DESCRIPTION STEEL MAIERIAL AND EXPANDABLE OIL COL]NTRY TUBULAR GOODS TECHNICAL FIELD [0001] The present invention relates to a steel material and expandable oil country tubular goods, and more particularly, to a steel material excellent in pipe expandability and sulfide stress cracking resistance, which is used in oil well and gas well environmenfs and the like environments containing hydrogen sulfide (HzS) and expandable oil country tubular goods using the same. BACKGROLINDART [0002] In dritling of oil wells and gas wells (hereinafter, collectively referred to simply as "oil wells"), a general method employed is to insert and bury casings after a drill hole reaches a predetermined depth in order to prevent a well wall from collapsing. Furthermore, the operation of inserting casings having smaller outside diameter one by one is repeated while performing the drilling. Therefore, conventionally, in the case where it is necessary to perform drilling up to a large depth, a drilling area of the oil well in a stratum-near-surface portion becomes larger in an outside-diameter direction because of the increase in the number of times a casing is inserted, which increases drilling cost and construction period, and is thus economically disadvantageous. Accordingly, in recent years, there has been proposed a method of construction in which casings inserted in an oil well are expanded in the oil well to reduce a drilling area in a strafum-nearsurface portion, so that a drilling construction period can be significantly shortened (for example, refer to Patent Document 1). [0003] ln oil welis of crude oil, natural gas, and the like containing HzS, sulfide stress cracking (hereinafter, referred to as "SSC") of steel in wet hydrogen sulfide environments poses a problem, and therefore steel pipes for casing excellent in SSC resistance are needed. In the above-described method of construction, casings are exposed to a corrosive environment after it is subjected to working for expansion without being subjected to heat treatment or the like. Therefore, a material used for casings has to be excellent in expandability and also in corrosion resistance after cold working. For example, Patent Documents 1 to 3 propose materials that are excellent in expansion capability and corrosion resistance. LIST OF PRIOR ART DOCUMENTS PATENT DOCUMENT [0004] Patent Document 1: JP2008-202128A Patent Document 2: JP2002-2660554 Patent Document 3: JP2006-90784 DISCLOSURE OF THE INVENTION PROBLEMS TO BE SOLVED BY THE INVENTION [000s] ln order to assure the expandability of steel pipes that is indispensable for use in the above-described process, a high uniform elongation is required. Patent Documents 1 and 2 disclose steel pipes that are excellent in SSC resistance but have room for improvement because no examination is made about uniform elongation. Also, Patent Document 3 discloses the value of uniform elongation. The value, howeve¡ indicates a result which is 2Io/o or less. In addition, no examination has been made about SSC resistance. In order to further increase application opportunities of steel pipes that are to be expanded in an oil well, it is necessary to have a uniform elongation of, for example, 40Yo or more and assure an SSC resistance after expansion. [0006] An objective of the present invention is to provide a steel material that has a high expandability, is excellent in SSC resistance after cold working and moreover has a high economic efficiency, and expandable oil country tubular goods using the same. MEANS FOR SOLVINGTHE PROBLEMS [0007] The present inventors examined the chemical composition of a steel materiai that satisfies the above-described conditions. As the result, the present inventors came to obtain the following findings. [0008] (A) In order to assure a high SSC resistance and uniform elongation, it is effective to contain Mn and C, which are austenite stabilizing elements. In particular, it is effective to contain a large amount of Mn. An austenitic