In this study, the effects of C and Mn contents on the evolution of microstructures in hot stamping processes have been investigated to find an alloy design for steels suitable for highly productive advanced hot stamping processes. A steel containing 0.28% C and 3% Mn was found suitable for advanced hot stamping technology and this steel shows a tensile strength of more than 2000MPa. An appropriate microstructural control of such steel exhibited a good strength-toughness balance and a high resistance to delayed fracture.
The AZX (X = calcium) series of magnesium alloys have recently been developed as lightweight incombustible alloys for use in automotive and railway vehicles. However, magnesium alloys containing Ca show inferior plastic deformation and poor formability. The tensile properties of rolled and annealed samples of Mg-3Al-1Zn-1Ca (AZX311) alloy were examined. Sheets of this alloy subjected to a rolling process of three passes at 473 K without reheating showed a 0.2% proof stress of 361 MPa and a tensile strength of 373 MPa. The high strength of the AZX311 alloy sheet was the result of the effective grinding of compounds and their fine dispersal in the magnesium phases, which occurred during rolling deformation and grain refinement. After annealing, the growth of compounds and grains was improved without extreme elongation, but the strength of the compounds was maintained owing to their fine dispersion. The thermal stability of this alloy was improved by adding only 1 mass% elemental Ca.
A new numerical simulation method that can be used to predict microstructure changes in ultrafine-grain generation in super short interval multi-pass rolling (SSMR) process is proposed. The SSMR process has been developed for manufacturing ultrafine-grained hot strips of about 1 μm grain size at ordinary rolling load. A new ferrite nucleation mechanism has been discussed in this process. Because multi-pass rolling and inter-pass and after-pass cooling are performed within a short time in a stable austenite temperature region, deformation strain is accumulated in the austenite. Then dislocation cells are formed inside a severely deformed and unrecrystallized austenite grain, and become the nucleation site of ferrite transformation. In this new simulation method, nucleation inside a deformed austenite grain was formulated in addition to nucleation at the austenite grain boundary.
The optimum conditions for manufacturing ultrafine-grained hot strips in super short interval multi-pass rolling (SSMR) process were discussed. As the first means of evaluation, the changes in austenite grain size and shape during multi-pass rolling were observed using experimental equipment. Deformed austenite grains could be observed with immediate quenching after each rolling pass. As the second means, a new numerical simulation method was used, which could predict the microstructure change of austenite, ferrite nucleation inside austenite grains, and ferrite grain growth. The effects of austenite grain size before finish rolling and also of temperature control between rolling passes on ferrite grain refinement were investigated. It was found that the temperature control between rolling passes is effective for grain refinement, whereas austenite grain refinement before finish rolling is not so effective. Therefore, deformation strain accumulation in austenite is important for ferrite grain refinement in SSMR process.
Using a crystal orientation database, a statistical estimation of springback was conducted in this study on the by basis of statistical analysis. Both springback in bending deformation and experimental data related to the crystal orientation show significant dispersion. Therefore, a probabilistic statistical approach was established for the proper quantification of these values. Correlation was examined among the parameters F(x) of springback, F(x) of the buildup fraction to three orientations after 92° bending, and F(x) at an as-received part on the basis of the three-parameter Weibull distribution. Consequent springback estimation using a crystal orientation database yielded excellent estimates compared with experimental values. Consequently, the estimation of springback using a crystal orientation database proposed in this study suggests promising possibilities toward the high-precision bending of high-strength materials.
Bainitic TRIP (transformation induced plasticity) -type hot-rolled strips with a tensile strength of 980 MPa were manufactured with an actual tandem hot strip mill. The importance of ultra high strength steel strips has been increasing with the weight reduction of automobiles. TRIP-type steel strips have both high strength and excellent workability for forming final products. Despite the popularity of continuous annealing for producing ultra high strength bainitic TRIP-type steel strips, few studies have been conducted with respect to the production of bainitic TRIP-type hot-rolled strips. The retarding effect of chromium on bainitic transformation was confirmed by experiments at low cooling rates using a laboratory mill and a salt bath, which simulated coiling after hot rolling.
