BUSINESS AND SUMMARY OF ACCOUNTING POLICIES (Policies)	6 Months Ended
Jun. 30, 2025
Accounting Policies [Abstract]
Basis of presentation	Basis of presentation and principles of consolidation The accompanying unaudited interim condensed consolidated financial statements include the accounts of Constellium SE and its controlled subsidiaries. All intercompany transactions and balances are eliminated. The accompanying unaudited interim condensed consolidated financial statements have been prepared by Constellium in accordance with U.S. generally accepted accounting principles ("GAAP") and the rules and regulations of the Securities and Exchange Commission ("SEC") applicable for interim periods and, therefore, do not include all information and footnotes required by GAAP for complete financial statements. In management’s opinion, all adjustments (which include normal recurring adjustments) considered necessary for a fair statement of its financial position at June 30, 2025, results of operations and cash flows for the three-month and six-month periods ended June 30, 2025 and 2024 have been included. The accompanying unaudited interim condensed consolidated financial statements should be read in conjunction with the Group’s audited consolidated financial statements and accompanying notes in its annual report on Form 10-K for the year ended December 31, 2024. The results of operations for our interim periods are not necessarily indicative of the results of operations that may be achieved for the entire 2025 fiscal year.
Principles of consolidation	Basis of presentation and principles of consolidation The accompanying unaudited interim condensed consolidated financial statements include the accounts of Constellium SE and its controlled subsidiaries. All intercompany transactions and balances are eliminated. The accompanying unaudited interim condensed consolidated financial statements have been prepared by Constellium in accordance with U.S. generally accepted accounting principles ("GAAP") and the rules and regulations of the Securities and Exchange Commission ("SEC") applicable for interim periods and, therefore, do not include all information and footnotes required by GAAP for complete financial statements. In management’s opinion, all adjustments (which include normal recurring adjustments) considered necessary for a fair statement of its financial position at June 30, 2025, results of operations and cash flows for the three-month and six-month periods ended June 30, 2025 and 2024 have been included. The accompanying unaudited interim condensed consolidated financial statements should be read in conjunction with the Group’s audited consolidated financial statements and accompanying notes in its annual report on Form 10-K for the year ended December 31, 2024. The results of operations for our interim periods are not necessarily indicative of the results of operations that may be achieved for the entire 2025 fiscal year.
Use of estimates and assumptions	Use of estimates and assumptions The preparation of the Group’s consolidated financial statements in accordance with U.S. GAAP requires management to make judgments, estimates and assumptions that affect the reported amounts of revenues, expenses, assets and liabilities, and the accompanying disclosures, and the disclosure of contingent liabilities. The principal areas of judgment relate to (1) impairment of assets; (2) actuarial assumptions related to pension and other postretirement benefit plans; (3) tax uncertainties and valuation allowances; and (4) assessment of loss contingencies, including environmental and litigation liabilities. These judgments, estimates and assumptions are based on management’s best knowledge of the relevant facts and circumstances, giving consideration to previous experience. Future events and their effects cannot be predicted with certainty, and accordingly, our accounting estimates require the exercise of judgment. The accounting estimates used in the preparation of our consolidated financial statements may change as new events occur, more experience is acquired, additional information is obtained, and our operating environment changes. The Group continuously reviews its significant assumptions and estimates in light of the uncertainty associated with the global geopolitical and macroeconomic conditions and their potential direct and indirect impacts on its business and its financial statements. There can be no guarantee that our assumptions will materialize or that actual results will not differ materially from estimates.
Recently adopted and recently issued accounting guidance	Recently adopted and recently issued accounting guidance In December 2023, the Financial Accounting Standards Board ("FASB") issued ASU 2023-09 - Improvements to Income Tax Disclosures. The standard requires disaggregated information about a reporting entity's effective tax rate reconciliation as well as information on income taxes paid. The standard is intended to benefit investors by providing more detailed income tax disclosures that would be useful in making capital allocation decisions and applies to all entities subject to income taxes. The new standard is effective for annual periods beginning after December 15, 2024. Early adoption is permitted. This accounting standard is effective for annual disclosures in fiscal year ended December 31, 2025. We are currently evaluating the impact of adoption on our annual financial disclosures. In November 2024, the FASB issued ASU 2024-03 - Income Statement — Reporting Comprehensive Income — Expense Disaggregation Disclosures, requiring public business entities to disclose, on an annual and interim basis, disaggregated information about certain income statement line items in a tabular format in the notes to the financial statements. The standard is intended to benefit investors by providing more detailed expense information notably on employee compensation, depreciation and amortization and purchase of inventory, which is critical to understanding an entity’s performance, assessing its prospects for future cash flows and comparing its performance both over time and with that of other entities. This accounting standard as updated in ASU 2025-01 - Income Statement-Reporting Comprehensive Income-Expense Disaggregation Disclosures (Subtopic 220-40): Clarifying the Effective Date which clarified the interim reporting effective date of ASU 2024-03 is effective for annual periods beginning after December 15, 2026, and interim periods within fiscal years beginning after December 15, 2027. Early adoption is permitted and the guidance may be applied prospectively or retrospectively. We are currently evaluating the impact of adoption on our financial disclosures. The Group plans to adopt these new standards, amendments and interpretations on their required effective dates and does not expect any material impact on its financial position, results of operations and cash flows as a result of their adoption.

