Keluli

Keluli, juga dikenali sebagai besi baja atau besi waja (Bahasa Inggeris: Steel) adalah sejenis aloi yang bahan utamanya ialah besi, dengan sedikit kandungan karbon di antara 0.02% dan 1.7 atau 2.04% mengikut berat (C:1000–10,8.67Fe), bergantung kepada gred. Karbon adalah bahan sebatian paling murah dan berkesan bagi besi, tetapi pelbagai unsur sebatian lain yang turut digunakan seperti manganese dan tungsten. Karbon dan unsur lain bertindak sebagai agen pengeras, menghalang kerawang kristal (crystal lattice) dalam atom besi berpisah dengan tergelincir sesama sendiri. Jumlah unsur sebatian yang berbeza dan bentuk kehadirannya dalam keluli (unsur solute, fasa precipitated) mengawal kualiti seperti kekerasan, kelenturan, dan kekenyalan keluli yang terhasil. Besi dengan peningkatan kandungan karbon mampu menjadi lebih kukuh dan kuat berbanding besi , tetapi ia juga lebih rapuh. Maksima kelarutan karbon dalam besi (di kawasan austenite) adalah 2.14% menurut berat, berlaku pada 1149 °C; kandungan karbon yang lebih tinggi atau suhu yang lebih rendah akan menghasilkan cementite. Sebatian besi dengan kandungan karbon lebih tinggi dari ini dikenali sebagai besi tuang kerana kadar leburnya yang lebih rendah. Keluli juga dibezakan dari besi tempa (wrought iron) dari segi kandungan yang mengandungi hanya sejumlah kecil unsur lain, tetapi mengandungi 1–3% slag menurut berat dalam bentuk partikel memanjang pada satu arah, memberikan ciri-ciri urat besi. Ia lebih tahan karat berbanding keluli dan lebih mudah dipetri. Tetapi pada masa kini istilah ini jarang digunakan dalam industri keluli. Ia merupakan perkara biasa pada masa kini bagi merujuk 'industri besi keluli' seolah-olah ia satu entiti, tetapi dalam sejarah ia merupakan keluaran yang berbeza.
Sungguhpun besi telah dihasilkan melalui pelbagai kaedah tidak efisen lama sebelum Renaissance, kegunaannya lebih biasa selepas kaedah lebih efisen dicipta pada abad ke-17. Dengan ciptaan proses Bessemer pada pertengahan abad ke-19, besi menjadi barangan keluaran pukal yang murah dari segi perbandingan. Peningkatan lanjut dalam proses tersebut, seperti penghasilan besi asas oksijen, menurunkan lagi kos penghasilan sementara pada masa yang sama meningkatkan kualiti logam. Hari ini, keluli merupakan salah satu bahan yang biasa didapati di dunia dan merupakan komponen utama dalam pembinaan bangunan, perkakasan, kereta, dan peralatan utama. Keluli moden biasanya dikenali menurut gred keluli yang ditakrifkan oleh pelbagai organisasi piawaian.

