Titanium microalloying of steel: A review of its effects on processing, microstructure and mechanical properties

Abstract

Carbon neutrality of the steel industry requires the development of high-strength steel. The mechanical properties of low-alloy steel can be considerably improved at a low cost by adding a small amount of titanium (Ti) element, namely Ti microalloying, whose performance is related to Ti-contained second phase particles including inclusions and precipitates. By proper controlling the precipitation behaviors of these particles during different stages of steel manufacture, fine-grained microstructure and strong precipitation strengthening effects can be obtained in low-alloy steel. Thus, Ti microalloying can be widely applied to produce high strength steel, which can replace low strength steels heavily used in various areas currently. This article reviews the characteristics of the chemical and physical metallurgies of Ti microalloying and the effects of Ti microalloying on the phase formation, microstructural evolution, precipitation behavior of low-carbon steel during the steel making process, especially the thin slab casting and continuous rolling process and the mechanical properties of final steel products. Future development of Ti microalloying is also proposed to further promote the application of Ti microalloying technology in steel to meet the requirement of low-carbon economy.

Keywords

titanium microalloying / precipitation / grain refinement / phase transformation / high-strength steel

References

[1] M. Ren, P.T. Lu, X.R. Liu, M.S. Hossain, Y.R. Fang, T. Hanaoka, B. O’Gallachoir, J. Glynn, and H.C. Dai, Decarbonizing China’s iron and steel industry from the supply and demand sides for carbon neutrality, Appl. Energy, 298(2021), art. No. 117209.

[2] Dong L, Miao GY, Wen WG. China’s carbon neutrality policy: Objectives, impacts and paths. East Asian Policy, 2021, 13(1): 5.

[3] Kah P, Pirinen M, Suoranta R, Martikainen J. Welding of ultra high strength steels. Adv. Mater. Res., 2013, 849, 357.

[4] Noren TM. Columbium as a Micro-alloying Element in Steels and Its Effect on Welding Technology, 1963, Washington, Ship Structure Committee

[5] Liu Y, Sun YH, Wu HT. Effects of chromium on the microstructure and hot ductility of Nb-microalloyed steel. Int. J. Miner. Metall. Mater., 2021, 28(6): 1011.

[6] Yang CF, Zhang YQ. Applications of V−N microalloying technology in HSLA steels. Iron Steel, 2002, 37(11): 42

[7] Fu JY. Development history of Nb-microalloying technology and progress of Nb-microalloyed steel. Iron Steel, 2005, 40(8): 1

[8] Misra RDK, Nathani H, Hartmann JE, Siciliano F. Microstructural evolution in a new 770 MPa hot rolled Nb−Ti microalloyed steel. Mater. Sci. Eng. A, 2005, 394(1–2): 339.

[9] Xu G, Gan XL, Ma GJ, Luo F, Zou H. The development of Ti-alloyed high strength microalloy steel. Mater. Des., 2010, 31(6): 2891.

[10] López-Chipres E, Mejía I, Maldonado C, Bedolla-Jacuinde A, El-Wahabi M, Cabrera JM. Hot flow behavior of boron microalloyed steels. Mater. Sci. Eng. A, 2008, 480(1–2): 49.

[11] Akben MG, Chandra T, Plassiard P, Jonas JJ. Dynamic precipitation and solute hardening in a titanium microalloyed steel containing three levels of manganese. Acta Metall., 1984, 32(4): 591.

[12] Chen SJ, Li LJ, Peng ZW, Huo XD, Sun HB. On the correlation among continuous cooling transformations, interphase precipitation and strengthening mechanism in Ti-microalloyed steel. J. Mater. Res. Technol., 2021, 10, 580.

[13] Yoda R, Tsukatani I, Inoue T, Saito T. Effect of chemical composition on recrystallization behavior and r-value in Ti-added ultra low carbon sheet steel. ISIJ Int., 1994, 34(1): 70.

[14] Charleux M, Poole WJ, Militzer M, Deschamps A. Precipitation behavior and its effect on strengthening of an HSLA-Nb/Ti steel. Metall. Mater. Trans. A, 2001, 32(7): 1635.

[15] Meng CF, Wang YD, Wei YH, Shi BQ, Cui TX, Wang YT. Strengthening mechanisms for Ti- and Nb−Ti-micro-alloyed high-strength steels. J. Iron Steel Res. Int., 2016, 23(4): 350.

