General Motors-AISI AAC Advanced High Strength Steel Repairability Study Phase I Final Report

Introduction The introduction of Advanced High Strength Steels (AHSS) to light vehicle body structure applications poses a significant challenge to organizations involved in the repair of vehicle structures. AHSS are typically produced by nontraditional thermal cycles and contain microstructural components whose mechanical properties can be altered by exposure to elevated temperatures. This temperature sensitivity could alter the mechanical behavior of AHSS after exposure to elevated temperatures during repair welding or flame straightening and could seriously affect the structural performance of AHSS components after repair. This study, requested by General Motors' Collision Repair Tech. Center, examined the mechanical behavior of two AHSS products, a 600 MPa tensile strength (TS) dual phase steel and a 1300 MPa TS Martensitic steel, after exposure to typical repair arc welding and flame straightening temperature cycles and developed recommended practices for repairing components made of these materials. This study is the first phase of and anticipated multi-phase effort to characterize AHSS and develop appropriate repair procedures for their various grades. Phase I objectives were to develop an appropriate method for characterizing the mechanical behavior of AHSS upon exposure to typical repair procedures and to develop recommendations for heat application during the repair of parts made from the dual phase and martensitic grades mentioned above. These AHSS grades are being specified for near-term General Motors vehicle platform components and require immediate repair procedure development. Subsequent phases of this activity will develop repair procedures for other AHSS grades as prioritized by General Motors or other auto OEM AHSS component introductions. Phase I was conducted by a team composed of General Motors, AISI, and AISI Automotive Applications Committee steel company representatives. Team members included: Member Affiliation

Brian Dotterer General Motors Corporation Jim Fekete General Motors Corporation David Anderson AISI (AISI Coordinator) David Hoydick United States Steel Corporation Steve Kelley International Steel Group Nassos Lazaridis Ispat Inland, Inc. Blake Zuidema International Steel Group The results, conclusions, and recommendations contained herein are the consensus views of the team members.

Procedure Materials Conventional and AHSS steel grades selected for this study are as follows: Steel Grade Description GMW2M-ST-S Grade 4 Interstitial-Free HDGI-coated mild steel GMW3032M-ST-S CR340 340 MPa yield strength HDGI-coated HSLA steel GMW3032M-ST-S CR 340 DP 340 MPa YS, 600 MPa TS HDGI-coated DP steel GMW3399M-ST-S 1300T/1030Y M 1300 MPa TS cold rolled (bare) martensitic steel The interstitial-free (IF) mild and high strength, low alloy (HSLA) steels are conventional products that have been used in body structures for many years and are well known to be repairable without substantial structural performance degradation by arc welding and flame straightening. The mechanical property degradation exhibited by these materials defined a baseline against which the degradation of AHSS properties will be judged. The dual phase (DP) and martensitic (Mart) steels are AHSS grades used in structural applications on General Motors vehicles scheduled to launch in the near future, components for which valid repair techniques must be available. All steels were of approximately 1.5 mm gauge and were tested in the longitudinal direction. Vehicle Manufacturing Simulation The sheet steel comprising vehicle body structure components does not exist in its as-produced state at the time of repair, but rather has been subjected to several mechanical deformations and thermal treatments during stamping, assembly and painting, and subsequent damage. These treatments could alter the response of a component to subsequent repair processes. To simulate the actual state of material at the time of repair, the mild steel, HSLA, and DP steels were first subjected to 8% strain in uniaxial tension (to simulate part forming) and heated to 170 °C for 20 minutes to simulate paint baking. Samples of DP600 steel were also subjected to the repair procedure thermal cycle in the as-received condition and with several combinations of low strain and paint bake. The martensitic steel was subjected only to the paint bake treatment, as martensitic steels rarely undergo substantial deformation during fabrication. All coated samples were stripped of zinc to prevent environmental problems during thermal treatment and testing. Simulated Repair Procedure Thermal Cycle Previous General Motors studies [1, 2] measured temperature histories at various distances from arc welds and flame straightening treatments on typical body structure components subjected to repair. A time-temperature test matrix was developed to represent the various thermal conditions encountered during repair welding and flame straightening, Table 1.

Table 1. Time-Temperature Test Matrix.

