Characteristics of Al_2O_3, MnS, and TiN inclusions in the remelting process of bearing steel

The Al_2O_3, MnS, and TiN inclusions in bearing steel will deteriorate the steel's mechanical properties. Therefore, elucidating detailed characteristics of these inclusions in consumable electrode during the electroslag remelting process is important for achieving a subsequently clean ingot. In this study, a confocal scanning violet laser microscope was used to simulate the remelting process and observe, in real time, the behaviors of inclusions. The obtained images show that, after the temperature exceeded the steel solidus temperature, MnS and TiN inclusions in the specimen began to dissolve. Higher temperatures led to faster dissolution, and the inclusions disappeared before the steel was fully liquid. In the case of an observed Al_2O_3 inclusion, its shape changed from angular to a smooth ellipsoid in the region where the solid and liquid coexisted and it began to dissolve as the temperature continued to increase. This dissolution was driven by the difference in oxygen potential between the inclusion and the liquid steel.

Effect of slag on oxide inclusions in carburized bearing steel during industrial electroslag remelting

Industrial experiments with three types of slags were performed to investigate the effect of slag on oxide inclusions during electroslag remelting(ESR) process. G20CrNi2Mo bearing steel was used as the consumable electrode and remelted using a 2400-kg industrial furnace. The results showed that most inclusions in the electrode were low-melting-point CaO-MgO-Al_2O_3. After ESR, all the inclusions in ingots were located outside the liquid region. When the slag consisted of 65.70 wt% CaF_2, 28.58 wt% Al_2O_3, and 4.42 wt% CaO was used, pure Al_2O_3 were the dominant inclusions in ingot, some of which presented a clear trend of agglomeration. When the ingot was remelted by a multi-component slag with 16.83 wt% CaO, a certain amount of sphere CaAl_4O_7 inclusions larger than 5 μm were generated in ingot. The slag with 8.18 wt% CaO exhibited greater capacity to control the inclusion characteristics. Thermodynamic calculations indicated that the total Ca and Mg in ingots were attributed from the relics in electrode and strongly influenced by the slag composition. The formation of ingot inclusions was calculated by FactSage^(TM) 7.0, and the results were basically in accordance with the observed inclusions, indicating that a quasi-thermodynamic equilibrium could be obtained in the metal pool.

A coupled model of TiN inclusion growth in GCr15SiMn during solidification in the electroslag remelting process

TiN inclusions observed in an ingot produced by electroslag remelting （ESR） are extremely harmful to GCrl5SiMn steel. Therefore, accurate predictions of the growth size of these inclusions during steel solidification are significant for clean ESR ingot production. On the basis of our previous work, a coupled model of solute microsegregation and TiN inclusion growth during solidification has been established. The results demonstrate that compared to a non-coupled model, the coupled model predictions of the size of TiN inclusions are in good agreement with experimental results using scanning electron microscopy with energy disperse spectroscopy （SEM-EDS）. Because of high cooling rate, the sizes of TiN inclusions in the edge area of the ingots are relatively small compared to the sizes in the center area. During the ESR process, controlling the content of Ti in the steel is a feasible and effective method of decreasing the sizes of TiN inclusions.

A process model for BOF process based on bath mixing degree

The process model for BOF process can be applied to predict the liquid steel composition and bath temperature during the whole steelmaking process. On the basis of the traditional three-stage decarburization theory, the concept of mixing degree was put forward, which was used to indicate the effect of oxygen jet on decarburization. Furthermore, a more practical process model for BOF steelmaking was developed by analyzing the effect of silicon, manganese, oxygen injection rate, oxygen lance height, and bath temperature on decarburization. Process verification and end-point verification for the process model have been carried out, and the verification results show that the predic- tion accuracy of carbon content reaches 82.6% （the range of carbon content at the end-point is less than 0. 1wt%） and 85.7% （the range of carbon content at end-point is 0. 1wt% -0.7wt%） when the absolute error is less than 0.02wt% and 0.05wt%, respectively.

Roles of titanium-rich precipitates as inoculants during solidification in low carbon steel

The effect of titanium on the as-cast structure and the growth form of titanium precipitates, and the effect of cooling rate on the size and distribution of titanium precipitates were studied. It is shown that Ti-rich precipitates acting as heterogeneous nucleation sites play an important role in refining the grain size and increasing the equiaxed grain ratio. Cooling rate has a great effect on the size and distribution of precipitates. The number of precipitates increases and the size decreases with the increase of cooling rate. Ti-rich particles acting as het- erogeneous nucleation sites at the onset of solidification are observed in the experiment. This result suggests that TiN nucleated on Ti2O3 is an effective inoculant for δ-ferrite during solidification in low carbon steel.

