Basic Research On Continuous Casting Process Of High Speed Steel(M2)

High speed steel has the advantages of high hardness,high wear resistance,high red hardness and so on.It is an indispensable basic material for manufacturing and processing industry.At present,China's high speed steel production still depends on ingot casting-electroslag process,the production efficiency is low,and the production cost is higher.The basic research on continuous casting process will help to realize the continuous casting of high speed steel and optimize the production process of high speed steel.Taking the realization of high speed steel continuous casting as the research object,the relationship between cooling rate and element segregation,carbide precipitation and grain size during solidification process of M2 steel are clarified through laboratory simulation experiment and relevant detection and analysis.The crack sensitivity and thermodynamics of carbide precipitation during solidification of M2 steel are analyzed.The influence of continuous casting process parameters on the quality of M2 steel continuous casting billet is studied by ProCAST software simulation and laboratory scale continuous casting experiment research.The suitable billet section and continuous casting process parameters for M2 steel continuous casting are proposed and optimized.The results are as follows:The relationship between cooling rate and element segregation,carbide precipitation and grain size during solidification of M2 steel shows that:(1)The peritectic reaction is fully carried out at the cooling rate of 1.5 K/s during M2 steel solidification,the reaction time is 32.7 s,and cracks exist on the y grain boundary;the solidification time was only 5.7 s at 10 K/s cooling rate,the peritectic reaction is inhibited,no peritectic reaction is observed,and no grain boundary crack is found.The relationship between grain size(DL)and cooling rate(CR)in as-cast solidification structure of M2 steel is:DL = 93.9 CR-0.33.(2)The eutectic carbide types of M2 steel are M2C.The change law of the eutectic carbide morphology is as follows with the decrease of cooling rate:single-layer lamellar ? rod-shaped ? honeycomb-shaped ? multi-layer curved lamellar ? multi-layer flat lamellar.Thermodynamic of carbide precipitation and crack sensitivity analysis during solidification of M2 steel shows that:(1)W2C and Mo2C have thermodynamic conditions of precipitation at 1504 K and 1503 K respectively during the solidification of M2 steel.The secondary carbides VC,Mo2C and W2C precipitate at 1475 K,549 K and 333 K,when the temperature drops below the solid line.(2)The crack sensitivity of M2 steel is high.when the cooling rate is 0.5 K/s-50 K/s the crack sensitivity index of M2 steel is greater than 0.68.The suitable billet section and continuous casting process parameters for M2 steel are as follows:(1)When the cross-section of M2 steel billet increased from ? 100 mm to ?150 mm,the average thickness of network carbide increases from 16.2 ?m to 27.9?m,and the standard deviation of average thickness decreases by 31.1%,the average grain diameter increased from 55.7 ?m to 113.2 ?m,and the standard deviation of the average grain size decrease by 23.8%.(2)Electromagnetic stirring(1000A/5Hz)in secondary cooling zone can effectively inhibit the subcutaneous porosity and shrinkage cavity of M2 steel continuous casting billet,refine and disperse the solidification structure.The uniformity of solidification structure distribution has a particularly significant effect,and the effect is better than the effect of increasing secondary cooling.The standard deviation of grain size and carbide thickness of ? 150 mm M2 steel continuous casting billet with and without electromagnetic stirring in secondary cooling zone decrease by 47.4%and 37.2%.(3)The suitable vertical continuous casting process for M2 steel with 135 mm×135 mm section is as follows:the casting speed is 1.0 m/min,the superheat is 30 ?,the mould electromagnetic stirring is less than 200A/2Hz.It is necessary to add electromagnetic stirring in secondary cooling zone,and the stirring intensity shall be less than 1000A/5Hz,and the temperature holding+slow cooling when the surface temperature of continuous casting billet drops to 943 ?.The solidification coefficient of continuous casting billet is 25.3 mm/min1/2 under these process conditions.