A. K. Srivastava - Microstructural and mechanical characterization of C–Mn–Al–Si cold-rolled TRIP-aided steel.pdf - Ang - rajmundos9

Materials Science and Engineering A 445–446 (2007) 549–557

Microstructural and mechanical characterization of C–Mn–Al–Si

cold-rolled TRIP-aided steel

Ashok Kumar Srivastava a , ∗ , D. Bhattacharjee b ,G.Jha b , N. Gope b , S.B. Singh a

a Department of Metallurgical and Materials Engineering, Indian Institute of Technology, Kharagpur, West Bengal 721302, India

b Research & Development, Tata Steel Limited, Jamshedpur 831001, India

Received 8 July 2006; received in revised form 4 September 2006; accepted 27 September 2006

Abstract

Retained austenite characteristics and tensile properties in a 0.21C–1.77Mn–1.2Al–0.283Si, high strength cold-rolled TRIP steel were inves-

tigated. The microstructure of this steel is comprised of ferrite, bainite and retained austenite, which is obtained by controlled cooling from the

intercritical annealing temperature to the isothermal bainitic holding temperature. The effects of cooling rate from intercritical annealing temper-

ature to isothermal transformation temperature, as well as effect of isothermal transformation time on microstructure and mechanical properties,

were studied using optical microscopy, SEM, TEM, XRD, dilatometry and mechanical testing.

These studies revealed that the microstructure of the steel mainly consisted of bainitic ferrite lath matrix, blocky martensites and stable retained

austenite ﬁlms of 7–8 vol%. When austempered at temperatures above M S temperature, the steel possessed high tensile strength of 600 MPa, total

elongation of 28–32% and n -value 0.2. The present paper deals with the effect of heat treatment on microstructure and mechanical properties of

C–Mn–Al–Si cold-rolled TRIP steel.

Keywords: TRIP steel; Microstructure; Mechanical properties; Retained austenite

1. Introduction

The nomenclature “AHSS” refers to dual phase (DP), trans-

formation induced plasticity (TRIP), complex phase (CP) and

martensitic (M) steels which, considering the current absence

of a clear classiﬁcation scheme for these steels, might be char-

acterized as steels with a yield strength >300 MPa and a tensile

strength >600 MPa. Despite their strength, most AHSS, with the

exception of the martensitic steels, have a superior formability

resulting from a high work hardening rate.

Of late, there has been a lot of research in the area

of relatively new steel grades, like (transformation induced

plasticity)—aided cold-rolled steel, which show higher strength

without much of a loss in ductility. TRIP steels are basically

cold-rolled C–Mn–Si steels heat treated to produce a com-

posite microstructure of ferrite, bainite and retained austenite

[1–4] . Gradual transformation of retained austenite to marten-

site during plastic deformation delays necking and results in

high uniform elongation. And as such, they have been receiving

increasing attention as new high formability cold-rolled steel

sheets for automobile bodies.

The original observations of transformation induced plas-

ticity in highly-alloyed homogeneous metastable austenitic

steels by Wassermann [5] and Zackay et al. [6] were used by

researchers at Nippon Steel Corporation to show that austenite

One of the basic rules of materials science is that an increase

in strength leads to a loss of ductility, and an increase in ductility

can only be achieved through sacriﬁces in strength. However, in

recent years advanced high strength steels (AHSS) have been

developed that seek to achieve good formability even at higher

strength levels. The three principal reasons for the current trend

to use more advanced high strength sheet steel (AHSS) in auto-

bodies are:

(1) The reduction of passenger car weight by the increased use

of high strength thinner gauge sheet steel, leading to reduced

fuel consumption and emissions;

(2) The improvement of the passive safety of vehicles, which

leads to a better passenger safety by an improved crash-

worthiness;

(3) The strong competition from the light-weight materials, in

particular Al and Mg alloys, and plastics.

∗ Corresponding author. Tel.: +91 3222 281706; fax: +91 3222 25503.

