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A kind of 9Cr18Mo multidiameter shaft part was fabricated through a designed thixoforging set-up. As compared with conventional heat treatment specimen, wear and corrosion behavior of 9Cr18Mo thixoforging specimen were studied. The results showed the surface of 9Cr18Mo thixoforging specimen demonstrated fine wear and corrosion resistance. Thixoforging specimen exhibited functionally graded property with inner austenite particles and fine dendrite eutectic skin layer. At the skin layer, the former solidified liquid led to the concentration of alloying elements such as Cr and Mo. Besides, eutectic structure of secondary austenite and M7C3carbide helped to improve the hardness. The high hardness of M7C3carbide and high concentration of Cr, Mo elements at the edge skin layer contributed to reducing the delamination wear, abrasive wear and the appearance of oxidation. Even at higher wear load, relative sliding occurred at the wear surface, which avoided crack formation and chunk spalling. As compared with conventional heat treatment specimen, corrosion current density of thixoforging surface reduced, while corrosion potential increased. Thixoforging surface also exhibited passivation behavior and the corrosion resistance increased slightly. High concentration of Cr and Mo at the skin layer avoided the formation of chromium depleted zone. In view that wear and corrosion failure tend to occur at the surface of specimen, the thixoforging specimen with a wear and corrosion skin layer might provide a possibility for expanding the application fields of 9Cr18Mo steel.

A new steel chemical composition is combined with a new press hardening process, in which die-quenching is interrupted by opening the forming tool to permit slow cooling of the hot formed part through the anisothermal bainitic ferrite transformation. This promotes carbon partitioning to austenite before the forming tool is re-closed and die-quenching is resumed to near-ambient temperature. The final microstructure is predominantly bainitic ferrite with dispersions of martensite and up to 11 % retained austenite. Retained austenite can undergo stress induced transformation to martensite in an automobile crash event. The steel exhibits up to 25 % elongation and 930 MPa tensile strength. In contrast to traditional cold formable Transformation Induced Plasticity assisted steels, where retained austenite is consumed during work hardening of cold forming, here, the desired microstructure is achieved after hot forming meaning the retained austenite is more uniformly distributed within the formed part, which enhances energy absorption. The new steel chemical composition is carefully designed to provide optimal microstructural evolution within the constraints of the new press hardening process, yet relatively lean and manufacturer friendly. The new press hardening process is energy efficient as secondary heating is not required since retarded cooling through the bainitic ferrite transformation is provided by residual heat accumulation of the newly developed titanium alloy forming tool. Development of the new technology is demonstrated by press hardening experiments, tensile testing, microstructural analysis, transversal & axial crush testing of formed parts and numerical simulation of crush testing, including a new modelling technique that more accurately simulates deformation of hot versus cold formed parts. Results show a 22 % increase to energy absorption under axial crushing compared to traditional cold formed Transformation Induced Plasticity assisted steels owing to greater work hardening capacity in formed radii of the part, which are shown to be exposed to the highest stresses during crushing.

A series of reheating-isothermal holding experiments and compression tests were conducted on pristine magnesium alloy AZ91 extruded by equal channel angular extrusion (ECAE) and SiC particles (a volume fraction of 15%) reinforced AZ91 composite (AZ91-SiCp) by regular extrusion. Dissolution of eutectic compounds and partial melting of the α-Mg matrix occurred during the reheating of these materials. Spherical semisolid slurries of these materials were obtained when the reheating temperature and isothermal holding time were 550 °C and 20 s, respectively. The presence of SiCp in AZ91-SiCp not only caused lower liquid fractions of semisolid slurries but also resulted in higher values of flow stress during semisolid compression tests. Both AZ91 alloy and AZ91-SiCp composite exhibited better thixoforming properties at high temperatures. Segregation of SiCp did not occur during thixoforming of AZ91-SiCp composite after an isothermal holding at semisolid temperatures for 20 s.

