RESEARCHES ON THE PRESENCE OF HYDROGEN DURING CONTINUOUS CASTING OF STEEL

Transcription

1 46 THE ANNALS OF UNIVERSITY DUNĂREA DE JOS OF GALAŢI Paper present at International Conference on Diagnosis and Prediction in Mechanical Engineering Systems (DIPRE 09) October 2009, Galati, Romania RESEARCHES ON THE PRESENCE OF HYDROGEN DURING CONTINUOUS CASTING OF STEEL Claudiu TELETIN, Ion CRUDU, Constantin SPÂNU University Dunarea de Jos, Galati, ROMANIA ABSTRACT Due to the negative effects that hydrogen has on steels, its content in steels has lately been given special attention. The paper presents the evolution of hydrogen content in steel during the manufacturing and continuous casting process. The sources that contribute to increasing the content of hydrogen in steel are identified, and are mentioned some measures for its reduction. Some experimental results are presented, obtained both during casting, as the measurements were performed in the charging tundish of the continuous casting machine, and after casting, from the slabs consequential to casting. KEYWORDS: Hydrogen, continuous casting. 1. INTRODUCTION Continuous casting has evolved during the latest 40 years, distinguishing itself by productivity, quality and cost efficiency. Through this method 750 million tons of steel are casted every year [1, 2]. The steel is casted from the ladle into the charging tundish and afterwards into the mold. After leaving the mold the fluid steel is surrounded by a solidified shell [2]. On the basis of the systems theory, in a previous paper [3], it has been designed a model of the casting process structure: mold, guidance slab rollers, guidance- traction rolls and guidancetraction- soft reduction rolls. The model, presented in figure 1, contains input-output parameters, command and control, noted with I, E, C and K. Output parameters from a sector constitutes input parameters for the next sector. Both input-output as well as command and control parameters can be measurable or non-measurable. It is to be noticed, for each sector, the quantity of heat Q released. Some metals dissolve the hydrogen at high temperatures: iron up to 19 times, and palladium up to 875 times its volume [4]. When the content in hydrogen surpasses a certain value, the material is cracked. Hydrogen can reach into the steel through various ways [5]: - introducing it during steel making and casting (internal or metallurgical hydrogen); - through the surface of the metal, when manufacturing the metallic part and after this, due to environment conditions and mechanical stresses. The authors shall further present: - some aspects related to the evolution of the hydrogen content in steel during steel making and continuous casting; - sources that contribute to the increase in hydrogen content in steel and measures for its diminution; - measurements of the hydrogen content in fluid steel, performed on various quality steels; - measurements of hydrogen content in slabs obtained after casting.

2 THE ANNALS OF UNIVERSITY DUNĂREA DE JOS OF GALAŢI 47 f [H] - parameter of thermodynamic activity of hydrogen; [%H ] - hydrogen concentration; P H 2 - partial pressure of hydrogen. The resulting law of Sievert is: K[H] [%H ] = PH (3) 2 f[h] Degradation mechanism of steel by hydrogen can be attributed to hydrogen solubility in steel. Hydrogen solubility in liquid iron is higher than in solid steel (fig. 2). During liquid-solid transition phase, solubility has a sudden fall. After that, in the solid steel, it decreases with temperature and, at a normal temperature it is almost zero. That is why, liquid iron dissolves large quantities of hydrogen out of humidity, scrap iron etc., and during solidification and temperature fall, hydrogen is trapped in the crystal lattice of steel. Fig. 1. Model of the continuous casting process [3]. 2. EVOLUTION OF THE HYDROGEN CONTENT DURING STEEL MAKING AND CONTINUOUS CASTING PROCESS Primary sources of hydrogen for steel making and continuous casting are the air and the humidity [6]. During the process of steel making process, the hydrogen may derive from: - metallic and non-metallic raw materials (cast iron, scrap iron, iron ore, chalkstone, lime etc.); - the composition of the air within the converter (air, fuel burning products, oxygen blasted into the steel etc.); - ferro-alloys used for deoxidation and alloying. Diatomic gas H 2 can dissolve itself into the fluid steel in atomic form [7, 8, 9]: 1/ 2 {H 2 } = [H] (1) The equilibrium constant of the reaction is: a[h] f[h] K [H] = = [%H ] (2) PH P 2 H2 where: a [H] - thermodynamic activity of the hydrogen dissolved in steel; Fig. 2. Hydrogen solubility in pure iron as function of temperature [7]. It can diffuse and create hydrogen accumulation, especially in places where lattice defects and non-metallic inclusions are located. In such accumulations, hydrogen atoms recombine, resulting hydrogen molecules with bigger volumes than atoms, and when the pressure created by hydrogen molecules exceeds a critical point, the steel cracking takes place. This cracking, in favourable conditions, can spread, leading to a degradation of steels sensible to cracking (fig. 3). Fig. 3. General damage mechanism [7].

