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鋼
一、普通碳素鋼
任何煉鋼方法都能煉出只有0.05%（甚至更少）碳的鋼。由于只有少量的碳，鋼的性能接近于純鐵，具有很高的塑性和很低的強(qiáng)度。從便于成形和使用角度看，高塑性和低強(qiáng)度是變形所需要的，然而，從產(chǎn)品設(shè)計(jì)角度來說，需要比這種低碳鋼更高的強(qiáng)度。增加強(qiáng)度最適用的方法是在鋼中增加或保留一些碳。然而，必須明白，強(qiáng)度的增加只在有損失塑性的情況下才能實(shí)現(xiàn)，因此，最終總是在塑性和強(qiáng)度之間形成某種折衷。因?yàn)槌煞挚刂坪驮鎏歼^程有一定難度，高碳高強(qiáng)度鋼的成本比低碳鋼高。
最常用的普通碳素鋼 因成本低，實(shí)際使用的大多數(shù)網(wǎng)是普通碳素鋼，它們由鐵和碳組成，普通碳素鋼的碳含量可分為低碳、中碳、高碳三類。除了用來控制硫的錳元素以外，其他元素只有很少量而被認(rèn)為是雜質(zhì)，有時(shí)它們對(duì)材料的性能可能有較小的影響。
低碳鋼 含碳大約0.06%-0.25%的鋼稱為低碳鋼，它們很難通過熱處理淬硬，因?yàn)樘嫉暮刻?，很難形成硬的馬氏體結(jié)構(gòu)，從而使熱處理相對(duì)不起作用。大量的低碳鋼被做成薄板材、帶材、棒材、板材、管材和線材等結(jié)構(gòu)。很多這類材料最后通過冷加工來提高硬度、強(qiáng)度和表面質(zhì)量。含碳小于等于20%的鋼可以經(jīng)受較大的塑性流動(dòng)，經(jīng)常用作深拉成形零件或可用作表面硬化材料的塑性心部。低碳鋼容易鋼焊、熔焊和鍛造。
中碳鋼 中碳鋼（0.25%-0.5%）含有足夠的碳，可通過熱處理得到所需強(qiáng)度、硬度、切削加工性或其它特性。比類普碳鋼的硬度不能顯著提高到滿意的作為切削刀具，但承載能力可提高很多，同時(shí)保留足夠的塑性和良好的韌性。大多數(shù)鋼在熱軋狀態(tài)提供，經(jīng)常需進(jìn)行切削加工。它能焊接，但比低碳鋼難得多，因?yàn)楹附訜崃吭诰植繀^(qū)域引起了組織結(jié)構(gòu)的變化。
高碳鋼 高碳鋼含有 0.5%-1.6%的碳，這類鋼稱為工具和模具鋼，硬度是這類鋼所需的主要性能。因?yàn)榻M織轉(zhuǎn)變快，淬透性低，這種鋼幾乎都是用水淬火。即使用這種激烈的處理方式，并有變形和開裂的危險(xiǎn)，這種鋼很少能完全淬透，淬硬層厚度不超過1英寸。實(shí)際上，在同樣強(qiáng)度下，熱處理淬硬的普通碳素鋼的塑性比合金鋼的低，但即使如此，因其成本低，仍堂使用碳素鋼。
二、合金鋼
普通碳素鋼可用于許多場(chǎng)合，也是最便宜的鋼種，因此使用得最多，但它們對(duì)某些工作要求不能完全滿足。這時(shí)可通過加入一些元素形成合金鋼的方式來提高鋼的某一項(xiàng)或幾項(xiàng)性能。即使是普通碳素鋼，也是鐵、碳、和錳的合成，但合金鋼中除了這些元素外，其他元素含量大于普通碳素鋼的雜質(zhì)含量，如錳含量要大于1.5%。
合金元素影響淬透性 人們對(duì)淬透性的興趣是間接的。淬透性通常與完全淬火時(shí)硬化深度的能力有關(guān)系。然而，隨著等溫曲線右移，即使在未完成硬化時(shí)，材料的性能也能顯著變化。在熱軋或鍛打后，材料通常采用空冷。所有合金鋼通常使等溫曲線右移，空冷時(shí)得到比普通鋼細(xì)的珠光體。這種細(xì)珠光體有較高的硬度和強(qiáng)度，可能會(huì)降低塑性，對(duì)切削加工性也有影響。
可焊性 總的來說，合金元素對(duì)可焊性產(chǎn)生壞影響，這里也影響淬透性的一種反應(yīng)，焊接區(qū)快冷里，合金會(huì)使焊接區(qū)形成硬的、韌性差的結(jié)構(gòu)，經(jīng)常導(dǎo)致開裂和變形。
三、低合金結(jié)構(gòu)鋼
市場(chǎng)上已有多種多樣的低合金結(jié)構(gòu)鋼，它們是屈服強(qiáng)度比普碳鋼高的低成本結(jié)構(gòu)材料。外加少量的一些合金元素不需經(jīng)過熱處理就可以提高熱軋鋼的屈服強(qiáng)度，比普碳鋼高30%-40%。在高應(yīng)力條件下，可減少橫截面尺寸25%-30%，同時(shí)增加成本15%-50%，這取決于合金元素的量和種類。
四、低合金AISI鋼
高性能高成本 低合金AISI（美國(guó)鋼鐵協(xié)會(huì)）鋼中的合金元素主要用于提高淬透性，它們比普碳鋼貴得多，通常只在必需時(shí)使用，用于熱處理硬化和回火條件下。與普碳鋼相比較，屈服強(qiáng)度高30%-40%，抗拉強(qiáng)度高10%-20%。同樣的拉伸強(qiáng)度和硬度時(shí)面積可減少30%-40%，沖擊強(qiáng)度大約提高兩倍。
