DESIGN AND PRODUCTION OF 550 MPa MICROALLOYED HIGH-STRENGTH STRUCTURAL STEEL
This paper introduces the design,development,and production practice of microalloyed high-strength steel with a yield strength of 550 MPa on the medium slab continuous casting and rolling production line.The comparison results of three experimental schemes show that the high manganese low niobium titanium system adopts low-temperature coiling,with a grain size of 11 levels and a yield strength of 668 MPa.Carbides are dispersed and precipitated,and the impact fracture surface shows a clear mixed fracture morphology,with clustered dimples and quasi cleavage fracture cross distribution.The impact absorption energy is relatively low.The composition system of medium manganese,medium niobium,and high titanium adopts high-temperature rolling and coiling,with a grain size of 10 levels and a yield strength of 649 MPa.Carbides precipitate fully at the grain boundaries,improving toughness but significantly reducing strength;After reducing the coiling temperature,the microstructure was significantly refined,the grain size increased to level 12,and the yield strength was 721 MPa;However,coarsening of strip like ferrite appears in the longitudinal organization,resulting in a decrease in the uniformity of the material structure;The fracture surface of the impact specimen taken from high-temperature coiling is generally characterized by ductile dimples,while the fracture surface of the specimen taken from low-temperature coiling is small and shallow with a small amount of cleavage fracture.Under the composition system of low manganese,high niobium,and medium titanium,the grain size is 11 grades,the yield strength is 620 MPa,and the carbide dispersion precipitation effect is the best.The pearlite content is low and the size is uniform and small;The fracture surface of the impact specimen also shows a significant ductile fracture morphology,with large and deep dimples in some positions,exhibiting a typical morphology of micro pore aggregation fracture,but a small amount of cleavage tearing morphology can still be observed.The experimental results show that by adding strengthening elements such as Nb and Ti,the material composition,size,and uniformity can be optimized,achieving the best match between the strength and plasticity toughness of 550 MPa grade product materials.
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