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2 SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Details) - shares | 12 Months Ended
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Dec. 31, 2019 | Dec. 31, 2018
SummaryOfSignificantAccountingPoliciesLineItems [Line Items] |  |  |
Number of shares held by CSN (in units) |  | 1,387,524,047 | 1,387,524,047
Direct [Member] | Ita Energetica S.A. [Member] | Joint Operation [member] |  |  |
SummaryOfSignificantAccountingPoliciesLineItems [Line Items] |  |  |
Number of shares held by CSN (in units) |  | 253,606,846 |
Core business |  | Electric power generation |
Companies, operation |  | Itá Energética S.A. |
Equity interests (%), operation |  | 48.75% | 48.75%
Direct [Member] | Consorcio Da Usina Hidreletrica De Lgarapava [Member] | Joint Operation [member] |  |  |
SummaryOfSignificantAccountingPoliciesLineItems [Line Items] |  |  |
Core business |  | Electric power consortium |
Companies, operation |  | Consórcio da Usina Hidrelétrica de Igarapava |
Equity interests (%), operation |  | 17.92% | 17.92%
Direct [Member] | MRS Logistica S.A. [Member] | Joint Venture [Member] |  |  |
SummaryOfSignificantAccountingPoliciesLineItems [Line Items] |  |  |
Number of shares held by CSN (in units) | [1] | 63,377,198 |
Core business | [1] | Railroad transportation |
Companies, joint venture | [1] | MRS Logística S.A. |
Equity interest (%), joint venture | [1] | 18.64% | 18.64%
Direct [Member] | Aceros Del Orinoco S.A. [Member] | Joint Venture [Member] |  |  |
SummaryOfSignificantAccountingPoliciesLineItems [Line Items] |  |  |
Core business |  | Dormant company |
Companies, joint venture |  | Aceros Del Orinoco S.A. |
Equity interest (%), joint venture |  | 31.82% | 31.82%
Direct [Member] | CBSI - Companhia Brasileira De Servicos De Lnfraestrutura [Member] | Joint Venture [Member] |  |  |
SummaryOfSignificantAccountingPoliciesLineItems [Line Items] |  |  |
Core business |  | Provision of services |
Companies, joint venture |  | CBSI - Companhia Brasileira de Serviços de Infraestrutura |
Equity interest (%), joint venture |  |  | 50.00%
Direct [Member] | Transnordestina Logistica S.A [Member] | Joint Venture [Member] |  |  |
SummaryOfSignificantAccountingPoliciesLineItems [Line Items] |  |  |
Number of shares held by CSN (in units) | [2] | 24,670,093 |
Core business | [2] | Railroad logistics |
Companies, joint venture | [2] | Transnordestina Logística S.A. |
Equity interest (%), joint venture | [2] | 47.26% | 46.30%
Direct [Member] | Arvedi Metalfer do Brasil S.A. [Member] | Associates [member] |  |  |
SummaryOfSignificantAccountingPoliciesLineItems [Line Items] |  |  |
Number of shares held by CSN (in units) |  | 46,994,971 |
Core business |  | Metallurgical and Equity interests |
Companies, associate |  | Arvedi Metalfer do Brasil S.A. |
Equity interest (%), associate |  | 20.00% | 20.00%
Indirect [Member] | MRS Logistica S.A. [Member] | Joint Venture [Member] |  |  |
SummaryOfSignificantAccountingPoliciesLineItems [Line Items] |  |  |
Core business |  | Railroad transportation |
Companies, joint venture |  | MRS Logística S.A. |
Equity interest (%), joint venture |  | 16.30% | 16.30%
Subsidiaries [member] | Direct [Member] | CSN Islands VII Corp. [Member] |  |  |
SummaryOfSignificantAccountingPoliciesLineItems [Line Items] |  |  |
Companies, subsidiary |  | CSN Islands VII Corp. |
Number of shares held by CSN (in units) |  | 20,001,000 |
Equity interests (%), subsidiary |  | 100.00% | 100.00%
Core business |  | Financial transactions |
Subsidiaries [member] | Direct [Member] | CSN Islands XI Corp. [Member] |  |  |