通常需熱處理 低合金AISI鋼的總合金元素含量小于8%，雖然工業(yè)上大多數(shù)重要鋼的合金元素的含量少于5%。碳含量可從很低變到很高，但大多數(shù)為中碳鋼， 可用最小成本進(jìn)行熱處理來有效改善性能。這種鋼廣泛用于汽車、機(jī)床、飛機(jī)，特別是用于制造承受高應(yīng)力且磨損大的動(dòng)力零件。
五、不銹鋼
大量使用且最重要的高合金鋼是一組抗化學(xué)腐蝕能力極高的高鉻鋼。這類鋼的大多數(shù)在高溫下有好的力學(xué)性能，這類鋼最早稱為不銹鋼，隨著在高溫下使用的增加，它們經(jīng)常也稱為耐熱耐腐蝕鋼。
馬氏體不銹鋼 在鋼中加入少量鉻，或在一些高鉻鋼中加入硅或鋁，這種鋼對(duì)熱處理的響應(yīng)像低合金鋼一樣強(qiáng)。這種鋼具有正常的г相向а相的轉(zhuǎn)變，可采用與普碳鋼和低碳鋼類似的熱處理方法硬化。這種鋼稱為馬氏體鋼，其中含鉻4%到6%的鋼最常用。
鐵素體不銹鋼 含鉻量達(dá)30%或更多時(shí)，鐵碳平衡相圖的奧氏體區(qū)縮小，鋼失去了用通常熱處理方法感化的能力。這種鋼稱為鐵素體鋼，特別適用于有高耐腐蝕性要求的冷加工產(chǎn)口。
奧氏體不銹鋼 高絡(luò)鋼再加上8%以上的鎳或鎳與錳，相圖的鐵素體區(qū)就會(huì)縮小。最典型的鋼含18%和8%鎳，稱為奧氏體不銹鋼。他們不能用通常鋼的熱處理方法硬化，但可附加少量的其他元素通過固溶強(qiáng)化使它們硬化。
六、工具模具鋼
大量的工具（與切削刀具不同）和模具用普碳鋼或低合金鋼制造，這只是因?yàn)樗鼈儍r(jià)格便宜，但這些材料有很多缺點(diǎn)。它們的淬透性差，硬度高而塑性低，溫度升高時(shí)不能很好地保持硬度。
錳鋼 錳工具模具鋼是油淬硬化鋼，在熱處理時(shí)很少變形或開裂。為提高淬透性，鋼中售有0.85%-1.00%的碳和1.5%-1.75%的錳，并有少量鉻、釩、鉬來提高硬度和韌性。
鉻鋼 高鉻工具模具鋼通常在油中淬硬，但有一些鉻鋼淬透性好，在空冷時(shí)就能淬硬。有組高絡(luò)鋼加有許多鎢、釩（有時(shí)還有鈷）來提高其高溫硬度，它們稱為高速鋼。
STEEl
PLAIN CARBON STEEL
Any steel-making process is capable of producing a product that has 0.05%or less carbon.With this small amount of carbon,the properties approach those of pure iron with maximum ductility and minimum strength.Maximum ductility is desirable from the standpoint of ease in deformation processing and service use.Minimum strength is desirable for deformation processing.However,higher strengths than that obtainable with this low carbon are desirable from the standpoint of product design.The most practical means of increasing the strength is by the addition or retention of some carbon.However ,it should be fully understood that any increase of strength over that of pure iron can be obtained only at the expense of some loss of ductility,and the final choice is always a compromise of some degree.Because of the difficulty of composition control or the additional operation of increasing carbon content,the cost of higher carbon,higher strength steel is greater than that of low carbon.
