Effects of Ti addition on low carbon hot strips produced by CSP process

Journal of University of Science and Technology BeijingMaterialsVolume I3, Number 3, June 2006, Page 230Effects of Ti addition on low carbon hot strips produced by CSP processMingzhuo Bai), Delu Liu), Yanzhi Lou", Xinping Mao?), Liejun LiP, and Xiangdong Huo2)1) Department of Materials Physics and Chemistry, University of Science and Technology Beiing, Bejing 100083, China2) Guangzhou Zhujiang Iron and Steel Co. Ltd, Guangzhou 510620, China(Received 2005-02-09)A bstract: Large quantity of fine Ti(C,N) particles, 15-30 nm in size, were observed in low carbon hot strips added to a small amount ofTi and produced by CSP process. The results showed that the precipitation of Ti(C,N) mostly took place during soaking and hot rolling,which is significantly different from that in the conventional production. These fine Ti carbonitride particles could be very effective onthe austenite grain refinement by hindering grain growth of recrystallized austenite. Their precipitation behavior was discussed andcompared with that of the steels produced in the conventional production.Key words: compact strip production (CSP); thin slab direct rlling; Ti(C,N) precipitation; austenite recrytallization; grain refinement[This work was financialy supported by the National Natural Science Foundation of China (No.50371009).]1. Introductionthe effects of microalloyed elements on the micro-The role and precipitation behavior of Ti addition instructure and properties of steels. However many tech-microalloyed steels have been studied and reportednical principles are needed to be studied urgently forextensively [1-6]. In the conventional hot rolling proc-full utilization of this technique. In the present work,the precipitation behavior of titanium carbonitrides iness, a small amount of Ti is often used to refine theCSP low carbon steel was studied. And the effects ofaustenite grain size of cast slabs during solidificationsmall amounts of Ti addition in CSP hot strips as welland in the reheating process by hindering grain growthas the differences between CSP steels and conventionalof the austenite. On the other hand, precipitation of Tiones were also discussed.carbonitrides in the direct charging and continuousrolling, i.e, TSCR (thin slab continuous casting and2. Experimental materials and methodsrolling) steels, has also been studied in recent years [7-Samples of the CSP low carbon steel with Ti addi-9]. Because the thermomechanical process or thermaltion were taken from the hot strips after coiling and thehistory of the steels by TSCR process is different fromcast slab before soaking. Its chemical composition isthat of the conventional cold charging one, significantlisted in Table 1. The slab samples were cut from a castdifferences in the precipitation behavior of some sec-slab before soaking and air cooled. The sampling wasond phase may appear between these two techniques.taken at near surface, 1/4 depth, and the central region,As one of the most important TSCR techniques CSPrespectively, along the depth direction of the slab. Ex-(compact strip production) process has been developedtract replica and thin foil specimens of the steel sam-very fast in China because of its obvious advantages.ples were prepared and observed by transmission elec-Much faster solidifying rate and cooling rate of thetron microscopes (TEM) with X-ray energy dispersiveslabs are the distinguishing feature of the CSP processspectroscope (XEDS). Statistical distribution of thecompared with the conventional techniques [10-11].particle size was also measured in more than 20 view-One of the major changes induced by these aspects ising fields of TEM observation.Table 1. Chemical composition of the experimental CSP steelwt%MnSiPSCuNiCr中国煤化工0.170 1.18 0.32 0.019 0.002 0.10 0.036 0.024HCNMH GCorresponding author: Delu Liu, E-mail: dlliu@mater.ustb edu.cn.M.Z. Bai et al, Effects of Ti addition on low carbon hot strips produced by CSP process2313. Results and discussions1/4 depth region, the particles are distinguished by cur-ved edges and rounded corners. Whereas, in the central3.1. Ti(C,N) precipitates in the hot stripsregion the particles are with sharp edges, thus implyingTEM observation shows a great deal of cuboid finethat they have already grown up. The difference inparticles precipitated in the hot strip samples (Fig.1quantity and shape of the particles results from the dif-(a)-()). They are mostly in regular shape with sharpferent cooling rates in the different regions of the slab.edges and corners. As