Improvement of steel cleanliness by controlling slag compositon

Relationship between average life and inclusion content: counts based on total inclusions observed (Ö 750) in 516 fields representing total area of ~9 mm2
© 2002 IoM Communications Ltd.
INTRODUCTION
Owing to its junction axis location in machinery, the bearing unit receives repeated stress and therefore may suVer from fatigue cracking after a certain period in service. Therefore, it is necessary to improve the quality of bearing steels so that they can be used for the entire life of the machinery. Generally, the bearing steels used in most products are mainly of high Cr –C system composition, and the bearing steel grades produced by POSCO are no exception to this, as can be seen in Table 1.
3(FeO)+2Al =(Al2 O3 )+ 3Fe . . . . . . . (1a) 3(MnO)+2Al=(Al2 O3 )+3Mn . . . . . . (1b) 3(SiO2 )+4Al=2(Al2 O3 )+ 3Si . . . . . . (1c)
From equations (1a) and (1b), when the contents of FeO and MnO in slag are high, non-metallic inclusions are produced, therefore it is essential to reduce the FeO and MnO contents in the slag. However, as shown in Fig. 3, the reoxidation behaviour of SiO2 is diVerent from that of FeO and MnO, because Si is a stronger deoxidant than Fe and Mn. Consequently, if the basicity of molten slag is controlled to a certain degree, the reoxidation of molten steel by the SiO2 in the slag does not occur, and it can therefore be prevented by slag composition control.
Improvement of steel cleanliness by controlling slag composition
B.-H. Yoon, K.-H. Heo, J.-S. Kim, and H.-S. Sohn
The properties of slag are very important in the
Ironmaking and Steelmaking 2002 Vol. 29 No. 3 215
216 Yoon et al. Improvement of steel cleanliness by controlling slag composition
3 Effect of slag basicity on SiO2 and hence total oxygen contents in steel
2 Control of slag composition of bearing steel
Table 1 Composition of bearing steels, wt-%
Standard/grade
C
Si
J/SUJ1 I/SUJ2 S/SUJ3
0·95–1·10 0·95–1·10 0·95–1·10
Non-metallic inclusions such as Al2 O3 , SiO2 , TiO2 , etc. are known to cause cracking. Figure 1a5 shows that the life of a bearing steel decreases as the sum of Al2 O3 and SiO2 inclusions in the steel increases. However, the sulphide, which exists in the form of MnS in the neighbourhood of inclusions, decreases the tensile stress due to inclusions, and so has a positive eVect on fatigue failure (see Fig. 1b). However, considering the general harmful eVects caused by the sulphur, its presence in steel should be limited to an appropriate range.
THEORETICAL BACKGROUND
Effect of slag composition on removal of Al2O3 inclusions
In bearing steels, the inclusions are mainly Al2 O3 . Therefore, it is necessary to control the slag composition to remove
I&S/1705
The authors are in the Technical Research Laboratories, POSCO, Pohang, 790–785, Korea. Based on a presentation at the 6th International Conference on ‘Molten slags, fluxes, and salts’ held in Stockholm and Helsinki on 12–16 June 2000.
The range of slag composition satisfying these conditions should be close to the Al2 O3 –CaO binary system considering both the slag composition used with the bearing steel and the reoxidation of molten steel by the SiO2 in the slag. If the former is considered to be more important, the appropriate composition range should be near the CaO saturation region marked as region A in Fig. 2. However, if the latter is considered to be the more important, the composition should lie in region B.
improvement of steel cleanliness. In particular, the
value of %CaO/%Al2O3 ratio, slag basicity, fluidity, and oxygen activity in the slag are considered to
Reoxidation of molten steel by low grade oxide in slag As shown in the following equations, low grade oxides in the slag, such as FeO and MnO, react with a deoxidant in the molten steel to create non-metallic inclusions
Effect of oxygen content at end of blow in BOF on steel cleanliness Since the oxygen content in the molten steel at the end of blow in BOF ( basic oxygen furnace) is the origin of nonmetallic inclusions, it aVects the cleanliness of steel, and should therefore be considered.
0·15–0·35 0·15–0·35 0·40–0·70
Mn
<0·50 <0·50
0·90–1·15
P
<0·025 <0·025 <0·025
S
<0·025 <0·025 <0·025
Cr
0·90–1·20 1·30–1·60 0·90–1·20