Preparation of ultra-fine MgO_Al2O3 spinel powder and its metallurg

Materials Science Forum Vols. 675-677 (2011) pp 291-294

Online available since 2011/Feb/21 at

© (2011) Trans Tech Publications, Switzerland

doi:10.4028//MSF.675-677.291

Preparation of ultra-fine MgO•Al

2

O

3

spinel powder and its metallurgy

behavior in low carbon steel

Yang Li

a

, Shuang-jiang Li

b

, Ying Zhuang

c

, Wei-jian Li

d

, Zhou-hua Jiang

e

School of Material and Metallurgy, Northeastern University, Shenyang, Liaoning, China

a

liy@,

b

two_river@,

c

zyyyykx@,

d

syy0309@,

e

jiangzh@

Keywords: MgO·Al

2

O

3

, ultra-fine powder, low carbon steel, mechanical property

Abstract: Micron, sub-micron and nanometer sized MgO•Al

2

O

3

ultra-fine powders were prepared

by gel precipitation, solid-phase synthesis, sol-gel and flame throwing pyrogenation methods. XRD

analysis showed that all of the ultra-fine powder is pure with the single MgO•Al

2

O

3

spinel phase.

The powder size was measured by laser granularity analyzer and the average size is 1780, 505 and

60 nm with a quite uniform distribution of particle size. MgO•Al

2

O

3

spinel powder with different

granularity were sprayed into molten low carbon steel in MgO crucible and MoSi

2

furnace at 1873

K. Quantitative microscopic examination showed that big particle inclusions reduce and small

particle inclusions increase, as a result, the average size reduce. Data comparison from spraying

powders with different size showed that spraying MgO•Al

2

O

3

of nanometer tends to cause more

small inclusions in molten steel. The sprayed steel samples were rolled and heat treated for the

mechanical properties tests, which showed spraying nanometer MgO•Al

2

O

3

is the best way to

improve mechanical property of steel.

Introduction

The strengthening mechanism of second-phase particles in steel mainly includes precipitation

strengthening and fine grain strengthening. At present, the research of adding ultra-fine particles

into molten steel is few. By this way, the demand on the cleanliness of molten steel is not higher;

and the process controlling is also easily, moreover, the properties of steel could be improved by

adding ultra-fine powders as the second-phase particles. The preparation methods of ultra-fine

powders include gel precipitation, solid-phase synthesis, sol-gel and flame throwing pyrogenation,

and so on.

Solid-phase synthesis method is using manufactured alumina and light burning magnesia as the

material, sintered under high temperature to control the size of ultra-fine powder [1]. Gel

precipitation method is metal-salt dissolved into solvent, under a certain condition, co-precipitation

forming uniform compound hydroxide or oxide gel. The precipitation is jelly, it’s difficult to wash

and filter, and precipitator is easy entraining as an impurity [2].

At present, the researches of preparing ultra-fine powder mainly focus on the sol-gel method.

The advantage is highly homogeneous mixture of raw materials, low temperature synthesis of

materials, easily to control the composition and relatively simple equipment. However, there are

still inadequacies that the reaction speed of precursor solution is not easy to control for the faster

solution and polycondensation of Mg-Al alkoxide; when the colloid in the dehydration, due to the

existence of surface tension, easily collapsing the pore structure, particle reunion took place [3].

Flame throwing pyrogenation method is injecting metal-salt solution into the heated reactor,

causing the evaporation and calcined of solvent immediately, thereby direct synthesis of oxide

powder, which is suitable for continuous operation.

As a kind of inclusions in steel, MgO·Al

2

O

3

spinel has great effect on the production process

and mechanical properties of steel, while the forming of which is inescapability under the current

steel-making process. To avoid the harm of MgO·Al

2

O

3

spinel, the former works almost focus on

decreasing the generation of which by controlling the deoxidizers and refining slags [4-7], whereas

the research of turning waste into wealth can not be found by controlling the size of MgO·Al

2

O

3

All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of TTP,

. (ID: 219.234.81.135-13/04/11,10:07:26)

292Advanced Material Science and Technology

spinel inclusions. In the current works, the metallurgy behavior in molten steel and the effect on the

mechanical properties of rolled steel were investigated by spaying different sized MgO·Al

2

O

3

spinel

powders.

