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Çelikte metalik olmayan kalıntıların oluşumu, etkileri ve mikroskopik yöntemlerle değerlendirilmesi

Formation of non-metallic inclusions in steels, their effects and evaluation with microscopical methods

  1. Tez No: 39392
  2. Yazar: FUAT ÖZEK
  3. Danışmanlar: PROF.DR. FERİDUN DİKEÇ
  4. Tez Türü: Yüksek Lisans
  5. Konular: Metalurji Mühendisliği, Metallurgical Engineering
  6. Anahtar Kelimeler: Belirtilmemiş.
  7. Yıl: 1993
  8. Dil: Türkçe
  9. Üniversite: İstanbul Teknik Üniversitesi
  10. Enstitü: Fen Bilimleri Enstitüsü
  11. Ana Bilim Dalı: Belirtilmemiş.
  12. Bilim Dalı: Belirtilmemiş.
  13. Sayfa Sayısı: 99

Özet

ÖZET Bu çalışmada; farklı üretim yöntemlerine göre türetilmiş çelik malzemelerde oluşan kalıntı türlerinde sıcak deformasyon oranının artışı ile birlikte kalıntı ların boyut ve dağılımındaki değişim incelenmiştir. Bu amaçla; EAF + Ingot döküm prosesine göre üretilmiş St 37 kalitesindeki (p 8 mm çapındaki beton çeliğinden haddele me sırasında alınan farklı deformasyon oranlarındaki nu muneler ile EAF + Pota fırını + Sürekli döküm prosesine göre üretilmiş aynı kalitedeki U 80 profili kullanılmış tır. Bu numunelerde; makroskopik kalıntı incelenmesi amacı ile mavi kırılma ve kükürt baskısı deneyleri yapı larak makro inklüzyonların dağılımı saptanmıştır. Mik- roskopik inceleme için de kalıntıların boyut ve dağılım larını belirlemek amacı ile DİN 50602 K4 ve ASTM E45 standartlarına göre kalıntı analizi yapılmış ve genel indeks K4 değerleri belirlenmiştir. Ayrıca yapıdaki metalik olmayan kalıntıların me kanik özellikler üzerindeki etkilerini belirlemek için; çekme, basma ve eğme deneyleri yapılmıştır. Numunelere uygulanan mekanik deneylerin sonuçla rına göre; malzemeler yüksek mukavemet ve süneklik özel likleri gösterdiğinden, bileşimdeki kükürt miktarının standartların içinde fakat oldukça yüksek görünmesine rağmen, yapıda mekanik özellikler açısından herhangibir olumsuzluğu görülmemiştir. Mavi kırılma makroskopik kalıntı analizlerinden elde edilen 0 ve 1 değerleri, makroskopik kalıntıların yapıda az sayıda ve küçük boyutlu olduğunu göstermiştir. DİN 50602 K4 testine göre yapılan kalıntı analiz lerinden elde edilen sonuçlara göre, çelikte sıcak de formasyon oranının artışı ile birlikte kalıntıların de forme olarak uzadıkları ve genel indeks K4 değerlerinin arttığı görülmüştür. Buna göre? sülfür kalıntılarının boyutları, kalıntı değerlendirme sayısı 4 olan 210-470 pm uzunluklarından kalıntı değerlendirme sayısı 7 olan 700-1580 pm. uzunluklara kadar değişmektedir. Sı cak deformasyon oranının artışı ile birlikte alümina ka lıntılarının uzunluklarının da 140-470 jum'den 70-105 jum mertebesine değiştiği görülmüştür. vı

Özet (Çeviri)

