The Microstructure Evolution of Nb,Ti Complex Microalloyed Steel During the CSP Process

Materials Science Forum Vols. 500-501 (2005) pp 229-236 Online available since 2005/Nov/15 at https://www.360docs.net/doc/cb5724122.html, ? (2005) Trans Tech Publications, Switzerland doi:10.4028/https://www.360docs.net/doc/cb5724122.html,/MSF.500-501.229
The Microstructure Evolution of Nb,Ti Complex Microalloyed Steel During the CSP Process Ruizhen Wang1,a, C. I. Garcia2,b, M. Hua2,c, Hongtao Zhang1,d and A.J. DeArdo2, e
1) Central Iron and Steel Research Institute, Beijing, 100081, China 2) Basic Metals Processing Research Institute, Department of Materials Science and Engineering, University of Pittsburgh, PA, 15261, USA a b c ruizhenwang86@https://www.360docs.net/doc/cb5724122.html, garcia@https://www.360docs.net/doc/cb5724122.html, mjhua+@https://www.360docs.net/doc/cb5724122.html, d e zhangwy@https://www.360docs.net/doc/cb5724122.html, deardo@ https://www.360docs.net/doc/cb5724122.html,
Keywords: CSP process, niobium, microalloyed steel, microstructure, precipitation
Abstract. The development of microstructure of Nb,Ti-bearing microalloyed steel during the CSP process was studied. Three samples were taken from the as-cast slab prior to tunnel furnace, intermediate bar after stand F2 and the hot band, respectively. In the as-cast slab, the average austenite grain size is 654 μm with a large size range from 150 to 2000 μm. In the intermediate bar after stand F2, the austenite grains are remarkably refined, but are heterogenous due to the incomplete recrystallization, which are in the size range of 23 to 116 μm. In the hot band is mainly non-polygonal ferrite. Microstructural heterogeneity exists in the hot band. It is attributed to the heterogeneous austenite grain size in the intermediate bar and the less rolling reduction after stand F2. With regards to precipitation, cubic TiN and fine precipitates less than 20nm are commonly observed in the as-cast slab and the intermediate bar. Some complex (Ti,Nb)(C,N) precipitates with a slightly larger size also exist. In the hot band, most particles are complex (Ti,Nb)(C,N) precipitates, in a shape of irregular or cruciform. The fine precipitates which can strengthen the ferrite matrix are seldom seen. These results are in good agreement with the size distribution of the precipitates determined using small angle X-ray scattering method. The chemical phase analysis reveals that 45%Nb of the total and 43%Ti of the total are still in solution in ferrite of the hot band. Introduction Compact strip production (CSP) technology has some significant difference when compared with conventional cold charge rolling (CCR) process [1]. In the CSP process, the as cast slab, from the caster, are directly hot charged into a tunnel furnace. The coarse austenite as cast microstructure is retained at the start of the rolling process. A smaller total rolling reduction is available in the CSP process since 50~70mm thick slabs are adopted. Particularly in the case of microalloyed steel, the constitution of the as-cast austenite prior to rolling differs from that of austenite of thick slabs reheated for CCR process [2]. These factors affect microstructural development during the CSP process, the final microstructure and properties of the CSP product. The purpose of this research is to investigate the microstructural evolution of Nb, Ti complex microalloyed steel during the CSP process, including solidification structure, austenite grain size change, general microstructure and precipitation behavior of microalloying elements.
