Ultrafine Ferrite Grains Research Articles

Ultra-Fine Grain Size by Dynamic Recrystallization in Strip Rolling of Nb Microalloyed Steel

The design of base chemistry and optimization of rolling schedule are the two important factors that influence large strain accumulation in multi-pass rolling in order to obtain ultra-fine grain size by dynamic recrystallization. A base chemistry of 0.03C-0.003N-0.08Nb-0.015Ti-1.8Mn (all in weight %) of HTP steel design was chosen in order to control the time evolution of strain induced precipitation of NbC and the strain accumulation through precipitate interaction with recovery and recrystallization at short inter-pass times characteristic of strip rolling. Experimental data on the critical strain for static and dynamic recrystallisation for HTP steel are used in a quantitative model to predict strain accumulation pass by pass and to achieve grain refinement by dynamic recrystallisation through large strain accumulation. The model is used to optimize the time-temperature-deformation schedule to prevent static recrystallization during the inter-pass times and to target ultra-fine grain size through dynamic recrystallization by large strain accumulation. The model predictions are validated by simulation of strip rolling of HTP steel on the thermo-mechanical simulator (WUMSI) to obtain a uniform ultra-fine ferrite grain size of about 1.5 micrometer diameter in final ferrite microstructure.

Development of an Ultrafine Grained Ferrite in a Low C-Mn Steel Intensely Deformed under Hot Torsion

In the last years, several studies concerning ultra refinement of ferrite grains have been conducted using different experimental techniques (ECAP, ARB, HPT). The aim of all investigations was to provide an optmized relationship between mechanical properties and microstructure of steels. The present work, likewise, deals with strain induced dynamic transformation of ferrite. Samples of low C-Mn steel were intensely deformed in hot torsion aiming at the production of ultrafine grains of ferrite thereby enhancing the mechanical properties when compared to hot rolled products. After soaking during 5min at 900°C, the samples were quenched and then reheated and submitted to hot torsion deformation at temperatures of 700 and 740°C. The torsion schedule consisted of 7 isothermal passes leading to a total strain of ≈1, generating an ultrafine microstructure with grain sizes of the order of 1µm. The shape of stress-strain curves so obtained suggested that ferrite refinement occurred by dynamic recrystallization. The various constituents present in the microstructure as well as ferrite grain size and morphology were examined by optical and scanning electron microscopy. Microhardness tests were performed to evaluate mechanical properties.

Grain boundary characterization and grain size measurement in an ultrafine-grained steel

Abstract Ultrafine ferrite grains in a plain C–Mn steel (0.3 mass% C) were produced by large-strain warm compression and subsequent annealing treatment in a temperature range between 773 K and 1003 K. The samples were investigated by means of high-resolution electron back-scatter diffraction. The resulting microstructures showed very fine ferrite grains and homogeneously distributed cementite particles. The majority of the grain boundaries (55–70 %) were classified as high-angle ones (≥ 15° misorientation). When considering only these high-angle grain boundaries, the average grain size changed from 0.9 mm at a deformation temperature of 773 K to 2.2 mm at a deformation temperature of 1003 K. For the same range the average subgrain sizes increased from 0.6 mm to 1.5 mm. The basic result of this study is that the grain size characterization of polycrystalline microstructures with ultrafine grains requires the use of the high-resolution electron back-scatter diffraction method in conjunction with a careful a...

