ABSTRACT In this paper the formation and transformation of non-metallic globular inclusions (Type-D inclusions ) in ball bearing steel production have been studied. There are different kinds of Type-D inclusions in the commercial specimens of bearing steel from SEM inspection. Content of Al have an important effect in inclusions formation in the experiment of the induction furnace. A great amount of MgO·Al 2 O 3 can be formed with higher Al deoxidation in MgO crucible melting. With high content of Ca, the inclusions of MgO · Al 2 O 3 spinel will become calcium-aluminates entirely. It is shown that controlling the content of Ca, Mg, Al in liquid steel have an important function on the Type-D inclusions. It is testified in the experiment of the steel production That Type-D inclusions can be improved by specific slag modification and composition optimization.
I. Introduction
Bearing control inclusions in steel metallurgy is a problem of great concern to researchers, currently used in the standard test of inclusions in steel is divided into A (sulfide), B (alumina), C (silicate), D ( Spherical non-deformed inclusions), the hazard of various types of inclusions on bearing life can be arranged in the order of D→B→C→A. For inclusions, spherical non-deformation inclusions are extremely harmful to bearing life. Calcium aluminate inclusions are one of the main types. The influence of inclusion size on bearing fatigue limit is obvious. The larger the size, the fatigue life. The shorter.
There are many reports on the study of calcium aluminate in bearing steel. The literature indicates that the formation of calcium aluminate inclusions is free to change. 
G) at steelmaking temperatures are negative [1], the high bearing steel refining slag basicity of production, always find calcium aluminates and magnesium aluminum spinel inclusions [4] [6], the control [ The content of Ca] and [O] is important for controlling the formation and compositional transformation of calcium aluminate inclusions [1] . This paper has carried out the precipitation morphology, size and distribution of D-type inclusions in bearing steel of a domestic factory. Observed, theoretically analyzed the formation possibility of calcium aluminate in the production process of bearing steel, calculated the influence of acid-soluble aluminum content in steel on the content of calcium and magnesium in steel, and investigated the effects in laboratory and factory experiments. The effect of factors on the likelihood of calcium aluminate formation.
Second, the shape of inclusions in bearing steel
Take GCr15 continuous casting 180×180mm billet produced by a special steel factory in China and after rolling The 45mm bar was prepared into an electron microscope. The morphology, size and distribution of the D-type inclusions were observed by SEM, and qualitative analysis was carried out by energy spectrum analysis. It was found that the D-type inclusions can be roughly divided into several types: independent calcium aluminate inclusions ( As shown in Figure 1), independent MgO•Al 2 O 3 inclusions (as shown in Figure 2), Ca-Mg-Al-O composite inclusions (as shown in Figure 3), and composites with sulfides on the outside. Inclusions (as shown in Figure 4). In the scanning observation, there are relatively few calcium aluminate and composite inclusions, while magnesium aluminate spinel is relatively more. The above analysis shows that the appearance of D-type inclusions in bearing steel is more complicated, and it is necessary to distinguish the properties of different inclusions, and then to understand the law of its formation. The literature on the formation behavior of magnesium-aluminum spinel in bearing steel [7] has been studied intensively. The focus of this paper is on the formation mechanism and control of calcium aluminate.
Figure 1 Independent calcium aluminate inclusions Figure 2 Independent MgO•Al 2 O 3 inclusions Figure 3 Ca-Mg-Al-O composite inclusions
Figure 4: The outer surface of the bread has sulfide composite spherical inclusions: (a) inclusion morphology; (b) central part energy spectrum; (c) peripheral part energy spectrum
Third, laboratory research
(1) Experimental methods
In this experiment, the bearing steel samples produced by a special steel plant were selected, and the slag of different compositions was designed. The smelting was carried out in a 10kg induction furnace to explore the formation law and influence conditions of the D-type inclusions in the bearing steel, focusing on the Al in the furnace steel. ,Ca,Mg content on the inclusions in bearing steel, Al content is adjusted in the form of metal pure aluminum added to molten steel, Ca content is controlled by adding silicon-calcium alloy, Mg content is mainly due to Al reduction MgO lining during smelting process Change into steel liquid. The experimental slag composition design range is: WCaO 50 to 60%, WAl 2 O 3 25 to 35%, WMgO 5 to 10%, WSiO 2 8 to 12%, and WCaF 2 0 to 15%.
For the experiment, the rolled Φ45mm bearing steel is about 7kg per furnace and the slag is about 500g. After the steel sample and the slag are melted together in the medium frequency induction furnace, the appropriate alloy is added, the slag sample is taken, the temperature is measured, and then The molten steel is poured into small steel ingots, cooled in the air after molding, and the composition analysis of the obtained slag samples and steel samples (the steel samples mainly analyze acid-soluble Al, all Ca, and all Mg), and the steel samples are prepared for macroscopic microstructure observation. And scanning electron microscope observation.
