Effect of Rare Earths on Corrosion Resisting Properties of Carbon-Manganese Clean Steels

Electrochemistry experiments were made on carbon-manganese clean steel with rare earths Ce and La respectively to observe corrosion parameters such as corrosion current i_(corr), and characteristic potential of pitting E_b. The results indicate that the rare earths have effect on corrosion resisting properties of carbon-manganese clean steel, and the optimum contents of La is about 0.011% (mass fraction) and Ce about 0.014% (mass fraction) respectively. The change of corrosion resistance is related to the action of rare earths on microstructure and effect on surface state of samples in the process of polarization.

Oxide Inclusions in Ferromanganese and Its Influence on the Quality of Clean Steels

Low and medium carbon ferromanganese produced by oxygen decarburization process and electric silicothermic process was briefly introduced, and the quality of products by these two processes was analyzed. Results showed that the total oxygen content in medium carbon ferromanganese by electric silicothermic process in China, which ranged from 0.039% to 0.171%, was between those of the common and refined products by oxygen decarburization process outside of China. The increments of total oxygen content in liquid steel were estimated when ferromanganese was added for the purpose of Mn element adjustment at the end of smelting. Refined low and medium carbon ferromanganese, which had low total oxygen content, was recommended for composition adjustment of clean steels during final stage of a heat. It is possible that the inclusions in the ferromanganese alloy greatly influenced the quality of clean steel indirectly by affecting the amount, size and composition of inclusions in steel.

Progress on statistical models of evaluating inclusions in clean steels

Non-metallic inclusions are critical for the fatigue failure of clean steels in service;especially,the large and hard inclusions are detrimental.Since it is not possible to measure all the inclusions in the large-volume clean steels,statistical models have been developed to evaluate inclusions,aiming at predicting the maximum inclusion size in the large volume from the data of inclusions,which are derived from the limited observations on small-volume specimens.Different statistical models were reviewed together with their supporting theories.In particular,the block maxima and the threshold types of models were discussed through a thorough comparison as they are both widely used and based on the extreme value theory.The predicted results not only are used to distinguish the different cleanliness levels of steels,but also help to estimate fatigue strength.Finally,future research is proposed to focus on tackling the present difficulties encountered by statistical models,including the sufficient credibility of obtained results and the robustness of models for applications.

Behaviors of Lanthanum and Cerium on Grain Boundaries in Carbon Manganese Clean Steel

The behaviors of La and Ce on gram boundaries in carbon manganese clean steel were investigated by high-reso- lution transmission elecetron microscope （HRTEM）, scanning elecetron microscopy（SEM ）, energy dispersive spectrometer （EDS） and X-ray diffraction（XRD） analysis. The existing forms of rare earths （RE） in clean steel were as follows： dissolved in sohd solution, forming inclusion or second phase containing RE （RE-Fe-P, La-P, Fe-La eutectic and Fe-Ce phase）. The dissolved La or Ce segregated at grain boundaries. The segregation of both S and P at gram boundaries was reduced with suitable RE content. The impact toughness of the steel was improved obviously. La and Ce had effecets on purifying molten steel and modifying inclusions in clean steel, whereas with excessive La or Ce, La-Fe-P, La-P and Fe-La eutecetic phase or Ce-Fe-P and Fe-Ce intermetallic compound would form along grain boundaries, causing the impact energy to decrease significantly.

Effects of Rare Earth on Structure and Mechanical Properties of Clean BNbRE Steel

The mechanisms of RE in clean BNbRE steel were studied by means of experimental measurement, microstructural observation and theoretical analysis. For BNbRE steel, the state and the content of RE were measured, and the effects and the mechanisms of RE on sulfide inclusions, microstructure and properties of steel were determined. On the condition of increasing the cleanliness of steel, the mechanisms of RE in steel were changed to certain degree. Small amount of RE has the effect of cleaning, modifying inclusions and alloying in clean steel, too. With increasing the cleanliness of BNbRE steel, addition of RE should be decreased properly. Under experimental conditions, the optimum addition of RE is -0.01 % （mass fraction） for clean BNbRE steel, while RE can evidently improve plasticity and impact toughness of BNbRE steel.