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Experimental steel and procedures Experimental steel. The chemical composition of the experimental steel is listed in Table 1. It is a low carbon Nb, Ti -containing microalloyed steel. The steel was produced in a 150-ton DC-EAF and final composition adjusted in the LF station. The average casting speed was 5.5 m/min. The slabs were placed into the tunnel furnace for 20 min. The average drop out temperature was 1150oC. Then the slabs were fed into a six stand rolling mill. Table 1. Chemical composition of the experimental steel (wt%) C 0.04 Si 0.24 Mn 1.26 Nb 0.07 Ti 0.016 V Al N S 0.001 P 0.018
0.006 0.034 80ppm
In this study, three samples were taken from different location of the CSP production line: the as cast slab prior to entering the tunnel furnace, the intermediate bar after stand F2 and the hot band. Their thicknesses are 53 mm, 16.5 mm and 7.5 mm, respectively. The first two pieces were water quenched. Some processing parameters are as follows: For stand F1: the entry temperature 1018 oC, exit temperature 1000 oC, rolling reduction 51%; for stand F2: the entry temperature 991 oC, exit temperature 970 oC, rolling reduction 36%. Optical microstructure. Full thickness metallographic samples were cut from 53 mm thick slabs parallel to the casting direction, 16.5mm thick intermediate bar and 7.5 mm thick hot band parallel to the rolling direction. The samples were etched with 2% Nital after grinding and polishing to reveal the overall microstructure. To reveal the solidification structure and austenite grain boundaries, the following etching reagent was used: 200ml of aqueous solution of picric acid, 2-4g of sodium dodecylbenzene sulfonate and 2-4ml of HCl. The assessments of the secondary dendrite arm spacing (SDAS) for the as cast slab and the average equivalent diameter of austenite grains for the as cast slab and the intermediate bar after stand F2 were evaluated using a computer control image analysis system, BioQuant IV, attached to an optical microscope. Precipitation analysis. The precipitation analysis was conducted using a TEM-JEM-200CX transmission electron microscope, to determine the type, size and location of precipitates. The samples were taken from surface and quarter thickness positions of the as cast slab, intermediate bar and the hot band. Thin foils in Φ3 mm diameter were prepared for TEM examination. Physical-chemical phase analysis for precipitates. This experiment was aimed at determining the distribution of microalloying elements in the steel, the amount and the size distribution of microalloyed carbonitride. The samples were taken from the as cast slab and the hot band. Specimens in shape of a bar, 6 mm in diameter and 100 mm in length, were machined. The specimens were first electrolytically extracted. The base metal was dissolved and all the remaining precipitates were extracted. Secondly, the particles containing Nb and Ti were isolated from the extracts. Nb, Ti contents in particles were then determined. The extracted particles were then identified by using X-ray diffraction. Finally, the particles containing Nb and Ti were subjected to a size distribution analysis by using a small angle X-ray scattering method. Results and Discussion Solidification structure. The solidification structure of as-cast slab consists of two regions. A region of about 4~5 mm in thickness, this zone is characterized by equiaxed grains. Below this zone, there is a columnar dendritic region which grows into the center of the slab. The width and the

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secondary dendrite arm spacing (SDAS) of the dendrite are smaller close to the surface and increases towards the center of the slab. Fig. 1a shows the typical dendrite morphology at the quarter point of the as cast slab. The quantitative evaluation of SDAS as a function of distance from the surface to the center is presented in Fig. 1b. For comparison purpose, the SDAS values from ref[3] are also shown in this figure. The results show that the SDAS of the experimental steel containing 0.07Nb-0.016Ti varies from 90 μm near the surface to 230 μm at the center of the as cast slab. These values are smaller than those of the steel containing 0.035Nb-0.004Ti, but very close to those of the steel bearing 0.063Nb-0.039Ti. This might mean that a certain increase of solute content such as Nb and Ti has an effect on reducing the width and the SDAS. However, the further increase of Ti content has no effect on reducing the size of SDAS.
a) Dendrite morphology at quarter point b) The SDAS vs. the slab thickness position Fig. 1 Dendrite morphology and SDAS in the as cast slab Austenite grain size As-cast slab. Fig. 2a shows the austenite grains exhibited in the as-cast slab. As expected, the austenite grain is large and has an elongated shape influenced by columnar dendrite. Fig. 2b shows its size distribution. The average equivalent diameter of austenite grains is 654±314 μm, which has a size range of 150~2000 μm. This figure is a little smaller than those of the steel containing 0.035Nb-0.004Ti and very close to those of the steel bearing 0.063Nb-0.039Ti [3]. It suggests that high Ti content in Nb-bearing steel has little influence on reducing austenite grain size of the as-cast slab.
