| The concept of “reforming steel to plastic” has become the mainstream of the automotive industry and one of the effective ways to reduce the weight of automobiles.Driven by the demand for large-scale complex plastic auto parts such as large-scale automobile bumpers,instrument panels,and door lights,plastic mold steel is developing toward large-scale and high-hardness trends.The basic research of the domestic mold steel industry is weak,and the steel used in the high-end market is heavily dependent on imports.Imported steel is not only expensive,but also restricts the development of the domestic mold industry.Therefore,it is very urgent and necessary to develop a new type of high-hardness pre-hardened plastic mold steel.The experimental steel composition design conforms to the development of the times.By reducing the carbon to solve the problem of cavity cracking caused by excessive internal stress and improving the alloy element content to compensate for the strength loss,the preparation of ?16×2000mm bar and 1200 mm wide and 600 mm thick experimental materials were carried out and analyzed changes in microstructure and properties.The experimental part of the thesis mainly carries out research on heat treatment technology,industrial large module analysis and precise regulation of tissue performance,and provides theory and basis for pre-hardening technology control and organization control principle.The main conclusions are as follows:1.The 2Cr2 MnNiMoV steel designed by reducing carbon and increasing the content of alloying elements,the alloying elements Mo and V are added to refine the size of the carbide,and the precipitation strengthening effect makes up for the lack of carbon reduction strength.880? oil quenching,550? tempering isothermal heat treatment processes have the best comprehensive mechanical properties: the tensile strength and yield strength reached 1409 MPa and 1192.7 MPa,respectively,and the area shrinkage and elongation reached 66.7% and 17.9%.The impact toughness was 100 J.Comparing the property of domestic popular 1.2738 steel under the same process: the tensile strength is 1326 MPa,the yield strength is 1180 MPa,the area shrinkage is 57.7%,the elongation is 14.7%,the impact toughness is 47 J,and the mechanical properties are greatly improved.2.The industrial production of the 1200 mm wide and 600 mm thick module 2Cr2 MnNiMoV steel ends are completely hardened by pre-hardening.The tempered martensite microstructure exists at the side and the tempered bainite is the main microstructure.The hardness of the section is uniform,and the hardness deviation is within the range of 2.5HRC,which satisfies the section hardness requirement of the high hardness pre-hardened plastic mold steel of 38~42HRC.3.There are mainly two phases of bainite and martensite in 2Cr2 MnNiMoV continuous cooling transition curve.The main product of phase transformation is bainite when the cooling rate is 0.03-0.15 ?/s;the cooling rate at 0.3-16 ?/s,the main product of phase transformation is martensite.At the cooling rate in the range of 0.03-0.3 ?/s,bainitic ferrite changes from equiaxed to lath,and finally eventually merged into massive ferrite.The M-A island is large size,small quantity,irregular arrangement in the massive ferrite formed by equiaxed ferrite;The M-A is small in size,large quanlity,and arranged in a parallel manner in the massive ferrite formed by lath ferrite,and the hardness increases with the rate of cooling.When the cooling rate is 0.3-16 ?/s,cooling rate has no effect on martensite transformation and hardness.4.The experimental steel was austenitized at 880? for 30 minutes and cooled to room temperature by oil cooling,air cooling,furnace cooling and isothermal,to obtain martensite,martensite and bainite,and all bainite.The results show that a small amount of bainite exists in martensite,which acts to split the austenite grains and refine the martensite microstructure increased toughness;Furnace cooling and isothermal for a long time,bainite lath is coarsened,and the hard phase of the M-A island is increased in size and located at the grain boundary to reduce the material toughness.5.Bainite tempering exhibits temper brittleness at 500?.The island of M-A island is decomposed with an increase in tempering temperature: on the one hand,the decomposed carbides and the refined M-A island provide strength and hardening peaks appear;on the other hand,carbides and high-carbon martensite formed after decomposition of the M-A island on the grain boundary deteriorate the toughness of the material. |
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