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1、MILLENNIUM STEEL 201277Development and application of dynamic secondary cooling and soft reduction for slab castingDynamic secondary cooling and soft reduction control models have been developed for a newly commissioned slab caster to improve the quality of the cast slab.The system comprises three p
2、arts:dynamic thermal tracking model,dynamic spray water optimisation model and a dynamic soft reduction control model.The system has been shown to be stable and robust for the production of ship plate grades and,following further calibration,will be applied to the whole product mix.Chinas steel prod
3、ucers are facing a series of severe challenges from users who require higher quality steels.In continuous casting,internal cracks,centre segregation and centre porosity are usually the main bottlenecks preventing development of high-strength steel grades with qualifi ed mechanical properties.For thi
4、s reason,dynamic secondary cooling and soft reduction technology have been two of the key technologies adopted by many steel plants throughout the world in the past few years to improve slab quality.In casting,the interdendritic liquid steel in the mushy zone of the fi nal solidifi cation stage fl o
5、ws to the centre area through convection and bulging.This leads to centreline segregation.The volume contraction of steel on solidifi cation can also lead to central porosity where there is poor feeding or compensation.Also,the cyclical cooling and reheating of the strand surface temperature during
6、casting is connected with the formation of broad face cracking,subsurface cracking and other defects.The appearance and intensity of centre segregation and centre porosity or other defects are closely related to steel grade,liquid steel superheat,casting speed and cooling history.Dynamic soft reduct
7、ion,combined with dynamic secondary cooling,has been shown to be an effective way to minimise centre segregation and porosity.In this paper,a newly developed thermal tracking,dynamic secondary cooling and dynamic soft reduction system for slab casting control is described.DYNAMIC THERMAL TRACKING MO
8、DELAccurate and timely access to the solidifi cation temperatures of the slab being cast are the basic requirements for the implementation of dynamic secondary cooling and soft reduction.A model for the determination of the temperature fi eld of the continuously cast slab was made for the casting pr
9、actices adopted in production and based on the basic Authors:Yunhe Chang,Ke Liu,Zhanguang Han and Jiaquan ZhangState Key Laboratory of Advanced Metallurgy and University of Science and Technology,BeijingCASTINGar Fig 1 Structure of dynamic thermal tracking modelMILLENNIUM STEEL 201278different tempe
10、ratures,are considered to be isotropic partitioned constants for liquid,mushy and solid states,respectively From the meniscus to the secondary cooling zone,symmetrical cooling is assumed for the loose and fi xed sides of the slab Effective heat transfer co-effi cients are used for the overall descri
11、ption of heat transfer of slab surface in the secondary cooling zones,which include the contribution from heat radiation,heat conduction through backup rolls and heat convection during water spraying.Based on these assumptions the control equation for the heat transfer and solidifi cation process ca
12、n be simplifi ed as:(1)Where,Ceff is equivalent heat capacity,J/(kgC)k is thermal conductivity,W/(m.C)t is time,sT is temperature,C is density,kg/m3 Initial condition:Temperature distribution is assumed to be even at the meniscus and equal to the pouring temperature of the molten steel,Tc.(2)Boundar
13、y conditions:Centre of the slab domain:(3)Surface of the slab domain:Mould region:(4)Secondary cooling zones:(5)Air cooling zone:(6)theory of solidifi cation and heat transfer(see Figure 1).The model uses real-time temperature and solidifi cation information from the mould meniscus to the end of the
14、 control region,and calculates periodically based on the transient casting conditions or technology parameters read from the Level 1 control system.In the model,the strand from mould meniscus to the end of control region is divided into 426 slices(see Figure 2).Each 100mm slice is an information sto
15、rage unit generating time-related data from the meniscus,present location and grid temperatures from the centre to the upper slab surface(ie,loose side of the slab strand).Data are stored from the start of casting production and updated periodically.Cartesian co-ordinates were adopted for each compu
16、tational slice,in which the X,Y and Z axes represent width,thickness and casting directions,respectively.Based on the characteristics of the continuous casting process,the basic assumptions used in the establishment of the heat transfer model of solidifi cation are given as follows:Compared with the
17、 casting speed,longitudinal heat transfer along with casting direction can be neglected The temperature gradient along the width direction of the slice domain is not dominant so heat transfer in that direction can be ignored Heat transfer of the slab is treated as a heat conduction process in which
