TRP 9946Properties of Galvanized and Galvannealed Advanced High Strength Hot Rolled Steels

Report atAISI / DOE TRP Industry Briefing Session

October 10, 2007Salt Lake City, Utah

Project Participants

Research Organizations:CANMET–Materials Technology Laboratory, Ottawa, Canada McGill University, Montreal, CanadaMcMaster University, Hamilton, Canada

Industry Participants:International Lead Zinc Research Organization (ILZRO)Nucor Steel, SCSeverStal N.A., Dearborn, MIUS Steel, Pittsburgh, PA

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Project Team

CANMET–Materials Technology LaboratoryVal Guertsman (Project Leader) Réal Bouchard Elhachmi Essadiqi (Program Manager) Pierre Martin Olga Dremailova Benoit Voyzelle

International Lead Zinc Research OrganizationFrank Goodwin

McGill UniversityJames Nemes, Wael Dabboussi

McMaster UniversityJoe McDermid, Richard Fourmentin

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Objectives

This project addresses three technical hurdles for implementation of galvanized and galvannealed advanced high strength hot rolled steels for automotive applications :

achievement of good quality coatings on the hot rolled steels while retaining target mechanical propertieslack of precise knowledge of the behavior of these steels in the various forming operations required by automobile parts manufacturersdevelopment of accurate user property data in the galvanized and galvannealed conditions

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Project Tasks

The project was scheduled to be completed in two years following the kick-off meeting (start date July 1, 2005).At the AISI Project Review Meeting on February 8, 2007 in Ottawa it was agreed to revise the work plan and schedule, and extend the end date to the end of December 2007.

Task/Sub-Task Description A Galvanizing Simulator Trials A.1. Obtaining the Steels A.2. Hot Dip Galvanizing A.3. Galvannealing B. Formability Evaluation B.1. Stretch-Flange-Formability B.2. Tube Welding B.3. Hydroforming CharacteristicsC. User Properties C.1. Fatigue C.2. Dynamic Tensile Testing D. Reporting

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Obtaining Steel SubstratesThree grades of advanced high strength hot rolled steel with thickness 2.0-2.5 mm were envisaged in the project planThe following materials were received for the project:

HSLA (composition similar to alternative HSLA grade) – delivered in February 2006DP (composition close to alternative DP grade) – delivered in December 2005TRIP (composition somewhat similar to alternative TRIP grade) – delivered March 30, 2007

Composition (wt.%) Target Properties Type

C Si Mn Nb Ti Other Y. S. (MPa)

T. S. (MPa)

Total El. (%)

HSLA 0.08 0.01 0.54 0.04 0.05 - 570 620 23 Alternative HSLA grade

0.06-0.08

0.01-0.14

0.5-1.4

Precipitation strengthening based on Ti, Nb and/or V

TRIP 0.18 1.5 1.4 0.025 0.005 - 500 900 20

Alternative TRIP grade 0.1-0.2 0.5-1.5 <1.5

0.05 to 1.0 Al (depending on Si content) Nb and/or P to obtain desired strength level

DP* 0.08 <0.3 1.5 0.03 0.002 0.25 Mo, 0.5 Cr 350 600 20

Alternative DP grade <0.1 1.6-

2.0 Nb, Cr and/or Mo additions

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HSLA Steel: As-received Hot Rolled Steel

Uniform microstructure:Fine-grained ferrite interspersed with a few pearlite grainsSmall volume fraction of pearliteFine GB carbidesComplex oxide and sulfide inclusions up to 10 microns in diameter

Mechanical properties conform to the target properties

Optical metallography (OM) Scanning electron microscopy (SEM)

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Hot Dip Galvanizing of HSLA Steel38 coupons were galvanized at McMaster University and delivered to CANMET for further tests

side 1 side 2

88 cm

20 c

m

Hot Dip Galvanizing Regimes

0.20%4s460°C5°C/s60/120s600°C5°C/s20%- 30°C

wt.% Al time (s) T Cooling Holding

timeHolding

T-reHeating %H2Dew Pt

Dipping ConditionsHeating ConditionsAtmosphere

5.02°C/s

Holding for 60s

5.07°C/s

5.03°C/s

5.32°C/s

Dipping Dipping

Holding for 120s

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Summary on Galvanizing Simulator Trials for HSLA Steel

Two thermal cycles were tested for the HSLA steel, 60s and 120s holding times at 600°C.