structure has a high resistance to SSC, and if the contents of C and Mn are properly selected, the austenitic structure is stable in cold working and difficult to cause strain induced martensitic transformation. Therefore, the occurrence of SSC, which is likely to occur in the presence of a BCC (body-centered cubic) micro-structure, can be suppressed. [000e] (B) Mn has a problem in that it brings about the deterioration in general corrosion resistance in wet hydrogen sulfrde environments. However, the deterioration of general corrosion resistance can be suppressed by containing Cu in a steel material. [0010] (C) When a C content is properly managed, in the case where V, which is a carbide-forming element, is contained, C is consumed to form carbides. Therefore, it is necessary to adjust the C content considering the amount of C consumed as carbides. [0011] The present invention has been accomplished on the basis of the above-described findings, and involves a steel material and expandable oil countrytubular goods described below. [0012] (1) A steel material having a chemical composition consisting, by mass percent, of C: 0.6 to l.8o/o, Si: 0.05 ro 1,00%, Mn: more than 25 .jYo and 45 .ÙYo or less, Al: 0.003 to 0.060/o, P: 0.03% or less, S: 0.03% or less, Cu: 0.5 to 3.0%o, N: 0.l0% or less, V: 0 to 2.0Vo, Cr: 0 to 3.0%, Mo: 0 to 3.ÙYo, Ni: 0 ro 1.5%, Nb: 0 ro 0.5%, Ta:0 to 0.5Y0, Ti: 0 to 0.5%, Zr:0 to 0.5o/o, Ca: 0 to 0.005.yo, Mg: 0 to 0.005%, REM: 0 to 0.01%, B: 0 to 0.015yo, the balance: Fe and impurities, and satisfying the following formula (i), wherein a metal micro-structure is consisting of an austenite single phase, a yield strength is 241 MPa or higher, and a uniform elongation is 40%o or higher; 0.6 70-0.06xYS (MPa) (ii) where, in the formula, uEl means the uniform elongation (%) of the steel material, and YS means the yield strength (MPa) thereof. [004e] In particular, if the yield strength is less than 500 MPa, it is also supposed that steel pipes having been subjected to solid solution heat treatment are strengthened by cold working in advance before shipment, and it is therefore desirable to satisfy the formula (ii). [00s0] 4. Application As described above, the steel material according to the present invention is excellent in expandability and, in addition, has a feature that the corrosion resistance thereof does not deteriorate after expansion even without being subjected to heat treatment. Therefore, the steel material according to the present invention is suitable to be used as expandable oil country tubular goods. The kind ofthe tubular goods is not specifically limited, and a seamless steel pipe, an electric resistance welded steel tube, an arc welded steel pipe, or the like can be used. [00s 1] Typically, in expansion, it is desirable to use steel pipes That are produced by processing steel strips or steel plates having uniform thicknesses into tubular shapes and thereafter joining them, rather than seamless steel pipes that have some variations in thickness. However, the steel material according to the present invention has characteristics of being considerably hardened by working. Therefore, in the case of expanding a steel pipe having variations in thickness, a thin portion is first expanded to be hardened, and the further elongation thereof is restricted. A thick portion is then expanded, and the steel pipe is uniformly expanded as a consequence. Therefore, the steel material according to the present invention can be suitably used for seamless steel pipes. tn addition, it is more desirable that seamless steel pipes include no weld zoteto stably exhibit a good SSC resistance. 16 .i::.'i.-;::,; -{': ::.