Plain Carbon Steels Most Used.Because of their low cost,the majority of steels used are plain carbon steels.These consist of iron combined with carbon concentrated in therr ranges classed as low carbon,medium carbon,and high carbon .With the exception of manganese used to control sulphur,other elements are present only in small enough quantities to be considered as impurityes ,though in some cases they may have minor effect on properties of the material.
Low Carbon. Steel with approximately 6 to 25 point of carbon(0.06%-0.25%) are rated as low carbon steels and are rarely hardened by heat treatment because the low carbon content pernmits so little formation of hard martensite that the process is relatively ineffective.Enormous tonnages of these low carbon steels are processed in such structural shapes as sheet ,strip ,rod,plate ,pipe ,and wire.A large portion of the material is cold worked grades containing 20 points or less of carbon are susceptilbe to considerable plastic flow and are frequently used as deep-drawn products or may be used as a ductile core for casehardened material.The low lpain carbon steels are readily brazed ,welded,and forged.
Medium Carbon. The medium carbon steels (0.25%-0.5%) contain sufficient carbon that they may be heat treated for desirable strength,hardness,machinability,or other properties.The hardness of plain carbon steels in this range connot be increased sufficiently for the material to serve satisfactorily as cutting tools ,but the load-carrying capacity of the steels can be raised considerably,while still retaining sufficient ductility for good toughness.The majority of the steel is furnished in the hot-rolled condition and is often machined for final finishing.It can be welded,but is more difficult to join by this method than the low carbon steel because of structural changes caused by welding heat in localized areas.
High Carbon. High carbon steel contains from 50 to 160 points of carbon (0.05%-1.6%) .This group of steels is classed as tool and die steel,in which hardness is the principal property desired.Because of the fast reaction time and resulting low hardenability,plan carbon steels nearly always must be waterquenched.Even with this drastic treatment and its accociated danger of distortion or cracking,it is seldom possible to develop fully hardened structure in material more than about 1 inch in thickness.In practice the ductility of heat-treat-hardened plain carbon steel is low compared to that of alloy steels with the same strength,but ,even so ,conbon steel is frequently used because of its lower cost.
ALLOY STEELS
Although plain carbon steels work well for many uses and are the cheapest steels and therefore the most used,they connot completely fulfill the requirements for some work.Individual or groups of properties can be improved by addition of various elements in the form of alloys. Even plain carbon steels are alloys of at least iron,carbon,and manganses,but the term blloy steel refers to steels containing elements other than these in controlled quantities greater than impurity concentration or ,in the case of manganese,greater than 1.5%.
Alloys Affect Hardenability. Interest in hardenability is indirect.Hardenability is usually thought of most in connection with depth-hardening ability in a full hardening operation. However,with the isothermal transformation curves shifted to the right,the properties forging ooperations ,the material usually air cools .Any alloy generally shifts the transformation curves to the right ,which with air cooling results in finer pearlite than would be formed in a plain carbon steel. This finer pearlite has higher hardness and strength,which has an effect
on machinability and higher hardness and strength,which has an effect on machinability and may lower ductility.
