Technology & Development

Premium Cleanliness Steel Manufacturing Technology

Technology that maximizes the cleanliness of steel Improves
service life and reliability of bearings, high-strength automotive parts, etc.

Bearing steel, one of the major products of Sanyo Special Steel, is mainly used as bearing materials. Bearings are subjected to severe service conditions including high load and high-speed rotation. High strength and wear resistance as well as superior fatigue strength are required of bearing steel used as bearing material. The fatigue life depends largely on the non-metallic inclusions in steel, which also largely affect the service life of bearings. Non-metallic oxide inclusions are extremely hard, causing a concentration of stress when the bearing rotates, which leads to fatigue fracture. The amount of inclusions is reduced by reducing the oxygen content in steel. Sanyo Special Steel has been producing high-cleanliness steel with a limited amount of inclusions by reducing the oxygen content in steel.

Steel production method that maximizes excellent performance of steel

We performed a quantitative research on non-metallic inclusions in steel, aiming to further improve fatigue strength. Through the research, we found that “the size of the largest inclusion,” as well as the amount of inclusions, is related closely to the strength and service life of steel materials. Based on a unique concept, which is to control the size of the largest inclusion in steel to maximize the performance of steel materials, we developed, a new steel production method. The premium cleanliness steel produced with this technology features extremely high fatigue strength and is highly evaluated by users related to the automobile industry, which are making efforts “to meet environmental regulations and improve performances through downsizing and weight reduction of parts.”

Incorporated in the world’s first toroidal CVT

Toroidal CVOur premium cleanliness steel was incorporated in the toroidal CVT (continuously variable transmission) that was commercialized for the first time in the world. The steel is effective for reducing the size and weight of automobile engines and transmission parts and improving the service life and reliability of bearings, etc. that are used under severe conditions. It can also replace special dissolving materials.

Sizes of inclusions are evaluated in the statistical analysis method

Sanyo Special Steel incorporates “the statistics of extreme value method” to evaluate “the size of the largest inclusion,” which is an important factor in manufacturing premium cleanliness steel. In the extreme value statistics method, the size of the largest inclusion in a certain area is estimated by statistical processing, based on the distribution of largest inclusions in multiple standard areas, to evaluate the cleanliness of steel.

Mass production technology incorporating the statistics of extreme value method

Sanyo Special Steel incorporated the statistics of extreme value method to develop a production technology that reduces the generation of large inclusions in steel. The production process is basically the same as the process at other manufacturers’ special steel plants, but we implement strict operation control to produce high-cleanliness steel. We place special priority on refining and prevention of contamination by air, slag and refractories. To ensure the cleanliness of premium cleanliness steel in the mass production process, for example, we have developed favorable environments to maintain the optimum conditions for improved refining and prevention of contamination, through optimization of slag components, improvement of air shutoff method, etc.

(1)Melting in electric furnace
Scraps are melted. Our electric furnace is an eccentric bottom tapping type, which is designed so that molten steel is poured into the ladle through the bottom of the furnace, and this prevents the oxidized slag, generated in the furnace, from mixing with the molten steel.
(2)Ladle furnace refining
Steel after oxidation refining is reduced and the component is adjusted by adding alloys. Impurities such as oxygen and sulfur are removed by reduction reaction.
(3)RH vacuum degassing
The molten steel is then circulated in the vacuum tank to remove unnecessary gas components such as oxygen and hydrogen.
(4)Continuous casting
Molten steel is cast (solidified) in this process. The curved type, which features low equipment cost, is the mainstream in continuous casting, but Sanyo Special Steel incorporates the vertical-type continuous caster. When the vertical type is used, steel materials are not bent in the solidification process, which means less cracks and uneven distribution of inclusions and creation of special steel of a higher cleanliness.

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