Grain Refinement Of Steel Research Articles

Microstructural Modifications of Dual‐Phase Steels: An Overview of Recent Progress and Challenges

Optimization of the microstructure and its effect on the strength and ductility of the steel is one of the main aspects of the researcher's effort toward production of advanced structural materials. There have been various investigations focusing on the thermomechanical treatment and grain refinement of steel while some researches are dealing with the modification of chemical composition. This article considering major parameters influencing the mechanical properties of steels aims to present an overview of the outstanding achievements in improvement of the dual‐phase steels. Following the obtained consensus, some advanced promising microstructural modification strategies are also suggested, which can bring new insights to shed light upon the further enhancement of the mechanical behavior of the dual‐phase steels.

Thermal Mechanisms of Grain Refinement in Steels: A Review

Thermal mechanisms of microstructural refinement in steels were reviewed. These include thermal cycling, martensite process, and static recrystallization (SRX), in which the dominant stage of microstructural refinement is governed by an annealing treatment of a deformed or an undeformed material. Recent progress in grain refinement by thermal cycling for the body-centered cubic, face-centered cubic, and dual phase (DP) steels was introduced. The application of the cold rolling and subsequent annealing of a martensite starting structure (martensite process) for grain refinement of low-carbon and DP steels was reviewed. The formation and reversion of strain-induced martensite in metastable austenitic stainless steels and their effects on the microstructural evolutions were critically discussed. Moreover, the repetition of the martensite process and its limitations were explained. Important findings on the SRX of ferrite and austenite for grain refinement as well as the recrystallization kinetics were presented. Finally, the concepts of controlled rolling for grain refinement and the interaction of austenite recrystallization and strain-induced precipitation in microalloyed steels during thermomechanical processing were also reviewed. This short overview presents the opportunities that the conventional and innovative processing routes can offer for grain refinement of steels.

Roles of Lanthanum and Cerium in Grain Refinement of Steels during Solidification

Refinement of as-cast structures is one of the most effective approaches to improve mechanical properties, formability, and surface quality of steel castings and ingots. In the past few decades, addition of rare earths (REs), lanthanum and cerium in particular, has been considered as a practical and effective method to refine the as-cast steels. However, previous reports contained inconsistent, sometime even contradictory, results. This review summaries the major published results on investigation of the roles of lanthanum or/and cerium in various steels, provides reviews on the similarity and difference of previous studies, and clarifies the inconsistent results. The proposed mechanisms of grain refinement by the addition of lanthanum or/and cerium are also reviewed. It is concluded that the grain refinement of steels by RE additions is attributed to either heterogeneous nucleation on the in-situ formed RE inclusions, a solute effect, or the combined effect of both. The models/theories for evaluation of heterogeneous nucleation potency and for solute effect on grain refinement of cast metals are also briefly summarized.

Grain refinement of non-magnetic austenitic steels during asymmetrical hot rolling process

Asymmetrical hot rolling (ASHR) was proposed to acquire productive grain refinement for Fe-20Mn-4Al-0.3C and Fe-18Cr-18Mn-0.5N non-magnetic austenitic steels. The intensive additional shear deformation caused by ASHR promotes the nucleation of recrystallization and grain refining of steel plates. With the speed ratio of 1.2, the austenitic grains were refined to ∼5μm on the surface, the recrystallization fraction was enhanced to ∼34.7%, and the thickness of fine-grained surface layer increases to ∼450μm for Fe-20Mn-4Al-0.3C steel. The Fe-18Cr-18Mn-0.5N steel also exhibited an effective surface grain refinement with an average size of ∼3μm, and the recrystallization fraction reached ∼76.9% at the speed ratio of 1.15.

