炉外精炼 铁水预处理2.ppt

1、3、 Fundmental principles for steel refining 3.1 Degassing 3.2 Inclusion Removal,3.1 Degassing 3.1.1 Basic concepts,Measures to be taken for hydrogen removal from molten steel (1)Dry materials (alloy,flux,deoxidizer) added ; (2) Steel tapping ladle to be heated more than 800； (3)High vacuum degree- v

2、acuum chamber pressure having to be reduced as lower as possible； (4)Strong stirring; (5) Air-sealed pouring and casting; (6) Weather,Measures to be taken for nitrogen removal from molten steel (1) Materials (alloy,flux,deoxidizer,recarbon agent) added in molten steel or slag being dryer and nitroge

3、n contained lower enough; (2) Steel tapping ladle to be heated more than 800； (3)High vacuum degree- vacuum chamber pressure having to be reduced as lower as possible； (4)Strong stirring; (5) Air-sealed pouring and casting; (6) Surfactant in steel ;(7) Weather,3.1.2 Degassing process (1) Hydrogen re

4、moval Hydrogen degassing is applied in the case of steel grades which are sensitive to flakes, i.e., forging ingots,rail steel,pipe steel and heavy plates. In case of hydrogen degassing,the pressure in the vacuum vessel and the amount of stirring/circulation gas are essential factors. The solubility

5、 of hydrogen in steel at 1mba is lower than 1ppmH. In practice,contents of less than 2ppm Hare usually requested. It is therefore of importance to keep the melt under very low pressure in order to achieve the target value.,The figure above illustrates the decrease in hydrogen in relation to time as

6、a function of different specific inert gas flow rates for VD process. Intensive inert gas stirring results in short time ranges for removal of hydrogen. Practical experience has shown that a maximum flow rate of 34L/(min.t) should be applied in order to avoid possible slag reactions resulting in unc

7、ontrolled foaming.,To compensate for such possible reaction, a steel freeboard exceeding 900mm within a new lined ladle is a precondition for hydrogen removal in a VD unit.In order to achieve reasonable treatment time,the amount of stirring gas,compared with the treatment for hydrogen removal is sub

8、stantially higher,resulting in a steel freeboard exceeding 1200mm within a new lined ladle.,The figure above demonstrates the time ranges required for different starting contents to achieve less than 2 ppm hydrogen as treatment target for RH process. The inert gas injection rate into the up-leg snor

9、kel in order to initiate steel circulation through the vacuum vessel depends on the chosen snorkel diameter.Usually approx. 810L/(min.t) are injected. There are no steel freeboard restrictions to be considered since all reactions take place inside the vacuum vessel.,Comparison VD/RH process Both pro

10、cesses are capable of reducing the hydrogen content to the desired level within similar treatment times. Due to lower investment and operation costs,the VD process is the first choice in case hydrogen removal is the primary treatment target and provided that the required freeboard can be made availa

11、ble.,(2) Nitrogen removal Apart from the fact that mass transfer occurs more slowly in the case of nitrogen than of hydrogen due to its lower coefficient of diffusion, even an only minimal oxygen and sulphur content causes a further deceleration of the removal of nitrogen as a result of the surfacta

12、nt properties of these elements. The precondition for effective denitrogenation is,therefore, a fully killed steel with a low sulphur content.,The solubility of nitrogen in steel at 1 mbar is 14ppmN. In practice, values below 50ppm N are requested for special applications,such as sour gas resistant

13、pipe qualities(HIC).For the production of low carbon and low nitrogen steels through decarburization (IF steels),the usual practice is to arrive at a low nitrogen content during operation of the melting unit.,Typical experimental results,Ar ,Nm3/t steel,VD process Research demonstrates the relation

14、between the stirring energy to improve the nitrogen diffusion,and the degree of nitrogen removal in a VD unit based on different sulphur contents. If a low nitrogen content is the desired result of the ladle treatment,the sulphur content has to be very low.Due to slag reaction during VD process,the

15、sulphur can be lowered such that nitrogen reduction is enabled.,RH process Due to the lack of slag reaction during the RH process,nitrogen removal is only achieved in cases where the steel has been desulphurization prior to RH treatment.Processes have been developed recently,which enable the injecti

16、on of powdered agents by means of a top blowing lance,also used for oxygen blowing,inside the degassing vessel.,Map of De-N mechanism in BOF,Air,VD/RH Treatment cycle times In order to integrate the vacuum processes between melting and casting units,especially to enable sequence casting procedure,it

17、 is important to evaluate the total cycle times of the ladle in operation before deciding on the plant design. A cycle time comparison between VD and RH processes for the different kinds of vacuum metallurgy processes is based on the following principle plant design: Both plants are placed between t

18、he melting and casting unit such that a vacuum tank car(VD) and a ladle car(RH) receive the ladle for transfer into the treatment station.After having finished the treatment,the ladle is then driven to the casting bay for further crane handling to the caster turret. The following investigations rega

19、rding cycle time have been made,based on a heat size of 200t for an evaluation of temperature losses.,Cycle time comparison for hydrogen removal The cycle times for removal of hydrogen in the VD and RH processes is important.In the VD route the longer handling times,which are mainly due to manual co

20、nnection of inert gas to the porous plugs are compensated by shorter treatment times. As a result, the VD and RH cycle times are similar and will suit normal sequence casting times. The temperature losses due to handing and treatment are similar.,Cycle times for nitrogen/sulphur removal Research sho

21、ws the cycle times for desulphurization to the lowest values,followed by nitrogen removal. As already indicated,with the RH process this procedure is not possible due to the lack of slag reaction. Due to the time-consuming treatment.including wire addition for better castability, the total cycle tim

22、e might exceed sequence casting times. In this case two treatment stations have to be installed. As the time under vacuum is only a part of the total treatment time,the installation of a common vacuum pump for alternative use is sufficient. A considerable temperature loss has to be taken into consid

23、eration.,3.2 Inclusion Removal Whenever a polished section of a steel, or almost any metal that has solidified from a liquid state, is carefully examined, small quantities of nonmetallic phase always will be found. Such nonmetallic matter may occur in a wide variety of contents, compositions, partic

24、le size, shape, and distribution, but no means have yet been developed to make commercial steel wholly free of it, despite extensive efforts have been accomplished till now.,Nature and Composition of Nonmetallic Inclusions Nonmetallic inclusions naturally fall into three general groups:(1) those tha

25、t originate by precipitation from molten or freezing steel; (2)those produced from mechanical or chemical erosion by the molten on refractories or other materials with which it comes in contact; (3)by direct air oxidation of the liquid metal, and entrained in the flowing steel.,Usually nonmetallic i

26、nclusions are distinctly differentiated on the basis of size,composition, and distribution.The first group is regarded as endogenous,natural,or indigenous while the latter are termed exogenous,accidental,adventitious or foreign inclusions.,The endogenous inclusions, in general, are small, numerous, rather uniformly dispersed in any part of an casting body. They are, i