An Experimental Study on Constraint Cooling Process of Hot-rolled CoilS

220J. Mater. Sci. Technol, Vol.19 Suppl.1, 2003An Experimental Study on Constraint CoolingProcess of Hot-rolled CoilsTG|ALijuan WANGI)t and Chunli ZHANG2)1) State Key Laboratory of Rolling and Automation, Northeastern University, Shenyang 10004, China2) Shanghai Baosteel Group Corporation, Shanghai 201900, China[ Manuscript received November 4, 2003]In order to master mechanical property, surface quality and microstructure of constraint cooling (CC) coils undervarious water cooling parameters, more than 100 coils cooling experiments were done with real production process,of which is designed a cooling experimental instrument in the end. The experiments show that high initial coolingtemperature, discontinuous cooling style, and long cooling time can improve mechanical property of cooling coilsand shorten cooling time. The CC coils experiments cover the different steel grades, so that CC process effects onhot-rolled coils may be predicted and controlled actively.KEY WORDS: Constraint cooling, Hot-rolled coils, Experiment1. IntroductionCoil lengthHot-olled coils usually unload in the coils store and na-tively cool until their temperatures satisfy the client requests.RollingdirectionCoil widhRecently, more and more clients require the coils to transferthem in short time, so that a cooling experimental instru-ment was designed in the end of hot rolling process line anddone with more than 100 coils. The industrial experinentsresearch the macroscopic and microscopic rules of CC coils un-。A-Splines ocoilenghgC 8- Sumple on coil widhder production environment and apply the fundamental CCparameters for production.The purpose of constraint cooling (CC) experiments is tomaster mechanical property, surface quality and microstruc-Fig.1 Intercepted sample position and sizeture of the CC coils under various cooling parameters, whichinclude initial temperature, cooling velocity, cooling style,period is 5 min water cooling and then 15 min air cooling.cooling time and so on.Based on real production, the main experiments of cool-ing parameters include discontinuous cooling of diferent ini-2.3 Picking sample datatial temperature and cooling time, and continuous cooling ofMeasured four uniform points on central circle of side facedifferent cooling time. The other mechanical property exper-of the coil with an infrared thermometer, the mean value ofiments of different steel size, coiling temperature and steelthe four temperatures tepresents as coil side surface temper-grade are done with 100 coils, so that the effects on the coilsature at sometime.used CC technology may be predicted actively.When cooling coil is under finishing requirement temper-ature, 60°C, it is uncoiled and uniformly intercepted 5 splines2. Experimental Instrument and Methodsat its length direction, each spline is uniformly intercepted 7samples at its width direction, see Fig.1. AIl samples are donewith tension tests and given intensity of tension, intensity of2.1 Experimental deviceTo testing the effects of water CC coils, we designed ayield and elongation ratio of picked position.Select the second sample of each spline of test 3, take theirwater jet equipment that nozzles spray water toward the heatmicrostructure photos and measure the oxygen scale thicknesscoil, resettled in the back of coil storehouse.For accelerating cooling process, the CC experiment coilupon them and observe the oxygen scale width on their sur-is set horizontally on the“V”shape base. Water is sprayedface.from nozzles assembled with vertical water pipes towards bothsides of the heat coil. According to the temperature field cal-3. Results and Analysesculation, when coil is cooling, its radial heat conduction islower than the axial direction. Therefore the exterior sur-3.1 Coil side surface cooling curvesface in axial direction is not sprayed water to avoid the fuidObserving the cooling curve of side face temperature oferosion.experimental coils, see Fig.2, we know that the curve chang-ing is related with cooling parameters.2.2 Coil materials and experiment conditionsTest cooling curves indicate that the coil temperature re-The chemical compositions of coil materials are listed induces to about 60°C in 35 h, and there is the bigger coolingTable 1, and the test groups coils are selected No.2 steel grade,rate at the higher cooling initial temperature. The more wC1.2 m width, 1.52 m strip thickness and 25 t coil weights.time, the lower end temperature of wC and apex temperatureWater cooling (WC) experiment conditions include three testof ascending process。