Papers by Keyword: Hot Ductility

Abstract: In order to evaluate the feasibility of 9SiCr alloy tool steel produced by thin slab casting, the high temperature mechanical properties of 9SiCr alloy tool steel were investigated by Gleeb-1500 thermal simulator. The morphologies of the tensile fracture at different temperatures were observed by scanning electron microscope (SEM), together with analysis of fracture mechanisms in different regions. The results showed that there were two brittle zones in the temperature range from 600 °C to 1200 °C. A melting fracture was characterized in the high temperature brittle zone of above 1170 °C, whereas a typical cleavage fracture was exhibited in the low temperature brittle zone from 820 °C to 600 °C, Meanwhile, a good hot ductility behavior characterized by typical dimple fracture was demonstrate at the temperature range from 1170 °C to 820 °C.Thus, the 9SiCr alloy tool steel with the final gauge of 1.5mm was produced by CSP, based on the optimal process parameters.

Abstract: Modern steel making and hot rolling processes like CSP^® thin slab technology require precise data on casting and rolling behavior of the produced steel grades. Up today only few data is available for the latest generations of advanced high strength steel (AHSS) grades. AHSS have developed by 3 generations [1, 2]. 1^st Generation AHSS as dual phase (DP), complex phase (CP), martensitic (MS) and transformation induced plasticity (TRIP) steel grades are currently applied in automotive industry. 2^nd and 3^rd Generation AHSS typically have elevated Mn-content as well as Al and Si content. High Mn-content of up to 30% seriously affects casting and forming properties of 2^nd Generation AHSS. In particular, the large solidification range of more than 100 K prevents commercial production of these steel grades by continuous casting [3]. 3^rd Generation AHSS with reduced Mn-content up to about 12% are currently under development [1-4]. Investigations have been carried out to assess the CSP^® thin slab process for the production of such grades. To this purpose solidification and hot forming properties of different alloys having Mn-content up to 10% have been examined by thermodynamic calculations and laboratory testing by hot forming dilatometry. The achieved flow curves match figures achieved on a hot rolling mill.

Abstract: The influence of aluminium content on the hot ductility behaviour in V-N steels was investigated. Cylindrical specimens were subjected to thermal cycles and strain rates approximating those experienced by a conventional slab surface during continuous casting. The resulting microstructures were examined using light optical and electron microscopy and correlated with measured reduction-in-area (RA) values, calculated precipitate chemistries and volume fractions, as well as the flow stress behaviour. It was found that removal of aluminium significantly improved the hot ductility. However, increasing the total [V][N] product in Al-free steels reduces RA. Poor hot ductility is caused by low austenite grain boundary mobility characterized by high work hardening rates. The fracture mode in brittle specimens is intergranular along thin ferrite films. AlN appears to inhibit austenite grain boundary mobility in V-high N steels when the cooling rate and strain rate are both very slow as experienced during unbending. SEM analysis of fracture surfaces revealed the presence of MnS-AlN particles in microvoids. TEM-EDAX spectra showed that the larger particles observed in the Al-containing steels are mostly a constitution of duplex/triplex grain boundary precipitates, i.e., MnS-AlN and V(C,N). Conversely, good ductility in austenite is associated with high grain boundary mobility that produces fine, recrystallised grains and subsequent dimple fracture after plastic tensile stress.

Abstract: Hot ductility of 20Cr13 steel at high temperature was investigated through tensile test. The main phases of the steel in temperature range of 600-1400 °C were calculated with thermodynamic software. The fracture morphologies and microstructure were observed by scanning electron microscopy. The steel showed good hot ductility in temperature range of 1000-1200 °C, and the area reduction was 82 % or more. In temperature range of 800-950 °C, the M₂₃C₆ precipitated at the grain boundary of austenite, and it fractured in the tensile process. Thus the micro-crack or micro-hole formed at the grain boundary of austenite, and the area reduction was 52-68 %. The ferrite precipitated at 800 or 1250 °C, and the micro-voids formed at interface of ferrite and austenite because of the discordant deformation of two phases, which was harmful to hot ductility. Moreover, quasi-cleavage fracture happened at 800 °C because of the ferrite cleavage, which further decreased hot ductility of the steel.

Abstract: The aim of this work was to observe the relationship between hot ductility and morphology, distribution and size of particles in TiNb IF steel after hot torsion testing at the critical temperatures of deformation with low as well as maximum values of plasticity. Transmission electron microscopy showed that the particles at all temperatures of deformation with minimum number of turns to failure e.g. 1132°C (33.72 rev.), 946°C (6.24 rev.), 637°C (5.54 rev.) as well as with maximum value of plasticity at 844°C (1726 rev.) were of globular, cuboid or elliptical shape. EDX analysis revealed that there were different types of particles such as carbides, sulfides, and carbonitrides of Ti and Nb, Al and Si oxides, Mn sulfides, and phosphides. Quantitative evaluation of particle size in the carbon extracted replicas showed 20% of total number of particles with size 2r = 30-39 nm and an average linear dimension = 42 nm at the deformation temperature of 1132°C. There were 28% of the particles with size 2r = 20-29nm and = 41nm at the temperature of 946°C while at the temperature of 637°C there were 29% of particles with size 2r = 20-29 and 26% with size 2r=10-19 nm and = 41 nm. In the case of maximum plasticity (1726 rev.) at 844°C, the presence of large particles was confirmed with = 105 nm size and 9% distribution in three size categories of 20-29, 30-39, 90-99 nm.

Abstract: One of the main challenges during continuous casting of microalloyed steel is to avoid the formation of transverse surface cracks on the steel slabs. These cracks occur due to severe mechanical and thermal stresses in the strand during the straightening operation. The reason for this phenomenon is a ductility loss of austenite in a typical temperature range of 700°C - 1100°C. One of the main mechanisms reducing the ductility is the precipitation of carbides and nitrides. In this work, we correlate ductility loss and precipitation state accompanying two model cooling strategies in a microalloyed steel grade. Continuously cooled samples show a minimum of ductility at temperatures around 750°C. With increasing temperature, deformability recovers again to reach full ductility again at 950°C. In contrast, samples treated with a fast intermediate cooling and reheating show constant low ductility in this entire temperature range. A transmission electron microscopy (TEM) investigation shows nanometer-sized NbC precipitates in the low ductility states. In contrast, in the samples with high ductility, larger NbC precipitates with lower number densities are observed. The experimental results show a good accordance with corresponding precipitation kinetics simulations carried out with the MatCalc software package.

Abstract: Twelve experimental steels with a base composition 1.5wt% Mn, 0.01 wt% V and 0.1 wt% Nb and varying C (0.05, 010 and 0.20 wt%), Ti (20 – 260 ppm) and B (0 – 100 ppm) contents have been systematically examined to quantify the effects of composition on precipitation behavio-ur and hot ductility during simulated continuous casting conditions. Nb-rich precipitates were present in the alloys with 0.10 wt-% C and 0.20 wt-% C. Alloys with 0.05, 010 and 0.20wt% C contained 50 – 100 nm size Ti-Nb carbonitrides. Boron was bound in 20 – 100 nm size boronitrides located in prior austenite grain boundaries. A Gleeble 3800 was used to study hot ductility and strain induced precipitation processes in the alloys. Alloys without B and Ti additions exhibited poor hot ductility at 850°C and 950°C, whereas the 0.05 wt-% C and 0.10 wt-% C alloys showed improved hot ductility (reduction in area 40-50%) by the addition of either >50 ppm B or 250 ppm Ti. The 0.2 wt-% C alloys showed no improvement from B or Ti additions. Examination of fracture surfaces of hot ductility specimens showed that boronitrides were located at prior austenite grain boundaries in alloys containing 80 – 100 ppm of B. Compression-relaxation tests showed that alloying with boron caused a noticeable decrease of the start temperature of strain-induced precipitation in the alloys.

Abstract: The hot ductility of Ti-Nb microalloyed steel has been investigated to evaluate the sensitivity to surface crack formation during the continuous casting process. Tensile samples were subjected to different thermal treatments and were tested at deformation temperatures ranging from 650°C to 1000°C using a strain rate of 10^-3s^-1. It has been found, that the investigated steel evinced poor ductility over almost the whole testing temperature range characterized by marked grain boundary cracking, irrespective of which thermal cycle has been utilized or whether the samples have been melted or only reheated. Microstructural examinations and supplementary thermo-kinetic computer simulations revealed distinct Ti-Nb precipitation throughout the microstructure being responsible for the deteriorated materials hot ductility.

Abstract: High temperature tensile tests of novel developed Nb-microalloyed weathering steels were carried out with a constant true strain rate of 0.001/s at 650°C-1300°C using Gleeble3500 thermo-mechanical simulator. The tensile strength (TS) and reduction in area (RA) were calculated afterwards to obtain hot ductility curve and hot strength curve of the steel. The hot ductility behaviors were studied in detail under optical microscope (OM), scanning electron microscope (SEM) and transition electron microscope (TEM). The third brittle zone of the studied steels was between 650°C-800°C. SEM fractographs and microstructures of the tensile specimen showed that the occurrence of the third brittle zone was mainly related to the formation of pro-eutectoid ferrite film along the prior austenite grain boundaries, and secondly to the precipitation of second phases. Therefore, it was recommended that the straightening temperature of the studied steel after casting should be kept over 800°C to get crack free continuous casting (CC) slabs.

Abstract: Hot ductility of HRB500E vanadium-containing steel rebar was study with thermal simulation test and the secondary cooling model was established based on the relationship between the cooling water and the casting speed. The solidification cooling curves and the temperature fields of HRB500E were simulated with the PROCAST software. The result of the hot ductility and the pin shooting experiment shows that the casting straightening temperature should be above 902 °C and the secondary cooling model was feasible and accurate. Finally the high-quality billet of HRB500E was obtained.