Accepted Manuscript

Effects of annealing cooling rates on mechanical properties, microstructure andtexture in continuous annealed IF steel

Ruwen Zheng, Renbo Song, Wuyan Fan

PII: S0925-8388(16)32753-0

DOI: 10.1016/j.jallcom.2016.09.018

Reference: JALCOM 38852

To appear in: Journal of Alloys and Compounds

Received Date: 19 June 2016

Revised Date: 14 August 2016

Accepted Date: 2 September 2016

Please cite this article as: R. Zheng, R. Song, W. Fan, Effects of annealing cooling rates on mechanicalproperties, microstructure and texture in continuous annealed IF steel, Journal of Alloys and Compounds(2016), doi: 10.1016/j.jallcom.2016.09.018.

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http://dx.doi.org/10.1016/j.jallcom.2016.09.018

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Effects of annealing cooling rates on mechanical properties,

microstructure and texture in continuous annealed IF steel

Ruwen Zheng, Renbo Song*, Wuyan Fan

School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing 100083, China 5

Abstract1

The mechanical properties, microstructure and texture of interstitial-free (IF) steel

processed through continuous annealing routes under five cooling rates (5 oC/s, 50

oC/s, 200 oC/s, 500 oC/s and 1000 oC/s) have been studied. The cooling rates of 200

oC/s, 500 oC/s and 1000 oC/s are defined as ultra-fast cooling rates. This paper focuses 10

on the differences of mechanical properties, microstructure and texture evolution

between ultra-fast and common cooling rates (5 oC/s, 50 oC/s) for the first time. The

overall structural features of specimens were observed by optical microscopy and

nano-sized precipitates were observed by transmission electron microscope (TEM) on

carbon extraction replicas. Texture measurement was carried out using D8 advance 15

X-Ray Diffraction (XRD) and the (110), (200) and (112) pole figures were

determined. The yield strength increases with increasing cooling rates, from 96 MPa

under the cooling rate of 5 oC/s to 136 MPa under the cooling rate of 1000 oC/s , while

the ultimate tensile strength changes very little (around 280 MPa). The samples

annealed under the cooling rates of 5 oC/s and 500 oC/s have low rave values (2.13 and 20

2.21 respectively). By contrast, the samples in cases of 50 oC/s, 200 oC/s and 1000

*Corresponding author. Tel:+86-10-82377990; e-mail: songrb@mater.ustb.edu.cn

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oC/s possess higher rave values (2.32, 2.40 and 2.51 respectively). Three morphologies

of TiN particles can be observed in all samples. TiN, with CaO and SiO2 acting as

heterogeneous nuclei, is prone to have big size. The size of TiN precipitates deceases

as increasing of cooling rates. FeTiP is found near TiC precipitates under the cooling 25

rate of 5 oC/s. The sample annealed under the cooling rate of 5 oC/s has the weakest

{111} and {111} orientations. The intensity of {111}

component increases as the cooling rate increases from common cooling rates of 5

oC/s and 50 oC/s to ultra-fast cooling rates of 200 oC/s, 500 oC/s and 1000 oC/s. The

intensity of {011} orientation decreases as the cooling rate increases from 5 30

oC/s to 500 oC/s. The relationship between r value, microstructure and texture

components has been studied in this paper.

Key Words: Interstitial-free steel; Precipitation; Texture; Cooling rates; Continuous

annealing

1. Introduction 35

Application of IF steels in the automotive industry has seen a significant increase

over the past couple of decades due to their excellent deep-drawing property[1].

Titanium (Ti) and niobium (Nb) are added to these steels, singly or in combination, to

scavenge carbon and nitrogen by the formation of carbides and nitrides and thus

remove interstices from the matrix[2-4]. This clear matrix is essential for increased 40

formability and is suitable for galvanneal coating, which is required for automotive

body and home appliances[5]. The main trends in the development of IF steels are

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towards high-strength and high-formability varieties. P and Mn have been added in IF

steels as solution strengthening elements to improve strength performance.

