Modeling for Formation of Proeutectoid Ferrite in Steel during Continuous Cooling

A novel model of the evolution of microstructure during continuous cooling with the formation of proeutectoid ferrite in steel was proposed from a Voronoi construction for the austenite grains, based on the Rappaz‘s integral nucleation model and the assumption that the ferrite nucleates at the edges of the original austenite grains and the successive growth of the ferrite grain is radial. The model can be used to calculate the fraction of ferrite as a function of time or temperature during continuous cooling, and to determine the microstructure of ferrite. The calculated results are in agreement with experimental results investigated in 0.38C-0.28Si-0.55Mn-0.92Cr-0.20Mo steel under continuous cooling using a Gleeble 1500 thermomechanical simulator.     

Coupling Thermomechanical and Microstructural FE Analysis in Plate Rolling Process

Using three-dimensional rigid-viscoplastic finite element method （FEM）, a coupling multivariable numerical simulation model for steel plate rolling has been established based on the physical metallurgy microstructural evolution rule and experiential equations. The effects of reduction, deformation temperature, and rolling speed on the deformation parameters and microstructure in plate rolling were investigated using the model. After a typical rolling process of steel plate 16Mn is simulated, the strain, temperature, and microstructure distributions are presented, as well as the ferrite grain transformation during the period of cooling. By comparing the calculated ferrite grain sizes with measured ones, the model is validated.     

Surface Ferrite Grain Refinement and Mechanical Properties of Low Carbon Steel Plates

Considering the specialities of the steel plate production, the TMCP study has been carried out with Gleeble 2000 tester to explore the possibility of fine grained ferrite in the low carbon steel plates with the chemical composition of C 0.13--0.18, Si 0.12-0.18, Mn 0.50-0. 65, P 0. 010-0. 025, and S 0. 005-0. 028. The plates with thickness of 8. 7 mm in which the ferrite grain size is smaller than 8μm have been produced by special de- formation process in the laboratory. Furthermore, the trial production of special plain carbon steel plates of 16-25 mm in thickness and 2 000- 2 800 mm in width with fine grained ferrite has been successfully carried out in the Shougang Steel Plate Rolling Plant. The ferrite grain size is 5.5-7μm in the surface layers and 9.5-15μm in the central layer respectively. The yield strength is 320- 360 MPa, tensile strength is 440-520 MPa and the elongation is 25%- 34 %. It is very important for the rolling plants to improve the low carbon steel plates＇ mechanical properties. The results show that the ferrite grains in the surface layer can be refined effectively by the appropriate rolling process, and the strength can be also increased.     

Effect of Tempering Temperature on Microstructure and Properties of E690 Offshore Plate Steel

 A new kind of Mn-Mo-Nb-Cu-B bainite steel which satisfied mechanical demands of E690 offshore plate steel was designed. The effect of two processes——thermomechanical control process (TMCP)+tempering (T) and thermomechanical control process (TMCP)+reheating and quenching (RQ)+tempering (T)——on microstructure and mechanical properties were studied by means of scan electron microscope (SEM), transmission electron microscope (TEM) and electron back scatter diffraction (EBSD). The results showed that optimal mechanical properties were available when tempering at 550 ℃ for both processes. The microstructure of the TMCP+T treated sample tempering in the range of 450 to 550 ℃ for 1 h did not change dramatically yet the lath in the TMCP+RQ+T treated sample merged together and transformed into polygonal ferrite. At the same time, the sub-structure of grain bainite transformed from lath to cell-shape to refine grains with tempering temperature mounting. Lots of sub-grain boundaries were located within bainite and adjacent bainite grain boundaries were high angle.     

Microstructure and Deformation Mechanics of X100 Line Pipe Steel

In present paper,microstructure is observed by means of OM,SEM,EBSD and TEM analysis on material obtained from X100 linepipe with wall thickness as 20.6mm and outside diameter as 1016mm diameter Grade 690.To study the deformation mechanics,microstructure transformation is also investigated on deformed specimens with different strain level as 0%,5% and 8%.Results show that X100 line pipe material consists mainly of acicular ferrite whose ratio in material reaches 64%,obvious cellular structures formed by tangled dislocation and M/A island distributed in acicular plates indicate abundant dislocation and substructure existed in material.It is measured that line pipe steel has a much finer effective grain size as 2.07μm,which can results in a higher strength and toughness for line pipe steel.Contrast analysis for Kikuchi Pattern reveals that the ratio of large-angle boundary is comparatively higher as 55%,implying that line pipe steel has a good toughness.It is observed that the proportion of small-angle grain boundary increases greatly after material deformed,which consequently results in the increment of dislocation and sub-structure.     

