keluli
TRANSCRIPT
Keluli
Kabel keluli yang digunakan di menara lombong batu arang
Keluli atau Besi waja 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.[1] 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 tuangkerana kadar leburnya yang lebih rendah.[1] 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
Templat:Steels Besi, sebagaimana kebanyakan logam, biasanya tidak dijumpai dalam
kerak Bumi dalam bentuk unsur.[2] 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).[3]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.[4]
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).
[5] 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.
Fail:Phase diag iron carbon-color temp.png
Diagram fasa besi-karbon, menunjukkan keadaan yang diperlukan bagi membentuk fasa berlainan.
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. [6]
Pelet bijih besi bagi penghasilan keluli.
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.[7]
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.[8] Some of the first steel comes from East Africa, dating back to 1400 BCE.[9] 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.[10]
[11] 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.[12] 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.[13] 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.[14] 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.[15]
[sunting]Early modern steel
A Bessemer converter in Sheffield, England.
[sunting]Blister steel
Rencana utama: Cementation process
Blister steel, produced by the cementation process was first made in Italy in the early 17th
century CE and soon after introduced to England. It was probably produced by Sir Basil
Brooke at Coalbrookdale during the 1610s. The raw material for this was bars of wrought iron.
During the 17th century it was realised that the best steel came from oregrounds iron from a
region of Sweden, north of Stockholm. This was still the usual raw material in the 19th century,
almost as long as the process was used.[16][17]
[sunting]Crucible steel
Rencana utama: Crucible steel
Crucible steel is steel that has been melted in a crucible rather than being forged, with the
result that it is more homogeneous. Most previous furnaces could not reach high enough
temperatures to melt the steel. The early modern crucible steel industry resulted from the
invention ofBenjamin Huntsman in the 1740s. Blister steel (made as above) was melted in
a crucible in a furnace, and cast (usually) into ingots.[17] Rencana utama: Penyimenan
berjalan Keluli lecur, dikeluarkan oleh penyimenan berjalan adalah pertama dibuat di Itali
dalam abad ke-17 dan awal CE dan tidak lama lagi sehabis diperkenalkan ke England. Ia
adalah mungkin dihasilkan Sir Basil Brooke yang dekat di Coalbrookdale sepanjang 1610s.
Bahan mentah untuk ini adalah batang-batang besi tempaan. Sepanjang ia abad ke-17
adalah sedar yang waja yang terbaik datang daripada oregrounds kuat daripada sebuah
rantau Sweden, utara Stockholm. Ini adalah masih bahan mentah biasa dalam abad ke-19,
hampir sebagai proses yang lama seperti telah digunakan
[sunting]Modern steelmaking
Oven keluli Siemens-Martin di Muzium Industri Brandenburg.
See also History of the modern steel industry.
Era moden dalam penghasilan besi bermula dengan pengenalan proses Bessemer
oleh Henry Bessemer pada akhir 1850-an. Ini membolehkan keluli dihasilkan dalam
jumlah yang besar dengan murah, dengan itu besi serdahana kini digunakan bagi
kebanyakaan tujuan yang sebelum ini besi tempa digunakan.[18] Ini hanyalah yang
pertama dalam kaedah penghasilan besi. Proses Gilchrist-Thomas (atau asas proses
Bessemer) merupakan peningkatan kepada proses Bessemer, melapik penukar
dengan bahan asas bagi menyingkir phosphorus. Satu lagi adalah proses Siemens-
Martin kaedah penghasilan besi relau terbuka, di mana proses Gilchrist-Thomas
seiring dan bukan menggantikannya, proses asal Bessemer.[17]
Ini dijadikan lapuk oleh proses Linz-Donawitz penghasilan besi oksijen asas,
dibangunkan pada tahun 1950-an, dan proses penghasilan besi oksijen yang lain.[19]
[sunting]Steel industry
Fail:Port talbot large.jpg
Tata Steel plant in the United Kingdom.
Fail:Steel (crude)1.PNG
Steel output in 2005
Because of the critical role played by steel in infrastructural and overall economic
development, the steel industry is often considered to be an indicative for economic
prowess.
The economic boom in China and India has caused a massive increase in the
demand for steel in recent years. Between 2000 and 2005, world steel demand
increased by 6%.[20] Since 2000, several Indian[21] and Chinese steel firms have rose
to prominence like Tata Steel (which bought Corus Group in 2007), Shanghai
Baosteel Group Corporation and Shagang Group. Arcelor-Mittal is however the
world's largest steel producer.[20]
The British Geological Survey reports that in 2005, China was the top producer of
steel with about one-third world share followed by Japan, Russia and the USA.
Lihat juga: List of steel producers dan Global steel industry trends
[sunting]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.[22] Recycling one ton of steel saves
1,100 kilograms of iron ore, 630 kilograms of coal, and 55 kilograms
of limestone.[23] 76 million tons of steel were recycled in 2005.[22]
Fail:Steel scrap.jpg
A pile of steel scrap in Brussels, waiting to be recycled.
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.[24] 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.[22]
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).
[sunting]Contemporary steel
Modern steels are made with varying combinations of alloy metals to fulfill many
purposes.[25] Carbon steel, composed simply of iron and carbon, accounts for
90% of steel production.[1] 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.[26] 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.[1] 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.[27]
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 ofprecipitation hardening and improves the
alloy's temperature resistance.[1] 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.[28]
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.
[29] 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.[30] 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.[31] Twinning Induced Plasticity (TWIP) steel uses a specific type
of strain to increase the effectiveness of work hardening on the alloy.[32] Eglin
Steel uses a combination of over a dozen different elements in varying amounts
to create a relatively low-cost metal for use inbunker 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.[33] 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.[34]
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 gradesdefining many types of steel ranging from
standard carbon steel to HSLA and stainless steel.[35] 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.[36]
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.[37]
[sunting]Modern production methods
White-hot steel pouring out of an electric arc furnace.
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.[38] 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.[39]
[sunting]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,
andscrews.[40] 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).
A piece of steel wool
[sunting]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
forsprings, including those used in clocks and watches.[17]
A carbon steel knife
[sunting]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.[41]