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Steels for Cold Formed Parts without Heat Treatment

Steels for Cold Formed Parts without Heat Treatment€¦ · Sidenor | Cold Forging Pre-Treated Steels 13 Pre-treated Steels • Low carbon cold formable tempered-martensitic steels

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Page 1: Steels for Cold Formed Parts without Heat Treatment€¦ · Sidenor | Cold Forging Pre-Treated Steels 13 Pre-treated Steels • Low carbon cold formable tempered-martensitic steels

Steels for Cold Formed Parts without Heat Treatment

Page 2: Steels for Cold Formed Parts without Heat Treatment€¦ · Sidenor | Cold Forging Pre-Treated Steels 13 Pre-treated Steels • Low carbon cold formable tempered-martensitic steels

Sidenor | Cold Forging Pre-Treated Steels

2

Conventional Process

Hot Rolling Cold Heading StraighteningSpheroidizing Annealing

Quenching & Tempering

Fastener

Page 3: Steels for Cold Formed Parts without Heat Treatment€¦ · Sidenor | Cold Forging Pre-Treated Steels 13 Pre-treated Steels • Low carbon cold formable tempered-martensitic steels

Sidenor | Cold Forging Pre-Treated Steels

3

Technical Challenge: Slender Bolts

• Quenching and tempering of slender long bolts is particularly challenging, due to distortions. A subsequent straightening is compulsory, what extends the leadtime, increases the number of rejected parts and the scattering of properties between bolts with different straightening deformation

• Fortunately, there are technological solutions that skip quench and tempering, but achieve the mechanical properties of grade 8.8

Page 4: Steels for Cold Formed Parts without Heat Treatment€¦ · Sidenor | Cold Forging Pre-Treated Steels 13 Pre-treated Steels • Low carbon cold formable tempered-martensitic steels

Sidenor | Cold Forging Pre-Treated Steels

4

Alternative Processes

• Deformation hardening (equivalent reduction 30-60%) of a microalloyed steel thermomechanically hot rolled

• Cold forming of a quench and tempered wire rod of a low carbon steel (DUCTIL) with high ductility

Thermomechanical Hot Rolling

Hot Rolling + Direct Quench +

Tempering

High Deformation Cold Forging

Cold Forging

Fastener

Fastener

Spheroidizing Annealing

Spheroidizing Annealing

Straightening

Straightening

Quenching & Tempering

Quenching & Tempering

Page 5: Steels for Cold Formed Parts without Heat Treatment€¦ · Sidenor | Cold Forging Pre-Treated Steels 13 Pre-treated Steels • Low carbon cold formable tempered-martensitic steels

5

Ferrite-Pearlite Steel with High Ductility

MICROALLOYED Steels

Sidenor | Cold Forging Pre-Treated Steels

Page 6: Steels for Cold Formed Parts without Heat Treatment€¦ · Sidenor | Cold Forging Pre-Treated Steels 13 Pre-treated Steels • Low carbon cold formable tempered-martensitic steels

Sidenor | Cold Forging Pre-Treated Steels

6

Microalloyed Steels are…

Hot Rolling

Annealing

Cold Forging Tempering

time

T (ºC)

Thermomechanical Hot Rolling and Controlled Cooling

Cold Forging

• Low-medium carbon steels that, by microalloying and thermomechanical hot rolling, can be cold forged to achieve the properties required for grade 8.8 fasteners.

Quenching

Page 7: Steels for Cold Formed Parts without Heat Treatment€¦ · Sidenor | Cold Forging Pre-Treated Steels 13 Pre-treated Steels • Low carbon cold formable tempered-martensitic steels

Sidenor | Cold Forging Pre-Treated Steels

7

Hardening Mechanisms in Steels

• The tensile strength of an alloyed steel of a certain carbon content can be increased by…- Grain size refinement

- Precipitation of carbonitrides of microalloying elements (B, Ti, V, Nb)

- Plastic deformation

• A high deformation during cold forging allows rising noticeably the yield and tensile strength looking out:- As much homogeneous deformation as possible

between extruded and stamped zones

- A narrow scatter of metallurgical and mechanical properties in the as-rolled wire rod

Deformation Hardening

Tens

ile S

tren

gth

(MPa

)

Precipitation Hardening

Grain Refining

Solid Solution (Mn, Si)

Plain Carbon Steel

Page 8: Steels for Cold Formed Parts without Heat Treatment€¦ · Sidenor | Cold Forging Pre-Treated Steels 13 Pre-treated Steels • Low carbon cold formable tempered-martensitic steels

Sidenor | Cold Forging Pre-Treated Steels

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Effect on the Mechanical Properties• Deformation Hardening leads to:

- An equivalent ductility loss- Heterogeneity of mechanical properties between

areas with very different deformation ratios

Rm, Re (MPa)

A(%), Z(%), KU (J)

