TMCP Technology And Strength-toughness Control Of Si-Mn-Cr Series Super High Strength Steel

With the rapid development of science,technology and economic,all walks of life has been gradually increased the demand for iron and steel materials.Because of its excellent strength properties,high strength steel is widely used in engineering machinery,automobile manufacturing,building structure and other fields.In order to meet higher demand of industrial and performance requirements,the future directions of high strength steel are ensuring the strength of materials,reducing cost,improving the toughness,plasticity and weldability.The works of this paper based on Si-Mn-Cr steel,a small amount of Mo and Ni and other elements were added.Then researched the influence of different controlled rolling and controlled cooling technological parameters on structure and properties of experimental steel,meanwhile researched on the effects of different off-line quenching and tempering processes and different tempering processes on the microstructure and properties of experimental steel The main works of this paper are as follows:(1)Fully considered the role of elements in steel and reasonably designed the chemical composition of experiment steel,strictly controlled the composition of harmful elements.On the premise of not harmed experiment steel's performance,added as little alloying elements as possible to reduce production cost,and made a variety of strengthening mechanisms to play a role to enhance the strength of experimental steel.(2)Measured the critical temperatures of the experimental steel,the critical temperatures Ac3 and Ac1 are 831.7? and 683?,the Are and Ar1 are 841.0? and 683.8?.The continuous transformation behaviors of experimental steel were researched.Changed the cooling rate and observed the microstructure,and drawn the static CCT curve and dynamic CCT curve.The results showed that the cooling process is austenite-martensite transformation.The compression deformation made CCT curve to upper-left side.And with the increasing of cooling rate,the martensite structure was refined.(3)By carrying out laboratory hot rolling experiments,the results showed when rolled under lower temperature,the experimental steel had excellent strength and toughness matching.The impact toughness decreased with the rolling temperature increased.When the second stage rolling temperature is 850?,the yield strength,tensile strength and elongation are 908MPa,1342MPa and 12.7%,respectively.The impact energy at 0? and-20? are 104.68J and 57.27J.By air cooling under 250? and quenching the experimental steel can obtained excellent properties,and obtained martensite microstructure.By air cooling under 250?,the yield strength,tensile strength and elongation of the experimental steel are 950MPa,13 59MPa and 13.3%,respectively.The impact energy at 0? and-20? are 173.32J and 167.16J.By quenching,the yield strength,tensile strength and elongation are 1060MPa,1413MPa and 14.7%,respectively.The impact energy at 0? and-20? are.150.53J and 104.91 J.(4)Through different tempering processes experiments,,when tempering between 200?400?,with the increasing of the tempering temperature,the tempered structure was coarsening and existed irregular carbides precipitation.The impact energy and tensile strength decreased,the yield strength increased.(5)Through different off-line quenching and tempering experiments,the results showed that the impact toughness of the experimental steel was significantly improved compared with the steel after the controlled rolling and controlled cooling.The yield strength and tensile strength were reduced.With the increase of austenitizing temperature during off-line quenching,the impact energy decreased slightly,the yield strength and tensile strength decreased,and elongation increased.