The inhomogeneous quality of products manufactured by semisolid forming is mainly attributed from the phase segregation of semisolid metal slurry during forming. Phase segregation can be inhibited by lower forming temperature, higher strain rate, and lower stroke. However, the realization of net-shape forming with above parameters is quite difficult. In this study, a multi-stage thixoforging strategy including a partial melting, a fast first compression, a partial solidification, and a secondary compression was proposed to weaken phase segregation of thixoformed samples. A physical simulation of multi-stage thixoforging was conducted on commercial rolled Cr-V-Mo tool steel SKD11, using a multistage hot compression test machine. The microstructural evolution and forming behaviors of the SKD11 steel slurries in different stages of multi-stage thixoforging with various forming parameters were investigated experimentally. Out flow of liquid phase does not occur throughout the thixoforming, and can be inhibited effectively by partial solidification and subsequent secondary compression. SKD11 steel sample more homogenous microstructure and smoother surface was manufactured by multi-stage thixoforging including a fast first compression with a stroke of 4 mm and a strain rate of 2.0/s at 1280 °C, a partial solidification to 1240 °C with a cooling rate of 2 °C/s, and a secondary compression with a stroke of 2 mm and a strain rate of 0.1/s at 1240 °C.

Reheating and rapid cooling experiments, and semisolid compression tests were conducted on extruded Mg–8.20Gd–4.48Y–3.34Zn–0.36Zr alloy using a multistage hot compression test machine. Affected by reheating temperature, heating rate and isothermal holding time, dissolution of eutectic compounds and partial melting occurred during reheating, dynamic arrangement of alloying elements and phase transformation took place during subsequent rapid cooling. These microstructural behaviors not only changed the anisotropy in microstructural morphology of the extruded Mg–8.20Gd–4.48Y–3.34Zn–0.36Zr alloy, but also affected the anisotropy in forming properties of it. When this alloy was reheated to 580 °C with a heating rate of 20 °C/s and held isothermally for 20 s, semisolid slurry without anisotropy in microstructural morphology and forming properties, exhibiting a homogeneous spherical microstructure was obtained.

The demand for new processes to produce high strength parts, under appropriate cost and productivity, has grown with weight reduction and crash safety improvements in automobile design. The hot stamping processes of quenchable steel sheets potentially offer not only small forming load and high formability, but also high strength and no springback by die quenching. This paper aims to provide an overview of the state-of-the-art in such hot stamping processes, including quenchability, formability, heating and cooling approaches and lubrication. The paper also includes a description of the mechanism of formability and quenching, tailoring, analysis of hot stamping processes and applicability.

Chain-die forming is a promising manufacturing technology that is emerging as an alternative to roll forming and stamping in fabricating Advanced High-Strength Steel (AHSS) products. The microstructure evolution is a critical factor for evaluating the severity of a fabrication process, especially the plastic deformation. This paper mainly attempts to understand the microstructural changes involved with the Chain-die forming process. Scanning Electron Microscopic (SEM) observation and Electron Backscatter Diffraction (EBSD) analysis were adopted to have a deep understanding on microstructure evolution. Microscopic results of the investigation show that there is almost no variation of grains, with respect to Kernel Average Misorientation (KAM) value and rotation of the grains, in the undeformed areas. Rotation of the grains is observed between inner edge and outer surface regions. The microscopic analysis also revealed that the KAM distribution shifts to higher values at the highly deformed region.

In-depth understanding of the impact mechanism of nitrogen on tempering behavior of hot-working die steel is still limited. In present work, the microstructural evolution and mechanical property change of new nitrogen-alloyed Cr–Mo–V hot-working die steel during long-term tempering were investigated by scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HRTEM), eletron backscattered diffraction technique (EBSD) and 3D atom probe tomography (3DAP). The impact mechanism of nitrogen on tempering stability were discussed in detail. The results reveal that whether nitrogen is beneficial to the tempering stability of steel varies with quenching temperature. Trace nitrogen exerts a significant effect on the allowed quenching temperature and metastable M3C carbides, affecting the coarsening rate and transformation of M23C6 carbides during tempering process. Tempering stability was improved by nitrogen mainly by enhancing precipitation hardening and grain refinement strengthening, which account for about 70% of total yield strength. Improving effect of nitrogen in precipitation hardening is more distinct with the increase of tempering time. These results are helpful for understanding the mechanisms of nitrogen in steels worked at high temperatures.

