CSN, Brazil's largest diversified steel group, said on Tuesday that net income jumped 38 percent in the first quarter from a year earlier thanks to robust sales of iron ore and lower sales costs. Sao Paulo-based CSN (CSNA3.SA)(SID.N) earned 616 million reais ($390 million) in the period, up from 447 million reais a year earlier, according to a securities filing. But this fell short of the 697 million reais profit estimate by five analysts in a Reuters poll.