Application of Microstructure Engineering in Steel Coil Cooling Process

Vol.12 No. 2J. Iron & Steel Res. , Int.Mar.2005Application of Microstructure Engineering in Steel Coil Cooling ProcessLIU Zheng-dong'，D Q Jin',I V Samarasekera3，J K Brimacombe'(1. Central Iron and Steel Research Institute, Bejing 100081, China; 2. The Timken Co, Ohio 44706, USA;3. University of British Columbia, Vancouver V6T 1Z4, Canada)describe and analyze the thermnal history and its impact on precipitation phenomena during coil cooling for plain car-bon, HSLA-V and HSLA-Nb steels. The predicted result of the thermal model was compared with that measuredfrom industrial coil. The efect of cooling condition and coil dimension on the thermal history and final mechanicalproperties of the steel strip was examined. The coiling temperature and cooling rate have crucial influence on theprecipitation strengthening.Key words: hot rolling; steel coil cooling; process modelingA conventional hot strip mill consists mainly ofTable 1 Composition of tested steels%a reheating furnace， descaling units， roughingElementA36DQSK HSLA-V HSLA-Nbstands, finishing stands, runout table and downcoil-c0.0380.0450. 082ers. Metallurgical processes of steel strip associatedMn0.740. 300. 450.48with hot rolling are austenite recovery and recrystal-0. 0090.0100.012lization in rolling stands, phase transformation dur-0.0080. 0080. 005si0. 0120. 0690. 045ing runout table cooling and precipitation during coiliu0. 0160.0150.026cooling. In the last two decades, microstructure en-Ni0. 0250.013gineering has been paid more and more attention asCr0.0190. 033o. 0220. 023an approach to quantitatively combining adjustableAo<0.005< <0. 005<0. 005processing parameters of hot strip mill with mechan-v<0. 002< CO. 0020. 080<0.002cal properties of steel strip'1. One emphasis 0Ti<0.002 <0. 0020.00<0. 002process modelling is the controlled runout tableNb0.036cooling where the austenite decomposition occurs.Al0. 0400. 0780. 024Considering the importance of coil cooling process to0.0047 0. 00520. 007 20. 005 4final mechanical properties, it should be followed by .precipitation in ferrite during the cooling of steelcoil. However, little information is available todayof cast steel expanded during winding of the coil andon the relationship among the thermal process, met-collapsed during its removal with the maximum di-allurgical feature and mechanical properties of a steelameter change being about 40 mm. The mandrel iscoil during the cooling of the coil. Thus, a thermaloften powered by a 367 749 W to 735 498 W, DCmodel was developed. Moreover, precipitationmotor operating under variable voltage to providestrengthening models were integrated into the ther-coiling speeds up to 20 m/s. Most modern coilers canmal model, and efforts were made to connect pro-handle coils of 762 mm in inside diarmeter and 2082 mmcessing parameters such as coiling temperature andin outside diameter with strip thickness rangingcooling rate with final mechanical properties for fourfrom1mmto13mm.Coilmasscanbeashighas40steel grades, as shown in Table 1.t. Coiling temperature ranges from 480 to870 C de-pending on the product being rolled and sometimes1 Review of Several Modelswater sprays are installed at the coilers to aid in con-A coiler is mechanically designed as an entire u-trolling strip temperature[2,3]. To achieve a desirednit so that it may be removed conveniently from thcoiling temperature, it is also important to under-line for maintenance. The overhung mandrel is made stan中国煤化工istory of strip on theBiography: LIU Zheng-dong(1966-), Male, Doctor, Professor; E-mail: liuzhfYHCNMHGate:Juy1,2003J. Iron & Steel Res., Int.Vol. 12runout table. Different cooling modes on the runoutcipitation and/or solid solution hardening， andtable may lead to varied phase transformation kinet-transformation hardening. Several regression equa-ics and coiling temperatures. Coiling temperaturetions were suggested based on experimental datahas a significant influence on microstructure evolu-which link the mechanical properties of steel to thetion of a steel coil as reported by K Kunishige etcomposition, microstructure and volume fraction ofa14]frtl1,1]. The following is the IRSID model[12] ，An accurate prediction of thermal history of apredicting mechanical properties for plain carbonsteel strip is essential to predict microstructural evo-steels:lution due to the fact that metallurgical changes areYS = 63+ 23ar Mn] + 53or[s] + 700orp +5 000w'N+thermally activated. Coiling generally lasts less than(15.4- 30mrn+o.8+orm)6. 