【正文】 s extremely effective in percentages greater than %,which leads to a separate class of alloy steels called stainless steels. Many tool steels,while not designed for the purpose, are in effect stainless steels because of the high percentage of chromium present.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 strengh than plain carbon steel. The addition of small amount of some alloying elements can raise the yield strength of hotrolled sections 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 kind of alloy.The low alloy structural steels are sold almost entirely in the form of hot rolled structural shapes. These materials have good weldability, ductility, better impact strength than that of plain carbon steel, and 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 STEELSImproved Properties at Higher Cost. The low alloy American iron and steel institute (AISI) steels are alloyed primarily for improved hardenability. They are more costly than plain carbon steels, and their use can generally be justified only when needed in the heattreathardened and tempered condition. Compared to plain carbon steels, they can have 30%～40% higher yield strength 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 mercially important steels contain less than 5%. The carbon content may very 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 Tonnagewise, the most important of the higher alloy steels are a group of these steels have much better mechanical properties at high temperatures. This group was first called stainless steel. With the emphasis on high temperature use, they are frequently referred to as heat and corrosionresistant steels.Martensitic Stainless Steel. With lower amounts of chromium or with silicon or aluminium added to some higher chromium steels, the material responds to heat treatment much as any low alloy steal. The gammatoalpha transformation in iron occurs normally, and the steel may be hardened by heat treatment similar to that 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.Ferritic stainless steel. With large amounts of chromium, as great as 30% or more,the austenite region of the ironcarbon equilibrium diagram is suppressed, and the steel loses its ability to be hardened by normal steel heattreating procedures. Steels of this type are called ferritic and are particularly useful when high corrosion resistance is necessary in coldworked products.Austenitic Stainless Steel. With high chromium and the addition of 8% or more of nickel or binations 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 solutionprecipitation reaction.TOOL AND DIE STEELS The greatest tonnage of tools (other than cutting tools) and dies are made from plain carbon or low alloy steels. This is true only because of the low cost these materials as their use has a number of disadvantages. They have low hardenability, low ductility associated with high hardness, and do not hold their hardness well at elevated temperature.Manganese 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, %～% of manganese to improve hardenability, and small amounts of chromium, vanadium, and molybdenum to improve hardness 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.鋼 普通碳素鋼%（甚至更少）碳的鋼。由于只有少量的碳，鋼的性能接近于純鐵，具有很高塑形和很低的強(qiáng)度。從便于成形和使用的角度看，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)度的增加只有在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^程有一定的難度，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?yàn)槌杀镜停瑢?shí)際使用的大多數(shù)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。除了用來控制硫和錳元素以外，其他元素只有很少而被認(rèn)為是雜質(zhì)，有時它們對材料的性能可能有較小的影響。低碳鋼 %～%的鋼稱為低碳鋼，他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，經(jīng)常用作深拉成形零件或可用表面硬化材料的塑性心部。低碳鋼容易銅焊、熔焊和鍛造。中碳鋼 中碳鋼(%～%)含有足夠的碳，可以通過熱處理得到所需強(qiáng)度、硬度、切削加工性和其他特性。此類普通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 %～%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，這種鋼很少能完成淬透，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，仍常使用碳素鋼。 合金鋼普通碳素鋼可用于許多場合，也是最便宜的鋼種，因此使用最多，但它們對某些工作要求