High-Strength and High-Plasticity TWIP Steel for Modern Vehicle

In this paper new high-strength and high-plasticity twinning induced plasticity （TWlP） steel for modern automobile body was investigated. Some basic experimental results were given. The results indicate the TWlP steel has excellent properties. It exhibits high ultimate tensile strength （600,--1100 MPa） and extremely large elongation of 60% to 90%.In the future it would be capable of satisfying the requirements of new generation of vehicle.

Effects of annealing temperature on the microstructure and properties of the 25Mn-3Si-3Al TWIP steel

Microstructures and mechanical properties of the 25Mn twinning induced plasticity （TWIP） steel at different annealing temperatures were investigated. The results indicated that when the annealing temperature was 1000℃, the 25Mn steel showed excellent comprehensive mechanical properties, the tensile strength was about 640 MPa, the yield strength was higher than 255 MPa, and the elongation was above 82%. The microstructure was analyzed by optical microscopy （OM）, X-ray diffraction （XRD）, and transmission electron microscopy （TEM）. Before deformation the microstructure was composed of austenitic matrix and annealing twins at room temperature; at the same time, a significant amount of annealing twins and stacking faults were observed by TEM. Mechanical twins played a dominant role in deformation and as a result the mechanical properties were found to be excellent.

Efect of high temperature and high strain rate on the dynamic mechanical properties of Fe-30Mn-3Si-4Al TWIP steel

The dynamic mechanical properties of Fe-30Mn-3Si-4A1 twinning induced plasticity （TWIP） steel were studied by the split-Hopkinson pressure bar （SHPB） at temperatures of 298-1073 K and strain rates of 700, 2500, and 5000 s-1. The TWIP steel indicates strain rate hardening effect between 700 and 2500 s-1, but it shows strain rate softening effect between 2500 and 5000 s-1. In addition, the strain rate softening effect enhances with an increase in deformation temperature. After deformation, the microstructures were studied by optical microscopy （OM）. It is shown that the deformation bands become more convergence, a part of which become interwoven with an increase in strain rate, and the dynamic recovery and recrystallization are enhanced with an increase in both temperature and strain rate.

In-situ observation on the deformation behaviors of Fe-Mn-C TWIP steel

The microstructure and crack behaviour of twinning induced plasticity （TWIP） steel during tensile deformation was investigated with in-situ scanning electron microscopy （SEM）. The results show that there are two modes of plastic deformation during tensile test in the Fe-Mn-C TWIP steel： dislocation gliding and deformation twins. During the process of tensile deformation, secondary deformed twins are found. Inclusions have played a role in the course of ductile fracture, and microcracks initiate from inclusions and twin-twin intersections.

Effect of annealing temperatures on dynamic impact mechanical properties of high manganese TWIP steel

Results presented in this paper contribute to the investigation of the effect of annealing temperatures ranging from 800 to 1000 ℃ on the dynamic impact mechanical properties of high manganese TWIP steel. The properties were measured by using an instrumented Charpy impact tester. The impact absorbing energy initially increases with the annealing temperature, climaxes at 900 ℃, and then decreases. The dynamic crack extension resistance curve indicates that the crack initiation force decreases with the annealing temperature, and the steel annealed at 900 ℃ has the best ability against crack expansion

Deformation behavior of Fe-Mn-C TWIP steel within the temperature range of hot rolling

The hot-deformation behavior of Fe-Mn-C twinning induced plasticity （TWIP） steel was investigated by conducting hot compression tests within a recommended hot rolling temperature range at various strain rates. Flow resistance curves during hot-deformation were obtained, and strain rate sensitivities and activation energies for plastic deformation were calculated using the power law. It is found that the addition of Al and Si clearly increases the peak stresses for the present alloys, especially at 950℃. But Mn has a minor effect on the stress-strain curves and activation energy when its content varies from 15 mass% to 22 mass% for the present alloys.

INFLUENCE OF COLD ROLLING REDUCTION ON MICROSTRUCTURE AND MECHANICAL PROPERTIES OF TWIP STEEL

The influence of cold rolling reduction on microstructure and mechanical properties of the TWIP （ttwinning induced plasticity） steel was investigated. The results＇ indicated that the steel had better comprehensive mechanical properties when cold rolling reduction was about 65.0% and the annealing temperature was 1000℃. The tensile strength of the steel is about 640MPa and the yield strength is higher than 255MPa, while the elongation is＇ above 82%. The microstructure is composed of austenitic matrix and annealing twins at room temperature, at the same time, a significant amount of annealing twins and stacking faults＇ are observed by transmission electron microscopy （TEM）. Mechanical twins play a dominant role during deformation, and result in exceUent mechanical properties.

