Key Process Technologies For Manufacture Of Ultra High Strength Steels

Ultra-high strength steel is the leadship of structural material strength,with high specific strength,.specific modulus,widely used in aviation,aerospace and defense and other fields,which is the preferred material for the key components of the aircraft,representing the highest level of a national steel material research and production,and being an important symbol of a national science and technology and national defense industry development level.Tensile strength of ultra-high strength steel at room temperature is more than 1400 MPa,and yield strength is greater than 1300 MPa,and its toughness requirements are relatively high,which is always in the challenge of limit of material strength and toughness.At the same time,ultra-high strength steel is very sensitive to cracks,inclusions,welding and surface defects.Therefore,reducing the defect rate of ultra-high strength steel and improving the toughness of steel are always an important research direction of international forefront technology development.Chinese researchers have made great achievements and progress in ultra-high strength steels in the last decades,to reduce the content of harmful impurities in steel and to modify the shape of inclusions for improvement of the toughness of steel.However,there is still a big gap in ultra-pure melting,solidification structure control,thermal processing and heat treatment between China and other developed countries,which seriously hamper development of China's aerospace and defense industry.In this paper,taking several typical ultra-high strength steels produced in China as the research object,the key process technologies for manufacture of ultra-high strength steels have been studied in order to improve the cleanliness,structure and mechanical properties,to enhance the technical level for production of ultra high strength steels for aerospace and defense industry,which is of important practical significance for padding the "short board" of key materials.The research status of ultra high strength steels at home and abroad has been comprehensively reviewed in the paper.Through the in-depth investigation on the actual production technology and quality level of domestic ultra high strength,taking 300M,A-100 and S53 as the research object,the key process technologies for manufacture of ultra high strength steels including melting,forming and heat treatment have been carried out by the laboratory vacuum induction furnace test,theoretical calculation and analysis,adding rare earth tests,industrial tests and other methods.The melting of refined steel,vacuum induction melting(VIM),the treatment of ultra high strength steel by rare earth,the removal and control of non-metallic inclusions in ultra high strength steel,double vacuum melting(VIM-VAR),triple melting(VIM-ESR-VAR),forging and the heat treatment have been performed with the help of the chemical analysis,OM,SEM,TEM,mechanical property tests and XRD analysis..Under the industrial conditions,the ultra high strength steel with high purity and high performance was produced in FuShun steel,and it was showed that the triple melting process was the best way to solve the above-mentioned technical problems.Baesd on the above research results,the innovations and the main conclusions are as follows:(1)The effect of La on the purification of ultra high strength steel was studied.The results showed that it was possible to keep the perfect,stable deoxidation and desulphurization by La in the good condition of furnace and precisely controled temperature during the vacuum induction melting.And the inclusions are mainly La2O2S when the content of oxygen and sulfur in the steel is very low.And the Mg-bearing and Al-bearing oxides can precipitate on the La2O2S surface forming compound inclusions.(2)The effects of single vacuum induction melting,double vacuum melting process and triple melting process on the inclusions in steel were studied.The results show that there are no differences on the types of inclusions in ultra high strength steels prepared by the three processes.Typical inclusions are MgO-Al2O3-CaS,MgO-Al2O3-SiO2,etc.,which are mainly magnesium-containing spinel.The average diameter of inclusions and the number of inclusions per unit area in the triple process are all smaller than that of single vacuum and double vacuum process.(3)The research results about the effect of vacuum arc remelting(VAR)on the ingot microstructure indicated that based on the steel ingot produced by high melting rate during vacuum arc remelting process,it is easy to form the radial segregation defect of the steel structure.And based on the steel ingot produced by the cooling ways of strong cold helium and high water-flow rates,it is easy to form the ring pattern defect of steel structure.Therefore,to reduce appropriately the melting rate without helium cooling,we can produce the bar with qualified macrostructure.(4)The research results about the effect of the forging process on the impact resistance of G13Cr4Ni4Mo4VA aircraft engine bearing steel bar indicated that the precipitation of?-ferrite distributed along the grain boundaries caused impact property lowness of G13Cr4Ni4Mo4VA bar.The forging temperature is reduced from 1160 K to 1110 K,which can effectively avoid the precipitation of 8-ferrite and improve the impact performance of steel bar.(5)The research results about the effect of forging and preheat treatment on the grain size of A-100 ultra high strength steel indicated that for the A-100 steel,when the deformation is 30%,the deformation temperature is controlled in the range of 1000?1140 K,the fine complete recrystallization structure can be obtained,and the microstructure uniformity also can be improved.Application of normalizing process for the heat treatment can improve the grain size level,uniform microstructrure.When the normalizing temperature controlled in the range of 900?950 K and the proper holding time can prevent excessive growth of grains.(6)The effect of tempering heat treatment on the microstructure and properties of S53 ultra high strength steel has been studied.The results show that the microstructure and properties of S53 steel after two times of heat treatment are better than that of primary one.The tensile strength of the sample with the tempering temperature of 490 K is up to 1955 MPa,the yield strength is about 1684 MPa,the hardness is up to 56(HRC)and the uniform elongation rate is about 10%.Secondary tempering after treatment of 490 K,compared with tempering once of 505 K,the martensite lath is finer and the boundary distortion degree is deeper than that of the first tempering,which is more effective to the movement of the dislocation and the strengthening effect is better.At the same time,after the two tempering,more fine carbides precipitated on the lath martensite matrix and dislocation,which improved the strength of the material.The tempering temperature is 490 K,and the tensile fracture surface has a lot of dimples.With two times the tempering temperature,the carbide becomes coarse,and the tensile fracture morphology of samples will also form the features of quasi cleavage.Therefore,the best tempering process for S53 steel should be performed at 505 K×3 h tempering and then at a temperature of 490 K×12 h.