FIELD OF THE INVENTION

This invention relates to interstitial free (IF) steels of high strength and ductility, and a process for the production thereof. More particularly, the invention relates to a process of producing ultra- fine grained interstitial free (IF) steel of high strength and ductility suitable for making long products.

BACKGROUND OF THE INVENTION;

Interstitial-free (IF) steels are an important class of industrial materials which are known for their excellent formability properties. The material contains very little amount of carbon (<50ppm) and sufficient amount of titanium combines with any excess carbon and nitrogen resulting in this improved formability.

These steels have very good ductility, however relatively lower strength. Therefore, an additional strengthening mechanism is highly desirable without changing the chemistry. Strengthening due to reduced grain size could be one possibility. In addition, there are many other advantages that can be derived out of reduction in -grain size to submicron levels, for example, enhanced superplastic performance. However, all the grain size refinement techniques would lead to some loss of ductility.

The processes based on severe plastic deformation, which are known to reduce grain size to submicron to nanometer scale, are the ones where no decrease in ductility is observed. In recent years, a number of innovative severe plastic deformation techniques have been developed -for deforming metals to very high degrees of plastic strains with the aim of producing highly refined grain structure in bulk materials without entailing requirements for expensive alloying additions or energy consuming multi-step thermo-mechanical treatments. These include equal channel angular extrusion (ECAE), accumulative roll bonding (ARB) and multi-axial forging (MAF). Severe plastic deformation process like ECAE can still produce materials in rod or bar form with grain size in the range 100-1000 nm. A distinct advantage of this process is that it can be scaled up to produce large-scale billets in industry and is a relatively simpler and cheaper process. The additional novel feature of all the severe plastic deformation processes is that the net shape of the final produce remains essentially the same as the starting material after any given number of passes or cycles; so there is no constraint on the strain in the material. In comparison to conventional metal working process, like rolling, extrusion, effective strain greater than 4 can only be obtained in foils or filaments which have limited structural applications. ECAE involves abrupt changes in the strain path. It has been shown that certain passes are more favourable towards rapid refinement of grain size over others. In the paper entitled 'Formation of sub-micron and nanocrystalline grain structure by severe plastic deformation' by P.B. Prangnell, J.R. Bowen and A. Gholinia published in the Proceedings of The 22 ^nd Risoe International Symposium on Materials Science, 2001, pp 105- 126, the definition of submicron or nanocrystalline grain size has been proposed as the structure where (a) the average spacing of the high angle grain boundaries (HAGBs), having micsorientations greater than 15°, must be less than 1 micron in all orientations, and (b) the proportion of the HAGB area with respect to the total boundary area in the material must be greater than 70%.

OBJECTS OF THE INVENTION;

It is therefore an object of this invention to propose a process of producing ultra-fine grained interstitial-free (IF) steel of high strength and ductility suitable for making long products which adapts as a starting material a coarse-grained IF steel billet.

Another object of this invention is to propose a process of producing ultra-fine grained interstitial-free (IF) steel of high strength and ductility suitable for making long products, which is cost-effective.

A still another object of this invention is to propose a process of producing ultra-fine grained interstitial free (IF) steel of high strength and ductility suitable for making long products, which is capable of avoiding any loss in ductility of the steel. A further object of this invention is to propose a process of producing ultra-fine grained interstitial free (IF) steel of high strength and ductility suitable for making long products, which can be scaled up to produce large-scale billets.

These and other objects and advantages of the invention will be apparent from the ensuing description, when read in conjunction with the accompanying drawings.

SUMMARY OF THE INVENTION:

Accordingly, there is provided a process of producing ultrafine grained interstitial-free (IF) steel of high strength and ductility from coarse grained IF-steel billets, the produced IF-steel being suitable for making long products in large-scale, the process comprising the steps of:-

-providing an Equal Channel Angular Extrusion (ECAE) apparatus for extruding the starting material, the apparatus comprising a piston and an inlet channel, having an oblong section, the inlet channel intersecting an outlet channel at an angle about 90° with sharp corners configured at the intersecting functions; at least one die having a body and a drawer, the body of the die provided with slots wherein at least four heating elements being disposed to maintain the die at a temperature around 300 ⁰ C, the temperature being monitored by means of an operably connected _, thermocouple, the process comprising the steps of: a) inputting a billet formed of coarse-grain IF-steel through the inlet channel of the die via a first port; b) heating the die including the billet by switching on the heating elements; c) applying a force on the billet by using the piston; d) extruding the billet and allowing the billet to exit through the outlet channel causing the billet to undergo a ^* severe plastic deformation due to the shearing force generated along the plane of intersection of the two channels; e) applying a lubricant at the interface of the billet and the tooling to reduce frictional effects; and f) repeating the steps (b) to (e) at least upto four passes to complete the extrusion of the billet. BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS;

Fig 1 shows a cross sectional schematic view of an interstitial free steel billet.

Fig 2 shows a three dimensional view of an ECAE apparatus.

Fig 3 shows a cross-sectional view of the ECAE apparatus.

Figs 4A and 4B shows the routes followed for extrusion of the billet in the ECAE apparatus.

Figs 5A to 5C shows the microstructure of (a) starting material and (b) representative views of the sub-micron grain size respectively for two different routes.

