In its essence, a kitchen knife is simply a piece of steel with a handle. A good kitchen knife is made of good steel. Good steel should be considered a basis, a potential, which through the processes of forging, tempering and blade shaping can be exploited by only the most experienced craftsmen.
There are 3 main categories of steel used in kitchen knife manufacturing. Every category has its pros and cons, depending on the purpose of the knife.
- High-carbon steels (traditional Japanese steels, eg. Aogami)
- Corrosion-resistant steels (eg. VG-10)
- Powder steels (eg. ZDP-189 or R2)
But before we dive into different types of steel, let's first explore the main chemical elements in steel and their effect on the characteristics of the kitchen knife.
Chemical Elements in Steel and Their Effect
Steel is made of iron (Fe), carbon (C) and smaller contents of other elements. Carbon (C) is the key element in steel. Without carbon, steel could not be forged or tempered. More carbon means harder steel. Steel can contain between 0.1 and 3% of carbon.
Chemical elements in steel:
- Iron (Fe): Main element in steel.
- Carbon (C): Key element in steel. It gives steel the possibility to be hardened during the process of heat treatment. It reduces resistance to corrosion and makes steel brittle.
- Chromium (Cr): strongly increases the corrosion resistance of steel, and to a certain level increases its hardness.
- Manganese (Mn): improves the structure of steel and increases the possibility for higher hardening of steel.
- Vanadium (V): the key element in increasing the hardness of steel. It increases the possibility to have a sharper blade and maintains the sharpness for a longer period of time.
- Molybdenum (Mo): increases the resistance to corrosion, it is frequently present in corrosion-resistant steel, and it helps maintain the hardness and the power of steel in case of temperature changes.
- Silicon (Si): increases the positive effects of carbon (C). It increases the hardness and the power of steel.
- Cobalt (Co): for higher hardness and corrosion resistance.
- Tungsten (W): highly increases the wear resistance of steel.
- Phosphorus (P): impurity, which is present in all types of steel in small quantities.
- Sulfur (S): impurity, which is present in all types of steel in small quantities.
High-Carbon Steels (traditional Japanese steels)
High-carbon steels are the preferred choice for Japanese chefs. Due to its high carbon (C) content, such steels can be forged to a high hardness (60+ HRC), yet are very easy to resharpen. Knives made of high-carbon steels need extra attention with maintenance, wiping them dry after every use and oiling the blade occasionally, and will develop a patina over time. Improper maintenance will develop corrosion/rust quite fast.
Japanese blacksmiths have always chosen steel carefully. Traditional katana is made of tamahagane steel, which is only produced in the western part of Japan. It is produced in traditional smelting furnaces called “tatara” from iron dust and pure coal using ancient techniques.
Traditional Japanese steel is made using similar techniques. This steel is used in the production of knives which are subjected to similar processing procedures as the katana. There are two main types of steel: shiro-ko (white steel #1, #2) and ao-ko (blue steel #1, #2 and Aogami steel).
White Steel or Shiro-ko
White steel is extremely pure steel with a high percentage of carbon and no additional ingredients (it can contain some phosphorus (F) and sulfur (S) as impurities). There are two types of white steel: white steel 1 and white steel 2.
- White steel #1 contains iron (Fe), carbon (C) 1.25 - 1.35 %, manganese (Mn) 0.20 - 0.30 %, phosphorus (P) 0.03 %, sulphur (S) 0.004 % and silicon (Si) 0.10 - 0.20 %.
- White steel #2 contains iron (Fe), carbon (C) 1 - 1.15 %, manganese (Mn) 0.20 - 0.30%, phosphorus (P) 0.03 %, sulphur (S) 0.004% and silicon (Si) 0.10 - 0.20 %.
Blue Steel or Ao-ko
If we add chromium and tungsten to extremely refined white steel, we get blue steel. Due to the additives, blue steel is more durable, slightly more corrosion-resistant, but above all, it has better “kirenaga”, which is a Japanese word for the duration of sharpness. Knives made of Blue steels are primarily used in Japanese restaurants, where the chef needs a knife that stays sharp for a long period of time.
- Blue steel #1 contains iron (Fe), carbon (C) 1.25 - 1.35 %, chromium (Cr) 0.20 - 0.50 %, manganese (Mn) 0.20 - 0.30 %, phosphorus (P) 0.03 %, sulphur (S) 0.004 % and silicon (Si) 0.10 - 0.20 %.
- Blue steel #2 contains iron (Fe), carbon (C) 1.05 - 1.15 %, chromium (Cr) 0.20 - 0.50 %, manganese (Mn) 0.20 - 0.30 %, phosphorus (P) 0.03 %, sulphur (S) 0.004 %, silicon (Si) 0.10 - 0.20 % and tungsten (W) 1.00 - 1.58 %.