3 48 THE ANNALS OF UNIVERSITY DUNĂREA DE JOS OF GALAŢI Thus, figure 4 presents the cracks occurring around an oxide inclusion, due to brittleness by hydrogen, in a 3.5% Ni steel. During steel making, the hydrogen can be partially eliminated, while decarburating, when bubbles of CO are formed, and the hydrogen is trapped in the CO bubbles until its partial pressure in the bubbles reaches at equilibrium with the partial pressure of the hydrogen in the fluid steel. In CO bubbles the hydrogen recombines to form molecules. Diminution of the hydrogen content can also be obtained by blasting high purity argon into the steel, through the refractory porous plug mounted in the inferior cover of the ladle, converter etc. The mechanism of degassing by argon bubbling is the same as degassing by CO bubbles, while decarburizing. The degree of degassing by argon bubbling is bigger as the quantity of blasted argon is bigger. - ensuring a high enough decarburizing speed; - use of slags having minimal admissible basicity with respect to other processes (dephosphorization, desulphuration etc.); - maintaining the steel in the converter, ladle, etc. after decarburizing, as shorter a period as possible; - vacuum steel treatment, with argon blasting. The evolution of hydrogen content in steel during steel making and casting is presented in figure 5. During steel making, the content increases due to hydrogen taken mostly from ferro-alloys (in slab it can be found in the range of up to 5 8 ppm, whereas in ferroalloys it can reach ppm). During refining, the hydrogen content decreases, to increase again during evacuation and casting, due to hydrogen contamination. During solidification, the hydrogen content abruptly decreases, because of the decrease of hydrogen solubility into steel with temperature. To understand the mechanism of the rise in hydrogen content during the continuous casting, several studies have been carried out [7]. It has been noticed that the potential sources of hydrogen impurity during the casting are: - the packing sand for the sliding gate at the inferior part of the ladle; - casting tube (made of refractory materials); - refractory cover of the; - charging tundish covering agents; - casting powder; - the surrounding air; - water used for cooling the slab. Fig. 4. Cracks occurring around an oxide inclusion, due to brittleness induced by hydrogen, into a 3.5% Ni steel [9]. The best results are obtained by simultaneous treatment of melted steel with argon and in vacuum. According to Sievert s law (3), the solubility of hydrogen decreases with the pressure. Thus, while decreasing the pressure of the metallic bath, the hydrogen solubility also decreases, the surplus being eliminated from steel. Vacuum secondary metallurgy is often used in metallurgical industry, because of: - high quality of steel obtained by reaching a low content of carbon, oxygen, hydrogen, nitrogen and sulphur, decreasing the number of non-metallic inclusions and air holes; - very short treatment time (10 15 min), due to acceleration of reactions taking place in vacuum; - pollution level significantly diminished, as compared to other methods; - low for investments and raw materials expenses (the quantity of ferro-alloys used for deoxidization is greatly diminished) thus resulting a high productivity. Conditions that ensure obtaining a minimum content of hydrogen during steel making are: - use of raw materials with low content of humidity or hydrogen; Fig. 5. Evolution of hydrogen content in steel during steel making and casting [6]. The main sources of hydrogen impurity are the refractory cover of the charging tundish and the casting tubes. To eliminate the hydrogen contamination of the steel during casting, the main measures that need to be taken are: - preheating the tubes prior to casting; - preheating the charging tundish prior to casting; - using a refractory cover of the charging tundish with a more adequate chemical composition. Damages that may occur during steel casting and solidifying, due to high hydrogen content, are: bubbles, flakes, fish eyes, air holes, cracks and in certain situations even the cleavage of the material.

