Research And Application Of Ultra Fast Cooling Technology For Hot Rolled Strip

The research was conducted based on the significant technology "The equipment and process technology of NG-TMCP for hot rolled strip" of "Green producton technology and application demonstration of steel industry" for "12th Five-Year Plan" National Key Technology R & D Program, and focused on the key generic technology of steel industry "The NG-TMCP technology". Deep research on ultra fast cooling process for hot rolled strip was conducted in order to achieve well-distributed cooling process with higher cooling capacity. The key technology and main problems on ultra fast cooling equipment designing and ultra fast cooling process development have been studied from temperature calculated method, nozzle spraying diffusivity and uniformity, optimal cooling plate shape achieving method, ultra fast cooling capacity analysis after rooling and temperature distribution along the thickness by analytic method and numerical method. The ultra fast cooling technology was developed by the research and favorable effect was obtained in factory. The main contents and results are as below:(1) The solvers of heat conduction partial differential equation were obtained with the three boundary conditions according to characteristic of strip cooling process, and the ranges of Biot with different error ranges were obtained compared to the solver obtained by umped parameter analysis method. Heat transfer coefficient can be obtained by direct calculated method, inverse calculated method and measured method and the three calculated method were introduced in the paper. The results can provide referencial role for temperature calculated model select, error prediction and heat transfer coefficient calculation.(2) Cooling water will be diffused after it was sprayed into the atmosphere and the spray diffusivity influencing law was researched. The changing law of aixial velocity, length of potential core zone and half decay distance were obtained with round and slot nozzle. Nozzle structure can also affect sparing diffusivity and the changing law was analysised in the paper. The optimal structural parameters of round and slot nozzles were obtained according to the velocity and its distribution in the cross section with different structural parameters which will provide referencial role for nozzle design and selection.(3) Fluid with constant turbulence can be sprayed out from a nozzle and impinging jet flow can be formed with three regions, including free spray zone, impinging jet zone and wall jet zone. Fluid velocity and its distribution in the impinging jet zone and wall jet zone with different outlet velocity, nozzle size and spray distance can be obtained and the the influencing law was obtained by comparation. Fluid velocity can be upto the peak when the distance of fluid and strip surface was 0.4mm and it will be decreased with increase of the distance. Fulid velocity can be improved obviously by decreasing spray distance.(4) Compact nozzle and slot nozzle with multi damping structure were developed according to the fluid dynamics and fluid equipment design theory. The fluid velocity and its distribution along the length with different inlet velocity were obtained by numerical method, and the optimal nozzle structure was obtained by analysis.(5) Deep research on plate shape changing law during ultra fast cooling was conducted. The optimal water ratio will provide effective role in decreasing strip warping deformation. Temperature along the width will not be well-distributed with weak cooling or strong cooling on the side of strip, but obvious side wave will not be appared when the top and bottom heat transfer coefficient was the same approximately. Consequently, the key to keep strip shape well was setting optimal water ratio.(6) Optimal water ratio of laminar cooling, compact cooling and ultra fast cooling with different top nozzle fluxes can be obtained by stuying heat transfer coefficient in top and bottom surface, and the equations used to calculate optimal water ratio was obtained by least square method. Heat transfer coefficient can be affected by spray distance, water temperature, spray angle and outlet velocity, and the influencing law was obtained by comparing heat transfer coefficient with various parameters calculated by finite volume method. The calculated results will provide instructive effect for designing ultra fast cooling equipment and setup ultra fast cooling process parameters. Laminar cooling, compact cooling, ultra fast cooling and air-atomized spray cooling can be used for strip cooling, and Strip cooling capacity of different cooling processes was analysised and the cooling capacity and its reason were obtained which will provide inferenced role in selection of strip cooling method after rolling.(7) Node temperature trends along the thickness for early and late ultra fast cooling process with different initial temperature, ultra fast cooling rate and strip thickness were obtained. Re-reddening temperature, re-reddening time and temperature deviation between center and surface changing law was obtained and equations used to calculate them were obtained by least square method. There is a boundary layer along the thickness and deep research on position of boundary layer and average temperature was conducted. The changing law about boundary layer and average temperature were obtained as below:the boundary layer stay in 1/4 strip thickness; node temperature inside of the boundary layer was the nearly the same, but node temperature deviation between neighbouring nodes outside of the boundary was great and it will be greater when the neighbouring nodes were closed to strip surface. Average temperature was nearly the same with 3/11 strip thickness during ultra fast cooling and laminar cooling process. Certain deviation may exist during during re-reddening process after ultra fast cooling and laminar cooling, but the temperature deviation was less than 10?. The position of boundary layer and average temperature kept constant and ultra fast cooling rate, initial temperature and strip thickness cannot affect them.(8) The control methods used to achieve water pressure, nozzle flux, ultra fast cooling temperature and coiling temperature were obtained according to the characteristic of ultra fast cooling process, and the ultra fast cooling control system was developed and industrial practice proved that the control system achieved steady and high precision control and statisfied requirement of ultra fast cooling.It will provide instructive effect for designing ultra fast cooling equipment and setup ultra fast cooling process parameters based on the research on achieving well-distributed cooling process with higher cooling capacity in this paper.