improvement of mechanical properties and microstructure of 22mnb5 steel by hot stamping and direct...

7/24/2019 Improvement of Mechanical Properties and Microstructure of 22MnB5 Steel by Hot Stamping and Direct Cooling -

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Improvement of Mechanical Properties and

Microstructure of 22MnB5 Steel by

Hot Stamping and Direct Cooling

Fernando Aurelio Flandoli* and Sergio Tonini Button*** Centro Universitario da FEI, So Bernardo do Campo SP Brazil 09850-901

** Universidade Estadual de Campinas Campinas SP Brazil 13083-970

[email protected]

[email protected]

Abstract: In this work it was studied how hot stamping and direct cooling couldimprove the mechanical properties of stamped parts made with the high strength

hardenable steel 22MnB5, Hot stamping of a B-pillar sector was first simulated by finite

element method to define the best conditions like blank geometry and stamping

temperature. Experimental tests were carried out with four initial temperatures: room (to

represent cold stamping), 900, 950 and 980oC, in a hydraulic press with tools cooled to

17oC. Stamped parts were tempered by direct cooling between the tools immediately

after hot stamping. Samples taken from stamped parts were analyzed by optical

microscopy and micro-hardness Vickers, and other samples were analyzed by tensile

tests. Tests results have shown that all hot stamped parts presented mechanical

properties higher than cold stamped parts, and that the microstructure and the

mechanical properties were obtained with tests carried out at 950oC.

Keywords: metal forming, numerical analysis, phase transformation

1. INTRODUCTION

Hot stamping presents a wide application in the automotive industry from external

components that define the body of the vehicle to internal structural components which

require durability, rigidity and impact resistance that conventional cold stamping cannot

match without subsequent heat treatment. Many recent researches have been published

analysing important aspects of this process, like materials and products quality.

[Yanagida, and Azushima, 2009] state that numerical simulation is still not

efficient to predict how process variables influence hot stamping because manymetallurgical data and especially tribological parameters are not well established. They

tested two high strength hardenable steels (SPHC and 22MnB5) and three furnace

temperatures and concluded that the new tribological test they developed were effective

to evaluate the friction coefficient in hot stamping.

[Geiger et al., 2008] present a cup drawing test to evaluate tribological conditions

within hot stamping and showed that a significant dependency of blank temperature on


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the friction coefficient could be detected. With increasing sheet temperature, decreasing

friction values were observed at the interaction contact area.

[Barcellona and Palmieri, 2009] considering that little knowledge exists on the

continuous cooling transformations (CCTs) that reproduce the typical work conditions

of the press quenching process, describe experimental methods they employed to obtain

the hardness and microstructural changes of pre-strained and thermally treated

microalloyed boron steel. They investigated strains, transformation temperatures,

microstructure and micro-hardness of 22MnB5 steel samples under uniaxial tensile tests

at temperatures between 873 and 1223 K with a constant strain rate of 0.08 s-1

, and

concluded that high values of hot deformation during hot stamping, especially at lower

temperatures, require a strict control of post-cooling to ensure cooling rates that will

result in stamped parts with good mechanical properties.

[Bardelcik et al., 2010] present a similar work to investigate the effect of cooling

rate on the high strain rate behavior of hardened boron steel. In quenching tests

22MnB5 steel samples were heated to 950 oC and quenched in three different media:water bath at 22

oC, heated oil bath at 85

oC, and compressed air at low and high flow

rates. They concluded that mechanical properties and microstructure are strongly

dependent on quenching rate, and that ideal conditions can be achieved with the proper

selection of furnace temperature and quenching rate.

[Lee et al., 2009] present a numerical model based on finite element method to

analyze hot pressing considering phase transformation plasticity (TRIP) when stamping

a high carbon steel. They also present an extensive experimental procedure to validate

the numerical analysis, and concluded that phase transformation significantly influence

part strengthening by transforming hard martensitic phase and reducing dimensional

change by additional plastic deformation during phase transformation.

Numerical simulation has been applied considering more reliable frictioncoefficients, and material constitutive equations [Naderi et al., 2008], resulting in more

effective models to represent hot stamping industrial conditions [Tekkaya et al., 2007],

and [Liu et al., 2009]. New procedures have been proposed to employ induction heating

of blanks instead of convective heating in continuous furnaces [Kolleck et al., 2009].

