Materials Science Forum Vol. 941

Abstract: The aim of this study was to determine the effect of non-isothermal tempering on microstructure evolution in large-size slabs. Using high-resolution dilatometry, three different cooling rates (from 0.08 to 3°C/s) representative of different regions from the surface to the core of the slab were experimentally simulated, and then tempering was carried out for each starting microstructure. A combination of light and electron microscopy and X-ray diffraction analyses were employed to accurately analyze different phenomena occurring during the tempering process, specially, the identification of different microstructures (bainite, martensite and retained austenite), and the determination of the percentage of retained austenite for each experimental condition were considered. Experimental results revealed that the microstructure after the cooling rate of 0.08°C/s consisted of bainite and some retained austenite. For the cooling rate of 0.3°C/s, martensite plus bainite was detected, and when the cooling rate was increased to 3°C/s, a martensitic microstructure was obtained. Analysis of dilatometry curves indicated that tempering behavior varied significantly with the starting microstructure. Martensite tempering was accompanied with a length decrease due to the decomposition of medium-carbon martensite to low-carbon martensite plus carbides. Tempering of bainite and retained austenite resulted in a remarkable length increase.

Abstract: Commercial production of high strength steel plates by the quenching and tempering (Q&T) route requires control of alloy design and heat treatment parameters to achieve the desired strength and toughness through thickness. Plates with different thicknesses (up to approximately 100 mm) are produced for applications in the energy and power or lifting and excavation sectors. For thick plate the difference in cooling rate through thickness affects the as-quenched microstructure with martensite, auto-tempered martensite and lower and/or upper bainite being present. The different as-quenched microstructures can show a different response to tempering which affects the final strength and toughness.In this study the starting microstructure of a low alloy 0.17 wt% C Q&T steel has been varied using isothermal heat treatment at 430 °C to create mixed martensite and lower bainite microstructures (nominally 25:75; 50:50 and 75:25 percentages). The effects of tempering at 600 °C for times between 0.5 and 16 hours on the carbide precipitates and hardness of the mixed microstructures have been investigated and compared to the tempering response of single phase (martensite and lower bainite) microstructures. It has been found that the hardness decrease due to tempering is larger in the martensitic structure than the bainitic structure due to more rapid carbide coarsening. The as-quenched hardness of the mixed microstructures can be predicted by a rule of mixtures using the single phase properties. The tempering response of the mixed microstructures is discussed.

Abstract: Development of high strength or even ultra-high strength steels is mainly driven by the automotive industry which strives to reduce the weight of individual parts, fuel consumption, and CO₂ emissions. Another important factor is the passenger safety which will improve by the use of these materials. In order to achieve the required mechanical properties, it is necessary to use suitable heat treatment in addition to an appropriate alloying strategy. The main problem of these treatments is the isothermal holding time. These holding times are technologically demanding which is why industry seeks new possibilities to integrate new processing methods directly into the production process. One option for making high-strength sheet metals is press-hardening which delivers high dimensional accuracy and a small spring-back effect. In order to test the use of AHSS steels for this technology, a material-technological modelling was chosen. Material-technological models based on data obtained directly from a real press-hardening process were examined on two experimental steels, CMnSi TRIP and 42SiCr. Variants with isothermal holding and continuous cooling profiles were tested. It was found that by integrating the Q&P process (quenching and partitioning) into press hardening, the 42SiCr steel can develop strengths of over 1800 MPa with a total elongation of about 10%. The CMnSi TRIP steel with lower carbon content and without chromium achieved a tensile strength of 1160 MPa with a total elongation of 10%.

Abstract: Low-density medium-manganese steels offer a vast development prospect for industrial application due to their outstanding combination of mechanical properties and density reduction. The microstructural evolution following tensile deformation of cold-rolled and annealed Fe-10Mn-10Al-0.7C steels was investigated by means of SEM and TEM microstructure analysis and XRD measurements. Annealing in the range of 700-1100 °C led to an austenite-ferrite dual-phase microstructure that was characterized by tensile strength of 700-1100 MPa and elongation of 6-34%. κ-carbides were observed in steels annealed at relatively low temperatures (700-850 °C). The steel exhibited the optimum combination of tensile strength of 930 MPa and elongation of 34% after annealing at 900 °C for 0.5 h. The stacking fault energy was estimated to be 69mJ/m2 considering the difference between average constituent and practical constituent of austenite caused by the high ferrite fraction. The deformed microstructures of the austenite exhibited the typical planer glide characteristics in sequence of dislocation array, Taylor lattice, Taylor lattice domain and microband. And the wavy glide occurs in ferrite was manifested by tangled dislocation and dislocation cells.

Abstract: In the present study, the effects of ausforming on the bainitic transformation, microstructure and mechanical properties of a low-carbon rich-silicon carbide-free bainitic steel have been investigated. Results show that prior ausforming shortens both the incubation period and finishing time of bainitic transformation during isothermal treatment at a temperature slightly above the M_s point. The thicknesses of bainitic ferrite laths are reduced appreciably by ausforming; however, ausforming increases the amount of large blocks of retained austenite/martenisite and decreases the volume fraction of retained austenite. And accordingly, ausforming gives rise to significant increases in both yield and tensile strengths, but causes noticeable decreases in ductility and impact toughness.