a) Austenite grains at the quarter position b) Austenite grain size distribution Fig. 2 Austenite grains in the as cast slab

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Intermediate bar after stand F2. During the rolling process of the first two stands, the austenite grain size is remarkably refined by recrystallization. As shown in Fig.3a, at the surface is very fine austenite grain size whose average grain size is about 23 μm. Towards the center along the thickness, the austenite grain size increases gradually and reaches several tensμm at the center position. This heterogeneity is related to both austenite grain size distribution between the surface and center position in the as cast slab and the deformation distribution along the thickness direction during the rolling process. However, it is noted that there is a few much coarse area which appears from quarter to center position. The austenite grain in this area is much larger, as shown Fig. 3b. The average size is 116 μm. From the morphology of the austenite grains, it is suggested that it is attributed to incomplete recrystallization of austenite of as cast slab. Thus, in the intermediate bar after stand F2, the austenite grain size is not uniform whose average is in a size range of 23~116 μm. This heterogeneity can impact the subsequent deformation, up to the transformation product.
a) Surface b) Coarse grain area Fig. 3 Austenite grain structure in the intermediate bar after stand F2 Hot band. As mentioned above, the exit temperature after stand F2 is 971oC. This means, for the experimental steel containing 0.07Nb-0.016Ti, the deformation of the next four passes was performed in the range of non-recrystallization temperature. The total reduction for these passes is 55%, calculated according to the thickness change. The sample was firstly etched with an aqueous solution of picric acid to try to reveal the prior austenite grain boundary. As shown in Fig. 4a, the apparent elongated austenite grain boundaries can be seen clearly. The width of austenite grains increases from the surface (several μm) to the center position (~25 μm). This result is corresponding to the data obtained in the intermediate bar. A few coarse elongated austenite grains were also observed from the quarter to center position, as seen in Fig. 4b. Their width varies from scores of μm to more than one hundred μm.
a) Elongated austenite grain at quarter point b) Coarse elongated austenite grain Fig. 4 Prior austenite grain boundary in the hot band

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Optical microstructure in the hot band. The sample was etched with 2% Nital. Fig. 5a shows the microstructures of the steel which are non-polygonal ferrites. The ferrite grain size increases and the ferrite morphology also changes slightly from the surface to the center positions because of different width of elongated austenite grains. A few banded microstructure areas were found from the quarter to center positions along the thickness. Their morphology is shown in Fig. 5b. Their geometrical characteristics suggest that their origin can be related to some coarse elongated austenite grains, as shown in Fig.4b.
a) Optical microstructure b) Banded microstructure Fig. 5 Optical microstructure of the hot band, 2% Nital From the processing parameters and the results obtained, it is thought that the microstructural heterogeneity in the hot band is mainly related to the following reasons. Firstly, the austenite grain size prior to the deformation in the range of the non-recrystallization temperature (Tnr), that is after stand F2, is heterogeneous. The average from different areas varies from 23 to 116μm. Secondly, the total reduction below temperature Tnr is not big enough, which is just 55%. The calculation indicates [4] that, for equiaxed austenite grains in the range of 23~116μm, the effective austenite interfacial area Sv is within the range of 130~26mm-1. After experiencing 55% rolling reduction, the Sv increases to a range of 175~45mm-1. It can be seen that the total reduction of 55% below the temperature Tnr cannot give much increase of Sv. The significant difference of Sv between the fine and coarse austenite grains is clearly observed. The ferrite grain sizes produced by recrystallized and unrecrystallized austenite at various Sv are clearly shown elsewhere [4]. When Sv is large, the austenite grain volume can be consumed quickly by the ferrite nucleation and growth and fine ferrite grains is obtained. On the contrary, the small Sv (or the coarse austenite grains) promote the formation of hard phase, such as bainitic structure. In the case of Nb-bearing steel, when Nb in solution in austenite at the transformation temperature, it also has an important effect on the formation of low temperature transformation products[5]. The Nb distribution in the hot band is determined by using chemical phase analysis for precipitates and discussed below. This kind of microstructural heterogeneity is usually observed in the CSP processed Nb microalloyed steels [6]. In order to avoid this problem in the final products, it seems to be very necessary to obtain fully recrystallized autenite with the uniform grain size during the rolling process of the first two stands. Optimizing the deformation schedule design is also important for a satisfactory product of Nb microalloyed steel in the CSP process. Precipitation analysis As-cast slab. In the as-cast slab, three types of precipitates were observed depending on their size and morphology. Typical cuboidal TiN particles are mostly arranged in rows, as shown in Fig.