The main objective of this work was to study how hot stamping and direct cooling

could improve the mechanical properties of a B-pillar sector made of 22MnB5. First,

hot stamping was simulated by finite element method to define the best process

conditions like blank geometry and stamping temperature to be applied in the following

experimental procedure were blanks were hot stamped, and then analysed by optical

microscopy, tensile tests, and micro-hardness test.

2. MATERIALS AND METHODS

2.1. Numerical analysisHot stamping of a B-pillar sector (Figure 1) was simulated with software Deform 6 3D

based on the finite element method to define the best process conditions, and to evaluate


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the forming load and variatio

the parameters used in the mo

The material simulated

experimentally, modelled wi

2008]. The constant frictiontribological conditions within

Based on the CCT cu

temperatures were chosen: 90

(80mm x 120 mm) were auste

Blanks were modeled w

considered rigid and modele

initial and final steps of sim

the lower die, and completely

Table I Process and m

Blank M

Wi

46Furnace tempe

Room temperaHydraulic pres

Resting time a

of blank temperature along the process. Tabl

dels simulated in this work.

Figure 1 B-pillar sector.

in all models was the 22MnB5 steel, the s

h the constitutive equations provided by [Na

factor model was adopted equal to 0.7 consthe blank-tools interface.

ves presented by [Naderi et al., 2008] thr

, 950 and 980oC. For all temperatures rectang

nitized for five minutes.

ith elasto-plastic tetrahedral elements and the

d with 45000 tetrahedral elements. Figure 2

lation respectively with the blank initially po

deformed within closed dies.

aterial parameters used in the numerical simul

terial: 22MnB5 steel as cold rolled

dth x length x thickness (mm): 80x124.5x1.9

13 tetrahedral elementsrature (

oC): 900, 950 and 980

ture: 20oC Transfer time to press: 15 s

s Speed: 8 mm/s Stroke: 25 mm

ter stamping: 4 s

I presents

ame tested

deri et al.,

idering the

e furnace

ular blanks

dies were

shows the

itioned on

tion.

egion X


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Figure 2 Nu

2.2 Experimental p

Some experimental tests wershown in Table II) and the s

condition cold rolled were e

were evaluated by optic micro

Table II Some chara

2.2.1. Tensile testinBased on ASTM E8 standard

axes oriented at 0, 45 and 90o

sheets were anisotropic or not

isothermally hot tested at two

Cold tensile tests were c

mechanical properties of t

[ArcelorMittal, 2010]. Hot te

hot stamping temperatures an

[Naderi et al., 2008].

2.2.2. Hot stampingStamping tests were carried o

(Table I). Each blank was

temperature (900, 950 or 9

transferred to the stamping to

Chemical comp

C Si

0.24 0.27

As received con

Sheet nominal t

(a)

Blank

erical simulation: (a) initial step, (b) final step

rocedure

carried out to evaluate the blank material (chaamped products. 22MnB5 steel sheets as rece

aluated by cold and hot tensile tests. Stampe

scopy, tensile test and micro-hardness measure

teristics of the blank material before hot stamp

of the 22MnB5 sheets as received[ASTM, 2009], samples were machined from

in respect to the rolling direction to evaluate

. These samples were cold tested at room temp

furnace temperatures: 900 and 950oC.

arried out to evaluate anisotropy and also to c

e sheets used in this work to those pr

nsile tests were also carried out to evaluate an

d to compare the flow stress curves to those

testsut with the same conditions used in numerical

eld in a furnace for five minutes at the a

0oC). Then, the blank was taken off the f

ling assembled in a hydraulic press (Figure 3-a

sition (in weight %):

Mn P (max) S (max) Cr Ti B

1.14 0.015 0.001 0.17 0.036 0.003

ition: cold rolled

ickness: 1.9 mm

(b)

Upper

die

Lower

die

.

racteristicsived in the

d products

ents.

ing.

sheets with

hether the

rature and

mpare the

sented by

isotropy at

btained by

simulation

stenitizing

rnace and

).


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The dies were lubricat

molybdenum grease to facilit

was measured during the pro

At least three stamping test

stamped the part rested duri

martensite transformation and

Figure 3. (a) Hot stamping

2.2.3. Analysis of hSamples were cut off the

microstructure and micro-har

Tensile tests were carri

Samples were machined fro

were replicated at least twicemodel 810.

