Aspects of microstructure in low carbon steels produced by the CSP process

Journal of University of Science and Technology BeijingReviewVolume 10, Number 4, August 2003, Page 1Aspects of microstructure in low carbon steels produced by the CSP processDelu Liu, Xiangdong Huo, Yuanli Wang, and Xianwen SunMaterials Science and Engineering School, University of Science and Technology Beijing. Beijing 100083, China(Received 2003-05-20)Abstract: The solidification structure, microstructure evolution during rolling and precipitates with nanometers in dimension of thelow carbon steels produced by CSP process with thin slabs have been studied in recent years. Important differences in microstructureand mechanical properties between the CSP products and the conventional one were observed. These differences may arise from themuch rapider solidification rate and cooling rate after casting of the thin slabs. Some aspects of the microstructure for the low carbonsteels of the CSP thin slabs are summarized and compared with the conventional one.Key words: low carbon steel; CSP hot strip; microstructure evolution; grain refinement[Supported by the State foundation for key project: New Generation of Stels (No: G1998061500)]1 IntroductionZhujiang Steel Ltd Co. [2, 3]. It is expected that morethan 160 million tons of hot strip products will be pro-Since the first CSP (compact strip production) steelduced with these thin slab casting and rolling proc-plant was commissioned at Nucor in 1989, 36 produc-esses in mini mills worldwide by the year 2013 [4-6].tion mills with total 54 streams have been establishedThe rapid development of these compact flat productin the world. The total capacity has reached to 55 Mtmills has taken place in the last decade owing to theirannually including the processes of CSP, ISP, DSP,obvious advantages such as low specific investment,QSP, FTSR and ConROLL[1]. In 1999 the first CSPlow energy consumption and high productivity etc. A .production line in China started up successfully intypical CSP line is illustrated in figure 1.Hot-strip millCSP moldCooling lineCoilerDescalerSoaking furnaceQ:QIO:Q888888(DFigure 1 Schematic ilustration of a typical CSP process.Mechanical properties of the hot strips produced byIn comparison with the conventional cold chargeCSP process are usually good. Compared with plainprocess, there exists a series of important differenceslow carbon steels such as Q195, the EAF-CSP pro-for the CSP process. Firstly, much faster solidificationducts have much higher strength and good ductility.and cooling rate are characteristics for the thin slabs,Above 380 MPa yield strength with elongation 35%-which lead to distinct in the solidification structure,40% is obtainable for the plain low carbon steel stripsinterdendritic segregation and precipitation of secondproduced by EAF-CSP process. Composition of thesephases during solidification and cooling process.steels is similar to that of Q195 (or SS-330), namely CSome of the parameters for conventional slabs with<0.06%，Sis0.10%， Mn<0.50%， S<0.035%， P250 mm thick and thin slabs with 50 mm in thickness<0.025%, Cu<0.20% and Al 0.025%-0.040% (massare summarized in table 1 [12]. Secondly, the hotfrcation). The mechanical properties are almost no dif-strips produced by CSP process had undergone differ-ference between the transverse and rolling direction ofent thermo-mechanical brocesses. No v-→Ql and rever-the strips [7]. Systematic investigations on the micro-se transformatil中国煤化工lling in thestructure evolution of CSP steels have been carried outCSP process. AYHCNMHG0Cthethinand reported [8-11].slabs with solidified structure are rolled directly [13].Corresponding author: Delu Liu, E-mail: dlliu @ mater.ustb.edu.cn.J. Univ. Sci. Technol. Beijing, Vol.10, No.4, Aug 2003This approach will bring remarkable effects on the2 Characteristics of solidified structuremicrostructure transformations and precipitations andhence the final microstructure and mechanical proper-Much faster solidification and cooling rate are theties. Thirdly, the capability of microstructure con-characteristics of the thin slabs. It is pointed out [12]trolling by cooling rate in the run out table is a veryby table 1 that the period of solidification for the thinimportant factor, which has been much improved inslabs is about one tenth of that for the conventionalthe impact production lines. Therefore, much attentionslabs, and cooling rate for the thin slabs is about thir-has been attracted to this new research field [14-16].teen times faster than that for the conventional slab inSome aspects of microstructure in the hot strips pro-temperature range from 