These two regimes resulted in similar characteristics in terms of the obtained microstructure, and the quality of the coating and the inhibition layer.

According to recommendations of the sponsors, the holding time of 60 s was chosen.

38 coupons were galvanized at McMaster University and delivered to CANMET-MTL for further tests.

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Galvannealing of HSLA Steel24 coupons were galvannealed at McMaster University and delivered to CANMET for further tests

side 1 side 2

118 cm

20 c

m

Optimal Galvannealing ParametersReheating atmosphere 20% H2 in N2 with a dew point of -30°CZinc bath with 0.123% effective Al and Fe saturatedGalvannealing temperature = 510°CGalvannealing time = 30 s

Example of thermal cycle

510°C – 30s

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Holding for 60s

Cross-Section SEM:

ζ

Three layers of Fe-Zn intermetallics (i.e. Γ, δ and ζ)

Γ

δ

Galvannealing (continued)

Well-developed galvannealed coatings with ~10 wt.% Fe according to glow-discharge optical emission spectroscopy (GDOES) and inductively coupled plasma (ICP) analysisDensity of Fe-Zn intermetallics ~ 65 g/m2

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Tensile Properties of HSLA Steel

Mechanical properties retained after galvanizing and galvannealing~ correspond to target properties

0.0726±1635±3598±9Longitudinal

0.0725±1646±1616±4TransverseGalvannealed

0.0829±1632±1590±5Longitudinal

0.0728±1647±2608±8TransverseGalvanized

0.0922±1633±2580±5Longitudinal

0.0719±1654±13613±12TransverseAs-received(hot-rolled)

Strain hardening

n

Total Elongation

(%)

Ultimate Tensile Stress(MPa)

Stress at 0.2% Offset

(MPa)OrientationCondition

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Stretch-Flange Formability

According to ISO/TS 16630:2003

Force a conical die through a pre-punched hole until any one crack extends through the test piece thickness

Hole Expansion Test

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Stretch-Flange Formability of Galvanized HSLA SteelSix galvanized HSLA coupons testedThe hole expansion ratio λ = 67.4 ± 5.2 %

Jan.29/07 60 Degree Cone PunchSERIAL Dia.-Org. Dia. - Final Avg. %Stretch GaugeHSLA 25 0.3975 0.6940 0.6980 0.6970 0.6965 0.6964 75.2 0.1031HSLA 36 0.3975 0.6485 0.6470 0.6480 0.6475 0.6478 63.0 0.1051HSLA 41 0.3975 0.6835 0.6830 0.6840 0.6835 0.6835 71.9 0.1045HSLA 60 0.3975 0.6575 0.6560 0.6565 0.6570 0.6568 65.2 0.1033HSLA 46 0.3970 0.6435 0.6415 0.6425 0.6420 0.6424 61.8 0.1029HSLA 32 0.3970 0.6640 0.6655 0.6645 0.6650 0.6648 67.4 0.1023

Avg. 0.6653 67.4St.Dev. 0.0210 5.24

Example of galvanized coupon after hole expansion test

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Fractography After Hole Expansion Tests (HSLA GI)

Ductile fracture

In some places, crack follows carbide particles

Micro-cracks appeared in the coating only in the immediate vicinity of the main crack

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Stretch-Flange Formability of Galvannealed HSLA SteelFive galvannealed HSLA coupons testedThe hole expansion ratio λ = 34.7 ± 2.5 %