Samples were cut off in a

2% to observe, with a optical

obtained after stamping and d

height) and R3 (lower fillet)

B-B in Figure 1. These sa

measurements with hardness t

Figure 4 Stamp

(a)

d before each test with a mixture of mine

ate the extraction of the stamped product. Sta

ess with a load cell and a digital data acquisiti

s were carried out at each furnace tempera

g four to five seconds within the tools to co

then extracted to cool to room temperature (Fi

tooling assembled in a hydraulic press (b) stam

t stamped productsstamped products to evaluate mechanical

ness.

ed out based on ASTM E8 standard [AS

strips cut off from the region X in Figure 1.

for each furnace temperature with testing ma

transversal plane, grinded, polished and etche

microscope Olympus model BX51M, the mic

irect cooling near to points R1 (upper fillet), R

hown in Figure 4 which represents a half of cr

ples were finally evaluated by Vickers mic

ester Buehler model 2100 and an indentation lo

ed part half cross section - regions R1, R2 and

(b)

R1

R2

R3

Upperdie

Lowerdie

al oil and

ping load

on system.

ture. After

mplete the

ure 3-b).

ed part.

properties,

M, 2009].

hese tests

hine MTS

with Nital

rostructure

(wall half

oss section

o-hardness

ad of 3 N.

3.


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3.

RESULTS AND DIS

3.1. Numerical analThe simulations with the re

irregular shape and edges w

problem the blank shape had

Figure 5-b was chosen becaus

All numerical and exp

simulations with this modifie

Figure 5. (a) Misshaped part

Figure 6 shows the load

stamping at furnace temperatpresented the highest loads.

the different temperatures si

yield stress (Table IV).

All temperatures present

was observed that each load v

shown in Figure 6: the high l

associated to the first contact

A first steady state is re

Then the load is reduced to

the lower tool. In the third st

slight increase of the stampi

stroke when the dies are close

(a)

CUSSION

ysisctangular blank (80x124.5x1.9) formed a pr

ith a wrong length as seen in Figure 5-a. To

to be modified and after many trials the shap

e it formed products with correct shape and dim

rimental results shown in this work were

blank.

tamped with a rectangular blank, (b) modifie

curves obtained in simulations of cold stampi

res of 900, 950 and 980

o

C. As expected, colot stamping load did not show a significant

ulated, what can be explained for the small di

ed curves with the same aspect, and during si

ariation was related to a specific deformation g

ad found in the beginning is merely a numeric

lank-tools and can be neglected in this analysi

lated to the free bending of the blank by the

second steady state corresponding to the ben

age, after the blank edges touch the upper die

ng load, and finally a rapid increase near to

d and the part completely formed.

(b)

duct with

solve this

shown in

ensions.

btained in

blank.

g, and hot

stampingariation at

fference in

mulation it

eometry as

al problem

.

upper tool.

ing inside

, there is a

the end of


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Figure 6 Stamping load x process time numerical results.

3.2. Experimental results

3.2.1. Tensile testing of the 22MnB5 sheets as receivedTable III shows the mechanical properties of the as received sheets in the directions

tested. Yield and ultimate tensile strength obtained at room temperature are similar to

those indicated by [Saltzgitter, 2005].

Sample orientationto rolling direction

Yield strength (0.2%offset) [MPa]

Ultimate strength[MPa]

Elongation[%]

0o 418 13 463 20 333

45o 424 17 468 16 353

90o 415 15 462 14 322

Table III Mechanical properties of 22MnB5 sheets as received cold tensile tests.

There are no significant differences among the three directions and therefore the

material is anisotropic so the blank may be cut off in any position of the sheet regardless

the rolling direction.

Table IV shows the results of hot tensile tests. The material is anisotropic for both

furnace temperatures. By increasing temperature the tensile properties decrease

significantly, what is expected considering that softening mechanisms are most effective

at higher temperatures. These results are significantly smaller than those obtained by

[Naderi et al., 2008], maybe because the smaller strain rate used in these tests.


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Sample orientationto rolling direction

Furnacetemperature

[oC]

Yield strength(0.2% offset)

[MPa]

Ultimatestrength [MPa]

Area reductionat fracture

[%]

0o

900 272 442 856950 232 383 785

45o

900 272 452 795

950 203 352 785

90o

900 292 462 766

950 222 352 less than 754

Table IV Mechanical properties of 22MnB5 sheets as received hot tensile tests.