Figs 6A and 6B shows the stress,- strain curves after deformation respectively through routes A and Bc.

DETAIL DESCRIPTION OF THE INVENTION;

In accordance with one aspect of this invention, is provided a process for producing ultra-fine grained Interstitial-free (IF) steel of sufficient strength and ductility suitable for making long products. The grain size and structure of a coarse grained IF steel billet is refined to an ultra-fine grain size by subjecting the coarse grained billet to repeated extrusion through an Equal Channel Angular Extrusion (ECAE) apparatus following two types of roμtes, namely A and B ₀ . The resulting ultra-fine grained IF steel has strength exceeding HSLA steel and can be used to manufacture.long products in the steel industry, requiring strength and ductility.

In accordance with a second aspect of the invention, there is provided an improved ECAE-apparatus for extruding coarse-grained IF-steel to produce ultra-fine grained IF-steel with high strength and ductility.

In this new improved version of the apparatus, an oblong section for the piston and an inlet channel where it glides, are provided to reduce the stress-concentration to its minimum. Again, the inlet channel at its lower part of an outlet channel is separated, which permitted access to various parts of the channel for maintenance as and when needed, which resulted into separation of the die into three discrete parts, namely a piston, a die, a body and a drawer. Through these design optimizations a net reduction of load requirement as compared to the conventional die has been achieved.

The invention provides a method of producing the ultr.a-fϊne grained IF steel for making long products. Fig 1 shows a cross sectional schematic view of an interstitial free steel billet (B) . Fig. 2 and Fig. 3 respectively shows a three dimensional view and a cross- sectional view of an ECAE apparatus (1) for extruding the billet (B). The apparatus (1) includes at least one die (2), and a piston (3) that are made from Inconel 718. The billet (B) enters a first channel Ci of the die (2) through an inlet (4). After heating the die (2) and the billet (B), a force is applied on the billet (B) and the material gets extruded through a second channel (C2). As the billet (B) is extruded, through an outlet (5), it experiences a shearing force along the plane of intersection of the two channels (Ci,C2) and undergoes a severe plastic deformation. The dimensions of the billet (B) remain largely unaffected after extrusions since the inlet and outlet channels (Ci, C2) are almost of the same cross section. In order to reduce the frictional effects, a lubricant, molybdenum disulphide (M0S2) powder mixed with grease, is applied at the interface of the billet- tooling along the channels (Ci, C2) of the die (2).

Figs 4A and 4B each shows respectively two routes that were followed for extrusions of the billets (B) in the ECAE apparatus j for example, route A and B _c . In route A, (Fig 4A), the billet (B) is fed into the entry channel (Ci) without changing its orientation till it comes out of the exit channel. Thus, in route 'A' there is no orientation. In route Bc (Fig. 4B), the billet (B) is rotated about its longitudinal axis in a clockwise direction by 90 degrees every time the billet (B) comes out of the exit channel (C2). The billet (B) is extruded upto at least four passes following the two routes (A, B _c ).

By way of a preferred embodiment,, the IF steel used during the development of the invention, has the chemical composition as shown in table 1.

TABLE 1:

Billets (B) having a 10 mm x 10 mm square cross-section, length 100 mm and chemical composition of Table 1 were used. This starting material had a coarse grained structure with an average grain diameter of about 225 microns. The billets (B) .were coated with molybdenum disulphide lubricant and placed inside the ECAE die (2) having channels (Ci, C2) intersecting at an angle of 90 degrees; the intersecting channels (Ci, C2) had sharp corners at their junction. The die (2) was maintained at a temperature of 300 ⁰ C (573K) by means of four cylindrical stainless steel heating elements placed in the slots (not shown) provided in the die body (2) itself. The billet (B), before extrusion was heated to attain the temperature same as that of the die (2) by keeping it inside the die (2) while the temperature was monitored by means of a calibrated thermocouple (not shown). The billets (B) were then extruded at the same temperature up to four passes following routes A and B _c separately. The billets (B) were then evaluated for their properties and the results are provided hereinbelow:

Figs 5A to 5C each shows the microstructure of the staring material (Fig. 5A) and a representative view of the sub-micron grain size obtained after two passes of extrusion for route A (Fig. 5B), and route Bc (Fig. 5C). After 4 passes, the grain size reduced by three orders of magnitude, from 225 microns in the starting material to 0.2 micron. After the first pass itself, there was a drastic reduction in the grain size to about 5 microns and submicron level was achieved after 2 passes. However, after the second pass, the average grain size attained a saturation point wherein the reduction was not so significant till the end of fourth pass. This observation was found to hold good for the two routes that were followed.

Figs. 6A and 6B shows the true stress vs. true stain curves under compression of samples for all the four passes following both the route A (Fig. 6A), and route B _c (Fig. 6B). The compression tests were conducted at a constant strain rate of 1O ³ per sec at room temperature. The starting material had yield strength of about 105 MPa. After the first pass this value of the yield strength improved drastically to about 400 MPa. However the enhancement of the yield point was found to be not so marked for the subsequent two passes, although it reached a value of around 500 MPa in the case of route A and around 550 MPa for route B _c at the end of four passes. Thus, we recorded an approximate five-fold increase in the yield strength of the interstitial-free steel that was extruded till four passes. The variation of yield strength as a function of various passes and routes is documented in Table 2.

TABLE 2: