Steels for Structural ApplicationsB.C. De Cooman, J.G. Speer, Fundamentals of Steel Product Physical...

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Lecture 5

Steels for Structural

Applications

Dr. Javad Mola

Institute of Iron and Steel Technology (IEST)

Tel: 03731 39 2407

E-mail: [email protected]

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Applications of Structural Steels

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Hot Finished Products

Example of H-beam (I-beam or double T) processing

C

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Hot Finished Products

Example of Q&T heavy plate processing

Slab reheating Descaling Hot rolling

Cutting Quenching and

Tempering

Anti-corrosion

paint

The Steel Book: SSAB Communications, Lena Westerlund, Printing: Henningsons Tryckeri AB, Borlänge 2012.

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Microstructure

Banded microstructure of ferrite and pearlite after hot rolling followed by slow cooling

Cooling rate after hot rolling controls the final microstructure for a given

composition. If necessary, martensitic microstructures may be produced by

accelerated cooling.

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Microstructure

Steel A Steel B

Homogenized and normalized microstructures after hot rolling

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Effect of Cooling Rate

Austenitized 1 min at 950 °C before

cooling at different rates

Homogenized and normalized

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Mechanical Properties

Proposed for CMn steels containing 0.1-0.2%C

Base value

Grain sizeBase value

Grain size

mm

B.C. De Cooman, J.G. Speer, Fundamentals of Steel Product Physical Metallurgy, Association for Iron and Steel Technology, Warrendale, 2011.

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Partitioning of Stress and Strain


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Historical Developments

The original 1961 bridge with welded joints collapsed

shortly after construction

Riveted

Riveted

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The trend for structural steels used in the construction of bridges and buildings is to replace mild steels with HSLA steels. For many years, ASTM A 7 (now ASTM A 283, grade D) was the most common type of structural steel. In about 1960, improved steelmaking methods resulted in the introduction of ASTM A 36, with improved weldability and slightly higher yield strength. Nowadays, HSLA steels often provide a superior substitute for ASTM A 36, because they have a higher yield strength and at the same time a good weldability. Weathering HSLA steels offer a better atmospheric corrosion resistance than carbon steels.

Structural steels can be classified on the basis of their chemistry and processing in the following categories:

Carbon Steels

High-Strength Low-Alloy Steels

Heat-Treated Carbon and HSLA Steels

Heat-Treated Constructional Alloy Steels

Types of Structural Steels

R. L. Brockenbrough, Structural Steel Designer’s Handbook: Properties Of Structural Steels And Effects Of Steelmaking And Fabrication, 3rd ed., McGraw-Hill Professional, 1999.

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A steel may be classified as a carbon steel if:1. the maximum content specified for alloying elements does not exceed the

following: 1.65% manganese, 0.60% silicon, 0.60% copper2. the specified minimum for copper does not exceed 0.40% 3. no minimum content is specified for other elements added to obtain a desired

alloying effect.

Carbon Steels

S235JRS275JR

S275JR

Approximate EN equivalent


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Low-alloy steels contain alloying elements, including C, up to a total alloy content of about 8 %. High-strength low-alloy (HSLA) steels have specified minimum yield points greater than 40 ksi (275 MPa) and achieve that strength in the hot-rolled condition, rather than by heat treatment. Because these steels offer increased strength levels with a moderate increase in price, they are economical for a variety of applications.


S355J2WP

S355J2W

S355JR


The most commonly-used beams in construction

S355JR


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Both carbon and HSLA steels can be heat treated (normalized/quenched and tempered) to provide yield points in the range of 50 to 75 ksi (345 to 520 MPa). This provides an intermediate strength level between as-rolled HSLA steels and heat-treated constructional alloy steels.

Heat-Treated Carbon and HSLA Steels

P275NL, S275NL

P355N

L485MB



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These are low-alloy steel with sufficient hardenability which are heat treated (quenched and tempered) to obtain a combination of high strength and toughness (also known as quenched and tempered low-alloy steels and constructional alloy steels). Having a yield strength of 100 ksi (690 MPa), these are the strongest steels in general structural use.