-':J ì .:i -,':i'-1: [00s2] 4. Production method The steel material according to the present invention can be manufactured, for example, by the method described below, but the method is not subject to any special restriction. [00s3] Concerning melting and casting, a method carried out in the method for producing general austenitic steel materials can be employed, and either ingot casting or continuous casting can be used. ln the case where seamless steel pipes are produced, a steel may be cast into a round billet form for pipe making by round continuous casting. [00s4] After casting, hot working such as forging, piercing, and rolling is performed. In the production of seamless steel pipes, in the case where a circular billet is cast by the round continuous casting, processes of forging, blooming, and the like for forming the circular billet are unnecessary. ln the case where the steel material is a seamless steel pipe, after the piercing process, rolling is performed by using a mandrel mill or a piug mill. Also, in the case where the steel material is a plate material, the process is such that, after a slab has been rough-rolled, finish rolling is performed. The desirable conditions of hot working such as piercing and rolling are as described below [00s5] The heating of billet may be performed to a degree such that hot piercing can be performed on a piercing-rolling mill; however, the desirable temperature range is 1000 to 1250'C, The piercing-rolling and the rolling using a mill such as a mandrel mill or a plug mill are also not subject to any special restriction, However, from the viewpoint of hot workability, specifically, to prevent surface defects, it is desirable to set the finishing temperature at 900oC or higher. The upper limit of finishing temperature is also not subject to any special restriction; however, the finishing temperatuïe is preferably lower than 1100oC. t7 [00s6] In the case where a steel plate is produced, the heating temperature of a slab or the like is enough to be in a temperature range in which hot rolling can be performed, for example, in the temperafure range of 1000 to 1250"C. The pass schedule of hot rolling is optional. However, considering the hot workability for reducing the occurrence of surface defects, edge cracks, and the like ofthe product, it is desirable to set the finishing temperature at 900'C or higher. The finishing temperature is preferably lower than 1 100'C as in the case of seamless steel pipe. [00s7] For the present steel material, aging heat treatment can be performed with the purpose of precipitation strengthening by mainly precipitating carbides and carbonitrides. In particular, it is effective in the case where one or more elements selected from V, Nb, Ta, Ti and Zr is contained. However, exceeding aging heat treatment induces formation ofexcess carbides and reduce C concentration in parent phase to lead destabilization of austenite. As a heating condition, it is preferable to heat the steel material about several ten min to several h at the temperature range of 600 to 800.C. [006r] Cold working may be performed as necessary for the steel material having been subjected to solid solution heat treatment or further aging heat treatment. A working ratio (reduction ofarea) is not subject to any special restriction but, in particular, in order to obtain a yield strength of 400 MPa or higher and lower than862 MPa, it is preferable 19 to make the working ratio about 10%. ln the case where the steel material of the present invention is used as expandable oil country fubular goods, it is not preferable to perform cold working excessively and a working ratio is preferably selr.o 25%o or less, in order to assure high expandability. Excessively high working ratio makes it diffrcult to expand the fubular goods uniformly in the oii wells because a uniform elongation is reduced and a strength is enhanced. 