Evolution of precipitates in the Nb–Ti–V microalloyedHSLA steels during reheating

S.G. Hong a,b, H.J. Jun a,*, K.B. Kang b, C.G. Park a

a Department of Materials Science and Engineering and Center for Advanced Aerospace Materials,

Pohang University of Science and Technology, Pohang 790-784, South Koreab Technical Research Laboratories, POSCO, Kwangyang 545-090, South Korea

Received 3 September 2002; received in revised form 18 November 2002; accepted 20 November 2002

Abstract

In as-cast slab steel, dendritic Nb-rich (Ti,Nb)(C,N) carbonitrides were observed which have a thermodynamically

stable chemistry at lower than 1000 �C. These dendritic carbonitrides were dissolved and then re-precipitated to two

kinds of carbonitrides, Ti- and N-rich and Ti- and C-rich (Ti,Nb)(C,N) carbonitrides during reheating.

� 2003 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved.

Keywords: Steels; Precipitates; Reheating; Transmission electron microscopy (TEM)

1. Introduction

There is a substantial interest in the develop-

ment and use of high-strength steels for automo-tive applications with the goal to decrease vehicle

weight. Over the last 30 years, high-strength low-

alloy (HSLA) steels containing microalloying ele-

ments such as Ti, Nb, and V have become widely

used in the automotive sector and also for pipeline

applications [1–3]. These microalloying elements

yield a significant improvement in mechanical

properties through grain refinement, solid solutionhardening, and precipitation hardening. Multi-

microalloying can lead to the formation of com-

pounds with complex chemical compositions,

which further influences the mechanical properties

of the steels [4–8]. Therefore, an improved under-

standing of the sequence of the precipitation of

complex precipitates is clearly important to mi-crostructural control during hot rolling processing.

The purpose of the present study is, thus, to

identify the evolution of precipitates in Nb–Ti–V

microalloyed steels during the reheating process,

which is an important process, based on both

TEM observation and chemical analysis of car-

bonitrides.

2. Experimental procedure

The chemical composition of the Nb–Ti–V steel

examined in this study is given in Table 1. The

specimens were supplied by POSCO (Pohang Iron

& Steel Co. Ltd.) in the form of slab. Reheatingsimulations were performed in the temperature

Scripta Materialia 48 (2003) 1201–1206

www.actamat-journals.com

* Corresponding author. Tel.: +81-542792826; fax: +81-

542792399.

E-mail address: jhj@postech.ac.kr (H.J. Jun).

1359-6462/03/$ - see front matter � 2003 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved.

doi:10.1016/S1359-6462(02)00567-5

range of 1100–1400 �C for 1 h after which reheatedspecimens were water-quenched for chemical and

TEM analysis.

The equilibrium composition of complex pre-

cipitates in austenite has been predicted assuming

compositional homogeneity in austenite by using

�Thermo Calc� program. The morphology and

distribution of precipitates, which were extracted

from the reheated specimens, were examined usingFEG-TEM (JEM2010F). In addition, chemical

analysis for the precipitates was performed by

using both nano-beam EDS analysis on Al-repli-

cas and induction coupled plasma (ICP) analysis

with dissolved and filtered precipitates.

3. Results

3.1. Analysis of carbonitrides observed in slab steel

Prior to the analysis of precipitates formed dur-

ing reheating, TEM observations for as-cast slab

steel were performed. Fig. 1 shows aluminum rep-licas extracted from the as-cast slab steel, showing

dendritic precipitates. These dendritic precipitates

were identified as (Ti,Nb)(C,N) carbonitrides. The

chemistries of both the center (marked ‘‘A’’) and

wing parts (marked as ‘‘B’’) in these dendritic car-

bonitrides were investigated by using nano-EDS

analysis (Fig. 1(c) and (d)). The center parts re-

vealed higher Ti and N contents than wing parts.Most of all, it is important that these dendritic

carbonitries were not Ti-rich carbonitrides but Nb-

rich carbonitrides. That is, it is generally believed

that these Nb-rich carbonitrides are stable in the

low-temperature range of austenite. Especially, it

was easily predicted that the chemistries of these

carbonitrides are stable at lower than 1000 �C as

shown in the equilibrium chemistries of complexcarbonitrides in Nb–Ti–V steel calculated by using

Thermo Calc (Fig. 2). That is, these dendritic

carbonitrides are no longer stable above 1000 �C.

Zhou and Priestner [9] reported that these Nb-rich

dendritic (Ti,Nb)(C,N) carbonitrides can precipi-

Fig. 1. Aluminum extraction replicas showing (a) the distribution and (b) morphology of precipitates in slab steel. Characteristic EDS

spectra of (c) ‘‘A’’ and (d) ‘‘B’’ parts.

Table 1

Chemical composition of the steel examined in the present study (wt.%)

C Si Mn V Nb Ti N

Slab steel 0.06 0.16 1.53 0.03 0.039 0.018 0.004

1202 S.G. Hong et al. / Scripta Materialia 48 (2003) 1201–1206

tate at the low-temperature (less than 1000 �C)

range of austenite during non-equilibrium cooling

of as-cast slab steel. In addition, any other pre-

cipitates containing V were not observed in as-cast

slab steel.

