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Drawability of Automobile Sheet from IF-steel

#2664


Methods of Improvement of Drawability of Automobile Sheet from IF-steel

Tech Area / Field

  • MAN-MAT/Engineering Materials/Manufacturing Technology
  • MAT-ALL/High Performance Metals and Alloys/Materials
  • SAT-SUF/Surface Transportation/Space, Aircraft and Surface Transportation

Status
3 Approved without Funding

Registration date
10.12.2002

Leading Institute
Severstal, Russia, Vologda reg., Cherepovets

Supporting institutes

  • All-Russian Scientific Research Institute of Non-Organic Materials named after A. Bochvar, Russia, Moscow\nNPO Mashinostroieniya, Russia, Moscow reg., Reutov\nMISIS (Steel and Alloys), Russia, Moscow

Collaborators

  • Thyssen Krupp AG / ThyssenKrupp Stahl AG, Germany, Duisburg

Project summary

Nowadays the leading metallurgical companies increase a production of IF-steel. IF-steel is steel without interstitial atoms. Products include not only traditional hot and cold rolled steel sheets, but also coated steel sheets (for example, by zinc or another kind of coating) for automotive industry. Moreover, the new versions of IF steels are created, for example: high strength IF-steel, stainless IF-steels and so on.

Vacuum-degassed interstitial-free (IF) sheet steel has very high formability; this kind of steel allows obtaining high level of drawability. It is possible to explain the high formability of this steel by following:

– On the one hand, plasticity is increasing by reduction of content of the interstitial element (C and N) up to ultra-low level.


– On the other hand, additions of stabilizing elements (for example: titanium and niobium) allow bonding lost of nitrogen and carbon, which not removed after steelmaking and vacuum-degassing.

The IF-steel has interstitial-free body-centered cubic ferrite matrix. After cold rolling with high level of deformation and annealing, this matrix transforms in structure with a strong {111} re-crystallization texture. This kind of texture is reason of high values of the average plastic strain ration r, which is associated with a good formability of ultra low-carbon IF steels.

The optimization of micro-alloying and technological conditions allows further increasing r and improving formability. That is why investigation of micro-alloying in combination with technological conditions is important task.

There are a lot of completed good researches about the mechanism of re-crystallization texture formation for achieving deep drawability. However, role of grain boundaries in re-crystallization texture formation is not investigated enough. Role of them in the plastic anisotropy of steel (r-values) is not enough studied too. The physical metallurgy of IF-steels still contains many non-solved problem regarding to precipitation, segregation, re-crystallization, transformation, texture-formation and misorientation of grains.

At the present time, an acceptable method for control grains boundaries in industrial condition is absent. Today it is not possible to study and controlling grains boundaries with special misorientations and contained the coincidence site lattice (CSL) in industrial materials with any grain size. Elaboration of X-Ray method for determination of quantity and texture of special boundaries by repere diffraction will allow developing the method of producing extra deep drawing sheet steel, which characterized by Lankford coefficient r > 2.5.

Aim of this project is development of method of producing IF-steel sheets with improved properties, for example, with the extraordinarily cold formability (r >> 2.5, tensile strength < 210 MPa) on the base of intensive physical and metallurgical research. The project includes study of influence of special grain boundaries on steel sheets properties in dependence of various technological conditions particularly for IS-steel microalloyed by Ti and Nb. Developed X-Ray method of repere diffraction allows investigating correlation between special grain boundaries and anisotropy of plasticity.

The new X-Ray method of controlling the special grain boundaries in the cubic polycrystalline materials with any grain size will be developed for IF steel in X-Ray laboratory of the metallurgical company " Severstal".

The project includes development of method of using roentgen difractometer for:

– analysis of a special misorientations in pairs of different grains;


– determinations of quantity and texture of boundaries with coincidence site lattice (CSL), which connected with the main texture components of the sheet steel;
– X-Ray method of controlling CSL parameters on the grain boundaries will be developed. This method will allow obtaining improved plasticity of industrial IF-steel sheets and, consequently, sheet steel formability.

Numerous investigations have been performed to analyze the effects of texture on the mechanical properties of steels with good deep drawability. It has generally been known, that a cold rolled and annealed IF (interstitial free) steels with excellent deep drawability must be manufactured by special technology of the production. This special technology includes:

– special methods of de-carbonizing steel during melting;


– optimum additions of microalloying elements Ti and Nb;
– special technological conditions on the all metallurgical stages of sheets production.

The latest papers show, that it is possible to obtain high level of Lankford r-coefficient for cold rolled and annealed sheet steels, for example, at the levels 2.5-2.8. Recently the Chine investigators D.L. Cui and X.J. Wang have reported about a new texture control method which allowed to increase r-values of IF-steel up to extreme significance 3.25. But no information about influence of the grain boundaries on mechanical properties of IF-steels. It is important reserve of the improvement of drawability of cold rolled and annealed sheet steel. The project will suggest a new method of work. On the basis of these methods of analysis of grains texture and them special boundaries, it is possible to achieve in commercial cold rolled and annealed sheet IF-steel a high level of r-value in range 2.8-3.2 and strain hardening exponent n = 0.21-0.25.

Scope of activities includes following basic stages:

– development of the method of controlling the special boundaries in steel;


– development of X-ray repere diffraction method;
– using of repere diffraction method in IF-steel technology;
– industry experienced IF-steels;
– investigation of influence of following parameters on structure and texture of IF-steel sheets;
– control of the mechanical properties of experience metal;
– analysis of obtained data.

Expected results of the project are the following:

– Correlation between drawability and special boundaries in cold-rolled IF-steel, micro-alloyed by Ti and Nb.


– For IF-steel: new X-ray method of analysis of grains texture and special boundaries
– Method of correction of technological conditions of sheets rolling on the basis of study of texture.
– Using X-ray method of diffraction, influence of special boundaries on mechanical properties of steel (for example, on value of normal r and tangential r plastic anisotropy) will be determined.
– Method of obtaining in commercial cold rolled and annealed soft sheet steel a high levels of r-value in range 2.8-3.2 and strain hardening exponent n = 0.21-0.25.

High-qualified specialists will carry out the project, including professors and doctors.

The project is correlated with ISTC Goals and Objectives.


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