Microstructure and Texture Evolution of Fe-33Mn-3Si-3Al TWIP Steel on Strain

The microstructure and texture evolution of Fe-33Mn-3Si-3Al twinning induced plasticity(TWIP) steel were studied by the scanning electron microscope(SEM) and X-ray diffraction(XRD) at room temperature. After quasi-static tensile, the texture evolution of different strain was observed. It was shown that the Goss and Brass components increased within the strain range of less than 0.6. Whereas, the main components were decreased when the strain levels were greater than 0.6. This behavior was attributed to the low stacking fault energy(SFE) and was related to the strain energy of this high manganese steel. At high strain levels, the high strain energy may contribute to the Brass components transition to the A(rot-Brass) components.

Grain Size Effect on the Martensite Formation in a High-Manganese TWIP Steel by the Rietveld Method

The aim of the present work was to study the effect of austenite grain size （AGS） on the martensite formation in a high-manganese twinning-induced plasticity （TWIP） steel. The results of a quantitative microstructural characterization of the steel by the whole X-ray pattern fitting Rietveld software, materials analysis using diffraction （MAUD）, indicated that the volume fraction of αbcc-martensite increases with increasing AGS. However, the value of the stacking fault probability （Psf） does not show a large variation for samples with different values of AGS under water-quenching conditions.

Microstructure and Mechanical Properties of Hot-Rolled Fe-Mn-C-Si TWIP Steel

Mechanical properties and microstructural evolution of the hot-rolled Fe-Mn-C-Si TWIP steel were investigated and the deformation mechanism was analyzed.The results showed that the tensile strength and elongation were about 1050 MPa and 60%,respectively.The hot-rolled steel had high specific energy absorption and impact toughness between-120 ℃ and 20 ℃.Some inhomogeneous dislocation zones were observed in the undeformed steel.Lots of deformation twins and twin-dislocation interactions were observed in the deformed steel.TWIP effect was the major deformation mechanism for the excellent mechanical properties.

Microstructure and Mechanical Properties of TWIP Steel Joints

As a new type of high manganese steel, the twinning induced plasticity （TWIP） steels have attracted a growing interest in the automotive industry due to their good performance. Thin plates of TWIP steel were welded by laser beam welding （LBW） and gas tungsten arc welding （GTAW）. The microstructure result shows that GTAW joint has obvious heat-affected zone （HAZ）, while the HAZ of LBW joint is almost invisible. The X-ray diffraction result shows that the phase compositions of both joints are austenitic and no phase transition occurs. Energy disper- sive spectrometry result shows that there is violent evaporation of Mn element in LBW joint, while the proportion of Mn element in GTAW joint is almost unchanged. Tensile tests and micro-hardness measurements were performed to take into account the mechanical properties of joints manufactured by the two different processes. The micro-hard- ness profiles of both joints present a typical saddle distribution, and the hardness of GTAW seam is lower than that of LBW seam. The failure positions of LBW joints are all located in base metal while the GTAW joints are all at the weld toe due to the softening of HAZ. By means of scanning electron microscopy, a typical ductile fracture is observed in LBW joint, while a brittle fracture with quasi-cleavage fracture characteristic is observed in GTAW joint.

Effects of Twin-Dislocation and Twin-Twin Interactions on the Strain Hardening Behavior of TWIP Steels

Tensile tests of Fe-30Mn-5Si-2Al steel were carried out for different strains of 0.05, 0.14, 0.26, and up to the strain-to-failure in order to observe the evolution of microstructure during deformation and investigating the strain hardening behavior. Three-stage strain hardening behavior was observed in this steel during tensile test. In stage I , planar dislocation structure was observed by TEM and regarded as main deformation mechanism, and low strain hardening rate exponent was exhibited. Primary deformation twinning occurred in stage II , and the strain hardening rate exponent increased due to the blockage of dislocations＇ motion by twin boundaries. In stage III , the strain hardening rate exponent had increased to a value higher than 0.5. The obstacle effect of twin boundaries and twin-twin interact＇ion had been observed by TEM, and the interactions between primary and secondary twins were found to cause the additional hardening in addition to the obstacle effect on dislocations＇ motion, which led to the twinning induced plasticity effect in the later stage of deformation.

