High-silicon (1.5–2.5 wt%) steels were designed to achieve carbide-free bainitic matrices with retained austenite through industrial hot rolling, with coiling temperatures of 310 °C and 350 °C. The resulting ultrahigh strength steels (1409–1644 MPa) were characterized through tensile testing, Charpy impact toughness, and Kahn tear tests, while microstructural analysis was performed using scanning electron microscopy and X-ray diffraction. Both yield and tensile strengths correlated strongly with bainitic matrix characteristics, including phase fraction, dislocation density, carbon content, and plate thickness. Ductility showed dependence on film-type austenite content and Md temperature, mechanical stability. The steels exhibited exceptional impact toughness meeting industrial requirements, with ductile fracture behaviour observed down to −100 °C, challenging previous findings. Crack resistance values matched or exceeded those of comparable ultrahigh strength steels. The lower coiling temperature (310 °C) produced retained austenite with higher mechanical stability, benefiting tensile properties and crack resistance, while impact toughness remained largely unaffected by austenite stability due to high strain rates.

Due to the ensuing shift toward adoption of increased levels of scrap during steel production to reduce the carbon footprint, it is essential that the effects of tramp elements entering the produced steel from the scrap are understood. In this context, effects of four major tramp elements (Ni, Cu, Cr, and Mo) on the phase transformation and the microstructure evolution of medium‐manganese steels are studied by theoretical predictions. The results indicate that Cu and Ni are beneficial, whereas Cr and Mo have mixed effects due to the formation of different equilibrium metal carbides. The Cu and Ni concentrations of 0.1–0.5 wt% marginally increase the maximum retained austenite content by 2 vol%. Cr reduces the achievable maximum amount of retained austenite more strongly (decrease by 8 vol% for 0.5 wt% Cr) than Mo (decrease by 4 vol% with 0.5 wt% Mo), and Cr gives a wider processing window for achieving a microstructure with a lower variation than Mo. Combined effects of the elements are to decrease the maximum retained austenite by 8 vol% with lowering of processing temperature by 20 °C. This work provides a guideline for the selection of the optimum processing window of medium‐manganese steels in a circular economy.</jats:p>

Product development routes for new generation ultrahigh-strength structural steels such as S1300 are currently not well-defined, warranting studies on their alloy design, processing, and microstructure–property correlations. This study aims to bridge the gap in understanding of these aspects by investigating a boron-treated, low-carbon, S1300-type alloy steel produced via the direct-quench and tempering (DQ&T) route. The study includes detailed characterization of microstructure development under widely varied tempering temperatures using scanning electron microscopy (SEM), electron backscatter diffraction (EBSD), X-ray diffraction (XRD), dilatometry, and hardness measurements. It reveals three tempering stages of martensite, based on changes in hardness and corroborated by full width at half maximum (FWHM), microstrain, crystallite size, and lattice distortion of martensite. A strong linear correlation between hardness and FWHM is found. The microstructure remained predominantly martensitic (BCT) from its DQ to all tempered states up to 700 °C. Furthermore, EBSD analysis reveals the presence of bainite alongside tempered martensite. Nucleation of fine η-Fe2C, formation and growth of Fe3C and other alloy carbides, and secondary hardening are responsible for the three distinct stages of tempering. Finally, a new parameter named “microstrain-crystallite size parameter” is proposed to establish an empirical relationship for predicting hardness changes during tempering of S1300-type steels.

Medium manganese steels offer a wide range of advantages in hot forming compared to conventional boron steels such as lower heating temperatures and critical quenching rates. Furthermore, the combination of increased strength and ductility makes them a promising material for lightweight components in the mobility sector. For an efficient process design of such components, the finite element simulation is an effective tool to identify a suitable process window and minimise experimental trial-and-error tests. However, a realistic modelling of the material behaviour is essential for reliable simulation results. Therefore, this research focuses on the characterisation and modelling of the flow behaviour of a novel medium manganese steel for the simulation of hot forming processes. Isothermal tensile tests are carried out in a forming and quenching dilatometer under varying forming temperatures and strain rates. To reproduce the heat treatment at the steel producer and the hot forming at the parts manufacturer, the specimens are heat-treated before forming in a process route consisting of annealing, cooling and reheating. After the tests, flow curves are evaluated and modelled using different hardening laws, which are verified by means of numerical simulations.</jats:p>

In this work, we provide with an overview of the main theories, metallurgical and microstructural concepts used for the design of carbide free bainitic steels adapted to industrial hot rolling and coiling practices. The selected alloys were cast and tested at pilot plant scale and the obtained good combinations of properties tend to validate the thorough and careful approach undertaken to predict the microstructures during the alloy design process.

