Xiao Dong Tan

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.

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.

Microstructure and Tensile Deformation Behavior of a Hot-Rolled Directly Quenched and Dynamically Partitioned Steel Containing Proeutectoid Ferrite

Hot-rolling direct quenching and dynamical partitioning (HDQ&DP) processes were applied to a low-carbon steel containing silicon and manganese based on thermo-mechanical control process (TMCP) technology and ultra-fast cooling (UFC) technology. The microstructures and phase compositions were characterized and analyzed using SEM, EBSD, TEM and XRD. The mechanical properties and tensile deformation behaviors were investigated by means of uniaxial tensile test. The microstructures and tensile deformation behaviors of both HDQ&DP steel with and without proeutectoid ferrite were comprehensively expounded by comparing with each other. Results show that the amount of retained austenite in the HDQ&DP steel with proeutectoid ferrite can reach up to 17.3%, which is higher than that in the HDQ&DP steel without proeutectoid ferrite (15.7%). The HDQ&DP steel without proeutectoid ferrite possesses extremely high ultimate tensile strength (UTS) up to 1700 MPa with yield ratio about 0.73 and elongation about 11.5%. The introduction of proeutectoid ferrite can result in a moderate decrease of UTS to 1240-1400 MPa, a drastic decrease of yield ratio to 0.51-0.69 and a certain increase of elongation to 13.0-13.7%. The existence of the proeutectoid ferrite can partly enhance the work hardening ability of the steel and may improve its formability. It is concluded that the HDQ&DP steel with relatively large amount of proeutectoid ferrite and certain amount of bainite has extensive application prospects.

Effect of Rolling Process on Impact Toughness in High Strength Low Carbon Bainitic Steel

Based on TMCP and UFC technology, the microstructures and impact toughness of low carbon bainitic steel were studied in this paper. The bainite morphology and fracture surfaces of Charpy impact specimens were observed by SEM, and mechanical properties of bainitic steel were measured by tensile and impact test. The results showed that the yield and tensile strengths of steel were 804MPa and 1015MPa, and elongation was 15.7% when the rolling was finished in the austenite recrystallization region. The steel rolled below Tnr temperature obtained tht yield strength of 930 MPa, tensile strength of 1090 MPa and elongation of 16.2%. However, the impact toughness was deteriorated in the steel rolled above Tnr temperature while the excellent impact toughness existed in the steel rolled below Tnr temperature. The impact toughness of steel rolled below Tnr temperature was 140J at-60°C, while the impact toughness of 15J at the same temperature was obtained for the steel rolled above Tnr temperature. The large cleavage fracture region on the fracture surface occured with the decrease of tested temperature in the steel rolled above Tnr temperature and inevitably reduced the impact toughness, while the main ductile fracture existed in the steel rolled below Tnr temperature at the same temperature. The rolling process of steel can strongly affect impact toughness of low carbon bainitic steel. Hence, the different rolling processes can adjust the occurrence of cleavage fracture and ductile fracture in order to improve the impact toughness.

TRIP Effect in a Hot-Rolled Low-Carbon High Strength Complex Phase Steel

In the present work, a study has been made of the hot-rolling process for a transformation induced plasticity (TRIP) steel Fe-0.12C-0.5Si-1.4Mn-0.5Cr (wt%). The volume fractions of retained austenite before and after a deformation were determined by X-ray diffraction (XRD). The microstructure was characterized by optical microscope, scanning electron microscope (SEM) equipped with electron backscattered diffraction (EBSD) and transmission electron microscope (TEM). A uniaxial tension text indicated that the steel possesses ultimate tensile strength of 748 MPa with yield ratio of 0.7 and elongation of 20%. The steel with the volume fraction of retained austenite of 12.5 % exhibits significant TRIP effect.