| Low-alloy high-strength wear resistant steel is widely used in various industries for its excellent strength and toughness.However,high-strength steel is prone to hydrogen embrittlement after flame cutting,with the increase of wear-resistant steel grade and the hardness,which induces delayed crack initiation.In addition,grain growth and decarburization will occur during the process of flame cutting.The smaller the grain size of steel,the stronger the resistance of steel to external deformation and the more resistant to crack propagation.The strength of steel decreases and the steel is easy to crack when subjected to external stress after decarburization.Therefore,it is of great significance to study the formation mechanism of delayed crack and the grain growth behavior and decarburization behavior of steel.The research results of this paper are as follows:(1)Delayed cracking appeared up to a few days after the flame cutting of the NM450 steel plate.Cracks were found to originate from an segregation zone that contained multiple high-hardness segregation bands and contain more micron Ti N inclusions.Due to the enrichment of alloying elements,the dislocation density in the segregation zone is higher than that in the non-segregation zone,increasing the hardness and strength of the segregation zone,and local stress concentration is formed,which leads to the initiation of cracks easily.The size and number of Ti N inclusions in the SZ were relatively greater than those in the NSZ,which induced more quasicleavage fractures.The quasi-cleavage surface of the fracture surface in the segregation zone is more than that in the non-segregation zone,and the total fracture elongation and reduction of area in segregation zone were less than those in the non-segregation zone.(2)The grain size of NM500 steel containing V and NM500 steel containing Nb increases with increasing temperature during heating.In the heating temperature range of 850-1210 ℃,with the increase of heating temperature,grain size increases and the proportion of large grain size increases,while the proportion of small grain size decreases.The critical temperatures of austenite grain coarsening of NM500 steel containing V and NM500 steel containing Nb are about 910 and 940 ℃,respectively,and the grains grow up obviously above these temperatures.The strength of steel containing V is higher than that of steel containing Nb,indicating that the precipitation strengthening effect of V is higher than the fine grain strengthening effect of Nb.The yield strength,total fracture elongation and reduction of area of Nb-containing steel increase with the increase of temperature before grain coarsing temperature(940 ℃),but the change of tensile strength is not obvious.After 940 ℃,the yield strength,total fracture elongation and reduction of area decrease with the increase of temperature,while the tensile strength decreases slightly.The strength and plasticity of V-containing steel are the best at 850 ℃.After 850 ℃,total fracture elongation and reduction of area decrease with the increase of temperature,while tensile strength and yield strength have little change.(3)NM500 steel is heated in the temperature range of 850-1150 ℃.When the heating temperature is less than 910 ℃,the ferric oxide layer on the surface is thin and the particles are fine.With the increase of temperature,the oxide film on the surface began to float,and some particles showed preferential growth,and iron grains appeared.Obvious cracks appeared on the surface of iron sheet,which easily caused the fall off of the surface iron sheet.In 15,30 and 45 min insulation system,the thickness of the oxide layer increases with the increase of heating temperature,when the temperature increases to a certain extent,the oxide layer falls off,and the oxide dot layer appears in all the three heat preservation systems.The thickness of complete decarbonization layer and total decarbonization layer increased first and then decreased. |
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