The study investigated the tensile properties of two low-carbon ferritic low-density steels at strain rates of 1 × 10−4 s−1, 1 × 10−3 s−1, and 1 × 10−2 s−1. These steels underwent cold and warm rolling as well as annealing. Various tensile properties were evaluated, including yield strength, ultimate tensile strength, strain hardening exponent, energy absorption up to 10 pct engineering strain, and strain rate sensitivity. The results showed that higher strain rates increased yield strength, ultimate tensile strength, and energy absorption in both steels. The strain hardening exponent was determined using the Hollomon and differential Crussard–Jaoul analysis. The electron backscattered diffraction (EBSD) and transmission electron microscopy (TEM) technique were employed to explain the strain hardening response in both steels. Both steels exhibited two distinct stages of deformation, describing their strain hardening behavior. The study observed a decrease in strain rate sensitivity with increasing true strain in both steels. Steel 1 displayed higher strain rate sensitivity than Steel 2, resulting in a delayed necking tendency and higher total elongation. Micrographs of fracture surfaces revealed the presence of quasi-cleavage facets and secondary cracks at strain rates of 1 × 10−3 s−1 and 1 × 10−2 s−1 in both steels. At a lower strain rate of 1 × 10−4 s−1, Steel 1 exhibited a dimple fracture due to its lower strength and higher total elongation, while Steel 2 displayed a quasi-cleavage fracture. The progression of voids in Steel 1 at a strain rate of 1 × 10−4 s−1 was characterized by establishing a relationship between actual thickness strain and the quantity of voids. This analysis provided insights into the steels void formation and growth mechanisms under specific conditions.

Herein, an investigation on the influence of the hot press forming (HPF) temperature cycles of a medium manganese, high ductility, zinc-coated steel with ≈1000 MPa final tensile strength (HPF1000-GI), with a specific emphasis on the risks of liquid metal embrittlement (LME) induced cracking during resistance spot welding (RSW) is presented. The HPF1000-GI steel is hot press formed using several temperature cycles, and subsequently the LME sensitivity of the variously hot press formed materials are investigated using both dedicated RSW experiments as well as high-temperature tensile testing. It is shown that the LME sensitivity of the HPF1000-GI steel is significantly reduced after hot forming at higher intercritical temperatures, and that this favorable behavior is caused by microstructural changes at the coating-substrate interface.

The microstructural modification and the effect of cold/warm rolling and annealing on tensile properties of ferritic low density steels containing different amounts of carbon (0.0035 mass%, Steel 1 and 0.04 mass%, Steel 2) and aluminium (6.8 mass%, Steel 1 and 9.7 mass%, Steel 2) were investigated with respect to amount of ferrite, grain size and formation of Fe3Al intermetallic. The Steel 1 was deformed by cold rolling at room temperature and Steel 2 was deformed through warm rolling at 250°C to avoid prior cracks formation. After rolling, samples of Steels 1 and 2 were annealed at 900°C for 5 minutes. The yield strength, ultimate tensile strength and ductility increase while yield ratio decreases with annealing. This is due to formation of Fe3Al intermetallic and reduction of ferrite grain size. Due to high interstitial Carbon in Steel 2, yield point phenomenon was observed which may reduce the formability of the steel. The cracks started during tensile test at the ferrite grains and ferrite grain boundaries and the fracture took place in the quasi cleavage mode in Steels 1 and 2. The grain refinement and the microstructural features due to rolling and annealing are accountable for the good combination of strength and ductility of the both steels.

Carbide free bainitic microstructures of steels in hot rolled condition have high potential for automotive and structural applications, where both high elongation and toughness at a high strength level are needed. However, achieving a combination of these properties remains a challenge due to difficulties in ensuring a high stability of retained austenite while maintaining industrial processability. Therefore, an attempt has been made in this work to achieve combined high toughness and high elongation in hot rolled carbide free bainitic steels.

