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EQUITY - Ownership of shares (Details) - shares | Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
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Capital |  |  |
Number of shares held | 1,719,658,728 | 1,719,658,728 | 1,719,658,728
Shares Issued, Percentage | 100.00% | 100.00% | 100.00%
Metalurgica Gerdau S.A. |  |  |
Capital |  |  |
Number of shares held | 653,370,032 | 652,519,280 | 702,554,138
Shares Issued, Percentage | 38.00% | 37.90% | 40.90%
Brazilian institutional investors |  |  |
Capital |  |  |
Number of shares held | 155,482,209 | 134,604,327 | 123,530,670
Shares Issued, Percentage | 9.00% | 7.80% | 7.20%
Foreign institutional investors |  |  |
Capital |  |  |
Number of shares held | 715,363,943 | 716,775,213 | 646,598,657
Shares Issued, Percentage | 41.60% | 41.80% | 37.60%
Other shareholders |  |  |
Capital |  |  |
Number of shares held | 185,040,945 | 205,049,695 | 213,991,018
Shares Issued, Percentage | 10.80% | 11.90% | 12.40%
Common shares |  |  |
Capital |  |  |
Number of shares held | 573,627,483 | 573,627,483 | 573,627,483
Shares Issued, Percentage | 100.00% | 100.00% | 100.00%
Common shares | Metalurgica Gerdau S.A. |  |  |
Capital |  |  |
Number of shares held | 557,898,901 | 449,712,654 | 449,712,654
Shares Issued, Percentage | 97.30% | 78.40% | 78.40%
Common shares | Brazilian institutional investors |  |  |
Capital |  |  |
Number of shares held | 254,785 | 41,883,032 | 49,834,446
Shares Issued, Percentage | 0.00% | 7.30% | 8.70%
Common shares | Foreign institutional investors |  |  |
Capital |  |  |
Number of shares held | 7,254,556 | 11,122,498 | 13,881,226
Shares Issued, Percentage | 1.30% | 1.90% | 2.40%
Common shares | Other shareholders |  |  |
Capital |  |  |
Number of shares held | 6,521,703 | 69,211,761 | 58,501,619
Shares Issued, Percentage | 1.10% | 12.10% | 10.20%
Preferred shares |  |  |
Capital |  |  |
Number of shares held | 1,146,031,245 | 1,146,031,245 | 1,146,031,245
Shares Issued, Percentage | 100.00% | 100.00% | 100.00%
Preferred shares | Metalurgica Gerdau S.A. |  |  |
Capital |  |  |
Number of shares held | 95,471,131 | 202,806,626 | 252,841,484
Shares Issued, Percentage | 8.30% | 17.70% | 22.10%
Preferred shares | Brazilian institutional investors |  |  |
Capital |  |  |
Number of shares held | 155,227,424 | 92,721,295 | 73,686,224
Shares Issued, Percentage | 13.50% | 8.10% | 6.40%
Preferred shares | Foreign institutional investors |  |  |
Capital |  |  |
Number of shares held | 708,109,387 | 705,652,715 | 632,717,431
Shares Issued, Percentage | 61.80% | 61.50% | 55.20%
Preferred shares | Other shareholders |  |  |
Capital |  |  |
Number of shares held | 178,519,242 | 135,837,934 | 155,489,399
Shares Issued, Percentage | 15.60% | 11.90% | 13.60%
Treasury Stocks |  |  |
Capital |  |  |
Number of shares held | 10,401,599 | 10,710,213 | 32,984,245
Shares Issued, Percentage | 0.60% | 0.60% | 1.90%
Treasury Stocks | Common shares |  |  |
Capital |  |  |
Number of shares held | 1,697,538 | 1,697,538 | 1,697,538
Shares Issued, Percentage | 0.30% | 0.30% | 0.30%
Treasury Stocks | Preferred shares |  |  |
Capital |  |  |
Number of shares held | 8,704,061 | 9,012,675 | 31,286,707
Shares Issued, Percentage | 0.80% | 0.80% | 2.70%

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