| As a transmission chain on coal mining and transportation equipment,the service environment of the round link chain for mining is harsh,and the load received is complicated and the strength is large.In recent years,the coal mine industry has developed towards a new type of low energy consumption and high efficiency,which has put forward higher requirements on the size and performance of the round link chain for mining,the existing round link chains with a size less than Φ43mm are not enough to meet the growing production needs.Therefore,through alloying and microalloying methods to improve the hardenability of the round link chain steel,it has important theoretical significance and practical application value for improving the comprehensive performance of the large-scale mineral round link chain matrix and welding joint and extending its service life.In this paper,23MnNiMoCr54(hereinafter referred to as 54 steel)of common specification round link chain steel for mining has been microalloyed with Nb.Through thermodynamic calculation,the influence law of Nb content on the precipitation curve of Nb(C,N)was analyzed,and each equilibrium phase was calculated;the influence law of different quenching heating temperatures on the hardenability of two Nb-containing steels was explored by end quenching test;the influence law of different process parameters on the mechanical properties of matrix and thermal / mechanical simulation joint was analyzed,and the mechanism is discussed by combining OM,SEM,EDS,tensile test and impact test results.The following main conclusions are obtained:The calculation results of pseudo-binary equilibrium phase diagrams of different Nb content(0-0.20wt%)for mineral round link chain steel show that with the increase of Nb content,the initial precipitation temperature of Nb(C,N)gradually increases,and the precipitation quantity increases linearly with the increase of Nb content.The relationship curve of the molar fraction of each phase in Nb-containing steel with temperature under equilibrium state was obtained.It was found that Nb(C,N)began to precipitate at 1230℃and AlN began to precipitate at 1068℃;the composition of each phase and its relation curve with temperature were obtained;the phase transition points Ms,Mf,AC1 and AC3 of Nb-containing steel used in the experiment are 360℃,170℃,694℃,841℃.Comparing the end-quenching curves of the two Nb-containing steels and 54 steel,it is found that when the quenching heating temperature is 880℃,the hardenability of the two Nb-containing steels is poor;when the quenching heating temperature is increased to1050℃,the hardenability is significantly improved,and is better than 54 steel;when the heating temperature of quenching continues to increase to 1150℃,the hardenability of the two steels shows a downward trend.When quenching at 1050℃,the hardenability of the two Nb-containing steels is the best,while slightly increasing the Si content makes the hardenability of the round link chain steel decrease.For 1# Nb-containing steel,when the upset pressure in the simulated butt welding test is 15-20 MPa,the tensile specimen of the welded joint fractures at the weld;when the upset pressure increases to 25-30 MPa,the fracture position is in the base area,and the tensile strength of the joint and the matrix is basically the same,930-960 mpa.When the upset pressure is 25 MPa,the weld and heat-affected zone have a uniform structure,mainly composed of martensite laths.Compared with the heat treatment process of 1050℃ quenching + 400℃ tempering for 60 min,raising the tempering temperature to 430℃,the precipitation of carbide of 1#Nb-containing steel increases,and the form changes from fine needles to short flakes or granules.Its yield strength is 1195MPa(increased by about 50MPa);tensile strength is1311MPa(increased by about 63MPa);elongation is basically unchanged,about 17%;impact energy is increased from 94 J to 102 J.The number of carbides in 2# Nb-containing steel has only increased,and there is no change in shape.Its yield strength and tensile strength are 1137 MPa and 1271 MPa,respectively,an increase of about 120MPa;the elongation is 15.6%,an increase of 2.4%;the impact energy is 87 J Increased to 96 J.When the upset forging pressure in the simulated butt welding test is 25-30 MPa,the strength of the 1# Nb-containing steel welded joint is basically the same as that of the substrate;after quenching at 1050℃,the 1# Nb-containing steel has the best hardenability,and after tempering at 430℃ for 60 min,its hardness,strength and impact toughness all meet the requirements for the use of the mineral round link chain steel,and are higher than 54 steel under the same conditions;while the Nb content is about 0.049wt%,theincrease in Si content does not significantly improve the mineral round link chain steel quenching permeability and mechanical properties. |
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