Study On Process And Microstructure-properties Of Q1030 Ultra-high Strength Steel

The construction machinery industry has always been an important part of the national economy.In recent years,with the development of the national economy,all industries have higher and higher requirements for construction machinery equipment.With the improvement of the design level of construction machinery,the requirements for materials which including higher strength and weldability are also higher and higher.However,the increase of strength often leads to the decrease of toughness,especially for the high strength steel with yield strength higher than 1 000MPa,the difficulty of product development mainly lies in the control of toughness.In order to match the strength and toughness of high strength steel,a Q1030 ultra-high strength steel with yield strength over 1000MPa has been developed.The CCT curve of Q1030 ultra-high Strength Steel,the rolling process and the precipitation behavior of microalloy second phase particles have been studied,the microstructure and properties of Q1030 ultra-high strength steel have been studied under different quenching temperature,holding time,tempering temperature and tempering time.Finally,the impact toughness of Q1030 ultra-high strength steel-20? was over 100J after optimizing the process,the main results were as follows:For the static CCT curve of the Q1030 steel,when the cold speed is 1?/s,the metallographic structure is a small amount of ferrite and bainite.When the cold speed is 3?/s,the ferrite disappears,the metallographic structure is dominated by bainite,and when the cold speed rises to 7?/s,the martensite begins to appear in the metallographic structure.For the dynamic CCT curve of the Q1030 steel,the initial temperature of the deformed austenite phase transformation and the end temperature of the phase transformation have increased,and the phase transformation temperature range has also increased.The thermal deformation activation energy of Q1030 high strength steel in austenitic region has been determined by regression method.The critical strain and peak strain of dynamic recrystallization of high strength Q1030 steel were predicted accurately by the method of cubic polynomial fitting for the inflection point,and the relationship between critical strain and Z parameter has been established.The precipitation behavior of Nb and Ti particles during low strain rate deformation has been studied.The precipitation phases in the experimental steel are rectangular TiN,nearly square(Nb,Ti)(C,N)carbonitrides,elliptical(Nb,Ti)C and NbC,the sequence of the second phase precipitation is TiN,TiC,NbC,NbN.In the controlled rolling Q1030 steel,there are three types of microalloy element particles precipitated:the first type is above 100 nm,and the core is formed during the solidification of the steel,it nucleates during solidification of steel,and gradually grows during the cooling process;The second type of size is 5 0nm?80nm,with a nucleation in the austenite phase area;The third category has a size of 10 nm to 20 nm and a nucleation in the ferrite phase area.The growth law of austenite grains has been studied.The average size of austenite grains increased exponentially with the increase of heating temperature,and the average grain size increases parabolic with the extension of the insulation time.During quenching at 880?950?,with the increase of austenitic temperature,the hardness and strength of Q1030 steel gradually increased,reaching a maximum value at 950?;During quenching at 950?1100?,the hardness and strength of Q1030 steel gradually decreased with the increase of austenitic temperature.During quenching at 880?950?,the Q1030 steel has a good impact toughness.With the increase of austenitic temperature,the impact toughness of the steel decreases slightly.When the temperature exceeds 950?,its toughness begins to drop significantly.When the Q1030 steel is heated at a slower heating speed of 0.25?/s,the Martensian-Austenitic phase transition of the Q1030 steel is carried out in two stages.The first low temperature stage is controlled by the diffusion process.At the second stage of high temperature,the phase transition is performed in a shear manner.When heating speed is 10?/s,the entire phase transition process is carried out continuously in a shear manner.When the Q1030 steel is heated to 730?,a needle-like austenite appears in the tissue.When the heating temperature reaches 760?,a granular austenite is formed on the martensite strip boundary and the original austenite grain boundary.When the heating temperature reaches 820?,The metallographic structure begin to be dominated by granular austenitic bodies.When it is not more than 400?,the Martensite strip(small angle grain boundary)is still clearly visible,and the frequency of the small angle grain boundary does not change significantly.The yield strength slowly decreases,and the extension rate slowly rises,at a time greater than 400?.The frequency of the small angle grain boundary decreases obviously,the yield strength decreases rapidly,and the extension rate rises rapidly.According to the study on the precipitation rule of Q1030,when the tempering temperature is lower than 400?,the decrease trend of solid dissolved carbon in martensite is more intense,but when the tempering temperature is higher than 400?,the amount of solid dissolved carbon in martensite decreases very slowly.In addition,as the tempering temperature increases,a large number of small and parallel precipitated ?-carbide dissolves and is eventually replaced by Cr carbides precipitated along the Martensite strip,and the compound carbon and nitrogen compounds of Nb,V,and Ti also gradually grow.The shape also evolved from a square to an oval shape.