[16] Chen YT, Guo AM, Li PH. Nitride and carbonitride precipitation behavior in a Nb−Ti microalloyed extra low carbon HSLA steel. Heat Treat. Met., 2007, 32(9): 51

[17] Hall EO. The lüders deformation of mild steel. Proc. Phys. Soc. London Sect. B, 1951, 64(12): 1085.

[18] Petch NJ. The cleavage strength of polycrystals. J. Iron Steel Inst., 1953, 174, 25

[19] Morrison WB. The effect of grain size on the stress-strain relationship in low-carbon steel. Trans. Am. Soc. Met., 1966, 59(4): 824

[20] Davenport AT, Brossard LC, Miner RE. Precipitation in microalloyed high-strength low-alloy steels. JOM, 1975, 27(6): 21.

[21] T. Gladman, D. Dulieu, and I.D. McIvor, Structure-property relationships in high-strength microalloyed steels, [in] Proc. of Symp. on Microalloying 75, New York, 1976.

[22] Y. Tanaka, Progress in TMCP technology and expansion of its range of application, [in] ASME 2005 Pressure Vessels and Piping Conference, Denver, 2005, p. 515.

[23] Wu ZH, Zheng W, Li GQ, Matsuura H, Tsukihashi F. Effect of inclusions’ behavior on the microstructure in Al−Ti deoxidized and magnesium-treated steel with different aluminum contents. Metall. Mater. Trans. B, 2015, 46(3): 1226.

[24] López B, Rodriguez-Ibabe JM. Some metallurgical issues concerning austenite conditioning in Nb-Ti and Nb−Mo microalloyed steels processed by near-net-shape casting and direct rolling technologies. Metall. Mater. Trans. A, 2017, 48(6): 2801.

[25] Lou YZ, Liu DL, Mao XP, Bai MZ. Titanium carbonitrides in Ti-microalloyed steel produced by CSP process. Iron Steel., 2010, 45(2): 70

[26] Rodriguez-Ibabe JM. Thin slab direct rolling of microalloyed steels. Mater. Sci. Forum, 2005, 500–501, 49.

[27] Chen CY, Yen HW, Kao FH, Li WC, Huang CY, Yang JR, Wang SH. Precipitation hardening of high-strength low-alloy steels by nanometer-sized carbides. Mater. Sci. Eng. A, 2009, 499(1–2): 162.

[28] Wang TP, Kao FH, Wang SH, Yang JR, Huang CY, Chen HR. Isothermal treatment influence on nanometer-size carbide precipitation of titanium-bearing low carbon steel. Mater. Lett., 2011, 65(2): 396.

[29] Yen HW, Huang CY, Yang JR. Characterization of interphase-precipitated nanometer-sized carbides in a Ti−Mo-bearing steel. Scripta Mater., 2009, 61(6): 616.

[30] deArdo AJ. Metallurgical basis for thermomechanical processing of microalloyed steels. Ironmaking Steelmaking, 2001, 28(2): 138.

[31] Jang JH, Lee CH, Heo YU, Suh DW. Stability of (Ti,M)C (M = Nb, V, Mo and W) carbide in steels using first-principles calculations. Acta Mater., 2012, 60(1): 208.

[32] Leyens C, Peters M. Titanium and Titanium Alloys. Fundamentals and Applications, 2003, Weinheim, Willey-VCH

[33] McQuillan AD, McQuillan MK. Titanium, 1956, London, Butterworths Scientific Publications

[34] Read HH. Rutley’s Elements of Mineralogy, 1953, 25th ed. London, Thomas Murby & Co.

[35] Mao XP. Titanium Microalloyed Steel, 2016, Beijing, Metallurgical Industry Press

[36] Orowan E. Discussion on internal stresses. Symposium on International Stresses in Metals and Alloys, 1948, London, Institute of Metals, 451

[37] Mao XP, Sun XJ, Kang YL, Lin ZY. Physical metallurgy for the titanium microalloyed strip produced by thin slab casting and rolling process. Acta Metall. Sin., 2006, 42(10): 1091

[38] Wang ML, Cheng GG, Qiu ST, Zhao P, Gan Y. Behavior of precipitation containing titanium during solidification. J. Iron Steel Res. Int., 2007, 19(5): 44

[39] Chen JX. Manual of Chart and Data in Common Use of Steelmaking, 1984, Beijing, Metallurgy Industry Press

[40] J.I. Takamura and S Mizoguchi, Role of oxides in steel performance, [in] Proceeding of the 6th International Iron and Steel Congress, Nagoya, 1990, p. 591.

