Study On Microstructure And Hot Workability Of Nb-lc Steel Produced By Strip Casting

The microstructure and precipitates of low-carbon steel and Nb micro-alloyed low-carbon steel (referred to as Nb-LC) produced by strip casting are studied in this paper. By choosing the appropriate thermal simulation parameters, the hot deformation behavior of Nb - LC steel are studied by using the hot compression test. And the effect hot-rolling on thin Nb-LC strip organization and performance are systematic investigated to provide theory evidence for manufacturing Nb-LC strip by the strip casting technology.The solidification structure of strip is composed of two columnar zones on the two sides of strip and a equiaxed zone located in the middle of the strip. The primary dendrite spacing is about 15-20μm and secondary dendrite arm spacing is 8-10μm. The microstructure at the room temperature is built up by bainite and polygonal ferrite. There are large of 0°¹5 low-angle grain boundaries in Nb-LC strip, above 70%, while low-angle grain boundaries in low carbon strip is about 50%. The main texture in Nb-LC strip is (111) <101>, (001) <101>. There are CuS, MnO, SiO₂, Al₂O₃ and other inclusions in the strip, ranging in size from a few hundred nanometers to several microns. Nb is solute in the Nb-LC strip.The hot simulator experiment is based on three points: (1) The austenite grain size of samples are the same with that of the strips. (2) Nb is fully in solution. (3) Deformation temperature, strain rate and other parameters are according with the actual conditions. The dynamic CCT curves of low carbon steel shows that the microstructure is mainly composed of by acicular ferrite and bainite, when the cooling rate is higher than 10℃/s. This microstructure is same with the strip. For the Nb-LC(0.08%Nb) steel, when the cooling rate is 1℃/s, the microstructure is mainly polygonal ferrite with a small amount of bainite. Nb reduces the phase transition starting temperature and promoets formation of bainite.Dynamic recrystallization experiments showed that when Compression at 1100℃, Nb-LC(0.08%Nb) steelεc is 0.3589, while the low-carbon steelεc is 0.2491. Niobium is obviously hindering the recrystallization process.The Nb-LC static recrystallization behavior is investigated by using double-pass compression and back-extrapolation method. The results show that the crystallization process is speed up with deformation. increasing and deformation temperature rising. However, the crystallization process is slow down with austenite grain size increasing. It shows the strong hinder effect of Nb on static recrystallization. The static recrystalliztion analytical model of Nb-LC(0.08%Nb) steel is established.According to the microstructure and hot deformation properties of Nb-LC strip, the appropriate parameters of hot rolling are chosen. The mechanical properties of hot-rolled Nb-LC strip approach those of Q345. The yield strength is beyond 400MPa, tensile strength is about 500Mpa, and the elongation is about 21%. The results of HRTEM showed that Nb (C, N) separated out after hot rolling and the precipitates size is between 5 20 nm.