Physical and Mechanical Properties of High Manganese Non-magnetic. Steel and Its Application to Various Products for Commercial Use*

Transcription

1 Physical and Mechanical Properties of High Manganese Non-magnetic Steel and Its Application to Various Products for Commercial Use* By Terufumi SASAKI,** Kenji WATANABE,** Kiyohiko NOHARA,** Yutaka ONO,** Nobuyuki KOND 0 * * * and Shuzo SAT 0 * * * * Synopsis In order to develop new high manganese non-magnetic steels that can be employed to extensive applications ranging from cryogenic to elevated temperature uses, the effects of C and Mn on their magnetic permeability, thermal expansion coefficient and mechanical properties are investigated. It is found that the relation between thermal expansion coefficient, js, and both C and Mn contents can be expressed by the following linear regression equation: p(x 10_6/ C) = C (%) Mn (%). Good mechanical properties are exhibited in the wide range of Mn contents between 18 % and 30 % at room temperature, while there is a tendency that this optimum range of Mn content is narrowed at cryogenic temperature. Then, H-shapes, round bars and deformed bars are manufactured at the workshops using 5 t vacuum melted ingots, aiming to establish the conditions for practical processes for final products and to study such various characteristics of the products as their physical and mechanical properties, machinability and weldability. As a result, it is shown that all of those products have excellent properties as non-magnetic steels. In addition, the manufacturing of non-magnetic pinch rolls attached to the electro-magnetic stirring equipment on the continuous casting machine is described in detail as one of the practical applications of the high Mn non-magnetic steels. I. Introduction The high manganese steel is well-known as Hadfield steel with a good wear-resistance property.1'2) It is also employed as non-magnetic steel along with austenitic stainless steel. Recently with the start of such big projects as nuclear fusion reactors and high speed magnetic levitation railway systems which are both equipped with superconducting magnets, high Mn non-magnetic steels are drawing wide attention in view of their application to structural steels.3) The reasons are as follows : (1) They can maintain non-magnetic state even when being worked or heat treated. (2) Their thermal expansion coefficient can be controlled from that of austenitic stainless steel (about l8 x 10-6 C-1) to that of low C steel (about 12x 10-6 C-1). (3) High strength can be obtained. (4) They show good mechanical properties at low temperature. (5) They are produced at low costs. But ordinary Hadfield steel (lc-13mn) has the defects of being unstable when worked or heat treated, of large thermal expansion coefficient, and of showing poor hot or cold workability. The demand has been growing for developing new non-magnetic structural steel suited to a wide range of uses at cryogenic temperatures (below liquid nitrogen temperature) and higher temperatures as well as at room temperature. To meet these requirements, a variety of new high Mn non-magnetic steels have been developed. The results are described below. II. Experiment The effect of such principal elements as C and Mn, and some other various alloying elements on magnetic permeability, thermal expansion coefficient and mechanical properties was studied to develop new type of non-magnetic steels widely applicable to the latest demands. 1. Experimental Procedures In the present experiment the study was carried out with small ingots weighing 1O-.3O kg. The properties of high manganese steels are characterized mainly by C and Mn contents so that the effect of C and Mn on magnetic permeability, thermal expansion coefficient and mechanical properties was investigated. The tests were conducted in both as-rolled and solution heat treated states. Thermal expansion coefficient was measured with 50 X 50 mm round bar by means of differential transformerr method. Magnetization measurements were carried out using a specimen of 5~b X 7 mm by vibrating sample magnetometer. Magnetic permeability (p) was calculated by the following formula, p =1 +4lrxp where x is magnetic susceptibility and p is density. The maximum value of magnetic field is 10 koe. 2. Experimental Results 1. Magnetic Permeability The change in u with C and Mn contents after solution heat treatment followed by water quenching is shown in Fig. 1 that corresponds to an equilibrium phase diagram of Fe-Mn-C system at C.4,5) It is shown that p is satisfactorily maintained below 1.02 that is usually considered as a criterion of nonmagnetic condition,6) in the region without a or a' phase (martensite). In this region, the effect of C and Mn on p is negligibly small and p is regarded actually constant. This is related with the fact that * ** *** **** Originally published in Kawasaki Steel Giho, 13 (1981), 381, in Japanese. English version received March 10, ISIJ Research Department II, Research Laboratories, Kawasaki Steel Corporation, Kawasaki-cho, Chiba 260. Mizushima Research Department, Research Laboratories, Kawasaki Steel Corporation, Kawasaki-dori, Mizushima, Kurashiki 712. Tenchnical Control Department, Mizushima Works, Kawasaki Steel Corporation, Kawasaki-dori, Mizushima, Kurashiki 712. (1010) Report