Effect of Temperature on Microstructure and Deformation Mechanism of Fe-30Mn-3Si-4Al TWIP Steel at Strain Rate of 700 s^(-1)

As twinning-induced plasticity （TWIP） steel is one potential material for shaped charge liner due to the combination of high strength and high plasticity, deformation mechanism at high strain rate and high temperature is required to study. Compression experiments of Fe 30Mn-3Si-4Al TWIP steel were conducted using a Gleeble 1500 thermal simulation machine and a split-Hopkinson pressure bar （SHPB） between 298 and 1073 K at strain rates of 10 3 and 700 s-1 , respectively. Microstructures were observed using optical microscopy （OM） and transmission e- lectron microscopy （TEM）. Results show that flow stress and densities of deformation twins and dislocations de crease with increasing deformation temperature at strain rates of 10 3 and 700 s -1. The stack fault energy （SFE） values （Г） of Fe-30Mn-3Si-4Al TWIP steel at different temperatures were calculated using thermodynamic data. Based on corresponding microstructures, it can he inferred that at 700 s -1 , twinning is the main deformation mecha- nism at 298-573 K for 30 mJ/m2≤Г≤63 mJ/m2 , while dislocation gliding is the main deformation mechanism above 1 073 K for Г≥145 mJ/m2. In addition, with increasing strain rate from 10-3 to 700 s -1 , the SFE range of twinning is enlarged and the SEF value of twinning becomes higher.

Formability of TRIP/TWIP Steel Containing Manganese of 18.8%

The mechanical property and forming limit experiments were carried out on a higli manganese TRIP/TWIP steels with manganese of 18.8%. And the forming /imit diagram was obtained. This hig：h manganese steel shows outstanding mechanical properties comhining high strength with good formability. Its limit plane strain corresponding to the lowest point in the plain strain condition is about 38%. The effects of conventional mechanical property on the limit plane strain were analyzed. With increasing the content of Mn, the limit plane strain increases, indicating that TWlP effect is advantageous to the increase of formability. Combining with three classical models and volume invaria ble principle, a FLD model was buih, which is suitable for high manganese steels very well.

Microstructural Development and Deformation Mechanisms during Cold Rolling of a Medium Stacking Fault Energy TWIP Steel

The magnetic response, microstructural and texture changes occurring during cold rolling of a Fe-14Mn-0.64C-2.4AI-0.25Si medium stacking fault energy TWlP （twinning induced plasticity） steel have been studied by X-ray diffraction and magnetic techniques. The changes in the sub-grain size （Ds）, probability of stacking fault formation （Psf） and microstrain in the material as cold rolling progressed were determined by using a modified version of the Williamson and Hall equation. A strong development of the crystallographic texture with increasing deformation was observed. Deformation-induced formation of a small fraction α＇-martensite was observed, indicating that the steel also exhibits γ→α＇-martensite transformation during cold rolling, which is discussed via the changes of the stacking-fault probability and the texture development during cold rolling.

Texture and Microstructure Evolution During Tensile Testing of TWIP Steels with Diverse Stacking Fault Energy

Texture and microstructure evolution in two kinds of the twinning induced plasticity （TWIP） steels （Fe-Mn- Si-AI and Fe-Mn-C） with diverse stacking fault energies during tensile testing were investigated by interrupted testing. The strain-hardening rate curves of the two steels were quite similar, but the texture characterization curves （maximum of pole density measured by X-ray diffraction） were varied. According to the curvature of max pole density curves, the evolution of the texture and the microstructure can be divided into three stages： low strain stage, medium stage and high stage. In low strain stage the difference of the microstructure came from the intensity of dislocation, which was much smaller in Fe-Mn-Si-AI. The main difference of the microstructure in medium and high strain stages originated from the numbers of activated twin systems. There were more than one twin systems activated in Fe-Mn-C, while only a single twin system activated in Fe-Mn-Si-AI. Texture showed various differences in the whole tensile process because it was affected by their micromechanism, such as concentration of the dislocation and the activation of twin systems. Texture in low strain stage was connected with annealing twin; the evolution ofthe texture was mainly induced by deformation twin generation. More than one activated twin systems in medium and high stages may counteract each other in the view of concentration of the grain orientations.