Friction stir welding (FSW) was applied to a low carbon alloyed steel in order to stabilize the austenite phase which can effectively maintain a preferable toughness. A Cr–Mo steel sheet (0.20 wt.%C–1.07%Cr–0.16%Mo–0.24%Si–0.61%Mn–bal. Fe) with a ferrite pearlite structure was friction stir welded under the conditions in which the sample was reheated above the A3 temperature at the maximum. The microstructure observations in the stir zone clarified martensite with several area fractions of retained austenite and elongated ferrite. The retained austenite provides a high ductility with a high strength in the stir zone. In order to clarify the formation mechanism of the austenite, compression tests were conducted at various strain rates up to 10/s using an ultrahigh-speed compression testing machine. When the sample was compressed at the high strain rate, the elongated ferrite grains and retained austenite were formed. This microstructure was similar to the microstructure in the FSW stir zone, suggesting that the efficient accumulation of plastic strain in the austenite during the FSW plays an important role in obtaining the retained austenite. Keywords: Friction stir welding, Steel, Strain rate, Phase transformation

The stamping of an ultra-thin stainless steel sheet into a wave shape for a finless heat exchanger was investigated. As an alternative to fins made by the extrusion or drawing, a bidirectionally corrugated shape is advantageous because of its higher efficiency in heat exchange with smaller resistance to an ambient air flow. A greater wave height is advantageous, but is suppressed by fracture during stamping. A new stamping method was developed to realize a high-efficiency heat exchanger using an ultra-thin stainless steel sheet with an optimized corrugated shape in terms of the plan-view pattern and cross-sectional profile.

Using a multistage hot compression test machine, reheating experiments and semisolid compression tests were conducted on two commercial rolled Cr–V–Mo tool steels, SKD11 and SKD61. Austenization, grain growth, carbide dissolution, and partial melting took place during the reheating of the tool steels, and resulting in spherical semisolid slurries at certain temperatures (1300 °C for SKD11, 1405 °C for SKD61). The microstructural evolution of SKD11 and SKD61 tool steels during reheating and rapid cooling was found to be different owing to their different alloying element contents and the different distributions and morphologies of the carbides inside them. The forming characteristics of the two tool steels in the semisolid state were discussed on the basis of the results of compression tests.

Hard materials were mixed with alloy powders and thermally sprayed at an ultra rapid cooling rate of more than 106°C/s. For example, 65Ni15Cr16P4B at% alloy powder containing tungsten carbide was thermally sprayed. SEM image showed tungsten carbide paticles were scattered in the amorphous alloy coatings. The resulting thermal coatings showed low friction and high wear resistance in a reciprocating-slide-type friction-testing machine because tungsten carbides exhibited cleavage fracture. The stress analysis of an amorphous alloy coating with low friction was also carried out by the finite element method.

A heavy-reduction controlled rolling process with approximately 75% thickness reduction was carried out to investigate the microstructural evolution including texture development, focusing on the formation of a bimodal structure of 0.2% carbon steel with heating temperatures of 700, 800, 900, and 1000 °C. Upon increasing the heating temperature from 700 to 900 °C, the microstructure was refined and precipitates such as Fe 3 C were uniformly distributed throughout the microstructure. For the microstructures control-rolled at heating temperatures of 900 and 1000 °C with average ferrite grain sizes of 1.34 and 1.63 μm, respectively, a bimodal structure could be observed by scanning electron microscopy (SEM), which was very similar to the result of a plane-strain compression (PSC) test. Moreover, the 900 and 1000 °C-heated specimens had less well developed textures primarily consisting of {113}–{4 4 11}〈110〉 and {332}〈113〉 components, which usually developed by the transformation (γ→α), and the 1000 °C-heated specimen exhibited various textures and a low intensity of the {100}〈011〉 component, which was generally transformed from the {100}〈001〉 component of the recrystallized austenite.

A growing number of studies suggest that the accumulation of unwanted elements is a growing problem in all recycled material streams. Thus, the separation of tramp elements from aluminium alloy scrap is very important in the aluminium recycling process. In this paper, a new technique of backward extrusion in the semisolid state is reported for separating the tramp elements from aluminium alloy scrap and obtaining high-purity aluminium easily and quickly. In this process, partial solidification is adopted to condense tramp elements in the liquid phase on the grain boundary. In experimental trials, four aluminium alloys were selected: cast aluminium alloy (AC4C), two kinds of wrought aluminium alloy (A7N01, A7075) and aluminium beverage can scraps. From the results of macro- and optical microscopic analysis, Rockwell hardness testing and electron probe micro-analyser (EPMA) analysis, it was confirmed that the backward extrusion in the semisolid state could separate the tramp elements from the aluminium alloy scrap easily and high-purity aluminium was obtained.

