The heart of Japanese knives: Steel

In its simplest essence, a knife is a piece of steel with a handle, so good steel makes a good knife. Good steel is key. Steel can be exploited in the best way by the most experienced blacksmiths, through the forging process, heat treatments and the creation of geometry.

Steels can be separated into 3 main categories:

Traditional Japanese high carbon steel (ex. Aogami)
Corrosion resistant steel (ex. VG10)
Powder steel (ex. ZDP-189 or SG2)

But to begin, let’s see what are the elements that come to compose a steel and what are their effects.

Chemical elements presents in steel

Steel is mainly composed of iron (fe) and carbon (c), and added to this, small amounts of other elements. Without carbon, a steel could not be forged or tempered. The higher the amount of carbon, the harder the steel will be.

Chemical elements in steel:

Iron (Fe): main element in steel.
Carbon (C): Key element in steel, which gives steel the ability to be hardened through heat treatments. It decreases the corrosion resistance and increases the breakage rate.
Chromium (Cr): Greatly increases corrosion resistance, and at certain levels also increases hardness.
Manganese (Mn): allows the formation of a better structure and increases the possibility of hardening a steel with a higher level.
Vanadium (V): key element to increase the hardness of steel. Increases the possibility of having a very sharp blade that holds its edge longer.
Molybdenum (Mo): increases resistance to corrosion, so it is frequently present in steels resistant to oxidation, and helps to maintain the hardness and capabilities of a steel in the event of temperature change (overheating).
Silicone (Si): increases the positive effects of carbon (C), and increases the hardness and strength of steel.
Cobalt (Co): Increases hardness and corrosion resistance.
Tungsten (W): greatly increases the wear resistance of steel.
Phosphorus (P): impurity, present in all steels in small amounts.
Sulfide (S): impurity, present in all steels in small amounts

Traditional Japanese high carbon steel

Traditional Japanese steels, thanks to their carbon (C) content, can be hardened to a hardness that exceeds 60 HRC (Hardness Rockwell scale C). Nevertheless, these steels remain very easy to sharpen, but they require more maintenance. Dry them well after use and store them in a dry place, and apply a thin layer of oil if they are not going to be used for an extended period of time. In the other case, corrosion and rust stains will develop quite quickly.

Japanese blacksmiths have always chosen their steels carefully. Katanas were forged with a steel called “tamahagane”. Obtained through special furnaces located on the ground called “tatara” with iron powder and pure coal using ancestral techniques.

Traditional Japanese steels are forged with similar techniques. The two main steels are Shiro-ko or Shirogami (Japanese white steel #2, #1) and Ao-ko or Aogami (Japanese blue steel #2, #1, super)

Shirogami

Shirogami steel is considered the purest steel, made of only iron and carbon (although it may contain small amounts of phosphorus and sulfide as impurities).

• Shirogami #1 contains iron (Fe), carbon (C) 1.25 – 1.35%, manganese (Mn) 0.20 – 0.30%, phosphorus (P) 0.03%, sulfide (S) 0.004% and silicone (Si) 0.10 – 0.20%.
• Shirogami #2 contains iron (Fe), carbon (C) 1 – 1.15%, manganese (Mn) 0.20 – 0.30%, phosphorus (P) 0.03%, sulfide (S) 0.004% and silicon (Si) 0.10 – 0.20%.

Aogami

If we add chromium and tungsten to a very refined Shirogami #1 steel we obtain Aogami steel. Thanks to the addition of these elements, Aogami steel is slightly more resistant to corrosion. Above all, it has better “kirenaga”, a Japanese term used to refer to better thread retention.

• Aogami #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%, sulfide (S) 0.004% and silicon (Si) 0.10 – 0.20%.
• Aogami #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%, Sulphide(S) 0.004%, silicon (Si) 0.10 – 0.20% and tungsten (W) 1.00 – 1.58%.
• Aogami Super 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% , sulfide (S) 0.004%, silicon (Si) 0.10-0.20%, tungsten (W) 2.00 – 2.50% and vanadium (V) 0.30 – 0.50%. An example of a super Aogami steel forged knife is the YU KUROSAKI FUJIN AS GYUTO 210MM

Corrosion resistant steel

Adding chromium to an alloy increases its resistance to corrosion. The chromium oxide creates a film around the steel to prevent direct contact with water and oxygen. An alloy that contains a chromium level equal to or higher than 12% is considered resistant to oxidation. That said, oxidation resistant knives should be wiped dry, especially if cutting acidic foods. The protective film is sensitive and the acid could damage it, even a corrosion resistant steel if left uncleaned for too long, it could end up oxidizing.

The advancement of technologies has allowed the creation of new steels which contain both the properties of high carbon steels and the properties of corrosion resistant steels. The most used steels of this type are:

• VG-10 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 (V) 0.10-0.3%. An example of a knife forged from VG-10 steel is YOSHIMI KATO NICKEL DAMASCUS VG10 GYUTO.
• Ginsan, or 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% , sulfide (S) 0.02% and silicon (Si) 0.35%.
• 19c27, Sandvik or Swiss steel, it can be hardened between 62-66 HRC, and it contains iron (Fe), carbon (C) 0.95%, chromium (Cr) 13.5%, manganese (Mn) 0.70%, phosphorus (P) 0.03%, sulphide (S) 0.01% and silicon (Si) 0.40%.

Powdered steel

Thanks to new metallurgical technologies, the Japanese were able to create new special steels with incredible performance. These powder steels are created, indeed, with powders of very rich chemical elements, following several processes which allows them to obtain a very fine grain at the level of the structure, and consequently a very homogeneous structure which gives to the higher hardness steel. This allows you to obtain a sharper edge for a longer time. The breakage rate is also reduced thanks to the homogeneity of the structure, despite the high hardness.

Knives forged with these steels are more rare, difficult and expensive to produce. Only the most experienced and knowledgeable blacksmiths are able to forge, roll and temper them properly to exploit the potential of these steels.

Advantages of powder steels

• Very high hardness, between 66-68 HRC, and very good solidity
• Very good corrosion resistance
• Fairly simple to sharpen, with a very fine microstructure that allows very fine sharpening with the ability to keep the edge longer than traditional steels .

The most common and suitable powder steels for the manufacture of kitchen knives

• ZDP-189 (Hitachi Metals Ltd.) Unfortunately due to the difficulty of producing this steel and the very specific heat treatments that are required, only a very limited number of blacksmiths are able to forge knives with this steel, which also makes dramatically increase their prices.
  C 3.00% | 20.00% | 0.60% | 0.10% | MB 1.40% | Min 0.50% | If 0.40%

• R2 (SG2) this super steel has become very famous thanks to its cutting capacities, its resistance to wear and the ability to keep its edge for a long time. Unlike the ZDP189, it is much easier to find on the market. EX. YU KUROSAKI SENKO SG2 PETTY 150MM
  C 1.25-1.45 % | Cr 14.00-16.00 % | Mo 2.3-3.3 % | V 1.8-2.2 %
• HAP-40 (Hitachi Metals Ltd). Its very rich chemical composition and a very fine microstructure, gives this steel an incredible balance between hardness, straightness 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 %