Influence of Deformation on Transformation of Low Carbon and High Nb-containing Steel during Continuous Cooling

 The effect of deformation in the nonrecrystallized region on the phase transformation for a low carbon and high Nb-containing steel with coarse austenite grain size was investigated by means of dilatometry measurement and microstructure observation. The results show that with the cooling rate increased, both the transformation start and finish temperatures measured by dilatometer are decreased, and the corresponding microstructure is changed from ferrite and pearlite to full granular bainite gradually. The dynamic CCT diagram is plotted according to the dilatometry measurements and microstructure observations. Dilatometry measurements also show that the transformation start and finish temperatures of the tested steel are raised with increasing strain, strain rate and deformation temperature, and the reasons for this are discussed.     

Modeling of Strain-Induced Precipitation Kinetics and Evolution of Austenite Grains in Nb Microalloyed Steels

Considering the effect of strain and chemical composition onprecipitation behavior, new models for the start and end time of Nb（C,N） precipitation in austenite under the conditions of different temperatures and strains have been investigated for Nb microalloyed steel. The value of n in the precipitation kinetic equation has been determined by using the available experimental data in literature, which indicated that n is a constant and independent of temperature. The values of the start and end time of the predicted precipitation are compared with the experimental values. Calculated results are in good agreement with the experimental results. Also, the evolution of austenite grains before ferrite transformation is simulated by taking the effect of precipitation into consideration. The measured austenite grain size is in good agreement with predicted one prior to ferrite transformation.     

Modeling of StrainInduced Precipitation Kinetics and Evolution of Austenite Grains in Nb Microalloyed Steels

Considering the effect of strain and chemical composition on precipitation behavior, new models for the start and end time of Nb(C,N) precipitation in austenite under the conditions of different temperatures and strains have been investigated for Nb microalloyed steel. The value of n in the precipitation kinetic equation has been determined by using the available experimental data in literature, which indicated that n is a constant and independent of temperature. The values of the start and end time of the predicted precipitation are compared with the experimental values. Calculated results are in good agreement with the experimental results. Also, the evolution of austenite grains before ferrite transformation is simulated by taking the effect of precipitation into consideration. The measured austenite grain size is in good agreement with predicted one prior to ferrite transformation.     

Analysis of Microstructure Characteristics and Precipitation Behavior of Automobile Beam Steels Produced by Compact Strip Production

The microstructure characteristics and precipitation behavior of automobile beam steels produced by compact strip production (CSP) were investigated by use of scanning electron microscopy (SEM),transmission electron microscopy (TEM) and X-ray energy dispersive spectroscopy. The result shows that the final microstructure is mainly composed of polygonal ferrite and small amount of pearlite,and the average ferrite grain size is about 3-6μm. Small amount addition Ti to aluminium-killed steel can help to refine the microstructure and improve the mechanical properties. A large number of fine precipitates have been observed in automobile beam steels. The mean particle size is about 10-30nm. Remarkable strengthening and grain refinement can be obtained by these nano-particles.     

Microstructure and Mechanical Properties of Nb-Microalloyed X100 High Deformability Pipeline Steel

A low carbon Nb-microalloyed high deformability pipeline steel with X100 grade has been processed by TMCP and followed two-stage cooling process.The microstructure is characterized by ferrite/bainite multiphase.The effective grain size is 1.85μm in average.The volume of ferrite is about 10-15% and the grains sizes are mostly less than 5μm.The bainite consists of granular-bainite and lath-bainite,with M/A islands finely dispersed.The longitudinal tensile yield strength,uniform elongation,yield ratio are 647MPa,7.6% and 0.78,respectively.Ferrite/bainite multiphase have large strain hardenability that resulting high strength and high deformability combination.Precipitation of Nb also improves the strength and uniform deformability by precipitation strengthening and grain refinement.     

Analysis on Microstructure Characteristics and Precipitation Behavior of Automobile Beam Steels Produced by Compact Strip Production

 The microstructure characteristics and precipitation behavior of automobile beam steels produced by Compact Strip Production (CSP) were investigated by use of scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X-ray energy dispersive spectroscopy. The result shows that the final microstructure is mainly of polygonal ferrite and small amount of pearlite, the average ferrite grain size is about 3～6μm. Small amount addition Ti to aluminium-killed steel can help to refine the microstructure and improve the mechanical properties. A large number of fine precipitates have been observed in automobile beam steels. The mean particle size is about 10～30nm. Remarkable strengthening and grain refinement can be obtained by these nano-particles.     