Abstract: In this work, to clarify the effect of carbide precipitation state on strength and toughness, Ti,V alloyed precipitation hardened ferrite single phase steel sheets with different carbide size were investigated. In order to change the precipitated particle size, cooling conditions after hot rolling were changed. Under condition A, steel sheets were cooled to 873K by water spray (for fine precipitation). Under condition B, steel sheets were air-cooled from 1053K for 20sec, then cooled by water spray to 873K (for coarse precipitation). The experimental results were following. The balance of tensile strength and Charpy absorbed energy was better in condition B. (Ti,V)C were observed in both conditions, but the size of (Ti,V)C were larger in condition B. From the above, it was suggested that as the carbide size become larger, the decrease in toughness per strengthening amount becomes smaller.

Abstract: Although welding results in premature failure by type IV fracture under high temperature creep conditions, the alloy design of light elements such as boron addition and nitrogen reduction enhances the creep lifetime of 9Cr heat resistant steel. In particular, the simulated heat affected zone (SHAZ) sample of new 9Cr steel (called TA steel) shows about 10 times longer creep lifetime than that of the standard Gr. 91 steel. The welded TA steel is thus expected to exhibit good creep properties because its SHAZ sample has coarser grains and suppresses type IV fracture. The preservation of base metal’s microstructure after welding results from the precipitate morphology, such as high grain boundary coverage by precipitates and low amount of MX being nucleation sites of ferrite grains during the a-g phase transformation. In addition, the increase of stability of M₂₃C₆ affects high pinning pressure toward grain boundary migration upon rapid heating during welding. First-principles calculations confirm the increased stability when boron is absorbed by M₂₃C₆. Moreover, the calculations reveals that boron decreases the coherency between matrix and M₂₃C₆, suppressing grain coarsening during creep tests in TA steel. It is concluded that the increased microstructural stability during welding and long high temperature exposure generates the elongated creep lifetime in welded TA steel including about 0.01 wt% boron and less than 0.01 wt% nitrogen.

Abstract: Medium manganese steels are nowadays energetically investigated as the third generation advanced high strength steels (AHSS) because of their excellent balance between material cost and mechanical properties. However, the phase transformation and microstructure evolution in medium manganese steels during various heat treatments and thermomechanical processing are still unclear. The present study firstly examined kinetics of static phase transformation behavior and microstructural change in a 3Mn-0.1C medium manganese steel. Hot compression tests were also carried out to investigate the influences of high-temperature thermomechanical processing on the microstructure evolution. It was found that ferrite transformation was quite slow in static conditions but greatly accelerated by hot compression in (austenite and ferrite) two phase region. Dual phase microstructures composed of martensite and ferrite with ferrite grain sizes of 1~2 μm were obtained, which exhibited superior mechanical properties.

Abstract: In cold forming for automotive lightweight design, advanced high strength steels (AHSS) lead to limited formability, high springback and press forces, low stretch flangeability, multiple operations for complex geometries and large scrap rates. Two sets of AHSS, namely ferritic-martensitic dual-phase (DP) steel and martensitic-bainitic complex-phase (CP) steel with some amounts of retained austenite (RA), were designed for the hot-forming route, which eliminates the above drawbacks and guarantees higher performance in the body-in-white (BIW). Design of four DP and four CP alloys was accomplished using JMatPro6.0 thermodynamic software and available literature. The alloys were manufactured in the laboratory in cold-rolled gauge of ~1.5 mm and subjected to hot-forming cycles including hot deformation (up to 20% strain), using a dilatometer and a Gleeble 3800 machine. The thermal cycles of the DP alloys included an intercritical reheating whereas in-situ austempering or slow continuous cooling followed by supercritical reheating was used for the CP alloys. The results showed that yield strength (YS) of 605MPa & 695MPa, ultimate tensile strength (UTS) of 1097MPa & 1242MPa with a total elongation (TE) of 12.6% & 14.1% can be achieved in the best performing DP alloys with a martensite content of 65% & 60 vol.%. The best CP alloys with austempering achieved YS of 673MPa & 699MPa, UTS of 983MPa & 1026MPa and TE of 9.2% & 13.6% with RA of 4%-12 vol.%. The continuously-cooled alloys achieved even better properties. Higher bendability at 1.0 mm gauge in the critical direction was achieved in the CP alloys (90^o&107^o) than in the DP alloys (73^o&76^o).

Abstract: This paper analyzed mechanical properties and splitting fracture performance of 46MnVS5 with different microstructures. It will provide support of date for new type of splitting fracture connecting rod materials. The results show that bainite has good strength and ductility, but its high ductility is bad for splitting fracture. Martensite and tempered martensite have high strength and low ductility, which is bad for using performance. Ductility of tempered sorbite is too high to meet splitting fracture performance. Tensile and yield strength, impact toughness of tempered troostite is 1569MPa, 1407MPa and 17.5J/cm², respectively. Tempered troostite has high strength and good splitting fracture performance. Bainite+martensite and tempered troostite maybe two good choices for better splitting fracture and using performance.