- Aogami Super steel contains iron (Fe), carbon (C) 1.40 - 1.50 %, chromium (Cr) 0.30 - 0.50 %, manganese (Mn) 0.20 - 0.30 %, molybdenum (Mo) 0.30 - 0.52 %, phosphorus (P) 0.03 %, sulphur (S) 0.004 %, silicon (Si) 0.10-0.20 %, tungsten (W) 2.00 - 2.50 % and vanadium (V) 0.30 - 0.50 %. A proud member of the Aogami super steel would be the KUROSAKI GYUTO KURO-UCHI 210MM (8.3")
By adding chromium (Cr) to steel we increase its resistance to corrosion. Chromium oxide forms a protection film on the surface of the steel, which prevents contact between iron and water or oxygen. If the basic steel alloy contains 12 % or more chromium, we call it stainless steel. Even stainless steel kitchen knives have to be wiped and dried after the use, especially if we cut fruit and vegetables containing acids (lemon, onions, tomatoes, etc.). The protective film is sensitive and acids can damage it. If we leave the knife uncleaned and wet for a longer period of time, even stainless steel can corrode.
Due to technological development, we witnessed the emergence of new steel types, which combine the quality of steel with a high percentage of carbon with the practicality of corrosion-resistant steel. High-carbon (C) and corrosion-resistant steels are nowadays very popular with professional chefs. The most famous types of steel of this kind are:
- VG-10 presents the top of the offer of corrosion-resistant steel. It contains iron (Fe), carbon (C) 0.95 - 1.15 %, chromium (Cr) 14.50 - 15.5 %, cobalt (Co) 1.30 - 1.50 %, manganese (Mn) 0.50 %, molybdenum (Mo) 0.90 - 1.20 %, phosphorus (P) 0.03 % and vanadium 0.10-0.3 %. A typical VG-10 knife would be the Bunka Black Damascus.
- Ginsan, Silver steel or Ginsan-ko, which contains iron (Fe), carbon (C) 0.92 - 1.10 %, chromium (Cr) 13.00 - 14.5 %, manganese (Mn) 0.60 - 1.00 %, phosphorus (P) 0.03 %, sulphur (S) 0.02 % and silicon (Si) 0.35 %.
- 19c27, Sandvik steel or Swedish steel is a very popular steel with Japanese producers, it can be hardened to 62- 66 HRC, and it contains iron (Fe), carbon (C) 0.95 %, chromium (Cr) 13.5 %, manganese (Mn) 0.70 %, phosphorus (P) 0.03 %, sulphur (S) 0.01 % and silicon (Si) 0.40 %.
Tamahagane is considered the mother of all steels. That used to be true, or still is when we talk about Japanese swords katanas, but nowadays modern science is producing new, special steels, far more suitable for kitchen blades, called Powder steels. Such steel is more advanced, harder, tougher, sharper, and has one thing in common with the ancient tamahagane - they are both made in Japan! Japanese hi-tech steel is being forged the old way from the same blacksmith families that were forging katanas. It is the best merger of the ancient and modern to produce the best, or better, the sharpest knives!
Powder Metallurgy steels, also known as Powdered High-Speed Tool steels are steels that are often used in industrial applications that require tools capable of cutting steel and also withstanding tremendous forces and high temperatures. Powder steels are made using a different manufacturing process, allowing richer chemical ingredients and a very fine-grained structure with excellent metallurgical properties.
Knives made from these steels are rare, hard and expensive to produce, and only the best knife makers are able to forge, laminate and heat-treat these steels. It's a very difficult process that demands a lot of experience, knowledge and a blacksmith inclined to perfection. Correctly made powder steel knives are the crème de la crème of what kitchen cutlery can offer.
Advantages of Powder steel kitchen knives:
- Very high hardness up to 67 on Rockwell C scale (HRC) and good toughness,
- Satisfying corrosion resistance,
- Easy to sharpen steel, with a fine microstructure for fine sharpness and ability to stay sharp longer than other traditional steels.
Most common and suitable powder steels for kitchen cutlery:
ZDP-189 (Hitachi Metals Ltd.) powder metallurgy ‘super steel’. It has a similar chemical composition to Cowry X and offers a similar level of performance. Unfortunately, due to the difficulty of producing ZDP-189 and the special forging and heat-treatment, only rare blacksmith are capable to process it. Our ZDP-189 knives are from a family smithy Yoshida Hamono, handmade by blacksmith master Osamu Yoshida. We visited Yoshida Hamono in 2016 to find out more about the process of forging ZDP-189 steel.
C 3.00 % | Cr 20.00 %
R2 (SG2) is powder metallurgy ‘super steel’ that has become popular knife steel because of its cutting performance, excellent edge retention and high corrosion resistance. Unlike Cowry X and ZDP-189, kitchen knives made of R2 steel are far more available on the market.
C 1.25-1.45 % | Cr 14.00-16.00 % | Mo 2.3-3.3 % | V 1.8-2.2 %
HAP-40 is a powder metallurgy High-Speed Tool Steel from Hitachi Metals Ltd. It’s rich chemical composition and fine microstructure allows production of knives that possess an outstanding balance of hardness, toughness and edge retention.
C 1.27-1.37 % | Cr 3.70-4.70 % | W 5.60-6.40 % | Mo 4.60-5.40 % | V 2.80-3.30 % | Co 7.50-8.50 %
Cowry X (Daido Steel Co., Ltd.) is powder metallurgy ‘super steel’ that has a very high Carbon and Chromium content. Unfortunately, it is expensive and technically difficult for knife-makers to use, so these superb knives are quite rare.
C 3.00 % | Cr 20.00 % | Mo 1.00 % | V 0.3 %