Among cooling modes, WC is obviouslygroups under various cooling parameters, each test choosesinfluenced of roolino hich temneratire coils, and wind cooling3 or 4 coils, of which 3 coils were labeled with various cool-ndgrature coils.ing mode, see Table 2, the other used air cooling through the中国煤化工sture cili crang freature, heawhole process to compare with WC coils. Discontinuous wCfrorYHCN M H Gich temperature becormesrise, reacnes a apex auu tnen uescends again after ending oft Engineer, to whom correspondence should be addressed,E mail: cral@mail.neu.edu.cn.8nourd Dewater cooling.J. Mater. Sci. Technol, Vol.19 Suppl.1, 2003221Table 1 Chernical compositions of coil materialsSteelChemical composition/wt pctgrade .CSiMnAltNo.1S021<0.30. <1.30<0.035S0.035No.2≤0.15≤0.05<0.60≤0.035≥0.01No.3<0.10≤0.5No.4<0.20<0.55≤1.60≤0.030No.5<0.16<0.5≤1.6<0.03≤0.025Nb<0.06No.6<0.120.25~0.750.20~0.500.07~0.15Cr0.30~1.25Table 2 WC experiment conditions (A- Discontinuous water cooling, B- Con-tinuous water cooling, C air cooling, D- wind cooling )Coll codenitialwC timeCoolingWaterjet quantitytermp./°C/minmode(left/right)/m2__1122990310045020A+C2.6/2.11229903200400A+D2.1/2.412299033003502.3/1.921233505600502.2/2.41233505700104.2/4.5512335058008.5/9.4531234800900B+D1.1/0.912348010002.04/2.1123480110004.6/4.550004125275000-30400310市20~一236310523Time/hFig.2 Coil side surface cooling curvesFig.3 CC coils microstructure photos (200x) in Test 3Table 3 Mechanicalproperties at diferent WC3.3 Mechanical properitiestime in TestCompared mechanical property of about 6000 normal coilse Chante ain 2001 with 100 water cooling coils, their mechanical meanCoil wC~ Intensity of Intensity of Elongationvalues are shown in Table 4.timetension/MPa yield/MParatio/%From Table 4, C, Si and Mn contents of No.1 coil are high;10 min3631620 min365254C contents of No.4 and No.5 coils is lower than that of No.1,40min96625614Si and Mn content of both coils is higher than that of No.1;40 min366C, Si and Mn content of No.2 and No.6 coils are lower thanNo.4 and No.5; C, Si and Mn content of No.3 are the lowest.We can conclude that the higher the C, Si and Mn content3.2 Microstructure photosof the coil, the lower the clongation ratio and the higher theSee Fig.3 and Table 3, the microstructure photos and me-strer I中国煤化工chanical values of Test 3 coils show that the grain size is in-3.4CNMHGcreased with the shortening water cooling time. Having theMHFrum tne uxygen scue tnuckness curves, see Fig.4, it ishigher C, Si and Mn content, the coil can have lower elonga-shown that the oxygen scale thickness of head-tail parts aretion ratio and higher intensity than the normal one.222J. Mater. Sci. Technol, Vol.19 Suppl.1, 2003Table 4 Mechanical properties of various cooling modeGrade2001 normal coilWater cooling coilDiferential between CC and normal coilsYield Elongation Tensile Yield Elongation Tensile Yield ElongationTensilestrengthstrength strength/MPa/%/MPa__ /MPaMPa%No.1311.632.3459.5303.934.3445.3-7.714.2444.528.8560.94409553.7-4.50.2。No.5446.128.9543.8441.629.4538.6).5-5.2No.2242.948.6355.747.1358.97.6-1.53.2No.6390.9 .36.1535.3422.034.8556.031.1-1.320.7No.3245.448.3 .356.231733.7415.371.6-14.659.4. Conclusions(1) After 35 h discontinuous water cooling and wind cool-ng, the coil side surface temperature is reduced to about60'C2(- r(2) The higher the cooling initial temperature, the biggerthe wC cooling rate.101、1234800900(3) The longer the water cooling, the lower the coil side123481000surface temperature at end of water cooling and apex tem-12348011000perature of ascending process.(4) The grain size increases with shortening water coolingtime.1222--- 3242(5) By having the higher C, Si and Mn content, the coilSample numbercan have lower elongation ratio and higher strength than thenormal one. .(6) The higher the cooling initial temperature and theFig.4 Oxygen scale thickness curves in Test 3longer the water cooling time of the coil, the stronger thestrength.30(7) the scale thickness and width increase with water cool-ing time.5. Outlook20The CC coil technology can improve mechanical property andsatisfy the requirement of short production period and high3 10quality. The next questions are how available to prevent scalesfrom generating and realize the cooling process automation.REFERENCES4C60Water cooling time 1 min[1 I Zhanxue WANG: Control Rolling and Cooling, MetallurgicalIndustry Press, 1988 . (in Chineses)Fig.5 Surface quality curves in Test 3 (A) light brown scale[2 ] Seong-Jun PARK and Byung-Hee HONG: ISIJ Int, 1998, 38,width, (B) deep brown scale width, (C) yellw scale1262width{3 ] V.L.Mazur and V.V.Kostyakov: Steel in the USSR, 1989, 19,162.scarcely different and increase with WC time. From the sur-[4 ] Thorn, Horst and Brockway, Carl: AISE Steel Technol, 2002,79, 36.face quality curve, see Fig5, suface scale width increases with [5 ] S.Theerakulpisut and S.Priprem: Inter Commun. in Heat andWC time.Mass Transfer, 1998, 25, 127.中国煤化工MYHCNMHG