Fiber texture and substructural features in the caliber-rolled low-carbon steels

Caliber rolling at the recrystallization temperatures of ferrite is a new process that was developed to fabricate an ultrafine-grained microstructure for low-carbon steels. In the present investigation, the electron backscattered diffraction (EBSD) measurement was carried out to characterize thoroughly the texture and substructural features in two caliber-rolled low-carbon steels, with special attention on the effects of a phosphorus addition to the steel and the annealing treatment after rolling. Finer ferrite grains appeared in the phosphorus-added steel under the same rolling condition. The phosphorus addition caused also the stronger //rolling direction (RD) fiber texture in the caliber-rolled steel bars and, hence, showed a larger average Taylor factor than the steel without phosphorus. Microband features within the ultrafine ferrite grains were characterized with both transmission electron microscopy (TEM) observation and orientation-imaging micrograph (OIM) analysis. Nearly half of the low-angle boundaries, whose kernel average misorientation was larger than 0.8 deg, were found to have the planar character and were specifically parallel to the {110} or {112} planes. In the as-rolled condition, the total volume fraction of the low-angle boundaries was 0.3 and 0.23 in the steels with and without a phosphorus addition, respectively. More {112}-type planar boundaries were observed than the {110} type boundaries in both steels. Annealing treatment increased the volume fraction and changed the type of the low-angle boundaries in both steels. Using the model proposed by Peeters et al., the critical resolved shear stress (CRSS) of the 110 and 112 slip systems was calculated by considering the contributions of both the randomly distributed dislocations and the oriented planar boundaries. We concluded that the contribution of the planar low-angle boundaries to the total CRSS was less than 2 pct. The texture features and dislocation structure in the ultrafine-grained steels influenced the mechanical behavior to some extent, in addition to the refined ferrite grains. Phosphorus added to the steels showed a larger influence on the formation of texture and the retained dislocation structure, than that on the grain refinement in the caliber-rolling process.

Effect of rolling reduction on ultrafine grained structure and mechanical properties of low-carbon steel thermomechanically processed from martensite starting structure

The present authors have invented a novel and simple thermomechanical processing to realize the ultrafine grained microstructure in carbon steels. The key of the process is to start from martensite structure. In the previous study, it has been clarified that conventional cold-rolling to a reduction in thickness of only 50% (equivalent strain of 0.8) and subsequent annealing at warm temperature around 500 °C fabricates the multi-phased ultrafine grained structure composed of the ultrafine ferrite grains with mean grain size of 180 nm, uniformly precipitated nano cementite and tempered martensite. In this study, the effect of the rolling reduction ranging from 25 to 70% (equivalent strains of 0.3–1.5) on the ultrafine grained structure and the mechanical properties of the plain low-carbon steel (Fe–0.13 wt% C) processed from martensite starting structure was studied. In the as-deformed specimen, the area fraction of the region showing the lamellar structure, which is typical for severely rolled metals, increased with increasing the rolling reduction and the strength also increased. After annealing at warm temperature around 500 °C, the multi-phased ultrafine grained microstructures were obtained in all the examined rolling reductions. The area fraction of the region showing the ultrafine ferrite grains increased with increasing the rolling reduction. At higher temperature, conventional recrystallization took place, and the recrystallization temperature became lower with increasing the reduction. Tensile test exhibited that the specimen rolled to the intermediate reduction (50%) performed the best strength-ductility balance (870 MPa of tensile strength and 9% of uniform elongation). The reason for the good strength-ductility balance of the specimen rolled to the intermediate reduction was discussed on the basis of the observed microstructures.q 2003 Elsevier Ltd. All rights reserved.

