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Carbon steel

Carbon steel, also called plain carbon steel, is steel where the main alloying constituent is carbon. The AISI defines carbon steel as: "Steel is considered to be carbon steel when no minimum content is specified or required for chromium, cobalt, columbium [niobium], molybdenum, nickel, titanium, tungsten, vanadium or zirconium, or any other element to be added to obtain a desired alloying effect; when the specified minimum for copper does not exceed 0.40 per cent; or when the maximum content specified for any of the following elements does not exceed the percentages noted: manganese 1.65, silicon 0.60, copper 0.60."[1]

The term "carbon steel" may also be used in reference to steel which is not stainless steel; in this use carbon steel may include alloy steels.

Steel with a low carbon content has properties similar to iron. As the carbon content rises, the metal becomes harder and stronger but less ductile and more difficult to weld. In general, higher carbon content lowers the melting point and its temperature resistance. Carbon content influences the yield strength of steel because carbon atoms fit into the interstitial crystal lattice sites of the body-centered cubic (BCC) arrangement of the iron atoms. The interstitial carbon reduces the mobility of dislocations, which in turn has a hardening effect on the iron. To get dislocations to move, a high enough stress level must be applied in order for the dislocations to "break away". This is because the interstitial carbon atoms cause some of the iron BCC lattice cells to distort.

85% of all steel used in the U.S. is carbon steel.[1]

Carbon steel is broken down in to four classes based on carbon content:

Types
Mild and low carbon steel
Mild steel is the most common form of steel as its price is relatively low while it provides material properties that are acceptable for many applications. Low carbon steel contains approximately 0.05–0.15% carbon[1] and mild steel contains 0.16–0.29%[1] carbon, therefore it is neither brittle nor ductile. Mild steel has a relatively low tensile strength, but it is cheap and malleable; surface hardness can be increased through carburizing.[2]

It is often used when large amounts of steel is needed, for example as structural steel. The density of mild steel is approximately 7.85 g/cm3 (0.284 lb/in3)[3] and the Young's modulus is 210,000 MPa (30,000,000 psi).[4]

Low carbon steels suffer from yield-point runout where the materials has two yield points. The first yield point (or upper yield point) is higher than the second and the yield drops dramatically after the upper yield point. If a low carbon steel is only stressed to some point between the upper and lower yield point then the surface may develop Lüder bands.[5]

Higher carbon steels
Carbon steels which can successfully undergo heat-treatment have a carbon content in the range of 0.30–1.70% by weight. Trace impurities of various other elements can have a significant effect on the quality of the resulting steel. Trace amounts of sulfur in particular make the steel red-short. Low alloy carbon steel, such as A36 grade, contains about 0.05% sulfur and melts around 1426–1538 °C (2600–2800 °F).[6] Manganese is often added to improve the hardenability of low carbon steels. These additions turn the material into a low alloy steel by some definitions, but AISI's definition of carbon steel allows up to 1.65% manganese by weight.

Medium carbon steel
Approximately 0.30–0.59% carbon content.[1] Balances ductility and strength and has good wear resistance; used for large parts, forging and automotive components.[7]

High carbon steel
Approximately 0.6–0.99% carbon content.[1] Very strong, used for springs and high-strength wires.[8]

Ultra-high carbon steel
Approximately 1.0–2.0% carbon content.[1] Steels that can be tempered to great hardness. Used for special purposes like (non-industrial-purpose) knives, axles or punches. Most steels with more than 1.2% carbon content are made using powder metallurgy. Note that steel with a carbon content above 2.0% is considered cast iron.

Steel can be heat treated which allows parts to be fabricated in an easily-formable soft state. If enough carbon is present, the alloy can be hardened to increase strength, wear, and impact resistance. Steels are often wrought by cold working methods, which is the shaping of metal through deformation at a low equilibrium or metastable temperature.

Heat treatment

The purpose of heat treating carbon steel is to change the mechanical properties of steel, usually ductility, hardness, yield strength, or impact resistance. Note that the electrical and thermal conductivity are slightly altered. As with most strengthening techniques for steel, Young's modulus is unaffected. Steel has a higher solid solubility for carbon in the austenite phase; therefore all heat treatments, except spheroidizing and process annealing, start by heating to an austenitic phase. The rate at which the steel is cooled through the eutectoid reaction affects the rate at which carbon diffuses out of austenite. Generally speaking, cooling swiftly will give a finer pearlite (until the martensite critical temperature is reached) and cooling slowly will give a coarser pearlite. Cooling a hypoeutectoid (less than 0.77 wt% C) steel results in a pearlitic structure with α-ferrite at the grain boundaries. If it is hypereutectoid (more than 0.77 wt% C) steel then the structure is full pearlite with small grains of cementite scattered throughout. The relative amounts of constituents are found using the lever rule. Here is a list of the types of heat treatments possible:

Spheroidizing: Spheroidite forms when carbon steel is heated to approximately 700 °C for over 30 hours. Spheroidite can form at lower temperatures but the time needed drastically increases, as this is a diffusion-controlled process. The result is a structure of rods or spheres of cementite within primary structure (ferrite or pearlite, depending on which side of the eutectoid you are on). The purpose is to soften higher carbon steels and allow more formability. This is the softest and most ductile form of steel. The image to the right shows where spheroidizing usually occurs.[9]
Full annealing: Carbon steel is heated to approximately 40 °C above Ac3 or Ac1 for 1 hour; this assures all the ferrite transforms into austenite (although cementite might still exist if the carbon content is greater than the eutectoid). The steel must then be cooled slowly, in the realm of 38 °C (100 °F) per hour. Usually it is just furnace cooled, where the furnace is turned off with the steel still inside. This results in a coarse pearlitic structure, which means the "bands" of pearlite are thick. Fully-annealed steel is soft and ductile, with no internal stresses, which is often necessary for cost-effective forming. Only spheroidized steel is softer and more ductile.[10]
Process annealing: A process used to relieve stress in a cold-worked carbon steel with less than 0.3 wt% C. The steel is usually heated up to 550–650 °C for 1 hour, but sometimes temperatures as high as 700 °C. The image rightward shows the area where process annealing occurs.
Isothermal annealing: It is a process in which hypoeutectoid steel is heated above the upper critical temperature and this temperature is maintained for a time and then the temperature is brought down below lower critical temperature and is again maintained. Then finally it is cooled at room temperature. This method rids any temperature gradient.
Normalizing: Carbon steel is heated to approximately 55 °C above Ac3 or Acm for 1 hour; this assures the steel completely transforms to austenite. The steel is then air-cooled, which is a cooling rate of approximately 38 °C (100 °F) per minute. This results in a fine pearlitic structure, and a more-uniform structure. Normalized steel has a higher strength than annealed steel; it has a relatively high strength and ductility.[11]
Quenching: Carbon steel with at least 0.4 wt% C is heated to normalizing temperatures and then rapidly cooled (quenched) in water, brine, or oil to the critical temperature. The critical temperature is dependent on the carbon content, but as a general rule is lower as the carbon content increases. This results in a martensitic structure; a form of steel that possesses a super-saturated carbon content in a deformed body-centered cubic (BCC) crystalline structure, properly termed body-centered tetragonal (BCT), with much internal stress. Thus quenched steel is extremely hard but brittle, usually too brittle for practical purposes. These internal stresses cause stress cracks on the surface. Quenched steel is approximately three to four (with more carbon) fold harder than normalized steel.[12]
Martempering (Marquenching): Martempering is not actually a tempering procedure, hence the term "marquenching". It is a form of isothermal heat treatment applied after an initial quench of typically in a molten salt bath at a temperature right above the "martensite start temperature". At this temperature, residual stresses within the material are relieved and some bainite may be formed from the retained ferrite which did not have time to transform into anything else. In industry, this is a process used to control the ductility and hardness of a material. With longer marquenching, the ductility increases with a minimal loss in strength; the steel is held in this solution until the inner and outer temperatures equalize. Then the steel is cooled at a moderate speed to keep the temperature gradient minimal. Not only does this process reduce internal stresses and stress cracks, but it also increases the impact resistance.[13]
Quench and tempering: This is the most common heat treatment encountered, because the final properties can be precisely determined by the temperature and time of the tempering. Tempering involves reheating quenched steel to a temperature below the eutectoid temperature then cooling. The elevated temperature allows very small amounts of spheroidite to form, which restore ductility, but reduces hardness. Actual temperatures and times are carefully chosen for each composition.[14]
Austempering: The austempering process is the same as martempering, except the steel is held in the molten salt bath through the bainite transformation temperatures, and then moderately cooled. The resulting bainite steel has a greater ductility, higher impact resistance, and less distortion. The disadvantage of austempering is it can only be used on a few steels, and it requires a special salt bath.[15]

碳素钢
　　含碳量小于1.35％，除铁、碳和限量以内的硅 、锰、磷、硫等杂质外，不含其他合金元素的钢。碳素钢的性能主要取决于含碳量。含碳量增加，钢的强度、硬度升高，塑性、韧性和可焊性降低。与其他钢类相比，碳素钢使用最早，成本低，性能范围宽 ，用量最大。 适用于公称压力PN≤32.0MPa，温度为-30-425℃的水、蒸汽、空气、氢、氨、氮及石油制品等介质。常用牌号有WC1、WCB、ZG25及优质钢20、25、30及低合金结构钢16Mn