1560 to 1400C. The castingduced by CSP process will be discussed in the fol-speed is also much faster [17, 18] but the soakinglowing text. .temperature is about 100-200 K lower than the re-Table 1 Comparison between the conventional and CSPheating temperature of conventional slabs. Differencesprocessesin these parameters will lead remarkable diversity inthe microstructure of the slabs and hence the strips.ConventionalThin slabProcessRef.slab (250mm)(50mm)Figure 2 shows the macrostructure of a longitudi-Solidification10-151[12]nal section from a thin slab with 50 mm in thickness,period / minwhich is a low carbon steel containing 0.18 C, 0.09 SiCooling rate (1560-9120 [12]and 0.30 Mn (mass fraction in %, so as the follows).1400C)1 (K-min |)The optical micrographs showing the microstructureTransformationbefore rollingNin as cast slab of the steel at room temperature arTotal deformation/given in figure 3. Prior austenite grain boundaries canbe distinguished in the pictures. Microstructure at its4.63.0surface layer and central region (at half thicknessMaximum rlling21(position of the slab) of the slab is given in figure 3(a)speed /(m.s" ")and 3(b), respectively. Solidification structure of theCasting speed /(m-min-)1.4-1.82.8-5.5[10]thin slabs is characterized by the following features.Slab soaking1150-12501050-[3],temperature /C1150[20]surface3 mmFigure 2 Macrostructure of a low carbon steel slab (50 mm thick) produced by CSP process.Figure 3 Micrographs showing austenite grains in as cast slab of a low carbon steel, (a) surface layer; (b) central region. .(1) Very thin equiaxed grain zone at the surfacemeters. The au中国煤化工1-2 mm forlayer of the slabs. It can be seen in figure 2 that thethe slab as shov| YHCNMHGthickness of this equiaxed grain zone is several milli-(2) The dominated structure of the thin slabs is co-.D.L. lin et al, Aspets of mirostructure in low carbon sees poduced by the csp proeslumnar dentritic zone with much smaller dendrite arm0.09 Si and 0.30 Mn) after 55% compression of r1l-spacing as can be seen in figure 2. The spacing of theing, which was taken from the same rlling piece asfirst dendrite arms ranges from 0.25 to 1.83 mm. Thethe slab of figure 3. Microstructure at surface layermean secondary dendrite arm spacing varies fromand a central region (at half thickness position) in theabout 90 to 125 um [19, 20]. In contrast with the thinrolling picce are shown in Figure 4(a) and 4(b), re-slabs, the secondary dendrite arm spacing of the con-spectively. It can be seen by the comparison of figuresventional slabs ranges between 200 and 500 μum [21,4 and 3 that the austenite grain size at the central re-22]. Therefore, the solidifcation structure of the thingion decreases from 1-2 mm in the slab to 100-200slab is much finer than that of traditional continuousμum after 55% compression. This is very close to thecasting thick slabs.austenite grain size of the conventional slabs after re-(3) Due to rather rapid coolingcasting of theheatig and soaking at 1150-1250"C, whichrangesthin slabs redistribution of the solute elements such asfrom 50 to 250 um for a C-Mn steel with a base com-Mn, Si elc. could be limited by insuficient difusionposition of about 0.12 C, 0.30 Si and 1.40 Mn (yieldtime. The dendrite arm spacing is the distance overstrength level Grade 355 MN.m 2) [19]. But at the sur-which atomic migration must occur during solidifica-face layer the prior austenite grains are elongated as ation. Thus the kinetics of homogenization in the thin“pancake" shape. This can be explained by the strainslabs is obviously dffrent from the conventional onedistributions in the low carbon steel during rlling[23]. Further study on both of theoretical and experi-[25]. The finite element simulation results show thatmental aspects in detail is required for better under-the deformation is not homogeneously distributedstanding of the solidification and its effects on thealong the thickness of a rlling piece during rlling.succedent phenomena such as δ→γ transformation,The maximum shear strain, which is 0.13, mainly dis-precipitation and segregation.tributes at the superficial layer of the rlling piece,while the compressive strain is in the central area. It3 Grain refinement during rlling of thinwas reported that the existence of shear strain may re-slabsfine the grain size more effectively [26].Austenite grain size and microstructure is a veryMathematical models have been used to estimateimportant factor for controlling the final microstruc-the rytallisation behavior of the austenite duringture of the strips. In the thin slabs of low carbon steelscontinuous rlling for a low carbon steel (with 0.07 C,austenite forms from δ phase through δ→y transfor-0.325 Mn, 0.03 Si, 0.14 Cu) produced by CSP processmation or peritectic reaction during cooling. The mi-[26, 27]. Without roughing the thin slab with 50 mmcrostructure evolution of the thin slabs during rollingin thickness was rolled through six finishing standswould be diferent from that of the slabs produced byinto 1.9 mm strip. The results showed that the dy-conventional cold charge processes at least in the fol-namic recrystallisation of the austenite took place inlowing aspects:the first pass of rlling, from the second to the ffth(1) Without y-→a- →γ transformations before rllingpass static recrystallisation of the austenite occurred.thin slabs with as cast microstructure are directly de-In the sixth pass the austenite is non-recrystallized. Informed by finishing rolling. The austenite grain size,general, the grain refinement of the austenite is sig-microstructure and segregation prior to the thermo-nificant from the frstto the third rlling pass owing tomechanical process of the thin slabs is quite differentthe high deformation temperature and heavier defor-mation (& 250% for the first and the second pass).from that of the conventional one [24].(2) In contrast with the conventional process theThese results have been confirmed by a series ofsoaking of CSP thin slabs is at much lower tempera-experimental observations [10, 11]. Samples of theture (see table 1) with shorter holding time (20-30steels include those taken from the same piece of themin). The grain growth of the austenite may be re-slab but undergone different rolling passes. The spe-stricted by this condition.cimens were cut from a rlling piece, which was ob- .(3) Various dfferences in precipitation of the non-tained by suddenly stopping the rlling operationmetallic compounds, segregation and the solidificationduring continuous rlling. Figures 5(a)-5(f) are a seri-structure resulted from rapider cooling rate would af-es of micrographs showing the microstructure of thefect hereafter the transformations and final structure ofspecimens at room temperature from this rlling piece.They were takthe steels.中国煤化工tral areas ofthe rlling piec/erage ferriteOptical micrographs given in figure 4 showed thegrain size forYHCN MH G to the sixthmicrostructure in the low carbon steel (with 0.18 C,pass is 41.6, 25.2, 21.4, 20.2, 13.1, 6.7 um, respec-J. Univ. Sci. Technol. Beijing, Vol.10, No.4, Aug 2003tively. Difference in the microstructure between thefirst and the second pass is obvious.50umFigure 4 Microstructure in the low carbon steel after 55 % compression of rolling, (a) surface layer; (b) central region in therolling piece.c)50μm、 50y吧d)e)750 m-20μ20μmFigure 5 Microstructure of the specimens deformed through different rolling pass from the same slab, after the (a) first pass;(b) second pass; (c) third pass; (d) fourth pass; (e) fifth pass; (f) sixth pass rolling.summarized in table 2. Figure 6 shows three mi-4 Accelerated cooling after rollingcrographs of the 2 mm thick strips (low carbon steelAccelerated cooling after hot rolling is also an im-ZJ330) obtained by different finishing rolling andportant factor for controlling the final microstructurecoiling temperature but the same continued cooling[28]. The cooling rate can be controlled by the laminarmode. Their mechanical properties are given in tablecooling system in conjunction with water sprays for3. It can be seen that increasing the cooling rate maythe CSP lines, which are computer controlled to en-suppress the pearlite reaction and result in a finalsure the designed cooling mode and uniformity of it instructure of ferrite plus bainite (figure 6(b)). Lowerstrips. It provides an additional grain refinement pro-finish rolling and coiling temperature leads to refine-cedure for the products. The cooling start temperaturement of ferrite grains (figure 6()). The strength canbe increased obviously by the refinement of theor rolling finish temperature is usually above the Ar3and the coiling temperature or finish cooling tem-equivalent grain size.perature ranges from about 500 to 670C dependingTable 2 Cooling rate determined on a CSP line underon the steel grades. Thus the parameters of cooling aredifferent cooling modevery important as austenite decomposition reactionsStripCooling rate 1Cooling modeusually take place during the cooling process of thethickness(C.s-strips. Very fine ferrite grain size, for example aboutContinued cooling75-904-5 um, may be obtained by heavy deformation com-2 mmAlternation cooling~60bined with appropriate cooling.Air cooling12The cooling rate of the low carbon strips with中国煤化工40-50thickness 2 and 4 mm in the CSP line at Zhujiang4 mmMHCNMH G0Steel Co. has been determined experimentally and8.D.L. Liu et al, Aspects of microstructure in low carbon steels produced by the CSP processFigure 6 Strip (2 mm thick) microstructure corresponding to different finishing rolling and coiling temperatures, Finishingrolling (F.R) and coiling at (a) 880 and 660; (b) 880 and 550; (c) 800 and 550, respectively (continued cooling mode).Table 3 Mechanical properties of the strips ZJ330 obtained by various cooling processThickness / mmF.R/CCoiling/CCooling mode σ./MPa σ。