超狭開先アーク溶接における超細粒鋼熱影響部の特性―超細粒鋼溶接熱影響部の軟化域とミクロ組織（第１報）―

800 MPa high strength steel with a low carbon equivalent has been developed by means of refining ferrite grains. In the welded joints of the 800 MPa steel with ultra-fine ferrite grains (less than 1 μm) and dispersed fine cementite particles, fine ferrite grains coarsen due to the welding heat input, and then remarkable softening occurs in the heat-affected zone. For the ultra-fine grained steel (UFG steel), ultra-narrow gap gas metal arc (UNG) welding was utilized as a low heat input welding process. By the UNG welding process with the cooling time from 1073 K to 773 K of about 4.5 sec, the width of the softened region in the heat-affected zone (HAZ) of the ultra-fine grained steel can be minimized to below 2 mm. In order to clarify the softening behavior in the HAZ of UFG steel, the effect of welding thermal cycle on the microstructures of UFG steel was discussed on both the HAZ of the welded joint of UNG welding and its synthetic HAZ, and the results were also compared with the HAZ of SM490 steel. The peak temperature at each point of HAZ was estimated from thermal conduction theory and the measured data. Slightly coarsened ultra-fine ferrites (fine grain A) and larger polygonal ferrites (fine grain B) can be observed in the regions of the peak temperatures from 920 K to Ac3 (1150 K). It was found that the softening in the HAZ of UFG steel occurs within the peak temperature range of about 920 K (below Ac1=980 K) to 1300 K, and the softening in the regions of the peak temperatures from 920 K to 990 K is dependent on the grain size of the larger polygonal ferrite grains (fine grain B). M-A constituents begin to form when the peak temperature exceeds Ac1, and its volume fraction increases with the peak temperature. As second hard phase, M-A constituents restrain the softening of the HAZ in the peak temperature range from about 1000 K to 1250 K. It is known from the results that the hardness in the HAZ has the lowest value at the peak temperature of 1100 K.

Effect of Heat Treatment on Microstructures and Tensile Properties of Ultrafine Grained C-Mn Steel Containing 0.34 mass% V

An ultrafine grained (UFG) C-Mn steel containing a relatively large amount of vanadium was fabricated by equal channel angular pressing (ECAP) and its microstructures and tensile properties were examined. This investigation was aimed at demonstrating the effect of precipitation stage of vanadium precipitates in the course of the material processing on the tensile properties of the ultrafine grained C-Mn steel, especially strength and strain hardening capability. For this purpose, the two different heat treatments were carried out on the present steel: (a) conventional normalization for vanadium precipitation before ECAP, and (b) isothermal transformation for vanadium precipitation during ECAP and subsequent annealing. The results showed that the second heat treatment was more effective on improving the thermal stability and the overall tensile properties of the steel by better uniform distribution of nano-sized vanadium precipitates which yielded an extensive interaction with lattice dislocations inside ultrafine ferrite grains. In addition, in this report, the feasibility enhancing the strain hardening capability of the UFG C-Mn steel was explored by comparing the microstructure and stress-strain curve of the steel prepared by the second heat treatment with those of the UFG C-Mn steel without vanadium.

Effect of Deformation Temperature and Strain Rate on Evolution of Ultrafine Grained Structure through Single-Pass Large-Strain Warm Deformation in a Low Carbon Steel

Ultrafine grained structure formed dynamically through a severe warm deformation in the temperature range from 773 K to 923 K has been investigated in a 0.16%C-0.4%Si-1.4%Mn steel. The effects of the deformation conditions such as deformation temperature and strain rate on microstructural evolution were examined using a single-pass compression technique with a pair of anvils. A large plastic strain up to 4 was imposed on the specimen interior at a strain rate of 1 or 0.01 s−1. Ultrafine ferrite grains surrounded by high angle boundaries, whose nominal grain size ranged from 0.26 to 1.1 μm, evolved when the equivalent plastic strain exceeded the critical value about 0.5 to 1, and increased with an increase in strain without any large-scale migration of high angle boundaries. The effects of deformation conditions on microstructural evolution of ultrafine grained structures can be summarized into the Zener-Hollomon(Z-H) parameter dependences. The average size and the volume fraction of newly evolved ultrafine grains depend on the Z-H parameter. Decreasing Z-H parameter enhances the formation of equiaxed ultrafine grains. These indicate that the mechanism forming ultrafine grained structures through the warm severe deformation in the present study is similar to “continuous recrystallization” or “in-situ recrystallization” and that some activation process during or after the deformation plays an important role in the microstructural evolution.

