Preparation,Microstructure And Mechanical Properties Of Multiphase Steels Based On Fine-Grained Ferrite/Carbide-Free Bainite

Compared with the traditional ferrite/martensite dual-phase steel,the ferrite/bainite dual-phase steel has more excellent ductility and formability,which has attracted extensive attention of researchers.In order to further optimize the properties of ferrite/bainite dual-phase steel,a thermomechanical method for preparing multiphase steel based on fine-grained ferrite/carbide-free bainite was proposed in the present thesis,namely martensitic tempering rolling+intercritical annealing in the dual-phase region+austempering near the Ms,and applied to two medium carbon（0.47wt.%,0.33wt.%）and two low carbon（0.21wt.%and 0.14wt.%）rich-Si low-alloy steels.According to the composition of the test steel,the heat treatment process was formulated to study the influence of process parameters on the microstructure,and the mechanical properties of the test steel after different processes were tested to analyze the relationship between the microstructure and the properties.The main research results are as follows:The carbon content had great influence on the pretreatment process of the test steel and the morphology of fine-grained ferrite.When 30 vol.%,15 vol.%and 5 vol.%ferrite were obtained for all test steels with different carbon contents,the required intercritical annealing temperature increased continuously with the decrease of C content.The exception was that the addition of Ni and Cu in the 0.14C steel expanded the austenite phase region,lowering the Ac₃ temperature,and leading to the intercritical annealing temperature lower than that of the 0.21C steel.With the decrease of C content,the intercritical annealing time was shortened.Fine equiaxed fully recrystallized ferrite grains were obtained by intercritical annealing at different temperatures for the 0.47C and 0.33C steels.However,for the 0.21C and 0.14C steels,the ferrite obtained was incompletely recrystallized,and some long-strip ferrite was retained.With the increase of intercritical annealing temperature,the recrystallization degree of both steels was improved.The difference of microstructures of low carbon steel treated by different processes was small.A matrix microstructure consisting of fine-grained ferrite with sizes of～1.2μm and～1.6μm and carbide-free bainite composed of bainitic ferrite laths and retained austenite films was obtained in the 0.47C and 0.33C steels,and the thickness range of bainitic ferrite laths was 74±6～165±30 nm and 113±15～198±73 nm,respectively.In addition,a small number of smaller martensite/austenite（M/A）constituents were obtained after the above-M_S austempering,while no obvious M/A constituents were observed after the below-M_S austempering,indicating that the bainite transformation was more complete,which was related to a small amount of pre-formed martensite in the microstructure.The intercritical annealing temperature and austempering temperature have significant effects on the tensile strength,yield strength and impact energy of medium carbon steel.With the increase of the intercritical annealing temperature,the tensile strength and yield strength of the two kinds of medium carbon steels were obviously improved due to the increase of bainite content.In particular,the increase of yield strength was greater than the tensile strength,leading to the increase of yield ratio,while the decrease of elongation was small.When the intercritical annealing temperature was constant,the tensile strength decreased with the increase of austempering temperature.The overall trend of the impact energy was increasing with the increase of the intercritical annealing temperature.The impact energy of the 0.47C steel was increased from 10-18 J to 34-53 J,and that of the 0.33C steel increased from 56-84 J to 81-126 J.When the intercritical annealing temperature was constant,the impact energy was increased with the increase of austempering temperature.This was because the retained austenite and fine M/A constituents played a positive role in improving the impact energy.The microstructures of low carbon steel treated by different processes were obviously different.In the 0.21C and 0.14C steels,the width ranges of long-strip ferrite obtained was 2.2–3.4μm and 2.0–2.4μm,respectively;the range of equiaxed ferrite grain size was 2.2–2.6μm and 1.5–1.9μm,respectively.The carbide-free bainite mainly obtained in the 0.21C steel was composed of bainitic ferrite laths and retained austenite films,and a small amount of granular bainite appeared when the austempering temperature was high.However,the carbide-free bainite obtained in 0.14C steel was mainly granular bainite.In addition,it was easier to obtain M/A constituents in both low carbon steels after austempering.And compared with below-M_S austempering,more number and larger size of M/A constituents were obtained by above-M_S austempering,and small angle grain boundaries were also significantly increased.The tensile strength of low carbon steel was less affected by the change of intercritical annealing temperature and austempering temperature,but the yield strength and impact energy changed obviously.With the increase of the intercritical annealing temperature,the tensile strength of the two kinds of low carbon steels increased slightly and the elongation decreased.However,the yield strength did not show a monotonic increase,because the formation of granular bainite and a large number of M/A constituents in the microstructure after above-M_S austempering limited the increase of yield strength.When the intercritical annealing temperature was constant,with the increase of austempering temperature,the tensile strength and yield strength decreased and the elongation increased.With the increase of the intercritical annealing temperature,the impact energy of the both low carbon steels decreased.When the intercritical annealing temperature was constant,the higher the austempering temperature,the more the decrease in impact energy.