Heat-Treated Low-Alloy Steels

P690QLS690QL



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Heat-Treated Low-Alloy Steels

ASM Handbook, Volume 1: Properties and Selection: Irons, Steels, and High-Performance Alloys, ASM International, Materials Park, Ohio

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Tensile Properties


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Tensile Properties

St E 690

St 37-3St 52-3

St E 460

Strain, %

Stre

ss, M

Pa

Room temperature tensile properties of round specimens (DIN grade designations)

St 37-3

St 52-3

St E 460

St E 690

Werkstoffkunde STAHL - Band 2: Anwendung | Springer, Verein Deutscher Eisenhüttenleute (Hrsg.), Düsseldorf, 1985.

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Section Sensitivity of Strength

For a given chemical composition, higher strength levels may be achieved in smaller sections.

Martin Anderson, Charles J. Carter, Are you properly specifying materials?, http://msc.aisc.org/globalassets/modern-steel/steelwise/022012_steelwise_spec.pdf

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High-strength low-alloy (HSLA) steels, also known as microalloyed steels, are used in applications where a high strength and a moderate level of formability is required. HSLA steels are basically low C structural CMn steels with less than 0.1 %C, 0.6-1.2 %Mn, and microalloying additions of Nb, Ti, or V which are strong carbide-and nitride-forming elements.

Role of microalloying elements:- Strengthening by grain refinement (ferrite grain sizes below 10 m)- Solid solution strengthening- Precipitation strengthening

Example applications: Plates for naval vessels and offshore structures, forgings and welded connectors in tension-leg platforms, pressure vessel steels used for transportation of compressed natural gas, steels for line-pipe, formable steels used for automotive & truck parts.

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CMn Steel vs. HSLA

Typical mechanical properties for standard S235 constructional steel (4 mm thick)

Typical mechanical properties for S460MC HSLA steel (4.55 mm thick)

Chemical composition of a typical constructional steel and its HSLA equivalent.


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CMn Steel vs. HSLA


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Controlled-Rolling Process for Grain Refinement

Controlled-rolling process and associated changes in the microstructure

Tnr

H. Sekine, T. Tanaka, C. Ouchi, eds., in: Thermomechanical Process. High-Strength Low-Alloy Steels, Butterworth-Heinemann, 1988

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Delayed Static Recrystallization of Austenite

The main effect of the microalloying elements is the retardation of the recrystallization of deformed which eventually leads to the development of a fine-grained microstructure. HSLA steels are hot rolled at relatively low temperatures in the homogenous -field. Below a composition-dependent temperature known as Tnr , there is a strong retardation of the static recrystallization of the deformed , so that the interpass time (interval between consecutive rolling passes) is not sufficiently long to allow for the recrystallization of austenite.

An empirical relationship for recrystallization stop temperature is as follows:

Tnr(°C)=887 + (6445%Nb) + (890%Ti) + (732%V) + (464%C) + (363%Al) - (357%Si

+ 644 %Nb + 230 %V)

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Effect of solute Nb on the softening at 900 °C of a 20 ppm C steel due

to static recrystallization of austenite.

Effect of microalloying elements Nb, Ti, and V on the recrystallization stop

temperature of a Fe-1.4Mn-0.25Si-0.07C steel.


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Interpass time, secSo

ften

ed f

ract

ion

Interpass time, sec

Soft

ened

fra

ctio

n

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Precipitation Strengthening by Microalloy Carbides

Effect of cooling rate on the increase in yield strength due to precipitation

strengthening in a 0.15%V steel.

Effect of Nb on yield strength for various

sizes of niobium carbide particles.


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Precipitation Strengthening by Microalloy Carbides

The reduced solubility of TiC and NbC carbides in ferrite compared to austenite results in further precipitation of these carbides during the transformation. The formation of fine carbides during transformation acts as an additional contribution to the strength by dispersion hardening. These precipitates which most often form at / interphase boundaries appear as well-defined rows of precipitates in the final ferritic microstructure.

100 nm

Rows of

vanadium

carbides/nitrides


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Hot-Rolled Microstructure

The final hot rolled microstructure can be adjusted by controlled cooling after hot rolling on the run-out table. A low coiling temperature inhibits the coarsening of fine precipitates.


