Q＆A

Difference between Steel and Stainless Steel？

Iron with 0.04% to 2% carbon is called steel. In addition to iron and carbon, steel also contains silicon, manganese, phosphorus, and sulfur. These four elements and carbon are called the five elements of steel. Among them, phosphorus and sulfur deteriorates steel properties, and the content of the two elements are limited to 0.04% or less when used for a knife. Stainless steel is obtained by adding chromium to steel. The addition of chromium to steel improves (12% or over) the resistance to corrosion, thus preventing rust. The reason for this is that an oxidized film is formed on the steel surface to prevent air or moisture from contacting the iron directly. The oxidized film advantageously recovers the steel surface even if the surface is damaged while sharpening the knife. However, when salt content, acids, or strong chemical agents are left on the surface, there is a possibility that the steel may rust since the efficacy of the oxidized film is deteriorated. Therefore, it is important to clean and dry the knife after use, irrespective of whether the knife is made of steel or stainless steel.

Mechanism of Iron Corrosion？

Iron exists in an oxidized (rusted) state in the natural world in the form of iron ore. By applying a reduction (rust removal) process, iron ore becomes the metal iron, which we typically use.Therefore, the rusting of iron is a natural phenomenon of returning to the natural state. In general, there are two types of rusting in the case of iron: a type is red rust in which the surface turns reddish brown and becomes brittle; and another type is black rust in which the surface loses luster and turns brownish. The red rust occurs after oxidization begins with exposure to moisture, salt, or acids, in addition to oxygen. Such ingredients are contained in food, and when you don’t clean your steel knife after use, rust will develop from the steel surface down to the deeper portion of the steel. When this happens, you need to remove rust as soon as possible. Otherwise, rust will adhere to food, and in the worst case, the knife could fracture at the handle.Black rust is where the steel surface is oxidized after exposure to the air forming a thin oxidized film. The rust, once it has developed over the steel surface, works to block the contact between oxygen and iron and will not develop further. Therefore, the rust will seldom transfer to food. After using a steel knife, it is important to clean it well, remove moisture, and store it in a dry area to keep it free of red rust.

Sharpness of Knife？

When cutting a tomato, have you ever smashed it? Have you cried when cutting an onion? These problems are caused not by the food but by a blunt knife. A blunt knife adds extra force to the cut surface, and the food cells collapse. For food, such as an onion, the lachrymator within the cells is released as the cell collapses, causing the tears in your eyes to flow. In addition, when cutting thin, soft food like fish or a tomato with a blunt knife, you have to move the blade back and forth for many times. When this happens, the color at the cut surface becomes dull, the luster appearance was gone, degrading the fresh look of the food. Moreover, oxidization develops rapidly at the cut surface, so the food loses firmness and taste. In other words, the sharpness of a knife is a decisively important issue for cooking.

Efficacy of Elements？

Carbon（C)

Since quenching is not possible unless the iron contains carbon, the element is essential for the steel used in knives.

Silicon (Si)/Manganese (Mn)

The elements subserve the efficacy of carbon to facilitate quenching.

Phosphorus (P)/Sulfur (S)

The elements are harmful to knives, since they make steel brittle when subjected to temperature changes.

Chromium (Cr)

The element makes steel more resistive to wear. By reacting with air, it creates an oxidized film, which shuts out moisture and restricts the development of rust.

Molybdenum (Mo)

The element prevents the deterioration of steel caused by temperature changes, improves resistance to corrosion, and facilitates better quenching. When used with stainless steel, it alleviates the disadvantages of chromium.

Tungsten (W)

The element prevents deterioration in the hardness of steel in the higher temperature ranges.

Vanadium (V)

The element prevents steel from softening when steel is subjected to quenching and strengthens steel by adding elasticity.

Note: The above-stated properties will not be expected unless the steel contains at least 12% chromium or at least 1% molybdenum/vanadium.

What is Yasugi steel？

The Japanese sword represents traditional Japanese craftsmanship. Made of Japanese steel using iron sand as the raw material, the edge of a Japanese sword is well known throughout the world. Hitachi Metals Co., Ltd., produces Yasugi steel from the raw materials for Japanese steel. In the past, iron sand with a high degree of purity could be excavated in the mountains in Tottori and Shimane prefectures and was refined to Tama Hagane using traditional processing (Tatara Processing). Hitachi Metals further improved the techniques, establishing their Yasugi Works to manufacture steel. The steel is called the blue steel, white steel, and yellow steel in the order of quality. Such names are said to be derived from the fact that blue and white papers were used to pack the finished steel for identification.