MACROSCOPIC INVESTIGATIONS OF THE PRIMARY STRUCTURE OF CONTINUOUS CASTING INGOTS

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1 Act Metllurgic Slovc, Vol. 20, 2014, No. 2, p MACROSCOPIC INVESTIGATIONS OF THE PRIMARY STRUCTURE OF CONTINUOUS CASTING INGOTS Zdzisłw Kudliński 1), Krystin Jniszewski 2)* 1) Silesin University of Technology, Deprtment of Mterils Science nd Metllurgy, Institute of Metllurgicl Technology Ktowice, Polnd 2) Silesin University of Technology, Deprtment of Mterils Science nd Metllurgy, Institute of Metllurgicl Technology, Ktowice, Polnd Received: Accepted: * Corresponding uthor: e-mil: krystin.jniszewski@polsl.pl, Tel.: , Deprtment of Metllurgy, Silesin University of Technology, Krsińskiego 8, Ktowice, Polnd. Astrct This pper presents the results of mcroscopic investigtions of the structure of continuous csting ingots depending on the kinetics of solidifiction nd speed of csting of low cron steel intended for welding wires. We hve determined the L k length of solidifiction pth of ingots of 140 x 140 mm cross-section. We hve lso performed mcroscopic investigtions of the ingot structures together with identifiction of shrinkge porosity defects nd xil porosities. microscopic investigtions of xil ingot zones, res of shrinkge defects nd xil porosity, hve demonstrted tht the non-metllic phse (silictes nd lumintes) presented there stnds in the wy of welding during the process of rolling of continuously cst ingots. Keywords: nlyticl electron microscopy, microstructure, continuous csting, solidifiction, ingot structure 1 Introduction Defects of the primry structure of continuous csting ingots, including porosity nd xil shrinkge defects, occurring concurrently with typicl zones of crystlliztions, re their importnt components, which form the non-continuity zones in the mteril. These defects hve especilly negtive impct in cse when continuous csting ingots re intended for production of metllurgicl steel products with very smll cross-section res. Such products include welding wires of dimeters from 1.5 to 3.0 mm. The occurrence of wire rekge oserved in the process of wire drwing from the rolled mteril produced of the continuous csting ingots, is the result of the shrinkge porosity defects nd xil porosity. The resons why the mentioned mcroscopic defects of continuous csting ingots occur, cn minly e looked for in improper selection of prmeters for the process of csting of the given steel sort, which prmeters determine the ingot solidifiction kinetics nd do not ssure proper moment of solidifiction in the continuous csting mchine, in the secondry cooling zone. This especilly regrds to the rdil nd curved (rced) pieces of equipment. Liquid steel filtrtion, rodly descried in literture, cn e n lterntive in the process of elimintion oserved in xil zone of continuous csting ingots of non-metllic phse [1-11].

2 Act Metllurgic Slovc, Vol. 20, 2014, No. 2, p Estimtion of solidifiction kinetics of 140 x 140 mm continuously cst ingots contriutions Essentil prt of the estimtion of solidifiction kinetics of the 140 x 140 mm ingot of G3Si1 steel (Tle 1) is the determintion of L k length of solidifiction pth in reltion to the design chrcteristics of the used CSC mchine: L m metllurgicl length of the mchine ( distnce etween steel tle level in the crystllizer nd the point of ingot cutting) nd L h length of the secondry cooling zone Fig. 1. Tle 1 Chemicl composition of G3Si1 steel, %. C Mn Si P S Cr Ni Al (c) C (m) O 2 N 2 min ,30 0,75 mx 0,09 1,55 0,95 0,02 0,02 0,15 0,10 0,02 0,001 0,01 0,01 Fig. 1 Illustrtion of solidifiction pth of continuously cst ingot [12] In prctice the solidifiction kinetics of continuously cst ingots cn e expressed with use of Stefn [13] formul: d w K (1) K where: d w [cm] thickness of the solidified lyer of the ingot, K [cm/min 0,5 ] solidifiction speed constnt, τ [min] - solidifiction time. When prmeters of the continuous csting process re tken into ccount, the formul (1) otins the following form: d L K w K (2) vodl where: L k [cm, m] - length of ingot solidifiction pth, V odl [m /min] - csting speed.

