Repairability of Advanced High-Strength Steel

Repairability of Advanced High-Strength Steel James R. Fekete Ph.D P.E. General Motors Corporation www.autosteel.org Range of Modern Automotive She...
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Repairability of Advanced High-Strength Steel James R. Fekete Ph.D P.E. General Motors Corporation

www.autosteel.org

Range of Modern Automotive Sheet Steel Products Low Strength Steels (700MPa)

0

300

CM

TR IP

n

HSL

DP , CP A

AHSS

MART

600 900 Tensile Strength (MPa)

1200

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1600

AHSS Repairability Concerns

• AHSS are strengthened by a combination of soft and hard phases • The hard phases are metastable, and want to revert to soft equilibrium phases when heated • Conventional repair procedures involve application of heat

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Sources of Heat in Conventional Repair Practices

• Flame Straightening • MIG welding

To address these concerns, AISI and General Motors performed a study to determine the mechanical property response of AHSS to conventional collision repair practices

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Simulated Flame Straightening Thermal Cycle

Temperature (oC)

Thermal histories of welds, flame straightening were determined previously by GM using thermocoupled coupons* 1000 850 750 650





Time *Stevenson, et. al, SAE Technical Paper No. 910292

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Materials Steel IF Grade 4

Description HDGI IF mild steel

HSLA 340

HDGI 340 MPa YS HSLA steel

DP 600 DP 780

HDGI 340 MPa YS, 600 MPa UTS DP steel HDGI 420 MPa YS, 780 MPa UTS DP steel

TRIP 580 TRIP 780

HDGA 380 MPa YS, 590 MPa UTS TRIP steel Uncoated 440 MPa YS, 780 MPa UTS TRIP steel

Mart 1300

Uncoated 1300 MPa UTS martensitic steel (note: all steels ~1.5 mm gauge)

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Experimental Procedure

•Test as-produced mechanical properties •Test as-fabricated mechanical properties •Strain 8% uniaxial tension, bake 170 oC/20 sec •Test as-repaired mechanical properties •Apply simulated flame straightening thermal cycles

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Experimental Procedure – MIG Welding

Welder:

Miller 190 MIG, 220V, Single Phase

Welder Settings: Gas = 20 PSI, 75% Ar/25% CO2 Wire Speed Setting = 53 Voltage Setting = 3 to 4 Filler Wire:0.023" Harris-Welco Welco-Matic ER70S-6

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Notes on Experimental Procedure

• • • • • •

Martensitic steel was baked only, not pre-strained All tensile tests ASTM longitudinal n-values calculated in 10%-UE range (YS-UE for mart.) Zinc stripped from GI, GA steels before thermal treatment 750 oC condition included 3x30sec interrupted heating AC1, AC3 temperatures provided by steel suppliers

For all steels tested: • 650 C was high in ferrite phase field • 750 C was low in ferrite + austenite phase field • 850 C was high in ferrite + austenite phase field • 1000 C was in austenite phase field www.autosteel.org

