The fatigue property is significantly affected by the inner inclusions in steel. Due to the inhomogeneity of inclusion distribution in the micro-scale, it is not straightforward to quantify the ...effect of inclusions on fatigue behavior. Various investigations have been performed to correlate the inclusion characteristics, such as inclusion fraction, size, and composition, with fatigue life. However, these studies are generally based on vast types of steels and even for a similar steel grade, the alloy concept and microstructure information can still be of non-negligible difference. For a quantitative analysis of the fatigue life improvement with respect to the inclusion engineering, a systematic and carefully designed study is still needed to explore the engineering dimensions of inclusions. Therefore, in this study, three types of bearing steels with inclusions of the same types, but different sizes and amounts, were produced with 50 kg hot state experiments. The following forging and heat treatment procedures were kept consistent to ensure that the only controlled variable is inclusion. The fatigue properties were compared and the inclusions that triggered the fatigue cracks were analyzed to deduce the critical sizes of inclusions in terms of fatigue failure. The results show that the critical sizes of different inclusion types vary in bearing steels. The critical size of the spinel is 8.5 μm and the critical size of the calcium aluminate is 13.5 μm under the fatigue stress of 1200 MPa. In addition, with the increase of the cleanliness of bearing steels, the improvement of fatigue properties will reach saturation. Under this condition, further increasing of the cleanliness of the bearing steel will not contribute to the improvement of fatigue property for the investigated alloy and process design.
—A statistical analysis is performed to determine the total oxygen content in pipe steel samples at the stage of continuous casting on a continuous casting machine (CCM) of the casting-and-rolling ...complex (CRC) at AO VMP. The microstructure of rolled steel samples is studied, and the chemical composition of nonmetallic inclusions is determined by electron-probe microanalysis. A laboratory analysis of samples of liquid steel and hot-rolled CRC steel is performed by the inert gas fusion method. The dependences between the time of metal residence in a ladle, the magnesium content in nonmetallic inclusions, and the average number of nonmetallic inclusions in rolled steel and the total oxygen content in liquid steel at the stage of continuous casting are built.
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The effect of total oxygen content on the machinability of low carbon resulfurized free cutting steel was studied by means of machinability tests, metallographic, and scanning electron microscopy ...observation. The results showed that the total oxygen content had a notable effect on the machinability of SAE 1215 steel. The best machinability of the steel in terms of tool life and surface roughness could be obtained by controlling the total oxygen content at 0.0105 and 0.0125 wt%. When total oxygen content in the steel was less than 0.0105 wt%, the machinability of the steel was increased with the increase of total oxygen content because of the significant reduction of the aspect ratio of MnS inclusions. However, when total oxygen content in the steel was more than 0.0125 wt%, the machinability of the steel was deteriorated dramatically with the increase of total oxygen content, which was attributed to that the amount and the size of oxide inclusions (MnO–Al2O3, MnO–SiO2, and 2MnO–SiO2) with high hardness and high melting point both had a significant increase, and some MnS inclusions were also wrapped by the MnO–SiO2 and 2MnO–SiO2 hard oxides. Furthermore, the tool wear would reach the maximum when total oxygen in the steel rose to 0.0150 wt%.
The best machinability of low carbon resulfurized free cutting steel could be obtained by controlling total oxygen content at 0.0105 and 0.0125 wt%. However, when the total oxygen content was more than 0.0125 wt%, the machinability of the steel would deteriorated dramatically with the increase of total oxygen content, which was attributed to the increase of hard oxide inclusions.
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Reactions between Mn of manganese steel and CO
2
, which derives from limestone containing refractories under high temperature, were studied via vacuum induction furnace. Refractory crucibles ...manufactured with limestone contents of 0, 10 and 20 wt-% were filled with manganese steel, which was melted in an Ar atmosphere. Samples were taken from the steel liquid at regular intervals by quartz tube during the experiment; the molten steel was poured out from crucible when the experiment was finished. Mn, C and total oxygen content of steel samples taken from the molten steel were tested, and also, the refractory crucible was investigated after the experiments. The results show that CO
2
, which comes from limestone containing refractories, can react with the Mn in the steel, the result being an increase in C content and total oxygen content of steel.
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Data on the metallurgical defects in pipe show that they may be due to nonmetallic oxide inclusions. Analysis of the formation of oxide inclusions from smelting to the continuous casting of steel ...shows that, in order to reduce the incidence of such defects, additional measures must be taken in the smelting and ladle treatment of the steel. Such measures reduce the content of oxide inclusions in the steel by half. The benefits of this approach are confirmed by the reduced rejection rate of the pipe on account of defects associated with the presence of nonmetallic inclusions.
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In the present work, the reoxidation of Al‐killed molten steel from the magnesia‐chromite refractory was investigated. Firstly, the composition and the phases of magnesia‐chromite bricks from three ...different steelmaking shops were analyzed. Secondly, the magnesia‐chromite bricks were immersed in Al‐killed molten steel for a determined time to observe the reaction mechanism. Finally, the reoxidation of Al‐killed molten steel by the magnesia‐chromite bricks was evaluated based on a comparison with the pure magnesia crucibles. The XRF showed the Fe2O3 content of the magnesia‐chromite bricks is about 6 mass%, and the Cr2O3 content is in a range of 13–19 mass%. The XRD pattern showed the phases in the bricks are mainly MgO and (Mg,Fe)(Cr,Al,Fe)2O4. The fracture surface of magnesia‐chromite bricks after immersion in Al‐killed molten steel indicated that there exists a reacted layer, and the reacted layer formed after immersion in the low Cr steel is more obvious than that formed after immersion in the high Cr steel. The SEM–EDS of magnesia‐chromite bricks showed that (Mg,Fe)(Cr,Al,Fe)2O4 in the reacted layer decreased after immersion in Al‐killed molten steel, especially in the low Cr molten steel. Compared with steel melted in pure magnesia crucibles, the total oxygen of steel melt in magnesia‐chromite crucibles showed a lower decreasing rate, and the rate of Cr increase and Al loss were much larger, especially in the low Cr molten steel.