3 Act Metllurgic Slovc, Vol. 20, 2014, No. 2, p If we define the ingot solidifiction pth length s distnce etween the level of steel tle in the crystllizer nd the point where the ingot ecomes solidified (Fig. 1), then its vlue will e expressed y the formul: L K 2 Dw vodl (3) 2 4 K where: D w [m] dimension of the squre ingot side (equl to 2d w ). The CSC mchine used in csting of the investigted squre ingots, hve hd the following design chrcteristics [14-17]: length of the secondry cooling zone L h = 9,5 m metllurgicl length L m = 29,7 m distnce of the ingot strightening 13,7 m from steel tle in the crystllizer, recommended ingot csting speeds for 140x140 mm products, V odl = 3,1-3,4 m/min. For this nlysis we hve ssumed the vlues of K solidifiction constnt nd csting speed, which result from the csting technology used worldwide. The results of clcultions of the solidifiction pth length re presented in Tle 2. The otined results indicte tht only in one cse (for K =3,0 cm/min 0,5 nd V odl =2,0 m/min) the ingot ecomes solidified in the terminl re of the secondry cooling zone. Ech tril to increse csting speed cuses the solidifiction pth length to e incresed. Tle 2 Pth of solidifiction of 140 mm squre ingot The decrese in csting speed, K, cm/min 0,5 Ingot csting speeds, 1,8 2,0 2,2 2,4 2,6 2,8 V odl, m/min Length of ingot solidifiction pth L K 3,0 2,0 30,25 24,50 20,25 17,01 14,50 12,50 10,88 2,2 32,27 26,95 22,27 18,72 15,95 13,75 11,98 2,4 26,30 29,40 24,30 20,42 17,40 15,00 13,07 2,6 39,32 31,85 26,32 22,12 18,55 16,25 14,15 2,8 42,34 34,30 28,35 23,82 20,29 17,50 15,24 3,0 45,37 36,75 30,37 25,52 21,75 18,75 16,33 3,2 48,39 39,20 32,40 27,22 23,19 20,00 17,42 3,4 51,42 41,65 34,42 28,92 24,65 21,25 18,51 3,6 54,44 44,10 36,45 30,62 26,09 22,50 19,06 3,8 57,42 46,55 38,47 32,33 27,55 23,75 20,69 L h = 9,5 m, L m = 29,7 m When continuously cst ingot with non-solidified internl prt psses to its horizontl position then formtion of xil porosity nd shrinkge porosity defects will lwys e the result. If mcrostructure of continuously cst ingots is the priority, then incresing csting speed cn not e deemed s the wy to increse the CSC mchine performnce. 3 Results of mcrostructure investigtion of 140x140 mm continuous csting ingots For these investigtions we hve selected ingots from melts chrcterized y significnt differentition of product rejections resulting from shrinkge defects nd xil porosity occurring during ingot rolling nd wire drwing. Selected exmples of mcrostructure re illustrted in Fig Increse of csting speed in ech cse hs negtively ffected the product rejection index.

4 Act Metllurgic Slovc, Vol. 20, 2014, No. 2, p Fig. 2 Mcrostructure of the crosswise () nd lengthwise () sections of continuously cst ingots (2/2). Melt 739, V odl = 2,8 m min -1, T p = 34ºC, level of rejections of steel per melt 3,88 %. Fig. 3 Mcrostructure of the crosswise () nd lengthwise () sections of continuously cst ingots (1/9). Melt 739, V odl = 2,8 m min -1, T p = 34ºC, level of rejections of steel per melt 3,88 %. Fig. 4 Mcrostructure of the crosswise () nd lengthwise () sections of continuously cst ingots (3/1). Melt 245, V odl = 3,0 m min -1, T p = 35ºC, level of rejections of steel per melt 11,04 %.

5 Act Metllurgic Slovc, Vol. 20, 2014, No. 2, p Fig. 5 Mcrostructure of the crosswise () nd lengthwise () sections of continuously cst ingots (2/20). Melt 245, V odl = 3,0 m min -1, T p = 34ºC, level of rejections of steel per melt 11,04 %. Fig. 6 Mcrostructure of the crosswise () nd lengthwise () sections of continuously cst ingots (1/9). Melt 244, V odl = 3,3 m min -1, T p = 23ºC, level of rejections of steel per melt 14,35 %. 4 Conclusions Following conclusions cn e drwn on the sis of the performed investigtions: the significnt level of rejections in rolled steel wires 5.5 mm in dimeter nd drwn wires hs een cused y shrinkge defects nd xil porosity of continuously cst ingots, the decrese in csting speed from 3.3 to 2.8 m/min hs positively influenced the level of rejections of steel products. References [1] K. Jniszewski: Steel Reserch Interntionl, Vol. 84, 2013, No. 3, p , DOI: /srin [2] K. Jniszewski: Archives of Metllurgy nd Mterils, Vol. 58,2013, No. 2, p [3] K. Jniszewski: Metlurgij, Vol. 52, 2013, No. 1, p [4] K. Jniszewski, Z. Kudliński: Steel Reserch Interntionl, Vol. 77, 2006, No. 3, p

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