094Fd.1/2 + (360+1 min. Thus, coil cooling after coiling is the major2 600wia)(1- F)(2)heat transfer process which is very complex. For asteel coil, there are different gap distances betweenUTS = 237 + 29a[Ma) + 79orsa + 700urp+5 369w'N+7. 24Fd.1/2+500(1- F)(3different neighboring windings. Oxide scale may oc-cur on the strip surface and heated air fills the gaps.where F is transformed ferrite fraction; d。is ferriteWhen water spray is introduced into the coiler, igrain size, mm; composition is in mass percent ;makes the thermal process more complex. Phasestrength is in MPa.In order to describe age hardening in aluminumtransformation heat may be involved if the strip isalloys, H R Shercliff et al[19,14] developed a processcoiled at a high temperature for a certain steelmodel which assumes the particle coarsening to begrade. Numerically, it is common to employ 1D or2D finite difference models to simulate the thermalrate controlling and takes the contributions of shear-history of coil cooling. Ch Plociennik et a15.6] and Aable and unshearable particles as well as solutionMonsalve et al(] recently developed 2D FEM modelsstrengthening into account. Recently, M Militzer etal[15,16] extended the Shercliff- Ashby model and IR-to describe the heat transfer of coil cooling.SID model to microalloyed steel grades to describePrecipitation is the most important metallurgi-the precipitation strengthening and predict mechani-cal process during coil cooling for plain carbon andcal properties, which were expressed as:microalloyed steel grades. The nature of precipitati-1.9[1-exp(-p" 1r)]/2(p" /4)/6 +on is the formation of fine precipitates from super-Orpr1+(p" /4)12saturated solid solution which significantly affectso.,oexp(-2p* /x)(4)the final mechanical properties of the steel strip.The strengthening mechanisms of precipitation havei exp[- Q/k0(')]gt' .5)0(t' )been systematically investigated and summarized byAJ ArdelI[8] and J D Embury et al19]. The contribu-p*= p/pp(6)tion of precipitation to the strength of steel lies inYS = YSTRsm(w'N=0) +aopethe combined effect of the volume fraction and meanUTS =UTSnasm(w'N=0) +Bopm(8)radius of precipitates. For plain carbon steels, G Awhere σ,o is initial solid solution strengthening valDuit et al[lo] proposed a model to predict AIN precip-ue; K is a constant; p is temperature-corrected time;itation for arbitrary thermal paths. The free nitro-p is p value corresponding to the point at which thegen in solution can be calculated as:contributions of shearable precipitate and bypassingprecipitate are the same; and a, β are material de-w'N =wn一5190o[An,pendent constants.exp-260kJ●mol1- expRO(1) 2 Mathematical Model of Coil Cooling4.3X10-10sThermal profiles of steel coil during coil coolingwhere w'N is free nitrogen amount, wN is total nitro-can be mathematically described as:gen amount (X10-°)，15X10‘< wN< <75X10-。;中国煤化工k器)+g=pcand orAn, is amount of acid soluble aluminum (%) ,0. 028%< w[An, <0. 052%, and θ is temperature.YHCNMHG(9)For microalloyed steels, alloying elements enhancewhere, 0= =0(r,中, z, t), k=k(),p≈p(0),cp=c,(0); gstrength by three routines: grain refinement， pre-is phase transformation latent heat if any during theNo.2Application of Microstructure Engineering in Steel Coil Cooling Process●39process.was 487 C at the head, 477 C at the middle and 432Due to the complexity of thermal process, someC at the tail, and the difference is more than 50 Creasonable assumptions were made to simplify thebetween the head and the tail. However, after cool-simulation. A coil was assumed to be a hollow soliding for 200 min， there is nearly no temperaturecylinder. The effect of gaps between wraps on thedifference between head and tail of the coil, but theheat transfer was considered by introducing a gaptemperature at the middle is slightly higher. Afterfactor to adjust the thermal conductivity. The initialcooling for 20 h, the temperature profile of the coilthermal data of coil cooling was the temperature