METALLURGICAL AND CHEMICAL QUALITY OF LOW-ALLOY CONSTRUCTIONAL CAST STEEL VS. MECHANICAL PROPERTIES

Transcription

1 METALLURGICAL AND CHEMICAL QUALITY OF LOW-ALLOY CONSTRUCTIONAL CAST STEEL VS. MECHANICAL PROPERTIES Dariusz BARTOCHA, Jerzy KILARSKI, Jacek SUCHOŃ, Czesław BARON, Jan SZAJNAR, Krzysztof JANERKA, Wojciech SEBZDA Abstract Foundry Department, Silesian University of Technology, ul. Towarowa 7, Gliwice, Polska Production of constructional casting competitive for welded structure of high-strength steel first of all required high metallurgical quality of cast steel. Assumptions, methodology and results of investigation which the aim was determination of the most advantageous: configuration and parameters of metallurgical treatments and ways to modify, in aspects of reach the low-alloy cast steel of the highest quality as possible, are presented. A series of low-alloy cast steel melts modeled on cast steel L20HGSNM was performed, the way of argoning in laboratory induction furnace with a capacity of 50kg was worked out, modifications with additions of FeNb, FeV and master alloy MgCe were performed. During each melts samples of cast steel direct from metal bath were get and series of experimental casting was made. Chemical compositions of melted cast steel, contents of O, N and H were determined. Moreover in the article the influence parameters of quenching on mechanical properties of low-alloy structural cast steel, are presented. An attempt to quantify this relationship was made. Keywords: Construction cast steel, metallurgical processing, modification, argon, microstructure, mechanical properties 1. INTRODUCTION, RESEARCH PLAN. Among the construction cast alloy (PN-90/H-83161) and resistant to abrasion steels (PN-88/H-83160) the highest mechanical properties i.e. Rm and Re has the L20HGSNM cast steel. High, according to the standards, Rm>1300 MPa and Re>1100 MPa, actually predisposes it to be used on heavily loaded machine parts. In contrast to steel, the only way to improve the properties and structure of cast steel is heat treatment with appropriate parameters, which makes this treatment extremely important in the case of construction castings. Not without significance is the chemical composition of metallic and non-metallic impurities and the structure of the original cast. The term "purity" in steelmaking is variable in time. It means something different for open-hearth process, another thing for the production of a few decades ago, and also something else for the present steel. Understanding of this term varies with the development of smelting and refining technology and materials used. The purity of steel is affected by such factors as: the content of harmful components (additives), chemical and metal phase inhomogeneity (segregation), content, form, location, size and type of nonmetallic inclusions. Currently, the prevailing tendency is to consider the purity in two matters: chemical purity, metallurgical purity.

2 Tab. 1. Melting plan Operation Taking sample Deoxidation Argon FeNb modification FeV modification Taking sample Pouring sample cast I Deoxidation Argon FeNb modification FeV modification Adding MgCe master alloy Taking sample Pouring sample cast II Chemical purity is bound to the chemical composition and the content of harmful elements. The concept of chemical purity increases with the increase of recycling of steel, particularly in the process carried out in an electric arc furnace. Metallurgical purity is associated with the occurrence of non-metallic inclusions, which have a negative impact on the process of casting, machining and the final properties. Of particular importance is whether the inclusions are "soft" or "hard" in comparison with the metallic matrix. The experimental plan included the implementation of four two-stage cast steel melts with the chemical composition of cast steel L20HGSNM. Planned activities within each melt from the overheating to the addition of ferro-alloys is presented in Table MATERIAL As mentioned above the tested steel was based on the L20HGSNM cast steel with the chemical composition as set out in the Polish standard, which was presented in Table 2. Basic charge materials were the two, previously prepared steel melts, Table 3 presents the chemical compositions of the charge materials and auxiliary inputs defined as Cast I and Cast II. The aim was to determine the effect of the main input material constituting almost 98% of the entire charge on the chemical composition and impurity content of the test cast steel. Tab. 2.Chemical composition of L20HGSNM cast steel [%] C Si Mn P S Cr Mo Ni 0,18-0,25 0,7-1,0 0,8-1,1 <0,035 <0,03 0,6-0,9 0,1-02 0,9-1,2 Tab. 3. Chemical composition of charge materials [%] C Si Mn P S Cr Mo Ni Al Cu V Nb C. steel I 0,18 0,6 0,18 0,02 0,008 5 CeM M Mg N [ppm] O [ppm] H [ppm] 1,6 0,77 1,69 0,03 0, <0,6 C. steel II 0,14 0,48 0,4 0,025 0,03 0,75 0,5 0,8 0,06 0, <0,6 FeMn- HC 7,00 1,50 75,00 0,250 0,030 FeMn-LC 1, ,93 0,1 0,001 FeNb 1,04 0,12 0,1 65,2 FeV 0,06 0,5 0,08 0,02 0, ,3 MgCe 42,6 0,7 1, RESEARCH Melts were carried out in an acid lining induction furnace with a capacity of 50 kg, the mass of melted cast steel was 40 kg. The individual melts were carried out in accordance with the adopted plan (Table 1) a detailed course of the casts was as follows. 1. The melting of the major charge materials Cast I and Cast II, together with slag-making materials,