Deformation ratio(%)

Page 9: Steels for Cold Formed Parts without Heat Treatment€¦ · Sidenor | Cold Forging Pre-Treated Steels 13 Pre-treated Steels • Low carbon cold formable tempered-martensitic steels

Sidenor | Cold Forging Pre-Treated Steels

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Adjustment of Forging Process• The current forging process generates great hardness divergences in areas with dissimilar deformation ratios

• An adequate balance of deformation, wire rod diameter and equilibrated mechanical features of the raw material allow minimizing the scattering of bolt properties

Page 10: Steels for Cold Formed Parts without Heat Treatment€¦ · Sidenor | Cold Forging Pre-Treated Steels 13 Pre-treated Steels • Low carbon cold formable tempered-martensitic steels

Sidenor | Cold Forging Pre-Treated Steels

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Direct-Use 20MnB5 for Grade 8.8• Microalloyed steel with high ductility ferrite-pearlite suitable for grade 8.8 after a 30-60% plastic

deformation

Mechanical properties and microstructure in as-supply condition

C Mn Si P S Al B

0,15 0,8 0,15 - - - 0,0010

0,25 1,5 0,5 0,03 0,03 0,05 0,0060

Rm (MPa) Z (%) A (%) Hardness (HB)

≤ 700 ≥ 65 ≥ 25 ≤180

Page 11: Steels for Cold Formed Parts without Heat Treatment€¦ · Sidenor | Cold Forging Pre-Treated Steels 13 Pre-treated Steels • Low carbon cold formable tempered-martensitic steels

Sidenor | Cold Forging Pre-Treated Steels

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24MnV6 for Grade 10.9

• Microalloyed steel with high ductility ferrite-pearlite suitable for grade 10.9 when ɛ>30%

• Nevertheless, fastening standard introduces some restrictions at the grades 10.9 and upper that limit the use of deformation-hardened steels to special applications (under particular agreements between supplier and end user)

C Mn Si P S Al V Ti

0,2 1 0,25 - - - 0,08 -

0,28 1,6 0,75 0,03 0,03 0,05 0,15 0,005

Page 12: Steels for Cold Formed Parts without Heat Treatment€¦ · Sidenor | Cold Forging Pre-Treated Steels 13 Pre-treated Steels • Low carbon cold formable tempered-martensitic steels

Cold Formable Quenched & Tempered Low Carbon Steels

DUCTIL Steels

Sidenor | Cold Forging Pre-Treated Steels

Page 13: Steels for Cold Formed Parts without Heat Treatment€¦ · Sidenor | Cold Forging Pre-Treated Steels 13 Pre-treated Steels • Low carbon cold formable tempered-martensitic steels

Sidenor | Cold Forging Pre-Treated Steels

13

Pre-treated Steels

• Low carbon cold formable tempered-martensitic steels• Quench and tempering are carried out on the wire rod, therefore final mechanical properties are

achieve at the raw material• Final microstructure is tempered martensite

Hot Rolling

Annealing

Cold Forging Tempering

Oil Quenching

time

T (ºC)

Direct Water Quenching

Cold Forging

Tempering

Page 14: Steels for Cold Formed Parts without Heat Treatment€¦ · Sidenor | Cold Forging Pre-Treated Steels 13 Pre-treated Steels • Low carbon cold formable tempered-martensitic steels

Sidenor | Cold Forging Pre-Treated Steels

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Ductile Low Carbon Martensite

• Lowering carbon content, the tetragonal lattice of martensite distorts less, leading to a microstructure of a ductile, deformable cubic martensite

Page 15: Steels for Cold Formed Parts without Heat Treatment€¦ · Sidenor | Cold Forging Pre-Treated Steels 13 Pre-treated Steels • Low carbon cold formable tempered-martensitic steels

Sidenor | Cold Forging Pre-Treated Steels

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Chemical Composition• A balanced alloying makes DUCTIL steels able to be direct quenched and ductile enough for

cold forging, achieving grades 8.8 and 10.9 without subsequent heat treatment

DUCTIL80 (grade 8.8)

DUCTIL100 (grade 10.9)

C Mn Si Cr Mo Ti Nb B

0,04 1,3 0,2 - - - - -

0,12 1,8 0,4 0,5 - 0,04 0,05 0,003

C Mn Si Cr Mo Ti Nb B

0,05 1 0,1 - - - - -

0,2 2 1 0,8 0,2 0,05 0,08 0,004

Page 16: Steels for Cold Formed Parts without Heat Treatment€¦ · Sidenor | Cold Forging Pre-Treated Steels 13 Pre-treated Steels • Low carbon cold formable tempered-martensitic steels