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S2017Proportion of HSLA steel used for different applications

relative to carbon and alloy steel (1986 statistics)

Application of HSLA Steels

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YS/UTS Ratio(Y

S/U

TS)x

10

0

Yield Strength, MPa

Ferrite + Pearlite

Bainite

Bainite Tempered Martensite

In conventional structural design the working stress is usually taken as a proportion of the yield stress; typical values are 60% YS in normal loading and up to 80% in severe loading. The YS/UTS ratio is largely irrelevant for such elastic cases. More recently, structures have been designed using plastic design concepts whereby the ability of the structure to yield and redistribute load without catastrophic failure is required. In such cases, the post-yield strain-hardening behavior of the steel is of increasing importance.

Werkstoffkunde STAHL -Band 2: Anwendung | Springer, Verein Deutscher Eisenhüttenleute (Hrsg.), Düsseldorf, 1985.

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Plastic Design ConceptC

rack

op

enin

g in

cra

cked

p

late

s, m

m

Total strain, %

LOW

HIGH

At temperatures where the material is relatively ductile and the development of a critical strain is required for fracture, a high strain hardening exponent (high n, low YS/UTS) increases the energy required to produce failure. In the DBTT transition regime and lower temperatures, however, a critical stress law is valid and a low n may enhance the resistance to crack propagation.

above DBTTA.C. Bannister and S.J. Trail, Structural Integrity Assessment Procedures For European Industry: The Significance Of The Yield Stress/Tensile Stress Ratio To Structural Integrity, British Steel plc Report

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Toughness: C

Normalized

steels, carbon

levels above

solubility limit


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Toughness: P and Si

Optimum Si in view of toughness


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Toughness: Mn, P, and C

Increased grain boundary cohesion

Containing Fe3C

Fe3C-free


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Toughness: Grain Size

Fe-C-Mn-Nb steel

DB

TT, °C

Ferrite Grain Size, µm

Lower strength

Easier crack propagation (lower chance

of crack deflection at grain boundaries)

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Toughness

Temperature, °C

Imp

act

en

erg

y, J

Toughness of some structural steels (DIN designations)

St E 690

St E 460

St 52-3 RSt 37-2

St E 355Werkstoffkunde STAHL -Band 2: Anwendung | Springer, Verein Deutscher Eisenhüttenleute (Hrsg.), Düsseldorf, 1985.

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Toughness

Positive effect on toughness Grain refinement

• by transformation at lower temperatures (Mn, Cr, Mo)

• by accelerated cooling after hot rolling

• by micro-alloying and controlled rolling

Alloying with Ni (Fe-9Ni martensitic steels for cryogenic applications)

Alloying with Mn

Negative effect on toughness Non-metallic inclusions Banded microstructure Presence of pearlite at higher C

contents Precipitation strengthening Prior deformation Si solid solution strengthening


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Anisotropy of Toughness

Temperature, °C

Imp

act

en

erg

y, J Anisotropy of toughness in St 52-3

steel with 0.02%S. l and t indicate longitudinal and transversespecimens, respectively.

Subscript 1: high rolling reduction ratioSubscript 2: low rolling reduction ratio

l1

t2

t1

l2


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Weathering Steels

Small amounts of Cu, Cr, Ni, Si or P can substantially improve the atmospheric corrosion resistance of structural steels by the formation of a stable protective rust layer on the steel surfaces exposed to the periodic wetting and drying cycles that characterize atmospheric corrosion. This allows them to be used in non-painted conditions as they slowly develop an adherent brownish patina. The addition of 0.35%Cu is very effective in improving the resistance to atmospheric corrosion.

Improvement of atmospheric corrosion resistance in presence of the above elements is because they catalyze the formation of a compact nanoscale rust layer containing the dense and stable -FeOOH and -FeOOH variants of iron oxy-hydroxide


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Atmospheric Corrosion Resistance

Time of Exposure, years

Ave

rag

e R

edu

ctio

n in

Th

ickn

ess,

mils

(1

0-3

in.)