Experimental Materials and Methods

The MoSi

2

resistance furnace, MgO crucible and PtRh thermocouple were used in spraying

experiments under Ar atmosphere at 1600 ºC. The composition of low carbon steel (mass %)

contains 0.18 - 0.21 C, 0.10 - 0.20 Si, 0.20 - 0.30 Mn and less than 0.035 S and P. Under an

atmosphere pressure, MgO·Al

2

O

3

spinel powder spraying experiments were carried out including

micron, sub-micron and nanometer sized powder. Due to comparing the mechanical properties, a

contrast heat without spraing powder was also done. The experimental operation was as follows:

(1) About 1 kg low carbon steel was added into a MgO crucible, and then it was put into MoSi

2

furnace, the protection of argon started at 600 ºC with the flow of 1 L/min, while the argon flow

changed to 3 to 5 L/min at 1400 ºC. When the steel temperature reached 1600 ºC, kept it for 10

minutes; (2) Taken 0 # steel sample (original steel sample); (3) Quartz tube inserted into the bottom

of molten steel, following a certain volume (about 0.2 L/min) argon as the carrying gas, a certain

sized of MgO·Al

2

O

3

spinel powder with 1.0 g was sprayed into the steel; (4) The beginning of time

(0min); (5) Taken 1 # steel sample after 1 min; (6) Taken 2 # steel sample after 10 min; (7) In

accordance with the above (3) - (6) steps to continue the spraying experiment, the steel samples

obtained by sequence number until the total spraying quantity reached to 5 g.

Steel samples obtained were cut, grinded and polished, the distribution of inclusions were

examined by quantitative microscopic, while the morphology and composition of inclusions were

observed by scanning electron microscopy.

Experimental Results and Discussion

Preparation Results of Ultra-fine MgO•Al

2

O

3

Spinel Powder. Alumina and magnesia were

mixing, milling 24 hours in a ball mill, insulated 1 hour at 1250 ºC, then the powder were milling

again and insulated 12 hour at 1600 ºC, finally, the micron sized powders was gained by plenty

mechanical milling. Fig. 1 is the XRD pattern of micron sized MgO·Al

2

O

3

powders prepared by

solid-phase synthesis method, which showed that the perfect spinel structure has formed, the

dissociative alumina and magnesia phase can not be found. The size distribution of MgO·Al

2

O

3

powders was measured by laser granularity analyzer (Malvern nano S90), and the average size of

which is 1780 nm.

40

)

%

30

(

y

t

i

s

n

20

e

t

n

I

10

0

110

Size ()

Fig. 1 XRD pattern and size distribution of micron sized MgO·Al

2

O

3

powder

NH

4

OH was chosen as precipitator, the gel could be gained by controlling the quantity and

addition speed. Fig. 2 is the XRD pattern of MgO·Al

2

O

3

powders sintered at 1000 ºC prepared by

gal precipitation method, and the average size of which is 505 nm.

Calcinated the powder at 800 ºC, 900 ºC, 1000 ºC, 1100 ºC, 1200 ºC, its found that the best

calcination temperature is 1000 ºC, relatively complete spinel phase and unremarkable

agglomeration of spinel powder can be gained at this temperature by sol-gel method.

Materials Science Forum Vols. 675-677293

Fig. 3 is SEM photos of powders prepared by the flame throwing pyrogenation method. As can

be seen from Figure 3, the spinel powders with the smaller particle size of 60 nm, and spherical

particles are distributed evenly, less reunion.

Fig. 2 XRD pattern of sub-micron sized powder Fig. 3 SEM photos of nanometer-sized powder

Quantitative Microscopic Examination Results. Table 1 is the size distribution of inclusions in

terminal samples sparyed MgO·Al

2

O

3

spinel powders with different particle size. The total number

of inclusions decreases with the increasing size of sparying powder, and the average particle size of

inclusion increases with the increasing size of sparying powder, while the total area of inclusions

changes little. Data comparison from spraying different size powders shows that spraying

MgO•Al

2

O

3

of nanometer tends to cause more small inclusions in steel.

Table 1 Size distribution of inclusion in terminal samples with spraying MgO·Al

2

O

3

powder with different particle size

Spraying powder

Amount of inclusions

Average

Total

Total area

0.65.4µm

diameter [µm]

amount

[µm

2

]

Nanometer sized 1055 899 244 6 1.708 2204 6847

Sub-micron sized 814 378 78 6 1.835 1276 4321

Micron sized 286 736 108 9 1.959 1139 4237

Morphology and Composition Analysis of Inclusions. The inclusions in the 0 # sample

mainly are Al

2

O

3

and silicate inclusions, and the Al

2

O

3

inclusions is irregular shape with the

particle size of about 2 µm, while the MgO·Al

2

O

3

spinel inclusions can not be found in the sample.

(a)

(b)

Fig. 4 Morphology of inclusions in steel by SEM

When sprayed into a certain amount of MgO·Al

2

O

3

spinel powders, a lot of MgO·Al

2

O

3

spinel

inclusions can be found instead of the pure Al

2

O

3

inclusions in steel, as shown in Fig. 4 (a), and (b).