Sidenor | Cold Forging Pre-Treated Steels

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Adjustment of Properties by Wire Rod Tempering• Depending on the reduction ratio applied during cold forging, it is possible to tune the wire rod

strength to attain the required properties for grade 8.8 fasteners

0 5 10 15 20 25 30 35 40 45 50 55 60

Deformation ratio (%) 700

800

900

1000

1100

1200

Te

nsile

Str

en

gth

(M

Pa

)

700

800

900

1000

1100

1200

High Temperature

TEMPERING

Low Temperature

TEMPERING

Page 17: Steels for Cold Formed Parts without Heat Treatment€¦ · Sidenor | Cold Forging Pre-Treated Steels 13 Pre-treated Steels • Low carbon cold formable tempered-martensitic steels

Sidenor | Cold Forging Pre-Treated Steels

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Wire Rod Online Quenching• Low carbon steel can be quenched directly after hot rolling in a water cooling bed, obtaining a

microstructure of cubic martensite, ductile and cold formable• Subsequent tempering allows to obtain the desired strength and ductility levels• Pretreated wire rod shows a microstructure of tempered martensite and about 800MPa of UTS

Page 18: Steels for Cold Formed Parts without Heat Treatment€¦ · Sidenor | Cold Forging Pre-Treated Steels 13 Pre-treated Steels • Low carbon cold formable tempered-martensitic steels

Sidenor | Cold Forging Pre-Treated Steels

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DUCTIL80 – Wire Rod Features

• Microstructure: Tempered Martensite• Mechanical Properties:

• Tolerances: Ø 5,5-15 (±0,2)• Surface Quality:

Defects below 0,03mm (Ø<10)/0,04mm (Ø>10)• Descarburizing:

- Total: nil

- Partial: 0,06mm (Ø<10) and 0,08mm (Ø>10)• Cleanliness:

UTS (MPa) RofA (%) Upsetting

>800 >65 >1/4

Jerkontoret Fine Coarse

A 1,5 1,5

B 1,5 1,5

C 1 1

D 2 1,5

Page 19: Steels for Cold Formed Parts without Heat Treatment€¦ · Sidenor | Cold Forging Pre-Treated Steels 13 Pre-treated Steels • Low carbon cold formable tempered-martensitic steels

Sidenor | Cold Forging Pre-Treated Steels

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Fastener Forging• Skin-pass, extrusion and cold heading increase fastener hardness and strength, but fit requirements

of standard EN20898

Head hardness (HB) 280-310 Shank hardness(HB) 235-280 Axial Strength (MPa) 800-910 Wedge Strength (MPa) 800-890 Toughness Shank/Head No cracks

Page 20: Steels for Cold Formed Parts without Heat Treatment€¦ · Sidenor | Cold Forging Pre-Treated Steels 13 Pre-treated Steels • Low carbon cold formable tempered-martensitic steels

Sidenor | Cold Forging Pre-Treated Steels

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Cost Savings

• Final heat treatment is avoided when DUCTIL steels are used, keepong a microstructure of tempered martensite

• As forged parts are not quenched, distortions are also avoided, particularly in slender bolts

• A total saving of 15% is estimated

WIRE ROD

CONVENTIONAL DUCTIL

0%0%

85%

100%

ANNEALING

COLD FORGING

STRAIGHTENING

QUENCHING & TEMPERING

COLD FORGING

SAVINGS

WIRE ROD

Q&T

Page 21: Steels for Cold Formed Parts without Heat Treatment€¦ · Sidenor | Cold Forging Pre-Treated Steels 13 Pre-treated Steels • Low carbon cold formable tempered-martensitic steels

Sidenor | Cold Forging Pre-Treated Steels

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DUCTIL 100 – Q&T Wire Rod Features

• A higher carbon and alloy content allows DUCTIL100 achieving properties required by grade 10.9 fasteners

0 5 10 15 20 25 30 35 40 45 50 55 60

Deformation ratio (%) 800

900

1000

1100

1200

1300

Tensile

Str

ength

(M

Pa)

800

900

1000

1100

1200

1300

High Tempering

TEMPERATURE

Low Tempering

TEMPERATURE

Page 22: Steels for Cold Formed Parts without Heat Treatment€¦ · Sidenor | Cold Forging Pre-Treated Steels 13 Pre-treated Steels • Low carbon cold formable tempered-martensitic steels

Sidenor | Cold Forging Pre-Treated Steels

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• Ability to manufacture long slender parts with final straightening

• Cost savings• Reduction of operations and

simplification of the manufacturing chain

• Lower process time

• Higher heterogeneity of properties• Higher tool wear• Higher forging stresses• Lower residual ductility• Higher susceptibility to hydrogen

embrittlement

PROS CONS

Pros and Cons of Direct Use steels

Page 23: Steels for Cold Formed Parts without Heat Treatment€¦ · Sidenor | Cold Forging Pre-Treated Steels 13 Pre-treated Steels • Low carbon cold formable tempered-martensitic steels

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