FORMATION OF NON-METALLIC INCLUSIONS IN STEELS, THEIR EFFECTS AND EVALUATION WITH MICROSCOPICAL METHODS SUMMARY Non-metallic inclusions are non-metallic com pounds which are present in steel. Non-metallic inclu sions in steel generally consist of oxides, silicates and sulfides, but also contain phosphates, selenites, nitrides and carbides. Non-metallic inclusions are basica two main groups. These are (1) inclusions consequence of reactions in the molten st of a change in temperature, and (2) inclu inadvertently entrapped in the steel and particles of refractories or other materi the molten steel comes in contact during from furnace to ladle to mold. Inclusion group, usually given the descriptive adje nous, natural or native. Those of the se which by their nature occur sporadically termed exogenous, adventitious, or accide nous inclusions include a generally large ic occurrence, preferred location in ingo irregular shapes and complex structure. lly fall into that form as a eel or because sions that are consist of als with which its passage s of the first ctives indige- cond group, are commonly ntal. Exoge- r size, sporad- t or casting, In open-heart and oxygen-steelmaking process, the metallic charge to the furnace consists of hot metal from the blast furnace and steel scrap. In most elec tric furnace steelmaking, the steel scrap is used en tirely as the metallic charge material. In both cases, the impurities in the metal, such as carbon, silicon, manganese, and phosphorus, as well as some iron are removed by oxidizing. A molten slag is formed by reac tion of the oxides of iron, silicon, manganese, and phosphorus with the calcium oxide (burnt lime) that is added to the furnace together with the metallic charges. vixinclusions of the MnS-type are considered to be of advantage for the machinability of the steel. Inclusions which are brittle as compared with the steel and retain their shape during working processes (V = 0) are much more detrimental to the fatigue proper ties than inclusions which deform plastically (V > 0). Inclusions which have an index of def ormability V = 1 are the least harmful. Sulfide inclusions have a index of def ormability at all temperatures. No cracks are developed at the inclusion/steel interface during working of the steel. Oxide inclusions are in general harmful to the fatigue properties, but differences exit between differ ent oxide inclusion phases and their size and position are also of importance: Those most detrimental to fatigue properties are the spherical, non-def ormable calcium aluminate inclusions and alumina inclusions, that is, oxide inclusions with an index of def ormability ?^ = 0. Both these inclusion types retain their shape during steel deformation. Cavities may be developed around the spherical calcium aluminate inclusion. The alumina inclusions are often angular and are the nuclei of cracks. Non-metallic inclusions may be introduced into weld metal from parent metal, from the electrode materi al, from the non-metallic coating of the electrode or by entrapment of the electrode slag. Possible effects are laminations in the unmelted heat-affected zone, burning and hot tearing from sulfides as well as crack formation from larger oxide inclusions. Non-metallic inclusions can also affect mechani cal properties of steel, badly. These effects are that the upper yield point is suppressed, elastic limit and maximum tensile strength decrease, ductility also de crease. The first stage in the formation of the new phase inside of a phase is the nucleation stage. The nuclea- tion is not only realized in a liquid metal homogeneous ly, but also heterogeneously realized on a stranger matter which is present in metal.inclusions of the MnS-type are considered to be of advantage for the machinability of the steel. Inclusions which are brittle as compared with the steel and retain their shape during working processes (V = 0) are much more detrimental to the fatigue proper ties than inclusions which deform plastically (V > 0). Inclusions which have an index of def ormability V = 1 are the least harmful. Sulfide inclusions have a index of def ormability at all temperatures. No cracks are developed at the inclusion/steel interface during working of the steel. Oxide inclusions are in general harmful to the fatigue properties, but differences exit between differ ent oxide inclusion phases and their size and position are also of importance: Those most detrimental to fatigue properties are the spherical, non-def ormable calcium aluminate inclusions and alumina inclusions, that is, oxide inclusions with an index of def ormability ?^ = 0. Both these inclusion types retain their shape during steel deformation. Cavities may be developed around the spherical calcium aluminate inclusion. The alumina inclusions are often angular and are the nuclei of cracks. Non-metallic inclusions may be introduced into weld metal from parent metal, from the electrode materi al, from the non-metallic coating of the electrode or by entrapment of the electrode slag. Possible effects are laminations in the unmelted heat-affected zone, burning and hot tearing from sulfides as well as crack formation from larger oxide inclusions. Non-metallic inclusions can also affect mechani cal properties of steel, badly. These effects are that the upper yield point is suppressed, elastic limit and maximum tensile strength decrease, ductility also de crease. The first stage in the formation of the new phase inside of a phase is the nucleation stage. The nuclea- tion is not only realized in a liquid metal homogeneous ly, but also heterogeneously realized on a stranger matter which is present in metal.inclusions of the MnS-type are considered to be of advantage for the machinability of the steel. Inclusions which are brittle as compared with the steel and retain their shape during working processes (V = 0) are much more detrimental to the fatigue proper ties than inclusions which deform plastically (V > 0). Inclusions which have an index of def ormability V = 1 are the least harmful. Sulfide inclusions have a index of def ormability at all temperatures. No cracks are developed at the inclusion/steel interface during working of the steel. Oxide inclusions are in general harmful to the fatigue properties, but differences exit between differ ent oxide inclusion phases and their size and position are also of importance: Those most detrimental to fatigue properties are the spherical, non-def ormable calcium aluminate inclusions and alumina inclusions, that is, oxide inclusions with an index of def ormability ?