Fig. 1 (d) shows, the size of mostIn the surface region, the precipitation kinetics is re-particles ranges between 15 and 30 nm, and the averagestrained by the higher cooling rate. Differently, growthsize is about 23 nm. Distribution of the particles is notof the precipitates takes place, but it is still insufficientvery uniform, but often presents the precipitation ten-in the 1/4 depth region for the relatively lower coolingdency along some directions (Fig. 1 (b)). There are norate. In the central region of the slab, the cooling rate isobvious differences in distribution and in size of thelowest so that the precipitates grow sufficiently andparticles between ferrite (Fig. 1 (a)) and pearlite (Fig. 1form the sharp cuboid shape.(C)) regions. Therefore, the particles have precipitatedCoarse particles (>50 nm in size) are also observedand grown up before the γ→a phase transformation.in the slab samples (Fig. 2 ()). There is no striking dif-The non-uniformity in distribution is the result of theference in morphology, composition, and distributiondendritic segregation [3]. According to the EDXSas compared with the coarse particles in the strip sam-analysis results, it is clear that these fine particles areples. As a result, it can be confirmed that these coarseall titanium carbonitride precipitates. Selected areaparticles are TiN precipitated during solidification or atelectron diffraction study also confirmed that the crys-a higher temperature range after solidification.talline structure of the particles is consistent with ti-tanium carbonitride as shown in Fig. 1 (g). The lttie3.3. Effects of Ti addition on the microstructure ofthe CSP steelsparameter deduced from the diffraction patterns isabout 0.43 nm, very close to that of TiC (0.432 nm) orIt is clear that the effects of Ti addition on micro-TiN (0.423 nm). It is already known that the ratio of Nstructure and mechanical properties of the steels de-and C in the carbonitrides varies depending on the pre-pend on the size, volume fraction and distribution ofcipitation temperature. Carbon content in the precipi-the precipitates. All these parameters are controlled bytates usually increases with the precipitation tempera-the precipitation dynamics of the TiN, Ti(N,C), andture decreasing. At the same time the lattice parametersTiC in the steels. The remarkable differences in heat-also vary with the ratio of N and C [12]. Therefore, thecycle history, cooling rate, and solidification periodparticles are considered as Ti(C,N).between the CSP and conventional process will lead toThere is another type of particle observed in the stripa significant change in precipitation behavior of thesesamples (Fig. 1 (). They are all more than 50-60 nmcarbonitrides.in size and much less in quantity than the fine particles.The thermodynamic temperature of TiN and TiCThese coarser particles are confirmed to be TiN pre-precipitation can be calculated by the following rela-cipitates with EDXS (Fig. 1 (i)). It should be pointedtions [1]: .out that no TiN inclusions larger than 1 um in size havebeen observed in the steel, suggesting that the micro-lg(%Ti[%N])L --17040+6.40,alloying element can be effectively utilized in the CSP15790lg([%Ti][%N)r=-- + 5.40,T3.2. Small particles in the cast slabSome small particles are also observed in the slablg([%Ti][%C])y =-7000+2.75.samples, but their amount is far less than that in the hotstrip samples. They are generally less than 20 nm inThe precipitation temperature is calculated, respec-size. Analyzed by EDXS, the particles are also con-tively, as 1363°C for TiNL, 1404°C for TiN, andfirmed to be Ti(C,N). A comparison of the particles1044°C for TiCy in the experimental steels. Because ofbetween the near surface, 1/4 depth and central regionsthe low content of Ti and N elements in the steel, theof the slab shows that a few particles are observed inthermodynamic condition of TiN precipitation can notthe extract replica of the near surface region, but quite abe matched in中国煤化工Under equi-quantity of particles precipitated in the 1/4 depth andlibrium conditi1HCN MH G higher tem-central regions of the slab (Figs. 2 (a) and (b)). At theperature range dlct suuTILallvll allui 1iC should ap-.232J. Univ. Sci. Technol. Bejing, Vol.13, No.3, Jun 2006pear at the lower temperature (1044°C or below). Be-tween them Ti(C,N) may form.(b100 nm40[()240(e280 (33.0台200240FR 3026.8弓160|8 20020e 20-会120十.百16010.880)-11080S3|0fo出凹010~15 20~25 30~35 40 45Precipitate size / nmFEnergy I keVEnergy / keV(g)280T200(200). (220)6020 IN10 t(020)50 nmEnergy1 keVFig. 