잔류오스테나이트를 포함하고 있는 강재의 레이저 용접부 파단특성

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2016. 03. 30.

강 민 정, 한 흥 남, 김 철 희

KITECH

Korea Institute of Industrial Technology

잔류오스테나이트를 포함하고 있는

강재의 레이저 용접부 파단특성

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Elongation(%)

Tensile Strength (MPa) 0

10 20 30 40 50 60 70

600 800 1200

400 1000 1400

DP, CP TRIP

MART IF

Mild

HSLA BH

CMn IS HS IF

200

HPF TSxEl=60,000

TWIP TSxEl=10,000

TSxEl=20,000 TSxEl=40,000

X-AHSS

Elongation(%)

Tensile Strength (MPa) 0

10 20 30 40 50 60 70

600 800 1200

400 1000 1400

DP, CP TRIP

MART IF

Mild

HSLA BH

CMn IS HS IF IF

Mild

HSLA BH

CMn IS HS IF

200

HPF TSxEl=60,000

TWIP TSxEl=10,000

TSxEl=20,000 TSxEl=40,000

X-AHSS

H.N. Han et al. Materials Science and Engineering A 499 (2009) 462–468

 Higher strength & elongation

 Lower density

• Development of steels for vehicle weight reduction

Lightweight steels !!

Recent trend for developing steels

 High Al contents !

 Austenite/Duplex steels 차체 안정성 & 경량화

 우수한 충격흡수능 & 연신율

 다양한 합금첨가

 용접성 저하

Weakness

: 높은 비저항으로 인해 용접가능영역 이 좁아짐

: 용접 중 가해지는 입열로 인한 금속

의 상변태

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KITECH Korea Institute of Industrial Technology

3

Density reduction by Al addition

Mass reduction (addition of substitutional light elements such as Al and Si)

Volume expansion

a_g (Å) = 3.5780 + 0.0330C + 0.00095Mn - 0.0002Ni + 0.0006Cr + 0.0056Al + 0.0220N - 0.0004Co + 0.0015Cu + 0.0031Mo + 0.0051Nb + 0.0039Ti

+ 0.0018V + 0.0018W (unit: wt.%)

(D.J.Dyson and B.Holmes, J. Iron Steel Inst., 1970)

Ferrite Austenite

 Lattice expansion by Al addition

Fe Al

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 Substitution: ~ 1.0%/wt.%Al

 Lattice expansion: ~ 0.5%/wt.%Al Total density reduction:

~ 1.5%/wt.%Al

Density reduction by Al addition

J. Herrmann et al, Acta Materialia 51, 2003, 2847-2857

Density reduction Fe-Al system

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Classification of lightweight steel

Matrix phase

C (wt.%)

Mn (wt.%)

Al (wt.%)

Other (wt.%)

UTS (MPa)

El

(%) Company Year

Ferritic

< 0.03 0.03~0.2 5~9 - 300~500 20~30 MPIE 2002

< 0.2 < 3.5 2.5~10 - 400~600 20~30 JFE 2007

< 0.004 0.01~3 2~10 Si 0.01~7 400~500 20~30 NSC 2010

Austenitic

&

Multi-phase

0.02~0.04 15~25 3 Si 3 600~1,100 60~90 MPIE 2003

0.8~1.2 10~30 8~12 - 700~900 60~80 NSC 2006

0.5~2.0 18~35 8~12 (Si 3~6) 600~1,100 25~70 MPIE 2007

0.3~0.4 3~4 5~6 Mn < Al 700~800 27~34 POSCO 2009

0.15~0.5 6~12 5~6 Si 0.05~0.5 700~1,000 35~60 KIMS 2012

0.8~0.9 15~17 8~10 Ni 4.9 1300~1500 20~35 GIFT 2015

Ref.) Scripta Materialia 66 (2012) 519–522

 Austempering

 Retained austenite at room temp.

Heat treatment

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Scope of study

 잔류오스테나이트를 포함하고 있는 강재를 대상으로 레이저

용접을 수행하고 기계적 물성 및 조직학적 거동을 확인

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• Properties of applied materials

Applied materials (Lightweight steel-LWS)

g phase (111)

a phase (110)

Pole figure

Phase map

● Austenite

● Ferrite

Max. 4.05 Max. 5.37

ND TD

RD

Phase fraction (Austenite:Ferrite)

Grain size (㎛, < 15^o) Austenite Ferrite

33 : 67 1.75 5.02

오스테나이트와 페라이트의 이상조직 (Dual phase) 구성

Chemical composition (Wt.%) Mechanical properties (RD)

C Mn Al W Ti Si Fe UTS (MPa) YS (MPa) E.L. (%)

LWS 1 0.33 3.64 5.40 0.16 0.027 - Bal. 778 503 34%

LWS 2 0.34 3.72 5.22 - - 0.11 Bal. 870 526 43%

LWS 3 0.33 3.68 5.10 - - 0.08 Bal.