The different forming behaviors of the solid and liquid phases in semisolid metal slurry during deformation result in products with inhomogeneous quality. A mechanical servo press with the capability of multistage compression was employed to forge A6061 aluminum alloy in the semisolid state. A time-dependent rheoforging strategy including mechanical stirring, a fast first compression, short holding, and a secondary compression was designed to improve the homogeneity of rheoforged samples. The distributions of the microstructure and mechanical properties, such as the solid fraction, hardness, and deformation resistance, of samples manufactured under various experimental conditions were investigated. When the stirred semisolid A6061 slurry was forged in the temperature range from 625 to 628 °C with a short holding time of 4 s and the upper die preheated to 300 °C, samples with a homogeneous microstructure and mechanical properties were manufactured. The homogeneity of rheoforged samples was attributed to the controllable free motion capability of the mechanical servo press and the adjustable fluidity and viscosity of the semisolid slurry.

In the seamless pipe rolling process, the pipe wall thickness is largely determined at the mandrel mill or plug mill. It is possible to obtain the target thickness at these mills by defining the gap of a grooved roll and an inside tool such as a plug. However, the thickness of the free deformation part, which is not in contact with the roll and tool, had generally been estimated by experimental techniques. Although a number of analytical studies of mandrel mill rolling had been reported, few reports had examined plug mill rolling. Therefore, in this research, a finite element analysis model for plug mill rolling was developed by extending the rigid plasticity finite element model "Computational Rolling Mill (CORMILL)." Good agreement between the calculated results and experimental results was obtained for the wall thickness, and it was found that the thickness of the flange part decreases with reduction of the wall thickness at the grooved bottom. These results suggested that the wall thickness distribution of rolled pipes can be controlled by using a suitable inside tool and roll shape in each rolling pass, and the necessary shapes can be obtained by using the newly-developed model.

A compression test simulating heavy-reduction single-pass rolling was conducted to investigate the microstructural evolution based on the formation of a bimodal structure and the mechanical properties of 0.01% and 0.1% carbon steels and niobium steel. When thermomechanical processing was conducted near and above the critical transformation temperature ( A c 3 ), microstructures of all steels were significantly refined and consisted of equiaxed grains without elongated grains. Nevertheless, these microstructures showed weak or no formation of the bimodal structure or coarse grains with decreasing carbon content, while they showed bimodal structure formation when 0.2% carbon steel was used in our previous research. The average grain size of Nb steel was about 2 μm and its microstructure was uniformly refined. These may be attributed to a decrease in the number of nucleation sites with decreasing carbon content in low-carbon steels and the occurrence of nucleation at grain boundaries as well as in grain interiors in Nb steel during processing. Mechanical properties of all steels deformed above the critical transformation temperature exhibited high performance characteristics with superior strength and marked elongation. Their fractographs indicated ductile fracture, which was revealed by SEM observation after a tensile test.

T.M.C.P.(Thermo Mechanical Control Processing) has been widely used to improveplastic formability in steel strips. We have produced interstitial free steel(IF steel) strips and ferriticstainless-steel strips through T.M.C.P. rolling method. Optimizing conditions of hot rolling, hotrolled annealing, cold rolling and cold rolled annealing, we developed texture prediction model. Wecan predict rolling texture accurately using the conventional Taylor model. Moreover, we preciselypredict recrystallization texture classifying the total number of microscopic slips which arecalculated using the Taylor model. We consider that these calculated results provednucleation-oriented model and two types of recrystallization and grain growth mechanisms exit inour studies. One mechanism is that grains which had the small total number of microscopic slips arepreferred orientation for the hot rolled and annealed ferritic stainless-steel strip. The othermechanism is that grains which had the high total number of microscopic slips are preferredorientation for the cold rolled and annealed IF steel strip.

