1. Thermomechanical Processing and Metallurgy of Medium–Mn Steels
The range of “medium Mn” is defined typically between 4 and 12 wt pct, i.e., lower compared to that of high Mn (≥15 wt pct) twinning-induced plasticity (TWIP) steels. Notably, “intercritically annealed” medium-Mn steels are capable of achieving a superior combination of strength, ductility, and work-hardening capability. Their high work-hardening rate is associated with the TRIP effect or a combination of two plasticity-enhancing mechanisms, the TWIP and TRIP effects. Despite the lower Mn contents compared to high Mn steels, medium-Mn steels achieve excellent mechanical properties, with a product of ultimate tensile strength and total engineering strain in the range of 5000 to 90,000 MPa∙pct [2
]. These steels have substantially higher yield strengths than their austenitic counterparts. Austenite stability can be modified by partitioning Mn during intercritical processing and strength can be enhanced by incorporating martensite into the microstructure, using a variety of processing approaches.
The production routes for medium-Mn steels and resulting microstructures are shown in the schematic of . Note that many of the microstructural variants presented in are developed through processing of a “cold-rolled” product. It is anticipated, however, that some of the processing and microstructural engineering concepts developed for the cold-rolled product can be extended to hot-rolled or plate-rolled products.
Mn alloying enhances hardenability, i.e., it delays ferritic/bainitic transformation during hot/plate rolling. Therefore, hot-rolled medium-Mn steels often have martensite matrix microstructures, which is one key difference from the ferrite matrix in as-rolled microstructures of conventional low alloyed steels. The evolution of the martensite matrix microstructure of medium-Mn steel is an important attribute to control during subsequent thermo-mechanical processing. For example, intercritical annealing (IA), i.e., annealing at a temperature between the Ac1
temperatures, of the martensitic starting microstructure of a medium-Mn steel results in the partial reversion of martensite to austenite. For specimens that are not cold-worked prior to annealing, the preferred sites for austenite formation are martensitic lath interfaces and prior austenite grain boundaries. Consequently, the austenite reversion heat–treatment (ART) or IA of undeformed martensitic starting microstructures produces a “lamellarized” duplex microstructure consisting of α′-martensite and austenite [1
] (first example in ). The “α′-martensite” after reversion heat-treatment may not preserve its martensitic nature because recovery and/or partial recrystallization take place in the martensitic regions during IA [8
]. On the other hand, IA of a deformed or cold-worked starting microstructure leads to a uniform distribution of “equiaxed” ferrite and austenite, usually characterized by an ultra-fine-grained (UFG) microstructure, resulting from the simultaneous recrystallization of ferrite and growth of austenite [1
] (second example in ). The two (lamellarized and equiaxed) microstructures are the most common forms amongst the different microstructural variants of medium-Mn steel grades. Note that the lamellarized microstructure does not have to be produced from a hot-/plate-rolled condition, e.g., it can be generated by two-stage annealing (consisting of austenitization annealing, quenching, and austenite reversion heat-treatment) of a cold worked microstructure. It should also be noted that two-stage annealing of a cold-rolled steel can develop either a lamellarized or equiaxed microstructure, depending on the initial Mn distribution and thermal cycle [9
]. One variant of the equiaxed microstructure is generated by a “double soaking” heat treatment, consisting of an IA stage to stabilize austenite through Mn partitioning, followed by a very short secondary soaking stage [11
]. The second soak is performed at a relatively high temperature in order to partially or fully replace intercritical ferrite with austenite that is converted to athermal martensite during quenching, and to create a higher-strength TRIP microstructure containing both martensite and austenite [11
]. This microstructure offers attractive strength/ductility combinations, but it is likely less attractive in applications requiring substantial resistance to HE, without an additional tempering step.
A key attribute of both lamellarized and equiaxed microstructures is the presence of either UFG or nanolaminate [13
] retained austenite, respectively, which contributes to enhanced work hardening and high ductility associated with the TRIP and/or TWIP effect. For medium-Mn steels, submicron-sized retained austenite grains, typically smaller than 500 nm, are obtained by a relatively simple IA step or austenite reversion annealing. It is also important to note that IA is essential to obtain a substantial amount of retained austenite with adequate stability as carbon (C) and Mn, both strong austenite stabilizers, partition from ferrite or martensite to austenite during IA. The origin of the development of a high fraction of UFG austenite in the intercritically annealed medium-Mn steel is largely associated with the Mn austenite-stabilizing effect [2
]. When the bulk Mn content is increased, the reverse transformation to austenite starts at a lower temperature and, thus, a relatively high fraction of austenite may be produced before recrystallization takes place. Suh and Kim [2
] pointed out that grain growth in the ultra-fine regions is restricted even at a relatively high IA temperature (~800 °C), presumably due to the nature of the two-phase microstructure.