3.2.2. Results of tests with the hot stamped productsTable V presents the mechanical properties obtained in the tensile tests with the samples

extracted from parts stamped at room temperature, 900, 950 and 980 oC. Cold stamped

parts present properties higher than the as received sheets (Table III) because of workhardening caused by cold stamping.

Furnace

Temperature

[oC]

Yield strength

(0.2% offset)

[MPa]

Ultimate strength [MPa]

Elongation

at fracture

[%]

Cold 436 24 491 16 26.6 5

900 1156 34 1543 24 6.4 3

950 1296 28 1700 27 6.1 4

980 1273 14 1734 31 7.5 5

Table VI Mechanical properties of stamped products.

Hot stamped samples presented the highest properties, especially when stamped intests with furnace temperature at 950 or 980 oC that presented similar results,

significantly higher than those presented by one producer of 22MnB5 steel which shows

that after quenching a cold rolled sheet (without hot stamping) this steel can reach 1100

MPa (yield strength) and 1500 MPa (ultimate strength) [ArcelorMittal, 2010].

Table VII presents the micro-hardness Vickers measured in the regions R1, R2 and

R3 of the stamped parts. Higher micro-hardness values were found in samples of tests

with furnace temperature at 950 and 980 oC confirming the best results found for yield

and ultimate strength of these samples.

Furnace

temperature [

o

C]

Region R1 Region R2 Region R3

900 41430 46827 32928

950 42135 52948 45537

980 404 49 549 26 46132

Table VII Micro-hardness Vickers indentation load 3 N.


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Each region presented

depending on local deforma

[Bardelick et al., 2010] micro

under 25 oC/s, not enough to

al., 2008], as observed

(martensite+bainite). Otherw

hardness near to 470 HV

microstructures only formed

at 950 and 980 oC.

Furnace temperature

[oC]

Re

900

950

Figure 7 Microstructure

4.

CONCLUSIONS

Numerical simulation proved

to choose the ideal processi

stress and temperature distrib

achieve a martensitic microstr

Experimental results s

properties higher than cold st

without bainite colonies) and

carried out at 950 and 980oC

the lower energy necessary an

5. ACKNOWLEDGME

Authors wish to thank FAPES

ifferent micro-hardness and microstructures

tion, and cooling rate during hot stamping.

-hardness less than 450 HV is obtained with c

form a microstructure completely martensitic

in Figure 7 for furnace temperature at

ise, those authors observed that samples w

re related to a cooling rate of 45 oC/s and

y martensite, as resulted in tests with temperat

gion R1 Region R2 Regi

of hot stamped and cooled samples Nital 2%

to be an important tool to design the best blan

g conditions. With simulation it is possible

ution and therefore define the best furnace tem

ucture and consequently the higher mechanical

ow that all hot stamped parts presented

amped parts, and that the best microstructure

the best mechanical properties were obtained

, being the lower furnace temperature preferre

d less surface oxidation.

NT

P and CNPq for the financial support to this w

(Figure 7)

According

oling rates

[Naderi et

900 oC

ith micro-

presented

re furnace

n R3

- 500X.

shape and

to analyze

perature to

properties.

echanical

(martensite

with tests

regarding

rk.


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[ArcelorMittal, 2010]ArcelorMittal Flat Carbon Europe S.A., A54 Quenchable boron

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[ASTM, 2009] ASTM International, ASTM E8 / E8M - 09 Standard Test Methods

for Tension Testing of Metallic Mateials; DOI: 10.1520/E0008_E0008M-09.

[Barcellona and Palmieri, 2009] Barcelllona, A. and Palmieri, D., Effect of Plastic

Hot Deformation on the Hardness and Continuous Cooling Transformations of

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[Bardelcik et al., 2010] Bardelcik, A., Salisbury, C.P., Winkler, S., Wells, M.A.,

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[Geiger et al., 2008] Geiger, M., Merklein, M. and Lechler, J.; Determination oftribological conditions within hot stamping; Production Engineering Research

Development., 2; pp. 269-276.

[Kolleck et al., 2009] Kolleck, R., Veit, R., Merklein, M., Lechler, J., Geiger, M.,

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[Liu et al., 2009]Liu, H.S., Xing, Z.W., Bao, J. and Song, B.Y.; Investigation of the

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improvement of mechanical properties and microstructure of 22mnb5 steel by hot stamping and direct...

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