Development of carbon–manganese–chromium steels for automotive hot stamping technologies

The hot stamping potential of three commercial carbon–manganese–chromium steels was demonstrated. The steels were selected with the intention of producing ultrahigh strength ferritic–martensitic dual phase microstructures from hot stamping heat treatment, exhibiting superior tensile properties for ‘impact energy absorptive’ crash performance compared to the conventional carbon–manganese–boron steel for automotive hot stamping technologies, 22MnB5. Compared to 22MnB5, each of the carbon–manganese–chromium steels provided at least twice the total elongation, yet with a relatively small loss to ultimate tensile strength. It was concluded that the carbon–manganese–chromium steels provide superior tensile properties for impact energy absorptive crash performance, while exhibiting ‘lean’ chemical compositions compared to previously investigated steels with similar aims and offer opportunities for use in the ‘tailor welded blank’ concept.     

Preparation of nanocrystalline Ni–Fe strip via mechanical alloying–compaction–sintering–hot rolling route

Nanocrystalline structures offer opportunity for the development of soft magnetic materials, such as 80 wt% Ni–20 wt% Fe, with superior properties. In recent years, nanocrystalline 80Ni–20Fe (wt%) alloy has been prepared by mechanical alloying of elemental powders. However, retention of nanocrystallinity during consolidation of powder is the key issue to take advantage of improved magnetic properties. In the present work, it has been shown that near-full density bulk nanocrystalline 80Ni–20Fe strip can be prepared via a route consisting of mechanical alloying, cold compaction, sintering, and multi-step unsheathed hot rolling. A crack-free strip of nanocrystalline 80Ni–20Fe, having 99% theoretical density and a grain size of approximately 55 nm, was successfully prepared by sintering and hot rolling of mechanically alloyed powder preforms at 1140 °C. The bulk nanocrystalline 80Ni–20Fe material resulted in a very narrow hysteresis loop indicating a very small hysteresis loss. The present study shows that mechanical alloying–sintering–hot rolling route can be a promising method for producing bulk nanocrystalline materials.     

The hot deformation behavior and processing map of Ti–47.5Al–Cr–V alloy

The high temperature flow behavior of as-extruded Ti–47.5Al–Cr–V alloy has been investigated at the temperature between 1100 °C and 1250 °C and the strain rate range from 0.001 s^− 1 to 1 s^− 1 by hot compression tests. The results showed that the flow stress of this alloy had a positive dependence on strain rate and a negative dependence on deformation temperature. The activation energy Q was calculated to be 409 kJ/mol and the constitutive model of this material was established. By combining the power dissipation map with instability map, the processing map was established to optimize the deformation parameters. The optimum deformation parameter was at 1150 °C–1200 °C and 0.001 s^− 1–0.03 s^− 1 for this alloy. The microstructure of specimens deformed at different conditions was analyzed and connected with the processing map. The material underwent instability deformation at the strain rate of 1 s^− 1, which was predicted by the instability map. The surface fracture was observed to be the identification of the instability.     

Influence of titanium and nitrogen on hot ductility of C–Mn–Nb–Al steels

Abstract

The influence of small additions of titanium on the hot ductility of C–Mn–Nb–Al steels has been examined. Titanium and nitrogen levels varied in the ranges 0·014–0·045 and 0·004–0·011 wt-%, respectively, so that a wide range of Ti/N ratios could be studied. The tensile specimens were cast and cooled at average cooling rates of 25, 100, and 200 K min^-1 to test temperatures in the range 1100–800°C and strained to failure at a strain rate of 2 × 10^-3 s^-1. It was found that ductility in the titanium containing niobium steels improved with a decrease in the cooling rate, an increase in the size of the titanium containing precipitates, and a decrease in the volume fraction of precipitates. Coarser particles could be obtained by increasing the Ti/N ratio above the stoichiometric ratio for TiN and by testing at higher temperatures. However, ductility was generally poor for these titanium containing steels and it was equally poor when niobium was either present or absent. For steels with ～0·005 wt-%N ductility was very poor at the stoichiometric Ti/N ratio of 3·4 : 1. Ductility was better at the higher Ti/N ratios but only two of the titanium containing niobium steels gave better ductility than the titanium free niobium containing steels and then only at temperatures below about 950–900°C. One of these steels had the lowest titanium addition (0·014 wt-%), thus limiting the volume fraction of fine Ti containing particles and the other had the highest Ti/N ratio of 8 : 1. However, even for these two steels ductility was worse than for the titanium free steels in the higher temperature range. The commercial implications of these results are discussed.     

