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1、毕 业 设 计 外 文 翻 译 专 业： 冶金工程 班 级： 冶金一班 Basic oxygen furnace based steel-making processes and cleanliness control at BaosteelL. Zhang*1, J. Zhi2, F. Mei2, L. Zhu2, X. Jiang2, J. Shen2, J. Cui2, K. Cai3 and B. G. Thomas4Optical microscopy, total oxygen measurements and slime tests have been conducted to
2、quantify the size distribution and amount of inclusions at various processing steps during basic oxygen furnace (BOF) based steel production at Baosteel. The effects on steel cleanliness of specific operational improvements during steel refining and continuous casting have been investigated. Such im
3、provements to these processes and the resulting level of steel cleanliness at Baosteel are summarised in the present paper. Ladle slag reduction lowers FeO + MnO in the slag to below 5%, decreasing steel reoxidation by the slag. Calcium treatment by CaSi wire injection during ladle furnace (LF) refi
4、ning is used to modify inclusions . Slag detection is employed at the ladle bottom during continuous casting. Flow control devices, CaO containing filters and high CaO based basic powder with CaO/Si024 are used in the tundish to remove more inclusions. With this BOF based steelmaking process, impuri
5、ty levels can be controlled to achieve-total oxygen (TO)16 ppm, S5 ppm, P35 ppm, N29 ppm, H1 ppm in line pipe steels, and C16 ppm, TO19 ppm, N15 ppm in interstitial free (IF) steels.Keywords: Clean steel, Inclusions, Impurity elements, Interstitial free steel, Line pipe steelIntroductionThe importan
6、ce of clean steel in terras of product quality is increasingly being recognised. Clean steel requires control of the size distribution, morphology and composition of non-metallic oxide inclusions in addition to the amount. Furthermore, sulphur, phosphorus, hydrogen, nitrogen and even carbon1,2 shoul
7、d also be controlled to improve the steel properties. For example, ,formability, ductility and fatigue strength worsen with increasing sulphide and oxide inclusion contents. Lowering the carbon and nitrogen enhances strain aging and increases ductility and toughness. Hardenability and resistance to
8、temper embrittlement can be enhanced by reducing phosphorus. The definition of clean steel varies with the steel grade and its end use. For example, interstitial free (IF) steel requires both carbon and nitrogen to be 30 ppm; line pipe steel requires sulphur, phosphorus and total oxygen (TO) all to
9、be 30 ppm，low hydrogen, low nitrogen and suitable Ca/S and bearing steel requires the total oxygen to be less than 10 ppm.3 In addition, many applications restrict the maximum size of inclusions 3,4 , so the size distribution of inclusions is also important. Baoshan Iron & Steel Co., Ltd (Baosteel)
10、is currently the largest steel company in China. Its annual steel production was 115 million tonnes in 2003, 119 million tonnes in 2004 and 14.0 million tonnes in 2005. With regard to the basic oxygen furnace (BOF) based steelmaking route, there are three 300 t and two 250 t BOFs; several steel refi
11、ning units, including one CAS-OB unit (controlled argon stirring-oxygen blow), two RH (Ruhrstahl-Heraeus) degassers and one ladle furnace (LF). Since 1990, efforts to improve steel cleanliness have focused on developing steelmaking practices to lower TO, N, S, P, H and C levels to achieve low carbon
12、 aluminium killed (LCAK) steel. For LCAK steel and IF steel, the production process is BOFRHcontinuous casting (CC), and for line pipe steel, the process is BOFRHLFCC.Experimental method and examination of inclusions in steelExperimental methodsLadle steel samples were taken 500-600 mm below the top
13、 slag in the ladle, tundish steel samples from 300 mm above its outlet and mould steel samples from 150 mm below the meniscus and 300 mm away from the submerged entry nozzle (SEN) outports. The sampler was a cylindrical steel cup with a cone shaped copper cover to protect it from slag entrainment du
14、ring immersion. Attached to a long bar, the sampler was immersed deep into the molten steel, where the copper melted and the cup was filled. Small steel samples , 80mm in length and 30mm in diameter, were machined into 5 (dia.) x 5 mm cylinders for TO and nitrogen analysis, and 20 (dia.) 15 mm cylin
15、ders for microscope examination. The steel powders resulting from machining were used for analysis of the carbon, phosphorus and sulphur contents. Large Steel samples from the ladle and tundish, 200 mm in length and 80 mm in diameter, were machined into 60 (dia.) 150 mm cylinders; as shown in Fig. 1
