Effect of cooling rate on phase transformation in the low-carbonboron-treated steel

Di Zhang • Yoshiaki Shintaku • Shuichi Suzuki •

Yu-ichi Komizo

Received: 5 October 2011 / Accepted: 23 March 2012 / Published online: 11 April 2012

� Springer Science+Business Media, LLC 2012

Abstract In this research, phase transformation process

under different thermal cycles corresponding to a low and a

high heat input welding in the heat-affected zone of low-

carbon boron-treated steel is systematically investigated by

a high temperature laser scanning confocal microscopy. The

effect of thermal cycles on the phase transformation process

is quantified by measuring the transformation start tem-

perature of each transformation product and the average

number of nucleation sites of intragranular acicular ferrite.

Introduction

The variation of post-cooling rates alters the strength of

steel in association with toughness and formability [1–5].

This is achieved through ferrite grain refinement and

transformation of low temperature products. The micro-

structure changes from martensite (M) and/or intragranular

bainitic ferrite (IBF) at a higher cooling rate, to Wid-

manstatten ferrite (WF) and/or intragranular acicular ferrite

(IAF) at a medium cooling rate, and to intragranular

polygonal ferrite (IPF) and/or perlite (P) at a lower cooling

rate [6–8]. Among all the microstructures, IAF with a

chaotic arrangement of laths and fine-grained interlocking

microstructure toward optimizing strength and toughness

both in the weld metals and in the heat-affected zone

(HAZ). A large volume fraction of IAF in medium carbon

steel, low carbon steel, and medium carbon vanadium

steels were observed at cooling rates 10 [9], 5 [10, 11], and

0.1 K/s [12], respectively.

The optimal cooling rate for the formation of IAF also

depends on the chemical composition of the steels. Potent

inclusions for the formation of IAF are known as titanium-

rich cores (TiN, TiO, and Ti2O3) [13], alumina-rich cores

(Al2O3) [6, 7], crystalline galaxite spinel (MnAl2O4) [14],

boron-rich cores (BN, Fe2B and Fe23CB6) [15], and MnS

[6, 7, 13]. Segregation of boron and sulfur to the austenite

grain boundary (AGB) is also an effective way to improve

the formation of IAF.

The effect of cooling rate is generally investigated by

observing the microstructure at ambient temperature. In our

previous paper, we systematically investigated the effects

of boron content (0–33 ppm) and austenite grain size on

the formation of IAF in the HAZ of low-carbon boron-

treated steels [16]. Here, we focus on the effect of cooling

rate on the formation of IAF in these steels. A high tem-

perature laser scanning confocal microscopy (LSCM) is

used to directly observe the phase transformation process.

The effect of cooling rate is quantified by measuring the

average number of potent nucleation sites and the trans-

formation start temperature of each transformation product.

Experimental procedure

The chemical compositions of the low-carbon boron-treated

steels used in the present research are shown in Table 1. The

D. Zhang (&) � Y. Komizo

Joining and Welding Research Institute, Osaka University,

11-1 Mihogaoka, Ibaraki, Osaka 567-0047, Japan

e-mail: zhangdi1211@hotmail.com

Y. Shintaku

Plate & Bar/Wire Rod Research & Development Department,

Sumitomo Metal Industries Ltd, 1-8 Fuso-cho, Amagasaki,

Hyogo 660-0891, Japan

S. Suzuki

Steel Sheet, Plate Titanium & Structural Seel Company,

Sumitomo Metal Industries Ltd, 8-11 Harumi 1-chome,

Chuo-ku, Tokyo 104-6111, Japan

123

J Mater Sci (2012) 47:5524–5528

DOI 10.1007/s10853-012-6444-9

contents of boron are 2 and 12 ppm in Sample 1 and Sample

2, respectively. The phase transformation processes were

directly observed using a high temperature LSCM and an

infrared imaging furnace (IIF), which was described in detail

in the previous research [17]. The materials for in situ

observation were machined into disks (5 mm diameter and

1 mm height) and mirror polished. The thermal cycles con-

sisted of three steps: (1) heating up to 1673 K, at 33 K/s, (2)

austenitizing at 1673 K for 5 s and 300 s, and (3) continu-

ously cooling to the ambient temperature, with different

cooling rates. The details of the thermal cycles are sche-

matically shown in Fig. 1. In the case of welding, the cooling

rate is also given by the cooling time from 1073 to 773 K,

Dt8/5 [18]. To simulate the cooling process in the high heat

input HAZ, Dt8/5 is chosen to be 4, 2, and 1 K/s for both

Sample 1 and Sample 2. The transformation start tempera-

ture of each transformation product and average number of

potent nucleation sites were directly measured from the in

situ microstructure.

Results and discussion

Figure 2 shows the austenite grain size measured at

ambient temperature for Sample 1 and Sample 2 for all

thermal cycles. The error bars are given on each data. The

figure shows that the austenite grain size increases with an

increase in holding time and a decrease in boron content.

The effect of cooling rate on the austenite grain size can be

neglected.

Table 1 Chemical compositions of the materials used in this paper (mass%)

C Si Mn P S Ti B N Al

Sample 1 0.15 0.20 1.49 0.006 0.002 0.010 0.0002 0.0050 0.004

Sample 2 0.15 0.19 1.47 0.006 0.002 0.010 0.0012 0.0051 0.005

0 400 800273

673

1073

1473

18735s

Thermal cycle 6

Thermal cycle 5

Thermal cycle 4

Time, s0 400 800

Thermal cycle 3

Thermal cycle 2

Time, s

Thermal cycle 1

300s

1673 1373 1373 1073 1073 773 773 473

Cycles 1 and 4 15 12 4 1

Cycles 2 and 5 10 7.5 2 0.5

Cycles 3 and 6 5 3.75 1 0.2

Temperature (K)

Cooling rate (K/s)

4K/s

Tem

pera

ture

, K

2K/s1K/s

4K/s 2K/s1K/s

Fig. 1 In situ thermal cycles set

by the IIF-controller system

1 2 4

Aus

teni

te g

rain

siz

e, μ

m

Cooling rate (Δ8/5) (K/s)

Sample 2

1 2 40

100

200

300

400

500

Cooling rate (Δ8/5) (K/s)

Sample 1

5s 300s

Fig. 2 Austenite grain size for

Sample 1 and Sample 2 under

all thermal cycles

J Mater Sci (2012) 47:5524–5528 5525

123

Figures 3 and 4 show the typical transformation prod-

ucts observed by in situ observation in the low-carbon

boron-treated steels. WF, also distinguished as WF side

plates or laths, first takes place at the AGBs and the pre-

existed grain boundary allotriomorphic ferrite (GBAF), and

grow into the austenite grain by sympathetic nucleation.

Another microstructure, IBFs, nucleate entirely in the

austenite grains. As shown in Fig. 3, IBF growth as intra-

granular needles or intragranular plates. IAFs, which

directly nucleated on the potent inclusions, contributed to

the formation of a fine-grained interlocking microstructure.

Transformation takes place both at the AGB and in the

austenite grain. The morphology of WF, IBF, and IAF were

detailed in our previous paper [16].

Figure 5 shows the transformation start temperatures of

WF, IBF, and IAF for Sample 1 and Sample 2 under all the

thermal cycles. The error bars for WF, IBF, and IAF are

given in the figure. With increasing holding time and

increasing boron content, the difference between the

transformation start temperatures of WF and IAF, expres-

sed by WF–IAF in the figure, decrease. The large area of

interface with smaller austenite grains is thought to have

induced a larger number of nucleation sites. With

increasing boron content, the boron segregation at the AGB

generally must have suppressed the transformation at the

AGBs. On the other hand, the formation of BN thin layer

on the TiN inclusions increased the potent of IAF nucle-

ation [16]. With increasing cooling rate, the transformation

start temperatures of WF, IBF, and IAF decrease. At the

same time, the difference of the transformation start tem-

peratures of WF and IAF decrease with increasing cooling

rate. It demonstrated that the potential for the formation of

IAF was greatly improved with increasing cooling rate. For

example, the transformation start temperatures of WF and

IAF of Sample 2 with a holding time of 300 s and a cooling

rate Dt8/5 of 4 K/s were almost the same. The experimental

50μm

50μm 50μm

IAF

(a)972K

(d)925K (e)900K

50μm 50μm

50μm

(b)951K (c)940K

(f)Final microstructure

IAF nucleationInclusion IAF

IAF

IAF

IAF

Fig. 4 IAF observed by in situ

observation. (Sample 2 under

thermal cycle 1)