Current Situation and Development of Refractories for Clean Steel Production

The low cost and high efficient clean steels metallur- gy has been focused much attention in China. Among the factors affecting the cleanliness and consequent quali- ty of steel products, refractories are one of the key fac- tors. Meanwhile the restriction of resource, energy and environment is getting stronger, thus the improvement of refractories becontes much more important. In this paper, refractories selection principle Jbr clean steel production according to their thermodynamic features was ex- plained; refractories for tundLsh and flow control in con- tinuous casting, which play important roles in prohibi- ring secondary contamination of molten steel, were intro- duced. Moreover, new development of low-carbon and carbon-free refractory materials was also described.

Microstructural Features During Strain Induced Ferrite Transformation in 08 and 20Mn Steels

The microstructure evolution during strain induced ferrite transformation was followed in thermal-simulation tests of clean 08 and 20Mn steels. The influences of carbon equivalence and initial austenite grain size on ferrite grain refinement and the volume fraction of ferrite during straining were inspected. The results revealed that the accelerating effect of ferrite transformation by strain was increased as the carbon equivalence decreased. However, finer ferrite grains were obtained at higher carbon content. At strain of similar to1.5 ferrite grains less than 3 mum and 2 mum can be obtained in 08 and 20Mn steels respectively. Whereas the ferrite grain refinement in 08 steel was due to both effects of strain induced transformation and ferrite dynamic recrystallization, that in 20Mn was mainly due to strain induced transformation. Heavy strain can produce fine ferrite grains in coarse austenite grained 08 steel, but it would lead to band microstructure in coarse austenite grained 20Mn.

Roles and Progress of Refractories for Clean Steel Technology

This paper presents a general survey of roles and progress of fundamental and fitnctional refractories applied in seco Mal：y steelmaking, tundish and mold tnetallurgy along with the evolution of clean steel technology, focusing on refractories that are to create clean environments, benefit cleanliness and productive efficiency for the clean steal production, in particular, reducing the total oxygen content and inclusion defects in steel. All that arguments have revealed in abundant that the interaction between the clean steel technology and inno vation of refractories is so intimate that the philosophy of clean steel technology would not be accomplished without functioning of the innovated state of the arts refractories in it.

Comparison of Ferrite Refinement Mechanisms by Different Processing Schedules in 08 and 20Mn Steels

The influence of deforming temperature on ferrite refinement was analyzed by comparing the microstructures obtained by deformation at above A(r3), in two-phase region of (alpha + gamma) and at below A(t) in clean 08 and 20Mn steels. The results indicate that ferrite refinement through strain induced transformation by deformation at above A(r3) is more effective than that by deformation simply through ferrite dynamic recrystallization. The main problem of ferrite refinement by deformation at below A(r3) is the inhomogeneity of microstructure which is controlled by the orientations and sizes of ferrite grains and the distribution of second phases. Ferrite dynamic recrystallization after strain induced transformation can further effectively refine ferrite.

RESEARCH ON AS-CAST GRAIN REFINING TECHNOLOGY BY INOCULATING CLEAN STEEL WITH TITANIUM OXIDE

Precipitation condition and nucleation of Ti2O3 during solidification of clean steel have been studied. The behavior of Ti2O3 during solidification as well as possibility of using Ti2O3 to refine as-cast grains as heterogeneous nucleation sites is discussed. It is shown that Ti2O3 can be formed provided that the steelmaking and solidification process is controlled strictly. And inoculating clean steel with Ti2O3 is an effective way to refine as-cast grains.

Kinetics Analysis of Pre-Eutectoid Transformation of Austenite in Carbon-Manganese Clean Steel with Rare Earths

By means of rapid cool dilatometry, the influence of rare earths on kinetics of pre-eutectoid transformation of austenite of carbon-manganese clean steel was studied. While the content of rare earths were proper, the nucleation of pre-eutectoid ferrite on the grain boundary of austenite was accelerated and the incubation period of pre-eutectoid transformation was shortened. But while rare earths were excessive, opposite actions appeared. Rare earths decreased transformation velocity constant in the process of transformation from austenite to ferrite, which meant that the growth of per-eutectoid ferrite became slow. The reason above varieties was related to effect of rare earth on grain boundary constitution of austenite and diffusion of carbon element.

