Strain Aging Behavior of Bake Hardening Steel for Automotive Application (Presentation, MST'2007)

Materials Science & Technology 2007; September 16-20, 2007 — Detroit, MI, USA

Strain Aging Behavior of Bake Strain Aging Behavior of Bake Hardening Steel for Hardening Steel for

Automotive Application Automotive Application

A.A. VasilyevVasilyev, N. , N. KuzminKuzminDepartment of Metal Physics and Computer Technologies in MateriaDepartment of Metal Physics and Computer Technologies in Materials Science,ls Science,

St. Petersburg State St. Petersburg State PolytechnicalPolytechnical University, University, St. Petersburg, Russian FederationSt. Petersburg, Russian Federation

H.H.--C. LeeC. LeeSchool of Materials Science and Engineering, Seoul National UnivSchool of Materials Science and Engineering, Seoul National University, ersity,

Seoul, KoreaSeoul, Korea


Objective of the StudyObjective of the Study

The present work was carried out to study the The present work was carried out to study the strain aging behavior in the industrially produced strain aging behavior in the industrially produced

ultra low carbon BHultra low carbon BH--steel for automotive steel for automotive application in order application in order to clarify its nature and try to to clarify its nature and try to

solve the problem concerning competition between solve the problem concerning competition between dislocations and grain boundaries for carbon.dislocations and grain boundaries for carbon.


Experimental MaterialExperimental Material

Investigated steel was taken from Investigated steel was taken from the industrial production line.the industrial production line.

Chemical Composition, Grain SizeChemical Composition, Grain Size0.002C, 0.60Mn, 0.052P, 0.013S, 0.039Al, 0.0006B, 0.0011N (wt.%)0.002C, 0.60Mn, 0.052P, 0.013S, 0.039Al, 0.0006B, 0.0011N (wt.%)

The average grain size at the surface of the sheet is close to The average grain size at the surface of the sheet is close to 13 13 µµмм..______________________________________________________________________________________________________________________________

The steel was subjected to approximately The steel was subjected to approximately five day natural prefive day natural pre--agingagingduring machining and transportation of the samples.during machining and transportation of the samples.


Experimental ProceduresExperimental ProceduresThe aging process at different temperatures was investigated byThe aging process at different temperatures was investigated by::

•• dynamic dynamic YoungYoung’’s modulus (YM)s modulus (YM) and dislocation amplitudeand dislocation amplitude--independent independent internal frictioninternal friction change measurements by acoustic methods;change measurements by acoustic methods;

•• tensile tensile mechanical propertiesmechanical properties testing.testing.

Vibrationgenerator

Pieso-electricaldetector andresonancefrequencydetectingsystem

f

ThreadSample

Vibrationgenerator

Pieso-electricaldetector andresonancefrequencydetectingsystem

f

ThreadSample

Acoustic methodsAcoustic methods

2 20

20 0

f ( t;T ) fE( t;T ) E( t;T )E f

N

1 A 1ln ln2N A N

Logarithmic decrement (LD)Logarithmic decrement (LD)::

Relative YM changeRelative YM change::


The Experimental ResultsThe Experimental ResultsThe YM relative change during the aging at different temperatureThe YM relative change during the aging at different temperaturess

The points corresponding to transitions between the aging stagesThe points corresponding to transitions between the aging stages are indicated by are indicated by empty circles.empty circles.


The YM and LD change during the natural (a) and 100 The YM and LD change during the natural (a) and 100 ººC (b) agingC (b) aging

Four aging stages may be allocated according with the obtained data on the YM and LD change.

Both characteristics demonstrate non-monotonous behavior, reflecting the complicate strain aging process developing in the steel.

11 22 33 44


The YPE (a) and YS (b) changes during the aging at different The YPE (a) and YS (b) changes during the aging at different temperaturestemperatures

At all investigated temperatures detectable change in the YPE and the YSappears only after the substantial incubation periods.


d d C Cl (0 ) / l ( t;T ) 1 n ( t;T ),

dl ( t;T )

Analysis of the Aging StagesAnalysis of the Aging StagesThe first stageThe first stage

is the average distance between is the average distance between dislocation pinning points;dislocation pinning points;

Cn ( t;T )is an additional number of the is an additional number of the pinning points created by the pinning points created by the carbon segregated to the carbon segregated to the dislocations.dislocations.

The first stage aging process follows CottrellThe first stage aging process follows Cottrell’’s kinetics with s kinetics with carbon volume diffusion AE close to 68.5 kJ/mol.carbon volume diffusion AE close to 68.5 kJ/mol.

Appearance of Appearance of additional amount of the carbon atomsadditional amount of the carbon atoms available for available for segregation on dislocations, which increase with aging temperatusegregation on dislocations, which increase with aging temperature is re is

rationalized with rationalized with thermally activated disassociation of the Cthermally activated disassociation of the C--MnMn complexes.complexes.


A A large additional amount of the interstitialslarge additional amount of the interstitials available for segregation on available for segregation on the dislocations the dislocations appears in solution at the second stageappears in solution at the second stage after the after the

incubation time incubation time due to operating of some source.due to operating of some source.

The second stageThe second stage

At the beginning of the stage LD At the beginning of the stage LD decreases a little and then decreases a little and then

remains unchanged.remains unchanged.

Substantial LD drop was Substantial LD drop was observed during further observed during further

aging only after the aging only after the incubation period.incubation period.

22


SSWt* t t (T ),

is a time corresponding to the beginning of the second segregation wave.

SSWt (T )

During the second segregation wave period a number of the additional pinning points on the dislocations follows Cottrell’s kinetics.