Hold Hold Temperature (°C) Time (s) 650 750 850 1000

5 X 10 X X 30 X X X X 60 X X X 90 X X

Two additional cycles were simulated. Samples were heated to 750 °C for 90 seconds, with intermediate cooling briefly to below 538 °C after 30 and 60 seconds (to simulate multiple flame heating cycles). Samples of IF, HSLA, and DP without prestrain and paint baking (as-received condition) were subjected to the 650 °C, 90 second thermal treatment. Mechanical Testing Standard ASTM tensile tests were conducted of samples in the as-received condition, after straining and paint baking (baking only for the martensitic steel), and after the indicated thermal treatments. Results for two samples were averaged and yield strength (YS), ultimate tensile strength (UTS), uniform elongation (UEL), total elongation (TEL), and n-value were reported and plotted against temperature for each hold time. n-values were calculated in the 10% to end of uniform elongation strain range (YS to end of uniform elongation (~3% strain) for martensitic steel) The AC1 and AC3 temperatures (phase boundaries) for each steel grade were indicated on the property-temperature plots. Results and Discussion Interstitial-Free Grade 4 Mild Steel Tensile test results for the Grade 4 IF mild steel are shown in Figures 1 through 5. Yield strength and TEL decreased slightly and USA and UEL increased slightly upon exposure to the lowest temperature thermal treatment, 650 °C. Yield strength and UTS monotonically decreased and UEL, TEL, and n-value monotonically increased upon exposure to all higher temperature thermal treatments. Strength remained above the as-received level at temperatures up to 850 °C, despite partial austenitization at 750 °C and 850 °C temperatures. There was no indication of embrittlement or significant loss in ductility at the 650 °C temperature. Only temperature affected mechanical properties; time at temperature had no significant effect. HSLA 340 Steel Tensile test results for the HSLA 340 steel are shown in Figures 6 through 10. Yield and ultimate tensile strength decreased from initial strain plus age levels up to a thermal treatment of 850 °C and then increased slightly at the highest thermal treatment, 1000 °C. UEL, TEL, and n-value all increased with increasing thermal treatment temperature up to 850 °C, and generally decreased as temperature increased thereafter to 1000 °C. Strength remained at or above as-

Figure 1. Yield strength of IF Grade 4 mild steel after exposure to several simulated repair thermal cycles.

IF Grade 4 Mild Steel

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IF Grade 4 Mild Steel

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Figure 2. Ultimate tensile strength of IF Grade 4 mild steel after exposure to several simulated repair thermal cycles.

Figure 3. Uniform elongation of IF Grade 4 mild steel after exposure to several simulated repair thermal cycles.

IF Grade 4 Mild Steel

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Figure 4. Total elongation of IF Grade 4 mild steel after exposure to several simulated repair thermal cycles.

Figure 5. n-value of IF Grade 4 mild steel after exposure to several simulated repair thermal cycles.

IF Grade 4 Mild Steel

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HSLA 340 Steel

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Figure 6. Yield strength of HSLA 340 steel after exposure to several simulated repair thermal cycles.

Figure 7. Ultimate tensile strength of HSLA 340 steel after exposure to several simulated repair thermal cycles.

HSLA 340 Steel

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Figure 8. Uniform elongation of HSLA 340 steel after exposure to several simulated repair thermal cycles.

Figure 9. Total elongation of HSLA 340 steel after exposure to several simulated repair thermal cycles.

HSLA 340 Steel

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HSLA 340 Steel

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Figure 10. n-value of HSLA 340 steel after exposure to several simulated repair thermal cycles.

received levels at temperatures up to 750 °C. There was no indication of embrittlement or significant loss of ductility at any test temperature. As with the mild steel, only temperature had a significant effect on mechanical properties; time at temperature had little effect. DP 600 Steel Tensile test results for the DP 600 steel are shown in Figures 11 through 15. Yield and ultimate tensile strength both decreased after exposure to the lowest thermal treatment temperature, 650 °C. Yield strength continued to decrease with exposure to higher temperatures, but the UTS increased substantially at the next higher temperature, 750 °C, then fell to an intermediate level at the highest thermal treatment temperatures. Ultimate tensile strength fell slightly below the as-received level when subjected to the 650 °C treatment. Yield strength remained above as-received levels at the 650 °C and 750 °C thermal treatment temperatures. Elongation and n-value generally increased with increasing thermal treatment temperature. There was no indication of embrittlement or substantial loss of ductility at any thermal treatment temperature. As with the two previous steels, the DP 600 steel was sensitive only to temperature; time at temperature had little effect on mechanical properties. When the DP 600 steel in the As Received condition (without the 8% prestrain plus paint bake thermal treatment) was exposed to the 650 °C/90 second simulated repair thermal cycle, substantial degradation in ultimate tensile strength was noted. UTS dropped from 642 to 515 MPa, a decline of 20% and well below the 600 MPa minimum UTS specified for this grade. YS and n-value were unchanged, and elongation improved slightly from the as-received condition. The drop in UTS is of substantial concern in crash performance. Dual phase steels are used increasingly in crashworthiness-critical auto body structural components because its high UTS for a given YS provides much greater energy absorption than in conventional HSS. Many structural components contain large areas with little or no strain imparted by the forming process, and these areas do not benefit from strain hardening and age hardening. The degradation in UTS from exposure to the repair thermal cycle could substantially reduce the ability of such components to absorb crash energy. Additional tests were performed on As Received DP 600 samples that were subjected to just the paint bake thermal cycle, and to a 1/2% uniaxial tensile prestrain plus the paint bake cycle to determine if low levels of strain and age hardening are sufficient to offset the strength loss from exposure to the repair thermal cycle in DP 600 components with little or no forming strain. Results of these supplemental tests and those of pertinent initial tests are listed in Table 2.