Grain Refinement and Deformation Behaviour of Medium Carbon Steel Processed by ECAP

The work presents the results on grains refinement of steel containing 0,45 wt pct carbon resulted from severe plastic deformation (SPD). Different steel structures from prior solutioning and/or thermomechanical treatment were prepared for deformation experimental. A coarse grain ferrite-pearlite structure was achieved applying solutioning. By application of thermomechanical (TM) controlled forging process, performing multistep open die forging, the refined ferrite-pearlite mixture was prepared. Final structure refinement of steel, having different initial structure, was then accomplished applying warm Equal Channel Angular Pressing (ECAP) at 400°C. Employment of this processing route resulted in extensive deformation of ferrite grains and cementite lamellae fragmentation. Applying the highest shear stress (εef - 4) the mixed structure of subgrains and ultrafine grains was present within the ferrite phase. In pearlite grains, modification of cementite lamellae due to shearing, bending, twisting and breaking was found efficient. The coarse cementite lamellae spheroidization was more efficient in prior TM treated steel. The tensile deformation records confirmed strength increase and diversity in strain hardening behaviour.

Prior Structure Modification on Grain Refinement and Deformation Behavior of Medium Carbon Steel Processed by ECAP

The present work, deals with grain refinement of medium carbon steel AISI 1045 (0.45% C), having different initial ferrite–pearlite microstructure resulted from thermal and thermomechanical treatment (TM). The purpose of TM steel processing was to refine ferrite and modify pearlite lamellae structure. The final grain refinement of steel structure was then accomplished during warm Equal Channel Angular Pressing (ECAP) at 400°C. Employment of this processing route, in dependence of the applied effective strain ϵef, resulted in extensive deformation of ferrite grains and cementite lamellae fragmentation. When applying higher shear stress (ϵef = 4) the mixed structure of subgrains and ultrafine grains was formed within ferrite phase, regardless the initial steel structure morphology. In pearlite grains, modification of cementite lamellae due to shearing, bending, twisting, and breaking was found efficient as straining increased. Processes of dynamic polygonization and recrystallization in deformed structure also contributed to submicrocrystalline grains formation in deformed structure. Comparing results the course of lamellae cementite spheroidization was then more efficient in prior TM treated steel. The tensile deformation results confirmed the strength increase, however deformation behavior and strain hardening generally for different initial structural conditions of steel, showed diversity.

The effect of bainite in producing nano/ultrafine grained steel by the martensite treatment

The martensite treatment including cold rolling and subsequent annealing of microstructures with high percentage of martensite is one of the most effective thermo-mechanical methods for grain refinement in steels. In the present work, the effect of bainite phase on the formation of nano/ultrafine grained structure in a low alloy 0.2% C steel was investigated. The martensitic and bainitic–martensitic microstructures were first produced in the specimens by quenching in water and oil from the austenitizing temperature, respectively. The specimens were subjected to 90% reduction in thickness by cold rolling and subsequent annealing. The formation of the nano/ultrafine grained structure was discussed from the viewpoint of characteristics of the martensite and bainite starting structure. It was found that nano/ultrafine grained structures with similar hardness and mean ferrite grain size can be produced in both specimens by annealing 90% cold rolled specimens at 550 °C for 80 min. However, due to the lower hardness of the bainite phase, cold rolling of the bainitic–martensitic microstructure was much more facile than the martensitic one, resulting in no transverse cracks in the cold rolled specimen.

Effect of Preliminary Treatment on Grain Refinement of Medium Carbon Steel Using ECAP at Increased Temperature

By application of thermomechanical controlled rolling and accelerated cooling, the carbon steel grain refinement is limited to levels of ~ 5 μm in steels. The strain assisted or strain induced transformation could be considered for the refining process. The present work, likewise, deals with grain refinement of medium carbon steel containing 0.45 wt pct carbon having different initial microstructure modified by either thermal and/or thermomechanical treatment (TM) prior severe plastic deformation. In case of TM treated steel, structure refinement was conducted in two steps. Preliminary structure refinement has been achieved due to multistep open die forging process which provided total strain of 3. Uniform and fine recrystallized ferrite structure with grain size of the order of 2-5 μm and with nest-like pearlite colonies was obtained. The further grain refinement of steel samples having different initial structure was accomplished during warm Equal Channel Angular Pressing (ECAP) at 400°C. The steel samples of different initial structure were then subjected to six ECAP pressing passes through die channel angle of 120°. The microstructure development was analyzed in dependence of effective strain introduced (εef ~ 2.5 - 4). Employment of this processing route resulted in extensive deformation of ferrite grains where mixture of subgrains and ultrafine grain was found regardless the preliminary treatment of steel. As straining increases the dynamic polygonization and recrystallization became active to form mixture of polygonized subgrains and submicrocrystalline grains having high angle boundaries. The straining and moderate ECAP temperature caused the partial cementite lamellae fragmentation and spheroidization as straining increased. The lamellae cementite spheroidization was more extensive in TM treated steel samples. The tensile behavior was characterized by strength increase for both structural steel states; however the work hardening behavior was modified in steel where preliminary TM treatment was introduced to modified coarse ferrite-pearlite structure.