Example of galvannealed coupon after hole expansion test

May14/07 60 Degree Cone PunchSERIAL Dia.-Org. Dia. - Final Avg. %StretchGA 30 0.3965 0.5440 0.5425 0.5435 0.5445 0.5436 37.1GA 33 0.3965 0.5260 0.5270 0.5255 0.5270 0.5264 32.8GA 35 0.3970 0.5315 0.5290 0.5315 0.5310 0.5308 33.7GA 36 0.3970 0.5460 0.5455 0.5470 0.5465 0.5463 37.6GA 37 0.3965 0.5255 0.5235 0.5265 0.5240 0.5249 32.4

Avg. 0.5344 34.7St.Dev. 0.0099 2.47

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Fractography After Hole Expansion Tests (HSLA GA)

Substrate material appears to be more brittle with GA coating than galvanized

GA coating is cracked all over bended parts

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Dynamic Tensile Tests (McGill University)

Direct Split Hopkinson Tension Bar

Tensile Samples in longitudinal (rolling) direction10 mm gage length, 5 mm width

Three strain rates:• 10-3 s-1 (quasi-static tests)• 900 s-1

• 1600 s-1

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Dynamic Tensile Testing:Examples of Stress-Strain Curves

0

100

200

300

400

500

600

700

800

900

1000

0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4

Eng. Strain

Eng.

Stre

ss (

MPa

)QS Ten HGZ 0.001 /s

Dyn Ten HGZ 1000 /s

Dyn Ten HGZ 1600 /s

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Summary Results of Dynamic Tensile Tests

287351600 s-1

27735900 s-1

3765810-3 s-1

HSLA Galvannealed

267401600 s-1

27698900 s-1

3663810-3 s-1

HSLA Galvanized

Total Elongation (%)UTS (MPa)Strain Rate

The results indicate an increase in stress at higher strain rates coupled with a small decrease in total elongation to failure. The energy absorbed at high strain rates is above that at quasi-static rate, and similar in the GI and GA conditions.

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14210770.914010872.31600 s-1

14310871.513810570.2900 s-1

12794.561.212390.659.310-3 s-1

at 20 % strainat 15 % strainat 10 % strainat 20 % strainat 15 % strainat 10 % strainStrain Rate

Galvannealed (N/m2)Galvanized (N/m2)

Absorbed Energy

Fractography After Dynamic Tensile Tests

Somewhat different appearance on macro-scale

Similar ductile fracture on micro-scale

Quasi-static tests Dynamic tests

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Several panels were provided, but not enough to finish all the tasks planned in the project.Additional quantity of this steel could not be received.In the as-received condition the steel did not have characteristic DP microstructure.Mechanical properties of the steel in the as-received condition did not conform to the requirements.

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Dual-Phase Steel

Representative stress-strain curve for the as-received hot rolled DP steel showing yield point elongation behavior

Microstructure of As-received Hot Rolled DP Steel

Ferrite + pearlite and/or bainite with very small volume fraction of martensiteSome microstructural bandingElongated MnS (up to 10 microns in length)Al2O3 attached to some MnS

OM SEM

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Additional Heat Treatment of DP Steel

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Dilatometer trials have determined reheating schedules producing ferrite + martensite microstructure:

188±14.4±0.3438688950°C/5 min 50°C/sDP438

201±24.6±0.3481761950°C/5 min 50°C/sDP255

177±24.8±0.3490716950°C/5 min 25°C/sDP253

168±35.8±0.4473749820°C/2 min 25°C/sDP248

169±44.7±0.4481744820°C/1 min 25°C/sDP249

179±27.4±0.6433674776°C/5 min 25°C/sDP440

148±13<2---Hot-Rolled

Vickers HardnessVolume % M.A.T FINISH (°C)T START (°C)ScheduleSample ID.