3.2. Evolution of precipitates during reheating

In order to investigate the dissolution and rep-recipitation behavior of these carbonitrides during

reheating process, we examined the distribution

and morphology of precipitates after reheating at

various temperatures. After reheating in the tem-

perature range of 1100–1400 �C for 1 h, the den-

dritic Nb-rich (Ti,Nb)(C,N) carbonitrides, which

resulted from non-equilibrium cooling, disap-

peared, and new cubic shape precipitates formedalong austenite grain boundaries (for the case of

reheating at 1100–1400 �C) and within austenite

grain (for the case of reheating at 1100–1250 �C)

(as shown in Fig. 3). At the 1100 �C reheating

600 800 1000 1200 1400 1600

0

10

20

30

40

NCN

CCN

VCN

TiCN

NbCN

Temperature(Wt.%

of a

lloy

elem

ents

in M

X c

arbo

nitr

ides

(x10

-3)

C)

NbCN

TiCN

VCN

CCN

NCN

Dendritic

(Ti,Nb)(C,N)

Fig. 2. The equilibrium chemistries of complex carbonitrides in

Nb–Ti–V steel at 600–1600 �C (calculated by using Thermo

Calc).

Fig. 3. Aluminum extraction replicas exhibiting the distribution of precipitates in slab steel reheated at (a,b) 1100 �C, (c,d) 1150 �C, (e,f)

1200 �C, and (g,h) 1250 �C.

Fig. 4. Aluminum extraction replicas exhibiting the distribution of precipitates in slab steel reheated at (a) 1300 �C and (b) 1400 �C.

S.G. Hong et al. / Scripta Materialia 48 (2003) 1201–1206 1203

condition, there were both cubic shape precipitates

and irregular shape precipitates. It is believed that

the former are re-precipitated carbonitrides, and

the latter are undissolved carbonitrides observedduring reheating process. After reheating above

1300 �C, however, only large cubic precipitates

were observed along grain boundaries as shown in

Fig. 4.

4. Discussion

It is well known that Ti added to Nb or Nb–V

microalloyed HSLA steels restricts effectively the

grain growth of austenite during reheating pro-

cess by forming stable nitrides or carbonitrides

and finally results in fine-grained steel. However,

the formation mechanism of these Ti-nitrides or

Ti-carbonitrides during reheating process is not

clearly identified. It is not clear when these cubicshape Ti-precipitates easily observed in hot rolled

steel form during cooling of slab steel or reheating

process. In addition, the distribution and size of

Ti-precipitates formed during reheating process

can play important roles not only on pinning of

austenite grain during reheating, but on precipi-

tation kinetics of Nb-precipitates [8]. Especially, in

the previous result [8], we found that these Ti-precipitates observed after reheating process acted

as preferred nucleation sites for NbC carbides

during hot deformation. That is, we can easily

predict that the precipitation kinetics of NbC

carbides, which are well known as effective pre-

cipitates for inhibition of recrystallisation of aust-

enite, can be influenced by the distribution and size

of Ti-rich precipitates after reheating process.Therefore, in order to identify systematic precipi-

tation process of cubic shape Ti-precipitates, the

chemical analysis for Ti-precipitates observed in

as-slab steel and after reheating at various tem-

peratures were conducted.

First, we examined the total amount of Ti and

Nb precipitated in as-slab and reheated steels by

using dissolution and filtration method. As shownin Fig. 5(a), electrolytic dissolution and filtration

results revealed that almost all the Nb and Ti

contents (0.039 and 0.017 wt.%, respectively) ad-

ded during steel making process precipitated as

dendritic Nb-rich (Ti,Nb)(C,N) carbonitrides. As

reheating temperature increased from 1050 to 1400

�C, Nb content within precipitates decreased,

while Ti content did not change to 1250 �C. Above1300 �C, Ti started to dissolve into the austenite

matrix.

Fig. 5(b) shows the atomic ratio of Ti or Nb

to total Ti and Nb content contained within

(Ti,Nb)(C,N) carbonitrides. The composition of

precipitates changed from Nb-rich (Ti,Nb)(C,N)

carbonitrides in as-cast slab steel to Ti-rich

(Ti,Nb)(C,N) carbonitrides in reheated steel. Inaddition, as reheating temperature increased,

atomic ratio of Ti to total Ti and Nb content in-

creased, simultaneously. As shown in thermody-

namic equilibrium of Ti, Nb and V containing

1000 1100 1200 1300 1400 15000.000

0.005

0.010

0.015

0.020

0.025

0.030

0.035

0.040

0.045

(a)

Slab

wt.%

Reheating temp.( C) Reheating temp.( C)

TiCNNbCN

1100 1200 1300 14000.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

Slab(b)

Ato

mic

ratio

(Tio

rNb/

(Ti+

Nb)

)

TiNb

Fig. 5. (a) Amount of Ti and Nb, and (b) atomic ratio of Ti and Nb within precipitates observed in slab steel reheated at 1050–1400 �C(dissolution and filtration results (ICP)).