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Welding

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Welding

Effect of a change in the peak temperature of weld thermal cycles from 1000 °C to 1400 °C on the hardenability characteristics

1000 °C (fine-grained austenite)1400 °C (coarse-grained austenite)

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Welding

Underbead crack Welding speed0.4 m/min

Welding Speed, m/min

Heat Input, J/cm

Position

Ha

rdn

ess

HV

10

HAZMeasurement points

0.08 %C

St 52-3

St E 480.7 TM

0.18 %C

Pea

k H

ard

nes

s H

V1

0


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WeldingCE=%C+(%Mn/6)

SteelPlate

Thicknessmm

Yield Strength

MPa %C %Mn %Nb %Mn/%C CE

Preheat Temperature, °C

Pea

k H

ard

nes

s in

th

e H

eat-

Aff

ecte

d Z

on

e, H

V3

An alternative crack-susceptibility parameter proposed by Ito (increased negative effect of C):

d: plate thickness in mm, H: H content in cm3/100g

St E 355

Mn-Nbsteel


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Welding

Ratio of bend angle (bending angle if welded divided by bending angle in the unwelded condition) for normalized steel plates. A high value of the ratio indicates proper weldability.

Thickness, in.

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Cold Cracking after Welding

Cold cracking is the term given to hydrogen-induced cracking in welds and weld heat-affected zones.

Hydrogen from moisture in the air, from fluxes or electrode coatings can readily diffuse into heat-affected zone surrounding weld metal.

The microstructure most sensitive to cold cracking is martensite. Trapping sites for hydrogen: alloying element atoms, dislocations, carbide

and inclusion interfaces, etc. A critical combination of hydrogen concentration and triaxial stress state is

required for crack initiation. An incubation period is necessary for crack initiation. This period is related to the time for hydrogen diffusion to the tri-axial stress field at the root of a notch or crack.

The low strength associated with hydrogen brittle fracture of steel is attributed to the weakening of the cohesive or bond strength between iron atoms by hydrogen.

It is important to estimate the tendency of base metal to form martensite, essentially hardenability, in heat-affected zones. Numerous empirical carbon equivalent formulae and cracking susceptibility parameters are available.

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Thermal Cutting

Distance from cut surface, mm

Surf

ace

ha

rdn

ess,

HV

10

St 52-3 (plate thickness=50 mm)

St 37-2 (plate thickness=45 mm)

Embrittlement caused by thermal cutting is mainly due to: Carbon pick-up in the vicinity of the cut edge Rapid cooling and heating (residual stress and hard microstructural phases)


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Preheating before Welding and Cutting

Preheating the steel prior to cutting and welding as well as decreasing the cutting/welding speed reduce the induced temperature gradients thereby serving to:

(1) decrease the migration of carbon to hotter regions, (2) decrease the hardness, (3) reduce distortion, (4) reduce or give more favorable distribution to the thermally-induced stresses,

and (5) reduce the formation of quench or cooling cracks.

The need for preheating increases with:- Increased carbon and alloy content of the steel- Increased thickness of the steel, and- For parts having geometries that act as stress raisers.

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Stress Relief

Effect of heat treatment for 1 hour at 580 °C on the strength and toughness of St E 355 steel cold formed to different levels:the negligible change in strength and toughness after heat treatment shows that the microstructural changes associated with stress relieving heat treatment are small enough to ensure that the strength and toughness are not compromised.

Cold forming strain, %

Tra

nsi

tio

n t

emp

era

ture

, °C

Ten

sile

or

yiel

d s

tren

gth

, MP

a

UTS

YS

DBTT (27J)

Cold formedCold formed +1 hr 580 °C


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Stress Relief

T(20+log t)10-3

Res

idu

al s

tres

s, M

Pa

T: temperature in Kelvint: time in hours

Example:t: 1 hrT=580 °C=853 KHollomon’s parameter=17060

St E 460

St E 315

St 37-3

(Hollomon’s parameter)


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Steel:Constructability: can be erected as soon as the material is delivered on site.Strength: High strength, stiffness, toughness, and ductile properties. The high specific strength makes it the preferred choice in high-rise buildings.Fire resistance: The strength and stiffness are significantly reduced when heated to temperatures encountered in a fire scenario. The International Building Code requires that the steel is enveloped in sufficient fire-resistant materials which adds to the costs.Corrosion resistance : Steel, when in contact with water, can corrode, creating a potentially dangerous structure. The steel can be painted, providing water resistance.