3 2. The addition of FeMn-HC and Ni, start of the protective argon blast at liquid metal mirror, 3. Overheating of the melt above 1600 o C, 4. Taking a sample with the T.O.S immersion probe from Heraeus Electro-Nite(Fig. 1), Fig. 1. A system for sampling of cast steel for the determination of the total oxygen content, Heraeus Electro-Nite company 5. Deoxidation of the melt by placing in the furnace 0.05 kg of AL and 0.06 kg FeMn-LC, 6. Purging out the melt with argon lance placed from the top at 90 degrees angle to the liquid metal surface (excluding melt No. 1 where at this stage argon was not applied). Based on the analysis of argon process parameters used in steelworks ([1]) and several attempts it was decided to use argon for 1 min with the flow of 10 l / min in all cases. In the melt No. 1 the use of argon was not performed to determine the impact of the deoxidation combined with argon and without argon, the decline the amount of O, N and H in the cast steel. The operation of argon use was conducted in small tanks as an independent refining process has a negligible effect on reducing the oxygen content. It reduces its content only when the initial amount of metal is high enough, in the other case the oxygen content increases as the nitrogen content [2]. 7. After overheating the melt above 1600 o C and the time of 2 minutes, the next sample was retrieved, 8. The introduction of modifying additives, a) Melt No.1 without additives, b) Melt No. 2 addition of 0,04kg FeNb, c) Melt No. 3 addition of 0,04kg FeV, d) Melt No. 4 addition of 0,04kg FeNb and 0,04kg FeV. Ferro-niobium and iron-vanadium were used as additives, due to the relatively high so-called modifying activity of niobium and vanadium [3], furthermore, even a small addition of these elements in combination with an appropriate heat treatment has a beneficial effect on the mechanical properties of low-alloy steel [4]. Alternating points 8bc and 14bc and at the same time point 8d, the introduction of additives, will help to determine their possible interactive activities. 9. Overheating of the melt above 1600 o C and the tapping of half of the metal in the ladle, plus deoxidation with 0.04 kg Al, 10. Casting the sample cast, Within each melt at a specific stage in the plan a sample casting was cast (Fig. 2). The weight of the casting was 15kg, and its geometry was based on industrial sample castings used in the test for determining the mechanical properties. It has been chosen so as to obtain maximum-proofness for casting defects of shrinkage origin, what was confirmed by computer simulation results presented in Figure Overheating the remaining melt above 1600 o C, 12. Deoxidation of the melt through the addition of 0,03kg Al and 0,03kg FeMn-LC to the furnace, only cast No. 1, 13. Argon use on the melt, only cast No. 1, 14. The introduction of modifying additives, a) Melt No. 1 no additives, b) Melt No. 2 addition of 0,04kg FeV, c) Melt No. 3 addition of 0,04kg FeNb, d) Melt No. 4 addition of MgCe master alloy ca. 0,03kg Mg. In the fourth melt it was decided to use magnesium-cerium master alloy used as spheroidizator for cast iron, because of the value of 'modifying activity' of these elements. This value for both magnesium and cerium is nearly one hundred times higher compared to niobium, and compared to vanadium almost fifty times higher [3].

