Precipitation Control Of Cold-rolled Enamel Steel And Study On Formability And Fish-scale Resistance

The microstructure characteristics,texture evolution, formability and fish-scale resistance of LC and ULC cold-rolled enamel steel were studied. The research was mainly focused on the influence of chemical compositions on microstructure and properties of LC cold-rolled enamel steel, and in ULC cold-rolled enamel steel, the precipitations of two kinds of second phase particles Ti4C2S2 and Ti(C.N), and their effects on texture evolution, formability and fish-scale resistance. Main works in this paper are as follows:(1) The chemical composition of LC cold-rolled enamel steel was optimized. The effect of S, Mn and B contents on microstructure and properties of LC cold-rolled enamel steel was intensively studied.There is a lack of hydrogen traps LC steel, the second phase such as pearlite, cementite, MnS and Ti/V carbonitride etc. were needed as hydrogen traps. Improving S, Mn contents in steel can increase the number of MnS, refine the ferrite and diffuse cementite. which improving fish-scale resistance of the enamel steel. However, the increase of S, Mn contents was not conducive to deep-drawing ability.Trace element of B can coarsen ferrite and push more cementite particles precipitate within ferrite grains; both r-value and elongation of annealed sheet were significantly improved. B and N atoms segregated in grain boundaries, cementite and MnS, increasing the lattice distortion degree of elastic stress field near the interface between ferrite matrix and cementite/MnS, making it easier for H atoms trapping and improving the activation energy threshold value for H atoms escape from hydrogen traps. Appropriately increasing S, Mn contents and adding trace element of B, the formability of steel sheet can be improved, at the same time, the fish-scale resistance of LC cold-rolled enamel steel be enhanced further.(2) The effect of hot-strip coiling and annealing process on the microstructure and properties of LC cold-rolled enamel steel was studied. The process parameters were optimized and the comprehensive performance of the products was improved.Hot-trip as raw material of cold-rolling is an important premise of the final product with good quality. Hot-trip using relatively high coiling temperature, both formability and fish-scale resistance of LC cold-rolled enamel steel can be improved.Uniform and coarse ferrite grains obtained by high temperature coiling were more conducive to reducing strength, increasing rm-value of annealed sheet. The number of degenerated pearlite chains in annealed sheet increased gradually with the increasing hot-strip coiling temperature, the spread of the H atoms along the thickness direction of steel sheet was more likely to be blocked by the chain like degenerated pearlite.The coarser ferrite grains in annealed steel obtained by using high temperature and short time continuous annealing process were more beneficial to the development of γ-fiber recrystallization texture, which improving formability. However, relatively finer ferrite grains +chain like degenerated pearlitic and more dispersed cementite particles obtained by using relatively low temperature and long time continuous annealing process were more advantageous to reducing the diffusion rate of H atoms in steel, improving the fish-scale resistance of the products.(3) The precipitations of two kinds of second phase particles Ti4C2S2 and Ti(C,N) were studied. The exploration was made on the effect mechanism of Ti4C2S2 and Ti(C,N) precipitates on texture evolution and ultra-deep drawing ability of ULC cold-rolled enamel steel.Ti4C2S2 precipitation kinetics curve in austenite was obtained through isothermal double-pass compression experiment, the PTT (precipitation-time-temperature) diagram was given. In hot-rolling, the temperature was controlled at/near the nose tip of Ps line,the Ti4C2S2 precipitated in austenite rapidly, which consumed a large amount of C atoms, the volume fraction of Ti(C,N) in hot-trip was reduced, and the adverse impact of pinning effect of fine dispersied Ti(C,N) particles on the migration of interface was mitigated, the solute drag effect of C atoms dissolved was significantly reduced.The y-fiber recrystallization texture in annealed sheet consisted of{111}<112> and{111}<110> with almost equal intension,rm-value reached above 2.4,the products had excellent ultra-deep drawing performance.The Ti(C,N)particles in ULC cold-rolled enamel steel hot-strip got coarse gradually with the increasing hot-strip coiling temperature, both in size and volume fraction.The annealed sheet had stronger y-fiber recrystallization texture when using 720℃ coiling, rm-value reached above 2.2; whereas, using hot-strip coiling below 660℃, the γ-fiber recrystallization texture in annealed sheet was weak, rm-value was lower than 1.8. Every annealed sheet was low in H diffusion coefficient using different hot-strip coiling processes, far less than critical value. In consideration of ultra-deep drawing performance of the products, hot-strip using relatively high temperature coiling was more preferable.(4) The effect of S contents on sizes and distributions of Ti4C2S2 and Ti(C,N) precipitates in ULC cold-rolled enamel steel was studied. The influence mechanism of Ti4C2S2 and Ti(C,N) precipitate particles on fish-scale resistance was analyzed.With the increase of S content, the nose tip of Ps line on PTT diagram of Ti4C2S2 moved gradually to the shorter time and higher temperature. Increasing S content promoted the adequately precipitation of Ti4C2S2 in austenite, the distribution of Ti4C2S2 was more dispersed, at the same time, causing less fine Ti(C,N) particles. The interface between (semi)coherent Ti(C,N) particle and ferrite matrix is the main irreversible hydrogen traps, therefore annealed sheet was low in H diffusion coefficient with lower S content using low temperature continuous annealing (lower than 850℃). When the redissolution of Ti(C,N) occurred resulting in the decrease of hydrogen traps after high temperature continuous annealing (above 850℃), TC4C2S2 precipitates would became the main source of hydrogen traps. To ensure the fish-scale resistance of ULC cold-rolled enamel steel adopting high temperature continuous annealing, appropriately raising S content and controlling of hot-rolling temperature should be applied in order to obtain dispersed Ti4C2S2 precipitates.