Study On The Microstructure Evolution And Softening Behavior Of SWRS82B During Heat Treatment

In view of the different heat treatment processes in the production of cold drawn pearlitic steel wires,this paper took SWRS82 B pearlitic steel and cold-drawn pearlitic steel wires as the research objects,and adopted different heat treatment processes to study the structural evolution of SWRS82 B pearlitic steel and cold-drawn pearlitic steel wires.And the relationship between the microstructure evolution of pearlite and the strength of pearlitic steel wires was studied.Using GX51 metallographic microscope(OM),scanning electron microscope(SEM),field emission transmission electron microscope(TEM),high-resolution transmission electron microscope(HRTEM)and electron backscatter diffraction(EBSD)and other test methods to explored the nucleation and growth of pearlite colony and its relationship with impact toughness in eutectoid steel,the effect of micro-parameters in the annealing process on the strength of the cold-drawn pearlite steel wires and the relationship between the microstructural evolution and mechanical properties of the cold-drawn pearlite steel wire after austenitizing heat treatment.The conclusions as follows:1)The size of austenite grains increased with the increase of austenitizing temperature,but the size of the pearlite colony remained constant.In the process of nucleation and growth of the pearlite colony,there are many nucleation sites at the eutectoid steel grain boundaries and the pearlite colonies randomly nucleated at the grain boundary and grew into the crystal.At the same time,the new pearlite colonies nucleated beside the existing pearlite colony.And a large number of Mn S inclusions in the crystal serve as heterogeneous nucleation points,which reduced the energy barrier of nucleation and provided a location for the nucleation of intragranular pearlite colonies.These nucleation particles increased the chance of growing pearlite colonies colliding with each other,and eventually lead to pearlite colonies of constant size.In terms of mechanical properties,the austenite grain size and strength conformed to the Hall-Petch relationship.The finer the grains,the higher the strength.The size of austenite grain had nothing to do with impact toughness,which was controlled by the size of pearlite colony.2)The cold-drawn pearlitic steel wires with the corresponding variable of 1.8 were annealed to analyze the influence of microscopic parameters on the strength during the annealing process.When annealed below 250 ?,the carbon atoms gathered at the ferrite / cementite interface enter the ferrite,resulting in an increase in the number of dissolved carbon atoms in the ferrite and a uniform distribution,pinning dislocation,resulting in increased strength and hardening behavior.When annealed at 250? ? 450?,the number of carbon atoms in ferrite decreased,the cementite gradually spheroidized and the dislocation density in ferrite decreased,resulting in reduced strength and softening behavior.When annealing at 450°C to 550°C,the sharp decrease in tensile strength was caused by the decrease in the number of carbon atoms in the ferrite,the spheroidization of cementite leads to the disappearance of the pearlite lamellar structure and the decrease in the dislocation density.In the whole annealing process,the ferrite did not recrystallize.3)The cold-drawn pearlitic steel wires with the strain variable of 1.8 was austenitized at different temperatures followed by isothermal treatment at 550°C,and its tensile strength would decreased sharply.And as the austenitizing temperature increased from 790°C to 850°C,the spherical cementite and eutectoid ferrite gradually disappeared.Simultaneously,the tensile strength of the steel wires gradually increased,and the hardening phenomenon appeared.However,when the temperature reached 850 ?,the tensile strength of the steel wires reached the maximum,and the microstructure was uniform and fine pearlite.When the temperature increased from 850°C to 880°C,the <110> texture strength of the heat-treated steel wires decreased with increasing temperature,and at the same time,its tensile strength also decreased significantly.