Microstructure Evolution and Mechanical Behavior of a Hot-Rolled High-Manganese Dual-Phase Transformation-Induced Plasticity/Twinning-Induced Plasticity Steel

The microstructures and deformation behavior were studied in a high-temperature annealed high-manganese dual-phase (28 vol pct δ-ferrite and 72 vol pct γ-austenite) transformation-induced plasticity/twinning-induced plasticity (TRIP/TWIP) steel. The results showed that the steel exhibits a special Lüders-like yielding phenomenon at room temperature (RT) and 348 K (75 °C), while it shows continuous yielding at 423 K, 573 K and 673 K (150 °C, 300 °C and 400 °C) deformation. A significant TRIP effect takes place during Lüders-like deformation at RT and 348 K (75 °C) temperatures. Semiquantitative analysis of the TRIP effect on the Lüders-like yield phenomenon proves that a softening effect of the strain energy consumption of strain-induced transformation is mainly responsible for this Lüders-like phenomenon. The TWIP mechanism dominates the 423 K (150 °C) deformation process, while the dislocation glide controls the plasticity at 573 K (300 °C) deformation. The delta-ferrite, as a hard phase in annealed dual-phase steel, greatly affects the mechanical stability of austenite due to the heterogeneous strain distribution between the two phases during deformation. A delta-ferrite-aided TRIP effect, i.e., martensite transformation induced by localized strain concentration of the hard delta-ferrite, is proposed to explain this kind of Lüders-like phenomenon. Moreover, the tensile curve at RT exhibits an upward curved behavior in the middle deformation stage, which is principally attributed to the deformation twinning of austenite retained after Lüders-like deformation. The combination of the TRIP effect during Lüders-like deformation and the subsequent TWIP effect greatly enhances the ductility in this annealed high-manganese dual-phase TRIP/TWIP steel.     

Austenite Stability Effects on Tensile Behavior of Manganese-Enriched-Austenite Transformation-Induced Plasticity Steel

Manganese enrichment of austenite during prolonged intercritical annealing was used to produce a family of transformation-induced plasticity (TRIP) steels with varying retained austenite contents. Cold-rolled 0.1C-7.1Mn steel was annealed at incremental temperatures between 848 K and 948 K (575 °C and 675 °C) for 1 week to enrich austenite in manganese. The resulting microstructures are comprised of varying fractions of intercritical ferrite, martensite, and retained austenite. Tensile behavior is dependent on annealing temperature and ranged from a low strain-hardening “flat” curve to high strength and ductility conditions that display positive strain hardening over a range of strain levels. The mechanical stability of austenite was measured using in-situ neutron diffraction and was shown to depend significantly on annealing temperature. Variations in austenite stability between annealing conditions help explain the observed strain hardening behaviors.     

TRIP-相变诱发塑性钢的研究进展

相变诱发塑性钢是一种汽车用钢，通过相变诱发塑性(TRIP)效应使钢板中残余奥氏体在塑性变形作用下诱发马氏体生核和形成，并产生局部硬化，继而变形不再集中在局部，使相变均匀扩散到整个材料以提高钢板的强度和塑性。典型TRIP钢c含量为0．2％，Mn 1％～2％，Si 1％～2％，通过热轧变形热处理或冷轧 热处理，TRIP钢的组织由50％～60％铁素体，25％～40％贝氏体或少量马氏体和5％-15％残余奥氏体组成。TRIP钢的强度和韧性高于双相钢和微合金钢。介绍了TRIP钢的生产工艺和性能，残余奥氏体、合金元素、热处理对TRIP效应的影响和TRIP钢研究趋势。     

Tetragonality of bcc Phases in a Transformation-Induced Plasticity Steel

In a low-alloyed multi-phase transformation-induced plasticity steel, solute carbon content in polygonal ferrite, bainitic ferrite, and martensite was characterized using site-specific atom probe tomography. Selected area diffraction patterns were obtained using transmission electron microscopy, and the geometric distortion thereof was determined. The results showed that the lattice distortion increased in a sequence of polygonal ferrite, lath-like bainitic ferrite, and martensite. This increasing distortion corresponded to an increase in carbon content of the phase.     