2 Transactions ISIT, Vol. 22, 1982 (1011) Fig. 1. Phase diagram of Fe-Mn-C at netic permeability of the alloys position balances C and magwith various corn- Fig. 2. Effect of solution annealing temperature and holding time on magnetic permeability after water quench. both r (austenite) and s phases show antiferromagnetic behavior7~ and that their magnetization is very small like that of paramagnetic materials. The effect of temperature for solution heat treatment before water quenching on magnetic properties is shown in Fig. 2. Steels whose composition comes near the boundary of a and r phases have a high magnetic permeability in as-rolled state and show a low value after solution heat treatment, while steels whose composition is within stable r phase show a low constant value in each state. Then, the selection of composition balance of C and Mn having stable r or (r+s) phase is essential to develop non-magnetic steels with much higher manganese contents which are subjected to various working, heat treatment and welding processes. 2. Thermal Expansion Coefficient High manganese steels are characteristic of large thermal expansion coefficient due to the presence of austenite phase. The effect of C and Mn on the average thermal expansion coefficient, f3, between 0 C and 100 C is presented in Fig. 3. This can be summarized as follows : (1) p decreases with decrease of C content. (2) Such a decrease of j3 depends on Mn content, being small in the range of lon 13 % Mn and around 50 % Mn but large in the intermediate range of Mn. (3) j3 decreases with increase of Mn content. (4) The extent of decrease in % Mn is smaller than that below 30 % Mn. The result leads to a following linear regression equation concerning ~3(x 10-6/ C), C and Mn, /3 = C(%)-O.22 Mn (%) where a percentage variation is 85 %. The equation may show that low thermal expansivity comparable to that of low C steels can be obtained by adopting high manganese steels with lower C and higher Mn contents. This thermal expansion behavior is closely related to the fact that high manganese steels show antiferromagnetism. The relation between art (the average thermal expansion coefficient between 0 C and 100 C) and TN (the transition temperature between paramagnetic state and antiferromagnetic state) is shown in Fig. 4. The figure shows a good correspondence between them and JSRT decreases with increase of TN. This originates from the same reason Fig. 3. Effect of C and Mn on thermal cient, j3, of high Mn steels. Fig. 4. Relation between Neel temperature mal expansion coefficient, js. expansion coeffi-, Tv, and ther- as to Invar alloys whose thermal expansion coefficient is extremely small.8-10) The reason is as follows : spontaneous volume magnetostriction, which is caused by magnetic interaction, becomes large with decreasing temperature below either TN or Tc (Curie temperature) which cancels the volume change due to the lattice vibration, thereby small thermal expansion being observed. 3. Mechanical Properties The mechanical properties at room temperature (RT) are shown in Fig. 5 and can be summarized as follows: (1) The values of strength and elongation in tensile test increase with C content.

