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2019年6月大学英语六级考试真题（三）
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Steelisvalued foritsreliability, .butnot when itgets .cold. Mostforms ofsteel 26 become
�rittle.(脆的） attemperatures belowabout :--25 'C. .unless theyare mixed withother metals. tho�gh, Now,
a-noveltypeofsteel developed ,that _ 27 .at much lower temperatures, whileretaining
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1 itsstrength·and toughnesswithout theneed forexpensive ' 28 .
,Steel's . fragilityat low ,temperatures first·becamea major ,concern •duringtheSecond World War.
AfterGerman U-boats torpedoed C用鱼雷攻击） numerous Britisha2, 700-strong fleetships,ofcheap:and­
cheerful"Liberty°ships" wasintroduced toreplace the lost vessels,· a lifeline providingforthe' 29
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，' steel shells·of hundreds ·ofthe ships · ·30·- inthe icy north Atlantic, and12 broke in
，T1 halfandsank.
lit Brittlenessaproblem remainswhenbuilding steel structures incold conditions, suchasoil rigs inthe
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；, have31 tofind asolution bymixing it with expensive metalssuch as nickel.
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_I · ·YuujiKimura�nd colleagues inJapan tried · amore physical 32 . Rather thanadding other
1 me��ls,theydevelope� a complexmechanical processinvolving repeatedheating and very severe
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14 mechanicaldeformation, �nownastempfom�ing. ..
_l ,;m eresulting steelappear� t�achi�ve� combinatioi:iofstrength, and toughness .that is 33·to
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j. thatofmodern .steelsthatare very richin alloy content and,th�refore, veryexpensive.
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I｀ Kimura'steamintends touse its tempformed �teeltomake ultra-high strengthpar�s, such .as bolts.
1 Theyhope toreduce boththe number of. 34- neededinaconstruction joband their ,weight-by
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