Mechanism of generation of large(Ti,Nb,V)(C,N)-type precipitates in H13 + Nb tool steel

The characteristics and generation mechanism of（Ti,Nb,V）（C,N） precipitates larger than 2 μm in Nb-containing H13 bar steel were studied. The results show that two types of（Ti,Nb,V）（C,N） phases exist—a Ti-V-rich one and an Nb-rich one—in the form of single or complex precipitates. The sizes of the single Ti-V-rich（Ti,Nb,V）（C,N） precipitates are mostly within 5 to 10 μm, whereas the sizes of the single Nb-rich precipitates are mostly 2–5 μm. The complex precipitates are larger and contain an inner Ti-V-rich layer and an outer Nb-rich layer. The compositional distribution of（Ti,Nb,V）（C,N） is concentrated. The average composition of the single Ti-V-rich phase is（Ti_（0.511）V_（0.356）Nb_（0.133））（CxNy）, whereas that for the single Nb-rich phase is（Ti_（0.061）V_（0.263）Nb_（0.676））（C_xN_y）. The calculation results based on the Scheil–Gulliver model in the Thermo-Calc software combining with the thermal stability experiments show that the large phases precipitate during the solidification process. With the development of solidification, the Ti-V-rich phase precipitates first and becomes homogeneous during the subsequent temperature reduction and heat treatment processes. The Nb-rich phase appears later.

Influence of refining process and utilization of different slags on inclusions, titanium yield and total oxygen content of Ti-stabilized 321 stainless steel

Ti-stabilized 321 stainless steel was prepared using an electric arc furnace, argon oxygen decarburization (AOD) furnace, ladle furnace (LF), and continuous casting processes. In addition, the effect of refining process and utilization of different slags on the evolution of inclusions, titanium yield, and oxygen content was systematically investigated by experimental and thermodynamic analysis. The results reveal that the total oxygen content (TO) and inclusion density decreased during the refining process. The spherical CaO–SiO2–Al2O3–MgO inclusions existed in the 321 stainless steel after the AOD process. Moreover, prior to the Ti addition, the spherical CaO–Al2O3–MgO–SiO2 inclusions were observed during LF refining pro-cess. However, Ti addition resulted in multilayer CaO–Al2O3–MgO–TiOx inclusions. Two different samples were prepared by conventional CaO–Al2O3-based slag (Heat-1) and -TiO2-rich CaO–Al2O3-based slag (Heat-2). The statistical analysis revealed that the density of inclusions and the -TiOx content in CaO–Al2O3–MgO–TiOx inclusions found in Heat-2 sample are much lower than those in the Heat-1 sample. Furthermore, the TO content and Ti yield during the LF refining process were controlled by using -TiO2-rich calcium aluminate synthetic slag. These results were consistent with the ion–molecule coexist-ence theory and FactSage?7.2 software calculations. When -TiO2-rich CaO–Al2O3-based slag was used, the -TiO2 activity of the slag increased, and the equilibrium oxygen content significantly decreased from the AOD to LF processes. Therefore, the higher -TiO2 activity of slag and lower equilibrium oxygen content suppressed the undesirable reactions between Ti and O.

Detection and analysis of magnetic particle testing defects on heavy truck crankshaft manufactured by microalloyed medium-carbon forging steel

An unqualifed six-cylinder heavy truck crankshaft has been studied to investigate the cause of magnetic particle testing defects on the rod journals.Large-sized long-striped MnS inclusions are regarded as the major cause for the magnetic particle testing failure because they have been detected in situ under the magnetic particle indications.Through the observation of macroscopic structures of the rod journals and corresponding counterweight blocks,it is found that for the 1#and 3#rod journals,the center metal of the original hot-rolled bar has been extruded to the inboard edge of the rod journals and large-sized long-striped MnS inclusions are exposed on the surface after fash removal,leading to the failure of magnetic particle testing.As for the 2#rod journal,the center metal of the original bar has not been extruded to the surface and MnS inclusions on the rod journal surface are small in size,few in number,resulting in passing the magnetic particle testing.If the quality of the hot-rolled bars fuctuates,it is more recommended to apply magnetic particle testing on samples at the center of bars before forging to evaluate the severity of defects caused by the long-striped MnS inclusions for fear of the scrap of the fnal crankshafts.