^ = 0. Both these inclusion types retain their shape during steel deformation. Cavities may be developed around the spherical calcium aluminate inclusion. The alumina inclusions are often angular and are the nuclei of cracks. Non-metallic inclusions may be introduced into weld metal from parent metal, from the electrode materi al, from the non-metallic coating of the electrode or by entrapment of the electrode slag. Possible effects are laminations in the unmelted heat-affected zone, burning and hot tearing from sulfides as well as crack formation from larger oxide inclusions. Non-metallic inclusions can also affect mechani cal properties of steel, badly. These effects are that the upper yield point is suppressed, elastic limit and maximum tensile strength decrease, ductility also de crease. The first stage in the formation of the new phase inside of a phase is the nucleation stage. The nuclea- tion is not only realized in a liquid metal homogeneous ly, but also heterogeneously realized on a stranger matter which is present in metal.There are two main mechanisms which are effective on the inclusion growth. These are growing as a result of diffusion and bonding to each other of inclusions. The contact angle (0) must be higher than ninety degree in order not to break the inclusions from each other. According to Stoke' s law, 1 r 'm v = density of inclusion density of metal bath viscosity radius of inclusion. 6m~6e n High dimensional inclusions rapidly rise in metal bath than small ones. So, alumina and calcium silicate inclusions (6A1 Q = 141°, 6Ca0 = 132°) are separated from steel bath easier than silica and silicate inclu sions. Bubbling of an inert gas (argon or nitrogen) is a simple and widely used method of stirring the metal to facilitate temperature and composition adjustment. The rising gas bubbles can trap non-metallic particles and carry them to the surface where they can be absorbed into a suitable top slag. At the same time, the column of gas bubbles sticks the bath and causes inclusion collision, growth and flotation. Macroscopic and microscopic methods are used for the examination of steels for non-metallic inclusions of sulfide and oxidic nature. Macroscopic methods include macroetch, fracture, step-down and magnetic particle tests. Microscopic methods are performed by means of DIN 50602 and ASTM E45 standards. The macroscopic methods are amply sensitive to reveal the larger inclusions. They don't distinguish between the different types of inclusions such as sul fides, silicates and oxides. They are not suitable for the detection of small globular inclusions or of chains of very fine elongated inclusions. They enable the examination of specimens with large surface areas. Microscopical methods are used to determine the size, distribution number, and type of inclusions. This XIcan be done by the usual procedure of examining speci mens with the microscope and describing the results of the metallographic study in a report which may be illus trated by representative photomicrographs. Extremely small inclusions can be revealed by microscopical meth ods. The aim of this study is to determine the effects of inclusions on mechanical properties of steels pro duced by different production processes. It was also studied that the type, quantity, distribution and dimen sion of inclusions affected by these different pro duction processes (EAF + Ladle furnace + Continuous casting and EAF + Ingot casting). In order to achieve this aim, test samples were subjected to experimental studies. One of these test samples which was produced by EAF + Ingot casting proc ess was called X. The other of these test samples, produced by EAF + Ladle furnace + Continuous casting process, was called Y. X test samples were taken from St 37 grade steel after the deformation at different deformation ratios. Y is U80 profile which is produced from St 37 grade. The specifications related to X and Y samples are as follows. Deformation Chemical Composition Sample Ratio (I) CI Mn% All Sil Hol Nil Crl Cul Snl PI SI Xi 63.0 lx 93.0 X X3 95.0 0.12 1.21 0.010 0.25 0.016 0.083 0.12 0.26 0.034 0.040 0.050 Xq 99.0 Is 99.9 Î 93.0 0.10 0.57 0.020 0.18 0.340 0.26 0.11 0.28 0.043 0.033 0.034 The mechanical tests, tensile, bending and com pression tests, were carried out in the laboratory. After that, blue embrittlement and sulfur development tests were also applied to determine the type and the distribution of macro-inclusions. According to DIN 50602 K4 and ASTM E45 standards, inclusion analyses were also performed and K4 values were obtained from these standards. xxigroup microscopic inclusion distribution. - It was found that with increasing deformation ratio of X samples the form factor of the inclusions was changed from 0.54 to 0.56. As a result, the shape of inclusions became an ellipse. - As the steel cools slowly, the inclusions concentrate on the top of ingots in ingot casting process. During the rolling process, blooms are cut from the both end. Because of this cutting process, the distribution of inclusions decreases. The molten steel cools during the stage of bloom in continuous casting process. So, the inclusions are dispersed in all steel structure. This point should be considered in the continuous casting process using plants. - Because of inclusions have inhomogeneity in steel, the method of sample preparation in DIN 50602 K4 standard does not show the distribution of inclusions in the whole cross-section of the materials. The inclusion analyses should be performed in the whole cross-section of materials. In addition, the inclusion analyses should be performed with samples as much as possible and, the evaluated areas of samples should be equal to each other. xivgroup microscopic inclusion distribution. - It was found that with increasing deformation ratio of X samples the form factor of the inclusions was changed from 0.54 to 0.56. As a result, the shape of inclusions became an ellipse. - As the steel cools slowly, the inclusions concentrate on the top of ingots in ingot casting process. During the rolling process, blooms are cut from the both end. Because of this cutting process, the distribution of inclusions decreases. The molten steel cools during the stage of bloom in continuous casting process. So, the inclusions are dispersed in all steel structure. This point should be considered in the continuous casting process using plants. - Because of inclusions have inhomogeneity in steel, the method of sample preparation in DIN 50602 K4 standard does not show the distribution of inclusions in the whole cross-section of the materials. The inclusion analyses should be performed in the whole cross-section of materials. In addition, the inclusion analyses should be performed with samples as much as possible and, the evaluated areas of samples should be equal to each other. xiv

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