Table 2. Supplemental tensile tests of DP 600 steel. Test Condition

YS (MPa)

UTS (MPa)

TEL(%)

UEL(%)

n-value (4-6%)

n-value (10-UEL)

YPE

As Received (AR) 404 642 26 16 0.18 0.14 0.0 AR+650 °C/90sec 412 515 30 18 0.21 0.16 2.8 AR+Bake+650 °C/90sec 419 526 28 16 0.20 0.16 2.6 AR+1/2%ε + Bake 471 658 25 16 0.16 0.15 0.0 AR+1/2%ε + Bake + 650 °C/90sec 427 527 27 16 0.20 0.16 2.8 AR+8%ε + Bake 658 683 15 4 n/a n/a 0.0 AR+8%ε + Bake + 650 °C/90sec 550 610 22 11 0.13 0.11 3.5

Clearly, bake hardening and low levels of strain plus bake hardening are insufficient to offset the UTS loss due to exposure to the lowest temperature repair thermal cycle in DP 600 steel. Structural components of this grade with low levels of forming strain will likely exhibit UTS well below the specified 600 MPa minimum UTS for DP 600. Structure evolution in the DP 600 steel was studied by metallography. Figures 16 through 18 show the microstructure of the DP 600 steel in the As Received, As Received + 8% Strain + 650 °C/60s, and As Received + 8% Strain + 750 °C/90s conditions, respectively, after etching in LePera' etchant. LePera's etchant stains the ferrite phase a light gray color and leaves untempered martensite white. The 650 °C/60s thermal treatment substantially tempered the martensite phase, as indicated by darkening of the martensite phase in Figure 17. The 750 °C/90s thermal treatment heated the steel back in to the ferrite plus austenite phase field and re-formed fresh untempered martensite, as indicated by the return of the light colored phase in Figure 18. 1300 MPa UTS Martensitic Steel Tensile test results for the 1300 MPa UTS martensite steel are shown in Figures 19 through 22. Martensitic steels generally exhibit very little uniform elongation; only total elongation is shown here. Yield and ultimate tensile strength were substantially reduced by all thermal treatments, falling to about half their as-received level at the lowest thermal treatment temperature, 650 °C. The loss in strength was accompanied by a rapid increase in elongation and n-value, particularly as temperature entered the two-phase region (above AC1 temperature). There was, however, no substantial embrittlement noted at any thermal treatment temperature. General Observations The IF mild and HSLA 340 materials are conventional steels, both relying on various combinations of grain size, solute, and precipitation strengthening to attain their requisite mechanical properties. These strengthening mechanisms are generally insensitive to exposure to temperatures below the two-phase region for short times. In this study, these conventional steels exhibited only slight decrease in strength upon exposure to the subcritical thermal treatment temperature, 650 °C. Strength decrease in these cases is attributed to recovery of the cold work imparted by the simulated forming step, slight grain coarsening, and in the case of the HSLA steel, coarsening of the strengthening precipitates. Higher thermal treatment temperatures caused these steels to enter or exceed the two-phase region, resulting in greater strength reductions due to recrystallization and grain growth. It was encouraging to find that these steels were not embrittled by exposure to temperatures just below the two-phase region (the so-called blue brittleness effect). This result is consistent with previous GM work [1, 2]. The DP 600 steel is strengthened largely by the presence of islands of hard martensite in a soft ferrite matrix. Strengthening is a function of both the volume fraction of martensite and the strength of the martensite phase. Prior to this study, concerns had been raised that exposure to