分类：
　　按含碳量分为低碳钢(碳含量为0.04％～0.25％) 、中碳钢（碳含量为0.25％～0.6％）、高碳钢（ 碳含量为0.6％～1.35％） 。
　　按质量分为普通碳素钢，其有害杂质磷 、硫含量均小于0.05％ ，包括甲类钢（A类钢，保证力学性能）、乙类钢（B类钢，保证化学成分）和特类钢（C类钢，保证力学性能和化学成分）－－如：Q235A,Q235B，A235C，Q235D，SS400等等；优质碳素钢，有害杂质磷、硫含量均小于0.04％；高级优质碳素钢，有害杂质磷、硫含量小于0.03％－－如：45，S50C，S45c，P20等等。
　　按用途又分为碳素结构钢和碳素工具钢，前者主要用于制造各种结构件和机器零件，一般属低碳钢和中碳钢；后者用于制造刀具、量具、模具等，一般属高碳钢。
　　碳素结构钢按照钢材屈服强度分为5个牌号：
　　Q195、Q215、Q235、Q255、Q275
　　每个牌号由于质量不同分为A、B、C、D等级，最多的有四种，有的只有一；另外还有钢材冶炼的脱氧方法区别。
　　脱氧方法符号：
　　F——沸腾钢
　　b——半镇静钢
　　Z——镇静钢
　　TZ——特殊镇静钢
叙述：
　　1. 概述
　　碳素钢是指碳含量低于2%，并有少量硅、锰以及磷、硫等杂质的铁碳合金。工业上应用的碳素钢碳含量一般不超过1.4%。这是因为含碳量超过此量后，钢表 现出很大的硬脆性，并且加工困难，失去生产和使用价值。碳素钢按其质量不同可分为普通碳素结构钢和优质碳素结构钢二类。优质碳素结构钢规定硫、磷的允许含量比普通碳素钢低 ，所以综合机械性能比普通碳素钢好。
　　(1)生产制造方法。碳素钢的冶炼通常在转炉、平炉中进行。转炉一般冶炼普通碳素钢，而 平炉可以冶炼各种优质钢。近年来氧气顶吹转炉炼钢技术发展很快，有趋势可代替平炉炼钢。将炼好的钢液注入钢锭模，就得到各种钢锭。钢锭经过锻压或轧制后便加工成各种形状的钢材和锻件。钢锭经过压力加工后，能够改善钢的内部组织和夹杂物分布，所以同样成分的钢材要比钢锭的性能优越一些。
　　(2)用途。碳素钢主要用来制造强度要求不高的机器零件和各种金属构件。广泛应用于机械制造的各个方面。如农业机械、机床、船舶等。它是一种应用最广、用途最大的钢材。
　　2. 主要生产及输往国家、地区
　　我国的鞍山钢厂、宝山钢厂、上海钢厂、太原钢厂、重庆钢厂、天津钢厂等是出口碳素钢的主要产地。一般碳素钢多加工成型材，如角钢、扁钢、工字钢等输往日本、香港、东南亚、中东等国家和地区。
　　3. 主要进口生产国家
　　我国主要从日本、俄罗斯、德国、东欧等国家进口。与其他钢类相比，碳素钢进口数量最多。进口到货后缺重问题较为突出。收用货部门要加强到货后重量的验收。
　　4. 种类
　　碳素钢按含碳量可划分为低碳钢、中碳钢和高碳钢。高碳钢属于工具钢，详见“工具钢”部分。低碳钢如20号钢一般多用来制作渗碳零件。热处理工艺是先进行渗碳处理，随后进行淬火和低温回火。经这样处理后零件表面具有较高的硬度而心部具有良好的塑性。而中碳钢如45号钢根据使用条件不同，热处理方式也不同。一般做热加工使用的要进行调质处理，即淬火后高温回火。其他条件使用的可进行正火处理。
　　5. 规格及外观质量
　　碳素钢的品种主要有圆钢、扁钢、方钢等。经冷、热加工后钢材的表面不得有裂缝、结疤、夹杂、折叠和发纹等缺陷。尺寸和允许公差必须符合相应品种国家标准的要求。
6. 化学成分
　　7. 物理性能
　　（6、7点详见参考资料）
　　8. 包装
　　裸装，国产钢按钢号在端部进行涂色，详见GB/T699-88标准规定。
　　9. 注意事项
　　碳素钢淬火时通常采用水冷，但对小尺寸的中碳钢，尤其是直径为8―12mm的45号钢淬火时容易产生裂纹，这是一个较为复杂的问题。目前采取的措施是淬火时试样在水中快速搅动，或者采用油冷，可避免出现裂纹。