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第 1 页 共 18 页英文原文: SteelsSteel is one of the most valuable metals known to man; approximately 200 million tons can be produced in the United States annually. In 1900, US capacity was but 21 million tons. Although the process of steelmaking is familiar to most engineers, a review of this process would be appropriate at this time.Iron ore, limestone, and coal are the principal raw materials used in making iron and steel. Coke is produced by heating bituminous coal in special ovens. Skip cars go up the skip hoist with loads of iron ore, coke, and limestone and dump them into the top of the blast furnace. Hot air from the stove is blown into the furnace near the bottom. This causes the coke to burn at temperatures up to 3000F. The ore is changed into drops of molten iron that settle to the bottom of the blast furnace. The limestone that has been added joins with impurities to form a slag that floats on top of the pool of liquid iron. Periodically , the molten iron is drained into a ladle for transporting to either the Bessemer converter, electric furnace or open-hearth furnace. The slag is removed separately so as mot to contaminate the iron.The making of steel from iron involves a further removal 第 2 页 共 18 页of impurities. Regardless of which process is used for making steel-open-hearth, Bessemer-converter, or electric-furnace-steel scrap is added along with desired alloying elements and the impurities are burned out.Liquid steel removed from the furnace is poured into ingot molds. The ingots are then removed to “soaking pits” where they are brought to a uniform rolling temperature.At the rolling mill, the white-hot steel passes through rolls that form the plastic steel into the desired shape: blooms, slabs, or billets. These three semifinished shapes then go to the finishing mills where they are rolled into finished forms as structural steel, plates and sheets, rods, and pipes.Steel is the basic and most valuable material used in apparatus manufactured today. Its application is based on years of engineering experience, which serves as a guide in choosing a particular type of steel. Each variable, such as alloy, heat treatment, and processes of fabrication has its influence on the strength, ductility, machinability, and other mechanical properties, and affects the type of steel selected. The following basic concepts also assist in determining which steel should be used:1. The modulus of elasticity in tension falls within the 第 3 页 共 18 页range of 28106to 30106lb/in2, regardless of composition or form; therefore, sizes as determined by deflection remain the same regardless of the steel chosen.2. Carbon content determines the maximum hardness of steel regardless of alloy content. Therefore, the strength desired, which is proportional to hardness, can determine the carbon content.3. The ability of the steel to be uniformly hardened throughout its volume depends on the amount and kind of alloy. This is more complex, but does not necessarily change the calculation of the size of the part. 4.Ductility decreases as hardness increases.The preliminary choice of steel for a part as well as for other factors, such as notch sensitivity, shrinkage, blowholes, corrosion, and wear, is simplified when based on the above principles. The final selection is made by matching the material with the process of manufacture used in order to obtain the shape, surface, and physical requirements of the part. The selection may be made from among low-carbon steels, low-alloy steels, high-carbon steels, and high-alloy steels.Steel is one of the few common metals that has an endurance limit. You will recall that fatigue is the failure of a 第 4 页 共 18 页material due to repeated loading. Most metals become tired as they are subjected