200μm

200μm

IBF

WFWF

IBF

(a)994K

(d)959K (e)935K

Austenite grain boundary

Inclusion

IBF nucleation Growth of IBF

WF

WF

200μm

200μm

200μm

200μm

(b)978K (c)966K

(f)Final microstructure

WF

Fig. 3 WF and IBF observed by in situ observation. (Sample 2 under thermal cycle 4)

5526 J Mater Sci (2012) 47:5524–5528

123

results of Sample 1 and Sample 2 are also presented in the

continuous cooling transformation (CCT) diagrams, as

shown in Fig. 6. Sample 2 with a holding time of 300 s and

a cooling rate Dt8/5 of 4 K/s had the highest nucleation

potency of IAF.

Figure 7 shows the average number of potent nucleation

sites under all the thermal cycles for both Sample 1 and

Sample 2. The measured average number of potent

nucleation sites for Sample 1 was almost the same. While

for Sample 2, the nucleation sites increased with increasing

WF IBF IAF WF-IAF

Transformation start temperature

273

303

963

993

1023

Tem

pera

ture

, KTe

mpe

ratu

re, K

273

303

963

993

1023

Sample 1 5s

1 2 4

Cooling rate (Δ8/5)(K/s)

Sample 1 300s

Sample 2 5s

1 2 4

Sample 2300s

Cooling rate (Δ8/5)(K/s)

Fig. 5 Transformation start

temperatures of WF, IBF, and

IAF for Sample 1 and Sample 2

under all thermal cycles

0 200 400 600

Time, t/s

300s

2K/s

5s

1K/s 1K/s2K/s4K/s4K/s

IAFIAF

IBFIBF

WFWF

Sample 2

0 200 400 600923

948

973

998

1023

Time, t/s

300s

2K/s

5s

1K/s 1K/s2K/s4K/s4K/s

IAFIAF

IBFIBF

WFWF

Sample 1

Tem

pera

ture

, K

Fig. 6 CCT diagrams of

Sample 1 and Sample 2

1 2 40.0

0.2

0.4

0.6

0.8

Num

ber

of n

ucle

atio

n si

tes,

/104 μm

2

5s 300s

1 2 4

Cooling rate (Δ8/5) Cooling rate (Δ8/5)

Sample 1 Sample 2

(K/s) (K/s)

Fig. 7 Average number of

nucleation sites of IAF for

Sample 1 and Sample 2 under

all thermal cycles

J Mater Sci (2012) 47:5524–5528 5527

123

cooling rate. Sample 2 with a holding time of 300 s and a

cooling rate Dt8/5 of 4 K/s had the highest number of potent

nucleation sites.

Conclusion

In general, small austenite grain promotes the grain

boundary transformation. While large austenite grain and

higher content of boron bring high number of potent

nucleation site. Increasing cooling rates result in the lower

transformation temperatures. In the boron-treated steel, at a

given austenite grain size and boron content, increasing

cooing rate (from Dt8/5 = 1 to Dt8/5 = 4 K/s) is possible to

switch the microstructure from grain boundary nucleation

(GBAF and WF) to intragranular nucleation (IBF and IAF).

It also should be pointed out that the optimal cooling rate

for the formation of IAF depends on the boron content and

the austenite grain size of boron-treated steels. In the

present research, the sample with an appropriate boron

content (12 ppm) and higher holding time (300 s) under a

proper cooling rate Dt8/5 = 4 K/s) shows the optimized

microstructure. The presented results show that the high

temperature in situ observation is able to quantitatively

investigate the phase transformation process in real time.

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