Non-recrystallization Region Research Articles

The Formation of Acicular Ferrite and Mechanical Properties Behaviors in a Low Welding Crack Susceptibility Steel

The nucleation and crystallographic characteristics of acicular ferrite in the as-rolled and tempered steel were investigated by using an optical microscopy (OM), transmission electron microscopy (TEM) and scanning electron microscopy (SEM) equipped with electron backscatter diffraction (EBSD). The results showed that acicular ferrite forms at nucleation sites such as dislocations within austenite grains under the heavy reduction in the austensite non-recrystallization region and accelerated cooling conditions, and the inclusions such as Al2O3•MnS may have promotion effect for the formation of acicular ferrite. Acicular ferrite has high misorientation angle boundaries and a number of sub-boundaries or high density of dislocations inside, which contributes to an excellent combination of high strength and toughness. And the morphology of acicular ferrite does not show obvious change in tempering microstructure due to high temperature thermal stability, which plays an important part in keeping good mechanical properties for tempered specimens.

Effect of Thermomechanical Control Processing on Microstructure and Mechanical Properties of Fe-0. 2C-1. 44Si-1. 32Mn Hot Rolled TRIP Steel

The effect of thermomechanical control processing (TMCP) on microstructure and mechanical properties of Fe-0. 2C-1. 44Si-1. 32Mn hot rolled TRIP steel was investigated through experiments. Strain-induced transformation and transformation-induced plasticity behavior of retained austenite were analyzed. The results show that with multipass deformation, reduction per pass of more than critical deformation in austenite recrystallization region and total reduction of more than 58% in non-recrystallization region and high temperature section of two-phase region, austenite can be refined before γ→α transformation. It is beneficial to obtain refined ferrite grain in final microstructure. To obtain fine and uniform microstructure and excellent strength-ductility balance, a three-stage cooling process (laminar cooling-air cooling-ultra-fast cooling) after hot rolling was conducted. The ultimate tensile strength and elongation of the investigated steel can reach 663 MPa and 41%, respectively.

Microstructural features controlling the deformation and recrystallization behaviour Fe–30%Mn and Fe–30%Mn–0.5%C

The deformation and recrystallization behaviour of Fe–30%Mn and Fe–30%Mn–0.5%C were investigated with special emphasis on the effect of carbon on the microstructure evolution. In the carbon-free alloy, epsilon martensite and to a lesser extent mechanical twinning are observed at large strains. By contrast, only mechanical twinning is observed in the carbon containing alloy. The transformation products (epsilon martensite, twins) exhibited very high thermal stability and were present in the non-recrystallized region of the microstructure even at high temperatures.

Influence of Deformation on Transformation of Low-Carbon and High Nb-Containing Steel During Continuous Cooling

The effect of deformation in the non-recrystallization region on the phase transformation for a low-carbon and high Nb-containing steel with coarse austenite grain size was investigated by means of dilatometric measurement and microstructure observation. The results show that with increasing the cooling rate, both the transformation start and finish temperatures measured by dilatometer are decreased, and the corresponding microstructure is transformed from ferrite and pearlite to full granular bainite gradually. The dynamic continuous cooling transformation diagram was plotted according to the dilatometric measurements and microstructure observations. Dilatometric measurements also show that the transformation start and finish temperatures of the test steel rise with increasing strain, strain rate and deformation temperature, and the reasons are discussed.

Macro–Micro Combined Texture Evolution Model for Hot Strip with High Reduction Rolling

Techniques for controlling the texture of polycrystal materials are very important, because texture anisotropy has a strong effect not only on the characteristics of a steel sheet but also on the mechanical and electrical properties. We have produced fine-grained hot steel strips industrially through high-reduction low-temperature rolling and these steels have the same texture as that of thermomechanically rolled steels. In this study, we first obtained the critical resolved shear stress of hot materials from functions of dislocation density and predicted the hot rolling texture in the nonrecrystallized austenite region using Taylor theory. Next, we predicted the transformation texture of fine-grained hot strips from the hot rolling texture of austenite in accordance with the Kurdjumov–Sachs orientation relationship. The accuracy of the new macro–micro combined model for texture evolution is excellent.

