Alaşımsız dual fazlı çeliklerin mekanik ve nokta kaynağı özellikleri

Abstract

ÖZET Otomotiv endüstrisinde son altı yıldan beri dual fazlı çelikler kullanılmaktadır. Genellikle dü şük karlbonlu ve az alaşımlı çelikler kritik sıcaklık lar* arası ışıl İşİemö; tâbi tutularak `errit matrisi içinde martensit adalarir -ihtiva eden ; değişik kimyasal bileşim ve türde dual faz mikîroyapisi elde edilmekte dir. Bu tezde, hakkında çok az bilgi yayınlanmış o- lan, hizlı soğutulan dual fazlı çelikler incelenmiş tir... Hızlı soğutulan dual fazlı çelikler düşük kajrbon oranı ile birlikte çok az alaşım, elementi İh tiva ettiklerinden sertleşme kaabiiiyetieri düşüktür. Bu sebeple kritik sıcaklıklar arasına ısıtıldıktan sonra hızla su verilerek dual faz mikröyapısı elde e- dilir* -üaha: sohra çeliğe yeteri kadar süneki'ik kazan dırmak için kisa süreli bir temperleme yapılır. Tezin hazırlanması sırasında, deneysel çalış malar, entegre bir demir çelik tesisinin sürekli tav lama hattında yapılacak endüstriyel uygulamaya ışık tutacak şekilde düzenlenmiştir. Kalınlıkları 1 mm civarında olan SAE 1010 ve kimyasal bileşimi iJAE 1010* a yakın olan düşük karbonlu çelikler üzerinde deneyler yapılmıştır. Isıl işlemden sonra, dual fazlı çelikle rin mekanik özelliklerini tayin etmek için çekme deneyi yapılmıştır. Kritik sıcaklıklar arası tavlama süresi, bütün deneylerde 1 dakika olarak sabit tutulmuştur. Nötr tuz banyosunun sıcaklığı değiştirilerek farklı hacım ora nında martensit elde edilmiştir.` Dual fazlı çelikler de, mukavemet ve süneklik arasında optimum ilişki için martensit hacım oranının %20* den daha âz olması ge rektiği tesbif edilmiştir. Su verilen çeliklerde şü- nekl iğin artması için 200-500°C arasında, I5-I8O sn. temperleme yapılmış ve temperleme sıcaklığının 300°^'. nin altında olması gerektiği bulunmuştur. Bu sıcaklık-*II.tan daha yüksek sıcaklıklarda akma uzamasının hızla arttığı görülmüştür. Temperleme süresinin 60-90 sn. arasında seçilmesinin yeterli olduğu bulunmuştur. Ayrıca, dual fazlı çeliklerin alüminyum ile söndürül müş.çeliklerden üretilmesinin gerektiği tesbit edil miştir.. -.'?': ;`-;, '?? Dual faz mikrpyapıöı ve teraperlemenin raeka- ' nik özelliklere tesirlerinin ygtnısıra, SAE 1010 cinsî.saçtan hızlı soğutma metodu' ile üretilen dual fazlı çeliğin elektrik direnç nokta kaynak özellikleri de araştırılmıştır. Temperlenmiş dual fazlı saçların nokta kaynak şartlarının aynı -kimyasal bileşimdeki peri- İt ik saçın kaynak şar ti arma. çok yakın olduğu bulunmuştur. ? Ancak bu iki çeliğin mukavemet ve sttnek- lik özelliklerine bağlı olarak kaynak dikiş özellik leri arasında bazı farklar vardır..JDual fazlı çe likte çekme-makaslaraa kuvveti daha yüksek olurken, U-ç ekme kuvveti ve kaynak sünekliliği daha düşük; olmaktadır. Dual fazlı çeliklerde, mukavemetli ve sağlam' bir kaynak yapılması için kaynak `işlemini takiben temleri eme yapma zorunluluğu yoktur. Bu tez, hızlı soğutulan dual fazlı çeliklerin nokta kaynak özellikleri hakkında yayınlanan ilk detaylı çalışma olmaktadır.

