Recently, POSCO proved its unrivaled steel material technology by becoming the first in Korea to obtain Korean Register (KR) certification for its new naval materials: high-ductility steel for naval vessels and armor steel for naval vessels. The application of these new materials is expected to drastically improve the performance of the Republic of Korea (ROK) Navy’s next-generation vessels. How were these steel plates, which withstand extreme water pressure and strong external impacts, born? We sit down with Lee Jae-ik and Lee Jin-woo, Principal Researchers at POSCO’s Steel Solution Research Group, to take a closer look at POSCO’s core competitiveness and future vision leading K-marine defense materials.

Lee Jae-ik: Hello. I research “application technology” to help our customers perfectly implement thin yet strong “high-strength steel” into their products. Recently, as reducing the weight of structural frames for carbon reduction has become a major topic, the demand for high-strength steel has increased significantly. However, as steel becomes stronger, welding becomes more difficult, and even after successful welding, the fatigue life of the joints often fails to match the strength of the steel itself. I solve this exact problem. My role is to dramatically extend the fatigue life of high-strength steel welded joints, providing optimal solutions so that customers can safely use lighter and stronger steel for a long time.

Lee Jin-woo: Nice to meet you. I am Lee Jin-woo, researching customized customer solutions at POSCO’s Steel Solution Research Group. Our group’s role does not end with simply making good steel. We provide a comprehensive “user manual” containing the optimal technology and guidelines so that the material can perform at 100% capacity at the customer’s site.

Currently, my main areas of focus are twofold. First is next-generation material solutions for land-based weapon systems and special vessels (surface combatants) that lead K-defense. Second is the heavy equipment and construction machinery sector. In particular, by applying the “extreme environment response technology” and “structural optimization know-how” accumulated in the defense sector to construction equipment, we create synergy to maximize the durability and efficiency of the equipment when our steel is applied. In short, my role can be described as a “technological bridge” that transforms raw materials into core components for high-tech industries.

▲ Conceptual rendering of a next-generation naval vessel applying new defense materials.

Lee Jin-woo: This KR certification is a decisive moment showing that the role of naval materials has evolved from simple “structural materials” to “core functional materials” that determine combat capability. It also demonstrates that Korea’s naval shipbuilding technology has entered a new dimension through material innovation. In modern warfare, where advanced weapons are becoming more sophisticated, the evolution of the hull that protects them is a necessity, not an option. POSCO’s new materials have secured both defensibility and mobility. By increasing elongation to 35% and impact absorption capacity by over 50%, we secured strong survivability, while reducing hull thickness by 30% to dramatically improve mobility and fuel efficiency. Our materials are completing the super-gap of K-defense as core functional materials supporting weapon systems. I am confident that this will serve as a solid foundation for POSCO to leap forward as a key partner in the global defense export market, going beyond being a “steel shield” for Korea’s next-generation vessels.

Lee Jin-woo: POSCO’s new naval materials are evolving in a direction to develop stronger and tougher steel materials so that submarines can operate safely deep under the sea. First, high-ductility steel, which applies technology that remains intact even when colliding with glaciers, stretches much better than conventional steel (35% elongation) while its capacity to absorb impact has become over 50% stronger. In addition, high-performance armor steel, which transplants solid ballistic technology from land-based weapon systems to the marine sector, maintains its strength while reducing the thickness of the steel plate by approximately 30% compared to existing ones. Reducing the thickness of the steel plate enables lightweighting of the hull, thereby improving both mobility and fuel efficiency. This advancement in material technology symbolizes a paradigm shift from structural materials to core functional materials, and will be POSCO’s core competitiveness to contribute to expanding K-defense exports and entering the global naval vessel market in the future.

Lee Jae-ik: Since they are used at sea, both general shipbuilding heavy plates and special steel for naval vessels require high reliability. However, the core purpose of the materials is different. General shipbuilding heavy plates focus on optimizing “economic efficiency and stability” according to the waves or temperatures of designated routes. On the other hand, marine defense materials, such as surface combatants and submarines, must perform operations in any ocean worldwide and survive extreme combat situations like shell hits or explosions. Therefore, they require the most stringent standards in existence. While general shipbuilding mainly uses “high-efficiency steel” with good work efficiency such as welding, marine defense uses “premium special steel” containing a large amount of special alloys to boast the highest level of strength and toughness. This is the biggest difference.

Lee Jae-ik: The pressure hull, which is the framework of a submarine, must maintain its original shape firmly without fatigue or cracking to protect the crew and sensitive equipment from intense water pressure, explosions, and impacts. To achieve this, steel must possess a combination of strength to withstand water pressure (yield strength), flexibility without breaking (elongation), impact absorption capacity (toughness), and ease of bending without heat (cold formability). Perfect welding that connects joints as strongly as the original steel plate and technology to eliminate post-welding stress are also essential.