(II) Experimental results and discussion
1. Influence and discussion of [Al] content
The analysis results of the steel samples show that the trace components of the steel without aluminum are not changed much compared with the initial sample, the [Al] content is about 0.01%, the [Mg] content is about 0.0005%, and the [Ca] content is 0.0005. % or less; after adding aluminum, the content of [Al] is 0.1% to 0.4%, the content of [Mg] is 0.0042 to 0.0057%, and the content of [Ca] is about 0.0005%. From the results of electron microscopy, it is found that induction The type of inclusions in the furnace remelted but not added with Al is similar to the actual production situation, in which many Al 2 O 3 inclusions are found, as shown in Figure 5; [Al] is significantly elevated in steels with Al smelting , all in the range of 0.1 to 0.4%, but no single Al 2 O 3 inclusions were found in the steel, but some MgO·Al 2 O 3 inclusions were found, and a large amount of MgO inclusions were found, as shown in Fig. 6 and Fig. 7; No calcium aluminate was found in these steels.
Fig. 5 Al 2 O 3 inclusions in low [Al] steel Fig. 6 MgO inclusions in high [Al] steel Fig. 7 MgO inclusions in high [Al] steel
The addition amount of Al has a great influence on the inclusions, and controls the change of the properties of the inclusions. When the [Al] in the steel is relatively low (about 0.01%), many Al 2 O 3 inclusions can be found; when [Al] is compared At high time (0.1-0.4%), Al can react with the MgO lining to displace the Mg into the molten steel. After the solidification of the molten steel, MgO·Al 2 O 3 and MgO inclusions are formed. At the same time, the excessive Mg content in the steel is inhibited. The formation of xCaO•yAl 2 O 3 .
2. Influence of CaF 2 and MgO in slag
Among the components in the slag, CaF 2 has a relatively large effect on the formation of inclusions. When the aluminum content is high or low, the addition of CaF 2 generates many MgO·Al 2 O 3 inclusions, which may be CaF 2 energy. Erosion of the furnace lining, so that MgO into the molten steel, which is conducive to the formation of MgO·Al 2 O 3 inclusions, as shown in Figure 8:
Figure 8 Addition of MgO·Al 2 O 3 in CaF 2 steel
The MgO content in the slag has little effect on the formation of MgO·Al 2 O 3 inclusions in the steel. In the steel with the highest MgO content in the slag system, the acid-soluble Al is not high, and no MgO·Al 2 O 3 and MgO inclusions are found. In the steel in which the MgO content in the slag is not high, since the acid-soluble Al is relatively high, many MgO·Al 2 O 3 and MgO inclusions can be found.
3. Influence and discussion of [Ca] content
In the steel without calcium treatment, the [Al] content is about 0.01%, the [Mg] content is about 0.0005%, and the [Ca] content is 0.0005% or less. It is found by electron microscopy that many Al 2 O 3 are present alone, such as Figure 9 shows that after the micro-calcium treatment, the [Al] content of the steel sample is about 0.01-0.08%, the [Mg] content is below 0.0005%, and the [Ca] content is 0.0018%-0.0037%. The regular spherical xCaO·yAl 2 O 3 is shown in Figure 10 and Figure 11:
9 non-calcium treatment of steel in FIG Al 2 O 3 inclusions of calcium treated steel 10 in FIG xCaO · yAl 2 O 3 inclusions
Figure 11 xCaO·yAl 2 O 3 inclusions in calcium treated steel
Calcium has a very obvious effect on the denaturation of inclusions. Bearings with high [Ca] content are denatured into spherical calcium aluminate inclusions, and no MgO·Al 2 O 3 is formed . This indicates that Mg and Ca have a certain effect. When Mg is dominant, MgO·Al 2 O 3 can be formed in a large amount, and in contrast, calcium aluminate inclusions are formed in a large amount.
It is found through experiments that the influence of Al, Ca and Mg content in steel on D-type inclusions is important. Therefore, it is necessary to control the content of Al, Ca and Mg in steel by controlling the inclusion of D. Studies have shown that with the increase of Al content in steel, the Mg content increases, and under the condition of high alkalinity slag (WCaO/WSiO 2 ≥3), the tendency of Mg content in steel increases more, which leads to MgO in bearing steel. The possibility of formation of Al 2 O 3 is increased. Similarly, as the acid-soluble Al in the steel increases, the Ca content also increases, especially under high alkalinity conditions, and the effect is more pronounced. In the case of low [Mg] in steel, spherical xCaO•yAl 2 is formed. O 3 inclusions.