Microstructural evolution during hot shear deformation of an extruded fine-grained Mg–Gd–Y–Zr alloy

Mg–Gd–Y–Zr alloys are among recently developed Mg alloys having superior mechanical properties at elevated temperatures. Dynamic recrystallization (DRX) and rare earth-rich particles play important roles in enhancing the high-temperature strength of these alloys. Accordingly, the microstructural evolution of a fine-grained extruded Mg–5Gd–4Y–0.4Zr alloy was investigated after hot shear deformation in the temperature range of 350–450 °C using the shear punch testing (SPT) method. The results reveal the occurrence of partial dynamic recrystallization at the grain boundaries at 350 °C while the fraction of DRX grains increases with increasing deformation temperature. A fully recrystallized microstructure was achieved after SPT at 450 °C. The Gd-rich and Y-rich cuboid particles, having typical sizes in the range of ~50 nm to ~3 μm, show excellent stability and compatibility after hot shear deformation, and these particles enhance the high-temperature strength during hot deformation at elevated temperatures. The textural evolution, examined using electron backscattered diffraction, revealed a non-fibrous basal DRX texture after SPT which is different from the conventional deformation texture.     

Tribological behaviour of hot extruded Al–Cu–Mg–Ag alloy reinforced by TiB2 particles

In the present investigation, tribological behaviour of the hot extruded Al–Cu–Mg–Ag (matrix) alloy and the effect of Ti and TiB₂ addition in matrix alloy have been studied. Hot extrusion was introduced to eliminate cast defects like porosity, voids and micro cracks. Addition of Ti and TiB₂ particles increased the hardness of the matrix by grain refinement and dispersion hardening, respectively. It has been observed that the increase in hardness had significantly improved the wear resistance of the material. Detail study of the wear surfaces and debris were carried out to understand the wear mechanism of the samples. It revealed a complex mechanism of micro-cutting, plastic deformation, abrasion and delamination of the wear samples.     

Fatigue properties of hot isostatically pressed Al–10Mg castings

Abstract

Constant amplitude fatigue tests have been carried out to assess the effect of hot isostatic pressing (HIP) on the fatigue properties of cast Al–10Mg (wt-%) alloy (AA 520.0). The results showed that HIP treatment can result in large increases in the fatigue life. At a test stress of 106 MN m^?2 (60% of proof stress) the average number of cycles to failure increased by 300% following HIP treatment. The magnitude of the increase was significantly greater at a lower test stress of 84 MN m^?2 (47% of proof stress). However, the scatter in the results from specimens fatigue tested at 106 MN m^?2 following HIP treatment was higher, resulting in a reduction in the safe fatigue life value that could be used for design purposes. Fracture surfaces of these specimens were examined to correlate fatigue life with defect levels. The high scatter can be attributed to the presence of oxide inclusions in the material.

MST/1621     

Microstructure characterisation of Fe–21Cr–15Ni–Nb–V non-magnetic austenitic stainless steel during hot deformation

Hot deformation behaviour of Fe–21Cr–15Ni–Nb–V stainless steel was investigated by isothermal compression in the temperature range of 950–1150°C with a strain rate of 0.01–10?s^?1. The results showed that complete recrystallisation occurred beyond 1050°C, resulting from the pinning effect of (Nb, V)(C, N). The nucleation of dynamic recrystallisation (DRX) was performed by the bulging, sub-grain swallowing and twinning mechanism. With increasing strain rate, new twinning was transformed into the Σ3 regeneration mechanism in the partial DRX region, while an opposite transformation was observed in the complete DRX region. In the partial recrystallisation region, grain rotation resulted in the formation of 110 orientation. In the complete recrystallisation region, the texture tended to distribute randomly at a high strain rate, and the grain growth was accompanied by the emergence of stable 100 orientation.     

Effect of Al content on hot tearing behaviour of Mg–xAl–2Ca–2Sm alloys

Mg–xAl–2Ca–2Sm (x?=?3, 5, 9 and 15) alloys were tested using an ‘L’-shaped sand mould serving as a hot tearing testing system. The experimental results showed that the solidification range of the Mg–xAl–2Ca–2Sm alloys first decreased and then increased as the Al content was increased. Furthermore, by increasing the Al content, the dendritic arms of the α-Mg phase become more developed, and the hot tearing tendency of the Mg–xAl–2Ca–2Sm alloys increased. In addition, the variety of precipitated phases was seen to be affected by the Al content and the tendency for hot tearing depended on the precipitated phase. The tendency of the Mg–xAl–2Ca–2Sm alloys for hot tearing first decreased and then increased with increasing Al content.     

