Research On Continuous Cooling Transformation And Tempering Mistructure And Mechanical Properties Of Modified Lager-size Link-chain Steels For Mining

For a long time,as a key component of coal mining equipment and transportation equipment,the round link chain for mining has been manufactured with 23MnNiCrM054steel?abbreviated as 54 steel?with high toughness,wear resistance and corrosion resistance due to the harsh service environment.However,as the coal mining industry develops toward the direction of new high-efficiency capacity optimization,the size specifications and performance requirements of the round link chain for mining become larger and larger,and the conventional 54 steel has failed to meet the requirements and the cost is high.Therefore,it is of great significance to develop a new large-size round link chain steel for mining that meets the requirements of production process performance and is economically reasonable.In this paper,based on 54 steel,two kinds of large-diameter of nickel free round link chain steels for mining were designed with adjusting the content of carbon and silicon which named C1 and C2.The continuous cooling transformation curves of two steels were determined by dilatation method;the hardenability curves of two steels was studied by end quenching method;By means of OM,SEM,XRD,tensile and impact tests the evolution of the structure and mechanical properties of two steels were tested that after quenching and tempering.The following main research results were obtained:The CCT curves of the C1 and C2 steels were obtained.For C1 steel,the A_c1 and A_c3are 721°C and 855°C,and M_s and M_f are 380°C and 240°C,respectively.For C2 steel,A_c1 and A_c3 are 730°C and 875°C,M_s and M_f are 362°C and 225°C,respectively.In the continuous cooling transition of the two test steels,the transition products from fast to slow cooling rate were martensite,martensite+small amount of bainite,mixed bainite,pro-eutectoid ferrite+Bainite+M/A composition,in which C2 steel does not have a complete bainite phase region.By CCT curve,the martensite critical cooling rate of the two test steels was obtained,C1 was 1°C/s,C2 was 5°C/s;According to the hardenability curve,the slope of the hardenability curve of C2 steel does not fluctuate greatly,and the decrease of hardness from the quenching end to the end of the sample does not exceed 6HRC,showing excellent hardenability.C1 and C2 steels are quenched at 880°C and tempered at 200⁴00°C for 2 h.,the proportion of lath is gradually reduced with the increase of temperature,and the retained austenite is gradually decomposed.When the temperature is 400°C,the complete plate cannot be observed.The presence of martensite and retained austenite.Due to the high silicon content of C2 steel,the amount of carbides precipitated from lath martensite is small;when tempered at 200°C for 2⁸h,the microstructure of the two test steels does not change much.After quenching at 880°C and tempering at 200⁴00°C for 2 h,the mechanical properties of the two steels decreased with increasing temperature..The yield strength and tensile strength of C1 steel are reduced from 1240 MPa and 1580 MPa at 200°C-2 h to1055 MPa and 1235 MPa at 400°C;impact absorption energy and hardness decrease from52 J,51 HRC to 23 J,44 HRC;yield strength and tensile strength of C2 steel decreased from 1250MPa and 1746MPa to 1078MPa and 1314MPa at 400°C respectively.The impact absorption energy and hardness decreased from 44J and 54HRC to 23J and46HRC.When tempering at 200°C for 2⁸h,the tempering time has little effect on the mechanical properties of C1 steel.With the extension of tempering time,the yield strength of C2 steel is not reduced to about 60MPa,and the tensile strength is rapidly decreased from 1746MPa at 2h to 1595MPa after tempering for 4h and then slowly decreased;the tempering time had little effect on the impact energy and hardness.Through the analysis of the microstructure and properties of C1 and C2 steels,the hardenability of the two teels has reached the production process requirements of?55mm link chain steel for mining;the tempering at 200⁴00°C,the yield strength and the tensile strength and impact toughness also meet the actual use requirements,but the elongation of test steel has yet to be improved,which is also the direction of subsequent experiments.