to stress over and over again. The stress a material can withstand under constant loading is much less than under static loading. As steel is continually loaded, it will reach a lower limit of strength. This property is quite pronounced in wire shapes. Common copper and aluminum wire can easily be broken by flexing the wire in a local spot. Normally after a few dozen flexes, the wire breaks. Steel wire, however, is very tough and flexing the wire simply cold works the material making the process futile for the unknowing person trying to break a steel wire. At some point steel will resist weakening due to repeated loading. This is known as an “endurance limit”. The endurance limit of steel is around 60% of its original strength.This property of having an endurance limit makes steel invaluable for use in structural applications like bridges, springs, struts, beams, etc. Of course, there are many factors that effect the endurance limit of a material. A primary factor is the surface quality of the material and/or the manufacturing process used to produce the specimen.Fatigue is attributable to the initial material mot being an ideal homogeneous solid. In each half cycle, irreversible 第 5 页 共 18 页minute strains are produced. Fatigue failure usually develops from:1.Repeated cyclic stresses that cause incremental slip and cold working locally in the material.2.Gradual reduction of ductility of the strain hardened areas that develop into cracks.3.A notching effect from submicroscopic cracks.The endurance limits of steels create some very desirable physical properties. These properties can be detrimental to the manufacturability of the material. For instance, in the cold rolling of steel the endurance limit creates a limitation on the amount of cold working that can be input to any part. After this limit has been reached the material must be heated above its critical temperature to permit further cold working.Plain carbon steels represent the major proportion of steel production. Carbon steels have a wide diversity of application, including castings, forgings, tubular products, plates, sheets and wire products, structural shapes, bars, and tools. Plain carbon steels, generally, are classified in accordance with their method of manufacture as basic open hearth, acid open hearth, or acid Bessemer steels, and by carbon content.第 6 页 共 18 页The principal factors affecting the properties of the plain carbon steels are the carbon content and the microstructure. The microstructure is determined by the composition of the steel (carbon, manganese, silicon, phosphorus, and sulfur, which are always present, and residual elements including oxygen, hydrogen, and nitrogen) and by the final rolling, forging, or heat-treating operation. However, most of the plain carbon steels are used without a final heat treatment and , consequently, the rolling and forging operations influence the microstructure.Carbon steels are predonminantly pearlitic in the cast, rolled, or forged conditions. The constituents of the hypoeutectoid steels are therefore ferrite and pearlite, and of the hypereutectoid steels are cementite and pearlite.Alloy steel is an alloy of iron and carbon containing alloying elements, one or more of which exceeds the following: manganese, 1.65 percent; silicon, 0.60 percent; copper, 0.60 percent; and/or specified amounts of other alloying elements, including aluminum, boron , and chromium up 3.99 percent; cobalt, niobium, molybdenum, nickel, tungsten, vanadium, zirconium, or other elements added in sufficient quantity to give the desired properties of the steel.