The material genome of type 316 austenitic stainless steel is presented. The material genome is a set of constitutive equations which describes the microstructure evolution during hot forming. Single- and double-compression tests at temperatures of 950–1150°C and with strain rates of 0.01–20 s−1 were performed to obtain the kinetics of dynamic and static events. Inverse analysis, which couples a thermomechanical finite element analysis with experimental data, is used to obtain the flow curves. It compensates the effect of inhomogeneous distributions of deformation and temperature during the compression tests. The sine-hyperbolic law, which relates the flow stress and the Zener–Hollomon parameter, is used to validate the accuracy of the solution. Regression analysis is applied on the coefficients of the flow curves in order to obtain the parameters of the constitutive equations. Microstructure investigations are used to demonstrate the validity of the newly obtained material genome.

The conventional route for tool steel manufacturing consumes large amounts of energy and time and results in a long process chain for the manufacture of products. Environmental consciousness is fuelling the development of an improved route with a shorter process chain and lower energy consumption that can replace the conventional one. An energy-efficient process route for manufacturing products with high quality was proposed based on recrystallization and partial melting (RAP) technology. To verify the feasibility of the RAP-based route, the effects of predeformation, partial melting, and post heat treatments on the microstructure characteristics and mechanical properties of cast Cr-V-Mo steel were studied experimentally. During the RAP process, plastic deformation, recrystallization, austenization, grain growth, and partial melting occurred. When the RAP processed specimen was quenched from 1050 o C after isothermal holding for 480 s and then tempered twice at 560 o C for 2 h, microstructural evolution occurred in both former solid-phase and liquid-phase regions and caused a good combination of mechanical properties.

The phase segregation of semisolid processed products resulted in an inhomogeneous microstructure and poor mechanical properties of such products. Optimal subsequent heat treatments including quenching and tempering with various processing parameters were conducted to improve the quality of RAP (recrystallization and partial melting) processed Cr–V–Mo steel. The microstructure characteristics and mechanical properties, such as hardness, tensile strength, elongation, impact toughness, and resistance to high-temperature wear, of specimens subjected to various heat treatments were investigated. When the RAP-processed specimen was quenched from 1050 °C after isothermal holding for 480 s and then tempered twice at 560 °C for 2 h, microstructural evolution took placed in both former solid-phase and liquid-phase regions. The weakening of phase segregation, the redistribution of carbides, and the release of residual stress occurred during this heat treatment strategy caused a good combination of mechanical properties.

In this study, multi-stage thixoforging and subsequent heat treatments of SKD11 tool steel samples were physically simulated by experiments. The microstructure and mechanical properties of various regions in the multi-stage thixoforged SKD11 samples before and after different heat treatments were investigated. The effects of the heat treatments on the former solid and liquid areas of the multi-stage thixoforged SKD11 sample were quite different, because the contents of alloying elements and microstructural morphologies in these areas were different. Static recrystallization of austenite, precipitation of carbides, growth of austenite, dissolution of carbides, and martensitic transformation occurred sequentially in the former solid areas of the multi-stage thixoforged SKD11 sample during annealing, quenching, and tempering. The values of the residual stress and volume fractions of austenite and martensite in the former solid areas changed significantly by the above microstructural evolution sequence. However, heat treatments slightly affected the volume fraction, distribution, and morphology of the carbides in the former liquid areas of the multi-stage thixoforged SKD11 sample. The mechanical properties of the centre regions of the multi-stage thixoforged SKD11 samples were adjustable during heat treatment because of the low volume fraction (approximately 10%) and dispersed distribution of the carbides in these regions. It is difficult to improve the mechanical properties of the edges of multi-stage thixoforged SKD11 samples by heat treatments owing to the high volume fraction (approximately 45%) and net-shaped morphologies of the carbides in these regions because they were abundant in the former liquid phase.

The accumulative roll-bonding (ARB) process, which is a severe plastic deformation process, was simulated using finite element analysis, including the influence of friction, stress–strain relations, and roll diameter. The complicated distributions of equivalent strain through the thickness of ARB-processed sheets were quantified. These quantitative strain analyses would be useful for analyzing the evolution of ultrafine-grained structures in the ARB process.