E-mail address: aksrivastava@metal.iitkgp.ernet.in (A.K. Srivastava).

doi: 10.1016/j.msea.2006.09.101

550

A.K. Srivastava et al. / Materials Science and Engineering A 445–446 (2007) 549–557

Fig. 1. Schematic of a continuous annealing cycle for cold rolled, intercritically annealed TRIP-aided steel (left). Pseudo-binary Fe–C diagram for Fe–1.5% Mn,

showing the changes in the C content of the austenite (middle) and the corresponding phase diagram indicating the position of the T 0 -line which indicates the

end-point for the bainitic transformation during austempering (right).

annealing of low alloy Si-bearing

medium–C (0.12–0.55%) C–Mn steel [7–18] .

In this new class of low alloy TRIP steels the austenite is

present as a dispersed phase. These TRIP-aided steels are par-

ticularly suitable for demanding automotive applications, which

require the combination of high strength (TS: 500–1000 MPa),

high formability (20–40%) and high dynamic energy absorp-

tion during high strain rate deformation. The processing of

cold-rolled and intercritically annealed TRIP-aided steel is

schematically shown in Fig. 1 . During the intercritical anneal-

ing (IA) the austenite C content is raised to

rite. A fast cooling rate after annealing is employed to avoid any

major ferrite formation and the ﬁnal transformation is carried out

isothermally in the bainite region (second stage heat treatment).

During bainite formation, the carbon diffuses into the austenite

islands. The enrichment of carbon in the austenite increases its

thermal stability and consequently, the austenite can be retained

upon cooling to the room temperature [21–25] .

Studies on currently available classical C–Mn–Al–Si TRIP-

aided cold-rolled steel sheets reveal that quite a large number of

researchers have carried their work on steels containing carbon

in the range of 0.10–0.15 wt% for better weldability. However,

in order to get a high volume fraction of metastable retained

austenite during intercritical annealing, a carbon content in the

range of 0.20–0.25 wt% was selected for the present study. The

purpose of the study was to establish an appropriate intercriti-

cal annealing and isothermal bainite transformation conditions.

Subsequently, the effect of intercritical annealing time on the

nature of retained austenite and mechanical properties were

evaluated.

0.3–0.4 wt% as a

result of the C-partitioning between ferrite and austenite. After

the intercritical annealing, the steel is quenched to a tempera-

ture in the bainitic transformation range and isothermally held

for several minutes. The isothermal bainite transformation (IBT)

temperature is below the T 0 temperature, where T 0 is the tem-

perature at which austenite and ferrite of the same composition

has the same free energy.

At the bainite transformation temperature, austenite is con-

sumed, the austenite region decrease in size and are further

enriched with C until the T 0 point is reached or until the isother-

mal treatment is interrupted; whichever occurs earlier. Alloying

elements like Si, Al or P raise the C-content of austenite after

the bainite transformation. Experiments show that for TRIP steel

compositions, the C is enriched in the range of 1.5–2.0% C in

the retained austenite after isothermal bainite transformations

in the 400–500 ◦ C temperature range. Long isothermal bainite

transformation times result in the decomposition of the austenite

into ferrite and carbides.

The microstructure of TRIP-aided steels is achieved by car-

rying out a two-stage heat treatment after cold rolling as shown

in Fig. 1 [19,20] . The ﬁrst stage of heat treatment is carried out

at slightly higher temperature in the

∼

2. Experimental procedure

The chemical composition of the TRIP steels used in this

study is given in Table 1 . The CMnAlSi TRIP-aided was pre-

pared as 50 kg ingot in an air induction furnace. The cast

ingot was forged into a 300 mm

120 mm

50 mm block.