Effect of TMCP parameters on the microstructure and properties of Q460qNH steel

The effects of TMCP parameters, consisting of finish cooling temperature and start rolling temperature in non-recrystallization region, on the final microstructure and mechanical properties of Q460 qNH steel were studied by tensile, Charpy impact tests and optical microscopy. The TMCP parameters for Q460 qNH steel were optimized by laboratory experiments. The results show that the yield strength and tensile strength increase with the finish cooling temperature, and the microstructure of Q460 qNH steel consists of ferrite and granular bainite.

Dynamic recrystallization behavior during compressive deformation in Mg–Al–Ca–RE alloy

Mg alloys have a high potential for reduction in CO2 emission because of their high specific strength and stiffness [1]. For more applications of Mg alloys, it is desirable to improve creep resistance because Mg alloys often show poor creep resistance. It has been reported that Mg alloys containing Ca showed high creep resistance and elevated temperature strength [2, 3]. Recently, Bohlen et al. [4] suggested that dynamic recrystallization (DRX) occurred due to particle-stimulated nucleation (PSN). Recrystallization due to PSN tends to occur in metals containing large-sized particles more than 1 lm [5]. Insoluble second phases such as Al2Ca and Mg2Ca are present in Mg–Al–Ca system alloys, so that DRX is expected to be enhanced due to the PSN mechanism in Mg–Al–Ca system alloys. Several studies [6–8] showed that DRX occurs during hot deformation in Mg–Al–Ca system alloys. However, DRX in Mg–Ca alloys has not been understood sufficiently. In the present paper, compression tests are conducted on Mg–6Al–2Ca–2RE (in mass%) alloy at 523–573 K with 10–1 s and its DRX behavior is investigated. A Mg–6Al–2Ca–2RE (in mass%) alloy ingot was prepared by die-casting (Mitsui Mining & Smelting Co., Ltd.). The chemical composition of the alloy is listed in Table 1. Annealing was carried out at 683 K for 108 h for homogenization. Microstructure of the annealed alloy is shown in Fig. 1. The grain size of the alloy was approximately 20 lm. The second-phase particles were observed at the grain boundaries. Energy-dispersive X-ray spectroscopy in a transmission electron microscope showed that the precipitates were (Al, Ca, RE) compounds. The cylindrical specimens with 10-mm diameter and 12-mm height were cut from the ingot after homogenization. Compression tests were carried out at 523 and 573 K with constant true strain rates of 10–1 s. Microstructure of the specimens after the compression tests was observed with an optical microscope. It took less than 5 seconds for the specimens to cool off to room temperature after stopping the test. The grain size was measured by the line intercept method (d = 1.74L, where d and L are the grain size and the line intercept length, respectively). Microstructures of the specimens deforming to e = 1.6 at 573 K with 10 1 s are shown in Fig. 2. DRX was completed throughout the specimen and the grains were almost equiaxed in these specimens. The average grain sizes were 8.8, 6.7, 7.1, and 7.1 lm at the true strain rates of 10, 10, 10, and 1 s, respectively. In Mg–Zn– Y–Zr alloy [9], the DRX grain size depended on the strain rate. In the Mg–Al–Ca–RE alloy, however, effect of strain rate on the DRX grain size was small. It appears that the distribution and size of the particles did not depend on the strain rate. Microstructure of the specimen deforming to e = 1.6 at 523 K with 10 s is shown in Fig. 3. DRX occurred at 523 K as well as 573 K, but non-recrystallized regions were partially observed at 523 K. The grain size of the recrystallized regions was 5.5 lm, which was a little lower than the grain sizes at 573 K. Some studies [10, 11] showed that the DRX grain size of Mg alloys is governed by the Z-parameter (=_ eexp(Q/RT)), where _ e is the strain rate, Q is the activation energy for M. Hakamada (&) A. Watazu N. Saito Materials Research Institute for Sustainable Development, National Institute of Advanced Industrial Science and Technology, 2266-98 Anagahora, Shimo-shidami, Moriyama-ku, Nagoya 463-8560, Japan e-mail: masataka-hakamada@aist.go.jp