Effects of Temperature and Alloying Elements on γ Phase Fraction of Grain-oriented Silicon Steel

The effects of temperature and alloying elements on γ phase fraction of grain-oriented silicon steel,which contained 2.97-3.42mass% Si and 0.028-0.058mass% C,were studied by microstructure observation and statistics.Furthermore,the quantitative relationships of temperature as well as C,Si,and Mn contents to γ phase fraction were obtained by numerical fitting.The experimental results show that γ phase fraction firstly increases with increasing temperature,reaches a maximum and then decreases in the temperature range of 900-1 250 ℃.The temperature corresponding to the maximum γ phase fraction is about 1 150-1 200℃.Meanwhile,the γ phase fractions in steels at the same temperature have some differences because of different contents of various alloying elements.The verification results show that the values of γ phase fractions to C,Si,and Mn contents at the specific temperatures,which were obtained by multiple linear regression method,agree well with the measured values.In addition,the values of γ phase fractions to C,Si,and Mn contents in the temperature range of 900-1 250℃,which were obtained by binomial regression method,agree with the measured values when the contents of Mn and soluble Al are not more than 0.320mass% and 0.034mass%,respectively.The obtained equations can carry out the approximate prediction of γ phase fractions of grain-oriented silicon steels during the hot rolling process.

Precipitation behavior of titanium nitride on a primary inclusion particle during solidification of bearing steel

Titanium nitride precipitation on a primary inclusion particle during solidification of bearing steel has been tracked by varying temperature in a confocal scanning violet laser microscope.Upon precipitation,an obvious growth of titanium nitride on a primary inclusion particle was observed due to the rapid solute diffusion in liquid steel.The onset of titanium nitride precipitation did not change with primary inclusion particle size,but the time of growth was greater for a smaller primary inclusion particle.Meanwhile,the particle size displayed little influence on the total precipitated amount of titanium nitride on it under the same conditions.At the later period of solidification,almost no change occurred in inclusion size,but the inclusion shape varied from circle to almost square in two-dimension,or cubic in three-dimension,to attain the equilibrium with steel.

Mathematical model for design of optimized multi component slag for electroslag remelting

Slag is the heart of electroslag remelting(ESR)process.A new mathematical model to design the optimized slag for ESR was developed based on slag–metal equilibrium theory,ion and molecule coexistence theory and modified Butler’s equation.It was assumed that an overall thermodynamic equilibrium did exist at electrode tip–slag interface.With this model,the equilibrium slag and its surface tension could be obtained quantitatively when the initial compositions of consumable electrode were given.An industrial experiment with four types of slags was carried out in a special steel plant in China.The variation of Al,Si and Mn corresponded well with the deviation of corresponding oxide from equilibrium,reflecting the reasonability of the model.Besides that,the effects of Al in electrode as well as CaO,CaF2 and MgO in slag on the equilibrium slag,dissolved oxygen and surface tension were discussed in detail.

Design of a submerged entry nozzle for optimizing continuous casting of stainless steel slab

To solve slag entrapment and casting slab defects in the process of stainless steel continuous casting,submerged entry nozzle(SEN)for slab casters operating at casting speed of 1 m/min was developed based on 3D numerical simulation and water modeling experiments by controlling the outlet shape and angle of original SEN with oval and 15°angle outlet under current industrial use.Mathematical simulations of fluid velocity at outlets with different shapes and angles of SENs have been carried out.The results showed that oval outlet with 5°and 15°angle led to asymmetric rotating flow pattern at outlet,as well as square outlet with 15°angle,but symmetric flow pattern formed at square outlet with 5°angle.The effect of these SENs on meniscus stability,flow field and slag entrapment behavior of stainless steel slab casting mold was further studied by water modeling experiments.The results showed that difficult floating fine droplets formed when the angle of outlet was 15°under the dual effect of vortex convection and shear force due to the strong swirling flow from outlet and rotating flow of outlet.However,outlet with 5°angle could lead to the formation of larger slag droplets,while the oval outlet with 5°angle could result in the scour to the mold wall.Thus,the square outlet with 5°angle was a relatively ideal solution for the submerged entry nozzle from the aspects of the stability of the mold and the slag entrapment behavior.After the design of a new SEN according to the experimental result,the solidification structure of continuous casting slab was obviously improved by industrial test.

Effect of Cu on Surface Microcrack of Austenitic Stainless Cold Rolled Coil

In order to find out the cause of surface microcrack on 304 austenitic stainless cold rolled coils which is produced in a steel plant of China, lots of studies have been carried out. The results indicated that the copper guide of steekle mill used in hot rolling process contacts directly with the hot rolled coil, so parts of copper melt and glued to the surface of the stainless steel plates due to a higher temperature of stainless steel plates than the copper melting temperature, which leads to deterioration of austenitic grain boundaries. Shear stress produced in the process of repeatrolling on finishing mill induces the surface microcracks and promotes it. After changing the copper guide to the cast steel one, such kinds of surface microcracks have never appeared.