For example, Japan’s Taigei-class submarines apply ultra-high-strength steel (NS110, yield strength of approx. 980 MPa) that withstands depths of 500 meters. The core technology here is overcoming the problem of reduced flexibility as steel becomes harder. On the other hand, Germany’s Type 212/214 submarines used “non-magnetic steel” that does not stick to magnets to avoid detection based on the enemy’s magnetic field, but because of the limitations in the material’s strength, their diving depth is limited to about 250 meters. Ultimately, for a submarine to survive intact in extreme crisis situations such as undersea collisions, torpedo evasion, or rapid surfacing, advanced steel materials that balance hardness and flexibility, along with reliable welding technology, must be supported.

Lee Jae-ik: To prove the reliability of submarine materials, we must go through three stages of evaluation. First, we quantify the basic alloy design and physical properties using fingernail-sized “micro-specimens.” Second, once the steel is produced, we manufacture “full-thickness specimens” to directly test the limits of strength and toughness of the base metal and welded joints. Finally, we conduct “structural mock-up testing,” assembling it similarly to an actual submarine shape. In this stage, we final-check residual stress generated during actual processing, fatigue life, and deformation during explosions. As a result, to develop a single steel material, hundreds of similar items are repeatedly verified double and triple, and only after passing this harsh process can we finally take responsibility for the submarine’s survivability.

Lee Jae-ik: That is the most difficult part of development. This is because a vicious cycle occurs where increasing the strength of the steel plate makes it weak against impact, and compensating for this warps the welding. POSCO is breaking through this difficult limitation with two innovations. We are researching to secure both strength and toughness simultaneously through advanced Thermo-Mechanical Control Process (TMCP) technology and fine microstructure control technology. Recently, technology that combines AI to find the optimal alloy ratio that captures both strength and toughness (flexibility) has been established. In addition, by package-developing the material and welding consumables as a single set from the beginning, we eliminated the trial and error of having to return to the starting point due to mismatching later, thereby dramatically shortening the overall development period.

Lee Jae-ik: Due to national security and diplomatic sensitivity, all technologies from design to operation in marine defense steel development are managed under strict security. Technology development is mainly carried out as national projects led by the Defense Acquisition Program Administration (DAPA), and the achievements belong to national assets. When a customer requests the development of general-purpose technology, development proceeds in the form of a “Joint Development Project (JDP).”

The most representative case is the “development of fatigue design technology for high-strength steel welded joints” project conducted in 2019 with Hanwha Ocean (formerly Daewoo Shipbuilding & Marine Engineering) and the Korean Register (KR). The design was initially carried out by applying design technology transferred from Germany’s HDW, but we eventually pursued localization. Since then, the three organizations have cooperated to secure localized design technology. In the future, when localized welding materials and welding solutions are applied, it is expected that a self-production foundation for high-performance large submarines will be established.

Lee Jin-woo: POSCO develops next-generation naval materials by establishing an “Early Supplier Involvement (ESI) framework,” collaborating closely from the initial design stage with the special ship business divisions of the two major heavy industries, which are key communication channels for defense agencies. The KR certification of the newly developed materials was also achieved through collaboration with the Special Ship Business Division of HD Hyundai Heavy Industries and communication with defense agencies. Defense steel has the difficult challenge of simultaneously satisfying complex conflicting conditions such as protective performance, weldability, and on-site assembly alignment, beyond simple high strength. Therefore, during the collaboration process, we closely exchange opinions on manufacturing technology, component feasibility, special protective performance verification methods, and final specification (Spec-in).

This field-oriented, close technical exchange (Spec-in) allowed us to reflect the “reality of the field” early on, which is easy to miss in the laboratory, thereby compensating for the limitations of the laboratory unit early. Consequently, it is becoming a core competitiveness that overcomes the structural limitations unique to defense R&D, which usually takes more than 5 to 10 years, and enables actual naval application within a short period. POSCO proactively shared the operational concepts of weapon systems, coordinated required performance, and led the “simultaneous evolution of material and structural design.” This is a symbolic case showing why the current collaboration system is so important.

Lee Jin-woo: POSCO has established itself as a core foundation of the domestic shipbuilding and naval industries by proving global top-tier quality and mass production capability in the field of heavy plates for hull structures. In the field of next-generation special steel, we are continuing strategic challenges to surpass advanced military nations such as the US and Germany, which still lead in some areas. To this end, rather than simply chasing technology, we are pursuing a strategy to build “POSCO’s unique independent development model” where “material development, manufacturing technology, and vessel design” move like a single organic body by establishing a “close joint research system” with national defense research institutes and special ship design departments of shipyards. We believe that this value chain-integrated collaborative ecosystem will be the most powerful core competitiveness for POSCO to leap forward as a first mover leading the paradigm of next-generation naval material technology in the future.