4 Fig. 2. Experimental casting geometry Fig. 3. Distribution of shrinkage defects in experimental casting, simulated 15. After overheating the melt above 1600 o C and the time of 2 minutes, the next sample was retrieved, 16. The tapping of rest of the metal with deoxidation by 0,04kg Al in the ladle, 17. Casting the sample cast. The sample castings were cut for further testing, sampling location is shown in Figure 2. From each casting after the initial selection, nine standard samples were obtained for testing the tensile strength, and nine pieces of standard V-shaped samples for the impact test. The samples obtained in this way were divided into three sets of three samples of each type from each melt. The prepared sets of samples were heat-improved according to the following plan: I stage. Quenching - heating of all samples to the temperature of 920 o C - austenitizing at this temperature for 20 min, - cooling in water. II stage. Tempering - heating of subsequent set of samples (three samples of impact resistance and 3 to test the tensile strength) respectively to a temperature of 600 o C, 650 o C, 700 o C - soaking at this temperature for 20 min, - cooling in the air, 4. RESULTS Analysis of chemical composition of tested steel was performed on samples from the sample castings. The results were presented in Table 4. Compared to L20HGSNM steel, chromium and nickel have been slightly raised. Molybdenum because of its beneficial effects on the mechanical properties was decided to be raised to the content of above 0.6%. Due to the acidic lining of the furnace it was difficult to control the content of manganese and silicon hence the relatively large fluctuations in the content of these elements. Their contents were pursued, as the lowest concentration of about 0.4% silicon, and manganese concentrations in the upper limit provided for L20HGSNM cast about 1.1%. The total oxygen, nitrogen, and hydrogen content was determined by melting the sample, placed in a graphite crucible for the nitrogen, oxygen and hydrogen content in analyzer measurement TCHEN600 by LECO using samples taken at specific phases of each of the melts. The results were presented in Table 5. For the third cast simultaneously with taking the samples 0 and 1 the active oxygen content in liquid cast steel was measured with the use of CELOX system by Heraeus Electro-Nite, the measured values were given in parentheses. Heat treated samples were subjected to standard tests to determine the mechanical properties of the tested cast steel. Static tensile test was performed on the machine VEB Leipzig with a nominal measuring range up to 100 [kn]. To determine the impact resistance, the Charpy hammer was used with a measuring range up to 50 [J]. The hardness measurements were made three times for every impact resistance sample on a

5 standard Brinnell stand using tungsten carbide balls with a diameter of 5 mm under the load of 750 [J]. The values of the results of the study were presented in Table 6. Tab. 4. Chemical compositions of melted cast steel Cast No. C Si Mn P S Cr Mo Ni Al Cu 1_1 0,27 0,35 0,80 0,029 0,029 1,02 0,67 1,31 0,06 0,16 1_2 0,25 0,46 0,74 0,029 0,026 1,02 0,66 1,30 0,10 0,16 2_1 0,22 0,47 0,86 0,027 0,022 1,08 0,64 1,33 0,19 0,17 2_2 0,22 0,49 0,80 0,028 0,026 1,08 0,65 1,33 0,15 0,16 3_1 0,22 0,44 0,96 0,028 0,022 1,05 0,63 1,30 0,20 0,16 3_2 0,24 0,46 0,93 0,029 0,025 1,11 0,64 1,30 0,04 0,16 4_1 0,23 0,47 0,92 0,030 0,023 1,11 0,65 1,35 0,02 0,17 4_2 0,25 0,67 0,88 0,029 0,022 1,16 0,66 1,36 0,05 0,17 Avg. 0,24 0,48 0,86 0,03 0,02 1,08 0,65 1,32 0,10 0,16 STD. 0,018 0,088 0,075 0,001 0,003 0,048 0,013 0,024 0,069 0,004 Tab. 5. Contents of O, N and H in cast steel Cast No. Sampl No. N O H <0, <0, <0, , <0, <0, (56) 0, (6) <0, <0, , <0, <0,6 In parentheses is the active oxygen content measured directly in the melt with a CELOX equipment by Heraeus Electro-Nite 5. SUMMARY AND CONCLUSIONS The chemical composition of the tested cast steel in each melt does not differ significantly, only variation of silicon content is above 10% for the other elements the value is below 10% for most of them less than 5%. Similar results are obtained by comparing the terminal oxygen and nitrogen values in the cast sample, the differences in the content of oxygen are less than 15% and for nitrogen less than 8%. Therefore it can be concluded that the differences in structure and properties of various steel melts are the result of a sequence of used metallurgical treatments, used modifying additives and solidification conditions. The last factor can also be regarded as insignificant because all the castings were made in identical forms prepared in the same way with the same molding, and the pouring temperature ranged o C. To sum up the results obtained the following conclusions: - The use of argon resulted in only slight variations of O and N content in the test cast steel confirming literature data [2], in each melt increment of N was recorded while the O content in melts 1 and 2, fell slightly and in melts 3 and 4 increased, - The use of low-carbon ferromanganese in combination with aluminum caused even a fourfold decrease in oxygen content, The fundamental general principle that the higher the tempering temperature the tensile strength, yield strength and hardness have a lower value and the impact strength, elongation and contraction, all the plastic properties are higher, has been confirmed. Based on the obtained results we can expect the following: - Tensile strength Rm of the tested cast steel decreases by nearly 30% with an increase in tempering temperature from 600 to 700 o C.