Reinforced concrete:Constructability: Consisting of portland cement, water, construction aggregate, and rebars, concrete is cheaper compared to structural steel. Concrete must be poured and left to set and cure (waiting time of 1-2 weeks). Off-site pre-cast concrete members may be used right after delivery to the construction site.Strength: Compared to structural steel, a larger volume is required for a structural concrete member to support the same load. This is a significant disadvantage in high-rise buildings. Reinforced concrete is often the preferred choice in low-rise buildings. Lack of tensile strength is compensated by reinforcing steel bars.Fire resistance: Excellent fire resistance propertiesCorrosion resistance: Excellent corrosion resistance properties (the steel reinforcement must not be exposed). Cracks in the concrete, for instance in regions under tension, may expose the steel in which case coating with epoxy resins is recommended (epoxy coated rebars). This however reduces the bond strength between rebars and concrete which in turn necessitates the use of larger and stronger reinforced concrete members.

Steel vs Reinforced Concrete

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Steels for Concrete Reinforcement (Rebar)

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Formability (bending and in the case of mesh bars drawing) Strengthening by

Microstructure control Solid solution strengthening Micro-alloying elements such as V (or Nb) and controlled

rolling Work hardening by cold drawing, torsion, etc.

Surface profile: Ribbed (superior bonding to concrete) Smooth

Common types: Reinforcing bar Reinforcing mesh

Steels for Concrete Reinforcement (Rebar)

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Cold-twisted rebar profiles

Rebar Profiles


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Mass-%Niobium or Vanadium

Ave

rag

e C

ha

ng

e in

Yie

ld S

tren

gth

, MP

a

VanadiumNiobium

Solubility product of various carbides and nitrides in austenite

HSLA Rebars

Micro-alloying with Nb and V


ASM Handbook, Volume 1: Properties and Selection: Irons, Steels, and High-Performance Alloys, ASM International, Materials Park, Ohio.

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Twisting Index Twisting Index

Twisting Index=Lay length of an intially longitudinal rib

Diameter of bar

Ten

sile

Str

eng

th, M

Pa

Yie

ld S

tren

gth

, MP

a

Strengthening of rebars by cold deformation (torsion)

Nominal UTS

values

Nominal YS values

Rebar Strengthening by Cold Deformation


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Cold deformation, %

Yie

ld R

p0

.2o

r te

nsi

le R

mst

ren

gth

, MP

a

Tota

l Elo

ng

ati

on

A1

0 ,

%

Change in the mechanical properties of a 0.15%C hot formed wire through cold deformation

Rebar Strengthening by Cold Deformation


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Type of Rebar

DIN Designation

EN Designation

YS, MPa UTS, MPaTotal

Elongation, %

Rebar BSt 420 S B 420 … 420 500 10

Rebar BSt 500 S B 500 … 500 550 10

Mesh Rebar

BSt 500 M B 500 … 500 550 8

Typical compositions: 0.22 C, 0.60 Si max., 1.30 Mn, Nb/V microalloying0.22 C, 0.60 Si max., 0.80 Mn, Nb/V microalloying0.15 C, 0.60 Si max., 0.50 Mn (cold forming mesh grade)

Example Compositions and Properties

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Cooling time, sec

Tem

per

atu

re, °

C

Ferrite+Pearlite

Temperature equalization

Bainite

Martensite

Large R (near surface), martensite

Small R (near core), coarse +P

R intermediate, fine +P

CCT diagram of a Fe-0.13C-1.21Mn steel rebar with a diameter of 20 mm. Cooling curves through the thickness are given. R indicates distance from the center of the bar.

Inducing Microstructure Gradient


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Why pre-stressing? To prevent the development of tensile stresses in concrete. Pre-stressing is a method for overcoming concrete's natural weakness in tension.

Types of pre-stressing steel bars: Quenched and tempered low-alloy steels Cold worked (drawn) unalloyed steels

Due to the high C contents and the high stresses to be carried by the pre-stressed steel, they may not be welded. This allows the use of higher C concentrations.

Pre-Stressing Steels

Example types and compositions:Quenched and tempered (Q&T) Wire: 0.5C, 1.6Si, 0.6Mn, 0.4CrCold-drawn wire: 0.8C, 0.2Si, 0.7MnRound bar: 0.7C, 0.7Si, 1.5Mn, 0.3V

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Due to the higher stresses in the steel-concrete interface, the surface pattern of pre-stressing steels plays a more important role than it does in normal rebar steels.

