In situ measured growth rates of bainite plates in an Fe-C-Mn-Si superbainitic steel

The growth rates of bainite plates in an Fe-C-Mn-Si superbainitie steel were investigated by in situ observation. The lengthening rates of ferrite bainite during both cooling and isothermal holding processes were observed and the growth rates of bainite plates nucleating at grain boundaries, within grains and on preformed bainite were measured. It is indicated that the lengthening rates of bainite plates during the cooling and isothermal processes were different, and that the growth rates of bainite plates nucleating at different types of sites also demon- strated diversity. The bainite plates initiating at [vain boundaries during cooling grew the fastest, while the plates nucleating on preformed bainite did the slowest. However, the growth rate of the bainite plates nucleating at grain boundaries during isothermal transformation de- creased the most, whereas the bainite plates initiating within grains grew the fastest. In addition, the growth rate of ferrite bainite in the study supported the diffusion transformation mechanism of bainite from the viewooint of ~rowth rate.

In situ observations of austenite grain growth in Fe-C-Mn-Si super bainitic steel

In situ observations of austenite grain growth in Fe-C-Mn-Si super bainitic steel were conducted on a high-temperature laser scanning confocal microscope during continuous heating and subsequent isothermal holding at 850, 1000, and 1100℃ for 30 min. A grain growth model was proposed based on experimental results. It is indicated that the austenite grain size increases with austenitizing temperature and holding time. When the austenitizing temperature is above 1100℃, the austenite grains grow rapidly, and abnormal austenite grains occur. In addition, the effect of heating rate on austenite grain growth was investigated, and the relation between austenite grains and bainite morphology after bainitic transformations was also discussed.

Influence of Ti(C,N)precipitates on austenite growth of micro-alloyed steel during continuous casting

Austenite grain size is an important influence factor for ductility of steel at high temperatures during continuous casting. Thermodynamic and kinetics calculations were performed to analyze the characteristics of Ti(C,N) precipitates formed during the continuous casting of micro-alloyed steel. Based on Andersen-Grong equation, a coupling model of second phase precipitation and austenite grain growth has been established, and the influence of second precipitates on austenite grain growth under different cooling rates is discussed. Calculations show that the final sizes of austenite grains are 2.155, 1.244, 0.965, 0.847 and 0.686 mm, respectively, under the cooling rate of 1, 3, 5, 7, and 10 ℃·s^(-1), when ignoring the pinning effect of precipitation on austenite growth. Whereas, if taking the pinning effect into consideration, the grain growth remains stable from 1,350 ℃, the calculated final sizes of austenite grains are 1.46, 1.02, 0.80, 0.67 and 0.57 mm, respectively. The sizes of final Ti(C,N) precipitates are 137, 79, 61, 51 and 43 nm, respectively, with the increase of cooling rate from 1 to 10 ℃·s^(-1). Model validation shows that the austenite size under different cooling rates coincided with the calculation results. Finally, the corresponding measures to strengthen cooling intensity at elevated temperature are proposed to improve the ductility and transverse crack of slab.

Kinetics Analyzing of Direction Reduction on Manganese Ore Pellets Containing Carbon

Using high temperature carbon tube furnace, reduction of manganese ore pellets containing carbon was investigated. The reaction was divided into two stages at five minutes after reaction, and the kinetics model of reduction process was established. The experimental results showed that, the reaction rate in the earlier stage was controlled by the chemical reactions between FeO, MnO and carbon reductant, and the activation energy was 28.85 KJ/mol. In the later stage, as the carbon reductant replaced by CO, the reaction rate was controlled by CO-diffusing in solid products, and the cor- responding activation energy was 86.56 KJ/mol. Reaction rate of the later stage was less than the earlier one.

Nitride-inclusion characterization in lightweight steel and re-precipitation behavior of AlN during heat treatment:effect of Al content

The composition,morphology,size,and number of inclusions in Fe-22Mn-xAl-0.7C(x=0.5%,5.2%,and 10.5%)lightweight steels after smelting and heat treatment experiments are characterized by scanning electron microscopy and energy-dispersive X-ray spectroscopy from both 2D and 3D perspectives.The inclusions are classified into eight categories according to chemistry and morphology.For the steel sample with a low Al content(0.5%),the inclusions are MnS,AlN-MnS,and Al_(2)O_(3)-MnS,among which Al_(2)O_(3)-MnS is the primary type.With the increase in Al content in the steel sample(to 5.2%),the populations of AlN and AlN-MnS inclusions of 1-3μm in diameter increase.A further increase in Al content(to 10.5%)leads to a significant decrease in the amount of AlN inclusions of 1-3μm in diameter,and an increase in the amount of AlN-MnS inclusions.The precipitation behavior during the phase transformation is also studied by FactSage 8.0 thermodynamic software,and a precipitation mechanism is proposed based on the calculation results.During the heat treatment,AlN inclusions re-precipitate out,due to the interactions between Al and dissolved N in the steel matrix.However,AlN inclusions cannot grow large because of unfavorable kinetic conditions.The re-precipitation phenomenon of AlN is predominant under low Al and high N conditions but not at high Al cases.

Dissolution kinetics of solid fuels used in COREX gasifier and its influence factors

Carbon dissolution from solid fuels used in a COREX gasifier was investigated in a high-temperature furnace to investigate the influences of temperature, carbon structure and ash properties of solid fuels into molten iron on carbon dissolution behavior. The results showed that the final carbon content of molten iron and dissolution reaction rate of carbon increased as the temperature increased. However, the dissolution behavior of different solid fuels varied with their properties. At the same temperature, the dissolution reaction rate of solid fuel from high to low was coke, semi-coke and lump coal. The apparent reaction rate constants of solid fuel were calculated using the piecewise fitting method based on the experimental data. The analyzed results showed that the dissolution rates of solid fuels had a good correlation with their microcrystalline structures. Moreover, the carbon crystallite structures of solid fuels used in COREX had greater influence on dissolution behavior than their ash properties.

Genetic Analysis for Large TiN Inclusions in Wire Rod for Tire Cord Steel of SWRH82A

The law of element segregation of Ti, N, Mn and S, and the sequence of selective precipitation of TiN and MnS inclusions during solidification of molten steel of SWRH82 A are studied on the basis of thermodynamics. The origin of large TiN inclusions which affect the titanium inclusions point penalty in SWRH82 A wire rod is analyzed based on the research on the distribution characteristics of MnS and large size of TiN inclusions observed on metallographic specimen of SWRH82 A steel wire rod. The solidification segregation ratio of Ti is far more than that of N, and the solidification segregation ratio of S is far more than that of Mn. In the range of cooling rate of the continuous casting production, the cooling rate of solidification has little effect on the segregation ratios of Ti, N, Mn and S. MnS inclusions will precipitate earlier than TiN inclusions during solidification of the molten steel of SWRH82 A. The large TiN inclusion which is wrapped by MnS in the SWRH82 A wire rod may be foreign inclusions and it is not precipitated product during solidification in the molten steel of SWRH82 A.

Effects of Al addition on bainite transformation and properties of high-strength carbide-free bainitic steels

The effects of aluminum addition on bainite transformation and properties of carbide-free bainitic steels containing 0.22 wt.% carbon were investigated by two different types of heat treatment processes: continuous cooling process (CCP) and isothermal transformation process (ITP). The results indicate that for the CCP treatment, Al addition significantly promoted the ferrite and bainite transformation;however, it did not significantly increase the product of tensile strength and total elongation (PSE). For the ITP treatment, Al addition significantly promoted the kinetics of bainite transformation, and thus, more bainite was formed with Al addition;however, it was found that Al addition resulted in a decrease in tensile strength and an increase in elongation of the tested bainitic steels. Moreover, the effects of Al addition on comprehensive property were profoundly dependent on austempering temperatures. When the austempering temperature was higher (430℃), PSE significantly increased with Al addition, whereas it decreased at the lower austempering temperature (400℃). Therefore, it can be concluded that the effects of Al on properties of bainitic steels were more significant at higher austempering temperatures.

