steels i. identification of terms 1. metal, alloy, iron, steel, cast iron, wrought iron, reinforcing...

SteelsI. Identification of terms1. Metal, alloy, Iron, steel, cast iron, wrought iron, reinforc

ing steel, reinforcement ， structural steel, tool steel, etc.2. Pure nonferrous metals are suitable for engineering appl

ications but not unalloyed iron3. Cheap iron alloys are used in larger quantities than the a

lloys of any other metal because steels and cast irons can be produced with a variety of useful properties, by variation of composition, heat treatment and working (hot and cold-rolling/drawing).

4. Heat treatment is the controlled heating and cooling of metals to alter their physical and mechanical properties without changing the product shape.

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SteelsII. Wrought steels Engineering components are produced in steels by ho

t- or cold-working processes, although some machining may be involved.

The component may be put directly into service or may be heat treated or surface treated depending on the grade of steel or the application.

The largest usage of wrought steels consists of plain carbon steels ( 普通碳钢 , 碳素钢 )or slightly modified plain carbon steels.

The strength and hardness of normalized plain carbon steels tend to increase with increasing carbon content whereas ductility decreases.


SteelsIII. Structural steels (plain carbon steels, notch ductile ste

els, grade 50, 55 steels) The design of steel structures is based primarily on th

e yield stress or proof stress( 弹限强度 , 保证强度 )of the steel.

Ductility, toughness (notch ductility) at normal and subzero temperatures, and weldability are often important properties.

Weldability, which for steels deteriorates with increasing carbon content is important because welded structures give a weight saving and ease of fabrication compared with bolted or riveted structures.


Steels4. Plain carbon steels

1) Plain carbon steel has good ductility and weldability if its carbon content is low and a high yield strength if its carbon content is high.

2) Consequently the carbon content is limited to 0.25 percent maximum in the basic structural steels to give a compromise between the opposing requirements.

3) These steels also contain such elements as manganese, silicon, sulphur, and phosphorus which arise and are controlled in the steelmaking process.

4) Manganese improves strength and notch ductility and 0.5 and 1.0 percent is normally present.


Steels5) Silicon improves the strength but if present in excessi

ve amounts may cause the carbon to occur as graphite flakes which reduce the strength and so the silicon content rarely exceeds 0.6 percent.

6) Sulphur and phosphorus embrittle the steel and are controlled to 0.06 percent maximum.

7) Deficiencies

(1) poor notch ductility since they become brittle at temperatures only slightly below room temperature;

(2) low yield strength;

(3) poor atmospheric corrosion resistance.


Steels5. Notch ductile steels( 耐冲击钢 , 韧性钢 )1) Improvements in notch ductility are achieved through

lowering the ductile-to-brittle impact transition temperature by a reduction in the carbon content or an increase in the manganese content of low carbon steels.

2) The result is the “notch ductile” mild steels with the manganese content raised from 0.5/1.0 percent to 1.5 percent.

6. Grade 50, 55 steels1) The strongest and toughest structural steels are devel

oped by controlling the factors that significantly influence the strength and notch ductility of low carbon steels.


Steels2) Within this very limited carbon range(1) an increase in the pearlite percentage gives an increas

e in tensile strength, a rise in the impact transition temperature and no significant effect on the yield strength;

(2) a decrease in the ferrite grain size 铁素体晶粒度 gives an increase in both the tensile strength and the yield strength and the impact transition temperature is lowered.

3) The desired combination of high yield strength and low impact transition temperature is best achieved by a reduction in both the pearlite percentage and the ferrite grain size.


SteelsIV. *Corrosion protection1. The atmospheric corrosion resistance of steel structur

es may be enhanced in several ways1) Protective coatings of paint, aluminum and zinc: the

metal coatings are more resistant to corrosion than steel in all but the most severe environments and are more abrasion-resistant than painted coatings.

2) Cathodic protection: for parts of structures continuously immersed in water but this method is not effective when only part of the structure is immersed, e.g., in a tidal situation.

3) Some steels can be supplied with copper content of 0.2/0.5 per cent.


Steels4) A number of weathering steels contain an increase am

ount of phosphorus and some chromium and copper, compared with normal steels.

V. Selection of structural steel

It is suggested that selection for a specific application is determined by the following factors

1. strength level required;

2. other mechanical properties required together with the strength;

3. steelmaking, heat treatment and other plant available;

4. arbitrary local conditions such as imposed by specifications and codes of practice.


SteelsVI. *Concrete reinforcement1. Concrete has low tensile and bending strengths and a

high compressive strength. 2. Steel reinforcement overcomes the deficiencies in the t

ensile and bending strengths.3. The reinforcing steel must have adequate tensile prop

erties and form a strong bond with the concrete. 4. The bond is purely mechanical and arises from surfac

e roughness and friction. 5. Mild steel with a maximum carbon content of .0.25 / 0.

40 percent is suitable and is supplied in three conditions: hot rolled, cold rolled and hard drawn.


Steels6. Reinforcing steels are supplied as plain round or squa

re rods, round ribbed bars 棱形（带筋）钢条 , square or round ribbed twisted bars 螺旋钢筋 and also woven or electrically welded mesh.

VII. *Prestressing steels1. Prestressing steels must have a high yield stress in ten

sion so that a high elastic strain can be induced in them.

2. A stress approaching the yield strength must be supported without the steel suffering excessive creep relaxation.

3. In addition pretensioning steels must form a good bond with the concrete.

4. For prestressing 0.6/0.9 percent carbon, 0.5/0.9 percent manganese steel is suitable.


Steels

5. It has better tensile properties than mild steel owing to the higher carbon content and these can be further enhanced by cold working.

6. The steel is supplied in various conditions and forms: cold drawn wire, strengthened and stress relieved wire with two relaxation classes, seven-wire strand in three strength ranges.

7. When exceptional corrosion resistance is essential stainless steel is specified.

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Steels

VIII. *Bridge wire

1. The suspension-bridge cables are of high strength, toughness and fatigue resistance to withstand the fluctuations of stress caused by loading of the bridge and climatic effects.

2. Corrosion resistance is also important. Suitable cables are produced from heavily galvanized, cold-drawn 0.75/0.85 percent carbon, 0.5/0.7 percent manganese steel with a minimum tensile strength of 1600 MN m-2.

Tuesday, April 18, 2023

steels i. identification of terms 1. metal, alloy, iron, steel, cast iron, wrought iron, reinforcing...

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