50 mm were cut from

forged stock. These 50 mm thick blocks were soaked at 1150 ◦ C

for 1 h, and hot rolled to a thickness of 3 mm in eight

passes. Hot rolled plates were annealed at 900 ◦ C for 1 h to

120 mm

Table 1

Chemical composition (wt%) of experimental heat

Trip steel

two-phase regions,

leading to a microstructure of about 50% austenite and 50% fer-

C–Mn–Al–Si

0.21

1.77

0.283

0.025

1.2

0.013

0.003

stabilization also occurred during an isothermal bainite trans-

formation, a process often referred to as “austempering”, which

followed the intercritical

Small blocks of 120 mm

A.K. Srivastava et al. / Materials Science and Engineering A 445–446 (2007) 549–557

551

Table 2

Heat treatment detail of samples

Table 3

Measured Ac 1 ,Ac 3 and M S values for CMnAl Si TRIP steel

TRIP steel

Sample

code

IA temperature

( ◦ C)

IA time

(min)

IBT temperature

( ◦ C)

IBT time

(min)

Ac 1 ( ◦ C)

Ac 3 ( ◦ C)

M S ( ◦ C)

C–Mn–Al–Si

740

1010

400

900

410

900

410

900

410

technique [26] . The volume fraction of retained austenite and

its carbon content after the heat treatment was measured using

X-ray diffraction.

The room temperature tensile properties (proof strength for

0.2% offset strain, ultimate tensile strength and elongation) of

heat-treated samples were evaluated on a gauge length of 50 mm

with 6.4 mm nominal width. Tensile tests were carried out in

an INSTRON universal testing machine using a load range of

5000 kgf. The crosshead speed was maintained at 5 mm/min.

The fractured surfaces of tensile tested specimens were exam-

ined using scanning electron microscopy (SEM).

Thin slices of 0.05 mm thickness were prepared from the

heat-treated specimens as discussed in the paper [27] . The foils

were examined in a JEOL 200 CX transmission electron micro-

scope (TEM) operating at 160 or 200 kV.

900

410

reduce the hardness for easier cold rolling. The hot-rolled air

annealed steel plates were pickled in a 25 vol% HCl solution

and subsequently cold-rolled to achieve a ﬁnal thickness of

1 mm.

The equilibrium Ac 1 and Ac 3 temperatures and the theo-

retical limit for bainitic transformation ( T 0 ) temperatures were

found from the phase diagram calculated using Thermo-Calc

Software. The intercritical annealing and isothermal bainitic

transformation temperatures were selected based on these calcu-

lations. The phase transformation behaviour was also studied by

means of dilatometry. Solid cylindrical samples with a diameter

of 3 mm and a length of 5 mm, cut from hot-rolled sheet, with

an initial pearlite-ferrite microstructure, were heated to 1100 ◦ C

at 1 ◦ Cs − 1 under vacuum. A dialatometer was used to measure

dilatation across the sample diameter.

Comparisons were made between thermo-calc results and

the dilatatometry experiments for CMnAlSi cold-rolled spec-

imens. Ac 1 ,Ac 3 and M S temperatures were also determined

by dilatatometry curves. In order to obtain a TRIP-aided steel,

a two-step heat treatment was used. The cold-rolled CMnAlSi

steel samples were heat treated in two separate salt baths kept at

two different temperatures to simulate intercritical annealing and

isothermal bainitie transformation, respectively. The salt solu-

tion consisted of a mixture of BaCl 2 (80%) and NaCl (20%).

The isothermal bainite treatment was carried out at a tempera-

ture 10 ◦ C higher than the nominal M S (400 ◦ C) temperature,

followed by air-cooling. The four sets of heat treatment are

summarised in Table 2 . Cold-rolled heat-treated samples were

prepared for microstructural study with LePera colour etching

3. Results

3.1. Determination of IA and IBT temperatures

The pseudo-binary equilibrium phase diagram and the

allotropic phase boundary ( T 0 line) at which the austenite

and ferrite of identical composition have equal Gibbs energy

were calculated with the help of thermodynamic Software

Thermocalc TM , using TCFE3 database for the CMnAlSi steel

under investigation. The pseudo-binary phase diagram for the

steel is given in Fig. 1 . Samples were heated to 1100 ◦ Cat

a rate of 1 ◦ Cs − 1 to determine Ac 1 and Ac 3 and found to

be 740 and 1010 ◦ C, respectively, as illustrated in Fig. 2 a.