1204 S.G. Hong et al. / Scripta Materialia 48 (2003) 1201–1206

precipitates (Fig. 2), we can predict that non-

equilibrium dendritic (Ti,Nb)(C,N) carbonitrides

are not stable any more during reheating andchanged to equilibrium carbonitrides.

In order to examine the composition of Ti-

precipitates observed both along austenite grain

boundaries and within austenite, respectively, the

nano-chemistries of cubic shape Ti-carbonitrides

were investigated by using nano-EDS analysis. As

shown in Fig. 6, the large carbonitrides along

austenite grain boundaries were identified as ni-trogen-rich carbonitrides except for 1100 �C and

the atomic ratio of nitrogen to carbon increased as

reheating temperature increased. The small car-

bonitrides within the austenite matrix, however,

were carbon-rich carbonitrides and the atomic

1100 1200 1300 1400

0.0

0.2

0.4

0.6

0.8

1.0

Slab

Reheating Temperature ( C) Reheating Temperature ( C)

Ato

mic

rat

io

Ato

mic

rat

io

TiNbCN

(a)1050 1100 1150 1200 1250

0.0

0.2

0.4

0.6

0.8

1.0

(b)

TiNbCN

Fig. 6. Atomic ratio of (a) coarse and (b) fine precipitates observed in slab steel reheated at various temperatures (TEM nano-EDS

results).

Fig. 7. (a) Carbon extraction replica showing the distribution of carbonitrides in slab after reheating at 1250 �C for 1 h, and schematic

diagrams showing (b) dendritic carbonitrides in slab, and (c,d) the formation process of cubic carbonitrides located in grain boundaries

and matrix during reheating treatments.

S.G. Hong et al. / Scripta Materialia 48 (2003) 1201–1206 1205

ration of nitrogen to carbon was not changed re-

gardless of reheating temperature.

As shown in Fig. 7(a), there was precipitates

free zone between the large N-rich (Ti,Nb)(C,N)and the small C-rich (Ti,Nb)(C,N). Therefore,

from the results of nano-chemistries and this

characteristic distribution of precipitates, it is be-

lieved that during reheating process, dendritic

carbonitrides which precipitated during non-equi-

librium cooling (Fig. 7(b)), dissolved and Ti and

Nb solutes near austenite grain boundaries dif-

fused primarily into austenite grain boundaries.In addition, N and C solutes diffused from the

austenite matrix into austenite grain boundaries,

and formed Ti- and N-rich (Ti,Nb)(C,N) carbo-

nitrides (Fig. 7(c)), which were stable chemistries at

reheating temperatures as known in Fig. 2. On the

other hand, Ti and Nb within austenite ma-

trix, which did not diffuse into austenite grain

boundaries, formed the small Ti- and C-rich(Ti,Nb)(C,N) carbonitrides (Fig. 7(d)). Especially,

these small carbonitrides did not reveal the

chemistries predicted from Thermo Calc. That is,

these precipitates were C-rich (Ti,Nb)(C,N), not

N-rich (Ti,Nb)(C,N). It is believed that because N

solutes within austenite matrix primarily diffused

into austenite grain boundaries during reheating

process, and formed large N-rich carbonitrides, Ncontent within the austenite matrix was too low to

form N-rich (Ti,Nb)(C,N) carbonitrides.

Consequently, we can conclude that fine Ti-

precipitates observed within austenite are at-

tributed to non-equilibrium precipitates, that

is, dendritic Nb-rich (Ti,Nb)(C,N) carbonitrides

formed during cooling of slab steel. Dendritic Nb-

rich (Ti,Nb)(C,N) carbonitrides dissolved intoaustenite during reheating process and reprecipi-

tated simultaneously as stable Ti-rich (Ti,Nb)

(C,N) carbonitrides. Especially, these fine (Ti,Nb)

(C,N) carbonitrides observed within austenite will

be not effective on restriction of grain growth of

austenite during reheating. From our previous

results [8], we would rather predict that these fine

(Ti,Nb)(C,N) showing uniform distribution within

austenite can have reverse effects on inhibition of

recrystallisation of austenite.

5. Conclusion

1. In as-cast slab steel, dendritic Nb-rich (Ti,Nb)

(C,N) carbonitrides were observed which have

a thermodynamically stable chemistry at lower

than 1000 �C.

2. These dendritic carbonitrides were dissolved

and then re-precipitated to two kinds of carbo-

nitrides, Ti- and N-rich and Ti and C-rich

(Ti,Nb)(C,N) carbonitrides during reheating.3. During the reheating process, stable Ti- and N-

rich (Ti,Nb)(C,N) precipitated primarily along

austenite grain boundaries, followed by precip-

itation of Ti- and C-rich (Ti,Nb)(C,N) within

austenite grain, where N solutes were depleted.

Acknowledgement

The authors thank Pohang Iron and Steel Co.(POSCO) for financial support.

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