土木工程材料 課件 Chapter 2 Building Steel

Materials in Civil

andConstructionEngineering土木工程材料Chapter

2Building SteelIntroductionProperties

of

Building

Steel2.3

Standards

andSelection2.4

Corrosion

and

ProtectionContent2.1Introduction2.1 IntroductionBuildingsteels

arefor

engineeringbuildings,

including

profiledbars,armor

plates,steels

and

steelwires.Buildingsteel

isthematerial

produced

under

strict

technical

conditions,and

ithasthe

following

advantages:high

strength-weightratio,betterplasticity

and

toughness,

flexibleprocessingandthe

propertiesto

bearimpacts

andvibration

loads;the

disadvantages

are:

easy

tobe

corrodedandhighcostofrepairs.Thesecharacteristics

determine

that

steel

isoneof

the

important

materials

needed

by

economicconstruction

departments.

In

construction,

the

steelstructuresconsistedby

steel

invarious

shapeshavehighsecurity

and

light

deadweight,

usedfor

large-spanandhigh-risestructures.

But

though

concretestructureshave

heavydeadweight,theusageof

steel

isdecreased

greatly,

anditcanovercome

the

corrosion

andhighcost

of

repairs

of

steel.

Thus,

steel

iswidely

used

inconcrete

structures.Thischapterfocuses

on

theproperties

of

building

steel

andit

introduces

thestandards

andselection

ofbuilding

steel.

It

simply

introduces

corrosionof

building

steel

and

themeasures

topreventcorrosion.2.1 IntroductionSteel

is

an

alloy

of

iron

with

typically

a

few

percent

of

carbon

to

improve

its

strength

and

fracture

resistancecompared

to

iron.

Many

other

additional

elements

may

be

present

or

added.

Stainless

steels

that

are

corrosion

andoxidation

resistant

need

typically

an

additional

11%

chromium.

Because

of

its

high

tensile

strength

and

low

cost,steel

isused

in

buildings,

infrastructure,

tools,

ships,

trains,cars,

machines,electrical.The

carbon

in

typical

steel

alloys

may

contribute

upto2.14%

ofitsweight.

Varying

the

amount

ofcarbon

andmany

other

alloying

elements,

aswell

as

controlling

their

chemical

andphysical

makeup

inthe

final

steel

(eitherassolute

elements,

oras

precipitated

phases),

slows

the

movement

ofthose

dislocations

that

make

pure

ironductile,

and

thus

controls

andenhances

itsqualities.

These

qualities

include

the

hardness,

quenching

behavior,need

forannealing,

tempering

behavior,

yield

strength,

andtensile

strength

ofthe

resulting

steel.The

increase

insteel's

strength

compared

topure

iron

is

possible

onlyby

reducing

iron'sductility.2.1 IntroductionSteel

wasproducedinblooming

furnaces

forthousands

ofyears,

butits

large-scale,

industrial

use

began

only

after

moreefficient

production

methodsweredevised

inthe

17thcentury,with

the

introduction

of

the

blast

furnaceandproduction

ofcrucible

steel.

This

wasfollowedby

theopen-hearth

furnaceandthen

theBessemer

process

inEnglandinthemid-19thcentury.

With

theinvention

of

the

Bessemer

process,aneweraof

mass-produced

steel

began.Mildsteel

replaced

wrought

iron.HistoryFurther

refinementsinthe

process,such

as

basicoxygen

steelmaking(BOS),

largely

replaced

earliermethodsby

furtherlowering

thecost

ofproduction

andincreasingthequality

ofthefinalproduct.Today,

steelis

oneofthemostcommonmanmade

materials

inthe

world,

with

more

than

1.6billion

tonsproducedannually.Modernsteelis

generallyidentified

byvarious

gradesdefined

byassortedstandardsorganizations.2.1 Introduction2.1.2 Classification1.By

Smelting

ProcessesDuring

smelting,theremovaldegrees

ofimpurities

bydifferent

smelting

methods

arenotthesame,so

thesteelqualities

aredifferent.