10062) The cold working method is not subject to any special restriction as far as the steel material can be worked evenly by the method. However, in the case where the steel material is a steel pipe, it is advantageous on an industrial basis to use a so-called cold draw bench using a holed die and a plug, a cold rolling mill called a cold Pilger rolling mill, or the like. Also, in the case where the steel material is a plate material, it is advantageous on an industrial basis to use a rolling mill that has been used to produce the ordinary cold rolled plate. [0063] After the cold working, annealing can be performed. In particular, annealing can be applied with a view to reducing a strength when the excess strength is obtained by the cold working, and recovering an elongation. As an annealing condition, it is preferable to heat the steel material about several min to t h at the temperature range of 300 to 500'C, 100641 Hereunder, the present invention is explained more specifically with reference to examples; however, the present invention is not limited to these examples, EXAMPLE I [006s] Twenty-three kinds of steels of A to P and AA to AG having the chemical compositions given in Table 1 were melted in a 50kg vacuum furnace to produce ingots. Each of the ingots was heated at 1180'C for 3 h, and thereafter v/as forged and cut by z0 electrical discharge cutting-off. Thereafter, the cut ingots were further soaked at I 150'C for I h, and were hot-rolled into plate materials having a thickness of 20 mm. Subsequently, the plate materials were subjected to solid solution heat treatment at 1100"C for I h to obtain test materials (test Nos. I to 23). Additionally, test materials produced in the same manner as test Nos. I To 23 are further cold-rolled at a working ratio of l0% to obtain strengthened test materials (test Nos. 24 to 46). [0066] fTable 1] Steel A B c c 7.21 D t.t9 Si E t. l6 0.29 F t.2l 0.30 (i 0.68 Mn 0.27 26.72 H 0.69 0.28 36.00 0.69 I 0.ì9 AI 25.95 0.9 J 0.033 0.23 N) ì-) K 26.13 t.03 0.036 022 32.11 o.62 P L 0.æ8 0.3 r 0.012 0.90 31.82 M 0.031 0.33 0.010 N 32.31 0.88 o.o32 S 0.31 0.014 0.006 o '36.12 t.02 0.020 0.20 0.013 0.006 36.38 P 0.99 0.033 0. l9 Cu 0.012 0.004 AA 35.87 t.D. Chcmical composition (in mass%, balancc: Fc and impurities) 0.021 t.48 0.20 0.012 0.m4 AB 29.69 0.90 0.026 o.22 l.5l 0.010 0.006 N AC 30.08 0.41 r 0.91 0.019 0.0t3 0.r5 0.013 AD 0.006 27.p, 1.18 2.13 0.028 o.25 0.012 0.0r I AE 0.004 n.9'l 1.18 0.82 0.020 0.0t3 0.31 0.0t3 AF 0.006 2ß.02 0.91 Tablc I 0.8t 0.029 0.012 028 0.011 AG 0.006 25.48 o.87 0.81 C¡ 0.016 0.034 0.28 0.012 0.005 30.24 0.70 + indicatcs that couditions do not satisfy thosc dcfirrcd by the prcsent invcntion 0.025 0.01I 0.20 l.l9 0.0il 0.00s o.u 8.12 r 0.ûtl 0.18 0.01I Lt7 0.0t3 0.005 Mo 2.6.28 0.028 0.013 o.32 [21 0.012 0.00s 28.12 0.019 0.21 l.m 0.01I 0,011 0.007 28.22 0.62 0.?8 0.021 0.012 0.010 Ni 0.006 31.94 O.Cú 0.026 0.011 0.012 0.006 15.88 r 0.u29 0.61 0.015 0.0r2 0.m5 Nb o.78 0.013 0.031 0.013 0.81 t.49 0.006 0.55 0.018 0.011 0.0t2 0.77 0.005 Îa 0.68 0.31 0.011 0.012 0.006 1.45 0.012 0.011 0.005 0.29 0.0t3 Ti 0.80 0.29 0.007 0.30 1.02 0.013 0.008 0.9s 0.011 Zr 0.7t 0.011 0. t8 0.58 0.011 Ca 0.012 0.011 0.57 0.1 I Ms 1.2 t.03 0.10 3.94 REM o.12 B 0.002 c-0.t8v 0.0æ 0.003 1.52 | 2t l9 l6 0.68 0.69 0.69 0.001 0.99 1.03 o.62 0.003 0.63 0.74 T.U2 0.99 t.17 0.90 o.4l * l.I8 lr8 0.9r 0_87 0.51 0.9 [0067] With use of the above-described test materials, mechanical properties and a metal micro-structure were examined. Thereafter, the test materials were subjected to cold working at working ratio of 25o/o simulating the expansion. And, mechanical properties, a metal micro-structure, SSC resistance and a corrosion rate were examined with use of the cold-worked test materials. Conceming the mechanical properties, yield strength and uniform elongation were measured. From each of the steeis, a round-bar tensile test specimen having a parallel part measuring 6 mm in outside diameter and 40 mm in length was sampled. A tension test was conducted at normal temperature (25"C), whereby the yield strength YS (0.2% yield stress) (MPa) and the elongation (o/o) were determined. [0068] In the present example, the test material that had a uniform elongation being40Yo or higher and satisfying the following formula (ii) in relation to a yield strength was evaluated so that the uniform elongation property is