Effect of Zn addition on hot tearing behaviour of Mg–0.5Ca–xZn alloys

The influence of Zn addition (0, 0.5, 1.5, 4.0 and 6.0 wt.%) on hot tearing behaviour of Mg–0.5 wt.% Ca alloy was investigated using a constrained rod casting (CRC) apparatus. The effects of mould temperature and grain refinement on the hot tearing susceptibility (HTS) were studied. Hot tears were observed with 3D X-ray tomography and the tear volumes were quantified. Results show that the Zn addition increases the HTS of Mg–0.5Ca alloys. At a mould temperature of 250 °C, all alloys investigated except Mg–0.5Ca–6Zn alloy show severe HTS. An increase in the mould temperature from 250 °C to 450 °C did not reduce the HTS in Mg–0.5Ca–1.5Zn and Mg–0.5Ca–4Zn alloys. Among all the investigated alloys, Mg–0.5Ca–4Zn alloy exhibits severe HTS as it completely broke away from the sprue–rod junction. The HTS of alloys was well correlated with the susceptible temperature range (ΔT_s). An increase in ΔT_s increased the HTS. The hot tears propagated along the grain boundaries through liquid film rupture. Grain refinement by Zr addition improved the hot tearing resistance of Mg–0.5Ca–4Zn alloy as the fine grain structure facilitated the easy feeding of liquid into the last area of solidification and accommodated the developed strain more effectively.     

Static recrystallisation and recrystallisation during hot deformation of Al–1Mg–1Mn alloy

Abstract

Plane strain compression tests at 5 s^?1 and at temperatures of 270–480°C have been carried out on an Al–1Mg–1Mn alloy containing a bimodal distribution of intermetallic particles and after a prior heat treatment to coarsen all particles to greater than 1 μm in size. During the heat treatment, recrystallisation of the initially hot worked material only proceeded with coarsening of the fine particles. During subsequent hot deformation, thin foil electron microscopy revealed that identical subgrain structures were developed in the two materials by dynamic recovery at temperatures below 450°C. At higher temperatures, the initially recrystallised material showed localised particle stimulated dynamic recrystallisation. The subsequent static recrystallisation rate was more than 10³ times faster in the material free from small particles.

MST/751     

Effects of Y and Zn/Y on hot tearing susceptibility of Mg–Zn–Y–Zr alloys

The effects of Y and Zn/Y on hot tearing susceptibility (HTS) of Mg–6.5Zn–xY–0.5Zr (x?=?4, 9, 12 and 18) and Mg–5Zn–13.5Y–0.5Zr alloys were investigated herein. The results illustrated that HTS of the investigated alloys decreased in the following order: Mg–6.5Zn–4Y–0.5Zr?>?Mg–6.5Zn–18Y–0.5Zr?>?Mg–6.5Zn–9Y–0.5Zr?>?Mg–6.5Zn–12Y–0.5Zr?>?Mg–5Zn–13.5Y–0.5Zr. The results also showed that HTS of the α-Mg-based alloy containing only LPSO phase was lower than that of the alloy containing only W-phase and (I+W) or (W+LPSO) mixed secondary phases. This was attributed to the coherence relationship between LPSO phase and α-Mg, and the bridging effect of LPSO phase.     

Plastic flow and microstructural evolution in Ti–6Al–2Zr–1Mo–1V alloys during hot deformation

Abstract

The hot deformation behaviour and microstructural evolution in Ti–6Al–2Zr–1Mo–1V alloys have been studied using isothermal hot compression tests. The processing map was developed at a true strain of 0·7 in the temperature range 750–950°C and strain rate range 0·001–10 s^?1. The corresponding microstructures were characterised by means of a metallurgical microscope. Globularisation of lamellae occurring to a greater extent in the range 780–880°C and 0·001–0·01 s^?1 had a peak power dissipation efficiency of 58% at about 850°C and 0·001 s^?1. The specimens deformed in 750–880°C and 0·01–10 s^?1 showed an instability region of processing map, whereas the specimens deformed in 880–950°C and 1–10 s^?1 indicated three kinds of flow instabilities, i.e. macro shear cracks, prior beta boundary cracks and flow localisation bands.     

