Metallus Inc.

UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

WASHINGTON, D.C. 20549

SCHEDULE 14A

(RULE

14a-101)

SCHEDULE 14A INFORMATION

Proxy Statement Pursuant to Section 14(a) of the Securities

Exchange Act of 1934 (Amendment No. )

Filed by the Registrant

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Metallus Inc.

(Name of Registrant as Specified In Its Charter)

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Dear Shareholders,

Metallus Inc. invites you to attend our 2026 annual meeting of shareholders at 10:00 a.m. EDT on April 30, 2026. We will consider matters that are important to our company and to you.

2025 marked a period of commercial recovery for our specialty steel and multi-metal solutions after a challenging 2024. Throughout the year, demand improved across most end markets, and Section 232 steel tariffs helped to ensure fair trade conditions for our domestic steel industry, which is critical for national defense and infrastructure.

We were successful in growing our position in aerospace and defense and gained strong momentum with vacuum arc remelt (VAR) steel. With an enhanced focus on this product offering in 2025, supported by our downstream processing capabilities and strategic supplier partnership, we delivered VAR products to meet increased customer demand. Alongside growing volumes with existing customers, the enhanced strength and durability of VAR steel enabled us to win new business in the aerospace, defense and industrial sectors. VAR-related revenue reached approximately $28 million in 2025, almost twice that of the previous year.

We maintained our unwavering focus on safety, with initiatives like zero incident planning, crew safety meetings, and our StandUP for Safety program, actively involving more than 1,000 employees and enhancing both hazard awareness and safe work practices. A focus on strong leading indicators underscores a more proactive, prevention-focused safety culture. Our dedication to preventing serious injuries paid off as we had zero serious injuries in 2025. We received industry recognition for our commitment to safety, earning the Safety Culture Improvement Award from the Metals Service Center Institute.

We also continued to invest in our people in order to foster a dynamic and resilient workforce that can adapt to the evolving needs of our industry. During the year we focused on enhancing bothjob-specific skills and leadership capabilities and expanded and strengthened ourco-op and apprenticeship programs. These initiatives reinforce our culture of Care, Communicate, Collaborate, Follow-Through, and Follow-Up, positioning us for sustainable growth.

We made significant strides in advancing our operational capabilities and supporting our long-term growth strategy, completing theramp-up for the automated grinding line at our Harrison plant in late 2025. We remain on track for the scheduled commissioning of the new bloom reheat furnace, roller hearth furnace, and automated saw lines in 2026. These state-of-the-art assets will strengthen our ability to serve all of our customers with high-quality specialty metals, enhance production capability, and improve first-time quality. At the same time, we are advancing organizational capabilities through implementation of a standardizedday-to-day manufacturing operating system to drive consistency of execution across our plants while reducing cost and enabling profitable growth.

The improved demand environment in 2025 resulted in profitability driven primarily by higher volume and raw material spread, as well as better fixed cost leverage on higher melt utilization, offset by lower base selling prices. For the full year 2025, we delivered net sales of $1,158.3 million, a net loss of $1.2 million, adjusted EBITDA of $75.6 million(1), and operating cash flow totaling $16.0 million. Liquidity remained strong at $389.2 million, reinforcing our ability to invest and return capital to shareholders.

We continue to execute a balanced capital allocation strategy, investing in critical assets and reliability improvements while maintaining our share repurchase program. We repurchased 0.9 million shares of common stock for $13.1 million in 2025 and also repaid the remaining convertible notes balance. In total (including share repurchase and convertible notes repurchase and settlement activities) we have reduced diluted shares outstanding by approximately 25 percent or 13.5 million shares since the inception of our share repurchase program in early 2022. Our strong through-cycle cash flow generation supports our strategic priorities.

Working together with the United Steelworkers (USW) Local 1123, we recently reached a mutually beneficial labor contract that recognizes and respects the dedicated workforce who drive our operations. This agreement delivers wage increases as well as comprehensive healthcare and retirement benefits for every USW member and positions us to continue to serve our customers well in 2026 and beyond.

Steady customer demand and our reputation for product quality in critical markets have provided a strong order book as we begin 2026. Our top priorities for 2026 center on further advancing our safety culture and performance and ensuring asset reliability and first-time quality in every shift and process. These actions will enable us to consistently meet customer commitments and pursue strategic growth with new and existing customers, capitalizing on momentum in the aerospace and defense market and more broadly in the U.S. steel industry as a domestic producer of high-quality products.

We are grateful to our employees for their dedication, our customers for their trust, our suppliers for their partnership, and our shareholders for your continued support.

Sincerely,

March 12, 2026

Enclosure

Notice of annual meeting

of shareholders

Annual meeting information

Date: April 30, 2026

Time:10:00 a.m. Eastern time

Place: The meeting will be held “virtually” over the internet atwww.cesonlineservices.com/mtus26_vm. There will be noin-person annual meeting.