SummaryOfSignificantAccountingPoliciesLineItems [Line Items] |  |  |
Companies, subsidiary |  | CSN Islands XI Corp. |
Number of shares held by CSN (in units) |  | 50,000 |
Equity interests (%), subsidiary |  | 100.00% | 100.00%
Core business |  | Financial transactions |
Subsidiaries [member] | Direct [Member] | CSN Islands XII Corp. [Member] |  |  |
SummaryOfSignificantAccountingPoliciesLineItems [Line Items] |  |  |
Companies, subsidiary |  | CSN Islands XII Corp. |
Number of shares held by CSN (in units) |  | 1,540 |
Equity interests (%), subsidiary |  | 100.00% | 100.00%
Core business |  | Financial transactions |
Subsidiaries [member] | Direct [Member] | CSN Steel S.L.U. [Member] |  |  |
SummaryOfSignificantAccountingPoliciesLineItems [Line Items] |  |  |
Companies, subsidiary |  | CSN Steel S.L.U. |
Number of shares held by CSN (in units) |  | 22,042,688 |
Equity interests (%), subsidiary |  | 100.00% | 100.00%
Core business |  | Equity interests and Financial transactions |
Subsidiaries [member] | Direct [Member] | TdBB S.A [Member] |  |  |
SummaryOfSignificantAccountingPoliciesLineItems [Line Items] |  |  |
Companies, subsidiary | [3] | TdBB S.A |
Equity interests (%), subsidiary | [3] | 100.00% | 100.00%
Core business | [3] | Equity interests |
Subsidiaries [member] | Direct [Member] | Sepetiba Tecon S.A. [Member] |  |  |
SummaryOfSignificantAccountingPoliciesLineItems [Line Items] |  |  |
Companies, subsidiary |  | Sepetiba Tecon S.A. |
Number of shares held by CSN (in units) |  | 254,015,052 |
Equity interests (%), subsidiary |  | 99.99% | 99.99%
Core business |  | Port services |
Subsidiaries [member] | Direct [Member] | Minerios Nacional S.A. [Member] |  |  |
SummaryOfSignificantAccountingPoliciesLineItems [Line Items] |  |  |
Companies, subsidiary |  | Minérios Nacional S.A. |
Number of shares held by CSN (in units) |  | 141,719,295 |
Equity interests (%), subsidiary |  | 99.99% | 99.99%
Core business |  | Mining and Equity interests |
Subsidiaries [member] | Direct [Member] | Companhia Florestal Do Brasil [Member] |  |  |
SummaryOfSignificantAccountingPoliciesLineItems [Line Items] |  |  |
Companies, subsidiary |  | Companhia Florestal do Brasil |
Number of shares held by CSN (in units) |  | 42,551,519 |
Equity interests (%), subsidiary |  | 99.99% | 99.99%
Core business |  | Reforestation |
Subsidiaries [member] | Direct [Member] | Estanho De Rondonia S.A. [Member] |  |  |
SummaryOfSignificantAccountingPoliciesLineItems [Line Items] |  |  |
Companies, subsidiary |  | Estanho de Rondônia S.A. |
Number of shares held by CSN (in units) |  | 195,454,162 |
Equity interests (%), subsidiary |  | 99.99% | 99.99%
Core business |  | Tin Mining |
Subsidiaries [member] | Direct [Member] | Companhia Metalurgica Prada [Member] |  |  |
SummaryOfSignificantAccountingPoliciesLineItems [Line Items] |  |  |
Companies, subsidiary |  | Companhia Metalúrgica Prada |
Number of shares held by CSN (in units) |  | 445,921,292 |
Equity interests (%), subsidiary |  | 99.99% | 99.99%
Core business |  | Manufacture of packages and distribution of steel products |
Subsidiaries [member] | Direct [Member] | CSN Gestao De Recursos Financeiros Ltda [Member] |  |  |
SummaryOfSignificantAccountingPoliciesLineItems [Line Items] |  |  |
Companies, subsidiary | [3],[4] | CSN Gestão de Recursos Financeiros Ltda. |
Equity interests (%), subsidiary | [3],[4] |  | 99.99%
Core business | [3],[4] | Management of funds and securities portfolio |
Subsidiaries [member] | Direct [Member] | CSN Mineracao S.A. [Member] |  |  |
SummaryOfSignificantAccountingPoliciesLineItems [Line Items] |  |  |