The shape of the MgO·Al

2

O

3

spinel inclusions is nearly rectangular with the size of about 2 to 8 µm,

but the heterogeneous phase, in accordance with energy spectrum analysis, magnesium and

aluminum molar ratio of 1 : 2 is not accurate, the mole percentage of aluminum content in the high

side. Considering of Al

2

O

3

inclusions in steel reduced, there could be a combining process between

the sprayed MgO·Al

2

O

3

spinel powder and the original Al

2

O

3

inclusions, which leads the

composition ratio is not consistent with the pure MgO·Al

2

O

3

spinel phase.

Mechanical Properties of steel Sprayed Powders. Small steel ingot was heated to 1150 ºC,

keeping for 4 hours. Hot rolled by the 450 mm hot rolling experimental unit, the 3 mm slab was

rolled through six passes, and the rolling deformation was 25%. The obtained rolled metal was

heated to 920 ºC and normalized treatment after keeping temperature for 20 min.

According the requirements of GB/T228-2002, the rolling sample was cut into tensile samples

by cutting machine. The tensile tests were done on the electronic universal tensile sample machine

(CMT5105, accuracy of 0.5) to the sample of the steel sprayed by nanometer, submicron, micron

294Advanced Material Science and Technology

sized MgO·Al

2

O

3

spinel powders and the steel without sprayed, respectively. Test results were

shown in Table 2.

Table 2 Mechanical properties of low carbon steel with different spraying powder conditions

Mechanical property Nanometer sized Submicron sized Micron sized Without sprayed

Tensile strength [Mpa]

350 295 265

255

Extensibility [%]

32.3 35.4 34.3 33.1

Table 2 shows that after spraying MgO·Al

2

O

3

spinel powder with different particle size, the

tensile strength of steel had a certain degree of improvement, and the nanometer size powders could

increase substantially in tensile strength, while the extensibility had little variation. That is to say,

spraying nanometer sized MgO·Al

2

O

3

spinel powders can improve the steel strength. One of the

reasons is due to the dispersion of nanometer size spinel particles in steel or the small enough

MgO·Al

2

O

3

spinel inclusion from the reaction with Al

2

O

3

in steel, which played a certain dispersion

strengthening or precipitation strengthening; the other is that spraying nanometer size spinel

particles provide the opportunity to grow up for the inclusions in steel, some of which gather up and

float to the surface of molten steel. Therefore, the molten steel is purified, and which improves the

mechanical properties of steel.

Summary

Ultra-fine MgO•Al

2

O

3

spinel powders were prepared by solid-phase synthesis, gel

precipitation, sol-gel and flame throwing pyrogenation methods with the average size of 1780, 505

and 60 nm.

Compared with the terminal sample of sprayed sub-micron and micro sized powder, spraying

nanometer sized MgO•Al

2

O

3

powder into steel tends to cause more small inclusions. The shape of

the MgO·Al

2

O

3

spinel inclusions is nearly rectangular with the size of about 2 to 8 µm, but

magnesium and aluminum molar ratio of 1 : 2 is not accurate in the heterogeneous phase.

The sprayed steel samples were rolled and heat treated for the mechanical properties tests,

which shows spraying nanometer sized MgO•Al

2

O

3

is the best way to improve mechanical property.

Acknowledgement

The authors acknowledge National Natural Science Foundation of China (No. 50704010) and

the Fundamental Research Funds for the Central Universities (N090402019) for the financial

support.

References

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[2] V.K. Singh and R.K. Sinha: Mater. Lett., Vol. 31 (1997), p. 281

[3] O. Varnier, N. Hovnanian and A. Larbot: Mat. Res. Bull., Vol. 39 (1994), p. 479

[4] S.K. Saxena, in: 79th Steelmaking Conference Proceedings, edited by Iron & Steel Soc, volume

79 of Steelmaking Proceedings, Iron & Steel Soc AIME (1996)

[5] H.X. Wang, Y. Li, Z.B. Li and Z.H. Jiang: Iron and Steel (Peking), Vol. 40 (2005), p. 25

[6] J.H. Park, D.S. Kim: Metall. and Mater. Trans. B: Proc. Metall. and Mater. Proc. Sci., Vol. 36

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Advanced Material Science and Technology

doi:10.4028//MSF.675-677

Preparation of Ultra-Fine MgO•Al₂O₃ Spinel Powder and

its Metallurgy Behavior in Low Carbon Steel

doi:10.4028//MSF.675-677.291