4 THE ANNALS OF UNIVERSITY DUNĂREA DE JOS OF GALAŢI 49 To prevent sudden cooling of slabs after casting and to ensure slow diffusion of the hydrogen in steel, these are cooled in stacks. solidification, the hydrogen analysis was performed during casting and after casting, on resulting slabs. 3. THE HYDROGEN CONTENT IN FLUID STEEL AND IN SLABS OBTAINED AFTER CONTINUOUS CASTING The hydrogen content was monitored for batches casted during one month, with the same casting machine, the measurements being performed using the Hydris unit, in the charging tundish. Usually, for each batch there have been performed two analyses. The results of the analyses are shown in table 1 and in the diagram in figure 6. Analysing the obtained results, it was found that the steels X70PSL2, X70 used in petroleum industry show a very low content in hydrogen (3.61 ppm, ppm), to prevent the pipes and oil plants damage, as a result of surpassing the critical concentration of hydrogen. For other steels, the average concentration of hydrogen is in the range of 6.35 ppm and ppm. Table 1. Average of the results for the hydrogen analyses for batches casted during one month period. Steel grade S355J2N S235J2N S235JRN X70 X70PSL2 ABSEH36 A537-CLS1 A516-GR70 H (ppm) To analyse the variation of the hydrogen content in the same steel batch, both in fluid phase and after Fig. 6. The hydrogen concentrations monitored for batches casted during one month. There were studied two X 65 steel batches. During casting there was performed the hydrogen analysis straight in the casting unit, by using the Hydris system. Samples of 80 X 25 X 20 mm were collected from various sections of slabs from the two batches, so as to carry out the hydrogen analyses in the solidified steel. After cutting the samples for hydrogen analysis, before performing the analysis proper, the margins of the samples were processed so as to obtain cylinders of 4 mm in diameter and of 20 mm in length. The samples were carefully monitored not to be impurified during the processing (with oils, grease etc) and not to be too much heated. To perform the hydrogen analysis, the cylindrical margins of the samples were cut. For each sample there have been performed two analyses, using the LECO TCH 600 automated analyser. The results of these analyses are shown in table 2. Slab and sample number Sample 21-1 Sample 21-2 Sample 21-3 Sample 13-1 Sample 13-2 Sample 13-3 Table 2. Experimental results on the hydrogen content for continuous casted slabs LECO Hydris analysis (%) analysis (%) Slab Slab

5 50 THE ANNALS OF UNIVERSITY DUNĂREA DE JOS OF GALAŢI Analysing the results obtained, it was found that the fluid steel contains a greater quantity of hydrogen than that observed in the steel from the tested slabs (fig. 7). In figure 7a it is presented the variation of the hydrogen content for slab coded with 21, and in figure 7b for slab coded with 13. Table 3. The concentration of the hydrogen for the samples taken from various section of the slab. Sample name Hydrogen (ppm) H H H a) For slab 21 Fig. 9. Hydrogen content for samples taken from various sections of the slab b) For slab 13 Fig. 7. Variation of hydrogen concentration for tested slabs In order to observe the hydrogen content from various sections of the slab, samples were taken from a S 355 steel slab. At the beginning three plates were transversally cut. Samples taken for this test are represented in figure 8: Fig. 8. Location of samples taken for hydrogen analysis into the transverse section of the slab. - sample in the middle (H28), from the slab central plate; - sample H26 located in the middle of the plate of the lateral side of the slab; - sample H21 located at the plate margin in the lateral side of the slab. The obtained results are shown in table 3 and are represented in the diagram in figure 9. The results of the analysis had shown that for the sample taken from the middle of the slab (H28), the hydrogen concentration is the lowest (1.2 ppm), whereas for the sample taken from the surface (H21), the hydrogen concentration is the highest (3.9 ppm). This difference can be the result of the hydrogen penetrating into the steel during casting upon contact with cooling water and with rollers, as well as the result of penetrating hydrogen within the slab surface right after solidifying. 4. CONCLUSIONS Researches have been carried out on the hydrogen content of the casted steels for one month period at a continuous casting machine, measurements being done in the charging tundish. For the X70PSL2 and X70 steel grades it has been found that the content of hydrogen is very low (3.61 ppm, ppm). For other steels, average concentration of hydrogen was in the range of 6.35 ppm and ppm. Studies carried out tested the presence of hydrogen, both at the beginning of casting as well as in the slabs obtained after casting. It was highlighted that for the samples taken from the fluid steel, the resulting concentrations are higher than those of the samples taken from the slab. The reason is because during solidification, the hydrogen solubility in steel decreased dramatically. If the cooling is done abruptly, there is a risk for the hydrogen accumulated in the slabs to produce its damage. Researches carried out related to hydrogen concentration in slabs (in different areas of the slab section). It was found that for the sample taken from

