Development And Strengthening Mechanism Of High Strength Sorbite Steels

In order to improve the properties of spring steels,the sorbite with ultrafine lamellar structure was utilized,and high-carbon steel strips were selected as the research objects.The sorbitizing has been successfully prepared by controlled cooling instead of lead bath patenting.The evolutions of the microstructures in sorbite steel strips under different cold rolling reductions were studied using X-ray diffraction(XRD),scanning electron microscopy(SEM),electron backscatter diffraction(EBSD)and transmission electron microscopy(TEM),and the strengthening mechanism of sorbite steel under cold rolling was revealed by testing the mechanical properties.An innovative approach was used by introducing Cu acting as the second phase strengthening particles into the sorbite structure,and the particles precipitated during the isothermal quenching process,thus the strength of the sorbite spring steel was further enhanced.The main achievements are summarized as follows:1.The phase transformation temperatures and the supercooled austenite transformation curves of SAE 1078 and SK 85 hot-rolled high-carbon steels were analyzed by using JMatPro metal phase diagram calculation software and Gleeble3800 thermal simulator.The result showed that the interlamellar spacing of pearlite and the thickness of cementite layer decreased gradually with the decreasing of isothermal temperature.The fine sorbites were obtained when isothermal quenching temperatures are below 450°C.In addition,when the isothermal temperatures continue to increase,the increments of interlamellar spacing are mainly contributed by cementites coarsening,and the thick rodlike cementites appeared in the conditions of higher isothermal temperatures.2.By considering the interface energy of the pearlite transformation,the shape of the nucleus in the initial nucleation and the positions of the nucleation,a model with respect to the pearlite transformation was established.The interlamellar spacings of the pearlites were measured by SEM.It was found that the calculated interlamellar spacings of the pearlite from the model were consistent with those of the actual measurements.3.The tensile properties of SAE 1078 and SK 85 steels corresponded with the microstructures at different isothermal processes.The sorbite obtained at lower isothermal temperature shows good strength and plasticity.The highest strengths of two steels were respectively reached to 1170 MPa and 1240 MPa isothermal at350°C.When the temperature higher than 500°C the interlamellar spacing of pearlite gradually became coarsening and the strength decreased,and the plasticity did not increase,showing characteristics of brittleness,which is not suitable to further cold rolling processing.4.Different cold rolling reductions for SAE 1078 and SK 85 at their best patenting processes were carried out.The interlamellar spacing of pearlite decreased with the increasing of cold rolling reduction,but the amount of decrease reduced with the increase of reduction.It was showed that the pearlite was obviously refined with the interlamellar spacing decreased to about 58 nm at the rolling reduction of90%,and the interlamellar spacing of each pearlite colony is similar to each other.With the increasing of cold rolling reduction,ferrite with body-centered cubic(bcc)structure was transformed into body-centered tetragonal(bct)structure at 90%rolling reduction.5.The yield strength and interlamellar spacing of cold-rolled sorbitic steel basically met the relationship of Hall-Petch.The mechanism of pearlite cold rolling conforms to the interface strengthening model in the cold-rolling process from 10%to 90%reduction ratios,and the yield stress(σ_y)and strain(ε)follows?_y(28)4563.(10)6488.exp(?/)2.Baking of sorbitic steel after large derfomation promote the precipitation of carbides in supersaturated ferrite and led to the increasing of hardness.The dislocation density decreased and carbides coarsened with the increasing baking temperature and time.The strength of SAE 1078 and SK 85 could be further improved by baking at 200 to 250°C for 20 minutes.6.Adding~1.5 wt%Cu into SAE 1078,the nano-scaleε-Cu particles precipitated in ferrites by controlling the isothermal temperature and holding time.The mechanical properties of different patenting processes have been tested.It was found that the hardness(380 HV)and tensile strength(1450 MPa)reach their peak values at 500°C for 1000 s.According to Ashby-Orowan precipitation strengthening mechanism the amount of precipitation strengthening was determined to be approximately 150 MPa whenε-Cu size is~20 nm,which is in good agreement with that of the actual measured value.7.With the increasing of cold rolling reductions,Cu-containing sorbite steel and other two Cu-free sorbite steels all experienced dislocation strengthening and solution strengthening respectively.However,dislocation strengthening and solution strengthening mechanism began at lower cold rolling reductions due to the precipitation ofε-Cu making the Cu-containin sorbite steel having a higher strength under lower cold rolling reductions and thus the manufacturability of steel is improved.