Formation of Ultrafine Grained Structure in Plain Carbon Steels Through Thermomechanical Processing

In the present study, wedge-shaped samples were used to determine the effect of nominal equivalent strain (between 0 and 1.2) and carbon content (0.06—sh0.35%C) on ferrite grain refinement through dynamic strain-induced transformation (DSIT) in plain carbon steels using single-pass rolling. The microstructural evolution of the transformation of austenite to ferrite has been evaluated through the thickness of the strip. The results showed a number of important microstructural features as a function of strain which could be classified into three regions; no DSIT region, DSIT region and the ultrafine ferrite (UFF) grain region. Also, the extent of these regions was strongly influenced by the carbon content. The UFF microstructure consisted of ultrafine, equiaxed ferrite grains (<2 μm) with very fine cementite particles. In the centre of the rolled strip, there was a conventional ferrite-pearlite microstructure, although ferrite grain refinement and the volume fraction of ferrite increased with an increase in the nominal equivalent strain.

Phosphorus-induced dislocation structure variation in the warm-rolled ultrafine-grained low-carbon steels

Dislocation structures in warm-rolled low-carbon steels with ultrafine ferrite grains (∼1 μm) are analyzed by both transmission electron microscopy (TEM) observation and the modified Warren–Averbach procedures for X-ray diffraction profiles. In good accordance with the TEM observation results, the dislocation density is derived by the X-ray diffraction profile analysis to be 1.06×10 14 and 1.98×10 14 m −2, in the as-rolled 0.15C and 0.15C–0.1P steel, respectively. In contrast to the straight dislocation segments in the 0.15C steel, tangled and curved dislocations are observed in the phosphorus steel at both the as-rolled and annealed conditions. The larger q value and smaller dislocation arrangement parameter, M=R e ρ , are observed in the phosphorus steel. Phosphorus in the low-carbon steels tends to prompt the ratio of screw dislocation component and the dislocation arrangement with a stronger interaction during plastic deformation. Not only the dislocation density, but also the dislocation interaction behaviors control the changes of yield strength during annealing the as-rolled steels at 723 K. The modified Warren–Averbach X-ray diffraction profile analysis may be useful in characterizing the dislocation interaction behavior quantitatively.

Processing of ultrafine ferrite steels

Recently, the grain refinement of metallic materials has been investigated intensively. There are two basic methods, i.e., severe plastic deformation and thermomechanical processing, to refine grain sizes of bulk steels for the ultrafine or submicron range. By severe plastic deformation submicron-grained microstructure (less than 1 μm) in steels can be obtained. For example, the microstructure with ultrafine grains of 0.3 μm in a commercial 0.15 wt.%C–1.1 wt.%Mn–0.25 wt.%Si steel was obtained after ECAP at an accumulative strain of about 4, and the microstructure with ultrafine grains of 0.4 μm in a Ti-added interstitial free (IF) steel was obtained after ARB at an accumulative strain of about 5.6. As a result of thermomechanical processing for the grain refinement, the ultrafine ferrite (UFF) grains (about 1 μm) of steels could be obtained through hot rolling in the intercritical region, warm rolling in the ferrite region, or strip rolling under the condition of strain-induced transformation from austenite to ferrite. A composite microstructure, consisting of UFF grains in the surface layers penetrated to about one-quarter to one-third of thickness and coarser ferrite (about 5–10 μm) and pearlite in the core of the strip, is introduced by strain-induced transformation. Finally, the relationship of strength and ductility is discussed. The strength of ultrafine-grained steels increases with decreasing ferrite grain sizes, following the Hall–Petch relationship. On the other hand, ductility decreases with decreasing ferrite grain sizes. The composite microstructure of UFF surface and coarser core in steels has an advantage to keep both high strength and high ductility.

