Fine Ferrite Grains Research Articles

EFFECT OF DEFORMATION-INDUCED TRANSFORMATION PROCESS ON FERRITE GRAIN SIZE AND VOLUME FRACTION IN STEEL 12CR1MOV

Effect of deformation induced transformation (DIT) process on ferrite grain size and ferrite transformation volume fraction in the refractory low alloy steel 12 Cr 1 MoV by controlled rolling and controlled cooling technology were experimentally studied. Simulation experiments of hot deformation were carried out with the Gleeble-1500 system. Single-pass and multi-pass hot rolling process with different deformation temperature, deformation reduction, strain rate and cooling rate were performed separately. The ferrite grain size decreased and the ferrite volume fraction increased with decreasing deformation temperature, and the extra-fine ferrite grain about 1.01µm was obtained when the deformation temperature reached 780°C. Higher deformation reduction resulted finer ferrite grain size and higher ferrite volume fraction. Both the ferrite grain size and ferrite volume fraction decreased with increasing strain rate. The ferrite grain size decreased but the ferrite volume fraction didn't change much when the cooling rate increased. The grain size and the ferrite volume fraction were improved more by three-passes than two-passes rolling. The mechanism of grain refinement by DIT was discussed.

表層微細粒組織を有する疲労特性に優れた線材製品の開発

表層微細粒組織を有する疲労特性に優れた線材製品の開発

Effect of transformation mechanism (static or dynamic) on final ferrite grain size

A simple series of test was developed to highlight and compare the difference between the static strain induced transformation (SSIT) and the dynamic strain induced transformation (DSIT) mechanism in grain refinement and also to investigate the origin of the difference between the two mechanisms. The results showed that while the SSIT sets up a two-dimensional impingement among the ferrite grains, it cannot avoid their coarsening (normal growth). However, the DSIT forms a group of grains with a three-dimensional impingement which does not coarsen and maintains their fine size throughout the transformation, thereby, reduces the final average grain size.

連続冷却過程における γ→α 相変態のモンテカルロ・シミュレーション

The γ→α transformation during continuous cooling was simulated by Monte Carlo method on conditions of ten stages of cooling rate and nine stages of austenite grain sizes. The authors have investigated effects of cooling rate and initial austenite grain size on γ→α transformation and ferrite grain growth. Main results obtained are as follows. The relationship between ferrite grain sizes and cooling rate showed that the ferrite grain sizes hyperbolically decreased against the cooling rate until 500°C/s and then the ferrite grain refinement hardly occurred. The ferrite grain sizes were decreased with an decrease of the initial austenite grain sizes. γ→α transformation rate was increased with the refinement of initial austenite grain. A linear relationship between initial austenite grain sizes and final ferrite grain sizes was obtained by simulation of continuous cooling.

Mechanical properties of fine-grained dual phase low-carbon steels based on dynamic transformation

The fine grained dual phase (FG-DP) steel with ferrite grains of 2-4.5 μm and martensite islands smaller than 3 μm was obtained through the mechanism of deformation-enhanced ferrite transformation (DEFT). Mechanical properties of the steel were tested at room temperature. The results indicated that with a similar volume fraction of martensite (about 20vol%), FG-DP steel exhibited a superior combination of higher strength and more rapid strain hardening at low strains compared with the coarse-grained dual phase (CG-DP) steel obtained by critical annealing. The combination of higher strength, large elongation, and more rapid strain hardening of FG-DP steel can be attributed to the fine ferrite grain and finely dispersed martensite islands. In addition, the uniformly distributed martensite islands in FG-DP steel have smaller interspacing compared with that of CG-DP steel. So, at the initial plastic deformation stage, the plastic deformation of ferrite was restrained and more pronounced load was transferred from ferrite to martensite. The plastic deformation of martensite in FG-DP steel started earlier.