[41] Liu ZZ, Kuwabara M. Recent progress in oxide metallurgy technology and its application. Steelmaking, 2007, 23(4): 1

[42] Yamamoto K, Hasegawa T, Takamura JI. Effect of boron on intra-granular ferrite formation in Ti-oxide bearing steels. ISIJ Int., 1996, 36(1): 80.

[43] Shim JH, Cho YW, Chung SH, Shim JD, Lee DN. Nucleation of intragranular ferrite at Ti₂O₃ particle in low carbon steel. Acta Mater., 1999, 47(9): 2751.

[44] Han LN, Bao YP, Liu JH, Li TQ. Research on nucleation mechanism of IGF of low carbon steel containing titanium. Wide Heavy Plate, 2008, 14(1): 1

[45] Takeuchi A, Inoue A. Classification of bulk metallic glasses by atomic size difference, heat of mixing and period of constituent elements and its application to characterization of the main alloying element. Mater. Trans., 2005, 46(12): 2817.

[46] Inoue K, Ohnuma I, Ohtani H, Ishida K, Nishizawa T. Solubility product of TiN in austenite. ISIJ Int., 1998, 38(9): 991.

[47] Narita K. Physical chemistry of the groups IVa (Ti, Zr), Va (V, Nb, Ta) and the rare earth elements in steel. Trans. ISIJ, 1975, 15(3): 145.

[48] Gómez M, Rancel L, Gómez PP, Robla JI, Medina SF. Simplification of hot rolling schedule in Ti-microalloyed steels with optimised Ti/N ratio. ISIJ Int., 2010, 50(6): 868.

[49] Kuziak R, Bołd T, Cheng YW. Microstructure control of ferrite-pearlite high strength low alloy steels utilizing microalloying additions. J. Mater. Process. Technol., 1995, 53(1–2): 255.

[50] Lee SY, Oh YJ, Yi KW. Effects of titanium and oxygen content on microstructure in low carbon steels. Mater. Trans., 2002, 43(3): 518.

[51] L.D. Xing, J.L. Guo, X. Li, Z.F. Zhang, M. Wang, Y.P. Bao, F.Z. Zeng, and B.T. Chen, Control of TiN precipitation behavior in titanium-containing micro-alloyed steel, Mater. Today Commun., 25(2020), art. No. 101292.

[52] Liu T, Long MJ, Chen DF, Duan HM, Gui LT, Yu S, Cao JS, Chen HB, Fan HL. Effect of coarse TiN inclusions and microstructure on impact toughness fluctuation in Ti micro-alloyed steel. J. Iron Steel Res. Int., 2018, 25(10): 1043.

[53] Fu J, Zhu J, Di L, Tong FS, Liu DL, Wang YL. Study on the precipitation behavior of TiN in the microalloyed steels. Acta Metall. Sin., 2000, 36(8): 801

[54] Yang X, Cheng GG, Wang ML, Li YL, Wang YG, Zhao P. Precipitation and growth of titanium nitride during solidification of clean steel. J. Univ. Sci. Technol. Beijing, 2003, 10(5): 24

[55] Akamatsu S, Hasebe M, Senuma T, Matsumura Y, Akisue O. Thermodynamic calculation of solute carbon and nitrogen in Nb and Ti added extra-low carbon steels. ISIJ Int., 1994, 34(1): 9.

[56] Yang XH, Vanderschueren D, Dilewijns J, Standaert C, Houbaert Y. Solubility products of titanium sulphide and carbosulphide in ultra-low carbon steels. ISIJ Int., 1996, 36(10): 1286.