18、possible effects on the heat transfer from the contribution of fl uid fl ow,both in the mushy zone and in the bulky liquid region,are dealt with through effective thermal conductivities according to the solidifying state The thermal physical properties of steel,such as heat capacity,thermal conducti
19、vity and density at r Fig 2 Schematic of slice generation and computational domainrFig 2 Schematic of slice generation and computational domainCASTINGMILLENNIUM STEEL 201279aWhere:is heat fl uxKJ/m2:A,B are experimental constants;h is the heat transfer coeffi cient in kW/m2C;Ts,Tw and To are tempera
20、tures of slab surface,cooling water and ambient in C;is the Stefan-Boltzmann constant and assumed to be 5.6710-8 W/(m2K4);is the radiation coeffi cient of slab surface(black degrees),and is 0.8 in the model.Equations 1 to 6 constitute a one-dimensional unsteady mathematical model of the computationa
21、l slice for the description of heat transfer and solidifi cation during casting.The fi nite difference method is used to solve the differential equations of heat transfer.In the actual production process,the slices at different times have different information for different locations,which includes
22、the contribution of the variable process conditions,such as casting speed,tundish temperature,cooling intensity of each cooling zone and steel grade.The information,temperature fi eld and location of slices,is dynamically tracked in the model.Thus a non-steady solidifi cation heat transfer model is
23、established for each slice,which is discretised and solved through the fi nite difference method.Accordingly,the model can describe the temperature fi eld of each slice at different times and in different locations.The updated temperature of each slice represents the local temperature fi eld of the
24、strand,and the collection of process parameters and calculation for the temperature fi eld of the slices are updated every 5 seconds.Thus,the real-time temperature distribution of the strand can be revealed through the information integration of all slices.DYNAMIC SECONDARY COOLING MODELThe database
25、 module of the secondary cooling model is responsible for providing the initial,maximum and minimum amount of secondary cooling water,its allocation and other process parameters corresponding to the casting speed for the online calculation module.This is the core of the model,which receives the slic
26、e information(time,location and temperature fi eld)provided by the thermal tracking model,calculates the effective casting speed of each cooling area for slab continuous casting and combines with the parameters of the database to determine the water fl ow rate of each zone.Spray water allocation is
27、determined via water meter databases related to the given target surface temperatures in every cooling zone,effective casting speed and the method of target surface temperature control.Thus,the surface temperatures of the slab are controlled online to optimise the temperature fi eld with the given t
28、arget surface temperatures and to minimise reheating(see Figure 3).In addition,the system can support users to add new steel grades and modify the original water spray table if necessary.The control system has various protective measures,such as limitation to maximum or minimum water fl ow rate.Base
29、d on the mechanical properties of steels at elevated temperatures,the surface target temperatures for the slab in the secondary cooling zones are determined for any given steel grade in the product mix,in which related metallurgical criteria are considered for control of thermal stress on the surfac
30、e of slabs.Offl ine simulation and calculation for cooling optimisation are carried out in advance by secondary cooling software to get the best relationship between casting speed and the related secondary cooling water allocation.The water fl owrates obtained under different casting speeds are stor
31、ed in the database as the basic table for the water sprays.To avoid an over response of water sprays to any fl uctuation of casting speed,we defi ne the concept of effective casting speed in the control model,as given in equation 7.(7)Where,vi is the actual casting speed;veff is the effective castin
32、g speed;vc is average casting speed and ki is a constant factor for each given cooling zone.Thus,the model can ensure a slower variation of the surface temperatures with increments in casting speed,and the frequently observed dramatic changes of surface temperatures under a normal control system are
33、 avoided during the change of casting speed,which should be benefi cial to steel surface quality.Finally,the amount of water is optimised by a target r Fig 3 Structure of dynamic secondary cooling modelrFig 3 Structure of dynamic secondary cooling modelMILLENNIUM STEEL 201280surface temperature cont