3 (1012) Transactions ISIJ, Vol. 22, 1982 Fig. 5. Effect of C and Mn on mechanical properties of high Mn steels at room temperature. Fig. 6. Effect of C and Mn on mechanical properties of high Mn steels at -196 C. (2) The strength, elongation and toughness increase with Mn content up to 18 % Mn, and remain almost constant up to nearly 30 % Mn and rather decrease over 30 % Mn. Thus high manganese steels for room temperature use may have an extensive possibility in determining the C and Mn composition balances. The results at -196 C are presented in Fig. 6 and can be summarized as follows : (1) The strength increases with C content but is strongly affected by Mn content. (2) When C content is constant, strength, elongation and toughness increase with Mn content between 10 % and 25 % Mn, but they are liable to decrease over 25 % Mn. Therefore preferable manganese content of low temperature steels shall be lower than that of RT steels.11 Thus it is desirable to select the suitable Mn content for the steels employed to a variety of applications. Mechanical properties below RT of several high manganese steels are shown in Fig. 7 with stainless steel which is conventionally used as low temperature steels. The 1 C-13Mn steel shows good properties of strength and toughness at RT, but exhibits a marked drop of ductility, toughness and strength at low temperatures. On the contrary, steels A and B have only a small fall of ductility and toughness at low temperatures.11,12~ Particularly steel B shows high strength, good ductility and excellent toughness at low temperature. The results mentioned above were utilized to manufacture high manganese non-magnetic steel products at works.

4 Transactions ISIJ, Vol. 22, 1982 (1013) III. Production of High Manganese Non-magnetic Steels and an Example of Application 1. Production of High Manganese Non-magnetic Steels and Their Properties Experiments were conducted on manufacturing high manganese non-magnetic steel products including 13 Mn steel of H shapes, deformed bars and plates at works. The experiment is aimed at the following : 1) Establishment of producing technique at works for high Mn steels containing over 13 % Mn 2) Application of high Mn non-magnetic steels to various types of products 3) Investigation of the properties of the products. Chemical compositions and product sizes are shown in Table 1. The size of H-shape is long, and deformed bar is used with concrete. The thermal expansion coefficient of these products is required to be Fig. 7. Temperature dependence of mechanical properties of high Mn non-magnetic steels and stainless steels. as low as that of plain low C steel. Then 24Mn- 5Cr and 30 Mn steels are chosen in view of earlier results. Another Mn steel of 17Mn-3Cr was selected to make round bars, which are used to produce bolts and nuts. The 5 t ingots were melted by vacuum induction furnace. H-shapes, round bars and deformed bars were produced by the following process at works. H-shape : Bloom Wide flange beams 120x 140x470-~H512x212x22x22 Round bar Deformed bar: Bloom Billet Steel bar Wire -115Li--~80c 180 -l o ~D25 ---D 13 (unit : mm) Heating and hot rolling conditions were decided from such material characteristics of steels as : 1) low melting point, that leads to easy burning on heating 2) small thermal conductivity 3) large hot deformation resistance 4) large thermal expansion. All the products were manufactured with the conventional facilities though these high Mn steels have a large hard-workability at high temperature. The results of tensile test between -196 C and 200 C are shown in Figs. 8 and 9. It is shown that all the steels are ductile even at -196 C. Especially Mn steels with higher Mn contents have good ductility and high strength. The temperature dependence of absorbed energy is obtained by V-notch Charpy impact test and the results of as-rolled products are presented in Fig. 10. It is noted that steel with the same composition has different absorbed energies from one product to another. This is apparently due to the difference in hot rolling conditions. Properties at various positions of the products are shown in Table 2. Each product exhibits the values which are satisfactory enough as non-magnetic steels. The results of H-shapes, namely steels HA and HB at various positions are presented in Fig. 11. The Table 1. Details of 5 t vacuum melted high Mn steel ingots.

5 (1014) Transactions ISIJ, Vol. 22, 1982 Fig. 8. Tensile properties of H-shapes as a function temperatures between -196 C and 200 C. of test Fig. 9. Tensile properties of round bar and deformed bar as a function of test temperatures between -196 C and 200 C. Fig. 10. Charpy impact energy of H-shapes (a), and round bar and deformed bar (b) as a function of test temperatures between -196 C and 200 C. Table 2. Properties of high Mn steels shown in Table 1.