Development of Ultra-Carbon Steel Prepared by Mechanical Alloying and Subsequent Hot Pressing

It is known that the most effective way to improve mechanical properties of steels is by microstructural refinement. Iron and graphite powders mixture with hypoeutectoid composition were mechanically alloyed (MAed) and subsequently hot pressed (HPed) to provide steels with fine structure. Mechanical Alloying (MA) was carried out using a planetary ball mill for 100, 200 and 300 h, while HP at 41 MPa at various temperatures: 610oC (below), 730oC (near) and 800oC (above the eutectoid transformation point A1). The mechanically alloyed (MAed) powders were characterized by X-ray diffraction (XRD), differential thermal analysis (DTA) and scanning electron microscopy (SEM). While, the HPed compacts were characterized by SEM, Vickers hardness and tensile test. During MA, refinement of crystallite, formations of super-saturated iron solid solution and Fe/C amorphous phase occured first. With further MA time, these phases might begin to transform to more stable phases such as carbides. In the case of HP at 610oC (below A1), very fine cementites were precipitated in fine ferrite grain with sub-micron meter in the size. However, the mechanical properties cannot be attained because of low sinterability. At 730oC (near A1), the strength reaches the maximum value. With further temperature increase (at 800oC), the sintering progressed well and the coarsening occured, resulting the decrease in strength. However, the fracture strain increased significantly. The steels obtained in the present study had mechanical properties comparable to those of standard (JIS) steels through the welle stablished heat treatment such as normalizing and thermal refining.

Influence of hot rolling conditions on the mechanical properties of hot rolled TRIP steel

Influence of hot rolling conditions on the mechanical properties of hot rolled TRIP steel was investigated. Thermomechanical control processing (TMCP) was conducted by using a laboratory hot rolling mill, in which three different kinds of finish rolling temperatures were applied. The results show that polygonal ferrite, granular bainite and larger amount of stabilized retained austenite can be obtained by controlled rolling processes. The finer ferrite grain size is produced through the deformation induced transformation during deformation rather than after deformation, which affects the mechanical properties of hot rolled TRIP steel. Mechanical properties increase with decreasing finish rolling temperature due to the stabilization of retained austenite. Ultimate tensile strength (UTS), total elongation (TEL) and the product of ultimate tensile strength and total elongation (UTS×TEL) reaches optimal values (791 MPa, 36% and 28 476 MPa%, respectively) when the specimen was hot rolled for 50% reduction at finish rolling temperature of 700 °C.

Ferrite Transformation during Deformation of Super-cooled Austenite

The research results achieved by the Korean national project, HIPERS-21, on the ferrite transformation during the deformation of super-cooled low carbon austenite were summarized. Fine ferrite grains formed during the deformation of austenite, i.e. dynamically. The rate of ferrite nucleation was estimated to be accelerated several hundred times by the deformation of austenite. However, the grain refinement could not be explained by the accumulated strain alone. The application of stress during the ferrite transformation is known to effectively weaken the orientation relationship of the ferrite grains with austenite, making the coalescence of the grain difficult during the growth of ferrite and effectively increasing the nominal ferrite nucleation rate. The application of the dynamic ferrite transformation to industrial hot rolling or plate milling was also summarized. The multi-pass rolling technique was introduced to produce a fine ferrite grain structure and controlled cooling was adopted to produce a multi-phase structure in order to improve the work hardening rate of fine grained steels.

Characteristics of Microstructure in Ultrahigh Carbon Steel Produced during Friction Stir Welding