Chemistry and Process Optimization of V-Microalloyed N80-Class Seamless Casing Tube

The complex effects of different nitrogen (N) contents and thermal routines on the microstructure and mechanical properties of 33Mn2V steels for N80-Class seamless casing tube application were investigated using Gleeble simulation technique. The results showed that the N additions of 0.014%-0.021% in the steels for the in-line normalization process (ILNP) increased the strength while the toughness remained at a high level as compared with the steel with N content of 0.005%. It was also revealed that the N addition of 0.021% could enhance the performance combination of strength and toughness in the steels by using 700℃ as the cooling interrupted temperature (CIT) for the non in-line normalizing process (NILNP). It was further evidenced that the toughness was improved at expense of strength to some degree in all the steels by decreasing reheating temperature for the ILNP,while an increase of CIT for NILNP severely impaired the toughness and slightly improve the strength in the high-N steel with N content of 0.021%. This can be attributed to the dissolution and precipitation behavior of V(C,N). The optimization of V(C,N) precipitation could be achieved by enhancing N. The precipitation of V(C,N) in austenite was promoted by cooling to a certain temperature lower than the Ar1 for the ILNP. The V(C,N) particles formed in austenite contributed to grain refinement by the VN-induced nucleation of intragranular ferrite,but as a result the effect of precipitation on strengthening would become weaker due to a decrease of the precipitation of V in ferrite.     

Effects of Processing Variables on Microstructure and Yield Ratio of High Strength Constructional Steels

The process of “controlled rolling+relaxation+ultra fast cooling (UFC)”for high strength constructional steel with low yield ratio was presented.Microstructure and corresponding relationship with low yield ratio were in-vestigated.The results showed that the constructional steels with multiphase microstructure of bainitic ferrite,mar-tensite-austenite (M-A)and lath bainite were obtained through the creative process.The grain size decreased with the decrease in finish rolling temperature,which enhanced the strength by the grain refinement strengthening.The proper relaxation treatment promoted the bainitic ferrite lath width and the formation of blocky M-A constituent.In addition,both the tensile and yield strength increased with the decrease in finish rolling temperature and UFC final temperature,but the yield strength increased more significantly than tensile strength,which caused the increase in yield ratio.By using the process of “controlled rolling+relaxation+ultra fast cooling”,the excellent comprehensive mechanical properties of 780 MPa grade constructional steels of 12-40 mm in thickness were achieved.     

Solidification Structure Refining of 430 Ferrite Stainless Steel With TiN Nucleation

The thermodynamics of TiN precipitation in liquid steel of 430 ferrite stainless steel has been calculated to find out the condition of TiN precipitation during the initial solidification stage. The difference in the solidification structure of 430 ferrite stainless steel has been discussed through comparative tests of vacuum induction furnace melting with different contents of Ti. It has been found that the equiaxed grain proportion can be increased from 20% to 69% as the content of Ti from 0.1% up to 0. 4%. The size of the TiN particles precipitated is 1-3 μm and the number of TiN particles is about （200- 300）/μm^2. It is found that the effect of using TiN to refine the solidification structure has been confirmed under the strict process condition used for 430 ferrite stainless steel.     

Influence of on-line tempering parameters on microstructure of medium-carbon steel

A new process involving ultra-fast cooling(UFC)and on-line tempering(OLT)was proposed to displace austempering process,which usually implements in a salt/lead bath and brings out serious pollution in the industrial application.The optimization of the new process,involving the evolution of the microstructure of medium-carbon steel during various cooling paths,was studied.The results show that the cooling path affected the final microstructure in terms of the fraction of pearlite,grain size and distribution of cementite in pearlite.Increasing the cooling rate or decreasing the OLT temperature contributes to restraining the transformation from austenite to ferrite,and simultaneously retains more austenite for the transformation of pearlite.It is also noted that bainite was observed in the microstructure at the cooling rate of 45°C/s and the OLT temperature of 500°C.Through either increasing the cooling rate or decreasing the OLT temperature,the distribution of cementite in pearlite is more dispersed and grain is refined.Taking the possibility of industrial applications into account,the optimal process of cooling at 45°C/s followed by OLT at 600°C after hot rolling was determined,which achieves a microstructure containing nearly full pearlite with an average grain size of approximately 7μm and a homogeneously dispersed distribution of cementite in pearlite.     