This circumstance proves that the second LD drop is caused by the second stage of the atmospheres’ formation, taking place due to

appearance of a significant additional amount of the free interstitials.


dgbs lE ( t;T ) E( t;T ) E ( t;T ),

gbsE ( t;T )

is the contribution caused by carbon atoms desegregation from the grain boundaries;

Experimentally detected YM change is a result of the superposition of the two large contributions, having the opposite signs.

The negative one is caused by carbon desegregation from the grain boundaries. This contribution is small at the beginning of aging and starts to grow simultaneously with developing of the second wave of interstitial

segregation to the dislocations.

dlE ( t;T )

is the contribution caused by carbon atoms segregation on the dislocations.


The third and forth stagesThe third and forth stages

Under natural and 100 Under natural and 100 ººC aging the C aging the LD tends to grow a little at the LD tends to grow a little at the beginning of the third stage.beginning of the third stage.

These results allow relating the These results allow relating the third and forth stagesthird and forth stages with a with a carbide/carbide/carbocarbo--nitride precipitation on the dislocationsnitride precipitation on the dislocations that begins that begins after formation of the after formation of the

saturated atmospheres.saturated atmospheres.

At the 100 At the 100 ººC aging the YM initial C aging the YM initial increase is replaced by its increase is replaced by its continuous decrease at the forth continuous decrease at the forth stage. One may suppose that the stage. One may suppose that the mechanism, providing negative mechanism, providing negative contribution in the YM change, contribution in the YM change, continues to operate during further continues to operate during further aging and causes the YM to aging and causes the YM to decrease at long aging times, when decrease at long aging times, when the LD falls down.the LD falls down.

33


11YPEt (T ) 1.46 10 exp( 83,039 / RT ) (min)

Analysis of the Tensile Testing ResultsAnalysis of the Tensile Testing Results

11SSWt (T ) 3.04 10 exp( 81,089 / RT ) (min)

The incubation time for The incubation time for YPE appearanceYPE appearance correlates very well with the time correlates very well with the time of beginning of the of beginning of the second segregation wave.second segregation wave.


There is a good correlation between There is a good correlation between gbsYPE and E ( t;T )

proving proving close interrelationship between the YPE behavior and the processclose interrelationship between the YPE behavior and the process of of carbon desegregation from the grain boundaries.carbon desegregation from the grain boundaries.

Correlation of the YPE behavior and carbon desegregation from GBCorrelation of the YPE behavior and carbon desegregation from GB


After the initial stage of the atmospheres formation dueAfter the initial stage of the atmospheres formation dueto consumption of the carbon atoms from solution the to consumption of the carbon atoms from solution the distribution of the dislocation sources over the critical distribution of the dislocation sources over the critical

stress of their operation is rather wide stress of their operation is rather wide (1). (1). As a result no YPE was detected after this stage.As a result no YPE was detected after this stage.

The carbon desegregation from the grain boundaries, The carbon desegregation from the grain boundaries, which occurs after some incubation time, provides thewhich occurs after some incubation time, provides the

second wave of segregation on the dislocations, second wave of segregation on the dislocations, covering first the dislocations located near the grain covering first the dislocations located near the grain

boundaries. boundaries.

This process This process narrowed down the distribution of the narrowed down the distribution of the dislocation sources (2,3)dislocation sources (2,3) differently located over a grain differently located over a grain

and resulted, finally, in the YPE return.and resulted, finally, in the YPE return.

11 3322

100 100 ººCC

The physical nature of the incubation period needed for the YPE The physical nature of the incubation period needed for the YPE returnreturn

Sketch illustration Sketch illustration for the change in for the change in distribution of the distribution of the

dislocation dislocation sources over the sources over the

critical stress critical stress (CS). (CS).

CS near GB

CS in GV


ConclusionsConclusions

1.1. The amplitudeThe amplitude--independent dislocation internal friction and the dynamic independent dislocation internal friction and the dynamic YoungYoung’’s modulus measurements provide together an effective tool for s modulus measurements provide together an effective tool for strain aging experimental investigation.strain aging experimental investigation.

2.2. A large additional amount of the interstitial atoms available foA large additional amount of the interstitial atoms available for r segregation on the dislocations appear in a solution after substsegregation on the dislocations appear in a solution after substantial antial incubation time. This fact is rationalized assuming that incubation time. This fact is rationalized assuming that the process of the process of the carbon segregation to the dislocations is accompanied by thethe carbon segregation to the dislocations is accompanied by their ir desegregation from the grain boundaries. The second wave of carbdesegregation from the grain boundaries. The second wave of carbon on segregation to the dislocations crucially affects the mechanicalsegregation to the dislocations crucially affects the mechanicalproperties change during strain aging of the investigated steel properties change during strain aging of the investigated steel and and deserves special attention due to the practical importance.deserves special attention due to the practical importance.

3.3. Despite of the meaningful trapping effect rendered by the manganDespite of the meaningful trapping effect rendered by the manganese ese atoms in solution the activation energy of the carbon volume difatoms in solution the activation energy of the carbon volume diffusion in fusion in the steel is substantially lower than in iron. the steel is substantially lower than in iron. The low value of the The low value of the activation energy may be related with additional effect caused bactivation energy may be related with additional effect caused by the y the phosphorus. phosphorus.


AcknowledgmentsAcknowledgments

The authors wish to acknowledge the financial support The authors wish to acknowledge the financial support received from the received from the

Korean Science and Engineering Foundation (KOSEF)Korean Science and Engineering Foundation (KOSEF)under under

"Overseas Laboratory Program""Overseas Laboratory Program"

Strain Aging Behavior of Bake Hardening Steel for Automotive Application (Presentation, MST'2007)

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