Microstructure and Mechanical Behavior of a Mini-Thixoformed Tool Steel

We report on mini-thixoforming of a hard and wear-resistant crucible particle metallurgy tool steel. Significant microstructural modifications associated with the special semisolid forming process are characterized in detail by scanning electron microscopy and X-ray diffraction. The mechanical performance of the material is assessed both pre-thixoforming and post-thixoforming by nanoindentation of the constituent phases. The novel microstructural changes that result from mini-thixoforming, which are discussed in this article for the first time, are beneficial in further improving the hardness of the steel.     

Influence of Al on the Microstructural Evolution and Mechanical Behavior of Low-Carbon,Manganese Transformation-Induced-Plasticity Steel

Microstructural design with an Al addition is suggested for low-carbon, manganese transformation-induced-plasticity (Mn TRIP) steel for application in the continuous-annealing process. With an Al content of 1 mass pct, the competition between the recrystallization of the cold-rolled microstructure and the austenite formation cannot be avoided during intercritical annealing, and the recrystallization of the deformed matrix does not proceed effectively. The addition of 3 mass pct Al, however, allows nearly complete recrystallization of the deformed microstructure by providing a dual-phase cold-rolled structure consisting of ferrite and martensite and by suppressing excessive austenite formation at a higher annealing temperature. An optimized annealing condition results in the room-temperature stability of the intercritical austenite in Mn TRIP steel containing 3 mass pct Al, permitting persistent transformation to martensite during tensile deformation. The alloy presents an excellent strength-ductility balance combining a tensile strength of approximately 1 GPa with a total elongation over 25 pct, which is comparable to that of Mn TRIP steel subjected to batch-type annealing.     

Austenite in Transformation-Induced Plasticity Steel Subjected to Multiple Isothermal Heat Treatments

The thermodynamic limit to the progress of the bainite reaction in steels containing a cementite inhibitor often leaves large quantities of thermally or mechanically unstable austenite. Such austenite is not effective in delaying the onset of plastic instabilities during the course of deformation. In such circumstances, it is useful to conduct isothermal transformation at a high temperature where the rate of reaction is relatively rapid, followed by a lower temperature step that permits more bainite to be generated. This in turn increases the stability of the refined austenite, which then transforms gently over a large range of strain during a tensile test. A significant corollary is that the two-step heat treatments are unnecessary in low-carbon steels, where the bainite reaction is able to proceed to a greater extent before reaching the thermodynamic limit. Furthermore, the two-step process can be counterproductive in low carbon steel, because the austenite content is reduced to a level below which it does not enhance the mechanical properties. Other circumstances in which multiple heat treatments are necessary are also discussed.     

Mechanical Behavior of CrMo4 Steel With Dual-Phase Microstructure

Multi-phase microstructures having good ductility may replace the conventional microstructures in different technological applications such as pressure vessels.Mechanical properties including the fracture behavior and the service life of pressure vessels are strongly affected by the microstructure.The objective of this study was to investigate the correlation between different dual-phase microstructures and mechanical properties of 42CrMo4 (AISI 4140) steel.To produce the martensite-bainite (M-B),the martensite-ferrite (M-F),and the ferrite-bainite (F-B) microstructures,the step quenching heat treatment was used.Mechanical properties of heat treated samples including the strength,ductility,and impact energy were measured.Tensile experiments revealed a discontinuous yielding in the F-B specimen with ferritic matrix.Fracto\graphic results showed high concentration bright facets (BFs) on broken specimen surfaces indicating the brittle cleavage fracture was the predominant mechanism in the dual-phase microstructures.     