Ciri-ciri bahan

Besi, sebagaimana kebanyakan logam, biasanya tidak dijumpai dalam kerak Bumi dalam bentuk unsur. Besi hanya boleh didapati dalam kerak Bumi dalam bentuk sebatian dengan oksigen dan belerang. Biasanya galian mengandungi besi termasuk Fe₂O₃—bentuk iron oxida yang terdapat dalam galian hematite, dan FeS₂—pyrite (emas dungu). Besi dikeluarkan dari bijih dengan menyingkir oksigen dengan mengabungkannya dengan pasangan kimia yang lebih digemari seperti karbon. Proses ini yang dikenali sebagai peleburan, pada awalnya digunakan dengan logam yang mempunyai tahap lebur rendah. Tembaga cair pada suhu lebih sedikit pada 1000 °C, sementara timah cair sekitar 250 °C. Besi tuang —besi sebatian dengan lebih dari 1.7% karbon—cair sekitar 1370 °C. Kesemua suhu ini mampu dicapai dengan kesemua kaedah kuno yang telah digunakan sekurang-kurangnya lebih 6,000 tahun (semenjak Zaman Gangsa). Disebabkan kadar pengoksidaan itu sendiri meningkat pada suhu melebihi 800 °C, ia penting bahawa peleburan dilakukan dikawasan rendah oksigen. Tidak seperti tembaga dan timah, besi cair menyerap karbon dengan mudah, oleh itu hasir peleburan menghasilkan sebatian yang mengandungi terlalu banyak karbon untuk dipanggi keluli.
Walaupun dalam julat kepekatan sempit yang menghasilkan keluli, campuran karbon dan besi boleh membentuk beberapa struktur berlainan, dengan ciri-ciri yang amat berbeza; memahami ini amat penting bagi menghasilkan keluli berkualiti. Pada suhu bilik, bentuk besi paling stabil adalah kubik pusat badan - (body-centered cubic - BCC) struktur besi ferrite atau besi-α, bahan logam yang agak lembut yang hanya mampu melarutkan sedikit kepekatan karbon (tidak melebihi 0.021 wt% pada 910 °C). Melebihi 910 °C ferrite melalui fasa perantaraan dari kubik pusat badan kepada struktur kubik pusat muka - (face-centered cubic - FCC), dikenali sebagai austenite atau besi-γ, yang sama logam dan lembut tetapi mampu melatutkan lebih banyak karbon (sehingga 2.03 wt% karbon pada 1154 °C). Ketika austenite yang kaya dengan karbon menyejuk, campuran itu cuba kembali kepada fasa ferrite, menyebabkan lebihan karbon. Satu cara bagi karbon meninggalkan austenite adalah bagi cementite untuk terpelowap (precipitate) keluar dari campuran, meninggalkan besi yang cukup tulin bagi membentuk ferrite, menghasilkan campuran cementite-ferrite. Cementite adalah fasa stoichiometri dengan formula kimia Fe₃C. Cementite terbentuk dalam kawasan kaya kandungan karbon sementara kawasan lain kembali kepada ferrite sekitarnya. Pola pengukuhan dir seringkali muncul dalam proses ini, mendorong kepada lapisan pola yang dikenali sebagai pearlite (Fe₃C:6.33Fe) disebabkan rupanya seperti mutiara, atau bainite yang serupa tetapi kurang cantik.
Kemungkinan allotrope yang paling penting adalah martensite, bahan yang metastabil secara kimia dengan empat hingga lima kali kekuatan ferrite. Kandungan minima Karbon 0.4 wt% (C:50Fe) diperlukan bagi membentuk martensite. Apabila austenite disejukkan bagi membentuk martensite, karbon di "kakukan" apabila struktur sel bertukar dari FCC kepada BCC. Atom karbon adalah terlalu besar untuk muat kedalam kekosongan interstitial dan dengan itu mengherotkan struktur sel menmbentuk struktur tetragonal pusat badan (BCT). Martensite dan austenite mempunyai komposisi kimia yang serupa. Dengan itu, ia memerlukan amat sedikit tenaga pengaktif haba bagi terbentuk.
Proses rawatan haba bagi kebanyakan keluli membabitkan memanaskan sebatian sehingga austenite terbentuk, kemudian merendam logam merah membara kedalam air atau minyak, menyejukkannya dengan pantas sehinggakan penukaran kepada ferrite atau pearlite tidak mempunyai masa yang mencukupi untuk berlaku. Penukaran kepada martensite, sebaliknya berlaku hampir serta merta, disebabkan tenaga pengaktif yang lebih rendah.
Martensite adalah kurang tumpat berbanding austenite, dengan itu penukaran antara mereka menyebabkan isipadu merosot. Dalam kes ini, pengembangan berlaku. Tekanan dalaman dari pengembangan ini mengambil bentuk pemampatan fizikal pada kristal martensite dan ketegangan pada baki ferrite, dengan sejumlah besar pengasingan (shear) pada kedua konstituent. Sekiranya rendaman tidak dilakukan dengan betul, ketegangan dalaman ini mampu menyebabkan ia berkecai ketika menyejuk; sekurang-kurangnya, ia menyebabkan pengerasan kerja (work hardening) dalaman dan kecacatan mikroskopik yang lain. Adalah perkara biasa bagi retakan rendaman berlaku apabila air digunakan, sungguhpun ia tidak selalunya kelihatan. 
Pada titik ini, sekiranya kandungan karbon cukup tinggi untuk menghasilkan ketumpatan martensite yang banyak, ia menghasilkan bahan yang amat keras tetapi rapuh. Seringkali keluli melalui rawatan haba berikut pada suhu lebih rendah untuk memusnahkan sebahagian dari martensite (dengan membenarkan cukup masa bagi pembentukan cementite.) dan membantu mengimbangi ketegangan dalaman dan menghapuskan kecacatan. Proses ini melembutkan keluli, menghasilkan logam yang lebih kenyal (ductile) dan tidak mudah patah. Disebabkan masa amat penting kepada hasil akhir, proses ini dikenali sebagai baja (tempering), yang membentuk keluli baja.
Bahan lain sering kali ditambah kepada campuran karbon-besi bagi mengawal ciri-ciri akhir. Nickel dan manganum dalam keluli menambah ketahanan kelenturan (tensile strength) dan menjadikan austenite lebih stabil dari segi kimia, chromium meningkatkan kekerasan dan tahap lebur, dan vanadium turut meningkatkan kekerasan disamping mengurangkan kesan kelesuan logam.

