Çok Düşük Karbonlu Çeliklerin Sac Şekillendirilebilirliği

Abstract

20. yüzyıldan farklı olarak 21. yüzyıl kütle üretiminden çok az miktarda ancak kaliteli mamullerin üretilmesinin ağırlık kazanacağı bir süreç olacaktır. Bunun ilk sinyalleri Avrupa ve Amerika' da demir çelik sektöründe yaşanan krizleri takiben çok sayıda çelik devinin tesislerini kapatmak zorunda kalmaları ile kendini göstermiştir. Ancak yüksek teknolojiye sahip, kontrol edilebilir kalitelerde mamuller üretebilen firmaların ayakta kalabileceği ağır tecrübelerle gözler önüne serilmiştir. Globalleşen dünyanın bir gereği olarak ülkeler, aralarındaki sınırlan hiçe sayarak üretim teknolojilerinde standardizasyona yönelmiş ve bu da, üreticileri son derece ağır toplam kalite gereklerine uymaya zorlamıştır. Özellikle otomotiv sektöründe ihtiyaç duyulan, yüksek şekillendirilebilirlik özelliğinin yanısıra tatminkar dayanım, düşük ağırlık ve maliyet özelliklerinin hepsini bünyesinde taşıyan malzeme olan ihtiyaç gündeme gelmiş ve araştırmalar bu ihtiyaçları karşılamak üzerine yoğunlaşmıştır. Öncelikle temiz çelik eldesi üzerinde durulmuş ve daha da ileri gidilerek gelişmiş ikincil metalürji prosesleri kullanılmaya başlanmıştır. Çeliğin bileşimi içerisindeki karbon miktarı neredeyse yok denilebilecek değerlere kadar indirilmiş ve bu suretle şekillendirilebilirliği yüksek çok düşük karbonlu (ULC) ve arayer atomları olabildiğince arındırılmış (IF) sac malzemeler üretilmiştir. Bu çalışmada, otomotiv sektörünün ihtiyaç duyduğu yeterli dayanım ve şekillendirilebilirlik özelliklerine sahip ERD 7114 (IF) ve ERD 7115 (ULC) kalite çelikleri esas hedef malzeme olarak ele alınmış ve deneyler sonucu elde edilen değerler geleneksel ERD 61 12 ve ERD 6114 derin çekme kalite sacları ile karşılaştırılmıştır. Öncelikle klasik mekanik deneyler (çekme, sertlik deneyi vb. ) ile malzemeler arasındaki fark basitçe görülmüş ve bunu takiben malzemelerin şekillendirilebilirlik kabiliyetlerini mukayese amacıyla hadde doğrultusu ile 0°, 45° ve 90°' lik açılar yapan doğrultularda çıkartılan numuneler kullanılarak Hecker Sac Çökertme, Gererek Bükme ve Delik Genişletme Deneyleri yapılmıştır. Deneyler sonucunda elde edilen verilere dayarak Şekillendirme Sınır Diyagramları elde edilmiştir.

The vacuum degassing of metals as performed by industry is a decades-old technology, with references to the process dating back to 1886 and worldwide commercial use dating back to the mid-1950s. In the steel industry, the initial application of vacuum degassing was the removal of hydrogen from heavy sections and forgings. Later, in the 1950s and 1960s, vacuum degasssing was applied to make cleaner steels with more precise and uniform chemistry control. Cleanliness was achieved through use of vacuum degassing for deoxidation, while improved methods for alloy additions and bath stirring under vacuum led to improvements in chemistry control and uniformity. The quality levels of the products offered by the steel industry rose sharply during this time. Most of these vacuum-degassed products were forging ingots, wrought bar and tube products, and large castings. Today, several producers of sheet steels in the United States and other countries have added vacuum degassing to their steel making process, thereby gaining the ability to make steels with carbon levels less than 50 ppm. This class of steel is known as ultra-low carbon (ULC) steels, and vacuum degassing is required for their practical production. The low carbon levels, extreme cleanliness, and chemistry uniformity and control made possible by vacuum degassing have been exploited to produce a variety of new flat-rolled products. These products include High strength steel grades have been developed in order to cope with the challenges presented by the automotive market. Examples of the chemical compositions and typical mechanical properties of these steels are presented in Table S. 1 and Table S.2, respectively. Table S.l: Typical Compositions of Cold-Rolled, High Strength Steels (wt.%) [1]. Table S.2: Typical Mechanical Properties of Cold-Rolled, High Strength Steels [1]. Re- yield strength, Rm- ultimate tensile strength, El.- elongation, r- anisotropy parameter, n- strain hardening effect, BH- bake-hardening effect. * Batch annealed, """Continuously annealed xxvi Temper-rolled steels with higher degrees of temper reductions are used to provide high-yield-strength steels for many applications. However, as formability is strongly reduced by this treatment, the application in the automotive industry is insignificant. Microalloyed and rephosphorized steels are the classic high-strength cold- rolled steels used for more demanding applications requiring formability. By adding small amounts of titanium, niobium, or phosphorus, strength is increased without significantly impairing formability. While microalloyed steels are mainly restricted to structural applications such as beams and pillars, rephosphorized steels are also used for panels. Both steel groups include grades with different yield-strength levels. Isotropic steels are a further development of microalloyed steels. The breakthrough for the use of high-strength steels for large size panels came about with the introduction of bake-hardening and high-strength, interstitial-free steels. These groups of steels combine formability comparable to deep-drawing qualities with moderately higher yield strength values. In many cases, ultra-low carbon contents form the basis for sophisticated alloy design. Multiphase steels such as dual-phase steels and the recently developed transformation-induced plasticity (TRIP) steels offer chances for the use of cold- formable materials with very high-yield-strength levels. Their unique forming behaviour, especially their strain-hardening behaviour, allows the pressing of complex shapes despite their high strength. On the other hand, metallurgical control of their manufacture and control of the forming operation is more demanding. The application of cold-rolled multiphase steels continues to be very limited. An extremely low interstitial-element content of less than 50 ppm carbon plus nitrogen is the main feature of super-ultra-low carbon steels. Together with stoichiometric microalloying, they exhibit good formability and coatability and offer bake-hardening properties. Developments in Traditional Methods. Low-carbon microalloyed steels containing niobium or titanium are strengthened by means of grain refinement and precipitation hardening. Their mechanical properties are not only determined by chemical composition, but also by their hot and cold-rolling parameters. In particular, xxvii the recrystallization annealing after cold rolling in a batch type or continuous annealing furnace influences microstructure and mechanical properties. While the yield strength of these high-strength low-alloy (HSLA) solid-solution strengthening elements, formability is strongly affected by the large number of fine precipitates, which retard recrystallization and texture formation and result in poor r values. A recent approach tries to overcome this drawback by a compromise solution. Small additions of titanium fix most of the nitrogen as relatively large TiN particles and part of carbon as small Ti (C,N) precipitates. With a proper selection of cold- rolling degree and batch annealing practice, the recrystallization texture can be controlled so that the planar anisotropy is minimized. The titanium nitrides precipitate during the strand casting; hence, a fine uniform grain size is developed by these particles as early as the hot-rolling process. After cold-rolling, the retarded