Companies, subsidiary |  | CSN Mineração S.A. |
Number of shares held by CSN (in units) |  | 158,419,480 |
Equity interests (%), subsidiary |  | 87.52% | 87.52%
Core business |  | Mining and Equity interests |
Subsidiaries [member] | Direct [Member] | CSN Energia S.A. [Member] |  |  |
SummaryOfSignificantAccountingPoliciesLineItems [Line Items] |  |  |
Companies, subsidiary |  | CSN Energia S.A. |
Number of shares held by CSN (in units) |  | 43,149 |
Equity interests (%), subsidiary |  | 99.99% | 99.99%
Core business |  | Sale of electric power |
Subsidiaries [member] | Direct [Member] | FTL - Ferrovia Transnordestina Logistica S.A. [Member] |  |  |
SummaryOfSignificantAccountingPoliciesLineItems [Line Items] |  |  |
Companies, subsidiary |  | FTL - Ferrovia Transnordestina Logística S.A. |
Number of shares held by CSN (in units) |  | 486,592,830 |
Equity interests (%), subsidiary |  | 92.38% | 91.69%
Core business |  | Railroad logistics |
Subsidiaries [member] | Direct [Member] | Nordeste Logistica S.A. [Member] |  |  |
SummaryOfSignificantAccountingPoliciesLineItems [Line Items] |  |  |
Companies, subsidiary |  | Nordeste Logística S.A. |
Number of shares held by CSN (in units) |  | 99,999 |
Equity interests (%), subsidiary |  | 99.99% | 99.99%
Core business |  | Port services |
Subsidiaries [member] | Direct [Member] | Aceros Mexico CSN [Member] |  |  |
SummaryOfSignificantAccountingPoliciesLineItems [Line Items] |  |  |
Companies, subsidiary | [5] | Aceros México CSN |
Equity interests (%), subsidiary | [5] |  | 0.08%
Core business | [5] | Commercial representation, steel sales and related activities |
Subsidiaries [member] | Direct [Member] | CSN Inova Ltd. [Member] |  |  |
SummaryOfSignificantAccountingPoliciesLineItems [Line Items] |  |  |
Companies, subsidiary |  | CSN Inova Ltd. |
Equity interests (%), subsidiary |  | 100.00% | 100.00%
Core business |  | Advisory and implementation of new development projects |
Subsidiaries [member] | Direct [Member] | CSN Equipamentos S.A [Member] |  |  |
SummaryOfSignificantAccountingPoliciesLineItems [Line Items] |  |  |
Companies, subsidiary | [6] | CSN Equipamentos S.A |
Number of shares held by CSN (in units) | [6] | 999 |
Equity interests (%), subsidiary | [6] | 99.99% |
Core business | [6] | Rental of commercial and industrial machinery and equipment |
Subsidiaries [member] | Direct [Member] | CBSI - Companhia Brasileira De Servicos De Infraestrutura [Member] |  |  |
SummaryOfSignificantAccountingPoliciesLineItems [Line Items] |  |  |
Companies, subsidiary | [7] | CBSI - Companhia Brasileira de Serviços de Infraestrutura |
Number of shares held by CSN (in units) | [7] | 3,752,292 |
Equity interests (%), subsidiary | [7] | 100.00% |
Core business | [7] | Provision of services |
Subsidiaries [member] | Indirect [Member] | Aceros Mexico CSN [Member] |  |  |
SummaryOfSignificantAccountingPoliciesLineItems [Line Items] |  |  |
Companies, subsidiary | [5] | Aceros México CSN |
Equity interests (%), subsidiary | [5] |  | 99.92%
Core business | [5] | Commercial representation, steel sales and related activities |
Subsidiaries [member] | Indirect [Member] | Lusosider Projectos Siderurgicos S.A. [Member] |  |  |
SummaryOfSignificantAccountingPoliciesLineItems [Line Items] |  |  |
Companies, subsidiary |  | Lusosider Projectos Siderúrgicos S.A. |
Equity interests (%), subsidiary |  | 100.00% | 100.00%
Core business |  | Equity interests and product sales |
Subsidiaries [member] | Indirect [Member] | Lusosider Acos Planos, S. A. [Member] |  |  |
SummaryOfSignificantAccountingPoliciesLineItems [Line Items] |  |  |