Coercivity enhancement of Ce-containing hot-deformed magnets by grain boundary diffusion of DyF3

The samples with full density were prepared by hot pressing the the melt-spun powders mixed with DyF₃powders of different mass fractions followed by hot-deformation process.The magnetic properties and temperature dependence of coercivity were obtained by BH tracer and VSM,respectively.The microstructure were analyzed by scanning electron microscopy(SEM)and transmission electron microscopy(TEM).The coercivity of Ce-containing hot-deformed magnets is increased from 1.41 to 1.95 T by grain boundary diffusion of 3 wt%DyF₃,and is further enhanced to 2.05 T after annealing treatment.The thermal stability of coercivity and remanence is improved.The annealing condition in this work crucially plays a role in thickening the grain boundary phase.Microstructure analysis reveals that the continuous and thick grain boundary phase formed after DyF₃diffusion can weaken the magnetic coupling between grains,and suppress the platelet shaped grain size and the aspect ratio.The Dycontaining shell structure formed by the partial diffusion of Dy into the main phase can increase the magnetic anisotropy field,which is the main reason for the coercivity improvement.After optimizing the structure by DyF₃diffusion,the"dendritic-like"reverse domain is transformed into the"dot scatteredlike"reverse domain.     

The Cellular Automata Model of Electroslag Remelting Ingot Structure

Electroslag remelting (ESR) is an advanced process for the production of high quality steels. The microstructure of remelted steel which affect the mechanical properties and the performance of the ingot was determined by the technological parameters. A two-dimensional axisymmetric geometry which was established in this paper was divided into macro-grid finite element in order to compute temperature field; then the grid was divided into more detailed and uniform cells, and at last the continuous nucleation model based on the Gaussian distribution and KGT growth model was established for nucleation and growth calculations using cellular automaton method (CA) on the solidification of molten steel. The results show that: a vertical columnar grain zone and a inverted V-shaped columnar crystal zone appeared in the ESR ingot. In addition, the temperature field with different electrode melting rate and slag pool temperature parameters and the microstructure with different average nucleation under cooling and maximum grain density were studied in this paper. The simulation results agree well with the experimental results, so it is proved that the model and calculation method is reliable. To produce ideal solidified ingot and achieve the purpose of optimizing the production process, the production process was adjusted according to the simulation results.     

Effects of rare earth oxide on hardfacing metal microstructure of medium carbon steel and its refinement mechanism

The electrodes for hardfacing medium carbon steel with six additions of rare earth oxide were developed in this work. By means of optical microscopy and scanning electron microscopy, the microstructure, inclusion and the fractograph of the hardfacing metal were observed. Then, the effects of rare earth oxide on microstructure and inclusions in hardfacing metal were analyzed. The effectiveness of rare earth oxide as heterogeneous nuclei of δ-Fe was calculated with the misfit theory. The results showed that, the microstructure of hardfacing metal was composed of ferrite and small amount of pearlite. The microstructure was refined at first and then coarsened with the increase of rare earth oxide addition. The fractograph was changed from brittle to equiaxed dimples, then became quasi-cleavage and cleavage gradually. The calculated results showed that, the ferrite grain size could be refined because that LaAlO3 as heterogeneous nuclei of δ-Fe was moderately effective, and the ferrite grain size was coarsened because the misfits between Ce2O3 and δ-Fe, Ce2O2S and δ-Fe were increased with futher increase of rare earth oxide addition.     

Model of temperature field for the preparation process of melt-spun NdFeB powders

Melt-spun ribbons which are the important raw material for hot-deformed magnets can be prepared by single-roller melt-spinning. In order to prepare well-structured ribbons, the model of temperature field for single-roller melt-spinning process was constructed in this work. The heat conduction in this process was simplified as one dimensional heat conduction problem. It was shown by modeling that, the temperature field in the melt-spinning before solidification in this model could be described as this equation T(x,t)=Tmoexp[–k(x–x0)–k2αt]+T0. The temperature T(x,t) of the alloy melts decreased with increased position x and cooling time t exponentially from the wheel-free surface to the wheel-side surface. The constant k determined the decrease speed of alloy temperature T(x,t), which was proportional to the interfacial heat transfer coefficient h and the interfacial area of heat conduction A0, but inversely proportional to the thermal conductivity K. x0 was the thickness of the alloy melt. With increased x0, the temperature difference between wheel-free surface and the wheel-side surface became larger, which would lead to larger difference in grain size. In experiments, the influence of melt-spinning process parameters on the temperature field model was discussed, such as cooling roller materials, wheel speed, and so on. Melt-spun ribbons prepared by single-roller melt spinning at different wheel speed were investigated and magnetic properties of die-upset magnets from melt-spun ribbons on different cooling roller were analyzed. The variation of grain size in the depth direction consisted with temperature field model. This model provided directions for the preparation of melt-spun ribbons with uniformly distributed fine grains, which were very necessary for producing hot-deformed magnets with high magnetic performance.