recrystallization during batch annealing again guarantees a fine-grained structure with the desired strength level about 250 MPa. In addition, the very fine Ti (C,N) precipitates increase the strain-hardening rate of these steels. The isotropic flow behaviour combined with a relatively high strain-hardening rate results in a good press-shop performance, especially in the area of stretch forming. Most cold-rolled steels have to be surface coated for automotive applications. As hot-dip galvanizing is widely used for inner parts, high-strength steels need to be developed and adjusted to the special features of hot-dip galvanizing lines. These lines are characterized by a short continuous-annealing cycle without overaging treatment in-line. Good formability of the material requires complete or at least very extensive recrystallization of the structure. The annealing temperature required for complete recrystallization is dependent upon the alloy content and the degree of cold rolling. Very high annealing temperatures are not practical, however, because precipitation hardening decrease as a result of particle coarsening. With the ability of efficient vacuum degassers in steel plants, ultra-low carbon steels are emerging as materials for large volume applications. There are four arguments for the usage of ULC contents in high-strength steels: xxviii . Formability: In many steels, cold formability is improved when carbon decreases. This is determined from elongation behavior, strain hardening behavior, and texture development.. Aging: Continuously annealed steels need a proper control of carbon in solid solution in order to prevent quench aging. Microalloyed ULC steels, either EF or stoichiometric, as well as super-ultralow carbon steels (less than 25ppm carbon) help to suppress the undesired aging phenomena.. Bake Hardening: Batch annealed steels with bake-hardening properties can be produced due to retarded precipitation kinetics of carbides in ULC steels.. Consistency: Strip-end effects during hot-strip rolling can be minimized by ULC steels. Furthermore, the effects of trace elements in IF steels are less critical than in unalloyed mild steels. Even though there are many variables that can influence sheet metal workability, we aimed to name and discuss only a few that are well recognized as the most important ones. If we broadly classify these variables as process variables and material variables, then factors such as die geometry, die material, blankholder pressure, lubrication, press speed etc. can be collectively regarded as process variables. In general, process variables determine the nature of external loading on the sheet, whereas material variables determine the kind of response, the material will exhibit to that loading. For a successful forming it is necessary to obtain a uniform strain distribution as possible under the limit established by a critical strain level of the material. If this limit exceeded, the stamping will break during the forming. These limit strains are best represented by the concept of a Forming Limit Diagram (FLD). FLD is obtained with the Hemispherical Dome Test. A 101.6mm punch is used and lock bead in combination with a hold-down force competely prevents drawing-in on the flanges. The FLD represents the acceptable limits of strain in a plot of two principal surface strains in a sheet over those combinations of strain where thinning occurs. Any combination of the two surface strains ei and e2 falling below the FLD is considered as acceptable, and any combination falling above it will produce failure. Failure is xxix defined as the appearance of localized thinning or necking, not necessarily final separation. It is possible to predict the FLD theoretically but the best and the common way is to predict it by practice. For experimental predictions, the surface of the sheet is covered by circle grids. These circle grids are photographically printed on the specimen blanks by using a photosensitive-resist method. Photoetched or electrochemically etched grids offer the advantage of not being erasable, but they may lead to premature failure in thin sheets if the etched depth exceeds a certain value. The circle diameter is important in measuring the deformations accurately. It must be small in relation to the strain gradient in the specimen. A general guide is to keep the ratio of punch diameter to circle diameter greater than or equal to 40. To construct the FLD deformed circles (ellipses), both close to and within necked or fractured areas are measured. Here, one can identify three types of ellipses: type I, fractured (the fracture passes through the ellipse); type II, necked or fracture- affected (the ellipse lies either within a necked area or within the fracture-affected zone identifiable by heavy surface granulation); and type III, acceptable (the ellipse falls outside the area affected by necking or fracture. The values of the three types of circles are taken and used to establish the FLD of the examined. Diagrams for fracture, necked and acceptable regions are found. These diagrams are usefull in sheet metal forming processes. Strains that have been obtained from the forming processes are compaired with the FLD. If the results lay under the FLD, the process will be considered as successful. If the results lay above the FLD, the process will result with failure. This