Ribbed and profiled bars and wires

Stranded wire

Pre-Stressing Steel Profiles and Types


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Example EN designations:

Y1770C: cold drawn wire for pre-stressing with a tensile strength of 1770 MPa

Y1770S7: 7-strand steel for pre-stressing with a tensile strength of 1770 MPa

Y1230H: Hot rolled or hot rolled and processed bar with a tensile strength of 1230 MPa

Strain, %

Stre

ss, M

Pa

Strength of Pre-Stressing Steels


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Stress relaxation test

Relaxation time, hours Years

Stre

ss d

rop

fro

m t

he

init

ial σ

iva

lue,

%

Initial stress (σi ) as percentage of tensile

strength

Test temperature 20 °C

40 °C

20 °C

20 °C

Stress relaxation of a 7mm-diameter wire made from St 1420/1570 steel grade (dashed regions are extrapolated values)

Stress Relaxation

60%

70%

80%


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Tempering Temperature, °C

Tota

l Elo

ng

ati

on

, %

Yie

ld S

tren

gth

/Pro

of

Stre

ss/T

ensi

le S

tren

gth

, MP

aR

edu

ctio

n o

f A

rea

, %

Control of mechanical properties by heat treatment

Effect of tempering on mechanical properties of

quenched St 1420/1570 pre-stressing steel

Strengthening by Q&T


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Degree of Deformation

Wire Diameter, mm

Reduction of Area, %

Ten

sile

Str

eng

th, M

Pa

Strengthening of a 0.8%C pre-stressing steel wire by drawing

Strengthening by Cold Drawing


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Pipeline Steels

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Weldability High strength for pressure containment High toughness even at subzero temperatures (particularly

important to pipelines in arctic regions) Resistance to hydrogen-related failure

Oil and gas pipelines are a classic application of HSLA steel, and one of the first applications involved the use of acicular ferrite steel for pipelines in the Arctic regions. The development of high-strength linepipe grades has permitted the use of large-diameter pipe operating at high pressures in excess of 11 MPa (1600 psi). Grades with minimum yield strengths up to 483 MPa (70 ksi) in thicknesses up to 25 mm (1 in.) are readily available. Tensile strength is a key requirement in linepipe steels. Other requirements include weldability, fracture toughness, and resistance to sour gas attack. In addition to higher strengths, HSLA steels can provide excellent toughness, good field weldability, resistance to ductile crack propagation, and, in some cases, resistance to sour gas and oil.

Major Requirements

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Development Timeline of Pipeline Steels

Development of pipeline steelsYS in ksi

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Mechanical Properties of Pipeline Steels


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Mechanical Properties of Pipeline Steels

Microstructural factors influencing strength and toughness of pipeline steels

Toughening

Stre

ng

then

ing

H.G. Hillenbrand, M. Gräf, C. Kalwa, Development and production of high strength pipeline steels, Niobium 2001, Orlando, FL, USA.

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Pipeline Steel Grades

X100 and X120 grades with a low carbon bainitic microstructure (superior strength and toughness compared to polygonal ferrite and pearlite) may be produced by a combination of thermomechanicalcontrol processing (TMCP) and accelerated cooling.


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Plastic Design Concept

Traditionally, the limit for allowable in-service stress is set at approximately 70-80% of the yield stress, pressure containment being the main focus. In this approach, yielding and plastic strain are avoided. In actual environmental conditions, however, linepipes are often subjected to strain from seismic activities, icebergs, permafrost melting, or ice formation in initially non-frozen soil. Installation of pipes may also cause small plastic strains. The pipeline must therefore be designed to tolerate in-service plastic deformation of a few percent. More recently, the strain-based design is becoming more popular. Strain-based design (plastic design concept) requires that a low ratio of YS/UTS is maintained. The yield strength of the steel must be low enough to ensure that the weld material (which has a rapidly solidified microstructure and is prone to the presence of defects which may act as local stress concentration sites) is not highly stressed.Many specifications list a maximum YS/UTS ratio of 0.85-0.90. A ratio of 0.85 can be easily achieved in CMn grades but maintaining a low YS/UTS ratio is difficult in microalloyed steels with high strength levels.

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Pipeline Steel Design Considerations


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Steels for Structural ApplicationsB.C. De Cooman, J.G. Speer, Fundamentals of Steel Product Physical...

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