Some alloy and microstructure variants of typical UFG duplex medium-Mn steel are also shown in . For example, many investigators considered aluminum (Al) and silicon (Si) as essential alloying elements for medium-Mn steels. These elements are useful in raising the intercritical temperature and, thus, increasing the solute partitioning kinetics. In addition, it has been recognized that a bimodal grain structure can be achieved in medium-Mn steels by the addition of Al and Si, when the amount of both alloying elements exceeds 3 wt pct [16
]. Fe–xC-6 wt pct Mn steels were employed in the initial studies of Al-added medium-Mn TRIP steels [16
]. shows the influence of Al and Si additions on the pseudo-binary phase diagram for an Fe–6.0Mn–xC alloy system (all in wt pct), calculated by using Thermo-Calc®
software. The Al and Si additions expand both the α-ferrite and δ–ferrite stability regions, resulting in the formation of layers of coarse δ–ferrite grains in the hot–/cold-rolled microstructures (third example in ). For specific alloy and heat-treatment conditions, a steel specimen with a UFG microstructure often experiences pronounced localized deformation and plastic instability, associated with dynamic strain aging [19
]. The introduction of coarse δ–ferrite grains in the UFG structure, resulting in a bimodal grain size distribution, may eliminate the localized deformation and plastic instability and enhance ductility [20
]. In another alloying and processing variation, “warm” rolling at an IA temperature has been applied to Al-free and Al-containing medium-Mn steels, leading to an even finer-grained, lamellarized microstructure, elongated along the rolling direction [21
] (fourth example in ).
Some investigations applied “conventional” quenching and tempering (Q&T) to medium-Mn steels in order to produce a tempered martensite matrix microstructure containing an adequate/high fraction of retained austenite, resulting from the effect of the Mn addition on lowering the martensite start (Ms
) and martensite finish (Mf
) temperatures [24
] (fifth example in ). Quenching and partitioning (Q&P) heat treatments can be used to increase and control the fraction of retained austenite [24
]. These Q&T and Q&P microstructures consist of metastable retained austenite within the martensitic matrix, somewhat similar to the lamellarized microstructure achieved by austenite-reversion heat-treatment (first example in ). However, tempering/partitioning treatments involve carbon redistribution at low temperatures, whereas austenite reversion typically requires a longer hold time at a sufficiently high “IA” temperature to partition Mn. Thus, the degrees of tempering and solute partitioning are lower in the Q&T and Q&P processes, which may lead to lower ductility, but ultrahigh strength and a higher “yield ratio” (the ratio of yield strength to ultimate tensile strength) compared to the austenite reversion treated steel. In addition, retained austenite in quenched and tempered (or partitioned) specimens can have both bulky and film-type morphologies and is metastable, resulting in a pronounced TRIP effect. In contrast, austenite reversion is expected to induce partitioning of substitutional alloying elements (e.g., Mn, Si, and Al) in addition to C, which can lead to both TRIP and TWIP effects. The use of medium-Mn steels in Q&P processing may offer several advantages in addition to the Mn austenite stabilizing effect. For Mn-alloyed steels, alloying and processing strategies can be developed to decrease the Mf
temperature slightly below room temperature; in this condition, one can apply “room temperature” quenching and partitioning, which is potentially easier to implement industrially compared to “conventional” Q&P processing [28
Similar to the Q&T/Q&P applications, the application of hot or “warm” stamping to medium-Mn steels produces microstructures with retained austenite in addition to martensite, which provides an ultrahigh strength (tensile strength up to 1.6 GPa) and a ductility (total elongation up to 44 pct) much higher than that of conventional 22MnB5 press-hardened steel (7–8 pct) [32
]. There are additional advantages of the application of medium-Mn steel to hot stamping over the conventional low-alloy boron-added steels; for example, Mn-associated enhanced hardenability reduces the cooling rate requirement and enables alternative cooling approaches [32
]. In addition, compared to low-alloy steels, lower reheating (austenitizing or IA) temperatures can be employed for hot stamping of medium-Mn steel, which reduces both the heating time and potentially production costs, and also influences the behavior of metallic coatings that may be employed.