Precipitation behavior in IF steels is quite complex and TiN, TiS, TiC and NbC 45

precipitates are quite common in Ti-IF steels[6-8]. Characterization of TiN, TiC and

Ti (C, N) in titanium-alloyed ferritic chromium steels focusing on different particle

morphologies has been studied by Michelic[9]. A detailed analysis of TiN morphology

and composition shows that there are four subcategories, pure TiN, TiN with an oxide

nuclei, TiN with a carbide or carbonitride layer and TiN clusters (agglomerated single 50

TiN), which differ significantly in size and morphology in ferrite stainless steels[9].

Ghosh and his group[10] have studied the thermodynamics of precipitation and

textual development in IF steels. Above 900 oC, the TiCN phase mainly contains N

apart from Ti, which is simply TiN. However, at around 900 oC, independent TiC

formation is likely to take place and the amount of carbon is considerably increased in 55

this phase below 900 oC[10]. The precipitation behavior of FeTiP-type precipitates in

IF steels has been studied[2-4, 11]. P has been added to normal IF steels as solid

solution strengthening elements and this leads to the formation of FeTiP precipitates

in annealing process. FeTiP prevents the growth of {111} recrystallization texture.

FeTiP usually precipitates at the grain boundary and the segregation of phosphorus 60

atoms at grain boundaries has been studied before[12]. During the annealing process,

non-equilibrium segregation of phosphorus atoms at the grain boundaries is related to

the moving dislocation lines[12-14].

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The strong {111} recrystallization textures are beneficial for deep-drawing

property and their formation mechanism and texture evolution have been studied in 65

several literatures[15-20]. Path of crystal rotation during rolling and the impacts of

alloying elements and second phase particles on texture formation in low carbon

steels have been studied[21]. It is reported that the precipitates increase the

recrystallization temperature and hinder the recrystallization texture[22]. In addition,

different texture components have different impacts on r and △r values. The {011} 70 component contributes to the increase of r value greatly in BCC materials, but

it also increases △r value. The {111} and {111} components can increase r value without increasing △r value[23, 24].

Previous studies focused on mechanical properties and microstructure under

different continuous annealing temperature and holding time. As for cooling rates, 75

few studies have been made, especially under ultra-fast cooling rates. The present

industrial production continuous annealing cooling rate is lower than 50 oC/s and

precipitation behavior and texture evolution under ultra-fast cooling rates have not

been studied. While apart from common cooling rates (5 oC/s and 50 oC/s), ultra-fast

cooling rates (200 oC/s, 500 oC/s and 1000 oC/s) were also carried out in this 80

experiment. This paper makes comparison studies between effects of common cooling

rates (5 oC/s and 50 oC/s) and ultra-fast cooling rates (200 oC/s, 500 oC/s and 1000

oC/s) on mechanical properties (focusing on r values), microstructure and texture

components. Considering the low rave values in cases of 5 C/s and 1000 C/s and their

different major texture components, a relationship between the r value, △r value and 85

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texture components in IF steel has been established. By controlling annealing process,

the preferred texture components, which increase r value and decrease △r value, can be obtained. Thus the formability of the material can be improved. Studying different

continuous annealing cooling rates also supplies the IF continuous annealing studies

and improves continuous annealing efficiency. 90

2. Experimental and analytical methods

The starting material was a Ti-IF steel of 1 mm in thickness and its chemical

compositions are listed in Tab.1. P has been added as solid solution strengthening

elements. The steel was prepared by hot rolling and cold rolling and the processing

parameters are given in Tab.2. 95

Tab. 1 Chemical composition of the IF steel (wt%)

C Si Mn S P Ti Nb 0.0015 0.022 0.09 0.0061 0.015 0.072 0.012

Tab. 2 Processing parameters of the IF steel

Hot finish rolling temperature

(oC)

Hot coiling temperature

(oC)

Cold rolling reduction

% 930 630 82

There were five stages in the annealing cycles: (1) HS (heating section): heating

from room temperature to soaking temperature (810 oC); (2) SS (soaking section): 100

holding at soaking temperature for 180 s; (3) CS (cooling section): cooling to ambient

temperature (cooling rates range from 5 oC/s to 1000 oC/s); (4) OA (over aging

section): holding at overaging temperature (240 oC) for 120 s; (5) AC (air cooling): air

cooling to room temperature. The continuous annealing process was simulated in the

CCT-AY heat treatment system under the cooling rates of 5 oC/s and 50 oC/s. As for 105

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the cooling rates of 200 oC/s, 500 oC/s and 1000 oC/s, the steels were cooled in

different mediums. The different annealing tests were performed on laboratory scale.