Recently,

therearethree

kindsof

steel,

including

Bessemer

steel(converter

steel),Siemens-Martinsteel,

andelectric

steel.BessemerSteelThesmelting

process

of

thissteel

isto

use

themoltenpigiron

as

the

raw

materialwithoutany

fuelandtomake

steel

withair

or

pure

oxygen

being

blownthroughthemolteniron(therawmaterial)from

thebottomorthesidesoftheconverter.Siemens-Martin

SteelUse

solidor

fluid

pig

iron,

ore

orwaste

steelas

theraw

materials

andcoal

gasorheavy

oilasthe

fuelandtoremove

the

impurities

from

theironbyoxidation

withthe

oxygeninoreorwastesteelor

theoxygenbeing

blown

through

the

iron.Electric

SteelTheprocess

of

electric

steelistomake

steel

by

electricheating.

Theheat

source

ishigh-tension

arc,

andthesmelting

temperature

ishighandcan

beadjustedfreely.2.1 IntroductionByPress-workingModesIn

the

process

of

smelting

andingot-casting,

there

will

beuneven

structures,

foams

or

other

defectshappening

to

the

steel,so

the

steel

used

in

industry

should

beprocessed

bypress

to

eliminate

thedefects.

Meanwhile,

there

isrequirement

for

shapes.

Thepress-working

modes

include

hot

workingand

cold

working.Hot-working

steel:

Hot

working

is

to

heat

the

steel

ingot

to

a

certain

temperature

and

toconduct

press-working

to

the

steel

ingot

in

the

plastic

state,

such

as

hot

rolling

and

hot

forging.Cold-working

steel:

Thesteel

is

processed

under

thenormal

temperature2.1 Introduction3.ByChemical

ElementsSteel

Classifications

(GB/T

13304-1991),

the

Chinese

standard,

recommends

two

classificationmethods:

one

is

to

classify

by

chemical

elements,

and

the

other

is

to

classify

by

quality

degrees.

Bychemicalelements,

there

is

non-alloy

steel,

lean-alloy

steel

and

alloy

steel.Non-alloysteel:

thatis

carbonsteel

with

fewalloyelements.Lean-alloy

steel:

that

isthe

steel

with

low

alloyelements.Alloy

steel:

thatisthesteeladded

with

more

alloyelements

to

improve

some

propertiesofthesteel.2.1 IntroductionByQuality

DegreesAccording

to

quality

degrees,

the

steel

can

be

classified

into:

common

steel,

quality

steel

andadvanced

quality

steel.BypurposesThe

steel

can

be

classified

by

purposes,

such

as

construction

steel,

railway

steel,

and

pressure

vesselsteel.

The

construction

steel

can

be

classified

by

purposes

into

the

steel

for

steel

structures

and

that

forconcrete

structures.

At

present,

the

steel

commonly

used

in

constructions

includes

carbon

structuralsteel

and

lean-alloy

andhigh-strength

structural

steel.2.2Properties

ofBuilding

Steel2.2 PropertiesofBuildingSteelThe

essential

properties

of

steels

in

steel

structure

and

reinforced

concrete

in

civilengineering

are:Mechanical

property:

tensile

strength,

impacttoughness,fatigue

strength.Processing

property:

cold

bending

and

welding

property.2.2.1 Mechanical

Properties1.

Tensile

StrengthIt

is

the

most

important

property

of

the

building

steels.

Thetensile

strength

of

construction

steel

includes:

yield

strength,ultimate,

tensile

strength,

and

fatiguestrength.(1)

Low

Carbon

Steel

Stress-strain

Curves

(Figure2.1)Ⅰ-The

elastic

stage,

expressed

by

p

;Ⅱ-Theyield

stage,

expressed

by

s;Ⅲ-Thereinforcement

stage,

expressed

by

b;Ⅳ-The

necking

stage.Figure

2.1Low

carbon

steel

stress-strain

curves2.2 PropertiesofBuildingSteel2)

Strength①

Yield

StrengthAs

shown

in

Figure

2.1,

at

the

yield

stage,

the

corresponding

stress

of

the

highest

point

on

the

hackle

is

calledthe

upper

yield

point

(B);

get

rid

of

theinitial

transient

effect,

thecorresponding

stress

of

the

lowest

point

iscalledthe

lower

yield

point

(B

).