16、. TO and nitrogen measurement. Analysis included the chemical composition of slag and steel samples, microscope examination of microinclusions, slime extraction of macroinclusions and SEM analysis of the morphology and composition of inclusions. Fig.1 Sampling locations for continuously cast slab: T
17、O total oxygenIn the present work, macroinclusions were those greater than 50 um in diameter. Most of these were detected in the residues extracted by electrolytic isolation (slime test) from the larger steel samples. The microinclusions data derive from microscopic assessments carried out on planar
18、 sections, most of which were smaller than 50 mMorphology and composition of typical inclusions The morphology ,composition and likely sources of typical inclusions found in LCAK steel samples form the ladle ,tundish and mound are shown in Figs.2 and 3 respectively.The morphologies included: (a) ang
19、ular aluminate(Fig.2 d and f and Fig.3b);(b)alumina cluster (Fig.2b and c);and (c) spherical silicate (Fig. 2a and c and Fig. 3a). a. ladle; b. tundish; c,d. mound; e,f. slab Fig.2 Typical inclusions from given samples examined by microscope (a) tundish (b) slabFig. 3 Typical inclusions from given s
20、amples extracted using slime method The possible sources were deoxidation products, reoxidation products or broken refractory lining bricks. In line pipe steel, besides these common inclusions, many nanoscale TiN inclusions were found along grain boundaries. These nano TiN changed from square to ell
21、ipsoid if combined with Ti2O3 , as shown in Fig. 4 5 a . compound inclusions with composition Ti2O3+MnS ; b. TiN inclusion Fig.4 Nanoprecipitates in line pipe steelTotal oxygen measurement is an indirect method of evaluating oxide inclusions in a steel.3 The total oxygen (TO) in the steel is the sum
22、 of the free oxygen (dissolved oxygen) and the oxygen combined as non-metallic inclusions. Free oxygen, or active oxygen, can be measured relatively readily using oxygen sensors. It is controlled mainly by equilibrium thermodynamics with deoxidation elements, such as aluminium. If %A1 =0.03-0-06, th
23、e free oxygen is 3-5 ppm at 1600C. Because the free oxygen does not vary much, the total oxygen is a reasonable indirect measure of the total amount of oxide inclusions in the steel. Owing to the small population of large inclusions in a steel and the small sample size for TO measurement (normally 2
24、0 g), it is rare to find a large inclusion in a sample. Even if a sample contains a large inclusion, it is probably discounted because of the anomalous high reading.Thus, the TO content actually represents the level of 50 um small oxide inclusions only. The current TO in IF and line pipe steel slabs
25、 at Baosteel is 16 ppm. The TO in the ladle, tundish, mould and slab in two typical sequences of LCAK steel is shown in Fig.5 , indicating that the TO decreased from the ladle to the tundish, to the mould and to the continuously cast slab. Fig.5 Total oxygen in steel from ladle to slab Ladle operati
26、ons to remove more inclusionsLadle slag reduction treatmentReoxidation to form alumina in the ladle during steel refining is mainly caused by Si02 in the slag and lining refractory, and MnO and FeO in the ladle slag, by the following reactions:3/2(Si02) + 2Al=(Al203) + 3/2Si 3(MnO) + 2Als = (Al203)
27、+ 3Mn 3(FeO) + 2Als = (A1203) +3FeSlag reduction treatment is carried out by adding aluminium and lime onto the top of the ladle slag to reduce its FeO and MnO content. The effect of ladle slag reduction treatment on the TO content in the steel is shown in Fig. 6. A larger FeO + MnO content in the l
28、adle slag corresponds to higher total oxygen. With the slag reduction treatment, MnO and FeO in the ladle slag were reduced to 5%, corresponding to 25 ppm in order to prevent solid alumina based inclusion clogs . Too much calcium can also generate CaS with a high melting point (2450C). Too much sulp
29、hur in the steel and too low a temperature also enables CaS generation. Baosteel practice indicates that 0.09 favours prevention of nozzle clogging (Fig. 17). Hence, Ca needs to be controlled within the range 25-50 ppm, and Ca/Al0-09, to avoid nozzle clogging problems.Control of nitrogen, carbon, su
