Investigation On Trial-production Of Low Alloyed Ultra-high Strength Steel For Engineering Machinery

In this paper, a low alloyed ultra-high strength steel for engineering machinerywas developed by the pilot-scale simulations of hot rolling and in-line heat treatmentin the pilot plant on the bases of designs of microstructure, chemical compositions andproduction processes, and heat treatment simulations of the hot rolled steel exampleswere conducted in the laboratory furnaces to improve the comprehension mechanicalproperties. By means of mechanical properties tests, and microstructure andprecipitation observations, the effects of processing parameters of controlled rolling,controlled cooling and in-line heat treatment on the microstructure and properties ofthe tested steel were investigated, and the optimal processing parameters for the testedsteels were determined. The evolutions of microstructure and changes of mechanicalproperties of the tested steel plate under different heat treatments were analyzed anddiscussed, and the optimal heat treatment processing parameters were also obtained.The test results show that:(1) The room temperature microstructure of tested steel produced by two stagesof controlled rolling, direct quenching and low temperature tempering technology ishigh dislocation density lath martensite with plenty of wavy ε-carbide which isprecipitated in the martensite lath, and between the martensite lath boundaries remainsa certain amount of retained austenite, so that the tested steel had the optimalcombination of strength and toughness, with tensile strength of1700MPa～1900MPa,yield strength of1400MPa～1500MPa, elongation of10％, reduction of area of40％～50％, impact energy of room temperature and-40℃of28J and24Jrespectively, and hardness value of50HRC.(2) The appropriate hot rolling and in-line heat treatment processes for theinvestigated steel are as followings: the reheating temperature is1280℃, holding timeis120min, start rough rolling temperature is1080℃, start finish rolling temperature is940℃~900℃, finishing rolling temperature is about880℃~860℃, then waterquenching rapidly, tempering after cooling to room temperature, the temperingtemperature is250℃, and air cooling after holding1.5h.(3) The microscopic structure of the test steel quenched at850℃~1000℃andtempering at250℃×2h is mainly martensite lath and a small amount of residualaustenite, and little difference has been found in mechanical properties when thequenching temperature changes at the above mention ranges. The tested steel has themaximal strength and certain plasticity and toughness after tempering at250℃. With the increasing of tempering temperature, for instance after tempering at350℃and500℃, the strength and toughness decrease. After tempering at620℃, plenty ofalloyed cementites composed of Fe, Cr carbide are precipitated, the strength of testedsteel decreases, while the impact energy at room temperature increases. Themorphology of precipitates in tested steel after different tempering temperature arewith ellipsoid, spherical, petal shaped and rectangular shapes.(4) The optimal heat treatment process is:900℃×1h water quench+250℃×2hair cooling.