Research On Thermo Mechanical Control Process,microstructure And Properties Of Ultra-low Carbon Bainite Steel For Hull Structure

In this thesis,we focused on the development of 690 MPa grade ultra-low carbon bainite(ULCB)easy to weld steel,designed three ULCB test steels based on Mn-Mo-Nb system.The phase transformation behavior of designed ULCB,the relationship between deformation process parameters and microtructure,laboratory controlled rolling and cooling process were studied.The main findings are as follows.The static CCT curves of Mn-Mo-Nb,Mn-Mo-Nb-B and Mn-Mo-Nb-B-Cu were measured,and the evolution of phase transformation microstructure and hardness under different cooling rates was analyzed.The results showed that the phase transformation temperatures of ultra-low carbon bainite steels under the cold rates within 0.03°C/s～44°C/s were below 520°C,that means only bainite transformation occurred at medium temperature.With the increase of the cooling rate,the microtructure of Mn-Mo-Nb steel transitioned from granular bainite to lath bainite,and the microhardness increased gradually.The addition of 16 pm B significantly improved the bainite hardenability,promoted the formation of lath bainite,lowered the bainite transformation start temperature,and increased microhardness.The addition of B+Cu further lowered the bainite transformation temperature,further increased the tendency to form lath bainite,and significantly increased microhardness.Thermal simulation compression deformation experiments(deformation temperatures,deformation degrees,cooling rates)were conducted on Mn-Mo-Nb-B steel,and the microstructure changes were studied.The results show that the deformation temperature has little effect on the microstructure and M/A island of Mn-Mo-Nb-B steel,indicating that Mn-Mo-Nb-B steel has a wide processing temperature range.With the increase of deformation degree in the finishing zone,the microstructure of the Mn-Mo-Nb-B steel became flat gradually and the grain size as well as the size of the M/A island were significantly refined.The increase of the cooling rate was beneficial to increase the content of lath bainite and promote the transformation of M/A islands from block-like with random distribution to short rod-like with parallel regular arrangement.Laboratory controlled rolling and cooling process were conducted on Mn-Mo-Nb,Mn-Mo-Nb-B,and Mn-Mo-Nb-B-Cu steels to investigate the microstructures and properties at different final rolling temperatures and cooling rates.The results showed that the Mn-Mo-Nb-B-Cu steels achieved good strength toughness matching and low yield strength ratio under sand-burying,air-cooling and wind-cooling conditions.The final rolling temperature within 900°C to 750°C had little effect on the strength and toughness as well as the microstructure of the test steels.In the range of 0.2-2.3℃/s,the cooling rate had little effect on the strength of the ultra-low carbon bainite steel;when the cooling speed was accelerated to 13.8℃/s,the strength increased significantly while KV2 decreased sharply.The yield strength of Mn-Mo-Nb-B-Cu steel reached 852 MPa and the tensile strength reached 1033.5MPa under water-cooling condition.With the increase of the cooling rate,the densities of high and low angle grain boundaries in the test steels increased,along with the refinement of grains and narrowing of bainite lath.