Ultrahigh carbon steels containing 1 to 2 wt% C have very poor fusion-weldability, but exhibit high strength-ductility balance, good wear resistance and superplasticity at high temperatures, when a microduplex structure consisting of a fine ferrite grain structure with a uniform distribution of spheroidized cementite is formed. Thus, ultrahigh carbon steels with the microduplex structure show promise for use in construction. In the present study, two kinds of initial microstructure, a fully pearlitic structure and a microduplex structure, were produced in an ultrahigh carbon steel with 1 wt% C by different thermal treatments, this steel then being subjected to friction stir welding (FSW) at different rotational speeds using a polycrystalline cubic boron nitride tool. The feasibility of friction stir welding for ultrahigh carbon steel and the effect of welding parameters and initial microstructure on the residual microstructure characteristics of the weld were systematically examined. FSW successfully yielded defect-free welds in the ultrahigh carbon steel at all welding parameters employed. All welds exhibited very high hardness in the weld center due to the martensitic transformation. The difference in characteristics of the martensitic structure among the welds was negligible, but the austenite substructure was affected by the rotational speed. Microstructural development of various regions in the welds was reasonably accounted for by the friction stir welding process, the thermal stability of the initial microstructure and the solid-state transformation during cooling cycle of FSW.

Phase Transformation from Fine-grained Austenite

Microstructure formed by diffusional or martensitic transformation from fine-grained austenite of which grain size is smaller than 5 μm was studied. Grain refinement of austenite was established through two kinds of reversion processes; (1) cyclic transformation between martensite and austenite and (2) reverse transformation from tempered and cold-rolled lath martensite (or pearlite). In the process of (1), the fine austenite structures whose grain sizes of 5-10 μm are obtained. Refinement of austenite grain size results in the increase of hardness. In the process of (2), austenite grain size can be refined down to about 2 μm in low-carbon Mn steels by microalloying through pinning of austenite grain growth by alloy carbides. The ferrite grain size after continuous cooling transformation becomes finer as austenite grain size is refined. However, the grain size ratio of austenite and ferrite, d α /d γ , increases by refining austenite grain size. For the austenite of grain size smaller than 5 μm, the ferrite grain size becomes coarser than that of austenite for slow cooling. A similar trend in the change of ferrite grain size by refinement of austenite was recognized for isothermal pearlite transformation in eutectoid alloys. Thus, it is suggested that extensive accelerated cooling is important to obtain fine-grained ferrite by diffusional transformations from the fine-grained austenite. Packet and block sizes of lath martensite in low carbon steels are also refined by decreasing the austenite grain size. Several packets and blocks are formed even from the austenite matrix of 2 μm in grain size.

Effect of Warm Deformation on Ferrite Microstructure Evolution in a Ti-Microalloyed Steel

Warm deformation is one of the promising hot rolling strategies for producing thin hot rolled steel strips. A better understanding of the microstructure evolution during warm deformation is important for a successful introduction of such processing into the industrial production. In the present research, the effect of deformation strain on the ferrite microstructure development in a low carbon Ti-microalloyed steel was investigated through warm torsion testing. Microstructural analysis with optical microscope and electron back-scattering diffraction was carried out on the warm deformed ferrite microstructures. The results show that at the early stage of deformation an unstable subboundaries network forms and low angle boundaries are introduced in the original grains. Then, with further straining, low angle boundaries transform into high angle boundaries and stable fine equiaxed ferrite grains form. It was considered that dynamic softening and dynamically formation of new fine ferrite grains, with high angle boundaries, were caused by continuous dynamic recrystallization of ferrite.

Formation of Ultrafine Ferrite Grains during Interrupted Multi-Pass Deformation under Hot Torsion

Strain induced transformation (SIT) of austenite into ferrite has been frequently used as a powerful ferrite grain refinement mechanism. Ordinarily ferrite grain sizes of the order of 1-3μm are achieved via mechanical testing such as compression and torsion. Nonetheless, most of the work done so far employed continuous deformation in the range of 0.8 for compression experiments and in excess of this for torsion. SIT is a promising technique which may be used during actual hot rolling processing. However, in this case, not only deformations are applied with time interrupts between them but also the amount of total deformation allowable is relatively low, in order to attend to flatness and final gauges requirements. This work explores the consequences on SIT microstructure of deformation given in multiple passes as opposite to the usual continuous deformation presented in the literature. Multiple pass deformation at high temperature led to partial dynamic recrystallization and to a mixture of coarse and fine ferrite grains. Multiple pass deformation at the vicinity of Ar3 produced, on the hand, finer ferrite grains indicating that SIT took place. In this case, ferrite grains in the range of 1-3μm were produced and a much more homogeneous distribution of these grains was present.