In the present work, the reoxidation of Al‐killed molten steel by the Fe2O3 and Cr2O3 in the magnesia‐chromite refractory is investigated. Compared with pure magnesia refractory (Heat 1 and Heat 2), total oxygen content of steel in the magnesia‐chromite refractory (Heat 3 and Heat 4) shows a lower decreasing rate, especially in low Cr molten steel (Heat 3). The reason may be attributed to the reoxidation by Fe2O3 and Cr2O3 in the magnesia‐chromite refractory, and low Cr content in the molten steel is favorable for the reduction of Cr2O3 in the magnesia‐chromite refractory.
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7.
Cleanliness of Alloying Structural Steel YU, Hui-xiang; WANG, Xin-hua; ZHANG, Jing ...
Journal of iron and steel research, international,
12/2011, Volume:
18, Issue:
12
Journal Article
Peer reviewed
Alloying structural steel used for mechanical structures has a high requirement for cleanliness because its failures are greatly affected by non-metallic inclusions and total oxygen content in ...steel.It has been reported by some steelmaking plants to have some problems in controlling total oxygen content and inclusions during alloying structural steel production.For this purpose,cleanliness control in 0.2C-0.3Si-0.6Mn-1Cr-0.2Mo steel was investigated.Firstly,low melting temperature zone（≤1873 K） of CaO-Al2O3-MgO system and formation condition of low melting temperature inclusions were investigated through thermodynamic equilibrium calculation.On this basis,industrial tests were carried out.Through sampling at different stages,transformation of oxide inclusions and change of total oxygen content in steel were studied.The results show that：in order to form CaO-Al2O3-MgO system inclusions with low melting temperature,mass percent of Al2O3,MgO and CaO in inclusions should be controlled from 37.6% to 70.8%,0 to 17.4% and 25.5% to 60.6%;For the condition of 1873 K and 0.05%（mass percent） dissolved aluminum in steel,the activities of dissolved oxygen,magnesium and calcium should be controlled as 0.298×10-4-2×10-4,0.1×10-5-40×10-5 and 0.8×10-8-180×10-8 respectively.With secondary refining proceeding,average total oxygen content and inclusion amount decrease,the type of most inclusions changes from Al2O3 after tapping to Al2O3-MgO after top slag is formed during ladle furnace refining and finally to CaO-Al2O3-MgO after RH treatment.In the final products,average total oxygen content was 12.7×10-6 and most inclusions were in spherical shape with size less than 5 μm.
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Inclusion has an important effect on quality of high speed rail steel. In consideration of the lower acceptance percentage of the inclusion and its constraint against the requirement for large scale ...production of 350 km/h high speed rail steel in Panzhihua Iron and Steel （Group） Co,the technology of nonmetallic inclusion control for 350 km/h high speed rail steel was studied. An optimized model of the argon-blowing in ladle furnace （LF）,the control of the components of the ladle slag,and the technique of calcium treatment for the molten steel was brought forward. Using the researched technology,the removal ratio of the inclusion was increased and the components,distribution,and shape of the inclusion in the rail steel were changed,which resulted in a reduction in the average total oxygen content to 10.17×10^-6 and an increase in the comprehensive acceptance percentage of the nonmetallic inclusion from 48.21% to 98.1%. Test has shown that this metallurgical technology can meet the requirement for large scale production of 350 km/h high speed steel in Panzhihua Iron and Steel （Group） Co.
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During the smelting process EAF-LF-VD-CC, the non-metallic inclusions from non-quenched and tempered steel with high quality are investigated in detail, and the formation temperature of the major ...inclusion MnS is got based on the thermodynamic calculation. The results show that the w(TO) is controlled to 0.0012%, which is a bit high, should be control to less than 0.0010%. From after vacuum treatment in VD to tundish casting, for inclusion composition, oxide occupies 20%~30%, sulfide occupies 70%~80%. While the length of Si-Ca wires fed is 400m, MnS inclusions showing string-like or slender distribution are not observed, but inclusion shapes are various, not showing spherical or spindle-shaped distribution, and the inclusion whose diameter is more than 10 mu m occupy a large proportion, about 50%~60%. Taking into account element segregation by Scheil's model, when the solid fraction f(s) is about 0.9, MnS inclusion starts to form, which is in reasonable agreement with the results from the typical literatures.
Samples of interstitial free (IF) steel buried in MgO castable bonded by calcium aluminate cement (CA) in graphite crucibles were heated at 1600°C for 90 min. Total oxygen content (TOC) of the steel ...was examined after heating and the refractory was investigated by SEM and EDS. It was found that TOC was higher in IF steel samples in contact with MgO castables containing 3 or 5 wt-% CA than with castables containing 7 wt-% CA or without CA. A liquid layer formed between refractory oxide and molten steel separates the refractory oxides from molten steel and inhibits direct dissolution of oxides in the molten steel. Transfer of oxygen between the refractory oxide and molten steel occurs by the formation of CaO.Fe
2
O
3
at the boundary between the refractory oxide and the liquid layer, diffusion of CaO.Fe
2
O
3
in the liquid phase layer, decomposition of CaO.Fe
2
O
3
and dissolution of FeO into the molten steel. W ith increasing CA content in MgO based castables the CaO content in molten steel increases, but iron oxide content decreases, leading to the result mentioned above.
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