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• 4 kW Disk Yb:YAG laser - Fiber diameter: 200 ㎛

- Beam diameter: 0.54 mm, 0.60 mm

[Disk Laser] [6-axis robot]

Equipment

Welding condition

- Joint: bead on plate (150 X 240 X 1 mm) - Focal position: 0 mm

- Shielding gas: None

- Variable: laser power, welding speed - Full penetration

Experimental setup

[Schematic diagram of set up]

Analysis method

- Mechanical properties

: Tensile test, Vickers hardness test - Morphology

: OM, SEM-EDS, EBSD, EPMA

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9 Welding condition

- 0.54 mm beam diameter - 3 kW laser power

- 6 m/min welding speed

-용융부에 주조조직 형성 -열영향부 형성 확인

Macro-sections

Dendritic

structure

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 SEM images of welds

Macro-sections

Base metal H.A.Z. Fusion zone

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11 Base metal H.A.Z. Fusion zone

ND RD

TD (WD)

● Austenite (γ)

● Ferrite (α/α’)

용접부에서 오스테나이트의 상분율 감소.

33 : 67 1.75 5.02

20 : 80 1.07 4.86

5 : 95 0.68 6.13

 Results of EBSD

EBSD phase map

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0.0 0.2 0.4 0.6 0.8 400

800 1200 1600

Temperature (o C)

Carbon content (w.t.%)

Ferrite

Ferrite + Austenite

Liquid Liquid + Ferrite

Austenite

Carbide

Temperature (℃)

Time (t) FZ

△T/ △t > 50 ℃/s

HAZ

△T/ △t < 50 ℃/s

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Phase diagram

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13 높은 입열이 가해지는 경우, 낮은 강도 측정됨.

 Fusion line failure

16 20 24 28

400 600 800 1000

F rac tu re st ren g th ( M P a)

Heat input (kJ/m)

P: 3kW v: 10 m/min

P: 3kW v: 9 m/min

P: 4kW

v: 10 m/min P: 4kW v: 9 m/min

Tensile test with various welding condition

0 10 20 30 40 50

0 200 400 600 800 1000

Tensile strength (MPa)

Engineering strain (%)

3kW, 18 kJ/m 3kW, 20 kJ/m

0 10 20 30 40 50

0 200 400 600 800 1000

Tensile strength (MPa)

Engineering strain (%)

4kW, 24 kJ/m 4kW, 26.7 kJ/m

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Hardness of commercial steels

-3 -2 -1 0 1 2 3

200 250 300 350 400 450 500

Hardness (Hv)

Distance from weld center (mm)

CR 780DP 1.8t

-3 -2 -1 0 1 2 3

200 250 300 350 400 450 500 550 600

Hardness (Hv)

SGAFC 1180 1.2t

-3 -2 -1 0 1 2 3

100 150 200 250 300 350 400 450 500

Hardness (Hv)

SAPH 440 1.8t

-2 -1 0 1 2

0 100 200 300 400 500

After tensile test Before tensile test

Ha rd n es s (Hv )

Distance from center (mm)

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Observation of failure specimen

HAZ

HAZ

TD ND

RD

FZ

HAZ FZ

FZ

FZ FZ FZ

TD RD

ND

HAZ HAZ HAZ

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Observation of failure specimen

A

균열부에서

낮은 Al 함량 측정

 상간 경계면 취약

 균열전파

B

0.17~1.86 at.% Al 4.45~4.80 at.% Mn 94.05~95.30 at.% Fe

10.44~13.34 at.% Al 3.74~3.94 at.% Mn 82.93~85.61 at.% Fe

12.38 at.% Al

4.04 at.% Mn

83.58 at.% Fe

7.54 at.% Al

3.75 at.% Mn

88.71 at.% Fe

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Hardness and retained austenite fraction

WM HAZ BM

0 100 200 300 400 500

Hardness (Hv)_before T.t.

Hardness (Hv)_After T.t.

Austenite fraction (%)

0 10 20 30 40 50

A u ste n ite fr a cti o n ( % )

WM HAZ BM

0 50 100 150

Increased hardness value Austenite fraction

In cr e a se d h a rdn e ss v a lu e ( H v)

0 10 20 30 40 50

A u s te n it e f ra c ti o n ( % )

Reference 에서는 ferrite가 변형을 하고, austenite는 변태를 한다고 언급

 BM에서는 micro crack이 확인되지 않음.

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EPMA analysis of BM failure specimen

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EPMA analysis of FL failure specimen

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 잔류오스테나이트를 포함하고 있는 강재를 대상으로 레이저 용접 수행.

- 일부 조건에서 모재파단 발생.

- 소재의 경도는 소성강화 거동으로 인해 인장시험 후 증가하며, 위치별로 변형율이 다르게 나타남.

-

파단은 Al rich phase 주변에서 발생되어 입계를 따라 전파.

Summary and future work

 공정최적화를 통해 안정적인 모재파단 조건 확보.

 파단에 대한 원인 분석.

Summary

Future works

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Equipment

Welder

+ ARC - MAG - MIG - TIG - SMAW + Laser

- Disk laser (Nd:YAG) - Micro spot laser - Green laser

+ Spot - AC - DC

- Delta spot + FSW

Upper plate

lower plate

Thermocouple

TIG heat source

+

-

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THANK YOU FOR YOUR ATTENTION !

KITECH Korea Institute of Industrial Technology

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23 WM

0.0 0.2 0.4 0.6 0.8 1.0

600 800 1000 1200 1400 1600

T e mp e ra tu re (

o

C )

Carbon content (w.t.%)

Ferrite

Ferrite + Austenite

Liquid Liquid + Ferrite

Austenite

Carbide

상온에서 austenite phase가 없는 조성이지만,

Austempering을 통해

Retained austenite를 형성시킴.

 Quenching (welding)

 Martensite transformation

Phenomenon in welds

Fe-C phase diagram

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