30、lphur and phosphorus in steelNitrogenNormally, a large nitrogen content at tapping tends to result in a large nitrogen content in the slab. Thus, the control of nitrogen should mainly focus on lowering the nitrogen content during BOF blowing and preventing nitrogen pickup during tapping, steel refin
31、ing and continuous casting. Currently, at Baosteel, nitrogen during BOF steelmaking fluctuates from 11 to 43 ppm. Plant experiments indicate that when N is less than 30 ppm before RH treatment, N cannot be lowered further by RH treatment. Oxygen pickup is always many times greater than the measured
32、nitrogen pickup, owing to its faster absorption kinetics at the air/steel interface.7 In addition, nitrogen pickup is faster when the oxygen and sulphur contents are low.8 Thus, to reduce nitrogen pickup, deoxidation is best carried out after tapping, which is the current practice for clean steel gr
33、ades at Baosteel. The current nitrogen content of IF steel and line pipe steel slabs is 15-30 ppm, and nitrogen pickup from ladle to tundish can be controlled below 1.5 ppm by optimised shrouding, argon gas injection and fibre sealing at the tundish and SEN.CarbonThe greatest decarburisation is for
34、IF steel by converter treatment which can reach to higher than 90% and further decarburization for IF is by RH. Techniques to improve this operation include:(1) optimising initial C and O before degassinginto the ranges of 500-650 ppm and 300-450 ppm, respectively.(2) enlarging the snorkel diameter
35、from 500 to 750 mm and increasing the argon flowrate from 1000 to 3000 NL minT-1. After this treatment, C can be lowered to10ppm.The C pickup occurring during continuous casting is controlled below 6 ppm by the following techniques: (4)using low carbon, high viscosity mould flux, decreasing carbon p
36、ickup at the continuous casting mould from 5.5 to 1.8 ppm. (5) using carbon free ladle refractory lining（6）using high basic, low carbon tundish flux (CaO/SiO24)SulphurThe initial sulphur content of the molten iron at Baosteel is 200 ppm. After hot metal desulphurisation by injection of CaC2 powder o
37、r magnesium based powder, the sulphur decreases to 10-30 ppm. It is important to remove the top slag quickly after desulphurisation in order to decrease sulphur pickup. During the BOF steelmaking process, there is 10-30 ppm sulphur pickup, mainly from lime and scrap. To achieve an ultralow sulphur c
38、ontent, especially for line pipe steels, three desulphurisation methods during the steel refining process have been developed:(1) CaO-CaF2 flux is added to the vacuum chamber through the alloy addition hoppers; slag carryover from the BOF is controlled carefully for these heats, and ladle slag reduc
39、tion treatment is carried out to decrease the FeO content in the slag before steel desulphurisation; S is lowered from 28.4 to 16.2 ppm (2) After strong deoxidation, CaO-CaF2 powder is injected into the molten steel in the ladle by a lance below the up snorkel; S can be lowered from 61.9 to 35.8 ppm
40、 (3) A suitable emulsification condition, improving the reaction between slag and molten steel; S can be lowered from 67.0 to 8.7 ppm. This method can lower the sulphur below 10 ppm, so is currently used for the production of ultraclean line pipe steel. It should be noted that the MgO-CaO refractory
41、 lining and tundish flux may also remove some sulphur by the following reaction:(CaO)+2/3Al+S=(CaS)+1/3Al2O3PhosphorusFive different processing routes are used to achieve low phosphorus steel at Baosteel: (1) de-Si, de-P and de-S at hot metal treatment,followed by BOF steelmaking with a small slagco
42、ntent, lowering P to 120 ppm .(2) de-S at hot metal treatment, then the BOFprocess with a large slag content, lowering P to100 ppm.(3) de-Si, de-P and de-S at hot metal treatment,followed by BOF steelmaking with a large slagcontent, lowering P to66 ppm .(4) de-P at hot metal treatment, followed by B
43、OFsteelmaking with a large slag content, lowering P to 58 ppm .(5) double BOF steelmaking process, achieving 20 ppm P in the steel. The control of impurity elements at Baosteel has improved considerably during the past 15 years, as indicated in Table 1. Baosteel steel can now achieve TO16ppm, S5 ppm
44、, P35 ppm, N29 ppm, H1 ppm in line pipe steel, and C16 ppm, TO19ppm, N15ppm in IF steel. Currently, S + P + TO + N + H in line pipe steel can be maintained below 85-5 ppm, and C + TO + N in IF steel can be kept below 50 ppm.Table 1 Impurity content of line pipe steel and interstitial free (IF)steel achieved at Baosteel,ppm Line pipe steel IF steel S P TO N H C N TO1996 32 134 35 47 2 50
