Ductile 2-GPa steels with hierarchical substructure

Abstract

Mechanically strong and ductile load–carrying materials are needed in all sectors, from transportation to lightweight design to safe infrastructure. Yet, a grand challenge is to unify both features in one material. We show that a plain medium-manganese steel can be processed to have a tensile strength >2.2 gigapascals at a uniform elongation >20%. This requires a combination of multiple transversal forging, cryogenic treatment, and tempering steps. A hierarchical microstructure that consists of laminated and twofold topologically aligned martensite with finely dispersed retained austenite simultaneously activates multiple micromechanisms to strengthen and ductilize the material. The dislocation slip in the well-organized martensite and the gradual deformation-stimulated phase transformation synergistically produce the high ductility. Our nanostructure design strategy produces 2 gigapascal–strength and yet ductile steels that have attractive composition and the potential to be produced at large industrial scales.

The Materials Science behind Sustainable Metals and Alloys

“…steels for the automotive sector. 170,172 These steels differ from other steel groups regarding chemical composition and microstructure features, exploiting a huge microstructure variety in terms of phases and microstructure constituents including (metastable) austenite, ferrite, martensite, bainite, pearlite, and carbides, with a wide tunability of their size, dispersion, morphology, distribution and percolation. 166 The interplay of these features allows us to exploit many kinds of strengthening and strain hardening mechanisms in versatile combinations.…”

“…In the latter approach processing gains higher momentum, as many of the requested properties of alloys can in part be achieved by equipping the materials with a range of lattice defects (such as dislocations, interfaces, second phases, etc. ) , instead of only through significant chemical changes.…”

“…Another aspect concerns the introduction of high-manganese and medium-manganese steels, ,,− whereby the latter class of materials in particular will gain in importance in the future but will also introduce significantly higher manganese contents into the previously rather manganese-poor iron-carbon scrap. , The new class of medium-manganese high-strength steels refers to alloys with an enhanced manganese content ranging between 3 wt % and 15 wt %. − The maturation of these materials is driven by the demand for compositionally lean yet very strong and formable steels for the automotive sector. , These steels differ from other steel groups regarding chemical composition and microstructure features, exploiting a huge microstructure variety in terms of phases and microstructure constituents including (metastable) austenite, ferrite, martensite, bainite, pearlite, and carbides, with a wide tunability of their size, dispersion, morphology, distribution and percolation . The interplay of these features allows us to exploit many kinds of strengthening and strain hardening mechanisms in versatile combinations.…”

“…In the latter approach processing gains higher momentum, as many of the requested properties of alloys can in part be achieved by equipping the materials with a range of lattice defects (such as dislocations, interfaces, second phases, etc.) 166,172 instead of only through significant chemical changes. furnace where coke-based CO reduces iron ores to a neareutectic Fe-C alloy; the shaft furnace for direct reduction, in which solid-state cm-sized ores or pellets are in static piles exposed to a gaseous reductant; fluidized bed or rotary furnaces where fines of sub-mm-sized ore particles are as dynamic agglomerates (particles in motion) exposed to a gaseous reductant; and the electric arc furnace, which is a standard industry furnace with an electric arc ignited between electrodes and solids (today usually scrap) or liquids.…”

One-step quenching and partitioning treatment of directly hot rolled low carbon steel

“…Ultrahigh strength steels with good ductility yet cost-efficient are actively sought for in the automotive industry [1][2][3][4][5]. Quench and Partitioning (Q&P) steel is considered to be a promising candidate, for it can obtain ultrahigh strength without compromising the ductility [6].…”