Mechanical Properties of Welded Joints of 780 MPa Grade Ultra-fine Grained Steels

Three kinds of 780 MPa grade ultra-fine grained steels with different chemical composition were produced by warm rolling. The steels were characterized by ultra-fine ferrite grains (less than 1 μm). With the steels strength-overmatching welded joints were prepared, and their mechanical properties were investigated. It is found that softening occurred in the heat-affected zone (HAZ) because of the coarsening of ferrite grains due to welding heat input. However, by using low welding heat input and strength-overmatching weld metal, the detrimental effect of softening on strength was restrained, and welded joints with strength equivalent to that of base metal were obtained. The deformability of welded joints was found to be related to the yield ratio (yield strength / tensile strength) of base metal. Low yield ratio is desirable to the deformability of welded joints. The HAZs in the welded joints of low welding heat input of 10 kJ/cm have good impact toughness for all the steels. Except for 0.14C-0.30Si-1.46Mn steel, the HAZs in the welded joints formed from the other two steels also have good impact toughness for welding heat input of 20 kJ/cm, and their fracture appearance transition temperature (vTrs) of the HAZs is lower than -40°C, and their Charpy impact energy at -40°C exceeds 200 J.

温間圧延によって製造した超微細フェライト-セメンタイト組織鋼板の特性

温間圧延によって製造した超微細フェライト-セメンタイト組織鋼板の特性

Formation of Ultrafine Ferrite by Strain-induced Dynamic Transformation in Plain Low Carbon Steel

Strain-induced dynamic transformation (SIDT) is of great advantage to obtaining ultrafine-grained ferrite in low carbon steels partly by early impingement of ferrite nuclei which are very rapidly and concurrently formed due to the deformation and partly by random crystallographic orientation distribution of ferrite grains. The SIDT fraction increases with the increase of strain. There is, however, a critical strain under which no SIDT occurs. In order to apply this refining mechanism to actual production lines, it is necessary to reduce the critical stain and enhance the kinetics of SIDT. It has been known that refining prior austenite grain is the most effective for this purpose. The influences of deformation temperature and strain rate on SIDT were also examined. Because SIDT is a kind of softening mechanism of strained austenite, it competes with dynamic recrystallization (DRX) at below Ae3 temperature. Comparing the critical strains of SIDT and DRX, SIDT is predominant softening mechanism, which enables us to utilize it for grain refinement. This provides an important clue to overcome the limitation of conventional thermomechanical control process (TMCP) in grain refinement area.

Formation of ultra-fine ferrite microstructure in warm rolled and annealed C–Mn steel

Abstract Laboratory simulations of warm rolling followed by intercritical annealing of a low carbon 0.15%C–1.39%Mn steel have been performed. The effects of austenitising temperature, amount of deformation and annealing time on microstructure and mechanical properties have been investigated. The results have shown that the homogeneous ultra-fine ferrite grain microstructure (∼1.1–1.2 μm average grain size) has been achieved after austenitising at 900 °C, warm rolling with 3 passes of 20% reduction each at 700 °C and annealing at 800 °C for 60 min. This correlates to the 20% improvement in mechanical properties compared to traditional industrial hot rolled steel.

The production of ultrafine ferrite during hot torsion testing of a 0.11 wt pct C steel

Ultrafine ferrite grain sizes were produced in a 0.11C-1.6Mn-0.2Si steel by torsion testing isothermally at 675 °C after air cooling from 1250 °C. The ferrite was observed to form intragranularly beyond a von Mises equivalent tensile strain of approximately 0.7 to 0.8 and the number fraction of intragranular ferrite grains continued to increase as the strain level increased. Ferrite nucleated to form parallel and closely spaced linear arrays or “rafts” of many discrete ultrafine ferrite grains. It is shown that ferrite nucleates during deformation on defects developed within the austenite parallel to the macroscopic shear direction (i.e., dynamic strain-induced transformation). A model austenitic Ni-30Fe alloy was used to study the substructure developed in the austenite under similar test conditions as that used to induce intragranular ferrite in the steel. It is shown that the most prevalent features developed during testing are microbands. It is proposed that high-energy jogged regions surrounding intersecting microbands provide potential sites for ferrite nucleation at lower strains, while at higher strains, the walls of the microbands may also act as nucleation sites.