Ultra-fast cooling rates could be applied on industrial scale and the Kobe Steel Group

Japan has already applied ultra-fast cooling rates on steel production. The continuous

annealing curve is shown in Fig.1. 110

–

Fig.1 Continuous annealing curve of the IF steel

The mechanical properties of the samples annealed under different cooling rates,

including yield strength (YS), ultimate tensile strength (UTS), elongation, n and r

values were investigated at room temperature in three directions of 0°, 45° and 90° in 115

the tension test. Further TEM equipped with an energy dispersive spectrometer (EDS)

was used to study the precipitation behavior. Specimens for TEM were prepared by

carbon extraction replicas. Macroscopic textures were measured on an X-ray

diffractometer and three pole figures {110}, {200}, and {112} were obtained.

Orientation distribution functions (ODFs) were calculated from these pole figures 120

using the Bunge method[25]. The α, γ, η, ζ, θ, and ε-fiber intensities were also plotted

in this study.

3. Results

3.1 Mechanical properties under different cooling rates

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It can be clearly seen that their mechanical properties (shown in Tab.3) are 125

different under different cooling rates. The yield strength turns out gradually upward

trend with the increase of cooling rates, while the ultimate tensile strength changes

little (around 280 MPa). The yield ratio increases with the increase of cooling rates.

The yield ratio is the ability to resist deformation from yielding to plastic instability

and the lake of adequate strain hardening results in high yield ratio[26, 27]. The 130

mechanical results also reveal that the cooling rates have little effect on elongation

and n values.

The r value is defined as true plastic strain ratio of width to thickness under

uniaxial tension and low r value indicates poor deep-drawing property [28].

rave=(r0+2r45+r90)/4 (3-1) 135

△r=(r0-2r45+r90)/2 (3-2) The rave values of specimens annealed under the cooling rates of 5

oC/s and 500

oC/s are significantly lower than others. The rave value is 2.40 when the cooling rate is

200 oC/s and it decreases to 2.21 under the cooling rate of 500 oC/s.

In addition, LDR (limiting drawing ratio) value is usually used to evaluate deep 140

drawability and theoretically can be expressed as follows:

LDR = �exp ��2fexp�−�����1 + �� 2� � + exp �2���1 + �� 2� � − 1 (3-3) Where f is the factor of drawing efficiency, and when f equals to 0.9, the calculated

results are in good agreement with the experimental results[26]. Based on the

equation, the LDR values of samples annealed under different cooling rates (form 5 145

oC/s to 1000 oC/s) are 2.53, 2.59, 2.61, 2.18 and 2.22 respectively. This result reveals

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that the deep property of sample annealed under the cooling rate of 200 oC/s is higher

than others.

Tab. 3 Mechanical properties of the IF steels

Steel Cooling

rate

(oC/s)

Direction

(deg)

r rave r n nave Elongation

(%)

YS

(MPa)

UTS

(MPa)

1#

5

0 2.41

2.13

0.81

0.29

0.30

52.4 96 284

45 1.72 0.31 50 103 282

90 2.66 0.31 54 100 271

2#

50

0 2.83

2.32

1

0.30

0.30

53.5 103 280

45 1.82 0.30 50 105 287

90 2.81 0.31 48 102 276

3#

200

0 2.93

2.40

0.91

0.30

0.30

50.2 119 277

45 1.94 0.29 49.3 126 283

90 2.77 0.30 51.2 118 277

4#

500

0 2.53

2.21

0.75

0.30

0.29

50.3 123 284

45 1.84 0.29 47.6 132 289

90 2.64 0.29 48 127 280

5#

1000

0 3.03

2.51

0.97

0.30

0.29

51.6 126 275

45 2.02 0.29 48.6 136 285

90 2.96 0.28 48 132 277

3.2 Microstructure under different cooling rates 150

3.2.1Grain sizes

The optical micrographs of IF steels annealed under different cooling rates are

presented in Fig.2 and the effect of cooling rates on the ferrite grain sizes is shown in