Precipitation and Solid Solution of Titanium Carbonitride Inclusions in Hypereutectoid Tire Cord Steel

The properties of titanium carbonitride Ti（CxN1-x） inclusions precipitated during solidification of tire cord steels and the thermodynamic conditions for their decomposition and solid solution during billet heating were investigated using a thermodynamics method. The solid solution of Ti（CxN1-x） inclusions during high-temperature heating was also studied experimentally. The results revealed that： （1） the higher the content of carbon in the tire cord steel is, the greater the value of .r in the Ti（CxN1-x） inclusions is; （2） the higher the content of carbon in the tire cord steel is, the earlier the Ti（CxN1-x） inclusions precipitated during the solidification process and the lower the solidification front temperature is during precipitation; （3） when an 82A steel sample was heated to 1087℃, the Ti（CxN1-x） inclusions possess the thermodynamic conditions of decomposition and solid solution; and （4） when 82A samples were heated to 1150 and 1 250℃, the total number of Ti（CxN1-x） inclusions larger than 5μm in diameter decreased by 55.0% and 70.3%, respectively. In addition, although smaller inclusions with diameter less than 2 μm continued to decompose when the sample was heated at 1 250℃ for 2 h and then cooled to 1000℃ in the furnace, the number of inclusions larger than 5 μm in diameter increased.

Analysis on Surface Longitudinal Cracking of Q345D Slab

The surface longitudinal cracking of peritectic steel Q345 D slab was studied by means of metallographic microscopy and scanning electron microscopy. The results show that the surface longitudinal cracking is caused by the shell repeatedly tearing and healing under the action of stress, which is due to disruption of the steel matrix continuity with inclusion aggregation on the shell surface. The relative precautions of reducing the cracking are raised according to the practical situation.

Effect of heat input on microstructure and toughness of rare earthcontained C-Mn steel

Gleeble-1500D thermal simulator was used to simulate the thermal cycle of different welding processes in C-Mn steel. The toughness of steel matrix and heat-affected zone （HAZ） was investigated, and the microstructure and inclusion were characterized as well. The results showed that the welding process has a great influence on the microstructure in HAZ. When t8/5 （the cooling time from 800 to 500℃ ） value of the welding process is less than 111 s, the microstructure in HAZ is mainly bainite/Widmanstatten in spite of the addition of rare earth. However, when t8/5 value is more than 250 s, there are a lot of acicular ferrites in the steel containing rare earth, while the main microstructures are grain boundary ferrite and bainite/Widmanstatten in the steel without rare earth. The impact toughness of the HAZ at ambient temperature first decreases and then increases with the increase in t8/5 value. The impact toughness is the worst when t8/5 value is 111 s. Rare earth can improve the impact performance of HAZ at ambient temperature, especially when t8/5 value of the welding process is 445 s. After the rare earth treatment, the cooling rate of forming acicular ferrite is about 0.5-7.5 ℃/s. Acicular ferrite can form even during the welding process with larger t8/5 value up to 600 s in rare earth-treated steel.

Three-dimensional structure and micro-mechanical properties of iron ore sinter

A new analysis method based on serial sectioning and three-dimensional（3 D）reconstruction was developed to characterize the mineral microstructure of iron ore sinter.Through the 3 Dreconstruction of two types of iron ore sinters,the morphology and distribution of minerals in three-dimensional space were analyzed,and the volume fraction of minerals in a 3 Dimage was calculated based on their pixel points.In addition,the microhardness of minerals was measured with a Vickers hardness tester.Notably,different mineral compositions and distributions are obtained in these two sinters.The calcium ferrite in Sinter 1 is dendritic with many interconnected pores,and these grains are crisscrossed and interwoven;the calcium ferrite in Sinter 2 is strip shaped and interweaves with magnetite,silicate and columnar pores.The calculated mineral contents based on a two-dimensional region are clearly different among various layers.Quantitative analysis shows that Sinter 1 contains a greater amount of calcium ferrite and hematite,whereas Sinter 2 contains more magnetite and silicate.The microhardness of minerals from highest to lowest is hematite,calcium ferrite,magnetite and silicate.Thus,Sinter 1 has a greater tumbler strength than Sinter 2.