dis-tends to be uniform and was approximately 100 C.tribution of the strip at the exit of the runout table.To calibrate the thermal model, the comparison of pre-The heat transfer from all surfaces of the coil wasdicted thermal data with industrially measured data[19]described by means of effective heat transfer coeffi-for the same processing conditions was made, as showncient combining convection and radiation. Ignoringin Fig. 2. R and R2 denote the inside radius and outsidethe thermal gradient in the φ direction, Eqn. (9) be-radius of steel coil respectively. There is a good agree-comes a 2D thermal model which can describe thement between predicted and measured temperature data.heat flow in the axial and radial directions of theThe gap factor is 1. 0 for the calculation.coil. Further ignoring the thermal gradient in axialdirection, Eqn. (9) becomes an 1D thermal model500R-0.2794m R2=0.5588mwhich can only delineate temperature distribution inradial direction. Both 2D and 1D thermal models of40050 min▼100 mincoil cooling were developed at UBC with the aid of◆150 min' 200 minfinite difference method. In the case of silicon steel,the thermophysical properties used are171 : .300k=33. 521 +o.026 1θ-9. 239 5X10~*g(Temperature range: 0- -900 C)(10)200p=7 846.31-0.101 77θ-8. 3576X10 *f(Temperature range: 0- - 800 C)(11)c,=417. 216+1.683 320- -0.012 2910100(Temperature range: 50- 700 C)(12)0510152025For plain carbon steels, the thermophysicalNode from inside radius to outside radiusproperties used in the study are as follows8J :Input data: Head 487 C; Middle477 C; Tail 432 Ck=(19. 093 14-17. 786 6wrc])+(0. 008 834 +0. 014 706or[c])θ(13)Fig 1 Calculated thermal profile of silicon steelc。=(657.455 3-414. 832wrc)+ (0.005852+0.357 83ow[c])θ(14)480甘Calculated dataIn current work, Eqn. (1) was used to calculate■Measured datanitrogen amount in solution in plain carbon steels ;440Eqn. (6) to Eqn. (8) were employed to determinethe precipitation strengthening in HSLA-V and Nbsteels; and Eqn. (2) to Eqn. (5) were applied to de-scribe the structure property relations of these steelgrades.1D FDM thermal model was used.Middle position3 Thermal Profile and Model VerificationSlicon steel320 b Coil No.e 51058 us SleelDue to the availability of industrial measure-ment, silicon steel was selected to validate the coil05150250350cooling thermal model. Fig. 1 shows a calculated中国煤化工thermal profile of the silicon steel coil by the estab-CNMHGnperature 20 C;lished thermal model. The inside radius of the sili-con steel coil was 0. 279 4 m, and its outside radiusFig. 2 Comparison of measured temperature profilewas 0. 558 8 m. The initial temperature of the stripwith predicted oneJ. Iron & Steel Res.，IntVol. 12Coil dimension greatly affects the heat transfermiddle almost reached room temperature after cool-of the coil during cooling. The effect of coil dimen-ing for 20 h. Meanwhile, the cooling rate for thesion on the thermal history and cooling rate of thecoil with an qutside radius of 0. 35 m is much highersteel strip is shown in Fig. 3. After cooling for 20 h,compared to the other two coils in Fig. 3.the temperature at the middle of the coil with an4 Influence of Cooling Rate and Coiling Tem-outside radius of 0. 85 m (R2/R=3.0) was approx-perature on Final Mechanical Propertyimately 275 C, higher than that with an outside ra-dius of 0. 35 m (R2/R=1. 25). For the coil with anThe main metallurgical process during coil cool-outside radius of 0. 35 m, the temperature at theing is the precipitation of AIN for plain carbon steels700(aR:=0.2794m0.06b)R=0.2794m600A500R2/R-=3.0r 0.048400- R/R=1.250.03300、 R/R=200.02200 I,R/R=2.0R/R=1.250.01\R/R,=3.00L800120001 200Time/minFig.3 Relationship of coil dimension and temperature profile (a) or cooling rate (b)and (Nb/V)(CN) for microalloyed steels. Some ni-perature and cooling rate during coil cooling, andtrogen in plain carbon steels is tied up by AIN pre-precipitation strengthening is of industrial import-cipitates, and the remaining nitrogen is free in solu-tion which contributes to the strength of steels. Ac-B27.74%6cording to proposed structure- property relations68.31%[Eqn. (2)，Eqn. (3), Eqn. (7)，Eqn. (8)]，three28.65%6UTS- 3889 MPafactors, i. e. the composition of steel, the grain sizeV5-303.MIPand fraction of ferrite and the precipitation strength-649%25.2%63.619%ening, contribute to final mechanical properties ofhot rolled steels. For four steel grades listed in Ta-ble 1, the contribution of composition, microstruc-ture and precipitation to the strength was estab-A- -Composition; B- Nitrogen in solution;C一 Ferrite grain size (5. 