The

Chinese

standard

regulates

that

the

stress

of

the

lower

yield

point

is

the

yieldstrength

of

thesteel,

expressed

by

s.attentionWhen

theactual

stressof

astructurereaches

theyield

point,

there

willbeirretrievable

deformationwhich

isnotallowed

in

constructions.

Thus,

yield

strength

isthemain

base

todeterminetheallowable

stress

of

thesteel.2.2 PropertiesofBuildingSteel②

Ultimate

Tensile

Strength

(Simply

Called

Tensile

Strength)It

is

the

ultimate

tensile

stress

that

the

steel

can

bear

under

the

role

of

tension,

shown

in

Figure

2.1,

the

highestpoint

of

stage

Ⅲ.

s/

b

can

reflect

the

availability

and

safety

of

steel.

The

smaller

theyield

ratio,

the

more

reliablethe

structure

is.

However,

if

the

ratio

istoo

small,the

availableutilization

ratio

ofthe

steel

will

be

toolow,

and

thereasonable

yield

ratio

should

lie

between

0.6-0.75.

Therefore,

the

yield

strength

and

the

tensile

strength

are

themajor

test

indexes

of

themechanical

propertiesof

steel.③

Fatigue

StrengthUnder

the

role

of

alternating

loads,

steel

will

be

damaged

suddenly

when

the

stress

is

far

below

the

yieldstrength,

and

this

damage

is

called

fatigue

failure.

The

value

of

stress

at

which

failure

occurs

is

called

fatiguestrength,

or

fatigue

limit.

The

fatigue

strength

is

the

highest

value

of

the

stress

at

which

the

failure

never

occurs.Generally,

the

biggest

stress

that

the

steel

bears

alternating

loads

for

106-107

times

and

no

failure

occurs

is

calledthefatigue

strength.Thesmaller

theyield

ratioThemorereliable

thestructure

is2.2 PropertiesofBuildingSteel2.PlasticityThe

construction

steel

should

have

good

plasticity.

In

projects,

the

plasticity

of

the

steel

is

usually

expressed

bythe

elongation.

Elongation

refers

to

the

ratio

of

the

increment

of

the

gauge

length

to

the

original

gauge

lengthwhen

thespecimenis

stretched

off,expressed

by

(%),

shown

in

Figure

2.2.

L1

L0L0

100%Plasticity

isan

importanttechnical

property

forsteel.

Thoughthestructures

areused

duringtheelastic

stage,

thepartwherethestressconverges

could

bebeyond

theyield

strength.

Andcertain

plasticity

canguarantee

theredistribution

ofthestress

toavoid

failure

ofstructures.Figure

2.2

Tensile

specimens

of

steel2.2 PropertiesofBuildingSteel3.Impact

DurabilityImpact

durability

refers

to

the

property

that

the

steel

resists

loads

without

being

damaged.

It

is

regulated

thatthe

impact

durability

is

expressed

by

the

work

spent

on

the

unit

area

of

the

damaged

notch

when

the

standardnotched

specimen

is

stricken

by

the

pendulum

of

the

impact

test,

with

the

sign

αK,

and

the

unit

J,

as

shown

inFigure

2.3.The

bigger

αK

is,

the

more

work

will

be

spent

in

damaging

the

specimen,

or

the

more

energy

the

steel

willabsorb

beforegetting

cracked,

and

the

better

the

durability

of

the

steelis.The

impact

durability

of

the

steel

is

related

to

its

chemical

elements,

smelting,

and

processing.

Generally,

Pand

S

contents

in

steel

are

high,

and

impurities

and

the

tiny

cracks

forming

in

smelting

will

lower

the

impactdurability.2.2 PropertiesofBuildingSteelIn

addition,

the

impactdurability

of

the

steelcan

be

influenced

by

temperature

and

time.

At

the

roomtemperature,

the

impactdurability

willdecline

little

with

the

temperature

falling,

andthedamaged

steelstructurereveals

theductile

fracture;

ifthetemperature

falls

into

arange,αk

declines

suddenly,

thesteel

revealsthebrittlefracture,

andthetemperature

is

verylow

when

cold

brittle

fracture

occurs.

In

north,

especially

thecold

places,

thebrittle

fracture

of

the

steelshouldbe

tested

when

the

steelis

used.