Ultragrain refinement of plain low carbon steel by cold-rolling and annealing of martensite

Simple cold-rolling and annealing of martensite starting structure can produce ultrafine grained structure in carbon steel. The microstructural evolution during the process was studied in a 0.13%C steel. The ultrafine lamellar dislocation cells (LDCs) with mean thickness of 60 nm were mainly observed in a 50% cold-rolled specimen as well as the irregularly bent lamellas (IBLs) and the kinked laths (KLs). The LDCs and the IBLs had large local misorientations. The specimens annealed at temperatures from 723 to 773 K showed the multiphased ultrafine structure composed of equiaxed ultrafine ferrite grains with the mean grain size of 180 nm, nano-carbides distributed uniformly and small blocks of tempered martensite. The formation of the ultrafine grained structure was discussed from the viewpoint of characteristics of the martensite starting structure. It was concluded that the fine grained structure of martensite play an important role for ultrafine grain subdivision during plastic deformation.

Important factors to obtain homogeneous and ultrafine ferrite–pearlite microstructure in low carbon steel

The microstructural evolutions during compressions at different temperatures and strains were investigated using a Gleeble-1500 test machine for a low carbon steel, with and without the formation of pre-eutectoid ferrite. The physical processes that occurred during heavy deformation were discussed by observing the microstructure and analyzing the true stress–true strain responses. The important factors to obtain the equiaxed, homogeneous and ultrafine ferrite, include the initial microstructure before deformation, the deformation temperature and the true strain imposed. The refinement mechanisms for attaining the ultrafine-grained ferrite were discussed in terms of the strain-assisted transformation (SAT) of austenite to ferrite and the dynamic recrystallization of ferrite. The adequate ferritic microstructure has been obtained by deforming the undercooling austenite with a sufficient strain in a reasonable temperature range. The SAT should preferentially be considered in obtaining the ultrafine ferrite because it occurs more easily than the dynamic recrystallization of ferrite. Decreasing the recovery rate of ferrite and increasing the defect concentration in austenite is beneficial to the formation of ultrafine ferrite.

WITHDRAWN: Effect of pressing temperature on microstructure and tensile behavior of low carbon steels processed by equal channel angular pressing

Abstract Two grades of low carbon steels, one containing vanadium and the other without vanadium, were subjected to equal channel angular pressing at a temperature range of 623–873 K. For steel without vanadium, the equal channel angular pressing at 623 K resulted in ultrafine (∼0.3 μm) ferrite grains with high angle boundaries. At higher pressing temperature, coarser grains with low angle boundaries were formed, which indicates that the recovery occurs at a significant rate during the pressing. For the steel containing vanadium, submicrometer order ferrite grains and high dislocation density were preserved up to pressing temperature of 873 K. The enhanced thermal stability of the steel containing vanadium was attributed to its peculiar microstructure consisted with fine ferrite grains with uniformly distributed nano-sized cementite particles. In addition, the tensile behaviors of the equal channel angular pressed steels were characterized and discussed based on the microstructure.

Ultrafine Grained Ferrite Formed by Interrupted Hot Torsion Deformation of Plain Carbon Steel.

A plain carbon steel was deformed using a hot torsion deformation simulator. A schedule known to produce strain-induced ferrite was used with the strain interrupted for increasing intervals of time to determine the effect of an isothermal hold on the final microstructure. Microscopy and electron back-scattered diffraction (EBSD) were used to analyse the changes that occurred in the partially transformed microstructure during the hold and the subsequent applied strain. The strain-induced ferrite coarsened during the hold and this coarsened ferrite was refined during the second deformation. These results were compared to those obtained for a different plain carbon steel deformed in single pass rolling close to the Ar3 temperature.