Results - HDGI IF Grade 4 Steel IF Grade 4 Mild Steel

IF Grade 4 Mild Steel As Received

400

50 As Received + 650C/90s

8% Strain + Bake 5sec 10sec 30sec 60sec 90sec 30x3sec

300 As Received

200

As Received + 650C/90s 100 AC1

0

40

5sec 10sec 30sec 60sec 90sec 30x3sec

8% Strain + Bake

30 20 10

AC3

AC1

0 0

200

400

600

800

1000

1200

0

200

Hold Temperature, C

0.25 As Received As Received +650C/90s

5sec 10sec 30sec 60sec 90sec 30x3sec

200

100

0

200

400

600

AC3 800

Hold Temperature, C

1000

1200

As Received + 650C/90s As Received

8% Strain + Bake

AC1

800

IF Grade 4 Mild Steel

400

0

600

Hold Temperature, C

IF Grade 4 Mild Steel

300

400

AC3

1000

0.2

5sec 10sec 30sec 60sec 90sec 30x3sec

0.15 0.1 0.05 AC1

0 1200

0

200

400

600

AC3 800

Hold Temperature, C

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1000

1200

Results - HDGI HSLA 340 Steel HSLA 340 Steel

HSLA 340 Steel

40

600

35

8% Strain + Bake

500

5sec 10sec 30sec 60sec 90sec 30x3sec

As Received + 650C/90s

400

As Received

300 200 100

As Received As Received + 650C/90s

30 25 20

8% Strain + Bake

15 10 5

AC1

0 0

200

400

600

AC3 800

1000

AC1

0 0

1200

200

400

0.2

600 8% Strain + Bake As Received As Received + 650C/90s

5sec 10sec 30sec 60sec 90sec 30x3sec

300 200 100 AC1

0 0

200

400

600

Hold Temperature, C

1000

1200

1000

As Received + 650C/90s As Received

0.15

5sec 10sec 30sec 60sec 90sec 30x3sec

0.1

0.05 8% Strain + Bake AC1

AC3 800

800

HSLA 340 Steel

HSLA 340 Steel

400

600

AC3

Hold Temperature, C

Hold Temperature, C

500

5sec 10sec 30sec 60sec 90sec 30x3sec

AC3

0 1200

0

200

400

600

800

Hold Temperature, C

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1000

1200

Results - HDGI DP 600 Steel DP 600 Steel

DP 600 Steel

800

800 8% Strain + Bake

8% Strain + Bake 5sec 10sec 30sec 60sec 90sec 30x3sec

600 As Received + 650C/90s 400 As Received 200 AC1

As Received

600

5sec 10sec 30sec 60sec 90sec 30x3sec

As Received + 650C/90s 400

200

AC3

AC1

0

AC3

0

0

200

400

600

800

1000

1200

0

200

Hold Temperature, C

400

600

1000

1200

Hold Temperature, C

DP 600 Steel

DP 600 Steel As Received + 650C/90s

30

800

0.2 As Received + 650C/90s

25

As Received

5sec 10sec 30sec 60sec 90sec 30x3sec

20 15 8% Strain + Bake 10 5 AC1

5sec 10sec 30sec 60sec 90sec 30x3sec

0.15 As Received 0.1

0.05

AC3

0

AC1

0 0

200

400

600

800

1000

Hold Temperature, C

1200

0

200

400

600

AC3 800

Hold Temperature, C

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1000

1200

Results - HDGI DP 780 Steel

DP 780

DP 780 1200

1200

5sec 10sec 30sec 60sec 90sec 30x3sec

As Received + 650C/90s As Received

400 200

AC1 200

400

600

800

600

As Received +650C/90s

400 200 AC1 0

1000

5sec 10sec 30sec 60sec 90sec 30x3sec

800

0

AC3

0 0

UTS, MPa

600

8% Strain + Bake As Received

1000

8% Strain + Bake

800

200

400

600

AC3 800

1000

1200

Hold Temperature, C

1200

Hold Temperature, C

DP 780

DP 780 0.25

50 40

5sec 10sec 30sec 60sec 90sec 30x3sec

30 As Received + 650C/90s As Received

20 10

8% Strain + Bake AC1 200

0.15

400

600

As Received

0.05 AC1

AC3

0 0

200

400

AC3 800

1000

5sec 10sec 30sec 60sec 90sec 30x3sec

As Received + 650C/90s

0.1

600

800

Hold Temperature, C

0 0

n-value

0.2

TEL, %

YS, MPa

1000

1200

Hold Temperature, C

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1000

1200

Results – HDGA TRIP 590 Steel TRIP 600

TRIP 600

1200

1200

5sec 10sec 30sec 60sec 90sec 30x3sec

8% Strain + Bake

600 As Received 400 As Received + 650C/90s 200 AC1

1000 UTS, MPa

800

800

0

200

400

600

1000

As Received +650C/90s

400 200

AC3 800

5sec 10sec 30sec 60sec 90sec 30x3sec

8% Strain + Bake As Received

600

AC1

0

0

0

1200

200

400

600

AC3 800

1000

1200

Hold Temperature, C

Hold Temperature, C TRIP 600

TRIP 600

50

0.25 As Received

As Received