A compression test simulating heavy-reduction single-pass rolling was carried out to investigate deformation behavior and to collect basic data on the formation of bimodal structures in 0.2% plain carbon steel at deformation temperatures between 700 and 850 °C. The mechanical properties of such bimodal structures and the likelihood of their formation were revealed by field-emission scanning electron microscopy (FE-SEM), electron backscattering diffraction (EBSD) and tensile tests. These microstructures were significantly refined and had a higher fraction of high-angle grain boundaries when the deformation temperature was increased from 700 to 850 °C. The average ferrite grain sizes at deformation temperatures of 800 and 850 °C were 1.2 and 1.4 μm, respectively. In particular, there was a high probability of bimodal structures being formed in the specimen deformed at 850 °C, for which the proportions of ferrite grains with sizes of less than 1.2 μm and from 1.2 to 4 μm were 37% and 63%, respectively. This specimen had a high strength (677 MPa) and its average uniform elongation (7–8%) was about twice that of the other specimens. Moreover, bimodal dimples consisting of submicron-size (

An energy-efficient process route for manufacturing machine tools and dies of high quality was proposed based on recrystallization and partial melting (RAP) method and semi-solid forming technology. To verify its feasibility, the effects of parameters such as predeformation, heating rate, and holding time on the microstructure and mechanical properties of cast Cr–V–Mo steel were studied experimentally. Recrystallization, austenization, grain growth and partial melting occur during heating of predeformed cast billet. These behaviors refine the microstructure and improve the mechanical properties. The refinement of microstructure and improvement of mechanical properties become more significant, when RAP is conducted with larger predeformation (50%), higher heating rate (50 °C/s) and shorter isothermal holding time (20 s).

Controlled forging of aluminium alloys is conducted using a mechanical servo press. Semisolid forging can reduce the number of forging passes and energy consumption to produce a product with suitable microstructure. In the proposed semisolid forging, solid fraction of the aluminium alloys being forged is controlled to enable the manufacture of products with homogeneous and heterogeneous microstructures. Semisolid forging experiments using a predetermined sequence represented by transient changes in stroke were systematically conducted. The relationship between the time-dependent semisolid forging sequence and product quality was discussed to reveal the semisolid forming condition for manufacturing products with homogeneous and heterogeneous microstructures. [doi:10.2320/matertrans.M2013098]

The microstructural evolution and basic refinement mechanism of the semi-solid-state SKD61 tool steel fabricated by recrystallization and partial melting (RAP) method were investigated experimentally. The effects of different parameters in RAP processing were systematically clarified to achieve the grain refinement of cast SKD61 steel in the semi-solid state. Results showed that, the microstructure of RAP processed specimen were finer and more globular than the microstructure of starting material. The distribution of finer solid particles in the RAP processed specimen was affected by the distribution of strain during predeformation. Uniform and spherical solid particles inclined to locate in the regions suffered from larger strain. The specimens subjected to a plastic predeformation at a lower temperature have a more uniform and spherical semi-solid microstructure than those specimens undergoing predeformation with the same reduction at higher temperatures. In addition, the grain refinement mechanism of SKD61 tool steel processed by RAP method was discussed.

In this study, the authors propose a new refining process different from the rheorefining (a screen method) or the melt zone method. This new refining process is based on the gradient of solidification speed of the molten metal poured into a container. The molten metal in the cylindrical container firstly solidifies to columnar crystals from the wall of the container and a liquid phase gradually changes to semisolid state and progresses to the central portion of the container. After an intended semisolid state is achieved, a backward extrusion is carried out to extract the liquid phase component. The liquid component is known to contain a lot of impurity substances. For the refining trials, three aluminium alloys were selected: wrought aluminium alloy A2011, cast aluminium alloy AC4C and aluminium beverage can scraps. Backward extrusion (extraction) was applied twice, and the amount of pure Al increased from 90.59 % in the initial ingot to 96.34 % in the refined material. This new refining process can easily achieve the purification level of the rheorefining or the melt zone method.

Semi-solid forming technology has several advantages over the conventional metal forming process owing to the unique behavior and characteristic microstructure of metals in the semi-solid state. With the aim of establishing a shorter process chain for manufacturing the high-quality tools required for forming, the microstructural evolution and basic deformation mechanism of the hot-working tool steel SKD61 in the semi-solid-state were studied experimentally. The feasibility of the recrystallization and partial melting (RAP) process for the fabrication of SKD61 stock with a uniform spherical microstructure was validated. The effects of parameters in RAP processing, such as the predeformation temperature and predeformation ratio, on grain refinement were systematically clarified. Finally, the effects of semi-solid forming parameters, such as the forming temperature, forming ratio and strain rate, on the morphology of the microstructure and the flow stress were also investigated.

