Materials Science Forum Vols. 706-709

Abstract: The behaviour of dual phase steel tubes at 600, 780, and 980 MPa strength grades undergoing rotary draw bending and pressure sequence hydroforming is examined. Bending was performed using three different bend ratios. Principal strains were measured at several locations on each tube. It was found that the level of strain experienced by the tubes was independent of steel grade. The outcome of the bending process was stochastic in nature; for a given steel grade and bend ratio, some tubes were successfully formed, while others experienced failure. The proportion of failed tubes was found to increase with higher strength steel grades and tighter bend ratios. Metallographic samples from the extrados of the bent tubes revealed that many of the same microstructural features affecting strength and elongation in uniaxial tensile testing, namely martensite banding and non-metallic inclusions, affected dual phase steel tubes undergoing rotary draw bending. Additionally, a nanoindentation technique was investigated to determine the potential for more detailed microstructural characterization.

Abstract: The thermo-mechanical behaviour of the hot rolled dual-phase steel 10MnSi7 has been determined as a function of temperature and time within the two-phase field α+γ as well as at lower deformation temperatures down to Ms-temperature. The corresponding microstructures, phase hardness, state of recrystallisation have been determined using quantitative metallography. Using this information, a finite element model of a representative volume element (RVE) has been set up. Each element represents a subdomain of a grain that transforms according to the kinetics found in the measurements. The RVE has been subjected to the same loading history as in the accompanying experiments and the overall stress-strain response is monitored during ongoing transformation. The effect of a preferred orientation of the two phase structure has been studied taking into account the effects of a significant plastic deformation. The model is validated by comparison with the experimental evidence.

Abstract: The precipitation of vanadium takes place mainly in ferrite by interphase precipitation or nucleation on dislocation line, which makes sense for the industry production due to the precipitation strengthening. The objective is to analysize the cooling process of V-steels to exert the precipitation strengthening of vanadium. The steels with 0.09%C-0.055%N/0.0107%N/0.0168%N/0.0193%N-0.08%V/0.085V steel are the researched steel grades. Using solid solubility products model and thermodynamic equation, the full solid solution temperature, nucleation rate curve and PTT curve of precipitation process are calculated. The effect of nitrogen on the precipitation behaviour of V(C,N) in γ and the precipitation of V(C,N) in α are simulated. Based on the calculation results the trial process is determined. The laboratorial trials are carried out with ultra fast cooling. The precipitate particles are observed by TEM. The solid solution amount increases monotonously and the size of precipitate particle decreases with the nitrogen content. The solid solution temperature of 0.055%N, 0.0107%N, 0.0168%N and 0.0193%N are 977.0°C, 1028.0°C, 1062.3 and 1078.9°C respectively. The laboratorial trial results shows that the tensile strength is improved about 100 MPa due to the precipitation strengthening. The relationship between the coiling temperature and the strength is parabolic curve downward and the relationship between the coiling temperature and the elongation is parabolic curve upward. This calculation can determine both the proper nitrogen content and the optimal cooling process. The trial results proves this method is feasible and efficiency.

Abstract: The proper balance between yield strength, YS, and ductile to brittle transition temperature, DBTT, has been the main concern during development of high strength engineering steels and the effect of microstructure on impact toughness has attracted a great attention during the last decades. In this paper a review concerning the relationship between strength and toughness in steels will be presented and the effect of different microstructural parameters will be discussed, aiming to improve such properties in designing new high strength steels. Complex microstructures, obtained by quenching and tempering (Q&T) and thermo-mechanical (TM) processing are considered. The steels are low/medium carbon steels (C=0.04%-0.40%) with yield strength in the range YS=500-1000 MPa. Results show that the strength and the impact toughness behaviour are controlled by different microstructural parameters and not, as in the case of polygonal ferritic steels, by the same structural unit (the grain size) and that a “fine” microstructure is required in order to achieve high levels of both strength and toughness. The metallurgical design of high strength steels with toughness requirements is discussed using the same approach for both Q&T and TMCP processes.

Abstract: The paper presents the results of the numerical modelling and industrial research of the cooling ability of the device for the round plain bars accelerated cooling process. Research were carried out for one of the bar rolling mill technological conditions in a few variants. The paper purpose was determination of the cooling ability of device for accelerated cooling process to checking possibility of the using this device in the rolling line, during normalizing rolling process. Investigation results elaborated in the paper made the basis for determination of the heat exchange coefficients between cooled band and water. In the next stage of this paper numerical modeling of the normalizing rolling process with accelerated cooling of band in the final stage of the rolling process was carried out. In this investigation heat exchange coefficients between band and cooling medium (water), which were determinated with allowance of cooling possibilities of investigated device for band accelerated cooling were used. From the obtained results it was found that in the analysed bar rolling mill it is possible to decrease the band average temperature to about 900 °C, which is required during the normalizing rolling process.