Aluminum has excellent specific strength, good workability, and light weight. Therefore, the number of applications of aluminum alloy sheets has increased along with that of light-weight machine parts. Laser cutting is conducted at high precision, speed, and efficiency. Unfortunately, heat-affected zones on the laser cutting surface show changes in crystal grains. Moreover, their textures differ from those of other regions. Therefore, we studied crystal grain and texture changes using 1.2-mm-thick laser-processed 1050 and 6022 aluminum alloys as experimental materials. Crystal orientation measurement was performed using an electron backscatter diffraction (EBSD) apparatus. Inverse pole figure (IPF) maps and image quality (IQ) maps were measured from the laser cutting center. Microstructures recrystallized by the thermal effect of laser cutting. Then the recrystallization texture developed significantly with increased cutting speed. In addition, 6022 aluminum alloy grains coarsened from the abnormal grain growth by particle stimulated nucleation (PSN). Results show that the thermal effects of laser cutting relaxed the strain of aluminum alloy sheets processed by cold-rolling.
The compressive torsion process (CTP) was applied to the preparation of Al-4wt%Fe casting alloy in order to refining large precipitates of Al-Fe intermetallic compounds and improving their mechanical properties. To examine the effects of CTP conditions on precipitate refinement and tensile property, CTP was carried out under different processing temperatures (373-573K) and different numbers of torsional revolutions (1-30 times). Large primary Al3Fe precipitates hundreds of micrometers in diameter are broken and refined to 20µm or below by the severe plastic flow in CTP. Such precipitate refinement and uniform dispersion were promoted with increasing number of torsional revolutions and with decreasing process temperature. The tensile property of the alloy processed by CTP was remarkably improved compared with that of the cast alloy before CTP. It was 1.7 times in tensile strength and about 8 times in total elongation. Thus, it was concluded that CTP is very effective in refining large precipitates and improving the ductility of Al-Fe alloy.
A material with comparable tensile properties to conventional TiAl intermetallic compounds was fabricated by compression shearing at room temperature. Pure Ti and pure Al powders were mixed by ball milling. Mixing time was varied between 10 and 540 min. The tensile properties of the formed samples were measured. The results reveal that the Ti/Al thin plates were not mechanically alloyed. Rather, they consisted of separate Ti and Al phases. Tensile strength increased from 100 to 500 MPa when the milling time was increased from 10 to 540 min. Thin Ti/Al plates that were milled for 360 and 540 min were found to have similar tensile strength to conventional TiAl intermetallic compounds.
High-strength materials are required for aerospace and transport applications. Severe plastic deformation results in grain refinement, which produces high-strength materials. In this study, thin titanium plates were formed by compression shearing at room temperature. Grain refinement in thin Ti plates occurred upon varying shear strain. Transmission electron microscopy observations revealed that the microstructure involved grain refinement. It was found that diffusion bonding occurs upon uniaxial compaction. Compression stress and shear strain concentrated stress at bonded interfaces and subgrains formed in these stress-concentration regions. In addition, subgrain area increased with increasing shearing strain. Plastic deformation resulted in a dense, homogeneous material.
Aluminum and its alloys are used in many applications because they are lightweight. However, their usefulness would be enhanced if they were stronger. In this study, thin Al plates were fabricated by compression shearing at room temperature with the aim of controlling their mechanical properties including ductility. Mechanical strength was related to grain size by the Hall-Petch relationship. The microstructures of the samples were analyzed by X-ray diffraction analysis and scanning electron microscopy. They were found to have an average grain size in the range of 0.5-1 μm irrespective of the raw material used. The average grain sizes of the samples were uniform. The crystals of all the samples were oriented in the (1 0 0) direction.