work aims to investigate the formability of ULC sheet steel in comparison with conventional deep drawing qualities at room temperature. In order to do this four sheet steel qualities have been used. The steel qualities, that we have used were ERD 6112 DIN EN 10130-91 (Fe P01), ERD 6114 DIN EN 10130-91 (Fe P04), ERD 7114 CAL/IF (DQ-2) and ERD 7115 CAL/IF (ULC/DDQ). They are all the sheet grades of ERDEMİR Iron & Steel Factory. In order to investigate the formability of the sheets, in addition to the conventional tension tests three different below mentioned formability tests; XXX 1) Hecker punch-stretch test 2) Stretch bending test 3) Hole expansion test have been conducted. In the first and the second tests, sheet specimens with different widths at angles of 0°, 45° and 90° respect to the milling direction have been used. In the hole expansion test, square specimens with holes of 26.7mm in the center have been used. A flat sheet specimen with a circular hole in the center is clamped between annular die plates and deformed by a punch, which expands and ultimately cracks the edge of the hole. Hemispherical punch have been used and die plates have been equipped with lock beads to prevent drawing-in of the flange. This test shows the effect of burr and cold worked metal of the edge of the punched hole. In most cases, removing the burr and cold worked metal from the edge of the punched hole increased the hole expansion considerably. The hole expansion also increased with increasing total elongation and rm value and decreased with increasing tensile strength. Inclusions were observed in crack locations, and inclusion shape control improved hole expansion performance. Stretch Bending Test is used to determine the forming limits of sheet metals under different tension conditions. A rectangular strip of sheet metal is clamped at its ends in lock beads and deformed in the center by a punch. The punch travel between initial contact and specimen fracture is measured. The effect of variations in specimen width were investigated. The test showed that the height at fracture increased with increasing width and with lubrication. Stretch-bending tests are usefull for material selection and for prediction the effects of material substitution and gage reduction in many forming operations. The results of this study can be summarized as follows: 1) Tensile test performed at 0°, 45° and 90° to the milling direction of the sheets showed, that the mechanical properties are very similar to each other. 2) At the end of the Hecker punch-stretch test the obtained FLD' s at angles of 0°, 45° and 90° to the milling direction differ slightly few from each other. XXXI 3) By using lubrication strains occured at the right side of the forming limit diagram anf better uniform strains are obtained. 4) The slope of the right side of the forming limit diagrams of ERD 7114 and ERD 7115 decreases. This result matches with the results obtained in other investigations. 5) The obtained FLDo values are plotted against the tensile strengths of the sheets. It is seen, that the formability of ERD 7114 and ERD 71 15 has higher limits. However, IF steels have better formability in planar strain conditions. 6) Theoretical FLD' s are obtained by using the results of the tensile tests. The results showed, that FLD1 s obtained at angles of 0°, 45° and 90° to the milling direction are not dependent on the direction. 7) The theoretical FLD0' s are dependent to the strain hardening exponent. FLDo' s are equal to the strain hardening exponent in theoretical FLD' s. 8) FLD' s obtained are higher than the theoretical FLD' s. At the same time it has been observed, that there is a small difference between the theoretical FLD' s and the FLD1 s based on the conditions in which no probability of necking is expected. 9) Obtained hole expansion test results showed, that the method of machining the hole, the hole surface quality and enclusion shape and dimensions are very important and influence the hole expansion value. 10) The diagram obtained by plotting the hole expansion factors against the tensile strengths shows, that the limit strains of ERD 7114 and ERD 71 15 are higher. 1 1) Hole expansion test results show, that the hole expansion factors increases with increasing normal anisotropy factors. 12) At the end of stretch bending test the punch travel values are obtained. Results show, that ERD 7114 and ERD 7115 are less dependent to the specimen width and gives even at very narrow specimen widths the best results. 13) The strecth bend test results are plotted against the tensile strengths. It has seen, that ERD 7114 and ERD 71 15 have greater limit strains in comparison to the convential deep drawing qualities. 14) Stretch bending test is specially in material selection and gage reduction effective. xxxii 15) Hole expansion results are plotted against stretch bending test results. The diagram shows, that ERD 71 14 and ERD 7115 have got higher limit strains. 16) The test results are plotted together into multi-axis diagrams with respect to the millimg direction. These diagrams show, that the areas of ERD 71 14 and ERD 7115 in comparison to the conventional sheets are greater. This is a advantage in forming processes. 17) It has seen, that the strength features of ERD 61 12 is highest, because of the greater carbon content, and that the formability is worst.