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.