The

critical

temperature

ofits

brittle

fractureshould

be

lower

than

thelowesttemperature

oftheplace.

Because

themeasurement

ofthecritical

temperature

iscomplicated,

what

is

regulated

instandards

istheimpact

values

atthenegative

temperature

-20

C

or

-40

C.(a)

specimen

size(b)

test

device(c)

working

principle

of

pendulum

tester1-Pendulum;

2-Specimen;

3-Test-bed;

4-Dial;

5-Needle.Figure

2.3

Thetest

principle

of

impact

durability2.2 PropertiesofBuildingSteel4.RigidityRigidity

is

the

property

to

resist

the

plastic

deformation

when

there

is

a

hard

object

press

into

the

steel

withinthe

partial

volume

of

the

surface,

often

related

to

the

tensile

strength.

Recently,

there

are

various

methods

tomeasure

the

rigidity

of

thesteel,

and

themost

common

one

is

Brinell

hardness,

expressed

by

HB.2.2.2 Process

Properties1.ColdBendingPropertyCold

bending

is

thepropertythat

thesteel

bears

thebending

deformationunder

thenormalconditions.The

cold

bending

is

tested

by

checking

whether

there

are

cracks,

layers,

squamous

drops

and

ruptures

on

thebending

partafter

thespecimen

goes

throughtheregulated

bending.2.2 PropertiesofBuildingSteelGenerally,

it

is

expressed

by

the

ratio

of

the

bending

angle

α

and

the

diameter

of

the

bending

heart

d

to

thethickness

of

thesteel

or

thediameterof

thesteel

a.Figure

2.4

showsthatthebigger

the

bending

angleis,

thesmaller

theratio

of

d

to

a

is,

and

thebetter

thecold

bending

property

is.Coldbending

test

ishelpful

toexpose

some

defects

ofsteel,

such

aspores,impurities

andcracks.

Inwelding,

thebrittleness

of

parts

andjoints

can

befound

bycold

bendingtest,

sothecoldbendingtestis

notonly

theindex

tocheckplasticity

andmachinability,

butalsoan

importantindex

toevaluate

the

welding

quality.Thecold

bending

test

for

thesteelusedinimportant

structures

orthebended

steel

shouldbequalified.Figure

2.4

Cold

bending

test

of

steel2.2 PropertiesofBuildingSteel2.

Weld

AbilityWelding

is

the

major

mode

for

the

combination

of

steel.

The

quality

of

welding

depends

on

the

weldingtechniques,

welding

materials

and

theweldability

of

steel.abilityWeld

ability

refersto

theproperty

that

under

acertain

welding

condition,

there

isnocrackor

hardruptureinor

around

welding

seamsand

themechanical

property

after

welding,

especially

thestrength,

shouldbenotlowerthantheoriginal

one.conditionTheweld

ability

will

decrease,

ifthecarbon

content

ismorethan

0.3%,

or

there

ismore

sulfur,or

theimpurity

content

ishigh,

andthealloy

elements

content

ishigh.materialsThesteel

used

forweldingis

theoxygen

converter

ortheSiemens-Martin

fully-killed

steelwithlower

carbon

content.

Forthehighcarbon

steel

andalloy

steel,

preheating

andheat

treatment

shouldbeadopted

respectively

before

andafterwelding

inorderto

improvethehardbrittleness

ofthesteelafter

welding.2.2 PropertiesofBuildingSteel2.2.3 AffectingFactors

ofProperties1.Influences

of

Chemical

Elements(1)

CarbonCarbon

is

the

major

element

that

determines

the

properties

of

steel.

With

the

increasing

of

carbon

content,

therigidity

and

thestrength

of

steel

willincrease,

and

its

plasticity

and

toughness

willdecrease.If

the

carbon

content

ismore

than

1

%,

the

ultimate

strength

of

the

steel

begins

to

fall.

In

addition,

if

the

carboncontent

is

too

high,

the

brittleness

and

aging

sensitivity

of

the

steel

will

rise,

which

reduce

its

ability

to

resist

thecorrosion

of

the

atmosphere

and

weld

ability.the

rigidity

and

thestrength

of

steel

willincreaseWith

the

increasing

ofcarbon

content2.2 PropertiesofBuildingSteel(2)

Phosphor

and

SulfurPhosphor

is

similar

with

carbon

that

can

improve

the

yield

point

and

bending

strength

of

steel,

lower

itsplasticity

and

toughness,

and

greatly

increase

its

cold

brittleness.