Companies, subsidiary |  | Lusosider Aços Planos, S. A. |
Equity interests (%), subsidiary |  | 99.99% | 99.99%
Core business |  | Steel and Equity interests |
Subsidiaries [member] | Indirect [Member] | CSN Resources S.A. [Member] |  |  |
SummaryOfSignificantAccountingPoliciesLineItems [Line Items] |  |  |
Companies, subsidiary |  | CSN Resources S.A. |
Equity interests (%), subsidiary |  | 100.00% | 100.00%
Core business |  | Financial transactions and Equity interests |
Subsidiaries [member] | Indirect [Member] | Companhia Brasileira De Latas [Member] |  |  |
SummaryOfSignificantAccountingPoliciesLineItems [Line Items] |  |  |
Companies, subsidiary |  | Companhia Brasileira de Latas |
Equity interests (%), subsidiary |  | 99.99% | 99.99%
Core business |  | Sale of cans and packages in general and Equity interests |
Subsidiaries [member] | Indirect [Member] | Companhia De Embalagens Metalicas MMSA [Member] |  |  |
SummaryOfSignificantAccountingPoliciesLineItems [Line Items] |  |  |
Companies, subsidiary |  | Companhia de Embalagens Metálicas MMSA |
Equity interests (%), subsidiary |  | 99.67% | 99.67%
Core business |  | Production and sale of cans and related activities |
Subsidiaries [member] | Indirect [Member] | Companhia De Embalagens Metalicas - MTM [Member] |  |  |
SummaryOfSignificantAccountingPoliciesLineItems [Line Items] |  |  |
Companies, subsidiary |  | Companhia de Embalagens Metálicas - MTM |
Equity interests (%), subsidiary |  | 99.67% | 99.67%
Core business |  | Production and sale of cans and related activities |
Subsidiaries [member] | Indirect [Member] | CSN Steel Holdings 1, S.L.U. [Member] |  |  |
SummaryOfSignificantAccountingPoliciesLineItems [Line Items] |  |  |
Companies, subsidiary |  | CSN Steel Holdings 1, S.L.U. |
Equity interests (%), subsidiary |  | 100.00% | 100.00%
Core business |  | Financial transactions, product sales and Equity interests |
Subsidiaries [member] | Indirect [Member] | CSN Productos Siderurgicos S.L. [Member] |  |  |
SummaryOfSignificantAccountingPoliciesLineItems [Line Items] |  |  |
Companies, subsidiary |  | CSN Productos Siderúrgicos S.L. |
Equity interests (%), subsidiary |  | 100.00% | 100.00%
Core business |  | Financial transactions, product sales and Equity interests |
Subsidiaries [member] | Indirect [Member] | Stalhwerk Thuringen GmbH [Member] |  |  |
SummaryOfSignificantAccountingPoliciesLineItems [Line Items] |  |  |
Companies, subsidiary |  | Stalhwerk Thüringen GmbH |
Equity interests (%), subsidiary |  | 100.00% | 100.00%
Core business |  | Production and sale of long steel and related activities |
Subsidiaries [member] | Indirect [Member] | CSN Steel Sections UK Limited [Member] |  |  |
SummaryOfSignificantAccountingPoliciesLineItems [Line Items] |  |  |
Companies, subsidiary | [3] | CSN Steel Sections UK Limited |
Equity interests (%), subsidiary | [3] | 100.00% | 100.00%
Core business | [3] | Sale of long steel |
Subsidiaries [member] | Indirect [Member] | CSN Steel Sections Polska Sp.Z.o.o [Member] |  |  |
SummaryOfSignificantAccountingPoliciesLineItems [Line Items] |  |  |
Companies, subsidiary |  | CSN Steel Sections Polska Sp.Z.o.o |
Equity interests (%), subsidiary |  | 100.00% | 100.00%
Core business |  | Financial transactions, product sales and Equity interests |
Subsidiaries [member] | Indirect [Member] | CSN Asia Limited [Member] |  |  |
SummaryOfSignificantAccountingPoliciesLineItems [Line Items] |  |  |
Companies, subsidiary | [8] | CSN Asia limited |
Equity interests (%), subsidiary | [8] |  | 100.00%
Core business | [8] | Commercial representation |
