Investigation Of Asymmetric Rolling On Microstructures And Mechanical Properties For TRIP Steel

With the development of economy and society,the society put forward higher requirements on the environment,energy,safety,etc.In this background,the TRIP steel became a hot research direction of automotive steel for its high.strength and good plasticity.And steel demand falling and the excess capacity spurring at this stage,iron and steel enterprises urgently need to reduce the production cost,reduced the energy consumption of products and improved product quality.Therefore,the new rolling technology had become the new trend of the enterprise.Asynchronous rolling can reduce energy consumption and raised productivity.So the asynchronous rolling attracted our attentions.But asynchronous rolling had not widely used in industrial.It was no wonder that developing the asynchronous technology was very necessary.In this paper,we tested hot-rolled and analyzed the hot rolling process of hot-rolled steel structure and the influence of performances.In addition,we also studied the heat treatment influented on the microstructures and properties.So we can establish suitable hot rolling process for the actual production condition and heat treatment of cold rolling.The study provided the theoretical foundation and experiment evidence for industrial production.The main results of this research are as follows:(1)The hot rolling microstructure was mainly included ferrite,bainite and retained austenite.The microstructure was gradient distribution along the thickness direction.The edge of the steel had smaller average grain size about 1.98?m,but in the center the average grain size was about 2.38pm.Through SEM,we can find hot rolled microstructures with plate ferrite,and retained austenite and bainite distributed between ferrites.To the further observe with TEM,we found retained austenite,which showed island(M/A island)shape.XRD results said that the retained austenite transfered martensite under the tensile.(2)The steel yield strength reduced with the increase of finish rolling temperatures,while the tensile strength increased with the increase of finish rolling temperatures.The yield strength and tensile strength increased with the increase of the coiling temperature and decreased after the rising,peaked at 420?.Elongation also had the increase of finish rolling temperatures and decrease after the rising,but the elongation peaked at 470 ?.Compared with different asynchronou ratios,the yield strength,tensile strength and elongation peaked in 1.05.The microstructure was gradient distribution along the thickness direction,and grain size increases from the edge to the center.Studying the gradient microstructure's influence on the mechanical properties,we would find the hardness of steel decreased from edge to center position,and the hardness of the fast roll is slightly higher than that of the slow roll..(3)When the steel finished rolling temperatures at 780? and water cooling to 420?coiling,the value of tensile strength,yield strength,elongation and tensile ×enlongation was the highest.The hightest value of yield strength,tensile strength,elongation and tensile ×enlongation of exmperimental steels was 690MPa,850MPa,24.68%and 20978MPa%respectively.(4)Before the hot treatment,the microstructure was ferrite and pearlite.After cold rolling and heat treatment was ferrite,bainite and retained austenite.With the increase of heat treatment time and heat treatment temperature increased,banded structures gradually disappeared.The ferrite showed as polygonal structure,bainite and retained austenite around the ferrite grain boundaries.Compared with hot rolled TRIP steel,TRIP steel had a smaller grain size,the banded structure in the hot rolling was completely disappeared,and the aggregation of the bainite slab was reduced.Microstructures had the polygonal ferrite structure,and retained austenite and bainite distributed in the ferrite grain boundary using the SEM.To the further observe with TEM,we found retained austenite,which showed island(M/A island)shape.After the tensile,we found the martensite appeared where the retained austenite stayed.The XRD results also proved that the retained austenitet transfered martensite under the tensile.(5)The yield strength,tensile strength,elongation and tensile × enlongation were all increased of isothermal bainite temperatures and dropped later,but yield ratio kept steady at 0.80.The yield strength of steel was increased in two-phase region temperature rising,moreover the yield strength had risen sharply in 780?.The tensile strength of steel with the increase of the two-phase region temperature dropped and rising following,but the elongation was decreased,fell sharply in 780 ?.Compared with two-phase region temperature heating time on yield strength,tensile strength,elongation and tensile X enlongation,the value reached the highest in 8min.(6)After the steel finished hot rolling and cold rolling,the steel heated to 760? and held for 8 min,subsequently was quickly moved into salt bath at 420? and held for 5min.After all of this,the value of tensile strength,yield strength,elongation and tensile × enlongation was the highest.The hightest value of yield strength,tensile strength,elongation and tensile ×enlongation of exmperimental steels was 518MPa,623MPa,41.42%and 25473.3MPa%respectively.