Ultrafine grained structure development in steel with different initial structure by severe plastic deformation

The present work deals with grain refinement of medium carbon steel, having different initial microstructure, modified by either thermal and/or thermomechanical treatment (TM) prior severe plastic deformation. In case of TM treated steel, structure refinement was conducted in two steps. Preliminary structure refinement has been achieved due to multistep open die forging process which provided total strain of 3. Uniform and fine recrystallized ferrite structure with grain size of the order of 2-5 μm and with nest-like pearlite colonies was obtained. The further grain refinement of steel samples having different initial structure was accomplished during warm Equal Channel Angular Pressing (ECAP) at 400°C. The microstructure development was analyzed in dependence of effective strain introduced (εef ~2.5 - 4). Employment of this processing route resulted in extensive deformation of ferrite grains where mixture of subgrains and ultrafine grain was found regardless the preliminary treatment of steel. The straining and moderate ECAP temperature caused the partial cementite lamellae fragmentation and spheroidization as straining increased. The cementite lamellae spheroidization was more extensive in TM treated steel samples. The tensile behavior was characterized by strength increase for both structural steel states; however the work hardening behavior was modified in steel where preliminary TM treatment was introduced to modified coarse ferrite-pearlite structure.

Ultrafine-grained steel fabricated using warm multiaxial forging: Microstructure and mechanical properties

Grain refinement of bulk metals using severe plastic deformation (SPD) is a popular approach to improve both strength and toughness. In this paper, grain refinement of steel processed by warm multiaxial forging (MAF) and its mechanical behavior has been investigated. Coarse-grained, plain low carbon steel was deformed using MAF at 500 °C. Microstructural evolution is characterized using electron backscattered diffraction and mechanical behavior has been studied. Fraction of low angle grain boundaries (LAB) is observed to increase with strain up to total engineering strain of 1.3 thereafter it starts decreasing whereas, high angle grain boundaries showed just the opposite trend. It appears that initially grain subdivision takes place with imposition of strain thereby increasing the fraction of LAB. After a critical strain these LAB transforms into the high angle boundaries (HAB). The initial coarser grains of average 30 μm size subdivided into grains of the size finer than 0.5 μm. This has been confirmed by TEM micrographs. Improved tensile strengths and hardness values are obtained after warm MAF.

Grain Refinement in Steels and the Application Trials in China

In order to control microstructure and performances of steel products, the investigation has been taken to the ways to refine grains in steels and the mechanisms, the availability in industrial production of ultrafine grain steel products, and the applications of ultrafine grain steel products. It is shown that strength of steels can be increased without detriment to ductility when ferrite grains are refined to less than 4 μm in low carbon steels. Based on the study of deformation induced transformation, a new rolling technology has been developed to refine ferrite grains in plain low carbon steel rebar and strip. Plain low carbon steel rebars of 400 MPa grade, with ferrite grains in the range of 4-8 μm, have been developed as the candidate to replace microalloyed steel rebars. High strength plain low carbon steel strips in the yield strength range from 345 to 420 MPa have been used for components conventionally made of microalloyed steels. Ultrafine grained microalloyed steel strip has also been developed. A new on-line cementite spheroidization softening technology has been studied for the wire rod production of medium carbon bolt steel. It is obvious that deformation could also induce pearlite transformation in eutectoid steel.