Fig.3. All samples are made up of equiaxed ferrite grains. The results show that grain

sizes decrease with increasing cooling rates. The mean grain size reaches its 155

maximum (15.13 µm) under the cooling rate of 5 oC/s and decreases to the minimum

value of 11.75 µm under the cooling rate of 1000 oC/s. According to statistics

calculation, the average grain aspect ratio (17:10) in steels under the cooling rates of 5

oC/s and 50 oC/s is a little higher than that (about 3:2) in steels annealed under the

ultra-fast cooling rates of 200 oC/s, 500 oC/s and 1000 oC/s. 160

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165

170

175

180

185

190

Fig. 2 Optical micrographs of IF steels under different cooling rates

(a) 5 oC/s; (b) 50 oC/s; (c) 200 oC/s; (d) 500 oC/s; (e) 1000 oC/s

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Fig. 3 Influence of cooling rates on ferrite grain sizes 195

3.2.2 Precipitation behavior

Fig.4 shows the presence of precipitates in samples annealed under different

cooling rates. A general observation made from the TEM micrographs is that cuboid

TiN is the most common precipitates under all different cooling rates and TiC

precipitates are in elliptical shape. Three morphologies of TiN precipitates: (1) pure 200

TiN; (2) TiN clusters (Fig.4d); (3) TiN with an oxide nuclei (Fig.5); can be observed.

TiN particles (with oxide nuclei), which grew under the cooling rates of 5 oC/s and 50

oC/s, are greatly larger than those grown under ultra-fast cooling rates. The sizes of

pure TiN observed under different cooling rates are shown in Tab.4. and they decrease

with increasing cooling rates. Fig.6 reveals elliptical TiC precipitates and the presence 205

of FeTiP which formed on the pre-existing TiC particle under the cooling rate of 5

oC/s only. The schematic development (including morphologies and forming stage) of

different precipitates is shown in Fig.7.

210

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215

220 225

230

235

Fig. 4 TEM extraction replica micrographs showing presences of different precipitates under

various cooling rates 240

(a) 5 oC/s; (b) 50 oC/s; (c) 200 oC/s; (d) 500 oC/s; (e) 1000 oC/s; (f) non-annealed sample

245

250

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255

260

265

270

Fig.5 TEM micrographs showing the presence of large size TiN precipitates formed under the 275

cooling rates of 5 oC/s (a) and 50 oC/s (b) and corresponding EDS spectra (c) and (d)

Tab.4 Different sizes of pure TiN precipitates under different cooling rates

Common cooling rates

Ultra-fast cooling rates Non-annealed sample

Cooling rates/ oC·s-1

5 50 200 500 1000 none

TiN sizes/nm 20~25 20 10~15 10~15 10~12 10

280

285

290

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295

300

305

310

Fig. 6 TEM extraction replica micrographs showing presences FeTiP and TiC precipitates formed under

the cooling rate of 5 oC/s (a) and corresponding EDS spectra FeTiP (b) and TiC (c)

Fig. 7 Schematic development of various morphologies of different precipitates over different process

315

3.3 Texture evolution under different cooling rates

The ideal orientations of texture components in BCC materials are shown

schematically for φ2=0° and 45° ODF sections in Fig.8 and Tab.5. For IF steel, the

vital fibers are the α-fiber (fiber axis parallel to the rolling direction including

components of {001} , {112} and {111} ), γ-fiber (fiber axis 320

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parallel to the normal direction including components of {111} and {111}

), ε-fiber (fiber axis parallel to the transverse direction including

components of {001} , {112} , {111} and {011} ), η-fiber

(fiber axis parallel to the rolling direction including components of {001}

and {011} ), θ-fiber (fiber axis parallel to the normal direction 325

including components of {001} and {001} ) and ξ-fiber (fiber axis

parallel to the normal direction including components of {011} , {011} ,

{011} and {011} [18].

