Interstitial free steel: Influence of α-phase hot-rolling and cold-rolling reduction to obtain extra-deep drawing quality

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Abstract

This paper shows the drawing properties of Ti-bearing interstitial free steel. The study was carried out on the basis of microstructure, mechanical properties and texture using X-ray diffraction techniques and orientation distribution function (ODF) analysis. The results obtained prove the influence of each stage of processing to achieve extra-deep drawing quality (EDDQ), focusing our attention on hot-rolling and cold-rolling reduction. It is demonstrated that the optimum cold-rolling reduction increases to 90% for both rm and Δr coefficients.

Introduction

The development of interstitial free steel has been a milestone in sheet steel drawability improvement, achieving extra-deep drawing quality (EDDQ) with a yield stress (YS) of about 150 MPa and an rm over 2. Besides, it is possible to obtain an excellent surface condition, thickness homogeneity and good flatness, allowing coating for some applications.

These properties require strict chemical composition control: very low carbon contents — extra-low carbon (ELC) and ultra-low carbon (ULC) steels — and reducing the contents of impurities such as O, N, P and S. Therefore, the manufacturing process is complex and has several stages [1], [2], [3] (see Table 1):

  • •

    The required production (several millions tons per year) and the desirable product homogeneity exclude the use of scrap (electric arc furnace route); it is necessary to employ iron ore (blast furnace+basic oxygen furnace route).

  • •

    The iron oxide reduction by C in the blast furnace provides a hot metal which must reduced of its P and S contents. Refining to produce steel in the converter reduces the C to about 0.2%.

  • •

    Ar vacuum degassing dramatically reduces the C and gas (H, N, O) content.

  • •

    Vacuum secondary metallurgy reduce impurities and permit fine chemical analysis control.

Subsequently, the steel is continuous cast, hot rolled, coiled, cold rolled and α-phase annealed (Ac1 greater than 900°C).

In a previous paper [4], we studied the development of microstructure, texture and mechanical properties of an IF steel with the following process conditions: hot-rolled, 70% cold-rolled reduction, and batch annealed. At present, the aim of this work is to provide a more careful research of the hot-rolled texture, and to increase the cold-rolling reduction to 90% maintaining the annealing temperature (700°C), with the purpose of determining the influence of texture and cold-rolling reduction in drawability [5], [6].

We use the same hot-rolled steel as in the previous article [4] — 2.8 mm thickness and chemical composition according to Table 2 — with finishing temperature at 900°C and coiled between 640°C and 690°C. The maximum cold-rolling reduction was 90%. The simulated batch-annealing parameters are in Table 3. It is important to highlight the following subjects:

  • •

    The very low carbon content (23 ppm, ULC steel) and the Ti addition (500 ppm) to scavenge N and C.

  • •

    The finishing temperature in α-phase — below Ar3, approximately 910°C. The recrystallization stop temperature is about 920°C.

  • •

    The elevated coiling temperature to favour C and N precipitation as Ti(C,N) and to obtain, thus, interstitial free ferrite. The Ti* (effective Ti) can be expressed by the formula Ti*=Ti−(4C+3.4N), and it is equal to 0.027.

Section snippets

Experimental techniques

The experimental methods employed were as follows.

• Tensile test, obtaining the following parameters: YS, tensile strength, elongation (50 mm gauge length), Lankford coefficient (r) and planar anisotropy coefficient (Δr) (for annealed condition) and work hardening coefficient (n).

• Vickers hardness test, to show the hardening rate with the cold reduction growth.

• Light optical microscopical observation: samples were cut in the transverse section, parallel to rolling plane. Subsequently, they

Hot-rolled condition

Fig. 1 shows a partially recovered and a partially recrystallised microstructure, bearing evidence of the α-region hot-rolling schedule. There is a close resemblance to the ferritic stainless steels and magnetic sheets microstructures that do not undergo (or only partially undergo) γ⇒α transformation [9], [10]. This can also be found in texture analysis. The {200} pole figure (Fig. 2) shows that the {110}〈001〉 (Goss texture) and {113}〈332〉 are the most intense components. The ϕ2=45° ODF

Conclusions

• Hot rolling was carried out below the recrystallization stop temperature and Ar3 temperature; this process produces a microstructure and texture novel in relation to that found in ferritic steels for tinplate and automobile applications [11], [12]. Recrystallization is only partial and points to the {113}〈332〉 component, common to hot-rolled ferritic steels without γ⇒α transformation.

• Component {110} — also present in the hot-rolled condition — does not vanish until rolling deformations over

Acknowledgements

The authors are grateful to Mr. Cesar Alonso, Quality Manager of Aceralia Corporación Siderurgica, for test material and technical information supplied. They would like to thank Drs. Carmen Garcı́a Rosales and Javier Gil Sevillano from the Research Institute CEIT, for helpful texture analysis. The technical help of Mr. José Ovidio Garcı́a and Mr. Germán Romano is also gratefully acknowledged. Appreciation is also extended to Mrs. Teresa Iglesias for the secretarial help.

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