Automotive steels

Hot-rolled, microalloyed, high strength low alloy (HSLA) steels were first developed in the 1970s. HSLA steels are used for heavier gauged applications such as chassis parts, anti-intrusion beams, wheels, and frame applications for light and heavy trucks. These steels cover a yield strength range from 340 to 700 MPa. They also show good cold formability and high fatigue strength, which makes them ideal for components subject to cyclic loading. HSLA steels’ impact resistance is excellent due to their fine-grained microstructure and low carbon content. The low level of alloying also makes these steels readily weldable with all common processes.

Adding molybdenum to microalloyed steel becomes relevant for yield strength levels above 500 MPa. Molybdenum’s two major metallurgical effects are promoting transformation into a bainitic microstructure and controlling the size of microalloy precipitates.

A combination of low carbon content (<0.1%), molybdenum alloying, and fast cooling retains a substantial amount of the microalloying elements in solid solution after finish rolling. This solute microalloy content is available for nano-precipitation under appropriate coiling conditions. The coiling temperature and, with that, the cooling rate determine the extent of precipitation hardening. A special variant of low-carbon ultra-high strength steel is nano-precipitation hardened ferritic steel (Nano-Hiten), characterized by superior cold forming properties.

In cold-rolled, annealed HSLA steels, molybdenum alloying is not applicable in the specified (VDA 239-100) yield strength range below 460 MPa.

However, recent developments of ‘super-HSLA steels’ with yield strengths of 500 MPa and above benefit immensely from molybdenum alloying. These steels are recovered or partially recrystallized during annealing after cold rolling. By retarding recrystallization, molybdenum’s solute effect enhances the processing window of such annealing treatments, resulting in a strength surplus of 50-100 MPa at 0.15% molybdenum.

These steels comprise sub-micrometer ferrite grain sizes and additional strengthening by MC-type (metal carbide-type) nano-precipitates with molybdenum participation. There are also variants with a yield strength of 800 MPa, 14% total elongation, and excellent local formability.

Multiphase steels can be divided into three groups: DP (Dual Phase) steels, TRIP (Transformation Induced Plasticity) steels, and Partially Martensitic (PM) steels. The superior strength-ductility balance of multiphase steels is due to the tailored combination of soft and hard phases, and in the case of TRIP steels, the presence of metastable retained austenite. The tensile strength of these steels ranges between 450 and 1000 MPa while total elongation (A80) values are between 7% and 27%.

In cold-rolled strip, the amount of ferrite in the multiphase microstructure is adjusted by intercritical annealing between the A_r1 and A_r3 temperature. Depending on the cooling program, the newly formed austenite fraction enriches in carbon and transforms into ferrite, bainite, martensite, or stays as retained austenite. The production of such steels requires a continuous annealing line (CAL) or a continuous galvanizing line (CGL).

In the DP production route, the carbon-enriched austenite is rapidly cooled below the martensite start temperature without forming any other phases. With DP production it is critical to apply a high enough cooling rate to avoid bainite formation. This becomes particularly relevant for low-carbon DP steels, which are preferred by carmakers due to their better weldability.

Adding hardenability elements such as chromium or molybdenum, or a combination of both is common practice. However, molybdenum is approximately three times more effective than chromium in reducing the critical cooling rate. With molybdenum alloying, DP steel can even be produced on older galvanizing lines lacking a high-power cooling section. A small addition of molybdenum also significantly widens the process window, making production more robust against line speed and cooling rate variations.

Recently, steelmakers have developed low-carbon variants (<0.1% C) of DP steels for improving weldability. Consequently, the carbon content in the intercritical austenite is markedly reduced as compared to standard DP steels, which increases the tendency of forming bainite rather than martensite during rapid cooling. The process window of applicable intercritical annealing temperatures is thus narrow.

Molybdenum alloying, combined with an increased manganese level, allows achieving the required strength properties within an acceptable window of processing conditions. Several synergetic effects, including grain refinement and delayed bainite formation, contribute to this improvement. Alloy concepts in industrial production mostly use molybdenum in combination with chromium. Microalloying with niobium or titanium is sometimes used to optimize local formability properties.

For producing TRIP steel, the strip is quenched from annealing temperature to an intermediate temperature to form carbide-free bainite during an isothermal holding period. The bainite formation partitions carbon into retained austenite. Molybdenum makes the progress of bainite transformation extremely sluggish. Accordingly, a larger amount of austenite is preserved to which carbon can partition. The reduced average concentration of carbon in the austenite also raises the martensite-start temperature.

Consequently, molybdenum alloying promotes the formation of martensite and hence increases the tensile strength. Simultaneously, the amount of retained austenite is reduced. For longer holding times, ‘TRIP aided DP steel’ can be obtained by producing retained austenite within a mostly ferritic-martensitic microstructure. A shorter holding time in the bainitic region produces less retained austenite and more transformation into martensite. The DP steel character, therefore, becomes more pronounced and the tensile strength level is markedly increased without raising the carbon equivalent too much, thus providing good weldability.

For producing components with strength above 1200 MPa, cold forming is not recommended due to lack of ductility in such steels. This level of strength is obtained with a fully martensitic microstructure with very limited plasticity and total elongation remaining below 10%. Because cold forming is not suitable, applying hot stamping to produce components requiring such high strength is popular.

The original material is supplied as hot or cold-rolled steel with a ferritic-pearlitic microstructure. Prior to forming, the steel is heated above the A_r3 temperature and austenitized for several minutes at around 950°C. The hot sheet is then placed into a stamping die and formed to shape. The steel has excellent formability at high temperatures. Upon closing the die, rapid heat transfer by direct contact between the hot sheet and the cold die surface transforms the austenitic microstructure into a martensitic one.

Typically targeted tensile strength levels range from 1300 MPa to 2000 MPa with carbon contents of 0.15 to 0.35%, respectively. The base alloy concept of press hardening steel (PHS) is quite simple, mainly involving alloying elements that provide hardenability.

For the standard 1500 MPa grade (22MnB5), these are manganese (1.2%), chromium, (0.15%) and boron (20 ppm). Under standard operating conditions, grade 22MnB5 requires a minimum cooling speed (800→500°C) of 25 K/s. Adding molybdenum to steels alloyed with boron in the range of 5 to 20 ppm has a beneficial effect on hardenability. The critical cooling rate to obtain at least 90% martensite is markedly lowered for the molybdenum-boron co-alloyed steel.

Regarding industrial hot forming operations, molybdenum-boron co-alloying provides a wider processing window, making it less vulnerable against variations in the cooling rate. The molybdenum-boron alloy concept is used in heavy gauged PHS, for example those used in truck components, as heavy gauge parts have a slower cooling rate in the stamping press. Due to molybdenum’s ability to increase the steel’s resistance against hydrogen cracking, molybdenum-boron alloy concepts have also been established for PHS with strength levels above 1500 MPa. Otherwise, sensitivity to hydrogen embrittlement increases with the strength of the steel.