6 THE ANNALS OF UNIVERSITY DUNĂREA DE JOS OF GALAŢI 51 the middle of the slab, the hydrogen concentration is the lowest (1.2 ppm), whereas for the sample taken from the surface, the hydrogen concentration is the highest (3.9 ppm). This difference can be the result, on the one hand, of the hydrogen penetrating into the steel during casting through the superficial layer of the slab, when making contact with the cooling water and with the rollers, hydrogen that was not eliminated during slab solidifying and cooling, and on the other hand to hydrogen penetrating on the slab surface after solidification. ACKNOWLEDGMENTS The research has been performed within the Research Programme CEEX no. 84/2005, in partnership with the University Al. I. Cuza Iassy, S.C. MITTAL STEEL S.A. Galati and S.C. UZINSIDER ENGINEERING S.A. Galati. REFERENCES 1. Thomas B. G., 2001, Continuous Casting of Steel, Chapter 15 in Modeling for Casting and Solidification Processings, O. Yu, editor, Marcel Dekker, New York, NY, pp Teletin C., Crudu I., 2007, Hydrogen presence in the continuous casting process of steel, The Annals of the University Dunărea de Jos of Galaţi, Fascicle II, pp Crudu I., Teletin C., Bendrea C., Spânu C., 2008, A tribological model for steel continuous casting equipment, 6 th European Conference on Continuous Casting, Riccione, Italy, 3-6 June Neniţescu C.D., 1980, Chimie generală, Editura Didactică şi Pedagogică, Bucharest (in Romanian). 5. Hurst Ch.R., Vergauwens Ir.M., 2004, Gas Analysis in Steel: Identifying, Quantifying, and Managing Hydrogen Pick, Annual Australian Foundry Institute Conference, Adelaide, pp Dragomir, I., 1985, Teoria proceselor siderurgice, Editura Didactică şi Pedagogică, Bucharest (in Romanian). 7. Lachmund H., Schwinn V., Jungblut H. A., 2000, Heavy plate production: demand on hydrogen control, Ironmaking and Steelmaking, Vol. 27, No. 5, pp Turkdogan E. T., Fruehan R. J., 1998, Fundamentals of Iron and Steelmaking, The AISE Steel Foundation, Pittsburgh, PA, USA. 9. Bernstein M., Pressouyre G. M., 1985, The Role of Traps in the Microstructural Control of Hydrogen Embrittlement of Steels, Cap. 25, Hydrogen Degradation of Ferrous Alloys, Edited by Richard A. Oriani, New Jersey, U.S.A. 10. Pumnea C., Sorescu Fl., Dima I. Niculescu T., Dumitru M., 1988, Tehnici speciale de analiză fizico-chimică a materialelor metalice, Editura Tehnică, Bucharest (in Romanian). 11. Glitscher W., 2005, Milestones of process control in ferrous metallurgy. Past, today and future, La metallurgia italiana, No , pp Wuytens D. R., 2006, Eindwerk Informaticabeheer, Heraeus Electro-Nite.