DP249 : 820°C/1 min + 25°C/s

SEM(Nital etch)

OM(LePera etch)

Examples of ferrite + martensite microstructure

However…The above regimes may not be compatible with industrial processing lines.Additional hold at the galvanizing temperature seems to destroy the DP microstructure:

153±10.8±0.1506760820°C/2 min 10°C/s + 460°C/1 minDP254

Vickers Hardness

Volume % M.A.T FINISH (°C)T START (°C)ScheduleSample Id.

SEM(Nital etch)

OM(LePera etch)

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DP Steel (continued)

Due to time constraints it has been decided at the project review meeting to wrap up further work on this grade

• Al-based hot-rolled TRIP steel• 30 plates 0.096” x 35” x 50” (= 2.44 mm x 889 mm x 1270 mm)• Surface condition – pickled• Mechanical properties do not correspond to TRIP

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TRIP Steel

As-Received Hot Rolled TRIP SteelSample with scale on: uneven scale on both sides, rather rough surface

SEM

No internal oxidation. Scale – Fe oxide, no Si

Microstructure of As-Received Hot Rolled TRIP SteelOM – LePera etch

Microstructural banding; Ferrite + Bainite + very little MA

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B

M-A

F

SEM

Volume fraction of retained austenite determined by XRD

= 1.7%

Microstructure of As-Received Hot Rolled TRIP SteelSEM – Inclusions

B

M-A

F

M-A

Inclusions:Elongated MnSAl2O3 attached to some MnS Al-N, TiC, …

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Additional Heat Treatment of TRIP SteelDilatometer and Gleeble trials have determined reheating schedules producing desired microstructure with retained austenite

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

600 700 800 900 1000

Temperature (ºC)

Frac

tion

of γ

For

med

on

Hea

ting

TEST 1

TEST 2

Intercritical annealing at 850ºC produces about 50% austenite

TRIP Steel: Heat Treatment Thermal Profiles

050

100150200250300350400450500550600650700750800850900

0 20 40 60 80 100 120 140 160 180 200 220 240 260 280 300 320

Time (s)

Tem

pera

ture

( C)

050

100150200250300350400450500550600650700750800850900

0 20 40 60 80 100 120 140 160 180 200 220 240 260 280 300 320 340 360 380

Time (s)

Tem

pera

ture

( C

)

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050

100150200250300350400450500550600650700750800850900

0 20 40 60 80 100 120 140 160 180 200 220 240 260 280 300 320 340 360 380 400

Time (s)

Tem

pera

ture

(ºC

)

HT1

HT2

HT3

Volume fraction of retainedaustenite determined by XRD:

12% (Gleeble sample)19% (Dilatometer sample)

11% (Gleeble sample)10-12% (Tube blancs)

13% (Dilatometer sample)

Examples of Microstructure of Heat-treated TRIP Steel

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OM – LePera etch

HT1 HT3

Tensile Properties of TRIP Steel

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0.20±0.0145±229±1634±1417±5Longitudinal

0.19±0.0149±130±1642±3453±8TransverseHT2850°C×1min +

460°C×1min

0.21±0.0147±231±1643±3407±4Longitudinal

0.20±0.0150±332±1646±4436±5TransverseHT1:850°C×2min +

460°C×1min

0.18±0.0165±436±3569±20398±18Longitudinal

0.16±0.0150±130±1595±1446±2TransverseAs-received(hot-rolled)

0.1527631478Unknown(longitudinal?)Supplier’s data

Strainhardening

n

Reductionof Area

(%)

TotalElongation

(%)

UltimateTensile Stress

(MPa)

Stress at0.2% Offset

(MPa)OrientationCondition

Remaining Experimental Work

Galvanize TRIP coupons (ongoing).Perform tensile tests on GI TRIP.Dynamic tests on GI TRIP.Hole expansion tests on GI TRIP.Fatigue tests on all the grades (ongoing).Tube welding and ring hoop tension tests on all the grades (ongoing).

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