Nitrogen can increase the strength of steels without weakening the toughness and improve the corrosion resistance at the same time. Compared with conventional nitrogen-free die steels, a new type of nitrogen-containing die steel was developed with many superior properties, such as high strength, high hardness, and good toughness. This paper focused on the effects of heat treatment on the microstructures and mechanical properties of the new type of nitrogen-containing die steel, which were investigated by the optimized deformation process and heat treatment. Isothermal spheroidal annealing and high-temperature quenching as well as high-temperature tempering were applied in the experiment by means of an orthogonal method after the steel was multiply forged. The mechanical properties of nitrogen-containing die steel forgings are better than the standard of NADCA #207-2003.

It was found that the mechanical properties of ultrafine grained copper wire fabricated by deep wire-drawing process were significantly changed by an additional slight deformation. When the deeply wire-drawn copper was subsequently deformed by draw-bending process, the flow stress of the material greatly decreased and the elongation clearly increased.

The method of acquisition of material data, which describes the kinetics of microstructure evolution, is presented in this paper. The obtained material data could be regarded as a ‘material genome’ as it gives us the basis of estimating microstructure evolution during hot forging or hot rolling. The material genome of high-alloyed steel such as Cr–Mo–V tool steel is newly obtained by a series of single and double compression tests, inverse analysis to obtain flow curves, micrography of compressed specimens and the regression analysis of coefficients in material data. Finally, the validity of the obtained material genome is demonstrated by the coupled simulation of plastic deformation and microstructure evolution.

A method of predicting the microstructure evolution and shape in the hot forming process is important for optimizing the process conditions. To develop a new analytical model that can be used to predict microstructure behavior in the actual process, a high-speed with multistage compression testing machine is required for physical simulation of the production process and to obtain material data for the prediction model. A high-speed uniform true strain rate (~300 s–1) was achieved by a servo-hydraulic computer-driven machine with a deviation control feedforward program. The effects of processing parameter and chemical composition of plane carbon and Nb alloyed steels on ferrite grain size were investigated. A grain size of about 1 μm was accomplished in this simulator by severe hot plastic deformation.

The changes in mechanical properties and microstructures of fine Cu­Sn alloy wires manufactured by deep wire drawing and heat treatment, and of the wires deformed by draw bending were systematically investigated. During draw bending, the wires were subjected to the compressive/tensile or tensile/compressive strain in the longitudinal direction. As a result, it was confirmed that the softening caused by plastic deformation was induced when easing up the wire drawing strain by heat treatment were few, such as the decreasing strength and increasing elongation. Work hardening was induced when easing up the wire drawing strain by heat treatment were large, such as the increasing strength and decreasing elongation. With decreasing strength of softening induced by plastic deformation, increase in grain size is smaller than the strength deterioration. Additionally, it was confirmed that the occupancy probability of high-angle grain boundaries increases. This can be understood by considering that the many dislocations formed inside the grains are piled up in one direction during draw bending. Additionally, it is considered that ductility enhancement is caused by the increase in work hardening rate induced by the piled up dislocations. [doi:10.2320/matertrans.MD201114]

The ferrite transformation kinetics of severely hot-deformed austenite has been studiedby considering ferrite nucleation from dislocation cell blocks inside austenite grains. The size ofdislocation cell blocks and ferrite grain size just after phase transformation are acknowledged to beinversely proportional to the square root of dislocation density. It is found that the ferrite nucleationrate in this area can reach the saturated state at a high temperature just under Ae3, and the ferritetransformation finishes within a very short time. The kinetics of ferrite volume fraction and theferrite grain growth after phase transformation for plain carbon (0.1%C, 0.2%Si, 1.0%Mn) steelhave been studied using a THERMECMASTER hot-compression testing machine. These modelscan be applied to the hot and warm forming processes of plain carbon steel to predict the ferritetransformation from severely deformed austenite.

New continuous bending-drawing process to manufacture ultrafine electric wire is proposed. This process is capable of manufacturing ultrafine wire with excellent electrical and mechanical properties, by controlling the microstructure and grain orientations affected by the loading directions during drawing and bending. The mechanical properties and microstructure evolution of copper alloy deformed by the process was systematically investigated. It was confirmed that change of grain size and deformation induced texture that was introduced by the process have great effects on the successful manufacturing of ultrafine wire of 210 μm diameter with excellent mechanical properties, such as good ultimate strength and total elongation.