1. Ti(C,N) particles observed in CSP hot strips: (a), (b) fine particles precipitated in ferrite region; (c) particles in pearlite .region; (d) statistical size distribution diagram of particles; (e) EDXS spectrum of fine particles; () EDXS spectrum of FezC; (g)one particle and its diffraction pattern; (h) a large particle; (i) corresponding EDXS spectrum of the particle in Fig. (h).In conventional production, the cooling rate of thickthan 25 nm in size would dissolve only at a temperatureslabs is slow (about 9 K/min for 250 mm thick slabshigher than 1300°C, and not dissolve completely until[13]). During this process, TiN will precipitate whenthe liquidus [7]. As a result, only a very small amountthe solution product of [Ti] and [N] matches an ap-of Ti could be retained in the solution condition of theproximate value with the temperature decreasing. Ow-steel before hot rolling. On the other hand, as the re-ing to the high temperature (near 1400°C) and the lowheating temperature is generally lower than the pre-cooling rate of the slabs, the nucleation rate of the pre-cipitation temperature of TiN, larger particles wouldcipitates is low but their growth is fast. Thus they willkeep growth during reheating. Therefore, these parti-precipitate with small quantity and large size. Obvi-cles will have lttle effect on the grain refinement of theously these large precipitates are not very efficient forrecrystallized austenite during hot rolling.hindering the austenite grain growth during coolingIn contrast with the conventional process, the CSPand reheating. The effect of Ti on the grain refinementprocess is characterized by thin slab casting and directis not fully explored. Furthermore, during the reheatinghot rolling without reheating as employed in the con-process, part of the smaller TiN precipitates will be dis-ventional one, the. roolino rate nf the slahs in the rangesolved but the other larger ones will keep growing, i.e,of 1560-1400°中国煤化工K/min for 50the coarsening of the particles takes place. It has beenmm thick slabsYHC N M H Gentional one.observed that the titanium carbonitride particles largerIt has been pointed out by Kunishige that TiN precipi-.M.Z. Bai et al, Effects of Ti addition on low carbon hot strips produced by CSP process233tation can not be detected in steels with a high coolingmarkably increased in the CSP process. Large quantiti-rate of 36 K/min or more [8]. Therefore the TiN pre-es of fine Ti(C,N) precipitates would take place duringcipitation before soaking could be hindered by thethe soaking and hot rolling processes. Because of thehigher cooling rate of the slabs. Only a few particlesrelatively low precipitation temperature and the highlarger than 50 nm were observed in the slabs, whichdegree of supersaturation during soaking and espe-would have been precipitated before or during soaking.cially the first two rolling passes, Ti(C,N) precipitatedTitanium mostly remains in solid solution of the slabs,intensively as fine particles along austenite grainand is precipitated largely during subsequent soakingboundaries and within the grains. These fine Ti car-and hot rolling. This can also be confirmed by thebonitride particles can be very effective on the aus-Kunishige's experimental findings that the dissolved Titenite grain refinement by pinning the boundaries andaccounts for only 20% after reheating and soaking athindering growth of the recrystallized austenite grains.1150°C for 20 min in the conventional process,Therefore, the effect of the small amount of Ti additionwhereas 80% in the solution for CSP process [8]. As ais quite different from that in the conventional process.consequence of the fast cooling, the dissolved Ti is re-10 nm10100 nm25 nmFig. 2. Ti(C,N) particles in CSP slabs before soaking: (a) precipitates in 1/4 depth region of the slab; (b) precipitates in centralregion of the slab; (C) larger particles in the slab.4. ConclusionsReferences(1) Large quantities of Ti(C,N) precipitates with cu-1] E.T. Turkdogan, Causes and effects of nitride and carboni-tride precipitation during continuous casting, lron Steel-boid shape were observed in CSP hot strip samplesmaker, 16(1989), No.5, p.61.with a small amount of Ti addition. Statistics showedthat the particle size mainly ranges between 15 and 30analysis of microstructures of low carbon sheet steel con-nm. The average size is about 23 nm. These fine pre-taining micro-amount of Ti, Mater. Sci. Technol. (in Chi-cipitates are mainly precipitated during soaking and hotnese), 4(1996), No.2, p.25.rolling.3] Y.L. Li and M.Z. 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