The M S temperature was found to be 400 ◦ C. The experimen-

tal results for the TRIP steel compositions are summarised in

Table 3 .

Fig. 2. (a) Dilatation vs. temperature for determination of Ac 1 and Ac 3 (heating rate 1 ◦ Cs − 1 ) of CMnAlSi cold-rolled TRIP steels and (b) comparison of transformation

temperatures of dilatatometry with thermo-calc calculation of CMnAlSi cold-rolled TRIP steels.

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A.K. Srivastava et al. / Materials Science and Engineering A 445–446 (2007) 549–557

The thermal dilatation was measured for both the ferrite and

the austenite and the transformed fraction was calculated from

the dilatation curve. The measured transformed fraction is com-

pared with the values calculated using Thermo-Calc Software

in Fig. 2 b. It may be seen from ﬁgure that a good match exists

between the calculated and experimental values for the steel

composition.

3.2. Microstructure

Microstructure of the samples at various stages of processing

before heat treatment is shown in Fig. 3 . Optical microstruc-

ture of the samples A and C, i.e., after heat treatment is shown

in Fig. 4 . Various phases in the microstructure are identiﬁed in

Fig. 4 : the areas displayed as yellowish are ferrite, the bright

white ones are retained austenite, tanned regions are martensite

and dark ones are bainite. Retained austenite phase is homo-

geneously distributed throughout the microstructure, and are

connected to adjacent ferrites or bainites. The volume frac-

tion of austenite is about 7% in sample A and about 8% in

sample C.

Fig. 5 shows TEM micrographs of heat-treated sample A.

It can be seen that bainite in this steels is free from car-

bides and shows alternate layers of ferrite and martensite.

Martensite is expected to be formed by transformation of carbon-

enriched austenite left over after the isothermal transformation

to bainite at 400 ◦ C. This appears to consist of an adjacent

pile up of laths. It is clear that a large amount of residual

austenite (the illuminated phases in the dark ﬁeld condition)

is retained between the laths of bainitic ferrite. The selective

area diffraction pattern, shown in Fig. 5 e and X-ray diffraction

Fig. 3. Microstructure of CMnAlSi TRIP steel: (a) forged sample (b) hot-rolled

and (c) cold-rolled samples, etchant: 2% nital for (a) and Le Pera for (b) and (c).

Fig. 4. Microstructure of CMnAlSi TRIP steel after cold-rolled and heat treat-

ment: (a) sample A and (b) sample C, etchant: Le Pera.

A.K. Srivastava et al. / Materials Science and Engineering A 445–446 (2007) 549–557

553

Fig. 5. TEM micrographs of CMnAlSi cold-rolled heat-treated sample A for (a), (b) and (c) bright ﬁeld;(d) dark ﬁeld and (e) selected area diffraction.

data shown in Fig. 6 , clearly conﬁrm the presence of retained

austenite.

calculated using the following equation [28] .

4 (

+ I

311

)

3.2.1. Measurement of retained austenite (RA)

The volume fraction of retained austenite in heat-treated sam-

ples was determined by X-ray diffraction (XRD) using Mo K

V =

4 (

(1)

I +

+ I

311

)

radiation. The integrated intensity of the (2 1 1) peak of fer-

rite and the (2 2 0) and (3 1 1) peaks of austenite were used for

this purpose and the volume fraction of retained austenite was

where I is the average integrated intensity obtained at the (2 2 0)

and (3 1 1) peaks, and I

220

211

220

is that obtained at the (2 1 1) peak. For

the calculation of the C content of the retained austenite, the

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