But

the

segregation

of

phosphor

is

serious

andthere

are

cracks

in

welding,

so

phosphor

is

one

of

the

elements

that

can

lower

the

weld

ability

of

steel.

Thus,

incarbon

steel,

the

phosphor

content

should

be

controlled

strictly;

but

in

alloy

steel,

it

can

improve

the

resistance

toatmospheric

corrosion

of

steel,

and

can

also

be

thealloy

element.In

steel,

sulfur

exists

in

the

mode

of

FeS.

FeS

is

a

kind

of

low

melting

compound

that

has

been

melted

whenthe

steel

is

processed

or

welded

in

the

state

of

glowing

red

and

will

lead

to

cracks

inside

the

steel,

called

hotbrittleness.The

hot

brittleness

greatly

reduces

the

process

ability

and

weld

ability

of

steel.

In

addition,

the

segregation

ofsulfur

is

serious

that

can

reduce

the

impact-resistance,

fatigue

strength

and

anti-corrosion

of

steel.

Thus,

the

sulfurcontent

should

also

be

controlled

strictly.2.2 PropertiesofBuildingSteelOxygen

andNitrogenOxygen

and

nitrogen

can

partly

dissolve

in

ferrite

and

most

of

them

exist

in

the

mode

of

compounds.

Thesenon-metals

contain

impurities

that

reduce

the

mechanical

properties

of

steel,

especially

the

toughness

of

steel,

andcan

accelerate

aging

and

lower

weld

ability.

Thus,

the

oxygen

and

nitrogen

should

be

controlled

strictly

in

steel.Silicon

and

ManganeseSilicon

and

manganese

are

the

elements

added

purposely

during

steelmaking

for

deoxygenation

anddesulphurization.

Because

silicon

can

combine

with

oxygen

greatly,

it

can

capture

the

oxygen

in

ferric

oxide

togenerate

silicon

dioxide

and

stay

in

the

steel

slag.

Most

of

the

remaining

silicon

will

dissolve

in

ferrite.

Andwhen

the

content

is

low

(less

than

1%),

it

can

improve

the

strength

of

steel

and

has

little

influence

on

plasticityand

toughness.Combining

force

of

manganese

with

oxygen

and

sulfur

is

higher

than

that

of

iron,

so

manganese

can

changeFeO

and

FeS

into

MnO

and

MnS

respective

19

and

stay

in

the

steel

slag.

And

the

remaining

manganese

dissolvesin

ferrite

and

twiststhecrystal

lattice

to

prevent

slippageand

deformation,

greatly

improving

thestrength

of

steel.concept2.2 PropertiesofBuildingSteel2.Influences

of

Cold

Working

andHeat

Treatment(1)

Cold

WorkingCold

working

is

the

process

that

steel

is

processed

at

the

room

temperature.

The

common

cold

working

modesfor

constructionsteel

include:

cold

stretching,

cold

drawing,cold

rolling,

cold

twisting,

notching.At

the

room

temperature,

beyond

the

elastic

range

of

the

steel,

the

plastic

deformation

strength

and

rigidity

ofthe

steel

have

increased

and

its

plasticity

and

toughness

have

decreased,

which

is

called

cold-workingstrengthening.

It

improvesthestrength

and

the

yield

strength

can

raise

20%-30%

after

cold

working.Within

a

certain

range,

the

bigger

the

cold-working

deformation

is,

the

greater

the

yield

strength

increases,

andthe

more

the

plasticity

and

the

toughness

decrease.2.2 PropertiesofBuildingSteel(2)

Heat

TreatmentSteel

can

be

hardened

or

softened

by

using

heat

treatment;

the

response

ofsteel

to

heat

treatment

depends

on

its

alloy

composition.

The

basic

process

is

toheat

the

steel

to

a

specific

temperature,

hold

the

temperature

for

a

specifiedperiod

of

time,

then

cool

the

material

at

a

specified