Record date: February 27, 2026

Agenda

Admission to the meeting

Only Metallus shareholders as of the close of business on February 27, 2026 (the record date for the annual meeting) are entitled to join the virtual meeting. To attend the virtual annual meeting, please follow the instructions set forth below under “Additional information.”

This proxy statement and the accompanying proxy card are being made available to shareholders beginning on or about March 18, 2026.

March 12, 2026

Kristine C. Syrvalin

Executive Vice President, General Counsel and

Chief Human Resources Officer

Your vote is important Please vote as soon as possible. Whether or not you plan to participate in the 2026 annual meeting of shareholders, please promptly vote on the internet, by phone or by mail. Voting early will help avoid additional solicitation costs and will not prevent you from voting electronically during the annual meeting if you wish to do so. How to vote:

Online: www.cesvote.com

Mail: If you received a printed version of the proxy materials, you may vote by mail.

During the annual meeting: If you participate in the virtual annual meeting, you may vote during the annual meeting by following the instructions set forth below under “Additional Information.”

Additional voting instructions are provided in this proxy statement and on the accompanying proxy card or on the notice of internet availability.

Important notice regarding the availability of proxy materials for the 2026 annual meeting of shareholders to be held on April 30, 2026: This proxy statement and our 2025 annual report to shareholders are available free of charge on the following website: www.proxydocs.com/mtus.

If you have not already done so, please consider signing up to receive proxy materials electronically by following the instructions when you vote your shares over the internet. Enrolling in electronic delivery reduces printing and mailing expenses and environmental impact.

Additional information

To register to attend the virtual annual meeting, please follow these instructions:

Registered Shareholders: If your shares are registered in your name with the company’s transfer agent or you are a participant holding Metallus shares in a 401(k) plan sponsored by Metallus and you wish to attend the virtual meeting, go towww.cesonlineservices.com/mtus26_vm. Please have your proxy card or notice of internet availability containing your 11- digit control number available and follow the instructions to complete your registration request. After registering, you will receive a confirmation email with a link and instructions for accessing the virtual annual meeting. Requests to register to participate in the virtual annual meeting must be received no later than 10:00 a.m. Eastern time on April 29, 2026.

Beneficial Shareholders (those holding shares through a stock brokerage account or by a bank or other holder of record): Beneficial shareholders who wish to attend the virtual meeting may register by visiting the websitewww.cesonlineservices.com/mtus26_vm. Please have your voting instruction form, notice or other communication containing your control number available and follow the instructions to complete your registration request. After registering, you will receive a confirmation email with a link and instructions for accessing the virtual annual meeting. Requests to register to participate in the virtual annual meeting must be received no later than 10:00 a.m. Eastern time on April 29, 2026.

If you do not register in advance following the instructions above, you will not be able to attend the annual meeting. If you need assistance registering, please contact our proxy solicitor, Innisfree M&A Incorporated, toll-free at (888)750-5834. The annual meeting will begin promptly at 10:00 a.m. Eastern time on April 30, 2026. We encourage you to access the annual meeting prior to the start time. Online access will begin at 9:30 a.m. Eastern time.

The virtual annual meeting platform is fully supported across browsers (Microsoft Edge, Firefox, Chrome and Safari) and devices (desktops, laptops, tablets and cell phones) running the most updated version of applicable software and plugins. Participants should ensure they have a strong internet connection wherever they intend to participate in the annual meeting. Participants also should allow plenty of time to log in and ensure that they can hear streaming audio prior to the start of the annual meeting.

In order to maintain the interactive nature of the virtual meeting, attendees will be able to vote during the annual meeting by clicking on the ‘Shareholder Ballot’ link located under the ‘Meeting Links’ section of the virtual meeting website.

If you wish to submit a question to be addressed at the annual meeting, please do so by submitting your question to MTUSRegister@Proxy-Agent.com in advance of the meeting. Questions will be accepted until 12:00 p.m. Eastern time on April 29, 2026.

Questions submitted by shareholders in accordance with the foregoing instructions and pertinent to the annual meeting will be answered during the annual meeting, subject to time constraints. We will respond to questions relevant to meeting matters that we do not have time to address during the meeting by posting the questions and answers on our investor relations website soon after the meeting. Questions regarding personal matters or matters not relevant to meeting matters will not be answered. Additional information regarding the ability of shareholders to ask questions during the annual meeting, related rules of conduct and other materials for the annual meeting will be available online during the meeting.

Even if you plan on attending the virtual annual meeting, we encourage you to vote your shares in advance using one of the methods described in this proxy statement to ensure that your vote will be represented at the annual meeting.

Technical Difficulties: If you experience any technical difficulties during the registration process, please access the ‘FAQ and System Test’ link on the login screen for support. If you experience technical difficulties during the virtual meeting, please access the ‘?’ link on the meeting website for support. In the event of technical difficulties with the virtual annual meeting, we expect that an announcement will be made onwww.cesonlineservices.com/mtus26_vm. If necessary, the announcement will provide updated information regarding the date, time and location of the annual meeting. Any updated information regarding the annual meeting will also be posted on our investor relations website atwww.metallus.com.