[57] Copreaux J, Gaye H, Henry J, Lanteri S. Relation Précipitation-Propriétés Dans Les Aciers Sans Intersticiels Recuits En Continu, 1997, Luxembourg, European Commission

[58] Yu H, Xiong XY, Kang YL, Liu X, Fang Y. Simulation of precipitation behaviors of the precipitates in Ti-IF steel produced by TSCR process. Heat Treat. Met., 2006, 31(5): 45

[59] Jing CN, Wang ZC, Han FT, Yi YH, Zhang WP. Study on the precipitates of Ti-IF steel hot-rolled in ferrite region. Heat Treat. Met., 2006, 31(1): 79

[60] Liu WJ, Jonas JJ, Bouchard D, Bale CW. Gibbs energies of formation of TiS and Ti₄C₂S₂ in austenite. ISIJ Int., 1990, 30(11): 985.

[61] Yoshinaga N, Ushioda K, Akamatsu S, Akisue O. Precipitation behavior of sulfides in Ti-added ultra low-carbon steels in austenite. ISIJ Int., 1994, 34(1): 24.

[62] Yan FY, Zhang XG. Application of Ti to automobile wheel steel and discussion on alloying technology. Iron Steel., 2001, 36(5): 47

[63] Whelan MJ. On the kinetics of precipitate dissolution. Met. Sci. J., 1969, 3(1): 95.

[64] Relander K. Austenitzerfall Eines 0.18%C−2%Mo-Stahles in Temperaturbereich der Perlitstufe, 1964, Helsinki, Teknillinen Korkeakoulu [Dissertation]

[65] Baker TN. Titanium microalloyed steels. Ironmaking Steel-making, 2019, 46(1): 1.

[66] Smith RM, Dunne DP. Structural aspects of alloy carbonitride precipitation in microalloyed steel. Mater. Forum, 1988, 11, 166

[67] Yen HW, Chen PY, Huang CY, Yang JR. Interphase precipitation of nanometer-sized carbides in a titanium-molybdenum-bearing low-carbon steel. Acta Mater., 2011, 59(16): 6264.

[68] Jang JH, Heo YU, Lee CH, Bhadeshia HKDH, Suh DW. Interphase precipitation in Ti-Nb and Ti−Nb−Mo bearing steel. Mater. Sci. Technol., 2013, 29(3): 309.

[69] Morrison WB, Woodhead JH. The influence of small niobium additions on mechanical properties of commercial mild steel. J. Iron Steel Inst., 1963, 201, 43

[70] Aaronson HI, Plichta MR, Franti GW, Russell KC. Precipitation at interphase boundaries. Metall. Trans. A, 1978, 9(3): 363.

[71] Gray JM, Yeo RBG. Columbium carbonitride precipitation in low-alloy steels with particular emphasis on precipitate-row formation. Trans. Am. Soc. Met., 1968, 61, 255

[72] McCann J, Ridal KA. High temperature decomposition of austenite in alloy steels. J. Iron Steel Inst., 1964, 202, 191

[73] Davenport AT, Honeycombe RWK. Precipitation of carbides at γ−α boundaries in alloy steels. Proc. R. Soc. London Ser. A, 1971, 322(1549): 191.

[74] Yen HW, Chen CY, Wang TY, Huang CY, Yang JR. Orientation relationship transition of nanometre sized interphase precipitated TiC carbides in Ti bearing steel. Mater. Sci. Technol., 2010, 26(4): 421.

[75] Okamoto R, Borgenstam A, Ågren J. Interphase precipitation in niobium-microalloyed steels. Acta Mater., 2010, 58(14): 4783.

[76] Honeycombe RWK, Mehl RF. Transformation from austenite in alloy steels. Metall. Trans. A, 1976, 7(7): 915.

[77] Kim YW, Hong SG, Huh YH, Lee CS. Role of rolling temperature in the precipitation hardening characteristics of Ti−Mo microalloyed hot-rolled high strength steel. Mater. Sci. Eng. A, 2014, 615, 255.

[78] Dunlop GL, Carlsson CJ, Frimodig G. Precipitation of VC in ferrite and pearlite during direct transformation of a medium carbon microalloyed steel. Metall. Trans. A, 1978, 9(2): 261.

[79] Batte AD, Honeycombe RWK. Precipitation of vanadium carbide in ferrite. J. Iron Steel Inst., 1973, 211(4): 284

[80] Todd JA, Li P, Copley SM. A new model for precipitation at moving interphase boundaries. Metall. Trans. A, 1988, 19(9): 2133.

[81] Li P, Todd JA. Application of a new model to the interphase precipitation reaction in vanadium steels. Metall. Trans. A, 1988, 19(9): 2139.

