Yun Bo Xu

Hot Deformation Behavior of a New-Generation 980 MPa Grade TRIP Steel for Automobiles

Using a Gleeble 1500 thermo-mechanical simulator, the behavior of the dynamic recrystallization (DRX) of a low-silicon TRIP steel containing phosphorus and vanadium for automobiles is studied systematically. By the analysis of ture stress-strain curves, critical stress(σc) and strain(εc) are determined at different deformation conditions. The deformation activation energy Qdef is 323 kJ/mol. The ratios of εc/εp and σc/σp of the tested TRIP steel are 0.553 and 0.89, respectively. The equation for describing the Zener–Hollomn formula for tested steel is derived. A linear equation between σc, σp, εc, εp and lnZ have been established through regression analysis, respectively.

Isothermal Bainite Transformation in Low Silicon TRIP Steel with Phosphorus Addition

The low silicon transformation-induced plasticity (TRIP) steel with phosphorus addition is consisted of the microstructure of retained austenite, ferrite and bainite which produced by intercritical annealing and isothermal bainite transformation followed by rapid cooling. In this study, isothermal bainite transformation of different experimental parameters in low silicon TRIP steel with phosphorus addition was investigated in laboratory. The addition of cheap element phosphorus, which has the same effect of silicon in retard the kinetics of carbide formation during isothermal bainite transformation, can reduce the silicon content, improve surface quality, solve galvanizing problem and reduce costs. The effect of bainite isothermal temperature, intercritical annealing temperature and heating rates were investigated by dilatometric experiment on a push-rod Formastor-FII high-speed dilatometer with radio frequency induction heating. The high resolution observations, composition analysis and elements distribution of microstructure were carried out using the electron scanning microscopy of a field emission electron probe microanalyzer (JEOL JXA-8530F) operating at 20KV accelerating voltage.

Microstructure and Mechanical Properties of a Cold-Rolled Medium Manganese Steel with Delta Ferrite

In this paper, a quenching and tempering process was applied to the cold-rolled medium Mn steel with the delta ferrite (Fe-0.18C-6.4Mn-2.8Al). Microstructure characterization was carried out by means of optical microscope, scanning electron microscope (SEM) equipped with electron backscattered diffraction (EBSD) and transmission electron microscope (TEM). Mechanical properties tests were carried out by uniaxial tension tests. The microstructure characterization results revealed that the steel possessed a complex microstructure composed of three phases (austenite, martensite and delta ferrite). The volume fractions of austenite before and after a deformation were determined by X-ray diffraction (XRD). The XRD results indicated that the amount of austenite reached up to 20 vol.% and the TRIP effect occurred quite apparently. The mechanical property results showed that the steel possessed adequate ultimate tensile strength of 800MPa and excellent elongation of 25%. The outstanding combination of strength and ductility with the product of strength and elongation (PSE) reaching up to over 20GPa% indicates that the steel has a bright application prospect.

Effect of Ferrite Status on Mechanical Properties of Hot-Rolled Directly Quenched and Partitioned Steel

Hot-rolling direct quenching and partitioning (HDQ&P) processes were applied to both low carbon high silicon vanadium micro-alloyed steel and low carbon low silicon steel with phosphorus addition. Proeutectoid ferrite with an area fraction of about 30% was introduced into some of the sheets. Microstructures were characterized using SEM, TEM and XRD. Mechanical properties were investigated by means of uniaxial tensile test. Austenite stabilization, retained austenite stability, tensile deformation and fracture were comprehensively analyzed by making the comparison between the two steels and between the sheets with and without ferrite considering the different precipitation statuses in ferrite. Experimental results showed that the high silicon vanadium micro-alloyed steel contained more retained austenite with higher stability compared with the low silicon phosphorus added steel. Filled with much more carbides with larger sizes, the martensite in the low silicon phosphorus added steel exhibited a much salient tempered feature. The high silicon vanadium added steel possessed higher strength and ductility than the low silicon phosphorus added steel. The introduction of ferrite can result in an extremely obvious post-necking elongation drop in the low silicon phosphorus added steel. The dispersed V-bearing precipitates and high silicon content in ferrite can increase the yield strength and simultaneously diminish the hardness difference between ferrite and martensite, which can improve their compatible deformation capability and then enhance the work hardening ability and finally raise both the UTS and elongation of the steel.

Structure and Precipitation of Strip As-Cast and Hot-Rolled by TSCR on Oriented Silicon Steel

The simulation studies were carried out on the oriented silicon steel produced by thin slab casting and rolling (TSCR) and twin-roll strip casting in the laboratory. The precipitation of inhibiter, formation of microstructure and texture were investigated before cold rolling. The inhomogeneous microstructure and texture gradient were observed in the 7-pass hot-rolled strip（2mm）for TSCR process, and texture gradient was not changed after normalizing, and the twin-roll strip casting directly supplied a strip with approx 2mm of thickness being same as that of hot-rolled strip by TSCR. The microstructure of twin-rolling casting strip was almost composed of all equiaxed grains which similar to the normalizing microstructure in TSCR process, but the random texture was obtained by twin-rolling strip casting. The dispersed and clustered precipitates were presented in hot-rolled strip when the ingots were soaked at 1200°C and 1150°C respectively for the TSCR process. And disperse and acicular precipitates were observed by TEM for air-cooling cast strips for process twin-rolling casting.