Effects of Homogenization Annealing on Dynamic Recrystallization in Mg-Al-Ca-RE (Rare Earth) Alloy

Compression tests were conducted at the temperature of 573 K with the true strain rates of 10 � 3 –1 s � 1 on as-cast and homogenized Mg6Al-2Ca-2RE (RE = rare earth) (in mass%) alloy specimens, and their dynamic recrystallization (DRX) behaviors were investigated. Strain hardening occurred after yielding, followed by strain softening. The flow stress of the as-cast specimen was higher than that of the homogenized specimen. The DRX grain size depended minimally on the Z-parameter in both of the as-cast and homogenized specimens. This is likely to be due to the particle-stimulated nucleation mechanism involving the second-phase particles. When the specimens were deformed to the true compressive strain of 1.6, non-recrystallized regions were not observed in the homogenized specimen; however, they were partially observed in the as-cast specimen. The grain size in the recrystallized region in the as-cast specimen was smaller than that in the homogenized specimen. Elemental analyses revealed Al segregation around the second-phase particles in the as-cast specimen. Therefore, it is suggested that DRX in the present Mg-Al-Ca-RE alloy is affected by not only the second-phase particles, but also the Al segregation. [doi:10.2320/matertrans.MC200783]

Effect of deformation and cooling rate on the transformation behavior and microstructure of X70 steels

The effects of the deformation in the non-recrystallization region of austenite and the cooling rate on the transformation behavior and microstructure of low-carbon low-alloy steel for pipeline application were studied on the thermal-mechanical simulator Gleeble-1500. It was shown that an increase in deformation amount can greatly increase the nucleation site of ferrite when deformed in the non-recrystallization region of austenite, and an increase in nucleation ratio can greatly refine grains. When the cooling rate is accelerated, the driving force of nucleation is increased and the nucleation rate also improves. Ultra-refine grains can be obtained by controlled rolling. The high density of ferrite nucleus, which forms along the austenite grain boundary, twin interface, and deformation band are introduced in the matrix of austenite by the control of hot rolling, after which the microstructure can be refined. It was found that the acicular ferrite has a very fine sub-structure, high dislocation density, and a thin slab with ultra-fine grains. Small M/A islands and cementite are precipitated on the matrix of the slabs by the analysis technique of TEM and SEM.

Transformation behavior and microstructural characteristics of acicular ferrite in linepipe steels

Transformation behavior and morphological characteristics of acicular ferrite in linepipe steels were investigated through the use of a dilatometer and EBSD. The results show that the volume fraction of acicular ferrite increased with an increase in the amount of hot-deformation in the austenite non-recrystallization region because acicular ferrite is formed at nucleation sites such as dislocations within austenite grains. This study found that acicular ferrite is formed at approximately 600 °C, where the transformation behavior can be characterized as a two-stage reaction: (i) nucleation of acicular ferrite and (ii) formation of polygonal ferrite between acicular ferrite grains. EBSD analyses show that an acicular ferrite grain consists of several sub-units misoriented by 1–2° and that a set of adjacent acicular ferrite grains with crystallographic misorientation below 15° makes up the crystallographic packet.

The Effect of Multiple Deformations on the Formation of Ultrafine Grained Steels

A C-Mn-Nb-Ti steel was deformed by hot torsion to study ultrafine ferrite formation through dynamic strain-induced transformation (DSIT) in conjunction with air cooling. A systematic study was carried out first to evaluate the effect of deformation temperature and prior austenite grain size on the critical strain for ultrafine ferrite formation (e C,UFF) through single-pass deformation. Then, multiple deformations in the nonrecrystallization region were used to study the effect of thermomechanical parameters (i.e., strain, deformation temperature, etc.) on e C,UFF. The multiple deformations in the nonrecrystallization region significantly reduced e C,UFF, although the total equivalent strain for a given thermomechanical condition was higher than that required in single-pass deformation. The current study on a Ni-30Fe austenitic model alloy revealed that laminar microband structures were the key intragranular defects in the austenite for nucleation of ferrite during the hot torsion test. The microbands were refined and overall misorientation angle distribution increased with a decrease in the deformation temperature for a given thermomechanical processing condition. For nonisothermal multipass deformation, there was some contribution to the formation of high-angle microband boundaries from strains at higher temperature, although the strains were not completely additive.

