Repairability of Advanced High-Strength Steel James R. Fekete Ph.D P.E. General Motors Corporation
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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