Abstract: Microstructure and mechanical properties of 65Mn steel hot-rolled plate with strip breakage during the first cold-rolling were analyzed by means of optical microscope (OM), scanning electron microscope (SEM), tensile and hardness tests. The results show that the grain of 65Mn steel strip breakage plate is coarse and unevenly distributed along the transverse direction. The grain size coarsens from grade 6.0 to 4.5 at the edge to 1/2 of the strip breakage plate, and the prior austenite grain size coarsens from grade 5.5 to 4.0. The lamellar spacing of pearlite increases from 759.0 nm to 1128.0 nm, and the pearlite colony size increases from 26.1 μm to 31.3 μm. The transverse distribution of mechanical properties is also uneven. From the edge to 1/2 of the cold-rolled plate, the yield strength decreases from 541 MPa to 529 MPa, and the tensile strength decreases from 669 MPa to 651 MPa. The elongation increases from 9.6% to 11.7%, and the hardness decreases from 65.0 HRB to 62.2 HRB. When subjected to external stress, the deformation of 65Mn steel strip breakage plate is not coordinated due to the uneven microstructure and grains, which eventually leads to the occurrence of strip breakage defect. The incidence of plastic and toughness defects such as cold rolling cracking and strip breakage of the 65Mn steel is reduced to less than 0.15% after optimizing the hot rolling process.

Key words: 65Mn steel, strip breakage, grain size, pearlite, uncoordinated deformation