The formability of multilayer metallic sheets is evaluated by tensile, V-bending, hat bending and hemming tests. A monolithic type-420J2 stainless steel sheet cannot be formed because of poor elongation as small as 1.7%. Marked enhancement of the bending formability was observed in the bending of type-420J2 stainless steel sheets when they are layered by type-304 stainless steel sheets and composed into a multilayer metallic sheet. The mechanism of the enhancement of the formability of type-420J2 stainless steel in a multilayer metallic sheet is investigated analytically by focusing on the delay of the initiation of necking, and by performing stress analysis by finite element method (FEM).

A new hot forming system for high-alloy metals with poor workability is developed. This forming system is a direct combination of a forming machine and continuous electric resistance heating, and is capable of the high-speed forming of metals with flexible and precise control of elevated temperature. The prototype of the new forming system is applied to the forming of high-alloy metals with poor formability, such as a Ti–6Al–4V bar, and the basic characteristics of this forming system are clarified. Using this system, sound, high speed forming of high alloys becomes possible, and the Ti–6Al–4V bar is successfully formed without fracture.

The importance of structural metals for industrial applications is based on their superior combination of mechanical properties—strength, elongation, toughness and corrosion resistance—achieved at the end of forming processes. A numerical analysis for the prediction of microstructure is strongly required for the optimization of hot forming process parameters, because the microstructure of structural metals, which has the significant effects on mechanical properties, is strongly dependent to forming process conditions as well as the chemical composition. The incremental dislocation density and microstructural evolution analysis method enables the prediction of the change in microstructure after forming. The outline of the analytical scheme is explained briefly, and the results of its application to strip rolling, bar rolling and shape rolling are presented. Finally, the remaining research topics in this field are discussed. [doi:10.2320/matertrans.MF200906]

A hot extrusion system with controlled cooling is investigated in this study. This system is used with the aim of manufacturing steel and other metal products with ultrafine microstructures by single-pass heavy deformation followed by controlled cooling. The continuous formation of bulk materials with the desired geometry and microstructure could be realized using the proposed system or by adding hot rolling stands after processing using the proposed system. Microstructure evolution during and after heavy single-pass deformation is examined through a series of experiments involving the hot extrusion of plain carbon steel followed by controlled cooling. The effects of severe plastic deformation on grain refinement and the mechanical properties of the formed products are discussed. It is clear that the use of the proposed system is helpful for the grain refinement of a cast microstructure. Also, ultrafine-grain steel with a grain diameter of 3 μm can be formed using the proposed system. Finally, the formability of the ultrafine-grain steel manufactured using the proposed system is examined by a cold-forging experiment.

The semi-solid metal processing involves reheating feedstock slugs that have a fine spherical microstructure obtained normally by continuous casting of round bars under the effect of electromagnetic stirring or SIMA (strain-induced melted activation). These methods require additional production processing to produce the feedstock billets previously. Through the research on the microstructural evaluation of the PR (peritectic reaction) stainless steels and the NPR (non-peritectic reaction) stainless steels a new approach of spheroidization is proposed in this paper. Type 310S stainless steel which melts in γ → L + γ → L transformation and type 430 stainless steel which melts in δ → L + δ → L transformation are employed as NPR steel in this investigation. The melting of the steels commences with the place of low melting point composition and develops towards the place of high melting point composition until the end of melting. On the other hand, the type 304, type 316 and AUS304J3-L stainless steel, which melt in γ + δ → L + γ + δ → L + δ → L transformation, are also observed as PR steel in this investigation. The result shows that a perfect and fine spherical structure of ferrite particles surrounded by liquid can be obtained through an inverse peritectic reaction γ ↔ L + δ although it exhibited a strong banded L +δ + γ structure in the prior half of partial remelting state. The new approach to microstructural spheroidization of semi-solid metal is so called inverse peritectic-induced spheroidization (IPIS). This method offers advantages over traditional semi-solid manufacturing techniques: the need for specialty, added-cost raw material is eliminated, capital cost required for material handling is reduced and the issue of oxide formation on billets during reheating is avoided.