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6a. The average equivalent diameter is 24nm with a size range of 15 to 37nm. Most of them are in the range of 20-30nm. Very fine precipitates are randomly distributed in the matrix, as shown in Fig. 6b. Their sizes are mostly in the size range of 5 to 20nm. A few irregular precipitates exist with slightly larger size, seen in Fig. 6c. It seems that a coating is formed surrounding the pre-existing particle which may have different compositions.
a) TiN precipitates
b) Fine precipitates c) Irregular precipitates Fig.6 Precipitates in the as cast slab, dark field
Intermediate bar after stand F2. As shown in Fig. 7a, it seems that the other phase grows around some cubic TiN particles and forms the coatings or short arms. These particles have a shape of cruciform, rhombic or irregular shape. Their diagonal length is usually in the range of 50 to 85nm. Fine precipitates with a size range of 3-20nm are observed, as shown in Fig. 7b. These precipitates seem to form at dislocation lines and subgrains.
a) Complex precipitates, bright field b) Fine precipitates, dark field Fig. 7 Precipitates in the intermediate bar
a) Irregular precipitates b) Cruciform precipitates Fig. 8 Irregular or cruciform complex precipitates in the hot band, bright field

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Hot Band. In the hot band, some cubic TiN particles still exist. Their sizes are in the range of 20~35nm. Complex particles are most commonly observed. They are either in an irregular shape, as shown in Fig. 8a, or in a cruciform (star-like) or rhombic shape, as shown in Fig. 8b. The average equivalent diameter of the complex particles is 66 nm with a size distribution range of 40 to 120 nm. The EDAX analysis indicates that these particles contain much Nb and less Ti. Fine precipitates less than 18nm which can strength ferrite matrix are seldom seen in the hot band. The evolution of precipitation in the steel appears to be in agreement with reports regarding the solubility temperature of Ti and Nb carbonitrides. The solubility temperature is higher for Ti carbonitrides compared with Nb carbonitrides. TiN particles come out of solution first. Their distribution suggests their association with the primary solidification structure. Niobium-rich particles which form in low austenite temperature can precipitate on the facets of pre-existing particles or on their own [7]. The complex particles, cruciform or irregular, are also observed in V, Ti or Nb, Ti complex microalloyed steels produced by thin slab casting and direct rolling process [8, 9]. It is thought this effectively removes Nb or V in solution in austenite and strongly decreases the probability of Nb or V as small particles precipitated in ferrite. The study [9] indicates that the average size of complex particles in this experimental steel is much smaller than those of high Ti steel produced by the CSP process. Physical-chemical phase analysis for precipitates Precipitation phase analysis. Table 2 shows the results of quantitative analysis of the precipitates in the as cast slab and the hot band. According to this table, the Nb, Ti contents in solid solution can also be calculated. Table 2 Nb, Ti contents in M(CxNy) (wt%) Sample As cast slab Hot band M(CxNy) 0.0640 0.0558 Nb 0.0468 0.0385 Ti 0.0081 0.0091 C 0.0023 0.0016 N 0.0068 0.0066
With respect to Ti, Ti in the precipitates in the hot band is slightly more than that in the as cast slab. About 57% of the total Ti is in the M(CxNy) precipitates in the hot band. It is well know that TiN particles are stable when the as cast slab is held in the tunnel furnace at a temperature of 1120~1150oC. With respect to Nb, it can be seen that Nb precipitation takes place both in the as-cast slab and in the hot band. It appears that more Nb is associated with the precipitates in the as-cast slab than those in the hot band. This result is unexpected. It might suggest that apparent dissolution