34、rol method based on the temperature fi eld from the dynamic thermal tracking model using equation 8.(8)Where,Qi is regulation amount of water,m3/min;K1 and K2 are adjustment co-effi cients determined by experience and in-situ production;Ti and Ti-1 are the deviation of the temperature at i and i-1 t
35、imes.The amount of water regulation is increased or decreased accordingly through the evaluation on Ti and the difference between Ti and Ti-1.The surface temperatures should approach the target temperatures at given set points as quickly as possible along with the water regulation.The dynamic second
36、ary cooling control model based on the strand surface temperature reheating feedback is described in equation 9.(9)Where Ti is the difference of surface temperature reheating in C/m;Ti and Ti-1 are surface temperature reheating values of import and export for I cooling zone in C/m;Taim is target val
37、ue of surface temperature reheating,0Taim100 in C/m;If TiTaim or Ti-1Taim,following the proportional integral derivative(PID)principle:(10)Where,Qk is regulation amount of water in m3/min,less than 10%of Qk;K is the sample number;Tk,Tk-1 are k and k-1 sample difference of surface temperature reheati
38、ng;Kp,Ki,Kd are the proportional co-effi cient,integral co-effi cient and differential co-effi cient,respectively.DYNAMIC SOFT REDUCTION MODELDue to volume contraction during solidifi cation,a shrinkage cavity and/or porosity can be produced at the crater of the fi nal solidifi cation region,and cen
39、treline segregation can be produced due to solute redistribution during solidifi cation and fl uid fl ow induced by thermal convection or bulging.For these reasons,a dynamic soft reduction control model has been developed to compensate for solidifi cation shrinkage by decreasing the roll gaps in the
40、 region of the centreline mushy zone.Dynamic soft reduction can minimise the formation of internal voids r Fig 5 Schematic of logical roll gap adjustment principler Fig 4 Structure of dynamic soft reduction control modelrFig 4 Structure of dynamic soft reduction control modelrFig 5 Schematic of logi
41、cal roll CASTINGMILLENNIUM STEEL 201281athe dynamic thermal tracking model,the location of the centreline mushy zone is determined by the online roll gap calculation module based on the centreline solid fraction.Combined with the preset parameters for the position of soft reduction provided by the d
42、atabase module,the online calculation module calculates the range of soft reduction and determines the initial segment and end segment for soft reduction implementation.The reduction needed is then allocated to the assigned segments.The initial soft reduction can be further modifi ed or optimised to
43、 follow the maximum soft reduction rate given by the modifi cation module.Finally,the roll gaps of segments are set up based on the basic roll gap,and distributed to the PLC control system.The dynamic soft reduction control system allows users to add new steel grades and modify appropriate process p
44、arameters.The system is designed with a function to optimise the roll gap process and the maximum/minimum soft reduction allowed in order to protect the equipment and enhance accurate control and equipment life by avoiding frequent movement of the segment hydraulic cylinders.Upper and lower threshol
45、d values are also used in the control model.As shown in Figure 5,the overall roll gaps of the strand will be adjusted in a controlled way;ie,when the roll gaps of segments for soft reduction begin to change,the roll gap of the following segments changes upon its arrival at the next roller.The gap ad
46、justments of the following segments take place step by step based on the casting speed,which improves the effectiveness of the soft reduction and avoids the excessive deformation resistance from the following solidifi ed slab.and prevent the interdendritic fl uid fl ow of the condensed liquid steel,
47、especially in the transverse direction.Additionally,the compression produced by the soft reduction can also promote liquid steel fl ow in the reverse direction of casting,which can lead to the redistribution of solute elements in the liquid steel.Thus the solidifi ed slab structure is usually more u
48、niform and compact under soft reduction,with less centreline segregation and porosity.The key technologies of dynamic soft reduction are to use the dynamic soft reduction control model accurately to predict the location of the centreline mushy zone,and to determine the position of soft reduction and
49、 the soft reduction required,giving consideration to the characteristics of different steels.As shown in Figure 4,the dynamic soft reduction control model comprises the database module of parameters,online roll gap calculation module and soft reduction modifi cation module.The maximum soft reduction
50、s for different steel grades are shown in Table 1.To avoid soft reduction in the crack-sensitive zone near the very end of the crater,we take the region of fs=0.3-0.7 as the operational zone for soft reduction(where fs is centreline solids fraction).This is an acceptable range to compensate for soli