temperatures high in the subcritical region would over-temper the martensite phase and substantially reduce the strength of the DP 600 steel. This study confirmed that strength loss upon exposure of the DP 600 steel to the 650 °C thermal treatment temperature was more significant than that of the conventional steel. A 20% reduction in ultimate tensile strength was observed DP 600 samples subjected to the 650 °C thermal treatment. Strength decrease in this material is attributed to two major factors - tempering of the martensite and recovery of cold work from the simulated forming operation. The ferrite in the DP 600 steel contained a much greater amount of cold work before the thermal treatment, (strain is concentrated in the softer ferrite phase) and driving force for recovery was much greater. Strength loss due to recovery is expected to be greater than that exhibited by the HSLA 340 steel. Furthermore, in this study, effects of strength loss due to recovery cannot be separated from those due to martensite tempering. For the DP 600 steel, the combined effects can be sufficient to reduce UTS to substantially below the 600 MPa UTS minimum for the grade, particularly in part regions that are not strained significantly by the press forming operation, and degrade structural performance. These data indicate that the DP600 should not be repaired by flame straightening or be exposed to heat during repair. The 1300 MPa UTS martensitic steel is strengthened by transformation to almost 100% volume fraction martensite. This steel was expected to exhibit significant strength loss due to tempering of the martensite by the repair thermal treatments. Indeed, this study confirmed that exposure to even the lowest welding and thermal straightening thermal treatments substantially degraded mechanical properties. These results indicate that 1300 MPa UTS martensitic steel components should not be welded or flame straightened in any manner, but be replaced in entirety if necessitated by repair requirements. This precaution can be extended to cover all martensitic grade steel components. The Mild and HSLA 340 steels also exhibited slight changes in mechanical properties upon exposure to the 650 °C thermal cycle. These changes could affect fatigue endurance, particularly in the case of UTS reduction. However, other durability degradations introduced by the damage and repair processes themselves are felt to be of a much greater magnitude, however, and durability degradation due to a slight reduction in fatigue endurance is not felt to be a significant consideration. The lowest thermal treatment (650 °C) employed by this study is tolerated by the mild and HSLA steels. This temperature is consistent with GM's currently recommended repair temperature range, up to 1200 °F (650 °C - note that GM's current repair policy references temperature in °F). This temperature is also practical to sense with the unaided eye - repair technicians need only keep the heat below a dull cherry red color to stay within the recommended temperature range. GM Position on Repairability of the Subject Steel Grades Per GM service policies, mild steel (including interstitial-free steel), high-strength-low-alloy steel (HSLA), high tensile strength steel (HSS) and bake hardenable steel are considered repairable after a collision. The policy supplies recommendations for the use of heat in collision

repair. The recommendation limits the maximum temperature to 1200 °F (650 °C), and the heating time to 90 seconds. The heating can be performed two times if needed. The purpose of this study was to re-validate these recommendations for today’s steels, and to begin evaluating repairability of advanced high strength steels (AHSS) The results shown here indicate that heat can be used in repair procedures for IF steel (GM6409M, GMW2M) and 340 MPa minimum yield strength HSLA (GM6208M, GM6218M, GMW3032M). The limitation that temperature must not exceed 1200 °F (650 °C) for a maximum of two cycles of 90 seconds each should remain in force. The maximum temperature is the most important factor affecting mechanical property degradation; time at temperature did not have a significant effect on properties at the times and temperatures studied. The data also shows that following the recommended repair thermal treatment limitations should not cause embrittlement or significant loss of ductility in the tested materials. These results also show that heat is not recommended for repair of dual phase materials (GMW3032M, GMW3399M). These results show that as-received DP steels with minimum tensile strength values of 600MPa experience a 20% drop in TS after the application of 650 °C. Strain and bake hardening response does not counteract this behavior. Results for the 190 KSI (1300 MPa) minimum tensile strength product (GM6123M) reinforce the previously drawn conclusion that martensitic products of this strength level must not be repaired. The only available repair procedure is removal and replacement. Future Work The industry is moving rapidly towards implementing AHSS materials with even higher strength levels than the 600 MPa minimum ultimate tensile strength dual phase steel studied in these experiments. Therefore, there is an urgent need to expand the range of materials tested here to include DP 780, DP 980 and TRIP steel in the range of 600 to 800 MPa TS. There is also a need to evaluate the combination of heat application and welding during collision repair on the properties of high strength steels and advanced high strength steels. This is critical because if heat cannot be used for straightening operations, there will be an additional reliance on sectioning and welding techniques for repair of components made from AHSS materials. Further collaborative work should be focused on these two objectives. Phase II of this study will address both.