BUSINESS AND SUMMARY OF ACCOUNTING POLICIES (Policies)

6 Months Ended

Jun. 30, 2025

Basis of presentation and principles of consolidation

The accompanying unaudited interim condensed consolidated financial statements include the accounts of Constellium SE and its controlled subsidiaries. All intercompany transactions and balances are eliminated.

The accompanying unaudited interim condensed consolidated financial statements have been prepared by Constellium in accordance with U.S. generally accepted accounting principles ("GAAP") and the rules and regulations of the Securities and Exchange Commission ("SEC") applicable for interim periods and, therefore, do not include all information and footnotes required by GAAP for complete financial statements. In management’s opinion, all adjustments (which include normal recurring adjustments) considered necessary for a fair statement of its financial position at June 30, 2025, results of operations and cash flows for the three-month and six-month periods ended June 30, 2025 and 2024 have been included. The accompanying unaudited interim condensed consolidated financial statements should be read in conjunction with the Group’s audited consolidated financial statements and accompanying notes in its annual report on Form 10-K for the year ended December 31, 2024. The results of operations for our interim periods are not necessarily indicative of the results of operations that may be achieved for the entire 2025 fiscal year.

Principles of consolidation

Basis of presentation and principles of consolidation

Use of estimates and assumptions

The preparation of the Group’s consolidated financial statements in accordance with U.S. GAAP requires management to make judgments, estimates and assumptions that affect the reported amounts of revenues, expenses, assets and liabilities, and the accompanying disclosures, and the disclosure of contingent liabilities. The principal areas of judgment relate to (1) impairment of assets; (2) actuarial assumptions related to pension and other postretirement benefit plans; (3) tax uncertainties and valuation allowances; and (4) assessment of loss contingencies, including environmental and litigation liabilities. These judgments, estimates and assumptions are based on management’s best knowledge of the relevant facts and circumstances, giving consideration to previous experience. Future events and their effects cannot be predicted with certainty, and accordingly, our accounting estimates require the exercise of judgment. The accounting estimates used in the preparation of our consolidated financial statements may change as new events occur, more experience is acquired, additional information is obtained, and our operating environment changes. The Group continuously reviews its significant assumptions and estimates in light of the uncertainty associated with the global geopolitical and macroeconomic conditions and their potential direct and indirect impacts on its business and its financial statements. There can be no guarantee that our assumptions will materialize or that actual results will not differ materially from estimates.

Recently adopted and recently issued accounting guidance

In December 2023, the Financial Accounting Standards Board ("FASB") issued ASU 2023-09 - Improvements to Income Tax Disclosures. The standard requires disaggregated information about a reporting entity's effective tax rate reconciliation as well as information on income taxes paid. The standard is intended to benefit investors by providing more detailed income tax disclosures that would be useful in making capital allocation decisions and applies to all entities subject to income taxes. The new standard is effective for annual periods beginning after December 15, 2024. Early adoption is permitted. This accounting standard is effective for annual disclosures in fiscal year ended December 31, 2025. We are currently evaluating the impact of adoption on our annual financial disclosures.

In November 2024, the FASB issued ASU 2024-03 - Income Statement — Reporting Comprehensive Income — Expense Disaggregation Disclosures, requiring public business entities to disclose, on an annual and interim basis, disaggregated information about certain income statement line items in a tabular format in the notes to the financial statements. The standard

is intended to benefit investors by providing more detailed expense information notably on employee compensation, depreciation and amortization and purchase of inventory, which is critical to understanding an entity’s performance, assessing its prospects for future cash flows and comparing its performance both over time and with that of other entities. This accounting standard as updated in ASU 2025-01 - Income Statement-Reporting Comprehensive Income-Expense Disaggregation Disclosures (Subtopic 220-40): Clarifying the Effective Date which clarified the interim reporting effective date of ASU 2024-03 is effective for annual periods beginning after December 15, 2026, and interim periods within fiscal years beginning after December 15, 2027. Early adoption is permitted and the guidance may be applied prospectively or retrospectively. We are currently evaluating the impact of adoption on our financial disclosures.

The Group plans to adopt these new standards, amendments and interpretations on their required effective dates and does not expect any material impact on its financial position, results of operations and cash flows as a result of their adoption.