REHEATING EFFECT ON THE STRENGTH AND FORMABILITY OF DP980

BACKGROUND

• Ultra-High-Strength steels (UHSS) are introduced to autobody structures to reduce

weight and increase safety, but the application is constrained by (1) the limitation of

material’s forming limit; (2) the limitation of available stamping press tonnage (for

large body panels).

• One possible solution is to reheat-soften certain local high-strain regions where

potential fracture may occur, and the total tonnage can also be reduced.

• Depending on thermal history microstructure products can be pearlite, bainite,

residual austenite, and martensite of different morphology .

OBJECTIVES

• This study is to investigate DP980’s response to thermal history, microstructure and

properties. To obtain the desired formability and strength.

• Prepare test coupons with various induction heating profiles

• Develop new tensile test coupon to investigate local properties

• Investigate the induction heating process window that could be applied for other

laboratory scale formability tests.

• Verify effect of heating via tensile and harness test.

• To achieve hopeful goals of total elongation improve 20% by induction softening and

less than 5% unevenness inside heat treated zone.

EXPERIMENTS SETUP & SAMPLE HOLDING

TEST AND METHODS

• Test coupons were made of ASP 1.2 mm DP980-GA

• A tensile test with an arc-sided coupon was used to measure elongation and R

values

• Advantage of this test over conventional tests with gauge length is the

measurement can be focused on a location where the heat profile is known

Measure w & t with two extensometers, or w & R where R from interrupt test:

R = ew/et = ln(w/wo)/ln(t/to)

True strain el :

el = - (ew + et) = - ew (1+ 1/R) = - (1+ 1/R) ln(w/w0)

True stress:

t/t0=(w/w0)1/R

sl = F/(wt)=F/[wt0(w/wo)1/R]

ABSTRACT: Ultra-High-Strength steels (UHSS) are introduced to

autobody structures to reduce weight and increase safety, but

UHSS has increased forming difficulties. One possible solution

is to reheat-soften certain local high-strain regions where

potential fracture may occur. This study is to investigate DP980’s

response to thermal history, microstructure and properties. To obtain the

desired formability and strength. The effects of this local softening

were studied through: tensile and hardness tests, and

microstructure analysis. A new local tensile property evaluation

technique was developed. Peak heating temperature and cooling

rate was found to be the two most important parameters

effecting the properties of the material. These were held constant

during the various heat treatment. Within the range of 800-

1000oC and in cooling rate of 2-20oC results showed reduced

strength and increased fracture strain relative to the as-received

DP980. The best result where seen at 900oC with a fast cooling

rate. Microstructure observation indicates that; cooling rate

affects reduction of martensite volume, which affect both

strength and formability.

RESULTS

• Approach asymptote value of 0.4

• Greater initial R value has a greater elongation

• No clear trend for low values of strain •Fast cooling shows high Hardness • Reduction in hardness from heating

• Martensite clusters have been dissolved via heating

• Fast cooled shows finer grain structure

• Samples heated to 800C have finer grain than 1,000C

CONCLUSION

•Testing technique proved sufficiently consistent for results to be valid.

•Peak heating temperature and cooling rates were found to be the two most important

heating parameters.

•Induction local heating achieved the goal: the strength is reduced, and formability is

significantly increased of DP980 steel sheet.

• Best result were seen at 900oC combined with a fast cooling rate. This combination

increased the elongation almost 100%, and has the highest initial R value

•Microstructure observation indicates that; cooling rate affects reduction of martensite

volume, which affect both strength and formability.

RESULTS

Stress Vs. Strain DP980 different cooling rates

• Similar stress vs. strain curve for all trials

• Small Decrease of UTS

• Increase in elongation for all trails

• Increase in temperature decrease UTS

• Fast cooling shows lower UTS

•Fast cooling shows high elongation before failure

ASME 7416

900°C 30s

400°C

Programmable

Temp. Controller

T/C Coil

Feedback

Induction Power Supply

DP980

PC

8-Channel T/C

BN coating to reduce oxidation

Induction Power Supply Controller

Heating Coil

Furnace Tube

Configuration:

Curved arc test coupon

Slow cooling (20oC/s)

0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.18

Fast cooling (2oC/s)

Tru

e S

tress (

MP

a)

Tmax

Blue: 800C

Green: 850C

Cyan: 900C

Magenta: 950C

Yellow: 1000C

No heating

True Strain

0

200

400

600

800

1000

1200

0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.18 0.2 0

200

400

600

800

1000

1200

True Strain

Tru

e S

tress (

MP

a)

No heating

Tmax

Blue: 800C

Green: 850C

Cyan: 900C

Magenta: 950C

Yellow: 1000C

Elongation vs. Tmax UTS vs. Tmax

800 820 840 860 880 900 920 940 960 980 1000 850

900

950

1000

1050

11100

1150

Peak Temp.

Str

ess M

ax (

MP

a)

True stress without heating

Slow cooling (2oC/s)

Fast cooling (20oC/s)

800 820 840 860 880 900 920 940 960 980 1000 0.1

0.15

0.2

0.25

Peak Temperature

ema

x

Reference without heating

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

1.1

1.2

0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.5

R V

alu

e

Strain Length (εl)

Elongation vs. R value

No heat

700C Slow

800C Slow

800C Fast

900C Slow

900C Fast

As-received (no heating)

Low mag High mag

MICROSTRUCTURE NO HEATING

MICROSTRUCTURE 900 C SLOW & FAST COOLING

Jorge Cisneros

Advisor: Dr. Xin Wu Wayne State University

Detroit, MI

jscisneros@gmail.com

0

50

100

150

200

250

300

350

400

450

0 200 400 600 800 1000 1200

Har

dn

ess

(H

V)

Tempurature (oC)

Slow vs Fast Cooling Rate

2 C/s

20 C/s