330

335

\ 340

Fig. 8 Schematic illustration of the important texture components in BCC materials

(a) φ2=0°(b) φ2=45°

345

350

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Tab.5 The important fibers and orientations for BCC materials

Fiber name Fiber axis Important components

α //RD {001}, {112}, {111}

γ //ND {111}, {111}

ε //TD {001}, {112}, {111}, {011}

η //RD {001}, {011}

θ //ND {001}, {001}

ξ //ND {011}, {011}, {011}, {011}

Fig.9 illustrates the ODFs of samples annealed under different cooling rates.

Changes of maximum intensity versus the annealing cooling rates for major texture

components are presented in Fig.10.It can be seen that the main texture orientations 355

were {111} , {111} and {011} with the maximum intensity of

4.09×R, 4.15×R and 2.59×R when the cooling rate was 5 oC/s. Fig.9(b) indicates that

the intensity of {111} orientation increased by 25%, from 4.09×R to 5.06×R,

under the cooling rate of 50 oC/s. There was also an increase in the intensity of {001}

orientation from 0.453×R to 1.54 ×R. It should be noted that the intensity of 360

{111} increased by 21%, from 4.49×R to 5.41×R, under the cooling rate of 200

oC/s. Fig. 9(d) indicates the intensity of {111} and {111} texture

components decreased by 9.8% and 6.5% under the cooling rate of 500 oC/s,

compared with those formed under the cooling rate of 200 oC/s. The intensity of {011}

orientation decreased a lot, from 1.77×R to 1.05×R. Finally, when the cooling 365

rate was 1000 oC/s, the intensity of {111} and {011} components

increased, while the intensity of {111} decreased.

370

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385

390

395

400

405

410

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415

420

Fig. 9 ODFs of the IF steel annealed under different cooling rates

(a) 5 oC/s; (b) 50 oC/s; (c) 200 oC/s; (d) 500 oC/s; (e) 1000 oC/s 425

430

435

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440

445

Fig. 10 The maximum intensity of main texture components versus annealing cooling rates

4. Discussion

4.1 Effects of cooling rates on mechanical properties 450

The mechanical properties of samples annealed under different cooling rates are

shown in Fig.11. The yield strength and rave values increase with increasing cooling

rates, except that the sample annealed under the cooling rate of 500 oC/s possesses

lower r value (2.21). The elongation rates, n values and UTS change little under

different cooling rates. 455

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Fig. 11 Mechanical properties of samples annealed under different cooling rates

According to the measured mechanical properties, the yield strength increases as 460

the increase of cooling rates. The grain boundary strengthening mechanism is usually

described by Hall-Petch equation as follows:

σy=σ0+ky*d-1/2 (4-1)

where σy is the yield strength, d is the average grain diameter, σ0 is the friction stress

and ky is the Hall-Petch slope[23]. According to the grain sizes obtained under 465

different cooling rates, the grain diameters decrease as the increase of cooling rates.

The grain size of the sample annealed under the cooling rate of 5 oC/s is 15.2µm,

while under the cooling rate of 1000 oC/s, the value decreases to 11.7µm. Based on the

Hall-Petch equation, the decrease of grain diameters will result in the increase of yield

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strength. This is consistent with the increase of yield strength values as the increasing 470

of cooling rates in this paper.

The little change of UTS, elongation and n values can be explained as follows.

Solution strengthening is the major method to increase UTS in this IF steel. All the

samples contain the same amount of P as solution strengthening atoms and thus the

UTS changes little in different steels. Ferrite structure and large grains are responsible 475

for retaining the ductility in low carbon steels[19, 29]. Since all samples are composed

of 100% ferrites and the difference of grain sizes formed under the cooling rates from

5 oC/s to 1000 oC/s is small, the elongation changes little. Strain hardening exponent

(n) is an important parameter to assess the deformation characteristic of steel and it

depends mainly on the structure types in low carbon steels[19]. The ferrites lead to 480

more hardening by large amount of deformation compared with other structure types.

