Microstructure Features with Ultrafined and Purified Grains for Si-Mn Hot-rolling TRIP Steel

The advanced hot-rolled transformation induced plasticity （TRIP） steel with high tensile strength of 775 MPa, high elongation of 29% and low ductile-brittle transmission temperature less than -80℃ has been developed in laboratory based on the common silicon and manganese chemical composition. The experimental results showed that the microstructures were featured with ultra fine grain size less than 5μm, appropriate retained austenite volume fraction around 11.8% and purified ferrite grain, which may result in well balanced mechanical properties for the steel.

Prediction of HAZ grain size in welding of ultra fine grained steel with different parameters

The temperature field and thermal cycling curve in the heat-affected zone during welding 400 MPa ultra fine grained steel by plasma arc were simulated using finite element method. The principle of grain growth kinetics was used to predict the grain size in the heat-affected zone under different welding parameters. The simulation results show that the growing tendency of HAZ grain could be controlled by adjusting the welding parameters, but the growth of HAZ grain could not be eliminated at all. The HAZ grain size became small with increasing of the cooling rate and added with increasing of welding current, arc voltage and welding speed.

Study on improving properties of welded joints of 400 MPa ultra fine grained steel by mechanical surface hardening

The welded joints of 400 MPa ultra fine grained steel in manual arc welding were treated by mechanical surface hardening. Microstructure and mechanical properties of the treated joints were compared with those of the untreated joints, based on which, primary study on the process and principle of mechanical surface hardening was carried out. The results shows that： Grain size of HAZ increases greatly and mechanical properties of welded joint decrease obviously compared with those of base martial, but grain size in the surface layer of HAZ can be refined （the grain size is about 100 nm or so） and mechanical properties of welded joints can be improved greatly by mechanical surface hardening.

Recrystallized microstructural evolution of UFG copper prepared by asymmetrical accumulative rolling-bonding process

Copper sheet with grain size of 30-60μm was processed by plastic deformation of asymmetrical accumulative rolling-bonding(AARB)with the strain of 3.2.The effects of annealing temperature and time on microstructural evolution were studied by means of electron backscattered diffraction(EBSD).EBSD grain mapping,recrystallization pole figure and grain boundary misorientation angle distribution graph were constructed,and the characteristics were assessed by microstructure,grain size,grain boundary misorientation and texture.The results show that ultra fine grains(UFG)are obtained after annealing at 250℃ for 30?40 min.When the annealing is controlled at 250℃for 40 min,the recrystallization is finished,a large number of small grains appear and most grain boundaries consist of low-angle boundaries.The character of texture is rolling texture after the recrystallization treatment,but the strength of the texture is faint.While second recrystallization happens,{110}<1ī2>+{112}<11ī> texture component disappears and turns into{122}<212>cube twin texture component.

Repetitive Thermomechanical Processing towards Ultra Fine Grain Structure in 301,304 and 304L Stainless Steels

Thermomechanical processing as a combination of cold rolling and annealing was performed on austenitic stainless steels 301,304 and 304L. Two cold rolling steps each one up to a reduction of 75% were combined with an intermediate annealing at 800℃ for 20 min. The final annealing was performed at.the same temperature and time. Cold rolling contributed to martensite formation at the expense of metastable austenite in the studied materials. Austenite in 301 was found to be less stable than that in 304 and 304L. Hence, higher strength characteristics in the as-quenched 301 stainless steels were attributed to the higher volume fraction of martensite. Both α′-martensite and ε-martensite were found to form as induced by deformation. However, the intensity of ε-martensite increased as the stability of austenite decreased. Annealing after cold rolling led to the reversion of austenite with an ultra fine grained structure in the order of 0.5-1 μm from the strain induced martensite. The final grain size was found to be an inverse function of the amount of strain induced martensite. The thermomechanical processing considerably improved the strength characteristics while the simultaneous decrease of elongation was rather low.