond those produced during forming. At higher carbon level the retained austenite remains in the final part. It transforms to martensite only during subsequent deformation.
TRIP steel can be engineered for providing very good formability during manufacturing. It shows high work hardening during crash deformation and provides excellent crash energy absorption. The additional requirement of alloying elements in TRIP steel lowers its resistance spot-welding behaviour.
Complex phase (CP) steel
This type of steel is associated with the transition to steel having very high ultimate tensile strength. The microstructure consists of small quantity of martensite, retained austenite and pearlite within the ferrite/ bainite matrix. In this steel, retarded recrystallization or precipitation of micro alloying elements like niobium or titanium causes extreme grain refinement. When compared with DP steel, CP steel shows considerably higher yield strength at equal tensile strength of equal to or more than 800 MPa. CP steel has high energy absorption and high capacity for residual deformation.
Martensitic (MS) steel
In MS steel, the entire amount of austenite in the steel microstructure existing during hot rolling or annealing is totally converted into martensite. MS steel has a martensitic matrix which contains small amount of bainite and/or ferrite. MS steel shows maximum tensile strength amongst multiphase steels. This type of steel offers maximum strength up to 1700 MPa. The structure of MS steel can also be developed by heat treatment after post forming. This type of steel is sometimes subjected to post quench tempering for improving its ductility. This steel can provide sufficient formality even at very high strength. The element carbon when added to MS steel not only increases its hardenability but also strengthens the martensite. Boron, chromium, manganese, molybdenum, nickel, silicon and vanadium are also added in various amounts to increase the hardenability. MS steel is produced with rapid quenching of the austenite phase with majority of the austenite transforming into martensite. Though cooling pattern is similar to the CP steel, but due to the adjustment of the chemistry there is less retained austenite and more fine precipitates in MS steel for strengthening the martensite and bainite phases.
Ferritic bainitic (FB) steel
FB steel has better edge stretch property and hence it is also sometimes known as stretch flangeable (SF) steel or high hole expansion (HHE) steel. The microstructure of the FB steel contains fine ferrite and bainite. Strengthening in the steel is achieved by grain refinement as well as by the hardening of the second phase with bainite. The product of FB steel is in as rolled condition. The FB steel has better stretchability of sheared edges as determined by the hole expansion test than high strength low alloy (HSLA) and DP steel. FB steel also exhibits increased total elongation and higher strain hardening exponent when compared with HSLA steel of equal strength. FB steel is having good fatigue properties and crash performance. It is used for tailored blank application since it has good weldability.
Twinning induced plasticity (TWIP) steel
The manganese content of TWIP steel is high and is in the range of 17 % to 24 %. High manganese con tent of this steel makes it fully austenitic at room temperature. Due to this the main deformation mode in this steel is twining inside the grain. The twinning inside the grain results into a high degree of instantaneous hardening rate (n value) as the microstructure of this steel becomes finer. The boundaries of these twins act as grain boundaries and strengthen the steel. The advantage of this steel is that it has extreme high strength with extreme high formability. The n value in this steel increases to 0.4 at around 30 % of engineering strain and then it remains constant until a total elongation of 50 %. The steel has a tensile strength which is higher than 1000 MPa.
Hot formed (HF) steel
This hot forming quench hardenable steel is used at temperatures above the austenitic range (900 -950 deg. C) for optimized part geometries with intricate shapes having no spring back issues. While processing this steel there are three important states with different mechanical properties.
- State 1 – In this state tensile strength of 600 MPa maximum is considered for blanking dies design.
- State 2 – In state 2, there is low strength and high elongation (up to 50 %) at forming temperatures which facilitates forming of complex shapes. To avoid surface oxidation after the forming operation special coating based on aluminium and silicon is used.
- State 3- In this state, strength above 1300 MPa is achieved after quenching in the die following the forming operation.
There are special processes which are to be taken into account while finishing the product. Typical press cycle time is 20 to 30 seconds. Hot forming boron steels are usually used for structural and safety parts.
Post forming heat treatable (PFHT) steel
In PFHT steel high strength in the steel is developed by the post forming heat treatment. Widespread use of high strength steel had been restricted since it is difficult to maintain the geometry of the part during and after the heat treatment process. Making of the part then heating with immediate quenching appear to be one of the solution. In this production process stamping is carried out at lower strength of the steel and after that steel strength is increased by heat treatment. In another process water quenching of cheaper steel with chemistry that results into tensile strength of the part in the range of 900 MPa to 1400 MPa. In this type of steel big co ordination is needed with the steel manufacturer to get the specific chemistry of steel for meeting the requirements of the part. Another process is the air hardening of alloyed tempering steel. This steel has good forming properties in soft stage and high strength after heat treatment. Besides sheet metals, air hardening steels are also suitable for tube welding. These tubes are usually used for hydro forming applications. The part is heat treated in a furnace in protective gas atmosphere (austenitized) and then hardened and tempered by natural cooling in air or in protective gas. The excellent hardenability and resistance to tempering is achieved by adding other alloying elements such as boron, chromium, molybdenum, titanium and vanadium in addition to carbon and manganese. The steel has good welding property in both its soft and air hardened state, as well as in the combination of soft/air-hardened. This steel also responds well to coating using standard coating methods.
New evolving advanced high strength steels
To meet the demands of the automotive industry for additional capabilities of AHSS, research in this area is continuing for development of new types of steels. These steels are designed to reduce density, to improve strength, and/or to increase elongation. For example, Nano steel is being designed for avoiding low value of edge stretch (local elongation) usually experienced in DP steel and TRIP steel. In this steel the ferrite matrix is strengthened with ultra fine nano sized (less than 10 Nm) particles instead of islands of martensite. This has been achieved in hot rolled high strength steel with a tensile strength around 750 MPa. The resulting steel has a high YS/TS ratio with an excellent balance of total elongation and local elongation (Hole expansion ratio). Other example of the new evolving steel is the ultra fine grain, low density and high Young’s modulus steel.