6 Transactions ISIJ, Vol. 22, 1982 (1015) Fig. 12. Illustration positions. of H-shape showing various specimen Fig. 11. Variation in properties of H-shapes with specimens at various positions. symbols T, B and M, denote the ingot positions of top, bottom and middle, respectively, and the notation X, Y, etc., are given in Fig. 12. The results indicate large values of strength at the positions T4, B4, MX and M4. The microstructure is shown in Photo. 1. This shows the existence of very fine structure which is not crystallized enough and contains some residual strain, leading to such high strength as indicated above. 2. Example Application of High Manganese Non-magnetic Steels to Non-magnetic Rolls for Continuous Casting The following is an example of application of high Mn steel to the non-magnetic rolls attached to electro-magnetic stirring (EMS) equipment on a continuous casting machine. 1. Specification and Chemical Composition The specification of mechanical properties and permeability of the steel for this application is shown in Table 3. Alloying compositions were determined in consideration of the following items: 1) relation between alloying element and strength 2) high temperature strength and high temperature wear 3) magnetic stability and thermal expansion of '- phase 4) precipitation hardening effect of additional elements 5) machinability and weldability 6) productivity (hot workability). The contribution of various elements to the strength of 0.5C-13Mn3"3-15~ steel is shown in Table 4 as a result of studies described earlier. Table 5 presents the recommendable chemical compositions and estimated strength at RT. It is a lower C and higher Photo. 1. Table Microstructures of H-shapes at each position shown in Fig Specification of non-magnetic EMS rolls for continuous casting.

7 (1016) Transactions Is", Vol. 22, 1982 Table 4. Change in strength of high Mn standard steel by addition of each element by 1 wt%. Table 5. Recommendable c hemical composition for non-magnetic EMS roll and its estimated strength at room temperature. Mn steel than Hadfield steel. 2. Production and Result of Test (1) Manufacturing Process Manufacturing process is as follows : Table 7. Mechanical properties of non-magnetic EMS roll (240 mmo) having nominal composition of 18Mn-5.5Cr after solution heat treatment at C for 6 h followed by water cooling. Melting and refining -* Blooming --~ (Electric Furnace, LRF (degassing)) [340 x 350] Rolling of large structural steel --f Solution heat treat- [ ~i5] (1 080 C x 6 h WQ) ment and water quenching -+ Material test -+ Machining --* Welding and finishing processing -> [220 ~5, 2700] Continuous casting shop (unit : mm) (2) Chemical Analysis of Molten Steel The chemical analysis of molten steel is shown in Table 6. (3) Test Results of Physical and Mechanical Properties Magnetic permeability, 1u, is shown to be N in as-rolled and solution treated states and is 1.08 on a gas melt surface of the same steel. These values satisfy the specification in Table 3. The average thermal expansion coefficient at RT is 14.5'-.' 15.2 x 10-6 C-1. Table 7 shows the results of tensile test and Charpy impact test at RT after solution heat treatment at C for 6 h. Mechanical properties at RT also fill the specification in Table 3 and their values well correspond with the estimated ones in Table 5 with the exception of the lower proof stresses at high temperature than those in Table Results in Application Two different sizes of rolls, 220 dia, and 270 dia. were used as EMS rolls and a trouble happened to cause an excessive bending of rolls. Since it was inferred that this is attributed to a little too low high temperature strength or proof stress, the EMS rolls were submitted to a precipitation hardening at 820 C for 8 h followed by furnace cooling. The results are presented in Table 8, showing the increase in proof stress by approximately 10 kgf/mm2. Practically both precipitation heat treatment and newly constructed installation of cooling water sprays on rolls largely reduced the bending of rolls. It should be noticed that no bending trouble happened on 270 dia. rolls. Thus several disadvantages are pointed out in case