Dynamic adjustment of ferrite grains during dynamic strain induced transformation

The dynamic adjustment of ferrite grains formed during 'dynamic strain induced transformation (DSIT)' is an important feature of this mechanism that has not been addressed previously. A novel experimental method was applied to follow the effect of deformation at different stages on ferrite formed initially through DSIT. It is shown that while the continuous dynamic recrystallisation (CDRX) appears to be an acceptable mechanism for re-refinement of coarser grain size (i.e. dα>2dDSIT), it cannot explain the steady state grain size for finer ferrite grains (i.e. dα<2dDSIT). Other potential mechanisms involved in this phenomenon are examined.

Dynamic recrystallization by large strain deformation with a high strain rate in an ultralow carbon steel

Microstructural evolution was observed during large strain deformation with a high strain rate using an ultralow carbon steel. Fine-grained ferrite was obtained by decreasing the deformation temperature and increasing the strain rate. A strong texture, such as ND∥〈1 0 0〉, 〈1 1 1〉 and RD∥〈1 1 0〉, was developed. The change of grain size could be analyzed by the Z parameter, whereas the duration had a large effect on the deviation of the predicted grain size in deformation with a high strain rate.

Microstructural characterization of a warm-deformed microalloyed steel

A warm deformation process using torsion testing was carried out on a low carbon Nb-microalloyed steel. The physical processes that occur during deformation were studied by analyzing the warm flow curves. The mechanisms of fine ferrite grain formation were studied by means of optical microscopy and an electron back-scattering diffraction technique. The results show that warm flow curves of ferrite are similar to those affected only by dynamic softening events. Microstructural analysis shows that, with increasing strain, the new fine equiaxed ferrite grains surrounded by high-angle boundaries are generated at the initial boundaries. During the early stages of deformation, as strain increases the grain size decreases and the grain aspect ratio rapidly increases. A further increase of the strain also leads to continuous decreases of both the grain size and the grain aspect ratio. The dynamic softening mechanism and dynamical formation of new fine grains, observed during warm deformation, were verified to be due to continuous dynamic recrystallization.

Microstructural development in a low carbon Ti-microalloyed steel during deformation within the ferrite region

Microstructural evolution of ferrite in a low carbon Ti-microalloyed steel, during deformation within the ferrite region, was investigated by using torsion testing. Warm deformation characteristics of ferrite were studied by analyzing of its flow curves, optical microstructures and electron back-scattered diffraction maps. The results show that an unstable sub-boundaries network forms and then these transform into high angle boundaries, with further straining. It was found that the occurrence of continuous dynamic recrystallization is responsible for the development of very fine ferrite grains with high angle boundaries.

Superplasticity in Ultrahigh Carbon (1.6 Pct C) Steel

Ultrahigh carbon steel containing 1.6 wt pct C was processed to create microduplex structure consisting of fine-spheroidized carbides and fine ferrite grains. Elongation-to-failure tests were conducted at strain rates from 10-4s-1 to 15×10-4s-1, and at temperatures from 600 °C to 850 °C. The steel exhibited superplasticity at and above 700 °C when testing at a strain rate of 10-4s-1, and at 800 °C when testing at strain rates of 7×10-4s-1 and slower. The grains retained the equiaxed shape and initial size during deformation; dynamic grain growth was not observed after superplastic deformation, whereas carbide coarsening was observed. It is concluded that the fine ferrite grains or austensite grains are stabilized by the grain boundary carbides, and grain-boundary sliding controlled by grain boundary diffusion is the principal superplastic deformation mechanism at temperatures in the range of 700-850 °C.

