Advanced High Strength Steels  

Emmanuel De Moor    Advanced Steel Processing and Products Research Center*

Colorado School of Mines   Global Automotive Lightweight Materials Detroit 2015

August 20th, 2015 Novi, MI  

*An NSF Industry/University Cooperative Research Center - Est. 1984

 

Outline

2

Provide an overview of advanced high strength sheet steels Metallurgical strategies enabling high strength and ductility Microstructures of interest to enable next generation AHSS properties

Overview

3

Higher strength with ductility/formability to enable downgauging. Processing and alloying to tailor microstructures resulting in attractive properties e.g. high strength phases can be introduced

AHSS Development

4 AISI: www.steel.org (2006)

Elo

ngat

ion

(%)

Tensile Strength (MPa)

0

10

20

30

40

50

60

70

0 600 1200 300 900 1600

HSLA

IF

Mild IF - HS

BH

Elo

ngat

ion

(%)

600

-

Elo

ngat

ion

(%)

Tensile Strength (MPa)

0

10

20

30

40

50

60

70

0 600 1200 300 900 1600

HSLA

IF

Mild IF - HS

BH ISO

Elo

ngat

ion

(%)

600

- ISO

AISI: www.steel.org (2006)

Dual Phase Steels

Dual Phase Steels  

“Typical Compositions:” C: 0.05 - 0.15 Mn: 1.0 - 2.0  Others: Si, Cr, Ni, Mo, Nb, V  

0.15C, 1.5 Mn, 1.5 Si   WQ from 775oC  

A. De et al., Adv. Mat. Proc., 2003  

“Dual Phase:” High Strength Martensite Ductile Ferrite Processing: Intercritical Annealing  

Strengthening in DP Steels

Davies (1978)

Strength increase follows rule of mixtures: σT = Vfσf + VMσM

Elo

ngat

ion

(%)

Tensile Strength (MPa)

0

10

20

30

40

50

60

70

0 600 1200 300 900 1600

HSLA

IF

Mild IF - HS

BH ISO

Elo

ngat

ion

(%)

600

- ISO

AISI: www.steel.org (2006)

TRIP Steels

TRIP Steels

“Typical Compositions:” C: 0.20 Mn: 1.5 Si, Al 1.5 TRansformation Induced Plasticity: Austenite transforms to martensite with deformation

Significant strain hardening resulting in high strength and ductility

Time

Tem

pera

ture

Ferrite-Bainite-Austenite

"TRIP"

"Dual Phase"Ferrite +Martensite

A1

A3

Elo

ngat

ion

(%)

Tensile Strength (MPa)

0

10

20

30

40

50

60

70

0 600 1200 300 900 1600

MART

HSLA

IF

Mild IF - HS

BH ISO

Elo

ngat

ion

(%)

600

- ISO

AISI: www.steel.org (2006)

Martensitic Steels

Hot Stamping-Press Hardening

Formability at elevated temperature Blanks heated to > 900 ºC Forming and accelerated cooling in dies Complex geometry and high strength Alloying for hardenability - Boron

Elo

ngat

ion

(%)

Tensile Strength (MPa)

0

10

20

30

40

50

60

70

0 600 1200 300 900 1600

MART

HSLA

IF

Mild IF - HS

BH ISO

600

- ISO

Before hardening Press-hardened

Hot Stamping

Elo

ngat

ion

(%)

Tensile Strength (MPa)

0

10

20

30

40

50

60

70

0 600 1200 300 900 1600

MART

HSLA

IF

Mild IF - HS

BH ISO

Elo

ngat

ion

(%)

600

-

First Generation AHSS

Second Generation AHSS

ISO

Properties: “1st and 2nd Generation AHSS”

AISI: www.steel.org (2006)

Advanced High Strength Sheet Steels

12

Elo

ngat

ion

(%)

Tensile Strength (MPa)

0

10

20

30

40

50

60

70

0 600 1200 300 900 1600

MART

HSLA

IF

Mild IF - HS

BH ISO

-

BH

“3rd Generation AHSS”

AISI: www.steel.org (2006) 13

Current Status of AHSS •  Conventional High Strength

Bake Hardenable (BH) HSLA

•  “1st Generation”: (ferrite-based) Dual Phase (DP) TRIP Complex Phase (CP) Martensitic

•  “2nd Generation”: (austenite-based) Austenitic stainless steels TWIP - Twinning Induced Plasticity (TWIP) L-IP® - Lighter Weight Steels with Induced Plasticity

•  “3rd Generation”: New (?) multiphase 14

Predictive Model: Ferrite + Martensite

Increase MVF Matlock and Speer: ICASS 2006

200 400 600 800 1000 1200 1400 1600Ultimate Tensile Strength (MPa)

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

Uni

form

Eng

inee

ring

Stra

inFerrite + Martensite

50%

40%

70%

% MVF

60% Constituent UTS (MPa)

Uniform True

Strain Ferrite 300 0.3

Martensite 2000 0.08

15

Composite modeling to predict properties of hypothetical mixtures Ferrite + Martensite

Constituent properties from the literature

Comparison to “3rd Generation” AHSS

Matlock and Speer: ICASS 2006 Tensile Strength (MPa)

Elo

ngat

ion

(%)

0

10

20

30

40

50

60

70

0 600 1200 300 900 1600

MART

IF

Mild IF - HS

BH ISO

-

BH

HSLA Ferrite + Martensite Ferrite + Martensite 50% 40% 60%

% MVF

16

Predictive Model: Austenite + Martensite

Increase MVF Matlock and Speer: ICASS 2006

Constituent UTS (MPa)