of Nb precipitates takes place in the tunnel furnace [3] and then Nb re-precipitats during the subsequent rolling and cooling process steps. The further study needs to be conducted. In the hot band, only 55% of the total Nb is tied up by precipitates, the rest is still in solution in ferrite. It means that these solute Nb exists in solution in austenite prior to γ→α transformation which has significant influence on obtaining non-polygonal ferrite in the final microstructure. Size distribution of the precipitates. The size distribution data of the precipitates is summarized in Fig.9. In the as cast slab, as shown in Fig. 9a, 70% of the total particles is in the size range of 1~36nm. Most of them are in the size range of 18-36nm. This result is in good agreement with the TEM observations that TiN particles are commonly observed in the as cast slab. Compared to that of the precipitates in the as cast slab, the size distribution of the precipitates in the

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hot band has changed. Most particles are in the size range of 36-96nm. This is corresponding to the formation of complex particles. Caps, foils or coatings of Nb-rich precipitates were formed on the surface of pre-existing TiN particles and led to the formation of larger irregular or cruciform particles. In the hot band, most of concern is about small particles which can strengthen ferrite matrix. It can be seen from Fig. 9b that their mass fraction (less than 18nm) is very small.
a) As cast slab b) Hot band Fig. 9 Mass fraction versus particle size Summary and Conclusions A systematic study of microstructure evolution of Nb-bearing microalloyed steel during the CSP process was conducted. The results of this study have shown that the austenite grain size in the as cast slab is rather coarse and has a large size range of 150 to 2000 μm. After the rolling of the first two stands, the austenite grain is apparently refined, but is heterogenous due to the incomplete recrystallization. The heterogeneous austenite grain size and less total reduction after stand F2 are the main reasons producing the microstructural heterogeneity in the hot band. Two major types of precipitates are in the as cast slab and the intermediate bar: cubic TiN particles and fine precipitates less than 20nm. Some complex (Ti,Nb)(C,N) precipitates are also found. In hot band, although 55% Nb of the total is tied up by microalloying carbonitrides, most particles are complex (Ti,Nb)(C,N) precipitates with a large size, in a shape of irregular or cruciform. The fine precipitates strengthening the ferrite matrix is very few in the hot band. 45% Nb of the total is still in solution in ferrite which is thought to have an important influence on obtaining non-polygonal ferrite in the final microstructure. References [1] [2] [3] [4] [5] [6] [7] [8] [9] P.J. Lubensky, S.L. Wigman and D.J. Johnson: Proc. Conf. on “Microalloying’ 95”, ISS, p. 225 R. Priestner: Materials Science Forum Vols. 284-286(1998), p. 95 C.I. Garcia, et al: Proc. Conf. on Thin Slab Casting and Rolling, Guangzhou, China, Dec. 3-5, 2002, The Chinese Society for Metals, p. 386 G.R. Speich, L.J. Cuddy, C.R. Gordon and A.J. DeArdo: Proc. Conf. on Phase Transformation in Ferrous Alloys, The Metallurgical Society of AIME, 1984, p. 341 A.J. Deardo: International Materials Reviews, Vol.48 (2003), No.6, p.371 P. Uranga,et al: 43rd MWSP Conf. Proc., Vol. XXXIX, 2001, ISS, p. 511 A.J. Craven, K. He, L.A.J. Garvie, T.N. Baker: Acta Materialia, Vol. 48 (2000), p. 3857 Y. Li, D.N. Crowther, P.S. Mitchell and T.N. Baker: ISIJ International Vol. 42 (2002), p.636 Ruizhen Wang: Research report, University of Pittsburgh, 2004

Microalloying for New Steel Processes and Applications doi:10.4028/https://www.360docs.net/doc/cb5724122.html,/MSF.500-501 The Microstructure Evolution of Nb, Ti Complex Microalloyed Steel during the CSP Process doi:10.4028/https://www.360docs.net/doc/cb5724122.html,/MSF.500-501.229