Influência da deformação na região de não recristalização da austenita sobre a transformação em resfriamento contínuo em um aço bainítico de baixo carbono

Estudaram-se os efeitos da deformação na região de não recristalização da austenita e da taxa de resfriamento sobre a temperatura Bi (temperatura de início de transformação bainítica) e sobre a microestrutura final, de um aço ARBL bainítico de baixo carbono submetido a resfriamento contínuo, usando ensaios de torção a quente e resfriamento a gás He. As amostras foram submetidas a quatro programas de ensaios de torção, sendo quatro taxas de resfriamento acelerado para cada um. A evolução microestrutural com a taxa de resfriamento foi estudada por microscopia eletrônica de varredura e por medição de dureza Vickers. Os resultados mostram que, para a taxa de resfriamento de 2ºC/s, a microestrutura é constituída de ferrita poligonal e bainita, independente do grau de deformação. Para taxas de resfriamento mais altas, a microestrutura é, predominantemente, bainítica, com uma pequena quantidade de ferrita poligonal fina. Verifica-se que, para um mesmo grau de deformação, uma maior taxa de resfriamento implica uma menor temperatura Bi, sendo esse efeito mais pronunciado para as amostras não deformadas; e que, para uma mesma taxa de resfriamento, uma maior deformação na região de não recristalização resulta em uma maior temperatura Bi.

Effect of TMCP Parameters on the Microstructure and Properties of an Nb-Ti Microalloyed Steel

Thermal mechanical control processing (TMCP), which includes combination of controlled rolling and controlled cooling, provides a powerful means of developing high-strength low alloy (HSLA) steels by intensive microstructural control. In the present investigation, the effects of TMCP parameters, consisting of the finish cooling temperature and the start rolling temperature in non-recrystallization region, on the final microstructure and mechanical properties of an Nb-Ti microalloyed steel has been studied by tensile, Charpy impact tests, optical microscopy and scanning electron microscopy. The TMCP parameters for Q460 grade steel have been optimized by laboratory experiments. The microstructure and properties of industrial product were coincident with the results of laboratory experiments.

Effects of Austenite Conditioning on Austenite/Ferrite Phase Transformation of HSLA Steel

Effects of surface area of prior austenite grain boundaries and deformation bands on austenite/ferrite phase transformation of high strength low alloy (HSLA) steel during hot working were investigated through simulated hot workings using Gleeble 1500. Effective surface areas were controlled by applying different austenite conditionings. It was found that the volume fraction of strain induced dynamic transformation (SIDT) ferrites increased with an increase in the effective surface area. The volume fraction of SIDT ferrite nucleated from the elongated prior austenite grains, however, was higher than that nucleated from equi-axed prior austenite grains at the same effective surface area. From the experiment results and discussions, it was concluded that diffusional austenite/ferrite phase transformation of HSLA steel by the interface nucleation and growth mechanism is accelerated by the deformation bands in the non-recrystallization region.

Effect of deformation schedule on the microstructure and mechanical properties of a thermomechanically processed C-Mn-Si transformation-induced plasticity steel

Thermomechanical processing simulations were performed using a hot-torsion machine, in order to develop a comprehensive understanding of the effect of severe deformation in the recrystallized and nonrecrystallized austenite regions on the microstructural evolution and mechanical properties of the 0.2 wt pct C-1.55 wt pct Mn-1.5 wt pct Si transformation-induced plasticity (TRIP) steel. The deformation schedule affected all constituents (polygonal ferrite, bainite in different morphologies, retained austenite, and martensite) of the multiphased TRIP steel microstructure. The complex relationships between the volume fraction of the retained austenite, the morphology and distribution of all phases present in the microstructure, and the mechanical properties of TRIP steel were revealed. The bainite morphology had a more pronounced effect on the mechanical behavior than the refinement of the microstructure. The improvement of the mechanical properties of TRIP steel was achieved by variation of the volume fraction of the retained austenite rather than the overall refinement of the microstructure.

