High Strength Low Alloy Steel (HSLA)

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    High Strength Low Alloy Steel (HSLA)

    Ultra Low Carbon Steel

    Advance High Strength Steel

    By

    Panya Buahombura

    School of Metallurgical EngineeringSuranaree University of Technology

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    Outline Overviews Low carbon structural steel

    High strength low alloy steel (HSLA)/Micro-alloy steeland Thermo-mechanical control process (TMCP)

    Low carbon strip steel

    Ultra-low carbon steel- Interstitial Free (IF) Steel

    - Bake Hardening (BH) Steel

    Advance high strength steel or Multi-phases steel

    - Dual Phase (DP) Steel

    - Transformation Induced Plasticity (TRIP) Steel

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    Overviews

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    Overviews: Low carbon structural steel

    and low carbon strip steel

    High strength low carbon steels?

    StrengthFFFhigh strength low carbon steel? High strength low carbon steelsstrengthening

    mechanismF,?

    High strength low carbon steels,?

    High strength low carbon steelsF?

    Physical metallurgyhigh strength low carbonsteelsF?

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    Steel

    Alloy Steel

    Low-C steel

    Plain Carbon Steel

    Low alloy steelHigh-C steelMedium-C steel High alloy steel

    C, Si (up to 0.40%), Mn (up to 1.20%), S, P Nb, Ti, V, Al, Cr, Ni, Mo, Co, Cu, Mo, W, Mn, Si and etc.

    C 0.2%

    Flat products (rolled)

    Structural (rolled)

    C = 0.2 0.5 %

    Machine parts

    (Heat treatable)

    C > 0.5%

    Tool steels

    (Wear, Abrasion, Heat resisting,

    Corrosion applications)

    Alloy elements 10%

    (some data: 5%)

    Alloy elements > 10%

    (some data: > 5%)

    Applications

    - Body parts in automotive industry

    - Construction of building, bridge, pipeline, etc.

    High strength low carbon steels

    Strengthening Mechanisms

    Produced lighter wt. and higher strength

    - Cold-reduced products: YS > 220 MPa, TS > 330 MPa

    - Hot rolled products: YS > 280 MPa, TS > 370 MPa

    - Solid solution strengthening

    - Precipitation strengthening

    - Dislocation strengthening (Work hardening)

    - Transformation strengthening (Heat treatment)

    - Refining the ferrite grain size (Grain size effects)

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    General Steel Production Process

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    General Steel Production Process

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    Iron and Steel Making Process

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    Semi Finished Products

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    Overview

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    Overview

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    Overview

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    Conventional high strength sheetsteel for automobiles used to besolid solution-hardened steel or

    precipitation-hardened steel withmicro-alloy added.

    Currently, high strength steelproducts whose microstructureis reinforced for greaterstrength have been used.

    (DP steel, TRIP steel)

    Relation between tensile strength and elongation of HSS

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    Chemical compositions (mass%) and mechanical

    properties of the steels

    Yield Tensile Elon-

    Type of steel C Si Mn Ti strength strength gation(Mpa) (Mpa) (%)

    A Mild steel 0.05 0.01 0.24 - 241 384 43

    B Solid solution 0.08 0.02 1.46 - 370 487 30hardened steel

    C DP steel 0.05 0.89 1.25 - 432 618 27

    D Precipitation 0.09 0.01 0.80 0.07 539 636 22

    hardened steel

    E TRIP steel 0.15 1.48 0.99 - 510 644 37

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    Overview

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    Overview

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    Strengthening Mechanisms Refining the ferrite grain size

    (Grain size effect)

    Solid solution strengthening

    Precipitation strengthening Dislocation strengthening/Work hardening

    Transformation strengthening

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    Refining the ferrite grain size

    (Grain size effect)

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    Refining the ferrite grain size

    (Grain size effect)

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    Solid solution strengthening

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    Precipitation strengthening

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    Low Carbon Structural Steel