MICROSTRUCTURE AND CRYSTALLOGRAPHIC TEXTURE OF STRIP-CAST FE-3.2% SI STEEL SHEET

Fe-3.2%Si steel strips were produced using vertical type twin casting process, and the changes of microstructure and texture trough thickness direction were analyzed. The equiaxed grains of approximately 44.6µm were observed in the center layer, the great mass of columnar dendrite was formed near the surface, and the dendrite truck mainly developed in the transverse direction with respect to the casting direction of about 45° or less. From the subsurface to the center, the volume fraction of the Goss texture (110)[001] gradually decreases. The Goss {110} components at the surface are two times those at the center layer, the {001} components are three times those at the center layer, and the overall texture components are similar to that of the hot-rolled oriented silicon steel strip. The minor α texture could be found from the φ2=45° sections of ODF, and there is no remarkable composition segregation of Si element in the thickness direction of thin strip.

Effect of Continuous Annealing Temperature on Microstructure and Mechanical Properties of a High Strength Cold-Rolled DP980 Steel

Continuous annealing processes were applied to a 980MPa cold-rolled dual phase steel (Fe-0.11C-2.5Mn-0.5Si-0.4Cr) and the effect of continuous annealing temperature on microstructure and mechanical properties was investigated. The microstructures were observed and analyzed by optical microscopy (OM), scanning electron microscopy (SEM), electron probe micro-analyzer (EPMA) and transmission electron microscopy (TEM). The mechanical properties were measured by uniaxial tensile tests. The results revealed that the steel is composed of a certain percentage of ferrite, martensite and perhaps a small amount of retained austenite as well. As the annealing temperature increased, the volume fraction of martensite reached to 67% from 48% and the morphology translated to lath-like from M/A island. As a consequence, the ultimate tensile strength (UTS) and yield strength had a moderate increase from 1070 to 1110 MPa and 580 to 640 MPa, respectively. Meanwhile, the fracture elongation rose to the maximum 12.6% firstly and then decreased to about 9.0%. The optimizing mechanical properties with UTS up to 1090 MPa, yield ratio about 0.54 and fracture elongation about 13% could be obtained at the annealing temperature of 790°C for 120s.

Microstructure and Mechanical Properties of Medium Manganese Steel Plate with High Strength and Toughness

An intercritical annealing process was applied to a medium manganese steel plate (Fe-0.01C-5.3Mn-1.53Si) after the thermo-mechanical controlled processing (TMCP) and ultrafast cooling (UFC). The microstructures were observed by scanning electron microscopy (SEM) equipped with electron backscatter diffraction (EBSD), electron probe micro-analyzer (EPMA) and transmission electron microscopy (TEM). The retained austenite was measured by XRD and mechanical properties were measured by uniaxial tensile and impact tests. The influence of different annealing temperature was compared and the relationship between microstructures and mechanical properties was investigated. Results showed that the microstructures of the medium manganese steel plate were characterized by ultrafine grained lath-like ferrite and retained austenite and the excellent mechanical properties could be obtained at the annealing temperature of 640°C for 5 h. The volume fraction of the retained austenite reached up to 21%, which could significantly increase the elongation compared with the traditional steel plate. The mechanical property results revealed that the steel possessed adequate ultimate tensile strength of 865MPa and excellent impact energy of 121J (-20°C). The outstanding combination of strength and toughness indicates that the steel has a bright application prospect.

Inhomogeneous Distributions of Sub-Grains and Precipitates in Strip-Cast 3wt% Si Steel Sheet

A Fe-3wt% Si strip was produced using a vertical type twin-roll casting process. Sub-grains with an inhomogeneous distribution through the thickness direction were firstly observed in the as-cast strip. It was found that the surface layer was characterized by columnar grains containing lineage sub-grains, the subsurface layer was dominated by numerous polygonal sub-grains inside columnar grains or fine equiaxed grains, and the center layer was composed of coarse equiaxed grains without sub-grains. In addition, the sub-grain boundaries provided additional nucleation sites for precipitates and the nanoscale MnS particles were successfully obtained. Moreover, the average size of MnS precipitates in the surface layer was smaller than that in the subsurface layer. The inhomogeneous distributions of sub-grains and precipitates can be attributed to the large temperature gradient through the thickness direction during the strip casting process.

Effect of Rapid Thermal Process on the Recrystallization and Precipitation in Non-Oriented Electrical Steels Produced by Twin-Roll Strip Casting

A Fe-1.3% Si non-oriented silicon steel strip was produced by twin-roll strip casting process, and subsequently treated with cold rolling and annealing. The effect of heating rates on the recrystallization and precipitation behavior of second phase particles (AlN and MnS) was investigated by MMS-200 thermal mechanical simulator. It was found that the recrystallization area fraction decreased obviously with the increase of heating rate. At the heating rate of 5 °C/s, the recrystallization rate gradually decreased with the extension of holding time, but it increased at the rapid heating rates. The particle’s sizes mainly concentrated in 50~200 nm at the heating rate of 5 °C/s during annealing. The number of particles under 50nm increased gradually and the number of precipitates between 50~400 nm reduced significantly when the heating rate was increased to 50~300 °C/s. The results indicated that the rapid heating rate could refine the size of precipitates and decrease the number of particles above 50nm.