Steel was known in antiquity, and may have been produced by managing the bloomery so that the bloom contained carbon. Some of the first steel comes from East Africa, dating back to 1400 BCE. In the 4th century BCE steel weapons like the Falcata were produced in the Iberian peninsula. The Chinese of the Han Dynasty (202 BCE – 220 CE) created steel by melting together wrought iron with cast iron, gaining ultimate product of a carbon intermediate—steel—by the 1st century CE. Along with their original methods of forging steel, the Chinese had also adopted the production methods of creating Wootz steel, an idea imported from India to China by the 5th century CE. Wootz steel was produced in India and Sri Lanka from around 300 BCE. This early steel-making method employed the use of a wind furnace, blown by the monsoon winds. Also known as Damascus steel, wootz is famous for its durability and ability to hold an edge. It was originally created from a number of different materials including various trace elements. It was essentially a complicated alloy with iron as its main component. Recent studies have suggested that carbon nanotubes were included in its structure, which might explain some of its legendary qualities, though given the technology available at that time, they were probably produced more by chance than by design. Crucible steel was produced in Merv by 9th to 10th century CE.
In the 11th century, there is evidence of the production of steel in Song China using two techniques: a "berganesque" method that produced inferior, inhomogeneous steel and a precursor to the modern Bessemer process that utilized partial decarbonization via repeated forging under a cold blast.

Recycling

Steel is the most widely recycled material in North America. The steel industry has been actively recycling for more than 150 years, in large part because it is economically advantageous to do so. It is cheaper to recycle steel than to mine iron ore and manipulate it through the production process to form 'new' steel. Steel does not lose any of its inherent physical properties during the recycling process, and has drastically reduced energy and material requirements than refinement from iron ore. The energy saved by recycling reduces the annual energy consumption of the industry by about 75%, which is enough to power eighteen million homes for one year. Recycling one ton of steel saves 1,100 kilograms of iron ore, 630 kilograms of coal, and 55 kilograms of limestone. 76 million tons of steel were recycled in 2005.
In recent years, about three quarters of the steel produced annually has been recycled. However, the numbers are much higher for certain types of products. For example, in both 2004 and 2005, 97.5% of structural steel beams and plates were recycled. Other steel construction elements such as reinforcement bars are recycled at a rate of about 65%. Indeed, structural steel typically contains around 95% recycled steel content, whereas lighter gauge, flat rolled steel contains about 30% reused material.
Because steel beams are manufactured to standardized dimensions, there is often very little waste produced during construction, and any waste that is produced may be recycled. For a typical 2000-square-foot two-story house, a steel frame is equivalent to about six recycled cars, while a comparable wooden frame house may require as many as 40–50 trees.
Global demand for steel continues to grow, and though there are large amounts of steel existing, much of it is actively in use. As such, recycled steel must be augmented by some first-use metal, derived from raw materials. Commonly recycled steel products include cans, automobiles, appliances, and debris from demolished buildings. A typical appliance is about 65% steel by weight and automobiles are about 66% steel and iron.
While some recycling takes place through the integrated steel mills and the basic oxygen process, most of the recycled steel is melted electrically, either using an electric arc furnace (for production of low-carbon steel) or an induction furnace (for production of some highly-alloyed ferrous products).

