Thermal Deformation Behaviors And Strengthening Mechanism Of The Steel Produced By Compact Strip Production (CSP)

CSP (Compact Strip Production) process is a promising procedure for steel making withadvantages of low cost, short production period and high efficiency. Compared withconventional products, the process has characteristics as follow:1) the continuous cast slab issent into finishing mills directly before undergoing γ→α phase transformation, resulting in thefact that initial hot-rolling microstructure is columnar crystal;2) CSP products experience highersolidification rate, which leads to less segregation and larger amount of soluted alloying elementin austenite;3) the part-recrystallization zone is an unavoidable stage during hot rolling.Therefore, recrystallization and phase-transformation of the steel is different from theconventional process. As a new CSP line was constructed in WISCO, Q235B and Q345B steelswere tried to understand recrystallization and phase-transformation by thermal simulationmethod. The chemical composition of the steel and rolling parameters were fristly designed.SEM and TEM were then applied to analyze the evolution of microstructure and precipitation,and finally, strengthening mechanisms were figured out. The prediction models of microstructureand mechanical properties were established.As the steel usually showed a finer microstructure with inclusions in nano-scale, thedesignation of steel composition is try to meet mechanical standards of steel Q235B and Q345Bby controll rolling through CSP line rather than by adding alloying elements. In most cases,mechanical properties of the designed steel appeared in the upper limit zone. However, somecracks were found in the steel strips, which were not caused by precipitation in a nano-size scalebut by mixed grains. The models of dynamical recrystallization and grain size for Q235B steelwere established as Z=εexp(289.58/RT) and d_d=24.08454·Z^-0.00727 respectively from thethermal simulation data. The recrystallization regional diagram was also sketched and the F3rolling was considered to locate in part-recrystallization zone for devoloping the mixed grains.Finally rolling procedure was adjusted as increasing the finish rolling entry temperature over1100℃, and the spheroidize process of recrystallization austenite grains should be completed inthe first two mills by increasing reducction rate.The thermal simulation results of Q235B and Q345B phase transformation proved that theaustenite of the steel mainly transformed to ferrite and pearlite. When the cooling rate was largerthan0.5℃/s, some bainite was found in the microstructure of the steel and a little of martensitewas found at rate above20℃/s. The higher cooling rate, the lower the critical temperature ofphase transformation. The hot deformation of steel is beneficial to the diffusion transformationand made the grains smaller. The hardening capacity of the steel is too poor to increase thestrength by bainite or martensite transformation. The high strength ferrite-pearlite steel ownedgreat stability on microstructure and a good comprehensive performance. Because the amount ofsolid-soluted alloying elements in the austenite is larger than the ones made by tranditionalprocess, the precipitation of TiN, Ti(C, N) and TiC can flexiblely be controlled by hot roolingprocedure. Reasonable rolling process is designed to make a new hot strip with yield strength over700MPa and with a thickness of1.2mm and with a plate width of1280mm, which is themaximum strength ever achieved for this specification in the world. The size scale ofprecipitation particles are100¹20nm and25⁴5nm. The bigger particles precipitated fromaustenite, hindering dynamical recrystallization. The smaller ones from ferrite or phasetransformation intervals, which give a strong strengthening for the strip. The strengtheningmechanisms of the700MPa high strength steel is composed of refinement strengthening,precipitation strengthening, solid-solution strengthening, dislocation strengthening and theinherent ferrite strength.