8 Transactions Is", Vol. 22, 1982 (1017) usual high Mn steels are subjected to high temperature application14,1s~ : 1) too large thermal expansion coefficient, Table 8. Mechanical properties of non-magnetic EMS roll (240 mm~b) having nominal composition of 18Mn-5.5Cr after age-hardening treatment. (1080 C X 6 h W.C. --~ 820 C X 8 h F.C.) 2) too small thermal conductivity, 3) too low high temperature strength (especially proof stress), 4) insufficient creep stress, and 5) inferior high temperature corrosion resistance. Much improvement in the items 3) and 5) has been established in the present study. Iv. Various Material Tests of High Manganese Non-magnetic Steels The steels (24Mn-5Cr, 0.6C-30Mn, 17Mn-3Cr, 18Mn-5.5Cr) mentioned in Sections III. 1 and are so typical as high Mn non-magnetic steels that various material tests were carried out on these steels in the following way. 1. Physical and Mechanical Properties after Solution Heat Treatment Physical properties are presented in Table 9. High Mn steels are characterized by small thermal conductivity. The results of tensile test are shown in Fig. 13. Proof stress and tensile strength of solution heat treated specimens are smaller by kgf/mm2 than those of as-rolled specimens. The results of V-notch Charpy impact test are shown in Fig. 14. The impact property of 24Mn-5C is much improved by solution heat treatment. Table 9. Physical properties of high Mn non-magnetic steels along with low carbon steel and austenitic stainless steel. Fig. 13. Temperature dependence of tensile properties of H-shapes (a), and CC- EMS roll (b), subjected to solution annealing and then water cooling.

9 (1018 Transactions ISIJ, Vol. 22, 1982 Fig. 14. Charpy impact energy subjected to solution cooling. of various annealing high and Mn then steels water Fig. 16. Change in tensile properties of H-shapes and CC- EMS rolls in the region of elevated temperatures. Fig. 15. Proof stress, tensile strength and hardness increase with cold reduction, whereas elongation decreases. Steels HA and HB show no change in magnetic susceptibility (x), but steel CC presents a certain increase when over 20 % cold reduction is given. 3. The Results of Tensile Test at High Temperature A few studies have been reported on the high temperature strength of high Mn steels. High temperature strength of steels HA, HB and CC after solution heat treatment is shown in Fig. 16. The strength of conventional high Mn steels is smaller than that of either stainless steel or heat resistance steel. High temperature strength (especially proof stress) is greatly dependent on the composition, as is presented in these results, and it is suggested that high Mn steels are applicable to high temperature use if the composition of steels was suitably chosen. Fig. 15. Effect of cold rolling on mechanical and magnetic properties of various products at room temperature. 2. Effect of Cold Rolling Tensile test and permeability measurement were conducted in the cold-rolled state after solution heat treatment. The results of RT test are presented in 4. Machinability High Mn steels exhibit a large work hardening and good wear resistivity, which results in a poor machinability. This is one of the reasons why the application of high Mn steels is limited though they have good mechanical and physical properties. Test in turning and drilling was performed with the steels produced at the works to improve their machinability.