Figure 11. Yield strength of DP 600 steel after exposure to several simulated repair thermal cycles.

DP 600 Steel

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DP 600 Steel

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Figure 12. Ultimate tensile strength of DP 600 steel after exposure to several simulated repair thermal cycles.

Figure 13. Uniform elongation of DP 600 steel after exposure to several simulated repair thermal cycles.

DP 600 Steel

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DP 600 Steel

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Figure 14. Total elongation of DP 600 steel after exposure to several simulated repair thermal cycles.

DP 600 Steel

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Figure 15. n-value of DP 600 steel after exposure to several simulated repair thermal cycles.

Figure 16. Microstructure of DP 600 steel in the As Received condition. LePera's etch, 1000X.

Figure 17. Microstructure of DP 600 steel after 8% strain in uniaxial tension, paint baking, and exposure to 650 C for 60 seconds. LePera's etch, 1000X.

Figure 18. Microstructure of DP 600 steel after 8% strain in uniaxial tension, paint baking, and

exposure to 650 C for 90 seconds. LePera's etch, 1000X.

1300 MPa UTS Martensitic Steel

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Figure 19. Yield strength of 1300 MPa martensitic steel after exposure to several simulated repair thermal cycles.

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Figure 20. Ultimate tensile strength of 1300 MPa martensitic steel after exposure to several simulated repair thermal cycles.

Figure 21. Total elongation of 1300 MPa martensitic steel after exposure to several simulated repair thermal cycles.

1300 MPA UTS Martensitic Steel

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Figure 22. n-value of 1300 MPa martensitic steel after exposure to several simulated repair thermal cycles.

Conclusions 1. The mechanical properties of IF grade 4 mild and HSLA 340 do not degrade substantially

upon exposure to temperatures of up to 650 °C (1200°F), for short duration. 2. The mechanical properties of the DP 600 and 1300 MPa UTS martensitic steels were

substantially degraded by exposure to 650 °C (1200°F) for any duration. 3. Exposure to 650 °C (1200°F) did not significantly embrittle any steel tested in this study. 4. Degradations of fatigue performance introduced by exposure of the steels tested to 650 °C

(1200 °F) for short duration should not be significant compared to degradations introduced by the damage and subsequent repair processes themselves.

5. Temperature was the most significant factor affecting mechanical properties of the mild, HSLA, and DP; properties were not significantly influenced by time at temperature.

6. Structural components made of mild, bake hardenable, solid solution-strengthened, and high strength low alloy, can be repaired by arc welding or flame straightening processes provided that the maximum temperature does not exceed 650 °C (1200 °F) for more than two cycles of 90 seconds each.

7. Structural components made of DP 600 and of martensitic steel of any strength should not be repaired by application of heat (flame straightening), but replaced in entirety.

Recommendation Following the results of Phase I of this study, the GM repair matrix is updated according to the recommendation below. In this update, consideration is given to higher strength grades of dual phase steel and other AHSS types referenced by GMW3399M-ST-S, grades that may be of interest in future studies.

Recommended GM Steel Repairability Matrix Grade Repairable? Use of Heat? Temp. Range Maximum Heat Mild Steel

Yes Yes Up to 1200 °F (650 °C)

90 sec. x 2

Bake Hardenable

Yes Yes Up to 1200 °F (650 °C)

90 sec. x 2

Solid Solution-Strengthened

Yes Yes Up to 1200 °F (650 °C)

90 sec. x 2

High Strength, Low Alloy

Yes Yes Up to 1200F (650 °C)

90 sec. x 2

Dual Phase <=600 MPa UTS

TBD No N/A N/A

Dual Phase - >600 MPa UTS

TBD TBD TBD TBD

TRIP

TBD TBD TBD TBD

Martensitic

No No N/A N/A

References 1. E. G. Brewer, K. Malstrom, R. Stevenson, and H. D. Pursel, "Effect of Simulated Repair

Heat Treatments on the Physical Properties of High Strength Steels," General Motors Corporation Research Report No. PH-1251, August 9, 1985.

2. R. Stevenson, E. G. Brewer, K. Malstrom, and H. D. Pursel, "Effect of Simulated Repair Heat Treatments on the Physical Properties of High Strength Steels," SAE Technnical Paper No. 910292, SAE, Warrendale, PA, 1991.