Steel is iron with carbon added. In the simplest of terms: cutlery grade steels will have carbon contents of at least .5% (the minimum of carbon to be considered “high carbon”) but higher quality steels will have much more. Often, non-stainless (chromium content of 13% in alloy) steels will be called carbon steel, technically, however all steels are ‘carbon steel’.
Cutlery steel technology has branched in many directions, and there are a huge variety of good quality steels with stainless or non-stainless properties. Before going into detail into some specific steels that we find in the Japanese knives at Bernal Cutlery carries it should be emphasized that much like the ingredients in cooking the end results depend on the skill of the cook, the end result in the properties of a steel depend on the skill and care of the treatment of the steel during forging and heat treatment. Many lose the forest for the trees in predicting a knife’s performance strictly based on the steel type and hardness. The following list is a brief overview of some of the steels we often find in our knives and is not intended to be a complete compendium of Japanese steels. Metallurgy is a very complex field and this is in no way to be taken as a finished exploration.
Non-Stainless ‘Carbon’ Steels
Shirogami / White Paper / Shiroko / White Steel
Named for the paper put on billets of steel at the Hitachi factory. Graded at #s 1, 2 and 3; #1 has the highest carbon content and #3 the lowest. Generally speaking #1 will hold it’s edge longest but be the most brittle, #3 is the toughest (chip resistant) with shortest edge life. #2 is the most commonly used white steel.
White steel is a very fine grained carbon steel made from a very low contaminate iron loved for its ease of sharpening and ability to take a very fine razor sharp edge quickly. White steel is a favorite of sushi chefs for knives in which a very fine finish is essential. Not the easiest steel to forge and temper properly, white steel done well is a testimony to a smith’s skill. White steel is fairly reactive and requires care to avoid rust as it has virtually no rust resistance.
Aogami / Blue Steel / Aoko
Named for blue paper put on steel at Hitachi factory. Graded at; super, #1 and #2;
All made from same iron stock as white steel, very low in contaminates. Blue steels have high carbon (1.1-1.5%), .5% chromium for carbide formation, tungsten for edge life with Super having added vanadium for wear resistance. Typically blue super has the longest edge life, #1 has the best edge formation and #2 the best toughness. Blue steels do not sharpen as easily as whites but they cut better as they dull.
Japanese tool steel very similar to American D2 steel, a semi-stainless steel (12% chromium) with very high carbon content, Yoshikane uses this steel with a stain resistant cladding, very fine grain, excellent edge life and very easy to sharpen, this is also due to skill in forging and heat treatment.
Stain-Resistant & Semi-Stainless Steels
No steel is truly rust proof, with the addition of 13% chromium in an alloy the chromium will bond with itself forming a film that will reduce oxidization. Often the more chromium the lower the performance of the steel. 30-40 years ago one could say with a good degree of accuracy that stainless steels were not as good as non-stainless ‘carbon’ steels. Stainless steel would be harder to sharpen, not get as sharp and dull quicker. there are many good stainless steels developed which offer good edge formation, edge life and sharpenability.
Swedish stainless steels: Sweden has some of the best iron deposits in the world, very low contaminants and fine grains. Many Swedish stainless alloys have been developed for making razor blades which require a very fine grain to get a good edge, Sandvik 12C27, 13C27 and Bohler-Uddeholm AEB-L are popular stainless steels.
Powder metal steels such as the Japanese SG2 and SRS-15 as well as American CPM154 and CPMS30V (among many others) are high carbon high alloy content stainless steels that have been powderized into a very fine grain and sintered back together. This process allows for both easier sharpening (especially in finishing) and increased consistency in production, powder metals are very homogenous one batch to the next allowing for consistent results.
Ginsanko, Gin3, Silver 3: Ginsanko is a stainless steel made by Hitachi and is an excellent stainless steel for forging. A fine grained high carbon content (1% + carbon) stainless steel with the cutting feel and ease of sharpening of a carbon steel. In the hands of a skilled smith ginsanko will be very similar to white steel’s edge formation and sharpenability.