Behavior of Inclusions in Deoxidation Process of Molten Steel with <i>in Situ</i> Produced Mg Vapor

In the deoxidation process of molten steel with magnesium vapor produced in situ by aluminothermic reduction of magnesium oxide, the formed inclusions are usually small in size and of spherical shape. They tend not to aggregate and form cluster. The MgO content in the inclusions decreases, while the SiO 2 content increases with the progress of experiment. Larger inclusions are removed by floating up with the rising bubbles more easily and the fine inclusions tend to remain in the melt. The oxygen concentrations originated from the inclusions can be reasonably explained from the experimental analysis ones. By use of the MgO porous immersion tube, deoxidation proceeds slowly at the initial stage, but it continues until the later stage of experiment, in contrast to that using the dense Al 2 O 3 immersion tube. The formed inclusion number per unit area has a larger value than that with the dense Al 2 O 3 tube. The number of inclusions also increases with increasing the initial oxygen concentration and dividing pellets charging into several portions. When the carrier gas flow rate is large, the number of inclusions tends to increase at the later stage of experiment due to the strong involvement of inclusions from the melt surface into the melt. In the present deoxidation process, it is considered that the rapid removal of larger inclusions by rising bubbles is beneficial to produce high cleanness steel, while the remaining fine inclusions is helpful for the grain refinement of steel during the solidification and phase transformation process.

Application of Severe Torsion Straining Process for Grain Refinement of Steel

Application of Severe Torsion Straining Process for Grain Refinement of Steel

Grain Refinement in Steels through Thermomechanical Processing

The development of ultrafine grained microstructures in steels has received considerable attention in recent times. In many cases the aim is to produce high strength structural steels with minimal alloying. It is well established that for an equiaxed ferrite with a uniform dispersion of second phase, both the strength and toughness will be markedly improved if the grain size can be reduced to 1-2 µm, from the typical range of 5-10 µm. Means of achieving this through dynamic strain induced transformation are examined here, following a brief overview of some of the key issues encountered when attempting to refine the austenite in existing mill configurations. A number of deformation microstructure maps are developed to aid the discussion.

Effect of Copper on Tensile Properties and Grain-Refinement of Steel and its Relation to Precipitation Behavior

Progress of Ferrous Nano-Metal Project is introduced in the present paper. In the project, maximum use of copper clusters and precipitates is pursued for achieving better strength-ductility balance than that of conventional high-tensile strength steels. Fundamental aspect of clustering and precipitation of Cu in Fe-Cu alloys was studied using Optical Tomographic Atom-Probe (OTAP). It was found that Cu precipitation during aging was enhanced by plastic deformation. The observed Cu precipitation behavior was well related to the age-hardening behavior, that is, aging started at lower temperature and maximum hardness was higher for plastically deformed and aged ferrite. Aging behavior and associated tensile properties were further examined for Fe-C-Mn-Cu martensitic steel. Higher value of tensile strength times elongation was achieved in FeC-Mn-Cu steel than Fe-C-Mn steel. Finally, effect of Cu precipitation on grain-refinement of ferrite was studied for Fe-C-Mn-Cu steel. Ferrite grain smaller than 1 mm was obtained in both processes of strain-assisted ferrite transformation from heavily deformed austenite and dynamic recrystallization of heavily deformed ferrite. Ferrite grain size was found to decrease by addition of Cu at the both processes. It was suggested that a simple additivity rule does not hold in terms of the strengthening by grain-refinement and that by precipitation, especially at grain-size range less than 1 mm.

Grain refinement of steel by oxidic second phase particles

Steels manufactured by a mechanical alloying and hot pressing route, to give a fine dispersion of alumina particles, have shown complete inhibition of grain growth after heating to 1350°C for 30 min, and in the near fusion line region of the heat affected zone of a bead on plate weldment. Comparison with theoretical predictions have shown that, with the 10 μm radius of the as transformed austenite grains, the observed grain growth inhibition would be predicted. The efficacy of these fine dispersions can be attributed to the high stability of the alumina which results in extremely small levels of dissolution of alumina in austenite, and which also severely curbs the coarsening of the dispersed alumina particles.

Grain refinement of steel by cyclic rapid heating

A recently developed technique of grain refinement by rapid cyclic heat treatment has been applied to a commercial medium carbon steel. The changes in the microstructure, microfracture and tensile behavior as related to the reduced grain size are reported. An ultimate refined grain size of about 4 μ is achieved after four cycles. The nature of dimple fracture as revealed by microfractography indicates that the increased yield stress is associated with improved ductile fracture behavior.