This study focuses on the effect of non-conventional annealing strategies on the micro-structure and related mechanical properties of austempered steels. Multistep thermo-cycling (TC) and ultrafast heating (UFH) annealing were carried out and compared with the outcome obtained from a conventionally annealed (CA) 0.3C-2Mn-1.5Si steel. After the annealing path, steel samples were fast cooled and isothermally treated at 400 °C employing the same parameters. It was found that TC and UFH strategies produce an equivalent level of microstructural refinement. Neverthe-less, the obtained microstructure via TC has not led to an improvement in the mechanical properties in comparison with the CA steel. On the other hand, the steel grade produced via a combination of ultrafast heating annealing and austempering exhibits enhanced ductility without decreasing the strength level with respect to TC and CA, giving the best strength–ductility balance among the studied steels. The outstanding mechanical response exhibited by the UFH steel is related to the formation of heterogeneous distribution of ferrite, bainite and retained austenite in proportions 0.09–0.78–0.14. The microstructural formation after UFH is discussed in terms of chemical hetero-geneities in the parent austenite.@en

The effect of ultra-fast heating on the microstructures of steel has been thoroughly studied over the last year as it imposes a suitable alternative for the production of ultra high strength steel grades. Rapid reheating followed by quenching leads to fine-grained mixed microstructures. This way the desirable strength/ductility ratio can be achieved while the use of costly alloying elements is significantly reduced. The current work focuses on the effect of ultra-fast heating on commercial dual phase grades for use in the automotive industry. Here, a cold-rolled, low-carbon, medium-manganese steel was treated with a rapid heating rate of 780 °C/s to an intercritical peak temperature (760 °C), followed by subsequent quenching. For comparison, a conventionally heated sample was studied with a heating rate of 10 °C/s. The initial microstructure of both sets of samples consisted of ferrite, pearlite and martensite. It is found that the very short heating time impedes the dissolution of cementite and leads to an interface-controlled α → γ transformation. The undissolved cementite affects the grain size of the parent austenite grains and of the microstructural constituents after quenching. The final microstructure consists of ferrite and martensite in a 4/1 ratio, undissolved cementite and traces of austenite while the presence of bainite is possible. Finally, it is shown that the texture is not strongly affected during ultra-fast heating, and the recovery and recrystallization of ferrite are taking place simultaneously with the α → γ transformation.@en

The influence of the heating rates from 10 to 1000 °C/s and annealing temperatures on the microstructure and mechanical properties of two 0.2%C, 1.9%Mn, 1.4%Si cold-rolled steels with and without the addition of carbide-forming elements (Mo, Nb, and Ti) have been investigated. Results show that the increase of the heating rate above 100 °C/s refines the parent austenitic grains in both alloys. The increment of the heating rate led to carbon heterogeneities in the austenite, which after subsequent cooling promoted the formation of a complex mixture of fine-grained constituents. As expected, at the lower heating rates the presence of Nb and Ti-rich carbides and carbonitrides controls the austenite grain growth during the annealing treatment. The tensile test results reveal that high heating rates do not have a significant influence on the tensile strength of the alloy with carbide-forming elements. On the other hand, both the ultimate tensile strength (UTS) and total elongation of the alloy without carbide-forming elements decrease, due to the formation of bands of ferrite and high carbon martensite. However, samples treated at heating rates above 100 °C/s show a combination of UTS in the range of 1400–1600 MPa, and 12–18% of total elongation. The results suggest that the microstructure heterogeneity obtained after high heating rates, especially the ferrite content, has the major effect on the mechanical behavior of the studied steels.@en