46 μm)lished by using above proposed structure propertyFig. 4 Contribution to strength of A36 steelrelations and maximizing the strengthening effect ofmicrostructure and precipitation, as shown in Fig. 4to Fig. 7. Based on Fig.4 to Fig. 7, the maximum73.2896contribution of precipitation to the yield strength of27.6696steels is listed in Table 2. The effect of precipitation| UTS 345.8 MPastrengthening on microalloyed steels is much greaterY5-279.7MPa18.6596than that on plain carbon steels. Since precipitation。Boccurs mainly in coil cooling, it is an important met-中国煤化工allurgical tool to control final mechanical propertiesof steel in a hot rolling mill.MHCNMHGA- Composition; B- Nitrogen in solution;Quantitative investigation of the relationshipc- - Ferrite grain size (12. 61 μm)between processing parameters, such as coiling tem-Fig5 Contribution to strength of DQSK steelNo.2Application of Microstructure Engineering in Steel Coil Cooling ProcessTable 3 Industrial cooling methods and associatedcooling rates24.959%623.67%55.12%Cooling modeCooling rate/Cooling rate/ Mass of coil/(C●s~1) (C ●h-1)UTS-478.8 MPaNatural air cooling0. 00725. 29-31Y5-360.8 MPa14.81%6Forced air cooling0.0111029-3151389630.07%6Air water cooling0. 050800. 01865Note: Temperature range of 800- 200 C ; Strip dimension ofA- Compositioon; B- - Precipitation strengthening;(4-6) X (1 450- 1550) mm? ; Thermocouple is atC- Ferrite grain size (10. 42 μm)center of coil and in middle part across coiled stripFig 6 Contribution to strength of HSLA-V steelC/s to 20 "C/s, which covers the typical coolingrates from natural air cooling to water quenching._AThe higher the cooling rate, the more nitrogen in26.896 A52.09%20.74%solution is， which strengthens the steel strength.UTS=505.5 MPaLikewise，the relationship of coiling temperatureY35-409 MPaand cooling rate to nitrogen amount in solution of17.34965237%630.66DQSK steel is plotted in Fig. 8. Fig. 9 shows the re-lationship between nitrogen amount in solution andcooling rate when coiling temperature is fixed at 500A- Composition; B- -Precipitation strengthening;C/600 C for DQSK steel. Increasing cooling rateC--Ferrite grain size (6. 83 pm)from 0.001 "C/s to 1 C/s, the nitrogen amount inFig, 7 Contribution to strength of HSLA-Nb steelsolution increases significantly and nearly all nitro-gen in solution is free when the cooling rate reachesTable 2 Maximum contribution of precipitation to1 C/s. From the standpoint of processing parame-yield strength of steelsters, different combination of cooling rate with coi-A36 DQSK HSLA-V HSLA-Nbling temperature can obtain the same AIN precipita-Yield strength/MPa03.5 279.7360.809tion strengthening effect both for A36 and for DQSKPercent of precipitation7.74 9. 324. 9526. 89grades, as shown in Fig. 10. In the case of A36，contribution/%complete nitrogen in solution can be achieved eitherPrecipitation23. 5269by coiling at 350 C and cooling at 0.001 C/s or by .strengthening/MPacoiling at 500 C and coolingat 1.0 C/s. Similarly,that all nitrogen is precipitated as AIN can be ob-ance. The cooling rate of a coil is directly associatedtained either by coiling at 630 c and cooling atwith its dimensions as aforementioned. In addition,0.001 "C/s or by coiling at 830 C and cooling at 1.0the cooling rate of the steel coil is also related to in-C/s (Fig. 10). Although there is 26 MPa at mostdustrial cooling methods as shown in Table 3[20].which can be adjusted by this processing to the yieldFor forced air cooling, the cooling rateof 15 t coil isstrength of A36 and DQSK grades, this is still a wayhigher than that of 30 t coil. Forced air cooling has ato control final mechanical properties of the steels.better cooling effect than natural air cooling. If wa-The relationships among coiling temperature,ter spray is introduced into the cooling process, thecooling rate and precipitation strengthening for HS-cooling rate of strip in water cooling zone may exceed 1LA-V and HSLA-Nb grades are plotted in Fig. 11.C/s. For A36 steel, the relationship among coilingThere is a precipitation strengthening peak at a cer-temperature, cooling rate and nitrogen amount intain cooling rate between 0. 