外文翻译宝钢碱性氧气转炉炼钢生产及洁净钢控制.docx

1、外文翻译宝钢碱性氧气转炉炼钢生产及洁净钢控制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 quantify the s

2、ize 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 improvements to

3、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) refining is used t

4、o 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, impurity levels can

5、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 importance of clean st

6、eel 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 should also be cont

7、rolled 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 temper embritt

8、lement 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 be 30 ppm，low

9、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) is currently t

10、he 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 refining units, in

11、cluding 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 aluminium kil

12、led (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 slag in the l

13、adle, 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 during immersion

14、. 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 cylinders for micro

15、scope 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. TO and nitro

16、gen 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: TO total oxygen

17、In 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 sections, mos

18、t 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) angular aluminate

19、(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 samples extract

20、ed 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 ellipsoid if comb

21、ined 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 of the free o

22、xygen (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, the free oxygen

23、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 20 g), it is ra

24、re 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 at Baosteel i

25、s 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 operations to remove

26、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) + 3Mn 3(FeO) +

27、 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 ladle slag corr

28、esponds 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 sulphur in the ste

29、el 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, sulphur and phosphorus i