钢铁是铁与碳、硅、锰、磷、硫以及少量的其他元素所组成的合金。其中除铁外，碳的含量对钢铁的机械性能起着主要作用，故统称为铁碳合金。它是工程技术中最重要、用量最大的金属材料。
Iron and steel are alloys of iron with carbon, silicon, manganese, phosphorus, sulfur, and a small amount of other elements. In addition to iron, the content of carbon plays a major role in the mechanical properties of steel, so it is collectively referred to as iron carbon alloy. It is the most important and the largest amount of metal material in engineering.
按含碳量不同，铁碳合金分为钢与生铁两大类，钢是含碳量为0.03%~2%的铁碳合金。碳钢是最常用的普通钢，冶炼方便、加工容易、价格低廉，而且在多数情况下能满足使用要求，所以应用十分普遍。按含碳量不同，碳钢又分为低碳钢、中碳钢和高碳钢。随含碳量升高，碳钢的硬度增加、韧性下降。合金钢又叫特种钢，在碳钢的基础上加入一种或多种合金元素，使钢的组织结构和性能发生变化，从而具有一些特殊性能，如高硬度、高耐磨性、高韧性、耐腐蚀性，等等。

According to the different carbon content, iron carbon alloy is divided into two categories of steel and pig iron, steel is a carbon content of 0.03%~2% of iron carbon alloy. Carbon steel is the most commonly used ordinary steel, smelting convenience, easy processing, low price, and in most cases can meet the use requirements, so the application is very common. According to the carbon content, carbon steel is divided into low carbon steel, medium carbon steel and high carbon steel. With the increase of carbon content, the hardness and toughness of carbon steel decrease. Alloy steel and special steel, adding one or more alloying elements in steel on the basis of the change of microstructure and properties of steel, it has some special properties, such as high hardness, high wear resistance, high toughness, corrosion resistance, etc..
生铁硬而脆，但耐压耐磨。根据生铁中碳存在的形态不同又可分为白口铁、灰口铁和球墨铸铁。白口铁中碳以Fe3C形态分布，断口呈银白色，质硬而脆，不能进行机械加工，是炼钢的原料，故又称炼钢生铁。碳以片状石墨形态分布的称灰口铁，断口呈银灰色，易切削，易铸，耐磨。若碳以球状石墨分布则称球墨铸铁，其机械性能、加工性能接近于钢。在铸铁中加入特种合金元素可得特种铸铁，如加入Cr，耐磨性可大幅度提高，在特种条件下有十分重要的应用。
Pig iron is hard and crisp, but it is resistant to pressure and wear. According to the existing iron carbon in different forms can be divided into white iron, gray iron and ductile iron. White iron in the carbon distribution in the form of Fe3C, the fracture was silver white, hard and crisp, can not be processed mechanically, is the raw material of steelmaking, it is also known as steelmaking pig iron. The distribution of carbon graphite in Gray said, the fracture was gray, easy cutting, easy casting, wear resistance. If the carbon is spherical graphite, it is called ductile iron. Its mechanical properties and processability are close to that of steel. Special alloy elements are added to cast iron to obtain special cast iron, such as adding Cr, the abrasion resistance can be greatly improved, and it has a very important application under special conditions.
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