Ordinary steel is carbon steel, that is, iron-carbon alloy. According to the level of carbon content, it is divided into low carbon steel (wrought iron), medium carbon steel and cast iron. Generally, those with a carbon content of less than 0.2% are called low-carbon steel, commonly known as wrought iron or pure iron; those with a content of 0.2-1.7% are called steel; those with a content of more than 1.7% are called pig iron.

1. The steel that contains more than 12.5% chromium in steel and has high resistance to corrosion by external media (acid, alkali salt) is called stainless steel. According to the internal structure of the steel, stainless steel can be divided into martensitic, ferritic, austenitic, ferritic-austenitic, precipitation hardening stainless steel. According to the national standard GB3280-92, there are a total of 55 regulations. .
In our daily life, we have more contact with austenitic stainless steel (some people call it nickel stainless) and martensite stainless steel (some people call it "stainless iron", but it is unscientific and easy to misunderstand and should be avoided) Two categories. The typical grades of austenitic stainless steel are 0Cr18Ni9, namely "304" and 1Cr18Ni9Ti. Martensitic stainless steels include, for example, stainless steel used to make knifes and scissors, and the main brands are 2Cr13, 3Cr13, 6Cr13, 7Cr17, etc.

2. Due to the difference in the composition of the two types of stainless steel, the microstructure of the metal inside is different.
 
3. Austenitic stainless steel is added with high chromium and nickel (with chromium about 18% and Ni above 4%), the internal structure of the steel presents a state called austenite. Tissues are not magnetic and cannot be attracted by magnets. Commonly used as decorative materials, such as stainless steel pipes, towel racks, tableware, stoves, etc.
 
4. Martensitic stainless steel should be used to make knife and scissors stainless steel. Because knives and scissors have the function of cutting objects, they must have sharpness, and to have sharpness, they must have a certain degree of hardness. This kind of stainless steel must undergo a heat treatment to make the internal structure change and increase the hardness before it can be used as a knife. However, the internal structure of this type of stainless steel is tempered martensite, which is magnetic and can be attracted by magnets. Therefore, we cannot simply use whether it is magnetic to explain whether it is stainless steel.