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    Overview: Low Carbon Structural Steel

    Predominantly C-Mn steels (Ferrite-Pearlitemicrostructures)

    Used in large quantities in civil and chemical engineering

    General Y.S. up to 500 N/mm2 (low alloy grades whichquenched & tempered, Y.S. up to 700 N/mm2)

    Applications: building, bridges, pressure vessels, ships,

    offshore oil & gas platforms, pipeline (for weldability andtoughness which required low-carbon)

    Early 1950s, designed of structural steel with concept ofrefinement of ferrite grain increase Y.S. & toughnessof ferrite-pearlite steels (Al-grain refined compositions

    Y.S. up to 300 N/mm2 which have good impact propertyand good welding characteristics)

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    Overview: Low Carbon Structural Steel

    For higher strength steel, required precipitationstrengthening by small addition of Nb, V, Ti tostructural steel Y.S. up to 500 N/mm2 (known

    as Micro-alloy steel or HSLA steel)

    After 1950s and 1960s, new technique toproduce structural steel Control Rolling

    (fine-grained in as rolled conditions whicheliminating of normalizing heat treatment)

    1970s and 1980s, Control Rolling + Controlled

    Cooling TMCP Improving history of structural steel for: Strength,

    Toughness, Weldability

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    High Strength Low Alloy Steel (HSLA)And

    Thermo-mechanical Processing (TMCP)

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    High Strength Low Alloy Steel (HSLA)

    (Precipitation strengthened/Grain refined steel) Addition of micro-alloy (carbide, nitride or carbo-nitride

    forming elements) such as Nb, V, Ti in structural steel

    and strip steel grades, the materials are known asHigh Strength Low Alloy (HSLA) steel

    At slab soaking temperature ~ 1200 C

    - undissolved particles (such as TiN, NbC and AlN)restricts the size of austenite grain (affect to inhibit

    recrystallization during hot rolling produces fine

    austenite grain size induces fine ferrite grain size)

    - a proportion of micro-alloys are dissolved to solid

    solution (affect to precipitate in later process in form of

    fine carbide/carbonitride/nitride at austenite-ferrite

    interface on cooling to room temperature)

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    Hot rolled materials can be strengthened by separate

    mechanisms of grain refine & precipitation strengthening

    Magnitude of effects depend on:- type and amount of elements added

    - base compositions

    - soaking temperatures

    - finishing and coiling temperatures

    - cooling rate to room temperature

    Strength increment up to 300 N/mm2 and Y.S. ~ 500-600

    N/mm2 can be produced in hot rolled state

    Y.S. ~ 350 N/mm2 are produced in cold-rolled strip

    containing 0.06-0.10 %Nb

    High Strength Low Alloy Steel (HSLA)

    (Precipitation strengthened/Grain refined steel)

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    High Strength Low Alloy Steel (HSLA)

    (Precipitation strengthened/Grain refined steel) Precipitate Ti growth austenite > 1250 C

    Precipitate Nb growth

    austenite 1150 C

    Precipitate V growth austenite 1000 C

    Precipitate Al growth austenite 1100 C

    HSLA steel precipitation strengthening ferrite grain refining

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    High Strength Low Alloy Steel (HSLA)

    (Precipitation strengthened/Grain refined steel)

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    High Strength Low Alloy Steel (HSLA)

    (Precipitation strengthened/Grain refined steel)

    High Strength Low Alloy Steel (HSLA)

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    High Strength Low Alloy Steel (HSLA)(Precipitation strengthened/Grain refined steel)

    Precipitation-Time-Temperature (PTT) Diagram Nb(CN) austenite F 50%

    %Mn precipitation (shiftPTT curve )

    Nb(CN) dynamicprecipitation ~ 900 C

    Ps : Precipitation start

    Pf: Precipitation finish

    High Strength Low Alloy Steel (HSLA)

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    High Strength Low Alloy Steel (HSLA)(Precipitati