Separation of Static Recrystallization and Reverse Transformation of Deformation-induced Martensite in an Austenitic Stainless Steel by Calorimetric Measurements

During plastic deformation of metals and alloys a small fraction, between 1 and 5%, of the energy employed in cold working, is stored inside the material in the form of defects, mainly dislocations. This stored energy derived from cold working is the driving force for recrystallization and is released during annealing. Despite recrystallization presents one of the smallest driving forces amongst the solid state transformations, for highly deformed metals, measuring stored energy due to deformation can be assessed by calorimetric methods. For austenitic stainless steels (ASSs), depending on steel composition and processing variables, plastic deformation apart from introducing crystal defects, can also cause the appearance of deformation-induced martensites, DIM, which has an important influence on the strain-hardening coefficient and on the formability. Two types of martensite may occur in the ASSs, namely: a -(bcc, ferromagnetic) and e-(hcp, paramagnetic). Some researches show that for low deformation levels the amount of e-martensite is predominant in relation to a . When the quantity of a grows continuously with the degree of deformation, the amount of e-martensite goes through a maximum and lowers thereafter, suggesting the following transformation sequence: g→e→a . Reversion of a -martensite of the DIM in the ASS is less studied than its formation. A good judgment about the thermal stabilities of the eand a martensites, as well as the deformation bands introduced by cold working, can be obtained from the work of Singh. He detected that during annealing (with duration of 1 h) of a cold rolled AISI 304, e-martensite remained stable up to 200°C, that a was stable up to 400°C, whereas deformation bands stayed in nonrecrystallized regions up to 800°C. Guy and co-authors studied the reversion of a induced by cooling and by deforming in two different steels (18%Cr–8%Ni and 18%Cr–12%Ni). In both cases, after 2 min at 600°C, practically all a -martensite reverted to austenite. Martins and co-authors studied reversion of a -martensite in the 304 and 304L steels and concluded that the reversion temperature was little sensitive to the type of steel, strain and the effect of precipitation treatments introduced prior to cold working. For the 12 studied conditions (2 steels 2 pretreatments and 3 strain levels), the temperature (annealing for 1 h) for 50% a (T50%RM) reversion was always within the 550 20°C range. Tavares and co-authors studied reversion of a -martensite and observed that the T50%RM depended only slightly on strain and heating rate. The temperatures, measured for 3 levels of strain and 3 heating rates, stayed between 549 and 573°C. Analysis of the above mentioned works shows that reversion of the strain induced martensites in the ASS occurs at temperatures much lower than the recrystallization temperature, despite the complete reversion of a -martensite, may reach temperatures of the order of 750°C. The main objective of this work is to verify if it is possible to separate, with the help of calorimetry, the phenomena of reversion of deformation-induced martensite and recrystallization in a AISI 304 cold worked austenitic stainless steel.

Influence of plastic deformation conditions on structure evolution in Nb–Ti microalloyed steel

To achieve the optimum strength of Nb–Ti microalloyed steel, TMT was applied by varying the reheating temperature, the percentage reduction in the recrystallization region, the percentage reduction in the non-recrystallization region and the coiling temperature. Specimens were reheated between 900 and 1250°C for 25min to select the appropriate reheating temperature of 1150°C. Then they were deformed at 1150°C in the range of 30–50%, subsequently were deformed at 860°C in the range of 40–70%, then cooled down to a temperature between 520 and 670°C and finally were furnace cooled over about 24h to ambient temperature. The prior austenite grain size, the recrystallized austenite grain size, the effective nucleation area (Sv), and the ferrite grain size were determined by optical microscopy. The mechanical properties of strips with different coiling ferrite–pearlite structure were determined by the tensile test. It was found that there is an abnormal grain growth of austenite grains in the reheating stage at 1050°C. The increase of percentage reduction in the non-recrystallization region from 40 to 70% results in increase of Sv due to the greater amount of elongated austenite grains and deformation bands present in the interior, of the grains consequently very fine ferrite structures were formed. Decrease of the coiling temperature from 670 to 520°C resulted in refining the ferrite grain size, thereby improving the tensile strength and yield-strength. Coiling at 550°C results in an additional yield-strength increase due to the precipitation-strengthening effect.