/MPa_ 81%_σ/σp4.0880600continued317398280.796Alternation302384300.7862.00.825803293940.835550440.834840357410250.8708006740900.8972.C6603860.8655 Nanometer precipitatesgrowth of the original and recrystallized austenite.The new results showed that oxides and sulfidesThis could be one of the important factors for themay precipitate as dispersive particles in low carbonaustenite grain refinement, which controls the finalsteels strips produced by EAF-CSP process. Largegrain size of the steels.number of fine oxide and sulfide precipitates withThe second group of the particles includes the dis-nanometers in dimension in the steels have been ob-persive precipitates with the size smaller than about 30served by using transmission electron microscopym. These particles seem to settle out at lower tem-(TEM) and X-ray energy dispersive spectroscopyperature in the strips and far from the equilibrium(XEDS) on both of extract replica and thin foil speci-conditions. The nature of these tiny precipitates aremens. The particles may be divided into two groupsnot very clear so far. It is very likely that deformationaccording to their size. Dimension of the particles induring rolling would have important effects on theirthe first group ranges from about 30 to 300 nm. Theyprecipitation. They may have significant influence onare mainly manganese and iron sulfides as well as ironthe grain refinement of ferrite and mechanical proper-oxides.ties of the steels [33].These precipitates exist in the thin slabs and all ofBesides the oxides and sulfides, small particles ofthe specimens in spite of the number of rolling pass. Itcarbide and nitride also exist in the hot strips. In cer-can be deduced that most of them precipitated in theain condition carbides seem to nucleate at the surfacesteels during soaking or even at higher temperature.of sulfide particles [34]. Some other precipitates couldThis is also consistent with the calculation from thealso appear. Figure 7 shows the TEM micrographs ofsolubility product of MnS in the steels [29-30]. Thehe particles in a thin foil specimen with a X-raysulfides include manganese sulfide, iron sulfide andspectrum from one particle. The composition andcopper sulfide [15,31]. The XEDS analysis confirmedstructure of these particles are not those, which usu-that the oxide particles are mainly Fe oxide, contain-ally occur in conventional plain carbon steels.ing small amount of Si, Al or Cr. The structure of theIt is expected that the effective grain refinement re-oxide precipitates is consistent with the cubic systemsulted from these nano-scaled particles and grainspinel structure with (F d 3 m) group as determinedboundary segregation of impurity elements would playby electron diffraction. Their lattice parameter a isvery important role for greatly improving theabout 0.82 nm.According to the Zener model migrating grainmechanical pror中国煤化工cteels.=ially the dy-In general,boundaries would be pinned by the sulfide and oxidenamics of theYHcNMHGlesaswellas.particles [32]. Thus these particles can clog graincarbides in a low carbon steel produced by thin slabs.J. Univ. Sci. Technol. Beijing, Vol.10, No.4, Aug 2003would have important differences from that in theconventional steels.0160。120|善8200 om100 nmE;/keVFigure 7 Precipitates in a low carbon steel strips produced by EAF-CSP process, (a) and (b) TEM micrographs of a thin foilspecimen, (C) XEDS spectrum from one of the particles.6 Summary[14] SJ. Cobo and C.M. Sellars, lronmaking Steelmaking [J]，29(2001), No.3, p.230.Rapider solidification and cooling rate of the thin[15]D.L. Liu,J. Fu, YL. Kang, et al., J. Mater: Sci. Technol.slabs followed by the particular thermo-mechanical[田], 18(2002), No.1, p.7.procedure in the CSP process would induce very im-[16] D.L. Liu, X.D. Huo, Y.L. Wang, et al., J. Univ. Sci. Tech-portant effects on the solidified structure, microstruc-ture evolution during rolling and precipitations in low[17]Z.Q. Zheng, Z.Y. Li, FM. Chen, et al, Iron Steel [],30(1995), No.1, p.23.carbon steels. 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