Dynamic spheroidisation behaviour of the lamellar Ti–22Al–25Nb alloy during hot compression

The dynamic spheroidisation behaviour of the lamellar Ti–22Al–25Nb alloy during hot compression at 970°C/0.01?s^?1 and strain of 30%, 50% and 70% was investigated. The results showed the lamellar-O phase that participated in the B2 matrix could be obtained by hot pressed sintering from pre-alloy powders. The corresponding scanning electron microscopy, electron backscattered diffraction and transmission electron microscopy observations indicated that the dynamic spheroidisation process can be divided into three stages, the first is the primary interphase high-energy defect stage, introduced by dislocation migration twinning and localised shearing. The second is the separation, and the last is the spheroidisation stages, which could be explained by boundary splitting and termination migration.     

Tensile properties of hot rolled Mg–3Sn–1Ca alloy sheets at elevated temperatures

Mechanical behavior of hot rolled Mg–3Sn–1Ca (TX31) magnesium alloy sheets were studied in the temperature range 25–350 °C. The microstructure of the alloy consisted of the eutectic structure of α-Mg + Mg₂Sn and a dispersion of needle-like CaMgSn. The highest room-temperature ductility of 18% was obtained by hot rolling of the cast slabs at 440 °C, followed by annealing at 420 °C. The high temperature tensile deformation of the material was characterized by a decrease in work hardening exponent (n) and an increase in strain rate sensitivity index (m). These variations resulted in respective drops of proof stress and tensile strength from 126.5 MPa and 220 MPa at room temperature to 23.5 MPa and 29 MPa at 350 °C. This was in contrast to the ductility of the alloy which increased from 18% at room temperature to 56% at 350 °C. The observed variations in strength and ductility were ascribed to the activity of non-basal slip systems and dynamic recovery at high temperatures. The TX31 alloy showed lower strength than AZ31 magnesium alloy at low temperatures, while it exhibited superior strength at temperatures higher than 200 °C, mainly due to the presence of thermally stable CaMgSn particles.     

An analysis of hot deformation of an Al–Cu–Mg alloy produced by powder metallurgy

The high-temperature plasticity of a 2014 aluminium alloy produced by powder metallurgy was investigated in a wide range of temperatures and strain rates. When the strain rate was plotted as a function of stress (either peak flow stress in torsion, or applied stress in tensile creep), the alloy exhibited the same threshold-like behaviour observed in similar materials. The microstructure of representative torsioned samples was analysed in a transmission electron microscope (TEM) and the characteristics of particles and precipitate distribution were estimated. The dependence on stress and temperature was analysed by means of the conventional constitutive equations used for describing the hot-working behaviour and by means of a modified form of the sinh-equation, where the stress was substituted by an effective stress i.e. by the difference between the actual stress and a threshold stress. This temperature-dependent threshold stress was found to be a constant fraction (15%) of the Orowan stress generated by the dispersion of alumina particles and of precipitated intermetallic phases.     

Changes of flow stress and microstructure during hot deformation of Al–1Mg–1Mn

Abstract

Plane strain compression tests have been carried out at strain rates between 0·5 and 10 s^?1 and temperatures in the range 275–510°C, both under nominally isothermal conditions and with temperature decreasing. Also, temperature or strain rate have been changed in the interval between two deformations. In all cases, the stress–strain curves obeyed a mechanical equation of state, described by constitutive relationships in terms of strain and instantaneous value of Zener–Hollomon parameter Z. When the value of Z varies slowly during deformation, flow stress is uniquely related to subgrain size and to dislocation density within subgrains, but these relationships break down in transition structures developed after a change of Z between two deformations. The existence of an equation of state for mechanical behaviour, but not for microstructure, is considered to result from important contributions of both dislocation velocity and density to hot strength.

MST/1066     

Flow stress and recrystallisation during hot deformation of Cu–9%Sn alloys

Abstract

Compression tests were carried out on two compositions of Cu–Sn bronze: Cu–9·2Sn and Cu–9·1Sn–0·26Zn (wt-%). The experiments were performed at temperatures from ambient up to 750°C and at nominal (initial) strain rates in the range 10^-3 to 10^-1 s^-1. The measured data were converted into true stress–true strain curves; these displayed yield drops as well as single peaks (or maxima) at higher temperatures and lower strain rates. The mean rate sensitivity applicable to the curves was 0·25. Optical metallography indicated that dynamic recrystallisation of the ‘grain refinement’ type was taking place at the higher temperatures and proceeded by necklace formation. Electron backscattered diffraction measurements were also carried out; these revealed that twinning plays an important role in these materials. The present results show that the progress of recrystallisation is considerably slower than in OFHC copper and that the recrystallised grain size is appreciably finer. These observations, taken together, all indicate that the high temperature flow behaviour of the tin bronzes is controlled by solute drag and is not of the conventional ‘pure metal’ type.