Subsidiaries [member] | Indirect [Member] | CSN Mining Holding, S.L [Member] |  |  |
SummaryOfSignificantAccountingPoliciesLineItems [Line Items] |  |  |
Companies, subsidiary |  | CSN Mining Holding, S.L |
Equity interests (%), subsidiary |  | 87.52% | 87.52%
Core business |  | Financial transactions, product sales and Equity interests |
Subsidiaries [member] | Indirect [Member] | CSN Mining GmbH [Member] |  |  |
SummaryOfSignificantAccountingPoliciesLineItems [Line Items] |  |  |
Companies, subsidiary |  | CSN Mining GmbH |
Equity interests (%), subsidiary |  | 87.52% | 87.52%
Core business |  | Financial transactions, product sales and Equity interests |
Subsidiaries [member] | Indirect [Member] | CSN Mining Asia Limited [Member] |  |  |
SummaryOfSignificantAccountingPoliciesLineItems [Line Items] |  |  |
Companies, subsidiary |  | CSN Mining Asia Limited |
Equity interests (%), subsidiary |  | 87.52% | 87.52%
Core business |  | Commercial representation |
Subsidiaries [member] | Indirect [Member] | Lusosider Lberica S.A. [Member] |  |  |
SummaryOfSignificantAccountingPoliciesLineItems [Line Items] |  |  |
Companies, subsidiary |  | Lusosider Ibérica S.A. |
Equity interests (%), subsidiary |  | 100.00% | 100.00%
Core business |  | Steel, commercial and industrial activities and equity interests |
Subsidiaries [member] | Indirect [Member] | CSN Mining Portugal, Unipessoal Lda. [Member] |  |  |
SummaryOfSignificantAccountingPoliciesLineItems [Line Items] |  |  |
Companies, subsidiary |  | CSN Mining Portugal, Unipessoal Lda. |
Equity interests (%), subsidiary |  | 87.52% | 87.52%
Core business |  | Commercial and representation of products |
Subsidiaries [member] | Indirect [Member] | Companhia Siderurgica Nacional, LLC [Member] |  |  |
SummaryOfSignificantAccountingPoliciesLineItems [Line Items] |  |  |
Companies, subsidiary |  | Companhia Siderúrgica Nacional, LLC |
Equity interests (%), subsidiary |  | 100.00% | 100.00%
Core business |  | Import and distribution/resale of products |
[1] | On December 31, 2019 and 2018, the Company directly owned 26,611,282 common shares, 2,673,312 preferred shares class A and 34,092,604 preferred shares class B, totaling 36,765,916 preferred shares of MRS Logistica S.A.
[2] | On May 10, 2019, 501,789 shares of shareholder FINOR, all class B preferred shares, were transferred to shareholder CSN. On December 31,2019, the Company had 24,168,304 ordinary shares, 501,789 preferred shares Class B (as of December 31,2018 had 24,168,304 ordinary shares and no preferred shares).
[3] | They are dormant Companies therefore they do not appear in the note 8.a, where is disclosed business information under the equity method and fair value through profit or loss and comprehensive income.
[4] | CSN Gestao de Recursos Financeiros was liquidated on June 13, 2019
[5] | On February 1, 2019, the Federal Taxpayers' Registry was canceled and, therefore, the settlement process of Aceros Mexico CSN was terminated, however, before third parties and for the purposes of commercial law, the settlement was retroactive to September 18, 2018
[6] | Company incorporated on August 22, 2019.
[7] | On November 29, 2019, a purchase and sale agreement for share was signed, whereby Companhia Siderurgica Nacional acquired the entire participation that CKTR Brasil Servicos Ltda held in CBSI - Companhia Brasileira de Servicos de Infraestrutura. As a result, as of the date mentioned, CSN now holds 100% of CBSI's share capital.
[8] | On August 6, 2019 CSN Asia Limited was liquidated

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