Experimental study on smelting reduction of carbon-bearing manganese briquettes in slag bath

The reduction of carbon-bearing manganese briquettes in a slag bath was experimentally investigated at temperatures ranging from 1550 to 1650 ℃. Both the internal temperature and the microstructure evolution of the briquettes were analyzed by differential thermal analysis, scanning electron microscopy and energy-dispersive spectrum analysis, and the smelting reduction mechanism of the carbon-bearing manganese briquettes in the slag bath was further elaborated. The results indicated that the smelting reduction of the briquettes in the slag bath could be divided into three stages, and the aggregation and growth of the metallic particles during the reduction were significantly affected by the slag temperature. Under the experimental conditions, the reduction speed at the initial stage of the carbon-bearing manganese briquettes smelting reduction was controlled by the chemical reaction, whereas the reaction speeds at both the middle and following stages were limited by gaseous diffusion.

Non-isothermal kinetics of synthesizing vanadium nitride by one-step method

Vanadium nitride was synthesized by one-step method using V2O5 and carbon black as raw materials in nitrogen atmosphere. The phases of different reaction products prepared in different reaction temperatures were analyzed by X-ray diffraction(XRD), and the dynamic behavior of the process of synthesizing vanadium nitride(VN) by one-step method was studied with non-isothermal thermogravimetry. The mechanism function and kinetic parameters of reaction process were calculated by thermal gravimetric analyses(TGA), and the reaction rate equation was established. The XRD results show that for the samples tested with minimal VN after holding for 4 h at 1273 K, the main phase of products is VN at 1476 K, while some vanadium nitrides transform into vanadium carbides again over 1573 K. It is found that N2 is beneficial to stimulate reduction and proceed carbonization reaction,and the reduction and nitridation reaction can occur simultaneously. The activation energy of preparing VN by one-step method is 104.005 kJ·mol-1, and the frequency factor is 470.52 at 1280–1358 K, and 150.052 kJ·mol-1 and 2.35 9 104 at 1358–1426 K, respectively.

Medium oxygen enriched blast furnace with top gas recycling strategy

Top gas recycling oxygen blast furnace（TGR-OBF）process is a promising ironmaking process.The biggest challenge of the TGR-OBF in operation is the dramatic decrease of top gas volume（per ton hot metal）,which once led to hanging-up and shutdowns in practice of the Toulachermet.In order to avoid this weakness,the strategy of medium oxygen blast furnace was presented.The maneuverable zone of the TGR-OBF was determined by the top gas volume,which should not be far from the data of the traditional blast furnace.The deviation of ±12.5% was used,and then the maneuverable blast oxygen content is from 0.30 to 0.47 according to the calculation.The flame temperature and the top gas volume have no much difference compared to those of the traditional blast furnace.The minimum carbon consumption of 357 kg per ton hot metal in the maneuverable zone occurs at the oxygen content of 0.30（fuel saving of 14%）.In the unsteady evolution,the N2 accumulation could approach nearly zero after the recycling reached 6 times.Thus far,some TGR-OBF industrial trials have been carried out in different countries,but the method of medium oxygen enriched TGR-OBF has not been implemented,because the accumulation of N2 was worried about.The presented strategy of medium oxygen enriched TGR-OBF is applicable and the strategy with good operational performance is strongly suggested as a forerunner of the full oxygen blast furnace.