[82] Todd JA, Su YJ. A mass transport theory for interphase precipitation with application to vanadium steels. Metall. Trans. A, 1989, 20(9): 1647.

[83] Irvine J, Baker TN. The influence of rolling variables on the strengthening mechanisms operating in niobium steels. Mater. Sci. Eng., 1984, 64(1): 123.

[84] Mao XP, Gao JX, Chai YZ. Development of thin slab casting and direct rolling process in China. Iron Steel, 2014, 49(7): 49

[85] Medina SF, Chapa M, Valles P, Quispe A, Vega MI. Influence of Ti and N contents on austenite grain control and precipitate size in structural steels. ISIJ Int., 1999, 39(9): 930.

[86] Palmiere EJ, Garcia CI, De Ardo AJ. Compositional and microstructural changes which attend reheating and grain coarsening in steels containing niobium. Metall. Mater. Trans. A, 1994, 25(2): 277.

[87] Funakawa Y. Mechanical properties of ultra fine particle dispersion strengthened ferritic steel. Mater. Sci. Forum, 2012, 706–709, 2096.

[88] DeArdo AJ. Niobium in modern steels. Int. Mater. Rev., 2003, 48(6): 371.

[89] Gordon P, Vandermeer RA. Grain boundary migration. Recrystallization, Grain Growth and Textures, 1966, Metals Park, Ohio, American Society for Metals, 205

[90] A.J. De Ardo, J.M. Gray, and L. Meyer, Fundamental metallurgy of niobium in steel, [in] H. Stuart, ed., Niobium — Proceedings of The International Symposium, San Francisco, CA, 1984, p. 685.

[91] Bhadeshia HKDH. Interpretation of The Microstructure of Steels, 2008, Cambridge, University of Cambridge

[92] J.S. Kirkaldy and D. Venugopalan, Prediction of microstructure and hardenability in low alloy steels, [in] A.R. Marder and J.I. Goldstein, eds., The International Conference on Phase Transformation in Ferrous Alloys, Philadelphia, 1983, p. 125.

[93] Voort GFV, Lucas GM. Microstructural characterization of carburized steels. Heat Treat. Prog., 2009, 9(5): 37

[94] Farrar RA, Harrison PL. Acicular ferrite in carbon-manganese weld metals: An overview. J. Mater. Sci., 1987, 22(11): 3812.

[95] Krauss G, Thompson SW. Ferritic microstructures in continuously cooled low- and ultralow-carbon steels. ISIJ Int., 1995, 35(8): 937.

[96] Ali A, Bhadeshia HKDH. Nucleation of Widmanstätten ferrite. Mater. Sci. Technol., 1990, 6(8): 781.

[97] Bhadeshia HKDH, Christian JW. Bainite in steels. Metall. Trans. A, 1990, 21(3): 767.

[98] Bhadeshia HKDH, Edmonds DV. The mechanism of bainite formation in steels. Acta Metall., 1980, 28(9): 1265.

[99] Choi JY, Seong BS, Baik SC, Lee HC. Precipitation and recrystallization behavior in extra low carbon steels. ISIJ Int., 2002, 42(8): 889.

[100] Toroghinejad MR, Dini G. Effect of Ti-microalloy addition on the formability and mechanical properties of a low carbon (ST14) steel. Int. J. Iron Steel Soc. Iran, 2006, 3(2): 1

[101] Wu D, Wang FM, Cheng J, Li CR. Effect of Nb and V on the continuous cooling transformation of undercooled austenite in Cr-Mo-V steel for brake discs. Int. J. Miner. Metall. Mater., 2018, 25(8): 892.

[102] Wan XL, Wu KM, Huang G, Wei R, Cheng L. In situ observation of austenite grain growth behavior in the simulated coarse-grained heat-affected zone of Ti-microalloyed steels. Int. J. Miner. Metall. Mater., 2014, 21(9): 878.

[103] Yu YS, Hu B, Gao ML, Xie ZJ, Rong XQ, Han G, Guo H, Shang CJ. Determining role of heterogeneous microstructure in lowering yield ratio and enhancing impact toughness in high-strength low-alloy steel. Int. J. Miner. Metall. Mater., 2021, 28(5): 816.