▲ Conceptual rendering of a submarine. (Image source: my posco)

Lee Jae-ik: Analyzing the aspects of modern warfare, such as the recent war in Ukraine, the paradigm of future submarine steel technology can be summarized as “expansion of unmanned systems” and “extreme stealth” such as stealth performance. To apply to unmanned drones that will disperse the risks of manned submarines, a higher level of high-strength steel is essential, and neighboring countries like Japan are also secretly continuing to improve material performance. In response to this, Korea must also move toward developing “ultra-high-strength steel” that withstands deeper seas, larger hulls to increase weapon payload, and “high-strength non-magnetic steel” that fundamentally blocks the enemy’s magnetic detection in shallow waters. Ultimately, we believe that the high-functional special steel equivalent to or surpassing that of neighboring countries, and the advancement of welding and processing technologies to perfectly implement it, are the clear future directions we must preoccupy.

Lee Jae-ik: Ultra-high-strength steel, which completes the “stealthiness” that is the life of a submarine, has doubled in strength compared to 50 years ago, turning what was once a “dream material” into reality. The important point is that these cutting-edge defense technologies will expand to civilian industries in the future, such as lightweighting ships and reducing carbon emissions, serving as a solid foundation to lead the upcoming low-carbon, high-strength steel era. I will make sure POSCO plays a leading role in that.

Lee Jin-woo: I believe that material innovation has true value when applied in the field, rather than remaining as an achievement in the laboratory. POSCO has turned that value into reality through close collaboration with partners such as shipyards and research institutes, as well as research, production, quality, and marketing. Going forward, we will continue to enhance the competitiveness of next-generation naval materials based on accumulated technology and execution capability. I want to responsibly contribute to laying the foundation for Korea’s defense material technology and securing global competitiveness.

With the Trump administration recently easing restrictions on LNG exports and actively leveraging tariffs in trade negotiations, the entire LNG value chain—production, storage, transportation, and utilization—is gaining attention. Back in 2008, POSCO anticipated a steady increase in LNG demand as a response to tightening global environmental regulations. POSCO recognized the need for new materials to secure a competitive edge in the materials market for LNG storage and transportation. For this reason, it turned its focus to manganese alloy steel, and thus began the development of high-manganese steel. We sat down with Senior Researcher Soon-gi Lee, who is at the heart of the development, certification, and commercialization of this next-generation material.

Q. POSCO’s high-value-added steel, developed in-house, is drawing attention. What exactly is POSCO’s high-manganese steel, and what makes its manufacturing process unique?

POSCO’s high-manganese steel is a new type of steel alloyed with a high content of manganese (Mn, 22.5–25.5%). Compared to conventional materials such as stainless steel, 9% nickel steel, and Invar alloy, POSCO’s high-manganese steel offers comparable performance, but significantly better cost competitiveness. Most importantly, it retains excellent mechanical properties even at cryogenic temperatures as low as -196°C, making it ideal for LNG storage tanks and carriers.

When manganese is added to steel, introducing solid manganese directly into the molten metal can cause a drop in temperature. To prevent this, we melt the manganese before adding it, rather than inputting it in solid form. Typically, adding manganese increases wear resistance and strength, but this comes at the cost of ductility (the property of stretching easily without breaking). However, thanks to POSCO’s decades of accumulated know-how in controlled rolling and cooling techniques, we have succeeded in producing a ductile product despite the high manganese content. Production is currently taking place at our heavy plate plant, and both the material composition and manufacturing method have been bundled into a single patented package.

Q. You said POSCO melts the manganese first to avoid lowering the temperature of the molten steel. But if your competitors build similar melting facilities, won’t they be able to catch up?

Though it sounds simple, POSCO’s proprietary technology is embedded throughout the process. Manganese alloys often contain impurities, so refining during the intermediate stages is critical. Predicting the behavior of molten manganese is also one of our hidden core technologies. Our facilities for producing molten manganese and storing it in insulated furnaces will not be easy for our competitors to replicate.

Q. Is POSCO the only company that can produce high-manganese steel?

Some steelmakers produce general-purpose high-manganese steel. However, in applications requiring higher manganese content, such as the cryogenic (24%) and slurry pipe (18%) grades used in the energy industry, POSCO leads in technological capability.

Q. How does POSCO’s high-manganese steel compare with competing materials?

POSCO’s high-manganese steel meets all the requirements for LNG transport and storage, and offers several advantages over existing materials. It exhibits high strength and excellent elongation* properties.

While most steel materials used for LNG transport and storage contain a high amount of expensive nickel, nickel has been completely replaced with manganese in POSCO’s high-manganese steel. The manganese used in high-manganese steel is abundant worldwide and relatively low in cost, making the final product approximately 30% cheaper than conventional alternatives. Its high elongation makes it easy to process, and it is also highly resistant to hydrogen embrittlement. Having recently been approved for ammonia applications, it can now be used for most liquefied gases, including natural gas, ammonia, and CO₂.