All samples formed under different cooling rates are all made up of ferrite structure

explains the little change of n values in this paper.

4.2 Precipitation behavior

According to the second phase solid solubility formula in steels, the TiN and TiC 485

solid solubility formulas based on phase analysis are[30]

lg{[Ti][N]} γ=3.94-15190/T (4-2)

lg{[Ti][C]} γ=5.33-10475/T (4-3)

Their precipitation temperatures are 1355 oC and 872 oC respectively based on

these two formulas. The Ti compounds precipitation temperature has been studied and 490

previous researchers has made schematic illustration regarding the sequence of

precipitates formation and their stability[31]. According to his research, TiN takes

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place at 1300 oC, while TiC and FeTiP at about 700 oC. It is also found that TiN is

more stable than TiC in their studies. Park has pointed out that some oxides can act as

heterogeneous nuclei for the formation of TiN in ferritic chromium during steel 495

making route[31]. Some TiN precipitation took place in liquid based on previous

studies and the process is controlled by solubility limit of Ti-N equilibrium. CaO and

SiO2 are slag particles and they formed at liquid stage. These oxides acted as

nucleation for TiN precipitates at high temperatures and these TiN present in both all

the cooling rates. 500

The formation of large size TiN (shown in Fig. 5), which has oxide nuclei, are

due to they have more time to grow under the common cooling rates. In order to

explain the different sizes of pure TiN formed under different cooling rates, diffusion

controlled growth mechanism needs to be considered. Free standing TiN forms during

re-heating and hot rolling and it is the interstitial nitrogen which is trapped first by Ti 505

and remained so. As for free standing TiC, they usually take place during coiling.

Comparing the sizes of pure TiN in non-annealed sample and the sample annealed

under the cooling rate of 1000 oC/s, we can learn that TiN grows during the holding

section. As the cooling rates decrease from 1000 oC/s to 5 oC/s, the pure TiN particles

have obvious tendency to grow bigger. The cooling rates may change the dimensions 510

growth number and ultimately affects the time exponents and transformation

process[32,33].

Apart from these, FeTiP precipitates are observed under the cooling rate of 5 oC/s.

The precipitation behavior of FeTiP has been studied before. In batch annealing, the

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size of FeTiP precipitate is big and the content of phosphorus atoms is high[2]. On the 515

contrary, in the continuous annealing process, FeTiP is observed on the pre-existing

TiC and the content of phosphorus is low[3]. It has been reported that FeTiP takes

place over TiC particles, suggesting that FeTiP forms only after the formation of

TiC[2]. This is consistent with the FeTiP precipitation behavior in this paper. It is

reported that phosphorus accumulates at the bottom of the edge dislocation since the 520

radius of phosphorus (0.109 nm) is smaller than that of α-Fe (0.124 nm)[13]. This is

illustrated in Fig.12. Phosphorus atoms are driven by moving dislocation lines and

accumulate at grain boundaries during continuous annealing process[13]. The FeTiP

precipitates are found near TiC particles (Fig.6) in this study. Based on the above

conclusions, we can assume that phosphorus, which concentrates at grain boundaries, 525

interacts with Ti and Fe atoms. Thus FeTiP forms on pre-existing TiC particles. This

mechanism of FeTiP formation is consistent with the previous result that FeTiP may

not be a strict stoichiometric compound [11]. It has been studied that the formation of

FeTiP precipitates strongly prevents the growth of {111} recrystallization texture

which is beneficial for deep-drawing property[2, 3]. 530

535

Fig. 12 Schematic of phosphorus and edge dislocation

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4.3 Effect of texture on r value