Uniform True

Strain Ferrite 300 0.3

Martensite 2000 0.08

Austenite 640 0.6

200 400 600 800 1000 1200 1400 1600Ultimate Tensile Strength (MPa)

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

Uni

form

Eng

inee

ring

Stra

inFerrite + Martensite

Austenite + Martensite

50%

30%

40%

60%

% MVF

17

Composite modeling to predict properties of hypothetical mixtures Martensite + Austenite

Comparison to “3rd Generation” AHSS

Matlock and Speer: ICASS 2006

Elo

ngat

ion

(%)

Tensile Strength (MPa)

0

10

20

30

40

50

60

70

0 600 1200 300 900 1600

MART

IF

Mild IF - HS

BH ISO

-

BH

HSLA

Austenite + Martensite

Ferrite + Martensite Ferrite + Martensite

50% MVF

Stable Austenite + Martensite

desired 3rd generation microstructures: High strength phase + ductile austenite 18

Quenching and Partitioning

19

Future AHSS: Martensite/Austenite µstr

Quenching and Partitioning (Q&P) process

Speer et al., 2003 53Speer et al., 2003 Acta Mater., 51, p. 2611

Stabilize austenite in a martensitic matrix Alloying to suppress cementite: Si additions

Example  Proper,es  Q&P  Steels  

20  

C Mn Si 0.20 3.00 1.60 0.29 2.95 1.59

0 2 4 6 8 10 12 14 16 18600700800900

100011001200130014001500160017001800

Engi

neer

ing

stre

ss, M

Pa

Engineering strain, %

450°C/10s400°C/100s

400°C/30s

400°C/10s

0.3C‑3Mn‑1.6Si QT of 200°C  

800 900 1000 1100 1200 1300 1400 1500 1600 1700 1800789

10111213141516171819202122

0.3C-3Mn-1.6Si

0.2C-3Mn-1.6Si

Tota

l elo

ngat

ion,

%Tensile strength, MPa

De  Moor  et  al.,  ISIJ,  vol.  51,  pp.  137-­‐144,  2011  

Q&P  Steels  Proper,es  Overview  

21

800 1000 1200 1400 1600 1800 2000 2400

5

10

15

20

25

30

35

40

45

50 Austempered TRIP Sakuma et al. Matsumura et al.

Bainite Bhadeshia-Edmonds Miihkinen-Edmonds Caballero et al.

Mixed Microstructures Sugimoto et al. Jun-Fonstein Cobo et al.

Quenching & Partitioning Jun-Fonstein Streicher et al. De Moor et al.Thomas et al. Li et al. Wang et al.- ind. trial Santofimia et al.Martins et al.

Lower Mn TWIP/TRIP Merwin Gibbs et al. Jun et al.

To

tal E

lon

gat

ion

, %

Tensile Strength, MPa

data plotted with literature-reported elongations

800 1000 1200 1400 1600 1800 2000 24002468

101214161820222426283032

Tota

l Elo

ngat

ion,

%

Tensile Strength, MPaReferences detailed in: E. De Moor, P.J. Gibbs, J.G. Speer, D.K. Matlock, J.G. Schroth, AIST Trans., Vol. 7, No.11, 2010, pp. 133-44

Model  predic-ons  Martensite  +  Austenite  

Strategies

Desired microstructures consisting of a high strength phase and ductile austenite through: Quenching and Partitioning Carbide free bainite TRIP bainitic ferrite-TBF Medium Manganese steels Others..

Medium Manganese Steels

Fine grained ferrite and austenite Austenite stabilization by manganese enrichment during intercritical annealing Amount and stability of austenite Example: 0.1C-7.1Mn

P. J. Gibbs, E. De Moor, M. J. Merwin, B. Clausen, J. G. Speer, D. K. Matlock, Met. and Mat. Trans., Vol. 42, pp. 3691-02 2011.

0.1C-7.1Mn-0.1Si Annealed for 1 week

Medium Manganese Steels

(33 %)

(26%)

(40%)

(43.5%) (< 2%) • ASTM E-8 sub-sized

samples 32 mm reduced section

• Initial austenite contents in (%)

• Annealing temperatures in ºC

P. J. Gibbs, E. De Moor, M. J. Merwin, B. Clausen, J. G. Speer, D. K. Matlock, Met. and Mat. Trans., Vol. 42, pp. 3691-02 2011.

•  Neutron diffraction in-situ, under load •  Deformation paused for each diffraction measurement

Strain-dependent Austenite Transformation

7.1 Mn 600 oC

P. J. Gibbs, E. De Moor, M. J. Merwin, B. Clausen, J. G. Speer, D. K. Matlock, Met. and Mat. Trans., Vol. 42, pp. 3691-02 2011.

Strain-dependent Austenite Transformation

P. J. Gibbs, E. De Moor, M. J. Merwin, B. Clausen, J. G. Speer, D. K. Matlock, Met. and Mat. Trans., Vol. 42, pp. 3691-02 2011.

Amount and stability of austenite

Summary

27

Overview of Advanced High Strength Steels

Multiphase microstructures consisting of a high strength and a ductile phase such as austenite of interest for next generation properties development Promising metallurgical strategies

Acknowledgements

28

Innovative Manufacturing Initiative by the Advanced Manufacturing Office, US Department of Energy Award Number DE-EE0005765 Disclaimer This report was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor any agency thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof.

Sponsors of the Advanced Steel Processing and Products Research Center, an industry-university cooperative research center at the Colorado School of Mines