001 C/s and 20 C/s forsolution is shown in Fig. 8. For a given cooling rate,both steels. This means that the same precipitationthe higher the coiling temperature, the lower the a-stre中国煤化工:ved under the condi-mount of nitrogen amount in solution which weakenstionaperatures and one ithe steel strength. On the other hand, for a givendentiMYHCN MH G.A-V seel coil with .coiling temperature, the effect of cooling rate 0Ia cooling rate of 0. 001 C/s, coiling at 550 C andprecipitation was investigated in the range from 0. 001coiling at 720 C are expected to have the same pre-●42●J. Iron & Steel Res.，Int.Vol. 120.00500.0060(1)0.0050 t0.00400.004 0信0.0030,20.0030110.002 0曼0.00 00.001 0 t3000030500900Coiling temperature/C1- -Cooling rate 0.001 C/s; 2- Cooling rate0.01 C/s; 3- Cooling rate 1.0 C/s; 4- Cooling rate 20 C/s(a) w=0. 004 7%;(b) wrx=0. 005 2%Fig. 8 Relationship between ciling temperature and nitrogen amount in solution of A36 steel (a) and DQSK (b)20+(a).500C0.0050 115X=0p=0.004 7% \0%=0600 C5-0.0020700。20|(b)0.0010 I0n~=0.005 29%615 t0j=),=0.005 2%1020Cooling rate(Cs ")0;=0、Fig. 9 Relationship between cooling rate and nitrogen5amount in solution of DQSKcipitation strengthening effect [Fig.11 (a)]. Thepeak precipitation strengthening effect corresponds400600800to a coiling temperature of 640 C at the cooling rateof 0.001 C/s. With increasing cooling rate fromFig. 10 Relationship between coiling temperature, cooling0.001 C/s to 20 C/s, the coiling temperature cor-rate and nitrogen amount in solution of A36responding to peak precipitation strengthening in中国煤化工creases for both steels. However, at the same cool-ing rate, for HSLA-V steel, the coiling temperatureMHCN M H G for the control of pre-which matches peak precipitation strengthening is cipitation strengthening in HSLA steel grades, lead-higher than that for HSLA-Nb steel. Fig. 11 pro- ing to yield strength adjustability up to 90 MPa forNo.2Application of Microstructure Engineering in Steel Coil Cooling Processto yield strength is approximately 25 MPa for plain360 F(a)carbon steels and approximately 100 MPa for micro-alloyed steels. Therefore, as the last processingstage ina hot strip mill, to control the coil cooling340 tprocess is a powerful practice to adjust final mechan-ical properties of a steel strip. The results show thatthe coiling temperature and cooling rate during coil320 Icooling have significant influence on the precipitationfor all four steel grades.300The authors are grateful to the American Ironand Steel Institute and the Department of Energy forfinancial support for this work. The development of45055065075050the models requires extensive input in physical met-420向)allurgy and the efforts of E B Hawbolt, W P Sun, .M Militzer, W J Poole, T R Meadowcraft, B Chau,40R Cardeno and P Wenman are gratefully acknowl-edged. Numerous steel companies involved in the380program have provided mill data and coil samples/and we are grateful for their contribution, which has360led to significant improvements in the model. Dr. JK Brimacombe had passed away in 1997 and this pa-340per is dedicated to him for his excellence.References:32[ 1 ] SamarasekeraI V, Jin D Q, Brimacombe J K. The Application40000700800 900of Microstructural Enginering to the Hot Rolling of Steel [A].Coiling temperature/C38th MWSP Conf Proe [C]. ISS, Vol. XXXIV, 1997. 313-327.1- Cooling rate 0. 001 C/s; 2- Cooling rate 0.01 C/s;[2] Roberts W L. Hot Rolling of Steel [M]. Marcel Dekker Inc,3-Cooling rate1.0 C/s; 4一- Cooling rate 20 C/s1983.(a) HSLA-V steel, Ferrite grain size= 10 pm;[3] Robson J E, Ghobarah A A. Handling of Coiled Strip[J]. lron(b) HSLA- Nb stel, Ferrite grain size=6. 8 pμmand Steel Engineer, 1975, 64-66.Fig. 11 Relationship among coiling temperature, cooling[4Kunishige K, Hayashi Y. Thermo Mechaicelly Treated Hot-rate and yield strengthRolled High Strength Sheet Steels [J]. The Sumitomo Search,1989, (39); 97-106.vanadium steel and 110 MPa for niobium steel,[5] Plociennik Ch, Sauer w, Seiler B, et al. 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