Effect of Cr on mechanical properties and corrosion behaviors of Fe-Mn-C-Al-Cr-N TWIP steels

By using scanning electron microscopy（SEM） equipped with electron back-scattered diffraction（EBSD）system, transmission electron microscopy（TEM） and Corr Test4 electrochemical workstation, effects of chromium content（1.35 wt% - 3.95 wt%） on the mechanical properties and anti-corrosion behaviours of high manganese Fe-Mn-C-Al-Cr-N twinning-induced plasticity（TWIP） steels were studied. The results show that Cr content has an obvious influence on the mechanical properties and fracture behaviors of the high manganese TWIP steels. The yield and ultimate tensile strengths of the steel sheets were improved with increasing Cr content while the elongation was reduced. In addition, with the increase of Cr content, the fracture mode changed from ductile fracture pattern with coarse dimples and tear ridges（Cr content ≤ 2.35%） to intergranular fracture（when Cr content is 3.95%）. Furthermore, Cr content has a tremendous effect on anti-corrosion behaviors of the high manganese TWIP steels. The increase of Cr content enhanced the corrosion resistance of the annealed steel sheets by improving the proportion of low-angle boundary.

Effect of the Deformation Twin Density on the Corrosion Behavior in TWIP Steel

Based on its excellent tensile strength-ductility property combination,twinning-induced plasticity (TWIP) steel shows great potential in applications for structural components in automobile industry.The aim of this research is to investigate the corrosion resistance properties and corrosion mechanism under room temperature in TWIP steel.The influence of the deformation twin density on corrosion property was primarily considered by salt spray test.The specimens used in the investigation are as-annealed and as-deformed respectively.The microstructure and corrosion resistance property were characterized by scanning electron microscope (SEM),optical microscope (OM) and so on.There are some annealing twins distributed randomly in austenitic grains in the as-annealed specimen.After the specimen was subjected to tensile experiment,the density of the deformation twins increased sharply,which are different from the annealing twins in size and morphology.It was found that the corrosion potential of the as-annealed is lower than that of the as-deformed and the corrosion current density behaves contrarily.After immersed in 5% NaCl solution salt spray for 48h,the as-deformed showed a bit better than the as-annealed in corrosion resistance.With the time prolonged,the gap between the two specimens in corrosion resistance increased rapidly.The corrosion morphologies varied in color and shape.Further investigation,carried out by SEM and EDS,indicated that as-annealed and the as-deformed followed pitting corrosion and uniform corrosion mechanism respectively.The reason for the difference in corrosion mechanism is possibly the presence of the deformation twins.The deformation twins formed during the tensile test refine grains by way of segmentation.The twin boundaries largely belong to the coincidence site lattice (CSL),which is on lower energy state.It suggests that the twins not only play a role in strengthening,but also improve effectively the corrosion resistance in TWIP steel.

Effect of Carbon Content on Stacking Fault Energy of Fe-20Mn-3Cu TWIP Steel

The influence of carbon content on the stacking fault energy （SFE） of Fe-20Mn-3Cu twinning-induced plasticity （TWIP） steel was investigated by means of X-ray diffraction peak-shift method and thermodynamic modeling. The experimental result indicated that the stacking fault probability decreases with increasing carbon addition, the SFE increases linearly when the carbon content in mass percent is between 0.23 M and 1.41%. The thermody namic calculation results showed that the SFE varied from 22.40 to 29.64 mJ ~ m 2 when the carbon content in mass percent changes from 0.23 % to 1.41%. The XRD analysis revealed that all steels were fully austenitic before and after deformation, which suggested that TWIP effect is the predominant mechanism during the tensile deformation process of Fe-20Mn-3Cu-XC steels.

Martensite Transformation and Its Control in DP,TRIP and TWIP Steels

Designing of alloy concept and process for DP,TRIP and TWIP steels stressing at martensite transformation are analyzed.For DP steel,austenite volume percent and its carbon content at different intercritical temperatures are calculated as well as the tensile strength of the steel,which meet well with the experimental result.The condition for dissolution of carbide is discussed by experiments and predicted by kinetic estimation.Several sample TRIP steels are prepared and their concentration profiles are calculated showing different diffusion characteristics of elements.Calculation also shows carbon enrichment is successful in this stage through the quick diffusion of carbon from ferrite to austenie.In order to maintain the austenite stability or to prevent precipitation of cementite,minimum cooling rate from the intercritical zone to over aging stage is obtained through kinetic simulation.Bainite transformation is estimated,which indicates the carbon rerichment from ferrite of bainite structure to austenite in this stage is also successful.Thermal HCP martensite transformation and the strain induced martensite transformation in TWIP steel is introduced.Relationship between transformation and mechanical properties in the steel is also mentioned.