Fe-Mn-Al-C steels, previously developed in the 1950s for replacing Fe-Cr-Ni steels as oxidation or corrosion resistance steels or cryogenic temperature steels, are currently revisited due to two reasons. From an engineering point of view, the specific weight of the steels is reduced when a large amount of light element Al is added, resulting in the so-called low-density or lightweight steels. From an academic point of view, the metallurgical theories of the steels are not well established. The low-density or lightweight steels are expected to have potential applications for structural parts in the automotive industry. This chapter discusses the basic metallurgy, processing strategies, strengthening mechanisms and mechanical properties of these steels from the published literature over a period of many years and suggests avenues for future applications of these alloys in the automotive sector.

Advanced high strength steels (AHSS) are being used in the automotive sector with the aim of supporting the current demands of reducing vehicle weight and structures, but cold forming of such steel grades shows some challenges like springback, high press forces or low stretch-flangeability. To overcome these drawbacks, different ferritic-martensitic dual phase (DP) alloys and martensitic-bainitic complex phase (CP) alloys with retained austenite were designed and novel hot forming routes were applied for processing them. The thermal cycle for the DP alloys included an intercritical reheating whereas in-situ austempering or slow continuous cooling preceded by supercritical reheating was used for the CP alloys. The objective of obtaining similar, or even better, post-formed mechanical properties than the current cold-formable DP1000-Low Yield and CP1000 grades in terms of yield strength (YS), ultimate tensile strength (UTS) and total elongation (TE), combining the new alloys and the proposed hot forming routes was investigated through an intensive testing campaign. First, cold rolled alloys were subjected to hot forming cycles including deformation levels up to 20% in a Gleeble machine and then formability tests at high temperatures (hole tensile tests, omega-shaped parts manufacture) were conducted to compare their performance against the current cold formable alloys. The promising results after hot press forming (YS ≈ 650 MPa; UTS ≈ 1150 MPa; TE ≈ 10% for DP alloys and YS ≈ 850 MPa; UTS ≈ 1250 MPa; TE ≈ 7% for CP alloys) pave the way to promote the use of these new alloys that will allow designing vehicle components with increased geometric complexity, while minimizing the springback effect, reducing the press forces and the material scrap inherent of cold stamping.

The effects of intercritical annealing, before and after cold rolling, on retained austenite volume fractions, austenite stability, and tensile properties of a 7.39Mn-0.14C-1.55Al-0.2Si (wt pct) medium Mn sheet steel were evaluated. The extent of hot band annealing at 650 °C prior to cold rolling is shown to be an important parameter to consider in the development of processing histories leading to high strength–ductility combinations required for third-generation advanced high-strength steels.

This paper highlights some recent efforts to extend the use of medium-Mn steels for applications other than intercritically batch-annealed steels with exceptional ductility (and strengths in the range of about 1000 MPa). These steels are shown to enable a range of promising properties. In hot-stamping application concepts, elevated Mn concentration helps to stabilize austenite and to provide a range of attractive property combinations, and also reduces the processing temperatures and likely eliminates the need for press quenching. The “double soaking” concept also provides a wide range of attractive mechanical property combinations that may be applicable in cold-forming applications, and could be implemented in continuous annealing and/or continuous galvanizing processes where Zn-coating would typically represent an additional austempering step. Quenching and partitioning of steels with elevated Mn concentrations have exhibited very high strengths, with attractive tensile ductility; and medium-Mn steels have been successfully designed for quenching and partitioning using room temperature as the quench temperature, thereby effectively decoupling the quenching and partitioning steps.

Pure martensitic steels have after hot forming limited performance in terms of rest ductility which limits the application in crash relevant parts. New steel grades were designed in the EU project HOTFORM including the corresponding process routes. These steel grades have ferritic-martensitic dual phase (DP) and martensitic-bainitic complex phase (CP) microstructures after hot forming process. The laboratory tests show an improved formability after hot forming. The basic concepts of the new alloys are explained. Furthermore, for validation of upscaling purposes a semi-industrial test is carried out and the results are discussed. The main application is for vehicle safety. This is evaluated by comparing the crash performance of these hot formed grades with cold rolled DP1000 and CP1000 for crash cans in a drop tower test.