The experiment also showed that there is a certain correlation between the formation of calcium aluminate and MgO•Al2O3, according to the reaction:
[Ca]+MgO•Al 2 O 3 (s) =CaO•Al 2 O 3(s) +[Mg] G 0 6 =43807-53.33TJ/mol (1)
According to the coexistence theory of slag structure [2] and thermodynamic equilibrium, the theoretical model of the influence of slag composition on [Ca] and [Mg] content in bearing steel is derived, and it is obtained at 1773K:
Lg[%Ca]/[%Mg]= lgNCaO•Al 2 O 3 -lgNMgO•Al 2 O 3 +0.57 (2)
It can be seen that under high alkalinity conditions, calcium aluminate and MgO•Al 2 O 3 are easily converted to each other. When [%Ca] is high, calcium aluminate is formed; on the contrary, when [%Mg] At higher temperatures, MgO•Al 2 O 3 is formed. The precise relationship between Ca and Mg needs further study. Therefore, in actual production, the Ca, Mg and Al contents in steel should be fully optimized to reach D. Minimization of inclusions.
Fourth, factory experimental research
(1) Experimental methods
Under the premise of ensuring that the original basic process is unchanged, the steel mill experiment emphasizes that the process, slag composition and aluminum content in the steel are stable. It is divided into three experimental schemes: one is to add composite slag-removing agent 1 (metal deoxidizer and oxide) on the slag surface after refining in LF furnace, and then enter VD after slag; secondly, micro-alloy treatment after VD (feed A small amount of composite alloy additive is added; third, after the LF furnace is finished refining, the composite slag addition agent 2 is added, and after the slag is formed, the VD is entered.
Each scheme was tested for one casting (7 furnaces) and 2 furnace samples were taken. Each furnace is taken before LF, LF furnace, and after VD, and the two furnaces treated by microalloy are sampled before and after vacuum treatment. After the experiment, the steel slag samples were taken for chemical composition analysis, and electron microscopy samples were prepared for inclusion morphology observation and energy spectrum analysis.
(II) Experimental results and discussion
From the results of the component analysis, it can be seen that with the first scheme, the alkalinity of the slag is obviously improved, and the content of Al 2 O 3 in the slag is also increased, while the content of calcium and magnesium in the steel is several ppm, which is not changed much; In the scheme, the MgO content in the slag is not high, but the calcium content in the steel is not uniform; using the third scheme, the alkalinity of the slag is increased, and the MgO content in the slag is not changed much, but the calcium and magnesium contents in the steel are significantly improved.
In order to further verify the type, morphology and size distribution of oxide inclusions in steel, this study used SEM observation and combined with energy spectrum analysis, found that the first and second schemes still exist in bearing steel inclusions. MgO•Al 2 O 3 , calcium aluminate and their composite brittle non-deformed inclusions are mostly spherical in shape and have a size of about 3 to 10 μm (see Figures 12 and 13). It shows that these two experimental schemes have little effect on reducing the D-type inclusions in bearing steel. The third scheme (see Figure 14) is used to increase the slag basicity and increase the fluidity and adsorption performance of the slag, so that the large particles of the D-type inclusions in the steel are reduced, and the finer magnesium-aluminum is formed in the main steel. The D-type inclusions, which are mainly composed of spinel and Ca-Mg-Al-O composite inclusions, have a significant improvement in the quality of bearing steel.
Figure 12 is the first scheme typical Figure 13 is the second scheme typical Figure 14 The third scheme is typical
Ca-Mg-Al-O composite inclusions calcium aluminate inclusions magnesium aluminum spinel inclusions
V. Conclusion
(1) Class D inclusions in bearing steel actually produced by a special steel plant in China can be roughly divided into four types: independent calcium aluminate inclusions, independent MgO•Al 2 O 3 inclusions, and Ca-Mg-Al-O composites. Inclusions, composite inclusions with sulfides on the outside; where the independent MgO•Al 2 O 3 inclusions are the most abundant in steel.
(2) The Al content has a great influence on the formation of inclusions in the steel. Under the condition of MgO lining, when there is enough Al, the MgO can be reduced, so that MgO·Al 2 O 3 is formed in the steel, and even MgO.
(3) The presence of Mg inhibits the formation of calcium aluminate, and also inhibits the formation of MgO·Al 2 O 3 when the calcium content is high. When Mg and Ca content are low, Al 2 is easily formed. O 3 is mixed, so it is necessary to control the appropriate Mg, Ca, and Al, so that the total amount of Al 2 O 3 , MgO·Al 2 O 3 , and xCaO·yAl 2 O 3 inclusions in the bearing steel is less fine and dispersed, and the formation and The control mechanism needs further research.
(4) By controlling the comprehensive performance of the VD furnace slag and increasing the adsorption of inclusions in the steel, it can effectively reduce the large particle inclusions in the steel and improve the D-type inclusions in the steel.
references
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3. Lauri Holappa. Inclusion Control For Castability of Resulphurized Steels. Steelmaking Conference Proceedings 2001: 765-777.
4, Wang Bo, Jiang Zhouhua, Gong Wei, Zhan Dongping, Li Daliang. Analysis of inclusions and total oxygen content control process of GCr15 bearing steel. Journal of Materials and Metallurgy, 2004, 6.vol.3 No.2.
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