Ill SUMMARY The OPEC oil crisis of 197*t resulted in a dra matic gasoline shortage all over the world. Since tlteir^ the automobile industry concentrated his researches to fuel-economy. Much researches have been done by all the automobile companies to reduce the weight of cars» More attention was paid on steel parts because until 1985 60% of the weight of an automobile is expected to be steel'.:' For many- decades most of the steels used in automobiles Have been plain-carbon steels, either cold* rolled *tö ifiiçkriess up to 2 mm or hot-rolled to the- thicker gauges. If higher strength steel is used, the gauge of sheet used for many components can be reduced. The high-strength steels considered by automobile manu facturers are microalloyed, controlled-r oiled, High Strength Low Alloy (H5LÂ) steels. Like plain-carbon steels, HSLA steels contain less than 0,1% carbon. In addition, they typically contain about 1,0% Manganese, less than 0,60% Silicon and 0,1$ carbide forming microalloying elements such as Titanium, Vanadium and/or Niobium. These steels con sist of mainly, pearlite and ferrite in the microstuc- ture. High strength is gained in thepe steels by grain refinement,.substitutional strengthening and/or preci pitation strengthening., r Although HSLA. steels have an excellent balance of mechanical properties, their forraability or ducti lity is not enough for the manufacture of many press formed components for various automotive applications. This problem was resolved in the mid-19701 s by the de velopment of Dual Phase Steels.IV Dual. Phase Steels ar© a new class of HSLA steels characterized by a microstructure consisting of a dispersion of about 20% of hard martensite par ticles in a soft ductile fer rite matrix. The term ` Dual Phase ` refers to the presence of essentially two phases, ferrite and martensite, in the jnicrostruc- ture, although small amounts of pearl it e and retained austenite may also be present. Dual phase steels ar,e produced by either of two methods: (i) Hot-Rolling, (ii) Cold-Rolling. Steels thicker than 2 mm are produced by the first method and thinner ones by the second. - In hot rolling method properly alloyed steels are used» The rolling schedule is chosen in. such a way that final rolling temperature falls to in ter critical, between A c^ and Ac t, temperatures. Cooling rşte permits f err it e formation on the way between, hot rolling and coiling. Martensite forms during the coiling. ':, In cold-rolling laethod cold rolled steels are annealed between Ari and Aj.* temperatures. The steel obtains « Austenite Ferrîte ` microstructure. Then the steel is cooled according to its hardenabi*.. lity so that some fraction of austenite transforms to martensite. These steels are called Cold-Rolled Dual Phase Steels or Dual Phase Steels. Inter critical heat treatment' is applied to the cold rolled steels in two ways: Batch annealing and continous annealing. The steels which have more than 2t% Mn are inter critically batch annealed. Then they are slowly cooled to room temperature. »They gain dual phase structure at this slow cooling rate because of their high hardenability. ^ i. ;_._.. r !l?he steels which are less alloyed are heat treated at continous annealing lines. They are heat ed to inter critical temperatures with a speed of 15 °C/second and then annealed about a minute. After annealing they are cooled. Cooling rate is chosen'accpı^^î^İo;^ the steel. '.. 'i'he.eçjjb'lc^r' ^ the formation of t^^.^u^ıshşıse miçrostructure changes linearly wijbh'.;tji`e,.maşğanes the steels Alloyed sWbJls:' -..are '{> cooled; by preşsure-air and unalloyed 'steels are` wj^jsr^u^n^h'edV. Slower cooling rates produce betted ^tredg^ji-ductility. combinations.. Highör cool ing ratefr.^ndi<:e a larger number of lattice defects and residual stresses into the matrix and reduce duc tility» However ductility can be improved in these > steels by tampering.,, Both IxQt-rplled and cold-rolled dual phase steels, have the following features: (1) Abeençe of yield j)öint elongation. (2).Low yield and high tensile strength. ^3 J High ductility and high work hardening rate, (4) Good elongation despite the hi/ft strength..., CfO,Non«aging at room temperature. _'.'.;.ClVXars'©' bake'/ha^denability. Most of the publications were about alloyed eithei? cp^.d or hpt-rplled dual phase steels. Although unalloyed dual phase steels are commercially produced very few data have been published. So unalloyed dual phase steels were chosen, the subject of this research. The aim of the present investigation is to gain an understanding of the water quenching temperature and tempering factors, and carbon percentage of the steel wHich. control mechanical and electrical resistance spot' welding properties of unalloyed dual phase steels, ?i'o do this tensile, shear- t ens ile and U- t ensile tests have: ft e en done for low carbon unalloyed steels which have been subject to a variety of thermal heat treat ments».... The steels used in this study contain 0,055` 0,1^5%' carbon and about 0,35% manganese. One of them was killed '? steel and others were rimmed steels, 'i'he thickness of the steels were about 1 mm. The speci mens were prepared before the heat treatments.VI All heat treatments with the exception of 200°C; and 250°C tempering were carried out in neutral salt pots» Low temperature temperings have been done in an oil bath. All the quenched specimens were held, for a minute at the annealing temperature, The anneal ing temperatures varied between 740°C and 920°C, Then, the specimens were brine quenched. Tempering tempe ratures varied between 200?C and 5009C, and' the tem pering1 period 1 5-180 seconds. Tempered specimens ; were cooled in still air. The welder employed for the experiments was a 49 Kva rat0d, electronic time controlled, pnomatic spot welding machine. The welding current' wa'e determined by a secondar ampermetre. The copper electrodes had 4,30 mt!) tip diameters. Squeeze and hold times; were ' kept constant for all welds. Current, current time and electrode forces were the factors^which varied one at a time.0 All the tension tests were carried in an uni versal Instron machine at room, temperature at a cross- head speed of 10 mm/minute.. - ' The following results are obtained from the experiments: 1- Water quenched dual phase steels must be produ ced from killed steels. 2- Quenching temperature of unalloyed dual phase steels has to be strictly controlled in order to have maximum 20% mar t en s it e volume fraction* 3- As the volume fraction of mart ens.it e increases, t ens 11 ean d yield strength of the dual phase steel increase but total and uniform elongation dec rease, 4- As the carbon content of mart ensit« increases, strength of th'e dual phase steel increases and ductility decreases.VII 5- Increasing carbon content of steels permits to ' produce dual phase steels with higher strength but low ductility. 6- A tempering period of 60-90 seconds Is sufficient for 1 mm thick, water quenched dual phase steels, 7- Tempering temperature of water quenched dual phase steels can vary between 200°C and 300°^,. Tempering temperature can be raised up to 4Q0°tJ to gain extra ductility. 8- In water quenched dual phase steels deep drawabi- lity is low. 9- Recommended practices for carbon steels can be utilized in spot welding of water- quenched un alloyed dual phase steels, of the same chemical composition and gauge, as below: (a) Electrode body and face diameter, and electrode force can be applied without a change. (b) Welding current is applied either with a small amount of reduction or as recommen ded. (c) Weld time must be kept at its minimum value to give higher tensile strength. 10- In spot welding of tempered unalloyed dual phase steels ductility ratio of welds are about twice the lower ductility ratio limit of ductile welds, so a post-welding heat treatment is not needed, 11- The relation between spot weld properties and ultimate tensile strength of steels is similiar for precipitation hardening HSLA steels and quench hardened mild steels.