5sec 10sec 30sec 60sec 90sec 30x3sec

As Received + 650C/90s

30

8% Strain + Bake

20 10

AC1

0 0

200

400

600

Hold Temperature, C

1000

5sec 10sec 30sec 60sec 90sec 30x3sec

As Received + 650C/90s 0.15 0.1

8% Strain + Bake

0.05 AC1

AC3 800

0.2 n-value

40 TEL, %

YS, MPa

1000

AC3

0

1200

0

200

400

600

800

Hold Temperature, C

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1000

1200

Results – Uncoated TRIP 780 Steel TRIP 780

TRIP 780

1200

1200

As Received

UTS, MPa

As Received + 650C/90s

400 200

5sec 10sec 30sec 60sec 90sec 30x3sec

As Received

800

As Received +650C/90s

600 400 200

AC1

AC3

0 200

400

600

800

1000

1200

AC3

AC1

0 0

0

200

Hold Temperature, C

400

600

800

1000

1200

Hold Temperature, C TRIP 780

TRIP 780 50

0.25

40

0.2

5sec 10sec 30sec 60sec 90sec 30x3sec

As Received

30

As Received + 650C/90s 20 8% Strain + Bake 10 AC1

0

n-value

As Received

TEL, %

YS, MPa

5sec 10sec 30sec 60sec 90sec 30x3sec

800 600

8% Strain + Bake

1000

8% Strain + Bake

1000

5sec 10sec 30sec 60sec 90sec 30x3sec

As Received + 650C/90s

0.15 0.1 0.05

AC1

AC3

AC3

0 0

200

400

600

800

Hold Temperature, C

1000

1200

0

200

400

600

800

Hold Temperature, C

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1000

1200

Results - Mart 1300 Steel 1300 MPA UTS Martensitic Steel

1300 MPa UTS Martensitic Steel 40

1600

35

1400 Bake As Received

1200 1000

5sec 10sec 30sec 60sec 90sec 30x3sec

800 600 400

25 20 15 As Received

10 5

200

AC1

0 0

200

400

600

AC3 800

1000

Bake 0

1200

200

400

5sec 10sec 30sec 60sec 90sec 30x3sec

1000 800 600 400 AC1 0

200

400

1000

1200

0.25

1200

0

800

1300 MPa UTS Martensitic Steel

Bake As Received

200

600

AC3

Hold Temperature, C

1300 MPa UTS Martensitic Steel

1400

AC1

0

Hold Temperature, C

1600

5sec 10sec 30sec 60sec 90sec 30x3sec

30

600

Hold Temperature, C

1000

5sec 10sec 30sec 60sec 90sec 30x3sec

0.15 0.1 0.05

As Received Bake AC1

AC3 800

0.2

AC3

0

1200

0

200

400

600

800

Hold Temperature, C

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1000

1200

Results – MIG Welding Tensile Tests

MIG welds on 8% strain + bake samples. Steel

As Received

8% + Bake

8% + Bake + MIG Welded

YS (MPa)

UTS (MPa)

TE (%)

YS (MPa)

UTS (MPa)

TE (%)

YS (MPa)

UTS (MPa)

TE (%)

Failure Location

HSLA 340

347

455

29.8

487

499

19.8

487

496

5.9

Base

DP 600

404

642

26.1

658

683

14.6

622

712

3.8

HAZ

DP 780

521

857

18.3

928

962

8.4

824

891

2.6

HAZ

TRIP 600

402

651

34.9

661

712

25.4

620

704

17.8

Base

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Results – MIG Welding Tensile Tests

MIG welds on 8% strain + bake + 650oC/9sec. samples Steel

As Received

8% + Bake

8% + Bake + Heat + MIG Welded

YS (MPa)

UTS (MPa)

TE (%)

YS (MPa)

UTS (MPa)

TE (%)

YS (MPa)

UTS (MPa)

TE (%)

Failure Location

HSLA 340

347

455

29.8

442

491

22.5

443

486

19.8

Base

DP 600

404

642

26.1

550

610

21.6

550

604

14.8

Base

DP 780

521

857

18.3

681

723

16.6

676

724

7.3

Base

TRIP 600

402

651

34.9

502

621

24.0

499

615

20.3

Base

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Conclusions



The mechanical properties of the tested AHSS materials were substantially degraded by exposure to 650°C (1200°F). However, the exposure did not significantly embrittle any of the tested steels



Temperature was the most significant factor affecting mechanical properties; time at temperature was not a significant factor

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Conclusions

• Structural components made from DP, TRIP or Martensitic steels of any tensile strength level should not be repaired by application of heat • MIG welding results were encouraging, but HAZ failures are a concern. Further work is required to validate use of MIG welding in repair. This work is under way at the Auto/Steel Partnership www.autosteel.org