Enhanced clean steel tapping on BOF vessels with the interstop converter gate type CG series

Trends in today' s steel production show increasing demands for highly-sophisticated steel grades. Therefore,the steel industry will eventually be required to install best performing technologies to meet governmental specifications designed to significantly reduce environmental emissions.Such efforts are supported by innovative new solutions in the entire steel making process.One process that will need to be addressed is the cleanliness of the steel in the liquid steel phase,especially during the BOF-tapping process. The new converter gate(CG)(Fig.1) best supports the slide gate tapping technology for converters in terms of clean steel,productivity,yield and availability-at reduced costs. The CG gate introduces a unique feature for converter slide gates:The bottom plate can be exchanged while the gate remains mounted on the converter.This new feature allows for an increased utilisation of the refractory parts and a significant reduction of the downtime of the converter.This 'designed efficiency' is achieved with a refractory set consisting of three identical plates-two bottom plates and one slider plate-and a nozzle.A number of other improvements such as automatic cylinder coupling using the Interstop SNAP technology and a two step tensioning system have also been implemented. The new system is designed to meet minimal space requirements and therefore will adapt to most BOF shops.To perform preparation,service and maintenance work the modular CG gate can be dismounted from the converter and simultaneously replaced by a ready-to-use unit.The degree of automation for dismounting and mounting as well as for off-line preparation has been enhanced and reduces manual efforts. This paper provides a solution to the obvious trend towards efficient and controlled production of clean steel,in general,and especially at BOF shops.Moreover,it specifically describes the operational results of this innovative BOF tapping equipment at Salzgitter Flachstahl GmbH,Germany.

Inclusion control of ultra clean ferritic stainless steel

The most important characteristics of the ultra clean ferritic stainless steel is that the carbon,nitrogen and other interstitial elements are very low.The ultra clean ferritic stainless steel has been widely used for household appliances,auto exhaust system,elevator,water treatment system,building roof and other various fields,because of its low cost,pro-environment,excellent properties.They can replace some traditional austenitic stainless steel.such 304 and 316L.The addition of titanium to liquid steel has become common for stabilizing nitrogen and carbon in steel.Titanium reacts with nitrogen,carbon,and oxygen to form titanium nitride,carbide, carbonitride and oxide.These inclusions may have a deleterious effect on the properties such as toughness, ductility,weldability and corrosion.In addition,the inclusions can also agglomerate and cause surface quality problem of the slab and clogging of the submerged entry nozzle during continuous casting process. The formation rules of inclusions in ultra clean ferritic stainless steel were investigated by the thermodynamic calculation,and methods of controlling inclusions were put forward to improve the quality of product.The composition,type,amount,size and distribution of the inclusions in the slab are investigated by optical microscope and scanning electron microscope.The results can be concluded as follows.(1) In the slab,the main original inclusions,with size of larger than 2μm,are Al_2O_3,TiN or Ti(CN) and complex TiN or Ti(CN) inclusion with core of MgO,MgO-Al_2O_3 and Ti_2o_3,which will not affect the performance of the steel if they are in diffusing distribution.The size of these inclusions are less than 10μm except some Al_2O_3 inclusions in size of 10-40μm.(2) Foreign inclusions are Ti_2O_3-Al_2O_3-MgO-SiO_2 in size above 50μm covered by TiN or Ti (CN).Although these inclusions are few,they do harm to the surface quality of stainless steel.It is able to reduce the risk of forming this kind of inclusion by aluminium deoxidation with increasing Al content to restrain the formation of Ti_2O_3.(3 ) TiN or Ti(CN) is easy to precipitate on inclusions such as MgO,MgO-Al_2O_3 and Ti_2O_3,except Al_2O_3.Controlling the content of[Ti]and[N]and the formation of the oxides can be used to control the precipitation of TiN or Ti(CN).

Repetitive cleaning of a stainless steel first mirror using radio frequency plasma

First mirrors（FMs） are crucial components of optical diagnostic systems in present-day tokamaks and future fusion reactors. Their lifetimes should be extremely limited due to their proximity to burning plasma, greatly influencing the safe operation of corresponding diagnostics.Repetitive cleaning is expected to provide a solution to the frequent replacement of contaminated FMs, thus prolonging their lifetimes. Three repetitive cleaning cycles using radio frequency plasma were applied to stainless steel（SS） FM samples, to evaluate the change of the mirrors＇ optical properties and morphology during each cycle. Amorphous carbon films were deposited on mirror surfaces under identical conditions in three cycles. In three cycles with identical cleaning parameters, the total reflectivity was restored at up to 95%. Nevertheless, with successive cleaning cycles, the FM surfaces gradually appeared to roughen due to damage to the grain boundaries. Correspondingly, the diffuse reflectivity increased from a few percent to 20%and 27% after the second and third cycles. After optimizing the cleaning parameters of the second and third cycles, the roughness showed a significant decrease, and simultaneously the increase of diffuse reflectivity was remarkably improved.