Microstructure and mechanical properties of 780 MPa high strength steels produced by direct-quenching and tempering process

Microstructure and mechanical properties of 780 MPa grade steel plate manufactured by conventional reheat-quenching and tempering (RQ-T) and direct-quenching and tempering (DQ-T) processes were investigated. The DQ process was found to enhance the hardenability of steel effectively so that tensile strengths of a range from 780 to 860 MPa have been achieved using DQ-T process, while tensile strength of about 770 MPa has been obtained from the RQ-T sample. In contrast, low temperature toughness of DQ-T samples was generally inferior to that of RQ-T sample, unless hot rolling and cooling processes were optimized in a controlled manner. For example, fracture appearance transition temperature (FATT) of DQ-T samples was varied in a range from −50°C to −120°C, while RQ-T specimens exhibited nearly constant FATT of about −80°C. The finish-rolling temperature (FRT) was one of potential process parameters to determine strength/toughness balance of the steel manufactured by DQ process, while the effect of FRT was closely associated with the cooling rate applied in the process. It has been demonstrated that, for the specimens quenched with a cooling rate higher than 20°C/sec, it may seem to be appropriate to adjust the FRT as low as possible in the non-recrystallization region. In contrast, for the specimens quenched with a low cooling rate of less than 10°C/sec, it may seem to be proper to apply higher FRT to obtain excellent strength/toughness balance of the steel.

A Quantitative Characterization of Austenite Microstructure after Deformation in Nonrecrystallization Region and Its Influence on Ferrite Microstructure after Transformation.

A mathematical description of austenite structure after deformation in nonrecrystallization region is given. The essential characteristic of austenite grain, after recrystallization before deformation in the nonrecrystallization region is the effective nucleation area of austenite grain boundaries. Theoretical results are compared with experimental values for a C–Mn–Nb–V steel. The connection of austenite microstructure state before phase transformation and the ferrite microstructure after transformation is given too.

未再結晶オーステナイト域で熱間圧延した低炭素鋼の方位組織形態

The application of an EBSD technique in an SEM reveals a new approach to analyze a morphology of low carbon steel rolled in the nonrecrystallized austenite region. The bainitic ferrite structure has a grain size of 5 to 20 microns and a strong {332}〈113〉 and {311}〈011〉 transformation texture is developed as observed by the conventional measuring method. (1) The grain observed in the etched steel with conventional microscopy is defined as a region where a misorientation between adjacent points is within 1°, surrounded by boundaries having a misorientation over 2° by means of EBSD. In this steel, some of the grain boundaries include subboundaries having a misorientation from 2 to 5°. (2) Orientation colonies, which consist of some grains having a misorientation between adjacent grains within 15°, are observed with EBSD. These colonies are flat-elliptical in shape corresponding to a deformed austenite grain before transformation, resulting in the transformation from deformed one austenite grain to some ferrite grains with variant selection. (3) The orientation colony is divided into some regions which consist of a few grains having a misorientation between adjacent grains within 5°, resulting in the transformation from divided austenite grain to some ferrite grains with the same variant or close variants of the K-S relation. (4) In an orientation colony, each grain has a misorientation within 15°, leading to the transformation from austenite to ferrite with a close variant of the K-S relation. (5) These experimental results suggest that a bainite transformation might have proceeded in such a way that the Bain strain or shear strain corresponding to the Bain strain as first occurred and the variant selection is controlled by this strain.