10 Transactions ISIJ, Vol. 22, 1982 (1019) Tool life curves in turning of steels 13Mn, 17Mn- 3Cr, 18Mn-5.5Cr(-lNi-0.5V), 24Mn-5Cr and SUS- 304 are given in Fig. 17 under the test conditions shown in the same figure. The steels tested can be arranged as follows in the order of machinability, the most superior being SUS304 and the most inferior 13 Mn alloy. SUS304>24Mn-5Cr> 17Mn-3Cr > 18Mn-5.5Cr (SHT)>_ 18Mn-5.5Cr (as-rolled)> 13Mn. Drill life curves in boring of as-rolled steel products of 13 Mn, 17Mn-3Cr, 24Mn-5Cr and 30 Mn are shown in Fig. 18. Cutting condition is presented in Fig. 17. Tool life curves in turning of high Mn steels. Table 10 and cutting oil was used during the test. The order of drill life can be arranged as follows with 30 Mn steel as the best: 30Mn>24Mn-5Cr> 17Mn-3Cr> 13Mn. This indicates substantially the same tendency in case of the tool life in turning test. It is notable that the 30 Mn steel has good drill life on SKH9 drill. 5. Properties of Weld Joints Welding tests were performed to study some welding properties of high manganese steels. Base metal and welding conditions are presented in Table 11 along with the mechanical properties and magnetic permeability of weld joints. No crack was detected in weld joints by X-ray test. The microstructure observation and side bend tests also showed no particular defects in weld joints. The magnetic permeability, p, was measured by " Magnetoscope " capable of nondestructive detection. p of the weld joints stayed almost constant regardless of position, and a small amount of increase of p was observed near the welded boundary between the base metal and SM50. Thus it can be said that high Mn steels may be welded with either the same grade of materials or different type of materials without any serious troubles, referring to further tests carried out to examine weldability in addition to the present test. V. Conclusion New high Mn non-magnetic steels that can be employed to extensive applications have been developed from the study of their physical and mechanical properties. Then, H-shapes, round bars and deformed bars were manufactured at works using 5 t vacuum melted Table 10. Cutting conditions for d rill ing test. Fig. 18. Drill life curves in boring of high Mn steels. Table 11. Base metal, welding condition and mechanical properties and magnetic permeability of weld joints.

11 (1020) Transactions ISIJ, Vol. 22, 1982 ingots, aiming at the study of their various properties as well as establishment of production processes. As a result, these products were confirmed to exhibit excellent properties. In addition, manufacture of non-magnetic rolls attached to an EMS equipment on a continuous casting machine was described together with the results of the mechanical and physical properties of those products. New high Mn non-magnetic steels are expected to be applied to a wide variety of uses ranging from cryogenic to high temperature besides at room temperature as structural steels. REFERENCES 1) Handbook of Iron and Steel (in Japanese), ed, by ISIJ, Maruzen, Tokyo, (1962), ) K. Hashiura: Bull. Japan Inst. Metals, 16 (1977), ) S. Sawa: Bull. Japan Inst. Metals, 18 (1979), ) T. Saito: J. Japan Inst. Metals, 27 (1963), ) H. Shumann: Neue Hutte,17 (1972), ) H. Takada: The Special Steel, 28 (1979), No. 5, 8. 7) S. Chikazumi, K. Ota, K. Adachi, N. Tsuya and Y. Ishikawa : Handbook of Magnetic Materials (in Japanese), Asakura Shoten, Tokyo, (1975), ) S. Chikazumi and T. Mizoguchi : Solid State Physics (in Japanese), 3 (1968), 67. 9) H. Saito and H. Fujimori: Bull. Japan Inst. Metals, 7 (1968), ) S. Chikazumi, K. Ota, K. Adachi, N. Tsuya and Y. Ishikawa: Handbook of Magnetic Materials (in Japanese), Asakura Shoten, Tokyo, (1975), ) T. Sasaki, K. Watanabe, K. Nohara, N. Kondo, Y. Ono, S. Sato and I. Ichise : Tetsu-to-Hagane, 67 (1981), A81. 12) T. Sasaki, K. Watanabe, K. Nohara, Y. Ono and N. Ohashi : Tetsu-to-Hagane, 66 (1980), ) K. J. Irvine, T. Gladman and F. B. Pickering: JISI, 207 (1969), ) Handbook of Alloy Steel (in Japanese), ed, by Soc. for the Research of Electric Furnace Steel, Rikagakusha, Tokyo, (1969), Chap ) T. Kato, M. Fujikura, S. Kawasaki and K. Ishida: Tetsuto-Hagane, 65 (1979), ) Metals Handbook, I, ed. by AMS, Ohio, (1961), 834.