TUNDISH METALLURGY AND ITS DEVELOPMENT

The research on tundish metallurgy has focused mainly on taking countermeasures to avoid and remove inclusions and the recent developments on tundish metallurgy were surveyed.The authers'advices for future development of tundish metallurgy were given in summary section.

Evaluation of the Electroslag Remelting Process in Medical Grade of 316LC Stainless Steel

This study is focused on the effects of electroslag remelting by prefused slag （CaO, Al2O3, and CaF2） on macrostructure and reduction of inclusions in the medical grade of 316LC （316LVM） stainless steel. Analysis of the obtained results indicated that for production of a uniform ingot structure during electroslag remelting, shape and depth of the molten pool should be carefully controlled. High melting rates led to deeper pool depth and interior radial solidification characteristics, while decrease in the melting rates caused more reduction of nonmetallic inclusions. Large shrinkage cavities formed during the conventional casting process in the primary ingots were found to be the cause of the fluctuation in the melting rate, pool depth and extension of equiaxed crystals zone.

Dephosphorization Thermodynamics of Crude Steel Using CaO-Based Flux

The experiments were carried out at 1 175-1 450 ℃ to study the phosphorus distribution ratio between molten Ca O-based slags and carbon saturated hot metal. The phosphate capacity of the slags and the activity coefficient of PO2. 5were calculated from the phosphorus distribution ratio. The effect of addition of Ca F2 and / or Ca Cl2 on the thermodynamic properties of slags was discussed. The correlation of optical basicity with phosphate capacity of slags was studied,and the dephosphorization ability of Ca O-based fluxes with various additives was compared with the results from different studies.

Influence of flow control devices on metallurgical effects in a large-capacity tundish

The influence of flow control devices on metallurgical effects in alarge-capacity tundish has been studied carefully with elements tracing, sampling and theoreticalprospecting. The results from the studies arc (1) in the continuous casting of clean steel, badcontrol of tundish operation may deteriorate the cleanliness of steel; (2) the cleanliness of steelis deteriorated mainly at the unsteady state; (3) large amount of macro inclusions come from the topslag and the refractory of tundish; (4) installing dam and weir can improve the cleanliness ofsteel and lighten the influence of steel fluctuation; and (5) the result of theoretical calculationsuggests that the inclusions larger than the critical size of 92.6 mu m can be floated out from thetundish bath completely.

Mold flux entrainment mechanisms

This paper presents a comprehensive and critical review of mold flux entrainment mechanisms in continuous casting of steel.Entrainment introduces inclusions into the final product,and thus greatly hinders clean steel production.By understanding the mechanisms that cause entrainment,the operating conditions of casters can be tuned to reduce the number of defects.Nine different mechanisms have been proposed over the last three decades,including vortex formation around the submerged entry nozzle(SEN),argon bubble interactions with the slag layer,shear-layer instability at the slag-steel interface,excessive upward flow impingement upon the meniscus, top surface level fluctuations,meniscus freezing and hook formation,top surface "balding",top surface standing wave instability,and slag crawling down the SEN.The previous work done for each of these mechanisms is presented,including both quantitative and qualitative descriptions of their behavior.

Innovative Lining of Unburnt Al_(2)O_(3)-MgO Brick for 300t Steel Ladle in Baosteel

The manufacture of clean steel needs high performance carbon-free bricks for ladle lining.Based on long term application experiences of the prefab Al_(2)O_(3)-MgO blocks,unburnt Al_(2)O_(3)-MgO brick has been developed by pressing a mix of alumina,spinel and magnesia as major raw materials and Al_(2)O_(3)-MgO gel powder as binder.In addition to low open porosity and high strength,the unburnt Al_(2)O_(3)-MgO brick shows superior corrosion resistance and thermal shock resistance to the prefab block.Field trials in a 300t steel ladle have indicated the residual lining of unburnt Al_(2)O_(3)-MgO bricks showed smooth surface without large spalling and seldom steel infiltration,consequently longer service life than the prefab blocks.The reliable,unburnt Al_(2)O_(3)-MgO bricks have more advantages of high efficiency,energy-saving and eco-friendly production.It will be one of the best options for ladle lining in the manufacture of high-purity steel.