第 7 页 共 18 页Since there are more elements , some expensive, to be kept within the specified ranges in alloy steel than are required in carbon steel , alloy steel requires more involved techniques of quality control and, consequently, is more expensive.Alloy steel can give better strength, ductility, and toughness properties than can be obtained in carbon steel. Consequently, the engineer should consider alloy steels I designs subject go high stresses and/or impact loading.Almost all alloy steels are produced with fine-grain structures. A steel is considered to be fine-grained if its grain size is rated 5, 6, 7, or 8. Number1 grain size shows 1 .5 grains/in. of steel area examined at 100diameters magnification. Fine-grain steels have less tendency to crack during heat treatment and have better toughness and shock-resistance properties. Coarse grained steels exhibit better machining properties and may be hardened more deeply than fine-grained steels.To select the alloy steel that is best suited for a given design, the effects of the principal alloying elements must be taken into account. They are as follows.1. Nickel provides toughness, corrosion resistance, and deep hardening.第 8 页 共 18 页2. Chromium improves corrosion resistance, toughness, and hardenability.3. Manganese deoxidizes, contributes to strength and hardness, decreases the critical-cooling rate.4. silicon deoxidizes, promotes resistance to high-temperature oxidation, raises the critical temperature for heat treatment, increases the susceptivity of steel to decarburization and graphitization.5. Molybdenum promotes hardenability, increases tensile and creep strengths at high temperatures.6. vanadium deoxidizes, promotes fine-grained structure.7. Copper provides resistance to corrosion and acts as strengthening agent.8. Aluminum deoxidizes, promotes fine-grained structure, and aids nitriding.9. boron increases hardenability.The term “stainless steel” denotes a large family of steels containing at least 11.5percent chromium. They are not resistant to all corroding media.Stainless steel competes with nonferrous alloys of copper 第 9 页 共 18 页and nickel on a corrosion-resistance and cost basis and with light metals such as aluminum and magnesium on the basis of cost and strength-weight ratio. Stainless steel has a number of alloy compositions and there are many supplies. Information on its properties and fabrication can be obtained readily. Sound techniques have been evolved for casting, heat treating, forming, machining, welding, assembling, and finishing stainless steel. It will be found that this material usually work-hardens(which makes machining, forming, and piercing more difficult), and it must be welded under controlled conditions and under inert gas. It has desirable high strength, corrosion resistance, and decorative properties.A bright, clean surface is essential for best corrosion resistance. Traces of scale and foreign matter should be removed by machining, pickling, or polishing. Dipping in nitric acid will ensure the formation of a good oxide film on new pieces. Stainless steels may be electroplated and electropolished, anodically etched covered with porcelain enamel, or given colored coatings through the dying of surface oxides. Highly polished sheets may be purchased directly from stainless-steel producers. A coating of plastic may be used to protect the surface during fabrication.第 10 页 共 18 页Stainless steel can be made very hard and its strength can be more than doubled by cooling to 300F and simultaneously rolling under high pressure, then heating to 750F for 24hours.Corrosion resistance is the most important single characteristic of the stainless steels. This quality is due to a thin transparent film of chromium oxide that forms on the surface. It will withstand oxidizing agents such as nitric acid , but will be attacked by reducing agents such as hydrochloric acid or any of the halogen salts. Scaling and corrosion are accelerated in applications in which the oxide layer is constantly