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Strength of IF-steels for Automobile Industry

#2723


Application of Microalloying for Increase Strength of Drawable IF-steels for Automobile Industry

Tech Area / Field

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

Status
3 Approved without Funding

Registration date
27.02.2003

Leading Institute
Severstal, Russia, Vologda reg., Cherepovets

Supporting institutes

  • VNIITF, Russia, Chelyabinsk reg., Snezhinsk\nMISIS (Steel and Alloys), Russia, Moscow

Collaborators

  • Thyssen Krupp AG / ThyssenKrupp Stahl AG, Germany, Duisburg\nUniversity of Michigan, USA, MI, Ann Arbor

Project summary

Aim of the present project is development of the method of cold-rolled sheet manufacture from the high strength and good-drawing IF-steel. The extra-low-carbon high strength sheet from IF-steel is required for decreasing weight of automobile panels in connection with global environment problems. The production of each kind of high-strength steel is based on solid solution hardening. The results of the present work may be applied to industrial production of the hot and cold rolled sheets. For example, for production of:

– uncoated IF-steel sheets;


– coated by Zn and Al-Si IF-steel sheets after continuous annealing in the aggregates of hot zinc-plating and aluminizing.

As result of the project implementation, production methods of minimum two kinds of steel will be developed for obtaining automobile body parts with various drawability degrees:

– sheet steel with bake hardenability (BH) – effect and with the yield strength 220-280 MPa; Lankford coefficient r must be higher than 1.8;


– sheet steel with yield strength higher 400 MPa without sacrificing a plastic deformation at press-formability, Lankford coefficient r must be higher than 1.4.

Elaborated methods of making sheets steel will allow reducing a car body weight at 5-10% and improving the level of car safety. On base of the new physical metallurgy conceptions and the pilot X-ray methods of controlling the texture and special grain boundaries, following parameters will be determined:

– chemical composition of high strength IF-steel with the calculated alloying elements, which cause solid solution-strengthening;


– temperature of heating slabs;
– temperature-velocity routs of the hot rolling and the cooling IF-steels bands, regulated cooling of the coils;
– summary reductions of cold rolling;
– routs of heating and soaking temperature for annealing IF-steels in bell-type furnace and continuous annealing at hot dip galvanizing;
– reduction of skin pass rolling.

As well, the project includes developing methods of ensuring:

– high flatness;


– high exactitude of geometry parameters of sheets after cold rolling at reductions 65% and more;
– high cleanliness of steel sheets surface (not less than 80% of reflected light stream).

These parameters will be obtained by formation of strong octahedral texture and regulation of quantity of special boundaries of grains in the presence of needed alloying additions, which lead to forming required crystallographic anisotropy.

High strength IF-steels will be melted and vacuum degassed at “Severstal”. This steel will have various alloying micro-additions for strengthening of solid solution [Nb (<0.08%), Ti (<0.12%), Mn (<1.5%), P (<0.08%), B (<0.03%) and others] with calculations of atomic ratio. It must be noted that kinds of alloying additions influence for needed type of annealing. For example, IF-steel complex alloyed by Nb and Mn must be annealed in bell furnace; IF-steel alloyed by P and B must be treated in furnace for continuous annealing. The project includes not only investigation of chemical composition, but study of dependence of steel sheet quality (complex of mechanical properties, re-crystallized grains texture, quantity of special boundaries of grains and the Lankfords coefficient r, Dr and level of earing) from temperature of slab heating, temperature of rolling finishing and coiling, volume of total cold deformation and re-crystallization heating conditions.

The additional bake hardenability (BH) of steel sheets after pressing and paint-baking treatment at automobile plants (at approximately 170 °C, 20 min) leads to improved combination of deep drawability and finished high strength. In the present research work, BH-effect will be used with taking into account the atomic ratio of elements in the chemical composition. The project includes application of widely known research methods and new developed repere X-ray method of analysis of special grain boundaries (coincidence site boundaries-CSL), witch will provide the way to improvement of mechanical properties of industry IF-steels sheets.

Experience-industrial lots of the various kinds of high strength steels will be produced using developed methods.

The project includes the following stages:

1. Development of the method of IF-steel precision micro-alloying.


2. Elaboration of the method relating to calculation of crystal-geometry for internal crystalline and between-grained surfaces in textured IF-steel.
3. Conduct of modeling process.
4. Production of high strength IF-steels by means of the high – precision technology.
5. Analysis and estimation of the developments relating to tasks 1-4, preparation of the offers on their improvement, optimization and application.
6. Control tests of mechanical properties of high strength IF-steel.
7. Study of the influence on structure and grains texture, quantity and texture of special boundaries with different coincidence site lattices (CSL), secondary phases in IF-steels.
8. Analysis of received data.
9. Rolling of trial metal lots.

The main expected results are following:

– Method of producing extra-low-carbon micro-alloyed uncoated sheets from IF-steel with using BH-effect, which will be characterized by yield strength on range 220-280 MPa and coefficient of normal plastic anisotropy (Lankford coefficient) r >1.8 (variant of bell furnace annealing).


– Method of producing IF-high-strength sheet steel with the various micro-additions of alloying elements, which will be characterized by yield strength above 400 MPa and Lankfordґs coefficient r >1.4 (variant of continuous annealing).
– Variants of production of galvanizing (coated by Zn and Al-Si) high strength IF-steel sheets.
– High flatness and exactness of cold rolling sheet from high strength IF-steel.
– Methods of management of the crystallography texture in cold rolled sheet from high strength IF-steel.
– Elaboration of the new method of repere X-ray diffraction to the control of special boundaries (CSL) of grains.
– Experience-industrial lots of high-strength IF-steel sheets using developed methods.

The project involves highly qualified participants, namely, specialists in materials science and metallurgy of steel. The goals and objectives of the project fully comply with those of ISTC.


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