Nitrogen-annealed tensile specimens of a cold rolled medium Mn (9.76 wt%) steel exhibited significantly reduced ductility due to a two-zone embrittled surface layer, imparted during annealing in a nitrogen atmosphere. The embrittlement was associated with high nitrogen contents at the surface.

The effects of hot deformation conditions in the stable austenite state on the transformation kinetics and morphology of bainite were examined using dilatometry, electron back scatter diffraction (EBSD) and X-ray diffraction (XRD) measurements in a carbide-free bainitic steel with a composition of Fe-0.34C-2Mn-1.5Si-1Cr (in wt.%). Both uniaxial tensile and compression tests were applied to study the effect of the deformation mode. The temperature, strain and strain rate of deformation were varied in the ranges of 820-1000 °C, 0.1-0.6 and 0.001-0.1/s respectively. It has been revealed that hot tensile deformation retards the austenite transformation to lower bainite. The overall transformation kinetics slows down and the final attained amount of bainite decreases after completion of the isothermal transformation at 350 °C. However, hot compression deformation accelerates the bainite transformation, increasing both the bainite transformation rate and the final amount of bainite formed. The total amount of bainite increases with decreasing the strain rate irrespective of the mode of deformation. The effect of the deformation temperature and strain on the bainite transformation is in a complicated manner depending on the deformation mode.

Current hot stamped steels can offer ultra high strength (1500 MPa) with limited elongation (∼5%). Research is ongoing to develop hot stamped steels with similar strength and improved ductility by using various microstructural approaches. The current work describes in depth the characterisation of microstructural features of a steel that was processed via a quenching and partitioning method in a modified hot stamping thermal cycle to deliver a good combination of strength and ductility. It was found that the microstructure thus produced is very complex comprising many different phases. As it is also beyond the ability of one single technique to fully characterise it, several techniques were employed in order to properly understand the various features of the different constituents of this microstructure.

In cold forming for automotive lightweight design, advanced high strength steels (AHSS) lead to limited formability, high springback and press forces, low stretch flangeability, multiple operations for complex geometries and large scrap rates. Two sets of AHSS, namely ferritic-martensitic dual-phase (DP) steel and martensitic-bainitic complex-phase (CP) steel with some amounts of retained austenite (RA), were designed for the hot-forming route, which eliminates the above drawbacks and guarantees higher performance in the body-in-white (BIW). Design of four DP and four CP alloys was accomplished using JMatPro6.0 thermodynamic software and available literature. The alloys were manufactured in the laboratory in cold-rolled gauge of ~1.5 mm and subjected to hot-forming cycles including hot deformation (up to 20% strain), using a dilatometer and a Gleeble 3800 machine. The thermal cycles of the DP alloys included an intercritical reheating whereas in-situ austempering or slow continuous cooling followed by supercritical reheating was used for the CP alloys. The results showed that yield strength (YS) of 605MPa & 695MPa, ultimate tensile strength (UTS) of 1097MPa & 1242MPa with a total elongation (TE) of 12.6% & 14.1% can be achieved in the best performing DP alloys with a martensite content of 65% & 60 vol.%. The best CP alloys with austempering achieved YS of 673MPa & 699MPa, UTS of 983MPa & 1026MPa and TE of 9.2% & 13.6% with RA of 4%-12 vol.%. The continuously-cooled alloys achieved even better properties. Higher bendability at 1.0 mm gauge in the critical direction was achieved in the CP alloys (90^o &107^o ) than in the DP alloys (73^o &76^o ).

The effects of adiabatic heating during deformation of a medium-manganese transformation-induced plasticity steel containing 10.1Mn-1.68Al-0.14C-0.2Si (wt.%) processed with initially 57 vol.% retained austenite were investigated over the temperature range from − 60°C to 100°C at strain rates from 0.002 s⁻¹ to 0.2 s⁻¹. Tensile tests were performed on specimens immersed in isothermal baths, which reduced but did not completely eliminate adiabatic heating. The specimen temperature depended on the extent of adiabatic heating, which increased with strain and strain rate. The measured properties primarily reflected the effects of temperature on austenite stability and the corresponding resistance of austenite transformation to martensite with strain. Changes in austenite stability were monitored by measurements of austenite fractions at a specific strain and observation of microstructures after deformation. The results of this study provide a basis to identify input material parameters required for numerical models applicable to sheet metal forming of medium-Mn steels.