Cov txheej txheem kho saum npoo ntawm cov qhov rooj steel hlau yog ntau haiv neeg. Tsis tas li ntawd, lub qhov rooj hluav taws steel tuaj yeem ua tiav nrog ntau yam khoom siv hluav taws xob, uas muaj peev xwm tiv thaiv hluav taws thiab pa taws zoo, nrog rau lub suab rwb thaiv tsev, thiab zoo dua rau cov tsev kawm ntawv, tsev kho mob thiab lwm qhov chaw uas muaj suab nrov heev.

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