Research On Production Process And Technology For Surface Layer With Ultra Fine Grains Of Low Carbon Nb-microalloyed Steel Plates

Controlled rolling and controlled cooling technology is one of the greatest achievements of the steel industry since the20th century, which also has great space to improve. Particularly, for the improvement of the controlled cooling technology, many advanced technologies emerged, such as the ultra-fast cooling technology, the fine controlled cooling path technology, etc. The core content of the "New Generation of Iron and Steel Materials" project is to achieve double strength of exiting iron and steel materials through ultra-fine grain technology. For the above considerations, low carbon Nb-microalloyed steel to be as research object to find how to improve the strength of steel by improving TMCP. The principal chemical compositions of the tested steel are0.14～0.17%C,1.35～1.45%Mn,0.30～0.40%Si,0.030～0.035%Al and0.024～0.030%Nb. The major research works and conclusions of this dissertation are as follows:1. Research and development of high strength low alloyed (HSLA) steel plate for industry production(1) Through the thermal simulation experiments, the deformation resistance model, the behaviors of dynamic and static recrystallization and the law of continuous cooling transformation of austenite were obtained. The laboratory hot-rolling simulation experiments were employed, the results show that with the cooling rate increasing, the finish cooling temperature decreasing or the start rolling temperature of finish rolling decreasing, the tested steel grain size and plasticity decreases, and strength and level of duplex grain increases. The lower start rolling temperature of finish rolling is benefit to improve the toughness.(2) The continuous velocity field of plate rolling was established in rectangular coordinate system, functions of rolling total power, load and torque were obtained by applied strain rate vector inner-product. A hot rolling pilot was introduced to verify the accuracy of the models, the result shows average deviations of load and torque are no more than8%.(3) According to plant condition and the previous research results, Q390grade HSLA plates with below50mm thickness and Q460grade HSLA plates with below40mm thickness were successfully developed by trimming chemical compositions of tested steel and reasonably designing TMCP. In addition, the production processes of these two grades plate were given.2. Research on sub-sectional controlled cooling combined of ultra-fast cooling and fast cooling(1) The thermal simulation experiments of sub-sectional cooling were employed, the results show that the fist stage controlled cooling of sub-sectional cooling is mainly to controll the content and grain size of proeutectiod ferrite; as increasing cooling rate, grain size refine and content decrease; as final cooling temperature decreasing, the content of ferrite transformation increase; the second stage controlled cooling is mainly to controll the phase composition; ferrite-pearlite microstrucrure is obtained with low cooling rate, while ferrite-bainite microstructure is obtained with high cooling rate.(2) The12mm thickness plates were obtained by using the "ultra-fast coolingâ†'fast cooling" model compared to using the "fast coolingâ†'ultra-fast cooling" model, the strength grade increased from500MPa to550MPa, yield ratio increased from0.81～0.85to0.85～0.86, variations of plasticity and toughness were little, elongation percentage was about23%, ferrite percentage of microstructure were lower and grain size was finer, the degenerate level of degenerated pearlite was weaker. The12mm thickness fine grain ferrite-pearlite steel plate and bainite steel plate were obtained by using one-stage controlled ultra-fast cooling to different final cooling temperatures, their strength grades reached to600MPa, yield ratios were higher than0.88, elongation percentages were about18%. Compared with the one-stage controlled ultra-fast cooling, the sub-sectional controlled cooling result in lower strength, but the higher plasticity and lower yield ratio are guaranteed. So this technology has more practicability.3. Research on manufacturing technology of surface layer with ultra fine grains (SUF) steel plate(1) The thermocycling-thermomechanical treatment simulation experiment was applied to study product SUF. The hyperbolic-sine constitutive equation for deformation with Q=430.984KJ/mol and α=0.0036was obtained. The results show that with deformation occurred in ferrite+austenite duplex phase field, ferrite grain was obviously ultrafined, the refinement mechanism contain original ferrite continuous dynamic recrystallization (CDRX) and strain induced ferrite transformation (SIFT), and reverse transformation occurr in the unstable ferrite temperature region. With deformation occurred in elevated temperature region of tempered bainite, spheroidization of cementite on ferrite boundary and precipitation and coarsening of intragranular cementite firstly occurr. Meanwhile, ferrite dynamic recrystallized occurs, which have two kinds of mechanism, discontinuous dynamic recrystallization (DDRX) with particle stimulated nuclcation (PSN) in cementite exiting region and continuous dynamic recrystallization (CDRX) in cementite non-exiting region. The fine precipitates of NbC with size below10nm dispersed on ferrite in the second reheating process have delaying effects on dynamic recrystallization.(2) The11mm and20mm thickness SUF steel plates was obtained by using the "accelerate cooling intermediate slab-rolling-post rolling accelerate cooling" process, the maximum thicknesses of ultrafine grains layers of11mm and20mm plates were1.2mm and3.0mm respectively, the grains with size below2μm predominated and a large of cementite dispersed on ferrite boundary. From surface layer to the center of the plate, the percentage of ultrafine grains gradually decreased, the grain size was5-10μm in the center. The maximum yield strengths of11mm and20mm SUF plates were640MPa and584MPa respectively, and all of the plates had good plasticity and toughness.(3) Ferrite ultrafining mechanism of surface layer are:dynamic recrystallization of ferrite which could be tempered bainite or proeutectiod ferrite or SIFT ferrite, strain induced ferrite transformation, and inhibiting ultrafine grains growth effects of cementite dispersed on their boundaries.(4) The technological measures for obtaining SUF steel plates are as follows:Before the accelerated cooling intermediate slab process, obtained recrystallization fine austenite grain through austenite recrystallization region rolling, or obtained deformation austenite grain by non-recrystallization region rolling and impelled precipitation of solution Nb; increasing the rolling compression ratio after the accelerated cooling intermediate slab process; increasing the undercooling degree and thickness for intermediate slab and rolling in the self-reheating process; decreasing the final cooling temperature or increasing the cooling rate in the accelerated cooling process after rolling.