Abstract: The influence of carbides diameter on the mechanical properties of the ferritic steel that was strengthened by the interface precipitated carbides in rows was investigated. Low carbon steel containing titanium as much as carbon in atomic concentration was induction-melt and hot-rolled, followed by the soaking at some temperatures to obtain completely transformed ferrite in which fine carbides precipitated. Lowering holding temperature resulted in fine carbide generation and reduction of the row spacing. Yield strength significantly increased with the decrease in the carbide diameter. The amount of the particle dispersion strengthening was close to the calculation result according to Ashby-Orowan Mechanism. On the other hand, elongation slowly decreased. The decrease in elongation is caused by the reduction of the uniform-elongation. Local elongation did not change by the change of the diameter of the fine carbides in the matrix since voids in the specimen were generated not besides fine carbides but beside large TiN during tensile test.

Abstract: The low cost Q460 and Q550 steel plates were produced in the 4200mm wide and heavy plate mill using Normal Hot Rolling and High Rate Cooling (NHR+HRC) process. The effect of both rolling processes on microstructure and mechanical property was studied. The results indicate that new type process can realize the good match between microstructure and mechanical properties. Meanwhile the new technology can short rolling time, reduce alloying addition, and improve the output of mill, it is also promote iron and steel industry developing continuously.

Abstract: The automotive industry applies pressure on the PM industry to produce components with superior mechanical properties at minimum cost. In this regard, sinter-hardenable powders are particularly well suited since they allow direct quenching of components at the end of the sintering cycle, thus eliminating the extra steps required for heat treating. This paper presents the results of the modeling of the influence of admixing and/or prealloying on the optimization of compressibility and hardenability of sinter-hardenable steel powders. A first design of experiments (DOE) was used to optimize the chemical composition and to study the interactions between prealloyed elements (Nickel, Chromium, Molybdenum and Manganese) and admixed elements (Nickel, Chromium, Manganese and Copper) on hardenability and compressibility. A second DOE was generated based on the results obtained in the first series. Results show that among all of the examined alloying elements, only prealloyed nickel, chromium and molybdenum had a significant effect on compressibility and hardenability. Moreover, within the range of concentrations under study, the optimum sinter-hardenable powder had the following (prealloyed) chemistry: 1.5 wt-% Ni, 0.55 wt-% Cr and 1.25 wt-% Mo.

Abstract: A systematic experimental investigation was conducted using lab processed low carbon 0.08C-2.0Mn-Cr-Mo steel microalloyed with Ti/Nb to evaluate the influence of initial hot-rolled microstructures on the kinetics of austenite formation and decomposition after cold-rolling and subsequent annealing. Coiling temperature as a major hot rolling parameter was used to obtain different types of hot-rolled microstructures. Dilatometer and continuous annealing simulator were employed for austenite formation studies and annealing simulations, respectively. It was found that the coiling temperature affects the processes occurring during heat treatment in continuous annealing lines of full hard material: ferrite recrystallization, austenite formation during continuous heating and austenite decomposition during cooling. A decrease in coiling temperature accelerates the recrystallization of ferrite and nucleation of austenite, which results in formation of refined ferrite-martensite structure. The effect of initial hot rolled structure on final mechanical properties after continuous annealing was also investigated.

Abstract: A series of anisothermal multipass hot torsion tests were carried out to simulate hot rolling on three high-strength low-carbon steels with different amounts of Mn, Mo, Nb and Ti and designed for pipeline construction. Mean Flow Stress was graphically represented against the inverse of temperature to characterize the evolution of austenite microstructure during rolling. The effect of austenite strengthening obtained at the end of thermomechanical processing on the final microstructure obtained after cooling was studied. Higher levels of austenite strengthening before cooling promote a refinement of final microstructure but can also restrict the fraction of low-temperature transformation products such as acicular ferrite. This combined effect gives rise to a wide range of final microstructures and mechanical properties depending on the composition, processing schedule and cooling rates applied. On the other hand, the precipitation state obtained at diverse temperatures during and at the end of hot rolling schedule was evaluated by means of transmission electron microscopy (TEM) in two microalloyed steels. It was found that two families of precipitates with different morphology, composition and mean size can coexist in microalloyed steels.