Research On Decarburization And Carbide Spheroidizing Annealing Behavior Of Medium-High Carbon Hot Rolled Steel

Medium-High Carbon steel produced by hot rolling is an important basic material necessary for China’s manufacturing industry.The surface decarburization of hot rolled Medium-High Carbon steel sheet plate ahects its surface properties.In addition,some varieties of Medium-High Carbon hot rolled sheet often need spheroidizing annealing at the temperature close to Ac₁ for about 15～30 hours after off-line it can promote the transformation of lamellar cementite into spherical cementite,so as to reduce the hard before cold working and improve the shaping and toughness.However,long-time spheroidizing annealing consumes a lot of energy and prolongs the production cycle.The process makes the formability of steel plate unstable.Therefore,this paper studies the methods to reduce the surface decarburization of Medium-High Carbon steel and accelerates the spheroidizing annealing speed of layered carbide from the perspective of hot rolling process control.Firstly,the annealing spheroidization behavior and the switching mechanism of layered cementite are discussed based on the actual production of hypoeutectoid steel with carbon content of 0.5%.The effects of 790～880℃hot rolling temperature and 450℃coiling temperature on cementite spheroidizing annealing effect of Medium-High Carbon steel after off-line were studied by Gleeble thermal simulation tester.The results show that the effect of annealing at 820℃and 790℃on the transformation of lamellar cementite into granular cementite is relatively better.The lower the coiling temperature,the better the spheroidization effect of cementite in terms of morphology.However,under the condition of rapid cooling,the simulated coiling temperature is lower than500℃,which will lead to the formation of bainite in the microstructure.Secondly,the decarburization of high carbon hot rolled sheet with carbon content of 0.86%,0.67%and 0.5%was studied.It is found that the decarburization degree of the tail,middle and head areas decreases successively after the production of the three kinds of hot-rolled plates.Due to the direct contact between the edge of the steel plate and the oxygen atmosphere in the environment,the decarburization degree is higher than 1/4 and 1/2 in the center.Then,the influence mechanism of heating temperature on the formation of decarburization layer was studied in laboratory furnace atmosphere.It is found that the morphology of decarburization layer of the three kinds of hot-rolled plates is basically the same after heating at 700～1200℃for 2h.When heating at 700℃,only complete decarburization layer is formed,and complete decarburization layer and partial decarburization layer are formed at 800℃.Only partial decarburization layer is formed above 900℃.In addition,previous studies generally believed that heating steel plate above 1100℃could inhibit decarburization.Because the surface oxidation rate will be faster than the decarburization rate.However,it is found that the oxidation rate of steel plate at 1200℃is not necessarily higher than that of decarburization.Therefore,the influence of the heating stage in the heating furnace above 1200℃on the defects of decarburization layer cannot be ruled out.Finally,the effect of the content of alloy elements（Si,Ni,Cr,Al,Mn）on the surface decarburization behavior of Medium-High Carbon steel was studied by vacuum-arc melting furnace.It is found that in the hypoeutectoid steel with 0.5%C content,Si increases the decarburization tendency,Si has the greatest effect on decarburization,Ni has little effect,and Cr reduces the decarburization tendency.In hypereutectoid steel with carbon content of 0.86,Al increases the decarburization tendency and Mn decreases the decarburization tendency.The influence mechanism of alloy elements on decarburization rate of steel plate is mainly reflected in the influence on carbon activity in steel plate,and there is a strong correlation between them.