[104] Xie ZJ, Shang CJ, Wang XL, Wang XM, Han G, Misra RDK. Recent progress in third-generation low alloy steels developed under M³ microstructure control. Int. J. Miner. Metall. Mater., 2020, 27(1): 1.

[105] Kashima T, Hashimoto S, Mukai Y. 780 N/mm² grade hot-rolled high-strength steel sheet for automotive suspension system. JSAE Rev., 2003, 24(1): 81.

[106] Kashima T, Mukai Y. Development of 780 MPa class high strength hot rolled steel sheet with super high flange formability. R&D Kobe Steel Eng. Rep., 2002, 52(3): 19

[107] Kamibayashi K, Tanabe Y, Takemoto Y, Shimizu I, Senuma T. Influence of Ti and Nb on the strength-ductility-hole expansion ratio balance of hot-rolled low-carbon high-strength steel sheets. ISIJ Int., 2012, 52(1): 151.

[108] Zhou J, Kang YL, Mao XP, Lin ZY, Li LJ, Chen W. Effect of Ti on the mechanical properties of high strength weathering steel. J. Univ. Sci. Technol. Beijing, 2006, 28(10): 926

[109] X.P. Mao, J.X. Gao, L.J. Li, Q.Y. Liu, Z.Y. Lin, and C.F. Xu, Development and research of 550 MPa high strength and high formability plate, Automob. Technol. Mater., 2006, No. 11, p. 1.

[110] X.P. Mao, X.D. Huo, Q.Y. Liu, Y.L. Kang, Z.Y. Lin, H.Z. Zhuang, X.J. Sun, J. Zhou, and J.X. Gao, Research and application of microalloying technology based on thin slab casting and direct rolling process, [in] International Symposium on Thin Slab Continuous Casting and Rolling, Guangzhou, 2006.

[111] Gao JX, Mao XP, Chen QL, Li LJ. Microstructure and property of 700MPa Ti microalloyed high strength steel produced by EAF-CSP. Adv. Mater. Res., 2011, 287–290, 961.

[112] Yong QL. Secondary Phases in Steels, 2006, Beijing, Metallurgical Industry Press

[113] Chun EJ, Do H, Kim S, Nam DG, Park YH, Kang N. Effect of nanocarbides and interphase hardness deviation on stretch-flangeability in 998 MPa hot-rolled steels. Mater. Chem. Phys., 2013, 140(1): 307.

[114] Lu JX, Wang GD. Study on the performance of carbonitride precipitation in Nb−Ti microalloyed steel. Iron Steel, 2005, 40(9): 69

[115] Wang XN, Di HS, Du LX. Effects of deformation and cooling rate on nano-scale precipitation in hot-rolled ultrahigh strength steel. Acta Metall. Sin., 2012, 48(5): 621.

[116] Wang XN, Du LX, Di HS. Study on fatigue property of new type hot-rolled nano precipitation strengthening ultrahigh strength automobile strip. J. Mech. Eng., 2012, 48(22): 27.

[117] Wang XN, Du LX, Di HS. Austenitic transformation behavior of hot-rolled 590 MPa grade wheel wteel. J. Iron Steel Res., 2013, 25(4): 33.

[118] Wang XN, Du LX, Zhang HL, Di HS. Industrial trial of 780 MPa grade heavy-duty truck beams steels. J. Iron Steel Res., 2011, 23(5): 45

[119] Funakawa Y, Seto K. Coarsening behavior of nanometer-sized carbides in hot-rolled high strength sheet steel. Mater. Sci. Forum, 2007, 539–543, 4813.

[120] Huang Y, Liu WN, Zhao AM, Han JK, Wang ZG, Yin HX. Effect of Mo content on the thermal stability of Ti−Mo-bearing ferritic steel. Int. J. Miner. Metall. Mater., 2021, 28(3): 412.

[121] Zhang K, Li ZD, Sun XJ, Yong QL, Yang JW, Li YM, Zhao PL. Development of Ti-V-Mo complex microalloyed hot-rolled 900-MPa-grade high-strength steel. Acta Metall. Sin. Engl. Lett., 2015, 28(5): 641.

[122] Zhang K, Yong QL, Sun XJ, Li ZD, Zhao PL. Effect of coiling temperature on micro-structure and mechanical properties of Ti−V−Mo complex microalloyed ultra-high strength steel. Acta Metall. Sin., 2016, 52(5): 371