Localized Deformation in Multiphase,Ultra-Fine-Grained 6 Pct Mn Transformation-Induced Plasticity Steel

Multiphase, ultra-fine-grained transformation-induced plasticity (MP UFG TRIP) steel containing 6 mass pct Mn was obtained by cold rolling and intercritical annealing of an initially fully martensitic microstructure. UFG microstructures with an average grain size less than 300 nm were obtained. The amount of austenite in the microstructures, speculated to be formed by diffusionless transformation, was controlled by changing the intercritical temperature. The tensile properties were strongly influenced by the volume amount and the stability of the reversely transformed austenite. The MP UFG TRIP steel was characterized by pronounced localization of the deformation. The deformation band properties were analyzed in detail.     

Steel Microstructure Effect on Mechanical Properties and Corrosion Behavior of High Strength Low Carbon Steel

Different thermomechanical treatments were applied to a high strength low carbon steel with a novel chemical composition. As a result, three different microstructures were produced with dissimilar mechanical and corrosion properties. Subsequently, a tempering heat treatment was applied to redistribute the phases in the steel. Microstructure A with 56 pct martensite and 32 pct bainite presented high strength but medium ductility; microstructure C with 95 pct ferrite and 3 pct martensite/austenite resulted in low strength and high ductility, and finally microstructure B with 98 pct bainite and 2 pct martensite/austenite resulted in high strength and ductility. Alternatively the corrosion behavior obtained by polarization curves was characterized in 0.1 M H₂SO₄, 3 M H₂SO₄, 3.5 wt pct NaCl, and NS4 solutions resulting in similar magnitudes, while the corrosion behavior acquired by electrochemical impedance spectroscopy had slightly differences in 3 M H₂SO₄.     

回火对Si-Mn-Mo系贝氏体钢组织性能的影响

研究了淬火后不同温度回火对Si-Mn-Mo系贝氏体钢显微组织与力学性能的影响.结果表明,采用淬火后回火的工艺可以显著提高Si-Mn-Mo系贝氏体钢的强度和塑性.淬火后300℃回火与350℃回火,该钢的力学性能相差不大,而450℃回火后强度、硬度相对较低,韧塑性略有提高.组织观察表明,该钢为贝氏体铁素体和残余奥氏体(片状和块状M-A岛)的复合组织,适当温度回火可以促进块状M-A岛分解,增加板条铁素体含量,提高残余奥氏体的机械稳定性,进而稳定其组织性能..     

Influence of Microstructure on the Mechanical Properties of a Pearlitic Steel

Metallurgical and Materials Transactions A - The effect of austenite grain size on microstructure and mechanical properties was studied for a pearlitic steel. The best combination of ultimate...     

Influence of Mo Content on Microstructure and Mechanical Properties of Fire-Resistant Construction Steel

In order to ensure fire-resistant property of the steel under the condition of the least molybdenum content,effects of molybdenum on microstructure and properties of the fire-resistant construction steel,specially on high temperature strength were investigated.The results show that the room temperature and high temperature strength of the steel is enhanced due to increased Mo content but the impact toughness deteriorates at the same time.The fraction of bainite in the microstructure increases with increasing Mo content while the fractions of polygonal ferrite and pearlite decrease.Comparing with the steel of adding Nb alone,it was obvious that combined addition of Mo and Nb could provide better effect in high temperature strengthening.Addition of 0.25mass% Mo in the steel containing Nb could meet fire-resistant requirements of the construction steel and provide better economical effectiveness.     

Simultaneous Increase of Both Strength and Ductility of Medium Mn Transformation-Induced Plasticity Steel by Vanadium Alloying

In general, microalloying such as vanadium (V) is considered to deteriorate the mechanical stability of austenite grains in medium Mn steel due to the consumption of C content. In this paper, we show that the mechanical stability of austenite grains could be optimized by V-alloying. This is because the V-alloying will refine the austenite grain size and tends to stabilize the austenite grains. The competition between the grain refinement and the reduced C content results in proper mechanical stability of austenite grains, providing continuous transformation-induced plasticity (TRIP) effect. In addition, the V-alloying suppresses the formation of intergranular cracks, leading to a ductile fracture morphology and a large non-uniform elongation.