being broken. Repeated heating and cooling, with the accompanying expansion and contraction, cracks off the oxide layers. Since the straight-chromium grades of stainless steel have lower thermal expansion than the chromium-nicket grades, they serve best where constant heating and cooling is involved. Most stainless steels show good short-time strength at 1500F and a few special types are good at 2000F. Compare this with ordinary carbon steels, which lose their usefulness above 900 to 950F. The heatconducting properties of stainless steel are poor, so copper cladding is often used in cooking utensils to distribute heat.第 11 页 共 18 页中文: 钢钢是人们所熟悉的最有用的金属材料之一；美国每年大约要生产 2 亿吨钢，1990 年美国的钢生产能力只有 2100吨。虽然大多数工程师都有熟悉炼钢的过程，然而这里有必要回顾一下钢的生产过程。铁矿石，石灰石和煤都是炼钢的主要原材料。焦炭是在特定的炉子里将烟煤加热燃烧后生产出来的。上料车通过箕斗提升机将铁矿石，焦炭及石灰石从鼓风炉的顶部倒入炉内。热风从底部吹入炉内，这就使焦炭在高达的温度下燃烧，铁矿石则变成熔化的铁水沉积在鼓风炉第 12 页 共 18 页的底部。加入的石灰石与杂质一起形成炉渣并漂浮在池中的铁水上。熔化的铁水定期地排入铁水包中然后再送到酸性转炉，电炉或者平炉中。为了不使铁含杂质炉渣则被分别排除。由换炼成钢则要进一步去除杂质，不论是用平炉炼负还是酸性转炉或是电炉炼钢，乾要将废钢与所需的合金元素一起加入以使杂质燃烧掉。钢水从炉中排出后便注入钢锭模中，然后将钢锭送到均热炉内以达到均匀的轧制温度。在轧制车间，白炽状的钢通过轧辊将塑性状态的钢轧制成所需的形状：钢坯，扁钢坯或坯段。这三种半成品通过精轧后被轧制成最终的形状，如结构用钢材，板材，薄板，棒料或管材。今天 在设备制造中钢是基本的也是最重要的材料，基应用是基于多年的工程经验，而这些经验也可作为选用钢材的规则。钢的合金元素，热处理及制造过程等都会影响其强度，塑性，可加工及其它机械性能，也影响钢的选择。下面列出有助于选取钢材的一些概念。无论钢的成份与结构形状如何，其拉伸弹性模量均在于 lbin 之间，因此，由变形所确定的结构尺寸都是相同的，而与所选择的钢无关。钢的最高硬度由含碳量决定，而与合金含量无关。第 13 页 共 18 页因而由所要求的强度可确定钢的含碳量。钢的均匀淬透能力取决于合金元素的种类与含量，这是个较复杂的问题，但这与零件尺寸的计算无关。随着硬度的增加其塑性降低。在上述原则的基础上，为某一零件以及考虑其他因素初步选择钢材时可以简单一些，但最终选择要使材料与为了得到零件的形状，表面和物理要求所采用的制造工艺相匹配。可以选取低碳钢，低合金钢，高碳钢及高合金钢。钢是少数几种具有疲劳极限的常用金属之一，人们所称的疲劳是由于重复加载而引起的材料失效。大多数金属材料由于反复承受应力而变得疲劳。材料在恒定载荷下所承受的应力比静载时的应力小得多。当对钢连续加载时，其强度极限变得较小，这个特性已在钢材加工中得到了完全的验证。普通的钢丝和钢线在某一局部反复弯曲则很容易折断。正常情况下，钢材经过数十次反复弯曲后就会折断。然而，钢丝的韧性很好，在简单的冷作条件下使钢丝弯曲并将其折断是不可能的。如果在某些点反复加载则钢会阻止弱化，这就是众所周知的“疲劳极限” 。钢的疲劳极限约为其强度的。钢具有疲劳疲劳极限的特性在结构应用中是非常重要的。当然，影响材料疲劳极限的因素很多，主要的因素是第 14 页 共 18 页材料的表面质量和或试件的制造工艺过程。疲劳可归于原始材料理想的均匀体，在每半个应力循环中不可避免地会产生微应变。疲劳断裂通常由下列原因所引起： 反复的交变应力会使材料的局部滑移增加或产生冷作现象。 应变硬化区的塑性将逐步下降而发展成裂纹。 由亚微裂 纹会形成缺口效应。钢的疲劳极限产生了某些非常有用的物理性能，这些性能对材料的可加工性是不利的。例如，在钢的冷轧过程中，其疲劳极限会限制对零件施加的冷作加工量，当达到疲劳极限后，材料就必须加热到临界温度以上，使之能进一步冷作加工。钢产品的大部分是普通碳素钢。碳素钢应用广泛，有铸件，锻件，管材，板材，薄钢板，带材，线材及钢丝制品，结构型材，棒料和工具。通常，碳素钢根据其制造方法可分成平炉钢，酸性平炉钢或酸性转炉钢，或者由含碳量来区分。影响碳素钢性能的主要因素是含碳量和显微组织。显微组织由钢的成分以及最终的轧制，锻造或热处理等方法所决定。但是大多数碳素钢不需最终的热处理，因此，轧制及锻造加工影响其显微组织。第 15 页 共 18 页在铸造，轧制或锻造条件下，碳素钢的主要组织是珠光体，亚共析钢是由铁素体和珠光体组成的，而过共析钢则是由渗碳体和珠光体组成的。合金钢是含有合金元素的铁碳合金，其中一种或多种元素超过下列含量：锰，.65%；硅，0.60%;铜，0.60%，和或一定量的其它合金元素，铝，硼和铬的含量达到3.99%；也可加入足够的钴、铌、钼、镍、钨、钒、锆、或其它元素以使钢得到所要求的性能。由于合金钢含有较多合金元素，与碳素钢相比，合金钢更需要把合金元素含量保持在规定范围内，且合金钢的质量控制技术更复杂，所以价格更高。与碳素钢相比，合金钢具有较好的强度、塑性和韧性。因此，工程师在设计承受大应力和/或冲击载荷的零件时应考虑选用合金钢。几乎所有合金钢的晶粒都较细，若其粒度为5、6、7、8 号则为细晶粒钢，在直径为 100 范围内检查，1 号粒度的晶粒大小为 1.5 粒/in 2。在热处理过程中细晶粒结构钢出现裂纹的趋势较小，且韧性和抗冲击性能都更好。粗晶粒钢具有较好机加工性，且其淬透性比细晶粒钢好。为了对某一设计选定最合适的合金钢，应考虑所含的主要合金元素的影响，它们有下列特性：1加入镍使钢的韧性好，耐腐蚀，淬硬性好。第 16 页 共 18 页2铬可改善钢的耐腐蚀性，韧性及淬硬性。3锰可以脱氧，提高钢的强度和硬度，降低其临界冷却速度。4硅可以脱氧，增强钢的抗高温氧化能力，提高热处理时的临界温度，增强钢对脱碳和石墨化的敏感性。5钼具有较好的淬透性，可提高高温时钢的抗拉强度和抗蠕变强度。6钒可以脱氧，细化钢的晶粒。7铜可改善钢的搞腐蚀性，起强化剂的作用。8铝可脱氧，细化钢的晶粒，有助于氮化处理。9硼可提高淬透性。术语“不锈钢”代表钢的一种在家族，其含铬量至少为11.5%但它并不是对所有的介质都有抗腐蚀的能力。从抗腐蚀性和价格两方面来衡量，不锈钢可与有色金属铜合金和镍相比；而从价格和强重比来衡量，不锈钢右与轻金属铝、镁相比。不锈钢含水量有许多合金元素，也有许多生产厂家，因此，很容易获得其性能与制造方面的资料。已经成功地开发了不锈钢的铸造、热处理、成形加工、机加工、焊接、装配及精加工等技术。现已发现这种材料通常会产生加工硬化，且这种材料只有在受控条件及惰性气体中才能焊接，它具有所希望的高强度、耐腐蚀性和装第 17 页 共 18 页饰。光亮清洁的表面是获得最好的抗腐蚀性的基本要求。划痕及杂质可通过机加工、酸洗或抛光等方法去除。在硝酸中浸渍可使新显露的表面形成好的氧化膜。不锈钢也可以进行电镀、电抛光、阳极侵蚀，用搪瓷涂覆或者去除表面氧化层后刷上彩色涂料。也可以从不锈钢厂家直接购买经过精细抛光的不锈钢薄板，在制造过程中还可使用塑料涂层来保护其表面。若将不锈钢冷却至-300，同时在高压下滚压，然后再加热到 750并保温柔 4 小时，则可获得很高的硬度，且其强度可增加一倍以上。耐腐蚀是不锈钢最重要的一个特性，这是由于在表面形成了一薄层透明的氧化铬膜所致。它也可经受如硝酸等氧化剂的侵蚀，但不能经受如盐酸或卤盐等还原剂的侵蚀。如果使用过程中氧化层不断地被去除的话，则会加速生成氧化皮用腐蚀。对其反复进行加热和冷却，则伴随着产生热膨胀和收缩，氧化层就会破裂脱落。由于纯铬不锈钢的热膨胀比镍铬不锈钢的小，因此，它最适合于不断加热和冷却的场合。大多数不锈钢在职 500温度下短时强度较高，而只有几种特殊不锈钢在 2000温度下短时强度好。而普通碳素钢在 900 到 950以上便不能使用。不锈钢的导热性差，所以为了导热常在其上包一层铜以制成炊具。第 18 页 共 18 页
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