Effect of carbon content on etching response for identification of plastic deformation zones in carbon steels

A recently developed etching technique to detect localized plastic deformation on a macro scale in 0.10–0.55% carbon steels has been studied. Etching was carried out on samples plastically deformed and then heated to 550 °C for a certain period of time. The plastic deformation zone was clearly seen in the low carbon steel (∼0.15%C), whereas it was mainly opaque in the medium carbon steel (0.25–0.35%C) and could not be seen in the high carbon steel (0.55%C). In the case of low carbon steels, severe chemical reaction occurred in the undeformed ferrite grains, while there was a weak reaction in the deformed ferrite. The plastic zone was detectable from the different surface states. Such a change in etching response in the above carbon steels was found to be related to their microstructural morphology (ferrite/pearlite), and a large amount of ferrite grain in the sample, i.e. the low carbon steels, was more suitable for revealing the plastic zone. This was due to the change in microstructure: (i) many tiny cementite particles (Fe 3C) are precipitated in the deformed ferrite matrix during heating process, and (ii) finer ferrite grains are produced in the plastic zone. Because the main reason for observation of the plastic zone is related to the change in microstructure in the ferrite matrix, the material having a large amount of pearlite (high carbon steel) did not show the plastic deformation zone. Based upon the change of microstructure in ferrite, details of the etching mechanism for identification of the plastic zone are discussed.

Effects of consolidation temperature, strength and microstructure on fracture toughness of nanostructured ferritic alloys

Fully consolidated nanostructured ferritic alloys (NFAs) were prepared by attritor milling pre-alloyed Fe–14Cr–3W–0.4Ti and 0.3 wt% Y 2O 3 powders, followed by hot isostatic pressing (HIPing) at 1000 °C or 1150 °C at 200 MPa for 4 h. Transmission electron microscopy (TEM) revealed similar bimodal distributions of fine and coarse ferrite grains in both cases. However, as expected, the alloy microhardness decreased with increasing in HIPing temperature. Three point bend tests on single edge notched specimens, with a nominal root radius ρ = 0.15 mm, were used to measure the notch fracture toughness, K ρ , as a function of test temperature. The K ρ curves were found to be similar for both processing conditions. It appears that the coarser ferrite grains control cleavage fracture, in a way that is independent of alloy strength and HIPing temperature.

Microstructural Development in Bearing Steel during Rolling Contact Fatigue

It is well known that microstructural changes occur in a steel bearing, when the bearing is operated under conditions involving high cyclic stresses. When combined with relatively high temperatures, such microstructural changes result in the flaking of the bearing raceway. In this paper, microstructural changes that occurred during rolling contact fatigue were investigated, and the relationship between these changes and fatigue life are discussed in association with the recrystallization behavior of martensite. Conventional bearing steel SUJ2 (SAE52100) was subjected to partial solution treatment at 1133K for 2.4ks followed by oil quenching. The quenched material with a martensitic structure was tempered at 443K for 7.2ks, and then subjected to rolling contact fatigue testing. The testing was performed at temperatures ranging from 373K to 443K and surface pressures of 4.6GPa or 5.5GPa. During testing at 373K, flaking occurred from the surface of the raceway due to non-metallic inclusion and without any marked microstructural changes. On the other hand, in the case of testing at 403K or more, flaking occurs after obvious microstructural changes. Firstly, dark etching constituent (DEC) formed around the area of maximum shear stress, which was followed by the formation of white etching constituent (WEC) within the DEC at 80 and 30 degrees to the rolling direction. TEM observations showed the change from martensite lath to dislocation cell structure within the DEC, and also the existence of fine ferrite grains of 20nm through 100nm within the WEC. Arrhenius plots for the fatigue life indicated that the activation energy of the fatigue process corresponded to that of carbon diffusion in bcc ferrite. These results suggest that rolling contact fatigue originated from the WEC is controlled by the diffusion of carbon in the ferrite matrix.