The microstructure and mechanical properties of an Fe-0.24C-1.4Mn-1.4Si steel were investigated after combining ultrafast heating (UFH) at a heating rate of 500 °C/s followed by fast cooling to room temperature (DQ) or quenching and partitioning processes (Q&P). Two peak temperatures were studied, annealing into the intercritical range and above the AC3 temperature. After ultrafast heating and quenching, the resulting microstructures revealed that intercritical annealing led to the formation of a banded ferritic-martensitic microstructure. On the other hand, heating above the intercritical range led to an even distribution of allotriomorphic ferrite grains upon fast cooling and a complex phase microstructure, consisting mainly of martensite, was produced. Q&P steel grades exhibit an enhanced mechanical behavior compared to their DQ counterparts, where yield strength, uniform elongation, and total elongation increased after partitioning at 400 °C. The ultimate tensile strength of the Q&P steels decreased compared to the DQ steels annealed at the same peak temperature. However, the final strength-ductility balance of the studied Q&P steels was superior to the DQ steel grades. Moreover, considerable strength and improved ductility were obtained through the combination of peak annealing above the AC3 temperature followed by Q&P. These results are attributed to an interplay between a sustainable TRIP effect and effective strain-stress partitioning among the microconstituents resulted after the Q&P process.@en

In this study an Fe-0.28C-1.91Mn-1.44Si cold-rolled steel was subjected to conventional (10 °C/s) and ultrafast (100 °C/s - 700 °C/s) heating peak annealing treatments, followed by quenching and partitioning (Q&P). The microstructural characterization results showed that grain refinement of the parent austenite and its transformation products occurred with the increment of the heating rate from 10 °C/s to 100 °C/s, without further refining at 700 °C/s. The formation of complex microstructures after the end of the thermal treatment, accompanied by the reduction in the retained austenite carbon content, suggested that local chemical heterogeneities in austenite appear upon ultrafast heating. Regardless of the prior heating rate, similar mechanical properties and strain hardening were measured, revealing that both, the microstructure development and the extent of austenite stabilization during quenching and partitioning stage play a fundamental role on the mechanical behavior of the peak annealed Q&P steels.@en

In this study an Fe-0.28C-1.91Mn-1.44Si cold-rolled steel was subjected to conventional (10 °C/s) and ultrafast (100 °C/s - 700 °C/s) heating peak annealing treatments, followed by quenching and partitioning (Q&P). The microstructural characterization results showed that grain refinement of the parent austenite and its transformation products occurred with the increment of the heating rate from 10 °C/s to 100 °C/s, without further refining at 700 °C/s. The formation of complex microstructures after the end of the thermal treatment, accompanied by the reduction in the retained austenite carbon content, suggested that local chemical heterogeneities in austenite appear upon ultrafast heating. Regardless of the prior heating rate, similar mechanical properties and strain hardening were measured, revealing that both, the microstructure development and the extent of austenite stabilization during quenching and partitioning stage play a fundamental role on the mechanical behavior of the peak annealed Q&P steels.@en

The effect of the microstructure on the principal strain paths (uniaxial, plane, and biaxial) in the formability processes of ferritic stainless steel AISI 430 sheets is studied. The Marciniak test (determination of the plastic strain of sheet metal with a flat tip punch) is applied to determine the forming limit curves and different strain levels in the strain paths by the digital image correlation technique. The formability is discussed in light of the microstructure, standard mechanical properties, work hardening behavior, and anisotropy measurements (R-value). Electron backscatter diffraction analysis is carried out to determine the texture of the selected strain paths. The texture evolution shows a marked γ (<111>// normal direction [ND]) fiber and cube ({001} <100>) texture component under the biaxial strain mode, whereas the α (<110>// rolling direction [RD]) fiber is somewhat favored under uniaxial plane strain. The results are compared with texture simulations performed under the fully constrained Taylor model, finding reasonable agreement with the experimentally measured main components.@en

The effect of the microstructure on the principal strain paths (uniaxial, plane, and biaxial) in the formability processes of ferritic stainless steel AISI 430 sheets is studied. The Marciniak test (determination of the plastic strain of sheet metal with a flat tip punch) is applied to determine the forming limit curves and different strain levels in the strain paths by the digital image correlation technique. The formability is discussed in light of the microstructure, standard mechanical properties, work hardening behavior, and anisotropy measurements (R-value). Electron backscatter diffraction analysis is carried out to determine the texture of the selected strain paths. The texture evolution shows a marked γ (<111>// normal direction [ND]) fiber and cube ({001} <100>) texture component under the biaxial strain mode, whereas the α (<110>// rolling direction [RD]) fiber is somewhat favored under uniaxial plane strain. The results are compared with texture simulations performed under the fully constrained Taylor model, finding reasonable agreement with the experimentally measured main components.@en