*Elongation: The ratio by which a metal can stretch before breaking; it indicates ductility.

Q. What are some current applications of POSCO’s high-manganese steel?

▲POSCO International’s Gwangyang LNG Terminal tank using POSCO high-manganese steel.

Currently, it is being used in Gwangyang LNG Terminal units 5 and 6, Hanwha Ocean’s VLCCs, container ship LNG fuel tanks, and onshore LNG storage tanks. We are working toward expanding its use in LNG carriers and other applications. Beyond LNG, POSCO aims to promote high-manganese steel across various industries as a symbol of Korea’s technological strength.

Q. Could this steel eventually be used in LNG carriers, not just LNG-fueled ships? Doesn’t using thick plates reduce capacity?

POSCO manufactures high-manganese steel as thick plates. Onshore storage tanks are built with thick plates, and LNG carriers can be built using either thin membrane sheets (about 0.7–1.2 mm) or thick plates (6–40 mm). Compared to membrane systems, thick-plate LNG carriers are structurally more robust and better withstand sloshing from LNG motion, providing superior safety under various conditions. Recently, President Trump mentioned Korea’s potential involvement in the Alaska LNG project. To navigate through thick Arctic ice, LNG carriers would need to be icebreaking LNG ships. In such cases, high-manganese steel made in thick plates offers a clear advantage.

There is no reduction in capacity. Shipbuilders simply redesign the hull based on the plate thickness. The standard capacity is 174,000 m³, and designs are adjusted accordingly.

▲The 2024 IMO General Assembly in London invited Korea to present POSCO’s achievements.

Q. We read that POSCO’s high-manganese steel is setting global standards. What does that mean?

Materials for energy applications must adhere to strict international standards. For example, ships sail through international waters, so they aren’t subject to just regional or national regulations—they must comply with global standards. Since POSCO’s high-manganese steel was the first of its kind, no existing standards applied, so we had to create them ourselves. Despite continued resistance from our competitors, we succeeded in registering the material with ASTM, API, ISO, and IMO standards. These international standards are based on POSCO’s technology, meaning that other companies must follow our specifications—effectively making POSCO’s technology the global benchmark.

In particular, the International Maritime Organization (IMO), a UN-affiliated body that sets standards for shipbuilding and operation, formally requested the Korean government to present at the 2023 General Assembly when POSCO’s high-manganese steel received final approval. The IMO viewed the 10-year approval journey led by POSCO and the Korean government as a highly successful and exemplary case, and wanted to share it with member states.

Q. There seems to be a longstanding connection between Chairman In-hwa Chang and high-manganese steel.

▲(Left) Hanwha Ocean’s crude oil carrier equipped with a high-manganese steel LNG fuel tank in 2022 (Right) Hanwha Ocean’s ultra-large container ship equipped with a high-manganese steel LNG fuel tank in 2024.

Chairman In-hwa Chang has a deep understanding of both shipbuilding and steelmaking, having majored in naval architecture and marine engineering as a student and later worked as a researcher in the steel industry. This unique background has allowed him to make significant contributions to the development and broader adoption of high-manganese steel.

He played a key role in our solid track record* by ensuring the material was actually used in onshore and marine storage tanks, which laid the foundation for future sales growth. Back in 2017, when POSCO was planning to construct the No. 5 LNG terminal in Gwangyang, the initial plan was to use conventional materials. However, as executive vice president at the time, Chairman Chang personally made the decision to use POSCO’s high-manganese steel instead.

His strategy was to create a proven reference case using POSCO’s own material, thereby paving the way to enter new markets. He also believed that involving POSCO E&C for construction and POSCO International for operations would maximize group-wide synergy. He further helped open a new market for ship applications by driving the use of high-manganese steel in LNG fuel tanks for LNG-powered vessels.

Hanwha Ocean (formerly DSME) was initially hesitant to adopt the new material for LNG-powered ships, as safety assurance is critical for such large-scale vessels. During his term as POSCO CEO in 2020, Chairman Chang met directly with Hanwha Ocean’s top management and strongly advocated for the safety and reliability of the steel. As a result, in 2022, Hanwha Ocean became the first in the world to use high-manganese steel in LNG fuel tanks for a very large crude carrier (VLCC), which was followed by its application in container vessels.

* Track Record: Operational data collected after applying newly developed technologies or products under real working conditions

Q. Beyond the current market, what are the growth prospects for high-manganese steel?

Outside the LNG value chain, POSCO is leveraging the wear resistance and non-magnetic properties of high-manganese steel to explore new markets. Despite severe deformation, its non-magnetic properties do not deteriorate, which enhances stealth capabilities when applied to submarines, warships, and military tanks, thereby pushing for demand expansion as a material for the defense industry.