Weldability. The generally bad influence of alloys on weldability is a
further reflection of the influence on hardenability.With alloys present during the rapid cooling taking place in the ewlding area, hard ,nonductile structures are formed in the steel and frequently lead to cracking and distortion.
LOW ALLOY STRUCTURAL STEELS
Certain low alloy steels sold under various trade names have been developed to provide a low cost structural material with higher yield strength than plain carbon steel. The addition without heat treatment to 30%-40% greater than that of plain carbon steels.Designing to higher working stresses may reduce the required section size by 25%-30% at an increased cost of 15%-50%,depending upon the amount and the king of alloy.
The low alloy structural steels are sold almost entirely in the form of
hot-rolled structural shapes.These materials have good corrosion resistance ,particularly to atmospheric exposure.Many building codes are based on the more conservative use of plain carbon steels,and the use of alloy structural steel often has no economic advantage in these cases.
LOW ALLOY AISI STEELS
Improved Properties at Higher Cost. The low alloy American Iron and Steel Institute (AISI) steels are alloyed primarily for improved
hardenability.They are more coslty than plain carbon steels ,and their use can generally be justified only when needed in the heat-treat-hardened and
tempered condition.Compared to plain carbon steels,they can have 30%-40%
higher yield strengh and 10%-20% higher tensile strength. At equivalent tensile strengths and hardnesses,they can have 30%-40% higher reduction of area and approximately twice the impact strength.
Usually Heat Treated. The low alloy AISI steels are those containing less than approximately 8% total alloying elements,although most commercially inportant steels contain less than 5%.The carbon content may vary form very low to very high,but for most steels it is in the medium range that effective heat treatment may be employed for property improvement at minimum costs.The steels are used widely in automobile ,machine tool,and aircraft construction,especially for the manufacture of moving parts that are subject to high stress and wear.
STAINLESS STEELS
Tonnage-wise,the most important of the higher alloy steels are a group of high chromium steels with extremely high corrosion and chemical resistance. Most of these steels have much better mechanical properties at high temperatures. This group was first called stainless .With the emphasis on high temperature use,they are frequently referred to as heat and corrosion-resistant steels.
Martensitic Stainless Steel . With lower amounts of chromium or with silicon or aluminum added to some higher chromium steels ,the material responds to heat treatment much as any low alloy steal. The gamma-to-alpha
transformation in iron occurs normally,and the steel may be hardened by heat treatment similar to theat used on plain carbon or low alloy steels.
Steels of this class are called martensitic,and the most used ones have 4% to 6% chromium.
Austenitic Stainless steel. With larger amounts of chromium,as great sa 30% or more ,the austenite region of the iron-carbon equilibrium diagram is suppressed,and the steel loses its ability to be hardened by normal steel heat-treating procedures.Steels of this type are called ferritic and are particularly useful whenhigh corrosion resistance is necessary in cold-wored products.
Austenitic Stainless Steel. With high chromium and the addition of 8% or more of nickel or combinations of nickel and manganese,the ferrite region of the diagram is suppressed. These steels ,the most typical of which contains 18% chromium and 8% nickel,
are referred to as austenitic stainless steels. They are not hardenable by normal steel heattreating procedures,but the addition of small amounts of other elements makes some of them hardenable by a solution-precipittion reaction.
TOOL AND DIE STEELS
The greates tonnage of tools (other than cutting tools ) and dies are
made from plain carbon or low alloy steels.This is true onlly because of the low cost of these materials as their use has a number of disadvantages.THey have low harden-ability,low ductility associated with high hardness,and do not hold their hardness well at elevatesd temperature.
Mangnaese Steels. Manganese tool and die steels are oil hardening and have a reduced tendency to deform or crack during heat treatment .They contain from 85-100 points of carbon,1.5%-1.75% of manganese to improve hardenability,and small amounts of chromium. vanadium,and molybdenum to improve hardnwss and toughness qualities.
Chromium Steels.High chromium tool and die steels are usually quenched
in oil for hardening ,but some have sufficient hardenability to develop
hardness with an air quench.One group of the high chromium steels,called high speed steel,has substantial additions of tungsten, vanadium,and sometimes cobalt to improve the hardness in the red heat range .