All steels possess high rave values except those annealed under the cooling rates

of 5 oC/s and 500 oC/s (2.13 and 2.21 respectively). It is well known that the forming 540

property can be characterized by the rave and △r values which are strongly affected by the texture components. In order to get good forming property, the material should

have high rave value and low △r value[34, 35]. D. Daniel[35] has studied the relationship between r values and texture components in low carbon steels and the

statistics are listed in Tab.6. It can be learnt that the {111} , {111} and 545

{011} components are the most favorable to increase the r value, so the

intensity of these three components formed under different cooling rates is plotted in

Fig.13. From Tab.6, we can learn that the {011} component contributes most

to the increase of r value, but it also increases △r value dramatically. The {111} and {111} components can increase r value significantly without increasing △r 550 value. Different texture components correspond to different rave and △r values. In addition to texture components, Chung[24] suggests that grain morphology also has

an effect on the mechanical property anisotropy and equaixed grains normally

correspond to the low △r value. Since all samples are made up of equiaxed ferrite grains and the difference of average grain aspect ratio is small, the effect of grain 555

morphology on △r value could be ignored.

560

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Tab. 6 The correlation between texture components and r values in BCC materials

Texture Components r r

{001} 0.4 -0.8

{112} 2.1 -2.7

{111} 2.6 0

{111} 2.6 0

{554} 2.6 1.1

{011} 5.1 8.9

The FeTiP precipitate was observed under the cooling rate of 5 oC/s and its

formation result in the weak intensity of {111} and {111} components

(4.09×R and 4.15×R respectively). According to the analysis, the {111} and

{111} components are favorable to increase r values. Then the weak texture 565

components of {111} and {111} are responsible for the low rave value

(2.13) in the sample annealed under the cooling rate of 5 oC/s.

The sample annealed under the cooling rate of 500 oC/s also has a lower rave

value (2.21) and lower △r value (0.75) compared with those annealed under the cooling rates of 200 oC/s and 1000 oC/s. As shown in the Fig.13, we can learn that the 570

sample annealed under 500 oC/s has the weakest {011} fiber. According to the

relationship between texture components and r value, the strong {011}

component significantly contributes to the increase of r and △r value. Thus, the lower r and △r value in the sample annealed under the cooling rate of 500 oC/s is attributed to the weak intensity of {011} . 575

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Fig. 13 The intensity of major texture components under different cooling rates

5. Conclusions

(1) The yield strength, r and △r values are strongly influenced by cooling rates. The yield strength increases as increasing of cooling rates from 5 oC/s to 1000 oC/s. The r

values also show a rising trend as the increase of cooling rates, except that the sample 600

annealed under the cooling rate of 500 oC/s possesses lower r value. In addition, in the

case of 500 oC/s, the sample also has the lowest △r value. The ultimate tensile strength, n values and elongation rates change little under all different cooling rates.

(2) The size of pure TiN precipitates decreases as increasing of cooling rates. The

oxides, like CaO and SiO2, act as heterogeneous nuclei for TiN and these TiN have 605

larger size under the cooling rates of 5 oC/s and 50 oC/s. FeTiP can be observed on the

pre-existing TiC particles under the cooling rate of 5 oC/s and the presence of FeTiP

precipitate prevents the growth of recrystallization {111} texture.

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(3) The intensity of {111} orientation increases as the cooling rate increases

from 5 oC/s to 1000 oC/s while the intensity of {011} decreases as the cooling 610

rate increases from 5 oC/s to 500 oC/s. There is a texture transition in the γ-fiber: from

{111} to {111} under the cooling rate of 1000 oC/s. The r and △r values have strong connection with texture components. The low rave under the cooling rate

of 5 oC/s is attributed to the weak {111} and {111} components. The

weak {011} orientation is responsible for the low rave and △r values under the 615 cooling rate of 500 oC/s.

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Highlights:

(1) The best formability steel can be obtained under the cooling rate of 200 oC/s.

(2) FeTiP precipitates on parts of pre-existing TiC under the cooling rate of 5 oC/s.

(3) The size of pure TiN decreases as increasing of cooling rates.

(4) The intensity of {011} orientation decreases with increasing cooling rates.

(5) The weak {011} orientation is responsible for low r and △r values.