good. In the following Table 2 is indicated required elongation (%) which is higher value of 40%o andi0 - 0.06 x yS. uEl (%) > 70-0.06xYS (MPa) (iÐ where, in the formula, uEl means the uniform elongation (%) of the steel material, and YS means the yield strength (MPa) thereof. [006e] The SSC resistance was evaluated as described below. A plate-shaped smooth test specimen was sampled, and a stress corresponding to 90% of yield stress was applied to one surface of the test specimen by four-point bending method. Thereafter, the test specimen was immersed in a test solution, that is, solution A (5%NaCl + O.S%CH¡COQH aqueous solution, 1-bar HzS saturated) specified in NACE TM0177-2005, and was held at 24"C for 336 h. Subsequently, it was judged whether or not rupture occurred. As the result, a not-ruptured steel material was evaluated so that the SSC resistance is good (refened to as "o" in Table 2), and a ruptured steel material was evaluated so that the SSC resistance is poor (referred to as "x" in Table 2). ZJ [0070] Also, to evaluate the general corrosion resistance, the corrosion rate was determined by the method described below. The above-described test material was immersed in the solution A at normal temperature for 336 h, the corrosion loss was determined, and the corrosion loss was converted into the average corrosion rate. In the present invention, the test material that showed the corrosion rate of lower than 1,5 g/(m2'h) was evaluated so that the general corrosion resistance is good. [0071] On the obtained test materials of test Nos. I to 46 before and after the cold working at the working ratio of 25%o, the total volume amounts offerrite and cx,'martensite having BCC structures were measured by using the ferrite meter. For all of the test materials before the cold working, the phases having BCC structures could not be detected and the metal micro-structures were austenite single phases. Therefore, the volume amounts of the phases having BCC structures for the test materials only after the cold working are shown as a BCC ratio by volume % in tables. The results are given in Tables 2 and 3. [0072] fTable 2] Tabþ,2 Test No. Steel After solid sot¡tbn hcat treatmerü After simuhted expambn (25% coH working) Ybrd sferigfh OrPa) Required elongatbn (%) Uniform ebngatbn (%) Ybrd strength (MPa) BCC ratio (%) SSC resistance Corrosbn mte @rÌltÐ I A 391 47 76 942 # o 0.9 lrventive example 2 B 378 47 69 921 # o t,0 3 c 385 47 78 945 fl o 0.8 4 D 391 41 80 9s6 H ô 0.9 5 E 298 52 1'' 838 H ô 1.0 6 F 307 52 74 u2 # o 0.9 7 G 30r 52 70 830 o 0.1 8 H 358 49 69 901 i o 0.8 9 362 '48 65 915 fl o 1.0 t0 J 295 52 6'.7 863 a o 0.9 ll K 342 49 62 899 o 0.8 T2 L 340 50 60 905 o t.2 t3 M 352 49 7t 910 þ o Ll T4 N 363 48 66 932 il o 1.3 l5 o 380 47 70 921 H o r.2 16 P 344 49 69 889 # o 1.0 1'7 AA 26'l 54 38* 618 0.04 * o 1.1 Conparative exarple 18 AB 325 5l 2g r( 813 il o 0.8 l9 AC 386 49 68 928 o I.5 20 AD 343 49 76 903 þ o 1.6 21 AE 32r 5l '77 862 t 0.9 22 AF 308 52 42 '782 0.03 * o t.l 23 AG 313 5l 49 81t I o l.l * indicates tlnt corditbrs do not saüsry those defined by the present inveruion # irdbates that measured value is bebw tlre detection limit (0.01%). [0073] [Table 3] 25 Tabb 3 Test No. Steel Affer l0% coH working After sinn¡hted e4arÌsion (25% coH u'orking) YþH slrength (MPa) Requùed ebngation (%) Uniform ebngation (%') YþH sEength (MPa) BCC ratb (%) SSC rcsistarce Conosion râte GlnÌth) 24 A 622 4n 67 t54 # o 0.9 Im,erúive example 25 B 6M 40 58 142 I o 1.1 26 c 609 4n 68 148 o 0.9 2'7 D 601 N 6 t70 # o 1.0 28 E 520 4i 62 1080 # o 0.9 29 F 53r 40 lo 1083 H o 0.8 30 G 524 40 & 1071 o 0.8 3l H 578 4Ð 58 I 120 # o 0.8 32 582 û 56 I 135 fl o 1.0 JJ 519 40 59 r070 fl o 0.9 34 K 552 40 50 I 120 Ê o 0.9 35 L 546 40 47 1083 H o I 36 M 5g 40 û I 100 o r.2 37 N 598 40 56 1t46 fl o J Jõ o 60r 40 58 tt24 il o , 39 P 588 4A 6l t089 0.9 û AA 480 40 26* 930 0.21 o I.0 Comparative cxample 4l AB+ \t) 40 19* 1c/.2 # o 0.8 42 AC 607 40 & I 193 il o 1.6 43 AD 562 40