6 Tab. 6. Mechanical properties of investigated cast steel No. Rm [MPa] Re [MPa] HB Z [%] KCV [J/cm2] A5 [%] 1_1_ ,7 872,1 298,3 43,3 25,4 4,1 1_1_ ,5 657,2 264,0 46,0 16,7 3,6 1_1_ ,8 508,6 242,3 30,8 18,9 7,6 1_2_ ,1 830,6 310,3 36,1 21,0 6,1 1_2_ ,0 701,2 285,3 52,6 25,7 6,2 1_2_ ,6 497,1 235,7 64,4 36,0 10,2 2_1_ ,7 0,0 331,7 7,6 2,5 1,2 2_1_ ,9 564,0 292,3 10,7 7,4 1,7 2_1_ ,6 511,3 261,7 16,0 2,5 2,6 2_2_ ,0 0,0 388,3 6,4 0,0 1,2 2_2_ ,9 941,0 348,3 8,3 2,5 2,0 2_2_ ,9 685,7 267,7 14,8 7,3 3,9 3_1_ ,0 0,0 358,0 8,0 0,0 1,5 3_1_ ,3 0,0 335,0 9,9 0,0 1,7 3_1_ ,5 613,5 267,7 10,2 2,5 2,9 3_2_ ,5 933,0 355,0 6,8 0,0 0,5 3_2_ ,1 803,7 327,7 8,2 3,7 1,5 3_2_ ,6 647,2 288,7 17,1 11,3 6,6 4_1_ ,8 0,0 394,3 8,6 3,7 2,0 4_1_ ,3 775,9 342,7 14,9 3,7 1,5 4_1_ ,0 563,7 289,0 18,6 12,8 6,3 4_2_600 0,0 0,0 363,3 7,3 0,0 0,0 4_2_650 0,0 0,0 346,0 8,7 0,0 0,0 4_2_ ,0 666,5 274,3 11,2 7,3 4,3 - The yield strength of the tested cast steel decreases by over 40% with increasing tempering temperature from 600 to 700 o C. - The elongation A 5 of the investigated cast steel increases by up to 200% with an increase in tempering temperature from 600 to 700 C (the result of over 1200% is to be rejected). - Constriction of the tested cast steel is increased by over 70% with an increase in tempering temperature from 600 to 700 o C. - KCV impact strength of tested cast cast steel increased by more than 150% with an increase in tempering temperature from 600 to 700 o C. - HB hardness of the test steel decreases by 30% with increasing in tempering temperature from 600 do 700 o C. ACKNOWLEDGEMENTS Scientific work financed from the budget for science in as a research project N N The authors sincerely thank the company Heraeus Electro-Nite Poland Sp. z o.o. from Sosnowiec for help and providing the measuring apparatus. LITERATURE [1] ANIOŁA-KUSIAK A., LUX A., MAMRO K., RZESZOWSKI M.: Biblioteka metalurga Metalurgia argonowa stali. Wydawnictwo Śląsk Katowice [2] K.J. Hubner, J. Głownia: Metody argonowania stali w małych kadziach a czystość staliwa. Krzepnięcie Metali i Stopów, Rocznik 2, nr 44, Pan Katowice [3] J. SZYMSZAL, E. KRZEMIEŃ, T. ZAJĄC: Modyfikacja metali i stopów. Wydawnictwo Politechniki Śląskiej, Gliwice [4] T. WACHELKO, M.S. SOIŃSKI, A. NOWAK: Podwyższenie własności staliwa niskostopowego przez wprowadzenia mikrododatków wanadu i niobu. Krzepnięcie Metali i Stopów, nr 2, Pan Katowice [5] D. BARTOCHA, J. KILARSKI, J. SUCHOŃ, C. BARON, J. SZAJNAR, K. JANERKA: Niskostopowe staliwo konstrukcyjne, Archives of Foundry Engineering SI 3/2011, Gliwice [6] D. BARTOCHA, J. KILARSKI, J. SUCHOŃ, C. BARON, J. SZAJNAR: Wpływ temperatury opuszczania na własności niskostopowego staliwa. Archives of Foundry Engineering SI 3/2011, Gliwice [7] D. BARTOCHA, J. SUCHOŃ: Struktura niskostopowego staliwa ilościowa analiza zanieczyszczeń Archives of Foundry Engineering SI 3/2011, Gliwice 2011.