Kinetic triplet from low-temperature carburization and carbon deposition reactions

Carbon deposition reaction is unfavorable for smooth operation of blast furnace,while the product of carburization reaction is a superior iron-bearing raw material in non-blast furnace routes.The kinetic triplet of these two reactions was obtained based on non-isothermal kinetic analysis.According to the Sharp–Wentworth method,the activation energy of the carburization reaction is 397.77 kJ/mol,and the activation energies of the carbon depositions on hematite and magnetite are 188.92 and 100.89 kJ/mol,respectively.The carburization reaction is controlled by the Jander mechanism,and the carbon depositions on hematite and magnetite are both controlled by the mechanism of Zhuravlev–Lesokhin–Tempelman.Based on Coats–Redfern method,the activation energies of the above three reactions are 360.65,149.29,and 102.36 kJ/mol,respectively.The carburization reaction is a first-order reaction,while the carbon depositions on hematite and magnetite are both third-order reaction.In particular,the negative activation energy is obtained if considering the anti-Arrhenius circumstance in the Sharp-Wentworth method.Based on above results,it is feasible to adopt non-isothermal kinetic method to study the kinetic triplet of a reaction.According to the obtained activation energies and reaction mechanism functions,the simulated kinetic data are in good agreement with the experimental values even using the negative activation energy.

Effect of ferromanganese additions on non-metallic inclusion characteristics in TRIP steel

The inclusion characteristics in extra low-carbon ferromanganese(ELC-FeMn),low-carbon ferromanganese(LC-FeMn),mid-carbon ferromanganese(MC-FeMn),and high-carbon ferromanganese(HC-FeMn)were evaluated by scanning electron microscopy coupled with energy dispersive spectroscopy.The effect of FeMn alloys on the cleanliness of transformation-induced plasticity(TRIP)steel was studied based on laboratory-scale experiments and thermodynamic calculations.The results show that the main inclusions in ELC-FeMn are MnO-MnS and MnS,and the dominant inclusions in LC-FeMn and MC-FeMn are MnO,MnO-SiO_(2),and MnO-SiO_(2)-MnS.The inclusions in HC-FeMn are diversified.Besides MnO-SiO_(2)inclusion,a certain number of Al-,Si-,and Ti-containing inclusions and enrichment phases of Pb,Sn,and P are observed in HC-FeMn.Before FeMn alloy addition,the main inclusion in steel is Al2O.After alloying with the four different grades of FeMn alloy,the main inclusions in TRIP steel are Al_(2)O_(3),AIN,MnS,AlO3-AIN,AINMnS,and Al_(2)O_(3)-MnS.The MnO and SiO_(2)inclusions from FeMn alloys are not detected in TRIP steel.

Reduction characteristics of molybdenum trioxide with aluminum and silicon

Thermodynamics for reduction of molybdenum oxides by aluminum and silicon were calculated, and the results show that reduction reaction is feasible at a certain temperature region. Compared to the presence of CaO or CaCO3, reduction products of molybdenum trioxide with aluminum and silicon at various temperatures were detected by X-ray diffraction （XRD）. Results show that molybdenum trioxide is reduced by aluminum or silicon step by step, and the intermediate product is MOO2. At 1000 ℃, molybdenum trioxide could be reduced to metal Mo by aluminum, and in the presence of CaO, metal Mo as the reduction product appears even at 800 ℃. In contrast, silicon could barely reduce molybdenum trioxide to metal Mo even at 1200℃. In the presence of CaO or CaCO3, reducibility of silicon increases significantly, and the reduction products are metal Mo and MoSi2. Altogether, CaO or CaCO3 performs two major roles in reduction process： restraining sublimation of MoO3 and decreasing the temperature of reducing MoO3 to metal Mo.

Self-reduction of Manganese-rich Slag Briquette Containing Carbon

The self-reduction experiment of manganese-rich slag briquette containing carbon was carried out in a high- temperature carbon tube furnace. The main factors affecting the reduction rate were analyzed, and the kinetic model of reduction was established. The results show that the increase of basicity of briquette has an obvious effect on im- proving reduction rate. When the carbon ratio of briquettes is 1.2 and its basicity is 1.0, the reduction rate can reach 90 %. It can accelerate reduction process and decrease reduction time when the appropriate flux CaF2 is added to the briquette. The apparent activation energy of chemical reaction is 24.07 kJ/mol, and the apparent activation energy of internal diffusion is 107.55 kJ/mol by calculation. Therefore, the reduction rate of briquette is determined by the mass transfer of CO in the product layer.