Ultra-low Carbon Plate High-strength And High-toughness Mechanism Studies

With development of iron & steel towards to the direction of high strength and toughness, the balance between high strength, toughness and weldability has become the eternal subject. In the development and production of medium and heavy plate, Composition design and microstructure control, reasonable use of micro-alloying elements, controlled rolling and cooling process have become the important technical and technological process. Hot deformation behavior is helpful to control the rolling and cooling process, and obtain the ideal microstructure and mechanical properties.The paper researched the dynamic recrystallization and static recrystallization behavior during hot deformation of low carbon medium plate. Laboratory rolling test was based on the study of recrystallization. The effects of rolling pressing and Nb contain on microstructures and mechanical properties in different were investigated. Through quantitative calculation, fine-grain refinement strengthening, the dislocation strengthening, solution strengthening and precipitation strengthening, their contribution to the yield strength were investigated. And the impact toughness factors were analyzed from microstructure, grain size, the impact fracture,etc. The results showed that:(1) Through the tests of two steel hot deformation process of thermal deformation behavior of the recrystallization, when the deformation temperature was higher than 950℃, recrystallization could take place and obtain equiaxed grains. While deformation temperature was below the recrystallization temperature, the austenitc grain size get was flat. For the intermittent deformation of two-pass process, when the deformation temperature was above 1050℃, it will basically take place entirely recrystallization in the 60s; When deformation temperature was 1000℃, as a result of strain-induced precipitation, the conduct of static recrystallization was restrained, and resulted that platform appeared in the softening curve.(2) In the component design of this experiment, the content of Nb in the test steel 1# and 2# is 0.10% and 0.21% respectively. The grain size of 2# test steel whose Nb content is higher is slightly smaller than 1# test steel. Niobium carbonitride plays a great role in the test steel:first, when deformation at the high temperature, the solid solution of micro-alloying elements will play the role of dragging as solute when they cluster at the grain boundary and the crystal defects; second, when deformation at the low temperature, carbonitride which separate out at the original austenite grain boundaries and dislocation line will pin the grain boundaries and dislocations, prevent the movement of the dislocation and grain boundary, so as to prevent the recrystallization.(3)Through laboratory rolling tests, the analysis of microstructures and mechanical properties showed that, microstructures were mainly bainite after rolling. With the beginning rolling temperature decreasing, flattening level of microstructures became more serious. At the beginning temperature 1050℃, there was a best match of strength and toughness. Nb is important in fine-grain strengthening and deposition strengthening, Carbon nitriding niobium further improved strength and toughness.(4)The strengthening and toughening analysis of tested steel showed that, dislocation strengthening was the greatest contribution to strength, their strength increment respectively 249MPa and 266MPa, and it was the most important way to strengthen, followed by solution strengthening (strength increments were respectively 200MPa and 214MPa) and fine-grain strengthening (strength increments were respectively 92MPa and 112MPa).Through the analysis of toughness influencing factors and the micro-morphologies of two test steel's impact fracture, a large number of toughening dimples and a small amount of cleavage, it can be showed both are ductile fracture.With the beginning rolling temperature increasing, the impact properties of the two test steels first increased and then decreased, overall, the impact value of 1# test steel is higher than 2# test steel, the temperature of ductile-to-brittle of 1# test steel is lower than 2# test steel; Effective structure uniformity and grain size were important parameters to characterize low-carbon medium and heavy plate. With the improvement of microstructures uniformity and effective grain size, low carbon medium and heavy plate showed more excellent strength and toughness match feature.