Contemporary steel

Modern steels are made with varying combinations of alloy metals to fulfill many purposes. Carbon steel, composed simply of iron and carbon, accounts for 90% of steel production. High strength low alloy steel has small additions (usually < 2% by weight) of other elements, typically 1.5% manganese, to provide additional strength for a modest price increase. Low alloy steel is alloyed with other elements, usually molybdenum, manganese, chromium, or nickel, in amounts of up to 10% by weight to improve the hardenability of thick sections. Stainless steels and surgical stainless steels contain a minimum of 10% chromium, often combined with nickel, to resist corrosion (rust). Some stainless steels are magnetic, while others are nonmagnetic.
Some more modern steels include tool steels, which are alloyed with large amounts of tungsten and cobalt or other elements to maximize solution hardening. This also allows the use of precipitation hardening and improves the alloy's temperature resistance. Tool steel is generally used in axes, drills, and other devices that need a sharp, long-lasting cutting edge. Other special-purpose alloys include weathering steels such as Cor-ten, which weather by acquiring a stable, rusted surface, and so can be used un-painted.
Many other high-strength alloys exist, such as dual-phase steel, which is heat treated to contain both a ferrite and martensic microstructure for extra strength. Transformation Induced Plasticity (TRIP) steel involves special alloying and heat treatments to stabilize amounts of austentite at room temperature in normally austentite-free low-alloy ferritic steels. By applying strain to the metal, the austentite undergoes a phase transition to martensite without the addition of heat. Maraging steel is alloyed with nickel and other elements, but unlike most steel contains almost no carbon at all. This creates a very strong but still malleable metal. Twinning Induced Plasticity (TWIP) steel uses a specific type of strain to increase the effectiveness of work hardening on the alloy. Eglin Steel uses a combination of over a dozen different elements in varying amounts to create a relatively low-cost metal for use in bunker buster weapons. Hadfield steel (after Sir Robert Hadfield) or manganese steel contains 12–14% manganese which when abraded forms an incredibly hard skin which resists wearing. Examples include tank tracks, bulldozer blade edges and cutting blades on the jaws of life. A special class of high-strength alloy, the superalloys, retain their mechanical properties at extreme temperatures while minimizing creep. These are commonly used in applications such as jet engine blades where temperatures can reach levels at which most other alloys would become weak.
Most of the more commonly used steel alloys are categorized into various grades by standards organizations. For example, the American Iron and Steel Institute has a series of grades defining many types of steel ranging from standard carbon steel to HSLA and stainless steel. The American Society for Testing and Materials has a separate set of standards, which define alloys such as A36 steel, the most commonly used structural steel in the United States.
Though not an alloy, galvanized steel is a commonly used variety of steel which has been hot-dipped or electroplated in zinc for protection against rust.

Modern production methods

Blast furnaces have been used for two millennia to produce pig iron, a crucial step in the steel production process, from iron ore by combining fuel, charcoal, and air. Modern methods use coke instead of charcoal, which has proven to be a great deal more efficient and is credited with contributing to the British Industrial Revolution. Once the iron is refined, converters are used to create steel from the iron. During the late 19th and early 20th century there were many widely used methods such as the Bessemer process and the Siemens-Martin process. However, basic oxygen steelmaking, in which pure oxygen is fed to the furnace to limit impurities, has generally replaced these older systems. Electric arc furnaces are a common method of reprocessing scrap metal to create new steel. They can also be used for converting pig iron to steel, but they use a great deal of electricity (about 440 kWh per metric ton), and are thus generally only economical when there is a plentiful supply of cheap electricity.

Uses of steel

Iron and steel are used widely in the construction of roads, railways, infrastructure and buildings. Most large modern structures, such as stadiums and skyscrapers, are supported by a steel skeleton. Even those with a concrete structure will employ steel for reinforcing. In addition to widespread use, in electrical appliances and motor vehicles (despite growth in usage of aluminium, it is still the main material for car bodies), steel is used in a variety of other construction-related applications, such as bolts, nails, and screws. Other common applications include shipbuilding, oil and gas pipelines, mining, aerospace, office furniture, steel wool, tools, and armour in the form of personal vests or vehicle armour (better known as rolled homogeneous armour in this role).

Historically

Before the introduction of the Bessemer process and other modern production techniques, steel was expensive and was only used where no cheaper alternative existed, particularly for the cutting edge of knives, razors, swords, and other items where a hard, sharp edge was needed. It was also used for springs, including those used in clocks and watches.

Since 1850

With the advent of faster and more efficient steel production methods, steel has been easier to obtain and much cheaper. It has replaced wrought iron for a multitude of purposes. However, the availability of plastics during the later 20th century allowed these materials to replace steel in many products due to their lower cost and weight.